<%BANNER%>

Photophysics of Platinum and Iridium Organometallic Materials

Permanent Link: http://ufdc.ufl.edu/UFE0021728/00001

Material Information

Title: Photophysics of Platinum and Iridium Organometallic Materials From Molecular Wires to Nonlinear Optics
Physical Description: 1 online resource (140 p.)
Language: english
Creator: Farley, Richard T
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: My research presents the results of an investigation into the photophysical properties of organometallic materials that contain platinum and iridium. Characterizing the excited-state properties of these types of compounds is necessary for a complete understanding of the materials which may have application in opto-electronics, as molecular wires, or as nonlinear optical materials, as well as help satisfy the need to gain insight into the fundamental nature of triplet excited states of conjugated organic systems. First, in order to partially characterize the excited states of these materials, a transient absorption apparatus was designed and constructed. Second, a series of platinum-containing molecules with two platinum atoms and different end groups were studied to examine interrupted conjugation through metal centers. Third, long carbon chains end-capped with platinum atoms were analyzed to determine the relationship between chain length and nonradiative decay rates. Lastly, an iridium complex with a known two-photon-absorbing ligand was studied to determine if excited-state absorption could be achieved through an initial multi-photon absorption process, and consequently exhibit nonlinear optical properties. The goal of this work was to further the understanding of triplet excited states in organometallic systems. The most important conclusions of this study are as follows: (i) the triplet excited states of small dinuclear platinum acetylides are more localized than the singlet excited states, and have disrupted conjugation through the platinum atom; (ii) long carbon chains display correlation to the energy gap law; (iii) the triplet excited state can be populated through two-photon absorption, which leads to nonlinear optical effects through enhanced dual-mechanism absorption.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Richard T Farley.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Schanze, Kirk S.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021728:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021728/00001

Material Information

Title: Photophysics of Platinum and Iridium Organometallic Materials From Molecular Wires to Nonlinear Optics
Physical Description: 1 online resource (140 p.)
Language: english
Creator: Farley, Richard T
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: My research presents the results of an investigation into the photophysical properties of organometallic materials that contain platinum and iridium. Characterizing the excited-state properties of these types of compounds is necessary for a complete understanding of the materials which may have application in opto-electronics, as molecular wires, or as nonlinear optical materials, as well as help satisfy the need to gain insight into the fundamental nature of triplet excited states of conjugated organic systems. First, in order to partially characterize the excited states of these materials, a transient absorption apparatus was designed and constructed. Second, a series of platinum-containing molecules with two platinum atoms and different end groups were studied to examine interrupted conjugation through metal centers. Third, long carbon chains end-capped with platinum atoms were analyzed to determine the relationship between chain length and nonradiative decay rates. Lastly, an iridium complex with a known two-photon-absorbing ligand was studied to determine if excited-state absorption could be achieved through an initial multi-photon absorption process, and consequently exhibit nonlinear optical properties. The goal of this work was to further the understanding of triplet excited states in organometallic systems. The most important conclusions of this study are as follows: (i) the triplet excited states of small dinuclear platinum acetylides are more localized than the singlet excited states, and have disrupted conjugation through the platinum atom; (ii) long carbon chains display correlation to the energy gap law; (iii) the triplet excited state can be populated through two-photon absorption, which leads to nonlinear optical effects through enhanced dual-mechanism absorption.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Richard T Farley.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Schanze, Kirk S.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021728:00001


This item has the following downloads:


Full Text





PHOTOPHYSICS OF PLATINUM AND IRIDIUM ORGANOMETALLIC MATERIALS:
FROM MOLECULAR WIRES TO NONLINEAR OPTICS
























By

RICHARD T. FARLEY


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2007

































2007 Richard T. Farley



































To my sister









ACKNOWLEDGMENTS

I would like to express gratitude to my advisor, Prof Kirk S. Schanze, whose support and

unending patience have allowed me to complete my studies. His scope of knowledge on

scientific material and laboratory equipment remain impressive. The thorough explanations and

advice he has given in the lab have led me to work on things I never imagined as a chemistry

graduate student. I would also like to thank my committee members, Dr John Eyler, Dr. Valeria

Kleiman, Dr. Nicolo Omenetto, and Dr. Elliott Douglas, for their time and support.

This work could not have been completed without the many collaborators with whom I

have interacted: Ranjani Narayanan and Dr. Stephen Hagen for their stimulating discussions

regarding the instrumentation discussed in Chapter 2; Thomas Cooper and the Air Force

Research Laboratory for providing the compounds studied in Chapter 3; Qinglin Zheng and Dr.

John Gladysz for their collaboration and providing the materials studied in Chapter 4; and Dr.

Kye-Young Kim of the Schanze group for the synthesis and characterization of the compound

studied in Chapter 5. Much of our custom lab equipment would not exist without the hard work

and creativity of Joe Shalosky, Brian Smith and Todd Prox of the UF Chemistry Machine Shop.

I would also like to thank Mike Sytsma and Benjamin Walker for their programming help in the

laser lab.

I thank of all the present and former members of the Schanze research group for accepting

the oddball physical chemist as (almost) one of their own. The have been great friends and

wonderful scientists. I appreciate the guidance of Dr. Ksenija Glusac-Haskins and Dr. Thomas

Cardolaccia who patiently introduced me to the laboratory and photophysical measurements

during my first year in the group. I hold dear the friendships I have made in the group,

especially with Dr. Kye-Young Kim. Her stories, support and sharing of secrets have helped

make being in the lab much more than just research. I need to thank the various members with









whom I have shared desk space throughout the years, and with whom I've become close friends:

Dr Xiaoming Zhao, Amir Faraji, and Dr. Katsu Ogawa. Amir was always there to offer a bizarre

yet intriguing opinion on the matter at hand. Special thanks go to Katsu; his unfortunate close

proximity in the lab put him at the receiving end of unending questions, whether technical,

chemical or metaphysical.

Survival would not have been possible without the support, knowledge and friendship of

Lori Clark in the Graduate Office. Her role in the administrative process from recruiting to

graduation is essential to the department, but it is her friendship and support outside of work that

have made her invaluable to me. I would like to thank her and Dr. Ben Smith for the opportunity

to help with recruiting. Hopefully they realize it was more fun than it was work.

I would also like to thank my many friends who have supported and encouraged me

throughout the last five years. Rob and Lindsay have been there from the beginning, through

classes and seminars, qualifiers and research, suffering and celebrating together. I value the

existence of MUSCO and all my friends there who have kept me sane.

My family has offered unconditional support through all my endeavors, for which I am

most grateful. Each member has shared his or her own style of encouragement, love and

understanding during my years as a graduate student. They are my inspiration to succeed in all

that I do.










TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ..............................................................................................................4

L IST O F T A B L E S .............................................................................................

LIST O F FIG U RE S ................................................................. 9

ABSTRAC T ................................................. ............... 12

CHAPTER

1 IN TR O D U C T IO N ................................................................................ 14

Wave-Particle Nature of Photons and Electrons ............ ............................14
A b so rp tio n o f L ig h t ......................................................................................................15
N nature of the E excited State ............................................................................. 17
R adiativ e D ecay ................................................................................ 18
Radiationless Transitions............................................ 19
Triplet Excited States in Conjugated Organic Systems ................................................... 25
M molecular W ires .................................................... ... ....... ................25
N onlinear Optics and Platinum Acetylides ........................................ ............... 27
O bjectiv e of P resent Study ............................................................................................... 3 5

2 TRANSIENT ABSORPTION SPECTROSCOPY AND INSTRUMENTATION ................37

B a ck g ro u n d .................................................................. .............................................3 7
Techniques ............................................... .. ...... ............................ 38
A p p aratu s ................... ...................4...................2..........
H a rd w a re .........................................................................................................................4 2
D ata Collection .................................................................... .. ............ 46
D ata P ro c e ssin g ............................................................................................................... 4 8
S ou rces of N oise ......................................................................................... 52
B baseline M easurem ents .................. .................. ............................. ....... 54
R esu lts ...........................................................................................54
Lim stations of Current System ............. ..................... ..................... ...............57

3 DELOCALIZATION IN EXCITED STATES OF DINUCLEAR PLATINUM
A C E TY L ID E S ............................ ............................. .........................................61

Introduction ......... ......... ................ .... .................61
E x p erim en tal .............................................................................................6 5
Results ........... ......... ......... ....................................67
D iscu ssio n .....................................................................................................7 5
C o n clu sio n ......... ..... .................................................. ...........................7 9



6









4 PHOTOPHYSICS OF DIPLATINUM POLYYNEDIYL OLIOGMERS: CHAIN
LENGTH DEPENDENCE ON THE TRIPLET STATE IN SP CARBON CHAINS ...........80

In tro d u ctio n ................... ...................8...................0..........
E x p erim en tal ................... ................... ...................2..........
R e su lts ................................................................................................8 4
U V -V is A bsorption Spectra .................................................. .............................. 84
Steady-State Photoluminescence Spectroscopy ......................... ......................... ....85
Phosphorescence Decay Kinetics: Radiative and Non-Radiative Decay Rates ..............91
Transient Absorption Spectroscopy: Triplet-Triplet Absorption of the Oligoynes........94
D isc u ssio n ............... .... ...... ...... ............ ................. ............... ................ 9 6
Variation of Triplet Energy with Carbon Chain Length ...........................................96
Energy Gap Law Correlation for the Oligoynes................................... ............... 99
Excited State Decay above the Solvent Glass Point.................... ........................... 105
Sum m ary and C onclu sion ............................................................................ ................... 105

5 AN IRIDIUM COMPLEX THAT EXHIBITS DUAL-MECHANISM NONLINEAR
AB SORPTION ............................... ................. ................. .......... 107

In tro d u ctio n ..........................................................................................................1 0 7
E x p erim en tal ...........................................................................................1 13
R e su lts .......................................................................................................... 1 1 5
D iscu ssio n ............................................................................................ 12 1
C o n clu sio n ........................................................................................... 12 3

6 CONCLUSIONS AND FUTURE WORK ..................................124

L IST O F R E FE R E N C E S ............. .. ............. .................................................................... 128

BIOGRAPHICAL SKETCH ........................................................................... ......... ..................140









LIST OF TABLES


Table page

3-1 Summary of spectroscopic data of dinuclear platinum-acetylide oligomers ...................67

4-1 Emission spectral fitting parameters for PtCnPt at 100 K...........................................90

4-2 Photophysical parameters for PtCnPt complexes.............................................................94









LIST OF FIGURES


Figure p e

1-1 Potential energy curves for electronic transitions................................ ................16

1-2 Effects of energy difference and nuclear distortion on vibrational overlap.....................20

1-3 Jabloski diagram illustrating possible transitions. .................................. .................23

1-4 Structures of molecules studied by Davis et al. ...............................................26

1-5 Conditions for reverse saturable absorption. ........................................ ............... 29

1-6 Structure of platinum-ethynyl and corresponding photophysical data............................30

1-7 Jabloski diagram illustrating possible transitions for platinum-ethynyl..........................32

1-8 Structures of platinum acetylide oligomers studied by Cooper, Rogers, et al ................... 33

2-1 Transient absorption apparatus developed by Lindqvist. .............................................38

2-2 CCD intensifier. .................................... ... .. ............ ......... .... 41

2-3 The transient absorption apparatus. ..............................................................................43

2-4 Spectrum of xenon lamp used as the probe. ........................................................ 44

2-5 Illustration of timing parameters for ICCD data collection...................... ................47

2-6 Change in intensity of transmitted light during the lifetime of a transient species............49

2-7 Construction of a time trace and transient absorption spectra from gated
m ultiw avelength m easurem ents ........... ................. ................................. ............... 51

2-8 Relative probe intensities for Io and I with the pump beam off. ....................................53

2-9 B baseline noise of the apparatus .......................... ................. ................... ............... 55

2-10 Transient absorption spectrum of 1.2x10-4 M anthracene solution in deoxygenated
benzene. .........................................................................56

2-11 Transient absorption spectrum of 1.4x10-4 M aqueous solution of Ru(bpy)32+.................57

3-1 Platinum-acetylide polymer and model compound studied by Beljonne et al. ................62

3-2 Platinum-acetylide oligomers studied by Liu et al. ................................. ...............63

3-3 Platinum acetylide oligomers studied by Glusac-Haskins et al ............... .... .......... 63









3-4 Platinum-acetylide oligomers studied by Cooper, Rogers, et al................. ........... 64

3-5 D nuclear platinum -acetylide oligom ers....................................... ......................... 66

3-6 Absorbance spectra of dinuclear platinum-acetylide oligomers collected at room
tem perature in T H F solution ...................................................................... ..................68

3-7 Transient absorption spectra of dinuclear platinum-acetylides. ......................................69

3-8 Phosphorescence spectra of dinuclear platinum-acetylide oligomers. ...........................71

3-9 Variable-temperature emission spectra of P-PE2 in MTHF. .........................................72

3-10 Excitation spectra of dinuclear platinum-acetylide oligomers .......................................73

3-11 Variable-temperature excitation spectra of P-PE2 in MTHF........................... .........74

4-1 Structures of the P tC nP t series. ............................................................... .....................81

4-2 Absorption spectra of PtCnPt complexes in THF solution. ....................... .............85

4-3 Photoluminescence spectra of PtCnPt complexes in 2-MTHF solvent glass at 80 K ......86

4-4 Emission spectra of PtCsPt in MTHF solvent (glass) over 77 298 K temperature
ran g e ......................................... .. ........ ........................ ................. 8 8

4-5 Overlay of absorption, emission excitation and emission spectra (red lines) for
PtCnPt complexes in MTHF solvent glass at 100 K..................... .............................. 89

4-6 Fit of emission spectrum of PtC6Pt using Equation 4-1 and parameters listed in
T able 4-1 ........................................................................................ 9 1

4-7 Temperature dependence of photoluminescence decay lifetimes for PtCnPt
com plexes in M TH F solution (glass)........................................... .......................... 92

4-8 Transient absorption spectra of PtCnPt complexes following pulsed excitation with a
355 nm pulse (10 ns, 5 m J-pulse 1). ........................................... ............................. 95

4-9 Correlation of E00 (triplet energy) vs. 1/n for 7t-conjugated oligomers, where n = #
carbons in the chain. ..................................... .. .. ........ .. ............97

4-10 Energy gap law correlation. ..................................................................... .................. 102

4-11 Plot of Huang-Rhys parameter (Sm) vs. the triplet energy (Eoo) for PtCnPt complexes
and for series of ruthenium polypyridyl complexes and plot of calculated Franck-
Condon factors vs. natural log of non-radiative rate for PtCnPt complexes.................104

5-1 Jabloiski diagram and corresponding representative absorption spectra illustrating
tw o-photon absorption. ........................ ........ .. .. ..... ............... 108









5-2 Jablonski diagram for a four-level system illustrating TPA with ESA. ..........................111

5-3 Structural motifs for two-photon absorbers. .......................... .....................112

5-4 Structures of the ligand, L, and iridium(III) complex, ML......................................113

5-5 Apparatus for measuring nonlinear transmittance. ... .... ............... ........................... 115

5-6 Absorption of ligand L and complex ML and one-photon emission of ligand L and
com plex M L ............. .......................................... ............. ...........116

5-7 Transient absorption of ML in deoxygenated THF solution obtained at 400 ns
increments following 355 nm excitation ..................... ................ 117

5-8 Two-photon induced emission of ML after pulsed excitation by 1064 nm. ................. 118

5-9 Photograph of 5 m M M L solution .......... ............................................... ............... 19

5-10 Emission area dependence of ML on incident laser energy at 1064 nm........................119

5-11 Transient absorption spectra of 5 mM deoxygenated THF solution of ML following
1064 nm pulsed excitation. ..................................................................... ...................120

5-12 Transmittance of 1064 nm pulsed beam at various concentrations of ML in THF.........121

6-1 Variations of molecular wires synthesized by Stahl, Owen et al. ..................................127









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

PHOTOPHYSICS OF PLATINUM AND IRIDIUM ORGANOMETALLIC MATERIALS:
FROM MOLECULAR WIRES TO NONLINEAR OPTICS

By

Richard T. Farley

December 2007

Chair: Kirk S. Schanze
Major: Chemistry

My research presents the results of an investigation into the photophysical properties of

organometallic materials that contain platinum and iridium. Characterizing the excited-state

properties of these types of compounds is necessary for a complete understanding of the

materials which may have application in opto-electronics, as molecular wires, or as nonlinear

optical materials, as well as help satisfy the need to gain insight into the fundamental nature of

triplet excited states of conjugated organic systems.

First, in order to partially characterize the excited states of these materials, a transient

absorption apparatus was designed and constructed. Second, a series of platinum-containing

molecules with two platinum atoms and different end groups were studied to examine interrupted

conjugation through metal centers. Third, long carbon chains end-capped with platinum atoms

were analyzed to determine the relationship between chain length and nonradiative decay rates.

Lastly, an iridium complex with a known two-photon-absorbing ligand was studied to determine

if excited-state absorption could be achieved through an initial multi-photon absorption process,

and consequently exhibit nonlinear optical properties. The goal of this work was to further the

understanding of triplet excited states in organometallic systems.









The most important conclusions of this study are as follows: (i) the triplet excited states of

small dinuclear platinum acetylides are more localized than the singlet excited states, and have

disrupted conjugation through the platinum atom; (ii) long carbon chains display correlation to

the energy gap law; (iii) the triplet excited state can be populated through two-photon absorption,

which leads to nonlinear optical effects through enhanced dual-mechanism absorption.









CHAPTER 1
INTRODUCTION

Wave-Particle Nature of Photons and Electrons

The interaction of light with matter is a topic that has caused interest and debate since the

early 1600s. Our understanding of the atomic world has developed almost concurrently with the

understanding of light. Light was theorized to be a wave by Huygens in 1678, although this

work was largely overshadowed by Newton's corpuscular theory that light consisted of particles.

In the early 1800s, Young and Fresnel completed the double-slit experiments that supported

Huygens' theory by showing interference patterns from light. In 1860, Maxwell described light

with a series of equations. Using the existing laws of magnetism and electricity developed by

Coulomb, Farady, Ampere, and Gauss, Maxwell's theory of electromagnetism defines an

electromagnetic wave that should propagate at a speed of 3x108 m/s. At this point, Newton's

laws of classical mechanics were still the basis for the atomic structures proposed by Dalton,

Thomson and Rutherford.

At the end of the 19th century, it was widely accepted that electrons were particles, and

light was a wave. Planck's blackbody experiment drastically changed these views at the

beginning of the 20th century. His findings concluded that blackbody radiation is limited to

discrete values; that is, the energy is quantized. Newtonian physics could not explain this

phenomenon. Soon after the blackbody experiment was completed, Einstein discovered the

photoelectric effect, crushing the idea that light is merely a wave. Although it is now accepted

that light exhibits a dual wave-particle nature, Maxwell's wave equations generally explain most

light-related phenomena.

Because of the wave-particle duality of light, scientists soon postulated that electrons could

also exhibit the same duality. The idea that all particles, not just electrons, act as waves was









theorized by de Broglie in 1924. Research done independently by Thomson and by Germer and

Davisson showed electron diffraction patterns and confirmed the de Broglie hypothesis for

electrons. In 1928, Schrodinger formulated mathematical equations to describe the wave nature

of electrons in atomic structure, similar to the classical treatment of sound and light waves. This

development of quantum theory has dramatically impacted our understanding of atomic and

molecular structure.

Absorption of Light

The interaction of light with matter has given us considerable insight into electronic

molecular structure. The electronic energy levels of a molecule are discrete, and can be

measured through their interactions with photons. The energy of a photon is related to the

frequency of its electromagnetic wave by the following equation:

he
E=hv=h- (1-1)
A

where E is the energy of the photon, h is Planck's constant, vis the frequency, c is the speed of

light, and A is the wavelength. When a molecule absorbs light (the energy of the photon), an

electron is promoted to a higher energy level. The difference in energy levels is equal to the

energy of the photon absorbed. A quantitative description of the amount of light absorbed for a

certain wavelength is given by Equation 1-2.

A = ebC (1-2)

where A is the absorbance, sis the molar absorptivity (units of M1 cm-1), b is the pathlength of

absorption and C is the molar concentration of the absorbing species. The molar absorptivity is a

measure of the probability that a transition will occur and is proportional to the transition dipole

moment between the initial and final states. A related term called the absorption cross section,










a, represents the area around a molecule which is accessible to collide with a photon,1 and is

related to eby Equation 1-3.


a= [(0.1)1n10] -=3.825* 1025e (1-3)
NA

where cohas units of m2 and NA is Avogadro's number.

Although the electronic energy levels of a molecule are discrete, absorption in molecules

does not appear as a sharp line, as is the case in atomic absorption. Rather, the absorption

appears as a broader band. The explanation lies in the fact that electronic excitation is usually

accompanied by vibrational transitions. The absorption of light takes place faster (10-15 s) than

the equilibration of nuclei (10-13 s).1 This is referred to as the Franck-Condon principle (Figure

1-1). To illustrate this phenomenon, the ground and excited electronic states are represented by




A B
6 6
5 5
4 4



















similar equilibrium geometry. (B) Transition between states of different equilibrium
S Egeometry.
2 1 2 1


v =0 V, = 0 =










Nuclear Coordinates Nuclear Coordinates

Figure 1-1. Potential energy curves for electronic transitions. (A) Transition between states of
similar equilibrium geometry. (B) Transition between states of different equilibrium
geometry.









potential energy curves as a function of their relative equilibrium geometry. The equilibrium

geometry for a particular vibrational mode (or combination of modes) is where the potential is at

a minimum. The most probable electronic transition occurs in a "vertical" manner, suggesting

that when an electron is excited, it is promoted to the higher state regardless of the equilibrium

geometry. That is, if the nuclear arrangement between the equilibrium ground state and the

equilibrium excited state are very different, excitation will promote the molecule to a higher

vibrational mode of the excited state, and then the molecule will relax to the lowest vibrational

excited state. The differences in energy between the lowest vibrational ground state (v = 0) and

some vibrational excited state (v' > 0) will always be larger than the 0-0 transition.

Nature of the Excited State

Following absorption and population of the electronic excited state, the molecule will first

relax to the lowest vibrational level (v = 0) through thermal (loss of heat) or collisional (with

other molecules, either solvent or solute) relaxation. For a molecule whose initial ground state

has paired electrons, the electrons will have opposite spin. This is called a singlet state. Upon

excitation, the electron does not change its spin due to spin restrictions effected by quantum

mechanics. The excited state therefore is also a singlet state. Although the process is forbidden,

certain conditions allow for the excited electron spin to flip, resulting in a triplet excited state.

This process is called intersystem crossing (ISC). Coupling of singlet and triplet vibrational

levels of the same energy is required to allow ISC to occur. As in population of the singlet

excited state, population the triplet excited state also involves vibrational relaxation to the lowest

vibrational level.

An important characteristic of excited states is that the lowest singlet excited state is

always higher in energy than the lowest triplet excited state. For singlet excited states, the









electrons are of opposite spin, and therefore can exist in the same region of space, as allowed by

quantum mechanics. In the triplet state, the electrons have the same spin, which prohibits them

from occupying the same region of space. With a larger distance between electrons in the triplet

state, there is less Coulombic repulsion energy than in the singlet state where electrons can be

near each other.

There are several pathways which an excited state can follow once it is populated. One

deactivation pathway is radiative decay by emission of a photon. Radiative decay can occur

from the singlet or triplet excited state. Nonradiative decay is another process for an excited

state to return to the ground state in which the energy is lost through dissipation of heat. In

addition, the excited state could return to the ground state by transferring energy or an electron to

another species.

Radiative Decay

Radiative decay involves the excited state emitting a photon and returning to the ground

state. The energy of the emitted photon corresponds to the difference in energy between the two

states. Emission from a singlet excited state to a (singlet) ground state is called fluorescence.

Because the transition is between states of the same multiplicity, it is an allowed transition. The

radiative rate for fluorescence is relatively large (kF -109 s-1). If the emission of a photon is from

a triplet excited state, the process is called phosphorescence. Because the transition involves

states of different multiplicity, the rate of emission is much lower (kp 102 104 S-1) than that of

fluorescence. As stated earlier, the triplet state is lower in energy than the singlet state, and as a

result, phosphorescence will occur at a lower energy than fluorescence.

Photons emitted by the excited state via fluorescence have a slightly lower energy than the

photons absorbed by the ground state. This is due to loss of absorbed energy from vibrational

relaxation in the excited state. The difference in energy between the absorbed photon and the









emitted photon is known as the Stokes shift. The amount of the Stokes shift for a molecule is a

representation of the degree of structural (nuclear) difference between the excited and ground

electronic states. That is, if the distortion for the excited state is large, the observed Stokes shift

will be large.

As is the case with absorption spectra, emission spectra also do not appear as sharp lines.

The "vertical" transitions as described in the Franck-Condon principle illustrate the most

probable transitions from one electronic state to another, even if their equilibrium nuclear

configurations are quite varied. The transition between electronic states takes longer than the

rearrangement time of the nuclei of the molecule. Because vibrational relaxation occurs very

rapidly, emission originates from the v = 0 vibrational level of the excited state. In contrast to

absorption, the transitions originating from vibrational energy levels will be lower in energy for

transitions from a higher electronic state to a lower electronic state.

Radiationless Transitions

All transitions between electronic states that do not involve absorption or emission of

photons are considered radiationless transitions.1 These processes, such as energy loss through

release of heat or through collisions, are referred to as internal conversion (IC) for transitions

between states of the same spin. Nonradiative transitions occur only when the potential energy

curves of two electronic states overlap (Figure 1-2). The shift will take place where the curves

intersect, and produce a vibrationally excited ground electronic state. Vibrational deactivation

through thermal or collisional loss will bring the molecule to the relaxed ground state at v = 0.

Figure 1-2 shows the potential energy curves as a function of nuclear distance. As illustrated,

there exists an intersection between the two states depending on the energy difference between

the two states and also the nuclear equilibrium configuration differences between the two states.










A B C






















Nuclear Coordinates

Figure 1-2. Effects of energy difference and nuclear distortion on vibrational overlap. (A) Small
AE, large overlap; (B) Large AE, small distortion, low overlap; (C) Large AE, large
distortion, large overlap.



If there is a large energy difference between the two electronic states, their potential energy

curves will not intersect, and the so-called Franck-Condon overlap will be small. A large

overlap will bring about a fast and efficient radiationless transition of the excited state. The

overlap of the two curves increases exponentially as the difference in energy levels between the

excited state and ground state decreases. Therefore, the decrease in energy difference facilitates

faster nonradiative decay between the two states.2 This effect is known as the energy gap law.

Additionally, for two systems that have the same difference in energy between the excited and









ground states, the overlap will be greatest for the system that has a higher degree of nuclear

distortion between the states.

A radiationless transition that occurs between states of different spin is called intersystem

crossing (ISC). The transition follows the Franck-Condon principle as described above where

the overlap of the two electronic potential energy surfaces is required for vibrational coupling.

For ISC from the excited singlet state to the (lower energy) excited triplet state, the triplet state

will initially be in a vibrationally excited state, and the transition will be followed by rapid

internal conversion to the lowest vibrational level of the triplet excited electronic state. The

transition from a singlet state to a triplet state (or vice versa) is a forbidden process as defined by

selection rules because it involves states of different spin multiplicity. Consequently, the triplet

state acts as an energy reservoir because radiative decay back to the ground state is spin

forbidden.3 The spin angular momentum and orbital angular momentum of electrons in a

particular state are separate from one another. This leads to the outcome that transitions between

states of different spin are forbidden by the momentum conservation rule. This effect is more

important for molecules comprised of lighter elements. As a result, ISC yields are typically low

for organic molecules. In molecules with heavy atoms (or light molecules in the vicinity of

heavy atoms), the conditions are different. The relative orientations of spin moment and orbital

moment are less important. Rather, the spin and orbital angular moment are coupled together.3

This is known as spin-orbit coupling, and accounts for the breakdown of selection rules in

singlet-triplet transitions. The change in electron spin magnetic momentum coupled with the

corresponding orbital change provides a mean of conserving total angular momentum.1 This

heavy-atom effect is responsible for increased ISC yields.









Other types of radiationless transitions are also possible, and will be mentioned briefly.

Energy transfer from an excited state to some suitable acceptor is one type of relaxation pathway,

as described by Equation 1-3.

D* +A D +A (1-3)

where D is an energy donor, A is an energy acceptor, and denotes an excited state for either.

Exchange energy transfer (collisional) and Coulombic energy transfer (dipole-dipole interaction)

are two mechanisms for energy transfer. Both require that the energy ofD* is higher than that of

A, and the rate of energy transfer from D* to A is greater than the decay rate of D*. Similarly, an

excited state donor can transfer an electron to a suitable acceptor.

D +A D + A (1-4)

Furthermore, triplet-triplet annihilation is another type of nonradiative decay involving two

triplet excited state species:

T*+T* S *+S (1-5)

where T* and S* are the excited triplet and excited singlet, respectively, and S is the ground-state

singlet. Triplet-triplet annihilation occurs when two molecules in the triplet state interact

(generally by collision) to create an excited singlet state and a ground singlet state. If the excited

singlet state returns to the ground state through emission of a photon, it is called delayed

fluorescence.

A Jablonski diagram (Figure 1-3) will be useful to summarize the probable potential

transitions between the common electronic states in a molecule as discussed above.

Before introducing a brief review of organometallic materials and their photophysical

behavior, there are two characteristic properties that should be discussed: the photoluminescence

quantum yield and lifetimes of the excited states. Quantum yield of emission is simply the ratio









I v=3
I IC v=2
1C v12
S1 v=1
-I I v=


So


ISC
'- .


v=3
v=2
v=1
v=l
v=O


~E I U


Figure 1-3. Jabloski diagram illustrating possible transitions. So: Ground singlet state; S1:
Excited singlet state; Ti: Excited triplet state; A: Absorption; F: Fluorescence; P:
Phosphorescence; IC: Internal conversion; ISC: Intersystem crossing.



of absorbed photons to emitted photons. A general expression for quantum yield1 is given by

Equation 1-6:

e = .k r (1-6)

where 4 e is the emission quantum yield, 4. is the formation efficiency of the emitting state, kj

is the rate constant for emission, and ris the measured experimental lifetime. The lifetime is

defined by Equation 1-7:


(1-7)


- + k,


where 2k, is the sum of all other deactivating rate constants. More specifically, we can define

the two terms 4 and zfor fluorescence and phosphorescence. For these values, 4, is assumed to


- IC
^____









be unity. For the processes indicated in Figure 1-3, the fluorescence quantum yield, 4F, is

described by Equation 1-8:

kF = kF S

where Ts is the lifetime of the singlet state, and is defined in Equation 1-9:

Ts kF + kis + kn, (1-9)

In Equation 1-9, kF is the rate of fluorescence, k1sc is the rate of intersystem crossing (to the first

excited triplet state), and kn is the rate of all other nonradiative processes from the singlet state.

Similarly, and assuming the triplet excited state is populated only through excitation of the

ground state to form the singlet excited state, followed by ISC, the phosphorescence quantum

yield can be taken as Equation 1-10:

kP ISCkprT (1-10)

where jisc is the intersystem crossing quantum yield, and T, the triplet lifetime, is defined in

Equation 1-11:

1
rT (1-11)
kp + k,

where kp is the rate of phosphorescence and knr is the rate of all other nonradiative processes

from the triplet excited state. Fluorescence lifetimes are generally short (1011 10-s s) because

they correspond to allowed transitions. In contrast, phosphorescence occurs on a much longer

timescale (10-6 101 s) due the forbidden nature of the triplet-to-singlet transition. As mentioned

earlier, heavy atoms may help facilitate intersystem crossing to populate the triplet excited state.

However, the heavy atom effect may also bring out spin-orbit coupling in the radiative decay

process, shortening the phosphorescence lifetime.









Triplet Excited States in Conjugated Organic Systems

Although slow ISC prevents detection of triplet states in organic materials, triplet excited

state properties are nevertheless important in these systems. The increasing use of organic

materials in organic light emitting diodes, field-effect transistors, electronic semiconducting

materials, light harvesters, photosensitizing and nonlinear applications makes the triplet state an

interesting research area.4-8

Molecular Wires

The concept of an organic molecular electronics device was first proposed over thirty years

ago by Aviram and Ratner.9 However, it was not until the measurement of electrical conduction

through a single benzene-1,4-dithiol molecule by Reed and coworkers10 in 1997 that molecular-

scale electronics attracted serious attention. Since then, the study of prospective molecular wires

for application in electronics has grown tremendously.11-13 Conjugated organic rigid-rod type

molecules offer potential for such use due to their delocalized nt structure which allows electronic

communication throughout the molecule.

Poly-(para-phenylenevinylene) (PPV) oligomers are a class of molecules that afford

electron delocalization through nT-conjugation. Davis and coworkers14'15 have used nT-conjugated

linkers as a bridge between electron donor and electron acceptor molecules to investigate charge

separation. These molecules, consisting of a tetracene donor and pyromellitimide acceptor

moieties connected by phenyl, stilbene, and PPV oligomers of varying length, are illustrated in

Figure 1-4. The usefulness of the bridge is to act as an electronic connector between the end-

group molecules under study. The understanding of the electronic properties of the bridge linker

is important to determine the mechanism of charge transfer between the donor and acceptor end-













BRIDGE


/ / B



R-O
R=2-ethylhexyl /


Figure 1-4. Structures of molecules studied by Davis et al.15 Linkers include (A) phenyl, (B)
trans-stilbene, and (C) an oligo-PPV.



groups. For example, in the short linkers, charge transfer was achieved through tunneling (due to

a high energy barrier in the bridge), whereas in the longer PPV chains, there existed larger

delocalization (i.e. a lower energy barrier) in order to help facilitate the movement of charge.14'15

The work completed by Davis and coworkers has encouraged the use of computational methods,

such as density functional theory, to further study the electronic structure of PPV systems as 7t-

bridges for molecular wire-like applications.16

Among the most versatile active components for establishing electronic communication

are transition metal centers17-19 and unsaturated organic systems have proven to be among the

most useful bridging units.20'21 Also of interest are polymers which contain metal atoms in the

backbone.22'23 The incorporation of heavy atoms, particularly metals, into organic structures

changes several other characteristics, including redox, optical and electronic properties. As

mentioned earlier, introducing heavy atoms into organic molecules induces spin-orbit coupling


C8H17









of the excited state. From a synthetic standpoint, the alteration of structure through ligands,

spacer units, and types of metals used can systematically vary the singlet and triplet energy

states.

Polyynes, e.g. -(C-C)n- oligomers or polymers, are distinctive in that they provide one of

the simplest possible linear 7t-conjugated organic units of variable length. With the addition of

metal atoms, such as platinum, to the ends of highly conjugated organic chains, the metal centers

not only terminate the extended 7t system, but become integral parts of them. Furthermore, metal

atoms as end-caps increase the stability of long carbon chains.24-26 In such a,co-dimetalla-7T-

systems the communication of electronic information between the metal centers is expected to be

particularly efficient. Additionally, luminescence properties become of interest.23'27 The spin-

orbit coupling increases population of the triplet state, therefore giving access to the triplet

manifold and allowing for further investigation of the electronic state energies.

Nonlinear Optics and Platinum Acetylides

Due to the advancement of laser technology in a variety of applications, the need for

protection from intentional or erratic laser pulses to prevent damage of human eyes, optical

sensors, and other sensitive optical components has become important to a number of fields. An

optical power limiter is a device that will allow the transmittance of light at low intensities, but

will strongly attenuate the incident energy at high intensities.28'29 It is also desirable for a

material which exhibits optical power limiting behavior to have a fast response time, reasonable

processability, good linear transparency, and a high damage threshold.29-31 Many materials

currently used to protect eyes or sensors from damaging light absorb strongly at only particular

wavelengths, restricting use to certain wavelengths while also limiting visibility at low

incidences.









Due to their high optical nonlinearities and fast response time, organic materials are

attractive for use as optical limiters. More specifically, metallophthalocyanines and

metalloporphyrins display excellent nonlinear optical behavior due to large ISC efficiencies from

spin-orbit coupling of the heavy atoms which populate the highly absorbing triplet excited

states.32'33 Unfortunately, poor solubility has led to difficulties in processing and hindering the

development of practical devices.

Reverse saturable absorption (RSA) is one mechanism by which nonlinear absorption can

occur. RSA is a relatively slow, sequential multiphoton process in which the ground electronic

state first absorbs a photon, and then an excited state absorbs a second photon. For RSA to

occur, the absorption cross section of the excited state must be larger than the absorption cross

section of the ground state (Figure 1-5). In transition metal-containing organic compounds, the

absorbing excited state is usually the triplet state, populated through ISC facilitated by the heavy-

atom effect. Transient absorption (TA) spectroscopy is a convenient way to probe the excited-

state absorption properties (a more detailed discussion about TA spectroscopy will be given in

Chapter 2).

Platinum-acetylide complexes have been shown to have highly absorbing triplet states with

long lifetimes.34-41 The large ISC yields afforded by the platinum atom create strongly

phosphorescent triplets, making these molecules excellent candidates for studying triplet-state

properties. Structural changes in conjugation length of the organic moieties allows for spectral

tunability in absorption and emission to create optical bandgaps in the UV, visible and near-IR.

McKay and coworkers3439 were the first to report on the nonlinear optical properties of a

platinum-acetylide compound and have since thoroughly investigated the optical properties of a

platinum-ethynyl complex (shown in Figure 1-6). The compound exhibits strong singlet









(es > gsTn

0ISC
S1 IS -es
-----T1
T 1
gs

So



t! Linear
c absorbance -

RSA




Input Intensity

Figure 1-5. Conditions for reverse saturable absorption.


absorption with the maximum at 355 nm. (Interestingly, the absorption maximum makes the
molecule convenient for optical excitation by the third harmonic of a Nd:YAG laser.) There is
emission from the compound at 410 nm, which is attributed to fluorescence, and another peak at
520 attributed to phosphorescence. The peaks were assigned mostly due to the short (330 ps)

and long (0.5 ts) lifetimes, respectively. At low temperatures, the phosphorescence lifetime

increases, up to 700 ts at 17 K, while the fluorescence lifetime is unchanged. The intensity of

phosphorescence emission increased as well at low temperatures, unlike the fluorescence. The









PBu3


PBu3


g- 2000-
i08M go





m) 400 450 500 550 600 650
2x14M G

3W 4W W 0 0
X(In) 400 450 500 550 600 650
Wavelength (nm)

Figure 1-6. Structure of platinum-ethynyl and corresponding photophysical data. Figures
adopted from McKay et al.36



unvarying fluorescence intensity and decay rate over a range of temperatures show that ISC

dominates the singlet state decay path and is independent of temperature, indicating that ISC

efficiency approaches unity even at room temperature. Excited-state absorption measurements

reveal that platinum-ethynyl demonstrates strong RSA over some wavelengths. For example, at

513 nm, the excited-state absorption cross section is 104 times larger than the ground state

absorption.

Further study of platinum-ethynyl by Staromlynska and coworkers35 leads to observation

of an alternate absorption pathway. Emission from the compound was detected when excited

where the compound has effectively no ground-state absorption at 595 nm, by either a 45 ps

pulse or 2 ns pulse. Emission spectra caused by excitation at 595 nm are identical to those

caused by excitation at 355 nm, indicating that the same state (the triplet excited state) is arrived

via two different absorption pathways: one photon absorption and two photon absorption (TPA),









each to the singlet excited state, followed by fast ISC to the triplet excited state. TPA will be

discussed in more detail in Chapter 5. Briefly, two-photon absorption occurs when a material

simultaneously absorbs two photons whose combined energy equals that of the energy difference

between the absorbing state and the excited state. An additional report by the same group36

shows that TPA dominates at wavelengths between 560 and 700 nm. There is no absorbance in

this wavelength region at low incident energy. The two-photon absorption coefficient is found to

be independent of excitation wavelength. For shorter wavelengths (< 560 nm), one-photon

excitation dominates. The transmission as a function of input intensity was also measured at 595

nm for 45 ps pulses and 2 ns pulses. The results demonstrate the same nonlinear behavior,

regardless of pulse length. Slightly stronger nonlinear behavior is observed for 2-ns pulses at

455 nm.

Two additional reports by McKay, Staromlynska and coworkers38'39 on platinum-ethynyl

provided several further findings and summarized the results of their work. Three main regions

for excitation are described: (i) blue (< 500 nm), where single photon excitation to the singlet

state, followed by rapid ISC to the triplet state, (ii) green (500 540 nm), where direct excitation

of the triplet state from the ground singlet state occurs, as spin-orbit coupling aids in the

forbidden transition, and (iii) red (540 700 nm), where no one-photon process occurs between

the singlet ground and either singlet or triplet excited states, but two-photon absorption

dominates to populate the singlet excited state, followed by ISC to the triplet excited state. The

triplet excited state in all three regions is highly absorbing throughout the visible. Furthermore,

the excited state absorption cross section is independent of the excitation pathway to the excited

triplet state. Absorption from the excited singlet was not studied as no direct evidence of this







transition has been found. The Jablonski diagram in Figure 1-7 summarizes the transitions
occurring in platinum-ethynyl.


S1


ISC


Ti


-1--


So I I I
Figure 1-7. Jabloski diagram illustrating possible transitions for platinum-ethynyl. (1) Blue (<
500 nm) single photon absorption followed by ISC. (2) Green (500 540 nm) -
direct excitation to the triplet state. (3) Red (540-700 nm) Two-photon absorption
followed by ISC. All three regions include triplet-state absorption.


Platinum-ethynyl shows promise as a broadband optical limiter in the visible region that is
effective over a range of pulse lengths from picoseconds to hundreds of nanoseconds. Between
530 and 650 nm, incident energies of greater than 500 jiJ were attenuated to -6 jiJ by a 0.08 M
solution of the compound. However the authors did note some degree of decomposition at
irradiances of > 6 J/cm2 at 532 nm and 1064 nm. Also, solutions of platinum-ethynyl showed
degradation over a period of several weeks. Such behavior is not acceptable for a practical
optical limiting device, and if stabilization is not possible, alternate approaches should be found.


N N '










Although the photophysics of platinum-ethynyl have been thoroughly investigated, the

lack of a series of related molecules makes any conclusions of structure-property relationships

difficult. Cooper, Rogers, and coworkers4045 have synthesized and studied the photophysical

behavior of a series of platinum(II)-containing phenyl-ethynyl oligomers (Figure 1-8). The goal

was to determine structure-property relationships in a well-defined system through IR, UV/Vis,

photoluminescence, and transient absorption spectroscopy and kinetics of some of the

transitions.


El (Ch3)3Si




PE1


PBU3
-Pt
PBu3

PBu3
Pt
PBu3


Si(Ch3)3


PBu3
Pt
PBu3


PPE




PE2




PE3


PBu3
-Pt
PBu3
PBu3


PBu3
PPt
PBu3


Figure 1-8. Structures of platinum acetylide oligomers studied by Cooper, Rogers, et a/.40-45


The PtCC acetylenic vibrational bands for El were 2033 cm-1 compared to 2100 cm-1 for

the other compounds, suggesting that the PtC and CC bonds have more double-bond character in

El than the compounds with the larger ligands.43 UV/Vis absorption maxima systematically red-









shifted from 308 nm for El to 376 nm for PE3, indicating that the increase ofligand length

decreases the energy between the ground and excited singlet states. As with PE2 as studied by

Staromlynska, McKay et al.,35 fluorescence quantum yields were small. El and PE1 show

minimal emission. PE2 and PE3 have fluorescence maxima near 400 nm, while PPE shows

fluorescence emission at 375 nm. The higher emission energy is attributed to the twisting of

phenyl groups interrupting conjugation. The dihedral angle was calculated43 to be 400, whereas

PE2 and PE3 have free rotation around the triple bond. Room temperature phosphorescence of

PPE, PE2 and PE3 is observed, with a 0-0 band preceding unresolved vibronic bands at lower

energy, due to combinations of CC single and triple bond stretching, out-of-plane CH stretching,

etc.43 Phosphorescence maxima systematically increase with increased conjugation length.

Triplet excited state absorption observed for PE1 decayed within the length of the laser pulse (<

15 ns), however, the other compounds exhibited transient molar absorptivities that increase and

red shift with increased conjugation length. The excited state lifetimes also show a clear trend

that lifetime increases as conjugation length increases, attributed to less spin-orbit coupling

effect from platinum as the excited state is more delocalized on the ligand.40

The group further complemented their study of the platinum oligomer series by direct

comparison with a set of all-carbon butadiyne analogs44. They found that conjugation through

the platinum center occurs in the singlet state, but the triplet states are more complex. The

lowest triplet state shows metal-to-ligand charge transfer character as is likely confined to one

ligand, whereas the higher excited triplet shows ligand-to-metal charge transfer.44 The shortest

compounds are affected greatest by the presence of platinum. Fluorescence emission spectra for

the butadiyne compounds are very similar to the platinum-containing compounds, but quantum

yields decreased dramatically when platinum is incorporated into the structure. For the shorter









all-organic structures, triplet emission is very weak and blue-shifted compared to the platinum

analogs. However, phosphorescence emission bands begin to appear the same between platinum

oligomers and butadiyne as conjugation length increases.

Based on these findings, the authors have concluded several things from the sensitivity of

state energies. The transitions involving absorption by the ground singlet state and by the triplet

excited state are more sensitive to molecular size than emission from the triplet excited state to

the ground excited state. Therefore, it follows that the ground singlet and first triplet excited

states are more confined than the excited singlet and higher excited triplet states which are

delocalized over the ligand.

Objective of Present Study

Defining structure-property relationships between molecules of interest and photophysical

behavior is paramount to increased development of organic and organometallic compounds for

application in light-emitting diodes, chemical sensors, molecular electronics, photovoltaics,

optical sensors, and nonlinear optics. Platinum- and iridium-containing conjugated compounds

offer the ability to study triplet excited state behavior in a controlled and deliberate manner

through synthetic capabilities.

Fundamental aspects of characterization, such as singlet and triplet state energies and

quantum yields, are measured using traditional techniques with commercially available

spectrophotometers. In addition to commercial instrumentation, a transient absorption apparatus

was designed and constructed to examine photoexcited states of metal-containing organic

compounds. Included along with arrangement of optical components, user-friendly software was

developed to control the timing parameters of the hardware involved.

Platinum acetylide complexes are excellent systems for probing a variety of triplet-state

phenomena such as ground state absorption to the triplet state, intersystem crossing, triplet state









absorbance, and phosphorescence.45 While the consequence of the platinum atom on

delocalization of the excited states has been studied, the effect is not completely understood for a

variety of compounds. The photophysics of a series of dinuclear platinum-acetylide oligomers

with phenylene-ethynylene end-caps has been investigated to provide insight into conjugation

through platinum atoms and the effect on excited state energies.

The understanding of polyyne is crucial for the progress of molecular electronics. While

direct application of molecular wires is not the focus of this work, it has been shown that

characterization of excited-state properties of potential molecules is fundamental in

understanding the nature of these materials. Incorporation of heavy metal atoms as end-caps to

polyyne chains allows for the population, and thereby detection, of the triplet state. A series of

long carbon chains end-capped with platinum has been photophysically characterized to probe

the effect of chain length on excited state behavior.

The need for optical limiting materials is increasing as the use of lasers in a variety of

applications is becoming more popular. The nonlinear optical behavior of many organic and

organometallic materials provides promise for protection from potentially damaging laser pulses.

The incorporation of iridium with a highly absorbing organic ligand has created a material with

alternate absorption pathways. The ground and excited state absorption pathways have been

probed by single and multiphoton spectroscopy, and preliminary nonlinear behavior has been

detected.









CHAPTER 2
TRANSIENT ABSORPTION SPECTROSCOPY AND INSTRUMENTATION

Background

The study of transient chemical species generated by various approaches such as chemical

reactions or the absorption of energy is necessary for the complete understanding of these

processes. As some events occur on timescales of minutes to days, the observation of the species

involved is less hindered by time sensitivity. However, many chemical reactions produce

reactive intermediates that last only milliseconds or less before their demise. Indeed, the

absorption of light or other energy to produce excited-state species and the transfer of energy or

electrons within a molecule or between molecules are processes which generally do not allow for

the separation of very long-lived, stable species. The dynamics of such processes are also of

interest to characterize mechanisms and species involved. A detection technique on an

equivalent timescale to the event being monitored is necessary for unhindered observation. Also,

it is imperative that the species to be studied be generated quickly as to have a suitable

concentration available for detection. In this chapter, the construction and operation of a new

transient absorption apparatus for the detection of excited-state species (such as those discussed

in the following chapters) is described.

Transient absorption (TA) spectroscopy, which is also called flash photolysis or simply

"pump-probe" spectroscopy, has remained one of the most important techniques for detection

and characterization of short-lived chemical species since its development in the late 1940s.46

Indeed, the importance of this work was recognized as half of the 1967 Nobel Prize was awarded

to Norrish and Porter for their development of the flash photolysis method. Their research

focused on studying reactive intermediates generated by excitation from a flashlamp.46 It was

not until the 1960s that better time resolution could be achieved with use of lasers as an








excitation source.47 The apparatus design developed by Lindqvist is still the basis of modem

instrumentation (Figure 2-1). The system consists of three main components: an excitation



laser light





lampE) ( <) mono. PMT

sample amplifier


oscilloscope

Figure 2-1. Transient absorption apparatus developed by Lindqvist.47



source (laser), a monitoring source (lamp), and a detector (monochromator/PMT). Computer

control and data acquisition was the next advancement, and such a system was developed in the

1970s by Small and Scaiano.48 In earlier instruments, the time resolution was limited not by the

electronics, but the duration of the excitation source. Pico- and femtosecond techniques are now

popular, allowing detection of short-lived species or events as short 10-15 s. However, the need

for nanosecond techniques remains prevalent.

Techniques

In the early stages of nanosecond timescale measurements, nitrogen and ruby lasers played

a defining role in the development of experimental ability.49 Currently, a variety of excitation

sources are employed for initiating events of the same time regime. Commercially available

excimer and Nd:YAG lasers offer the ability to excite samples at a large range of wavelengths









from the ultraviolet into the infrared. Tunable optical parametric oscillators (which are used in

combination with such pump sources as Nd:YAG lasers) offer even greater flexibility in

excitation wavelengths.

The probe beam for transient absorption is commonly one of two types of xenon lamps.

Xenon lamps are convenient due to their broadband spectral output and high intensities. A

continuous output lamp can be used, often paired with mechanical shutters to limit sample

exposure between acquisitions. The lamp intensity can be increased by a factor of 5-100, or

greater, over a several milliseconds by pulsing the lamp.50 This technique is sometimes

necessary for short timescales, and although it reduces the lifetime of the lamp, the signal-to-

noise is significantly improved. Alternatively, strobe flashlamps with high-voltage power

supplies are employed as the monitoring source. These lamps are designed for high-repetition

operation (to coincide with the pulsed excitation source) and can offer excellent arc stability,

making them ideal for use in such an application.

A number of detectors can be used for measuring and recording the probe beam as long as

the basic requirements of sensitivity in the desired spectral range and being responsive on the

timescale of the events being measured are satisfied. A common detector is comprised of a

monochromator and a photomultiplier tube (PMT). The PMT should be wired for a fast time

response. With the range of PMTs available, detection from the UV into the near IR is possible.

Using such a detector leads to collecting a time-resolved measurement of the lamp intensity at a

single wavelength, and then extracting the transient absorption data. A complete spectrum is

then constructed for a particular time within the decay of the transient videe infra).

Alternatively, transient absorbance can be measured using a spectrally resolved detector

and gating the collection temporally. Such a detector could be a photodiode array. A diode









array can measure polychromatic light from a dispersive element such as a prism or grating, and

then the information from each diode is processed by a computer. The data collected from this

type of detector provides a complete spectrum from which a time trace can be constructed videe

infra). Although multiwavelength detection is convenient, there are some limitations with diode

detectors. The main disadvantage of using diode arrays is their lack of sensitivity. Diode

detectors are generally less sensitive than PMT detectors because diode detectors do not have the

electronic gain that PMTs do, and therefore produce only one charge per photon. Diode arrays

are also noisier than PMT detectors.

A major advancement to simultaneous multiwavelength acquisition was the introduction of

the charge-coupled device (CCD) camera. Simply stated, a CCD is a two-dimensional matrix of

photosensors, or pixels.51 Each pixel will convert photons into charge, which is then converted

into a digital signal read and processed by a computer. Coupling with a dispersion element such

as a prism or grating is necessary to separate wavelengths along a particular axis of the CCD

chip. The relative intensities of incident photons will determine the amount of relative charge

that is stored in the CCD chip and subsequently read to a computer. A CCD is similar to a

photodiode array in that each of the individual pixels of the CCD responds independently to the

photons which come into contact with it. Generally, a silicon-based photosensor is used, and

therefore the response is not wavelength dependent. For imaging-type CCD cameras, filters are

used to discern colors between pixels on a CCD chip. In spectroscopy, however, such two-

dimensional wavelength dependence is not necessary. By coupling the CCD with a dispersion

element such as a spectrograph, a range of wavelengths (along one axis of the chip) can be

collected simultaneously.









While CCD cameras are effective instruments for recording images in a variety of

applications, the response times of early devices were not adequate for use in kinetic

measurements. The invention of the image intensifier has circumvented this problem.

Intensified charge-coupled devices (ICCDs) are often used in low-light conditions due to the

increased sensitivity of the CCD. More importantly for kinetic measurements, the intensifier can

be used as a fast electronic gate to increase temporal resolution. The intensifier is mounted



Applied Voltage

Input Window b
Fiber Optic
/Output Window

Photocathode /




Incident
Photons .


Microchannel .
Plate II

SPhosphor Screen

Applied Voltage

Figure 2-2. CCD intensifier. Figure adapted from www.andor.com.51



within the camera before the CCD chip, and consists of three main components: a photocathode,

a microchannel plate (MCP), and an output phosphor screen. A schematic representation of the

intensifier is shown in Figure 2-2. When incident photons collide with the photocathode,









electrons are ejected towards the MCP. The MCP consists of millions of parallel channels with

secondary electron emitters contained within their inner walls.52 When a sufficiently high

voltage is applied across the MCP, the ejected photoelectrons are drawn through the channels

ejecting other electrons, effectively acting as a photomultiplier. The electrons exiting the MCP

collide with the phosphor screen, creating an intensified image to be projected onto the CCD. A

fiber optic typically connects the intensifier to the CCD. By appropriately controlling the

voltage across the MCP, the intensifier becomes an effective electronic shutter capable of very

fast gate times (-10-9 s). The transient absorption spectra and time decay profiles from data

collected by ICCD detectors are constructed in a similar fashion as diode array detectors.

Apparatus

Hardware

A diagram of the final apparatus is shown in Figure 2-3. The three main components as

described earlier are the pump, the probe, and the detector. The pump beam (blue path) is

generally created by the third harmonic of a Nd:YAG laser (355 nm, Continuum Surelite II, 5 ns

fwhm). An alternate excitation source, which provides greater flexibility in excitation

wavelengths, is generated by using an optical parametric oscillator (Continuum Surelite OPO

PLUS) pumped by the laser described above. The pump beam is set to an energy of

approximately 10 mJ/pulse, and directed to the sample using conventional laser optics. The

beam is shaped prior to the sample using a cylindrical plano-concave lens (not shown in figure)

such that the beam hitting the sample spans the width of the cuvette along the optical axis of the

probe beam. An anodized aluminum sample holder masks the pump beam to a vertical thickness

of 4.0 mm, restricting the excitation to a specific region within the sample cuvette.

The probe beam (red path in Figure 2-3) is generated by a xenon flashlamp (Perkin Elmer

Flashpac LS-1130 with an FX-1150 bulb). The spectrum of the lamp as provided by the














BS1 M1 Prism


o-4-.


M2 BS2


Sample L5 L6 Spectrograph
& CCD Camera


Figure 2-3. The transient absorption apparatus. Lenses, mirrors, and beamsplitters are abbreviated by L, M, and BS and described in
the text. The blue line (---) is the pump; red line (-) is the sample probe, I; and black line (---) is the reference probe, Io.


Lamp L1 L2 Iris L3


OPO Nd:YAG laser



























I I I
300 400 500 600 700 800 900
Wavelength nanometers

Figure 2-4. Spectrum of xenon lamp used as the probe. (Taken from 1100 series flashlamp
datasheet, PerkinElmer.)



manufacturer is show in Figure 2-4. The red region of the measured spectrum is relatively much

weaker, and the intense peaks between 800 nm and 850 nm are not observed mostly due to the

coating on the lenses as well as the weak detector response at those wavelengths videe infra). All

optics are antireflective coated 350-700 nm, 50.8 mm diameter plano-convex BK7 lenses, unless

otherwise noted. The beam is first collimated by L1 (focal length = 3.45 cm) and then refocused

by L2 (fl = 30 cm) through an adjustable iris which serves as the primary adjustment for the final

beam size. The beam is then collimated again by L3 (fl = 30 cm) and passed through a 50/50

beamsplitter. The transmitted beam is reflected first from a mirror and then a second

beamsplitter. The reflected beam from the first beamsplitter is reflected from a mirror and then

passed through a second beamsplitter. The distance between BS1 and Ml and between BS2 and

M2 is 8.5 cm, and the distance between BS1 and M2 and between Ml and BS2 is 19 cm. This









arrangement of beamsplitters and mirrors allows for the creation of two beams of equal intensity.

By creating two beams from a single source, one beam can be slightly offset to provide an

essentially identical reference beam. Both beams are focused through the sample using L4 (fl =

10 cm, diameter = 38.1 mm). The main probe beam (sample beam) is focused through the

portion of the sample that has been excited by the laser. The reference beam is focused

approximately 3 mm directly above the sample beam focus. If necessary, the focused beams can

be aligned relative to each other by adjusting the position of either mirror. After passing through

the sample, both beams are collected and collimated by L5 (fl = 17.5 cm) and focused onto the

entrance slits of the spectrograph by L6 (fl = 20 cm).

The detector is comprised of a spectrograph (Acton SpectroPro 150) coupled to an

intensified CCD camera (Princeton Instruments PI-Max iCCD). The spectrograph grating

provides a spectral range of -350 850 nm. A higher resolution grating can alternatively be

used to give a spectral window of roughly 250 nm. However, the transient absorption spectra of

the compounds typically studied in our lab do not have the fine structure to necessitate gratings

of this resolution. The CCD camera settings are controlled primarily by commercially available

software (WinSpec/32 version 2.5.12.2, Roper Scientific) supplied by the camera manufacturer.

The CCD chip is 512 x 512 pixels, with the x-axis corresponding to wavelength and the y-axis

corresponding to vertical position on the entrance slits of the spectrograph. Two regions of

interest of on the chip have been defined to identify the sample beam and the reference beam.

The CCD camera and software collect and record the raw data, which are intensities of the probe

for the sample beam and the reference beam as a function of wavelength. This information must

be converted to A absorbance values in a separate step.









Data Collection

There are several parameters that can be adjusted in order to obtain meaningful, quality

time-resolved transient absorption spectra. Following excitation, a sample relaxes to the ground

state, for example, through first order decay. The most important factor in a transient absorption

apparatus is timing; it is imperative for meaningful data to be collected that all components of the

apparatus perform their functions at intended and known times throughout the experiment. In

order to simplify this potentially complex problem, a four-channel pulse generator (Berkeley

Nucleonics Corporation model 555 pulse/delay generator) is used to send electronic triggers to

each of the 3 components (laser flashlamps and Q-switch, xenon flashlamp, and camera). To

increase the ease of operation and expand the flexibility of experiments and maintenance, a

LabVIEW-based virtual instrument (VI) was written in-house to provide a computer interface for

the pulse generator. The VI is used in collaboration with the WinSpec/32 software for data

collection. By using the pair of programs, all necessary timing parameters can be optimized.

An observer using a CCD detector has several choices to make after exciting a sample:

when to begin monitoring, for how long to integrate each image, how many images to take, and

over what timescale to make the measurements. A graphical representation of these parameters

is illustrated in Figure 2-5.

* to (time zero) This is the time at which the laser fires. Assuming fast excitation to the
singlet state (10-15 s) followed by rapid intersystem crossing (10-9 s), this is also the time at
which there exists the greatest population of triplet excited molecules.

* Camera delay This is the time between the laser fire (to) and the onset of the first
recorded transient. Camera delay is useful when there is a short timescale event, such as
strong fluorescence or scattered laser light, which can cause artifacts in the TA spectrum.

* Gate width This is the amount of time the CCD is exposed to light. The maximum
amount of light in the shortest amount of time is ideal for greater time resolution and
signal-to-noise. The gate width is determined by the amount of probe light reaching the
detector, i.e. a more intense probe will need a shorter gate width.









S Camera delay increment This is the time between the beginnings of each image
acquisition. The camera delay increment should be equal to or longer than the gate width
so that no overlap of signal is measured.


*


t Time
to Time


to Time


Figure 2-5. Illustration of timing parameters for ICCD data collection. (A) shows a
comparatively long camera delay (-4 ), short gate width (......) and long camera
delay increment (- -,) with respect to (B).









The length of the measurement will equal the number of transients collected multiplied by the

camera delay increment. Ideally, the total time should exceed the decay time of the excited state.

Also, the greater the number of transient spectra collected, the easier it will be to accurately fit

the set of spectra to a kinetic model. The parameters above, as well as some additional settings,

will greatly affect the signal-to-noise ratio of the system, and will be discussed in more detail

later in the chapter.

Data Processing

To look at how a transient signal is measured and calculated, it can help to begin with the

definition of absorbance and transmittance,

A(A)= -log T(A) (2-1)

J(A)
T(A) = (2-2)
10 (A)

where lo(A) and I(A) are the intensities of radiation prior to passing through the sample and after

passing through the sample, respectively. Substituting Equation 2-2 into Equation 2-1, an

expression for absorbance in terms of intensities can be given as Equation 2-3, shown below.


A(A) log= 1 (2-3)


This expression is a convenient way to convert measured intensities of light into

absorbance values. An important part of this equation is that 1o must be measured without the

sample in the optical path (either by physical placement or by using a dual-beam instrument).

However, in transient absorption experiments, the intensity of the lamp itself is never measured

directly. Instead, 1o is defined as the intensity of the light that passes through the ground state

(i.e. before optical pumping and creation of excited states). After populating the excited state,

there will be some change in intensity of the transmitted light,









I(A)= ,1(A)+ A(A). (2-4)

Figure 2-6 shows a graphical representation of the change in intensity of the transmitted light

throughout the lifetime of a transient at a single wavelength. Io is an arbitrary value determined


>,
U)

c:
(DJ

"


Time


Figure 2-6. Change in intensity of transmitted light during the lifetime of a transient species.



by the probe lamp, alignment, and camera settings. The laser fire defines to. Equation 2-4 can be

substituted into Equation 2-3 to define AA as


AA(A) = -logI, () +7 ()
I0 (A)

and simplifying to


AA() = -log I)


(2-5)


(2-6)









In many transient absorption apparatus, a continuous-wave monitoring source is used (such

as a PMT connected to an oscilloscope) for transient absorption spectroscopy. Therefore Io can

be measured prior to to, and stored for calculation later. This method would also be possible

using a pulsed monitoring source such as a CCD camera, although it would not necessarily

simplify any of the calculations. For this reason, and more importantly to try to minimize shot-

to-shot fluctuations of the flashlamp, the probe beam in the current apparatus has been split into

a sample beam and a reference beam. The probe beam passes through the region of the sample

that has been excited by the laser. The reference beam passes through an area of the sample

which has not been excited (i.e. the ground state). This allows for 1o and Ito come from the

same flash of the probe (minimizing noise), as well as the simultaneous collection of each

(minimizing experiment time). In order to normalize the sample and reference beams, a simple

correction factor, k(A), can be calculated:

(,(27
k(A) = I when pump is off. (2-7)


When measured prior to to (i.e. with the laser off), k(A) will correct for any slight differences in

intensity of the two beams caused by the detector or alignment. The correction factor will be

different for each experiment, and therefore should be collected at the beginning of each

measurement. Placing the correction factor into Equation 2-6,


AA(A) = -log k()I() (2-8)
I (A) )

a relationship between raw data from the camera software and a presentable set of spectra is

easily made.

Using a combination of full spectral range detection, along with pulsed delay detection,

time-resolved spectral data can be constructed. Figure 2-7 shows how the light intensities of










I .. *
0)



St3 -
t2 Time
tl
t ,
Wavelength to







0













Wavelength

Figure 2-7. Construction of a time trace and transient absorption spectra from gated
multiwavelength measurements.



the sample and reference beams at a single wavelength vary throughout the excited state decay

using a pulsed monitoring source. By having this data for many wavelengths, time-resolved

transient absorbance data for a particular wavelength can be constructed as shown.

It is convenient to use temporally resolved data to characterize the nature of the excited

state of a material. Often in combination with spectral information, lifetimes can help discern









between simple excited-state decay, electron transfer, energy transfer or even creation of another

species. In the most general case of monitoring the relaxation of an excited (triplet) state, kinetic

information can be elucidated quite easily. As described earlier, transient absorption intensity as

a function of time can be plotted for a particular wavelength. These data can then be fitted

following first-order kinetics. That is, assuming one excited state and one ground state, the

kinetics follow that of reactive chemistry:


lnF[A 1] ln(A) = -kt (2-9)
\[A],) [(AA), )

where [A]t and [A]o are the concentrations of a species A at some time t, and the initial

conditions at time to, and (AA)t and (AA)o are the transient absorbance at some time t, and the

initial time to. This relationship can be made due to the linear relationship between sample

concentration and absorbance. For more complicated kinetic systems, several wavelengths could

be monitored independently, or commercially available software could be used for analysis.

Sources of Noise

A majority of the noise in the current system can be ascribed to the inherent stabilities of

each of the components that make up the apparatus, which are described in further detail below.

A smaller part of the noise is attributed to the sample itself, in which photodegradation can

occur, causing potentially severe changes in the signal. This can generally be controlled by

using an appropriate excitation energy and properly mixing the solution throughout the course of

the experiment; however, some samples are more prone to photodegradation, and little can be

done to prevent it. Repetitive sampling for each transient greatly affects the signal-to-noise ratio,

and some examples are given below.

Each component has inherent fluctuations which contribute to the noise of the system. For

example, the stability of the laser power output will influence the excited-state population and









affect the intensity of the transient absorbance signal. The probe beam stability, while not

affecting the intensity, contributes to the shot-to-shot fluctuations that limit reproducibility and

causes baseline error. The camera has excellent stability at full-well capacity; the wavelength

range at which this happens is dependent on the output of the xenon lamp. In other words, based

on the spectral distribution of the probe, the noise is much lower between 400 and 650 nm where

the measured lamp output is strongest, and the noise is larger at the tail ends of the spectrum,

especially around 800-850 nm where the measured output is weakest. A spectrum of the xenon

probe source used, in terms of and Io as measured by the detector, is shown in Figure 2-8. The

discrepancy around 500 nm is due to a defect in the CCD chip (presumably from damage);

however, this has minimal effect on the transient absorption signal.










-- I I I I'
_I

a)
I i
-i I f \ i

ICU






400 500 600 700 800

Wavelength / nm

Figure 2-8. Relative probe intensities for Io (-) and I (----) with the pump beam off









Baseline Measurements

Baseline measurements were made using the apparatus and the representative results are

displayed in Figure 2-9. In this series of tests, the primary goal was to ascertain the overall noise

of the system as a whole. Using a blank sample (i.e. an empty cuvette), spectra were collected

under the same conditions while the number of averaged images were changed. For each set, a

collection often "transients" was collected, each at to, using a gate width of 10 ns. The camera

software was set to accumulate 10, 25, 100, or 500 images. As shown clearly in Figure 2-9A and

B, the extremes of the spectrum are the noisiest, due to the low output of the probe at those

wavelengths. Increasing the number of averaged images from 10 to 100 improves the limit of

detection from greater than 15 mO.D. to roughly 5 mO.D. While increasing the number of

averaged images from 100 to 500 results in slightly better reproducibility, the overall noise is not

improved greatly (Figure 2-9C and D). In order to limit the amount of laser energy to which the

sample is exposed and reduce the experiment time, as well as maintain a satisfactory baseline,

averaging 100 images per transient delay time is the most suitable for data collection.

Results

A 1.2x10-4 M solution of anthracene (Acros) was prepared in HPLC grade benzene (Sigma

Aldrich). The solution was deoxygenated by bubbling with argon for 40 minutes. The camera

was set to collect with a 10-ns gate width with a 3-[ts delay between transients. Prior to

excitation, the spectrograph slits were adjusted (130 [tm) so that the peak probe intensity was

near the maximum for the detector. The laser energy was set to -10 mJ per pulse. The transient

absorbance of the anthracene solutions under these conditions is shown in Figure 2-10. The Xmax

at 432 nm, as well as the intensity, matches well with previously published data.53 The lifetime











15
10
5
0
-5
-10
-15 A
15
10
5


o -5
-10

15:
o



-5



-10
0










S10 r
,< 5
10
-5




-10
-15 C
15
10



-5
-10
-15 D

400 500 600 700 800

Wavelength / nm

Figure 2-9. Baseline noise of the apparatus. Conditions: 10 ns gate width, 10 transients, camera
delay and delay increment 0 ns, number of averages (A) 10, (B) 25, (C) 100 and (D)
500.









0.4


0.3



c 0.2
-Q
0
C,
S0.1



0.0



-0.1 .
400 450 500 550 600 650 700

Wavelength / nm


Figure 2-10. Transient absorption spectrum of 1.2x10-4 M anthracene solution in deoxygenated
benzene. Camera delay 50 ns, delay increment 3 |ts, 100 images averaged.



of the triplet was extracted and shown to be 9.1 hts, which corresponds with the literature.54

Lifetimes were calculated by using global analysis of the spectral-kinetics data using the

SPECFIT/32 software package (Bio-logic SAS, Grenoble, France, www.bio-logic.info).

A 1.4x10-4 M aqueous solution of tris(2,2'-bipyridine)ruthenium(II) chloride (Ru(bpy)32+)

(Aldrich) was also prepared and deoxygenated by argon purging for 40 minutes. The camera

delay was set to 10 ns, and a small bleaching peak at 710 nm can be seen from the second order

of the spectrograph grating. A 250-ns delay between transients was used. The transient

absorbance is shown in Figure 2-11. The signal is quite noisy between 420 nm and 470 nm, due









0.15


0.10


S 0.05





-0.05


-0.10


-0.15
400 500 600 700 800

Wavelength / nm

Figure 2-11. Transient absorption spectrum of 1.4x10-4 M aqueous solution of Ru(bpy)32+. The
solution was deoxygenated by bubbling with argon for 40 minutes. Camera delay 10
ns, delay increment 250 ns, 100 images averaged. The small peak at 710 nm is from
scattered excitation light.


to the large absorbance of Ru(bpy)32+ in that region (therefore giving a very weak lo). However,

the spectrum and excited-state lifetime of 724 ns are in good agreement with the literature.5556

Limitations of Current System

While the current system described provides the ability to study the excited state properties

of many materials, there are some limitations. These restrictions arise from several sources,

namely the spectral range of the components used, including the pump, probe, detector and

optics used therein. Another source of limitation comes from the temporal nature of the









electronics and hardware used in the apparatus. Lastly, the optical geometry is such that

particular attention needs to be made while preparing samples to be studied.

The excitation source used can limit the types of chromophores available for study. The

primary excitation source is the third harmonic of a Nd:YAG laser at 355 nm. This output can

also be used to pump an OPO that produces wavelengths from 420 nm to 1200 nm. Currently,

excitation cannot be produced at wavelengths shorter than 355 nm and between 355 nm and 420

nm. Another excitation source, such as different laser or OPO system, could be incorporated into

the apparatus to provide a larger range of excitation wavelengths. However, the types of

materials studied in our lab generally provide a fairly broad absorption throughout the UV and

blue region (excitation by 355 nm) or further into the red (excitation via the OPO).

The detector range is limited by a combination of the probe output wavelength,

spectrograph range and the chip used in the CCD camera. As shown earlier in Figure 2-4, the

xenon flashlamp is not a true white light source. Other probe sources may offer a wider or more

specific wavelength range suitable for different chromophores. A variety of CCD cameras and

corresponding spectrographs are available from commercial manufacturers to expand detection

wavelengths.

Currently, the apparatus has been optimized for use with solution samples which can be

contained in a 1-cm x 1-cm square cuvette. For most cases, sample concentrations must be

adjusted such that intermolecular interactions, such as self-absorption or triplet-triplet

annihilation, do not disrupt the signal. The sample concentrations are then adjusted such that the

absorbance is optimal for excitation and detection (that is, A -0.8 at the excitation wavelength).

If highly absorbing or concentration-dependent samples are to be studied, various sample cells

could be used, including small or large pathlength cuvettes. As shown in Figure 2-3 earlier, the









sample is placed in a right-angle geometry with respect to the pump and the probe. Therefore,

the studying of thin films poses a problem because the beams are not colinear. Films are also

damaged easily under the excitation conditions used for solution samples.

The kinetic resolution is limited almost entirely by the electronics of the system. The

repetition rate of each measurement is limited by the occurrence of to, generated at 10 Hz by the

Nd:YAG laser. The pulse generator used has a limit near 4 ns. Similarly, the camera can not

record images with a gate width less than 5 ns, although the resolution is reported at 40 ps.

Neither of these timings is limited by the 10 Hz repetition rate of the laser. Even so, events on a

timescale of less than 100 ns are not able to be monitored with a high degree of accuracy. This

does not pose a problem when studying triplet states, which are generally long-lived.

There are several other concerns that affect ease of use and presentation of data. The first

issue involves the nature of data collection, as described above. Because of the type of the

information recorded (lo and I), and that those data must be processed after the experiment, there

is no ability for real-time monitoring of the transient absorption signal. An experimental run is

necessary to determine if the sample exhibits transient absorption and under what time

conditions; parameters can not be adjusted on the fly. Therefore, if the user has little knowledge

of the sample behavior, several runs may be necessary before the appropriate parameters can be

determined. Secondly, for highly emissive species, fluorescence from the sample can cause

bleaching in the transient absorption signal. The fluorescence will sometimes obscure the

absorbance of the excited state. This problem can be remedied by collecting data with the pump

on and the probe off, resulting in what is essentially an emission spectrum of the sample. If this

is done at each time delay, the emission can be corrected simply by subtraction for each time









measurement of the transient. A LabVIEW VI and Matlab application are under development to

address this problem.









CHAPTER 3
DELOCALIZATION IN EXCITED STATES OF DINUCLEAR PLATINUM ACETYLIDES

Introduction

The interesting properties of materials with extended 7t-conjugated systems have made

such materials potential candidates for use in a variety of applications, including nonlinear

optics,57 light emitting diodes,6'58 and conductance switches.59 The incorporation of transition

metals into the organic structure changes the redox, electronic, and optical properties of the

molecule. The organized combination of metals and organic moieties through deliberate

synthesis allows for tunability of these properties.

Photophysics of transition-metal acetylide complexes have garnered considerable interest

throughout the last decade.60-66 Platinum acetylides are square-planar molecules having the

general formula trans-Pt(PR3)2(C-CR')2. Typical R groups are methyl, ethyl, butyl, or phenyl,

which give the compound crystalline properties, aid in solubility, or form glasses41'67 or gels.68

The R' group is normally an aromatic substituent, such as phenyl, thienyl, or fluorenyl. There

has been a wide range of structures synthesized, including oligomers,69 polymers,70 and

dendrimers.71 The spin-orbit coupling that is a result of the heavy platinum atom allows for

formation of triplet states in high yield. Hence, platinum-acetylides afford the ability to study

spin-forbidden phenomena such as singlet-to-triplet-state absorption (So--T1), intersystem

crossing of the singlet excited state to the triplet excited state (S1--T1), and triplet-state emission

(phosphorescence, T1--So). Population of the triplet state also provides an opportunity to probe

triplet-state absorption (T1--Tn).

A key point in the advancement of use of these types of materials is to make distinct

relationships between chemical structure and spectroscopic properties. The fundamental nature

of this type of structure-property relationship is also of importance to the general understanding









of interaction of light with matter and the information that can be elicited from experiments.

One such issue is the relation between structure and the delocalization of the singlet and triplet

excitons. Beljonne and coworkers72 have carried out an experimental and theoretical

investigation on a platinum-containing acetylide polymer, the structure of which is shown in

Figure 3-1. Using various spectroscopic techniques, such as absorption, emission, and excited-

state absorption, along with quantum chemical calculations, the spatial extent of singlet and

triplet excited states were determined. Transitions including So--S1, So--T1, and T1--Tn have



PBu3 PBu3 PBu3 PBu3

CI Pt Pt-CI CI- t Pt-CI

PBu3 nPBu3 PBu3 PBu3

Figure 3-1. Platinum-acetylide polymer and model compound studied by Beljonne et al.72



shown chain-length dependencies on these excitation energies. The authors suggest that the

lowest triplet excited state, T1, is localized on one phenylene ring, while the Si excited state is

confined to two repeat units.72 However, the higher Tn excited state is less restricted, and spread

out over at least three repeat units. The use of model compounds (Figure 3-1) through Huang-

Rhys fitting of phosphorescence emission spectra and theoretical modeling also indicates

considerable distortion between the lowest triplet excited state and the singlet ground state.72

Our group has more recently studied delocalization in singlet and triplet states of platinum-

acetylide oligomers,69 the structures of which are shown in Figure 3-2. It was found that the

ground-state absorption and fluorescence have significant dependence on the length of the

oligomer. As the chain length increases, the ground-state absorption and singlet emission red-

shift systematically. However, phosphorescence maxima are less affected by chain length, as the









PBu3

Pt


PBu3 n

Figure 3-2. Platinum-acetylide oligomers studied by Liu et al.69 Oligomer length varies from n
= 1-5, 7.



peaks are only slightly red-shifted with respect to longer oligomer length.69 The findings of this

report further support the idea that the triplet state is more localized than the singlet state.

Furthermore, another investigation by our group73 involving the probing of delocalization was





PBu3


Figure 3-3. Platinum acetylide oligomers studied by Glusac-Haskins et al.73









completed on platinum-acetylide stilbene compounds, the structures of which are shown in

Figure 3-3. Similar results in the absorption spectra indicate delocalization over the entire 7t-

conjugated system. That is, the absorption maxima are affected by the conjugation length of the

molecule. Also, 7t-conjugation is more efficient when the ligands are substituted trans to

platinum, indicating delocalization through the d orbitals of the platinum atom. Moreover, the

triplet-state properties of the ligand and platinum-containing molecules are notably similar,

suggesting that there is no significant delocalization through the platinum atom in the triplet state

and that the state is localized on the ligand.

The photophysics of a short platinum-acetylide oligomer, PE2, was described in Chapter 1,

and a more complete series studied by the Air Force Research Laboratory40'42-45'74'75 was also

introduced. In addition to the symmetrical molecules shown in Figure 1-8, the group also

synthesized mono-substituted45 and asymmetrically-substituted75 platinum-acetylide complexes

and all-organic butadiynes.44 The complete series of compounds is shown in Figure 3-4. As




PE1 a = b = L1 L1
PE2 a = b = L2
PE3 a = b = L3PBu3
a- Pt b
PE1-2 a = L1, b = L2 L2
PE2-3 a = L2, b = L3 PBu3
PE1-3 a = L1, b = L3

half-PEn a = CI, b = Ln
L3



PE1-BD m = 0
PE2-BD m = 1
PE3-BD m = 2
m m


Figure 3-4. Platinum-acetylide oligomers studied by Cooper, Rogers, et a/.40,42-45'74'75









mentioned in Chapter 1, the symmetrically-substituted complexes show similar fluorescence

spectra, but absorption and triplet emission spectra red-shift with increased conjugation length.40

The mono-substituted complexes also exhibit greater dependence on oligomer length for ground-

state absorption and triplet-state emission than for singlet-state emission.45 Also, the

phosphorescence spectra for the mono- and di-substituted oligomers are nearly identical,45

further indicating localization of the triplet state. Transient absorption spectra match for each

oligomer pair as well. The photophysics of the asymmetrical platinum-acetylide complexes7

reveal more support on the confinement of the triplet excited state and delocalization of the

singlet excited state. It was observed that the singlet exciton is delocalized through the platinum

atom, but phosphorescence comes from the lowest energy (largest) ligand.

Most of the work to date has considered platinum acetylide oligomers with a single

platinum atom and various ligands, or oligomers and polymers with identical repeat units. The

study of delocalization in excited states is therefore limited. A set of dinuclear platinum-

acetylides with various short spacers and end-capped substitution of either chloride or a

conjugated phenyl-ethynyl-type substituent are presented in Figure 3-5. The photophysics of the

series have been investigated and are presented in the following sections. The effects of the end

group, platinum atom, and spacer are considered, and some conclusions are made regarding the

delocalization of singlet and triplet states.

Experimental

Steady-state absorption measurements were recorded on a Varian Cary 100 dual-beam

spectrophotometer. Corrected steady-state emission measurements were conducted on a either a

FluoroMax-3 or SPEX Fluorolog-3 spectrophotometer. All sample solutions were prepared in

tetrahydrofuran (THF) or 2-methyltetrahydrofuran (MTHF). Solvents were distilled over CaH2









immediately prior to use. Room-temperature measurements were carried out in 1-cm square

quartz cuvettes. Solutions were deoxygenated by purging with argon for 30 minutes and sample


PE2: R 2


PBu3 PBu3
P: R = C1
P-PE2: R = PE2 RPt Pt-R
PBu3 PBu3

PBu3 PBu3
BP: R = C1 R- F^
p Dt- / \\- t-
BP-PE2: R = PE2 Pt Pt
PBu3 PBu3

PBU3 PBU3
PEP: R =C1 PBu3 PBu3
PEP-PE2: R = PE2 R-Pt Pt-R
PBu3 PBu3

Figure 3-5. Dinuclear platinum-acetylide oligomers.



concentrations were made such that the solutions were optically dilute (Amax < 0.20). Low-

temperature emission measurements were conducted in 1-cm i.d. borosilicate glass tubes in a

liquid nitrogen-cooled Oxford Instruments OptistatDN cryostat connected to an Omega

CYC3200 autotuning temperature controller. Samples were degassed by three consecutive

freeze-pump-thaw cycles on a high vacuum (10-5 torr) line. Transient absorption measurements

were conducted on the apparatus described in Chapter 2. Samples were contained in a cell with a

total volume of 10 mL and the contents were continuously circulated through the pump-probe

region of the cell. Solutions were deoxygenated by purging with argon for 40 minutes and

concentrations adjusted so that A355 ~ 0.8. Excitation was generated using the third harmonic

output of a Nd:YAG laser (355 nm, Continuum Surelite). Typical excitation energies were 8 mJ-









pulse-', corresponding to a fluence of -20 mJ-cm-2. Transient absorption spectra were generated

by using software developed in-house following the calculations described in Chapter 2. Kinetic

data were calculated by using global analysis of the spectral-kinetics data using the SPECFIT/32

software package (Bio-logic SAS, Grenoble, France, www.bio-logic.info).

Results

In Figure 3-6, the absorption maxima for all compounds in THF solution are shown to be

within 15 nm of each other. BP has the most blue-shifted maximum at 346 nm, while P-PE2,

BP-PE2 and PEP-PE2 have maxima at 358 nm. The absorption of PEP-PE2 is slightly broader

than the others, suggesting there may be a secondary band red-shifted from the maximum. The

absorption spectra for P and PEP are nearly identical with two closely-spaced bands near their

absorption maxima at 351 nm. For the PE2-end-capped molecules, the molar absorptivity in

benzene is an order of magnitude larger than the chloride-end-capped molecules, as given in

Table 3-1. (No data for P was obtained, although presumably its molar absorptivity is

considerably lower than that of P-PE2.)



Table 3-1. Summary of spectroscopic data of dinuclear platinum-acetylide oligomers.
a b c c a d e
..max, abs a b 'max, em c max, ex c max, em d max, ex d max, TA a TT
nm M-1 cm'1 nm nm nm nm nms
P 351 -J 558 372 545 387 608 28.3
P-PE2 360 97 100 532 365 524 389 576 42.7
BP 346 12900 556 351 541 366 569 27.1
BP-PE2 358 123 000 559 363 543 388 590 94.8
533 359 522 381
PEP 351 13 900 554 373 552 386 611 63.0
PEP-PE2 358 117000 559 375 550 396 629 82.9
533 358 523 381
a THF solution. Benzene solution. c THF solution, room temperature. dMTHF solution, 80 K.
e Extracted from TA data. fNot measured.





















0 B





n C




















300 350 400 450 500




temperature in TfW solution. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and
(F) PEP-PE2.
-Q \
O
.Q



D













300 350 400 450 500
Wavelength I nm

Figure 3-6. Absorbance spectra of dinuclear platinum-acetylide oligomers collected at room
temperature in THF solution. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and
(F) PEP-PE2.











Fluorescence is observed for P at room temperature, although the maximum at 395 nm is


weak compared to the phosphorescence. Some broad vibronic structure is seen, though three

bands are shown and it does not quite mirror the absorption. Weak fluorescence is also shown


for BP, and although broader than P, it exhibits similar structure at the same wavelengths. The


photoluminescence of the remaining compounds is dominated by phosphorescence, even at room


temperature.


Transient absorption data are shown in Figure 3-7. Intense, broad signals throughout the


visible are seen from the chloride-end-capped compounds with strong peaks at 608 nm, 596 nm,






--- P-PE2






C.)

BP
-0 --- BP-PE2



N /
U,


/

-PEP
/ "- --- PEP-PE2
.................


i . ... . .



400 500 600 700 800

Wavelength / nm


Figure 3-7. Transient absorption spectra of dinuclear platinum-acetylides. Samples prepared in
deoxygenated THF solution and spectra collected 20 ns after excitation by 355 nm
pulse (10 ns, 8 mJ pulse-'). The horizontal dotted line represents AA = 0.









and 611 nm for P, BP and PEP, respectively. The PE2-end-capped molecules exhibit much

broader TA signals throughout the visible. For the BP and PEP series, the PE2-substituted

molecules have about a 20 nm red-shift from their analogs. P-PE2 is actually blue-shifted 32 nm

from the P peak. The PE2 analogs red-shift following the order P-PE2 < BP-PE2 < PEP-PE2.

Triplet-state lifetimes calculated from transient absorption decay are listed in Table 3-1. The

relative error in measurement of lifetimes approaches almost 20%; however, it is clear that the

lifetimes are considerably longer for the larger compound of each pair.

Low-temperature and room-temperature phosphorescence spectra were collected and are

shown in Figure 3-8. The room-temperature phosphorescence for P, P-PE2, BP, and PEP show

a single peak with an unresolved vibronic progression. BP-PE2 and PEP-PE2 show similar

structure, with the addition of a smaller peak blue-shifted from the main emission band. The

smaller peaks of the two PE2-substituted compounds correspond in wavelength with the main

peak of P-PE2 at 532 nm. The main peaks of the chloride-end-capped compounds all have

phosphorescence maxima near 557 nm. At 80 K, the emission spectra of all of the compounds

are blue-shifted and the vibronic progressions become slightly more resolved. The resolution

increase is most notable for P and PEP, although more structure is observed for P-PE2 and

PEP-PE2 as well. BP and BP-PE2 show very little fine structure at either temperature.

Representative variable-temperature emission is shown by P-PE2 in Figure 3-9. As the

temperature is increased from 80 K, the emission intensity decreases steadily. Upon the glass

transition temperature of the solvent, the peak red-shifts 7 nm, and also increases intensity. After

further heating of the sample from 120 K, the intensity decreases and vibronic structure is lost.

Similar behavior is seen from the entire series.










1.2 F-
1.0
0.8
0.6
0.4
0.2 A
0.0 .Lr .

0.8
0.6
0.4
0.2 B
0.0
1.0
0.8
0.6
c 0.4
S0.2 C
o 0.0

m 0.8
E 0.6 71 it
z 0.4
0.2 / D
0.0 ---- \~7-- r----------^1-i
0.0
1.0
0.8
0.6
0.4
0.2 E
0.0
1.0 -
0.8
0.6 i /
0.4 /1i -
0.2 / F
0.0
300 400 500 600 700

Wavelength / nm

Figure 3-8. Phosphorescence spectra of dinuclear platinum-acetylide oligomers. Room-
termperature (-) spectra collected in THF solution and 80 K spectra (----) in MTHF
glass. Excitation wavelength set at absorption maximum for each compound. (A) P,
(B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2.
















jI 6Ul
140K
c 160K
S- 180K
Sl I 200K
C 220K
(D 240K
C -/ 260K








500 550 600 650

Wavelength / nm

Figure 3-9. Variable-temperature emission spectra of P-PE2 in MTHF. Excitation wavelength
= 360 nm.



Excitation spectra for the set of compounds are shown in Figure 3-10. Room-temperature

excitation spectra collected at the maxima of emission are displayed as featureless bands shifted

a few nanometers lower in energy than the absorbance maxima. However, P and PEP exhibit a

blue-shifted shoulder that corresponds with the absorption spectra. Excitation spectra collected

at the smaller higher-energy peak in BP-PE2 and PEP-PE2 have blue-shifted maxima which

match the absorbance maxima. At 80 K, the excitation spectra are more structured, and

significantly (-20 nm) red-shifted from the room-temperature data. For the chloride end-capped

series, the maxima are shifted approximately 15 nm to the red compared to










1.2
1.0 551
_--_- 558
0.8
0.6
0.4
0.2 A
0.0 . .
1.0- 524
S-- 532
0.8
0.6
0.4
0.2 B
0.0 :5
S1.0 536
--- 556
(d 0.8
0.6
"F 0.4
S0.2 C
.c 0.0 .. .. ... ... -. i.|...
.0i 544
1.0 --- 522
0.8 \ 559
0.6 532
O 0.4 \
Z 0.2 -
0.0
1.0 550
0.8




-- 550
1.0 550
/ --- 523
0.8- \ 560
0.6 -- 533
0.4\ \
0.2 F
0.0
300 320 340 360 380 400 420 440

Wavelength / nm

Figure 3-10. Excitation spectra of dinuclear platinum-acetylide oligomers. Room-temperature
spectra (-) collected in deoxygenated THF and low-temperature spectra (-)
collected at 80 K in MTHF glass at emission wavelengths denoted in the figure. The
dotted lines correspond to the higher-energy bands in the PE2-end-capped oligomers.
(A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2.








room-temperature excitation, whereas the PE2 end-capped molecules show a larger shift of about

24 nm. The excitation spectra of the higher-energy peak for BP-PE2 and PEP-PE2 have

maxima blue-shifted from the excitation peak measured for the main emission band.

Representative variable-temperature excitation spectra for P-PE2 are shown in Figure 3-11. At

80 K, the maximum peak is at 389 nm. As the temperature increases, the peak decreases and

blue-shifts. As seen in the emission spectra, the vibronic structure is lost as the solvent glass is

warmed. The excitation maximum shifts a total of 24 nm. A comparable trend throughout the

series is observed as the temperature is increased from 80 K to room temperature.


300


320 340 360 380

Wavelength / nm


400


420


Figure 3-11. Variable-temperature excitation spectra of P-PE2 in MTHF. Emission monitored
at 532 nm.









Discussion

The properties of 7t-conjugated materials attract interest due to the potential applications of

these types of compounds. By introducing platinum atoms into the 7t-conjugated structure, the

triplet-state properties are more easily investigated due to population through spin-orbit coupling

of the heavy atom. The results of previous work have uncovered more questions as to the nature

of the singlet and triplet excited states and the role of platinum in the chromophore. The

photophysical characterization of a series of dinuclear platinum-acetylide oligomers that consist

of different spacer units and a well-studied end-cap will further the understanding of this type of

molecule.

The ground-state absorption of platinum-acetylide complexes originates from a 77rT*

transition with some metal-to-ligand charge transfer (MLCT) character.44 Theoretical

calculations of the highest occupied molecular orbital (HOMO) and lowest unoccupied

molecular orbital (LUMO) have contributed to the understanding of electronic transitions in this

type of compound.63'74 A combination of the platinum d orbital and the aryl '7 orbitals

contributes to the HOMO, while the LUMO consists of only 7t* orbitals and no contribution from

the platinum d orbital. Without quantum chemical methods to study the geometry and orbital

configurations of this series, limited conclusions can be made. The central phenyl rings in BP

and BP-PE2 are most likely twisted, which disrupts the conjugation. Cooper and coworkers43

have calculated the dihedral angle in the related PPE to be 400. Therefore, it is not possible to

determine trends based purely on conjugation length as the exact conjugation length is not

known. However, direct comparison between the chloride and PE2 end-capped molecules in

each pair is more legitimate. The absorption maxima of the PE2 end-capped molecules are red-

shifted 7-12 nm from the chloride analogs. The asymmetric PE2-3, which has a similar









absorption maximum to the PE2 end-capped oligomers, has a longer conjugation length if

comparing only the "ligand-metal-spacer" as the chromophore. The increase in conjugation

length between P and PEP predicts a greater red-shift than what is observed. However, the

second peak at 375 nm observed in the absorbance of PEP occurs near the absorbance maximum

of PE3. Interestingly, the main absorption band occurs at same wavelength as the asymmetrical

PE2-3. This suggests that there maybe conformational twisting near the platinum that interrupts

the conjugation to a "ligand-metal-spacer" chromophore, but also there exists delocalization

through the platinum. The PE2 end-cap contributes heavily to the oscillator strength, increasing

the value of the chloride end-capped molecules by an order of magnitude. The values are higher

than the reported value75 for the related PE2 (; = 89 000 M-1 cm-1). The increased molar

absorptivity and absorption maxima suggest that there is delocalization through both platinum

atoms in the singlet state. Due to the small size of the spacers, it is inconclusive at this point as

to the origin of absorbance and relationship of the platinum in the HOMO and LUMO.

The trends observed for the transient absorption spectra are different for the chloride- and

PE2-end-capped series. P and PEP have similar TA spectra, and BP has a peak 40 nm higher in

energy. However, the maxima of the broad TA spectra of the PE2-end-capped oligomers

increase in the order P-PE2 < BP-PE2 < PEP-PE2. The trend in the latter can be attributed to

the delocalization through the platinum atoms into the conjugated end-caps. It has been shown

for the related platinum-acetylide oligomers that the Tn excited state is delocalized through the

metal d orbitals. Additionally, as stated earlier, theoretical calculations have shown that the

triplet exciton in platinum-acetylide polymers is delocalized onto at least three repeat units.72 In

contrast to their study on films, the experimental triplet absorption maximum of P is 608 nm for

the solution sample, nearly 20 nm red-shifted from their film sample. The PE2 end-capped









oligomers also show much broader TA signals. The Tn state is likely delocalized throughout the

molecule, as suggested by previous reports of similar molecules.45

The lack of fluorescence in the emission spectra of the series, even at room temperature,

indicates that intersystem crossing via spin-orbit coupling dominates decay of the singlet excited

state. In other platinum-acetylide oligomers, the reported ISC quantum yields approach

unity.35'36 Only P shows reasonable fluorescence emission at room temperature; the maximum

value occurs at a lower energy (395 nm) than related oligomers that contain only a single phenyl

group per platinum atom. PE1, which has a single phenyl-ethynyl group on each side of the

platinum atom has a fluorescence maximum at 364 nm, whereas maximum emission near 395

comes from PE2 (385 nm) or PE3 (400 nm).44 It has been shown that the excited singlet states

have delocalized character. It would seem reasonable then that there is a strong contribution

from the metal orbitals in the singlet state, extending the conjugation from the phenyl 7 orbitals

through the platinum d orbitals. The red-shift in the expected fluorescence is attributed to

delocalization of the singlet excited state into the d orbitals of both platinum atoms.

As stated earlier, definitive trends on conjugation length cannot be determined by structure

alone. Low-temperature emission spectra for all compounds, excluding P-PE2, have

phosphorescence maxima within 5 nm of 546 nm. P-PE2 phosphorescence is at higher energy

of 524 nm. Similarly, the other PE2 end-capped oligomers, BP-PE2 and PEP-PE2, exhibit a

higher energy peak at the same wavelength. In the asymmetrical series of platinum complexes,

phosphorescence was observed by emission from the lowest energy (largest) ligand. Only in

PE2-3 was very weak emission from the PE2 ligand observed at 77 K. In the dinuclear platinum

series, this higher energy peak in the dinuclear platinum compounds is ascribed to triplet

localization on the PE2 ligand. In P-PE2, the entire triplet is localized on the PE2 end-cap, and









no emission is observed from the spacer group. However, in BP-PE2 and PEP-PE2, the triplet

can be localized on the end-cap or the central spacer. An explanation for this resides in

intramolecular triplet energy transfer between the end-cap chromophore and the spacer

chromophore through Dexter coupling.7 While emission is expected to come from only the

lowest energy ligand according to Kasha's rule, our group has previously reported a contrasting

study76 on a platinum acetylide polymer containing both phenyl and thienyl monomer units.

Emission from the polymer is observed from thiophene units as well as the higher-energy phenyl

groups.

At low temperatures, the vibronic structure of the emission is more defined, showing

several peaks in the progression. P and PEP have the clearest progression of small peaks, while

the BP pair is fairly broad with two large bands. The lower resolution of vibronic structure in P-

PE2 and PEP-PE2 can be attributed to a greater amount of vibrational modes stemming from

the large end-cap, which results in overlap of the emission bands. Also, at low temperatures the

vibrational modes are more restricted by the solvent glass, which helps resolve the progression.

At room temperature, more vibrational modes are accessed, leading to broadening of the bands.

This effect is also responsible for the red-shift of the phosphorescence spectra as the sample goes

from glass to solution.

Similarly, the excitation spectra are much more resolved at low temperature. P and P-PEP

show the greatest amount of structure, while the rest of the series show very broad shoulders at

higher energy from the peaks. The red-shift of the spectra is also attributed to the restricted

access of vibrational modes when the sample is frozen in the glass. However, the shift in

phosphorescence energy is much smaller than the change in excitation energy. The dramatic









red-shift and strong broadening of the excitation spectra further support the delocalization of the

singlet state, and the localization of the triplet state.

Conclusion

In this chapter, a series of dinuclear platinum-acetylide oligomers has been characterized

by using photophysical methods. The molecules in the series consist of small aryl spacer groups

spanning two platinum atoms. The oligomer chain is end-capped with either chloride or a

phenyl-ethynyl 7t-conjugated group. It has been shown that the ground-state and triplet-state

absorbance are affected by the addition of the PE2 end-cap, whereas the triplet emission energies

are less dependent on end-cap substitution. In addition, phosphorescence is observed from the

spacer group as well as the higher energy end-cap. The results lead to conclusion that the singlet

So ground state, singlet Si excited state and triplet Tn excited state are delocalized throughout the

molecule. The T1 states, however, are localized on the ligand and spacer groups.

Phosphorescence originates from both the spacer group and the PE2 end-cap, indicating that

there is no significant delocalization through the platinum atom in the triplet state.









CHAPTER 4
PHOTOPHYSICS OF DIPLATINUM POLYYNEDIYL OLIOGMERS:
CHAIN LENGTH DEPENDENCE ON THE TRIPLET STATE IN SP CARBON CHAINS

Introduction

Over the past few decades there has been increasing interest in the study of organic and

organometallic oligomers that feature extended carbon chains, e.g., R-(C-C)n-R and LyM-

(C=C)n-MLy.25,62,77-88 The extended 7c-systems and rigid linear structure characteristic of these

carbon chain oligomers make them potentially useful as molecular wires for transport of charge

(polarons) or excitons on the nanoscale.89 On a fundamental level, oligoynes are unique in that

they provide one of the simplest possible linear 7t-conjugated organic units of variable length.

Such systems are particularly amenable for experimental and theoretical studies focused on

issues related to charge and exciton structure and delocalization in linear 7t-conjugated organic

systems.90-94

While there have been a number of experimental and theoretical reports concerning the

properties of linear carbon chains and related structures, most of this work focused on vibrational

and optical absorption spectroscopy.81'87'92-94 Although several studies have reported

fluorescence emission from carbon chain structures, there are only a few reports concerning the

phosphorescence of oligoynes.93'94 In an effort to increase the stability of linear carbon chain

compounds, as well as extend the study of their photophysics to the triplet state, transition metals

have been incorporated as end-caps to the oligoyne chains.62'88 Notable examples of this work

include a spectroscopic study of 2- and 4-carbon chains spanning two gold atoms reported by

Che and co-workers,95 and a spectroscopic study of a series of platinum(II)-terpyridyl-capped

carbon chains reported by Yam and co-workers.96 Both of these studies provide evidence that

the heavy metal end-groups induce efficient population of a 37r,7* state that is localized on the










carbon chain. The triplet is clearly distinguished by its characteristic phosphorescence emission,

which appears as a well-defined vibronic progression with spacing of ca. 2100 cm1.

One of our collaborators, the Gladysz group at Friedrich-Alexander-Universitat Erlangen-

Nirnberg, in Erlangen, Germany, has developed synthetic methods that allow routine

construction of variable length carbon chains capped on both ends by transition metal units. In

particular, they have recently reported the synthesis and structural characterization of a broad

series of linear carbon chain oligomers of the type Ar-Pt(P2)-(C-C)n-Pt(P2)-Ar, where Ar = aryl,

P = a phosphine ligand and n = 2 14.85,86 The structural, optical and electronic properties of

these oligomers have been probed by x-ray crystallography, UV-visible absorption and density

functional theory (DFT) calculations.86,90 While the optical and theoretical investigations

provide considerable insight concerning electronic structure and 7t-conjugation in the carbon

chains, little work has been done previously to probe the structure and dynamics of the long-

lived excited states (e.g., singlet and/or triplet excitons) in the oligoyne systems.




Pptol3 Pptol3
PtC6Pt Pt Pt
Pptol3 Pptol3

Pptol3 Pptol3
PtCsPt --t Pt
Pptol3 Pptol3

Pptol3 Pptol3
PtCloPt -t t-
Pptol3 Pptol3

Pptol3 Pptol3
PtC12Pt t Pt-
Pptol3 Pptol3

Figure 4-1. Structures of the PtCnPt series.









In this chapter a detailed study of the photophysics of the series of platinum end-capped

oligoynes, PtCnPt (n = 6 12, Figure 4-1) is presented. This study has focused on the properties

of the 37r,7* state localized on the carbon chains that span the two platinum centers. The triplet

state is produced in relatively high yield in these oligoynes due to the strong spin-orbit coupling

induced by the platinum centers. The spectroscopy and dynamics of the triplet state are probed

using variable temperature luminescence, transient absorption, and time-resolved emission

spectroscopy. The results provide clear evidence for an energy gap law dependence of the non-

radiative decay rate of the triplet state.

Experimental

Steady-state absorption measurements were recorded on a Varian Cary 100 dual-beam

spectrophotometer. Corrected steady-state emission measurements were conducted on a SPEX

Fluorolog-3 spectrophotometer. All sample solutions were prepared in tetrahydrofuran (THF) or

2-methyltetrahydrofuran (MTHF). Solvents were distilled over CaH2 immediately prior to use.

Room-temperature measurements were carried out in 1-cm square quartz cuvettes. Solutions

were deoxygenated by purging with argon for 30 minutes and sample concentrations were made

such that the solutions were optically dilute (Amax < 0.20). Low-temperature emission

measurements were conducted in 1-cm i.d. borosilicate glass tubes in a liquid nitrogen-cooled

Oxford Instruments OptistatDN cryostat connected to an Omega CYC3200 autotuning

temperature controller. Samples were degassed by three consecutive freeze-pump-thaw cycles

on a high vacuum (105 torr) line.

Photoluminescence quantum yields were determined by relative actinometry, with

Ru(bpy)32+ as an actinometer (Pp = 0.055 in water). Low temperature quantum yields were

calculated by appropriately scaling the integrated emission area with temperature.97









The phosphorescence spectra for PtC6Pt, PtCsPt, and PtCloPt at 100 K were fitted using a

single-mode Franck-Condon expression,97'98


I(V) y O m (Sm)v exp -41n2 oo+ mm (4-1)
V.=0 E0oo Vm! A A0,1/2

Where I(F) is the relative emission intensity at energy v, Eoo is the energy of the zero-

zero transition, Vm is the quantum number of the average medium-frequency vibrational mode,

h(wm is the average medium-frequency acceptor mode coupled to the triplet-excited state to

ground state transition, Sm is the Huang-Rhys factor, and AF0,1/2 is the half-width of the

individual vibronic bands.

Time-resolved emission measurements were carried out on a home-built apparatus

consisting of a Continuum Surelite series Nd:YAG laser as the excitation source (X = 355 nm, 10

ns fwhm, < ImJ/pulse) and detection measured with a Princeton Instruments PI-MAX intensified

CCD camera detector coupled to an Acton SpectraPro 150 spectrograph. Lifetimes were

calculated by using global analysis of the spectral-kinetics data using the SPECFIT/32 software

package (Bio-logic SAS, Grenoble, France, www.bio-logic.info).

Transient absorption measurements were conducted on a previously described home-built

apparatus.99 Samples were contained in a cell with a total volume 10 mL and the contents were

continuously circulated through the pump-probe region of the cell. Solutions were deoxygenated

by argon purging for 40 minutes and concentrations adjusted so that A355 -0.8. Excitation was

generated using the third harmonic output of a Nd:YAG laser (355 nm, Spectra Physics GCR-

14). Typical excitation energies were 5 mJ-pulse-1, corresponding to a fluence of -20 mJ-cm2.

Transient absorption spectra and decay lifetimes were generated by using software developed in-

house.









Results


UV-Vis Absorption Spectra

The absorption spectra for the series of PtCnPt oligomers in CH2C2 have recently been

reported.86 However, to facilitate comparison with the photophysical data presented herein, the

absorption spectra of the series in were measured THF and the results are presented in Figure 4-

2. The spectra in THF solution are nearly identical to those reported in CH2Cl2;86 however, the

molar absorptivity values are slightly larger in THF. With the exception of PtC6Pt, the

absorption spectra of the oligoynes feature two primary transitions which each appear as a well-

defined vibronic progression that shift to lower energy with increasing carbon chain length. The

low energy transition (inset of Figure 4-2) is comparatively weak (g < 5,000 M^cm-1), whereas

the higher energy transition is considerably more intense, with the molar absorptivity increasing

with carbon chain length. On the basis of DFT computations, both of these bands are assigned as

arising from 7t-t* transitions between orbitals that are concentrated on the -(C-C)n- chains.90

As noted above, the absorption of PtC6Pt does not follow the same pattern seen for the

longer oligomers. The absorption of PtC6Pt is considerably weaker compared to the other

complexes, and it features a broad absorption in the 320 360 nm region, with a second weaker

band with max = 381 nm. While the previously reported DFT calculations suggest that there

should not be a difference in the nature of the low energy transitions for PtC6Pt compared to the

other complexes, it is possible that the absorption of this oligomer is more complex due to

mixing of the 7 and t* levels of the C6 chain with orbitals on the -Pt(P2)(Ar) end groups.











I"
I
I





S I
I )\ I
I
I'\ /
I
~ \ I
I \ I3

I I 'A
l I3
'\'

-/
\^-X \


x 50


0 4 4 5



50 400 450 500 550-


U
300


350


400

Wavelength / nm


450


500


Figure 4-2. Absorption spectra of PtC.Pt complexes in THF solution. In order of increasing
molar absorptivity: n = 6, 8, 10 and 12.


Steady-State Photoluminescence Spectroscopy

Photoluminescence spectra for all of the PtCnPt complexes were studied in MTHF

solution at temperatures ranging from 80 300 K over the wavelength interval 400 800 nm.

The low energy limit in the spectra is determined by the fall-off in the photomultiplier detector

response. As shown in Figure 4-3, at 80 K in the MTHF glass all of the oligoynes display a well-

defined photoluminescence that can be assigned to phosphorescence from a 37,7* excited state.

Note that little or no fluorescence emission is observed in the spectra. (The spectra shown in

Figure 4-3 were obtained with excitation at the absorption maximum for each complex; however,


E
0


0

o:


I I I I I I I I I I 1 I


nI










400 500 600 700 800

1.0 0-0

0.8 0-1

0.6

0.4

0.2 0-2
0-3 A
0.0 1

1.0 0-0

0.8

0.6
i0-1
c 0.4

c 0.2

N 0.0 L

E 1.0 0-0
0
z 0.8

0.6

0.4 -

0.2

0.0 ; -
1.00-0

0.8

0.6

0.4

0.2
D
0.0 ---* ---- *--------
400 500 600 700 800

Wavelength / nm


Figure 4-3. Photoluminescence spectra of PtC.Pt complexes in 2-MTHF solvent glass at 80 K.
(A) n = 6, Xex = 335 nm. (B) n = 8, Xex = 335 nm. (C) n = 10, Xex = 350 nm. (D)n =
12, Xex = 368 nm.









the observed emission spectra did not vary with excitation wavelength.) For the C6, Cs and Co1

oligomers, the phosphorescence appears as a narrow 0-0 band followed by a series of vibronic

sub-bands at lower energy separated by ca. 2100 cm-1. The 0-0 bands of the phosphorescence

red-shift systematically with increasing carbon chain length, and the max value for the

phosphorescence origin for each complex is listed in Table 4-1. The vibronic progression in

each of the spectra arises due to coupling of the triplet excitation to the C-C stretch of the carbon

chain. For PtC6Pt the 0-1, 0-2 and 0-3 sub-bands are resolved, for PtCsPt the 0-1 and 0-2 sub-

bands are seen, and for PtCo1Pt only the 0-1 sub-band appears in the accessible spectral range.

The phosphorescence of PtC12Pt is weaker than for the other oligomers, and it is so red-shifted

that it is only possible to observe the origin of the phosphorescence.

With increasing temperature, the phosphorescence spectra do not change in energy or

bandshape; however, the emission intensity steadily decreases. An example of this is shown in

Figure 4-4. The emission from all of the complexes is much weaker at ambient temperature (300

K), and for PtC6Pt and PtC12Pt the phosphorescence is too weak to detect at 300 K. The room-

temperature phosphorescence quantum yields were measured for PtCsPt and PtCloPt (0.003 and

0.0012, respectively). Using these values, the phosphorescence quantum yields at 100 K were

estimated by scaling the room-temperature quantum yields by the integrated emission area

(which increases with decreasing temperature). These extrapolated low temperature quantum

yields are 0.018 and 0.0053 for PtCsPt and PtCo1Pt, respectively. Because there was no

phosphorescence at ambient temperature for C6 and C12, the low temperature emission quantum

yields could not be determined.








1.2


1.0 I. u r\
120 K
140 K
=- 0.8 165 K
o 180 K

0.6 200 K
230 K
S : 265 K
0.4 298 K


0.2


0.0 .
500 550 600 650 700

Wavelength / nm

Figure 4-4. Emission spectra of PtCsPt in MTHF solvent (glass) over 77 298 K temperature
range. Excitation wavelength 335 nm. Spectra decrease in intensity (quantum
efficiency) with increasing temperature.


Excitation spectra obtained for each of the complexes while monitoring the

phosphorescence at 80 K are shown in Figure 4-5. In general, the excitation spectra are closely

similar to the absorption spectra, except the bands are better resolved due to the low temperature

glass.

In order to provide more information concerning the nature of the triplet state that gives

rise to the phosphorescence, the low temperature (100 K) emission spectra of PtC6Pt, PtCsPt

and PtCloPt were fitted by using a Franck-Condon bandshape analysis according to Equation 4-

1.97'98 The spectrum of PtC12Pt could not be fitted since only the origin is observed. The fits










300 400 500 600 700 800
1.2
1.0
0.8
0.6
0.4
0.2 \ A
0.0
1.0
0.8
0.6
0.4
0.2 /\,J B
0 0.0 '
1.0
E 0.8
0
Z 0.6


C
0.4




1.0


0.6
0.4 I
0.2 D
0.0
300 400 500 600 700 800

Wavelength / nm


Figure 4-5. Overlay of absorption (black lines), emission excitation (green lines) and emission
spectra (red lines) for PtCnPt complexes in MTHF solvent glass at 100 K. Excitation
spectra monitored at peak of 0-0 emission line. (A) n = 6, (B) n = 8, (C) n = 10, (D) n
= 12.









provide estimates for the 0-0 energy (Eoo, the triplet energy), the frequency of the dominant

vibrational mode coupled to the excitation (ho)), the bandwidth (A 1/2), and the Huang-Rhys

parameter (Sm) which is a quantitative measure of the geometric distortion between the ground

and triplet excited states. A listing of the parameters recovered from the fits is shown in Table 4-

1, and an example of the excellent quality of the fit of the experimental spectra is shown in

Figure 4-6.



Table 4-1. Emission spectral fitting parameters for PtCPt at 100 K.
Xmax, em Eoo / cm-1 ho) / cm-1 A 0,12 / cm-1 Sm AST / eV
C6 497 20121 2150 190 1.05 0.53
C8 580 17241 2120 200 0.90 0.71
C10 657 15221 2060 250 0.78 0.75
C12 727 13755 2020 260 a 0.71b 0.68
aEstimated values. Estimated from linear plot of Sm vs Eoo00.



Several features are of interest with respect to the parameters recovered from the fits.

First, as noted above, E00 decreases systematically with increasing carbon chain length, reflecting

the decrease in triplet energy with increasing length of the 7t-conjugated electron system.

Second, as noted above, it is evident that the triplet excitation is coupled to only one high

frequency vibration that corresponds to the stretching mode. Interestingly, the frequency of the

mode decreases slightly with increasing oligomer length, consistent with previous theoretical

studies which indicate that the oligoyne stretching mode decreases in frequency with length.92

Third, it is quite evident that the Huang-Rhys parameter decreases with increasing oligomer

length videe infra). This is consistent with the triplet exciton becoming more delocalized as the

number of carbons in the sp chain increases.








1.2 .


1.0


0.8


0.6
N

E 0.4
0
z
0.2

0i 0S 0 ( : I ,
0.0
14000 16000 18000 20000 22000

Energy / cm-1

Figure 4-6. Fit of emission spectrum of PtC6Pt using Equation 4-1 and parameters listed in
Table 4-1. Dots are experimental data and red solid line is calculated fit.



Phosphorescence Decay Kinetics: Radiative and Non-Radiative Decay Rates

Variable-temperature time-resolved emission spectra were measured for the PtCnPt series

in degassed MTHF solution (glass) over the 80 300 K temperature range and the decay

lifetimes recovered from fits of the emission decays are plotted vs. temperature in Figure 4-7.

Several features are of interest with respect to the temperature-dependent lifetimes. First, for all

of the complexes the emission lifetimes decrease with increasing temperature. This lifetime

decrease is especially large in the temperature region corresponding to the glass-to-fluid

transition of MTHF (120 140 K). Specifically, for PtC6Pt, the lifetime decreases by nearly a

factor of 20 between 110 and 150 K. For the other complexes, the decrease in lifetime is less but










C6 **
25


20


0 15
E

4-I
10


5 C8 xxxc
C10 **X*.4 x
0 C12 vvvvv v*voovevr v v X

50 100 150 200 250 300

Temperature / K

Figure 4-7. Temperature dependence of photoluminescence decay lifetimes for PtCnPt
complexes in MTHF solution (glass). Carbon chain length (n) indicated in plot.


nevertheless it is evident. Second, below the glass point of the solvent (e.g., T = 100 K) the

emission lifetime increases with decreasing carbon chain length. For example, at 100 K, the

emission lifetime increases by approximately a factor of 3 between PtC12Pt, PtCjoPt and PtCsPt

and then it increases by a further factor of 5 for PtC6Pt. As discussed in more detail below, the

overall trend of increasing triplet lifetime with chain length arises due to the energy gap law.

By using the emission quantum yield and lifetime data at low temperature it is possible to

estimate the radiative and non-radiative decay rate constants (kr and knr, respectively). The triplet









lifetime (ZT), phosphorescence quantum yield (4p), and intersystem crossing efficiency (4Isc) are

related to kr and knr by the following expressions:


TT (4-2)
(k, + kp)

kP = ISCk'T (4-3)

Equations 4-2 and 4-3 can be rearranged to expressions that allow knr and kr to be

computed directly from the experimental parameters:


k, =1- --sc 1 1, (4-4)
'ISC TT


kr = c 1 (4-5)


In order to apply these expressions to compute kr and knr, values of TT, 4p and Ilsc are

needed, but we note that 4p is only known with certainty for two of the complexes, and lisc is

not known. However, given the lack of fluorescence from any of the complexes and the

observation of relatively strong triplet-triplet absorption, it is likely that 4jsc is significantly

larger than 0.1, and for the shorter oligomers it is likely close to unity. Thus, it is safe to

conclude that 4p << 4Isc and Equation 4-4 reduces to Equation 4-6. We further rearrange

Equation 4-5 to the usable form Equation 4-7 which emphasizes that there is some uncertainty in

the computed radiative rate constants.

knr =-- (4-6)
TT


isckr = (4-7)
TT









The values of knr and 4Isck, computed from the experimental data at 100 K are listed in

Table 4-2. Several features are of note with respect to this data. First, for PtCsPt and PtC1oPt,

knr exceeds Ilsckr by nearly 100-fold, which indicates that the oligoynes decay predominantly via

non-radiative pathway(s), even in the low temperature glass. Second, it is evident that there is a

systematic increase in knr with increasing carbon chain length. As noted above, this trend is a

manifestation of the energy gap law. Finally, while there is limited data, it is also interesting that

4Isckr decreases from PtCsPt and PtC1oPt; this effect probably arises because spin-orbit

coupling induced by the Pt centers decreases with increasing chain length.



Table 4-2. Photophysical parameters for PtCnPt complexes.

IT / 10-6 S p pa rISCkr / 103 -1 b knr/ 10 s-b / S c
C6 26.85 d 0.372e 1.20
C8 5.24 0.003 0.018 3.44 1.87 1.41
C10 2.92 0.0012 0.0053 1.82 3.41 1.20
C12 0.54 d 18.5 e 1.47
a Adjusted for 100 K. Measured at 100 K. c Extracted from room temperature
transient absorption. d Not observed. e knr = l/T, est.



Transient Absorption Spectroscopy: Triplet-Triplet Absorption of the Oligoynes

In order to provide additional data concerning the properties of the triplet state of the

oligoynes, nanosecond transient absorption spectra were measured at room temperature in

deoxygenated THF solution. As shown in Figure 4-8, near-UV excitation of all of the complexes

produces strongly absorbing transients which decay on a timescale of a few |ts (computed decay

lifetimes are listed in Table 4-2). The transient which gives rise to the absorption is the triplet

excited state. In each case the transient difference-absorption spectra are characterized by

bleaching of the near-UV ground state absorption bands, combined with moderately intense










400 500 600 700


0.02

0.00

-0.02

-0.04
A

0.20

0.15

0.10

0.05

c 0.00
(0 B
0

< 0.20

0.00

-0.20

-0.40
C


0.20


0.00


-0.20
D

300 400 500 600 700 800
Wavelength / nm


Figure 4-8. Transient absorption spectra of PtC.Pt complexes following pulsed excitation with
a 355 nm pulse (10 ns, 5 mJ-pulse-1). (A) n = 6, (B) n = 8, (C) n = 10, and (D) n = 12.
For (A) (C) the first spectrum was obtained 20 ns following excitation, and
subsequent spectra at 400-ns delay increments. For (D) the first spectrum was
obtained at 20 ns following excitation, and subsequent spectra at 80-ns delay
increments.









triplet-triplet absorption in the visible region. Interestingly, for each complex, the triplet-triplet

absorption appears to arise in two distinct bands; one stronger band which is in the near-UV and

blue of the visible, and a second weaker band that is centered in the mid-visible. For PtC12Pt the

visible band is apparently split into vibronic bands. While assignment of these transitions will

require further (theoretical) study, it is evident from the transient absorption study that

intersystem crossing is efficient in all of the oligoynes. Furthermore, it is clear that the triplet-

triplet absorption of the carbon chains is strongly allowed.

Discussion

Variation of Triplet Energy with Carbon Chain Length

A key result of this investigation is that it provides the 0-0 triplet energy as a function of

oligoyne chain length for the PtCnPt complexes. In order to analyze the chain-dependent triplet

energy data, the correlation in Figure 4-9 has been constructed, which shows a plot of E00 vs. 1/n,

where n is the number of carbon atoms in the oligoyne chain. Interestingly, this plot features an

excellent linear correlation and it provides an intercept of 7670 cm-1 (0.95 eV) which

corresponds to the triplet energy in the of the -(C-C)n- polymer. It is noteworthy that there is

substantial variation of the triplet energy across the PtCnPt series, a fact which implies that the

triplet state is delocalized across the entire 7t-conjugated system defined by the carbon chain for

all members of the PtCnPt series. There is no sign of leveling off of the triplet energy with

increasing chain length, suggesting that for chains longer than twelve carbons the triplet energy

will continue to decrease (i.e., the effective "conjugation length" for the triplet has not been

reached in this series).

In a recent study, Nagano and co-workers reported the evolution of the absorption,

fluorescence and phosphorescence of the series of diphenyl-end capped oligoynes Ph-(C-C)n-Ph








(n = 1 6).94,100 Using their reported phosphorescence 0-0 band energies, we have included their

data in Figure 4-9 for comparison with that of the PtCnPt series. Interestingly, the correlation of

E00 vs. 1/n defined by the Ph-(C-C)n-Ph series is curved. For the shorter oligoynes, the trend

defined by the Ph-(C-C)n-Ph series deviates to lower energy relative to the correlation for the

PtCnPt series; however, for longer carbon chain length (n = 4 and 6) the data for Ph-(C-C)n-Ph

series nearly converges to the correlation defined by the PtCnPt series. The curvature in the


30000



25000


E
0

o
LU


20000



15000


10000


5 0 0 0 1 .'I ..'' '.'' I .'' '''. '' I
0.00 0.05 0.10 0.15 0.20 0.25 0.30

1/n

Figure 4-9. Correlation of E00 (triplet energy) vs. 1/n for nT-conjugated oligomers, where n = #
carbons in the chain. (e) : PtCnPt complexes. (x) : Ph-(C-C)x-Ph oligomers ((from
Nagano et al.94). (A) : a-thiophene oligomers (from de Melo et al.101). The dashed
line is best fit to the PtCnPt data with R2 = 0.997.


7
/
/
/
/
/
/


7 x



-7
)17X









correlation for the Ph-(C-C)n-Ph series likely results from the fact that for short oligoyne chain

length, the triplet is delocalized significantly into the phenylene end-groups. This hypothesis is

supported by the observation of significant vibronic coupling to phenyl-based modes in the

phosphorescence spectra of Ph-C-C-Ph and Ph-(C-C)2-Ph.94 At longer chain length, the effect

of the phenylene end groups on the triplet energy in the Ph-(C-C)n-Ph series becomes less

pronounced, and consequently the two correlations converge. This comparison suggests that

correlation of E00 vs. 1/n for the PtCnPt series more accurately reflects the trend in triplet state

energy for the "pure" -(C-C)n- carbon chain (i.e., without interference from the end-groups).

Importantly, this comparison also shows that the triplet energy for the -(C-C)n- polymer which

is estimated by extrapolation of the correlation for the PtCnPt series is likely a much better

estimate than would be obtained from the Ph-(C-C)n-Ph series.

A number of recent studies have explored the variation of the singlet and triplet energies

with oligomer length for nT-conjugated oligomers such as oligothiophene,101-104 oligo(phenylene

vinylene)105'106 and oligo(fluorene).107 Of particular relevance to the present work are studies of

thiophene and substituted thiophene oligomers, where very clear correlations of excited state

energy with oligomer length emerge.101103'107 Figure 4-9 also includes E00 values for the triplet

state in the series of ca-oligothiophene oligomers (for n = 2 7) reported by de Melo and co-

workers.101 In the figure the oligothiophene triplet energies are plotted vs. the 1/n, where n is the

number of carbon atoms in the nT-conjugated chain (there are 4 carbons per thiophene repeat

unit). Interestingly, it is clear that the trend of E00 vs. 1/n for the oligothiophene series runs

nearly parallel to the correlation defined by the PtCnPt series. This agreement indicates that the

degree of triplet delocalization is similar in oligoynes compared to the oligothiophenes. Note,









however, that the extrapolated value of the triplet energy for polythiophene is ca. 2500 cm-1

higher than that of the -(C-C)n- polymer.

Since significant fluorescence was not observed for the PtCnPt series, it is only possible to

estimate the Si energy based on the onset of the absorption bands. Using this as an

approximation for the Si state energy, we have computed estimated singlet-triplet splitting

(AEST) for the PtCnPt series and the values are listed in Table 4-1. Interestingly, for the three

longer oligoynes the AEST cluster around a value of 0.7 eV, which is in good accord with the

value observed for a variety of other long 7t-conjugated systems, including poly(thiophene) and

poly(phenylene vinylene).60

Energy Gap Law Correlation for the Oligoynes

The energy gap law was first suggested by Robinson and Frosch to explain the systematic

variation of triplet state non-radiative decay rates in polycyclic aromatic hydrocarbons.108

Shortly thereafter, Siebrand developed a quantitative theory for the energy gap law.109 The

energy gap law predicts that the rate of non-radiative decay of an excited state will increase as

the excited state energy decreases. The effect arises because of the relationship between the

Franck-Condon factors for vibrational overlap of the excited- and ground-electronic states and

the energy gap (and geometric distortion) between the potential surfaces for the two electronic

states (see below, Equation 4-8). The most comprehensive study of the energy gap law carried

out to date lies in the work of Meyer and co-workers who showed the existence of a quantitative

relationship between excited state energy and the non-radiative rate for the metal-to-ligand

charge transfer state in a series of Ru(II) and Os(II) polypyridine complexes.110-112 More

recently, Kohler and co-workers showed that the energy gap law holds for triplet decay in a

series of platinum-acetylide oligomers and polymers.113









A focus of the present investigation is to explore whether the energy gap law holds for

decay of the triplet excited state in the series of platinum end-capped oligoynes. This study is of

interest due to the remarkable simplicity of the carbon chains: they consist of a linear, 7r-

conjugated electronic system in which the excitation is coupled to a single, very well-defined

high frequency vibrational mode. In short, the oligoynes seem to provide an ideal platform to

examine the correlation between spectroscopy, triplet energy and non-radiative decay rate.

The following expressions provide the complete relationship between the non-radiative

decay rate and parameters accessible from spectroscopy,98'110


ln k = ln Sm + 0 +1)2 (AO,1/2/1m )2- 0.51n( )mEo 1yE (4-8)
161n2 1000 cm -) TOm


S= Ck 1000 cm-' (4-9)



70 =ln( E -1 (4-10)


where Sm, Eoo, AFO,1/2, and hom, are the same terms defined in Equation 4-1, Ao is the

vibronically induced electronic coupling term, Ck is the vibronic coupling matrix element, and )Ok

is the frequency of the promoting vibrational mode. These equations can be simplified in order

to facilitate analysis of experimental data. The assumption that all terms on the right hand side

of Equation 4-8 except the last one are independent of changes in E00 leads to Equation 4-11.112

This equation predicts a linear relationship between the log of the non-radiative decay rate and

E00, with a slope equal to Yo/hco (this is the mathematical statement of the energy gap law). Also,

since In /o is the only term in Equation 4-8 that cannot be computed using parameters derived

from a Franck-Condon fit of an emission spectrum, this equation can be re-cast as Equation 4-12









where ln[FCF(calcd)] is an abbreviation for "calculated Franck-Condon factors". Note that

ln[FCF(calcd)] is computed by substituting the parameters recovered from the emission spectral

fits (Table 4-1) into the last 4 terms on the right hand side of Equation 4-8. The latter expression

implies that a plot of ln[FCF(calcd)] vs. In k, will be a linear correlation with a slope equal to

unity.


n knr= a (4-11)


ln[FCF(calcd] = Ink, In/, (4-12)

Figure 4-10 illustrates plots of In knr vs. E00 for the PtCnPt series (filled circles, values at

100 K, see Table 4-2). Also included in the correlation are data from the series of platinum

acetylide oligomers reported by Kohler.113 Several features are evident from this presentation.

First, the non-radiative decay rates for the PtCnPt series clearly follows the correlation predicted

by the energy gap law. This result clearly shows that in the frozen solvent glass non-radiative

decay of the triplet state in the oligoynes is controlled by the extent of vibronic coupling of the

exciton to the high frequency mode (the -C-C- stretch), which varies exponentially on the

excited state energy. The slope of the energy gap correlation for the PtCnPt series is almost the

same as that defined by the platinum-acetylide oligomers reported by Kohler and co-workers.113

This correspondence in the energy gap law dependence likely arises because in both of the

platinum-containing systems the -C-C- stretching mode is likely the dominant acceptor mode

coupled to non-radiative decay. The correlation for the PtCnPt series has a larger intercept

value, which reflects the fact that non-radiative decay is faster at equal excited-state energy in the

two series. This larger "intrinsic" knr (a in Equation 4-11) for the PtCnPt series may be due to

the fact that in the platinum-acetylides studied by Kohler medium frequency modes (aromatic












16-


14


12 -


10 X

S--x
8



10000 12000 14000 16000 18000 20000 22000

Eoo / cm-1

Figure 4-10. Energy gap law correlation, see text for details. (e) : PtCnPt complexes. The solid
line is best fit with R2 = 0.951. (x) : Platinum-acetylide oligomers (from Wilson et
al.113) The dashed line is best fit with R2 = 0.946.



ring C-C stretching) are also coupled to the triplet exciton. This would effectively decrease the

Frank-Condon factors for non-radiative decay (note that ok appears in the /o term in Equation 4-

8).

Also of interest is the correlation of the Huang-Rhys parameter (Sm, calculated from the fits

of the emission spectra of the PtCnPt oligomers for n = 6, 8 and 10, values in Table 4-1) with the

triplet energy shown in Figure 4-11A. As can be seen from this correlation, Sm decreases with

E00, with a very good linear correlation being defined by the series. (By extrapolating the linear

correlation defined for PtC6Pt, PtCsPt and PtCjoPt the Sm value for PtC12Pt has been









estimated). The decrease in Sm with increasing carbon chain length is consistent with the triplet

excited state becoming more delocalized, which has the effect of decreasing the electron-

vibrational coupling. A similar correlation for Sm vs. Eoo was reported by Meyer and co-workers

for a metal-to-ligand charge transfer states in a series of Ru-polypyridine complexes, and their

data is shown for comparison in Figure 4-11A.112 Despite the significant difference in the nature

of the excited states for the Ru-polypyridine and PtCnPt complexes (metal-to-ligand charge

transfer and 37 ,7*, respectively), the correlations are remarkably similar.

Finally, as noted above, it is possible to compute the Franck-Condon factors (FCF) for

non-radiative decay using the parameters obtained from the emission fits. As suggested by

Equation 4-12, the calculated FCF values should vary linearly with In knr, with a slope of unity

and a single adjustable parameter which corresponds to In fl.98'112 Figure 4-11B shows the

correlation of In FCF and In knr, where the In FCF values for the PtCnPt complexes were

computed using the parameters recovered from the spectral fits (Table 4-1) and Equation 4-8.

Since it was not possible to fit the emission spectrum for PtC12Pt, the Sm needed to compute In

FCF for this complex was extrapolated using the observed E00 value in the correlation shown in

Figure 4-11A. The dashed line in Figure 4-11B has a slope of unity and an intercept of -24.7,

which affords a value of f = 5.3 x 1010 cm-1. Given the relatively large range in Eoo00 and kn

values for the series (6000 cm-1 and a factor of 50) the correlation of the calculated Franck-

Condon factors and the non-radiative decay rates is impressive. The quality of the correlation

underscores the quality of the energy gap law correlation for the PtCnPt series, and that in the

frozen solvent glass non-radiative decay is controlled by coupling to the -C-C- mode of the

carbon chain.










1.6


1.4


1.2


E 1.0


0.8


0.6


0.4
12000




-9 i


-10 k


- -11
0

-12
LL

_ -13


-14


-15


14000 16000 18000 20000 22000

E00 / cm-1


In knr


Figure 4-11. (A) Plot of Huang-Rhys parameter (Sm) vs. the triplet energy (Eoo) for PtC.Pt
complexes (*) and for series of ruthenium polypyridyl complexes (7). Data for the
ruthenium complexes are from Barqawi et a.112 Lines are best fit to data, R2= 0.999
for PtCnPt complexes and R2 = 0.963 for ruthenium polypyridyl complexes. (B) Plot
of calculated Franck-Condon factors vs. natural log of non-radiative rate for PtC.Pt
complexes. The line has a slope of unity. In both plots, the open circle data point
(0) is for PtC12Pt. The Sm value for PtC12Pt in plot (A) is extrapolated using the
best fit line (R2 = 0.960) and the experimental E00 value and the In FCF(calcd) value
for this complex in plot (B) is computed using the extrapolated Sm value.


/ 0

0^--


F


I I I I I I I I I I I I I I I I I I I I I I I I









Excited State Decay above the Solvent Glass Point

As noted above, the energy gap correlation for the PtCnPt series holds only at

temperatures below the solvent glass point. It is evident that in fluid solution an additional non-

radiative decay pathway is active that dominates decay of the triplet state in the complexes.

Activation of this pathway as the glass melts is very evident in PtC6Pt; for this complex the

lifetime decreases by nearly a factor of 100 as the temperature is increased from 100 160 K.

Given that this decay pathway is activated by melting of the solvent matrix, it is reasonable to

assume that it is related to the increase in the ability of the carbon chain to undergo fluxional

motion. Previous theoretical and experimental studies of-(C-C)n- oligomers indicate that in

some cases the optimum geometry in the excited state involves a transoid conformation in which

the triple bond length is significantly increased (i.e., the carbons become sp2 hybrids).94'114

Although there is insufficient experimental data from the present photophysical studies to prove

that a bending mode of the -(C-C)n- chain is the origin of the accelerated triplet decay in fluid

solution, it is believed that it is a very reasonable hypothesis. Future DFT calculations which

probe the geometric energy landscape for the triplet state of the PtCnPt series may provide

insight into this issue.

Summary and Conclusion

A detailed photophysical investigation of a homologous series of carbon chain oligomers

that are end-capped with Pt-acetylide units has been carried out. The photophysics of the

complexes is dominated by a 3x,7* that is concentrated on the -(C-C)n- chain, and the triplet

state is produced in comparatively high yield due to the strong spin-orbit coupling induced by the

heavy metal platinum centers. At low temperature (T < 130 K) each of the complexes exhibits a

unique, highly structured phosphorescence that is characterized by a series of narrow (< 200 cm-1









fwhm) vibronic bands separated by ca. 2100 cm-1. The appearance of the spectrum signals that

the triplet state is coupled to a single, high frequency vibrational mode that corresponds to the

stretch of the -(C-C)n- chain. Analysis of the phosphorescence spectra using a single-mode

Franck-Condon bandshape analysis indicates that the 0-0 energy of the triplet state decreases by

ca. 6000 cm-1 as the length of the carbon chain increases from 6 to 12 atoms, and the electron-

vibrational coupling constant (Sm) decreases with increasing chain length. At room temperature

the triplet state can be observed by transient absorption spectroscopy, and each of the complexes

exhibits moderately intense triplet-triplet absorption throughout the visible region (400 700

nm).

Phosphorescence lifetime studies were carried out on the series to study the decay kinetics

of the triplet state. At low temperature in a frozen solvent glass, the triplet lifetime

systematically increases with decreasing length of the carbon chain, with the lifetimes ranging

from 27 |ts (PtC6Pt) to 500 ns (PtC12Pt). Given that the phosphorescence quantum yields are

comparatively low, the lifetimes are controlled by the rate of non-radiative decay, and the

observed lifetime variation at low temperature is attributed to the energy gap law. Consistent

with this hypothesis, the non-radiative decay rates of the complexes at T = 100 K are shown to

exhibit a quantitative energy gap law correlation.

The properties of the triplet state in the PtCnPt series are believed to be minimally

perturbed by the presence of the platinum complex end-capping units. Consequently the results

of this study provide the best available quantitative insight concerning the structure and

dynamics of the triplet state in carbon chains of extended length.









CHAPTER 5
AN IRIDIUM COMPLEX THAT EXHIBITS
DUAL-MECHANISM NONLINEAR ABSORPTION

Introduction

Reverse saturable absorption (RSA) as a sequential, multi-step process involving two

photons as a mechanism for optical limiting was described in detail in chapter 1. When a

material has an excited state absorption cross section larger than the ground state absorption

cross section at a particular wavelength, it is said to be a reverse saturable absorber. RSA is a

slow (-10-10 10-8 s) process requiring first the ground state absorption of a photon to populate

the excited state, followed by excited state absorption of a second photon. While this has been

shown as an effective method of nonlinear absorption, there are several limiting factors. Most

notably is that RSA is generally a slow process requiring long-lived excited states (usually triplet

excited states populated by ground state absorption to the singlet excited state followed by

intersystem crossing). Due to the multi-step nature of the absorbing mechanism, RSA is an

ineffective pathway for limiting laser pulses below the nanosecond time regime.

Another mechanism for nonlinear absorption involves the instantaneous absorption of two

photons. Consequently, two-photon absorption (TPA) is mostly effective for optical limiting

only at very short timescales (< 100 ps).115 A second benefit of using TPA as an optical limiting

mechanism is that the material can be completely transparent at the active wavelength, whereas a

RSA material must have some ground state absorption.

One method of extending the time regime of an optical limiter is to combine absorption via

several mechanisms. By coupling slow timescale reverse saturable absorption-or more

generally, excited state absorption-with fast timescale two-photon absorption, a broad temporal

response to optical limiting can be achieved. The compound characterized in this chapter

exhibits such absorption pathways.









The theoretical prediction of two-photon absorption was first described by Goeppert-

Mayer in 1931.116 However, two-photon absorption processes were predicted thirty years before

they could be experimentally verified. It took until the advent of the laser for the first

observation of TPA in the lab by Kaiser and Garrett in 1961.17

Figure 5-1 shows a Jablonski diagram with corresponding representative absorbance

spectra illustrating two-photon absorption. As shown, a material can absorb a single photon

corresponding to an energy, E, which will promote an electron to a higher state. Alternatively,

certain materials can simultaneously absorb two photons, whose energies sum to equal the



One-photon Two-photon
absorption absorption



: 2hv

w hv A 0
Y2hv <



X 2X
Wavelength

Figure 5-1. Jablofski diagram and corresponding representative absorption spectra illustrating
two-photon absorption.



energy between the two states. One- and two-photon excitation will not necessarily promote

electrons to the same state. That is, one- and two- photon excitation follow different selection

rules.18 Selection rules for two-photon excitation will not be explored here, as the results

presented later will be given in a more qualitative presentation. One-photon absorbance spectra









for a material theoretically should be different from two-photon absorbance spectra. While the

amount of data on two photon absorption spectra is increasing, it is still a difficult measurement.

Measured values for two-photon absorption coefficients can vary significantly based upon

solvent, laser intensity, pulse duration of the laser and other experimental conditions.119

In order to understand the concept of nonlinear absorption through a multi-photon process,

it is necessary to first understand the linear, one-photon absorption process. From Beer's law,

one can consider a sample of some thickness along the z-axis. For incident light propagating

along the z-axis through a sample, the light changes according to Equation 5-1.

S-al (5-1)
dz

where a is the absorption coefficient. For a one-photon process, no other terms apply.

However, for two-photon absorption, the absorption coefficient is defined by Equation 5-2.

a = ao + fl (5-2)

where ao is the one-photon absorption coefficient and /is the two-photon absorption coefficient.

Substituting a in Equation 5-1,

=-aI P2 (5-3)
dz

an expression is obtained illustrating the (linear) one- and (nonlinear) two-photon absorption

dependence on incident intensity.120

To further the nonlinear optical properties of a two-photon absorbing material, a second

absorbing mechanism can take place. After populating the excited state of a material, the

electron can either return to the ground state, or absorb an additional photon. The initial

absorbing state can be excited by either one- or two-photon absorption, as the resulting excited









state is independent of the excitation pathway. Excited state absorption (ESA) following TPA

leads to enhanced nonlinear optical properties.

If excited-state absorption occurs, a third term applies in Equation 5-3, causing the change

in intensity through the sample to become

dl
= -aoI- Jf2 cTTTI (5-4)
dz

where rTT is the absorption cross-section of the T1 triplet excited state, and T1 is the population

of the first triplet excited state. Equation 5-4 holds only within the lifetime of the triplet state.

For many compounds, and in particular the compound discussed in this chapter, the

absorbing excited state will be the triplet state. A four-level Jablofski diagram illustrating the

transitions involved in TPA followed by ESA is shown in Figure 5-2. The process is initiated by

simultaneous absorption of two photons via TPA by the ground state promoting an electron to

the Si excited state. Intersystem crossing then populates the first triplet excited state, Ti. The

last step involves the absorption of a photon to populate some higher triplet state, Tn.

(Intersystem crossing is not necessary for enhanced TPA; the Si state could alternatively be an

absorbing species, although this mechanism will not be the focus of this chapter.) Rate equations

for each state can be written as shown in Equations 5-5, 5-6, 5-7 and 5-8. The following rate

equations are described assuming only the transitions illustrated in Figure 5-2 will occur.

dS0 = I2+--+-- (55)
S # _2 + + T(5-5)
dt 2hv 1s TT1


dS, p I2 k-s- c S (5-6)
dt 2hv rTs

d T IIT T T,
= kiscS S 1 + (5-7)
dt hv rT, rT,








dT, ,T, T
= 1+j (5-8)
dt hv Tr


T-


Si






So


[n
ri
ISC

T 1


CoTT hv


hv




hv


Figure 5-2. Jablonski diagram for a four-level system illustrating TPA with ESA. The excited
triplet state is populated by two-photon absorption of the ground state followed by
intersystem crossing. The excited triplet state then absorbs an additional photon.


where So, Si, T1 and T, are the populations of the ground and first excited singlet states and first

and some higher triplet excited states, respectively, ris the lifetime of the labeled state, and kisc

is the intersystem crossing rate. Integrating the equations to find an expression for T1, and

substituting into Equation 5-4, an expression for the intensity at the end of the laser pulse is

given:

= fl2 TT 13 (5-9)
dz 2hv

where ris the duration of the laser pulse. Several assumptions are made for Equation 5-9 to be

valid:121 (i) one-photon absorbance at the two-photon absorbing wavelength is negligible for the

So state (ii) the lifetime of the T1 state is longer than the length of the laser pulse, (iii) intersystem









crossing efficiency approaches unity and is shorter than the length of the laser pulse, and (iv) the

population of the So ground state and is not sufficiently depleted, i.e. Si, T1, T, << So. From

Equation 5-9, it is shown that reverse saturable absorption occurs when rTT > 0.121 Therefore, a

material which exhibits excited-state absorption at the two-photon absorbing wavelength is a

reverse saturable absorber.

Two-photon absorbing materials commonly have donor (D) or acceptor (A) groups linked

by a 7t-conjugated bridge.122-124 Different types of architectures are shown in Figure 5-3. More

complex motifs, including branched structures, with D-1t-A-7t-D type conjugation are also

possible, allowing for increased TPA cross sections. There have been numerous reports

concerning TPA or RSA in organic and organometallic molecules, but currently there is not

much in the literature about chromophores that exhibit both TPA and RSA.125-127 Our group is

interested in the design, synthesis and characterization of materials that exhibit "dual-mode"



Type I


Acceptor 71 Acceptor


Type II


Donor 71 Acceptor


Type III


Donor 1 Donor

Figure 5-3. Structural motifs for two-photon absorbers.









TPA/RSA nonlinear absorption. In particular, oligomers that contain a 7t-conjugated

chromophore with a large TPA cross section that is strongly coupled to a transition metal

chromophore that promotes intersystem crossing to a long-lived triplet excited state with a large

triplet-triplet absorption cross section.

The photophysical characterization of such a chromophore will be described in this

chapter. The material of interest, the Ir(III) complex ML, is shown in Figure 5-4. This complex

incorporates a heavy metal center into a D-7T-D type 7t-conjugated oligomer L. 7t-Conjugated

molecular structures containing donors or acceptors at both ends of a delocalized, 7t-electron

system exhibit large TPA cross sections.124,128 In addition, iridium has the largest spin-orbit

coupling constant of all the transition metals, and consequently it is able to facilitate intersystem

crossing to afford a large triplet yield.129




Hex2N \-N NHex2 L
N N



Hex2N NHex2 ML
N N
M

M = Ir(ppy)2+[PF6-]; ppy = 2-phenylpyridine

Figure 5-4. Structures of the ligand, L, and iridium(III) complex, ML.



Experimental

All photophysical measurements were carried out in 1 cm x 1 cm quartz cuvettes, unless

otherwise noted. All sample solutions were prepared in dry THF and deoxygenated by bubbling









with argon unless otherwise noted. UV-visible absorption spectra were recorded on a Varian

Cary 100 dual-beam spectrophotometer. For emission measurements, sample concentrations

were adjusted to produce optically dilute solutions (Amax < 0.2). Corrected steady-state emission

measurements were performed on a SPEX F-112 fluorescence spectrometer equipped with a

Hamamatsu R928 PMT for visible measurement. Emission quantum yields were measured

relative to Ru(bpy)3C12 in air-saturated H20, where 4 = 0.0379.130 Transient absorption

measurements were carried out with solutions having an absorbance of 0.8-1.0 at 355 nm.

Transient absorption spectra were obtained on an instrument that has previously been described99

that uses the third harmonic of a Nd:YAG laser (Spectra Physics GCR-14, X = 355 nm, 10 ns

fwhm, 10 mJ pulse-1, 20 mJ-cm-2 fluence) as the excitation source.

Two-photon-induced emission spectra were recorded on an apparatus that relies on a

Continuum Surelite series Nd:YAG laser as the excitation source. The fundamental output of the

laser (k = 1064 nm, 10 ns fwhm) was passed through a telescope providing a final beam diameter

of 0.7 cm. This beam was focused with a 10-cm focal length lens (Newport Optics) and the

sample was positioned at the focal plane of the lens. The luminescence was detected by an

intensified CCD detector (Princeton Instruments, PI-MAX) coupled to an Acton SpectraPro 150

spectrograph. The sample was contained in a 5 mm i.d. round borosilicate tube. Nonlinear

transmittance was measured using the same apparatus (shown in Figure 5-5), with samples

contained in a 10-mm pathlength quartz optical cell that was positioned at the focal plane.

Nuetral density filters were used to attenuate the beam fluence. A 10% beamsplitter was used to

direct part of the incident energy to the reference detector, DI, (Ophir analog power meter). The

transmitted beam was passed through a 10-cm focal length lens (L1) before reaching the sample.









A second lens (L2) collected the beam before reaching the detector, D2 (Scientech P09 energy

meter with a Vector S310 digital display).

Transient absorption arising from two-photon excitation was carried out using an

unfocused beam from the Continuum Surelite laser as the excitation source, a Hamamatsu xenon

flash lamp, and the previously mentioned spectrograph and CCD detector.

Fluence-dependence emission intensity arising from two-photon excitation was measured

using an unfocused 0.8 cm diameter beam from the fundamental of a QuantaRay GCR series

Nd:YAG laser as the excitation source (k = 1064 nm, 10 ns fwhm). Detection was provided by a

Hamamatsu R928 PMT coupled to an Oriel 1/8 m monochromator set at the emission

wavelength maximum.



Nd:YAG laser NDF BS L1 Sample L2 D2











D1

Figure 5-5. Apparatus for measuring nonlinear transmittance. NDF: neutral density filter; BS:
beamsplitter; L: lens; D: detector.



Results

One-photon ground-state absorption and photoluminescence spectra for ligand L and Ir

complex ML in THF solution are shown in Figure 5-6. Ligand L exhibits absorption bands at









280 and 400 nm, arising from short- and long-axis 7t-7t* transitions, respectively. Ir complex

ML features a broad absorption band at 500 nm, arising from a combination of the long-axis

polarized 7t-7t* transition and Ir--ligand metal-to-ligand charge transfer (MLCT) transition.

Intense fluorescence from ligand L is observed with a band maximum at 520 nm. The emission

of ML is dominated by a broad featureless band having an emission maximum at 730 nm. The

quantum yield at this wavelength is 0.003, with an emission lifetime of 513 ns.



10 .....................1.2
A C
8 1.0

0.8
6
0.6
4
0.4 -

2
E 2 0.2



--- 1.0
J 8- 0
Z
0.8
6
0.6
4
0.4

2 0.2
B D
0 ** 0.0
300 400 500 600 700 300 400 500 600 700 800

Wavelength / nm

Figure 5-6. Absorption of (A) ligand L and (B) complex ML and one-photon emission of (C)
ligand L and (D) complex ML.








Transient absorption spectra of ML in THF solution obtained under one-photon excitation

conditions (5 ns pulse, 355 nm) are show in Figure 5-7. The TA spectrum features a ground-

state bleaching centered near 500 nm combined with a strong absorption band that extends

throughout the visible and into the near infrared region. The absorption wavelength maximum of

the transient is approximately 875 nm with a As value of 6.1 x 104 M1 cm-1. The transient

absorption decays with a lifetime of 1.2 [Is.


0.3


0.2

-0.1
0
S0.0

< -0.1


-0.2

-0.3


400 600 800 1000 1200 1400

Wavelength / nm

Figure 5-7. Transient absorption of ML in deoxygenated THF solution obtained at 400 ns
increments following 355 nm excitation.








Given that the tn-t* ground state absorption in complex ML is at 500 nm, it seemed

reasonable that two-photon absorption would occur in the near-infrared at 1064 nm, i.e. the

fundamental of a Q-switched Nd:YAG laser. The two-photon upconverted photoluminescence

spectrum of ML produced by excited with the 1064 nm beam is shown in Figure 5-8. The

emission band shape and wavelength maximum are the same as those observed under one-

photon absorption conditions. The upconverted luminescence can easily be seen by eye as

shown in Figure 5-9. A plot of the integrated emission area as a function of infrared excitation

intensity is shown in Figure 5-10, and a fit shows that the emission is dependent on the square of



2500



2000



O 1500

C,

1000



500




400 500 600 700 800
Wavelength / nm

Figure 5-8. Two-photon induced emission of ML after pulsed excitation by 1064 nm.






















Figure 5-9. Photograph of 5 mM ML solution. (A) Under ambient conditions and (B) showing
visible emission after excitation by 1064 nm laser light.


0.0


0.5 1.0 1.5


Laser fluence / mJ cm-2

Figure 5-10. Emission area dependence of ML on incident laser energy at 1064 nm.









the of the excitation intensity. According to Equation 5-3, this indicates a two-photon absorbing

mechanism.

Transient absorption spectra collected after excitation by 1064 nm were also observed

(Figure 5-11). The band shape of the transient absorption spectrum measured under two-photon

excitation conditions is similar to that obtained under one-photon excitation. The two-photon

excited transient absorption spectrum was obtained on a different instrument than the one-photon

excited spectrum shown in Figure 5-6(D). The detector used for the two-photon excited studies

is not sensitive beyond 850 nm, and therefore, it is not possible to observe the maximum of the

triplet-triplet absorption that occurs near 875 nm.





0.03


0.02


0.01
0


< 0.00


-0.01


500 600 700 800 900

Wavelength / nm

Figure 5-11. Transient absorption spectra of 5 mM deoxygenated THF solution of ML
following 1064 nm pulsed excitation. Excitation fluence in order of decreasing
transient absorbance signal: 0.35 J cm-2; 0.25 J cm-2; 0.20 J cm-2; 0.12 J cm-2; and
baseline (no laser).








Nonlinear transmittance measurements of ML were carried out at various concentrations

using 1064 nm, 5 ns pulses. Figure 5-12 shows the laser fluence dependence of the transmittance

of ML at a range of concentrations. While the blank THF solution responds linearly to the

incident energy, it is evident that the THF solutions of the Ir complex ML exhibit nonlinear

absorption, the extent of which increases with the concentration of ML. Notably, for a 20 mM

solution of ML, the transmitted energy is significantly reduced above an input energy of 0.8 mJ.




1.2 THF only
2.5 mM
1.0 5.0 mM .
v 10 mM
E 0.8 20 mM v
0
-,0.6


l0.4 o -
0.2
0 V


0.4 *
0.2 09 4*


0.0
0.0 0.5 1.0 1.5 2.0
Ein / J cm-2

Figure 5-12. Transmittance of 1064 nm pulsed beam at various concentrations of ML in THF.



Discussion

The absorption assignment for the Ir complex ML is slightly more complex than for the

ligand L. The related complex Ir(ppy)2(bpy)+ (where bpy = 2,2'-bypyridine), in which the visible









absorption is exclusively due to an MLCT transition, has previously been reported.131'132 The

molar absorptivity of 104 M-1 cm1, as reported by Glusac and co-workers,131 which is much

smaller than that of ML, indicates that the visible absorption of ML is dominated by the

intraligand 7t-7t* transition. The absorption maximum for ML is red-shifted by approximately

100 nm compared to ligand L. The red shift arises due to the effect of the cationic Ir(III) center

on the 7t-electron system of the conjugated bypyridine ligand system. In particular, the metal

reduces the LUMO energy of the conjugated ligand, in part due to its positive charge, and in part

due to the effect of coordination which forces the bipyridine unit into a planar conformation.133

The emission of ML arises from an excited state having mixed character, consisting of

intraligand 3 7,7t* and 3MLCT configurations.131'134'135 This assignment is supported by the fact

that the emission is strongly Stokes-shifted from the absorption, and it is comparatively long-

lived.

An important question concerns the nature of the excited state that is responsible for the

strong transient absorption. As noted earlier, on the basis of previous investigations of d6

transition metal complexes with 7t-conjugated ligands,131'134'135 the long-lived excited state of

ML is tentatively assigned to a state having mixed 3MLCT-37x,7t* character. This assignment is

based on previous studies of Ir(III) and Ru(II) complexes with 7t-conjugated diimine ligands of

varying conjugation length.131'134 In this work, it has been demonstrated that as the energies of

the 3MLCT and 37r,7* states converge, the lowest excited state exhibits mixed 3MLCT-3 7,t*

character. In ML, the 3MLCT-37,7rt* are within 0.25 eV relative to one another,131'134 and

therefore, the condition is correct for configuration mixing of the two states.

The emission band shape and wavelength maximum arising from absorption at 1064 nm

are the same as those observed under one-photon excitation conditions. The observation of









emission at 700 nm under 1064 nm excitation clearly indicates that ML can be excited by two-

photon absorption. That is, excitation at a particular wavelength would not cause emission at a

higher-energy wavelength unless multiphoton absorption is the mechanism by which the excited

state is populated.

Although instrument limitations restricted detection of the full transient absorption

spectrum under 1064 excitation conditions, the portion of the observed spectrum matches that of

the spectrum obtained after excitation by 355 nm. The fact that strong transient absorption is

seen for the triplet state under two-photon excitation conditions further indicates that a relatively

large triplet excited-state population can be produced via two-photon absorption, even with a

nanosecond laser.

Conclusion

In this chapter, the photophysical characterization of a two-photon absorbing iridium

complex has been described. The ligand in the complex provides a large absorption cross-

section for one- and two-photon excitation. The long-lived triplet excited state affords the

opportunity for absorption of an additional photon. This combination leads to enhanced

nonlinear optical absorption.

The photophysics of this complex have shown that the triplet state can be populated

independently by one-photon and two-photon absorption. Incident-energy-dependent emission

intensity illustrates the two-photon absorbing nature of the complex. The iridium atom

contributes to strong spin-orbit coupling of the ligand to reach the triplet excited state through

intersystem crossing from initial excitation. This is demonstrated by one- and two-photon

induced phosphorescence, as well as transient absorption. In addition, the nonlinear optical

properties are clearly shown by the trend seen with incident energy dependence of the

transmittance.









CHAPTER 6
CONCLUSIONS AND FUTURE WORK

In previous chapters, various aspects of the photophysical properties of some platinum-

and iridium-containing organometallic compounds have been presented. Additionally, a

transient absorption apparatus was designed and constructed to probe the excited state

photophysics in supplement to techniques possible with commercial instrumentation. The

platinum oligomers and iridium complex were designed to gain some insight into the

photophysical behavior of these novel materials, but also to gain knowledge about the triplet

state in general as it is less studied than the singlet state. The triplet state properties of all-

organic systems are less understood as access into the triplet manifold is not easily facilitated.

The studies presented in previous chapters may therefore help the understanding of the

photophysical properties of triplet excited states in organic and metal-organic conjugated

systems.

The transient absorption apparatus was designed with the primary goal of incorporating an

intensified CCD camera detector into the system. Along with a white-light probe source, a CCD

detector allows for the simultaneous collection of all wavelengths in the visible spectrum. The

benefits of using a CCD camera as opposed to another detector such as a photomultiplier tube

include reduced sample exposure to the excitation source and reduced experiment time. The

inclusion of an intensifier as part of the CCD system allows for gate times as short as 10 ns,

therefore making it possible to extract kinetics from species with lifetimes as short at -100 ns.

Data conversion software was also developed to present the time-resolved transient absorption

spectra in a conventional manner. While the excitation and probe source, as well as the detector,

can be replaced or modified to expand the range of molecules that are possible to study, a major

limitation of the apparatus is defined by the nature of the sample. Specifically, if a material









exhibits strong photoluminescence, it can be collected by the detector. This may result in a

negative signal that can obscure the transient absorption. Consequently, the software used to

control the hardware and calculate the spectra are being modified to create an option for

emission correction.

The photophysical characterization of a series of dinuclear platinum oligomers reveals

several things concerning the delocalization of singlet and triplet states in these types of

molecules. Most significantly, the phosphorescence of the PE2 end-capped oligomers show

distinct emission from the spacer and the end-cap, which indicates limited delocalization of the

triplet state through the platinum centers. Temperature-dependent emission spectra also support

the idea of localization of the triplet state. In contrast, variable temperature excitation indicate

less temperature dependence of the singlet ground and excited states, suggesting more

delocalization of these states compared to the triplet excited state. It is not known if the emission

occurs simultaneously from the spacer and end-cap, or if there is energy transfer from one

chromophore to another during the lifetime of the triplet state. Time-resolved emission studies

could provide better understanding on the nature of the excited state. More definitive trends

could be assigned by characterizing a larger series of dinuclear platinum oligomers. The spacer

and end-caps should be modified systematically in order to gain better understanding of the role

of platinum in the triplet state.

In a second study, a series of molecules in which carbon chains of different lengths span

two platinum atoms was characterized by photoluminescence, lifetime and quantum yield

measurements. The low-temperature emission spectra reveal intense vibrational coupling to the

-C-C- stretching mode. The emission spectra were subjected to Franck-Condon analysis, and a

linear relationship between emission energy and chain length (from 6 to 12 carbon atoms) was









revealed. The electron-vibrational coupling constant decreases with increasing chain length.

Triplet lifetimes are governed by the nonradiative decay rate, and it was found that the triplet

excited states at low temperature (i.e. in a solvent glass) are governed by the energy gap law. It

is believed that the triplet properties are minimally perturbed by the presence of the platinum

atom in the end-cap, and therefore the results of this study provide quantitative insight

concerning the structure and dynamics of the triplet state in long carbon chains.

Finally, the photophysical characterization of a two-photon absorbing iridium complex has

been described. A large absorption cross-section for one- and two-photon excitation is provided

by the ligand, and the iridium atom provides access to the triplet manifold. The long-lived triplet

excited state affords the opportunity for absorption of an additional photon. This combination

leads to enhanced nonlinear optical absorption. Incident-energy-dependent emission intensity

illustrates the multi-photon absorbing nature of the complex. In addition, the nonlinear optical

properties are clearly shown by the trend seen with incident energy dependence of the

transmittance.

In order to gain further understanding of triplet state behavior in these types of systems, it

is necessary to characterize a broad range of related molecules. In a sythentic sense, the

possibilities are nearly endless. Gladysz and coworkers136-138 have already synthesized related

molecules, and some examples are shown in Figure 6-1. These compounds would be helpful in

understanding intermolecular interactions. For example, the series shown in Figure 6-1A could

be thought of as an insulated molecular wire, with the carbon chain as the wire surround by the

alkyl chain. Depending on conformation, the alkyl chain could wrap around the center chain and

perhaps protect it from intermolecular interactions. In contrast, the carbon chains in the dimers









shown in Figure 6-1B are forced to be near each other, facilitating the study of aggregate-like

interactions.


-F A










B


Figure 6-1. Variations of molecular wires synthesized by Stahl, Owen et al.136-138 (A) A
shielded molecular wire. (B) A wire dimer.



As with the platinum oligomers, further knowledge would increased through

characterization of a broader range of molecules. However, the photophysics of the molecule

presented in Chapter 5 could be more thoroughly characterized. For example, using a variable-

wavelength excitation source, such as an optically parametric oscillator (OPO), a greater

understanding of the multiphoton absorption properties would be gathered. The multi-photon

absorption properties could be further examined on a temporal scale, that is, through excitation

by a short (ps) laser pulse. Such information is essential in the development of materials for use

as broad-band optical limiters.









LIST OF REFERENCES


(1) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Sausilito,
California, 1991.

(2) Henry, B. R.; Siebrand, W. In Organic Molecular Photophysics; Birks, J. B., Ed.; John
Wiley and Sons, 1973; Vol. 1.

(3) Shriver, D. F.; Atkins, P. W. Inorganic Chemistry; 3rd ed.; W. H. Freeman and
Company: New York, 1999.

(4) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R., Eds. Handbook of Conducting
Polymers; 2nd ed.; Marcel Dekker: New York, 1998.

(5) McGehee, M. D.; Miller, E. K.; Moses, D.; Heeger, A. J. In Advances in Synthetic
Metals. Twenty Years of Progress in Science and Technology.; Bernier, P., Lefrant, S.,
Bidan, G., Eds.; Elsevier: Amsterdam, 1999.

(6) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R.
H.; Burns, P. L.; Holmes, A. B. Light-Emitting Diodes Based on Conjugated Polymers.
Nature 1990, 347, 539-541.

(7) Gunder, P. Nonlinear Optical Effects and Materials; Springer: New York, 2000.

(8) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater.
2001, 11, 15-26.

(9) Aviram, A.; Ratner, M. A. Molecular Rectifiers. Chem. Phys. Lett. 1974, 29, 277-283.

(10) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Conductance of a
Molecular Junction. Science 1997, 278, 252-254.

(11) Ashwell, G. J.; Chwialkowska, A. Controlled Alignment of Molecular Diodes via Ionic
Assembly of Cationic Donor-(7r-Bridge)-Acceptor Molecules on Anionic Surfaces.
Chem. Commun. 2006, 1404-1406.

(12) Ashwell, G. J.; Urasinska, B.; Wang, C.; Bryce, M. R.; Grace, I.; Lambert, C. J. Single-
Molecule Electrical Studies on a 7 nm Long Molecular Wire. Chem. Commun. 2006,
4706-4708.

(13) Ashwell, G. J.; Tyrrell, W. D.; Urasinska, B.; Wang, C.; Bryce, M. R. Organic Rectifying
Junctions from an Electron-Accepting Molecular Wire and an Electron-Donating
Phthalocyanine. Chem. Commun. 2006, 1640-1642.

(14) Davis, W. B.; Ratner, M. A.; Wasielewski, M. R. Conformational Gating of Long
Distance Electron Transfer through Wire-like Bridges in Donor-Bridge-Acceptor
Molecules. J. Am. Chem. Soc. 2001, 123, 7877-7886.










(15) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Molecular-Wire Behavior
in p-Phenylenevinylene Oligomers. Nature 1998, 396, 60-63.

(16) Filatov, I.; Larsson, S. Electronic Structure and Conduction Mechanism of Donor-Bridge-
Acceptor Systems where PPV Acts as a Molecular Wire. Chem. Phys. 2002, 284, 575-
591.

(17) Barybin, M. V.; Chisholm, M. H.; Dalal, N. S.; Holovics, T. H.; Patmore, N. J.;
Robinson, R. E.; Zipse, D. J. Long-Range Electronic Coupling of MM Quadruple Bonds
(M = Mo or W) via a 2,6-Azulenedicarboxylate Bridge. J. Am. Chem. Soc. 2005, 127,
15182-15190.

(18) Xu, G.-L.; Crutchley, R. J.; DeRosa, M. C.; Pan, Q.-J.; Zhang, H.-X.; Wang, X.; Ren, T.
Strong Electronic Couplings between Ferrocenyl Centers Mediated by Bis-
Ethynyl/Butadiynyl Diruthenium Bridges. J. Am. Chem. Soc. 2005, 127, 13354-13363.

(19) Sheng, T.; Vahrenkamp, H. Long Range Metal-Metal Interactions Along Fe-NC-Ru-CN-
Fe Chains. Eur. J. Inorg. Chem. 2004, 1198-1203.

(20) Liu, S. H.; Chen, Y.; Wan, K. L.; Wen, T. B.; Zhou, Z.; Lo, M. F.; Williams, I. D.; Jia, G.
Synthesis and Characterization of Linear (CH)s-Bridged Bimetallic Ruthenium
Complexes. Organomettalics 2002, 21, 4984-4992.

(21) Chung, M.-C.; Gu, X.; Etzenhouser, B. A.; Spuches, A. M.; Rye, P. T.; Seetharaman, S.
K.; Rose, D. J.; Zubieta, J.; Sponsler, M. B. Intermetal Coupling in [(r5-
CsR5)Fe(dppe)]2(m-CH:CHCH:CH) and in Their Dicationic and Monocationic Mixed-
Valence Forms. Organomettalics 2003, 22, 3485-3494.

(22) Wong, W.-Y.; Ho, C.-L. Di-, Oligo- and Polymetallaynes: Syntheses, Photophysics,
Structures and Applications. Coord. Chem. Rev. 2006, 250, 2627-2690.

(23) Wong, W.-Y. Luminescent Organometallic Poly(aryleneethynylene)s: Functional
Properties towards Implications in Molecular Optoelectronics. Dalton Trans. 2007, 4495-
4510.

(24) Peters, T. B.; Bohling, J. C.; Arif, A. M.; Gladysz, J. A. C8 and C12 sp Carbon Chains
That Span Two Platinum Atoms: The First Structurally Characterized 1,3,5,7,9,11-
Hexayne. Organomettalics 1999, 18, 3261-3263.

(25) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. Toward Metal-Capped
One-Dimensional Carbon Allotropes: Wirelike C6-C20 Polyynediyl Chains That Span
Two Redox-Active (q5-C5Me5)Re(NO)(PPh3) Endgroups. J. Am. Chem. Soc. 2000, 122,
810-822.









(26) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Bent and Stretched but Not Yet to the
Breaking Point: C8-C16 sp Carbon Chains That Span Two Platinum Atoms and the First
Structurally Characterized 1,3,5,7,9,11,13,15-Octayne. Inorg. Chem. 2001, 40, 3263-
3264.

(27) Yam, V. W.-W.; Wong, K. M.-C. Luminescent Molecular Rods Transition-Metal
Alkynyl Complexes. Top. Curr. Chem. 2005, 257, 1-32.

(28) Spangler, C. W. Recent Development in the Design of Organic Materials for Optical
Power Limiting. J. Mater. Chem. 1999, 9, 2013-2020.

(29) Tutt, L. W.; Boggess, T. F. A Review of Optical Limiting Mechanisms and Devices
Using Organics, Fullerenes, Semiconductors and Other Materials. Prog. Quantum
Electron. 1993, 17, 299-338.

(30) Hollins, R. C. Materials for Optical Limiters. Curr. Opin. Solid State Mater. Sci. 1999, 4,
189-196.

(31) Sun, Y.-P.; Riggs, J. E. Organic and Inorganic Optical Limiting Materials. From
Fullerenes to Nanoparticles. Int. Rev. Phys. Chem. 1999, 18, 43-90.

(32) McEwan, K.; Lewis, K.; Yang, G.-y.; Chng, L.-1.; Lee, Y.-w.; Lau, W.-p.; Lai, K.-s.
Synthesis, Characterization, and Nonlinear Optical Study of Metalloporphyrins. Adv.
Funct. Mater. 2003, 13, 863-867.

(33) Perry, J. W.; Mansour, K.; Lee, I. Y. S.; Wu, X. L.; Bedworth, P. V.; Chen, C. T.; Ng, D.;
Marder, S. R.; Miles, P.; Wada, T.; Tian, M.; Sasabe, H. Organic Optical Limiter with a
Strong Nonlinear Absorptive Response. Science 1996, 273, 1533-1536.

(34) Staromlynska, J.; Chapple, P. B.; Davy, J. R.; McKay, T. J. A Platinum Ethynyl
Compound for Optical Limiting. Proc. SPIE-Int. Soc. Opt. Eng. 1994, 2229, 59-66.

(35) Staromlynska, J.; McKay, T. J.; Bolger, J. A.; Davy, J. R. Evidence for Broadband
Optical Limiting in a Pt:ethynyl Compound. J. Opt. Soc. Am. B 1998, 15, 1731-1736.

(36) McKay, T. J.; Bolger, J. A.; Staromlynska, J.; Davy, J. R. Linear and Nonlinear Optical
Properties ofPlatinum-Ethynyl. J. Chem. Phys. 1998, 108, 5537-5541.

(37) McKay, T. J.; Staromlynska, J.; Wilson, P.; Davy, J. Nonlinear Luminescence
Spectroscopy in a Pt:ethynyl Compound. J. Appl. Phys. 1999, 85, 1337-1341.

(38) Staromlynska, J.; McKay, T. J.; Wilson, P. Broadband Optical Limiting Based on Excited
State Absorption in Pt:ethynyl. J. Appl. Phys. 2000, 88, 1726-1732.

(39) McKay, T. J.; Staromlynska, J.; Davy, J. R.; Bolger, J. A. Cross Sections for Excited-
State Absorption in a Pt:ethynyl Complex. J. Opt. Soc. Am. B 2001, 18, 358-362.










(40) Rogers, J. E.; Cooper, T. M.; Fleitz, P. A.; Glass, D. J.; McLean, D. G. Photophysical
Characterization of a Series of Platinum(II)-Containing Phenyl-Ethynyl Oligomers. J
Phys. Chem. A 2002, 106, 10108-10115.

(41) Cooper, T. M.; Hall, B. C.; Burke, A. R.; Rogers, J. E.; McLean, D. G.; Slagle, J. E.;
Fleitz, P. A. Glass-Forming Liquid Platinum Acetylides. Chem. Mater. 2004, 16, 3215-
3217.

(42) Cooper, T. M.; Blaudeau, J.-P.; Hall, B. C.; Rogers, J. E.; McLean, D. G.; Liu, Y.;
Toscano, J. P. The Triplet State of a Platinum Acetylide Chromophore Examined by
Time-Resolved Infrared Spectroscopy and Density Functional Theory. Chem. Phys. Lett.
2004, 400, 239-244.

(43) Cooper, T. M.; McLean, D. G.; Rogers, J. E. Molecular Structure-Spectroscopic Property
Relationships in a Series of Transition Metal-Containing Phenylacetylene Oligomers.
Chem. Phys. Lett. 2001, 349, 31-36.

(44) Rogers, J. E.; Hall, B. C.; Hufnagle, D. C.; Slagle, J. E.; Ault, A. P.; McLean, D. G.;
Fleitz, P. A.; Cooper, T. M. Effect of Platinum on the Photophysical Properties of a
Series of Phenyl-Ethynyl Oligomers. J. Chem. Phys. 2005, 122, 214708/214701-
214708/214708.

(45) Cooper, T. M.; Krein, D. M.; Burke, A. R.; McLean, D. G.; Rogers, J. E.; Slagle, J. E.;
Fleitz, P. A. Spectroscopic Characterization of a Series of Platinum Acetylide Complexes
Having a Localized Triplet Exciton. J. Phys. Chem. A 2006, 110, 4369-4375.

(46) Norrish, R. G. W.; Porter, G. Chemical Reactions Produced by Very High Light
Intensities. Nature 1949, 164, 658.

(47) Lindqvist, L. Utilization of a Laser with Pulsed Ultraviolet Emission in Flash Photolysis.
Triplet State of Acridine. Hebd. Seances Acad. Sci., Ser. C 1966, 263, 852-854.

(48) Small, R. D., Jr.; Scaiano, J. C. Absolute Rates of Hydrogen Abstraction by tert-Butoxy
Radicals. J Am. Chem. Soc. 1978, 100, 296-298.

(49) Scaiano, J. C. In Reactive Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M. J.,
Eds.; John Wiley & Sons, Inc.: Hoboken, New Jersey, 2004.

(50) Scaiano, J. C. Solvent Effects in the Photochemistry of Xanthone. J. Am. Chem. Soc.
1980, 102, 7747-7753.

(51) How Intensified CCD Cameras Operate. http://www.andor.com/library/digitalcameras/
(accessed August 2007).

(52) Princeton Instruments Homepage. http://www.piacton.com/ (accessed August 2007).










(53) Zimmermann, C.; Mohr, M.; Zipse, H.; Eichberger, R.; Schnabel, W. Photoionization of
Anthracene and Anthracene Derivatives. J. Photochem. Photobiol., A 1999, 125, 47-56.

(54) Haarmann, K.; Haselbach, E.; Suppan, P. Triplet State Lifetime of Anthracene in
Benzene Solution. Chimia 1987, 41, 198-200.

(55) Ohno, T.; Yoshimura, A.; Prasad, D. R.; Hoffman, M. Z. A Weak AG Dependence of
Back Electron Transfer within the Geminate Redox Pairs Formed in the Quenching of
Excited Ruthenium(II) Complexes by Methyl Viologen. J. Phys. Chem. 1991, 95, 4723-
4728.

(56) Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N. Lifetimes, Spectra, and
Quenching of the Excited States of Polypyridine Complexes of Iron(II), Ruthenium(II),
and Osmium(II). J. Am. Chem. Soc. 1980, 102, 1309-1319.

(57) Tessler, N.; Denton, G. J.; Friend, R. H. Lasing from Conjugated-Polymer Microcavities.
Nature 1996, 382, 695-697.

(58) Brown, A. R.; Pichler, K.; Greenham, N. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A.
B. Optical Spectroscopy of Triplet Excitons and Charged Excitations in Poly(p-
phenylenevinylene) Light-Emitting Diodes. Chem. Phys. Lett. 1993, 210, 61-66.

(59) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton,
J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S.
Conductance Switching in Single Molecules through Conformational Changes. Science
2001, 292, 2303-2307.

(60) Kohler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan, M. S.; Gerhard, A.;
Bassler, H. The Singlet-Triplet Energy Gap in Organic and Pt-Containing Phenylene
Ethynylene Polymers and Monomers. Journal of Chemical Physics 2002, 116, 9457-
9463.

(61) Bruce, M. I.; Davy, J.; Hall, B. C.; Van Galen, Y. J.; Skelton, B. W.; White, A. H. Some
Platinum(II) Complexes Derived from Aromatic Alkynes. Appl. Organomet. Chem. 2002,
16, 559-568.

(62) Szafert, S.; Gladysz, J. A. Carbon in One Dimension: Structural Analysis of the Higher
Conjugated Polyynes. Chem. Rev. 2003, 103, 4175-4205.

(63) Emmert, L. A.; Choi, W.; Marshall, J. A.; Yang, J.; Meyer, L. A.; Brozik, J. A. The
Excited-State Symmetry Characteristics of Platinum Phenylacetylene Compounds. J
Phys. Chem. A 2003, 107, 11340-11346.

(64) Yam, V. W.-W.; Kam-Wing Lo, K.; Man-Chung Wong, K. Luminescent Polynuclear
Metal Acetylides. J. Organomet. Chem. 1999, 578, 3-30.










(65) Nguyen, P.; Gomez-Elipe, P.; Manners, I. Organometallic Polymers with Transition
Metals in the Main Chain. Chem. Rev. 1999, 99, 1515-1548.

(66) Ley, K. D.; Schanze, K. S. Photophysics of Metal-Organic 7t-Conjugated Polymers.
Coord. Chem. Rev. 1998, 171, 287-307.

(67) Slagle, J. E.; Cooper, T. M.; Krein, D. M.; Rogers, J. E.; McLean, D. G.; Urbas, A. M.
Triplet Excimer Formation in a Platinum Acetylide. Chem. Phys. Lett. 2007, 447, 65-68.

(68) Tam, A. Y.-Y.; Wong, K. M.-C.; Wang, G.; Yam, V. W.-W. Luminescent Metallogels of
Platinum(II) Terpyridyl Complexes: Interplay of Metal-Metal, '7-7K and Hydrophobic-
Hydrophobic Interactions on Gel Formation. Chem. Commun. 2007, 2028-2030.

(69) Liu, Y.; Jiang, S.; Glusac, K.; Powell, D. H.; Anderson, D. F.; Schanze, K. S.
Photophysics of Monodisperse Platinum-Acetylide Oligomers: Delocalization in the
Singlet and Triplet Excited States. J. Am. Chem. Soc. 2002, 124, 12412-12413.

(70) Khan, M. S.; Al-Mandhary, M. R. A.; Al-Suti, M. K.; Al-Battashi, F. R.; Al-Saadi, S.;
Ahrens, B.; Bjernemose, J. K.; Mahon, M. F.; Raithby, P. R.; Younus, M.; Chawdhury,
N.; Kohler, A.; Marseglia, E. A.; Tedesco, E.; Feeder, N.; Teat, S. J. Synthesis,
Characterization and Optical Spectroscopy of Platinum(II) Di-ynes and Poly-ynes
Incorporating Condensed Aromatic Spacers in the Backbone. Dalton Trans. 2004, 2377-
2385.

(71) Onitsuka, K.; Takahashi, S. Metallodendrimers Composed of Organometallic Building
Blocks. Top. Curr. Chem. 2003, 228, 39-63.

(72) Beljonne, D.; Wittmann, H. F.; Kohler, A.; Graham, S.; Younus, M.; Lewis, J.; Raithby,
P. R.; Khan, M. S.; Friend, R. H.; Bredas, J. L. Spatial Extent of the Singlet and Triplet
Excitons in Transition Metal-Containing Poly-ynes. J. Chem. Phys. 1996, 105, 3868-
3877.

(73) Haskins-Glusac, K.; Ghiviriga, I.; Abboud, K. A.; Schanze, K. S. Photophysics and
Photochemistry of Stilbene-Containing Platinum Acetylides. J. Phys. Chem. B 2004, 108,
4969-4978.

(74) Cooper, T. M.; Hall, B. C.; McLean, D. G.; Rogers, J. E.; Burke, A. R.; Turnbull, K.;
Weisner, A.; Fratini, A.; Liu, Y.; Schanze, K. S. Structure-Optical Property Relationships
in Organometallic Sydnones. J. Phys. Chem. A 2005, 109, 999-1007.

(75) Cooper, T. M.; Krein, D. M.; Burke, A. R.; McLean, D. G.; Rogers, J. E.; Slagle, J. E.
Asymmetry in Platinum Acetylide Complexes: Confinement of the Triplet Exciton to the
Lowest Energy Ligand. J. Phys. Chem. A 2006, 110, 13370-13378.









(76) Schanze, K. S.; Silverman, E. E.; Zhao, X. Intrachain Triplet Energy Transfer in
Platinum-Acetylide Copolymers. J. Phys. Chem. B 2005, 109, 18451-18459.

(77) Jones, E. R. H.; Lee, H. H.; Whiting, M. C. Acetylenic Compounds. LXIV. The
Preparation of Conjugated Octa- and Decaacetylenic Compounds. J. Chem. Soc. 1960,
3483-3489.

(78) Johnson, T. R.; Walton, D. R. M. Silylation as a Protective Method in Acetylene
Chemistry. Polyyne Chain Extensions Using 1-(Triethylsilyl)acetylene, -butadiyne and -
octatetrayne in Mixed Oxidative Couplings. Tetrahedron 1972, 28, 5221-5236.

(79) Eastmond, R.; Johnson, T. R.; Walton, D. R. M. Silylation as a Protective Method for
Terminal Alkynes in Oxidative Couplings. General Synthesis of the Parent Polyynes H(C
= C)nH (n = 4-10, 12). Tetrahedron 1972, 28, 4601-4616.

(80) Schermann, G.; Grosser, T.; Hampel, F.; Hirsch, A. Dicyanopolyynes: A Homologous
Series of End-Capped Linear sp Carbon. Chem.-Eur. J. 1997, 3, 1105-1112.

(81) Gibtner, T.; Hampel, F.; Gisselbrecht, J.-P.; Hirsch, A. End-Cap Stabilized Oligoynes:
Model Compounds for the Linear sp Carbon Allotrope Carbyne. Chem.-Eur. J 2002, 8,
408-432.

(82) Klinger, C.; Vostrowsky, O.; Hirsch, A. Synthesis of Alkylene-Bridged Diphenyl-
Oligoynes. Eur. J. Org. Chem. 2006, 1508-1524.

(83) Eisler, S.; Slepkov, A. D.; Elliott, E.; Luu, T.; McDonald, R.; Hegmann, F. A.;
Tykwinski, R. R. Polyynes as a Model for Carbyne: Synthesis, Physical Properties, and
Nonlinear Optical Response. J. Am. Chem. Soc. 2005, 127, 2666-2676.

(84) Luu, T.; Elliott, E.; Slepkov, A. D.; Eisler, S.; McDonald, R.; Hegmann, F. A.;
Tykwinski, R. R. Synthesis, Structure, and Nonlinear Optical Properties of
Diarylpolyynes. Org. Lett. 2005, 7, 51-54.

(85) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Synthesis, Structure, and Reactivity of sp
Carbon Chains with Bis(phosphine) pentafluorophenylplatinum Endgroups:
Butadiynediyl (C4) through Hexadecaoctaynediyl (C16) Bridges, and Beyond. Chem.-
Eur. J. 2003, 9, 3324-3340.

(86) Zheng, Q.; Bohling, J. C.; Peters, T. B.; Frisch, A. C.; Hampel, F.; Gladysz, J. A. A
Synthetic Breakthrough into an Unanticipated Stability Regime: A Series of Isolable
Complexes in Which C6, C8, C10, C12, C16, C20, C24, and C28 Polyynediyl Chains
Span Two Platinum Atoms. Chem.-Eur. J 2006, 12, 6486-6505.

(87) Paul, F.; Lapinte, C. In Unusual Structures and Physical Properties in Organometallic
Chemistry; Geilen, M., Willem, R., Wrackmyer, B., Eds.; Wiley: New York, 2002, pp
220-291.










(88) Bruce, M. I.; Low, P. J. Transition Metal Complexes Containing All-Carbon Ligands.
Adv. Organomet. Chem. 2004, 50, 179-444.

(89) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat,
P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M.
E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.;
Zhu, X. Charge Transfer on the Nanoscale: Current Status. J. Phys. Chem. B 2003, 107,
6668-6697.

(90) Zhuravlev, F.; Gladysz, J. A. Electronic Structure and Chain-Length Effects in
Diplatinum Polyynediyl Complexes trans,trans-[(X)(R3P)2Pt(C.tplbond.C)nPt(PR3)2(X)]:
A Computational Investigation. Chem.-Eur. J. 2004, 10, 6510-6522.

(91) Yang, S.; Kertesz, M. Bond Length Alternation and Energy Band Gap of Polyyne. J.
Phys. Chem. A 2006, 110, 9771-9774.

(92) Yang, S.; Kertesz, M.; Zolyomi, V.; Kuerti, J. Application of a Novel Linear/Exponential
Hybrid Force Field Scaling Scheme to the Longitudinal Raman Active Mode of Polyyne.
J. Phys. Chem. A 2007, 111, 2434-2441.

(93) Nagano, Y.; Ikoma, T.; Akiyama, K.; Tero-Kubota, S. Symmetry Switching of the
Fluorescent Excited State in a,co-Diphenylpolyynes. J. Am. Chem. Soc. 2003, 125,
14103-14112.

(94) Nagano, Y.; Ikoma, T.; Akiyama, K.; Tero-Kubota, S. Electronic Structures and
Dynamics of the Excited Triplet States of a,co-Diphenylpolyynes. J. Chem. Phys. 2001,
114, 1775-1784.

(95) Che, C.-M.; Chao, H.-Y.; Miskowski, V. M.; Li, Y.; Cheung, K.-K. Luminescent [t-
Ethynediyl and [t-Butadiynediyl Binuclear Gold(I) Complexes: Observation of 3(7n*)
Emissions from Bridging Cn2- Units. J. Am. Chem. Soc. 2001, 123, 4985-4991.

(96) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Luminescent Platinum(II) Terpyridyl-Capped
Carbon-Rich Molecular Rods-an Extension from Molecular- to Nanometer-Scale
Dimensions. Angew. Chem. Int. Ed. Engl. 2003, 42, 1400-1403.

(97) Caspar, J. V. Excited State Decay Processes in Osmium(II), Ruthenium(II) and
Rhenium(I) Polypyridyl Complexes. Ph.D. Dissertation, University of North Carolina,
Chapel Hill, NC, 1982.

(98) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Photophysics of Diimine
Platinum(II) Bis-Acetylide Complexes. Inorg. Chem. 2001, 40, 4053-4062.

(99) Wang, Y.; Schanze, K. S. Photochemical Probes of Intramolecular Electron and Energy
Transfer. Chem. Phys. 1993, 176, 305-319.










(100) Maddux, T.; Li, W.; Yu, L. Stepwise Synthesis of Substituted Oligo(phenylenevinylene)
via an Orthogonal Approach. J. Am. Chem. Soc. 1997, 119, 844-845.

(101) de Melo, J. S.; Silva, L. M.; Arnaut, L. G.; Becker, R. S. Singlet and Triplet Energies of
a-Oligothiophenes: A Spectroscopic, Theoretical, and Photoacoustic Study:
Extrapolation to Polythiophene. J. Chem. Phys. 1999, 111, 5427-5433.

(102) Becker, R. S.; de Melo, J. S.; Macanita, A. L.; Elisei, F. Comprehensive Evaluation of the
Absorption, Photophysical, Energy Transfer, Structural, and Theoretical Properties of a-
Oligothiophenes with One to Seven Rings. J. Phys. Chem. 1996, 100, 18683-18695.

(103) Wasserberg, D.; Marsal, P.; Meskers, S. C. J.; Janssen, R. A. J.; Beljonne, D.
Phosphorescence and Triplet State Energies of Oligothiophenes. J Phys. Chem. B 2005,
109, 4410-4415.

(104) Wasserberg, D.; Meskers, S. C. J.; Janssen, R. A. J.; Mena-Osteritz, E.; Baeuerle, P.
High-Resolution Electronic Spectra of Ethylenedioxythiophene Oligomers. J Am. Chem.
Soc. 2006, 128, 17007-17017.

(105) Martin, R. E.; Diederich, F. Linear Monodisperse nT-Conjugated Oligomers: Model
Compounds for Polymers and More. Angew. Chem. Int. Ed. Engl. 1999, 38, 1351-1377.

(106) Candeias, L. P.; Wildeman, J.; Hadziioannou, G.; Warman, J. M. Pulse Radiolysis-
Optical Absorption Studies on the Triplet States of n-Phenylenevinylene Oligomers in
Solution. J. Phys. Chem. B 2000, 104, 8366-8371.

(107) Wasserberg, D.; Dudek, S. P.; Meskers, S. C. J.; Janssen, R. A. J. Comparison of the
Chain Length Dependence of the Singlet- and Triplet-Excited States of Oligofluorenes.
Chem. Phys. Lett. 2005, 411, 273-277.

(108) Robinson, G. W.; Frosch, R. P. Electronic Excitation Transfer and Relaxation. J. Chem.
Phys. 1963, 38, 1187-1203.

(109) Siebrand, W. Radiationless Transitions in Polyatomic Molecules. II. Triplet-Ground-State
Transitions in Aromatic Hydrocarbons. J. Chem. Phys. 1967, 47, 2411-2422.

(110) Caspar, J. V.; Meyer, T. J. Application of the Energy Gap Law to Nonradiative, Excited-
State Decay. J. Phys. Chem. 1983, 87, 952-957.

(111) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. Application of the Energy Gap
Law to Excited-State Decay of Osmium(II)-Polypyridine Complexes: Calculation of
Relative Nonradiative Decay Rates from Emission Spectral Profiles. J. Phys. Chem.
1986, 90, 3722-3734.









(112) Barqawi, K. R.; Murtaza, Z.; Meyer, T. J. Calculation of Relative Nonradiative Decay
Rate Constants from Emission Spectral Profiles: Polypyridyl Complexes of
Ruthenium(II). J. Phys. Chem. 1991, 95, 47-50.

(113) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus, M.; Khan, M. S.;
Raithby, P. R.; Kohler, A.; Friend, R. H. The Energy Gap Law for Triplet States in Pt-
Containing Conjugated Polymers and Monomers. J. Am. Chem. Soc. 2001, 123, 9412-
9417.

(114) Ishibashi, T.-a.; Hamaguchi, H.-o. Structure and Dynamics of S2 and S1
Diphenylacetylene in Solution Studied by Picosecond Time-Resolved CARS
Spectroscopy. J. Phys. Chem. A 1998, 102, 2263-2269.

(115) Baur, J. W.; Alexander, M. D., Jr.; Banach, M.; Denny, L. R.; Reinhardt, B. A.; Vaia, R.
A.; Fleitz, P. A.; Kirkpatrick, S. M. Molecular Environment Effects on Two-Photon-
Absorbing Heterocyclic Chromophores. Chem. Mater. 1999, 11, 2899-2906.

(116) Goppert-Mayer, M. Elementary File with Two Quantum Fissures. Ann. Phys. (Leipzig)
1931, 9, 273-294.

2+
(117) Kaiser, W.; Garrett, C. G. B. Two-Photon Excitation in CaF2: Eu Phys. Rev. Lett. 1961,
7, 229 LP 231.

(118) Callis, P. R. Two-Photon-Induced Fluorescence. Annu. Rev. Phys. Chem. 1997, 48, 271-
297.

(119) Lackowicz, J. R. Principles ofFluorescence Spectroscopy; 2nd ed.; Kluwer Academic /
Plenum Publishers: New York, 1999.

(120) Boyle, A.; Blau, W. J. In Handbook ofAdvanced Electronic andPhotonic Materials and
Devices; Nalwa, H. S., Ed.; Academic Press: New York, 2001; Vol. 9.

(121) Fakis, M.; Tsigaridas, G.; Polyzos, I.; Giannetas, V.; Persephonis, P.; Spiliopoulos, I.;
Mikroyannidis, J. Intensity Dependent Nonlinear Absorption of Pyrylium Chromophores.
Chem. Phys. Lett. 2001, 342, 155-161.

(122) Lin, T.-C.; Chung, S.-J.; Kim, K.-S.; Wang, X.; He, G. S.; Swiatkiewicz, J.; Pudavar, H.
E.; Prasad, P. N. Organics and Polymers with High Two-Photon Activities and Their
Applications. Adv. Polym. Sci. 2003, 161, 157-193.

(123) Ehrlich, J. E.; Wu, X. L.; Lee, I. Y. S.; Hu, Z. Y.; Rockel, H.; Marder, S. R.; Perry, J. W.
Two-Photon Absorption and Broadband Optical Limiting with Bis-Donor Stilbenes. Opt.
Lett. 1997, 22, 1843-1845.









(124) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt, J. C.; Kannan, R.;
Yuan, L.; He, G. S.; Prasad, P. N. Highly Active Two-Photon Dyes: Design, Synthesis,
and Characterization Toward Application. Chem. Mater. 1998, 10, 1863-1874.

(125) Powell, C. E.; Morrall, J. P.; Ward, S. A.; Cifuentes, M. P.; Notaras, E. G. A.; Samoc,
M.; Humphrey, M. G. Dispersion of the Third-Order Nonlinear Optical Properties of an
Organometallic Dendrimer. J. Am. Chem. Soc. 2004, 126, 12234-12235.

(126) Joshi, M. P.; Swiatkiewicz, J.; Xu, F.; Prasad, P. N.; Reinhardt, B. A.; Kannan, R. Energy
Transfer Coupling of Two-Photon Absorption and Reverse Saturable Absorption for
Enhanced Optical Power Limiting. Opt. Lett. 1998, 23, 1742-1744.

(127) McIlroy, S. P.; Clo, E.; Nikolajsen, L.; Frederiksen, P. K.; Nielsen, C. B.; Mikkelsen, K.
V.; Gothelf, K. V.; Ogilby, P. R. Two-Photon Photosensitized Production of Singlet
Oxygen: Sensitizers with Phenylene-Ethynylene-Based Chromophores. J. Org. Chem.
2005, 70, 1134-1146.

(128) Kannan, R.; He, G. S.; Lin, T.-C.; Prasad, P. N.; Vaia, R. A.; Tan, L.-S. Toward Highly
Active Two-Photon Absorbing Liquids. Synthesis and Characterization of 1,3,5-Triazine-
Based Octupolar Molecules. Chem. Mater. 2004, 16, 185-194.

(129) Cotton, F. A.; Wilkinson, G. Advanced norganic Chemistry: A Comprehensive Text;
Wiley: New York, 1980.

(130) Thorton, N. B. Chromophore Quenched Based Luminescence Probes for DNA. Ph.D.
Dissertation, University of Florida, Gainesville, FL, 1995.

(131) Glusac, K. D.; Jiang, S.; Schanze, K. S. Photophysics of Ir(III) Complexes with
Oligo(arylene ethynylene) Ligands. Chem. Commun. 2002, 2504-2505.

(132) Ichimura, K.; Kobayashi, T.; King, K. A.; Watts, R. J. Excited-State Absorption
Spectroscopy of ortho-Metalated Iridium(III) Complexes. J. Phys. Chem. 1987, 91, 6104-
6106.

(133) Manas, E. S.; Chen, L. X. Electronic Interactions in Metal Complexed Photoconducting
Polymers: A ZINDO Study. Chem. Phys. Lett. 2000, 331, 299-307.

(134) Walters, K. A.; Ley, K. D.; Cavalaheiro, C. S. P.; Miller, S. E.; Gosztola, D.;
Wasielewski, M. R.; Bussandri, A. P.; van Willigen, H.; Schanze, K. S. Photophysics of
7t-Conjugated Metal-Organic Oligomers: Aryleneethynylenes that Contain the
(bpy)Re(CO)3C1 Chromophore. J. Am. Chem. Soc. 2001, 123, 8329-8342.

(135) Liu, S.; Schanze, K. S. Solvent Tuned Excited State Configuration Mixing in a 7t-
Conjugated Metal-Organic Oligomer. Chem. Commun. 2004, 1510-1511.









(136) Stahl, J.; Bohling, J. C.; Bauer, E. B.; Peters, T. B.; Mohr, W.; Martin-Alvarez, J. M.;
Hampel, F.; Gladysz, J. A. sp Carbon Chains Surrounded by sp3 Carbon Double Helices:
A Class of Molecules that are Accessible by Self-Assembly and Models for "Insulated"
Molecular-Scale Devices. Angew. Chem. Int. Ed. Engl. 2002, 41, 1871-1876.

(137) Owen, G. R.; Stahl, J.; Hampel, F.; Gladysz, J. A. Syntheses and Structures of
Diplatinum Hexatriynediyl Complexes, in Which the sp Carbon Chains are Shielded by
sp3 Carbon Chains. Organomettalics 2004, 23, 5889-5892.

(138) Owen, G. R.; Hampel, F.; Gladysz, J. A. Syntheses and Structures of Tetraplatinum
Bis(polyynediyl) Complexes with Laterally Arrayed sp Carbon Chains. Organomettalics
2004, 23, 5893-5895.









BIOGRAPHICAL SKETCH

Richard T. Farley was born in Rochester, New York, and grew up in the suburbs of

Orlando, Florida. He spent a lively childhood with his grandparents, parents and sister. Richard

started his education in chemistry at the University of Central Florida in 1998. He took a strong

interest in analytical and organic chemistry. It was not until he began his graduate studies at the

University of Florida that Richard discovered his curiosity for the fundamental nature of physical

chemistry. He took advantage of the opportunity to merge physical and instrumental work in a

primarily organic atmosphere, working under the guidance of Prof. Kirk S. Schanze to pursue a

doctoral degree in physical chemistry.





PAGE 1

PHOTOPHYSICS OF PLATINUM AND IRIDIUM ORGANOMETALLIC MATERIALS: FROM MOLECULAR WIRES TO NONLINEAR OPTICS By RICHARD T. FARLEY A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007 1

PAGE 2

2007 Richard T. Farley 2

PAGE 3

To my sister 3

PAGE 4

ACKNOWLEDGMENTS I would like to express gratitude to my advisor, Prof. Kirk S. Schanze, whose support and unending patience have allowed me to complete my studies. His scope of knowledge on scientific material and laboratory equipment remain impressive. The thorough explanations and advice he has given in the lab have led me to work on things I never imagined as a chemistry graduate student. I would also like to thank my committee members, Dr John Eyler, Dr. Valeria Kleiman, Dr. Nicolo Omenetto, and Dr. Elliott Douglas, for their time and support. This work could not have been completed without the many collaborators with whom I have interacted: Ranjani Narayanan and Dr. Stephen Hagen for their stimulating discussions regarding the instrumentation discussed in Chapter 2; Thomas Cooper and the Air Force Research Laboratory for providing the compounds studied in Chapter 3; Qinglin Zheng and Dr. John Gladysz for their collaboration and providing the materials studied in Chapter 4; and Dr. Kye-Young Kim of the Schanze group for the synthesis and characterization of the compound studied in Chapter 5. Much of our custom lab equipment would not exist without the hard work and creativity of Joe Shalosky, Brian Smith and Todd Prox of the UF Chemistry Machine Shop. I would also like to thank Mike Sytsma and Benjamin Walker for their programming help in the laser lab. I thank of all the present and former members of the Schanze research group for accepting the oddball physical chemist as (almost) one of their own. The have been great friends and wonderful scientists. I appreciate the guidance of Dr. Ksenija Glusac-Haskins and Dr. Thomas Cardolaccia who patiently introduced me to the laboratory and photophysical measurements during my first year in the group. I hold dear the friendships I have made in the group, especially with Dr. Kye-Young Kim. Her stories, support and sharing of secrets have helped make being in the lab much more than just research. I need to thank the various members with 4

PAGE 5

whom I have shared desk space throughout the years, and with whom Ive become close friends: Dr Xiaoming Zhao, Amir Faraji, and Dr. Katsu Ogawa. Amir was always there to offer a bizarre yet intriguing opinion on the matter at hand. Special thanks go to Katsu; his unfortunate close proximity in the lab put him at the receiving end of unending questions, whether technical, chemical or metaphysical. Survival would not have been possible without the support, knowledge and friendship of Lori Clark in the Graduate Office. Her role in the administrative process from recruiting to graduation is essential to the department, but it is her friendship and support outside of work that have made her invaluable to me. I would like to thank her and Dr. Ben Smith for the opportunity to help with recruiting. Hopefully they realize it was more fun than it was work. I would also like to thank my many friends who have supported and encouraged me throughout the last five years. Rob and Lindsay have been there from the beginning, through classes and seminars, qualifiers and research, suffering and celebrating together. I value the existence of MUSCO and all my friends there who have kept me sane. My family has offered unconditional support through all my endeavors, for which I am most grateful. Each member has shared his or her own style of encouragement, love and understanding during my years as a graduate student. They are my inspiration to succeed in all that I do. 5

PAGE 6

TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES...........................................................................................................................8 LIST OF FIGURES.........................................................................................................................9 ABSTRACT...................................................................................................................................12 CHAPTER 1 INTRODUCTION..................................................................................................................14 Wave-Particle Nature of Photons and Electrons....................................................................14 Absorption of Light................................................................................................................15 Nature of the Excited State.....................................................................................................17 Radiative Decay......................................................................................................................18 Radiationless Transitions........................................................................................................19 Triplet Excited States in Conjugated Organic Systems..........................................................25 Molecular Wires..............................................................................................................25 Nonlinear Optics and Platinum Acetylides.....................................................................27 Objective of Present Study.....................................................................................................35 2 TRANSIENT ABSORPTION SPECTROSCOPY AND INSTRUMENTATION................37 Background.............................................................................................................................37 Techniques..............................................................................................................................38 Apparatus................................................................................................................................42 Hardware.........................................................................................................................42 Data Collection................................................................................................................46 Data Processing...............................................................................................................48 Sources of Noise..............................................................................................................52 Baseline Measurements...................................................................................................54 Results.............................................................................................................................54 Limitations of Current System........................................................................................57 3 DELOCALIZATION IN EXCITED STATES OF DINUCLEAR PLATINUM ACETYLIDES........................................................................................................................61 Introduction.............................................................................................................................61 Experimental...........................................................................................................................65 Results.....................................................................................................................................67 Discussion...............................................................................................................................75 Conclusion..............................................................................................................................79 6

PAGE 7

4 PHOTOPHYSICS OF DIPLATINUM POLYYNEDIYL OLIOGMERS: CHAIN LENGTH DEPENDENCE ON THE TRIPLET STATE IN SP CARBON CHAINS...........80 Introduction.............................................................................................................................80 Experimental...........................................................................................................................82 Results.....................................................................................................................................84 UV-Vis Absorption Spectra............................................................................................84 Steady-State Photoluminescence Spectroscopy..............................................................85 Phosphorescence Decay Kinetics: Radiative and Non-Radiative Decay Rates..............91 Transient Absorption Spectroscopy: Triplet-Triplet Absorption of the Oligoynes........94 Discussion...............................................................................................................................96 Variation of Triplet Energy with Carbon Chain Length.................................................96 Energy Gap Law Correlation for the Oligoynes..............................................................99 Excited State Decay above the Solvent Glass Point......................................................105 Summary and Conclusion.....................................................................................................105 5 AN IRIDIUM COMPLEX THAT EXHIBITS DUAL-MECHANISM NONLINEAR ABSORPTION.....................................................................................................................107 Introduction...........................................................................................................................107 Experimental.........................................................................................................................113 Results...................................................................................................................................115 Discussion.............................................................................................................................121 Conclusion............................................................................................................................123 6 CONCLUSIONS AND FUTURE WORK...........................................................................124 LIST OF REFERENCES.............................................................................................................128 BIOGRAPHICAL SKETCH.......................................................................................................140 7

PAGE 8

LIST OF TABLES Table page 3-1 Summary of spectroscopic data of dinuclear platinum-acetylide oligomers.....................67 4-1 Emission spectral fitting parameters for PtC n Pt at 100 K.................................................90 4-2 Photophysical parameters for PtC n Pt complexes..............................................................94 8

PAGE 9

LIST OF FIGURES Figure page 1-1 Potential energy curves for electronic transitions..............................................................16 1-2 Effects of energy difference and nuclear distortion on vibrational overlap.......................20 1-3 Jabloski diagram illustrating possible transitions..............................................................23 1-4 Structures of molecules studied by Davis et al..................................................................26 1-5 Conditions for reverse saturable absorption......................................................................29 1-6 Structure of platinum-ethynyl and corresponding photophysical data..............................30 1-7 Jabloski diagram illustrating possible transitions for platinum-ethynyl............................32 1-8 Structures of platinum acetylide oligomers studied by Cooper, Rogers, et al...................33 2-1 Transient absorption apparatus developed by Lindqvist...................................................38 2-2 CCD intensifier..................................................................................................................41 2-3 The transient absorption apparatus....................................................................................43 2-4 Spectrum of xenon lamp used as the probe.......................................................................44 2-5 Illustration of timing parameters for ICCD data collection...............................................47 2-6 Change in intensity of transmitted light during the lifetime of a transient species............49 2-7 Construction of a time trace and transient absorption spectra from gated multiwavelength measurements.........................................................................................51 2-8 Relative probe intensities for I 0 and I with the pump beam off.........................................53 2-9 Baseline noise of the apparatus..........................................................................................55 2-10 Transient absorption spectrum of 1.2x10 -4 M anthracene solution in deoxygenated benzene..............................................................................................................................56 2-11 Transient absorption spectrum of 1.4x10 -4 M aqueous solution of Ru(bpy) 3 2+ .................57 3-1 Platinum-acetylide polymer and model compound studied by Beljonne et al..................62 3-2 Platinum-acetylide oligomers studied by Liu et al............................................................63 3-3 Platinum acetylide oligomers studied by Glusac-Haskins et al.........................................63 9

PAGE 10

3-4 Platinum-acetylide oligomers studied by Cooper, Rogers, et al........................................64 3-5 Dinuclear platinum-acetylide oligomers............................................................................66 3-6 Absorbance spectra of dinuclear platinum-acetylide oligomers collected at room temperature in THF solution..............................................................................................68 3-7 Transient absorption spectra of dinuclear platinum-acetylides.........................................69 3-8 Phosphorescence spectra of dinuclear platinum-acetylide oligomers...............................71 3-9 Variable-temperature emission spectra of P-PE2 in MTHF.............................................72 3-10 Excitation spectra of dinuclear platinum-acetylide oligomers...........................................73 3-11 Variable-temperature excitation spectra of P-PE2 in MTHF............................................74 4-1 Structures of the PtC n Pt series..........................................................................................81 4-2 Absorption spectra of PtC n Pt complexes in THF solution...............................................85 4-3 Photoluminescence spectra of PtC n Pt complexes in 2-MTHF solvent glass at 80 K.......86 4-4 Emission spectra of PtC 8 Pt in MTHF solvent (glass) over 77 298 K temperature range...................................................................................................................................88 4-5 Overlay of absorption, emission excitation and emission spectra (red lines) for PtC n Pt complexes in MTHF solvent glass at 100 K.........................................................89 4-6 Fit of emission spectrum of PtC 6 Pt using Equation 4-1 and parameters listed in Table 4-1............................................................................................................................91 4-7 Temperature dependence of photoluminescence decay lifetimes for PtC n Pt complexes in MTHF solution (glass).................................................................................92 4-8 Transient absorption spectra of PtC n Pt complexes following pulsed excitation with a 355 nm pulse (10 ns, 5 mJ-pulse -1 )....................................................................................95 4-9 Correlation of E 00 (triplet energy) vs. 1/n for -conjugated oligomers, where n = # carbons in the chain...........................................................................................................97 4-10 Energy gap law correlation..............................................................................................102 4-11 Plot of Huang-Rhys parameter (S m ) vs. the triplet energy (E 00 ) for PtC n Pt complexes and for series of ruthenium polypyridyl complexes and plot of calculated Franck-Condon factors vs. natural log of non-radiative rate for PtC n Pt complexes...................104 5-1 Jaboski diagram and corresponding representative absorption spectra illustrating two-photon absorption.....................................................................................................108 10

PAGE 11

5-2 Jablonski diagram for a four-level system illustrating TPA with ESA...........................111 5-3 Structural motifs for two-photon absorbers.....................................................................112 5-4 Structures of the ligand, L, and iridium(III) complex, ML.............................................113 5-5 Apparatus for measuring nonlinear transmittance...........................................................115 5-6 Absorption of ligand L and complex ML and one-photon emission of ligand L and complex ML....................................................................................................................116 5-7 Transient absorption of ML in deoxygenated THF solution obtained at 400 ns increments following 355 nm excitation..........................................................................117 5-8 Two-photon induced emission of ML after pulsed excitation by 1064 nm....................118 5-9 Photograph of 5 mM ML solution...................................................................................119 5-10 Emission area dependence of ML on incident laser energy at 1064 nm.........................119 5-11 Transient absorption spectra of 5 mM deoxygenated THF solution of ML following 1064 nm pulsed excitation...............................................................................................120 5-12 Transmittance of 1064 nm pulsed beam at various concentrations of ML in THF.........121 6-1 Variations of molecular wires synthesized by Stahl, Owen et al....................................127 11

PAGE 12

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PHOTOPHYSICS OF PLATINUM AND IRIDIUM ORGANOMETALLIC MATERIALS: FROM MOLECULAR WIRES TO NONLINEAR OPTICS By Richard T. Farley December 2007 Chair: Kirk S. Schanze Major: Chemistry My research presents the results of an investigation into the photophysical properties of organometallic materials that contain platinum and iridium. Characterizing the excited-state properties of these types of compounds is necessary for a complete understanding of the materials which may have application in opto-electronics, as molecular wires, or as nonlinear optical materials, as well as help satisfy the need to gain insight into the fundamental nature of triplet excited states of conjugated organic systems. First, in order to partially characterize the excited states of these materials, a transient absorption apparatus was designed and constructed. Second, a series of platinum-containing molecules with two platinum atoms and different end groups were studied to examine interrupted conjugation through metal centers. Third, long carbon chains end-capped with platinum atoms were analyzed to determine the relationship between chain length and nonradiative decay rates. Lastly, an iridium complex with a known two-photon-absorbing ligand was studied to determine if excited-state absorption could be achieved through an initial multi-photon absorption process, and consequently exhibit nonlinear optical properties. The goal of this work was to further the understanding of triplet excited states in organometallic systems. 12

PAGE 13

The most important conclusions of this study are as follows: (i) the triplet excited states of small dinuclear platinum acetylides are more localized than the singlet excited states, and have disrupted conjugation through the platinum atom; (ii) long carbon chains display correlation to the energy gap law; (iii) the triplet excited state can be populated through two-photon absorption, which leads to nonlinear optical effects through enhanced dual-mechanism absorption. 13

PAGE 14

CHAPTER 1 INTRODUCTION Wave-Particle Nature of Photons and Electrons The interaction of light with matter is a topic that has caused interest and debate since the early 1600s. Our understanding of the atomic world has developed almost concurrently with the understanding of light. Light was theorized to be a wave by Huygens in 1678, although this work was largely overshadowed by Newtons corpuscular theory that light consisted of particles. In the early 1800s, Young and Fresnel completed the double-slit experiments that supported Huygens theory by showing interference patterns from light. In 1860, Maxwell described light with a series of equations. Using the existing laws of magnetism and electricity developed by Coulomb, Farady, Ampere, and Gauss, Maxwells theory of electromagnetism defines an electromagnetic wave that should propagate at a speed of 3x10 8 m/s. At this point, Newtons laws of classical mechanics were still the basis for the atomic structures proposed by Dalton, Thomson and Rutherford. At the end of the 19 th century, it was widely accepted that electrons were particles, and light was a wave. Plancks blackbody experiment drastically changed these views at the beginning of the 20 th century. His findings concluded that blackbody radiation is limited to discrete values; that is, the energy is quantized. Newtonian physics could not explain this phenomenon. Soon after the blackbody experiment was completed, Einstein discovered the photoelectric effect, crushing the idea that light is merely a wave. Although it is now accepted that light exhibits a dual wave-particle nature, Maxwells wave equations generally explain most light-related phenomena. Because of the wave-particle duality of light, scientists soon postulated that electrons could also exhibit the same duality. The idea that all particles, not just electrons, act as waves was 14

PAGE 15

theorized by de Broglie in 1924. Research done independently by Thomson and by Germer and Davisson showed electron diffraction patterns and confirmed the de Broglie hypothesis for electrons. In 1928, Schrdinger formulated mathematical equations to describe the wave nature of electrons in atomic structure, similar to the classical treatment of sound and light waves. This development of quantum theory has dramatically impacted our understanding of atomic and molecular structure. Absorption of Light The interaction of light with matter has given us considerable insight into electronic molecular structure. The electronic energy levels of a molecule are discrete, and can be measured through their interactions with photons. The energy of a photon is related to the frequency of its electromagnetic wave by the following equation: hchE (1-1) where E is the energy of the photon, h is Plancks constant, is the frequency, c is the speed of light, and is the wavelength. When a molecule absorbs light (the energy of the photon), an electron is promoted to a higher energy level. The difference in energy levels is equal to the energy of the photon absorbed. A quantitative description of the amount of light absorbed for a certain wavelength is given by Equation 1-2. bCA (1-2) where A is the absorbance, is the molar absorptivity (units of M -1 cm -1 ), b is the pathlength of absorption and C is the molar concentration of the absorbing species. The molar absorptivity is a measure of the probability that a transition will occur and is proportional to the transition dipole moment between the initial and final states. A related term called the absorption cross section, 15

PAGE 16

, represents the area around a molecule which is accessible to collide with a photon, 1 and is related to by Equation 1-3. 2510825.310ln)1.0(AN (1-3) where has units of m 2 and N A is Avogadros number. Although the electronic energy levels of a molecule are discrete, absorption in molecules does not appear as a sharp line, as is the case in atomic absorption. Rather, the absorption appears as a broader band. The explanation lies in the fact that electronic excitation is usually accompanied by vibrational transitions. The absorption of light takes place faster (10 -15 s) than the equilibration of nuclei (10 -13 s). 1 This is referred to as the Franck-Condon principle (Figure 1-1). To illustrate this phenomenon, the ground and excited electronic states are represented by AB AB Figure 1-1. Potential energy curves for electronic transitions. (A) Transition between states of similar equilibrium geometry. (B) Transition between states of different equilibrium geometry. 16

PAGE 17

potential energy curves as a function of their relative equilibrium geometry. The equilibrium geometry for a particular vibrational mode (or combination of modes) is where the potential is at a minimum. The most probable electronic transition occurs in a vertical manner, suggesting that when an electron is excited, it is promoted to the higher state regardless of the equilibrium geometry. That is, if the nuclear arrangement between the equilibrium ground state and the equilibrium excited state are very different, excitation will promote the molecule to a higher vibrational mode of the excited state, and then the molecule will relax to the lowest vibrational excited state. The differences in energy between the lowest vibrational ground state ( = 0) and some vibrational excited state (' > 0) will always be larger than the 0-0 transition. Nature of the Excited State Following absorption and population of the electronic excited state, the molecule will first relax to the lowest vibrational level ( = 0) through thermal (loss of heat) or collisional (with other molecules, either solvent or solute) relaxation. For a molecule whose initial ground state has paired electrons, the electrons will have opposite spin. This is called a singlet state. Upon excitation, the electron does not change its spin due to spin restrictions effected by quantum mechanics. The excited state therefore is also a singlet state. Although the process is forbidden, certain conditions allow for the excited electron spin to flip, resulting in a triplet excited state. This process is called intersystem crossing (ISC). Coupling of singlet and triplet vibrational levels of the same energy is required to allow ISC to occur. As in population of the singlet excited state, population the triplet excited state also involves vibrational relaxation to the lowest vibrational level. An important characteristic of excited states is that the lowest singlet excited state is always higher in energy than the lowest triplet excited state. For singlet excited states, the 17

PAGE 18

electrons are of opposite spin, and therefore can exist in the same region of space, as allowed by quantum mechanics. In the triplet state, the electrons have the same spin, which prohibits them from occupying the same region of space. With a larger distance between electrons in the triplet state, there is less Coulombic repulsion energy than in the singlet state where electrons can be near each other. There are several pathways which an excited state can follow once it is populated. One deactivation pathway is radiative decay by emission of a photon. Radiative decay can occur from the singlet or triplet excited state. Nonradiative decay is another process for an excited state to return to the ground state in which the energy is lost through dissipation of heat. In addition, the excited state could return to the ground state by transferring energy or an electron to another species. Radiative Decay Radiative decay involves the excited state emitting a photon and returning to the ground state. The energy of the emitted photon corresponds to the difference in energy between the two states. Emission from a singlet excited state to a (singlet) ground state is called fluorescence. Because the transition is between states of the same multiplicity, it is an allowed transition. The radiative rate for fluorescence is relatively large (k F ~10 9 s -1 ). If the emission of a photon is from a triplet excited state, the process is called phosphorescence. Because the transition involves states of different multiplicity, the rate of emission is much lower (k P ~ 10 2 10 4 s -1 ) than that of fluorescence. As stated earlier, the triplet state is lower in energy than the singlet state, and as a result, phosphorescence will occur at a lower energy than fluorescence. Photons emitted by the excited state via fluorescence have a slightly lower energy than the photons absorbed by the ground state. This is due to loss of absorbed energy from vibrational relaxation in the excited state. The difference in energy between the absorbed photon and the 18

PAGE 19

emitted photon is known as the Stokes shift. The amount of the Stokes shift for a molecule is a representation of the degree of structural (nuclear) difference between the excited and ground electronic states. That is, if the distortion for the excited state is large, the observed Stokes shift will be large. As is the case with absorption spectra, emission spectra also do not appear as sharp lines. The vertical transitions as described in the Franck-Condon principle illustrate the most probable transitions from one electronic state to another, even if their equilibrium nuclear configurations are quite varied. The transition between electronic states takes longer than the rearrangement time of the nuclei of the molecule. Because vibrational relaxation occurs very rapidly, emission originates from the = 0 vibrational level of the excited state. In contrast to absorption, the transitions originating from vibrational energy levels will be lower in energy for transitions from a higher electronic state to a lower electronic state. Radiationless Transitions All transitions between electronic states that do not involve absorption or emission of photons are considered radiationless transitions. 1 These processes, such as energy loss through release of heat or through collisions, are referred to as internal conversion (IC) for transitions between states of the same spin. Nonradiative transitions occur only when the potential energy curves of two electronic states overlap (Figure 1-2). The shift will take place where the curves intersect, and produce a vibrationally excited ground electronic state. Vibrational deactivation through thermal or collisional loss will bring the molecule to the relaxed ground state at = 0. Figure 1-2 shows the potential energy curves as a function of nuclear distance. As illustrated, there exists an intersection between the two states depending on the energy difference between the two states and also the nuclear equilibrium configuration differences between the two states. 19

PAGE 20

A B C Figure 1-2. Effects of energy difference and nuclear distortion on vibrational overlap. (A) Small E, large overlap; (B) Large E, small distortion, low overlap; (C) Large E, large distortion, large overlap. If there is a large energy difference between the two electronic states, their potential energy curves will not intersect, and the so-called Franck-Condon overlap will be small. A large overlap will bring about a fast and efficient radiationless transition of the excited state. The overlap of the two curves increases exponentially as the difference in energy levels between the excited state and ground state decreases. Therefore, the decrease in energy difference facilitates faster nonradiative decay between the two states. 2 This effect is known as the energy gap law. Additionally, for two systems that have the same difference in energy between the excited and 20

PAGE 21

ground states, the overlap will be greatest for the system that has a higher degree of nuclear distortion between the states. A radiationless transition that occurs between states of different spin is called intersystem crossing (ISC). The transition follows the Franck-Condon principle as described above where the overlap of the two electronic potential energy surfaces is required for vibrational coupling. For ISC from the excited singlet state to the (lower energy) excited triplet state, the triplet state will initially be in a vibrationally excited state, and the transition will be followed by rapid internal conversion to the lowest vibrational level of the triplet excited electronic state. The transition from a singlet state to a triplet state (or vice versa) is a forbidden process as defined by selection rules because it involves states of different spin multiplicity. Consequently, the triplet state acts as an energy reservoir because radiative decay back to the ground state is spin forbidden. 3 The spin angular momentum and orbital angular momentum of electrons in a particular state are separate from one another. This leads to the outcome that transitions between states of different spin are forbidden by the momentum conservation rule. This effect is more important for molecules comprised of lighter elements. As a result, ISC yields are typically low for organic molecules. In molecules with heavy atoms (or light molecules in the vicinity of heavy atoms), the conditions are different. The relative orientations of spin momenta and orbital momenta are less important. Rather, the spin and orbital angular momenta are coupled together. 3 This is known as spin-orbit coupling, and accounts for the breakdown of selection rules in singlet-triplet transitions. The change in electron spin magnetic momentum coupled with the corresponding orbital change provides a mean of conserving total angular momentum. 1 This heavy-atom effect is responsible for increased ISC yields. 21

PAGE 22

Other types of radiationless transitions are also possible, and will be mentioned briefly. Energy transfer from an excited state to some suitable acceptor is one type of relaxation pathway, as described by Equation 1-3. **ADAD (1-3) where D is an energy donor, A is an energy acceptor, and denotes an excited state for either. Exchange energy transfer (collisional) and Coulombic energy transfer (dipole-dipole interaction) are two mechanisms for energy transfer. Both require that the energy of D* is higher than that of A, and the rate of energy transfer from D* to A is greater than the decay rate of D*. Similarly, an excited state donor can transfer an electron to a suitable acceptor. ADAD* (1-4) Furthermore, triplet-triplet annihilation is another type of nonradiative decay involving two triplet excited state species: SSTT*** (1-5) where T* and S* are the excited triplet and excited singlet, respectively, and S is the ground-state singlet. Triplet-triplet annihilation occurs when two molecules in the triplet state interact (generally by collision) to create an excited singlet state and a ground singlet state. If the excited singlet state returns to the ground state through emission of a photon, it is called delayed fluorescence. A Jaboski diagram (Figure 1-3) will be useful to summarize the probable potential transitions between the common electronic states in a molecule as discussed above. Before introducing a brief review of organometallic materials and their photophysical behavior, there are two characteristic properties that should be discussed: the photoluminescence quantum yield and lifetimes of the excited states. Quantum yield of emission is simply the ratio 22

PAGE 23

= 0 = 1 = 2 = 3 IC S1 ISC = 0 = 1 = 2 = 3 IC T1 A IC F P IC S0 Figure 1-3. Jabloski diagram illustrating possible transitions. S 0 : Ground singlet state; S 1 : Excited singlet state; T 1 : Excited triplet state; A: Absorption; F: Fluorescence; P: Phosphorescence; IC: Internal conversion; ISC: Intersystem crossing. of absorbed photons to emitted photons. A general expression for quantum yield 1 is given by Equation 1-6: 0eek (1-6) where e is the emission quantum yield, is the formation efficiency of the emitting state, k is the rate constant for emission, and is the measured experimental lifetime. The lifetime is defined by Equation 1-7: 0e iekk01 (1-7) where k i is the sum of all other deactivating rate constants. More specifically, we can define the two terms and for fluorescence and phosphorescence. For these values, is assumed to 23

PAGE 24

be unity. For the processes indicated in Figure 1-3, the fluorescence quantum yield, F is described by Equation 1-8: SFF k where S is the lifetime of the singlet state, and is defined in Equation 1-9: nrISCFSkkk (1-9) In Equation 1-9, k F is the rate of fluorescence, k ISC is the rate of intersystem crossing (to the first excited triplet state), and k nr is the rate of all other nonradiative processes from the singlet state. Similarly, and assuming the triplet excited state is populated only through excitation of the ground state to form the singlet excited state, followed by ISC, the phosphorescence quantum yield can be taken as Equation 1-10: TPISCP k (1-10) where ISC is the intersystem crossing quantum yield, and T the triplet lifetime, is defined in Equation 1-11: nrPT1kk (1-11) where k P is the rate of phosphorescence and k nr is the rate of all other nonradiative processes from the triplet excited state. Fluorescence lifetimes are generally short (10 -11 10 -8 s) because they correspond to allowed transitions. In contrast, phosphorescence occurs on a much longer timescale (10 -6 10 1 s) due the forbidden nature of the triplet-to-singlet transition. As mentioned earlier, heavy atoms may help facilitate intersystem crossing to populate the triplet excited state. However, the heavy atom effect may also bring out spin-orbit coupling in the radiative decay process, shortening the phosphorescence lifetime. 24

PAGE 25

Triplet Excited States in Conjugated Organic Systems Although slow ISC prevents detection of triplet states in organic materials, triplet excited state properties are nevertheless important in these systems. The increasing use of organic materials in organic light emitting diodes, field-effect transistors, electronic semiconducting materials, light harvesters, photosensitizing and nonlinear applications makes the triplet state an interesting research area. 4-8 Molecular Wires The concept of an organic molecular electronics device was first proposed over thirty years ago by Aviram and Ratner. 9 However, it was not until the measurement of electrical conduction through a single benzene-1,4-dithiol molecule by Reed and coworkers 10 in 1997 that molecular-scale electronics attracted serious attention. Since then, the study of prospective molecular wires for application in electronics has grown tremendously. 11-13 Conjugated organic rigid-rod type molecules offer potential for such use due to their delocalized structure which allows electronic communication throughout the molecule. Poly-(para-phenylenevinylene) (PPV) oligomers are a class of molecules that afford electron delocalization through -conjugation. Davis and coworkers 14,15 have used -conjugated linkers as a bridge between electron donor and electron acceptor molecules to investigate charge separation. These molecules, consisting of a tetracene donor and pyromellitimide acceptor moieties connected by phenyl, stilbene, and PPV oligomers of varying length, are illustrated in Figure 1-4. The usefulness of the bridge is to act as an electronic connector between the end-group molecules under study. The understanding of the electronic properties of the bridge linker is important to determine the mechanism of charge transfer between the donor and acceptor end25

PAGE 26

A B C Figure 1-4. Structures of molecules studied by Davis et al. 15 Linkers include (A) phenyl, (B) trans-stilbene, and (C) an oligo-PPV. groups. For example, in the short linkers, charge transfer was achieved through tunneling (due to a high energy barrier in the bridge), whereas in the longer PPV chains, there existed larger delocalization (i.e. a lower energy barrier) in order to help facilitate the movement of charge. 14,15 The work completed by Davis and coworkers has encouraged the use of computational methods, such as density functional theory, to further study the electronic structure of PPV systems as -bridges for molecular wire-like applications. 16 Among the most versatile active components for establishing electronic communication are transition metal centers 17-19 and unsaturated organic systems have proven to be among the most useful bridging units. 20,21 Also of interest are polymers which contain metal atoms in the backbone. 22,23 The incorporation of heavy atoms, particularly metals, into organic structures changes several other characteristics, including redox, optical and electronic properties. As mentioned earlier, introducing heavy atoms into organic molecules induces spin-orbit coupling 26

PAGE 27

of the excited state. From a synthetic standpoint, the alteration of structure through ligands, spacer units, and types of metals used can systematically vary the singlet and triplet energy states. Polyynes, e.g. (CC) n oligomers or polymers, are distinctive in that they provide one of the simplest possible linear -conjugated organic units of variable length. With the addition of metal atoms, such as platinum, to the ends of highly conjugated organic chains, the metal centers not only terminate the extended system, but become integral parts of them. Furthermore, metal atoms as end-caps increase the stability of long carbon chains. 24-26 In such ,-dimetalla--systems the communication of electronic information between the metal centers is expected to be particularly efficient. Additionally, luminescence properties become of interest. 23,27 The spin-orbit coupling increases population of the triplet state, therefore giving access to the triplet manifold and allowing for further investigation of the electronic state energies. Nonlinear Optics and Platinum Acetylides Due to the advancement of laser technology in a variety of applications, the need for protection from intentional or erratic laser pulses to prevent damage of human eyes, optical sensors, and other sensitive optical components has become important to a number of fields. An optical power limiter is a device that will allow the transmittance of light at low intensities, but will strongly attenuate the incident energy at high intensities. 28,29 It is also desirable for a material which exhibits optical power limiting behavior to have a fast response time, reasonable processability, good linear transparency, and a high damage threshold. 29-31 Many materials currently used to protect eyes or sensors from damaging light absorb strongly at only particular wavelengths, restricting use to certain wavelengths while also limiting visibility at low incidences. 27

PAGE 28

Due to their high optical nonlinearities and fast response time, organic materials are attractive for use as optical limiters. More specifically, metallophthalocyanines and metalloporphyrins display excellent nonlinear optical behavior due to large ISC efficiencies from spin-orbit coupling of the heavy atoms which populate the highly absorbing triplet excited states. 32,33 Unfortunately, poor solubility has led to difficulties in processing and hindering the development of practical devices. Reverse saturable absorption (RSA) is one mechanism by which nonlinear absorption can occur. RSA is a relatively slow, sequential multiphoton process in which the ground electronic state first absorbs a photon, and then an excited state absorbs a second photon. For RSA to occur, the absorption cross section of the excited state must be larger than the absorption cross section of the ground state (Figure 1-5). In transition metal-containing organic compounds, the absorbing excited state is usually the triplet state, populated through ISC facilitated by the heavy-atom effect. Transient absorption (TA) spectroscopy is a convenient way to probe the excited-state absorption properties (a more detailed discussion about TA spectroscopy will be given in Chapter 2). Platinum-acetylide complexes have been shown to have highly absorbing triplet states with long lifetimes. 34-41 The large ISC yields afforded by the platinum atom create strongly phosphorescent triplets, making these molecules excellent candidates for studying triplet-state properties. Structural changes in conjugation length of the organic moieties allows for spectral tunability in absorption and emission to create optical bandgaps in the UV, visible and near-IR. McKay and coworkers 34-39 were the first to report on the nonlinear optical properties of a platinum-acetylide compound and have since thoroughly investigated the optical properties of a platinum-ethynyl complex (shown in Figure 1-6). The compound exhibits strong singlet 28

PAGE 29

S1S0T1gs ISCTn es es> gs Input IntensityOutput Intensity Linear absorbance RSA Figure 1-5. Conditions for reverse saturable absorption. absorption with the maximum at 355 nm. (Interestingly, the absorption maximum makes the molecule convenient for optical excitation by the third harmonic of a Nd:YAG laser.) There is emission from the compound at 410 nm, which is attributed to fluorescence, and another peak at 520 attributed to phosphorescence. The peaks were assigned mostly due to the short (330 ps) and long (0.5 s) lifetimes, respectively. At low temperatures, the phosphorescence lifetime increases, up to 700 s at 17 K, while the fluorescence lifetime is unchanged. The intensity of phosphorescence emission increased as well at low temperatures, unlike the fluorescence. The 29

PAGE 30

Pt PBu3 PBu3 Figure 1-6. Structure of platinum-ethynyl and corresponding photophysical data. Figures adopted from McKay et al. 36 unvarying fluorescence intensity and decay rate over a range of temperatures show that ISC dominates the singlet state decay path and is independent of temperature, indicating that ISC efficiency approaches unity even at room temperature. Excited-state absorption measurements reveal that platinum-ethynyl demonstrates strong RSA over some wavelengths. For example, at 513 nm, the excited-state absorption cross section is 10 4 times larger than the ground state absorption. Further study of platinum-ethynyl by Staromlynska and coworkers 35 leads to observation of an alternate absorption pathway. Emission from the compound was detected when excited where the compound has effectively no ground-state absorption at 595 nm, by either a 45 ps pulse or 2 ns pulse. Emission spectra caused by excitation at 595 nm are identical to those caused by excitation at 355 nm, indicating that the same state (the triplet excited state) is arrived via two different absorption pathways: one photon absorption and two photon absorption (TPA), 30

PAGE 31

each to the singlet excited state, followed by fast ISC to the triplet excited state. TPA will be discussed in more detail in Chapter 5. Briefly, two-photon absorption occurs when a material simultaneously absorbs two photons whose combined energy equals that of the energy difference between the absorbing state and the excited state. An additional report by the same group 36 shows that TPA dominates at wavelengths between 560 and 700 nm. There is no absorbance in this wavelength region at low incident energy. The two-photon absorption coefficient is found to be independent of excitation wavelength. For shorter wavelengths (< 560 nm), one-photon excitation dominates. The transmission as a function of input intensity was also measured at 595 nm for 45 ps pulses and 2 ns pulses. The results demonstrate the same nonlinear behavior, regardless of pulse length. Slightly stronger nonlinear behavior is observed for 2-ns pulses at 455 nm. Two additional reports by McKay, Staromlynska and coworkers 38,39 on platinum-ethynyl provided several further findings and summarized the results of their work. Three main regions for excitation are described: (i) blue (< 500 nm), where single photon excitation to the singlet state, followed by rapid ISC to the triplet state, (ii) green (500 540 nm), where direct excitation of the triplet state from the ground singlet state occurs, as spin-orbit coupling aids in the forbidden transition, and (iii) red (540 700 nm), where no one-photon process occurs between the singlet ground and either singlet or triplet excited states, but two-photon absorption dominates to populate the singlet excited state, followed by ISC to the triplet excited state. The triplet excited state in all three regions is highly absorbing throughout the visible. Furthermore, the excited state absorption cross section is independent of the excitation pathway to the excited triplet state. Absorption from the excited singlet was not studied as no direct evidence of this 31

PAGE 32

transition has been found. The Jaboski diagram in Figure 1-7 summarizes the transitions occurring in platinum-ethynyl. S1S0T1 ISC Tn123 S1S0T1 ISC Tn12 3 Figure 1-7. Jabloski diagram illustrating possible transitions for platinum-ethynyl. (1) Blue (< 500 nm) single photon absorption followed by ISC. (2) Green (500 540 nm) direct excitation to the triplet state. (3) Red (540-700 nm) Two-photon absorption followed by ISC. All three regions include triplet-state absorption. Platinum-ethynyl shows promise as a broadband optical limiter in the visible region that is effective over a range of pulse lengths from picoseconds to hundreds of nanoseconds. Between 530 and 650 nm, incident energies of greater than 500 J were attenuated to ~6 J by a 0.08 M solution of the compound. However the authors did note some degree of decomposition at irradiances of > 6 J/cm 2 at 532 nm and 1064 nm. Also, solutions of platinum-ethynyl showed degradation over a period of several weeks. Such behavior is not acceptable for a practical optical limiting device, and if stabilization is not possible, alternate approaches should be found. 32

PAGE 33

Although the photophysics of platinum-ethynyl have been thoroughly investigated, the lack of a series of related molecules makes any conclusions of structure-property relationships difficult. Cooper, Rogers, and coworkers 40-45 have synthesized and studied the photophysical behavior of a series of platinum(II)-containing phenyl-ethynyl oligomers (Figure 1-8). The goal was to determine structure-property relationships in a well-defined system through IR, UV/Vis, photoluminescence, and transient absorption spectroscopy and kinetics of some of the transitions. Pt PBu3 PBu3 Pt PBu3 PBu3 Pt PBu3 PBu3 Pt PBu3 PBu3 (Ch3)3Si Pt Si(Ch3)3 PBu3 PBu3 E1PE1PPEPE2PE 3 Figure 1-8. Structures of platinum acetylide oligomers studied by Cooper, Rogers, et al. 40-45 The PtCC acetylenic vibrational bands for E1 were 2033 cm -1 compared to 2100 cm -1 for the other compounds, suggesting that the PtC and CC bonds have more double-bond character in E1 than the compounds with the larger ligands. 43 UV/Vis absorption maxima systematically red33

PAGE 34

shifted from 308 nm for E1 to 376 nm for PE3, indicating that the increase of ligand length decreases the energy between the ground and excited singlet states. As with PE2 as studied by Staromlynska, McKay et al., 35 fluorescence quantum yields were small. E1 and PE1 show minimal emission. PE2 and PE3 have fluorescence maxima near 400 nm, while PPE shows fluorescence emission at 375 nm. The higher emission energy is attributed to the twisting of phenyl groups interrupting conjugation. The dihedral angle was calculated 43 to be 40, whereas PE2 and PE3 have free rotation around the triple bond. Room temperature phosphorescence of PPE, PE2 and PE3 is observed, with a 0-0 band preceding unresolved vibronic bands at lower energy, due to combinations of CC single and triple bond stretching, out-of-plane CH stretching, etc. 43 Phosphorescence maxima systematically increase with increased conjugation length. Triplet excited state absorption observed for PE1 decayed within the length of the laser pulse (< 15 ns), however, the other compounds exhibited transient molar absorptivities that increase and red shift with increased conjugation length. The excited state lifetimes also show a clear trend that lifetime increases as conjugation length increases, attributed to less spin-orbit coupling effect from platinum as the excited state is more delocalized on the ligand. 40 The group further complemented their study of the platinum oligomer series by direct comparison with a set of all-carbon butadiyne analogs 44 They found that conjugation through the platinum center occurs in the singlet state, but the triplet states are more complex. The lowest triplet state shows metal-to-ligand charge transfer character as is likely confined to one ligand, whereas the higher excited triplet shows ligand-to-metal charge transfer. 44 The shortest compounds are affected greatest by the presence of platinum. Fluorescence emission spectra for the butadiyne compounds are very similar to the platinum-containing compounds, but quantum yields decreased dramatically when platinum is incorporated into the structure. For the shorter 34

PAGE 35

all-organic structures, triplet emission is very weak and blue-shifted compared to the platinum analogs. However, phosphorescence emission bands begin to appear the same between platinum oligomers and butadiyne as conjugation length increases. Based on these findings, the authors have concluded several things from the sensitivity of state energies. The transitions involving absorption by the ground singlet state and by the triplet excited state are more sensitive to molecular size than emission from the triplet excited state to the ground excited state. Therefore, it follows that the ground singlet and first triplet excited states are more confined than the excited singlet and higher excited triplet states which are delocalized over the ligand. Objective of Present Study Defining structure-property relationships between molecules of interest and photophysical behavior is paramount to increased development of organic and organometallic compounds for application in light-emitting diodes, chemical sensors, molecular electronics, photovoltaics, optical sensors, and nonlinear optics. Platinumand iridium-containing conjugated compounds offer the ability to study triplet excited state behavior in a controlled and deliberate manner through synthetic capabilities. Fundamental aspects of characterization, such as singlet and triplet state energies and quantum yields, are measured using traditional techniques with commercially available spectrophotometers. In addition to commercial instrumentation, a transient absorption apparatus was designed and constructed to examine photoexcited states of metal-containing organic compounds. Included along with arrangement of optical components, user-friendly software was developed to control the timing parameters of the hardware involved. Platinum acetylide complexes are excellent systems for probing a variety of triplet-state phenomena such as ground state absorption to the triplet state, intersystem crossing, triplet state 35

PAGE 36

absorbance, and phosphorescence. 45 While the consequence of the platinum atom on delocalization of the excited states has been studied, the effect is not completely understood for a variety of compounds. The photophysics of a series of dinuclear platinum-acetylide oligomers with phenylene-ethynylene end-caps has been investigated to provide insight into conjugation through platinum atoms and the effect on excited state energies. The understanding of polyyne is crucial for the progress of molecular electronics. While direct application of molecular wires is not the focus of this work, it has been shown that characterization of excited-state properties of potential molecules is fundamental in understanding the nature of these materials. Incorporation of heavy metal atoms as end-caps to polyyne chains allows for the population, and thereby detection, of the triplet state. A series of long carbon chains end-capped with platinum has been photophysically characterized to probe the effect of chain length on excited state behavior. The need for optical limiting materials is increasing as the use of lasers in a variety of applications is becoming more popular. The nonlinear optical behavior of many organic and organometallic materials provides promise for protection from potentially damaging laser pulses. The incorporation of iridium with a highly absorbing organic ligand has created a material with alternate absorption pathways. The ground and excited state absorption pathways have been probed by single and multiphoton spectroscopy, and preliminary nonlinear behavior has been detected. 36

PAGE 37

CHAPTER 2 TRANSIENT ABSORPTION SPECTROSCOPY AND INSTRUMENTATION Background The study of transient chemical species generated by various approaches such as chemical reactions or the absorption of energy is necessary for the complete understanding of these processes. As some events occur on timescales of minutes to days, the observation of the species involved is less hindered by time sensitivity. However, many chemical reactions produce reactive intermediates that last only milliseconds or less before their demise. Indeed, the absorption of light or other energy to produce excited-state species and the transfer of energy or electrons within a molecule or between molecules are processes which generally do not allow for the separation of very long-lived, stable species. The dynamics of such processes are also of interest to characterize mechanisms and species involved. A detection technique on an equivalent timescale to the event being monitored is necessary for unhindered observation. Also, it is imperative that the species to be studied be generated quickly as to have a suitable concentration available for detection. In this chapter, the construction and operation of a new transient absorption apparatus for the detection of excited-state species (such as those discussed in the following chapters) is described. Transient absorption (TA) spectroscopy, which is also called flash photolysis or simply pump-probe spectroscopy, has remained one of the most important techniques for detection and characterization of short-lived chemical species since its development in the late 1940s. 46 Indeed, the importance of this work was recognized as half of the 1967 Nobel Prize was awarded to Norrish and Porter for their development of the flash photolysis method. Their research focused on studying reactive intermediates generated by excitation from a flashlamp. 46 It was not until the 1960s that better time resolution could be achieved with use of lasers as an 37

PAGE 38

excitation source. 47 The apparatus design developed by Lindqvist is still the basis of modern instrumentation (Figure 2-1). The system consists of three main components: an excitation samplemono.PMTamplifieroscilloscope lamplaser light samplemono.PMTamplifieroscilloscope lamplaser light lamp samplemono.PMTamplifieroscilloscope lamplaser light samplemono.PMTamplifieroscilloscope lamplaser light lamp Figure 2-1. Transient absorption apparatus developed by Lindqvist. 47 source (laser), a monitoring source (lamp), and a detector (monochromator/PMT). Computer control and data acquisition was the next advancement, and such a system was developed in the 1970s by Small and Scaiano. 48 In earlier instruments, the time resolution was limited not by the electronics, but the duration of the excitation source. Picoand femtosecond techniques are now popular, allowing detection of short-lived species or events as short 10 -15 s. However, the need for nanosecond techniques remains prevalent. Techniques In the early stages of nanosecond timescale measurements, nitrogen and ruby lasers played a defining role in the development of experimental ability. 49 Currently, a variety of excitation sources are employed for initiating events of the same time regime. Commercially available excimer and Nd:YAG lasers offer the ability to excite samples at a large range of wavelengths 38

PAGE 39

from the ultraviolet into the infrared. Tunable optical parametric oscillators (which are used in combination with such pump sources as Nd:YAG lasers) offer even greater flexibility in excitation wavelengths. The probe beam for transient absorption is commonly one of two types of xenon lamps. Xenon lamps are convenient due to their broadband spectral output and high intensities. A continuous output lamp can be used, often paired with mechanical shutters to limit sample exposure between acquisitions. The lamp intensity can be increased by a factor of 5-100, or greater, over a several milliseconds by pulsing the lamp. 50 This technique is sometimes necessary for short timescales, and although it reduces the lifetime of the lamp, the signal-to-noise is significantly improved. Alternatively, strobe flashlamps with high-voltage power supplies are employed as the monitoring source. These lamps are designed for high-repetition operation (to coincide with the pulsed excitation source) and can offer excellent arc stability, making them ideal for use in such an application. A number of detectors can be used for measuring and recording the probe beam as long as the basic requirements of sensitivity in the desired spectral range and being responsive on the timescale of the events being measured are satisfied. A common detector is comprised of a monochromator and a photomultiplier tube (PMT). The PMT should be wired for a fast time response. With the range of PMTs available, detection from the UV into the near IR is possible. Using such a detector leads to collecting a time-resolved measurement of the lamp intensity at a single wavelength, and then extracting the transient absorption data. A complete spectrum is then constructed for a particular time within the decay of the transient (vide infra). Alternatively, transient absorbance can be measured using a spectrally resolved detector and gating the collection temporally. Such a detector could be a photodiode array. A diode 39

PAGE 40

array can measure polychromatic light from a dispersive element such as a prism or grating, and then the information from each diode is processed by a computer. The data collected from this type of detector provides a complete spectrum from which a time trace can be constructed (vide infra). Although multiwavelength detection is convenient, there are some limitations with diode detectors. The main disadvantage of using diode arrays is their lack of sensitivity. Diode detectors are generally less sensitive than PMT detectors because diode detectors do not have the electronic gain that PMTs do, and therefore produce only one charge per photon. Diode arrays are also noisier than PMT detectors. A major advancement to simultaneous multiwavelength acquisition was the introduction of the charge-coupled device (CCD) camera. Simply stated, a CCD is a two-dimensional matrix of photosensors, or pixels. 51 Each pixel will convert photons into charge, which is then converted into a digital signal read and processed by a computer. Coupling with a dispersion element such as a prism or grating is necessary to separate wavelengths along a particular axis of the CCD chip. The relative intensities of incident photons will determine the amount of relative charge that is stored in the CCD chip and subsequently read to a computer. A CCD is similar to a photodiode array in that each of the individual pixels of the CCD responds independently to the photons which come into contact with it. Generally, a silicon-based photosensor is used, and therefore the response is not wavelength dependent. For imaging-type CCD cameras, filters are used to discern colors between pixels on a CCD chip. In spectroscopy, however, such two-dimensional wavelength dependence is not necessary. By coupling the CCD with a dispersion element such as a spectrograph, a range of wavelengths (along one axis of the chip) can be collected simultaneously. 40

PAGE 41

While CCD cameras are effective instruments for recording images in a variety of applications, the response times of early devices were not adequate for use in kinetic measurements. The invention of the image intensifier has circumvented this problem. Intensified charge-coupled devices (ICCDs) are often used in low-light conditions due to the increased sensitivity of the CCD. More importantly for kinetic measurements, the intensifier can be used as a fast electronic gate to increase temporal resolution. The intensifier is mounted Applied Voltage Applied Voltage Photocathode Incident Photons MicrochannelPlate Phosphor Screen Fiber Optic Output Window Input Window Applied Voltage Applied Voltage Photocathode Incident Photons MicrochannelPlate Phosphor Screen Fiber Optic Output Window Input Windo w Figure 2-2. CCD intensifier. Figure adapted from www.andor.com. 51 within the camera before the CCD chip, and consists of three main components: a photocathode, a microchannel plate (MCP), and an output phosphor screen. A schematic representation of the intensifier is shown in Figure 2-2. When incident photons collide with the photocathode, 41

PAGE 42

electrons are ejected towards the MCP. The MCP consists of millions of parallel channels with secondary electron emitters contained within their inner walls. 52 When a sufficiently high voltage is applied across the MCP, the ejected photoelectrons are drawn through the channels ejecting other electrons, effectively acting as a photomultiplier. The electrons exiting the MCP collide with the phosphor screen, creating an intensified image to be projected onto the CCD. A fiber optic typically connects the intensifier to the CCD. By appropriately controlling the voltage across the MCP, the intensifier becomes an effective electronic shutter capable of very fast gate times (~10 -9 s). The transient absorption spectra and time decay profiles from data collected by ICCD detectors are constructed in a similar fashion as diode array detectors. Apparatus Hardware A diagram of the final apparatus is shown in Figure 2-3. The three main components as described earlier are the pump, the probe, and the detector. The pump beam (blue path) is generally created by the third harmonic of a Nd:YAG laser (355 nm, Continuum Surelite II, 5 ns fwhm). An alternate excitation source, which provides greater flexibility in excitation wavelengths, is generated by using an optical parametric oscillator (Continuum Surelite OPO PLUS) pumped by the laser described above. The pump beam is set to an energy of approximately 10 mJ/pulse, and directed to the sample using conventional laser optics. The beam is shaped prior to the sample using a cylindrical plano-concave lens (not shown in figure) such that the beam hitting the sample spans the width of the cuvette along the optical axis of the probe beam. An anodized aluminum sample holder masks the pump beam to a vertical thickness of 4.0 mm, restricting the excitation to a specific region within the sample cuvette. The probe beam (red path in Figure 2-3) is generated by a xenon flashlamp (Perkin Elmer Flashpac LS-1130 with an FX-1150 bulb). The spectrum of the lamp as provided by the 42

PAGE 43

LampL1L2L3BS1M1OPONd:YAGlaserIrisPrism L4L5L6BS2M2SampleSpectrograph & CCD Camera LampL1L2L3BS1M1OPONd:YAGlaserIrisPrism L4L5L6BS2M2SampleSpectrograph & CCD Came ra 43 Figure 2-3. The transient absorption apparatus. Lenses, mirrors, and beamsplitters are abbreviated by L, M, and BS and described in the text. The blue line (--) is the pump; red line () is the sample probe, I; and black line (---) is the reference probe, I 0

PAGE 44

Figure 2-4. Spectrum of xenon lamp used as the probe. (Taken from 1100 series flashlamp datasheet, PerkinElmer.) manufacturer is show in Figure 2-4. The red region of the measured spectrum is relatively much weaker, and the intense peaks between 800 nm and 850 nm are not observed mostly due to the coating on the lenses as well as the weak detector response at those wavelengths (vide infra). All optics are antireflective coated 350-700 nm, 50.8 mm diameter plano-convex BK7 lenses, unless otherwise noted. The beam is first collimated by L1 (focal length = 3.45 cm) and then refocused by L2 (fl = 30 cm) through an adjustable iris which serves as the primary adjustment for the final beam size. The beam is then collimated again by L3 (fl = 30 cm) and passed through a 50/50 beamsplitter. The transmitted beam is reflected first from a mirror and then a second beamsplitter. The reflected beam from the first beamsplitter is reflected from a mirror and then passed through a second beamsplitter. The distance between BS1 and M1 and between BS2 and M2 is 8.5 cm, and the distance between BS1 and M2 and between M1 and BS2 is 19 cm. This 44

PAGE 45

arrangement of beamsplitters and mirrors allows for the creation of two beams of equal intensity. By creating two beams from a single source, one beam can be slightly offset to provide an essentially identical reference beam. Both beams are focused through the sample using L4 (fl = 10 cm, diameter = 38.1 mm). The main probe beam (sample beam) is focused through the portion of the sample that has been excited by the laser. The reference beam is focused approximately 3 mm directly above the sample beam focus. If necessary, the focused beams can be aligned relative to each other by adjusting the position of either mirror. After passing through the sample, both beams are collected and collimated by L5 (fl = 17.5 cm) and focused onto the entrance slits of the spectrograph by L6 (fl = 20 cm). The detector is comprised of a spectrograph (Acton SpectroPro 150) coupled to an intensified CCD camera (Princeton Instruments PI-Max iCCD). The spectrograph grating provides a spectral range of ~350 850 nm. A higher resolution grating can alternatively be used to give a spectral window of roughly 250 nm. However, the transient absorption spectra of the compounds typically studied in our lab do not have the fine structure to necessitate gratings of this resolution. The CCD camera settings are controlled primarily by commercially available software (WinSpec/32 version 2.5.12.2, Roper Scientific) supplied by the camera manufacturer. The CCD chip is 512 x 512 pixels, with the x-axis corresponding to wavelength and the y-axis corresponding to vertical position on the entrance slits of the spectrograph. Two regions of interest of on the chip have been defined to identify the sample beam and the reference beam. The CCD camera and software collect and record the raw data, which are intensities of the probe for the sample beam and the reference beam as a function of wavelength. This information must be converted to absorbance values in a separate step. 45

PAGE 46

Data Collection There are several parameters that can be adjusted in order to obtain meaningful, quality time-resolved transient absorption spectra. Following excitation, a sample relaxes to the ground state, for example, through first order decay. The most important factor in a transient absorption apparatus is timing; it is imperative for meaningful data to be collected that all components of the apparatus perform their functions at intended and known times throughout the experiment. In order to simplify this potentially complex problem, a four-channel pulse generator (Berkeley Nucleonics Corporation model 555 pulse/delay generator) is used to send electronic triggers to each of the 3 components (laser flashlamps and Q-switch, xenon flashlamp, and camera). To increase the ease of operation and expand the flexibility of experiments and maintenance, a LabVIEW-based virtual instrument (VI) was written in-house to provide a computer interface for the pulse generator. The VI is used in collaboration with the WinSpec/32 software for data collection. By using the pair of programs, all necessary timing parameters can be optimized. An observer using a CCD detector has several choices to make after exciting a sample: when to begin monitoring, for how long to integrate each image, how many images to take, and over what timescale to make the measurements. A graphical representation of these parameters is illustrated in Figure 2-5. t 0 (time zero) This is the time at which the laser fires. Assuming fast excitation to the singlet state (10 -15 s) followed by rapid intersystem crossing (10 -9 s), this is also the time at which there exists the greatest population of triplet excited molecules. Camera delay This is the time between the laser fire (t 0 ) and the onset of the first recorded transient. Camera delay is useful when there is a short timescale event, such as strong fluorescence or scattered laser light, which can cause artifacts in the TA spectrum. Gate width This is the amount of time the CCD is exposed to light. The maximum amount of light in the shortest amount of time is ideal for greater time resolution and signal-to-noise. The gate width is determined by the amount of probe light reaching the detector, i.e. a more intense probe will need a shorter gate width. 46

PAGE 47

Camera delay increment This is the time between the beginnings of each image acquisition. The camera delay increment should be equal to or longer than the gate width so that no overlap of signal is measured. Intensity A Intensity Time t0 B Time t0 Figure 2-5. Illustration of timing parameters for ICCD data collection. (A) shows a comparatively long camera delay ( ), short gate width ( ) and long camera delay increment ( ) with respect to (B). 47

PAGE 48

The length of the measurement will equal the number of transients collected multiplied by the camera delay increment. Ideally, the total time should exceed the decay time of the excited state. Also, the greater the number of transient spectra collected, the easier it will be to accurately fit the set of spectra to a kinetic model. The parameters above, as well as some additional settings, will greatly affect the signal-to-noise ratio of the system, and will be discussed in more detail later in the chapter. Data Processing To look at how a transient signal is measured and calculated, it can help to begin with the definition of absorbance and transmittance, )(log)( TA (2-1) )()()(0 IIT (2-2) where I 0 () and I() are the intensities of radiation prior to passing through the sample and after passing through the sample, respectively. Substituting Equation 2-2 into Equation 2-1, an expression for absorbance in terms of intensities can be given as Equation 2-3, shown below. )()(log)(0IIA (2-3) This expression is a convenient way to convert measured intensities of light into absorbance values. An important part of this equation is that I 0 must be measured without the sample in the optical path (either by physical placement or by using a dual-beam instrument). However, in transient absorption experiments, the intensity of the lamp itself is never measured directly. Instead, I 0 is defined as the intensity of the light that passes through the ground state (i.e. before optical pumping and creation of excited states). After populating the excited state, there will be some change in intensity of the transmitted light, 48

PAGE 49

)()()(0 III (2-4) Figure 2-6 shows a graphical representation of the change in intensity of the transmitted light throughout the lifetime of a transient at a single wavelength. I 0 is an arbitrary value determined Intensity It0 I0 Time Intensity It0 I0 Tim e Figure 2-6. Change in intensity of transmitted light during the lifetime of a transient species. by the probe lamp, alignment, and camera settings. The laser fire defines t 0 Equation 2-4 can be substituted into Equation 2-3 to define A as )()()(log)(00IIIA (2-5) and simplifying to )()(log)(0IIA (2-6) 49

PAGE 50

In many transient absorption apparatus, a continuous-wave monitoring source is used (such as a PMT connected to an oscilloscope) for transient absorption spectroscopy. Therefore I 0 can be measured prior to t 0 and stored for calculation later. This method would also be possible using a pulsed monitoring source such as a CCD camera, although it would not necessarily simplify any of the calculations. For this reason, and more importantly to try to minimize shot-to-shot fluctuations of the flashlamp, the probe beam in the current apparatus has been split into a sample beam and a reference beam. The probe beam passes through the region of the sample that has been excited by the laser. The reference beam passes through an area of the sample which has not been excited (i.e. the ground state). This allows for I 0 and I to come from the same flash of the probe (minimizing noise), as well as the simultaneous collection of each (minimizing experiment time). In order to normalize the sample and reference beams, a simple correction factor, k(), can be calculated: )()()(0IIk when pump is off. (2-7) When measured prior to t 0 (i.e. with the laser off), k() will correct for any slight differences in intensity of the two beams caused by the detector or alignment. The correction factor will be different for each experiment, and therefore should be collected at the beginning of each measurement. Placing the correction factor into Equation 2-6, )()()(log)(0IIkA (2-8) a relationship between raw data from the camera software and a presentable set of spectra is easily made. Using a combination of full spectral range detection, along with pulsed delay detection, time-resolved spectral data can be constructed. Figure 2-7 shows how the light intensities of 50

PAGE 51

Absorbance Wavelength Absorbance Wavelength IntensityWavelengtht0t1t2t3 IntensityWavelengtht0t1t2t3 Wavelengtht0t1t2t3 Absorbance TimeAbsorbance Tim e Figure 2-7. Construction of a time trace and transient absorption spectra from gated multiwavelength measurements. the sample and reference beams at a single wavelength vary throughout the excited state decay using a pulsed monitoring source. By having this data for many wavelengths, time-resolved transient absorbance data for a particular wavelength can be constructed as shown. It is convenient to use temporally resolved data to characterize the nature of the excited state of a material. Often in combination with spectral information, lifetimes can help discern 51

PAGE 52

between simple excited-state decay, electron transfer, energy transfer or even creation of another species. In the most general case of monitoring the relaxation of an excited (triplet) state, kinetic information can be elucidated quite easily. As described earlier, transient absorption intensity as a function of time can be plotted for a particular wavelength. These data can then be fitted following first-order kinetics. That is, assuming one excited state and one ground state, the kinetics follow that of reactive chemistry: ktAAtt)()(ln]A[]A[ln00 (2-9) where [A] t and [A] 0 are the concentrations of a species A at some time t, and the initial conditions at time t 0 and (A) t and (A) 0 are the transient absorbance at some time t, and the initial time t 0 This relationship can be made due to the linear relationship between sample concentration and absorbance. For more complicated kinetic systems, several wavelengths could be monitored independently, or commercially available software could be used for analysis. Sources of Noise A majority of the noise in the current system can be ascribed to the inherent stabilities of each of the components that make up the apparatus, which are described in further detail below. A smaller part of the noise is attributed to the sample itself, in which photodegradation can occur, causing potentially severe changes in the signal. This can generally be controlled by using an appropriate excitation energy and properly mixing the solution throughout the course of the experiment; however, some samples are more prone to photodegradation, and little can be done to prevent it. Repetitive sampling for each transient greatly affects the signal-to-noise ratio, and some examples are given below. Each component has inherent fluctuations which contribute to the noise of the system. For example, the stability of the laser power output will influence the excited-state population and 52

PAGE 53

affect the intensity of the transient absorbance signal. The probe beam stability, while not affecting the intensity, contributes to the shot-to-shot fluctuations that limit reproducibility and causes baseline error. The camera has excellent stability at full-well capacity; the wavelength range at which this happens is dependent on the output of the xenon lamp. In other words, based on the spectral distribution of the probe, the noise is much lower between 400 and 650 nm where the measured lamp output is strongest, and the noise is larger at the tail ends of the spectrum, especially around 800-850 nm where the measured output is weakest. A spectrum of the xenon probe source used, in terms of I and I 0 as measured by the detector, is shown in Figure 2-8. The discrepancy around 500 nm is due to a defect in the CCD chip (presumably from damage); however, this has minimal effect on the transient absorption signal. Wavelength / nm 400500600700800 Relative Intensity / a.u Figure 2-8. Relative probe intensities for I 0 () and I (----) with the pump beam off. 53

PAGE 54

Baseline Measurements Baseline measurements were made using the apparatus and the representative results are displayed in Figure 2-9. In this series of tests, the primary goal was to ascertain the overall noise of the system as a whole. Using a blank sample (i.e. an empty cuvette), spectra were collected under the same conditions while the number of averaged images were changed. For each set, a collection of ten transients was collected, each at t 0 using a gate width of 10 ns. The camera software was set to accumulate 10, 25, 100, or 500 images. As shown clearly in Figure 2-9A and B, the extremes of the spectrum are the noisiest, due to the low output of the probe at those wavelengths. Increasing the number of averaged images from 10 to 100 improves the limit of detection from greater than 15 mO.D. to roughly 5 mO.D. While increasing the number of averaged images from 100 to 500 results in slightly better reproducibility, the overall noise is not improved greatly (Figure 2-9C and D). In order to limit the amount of laser energy to which the sample is exposed and reduce the experiment time, as well as maintain a satisfactory baseline, averaging 100 images per transient delay time is the most suitable for data collection. Results A 1.2x10 -4 M solution of anthracene (Acros) was prepared in HPLC grade benzene (Sigma Aldrich). The solution was deoxygenated by bubbling with argon for 40 minutes. The camera was set to collect with a 10-ns gate width with a 3-s delay between transients. Prior to excitation, the spectrograph slits were adjusted (130 m) so that the peak probe intensity was near the maximum for the detector. The laser energy was set to ~10 mJ per pulse. The transient absorbance of the anthracene solutions under these conditions is shown in Figure 2-10. The max at 432 nm, as well as the intensity, matches well with previously published data. 53 The lifetime 54

PAGE 55

-15-10-5051015 Absorbance / 10-3 -15-10-5051015 -15-10-5051015 Wavelength / nm 400500600700800 -15-10-5051015 ABC D Figure 2-9. Baseline noise of the apparatus. Conditions: 10 ns gate width, 10 transients, camera delay and delay increment 0 ns, number of averages (A) 10, (B) 25, (C) 100 and (D) 500. 55

PAGE 56

Wavelength / nm 400450500550600650700 Absorbance -0.10.00.10.20.30. 4 Figure 2-10. Transient absorption spectrum of 1.2x10 -4 M anthracene solution in deoxygenated benzene. Camera delay 50 ns, delay increment 3 s, 100 images averaged. of the triplet was extracted and shown to be 9.1 s, which corresponds with the literature. 54 Lifetimes were calculated by using global analysis of the spectral-kinetics data using the SPECFIT/32 software package (Bio-logic SAS, Grenoble, France, www.bio-logic.info). A 1.4x10 -4 M aqueous solution of tris(2,2'-bipyridine)ruthenium(II) chloride (Ru(bpy) 3 2+ ) (Aldrich) was also prepared and deoxygenated by argon purging for 40 minutes. The camera delay was set to 10 ns, and a small bleaching peak at 710 nm can be seen from the second order of the spectrograph grating. A 250-ns delay between transients was used. The transient absorbance is shown in Figure 2-11. The signal is quite noisy between 420 nm and 470 nm, due 56

PAGE 57

Wavelength / nm 400500600700800 Absorbance -0.15-0.10-0.050.000.050.100. 15 Figure 2-11. Transient absorption spectrum of 1.4x10 -4 M aqueous solution of Ru(bpy) 3 2+ The solution was deoxygenated by bubbling with argon for 40 minutes. Camera delay 10 ns, delay increment 250 ns, 100 images averaged. The small peak at 710 nm is from scattered excitation light. to the large absorbance of Ru(bpy) 3 2+ in that region (therefore giving a very weak I 0 ). However, the spectrum and excited-state lifetime of 724 ns are in good agreement with the literature. 55,56 Limitations of Current System While the current system described provides the ability to study the excited state properties of many materials, there are some limitations. These restrictions arise from several sources, namely the spectral range of the components used, including the pump, probe, detector and optics used therein. Another source of limitation comes from the temporal nature of the 57

PAGE 58

electronics and hardware used in the apparatus. Lastly, the optical geometry is such that particular attention needs to be made while preparing samples to be studied. The excitation source used can limit the types of chromophores available for study. The primary excitation source is the third harmonic of a Nd:YAG laser at 355 nm. This output can also be used to pump an OPO that produces wavelengths from 420 nm to 1200 nm. Currently, excitation cannot be produced at wavelengths shorter than 355 nm and between 355 nm and 420 nm. Another excitation source, such as different laser or OPO system, could be incorporated into the apparatus to provide a larger range of excitation wavelengths. However, the types of materials studied in our lab generally provide a fairly broad absorption throughout the UV and blue region (excitation by 355 nm) or further into the red (excitation via the OPO). The detector range is limited by a combination of the probe output wavelength, spectrograph range and the chip used in the CCD camera. As shown earlier in Figure 2-4, the xenon flashlamp is not a true white light source. Other probe sources may offer a wider or more specific wavelength range suitable for different chromophores. A variety of CCD cameras and corresponding spectrographs are available from commercial manufacturers to expand detection wavelengths. Currently, the apparatus has been optimized for use with solution samples which can be contained in a 1-cm x 1-cm square cuvette. For most cases, sample concentrations must be adjusted such that intermolecular interactions, such as self-absorption or triplet-triplet annihilation, do not disrupt the signal. The sample concentrations are then adjusted such that the absorbance is optimal for excitation and detection (that is, A ~0.8 at the excitation wavelength). If highly absorbing or concentration-dependent samples are to be studied, various sample cells could be used, including small or large pathlength cuvettes. As shown in Figure 2-3 earlier, the 58

PAGE 59

sample is placed in a right-angle geometry with respect to the pump and the probe. Therefore, the studying of thin films poses a problem because the beams are not colinear. Films are also damaged easily under the excitation conditions used for solution samples. The kinetic resolution is limited almost entirely by the electronics of the system. The repetition rate of each measurement is limited by the occurrence of t 0 generated at 10 Hz by the Nd:YAG laser. The pulse generator used has a limit near 4 ns. Similarly, the camera can not record images with a gate width less than 5 ns, although the resolution is reported at 40 ps. Neither of these timings is limited by the 10 Hz repetition rate of the laser. Even so, events on a timescale of less than 100 ns are not able to be monitored with a high degree of accuracy. This does not pose a problem when studying triplet states, which are generally long-lived. There are several other concerns that affect ease of use and presentation of data. The first issue involves the nature of data collection, as described above. Because of the type of the information recorded (I 0 and I), and that those data must be processed after the experiment, there is no ability for real-time monitoring of the transient absorption signal. An experimental run is necessary to determine if the sample exhibits transient absorption and under what time conditions; parameters can not be adjusted on the fly. Therefore, if the user has little knowledge of the sample behavior, several runs may be necessary before the appropriate parameters can be determined. Secondly, for highly emissive species, fluorescence from the sample can cause bleaching in the transient absorption signal. The fluorescence will sometimes obscure the absorbance of the excited state. This problem can be remedied by collecting data with the pump on and the probe off, resulting in what is essentially an emission spectrum of the sample. If this is done at each time delay, the emission can be corrected simply by subtraction for each time 59

PAGE 60

measurement of the transient. A LabVIEW VI and Matlab application are under development to address this problem. 60

PAGE 61

CHAPTER 3 DELOCALIZATION IN EXCITED STATES OF DINUCLEAR PLATINUM ACETYLIDES Introduction The interesting properties of materials with extended -conjugated systems have made such materials potential candidates for use in a variety of applications, including nonlinear optics, 57 light emitting diodes, 6,58 and conductance switches. 59 The incorporation of transition metals into the organic structure changes the redox, electronic, and optical properties of the molecule. The organized combination of metals and organic moieties through deliberate synthesis allows for tunability of these properties. Photophysics of transition-metal acetylide complexes have garnered considerable interest throughout the last decade. 60-66 Platinum acetylides are square-planar molecules having the general formula trans-Pt(PR 3 ) 2 (CCR') 2 Typical R groups are methyl, ethyl, butyl, or phenyl, which give the compound crystalline properties, aid in solubility, or form glasses 41,67 or gels. 68 The R' group is normally an aromatic substituent, such as phenyl, thienyl, or fluorenyl. There has been a wide range of structures synthesized, including oligomers, 69 polymers, 70 and dendrimers. 71 The spin-orbit coupling that is a result of the heavy platinum atom allows for formation of triplet states in high yield. Hence, platinum-acetylides afford the ability to study spin-forbidden phenomena such as singlet-to-triplet-state absorption (S 0 T 1 ), intersystem crossing of the singlet excited state to the triplet excited state (S 1 T 1 ), and triplet-state emission (phosphorescence, T 1 S 0 ). Population of the triplet state also provides an opportunity to probe triplet-state absorption (T 1 T n ). A key point in the advancement of use of these types of materials is to make distinct relationships between chemical structure and spectroscopic properties. The fundamental nature of this type of structure-property relationship is also of importance to the general understanding 61

PAGE 62

of interaction of light with matter and the information that can be elicited from experiments. One such issue is the relation between structure and the delocalization of the singlet and triplet excitons. Beljonne and coworkers 72 have carried out an experimental and theoretical investigation on a platinum-containing acetylide polymer, the structure of which is shown in Figure 3-1. Using various spectroscopic techniques, such as absorption, emission, and excited-state absorption, along with quantum chemical calculations, the spatial extent of singlet and triplet excited states were determined. Transitions including S 0 S 1 S 0 T 1, and T 1 T n have Pt Cl Pt Cl PBu3 PBu3 PBu3 PBu3 Pt Cl Pt Cl PBu3 PBu3 PBu3 PBu3 n Pt Cl Pt Cl PBu3 PBu3 PBu3 PBu3 Pt Cl Pt Cl PBu3 PBu3 PBu3 PBu3 n Figure 3-1. Platinum-acetylide polymer and model compound studied by Beljonne et al. 72 shown chain-length dependencies on these excitation energies. The authors suggest that the lowest triplet excited state, T 1 is localized on one phenylene ring, while the S 1 excited state is confined to two repeat units. 72 However, the higher T n excited state is less restricted, and spread out over at least three repeat units. The use of model compounds (Figure 3-1) through Huang-Rhys fitting of phosphorescence emission spectra and theoretical modeling also indicates considerable distortion between the lowest triplet excited state and the singlet ground state. 72 Our group has more recently studied delocalization in singlet and triplet states of platinum-acetylide oligomers, 69 the structures of which are shown in Figure 3-2. It was found that the ground-state absorption and fluorescence have significant dependence on the length of the oligomer. As the chain length increases, the ground-state absorption and singlet emission red-shift systematically. However, phosphorescence maxima are less affected by chain length, as the 62

PAGE 63

Pt PBu3 PBu 3 H n Figure 3-2. Platinum-acetylide oligomers studied by Liu et al. 69 Oligomer length varies from n = 1-5, 7. peaks are only slightly red-shifted with respect to longer oligomer length. 69 The findings of this report further support the idea that the triplet state is more localized than the singlet state. Furthermore, another investigation by our group 73 involving the probing of delocalization was Figure 3-3. Platinum acetylide oligomers studied by Glusac-Haskins et al. 73 63

PAGE 64

completed on platinum-acetylide stilbene compounds, the structures of which are shown in Figure 3-3. Similar results in the absorption spectra indicate delocalization over the entire -conjugated system. That is, the absorption maxima are affected by the conjugation length of the molecule. Also, -conjugation is more efficient when the ligands are substituted trans to platinum, indicating delocalization through the d orbitals of the platinum atom. Moreover, the triplet-state properties of the ligand and platinum-containing molecules are notably similar, suggesting that there is no significant delocalization through the platinum atom in the triplet state and that the state is localized on the ligand. The photophysics of a short platinum-acetylide oligomer, PE2, was described in Chapter 1, and a more complete series studied by the Air Force Research Laboratory 40,42-45,74,75 was also introduced. In addition to the symmetrical molecules shown in Figure 1-8, the group also synthesized mono-substituted 45 and asymmetrically-substituted 75 platinum-acetylide complexes and all-organic butadiynes. 44 The complete series of compounds is shown in Figure 3-4. As bPt a PBu3 PBu3 PE1PE2PE3PE1-2PE2-3PE1-3half-PEnL1L2L3a = b = L1 a = b = L2a = b = L3a = L1, b = L2a = L2, b = L3a = L1, b = L3a = Cl, b = Ln PE1-BDPE2-BDPE3-BDm = 0 m = 1m = 2 mm Figure 3-4. Platinum-acetylide oligomers studied by Cooper, Rogers, et al. 40,42-45,74,75 64

PAGE 65

mentioned in Chapter 1, the symmetrically-substituted complexes show similar fluorescence spectra, but absorption and triplet emission spectra red-shift with increased conjugation length. 40 The mono-substituted complexes also exhibit greater dependence on oligomer length for ground-state absorption and triplet-state emission than for singlet-state emission. 45 Also, the phosphorescence spectra for the monoand di-substituted oligomers are nearly identical, 45 further indicating localization of the triplet state. Transient absorption spectra match for each oligomer pair as well. The photophysics of the asymmetrical platinum-acetylide complexes 75 reveal more support on the confinement of the triplet excited state and delocalization of the singlet excited state. It was observed that the singlet exciton is delocalized through the platinum atom, but phosphorescence comes from the lowest energy (largest) ligand. Most of the work to date has considered platinum acetylide oligomers with a single platinum atom and various ligands, or oligomers and polymers with identical repeat units. The study of delocalization in excited states is therefore limited. A set of dinuclear platinum-acetylides with various short spacers and end-capped substitution of either chloride or a conjugated phenyl-ethynyl-type substituent are presented in Figure 3-5. The photophysics of the series have been investigated and are presented in the following sections. The effects of the end group, platinum atom, and spacer are considered, and some conclusions are made regarding the delocalization of singlet and triplet states. Experimental Steady-state absorption measurements were recorded on a Varian Cary 100 dual-beam spectrophotometer. Corrected steady-state emission measurements were conducted on a either a FluoroMax-3 or SPEX Fluorolog-3 spectrophotometer. All sample solutions were prepared in tetrahydrofuran (THF) or 2-methyltetrahydrofuran (MTHF). Solvents were distilled over CaH 2 65

PAGE 66

immediately prior to use. Room-temperature measurements were carried out in 1-cm square quartz cuvettes. Solutions were deoxygenated by purging with argon for 30 minutes and sample P: R = ClP-PE2: R = PE2BP: R = ClBP-PE2: R = PE2PEP: R = ClPEP-PE2: R = PE2PE2: Pt Pt R R Pt R Pt R Pt Pt R R PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 P: R = ClP-PE2: R = PE2BP: R = ClBP-PE2: R = PE2PEP: R = ClPEP-PE2: R = PE2PE2: Pt Pt R R Pt R Pt R Pt Pt R R PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu3 PBu 3 Figure 3-5. Dinuclear platinum-acetylide oligomers. concentrations were made such that the solutions were optically dilute (A max < 0.20). Low-temperature emission measurements were conducted in 1-cm i.d. borosilicate glass tubes in a liquid nitrogen-cooled Oxford Instruments OptistatDN cryostat connected to an Omega CYC3200 autotuning temperature controller. Samples were degassed by three consecutive freeze-pump-thaw cycles on a high vacuum (10 -5 torr) line. Transient absorption measurements were conducted on the apparatus described in Chapter 2. Samples were contained in a cell with a total volume of 10 mL and the contents were continuously circulated through the pump-probe region of the cell. Solutions were deoxygenated by purging with argon for 40 minutes and concentrations adjusted so that A 355 ~ 0.8. Excitation was generated using the third harmonic output of a Nd:YAG laser (355 nm, Continuum Surelite). Typical excitation energies were 8 mJ66

PAGE 67

pulse -1 corresponding to a fluence of ~20 mJ-cm -2 Transient absorption spectra were generated by using software developed in-house following the calculations described in Chapter 2. Kinetic data were calculated by using global analysis of the spectral-kinetics data using the SPECFIT/32 software package (Bio-logic SAS, Grenoble, France, www.bio-logic.info). Results In Figure 3-6, the absorption maxima for all compounds in THF solution are shown to be within 15 nm of each other. BP has the most blue-shifted maximum at 346 nm, while P-PE2, BP-PE2 and PEP-PE2 have maxima at 358 nm. The absorption of PEP-PE2 is slightly broader than the others, suggesting there may be a secondary band red-shifted from the maximum. The absorption spectra for P and PEP are nearly identical with two closely-spaced bands near their absorption maxima at 351 nm. For the PE2-end-capped molecules, the molar absorptivity in benzene is an order of magnitude larger than the chloride-end-capped molecules, as given in Table 3-1. (No data for P was obtained, although presumably its molar absorptivity is considerably lower than that of P-PE2.) Table 3-1. Summary of spectroscopic data of dinuclear platinum-acetylide oligomers. max, abs a nm b M-1 cm-1 max, em c nm max, ex c nm max, em d nm max, ex d nm max, TA a nm T e s P 351 f 558 372 545 387 608 28.3 P-PE2 360 97 100 532 365 524 389 576 42.7 BP 346 12 900 556 351 541 366 569 27.1 BP-PE2 358 123 000 559 363 543 388 590 94.8 533 359 522 381 PEP 351 13 900 554 373 552 386 611 63.0 PEP-PE2 358 117 000 559 375 550 396 629 82.9 533 358 523 381 a THF solution. b Benzene solution. c THF solution, room temperature. d MTHF solution, 80 K. e Extracted from TA data. f Not measured. 67

PAGE 68

A B C D Absorbance E FWavelength / nm 300350400450 500 Figure 3-6. Absorbance spectra of dinuclear platinum-acetylide oligomers collected at room temperature in THF solution. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2. 68

PAGE 69

Fluorescence is observed for P at room temperature, although the maximum at 395 nm is weak compared to the phosphorescence. Some broad vibronic structure is seen, though three bands are shown and it does not quite mirror the absorption. Weak fluorescence is also shown for BP, and although broader than P, it exhibits similar structure at the same wavelengths. The photoluminescence of the remaining compounds is dominated by phosphorescence, even at room temperature. Transient absorption data are shown in Figure 3-7. Intense, broad signals throughout the visible are seen from the chloride-end-capped compounds with strong peaks at 608 nm, 596 nm, P P-PE2 Normalized Absorbance BP BP-PE2 Wavelength / nm 400500600700800 PEP P EP-PE2 Figure 3-7. Transient absorption spectra of dinuclear platinum-acetylides. Samples prepared in deoxygenated THF solution and spectra collected 20 ns after excitation by 355 nm pulse (10 ns, 8 mJ pulse -1 ). The horizontal dotted line represents A = 0. 69

PAGE 70

and 611 nm for P, BP and PEP, respectively. The PE2-end-capped molecules exhibit much broader TA signals throughout the visible. For the BP and PEP series, the PE2-substituted molecules have about a 20 nm red-shift from their analogs. P-PE2 is actually blue-shifted 32 nm from the P peak. The PE2 analogs red-shift following the order P-PE2 < BP-PE2 < PEP-PE2. Triplet-state lifetimes calculated from transient absorption decay are listed in Table 3-1. The relative error in measurement of lifetimes approaches almost 20%; however, it is clear that the lifetimes are considerably longer for the larger compound of each pair. Low-temperature and room-temperature phosphorescence spectra were collected and are shown in Figure 3-8. The room-temperature phosphorescence for P, P-PE2, BP, and PEP show a single peak with an unresolved vibronic progression. BP-PE2 and PEP-PE2 show similar structure, with the addition of a smaller peak blue-shifted from the main emission band. The smaller peaks of the two PE2-substituted compounds correspond in wavelength with the main peak of P-PE2 at 532 nm. The main peaks of the chloride-end-capped compounds all have phosphorescence maxima near 557 nm. At 80 K, the emission spectra of all of the compounds are blue-shifted and the vibronic progressions become slightly more resolved. The resolution increase is most notable for P and PEP, although more structure is observed for P-PE2 and PEP-PE2 as well. BP and BP-PE2 show very little fine structure at either temperature. Representative variable-temperature emission is shown by P-PE2 in Figure 3-9. As the temperature is increased from 80 K, the emission intensity decreases steadily. Upon the glass transition temperature of the solvent, the peak red-shifts 7 nm, and also increases intensity. After further heating of the sample from 120 K, the intensity decreases and vibronic structure is lost. Similar behavior is seen from the entire series. 70

PAGE 71

0.00.20.40.60.81.01.2 0.00.20.40.60.81.0 Wavelength / nm 300400500600700 0.00.20.40.60.81.0 0.00.20.40.60.81.0 Normalized Intensity / a.u. 0.00.20.40.60.81.0 0.00.20.40.60.81.0 ABCDE F Figure 3-8. Phosphorescence spectra of dinuclear platinum-acetylide oligomers. Room-termperature () spectra collected in THF solution and 80 K spectra (----) in MTHF glass. Excitation wavelength set at absorption maximum for each compound. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2. 71

PAGE 72

Wavelength / nm 500550600650 Intensity / a.u. 80K 90K 100K 110K 120K 130K 140K 160K 180K 200K 220K 240K 260K Figure 3-9. Variable-temperature emission spectra of P-PE2 in MTHF. Excitation wavelength = 360 nm. Excitation spectra for the set of compounds are shown in Figure 3-10. Room-temperature excitation spectra collected at the maxima of emission are displayed as featureless bands shifted a few nanometers lower in energy than the absorbance maxima. However, P and PEP exhibit a blue-shifted shoulder that corresponds with the absorption spectra. Excitation spectra collected at the smaller higher-energy peak in BP-PE2 and PEP-PE2 have blue-shifted maxima which match the absorbance maxima. At 80 K, the excitation spectra are more structured, and significantly (~20 nm) red-shifted from the room-temperature data. For the chloride end-capped series, the maxima are shifted approximately 15 nm to the red compared to 72

PAGE 73

0.00.20.40.60.81.01.2 551 558 0.00.20.40.60.81.0 524 532 Wavelength / nm 300320340360380400420440 0.00.20.40.60.81.0 550 523 560 533 0.00.20.40.60.81.0 550 554 Normalized Intensity / a.u. 0.00.20.40.60.81.0 536 556 0.00.20.40.60.81.0 544 522 559 532 ABCDE F Figure 3-10. Excitation spectra of dinuclear platinum-acetylide oligomers. Room-temperature spectra () collected in deoxygenated THF and low-temperature spectra () collected at 80 K in MTHF glass at emission wavelengths denoted in the figure. The dotted lines correspond to the higher-energy bands in the PE2-end-capped oligomers. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2. 73

PAGE 74

room-temperature excitation, whereas the PE2 end-capped molecules show a larger shift of about 24 nm. The excitation spectra of the higher-energy peak for BP-PE2 and PEP-PE2 have maxima blue-shifted from the excitation peak measured for the main emission band. Representative variable-temperature excitation spectra for P-PE2 are shown in Figure 3-11. At 80 K, the maximum peak is at 389 nm. As the temperature increases, the peak decreases and blue-shifts. As seen in the emission spectra, the vibronic structure is lost as the solvent glass is warmed. The excitation maximum shifts a total of 24 nm. A comparable trend throughout the series is observed as the temperature is increased from 80 K to room temperature. Wavelength / nm 300320340360380400420 Intensity / a.u. 80K 90K 100K 110K 120K 130K 140K 160K 180K 200K 220K 240K 260K Figure 3-11. Variable-temperature excitation spectra of P-PE2 in MTHF. Emission monitored at 532 nm. 74

PAGE 75

Discussion The properties of -conjugated materials attract interest due to the potential applications of these types of compounds. By introducing platinum atoms into the -conjugated structure, the triplet-state properties are more easily investigated due to population through spin-orbit coupling of the heavy atom. The results of previous work have uncovered more questions as to the nature of the singlet and triplet excited states and the role of platinum in the chromophore. The photophysical characterization of a series of dinuclear platinum-acetylide oligomers that consist of different spacer units and a well-studied end-cap will further the understanding of this type of molecule. The ground-state absorption of platinum-acetylide complexes originates from a transition with some metal-to-ligand charge transfer (MLCT) character. 44 Theoretical calculations of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have contributed to the understanding of electronic transitions in this type of compound. 63,74 A combination of the platinum d orbital and the aryl orbitals contributes to the HOMO, while the LUMO consists of only orbitals and no contribution from the platinum d orbital. Without quantum chemical methods to study the geometry and orbital configurations of this series, limited conclusions can be made. The central phenyl rings in BP and BP-PE2 are most likely twisted, which disrupts the conjugation. Cooper and coworkers 43 have calculated the dihedral angle in the related PPE to be 40. Therefore, it is not possible to determine trends based purely on conjugation length as the exact conjugation length is not known. However, direct comparison between the chloride and PE2 end-capped molecules in each pair is more legitimate. The absorption maxima of the PE2 end-capped molecules are red-shifted 7-12 nm from the chloride analogs. The asymmetric PE2-3, which has a similar 75

PAGE 76

absorption maximum to the PE2 end-capped oligomers, has a longer conjugation length if comparing only the ligand-metal-spacer as the chromophore. The increase in conjugation length between P and PEP predicts a greater red-shift than what is observed. However, the second peak at 375 nm observed in the absorbance of PEP occurs near the absorbance maximum of PE3. Interestingly, the main absorption band occurs at same wavelength as the asymmetrical PE2-3. This suggests that there maybe conformational twisting near the platinum that interrupts the conjugation to a ligand-metal-spacer chromophore, but also there exists delocalization through the platinum. The PE2 end-cap contributes heavily to the oscillator strength, increasing the value of the chloride end-capped molecules by an order of magnitude. The values are higher than the reported value 75 for the related PE2 ( = 89 000 M -1 cm -1 ). The increased molar absorptivity and absorption maxima suggest that there is delocalization through both platinum atoms in the singlet state. Due to the small size of the spacers, it is inconclusive at this point as to the origin of absorbance and relationship of the platinum in the HOMO and LUMO. The trends observed for the transient absorption spectra are different for the chlorideand PE2-end-capped series. P and PEP have similar TA spectra, and BP has a peak 40 nm higher in energy. However, the maxima of the broad TA spectra of the PE2-end-capped oligomers increase in the order P-PE2 < BP-PE2 < PEP-PE2. The trend in the latter can be attributed to the delocalization through the platinum atoms into the conjugated end-caps. It has been shown for the related platinum-acetylide oligomers that the T n excited state is delocalized through the metal d orbitals. Additionally, as stated earlier, theoretical calculations have shown that the triplet exciton in platinum-acetylide polymers is delocalized onto at least three repeat units. 72 In contrast to their study on films, the experimental triplet absorption maximum of P is 608 nm for the solution sample, nearly 20 nm red-shifted from their film sample. The PE2 end-capped 76

PAGE 77

oligomers also show much broader TA signals. The T n state is likely delocalized throughout the molecule, as suggested by previous reports of similar molecules. 45 The lack of fluorescence in the emission spectra of the series, even at room temperature, indicates that intersystem crossing via spin-orbit coupling dominates decay of the singlet excited state. In other platinum-acetylide oligomers, the reported ISC quantum yields approach unity. 35,36 Only P shows reasonable fluorescence emission at room temperature; the maximum value occurs at a lower energy (395 nm) than related oligomers that contain only a single phenyl group per platinum atom. PE1, which has a single phenyl-ethynyl group on each side of the platinum atom has a fluorescence maximum at 364 nm, whereas maximum emission near 395 comes from PE2 (385 nm) or PE3 (400 nm). 44 It has been shown that the excited singlet states have delocalized character. It would seem reasonable then that there is a strong contribution from the metal orbitals in the singlet state, extending the conjugation from the phenyl orbitals through the platinum d orbitals. The red-shift in the expected fluorescence is attributed to delocalization of the singlet excited state into the d orbitals of both platinum atoms. As stated earlier, definitive trends on conjugation length cannot be determined by structure alone. Low-temperature emission spectra for all compounds, excluding P-PE2, have phosphorescence maxima within 5 nm of 546 nm. P-PE2 phosphorescence is at higher energy of 524 nm. Similarly, the other PE2 end-capped oligomers, BP-PE2 and PEP-PE2, exhibit a higher energy peak at the same wavelength. In the asymmetrical series of platinum complexes, 75 phosphorescence was observed by emission from the lowest energy (largest) ligand. Only in PE2-3 was very weak emission from the PE2 ligand observed at 77 K. In the dinuclear platinum series, this higher energy peak in the dinuclear platinum compounds is ascribed to triplet localization on the PE2 ligand. In P-PE2, the entire triplet is localized on the PE2 end-cap, and 77

PAGE 78

no emission is observed from the spacer group. However, in BP-PE2 and PEP-PE2, the triplet can be localized on the end-cap or the central spacer. An explanation for this resides in intramolecular triplet energy transfer between the end-cap chromophore and the spacer chromophore through Dexter coupling. 75 While emission is expected to come from only the lowest energy ligand according to Kashas rule, our group has previously reported a contrasting study 76 on a platinum acetylide polymer containing both phenyl and thienyl monomer units. Emission from the polymer is observed from thiophene units as well as the higher-energy phenyl groups. At low temperatures, the vibronic structure of the emission is more defined, showing several peaks in the progression. P and PEP have the clearest progression of small peaks, while the BP pair is fairly broad with two large bands. The lower resolution of vibronic structure in P-PE2 and PEP-PE2 can be attributed to a greater amount of vibrational modes stemming from the large end-cap, which results in overlap of the emission bands. Also, at low temperatures the vibrational modes are more restricted by the solvent glass, which helps resolve the progression. At room temperature, more vibrational modes are accessed, leading to broadening of the bands. This effect is also responsible for the red-shift of the phosphorescence spectra as the sample goes from glass to solution. Similarly, the excitation spectra are much more resolved at low temperature. P and P-PEP show the greatest amount of structure, while the rest of the series show very broad shoulders at higher energy from the peaks. The red-shift of the spectra is also attributed to the restricted access of vibrational modes when the sample is frozen in the glass. However, the shift in phosphorescence energy is much smaller than the change in excitation energy. The dramatic 78

PAGE 79

red-shift and strong broadening of the excitation spectra further support the delocalization of the singlet state, and the localization of the triplet state. Conclusion In this chapter, a series of dinuclear platinum-acetylide oligomers has been characterized by using photophysical methods. The molecules in the series consist of small aryl spacer groups spanning two platinum atoms. The oligomer chain is end-capped with either chloride or a phenyl-ethynyl -conjugated group. It has been shown that the ground-state and triplet-state absorbance are affected by the addition of the PE2 end-cap, whereas the triplet emission energies are less dependent on end-cap substitution. In addition, phosphorescence is observed from the spacer group as well as the higher energy end-cap. The results lead to conclusion that the singlet S 0 ground state, singlet S 1 excited state and triplet T n excited state are delocalized throughout the molecule. The T 1 states, however, are localized on the ligand and spacer groups. Phosphorescence originates from both the spacer group and the PE2 end-cap, indicating that there is no significant delocalization through the platinum atom in the triplet state. 79

PAGE 80

CHAPTER 4 PHOTOPHYSICS OF DIPLATINUM POLYYNEDIYL OLIOGMERS: CHAIN LENGTH DEPENDENCE ON THE TRIPLET STATE IN SP CARBON CHAINS Introduction Over the past few decades there has been increasing interest in the study of organic and organometallic oligomers that feature extended carbon chains, e.g., R-(CC) n -R and L y M-(CC) n -ML y 25,62,77-88 The extended -systems and rigid linear structure characteristic of these carbon chain oligomers make them potentially useful as molecular wires for transport of charge (polarons) or excitons on the nanoscale. 89 On a fundamental level, oligoynes are unique in that they provide one of the simplest possible linear -conjugated organic units of variable length. Such systems are particularly amenable for experimental and theoretical studies focused on issues related to charge and exciton structure and delocalization in linear -conjugated organic systems. 90-94 While there have been a number of experimental and theoretical reports concerning the properties of linear carbon chains and related structures, most of this work focused on vibrational and optical absorption spectroscopy. 81,87,92-94 Although several studies have reported fluorescence emission from carbon chain structures, there are only a few reports concerning the phosphorescence of oligoynes. 93,94 In an effort to increase the stability of linear carbon chain compounds, as well as extend the study of their photophysics to the triplet state, transition metals have been incorporated as end-caps to the oligoyne chains. 62,88 Notable examples of this work include a spectroscopic study of 2and 4-carbon chains spanning two gold atoms reported by Che and co-workers, 95 and a spectroscopic study of a series of platinum(II)-terpyridyl-capped carbon chains reported by Yam and co-workers. 96 Both of these studies provide evidence that the heavy metal end-groups induce efficient population of a 3 ,* state that is localized on the 80

PAGE 81

carbon chain. The triplet is clearly distinguished by its characteristic phosphorescence emission, which appears as a well-defined vibronic progression with spacing of ca. 2100 cm -1 One of our collaborators, the Gladysz group at Friedrich-Alexander-Universitt Erlangen-Nrnberg, in Erlangen, Germany, has developed synthetic methods that allow routine construction of variable length carbon chains capped on both ends by transition metal units. In particular, they have recently reported the synthesis and structural characterization of a broad series of linear carbon chain oligomers of the type Ar-Pt(P 2 )-(CC) n -Pt(P 2 )-Ar, where Ar = aryl, P = a phosphine ligand and n = 2 14. 85,86 The structural, optical and electronic properties of these oligomers have been probed by x-ray crystallography, UV-visible absorption and density functional theory (DFT) calculations. 86,90 While the optical and theoretical investigations provide considerable insight concerning electronic structure and -conjugation in the carbon chains, little work has been done previously to probe the structure and dynamics of the long-lived excited states (e.g., singlet and/or triplet excitons) in the oligoyne systems. Pt Pt Pt Pt Pt Pt Pt Pt PtC6PtPtC8PtPtC10PtPtC12PtP p tol3 Pptol3 P p tol3 Pptol3 Pptol3 Pptol3 Pptol3 Pptol3 Pptol3 Pptol3 Pptol3 Pptol3 Pptol 3 Pptol3 Pptol 3 Pptol3 Figure 4-1. Structures of the PtC n Pt series. 81

PAGE 82

In this chapter a detailed study of the photophysics of the series of platinum end-capped oligoynes, PtC n Pt (n = 6 12, Figure 4-1) is presented. This study has focused on the properties of the 3 ,* state localized on the carbon chains that span the two platinum centers. The triplet state is produced in relatively high yield in these oligoynes due to the strong spin-orbit coupling induced by the platinum centers. The spectroscopy and dynamics of the triplet state are probed using variable temperature luminescence, transient absorption, and time-resolved emission spectroscopy. The results provide clear evidence for an energy gap law dependence of the non-radiative decay rate of the triplet state. Experimental Steady-state absorption measurements were recorded on a Varian Cary 100 dual-beam spectrophotometer. Corrected steady-state emission measurements were conducted on a SPEX Fluorolog-3 spectrophotometer. All sample solutions were prepared in tetrahydrofuran (THF) or 2-methyltetrahydrofuran (MTHF). Solvents were distilled over CaH 2 immediately prior to use. Room-temperature measurements were carried out in 1-cm square quartz cuvettes. Solutions were deoxygenated by purging with argon for 30 minutes and sample concentrations were made such that the solutions were optically dilute (A max < 0.20). Low-temperature emission measurements were conducted in 1-cm i.d. borosilicate glass tubes in a liquid nitrogen-cooled Oxford Instruments OptistatDN cryostat connected to an Omega CYC3200 autotuning temperature controller. Samples were degassed by three consecutive freeze-pump-thaw cycles on a high vacuum (10 -5 torr) line. Photoluminescence quantum yields were determined by relative actinometry, with Ru(bpy) 3 2+ as an actinometer ( P = 0.055 in water). Low temperature quantum yields were calculated by appropriately scaling the integrated emission area with temperature. 97 82

PAGE 83

The phosphorescence spectra for PtC 6 Pt, PtC 8 Pt, and PtC 10 Pt at 100 K were fitted using a single-mode Franck-Condon expression, 97,98 50221,0mm00mm300mm00mm2ln4exp!)(ESEEI (4-1) Where I( ) is the relative emission intensity at energy E 00 is the energy of the zero-zero transition, m is the quantum number of the average medium-frequency vibrational mode, m is the average medium-frequency acceptor mode coupled to the triplet-excited state to ground state transition, S m is the Huang-Rhys factor, and 0,1/2 is the half-width of the individual vibronic bands. Time-resolved emission measurements were carried out on a home-built apparatus consisting of a Continuum Surelite series Nd:YAG laser as the excitation source ( = 355 nm, 10 ns fwhm, < 1mJ/pulse) and detection measured with a Princeton Instruments PI-MAX intensified CCD camera detector coupled to an Acton SpectraPro 150 spectrograph. Lifetimes were calculated by using global analysis of the spectral-kinetics data using the SPECFIT/32 software package (Bio-logic SAS, Grenoble, France, www.bio-logic.info). Transient absorption measurements were conducted on a previously described home-built apparatus. 99 Samples were contained in a cell with a total volume 10 mL and the contents were continuously circulated through the pump-probe region of the cell. Solutions were deoxygenated by argon purging for 40 minutes and concentrations adjusted so that A 355 ~0.8. Excitation was generated using the third harmonic output of a Nd:YAG laser (355 nm, Spectra Physics GCR-14). Typical excitation energies were 5 mJ-pulse -1 corresponding to a fluence of ~20 mJ-cm -2 Transient absorption spectra and decay lifetimes were generated by using software developed in-house. 83

PAGE 84

Results UV-Vis Absorption Spectra The absorption spectra for the series of PtC n Pt oligomers in CH 2 Cl 2 have recently been reported. 86 However, to facilitate comparison with the photophysical data presented herein, the absorption spectra of the series in were measured THF and the results are presented in Figure 4-2. The spectra in THF solution are nearly identical to those reported in CH 2 Cl 2 ; 86 however, the molar absorptivity values are slightly larger in THF. With the exception of PtC 6 Pt, the absorption spectra of the oligoynes feature two primary transitions which each appear as a well-defined vibronic progression that shift to lower energy with increasing carbon chain length. The low energy transition (inset of Figure 4-2) is comparatively weak ( < 5,000 M -1 cm -1 ), whereas the higher energy transition is considerably more intense, with the molar absorptivity increasing with carbon chain length. On the basis of DFT computations, both of these bands are assigned as arising from transitions between orbitals that are concentrated on the -(CC) n chains. 90 As noted above, the absorption of PtC 6 Pt does not follow the same pattern seen for the longer oligomers. The absorption of PtC 6 Pt is considerably weaker compared to the other complexes, and it features a broad absorption in the 320 360 nm region, with a second weaker band with max = 381 nm. While the previously reported DFT calculations suggest that there should not be a difference in the nature of the low energy transitions for PtC 6 Pt compared to the other complexes, it is possible that the absorption of this oligomer is more complex due to mixing of the and levels of the C 6 chain with orbitals on the -Pt(P 2 )(Ar) end groups. 84

PAGE 85

Wavelength / nm 300350400450500 / 105 M-1 cm-1 012345 350400450500550 x 50 Figure 4-2. Absorption spectra of PtC n Pt complexes in THF solution. In order of increasing molar absorptivity: n = 6, 8, 10 and 12. Steady-State Photoluminescence Spectroscopy Photoluminescence spectra for all of the PtC n Pt complexes were studied in MTHF solution at temperatures ranging from 80 300 K over the wavelength interval 400 800 nm. The low energy limit in the spectra is determined by the fall-off in the photomultiplier detector response. As shown in Figure 4-3, at 80 K in the MTHF glass all of the oligoynes display a well-defined photoluminescence that can be assigned to phosphorescence from a 3 ,* excited state. Note that little or no fluorescence emission is observed in the spectra. (The spectra shown in Figure 4-3 were obtained with excitation at the absorption maximum for each complex; however, 85

PAGE 86

400500600700800 0.00.20.40.60.81.0 0.00.20.40.60.81.0 Normalized Intensity 0.00.20.40.60.81.0 Wavelength / nm 400500600700800 0.00.20.40.60.81.0 ABCD0-00-10-20-30-00-00-00-10-102 Figure 4-3. Photoluminescence spectra of PtC n Pt complexes in 2-MTHF solvent glass at 80 K. (A) n = 6, ex = 335 nm. (B) n = 8, ex = 335 nm. (C) n = 10, ex = 350 nm. (D) n = 12, ex = 368 nm. 86

PAGE 87

the observed emission spectra did not vary with excitation wavelength.) For the C 6 C 8 and C 10 oligomers, the phosphorescence appears as a narrow 0-0 band followed by a series of vibronic sub-bands at lower energy separated by ca. 2100 cm -1 The 0-0 bands of the phosphorescence red-shift systematically with increasing carbon chain length, and the max value for the phosphorescence origin for each complex is listed in Table 4-1. The vibronic progression in each of the spectra arises due to coupling of the triplet excitation to the CC stretch of the carbon chain. For PtC 6 Pt the 0-1, 0-2 and 0-3 sub-bands are resolved, for PtC 8 Pt the 0-1 and 0-2 sub-bands are seen, and for PtC 10 Pt only the 0-1 sub-band appears in the accessible spectral range. The phosphorescence of PtC 12 Pt is weaker than for the other oligomers, and it is so red-shifted that it is only possible to observe the origin of the phosphorescence. With increasing temperature, the phosphorescence spectra do not change in energy or bandshape; however, the emission intensity steadily decreases. An example of this is shown in Figure 4-4. The emission from all of the complexes is much weaker at ambient temperature (300 K), and for PtC 6 Pt and PtC 12 Pt the phosphorescence is too weak to detect at 300 K. The room-temperature phosphorescence quantum yields were measured for PtC 8 Pt and PtC 10 Pt (0.003 and 0.0012, respectively). Using these values, the phosphorescence quantum yields at 100 K were estimated by scaling the room-temperature quantum yields by the integrated emission area (which increases with decreasing temperature). These extrapolated low temperature quantum yields are 0.018 and 0.0053 for PtC 8 Pt and PtC 10 Pt, respectively. Because there was no phosphorescence at ambient temperature for C6 and C12, the low temperature emission quantum yields could not be determined. 87

PAGE 88

Wavelength / nm 500550600650700 Intensity / a.u. 0.00.20.40.60.81.01.2 77 K 100 K 120 K 140 K 165 K 180 K 200 K 230 K 265 K 298 K Figure 4-4. Emission spectra of PtC 8 Pt in MTHF solvent (glass) over 77 298 K temperature range. Excitation wavelength 335 nm. Spectra decrease in intensity (quantum efficiency) with increasing temperature. Excitation spectra obtained for each of the complexes while monitoring the phosphorescence at 80 K are shown in Figure 4-5. In general, the excitation spectra are closely similar to the absorption spectra, except the bands are better resolved due to the low temperature glass. In order to provide more information concerning the nature of the triplet state that gives rise to the phosphorescence, the low temperature (100 K) emission spectra of PtC 6 Pt, PtC 8 Pt and PtC 10 Pt were fitted by using a Franck-Condon bandshape analysis according to Equation 4-1. 97,98 The spectrum of PtC 12 Pt could not be fitted since only the origin is observed. The fits 88

PAGE 89

Wavelength / nm 300400500600700800 0.00.20.40.60.81.0 0.00.20.40.60.81.0 0.00.20.40.60.81.0 300400500600700800 0.00.20.40.60.81.01.2 Normalized IntensityABCD Figure 4-5. Overlay of absorption (black lines), emission excitation (green lines) and emission spectra (red lines) for PtC n Pt complexes in MTHF solvent glass at 100 K. Excitation spectra monitored at peak of 0-0 emission line. (A) n = 6, (B) n = 8, (C) n = 10, (D) n = 12. 89

PAGE 90

provide estimates for the 0-0 energy (E 00 the triplet energy), the frequency of the dominant vibrational mode coupled to the excitation (), the bandwidth ( 1/2 ), and the Huang-Rhys parameter (S m ) which is a quantitative measure of the geometric distortion between the ground and triplet excited states. A listing of the parameters recovered from the fits is shown in Table 4-1, and an example of the excellent quality of the fit of the experimental spectra is shown in Figure 4-6. Table 4-1. Emission spectral fitting parameters for PtC n Pt at 100 K. max, em E 00 / cm -1 / cm -1 0,1/2 / cm -1 S m E ST / eV C6 497 20121 2150 190 1.05 0.53 C8 580 17241 2120 200 0.90 0.71 C10 657 15221 2060 250 0.78 0.75 C12 727 13755 2020 a 260 a 0.71 b 0.68 a Estimated values. b Estimated from linear plot of S m vs E 00 Several features are of interest with respect to the parameters recovered from the fits. First, as noted above, E 00 decreases systematically with increasing carbon chain length, reflecting the decrease in triplet energy with increasing length of the -conjugated electron system. Second, as noted above, it is evident that the triplet excitation is coupled to only one high frequency vibration that corresponds to the stretching mode. Interestingly, the frequency of the mode decreases slightly with increasing oligomer length, consistent with previous theoretical studies which indicate that the oligoyne stretching mode decreases in frequency with length. 92 Third, it is quite evident that the Huang-Rhys parameter decreases with increasing oligomer length (vide infra). This is consistent with the triplet exciton becoming more delocalized as the number of carbons in the sp chain increases. 90

PAGE 91

Energy / cm-1 1400016000180002000022000 Normalized Intensity 0.00.20.40.60.81.01.2 Figure 4-6. Fit of emission spectrum of PtC 6 Pt using Equation 4-1 and parameters listed in Table 4-1. Dots are experimental data and red solid line is calculated fit. Phosphorescence Decay Kinetics: Radiative and Non-Radiative Decay Rates Variable-temperature time-resolved emission spectra were measured for the PtC n Pt series in degassed MTHF solution (glass) over the 80 300 K temperature range and the decay lifetimes recovered from fits of the emission decays are plotted vs. temperature in Figure 4-7. Several features are of interest with respect to the temperature-dependent lifetimes. First, for all of the complexes the emission lifetimes decrease with increasing temperature. This lifetime decrease is especially large in the temperature region corresponding to the glass-to-fluid transition of MTHF (120 140 K). Specifically, for PtC 6 Pt, the lifetime decreases by nearly a factor of 20 between 110 and 150 K. For the other complexes, the decrease in lifetime is less but 91

PAGE 92

Temperature / K 50100150200250300 Lifetime / s 051015202530 C6C10C12 C8 Figure 4-7. Temperature dependence of photoluminescence decay lifetimes for PtC n Pt complexes in MTHF solution (glass). Carbon chain length (n) indicated in plot. nevertheless it is evident. Second, below the glass point of the solvent (e.g., T = 100 K) the emission lifetime increases with decreasing carbon chain length. For example, at 100 K, the emission lifetime increases by approximately a factor of 3 between PtC 12 Pt, PtC 10 Pt and PtC 8 Pt and then it increases by a further factor of 5 for PtC 6 Pt. As discussed in more detail below, the overall trend of increasing triplet lifetime with chain length arises due to the energy gap law. By using the emission quantum yield and lifetime data at low temperature it is possible to estimate the radiative and non-radiative decay rate constants (k r and k nr respectively). The triplet 92

PAGE 93

lifetime ( T ), phosphorescence quantum yield ( P ), and intersystem crossing efficiency ( ISC ) are related to k r and k nr by the following expressions: nrrT1kk (4-2) TrISCPk (4-3) Equations 4-2 and 4-3 can be rearranged to expressions that allow k nr and k r to be computed directly from the experimental parameters: TISCPnr11k (4-4) TISCPr1k (4-5) In order to apply these expressions to compute k r and k nr values of T P and ISC are needed, but we note that P is only known with certainty for two of the complexes, and ISC is not known. However, given the lack of fluorescence from any of the complexes and the observation of relatively strong triplet-triplet absorption, it is likely that ISC is significantly larger than 0.1, and for the shorter oligomers it is likely close to unity. Thus, it is safe to conclude that P << ISC and Equation 4-4 reduces to Equation 4-6. We further rearrange Equation 4-5 to the usable form Equation 4-7 which emphasizes that there is some uncertainty in the computed radiative rate constants. Tnr1k (4-6) TPrISCk (4-7) 93

PAGE 94

The values of k nr and ISC k r computed from the experimental data at 100 K are listed in Table 4-2. Several features are of note with respect to this data. First, for PtC 8 Pt and PtC 10 Pt, k nr exceeds ISC k r by nearly 100-fold, which indicates that the oligoynes decay predominantly via non-radiative pathway(s), even in the low temperature glass. Second, it is evident that there is a systematic increase in k nr with increasing carbon chain length. As noted above, this trend is a manifestation of the energy gap law. Finally, while there is limited data, it is also interesting that ISC k r decreases from PtC 8 Pt and PtC 10 Pt; this effect probably arises because spin-orbit coupling induced by the Pt centers decreases with increasing chain length. Table 4-2. Photophysical parameters for PtC n Pt complexes. T / 10 -6 s P P a ISC k r / 10 3 s -1 b k nr / 10 5 s -1 b / s c C6 26.85 d 0.372 e 1.20 C8 5.24 0.003 0.018 3.44 1.87 1.41 C10 2.92 0.0012 0.0053 1.82 3.41 1.20 C12 0.54 d 18.5 e 1.47 a Adjusted for 100 K. b Measured at 100 K. c Extracted from room temperature transient absorption. d Not observed. e k nr = 1/, est. Transient Absorption Spectroscopy: Triplet-Triplet Absorption of the Oligoynes In order to provide additional data concerning the properties of the triplet state of the oligoynes, nanosecond transient absorption spectra were measured at room temperature in deoxygenated THF solution. As shown in Figure 4-8, near-UV excitation of all of the complexes produces strongly absorbing transients which decay on a timescale of a few s (computed decay lifetimes are listed in Table 4-2). The transient which gives rise to the absorption is the triplet excited state. In each case the transient difference-absorption spectra are characterized by bleaching of the near-UV ground state absorption bands, combined with moderately intense 94

PAGE 95

300400500600700800 -0.04-0.020.000.02 0.000.050.100.150.20 Absorbance -0.40-0.200.000.20 Wavelength / nm 300400500600700800 -0.200.000.20 ABCD Figure 4-8. Transient absorption spectra of PtC n Pt complexes following pulsed excitation with a 355 nm pulse (10 ns, 5 mJ-pulse -1 ). (A) n = 6, (B) n = 8, (C) n = 10, and (D) n = 12. For (A) (C) the first spectrum was obtained 20 ns following excitation, and subsequent spectra at 400-ns delay increments. For (D) the first spectrum was obtained at 20 ns following excitation, and subsequent spectra at 80-ns delay increments. 95

PAGE 96

triplet-triplet absorption in the visible region. Interestingly, for each complex, the triplet-triplet absorption appears to arise in two distinct bands; one stronger band which is in the near-UV and blue of the visible, and a second weaker band that is centered in the mid-visible. For PtC 12 Pt the visible band is apparently split into vibronic bands. While assignment of these transitions will require further (theoretical) study, it is evident from the transient absorption study that intersystem crossing is efficient in all of the oligoynes. Furthermore, it is clear that the triplet-triplet absorption of the carbon chains is strongly allowed. Discussion Variation of Triplet Energy with Carbon Chain Length A key result of this investigation is that it provides the 0-0 triplet energy as a function of oligoyne chain length for the PtC n Pt complexes. In order to analyze the chain-dependent triplet energy data, the correlation in Figure 4-9 has been constructed, which shows a plot of E 00 vs. 1/n, where n is the number of carbon atoms in the oligoyne chain. Interestingly, this plot features an excellent linear correlation and it provides an intercept of 7670 cm -1 (0.95 eV) which corresponds to the triplet energy in the of the (CC) n polymer. It is noteworthy that there is substantial variation of the triplet energy across the PtC n Pt series, a fact which implies that the triplet state is delocalized across the entire -conjugated system defined by the carbon chain for all members of the PtC n Pt series. There is no sign of leveling off of the triplet energy with increasing chain length, suggesting that for chains longer than twelve carbons the triplet energy will continue to decrease (i.e., the effective conjugation length for the triplet has not been reached in this series). In a recent study, Nagano and co-workers reported the evolution of the absorption, fluorescence and phosphorescence of the series of diphenyl-end capped oligoynes Ph(CC) n Ph 96

PAGE 97

(n = 1 6). 94,100 Using their reported phosphorescence 0-0 band energies, we have included their data in Figure 4-9 for comparison with that of the PtC n Pt series. Interestingly, the correlation of E 00 vs. 1/n defined by the Ph(CC) n Ph series is curved. For the shorter oligoynes, the trend defined by the Ph(CC) n Ph series deviates to lower energy relative to the correlation for the PtC n Pt series; however, for longer carbon chain length (n = 4 and 6) the data for Ph(CC) n Ph series nearly converges to the correlation defined by the PtC n Pt series. The curvature in the 1 / n 0.000.050.100.150.200.250.30 E00 / cm-1 50001000015000200002500030000 Figure 4-9. Correlation of E 00 (triplet energy) vs. 1/n for -conjugated oligomers, where n = # carbons in the chain. () : PtC n Pt complexes. (x) : Ph-(CC) x -Ph oligomers ((from Nagano et al. 94 ). () : -thiophene oligomers (from de Melo et al. 101 ). The dashed line is best fit to the PtC n Pt data with R 2 = 0.997. 97

PAGE 98

correlation for the Ph(CC) n Ph series likely results from the fact that for short oligoyne chain length, the triplet is delocalized significantly into the phenylene end-groups. This hypothesis is supported by the observation of significant vibronic coupling to phenyl-based modes in the phosphorescence spectra of PhCCPh and Ph(CC) 2 Ph. 94 At longer chain length, the effect of the phenylene end groups on the triplet energy in the Ph(CC) n Ph series becomes less pronounced, and consequently the two correlations converge. This comparison suggests that correlation of E 00 vs. 1/n for the PtC n Pt series more accurately reflects the trend in triplet state energy for the pure (CC) n carbon chain (i.e., without interference from the end-groups). Importantly, this comparison also shows that the triplet energy for the (CC) n polymer which is estimated by extrapolation of the correlation for the PtC n Pt series is likely a much better estimate than would be obtained from the Ph-(CC) n -Ph series. A number of recent studies have explored the variation of the singlet and triplet energies with oligomer length for -conjugated oligomers such as oligothiophene, 101-104 oligo(phenylene vinylene) 105,106 and oligo(fluorene). 107 Of particular relevance to the present work are studies of thiophene and substituted thiophene oligomers, where very clear correlations of excited state energy with oligomer length emerge. 101-103,107 Figure 4-9 also includes E 00 values for the triplet state in the series of oligothiophene oligomers (for n = 2 7) reported by de Melo and co-workers. 101 In the figure the oligothiophene triplet energies are plotted vs. the 1/n, where n is the number of carbon atoms in the -conjugated chain (there are 4 carbons per thiophene repeat unit). Interestingly, it is clear that the trend of E 00 vs. 1/n for the oligothiophene series runs nearly parallel to the correlation defined by the PtC n Pt series. This agreement indicates that the degree of triplet delocalization is similar in oligoynes compared to the oligothiophenes. Note, 98

PAGE 99

however, that the extrapolated value of the triplet energy for polythiophene is ca. 2500 cm -1 higher than that of the (CC) n polymer. Since significant fluorescence was not observed for the PtC n Pt series, it is only possible to estimate the S 1 energy based on the onset of the absorption bands. Using this as an approximation for the S 1 state energy, we have computed estimated singlet-triplet splitting (E ST ) for the PtC n Pt series and the values are listed in Table 4-1. Interestingly, for the three longer oligoynes the E ST cluster around a value of 0.7 eV, which is in good accord with the value observed for a variety of other long -conjugated systems, including poly(thiophene) and poly(phenylene vinylene). 60 Energy Gap Law Correlation for the Oligoynes The energy gap law was first suggested by Robinson and Frosch to explain the systematic variation of triplet state non-radiative decay rates in polycyclic aromatic hydrocarbons. 108 Shortly thereafter, Siebrand developed a quantitative theory for the energy gap law. 109 The energy gap law predicts that the rate of non-radiative decay of an excited state will increase as the excited state energy decreases. The effect arises because of the relationship between the Franck-Condon factors for vibrational overlap of the excitedand ground-electronic states and the energy gap (and geometric distortion) between the potential surfaces for the two electronic states (see below, Equation 4-8). The most comprehensive study of the energy gap law carried out to date lies in the work of Meyer and co-workers who showed the existence of a quantitative relationship between excited state energy and the non-radiative rate for the metal-to-ligand charge transfer state in a series of Ru(II) and Os(II) polypyridine complexes. 110-112 More recently, Khler and co-workers showed that the energy gap law holds for triplet decay in a series of platinum-acetylide oligomers and polymers. 113 99

PAGE 100

A focus of the present investigation is to explore whether the energy gap law holds for decay of the triplet excited state in the series of platinum end-capped oligoynes. This study is of interest due to the remarkable simplicity of the carbon chains: they consist of a linear, -conjugated electronic system in which the excitation is coupled to a single, very well-defined high frequency vibrational mode. In short, the oligoynes seem to provide an ideal platform to examine the correlation between spectroscopy, triplet energy and non-radiative decay rate. The following expressions provide the complete relationship between the non-radiative decay rate and parameters accessible from spectroscopy, 98,110 m000100m2m21,020monrcm 1000ln5.02ln161lnlnEESk (4-8) 12ocm 10002kkC (4-9) 1lnmm000SE (4-10) where S m E 00 0,1/2 and m are the same terms defined in Equation 4-1, o is the vibronically induced electronic coupling term, C k is the vibronic coupling matrix element, and k is the frequency of the promoting vibrational mode. These equations can be simplified in order to facilitate analysis of experimental data. The assumption that all terms on the right hand side of Equation 4-8 except the last one are independent of changes in E 00 leads to Equation 4-11. 112 This equation predicts a linear relationship between the log of the non-radiative decay rate and E 00 with a slope equal to o / (this is the mathematical statement of the energy gap law). Also, since ln o is the only term in Equation 4-8 that cannot be computed using parameters derived from a Franck-Condon fit of an emission spectrum, this equation can be re-cast as Equation 4-12 100

PAGE 101

where ln[FCF(calcd)] is an abbreviation for calculated Franck-Condon factors. Note that ln[FCF(calcd)] is computed by substituting the parameters recovered from the emission spectral fits (Table 4-1) into the last 4 terms on the right hand side of Equation 4-8. The latter expression implies that a plot of ln[FCF(calcd)] vs. ln k nr will be a linear correlation with a slope equal to unity. 00m0nrlnEak (4-11) onrlnln]FCF(calcd)ln[ k (4-12) Figure 4-10 illustrates plots of ln k nr vs. E 00 for the PtC n Pt series (filled circles, values at 100 K, see Table 4-2). Also included in the correlation are data from the series of platinum acetylide oligomers reported by Khler. 113 Several features are evident from this presentation. First, the non-radiative decay rates for the PtC n Pt series clearly follows the correlation predicted by the energy gap law. This result clearly shows that in the frozen solvent glass non-radiative decay of the triplet state in the oligoynes is controlled by the extent of vibronic coupling of the exciton to the high frequency mode (the CC stretch), which varies exponentially on the excited state energy. The slope of the energy gap correlation for the PtC n Pt series is almost the same as that defined by the platinum-acetylide oligomers reported by Khler and co-workers. 113 This correspondence in the energy gap law dependence likely arises because in both of the platinum-containing systems the CC stretching mode is likely the dominant acceptor mode coupled to non-radiative decay. The correlation for the PtC n Pt series has a larger intercept value, which reflects the fact that non-radiative decay is faster at equal excited-state energy in the two series. This larger intrinsic k nr (a in Equation 4-11) for the PtC n Pt series may be due to the fact that in the platinum-acetylides studied by Khler medium frequency modes (aromatic 101

PAGE 102

E00 / cm-1 10000120001400016000180002000022000 ln knr 681012141618 Figure 4-10. Energy gap law correlation, see text for details. () : PtC n Pt complexes. The solid line is best fit with R 2 = 0.951. (x) : Platinum-acetylide oligomers (from Wilson et al. 113 ) The dashed line is best fit with R 2 = 0.946. ring C-C stretching) are also coupled to the triplet exciton. This would effectively decrease the Frank-Condon factors for non-radiative decay (note that k appears in the o term in Equation 4-8). Also of interest is the correlation of the Huang-Rhys parameter (S m calculated from the fits of the emission spectra of the PtC n Pt oligomers for n = 6, 8 and 10, values in Table 4-1) with the triplet energy shown in Figure 4-11A. As can be seen from this correlation, S m decreases with E 00 with a very good linear correlation being defined by the series. (By extrapolating the linear correlation defined for PtC 6 Pt, PtC 8 Pt and PtC 10 Pt the S m value for PtC 12 Pt has been 102

PAGE 103

estimated). The decrease in S m with increasing carbon chain length is consistent with the triplet excited state becoming more delocalized, which has the effect of decreasing the electron-vibrational coupling. A similar correlation for S m vs. E 00 was reported by Meyer and co-workers for a metal-to-ligand charge transfer states in a series of Ru-polypyridine complexes, and their data is shown for comparison in Figure 4-11A. 112 Despite the significant difference in the nature of the excited states for the Ru-polypyridine and PtC n Pt complexes (metal-to-ligand charge transfer and 3 *, respectively), the correlations are remarkably similar. Finally, as noted above, it is possible to compute the Franck-Condon factors (FCF) for non-radiative decay using the parameters obtained from the emission fits. As suggested by Equation 4-12, the calculated FCF values should vary linearly with ln k nr with a slope of unity and a single adjustable parameter which corresponds to ln o 98,112 Figure 4-11B shows the correlation of ln FCF and ln k nr where the ln FCF values for the PtC n Pt complexes were computed using the parameters recovered from the spectral fits (Table 4-1) and Equation 4-8. Since it was not possible to fit the emission spectrum for PtC 12 Pt, the S m needed to compute ln FCF for this complex was extrapolated using the observed E 00 value in the correlation shown in Figure 4-11A. The dashed line in Figure 4-11B has a slope of unity and an intercept of -24.7, which affords a value of o = 5.3 x 10 10 cm -1 Given the relatively large range in E 00 and k nr values for the series (6000 cm -1 and a factor of 50) the correlation of the calculated Franck-Condon factors and the non-radiative decay rates is impressive. The quality of the correlation underscores the quality of the energy gap law correlation for the PtC n Pt series, and that in the frozen solvent glass non-radiative decay is controlled by coupling to the CC mode of the carbon chain. 103

PAGE 104

E00 / cm-1 120001400016000180002000022000 Sm 0.40.60.81.01.21.41.6 ln knr 101112131415 ln FCF(calcd) -15-14-13-12-11-10-9 AB Figure 4-11. (A) Plot of Huang-Rhys parameter (S m ) vs. the triplet energy (E 00 ) for PtC n Pt complexes () and for series of ruthenium polypyridyl complexes (). Data for the ruthenium complexes are from Barqawi et al. 112 Lines are best fit to data, R 2 = 0.999 for PtC n Pt complexes and R 2 = 0.963 for ruthenium polypyridyl complexes. (B) Plot of calculated Franck-Condon factors vs. natural log of non-radiative rate for PtC n Pt complexes. The line has a slope of unity. In both plots, the open circle data point () is for PtC 12 Pt. The S m value for PtC 12 Pt in plot (A) is extrapolated using the best fit line (R 2 = 0.960) and the experimental E 00 value and the ln FCF(calcd) value for this complex in plot (B) is computed using the extrapolated S m value. 104

PAGE 105

Excited State Decay above the Solvent Glass Point As noted above, the energy gap correlation for the PtC n Pt series holds only at temperatures below the solvent glass point. It is evident that in fluid solution an additional non-radiative decay pathway is active that dominates decay of the triplet state in the complexes. Activation of this pathway as the glass melts is very evident in PtC 6 Pt; for this complex the lifetime decreases by nearly a factor of 100 as the temperature is increased from 100 160 K. Given that this decay pathway is activated by melting of the solvent matrix, it is reasonable to assume that it is related to the increase in the ability of the carbon chain to undergo fluxional motion. Previous theoretical and experimental studies of (CC) n oligomers indicate that in some cases the optimum geometry in the excited state involves a transoid conformation in which the triple bond length is significantly increased (i.e., the carbons become sp 2 hybrids). 94,114 Although there is insufficient experimental data from the present photophysical studies to prove that a bending mode of the (CC) n chain is the origin of the accelerated triplet decay in fluid solution, it is believed that it is a very reasonable hypothesis. Future DFT calculations which probe the geometric energy landscape for the triplet state of the PtC n Pt series may provide insight into this issue. Summary and Conclusion A detailed photophysical investigation of a homologous series of carbon chain oligomers that are end-capped with Pt-acetylide units has been carried out. The photophysics of the complexes is dominated by a 3 that is concentrated on the (CC) n chain, and the triplet state is produced in comparatively high yield due to the strong spin-orbit coupling induced by the heavy metal platinum centers. At low temperature (T < 130 K) each of the complexes exhibits a unique, highly structured phosphorescence that is characterized by a series of narrow (< 200 cm -1 105

PAGE 106

fwhm) vibronic bands separated by ca. 2100 cm -1 The appearance of the spectrum signals that the triplet state is coupled to a single, high frequency vibrational mode that corresponds to the stretch of the (CC) n chain. Analysis of the phosphorescence spectra using a single-mode Franck-Condon bandshape analysis indicates that the 0-0 energy of the triplet state decreases by ca. 6000 cm -1 as the length of the carbon chain increases from 6 to 12 atoms, and the electron-vibrational coupling constant (S m ) decreases with increasing chain length. At room temperature the triplet state can be observed by transient absorption spectroscopy, and each of the complexes exhibits moderately intense triplet-triplet absorption throughout the visible region (400 700 nm). Phosphorescence lifetime studies were carried out on the series to study the decay kinetics of the triplet state. At low temperature in a frozen solvent glass, the triplet lifetime systematically increases with decreasing length of the carbon chain, with the lifetimes ranging from 27 s (PtC 6 Pt) to 500 ns (PtC 12 Pt). Given that the phosphorescence quantum yields are comparatively low, the lifetimes are controlled by the rate of non-radiative decay, and the observed lifetime variation at low temperature is attributed to the energy gap law. Consistent with this hypothesis, the non-radiative decay rates of the complexes at T = 100 K are shown to exhibit a quantitative energy gap law correlation. The properties of the triplet state in the PtC n Pt series are believed to be minimally perturbed by the presence of the platinum complex end-capping units. Consequently the results of this study provide the best available quantitative insight concerning the structure and dynamics of the triplet state in carbon chains of extended length. 106

PAGE 107

CHAPTER 5 AN IRIDIUM COMPLEX THAT EXHIBITS DUAL-MECHANISM NONLINEAR ABSORPTION Introduction Reverse saturable absorption (RSA) as a sequential, multi-step process involving two photons as a mechanism for optical limiting was described in detail in chapter 1. When a material has an excited state absorption cross section larger than the ground state absorption cross section at a particular wavelength, it is said to be a reverse saturable absorber. RSA is a slow (~10 -10 10 -8 s) process requiring first the ground state absorption of a photon to populate the excited state, followed by excited state absorption of a second photon. While this has been shown as an effective method of nonlinear absorption, there are several limiting factors. Most notably is that RSA is generally a slow process requiring long-lived excited states (usually triplet excited states populated by ground state absorption to the singlet excited state followed by intersystem crossing). Due to the multi-step nature of the absorbing mechanism, RSA is an ineffective pathway for limiting laser pulses below the nanosecond time regime. Another mechanism for nonlinear absorption involves the instantaneous absorption of two photons. Consequently, two-photon absorption (TPA) is mostly effective for optical limiting only at very short timescales (< 100 ps). 115 A second benefit of using TPA as an optical limiting mechanism is that the material can be completely transparent at the active wavelength, whereas a RSA material must have some ground state absorption. One method of extending the time regime of an optical limiter is to combine absorption via several mechanisms. By coupling slow timescale reverse saturable absorptionor more generally, excited state absorptionwith fast timescale two-photon absorption, a broad temporal response to optical limiting can be achieved. The compound characterized in this chapter exhibits such absorption pathways. 107

PAGE 108

108 The theoretical prediction of two-photon absorption was first described by GoeppertMayer in 1931.116 However, two-photon absorption processes were predicted thirty years before they could be experimentally verified. It took until the advent of the laser for the first observation of TPA in the lab by Kaiser and Garrett in 1961.117 Figure 5-1 shows a Jab o ski diagram with corresponding representative absorbance spectra illustrating two-photon absorption. As shown, a material can absorb a single photon corresponding to an energy, E which will promote an electron to a higher state. Alternatively, certain materials can simultaneously absorb two photons, whose energies sum to equal the Figure 5-1. Jab o ski diagram and corresponding representative absorption spectra illustrating two-photon absorption. energy between the two states. Oneand two-photon excitation will not necessarily promote electrons to the same state. That is, onea nd twophoton excitation follow different selection rules.118 Selection rules for two-photon excitation will not be explored here, as the results presented later will be given in a more qualitative presentation. One-photon absorbance spectra WavelengthAbsorbance 2 One-photon absorption Two-photon absorption Energy hhh

PAGE 109

for a material theoretically should be different from two-photon absorbance spectra. While the amount of data on two photon absorption spectra is increasing, it is still a difficult measurement. Measured values for two-photon absorption coefficients can vary significantly based upon solvent, laser intensity, pulse duration of the laser and other experimental conditions. 119 In order to understand the concept of nonlinear absorption through a multi-photon process, it is necessary to first understand the linear, one-photon absorption process. From Beers law, one can consider a sample of some thickness along the z-axis. For incident light propagating along the z-axis through a sample, the light changes according to Equation 5-1. IdzdI (5-1) where is the absorption coefficient. For a one-photon process, no other terms apply. However, for two-photon absorption, the absorption coefficient is defined by Equation 5-2. I 0 (5-2) where 0 is the one-photon absorption coefficient and is the two-photon absorption coefficient. Substituting in Equation 5-1, 20IIdzdI (5-3) an expression is obtained illustrating the (linear) oneand (nonlinear) two-photon absorption dependence on incident intensity. 120 To further the nonlinear optical properties of a two-photon absorbing material, a second absorbing mechanism can take place. After populating the excited state of a material, the electron can either return to the ground state, or absorb an additional photon. The initial absorbing state can be excited by either oneor two-photon absorption, as the resulting excited 109

PAGE 110

state is independent of the excitation pathway. Excited state absorption (ESA) following TPA leads to enhanced nonlinear optical properties. If excited-state absorption occurs, a third term applies in Equation 5-3, causing the change in intensity through the sample to become ITIIdzdITT120 (5-4) where TT is the absorption cross-section of the T 1 triplet excited state, and T 1 is the population of the first triplet excited state. Equation 5-4 holds only within the lifetime of the triplet state. For many compounds, and in particular the compound discussed in this chapter, the absorbing excited state will be the triplet state. A four-level Jaboski diagram illustrating the transitions involved in TPA followed by ESA is shown in Figure 5-2. The process is initiated by simultaneous absorption of two photons via TPA by the ground state promoting an electron to the S 1 excited state. Intersystem crossing then populates the first triplet excited state, T 1 The last step involves the absorption of a photon to populate some higher triplet state, T n (Intersystem crossing is not necessary for enhanced TPA; the S 1 state could alternatively be an absorbing species, although this mechanism will not be the focus of this chapter.) Rate equations for each state can be written as shown in Equations 5-5, 5-6, 5-7 and 5-8. The following rate equations are described assuming only the transitions illustrated in Figure 5-2 will occur. 1111202ddTSTSIhtS (5-5) 1ISC12112ddSSkSIhtS (5-6) 1111ISC1ddTTnTTTTIhTSktTn (5-7) 110

PAGE 111

nTnTTnTIhTtT1dd (5-8) h hhS1S0T1Tn ISCTT Energy Figure 5-2. Jablonski diagram for a four-level system illustrating TPA with ESA. The excited triplet state is populated by two-photon absorption of the ground state followed by intersystem crossing. The excited triplet state then absorbs an additional photon. where S 0 S 1 T 1 and T n are the populations of the ground and first excited singlet states and first and some higher triplet excited states, respectively, is the lifetime of the labeled state, and k ISC is the intersystem crossing rate. Integrating the equations to find an expression for T 1 and substituting into Equation 5-4, an expression for the intensity at the end of the laser pulse is given: 322IhIdzdITT (5-9) where is the duration of the laser pulse. Several assumptions are made for Equation 5-9 to be valid: 121 (i) one-photon absorbance at the two-photon absorbing wavelength is negligible for the S 0 state (ii) the lifetime of the T 1 state is longer than the length of the laser pulse, (iii) intersystem 111

PAGE 112

crossing efficiency approaches unity and is shorter than the length of the laser pulse, and (iv) the population of the S 0 ground state and is not sufficiently depleted, i.e. S 1 T 1 T n << S 0 From Equation 5-9, it is shown that reverse saturable absorption occurs when TT > 0. 121 Therefore, a material which exhibits excited-state absorption at the two-photon absorbing wavelength is a reverse saturable absorber. Two-photon absorbing materials commonly have donor (D) or acceptor (A) groups linked by a -conjugated bridge. 122-124 Different types of architectures are shown in Figure 5-3. More complex motifs, including branched structures, with D--A--D type conjugation are also possible, allowing for increased TPA cross sections. There have been numerous reports concerning TPA or RSA in organic and organometallic molecules, but currently there is not much in the literature about chromophores that exhibit both TPA and RSA. 125-127 Our group is interested in the design, synthesis and characterization of materials that exhibit dual-mode DonorDonorAcceptorAcceptor DonorAcceptor Type IType IIType III DonorDonor DonorDonorAcceptorAcceptor AcceptorAcceptor DonorAcceptor DonorAcceptor Type IType IIType II I Figure 5-3. Structural motifs for two-photon absorbers. 112

PAGE 113

TPA/RSA nonlinear absorption. In particular, oligomers that contain a -conjugated chromophore with a large TPA cross section that is strongly coupled to a transition metal chromophore that promotes intersystem crossing to a long-lived triplet excited state with a large triplet-triplet absorption cross section. The photophysical characterization of such a chromophore will be described in this chapter. The material of interest, the Ir(III) complex ML, is shown in Figure 5-4. This complex incorporates a heavy metal center into a D--D type -conjugated oligomer L. -Conjugated molecular structures containing donors or acceptors at both ends of a delocalized, -electron system exhibit large TPA cross sections. 124,128 In addition, iridium has the largest spin-orbit coupling constant of all the transition metals, and consequently it is able to facilitate intersystem crossing to afford a large triplet yield. 129 N N NHex2 Hex2N N N NHex2 Hex2N M LMLM = Ir(ppy)2+[PF6-]; ppy= 2-phenylpyridi ne Figure 5-4. Structures of the ligand, L, and iridium(III) complex, ML. Experimental All photophysical measurements were carried out in 1 cm x 1 cm quartz cuvettes, unless otherwise noted. All sample solutions were prepared in dry THF and deoxygenated by bubbling 113

PAGE 114

with argon unless otherwise noted. UV-visible absorption spectra were recorded on a Varian Cary 100 dual-beam spectrophotometer. For emission measurements, sample concentrations were adjusted to produce optically dilute solutions (A max < 0.2). Corrected steady-state emission measurements were performed on a SPEX F-112 fluorescence spectrometer equipped with a Hamamatsu R928 PMT for visible measurement. Emission quantum yields were measured relative to Ru(bpy) 3 Cl 2 in air-saturated H 2 O, where = 0.0379. 130 Transient absorption measurements were carried out with solutions having an absorbance of 0.8-1.0 at 355 nm. Transient absorption spectra were obtained on an instrument that has previously been described 99 that uses the third harmonic of a Nd:YAG laser (Spectra Physics GCR-14, = 355 nm, 10 ns fwhm, 10 mJ pulse -1 20 mJ-cm -2 fluence) as the excitation source. Two-photon-induced emission spectra were recorded on an apparatus that relies on a Continuum Surelite series Nd:YAG laser as the excitation source. The fundamental output of the laser ( = 1064 nm, 10 ns fwhm) was passed through a telescope providing a final beam diameter of 0.7 cm. This beam was focused with a 10-cm focal length lens (Newport Optics) and the sample was positioned at the focal plane of the lens. The luminescence was detected by an intensified CCD detector (Princeton Instruments, PI-MAX) coupled to an Acton SpectraPro 150 spectrograph. The sample was contained in a 5 mm i.d. round borosilicate tube. Nonlinear transmittance was measured using the same apparatus (shown in Figure 5-5), with samples contained in a 10-mm pathlength quartz optical cell that was positioned at the focal plane. Nuetral density filters were used to attenuate the beam fluence. A 10% beamsplitter was used to direct part of the incident energy to the reference detector, D1, (Ophir analog power meter). The transmitted beam was passed through a 10-cm focal length lens (L1) before reaching the sample. 114

PAGE 115

A second lens (L2) collected the beam before reaching the detector, D2 (Scientech P09 energy meter with a Vector S310 digital display). Transient absorption arising from two-photon excitation was carried out using an unfocused beam from the Continuum Surelite laser as the excitation source, a Hamamatsu xenon flash lamp, and the previously mentioned spectrograph and CCD detector. Fluence-dependence emission intensity arising from two-photon excitation was measured using an unfocused 0.8 cm diameter beam from the fundamental of a QuantaRay GCR series Nd:YAG laser as the excitation source ( = 1064 nm, 10 ns fwhm). Detection was provided by a Hamamatsu R928 PMT coupled to an Oriel 1/8 m monochromator set at the emission wavelength maximum. L1L2NDFBSNd:YAGlaserSampleD1D2 L1L2NDFBSNd:YAGlaserSampleD1 D2 Figure 5-5. Apparatus for measuring nonlinear transmittance. NDF: neutral density filter; BS: beamsplitter; L: lens; D: detector. Results One-photon ground-state absorption and photoluminescence spectra for ligand L and Ir complex ML in THF solution are shown in Figure 5-6. Ligand L exhibits absorption bands at 115

PAGE 116

280 and 400 nm, arising from shortand long-axis -* transitions, respectively. Ir complex ML features a broad absorption band at 500 nm, arising from a combination of the long-axis polarized -* transition and Irligand metal-to-ligand charge transfer (MLCT) transition. Intense fluorescence from ligand L is observed with a band maximum at 520 nm. The emission of ML is dominated by a broad featureless band having an emission maximum at 730 nm. The quantum yield at this wavelength is 0.003, with an emission lifetime of 513 ns. / 104 M-1 cm-1 0246810 0.00.20.40.60.81.01.2 Wavelength / nm 300400500600700 02468 300400500600700800 Normalized Intensity 0.00.20.40.60.81.0 ADCB / 104 M-1 cm-1 0246810 0.00.20.40.60.81.01.2 Wavelength / nm 300400500600700 02468 300400500600700800 Normalized Intensity 0.00.20.40.60.81.0 ADCB Figure 5-6. Absorption of (A) ligand L and (B) complex ML and one-photon emission of (C) ligand L and (D) complex ML. 116

PAGE 117

Transient absorption spectra of ML in THF solution obtained under one-photon excitation conditions (5 ns pulse, 355 nm) are show in Figure 5-7. The TA spectrum features a ground-state bleaching centered near 500 nm combined with a strong absorption band that extends throughout the visible and into the near infrared region. The absorption wavelength maximum of the transient is approximately 875 nm with a value of 6.1 x 10 4 M -1 cm -1 The transient absorption decays with a lifetime of 1.2 s. Wavelength / nm 400600800100012001400 Absorbance -0.3-0.2-0.10.00.10.20. 3 Figure 5-7. Transient absorption of ML in deoxygenated THF solution obtained at 400 ns increments following 355 nm excitation. 117

PAGE 118

Given that the -* ground state absorption in complex ML is at 500 nm, it seemed reasonable that two-photon absorption would occur in the near-infrared at 1064 nm, i.e. the fundamental of a Q-switched Nd:YAG laser. The two-photon upconverted photoluminescence spectrum of ML produced by excited with the 1064 nm beam is shown in Figure 5-8. The emission band shape and wavelength maximum are the same as those observed under one-photon absorption conditions. The upconverted luminescence can easily be seen by eye as shown in Figure 5-9. A plot of the integrated emission area as a function of infrared excitation intensity is shown in Figure 5-10, and a fit shows that the emission is dependent on the square of Wavelength / nm 400500600700800 Intensity / a.u. 0500100015002000 2500 Figure 5-8. Two-photon induced emission of ML after pulsed excitation by 1064 nm. 118

PAGE 119

A B Figure 5-9. Photograph of 5 mM ML solution. (A) Under ambient conditions and (B) showing visible emission after excitation by 1064 nm laser light. Laser fluence / mJ cm-2 0.00.51.01.5 Emission area / a.u. Figure 5-10. Emission area dependence of ML on incident laser energy at 1064 nm. 119

PAGE 120

the of the excitation intensity. According to Equation 5-3, this indicates a two-photon absorbing mechanism. Transient absorption spectra collected after excitation by 1064 nm were also observed (Figure 5-11). The band shape of the transient absorption spectrum measured under two-photon excitation conditions is similar to that obtained under one-photon excitation. The two-photon excited transient absorption spectrum was obtained on a different instrument than the one-photon excited spectrum shown in Figure 5-6(D). The detector used for the two-photon excited studies is not sensitive beyond 850 nm, and therefore, it is not possible to observe the maximum of the triplet-triplet absorption that occurs near 875 nm. Wavelength / nm 500600700800900 Absorbance -0.010.000.010.02 0.03 Figure 5-11. Transient absorption spectra of 5 mM deoxygenated THF solution of ML following 1064 nm pulsed excitation. Excitation fluence in order of decreasing transient absorbance signal: 0.35 J cm -2 ; 0.25 J cm -2 ; 0.20 J cm -2 ; 0.12 J cm -2 ; and baseline (no laser). 120

PAGE 121

Nonlinear transmittance measurements of ML were carried out at various concentrations using 1064 nm, 5 ns pulses. Figure 5-12 shows the laser fluence dependence of the transmittance of ML at a range of concentrations. While the blank THF solution responds linearly to the incident energy, it is evident that the THF solutions of the Ir complex ML exhibit nonlinear absorption, the extent of which increases with the concentration of ML. Notably, for a 20 mM solution of ML, the transmitted energy is significantly reduced above an input energy of 0.8 mJ. E in / J cm-2 0.00.51.01.52.0 E out / J cm-2 0.00.20.40.60.81.01.2 THF only 2.5 mM 5.0 mM 10 mM 20 m M Figure 5-12. Transmittance of 1064 nm pulsed beam at various concentrations of ML in THF. Discussion The absorption assignment for the Ir complex ML is slightly more complex than for the ligand L. The related complex Ir(ppy) 2 (bpy) + (where bpy = 2,2'-bypyridine), in which the visible 121

PAGE 122

absorption is exclusively due to an MLCT transition, has previously been reported. 131,132 The molar absorptivity of ~10 4 M -1 cm -1 as reported by Glusac and co-workers, 131 which is much smaller than that of ML, indicates that the visible absorption of ML is dominated by the intraligand -* transition. The absorption maximum for ML is red-shifted by approximately 100 nm compared to ligand L. The red shift arises due to the effect of the cationic Ir(III) center on the -electron system of the conjugated bypyridine ligand system. In particular, the metal reduces the LUMO energy of the conjugated ligand, in part due to its positive charge, and in part due to the effect of coordination which forces the bipyridine unit into a planar conformation. 133 The emission of ML arises from an excited state having mixed character, consisting of intraligand 3 ,* and 3 MLCT configurations. 131,134,135 This assignment is supported by the fact that the emission is strongly Stokes-shifted from the absorption, and it is comparatively long-lived. An important question concerns the nature of the excited state that is responsible for the strong transient absorption. As noted earlier, on the basis of previous investigations of d 6 transition metal complexes with -conjugated ligands, 131,134,135 the long-lived excited state of ML is tentatively assigned to a state having mixed 3 MLCT3 ,* character. This assignment is based on previous studies of Ir(III) and Ru(II) complexes with -conjugated diimine ligands of varying conjugation length. 131,134 In this work, it has been demonstrated that as the energies of the 3 MLCT and 3 ,* states converge, the lowest excited state exhibits mixed 3 MLCT3 ,* character. In ML, the 3 MLCT3 ,* are within 0.25 eV relative to one another, 131,134 and therefore, the condition is correct for configuration mixing of the two states. The emission band shape and wavelength maximum arising from absorption at 1064 nm are the same as those observed under one-photon excitation conditions. The observation of 122

PAGE 123

emission at 700 nm under 1064 nm excitation clearly indicates that ML can be excited by two-photon absorption. That is, excitation at a particular wavelength would not cause emission at a higher-energy wavelength unless multiphoton absorption is the mechanism by which the excited state is populated. Although instrument limitations restricted detection of the full transient absorption spectrum under 1064 excitation conditions, the portion of the observed spectrum matches that of the spectrum obtained after excitation by 355 nm. The fact that strong transient absorption is seen for the triplet state under two-photon excitation conditions further indicates that a relatively large triplet excited-state population can be produced via two-photon absorption, even with a nanosecond laser. Conclusion In this chapter, the photophysical characterization of a two-photon absorbing iridium complex has been described. The ligand in the complex provides a large absorption cross-section for oneand two-photon excitation. The long-lived triplet excited state affords the opportunity for absorption of an additional photon. This combination leads to enhanced nonlinear optical absorption. The photophysics of this complex have shown that the triplet state can be populated independently by one-photon and two-photon absorption. Incident-energy-dependent emission intensity illustrates the two-photon absorbing nature of the complex. The iridium atom contributes to strong spin-orbit coupling of the ligand to reach the triplet excited state through intersystem crossing from initial excitation. This is demonstrated by oneand two-photon induced phosphorescence, as well as transient absorption. In addition, the nonlinear optical properties are clearly shown by the trend seen with incident energy dependence of the transmittance. 123

PAGE 124

CHAPTER 6 CONCLUSIONS AND FUTURE WORK In previous chapters, various aspects of the photophysical properties of some platinumand iridium-containing organometallic compounds have been presented. Additionally, a transient absorption apparatus was designed and constructed to probe the excited state photophysics in supplement to techniques possible with commercial instrumentation. The platinum oligomers and iridium complex were designed to gain some insight into the photophysical behavior of these novel materials, but also to gain knowledge about the triplet state in general as it is less studied than the singlet state. The triplet state properties of all-organic systems are less understood as access into the triplet manifold is not easily facilitated. The studies presented in previous chapters may therefore help the understanding of the photophysical properties of triplet excited states in organic and metal-organic conjugated systems. The transient absorption apparatus was designed with the primary goal of incorporating an intensified CCD camera detector into the system. Along with a white-light probe source, a CCD detector allows for the simultaneous collection of all wavelengths in the visible spectrum. The benefits of using a CCD camera as opposed to another detector such as a photomultiplier tube include reduced sample exposure to the excitation source and reduced experiment time. The inclusion of an intensifier as part of the CCD system allows for gate times as short as 10 ns, therefore making it possible to extract kinetics from species with lifetimes as short at ~100 ns. Data conversion software was also developed to present the time-resolved transient absorption spectra in a conventional manner. While the excitation and probe source, as well as the detector, can be replaced or modified to expand the range of molecules that are possible to study, a major limitation of the apparatus is defined by the nature of the sample. Specifically, if a material 124

PAGE 125

exhibits strong photoluminescence, it can be collected by the detector. This may result in a negative signal that can obscure the transient absorption. Consequently, the software used to control the hardware and calculate the spectra are being modified to create an option for emission correction. The photophysical characterization of a series of dinuclear platinum oligomers reveals several things concerning the delocalization of singlet and triplet states in these types of molecules. Most significantly, the phosphorescence of the PE2 end-capped oligomers show distinct emission from the spacer and the end-cap, which indicates limited delocalization of the triplet state through the platinum centers. Temperature-dependent emission spectra also support the idea of localization of the triplet state. In contrast, variable temperature excitation indicate less temperature dependence of the singlet ground and excited states, suggesting more delocalization of these states compared to the triplet excited state. It is not known if the emission occurs simultaneously from the spacer and end-cap, or if there is energy transfer from one chromophore to another during the lifetime of the triplet state. Time-resolved emission studies could provide better understanding on the nature of the excited state. More definitive trends could be assigned by characterizing a larger series of dinuclear platinum oligomers. The spacer and end-caps should be modified systematically in order to gain better understanding of the role of platinum in the triplet state. In a second study, a series of molecules in which carbon chains of different lengths span two platinum atoms was characterized by photoluminescence, lifetime and quantum yield measurements. The low-temperature emission spectra reveal intense vibrational coupling to the CC stretching mode. The emission spectra were subjected to Franck-Condon analysis, and a linear relationship between emission energy and chain length (from 6 to 12 carbon atoms) was 125

PAGE 126

revealed. The electron-vibrational coupling constant decreases with increasing chain length. Triplet lifetimes are governed by the nonradiative decay rate, and it was found that the triplet excited states at low temperature (i.e. in a solvent glass) are governed by the energy gap law. It is believed that the triplet properties are minimally perturbed by the presence of the platinum atom in the end-cap, and therefore the results of this study provide quantitative insight concerning the structure and dynamics of the triplet state in long carbon chains. Finally, the photophysical characterization of a two-photon absorbing iridium complex has been described. A large absorption cross-section for oneand two-photon excitation is provided by the ligand, and the iridium atom provides access to the triplet manifold. The long-lived triplet excited state affords the opportunity for absorption of an additional photon. This combination leads to enhanced nonlinear optical absorption. Incident-energy-dependent emission intensity illustrates the multi-photon absorbing nature of the complex. In addition, the nonlinear optical properties are clearly shown by the trend seen with incident energy dependence of the transmittance. In order to gain further understanding of triplet state behavior in these types of systems, it is necessary to characterize a broad range of related molecules. In a sythentic sense, the possibilities are nearly endless. Gladysz and coworkers 136-138 have already synthesized related molecules, and some examples are shown in Figure 6-1. These compounds would be helpful in understanding intermolecular interactions. For example, the series shown in Figure 6-1A could be thought of as an insulated molecular wire, with the carbon chain as the wire surround by the alkyl chain. Depending on conformation, the alkyl chain could wrap around the center chain and perhaps protect it from intermolecular interactions. In contrast, the carbon chains in the dimers 126

PAGE 127

shown in Figure 6-1B are forced to be near each other, facilitating the study of aggregate-like interactions. AB A B Figure 6-1. Variations of molecular wires synthesized by Stahl, Owen et al. 136-138 (A) A shielded molecular wire. (B) A wire dimer. As with the platinum oligomers, further knowledge would increased through characterization of a broader range of molecules. However, the photophysics of the molecule presented in Chapter 5 could be more thoroughly characterized. For example, using a variable-wavelength excitation source, such as an optically parametric oscillator (OPO), a greater understanding of the multiphoton absorption properties would be gathered. The multi-photon absorption properties could be further examined on a temporal scale, that is, through excitation by a short (ps) laser pulse. Such information is essential in the development of materials for use as broad-band optical limiters. 127

PAGE 128

LIST OF REFERENCES (1) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Sausilito, California, 1991. (2) Henry, B. R.; Siebrand, W. In Organic Molecular Photophysics; Birks, J. B., Ed.; John Wiley and Sons, 1973; Vol. 1. (3) Shriver, D. F.; Atkins, P. W. Inorganic Chemistry; 3rd ed.; W. H. Freeman and Company: New York, 1999. (4) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R., Eds. Handbook of Conducting Polymers; 2nd ed.; Marcel Dekker: New York, 1998. (5) McGehee, M. D.; Miller, E. K.; Moses, D.; Heeger, A. J. In Advances in Synthetic Metals. Twenty Years of Progress in Science and Technology.; Bernier, P., Lefrant, S., Bidan, G., Eds.; Elsevier: Amsterdam, 1999. (6) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-Emitting Diodes Based on Conjugated Polymers. Nature 1990, 347, 539-541. (7) Gunder, P. Nonlinear Optical Effects and Materials; Springer: New York, 2000. (8) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15-26. (9) Aviram, A.; Ratner, M. A. Molecular Rectifiers. Chem. Phys. Lett. 1974, 29, 277-283. (10) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Conductance of a Molecular Junction. Science 1997, 278, 252-254. (11) Ashwell, G. J.; Chwialkowska, A. Controlled Alignment of Molecular Diodes via Ionic Assembly of Cationic Donor-(-Bridge)-Acceptor Molecules on Anionic Surfaces. Chem. Commun. 2006, 1404-1406. (12) Ashwell, G. J.; Urasinska, B.; Wang, C.; Bryce, M. R.; Grace, I.; Lambert, C. J. Single-Molecule Electrical Studies on a 7 nm Long Molecular Wire. Chem. Commun. 2006, 4706-4708. (13) Ashwell, G. J.; Tyrrell, W. D.; Urasinska, B.; Wang, C.; Bryce, M. R. Organic Rectifying Junctions from an Electron-Accepting Molecular Wire and an Electron-Donating Phthalocyanine. Chem. Commun. 2006, 1640-1642. (14) Davis, W. B.; Ratner, M. A.; Wasielewski, M. R. Conformational Gating of Long Distance Electron Transfer through Wire-like Bridges in Donor-Bridge-Acceptor Molecules. J. Am. Chem. Soc. 2001, 123, 7877-7886. 128

PAGE 129

(15) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Molecular-Wire Behavior in p-Phenylenevinylene Oligomers. Nature 1998, 396, 60-63. (16) Filatov, I.; Larsson, S. Electronic Structure and Conduction Mechanism of Donor-Bridge-Acceptor Systems where PPV Acts as a Molecular Wire. Chem. Phys. 2002, 284, 575-591. (17) Barybin, M. V.; Chisholm, M. H.; Dalal, N. S.; Holovics, T. H.; Patmore, N. J.; Robinson, R. E.; Zipse, D. J. Long-Range Electronic Coupling of MM Quadruple Bonds (M = Mo or W) via a 2,6-Azulenedicarboxylate Bridge. J. Am. Chem. Soc. 2005, 127, 15182-15190. (18) Xu, G.-L.; Crutchley, R. J.; DeRosa, M. C.; Pan, Q.-J.; Zhang, H.-X.; Wang, X.; Ren, T. Strong Electronic Couplings between Ferrocenyl Centers Mediated by Bis-Ethynyl/Butadiynyl Diruthenium Bridges. J. Am. Chem. Soc. 2005, 127, 13354-13363. (19) Sheng, T.; Vahrenkamp, H. Long Range Metal-Metal Interactions Along Fe-NC-Ru-CN-Fe Chains. Eur. J. Inorg. Chem. 2004, 1198-1203. (20) Liu, S. H.; Chen, Y.; Wan, K. L.; Wen, T. B.; Zhou, Z.; Lo, M. F.; Williams, I. D.; Jia, G. Synthesis and Characterization of Linear (CH) 8 -Bridged Bimetallic Ruthenium Complexes. Organomettalics 2002, 21, 4984-4992. (21) Chung, M.-C.; Gu, X.; Etzenhouser, B. A.; Spuches, A. M.; Rye, P. T.; Seetharaman, S. K.; Rose, D. J.; Zubieta, J.; Sponsler, M. B. Intermetal Coupling in [( 5 -C 5 R 5 )Fe(dppe)] 2 (m-CH:CHCH:CH) and in Their Dicationic and Monocationic Mixed-Valence Forms. Organomettalics 2003, 22, 3485-3494. (22) Wong, W.-Y.; Ho, C.-L. Di-, Oligoand Polymetallaynes: Syntheses, Photophysics, Structures and Applications. Coord. Chem. Rev. 2006, 250, 2627-2690. (23) Wong, W.-Y. Luminescent Organometallic Poly(aryleneethynylene)s: Functional Properties towards Implications in Molecular Optoelectronics. Dalton Trans. 2007, 4495-4510. (24) Peters, T. B.; Bohling, J. C.; Arif, A. M.; Gladysz, J. A. C8 and C12 sp Carbon Chains That Span Two Platinum Atoms: The First Structurally Characterized 1,3,5,7,9,11-Hexayne. Organomettalics 1999, 18, 3261-3263. (25) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. Toward Metal-Capped One-Dimensional Carbon Allotropes: Wirelike C 6 -C 20 Polyynediyl Chains That Span Two Redox-Active ( 5 -C 5 Me 5 )Re(NO)(PPh 3 ) Endgroups. J. Am. Chem. Soc. 2000, 122, 810-822. 129

PAGE 130

(26) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Bent and Stretched but Not Yet to the Breaking Point: C8-C16 sp Carbon Chains That Span Two Platinum Atoms and the First Structurally Characterized 1,3,5,7,9,11,13,15-Octayne. Inorg. Chem. 2001, 40, 3263-3264. (27) Yam, V. W.-W.; Wong, K. M.-C. Luminescent Molecular Rods Transition-Metal Alkynyl Complexes. Top. Curr. Chem. 2005, 257, 1-32. (28) Spangler, C. W. Recent Development in the Design of Organic Materials for Optical Power Limiting. J. Mater. Chem. 1999, 9, 2013-2020. (29) Tutt, L. W.; Boggess, T. F. A Review of Optical Limiting Mechanisms and Devices Using Organics, Fullerenes, Semiconductors and Other Materials. Prog. Quantum Electron. 1993, 17, 299-338. (30) Hollins, R. C. Materials for Optical Limiters. Curr. Opin. Solid State Mater. Sci. 1999, 4, 189-196. (31) Sun, Y.-P.; Riggs, J. E. Organic and Inorganic Optical Limiting Materials. From Fullerenes to Nanoparticles. Int. Rev. Phys. Chem. 1999, 18, 43-90. (32) McEwan, K.; Lewis, K.; Yang, G.-y.; Chng, L.-l.; Lee, Y.-w.; Lau, W.-p.; Lai, K.-s. Synthesis, Characterization, and Nonlinear Optical Study of Metalloporphyrins. Adv. Funct. Mater. 2003, 13, 863-867. (33) Perry, J. W.; Mansour, K.; Lee, I. Y. S.; Wu, X. L.; Bedworth, P. V.; Chen, C. T.; Ng, D.; Marder, S. R.; Miles, P.; Wada, T.; Tian, M.; Sasabe, H. Organic Optical Limiter with a Strong Nonlinear Absorptive Response. Science 1996, 273, 1533-1536. (34) Staromlynska, J.; Chapple, P. B.; Davy, J. R.; McKay, T. J. A Platinum Ethynyl Compound for Optical Limiting. Proc. SPIE-Int. Soc. Opt. Eng. 1994, 2229, 59-66. (35) Staromlynska, J.; McKay, T. J.; Bolger, J. A.; Davy, J. R. Evidence for Broadband Optical Limiting in a Pt:ethynyl Compound. J. Opt. Soc. Am. B 1998, 15, 1731-1736. (36) McKay, T. J.; Bolger, J. A.; Staromlynska, J.; Davy, J. R. Linear and Nonlinear Optical Properties of Platinum-Ethynyl. J. Chem. Phys. 1998, 108, 5537-5541. (37) McKay, T. J.; Staromlynska, J.; Wilson, P.; Davy, J. Nonlinear Luminescence Spectroscopy in a Pt:ethynyl Compound. J. Appl. Phys. 1999, 85, 1337-1341. (38) Staromlynska, J.; McKay, T. J.; Wilson, P. Broadband Optical Limiting Based on Excited State Absorption in Pt:ethynyl. J. Appl. Phys. 2000, 88, 1726-1732. (39) McKay, T. J.; Staromlynska, J.; Davy, J. R.; Bolger, J. A. Cross Sections for Excited-State Absorption in a Pt:ethynyl Complex. J. Opt. Soc. Am. B 2001, 18, 358-362. 130

PAGE 131

(40) Rogers, J. E.; Cooper, T. M.; Fleitz, P. A.; Glass, D. J.; McLean, D. G. Photophysical Characterization of a Series of Platinum(II)-Containing Phenyl-Ethynyl Oligomers. J. Phys. Chem. A 2002, 106, 10108-10115. (41) Cooper, T. M.; Hall, B. C.; Burke, A. R.; Rogers, J. E.; McLean, D. G.; Slagle, J. E.; Fleitz, P. A. Glass-Forming Liquid Platinum Acetylides. Chem. Mater. 2004, 16, 3215-3217. (42) Cooper, T. M.; Blaudeau, J.-P.; Hall, B. C.; Rogers, J. E.; McLean, D. G.; Liu, Y.; Toscano, J. P. The Triplet State of a Platinum Acetylide Chromophore Examined by Time-Resolved Infrared Spectroscopy and Density Functional Theory. Chem. Phys. Lett. 2004, 400, 239-244. (43) Cooper, T. M.; McLean, D. G.; Rogers, J. E. Molecular Structure-Spectroscopic Property Relationships in a Series of Transition Metal-Containing Phenylacetylene Oligomers. Chem. Phys. Lett. 2001, 349, 31-36. (44) Rogers, J. E.; Hall, B. C.; Hufnagle, D. C.; Slagle, J. E.; Ault, A. P.; McLean, D. G.; Fleitz, P. A.; Cooper, T. M. Effect of Platinum on the Photophysical Properties of a Series of Phenyl-Ethynyl Oligomers. J. Chem. Phys. 2005, 122, 214708/214701-214708/214708. (45) Cooper, T. M.; Krein, D. M.; Burke, A. R.; McLean, D. G.; Rogers, J. E.; Slagle, J. E.; Fleitz, P. A. Spectroscopic Characterization of a Series of Platinum Acetylide Complexes Having a Localized Triplet Exciton. J. Phys. Chem. A 2006, 110, 4369-4375. (46) Norrish, R. G. W.; Porter, G. Chemical Reactions Produced by Very High Light Intensities. Nature 1949, 164, 658. (47) Lindqvist, L. Utilization of a Laser with Pulsed Ultraviolet Emission in Flash Photolysis. Triplet State of Acridine. Hebd. Seances Acad. Sci., Ser. C 1966, 263, 852-854. (48) Small, R. D., Jr.; Scaiano, J. C. Absolute Rates of Hydrogen Abstraction by tert-Butoxy Radicals. J. Am. Chem. Soc. 1978, 100, 296-298. (49) Scaiano, J. C. In Reactive Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M. J., Eds.; John Wiley & Sons, Inc.: Hoboken, New Jersey, 2004. (50) Scaiano, J. C. Solvent Effects in the Photochemistry of Xanthone. J. Am. Chem. Soc. 1980, 102, 7747-7753. (51) How Intensified CCD Cameras Operate. http://www.andor.com/library/digital_cameras/ (accessed August, 2007). (52) Princeton Instruments Homepage. http://www.piacton.com/ (accessed August 2007). 131

PAGE 132

(53) Zimmermann, C.; Mohr, M.; Zipse, H.; Eichberger, R.; Schnabel, W. Photoionization of Anthracene and Anthracene Derivatives. J. Photochem. Photobiol., A 1999, 125, 47-56. (54) Haarmann, K.; Haselbach, E.; Suppan, P. Triplet State Lifetime of Anthracene in Benzene Solution. Chimia 1987, 41, 198-200. (55) Ohno, T.; Yoshimura, A.; Prasad, D. R.; Hoffman, M. Z. A Weak G Dependence of Back Electron Transfer within the Geminate Redox Pairs Formed in the Quenching of Excited Ruthenium(II) Complexes by Methyl Viologen. J. Phys. Chem. 1991, 95, 4723-4728. (56) Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N. Lifetimes, Spectra, and Quenching of the Excited States of Polypyridine Complexes of Iron(II), Ruthenium(II), and Osmium(II). J. Am. Chem. Soc. 1980, 102, 1309-1319. (57) Tessler, N.; Denton, G. J.; Friend, R. H. Lasing from Conjugated-Polymer Microcavities. Nature 1996, 382, 695-697. (58) Brown, A. R.; Pichler, K.; Greenham, N. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Optical Spectroscopy of Triplet Excitons and Charged Excitations in Poly(p-phenylenevinylene) Light-Emitting Diodes. Chem. Phys. Lett. 1993, 210, 61-66. (59) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Conductance Switching in Single Molecules through Conformational Changes. Science 2001, 292, 2303-2307. (60) Kohler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan, M. S.; Gerhard, A.; Bassler, H. The Singlet-Triplet Energy Gap in Organic and Pt-Containing Phenylene Ethynylene Polymers and Monomers. Journal of Chemical Physics 2002, 116, 9457-9463. (61) Bruce, M. I.; Davy, J.; Hall, B. C.; Van Galen, Y. J.; Skelton, B. W.; White, A. H. Some Platinum(II) Complexes Derived from Aromatic Alkynes. Appl. Organomet. Chem. 2002, 16, 559-568. (62) Szafert, S.; Gladysz, J. A. Carbon in One Dimension: Structural Analysis of the Higher Conjugated Polyynes. Chem. Rev. 2003, 103, 4175-4205. (63) Emmert, L. A.; Choi, W.; Marshall, J. A.; Yang, J.; Meyer, L. A.; Brozik, J. A. The Excited-State Symmetry Characteristics of Platinum Phenylacetylene Compounds. J. Phys. Chem. A 2003, 107, 11340-11346. (64) Yam, V. W.-W.; Kam-Wing Lo, K.; Man-Chung Wong, K. Luminescent Polynuclear Metal Acetylides. J. Organomet. Chem. 1999, 578, 3-30. 132

PAGE 133

(65) Nguyen, P.; Gomez-Elipe, P.; Manners, I. Organometallic Polymers with Transition Metals in the Main Chain. Chem. Rev. 1999, 99, 1515-1548. (66) Ley, K. D.; Schanze, K. S. Photophysics of Metal-Organic -Conjugated Polymers. Coord. Chem. Rev. 1998, 171, 287-307. (67) Slagle, J. E.; Cooper, T. M.; Krein, D. M.; Rogers, J. E.; McLean, D. G.; Urbas, A. M. Triplet Excimer Formation in a Platinum Acetylide. Chem. Phys. Lett. 2007, 447, 65-68. (68) Tam, A. Y.-Y.; Wong, K. M.-C.; Wang, G.; Yam, V. W.-W. Luminescent Metallogels of Platinum(II) Terpyridyl Complexes: Interplay of Metal-Metal, and Hydrophobic-Hydrophobic Interactions on Gel Formation. Chem. Commun. 2007, 2028-2030. (69) Liu, Y.; Jiang, S.; Glusac, K.; Powell, D. H.; Anderson, D. F.; Schanze, K. S. Photophysics of Monodisperse Platinum-Acetylide Oligomers: Delocalization in the Singlet and Triplet Excited States. J. Am. Chem. Soc. 2002, 124, 12412-12413. (70) Khan, M. S.; Al-Mandhary, M. R. A.; Al-Suti, M. K.; Al-Battashi, F. R.; Al-Saadi, S.; Ahrens, B.; Bjernemose, J. K.; Mahon, M. F.; Raithby, P. R.; Younus, M.; Chawdhury, N.; Khler, A.; Marseglia, E. A.; Tedesco, E.; Feeder, N.; Teat, S. J. Synthesis, Characterization and Optical Spectroscopy of Platinum(II) Di-ynes and Poly-ynes Incorporating Condensed Aromatic Spacers in the Backbone. Dalton Trans. 2004, 2377-2385. (71) Onitsuka, K.; Takahashi, S. Metallodendrimers Composed of Organometallic Building Blocks. Top. Curr. Chem. 2003, 228, 39-63. (72) Beljonne, D.; Wittmann, H. F.; Khler, A.; Graham, S.; Younus, M.; Lewis, J.; Raithby, P. R.; Khan, M. S.; Friend, R. H.; Bredas, J. L. Spatial Extent of the Singlet and Triplet Excitons in Transition Metal-Containing Poly-ynes. J. Chem. Phys. 1996, 105, 3868-3877. (73) Haskins-Glusac, K.; Ghiviriga, I.; Abboud, K. A.; Schanze, K. S. Photophysics and Photochemistry of Stilbene-Containing Platinum Acetylides. J. Phys. Chem. B 2004, 108, 4969-4978. (74) Cooper, T. M.; Hall, B. C.; McLean, D. G.; Rogers, J. E.; Burke, A. R.; Turnbull, K.; Weisner, A.; Fratini, A.; Liu, Y.; Schanze, K. S. Structure-Optical Property Relationships in Organometallic Sydnones. J. Phys. Chem. A 2005, 109, 999-1007. (75) Cooper, T. M.; Krein, D. M.; Burke, A. R.; McLean, D. G.; Rogers, J. E.; Slagle, J. E. Asymmetry in Platinum Acetylide Complexes: Confinement of the Triplet Exciton to the Lowest Energy Ligand. J. Phys. Chem. A 2006, 110, 13370-13378. 133

PAGE 134

(76) Schanze, K. S.; Silverman, E. E.; Zhao, X. Intrachain Triplet Energy Transfer in Platinum-Acetylide Copolymers. J. Phys. Chem. B 2005, 109, 18451-18459. (77) Jones, E. R. H.; Lee, H. H.; Whiting, M. C. Acetylenic Compounds. LXIV. The Preparation of Conjugated Octaand Decaacetylenic Compounds. J. Chem. Soc. 1960, 3483-3489. (78) Johnson, T. R.; Walton, D. R. M. Silylation as a Protective Method in Acetylene Chemistry. Polyyne Chain Extensions Using 1-(Triethylsilyl)acetylene, -butadiyne and -octatetrayne in Mixed Oxidative Couplings. Tetrahedron 1972, 28, 5221-5236. (79) Eastmond, R.; Johnson, T. R.; Walton, D. R. M. Silylation as a Protective Method for Terminal Alkynes in Oxidative Couplings. General Synthesis of the Parent Polyynes H(C = C) n H (n = 4-10, 12). Tetrahedron 1972, 28, 4601-4616. (80) Schermann, G.; Grosser, T.; Hampel, F.; Hirsch, A. Dicyanopolyynes: A Homologous Series of End-Capped Linear sp Carbon. Chem.-Eur. J. 1997, 3, 1105-1112. (81) Gibtner, T.; Hampel, F.; Gisselbrecht, J.-P.; Hirsch, A. End-Cap Stabilized Oligoynes: Model Compounds for the Linear sp Carbon Allotrope Carbyne. Chem.-Eur. J. 2002, 8, 408-432. (82) Klinger, C.; Vostrowsky, O.; Hirsch, A. Synthesis of Alkylene-Bridged Diphenyl-Oligoynes. Eur. J. Org. Chem. 2006, 1508-1524. (83) Eisler, S.; Slepkov, A. D.; Elliott, E.; Luu, T.; McDonald, R.; Hegmann, F. A.; Tykwinski, R. R. Polyynes as a Model for Carbyne: Synthesis, Physical Properties, and Nonlinear Optical Response. J. Am. Chem. Soc. 2005, 127, 2666-2676. (84) Luu, T.; Elliott, E.; Slepkov, A. D.; Eisler, S.; McDonald, R.; Hegmann, F. A.; Tykwinski, R. R. Synthesis, Structure, and Nonlinear Optical Properties of Diarylpolyynes. Org. Lett. 2005, 7, 51-54. (85) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Synthesis, Structure, and Reactivity of sp Carbon Chains with Bis(phosphine) pentafluorophenylplatinum Endgroups: Butadiynediyl (C4) through Hexadecaoctaynediyl (C16) Bridges, and Beyond. Chem.-Eur. J. 2003, 9, 3324-3340. (86) Zheng, Q.; Bohling, J. C.; Peters, T. B.; Frisch, A. C.; Hampel, F.; Gladysz, J. A. A Synthetic Breakthrough into an Unanticipated Stability Regime: A Series of Isolable Complexes in Which C6, C8, C10, C12, C16, C20, C24, and C28 Polyynediyl Chains Span Two Platinum Atoms. Chem.-Eur. J. 2006, 12, 6486-6505. (87) Paul, F.; Lapinte, C. In Unusual Structures and Physical Properties in Organometallic Chemistry; Geilen, M., Willem, R., Wrackmyer, B., Eds.; Wiley: New York, 2002, pp 220-291. 134

PAGE 135

(88) Bruce, M. I.; Low, P. J. Transition Metal Complexes Containing All-Carbon Ligands. Adv. Organomet. Chem. 2004, 50, 179-444. (89) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. Charge Transfer on the Nanoscale: Current Status. J. Phys. Chem. B 2003, 107, 6668-6697. (90) Zhuravlev, F.; Gladysz, J. A. Electronic Structure and Chain-Length Effects in Diplatinum Polyynediyl Complexes trans,trans-[(X)(R 3 P) 2 Pt(C.tplbond.C) n Pt(PR 3 ) 2 (X)]: A Computational Investigation. Chem.-Eur. J. 2004, 10, 6510-6522. (91) Yang, S.; Kertesz, M. Bond Length Alternation and Energy Band Gap of Polyyne. J. Phys. Chem. A 2006, 110, 9771-9774. (92) Yang, S.; Kertesz, M.; Zolyomi, V.; Kuerti, J. Application of a Novel Linear/Exponential Hybrid Force Field Scaling Scheme to the Longitudinal Raman Active Mode of Polyyne. J. Phys. Chem. A 2007, 111, 2434-2441. (93) Nagano, Y.; Ikoma, T.; Akiyama, K.; Tero-Kubota, S. Symmetry Switching of the Fluorescent Excited State in ,-Diphenylpolyynes. J. Am. Chem. Soc. 2003, 125, 14103-14112. (94) Nagano, Y.; Ikoma, T.; Akiyama, K.; Tero-Kubota, S. Electronic Structures and Dynamics of the Excited Triplet States of ,-Diphenylpolyynes. J. Chem. Phys. 2001, 114, 1775-1784. (95) Che, C.-M.; Chao, H.-Y.; Miskowski, V. M.; Li, Y.; Cheung, K.-K. Luminescent -Ethynediyl and -Butadiynediyl Binuclear Gold(I) Complexes: Observation of 3 ( ) Emissions from Bridging C n 2Units. J. Am. Chem. Soc. 2001, 123, 4985-4991. (96) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Luminescent Platinum(II) Terpyridyl-Capped Carbon-Rich Molecular Rods-an Extension from Molecularto Nanometer-Scale Dimensions. Angew. Chem. Int. Ed. Engl. 2003, 42, 1400-1403. (97) Caspar, J. V. Excited State Decay Processes in Osmium(II), Ruthenium(II) and Rhenium(I) Polypyridyl Complexes. Ph.D. Dissertation, University of North Carolina, Chapel Hill, NC, 1982. (98) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Photophysics of Diimine Platinum(II) Bis-Acetylide Complexes. Inorg. Chem. 2001, 40, 4053-4062. (99) Wang, Y.; Schanze, K. S. Photochemical Probes of Intramolecular Electron and Energy Transfer. Chem. Phys. 1993, 176, 305-319. 135

PAGE 136

(100) Maddux, T.; Li, W.; Yu, L. Stepwise Synthesis of Substituted Oligo(phenylenevinylene) via an Orthogonal Approach. J. Am. Chem. Soc. 1997, 119, 844-845. (101) de Melo, J. S.; Silva, L. M.; Arnaut, L. G.; Becker, R. S. Singlet and Triplet Energies of -Oligothiophenes: A Spectroscopic, Theoretical, and Photoacoustic Study: Extrapolation to Polythiophene. J. Chem. Phys. 1999, 111, 5427-5433. (102) Becker, R. S.; de Melo, J. S.; Macanita, A. L.; Elisei, F. Comprehensive Evaluation of the Absorption, Photophysical, Energy Transfer, Structural, and Theoretical Properties of -Oligothiophenes with One to Seven Rings. J. Phys. Chem. 1996, 100, 18683-18695. (103) Wasserberg, D.; Marsal, P.; Meskers, S. C. J.; Janssen, R. A. J.; Beljonne, D. Phosphorescence and Triplet State Energies of Oligothiophenes. J. Phys. Chem. B 2005, 109, 4410-4415. (104) Wasserberg, D.; Meskers, S. C. J.; Janssen, R. A. J.; Mena-Osteritz, E.; Baeuerle, P. High-Resolution Electronic Spectra of Ethylenedioxythiophene Oligomers. J. Am. Chem. Soc. 2006, 128, 17007-17017. (105) Martin, R. E.; Diederich, F. Linear Monodisperse -Conjugated Oligomers: Model Compounds for Polymers and More. Angew. Chem. Int. Ed. Engl. 1999, 38, 1351-1377. (106) Candeias, L. P.; Wildeman, J.; Hadziioannou, G.; Warman, J. M. Pulse Radiolysis-Optical Absorption Studies on the Triplet States of -Phenylenevinylene Oligomers in Solution. J. Phys. Chem. B 2000, 104, 8366-8371. (107) Wasserberg, D.; Dudek, S. P.; Meskers, S. C. J.; Janssen, R. A. J. Comparison of the Chain Length Dependence of the Singletand Triplet-Excited States of Oligofluorenes. Chem. Phys. Lett. 2005, 411, 273-277. (108) Robinson, G. W.; Frosch, R. P. Electronic Excitation Transfer and Relaxation. J. Chem. Phys. 1963, 38, 1187-1203. (109) Siebrand, W. Radiationless Transitions in Polyatomic Molecules. II. Triplet-Ground-State Transitions in Aromatic Hydrocarbons. J. Chem. Phys. 1967, 47, 2411-2422. (110) Caspar, J. V.; Meyer, T. J. Application of the Energy Gap Law to Nonradiative, Excited-State Decay. J. Phys. Chem. 1983, 87, 952-957. (111) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. Application of the Energy Gap Law to Excited-State Decay of Osmium(II)-Polypyridine Complexes: Calculation of Relative Nonradiative Decay Rates from Emission Spectral Profiles. J. Phys. Chem. 1986, 90, 3722-3734. 136

PAGE 137

(112) Barqawi, K. R.; Murtaza, Z.; Meyer, T. J. Calculation of Relative Nonradiative Decay Rate Constants from Emission Spectral Profiles: Polypyridyl Complexes of Ruthenium(II). J. Phys. Chem. 1991, 95, 47-50. (113) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus, M.; Khan, M. S.; Raithby, P. R.; Khler, A.; Friend, R. H. The Energy Gap Law for Triplet States in Pt-Containing Conjugated Polymers and Monomers. J. Am. Chem. Soc. 2001, 123, 9412-9417. (114) Ishibashi, T.-a.; Hamaguchi, H.-o. Structure and Dynamics of S2 and S1 Diphenylacetylene in Solution Studied by Picosecond Time-Resolved CARS Spectroscopy. J. Phys. Chem. A 1998, 102, 2263-2269. (115) Baur, J. W.; Alexander, M. D., Jr.; Banach, M.; Denny, L. R.; Reinhardt, B. A.; Vaia, R. A.; Fleitz, P. A.; Kirkpatrick, S. M. Molecular Environment Effects on Two-Photon-Absorbing Heterocyclic Chromophores. Chem. Mater. 1999, 11, 2899-2906. (116) Goppert-Mayer, M. Elementary File with Two Quantum Fissures. Ann. Phys. (Leipzig) 1931, 9, 273-294. (117) Kaiser, W.; Garrett, C. G. B. Two-Photon Excitation in CaF 2 : Eu 2+ Phys. Rev. Lett. 1961, 7, 229 LP 231. (118) Callis, P. R. Two-Photon-Induced Fluorescence. Annu. Rev. Phys. Chem. 1997, 48, 271-297. (119) Lackowicz, J. R. Principles of Fluorescence Spectroscopy; 2nd ed.; Kluwer Academic / Plenum Publishers: New York, 1999. (120) Boyle, A.; Blau, W. J. In Handbook of Advanced Electronic and Photonic Materials and Devices; Nalwa, H. S., Ed.; Academic Press: New York, 2001; Vol. 9. (121) Fakis, M.; Tsigaridas, G.; Polyzos, I.; Giannetas, V.; Persephonis, P.; Spiliopoulos, I.; Mikroyannidis, J. Intensity Dependent Nonlinear Absorption of Pyrylium Chromophores. Chem. Phys. Lett. 2001, 342, 155-161. (122) Lin, T.-C.; Chung, S.-J.; Kim, K.-S.; Wang, X.; He, G. S.; Swiatkiewicz, J.; Pudavar, H. E.; Prasad, P. N. Organics and Polymers with High Two-Photon Activities and Their Applications. Adv. Polym. Sci. 2003, 161, 157-193. (123) Ehrlich, J. E.; Wu, X. L.; Lee, I. Y. S.; Hu, Z. Y.; Rockel, H.; Marder, S. R.; Perry, J. W. Two-Photon Absorption and Broadband Optical Limiting with Bis-Donor Stilbenes. Opt. Lett. 1997, 22, 1843-1845. 137

PAGE 138

(124) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt, J. C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Highly Active Two-Photon Dyes: Design, Synthesis, and Characterization Toward Application. Chem. Mater. 1998, 10, 1863-1874. (125) Powell, C. E.; Morrall, J. P.; Ward, S. A.; Cifuentes, M. P.; Notaras, E. G. A.; Samoc, M.; Humphrey, M. G. Dispersion of the Third-Order Nonlinear Optical Properties of an Organometallic Dendrimer. J. Am. Chem. Soc. 2004, 126, 12234-12235. (126) Joshi, M. P.; Swiatkiewicz, J.; Xu, F.; Prasad, P. N.; Reinhardt, B. A.; Kannan, R. Energy Transfer Coupling of Two-Photon Absorption and Reverse Saturable Absorption for Enhanced Optical Power Limiting. Opt. Lett. 1998, 23, 1742-1744. (127) McIlroy, S. P.; Clo, E.; Nikolajsen, L.; Frederiksen, P. K.; Nielsen, C. B.; Mikkelsen, K. V.; Gothelf, K. V.; Ogilby, P. R. Two-Photon Photosensitized Production of Singlet Oxygen: Sensitizers with Phenylene-Ethynylene-Based Chromophores. J. Org. Chem. 2005, 70, 1134-1146. (128) Kannan, R.; He, G. S.; Lin, T.-C.; Prasad, P. N.; Vaia, R. A.; Tan, L.-S. Toward Highly Active Two-Photon Absorbing Liquids. Synthesis and Characterization of 1,3,5-Triazine-Based Octupolar Molecules. Chem. Mater. 2004, 16, 185-194. (129) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry: A Comprehensive Text; Wiley: New York, 1980. (130) Thorton, N. B. Chromophore Quenched Based Luminescence Probes for DNA. Ph.D. Dissertation, University of Florida, Gainesville, FL, 1995. (131) Glusac, K. D.; Jiang, S.; Schanze, K. S. Photophysics of Ir(III) Complexes with Oligo(arylene ethynylene) Ligands. Chem. Commun. 2002, 2504-2505. (132) Ichimura, K.; Kobayashi, T.; King, K. A.; Watts, R. J. Excited-State Absorption Spectroscopy of ortho-Metalated Iridium(III) Complexes. J. Phys. Chem. 1987, 91, 6104-6106. (133) Manas, E. S.; Chen, L. X. Electronic Interactions in Metal Complexed Photoconducting Polymers: A ZINDO Study. Chem. Phys. Lett. 2000, 331, 299-307. (134) Walters, K. A.; Ley, K. D.; Cavalaheiro, C. S. P.; Miller, S. E.; Gosztola, D.; Wasielewski, M. R.; Bussandri, A. P.; van Willigen, H.; Schanze, K. S. Photophysics of -Conjugated Metal-Organic Oligomers: Aryleneethynylenes that Contain the (bpy)Re(CO) 3 Cl Chromophore. J. Am. Chem. Soc. 2001, 123, 8329-8342. (135) Liu, S.; Schanze, K. S. Solvent Tuned Excited State Configuration Mixing in a -Conjugated Metal-Organic Oligomer. Chem. Commun. 2004, 1510-1511. 138

PAGE 139

(136) Stahl, J.; Bohling, J. C.; Bauer, E. B.; Peters, T. B.; Mohr, W.; Martin-Alvarez, J. M.; Hampel, F.; Gladysz, J. A. sp Carbon Chains Surrounded by sp 3 Carbon Double Helices: A Class of Molecules that are Accessible by Self-Assembly and Models for "Insulated" Molecular-Scale Devices. Angew. Chem. Int. Ed. Engl. 2002, 41, 1871-1876. (137) Owen, G. R.; Stahl, J.; Hampel, F.; Gladysz, J. A. Syntheses and Structures of Diplatinum Hexatriynediyl Complexes, in Which the sp Carbon Chains are Shielded by sp 3 Carbon Chains. Organomettalics 2004, 23, 5889-5892. (138) Owen, G. R.; Hampel, F.; Gladysz, J. A. Syntheses and Structures of Tetraplatinum Bis(polyynediyl) Complexes with Laterally Arrayed sp Carbon Chains. Organomettalics 2004, 23, 5893-5895. 139

PAGE 140

BIOGRAPHICAL SKETCH Richard T. Farley was born in Rochester, New York, and grew up in the suburbs of Orlando, Florida. He spent a lively childhood with his grandparents, parents and sister. Richard started his education in chemistry at the University of Central Florida in 1998. He took a strong interest in analytical and organic chemistry. It was not until he began his graduate studies at the University of Florida that Richard discovered his curiosity for the fundamental nature of physical chemistry. He took advantage of the opportunity to merge physical and instrumental work in a primarily organic atmosphere, working under the guidance of Prof. Kirk S. Schanze to pursue a doctoral degree in physical chemistry. 140