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Dual Mechanism Nonlinear Response of Selected Metal-Organic Chromophores

Permanent Link: http://ufdc.ufl.edu/UFE0021546/00001

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Title: Dual Mechanism Nonlinear Response of Selected Metal-Organic Chromophores
Physical Description: 1 online resource (145 p.)
Language: english
Creator: Peak, John Dale
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: dual, metal, nonlinear, optical, two
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

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Abstract: The dissertation presented here is meant to further the basic science and knowledge in the area of organic chemistry and laser photophysics. Development of new compounds and materials to aid in the modulation of laser energy can and will have dramatic enhancements in the safety and productivity of lasers. In conjunction with other research of this type, future applications of compounds capable of nonlinear transmission of laser energy have the potential to broaden the scope for the use of lasers in materials production, medicine and myriad of other applications.
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 John Dale Peak.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Schanze, Kirk S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-12-31

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Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021546:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021546/00001

Material Information

Title: Dual Mechanism Nonlinear Response of Selected Metal-Organic Chromophores
Physical Description: 1 online resource (145 p.)
Language: english
Creator: Peak, John Dale
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: dual, metal, nonlinear, optical, two
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: The dissertation presented here is meant to further the basic science and knowledge in the area of organic chemistry and laser photophysics. Development of new compounds and materials to aid in the modulation of laser energy can and will have dramatic enhancements in the safety and productivity of lasers. In conjunction with other research of this type, future applications of compounds capable of nonlinear transmission of laser energy have the potential to broaden the scope for the use of lasers in materials production, medicine and myriad of other applications.
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 John Dale Peak.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Schanze, Kirk S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021546:00001


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1 DUAL MECHANISM NONLINEAR RESPONSE OF SELECTED METAL ORGANIC CHROMOPHORES By JOHN D. PEAK A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGRE E OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007

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2 Copyright 2007 by John D. Peak

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3 The views expressed in this dissertation are those of the author and do not reflect the official policy or position of the United States Air Force, Departmen t of Defense, or the U.S. Government.

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4 This dissertation is dedicated to Cameron and Kelsey for their unconditional love and support

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5 ACKNOWLEDGMENTS First, I would like to a cknowledg e Cameron and Kelsey I am extremely blessed to have two such wonderful children. Being apart from you for so long while completing this part of my education was not easy. Both of you have grown fast and have matured well beyond your years. rstand what I do or why I do it, but you have always freely given your prayers and provided moral support for my success. It is for that faith in me that I thank you. You always said I could be anything I wanted to be and I have truly appreciated that fr eedom of choice. I promise that you will be the first to know when I figure out what I want to be. A special thanks to Dr. Sophie Klein for her friendship and assistance in the preparation of this dissertation, without your help, understanding and excelle nt salads I never would have been able to finish. Lastly, I cannot thank Dr. Kirk Schanze enough for his mentorship throughout this project. It was refreshing to work for and with a professional of such high caliber. Without your guidance and extreme p atience, I never would have been able to complete such a demanding task

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ............... 5 LIST OF TABLES ................................ ................................ ................................ ........................... 9 LIST OF FIGURES ................................ ................................ ................................ ....................... 10 ABSTRACT ................................ ................................ ................................ ................................ ... 13 CHAPTER 1 PHOTOPHYSICAL BACKGROUND ................................ ................................ .................. 14 Introduction ................................ ................................ ................................ ............................. 14 Basic Photophysical Properties of Ruthenium Polypyridine Complexes ............................... 14 Electronically Excited States of Ru(bpy) 3 2+ ................................ ................................ ... 15 Photophysical Effects of Substitution in Metal Polypyridine Complexes ...................... 17 Non linear Optical Mechanisms ................................ ................................ .............................. 23 Two Photon Absorption ................................ ................................ ................................ .. 23 Excited State Absorption ................................ ................................ ................................ 26 Project Objective ................................ ................................ ................................ ............. 28 2 DETERMINATION OF NONLINEAR RESPONSE ................................ ............................ 31 Introduction ................................ ................................ ................................ ............................. 31 Linear Absorption ................................ ................................ ................................ ............ 31 Nonlinear Optical Behavior ................................ ................................ ............................. 32 Second order NLO processes. ................................ ................................ .................. 35 Third order NLO Processes ................................ ................................ ..................... 37 Optical Limiting ................................ ................................ ................................ ...................... 39 Background ................................ ................................ ................................ ...................... 39 Two Photon Absorption ................................ ................................ ................................ .. 40 Excited State Absorption and Dual Mode Limiting ................................ ........................ 43 Exp erimental Determination of Nonlinear Response ................................ ............................. 47 Z scan ................................ ................................ ................................ .............................. 47 Nonlinear Transmission ................................ ................................ ................................ ... 49 Nonlinear Transmission Test Case ................................ ................................ ......................... 51 Test Objective ................................ ................................ ................................ .................. 51 Apparatus Setup ................................ ................................ ................................ ............... 52 NLO Chromophore Test Series ................................ ................................ ....................... 54 Photophysical Properties ................................ ................................ ................................ 56 Ground State Absorption ................................ ................................ .......................... 56 Transient Absorption ................................ ................................ ................................ 56 Nonlinear Absorbance Determination ................................ ................................ ............. 59 Test Results ................................ ................................ ................................ ..................... 63

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7 Instrumentation ................................ ................................ ................................ ................ 65 3 SYNTHESIS AND PHOTOPHYSICS OF NLO CHROMOPHORES ................................ 66 Introduction ................................ ................................ ................................ ............................. 66 Synthesis ................................ ................................ ................................ ................................ 66 Organic Chromophore Synthesis ................................ ................................ ..................... 66 Metal organic complex synthesis ................................ ................................ .................... 69 Results and Discussion ................................ ................................ ................................ ........... 70 Photophysical Properties ................................ ................................ ................................ 70 UV Visible Absorption Spectroscopy ................................ ................................ ...... 70 Emission Spectra ................................ ................................ ................................ ...... 74 Transient Absorption ................................ ................................ ................................ 77 Nonlinear Absorption Determination ................................ ................................ .............. 81 Two Photon Emission ................................ ................................ .............................. 81 Nonlinear Absorpt ion of C 60 ................................ ................................ .................... 86 Nonlinear Absorbance Determination ................................ ................................ ...... 89 Discussion ................................ ................................ ................................ ...................... 102 Experimental ................................ ................................ ................................ ......................... 105 Instrumentation ................................ ................................ ................................ .............. 105 Materials ................................ ................................ ................................ ........................ 107 Synthesis ................................ ................................ ................................ ........................ 107 Protonated 2,2' bipyridine (1) ................................ ................................ ................ 107 5,5' Dibromo 2,2' bipyridine (2) ................................ ................................ ............ 107 Bis 5,5' trimethylsilylethynyl 2,2' bipyridine ................................ ........................ 108 5,5' Diethynyl 2,2' bipyridine ................................ ................................ ................ 109 4 Bromo N,N diphenylaniline (3 )77 ................................ ................................ ..... 109 4 (4 (Diphenylamino)phenyl) 2 methyl 3 butyn 2 ol (4)77 ................................ 110 4 Ethynyl N,N diphenylaniline (5)77 ................................ ................................ .... 111 4,4' (2,2' bipyridine 5,5' diylbis(ethyne 2,1 diyl))bis(N,N diphenylaniline) (TPA 1) (6) ................................ ................................ ................................ ......... 111 Ru(TPA 1)(bpy) 2 2+ 2PF 6 (Ru 1) (7) ................................ ................................ .. 112 Re(TPA 1)(CO) 3 Cl (Re 1) (8) ................................ ................................ .............. 113 Ir 2 (ppy) 4 Cl 2 (9) ................................ ................................ ................................ ....... 113 Ir(TPA 1)(ppy) 2 + PF 6 (I r 1) (10) ................................ ................................ ........ 114 4 CONCLUSION ................................ ................................ ................................ ..................... 116 APPENDIX A 1 H AND 13 C SPECTRA ................................ ................................ ................................ ........ 120 B NONLINEAR TRANSMISSION MANUAL ................................ ................................ ...... 124 Safety Notes ................................ ................................ ................................ .......................... 125 Sample Preparation ................................ ................................ ................................ ............... 125 Laser Table Preparation ................................ ................................ ................................ ........ 125

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8 Laser Wavelength Selection and Alignment ................................ ................................ ........ 126 Quanta Ray ................................ ................................ ................................ .................... 126 Surelite II/OPO ................................ ................................ ................................ .............. 127 Nonlinear transmission setup and alignment ................................ ................................ ........ 128 Setup ................................ ................................ ................................ .............................. 129 Alignment ................................ ................................ ................................ ...................... 130 Energy meter setup ................................ ................................ ................................ ........ 131 Performing the nonlinear experiment ................................ ................................ ................... 132 Startup ................................ ................................ ................................ ............................ 132 Collecting data ................................ ................................ ................................ ............... 133 Shut down ................................ ................................ ................................ ...................... 135 Plotting data ................................ ................................ ................................ ................... 135 LIST OF REFERENCES ................................ ................................ ................................ ............. 138 BIOGRAPHICAL SKETCH ................................ ................................ ................................ ....... 145

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9 LIST OF TABLES Table page 3 1 Near UV visible absorption bands of target ligand and metal organic complexes. .......... 73 3 2 Photophysical properties of target ligand and metal organic complexes. ......................... 75 3 3 Optical limiting properties of C 60 ................................ ................................ ..................... 87

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10 LIST OF FIGURES Figure page 1 1 Simplified molecular orbital diagram for a d 6 metal complex ................................ .......... 15 1 2 UV visible absorption and emission spectra of tris bipyridyl) ruthenium(II) ......... 16 1 3 Ruthenium(II) ............... 19 1 4 5 ................................ .. 20 1 5 Near UV/visible/near infrared transient absorption spectra of metal complexes ............. 21 1 6 Jablonski diagram of relative energies for excited states of in a free ligand and metallated ligand, and MLCT excited states of Re, Ru and Ir complexes. ........................ 22 1 7 Jablonski diagram visualizing two photon absorption. ................................ ..................... 23 1 8 General chromophore structures which exhibit high TPA cross sections. ........................ 25 1 9 High cross section TPA chromophore AF 455. 24 ................................ .............................. 26 1 10 Jablonski diagram of a dual mode nonlinear absorption (TPA/ESA) mechanism. ........... 27 1 11 Bipyridine centered target two photon absorbing chromophore. ................................ ....... 28 1 12 Target metal centered dual mode nonlinear absorbing chromophores. ............................. 30 2 1 ................................ ................................ .. 31 2 2 Structure of the typical dipolar model compound 4 ( N N dimethylamino) nitrostilbene. ................................ ................................ ................................ ....................... 36 2 3 Structure of an enhanced donor acceptor chromophore. ................................ ................ 37 2 4 Structures of diphenylbenzobisoxazole (PB O) and diphenylbenzobisthiazol (PBT). ....... 38 2 5 Chromophore structures for enhanced two photon absorption (Reinhart). ........................ 41 2 6 Struct ures of quad u polar and oct u polar chromophores with large two photon absorption cross sections. ................................ ................................ ................................ ... 42 2 7 Five level energy diagram for two photon induced excited state absorption model. ........ 43 2 8 Illustration of transmission loss difference by TPA and by TPA/ESA in AF 455 ........... 46 2 9 Comparison of theoretical vers u s experimental nanosecond nonlinear transmittance as a function of laser pulse energy for AF 455 at 800 nm. ................................ ................ 46

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11 2 10 Generalized illustration of a Z scan experimental setup. ................................ ................... 47 2 11 Z scan curves of C 60 in toluene at 1064 nm. a) closed aperture b) open aperture. ........... 48 2 12 Schematic illustration of the nonlinear transmission (NLT) expe rimental setup. ............. 49 2 13 Example NLT curve plot of input energy vers u s output energy depicting a clamping state energy. ................................ ................................ ................................ ....................... 50 2 14 E xample NLT curve plot depicting input energy vers u s transmittance. ............................ 51 2 15 Photo schematic of OPO based nonlinear transmission apparatus. ................................ ... 53 2 16 Platinum acetylide dimers, Pt2 Ar utilized for the NLT test study. ................................ 55 2 18 Transient absorption spectra of Pt2 Ar series in deoxygenated THF solution. ................ 58 2 19 Nonlinear determination results of Pt2 Ar series in 60 mM THF solutions. .................... 61 2 20 Nonlinear determination results of Pt2 Ar series in 20 mM THF solutions. .................... 62 2 21 Nonlinear transmission curve of the Pt2 Ar series in 20 mM benzene solutions. ............ 63 3 1 Synthesis of TPA 1 chro mophore ligand central core. ................................ ..................... 67 3 2 Synthesis of chromophore ligand end caps. ................................ ................................ ....... 68 3 3 Synthesis of TPA chromophore. ................................ ................................ ........................ 69 3 4 Metallation of TPA chromophore. ................................ ................................ ..................... 70 3 5 UV visible absorption and emissions spectra of TPA 1 Re 1 Ru 1 and Ir 1 ............... 72 3 6 Transient absorption spectra of metal organic complexes. ................................ ................ 78 3 7 Excited state lifetimes of metal organic complexes. ................................ ......................... 80 3 8 Two photon emission instrument modification ................................ ................................ 82 3 9 Two Photon Emission Spectra of TPA 1 ................................ ................................ ......... 83 3 10 Two photon emission spectra of metal complexes. ................................ ........................... 84 3 11 Power dependence, two photon generated optical limiting determination of C 60 in toluene at 1064 nm. ................................ ................................ ................................ ............ 88 3 12 Nonlinear determination of TPA 1 ................................ ................................ ................... 90

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12 3 13 Ground state absorption spectra for the near IR of 2.0 mM metal organic complex solutions in CH 2 Cl 2 ................................ ................................ ................................ ........... 92 3 14 Nonlinear determination results of metal organic chromophores at 970 nm. ................... 94 3 15 Nonlinear transmission results of me tal organic chromophores at 970 nm. ..................... 95 3 16 Photo schematic of modified nonlinear transmission setup. ................................ .............. 97 3 17 Schematic drawing of the modified nonlinear transmission setup. ................................ ... 97 3 18 Nonlinear determination results of metal organic chromophores at 1064 nm. ................. 98 3 19 Nonlinear transmission of metal organic chromophores at 1064 nm. ............................. 100 3 20 Nonlinear transmission for 10 and 20 mM, CH 2 Cl 2 solutions of Ir 1 at 1064 nm. ......... 10 2 3 21 Nonlinear determination for 10 and 20 mM CH 2 CL 2 solutions of Ir 1 at 1064 nm. ...... 103 A 1 1 H NMR spectrum of TPA 1 in CDCl3. ................................ ................................ ......... 120 A 2 1 H NMR spectrum of Re 1 in CD 2 Cl 2 ................................ ................................ ............ 120 A 3 1 H NMR spectrum of Ru 1 in CD 2 Cl 2 ................................ ................................ ............ 121 A 4 1 H NMR spectrum of Ir 1 in CD 2 Cl 2 ................................ ................................ ............. 121 A 5 13 C NMR spectrum of TPA 1 in CDCl 3 ................................ ................................ ......... 122 A 6 13 C NMR spectrum of Ru 1 in (CD 3 ) 2 CO. ................................ ................................ ...... 122 A 7 13 C NMR spectrum of Ir 1 in CD 2 Cl 2 ................................ ................................ ............ 123 B 1 Control rod configuration for wavelength selection (Quanta Ray). ................................ 126 B 2 Beam pickoff configuration for Quanta Ray. ................................ ................................ .. 127 B 3 Dual prism alignment for Surelite/OPO output. ................................ .............................. 128 B 4 General schematic diagram of nonlinear transmission setup. ................................ .......... 130 B 5 LaserStar energy meter. ................................ ................................ ................................ ... 131 B 6 Example of data acquired from a CH 2 Cl 2 solvent only sample. ................................ ...... 134 B 7 Example data for a sample exhibiting a nonlinear response. ................................ ........... 134 B 9 Sample graph of nonlinear transmission data. ................................ ................................ 137

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13 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the De gree of Doctor of Philosophy DUAL MECHANISM NONLINEAR RESPONSE OF SELECTED METAL ORGANIC CHROMOPHORES By John D. Peak December 2007 Chair: Kirk S. Schanze Major: Chemistry The goal for the research described herein is the development of a series of trans ition metal based metal organic chromophores that display both two photon and excited state absorption (TPA/ESA) character. With this goal in mind, we present the preparation and photophysical characterization for a series of metal organic chromophores co ntaining a two photon absorbing bipyridine core combined with a transition metal component which yields a long lived triplet excited state. The combination of these two photophysical properties represents a dual mode nonlinear optical (NLO) mechanism. Thre e major areas of interest for this project are addressed here. First, to develop and instrument an in house photophysical apparatus with the ability to evaluate and measure two photon activity as well as detect nonlinear optical responses. Second, to syn thesize, characterize and evaluate an all organic chromophore system, centered on a bipyridine core, which utilizes known TPA architecture. Lastly, to synthesize, characterize and evaluate the metal organic analogs of the TPA chromophore system utilizing t ransition metals with high spin orbit coupling values which help create long lived triplet excited states leading to a possible ESA. The metal organic analogs in turn should exhibit a dual mechanism for NLO response comprised of both TPA and ESA.

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14 CHAPTER 1 PHOTOPHYSICAL BACKGR OUND Introduction The major focus of this work is the synthesis and photophysical investigation of a series of transition metal based metal organic chromophores with the potential to exhibit a dual mode nonlinear optical response mec hanism. A system utilizing a dual mode nonlinear response will ideally incorporate the advantages of two separate nonlinear pathways while diminishing the drawbacks of each. A recent publication from our group presented this dual mechanism nonlinear abs orption for a transition metal centered metal organic complex and can be thought of as a proof of concept. 1 The nonlinear absorbing pathways utilized to obtain the dual mechanism nonlinear response are the same as those utilized herein, those being two photon absorption and reverse saturable absorption. As background for the work presented in this dissertation, the following subjects are reviewed in this chapter: (1) basic photophysical properties of rut henium polypyridine complexes; (2) the photophysical effects for substitution of bipyridine in metal polypyridine complexes; (3) basic nonlinear optical processes; (4) structure property relationships of nonlinear absorption as it relates to two photon abs orption and reverse saturable absorption. Basic Photophysical Properties of Ruthenium Polypyridine Complexes Ruthenium(II) polypyridine complexes are molecules in which ruthenium is in a d 6 electron configuration and is complexed with three neutral biden tate bipyridine ligands and configured in an octahedral symmetry. The simplest and most widely studied compound of this category is the complex Ru(bpy) 3 2+ bipyridine) 2 7 Discussion of the basic photophysical properties of Ru(bpy) 3 2+ is deemed beneficial in understanding other ruthenium polypyridine complexes as well as systems with similar electronic and structural configuration s

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15 These common features can be exploited in predicting and interpreting excited state photophysical properties across the transition metal family for the metal polypyridine architecture. Keeping in mind the goal of exploring the nonlinear nat ure of a select series of metal orga nic complexes, a general understanding the nature of the excited state manifold of th e s e system s is desirable. Electronically Excited States of Ru(bpy) 3 2+ Examination of the ground and excited states of Ru(bpy) 3 2+ primarily concerns the involvement of the bonding and antibonding orbitals centered on the bipyridine ligands and the 4d orbitals of the ruthenium metal center. For ruthenium, the d ( t 2g ) and d ( e g ) levels of the 4d orbitals are of particular interest. When in the ground state, only the b ipyridine bonding and ruthenium d are filled. A representative molecular orbital diagram is shown in Figure 1 1 for a typical d 6 polypyridine complex. Figure 1 1. Simplified molecular orbital diagram for a d 6 metal compl ex of octahedral symmetry with representative energy transitions from a ground state to potential excited states. d 1 d 2 d 3 1 d 1 d 2 2 1 (GS) t 2g d 6 (d ) 5 ( 1 ) 1 e g (d ) 5 (d ) 1 1,3 (d,d) MLCT dd 1,3 (MLCT) 1,3 ( *)

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16 Excited states for a system of these types may be manifested in three distinctly separate ways: (1) a metal centered d d ligand field transit ion; (2) a relatively high energy intraligand transition; and (3) a metal to ligand charge transfer (MLCT) configuration from a d to transition. The first type of transition, the metal centered d d ligand field transition, is a weak ( 1 cm 1 ) Laporte forbidden absorption. This symm etry forbidden ( t e ) transition would lead to a short lived excited state that would be present in a low energy region and should not be a prominent feature to our discussion. The relatively high energy ligand centered transition is obtained throug h the promotion of an electron from a orbital to a orbital which is localized on a single bipyridine ligand. 8 The energies of these transi tions vary only slightly as the metal and its oxidation state are changed and occur around 300 nm ( 1 *) and 240 nm ( 2 *) with very large molar absorptivity values. The final type of excited state involves the promotion of an electron from a d ( t 2g ) metal orbital to a ligand centered antibonding orbital Figu re 1 2 UV visible absorption and emission spectra of t ris (2,2 bipyridyl) ruthenium(II) in water ( ex 450 nm). 10

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17 and is considered a symmetry allowed MLCT transition with moderate molar absorptivity ( 15,000 M 1 cm 1 ). 9 For Ru(bpy) 3 2+ the excited states that have been shown to occur are and MLCT in nature. In the UV visible spectrum o f Ru(bpy) 3 2+ a relatively weak absorption above 500 nm is observed as well as a broad emission centered around 600 nm. (Figure 1 2) Both features have been clearly assigned to a spin forbidden charge transfer ( 3 MLCT) excited state. 3, 11 14 3 MLCT character is attributed to the interactions of the d ( t 2g ) orbital of the metal and the orbital of the ligand. These orbital interactions combined with a larg e spin orbit coupling of ruthenium lead to efficient singlet to triplet state conversion. Broadness in the emission spectrum is attributed to the emission emanating from a manifold of spin orbit coupled triplet states rather than a single triplet state. Excitation of Ru(bpy) 3 2+ to an allowed singlet excited state is quickly fo llowed by efficient intersystem crossing to a triplet excited state (less than 300 fs) with a lifetime of approximately 600 ns 15 In th is complex and systems like it th e intersystem crossing yield s, brought about by spin orbit coupling are very high, often assumed to be unity. Photophysical Effects of Substitution in Metal Polypyridine Complexes Photophysical investigation of metal containing polypyridine complexes as well as their non Wasielewski, Yellowless et al ., as well as the Schanze group. 8, 16, 17 Detailed studies of the photophysical effects of substitution of the bipyridine backbone as well as the effects generated fr om the change of the coordinating metal in these polypyridine systems have elucidated much about the complexity of their excited state manifolds. These studies have incorporated the tris (2,2' bipyridyl) metal moiety into conjugated organic molecules and polymers and have yielded compounds with varied photophysical characteristics. Among the key methods in determining

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18 the substitution effects that have been investigated are absorption and emission spectra which elucidates changes in ground state electroni c nature as well as their transient absorption spectrum which are utilized to reveal the electronic nature of the excited states. A key detail of the work that has been performed by previous group members is the determination of the nature of the interac tion between conjugated bipyridine systems and MLCT chromophores with regard to the photophysics of substituted systems. 18, 19 The main focus of several of the studies was the equilibrium between the 3 MLCT generated by d charge transfer of the metal to ligand and the ligand centered 3 states. Through investigations from our group it was found that addition of electron withdrawing substituent groups to bipyridine could sufficiently influence the LU MO energies of the ligand centered orbitals and provide the possibility for the reduction of MLCT excited state energy relative to the 3 state. The basis for this reasoning is that the excited electron would be promoted to the lowest energy electro n acceptor and, if bipyridine is provided with a strong enough electron withdrawing groups, in the proper position, energy transfer would tend toward the lower energy MLCT excited state. It has also been found that the MLCT energy can be lowered by modify ing the position of the substituent on the bipyridine ring 20 as well as changing the coordinated metal chromophore. 9 A comprehensive photophysical study of substituent position on the bipyridine ligand core with regard to excited state energies has been conducted by our group which revealed a significant positional impact on both electron delocalization and MLCT excited state energy. 20 For the ruthenium(II) bipyridine system containing identical phenyleneethynylene (PE) substituents containing electron withdrawing groups (Figure 1 several major findings were noted. First, the lowest excited state transition was definitely based

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19 on a d metal to ligand charge transfer. Second, evidence supported the conclusion that the exc ited electron in the MLCT state was significantly Figure 1 3. Ruthenium(II) M =Ru(bpy) 2 2+ counterion = PF 6 delocalized into the PE substituent. Furtherm ore, data supported the counterintuitive conclusion substituted positions of the bipyridine. substituted system the energy of the LUMO level was lower, which would indicate a larger intraligand delocalization. This conclusion would seem to follow the logical consequence of a conjugation enhancing para substituted architecture. substitute d complex was found substituted and parent Ru(bpy) 3 2+ systems. This substituted system it is important to note that the overall transition dipole is not expected to be enhanced by conjugation into the PE substituent since the axis of conjugation is perpendicular to that of the substit uted system is a key factor for its choice as a potential third order nonlinear material and will be discussed in subsequent sections. Overall, the definitive conclusion was reached by molecular orbital calculations which implied the extent of delocalizati on in the MLCT state was directly related to efficiency of the overlap of the low energy orbital localized on the ligand and the d

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20 orbitals of the metal. This overlap was strongly dependent on the size of the orbital coefficients of the nitrogen in the bipyridine which were in turn affected by the pattern of substitution of the bipyridi ne units. In separate follow on studies, the influence of altering the coordinating metal center on the nature of the excited state was examined. 21, 22 Electron donatin g (bis)dimethoxy substituted PE 5, 2) were independently studied to examine the nature of the excited state manifolds utilizing a series of transition metals which included Ru, Re and Ir. It was determined the 3 and 3 MLCT levels were very close in energy and the nature of the metal center did have a slight effect on which triplet state has the lowest energy. However, in all Figure 1 4. 5 M =Ru(bpy) 2 2+ R eCO 3 Cl, Ir(ppy) 2 + counterion = PF 6 (where applicable). cases moderately increasing the conjugation length of the associated ligand served to effectively lower the 3 excited state energy to a level at or below the excited state energy of the 3 MLCT exci ted state. Overall, it was noted that the excited state nature was photophysically very similar regardless of whether the lowest excited state energy was 3 or 3 MLCT. Key differences, as noted in these studies, in the determination of the lowest excit ed state being either 3 or 3 MLCT are: (1) the presence, placement and strength of a transient absorption (TA) band in the visible region around 500 520 nm associated with 3 MLCT and (2) the presence and magnitude of a broad transient absorption in the n ear IR in the region beyond 700 nm associated with 3 *. In the case of the three metal complexes of the ligand shown in Figure 1 4 a strong and relatively narrow TA band was present in the region of 500 520 nm and a significantly weaker

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21 Figure 1 5. Near UV/ visible/near infrared transient absorption spectra of metal complexes of oligo(arylene ethynylene) ligands 2 and 3 L eft : metal complex is Re(CO) 3 Cl; right : metal complex is Ru(bpy) 2 2+ excited state absorbance was obse rved beyond 700 nm in the near IR region. In Figure 1 5, conjugation extended forms of the ligand bound to Re (left) exhibit TA spectra with prominent near IR absorption and a less pronounced visible absorption. In contrast, for the same ligands bound to Ru (right) exhibit prominent visible absorbance which is narrower and a near IR absorption that is blue shifted and much weaker. These complexes were selected because in previous work it has been demonstrated that when M = Re(CO) 3 Cl the lowest excited s tate is OAE based 3 *, whereas when M = Ru(bpy) 2 2+ the lowest state is MLCT. 23 These experiments show quite clearly that, at least for the family of bipyridine substituted oligo(arylene 2 2 3 3 M = Re(CO) 3 Cl M = Ru(bpy) 2 2+ 2 3

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22 ethynylene)s, the 3 state can be identified on the basis of the strong near infrared transient absorption. Also in these same studies, when conjugation was extended there was a lowering in the 3 excited state energy. The energy decrease was enough to bring it on par wit h that of the 3 MLCT excited state energy. The conclusion reached was that the triplet excited state absorptions for the systems and others of similar architecture could be attributed to a triplet manifold where both 3 and 3 MLCT excited states are in e nergetic equilibrium (Figure 1 6). Figure 1 6 Jablonski diagram of relative energies for excited states of in a free ligand and metallated ligand, and MLCT excited states of Re, Ru and Ir complexes. Even though the under standing of the intricacies of this equilibrium is not critical for the success of this project, the background that has been provided has been extremely useful in understanding the basic photophysical nuances of the excited states being studied and utiliz ed herein. Whereas in previous studies it was desirable to attempt to energetically separate the 3 MLCT and 3 states to allow for independent excited state manifold examination, in this Energy / eV 3 3 1 1 1 MLCT 3 MLCT 3 MLCT 1 MLCT 3 MLCT 1 MLCT M = Ru(bpy) 2 2+ M = ReCO 3 Cl M = Ir(ppy) 2 +1

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23 study, a combination or overlap of excited states into an equilibrated manifold is not problematic and, as will be discussed in the next chapter, can be considered a benefit. Nonlinear Optical Mechanisms Two Photon Absorption Two photon absorption occurs when a molecule simultaneously absorbs two photons of lower energy whose energy sum is equivalent to the energy needed to produce an excited state equivalent to one photon abso rption. (Figure 1 7a) TPA does not require the material to exhibit ground state Figure 1 7 a) Illustration of two photon absorption from a region with zero ground state absorption. b) Jablonski diagram visualizing two pho ton absorption. absorption at the two photon active wavelength, however it does require a high photon flux. (Figure 1 7b) For a material with no ground state absorption at the two photon active wavelength, it will remain optically transparent at low photo n intensities. At high photon flux, however, a two photon active chromophore can access one photon excited states without being subjected to the energy absorption associated with a one photon transition, thus limiting potential damage to the material. One photon absorption Two photon absorption S o S 1 Two photon abs +h 1 h 2 Fluorescence +h 1 b) a)

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24 C onsiderable interest in the development of two photon absorbing (TPA) materials has grown in the past decade and significant advances in the design of these materials have been recently made, but it is thought that there is still progress that can be made. 24 Le Bozec 25 and Prasad 26 have extensively studied the enhanced TPA nonlinear optical (NLO) properties of conjugated transition metal bipyridine and 1,10 phenanthroline based chromophores. In these studi es it was found the metal plays several important roles contributing to increased NLO activity. First, the metal is a template to configure ligands in a predictable octupolar arrangement. Second, the metal can induce a low energy metal to ligand charge t ransfer (MLCT) transition. Third, the Lewis acid character of the metal can induce a strong intraligand charge transfer (ILCT) transition that is red shifted from the free ligand. With the addition of an MLCT transition and a red shift of the ILCT transit ion, enhanced two photon activity was achieved in the far red and near IR regions where the complexes are transparent. Several series of conjugated metal organic chromophores containing similar polypyridine architectures have been synthesized by our gro up and have the potential to display enhanced two photon absorption. 9, 21 With a project goal for this research being the development of a series of transition metal based metal organic chromophores that simultaneo usly display both two photon and excited state absorption (TPA/ESA) in a dual mode nonlinear response; these studies provide the base model for the metal organic chromophores to be studied. To further enhance the TPA character of the proposed organic chr omophore, recent work revealing important structure property relationships leading to high TPA cross sections was utilized. 27 33 Chromophores containing both electron donating and el ectron accepting moieties, linked by a conjugated bridge which is polarizable, have been shown to display very large

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25 TPA cross sections. 34 Over t he past decade different arrangements of these key elements have been explored with the intention of maximizing the overall TPA cross section of the D = donor species, A = acceptor species, = conjugated link Figure 1 8 General chromophore structures which exhibit high TPA cross sections. chromophore. 27 33 Essential structural elements of the architecture of these co mpounds, which lead to a high TPA cross section, are the presence of donor and/or acceptor units separated by a highly polarizable conjugated bridge. As shown in Figure 1 8, the arrangement can vary from, but not limited to, donor acceptor, acceptor donor and donor donor. An example of a structure which has proven to ex hibit a large TPA cross section infrared and that followed this morphology is shown in Figure 1 9 It has also been suggested the branching contained in these comp ounds provides a cooperative interaction between the donor and acceptor units giving rise to an overall enhancement in the performance of the chromophore. 24, 35, 36 The configuration for the organic chromophore used in this project (Figure 1 11) contains an electron donating species on the periphery linked to a central electron accepting core by a polarizable conjugated bridge. Using the nomenclature discussed above this would be referred to as a donor acceptor donor chromophore. The electronic configuration of this compound is d A D D D D A A A A D D A D D

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26 Figure 1 9 High cross section TPA chromophore AF 455. 24 Excited State Absorption Enhancement of nonlinear activity with the addition of e xcited state absorption in a multiphoton process has been effectively demonstrated in recent studies conducted by Silly, 34 Sutherland 37 and Gao. 38 By combining a large one photon excited state cross section with a large TPA cross section in the same regions of the visible or near IR, a cumulative multiphoton nonlinear absorption is generated (Figure 1 10). This dual mode nonlinear process can achieve high transparency in at low intensities while maintaining efficient and instantaneous protection from high intensity light. For this project a further nonlinear enhancement through the incorporation of a strongly absorbing and long lived excited state is desired. This synergistic combination of instantaneous response and lo ng lived nonlinear absorption is ideal for optical limiting. Oligomers and polymers incorporating transition metals have been synthesized in an orbit coupling ability to enhance the rate of intersystem crossi ng to generate long lived triplet excited states. Several of the transition metals used are ruthenium, rhenium, osmium, iridium and platinum. In these compounds the photophysical properties are dominated by a long lived triplet excited state which are st rongly

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27 absorbing. Materials such as these, if modified to include an NLO chromophore, have the potential to display nonlinear absorption enhanced from an excited state absorption (ESA) mechanism. For ESA to be most effective at enhancing an NLO response, a strongly absorbing excited state must be rapidly and efficiently generated. Figure 1 1 0 Jablonski diagram of a dual mode nonlinear absorption (TPA/ESA) mechanism. Again, several series of conjugated metal organic chrom ophores containing polypyridine architectures and long lived triplet states have been synthesized by our group. 9, 21 Utilizing similar synthesis techniques, metal organic chromophores with similar triplet character were produced which display this enhanced NLO response from ESA. It will be shown that with strong triplet absorption at the TPA active wavelength, a decrease in transmittance is observed as photon intensity increases brought about by a TPA/ESA dual mode mechanism (Figure 1 10). This characteristic therefore presents this class of metal organic compounds as potential optical limiting materials. 30, 31, 33 +h S 1 Two Photon Absorption +h +h 1 Excited State Absorption T o T n ISC S o

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28 Project Objective The goal of this research is to explore the potential for a dual mode nonlinear response in a novel series of metal organic chromophores. The system of interest contains an all organic two photon absorbing chromophore complexed with a transition metal chromophore capable of inducing a rapid intersystem crossing of singlet excited states to long lived triplet excited states. The desire for the inclusion of two photon absorption is two fold. First, utilizing a two photo n excitation source to produce an initial excited state limits the potential for high energy induced chromophore damage by addressing wavelengths which are optically transparent at low photon fluxes. Second, introduction of a two photon excitable chromoph ore incorporates a near that is very short (femtosecond to picosecond). The inclusion of excited state absorption is desired to take advantage of available long lived triplet states allowi ng for nonlinear absorption on a considerably longer time scale (nanoseconds to microseconds). Coupled together in the same system, the complementary temporal responses can be combined to generate a nonlinear Central to the investigation is an organic conjugated chromophore (Figure 1 11) with the potential to exhibit two photon absorbing character as well as the ability to complex with a range Fi gure 1 1 1 Bipyridine centered target two photon absorbing chromophore. of transition metals that have large spin orbit coupling potential. The functionality being integrated into the core structure of the chromophore to support metal complexation is a 5

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29 bipyridine comes from its electron accepting nature as well as an ability to strongly bind to a wide variety of late transition metals. It has been shown that when coupled to transition met als such as ruthenium, rhenium and bipyridine can be manipulated in tandem to enhance the spectral response of a chromophore. When combined with a transition metal, bipyridi ne exhibits a conjugation enhancement from a conformational change as well as an added metal to ligand charge transfer mechanism which adds absorption character to the chromophore. This ability to bind with a variety of metals yields several possible meta l organic chromophores. (Figure 1 12) Photophysical investigation of these transition metal based analogs is centered on their TPA nonlinear response and the generation of long lived triplet excited states from both a single and a two photon excitation so urce. Rapid intersystem crossing brought about by the incorporation of selected metal s with large spin orbit coupling value s are utilized to efficiently generate a large population of highly absorbing triplet states. Selection of ruthenium, rhenium and ir idium as the target metals is based on the previous synthetic and photophysical findings available in our group. It has been well documented that substituted bipyridine complexes of these target metals efficiently and rapidly produce triplet excited state s that are highly absorptive and long lived. The utilization of these triplet states to further absorb the excitation wavelength will enhance nonlinear absorption in this system through excited state absorption. Overall, the primary objective of this wor k is the two photon generation of excited states in a series of the metal organic complexes followed by rapid intersystem crossing to a long lived triplet excited state manifold comprised of 3 and/or 3 MLCT excited states to generate a temporally broad and wavelength significant dual mode nonlinear response

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30 Figure 1 1 2 Target metal centered dual mode nonlinear absorbing chromophores.

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31 CHAPTER 2 DETERMI NATION OF NONLINEAR RESPONSE Introduction Linear Absorption As a normal course, the absorption of light by a substance follows a process that can be easily understood and explained. Most optical interaction phenomena can be expressed as a linear relation ship of the absorption of light as it travels through a medium to the amount of light entering. The most widely recognized relationship dealing with linear absorption of light is that relates to the absorption or transmission of light through a material. In Equation 1, the linear relationships of absorbance to molar absorptivity ( ), path length ( l ), and concentration ( c ), are clearly evident. Also included is the mathematical and gra phical relationship of transmittance to absorbance (Equation 2 and Figure 2 1). A = l c (1) A = log 10 (1/T) (2) Figure 2 1. Beer It is important to point out that this linear relationship does not include a dependence on incident light intensity. For normal intensities of incident light energy, these linear relationships are the standard; however at very high intensities of incident light energy, materials can exhibit nonlinear optical behavior and vary their absorbance as a function of the incident light intensity.

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32 Nonlinear Optical Behavior To examine the photophysical ramifications of absorption which occurs in a manner that much more difficult. With regard to the interaction of light with an organic medium, one can examine this interaction by vie wing the system as a dielectric medium subject to an electric field. 39 The introduced electric field originates as the electric component of the electromagnetic field. As light is introduced to the medium an induced dipole moment is created by the applied field. In turn, an ind uced polarization ( P ind ) of the medium results from this newly established (1 ) to the applied electric field, E as shown in Equation 3. This susceptibility is related to the optical P ind ( E ) = (1) E (3) response of a medium at a given optical frequency ( ) and is linked, in turn, to a complex refractive index ( n c ). Expression of this complex refractive index exists in real ( n ) and n c 2 ( ) = 1+4 (1) ( ) (4) n c = n + i k (5) imaginary ( k ) parts corresponding to the dispersion of the refractive index and electronic absorption components, respectively. Viewing linear absorbance utilizing this model, the medium is observed as a collection of harmonic oscil lators forced by a sinusoidal optical field. For a system where the introduced field strength is of relatively low intensity and the oscillations of the medium remain harmonic and in equilibrium with small polarization/dipole displacement, the medium abso rption will remain a linear process and the oscillators will exhibit sinusoidal motion with the frequency of the driving optical field. In this case, the oscillators re emit the light but with a lag in phase. The

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33 cumulative phase lag in the medium is due to the reduced group velocity of the light waves in the medium. In turn, the oscillators experience a damping force by the medium which is brought about by an exchange of energy between the optical field and the medium and which can be expressed classica lly as the wavelength difference between absorption and emission or the Stokes shift. In the case where the electric field driving force is strong, anharmonic oscillations begin to occur in the medium. With the addition of these inharmonic oscillations to the system, the polarization expression (3) above expands to include nonlinear factors. Addition of higher order P = (1) E+ (2) E 2 + (3) E 3 +... ( 6 ) terms, such as (2) (3) etc, represent s high er order hyperpolarizabilty factors which give rise to and are re s ponsible for nonlinear optical effects. Nonlinear optic ( NLO) phenomena are generally characterized as second order or third order depending on whether they are described through the (2) or (3) terms though higher terms do exist. Second order nonlinear susceptibility, (2) represents and is responsible for second order effects such as second harm onic generation (SHG) and the electro optic effect. SHG occurs when two photons of angular frequency combine to produce a third photon with an angular frequency of 2 This doubling of frequency + generates a photon whose wavelength is half the original. This second order effect is the major mechanism utilized in producing frequency doubled wavelengths in laser applications such as producing 532 nm from a 1064 nm fundamental beam of a n Nd : YAG laser. The electro optic effect is the modifica tion of the refractive index of a medium brought about by an externally applied electric field. Two examples of this are the Pockels effect and the optical Kerr effect. The Pockels effect occurs in crystal material which is non centrosymmetric and is ref erred to as the linear electro optic effect

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34 because the induced refractive index change is directly proportional to the external electric field strength applied. Typical crystalline material s which produce the Pockels effect, such as potassium dideutrium phosphate (KDP) and beta barium borate (BBO) are used as electro optic modu lators for Q switching in laser applications The optical Kerr effect is also a change in refractive index brought about by the application of an electric field however the induced refractive index change is proportional to the square of the electric field intensity. For the optical Kerr effect the electric field acting on the medium is the incident light itself, therefore the variance in the index of refraction becomes proportiona l to the square of the irradiance of the incident light. This nonlinear optical effect is commonly used to produce ultrashort laser pulses in the range of a few femtosecond s Third order nonlinear effects are governed by the (3) susceptibility term and represent the nonlinear factors that are of concern for this body of work. This third order susceptibility factor is responsible for effects that include third harmonic generation (THG), and two photon absorption (TPA). THG, like that of SHG, occurs as a multiphoton event that combines three photons of angular frequency to generate a fourth photon with an angular frequency 3 and a wavelength equal to one third the fundamental. THG, however, occurs as a cascade of two seco nd order nonlinear events where + and 2 + 40 It is also noteworthy that it has been demonstrated that even order susceptibility factors ( (2) (4) .etc ) approach ze ro or are nullified in molecules that contain a center of inversion. This means that in molecules containing second and third order nonlinear structural features, but also possess a center of inversion will exhibit third order nonlinear effects almost exc lusively. These factors point to the influence and importance of molecular symmetry in the preparation of model compounds for studying second and third order nonlinear effects and will be addressed during the course of this

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35 dissertation. Since the focus o f this work is to evaluate and take advantage simultaneously of two very different nonlinear effects, a familiarity with the structure property relationships affecting excited state absorption and the nonlinear susceptibility term that governs two photon a bsorption is useful Second order NLO processes. T saw considerable research focused on second order nonlinear optical effects of non centrosymmetric molecules. Dipolar molecules like the one shown in Figure 2 2 were one of the classifications of molecules which attracted significant attention. These molecules exhibit preferential polarization with greater e fficiency in one direction than the opposite direction. In such systems, Oudar and Chemla 41 expressed the origi n of the first order hyperpolarizabilty (7) term, as a single resonance frequency, two state model consisting of the ground and first allowed charge transfer excited state dipole, and the transition energy, E In this m odel, the first order hyperpolarizability coefficient, unidirectional susceptibilit y and the molecules transition energy Inclusion of is meant to account for the preference of electron interaction with the optical field in one direction relative to the other. From this model, molecules designed for second order NLO applications were primarily unsymmetrically endcapped, aromati c electron systems containing electron donating moieties at one end and electron withdrawing groups at the other. This separation of electron donating and withdrawing endcaps was designed to incorporate the desired electronic bias to the system. A typica l dipolar model compound in these early studies was the NLO chromophore 4 ( N N dimethylamino) nitrostilbene (Figure 2 2) which contains

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36 two benzene rings linked by a double bond in a conjugated system to provide a polarizable bridge of electrons for the donor and acceptor groups dimethylamino and nitro to act Figure 2 2. Structure of the typical dipolar model compound 4 ( N N dimethylamino) nitrostilbene. upon. This compound and others of similar architecture h ave been studied and reviewed 39, 42 Following the rational e advancements in the understanding of the structure prope rty relationship of charge transfer in dipolar molecules and its effect on the first hyperpolarizability coefficient Studies by Ma r der and Perry 29 explored how the de gree of ground state charge separation (induced polarization) was a ffected by chemical structure and the medium surrounding the compound. In the course of these studies chemical structure property variables such as structure of the conjugated system an d the strength of the donor and acceptor substituents were examined. A positive correlation to increased hyperpolarizabil i ty was observed with the increase in bond length alternation (BLA) in polyenes having alternating double and single bonds which aid ed in charge transfer and charge separation. It was also shown that higher order polarizabilit y terms could also be correlated in a predicable manner to this charge transfer model. Initially predicted enhancements associated wit h increasing donor and accep tor strengths surprisingly yielded significantly lower hyperpolarizability gains than predicted. Jen Dalton et al 43 determined that for molecules with large dipole moments (>10D), chromophore chromophore interactions at high chromophore concentrations diminished polarizablity as electrostatic interactions increased. Subsequent work by Dalton and the Lockheed Martin Corporation 44 focused on highly electron deficient acceptor groups as well as minimizing chromophore interactions, Figure 2 3.

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37 Figure 2 3. Structure of an enhanced donor acceptor chromophore Thes e studies also afforded c onsiderable attention on developing a conjugated bridge that provided a good balance between enhancing charge transfer, preventing aggregation and promoting thermal and photochemical stability. Third o rder NLO P rocesses The acti ve presence of third order susceptibility, (3) in a substance leads to third order NLO interaction and its possible uses in such applications as optical switching for optical communications or optical information processing, and optical limiting for the protection of optical sensors from damage induced by intense laser energies. Increasingly third order nonlinear processes have proven valuable as tools for nonlinear applications as well as other technologically significant processes. The applications me ntioned are among the most important and desir able uses for third order NLO processes. Several different classes of conjugated organic molecules and polymers have been investigated to identify key structure property relationships in the effort to develop materials with large (3) nonlinearities for use in these applications. 45

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38 Considerable study into structure property relationships influencing the third order nonlinear processes has been performed. Early research associat ing chemical structure property relationships to the strength of the sec ond order hyperpolarizability coefficient, conducted by Prasad et al 46 developed a coupled anharmonic oscillator (CAO) model based on a single resonance frequency, two level model similar to that used to explain the first order hyperpolarizability coefficient, This CAO model correctly accounted for the observed dependence of the band gap, linear polarizability and second order hyperpolarizability on the number of repeat units contained in a conjugated organic molecule or polymer with structurally large repeat units. This model was found to be less accurate however for short chain polyenes for which a two level approximation model was deemed not to be valid. Further 47 proposed an inter chain mechanism for third order nonlinearity in conjugated polymers. They noted that NLO susceptibility for third harmonic ge neration had a contribution from inter chain stacking and yielded a large component in the direction of the stacking. Continued research by Prasad 48 in this area determined that the molecular geometry of repeat units ha s a significant effect on (3) values. I n theoretical studies of planar geometry oligomeric diphenylbenzobisoxazole (PBO) and diphenylbenzobisthiazol (PBT) i t was found they showed significantly larger values than similar molecules of nonplanar geometry. This increase in NLO s usceptibility was attributed to Figure 2 4. Structures of diphenylbenzobisoxazole (PBO) and diphenylbenzobisthiazol (PBT).

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39 a geometric enhancement lending to more of a free electron structure vers u s a twisted chain whi ch introduces a break in conjugation and restricts the free movement of electrons. In this study it was also noted that geometrical isomerism is an important factor influencing third order NLO effects. In general, it was found that an isomer allowing the greatest number of resonance structures and therefore better electron delocalization had a significantly higher value of over those having a smaller number of resonance structures. In the area of third order nonlinear responses, there are several mech anistic choices that are thought of as the common modes of this type of nonlinear behavior. Among them is two photon absorption (TPA), which we will examine in the course of this chapter. Optical Limiting Background By definition, optical limiters are N LO materials which allow normal intensity light to be transmitted but attenuate light at high incident power (high photon flux ) 49 Due to recent laser based technological advancements of power, freque ncy and wavelength agility as well as improvement in efficiency, portability and operational robustness, lasers have become commonplace and incredibly important to manufacturing, medicine, entertainment and a host of other sectors including the military. or the digital reading device on our home entertainment center, but concern for protection fro m the light energy produced by a laser is of serious concern. Damage to the retina of the eye has been shown to occur with an extraordinarily low amount of laser energy. A study conducted by the Human Effectiveness Directorate of the Air Force Research L aboratory found that for a few common wavelengths, such as 1064 and 532 nm, eye damage can occur at energies as low as 0.25 J at 532 nm and 5.5 J at 1064 nm with a 5 ns pulse duration. 50, 51 Items such as laser

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40 pointers are designed to operate as Class 3a devices with an output of less than 1.0 mW which, according to the ANSI Z136.1 1993 American National Standards for Safe Use of Lasers, is of sufficiently low power that the blink reflex normally affords adequate eye protection. However, even for laser pointers, recent technological a dvances in compact diode lasers used in these devices are producing up to 5.0 mW at 533 nm which are, according to the Food and Drug Administration, capable of producing flash blindness, glare and after images. 52 With the threshold for safe ocular laser exposure being so low and the need to protect other sensitive mechanical devices, such as optical sensors, from high power laser exposure, the need for efficient and flexible optical limiting material is growing ever more important. The development and testing materials for optical limiting has been ongoing for as long as laser research and advances have been made but there is still a great deal more that can be done. It is possible to however, assumes the laser wavelength being utilized can be accurately anticipated. Static filtering therefore is not an adequate defense against such high energy light sources as agile wavelength pulsed lasers. To properly protect the eye or an optical sensor from such a system, the sufficient period of time to allow for h uman (blink) or mechanical (shutter) reaction time to intervene and provide additional protection has become a necessity. Two Photon Absor ption Two photon absorption (TPA) is a third order nonlinear process in which a molecule has the ability to simultan eously absorb two identical photons to produce an excited state equivalent to that produced by the absorption of a single photon of equivalent energy. In the presence of

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41 normal intensity light the probability of TPA is low even for molecules with a large two photon cross section. In the presence of intense laser pulses however, a molecule with a large two photon cross section can simultaneously absorb two identical photons with a transition probability proportional to the square of the intensity of the la ser pulse. The necessity for a high intensity photon source presents an additional key feature of two photon absorption with potential for utilization in applications: spatial confinement. Spatial confinement of the TPA process arises not only due to the quadratic dependence of incident two photon resonant light but also from the quadratic decrease of the intensity of focused laser light as distance from its focal point increases. Together these factors lead to the probability of TPA occurrence decreasin g by the fourth power of distance from the focus of a suitably intense laser source. property relationships were developed that perm itted the deliberate design of new chromophores with enhanced two photon cross sections. 29, 32, 53 Several key studies by groups such as Reinhart, Perry, Marder, and Prasad have established basic design concepts which have successfully enhanced two photon absorption in different seri es of molecules. 29, 32 et al proposed two classes of chromophores which could exhibit enhanced two photon cross sections (Figure 2 5) During their study several key s tructural Figure 2 5. Chromophore structures for enhanced two photon absorption (Reinhart).

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42 cross section: (1) conjugation lengt h, (2) nature of the polarizable conjugated bridge (3) donor strength, (4) number of polarizable double bonds, and (5) the increase of planarity in the chromophore. Along similar lines Perry and Marder, working with Jean Luc Bredas 29 found a positive correlation between large two photon cross section values and a ability to undergo a large change in quadupole moment upon excitation. Successful enhancements of two photon cross sections where achieved in chromophores utilizing these architectures which promoted the ability to induce large quadupole moment changes with structural motifs containing acceptor donor acceptor (A D A), donor acceptor donor (D A D), and donor donor (D D) moieties (Figure 2 6, left). Figure 2 6. Structures of quadupolar (left) and octupolar chromophores (right) found to have large two photon absorption cross sections. D represents electron donating group. This strategy was further expanded to octupolar chromophores by Cho et al 54 (Figure 2 6, right) with considerable success. This structure property strategy to enhance two photon absorption has persisted and was a primary consideration in the design of the organic two photon absorbing chromophore in the work described herein.

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43 In optical limiting a pplications TPA is a desirable quality due to its high linear transparency at low light intensities as well as its fast temporal response. Utilization of TPA as a complementing NLO mechanism to RSA in a dual mode optical limiter allows for an ultra short picosecond in nature and lasting for several nanoseconds. Therefore by implementing TPA as the ground state excitation pathway with its inherent nonlinear absorption, though short in duration, covers the gap betw een exposure and excited state absorption which can be in the order of a nanosecond. Excited State Absorption and Dual Mode Limiting When excited state absorption (ESA) is combined with another NLO mechanism it acts as a cumulative nonlinear absorption eff ect. As an optical field interacts in a nonlinear manner with a medium or material to produce an excited state, a subsequent absorption by the excited state leads to further reduction of transmission. This additional absorption pathway leads to an enhanc ement of the initial NLO response. In this project a combined TPA/ESA process was utilized to further enhance the nonlinear character of the target TPA chromophore. A five level energy diagram, shown in Figure 2 7, was used to model the process. Two pho ton induced ESA is a three photon process, but the photons are not all absorbed simultaneously. Initiation of Figure 2 7 Five level energy diagram for two photon induced excited state absorption mo del. +h 2 S 1 Two Photon Absorption +h 1 +h 1 Excited State Absorption T o T n ISC S o

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44 the process is the simultaneous absorption of two photons from a TPA mechanism to generate a singlet excited state. Rapid intersystem crossing subsequently populates the lowest triplet state. A third photon is then absorbed by the triplet state. Th ough the possibility exists that the singlet excited state may be absorptive, its lifetime is significantly shorter than the nanosecond laser pulse due to rapid intersystem crossing (~300 fs) 15 and its effects can be con sidered negligible. As the efficiency of TPA increases, light transmittance at low input intensities remains high whereas light transmittance at high input intensities decreases. Chromophores exhibiting highly efficient TPA/ESA absorption are considered e xcellent candidates for use as optical limiters. In addition to high linear transmission at low intensities, other key factors in favor of the utilization of the TPA/ESA mechanism to achieve power attenuation include source absorption, temporal duration, and spectral range. For a limiting system utilizing this dual mode mechanism, optical energy is absorbed and converted to heat as opposed to being redirected as is the case in nonlinear scattering or reflective materials. This facilitates a response whic h is reliable, more predictable, and limits unwanted exposure to the surrounding area. In addition, a dual mode TPA/ESA chromophore with an efficient and rapid intersystem crossing mechanism producing a long lived triplet excited state is ideal for power attenuation for laser pulse widths ranging from sub nanosecond to several microseconds. Lastly, chemical modification of metal organic based TPA/ESA chromophores has the potential to produce systematic structural manipulations to attain the desired triple t absorption properties directly related to the enhancement of optical limiting responses favorable to the protection of a system of interest. Recent studies by Sutherland 37 and Li 55 have modeled and experimentally verified the intensity dependence of the TPA/ESA mechanism. A qualitative comparison of the

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45 enhancement from ESA was noted. In a modification of their model, the attenuation of intensity I as light passes through a sample can be described by Equation 8 where and represent (8) the molecular densities of molecules in the and states and and represent the two photon and triplet absorption cross sections respectively. Describing the TPA/ESA mechanism in this fashion illustrates the cubic dependence on incident light intensity of this dua l mode process. These studies also noted this cubic dependence can be used to differentiate TPA/ESA from other NLO processes such as TPA or reverse saturable absorption (RSA). An example where TPA alone was clearly differentiated by its quadratic relatio nship between attenuated intensity and incident light intensity versus a cubic dependence for TPA/ESA is shown in Figure 2 8. Assessing the magnitude of the NLO enhancement gained through ESA in AF 455 shown in Figure 2 8 was also compared versus the exc ited state absorption cross sections. 34 The excited state absorption cross sections were reported as follows : (1) two photon cross section ( ), 0.51 x 10 20 cm 4 /GW, (2) singlet state ( ), 1.68 x 10 17 cm 2 and (3) triplet state ( ), 17.1 x 10 17 cm 2 Initial studies of the excited states of AF 455 identified the triplet state as a potential source of enhanced nanosecond nonlinear absorption because of its large triplet triplet absorption cross section. Subsequent determination of a modest triplet quantum yields (<10%) led to singlet state absorpti on being identified as the dominant ESA pathway but with a significant contribution from triplet triplet absorption (due to its large absorption cross section). Figure 2 9 represent the comparison of experimental data versus a numerical model based on Equ ation 8 and the absorption cross sections reported above. A second theoretical curve was added using a different

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46 Figure 2 8. Illustration of the difference in transmission loss by TPA alone and by TPA/ESA in AF 455. 37 two photon cross section of AF 455 reflecting an alternate experimental method ( = 1.7 x 10 20 cm 4 /GW). It was concluded that the three photon absorption model provides a reasonably accurate estimate ( 10%) of the nanosecond nonlinear transmittance in this system. It was noted that the largest source of error in making the estimations was the variation in exp erimental measurements of the two photon cross sections. Also noteed was that excited state absorption is Figure 2 9. Comparison of theoretical (curves) versus experimental (circles) nanosecond nonlinear transmittance as a function of laser pulse ener gy for AF 455 at 800 nm. 37

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47 the dominant contribution to nonlinear transmission in AF 455 for the TPA/ESA mechanism on a nanosecond time s cale. It was concluded that two photon absorption did not contribute significantly to overall transmission loss despite a reasonably large two photon absorption cross section, but was an essential process for producing the excited states. Experimental Det ermination of Nonlinear Response Z scan Many experimental techniques have been developed to measure third order nonlinearity over the past two decades. Among the most popular is Z scan which was developed in 1989 by Van Stryland. 56 The experimental methodology is based on the observation of beam distortion generated by nonlinear refraction in conjuncti on with the overall change of transmitted intensity due to nonlinear absorption. Researchers utilizing this technique have described it as a useful spectroscopic tool due to its simplicity and high sensitivity. A typical Z scan experimental setup used to investigate third order nonlinear refraction and absorption is presented in Figure 2 10. Figure 2 10. Generalized illustration of a Z scan experimental setup. In general the Z scan setup contains a single laser beam passing t hrough a focusing lens and is utilized as the incident light source. The beam can be attenuated prior to the focus to achieve the desired input energy and is kept constant throughout the experiment. The sample is Detector 2 Laser source Energy Meter PC Attenuator Lens BS Sample Detector 1 Aperture Z +

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48 placed in a sample holder mounted on a si ngle axis translation stage positioned at the focus of the laser source with the axis of movement parallel to the laser source. Calibration of position with regard to the focus and precision of movement is of primary importance to obtain accurate results and is why the translation stage is ideally controlled by a computer. With the incident energy kept constant, the sample is moved from a point along the optical axis of source light prior to the focus ( Z) through the focus (Z = 0) to a position beyond th e focus (+Z). The extremes of positive and negative Z are positions relative to the focal point where only linear absorbance is present. Data collection involves recording the resulting change in transmitted energy by detectors 1 and 2 and correlated ver sus change in the Z position. Plotting normalized transmission versus Z position produces the typical open aperture Z scan curve (Figure 2 11b) which graphically depicts the nonlinear absorption character of the sample. A variation of this technique, kno wn as closed aperture Z scan, places an aperture in front of transmission detector 2. Utilizing an aperture prior to detector 2 allows for the determination of the sign and magnitude of third order nonlinear refraction. Nonlinear refraction occurs in the presence of self focusing and is detected as the sample is translated through the focus. Figure 2 11. Z scan curves of C60 in toluene (0.5% wt) at an excitation wavelength of 1064 nm. a) closed aperture b) open aperture. 57

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49 For molecules presenting negative nonlinear refraction values, as shown below for C 60 self focusing occurs prior to the point of focus and is detected as an apparent increase of transmittance above the normalized value of 1.0. As the sample passes the focus, transmission drops off to values less than 1.0 due to defocusing by the same mechanism (Figure 2 11a). Nonlinear Transmission Another common and often used nonlinear abso rption determination technique is nonlinear transmission (NLT). NLT is an experimental methodology conceptually similar to that described above for Z the technique chosen for nonlinear response detection for this project due to its ease of setup, relatively minimal equipment requirements and low startup cost. A Figure 2 12. Schematic illustration of the nonlinear transmi ssion (NLT) experimental setup. schematic illustration of the NLT experimental setup developed for this project is depicted in Figure 2 12. Important differences in this experimental setup versus a Z scan apparatus are as follows. First, the sample remai ns stationary with respect to the spot of laser focus. Second, the beam energy is sampled prior to the sample and compared to the energy after passing through the Laser source variable OPO Energy Meter PC ND filter optional ND filter 0.3 100% T Plano convex lens, f = 13 cm cm BS 50:50 Sample Detector 1 Detector 2

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50 sample. This arrangement allows for the determination of absolute energy input to the sampl e by a simple differential determination in the presence of a sample cell containing solvent only. Lastly, utilization of the ratio of transmitted and incident energies in this manner eliminates the influence of any inherent laser radiation instability wh ich has the potential to vary results between experiments. Two different representations of the information gathered by an experimental nonlinear transmission setup are easily depicted. First, directly plotting input energy versus output energy yields a graphical representation visualizing any nonlinearity of the obtained signals. In Figure 2 13. Example NLT curve plot of input energy versus output energy and depicting a clamping state energy in the sample. additi on, this method clearly represents the limited output energy (clamping energy) of the system if one is present (Figure 2 13). For the low energy region in this presentation style, it is difficult to distinguish nonlinear absorption from low initial transm ission. For this reason a second representation is utilized to better highlight the low energy region of NLT data. It is also

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51 noted that there is a spreading out of data as light intensity increases. As the material transitions into regions of increased nonlinear absorption the transmission error in the system also increases with a quadratic and cubic dependence and generates data that is more dispersed. In the second representation of NLT data (Figure 2 14), input energy (plotted on a common logarithmi c scale) is plotted versus energy transmission through the sample (output energy divided by input energy). This representation clearly indicates transmittance at low light intensities as well as the nonlinear threshold energy. For this data presentation linear absorption is depicted as a data set of zero slope and any nonlinearity is represented as a data set with a nonzero slope. Figure 2 14. Example NLT curve plot depicting input energy versus transmittance. Nonlin ear Transmission Test Case Test Objective In order to verify the experimental capabilities of the newly designed and built nonlinear transmission (NLT) apparatus, it was determined a survey study should be conducted. The tablish that the apparatus was capable of detecting a nonlinear

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52 response in a consistent and repeatable manner, (2) establish an experimental method for apparatus setup, data acquisition and data presentation, and (3) determine the capability of the appara tus to acquire data from a nonlinear chromophore of known nonlinear character and compare the findings to a similar chromophore series in a semi quantitative manner. The last criterion is considered the most important because the apparatus was not designed nor intended to measure an absolute value associated with a nonlinear response, such as absorptive cross section or nonlinear coefficients. The desired capabilities of the NLT apparatus were to detect the presence and judge the relative strength of a non linear response in an NLO active system. Therefore, the capability to gather consistent and repeatable data from multiple samples under a standard set of experimental conditions was considered a key objective. Apparatus Setup Nonlinear transmission (NLT) experiments were performed on an in house built setup similar to in the setup and operation of the NLT apparatus that is described below. A photo rendered sch ematic is presented in Figure 2 15. The NLT apparatus used for the test consisted of a variable wavelength laser source provided by a Continuum Surelite II 10 Nd:YAG laser (355 nm, third harmonic of the 1064 nm fundamental) augmented with a Surelite OPO P lus (wavelength range: 420 570 and 590 1200 nm). A single laser beam passing through a focusing lens was utilized as the incident focused light source. This beam could be attenuated prior to the focus (if desired) to achieve the desired beam energy and s tability. Upon achieving the adequate beam stability and maximum laser energy desired for the experiment, beam attenuation for power dependent measurements is achieved with the addition of a continuously variable neutral density (ND) filter placed prior t o the focusing lens. Nominal laser energy requirements for the experiments ranged from sample input energies of 50 J to approximately 5.0 mJ. These were

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53 achieved utilizing an ND filter capable of attenuations ranging from 0.1 to 100% transmittance. A 5 0.8 mm plano convex lens with a focal length of 10 cm was positioned in the incident Figure 2 15. Photo schematic of OPO based nonlinear transmission apparatus. laser pathway. The beam diameter at the lens was 14 mm. Utilizin g Equation 9 58 a calculated (9) beam waist of 63 m was attained assuming a Gaussian beam shape with an energy density range of 0.01 1.10 J/cm 2 f and represent the wavelength, lens focal length and half the Gaussian beam diameter at the lens respectively. The focusing lens was positioned so the tightly focused beam falls within the confines of the sample holder. Adjustment of the focus to the Plano C onvex Lens f = 100 mm ; dia. = 50.8 mm Beamsplitter 50:50 Sample Holder 1 cm cell Low Energy Pyroelectric Detector 1 Neutral Density Filter 0.1 100% Laser S ource (variable) 420 1200 nm Low Energy Pyroelectric Detector 2

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54 exact center of the sample holder is facilitated by placing the focusing lens on single axis translation stage. A 50:50 beamsplitter is placed prior to the focus and directed towards detector 1. Detector 2 is placed at an equivalent distance from the beamsplitter behind the sample holder. Both detectors are positioned so that the entire beam occupied no more than one half the detector. It is important to note that the exact linear distance from the 50:50 beamsplitter to each detector was maintained to insure an identical s pot size on each of the matched detectors. SAFETY NOTE: Special attention must be given to safety during the course of the experiment due to, as depicted in Figure 2 12, the generation of an additional spot of focused laser energy between the 50:50 beams plitter and detector 1. This area must be avoided at all times during the course of the experiment as the potential for serious equipment damage or personal injury due to the presence of high energy density laser light. NLO Chromophore Test Series The in vestigation of a series of platinum acetylide dimers containing large two photon cross section chromophores linked to a variety of arylene core units of differing electron donating abilities which can provide different triplet excited state energies was un dertaken to determine the presence of enhanced nonlinear absorption. The combination of two photon excitation and platinum induced intersystem crossing to afford triplet excited states which are strongly absorptive was proposed to have a dual mode (TPA/ES A) nonlinear mechanism for enhanced nonlinear absorption. The platinum acetylide dimers presented here were synthesized by Dr Kye Young Kim and she also assisted in the collection of data for this study. The series of platinum acetylide dimers, Pt2 Ar c ontaining different triplet chromophores are shown in Figure 2 16 along with the parent model chromophore E1 DPAF which has been shown to have

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55 Figure 2 16. Platinum acetylide dimers, Pt2 Ar utilized for the NLT test study. significant nonlinear character. 59 These platinum acetylide dimers were the subject of a test study for the newly constructed nonlinear transmission apparatus examining for the evidence of nonlinear activity. E1 DPAF was utilized as a control due to its documented large TPA cross section and efficient intersystem crossing to afford strongly absorbing long lifetime triplet states. 59 Because of these factors, E1 DPAF has the potential to exhibit a dual mode (TPA/ESA) nonlinear mechanism. These factors make E1 DPAF a suitable model to judge the relative strength of the nonlinear response in the Pt 2 Ar series and to verify the relative effectiveness of the detection capability of the experimental setup. The reason for conducting the test study was two fold. The primary goal was the desire to evaluate the nonlinear detection capability of the newly designed and constructed NLT apparatus and evaluate its ease of use, quality of data obtained and its flexibility of use with a variable wavelength source. Central to this goal was the ability to integrate the NLT apparatus with our

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56 optical parametric oscillator (OPO) which is the source of the variable wavelength laser energy. Second was the desire to evaluate the nonlinear response of the Pt2 Ar series for evidence of optical limiting character. Photophysical Properties A comprehe nsive photophysical study of the Pt2 Ar series was accomplished by Dr Kye Young Kim but only the key photophysical features affecting the experimental parameters of the nonlinear determination will be presented here. Ground State Absorption Ground state ab sorption spectra were obtained for the Pt2 Ar series in optically dilute THF solution and are shown in Figure 2 17. The main absorption feature of the Pt2 Ar series is a strong ligand based transition which varied only slightly from 382 nm to 387 nm. Additional absorption bands present as the arylene core units are varied and appear red shifted to the main absorption feature. As the arylene core units are varied, the red shift of these absorption bands indicates the energy change of the lowest excited state. Therefore the lowest excited energy state of the series can be ordered as: P1 > T1 > EDOT > T2 > BTD > TBTDT In general, as the energy of the core unit decreases so does the ener gy of the lowest excited state. In addition, for BTD and TBTDT it was determined the addition of an 1 MLCT component to the transition attributes to the dramatic red shift in these compounds. Transient Absorption Nanosecond transient absorption measurements of the Pt2 Ar series were conducted in deoxygenated THF solution and are shown in Figure 2 18. For the series each chromophore displays strong triplet triplet absorption in the visible and near IR regions. The general excited state absorption characteristics of P1 T1 T2 and EDOT are quite similar with only minor

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57 differences while BTD and TBTDT present transient abs orption spectra which are significantly different in shape. Figure 2 17. UV visible ground state absorption spectra of PT2 Ar series in THF solutions. P1 exhibits a broad transient absorption ranging from 400 to 900 nm with t he longest transient lifetime of 17 s. T1 also presents a broad transient absorption which is only slightly narrower and weaker than that of P1 with a range of 420 to 750 nm and lifetime of 8.4 s. The transient absorption range of EDOT is identical to that of T1 but is slightly stronger with a lifetime 3.6 s. T2 exhibits a narrower transient absorption with a range of 450 to 720 nm but has the strongest triplet triplet absorption. P1 T1 T2 EDOT BTD TBTDT

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58 Figure 2 18. Transient absorption spect ra of Pt2 Ar series in deoxygenated THF solution. BTD and TBTDT transient absorption shapes are significantly different than those of P1 T1 T2 and EDOT BTD exhibits a bimodal shape with a strong transient absorption from 375 450 nm and weaker absorptio n which begins at 520 nm and extends well into the near IR. TBTDT also exhibits a bimodal shape with a relatively weak transient absorption band in the visible region with a range of 420 530 nm and a modest transient absorption from 600 to the near IR reg ion. It was noted the transient absorption decay lifetimes mirrored the phosphorescence lifetimes, indicating excited state absorption for the Pt2 Ar series originates primarily from the lowest energy excited states and its absorption character is determi ned by the arylene contained at its core. Due to the fact that the Pt2 Ar series utilizes the same TPA Increment : 10 s Increment : 4 s Increment : 2 s Increment : 2 s Increment : 1 s Increment : 1 s P1 T1 T2 EDOT BTD TBTDT

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59 chromophore and should show similar excited state absorption cross sections it become evident the differences in the triplet triplet absorption manifest from the differences in the arylene units. Nonlinear Absorbance Determination To detect the nature and strength of a dual mode nonlinear response comprised of two photon excitation followed by excited state absorption from a highly absorbing excited sta te, it was necessary to first determine the appropriate wavelengths and conditions to obtain the desired response. The first consideration was the determination of range of excitation wavelengths which could potentially elicit a two photon induced excitat ion of the chromophore to a singlet excited state. For this information the ground state absorption data were used to choose a potentially suitable wavelength range. For the series a generous range of strong ground state absorption exists for all the com plexes from 250 nm to at least 400 nm. This equates to a potential two photon active range for the TPA chromophore of 500 800 nm. This correlated well with the reported two photon absorption maximum wavelength of 612 nm for E1 DPAF which contains the sam e TPA chromophores as the Pt2 Ar series. 59 The effort to o ptimize the TPA excitation conditions was deemed essential to generate a sufficient singlet excited state population which could in turn be converted through intersystem crossing to triplet excited states. A systematic survey of wavelengths and chromophor e concentrations was carried out to determine the optimum conditions with which to proceed. For the survey, a variety of concentrations were explored from 60 mM to 20 mM to establish a viable concentration range. It was determined that for concentration s of 40 mM and higher, initial energies needed to transmit enough energy to be detected were above the nonlinear thershold or the sample was undergoing optical scattering due to the high solute concentration. As shown in Figure 2 19 for concentrations of 60 mM, 800 nm excitation seemingly did not produce a two photon excitation of the Pt2 Ar series. Also at 60 mM concentrations, it can be

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60 noted nonlinear response was evident at both 600 and 625 nm. For both sets of data near saturation clamping of the ou tput energy was discovered. Given these results, a systematic reduction in concentration was performed. As the concentration of chromophore was reduced, the input energy necessary to detect output energy was greatly reduced and positive evidence of a non linear response was seen. Up on reaching concentrations of 20 mM, sufficient transmission of lower intensity light was noted and it was determined a region of near linear transmission was present. For the same samples it was noted that as the input energy increased the ratio of input energy to output energy began to change and thus the absorption was becoming nonlinear. Since it was deemed desirable to observe the region where transmission of light changed from linear to nonlinear the concentration of 20 mM was chosen as optimum. The determination of wavelength was decided in much the same manner. At both 600 and 625 nm a region of linear absorption was evident, however it was noted that departure from linear absorption occurred at slightly lower energie s for 600 nm than for 625 nm. In addition, the OPO was able to produce more power at 600 nm than at 625 nm and the availability of output energy from the OPO would mean the ability to sample a much wider energy range. Final determination of the nature o f the nonlinear response for the Pt2 Ar series was conducted at the two photon excitation wavelength of 600 nm and 20 mM in a solution of benzene (Figure 2 21). Benzene was used as the final solvent as it was determined to be the best for complex solubili ty and had the overall effect of raising the initial light transmission through the sample. To effectively evaluate the relative strength of the nonlinear response in the Pt2 Ar series the model complex E1 DPAF for which the two photon and excited state absorption properties are known, 59 was also measured at 20 mM in benzene. In Figure 2 21 a plot of input energy versus transmittance yields the nonlinear transmission results for the Pt2 Ar series.

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61 Figure 2 19. Nonlinear determination results of Pt2 Ar series in 60 mM THF solutions at two photon excitation wavelengths of: 600 nm (top), 625 nm (center), and 800 nm (bottom).

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62 Figure 2 20. Nonlinear determination results of Pt2 Ar series in 20 mM THF solutions at two photon excitation wav elengths of: 600 nm (top), 625 nm (center), and 800 nm (bottom).

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63 Figure 2 21. Nonlinear transmission curve of the Pt2 Ar series in 20 mM benzene under two photon excitation condition at 600 nm. Test Results The nonlinear trans mission apparatus was designed, tested and built in house for determining the presence and relative strength of a nonlinear response and optical limiting. The NLT setup was able to detect and render data to depict the desired nonlinear responses of intere st. In the current configuration the NLT apparatus is capable of a repeatable semi quantitative representation of nonlinear response. Ease of setup and data collection presents this NLT technique as an experiment that can be utilized with only a minimum of prior training and orientation. The quality of data gathered is excellent and very easy to compile into useable nonlinear absorption and optical limiting curves. Complete details on data collection and presentation are contained in the user manual fou nd in Appendix B. A key goal in this test study was the ability to measure the relative strength of a detected NLO response. This goal was considered only a marginal success. Despite detailed two photon cross section data and excited state absorption d ata being available for E1 DPAF 59 the lack of NLT or other nonlinear absorption data such as Z scan makes direct NLT comparison difficult.

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64 Future use of this setup in conjunction with experiments capable of determining nonlinear absorptive cross sections will allow for direct quantitative comparisons of nonlinear data results with those from other studies with little need for exhaustive model compound synthesis or standard sample preparations. A positive result from the testing and utilization of the nonlinear transmission setup was the successful collection of nonlinear data for the Pt2 Ar series of platinum acetylide dimers as well as the confirmation of an enhanced dual mode nonlinear response. Successful two photon excitation of the TPA followed by excited stated absorption was also confirmed and verified. Excitation at the TPA wavelengths of 600 and 625 nm was successfully accomplished in a wavelength region where no ground state absorption is present, eliminating the possibility of contribution from a one photon absorption process. Evidence of further enh ancement from an ESA mechanism was also determined to be present. As noted previously, TPA alone affords a quadratic dependence on incident light intensity where as a TPA/ESA system affords a cubic dependence. For E1 DPAF the cubic nature of the data is fairly pronounced. Interestingly as the series progresses in the rank order of the observed strength of triplet triplet absorption as found by transient absorption spectra reported in a previous section, the cubic nature lessens to a nearly quadratic dep endence for Pt2 EDOT and Pt2 T2 This trend would seem to contradict the previous conclusion and indicate a decreased contribution from an ESA mechanism, possibly due to smaller absorption cross sections, and a more dominate nonlinear contribution from TP A. Despite this inconsistency the Pt2 Ar series appears to display an overall enhanced NLT response when compared to the model E1 DPAF chromophore. A reason for this difference in nonlinear response can be attributed to the variation in the core arylene units. This is logical since the TPA chromophore is identical for each complex in the series. With the TPA nonlinear

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65 excitation pathway identical for each compound the differences seen are deemed to originate from the ESA active portion of the chromopho re. Preliminary analysis of the data found here point to the strength of triplet triplet absorption as the main variable that correlates to the strengthening found in the nonlinear response in the Pt2 Ar series. Instrumentation Nonlinear transmission de termination experiments were performed on an in house built optical limiting setup which was described in a previous section. A variable wavelength laser source was provided by a Continuum Surelite II 10 Nd:YAG laser (355 nm, third harmonic of the 1064 nm fundamental) augmented with a Surelite OPO Plus (wavelength range: 420 570 and 590 1200 nm). Dual one inch uncoated BK7 glass prisms from Thor Labs were utilized to position the laser source at the entry of the optical limiting setup. A variable neutral density filter capable of transmission values ranging from 0.3% to 100% obtained from Thor Labs was utilized to attenuate the laser source to obtain a variable power profile. An uncoated plano convex lens obtained from Thor Labs with a nominal focal leng th of 130 mm and a 50.8 mm diameter was used to achieve a tightly focused beam at the sample. Prior to the focus, an economy, laser stable 50:50 beamsplitter from Thor Labs was used to redirect a portion of the incident laser source toward detector 1 and to permit the remaining energy to continue through the sample to detector 2. Change in transmittance as a function of laser power was monitored directly utilizing an Ophir Laserstar dual channel power meter configured with a pair of matching Ophir OPH PE1 0 SH V2 pyroelectric detectors (detectors 1 and 2) having a sensitivity range of 10 J to 10 mJ.

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66 CHAPTER 3 SYNTHESIS AND PHOTOP HYSICS OF NLO CHROMO PHORES Introduction The main focus of this project was the synthesis and photophysical investigation of a selected series of metal organic chromophores with the potential to exhibit an enhanced dual mode nonlinear response. The dual mode mechanism utilized is a combination of two photon excitation followed by an NLO enhancing excited state absorption. The characteristics of this mechanism have been explained in detail in previous chapters. The experimental method employed to detect and identify this dual mode NLO response utilized the nonlinear transmission apparatus developed in Chapter 2. To achieve th e desired enhanced TPA/ESA mechanism a series of metal organic chromophores were developed utilizing established large cross section TPA architecture in concert with moieties known to afford long lived triplet excited states with strong excited state absor ptions. It was found that the metal organic complexes produced exhibited an enhanced TPA/ESA nonlinear response consistent with the models presented previously and in the literature. Also in this chapter, modifications to the NLT apparatus along with an excitation source modification on an emissions instrument to allow for detection of two photon induced emissions will be presented. Synthesis Organic Chromophore Synthesis The synthesis of the two photon absorbing chromophore ligand, 4,4' (2,2' bipyridine 5,5' diylbis(ethyne 2,1 diyl))bis(N,N diphenylaniline) TPA 1 is shown in Figures 3 1 through 3 3. A general synthesis for the metallated complexes of the TPA 1 chromophore is shown in Figure 3 4. Initial attempts to synthesize TPA 1 o

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67 the approach used in previous work done in our group on similar compounds. 1, 9 17, 20 22, 60, 61 However this methodology proved to be inefficient due to synthetic limitations of material purification necessary for the final coupling step. The initially proposed final step involved a Sonogashira coupling of 5,5' diethynyl 2,2' b ipyridine and 4 bromo N,N diphenylaniline to produce TPA 1 (not shown). Under Sonogashira conditions this coupling was only effective in producing pure TPA 1 in good yield if the mono brominated triphenylamine starting material was very pure. Specificall y, the reaction of triphenylamine with N bromosuccinimide (NBS) in MeCl 2 at 0 C (Figure 3 2, step i) i. Acetyl bromide, MeOH; ii. Br 2 180 C, 4 days. Figure 3 1. Synthesis of TPA 1 chromophore ligand central core. resul ted in a majority product of 4 bromo N,N diphenylaniline but also returned starting material f < 0.10) of these products required multiple elutions on silica gel to retrieve a product pure enough to proceed. The drawbacks initially lead to a disappointing 18% yield of the mono brominated triphenylamine. An improved route to the synthesis of 4 ethynyl N,N diphenylaniline 5 was synthetically explored which, in turn, lead to th e exploration of a more convergent approach (Figure 3 2 and 3 3) to synthesizing TPA 1

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68 i N bromosuccinimide, MeCl 2 0 C, 6hrs; ii. 2 methyl 3 butyn 2 ol, Et 3 N, Pd/CuI (cat.) 70 C, 6 hrs; iii. KOH, toluene, reflux, 6 hrs. Figure 3 2. Synthesis of chromophore ligand end caps. The Sonogashira coupling of mono brominated triphenylamine with 2 methyl 3 butyn 2 ol (Figure 3 3) incorporates a highly polar protecting group (along with the desired ethynyl moiety) onto the t riphenylamine framework. This greatly simplifies the chromatographic separation and purification of the alcohol protected ethynyl product and eliminates the need for multiple purification steps of mono bromo triphenylamine prior to a subsequent reaction. Following the coupling reaction, 4 can easily be separated from the starting material and its dibromo analog via flash chromatography on silica gel utilizing 2:1 hexane/Et 2 f values > 0.50. Deprotection of 4 with KOH in toluene at reflux produces 5 in a 77% yield. Overall, the revised synthetic route was successful in producing 5 in three steps with an overall yield of approximately 26%. The final synthetic step in the generation of TPA 1 (Figure 3 3) involves the coupling of 2 and 5 under Sonog ashira conditions and proceeds with a 50% yield. This final yielded only 29% upon the Sonogashira coupling of 5,5' diethynyl 2,2' bipyridine and 4

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69 i. Et 3 N, Pd/CuI (cat.) 70C. Figure 3 3. Synthesis of TPA chromophore. bromo N,N diphenylaniline (not shown). Overall, the synthesis of TPA 1 could be completed in six steps with a combined yield approaching 13%. Metal organic complex synt hesis Metallation of TPA 1 with a slight excess of Ru(bpy) 2 Cl 2 in refluxing THF/MeOH affords Ru( TPA 1 )(bpy) 2 2+ 2Cl A saturated solution of NH 4 PF 6 was added drop wise to afford the PF 6 analog Ru 1 Flash chromatography on silica gel, yielded pure Ru 1 a s a dark red powder. Metallation of TPA 1 with a slight excess of Re(CO) 5 Cl in refluxing toluene affords Re( TPA 1 )(CO) 3 Cl. Solvent evaporation and repeated rising with acetone yields pure Re 1 as a bright red powder. Metallation of TPA 1 with Ir 2 (ppy) 4 C l 2 (ppy = phenylpyridine) was accomplished in refluxing 2 methoxyethanol under an argon atmosphere. Upon cooling, a saturated solution of NH 4 PF 6 was added drop wise to generate the PF 6 analog Ir 1 Flash chromatography on neutral alumina, yielded pure I r 1 as a dark yellow powder. The synthetic scheme is depicted in Figure 3 4.

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70 i. Re(CO) 5 Cl, toluene, reflux, 45 min. ii. cis Ru(bpy) 2 Cl 2 MeOH/THF (4:1), reflux, 18 hrs. iii. Ir 2 (ppy) 4 Cl 2 2 methoxyethanol, reflux, 26 hrs iv. Sat. aq. NH 4 PF 6 Figure 3 4. Metallation of TPA chromophore. Results and Discussion Photophysical Properties UV Visible Absorption Spectroscopy Absorption spectra of each compound were obtained utilizing optically dilute CH 2 Cl 2 or THF solutions (OD ~ 0.1). The absorption spectra for the series are presented in Figure 3 5. Table 3 1 presents the list of absorption maxima, extinction coefficients / molar absorptivity values and absorption band assignments. Molar absorptivity values ( ) were calculate d based on Absorbance (A) = b c (1)

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71 Equation 3.1 where (A) equates to the absorbance of the sample, b is the cell path length (1 cm), and c is the sample concentration (M). The free ligand ( TPA 1 ) absorption is dominated by high energy short axis polarized transitions of the bipyridine core at 301 nm as well as a long axis polarized transition feature incorporating both the bipyridine core and the triphenylamine end caps that appear at 399 nm. It should be noted that in the free ligand t he bipyridine unit is not in a coplanar twisted confirmation in its unbound low energy state. As seen below, this has an impact on the energy of the tra nsitions. The absorption of each of the metal complexes is very similar relative to the free ligand. In the condition where the ligand is complexed with a metal, however, the geometric constraints of the coordinated metal dictate the bipyridine central co re of the ligand are directed into a coplanar configuration relative the central axis of the ligand. With this planarity as a core platform, the ligand generates a longer effective conjugation length and one would expect an absorption red shift of the mai n features of the parent ligand. There is in fact a red shift of the 301 nm absorption bands to 334 nm for the bipyridine core in both Re 1 and Ru 1 and a shift to 336 nm in Ir 1 The red shift of the absorption associated with the long axis transition of the ligand moves from 399 nm to 460 nm for Re 1 and Ru 1 and 471 nm for Ir 1 This band, in addition to red shifting, becomes broader than in the free ligand system. The extinction coefficient values are consistent with those of other similarly metallated complexes. 17,20 22 Despite their

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72 Figure 3 5. UV visible absorption and emissions spectra. OD 0.1, solvent noted above (deoxygenated for emission). ex : TPA 1 399 nm, Re 1 460 nm, Ru 1 463 nm, and Ir 1 470 nm.

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73 Table 3 1: Near UV visible absorption bands of target ligand and metal organic complexes. Complex max / nm ( max / M 1 cm 1 ) Assignment TPA 1 b 301 (48,200) 399 (73,900) bpy (short axis) ligand (long axis) Re 1 a 334 (55,000) 460 (49,300) TPA 1 (short axis) ligand (long axis) & MLCT Ru 1 a 290 (74,300) 334 (5 7,600) 462 (46,500) bpy TPA 1 (short axis) ligand (long axis) & MLCT Ir 1 b 297 (52,600) 336 (59,500) 470 (45,600) ppy TPA 1 (short axis) ligand (long axis) & MLCT a Measure ments were collected with optically dilute (OD~0.1) CH2Cl2 a or THF b solutions at 25 C. absolute values being lower than the 74,000 cm 1 M 1 extinction coefficient value for the band in the free TPA 1 ligand it appears overall oscillator strength i s maintained. As the electronic nature of these bands change, variations of the individual values of the extinction coefficients should be expected. Addition of the metals Re, Ru, and Ir to the ligand affords an electronic transition pathway between the metal and the ligand with the potential to transfer charge density from the metal to the ligand. This metal to ligand charge transfer (MLCT) represents an added energy transition which, as previously discussed, is similar in energy to the transition and therefore should manifest its presence by introducing absorption in the general vicinity of the transitions. The MLCT absorption arises as a result of the transfer of an electron from the d orbital of the

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74 metal to the orbital of the ligand. If the absorption bands centered at 460 nm (471 nm for Ir 1 ) are assigned as pure spin allowed MLCT bands, the extinction coefficients would be expected to be much smaller and in the range of 5,000 for typical (diimine)Re I (CO) 3 Cl 62, 63 and 15,000 for polypyridine Ru II compounds, 9 therefore some of the absorption presented fo r the metal complexes are considered to contain both and MLCT character. As an example, in the absorption band of Ru 1 centered around 460 nm, a hint of a shoulder exists at 405 nm which could be evidence of MLCT absorption obscured beneath the main absorption band of stronger overall oscillator s trength. This implies the overall nature of these absorption bands is made up of the combination of red shifted intraligand charge transfer (ILCT) character rising from the conjugation enhancement of the bipyridine rings at the core of the TPA 1 ligan d and MLCT character of the d to transitions from the coordinated metals. Emission Spectra The mission spectrum of each compound was obtained at room temperature utilizing an optically dilute, deoxygenated THF solution. Lifetimes were obtained with tim e correlated single photon counting utilizing 375 and 450 nm excitation sources. Emission quantum yields were measured by relative actinometry with 9,10 diphenylanthracene in cyclohexane ( em = 0.90) 64 and Ru(bpy) 3 in water ( em = 0.037) 1, 65 as the actinometers for TPA 1 and the metal complexes respectively. The emission spectra for the series are presented in Figure 3 5. Table 3 2 presents the list of emission maxima, quantum yields, and lifetimes. Excitation of TPA 1 at 301 and 399 nm yielded identical results and presented a very intense, short lived, broad emission at 512 nm with a quantum yield of unity ( fl = 1.0). Lifetimes of both aerated and deoxygenated solutions are both less than 5 ns.

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75 Each metallated complex, in contrast, displayed very weak emissions and long lifetimes. Emission spectra for Re 1 and Ir 1 afforded broad and featureless emissions while Ru 1 afforded a narrower emission band with a hint of structure. Re 1 emission was observed at Table 3 2: Ph otophysical properties of target ligand and metal organic complexes.a Complex em nm em b ns em 10 3 k r c s 1 10 6 k nr c s 1 TA d ns TPA 1 2 512 2 .3 1.0 Re 1 1 696 3 300 0.0051 1.5 0.3 441 Ru 1 1 700 796 0.0018 2.3 1.3 431 Ir 1 2 742 1600 0.0009 0.5 0.6 428 a Measurements were conducted on Ar deoxygenated CH 2 Cl 2 1 or THF 2 solutio n at 298 K. b Lifetimes were calculated with a single exponential decay fit. c k r = em / em ; k nr = 1/ em (1 em ). d Triplet lifetimes were calculated transient absorption spectra with a single exponential decay fit. excitation wavelengths of 334 and 460 nm. Upon excitation at 334 nm, Re 1 generated two broad and weak emission bands at 520 and 696 nm ( fl = 0.0051), presenting at relatively the same intensity. When observed at an excitation wavelength of 460 nm, emission at 520 nm was negligible and the broad emission at 700 nm was the sole emission feature. The lifetime of the 520 nm emission is very short, less than 5 ns, and can be assigned as fluorescence generated from the state of the ligand. The short lifetime and low quantum yield of this f luorescence is due to quenching brought about by rapid intramolecular energy transfer to the MLCT manifold and intersystem crossing resulting in energy states residing at lower energies. It has been noted in previous studies, that even though fluores cence attributed to a ligand complexed with a transition metal is not typically observed, a ligand with a large radiative decay rate allows state is short lived. 60 The broad emission of Re 1 centered at 696 nm ha ve the lifetime s of 590 ns for an aerated solution and 3.3 s for a solution which has been deoxygenated with Ar for 30 minutes. Long emission lifetimes that are prolonged with the removal of oxygen strongly suggests that it can be assigned to emission originating from a triplet manifold comprised of 3 and 3 M LCT excited states.

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76 Ru 1 emission was observed with excitation at wavelengths of 336 and 463 nm. Upon excitation at 336 nm, Ru 1 also generated two broad and weak emission bands at 520 and 700 nm ( fl = 0.0018), however relative intensity of the two emi ssions differed significantly. The intensity of the 700 nm emission was on the order of six times larger than that of the 520 nm 463 nm only emission at 700 nm was observed with an overall shape identical to that of the 334 nm excitation. As before the lifetime of the 520 nm emission was less than 5 ns and the emission can be attributed to fluorescence generated from the state of the TPA 1 ligand, and the broad emission centered on 700 nm, aerated = 37 0 ns and deoxy = 796 ns, again suggesting assignment as 3 and 3 MLCT emission. Ru 1 was the only complex to exhibit emission with a 3 MLCT feature. The overall emi ssion shape was broad but a narrow prominence at 700 nm was observed. This narrow feature is assigned as a 3 MLCT emission superimposed on the much broader and weaker 3 emission suggesting Ru 1 may exhibit more 3 MLCT character than either Re 1 or Ir 1 Ir 1 emission was observed at excitation wavelengths of 335 and 470 nm. Ir 1 similar to the other two complexes, yielded very weak emission. The quantum yield ( fl = 0.0009) was the lowest of the three complexes, and unlike Re 1 and Ru 1 only a broad emission centered at 742 nm dominated the spectrum with no indication of fluorescence from the ligand. This suggests a more efficient energy transfer or a faster intersystem crossing brought about by iridium. Lifetime s of the 7 42 nm emission are: aerated = 44 0 ns and deoxy = 1.6 s and are also assigned to a mixed 3 / 3 MLCT manifold.

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77 Transient Absorption Transient absorption spectroscopy was performed for all compounds in an effort to provide further information on the electronic nature of their exci ted states. Deoxygenated CH 2 Cl 2 or THF solutions with optical densities of approximately 0.8 at the excitation wavelength of 355 nm were used for this study. Transient absorption spectra following 5 ns pulsed laser excitation at 355 nm are presented in F igure 3 6. Excited state absorption was monitored in both the visible (350 nm to 800 nm) and near IR (800 nm to 1600 nm) regions utilizing two separate detection sources. Data from the two sources were interlaced, without modification, to present a contin uous spectrum. A transient absorption spectrum of TPA 1 was attempted but insufficient triplet population was generated to record triplet triplet absorption. The transient absorption spectra for the metal organic complexes feature ground state bleaching at 360 and 440 nm. These correlated well with the short and long axis ground state absorption of each complex. Each complex presents moderately broad excited state absorption in the visible region as well as a very broad and strong excited state absorbance in the near IR (Figure 3 6). The excited state absorption band s in the visible and near IR present moderately long lifetimes of just under 0.5 s in duration. (Table 3 2) Similar to the emission spectra, the transient absorption spectra for Re 1 and Ir 1 were similar and Ru 1 was much different. In the TA spectrum, the relationship of relative Absorbance difference between the excited state absorbance in the visible region versus that of

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78 Figure 3 6. Transient absorption spectra of metal organic complexes. Solvent (deoxygenated ) THF ( TPA 1 Ru 1 Ir 1 ) and CH 2 Cl 2 ( Re 1 ), OD 0.8, laser excitation 355 nm, 5 ns pulse.

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79 the excited state absorbance in the near IR region provided information on the triplet character of the excited states. As described in Chapter 1, a prominent narrow visible excited state absorption equal to or exceeding the strength of a broader near IR excited state absorption (Figure 1 5a) is indicative of an excited state of mostly 3 MLCT character. The TA spectra for Ru 1 presents this type of relationship and is therefore afforded a triplet triplet absorption assignment that is mostly 3 MLCT in nature. In contrast, the transient absorption spectra of Re 1 and Ir 1 are identical in character. Both exhibit relatively weak visible excited state absorption and strong near IR excited state absorption which is an indication of an excited state of mostly 3 character (Figure 1 5b). Further correlation of these assignments is based on the comparison of emission and excited state lifetimes. In systems whose excited states are mainly 3 MLCT, triplet excited state lifetimes are roughly equivalent to the em ission lifetime. This is the case for Ru 1 In the case of Re 1 and Ir 1 their triplet excited state lifetimes are mush different than their respective emission lifetimes. This is an indication that the excited state is dominated by 3 character. D espite the difference in excited state character, it is interesting to note that all three complexes afford nearly identical excited state lifetimes and decay profiles. This similarity is likely due to the mixed 3 / 3 MLCT triplet manifold proposed ear lier for in these types of systems. In the excited state decay profiles in Figure 3 7 each complex displays a fast decay component which is constant and reproducible. Due its extremely fast decay and the timescale limitations of the instrument further inv estigation into its origin was not possible. However a logical explanation for this behavior would be a preferential population of a higher energy triplet excited state of either 3 or 3 MLCT character and a subsequent relaxation of the excited state into the lower lying manifold of mixed 3 or 3 MLCT character. To address the equivalent

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80 Figure 3 7 Excited state lifetimes of metal organi c complexes. Solvent (deoxygenated) THF ( TPA 1 Ru 1 Ir 1 ) and CH 2 Cl 2 ( Re 1 ), OD 0.8, laser excitation 355 nm, 5 ns pulse. Spectra fit with single exponential decay fit.

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81 triplet excited state lifetimes, the model of an equilibrated 3 / 3 MLCT manifold combined with a common decay pathway is also logical. Even when either the 3 or 3 MLCT excited state is dominant, if a relative excited state energy equilibrium is present, the most efficient decay pathway would be preferred and equivalent ex cited state lifetime would be afforded. Nonlinear Absorption Determination Two Photon Emission Two photon excited emission spectra of each compound were obtained at room temperature utilizing aerated solutions at solution concentrations of 10 mM for all co mpounds except Re 1 which was 2.0 mM. A comparison of the one and two photon emission spectra are presented in Figures 3 9 and 3 10. Two photon excitation was accomplished utilizing only minor modifications of the time resolved emission instrumentation se tup currently in use. Directing of the optical path and the addition of focusing optics were the major modifications that allowed the current instrumentation setup to be utilized for two photon excited emission detection. Two factors became immediately a pparent as key to the success of this instrument being able to detect the presence of emission of a two photon excited species. These two factors were a tightly focused beam and sufficient laser energy. First, the need to focus the incident laser pulse t o provide the needed excitation intensity to produce two photon excitation was vital since the probability of two photon excitation is proportional to the square of the intensity of the incident photon flux. In order to provide the necessary intensity of photon flux, a plano convex lens with a nominal focal length of 9.5 cm was placed on single axis translation stage in an appropriate position in the beam path. Beam diameter at the lens was measured at 28 mm. A calculated beam waist of 30 m was attained assuming a Gaussian beam shape and energy densities of approximately 1.5 J/cm 2 at 5.0 mJ were

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82 generated. The translation stage was included to insure the tightly focused beam was centered (front to back) in the 1 cm sample cell and to aid in focusing of the input beam. As the excitation wavelength was changed the translation stage was adjusted to compensate for minute changes in the final focus of the excitation beam, Figure 3 8. Figure 3 8. Two photon emissi on instrument modification showing focusing optics, sample holder, emission collecting mirrors and spectrograph entrance slits. Tunable wavelength laser energy was provided by an optical parametric oscillator (OPO) pumped by the third harmonic of a 1064 nm Nd:YAG laser which provides an available wavelength range of 420 570 nm and 590 1200 nm. Despite the continuum of wavelengths available from the OPO equipped laser source, it is noted that available laser power at any given Plano C onvex lens f = 95 mm dia.= 50.8 mm Sample Holder 1 cm cell Con cave mirriors f = 100 mm dia.= 50.8 mm Turning M irrior dia .= 24.8 mm Spectrograh entry slits Laser S ource (variable) 420 1200 nm

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83 Figure 3 9 Two Photon Emission Spectra of TPA 1 (10 mM in aerated THF, 1 cm cell) following focused laser excitation at 790 nm, 6 mJ / 5 ns pulse (top) and 640 nm, 5 mJ / 5 ns pulse (bottom). The comparative one photon excited steady state e mission spectra is the same as presented in Figure 3 5. wavelength is highly variable. This is especially true near the limits of the wavelength ranges, where the available power drops off dramatically. Careful consideration of available wavelengths with sufficient energy led to the choice of utilizing the wavelengths 640, 790, 940

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84 Figure 3 10 Two photon emission spectra of metal complexes (concentrations noted) in deoxygenated solvent, 1 cm cell) following focused laser excitation at 970 nm, 5 mJ / 5 ns pulse. The comparative one photon steady state emission spectra are the same as Figure 3 5.

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85 and 970 nm for the excitation wavelengths for this experiment. For the excitation wavelengths of 640 and 790 nm, a 5 ns d elay of the CCD detector was utilized to avoid detector saturation. It has been well documented that the peak of two photon absorption occurs at wavelengths considerably shorter (more red) than simply two times the one photon maximum. 66 This, coupled with moderately broad and reasonably intense absorption bands of the ligand and metal organic complexes, allowed for utilization of wavelengths 20 nm more blue and greater than 40 nm more red than the calculated desire d wavelength of twice the one photon absorption maximum. Successful two photon excitation of TPA 1 w as conducted at the projected two photon wavelengths associated with the major absorption bands of 301 nm and 399 nm. Despite the extremely short lifetime of the fluorescence from TPA 1 and the needed gating of the detector to avoid saturation, sufficient emission signal was captured due to the intense spectral shap e and lifetime similar to those found upon one photon excitation. One photon emission spectra were acquired on a different corrected instrument (PTI Felix 32I fluorometer), therefore the spectral shift of the uncorrected two photon data is difficult to di rectly compare. However, it was noted the emission afforded by two photon excitation did not shift in wavelength with a change in excitation wavelength. Two photon excitations of Re 1 Ru 1 and Ir 1 were also found to produce detectable, albeit weak, emis sion. Despite the longer emission lifetime of the complexes compared to the free ligand TPA 1 insufficient emission intensity was present in all complexes to accomplish a time resolved emission determination of lifetime. Because of the weak emission int ensity, all two photon excitation emissions were collected with an extended CCD gate time of 100 ns. The

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86 resulting emissions again presented a spectral shape similar to those found upon one photon excitation. Since the one photon emission spectra were a cquired on a different instrument (PTI Felix 32I fluorometer) which is corrected, the spectral shift is again unresolved. Emission wavelength afforded by two photon excitation was again not altered with a change in excitation wavelength, identical to the behavior found in the free ligand. Nonlinear Absorption of C 60 C 60 has been considered among the best of all materials for optical limiting of laser radiation at 532 nm since the mid 67 More recently, experimental evidence of C 60 ability to present a nonlinear response at 1064 nm has again focused attention on this molecule as viable optical limiter. 57 The nonlinear absorption and optical limiting potential of C 60 in a toluene solution was first reported by Tutt and Kost in 1992 68 utilizing a nanosec ond pulsed Nd:YAG laser at 532 nm. Since that time, extensive investigation of the optical limiting potential of [60]fullerene and [60]fullerene derivatives has been pursued. Initial investigations into the origin of the NLO response of C 60 at 532 nm focu sed on reverse saturable absorption (RSA) as the primary nonlinear mechanism responsible for its NLO activity. Recent NLO evaluations of C 60 at 1064 nm focused on two photon absorption as the mechanism responsible for its NLO activity. For these reasons and due to its commercial availability, C 60 was examined in the present study as a benchmark compound for the newly developed nonlinear transmission apparatus, and to establish the potential utility of C 60 as an optical limiting standard for use to judge the strength of optical limiting in the series of dual mechanism limiting metal organic chromophores discussed here. Utilizing room temperature solutions of C 60 in toluene, McLean et al 69 used a five level modeling of RSA to correlate the optical limiting responses with ground and excited state absorption cross sections in C 60 Excellent correlation of experimental optica l limiting results

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87 towards nanosecond pulsed 532 nm were obtained utilizing light fluences up to ~ 1 J / cm 2 and their data were subsequently supported by study findings of other researchers. 70 Despite these strong correlations provided by a five state model supporting RSA based limiting due to strong triplet triplet absorption, the overall mechanistic details on the optical limiting properties of fullerenes remains a subject of debated. Subsequent studies have suggested additional NLO mechanisms provide contributions to the overall efficiency of C 60 as an optical limiter. 71 73 Thes e studies point to experimental evidence that optical limiting performance of C 60 is different in a solid matrix when compared to that of a solution. The differences in performance was determined not to be attributed to a change in nonlinear absorption be havior since it was shown ground state and triplet triplet absorptions of both a solid matrix and solutions remained relatively unchanged. This presents the possibility that contributions from other nonlinear processes, such as optical scattering, may pla y an important role in the optical limiting response in C 60 Table 3 3: Optical limiting properties of C60 67 Transmit tance Limiting threshold a (J/cm 2 ) I out at saturation (J/cm 2 ) 55% 0.18 65% 0.20 70% 0.31 82% 0.45 a The optical limiting threshold is defined as the input fluence at which output fluence is 50% of that predicted by linear transmittance. In a comparison study accomplished by Sun and Riggs 67 baseline C 60 optical limiting data was presented and is shown in Table 3 3. For the study, an Nd:YAG laser generating the second h armonic, 532 nm, was used. The input beam diameter was reported as 6 mm and output ranged from 5 160 mJ with yielded corresponding energy densities of 0.02 0.60 J/cm 2

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88 In addition to RSA based optical limiting of C 60 at 532 nm, studies have also shown C 60 has the capability of two photon absorption at 1064 nm. Investigations of the nonlinear refractive indices of C 60 and C 7 0 in toluene were performed by Usmanov et al 57 utilizing Z scan and the resultant data for C 60 was presented in Figure 2 11. In addition, optical limiting experiments utilizing similar apparatus methodology as was used herein discovered only the toluene solution of C 60 presented two photon nonlinear absorption (Figure 3 11). 57 Figure 3 11. Power dependence, two photon generated optical limiting determination of 0.5% wt C 6 0 in toluene at 1064 nm. 57 It has been observed during the course of our investigations while using C 60 as a benchmark, extreme diligence must be tak en to ensure C 60 has been completely dissolved into solution. Incomplete solubility, even at concentrations of less than 1.0 mM, was found to lead to linear transmissions well below the values stated in the literature as well as evidence of optical scatte ring leading to extreme variations in the transmitted energy through the sample. Additionally, comparison of C 60 NLO data was, at times, difficult given the relatively low initial transmittance of C 60 solutions. A decision to use a concentration which ma tched the initial transmittance value shown in Figure 3 11 was made based on previous experimental techniques

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89 used by other groups at different wavelengths (Table 3 3). The concentration that yielded initial transmittances most representative to the previ ous 1064 nm data was 3.0 mM in toluene. This concentration is one better NLT responses. Despite these potential drawbacks, utilization of C 60 as a benchmark for comparison of compound s without suitable model compounds of known nonlinear optical character was deemed a valid choice due to its well documented nonlinear optical properties at wavelengths utilized in this project. Nonlinear Absorbance Determination Following positive confirm ation of the ability to two photon excite TPA 1 and each of the metal organic complexes during the two photon excited emission experiment, examination of the optical limiting character of this series was able to proceed. With the goal of detecting the nat ure and strength of a dual mode nonlinear response comprised of two photon excitation followed by excited state absorption from a strongly absorbing excited state it was deemed necessary to determine the appropriate wavelengths and concentration conditions which would exhibit the clearest evidence of the desired NLO responses. Initial efforts were focused on the determination of the nonlinear response and optical limiting potential of the free ligand TPA 1 The experimental determination of nonlinear resp onse utilized the setup as previously described in Chapter 2 and visually represented in Figure 2 15. For this portion of the study it was noted that only the TPA mechanism would present as the nonlinear absorption mechanism. Isolating and separately vie wing the nature and

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90 Figure 3 12. Nonlinear determination of TPA 1 at 645 nm (top) and nonlinear transmission curve of TPA 1 at 645 nm (bottom). strength of the TPA response for the free ligand was a deliberate attem pt to obtain two photon only NLO data with which to judge the magnitude of enhanced NLT response gained by the addition of the ESA capable metal based chromophore. Nonlinear response determinations for TPA 1 were accomplished at 645 nm (Figure 3 12) and 9 70 nm (not shown) for concentrations

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91 of 10 mM and 20 mM. Despite acquiring effective two photon excited emissions at both 645 and 970 nm, only 645 nm elicited a nonlinear response detectable in the energy ranges utilized for these experiments. At 645 nm, TPA 1 exhibited a moderate departure from linear absorption which correlates to an observed 12% and 23% reduction in transmittance for 10 and 20 mM respectively. At 970 nm, however, no departure from linear absorbance was noted for sample input energies up to approximately 5.0 mJ. A possible explanation of this apparent inconsistency in findings is that 970 nm is near the far red extreme of the two photon excitation envelope of TPA 1 Most likely, two photon excited emission was observed due to the high quantum efficiency of the free ligand despite having a relatively low two photon cross section at this wavelength. Additionally, it was confirmed that TPA 1 presented as optically transparent at both 645 and 970 nm even for concentrations as high as 20 m M. Concentrations higher than 20 mM are possible but were not attempted since concentrations utilized for the metal complexes that this data is meant to be compared with were limited to this concentration or less and will be discussed later in this sectio n. Referring to the quadratic and cubic dependence NLT models presented in Chapter 2, it is clear that TPA 1 follows a quadratic relationship between attenuated intensity and incident light intensity for both 10 and 20 mM, fitting the TPA only model profi le. Once nonlinear data from two photon only excitation of the free ligand TPA 1 was successfully obtained tests were performed to determine the range of excitation wavelengths which could elicit a two photon induced excitation of the metal organic chromo phores Re 1 Ru 1 and Ir 1 to the appropriate singlet excited state. Information from ground state absorption data was used to choose a potentially suitable wavelength range. For the series, a generous range of strong ground state absorption exists for a ll the complexes from 250 nm to 550 nm. This equates

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92 Figure 3 13. Ground state absorption spectra for the near IR of 2.0 mM metal organic complex solutions in CH 2 Cl 2 to a potential two photon active range for the T PA chromophore of 500 1100 nm. Optimization of TPA excitation conditions was deemed essential due to the need for a sufficient singlet excited state population which could in turn be converted through intersystem crossing to triplet excited states availab le for enhanced nonlinear absorbance through an ESA mechanism. In addition to

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93 the ground state absorption data, a great deal of consideration was given to the two photon excited emission data discussed in a pervious section in choosing an effective wavele ngth to utilize. A systematic survey of wavelengths and chromophore concentrations was carried out to determine the optimum conditions with which to proceed. The wavelength range examined was 800 to 970 nm. Preliminary test results of the surveyed wavel engths revealed positive nonlinear results for the wavelength range. During this conditions survey, acute solubility limitations for the series became a significant factor. As concentrations were increased each metal complex exhibited very different solu bility and optical transparency characteristics. For Re 1 it was noted that solution saturation occurred at chromophore concentrations of just over 3.0 mM in CH 2 Cl 2 In THF, benzene, toluene, and DMSO were tested for evidence of higher solubility but it was determined none would support higher concentrations than that of methylene chloride. Despite the low solubility of Re 1 it was noted that optical transparency remained high even at near saturated solution concentrations. For Ru 1 solubility initia lly seemed less critical and solution concentrations of up to 10 mM were obtainable. Optical transparency however dropped off very rapidly as concentrations increased. Even at a concentration of 3.0 mM, initial transmission values of 80% were observed (n ot shown) .To eliminate ground state absorption as the reason for this lack of transparency; absorption spectra were obtained in the near IR region out to a wavelength of 1400 nm. Ground state absorption data obtained at concentrations of 2.0 mM for all t hree complexes showed no absorbance for wavelengths ranging from 550 nm through 1400 nm as shown in Figure 3 13. The reason for this lack of optical transparency at higher concentrations of Ru 1 is presumed to be due optical scattering. Previous NLO stud ies, where low optical transparencies were experienced, such as C 60 the reason given is optical scattering. For this reason it is suspected that incomplete solubility at higher concentrations of

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94 Ru 1 may indeed be a factor. In stark contrast to Re 1 and Ru 1 Ir 1 exhibited very high solubility in conjunction with high optical transparency for solution concentration up to 20 mM. The test survey at 970 nm concluded with concentrations of 10.0 mM for Ir 1 7.5 mM for Ru 1 and 2.0 mM for Re 1 showing the m ost consistent and greatest magnitude NLT responses. The results of the nonlinear determination at 970 nm are shown in Figure 3 14. Assembled in Figure 3 15 are the nonlinear transmission plots for the series at 970 nm at the same concentrations. Notewor thy is that despite a variation of chromophore concentration the energy of nonlinear onset appears to remain at 100 J for each complex. This is the energy that was observed for the onset of limiting for the free ligand TPA 1 (Figure 3 12). This correlat ion of onset energies Figure 3 14. Nonlinear determination results of metal organic chromophores; Re 1 Ru 1 and Ir 1 at 970 nm in CH 2 Cl 2 solutions (concentrations noted). suggests the nonlinear threshold is controll ed more by the nature of the TPA ligand and less by the character of the ESA chromophore. As expected overall limiting response was greater in Ir 1 which had the highest chromophore concentration and achieved the greatest reduction in

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95 transmission (100% t o 62 %). Ir 1 also remained completely transparent until the point of limiting onset. Ru 1 displayed an overall limited transmittance of 60% at 2 mJ but had an initial Figure 3 15. Nonlinear transmission results of metal organic chromophores; Re 1 Ru 1 and Ir 1 at 970 nm in CH 2 Cl 2 solutions (concentrations noted). transmission of 87% in the low energy linear region. Re 1 which had the lowest concentration and was near solution saturation retained a transmittance of 97% and still managed to attenuate the transmittance to 75% at 2 mJ. A second experimental study of the metal organic series was undertaken at 1064 nm. From results gathered in the NLT test series in Chapter 2 where a cubic relationship between attenuate d intensity and incident light intensity can be used to confirm the presence of a dual mode TPA/ESA mechanism, it was determined a study of limiting characteristics at higher incident energies may provide more insight into the overall character of the comp mechanism. Each of the complexes present strong excited state absorption around 1064 nm as seen in Figure 3 6 and since this wavelength is available as the fundamental output from the Nd:YAG laser it was decided to modify the nonlinear dete rmination apparatus to accept higher

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96 flux 1064 nm laser energy. Figure 3 16 shows a photo rendered schematic of the modified nonlinear transmittance setup for 1064 nm. A key modifications to the apparatus involved the attenuation of the high energy fundam ental beam emanating from the Nd:YAG laser. Laser outputs greater than 15 mJ at 1064 nm were necessary to achieve a stable beam profile which is well beyond the damage threshold of the sample cells and the energy detectors being utilized. Figure 3.17 dep icts the schematic of the system and clearly shows the addition of a 3:97 beamsplitter placed in the pathway of the main source beam. With the addition of this beamsplitter only 3% of the main beam is diverted to the main NLT apparatus. Additionally a st atic 30% transmission filter was placed prior to the continuously variable neutral density filter for further protection and control of the incident beam at higher incident powers. The focusing optic was also replaced by a 50.8 mm plano convex lens with a nominal focal length of 13 cm. The beam diameter at the lens was 10mm. Again utilizing Equation 9 from Chapter 2, a calculated beam waist of 130 m was attained assuming a Gaussian beam shape with an energy density range of 0.005 0.90 J/cm 2 The final determination of the nature of the response for the series Re 1 Ru 1 and Ir 1 was conducted at the two photon excitation wavelength of 1064 nm. Concentrations of 1.0, 3.0 and 5.0 mM in CH 2 Cl 2 for Ru 1 and Ir 1 and 1.0, 2.0 and 3.0mM in CH 2 Cl 2 for Re 1 were prepared and examined in an effort to compare overall optical limiting character for common chromophore densities. In Figure 3 18 the NLT res ults are presented for each complex at

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97 Figure 3 16. Photo schematic of modified nonlinear transmission setup. Figure 3 17. Schematic drawing of the modified nonlinear transmission setup. 1064 nm Beamsplitter 3:97 Beam Dump Beamsplitter 25:75 Low Energy Pyroelectric Detector Channel A Neutral Density Filter : 30%T Sample Holder 1 cm cell Plano C onvex Lens f = 130 mm dia. = 50.8 mm Neutral Density Filter 0.1 100% Low Energy Pyroelectric Detector Channel B Laser source Nd:YAG 1064 nm Energy Meter PC ND filter optional ND filter 0.3 100% T Plano convex lens, f = 13 cm BS 3:97 BS Sample Dump D etector 1 Detector 2

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98 Figure 3 18. Nonlinear determination results for Re 1 in 1, 2, 3 mM, Ru 1 and Ir 1 in 1, 3, and 5 mM CH 2 Cl 2 solutions at the two photon excitation wavelength of 1064 nm.

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99 varying concentrations. Along with the complex data, a comparative plot of the nonlinear response for a 3.0 mM solution of C 60 in toluene complied under identical experimental conditions is included. Also included with the nonlinear determination plot of Ir 1 is an additional comparative plot for a 1.0 mM so lution of C 60 in toluene and presents overlaid on the solvent plot. It of interest to note that for C 60 no detectable nonlinear response is obtained at a 1.0 mM concentration at 1064 nm under these experimental conditions with the energy densities approa ching 1 J/cm 2 For the 3.0 mM solution of C 60 a larger magnitude NLO response compared to the literature, was noted (Figure 3 11). Despite beginning at near equal transmittances (~85%), a final transmittance of 25% was attained at 9 mJ versus 65 70% for the previously sited study. For each complex excellent nonlinear response was noted at all concentrations and with the exception of Re 1 at 1.0 mM all exhibited an output energy saturation level referred to as energy clamping or limiting saturation. Comp ared to the free ligand TPA 1 with only two photon absorbing abilities where no clamping was noted for the energy range examined, a significant enhancement in NLO response brought about by the addition of the ESA active chromophore is evident. The limitin g response is seen more clearly in the optical limiting plots of the same data. Figure 3 19 presents the dramatic NLT character of this series even at relatively low chromophore concentrations. Once again the data obtained for the 3.0 mM C 60 solution is shown as a relative comparison of optical limiting strength. For all the complexes, the highest energy limiting data for 3.0 mM concentration shows equivalent or better limiting and is trending to far surpass the C 60 limiting response for energies just be yond those sampled in these experiments.

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100 Figure 3 19. Nonlinear transmission of Re 1 Ru 1 and Ir 1 CH 2 Cl 2 solutions at various concentrations at 1064 nm. 3.0 mM C 60 in toluene data is included as a relative standa rd.

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101 Also evident from the NLT plots in Figure 3 19, is the stark difference in relationship between attenuated intensity and incident light intensity. Clearly the C 60 model exhibits a quadratic dependence which is what would be expected for the proposed T PA nonlinear absorption mechanism for this molecule. In contrast, each metal organic complex exhibits a definitive cubic dependence between attenuated intensity and incident light intensity. This result is considered benchmark evidence of an enhanced dua l mode TPA/ESA mechanism present in the metal organic chromophore series. Even more dramatic is the comparison of higher concentrations of Ir 1 versus C 60 with regards to overall optical limiting. Compared to the relatively low solubility of Re 1 and the low optical transparency of Ru 1 the high solubility and high transparency of Ir 1 proved to be a great advantage for the further enhancement of the optical limiting ability of this complex. As seen in Figure 3 20, even at concentrations of 20 mM Ir 1 r emains optically transparent at low laser powers for 1064 nm. In stark contrast, 3.0 mM C 60 at best transmits 85% at low laser energies at a 3.0 mM concentration for 1064 nm. In addition, both 10 and 20 mM solutions of Ir 1 surpassed the optical limiting capabilities of C 60 at high energies. The 10 mM sample of Ir 1 out performs C 60 at input energies approaching 5 mJ and 40% transmittance where as the 20 mM surpassed the limiting capabilities of C 60 at only 175 J at a 80% transmittance. A definitive cu bic dependence between the attenuated intensity and incident light intensity is present in the 10 mM solution of Ir 1 For the 20 mM Ir 1 solution NLT data, the cubic dependence seemingly breaks down. This inconsistency is likely due to the onset of an a dditional NLO mechanism or effect. Several times during the course of this NLT study it was noted that an unexpected and inconsistent decrease of transmittance would present at higher energy densities. Examples of this can be seen in Figures 3 18 for Re 1 at 2.0 mM and Ru 1 at

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102 3.0 mM as well as in Figure 3 21 for Ir 1 at 10 mM. Occasionally these reductions in transmission were accompanied by an audible popping noise. The source of the noise was confirmed not to be from material degradation of the sampl e cell, but the possibility of solution heating in the vicinity of the beam focus can not be ruled out. If this is the case, the apparent Figure 3 20. Nonlinear transmission for 10 and 20 mM, CH 2 Cl 2 solutions of Ir 1 at 1064 nm. 3.0 mM C 60 in toluene data is included as a relative standard. transmission reduction could be due to a thermal lensing effect as opposed to an intrinsic NLO mechanism of the complexes. Despite these occasional anomalies the 10 and 20 mM so lutions consistently limit incident energies of up to 9 mJ to less than 200 and 100 J respectively by means of an enhanced dual mode NLO mechanism. Discussion The desired goal of developing a novel series of metal organic chromophores that exhibit an enhanced dual mode nonlinear response utilizing both two photon excitation and excited state absorption was successfully realized. Also realized was the ability to detect and semi

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103 quantitatively characterize the nature and relative strength of an NLT active chromophore utilizing an in house developed nonlinear transmission apparatus. Figure 3 21. Nonlinear determination results for Ir 1 in 10 and 20 mM CH 2 CL 2 solutions at a two photon excitation wavelength of 1064 nm. The highly soluble TPA chromophore TPA 1 was confirmed to elicit a robust two photo n response capable of optical limiting by a two photon mechanism at 645 nm. Complexation of TPA 1 with Re(CO) 3 Cl, Ru(bpy) 2 2+ and Ir(ppy) 2 + yielded the metal organic complexes Re 1 Ru 1 and Ir 1 which were shown to enhance the nonlinear capabilities of T PA 1 with the addition of strong triplet triplet absorptions leading to the establishment of an NLO enhancing excited state absorption mechanism. A systematic study of the presence and relative strengths of enhanced dual mode nonlinear absorption in the t arget metal organic series was successfully compared to a solution of C 60 for which the nonlinear characteristics and mechanisms are known. This study was accomplished utilizing the NLT apparatus for detecting: the presence of

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104 nonlinear absorption, the de termination of the relative strength of optical limiting, and the differentiation of a single verse dual mode optical limiting mechanism, During the photophysical investigation of the target compounds; ground state absorption, emission and transient absorp tion (TA) spectra were used to identify the triplet excited character of the metal organic chromophores. The triplet excited state of Ru 1 was identified as having mostly 3 MLCT character based on the overall structure of its TA spectrum. Ru 1 affords a s trong and relatively narrow visible excited state absorption as well as a broad near IR excited state absorption of equal intensity. This general TA profile along with typical MLCT features such as structured excited state emission and evidence of a shoul der on the lowest energy ground state absorption spectra, all point to a definitive classification of the excited state of Ru 1 having mostly 3 MLCT character. Similar characterization methodology was used to identify the triplet excited states of Re 1 and Ir 1 as having mainly 3 character. Both complexes exhibited broad and featureless excited state emission and absorption spectra. Additionally, their TA spectra clearly exhibited features previously shown to be characteristic of a triplet excited state having mostly 3 c haracter. These features are a broad and weakly absorbing visible excited state absorption and a very strong near IR excited state absorption. During the course of the NLT investigations, validation of the detection capability and establishment of experim ental reliability of the NLT apparatus was accomplished. Additionally, it was shown that the NLT apparatus could be used to distinguish the difference between a system having a TPA or RSA only NLO mechanism and an enhanced dual mode TPA/RSA mechanism. Ut ilizing a published model meant to characterize NLO mechanisms in conjunction with data obtained in the course of these investigations, the differentiation of a single NLO process from a dual mode mechanism was accomplished successfully. For a single NLO

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105 process, such as TPA or RSA, a quadratic dependence between the attenuated intensity and incident light intensity is easily differentiated from the cubic dependence exhibited for a TPA/ESA dual mode mechanism. The later finding will prove to be extremely useful in subsequent experimental studies conducted by our group to aid in distinguishing the nature of NLO responses in nonlinear chromophores with TPA, RSA, and dual mode NLO mechanisms. Lastly, it was successfully shown that the target chromophores TPA 1 Re 1 Ru 1 and Ir 1 all exhibit significant NLO activity. TPA 1 successfully achieved two photon excitation to produce an excited state equivalent to excited states produced from one photon ground state excitation. Being a strictly TPA chromophore, TP A 1 exhibited a quadratic dependence between the attenuated intensity and incident light intensity during the course of NLT experiments. Target metal organic chromophores Re 1 Ru 1 and Ir 1 were shown to be successfully two photon excited to produce equi valent excited states to those produced by one photon ground state absorption. Additionally, all three complexes exhibited enhanced TPA/ESA dual mode nonlinear absorption. Confirmation of the dual mode nature of an enhanced nonlinear absorption mechanism was verified by not only a relative strengthening of the overall NLO response but also by a representative cubic dependence between the attenuated intensity and incident light intensity. Experimental Instrumentation Nuclear Magnetic Resonance (NMR) spect ra were obtained with either a Varian Gemini 300 or a Varian VXR 300. No significant difference in data quality or resolution was noted between these two instruments. Mass Spectrometry data was provided by the University of Florida Mass Spectrometry Serv ices from a Bruker APEX II FTICR. UV Visible absorption spectra were obtained on a Varian Cary 100 dual beam spectrophotometer utilizing 1 cm quartz

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106 cells. Corrected steady state one photon emission spectra were obtained on a PTI Felix 32I fluorometer. Emission quantum yields were measured by relative actinometry with 9,10 diphenylanthracene in cyclohexane ( em = 0.90) 64 and Ru(bpy) 3 in water ( em = 0.037) 1, 65 as the actinometers for TPA 1 and the metal complexes, respectively. Time resolved emission decays were obtained with a Photochemical Research Associates FLT by time cor related single photon counting utilizing 375 and 450 nm IBH NanoLEDs as excitation sources and monitored without filters. Two photon emission spectra were obtained on an in house instrumented setup utilizing a Continuum Surelite II 10 Nd:YAG laser (355 nm third harmonic of the 1064 nm fundamental) augmented with a Surelite OPO Plus (420 1600 nm wavelength range) as the sample excitation source and directed through an Acton Research SpectraPro 150 dual grating spectrograph to a Princeton Instruments PI Ma x intensified CCD camera. Transient absorption spectra were obtained utilizing an in house instrumented setup 74 utilizing a Quanta Ray Nd:YAG laser (355 nm, third harmonic of 1064 nm fundamental) as the sample excitation source. Nonlinear absorption / transmittance spectra was obtained utilizing two optically similar in house instrumented setups utilizing either a Quanta Ray Nd:YAG laser (1064 nm fundamental) or the Continuum Surelite II 10 Nd:YAG laser augmented with a Surelite OPO Plus (420 1600 nm wavelength range) as a high intensity photon source. The laser source was attenuated by both fixed transmission and continuously variable neutral density filters to achieve the desired sample input energy and subsequently focused on the sample solution by 50.8 cm diameter plano convex lens of either 10 cm (for Surelite/OPO setup) or 13 cm (Quanta Ray 1064 nm setup). Change in transmittance as a function of laser power was monitored directly utilizing an Ophir Laserstar dual channel power meter configured with a pair of matching OPH PE10 SH V2 pyroelectric detectors (sensitivity range 10 J to 10 mJ).

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107 Materials Triphenylamine, 2 methyl 3 butyn 2 ol, N bromosuccinimide, and 2 phenylpyridine were purchased from Acros Organics and used in the condition received. Trans dichloro bis(triphenylphosphine) palladium (II), cis dichloro bis(2,2' b ipyridine) ruthenium(II) dihydrate, Re(CO) 5 Cl, and IrCl 3 hydrate were purchased from Strem Chemicals and used in the condition received. Trimethylsilylacetylene was purchased from GFS Chemicals and used in the condition received. 5,5' dibromo 2,2' bipyr idine, 5,5' trimethylsilylethynyl 2,2' bipyridine, and 5,5' diethynyl 2,2' bipyridine were synthesized using a significantly modified literature preparation. Full synthetic details are provided for clarity. All other chemicals/solvents not mentioned ab ove were purchased from commercial sources and used in the condition received. Synthesis Protonated 2,2' bipyridine (1) 75 Acetyl bromide (35.5 mL, 0.480 mol) was added dropwise (over a period of 30 minutes) into a solution containing 2,2' bipyridine (15.0 g, 0.096 mol) in 400 mL of MeOH. Upon addition of approximately 70% of the acetyl b romide, a pale yellow precipitate formed. The reaction solution was stirred for an additional hour, filtered, and the precipitate was washed with cold acetone yielding 27.46 g (91.7%) of 1 The NMR spectrum values were identical to values found in the li terature. 75 1 H NMR (300 MHz, D 2 0) 7.80 (m, 2H), 8.30 (m, 4H), 8.71 (dt, 2H). 13 C (75.4 MHz, D 2 0) 124.04, 127,36, 143.61,146.01, 147.12. 5,5' Dibromo 2,2' bipyridine (2) 75, 76 Protonated bipyridine 1 (5.0 g, 0.015 mol) and Br 2 (11.3 g, 0.071 mol) were combined in a stainless steel pressure reaction ves sel that was fitted with a Teflon sleeve and gasket. The apparatus was sealed pressure tight and heated to 180 C for 4 days. Heat was removed and the reaction vessel was allowed to cool to room temperature. The reaction vessel was placed on ice

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108 for 15 minutes before opening to ensure internal vessel pressure was eliminated. The dark orange material was removed and placed on a glass dish overnight to allow excess Br 2 to evaporate. The remaining solid was stirred in 1 M NaOH solution to deprotonate foll owed by extraction with CHCl 3 The fractions were combined, dried with MgSO 4 and the chloroform was removed under vacuum yielding a light tan solid. The solid was added to 40 ml of acetone and stirred for 30 minutes. The solid which did not dissolve was vacuum filtered yielding 5,5' dibromo 2,2' bipyridine as a white solid. The product was washed with two 10 mL aliquots of cold acetone and allowed to dry, yielding pure 5,5' dibromo 2,2' bipyridine (1.48 g, 4.69 mmol, 31.2%). The NMR spectrum values were identical to values found in the literature. 75, 76 1 H NMR (300 MHz, CDCl 3 ) 7.93 (qd, 2H), 8.29 (dd, 2H), 8.70 (dd, 2H). 13 C (75.4 MHz, CDCl 3 ) 121.45, 122.22, 139.60, 150.28 153.66. Bis 5,5' trimethylsilylethynyl 2,2' bipyridine 75, 76 5,5' Dibromo 2,2' bipyridine (1.60 g, 5.10 mmol), trimethylsilylacetylene (2.00 g, 1.4 mL, 20.40 mmol), THF (10 mL), and i Pr 2 NH (10 mL) were combined and degassed under argon for 30 minutes. 5 mol% Pd(PPh 3 ) 2 Cl 2 (0.180 g, 0.26 mmol) and 5 mol% CuI (0.05 g, 0.26 mmol) were added and the solution was dega ssed further for an additional 5 minutes. The reaction then heated to 70 C and refluxed under an argon balloon for 17 hrs. The solution was allowed to cool and filtered through a bed of sand/celite to remove CuI and ammonium bromide salts. The solution was vacuum evaporated to remove THF and i Pr 2 NH. The resulting solid was taken up in CHCl 3 washed with H 2 O, dried with MgSO 4 and solvent removed under vacuum. The solid was purified by flash chromatography on silica gel with 50:1 hexane ethyl acetate solution yielding bis 5,5' trimethylsilylethynyl 2,2' bipyridine as a flaky light tan solid (1.61 g, 4.61 mmol, 90.5%). The NMR spectrum values were identical to values found in the literature. 75, 76

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109 1 H NMR (300 MHz, CDCl 3 ) 0.27 (s, 9H), 7.86 (dd, 1H), 8.36 (dd, 1H), 8.72 (dd, 1H). 13 C (75.4 MHz, CDCl 3 0.18, 99.46, 101.74, 120.33, 120.47, 139.77, 152.06, 154.19. 5,5' Diethynyl 2,2' bipyridine 75, 76 5,5' Trimethylsilylethynyl 2,2' bipyridine (0.43 g, 1.23 mmol) was dissolved in a solu tion containing 20 mL THF and 10 mL MeOH. Sodium hydroxide (8ml, 1 M) was added and the reaction stirred at room temperature for 4 hrs. The solution was vacuum evaporated to remove THF and MeOH. The remaining slurry was diluted with 20 mL of H 2 O and ex tracted three times with CHCl 3 The extract was dried with MgSO 4 and vacuum evaporated to yield (0.23 g, 1.13 mmol, 91%) of pure 5,5' diethynyl 2,2' bipyridine as shiny cooper colored flakes that are very electrostatic. The NMR spectrum values were identi cal to values found in the literature. 75, 76 1 H NMR (300 MHz, CDCl 3 ) 3.30 (s, 2H), 7.90 (dd, 2H), 8.39 (dd, 2H), 8.78 (dd, 2H). 13 C (75.4 MHz, CDCl 3 4 Bromo N,N diphenylaniline (3) 77 Methylene chloride (25 mL) was cooled to 0 C and flushed with Ar for 20 min. N bromosuccinimide (NBS) (3.60 g, 20.4 mmol) was added and the solution was allowed to stir for 5 min. A pale yellow color was observed and a slight amount of NBS remained undissolved. Upon addition of triphenylamine (5.00 g, 20.4 mmol) t he solution turned dark leafy green and a slight rise in temperature was noted. Within 5 min. the majority of the remaining NBS entered into solution. The solution was allowed to slowly warm to room temperature and stirred overnight. The reaction mixtur e was added to 150 ml of water and the organic layer was separated, dried over MgSO 4 and evaporated under reduced pressure to yield a viscous oil with a slight green tint. 20 ml of MeOH was added to the oil and rapidly stirred. The MeOH developed a ligh t blue tint and the product precipitated as an off white solid. The solid was washed two

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110 additional times with fresh MeOH. Inspection by TLC (silica, 100% hexane) showed the solid contained a mixture of starting material and two other compounds believed to be the mono and dibromo species. Flash chromatography on silica gel with 100% hexane proved difficult due to f values being < 0.10. Initial purification of the product for spectroscopic identification was by recrystallization from hexane over a per iod of two days to a white solid. (3.91 g, 12.1 mmol, 58.3%) Subsequent synthetic pathway modification eliminated the need to purify the product. In turn, all other preparations of this compound were used without further purification. The NMR spectrum v alues were identical to values found in the literature. 77 1 H NMR (300 MHz, CDCl 3 ) 7.00 (m, 2H), 7.07 (m, 6H), 7.21(m, 6H). 13 C (75.4 MHz, CDCl 3 ) 122.54, 123.20, 124.18, 125.18, 129.19, 137.92, 147.08, 147.68. 4 (4 (Diphenylamino)phenyl) 2 methyl 3 butyn 2 ol (4) 77 4 Bromo N,N Diphenylaniline 3 (6.50 g, 20.0 mmol), 2 methyl 3 butyn 2 ol (3.37g, 0.860 g/ml, 40.0 mmol), 10 mL THF and 100 mL diisopropylamine were combined and degassed under argon for 30 minutes. Pd(PPh 3 ) 2 Cl 2 (0.704 g, 0.10 mmol, 5 mol%) and CuI (0.16 g, 0.10 mmol, 5 mol%) were added and the solution was degassed further for an additional 5 minutes. The reaction then stirred at 70 C under an argon balloon for 6 hrs. The solution was filtered through a bed of sand/celite to remove CuI and amine salts. The solvent was evaporated under vacuum to a minimum of solvent. The produc t was purified easily by flash f values > 0.50) with 2:1 hexane diethyl ether solution yielding pure 4 (4 (diphenylamino)phenyl) 2 methyl 3 butyn 2 ol 4 as a flaky light caramel colored solid (3.80 g, 11.6 mmol, 58.0%). The NMR spectrum values were identical to values found in the literature. 77 1 H NMR (300 MHz, CDCl 3 ) 1.60 (s, 6H), 2.11 (s, 1H), 6.96 (dd, 2H), 7.03 (dd,

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111 2H), 7.07 (d, 4H), 7.25 (m, 6H). 13 C (75.4 MHz, CDCl 3 ) 31.55, 65.63, 82.18, 92.91, 115.60, 122.35, 123.41, 124.82, 129.33, 132.52, 14 7.18, 147.84. 4 Ethynyl N,N diphenylaniline (5) 77 4 (4 (Diphenylamino)phenyl) 2 methyl 3 butyn 2 ol 4 (2.00 g, 6.10 mmol) was dissolved in 20 mL toluene. S olid KOH (2.50 g) was crushed inside a sealed and doubled plastic bag to maintain anhydrous conditions and added quickly to the reacti on vessel. The solution was stirred at reflux for 8 hrs. The solution was vacuum evaporated to remove excess toluene. The remaining slurry was diluted with 20 mL of H 2 O and extracted three times with CHCl 3 The extract was dried with MgSO 4 and vacuum e vaporated. Flash chromatography on silica gel with 100% hexane yielded (1.27 g, 4.71 mmol, 77.2%) of 4 ethynyl N,N diphenylaniline as a fine yellow powder. The NMR spectrum values were identical to values found in the literature. 77 1 H NMR (300 MHz, CDCl 3 ) 3.02 (s, 1H), 6.95 (dd, 2H), 7.05 (m, 2H), 7.10 (m, 4H), 7.30 (m, 6H). 13 C (75.4 MHz, CDCl 3 ) 76.15, 83.91, 114.73, 122.03, 123.62, 125.03, 129.40, 133.04, 147.11, 148.34. 4,4' (2,2' bipyridine 5,5' diylbis(ethyne 2,1 diyl))bis(N,N diphenylaniline) (TPA 1) (6) 4 Ethynyl N,N diphenyl aniline 5 (1.00 g, 3.70 mmol), 5,5' dibromo 2,2' bipyridine 2 (0.583 g, 0.185 mmol), THF (50 mL), and triethylamine (50 mL) were combined and degassed under argon for 30 minutes. Pd(PPh 3 ) 2 Cl 2 (0.130 g, 0.185 mmol, 5 mol%) and CuI (0.035 g, 0.184 mmol, 5 m ol%) were added and the solution was degassed further for an additional 5 minutes. The reaction was stirred at 70 C under an argon balloon for 20 hrs. The solution was allowed to cool and filtered through a bed of sand/celite to remove CuI and amine sal ts. The solvent was then evaporated under vacuum. The resulting solid was taken up in CHCl 3 washed with H 2 O, dried with MgSO 4 and solvent removed under vacuum. The solid was purified by flash chromatography on silica gel with 2:1 hexane/methylene chlo ride yielding pure TPA 1 as a

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112 bright yellow powder (0.641 g, 0.928 mmol, 50.1%). 1 H NMR (300 MHz, CDCl 3 ) 7.03 (m, 2H), 7.14 (m, 4H), 7.29 (m, 4H), 7.41 (m, 4H), 7.90 (dd, 2H), 8.40 (dd, 2H), 8.78 (dd, 2H)7.90 (dd, 2H), 8.40 (dd, 2H), 8.78 (dd, 2H). 13 C (75.4 MHz, CDCl 3 120.82, 121.93, 123.77, 125.18,129.44, 132.67, 139.0 6, 147.03, 148.43, 151.54, 153.81. MS calculated for C 50 H 34 N 4 690.28, M+1 found 691.29. Ru(TPA 1)(bpy) 2 2+ 2PF 6 (Ru 1) (7) c is Ru(bpy) 2 Cl 2 (0.133 g, 0.255 mmol) was dissolved in 20 mL of MeOH and refluxed under Ar for 2 hrs. Solution color turned a deep wine color immediately upon reflux. TPA 1 (0.0160 g, 0.232 mmol) was dissolved in 5 mL THF and added via syringe and the solution was refluxed for an additional 18 hrs. Solution fluorescence subsided slowly over the first 8 hrs. Upon cooling to room tem perature a saturated aqueous solution of NH 4 PF 6 was added dropwise producing a brick red precipitate. Approximately 5 mL of NH 4 PF 6 was added before precipitate no longer formed. The precipitate was filtered utilizing a medium porosity fritted filter. The product was reprecipitated by dissolving the product in a minimum of CH 2 Cl 2 and adding dropwise slowly into 50 mL of diethyl ether. The precipitate was filtered as before, rinsed with three 10 mL aliquots of diethyl ether and dried yielding a dark red pow der. The product was further purified on an alumina (6% H 2 O added) column eluting with 4:1 CH 2 Cl 2 /CH 3 CN. Pure metallated Ru 1 (0.051g, 0.037 mmol, 16%) was obtained as a red powder. 1 H NMR (300 MHz, CDCl 3 ) 6.91 (m, 4H), 7.08 (m, 12H), 7.27 (m, 12H), 7. 49 (m, 4H), 7.57 (d, 2H), 7.71 (m, 4H), 8.08 (m, 6H), 8.50 (m, 6H). 13 C (75.4 MHz, CDCl 3 125.55, 126.65, 128.91,129.44, 130.69, 133.84, 139.23, 140.41, 147.63, 150.44, 152.87, 153.24, 154.00, 156.20, 158.15, 161.8 5. MS calculated for C 70 H 50 N 8 Ru (M 2PF 6 ) 1104.32, M found 1104.32.

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113 Re(TPA 1)(CO) 3 Cl (Re 1) (8) Re(CO) 5 Cl (0.121g, 0.276 mmol) and TPA 1 (0.231g, 0.334 mmol) were combined in 40 ml of toluene and the solutions was deoxygenated with argon for 20 min then heated to reflux (110 C) with rapid stirring. Very quickly after the reaction temperature passed 100 C the bright blue green fluorescence of TPA 1 disappeared and was replaced by a bright reddish pink solution which was not fluorescent. Heat was remov ed after 45 minutes when it was noted that all evidence of fluorescence was gone. Stirring was continued until well after the reaction solution reached room temperature. A small amount of bright red precipitate was observed prior to removal of the solven t. Solvent was evaporated under vacuum and the product was rinsed with fresh acetone until rinsing solvent remained clear. Pure metallated Re 1 was recovered as a light red power (0.115g, 0.117 mmol, 42.4%). Multiple attempts to obtain a usable 13 C spect rum solubility in all available NMR solvents. Photophysical data is included to supplement the characterization data for Re 1 1 H NMR (300 MHz, CDCl 3 ) 7.99 (d, 4H ), 7.15 (m, 12H), 7.33 (m, 8H), 7.44 (m, 4H), 8.10 (m, 4H), 9.09 (bs, 2H). 13 max / max 334 nm / 5.5 x 10 4 M 1 cm 1 460 nm / 4.9 x 10 4 M 1 cm 1 em 696 nm ( fl 0.0051). MS calculated for C 53 H 34 ClN 4 O 3 Re (M) 996.19, M found 996 .19. Ir 2 (ppy) 4 Cl 2 (9) 78 IrCl 3 (hydrate) (0.500g, 1.67 mmol), and 2 phenylpyridine, (1.00g, 6.44 mmol, = 1.086 g/mL), were dissolved in a mixture of 2 methoxyethanol (40 mL) and water (20 mL). The reaction mixture was refluxed at 110 C for 18 hrs. The product solution was cooled to room temperature at which time a fine yellow precipitate was formed. T he precipitate was collected using vacuum filtration with a medium porosity glass fritted filter. The precipitate was washed

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114 three times with each 95% ethanol (15 mL each) and acetone (30 mL). The crude yellow product was dissolved in 3 mL of CH 2 Cl 2 and a layer of hexane was carefully added to top of the CH 2 Cl 2 solution. After one hour at room temperature, the solution began to precipitate a fine bright yellow powder. The solution was cooled overnight. The final product was collected by vacuum filtrati on with a medium glass fritted filter as a bright yellow crystalline powder (0.695g, 0.648 mmol, 77.6%). Ir(TPA 1)(ppy) 2 + PF 6 (Ir 1) (10) Ir 2 (ppy) 4 Cl 2 (0.164g, 0.153 mmol) and TPA 1 (0.211g, 0.305 mmol) were added to 7 mL of 2 methoxyethanol. The sta rting materials were only slightly soluble. The reaction mixture was heated to 100 C along with rapid stirring under an argon atmosphere for a period of 26 hrs. Reaction progress was monitored over the reaction period utilizing TLC (silica, 100% CH 2 Cl 2 ) and a UV lamp. It was noted that as the reaction proceeded, the solution turned a deep red and the fluorescence intensity decreased. The reaction was cooled to room temperature when fluorescence was no longer detected visually with a UV lamp and only a faint trace of fluorescent starting material remained on TLC. To the reaction mixture was added a saturated aqueous solution of NH 4 PF 6 (approx 6 mL) and a bright red precipitate formed immediately. Stirring was continued for 30 minutes at which time 50 m L of water was added and stirred for an additional 15 minutes. The precipitate was collected by vacuum filtration with a medium glass fritted filter and dried overnight. The product was further purified by flash chromatography on neutral alumina using 10 0% CH 2 Cl 2 First off the column was unreacted TPA 1 starting material followed by product. The product eluted as a medium red band. Pure metallated Ir 1 (0.304g, 0.255 mmol, 83.5%). was obtained after vacuum evaporation as a dark yellow powder. 1 H NMR ( 300 MHz, (CD 3 ) 2 CO) 6.29 (dd, 2H), 6.94 (m, 6H), 7.02 (m, 4H), 7.15 (m, 12H), 7.30 (m, 12H), 7.57 (dd, 2H), 7.79 (m, 4H), 8.00 (m, 4H), 8.12 (dd, 2H), 8.00 (d, 4H). 13 C (75.4

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115 MHz, (CD 3 ) 2 125.88, 126.02, 126.66, 127.29, 130.69, 131.47, 132.58, 133.89, 139.78, 141.91, 145.05, 147.64, 150.49, 150.66, 152.90, 154.85, 168.53. MS calculated for C 72 H 50 N 6 Ir (M PF 6 ) 1191.37, M+ found 1191.37.

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116 CHAPTER 4 CONCLUSION In the preceding chapters, f indings are presented from the study of the synthesis and photophysical investigation of a series of transition metal based metal organic chromophores with the potential to exhibit an enhanced dual mode nonl inear optical mechanism. The methodology involved the combination of two photon absorption (TPA) as the excitation pathway followed by intersystem crossing to produce strongly absorbing triplet excited states capable of nonlinear absorption through an exc ited state absorption (ESA) mechanism The initial focus of this project centered on the synthesis of the two photon active organic chromophore TPA 1 Utilizing known two photon structure property architecture, the TPA 1 chromophore was designed with elec tron donating triphenylamine end caps capable of producing a large two photon absorption cross section and linked with an electron accepting bipyridine core by highly polarizable acetylene linkages to produce a two photon active chromophore with a D A D architecture. Upon investigation of the photophysical characteristics of TPA 1 it was determined the chromophore exhibited strong ground state absorption over a wide spectral range with both one and two photon excitations as well as an emission quantum yield of unity. Further investigation of the nonlinear character of TPA 1 revealed significant nonlinear absorbance and optical limiting from a two photon excitation source and a fast temporal profile of less than 5 ns. For the subsequent synthetic porti on of the project, complexation of the TPA 1 chromophore with a series of transition metal based chromophores was undertaken. To achieve the final target metal organic chromophores the Re(CO) 3 Cl, Ru(bpy) 2 2+ and Ir(ppy) 2 + moieties were complexed with TPA 1 utilizing its bipyridine core by known reaction procedures. The resulting complexes Re 1 Ru 1 and Ir 1 were synthesized and photophysically characterized prior to the investigation for dual mode nonlinear absorption.

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117 The next phase of research involve d the development of an experimental apparatus capable of detecting nonlinear absorption. The desired capabilities of this nonlinear transmission (NLT) apparatus included the need to observe, acquire and record data of nonlinear response in a solution of known chromophore concentrations and be capable of comparing the data obtained to results obtained under similar experimental conditions for samples whose nonlinear character is well known. It was also desired to create an experimental technique that coul d utilize a broad spectrum of laser wavelengths as its excitation source. A survey of applicable nonlinear absorption characterization techniques and their needed equipment requirements led to the determination that a nonlinear transmission or optical lim iting apparatus could be developed with the least amount of new equipment required. Testing of the NLT setup was performed utilizing a series of platinum acetylide dimers ( Pt2 Ar ) that were synthesized by Dr Kye Young Kim and compared to the NLO response found in E1 DPAF for which nonlinear character is known. It was concluded the NLT apparatus was able to detect and render data to depict the desired nonlinear responses of interest. With the current configuration of the NLT apparatus, a repeatable semi q uantitative representation of nonlinear response can be achieved. Ease of setup and data collection presents this nonlinear detection technique as an experiment that can be utilized with only a minimum of prior training and orientation on the apparatus. The quality of data gathered is excellent and very easy to compile into useable nonlinear absorption and optical limiting plots. Future use of this setup in conjunction with experiments capable of determining nonlinear absorptive cross sections will allow for quantitative comparisons of nonlinear data to results from other studies with little need for exhaustive model compound synthesis or standard sample preparations and comparisons.

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118 The results from testing the NLT apparatus also led to the successful collection of nonlinear data for the Pt2 Ar series of platinum acetylide dimers. Successful two photon excitation followed by excited state absorption was confirmed and verified. With the comparison of the Pt2 Ar series results to the model compound E1 D PAF it was determined the Pt2 Ar series has a greater overall nonlinear response. The reason for this difference can be attributed to the variation in the core arylene units. This is logical since the TPA chromophore is identical for each complex in the series. With the TPA nonlinear excitation pathway identical for each compound, the differences seen are deemed to originate from the ESA active portion of the chromophore. Initial analysis of the preliminarily data found here points to the strength of t riplet triplet absorption as the main variable that correlates with the enhancement found in the nonlinear response in the Pt2 Ar series. The final portion of this research incorporated nonlinear transmission techniques and apparatus to investigate the r elative strength of an enhanced dual mode nonlinear response in the metal organic complexes Re 1 Ru 1 and Ir 1 which were synthesized earlier in this project. The goal of developing a series of metal organic chromophores that exhibit dual mode nonlinear response utilizing both two photon excitation and excited state absorption was successfully realized. TPA 1 was confirmed to elicit a robust two photon excitation capable of optical limiting by a two photon mechanism at 645 nm while maintaining excellent optical transparency at the two photon active wavelengths. Complexation of TPA 1 with the Re(CO) 3 Cl, Ru(bpy) 2 2+ and Ir(ppy) 2 + moieties yielded the metal organic complexes Re 1 Ru 1 and Ir 1 which were shown to enhance the nonlinear response of TPA 1 wi th the addition of triplet triplet absorption leading to of a excited state absorption (ESA) mechanism. Utilizing the NLT apparatus, a systematic study of the presence and relative strengths of dual mode nonlinear absorption in the

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119 metal organic series wa s successfully compared to solutions of C 60 whose nonlinear characteristics have been previously studied. In the process of conducting the nonlinear investigations, validation of the detection capability and establishment of the experimental reliability of the NLT apparatus was verified. It was also shown the NLT apparatus could be used to distinguish between a system having a single NLO mechanism and a dual mode TPA/RSA mechanism. Utilizing an established model developed to characterize NLO mechanisms in conjunction with data obtained during these investigations, the differentiation of a TPA only process from a dual mode mechanism was accomplished. A single NLO process, such as TPA or RSA exhibits a quadratic dependence between the attenuated intensity and incident light intensity and is easily differentiated from the cubic dependence of the attenuated intensity and incident light intensity for a TPA/ESA dual mode mechanism. The later finding will prove to be extremely useful in subsequent experimental studies conducted by our group. The ability to distinguishing the nature of an NLO response in nonlinear chromophores exhibiting a TPA, or RSA from a dual mode NLO mechanisms is a potentially powerful characterization tool.

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120 APPENDIX A 1 H AND 13 C SPECTRA Figure A 1. 1 H NMR spectrum of TPA 1 in CDCl3. Figure A 2. 1 H NMR spectrum of Re 1 in CD 2 Cl 2

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121 Figure A 3. 1 H NMR spectrum of Ru 1 in CD 2 Cl 2 Figure A 4. 1 H NMR spectrum of Ir 1 in CD 2 Cl 2

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122 Figure A 5. 1 3 C NMR spectrum of TPA 1 in CDCl 3 Fi gure A 6. 1 3 C NMR spectrum of Ru 1 in ( CD 3 ) 2 C O.

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123 Figure A 7. 1 3 C NMR spectrum of Ir 1 in CD 2 Cl 2

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124 APPENDIX B NONLINEAR TRANSMISSI ON MANUAL Manual for Nonlinear Transmission Prepared by John Peak Department of Chemistry Uni versity of Florida September 2007

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125 Safety Notes If at any time you are unsure of how to operate or modify the instrument layout, contact the laser lab manager or Dr. Schanze. Always know where the laser beam path will travel. Never lean over or into a n energized laser beam path. Proper eye protection is mandatory when the laser beam is energized. Ensure all reflected laser energy is suppressed prior to starting the experiment. Confirm all optics are clean and free of defects or damage prior to energiz ing the laser. Sample Preparation Any solvent may be used effectively for this experiment, however ground state absorption of your solvent at the desired wavelength should be evaluated and minimized as much as possible. Solvent choice should be based on c omplete solvation of your sample. Incomplete solvation will lead to potentially unwanted optical scattering and reduction of sample transmission. 2 to 3 ml of solution is needed to perform this experiment and should not be prepared in the sample cell. There are two reasons for this: first, a blank solvent measurement in the sample cell being used is necessary to compensate for the optical effects of cell containing the solvent, and second, any undissolved sample on the surface of the cell has the potent ial to damage the cell during the experiment. Laser Table Preparation Remove all equipment and material that is not needed for the experiment from the vicinity of the laser beam path.

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126 Remove all plastic bags from the prisms, lens and filters prior to tur ning on the laser and associated equipment. Laser Wavelength Selection and Alignment Quanta Ray Available wavelengths 266, 355, 532, and 1064 nm. If you need to change the laser wavelength, set the prism control rods to the proper configuration as listed below. These rods are located on the silver box at the front of the laser head. Never move the control rods when the laser is in operation. Prism 1 enables conversion of the 1064 nm fundamental to the second harmonic of 532 nm. Prism 2 allows generati on of the third and fourth harmonics (355 and 266 nm respectively). Do not place Prism 2 in either the T or F positions if Prism 1 is not in the II position as this will damage Prism 2. Figure B 1. Prism control rod configuration for wavelength selecti on (Quanta Ray). Remember that all wavelengths longer than the selected wavelength are emitted by the laser. Currently a dispersing prism is being utilized to separate wavelengths and multiple beam dumps are positioned to suppress unwanted wavelengths. I f you are unsure how to properly isolate and utilize the wavelength you need for your experiment contact the laser lab manager or Dr. Schanze. Alignment of the Quanta Ray output beam should be perpendicular to the optical axis of the nonlinear transmission apparatus. Due to the high energy output of this laser, a glass slide

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127 should be used to redirect a portion of the beam to the apparatus. A beam dump must be placed after the pickoff to capture the unused portion of the beam. In Figure B 2, a photo sche matic depicting the main output beam, pickoff and beam dump is presented as a visual guide. Figure B 2. Beam pickoff configuration for Quanta Ray. Surelite II/OPO Laser output from the optical parametric oscillator (OPO) is di vided into two different beams, the signal and idler. With respect to the direction of beam propagation, the left output is the signal and the right output is the idler. The available wavelengths from the signal output are approximately 420 to 570 nm and 600 to 1100 nm from the idler output. Energy output from the OPO is highly wavelength dependant and it should be determined if enough output energy is

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128 available at the desired wavelength prior to starting your experiment. Typical output energies needed range from 5 to 9 mJ. Figure B 3. Dual prism alignment for Surelite/OPO output. Initial alignment of the OPO output beam to the nonlinear transmission apparatus is best accomplished by a pair of right angle prisms. This two prism configuration allows for easier beam steering and aides in adjusting the output beams so they are parallel to the laser table. This configuration also simplifies switching from signal to idler beams since only the first prism needs to be moved to t he appropriated output position. Nonlinear transmission setup and alignment Regardless of laser output (Quanta Ray, or Surelite/OPO) the setup and alignment are similar. For this section, the laser source refers to the laser beam path as it exits the fi nal turning

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129 optic as described above in the laser wavelength selection and alignment section. To aid in the following alignment, a strip of colored tape may be placed on the surface of the laser table from the laser source to point beyond the estimated po sition of the sample holder and detector 2 (see Figure B 4) to help visualize the optical axis of the apparatus and aid in placement of the optics. Setup Refer to Figure B 4 for general configuration of apparatus setup. (1) Measure the height from the laser so urce to the surface of the laser table. This will be the height above the table surface you will use to place all the other optics, filters and detectors for the setup. (2) Place detector 2 along the optical axis at a distance of approximately 2 to 3 feet from the laser source at the height noted above. This distance should be sufficient to allow for proper placement of all the optics and minimize the setup footprint. A longer distance may be used if it is deemed necessary. (3) Center the sample holder on the opt ical axis approximately 5 cm in front of detector 2. (4) Place the focusing lens, centered on the beam path, in front of the sample holder at a distance equivalent to its focal length. Plano convex lenses, 50.8 mm (2") in diameter, with focal lengths of 10 to 14 cm have been used effectively. Lenses of different focal lengths may be substituted; however caution should be exercised when utilizing shorter focal length lenses. Shorter focal lengths will significantly increase energy densities and damage to the sample cell will be more likely. (5) Place a 50:50 or 30:70 beamsplitter 5 cm prior to the sample holder at a 45 angle to the optical axis and at the height of the laser source. The type of beamsplitter to be used is determined by the available energy from t he laser and the desired input energy to the sample. (6) Position detector 1 10 cm from the beamsplitter perpendicular to the optical axis of the apparatus. It is important to position both detectors at equal distances from the beamsplitter and focusing lens. By making these distances equivalent, the spot size hitting each detector will be equivalent and data gathered will be much more consistent.

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130 Figure B 4. General schematic diagram of nonlinear transmission setup. (7) A continuous ly variable neutral density filter mounted on a single axis translation stage is placed prior to the focusing lens. Note : a significant amount of laser energy is by slightly rotating the filter off axis (~10). The filter and beam can be clearly seen in Figure B 3. Alignment During alignment, the laser power should be set at just over threshold power to minimize the potential of equipment damage. Eye protection sho uld be worn anytime the laser is energized. (1) Adjust the laser source beam optics (prisms or pickoff) to ensure the beam is parallel to the laser table and at a constant height over the course of the setup. (2) Adjustment of the beamsplitter and detector 1 may b e needed to place the reflected energy on the exact center of detector 1. Inspect the size of the laser spot hitting detector 1. The spot should not be larger than one half the available area of the detection pad. (3) Adjust the focusing lens and/or the samp le holder placement as needed to position the beam focus on the exact center of the sample holder (front to back). Burn paper can be used as a guide to position the focus. An alternate method is to place a cell containing a fluorescent dye solution in th e sample holder which will allow for a visual alignment of the focus. Laser source variable OPO Energy Meter PC ND filter optional ND filter 0.3 100% T Plano convex lens, f = 13 cm BS Sample Detector 1 Detector 2

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131 (4) Adjust the position of detector 2 and inspect the size of the laser spot hitting the detection pad. The spot should not be lager than one half the available area of the detection pad. Recheck the distance of detectors 1 and 2 from the beamsplitter to ensure they are equal. Energy meter setup Prior to operation, familiarize yourself with the operation of the LaserStar dual channel energy meter by reading the operators manual. Note : the PE10 V2 pyroelectric detector heads have a sensitivity range of 10 J to 9 mJ. Laser energies in excess of 9 mJ will damage the detectors. All applicable operating software for the energy meter is installed on the laser lab laptop and can be accessed on the desktop with the StarCom32 icon. Installation instructions ar e available in operators manual. An RS 232 to USB converter is supplied for computer connection. The appropriate driver software must be installed to use this connector. The disk is stored in the LaserStar file and should be returned after installation. Figure B 5. LaserStar e nergy m eter. Channel A Channel B Wavelength Selector Energy range Selector Detector Rati o Detector Wavelength

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132 (1) With the energy m eter off connect detector 1 as channel A and detector 2 as channel B. (2) Connect the energy meter to the computer utilizing the RS 232/USB connector. (3) Turn energy meter on. Channels A and B should read zero, if not then recycle power. (4) Select the StarCom32 icon on the computer desktop. (5) From the drop down menu, choose the connect function. (6) Select ratio A/B from the drop down menu. (7) On the energy meter, select the wavelength for each detector that matches the wavelength to be used (see Figure B 5, middle red button). Press esc when complete. (8) Select the detector energy range most appropriate for each detector (see Figure B 5, second red button from right). Each range represents t he maximum energy to be read. The ranges 200 J, 2 mJ and 10 mJ are the three most common ranges you will use. The nonlinear transmission apparatus is now aligned and configured for data acquisition. Performing the nonlinear experiment Startup With th e variable neutral density filter completely out of the beam path, adjust the laser power to the highest energy desired for the experiment. Detector 2 is reading the actual energy the sample cell will be exposed to and caution must be used to not exceed 9 mJ during this adjustment. The ratio value observed on the energy meter should remain a constant value; if it is not; recheck the beam alignment and detector distances. You may notice the amount of energy redirected by the beamsplitter does not represen t the ratio value expected (i.e. 50:50) but this is normal. The beamsplitters will redirect different amounts of energy depending on the wavelength being used and its orientation in the beam path. This difference does not necessitate readjustment. If mo re energy is needed to the sample, replace the beamsplitter with one that has a higher throughput value.

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133 Block the laser source and reposition the variable neutral density filter into the beam path. Leave the beam block in place until data acquisition is desired. For the Surlite/OPO the manual shutter works the best for this step. A beam dump works best for the Quanta Ray. Collecting data (1) Place a sample cell (due to their expense, quartz is not needed) containing solvent in the sample holder. 2 to 3 ml of solution is optimum. Take note of which side is facing the laser source, as you will want to configure you sample solution in the same manner. (2) (3) Move the variable neutral density filter across the beam path using the adjustment knob on the bottom of the translation stage. Note : watch the energy readout closely and BEFORE the readings reach their maximum range value, press stop on the program. Select a new range and press start when the reading stab ilizes. Unless you press the clear tab, data will continue to be added to the current data file. Repeat this as often as needed to achieve the desired range of data. (4) When the desired range of data is collected, press stop. Replace the beam block. Immed iately save the data file using the drop down menu under file. If the data is not saved soon after acquisition, the meter may reset automatically and the data will be lost. For a solvent, no nonlinear response should be present and a constant ratio will be presented (see Figure B 6). The average ratio will be annotated at the top of the data page. For the example below the ratio is 0.291. This will be the factor you will use to convert the Channel A readings to input energy to your sample. (5) Reset the n eutral density filter and press clear on the program. (6) Remove the solvent from the sample cell and replace with a sample of known concentration. 2 to 3 ml of solution is sufficient for measurement. (7) Repeat steps 1 through 4. For samples that exhibit a nonli near response, a data plot similar to Figure B 7 will be collected. Remember that the ratio found for the solvent only sample is the conversion factor that is used for input energy calculation.

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134 Figure B 6. Example of data ac quired from a CH 2 Cl 2 solvent only sample. Figure B 7. Example data for a sample exhibiting a nonlinear response.

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135 Shut down (1) Power down laser. If unsure of laser power down sequence, contact the laser lab manager prior to start ing the experiment for a full laser operation orientation. (2) Turn off energy meter. Disconnect both detectors and computer interface. Return the (3) Cover all optics and filters with plastic bags. (4) Clean experiment area, remove and properly dispose of all samples and return sample cells. Plotting data The data file generated from the Ophir StarCom32 software will be saved as a .dnr file and is easily imported directly into a graphi ng program such as Sigmaplot. The raw data contains two rows, one for each channel. The first row is channel A which represents detector 1 and the second row is channel B and represents detector 2. The following is an example of how to create a plot of input energy versus output energy. (1) Insert a blank column after channel A. (2) Select quick transform from the drop down menu. Enter col(2) = col(1) / 0.291 then press run. The correction factor 0.291 is the correction factor found earlier and is used here o nly as an example. It is important that you use the proper value as found for your experiment. Col(2) will contain the energy values equivalent to the energy input to the sample corrected for the solvent and the cell. (3) Column 3 contains the actual energy t ransmitted through the sample and can be used directly as the energy output values. (4) Create a graph selecting column 2 as the x axis and column 3 as the y axis. Repeat as necessary for solvent and samples data. The graph should resemble Figure B 8.

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136 Figure B 8. Sample graph of nonlinear absorbance data. When the graph is complete, the solvent data will represent 100% transmittance and be presented as a straight line (line of constant slope). For samples that exhibit a nonlinear For samples that do not exhibit 100% transmittance from the start of the experiment but have initial absorbance that is linear, the slope will be constant but offset from the solvent line. The following is an example of how to create a plot of transmission (y axis) versus input energy (x axis). (1) Insert a blank column after channel A. (2) Select quick transform from the drop down menu. Enter col(2) = co l(1) / 0.291, press run. Col(2) will then contain the energy values equivalent to the energy input to the sample corrected for the solvent and the cell. (3) Select quick transform from the drop down menu. Enter col(4) = col(3) / col(2), press run. (4) Column 4 r epresents transmission through the sample.

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137 (5) Create a graph selecting column 2 as the x axis and column 4 as the y axis. Repeat as necessary for solvent and sample data. The graph should resemble Figure B 9. Often the input energy is plotted on a logarith mic scale to better represent the nonlinear nature of the data. When the graph is complete, the solvent data will represent 100% transmittance and be presented as a straight line (line of zero slope). For samples that exhibit a nonlinear response, the da samples which do not present 100% transmittance from the start of the experiment but have initial absorbance that is linear, the slope will initially be zero and or iginating from a value equal to the initial transmittance then deviate as the transmission becomes nonlinear. Figure B 9. Sample graph of nonlinear transmission data.

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141 36. 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. Chemistry of Materials 2004, 16, (1), 185 194. 37. Sutherland, R. L.; Brant, M. C.; Heinrichs, J.; Rogers, J. E.; Slagle, J. E.; McLean, D. G.; Fleitz, P. A., Excited state characterization and effective three photon absorption model of two photon induced excited state absorption in organic push pull charge transfer chromophores. Journal of the Optical Society of America B Optical Physics 2005, 22, (9), 1939 1948. 38. Gao, Y. W.; Potasek, M. J., Effects of excit ed state absorption on two photon absorbing materials. Applied Optics 2006, 45, (11), 2521 2528. 39. Prasad P N. Williams D.J., Introduction to N onlinear O ptical E ffects in M olecules and P olymers Wiley: New York, 1991; p 307. 40. Powell, C. E.; Humphrey, M. G., Nonlinear optical properties of transition metal acetylides and their derivatives. Coordination Chemistry Reviews 2004, 248, (7 8), 725 756. 41. Oudar, J. L.; Chemla, D. S., Hyperpolarizabilities of nitroanilines and their relations to excited state dipole moment. Journal of Chemical Physics 1977, 66, (6), 2664 2668. 42. Chemla D.S. Zyss J., Nonlinear O ptical P roperties of O rganic M olecules and C crystals Academic Press: Orlando, 1987. 43. Dalton, L. R.; Harper, A. W.; Robinson, B. H., The role of Lo ndon forces in defining noncentrosymmetric order of high dipole moment high hyperpolarizability chromophores in electrically poled polymeric thin films. Proceedings of the National Academy of Sciences of the United States of America 1997, 94, (10), 4842 48 47. 44. Liao, Y.; Eichinger, B. E.; Firestone, K. A.; Haller, M.; Luo, J. D.; Kaminsky, W.; Benedict, J. B.; Reid, P. J.; Jen, A. K. Y.; Dalton, L. R.; Robinson, B. H., Systematic study of the structure property relationship of a series of ferrocenyl nonli near optical chromophores. Journal of the American Chemical Society 2005, 127, (8), 2758 2766. 45. Marder, S. R., Organic nonlinear optical materials: where we have been and where we are going. Chemical Communications 2006 (2), 131 134. 46. Prasad, P. N.; Perrin, E.; Samoc, M., A coupled anharmonic oscillator model for optical nonlinearities of conjugated organic structures. The Journal of Chemical Physics 1989, 91, (4), 2360.

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145 BIOGRAPHICAL SKETCH Being born in Victorville, California and moving to Aviano, Italy at the age of seven weeks was a foreshadowing of things to come. As the son of a helicopter pilot in the United States A ir Force, I was blessed to be able to see more of the world before I was ten than most people get to see in a lifetime. I spent most of my primary and secondary school years in Marion, Ohio and went on to accomplish my undergraduate and masters degrees at Wright State University in Dayton, Ohio. It was at Wright State where I developed a love for teaching and chemistry. My penchant for not making things easy took me into the United States Air Force. I earned my aeronautical rating as a navigator and wha t followed was a wonderful thirteen year flying career which brought me to many wonderful and not so wonderful places. I married at about the same time I started my aviation career and was blessed with two gorgeous children Cameron and Kelsey. Sadly, mar riage ended at about the same time as flying but the change prompted my return to teaching and chemistry. I am fortunate to know that I will be returning to the United States Air Force Academy to teach, but more importantly I will be returning to be with my children.