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From Molecular Oligomers to Supramolecular Gels: Photophysics of Conjugated Metal-Organic Systems


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FROM MOLECULAR OLIGOMERS TO SUPRAMOLECULAR GELS: PHOTOPHYSICS OF CONJUGATED METAL-ORGANIC SYSTEMS By THOMAS CARDOLACCIA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 By Thomas Cardolaccia

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Dedicated to my pare nts for their support Dedicated to my wife for her love And vice versa.

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ACKNOWLEDGMENTS These years as a graduate student have been without a doubt the most stimulating and rewarding years of my life. Many people have contributed to make this journey a positive and gratifying adventure and they are hereby acknowledged. Some many things to say, so little space . First I would like to thank my advisor, Dr. Kirk S. Schanze, for his guidance and support throughout those years. His constant encouragements and motivation have been an incredible source of strength in many o ccasions. His genuine care for the intellectual development of his students is evidenced in the time he often took to explain new concepts to me or demonstrate the operat ion of some instruments. Dr. Schanze has always been willing to let me go my way, gi ving me a significant degree of freedom on my research. More importantly, he never ma de me feel bad for mistakes and failed experiments. Through these times of failure a nd success, I have grown as a scientist and a person, inspired by his creativity and approach to sciences. I would like to thank my committee members, Dr. John Reynolds, Dr. William Dolbier, Dr. Michael Scott and Dr. Bruce Ca rroll. Special gratitude goes to Dr. Dolbier for organizing and managing the Thursday night Bull Sessions, where I have been exposed to some aspects of organic chemistr y I would have never encountered otherwise. Many people have been involved with my research and I would like to thank Dr. Xiaoming Zhao for synthesizing polymers fa ster than I could characterize them spectroscopically, Dr. Alison M. Funston and Dr. John R. Miller for carrying out the iv

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pulse radiolysis in Brookhaven National Labor atory, Dr. Stephen Hagen for the use of his CD spectrometer, and Dr. Richard Weiss for carrying out the polarizing microscopy experiments. My experience in the laboratory has been particularly enriching due to several exceptional individuals willing to share their knowledge and time. Thanks go to Dr. Ben Harrison, Dr. Yiting Li, Dr. Mauricio R. Pint o, Dr. Yao Liu, Dr. Ksenija Haskins-Glusac, Dr. Eric Silverman and all presen t members of the Schanzes group. I thank my parents for making me what I am today and my family for the love I was always surrounded with. v

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TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................iv LIST OF TABLES .............................................................................................................ix LIST OF FIGURES .............................................................................................................x ABSTRACT .......................................................................................................................xv CHAPTER 1 INTRODUCTION........................................................................................................1 Introduction to Photophysics........................................................................................1 Absorption of Light ...............................................................................................1 Nature of the Excited State ....................................................................................4 Relaxation of Excited States ..................................................................................6 Energy Transfer .....................................................................................................9 Singlet and Triplet Excimers ...............................................................................12 Exciton Coupling in Molecular Aggregates ........................................................15 Triplet Excited States in Conjugated Systems .....................................................20 Platinum Acetylides ....................................................................................................22 Structure and Synthesis .......................................................................................22 Excited State........................................................................................................24 Objective of Present Study .........................................................................................30 2 TRIPLET EXCITED STATES IN BICHROMOPHORIC PLATINUM ACETYLIDE OLIGOMERS......................................................................................34 Introduction .................................................................................................................34 Synthesis .....................................................................................................................36 Results .........................................................................................................................39 UV-Vis Absorption .............................................................................................39 Steady-State Photoluminescence .........................................................................41 Transient Absorption ...........................................................................................49 Time-Resolved Photoluminescence ....................................................................50 Discussion ...................................................................................................................53 Energy of the Triplet Excited State in Pt4T3. .....................................................53 The Absence of Phosphorescence in Pt4T3 ........................................................55 Excited State Dynamics .......................................................................................57 vi

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Conclusion ..................................................................................................................60 Experimental ...............................................................................................................61 Photophysical Measurements ..............................................................................61 Mass Spectrometry of Pt Oligomers ....................................................................62 Synthesis ..............................................................................................................62 3 DELOCALIZATION OF CHARGE CARRIERS IN PLATINUM ACETYLIDE OLIGOMERS.............................................................................................................71 Introduction .................................................................................................................71 Results .........................................................................................................................73 Electrochemistry ..................................................................................................73 Pulse Radiolysis Ion Radical Spectra ...............................................................78 Discussion ...................................................................................................................80 Delocalization of Charge Carriers ................................................................80 Electronic Transitions of the Radical Ions ...................................................83 Conclusion ..................................................................................................................87 Experimental ...............................................................................................................88 Electrochemistry ..................................................................................................88 Pulse Radiolysis ...................................................................................................88 Synthesis ..............................................................................................................89 4 CONSEQUENCES OF AGGREGATION ON THE TRIPLET EXCITED STATE IN PLATINUM ACETYLIDE OLIGOMERS...........................................................90 Introduction .................................................................................................................90 Synthesis .....................................................................................................................95 Results .......................................................................................................................100 Gel Formation ....................................................................................................100 Thermal Properties ............................................................................................101 UV-Vis Absorption ...........................................................................................103 Circular Dichroism ............................................................................................108 Steady-State Photoluminescence .......................................................................110 Time-Resolved Photoluminescence ..................................................................119 Discussion .................................................................................................................127 Nature of Aggregates .........................................................................................127 Photoluminescence of Aggregates ....................................................................132 Molecular Exciton Modeling .............................................................................137 Conclusion ................................................................................................................140 Experimental .............................................................................................................142 Thermal Properties ............................................................................................142 Photophysical Measurements ............................................................................143 Calculation of Exciton Interaction Energy ........................................................144 Synthesis ............................................................................................................145 5 CONCLUSION.........................................................................................................158 vii

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APPENDIX NMR SPECTRA ..............................................................................................................161 LIST OF REFERENCES .................................................................................................172 BIOGRAPHICAL SKETCH ...........................................................................................191 viii

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LIST OF TABLES Table page 2-1. Photophysical data for oligomers Pt4 and Pt4Tn......................................................54 3-1. Redox potentials (V vs SCE) for Ptn and Pt4Tn oligomers series in CH2Cl2 containing 0.1 M TBAH.a........................................................................................74 4-1. Lifetimea of photoluminescence of self-assem bling platinum acetylide oligomers.b127 4-2. Absorption data of self-assemb ling platinum acetylide oligomers. .........................139 4-3. Some angles and dipole-dipole distances calculated with the molecular exciton model. .....................................................................................................................141 ix

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LIST OF FIGURES Figure page 1-1. Potential energy curves for electronic transitions. ........................................................5 1-2. Jablonski diagram representing the possible transitions after absorption. ...................8 1-3. Diagram for the exchange energy transfer mechanism.............................................. 10 1-4. Diagram for the Coulombic energy transfer mechanism. ...........................................12 1-5. Fluorescence spectra of pyren e solutions in cyclohexane.......................................... 13 1-6. Potential energy curves for monomer and excimer.................................................... 14 1-7. Schematic representation of the energy le vels of the excited state of the monomer and of aggregates in pa rallel (left) and head-to-tail (right) geometry. .....................16 1-8. Exciton band splitting energy diagram for a co-planar molecular dimer as a function of the angle ..............................................................................................18 1-9. Absorption and fluorescence spectra for cyclohexane solution (dotted line) and multilayers of fatty acid derivative of trans -stilbene (solid line). ............................19 1-10. Absorption spectra of a carbocyanine derivative in 10-2 M aqueous sodium hydroxide solution at different conc entrations and room temperature. ....................20 1-11. Schematic pictures of conjugated polymers studied by Monkman and Burrows.44.21 1-12. Plot of triplet energy against single t energy for the conjugated polymers studied by Monkman and Burrows. ......................................................................................22 1-13. General structure of a platinum acetylide polymer. .................................................23 1-14. Splitting of d orbital levels in square-planar Pt(II) complexes. ................................25 1-15. Structures of platinum acetylide dimers and polymers studied by Chawdhury et al.63...........................................................................................................................26 1-16. Platinum acetylide dimers and polymers studied by Wilson et al.64 from which figure was adopted. ..................................................................................................27 x

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1-17. Absorption spectra (high energy dotte d lines) and photolumin escence spectra (at 300 K dotted lines, at 20 K solid lines) of films of polymers P1 P8 .......................28 1-18. Energy levels of the S1 and T1 excited states and sing let-triplet energy gap for the Pt-containing and organic polymers. ..................................................................29 1-19. Platinum acetylide oligomers studied by Rogers et al.66 from which figure was adopted. ....................................................................................................................29 1-20. Platinum oligomers Pt-n (n = 1-5,7) studied by Liu et al.67.....................................30 1-21. Absorption (a) and photoluminescen ce (b) spectra of Pt-n oligomers. ....................31 1-22. Triplet exciton confinement in platinum acetylide oligomers. .................................31 2-1. The structures of platinum acetylide oligomers Pt4 and Pt4Tn (n = 1-3). .................36 2-2. Synthesis of platinum acetylide complexes 5a-d .......................................................37 2-3. Synthesis of platinum acetylide complex intermediate 11 .........................................38 2-4. Synthesis of oligomers Pt4 and Pt4Tn (n = 1-3). .......................................................39 2-5. Absorption spectra of oligomers in THF. ...................................................................40 2-6. Photoluminescence spectra of oligomer s in deoxygenated THF with an excitation = 352 nm. ...............................................................................................................42 2-7. Photoluminescence spectra of oligomers Pt4Tn in deoxygenated THF. ....................45 2-8. Excitation spectra of oli gomers Pt4Tn in deoxygenated THF. ..................................47 2-9. Low-temperature photoluminescence spectrum of Pt4T1 in deoxygenated MeTHF with an excitation = 352 nm. ...................................................................48 2-10. Low-temperature photoluminescence sp ectrum in deoxygenated MeTHF with an excitation = 352 nm at T = 90 K ( ) and T = 300 K ( ).. .................................50 2-11. Transient absorption spectra of o ligomers in deoxygenated THF following 355 nm excitation. ...........................................................................................................51 2-12. Time-resolved photoluminescence spectra of oligomers Pt4Tn in deoxygenated THF following 355 nm excitation. ...........................................................................52 2-13. Energy diagram representing the photophy sical processes involved in the Pt4Tn oligomers. .................................................................................................................59 3-1. Structure of platinum acetylide oligomers Ptn (n = 1-5). ...........................................73 xi

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3-2. Structure of platinum acetylide oligomers Pt4Tn (n = 1-3). ......................................73 3-3. Cyclic voltammetry (CV, left) and di fferential pulse voltammetry (DPV, right) of oligomers Ptn. ...........................................................................................................76 3-4. Cyclic voltammetry (CV) of oligomers Pt4Tn. ..........................................................77 3-5. Radical cation spectra for Ptn (top) and Pt4Tn (bottom) oligomers series.. ..............79 3-6. Radical anion spectra for Ptn (top) and Pt4Tn (bottom) oligomers series. ................81 4-1. Structures of phenyleneethynylene and platinum acetylide oligomers synthesized for the preliminary study. .........................................................................................93 4-2. Structures of self-assembling platinum acetylide oligomers. .....................................95 4-3. Synthesis of platinum complex intermediate 18.........................................................96 4-4. Synthesis of phenyl eneethynylene derivative 20........................................................97 4-5. Synthesis of chiral intermediate 23............................................................................98 4-6. Synthesis of Pt2M as a representative reaction of the oligomer series. .....................98 4-7. Synthesis of oligomer PE3 .........................................................................................99 4-8. Picture of deoxygenated dodecane gel of Pt2M (10-3 M) under illumination with a UV light. ................................................................................................................100 4-9. Differential scanning calorimetry th ermograms for second heating and cooling cycle at a10 oC/min scan rate. ................................................................................102 4-10. Pictures of liquid-crystal phases under a polarized optical microscope. ................104 4-11. Absorption spectrum of Pt2M in dodecane. ..........................................................105 4-12. Absorption spectrum of Pt2MT in dodecane. ........................................................106 4-13. Absorption spectrum of Pt2MP3 in dodecane. ......................................................107 4-14. Absorption spectrum of Pt2MC at room temperature in dodecane. ......................109 4-15. Circular dichroism (CD) absorption spectrum of Pt2MC in dodecane. ................109 4-16. Photoluminescence spectrum of PE3 with ex = 354 nm. ......................................111 4-17. Photoluminescence spectrum of Pt2M ...................................................................112 xii

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4-18. Photoluminescence spectrum of Pt2M in MeTHF at C = 7 x 10-6 M with ex = 326 nm from -63 oC to -23 oC. ...............................................................................113 4-19. Low-temperature photoe xcitation spectra of Pt2M in deoxygenated MeTHF monitoring em = 494 nm ( ) and em = 516 nm ( ). ......................................114 4-20. Photoluminescence spectrum of Pt2MT in deoxygenated dodecane with ex = 340 nm at C = 10-3 M () and 10-4 M ( ). ......................................................116 4-21. Photoluminescence spectrum of Pt2MP3..............................................................117 4-22. Absorption spectrum of Pt2M ( ) and Pt2MT ( ) in dodecane at C = 10-3 M. ...........................................................................................................................119 4-23. Photoluminescence spectrum of Pt2M Pt2MT mixed-oligomer system. .............120 4-24. Time-resolved photoluminescence spectrum of Pt2M in deoxygenated dodecane following ex = 337 nm. ..........................................................................................121 4-25. Principal components of emission decay of Pt2M at 10-3 M in dodecane for slow component = 59 s ( ) and fast component = 9 s ( ). ...............................123 4-26. Time-resolved photoluminescence spectrum of Pt2MP3 in deoxygenated dodecane following ex = 355 nm. .........................................................................124 4-27. Time-resolved photoluminescence spectrum of Pt2M with 5 mol% Pt2MT in deoxygenated dodecane following ex = 337 nm. ..................................................126 4-28. Principal components of emission decay of Pt2M at 10-3 M with 5 mol% Pt2MT in dodecane for slow component = 41 s ( ), fast component = 5 s ( ). .......................................................................................................................126 4-29. Plot of emission decay of Pt2M at 494 nm (a) and 516 nm (b) with x mol% doping levels of Pt2MT .......................................................................................128 4-30. Energy diagram for monomer and aggregates in self-assembling platinum acetylide oligomers. ...............................................................................................134 4-31. Proposed conformation of th e triplet excited state of Pt2M ..................................135 4-32. Plot of angle versus = agg m for different dipole-dipole distance R. ........140 A-1. 1H NMR (300 MHz, CDCl3) spectrum of Pt4 .........................................................161 A-2. 31P NMR (121 MHz, CDCl3) spectrum of Pt4 ........................................................161 A-3. 1H NMR (300 MHz, CDCl3) spectrum of Pt4T1 ....................................................162 A-4. 31P NMR (121 MHz, CDCl3) spectrum of Pt4T1...................................................162 xiii

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A-5. 1H NMR (300 MHz, CDCl3) spectrum of Pt4T2 ....................................................163 A-6. 31P NMR (121 MHz, CDCl3) spectrum of Pt4T2...................................................163 A-7. 1H NMR (300 MHz, CDCl3) spectrum of Pt4T3 ....................................................164 A-8. 31P NMR (121 MHz, CDCl3) spectrum of Pt4T3...................................................164 A-9. 1H NMR (300 MHz, CDCl3) spectrum of Pt2M.....................................................165 A-10. 13C NMR (75 MHz, CDCl3) spectrum of Pt2M ....................................................165 A-11. 31P NMR (121 MHz, CDCl3) spectrum of Pt2M ..................................................166 A-12. 1H NMR (300 MHz, CDCl3) spectrum of Pt2MT ................................................166 A-13. 13C NMR (75 MHz, CDCl3) spectrum of Pt2MT .................................................167 A-14. 31P NMR (121 MHz, CDCl3) spectrum of Pt2MT ................................................167 A-15. 1H NMR (300 MHz, CDCl3) spectrum of Pt2MP3..............................................168 A-16. 13C NMR (75 MHz, CDCl3) spectrum of Pt2MP3...............................................168 A-17. 31P NMR (121 MHz, CDCl3) spectrum of Pt2MP3..............................................169 A-18. 1H NMR (300 MHz, CDCl3) spectrum of Pt2MC ................................................169 A-19. 13C NMR (75 MHz, CDCl3) spectrum of Pt2MC .................................................170 A-20. 31P NMR (121 MHz, CDCl3) spectrum of Pt2MC ...............................................170 A-21. 1H NMR (300 MHz, CDCl3) spectrum of PE3 .....................................................171 A-22. 13C NMR (75 MHz, CDCl3) spectrum of PE3 ......................................................171 xiv

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Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy FROM MOLECULAR OLIGOMERS TO SUPRAMOLECULAR GELS: PHOTOPHYSICS OF CONJUGATED METAL-ORGANIC SYSTEMS By Thomas Cardolaccia August, 2005 Chair: Kirk S. Schanze Major Department: Chemistry In this dissertation, several series of platinum acetylide oligomers have been prepared and studied by photophysical methods The motivation for the research stems from direct opto-electronic applications that platinum acetylide materials may be used for, as well as from a more fundamental need to gain a better understanding on the triplet excited state in conjugated systems. First, platinum acetylide oligomers contai ning energy traps were prepared in order to investigate their effect on the triplet excited state. Sec ond, the delocalization of charge carriers (radical anions and cations) was st udied to determine the charge transport properties of these materials and the effect of platinum on the charge carriers. Third, a series of platinum acetylide oligomer was designed to self-assemble in solution with the goal of determining the consequences of aggreg ation on the triplet excited state. The goal of this work was to gain an insight into the dynamics of the triplet excited state in conjugated systems. xv

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The most significant findings of the study are as follows: (i) although more localized than the singlet exciton, the triplet exciton is also sensitive to the presence of energy traps, which can have a significant impact on the photophysical properties of the materials; (ii) charge carri ers are relatively localized on the oligomer chain and the estimated delocalization of the radical cation is no more than two repeat units; (iii) the consequences of aggregation on the triplet exci ted state may be very limited or relatively important, depending on the mode of aggregation. xvi

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CHAPTER 1 INTRODUCTION The interaction of light with matter is an elementary process in nature responsible for the life of plants and many other species. Absorption of light is the source of energy for plants, of heat for some animals. It is the process by which humans and many other animals see their environment and are able to in teract with it and with each other. In the next chapter, some concepts relying on the in teraction of light with molecules will be presented. It is therefore important to review several fundamental photophysical processes before. This introduc tory chapter is divided into two parts. The first part reviews several important concepts for phot ophysical studies such as absorption and emission of light. Then a clos er look is taken of the gene ral structure and photophysical properties of platinum acetylide polymers and oligomers. Introduction to Photophysics Absorption of Light The understanding of the interaction of li ght with matter has been considerably changed with the notion of the dual nature of light and the non-classical description of atomic structure. Maxwells theory of electromagnetism in 1860 and the development of quantum mechanics started by Schdinger in 1928 have provided scientists with mathematical equations describing these phenomena. 1 Light is often referred to as an elementary particle called a photon but can also be thought of as an electromagnetic wave. Electrons also possess th is dual nature and can be vi sualized as an elementary 1

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2 particle or a wave. The energy of the photon and the frequency of its electromagnetic wave are related by the following equation: hc hE (1) where E is the energy in Joules (J), is the frequency (Hz or s -1 ), h is Plancks constant (6.63 x 10 -34 J.s), c is the speed of light (3.00 x 10 8 m.s -1 in a vacuum) and is the wavelength (m). When a molecule interacts with an optical field, the outer valence electrons of the molecule interact with the light and can be promoted to higher energy levels. For this to occur, the light, or the photon, must have the appropriate energy (or quantum) that corresponds to the difference in energy between the two energy levels involved. This is because the energy levels of the electrons in an atom or a molecule are not continuous but discrete. Therefore the wavele ngth of the light absorbed pr ovides the energy difference between these energy levels. For many conditions, the absorption of radiation follows Beers law lCTA log (2) where A is the absorbance, T is the transmittance, l is the pathlength of absorption (cm), is the molar absorptivity (L.mol -1 .cm -1 ), and C is the concentration of the absorbing species (mol.L -1 ). The molar absorptivity represents the probability of the transition to occur and is related to the transition dipole moments between the initi al and final states. The molar absorptivity is a function of the radiation frequency and is usually reported for max as max However, a better measure of th e transition intensity is obtained by integrating over the whole absorption spectrum, 2 which gives the integrated absorption coefficient :

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3 ~ ) ~ ( dband (3) The integrated absorption coefficient provides a connection between the experimental spectrum and a theoretical quantity known as the oscillator strength, f nm This latter is a measure of the strength of an electric dipole transition between electronic states n and m compared to that of a free elec tron oscillating in three dimensions. It is given by 2 2 0)10ln(4 eN cm fA e nm (4) where 0 is the permittivity of vacuum, m e is the mass of the electron, c is the speed of light in vacuum, N A is the Avogadros number and e is the elementary charge. The collection of fundamental constant has a value of 4.319 x 10 -9 mol.L -3 .cm 2 Assuming a Lorentzian band profile for the absorption band, the integrated absorption coefficient can be calculated from the experimental absorption spectrum using max2 1 (5) where is the full width at half maximum (F WHM). The oscillator strength equation then becomes max 9 max 910784.6 2 10319.4 nmf (6) Moreover, the oscillator strength can also be related to the transition dipole moment nm 2 7 2 2 2~ 10702.4 ~ 3 8nm nm e nmhe cm f (7) where h is Plancks constant and the collection of fundament al constant has a value of 4.226 x10 52 C -2 .m -2 .cm, or 4.702 x 10 -7 D -2 .cm.

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4 Even though the energy involved in an elec tronic transition is discrete, absorption bands in molecules do not appear as sharp lines, but usually as more or less broad bands. The reason for this is that electronic tran sitions are usually accompanied by vibrational transitions. The explanation for this lies in th e fact that electronic transitions occur very rapidly (10 -15 s) with respect to the re-adjusteme nt time of the inter-atomic distance (10 -13 s) and this is referred to as the Franck Condon principle. This can be illustrated by representing the potentia l energy curves of the ground and ex cited states as a function of their respective equilibrium geometry, as show n in Figure 1-1. Elect ronic transitions are termed vertical with respect to the equilibrium geometry conveying the idea that the electron is excited to the upper state before the nuclei have had the time to re-equilibrate. Nature of the Excited State Following excitation and creation of an electronic excited state, the molecule will first relax to the lowest vibrational level by thermal (emission of heat) or collisional (collision with solvent or solute molecules) relaxation. When the initial state is a ground neutral state, the electrons ar e paired and of opposite spin, according to Hunds rule. Due to spin restrictions imposed by quantum mechanics, the electron promoted to a higher energy level does not change its spin during excitation and the excited state formed is called a singlet excited state (S 1 ). In certain cases, however, the spin of the promoted electron can flip and the resulting overall spin momentum of this excited state becomes equal to three. This process is called inters ystem crossing (ISC) and the resulting state is called a triplet excited state (T 1 ). 3 Similarly to the excitation into a singlet excited state, the triplet excited state formed is first vibra tionnally excited and then relaxes to its lowest vibrational level. In organic molecules, the ra te of ISC is slow and consequently the yield of the triplet excited state is usually low. Cert ain factors can greatly increase the ISC rate

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5 Figure 1-1. Potential energy curves for el ectronic transitions. (a) Transition between states of similar equilibri um nuclear geometry. (b) Transition between states of different equilibrium nuclear geometry. The figure was adopted from Gilbert and Baggot. 2 and the yield of the triplet excited state is then increased. The process of ISC relies on spin-orbit coupling and it is facilitated th rough the heavy-atom ef fect (internal via valence bond or external via solvent). In heavy atoms, the spin angular momentum and the orbital angular momentum of the elec tron can interact and are not separately

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6 conserved. Therefore as long as the total orbi tal momentum is conserved, the spin of the electron can be changed. Rapid intersystem crossing and efficient creation of triplet excited states are thus common in inorga nic or organometallic molecules, and the platinum acetylide systems that are the focus of this study are among them. Another important feature of excited states is the singlet -triplet splitting energy (E ST ). The first singlet excited st ate is always higher in energy than the first triplet excited state and the reasoning for this is as follows: In the singlet excited state, the electrons are of opposite spin and are therefore not prevente d by quantum mechanics to be in the same region of space. In the triplet excited state, the electrons ar e of the same spin and are therefore forbidden from being in the same region. This leads to a higher coulombic repulsion energy in the case of the singlet ex cited state compared to the triplet excited state. In a small molecule, this repulsive energy is large and the E S-T is therefore also large. In large molecules such as a conjuga ted polymer, the repulsive energy may not be as large and thus E S-T may not be large either. Relaxation of Excited States The excited state is metastable and the electrons will return to their initial configuration (ground state) by one of two self-re laxation mechanisms: radiative decay and nonradiative decay. Radiative decay. In radiative decay, the excited electron will relax to the ground state by emission of a photon. This photon w ill carry a quantum of energy corresponding to the energy difference betw een the geometrically relaxed excited state and ground state, similar to the absorption process. If the excite d state is a singlet, th is emission of light is called fluorescence whereas it is called phosphor escence if the excited state is a triplet. Since fluorescence is a transition between states of same spins, it is allowed by quantum

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7 mechanics and the radiative rate of th e singlet excited state is fast (~10 8 s -1 ). 4 Phosphorescence on the other hand is a transi tion between states of opposite spin and although facilitated by the presence of heavy atoms in the molecules, it is not as fast as fluorescence and the radiative rate of triplet ex cited states is typical ly much slower (~10 5 10 2 s -1 ). Fluorescence caused by di rect excitation to S 1 is called more precisely prompt fluorescence. Delayed fluorescence has a l onger lifetime than pr ompt fluorescence because S 1 is populated by indirect m echanisms. This alternate S 1 population can proceed through a thermally-assisted ISC back to S 1 from T 1 (T 1 S 1 E-type delayed fluorescence) or through a bimolecula r triplet-triplet annihilation (T 1 + T 1 S 0 + S 1 Ptype delayed fluorescence). Before the radiative decay, the electron has relaxed to the v = 0 vibrational level so the energy of this transition will be less than that of the absorption. This results in fluorescence bands appearing at a longer wave length than the absorption and this is called the Stokes shift. The extent of the Stokes shift is then a representation of the structural differences between the ground and exc ited states. If the exci ted state is largely distorted, a large Stokes shift will be observed. Nonradiative decay. Another type of relaxation mechanism is nonradiative decay. In this case, energy is releas ed to the system as heat a nd does not involve a photon. This process, as well as the vibrati onal relaxation, is also referred to as internal conversion. The relative rate of non-radiative decay is governed by the energy gap law, which states that as the energy of the excited state decreases, the rate of non-radiative decays will increase exponentially. 5-7 The triplet excited state being lower in energy than the singlet

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8 for the reason provided above, it is th erefore sometimes difficult to observe phosphorescence, even though the trip let excited state was formed. It is helpful to look at all these transitions in a represen tative Jablonski diagram, as shown in Figure 1-2 below. There are two characteristics of the excited state that will be encountered in the next chapters that are worth mentioning at this stage: the photoluminescence quantum yield and the lifetime The fluorescence quantum yield is the ratio of the number of emitted photons to the number of photons absorbed 4 and it is given by nrf f Fkk k (8) where F is the quantum yield of fluorescence, k f is rate constant of fluorescence and k nr ISC v= 0 S0 IC P IC F A S1 T1 IC IC v = 1 v= 0 v= 2 v= 3 v = 1 v= 2 v= 3 A: absorption F: fluorescence P: phosphorescence S0: ground state S1: singlet excited state T1: triplet excited state IC: internal conversion ISC: intersystem crossing Figure 1-2. Jablonski diagram representing the possible transitions after absorption.

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9 is the rate constant of non-radiative decay for S 1 In the case of phosphorescence, the quantum yield is given by nrP P nrF ISC Pkk k kk k (9) where P is the quantum yield of phosphorescence, k ISC is the rate constant of intersystem crossing, k P is the rate constant of phosphorescence and k nr is the rate constant of non-radiative decay for T 1 The luminescence lifetime is defined as the time for the luminescence signal to decay to 1/ e of its initial value. 4 The lifetimes for fluorescence F and phosphorescence P are related to the rate constants for d eactivation with the following equations nrF Fkk 1 (10) nrP Pkk 1 (11) Energy Transfer Other than by self-relaxation, excite d states may relax to the ground state by transferring the excitation to other molecule s present in the system by a bimolecular process as in equation 12 below. * ADAD (12) where D is the energy donor, A is the energy acceptor and denotes an excited state. Different mechanisms for energy transfer can occur, depending on the environment, but some conditions require (1) that the energy of D* is higher than the energy of A*; (2) that the energy transfer rate is more rapid than the decay rate of D*. Two mechanisms for energy transfer can be distinguished: exchange and Coulombic energy transfer.

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10 Exchange energy transfer Also called collisional or Dexter energy transfer, 8 this mechanism requires contact or a short sepa ration (6-10 ) between the donor and the acceptor. The rate of energy transfer is ther efore diffusion controll ed and depends on the temperature and the viscosity of the solvent. It also requires an overlap between the orbitals of the donor and the acceptor. Both S-S and T-T energy transfer processes are allowed and this is in fact the dominant mechanism in triplet-triplet energy transfer. The mechanism can be represented schematically as in Figure 1-3 below. The transfer rate constant k ET is given by LrJPhkET/2exp2/(2 (13) D* A D A* Figure 1-3. Diagram for the exchan ge energy transfer mechanism. where r is the distance between D and A, L and P are constant not easily related to experimentally determinable quantities and J is the spectral overlap integral. The decrease in emission intensity due to collisional quenching 9 is described by the Stern-Volmer equation ][1][10 00QkQK F Fq q (14)

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11 where F and F 0 are the fluorescence intensities in the presence and in the absence of the quencher (or acceptor), respectively, and 0 are the quantum yields of fluorescence in the presence and in the absen ce of quencher, respectively, K q is the Stern-Volmer quenching constant, k q is the bimolecular quenching constant, 0 is the lifetime in the absence of quencher, and [Q] is the quencher (or acceptor) concentration. A similar Stern-Volmer equation can be written in the case of phosphorescence. However, as can be seen from equation 9, excited species wi th long lifetimes (such as triplet excited states) are more prone to que nching than species with shor t lifetimes (such as singlet excited states). Oxygen is an efficient quencher of triplet excited states and for this reason, phosphorescence measurements are usually carried out in deoxygenated solutions or frozen matrix. Coulombic energy transfer Also called the dipole-dipole or Frster energy transfer mechanism, 10 this long-range interaction does not require contact between the donor and the acceptor. Efficient long-range en ergy transfer is favored in situations where the emission spectrum of the donor and the absorption spectrum of the acceptor overlap. It is important to note that no photon is involved. As opposed to the exchange energy transfer, only S-S energy transfer is allo wed. This type of energy transfer can be considered to be due to dipole-dipole c oupling between the donor and the acceptor and can be represented as in Figure 1-4 below. The rate of energy transfer 11 in this case is given by 6 01 R R kD ET (15) where D is the lifetime of the donor, R is th e average distance between donor and acceptor and R 0 is the Frster distance (which is a measure of the spectral overlap).

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12 D* A D A* Figure 1-4. Diagram for the Coulom bic energy transfer mechanism. Frster distances are in the range of 20 to 50 and can be as large as 100 for efficient acceptors. This is comparable in size to biological macromolecules and for this reason energy transfer has been used as a spectro scopic ruler for measurements of distances between sites on proteins. 12-14 Singlet and Triplet Excimers Under certain conditions, there is another pos sible fate for the excited state, which is to form an excited complex, either with a different analyte (to form an exciplex) or with another like molecule (to form an excimer, or exited state dimer). 15 The excimer formation mechanism can be represented by 1,3 M* + 1 M 1,3 E* (16) where the excimer E* can be a singlet or a triplet depending on the spin multiplicity of the excited molecule M* from which it is fo rmed. The concentration of the analyte must be relatively high for excimer formation to be likely. Alternatively, a poor solvent or a restricted environment may induce the formation of excimers. A very-well known example of a molecule that can form an ex cimer is pyrene (first discovered by Frster and Kasper in 1954) and its fluorescence sp ectrum is shown in Figure 1-5 below.

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13 Figure 1-5. Fluorescence spectra of pyrene solu tions in cyclohexane. Intensities are normalized to a common value of FM Concentrations decrease from A (10 -2 M) to G (10 -4 M). Figure was adopted from Birks. 15 At low concentration, pyrene displays a highly structured emission. At higher concentrations, a broad struct ureless band appears at long er wavelengths, due to the excimer luminescence. This is a general char acteristic of excimer luminescence which is usually broader and red-shifted from the monomeric emission. This is due to the fact that the ground state of the excimer is unstable and therefore the potential energy of the ground state dimer increases with decreas ing intermolecular distance. Another consequence of the absence of a bound dimer in the ground state is that a longer lifetime is usually observed for excimer emission compared to monomer emission. This is illustrated in Figure 1-6. Studies of fluorescence in crystals, 16 sandwich dimers 17 and diarylalkanes 18 all indicate that the preferred confor mation of singlet excimers is close to a symmetrical sandwich structure with a sepa ration of 3-4 The binding energy in the

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14 Figure 1-6. Potential energy curv es for monomer and excimer. This figure was adapted from Gilbert and Baggot. 2 singlet excimer comes mainly from the exc iton resonance and to a lesser extent from charge resonance. Triplet excimers, which are more related to this study, have been less studied due to the experimental difficulties associated wi th their detection. In fact there has been much debate on their existence and identification. 19,20 But an extensive and pioneering work by Lim 21 greatly contributed to establish trip let excimers as physical species. Experimental evidences point towards a different structure for the triplet excimer than for the singlet. In a spectro scopic study on a series of 1,n-di-naphtylalkanes (n=14), Subudhi and Lim 22 have concluded that the tr iplet excimer a dopts a skewed

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15 conformation, where the short axis of the naphthalene are highly nonparallel while the long axes are parallel. The angle between th e short axes was found to be 100-120. As a result, the activation energy of triplet excimer formation is higher than for singlet excimer and the rate constant of formation of triplet ex cimer is smaller than for a singlet excimer. More recently, triplet excimers have also been observed in polymers 23 and fullerenes 24 and have been used as the main source of white light emission in electrophosphorescent organic light emitting devices. 25 However, they remain rather elusive and more work is needed to fully comprehend their photophysical properties. Exciton Coupling in Molecular Aggregates In 1962, the molecular exciton model was developed by Davydov 26 to provide a theory describing the effects induced by the strong coupling of the collective excited states in organic crystals. Later, Kasha and co-workers 27,28 provided chemists with a model derived from the molecular exciton m odel that would provide simple tools to predict some of the photophysical properties of non-crystalline molecular aggregates. In particular, the molecular exciton model has proven useful in e xplaining the photophysical properties of porphyrins 29,30 and different dyes 31,32 in aggregates. One of the important features of this model is the ability to explain the spectral shift of the absorption band observed in aggregates. Th is spectral shift is due to the splitting of the monomer excited state into two excitonic levels. The exciton band splitting can be derived from detailed quantum calculations but an approximation of the excited state interaction can be made by considering the electrostatic interaction of the transition dipole moments. This is illustrated in Figure 1-7 for co-planar dimers arranged in a parallel and head-to-tail geometry. In the case of parallel geometry, the out-of-phase arrangement corresponds to a lowering of energy, so E lies lower than E (excited state of

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16 monomer), whereas the in-phase dipole interac tion gives repulsion, so E lies higher than E. Since the transition dipole moment is gi ven by the vector sum of the individual transition dipole moments, transitions from gr ound-state to exciton st ate E are forbidden whereas those from ground-state to E are allowed. The spectroscopic consequence of the exciton splitting will therefore be observed as blue-shift of the absorption in the aggregate in parallel arrangement compared to the monomer. For the head-to-tail geometry, the situation is opposite. The in -phase arrangement of individual dipole moments leads to an electrostatic attraction, whereas the out-of-phase arrangement causes electrostatic repulsion. However, transitions to E are allowed whereas transitions to E are forbidden. In this case, the spectroscopi c consequence of the ex citon splitting will be observed as a red-shift of the absorption in the head-to-tail aggreg ate compared to the monomer. In the literature, aggr egates in parallel arrangement are often referred to as Jaggregate, and the head-to-tail as H-aggregates. Figure 1-7. Schematic representation of the energy levels of the excited state of the monomer and of aggregates in parallel (l eft) and head-to-tail (right) geometry. Solid arrows and dashed arrows repres ent allowed transitions and forbidden transitions, respectively. This figure was adopted from Kasha. 33

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17 For planar aggregates co mposed of N monomers and in the point-dipole approximation, the exciton splitting energy E is given by 2 3 2 0cos31 4 11 4 R N N E (17) where E is in joules, is the electronic transition mome nt of the monomer in Coulomb meter, R is the point-dipolepoint -dipole distance in meter and is the angle between the long axis of the molecule and the line of molecular cente rs. This equation shows that the exciton splitting energy depe nds on the number of aggregat es, that it is proportional to the square of the transition moment of the monomer and proportional to the inverse cube of the distance between monomers. The theory also predicts that for an angle = arcos(1/ 3) = 54.7 o the exciton splitting energy is equal to zero and therefore no spectral shift may be observed in the absorption sp ectrum. This is the angle for which the aggregate will shift from J-aggregate to H-aggregate, as shown in Figure 1-8 below. While the spectroscopic consequence of aggreg ation can be easily identified in the absorption spectrum, there is also a cons equence on the photoluminescence. In the case of H-aggregates, where the excited state is the higher excited state E, there is usually a quenching of fluorescence observed in the emission spectrum of aggregates. After excitation to E, there is a rapid internal conversion to th e lower exciton level E. Since the transition from E to the ground-state is not allowed, the system goes back to groundstate via nonradiative decay or intersystem crossing through the triplet excited state. Experimentally, the fluorescence detected is red-shifted (as it originates from the lower excitonic level E) and longer lived (as th e transition from E to ground state is forbidden) compared to the fluoresce nce of the monomer. A phosphorescence enhancement was observed by several authors 34,35 in the 1950s and was later rationalized

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18 Figure 1-8. Exciton band splitting energy diagram for a co-planar molecular dimer as a function of the angle This figure was adopted from Kasha. 33 by Kasha using the molecular exciton model. 28,36 In the case of J-aggregates, emission occurs from the lowest exciton level E, at lower energy than th e corresponding monomer emission. The emission from J-aggregates may be enhanced compared to the emission from the monomer although the interplay of interand intramolecular effects are often difficult to discern. 37 Note that in this model and in the dipole-dipole approximation, the triplet excited state is consider ed to remain degenerate sinc e the oscillator strength (and hence the transition dipole moment) for singlet-triplet transition is zero. More recently, Jand H-aggregates have been observed in many different systems, some of them closely related to the molecule s studied in the next chapters. Whitten has studied fatty-acid derivatives of trans -stilbene in Langmuir-Blodge t films, in order to study the effect of aggregation on th e photophysical properties of stilbene. 38,39 The absorption and fluorescence spectra of a trans -stilbene derivative in solution and in multilayers in shown in Figure 1-9. The cons equences of the aggregate formation are

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19 found in both spectra. The absorption spectrum shows a hypsochromic shift compared to the monomer while the fluorescence spectrum shows a bathochromic shift compared to the monomer. This is consis tent with the presence of H-aggregates in the supported multilayers system. In this case, fluorescence is observed from the lower excitonic state E even though it is a forbidden process. Bu t the lifetime of the fluorescence band in the multilayers is four times longer (3.3 ns) than the fluorescence lifetime of trans -stilbene (0.8 ns), consistent with a forbidden radiative decay. 38 Examples of J-aggregates are found in a recent study of carbocyanine dyes by Pawlik et al. 40 Cyanines are strongly aggregating systems and the absorption spectrum (Figure 1-10) shows the presence of aggregates even in dilute solution. However, it is dependent on the concentration and increases wi th concentration. The absorption band for the aggregate is sharp and red-shifted fr om the absorption band of the monomer, consistent with a J-aggregate. Figure 1-9. Absorption and fl uorescence spectra for cyclohe xane solution (dotted line) and multilayers of fatty acid derivative of trans -stilbene (solid line). Figure was adopted from Whitten. 39

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20 Figure 1-10. Absorption spectra of a carbocyanine derivative in 10 -2 M aqueous sodium hydroxide solution at different concentrations and room temperature. (a) 1.7 x 10 -5 M; (b) 2.2 x 10 -5 M; (c) 4.4 x 10 -4 M. Figure was adopted from Pawlik et al 40 Triplet Excited States in Conjugated Systems While their presence usually goes undetected in organi c conjugated polymers (CPs) for reasons mentioned previously (slow ISC), tr iplet excited states are still important in these systems. This is particularly true for organic light-emitting devices (OLEDs) where electroluminescence is generate d by the recombination of el ectrons and holes injected from the electrodes. If conventional spin statis tics were applied, only 25% of all recombination events would lead to potential ly emissive singlet states, and 75% would lead to non-emissive triplet states. Ho wever, a number of recent experimental 41 and theoretical 42 studies point to the existence of a chain-length dependence on the exciton spin formation. It is therefore critical fo r the optimization of OLEDs to understand the factors controlling the formation of triplet excited states. The quantum yield of ISC in some represen tative organic CPs has been measured by Burrows et al. 43 using photoacoustic calorimetry and the values varied from 0.50-0.80

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21 for some polythiophenes to 0. 01-0.04 for poly(p-phenylene vinylene)s. The higher value of ISC obtained for the polythiophene are attr ibuted to the efficient spin-orbit coupling induced by the sulfur atom. Monkman and Burrows have also carried out an extensive study on a broad range of organic conjugated polymers and measured their singlet and trip let energies by pulse radiolysis and energy transfer. 44 The polymers studied are pr esented in Figure 1-11 and a plot of triplet energy gap against singlet en ergy gap is shown in Figure 1-12. As can be seen, there is a linear correlation between the triplet and the singlet energy gaps for the very different polymers studied. Above the tr end line are polymers with rigid and planar backbone structures while twisted polymers lie below the trend line. From this, it appears that while planarity enhances the delocalizati on of singlet excited st ate, triplets do not Figure 1-11. Schematic pictures of conjugated polymers studied by Monkman and Burrows. 44

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22 Figure 1-12. Plot of triplet energy agains t singlet energy for the conjugated polymers studied by Monkman and Burrows. Fi gure was adopted from Monkman et al 44 benefit as much from delocalization enhan cements and while torsion angles tend to localize singlet excited states, this has les im pact on the localization of triplet excited states. It follows then that triplet excitons must be more localized than singlet excitons, for which it is commonly agreed that their de localization extends over 7-8 repeat units. Khler et al. 45 have carried out a systematic study of singlet-triplet splitting energy (E S-T ) in organic poly(phenylene ethynylene) polymers, varying the optical bandgap by changing the nature of the spacer. They found that the E S-T was always 0.7.1 eV and this is similar to what has been reported in other organic CPs. 46,47 Monomers, oligomers and twisted polymers have higher E S-T because of the exciton confinement, which impacts the singlet more than the triplet exciton. 48 Platinum Acetylides Structure and Synthesis Platinum acetylide materials are a class of compounds which have recently attracted significant attenti on due to their non-linear optic al properties and potential application in optical limiting. The general structure of a platinum acetylide polymer is

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23 presented in Figure 1-13. The aryl group (Ar) is usually a phenyl or a thienyl group, while the R is typically a short to medium alkyl chain (1 to 8 car bons) or even a phenyl group. Figure 1-13. General structure of a platinum acetylide polymer. The synthesis of the first Pt-acetylide complex was accomplished by Hagihara and co-workers 49 in 1978 and it involves th e coupling of a platinum chloride complex with acetylene. The reaction is carried out in the presence of a base such as an alkylamine needed for the deprotonation of the aryla cetylene while a catalyst such as copper iodide(I) is used to activate the acetylene function towards deprotonation. To avoid unwanted oxidation of the catalyst and of the platinum complex, the reaction is best carried out under inert atmosphere. Recently, it has been found that a trans -platinum complex is not necessary to obtain trans -platinum acetylide products, as the cis -platinum complex rapidly isomerizes in the presen ce of an amine. The two isomers can be identified by 31 P NMR, from the coupling constant between the phosphorous and one of the NMR active platinum isotope ( 195 Pt, I = 33.8% natural abundance). While a trans isomer will typically have a J Pt-P below 2500 Hz, the J Pt-P for the cis isomer is usually higher than 2500 Hz. 50-52 Due to their attractive non-linear optical properties, first reported by Davy and Staromlynska 53,54 in 1994, different architectures of platinum acetylide have been prepared in the last few years. These include dendrimers 55-57 (one of them containing up to 189 platinum atoms), liquid-forming oligomers, 58 and metallamacrocycles. 59

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24 Excited State The photophysics of transition metal comp lexes involve the electrons in the dorbitals, which are split into different energy levels depending on the electronic environment provided by the ligands. In th e case of Pt(II) with four ligands, a d 8 complex, the ligand field stabilizes the d yz d xz d z 2 and d xy oriented away from the ligands and destabilizes the d x 2 y 2 pointing towards the ligands. The resulting orbital diagram is shown in Figure 1-14. As can be seen, the HOMO orbital is the d xy and the LUMO is the d x 2 y 2 so the photophysics of Pt(II) are expected to involve those two orbitals. Optical transitions encountered in organometallic complexes may be purely metallic (d-d transitions) or metal-to-ligand charge-tra nsfer (MLCT). When the ligands have a system, transitions localized on the ligand may also be observed. In platinum acetylide oligomers and pol ymers there is a mixing between the platinum d-orbitals and the -system and the optical transitions do not clearly fall in one or the other category. The extent of mixi ng depends on the overlap between the ligand and metal orbitals, the size of the spacer and the extent of conjugati on in the ligand. The emission of platinum acetylide complexes typically shows vibronic bands, which is consistent with a MLCT or excited state, but not w ith a d-d transition. Also, phosphorescence lifetimes of are often found to be intermediate between a typical 3 or 3 MLCT. Theoretical work 60-62 has shown that the HOMO of platinum acetylide complexes is composed mostly of the orbitals of the acetyl ide ligands with some contribution from the d xy metal orbital, while the LUMO consists only of the ligand orbitals. Therefore the phosphorescence in pla tinum acetylide complexes is best thought of as originating from a 3 */ 3 MLCT manifold with predominantly intraligand character. Platinum acetylide complexes and polymers absorb light into the S 1 state between 300

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25 5 dxz, dyz5 dz 25 dxy5 dx 2 -y 2 Figure 1-14. Splitting of d orbital levels in square-planar Pt(II) complexes. and 400 nm. Rapid and efficient intersyste m crossing (with close to unite quantum efficiency) converts the singlet excited state into triplets, so that their emission spectrum may show some fluorescence (decay of S 1 ) between 370 and 450 nm and phosphorescence (decay of T 1 ) between 500 and 600 nm. The tr iplet excited state absorbs light into higher triplet excited states (T n ) between 600 and 800 nm. Several groups, including ours, are actively working on the photophysics of platinum acetylide complexes and in the next section, the state of the research on these materials is described. Chawdhury et al. 63 have studied the evolution of the singlet and triplet excited states with the number of thienyl rings in platinum-acetylide dimers and polymers (shown in Figure 1-15). From absorption measurem ents of the polymers and dimers and comparison with the 2,5-diethynyl-oligothiophenes, they observed th at the introduction of platinum lowers the energy of the S 0 S 1 transition. This shows that conjugation is

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26 preserved through the platinum centers and that they do not confine the singlet exciton. The photoluminescence spectra of the pol ymers show a phosphorescence band between 650 and 740 nm, however phosphorescence was not observed for the terthienylcontaining polymer. They attributed th e absence of phosphorescence to the reduced influence of the heavy-metal center responsib le for ISC when increasing the number of thiophene rings. Figure 1-15. Structures of platinum acetylid e dimers and polymers studied by Chawdhury et al. 63 From a closer inspection of the fluorescence bands in the polymers, they observed increasing intensity in the vibr onic bands as the number of thiophene rings increased. This is an indication that that the excited state geometry differs more from the ground state geometry and that the excited state takes more of a thiophene-based character with increasing thiophene rings. Th ey also observed a constant S 1 -T 1 splitting energy of 0.7 eV across the polymer series. This constant S 1 -T 1 gap of 0.7 eV was also exhibite d in a larger range of platinum acetylide polymers (shown in Figure 1-16) studied by Wilson et al. 64 The polymers were chosen so that the triplet energy level woul d be tuned between 2.5 and 1.3 eV and this trend could be related to changes in nonradia tive decay rates. Ind eed, shorter lifetimes and lower quantum yields of phosphorescence were observed as the triplet energy level decreased, as illustrated in Figure 1-17. Calculations of the radiative and nonradiative

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27 rates showed that as the triplet energy level decreases, the rate of nonradiative decay rates increases exponentially. This is the consequence of the energy gap law, which relates the rate of nonradiative decay rate of a transiti on rate to the energy gap between the states involved. In simple terms, the energy gap law can be written as M nrE k exp (18) where E is the energy gap between the states involved, is a term expressed in terms of molecular parameters and M is the maximum and domina nt vibrational frequency available in the system. 65 Plots of ln(k nr ) against E(T 1 -S 0 ) gave straight lines for the monomers and the polymers studied, implyi ng that platinum acetylide polymers and monomers follow the energy gap law. Anothe r finding of this study is that radiative decay rates will be large enough to compete with nonradiative decay rates for materials with T 1 -S 0 gaps of 2.4 eV or above. Figure 1-16. Platinum acetylide dime rs and polymers studied by Wilson et al. 64 from which figure was adopted.

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28 Figure 1-17. Absorption spect ra (high energy dotted li nes) and photoluminescence spectra (at 300 K dotted lin es, at 20 K solid lines) of films of polymers P1-P8. Figure was adopted from Wilson et al 64 Extending the series of platinum-containi ng polymers studied and comparing them with their respective organic counterparts, Khler et al. 45 proved that conclusions drawn on the triplet excited state in platinum acetylide polymers could be carried to the corresponding organic polymers. In a seri es of 15 platinum-containing and organic polymers with various optical bandgaps, a constant singlet-triplet en ergy gap of 0.7 0.1 eV was found in both series (Figure 1-18). In a study concerned with the influence of the size of the ligand in platinum acetylide complexes (shown in Figure 1-19), Rogers et al. 66 have also greatly contributed to the field. The study found that the effect of increased conj ugation is a red-shift and an increase in the molar absorption coefficient of S 0 -S 1 and T 1 -T n absorption bands and an

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29 increase in the singlet-triplet energy gap. The authors concluded that as the conjugation increased, the S 0 -S 1 transition is more localized on the ligand with less metal character Figure 1-18. Energy levels of the S 1 and T 1 excited states and si nglet-triplet energy gap for the Pt-containing and organic poly mers. Figure was adopted from Khler et al 45 and therefore slower inters ystem crossing occurs. This was supported by the longer lifetimes of T 1 and lower quantum yield of phosphorescence observed. Finally, our group carried out a systematic study of the delocalization of the singlet and triplet excited state in a series of mono-disperse platinum acetylide oligomers 67 (Figure 1-20). While the previous studies had some indications that the triplet exciton Figure 1-19. Platinum acetylide oligomers studied by Rogers et al. 66 from which figure was adopted.

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30 Figure 1-20. Platinum oligomers Pt -n (n = 1-5,7) studied by Liu et al. 67 may be more localized than th e singlet exciton, no definite conclusions could be drawn by comparing long conjugated polymers a nd short monomers. Polymers have a distribution of chain lengths and it is therefore not possible to relate the molecular weight of the polymers to the optical properties. The absorption a nd emission spectra measured for this series of oligomers is shown in Figure 1-21. The main absorption band found between 320 and 370 nm red-shif ts across the series but th e change is small between Pt-5 and Pt-7, indicating that the effective and maximum conjugation length of the S 1 state is approximately 6 repeat units. The emission spectra of the oligomers displayed weak fluorescence between 370 and 390 nm and wa s dominated by a strong phosphorescence band at = 518 nm. The fluorescence red-shifted as the oligomer size increased but leveled off between Pt-5 and Pt-7, consistent with the absorption data. However, the phosphorescence was found to be independent of oligomer length and th is clearly showed that the triplet exciton is only delocalized over one or two repeat units. Figure 1-22 shows the estimated size of the triple t exciton based on these experime nts. Consistent with this idea and the observations made in previous studies, the singlet-triplet energy gap was not constant in this oligomer series and varied from 0.91 to 0.78 eV as the oligomers length increased. Objective of Present Study Platinum acetylide oligomers offer a unique framework to study triplet excited state in conjugated system. Their monodisperse lengt h and precise chemical structure allows structure-property relationship to be established by the synthesi s of different derivatives.

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31 Figure 1-21. Absorption (a) and photolumine scence (b) spectra of Pt-n oligomers. Fluorescence (F) intensity scale is magnified 100X compared to phosphorescence (P). Figure was adopted from Liu et al 67 Pt PBu3 PBu 3 Pt PBu3 PBu 3 Pt PBu3 PBu 3 Pt PBu3 PBu 3 Pt PBu3 PBu 3 Pt PBu3 PBu 3 Pt PBu3 PBu 3 Figure 1-22. Triplet exciton confinem ent in platinum acetylide oligom ers. Location of exciton is a r bitr ary. The inform a tion gained from the study of platinu m acetylid e oligom ers can be extrapo l ated to m e tal-organic 68 and even all-org a nic 45 polym ers, where the triplet excited state is just as cruc ial as the sing let e x cite d state. The need for m o re research on these unique m o lecules is therefore at a fundam e ntal level. There is also a direct m o tivation as pl atinum acetylide o ligomers are prom ising candidates for optical lim iting a pplications. McKay and co-workers 53, 69 have found evidence that platinum acetylid e olig om ers could be used as the active co mponent of a broadband, frequency agile optical lim iter. Th is is a crucially im portant technology for

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32 protection against intrusive, possibly damaging, laser radi ations that have become increasingly present. While the study of platinum acetylide oligomers and polymers carried thus far have significantly improved general knowledge on trip let excited states in these systems and related conjugated systems, much work re main to be done. In particular, several questions still need to be addresse d in platinum acetylide materials: What is effect of a low energy site on triplet excitons? While these effects are known in the case of the singlet exciton, 70,71 the situation may be different given the relative localization of the triplet excited stat e compared to the delocalized singlet excited state. For this, a series of platinum acetyl ide oligomers Pt4Tn with triplet energy traps were synthesized. The traps c onsisted of oligothiophenes (n = 1-3), which are known to have a lower triplet excited than benzene. The effect of these traps on the photophysical properties of platinum acetylide oligomer was probed and compared to a trap-free oligomer. What is the extent of delocalization of charge carriers? In all-organic conjugated polymers, the charge carriers are belie ved to extend over several repeat units 72 but the effect of a metal center on the delocalization of charge carriers remains unknown. In order to determine the extent of delocalization of charge carriers, ol igomer series Ptn and oligomer series P4Ttn were st udied with electrochemistry a nd pulse radiolysis. The first series of oligomers Ptn (n = 1-5) provided a system to study the delocalization of charge carriers as a function of conj ugation length while the second series of oligomers Pt4Tn probed the effect of low energy sites on charge carriers.

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33 What are the consequences of aggregation? While these consequences can have a dramatic effect on the singlet excito n and are now fairly well understood, 73,74 almost no information is available for the triplet ex citon in conjugated sy stems. A parallel to organic crystals has to be made and it remain s unclear whether this is reasonable or not. With the concepts of supramolecular chem istry, complex conjugated systems have appeared in the literature in the past few years 75 and it appeared that platinum acetylide oligomers could be easily modified and desi gned to self-assemble. A series of short platinum acetylide oligomers where the phe nylene end-group was tri-substituted with dodecanoxy chains were synthesized and thei r photophysical propert ies in molecular aggregates studied by stea dy-state and time-resolved spectroscopy using standard solution techniques.

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CHAPTER 2 TRIPLET EXCITED STATES IN BICHRO MOPHORIC PLATINUM ACETYLIDE OLIGOMERS Introduction Conjugated systems have received tremendous attention in the past fifteen years. Conjugated polymers in particular have been th e subject of an enormous research effort, as they combine attractive photophysical a nd mechanical properties. Much of the research efforts are aimed towards unde rstanding the dynamics of the different photophysical processes involved in an opto-electronic device s. For example in a light emitting diode, an exciton is formed followi ng recombination of an electron and a hole. 7678 The radiative decay of this exciton will give rise to luminescence. But the exciton may encounter a defect or low-energy site within the time of its lifetim e, which will usually lead to nonradiative decay and a redu ction of the device luminescence and performance. 79-84 It is therefore important to unders tand the susceptibility of the exciton to these low-energy traps, which are almo st inevitable in polymer systems. Most of the conjugated polymers studi ed are all-organic polymers, the photophysics of which are dominated by singlet excited states and fluorescence. The extent of delocalizaton of the singlet exciton 85,86 and its migration ability 70,87 are now fairly well understood. The tr iplet excited state on the othe r hand, being elusive and not directly active in all-organic conjugated polym ers, has been less studied and is therefore less understood. Our group has a special intere st in platinum acetylide polymers and oligomers. The introduction of the platinum cen ter in the conjugated backbone increases 34

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35 the ISC yield by spin-orbit coupling due to th e heavy atom effect induced by platinum. The photophysics of these platinum oligomer s and polymers are therefore dominated by the triplet excited state a nd intense phosphorescence emission. These systems provide access to the triplet excited state and allo w its study through standard photophysical techniques. Using a series of platinum acetylide oligomers of increasing length, our group has successfully determined th e extent of delocalization of the triplet excited state. 67 While the singlet excited state is sensitive to changes in conjugation length, the triplet excited state proved to be rather insensitive to the changes in conjugation length, implying that it is rather localized. The study poi nts out to a triplet ex cited state localized between two platinum centers with some el ectronic density on the outer ligands as well. Since the triplet excited state is beli eved to be much more localized than the singlet excited state, we wondered whether it would also be sensitive to the presence of low-energy traps in the conjugated backbone. For this we prepared the series of platinum acetylide oligomer Pt4Tn containing oligothie nyl units in the cente r. Oligothiophenes are known to have lower triplet energies than benzene. 88,89 It is therefore anticipated that the oligothiophenes would act as low-energy traps. Thus their influence on the platinum acetylide photophysics could be observed a nd compared to a trap-free oligomer Pt4. The structures of the oligomers are shown in Figure 2-1 below. Each oligomer contains four platinum centers spaced out by phenyleneethynylenes linkages. Pt4 is the trap-free reference oligomer and Pt4T1 to Pt4T3 are the oligomers with an oligothienyl unit energy trap of decreasing singlet and triplet energy. In the present study, we will first briefly look at the synthetic pathway to the o ligomer series. We will then examine their

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36 Figure 2-1. The structures of platinum acetylide oligomers Pt4 and Pt4Tn (n = 1-3). photophysical properties measured by steady-st ate and time-resolved techniques. Finally a discussion will provide some explanations for the results observed and what has been learned on the dynamics of triplet exciton in these metal-organic conjugated systems. Synthesis The synthetic strategy chosen for this se ries of oligomers is convergent, which allows the same platinum acetylide complex inte rmediate to be used for the synthesis of all four oligomers. The synthesis of th e different platinum acetylide complexes 5a-d is shown in Figure 2-2 below. It started with the iodination of o ligothiophenes with Niodosuccimide, in the presence of ace tic acid and a co-solvent. The diiodooligothiophenes 2b-d were then reacted with 2.1 equi valents of trimethylsilylacetylene under Sonogashira coupling conditions 90 to give the protected diacetylene

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37 oligothiophenes 3b-d in good yields. After deprotecti on of the acetylenes under basic conditions, the diacetylene-oligothiophene 4b-d are reacted with 3 equivalents of cis dichloro-bis-(trin -butylphosphine)platinum(II) in the presence of diethylamine to give the four different platinum acetylide complexes 5a-d in moderate to good yields. Figure 2-2. Synthesis of pl atinum acetylide complexes 5a-d. The synthesis of the common platin um acetylide complex intermediate 11 is shown in Figure 2-3 below. This was achieved by subjecting 1,4-diiodobenzene to a one-pot double Sonogashira coupli ng reaction with triiso -propylsilylacetylene and propargyl alcohol to give the unsymmetrical ly protected bis-acetylenebenzene 7. Selective deprotection of one acetylene function with manganese oxide and potassium hydroxide gave the unsymmetrically prot ected bis-acetylene compound 8. In parallel, phenylacetylene was reacted with cis -dichloro-bis-(trin -butylphosphine)platinum(II) to give platinum complex 9 in excellent yield. This latter was then reacted with compound 8, affording the platinum complex intermediate 10 in very good yield, which after deprotection with TBAF, gave the desired platinum acetylide complex intermediate 11 in good yield.

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38 Figure 2-3. Synthesis of platinum acetylide complex intermediate 11. The synthesis of the oligomers was finally achieved by reacting 2 equivalents of the platinum complex 11 and one equivalent of the re quired platinum acetylide complex 5a-d in diethylamine under mild reflux. The oli gomers were obtained in moderate to good yields. The reaction is illustr ated in Figure 2-4 below. The oligomers were characterized by 1 H and 31 P NMR, MALDI-DIOS and elemental analysis. The 1 H NMR allows the identification of the oligothiophene unit in each oligomer, their protons appearing downf ield compared to the phenylene protons. The 31 P NMR shows two peaks, due to the two di fferent magnetic environments of the phosphorus atoms in the oligomers. The value of the J Pt-P coupling constant (2340-2360 Hz) indicates that all platinum centers have a trans geometry in all oligomers.

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39 Figure 2-4. Synthesis of oligomers Pt4 and Pt4Tn (n = 1-3). Results UV-Vis Absorption The absorption spectra of the oligomers we re recorded in THF and the results are presented in Figure 2-5 below. Pt4 displays a strong absorption band at max = 352 nm and some weaker bands at hi gher energy. The strong band is believed to arise from the long-axis polarized transition, whereas the weaker low-energy bands arise from short-axis polarized transitions. 67,91 These transitions are comonly agreed to be mostly ligand-based and have only little metal-base d character. The influence of the low energy thiophene in Pt4T1 can be observed from the broadening of the main absorption band in this oligomer. The effect is more pronounced in Pt4T2 and Pt4T3 which both exhibit an extra thienyl-based band at max = 410 nm and max = 440 nm in addition to the phenylbased absorption. The red-shifting of the thie nyl-based absorption is consistent with the idea that the increasing size a nd conjugation length of the ol igothienyl leads to greater stabilization of the singlet excited state. From these spectra, it appears that while the phenyl-based and thienyl-based chromophores are too close in energy to be give two

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40 Figure 2-5. Absorption spectra of oligomers in THF. (a) Pt4; (b) Pt4T1; (c) Pt4T2; (d) Pt4T3. distinct states in Pt4T1, the corresponding larger difference in energy in Pt4T2 and Pt4T3 allows the identification of two distinct ly localized phenyl-based and oligothienylbased excited states. However, the thienyl singlet exciton is not experiencing its optimum delocalization and stabilization in these oligomers. In a se ries of thienyl-containing platinum acetylide polymers, Chawdhury et al. 63 reported absorption bands at lower energy ( max = 457 nm and max = 469 nm for the bithienyl and tert hienyl-containing platinum polymers, respectively) than the thienyl-based abso rption here. This is probably due to the

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41 confinement of the singlet exciton and to the presence of the higher-energy phenyl chromophores. Interestingly, the corres ponding thienyl-containing platinum acetylide monomers end-capped with phenylacetylene we re found to have absorption bands at max = 406 nm and max = 433 nm, very similar in energy to the thienyl-based absorption in Pt4T2 and Pt4T3. This would imply that the singlet confinement is not the largest contribution to the difference in stability between the Pt4Tn oligomers series and the thienyl-containing platinum acetylide polymers. Steady-State Photoluminescence The photoluminescence of the oligomers was recorded in deoxygenated THF solutions with an excitation wavelength = 352 nm for all oligomers and the spectra are shown in Figure 2-6. The excitation wavele ngth was chosen on the blue edge of the absorption band of Pt4 to minimize direct excitation of the thienyl units in the Pt4Tn oligomers. The dotted line across the spectra id entifies the position of the main emission band observed in Pt4. This band is centered at = 520 nm and it arises from the relaxation of a well-studied triplet excited state. 45,66,67 The delocalization of the triplet exciton is believed to be 1-2 repe at units, based on a previous study. 67 Very little fluorescence is seen in the spectrum, implying that intersystem crossing is very efficient in Pt4. The emission spectrum of Pt4T1 appears very different from that of Pt4. The spectrum is dominated by a br oad band with a maximum at = 604 nm, while the phosphorescence band at =520 nm observed in Pt4 is relatively weak. This broad band is believed to be originating from a thienyl-based triplet excited state. This assignment is made by comparison with the br oad phosphorescence observed around = 604 nm in a platinum acetylide polymer with a thiophene unit as a spacer. 63 Some fluorescence is also

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42 Figure 2-6. Photoluminescence spectra of oligomers in deoxygenated THF with an excitation = 352 nm. (a) Pt4; (b) Pt4T1; (c) Pt4T2; (d) Pt4T3. Dotted line indicates the position of main emission band in Pt4. Solid and dashed lines are for deoxygenated and air-satu rated solutions, respectively. seen in this spectrum ( = 415 nm) and it is believed to be originating from the thienyllocalized singlet excited state, as it is not phenyl-based 67 and it corresponds to a slightly less stable singlet excited state than in the thiophene-containing platinum acetylide polymer reported by Khler and Beljonne 48 ( = 435 nm). This is cons istent with the idea that the singlet excited state in these oligomers will experience a destabilizing confinement effect not found in polymers and thus will be found at higher energy than in the corresponding polymers. The observed maximum delocalization of the singlet excited

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43 state in platinum acetylide oligomers was found to be about six repeat units by Liu et al 67 In the present oligomers series, there are only fo ur repeat units, so th at the singlet excited state is confined and experiencing a de stabilization compared to the corresponding polymers. The emission spectra of Pt4T2 and Pt4T3 are similar. The emission is dominated by strong fluorescence ( = 457 nm and = 491 nm for Pt4T2 and Pt4T3, respectively) arising from a thienyl-based singlet excited state. The fluores cence peak shifts to lower energies from Pt4T1 to Pt4T3, consistent with the bathochromic shift of the thienylbased absorption band in the absorption spectr a. The singlet excite d state energies of Pt4T2 and Pt4T3 are again found at higher energies than those found in platinum acetylide polymers with the same oligothienyl unit spacing units. 48 Some phenyllocalized phosphorescence is al so observed buried under the fluorescence emission. The phenyl-based phosphorescence can be identified by comparison with the air-saturated emission spectrum from which it is not observed. In addition, Pt4T2 displays a very weak band around = 728 nm which arises from the b ithienyl-based triplet excited state. This is consistent with the phosphorescence observed around = 746 nm for a platinum acetylide polymer with a bithiophene spacer. 48,63 No terthienyl-based phosphorescence emission was observed at room temperature in Pt4T3, even in the near-IR up to 1600 nm. It is interesting to note that Chawdhury et al. 63 did not observed phosphorescence from the terthiophene-containing platinum acetylide polymer at room temperature. They did however observe this phosphorescence at = 816 nm when photoluminescence was measured at T = 10 K.

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44 The overall quantum yields of photoluminescence (fluorescence and phosphorescence) were measured for Pt4 ( = 6.8%), Pt4T1 ( = 1.8%) and Pt4T3 ( = 0.9%). These values are an important characte ristic of this series and will be discussed further in the discussion. But it is important to notice the decreasing trend of the quantum yield of photoluminescence as the triplet exciton energy decrease s. This trend is in fact expected as it is a predic tion of the energy gap law. 64 The effect of the excitation wavelength wa s studied in order to determine if direct excitation of the oligothienyl moieties contri butes to their observe d luminescence (Figure 2-7). We therefore examined the photolumin escence spectrum in THF solutions of each oligomer at two excitation wavelengths : one is the phenylen e-based chromophore absorption wavelength ( = 352 nm) and the other is the oligothienyl-based chromophore absorption wavelength ( = 369, 409 and 438 nm for Pt4T1, Pt4T2 and Pt4T3, respectively). The dependence of the photolum inescence on the excitation wavelength is not dramatic but significant nonetheless. When exciting the oligothienyl units directly, the phosphorescence observed at = 518 nm decreases (in Pt4T1) or disappears (in Pt4T2 and Pt4T3). It is not entirely unexpect ed to observe phenyl-based phosphorescence in Pt4T1 even when exciting at = 369 nm since the chromophores are not resolved in the absorption spectrum due to the proximity of their energy levels. The absorption spectrum of Pt4 shows that absorption is possible even at = 369 nm (see Figure 2-5) given that 367 160,000 M -1 .cm -1 67 More importantly, the phosphorescence arising from the ol igothienyl group in Pt4T1 ( = 604 nm) and Pt4T2 ( = 723 nm) does not increase significantly upon direct excitation of the thienyl-based chromophore. While it could be argued that the same amount of thienyl chromophores could be directly

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45 Figure 2-7. Photoluminescence spectra of oligomers Pt4Tn in deoxygenated THF. (a) Pt4T1, ex = 352 nm ( ), ex = 369 nm ( ); (b) Pt4T2 ex = 352 nm ( ), ex = 369 nm ( ); (c) Pt4T3 ex = 352 nm ( ), ex = 369 nm ( ). excited at = 352 nm and = 369 nm in Pt4T1, it does not hold for Pt4T2, where the absorption bands of both chromophores are resolved in the absorp tion spectrum. This implies that the phosphorescence of the bithienyl units is limited by other factors. One possible reason is that the nonradiative decay is much faster than the radiative decay of

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46 the bithienyl-based triplet excited state and inherently limits the luminescence. Another reason could be that an equilibrium exists between the phenyl-based and the bithienylbased excited state. In this case, no matter how many more bithienyl excited states are produced by direct excitation, the equilibrium will effectively balance the excitation on both chromophores. In order to clarify the exact origin of the bands observed in the emission spectra of the oligomers, we now turn to the excita tion spectra of the Pt4Tn series measured in THF solutions (Figure 2-8). The oligothienyl fluorescence bands ( = 415, 457 and 491 nm for Pt4T1, Pt4T2 and Pt4T3, respectively), the phenyl-based phosphorescence ( =520 nm) and oligothienyl phosphorescence bands ( = 604 and 723 nm for Pt4T1 and Pt4T2, respectively) are monitored while scanni ng for excitation and the results are very revealing. It is not surpri sing to see some of the phe nyl-based absorption in the fluorescence of the oligothienyl since the oligot hienyl-based singlet excited state is lower in energy than the phenyl-based and excitati on transfer is therefore downhill. It is however striking that some of the thienyl-bas ed absorption is seen in the phenyl-based phosphorescence, as this is uphi ll and therefore not energetically favorable. This supports the idea of an equilibrium between the exci ted states located on the phenylene and the thienyl, whereby a thienyl-based excited state can transfer some of the excitation back to the phenyl-based chromophore. The dynamics of the excited state in these oligomers is therefore more complex than it may have appe ared until now and this will be discussed further in the discussion.

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47 Figure 2-8. Excitation spect ra of oligomers Pt4Tn in deoxygenated THF. (a) Pt4T1 excitation for em = 425 nm ( ), em = 520 nm ( ) and em = 604 nm ( ); (b) Pt4T2 excitation for em = 457 nm ( ), em = 520 nm ( ) and em = 723 nm ( ); (c) Pt4T3 excitation for em = 500 nm ( ), em = 520 nm ( ). If an equilibrium is indeed present betw een the phenyl and thienyl-based excited state, it may be possible to influence it by changing the temperature. Moreover, lowering the temperature may reveal the terthienyl phos phorescence that has been elusive so far.

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48 Therefore, the photoluminescence spectrum of Pt4T1 was recorded in deoxygenated MeTHF at low temperature (Figure 2-9). As the temperature decreases, the phenyl-based emission intensity increases relative to the th ienyl-based emission. The change levels off at 220 K and only an overall intensity change is observed at lower te mperatures. This is consistent with the previously proposed equilibrium concept be tween the phenyl and thienyl-based triplet excited state. The thieny l-based triplet excited being lower in energy Figure 2-9. Low-temperature pho toluminescence spectrum of Pt4T1 in deoxygenated MeTHF with an excitation = 352 nm. Direction of arrow indicates effect of decreasing temperature from 300 K to 220 K. Region between 730 and 750 nm is removed because a strong scatteri ng peak appears at this wavelength. than the phenyl-based, the en ergy transfer to the phenyl-based is energetically not favorable and is slowed down at lower temperat ure. However, this could also be due to an activation energy associated with the energy transfer from the phenyl to the thienylbased triplet excited state.

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49 Photoluminescence of Pt4T2 and Pt4T3 (Figure 2-10) were recorded under similar conditions and the emission at 90 K and 300 K ar e shown only. The effect of temperature appears to be the same as the one observed in Pt4T1. Compared to the emission spectrum at room temperature, the phenyl-based phosphorescence is stronger and dominates the emission in Pt4T2 and Pt4T3 at T = 90 K. In fact, the phenyl-based phosphorescence band at = 520 nm in Pt4T3 is almost identical to the phosphorescence of Pt4 at room temperature (see Figure 2-6) No terthienyl-based phosphorescence was detected at T = 90 K in Pt4T3, even in the near-IR region up to 1600 cm (not shown). Transient Absorption In order to better understand the nature of the excited state present following the excitation of the oligomers, their transient absorption spectrum was recorded in deoxygenated THF (Figure 2-11). All oligom ers feature a strong triplet-triplet (T 1 -T n ) absorption band between 600 and 700 nm, and the lifetimes extracted from the decay of this band are presented in Table 2-1. The life time of the triplet excited state present is longest for Pt4 (17.3 s) and decreases from Pt4T1 (9.4 s) to Pt4T2 and Pt4T3 (4.4 and 4.6 s, respectively), which have a similar lifetime. All oligomers also feature a bleaching band between 360 and 440 nm, where the singlet ground state absorbs. The red-shift and the narrowing of the T 1 -T n band from Pt4T1 to Pt4T3 is a clear indication of a thienylbased triplet excited state. This is particularly significant for Pt4T3, where no terthienylbased phosphorescence was observed in the steady-state photoluminescence study. The red-shift of the triplet-triplet absorption band is consistent with a more stable triplet exciton as the size of the oligothienyl increa ses. The narrowing of the band implies that the triplet exciton is also better defined as the size of the oligothienyl increases.

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50 Figure 2-10. Low-temperature photolumin escence spectrum in deoxygenated MeTHF with an excitation = 352 nm at T = 90 K ( ) and T = 300 K ( ). (a) Pt4T2; (b) Pt4T3. Time-Resolved Photoluminescence In order to gain insight into the dynamics and decays of the triplet excited states, time-resolved emission measurements were carried out in deoxygenated THF on the three oligothienyl-containing oligom ers (Figure 2-12). The life times extracted from these measurements, as well as other relevant data for the platinum acetylide oligomers are presented in Table 2-1. The lifetimes are on the order of microseconds in support of our

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51 Figure 2-11. Transient absorp tion spectra of oligomers in deoxygenated THF following 355 nm excitation. (a) Pt4, 4 s delay; (b) Pt4T1, 1.6 s; (c) Pt4T2, 1.6 s delay; (d) Pt4T3, 1.6 s delay. assignment as emission from a triplet ex cited state for the emission bands at = 606 nm and = 723 nm for Pt4T1 and Pt4T2, respectively. It also supports the assignment as fluorescence for the emission bands observed between = 415 and 491 nm in Pt4T1 to Pt4T3. These bands decayed within ~ 200 ns and are typical of a si nglet excited state lifetime. Since any prompt fluorescence has b een gated out in these experiments, the phenyl-based phosphorescence is now clearly id entified in Pt4T2 and Pt4T3. However once again, no phosphorescence from the tert hienyl was detected by time-resolved measurements. All emission decays were found to give better fit with a bi-exponential function than with a mono-exponential function. Therefore, the lifetimes extracted from this sets of measurements shown in Table 2-1 give the lifetime and the corresponding contribution of each exponential decay.

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52 Figure 2-12. Time-resolved photolumines cence spectra of oligomers Pt4Tn in deoxygenated THF following 355 nm excitation. (a) Pt4T1, camera delay 0.4 s, delay increment 1.3 s; (b) Pt4T2, camera delay 0.5 s, delay increment 2.0 s; (c) Pt4T3, camera delay 0.4 s, delay increment 2.5 s. The presence of the oligothienyl units is reflected in the lower lifetimes for the phenylene emission in all Pt4Tn oligomers compared to Pt4. The oligothienyl units act as energy acceptors and provide an alternate relaxation pathway for the phenyl-based triplet excited state, resulting in a s horter phenyl-based triplet excited state lifetime in all Pt4Tn

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53 oligomers. For the phenylene emission decay, it is difficult to make a prediction on what should be observed. If energy transfer ta kes place through an exchange mechanism, which depends on the orbital spectral overlap between donor and acceptor, the rate of energy transfer and thus the phenylene emission lifetime should not change across the Pt4Tn oligomers series. Indeed, the orbita l overlap between the donor (phenyl-based triplet excited state) and the acceptor (oligothienyl-based trip let state) should not change across the Pt4Tn oligomers series as the same orbitals are most likely involved in all cases ( 3 with some metal character). However, other unidentified effects may also have an influence on the energy transfer rate. As can be seen from Table 2-1, while the phenylene lifetime drops from Pt4 to Pt4T1, it is slightly longer for Pt4T2 and about the same for Pt4T3. For the thienyl emission decay, for which one would expect the lifetime to get shorter as the thienyl-based triple t excited state energy level decreases, the lifetimes for Pt4T2 and Pt4T3 seem to follow the prediction based on the energy gap law. However it is difficult to interpret this as an energy gap law effect since only two decay lifetimes are available. These results onl y seem to indicate that the dynamics of the triplet exciton are not well-be haved and do not involve only forward energy transfer from a phenyl-based to a thienyl-based triplet excited state. Discussion Energy of the Triplet Excited State in Pt4T3. It is possible to estimate the energy of the terthienyl-based triplet excited state present in Pt4T3. As discussed in the first chapter, th e singlet-triplet splitting energy is constant in a family of platinum acetylid e polymers, regardless of the energy of the singlet excited state. In m onomers and oligomers, the S 1 -T 1 gap is larger than for the

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54 Table 2-1. Photophysical data for oligomers Pt4 and Pt4Tn. UV-vis PL f max / nm max / nm / % TA g / s PL h / s F d P e P i Tn j Pt4 366 389 517 6.8 17.3 18.6 b Pt4T1 369 415 516, 606 1.8 9.4 0.8 (64%) 1.1 (43%) 8.4 (36%) 7.4 (57%) Pt4T2 354, 409 457 515, 723 a 4.4 2.7 (51%) 1.6 (51%) 11.8 (49%) 4.7 (49%) Pt4T3 353, 438 491 511 0.9 4.6 1.4 (56%) b 12.7 (44%) a Not measured; b Not applicable; c From ref. 67 ; d F: fluorescence; e P: phosphorescence; f Total quantum yield of photoluminescence (F+P); g Transient absorption decay lifetime; h Photoluminescence decay lifetime (number in pa rentheses indicates relative amplitude of component); i P: decay at 517 nm; j Tn: decay at 606 nm for Pt4T1, decay at 723 nm for Pt4T2. corresponding polymer. This difference has been attributed to the f act that the triplet exciton is more localized than the singlet exciton and is therefore less sensitive to the confinement effect experienced by the singl et exciton in monomers and oligomers. Polymers with either thiophene bithiophene or terthiophene as the spacer in platinum acetylide polymers have been studied by Khler et al. 45 and a constant S 1 -T 1 energy gap of 0.7 0.1 eV was found for these three polym ers. It is therefore expected that the S 1 -T 1 gap should be close but higher than this in our o ligomers series. From the steady-state emission fluores cence and phosphorescence of the monoand bithienyl chromophores in Pt4T1 and Pt4T2, the S 1 -T 1 energy gap can be calculated: Pt4T1: E S-T = E S E T = 0.94 eV Pt4T2: E S-T = E S E T = 1.00 eV

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55 As can be seen, these values of S 1 -T 1 are very close. If we assume a value of 1.0 0.1 eV for the series of Pt4Tn oligomers, the energy of the terthienyl-based triplet excited state should be: Pt4T3: E T = E S E S-T = 1.52 eV This value is similar to the S-T splitti ng energy found in a platinum acetylide polymer based on the terthienyl spacer. 64 This means that a terthienyl-based phosphorescence would appear at = 816 nm and that it should have been detected in our measurements if this state was sufficiently emissive. In fact this emission has been detected by Chawdhury et al. at a temperature T =10 K at the exact same position = 816 nm in a terthiophenecontaining platinum acetylide polymer. 63 The Absence of Phosphorescence in Pt4T3 Now that the energy of the terthienyl-based triplet excited state and the wavelength at which it should have appeared have been determined, some possible explanations for the absence of phosphorescence from this excited state will be provided. While phosphorescence from the terthienyl chromophore in Pt4T3 was not successfully detected, it is important to bear in mind that such a state exists in this molecule. Evidence for this lies in its tr ansient absorption spectrum which clearly identifies a triplet excited stat e fitting the trend started in Pt4T1 and Pt4T2. The redshifting and the na rrowing of the T 1 -T n band is consistent with a terthienyl-based triplet excited state. As seen in Chapter 1, radi ative decays are always in competition with nonradiative decay processes and the observati on of light emission only means that the radiative rate is fast enough to compete with the nonradiative decay rate. In the case of Pt4T3, the absence of terthienyl-based phosphores cence only means that for this excited state, the radiative rate is too slow and that the relaxation of the terthienyl-based triplet

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56 excited state occurs only through nonradiative channels. One reas on for that is that as we move from Pt4T1 to Pt4T3, the organic content of th e oligothienyl chromophore increases and the metal content decreases. Ther efore the spin-orbit coupling must be less efficient so ISC should be slower. The conseque nces are a slower rate of radiative decay for the T 1 -S 0 transition and a lower quantum yield of phosphorescence. Cryogenic temperatures in a frozen solvent matrix are often used to de tect phosphorescence too weak at room temperature. However, the photoluminescence spectrum of Pt4T3 recorded under these conditions (although T = 70 K wa s the lowest temperature where the measurement was carried) did not reveal the terthi enyl-based phosphorescence. Looking at the low-temperature photoluminescence of Pt4T1 at low-temperature, it appears that lowering the temperature favors the phenyl-bas ed emission more than the thienyl-based. Therefore, the same effect can be expected in Pt4T3 and it should not be surprising that the low-temperature emission spectrum did not display any terthienyl-based phosphorescence. While a slower ISC is probably partly responsible for the absence of phosphorescence from the terthienyl-based triple t excited state, there is almost certainly another factor playing a part. As already discussed in Chapter 1, the energy gap law has been shown to apply to platinum acetylide polymers and monomers. 64 It had previously been shown to apply to metal complexes 7,92 Simply stated, the energy gap law predicts that as the energy of the trip let excited state decreases, the rate of nonradiative decay increases exponentially. In fact the calculated radiative and nonradiative rates for the terthiophene platinum acetylide polymer 64 were found to be k r = 3 x 10 3 s -1 and k nr = 4.2 x 10 6 s -1 at 300 K. At 20 K, they were k r = 0.4 x 10 3 s -1 and k nr = 6 x 10 5 s -1 These values

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57 mean that at room temperature, the rate of radiative decay is too small to compete with nonradiative decay for a terthienyl-based trip let excited state. Th e authors concluded from their study that rates of nonradiative d ecay are only small enough for materials with a triplet energy level of 2.4 eV and more, which is well above our estimate of the energy level of terthienyl-based trip let excited stat e (1.52 eV). Excited State Dynamics So far, it has been shown that the presence of a low-energy trap can have a dramatic effect on the photophysical prope rties of platinum acetylide oligomers. Introduction of a thiophene unit results in a bathochromic shift of 90 nm with lower quantum yield of photoluminescence. Introducti on of even lower energy trap such as bithiophene and terthiophene results in an emission spect rum dominated by fluorescence from the trap and the long-liv ed phosphorescence is lost. Th is could potentially be very problematic for optical applications relying on the triplet excited state of this type of compound. 53,69 It is therefore critical to understand the dynamics of the tr iplet excited state in these systems and the interplay of low-energy site s. The photophysical study of this series of oligomers with energy traps of decreasing en ergy has shown that the dynamics of the triplet exciton in these systems is rather comp licated. There appears to be more than just a simple energy migration to the low-energy trap s and subsequent decay from those traps. Several experiments seem to indicate the presence of an equilibrium between the phenylene and thienyl-localized excited states Evidence for the equilibrium lies in the excitation spectra of the Pt4Tn oligomers which shows that the phenylene emission is arising from excited states localized both on the phenylene and on the oligothienyl moieties, with equal contributions. This can be explained by an e quilibrium between the

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58 phenyl-based and thienyl-based excited stat e. Also, the photoluminescence collected upon direct excitation of the oli gothienyl fragment did not sh ow a significant increase in thienyl luminescence, either from the singlet or the triplet excited state localized on the oligothiophenes. This is also consistent w ith an equilibrium effectively balancing the proportion of excited states and their respectiv e luminescence, which is shown to be quite independent of the location of the initially formed excited state. The low-temperature photoluminescence of Pt4T1 revealed that the energy transf er is slowed down at lower temperature and relatively more phosphorescence from the phenyl-based triplet excited state is observed. This is also consistent with an equilibrium where the least energetically favorable process (back energy transfer to highe r excited state is endothermic) is slowed down at lower temperature. However, this co uld also be explained by a simple thermallyactivated energy transfer from the phenyl-based to the thienyl-based triplet excited state. The lifetime data extracted from the emi ssion decays are not easily interpreted and rationalized. While the thienyl-based phosphores cence seems to be in agreement with the expected decreasing trend with the decrease in triplet energy level, the phenylene-based phosphorescence does not really follow any trend. Although this does not support the presence of an equilibrium, it does not disprove it either. To illustrate this equilibrium, the photophysical processes involved in the oligomers series is presented in Figure 213 below. The equilibrium would be brought in by relatively efficient processes 11 or 9, in addition to the forward processes 10 and 8. The experiments carried out in this study do not allow distin guishing whether the forward energy transfer takes place between the singlets (process 8) or the triplets (process 10). In fact both the single t and the triplet energy transfer c ould be operating in this system.

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59 P Tn S0S1T1 Ener gy S1T1 1 2 6 3 7 8 9 10 11 12 4 13 5 Figure 2-13. Energy diagram representing th e photophysical proce sses involved in the Pt4Tn oligomers. For the singlets to be in equilibrium, this would require singlet energy transfer (processes 8 and 9) to be faster than ISC (processes 5 and 12). Both of these processes have been shown to be fast 93,94 therefore it is not possible to rule out the singlet or the triplet energy transfer without exact calculations of the ra te constants for the processes involved. Although not commonly observe d and studied, the presen ce of an excited state equilibrium is not unprecedented. In fact such a dynamic equilibrium has been observed recently in related metal-organic oligomers 95-97 The absence of a clear trend in the lifetimes of emission suggests that while the equilibrium might be operating in Pt4T1, it does not seem to be the case in Pt4T2 and Pt4T3. Indeed, it appears that in Pt4T1, the phenyl-based and the thienyl-based triplet ex cited states decay at similar rates (7-8 s), close to the lifetime extracted from transient absorption (9 s). However, in Pt4T2, the phenyl-based and the bithienyl-based triplet ex cited states do not decay at the same rate.

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60 The lifetime of the phenyl-based tr iplet excited state increases (12 s), closer to the lifetime observed in Pt4 (18 s), while the lifetime of the bithienyl-based triplet excited state decreases further (5 s). It is however in accordance with the lifetime of triplet excited state detected by transient absorption, supporting the idea that this latter is localized on the bithiophene. Th erefore if there are strong evidences for an excited state equilibrium in Pt4T1, it appears to be dependent on th e energy gap and does not operate in the other oligomers of the series. Conclusion In this chapter, a series of platinum acetylide oligomers has been synthesized and their photophyisical properties studied. The o ligomers incorporated oligothiophene units in the main chain to act as energy trap s. The increasing length the oligothiophenes provided a series of oligomer s with decreasing energy trap. The photophysics of these oligomers have shown that the presence of an energy trap can have dramatic consequence on the photophysical properties of the oligomers. Efficient energy transfer creates excited st ate on the low energy sites. From there, radiative decay can occur and red-shifted em ission can be observed. Alternatively, the radiative decay rate may become too small to compete with nonradiative decay as the energy of the trap decreases and the excited state created by energy tr ansfer will relax by nonradiative decay processes. It has been show n that the energy of the trap can determine the dominant relaxation mechanism, due to th e energy gap law. Finally, the presence of an excited state equilibrium has been s uggested to explain the photophysical data collected on this oligomer series. However, the equilibrium seems to depend upon in the energy gap between the excited states a nd may be shut down by large energy gaps between excited states.

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61 Experimental Photophysical Measurements Steady-state absorption spect ra were recorded on a Va rian Cary 100 dual-beam spectrophotometer. Corrected steady-state em ission measurements were conducted on a SPEX F-112 fluorescence spectrometer. Samp les were degassed by argon purging for 30 min and concentrations were adjusted to produce optically dilute solutions (i.e., A max < 0.20). Low-temperature fluorescence measuremen ts were made by cooling the samples in a LN 2 cooled Oxford Instruments DN-1704 opti cal cryostat connected to an Omega CYC3200 temperature controller. Samples were degassed by 4 repeated cycles of freezepump-thaw on a high vacuum line. Photoluminescence quantum yields were de termined according to the optically dilute method described by Demas and Crosby, with the quantum yield being computed according to eq. 14 in their paper. 98 fac-Ir(2-phenylpyridine) 3 was used as an actinometer ( F = 0.40 in THF) Transient absorption measurements were conducted on a home-built apparatus, which has been described elsewhere. 99 Samples were contained in a cell holding a total volume of 10 mL and the contents were co ntinuously circulated through the pump-probe region of the cell. Samples were degasse d by argon purging for 30 mn. Excitation was provided by the 3 rd harmonic output of a Nd :YAG laser (355 nm, Spectra Physics, GCR14). Typical pulse energies were 5 mJ/pulse which corresponds to an irradiance in the pump-probe region of 20 mJ/cm. Sample c oncentration were adjusted so that A 0.8. Time-resolved emission measurements were recorded on a home-built apparatus consisting of a Quanta Ray GCR Series Nd:YAG laser as a source ( = 355nm, 10 ns fwhm), with time-resolved detection provide d by an intensified CCD detector (Princeton

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62 Instruments, PI-MAX iCCD) coupled to an Acton SpectraPro 150 spectrograph. Optically dilute solutions were used. Mass Spectrometry of Pt Oligomers The samples were analyzed using a Bruker Reflex II time-of-flight mass spectrometer equipped with delayed extracti on (Bruker Daltonics, Billerica, MA). The analysis mode was desorption ionization on silicon (DIOS). 100 The samples were dissolved in dichloromethane at 1-10 M a nd 1 L was spotted on a DIOS plate that had been electrochemically etched with HF under tungsten light illumination. 101 Desorption was achieved using a nitrogen laser at 337 nm. Synthesis General. All chemicals used for synthesis were of reagent grade and used without purification unless noted. R eactions were carried out unde r an argon atmosphere with freshly distilled solvents unless otherwise noted. 1 H, 13 C and 31 P NMR spectra were recorded on a Varian Gemini 300, VXR 300 or Mercury 300 spectrometer and chemical shifts are reported in ppm relative to TMS. cis -Dichloro-bis(trin -butylphosphine)platinum(II) 102 and 1,4-diethynylbenzene 67 were prepared by literature methods. 2,5-diiodothiophene (2b). Thiophene (1.0 g, 11.9 mmol) and N-iodosuccinimide (5.48 g, 24.4 mmol) were dissolved in acetic acid (6 mL) and chloroform (8 mL). The flask was covered with aluminum foil and th e mixture was stirred at room temperature overnight. The mixture was then washed with 10% sodium thiosulfate and water, the organic phase was dried on MgSO 4 filtered and the solvent was removed. The dark red oil obtained was further purified by column ch romatography (silica gel, hexane) giving the desired product 2b as a yellow oil (3.0 g, 75%). 1 H NMR (C 6 D 6 300 MHz) 7.23 (s, 2H); 13 C NMR (C 6 D 6 75 MHz) 77.0, 139.3.

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63 5,5-diiodo-2,2-bithiophene (2c). 2,2-Bithiophene (0.5 g, 3 mmol) and Niodosuccinimide (1.67 g, 7.5 mmol) were di ssolved in methanol (45 mL). To this solution, acetic acid (0.5 mL) was added. After stirring for 2 hours, a precipitate had formed and the flask was placed in freezer over night to ensure complete precipitation of product. The white solid was then filtered by suction filtration and washed with cold methanol. After drying under vacuum, product 2c was obtained as a white solid (0.80 g, 64 %). 1 H NMR (CDCl 3 300 MHz) 6.80 (d, 2H), 7.18 (d, 2H). 5,5-diiodo-[2,2-5,2]-terthiophene (2d). [2,2-5,2]-terthiophene (0.1 g, 0.4 mmol) and N-iodosuccinimide ( 0.199 g, 0.89 mmol) were dissolved in dichloromethane (6 mL) and acetic acid (0.05 mL) and flask was purged with nitrogen. The mixture was stirred in a water/ice bath for 2 hours after which time a ye llow solid had precipitated. The solid was collected by suction filtration, washed with cold methanol and dried under vacuum giving product 2d as a yellow solid (0.15 g, 73 %). 1 H NMR (C 6 D 6 300 MHz) 6.39 (d, 2H), 6.58 (s, 2H), 6.74 (d, 2H). 2,5-Bis-[(trimethylsilyl)-e thynyl]-thiophene (3b). 2,5-diiodothiophene 2b (2.0 g, 5.96 mmol) and trimethylsilylacetylene (1. 22 g, 12.5 mmol) were dissolved in Et 2 NH (8 mL) and the solution was degassed with argon for 30 min. Then, Pd(PPh 3 ) 2 Cl 2 (0.21 g, 5% eq., 0.298 mmol) and CuI (0.114 g, 10 % eq., 0.596 mmol) were added and mixture was stirred at room temperature for 12 hours. The mixture was then passed through a bed of Celite, washed with 10 % ammonium hydroxide, water, the organic phase dried on MgSO 4 filtered and the solvent was removed. Chromatography (silica gel, hexane) gave the desired product 3b as a yellow solid (1.15 g, 70 %). 1 H NMR (CDCl 3 300 MHz)

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64 0.22 (s, 18H), 7.05 (s, 2H); 13 C NMR (CDCl 3 75 MHz) 0.01, 97.1, 100.1, 124.7, 132.5, 169.0. 5,5-Bis-[(trimethylsilyl)-eth ynyl]-2,2-bithiophene (3c). This compound was synthesized according to the same procedure used for 2,5-bis-[(tr imethylsilyl)-ethynyl]thiophene 3b except 5,5-diiodo-2,2-bithiophene 2c (0.2 g, 0.48 mmol) was used and the reaction was completed in 4 hours. The desired product 3c was obtained as a yellow solid (0.129 g, 75 %). 1 H NMR (CDCl 3 300 MHz) 0.22 (s, 18H), 7.00 (d, 2H), 7.10 (d, 2H); 13 C NMR (CDCl 3 75 MHz) 0.1, 97.4, 100.7, 122.7, 123.9, 133.6, 138.2. 5,5-Bis-[trimethylsilyl)ethynyl] -[2,2,5]-terthiophene (3d). This compound was synthesized according to the same pro cedure used for 2,5-bis-[(trimethylsilyl)ethynyl]-thiophene 3b except 5,5-diiodo-[2,2 -5,2]-terthiophene 2d (0.135 g, 0.27 mmol) was used and the reaction was stirred at 45 C for 2 hours. The desired product 3d was obtained as a bright yellow solid (0.096 g, 81 %). 1 H NMR (CDCl 3 300 MHz) 0.24 (s, 18H), 7.01 (d, 2H), 7.08 (s, 2H), 7.14 (d, 2H). 2,5-Diethynyl-thiophene (4b). 2,5-bis-[(Trimethylsily l)-ethynyl]-thiophene 3b (0.392 g, 1.42 mmol) was dissolved in meth anol (24 mL) and the solution was degassed with nitrogen. To this solution, 0.1 mL of a 0.5 M KOH solution was added and the mixture was stirred at room temperature for 2 hours. After this time, water (50 mL) was added and the mixture was extracted with pentane, the organic phase dried on Na 2 SO 4 filtered, and the solvent was removed under re duced pressure at room temperature. The desired product 4b was obtained as a colorl ess oil (0.15 g, 80 %). 1 H NMR (CDCl 3 300 MHz) 3.5 (s, 2H), 7.22 (s, 2H); 13 C NMR (CDCl 3 75 MHz) 76.4, 82.3, 123.8, 132.8.

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65 5,5-Diethynyl-2,2-bithiophene (4c). 5,5-bis-[(Trimethyl silyl)-ethynyl]-2,2bithiophene 3c (30.7 mg, 0.086 mmol) was dissolved in THF (2 mL) and the solution was degassed with nitrogen. Then tetrabutylammonium fluorid e (0.35 mL of a 1 M THF solution, 0.35 mmol) was added via a syri nge and the mixture was stirred at room temperature protected from light for 3 hours. After this time, the solvent was removed and chromatography (silica gel, he xane) gave the desired product 4c as a yellow solid (14.4 mg, 78 %). 1 H NMR (CDCl 3 300 MHz) 3.4 (s, 2H), 7.04 (d, 2H), 7.19 (d, 2H); 13 C NMR (CDCl 3 75 MHz) 82.9, 121.6, 124.1, 134.2, 138.3. 5,5-Diethynyl-[2,2,5,5]-terthiophene (4d). 5,5-Bis-[trimethylsilyl)ethynyl][2,2,5]-terthiophene 3d (93 mg, 0.21 mmol) and K 2 CO 3 (29 mg, 0.21 mmol) were dissolved in MeOH (9 mL) and THF (4 mL ) and solution degassed with argon. The mixture was stirred at room temperature overnight after which time the solvent was removed. The brown residue obtained was dissolved in CH 2 Cl 2 washed with 10% HCl, water, the organic phase dried on MgSO 4 filtered and the solvent was removed. Chromatography (silica gel, he xane) gave the desired product 4d as a yellow solid (43.5 mg, 70 %). 1 H NMR (CDCl 3 300 MHz) 3.42 (s, 2H), 7.02 (d, 2H), 7.10 (s, 2H), 7.20 (d, 2H). Complex 5a. 1,4-diethynylbenzene ( 46.2 mg, 0.373 mmol) and cis -dichloro-bis(trin -butylphosphine)platinum(II) were dissolved in Et 2 NH (15 mL) and the solution was degassed with nitrogen. The mixture was stirred under reflux for 8 hours. The solvent was removed and the crude product purified by flash chromatography (silica gel, hexane then 7:3 hexane/CH 2 Cl 2 ) giving the desired product 5a as a yellow solid (210 mg, 40 %). 1 H NMR (CDCl 3 300 MHz) 0.85-1.0 (t, 36H), 1.40-1.63 (m, 48H), 1.9-2.0 (m, 24H),

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66 7.05 (s, 4H); 13 C NMR (CDCl 3 75 MHz) 14.0, 22.1, 24.5, 26.3, 101.6, 125.3, 130.5; 31 P NMR (CDCl 3 121 MHz) 7.89 (J Pt-P = 2390.8 Hz). Complex 5b. This compound was synthesized a ccording to the same procedure used for complex 5a, except 2,5-diethynyl-thiophene 4b (64 mg, 0.483 mmol) and cis dichloro-bis(trin -butylphosphine)platinum(II) (0.648 g, 0.967 mmol) were used. Flash chromatography (silica gel, hexane then 7:3 hexane/CH 2 Cl 2 ) gave the desired product 5b as a dark orange oil (288 mg, 43 %). 1 H NMR (CDCl 3 300 MHz) 0.85-1.0 (t, 36H), 1.40-1.60 (m, 48H), 1.9-2.0 (m, 24H), 6.6 (s, 2H); 13 C NMR (CDCl 3 75 MHz) 13.9, 22.1, 24.4, 26.2, 53.5, 88.5, 93.9, 127.0; 31 P NMR (CDCl 3 121 MHz) 8.03 (J Pt-P = 2364.0 Hz). Complex 5c. This compound was synthesized acco rding to the same procedure used for complex 5a, except 5,5-diethynyl-2,2-bithiophene 4c (14.4 mg, 0.067 mmol) and cis -dichloro-bis(trin -butylphosphine)platinum(II) (134. 1 mg, 0.2 mmol) were used. Flash chromatography (silica gel, hexane then 9:1 hexane/CH 2 Cl 2 ) gave the desired product 5c as a yellow film-forming oil (82.8 mg, 83 %). 1 H NMR (CDCl 3 300 MHz) 0.9-1.0 (t, 36H), 1.40-1.65 (m, 48H), 1.9-2. 0 (m, 24H), 6.70 (d, 2H), 6.85 (d, 2H); 13 C NMR (CDCl 3 75 MHz) 14.0, 22.2, 24.5, 26.2, 91.5, 93.7, 122.6, 128.2, 128.4, 134.9; 31 P NMR (CDCl 3 121 MHz) 8.15 (J Pt-P = 2364.6 Hz). Complex 5d. This compound was synthesized acco rding to the same procedure used for complex 5a, except 5,5-diethynyl-[2 ,2,5,5]-terthiophene 4d (43.5 mg, 0.15 mmol) and cis -dichloro-bis(trin -butylphosphine)platinum(II) (201.2 mg, 0.30 mmol) were used. Flash chromatography (sili ca gel, hexane then 7:3 hexane/CH 2 Cl 2 ) gave the desired product 5d as a yellow film-forming oil (135 mg, 57 %). 1 H NMR (CDCl 3 300

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67 MHz) 0.8-1.0 (t, 36H), 1.4-1.6 (m, 48H), 1.9-2.0 (m, 24H), 6.75 (d, 2H), 6.94 (d, 2H), 6.98 (s, 2H); 13 C NMR (CDCl 3 75 MHz) 14.0, 22.2, 24.5, 26.3, 92.5, 93.8, 123.2, 123.7, 128.3, 129.1, 134.1, 136.2; 31 P NMR (CDCl 3 121 MHz) 8.19 (J Pt-P = 2347.6 Hz). 3-{4-[(Triisopropylsilyl)-ethyn yl]-phenyl}-prop-2-yn-1-ol (7). 1,4Diiodobenzene (5.0 g, 15.16 mmol) was dissolved in THF (60 mL) and iPr 2 NH (40 mL) in a Schlenk flask and the solution was degassed with argon for 30 min. Then, triiso propylsilylacetylene (2.76 g, 15.16 mmol), Pd(PPh 3 ) 2 Cl 2 (0.642g, 0.9 mmol) and CuI (0.346 g, 1.8 mmol) were added. The mixture was stirred at 70 C for 3 hours, after which time prop-2-yn-1-ol was added dropwise via a syringe. The mi xture was stirred overnight at 70 C. After cooling down, the mixture was passed through a bed of Celite, washed with 10% NH 4 OH (3 x 50 mL) and water (3 x 50 mL), the organic phase dried on MgSO 4 filtered and the solvents were removed. Chromatography on silica (hexane first, then 9:1 hexane/CH 2 Cl 2 ) gave the desired product 7 as a red oil (1.65 g, 35 %). 1 H NMR (CDCl 3 300 MHz) 1.20 (s, 21H), 4.40 (br, 1H), 4.58 (s, 2H), 7.40 (m, 4H); 13 C NMR (CDCl 3 75 MHz) 11.4, 18.7, 51.1, 85.1, 89.3, 92.5, 106.7, 122.7, 123.6, 131.5, 131.9. 1-Ethynyl-4-(tri-iso-propylsilylethynyl)-benzene (8). 3-{4-[(Triiso -propylsilyl)ethynyl]-phenyl}-prop-2-yn-1ol (1.43 g, 4.58 mmol) was dissolved in Et 2 O (80 mL) and degassed with nitrogen for 15min. Then, activated MnO 2 (6.37 g, 73.3 mmol) and KOH (2.05 g, 36.6 mmol) were added in four fract ions every hour and mi xture was stirred at room temperature for 4 hours protected from light. After this time, mixture was washed with 5% HCl (3 x 50 mL), water (3 x 50 mL), dried on MgSO4, filtered and the solvent was removed. Chromatography (silica ge l, hexane) gave the desired product 8 as a red oil

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68 (0.99 g, 86 %). 1 H NMR (CDCl 3 300 MHz) 0.98 (s, 21H), 2.98 (s, 1H), 7.22 (s, 4H); 13 C NMR (CDCl 3 75 MHz) 11.3, 18.6, 78.8, 83.2, 92.9, 106.4, 121.9, 124.0, 131.9. Complex 9. cis -dichloro-bis(trin -butylphosphine)platinum(II) (0.500 g, 0.74 mmol) and phenylacetylene (76.5 mg, 0.75 mmol) were dissolved in Et 2 NH (20 mL) and degassed with nitrogen for 15 min. The mixture was then stirred under gentle reflux for 8 hours after which time all phenylacetylene ha s been consumed. Mixture was allowed to cool down, solvents removed and crude pr oduct was purified by fl ash chromatography (silica gel, hexane) giving the desired product 9 as yellow solid (522.0 mg, 96 %). 1 H NMR (CDCl 3 300 MHz) 0.95 (t, 18H), 1.45 (m, 12H), 1.60 (m ,12H), 2.03 (m, 12H), 7.21 (m, 5H); 13 C NMR (CDCl 3 75 MHz) 14.02, 21.94, 22.16, 22.38, 24.08, 24.43, 24.52, 24.61, 26.01, 26.29, 26.42, 26.56, 31.79, 125.27, 128.03, 128.01, 130.97, 130.99; 31 P NMR (CDCl 3 121 MHz) 7.95 (J Pt-P = 2395.2 Hz). Complex 10. Platinum complex 9 (0.235 mg, 0.32 mmol ) and 1-ethynyl-4-(triiso propylsilylethynyl)-benzene (100.0 mg 0.35 mmol) were placed in Et 2 NH (6 mL) and the solution was degassed with nitrogen fo r 15 min. Mixture was stirred under gentle reflux for 8h. After cooling down, the solven ts were removed and the crude product was purified by flash chromatography (silica gel, hexane then 4:1 hexane/CH 2 Cl 2 ) giving the desired product as yellow solid (258.0 mg, 82%). 1 H NMR (300 MHz, CDCl3) 0.98 (t, 18H), 1.18 (s, 21H), 1.50 (m, 12H), 1.64 (m,12H), 2.19 (m, 12H), 7.25 (m, 9H); 13 C NMR (75 MHz, CDCl 3 ) 11.56, 14.01, 14.32, 15.57, 18.88, 22.86, 23.88, 24.11, 24.34, 24.51, 24.61, 24.69, 24.48, 25.48, 26.56, 31.80, 34.87, 90.67, 107.97, 119.68, 125.03, 128.03, 130.68, 130.95, 130.95, 131.82; 31P NMR (121 MHz, CDCl2) 4.21 (J Pt-P = 2395.4 Hz).

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69 Complex 11. Platinum complex 10 (340.0 mg, 0.35 mmol) was dissolved in THF (6 mL) and the solution was degassed with nitr ogen for 15 min. Then, TBAF (0.70 mL of a 1M solution in THF, 0.70 mmol) was a dded and the mixture was stirred at room temperature protected from light for 4 hours. Then the mixtur e was diluted to 50 mL with CH 2 Cl 2 washed with brine (2 x 30 mL) and water (2 x 30 mL), dried on MgSO4, filtered and the solvents removed. The crude oil was purified by flash chromatography (silica gel, hexane then 4:1 hexane/CH 2 Cl 2 ) giving the desired product as a yellow solid (240.0 mg, 83%). 1 H NMR (CDCl 3 300 MHz) 0.98 (t, 18H), 1.47 (m, 12H), 1.63 (m, 12H), 2.18 (12H), 7.22 (m, 9H); 13 C NMR (CDCl 3 75 MHz) 14.07, 23.90, 24.13, 24.35, 24.56, 24.65, 24.74, 26.44, 26.59, 26.74, 84.49, 107.86, 109.16, 109.34, 112.59, 112.79, 123.64, 125.03, 125.11, 128.10, 129.22, 129.98, 130.83, 131.02, 131.96; 31 P NMR (CDCl 3 121 MHz) 4.22 (J Pt-P = 2357.0 Hz). Pt4. Complex 5a (67.5 mg, 0.048 mmol) and complex 10 (0.08 g, 0.097 mmol) were dissolved in Et 2 NH (9 mL) and the solution was de gassed with nitrogen for 15 min. The mixture was stirred under reflux overnig ht. After cooling down, solvents were removed and crude product purified by flash chro matography (silica gel, hexane then 7:3 hexane/CH 2 Cl 2 ) gave Pt4 as a yellow solid (65 mg, 46 %). 1 H NMR (CDCl 3 300 MHz) 0.95 (t, 72H), 1.47 (m, 48H), 1.65 (m, 48H), 2.15 (m ,48H), 7.20 (m, 22H); 31 P NMR (CDCl 3 121 MHz) 3.98, 4.05 (J Pt-P = 2363.2 Hz). Pt4T1. This compound was synthesized according to the same procedure used for Pt4, except complex 5b (87 mg, 0.062 mmol) and complex 10 (0.103 mg, 0.125 mmol) were used. Flash chromatography (sili ca gel, hexane then 7:3 hexane/CH 2 Cl 2 ) gave Pt4T1 as a yellow solid (110 mg, 59 %). 1 H NMR (CDCl 3 300 MHz) 0.95 (t, 72H),

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70 1.35 (m ,48H), 1.62 (m, 48H), 2.12 (m, 48H), 6.66 (s, 2H), 7.27 (m, 18H); 31 P NMR (CDCl 3 121 MHz) 4.05, 4.08 (J Pt-P = 2365.5 Hz); Mass spec. (MALDI-TOF) calcd for C 140 H 236 P 8 Pt 4 S 2979.54, found 2978; Elemental anal. calcd C 56.43, H 7.98, found C 56.58, H 8.06. Pt4T2. This compound was synthesized according to the same procedure used for Pt4, except complex 5c (82.8 mg, 0.056 mmol) and complex 10 (101.5 mg, 0.123 mmol) were used. Flash chromatography (sili ca gel, hexane then 7:3 hexane/CH 2 Cl 2 ) gave Pt4T2 as a yellow solid (98.4 mg, 57 %). 1 H NMR (CDCl 3 300 MHz) 1.0 (m, 72H), 1.55 (m, 96H), 2.12 (m, 48H), 6.7 (d, 2H), 6.9 (d, 2H), 7.22 (m, 18H); 31 P NMR (CDCl 3 121 MHz) 4.07, 4.21 (J Pt-P = 2356.2 Hz); Mass spec. (MALDI-TOF) calcd for C 144 H 238 P 8 Pt 4 S 2 3061.66, found 3058; Elemental anal. calcd C 56.49, H 7.84, found C 56.45, H 8.10. Pt4T3. This compound was synthesized according to the same procedure used for Pt4, except complex 5d (102 mg, 0.065 mmol) and complex 10 (107.4 mg, 0.13 mmol) were used. Chromatography on silica (hexane first, 7:3 hexane/CH 2 Cl 2 ) gave Pt4T3 as a red film-forming solid (106 mg, 51 %). 1 H NMR (CDCl 3 300 MHz) 0.95 (m, 72H), 1.48 (m, 48H), 1.65 (m, 48H), 2.13 (m, 48H), 6.74 (d, 2H), 6.94 (s, 2H), 6.97 (s, 2H), 7.25 (m, 18H); 31 P NMR (CDCl 3 121 MHz) 4.07, 4.25 (J Pt-P = 2362.8 Hz); Mass spec. (MALDI-TOF) calcd for C 148 H 240 P 8 P 4 S 3 3143.79, found 3141; Elemental anal. calcd C 56.54, H 7.69, found C 56.43, H 8.00.

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CHAPTER 3 DELOCALIZATION OF CHARGE CARRIERS IN PLATINUM ACETYLIDE OLIGOMERS Introduction Conjugated polymers are promising active ma terials for use in light-emitting diodes (LEDs), 76-78,103 field-effect transistors 104,105 and photovoltaic devices. 106,107 All of these applications rely on charge carriers for charge transport. 108 Much debate arose over the last twenty years concerning the exact nature of these charge carriers, and whether they were solitons, 109 polarons 110 or bipolarons. 111 It is now believed that the charge carriers in most nondegenerate conjugated polyme rs such as poly(thiophene), poly( p -phenylene) and poly( p -phenylenevinylene) 112 are polarons, essentially ra dical ions. These charge carriers and the charge transport properties of conjugated polymers have therefore received a lot of attention in order to determine structure property-relationships 113,114 and improve the performance of devices based on conjugated polymers. 115 In organic light-emitting diodes (OLEDs), the internal electroluminescent quantum efficiency is limited by the proportion singlet excitons formed after recombination of an electron and a hole on the polymer chain 78 (a negative and a positive polaron). Based on spin statistics and assuming electron-hole recombination is spin-independent, the electron-hole recombination should give 25% of singlet excitons and 75% of triplet excitons. Since the triplet exciton is usually no t emissive in organic molecules, this would mean that the maximum efficiency of O LEDs would be 25%. However, several groups have now independently demonstrated that th e exciton formation is spin-dependent and 71

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72 internal quantum efficiencies up to 63% have been reported. 116-119 The reason for this is currently under strong theoretical and experi mental investigation. 42,120 However, it is likely that triplet forma tion will always limit the efficiency of OLEDs so other groups have pr oposed and successfully demonstr ated different strategies, such as using transition metal complexes. 121-123 The idea is that due to the heavy atoms inducing mixing of singlet and triplet excito ns, all excitons formed will luminescence through rapid radiative decay of the trip let exciton and thus all electron-hole recombination leads to an emissive exciton. In fact, there are examples of singletharvesting platinum(II) complexes used in light -emitting devices with external efficiency up to 11%. 124-126 There is therefore clearly a techno logical need for a better understanding of the dynamics of charge carriers in metalorganic systems such as platinum acetylide and this is one of the motivations for the work presented in this chapter. On a more fundamental level, our group also has an interest in conjugation through metal. After having successfully demonstrated the difference in delo calization of singlet and triplet excited states in platinum acetylide oligomers, 67 it appeared that this platinum oligomer series of increasi ng length would provide an in teresting system to study the effect of chain length on the delocaliza tion of charge carriers. Moreover, the oligothiophene-containing platinum acetylid e oligomers studied in Chapter 2 would provide a system to study the effects of lowenergy sites on charge ca rriers in platinum acetylide oligomers. Therefore the conjugation and delocalization effects of charge carriers have been explored through electrochemistry a nd pulse radiolysis. These techniques 93,113,127,128 and other time-of-flight techniques 129-131 have been used extensively and successfully to study

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73 charge transport in organi c conjugated oligomers and polymers. Among them, thiophene-based oligomers 132,133 and polymers 134,135 in particular have received considerable attention due to their rich and promising electrochemical properties. Conjugated metal-organic systems are less co mmon than all-organic conjugated systems but are gaining increasing attention due to the unique electroc hemical properties of transition metal complexes. 136-139 The oligomers studied here ar e presented in Figure 3-1 and Figure 3-2 below. The first series (Ptn), consists of platinum acetylide oligomers containing one to five platinum centers in the conjugated phenyleneethynylene backbone. Th eir synthesis has been described elsewhere 67 and their NMR spectra were iden tical to the oligomers prepared previously. The second series (PT4Tn) c onsists of platinum acetylide oligomers containing four platinum centers where the cen tral benzene ring is replaced by thiophene, bithiophene or terthiophene. Their synthesis and characterization has been described in Chapter 2. Figure 3-1. Structure of platinum acetylide oligomers Ptn (n = 1-5). Figure 3-2. Structure of platinum acetylide oligomers Pt4Tn (n = 1-3). Results Electrochemistry The oxidation and reduction of the plati num acetylide oligomers were explored using cyclic voltammetry (CV) and differen tial pulse voltammetry (DPV). Measurements

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74 were carried out in nitrogen-degassed methyl ene chloride solutions with TBAH (0.1 M) as the supporting electrolyte. The reduction of all oligomers gave only irreversible waves around -1.0 and -1.3 V and this data will not be discussed. However, reversible (or quasireversible) waves were observed between +0.6 and +1.1 V for all oligomers and all the electrochemical data is su mmarized in Table 3-1. Table 3-1. Redox potentials (V vs SCE) fo r Ptn and Pt4Tn oligomers series in CH 2 Cl 2 containing 0.1 M TBAH. a Oligomer E 1, red E 1, ox E 2, ox E 3, ox Pt1 -1.29 (b) 1.11 (b) Pt2 -1.27 (b) 0.89 (b) Pt3 -1.19 (b) 0.85 (1 e ) 1.06 (1 e ) Pt4 -1.30 (b) 0.81 (1 e ) 0.88 (1 e ) Pt5 -1.29 (b) 0.98 (1 e ) 1.16 (1 e ) Pt4T1 -1.08 (b) 0.71 (1 e ) 1.09 (2 e ) Pt4T2 -1.02 (b) 0.63 (1 e ) 1.01 (2 e ) Pt4T3 -1.01 (b) 0.64 (1 e ) 0.88 (1 e ) 1.08 (2 e-) a Number of electrons shown between parenthesis are only estimates from the current passed for each wave; b Irreversible wave. The results of the CV and DPV measur ements are complimentary and both instructive in the Pt oligomer series theref ore all electrochemistry spectra are presented fro the Ptn oligomers series (Figure 3-3) but only CV voltammograms for the Pt4Tn oligomers series (Figure 3-4). Starting with the Ptn oligomers series, it can be seen that Pt1 and Pt2 show only one irreversible wave at +1. 11 V and +0.89 V, respectively. This implies that the radical

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75 cation formed from Pt1 and Pt2 is not stable on the electrochemical timescale. In Pt3, two reversible waves are obser ved at +0.85 and +1.06 V. This is evidenced in the CV spectrum where the typical half-w ave shape of a reversible elect ron process is present. In Pt4, it appears that the two waves observed in Pt3 are merged into a single reversible wave centered at +0.85 V, while a new quasi-reversible wave appears at higher potentials. This merging of the two waves is clearly observed in the DPV spectrum of Pt4 where the broad peak shows a shoulder due to the second oxidation pr ocess next to the maximum peak. The merging of these waves suggests charge localization on an electrophore. In Pt3, each reversible wave is attributed to the formation of radical cation centers on two sites in close proximity of each other on the oligomer. Due to this proximity, the second radical cati on formed is affected by the presence of the first center already present. When the oligomer become s longer, two radical cation centers can be formed without interacting with each other. This is almost entirely possible in Pt4 where the waves have merged because the presence of a radical cation center does not influence the formation of a second center. In Pt5, the merging is complete and essentially one broad band is observed in the DPV spectrum. However, close inspection of the CV spectrum shows that the radical cation formed is not stable here again, as evidenced by the mostly irreversible wave observed. Severa l cycles revealed a loss of current after each cycle indicating that the oli gomer was probably being deposit ed on the electrode during the oxidation. All in all, the elec trochemistry data on the Ptn oli gomer series suggests that the radical cation electrophore is rather localized since two such sites can be created on Pt4 with almost no electronic coupling or interaction between them.

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76 Figure 3-3. Cyclic voltammetry (CV, left) and differential pulse voltammetry (DPV, right) of oligomers Ptn. In the Pt4Tn voltammograms, it can be s een that all Pt4Tn oligomers show two reversible bands around +1.0 V and +0.65 V. Based on the previous Ptn electrochemistry study and the lower oxidation potential of thiophene compared to benzene, 140 the high potential at +1.0 V is attributed to an oxi dation of a phenyl-based electrophore and the

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77 Figure 3-4. Cyclic voltammetry (CV) of oligomers Pt4Tn. wave at +0.65 V to an oxidation of a thienyl-based electrophore. But while the first thienyl-based oxidation is attributed to a one-e lectron process, it appe ars that twice the current is passed in the phenyl-based oxidation in Pt4T1 and Pt4T2. It is therefore believed that the phenyl-based wave is due to an oxidation on both end-phenylene electrophores. In addition, the DPV spectrum of Pt4T3 (not shown) revealed a third band

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78 at +0.88 V. This oxidation probably arises from the formation of a second oxidation on the terthiophene and formation of a dicationic terthienyl spec ies, as observed in other oligothiophenes studies. 141 Pulse Radiolysis Ion Radical Spectra Ion radicals were generated by pulse radi olysis at Brookhaven National Laboratory. The spectra were measured at 0.1-1.0 s delay time following the growth of the radical ion species. Radical ions we re produced from transfer of electron or hole from the solvated electrons and solvent-base d holes created by the electron pulse. Radical cations The spectra of the radical cat ion obtained for both oligomer s series are presented in Figure 3-5. All radical cation oligomers feature a strong ba nd in the visible spectrum (400-800 nm) and a weaker and broader band in the near-IR (800-1600 nm). Looking at the Ptn oligomers first, it appears that the visible band around between = 360 and = 520 nm red-shifts strongly from Pt1 to Pt2, but does not move after that. This is clearly evidenced in the inset of the visible region shown in Figure 3-5. In fact, the change levels off at Pt3, which has the same absorption band as Pt4 and Pt5. The same trend is observed for the band in the near-IR, where the change levels off at Pt2 in this case. Consistent with the electrochemistry data, this suggests a relatively localized radical cation species in these oligomers. Since no apparent stabilization energy is gained for oligomers longer than Pt3, it appears that the radical cati on is delocalized over ca. three repeat units. Turning to the Pt4Tn oligomers series, si milar spectra consisting of two absorption bands in the visible and near-IR are observed. The spectra suggest that the radical cations

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79 Figure 3-5. Radical cation spect ra for Ptn (top) and Pt4Tn ( bottom) oligomers series. In Ptn: Pt1 ( ), Pt2 ( ), Pt3 ( ), Pt4 ( ) and Pt5 ( ). In Pt4Tn: Pt4T1 ( ), Pt4T2 ( ), Pt4T3 ( ). formed are essentially localized on the oligothi enyl electrophore, as the visible bands are at much lower energies than the absorption bands in the Ptn oligomers series. Both of the bands undergo a significant bathochromic shift from Pt4T1 to Pt4T3. This is attributed to the increasing stability of radical cation centers on the oli gothienyl units of increasing length present in the oligomer.

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80 Radical anions The absorption spectra of the radical an ions are presented in Figure 3-6 and all oligomers display two bands, one in the visible between = 380 and = 580 nm and one extending far in the IR (except for Pt1). Apart from Pt1 which is blue-shifted from the rest of the series, the visibl e absorption band of all Ptn o ligomers radical anions are almost superimposable. This suggests the presence of a very local ized radical anion, which is not sensitive to an increase in chain length after Pt2. The anion radicals of the Pt4Tn again di splay two bands as well, again in the visible between = 520 and = 800 nm and one extending in the IR. The visible band shows the same trend observed in the radical cation spectra, that is a bathochromic shift with increasing oligothiophene size. The second IR band however does not seem to follow any particular trend, although it is diffi cult to interpret as the bands are beyond the range of the instrument. Discussion Except from bulk conductivity of oligoPPE -based self-assembled films studied by Tour and co-workers, 142 there is no literature on the electrochemistry of phenyleneethynylene-based conjugated syst ems. However, platinum acetylide complexes, oligo(phenylenevinylene)s and oligo(thiophene)s have been studied by several groups and this provide a basis for the discussion. Delocalization of Charge Carriers The oxidation waves observed in the CV and DPV spectra of the Ptn and Pt4Tn oligomers are assigned to oxida tion occurring on the phenyl or thienyl ligand rather than oxidation of the metal because of the oxidati on potential and their reversibility. It is known that the one-electron oxida tion of Pt(II) to Pt(III) is usua lly irreversible as the

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81 Figure 3-6. Radical anion spect ra for Ptn (top) and Pt4Tn ( bottom) oligomer s series. In Ptn: Pt1 ( ), Pt2 ( ), Pt3 ( ), Pt4 ( ) and Pt5 ( ). In Pt4Tn: Pt4T1 ( ), Pt4T2 ( ), Pt4T3 ( ). radical cation formed readily undergoes rapid interaction with the solvent, leading to decomposition products. 143 When observed, 144 the platinum oxidation has been reported at higher potential (~1.2 V) than all oxidation potential observed here. Moreover, the HOMO of platinum acetylide oligomers is beli eved to be located mostly on the aryl ligands with little metal character while the LUMO is located essentially on the aryl

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82 ligands. It is therefore safe to rule out a metal oxidation for the reversible waves. The oxidations are thus referred to as ligand-based but bearing in mind that some metal character is probably mixed in as well. Ho wever, a Pt(II) to Pt(III) oxidation with decomposition could be responsible for the irreversible waves observed in Pt1 and Pt2. The merging of the two oxidation waves in Pt3 into one wave in Pt4 and the redshift of the radical cation ab sorption bands leveling off for Pt3 give an indication on the delocalization of the ra dical cation of platinum acetylid e oligomers. The data suggest a fairly localized radical cation species, one th at is probably localized over 2 to 3 repeat units. With 4 repeat units, two radical cation centers can form with little interaction or electronic coupling between them The radical anions are more localized than the radical cations. The absorption spectra of radical anions of the Ptn series show no red-shift after Pt2 for the visible band. Moreover, the elec trochemistry of these platinum acetylide oligomers displayed only irre versible waves, implying that the radical anions are not stable. This means that these materials would probably perform better when p-doped rather than n-doped. The extent of delocalizat ion of the radical ions observed in the Ptn series is somewhat smaller but comparable to the extent of delo calization of charge carriers in related all-organic conj ugated oligomers studied recently. 72,145 The merging of the oxidation waves in Pt4 is reminiscent of mixed-valence systems which have been known for a long time. 146 And while it was first established for polymetallic complexes, the concept of mixed va lence is also valid for organic molecules, as shown in severa l recent examples. 147-151 If two M II metal centers are coupled by a strong electronic coupling, removal of one el ectron on one of the metal may lead to a delocalized cation with two equi valents metal each in a +2.5 oxidation state. If there is no

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83 electronic coupling, the one-electron oxida tion will lead to a mixed valence cation species with M II and M III metal centers. These two extreme cases represent the class III and I of the Robin and Days classi fication of mixed-valence species. 152 In between those two cases lies a wide range of intermediate species with different degree of electronic coupling. Electronic coupling can be signifi cantly improved by conjugation but it is also distance-dependent. Therefore it is probabl y favored and effective in the shorter oligomers of the Ptn series. For oligomers longer than Pt3, the distance between two radical cation centers becomes too great and the electronic c oupling is greatly reduced. The electrochemistry of thiophene-based conjugated materials is well documented in the literature. The oxidation potentials of unsubstituted oligothiophenes reported by Meerholz and Heinze 140 in methylene chloride are highe r than the poten tial observed in the Pt4Tn oligomers series studied here. Valu es reported for the first oxidation are +1.7, +1.25 and +0.87 V for thiophene, bithiophene n and terthiophene, re spectively, whereas they are +0.71, +0.63 and +0.64 V across the Pt4Tn oligomers series. Oxidation potentials for bis-(thiomethyl)b ithiophene and bis-(thiomethyl )terthiophene as reported by Hill et al. 153 in acetonitrile are +0 .93 and +0.89 V. Even with a good electron-donating substituent such as thiomethyl group, it appears the oligothiop henes located on the oligothiophenes of the Pt4Tn oligomers are easier to oxidize and th eir radical cations more stable. This is an indication that the platinum center is an excellent electron donor, that can bring a large stabilizat ion energy to the radical cation. 141 Electronic Transitions of the Radical Ions Charge carriers are a key to opto-electrical processes in conjugated materials. The most promising polymers for thes e applications, poly(thiophene), 154 poly( p phenylenevinylene) 130 and poly( p -phenylene) 155 have been studied extensively to

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84 understand the nature and the dynamics of the charge carriers pres ent. The absorption spectra of these extentded -conjugated systems have been interpreted in terms of the polaron-bipolaron model, based on band theory. However, the oligomers such as the one studied here do not form real electronic band structures a nd it has been suggested that their spectra are better interpreted within the MO theory. 156,157 Since the electrochemistry study showed irreversible processes for the re duction of the platinum acetylide oligomers, the discussion will be focused on the radical cations. Although the absorption spectra of the radical cations of oligo(phenyleneethynylene)s have not been reported, the related oligo(phenylenevinylene) has been investigated. Two bands are observed for oligomers up to the tetramer, one in the visible ( = 500-700 nm) and one in the near-IR ( = 8001500 nm). 158 Both of these bands were shown to sh ift systematically to lower energy with increasing conjugation length. Th is implies an increasing de localization of the radical cation in these all-organic oligomers and this is opposite to our Ptn oligomers series, where measurements have shown no shift of th e absorption bands to lower energies after the dimer or the trimer. While the platinum center preserves conjugation in the neutral state, it is possible that c onjugation becomes much more lim ited in the radical cation of these metal-organic oligomers. It is however interesting that the all-organic oligomers also exhibited two absorption bands, in the visible and in the near-IR. This is an indication that the metal centers do not drastically change the nature of the optical transitions of radical cations in me tal-organic conjugated oligomers. Different unsubstituted and substituted oligothiophene radical cations have been prepared and their absorption spec tra recorded by several groups. 153,156,159,160 It is now

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85 generally accepted that oxidized o ligothiophenes can form intermolecular -dimers and their spectra have been identified from the monomeric, non-aggregated oxidized species. Typically, the monomer ra dical cation shows two absorption bands (due to excitation from HOMO to SOMO and from SOMO to LUMO) at lower energy than the neutral oligomers, whereas the radical cation of the -dimer displays two bands broader and blue-shifted compared to the m onomeric radical cation, as well as a third charge transfer band. Experime ntal conditions and factors such as a high concentration or a low temperature, 160 use of a polar solvent and the oligomer length 159 have been shown to favor the formation of -dimers. Although one author has claimed that steric hindrance may prevent the formation of -dimers, 161,162 these have been unambiguously identified in many examples of sterically hindered ol igothiophenes. Therefor e it is not clear whether steric hindrance has an effect on the monomer-dimer equilibrium constant. The high energy transition observed in the visible for the radical cation of the Pt4Tn oligomers series is red-shifted comp ared to the visible absorption band observed for unsubstituted thiophene to terthiophene oligomers. The UV-vis band reported for this series is found between = 250 and = 545 nm, 156 whereas it is observed between = 530 and = 750 in the present Pt4Tn oligomers se ries. The visible absorption band also occurs at lower energy than va rious substituted oligothiophenes. 153,159,160 As mentioned earlier, this is consistent with the electrochemistry data a nd further supports the idea of a relatively stable radical cation in the plati num acetylide oligomers. The nature of the radical cation formed upon oxidation of the oligothienyl electrophore is confidently assigned to a monomeri c, non-aggregated species, rather than a -dimer for several reasons: 1) Measurements were carried out in 1,2-dichloroethane (not as polar as

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86 acetonitrile); 2) The radical ion spectra we re acquired on a short timescale (few s) after the laser pulse, leaving little time for -dimers to form; 3) The bis-(trin -butylphosphine) ligands may prevent -dimer formation; 4) The equilibrium constant for the formation of -dimers is probably too low for the short oligothiophenes investigated here. It is difficult to unambiguously assign the absorption bands displayed by the platinum acetylide oligomers. The high energy band in the visible region probably is a ligand-localized transition. The nature of the broad band observed in the near-IR could be due to several possi ble transitions: 1) A second transition as in all-organic oligo(phenylene vinylene)s; 2) A Pt (acetylide) + charge transfer tr ansition (CT); 3) An intervalence charge transfer transition (IVC T). These last two transitions were observed in a platinum acetylide-bridged organic mixed valence compound recently studied by Jones et al. 163 at = 1000 nm (CT) and = 1530 nm. A triarylaminium ligand-based transition was also observed at = 670 nm in this study. Close inspection of the radical cation spectra of Ptn and Pt4Tn oligomers actua lly reveals the presence of two bands in the near-IR, in addition to the visible band for Pt1 and Pt4T1 only. Longer members of the Ptn and Pt4Tn series onl y display one broad near-I R band which extends beyond 1600 nm. Therefore it is not possible to tell whether a third transi tion occurs after 1600 nm or whether it is just the extension of th e second band. A theoretical treatment of the radical cations would be needed to identify the exact orbitals involved in the electronic absorption of these species. Intervalence charge transfer transitions were first observed in mixed-valence inorganic systems and a theo retical model was developed by Hush in 1967. 164 More recently, intervalence charge transf er transitions have also been observed

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87 in organic systems 147-149,165 and their presence can not be ruled out in platinum acetylide oligomers. Conclusion Two series of platinum acetylide oligomer s have been studied by electrochemistry and pulse radiolysis. The first series of oligomers (Ptn) prob ed the effect of increasing chain length on the charge carriers while the ef fect of low-energy sites on these charge carriers was studied on the second se ries of oligomers (Pt4Tn). The reduction of all oligomers gave irreve rsible waves between -1.0 and -1.3 V, indicating that the radi cal anions of these oligomers are not stable and probably fairly reactive. The oxidation however displayed a rich electrochemi stry with reversible or quasi-reversible oxidations be tween +0.6 and +1.1 V. The electrochemical and pulse radiolysis data suggests the existence of a fair ly localized radical i on in these oligomers, more so for the radical anions than for the radical cations. The extent of delocalization of the radical cation is estimated to be no more th an three repeat units. This is comparable to the delocalization in all-organic conjugated oligomers and seems to indicate that the presence of platinum does not confine the charge carriers. By comparison with the elec tronic absorption data ava ilable for related compound, the transitions observed by pulse radiolysis have been discussed. While the visible transition is almost certainly ligand-based, the origin of the near-IR transition is not clear and can not be assigned unamb iguously. Several possibilitie s are envisaged such as charge transfer or intervalence charge transfer.

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88 Experimental Electrochemistry Electrochemical measurements were perfor med in dry methylene chloride solutions containing 0.1 M tetra-n-butyl ammonimum hexafluorophosphate (TBAH, Aldrich) as the supporting electrolyte. The three-electrode set up consisted of a platinum microdisk (2 mm 2 ) working electrode, a platinum wire auxi liary electrode and a silver wire quasireference electrode. Solutions were bubble-degassed with nitr ogen prior to measurements and a positive pressure of nitrogen was maintained during the measurements. The concentrations of oligomers in the solutions were all around 10 M. Cyclic voltammetry (CV) was performed with a s can rate of 100 mV/s. Differe ntial pulse voltammetry (DPV) was performed with a scan rate of 4 mV/s, a pulse amplitude of 50 mV and a pulse width of 50 ms. All potentials were internally calibrated against the ferrocene/ferricinium couple (E = 0.43 V vs SCE in methylene chloride 137 ). Pulse Radiolysis This work was carried out at the Broo khaven National Laboratory Laser-Electron Accelerator Facility (LEAF). The facility has been described elsewhere. 93,166,167 The electron pulse ( 120 ps duration) was focused into a quartz cell with an optical path length of 20 mm containing the solution of interest. The concentration of oligomers was typically 0.2-2 mM. The monitoring light source was a 75 W Osram xenon arc lamp pulsed to a few hundred times its normal intensity. Wavelengths were selected using either 40 or 10 mm band-pass interference filters. Transient absorption signals were detected with either FND-100G silicon ( 1000 nm) or GAP-500L InGas ( 1100 nm) diodes and digitized with a Tekt ronix TDS-680B os cilloscope.

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89 Synthesis The synthesis of the Pt4Tn oligomers se ries was described in Chapter 2. The synthesis of the Ptn oligomers series was described elsewhere and all NMR data was identical to the data published previously. 67

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CHAPTER 4 CONSEQUENCES OF AGGREGATION ON THE TRIPLET EXCITED STATE IN PLATINUM ACETYLIDE OLIGOMERS Introduction The performance of electronic devices (LEDs, photovoltaic cells, field-effect transistors) made out of c onjugated polymers depends on two main factors: the intrinsic photophysical properties of molecules, and th e morphology of the molecules in the solidstate device. While the former has received the most attention in this field, the latter is generally accepted as being the most cri tical issue for device performance. For conjugated systems-based devices, the interc hain electronic coup ling determines the performance and is difficult to tune. Some studi es have provided insights into the relationship between microstruc ture and device performance 115,168-170 but much work remains to be done in this particular area. Conjugated oligomers have an important role to play in the fabrication of efficient devices because their precise chemical struct ure and conjugation lengt h gives rise to welldefined functional properties and facili tates control over their supramolecular organization. 86,171 Controlled mesoscopic order can be created using supramolecular chemistry, the chemistry of molecu lar assemblies using noncovalent bonds. 172 Two important secondary interacti ons available for the supramol ecular assembly of conjugated systems are and hydrogen-bond interactions. 75 These interactions, individually or combined, 173 can provide a strong driving for ce for solution self-assembly, which contributes greatly to long-range macrosc opic order in the final solid form. 90

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91 The self-assembly process can take place in solution or in a liquid-crystalline phase. Conjugated oligomers such as derivatives of oligo( m -phenyleneethynylene) 174-176 and oligo( p -phenylenevinylene) 177-179 have shown very interesting self-assembling properties in solution, formi ng supramolecular helical or lamellar structures. In both systems, the expression of chirality was achie ved over large supramolecular structures, as evidenced by circular dichroism experiments. Meijer and co-workers 177 use the sergeants-and-soldiers principle to convey th e idea that chirality is achieved through a large number of achiral units (the soldiers) cooperating with a few chiral units (the sergeants). The supramolecular aggregat es of phenylene vinylene oligomers have provided a system to study in moderately concentrated solutions phenomena usually encountered in the solid-s tate. Energy transfer, 180,181 exciton diffusion 182,183 and exciton annihilation 184 were hence studied with st andard solution techniques. More recently, Stupp and co-workers have prepared oligo(pphenylene vinylene) amphiphiles and studied their luminescence properties. 179 These oligomers exhibited a liquid-crystalline mesophase and formed gels in water and DMSO (~30 wt%). The absorption spectrum of films displayed a blue -shifted absorption compared to solution, which was attributed to the presence of H-aggregates. A bilayer lamellar structure in films was proposed as a supramolecular architecture, brought upon by the amphiphilic character of the molecules. Being purely organic systems, however onl y the singlet exciton was explored in these studies and no information regard ing the triplet exciton was gained. Recently, Cooper et al. 58 described a platinum acetylid e oligomer exhibiting a glass phase at low temperature and a liquid stat e under ambient conditions. This was achieved

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92 by using trin -octylphosphine ligands on the plati num complex instead of the more commonly used trin -butylphosphine ligand. In the liqui d state, the chromophore density is increased 10 times compared to the ch romophore density of a similar platinum acetylide oligomer (with trin -butylphosphine ligands) at th e maximum concentration in solution. This is an interes ting approach that should allo w the study of tr iplet-triplet annihilation and triplet exciton dynamics in aggregated platinum acetylide oligomers. However to this date, no data on the triple t exciton dynamics in aggregated platinum oligomers is available on this system. This knowledge is nonetheless crucial for the design of efficient devices based on plati num acetylide oligomers, particularly in nonlinear optics and optical limiting. 69 While the properties of trip let excitons in platinum acetylide oligomers have been studied mostly in dilute solutions, questions remain on its fate in solid-state devices. There is a strong concern about this, as it is well-known that large and detrimental differences (red-shift ed excimer emission, with lower quantum yield of emission) are observed between th e solution and the solid-state properties of singlet excitons in organic c onjugated polymers and oligomers. 74,185-187 Inspired by the solution self-aggregating properties of the oligo(pphenylenevinylene) derivatives designed by Me ijer and co-workers, similar platinum acetylide oligomers have been prepared in hope of gaining some insight into the triplet exciton dynamics in aggregated states usi ng standard photophysical solution techniques. Before plunging into the synthesis of relativ ely expensive platinum-containing oligomers, organic oligo(phenyleneet hynylene) oligomers were prepared to assess the potential for self-assembling (Figure 4-1) in a preliminary study.

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93 The first oligomer prepared, PE1, contained three phenylen eethynylene repeat units with both end-phenyl groups tri-substituted with n -dodecyloxy hydrocarbon chains, similar to the oligomers studi ed by Meijer and co-workers 177 as well as other aggregating systems studied in other groups. 188-190 However, no aggregates were clearly detected, either in concentrated solutions (up to 1 mM) or in the solid-state (drop-cast films). Since -stacking is recognized to be an important driving force for the aggregation, a longer organic oligomer PE3 was prepared, containing five phenyleneethynylene repeat units end-capped by the same 1,2,3-tri-dodecyloxypheny l motif. In this case, clear evidence of aggregation was obtained from photoluminescen ce studies, both on thin films and in concentrated solutions (although only between 1 and 10 Figure 4-1. Structures of phenyleneet hynylene and platinum acetylide oligomers synthesized for the preliminary study.

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94 mM). Encouraged by this result, the synthesi s of similarly end-capped platinum acetylide oligomers Pt2B and Pt4B was carried out. However, no sign of aggregation was observed from the photoluminescence of both oligomers. Recognizing that the bulky trin -butylphosphine ligands of the platinum complex may be sterically hindering the aggregation, a platinum acetylide oligom er with trimethylphosphine ligands on the platinum complex Pt2M (Figure 4-2) was prepared and this time, evidence for aggregation was obtained from photolumines cence and the immediate observation that this oligomer could form a gel in a dodecane solution (at 1 mM). With a desire to probe some structure-property rela tionships in the photophysics of aggregates of platinum acetylide oligomers, several variations of Pt2M were also prepared. As the photophysical evidence of aggregation was not entirely clear at first and not as dramatic as the eximer emission seen in PE3, an oligomer with a longer -system Pt2MP3 was prepared, the idea being that the longer -conjugated segment would pr ovide a strong driving force towards solution aggregation. A thiophene-containing oligomer Pt2MT was prepared to allow the study of the effect of a low-energy site and to probe energy transfer processes in aggregates. Finally, an oligomer with chiral side chains extending from the idle phenyl ring Pt2MC was also prepared to study the presence of chiral aggregates. These three oligomers are presented in Figure 4-2 along with Pt2M. In the following, the synthesis of the oligomers presented in Figure 4-2 will be discussed. A brief look at some of the physical properties of these oligomers will be taken before presenting the photophys ical properties of the aggregates of the oligomers. Finally, a discussion will propose a rationa le for the photophysical observations made during the study.

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95 Figure 4-2. Structures of self-asse mbling platinum acetylide oligomers. Synthesis The synthesis of the platinum acetylid e oligomers described above relies on a convergent strategy, as all oligomer s share a common intermediate in trans -(tri-[3,4,5dodecyloxy]phenylethynyl)-chloride-bis-(t rimethylphosphine)-platinum(II) complex (Figure 4-3). This synthesis started with commercially available 3,4,5-trimethoxyaniline 12 which was converted to 1-iodo-3,4,5-trimethoxybenzene 13 in 83% yield through the in-situ formation of the diazonium salt and s ubsequent reaction with KI. The latter was treated with 4 eq. of boron tribromide 191 in CH 2 Cl 2 to give 1-iodo-3,4,5trihydroxybenzene 14 in a moderate 53% yield. Dodecyl chains were introduced by the

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96 reaction of 14 with 4.5 eq. of 1-bromododecane in the presence of K 2 CO 3 in DMF 188 to give 15 in 90% yield. Sonogashira coupling 90 of 15 and trimethylsilylacetylene (TMSA) gave the protected acetylene derivative 16 in good yield which was then treated with K 2 CO 3 to afford the free acetylen e derivative intermediate 17 in 91% yield.. Finally, the reaction of the latter with cis -dichloro-bis-(trimethylphosphine)platinum(II) gave the desired platinum complex 18 in excellent yield. The pres ence of a trans geometry around the platinum complex was assessed by 31 P NMR, which showed a single peak for the phosphorus resonance at = -13.6 ppm and the satellites arising from the coupling with 195 Pt with 1 J Pt-P = 2332 Hz typical of trans platinum complexes. Figure 4-3. Synthesis of platinum complex intermediate 18. The synthesis of Pt2MP3 required the preparation of a phenyleneethynylene trimer intermediate 20. The latter was prepared from the Sonogashira coupling of 1,4diiodobenzene and 1-ethynyl-4-(triiso -propylsilylethynyl)benzene 8 to give the protected

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97 intermediate 19 in 69% yield. After desilylation with TBAF in THF, the desired free acetylene trimer 20 was obtained in 66% yield. Th ese last two reactions were complicated by the limited solubility of compounds 19 and 20 but enough materials were obtained for the synthesis of Pt2MP3. Figure 4-4. Synthesis of phenyleneethynylene derivative 20. The synthesis of Pt2MC necessitated the synthesis of a central benzene ring with chiral side chains. This began with co mmercially availabl e 2,5-diiodohydroquinone which was reacted with (S)-(+)-1-bromo -2-methylbutane in the presence of K 2 CO 3 to give 1,4-diiodo-2,5-bis-[(S)-(+)-2-methylbutanoxy]benzene 21. The latter was then subjected to Sonogashira coupling with trimet hylacetylene, which after deprotection with TBAF in THF, afforded the desired 1, 4-bis-[(S)-(+)-2-methylbutanoxy]-2,5-bistrimethylsilyle thynylbenzene 22 in good yield. No loss of chirality was observed from 1 H NMR, as evidenced by the absence of diastereomers in the NMR spectrum of 23. Finally, the synthesis of th e platinum acetylide oligomers was achieved by reacting 2 eq. of platinum complex 18 with the required bis-acetylene-aryl unit (1,4diacetylenebenzene, 2,5-di acetylenethiophene, compound 20 and compound 23 for

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98 Figure 4-5. Synthesis of chiral intermediate 23. Pt2M, Pt2MT, Pt2MP3 and Pt2MC, respectively) under Hagihara conditions. 49 The synthesis of Pt2M only is shown in Figure 4-6 as a representative reaction of the series. Figure 4-6. Synthesis of Pt2M as a representative reaction of the oligomer series. Reactions were completed at room temperatur e within a few hours and the materials were obtained after column chromatography and recr ystallization in moderate to excellent yields. All NMR spectra ( 1 H, 13 C and 31 P) were consistent with the assumed structures. In particular, 31 P NMR showed one peak only around = 19.2 ppm along with the platinum satellites exhibiting a 1 J Pt-P value between 2290 and 2310 Hz typical of trans geometry of such complexes.

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99 Since the all-organic oligomer PE3 will be mentioned in the following study, the synthesis is described here as well (Figure 4-7). The synthesi s started with 1,4diiodobenzene which was reacted with 0.67 eq. of propargyl alcohol under Sonogashira conditions 90 to afford the iodophenylpropynol intermediate 24 in 61% yield. The latter was then reacted with the previously prepared 3,4,5tris-(dodecyloxy)ethynylbenzene 17 in a Sonogashira coupling reac tion giving 25 in 87%, which after deprotection with MnO 2 and KOH gave the intermediate 26 in 50%. This deprotec tion step which usually proceeds in good yield was hampered here by the poor solubility of the starting material. Finally, the oligomer PE3 was obtained by reacting 1,4diiodobenzene w ith 2 eq. of intermediate 26 under Sonogashira coupling c onditions giving oligomer PE3 in 86% yield. Figure 4-7. Synthesis of oligomer PE3.

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100 Results Gel Formation All of the oligomers presen ted in Figure 4-2 except Pt2MP3 are able to gel hydrocarbon solvents such as n -dodecane, hexane and cyclohexane. The critical gel concentration was determined to be around 1 mM in dodecane and hexane for Pt2M and Pt2MT (~2 wt %), while it is 4 mM for Pt2MC. Moreover, while the gels are formed upon cooling of a hot concentrated solution within a few minutes for Pt2M and Pt2MT, the gel takes about 3 days to form for Pt2MC. This prevented advanced photophysical characterization of Pt2MC gels since solutions need to be oxygen-free for triplet excited state studies. Upon heating to ~50 o C, the gels melted to an isotropic liquid that could be reversibly brought back to the gel upon cooling. The gels are stiff e nough that the flow of entrapped solvent molecules is frozen when a vial containing the gel is tilted or turned upside-down (Figure 4-8). a b Figure 4-8. Picture of deoxygenated dodecane gel of Pt2M (10 -3 M) under illumination with a UV light. (a) At room temperature. (b) At 50 o C. Notice the nonhorizontal level of the solution in (a ) due to the presence of the gel. It is important to realize that the ge l formation of these platinum acetylide oligomers is rather unique. While ex amples of luminescent organogels and

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101 organometallic gels are numerous, 192,193 examples of low molecular weight luminescent gel-forming systems with extended conjugati on are rare. In fact, there are only three examples found in the literature of fl uorescent organogels based on a conjugated oligomer. 173,194,195 The platinum acetylide oligomer series studied here truly makes a remarkable system, which could contribute to make platinum acetylide oligomers into more attractive materials for technological applications. Thermal Properties The thermal properties of the oligomers we re investigated by differential scanning calorimetry between -50 o C and 150 o C and the plot obtained fo r the second heating and second cooling are presented in Figure 49. The oligomers were also studied under polarized optical microscopy, both as neat samples and in their dodecane gels. The thermogram of oligomer Pt2M shows one endotherm at 58 o C (19 J.g -1 ) upon heating and a correspon ding exotherm at 37 o C (20 J.g -1 ) upon cooling. However, no transition was observed under polarized microscopy upon heating up to 190 o C. Upon cooling to room temperature, a glass phase wa s observed. Some fibers were seen in the dodecane gel sample, indicating the pr esence of a network structure. For oligomer Pt2MT, two small endotherms are found at 54 o C and 79 o C (2.7 and 1.0 J.g -1 respectively) and a larger one at 115 o C (11 J.g -1 ). Upon cooling, three corresponding exotherms are observed. In th is case, a liquid-crystalline phase was observed under polarized microscopy between 124 o C and 144 o C (Figure 4-10a), at which temperature the liquid-crystal melted into the isotropic liquid. Some fibers were also observed in the dodecane gel sample. U pon cooling to room temperature, no obvious change in appearance was observed, indicat ing a possible decompos ition of the sample during the first heating.

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102 Figure 4-9. Differential s canning calorimetry thermogr ams for second heating and cooling cycle at a10 o C/min scan rate. (a) Pt2M. (b) Pt2MT. (c) Pt2MC. In Pt2MC, the thermogram displays two endotherms, at -19 o C (7.1 J.g -1 ) and 55 o C (17.1 J.g -1 ), with corresponding exotherms upon coo ling. Under polarized microscopy, the oligomer started melting at 84 o C and a liquid-crystal phase was observed between 114 o C and 181 o C (Figure 4-10b). No change wa s observed upon cooling to room temperature, again indicating a possible d ecomposition during the first heating. Some fibers were also observed in the dodecane gel sample.

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103 Finally, Pt2MP3 was only investigated with polarized microscopy where the oligomer started melting at 102 o C. A liquid-crystal phase was observed between 154 o C and 181 o C (Figure 4-10c), but no clear ing point was detected at this temperature where the liquid-crystal phase remains. Upon cooli ng, the liquid-crystal phase disappeared at 143 o C and no further change was observed. On second heating, the mesoscopic phase was again observed at 143 o C but some crystals remained even up to 180 o C, indicating a possible decomposition during the first heating. The mesoscopic properties of the platinum acetylide oligomers is not the main concern of this study, hence observations with polarized microscopy were not carried out extensively. Therefore, it is not possible to identify the type of mesophase (nematic, smectic) displayed by these materials. However, they are included to illustrate the potential of these oligomers as active elements in opto-electronic devices. UV-Vis Absorption The absorption spectra of the oligomer s were recorded in dodecane solutions, where a concentration and a temperature-de pendence were observed for all oligomers. The absorbance was kept below a value of one by using short path length (1 cm, 1 mm, or 0.1 mm) cells when necessary. The absorption spectrum of Pt2M in dilute dodecane solutions (C = 10 -4 M and below) is dominated by a strong band centered around = 358 nm, with some weaker transitions at higher energy (Figure 4-11a). This is similar to the absorption spectrum of Pt4 from Chapter 2 and to Ptn o ligomers from a previous study, 67 and this is confidently assigned to a monomeric chromophore abso rption since the spectrum does not undergo any further change at lower concentrations. When the concentration is increased from 10 4 M, the spectrum broadens due to the decrease of the main absorption band to be finally

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104 (a) (b) (c) Figure 4-10. Pictures of liquid-crystal phases under a polariz ed optical microscope. (a) Pt2MT at 124 o C. (b) Pt2MC at 84 o C. (c) Pt2MP3 at 144 o C. The black space bar indicates 100 m. The presence of liquid-crystal phases was confirmed by lateral displacement of one c over slip with respect to the other. dominated by a blue-shifted band at = 300 nm in the gel phase (C = 10 -3 M). A similar trend is observed in the temperatur e-dependent absorption spectrum of Pt2M (Figure 411b). Starting in the gel phase at room temperature and increa sing the temperature up to ~60 o C results in the melting of the gel into th e isotropic solution and the recovery of the monomeric absorption at = 358 nm.

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105 Figure 4-11. Absorption spectrum of Pt2M in dodecane. (a) Room temperature, C = 10 -3 M and 0.1 mm pathlength ( ), C = 5x10 -4 M and 1 mm pathle ngth ( ), C = 10 -4 M and 1 mm pathlength ( ). (b) C = 10 -3 M and 0.1 mm pathlength, arrow indicates effect of increasing temp erature from 21 o C to 66 o C. A very similar behavior was observed for Pt2MT, for which only the temperaturedependent absorption is shown he re (Figure 4-12). As can be seen, the main absorption band of the hot isotropic solution at = 378 nm decreases with decreasing temperature and at room temperature the spectrum is dominated by a higher energy transition at = 338 nm. Again, the monomeric absorption of Pt2MT disappears when the gel is formed, to be replaced by higher energy transitions in the gel phase, as in Pt2M. This

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106 Figure 4-12. Absorption spectrum of Pt2MT in dodecane. C = 10 M and 0.1 mm pathlength. Arrow indicates effect of increasing temperature from 23 C to 63 C. -3 o o blue-shifting of the absorption in aggregated states is an indication of the presence of Haggregates and this will be examined more closely in the discussion. The absorption of Pt2MP3 (Figure 4-13) showed a very different trend than the two previous oligomers. Note that in this case no gel is formed in concentrated solutions but turbidity appears instead at a concentration of ~10 -3 M. In the concentrationdependent absorption spectrum (Figure 4-13a), it can be seen th at in the dilute solutions (10 -5 10 -4 M), the spectrum is dominated by a band at = 368 nm. This is again similar to what has been seen so far on the pla tinum acetylide absorption data and this is attributed to the absorption of a monomeric chromophore. However in concentrated solutions (10 -3 M), this band becomes narrower, with a maximum at = 373 nm, and a new sharp band appears at = 405 nm. The monomeric ab sorption in concentrated solution can be restored by heating the solution to ~ 60 o C, where aggregates are broken

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107 Figure 4-13. Absorption spectrum of Pt2MP3 in dodecane. (a) Room temperature, C = 10 -3 M and 0.1 mm pathlength ( ), 10 -4 M and 1 mm pathlength ( ), 10 -5 M and 1 cm pathlength ( ). (b) C = 10 -3 M and 0.1 mm pathlength, arrow indicates the effect of in creasing temperature from 23 o C to 58 o C. down and the same absorption spectrum as obs erved in dilute solutions is observed (Figure 4-13b). In Pt2MP3, the effect of aggregation on th e absorption of the oligomers is therefore opposed to what they are in Pt2M and Pt2MT. While the latter both show a broadening and a large blue shif t of the absorption maximum, Pt2MP3 show a small redshift and a new sharp red-shifted peak. And while the change in absorption observed for

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108 Pt2M and Pt2MT is often attributed to H-aggregates, the trend observed in Pt2MP3 is usually assigned to J-aggregates. As mentioned earlier, while Pt2MC also has the ability to gel dodecane, this process takes more time than Pt2M and Pt2MT oligomers. This is apparent in the absorption spectrum of Pt2MC (Figure 4-14), which is typical of platinum acetylide oligomers with a maximum at = 368 nm and two higher energy transitions. The spectrum is almost unchanged under dilute and concentrated conditions, except for a small increase in the higher energy bands absorption. However, if the solution is left sitting for several days and a gel has had time to settle, a large hypsoc hromic shift of the maximum of absorption to = 345 nm occurs. The higher energy bands are now stronger than the main absorption band and the reason for this is not clear. Circular Dichroism While the UV-vis absorption spectrum of Pt2MC offers some proof of aggregation, more evidence for the presen ce of aggregates in dodecane gels of Pt2MC is found in circular dichroism (CD) experi ments (Figure 4-15). The CD spectrum of Pt2MC only shows a very weak ne gative signal in freshly prepared dodecane solutions at 4 x 10 -3 M. However, after this solution is allowe d to gel (over a period of a few days), a strong negative signal is observed in the CD spectrum in the region where Pt2MC absorbs. Since Meijer and co-workers 177 observed bisignate si gnals in their H-bonded helical self-assembled oligo(p-phenylenevinylene)s, this indicates that the chiral assemblies formed in this gel are not s upramolecular helices. Th e possibility of a supramolecular lamellar structure is more likely in view of this result.

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109 Figure 4-14. Absorption spectrum of Pt2MC at room temperature in dodecane. C = 4 x 10 -3 M and 0.1 mm pathlength gelled for 40 days ( ), C = 4 x 10 -3 M and 0.1 mm pathlength freshly prep ared ( ), C = 10 -3 M and 0.1 mm pathlength ( ), C = 10 -4 M and 1 mm pathlength ( ). Figure 4-15. Circular dichrois m (CD) absorption spectrum of Pt2MC in dodecane. C = 4 x 10 M and 0.3 mm pathlength freshly prepared ( ), C = 4 x 10 M and 0.3 mm pathlength gelled after 4 days ( ). -3 -3

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110 Steady-State Photoluminescence The photoluminescence of the platinum acetylide oligomers were studied in dodecane. For gels, the solutions were bubbledegassed with argon from a hot isotropic state and heated regularly to prevent gel formation and ensure adequate degassing. In order to determine what emission band may appear in the photoluminescence of self-assembled oligomers in the solution, the photoluminescence of drop-cast films of the oligomers were measured as well. Moreover, in order to stress the difference between the aggregate emission of an organic system and the aggregate emission of the metal-organic systems, the emission spectrum of PE3 is presented in Figure 4-16. The effect of aggregation on the emission of PE3 is typical of many organic conjugated systems. Whereas the emission in dilute solutions is dominated by sharp fluorescence emission, the emission in thin films often exhibits a red-shifted band due to excimer formation. In fact, the excimer emission entirely dominates the spectrum in thin film of PE3 and no monomeric fluorescence is observed. A c oncentration study supports the excimer origin of the broad red-shifted emission band, which decreases as concentration decreases and is no longer observed for c oncentrations lower than 10 -3 M. The photoluminescence spectrum of Pt2M in gel (10 -3 M) is excitation wavelengthdependent and is dominated by phosphorescence emission (Figure 4-17a). When exciting at = 326 nm, close to the maximum of ab sorption found in the gel, the emission spectrum displays a phosphorescence peak at = 495 nm. When exciting at = 366 nm, where the maximum of absorption is found in the dilute solution, the peak of maximum intensity is = 516 nm. With an intermediate excitation wavelength = 346 nm, a phosphorescence band exhibiting both peaks at = 495 and 516 nm is observed. Both bands are assigned to emission fr om a triplet excited state due to their similarity to the

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111 Figure 4-16. Photoluminescence spectrum of PE3 with = 354 nm. (a) C = 10 M deoxygenated dodecane ( ), drop-cast film ( ). (b) From 10 M to 10 M in deoxygenated dodecane. Arrow indicates effect of decreasing concentration. ex -2 -2 -3 Pt4 oligomer emission spectrum (see Chapter 2). Moreover, the blue emission band at = 495 nm is attributed to an aggregated state, while the normal emission band at = 516 nm is assigned to a monomeric state. Se veral experiments support these assignments: 1) The blue band is also present in the em ission spectrum of thin films (Figure 4-17b); 2) The blue band is absent from the dilu te solution emission spectrum (Figure 4-17c); 3) The blue band intensity decreases rela tive to the normal emission band as

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112 Figure 4-17. Photoluminescence spectrum of Pt2M. (a) C = 10 -3 M with ex = 326 nm ( ), ex = 346 nm ( ) and ex = 366 nm ( ) in deoxygenated dodecane. (b) With ex = 326 nm in deoxygenated dodecane at C = 10 -3 M () and in dropcast film ( ). (c) With ex = 326 nm in deoxygenated dodecane at C = 10 -3 M (), C = 10 -4 M ( ), C = 10 -6 M ( ). (d) In deoxygenated dodecane at C = 10 -3 M, from 23 o C to 61 o C. The arrow indicates the effect of increasing temperature. temperature increases (Figure 4-17d). All of these observati ons support the idea that the emission band observed at = 495 nm arises from a triplet excited state in an aggregated environment. Note that this is very different from organic systems, as illustrated with PE3 in Figure 4-16, where aggregation is often accompanied by the appearance of redshifted excimer emission. Low-temperature photoluminescence in di lute MeTHF solutions (Figure 4-18) are also revealing as the blue emission band also appears at -53 o C, increases relatively to the normal emission until -33 o C, and then decreases to finally disappear at higher

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113 temperatures. While the presence of the b lue band only in this narrow temperature range is not entirely clear, it is interesting that it is present at all si nce the concentration is only ~7 M. Aggregation of similar o ligomers in dilute solutions at low temperature have been observed in our group before 196 and there is little doubt th at the appearance of a blue-shifted emission band in th is experiment has the same or igin as the one observed in concentrated solutions. Figure 4-18. Photoluminescence spectrum of Pt2M in MeTHF at C = 7 x 10 -6 M with ex = 326 nm from -63 o C to -23 o C. The arrow indicates the effect of increasing temperature. While the photoexcitation spectra obtained in concentrated solutions should be viewed with caution and ther efore are not discussed here the photoexcitation spectra obtained in dilute solutions (F igure 4-19) are not be plague d by high concentration effects and are hence useful in asse ssing the origin of the emi ssion bands observed. At -63 o C, there is no emission at = 494 nm and no photoexcitation signal when this emission wavelength is monitored. The emission spect rum displays the normal emission at =

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114 Figure 4-19. Low-temperature photoexcitation spectra of Pt2M in deoxygenated MeTHF monitoring em = 494 nm ( ) and em = 516 nm ( ). (a) -63 o C; (b) -53 o C; (c) -43 o C; (d) -33 o C. 516 nm and the photoexcitation spectrum shows the same band as the one observed in the dilute absorption spectrum of Pt2M. At temperatures of -53 o C and -43 o C, where the emission spectrum shows the blue emission band, the photoexcitation spectrum is

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115 indeed revealing. While the blue emission band at = 494 nm seems to be coming from a state absorbing at = 340 nm, the normal emission now has contributions from both excited states. This imp lies that in the presence of aggregates, the higher-energy excited state can transfer the excitation to the lower normal excited state from which emission is then observed. At -43 o C, there is probably too much thermal energy for aggregates to form, the blue emission is almost entirely absent from the emission spectrum and the photoexcitation of the normal emission at = 516 nm originates now almost entirely from the normal non-aggregated excited state. Although Pt2MT was not studied as extensively as Pt2M, there are indications that much of the same photophysics are found in this compound. For instance, the photoluminescence shows a blue shift of the emission band in dodecane gel (Figure 4-20) as well, although not as strong as in Pt2M. For PtMP3, while the absorption spectrum displa yed signs of possible aggregates absorption (see Figure 4-13), the photolumin escence spectrum does not show direct evidence of aggregates (Figure 4-21). The sp ectra of concentrated solutions and drop-cast films are very similar (Figure 4-21a), showing a strong fluorescence band at = 440 nm and a comparatively weak phosphorescence at = 573 nm. The emission band appearing at lower energy ( = 622 nm) is a scattering peak, as evidenced by its strong dependence on the excitation wavelength (not shown he re). The photoluminescence spectrum of Pt2MP3 at different concentratio ns (Figure 4-21b) shows th at under dilute conditions, the phosphorescence dominates the spectrum, as in all platinum acetylide oligomers studied here. Note the presence of the scattering peak at 670 nm, clearly dependent on the excitation wavelength used ( ex = 366 nm in Figure 4-21a, ex = 393 nm in Figure 4

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116 Figure 4-20. Photoluminescence spectrum of Pt2MT in deoxygenated dodecane with ex = 340 nm at C = 10 -3 M () and 10 -4 M ( ). 21b). While the relative increase in fluorescen ce at high concentrati ons can certainly be interpreted as originating from increased trip let-triplet annihilation, this has not been shown to be particularly more efficient in the high concentration photoluminescence spectra of Pt2M and Pt2MT where phosphorescence always dominates the emission spectra. However, this is consistent with the presence of J-aggregates, where increased fluorescence compared to monomer emission is usually observed. Further support for the presence of aggregates in concentrated dodecane solutions of Pt2MP3 comes from the temperature dependent photoluminescence spectr um (Figure 4-21c), where it can be seen that the phosphorescence ba nd increases as temperature increa ses. This is at first counterintuitive, as nonradiative decay channels are typically activated at high temperature but it is in fact consistent with a breakdown of the aggregates at elevated temperatures and a return to a monomeric photolumines cence dominated by the phosphorescence.

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117 Figure 4-21. Photoluminescence spectrum of Pt2MP3. (a) With ex = 366 nm from dropcast film ( ) and from deoxygenated dodecane at 10 -3 M ( ). (b) With ex = 393 nm in deoxygenated solution at C = 10 -3 M ( ), 10 -4 M ( ), 10 -5 M ( ), 10 -6 M ( ). (c) With ex = 366 nm from deoxygenated dodecane at 10 -3 M. Arrow indicates the effect of increasing temperature from 23 o C to 48 o C.

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118 As the photophysical data obtained so far for Pt2M indicated that emission from aggregates could be detected and identified, the possibility of observing energy transfer within those aggregates was envi saged. For this, doping levels of Pt2MT were mixed in a gel of Pt2M and the photoluminescence was measured. In order to minimize direct excitation of Pt2MT, the excitation wavelength us ed for photoluminescence was ex = 300 nm. This is where the least direct absorption of Pt2MT will occur while still allowing the observation of the blue emission band of Pt2M (Figure 4-22). Note that in the following photoluminescence energy transf er experiments, the concentration of Pt2MT is at doping levels, so its effective abso rption is of a few percent to what is apparent in Figure 4-22. Energy transfer from Pt2M to Pt2MT at doping levels was indeed observed in dodecane gels (Figure 4-23a). At only 3 mol% doping of Pt2MT in Pt2M at 10 -3 M, the photoluminescence displays equally intense phosphorescence from both oligomers. At 5 mol% doping level, the phosphorescence of Pt2MT dominates the emission spectrum. Note that the photoluminescen ce spectrum in Figure 4-23a is normalized to the PL intensity at = 494 nm. The slight incr ease in the phosphorescence at = 526 nm as the Pt2MT doping level increases suggests a stronger quenching of the blue emission band than of the normal emission. This is consis tent with the idea that energy transfer will occur more efficiently in aggregates than in non-aggregated domains. In order to provide evidence that the emission from Pt2MT is not due to its direct excitation and that energy transfer occurs preferentially in aggregates, the photoluminescence of Pt2M at different concentrations with a cons tant 5% doping level of Pt2MT was measured (Figure 4-23b). As expected, the phosph orescence emission from Pt2MT decreases with concentration.

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119 Figure 4-22. Absorption spectrum of Pt2M ( ) and Pt2MT ( ) in dodecane at C = 10 -3 M. At a Pt2M concentration of 10 -4 M, where the solution is in a liquid state a nd aggregates formation is limited, almost no phosphorescence from Pt2MT is detected and the emission is essentially that of Pt2M. These experiments point to a very efficien t energy transfer when aggregates are present, especially considering that the quantum yield of phosphorescence of Pt2MT is much less than that of Pt2M. Indeed, while p = 9.1% for Pt2M, p = 0.7% for Pt2MT in dilute dodecane solutions. This is consistent with what has been observed in Chapter 2, where lower phosphorescence quantum yields we re obtained in the thiophene-containing oligomers due in part to their lower triplet ex cited state energy. Therefore, from the ratio of the phosphorescence quantum yields of Pt2M and Pt2MT, approximately one in ten excitons transferred results in detectable phosphorescence from Pt2MT. Time-Resolved Photoluminescence The time-resolved photoluminescence of the platinum acetylide oligomers were investigated in order to extr act some information regarding the dynamics of their excited state in aggregates. All lifetimes obtained from the decay of the emission bands are

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120 Figure 4-23. Photoluminescence spectrum of Pt2M-Pt2MT mixed-oligomer system. (a) With [Pt2M] = 10 -3 M and [Pt2MT] = 0 mol% ( ), 1 mol% ( ), 2 mol% ( ), 3 mol% ( ), 4 mol% ( ) and 5 mol% ( ) in deoxygenated dodecane, with ex = 300 nm. (b) Pt2M with 5 mol% Pt2MT mixed-oligomer system in deoxygenated dodecane with [Pt2M] = 10 -3 M ( ), 5x10 -4 M ( ), 10 -4 M ( ), with ex = 300 nm. summarized in Table 4-1. When the decay wa s better fitted with a bi-exponential, the lifetime and the relative contribu tion of each component are shown. The time-resolved photoluminescence of Pt2M (Figure 4-24) shows a much slower decay for the blue emission ( = 494 nm) than for normal emission ( = 516 nm) in aggregates. While the lifetime of the emission at = 516 m is about 10 s at all concentrations studied, the lifetime of the emission at = 494 nm is about 59 s. The lifetime of the normal emission band is somewhat smaller but comparable to the

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121 Figure 4-24. Time-resolved photoluminescence spectrum of Pt2M in deoxygenated dodecane following = 337 nm. (a) C = 10 M, 100 ns camera delay, 10.5 s delay increment; (b) C = 10 M, 100 ns camera delay, 5.2 s delay increment; (c) C = 10 M, 100 ns camera delay, 5.2 s delay increment ex -3 -4 -5 emission lifetime of Pt4 (see Chapter 2). However, the lif etime of the blue emission is surprisingly longer. This is at first unexpected because triplet-triplet annihilation could have been thought to be efficient in aggregates and therefore shorte r lifetimes could have been expected in aggregates. Th is is clearly not the case for Pt2M and possible

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122 explanations for this will be provided in the discussion. The photoluminescence decay of Pt2M was deconvoluted into two components (F igure 4-25). The decay of the blue emission band, while longer, contributes onl y 22% of the emission decay observed. The decay of the normal emission band contributes 78% of the emission decay observed. This seems to imply that a significant proportion of photoluminescence is arising from nonaggregated domains of the gel. While the time-resolved photoluminescence spectrum of Pt2MT (not shown here) does not display two emission bands as Pt2M, the lifetime obtained in concentrated solution is also much longer than what could have expected ( = 23 s). As a comparison, the lifetime of the thiophene-based excited state in Pt4T1 in dilute solutions was much shorter ( = 7 s, see Chapter 2) than the lifetime of emission of Pt2MT in concentrated solutions, which may appear counter-intuitive but is consistent with the trend observed in the lifetimes of Pt2M. The time-resolved photoluminescence of Pt2MP3 (Figure 4-26) shows trends different than those observed in Pt2M, further supporting the different structure of the aggregates formed in Pt2MP3. The first noticeable feature is the presence of some delayed fluorescence in the photolumines cence at high concentr ation solution (10 -3 and 10 -4 M) acquired 1 s after laser pulse, indicating that tr iplet-triplet annihilation is taking place in aggregates of Pt2MP3. However, it does not a ppear as though the delayed fluorescence observed here can account for the strong fluorescence observed in the steady-state spectrum (Figure 4-21). This im plies that the strong fluorescence in the steady-state emission is prom pt-type fluorescence, rather than a delayed fluorescence originating from triplet-triplet annihilation.

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123 Figure 4-25. Principal components of emission decay of Pt2M at 10 -3 M in dodecane for slow component = 59 s ( ) and fast component = 9 s ( ). Another noteworthy feature extracted from the time-resolved emission is the lifetime of the emission band. As can be seen from Table 4-1, the lifetime of the phosphorescence band was fitted well with a mono-exponential decay function. Looking at the overall evolution of these lifetimes as concentration increases, it appears that in this case, the trend follows the e xpected lifetime increase as concentration decreases and excited states quenching becomes less prone to occur. Hence, the lifetimes vary from 37 s at 10 -3 M to 148 s at 10 -6 M. However, experiments carri ed out with increasing laser intensity (not shown here) did not show a decrease in lifetime, as expected if quenching is due to triplet-triplet annihilation at higher lase r energies. This again supports the idea that delayed fluorescence is not the main origin for the strong fluore scence observed in the steady-state experiment. The decay of the phosphorescence band in Pt2MP3 was also fitted well with a bi-exponential function (not shown here) for all concentrations, consisting of a short (9-25 s) and a long (52-160 s) component. The relative

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124 Figure 4-26. Time-resolved photoluminescence spectrum of Pt2MP3 in deoxygenated dodecane following = 355 nm. (a) C = 10 M, 1 s camera delay, 10 s delay increment. (b) C = 10 M, 1 s camera delay, 20 s delay increment. (c) C = 10 M, 1 s camera delay, 20 s delay increment. (d) C = 10 M, 1 s camera delay, 40 s delay increment. ex -3 -4 -5 -6 contribution of the short com ponent decreased from 42% at 10 -3 M to 13% at 10 -6 M, at the expense of the longer component. The trend displayed by these two components would be consistent with trip let-triplet annihilation respons ible for the short component, however again power-dependent lifetime meas urements did not reveal any effect on the lifetime. The reason for this is not clear at this time. The Pt2M-Pt2MT mixed oligomer system was also examined with time-resolved photoluminescence in order to gain some understanding on the dynamics of energy transfer. The time-resolved photoluminescence was measured in a dodecane gel of Pt2M containing 5 mol% Pt2MT (Figure 4-27) 1 s after laser excitation ( ex = 337 nm). The spectrum clearly shows a much shorter decay for the phosphorescence bands of Pt2M,

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125 resulting from the energy transfer to Pt2MT. The fitting of the decay curves did not deconvolute both bands from Pt2M, but gives an overall lifetime of 5 s, much shorter than the lifetimes of pure Pt2M in dodecane ( 1 = 9 s, 2 = 59 s). The energy transfer is also reflected in the lifetime of Pt2MT in the mixed-oligomers system ( = 62 s, with some contribution from the blue band of Pt2M), which is considerably longer than the lifetime of pure Pt2MT ( = 23 s). The photoluminescence decay was deconvoluted into phosphorescence components of Pt2M (71%) and Pt2MT (29%), although the component from Pt2MT has some contribution from the blue emission of Pt2M (Figure 4-28). A control experiment under dilute concentration (10 -5 M) at with 1:1 Pt2M/Pt2MT (not shown here) shows no change on the lifetime of Pt2M emission. This supports the idea that the presence of aggregat es is necessary for efficient energy transfer and that quenching does not occu r by a diffusional process. In an effort to determine whether both emission bands of Pt2M are quenched to the same extent by Pt2MT, measurements were carried out in dodecane gel of Pt2M with different doping levels of Pt2MT ranging from 0.25 to 4 mol%. The decay of the each emission band is plotted as a function time and can be at least qualitatively commented (Figure 4-29). First, it is clear that Pt2MT has an effect on both emission bands, as the lifetime of both bands gets shorter with increasing content of Pt2MT. However, it appears that the effect is somewhat more pronounced for the blue emission band at = 494 nm (Figure 4-29a) than for the normal emission at = 516 nm (Figure 4-29b). Since several pieces of evidence point out to effici ent energy transfer only in the aggregates. This is evidenced by the apparent faster decays at = 494 nm than at = 516 nm as well as by the relative

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126 Figure 4-27. Time-resolved photoluminescence spectrum of Pt2M with 5 mol% Pt2MT in deoxygenated dodecane following = 337 nm. [Pt2M] = 10 M, 1 s camera delay, 10.5 s delay increment. ex -3 Figure 4-28. Principal components of emission decay of Pt2M at 10 -3 M with 5 mol% Pt2MT in dodecane for slow component = 41 s ( ), fast component = 5 s ( ).

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127 quenching of instantaneous amplitude (t = 0) This further supports the idea that the blue emission band is the emission of Pt2M in aggregates. This is not contradictory with an energy transfer working also for the normal emission as th ere appears to be a significant proportion of oligomers in liquid non-aggregated domains in the gel. Table 4-1. Lifetime a of photoluminescence of self -assembling platinum acetylide oligomers. b Pt2M Pt2MT Pt2MP3 Pt2M/Pt2MT 10 -3 M 10 -4 M 10 -5 M 10 -3 M 10 -3 M 10 -4 M 10 -5 M 10 -6 M 10 -3 M Pt2M + 5% Pt2MT 59 (22%) c 5 (71%) e 9 (78%) d 11 10 23 37 70 119 148 41 (29%) f a : all lifetimes are in s. b : number in parenthesis is th e relative contri bution of the individual decay component; c : decay of blue band; d : decay of normal band; e : decay of Pt2M only; f : decay of Pt2MT and blue band of Pt2M. Discussion Nature of Aggregates The photophysical properties of platinum acetylide oligomers studied here have clearly shown signs of aggregation in dod ecane solution. For all oligomers except Pt2MP3, the solutions of oligomers at concentra tions of ~1 mM were found to form a gel in saturated hydrocarbon solvents. The observation of the gelation effect is a clear indication that some network stru ctures are present, such that the flow of the solvent is frozen. While the concentration necessary to reach gelation may appear high when compared to concentrations typically used in photophysical studies in solution, they are not. A concentration of 1 mM of Pt2M in dodecane corresponds to about 2 wt%, which is in fact on the low end of concentrati on range of efficient low molecular mass organogelators. 192 This is an important finding that would allow these materials to be

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128 Figure 4-29. Plot of emission decay of Pt2M at 494 nm (a) and 516 nm (b) with x mol% doping levels of Pt2MT. Doping x = 0% ( ), 0.25% ( ), 0.50% ( ), 1% ( ), 2% ( ) and 3% ( ). Lines are only an indication of the decays and do not represent an actual fit of the data points. easily processed, when most short oligom ers do not usually have the advantageous processable properties of long chain polymers. Their apparent liquid-crystalline behavior is also a clear advantage in terms of processability and technological application. Moreover, organogels based on -conjugated systems are relatively very few and therefore makes these platinum acetylide ol igomers an elegant and original system.

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129 While a direct characterization of the ge ls and the solution aggregates could be desirable, the photophysical st udy conducted here can give a wealth of information regarding the structure of aggregates. As brie fly introduced in Chapter 1, the interaction of aggregated chromophores has been fair ly accurately described by McRae and Kasha in terms of the molecular exciton model. 27 The model predicts a sp litting of the excited state in the aggregat e due to the interaction betwee n neighboring transition dipole moments. The exciton interaction energy ( E) depends on the tran sition dipole moment of the molecule ( E M 2 ), the distance between them ( E R -3 ) and their relative orientation. Depending on the pos ition of the absorption band of the aggregate relative to the monomer, the aggregates are called J-aggreg ates (for an observed bathochromic shift) or H-aggregates (for an obs erved hypsochromic shift). From the absorption spectra of the oligomers, it appears that Pt2M and Pt2MT form H-aggregates in concentrated dodecan e solutions. The hypsochromic shift of the absorption band compared to the one observed in dilute solutions is a typical signature of H-aggregates, as observed in various systems such as trans-stilbene, 38 squaraine dye 197 perylene bisimide polymers, 198 terthiophene 199 and different dyes. 31,200,201 In this case, the interaction of the transition dipoles splits the ex cited state into two excitonic levels E and E (see Chapter 1). Transition between ground st ate and E are forbi dden, so excitation occurs to the higher excitonic level E, re sulting in absorption at higher energy and shorter wavelength. The spectral shift of the absorption bands observed here ( = 58 nm for Pt2M, = 40 nm for Pt2MT with = agg m ) are comparable though somewhat smaller than those observed in th e examples cites above. For instance in the trans -stilbene system, 38 a spectral blue shift of 70 nm was observed upon aggregation,

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130 while the squaraine system 197 exhibited a strong blue shift of 130 nm in aggregates. This indicates that the exciton coupling in the a ggregates of platinum acetylide oligomers may not be as strong as in these other systems. The absorption spectrum of Pt2MP3 displays a red-shift that could be attributed to J-aggregates. In this case, absorption is onl y allowed from the ground state to the lower excitonic level E. This resu lts in absorption at lower en ergy and longer wavelength for the aggregate. J-aggregates have also been observed in many different systems such as bis-(biphenyl)-ethylene 37 and diphenylbutadiene 202 derivatives, carbocyanine dyes 40,203 as well as H-bonded oligo( p -phenylenevinylene)s. 177 The spectral shift of the absorption band in Pt2MP3 ( = 40 nm, with = agg m ) is also smaller than those observed in the examples cited above, again indicating that exciton coupling in aggregates of Pt2MP3 may relatively weak. The bathochromic shift of the ab sorption band in the aggregate of Pt2MP3 can also be explained by an intramolecular phenomenon, rather than an intermolecular one in the form of J-aggregates. Indeed, Bunz and co-workers 204 have studied the aggregation behavior of poly( p -phenenyleneethynylene)s (PPEs) and observed a sharp red-shifted absorption in films, very similar to the one observed in Pt2MP3. The red-shifted band disappeared once the polymer reached an is otropic state. The same behavior was observed more recently by Li and Wang 205 in a polystyrene-oligo( p phenyleneethynylene)-polystyrene triblock copo lymer. Upon addition of a poor solvent to the dissolved polymer, a red-shif ted shoulder appeared in the absorption spectrum, almost superimposable to the absorption spectrum in thin films. The genera l consensus about the bathochromic shift in these aggregates is that it is due to a planarization of the conjugated

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131 backbone induced by aggregation. 206 Therefore, it is not ex cluded that the red-shifted absorption in aggregates of Pt2MP3 could also be simply due to a planarization of the phenyleneethynylene segment of the oligomer. However, there are some indications that J-aggregates are indeed formed in Pt2MP3. Firstly, H-aggregates are clearly formed in Pt2M and Pt2MT so it appears likely that sim ilar aggregates should form in Pt2MP3. More importantly, when aggregate formation was observed in thin films of PPEs, the photoluminescence spectrum was entirely dominat ed by a broad strongly red-shifted band attributed to excimer emission. The intensit y of the excimer band was also weaker than the emission of non-aggregated PPEs. Fr om the photoluminescence experiments of Pt2MP3 (Figure 4-21), it does not appear as though emission fr om excimers is observed. While the fluorescence of Pt2MP3 in aggregates does seem broader and red-shifted compared to the photoluminescence in dilute so lutions, it is clearly not similar to the excimer emission at = 516 nm observed in PE3 (Figure 4-16). More over, thin films of PE3 exhibiting excimer emission did not displa y the sharp red-shif ted absorption band observed in Pt2MP3 but only a slight broadening of the monomeric absorption band (not shown here). Time-resolved photoluminescence measurements of Pt2MP3 at early times after laser excitation (not shown) did not reveal excimer emission either. Moreover, the fluorescence of Pt2MP3 seems stronger in aggregates th an it is in dilute solutions, further supporting the idea that J-ag gregates are indeed formed for Pt2MP3. Increased red-shifted fluorescence in J-aggregates has been observed before in similar systems, 37,194 and is attributed to the synergic effects of intramolecular planariza tion (which leads to a slight increase in oscillat or strength), restricted exci mer formation and increased transition dipole moment of the E S 0 transition. 27,207

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132 Although it appears that the ab sorption properties of the o ligomers studied here can be rationalized in terms of face-to-face Ha nd J-aggregates, other structures have been proposed for aggregates consisting of sma ll aromatic systems. Whitten and co-workers 208 have studied a wide variety of aroma tic-functionalized amphiphiles-fatty acid and phospholipids derivatives and th e results of their experime ntal and simulation studies indicate that in many cases, aggregates are characterized by strong noncovalent edge-toface interactions. Pinwheel and herringbone structures based on a chiral tetramer unit 209 were proposed for the aggregates which exhibi ted blue-shifted abso rption and red-shifted fluorescence compared to the monomer. Close energetic balance and competing factors such as topology and steric constraints do not allow an easy identification between edgeto-face and face-to-face. The rema ining of the discussion will therefore assume a face-toface arrangement for the aggregates of plati num acetylide oligomers studied here, bearing in mind that other supramolecular arrangement are also possible. Photoluminescence of Aggregates The photoluminescence of Hand J-aggregat es is rationalized with the molecular exciton model introduced in Chapter 1. For organic H-aggregates, quenching of fluorescence and long lifetimes are usually observed. After excitation to the upper excitonic level E (excitation to lower excit onic level E is forbidden), rapid internal conversion occurs to the E level. The radi ative relaxation from E to the ground state being forbidden, radiationless transitions are necessary to return to ground state. Two possibilities are nonradiativ e decay from E to S 0 and intersystem crossing to triplet excited state. The latter is considered to be nondegenera te in the molecular exciton model, since the oscillator strength of the S 0 -T 1 is zero. The consequence on photoluminescence are therefore quenching of fluorescence with long lifetime and

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133 phosphorescence enhancement in some cases. This effect was observed by Kasha and coworkers 28,36 as early as 1958. For J-aggregates, absorption from ground state to the lower excitonic level E is allowed, therefor e red-shifted fluorescence is observed. Platinum acetylide oligomers aggregates f it into the molecular exciton model, the only difference being that phosphor escence is intrinsically favored due to the presence of strong spin-orbit coupling induced by plat inum. A simplified energy diagram for monomers and aggregates is shown in Figure 4-30. The diagram shows the different photophysic al processes involved in monomers and aggregates. For monomers, excitation of the oligomer to S 1 results in radiative decay from S 1 and T 1A via ISC. From the photoluminescence in dilute solutions, for which almost no fluorescence is observed, ISC is fast and rela xation occurs by phosphorescence from T 1A For Pt2M and Pt2MT, which are believed to form H-aggregates, excitation of the oligomers occurs to the upper excitonic le vel E, as evidenced by the blue-shifted absorption in gels. Rapid internal conversion relaxes the molecule to the lower excitonic level E, from which the radiative decay is fo rbidden. Nonradiative decays can provide a channel for relaxation to ground state but fast intersystem crossing provide an alternate pathway to the triplet excited state T 1B The triplet excited state in H-aggregate is denoted T 1B and shown slightly higher in energy than T 1A (triplet excited state for monomer). As photoluminescence experiments have show n, phosphorescence from the aggregates displays two bands. The normal phosphorescence band at = 516 nm (similar to the one observed for the monomer), as well as a higher-energy, blue phosphorescence band at = 494 nm, only observed when aggregates are present. Preliminary theoretical calculations carried out in our group indicate that the pref erred conformation of the triplet

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134 Monomer H-aggregates J-aggregates S0S0S0S1T1AT1AT1BE E E E A IC ISC P A A ISC ISC P P F F E Figure 4-30. Energy diagram for monomer and aggregates in self-assembling platinum acetylide oligomers. Energy levels are only relative. A: absorption; F: fluorescence; P: phosphorescence; IC: inte rnal conversion; ISC: intersystem crossing; S 0 : singlet ground state; S 1 : singlet excited state; T 1A B : triplet excited state; E,E: excitonic levels. excited state in a Pt2 oligomer has the central phenyl ri ng co-planar to the planes of the platinum square complex, while the two outer phenyl rings are perpendicular to the plane of the platinum complex. Other conf ormations (obtained by a rotation of 90 o of one or several of the phenyl rings) were found to have energies higher by as much as 3 kcal/mol. It is proposed that the triplet excited in H-aggregates is a conformer higher in energy than the conformation of the triplet excited state in dilute solution. Indeed, for H-aggregates and even if the slippage angle is significant, there must be some planarization to allow the o ligomers to come close to each other. A proposed conformation for the triplet excited state in H-aggregates of Pt2M is depicted in Figure 4-31, along with the idea l triplet excited state conformation found in solution. The difference in energy between the blue and the normal emission band is ~2.5

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135 kcal/mol, therefore it is within the limit of the conformer of highest energy calculated (with all phenyl rings co-planar and perpendicular to the planes of the square platinum complex). Note that the photoluminescence of dodecane gel of Pt2M also shows normal phosphorescence. This implies that ev en in the gel, there must be domains where the oligomer is in a dodecane solven t cage and retains some configurational freedom. (a) (b) Figure 4-31. Proposed conformation of the triplet excited state of Pt2M. (a) All-planar high energy conformation in H-aggregates. (b) Ideal confor mation in dilute solution. Hydrogen atoms and dodecanoxy end-chains have been omitted for clarity For Pt2MP3, the photoluminescence spectrum can also be explained using the energy diagram in Figure 4-30. In this case, th e oligomer is excited to the lower excitonic

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136 level E. The radiative decay from E to S 0 is allowed so the oligomer can relax to ground state by emission of fluorescence. Efficient in tersystem crossing due to the presence of platinum may also provide access to the triplet excited T 1A from which relaxation occurs by emission of phosphorescence. Time-resolv ed photoluminescence experiments have shown that strong fluorescence observed in concentrated dodecane solution is prompt fluorescence (fluorescence lifetime on the orde r of the ns) and that triplet-triplet annihilation does not play a significant part (no excitati on power dependence). This reinforces the proposed J-aggregation of Pt2MP3 (as opposed to a simple planarization induced by aggregation) because increased fl uorescence is expected from J-aggregates (intensity scales as N 1/2 for aggregates smaller than the emission wavelength, with N the number of monomers in the aggregate). 33 Fluorescence enhancement in aggregates has been observed previously in related systems a nd is usually attributed to a combination of interand intramolecular effects. 210-212 A brief mention of the phosphorescence lifetimes measured by time-resolved photoluminescence experiments should be made at this point. The in terpretation of the lifetimes is speculative as no example of phosphorescence lifetime from aggregates in similar systems is found in the literature. As mentioned earlier, phosphorescence enhancement may observed in an H-aggregate, because fluorescence from the lower excitonic level E is forbidden. Howeve r in H-aggregates of platinum acetylide oligomers, phosphorescence is always the dom inant radiative process, regardless of concentration. The longer lif etime of the blue band ( = 59 s) of Pt2M (attributed to triplet excited states in aggregates) compared to the normal band ( = 9 s) is hypothetically attributed to tw o factors: 1) Constrained environment in aggregate may

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137 limit vibrational coupling through which nonradiative decays occur. 7,64 This situation is similar to phosphorescence measurements ma de in frozen solvent matrix, where increased phosphorescence is usually observed du e to decreased nonradiative decay rates; 2) Phosphorescence from the aggregated triplet excited state appears to arise from a highenergy conformer, which may slow down the rate of radiative decay. The case of Pt2MP3, where lifetimes decreases with concentration, is not clear at this time. While it may appear as though incr easing concentration should increase triplettriplet annihilation and therefore reduce the lifetime, no evidence for this process which should result in delayed fluor escence was observed. The obser ved trend can be explained by assuming the presence of J-aggregates. As concentration decreases, less aggregates are present in solution, which limits highly effi cient fluorescence from lower excitonic level to ground state. This would imply that aggreg ates are present even in relatively dilute solution as the lifetime increases regularly from 10 -3 M ( = 37 s) to 10 -6 M (148 s). While the aggregate band disappears fr om the absorption spectrum below 10 -4 M, it is not impossible that aggregates are still present below 10 -4 M. Molecular Exciton Modeling In an effort to further characterize the supramolecular arrangement of the aggregates, the molecular exciton model is used to predict the geometry of aggregates of platinum acetylide oligomers. For the sake of simplicity, it is assumed that spectral changes in the absorption spectra of the oligom ers are due to dimers of Jor H-aggregates and the point-dipole approximati on introduced in Chapter 1 is used. This model has been used recently on studies concerne d with aggregates of porphyrins, 213 sapphyrins 214 and lutein, 32 and provides a simple, semi-quantitativ e analysis of aggregate effects. Parameters needed are the interaction energy E (obtained from the energy difference

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138 between monomer absorption and aggregate ab sorption maxima), the transition moment of the monomer absorption (obtained through the calculation of the oscillator strength of monomer absorption), and the distance R between point-dipoles of oligomers in aggregates. The latter is critical and will have a large influence on the result of the calculation since the exciton interaction energy is proportional to the inverse cube of R. Since no crystallographic data on the oligomers was obtained in this study, the distance R has to be estimated. A maximum limit on R can be obtained by taking the intermolecular distance obtained from a non-aggregating platinum acetylide complex bearing bulky iptycene unit studied recently in our group, 215 which was found to have R = 11.782 Other crystallographic data available for a stilbene platinum acetylide complex 216 and a simple platinum acetylide complex 217 are R = 12.812 and 10.032 respectively. Note that in both cases, trin -butylphosphine ligands are used which most likely creates significant steric hindrance compared to the trimethylphos phine ligand used in the present study. Relevant parameters for the ca lculation are presented in Table 4-2 below. The evolution of the spectral shift is plotted as a function of the angle in Figure 428. Several possible distances are examined and calculated angles for the experimental spectral shifts are presented in Table 4-3 (see Experimental section for detail). From the plots of the angle versus (Figure 4-32), some limits can be assigned on the dipole-dipole distance in the aggregates. For Pt2M and Pt2MT, it can be seen that only distances shorter than 8 can account for the exciton splitting observed. Above 8 exciton splitting is too sm all to induce spectral shifts of 58 and 40 nm found in the absorption study of Pt2M and Pt2MT, respectively. Since the model assumes no ground

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139 Table 4-2. Absorption data of self-assembling platinum acetylide oligomers. Pt2M Pt2MT Pt2MP3 m (nm) a 358 378 365 max (M -1 .cm -1 ) b 100,520 52,500 123,000 agg (nm) c 300 338 405 (nm) d 58 40 40 FWHM (cm -1 ) e 4709 4673 4895 f f 3.21 1.66 4.08 (D) g 15.6 11.5 17.8 a : absorption maximum of monomer; b : molar extinction coefficient of monomer at absorption maximum; c : absorption maximum of aggregate; d : absorption spectral shift ( agg m ); e : full width at half maximum for monomer; f : oscillator strength; g : transition dipole moment for absorption of monomer. state interaction, R must be larger than van der Waalls distances, which are about 3-4 in the -stacking of neutral aromatic compounds. 214 Therefore the dipole-dipole distance in the aggregates of Pt2M can be estimated to be between 8 and 4 The angle calculated for some possible distances R are pr esented in Table 4-3. For a perfect face-toface arrangement with = 90 o the calculated distance R is 7.7 for Pt2M, while it is 7.5 for Pt2MT. For Pt2MP3, a large exciton splitting is obtained for a dipole-dipole distance R = 12 that can account for the ab sorption spectral shift observe d in the aggregate. For a perfectly arranged head-to-tail geometry with = 0 o the calculated dipole-dipole distance is 13.3 (Table 4-3).

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140 Figure 4-32. Plot of angle versus = agg m for different dipoledipole distance R. (a) Pt2M. (b) Pt2MT. (c) Pt2MP3; ( ) R = 12 ; () R = 10 ; ( ) R = 9 ; ( ) R = 8 ; ( ) R = 7 ; ( ) R = 6 Conclusion A series of platinum acetylide oligomers designed to self-assemble in solution have been synthesized in order to study the consequences of aggr egation on the triplet excited state. The oligomers were indeed found to form aggregates in dodecane solution, as evidenced physically by the form ation of gels at relatively low concentrations (~2 wt%).

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141 Table 4-3. Some angles and dipole-dipole distances calculate d with the molecular exciton model. Pt2M Pt2MT Pt2MP3 R () 7.7 7 6 7.5 7 6 13.3 10 8 (degree) 90 73 65 90 75 66 0 38 46 A preliminary study of the mesoscopic prope rties of the oligomers revealed liquidcrystalline phases between 120 o C and 180 o C. This is important for opto-electronic applications because it may improve the pro cessability of these oligomers, which are often not as processable as polymers. The photophysical study carried out on thes e oligomers revealed very different behavior among the oligomer se ries. The presence of aggreg ates has been identified spectroscopically, and the forma tion of Hand J-aggregates has been proposed from the direction of spectral shifts observed in absorption spectrometry of the aggregates. The mode of aggregation was found to have very different consequences on the photoluminescence. Oligomers that formed Haggregates showed intact phosphorescence upon aggregation. The emission spectrum of H-aggregates was dependent on the excitation wavelength, and exciting on the Jaggregate absorption band showed a blueshifted phosphorescence band in addition to the phosphorescence usually observed. The blue phosphorescence band is assumed to arise from a high energy conformer present in aggregate, where the restrained envir onment forces the molecule to adopt an unfavorable conformation. Overall, the phot ophysical properties of the oligomer are preserved in H-aggregates, whic h is particularly important fo r technological applications relying on the triplet ex cited state. On the other hand, the photoluminescence spectrum of

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142 an oligomer believed to form J-aggregate showed a drastic change upon aggregation, where strong fluorescence was observed in stead of phosphorescence, as in dilute solutions. Although shorter lifetime of phos phorescence are observed in aggregates compared to those in dilute solution, triplet-triplet annihilation is not believed to be significatly responsible for the relative increase of fluorescence in aggregates. Efficient energy transfer has been observed in mixed-oligomer systems, where one oligomer was used a dopant. Donor emission was significantly quenched with only 5% of dopant in gel. Both blue and normal emission bands were found to be affected by the presence of the dopant, although to a lesser extent for the latter. This implies that the gel contains domains where oligomer are not aggreg ated but rather trapped in a solvent cage with some conformational freedom. Finally, the molecular exciton model ha s been used under the dipole-dipole approximation on the absorption data collected For H-aggregates, the maximum dipoledipole distance was found to be ~7.5 while it was found to be 13.3 for a J-aggegate. Experimental Thermal Properties Differential scanning calorimetry (DSC) an alysis was performed using a PerkinElmer DSC 7 equipped with a controlled co oling accessory (CCA-7) at a heating and cooling rate of 10 o C/min. Calibrations were made using indium and freshly distilled noctane as the standards for peak temperature transitions and indium for the enthalpy standard. All samples were prepared in hermetically sealed pans (5-10 mg/sample) and were run using an empty pan as a reference a nd an empty cells as a subtracted baseline. Optical microscopic images of organogela tors and organogels were observed on a Leitz 585 SM-LUX-POL microsco pe equipped with crossed polars, a Leitz 350 heating

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143 stage, a Photometrics CCD camera inte rfaced to a computer, and an Omega HH503 microprocessor thermometer connected to a J-K-T thermocouple. All solids were sandwiched between thin cover slides. Or ganogel samples were smeared between two glass slides. Temperature was increased approximately 3 4 C/min from room temperature. All micrograph images we re taken with a full-wave plate. Photophysical Measurements Steady-state absorption spect ra were recorded on a Va rian Cary 100 dual-beam spectrophotometer. Samples were placed in adequate short path length cell (1 or 0.1 mm) and absorbance was kept below 1. Co rrected steady-state photoluminescence measurements were conducted on a SPEX F-112 fluorescence spectrometer. Samples were degassed by argon purging for 30 min and heated to an isotropic state regularly during this time for adequate degassing of gel-forming soluti ons. Samples were placed in a triangular-shaped cell and spectra recorded under pseudo front-face geometry to limit self-absorption. The sample cell was positioned so that the incident beam was at 45 o from the face of the cell and emission detected at 45 o Low-temperature fluorescence measurements were carried out similarly to the procedure described in Chapter 2. Time-resolved photoluminescence measurem ents were carried out on the same apparatus described in Chapter 2, except a N 2 laser ( = 337 nm, 10 ns fwhm) was used as well for some oligomers. Photolumines cence was measured by front-face detection to limit self-absorption. Circular dichroism (CD) measurements we re carried out on an Aviv-202 circular dichroism spectrometer with a cell path length of 0.3 mm.

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144 Calculation of Exciton Interaction Energy The oscillator strength f of the monomer absorption was calculated using the following equation: 2 max 9 max 910784.6 2 10319.4 nmf (19) where the constant is in cm 2 .mol.L -1 is the molar extinction coefficient in L.mol -1 .cm -1 and is the full width at half maximum in cm -1 The transition dipole moment of the monomer absorption band was calculated by v f ~ 10702.47 (20) where f has no dimension, the constant is in D -2 .cm and the absorption maximum is in cm v ~ -1 The dipole moment was converted to C.m using: 1 D = 3.33564 x 10 -30 C.m The angle between the dipole moment of the m onomers and the line of molecular centers was calculated acco rding to Kashas model 27,28 of the molecular exciton in the point-dipole approximation, assuming co-plana r monomers in the aggregates and using the following equation: 2 3 2 0cos31 4 11 4 R N N E (21) For dimers (N = 2), this equation becomes: 2 3 2 0cos31 2 1 R E (22) where E is in Joules, 0 is the permittivity of free space ( 0 = 8.8542 x 10 -12 C 2 .N -1 .m -2 ), is the transition dipole moment in C.m, R is the point-dipole point-dipole distance in

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145 m and is the angle between the dipole moment of the monomer and the line of molecular centers in degree. The interaction energy was obtained from the energy difference of the monomer and aggregate maximum of absorption using m agghchc E (23) where h is Planks constant (h = 6.626 x 10 -34 J.s), c is the speed of light (c = 2.998 x10 8 m.s -1 agg and m are the wavelengths in nm of abso rption maximum of the aggregate and of the monomer, respectively. Synthesis General The same procedures as described in Chapter 2 were used. Compounds were loaded on column for flash chromat ography by dry-loading. For this, the crude product was first dissolve d in a few mL of CH 2 Cl 2 silica (1:1 crude/silica by weight) was added and the solvent removed. Crude adso rbed on silica was then loaded on column. cis Dichloro-bis-(trimethylphosphine)-platinum(II) was prepared in two steps by an adapted literature method. 218,219 cis -Bis(trimethylphosphine)dichloroplatinum(II) Potassium tetrachloroplatinate (3.0 g, 7.27 mmol) was placed in water (60 mL) and the suspension degassed with nitrogen. Then, diethyl sulf ide (1.97 g, 21.8 mmol) was added via syringe and the mixture stirred for 2 hours under reflux, during which time the mixture turned clear and yellow. After cooling down, the solution was extracted with CH 2 Cl 2 (3 x 60 mL), the organic phase dried on MgSO 4 and the solvent was removed to give the desired product as a bright yellow solid. Yiel d = 2.815 g (87%). The next step was carried out without characterization and cis -bis(diethylsulfide)dichloropla tinum(II) (2.815 g, 6.3 mmol) was

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146 dissolved in CH 2 Cl 2 and the solution degassed with nitrogen. Then trimethylphosphine (13.9 mL of 1 M THF sol., 13.9 mmol) wa s added via syringe. A fine white solid precipitated immediately and the mixture was further stirred for 30 min at room temperature. After this time, the white solid was collected by filtration, washed with cold Et 2 O, recrystallized from Et 2 O and dried under vacuum for 2 days. After this time, a faint smell of diethyl sulfide could still be detected, although NMR does not reveal the presence of diethyl sulfid e. Yield = 2.483 g (94%). 1 H NMR (300 MHz, CDCl 3 ) 1.80 (m, 18H); 31 P NMR (75 MHz, CDCl 3 ) -23.53 (J Pt-P = 3474.6 Hz). 5-Iodo-1,2,3-trimethoxybenzene (13). 3,4,5-Trimethoxyaniline (10.0 g, 54.6 mmol) was placed in a 500 mL beaker and wate r (120 mL) and sulfuric acid (8 mL) were added. The beaker was placed in an ice-bath and stirred with a mechanical stirrer. Then sodium nitrite (37.7 g, 564.6 mmol) in 60 mL of water was added dropwise and the temperature was maintained between -5 o C and 0 o C. After addition of sodium nitrite was complete, the reaction mixture was poured onto a 30 mL aqueous solution of potassium iodide (14.0 g, 84.3 mmol) at 50 o C under strong magnetic stirri ng. After the addition of the diazonium salt solution was complete, th e mixture was stirred for 30 min. After cooling down, an aqueous solution of sodi um sulfite was added to neutralize excess iodine until no further change in color was observed. The resulting aqueous solution was extracted with Et 2 O (3 x 100 mL), the organic phase dried on MgSO 4 and the solvent removed to give the desired product as a ye llow-orange solid. Yield = 13.3 g (83%). 1 H NMR (300 MHz, CDCl 3 ) 3.82 (s, 3H), 3.84 (s, 6H), 6.89 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 56.5, 56.6, 86.4, 115.1, 147.4, 154.2.

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147 5-Iodo-1,2,3-trihydroxybenzene (14). 5-Iodo-1,2,3-trimethoxybenzene (9 g, 30.6 mmol) was placed in a 250 mL three-neck round-bottom flask fitted with a condenser and CaCl2 drying tube, dissolved in 60 mL of fr eshly distilled methylen e chloride, degassed with nitrogen and the solution cooled to -78 o C. Then boron tribromide (91.8 mL of 1M CH 2 Cl 2 solution, 91.8 mmol) was added slowly via syringe. After the addition was complete, the mixture was allowed to warm to room temperature and further stirred for 24 h. The reaction was then carefully quenche d with ice-water (40 mL) and stirred for 30 min. The mixture was then extracted with et hyl acetate (3 x 100 mL ), the organic phase washed with aqueous Na 2 SO 3 dried on MgSO 4 and solvent removed to give a brown oil. Finally, the product was precipitated fr om the crude oil by addition of CHCl 3 collected by filtration, washed with cold CHCl 3 and dried overnight to gi ve the desired product as a white solid. Yield = 4.06 g (53%). 1 H NMR (300 MHz, acetone-d 6 ) 6.78 (s, 2H), 7.56 (s, 1H), 8.22 (s, 2H); 13 C NMR (75 MHz, acetone-d 6 ) 60.7, 80.5, 117.2, 147.6. 1,2,3-Tris(dodecyloxy)-5-iodo-benzene (15). 5-Iodo-1,2,3-trihydroxybenzene (4.0 g, 15.9 mmol) was dissolved in DMF (80 mL) and the solution was degassed with nitrogen for 15 min. Then K 2 CO 3 (17.5 g, 127 mmol) was added and the mixture stirred at room temperature for 30 min. Then bromododecane (17.1 mL, 17. 8 g, 71.4 mmol) was added and the mixture stirred at 60 o C for 7 h. After cooling down, the brown crude solid was filtered and washed with ice-cold water. Chromatography (silica gel, hexane then 4:1 hexane/CH 2 Cl 2 ) gave the desired product as a wh ite solid. Yield = 10.79 g (90%). 1 H NMR (300 MHz, CDCl 3 ) 0.88 (t, 9H), 1.26 (s, 48H), 1.45 (m, 6H), 1.78 (m, 6H), 3.92 (m, 6H), 6.84 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 14.34, 22.92, 26.24, 26.31, 29.49,

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148 29.58, 29.61, 29.80, 29.84, 29.88, 29.91, 29.95, 30.48, 32.15, 69.50, 73.64, 85.86, 116.39, 138.56, 154.15. 3,4,5-Tris(dodecyloxy)trimethylsilylethynylbenzene (16). 1,2,3Tris(dodecyloxy)-5-iodo-benzene (10.70 g, 14. 14 mmol) was dissolved in THF (60 mL) and iPr 2 NH (40 mL) and the solution was degassed with argon. Then trimethylsilylacetylene (3.0 mL, 2.08 g, 21.2 mmol), Pd(PPh 3 ) 4 (489.6 mg, 0.42 mmol) and CuI (80.7 mg, 0.42 mmol) were added. Th e mixture turned dark right away and was stirred at 60 o C overnight. After cooling down, the mixture was passed through a bed of Celite and the solvents were removed. The crude mixture was dissolved in CH 2 Cl 2 (100 mL), washed with 10% aqueous NH 4 OH (2 x 100 mL), water (2 x 200 mL), dried on MgSO 4 and the solvent removed. The cr ude product was purified by flash chromatography (silica gel, hexane then 4:1 hexane/CH 2 Cl 2 ) to give the desired product as a yellow solid. Yield = 6.0 g (58%). 1 H NMR (300 MHz, CDCl 3 ) 0.24 (s, 9H), 0.88 (t, 9H), 1.28 (s, 48H), 1.46 (m, 6H), 1. 78 (m, 6H), 3.94 (m, 6H), 6.66 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 0.19, 14.27, 22.90, 26.28, 26.32, 29.55, 29.58, 29.61, 29.79, 29.85, 29.88, 29.91, 29.94, 29.97, 30.52, 32.14, 32.16, 53.50, 69.23, 73.32, 73.57, 92.55, 105.71, 110.68, 117.06, 117.69, 139.53, 153.01. 3,4,5-Tris(dodecyloxy)ethynylbenzene (17). 3,4,5-Tris(dodecyloxy)trimethylsilylethynylbenzene (5.58 g, 7.68 mmol) was dissolved in CH 2 Cl 2 (40 mL) and MeOH (40 mL). Then K 2 CO 3 (3.18 g, 23 mmol) was added and the mixture stirred at room temperature for 4 h. Then, the mixture was tr ansferred to a separa tory funnel and water (100 mL) was added. The aqueous phase was extracted with CH 2 Cl 2 (2 x 50 mL), the organic phase washed with water (3 x 50 mL), dried on MgSO 4 and the solvent removed

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149 to give the desired product as an o ff-white solid. Yield = 4.57 g (91%). 1 H NMR (300 MHz, CDCl 3 ) 0.88 (t, 9H), 1.28 (s, 48H), 1.46 (m, 6H), 1.78 (m, 6H), 2.98 (s, 1H), 3.94 (m, 6H), 6.68 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 14.29, 22.90, 22.26, 26.26, 29.49, 29.59, 29.84, 29.91, 30.50, 32.13, 69.19, 73.62, 75.91, 75.98, 76.81, 84.16, 110.70, 116.64, 139.55, 153.08. trans-[3,4,5-Tris(dodecyloxy)phenylethynyl ]-chloro-bis(trimethylphosphine)platinum(II) (18). cis -Dichloro-bis(trimethylphosphine)platinum(II) (382.2 mg, 0.91 mmol) was placed in THF (3 mL) and Et 2 NH (3 mL) and the solution was degassed with nitrogen. Then 17 (500 mg, 0.76 mmol) was added a nd the reaction stirred at room temperature for 24 h after which ti me TLC showed no more organic 17 (R F = 0.78, 1:1 hexane/CH 2 Cl 2 ) and a new spot (R F = 0.45, 1:2 hexane/CH 2 Cl 2 ) was apparent. The solvents were removed and the crude pr oduct was purified by flash chromatography (silica gel, hexane then 7:3, 1:1, 3:7 hexane/CH 2 Cl 2 and finally pure CH 2 Cl 2 ) to give the desired product as a pale yellow solid. Yield = 724.1 mg (96%). 1 H NMR (300 MHz, CDCl 3 ) 0.85 (t, 9H), 1.28 (s, 48H), 1.48 (m, 6H ), 1.63 (t, 18H), 1.76 (m, 6H), 3.93 (m, 6H), 6.51 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 13.12, 13.37, 13.63, 13.99, 22.55, 25.98, 29.25, 29.27, 29.53, 29.57, 29.60, 29.62, 30.20, 31.80, 68.74, 73.13, 80.43, 101.75, 109.47, 122.65, 137.12, 152.45; 31 P NMR (121 MHz, CDCl 3 ) -13.66 (J Pt-P = 2332.2 Hz). Compound (19). 1,4-Diiodobenzene (33.0 mg, 0.1 mmol) was dissolved in THF (6 mL) and iPr 2 NH (4 mL) in a Schlenk flask and th e solution degassed with argon. Then 1-ethynyl-4-(triiso -propylsilylethynyl)benzene ( 59.2 mg, 0.21 mmol), Pd(PPh 3 ) 4 (11.5 mg, 0.01 mmol) and CuI (1.9 mg, 0.01 mmol) we re added and the mixture was stirred at

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150 room temperature for 2 days. After the solv ents were removed, the crude product was purified by flash chromatography (silica gel, hexane then 4:1 hexane/CH 2 Cl 2 ) to give the desired product as a white solid. Yield = 43.9 mg (69%). 1 H NMR (300 MHz, CDCl 3 ) 1.15 (s, 42H), 7.45 (s, 8H), 7.50 (s, 4H); 13 C NMR (75 MHz, CDCl 3 ) 11.51, 18.88, 91.06, 91.31, 93.19, 106.79, 123.03, 123.23, 123.76, 131.57, 131.78, 132.22, 147.43. Compound (20). Compound 19 (43.9 mg, 0.07 mmol) wa s dissolved in THF (3 mL) and the solution degassed with argon. Th en TBAF (0.40 mL of 1 M THF solution, 0.40 mmol) was added via syringe. A white solid precipitated im mediately and the mixture was left stirring at room temperature for 5 h. After this time, the solvent was removed and the crude redissolved in CH 2 Cl 2 (50 mL), transferred to a separatory funnel, washed with brine (2 x 50 mL) and water (2 x 50 mL), dried on MgSO 4 and the solvent was removed. The crude product was further pur ified by flash chromatography (silica gel, hexane then 4:1, 7:3 hexane/CH 2 Cl 2 to give the desired product (R F = 0.34 in 4:1 hexane/CH 2 Cl 2 ) as a white crystalline so lid. Yield = 14.7 mg (66%). 1 H NMR (300 MHz, CDCl 3 ) 4.00 (s, 2H), 7.45 (s, 8H), 7.50 (s, 4H). 1,4-Diiodo-2,5-bis-[(S)-(+)-2-methylbutanoxy]benzene (21). 2,5Diiodohydroquinone (200 mg, 0.55 mmol) was dissolved in DMF (3 mL) and the solution degassed with argon for 15 min. Then K 2 CO 3 (455 mg, 3.3 mmol) was added and the mixture stirred at room temperat ure for 30 min. Then (S)-(+)-1-bromo-2methylbutane (0.2 mL, 249.2 mL, 1.65 mmol) was added via syringe and the mixture was stirred at 60 o C for 3 h. After this time, TLC (1:1 hexane/CH 2 Cl 2 ) showed no more starting material (R F = 0.05) and a new spot (R F = 0.25) was apparent. After cooling to room temperature, water (10 mL) was added and mixture transferre d to a separatory

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151 funnel. The aqueous phase was extracted with CH 2 Cl 2 (3 x 20 mL), the organic phase dried on MgSO 4 and the solvent was removed. The crude red oil obtained was further purified by flash chromatography (silica ge l, hexane then 4:1, 3:2 hexane/CH 2 Cl 2 ) to give the desired product as white crysta lline solid. Yield = 176 mg (64%). 1 H NMR (300 MHz, CDCl 3 ) 0.97 (t, 6H), 1.08 (d, 6H), 1.32 (m, 2H ), 1.62 (m, 2H), 1.90 (m, 2H), 3.80 (m, 4H), 7.17 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 11.61, 16.90, 26.26, 34.96, 74.96, 86.24, 122.48, 152.87. 1,4-Bis-[(S)-(+)-2-methylbutanoxy]-2,5-bis(trimethylsily lethynyl)benzene (22). 1,4-Diiodo-2,5-bis-[(S)-(+)-2-methylbut anoxy]benzene (175 mg, 0.35 mmol) was dissolved in THF (6 mL) and iPr 2 NH (4 mL) in a Schlenk flask and the solution was degassed with argon for 15 min. Then trimet hylsilylacetylene (75.2 mg, 0.11 mL, 0.76 mmol), Pd(PPh 3 ) 4 (40.1 mg, 0.035 mmol) and CuI (3.8 mg, 0.02 mmol) were added and the reaction mixture stirred at 50 o C for 4 h. After this time, TLC (2:1 CH 2 Cl 2 /hexane) showed no more starting material (R F = 0.36) and a new blue fluorescent spot (R F = 0.19) was apparent. After cooling down, the solven ts were removed and the crude product was purified by flash chromatography (silica gel, hexane then 4:1, 3:2, 1:1, 1:2, 1:4 hexane/CH 2 Cl 2 and finally pure CH 2 Cl 2 ) to give the desired pr oduct as a white solid. Yield = 151.7 mg (98%). 1 H NMR (300 MHz, CDCl 3 ) 0.26 (s, 18H), 0.96 (t, 6H), 1.08 (d, 6H), 1.30 (m, 2H), 1.64 (m, 2H), 1. 88 (m, 2H), 3.78 (m, 4H), 6.90 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 0.15, 11.66, 16.71, 26.30, 35.16, 74.27, 100.15, 101.25, 114.01, 116.96, 154.29. 1,4-Bis-[(S)-(+)-2-methylbutan oxy]-2,5-bis-ethynylbenzene (23). 1,4-Bis-[(S)(+)-2-methylbutanoxy]-2,5-bis-trimethylsily lethynylbenzene (151 mg, 0.34 mmol) was

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152 dissolved in THF (8 mL) and the solution was degassed with argon. Then TBAF (2 mL of 1 M THF solution, 2.04 mm ol) was added via syringe and the mixture stirred at room temperature protected fro m light for 2 h. After this time, TLC (CH 2 Cl 2 ) showed no more starting material (R F = 0.5) and a new spot below it (R F = 0.36). The solvent was removed at room temperature and th e crude product was purified by flash chromatography (silica gel, hexane then 1:1 and 2:3 hexane/CH 2 Cl 2 ) to give the desired product as a brown crystalline so lid. Yield = 78.0 mg (77%). 1 H NMR (300 MHz, CDCl 3 ) 0.95 (t, 6H), 1.03 (d, 6H), 1.28 (m, 2H), 1.58 (m, 2H), 1.88 (m, 2H), 3.33 (s, 2H), 3.78 (m, 4H), 6.95 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 11.55, 16.71, 26.26, 34.88, 75.48, 79.92, 82.57, 113.32, 117.66, 154.24. 3-(4-iodophenyl)-prop-2-yn-1-ol (24). 1,4-Diiodobenzene (1.13 g, 3.44 mmol) was dissolved in THF (12 mL) and i -Pr 2 NH (8 mL) in a Schlenk flask and the solution was degassed with argon. Then propargyl alcohol (128 mg, 2. 29 mmol), Pd(PPh 3 ) 4 (198 mg, 0.17 mmol) and CuI (65 mg, 0.34 mmol) were added and the mixture stirred at room temperature for 20 h. Then, water (50 mL) and Et 2 O (50 mL) were added and the mixture transferred to a separatory funnel. Th e aqueous phase was extracted with Et 2 O (3 x 50 mL), the combined organic phase washed with brine (3 x 50 mL), dried on MgSO 4 and the solvent was removed. The crude product was purified by flash chromatography (silica gel, 4:1, 3:2, 2:3, 1:4 hexane/CH 2 Cl 2 and finally pure CH 2 Cl 2 ) to give the desired product as a light yellow solid. Yield = 360.7 mg (61%). 1 H NMR (300 MHz, CDCl 3 ) 2.30 (s, 1H), 4.48 (s, 2H), 7.40 (dd, 4H); 13 C NMR (75 MHz, CDCl 3 ) 51.69, 84.91, 88.79, 94.64, 122.15, 133.29, 137.62.

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153 3-[4-(3,4,5-tris(dodecyloxy)ethynylben zene)-phenyl]-prop-2-yn-1-ol (25) 3-(4iodophenyl)-prop-2-yn-1-ol (360. 7 mg, 1.4 mmol) was dissolved in THF (9 mL) and i Pr 2 NH (6 mL) in a Schlenk flask and the solu tion was degassed with argon for 30 min. Then, 3,4,5-tris-(dodecyloxy)ethynylbenzen e (962.2 mg, 1.47 mmol), Pd(PPh 3 ) 4 (80.8 mg, 0.07 mmol) and CuI (26.6 mg, 0.14 mmol) were added and the mixture stirred at 70 o C for 2 hours. After this time, TLC (CH 2 Cl 2 ) showed no more starting material and a new bright blue fl uorescent spot (R F = 0.52) was apparent. Therefore, Et 2 O (20 mL) and water (20 mL) were added and the mixture transferred to a sepa ratory funnel. The aqueous phase was extracted with Et 2 O (3 x 20 mL), the combined organic phase was washed with 5% HCl (2 x 50 mL) a nd brine (2 x 50 mL), dried on MgSO 4 and the solvent was removed. The crude product wa s purified by flash chromatography (silica gel, 4:1, 3:2, 2:3, 1:4 hexane/CH 2 Cl 2 and finally pure CH 2 Cl 2 ) to give the desired product a sticky orange solid. Yield = 956.6 mg (87%). 1 H NMR (300 MHz, CDCl 3 ) 0.88 (t, 9H), 1.26 (s, 48H), 1.46 (m, 6H), 1.78 (m, 6H), 2.66 (s, 1H), 3.96 (m, 6H), 4.47 (s, 2H), 6.72 (s, 2H), 7.40 (dd, 4H); 13 C NMR (75 MHz, CDCl 3 ) 14.27, 22.87, 26.26, 29.50, 29.55, 29.58, 29.77, 29.83, 29.84, 29.88, 29.91, 29.93, 30.47, 32.10, 51.57, 69.26, 73.74, 85.18, 87.86, 89.39, 91.81, 110.26, 117.57, 122.42, 123.54, 131.46, 131.70, 153.09. 5-(4-ethynyl-phenylethynyl)1,2,3-tris(dodecyloxy)benzene (26) 3-[4-(3,4,5Tris(dodecyloxy)ethynylbenzene)-phenyl]-prop2-yn-1-ol (956.6 mg 1.22 mmol) was dissolved in dry Et 2 O and the solution degassed with nitrogen. Then, activated MnO 2 (1.70 g, 19.52 mmol) and KOH (548 mg, 9.76 mmol) were adde d in four fractions every hour and mixture was stirred at room temperature for 22 hours protected from light. After this time, the mixture was passed through a bed of Celite and transferred to a separatory

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154 funnel. The solution was washed with 5% HC l (2 x 100 mL) and water (2 x 50 mL), dried on MgSO 4 and the solvent was removed. Th e crude product was purified by flash chromatography (silica gel, hexane, then 7:3, 3:2 hexane/CH 2 Cl 2 ) to give the desired product as an orange solid. Yield = 457.9 mg (50 %). 1 H NMR (300 MHz, CDCl 3 ) 0.88 (t, 9H), 1.26 (s, 48H), 1.46 (m, 6H), 1.78 (m, 6H), 3.14 (s, 1H), 3.96 (m, 6H), 6.72 (s, 2H), 7.44 (s, 4H); 13 C NMR (75 MHz, CDCl 3 ) 14.28, 22.89, 26.27, 26.30, 29.52, 29.59, 29.60, 29.80, 29.85, 29.87, 29.91, 19.94, 30.52, 32.13, 69.22, 73.64, 79.05, 83.39, 87.74, 92.05, 110.24, 117.48, 121.89, 124.05, 131.48, 132.16, 139.42, 153.17. Pt2M trans -[3,4,5-Tris(dodecyloxy) phenylethynyl]-chlorobis(trimethylphosphine)platinum(II) 18 (297.2 mg, 0.287 mmol) was placed in THF (10 mL) and Et 2 NH (10 mL) in a Schlenk flask and the solution was degassed with argon for 15 min. Then, 1,4-diethynylbenzene (18.0 mg, 0.14 mmol) and CuI ( 2.6 mg, 0.014 mmol) were added and the mixture stirred at room temperature for 3 h. After this time, TLC showed no more platinum complex starting material and a new yellow-green phosphorescent spot (R F = 0.65, CH 2 Cl 2 ) was apparent. Therefore the solvents were removed and the crude product purified by flash chromatography (silica gel, 2:3, 3:2, 4:1 CHCl 3 /hexane, CHCl 3 and finally CHCl 3 + 3% MeOH) to give a yellow solid. The product was precipitated from 10:1 acetone/CH 2 Cl 2 to give the desired product as an offwhite solid. Yield = 275.5 mg (92%). 1 H NMR (300 MHz, CDCl 3 ) 0.90 (t, 18H), 1.35 (s, 96 H), 1.50 (m, 12H), 1.80 (m, 48H), 3.98 (m, 12H), 6.55 (s, 4H), 7.18 (s, 4H); 13 C NMR (75 MHz, CDCl 3 ) 14.24, 15.29, 15.55, 15.81, 22.81, 26.24, 29.49, 29.55, 29.78, 29.82, 29.86, 30.44, 32.05, 69.16, 73.54, 109.07, 109.19, 109.96, 123.02, 125.37, 130.78,

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155 137.34, 152.76; 31 P NMR (121 MHz, CDCl 3 ) 19.39 (J Pt-P = 2293.0 Hz); Elemental anal. calcd C 62.12, H 9. 19, found C 62.37, H 9.48. Pt2MT trans -[3,4,5-Tris(dodecyloxy )phenylethynyl]-chlorobis(trimethylphosphine)platinum(II) 18 (127.0 mg, 0.122 mmol) was placed in Et 2 NH (5 mL) in a Schlenk flask and the solution wa s degassed with argon for 15 min. Then, 2,5diethynylthiophene (7.9 mg, 0.06 mmol) and CuI (1.1 mg, 0.006 mmol) were added and the mixture stirred at room temperature for 4 h. After this time, TLC showed no more evolution, with some platinum complex st arting material remaining, and a new red phosphorescent spot (R F = 0.43, CH 2 Cl 2 ) was apparent. Therefore the solvents were removed and the crude product purified by flash ch romatography (silica gel, 1:1, 3:2, 7:3, 4:1 CHCl 3 /hexane and finally pure CHCl 3 ) to give a yellow solid. The product was precipitated from 10:1:1 acetone/CH 2 Cl 2 /MeOH to give the desired product as an offwhite solid. Yield = 69.3 mg (54%). 1 H NMR (300 MHz, CDCl 3 ) 0.88 (t, 18H), 1.26 (s, 96H), 1.46 (m, 12H), 1.74 (m, 48H), 3.92 (m, 12H), 6.53 (s, 4H), 6.68 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 14.32, 15.36, 15.62, 15.89, 22.88, 26.29, 29.56, 29.58, 29.61, 29.85, 29.89, 29.93, 30.49, 32.11, 53.62, 69.18, 73.62, 109.85, 122.97, 127.86, 137.32, 152.80; 31 P NMR (75 MHz, CDCl 3 ) 19.24 (J Pt-P = 2296.3 Hz); Elemental anal. calcd 60.82, H 9.07, found C 61.22, H 9.41. Pt2MP3 Compound 20 (14.7 mg, 0.045 mmol) and trans -[3,4,5tris(dodecyloxy)phenylethynyl]-chloro -bis(trimethylphosphine)platinum(II) 18 (95.7 mg, 0.092 mmol) were placed in THF (3 mL) and Et 2 NH (3 mL) in a Schlenk flask and the solution was degassed with argon for 15 min. Then CuI (0.8 mg, 0.004 mmol) was added and the mixture stirred at room temperature fo r 6 h. After this time, TLC showed no more

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156 starting material 20 and a new orange phosphorescent spot (R F = 0.35, 2:1 CH 2 Cl 2 /hexane) was apparent. Therefore the solvents were removed and the crude product was purified by flash chromatography (silica gel, hexane then 4:1, 3:2, 2:3, 1:4 hexane/CH 2 Cl 2 pure CH 2 Cl 2 and finally CH 2 Cl 2 + 3% MeOH) to give a yellow solid. The product was precipitated from 10:1:1 acetone/CH 2 Cl 2 /MeOH to give the desired product as a sticky orange so lid. Yield = 64.0 mg (61%). 1 H NMR (300 MHz, CDCl 3 ) 0.90 (t, 18H), 1.25 (s, 96H), 1.45 (m, 12H), 1.75 (m, 48H), 3.90 (m, 12H), 6.55 (s, 4H), 7.30 (dd, 8H), 7.45 (s, 4H); 13 C NMR (75 MHz, CDCl 3 ) 14.34, 15.37, 15.64, 15.90, 22.90, 26.32, 29.58, 29.60, 29.63, 29.87, 29.91, 29.95, 30.52, 32.13, 69.22, 73.65, 109.94, 131.22, 131.48, 131.58, 152.84; 31 P NMR (121 MHz, CDCl 3 ) 19.23 (J Pt-P = 2294.1 Hz); Elemental anal. calcd C 65. 03, H 8.75, found C 65.24, H 9.07. Pt2MC trans -[3,4,5-Tris(dodecyloxy )phenylethynyl]-chlorobis(trimethylphosphine)platinum(II) 18 (168.2 mg, 0.162 mmol) was placed in THF (5 mL) and Et 2 NH (5 mL) in a Schlenk flask and the solution was degassed with argon for 15 min. Then 1,4-bis-[(S)-(+)-2-methylbuta noxy]-2,5-bis(ethynyl)benzene (23.6 mg, 0.08 mmol) and CuI ( 1.0 mg, 0.008 mmol) were added and the mixture stirred at room temperature for 3 h. After this time, TLC showed no more starting material and a new yellow phosphorescent spot (R F = 0.47, CH 2 Cl 2 ) was apparent. Therefore the solvents were removed and the crude product was purified by flash chromatography (silica gel, hexane then 3:2, 2:3, 1:4 hexane/CHCl 3 and finally pure CHCl 3 ) to give a red solid. The product was precipitated from 10:1:1 acetone/CH 2 Cl 2 /MeOH to give the desired product as yellow solid. Yield = 140.7 mg (78%). 1 H NMR (300 MHz, CDCl 3 ) 0.88 (m, 24H), 0.98 (d, 6H), 1.26 (s, 98H), 1.46 (m, 12H), 1. 62 (m, 2H), 1.76 (m, 50H), 3.74 (dt, 4H),

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157 3.92 (m, 12H), 6.50 (s, 4H), 6.76 (s, 2H); 13 C NMR (75 MHz, CDCl 3 ) 11.50, 14.28, 15.37, 15.63, 15.89, 16.81, 22.83, 26.24, 26.30, 29.51, 29.29.80, 29.84, 29.89, 30.44, 32.06, 34.62, 69.08, 73.54, 74.24, 109.42, 109.83, 117.25, 123.04, 136.38, 137.17, 147.35, 147.37, 152.72, 153.08; 31 P NMR (121 MHz, CDCl 3 ) 19.10 (J Pt-P = 2309.5 Hz); Elemental anal. calcd C 62.69, H 9.38, found C 63.20, H 9.73. PE3 1,4-Diiodobenzene (97.6 mg, 0.3 mmol ) was dissolved in THF (18 mL) and i Pr 2 NH (12 mL) in a Schlenk flask and the solu tion was degassed with argon. Then, 5-(4ethynyl-phenylethynyl)-1,2,3-tris(dodecyl oxy)benzene (457.9 mg, 0.61 mmol), Pd(Ph 3 ) 4 (17.1 mg, 0.015 mmol) and CuI (5.6 mg, 0.03 mmol) were added and the mixture stirred at 50 o C for 3 hours. After this time, TLC (2:1 hexane/CH 2 Cl 2 ) showed no more 1,4diiodobenzene (R F = 0.87) and a new bright blue fluorescent spot (R F = 0.44) was apparent. The mixture was diluted to 6 mL with CH 2 Cl 2 transferred to a separatory funnel, washed with water (3 x 40 mL), dried on MgSO 4 and the solvents were removed. The crude product was purified by flash chromat ography (silica gel, hexane then 4:1, 7:3 and 3:2 hexane/CH 2 Cl 2 ) to give the desired product as a bright yellow solid. Yield = 405.1 mg (86%). 1 H NMR (300 MHz, CDCl 3 ) 0.88 (t, 18H), 1.26 (s, 96H), 1.46 (m, 12H), 1.78 (m, 12H), 3.96 (m, 12 H), 6.73 (s, 4H), 7.48 (s, 12H); 13 C NMR (75 MHz, CDCl 3 ) 14.30, 22.89, 26.28, 29.52, 29.58, 29.60, 29.79, 29.85, 29.91, 29.95, 30.52, 32.13, 69.22, 73.65, 87.99, 91.05, 91.31, 92.13, 110.23, 117.53, 122.76, 123.20, 123.64, 131.70, 132.64, 139.39, 153.17; Elemental anal. calcd C 83.38, H 10.56, found C 84.48, H 11.03.

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CHAPTER 5 CONCLUSION In previous chapters, different aspects of the photophysical properties of platinum acetylide oligomers have been presented. Th e oligomers were designed to gain some insight into the photophysical pr operties of these promising materials, but also on the triplet excited state in general as it has been studied less than the singlet excited state. The importance of the triplet excited state in allorganic systems where it is usually silent has only recently been recognized. The studies pr esented in previous chapters may therefore help understand the photophysical properties of tr iplet excited states in all-organic and metal-organic conjugated systems. A series of platinum acetylide oligomers containing oligothiophene units incorporated in the main chain have been prep ared in an effort to examine the effect of low-energy traps on the triplet excited state. Although more localized than the singlet exciton, the triplet exciton wa s found to be very sensitive to the presence of low-energy sites. Triplet excitons were efficiently trapped by the oligothienyl-based units, from which relaxation occurred. Depending on the ener gy level of the trap, two scenarios were observed. In one case where the trap is relatively close in energy to the normal energy level, relaxation occurred by ra diative decay of the triplet excited state of the trap. Evidence for an equilibrium between two excited states was observed and phosphorescence remained the main channel of deactivation. In a second case, where the energy of the trap is much lower than the normal triplet exc ited, phosphorescence was almost entirely lost. Governed by the energy gap law, the triplet excited state of the 158

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159 lowest energy traps relaxed through nonrad iative channels and th eir emission spectra were dominated by short lived fluorescence. Moreover, experimental evidence suggests that the equilibrium is not operating when th e energy gap between excited state is large. This study suggests that the triplet exciton can migrate as well as the singlet exciton and is therefore subject to being quenched by low energy sites. In a second study, the same oligothiophene -containing platinum acetylide oligomer series, as well as a series of oligomers of increasing chain length were studied by electrochemistry and pulse ra diolysis. The radical anions of the platinum acetylide oligomers were found to be relatively unstable and very localized. On the other hand, the radical cations displayed reversible oxidations and seemed more stable on the electrochemical timescale. The radical cati ons seemed relatively localized, although not as much as the radial anions. Evidence for possible multiple oxidations was found, suggesting a relatively weak elec tronic coupling. Electronic ab sorption of the radical ions displayed two transition bands, one in the visible attributed to a transition from ligandlocalized HOMO to SUMO and a second transi tion in the near-IR which origin was not clearly determined. Finally, a third study was devoted to self -assembling platinum acetylide oligomers in order to probe the consequences of aggrega tion on the triplet excited state. A series of different oligomers sharing long alkyl end-chai ns were synthesized to ward this goal. The materials displayed rich physical properties with liquid-crystalline mesophases and the formation of gel in saturated hydrocarbon so lvents. The consequences of aggregation were found to fall into one of two categories. In one group, absorpti on of aggregates was manifested by a blue-shifted absorption, at tributed to H-aggregates. The photophysical

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160 properties were almost intact and strong phosphorescence was observed even in aggregates. Phosphorescence from a high-en ergy triplet excited state conformer was observed but no dramatic change on the photol uminescence was otherwise observed. In a second group, aggregation resulted in the a ppearance of a sharp red-shifted peak in the absorption spectrum. The possibi lity that this could be due to a simple planarization induced by aggregation was cons idered but other signs point out to the presence of Jaggregates. In this case, the phosphorescence wa s seen to disappear almost completely from the photoluminescence of aggregates, at the expense of a strong fluorescence emission. The molecular exciton model was used assuming co-planar dimers and some geometrical parameters of the aggregates were calculated. This study highlights the fact that while aggregation may have practically no impact on the triplet excited state, in some cases, the consequences are dramatic and th e photophysical properties of the material may be entirely compromised.

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APPENDIX NMR SPECTRA Figure A-1. H NMR (300 MHz, CDCl ) spectrum of Pt4. 1 3 Figure A-2. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt4. 161

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162 Figure A-3. 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt4T1. Figure A-4. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt4T1.

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163 Figure A-5. 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt4T2. Figure A-6. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt4T2.

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164 Figure A-7. 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt4T3. Figure A-8. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt4T3.

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165 Figure A-9. 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2M. Figure A-10. 13 C NMR (75 MHz, CDCl 3 ) spectrum of Pt2M.

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166 Figure A-11. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2M. Figure A-12. 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2MT.

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167 Figure A-13. 13 C NMR (75 MHz, CDCl 3 ) spectrum of Pt2MT. Figure A-14. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2MT.

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168 Figure A-15. 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2MP3. Figure A-16. 13 C NMR (75 MHz, CDCl 3 ) spectrum of Pt2MP3.

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169 Figure A-17. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2MP3. Figure A-18. 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2MC.

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170 Figure A-19. 13 C NMR (75 MHz, CDCl 3 ) spectrum of Pt2MC. Figure A-20. 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2MC.

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171 Figure A-21. 1 H NMR (300 MHz, CDCl 3 ) spectrum of PE3. Figure A-22. 13 C NMR (75 MHz, CDCl 3 ) spectrum of PE3.

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BIOGRAPHICAL SKETCH Thomas Cardolaccia was born on February 10th, 1976, in Clamart, France, in the south suburbs of Paris. He enjoyed a lively childhood living n ear Marseille and then in Bordeaux where his parents moved when he was nine years old. During high school, he liked biology but during his last year in high school realized that chemistry was far more fascinating. He graduated from Lyce Cam ille-Julian with Mention Assez Bien in 1994. He obtained his Matrise de Chimie from Universit Bordeaux I in June 2000. During his time as an undergraduate, he spen t one year at the University of Reading, England, as an ERASMUS stude nt. During the summer of 1999, he also took part in the France-United States REU exch ange program and worked for three months in Dr. Schanzes group at the University of Florida where he was first exposed to the field of conjugated polymers. After retu rning to France to finish hi s degree, Thomas came back to the University of Florida and joined Dr. Schanzes group as a gra duate student to work toward his PhD. While there, he met his wi fe Joanne, who was teaching French in a high school in Gainesville and they got married in October 2003. After his PhD, Thomas will continue his education as a postdoctoral associate with Dr Thomas J. Meyer at the University of North Carolina at Chapel Hill. 191


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Permanent Link: http://ufdc.ufl.edu/UFE0011520/00001

Material Information

Title: From Molecular Oligomers to Supramolecular Gels: Photophysics of Conjugated Metal-Organic Systems
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0011520:00001

Permanent Link: http://ufdc.ufl.edu/UFE0011520/00001

Material Information

Title: From Molecular Oligomers to Supramolecular Gels: Photophysics of Conjugated Metal-Organic Systems
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0011520:00001


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FROM MOLECULAR OLIGOMERS TO SUPRAMOLECULAR GELS:
PHOTOPHYSICS OF CONJUGATED METAL-ORGANIC SYSTEMS















By

THOMAS CARDOLACCIA


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

UNIVERSITY OF FLORIDA


2005



































Copyright 2005

By

Thomas Cardolaccia



































Dedicated to my parents for their support
Dedicated to my wife for her love
And vice versa....















ACKNOWLEDGMENTS

These years as a graduate student have been without a doubt the most stimulating

and rewarding years of my life. Many people have contributed to make this journey a

positive and gratifying adventure and they are hereby acknowledged. Some many things

to say, so little space ...

First I would like to thank my advisor, Dr. Kirk S. Schanze, for his guidance and

support throughout those years. His constant encouragements and motivation have been

an incredible source of strength in many occasions. His genuine care for the intellectual

development of his students is evidenced in the time he often took to explain new

concepts to me or demonstrate the operation of some instruments. Dr. Schanze has

always been willing to let me go my way, giving me a significant degree of freedom on

my research. More importantly, he never made me feel bad for mistakes and failed

experiments. Through these times of failure and success, I have grown as a scientist and a

person, inspired by his creativity and approach to sciences.

I would like to thank my committee members, Dr. John Reynolds, Dr. William

Dolbier, Dr. Michael Scott and Dr. Bruce Carroll. Special gratitude goes to Dr. Dolbier

for organizing and managing the Thursday night Bull Sessions, where I have been

exposed to some aspects of organic chemistry I would have never encountered otherwise.

Many people have been involved with my research and I would like to thank Dr.

Xiaoming Zhao for synthesizing polymers faster than I could characterize them

spectroscopically, Dr. Alison M. Funston and Dr. John R. Miller for carrying out the









pulse radiolysis in Brookhaven National Laboratory, Dr. Stephen Hagen for the use of his

CD spectrometer, and Dr. Richard Weiss for carrying out the polarizing microscopy

experiments.

My experience in the laboratory has been particularly enriching due to several

exceptional individuals willing to share their knowledge and time. Thanks go to Dr. Ben

Harrison, Dr. Yiting Li, Dr. Mauricio R. Pinto, Dr. Yao Liu, Dr. Ksenija Haskins-Glusac,

Dr. Eric Silverman and all present members of the Schanze's group.

I thank my parents for making me what I am today and my family for the love I

was always surrounded with.















TABLE OF CONTENTS

page

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

LIST O F TA B LE S ......................................................... ..... ...... .... ...... ....... ix

LIST OF FIGURES ............................... ... ...... ... ................. .x

A B S T R A C T .........x.................................... ....................... ................. xv

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

Introduction to Photophysics ................................................... ....... .. ...............
A bsorption of Light .................. ................................ .......... .............. ..
N ature of the E excited State ......................................................................... .... 4
R elaxation of E excited States ...................................................................... .....
E energy T transfer ................................................................. .......... .. 9
Singlet and Triplet Excim ers .................................................... ...... ......... 12
Exciton Coupling in Molecular Aggregates.................................................. 15
Triplet Excited States in Conjugated Systems...................................................20
Platinum A cetylides.......... ..... ...................................................... ...... ..... 22
Structure and Synthesis ............................................... ............................ 22
E excited State .............................................................................................. ........24
O bjectiv e of P resent Stu dy .............................................................. .....................30

2 TRIPLET EXCITED STATES IN BICHROMOPHORIC PLATINUM
ACETYLIDE OLIGOMERS.............................. ................ 34

Introduction ................................................. 34
S y n th e sis ............................................................................................................... 3 6
R results ............... ............ .................... ............................. 39
U V -V is A b sorption ....................................................................................... 3 9
Steady-State Photoluminescence .................. ................................................41
T ran sient A b sorption ..................................................................................... 4 9
Tim e-Resolved Photolum inescence .............................................. ......50
D iscussion.........................................................................................53
Energy of the Triplet Excited State in Pt4T3 ..................................................53
The Absence of Phosphorescence in Pt4T3 ................................................ 55
E x cited State D ynam ics................................................................................. 57









C o n c lu sio n ........................................................................... 6 0
Experimental ...................................................................................................................61
Photophysical M easurem ents ........................................ ......................... 61
M ass Spectrom etry of Pt Oligom ers......................................... ............... 62
Synthesis .............................................. 62

3 DELOCALIZATION OF CHARGE CARRIERS IN PLATINUM ACETYLIDE
O L IG O M E R S ....................................................... 7 1

In tro d u ctio n .......................................................................................7 1
R e su lts ...........................................................................................7 3
E lectro ch em istry ................. .............................................................. 7 3
Pulse Radiolysis Ion Radical Spectra ........................................................78
D iscu ssio n .............................. ............................................................................... 8 0
Delocalization of Charge Carriers .................................. ............ ........ 80
Electronic Transitions of the Radical Ions .....................................83
C o n c lu sio n ............. ..... ............ ................. .................................................8 7
Experim mental ............. ...................................... ......... ....... ......... 88
Electrochemistry .......................................... .................. .... ...... 88
P u lse R a d io ly sis............................................................................................. 8 8
Synthesis .............................................. 89

4 CONSEQUENCES OF AGGREGATION ON THE TRIPLET EXCITED STATE
IN PLATINUM ACETYLIDE OLIGOMERS .......................................................... 90

Intro du action ............. ................. .................................................................... 9 0
S y n th e sis ............................................................................................................... 9 5
Results ............... ...... .........................................100
Gel Formation........................................ 100
T h erm al P ro p erties ...................................................................................... 10 1
U V -V is A absorption ................................. .............................. .............. 103
Circular Dichroism ............... ......... .................108
Steady-State Photolum inescence .... ........................................ ......... 110
Time-Resolved Photoluminescence ............................................................119
D isc u ssio n ........................................................................................................... 1 2 7
N nature of A ggregates ........................................ ................. ................ 127
Photoluminescence of Aggregates ..........................................................132
Molecular Exciton Modeling ......................................... ........... 137
Conclusion ........................ ......................................... ......140
Experimental ............. ...................................... ................ ........ 142
Therm al Properties ................................................... ............ ............. 142
Photophysical Measurements ...................................... 143
Calculation of Exciton Interaction Energy ............................ .............. 144
S y n th e sis ................................................................................. 14 5

5 CON CLU SION ........................................ .. .. .. ............... 158









APPENDIX

NM R SPECTRA ............. ...... ....................... ........... 161

L IST O F R E FE R E N C E S ......... ................. ...................................... ..........................172

B IO G R A PH ICA L SK ETCH ......... ................. ..........................................................191
















































viii
















LIST OF TABLES


Table p

2-1. Photophysical data for oligomers Pt4 and Pt4Tn ............................................... 54

3-1. Redox potentials (V vs SCE) for Ptn and Pt4Tn oligomers series in CH2C12
containing 0.1 M TBAH.a ........................... .................... .............. 74

4-1. Lifetime of photoluminescence of self-assembling platinum acetylide oligomers.b127

4-2. Absorption data of self-assembling platinum acetylide oligomers. .......................139

4-3. Some angles and dipole-dipole distances calculated with the molecular exciton
m odel. .............................................................................. 14 1
















LIST OF FIGURES


Figure p

1-1. Potential energy curves for electronic transitions....................... ............... 5

1-2. Jablonski diagram representing the possible transitions after absorption. ...............8

1-3. Diagram for the exchange energy transfer mechanism. ...........................................10

1-4. Diagram for the Coulombic energy transfer mechanism............... .......................12

1-5. Fluorescence spectra of pyrene solutions in cyclohexane....................................13

1-6. Potential energy curves for monomer and excimer. .................................................14

1-7. Schematic representation of the energy levels of the excited state of the monomer
and of aggregates in parallel (left) and head-to-tail (right) geometry ....................16

1-8. Exciton band splitting energy diagram for a co-planar molecular dimer as a
fu n action of th e an g le ............... .................................................. .......... .... .18

1-9. Absorption and fluorescence spectra for cyclohexane solution (dotted line) and
multilayers of fatty acid derivative of trans-stilbene (solid line)..........................19

1-10. Absorption spectra of a carbocyanine derivative in 10-2 M aqueous sodium
hydroxide solution at different concentrations and room temperature...................20

1-11. Schematic pictures of conjugated polymers studied by Monkman and Burrows.44.21

1-12. Plot of triplet energy against singlet energy for the conjugated polymers studied
by M onkm an and B urrow s. ........................................ ........................................22

1-13. General structure of a platinum acetylide polymer. ............................................23

1-14. Splitting of d orbital levels in square-planar Pt(II) complexes.............................25

1-15. Structures of platinum acetylide dimers and polymers studied by Chawdhury et
al. 63 ......................................... ................. 26

1-16. Platinum acetylide dimers and polymers studied by Wilson et a.64 from which
figure w as adopted. ........................................... .. .... ......... ......... 27









1-17. Absorption spectra (high energy dotted lines) and photoluminescence spectra (at
300 K dotted lines, at 20 K solid lines) of films of polymers P1-P8 .....................28

1-18. Energy levels of the Si and T1 excited states and singlet-triplet energy gap for
the Pt-containing and organic polymers............... .... .......... .. .............. 29

1-19. Platinum acetylide oligomers studied by Rogers et al.66 from which figure was
adopted. ...................................................................29

1-20. Platinum oligomers Pt-n (n = 1-5,7) studied by Liu et al.67 ................................. 30

1-21. Absorption (a) and photoluminescence (b) spectra of Pt-n oligomers...................31

1-22. Triplet exciton confinement in platinum acetylide oligomers..............................31

2-1. The structures of platinum acetylide oligomers Pt4 and Pt4Tn (n = 1-3) ................36

2-2. Synthesis of platinum acetylide complexes 5a-d. ........... .......................... 37

2-3. Synthesis of platinum acetylide complex intermediate 11......................................38

2-4. Synthesis of oligomers Pt4 and Pt4Tn (n = 1-3)........................................... ........... 39

2-5. Absorption spectra of oligomers in THF................................... ..............40

2-6. Photoluminescence spectra of oligomers in deoxygenated THF with an excitation
X = 3 5 2 n m ....................................................................... 4 2

2-7. Photoluminescence spectra of oligomers Pt4Tn in deoxygenated THF .................45

2-8. Excitation spectra of oligomers Pt4Tn in deoxygenated THF. .................................47

2-9. Low-temperature photoluminescence spectrum of Pt4T1 in deoxygenated
M eTHF with an excitation = 352 nm ................. ..........................................48

2-10. Low-temperature photoluminescence spectrum in deoxygenated MeTHF with an
excitation X = 352 nm at T = 90 K (-) and T = 300 K (- -)..............................50

2-11. Transient absorption spectra of oligomers in deoxygenated THF following 355
nm ex citation ..................................................................... ... 5 1

2-12. Time-resolved photoluminescence spectra of oligomers Pt4Tn in deoxygenated
THF following 355 nm excitation. ........................................ ........................ 52

2-13. Energy diagram representing the photophysical processes involved in the Pt4Tn
oligom ers. .............................................................................59

3-1. Structure of platinum acetylide oligomers Ptn (n = 1-5). ....................................73









3-2. Structure of platinum acetylide oligomers Pt4Tn (n = 1-3). ................ ..............73

3-3. Cyclic voltammetry (CV, left) and differential pulse voltammetry (DPV, right) of
oligom ers Ptn ................ .......... ...... ...... .... ........... .........76

3-4. Cyclic voltammetry (CV) of oligomers Pt4Tn .................. .............................. 77

3-5. Radical cation spectra for Ptn (top) and Pt4Tn (bottom) oligomers series.. ..............79

3-6. Radical anion spectra for Ptn (top) and Pt4Tn (bottom) oligomers series. ...............81

4-1. Structures of phenyleneethynylene and platinum acetylide oligomers synthesized
for the prelim inary study. ............................................... .............................. 93

4-2. Structures of self-assembling platinum acetylide oligomers ....................................95

4-3. Synthesis of platinum complex intermediate 18....................................................96

4-4. Synthesis of phenyleneethynylene derivative 20 ............ ..................................97

4-5. Synthesis of chiral interim ediate 23. ........................................ ....................... 98

4-6. Synthesis of Pt2M as a representative reaction of the oligomer series...................98

4-7. Synthesis of oligom er PE3. ..... ........................... ........................................ 99

4-8. Picture of deoxygenated dodecane gel of Pt2M (10-3 M) under illumination with a
UV light. ............. ............................... ...............100

4-9. Differential scanning calorimetry thermograms for second heating and cooling
cycle at al0 C/m in scan rate. ...........................................................................102

4-10. Pictures of liquid-crystal phases under a polarized optical microscope...............104

4-11. Absorption spectrum of Pt2M in dodecane ........................................................105

4-12. Absorption spectrum of Pt2M T in dodecane................................. ... ................ 106

4-13. Absorption spectrum of Pt2MP3 in dodecane.............. ..... .................107

4-14. Absorption spectrum of Pt2MC at room temperature in dodecane.......................109

4-15. Circular dichroism (CD) absorption spectrum of Pt2MC in dodecane. ................109

4-16. Photoluminescence spectrum of PE3 with x, = 354 nm.................. .......... 11

4-17. Photoluminescence spectrum of Pt2M..................................................112









4-18. Photoluminescence spectrum of Pt2M in MeTHF at C = 7 x 10-6 M with ex=
326 nm from -63 0C to -23 0C. .......... .... ........ .......... 113

4-19. Low-temperature photoexcitation spectra of Pt2M in deoxygenated MeTHF
monitoring em = 494 nm (-) and em = 516 nm (---) .................................... 114

4-20. Photoluminescence spectrum of Pt2MT in deoxygenated dodecane with )ex =
340 nm at C = 10-3 M (-) and 104 M (- -). .... ...... .......................... 116

4-21. Photoluminescence spectrum of Pt2M P3. ............................. .................117

4-22. Absorption spectrum of Pt2M (-) and Pt2MT ( -) in dodecane at C = 10-3
M .................. ......................................................... ................ 1 1 9

4-23. Photoluminescence spectrum of Pt2M-Pt2MT mixed-oligomer system..............120

4-24. Time-resolved photoluminescence spectrum of Pt2M in deoxygenated dodecane
follow ing ex = 337 nm ................................................................ ............... 12 1

4-25. Principal components of emission decay of Pt2M at 10-3 M in dodecane for slow
component T = 59 pts (-) and fast component T = 9 pts (- -). ...........................123

4-26. Time-resolved photoluminescence spectrum of Pt2MP3 in deoxygenated
dodecane following ex = 355 nm .............................................. ............... 124

4-27. Time-resolved photoluminescence spectrum of Pt2M with 5 mol% Pt2MT in
deoxygenated dodecane following Xex = 337 nm. .............................................126

4-28. Principal components of emission decay of Pt2M at 10-3 M with 5 mol%
Pt2MT in dodecane for slow component T = 41 ps (-), fast component T = 5 ps
(- -) ............................................................................... 126

4-29. Plot of emission decay of Pt2M at 494 nm (a) and 516 nm (b) with x mol%
doping levels of Pt2M T ............................................................ ............... 128

4-30. Energy diagram for monomer and aggregates in self-assembling platinum
acetylide oligom ers. ........................ ...................... ... .. ....... .... ...........134

4-31. Proposed conformation of the triplet excited state of Pt2M. .............................135

4-32. Plot of angle 0 versus AX = kagg mn for different dipole-dipole distance R. ........140

A- 1H NM R (300 M Hz, CDC13) spectrum of Pt4........................................................161

A-2. 31P NM R (121 M Hz, CDC13) spectrum of Pt4................................................... 161

A-3. 1H NMR (300 MHz, CDC13) spectrum of Pt4T1...................................................162

A-4. 31P NMR (121 MHz, CDC13) spectrum of Pt4T1 .............................................162









A-5. 1H NMR (300 MHz, CDC13) spectrum of Pt4T2................................................. 163

A-6. 31P NMR (121 MHz, CDC13) spectrum of Pt4T2. ................................................163

A-7. 1H NMR (300 MHz, CDC13) spectrum of Pt4T3................................................. 164

A-8. 31P NMR (121 MHz, CDC13) spectrum of Pt4T3. ................................................164

A-9. 1H NMR (300 MHz, CDC13) spectrum of Pt2M ...................................................165

A-10. 13C NMR (75 MHz, CDC13) spectrum of Pt2M ............. ...........165

A-11. 31P NMR (121 MHz, CDC13) spectrum of Pt2M. .................. ...................166

A-12. 1H NMR (300 MHz, CDC13) spectrum of Pt2MT. ......................... ..............166

A-13. 13C NMR (75 MHz, CDC13) spectrum of Pt2MT .............................................167

A-14. 31P NMR (121 MHz, CDC13) spectrum of Pt2MT.......................... ............167

A-15. 1H NMR (300 MHz, CDC13) spectrum of Pt2MP3 ...........................168

A-16. 13C NMR (75 MHz, CDC13) spectrum of Pt2MP3. ............................................168

A-17. 31 NMR (121 MHz, CDC13) spectrum of Pt2MP3 ................. ...................... 169

A-18. H NMR (300 MHz, CDC13) spectrum of Pt2MC.............................169

A-19. 13C NMR (75 MHz, CDC13) spectrum of Pt2MC....................... .............170

A-20. 31P NMR (121 MHz, CDC13) spectrum of Pt2MC. ............ ....... ............170

A-21. 1H NMR (300 MHz, CDC13) spectrum of PE3. ....................................................171

A-22. 13C NMR (75 MHz, CDC13) spectrum of PE3 ............................................. 171

















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

FROM MOLECULAR OLIGOMERS TO SUPRAMOLECULAR GELS:
PHOTOPHYSICS OF CONJUGATED METAL-ORGANIC SYSTEMS

By

Thomas Cardolaccia

August, 2005

Chair: Kirk S. Schanze
Major Department: Chemistry

In this dissertation, several series of platinum acetylide oligomers have been

prepared and studied by photophysical methods. The motivation for the research stems

from direct opto-electronic applications that platinum acetylide materials may be used

for, as well as from a more fundamental need to gain a better understanding on the triplet

excited state in conjugated systems.

First, platinum acetylide oligomers containing energy traps were prepared in order

to investigate their effect on the triplet excited state. Second, the delocalization of charge

carriers (radical anions and cations) was studied to determine the charge transport

properties of these materials and the effect of platinum on the charge carriers. Third, a

series of platinum acetylide oligomer was designed to self-assemble in solution with the

goal of determining the consequences of aggregation on the triplet excited state. The goal

of this work was to gain an insight into the dynamics of the triplet excited state in

conjugated systems.









The most significant findings of the study are as follows: (i) although more

localized than the singlet exciton, the triplet exciton is also sensitive to the presence of

energy traps, which can have a significant impact on the photophysical properties of the

materials; (ii) charge carriers are relatively localized on the oligomer chain and the

estimated delocalization of the radical cation is no more than two repeat units; (iii) the

consequences of aggregation on the triplet excited state may be very limited or relatively

important, depending on the mode of aggregation.














CHAPTER 1
INTRODUCTION

The interaction of light with matter is an elementary process in nature responsible

for the life of plants and many other species. Absorption of light is the source of energy

for plants, of heat for some animals. It is the process by which humans and many other

animals see their environment and are able to interact with it and with each other. In the

next chapter, some concepts relying on the interaction of light with molecules will be

presented. It is therefore important to review several fundamental photophysical

processes before. This introductory chapter is divided into two parts. The first part

reviews several important concepts for photophysical studies such as absorption and

emission of light. Then a closer look is taken of the general structure and photophysical

properties of platinum acetylide polymers and oligomers.

Introduction to Photophysics

Absorption of Light

The understanding of the interaction of light with matter has been considerably

changed with the notion of the dual nature of light and the non-classical description of

atomic structure. Maxwell's theory of electromagnetism in 1860 and the development of

quantum mechanics started by Schodinger in 1928 have provided scientists with

mathematical equations describing these phenomena.1 Light is often referred to as an

elementary particle called a photon but can also be thought of as an electromagnetic

wave. Electrons also possess this dual nature and can be visualized as an elementary









particle or a wave. The energy of the photon and the frequency of its electromagnetic

wave are related by the following equation:


E=hv= hc(1)
A

where E is the energy in Joules (J), v is the frequency (Hz or s-1), h is Planck's constant

(6.63 x 10-34 J.s), c is the speed of light (3.00 x 108 m.s-1 in a vacuum) and X is the

wavelength (m).

When a molecule interacts with an optical field, the outer valence electrons of the

molecule interact with the light and can be promoted to higher energy levels. For this to

occur, the light, or the photon, must have the appropriate energy (or quantum) that

corresponds to the difference in energy between the two energy levels involved. This is

because the energy levels of the electrons in an atom or a molecule are not continuous but

discrete. Therefore the wavelength of the light absorbed provides the energy difference

between these energy levels. For many conditions, the absorption of radiation follows

Beer's law

A = -log T = lC (2)

where A is the absorbance, T is the transmittance, 1 is the pathlength of absorption (cm), s

is the molar absorptivity (L.moll.cm-1), and C is the concentration of the absorbing

species (mol.L-1). The molar absorptivity represents the probability of the transition to

occur and is related to the transition dipole moments between the initial and final states.

The molar absorptivity is a function of the radiation frequency and is usually

reported for Xmax as Smax. However, a better measure of the transition intensity is obtained

by integrating s over the whole absorption spectrum,2 which gives the integrated

absorption coefficient a:









a= fJ(u)dU (3)
band

The integrated absorption coefficient provides a connection between the

experimental spectrum and a theoretical quantity known as the oscillator strength, fnm.

This latter is a measure of the strength of an electric dipole transition between electronic

states n and m compared to that of a free electron oscillating in three dimensions. It is

given by


nm 4comec2 1n(1O)
fn. = 4E C2 0 a (4)
NAe 2

where 0s is the permittivity of vacuum, me is the mass of the electron, c is the speed of

light in vacuum, NA is the Avogadro's number and e is the elementary charge. The

collection of fundamental constant has a value of 4.319 x 10-9 mol.L-3.cm2

Assuming a Lorentzian band profile for the absorption band, the integrated

absorption coefficient can be calculated from the experimental absorption spectrum using

1
a = 2Tmax F (5)


where F is the full width at half maximum (FWHM). The oscillator strength equation

then becomes


fn = 4.319 x10-9 max = 6.784 x10- Emax (6)
2

Moreover, the oscillator strength can also be related to the transition dipole moment Unm


fnm = 8rm c 2 = 4.702 x 10 7 n 2 (7)
3e 2h )

where h is Planck's constant and the collection of fundamental constant has a value of

4.226 x1052 C-2.m-2.cm, or 4.702 x 107 D-2.cm.









Even though the energy involved in an electronic transition is discrete, absorption

bands in molecules do not appear as sharp lines, but usually as more or less broad bands.

The reason for this is that electronic transitions are usually accompanied by vibrational

transitions. The explanation for this lies in the fact that electronic transitions occur very

rapidly (10-15 s) with respect to the re-adjustement time of the inter-atomic distance (10-13

s) and this is referred to as the Franck Condon principle. This can be illustrated by

representing the potential energy curves of the ground and excited states as a function of

their respective equilibrium geometry, as shown in Figure 1-1. Electronic transitions are

termed "vertical" with respect to the equilibrium geometry, conveying the idea that the

electron is excited to the upper state before the nuclei have had the time to re-equilibrate.

Nature of the Excited State

Following excitation and creation of an electronic excited state, the molecule will

first relax to the lowest vibrational level by thermal (emission of heat) or collisional

(collision with solvent or solute molecules) relaxation. When the initial state is a ground

neutral state, the electrons are paired and of opposite spin, according to Hund's rule. Due

to spin restrictions imposed by quantum mechanics, the electron promoted to a higher

energy level does not change its spin during excitation and the excited state formed is

called a singlet excited state (Si). In certain cases, however, the spin of the promoted

electron can flip and the resulting overall spin momentum of this excited state becomes

equal to three. This process is called intersystem crossing (ISC) and the resulting state is

called a triplet excited state (Ti).3 Similarly to the excitation into a singlet excited state,

the triplet excited state formed is first vibrationnally excited and then relaxes to its lowest

vibrational level. In organic molecules, the rate of ISC is slow and consequently the yield

of the triplet excited state is usually low. Certain factors can greatly increase the ISC rate












potential
energy


v"= 0


nuclear configuration


potential
energy


nuclear configuration


Figure 1-1. Potential energy curves for electronic transitions. (a) Transition between
states of similar equilibrium nuclear geometry. (b) Transition between states
of different equilibrium nuclear geometry. The figure was adopted from
Gilbert and Baggot.2

and the yield of the triplet excited state is then increased. The process of ISC relies on

spin-orbit coupling and it is facilitated through the heavy-atom effect (internal via

valence bond or external via solvent). In heavy atoms, the spin angular momentum and

the orbital angular momentum of the electron can interact and are not separately









conserved. Therefore as long as the total orbital momentum is conserved, the spin of the

electron can be changed. Rapid intersystem crossing and efficient creation of triplet

excited states are thus common in inorganic or organometallic molecules, and the

platinum acetylide systems that are the focus of this study are among them.

Another important feature of excited states is the singlet-triplet splitting energy (Es-

T). The first singlet excited state is always higher in energy than the first triplet excited

state and the reasoning for this is as follows: In the singlet excited state, the electrons are

of opposite spin and are therefore not prevented by quantum mechanics to be in the same

region of space. In the triplet excited state, the electrons are of the same spin and are

therefore forbidden from being in the same region. This leads to a higher coulombic

repulsion energy in the case of the singlet excited state compared to the triplet excited

state. In a small molecule, this repulsive energy is large and the ES-T is therefore also

large. In large molecules such as a conjugated polymer, the repulsive energy may not be

as large and thus ES-T may not be large either.

Relaxation of Excited States

The excited state is metastable and the electrons will return to their initial

configuration (ground state) by one of two self-relaxation mechanisms: radiative decay

and nonradiative decay.

Radiative decay. In radiative decay, the excited electron will relax to the ground

state by emission of a photon. This photon will carry a quantum of energy corresponding

to the energy difference between the geometrically relaxed excited state and ground state,

similar to the absorption process. If the excited state is a singlet, this emission of light is

called fluorescence whereas it is called phosphorescence if the excited state is a triplet.

Since fluorescence is a transition between states of same spins, it is allowed by quantum









mechanics and the radiative rate of the singlet excited state is fast (-108 s-1).4

Phosphorescence on the other hand is a transition between states of opposite spin and

although facilitated by the presence of heavy atoms in the molecules, it is not as fast as

fluorescence and the radiative rate of triplet excited states is typically much slower (-105

- 102 s-1). Fluorescence caused by direct excitation to Si is called more precisely prompt

fluorescence. Delayed fluorescence has a longer lifetime than prompt fluorescence

because Si is populated by indirect mechanisms. This alternate Si population can proceed

through a thermally-assisted ISC back to Si from Ti (Ti -- Si, E-type delayed

fluorescence) or through a bimolecular triplet-triplet annihilation (Ti + Ti -* So + Si, P-

type delayed fluorescence).

Before the radiative decay, the electron has relaxed to the v = 0 vibrational level so

the energy of this transition will be less than that of the absorption. This results in

fluorescence bands appearing at a longer wavelength than the absorption and this is

called the Stokes shift. The extent of the Stokes shift is then a representation of the

structural differences between the ground and excited states. If the excited state is largely

distorted, a large Stokes shift will be observed.

Nonradiative decay. Another type of relaxation mechanism is nonradiative decay.

In this case, energy is released to the system as heat and does not involve a photon. This

process, as well as the vibrational relaxation, is also referred to as internal conversion.

The relative rate of non-radiative decay is governed by the energy gap law, which states

that as the energy of the excited state decreases, the rate of non-radiative decays will

increase exponentially.57 The triplet excited state being lower in energy than the singlet










for the reason provided above, it is therefore sometimes difficult to observe

phosphorescence, even though the triplet excited state was formed.

It is helpful to look at all these transitions in a representative Jablonski diagram, as

shown in Figure 1-2 below.

There are two characteristics of the excited state that will be encountered in the

next chapters that are worth mentioning at this stage: the photoluminescence quantum

yield P and the lifetime T. The fluorescence quantum yield is the ratio of the number of

emitted photons to the number of photons absorbed4 and it is given by

ks
D f (8)
kf +kn

where OF is the quantum yield of fluorescence, kf is rate constant of fluorescence and knr

A: absorption
F: fluorescence
P: phosphorescence
So: ground state
Si: singlet excited state
v = 3 TI: triplet excited state
S1 c \ v 2 IC: internal conversion
= 1 ISC: intersystem crossing
v.- 0, ISC
"- ---- V=3
I 1
T V 2

I I
-z --- v-,

I I
A F IC
P IC
I I
I I
I I

So


Figure 1-2. Jablonski diagram representing the possible transitions after absorption.









is the rate constant of non-radiative decay for S1. In the case of phosphorescence, the

quantum yield is given by


P rksc ) kp (9)
k, +k,, kP +k',,

where Op is the quantum yield of phosphorescence, kisc is the rate constant of inter-

system crossing, kp is the rate constant of phosphorescence and k'nr is the rate constant of

non-radiative decay for T1.

The luminescence lifetime T is defined as the time for the luminescence signal to

decay to 1/e of its initial value.4 The lifetimes for fluorescence TF and phosphorescence rp

are related to the rate constants for deactivation with the following equations


TF = (10)
kF + k,,


k +k'nr


Energy Transfer

Other than by self-relaxation, excited states may relax to the ground state by

transferring the excitation to other molecules present in the system by a bimolecular

process as in equation 12 below.

D*+A->D+A* (12)

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

Different mechanisms for energy transfer can occur, depending on the environment, but

some conditions require (1) that the energy of D* is higher than the energy of A*; (2) that

the energy transfer rate is more rapid than the decay rate of D*. Two mechanisms for

energy transfer can be distinguished: exchange and Coulombic energy transfer.






10

Exchange energy transfer. Also called collisional or Dexter energy transfer,8 this

mechanism requires contact or a short separation (6-10 A) between the donor and the

acceptor. The rate of energy transfer is therefore diffusion controlled and depends on the

temperature and the viscosity of the solvent. It also requires an overlap between the

orbitals of the donor and the acceptor. Both S-S and T-T energy transfer processes are

allowed and this is in fact the dominant mechanism in triplet-triplet energy transfer. The

mechanism can be represented schematically as in Figure 1-3 below. The transfer rate

constant kET is given by

kET c [h/(2]P 2J exp(- 2r / L) (13)



] I > I







D* A D A*


Figure 1-3. Diagram for the exchange energy transfer mechanism.

where r is the distance between D and A, L and P are constant not easily related to

experimentally determinable quantities and J is the spectral overlap integral.

The decrease in emission intensity due to collisional quenching9 is described by the

Stern-Volmer equation

Fo o 1+K [Q]= 1+ kqo[Q] (14)
F (D









where F and Fo are the fluorescence intensities in the presence and in the absence of the

quencher (or acceptor), respectively, D and (o are the quantum yields of fluorescence in

the presence and in the absence of quencher, respectively, Kq is the Stern-Volmer

quenching constant, kq is the bimolecular quenching constant, to is the lifetime in the

absence of quencher, and [Q] is the quencher (or acceptor) concentration. A similar

Stern-Volmer equation can be written in the case of phosphorescence. However, as can

be seen from equation 9, excited species with long lifetimes (such as triplet excited

states) are more prone to quenching than species with short lifetimes (such as singlet

excited states). Oxygen is an efficient quencher of triplet excited states and for this

reason, phosphorescence measurements are usually carried out in deoxygenated solutions

or frozen matrix.

Coulombic energy transfer. Also called the dipole-dipole or Forster energy

transfer mechanism,10 this long-range interaction does not require contact between the

donor and the acceptor. Efficient long-range energy transfer is favored in situations

where the emission spectrum of the donor and the absorption spectrum of the acceptor

overlap. It is important to note that no photon is involved. As opposed to the exchange

energy transfer, only S-S energy transfer is allowed. This type of energy transfer can be

considered to be due to dipole-dipole coupling between the donor and the acceptor and

can be represented as in Figure 1-4 below.

The rate of energy transfer1 in this case is given by


kET (15)


where TD is the lifetime of the donor, R is the average distance between donor and

acceptor and Ro is the Forster distance (which is a measure of the spectral overlap).










I F





D* A D A*

Figure 1-4. Diagram for the Coulombic energy transfer mechanism.
Forster distances are in the range of 20 to 50 A and can be as large as 100 A for efficient

acceptors. This is comparable in size to biological macromolecules and for this reason

energy transfer has been used as a "spectroscopic ruler" for measurements of distances

between sites on proteins.12-14

Singlet and Triplet Excimers
Under certain conditions, there is another possible fate for the excited state, which

is to form an excited complex, either with a different analyte (to form an exciplex) or

with another like molecule (to form an excimer, or exited state dimer).15 The excimer

formation mechanism can be represented by
1,3M* + 1M 1,3E* (16)

where the excimer E* can be a singlet or a triplet depending on the spin multiplicity of

the excited molecule M* from which it is formed. The concentration of the analyte must

be relatively high for excimer formation to be likely. Alternatively, a poor solvent or a

restricted environment may induce the formation of excimers. A very-well known

example of a molecule that can form an excimer is pyrene (first discovered by Forster

and Kasper in 1954) and its fluorescence spectrum is shown in Figure 1-5 below.





















0I









400 450 500
Wavelength (nm)

Figure 1-5. Fluorescence spectra of pyrene solutions in cyclohexane. Intensities are
normalized to a common value of ODFM. Concentrations decrease from A (10-2
M) to G (10-4 M). Figure was adopted from Birks.15

At low concentration, pyrene displays a highly structured emission. At higher

concentrations, a broad structureless band appears at longer wavelengths, due to the

excimer luminescence. This is a general characteristic of excimer luminescence which is

usually broader and red-shifted from the monomeric emission. This is due to the fact that

the ground state of the excimer is unstable and therefore the potential energy of the

ground state dimer increases with decreasing intermolecular distance. Another

consequence of the absence of a bound dimer in the ground state is that a longer lifetime

is usually observed for excimer emission compared to monomer emission. This is

illustrated in Figure 1-6. Studies of fluorescence in crystals,16 sandwich dimers17 and

diarylalkanes18 all indicate that the preferred conformation of singlet excimers is close to

a symmetrical sandwich structure with a separation of 3-4 A. The binding energy in the












potential
energy








M*+ M
(MM)*







E
(" vibrational
Levels of M*



E M+M

intermolecular
Separation
C


Figure 1-6. Potential energy curves for monomer and excimer. This figure was adapted
from Gilbert and Baggot.2

singlet excimer comes mainly from the exciton resonance and to a lesser extent from

charge resonance.

Triplet excimers, which are more related to this study, have been less studied due


to the experimental difficulties associated with their detection. In fact there has been

much debate on their existence and identification.19,20 But an extensive and pioneering

work by Lim21 greatly contributed to establish triplet excimers as physical species.

Experimental evidences point towards a different structure for the triplet excimer

than for the singlet. In a spectroscopic study on a series of 1,n-di-a-naphtylalkanes (n=l-

4), Subudhi and Lim22 have concluded that the triplet excimer adopts a skewed









conformation, where the short axis of the naphthalene are highly nonparallel while the

long axes are parallel. The angle between the short axes was found to be 100-120. As a

result, the activation energy of triplet excimer formation is higher than for singlet excimer

and the rate constant of formation of triplet excimer is smaller than for a singlet excimer.

More recently, triplet excimers have also been observed in polymers23 and

fullerenes24 and have been used as the main source of white light emission in

electrophosphorescent organic light emitting devices.25 However, they remain rather

elusive and more work is needed to fully comprehend their photophysical properties.

Exciton Coupling in Molecular Aggregates

In 1962, the molecular exciton model was developed by Davydov26 to provide a

theory describing the effects induced by the strong coupling of the collective excited

states in organic crystals. Later, Kasha and co-workers27'28 provided chemists with a

model derived from the molecular exciton model that would provide simple tools to

predict some of the photophysical properties of non-crystalline molecular aggregates. In

particular, the molecular exciton model has proven useful in explaining the photophysical

properties of porphyrins29'30 and different dyes31'32 in aggregates.

One of the important features of this model is the ability to explain the spectral shift

of the absorption band observed in aggregates. This spectral shift is due to the splitting of

the monomer excited state into two excitonic levels. The exciton band splitting can be

derived from detailed quantum calculations but an approximation of the excited state

interaction can be made by considering the electrostatic interaction of the transition

dipole moments. This is illustrated in Figure 1-7 for co-planar dimers arranged in a

parallel and head-to-tail geometry. In the case of parallel geometry, the out-of-phase

arrangement corresponds to a lowering of energy, so E' lies lower than E (excited state of









monomer), whereas the in-phase dipole interaction gives repulsion, so E" lies higher than

E. Since the transition dipole moment is given by the vector sum of the individual

transition dipole moments, transitions from ground-state to exciton state E' are forbidden

whereas those from ground-state to E" are allowed. The spectroscopic consequence of

the exciton splitting will therefore be observed as blue-shift of the absorption in the

aggregate in parallel arrangement compared to the monomer. For the head-to-tail

geometry, the situation is opposite. The in-phase arrangement of individual dipole

moments leads to an electrostatic attraction, whereas the out-of-phase arrangement causes

electrostatic repulsion. However, transitions to E' are allowed whereas transitions to E"

are forbidden. In this case, the spectroscopic consequence of the exciton splitting will be

observed as a red-shift of the absorption in the head-to-tail aggregate compared to the

monomer. In the literature, aggregates in parallel arrangement are often referred to as J-

aggregate, and the head-to-tail as H-aggregates.


PARALLEL HEAD-TO-TAIL




E")E"
EE' EE

I


Ee EG
MONOMER DIMER MONOMER DIMER

Figure 1-7. Schematic representation of the energy levels of the excited state of the
monomer and of aggregates in parallel (left) and head-to-tail (right) geometry.
Solid arrows and dashed arrows represent allowed transitions and forbidden
transitions, respectively. This figure was adopted from Kasha.33









For planar aggregates composed of N monomers and in the point-dipole

approximation, the exciton splitting energy AE is given by


AE = 4N 1I (1(l- 3cos2 0) (17)
N }4taR3E R

where AE is in joules, t is the electronic transition moment of the monomer in Coulomb

meter, R is the point-dipole-point-dipole distance in meter and 0 is the angle between

the long axis of the molecule and the line of molecular centers. This equation shows that

the exciton splitting energy depends on the number of aggregates, that it is proportional

to the square of the transition moment of the monomer and proportional to the inverse

cube of the distance between monomers. The theory also predicts that for an angle 0 =

arcos(1//3) = 54.70, the exciton splitting energy is equal to zero and therefore no spectral

shift may be observed in the absorption spectrum. This is the angle for which the

aggregate will shift from J-aggregate to H-aggregate, as shown in Figure 1-8 below.

While the spectroscopic consequence of aggregation can be easily identified in the

absorption spectrum, there is also a consequence on the photoluminescence. In the case

of H-aggregates, where the excited state is the higher excited state E", there is usually a

quenching of fluorescence observed in the emission spectrum of aggregates. After

excitation to E", there is a rapid internal conversion to the lower exciton level E'. Since

the transition from E' to the ground-state is not allowed, the system goes back to ground-

state via nonradiative decay or intersystem crossing through the triplet excited state.

Experimentally, the fluorescence detected is red-shifted (as it originates from the lower

excitonic level E') and longer lived (as the transition from E' to ground state is

forbidden) compared to the fluorescence of the monomer. A phosphorescence

enhancement was observed by several authors34'35 in the 1950s and was later rationalized

















E-- E
I






EG 0* 9 54.7* 90*
MONOMER DIMER
LEVELS LEVELS


Figure 1-8. Exciton band splitting energy diagram for a co-planar molecular dimer as a
function of the angle 0. This figure was adopted from Kasha.33

by Kasha using the molecular exciton model.28'36 In the case of J-aggregates, emission

occurs from the lowest exciton level E', at lower energy than the corresponding monomer

emission. The emission from J-aggregates may be enhanced compared to the emission

from the monomer although the interplay of inter- and intramolecular effects are often

difficult to discern.37 Note that in this model and in the dipole-dipole approximation, the

triplet excited state is considered to remain degenerate since the oscillator strength (and

hence the transition dipole moment) for singlet-triplet transition is zero.

More recently, J- and H-aggregates have been observed in many different systems,

some of them closely related to the molecules studied in the next chapters. Whitten has

studied fatty-acid derivatives of trans-stilbene in Langmuir-Blodget films, in order to

study the effect of aggregation on the photophysical properties of stilbene.38'39 The

absorption and fluorescence spectra of a trans-stilbene derivative in solution and in

multilayers in shown in Figure 1-9. The consequences of the aggregate formation are









found in both spectra. The absorption spectrum shows a hypsochromic shift compared to

the monomer while the fluorescence spectrum shows a bathochromic shift compared to

the monomer. This is consistent with the presence of H-aggregates in the supported

multilayers system. In this case, fluorescence is observed from the lower excitonic state

E' even though it is a forbidden process. But the lifetime of the fluorescence band in the

multilayers is four times longer (3.3 ns) than the fluorescence lifetime of trans-stilbene

(0.8 ns), consistent with a forbidden radiative decay.38

Examples of J-aggregates are found in a recent study of carbocyanine dyes by

Pawlik etal.40 Cyanines are strongly aggregating systems and the absorption spectrum

(Figure 1-10) shows the presence of aggregates even in dilute solution. However, it is

dependent on the concentration and increases with concentration. The absorption band for

the aggregate is sharp and red-shifted from the absorption band of the monomer,

consistent with a J-aggregate.



e r
A It
SI ,


.



i / ;

o o
250 300 350 400 450 500
Wavelength (nm)

Figure 1-9. Absorption and fluorescence spectra for cyclohexane solution (dotted line)
and multilayers of fatty acid derivative of trans-stilbene (solid line). Figure
was adopted from Whitten.39










200


150-

E
E
r 100-

Sb




0-
400 500 600 700
X/nm


Figure 1-10. Absorption spectra of a carbocyanine derivative in 10-2 M aqueous sodium
hydroxide solution at different concentrations and room temperature. (a) 1.7 x
105 M; (b) 2.2 x 105 M; (c) 4.4 x 10-4 M. Figure was adopted from Pawlik et
al. 40

Triplet Excited States in Conjugated Systems

While their presence usually goes undetected in organic conjugated polymers (CPs)

for reasons mentioned previously (slow ISC), triplet excited states are still important in

these systems. This is particularly true for organic light-emitting devices (OLEDs) where

electroluminescence is generated by the recombination of electrons and holes injected

from the electrodes. If conventional spin statistics were applied, only 25% of all

recombination events would lead to potentially emissive singlet states, and 75% would

lead to non-emissive triplet states. However, a number of recent experimental41 and

theoretical42 studies point to the existence of a chain-length dependence on the exciton

spin formation. It is therefore critical for the optimization of OLEDs to understand the

factors controlling the formation of triplet excited states.

The quantum yield of ISC in some representative organic CPs has been measured

by Burrows et al.43 using photoacoustic calorimetry and the values varied from 0.50-0.80











for some polythiophenes to 0.01-0.04 for poly(p-phenylene vinylene)s. The higher value


of ISC obtained for the polythiophene are attributed to the efficient spin-orbit coupling


induced by the sulfur atom.


Monkman and Burrows have also carried out an extensive study on a broad range


of organic conjugated polymers and measured their singlet and triplet energies by pulse


radiolysis and energy transfer.44 The polymers studied are presented in Figure 1-11 and a


plot of triplet energy gap against singlet energy gap is shown in Figure 1-12. As can be


seen, there is a linear correlation between the triplet and the singlet energy gaps for the


very different polymers studied. Above the trend line are polymers with rigid and planar


backbone structures while twisted polymers lie below the trend line. From this, it appears


that while planarity enhances the delocalization of singlet excited state, triplets do not







9. poly(dloctyinourene) 13. poly3-hexy1-25-pyridincdiy) 11. paoy(2S5-hexyloxyphenyhcneviny]Cnle)
(PFO) (HPPY) 2 poly(3-octyllhicphnc) I. poly(3-ocly-4-methylthiophenc) (DHOPPV)
(P30T) (PMOT)





5 poy3-2-ethyth xy iop ) 6, poly(2-mothoxy5-(2 -cthylhcxyoxy)
\ 1 0. poly(2,5.ociyloxyphnylcnevinylne) (PEHT) -p-phenylenvcyanoinylee)
(DOOPPV) (CN-MEHPPV}
3. poly(2-buityloy-5-ryip henyl H .
-3-Ihtophne) -o
(PBOPT)
H .12 poly(2,S-pyridincdiyl)
17. polyencraldinc {PPY
(PANWi 8. poly(2-methoxy*-5(2'-ethylhcxyoxy)-p-phenyLcnevEnyl ie)
(MEHPPV)
15 po]y(6BCMU) ,--r- ,, r-" ,

= 7. poly(3.yclohexyl-4-mcthyllhOphcne))
(PCHMT)

R 6. poly(3-cyclhehylthipbhene) RI o -C H
R.HR2= 1)~ROC 2H4C)-OC0 Noj2H
R- (CH2)0CONHCH'COOC4H P4. MS-Y 14 MeLPPP

Figure 1-11. Schematic pictures of conjugated polymers studied by Monkman and
Burrows.44











2.6-
9 12
24-



2. 12 14
2.2




0.68 1 ---
c 10 4
1-4-


17
0-8

1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25
Singlet energy (eV)


Figure 1-12. Plot of triplet energy against singlet energy for the conjugated polymers
studied by Monkman and Burrows. Figure was adopted from Monkman et
al.44

benefit as much from delocalization enhancements and while torsion angles tend to

localize singlet excited states, this has les impact on the localization of triplet excited

states. It follows then that triplet excitons must be more localized than singlet excitons,

for which it is commonly agreed that their delocalization extends over 7-8 repeat units.

Kohler et al.45 have carried out a systematic study of singlet-triplet splitting energy (Es-T)

in organic poly(phenylene ethynylene) polymers, varying the optical bandgap by

changing the nature of the spacer. They found that the ES-T was always 0.70.1 eV and

this is similar to what has been reported in other organic CPs.46'47 Monomers, oligomers

and twisted polymers have higher ES-T because of the exciton confinement, which

impacts the singlet more than the triplet exciton.48

Platinum Acetylides

Structure and Synthesis

Platinum acetylide materials are a class of compounds which have recently

attracted significant attention due to their non-linear optical properties and potential

application in optical limiting. The general structure of a platinum acetylide polymer is









presented in Figure 1-13. The aryl group (Ar) is usually a phenyl or a thienyl group,

while the R is typically a short to medium alkyl chain (1 to 8 carbons) or even a phenyl

group.



LPR.n

Figure 1-13. General structure of a platinum acetylide polymer.

The synthesis of the first Pt-acetylide complex was accomplished by Hagihara and

co-workers49 in 1978 and it involves the coupling of a platinum chloride complex with

acetylene. The reaction is carried out in the presence of a base such as an alkylamine

needed for the deprotonation of the arylacetylene while a catalyst such as copper

iodide(I) is used to activate the acetylene function towards deprotonation. To avoid

unwanted oxidation of the catalyst and of the platinum complex, the reaction is best

carried out under inert atmosphere. Recently, it has been found that a trans-platinum

complex is not necessary to obtain trans-platinum acetylide products, as the cis-platinum

complex rapidly isomerizes in the presence of an amine. The two isomers can be

identified by 31P NMR, from the coupling constant between the phosphorous and one of

the NMR active platinum isotope (195Pt, I = 12, 33.8% natural abundance). While a trans

isomer will typically have a Jpt-p below 2500 Hz, the Jpt-p for the cis isomer is usually

higher than 2500 Hz.50-52

Due to their attractive non-linear optical properties, first reported by Davy and

Staromlynska53'54 in 1994, different architectures of platinum acetylide have been

prepared in the last few years. These include dendrimers5557 (one of them containing up

to 189 platinum atoms), liquid-forming oligomers,58 and metallamacrocycles.59









Excited State

The photophysics of transition metal complexes involve the electrons in the d-

orbitals, which are split into different energy levels depending on the electronic

environment provided by the ligands. In the case of Pt(II) with four ligands, a ds

complex, the ligand field stabilizes the dyz, dxz, dz2 and dxy oriented away from the ligands

and destabilizes the dx2 y2 pointing towards the ligands. The resulting orbital diagram is

shown in Figure 1-14. As can be seen, the HOMO orbital is the dxy and the LUMO is the

dx2 2 so the photophysics of Pt(II) are expected to involve those two orbitals. Optical

transitions encountered in organometallic complexes may be purely metallic (d-d

transitions) or metal-to-ligand charge-transfer (MLCT). When the ligands have a 7n-

system, 7n-7n* transitions localized on the ligand may also be observed.

In platinum acetylide oligomers and polymers there is a mixing between the

platinum d-orbitals and the nt-system and the optical transitions do not clearly fall in one

or the other category. The extent of mixing depends on the overlap between the ligand

and metal orbitals, the size of the spacer and the extent of conjugation in the ligand. The

emission of platinum acetylide complexes typically shows vibronic bands, which is

consistent with a MLCT or 7n-7* excited state, but not with a d-d transition. Also,

phosphorescence lifetimes of are often found to be intermediate between a typical 3 7n-*

or 3MLCT. Theoretical work60-62 has shown that the HOMO of platinum acetylide

complexes is composed mostly of the 7n orbitals of the acetylide ligands with some

contribution from the dxy metal orbital, while the LUMO consists only of the ligand 7n*

orbitals. Therefore the phosphorescence in platinum acetylide complexes is best thought

of as originating from a 3n-n*/3MLCT manifold with predominantly intraligand character.

Platinum acetylide complexes and polymers absorb light into the S1 state between 300











5 dx2 y2





5 dxy


-4- 5 d2

t I 5 d, dyz


Figure 1-14. Splitting of d orbital levels in square-planar Pt(II) complexes.

and 400 nm. Rapid and efficient intersystem crossing (with close to unite quantum

efficiency) converts the singlet excited state into triplets, so that their emission spectrum

may show some fluorescence (decay of Si) between 370 and 450 nm and

phosphorescence (decay of Ti) between 500 and 600 nm. The triplet excited state absorbs

light into higher triplet excited states (Tn) between 600 and 800 nm.

Several groups, including ours, are actively working on the photophysics of

platinum acetylide complexes and in the next section, the state of the research on these

materials is described.

Chawdhury et al. 63 have studied the evolution of the singlet and triplet excited

states with the number of thienyl rings in platinum-acetylide dimers and polymers (shown

in Figure 1-15). From absorption measurements of the polymers and dimers and

comparison with the 2,5-diethynyl-oligothiophenes, they observed that the introduction

of platinum lowers the energy of the So--S1 transition. This shows that conjugation is









preserved through the platinum centers and that they do not confine the singlet exciton.

The photoluminescence spectra of the polymers show a phosphorescence band between

650 and 740 nm, however phosphorescence was not observed for the terthienyl-

containing polymer. They attributed the absence of phosphorescence to the reduced

influence of the heavy-metal center responsible for ISC when increasing the number of

thiophene rings.

/ PE3 /=\ Bu3
I R t R-=
PEt3 PEt3 PBu- n


R= I m= 1-3


Figure 1-15. Structures of platinum acetylide dimers and polymers studied by Chawdhury
et al.63

From a closer inspection of the fluorescence bands in the polymers, they observed

increasing intensity in the vibronic bands as the number of thiophene rings increased.

This is an indication that that the excited state geometry differs more from the ground

state geometry and that the excited state takes more of a 7t-7t* thiophene-based character

with increasing thiophene rings. They also observed a constant S1-T1 splitting energy of

0.7 eV across the polymer series.

This constant S1-T1 gap of 0.7 eV was also exhibited in a larger range of platinum

acetylide polymers (shown in Figure 1-16) studied by Wilson et al.64 The polymers were

chosen so that the triplet energy level would be tuned between 2.5 and 1.3 eV and this

trend could be related to changes in nonradiative decay rates. Indeed, shorter lifetimes

and lower quantum yields of phosphorescence were observed as the triplet energy level

decreased, as illustrated in Figure 1-17. Calculations of the radiative and nonradiative









rates showed that as the triplet energy level decreases, the rate of nonradiative decay rates

increases exponentially. This is the consequence of the energy gap law, which relates the

rate of nonradiative decay rate of a transition rate to the energy gap between the states

involved.

In simple terms, the energy gap law can be written as


kc exp (18)


where AE is the energy gap between the states involved, y is a term expressed in terms of

molecular parameters and coM is the maximum and dominant vibrational frequency

available in the system.65 Plots of ln(kn) against AE(Ti-So) gave straight lines for the

monomers and the polymers studied, implying that platinum acetylide polymers and

monomers follow the energy gap law. Another finding of this study is that radiative

decay rates will be large enough to compete with nonradiative decay rates for materials

with T1-So gaps of 2.4 eV or above.

PfC2H.33 Ph PAHO

rt r -- -O +
P(C b P(C( b3
Monomer Polymer





5. 3.4.
R'= -/- 8



Figure 1-16. Platinum acetylide dimers and polymers studied by Wilson et a.64 from
which figure was adopted.

















_T -L----g 11--
S P4 -


SPS Ii



T
T $ /'"x,... ..



1.2 1.6 2.0 2.4 2.8 3.2
Energy (eV)

Figure 1-17. Absorption spectra (high energy dotted lines) and photoluminescence
spectra (at 300 K dotted lines, at 20 K solid lines) of films of polymers P1-P8.
Figure was adopted from Wilson et al.64

Extending the series of platinum-containing polymers studied and comparing them

with their respective organic counterparts, Kohler et al.45 proved that conclusions drawn

on the triplet excited state in platinum acetylide polymers could be carried to the

corresponding organic polymers. In a series of 15 platinum-containing and organic

polymers with various optical bandgaps, a constant singlet-triplet energy gap of 0.7 + 0.1

eV was found in both series (Figure 1-18).

In a study concerned with the influence of the size of the ligand in platinum

acetylide complexes (shown in Figure 1-19), Rogers et al.66 have also greatly contributed

to the field. The study found that the effect of increased conjugation is a red-shift and an

increase in the molar absorption coefficient of So-Si and T1-Tn absorption bands and an










increase in the singlet-triplet energy gap. The authors concluded that as the conjugation

increased, the So-Si transition is more localized on the ligand with less metal character

3.5 I i
S8, Pi polymer
3.0 T1v 0 T ppolyr -
SV* Sog,9 polymer
2.5 v A T, rg. polymer
2 0 o v

1,5 O 0 0
L5 AES,*T,
1, -e--- PtPoi~ler
1.0 oo org. Poner
0.5 .,..1 .-
0.0 I I
0 5 10 15 20
Spacer label


Figure 1-18. Energy levels of the Si and T1 excited states and singlet-triplet energy gap
for the Pt-containing and organic polymers. Figure was adopted from Kohler
et al.45

and therefore slower intersystem crossing occurs. This was supported by the longer

lifetimes of T1 and lower quantum yield of phosphorescence observed.

Finally, our group carried out a systematic study of the delocalization of the singlet

67
and triplet excited state in a series of mono-disperse platinum acetylide oligomers6

(Figure 1-20). While the previous studies had some indications that the triplet exciton


PE1 .-CEC-Pt-CE=C-Q



PBu3

IUU3



PE2 c -c-c -cc- -cct.c--J-cc- c
PBu3




Figure 1-19. Platinum acetylide oligomers studied by Rogers et al.66 from which figure
was adopted.










G '6 Pt---. H

Figure 1-20. Platinum oligomers Pt-n (n = 1-5,7) studied by Liu et al.67

may be more localized than the singlet exciton, no definite conclusions could be drawn

by comparing long conjugated polymers and short monomers. Polymers have a

distribution of chain lengths and it is therefore not possible to relate the molecular weight

of the polymers to the optical properties. The absorption and emission spectra measured

for this series of oligomers is shown in Figure 1-21. The main absorption band found

between 320 and 370 nm red-shifts across the series but the change is small between Pt-5

and Pt-7, indicating that the effective and maximum conjugation length of the Si state is

approximately 6 repeat units. The emission spectra of the oligomers displayed weak

fluorescence between 370 and 390 nm and was dominated by a strong phosphorescence

band at X = 518 nm. The fluorescence red-shifted as the oligomer size increased but

leveled off between Pt-5 and Pt-7, consistent with the absorption data. However, the

phosphorescence was found to be independent of oligomer length and this clearly showed

that the triplet exciton is only delocalized over one or two repeat units. Figure 1-22 shows

the estimated size of the triplet exciton based on these experiments. Consistent with this

idea and the observations made in previous studies, the singlet-triplet energy gap was not

constant in this oligomer series and varied from 0.91 to 0.78 eV as the oligomers length

increased.

Objective of Present Study

Platinum acetylide oligomers offer a unique framework to study triplet excited state

in conjugated system. Their monodisperse length and precise chemical structure allows

structure-property relationship to be established by the synthesis of different derivatives.










3 a J P b .o

250
E 0.8"
a P



150 1 .






20 300 30 4 400 00 00 00
250 300 360 400 400 500 N00 700
Wavelength I nm

Figure 1-21. Absorption (a) and photoluminescence (b) spectra of Pt-n oligomers.
Fluorescence (F) intensity scale is magnified 100X compared to
phosphorescence (P). Figure was adopted from Liu et al.67

PBu PBu PBu PBu PBu PBu PBu
PBu PBu PBu PBu PBu' PB PBu

Figure 1-22. Triplet exciton confinement in platinum acetylide oligomers. Location of
exciton is arbitrary.

The information gained from the study of platinum acetylide oligomers can be

extrapolated to metal-organic68 and even all-organic45 polymers, where the triplet excited

state is just as crucial as the singlet excited state. The need for more research on these

unique molecules is therefore at a fundamental level.

There is also a direct motivation as platinum acetylide oligomers are promising

candidates for optical limiting applications. McKay and co-workers53'69 have found

evidence that platinum acetylide oligomers could be used as the active component of a

broadband, frequency agile optical limiter. This is a crucially important technology for









protection against intrusive, possibly damaging, laser radiations that have become

increasingly present.

While the study of platinum acetylide oligomers and polymers carried thus far have

significantly improved general knowledge on triplet excited states in these systems and

related conjugated systems, much work remain to be done. In particular, several

questions still need to be addressed in platinum acetylide materials:

What is effect of a low energy site on triplet excitons? While these effects are

known in the case of the singlet exciton,70'71 the situation may be different given the

relative localization of the triplet excited state compared to the delocalized singlet excited

state. For this, a series of platinum acetylide oligomers Pt4Tn with triplet energy traps

were synthesized. The traps consisted of oligothiophenes (n = 1-3), which are known to

have a lower triplet excited than benzene. The effect of these traps on the photophysical

properties of platinum acetylide oligomer was probed and compared to a "trap-free"

oligomer.

What is the extent of delocalization of charge carriers? In all-organic conjugated

polymers, the charge carriers are believed to extend over several repeat units72 but the

effect of a metal center on the delocalization of charge carriers remains unknown. In

order to determine the extent of delocalization of charge carriers, oligomer series Ptn and

oligomer series P4Ttn were studied with electrochemistry and pulse radiolysis. The first

series of oligomers Ptn (n = 1-5) provided a system to study the delocalization of charge

carriers as a function of conjugation length while the second series of oligomers Pt4Tn

probed the effect of low energy sites on charge carriers.









What are the consequences of aggregation? While these consequences can have a

dramatic effect on the singlet exciton and are now fairly well understood,73'74 almost no

information is available for the triplet exciton in conjugated systems. A parallel to

organic crystals has to be made and it remains unclear whether this is reasonable or not.

With the concepts of supramolecular chemistry, complex conjugated systems have

appeared in the literature in the past few years7 and it appeared that platinum acetylide

oligomers could be easily modified and designed to self-assemble. A series of short

platinum acetylide oligomers where the phenylene end-group was tri-substituted with

dodecanoxy chains were synthesized and their photophysical properties in molecular

aggregates studied by steady-state and time-resolved spectroscopy using standard

solution techniques.














CHAPTER 2
TRIPLET EXCITED STATES IN BICHROMOPHORIC PLATINUM ACETYLIDE
OLIGOMERS

Introduction

Conjugated systems have received tremendous attention in the past fifteen years.

Conjugated polymers in particular have been the subject of an enormous research effort,

as they combine attractive photophysical and mechanical properties. Much of the

research efforts are aimed towards understanding the dynamics of the different

photophysical processes involved in an opto-electronic devices. For example in a light

emitting diode, an exciton is formed following recombination of an electron and a hole.76-

78 The radiative decay of this exciton will give rise to luminescence. But the exciton may

encounter a defect or low-energy site within the time of its lifetime, which will usually

lead to nonradiative decay and a reduction of the device luminescence and

performance.79-84 It is therefore important to understand the susceptibility of the exciton

to these low-energy traps, which are almost inevitable in polymer systems.

Most of the conjugated polymers studied are all-organic polymers, the

photophysics of which are dominated by singlet excited states and fluorescence. The

extent of delocalizaton of the singlet exciton85,86 and its migration ability70'87 are now

fairly well understood. The triplet excited state on the other hand, being elusive and not

directly active in all-organic conjugated polymers, has been less studied and is therefore

less understood. Our group has a special interest in platinum acetylide polymers and

oligomers. The introduction of the platinum center in the conjugated backbone increases









the ISC yield by spin-orbit coupling due to the heavy atom effect induced by platinum.

The photophysics of these platinum oligomers and polymers are therefore dominated by

the triplet excited state and intense phosphorescence emission. These systems provide

access to the triplet excited state and allow its study through standard photophysical

techniques. Using a series of platinum acetylide oligomers of increasing length, our group

has successfully determined the extent of delocalization of the triplet excited state.67

While the singlet excited state is sensitive to changes in conjugation length, the triplet

excited state proved to be rather insensitive to the changes in conjugation length,

implying that it is rather localized. The study points out to a triplet excited state localized

between two platinum centers with some electronic density on the "outer" ligands as

well.

Since the triplet excited state is believed to be much more localized than the

singlet excited state, we wondered whether it would also be sensitive to the presence of

low-energy traps in the conjugated backbone. For this we prepared the series of platinum

acetylide oligomer Pt4Tn containing oligothienyl units in the center. Oligothiophenes are

known to have lower triplet energies than benzene.88'89 It is therefore anticipated that the

oligothiophenes would act as low-energy traps. Thus their influence on the platinum

acetylide photophysics could be observed and compared to a "trap-free" oligomer Pt4.

The structures of the oligomers are shown in Figure 2-1 below. Each oligomer contains

four platinum centers spaced out by phenyleneethynylenes linkages. Pt4 is the "trap-free"

reference oligomer and Pt4T1 to Pt4T3 are the oligomers with an oligothienyl unit

energy trap of decreasing singlet and triplet energy. In the present study, we will first

briefly look at the synthetic pathway to the oligomer series. We will then examine their









/ PBu, PBu3 z PBu3 / BU3
P- t Pt Pt t
PBuI PBu3 PB'u PBu3
Pt4




P, BU3 P- BU3 S PBU3 P= PBU3
PBu P3Bu3 PBu3 PBU3
Pt4T1



P- ?Bu P Bu- $ S PBu .3 F PBu3 y
PBu3 PBu3 S PBu3 PBu3

Pt4T2




-, P, u PBu .S S S P, Buu- / P ,PB3
Pt '1- PU 1 -P,
P '* F-fI L.'' -Pi "
PBu3 PBu PBu3 PBu7
Pt4T3

Figure 2-1. The structures of platinum acetylide oligomers Pt4 and Pt4Tn (n = 1-3).

photophysical properties measured by steady-state and time-resolved techniques. Finally

a discussion will provide some explanations for the results observed and what has been

learned on the dynamics of triplet exciton in these metal-organic conjugated systems.

Synthesis

The synthetic strategy chosen for this series of oligomers is convergent, which

allows the same platinum acetylide complex intermediate to be used for the synthesis of

all four oligomers. The synthesis of the different platinum acetylide complexes 5a-d is

shown in Figure 2-2 below. It started with the iodination of oligothiophenes with N-

iodosuccimide, in the presence of acetic acid and a co-solvent. The diiodo-

oligothiophenes 2b-d were then reacted with 2.1 equivalents of trimethylsilylacetylene

under Sonogashira coupling conditions90 to give the protected diacetylene-










oligothiophenes 3b-d in good yields. After deprotection of the acetylenes under basic

conditions, the diacetylene-oligothiophene 4b-d are reacted with 3 equivalents of cis-

dichloro-bis-(tri-n-butylphosphine)platinum(II) in the presence of diethylamine to give

the four different platinum acetylide complexes 5a-d in moderate to good yields.

i ii
Ar I--Ar--1 TMS Ar TMS
64-75% 70-81%
2b-d 3b-d
iii 70-80%

a: Ar= benzene
b: Ar = 2,5-thiophene _PBu, F Bu3 iv
c: Ar = 5.5'-2,2'-bithiophene CI-Pit Ar it-CI -- 40-8% _- Ar
d: Ar = 5,5"-[2,2':5',2"]-terthiophene PBu3 PBu3
5a-d 4a-d

i. NIS, CH3CO2H/co-solvent, RT; ii. Trimeth> Isily)accty lenc. Pd/Cul (cat.), THF/i-PrN H. heat;
iii. base, THF/MeOH, RT; iv. cis-Pt(PBu3)2C12, Et-NH, heat.

Figure 2-2. Synthesis of platinum acetylide complexes 5a-d.

The synthesis of the common platinum acetylide complex intermediate 11 is shown

in Figure 2-3 below. This was achieved by subjecting 1,4-diiodobenzene to a one-pot

double Sonogashira coupling reaction with tri-iso-propylsilylacetylene and propargyl

alcohol to give the unsymmetrically protected bis-acetylenebenzene 7. Selective

deprotection of one acetylene function with manganese oxide and potassium hydroxide

gave the unsymmetrically protected bis-acetylene compound 8. In parallel,

phenylacetylene was reacted with cis-dichloro-bis-(tri-n-butylphosphine)platinum(II) to

give platinum complex 9 in excellent yield. This latter was then reacted with compound

8, affording the platinum complex intermediate 10 in very good yield, which after

deprotection with TBAF, gave the desired platinum acetylide complex intermediate 11 in

good yield.










I I R -TIPS

6 7: R = CH2OH
ii 86%




iii P BU3 iv PBU- /
% PIt- t R
PBu3 PBu3
9 10: R=TIPS
v 71%
l11: R =H


i. 1) tri-iso-propylsilylacetylene, 2) propargyl alcohol, Pd/Cul (cat.), THF/i-PrN H. heat; ii. MnO2,
KOH, MeOH, RT; iii. cis-Pt(PBu3)2Cl2, EtNH. heat; iv. 8, EtNH. heat; v. TBAF, THF, RT.

Figure 2-3. Synthesis of platinum acetylide complex intermediate 11.

The synthesis of the oligomers was finally achieved by reacting 2 equivalents of the

platinum complex 11 and one equivalent of the required platinum acetylide complex 5a-d

in diethylamine under mild reflux. The oligomers were obtained in moderate to good

yields. The reaction is illustrated in Figure 2-4 below.

The oligomers were characterized by 1H and 31P NMR, MALDI-DIOS and

elemental analysis. The 1H NMR allows the identification of the oligothiophene unit in

each oligomer, their protons appearing downfield compared to the phenylene protons.

The 31P NMR shows two peaks, due to the two different magnetic environments of the

phosphorus atoms in the oligomers. The value of the Jpt-p coupling constant (2340-2360

Hz) indicates that all platinum centers have a trans geometry in all oligomers.










/. P Bu' /= PFBu3 FPBU3
P-- t +CI- It Ar t -CI
PBu, PBu3 PBu3
10 5a-d
Et2NH, heat 40-80%



SBU3 PBu, PBu- PIBU
Ot I \----- Pt Ar -- P -t P, I
I I
PBu3 PBu3 PBu3 PBu3


Pt4: Ar = benzene
Pt4T1: Ar = 2,5-thiophene
Pt4T2: Ar = 5,5'-2,2'-bilhrophene
Pt4T3: Ar = 5,5"-[2.2'5',2']-terthiophene

Figure 2-4. Synthesis of oligomers Pt4 and Pt4Tn (n = 1-3).

Results

UV-Vis Absorption

The absorption spectra of the oligomers were recorded in THF and the results are

presented in Figure 2-5 below. Pt4 displays a strong absorption band at Xmax = 352 nm

and some weaker bands at higher energy. The strong band is believed to arise from the

long-axis polarized 71,71* transition, whereas the weaker low-energy bands arise from

short-axis polarized transitions.67'91 These transitions are commonly agreed to be mostly

ligand-based and have only little metal-based character. The influence of the low energy

thiophene in Pt4T1 can be observed from the broadening of the main absorption band in

this oligomer. The effect is more pronounced in Pt4T2 and Pt4T3 which both exhibit an

extra thienyl-based band at max = 410 nm and max = 440 nm in addition to the phenyl-

based absorption. The red-shifting of the thienyl-based absorption is consistent with the

idea that the increasing size and conjugation length of the oligothienyl leads to greater

stabilization of the singlet excited state. From these spectra, it appears that while the

phenyl-based and thienyl-based chromophores are too close in energy to be give two







40



250 300 350 400 450 500
10
a
0.8
0.6
0.4
0.2

S10 b
0o b
0.8


0 0.4
-1 0.2

"3 1.0
N 0.8
M 0.6
E
E 0.4
O
Z 0.2
1.0
0.8
0.6
0.4
0.2

250 300 350 400 450 500

Wavelength / nm

Figure 2-5. Absorption spectra of oligomers in THF. (a) Pt4; (b) Pt4T1; (c) Pt4T2; (d)
Pt4T3.

distinct states in Pt4T1, the corresponding larger difference in energy in Pt4T2 and

Pt4T3 allows the identification of two distinctly localized phenyl-based and oligothienyl-

based excited states.

However, the thienyl singlet exciton is not experiencing its optimum delocalization

and stabilization in these oligomers. In a series of thienyl-containing platinum acetylide

polymers, Chawdhury et al63 reported absorption bands at lower energy (max = 457 nm

and Xmax = 469 nm for the bithienyl and terthienyl-containing platinum polymers,

respectively) than the thienyl-based absorption here. This is probably due to the









confinement of the singlet exciton and to the presence of the higher-energy phenyl

chromophores. Interestingly, the corresponding thienyl-containing platinum acetylide

monomers end-capped with phenylacetylene were found to have absorption bands at Xmax

= 406 nm and Xmax = 433 nm, very similar in energy to the thienyl-based absorption in

Pt4T2 and Pt4T3. This would imply that the singlet confinement is not the largest

contribution to the difference in stability between the Pt4Tn oligomers series and the

thienyl-containing platinum acetylide polymers.

Steady-State Photoluminescence

The photoluminescence of the oligomers was recorded in deoxygenated THF

solutions with an excitation wavelength X = 352 nm for all oligomers and the spectra are

shown in Figure 2-6. The excitation wavelength was chosen on the blue edge of the

absorption band of Pt4 to minimize direct excitation of the thienyl units in the Pt4Tn

oligomers. The dotted line across the spectra identifies the position of the main emission

band observed in Pt4. This band is centered at X = 520 nm and it arises from the

relaxation of a well-studied triplet excited state.45'66'67 The delocalization of the triplet

exciton is believed to be 1-2 repeat units, based on a previous study.67 Very little

fluorescence is seen in the spectrum, implying that intersystem crossing is very efficient

in Pt4. The emission spectrum of Pt4T1 appears very different from that of Pt4. The

spectrum is dominated by a broad band with a maximum at X = 604 nm, while the

phosphorescence band at X =520 nm observed in Pt4 is relatively weak. This broad band

is believed to be originating from a thienyl-based triplet excited state. This assignment is

made by comparison with the broad phosphorescence observed around X = 604 nm in a

platinum acetylide polymer with a thiophene unit as a spacer.63 Some fluorescence is also











400 500 600 700 800
1.0
0.8
0.6
0.4
0.2

1.0 b
> 0.8
N 0.6
*- 0.4/ \
E0.2

0
Z- 1.0 r 7
-~ 0.8
N 0.6
S0.4
E 0.2

Z 1.0 d
0.8
0.6
0.4
0.2

400 500" 600 700 800

Wavelength / nm

Figure 2-6. Photoluminescence spectra of oligomers in deoxygenated THF with an
excitation X = 352 nm. (a) Pt4; (b) Pt4T1; (c) Pt4T2; (d) Pt4T3. Dotted line
indicates the position of main emission band in Pt4. Solid and dashed lines
are for deoxygenated and air-saturated solutions, respectively.

seen in this spectrum (X = 415 nm) and it is believed to be originating from the thienyl-

localized singlet excited state, as it is not phenyl-based67 and it corresponds to a slightly

less stable singlet excited state than in the thiophene-containing platinum acetylide

polymer reported by Kohler and Beljonne48 (X = 435 nm). This is consistent with the idea

that the singlet excited state in these oligomers will experience a destabilizing

confinement effect not found in polymers and thus will be found at higher energy than in

the corresponding polymers. The observed maximum delocalization of the singlet excited









state in platinum acetylide oligomers was found to be about six repeat units by Liu et al.67

In the present oligomers series, there are only four repeat units, so that the singlet excited

state is confined and experiencing a destabilization compared to the corresponding

polymers.

The emission spectra of Pt4T2 and Pt4T3 are similar. The emission is dominated

by strong fluorescence (X = 457 nm and X = 491 nm for Pt4T2 and Pt4T3, respectively)

arising from a thienyl-based singlet excited state. The fluorescence peak shifts to lower

energies from Pt4T1 to Pt4T3, consistent with the bathochromic shift of the thienyl-

based absorption band in the absorption spectra. The singlet excited state energies of

Pt4T2 and Pt4T3 are again found at higher energies than those found in platinum

acetylide polymers with the same oligothienyl unit spacing units.48 Some phenyl-

localized phosphorescence is also observed buried under the fluorescence emission. The

phenyl-based phosphorescence can be identified by comparison with the air-saturated

emission spectrum from which it is not observed. In addition, Pt4T2 displays a very

weak band around X = 728 nm which arises from the bithienyl-based triplet excited state.

This is consistent with the phosphorescence observed around X = 746 nm for a platinum

acetylide polymer with a bithiophene spacer.48'63 No terthienyl-based phosphorescence

emission was observed at room temperature in Pt4T3, even in the near-IR up to 1600 nm.

It is interesting to note that Chawdhury et al. 63 did not observed phosphorescence from

the terthiophene-containing platinum acetylide polymer at room temperature. They did

however observe this phosphorescence at X = 816 nm when photoluminescence was

measured at T = 10 K.









The overall quantum yields of photoluminescence (fluorescence and

phosphorescence) were measured for Pt4 (( = 6.8%), Pt4T1 (( = 1.8%) and Pt4T3 (( =

0.9%). These values are an important characteristic of this series and will be discussed

further in the discussion. But it is important to notice the decreasing trend of the quantum

yield of photoluminescence as the triplet exciton energy decreases. This trend is in fact

expected as it is a prediction of the energy gap law.64

The effect of the excitation wavelength was studied in order to determine if direct

excitation of the oligothienyl moieties contributes to their observed luminescence (Figure

2-7). We therefore examined the photoluminescence spectrum in THF solutions of each

oligomer at two excitation wavelengths: one is the phenylene-based chromophore

absorption wavelength (X = 352 nm) and the other is the oligothienyl-based chromophore

absorption wavelength (X = 369, 409 and 438 nm for Pt4T1, Pt4T2 and Pt4T3,

respectively). The dependence of the photoluminescence on the excitation wavelength is

not dramatic but significant nonetheless. When exciting the oligothienyl units directly,

the phosphorescence observed at X = 518 nm decreases (in Pt4T1) or disappears (in

Pt4T2 and Pt4T3). It is not entirely unexpected to observe phenyl-based

phosphorescence in Pt4T1 even when exciting at X = 369 nm since the chromophores are

not resolved in the absorption spectrum due to the proximity of their energy levels. The

absorption spectrum of Pt4 shows that absorption is possible even at X = 369 nm (see

Figure 2-5) given that ,367 160,000 M^.cm .67 More importantly, the phosphorescence

arising from the oligothienyl group in Pt4T1 (X = 604 nm) and Pt4T2 (X = 723 nm) does

not increase significantly upon direct excitation of the thienyl-based chromophore. While

it could be argued that the same amount of thienyl chromophores could be directly











400


2.5e+5

2.0e+5

1.5e+5

1.0e+5

5.0e+4

0.0

2.5e+4

2.0e+4

1.5e+4

1.0e+4

5.0e+3

0.0


4e+4


3e+4


2e+4


1e+4


0


700 800


400 500 600 700 800

Wavelength / nm


Figure 2-7. Photoluminescence spectra of oligomers Pt4Tn in deoxygenated THF. (a)
Pt4T1, Xex = 352 nm (-), Xx = 369 nm (- -); (b) Pt4T2 X, = 352 nm (-),
X, = 369 nm (- -); (c) Pt4T3 X, = 352 nm (-), Xex = 369 nm (- -).

excited at X = 352 nm and X = 369 nm in Pt4T1, it does not hold for Pt4T2, where the

absorption bands of both chromophores are resolved in the absorption spectrum. This

implies that the phosphorescence of the bithienyl units is limited by other factors. One

possible reason is that the nonradiative decay is much faster than the radiative decay of









the bithienyl-based triplet excited state and inherently limits the luminescence. Another

reason could be that an equilibrium exists between the phenyl-based and the bithienyl-

based excited state. In this case, no matter how many more bithienyl excited states are

produced by direct excitation, the equilibrium will effectively balance the excitation on

both chromophores.

In order to clarify the exact origin of the bands observed in the emission spectra

of the oligomers, we now turn to the excitation spectra of the Pt4Tn series measured in

THF solutions (Figure 2-8). The oligothienyl fluorescence bands (X = 415, 457 and 491

nm for Pt4T1, Pt4T2 and Pt4T3, respectively), the phenyl-based phosphorescence (X

=520 nm) and oligothienyl phosphorescence bands (X = 604 and 723 nm for Pt4T1 and

Pt4T2, respectively) are monitored while scanning for excitation and the results are very

revealing. It is not surprising to see some of the phenyl-based absorption in the

fluorescence of the oligothienyl since the oligothienyl-based singlet excited state is lower

in energy than the phenyl-based and excitation transfer is therefore downhill. It is

however striking that some of the thienyl-based absorption is seen in the phenyl-based

phosphorescence, as this is uphill and therefore not energetically favorable. This supports

the idea of an equilibrium between the excited states located on the phenylene and the

thienyl, whereby a thienyl-based excited state can transfer some of the excitation back to

the phenyl-based chromophore. The dynamics of the excited state in these oligomers is

therefore more complex than it may have appeared until now and this will be discussed

further in the discussion.







47



250 300 350 400 450 500


5e+5
/ \
4e+5

3e+5 I

2e+5 /

le+5

0
b
) 5e+5

-4e+5

U) 3e+5
a /
S2e+5

Sle+5 /



6e+4 C


4e+4


2e+4



0
250 300 350 400 450 500

Wavelength / nm

Figure 2-8. Excitation spectra of oligomers Pt4Tn in deoxygenated THF. (a) Pt4T1
excitation for kem = 425 nm (-), m = 520 nm (- -) and em = 604 nm (- -
); (b) Pt4T2 excitation for em = 457 nm (-), em = 520 nm (- -) and em =
723 nm (- ** -); (c) Pt4T3 excitation for em = 500 nm (-), em = 520 nm (-
-).

If an equilibrium is indeed present between the phenyl and thienyl-based excited

state, it may be possible to influence it by changing the temperature. Moreover, lowering

the temperature may reveal the terthienyl phosphorescence that has been elusive so far.









Therefore, the photoluminescence spectrum of Pt4T1 was recorded in deoxygenated

MeTHF at low temperature (Figure 2-9). As the temperature decreases, the phenyl-based

emission intensity increases relative to the thienyl-based emission. The change levels off

at 220 K and only an overall intensity change is observed at lower temperatures. This is

consistent with the previously proposed equilibrium concept between the phenyl and

thienyl-based triplet excited state. The thienyl-based triplet excited being lower in energy



2e+5 -



2e+5












0
le+5l



e+4 .




400 450 500 550 600 650 700 750 800

Wavelength / nm

Figure 2-9. Low-temperature photoluminescence spectrum of Pt4T1 in deoxygenated
MeTHF with an excitation X = 352 nm. Direction of arrow indicates effect of
decreasing temperature from 300 K to 220 K. Region between 730 and 750
nm is removed because a strong scattering peak appears at this wavelength.

than the phenyl-based, the energy transfer to the phenyl-based is energetically not

favorable and is slowed down at lower temperature. However, this could also be due to

an activation energy associated with the energy transfer from the phenyl to the thienyl-

based triplet excited state.









Photoluminescence of Pt4T2 and Pt4T3 (Figure 2-10) were recorded under similar

conditions and the emission at 90 K and 300 K are shown only. The effect of temperature

appears to be the same as the one observed in Pt4T1. Compared to the emission spectrum

at room temperature, the phenyl-based phosphorescence is stronger and dominates the

emission in Pt4T2 and Pt4T3 at T = 90 K. In fact, the phenyl-based phosphorescence

band at X = 520 nm in Pt4T3 is almost identical to the phosphorescence of Pt4 at room

temperature (see Figure 2-6). No terthienyl-based phosphorescence was detected at T =

90 K in Pt4T3, even in the near-IR region up to 1600 cm (not shown).

Transient Absorption

In order to better understand the nature of the excited state present following the

excitation of the oligomers, their transient absorption spectrum was recorded in

deoxygenated THF (Figure 2-11). All oligomers feature a strong triplet-triplet (Ti-Tn)

absorption band between 600 and 700 nm, and the lifetimes extracted from the decay of

this band are presented in Table 2-1. The lifetime of the triplet excited state present is

longest for Pt4 (17.3 ts) and decreases from Pt4T1 (9.4 ts) to Pt4T2 and Pt4T3 (4.4 and

4.6 as, respectively), which have a similar lifetime. All oligomers also feature a bleaching

band between 360 and 440 nm, where the singlet ground state absorbs. The red-shift and

the narrowing of the T1-Tn band from Pt4T1 to Pt4T3 is a clear indication of a thienyl-

based triplet excited state. This is particularly significant for Pt4T3, where no terthienyl-

based phosphorescence was observed in the steady-state photoluminescence study. The

red-shift of the triplet-triplet absorption band is consistent with a more stable triplet

exciton as the size of the oligothienyl increases. The narrowing of the band implies that

the triplet exciton is also better defined as the size of the oligothienyl increases.










400 500 600 700 800

a
1.5e+5



1.0e+5



S5-0e+4
S/ \














5.0e+4 y
S0.0
r b
T 2.0Pe+5e-


1.5e+5


1.0e+5


5.0e+4 /\


0.0 -
400 500 600 700 800

Wavelength / nm

Figure 2-10. Low-temperature photoluminescence spectrum in deoxygenated MeTHF
with an excitation K = 352 nm at T = 90 K (-) and T = 300 K (- -). (a)
Pt4T2; (b) Pt4T3.

Time-Resolved Photoluminescence

In order to gain insight into the dynamics and decays of the triplet excited states,

time-resolved emission measurements were carried out in deoxygenated THF on the three

oligothienyl-containing oligomers (Figure 2-12). The lifetimes extracted from these

measurements, as well as other relevant data for the platinum acetylide oligomers are

presented in Table 2-1. The lifetimes are on the order of microseconds in support of our











400 500 600 700 400 500 600 700
0.3 0.10
0.2 a b
0.2

-0.30 --- -- --- 0.05
c0.1
0.0 -0.00
-0.1 -0.05
S-0.2 C
-0.3 -0.10

d 0.15 d 0.03
,e 0.10 \ /
< 0.05 0.02
Fgr0.00 --1 ---o-------- ---- "*^--- yf \ 0.01
-0.05 0.00
-0.10,
-0.15 -001
-0.20 -0.02
400 500 600 700 400 500 600 700

Wavelength / nm

Figure 2-11. Transient absorption spectra of oligomers in deoxygenated THF following
355 nm excitation. (a) Pt4, 4 [ts delay; (b) Pt4T1, 1.6 j[s; (c) Pt4T2, 1.6 [ts
delay; (d) Pt4T3, 1.6 [ts delay.

assignment as emission from a triplet excited state for the emission bands at X = 606 nm

and X = 723 nm for Pt4T1 and Pt4T2, respectively. It also supports the assignment as

fluorescence for the emission bands observed between X = 415 and 491 nm in Pt4T1 to

Pt4T3. These bands decayed within 200 ns and are typical of a singlet excited state

lifetime. Since any prompt fluorescence has been gated out in these experiments, the

phenyl-based phosphorescence is now clearly identified in Pt4T2 and Pt4T3. However

once again, no phosphorescence from the terthienyl was detected by time-resolved

measurements. All emission decays were found to give better fit with a bi-exponential

function than with a mono-exponential function. Therefore, the lifetimes extracted from

this sets of measurements shown in Table 2-1 give the lifetime and the corresponding

contribution of each exponential decay.












8000
7000
6000
5000
4000
3000
2000
1000


2500

2000

1500

1000

500


7000
6000
5000
4000
3000
2000
1000


450 500 550 600 650 700 750


450 500 550 600 650 700 750

Wavelength / nm


Figure 2-12. Time-resolved photoluminescence spectra of oligomers Pt4Tn in
deoxygenated THF following 355 nm excitation. (a) Pt4T1, camera delay 0.4
[as, delay increment 1.3 [as; (b) Pt4T2, camera delay 0.5 [as, delay increment
2.0 [as; (c) Pt4T3, camera delay 0.4 [as, delay increment 2.5 [as.

The presence of the oligothienyl units is reflected in the lower lifetimes for the


phenylene emission in all Pt4Tn oligomers compared to Pt4. The oligothienyl units act as

energy acceptors and provide an alternate relaxation pathway for the phenyl-based triplet

excited state, resulting in a shorter phenyl-based triplet excited state lifetime in all Pt4Tn









oligomers. For the phenylene emission decay, it is difficult to make a prediction on what

should be observed. If energy transfer takes place through an exchange mechanism,

which depends on the orbital spectral overlap between donor and acceptor, the rate of

energy transfer and thus the phenylene emission lifetime should not change across the

Pt4Tn oligomers series. Indeed, the orbital overlap between the donor (phenyl-based

triplet excited state) and the acceptor (oligothienyl-based triplet state) should not change

across the Pt4Tn oligomers series, as the same orbitals are most likely involved in all

cases (37, with some metal character). However, other unidentified effects may also

have an influence on the energy transfer rate. As can be seen from Table 2-1, while the

phenylene lifetime drops from Pt4 to Pt4T1, it is slightly longer for Pt4T2 and about the

same for Pt4T3. For the thienyl emission decay, for which one would expect the lifetime

to get shorter as the thienyl-based triplet excited state energy level decreases, the

lifetimes for Pt4T2 and Pt4T3 seem to follow the prediction based on the energy gap

law. However it is difficult to interpret this as an energy gap law effect since only two

decay lifetimes are available. These results only seem to indicate that the dynamics of the

triplet exciton are not well-behaved and do not involve only forward energy transfer from

a phenyl-based to a thienyl-based triplet excited state.



Discussion

Energy of the Triplet Excited State in Pt4T3.

It is possible to estimate the energy of the terthienyl-based triplet excited state

present in Pt4T3. As discussed in the first chapter, the singlet-triplet splitting energy is

constant in a family of platinum acetylide polymers, regardless of the energy of the

singlet excited state. In monomers and oligomers, the S1-T1 gap is larger than for the









Table 2-1. Photophysical data for oligomers Pt4 and Pt4Tn.

UV-vis PL f__
,max / nm ,max / nm /% TAg / ts
Fd pe


PLh / tr
Pi Tn9


Pt4 366 389 517 6.8 17.3 18.6 b

Pt4T1 369 415 516, 606 1.8 9.4 0.8 (64%) 1.1 (43%)
8.4 (36%) 7.4 (57%)
Pt4T2 354, 409 457 515, 723 a 4.4 2.7 (51%) 1.6 (51%)
11.8(49%) 4.7 (49%)
Pt4T3 353,438 491 511 0.9 4.6 1.4(56%) b
12.7 (44%)

a Not measured; b Not applicable; C From ref.67; d F: fluorescence; e P: phosphorescence; f
Total quantum yield of photoluminescence (F+P); g Transient absorption decay lifetime; h
Photoluminescence decay lifetime (number in parentheses indicates relative amplitude of
component); i P: decay at 517 nm;J Tn: decay at 606 nm for Pt4T1, decay at 723 nm for
Pt4T2.


corresponding polymer. This difference has been attributed to the fact that the triplet

exciton is more localized than the singlet exciton and is therefore less sensitive to the

confinement effect experienced by the singlet exciton in monomers and oligomers.

Polymers with either thiophene, bithiophene or terthiophene as the spacer in platinum

acetylide polymers have been studied by Kohler et a.45 and a constant S1-T1 energy gap

of 0.7 0.1 eV was found for these three polymers. It is therefore expected that the S1-T1

gap should be close but higher than this in our oligomers series.

From the steady-state emission fluorescence and phosphorescence of the mono-

and bithienyl chromophores in Pt4T1 and Pt4T2, the Si-T1 energy gap can be calculated:

Pt4T1: ES-T = Es- ET = 0.94 eV

Pt4T2: ES-T = Es- ET = 1.00 eV









As can be seen, these values of S1-T1 are very close. If we assume a value of 1.0 + 0.1 eV

for the series of Pt4Tn oligomers, the energy of the terthienyl-based triplet excited state

should be:

Pt4T3: ET = Es E-T = 1.52 eV

This value is similar to the S-T splitting energy found in a platinum acetylide polymer

based on the terthienyl spacer.64 This means that a terthienyl-based phosphorescence

would appear at X = 816 nm and that it should have been detected in our measurements if

this state was sufficiently emissive. In fact this emission has been detected by Chawdhury

et al. at a temperature T =10 K at the exact same position X = 816 nm in a terthiophene-

containing platinum acetylide polymer.63

The Absence of Phosphorescence in Pt4T3

Now that the energy of the terthienyl-based triplet excited state and the wavelength

at which it should have appeared have been determined, some possible explanations for

the absence of phosphorescence from this excited state will be provided.

While phosphorescence from the terthienyl chromophore in Pt4T3 was not

successfully detected, it is important to bear in mind that such a state exists in this

molecule. Evidence for this lies in its transient absorption spectrum which clearly

identifies a triplet excited state fitting the trend started in Pt4T1 and Pt4T2. The red-

shifting and the narrowing of the T1-Tn band is consistent with a terthienyl-based triplet

excited state. As seen in Chapter 1, radiative decays are always in competition with

nonradiative decay processes and the observation of light emission only means that the

radiative rate is fast enough to compete with the nonradiative decay rate. In the case of

Pt4T3, the absence of terthienyl-based phosphorescence only means that for this excited

state, the radiative rate is too slow and that the relaxation of the terthienyl-based triplet









excited state occurs only through nonradiative channels. One reason for that is that as we

move from Pt4T1 to Pt4T3, the organic content of the oligothienyl chromophore

increases and the metal content decreases. Therefore the spin-orbit coupling must be less

efficient so ISC should be slower. The consequences are a slower rate of radiative decay

for the T1-So transition and a lower quantum yield of phosphorescence. Cryogenic

temperatures in a frozen solvent matrix are often used to detect phosphorescence too

weak at room temperature. However, the photoluminescence spectrum of Pt4T3 recorded

under these conditions (although T = 70 K was the lowest temperature where the

measurement was carried) did not reveal the terthienyl-based phosphorescence. Looking

at the low-temperature photoluminescence of Pt4T1 at low-temperature, it appears that

lowering the temperature favors the phenyl-based emission more than the thienyl-based.

Therefore, the same effect can be expected in Pt4T3 and it should not be surprising that

the low-temperature emission spectrum did not display any terthienyl-based

phosphorescence.

While a slower ISC is probably partly responsible for the absence of

phosphorescence from the terthienyl-based triplet excited state, there is almost certainly

another factor playing a part. As already discussed in Chapter 1, the energy gap law has

been shown to apply to platinum acetylide polymers and monomers.64 It had previously

been shown to apply to metal complexes7'92. Simply stated, the energy gap law predicts

that as the energy of the triplet excited state decreases, the rate of nonradiative decay

increases exponentially. In fact, the calculated radiative and nonradiative rates for the

terthiophene platinum acetylide polymer64 were found to be kr = 3 x 103 s-1 and knr = 4.2

x 106 s-1 at 300 K. At 20 K, they were kr = 0.4 x 103 s-1 and knr = 6 x 105 s- These values









mean that at room temperature, the rate of radiative decay is too small to compete with

nonradiative decay for a terthienyl-based triplet excited state. The authors concluded

from their study that rates of nonradiative decay are only small enough for materials with

a triplet energy level of 2.4 eV and more, which is well above our estimate of the energy

level of terthienyl-based triplet excited state (1.52 eV).

Excited State Dynamics

So far, it has been shown that the presence of a low-energy trap can have a

dramatic effect on the photophysical properties of platinum acetylide oligomers.

Introduction of a thiophene unit results in a bathochromic shift of 90 nm with lower

quantum yield of photoluminescence. Introduction of even lower energy trap such as

bithiophene and terthiophene results in an emission spectrum dominated by fluorescence

from the trap and the long-lived phosphorescence is lost. This could potentially be very

problematic for optical applications relying on the triplet excited state of this type of
53 69
compound.5369

It is therefore critical to understand the dynamics of the triplet excited state in these

systems and the interplay of low-energy sites. The photophysical study of this series of

oligomers with energy traps of decreasing energy has shown that the dynamics of the

triplet exciton in these systems is rather complicated. There appears to be more than just a

simple energy migration to the low-energy traps and subsequent decay from those traps.

Several experiments seem to indicate the presence of an equilibrium between the

phenylene and thienyl-localized excited states. Evidence for the equilibrium lies in the

excitation spectra of the Pt4Tn oligomers which shows that the phenylene emission is

arising from excited states localized both on the phenylene and on the oligothienyl

moieties, with equal contributions. This can be explained by an equilibrium between the









phenyl-based and thienyl-based excited state. Also, the photoluminescence collected

upon direct excitation of the oligothienyl fragment did not show a significant increase in

thienyl luminescence, either from the singlet or the triplet excited state localized on the

oligothiophenes. This is also consistent with an equilibrium effectively balancing the

proportion of excited states and their respective luminescence, which is shown to be quite

independent of the location of the initially formed excited state. The low-temperature

photoluminescence of Pt4T1 revealed that the energy transfer is slowed down at lower

temperature and relatively more phosphorescence from the phenyl-based triplet excited

state is observed. This is also consistent with an equilibrium where the least energetically

favorable process (back energy transfer to higher excited state is endothermic) is slowed

down at lower temperature. However, this could also be explained by a simple thermally-

activated energy transfer from the phenyl-based to the thienyl-based triplet excited state.

The lifetime data extracted from the emission decays are not easily interpreted and

rationalized. While the thienyl-based phosphorescence seems to be in agreement with the

expected decreasing trend with the decrease in triplet energy level, the phenylene-based

phosphorescence does not really follow any trend. Although this does not support the

presence of an equilibrium, it does not disprove it either.

To illustrate this equilibrium, the photophysical processes involved in the

oligomers series is presented in Figure 2-13 below. The equilibrium would be brought in

by relatively efficient processes 11 or 9, in addition to the forward processes 10 and 8.

The experiments carried out in this study do not allow distinguishing whether the forward

energy transfer takes place between the singlets (process 8) or the triplets (process 10).

In fact both the singlet and the triplet energy transfer could be operating in this system.






59


P Tn
S1. .-J::::: ::
STI
%6% 9"- S1
Ti
'';,10 12
T1
2l12

1 2 11" T1
5

3 7
4 13






Figure 2-13. Energy diagram representing the photophysical processes involved in the
Pt4Tn oligomers.

For the singlets to be in equilibrium, this would require singlet energy transfer (processes

8 and 9) to be faster than ISC (processes 5 and 12). Both of these processes have been

shown to be fast93'94, therefore it is not possible to rule out the singlet or the triplet energy

transfer without exact calculations of the rate constants for the processes involved.

Although not commonly observed and studied, the presence of an excited state

equilibrium is not unprecedented. In fact such a dynamic equilibrium has been observed

recently in related metal-organic oligomers95-97. The absence of a clear trend in the

lifetimes of emission suggests that while the equilibrium might be operating in Pt4T1, it

does not seem to be the case in Pt4T2 and Pt4T3. Indeed, it appears that in Pt4T1, the

phenyl-based and the thienyl-based triplet excited states decay at similar rates (7-8 ps),

close to the lifetime extracted from transient absorption (9 ps). However, in Pt4T2, the

phenyl-based and the bithienyl-based triplet excited states do not decay at the same rate.









The lifetime of the phenyl-based triplet excited state increases (12 p[s), closer to the

lifetime observed in Pt4 (18 [ps), while the lifetime of the bithienyl-based triplet excited

state decreases further (5 p[s). It is however in accordance with the lifetime of triplet

excited state detected by transient absorption, supporting the idea that this latter is

localized on the bithiophene. Therefore if there are strong evidences for an excited state

equilibrium in Pt4T1, it appears to be dependent on the energy gap and does not operate

in the other oligomers of the series.

Conclusion

In this chapter, a series of platinum acetylide oligomers has been synthesized and

their photophyisical properties studied. The oligomers incorporated oligothiophene units

in the main chain to act as energy traps. The increasing length the oligothiophenes

provided a series of oligomers with decreasing energy trap.

The photophysics of these oligomers have shown that the presence of an energy

trap can have dramatic consequence on the photophysical properties of the oligomers.

Efficient energy transfer creates excited state on the low energy sites. From there,

radiative decay can occur and red-shifted emission can be observed. Alternatively, the

radiative decay rate may become too small to compete with nonradiative decay as the

energy of the trap decreases and the excited state created by energy transfer will relax by

nonradiative decay processes. It has been shown that the energy of the trap can determine

the dominant relaxation mechanism, due to the energy gap law. Finally, the presence of

an excited state equilibrium has been suggested to explain the photophysical data

collected on this oligomer series. However, the equilibrium seems to depend upon in the

energy gap between the excited states and may be shut down by large energy gaps

between excited states.









Experimental

Photophysical Measurements

Steady-state absorption spectra were recorded on a Varian Cary 100 dual-beam

spectrophotometer. Corrected steady-state emission measurements were conducted on a

SPEX F-112 fluorescence spectrometer. Samples were degassed by argon purging for 30

min and concentrations were adjusted to produce "optically dilute" solutions (i.e., Amax <

0.20). Low-temperature fluorescence measurements were made by cooling the samples in

a LN2 cooled Oxford Instruments DN-1704 optical cryostat connected to an Omega

CYC3200 temperature controller. Samples were degassed by 4 repeated cycles of freeze-

pump-thaw on a high vacuum line.

Photoluminescence quantum yields were determined according to the "optically

dilute" method described by Demas and Crosby, with the quantum yield being computed

according to eq. 14 in their paper.98fac-Ir(2-phenylpyridine)3 was used as an actinometer

(OF = 0.40 in THF) .

Transient absorption measurements were conducted on a home-built apparatus,

which has been described elsewhere.99 Samples were contained in a cell holding a total

volume of 10 mL and the contents were continuously circulated through the pump-probe

region of the cell. Samples were degassed by argon purging for 30 mn. Excitation was

provided by the 3rd harmonic output of a Nd:YAG laser (355 nm, Spectra Physics, GCR-

14). Typical pulse energies were 5 mJ/pulse which corresponds to an irradiance in the

pump-probe region of 20 mJ/cm. Sample concentration were adjusted so that A 0.8.

Time-resolved emission measurements were recorded on a home-built apparatus

consisting of a Quanta Ray GCR Series Nd:YAG laser as a source (X = 355nm, 10 ns

fwhm), with time-resolved detection provided by an intensified CCD detector (Princeton









Instruments, PI-MAX iCCD) coupled to an Acton SpectraPro 150 spectrograph.

Optically dilute solutions were used.

Mass Spectrometry of Pt Oligomers

The samples were analyzed using a Bruker Reflex II time-of-flight mass

spectrometer equipped with delayed extraction (Bruker Daltonics, Billerica, MA). The

analysis mode was desorption ionization on silicon (DIOS).100 The samples were

dissolved in dichloromethane at 1-10 [M and 1 iL was spotted on a DIOS plate that had

been electrochemically etched with HF under tungsten light illumination.101 Desorption

was achieved using a nitrogen laser at 337 nm.

Synthesis

General. All chemicals used for synthesis were of reagent grade and used without

purification unless noted. Reactions were carried out under an argon atmosphere with

freshly distilled solvents, unless otherwise noted. 1H, 13C and 31P NMR spectra were

recorded on a Varian Gemini 300, VXR 300 or Mercury 300 spectrometer and chemical

shifts are reported in ppm relative to TMS. cis-Dichloro-bis(tri-n-butylphosphine)-

platinum(II)102 and 1,4-diethynylbenzene67 were prepared by literature methods.

2,5-diiodothiophene (2b). Thiophene (1.0 g, 11.9 mmol) and N-iodosuccinimide

(5.48 g, 24.4 mmol) were dissolved in acetic acid (6 mL) and chloroform (8 mL). The

flask was covered with aluminum foil and the mixture was stirred at room temperature

overnight. The mixture was then washed with 10% sodium thiosulfate and water, the

organic phase was dried on MgSO4, filtered and the solvent was removed. The dark red

oil obtained was further purified by column chromatography (silica gel, hexane) giving

the desired product 2b as a yellow oil (3.0 g, 75%). 1H NMR (C6D6, 300 MHz) 6 7.23 (s,

2H); 13C NMR (C6D6, 75 MHz) 6 77.0, 139.3.









5,5'-diiodo-2,2'-bithiophene (2c). 2,2'-Bithiophene (0.5 g, 3 mmol) and N-

iodosuccinimide (1.67 g, 7.5 mmol) were dissolved in methanol (45 mL). To this

solution, acetic acid (0.5 mL) was added. After stirring for 2 hours, a precipitate had

formed and the flask was placed in freezer overnight to ensure complete precipitation of

product. The white solid was then filtered by suction filtration and washed with cold

methanol. After drying under vacuum, product 2c was obtained as a white solid (0.80 g,

64 %). H NMR (CDC13, 300 MHz) 6 6.80 (d, 2H), 7.18 (d, 2H).

5,5"-diiodo-[2,2'-5',2"]-terthiophene (2d). [2,2'-5',2"]-terthiophene (0.1 g, 0.4

mmol) and N-iodosuccinimide ( 0.199 g, 0.89 mmol) were dissolved in dichloromethane

(6 mL) and acetic acid (0.05 mL) and flask was purged with nitrogen. The mixture was

stirred in a water/ice bath for 2 hours after which time a yellow solid had precipitated.

The solid was collected by suction filtration, washed with cold methanol and dried under

vacuum giving product 2d as a yellow solid (0.15 g, 73 %). 1H NMR (C6D6, 300 MHz) 6

6.39 (d, 2H), 6.58 (s, 2H), 6.74 (d, 2H).

2,5-Bis-[(trimethylsilyl)-ethynyl]-thiophene (3b). 2,5-diiodothiophene 2b (2.0 g,

5.96 mmol) and trimethylsilylacetylene (1.22 g, 12.5 mmol) were dissolved in Et2NH (8

mL) and the solution was degassed with argon for 30 min. Then, Pd(PPh3)2C12 (0.21 g,

5% eq., 0.298 mmol) and Cul (0.114 g, 10 % eq., 0.596 mmol) were added and mixture

was stirred at room temperature for 12 hours. The mixture was then passed through a bed

of Celite, washed with 10 % ammonium hydroxide, water, the organic phase dried on

MgSO4, filtered and the solvent was removed. Chromatography (silica gel, hexane) gave

the desired product 3b as a yellow solid (1.15 g, 70 %). 1H NMR (CDC13, 300 MHz) 6









0.22 (s, 18H), 7.05 (s, 2H); 1C NMR (CDC13, 75 MHz) 6 0.01, 97.1, 100.1, 124.7, 132.5,

169.0.

5,5'-Bis-[(trimethylsilyl)-ethynyl]-2,2'-bithiophene (3c). This compound was

synthesized according to the same procedure used for 2,5-bis-[(trimethylsilyl)-ethynyl]-

thiophene 3b except 5,5'-diiodo-2,2'-bithiophene 2c (0.2 g, 0.48 mmol) was used and the

reaction was completed in 4 hours. The desired product 3c was obtained as a yellow solid

(0.129 g, 75 %). 1HNMR (CDC13, 300 MHz) 6 0.22 (s, 18H), 7.00 (d, 2H), 7.10 (d, 2H);

13C NMR (CDCl3, 75 MHz) 60.1, 97.4, 100.7, 122.7, 123.9, 133.6, 138.2.

5,5"-Bis-[trimethylsilyl)ethynyl]-[2,2',5'2"]-terthiophene (3d). This compound

was synthesized according to the same procedure used for 2,5-bis-[(trimethylsilyl)-

ethynyl]-thiophene 3b except 5,5"-diiodo-[2,2'-5',2"]-terthiophene 2d (0.135 g, 0.27

mmol) was used and the reaction was stirred at 45 C for 2 hours. The desired product 3d

was obtained as a bright yellow solid (0.096 g, 81 %). 1H NMR (CDC13, 300 MHz) 6

0.24 (s, 18H), 7.01 (d, 2H), 7.08 (s, 2H), 7.14 (d, 2H).

2,5-Diethynyl-thiophene (4b). 2,5-bis-[(Trimethylsilyl)-ethynyl]-thiophene 3b

(0.392 g, 1.42 mmol) was dissolved in methanol (24 mL) and the solution was degassed

with nitrogen. To this solution, 0.1 mL of a 0.5 M KOH solution was added and the

mixture was stirred at room temperature for 2 hours. After this time, water (50 mL) was

added and the mixture was extracted with pentane, the organic phase dried on Na2SO4,

filtered, and the solvent was removed under reduced pressure at room temperature. The

desired product 4b was obtained as a colorless oil (0.15 g, 80 %). H NMR (CDC13, 300

MHz) 6 3.5 (s, 2H), 7.22 (s, 2H); 13C NMR (CDC13, 75 MHz) 6 76.4, 82.3, 123.8, 132.8.









5,5'-Diethynyl-2,2'-bithiophene (4c). 5,5'-bis-[(Trimethylsilyl)-ethynyl]-2,2'-

bithiophene 3c (30.7 mg, 0.086 mmol) was dissolved in THF (2 mL) and the solution was

degassed with nitrogen. Then, tetrabutylammonium fluoride (0.35 mL of a 1 M THF

solution, 0.35 mmol) was added via a syringe and the mixture was stirred at room

temperature protected from light for 3 hours. After this time, the solvent was removed

and chromatography (silica gel, hexane) gave the desired product 4c as a yellow solid

(14.4 mg, 78 %). 1HNMR (CDC13, 300 MHz) 6 3.4 (s, 2H), 7.04 (d, 2H), 7.19 (d, 2H);

13C NMR (CDCl3, 75 MHz) 6 82.9, 121.6, 124.1, 134.2, 138.3.

5,5"-Diethynyl-[2,2',5',5"]-terthiophene (4d). 5,5"-Bis-[trimethylsilyl)ethynyl]-

[2,2',5'2"]-terthiophene 3d (93 mg, 0.21 mmol) and K2CO3 (29 mg, 0.21 mmol) were

dissolved in MeOH (9 mL) and THF (4 mL) and solution degassed with argon. The

mixture was stirred at room temperature overnight after which time the solvent was

removed. The brown residue obtained was dissolved in CH2C12, washed with 10% HC1,

water, the organic phase dried on MgSO4, filtered and the solvent was removed.

Chromatography (silica gel, hexane) gave the desired product 4d as a yellow solid (43.5

mg, 70 %). H NMR (CDC13, 300 MHz) 6 3.42 (s, 2H), 7.02 (d, 2H), 7.10 (s, 2H), 7.20

(d, 2H).

Complex 5a. 1,4-diethynylbenzene (46.2 mg, 0.373 mmol) and cis-dichloro-bis-

(tri-n-butylphosphine)platinum(II) were dissolved in Et2NH (15 mL) and the solution was

degassed with nitrogen. The mixture was stirred under reflux for 8 hours. The solvent

was removed and the crude product purified by flash chromatography (silica gel, hexane

then 7:3 hexane/CH2Cl2) giving the desired product 5a as a yellow solid (210 mg, 40 %).

1HNMR (CDC13, 300 MHz) 6 0.85-1.0 (t, 36H), 1.40-1.63 (m, 48H), 1.9-2.0 (m, 24H),









7.05 (s, 4H); 13C NMR (CDC13, 75 MHz) 6 14.0, 22.1, 24.5, 26.3, 101.6, 125.3, 130.5;

31P NMR (CDC13, 121 MHz) 6 7.89 (Jpt-p = 2390.8 Hz).

Complex 5b. This compound was synthesized according to the same procedure

used for complex 5a, except 2,5-diethynyl-thiophene 4b (64 mg, 0.483 mmol) and cis-

dichloro-bis(tri-n-butylphosphine)platinum(II) (0.648 g, 0.967 mmol) were used. Flash

chromatography (silica gel, hexane then 7:3 hexane/CH2Cl2) gave the desired product 5b

as a dark orange oil (288 mg, 43 %). 1H NMR (CDC13, 300 MHz) 6 0.85-1.0 (t, 36H),

1.40-1.60 (m, 48H), 1.9-2.0 (m, 24H), 6.6 (s, 2H); 13C NMR (CDC13, 75 MHz) 6 13.9,

22.1, 24.4, 26.2, 53.5, 88.5, 93.9, 127.0; 31P NMR (CDC13, 121 MHz) 6 8.03 (Jpt-p =

2364.0 Hz).

Complex 5c. This compound was synthesized according to the same procedure

used for complex 5a, except 5,5'-diethynyl-2,2'-bithiophene 4c (14.4 mg, 0.067 mmol)

and cis-dichloro-bis(tri-n-butylphosphine)platinum(II) (134.1 mg, 0.2 mmol) were used.

Flash chromatography (silica gel, hexane then 9:1 hexane/CH2Cl2) gave the desired

product 5c as a yellow film-forming oil (82.8 mg, 83 %). 1H NMR (CDC13, 300 MHz) 6

0.9-1.0 (t, 36H), 1.40-1.65 (m, 48H), 1.9-2.0 (m, 24H), 6.70 (d, 2H), 6.85 (d, 2H); 13C

NMR (CDC13, 75 MHz) 6 14.0, 22.2, 24.5, 26.2, 91.5, 93.7, 122.6, 128.2, 128.4, 134.9;

31P NMR (CDC13, 121 MHz) 6 8.15 (Jpt-p = 2364.6 Hz).

Complex 5d. This compound was synthesized according to the same procedure

used for complex 5a, except 5,5" -diethynyl-[2,2',5',5"]-terthiophene 4d (43.5 mg, 0.15

mmol) and cis-dichloro-bis(tri-n-butylphosphine)platinum(II) (201.2 mg, 0.30 mmol)

were used. Flash chromatography (silica gel, hexane then 7:3 hexane/CH2Cl2) gave the

desired product 5d as a yellow film-forming oil (135 mg, 57 %). 1H NMR (CDC13, 300









MHz) 6 0.8-1.0 (t, 36H), 1.4-1.6 (m, 48H), 1.9-2.0 (m, 24H), 6.75 (d, 2H), 6.94 (d, 2H),

6.98 (s, 2H); 13C NMR (CDC13, 75 MHz) 6 14.0, 22.2, 24.5, 26.3, 92.5, 93.8, 123.2,

123.7, 128.3, 129.1, 134.1, 136.2; 31P NMR (CDC13, 121 MHz) 6 8.19 (Jpt-p = 2347.6

Hz).

3-{4-[(Triisopropylsilyl)-ethynyl]-phenyl}-prop-2-yn-l-ol (7). 1,4-

Diiodobenzene (5.0 g, 15.16 mmol) was dissolved in THF (60 mL) and i-Pr2NH (40 mL)

in a Schlenk flask and the solution was degassed with argon for 30 min. Then, tri-iso-

propylsilylacetylene (2.76 g, 15.16 mmol), Pd(PPh3)2Cl2 (0.642g, 0.9 mmol) and Cul

(0.346 g, 1.8 mmol) were added. The mixture was stirred at 70C for 3 hours, after which

time prop-2-yn-1-ol was added dropwise via a syringe. The mixture was stirred overnight

at 70C. After cooling down, the mixture was passed through a bed of Celite, washed

with 10% NH40H (3 x 50 mL) and water (3 x 50 mL), the organic phase dried on

MgSO4, filtered and the solvents were removed. Chromatography on silica (hexane first,

then 9:1 hexane/CH2Cl2) gave the desired product 7 as a red oil (1.65 g, 35 %). 1H NMR

(CDC13, 300 MHz) 6 1.20 (s, 21H), 4.40 (br, 1H), 4.58 (s, 2H), 7.40 (m, 4H); 13C NMR

(CDC13, 75 MHz) 6 11.4, 18.7, 51.1, 85.1, 89.3, 92.5, 106.7, 122.7, 123.6, 131.5, 131.9.

1-Ethynyl-4-(tri-iso-propylsilylethynyl)-benzene (8). 3-{4-[(Tri-iso-propylsilyl)-

ethynyl]-phenyl}-prop-2-yn-l-ol (1.43 g, 4.58 mmol) was dissolved in Et20 (80 mL) and

degassed with nitrogen for 15min. Then, activated MnO2 (6.37 g, 73.3 mmol) and KOH

(2.05 g, 36.6 mmol) were added in four fractions every hour and mixture was stirred at

room temperature for 4 hours protected from light. After this time, mixture was washed

with 5% HC1 (3 x 50 mL), water (3 x 50 mL), dried on MgSO4, filtered and the solvent

was removed. Chromatography (silica gel, hexane) gave the desired product 8 as a red oil









(0.99 g, 86 %). H NMR (CDCl3, 300 MHz) 6 0.98 (s, 21H), 2.98 (s, 1H), 7.22 (s, 4H);

13C NMR (CDC13, 75 MHz) 6 11.3, 18.6, 78.8, 83.2, 92.9, 106.4, 121.9, 124.0, 131.9.

Complex 9. cis-dichloro-bis(tri-n-butylphosphine)platinum(II) (0.500 g, 0.74

mmol) and phenylacetylene (76.5 mg, 0.75 mmol) were dissolved in Et2NH (20 mL) and

degassed with nitrogen for 15 min. The mixture was then stirred under gentle reflux for 8

hours after which time all phenylacetylene has been consumed. Mixture was allowed to

cool down, solvents removed and crude product was purified by flash chromatography

(silica gel, hexane) giving the desired product 9 as yellow solid (522.0 mg, 96 %). 1H

NMR (CDC13, 300 MHz) 6 0.95 (t, 18H), 1.45 (m, 12H), 1.60 (m ,12H), 2.03 (m, 12H),

7.21 (m, 5H); 13C NMR (CDC13, 75 MHz) 6 14.02, 21.94, 22.16, 22.38, 24.08, 24.43,

24.52, 24.61, 26.01, 26.29, 26.42, 26.56, 31.79, 125.27, 128.03, 128.01, 130.97, 130.99;

31P NMR (CDC13, 121 MHz) 6 7.95 (Jpt-p = 2395.2 Hz).

Complex 10. Platinum complex 9 (0.235 mg, 0.32 mmol) and 1-ethynyl-4-(tri-iso-

propylsilylethynyl)-benzene (100.0 mg, 0.35 mmol) were placed in Et2NH (6 mL) and

the solution was degassed with nitrogen for 15 min. Mixture was stirred under gentle

reflux for 8h. After cooling down, the solvents were removed and the crude product was

purified by flash chromatography (silica gel, hexane then 4:1 hexane/CH2Cl2) giving the

desired product as yellow solid (258.0 mg, 82%). 1H NMR (300 MHz, CDC13) 6 0.98 (t,

18H), 1.18 (s, 21H), 1.50 (m, 12H), 1.64 (m,12H), 2.19 (m, 12H), 7.25 (m, 9H); 13C

NMR (75 MHz, CDC13) 6 11.56, 14.01, 14.32, 15.57, 18.88, 22.86, 23.88, 24.11, 24.34,

24.51, 24.61, 24.69, 24.48, 25.48, 26.56, 31.80, 34.87, 90.67, 107.97, 119.68, 125.03,

128.03, 130.68, 130.95, 130.95, 131.82; 31P NMR (121 MHz, CDC12) 6 4.21 (Jpt-p

2395.4 Hz).









Complex 11. Platinum complex 10 (340.0 mg, 0.35 mmol) was dissolved in THF

(6 mL) and the solution was degassed with nitrogen for 15 min. Then, TBAF (0.70 mL of

a 1M solution in THF, 0.70 mmol) was added and the mixture was stirred at room

temperature protected from light for 4 hours. Then the mixture was diluted to 50 mL with

CH2C2 washed with brine (2 x 30 mL) and water (2 x 30 mL), dried on MgSO4, filtered

and the solvents removed. The crude oil was purified by flash chromatography (silica gel,

hexane then 4:1 hexane/CH2Cl2) giving the desired product as a yellow solid (240.0 mg,

83%). H NMR (CDC13, 300 MHz) 6 0.98 (t, 18H), 1.47 (m, 12H), 1.63 (m, 12H), 2.18

(12H), 7.22 (m, 9H); 13C NMR (CDC13, 75 MHz) 6 14.07, 23.90, 24.13, 24.35, 24.56,

24.65, 24.74, 26.44, 26.59, 26.74, 84.49, 107.86, 109.16, 109.34, 112.59, 112.79, 123.64,

125.03, 125.11, 128.10, 129.22, 129.98, 130.83, 131.02, 131.96; 31P NMR (CDC13, 121

MHz) 6 4.22 (JPt-P = 2357.0 Hz).

Pt4. Complex 5a (67.5 mg, 0.048 mmol) and complex 10 (0.08 g, 0.097 mmol)

were dissolved in Et2NH (9 mL) and the solution was degassed with nitrogen for 15 min.

The mixture was stirred under reflux overnight. After cooling down, solvents were

removed and crude product purified by flash chromatography (silica gel, hexane then 7:3

hexane/CH2Cl2) gave Pt4 as a yellow solid (65 mg, 46 %). 1H NMR (CDC13, 300 MHz) 6

0.95 (t, 72H), 1.47 (m, 48H), 1.65 (m, 48H), 2.15 (m ,48H), 7.20 (m, 22H); 31P NMR

(CDC13, 121 MHz) 6 3.98, 4.05 (JPt-P = 2363.2 Hz).

Pt4T1. This compound was synthesized according to the same procedure used for

Pt4, except complex 5b (87 mg, 0.062 mmol) and complex 10 (0.103 mg, 0.125 mmol)

were used. Flash chromatography (silica gel, hexane then 7:3 hexane/CH2Cl2) gave

Pt4T1 as a yellow solid (110 mg, 59 %). 1H NMR (CDC13, 300 MHz) 6 0.95 (t, 72H),









1.35 (m ,48H), 1.62 (m, 48H), 2.12 (m, 48H), 6.66 (s, 2H), 7.27 (m, 18H); 31P NMR

(CDC13, 121 MHz) 6 4.05, 4.08 (JPt- = 2365.5 Hz); Mass spec. (MALDI-TOF) calc'd for

C140H236P8Pt4S 2979.54, found 2978; Elemental anal. calc'd C 56.43, H 7.98, found C

56.58, H 8.06.

Pt4T2. This compound was synthesized according to the same procedure used for

Pt4, except complex 5c (82.8 mg, 0.056 mmol) and complex 10 (101.5 mg, 0.123 mmol)

were used. Flash chromatography (silica gel, hexane then 7:3 hexane/CH2Cl2) gave

Pt4T2 as a yellow solid (98.4 mg, 57 %). 1H NMR (CDC13, 300 MHz) 6 1.0 (m, 72H),

1.55 (m, 96H), 2.12 (m, 48H), 6.7 (d, 2H), 6.9 (d, 2H), 7.22 (m, 18H); 31P NMR (CDC13,

121 MHz) 6 4.07, 4.21 (JPt- = 2356.2 Hz); Mass spec. (MALDI-TOF) calc'd for

C144H238P8Pt4S2 3061.66, found 3058; Elemental anal. calc'd C 56.49, H 7.84, found C

56.45, H 8.10.

Pt4T3. This compound was synthesized according to the same procedure used for

Pt4, except complex 5d (102 mg, 0.065 mmol) and complex 10 (107.4 mg, 0.13 mmol)

were used. Chromatography on silica (hexane first, 7:3 hexane/CH2Cl2) gave Pt4T3 as a

red film-forming solid (106 mg, 51 %). 1H NMR (CDC13, 300 MHz) 6 0.95 (m, 72H),

1.48 (m, 48H), 1.65 (m, 48H), 2.13 (m, 48H), 6.74 (d, 2H), 6.94 (s, 2H), 6.97 (s, 2H),

7.25 (m, 18H); 31P NMR (CDC13, 121 MHz) 6 4.07, 4.25 (Jpt-p = 2362.8 Hz); Mass spec.

(MALDI-TOF) calc'd for C148H240P8P4S3 3143.79, found 3141; Elemental anal. calc'd C

56.54, H 7.69, found C 56.43, H 8.00.














CHAPTER 3
DELOCALIZATION OF CHARGE CARRIERS IN PLATINUM ACETYLIDE
OLIGOMERS

Introduction

Conjugated polymers are promising active materials for use in light-emitting diodes

(LEDs),76-78'103 field-effect transistors104'105 and photovoltaic devices.106,107 All of these

applications rely on charge carriers for charge transport.108 Much debate arose over the

last twenty years concerning the exact nature of these charge carriers, and whether they

were solitons,109 polarons110 or bipolarons.111 It is now believed that the charge carriers in

most nondegenerate conjugated polymers such as poly(thiophene), poly(p-phenylene)

and poly(p-phenylenevinylene)112 are polarons, essentially radical ions. These charge

carriers and the charge transport properties of conjugated polymers have therefore

received a lot of attention in order to determine structure property-relationships113'114 and

improve the performance of devices based on conjugated polymers.115

In organic light-emitting diodes (OLEDs), the internal electroluminescent quantum

efficiency is limited by the proportion singlet excitons formed after recombination of an

electron and a hole on the polymer chain78 (a negative and a positive polaron). Based on

spin statistics and assuming electron-hole recombination is spin-independent, the

electron-hole recombination should give 25% of singlet excitons and 75% of triplet

excitons. Since the triplet exciton is usually not emissive in organic molecules, this would

mean that the maximum efficiency of OLEDs would be 25%. However, several groups

have now independently demonstrated that the exciton formation is spin-dependent and









internal quantum efficiencies up to 63% have been reported.116-119 The reason for this is

currently under strong theoretical and experimental investigation.42'120

However, it is likely that triplet formation will always limit the efficiency of

OLEDs so other groups have proposed and successfully demonstrated different strategies,

such as using transition metal complexes.121123 The idea is that due to the heavy atoms

inducing mixing of singlet and triplet excitons, all excitons formed will luminescence

through rapid radiative decay of the triplet exciton and thus all electron-hole

recombination leads to an emissive exciton. In fact, there are examples of singlet-

harvesting platinum(II) complexes used in light-emitting devices with external efficiency

up to 11%.124-126 There is therefore clearly a technological need for a better understanding

of the dynamics of charge carriers in metal-organic systems such as platinum acetylide

and this is one of the motivations for the work presented in this chapter.

On a more fundamental level, our group also has an interest in conjugation through

metal. After having successfully demonstrated the difference in delocalization of singlet

and triplet excited states in platinum acetylide oligomers,67 it appeared that this platinum

oligomer series of increasing length would provide an interesting system to study the

effect of chain length on the delocalization of charge carriers. Moreover, the

oligothiophene-containing platinum acetylide oligomers studied in Chapter 2 would

provide a system to study the effects of low-energy sites on charge carriers in platinum

acetylide oligomers.

Therefore the conjugation and delocalization effects of charge carriers have been

explored through electrochemistry and pulse radiolysis. These techniques93'113,127,128 and

other time-of-flight techniques129-131 have been used extensively and successfully to study









charge transport in organic conjugated oligomers and polymers. Among them,

thiophene-based oligomers132,133 and polymers134,135 in particular have received

considerable attention due to their rich and promising electrochemical properties.

Conjugated metal-organic systems are less common than all-organic conjugated systems

but are gaining increasing attention due to the unique electrochemical properties of

transition metal complexes.136-139

The oligomers studied here are presented in Figure 3-1 and Figure 3-2 below. The

first series (Ptn), consists of platinum acetylide oligomers containing one to five platinum

centers in the conjugated phenyleneethynylene backbone. Their synthesis has been

described elsewhere67 and their NMR spectra were identical to the oligomers prepared

previously. The second series (PT4Tn) consists of platinum acetylide oligomers

containing four platinum centers where the central benzene ring is replaced by thiophene,

bithiophene or terthiophene. Their synthesis and characterization has been described in

Chapter 2.



PBu3 n

Figure 3-1. Structure of platinum acetylide oligomers Ptn (n = 1-5).

PPBu3 /=P PBu3 s FBu3 /=- PBu3
B Pt P-t Pt
PBu3 PBu, n PBu, PBu3

Figure 3-2. Structure of platinum acetylide oligomers Pt4Tn (n = 1-3).

Results

Electrochemistry

The oxidation and reduction of the platinum acetylide oligomers were explored

using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Measurements









were carried out in nitrogen-degassed methylene chloride solutions with TBAH (0.1 M)

as the supporting electrolyte. The reduction of all oligomers gave only irreversible waves

around -1.0 and -1.3 V and this data will not be discussed. However, reversible (or quasi-

reversible) waves were observed between +0.6 and +1.1 V for all oligomers and all the

electrochemical data is summarized in Table 3-1.

Table 3-1. Redox potentials (V vs SCE) for Ptn and Pt4Tn oligomers series in CH2C12
containing 0.1 M TBAH.a

Oligomer E1, red E1, ox E2, ox E3, ox

Ptl -1.29(b) .()

Pt2 -1.27(b) 0.89(b)

Pt3 -1.19() 0.85 (1 e-) 1.06 (1 e")

Pt4 -1.30(b) 0.81 (1 e-) 0.88 (1 e")

Pt5 -1.29(b) 0.98 (1 e-) 1.16 (1 e)

Pt4T1 -1.08(b) 0.71 (1 e") 1.09 (2 e")

Pt4T2 -1.02(b) 0.63 (1 e") 1.01 (2 e")

Pt4T3 -1.01(b) 0.64 (1 e-) 0.88 (1 e") 1.08 (2 e-)

aNumber of electrons shown between parenthesis are only estimates from the current
passed for each wave; b Irreversible wave.

The results of the CV and DPV measurements are complimentary and both

instructive in the Pt oligomer series therefore all electrochemistry spectra are presented

fro the Ptn oligomers series (Figure 3-3) but only CV voltammograms for the Pt4Tn

oligomers series (Figure 3-4).

Starting with the Ptn oligomers series, it can be seen that Ptl and Pt2 show only

one irreversible wave at +1.11 V and +0.89 V, respectively. This implies that the radical









cation formed from Ptl and Pt2 is not stable on the electrochemical timescale. In Pt3,

two reversible waves are observed at +0.85 and +1.06 V. This is evidenced in the CV

spectrum where the typical half-wave shape of a reversible electron process is present. In

Pt4, it appears that the two waves observed in Pt3 are "merged" into a single reversible

wave centered at +0.85 V, while a new quasi-reversible wave appears at higher

potentials. This merging of the two waves is clearly observed in the DPV spectrum of Pt4

where the broad peak shows a shoulder due to the second oxidation process next to the

maximum peak. The merging of these waves suggests charge localization on an

electrophore. In Pt3, each reversible wave is attributed to the formation of radical cation

centers on two sites in close proximity of each other on the oligomer. Due to this

proximity, the second radical cation formed is affected by the presence of the first center

already present. When the oligomer becomes longer, two radical cation centers can be

formed without interacting with each other. This is almost entirely possible in Pt4 where

the waves have merged because the presence of a radical cation center does not influence

the formation of a second center. In Pt5, the merging is complete and essentially one

broad band is observed in the DPV spectrum. However, close inspection of the CV

spectrum shows that the radical cation formed is not stable here again, as evidenced by

the mostly irreversible wave observed. Several cycles revealed a loss of current after each

cycle indicating that the oligomer was probably being deposited on the electrode during

the oxidation.

All in all, the electrochemistry data on the Ptn oligomer series suggests that the

radical cation electrophore is rather localized since two such sites can be created on Pt4

with almost no electronic coupling or interaction between them.








76



200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400
4 PT1 CV -1
2 Ptl DPV 2

2 -4
-4 -5
--
-7
10
4 Pt2 CV
2 Pt2DPV
0 2
2
3
4
-6

S10 Pt3CV 1 <











2
Pt3 DPV







-4




-6
-1 0
-5 -5
Pt4 CV ~ 1










Potential / Potential












Figure 3-3. Cyclic voltammetry (CV, left) and differential pulse voltammetry (DPV,
-10










right) of oligomers Ptn.

In the Pt4Tn voltammograms, it can be seen that all Pt4Tn oligomers show two
200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400

Potential/ mV Potential/ mV


Figure 3-3. Cyclic voltammetry (CV, left) and differential pulse voltammetry (DPV,
right) of oligomers Ptn.


In the Pt4Tn voltammograms, it can be seen that all Pt4Tn oligomers show two


reversible bands around +1.0 V and +0.65 V. Based on the previous Ptn electrochemistry


study and the lower oxidation potential of thiophene compared to benzene,140 the high


potential at +1.0 V is attributed to an oxidation of a phenyl-based electrophore and the






77


200 400 600 800 1000 1200 1400


200 400 600 800 1000 1200 1400
Potential / mV

Figure 3-4. Cyclic voltammetry (CV) of oligomers Pt4Tn.

wave at +0.65 V to an oxidation of a thienyl-based electrophore. But while the first

thienyl-based oxidation is attributed to a one-electron process, it appears that twice the

current is passed in the phenyl-based oxidation in Pt4T1 and Pt4T2. It is therefore

believed that the phenyl-based wave is due to an oxidation on both end-phenylene

electrophores. In addition, the DPV spectrum of Pt4T3 (not shown) revealed a third band









at +0.88 V. This oxidation probably arises from the formation of a second oxidation on

the terthiophene and formation of a dicationic terthienyl species, as observed in other

oligothiophenes studies.141

Pulse Radiolysis Ion Radical Spectra

Ion radicals were generated by pulse radiolysis at Brookhaven National Laboratory.

The spectra were measured at 0.1-1.0 0s delay time following the growth of the radical

ion species. Radical ions were produced from transfer of electron or hole from the

solvated electrons and solvent-based holes created by the electron pulse.

Radical cations

The spectra of the radical cation obtained for both oligomers series are presented in

Figure 3-5. All radical cation oligomers feature a strong band in the visible spectrum

(400-800 nm) and a weaker and broader band in the near-IR (800-1600 nm). Looking at

the Ptn oligomers first, it appears that the visible band around between X = 360 and X =

520 nm red-shifts strongly from Ptl to Pt2, but does not move after that. This is clearly

evidenced in the inset of the visible region shown in Figure 3-5. In fact, the change levels

off at Pt3, which has the same absorption band as Pt4 and Pt5. The same trend is

observed for the band in the near-IR, where the change levels off at Pt2 in this case.

Consistent with the electrochemistry data, this suggests a relatively localized radical

cation species in these oligomers. Since no apparent stabilization energy is gained for

oligomers longer than Pt3, it appears that the radical cation is delocalized over ca. three

repeat units.

Turning to the Pt4Tn oligomers series, similar spectra consisting of two absorption

bands in the visible and near-IR are observed. The spectra suggest that the radical cations











7e+4
6e+4
6e+4 Ptn 5e+.
4e+4
5e+4 3+4
3e+4
4e+4 2e+4 \

S3e4 le+4
3e+4 \f _--!,-
S400 450 500 550 600
2e+4 -


E0 J" I
"T le+4 '" "- 4-


i le+5 Pt4Tn

8e+4

6e+4

4e+4 -

2e+4 A

0 L ... . ..... ... .i .... .... .
400 600 800 1000 1200 1400 1600

Wavelength / nm

Figure 3-5. Radical cation spectra for Ptn (top) and Pt4Tn (bottom) oligomers series. In
Ptn: Ptl (e), Pt2 (m), Pt3 (A), Pt4 (V) and Pt5 (+). In Pt4Tn: Pt4T1 (I),
Pt4T2 (m), Pt4T3 (A).

formed are essentially localized on the oligothienyl electrophore, as the visible bands are

at much lower energies than the absorption bands in the Ptn oligomers series. Both of the

bands undergo a significant bathochromic shift from Pt4T1 to Pt4T3. This is attributed

to the increasing stability of radical cation centers on the oligothienyl units of increasing

length present in the oligomer.









Radical anions

The absorption spectra of the radical anions are presented in Figure 3-6 and all

oligomers display two bands, one in the visible between X = 380 and X = 580 nm and one

extending far in the IR (except for Ptl). Apart from Ptl which is blue-shifted from the

rest of the series, the visible absorption band of all Ptn oligomers radical anions are

almost superimposable. This suggests the presence of a very localized radical anion,

which is not sensitive to an increase in chain length after Pt2.

The anion radicals of the Pt4Tn again display two bands as well, again in the

visible between X = 520 and X = 800 nm and one extending in the IR. The visible band

shows the same trend observed in the radical cation spectra, that is a bathochromic shift

with increasing oligothiophene size. The second IR band however does not seem to

follow any particular trend, although it is difficult to interpret as the bands are beyond the

range of the instrument.

Discussion

Except from bulk conductivity of oligoPPE-based self-assembled films studied by

Tour and co-workers,142 there is no literature on the electrochemistry of

phenyleneethynylene-based conjugated systems. However, platinum acetylide

complexes, oligo(phenylenevinylene)s and oligo(thiophene)s have been studied by

several groups and this provide a basis for the discussion.

Delocalization of Charge Carriers

The oxidation waves observed in the CV and DPV spectra of the Ptn and Pt4Tn

oligomers are assigned to oxidation occurring on the phenyl or thienyl ligand rather than

oxidation of the metal because of the oxidation potential and their reversibility. It is

known that the one-electron oxidation of Pt(II) to Pt(III) is usually irreversible as the







81



1.4e+5 .
Ptn 14e+5
1.2e+5 1- 12e+5 /
11.0e+5
1.0e+5 8- 0e+4
6 0e+4
8.0e+44 0+4
2 0&+4
6.0e+4 o.o
S400 450 500 550 600
4.0e+4

E 2.0e+4 ,--

0.0
le+5 Pt4Tn


8e+4


6e+4


4e+4 -
1. f _
2e+4 h
T .. ... .


0
400 600 800 1000 1200 1400 1600

Wavelength / nm

Figure 3-6. Radical anion spectra for Ptn (top) and Pt4Tn (bottom) oligomers series. In
Ptn: Ptl (e), Pt2 (m), Pt3 (A), Pt4 (V) and Pt5 (+). In Pt4Tn: Pt4T1 (*),
Pt4T2 (m), Pt4T3 (A).

radical cation formed readily undergoes rapid interaction with the solvent, leading to

decomposition products.143 When observed,144 the platinum oxidation has been reported

at higher potential (-1.2 V) than all oxidation potential observed here. Moreover, the

HOMO of platinum acetylide oligomers is believed to be located mostly on the aryl

ligands with little metal character while the LUMO is located essentially on the aryl









ligands. It is therefore safe to rule out a metal oxidation for the reversible waves. The

oxidations are thus referred to as ligand-based but bearing in mind that some metal

character is probably mixed in as well. However, a Pt(II) to Pt(III) oxidation with

decomposition could be responsible for the irreversible waves observed in Ptl and Pt2.

The merging of the two oxidation waves in Pt3 into one wave in Pt4 and the red-

shift of the radical cation absorption bands leveling off for Pt3 give an indication on the

delocalization of the radical cation of platinum acetylide oligomers. The data suggest a

fairly localized radical cation species, one that is probably localized over 2 to 3 repeat

units. With 4 repeat units, two radical cation centers can form with little interaction or

electronic coupling between them. The radical anions are more localized than the radical

cations. The absorption spectra of radical anions of the Ptn series show no red-shift after

Pt2 for the visible band. Moreover, the electrochemistry of these platinum acetylide

oligomers displayed only irreversible waves, implying that the radical anions are not

stable. This means that these materials would probably perform better when p-doped

rather than n-doped. The extent of delocalization of the radical ions observed in the Ptn

series is somewhat smaller but comparable to the extent of delocalization of charge

carriers in related all-organic conjugated oligomers studied recently.72'145

The merging of the oxidation waves in Pt4 is reminiscent of mixed-valence

systems which have been known for a long time.146 And while it was first established for

polymetallic complexes, the concept of mixed valence is also valid for organic molecules,

as shown in several recent examples.147-151 If two MI metal centers are coupled by a

strong electronic coupling, removal of one electron on one of the metal may lead to a

delocalized cation with two equivalents metal each in a +2.5 oxidation state. If there is no









electronic coupling, the one-electron oxidation will lead to a mixed valence cation

species with M" and M1" metal centers. These two extreme cases represent the class III

and I of the Robin and Day's classification of mixed-valence species.152 In between those

two cases lies a wide range of intermediate species with different degree of electronic

coupling. Electronic coupling can be significantly improved by conjugation but it is also

distance-dependent. Therefore it is probably favored and effective in the shorter

oligomers of the Ptn series. For oligomers longer than Pt3, the distance between two

radical cation centers becomes too great and the electronic coupling is greatly reduced.

The electrochemistry of thiophene-based conjugated materials is well documented

in the literature. The oxidation potentials of unsubstituted oligothiophenes reported by

Meerholz and Heinze140 in methylene chloride are higher than the potential observed in

the Pt4Tn oligomers series studied here. Values reported for the first oxidation are +1.7,

+1.25 and +0.87 V for thiophene, bithiophenen and terthiophene, respectively, whereas

they are +0.71, +0.63 and +0.64 V across the Pt4Tn oligomers series. Oxidation

potentials for bis-(thiomethyl)bithiophene and bis-(thiomethyl)terthiophene as reported

by Hill et al. 153 in acetonitrile are +0.93 and +0.89 V. Even with a good electron-donating

substituent such as thiomethyl group, it appears the oligothiophenes located on the

oligothiophenes of the Pt4Tn oligomers are easier to oxidize and their radical cations

more stable. This is an indication that the platinum center is an excellent electron donor,

that can bring a large stabilization energy to the radical cation.141

Electronic Transitions of the Radical Ions

Charge carriers are a key to opto-electrical processes in conjugated materials. The

most promising polymers for these applications, poly(thiophene),154 poly(p-

phenylenevinylene)130 and poly(p-phenylene)155 have been studied extensively to









understand the nature and the dynamics of the charge carriers present. The absorption

spectra of these extended 7t-conjugated systems have been interpreted in terms of the

polaron-bipolaron model, based on band theory. However, the oligomers such as the one

studied here do not form real electronic band structures and it has been suggested that

their spectra are better interpreted within the MO theory.156,157 Since the electrochemistry

study showed irreversible processes for the reduction of the platinum acetylide oligomers,

the discussion will be focused on the radical cations.

Although the absorption spectra of the radical cations of

oligo(phenyleneethynylene)s have not been reported, the related

oligo(phenylenevinylene) has been investigated. Two bands are observed for oligomers

up to the tetramer, one in the visible (X = 500-700 nm) and one in the near-IR (X = 800-

1500 nm).158 Both of these bands were shown to shift systematically to lower energy with

increasing conjugation length. This implies an increasing delocalization of the radical

cation in these all-organic oligomers and this is opposite to our Ptn oligomers series,

where measurements have shown no shift of the absorption bands to lower energies after

the dimer or the trimer. While the platinum center preserves conjugation in the neutral

state, it is possible that conjugation becomes much more limited in the radical cation of

these metal-organic oligomers. It is however interesting that the all-organic oligomers

also exhibited two absorption bands, in the visible and in the near-IR. This is an

indication that the metal centers do not drastically change the nature of the optical

transitions of radical cations in metal-organic conjugated oligomers.

Different unsubstituted and substituted oligothiophene radical cations have been

prepared and their absorption spectra recorded by several groups.153,156,159,160 It is now