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Photophysics of Conjugated Organometallic Systems

Permanent Link: http://ufdc.ufl.edu/UFE0024139/00001

Material Information

Title: Photophysics of Conjugated Organometallic Systems Photoinduced Electron and Energy Transfer, Triplet Exciton Delocalization, and Phosphorescent Organogelators
Physical Description: 1 online resource (176 p.)
Language: english
Creator: Li, Yongjun
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: absorption, acceptor, acetylide, aggregation, conjugation, delocalization, donor, electron, emission, energy, exciton, gel, morphology, ndi, oligomer, organogel, organogelator, organometallics, phosphorescence, photoinduced, photophysics, platinum, singlet, thiophene, transfer, transient, triplet
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: We designed and synthesized several series of platinum acetylide oligomers. Their photophysical properties were studied by steady-state and time resolved methods towards the objective of studying exciton and charge transfer. The motivation of this research is to provide a clear understanding of the structure and dynamics of triplet excitons and charged states within conjugated systems that feature organometallic repeats. First, a series of platinum acetylide oligomers that feature a donor-spacer-acceptor architecture was prepared to investigate photoinduced electron transfer dynamics. The thienylene or bithienylene segments (T1 or T2) were used as electron donors, and naphthalene diimides (NDI) were used as electron acceptors. The donor and the acceptor were connected by several platinum acetylide repeat units. Second, photoinduced energy transfer was studied within anthracene-based platinum acetylide oligomers. Third, triplet exciton delocalization was explored in a series of platinum tetrayne oligomers by varying the carbon chain length. Finally, the properties of triplet excitons and charged states were investigated in supramolecular aggregates (and gels) generated by self-assembly of tailored platinum acetylide oligomers that structurally feature long alkyl chains and amide functional groups. The most important findings of this research are as follows: (i) Photoinduced electron transfer occurs via a direct or an indirect mechanism depending on the excitation wavelength;(ii) photoinduced energy transfer in the anthracene-based platinum acetylide oligomer occurs via a mechanism that starts from a singlet-triplet energy transfer followed by a triplet-triplet energy transfer; (iii) triplet excitons of the platinum tetrayne oligomers are localized and confined within two repeat units; (iv) in the triplet excited states of the aggregated oligomers, the chromophores interact only weakly and the rate of electron transfer is very rapid likely due to triplet exciton diffusion within closely packed oligomers.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Yongjun Li.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Schanze, Kirk S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024139:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024139/00001

Material Information

Title: Photophysics of Conjugated Organometallic Systems Photoinduced Electron and Energy Transfer, Triplet Exciton Delocalization, and Phosphorescent Organogelators
Physical Description: 1 online resource (176 p.)
Language: english
Creator: Li, Yongjun
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: absorption, acceptor, acetylide, aggregation, conjugation, delocalization, donor, electron, emission, energy, exciton, gel, morphology, ndi, oligomer, organogel, organogelator, organometallics, phosphorescence, photoinduced, photophysics, platinum, singlet, thiophene, transfer, transient, triplet
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: We designed and synthesized several series of platinum acetylide oligomers. Their photophysical properties were studied by steady-state and time resolved methods towards the objective of studying exciton and charge transfer. The motivation of this research is to provide a clear understanding of the structure and dynamics of triplet excitons and charged states within conjugated systems that feature organometallic repeats. First, a series of platinum acetylide oligomers that feature a donor-spacer-acceptor architecture was prepared to investigate photoinduced electron transfer dynamics. The thienylene or bithienylene segments (T1 or T2) were used as electron donors, and naphthalene diimides (NDI) were used as electron acceptors. The donor and the acceptor were connected by several platinum acetylide repeat units. Second, photoinduced energy transfer was studied within anthracene-based platinum acetylide oligomers. Third, triplet exciton delocalization was explored in a series of platinum tetrayne oligomers by varying the carbon chain length. Finally, the properties of triplet excitons and charged states were investigated in supramolecular aggregates (and gels) generated by self-assembly of tailored platinum acetylide oligomers that structurally feature long alkyl chains and amide functional groups. The most important findings of this research are as follows: (i) Photoinduced electron transfer occurs via a direct or an indirect mechanism depending on the excitation wavelength;(ii) photoinduced energy transfer in the anthracene-based platinum acetylide oligomer occurs via a mechanism that starts from a singlet-triplet energy transfer followed by a triplet-triplet energy transfer; (iii) triplet excitons of the platinum tetrayne oligomers are localized and confined within two repeat units; (iv) in the triplet excited states of the aggregated oligomers, the chromophores interact only weakly and the rate of electron transfer is very rapid likely due to triplet exciton diffusion within closely packed oligomers.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Yongjun Li.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Schanze, Kirk S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024139:00001


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1 PHOTOPHYSICS OF CONJUGATED ORGANOMETALLIC SYSTEM S : PHOTOINDUCED ELECTRON AND ENERGY TRANSFER, TRIPLET EXCITON DELOCALIZATION, AND PHOSPH ORE S C ENT ORGANOGELATORS By YONGJUN L I 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 2009

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2 2009 Yongjun Li

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3 To my family

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4 ACKNOWLEDGMENTS I would never have been able to finish my dissertation without the guidance of my committee members, helps from group members and friends, and support from my family and wife. First, I would like to express my deepest gratitude to my advisor, Dr. Kirk S. Schanze, for hi s excellent guidance, caring, patience and providing me with an excellent atmosphere for doing my research I also would like to thank my committee members, Dr. Ronald Castellano Dr. Ion Ghiviriga, Dr. Daniel Talham and Dr. Elliot Douglas for their suppo rt and assistance Many people have been involved with my research and I would like to thank Dr. Richard Farley for teach ing me instrumentation Dr. Paiboon Sreearunothai and Dr. John R. Miller for carrying out the pulse radiolysis in Brookhaven National L a boratory Dr. John A. Gladysz and his lab for supplying the platinum tetrayne complexes in Texas A&M University Dr. Erkan M. K o se for carrying out the DFT calculations in National Renewable Energy Laboratory and Dr. Yan Liu and Zhuo Chen for helping me with TEM and AFM Many thanks go to my labmates in Sisler Hall 420, Julia, Key Y o u ng, Emine and Abby Special thanks go to Julia for sharing the hood with me. Most my synthesis was done in our sharing hood and it was always a great time. I also would like to thank all present members of the Schanze s group. Finally, I thank my parents, my family and m y wife Jing for her love and support

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ............... 4 LIST OF TABLES ................................ ................................ ................................ ........................... 8 LIST OF FIGURES ................................ ................................ ................................ ......................... 9 ABSTRACT ................................ ................................ ................................ ................................ ... 14 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .................. 16 Basic Concepts of Photophysics ................................ ................................ ............................. 16 Light A bsorption ................................ ................................ ................................ ............. 16 Excited S tates ................................ ................................ ................................ .................. 18 Electron T ransfer ................................ ................................ ................................ ............. 20 Energy T ransfer ................................ ................................ ................................ ............... 25 Photophysical P roperties of M olecular A ggregates ................................ ........................ 28 Platinum Acetylides ................................ ................................ ................................ ................ 30 Synthetic M ethod ................................ ................................ ................................ ............. 30 Triplet E xcited S tates ................................ ................................ ................................ ...... 31 Object ive of P resent S tudy ................................ ................................ ................................ ..... 37 2 PHOTOINDUCED ELECTRON TRANSFER IN TRIBLOCK PLATINUM ACETYLIDE OLIGOM ERS ................................ ................................ ................................ .. 39 Introduction ................................ ................................ ................................ ............................. 39 Synthesis ................................ ................................ ................................ ................................ 42 Results ................................ ................................ ................................ ................................ ..... 47 Electrochemistry ................................ ................................ ................................ .............. 47 UV Vis Absorption ................................ ................................ ................................ ......... 49 Steady State Photoluminescence ................................ ................................ ..................... 50 Transient Absorption ................................ ................................ ................................ ....... 53 Discussion ................................ ................................ ................................ ............................... 58 Phosphorescence Quenching in Pt4T1A and Pt4T2A ................................ ..................... 58 Photoinduced Electron Transfer Mechanism in PtnT1A Series with 355 nm Laser Excitation ................................ ................................ ................................ ..................... 62 Photoinduced Electron Transfer Mechanism in PtnT 2 A Se ries with 355 and 420 nm Laser Excitation ................................ ................................ ................................ ........... 65 Conclusion ................................ ................................ ................................ .............................. 67 Experimental ................................ ................................ ................................ ........................... 68 Electrochemistry ................................ ................................ ................................ .............. 68 Photophysical M easurements ................................ ................................ .......................... 68 Synthesis ................................ ................................ ................................ .......................... 69

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6 3 P HOTOI NDUCED ENERGY TRANSFER IN BIBLOCK PLATINUM ACETYLIDE OLIGOMERS ................................ ................................ ................................ ......................... 80 Introduction ................................ ................................ ................................ ............................. 80 Synthesis ................................ ................................ ................................ ................................ 82 Results and Discussion ................................ ................................ ................................ ........... 84 UV Vis Absorption ................................ ................................ ................................ ......... 84 Steady State Photoluminescence ................................ ................................ ..................... 86 Time R esolved Photoluminescence ................................ ................................ ................ 90 Transient Absorption ................................ ................................ ................................ ....... 91 Density Functional Theory Calculations ................................ ................................ ......... 92 Energy Transfer Dynamics ................................ ................................ .............................. 94 Conclusion ................................ ................................ ................................ .............................. 97 Experimental ................................ ................................ ................................ ........................... 98 Photophysical Measurements ................................ ................................ .......................... 98 DFT Calculations ................................ ................................ ................................ ............. 99 Synthesis ................................ ................................ ................................ .......................... 99 4 PHOTOPHYSICS OF PLATINUM TETRAYNE OLIGOMERS: DELOCALIZATION OF TRIPLET EXCITON ................................ ................................ ................................ ...... 102 Introduction ................................ ................................ ................................ ........................... 102 Results ................................ ................................ ................................ ................................ ... 104 UV Vis Absorption ................................ ................................ ................................ ....... 104 Steady State Photoluminescence ................................ ................................ ................... 105 Tra nsient Absorption ................................ ................................ ................................ ..... 109 Phosphorescence Decay Kinetics ................................ ................................ .................. 112 Discussion ................................ ................................ ................................ ............................. 114 E ffect of Spacer on Delocalization of Triplet Exciton ................................ .................. 114 Effect of Platinum on Delocalization of Triplet Exciton ................................ ............... 116 Conclusion ................................ ................................ ................................ ............................ 117 Experimental ................................ ................................ ................................ ......................... 117 Materials ................................ ................................ ................................ ........................ 117 Photophysical Measurements ................................ ................................ ........................ 117 Photoluminescence Spectral Fitting ................................ ................................ .............. 118 5 PHOSPHORESCENT ORGANOGELATORS ................................ ................................ ... 119 Introductio n ................................ ................................ ................................ ........................... 119 Synthesis ................................ ................................ ................................ ............................... 123 Results and Discussion ................................ ................................ ................................ ......... 125 Photoinduced Electron Tran sfer in Pt2M/NDI 1 and Pt2MT/NDI 1 M ixed G els ........ 12 5 Morphology and Photophysics of Pt2MAM Gelator ................................ .................... 127 Gel formation ................................ ................................ ................................ ......... 127 Morphological characterization ................................ ................................ .............. 128 UV Vis absorption ................................ ................................ ................................ 129 Steady state photoluminescence ................................ ................................ ............. 131

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7 Transient absorption ................................ ................................ ............................... 134 Pulse Radiolysis in Pt2M Pure and Pt2M/NDI 1 Mixed Gels ................................ ...... 135 Conclusion ................................ ................................ ................................ ............................ 143 Experimental ................................ ................................ ................................ ......................... 144 Photophysical Measurements ................................ ................................ ........................ 144 Pulse Radiolysis ................................ ................................ ................................ ............. 145 Synthesis ................................ ................................ ................................ ........................ 145 6 CONCLUSION ................................ ................................ ................................ ..................... 152 APPENDIX A SUPPO RTING INFORMATION FOR CHAPTER 2 ................................ .......................... 155 B SUPPORTING INFORMATION FOR CHAPTER 4 ................................ .......................... 157 C NMR SPECTRA ................................ ................................ ................................ ................... 158 LIST OF REFERENCES ................................ ................................ ................................ ............. 168 BIOGRAPHICAL SKETCH ................................ ................................ ................................ ....... 176

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8 LIST OF TABLES Table page 2 1 Photophysical data for oligomers and model compounds. ................................ ................ 53 3 1 Photophysical data of Pt2An and Pt4An ................................ ................................ ......... 88 3 2 Energies of P t2An and Pt4An. ................................ ................................ .......................... 94 4 1 Emission spectra fitting parameters for (PtC 8 ) n complexes at 100 K. ............................ 109 4 2 Photophysical parameters for (PtC 8 ) n complexes. ................................ .......................... 114 5 1 Gelation tests of Pt2MAM. ................................ ................................ ............................. 127

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9 LIST OF FIGURES Figure page 1 1 Fra nck Condon principle ................................ ................................ ................................ ... 18 1 2 Jablonski diagram showing the photophysical processes. ................................ ................. 19 1 3 P hotoinduced electron transfer processes ................................ ................................ ......... 20 1 4 D ecay pathways of photoinduced electron transfer ................................ ........................... 21 1 5 E xchange interaction s (J) ................................ ................................ ................................ ... 23 1 6 Energy diagram of hyperfine coupling and the Zeeman splitting. ................................ ..... 24 1 7 Coulombic energy transfer mechanism. ................................ ................................ ............ 26 1 8 E lectron exchange energy transfer mechanism. ................................ ................................ 27 1 9 Exciton band structures in dimers with several orientations of transition dipoles ............ 29 1 10 M olecular structures s tudied by Shinkai ................................ ................................ ............ 30 1 11 General structure of platinum acetylides ................................ ................................ .......... 30 1 12 H a g i hara reaction for synth esi s of the platinum acetylide monomer or polymer ............. 31 1 13 Splitting of d orbital levels in Pt(II) complexes ................................ ................................ 32 1 14 G eneral structur e s of the polymers studied by Wilson ................................ ...................... 33 1 15 General structures of platinum containing polymers and monomers studied by Khler ................................ ................................ ................................ ................................ 34 1 16 Photoluminescence spe c tra at 10 K and absorption spectra at room temperature of thin films of the platinum containing polymer and monomer with spacer 2 ..................... 35 1 17 Energy levels of singlet S 1 and tr iplet T 1 excited states and singlet triplet splitting S1 T1 ) ................................ ................................ ................................ ................. 35 1 18 Platinum acetyl ide oligomers studied by Rogers ................................ ............................... 36 1 19 Platinum acetylide oligomers studied by Liu ................................ ................................ ..... 36 1 20 UV Vis absorption and p hotoluminescence s pectra of Pt n oligomers ............................ 37 2 1 Chemical structures and pulse radiolysis transient absorption spect ra for radical cations ................................ ................................ ................................ ................................ 40

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10 2 2 Chemical structures of platinum acetylide oligomers. ................................ ....................... 41 2 3 Synthesis of the platinum acetylide complexes 6a b ................................ ........................ 43 2 4 Synthesis of the platinum acetylide complexes 10a b ................................ ...................... 44 2 5 S ynthesis of the NDI end group ................................ ................................ ....................... 45 2 6 Synthesis of Pt4TmA Pt6TmA and Pt2 TmA oligomers ................................ ............... 46 2 7 Cyclic voltammograms of Pt4T1A and Pt4T 2A ................................ ............................. 48 2 8 Absorption spectra of platinum acetylide oligomers in THF solution .............................. 50 2 9 Photoluminescence spectra of p latinum acetylide oligomers in deoxygenated THF solution ................................ ................................ ................................ .............................. 51 2 1 0 Transient absorption spectra of PtnT1 A series in deoxygenated THF soution following 355 nm laser excitation ................................ ................................ ..................... 55 2 1 1 Transient absorption spectra of PtnT2 A serie s in deoxygenated THF solution following 355 nm laser excitation ................................ ................................ ..................... 56 2 12 Transient absorption spectra of PtnT2A series in deoxygenated THF solution following 420 nm laser excitation ................................ ................................ ..................... 58 2 1 3 P hotophysical processes of Pt4T1 ................................ ................................ .................... 60 2 14 P hotophysical proces ses of PtnT1 A series following 355 nm laser ex citation ................ 64 2 1 5 P hotophysical processes of PtnT2A series following 355 and 420 nm laser excitation ... 66 2 16 E lectronic coupling in Pt2T2A ................................ ................................ ......................... 67 3 1 Chemical structures of anthracene based platinum acetylide oligomers and reference complexes. ................................ ................................ ................................ ......................... 82 3 2 Synthesis of Pt2An ................................ ................................ ................................ ........... 83 3 3 Synthesis of Pt4An ................................ ................................ ................................ ........... 84 3 4 Absorption spectra in THF solution ................................ ................................ ................... 84 3 5 Photoluminescence spectra of Pt2An and Pt4An in THF solution. ................................ .. 87 3 6 Normalized photoluminescence spectra of Pt2An and Pt4An in THF solution ............... 87 3 7 Excitation spectra of Pt2An and Pt4An monitored at 5 36 nm ................................ ........ 88

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11 3 8 Low temperature emission spectra of Pt4An in Me THF solution ................................ .. 89 3 9 Time resolved emission spectra of Pt4An and Pt2An ................................ ..................... 90 3 10 Tra nsient absorption spectra of Pt2An and Pt4An ................................ .......................... 92 3 11 Energies of Pt2An and Pt4An by DFT calculations. ................................ ........................ 93 3 12 Energy diagram of photophysical processes in Pt4An ................................ ..................... 95 3 13 Energy diagram of photophysical processes in Pt2An ................................ ..................... 97 4 1 Platinum end capped polyynes studied by Farley. ................................ .......................... 103 4 2 Platinum tetrayne oligomers of the current study. ................................ ........................... 104 4 3 Absorption spectra of (PtC 8 ) n complexes in Me THF solution. ................................ ..... 105 4 4 Photoluminescence spectra of (PtC 8 ) n complexes in Me THF solvent glass at 100 K .. 106 4 5 Variable temperature photoluminescence spectra of (PtC 8 ) 2 in Me THF. ..................... 107 4 6 Excitation spectra of (PtC 8 ) 1 (PtC 8 ) 2 and (PtC 8 ) 3 ................................ ........................ 108 4 7 Triplet absorption spectra of (PtC 8 ) n complexes following 355 nm la ser excitation ...... 110 4 8 Transient absorption spectra at t = 0 s and d ecay profiles. ................................ ............ 111 4 9 Temperature dependence of photoluminescence lifetimes for (PtC 8 ) n complexes in Me THF solution (glass). ................................ ................................ ................................ 112 4 10 Platinum ace tylide oligomers studied by Liu ................................ ................................ ... 115 5 1 Pt2M/NDI 1 and Pt2MT/ NDI 1 donor acceptor systems. ................................ ............ 121 5 2 Pt2MAM / NDI 2 donor acceptor system. ................................ ................................ ....... 122 5 3 Synthesis of Pt2MAM and NDI 2 ................................ ................................ .................. 124 5 4 Synthesis of NDI 1 ................................ ................................ ................................ .......... 125 5 5 Photoluminenscence spectra of pure and mixed dod ecane gels ................................ ..... 126 5 6 S ol gel transitions of Pt2MAM ................................ ................................ ...................... 128 5 7 Mor phological features of Pt2MAM xero gels ................................ ............................... 129 5 8 Absorption spectra of Pt2MAM in solution/gel ................................ ............................. 130 5 9 Photoluminescence spectra of Pt2 MAM ................................ ................................ ........ 132

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12 5 10 P ossible packing modes in Pt2M and Pt2MAM aggregates. ................................ ......... 132 5 11 Photoluminescence spectra of Pt2MAM / NDI 2 mi xed gels in d eoxygenated DMSO 133 5 12 Transient absorption spectra of Pt2MAM and mi xed DMSO gels ................................ 134 5 13 Decay profiles of Pt2MAM in DMSO pure a nd mixed gels at = 700 nm. .................. 135 5 14 T he LEAF facility in Brookhaven National Laboratory ................................ ................. 136 5 15 P ossible electron transfer process es in the Pt2M / NDI 1 mixed gel. ............................... 137 5 16 Pulse radiolysis transient absorption spectra and decay profiles for radical anions of Pt2M solution/gel ................................ ................................ ................................ ............ 139 5 17 Pulse radiolysis transient absorption spectra and decay profiles of Pt2M /NDI 1 mixed gel and pure NDI 1 solution ................................ ................................ ................ 141 5 1 8 Pulse radiolysis t ransient absorption spectra and d ecay profiles of Pt2M and Pt2M / NDI 1 solution ................................ ................................ ................................ ..... 142 A 1 Chemical structure of the NDI model compound. ................................ ........................... 155 A 2 Emission spectra of plati num acetylide oligomers. ................................ ......................... 1 55 A 3 Emission spectrum of the NDI model compound. ................................ ........................... 156 A 4 Chemical structure and absorption spectra of Pt2 T2 ................................ ..................... 156 B 1 Fitting spectra of (PtC 8 ) 1 (PtC 8 ) 2 and (PtC 8 ) 3 (100 K in Me THF glass ) ..................... 157 C 1 T h e 1 H NMR (300 MHz, CDCl 3 ) spectrum of P t2T1A ................................ .................. 158 C 2 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2T1A ................................ ................. 158 C 3 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2T2A ................................ .................. 159 C 4 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2T2A ................................ ................. 159 C 5 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt4T1A ................................ .................. 160 C 6 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt4T1A ................................ ................. 160 C 7 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt4T2A ................................ .................. 161 C 8 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt4T2A ................................ ................. 161 C 9 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt6T1A ................................ .................. 162

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13 C 10 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt6T1A ................................ ................. 162 C 11 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt6T2A ................................ .................. 163 C 12 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt6T2A ................................ ................. 163 C 13 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2An ................................ ..................... 164 C 14 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2An ................................ .................... 164 C 15 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt4An ................................ ..................... 165 C 16 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt4An ................................ .................... 165 C 17 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2MAM ................................ ............... 166 C 18 The 31 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2MAM ................................ ............... 166 C 19 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of NDI 2 ................................ ..................... 167 C 20 The 1 H NMR (300 MHz, CDCl 3 ) spectrum of NDI 1 ................................ ..................... 167

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1 4 Abstract of Dissertation Presented to th e Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PHOTOPHYSICS OF CONJUGATED ORGANOMETALLIC SYSTEM S : PHOTOINDUCED ELECTRON AND ENERGY TRANSFER, TRIPLET EXCITON DELOCALIZATION, AND PHOSPH ORE S C ENT ORGANOGELATORS By Yongjun Li May 2009 Chair: Kirk S. Schanze Major: Chemistry We designed and synthesized several series of platinum acetylide oligomers. Their photophysi cal properties were studied by steady state and time resolved methods towards the objective of studying exciton and charge transfer T he motivation of this research is to provide a clear understanding of the structure and dynamics of triplet excitons and charged states within conjugated systems that fea ture organometallic repeats. First, a series of platinum acetylide oligomers that feature a donor spacer acceptor architecture was prepared to investigate photoinduced electron transfer dynamics. The thienylene or bithienylene segments (T1 or T2) were use d as electron donors, and naphtha lene diimides (NDI) were used as electron acceptors The donor and the acceptor were connected by several platinum acetylide repeat units. Second, photoinduced energy transfer was studied within anthracene based platinum ac etylide oligomers T hird, triplet exciton delocalization was explored in a series of platinum tetrayne oligomers by varying the carbon chain length. Finally, the properties of triplet excitons and charged states were investigated in supramolecular aggregat es (and gels) generated by self assembly of tailored platinum acetylide oligomers that structurally feature long alkyl chains and amide fu n ctional groups

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15 The most important findings of this research are as follows: (i) Photoinduced electron transfer occu rs via a direct or an indirect mechanism dependi ng on the excitation wavelength ; (ii) p hotoinduce d energy transfer in the anthracene based platinum acetylide oligomer occurs via a mechanism that starts from a singlet triplet energy transfer followed by a triplet triplet energy transfer; (iii) t riplet exciton s of the platinum tetrayne oligomers are localized and confined within two repeat units; (iv) i n the triplet excited state s of the aggregated oligomers, the chromophores interact only weakly and the rat e of electron transfer is very rapid likely due to triplet exciton diffusion within closely packed oligomers.

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16 CHAPTER 1 INTRODUCTION This chapter is divided into two sections The first section is an overview of basic concepts of photophysics, such as l ight absorption and emission, and photoinduced electron and energy transfer. The second section introduce s platinum acetylides including synthetic methods and photophysical properties. Basic Concepts of Photophysics Light A bsorption T he understanding of t he nature of light started from Newton s particle theory in the 17 th century 1 Huygens first came up w ith the wave theory, but it was not accepted until the middle of the 19 th century when Maxwell developed a new electromagnetic theory based on a set of equations. To explain his blackbody experiment, in 1900 Planck proposed a new theory in which h e suggested that the distribution of energy was not continuous, but was restricted to certain values. Then, Einstein used the principle of quantization that was introduced by Planck to explain the photoelectric effects and concluded that radiation energy w as localized in discrete packets (photons). Today, we understand that the nature of light is electromagnetic waves and has wave particle duality. 2 When an atom or a molecule absorb s light, an electron on the outermost shell of the molecule is promoted to a higher energy level. T he light frequency must match the molecular resonant frequency to promote the outmost electron. In other words, the energy of the absorbed light must equal to the energy difference of the two orbital lev els involved in this transition T h is is because the energy levels of the ele ctrons in an atom or a molecule are not continuous but discrete T he relationship between the photon energy and its frequency is expr essed by E quation 1 1

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17 E = h = hc ( 1 1) s constant (6.63 x 10 34 J.s), is the frequency (sec 1 ) at which absorption occurs, c is the speed of light (3.0 x 10 8 m/s), and is the wavelength (m). The absorption of light by materials follows L ambert Beer s law (Equation 1 2) A = log T = lC ( 1 2 ) where A is the absorbance (the intensity of light absorbed by molecules at a certain frequ ency) T is the transmittanc e, is the molar absor ptivi ty with units of L mol 1 cm 1 l is the light pathlength (cm), and C is the concentration of the absorbing species (mol L 1 ). Transmittance is the fraction of incident light at a specific wavelengt h that passes through a sample (Equation 1 3), T = I I 0 ( 1 3) where I 0 is the intensity of the incident light and I is the intensity of the light coming out of the sample. Lambert Beer s law indicates the linear relationship between the absorbance and the concentration of an absorbing species. From E quation 1 1 only photons with certain frequency can be absorbed by the molecule. Therefor e, the absorption spectra should appear as sharp lines. In fact, the absorption bands in molecules usually appear as broad bands. T he interpretation for this is given by the Franck Condon principle. 3,4 It states that electronic transitions (10 15 s) occur more rapidly than nucl ear motion (10 13 s). The nucl ear geometry re adjustment takes place after the electronic transitions have occurred (Figure 1 1). Electronic transitions are vertical with respect to the nuclear geometry, meaning that the electron is excited to the upper state before the nuclei have had a chance to respond to the new electronic structure. As a result, the transition from the lowest vibrational level of the ground state to vibrational levels of the excited state occurs before the nuclei geometry re equilibrate s The Franck Condon principl e is reflected in absorption spectra

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18 as a series of vibrational bands. However, in solution in particular in a polar solvent, these vibrational bands are not well resolved due to strong interactions between the excited molecul es and solvent molecules, res ulting in the loss of the vibronic structure and a broad featureless absorption band seen in the absorption spectrum. Figure 1 1 Franck Condon principle. T he figure was adopted from Atkins 5 Excited S tates The absorption of light by a molecule causes the excitation of the molecule from it s ground state to its first excited state. 6 From the excited state, the molecule first relax es to the lowest vibrational level through thermal relaxation. In the ground state, the electrons are paired and opposite spin. When the electron is promoted to its excited state, its spin does not change due to spin restrictions. Consequently, the excited state formed is called singlet excited state (S 1 ). If the

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19 spin of the excited electron flips, the spin momentum for the excited state becomes three and this process is called intersystem crossing. The res ulting state is triplet excited state (T 1 ). Intersystem crossing is a forbidden process due to spin restrictions. However, strong spin orbit coupling caused by heavy metals in organometallic molecules significantly increases the intersystem crossing yield and results in the increase of the triplet state yield. Platinum acetylide systems that are the focus of the present study are one of the examples. Several pathways are involved in the excited state relaxation. A singlet excited state decays to the ground state with emission of light and this process is called fluorescence Similarly, radiative decay of a triplet excited state is called phosphorescence Non radiative decay from excited states to the ground state releases energy by the form of heat and this process is called internal conversion. These processes can be illustrated in a Jablonski diagram (Figure 1 2). Moreover, excited state quenching via electron or energy transfer is another pathway of the deactivation of excited states, which will be address ed in detail below. Figure 1 2 Jablonski diagram showing the photophysical processes

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20 Electron T ransfer Excited states may relax to the ground state by photoinduce d electron transfer in the systems that have donor acceptor char acteristic. O n the other hand, photoinduced electron transfer reactions are the most fundamental processes in artificial and natural systems. Photosynthesis of the living plant converts the solar energy to chemical energy and produces the oxygen in the Ear th s atmosphere. To mimic photochemical charge separation, synthetic electron donor acceptor systems have been prepared to study photoinduced electron transfer. 7 I n p hotoinduced electron transfer, an electron migrates between a photoexcited molecule and a ground state molecule as shown in Figure 1 3. I n this figure, D is the electron donor, Figure 1 3 P hotoinduced electron transfer proces ses A is the electron acceptor and denotes an excited state. The excited state can be an electron donor or acceptor. T he energetics of electron transfer are determined by the redox potentials of D and A as well as the energy of the excited state. Figure 1 4 indicates an energy diagram that describes electron transfer processes and decay pathways. First, either the donor or the acceptor is excited to its singlet excited state followed by rapid electron transfer to form a contact ion pair. T he individual s pin of the positive ion or negative ion is a doublet, but the overall spin of the ion pair is singlet or triplet. T he contact ion pair decays via three pathways: (1) recombin ation and

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21 return to the ground state; (2) form triplet state ( 3 D* or 3 A*) ; (3) sep aration to form isolated ion pair. The isolated ion pair eventually recombines again if no chemical reactions take place. 8,9 Figure 1 4. D ecay pathways of photoinduced electron transfer 8,9 Solution d ynamics of photoinduced elect ron transfer. Most studies on photoinduced electron transfer have been performed in solution phase In solution, when either the donor or the acceptor is photoexcited, a series of short lived ion pair intermediates is created due to the interaction between an excited state and the ground state. The stability of the ion pair intermediates depends on electrostatic and s o lvation effects. I f an ion pair is separated by several solvent molecules, the resulting pair of geminate ions is called a solvent separated ion pair (SSIP). T he driving force of electron transfer resulting in a SSIP is given by Rehm Weller equation (Equation 1 4) G SSIP = E 0 (D + /D) E 0 (A/A ) G* + w p w R ( 1 4) where E 0 (D + /D) is the oxidation potential of the donor, E 0 (A/A ) is the reduction potential of the acceptor, G* is the free energy of the equilibrated excited state, and w P and w R are the work

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22 terms for electrostatic interaction in the product (P) and in the reactant (R) states, respectively. The work term w is expressed by E quation 1 5 w = Z D Z A e 2 d cc s ( 1 5) where Z D and Z A are the charges on the donor and the acceptor, d cc is the centre centre distance between the donor and the acceptor molecules, and s is the static dielectric constant or permittivity of the solvent. Rehm Weller equation implies that electrostatic effects are more important in nonpolar solvents. I n polar solvents, Coulombic attraction is reduced because the electrostatic attraction of a pair of ions is shielded by solvent molecules. Therefore, polar solvents are more favorable for ion dissociation into free ions. I n the conventional view of photoinduced electron transfer, the collision of neutral reactants within the solvent cage may fo rm a contact ion pair (CIP) or an intimate charge transfer complex (exciplex) depending on the structure of the reactants and the polarity of the solvent medium. A n exciplex displays light emission that can be observed in the emission spectr um In nonpolar solvents because of favorable Coulombic and solvent effects, the collision of reactants within the solvent cage is usually favorable exciplex formation. Spin Dynamics of photoinduced electron transfer. Ph otoinduced electron transfer can occur from both singlet and triplet excited states. T he electron spin is usually conserved due to spin restrictions, that is, the overall spin of an ion pair will be singlet if electron transfer occurs from a singlet excited state and a triplet ion pair form s if electron transfer takes place from a triplet excited state. However, in certain cases, the singlet triplet intersystem crossing of an ion pair can occur. The major factors that contribute to S T conversion are hyperfine coupling, the Zeeman splitting, and spin orbi t coupling. 10

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23 T he energy separation or exchange interaction between singlet and triplet states (J) decreases exponentially with the distance between ions (Figure 1 5). T he exchange interaction (J) tends to preserve the o riginal spin orientation W hen a force whose energy is stronger than J operates during electron transfer process, electron spin multiplicity changes and intersystem crossing occurs. Figure 1 5. E xchange interaction s ( J ). T = Tr iplet state; S = Singlet state. The figure was adopted from Turro 10 Hyperfine coupling is an electron nuclear interaction. Spin orbit coupling is an interaction that occurs when the magnetic moment changes by a magnetic field that is generated due to the orbital motion of electrons. Spin orbit coupling becomes very important when the interacting electrons are on a single atomic nucleus. Therefore, spin orbit coupling is usually not an important mechanism for intersystem c rossing of an ion pair. T he Zeeman splitting is an interaction due to the strength of an external magnetic field and it can induce a spin flip. With the increase of the strength of an external magnetic field, the Zeeman interaction splits the T + and T sub levels away from T 0 (Figure 1 6). When the energy gap between singlet and triplet (J) become smaller, that is, E hf > J, a spin flip takes place by hyperfine coupling. In the cases of solvent separated ion pair and free ions, the intersystem crossing is operated by hyperfine coupling mechanism because J is

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24 negligible at large distance. In the cases of contact ion p air or exciplexes, J becomes large due to small distance. Th us spin orbit coupling is the dominant mechanism for intersystem crossing. Figure 1 6 Energy diagram of hyperfine coupling and the Zeeman splitting. The figure was a dopted from Kavarnos 8 With the increase of t he strength of an external magnetic field, the Zeeman splitting inhibits intersystem crossing between S and T + or T and enhances intersystem crossing between S and T 0 T he external magnetic effects can be useful in identifying the charged species (SSIPs, CIPs and exciplexes) in photoinduced electron transfer. 11 W hen hyperfine coupling mechanism dominates intersystem crossing, the quantum yield of the triplet state that is produced by the recombination of the dissociated ion pair decreases with the strength of the external magnetic field. A s a consequence, the precursor ion pair must sli ghtly separate, so that E hf > J, such as SSIPs. O n the other hand, when the triplet yield has no discernible effects by the change in magnetic field strength, we can deduce that hyperfine coupling is not an important mechanism for intersystem crossing, th at is, E hf < J. T hus the precursor ion pair must be in close proximity such as CIPs or exciplexes.

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25 Marcus theory in rigid donor acceptor molecular system s The dependence of the rate of electron transfer within rigid donor acceptor systems on the free e nergy of reaction and the electronic interaction between the donor and the acceptor are described wel l by Marcus theory as shown in E quation 1 6 12 ET = 2 DA 2 1 4 1 2 e G 0 + 2 4 ( 1 6) G 0 is the free energy of reaction, V DA is the electronic coupling between the donor and acceptor and is the total energy of t he nuclear reorganization (structure change) within the donor, acceptor and solvent required for the reaction to occur. Marcus theory predicts that the rate of electron transfer decrease s when the driving force ( G 0 ) for electron transfer becomes very larg e (i.e., G 0 <<0) This is important for maximizing the rate of charge separation, while at the same time minimizing the rate of the energy wasting charge recombination. E xperimental evidence for the so called Marcus inverted region ha s been obtained in rigid intramolecular systems. 13 Energy T ransfer P hotoinduced e nergy transfer is a n important pathway for the quenching of molecular emission in a bimolecular system. Unlike electron transfer, the excited state in energy transfer i s exclusively an energy donor ( E quation 1 7). D* + A D + A* ( 1 7) where D is an energy donor, A is an energy acceptor and denotes the excited state. The energy of D* must be higher than the energy of A* for energy transfer to be ther modynamically favorable. In general, t here are four types of energy transfer according to the initial spin multiplicity of D* and final spin multiplicity of A*: singlet singlet, triplet triplet, triplet singlet

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26 and singlet triplet. E nergy transfer can oper ate by two mechanisms: dipole dipole (Coulombic) mechanism and electron exchange mechanism. Coulombic (F rster) energy transfer involves the mutual interaction of electrons and is dominated by long range dipole dipole interactions. 14 T he dipole d ipole interactions cause perturbation of the electronic structures of the energy donor and acceptor. Therefore, the oscillating dipole of excited energy donor interacts with the dipole of the acceptor and induces a corresponding dipole oscillation of the a cceptor. T he oscillation may finally lead to the excitation of the electron on the acceptor and a corresponding de excitation of the excited electron on the donor (Figure 1 7). Since mutual contact between the donor and the acceptor is not required, the di pole dipole mechanism can be operative over large distance, sometimes greater than 50 (up to 100 ). T he Coulomic mechanism of energy transfer does not involve physical contact between the donor and the acceptor. Therefore, the change of the spin multi plicity of both the donor and the acceptor is not allowed. Only singlet singlet energy transfer can be operative under this mechanism. Figure 1 7. Coulombic energy transfer mechanism Electron exchange (Dexter) energy transfer r equires much closer contact between the donor and acceptor to allow for transfer of electrons. 15 Energy transfer rate is limited by

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27 molecular diffusion of D* and A to within collision separations. Th us the rate of e nergy transfer by electron exchange mechanism is diffusion controlled and is sensitive to the viscosity of the solvent. T he mechanism is schematically represented as in Figure 1 8. Figure 1 8 E lectron exchange energy transfer mechanism These two mechanisms may be distinguished by comparison of the rate constants for energy transfer (k ET ) and for diffusion (k diff ), then measuring the rate constant for energy transfer versus solvent viscosity. If k ET is comparable to or less than k diff and k ET is sensitive to solvent viscosity, then energy transfer is dominated by electron exchange mechanism. On the other hand, if k ET is significantly greater than k diff and k ET is insensitive to solvent viscosity, then a dipole dipole energy trans fer mechanism is applied. Triplet triplet energy transfer is forbidden by the dipole dipole mechanism, but allowed by the electron exchange mechanism. 2 In triplet triplet energy transfer, an electronically excited donor in its triplet state produces an electronically excited acceptor in its trip let state as shown in E quation 1 8 D*(T 1 ) + A(S 0 ) D(S 0 ) + A*(T 1 ) ( 1 8 ) T riplet triplet energy transfer is the most common and most important type of energy transfer in organic phot ochemistry. It has been used to generate the triplet excited state of molecules which possess a low quantum yield for triplet formation. T he long lifetimes of the triplet excited states

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28 facilitate energy transfer processes. The rate constant for triplet tr iplet energy transfer depends on the triplet energy difference between the donor and acceptor ( E T T ). Photophysical P roperties of M olecular A ggregates Small molecules s elf assemble into supramolecul ar assembles by three noncovalent interactions: stac king, van der Waals interaction and hydrogen bonding. In certain cases, a low mass molecule can form extended structures leading to the gelation of the solvent in which the molecule is dissolved T he process of gelation initiates from self association of t he gelator molecules to form long, polymer like fibrous aggregates. T hen these fibers become entangle d to form a matrix that traps the solvent by surface tension 16 Compared with small molecules, molecular aggregates feature very different photophysical properties depending on their aggregation geometries, which can be explained by the molecular exciton model proposed by Kash a and c oworkers. 17 Accor ding to Kasha s molecular exciton model, the two most extreme aggregation geometries are the card pack H aggrega te and the head to tail J aggregat e In the H aggregat e stacking of transition dipole moments leads to an allowed transition shifted to h igher energy and a forbidden transition shift ed to lower energy (Figure 1 9 A ). I n contrast, the allowed transition dipole moments in the J aggregat e are shifted to lower energy and higher energy transition dipole mom ents are forbidden (Figure 1 9 B ). The transition dipole moments from 0 o (H aggregat e ) to 180 o (J aggregat e ) lead to the allowed transition shifted from blue to red (Figure 1 9 C ). The transition dipole moment is in the same energy with the isolated chromophore at 54 o The absorption spectra in H aggregates blue shift because the lowest energy singlet singlet transition is forbidden according to Kasha s molecular exciton model. On the other hand, the

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29 absorption spectra in J aggregates red shift because the transitions to the ground state from the lowest singlet state are allowed. Correspondingly, the fluorescence emission in H aggregates may be quenched due to non radiative decay from the higher singlet state. However, intersystem crossing that competes with other singlet decay processes should be more probable from H aggregates. In the case of J aggregates, the fluorescence emission may be enhanced compared to the emission from the monomer. Figure 1 9 Exciton band structures in dimers with several orientations of tra nsition dipoles. Dotted arrows indicate dipole forbidden transitions. The figure was adopted from Kasha 17 R ecently, J and H aggregates have been investigated in organogel systems. Shinkai and coworkers studied the relationship between the structure and the aggregation mode in an organogel system which consists of a porphyrin moiety bearing the amide groups (Figure 1 10). 18 The U V V is absorption band of the gelators w ith the structures of the amide groups at the 4 position of the meso phenyl groups is shifted to short wav elength. In contrast, the U V V is absorption band of the gelators with the structures of the amide groups at the 3, 5 positions and 3 position is shifted to longer wavelength. The shifts of the absorption bands indicate that the galators 3a and 3b adopt the H aggregation mode but 2a and 4a adopt J aggregation mode. The

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30 finding reveals that the aggregation modes of porphyrin stacks can be controlled by the peripheral hydrogen bonding interactions. Figure 1 10 M olecular structure s studied by Shinkai 18 Platinum Acetylides Platinum acetylide compounds have attracted recent interest due to their non linear optical properties 19 and potential ap plications as materials for the fabrication of high efficiency organic electroluminescent devices. 20 T he general structure of platinum acetylide polymers or oligomers is given in Figure 1 11 Figure 1 11 General structure of platinum acetylides Synthetic M ethod T he general synthetic method of pla tinum acetylide complexes is the Hagihara reaction, which was developed by Hagihara and coworkers in 1978 (Figure 1 1 2 ). 21 T he coupling reaction between the platinum chloride complex and a n aryl acetylide is carried out in an alkylamine base (e.g. diethylamine). Copper iodid e is used as a catalyst to deprotonat e the aryl acetylide. T he reaction needs to be carr ied out under an inert atmosphere to avoid the oxidation of copper iodide

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31 catalyst. Both cis and trans isomers of the platinum chloride complex give trans platinum ac etylide products because the cis platinum chloride quickly isomerizes in the presence of an amine. The cis and trans platinum isomers can be identified by 31 P NMR spectroscopy. T he coupling constant (J Pt P ) between 31 P and 195 Pt is usually below 2500 Hz f or the trans isomer and higher than 2500 Hz for the cis isomer. Figure 1 1 2 H a g i hara reaction for synthesi s of the platinum acetylide monomer or polymer Triplet E xcited S tates T riplet excited states in conjugated polymers are hard to detect due to the very slow intersystem crossing rate and small phosphorescence quantum yield ca u sed by spin forbidden rule. They can only be investigated by indirect measurements, such as photoinduced absorption 22 optically detected magnetic resonance, 23 or energy transfer. 24 However, a thorough understanding of triplet photophysics in conjugated polymers is essential for the development of their applications in optical and optoelectronic devices. O n the other hand, heavy metals, such as platinum, induce intersystem crossing du e to large spin orbit coupling. I ncorporation of platinum into c onjugat ed polymers significantly increase s intersystem crossing and the triplet excited state yield as well as provide s a direct way to study the triplet excited state in conjugated polymers. Due to this reason, platinum acetylide polymers and oligomers have been extensively studied.

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32 Pt(II) adopts a square planar configuration and forms stable bonds with ethynylen es. A ccording to the crystal field model, 25 those d orbitals that point towards the ligand groups (d x 2 y 2 in Pt(II)) are destabilized and move to higher energy level, and those that point away from ligand groups (d xy d z 2 d yz and d xz ) are less destabilized. T his results in the crystal field splitting as shown in Figure 1 1 3 T herefore, the HOMO and LUMO orbitals of Pt(II) are d xy and d x 2 y 2 respectively. Figure 1 1 3 Splitting of d orbital levels in Pt(II) complexes I n platinum acetylide polymer s an d oligomers, the HOMO and L U MO involve both platinum d orbitals and ligand p orbitals. 26,27 T he mixing betwe en platinum d orbitals and the ligands system preserves ligand s conjugation through the metal site. The extent of the mixing depends on the overlap between the ligand and metal orbitals, the size of the spacer and the extent of conjugation in the ligand Many studies have shown that the fundamental optical transition in platinum acetylide complex es mostly originates from transition of conjugated ligand with some contributions from metal d orbitals, but not from intraplatinum d d transitions. 28 This conclusion is consistent with the appearance of vibronic progressions in the emission spectra of platinum acetylide complexes. The main ground absorption bands of platinum acetylid e polymers and oligomers usually appear between 300 and 400 nm. T heir emission spectra show strong phosphorescence bands

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33 (T 1 S 0 ) between 500 and 600 nm, and weak fluorescence bands (S 1 S 0 ) between 370 and 450 nm. T he triplet triplet absorption bands (T 1 T n ) are between 600 and 800 nm. P hotophysics of platinum acetylide polymers and oligomers have been extensively studied by several groups. T he next section give s an overview of the up to date research in this topic. Wilson and coworkers investigated the ev olution of the triplet excited state in a series of platinum acetylide polymers (Figure 1 1 4 ). 29 T hey systematically varied the spacer so that the Figure 1 1 4 G eneral structure s of the polymers studied by Wilson 29 onset of the singlet absorption was tuned from 1.7 to 3 .0 eV. T hey found that the intensities and lifetimes of the triplet state emission are dramatically reduced with decreasing triplet energy as a result of increasing non radiative decay rate. In a more detailed study, 30 they concluded that the non radiative decay of the triplet states in a series of platinum containing conjugated polymers and monomers obey s the energy gap law. That is, the non radiative decay rate is sensitive to the triplet en ergy and increases exponentially as the triplet energy decreases. T he simplest form of energy gap law is expressed by E quation 1 9 nr exp ( E / M ) ( 1 9)

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34 where E is the energy gap separation between the potential minima of the states involved, is a term that can be expressed in terms of molecular parameters, and M is the maximum and dominant vibrational frequency available in the system. Another find ing of this study is that the singlet triplet ( S 1 T 1 ) energy splitting is constant ( ~ 0.7 eV) through the series and independent of the spacer R To further investigate this finding in detail they extended th e study in an extensive series of platinum conta ining conjugat ed polymers and monomers with varying the spacer R (Figure 1 1 5 ). 31 They observed that the Figure 1 1 5 General structures of platinum containing polymers and monomers studied by K hler 31

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35 single t state energy is significantly lower in the polymer than in the monomer, indicating that the singlet state is more delocalized in the polymer. I n contrast, the triplet energy difference between the polymer and the monomer is very small, demonstrating that the triplet state is more localized in the polymer (Figure 1 1 6 ). W hile the size and the electronic character of the polymer repeat units are variable with varying the spacer R, the singlet triplet splitting stay s constant ( ~ 0.7 eV) (Figure 1 1 7 ). This fi nding provides very useful information for estimating singlet triplet energy gap in the analog ous platinum containing conjugated systems. Figure 1 1 6 Photoluminescence spe c tra at 10 K and absorption spectra at room temperature of thin films of the platinum containing polymer and monomer with spacer 2 The f igure was adopted from K hler 31 Figure 1 1 7 Energy levels of singlet S 1 and triplet T 1 excited states and singlet triplet splitting energy ( E S1 T1 ). The f igure was adopted from K lher 31

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36 T o understand structure property relationship s for platinum acetylide conjugated systems, Rogers and coworkers investigated the effects of the conjugation length of the ligands on photophyiscal p roperties of a series of platinum containing oligomers (Figure 1 1 8 ). 32 T hey found that with increasing conjugation length, both ground state absorption (S 0 S 1 ) and triplet state absorption (T 1 T n ) red shif t indicating that spin orbit coupling effect of platinum is reduced. Figure 1 1 8 Platinum acetylide o ligomers studied by Rogers 32 Liu 33 a nd Glusac 34 recently reported photophysical studies of a series of platinum acetylide oligomers with varying the oligomer chai n length (Figure 1 1 9 ). The result of these Figure 1 19. Platinum acetylide oligomers studied by Liu 33 studies indicates tha t the singlet excited states red shift with increasing the oligomer chain length suggest ing that the singlet exciton is more delocalized (Figure 1 20 ). In contrast, th e triplet excited states show little or no shift with increasing the oligomer chain length,

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37 demonstrating that the triplet exciton is more localized. DFT calculations also support the notion that the triplet states of these series of oligomers are restrict ed within one or two repeat units. Figure 1 20 UV Vis absorption (a) and p hotoluminescence (b) s pectra of Pt n oligomers. Fluorescence (F) intensity scale is magnified 100 compared to phosphorescence (P). The f igure was adop ted from Liu 33 Object ive of P resent S tudy Incorporation of platin um into conjugated organic systems largely enhances the triplet quantum yield due to spin orbit coupling caused by the heavy metal. Thus platinum acetylide conjugated polymers and oligomers provide a unique platform for studying triplet excited state prop erties. Compared with platinum acetylide polymers, platinum acetylide oligomers have precise chemical structure and monodisperse property, which makes them better candidate s for investigating structure property relationship s by designing different derivati ves. From previous studies, we conclude that the triplet excited state of platinum acetylide oligomers is more localized and restricted within one or two repeat units. I n continuation of our investigation of triplet excited state photophysical properties i n platinum acetylide systems, we currently incorporate a donor acceptor moiety into platinum acetylide oligomers. This design

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38 allow s us to investigate photoinduced electron transfer and energy transfer dynamics in platinum acetylide oligomers. On the other hand, the question on how the triplet excited state is influenced by these processes will also be answered. For photoinduced ele ctron transfer study, oligothiophenes were used as electron donors due to their low oxidation potentials and conjugation proper ty. Naphthalene diimi des are easily reduced and form stable radical anions, which were used as electron acceptors in this study. For investigating photoinduced energy transfer, we incorporate d an anthracene moiety into a platinum acetylide backbone W e exp ect ed that triplet triplet energy transfer from the platinum acetylide segment to the anthracene segment w ould occur in these systems. T he extended conjugated linear carbon chain oligomers (oligoynes) have attracted increasing interest due to their poten tial use as molecular wires for transport of charge (polarons) or excitons on the nanoscale. 35 Incorporation of platinum into oligoyne s increases the stability of linear carbon chain compounds, as well as promotes the formation of the triplet excited state. I n the current study, the detailed photophysics o f a series of platinum tetrayne oligomers were investigated. To further extend our study i n platinum acetylide oligomers, we designed and synthesized a series of platinum acetylide oligomers with the ability of aggregation in solvents. I t is well known that three major non bonding interactions contribute to molecular aggregation: st acking, h ydrogen bonding and van der Waals interactions. O ur newly designed platinum acetylide oligomer features long alkyl chains (van der Waals interaction) and amide functional groups ( H bonding) that initiate molecular aggregation and even solvent gela tion in both nonpolar and polar solvents. W e investigate d photophysics and molecular morphology of these platinum acetylide aggregates.

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39 CHAPTER 2 PHOTOINDUCED ELECTRO N TRANSFER IN TRIBLO CK PLATINUM ACETYLID E OLIGOMERS Introduction Platinum acetylide bas ed conjugated polymers and ol igomers have attracted interest due to their possible use in optical and optoelectronic applications. 36 38 Photophysics o f platinum acetylide systems feature dominant long lived triplet excited s tates due to strong spin orbit c oupling caused by platinum 28,33 Compared to platinum acetylide polymers, platinum acetylide oligomers have mono disperse length and precise chemical structure, which provides a unique platform for investigation of s tructure photophysical property relationship s in particular how the triplet exciton is influenced b y the factors such as conjugat ion length and energy by designing different derivatives. 39 41 We recently reported photophysical studies of a series of platinum acetylide oligomers with the repeat unit [ C C Pt(PBu 3 ) 2 C C P h ] (P h = 1, 4 phenylene). 33,34 This investigation provides very clear experimental and theoretical evidence that the triplet state of these platinum acetylide oligomers is localized on a chro mophore consisting approximately of a single [ Pt(PBu 3 ) 2 C C P h C C Pt(PBu 3 ) 2 ] repeat unit. However, their singlet states are delocalized over a chromophore consisting of five or more repeat units. Charge carriers (polarons or radical ions) are responsib le for charge transport in optoelectronic devices, such as light emitting diodes, field effect transistors and photovoltaic cell s. T o investigate radical ion behavior in conjugated platinum acetylide systems, we u sed a pulse radiolysis method to attach i ons onto an extended series of platinum acetylide oligomers. 42 The structures of this investigation are shown in F igure 2 1. The study gives insight into the spectroscopy, electronic structure, and delocalization in the ion radical states of the oligomers.

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40 One of the important key findings of this investigation is that the radical cations are concentrated on a co mparatively short oligomer segment, in particular for Pt4Tn series oligomers (T = 2, 5 thienylene, n = 1, 2, 3) radical cations are restricted to near and on the thiophene segme nt s in the core of the oligomers (Figure 2 1). Fig ure 2 1. Chemical structures and pulse radiolysis transient absorption spectra for radical cations. The figure wa s adopted from Cardolaccia 42 In the present investiga tion, we designed and synthesized a series of platinum acetylide oligomers that are structurally similar to the Pt4Tn series but feature a donor spacer acceptor architecture as shown in Figure 2 2 Pt4T1 and Pt4T2 are also present as reference compounds.

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41 We chose naphthalene 1,8:4,5 tetracarboxydiimide ( NDI ) as the electron acceptor because it ha s been widely used as an electron acceptor in many fundamental s tudies of photoinduced electron transfer. 43 47 It is also well known that the NDI acceptor s undergo reversible one electron r eduction at moderate potentials to form stable radical anions. In addition, the radical anions of the NDI feature intense and characteristic absorption band s in both visible and near infrared region s which allows one to spectroscopically identify the form ation of charge separated states. Figure 2 2. Chemical structures of platinum acetylide oligomers O n the other hand, t hiophene based oligomers and polymers have received considerable attention due to their promising optical, e lectrochemical, and electronic properties. 48 50 Recently,

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42 organometallic systems consisting of oli gothiophenes are gaining increasing int erest 51 In th e present investigation, we incorporate d oligothiophenes into conjugated platinum acetylide systems. The resulting core units of [ Pt T n Pt ] ( T = 2, 5 thienyl ene ; n = 1, 2) act as electron donors due to their low oxidation potentials according to our previous stud y 42 The electron donor and the electron acceptor are linked by several platinum acetylide repe at units that also act as spacers. T he objective of this work is to study photophysical properties and intrachain charge separation dynamics of the platinum acetylide oligomers that feature a donor spacer acceptor architecture Synthesis T he synthetic st rategy we utilized for this series of platinum acetylide oligomers is an iterative convergent approach according to methods similar to those we have previously reported. 33,34 Briefly, the structures were assembled from the core o utward, where the platinum acetylide segments were constructed iteratively and the NDI end groups were prepared separately and attached to the oligomers in the final step T he synthesis of the core units 6a b is shown in Figure 2 3. I t started from the iod ination of 2, 2 bithiophene with N iodosucci ni mide (NIS) in a mixed solvent system consistin g of methanol and acetic acid. Then the Sonogashira coupling between 5, 5 diiodo 2, 2 bithiophene ( or 2, 5 dibromothiophene ) and trimethylsilyl acetylene (TMS A) gav e the protected diacetylene oligothiophene s 2a b in good yield s After deprotection under basic condition s the resulting compounds 3a b were reacted with cis dichloro bis (tr i n butylphosphine)platinum(II) in diethylamine to give the platinum complex es 4a b in ~88% yields. T h e c omplexes 4a b were further coupled with 1 et hynyl 4 (trimethylsilylethynyl) benzene in refluxing diethylamine to

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43 give the TMS protected platinum complex es 5a b D eprotection of the complexes 5a b in the presence of potassium hydroxid e at room tem perature gave the core units 6a b in high yields Figure 2 3. Synthesis of the platinum acetylide complexes 6a b The synthesis of the cores containing four platinum ac e tylide repeats is shown in Figure 2 4. First, the TMS protected platinum complex 8 was synthesized in two steps that started from

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44 the preparation of 1 et hynyl 4 (trimethylsilylethynyl) benzene followed by the coupling reaction with cis dichloro bis (tr i n butylphosphine )platinum (II) The overall yield of two steps is ~ 50%. T he Ha gihara coupling reaction s between the complexes 6a b and the complex 8 in diethylamine with copper iodide as catalyst gave the TMS protected platinum complex es 9 a b in ~82% yields. After deprotection in basic condition s the co res containing four platinum acetylide repeats 10 a b were obtained in high yields. Figure 2 4. Synthesis of the platinum acetylide complexes 10 a b The synthesis of the NDI end group s is shown in Figure 2 5. C ondensation of 1,4, 5,8 naphthalenetetracarboxy lic dianhydride with 1 aminooctane in reflux ing DMF gave N (n octyl) naphthalene 1,8 dicarboxyanhydride 4,5 dicarboximide 11 in 36% yield after separation from its di substitu ted side product. T hen compound 11 was condensed with 4 iodoaniline in reflu x ing

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45 DMF to yield the corresponding iodide 1 2 T he Sonogashira reaction between 1 2 and triisopropylsilylacetylene (TIPS A) gave the TIPS protected acetylene 1 3 After deprotection with tetra n butyl ammonium fluoride ( TBAF ) at room te mperature, the resulting acetylene 1 4 was coupled with cis dichloro bis (tri n butylphosphine)platinum(II) to give the NDI end group 1 5 in 65% yield. Figure 2 5. Synthesis of the NDI end group For the synthesis of the oligomers Pt4 TmA and Pt6 TmA (m = 1, 2) the final step was the coupling reaction s between the cores and the end groups as shown in Figure 2 6A The ol igomers Pt2Tm A (m = 1, 2) were synthesized by coupling the platinum complex 4a b with compound 1 4 (Figure 2 6B) T he se reactions gave moderate to high yields

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46 Figure 2 6. A) Synthesis of Pt4Tm A and Pt6Tm A (m = 1, 2) oligomers; B) Synthesis of Pt2 Tm A oligomers

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47 The oligomers were characterized by 1 H and 31 P NMR, and elemental analysis. T he J P t P coupling constant (2340 2360 Hz) in 31 P NMR is below 2500 Hz, indicating that the platinum center s adopt trans configuration in th is series of oligomers. Results Electrochemistry In a previous report, 42 we carried out an extensive electrochemical and pulse radiolysi s study of a series of platinum acetylide oligomers which are structurally similar to the comp lexes that are the focus of this i nvestigation. Some key findings o f this work are the following. First, one electron oxidation of ol i gomers that contain both thienylene and phenylene repeat units exhibit s t wo reversible oxidation waves. The first oxidatio n is cente red on the thienylene segment ( E 1/2 0.7 V) and the second is centered on th e phenylene repeats, (Pt P h ) n (E 1/2 Pulse radiolysis was also used to study the absorption spectra of the radical cations, and these data wi ll be described below as we discuss the transient absorption properties of these series of oligomers The electrochemical properties of Pt4T1 A and Pt4T2 A were investigated by cyclic voltammetry (CV ) using nitrogen degassed methylene chloride solutions wi th tetra n butylammonium hexafluorophosphate ( TBAH 0.1 M) as a supporting electrolyte. T he cyclic voltammograms are shown in Figure 2 7 Both complexes feature two reversible waves in t he reductive branch of the CV. These waves appear at 0. 6 4 and 1. 05 V and they are believed to arise from sequential one electron reductions centered on the NDI end groups. This assignment is supported by literature studies of other compounds that contain the NDI unit which show very similar reductive electrochemical respon se. 52,53 The ox i dative branch of the CV is more complicated, but on the basis of our previous work 42 we are able to assign the observed waves to

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48 sequential oxidations centered on the thienylene core segment and the (Pt P h ) n Pt4T1 A and Pt4T2 A appears at 0.63 V and 0.60 V, respectively. These waves are due to one electron oxidation of electrophores that are concentrated on the thienylen The oxidation is slightly less positive in Pt4T2 A which reflects the fact that the radical cation is slightly more stable on the bithiophene unit. Sweeping to more positive potent ials reveals additional waves. For Pt4T1 A the more positive wave is due to oxida tion centered on the (Pt P h ) n segments, and for Pt4T2 A several additional waves are observed which are ascribed to further oxidation of the bithiophene core and oxidation of the (Pt P h ) n segments. 42 Figure 2 7. Cyclic voltammogram s A ) Pt4T1 A B ) Pt4T2 A (in methylene chloride with 0.1 M TBAH as electrolyte, vs SCE)

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49 UV V is Absorption The a bsorption spectra of all of the oligomers were recorded in THF solutions (Figure 2 8 ). First, qualitatively the molar absorption coefficient s increase with the chain length consistent with the notion that each platinum acetylid e repeat unit acts as a chromophore to increase the absorption cross section of the oligomer. There are specific features in the spectra that can be assigned to the individual chrom ophore units in the oligomers. First, for PtnT1 A series (Figure 2 8 A n = 2 4, 6 ) the near UV absorption is dominated by a strong band with several distinc t maxima. The origin of these bands is understood by reference to the spectra of the model complex Pt4T1 and a model for the NDI (see Figure A 1 in the appendix A for the stru cture) chromophore which are shown in the inset to Figure 2 8 A In particular, the near UV absorption of PtnT1 A series arises from a superposition of the transition localized on the platinum acetylide segments ( max 33 and the transition of the NDI chromophores which appear as a band with max defined vibronic sub bands that appear at shorter wavelength. Note that the relative intensity of the structured absorption band due to the NDI chromophores dim inishes along the series Pt2T1 A < Pt4T1 A < Pt6T1 A consistent with the fact that the length of the platinum acetylide spacer is increasing along the series while the number of ND I end groups remains constant. A second important feature in the spectra is th e distinct shoulder that appears on the long wavelength side of the near UV band ( On the basis of our previous work it is clear that this weak band is due to a transition concentrated on the core thienylene segment, ( Pt T 1 Pt ). 54 The absorption spectra for the PtnT2 A (n = 2, 4, 6) series are illustrated in Figure 2 8 B The spectra for these complexes are similar in the near UV region compared to those of PtnT 1 A series ; however, there is a distinct, broad band that app ears in the visible region (400 450 nm)

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50 which is clearly due to the bithie nylene core segment, ( Pt T 2 Pt ). The appearance of the distinct long wavelength band which is due to the core bithienylene segment is significant; as shown below, this allows us to selectively photoexcite this segment of the oligomer in transient absorption experiments. Figure 2 8. Absorption spectra of p latinum acetylide oligomers in THF solution I nset of A : Pt4T1 NDI (see Figure A 1 in the appendix A for the structure ) ; Inset of B : Pt4T2 Stea d y State Photolumine scence The photoluminescence spectra of all of the oligomers and model compounds were recorded in deoxygenated THF solutions with the excitation wavelength at = 378 nm. Figure 2 9 shows the spectra of Pt4T1 A Pt4T2 A and their corresponding model compound s. The

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51 photoluminescence spectra of other oligomers are shown in the appendix A as Figure A 2 F or the model compound Pt4T1 the spectrum features a broad, dominant band centered at = 610 nm along with a vibronic band These bands are attributed to the ph osphorescence emission Figure 2 9 Photoluminescence spectra of p latinum acetylide oligome rs in deoxygenated THF solution All of spectra were obtained with a = 378 nm excitation. The emission spectrum of Pt4T1 A was normaliz ed at 420 nm arising from the thienylene segment. 54 A relatively weak band centered at = 520 nm arises from t he phosphorescence emission of the platinum ace tylide segment consistent with our previous report. 33 The relative intensity of this band is smaller than the phosphorescence band of the thienylene segment by over an order of magnitude A f luorescence emission band of the thienylene segment appears at max = 420 nm. Compared with its model compound Pt4T1 Pt4T1 A only s hows a fluorescence emission band centered at max = 420 nm, which is attributed to the superposition of the fluorescence emission from the thien ylene segment and the NDI end

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52 group s (See Figure A 3 in the appendix A for the emission spectrum of NDI) Both phosphorescence emission bands originating from the thienylene and the platinum acetylide segments are very weak, indicating that the phosphorescence emission is effectively quenched in Pt4T1 A This is also confirmed by the decrease of the quantum yield in Pt4T1A relative to its model compound Pt4T1 ( Table 2 1 ) For the model compound Pt4T2 the photoluminescence spectrum feature s a broad, dominant band centered at = 460 nm which is attributed to the fluorescence emission of the bithienylene segment. Not e that a shoulder next to the main emission band arising from the platinum acetylide segment appears at = 520 nm consistent with the moderate emission band in Pt4T1 A weak emission band appears at = 720 nm which is assigned to the phosphorescence emi ssion of the bithenylene segment according to our previous report 54 Compared with the strong phosphorescence emission of the thienylene segment in Pt4T1 the phosphorescence emission of the bithienylene segment in Pt4T2 is dra matically weak. Based on the previous study by Chawdhury and coworkers in the platinum thienylene based polymers 55 a pos sible explanation for the weak phosphorescence emission of the bithenylene segment in Pt4T2 is the reduced intersystem crossing caused by the increase of the number of thiophene rings which reduces the influence of the platinum metal, which is mainly respo nsible for the intersystem crossing. For the NDI acceptor end capped oligomer Pt4T2 A the photoluminescence spectrum is dominated by the fluorescence emission of the bithienylene segment c entered at = 460 nm. T he shoulder of the phosphorescence emission arising from the platinum acetylide segment disappears in Pt4T2 A Note that a band on the left of the main fluorescence emission band c entered at = 4 2 0 nm is assigned to the fluorescence emission of the NDI en d groups by

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53 comparing with the NDI photolumi nescence spectrum ( see Figure A 3 in the appendix A ). The weak phosphorescence emission of the bithi enylene segment decreases and is almost invisible in the Pt4T2A spectrum. C ompared with its model compound Pt4T2 the phosphorescence emission is effectivel y quenched in the Pt4T2A oligomer The photophysical data are shown in Table 2 1 The detailed quenching mechanism will be analyzed in the discussion section. Table 2 1 Photophysical data for oligomers and model compounds UV VIS / max/nm / M 1 cm 1 10 5 PL/ max/nm /% / ns F P CSS S Pt2T1 A 360, 378 1.33 420 0.058 20 Pt4T1 A 360, 378 1.99 420 610 0.060 47 Pt6T1 A 360, 378 2.91 420 610 0.200 65 Pt2T2 A 360, 378, 410 0.93 460 0.189 20 Pt4T2 A 360, 378, 410 1.47 460 720 0 .280 35 1820 Pt6T2 A 360, 378, 410 2.82 460 720 0.330 191 3690 Pt4T1 369 420 520,610 1.640 8818 Pt4T2 354, 420 460 520,720 0.530 4400 Transient Absorption In order t o explor e detailed electron transfer processes, nanosecond transient absorption spectra of the series of oligomers were record ed in deoxygenated THF solutions. PtnT1 A series with 355 nm laser excitation. Several important features can be seen from the transient a bsorption spectra of PtnT1 A series following 355 nm laser excitation (Figure 2 10 ) T he excitation wavelength corresponds to the absorption band of the platinum acetylide segment ( Pt P h Pt ) First, an intense transient absorption band in the visi ble region centered at = 480 nm appears on all of the three oligomers. T his band is assigned to the characteristic absorption of the NDI radical anion by comparing with the spectra of the NDI

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54 derivatives in the literature reports. 46,52 Second, a less intense band appears in the red region centered at = 610 nm, which is also attributed to the optical absorption of the NDI radical anion consistent with literature reports. 46,52 Third, an intense and broad band between 500 and 600 nm superpose s with the absorption bands of the NDI radical anion In order to assign thi s band, we refer to our recent study in radical ion states of platinum acetylide oligomers. 42 In this study, a pulse radiolysis method was utilized to produce radical ions in an extended series of platinum acetylide oligomers. The strctures of the oligomer s and corresponding pulse radiolysis transient absorption spectra of radical cations are shown in Figure 2 1. The radical cation of the model compound Pt4T1 exhibit s two absorption bands : one in the visible and the second in the near IR. T he broad visible band with a maximum at = 52 0 nm a ppear s between 450 and 640 nm. Both visible and near IR bands were assigned to the radical cation absorption concentrated on the thienylene segment. The other oligomer Pt4 that structurally consists of all platinum acetylide repeats was also stu died by the pulse radiolysis method. The radical cation absorption in Pt4 features an intense absorption band in the visible and a weak absorption band in the near IR. T he visible band with a maximum at 50 0 nm appear s between 400 and 540 nm. Both visib le and near IR bands in Pt4 were assigned to the radical cation absorption concentrated on the platinum acetylide segment. By comparing the transient absorption spectrum of Pt4T1A in the current study with the radical cation absorption spectra of Pt4T1 and Pt4 we assigned the absorption band between 500 and 600 nm in Pt4T1A to the radical cation absorption of the thienylene segment. It can be seen that both visible absorption bands in Pt4T1A and Pt4T1 have similar ly broad feature and the absorption regions match very well. The lifetimes recovered from the transient absorption spectra indicate a sufficientl y long lived charge separation i n these oligomers (See Table 2 1). Finally, note that the lifetimes increase with the oligomer chain

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55 length indicating th at charge recombination rates decrease with increasing the distance between the radical anion and the radical cation. Figure 2 1 0 Transient absorption spectra of PtnT1 A series in deoxygenated THF soution All of the spectra we re obtained following 355 nm laser excitation PtnT2 A series with 355 nm laser excitation. The transient absorption spectra for PtnT2 A series with 355 nm laser excitation are similar to those of PtnT1 A series (Figure 2 11 ). T he characteristic absorption of the NDI radical anion appears as an intense band centered at = 480 nm. However, this band is more distinct and separated from the radical cation absorption. A n

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56 intense and broad band centered at = 650 nm can be unambiguously assigned to the absorption of the bithienylene radical cation This assignment is confir med by referring to the radical cation absorption of the model compound Pt4T2 in our previous study of radical ion states of platinum Figure 2 1 1 Transient absorption spectra of PtnT2 A seri es in deoxygenated THF solution A ll of the spectra were obtained following 355 nm laser excitation acetylide oligomers that has been described in the introduction part of this chapter 42 The study indicates that the radical cation of Pt4T2 features an intense absorption band with a maximum at

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57 = 650 nm in the visible region. The band was assigned to the radical cation absorption centered on the bithienylene segment. The structure and absorption region of this band match very well with the absorption band in Pt4T2 A The red shift of the radical cation absorption of the bithienylene segment relative to that of the thienylene segment indicates that the radical cations are more stable in PtnT2A series of oligomers. The absorption band of the NDI radical anion at = 610 nm is n o t well resolved due to its superposition with the absorption of the bithienylene radical cation. T he lifetime s recovered from transient absorption spectra increase with the oligomer chain length, indicating the same trend as in the PtnT1 A series. However, the lifetime s increa se more than four times from Pt4T2 A (35 ns) to Pt 6 T 2 A (191 ns), compared with less than one time increase from Pt4T 1 A (47 ns) to P t6 T 1 A (65 ns). T he striking long lived charge separated state in Pt 6 T 2 A indicates that the radical ions are sufficiently stabl e and the charge recombination rate is largely reduced. PtnT2 A series with 420 nm laser excitation. One important feature for PtnT2 A series of oligomers is that a distinct absorption band centered at = 410 nm arising from the bithienylene segment is separated from the absorption of the platinum acetylide segment. This feature allows us to s electively photo excit e the bithienylene chromophore in the PtnT2 A series W e utilized 420 nm laser as an excitat ion source and t he corresponding transient absorption spectra were obtained as shown in Figure 2 12 For the oligomer Pt2T2 A the charge separated state with same spectroscopic feature s as excited at 355 nm was observed. H owever, for Pt4T2 A and Pt6T2 A the spectra feature broad and dominant bands between 600 and 700 nm. By comparing with the transient absorption spectrum of the model compound Pt4T2 we are able to assigned these bands to the T 1 T n absorption of the bithienylene segment consistent with our pr evious study 54 The lifetimes recovered from the transient absorption spectra are in the microsecond

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58 range, which are typical triplet state lifetimes (Table 2 1). Note that the lifetime for Pt4T2A is less than that of Pt6T2A suggesting that there is a quenching process active that depends on the length of the spacer. Figure 2 12 Transient absorption spectra of PtnT2 A series in deox ygenated THF solution A ll of the spectra were obtained following 420 nm laser excitation Discussion Phosphorescence Quenching in Pt4T1A and Pt4T2A In order t o gain insight into the phosphorescence quenching mechanism in Pt4T1 A we first analyze the photop hysical processes of the model compound Pt4T 1 Note that the model compound Pt4T1 consists of two distinct chromophores: the core thienylene segment ( Pt

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59 T 1 Pt ), and the platinum acetylide segment ( Pt P h Pt ) T h e absorption spectrum of Pt4T1 (see inset of Figure 2 8a ) feature s a broad, dominant band centered at = 370 nm which is attribut ed to the absorption superposition of its two distinct chromophores. According to our previous report 33 the ab sorption maximum of the platinum acetylide chromophore is centered at max = 367 nm. Guo and coworkers reported the absorption maximum ( max 385 nm ) of a thienylene based compound Pt2T 1 with the structure of [ Ph 1 Pt(PBu 3 ) 2 T 1 Pt(PBu 3 ) 2 Ph 1] (Ph 1 = 1 phenylene). 38,56 This allows us to assign the absorption maximum of the thienylene segment. A thienylene based platinum acetylide polymer with the absorption maximum max = 406 nm was also reported by Chawdhury and coworkers 55 The blue shift of the absorption maximum of the thienylene chrom o phore in Pt2T 1 is likely due to the singlet exciton destabi liz ation in the oligomer compared with the polymer. In order to avoid the direct excitation of the thienylene segment, the model compound Pt4T1 was first excited at = 3 60 nm. This wavelength corresponds to the absorption of the platinum acetylide segment. The resulting photoluminescence spectrum is similar to the spectrum obtained with ex = 378 nm (see Figure 2 9) The spectrum is dominated by the phosphorescence e mission of the thienylene segment According to our previous study, 54 this can be rationalized by effective triplet energy transfer from the platinum acetylide segment to the thienylene segment. W ith = 378 nm photoexcitation that corresponds to the main absorption of the thienylene segment and a small portion absorption of the platinum acetylide segment the phosphorescence emission of the platinum acetylide segment still appears in the photoluminescence spectrum. Its intensity does not significant ly decr ease compared with the phosphorescence with direct excitation of the platinum acetylide segment at = 3 60 nm. A mechanism involving an equilibrium of energy transfer was proposed as shown in Figure 2 13 according to our previous study. 54

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60 Figure 2 1 3 P hotophysical processes of Pt4T1 ET = Energy Transfer; S 1 = Singlet exci ted state; T 1 = Triplet excited state For the NDI end capped oligomer Pt4T1 A more than 95% of the phosphorescence emission from both the platinum acetylide segment and the thienylene segment is quenched and only fluorescence and very weak phosphorescence e mission were observed in its photoluminescence spectrum. T he phosphorescence quenching can be rationalized by photoinduced electron transfer in this system. T he NDI moiety is easily reduced and widely used as an electron acceptor. 43 46,52,57 When the NDI moieties are attached to the platinum oligomers, the donor spacer acceptor system form s where the donor is the thienylene segment, the spacer is the platinum acetylide segment and the acceptor is the NDI end group We observed efficient phosphorescence quenching in Pt4T1A with the excitation at both = 360 and 378 nm With the excitation at = 360 nm, first, the platinum acetylide segment is excited to its singlet state

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61 followed by rapid intersystem cros sing to decay to its triplet state Photoinduced electron transfer occurs from the triplet state of the platinum acetylide segment to the NDI acceptor. While the electron transfer rate was not obtained in our nanosecond transient absorption experiments th e efficient phosphorescence quenching indicates that energy transfer from the platinum acetylide segment to the thienylene segment is significantly suppressed by the electron transfer process W ith the excitation at = 378 nm, the thienylene segment is ex cited to its singlet state. Because the equilibri um mechanism for energy transfer exists in these systems the final triplet states are indepen den t on the excitation wavelength. Consequently, electron transfer process es are similar to those with the excita tion at = 360 nm. Note that direct electron transfer from the triplet state of the thienylene segment to the NDI acceptor may also occur This assumption is supported by the increasing quantum yield with increasing the oligomer chain length. Direct elect ron transfer from the thienylene segment to the NDI acceptor is affected by the electronic coupling between the two segments. With increasing the oligomer chain length, th e electronic coupling decrease s due to the separation by increased platinum acetylide spacers. The efficiency of the phosphorescence quenching for the thienylene segment decreases, and consequently the quantum yield increases along the order of Pt2T1A < Pt4T1A < Pt6T1A T he model compound Pt4T2 features weak phosphorescence emission arising from the bithienylene segment. According to our previous study, 54 there is no significant difference for the intensit y of the T 1 T n absorption between Pt4T1 and Pt4T2 as shown in their transient absorption spectra suggest ing that the efficiency of the S T intersystem crossing is similar for the two compounds. Thus, t he very low phosphoresce nce emission in Pt4T2 may be caused by low radiative rate. In addition the triplet energy level of the bithienylene segment lies ca. 0.3 eV lower than that of the thien ylene segment According to the energy gap law, the non radiative decay rate increases

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62 exponentially with decreasing the triplet state energy. The low radiative rate can not compete with the rapid non radiative decay rate and as a result, the phosphoresce nce is weak in Pt4T2 W hile the phosphorescence quenching in Pt4T2A is not as significant as in Pt4T1A it is still evident. In particular, the phosphorescence emission of the platinum acetylide segment is almost completely quenched. Partially quenched p hosphorescence of the bithieylene segment was also observed. O nce again, the quenching is due to photoinduced electron transfer from the triplet state of the platinum acetylide segment to the NDI acceptor. Chance s of direct electron transfer from the bithi enylene segment to the NDI acceptor are small considering the rapid non radiative decay rate in the triplet state, meaning the triplet relaxation of the bithienylene segment mainly adopts non radiative pathway. Photoinduced Electron Transfer Mechanism in P tnT1A Series with 355 nm Laser Excitation Nanosecond transient absorption spectra clearly show the formation of the charge separa ted state upon photoexcitation i n the platinum acetylide segment ( = 355 nm). For the PtnT1 A series with the thienylene segment in the core, the characteristic absorption of the NDI radical anion appears as an intense band centered at = 480 nm and a less intense band concentrated at =610 nm. T hese absorption bands a rising from the NDI radical anion are in excellent agreement with the literature reports of the corresponding NDI derivatives 46,52 The absorption of the thienylene radical cation appears as an inte nse and broad band between 500 and 600 nm consistent with our previous study of radical ion state s of platinum acetylide oligomers. 42 Note that the excitation wavelength ( 355 nm ) is located in the blue edge of absorption of the platinum acetylide segment. T he reason for choosing the wavelength is to exclusively excite the platinum acetylide chromophore and to av oid direct excitation of the thienylene chromophore Thus with 355 nm laser excitation the platinum acetylide chromophore is excited to its singlet

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63 excited state. Due to the very efficient S T intersystem crossing (close to unity because of the efficient spin orbit coupling caused by platinum metal) 33 the singlet excite d state rapidly decays to its triplet state and the rate of the intersystem crossing is close to 10 12 s 1 cor responding to 1 ps fluorescence lifetime according to our previous report. 38,56 T he lifetime of the triplet state is long enough to allow electron transfer to the NDI acceptor. Thus electron transfer becomes the dominant pathway for the triplet st ate relaxation and the charge separated state consisting of the NDI radical anion and the platinum acetylide radical cation is born Unfortunatel y, we are not able to detect th ese processes in our nanosecond transient absorption experiments because these events are too fast and beyond the nanosecond range. T he electrochemistry study indicates that the oxidation potential of the thienylene segment i s ~ 0.2 V less positive than that o f the platinum ac e tylide segment This illustrates that the thienylene based radical cation is more stable than the platinum acetylide based radical cation. As a result a consequent hole shift from the platinum acetylid e segment to the thienylene segment follows the initial charge separation to create the second charge separated state, which is clearly shown in the nanosecond transient absorption spectr a Taken together, photoinduced electron transfer with 355 nm laser excitat ion in the Pt4T1 A series of oligomers adopts a stepwise mechanism (Figure 2 14 A ) T he initial charge separated state forms by electron transfer from the triplet state of the platinum acetylide segment to the NDI acceptor followed by consequent hole shift to the thienylene segment to create the final charge separated state, which relaxes to the ground state by charge recombination The electron transfer process may suppress the energy transfer process from the platinum acetylide segment to the thienyl ene segment. However, direct electron transfer from the triplet state of the thienylene segment may also occur, which has been indicated by the efficient phosphorescence quenching

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64 of the thienylene segment in the photoluminescence spectra. Nevertheless, el ectron transfer from the triplet state of the platinum acetylide segment is the major pathway considering the electronic coupling. Also note that from Pt2T1 A to Pt6T1 A the lifetimes of the charge separated state increase, which is consistent with the noti on that the rate of charge recombination is distance dependent 58 The detailed photophysical p rocesses of PtnT1 A series are shown in Figure 2 1 4 B A B Figure 2 14 P hotophysical processes of PtnT1 A series follo wing 355 nm laser excitation A) Photoinduced electron transfer processes; B) E nergy level diagra m, e nergies of charge separated states were estimated from electrochemistry measurements. Blue lines indicate the dominant pathway

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65 Photoinduced Electron Transf er Mechanism in PtnT 2 A Series with 355 and 420 nm Laser Excitation F irst we analyze the electron transfer processes in the PtnT2A series of oligomers following 355 nm laser excitation The compound Pt2T2 with the structure of [ Cl Pt T 2 Pt Cl] ( See Figure A 4 in the appendix A for the structure ) was used as a model to locate absorption of the bithienylene segment. The result shows that the absorption maximum o f the bithienylene segment is centered at = 410 nm and the red re gion (from 400 470 nm) is well overlapped with the corresponding absorption region of Pt2T2 A ( see Figure A 4 in the appendix A ). While the bithienylene segment has the minimum absorption at = 355 nm, the absorption of the platinum acetylide segment is cl ose to maxim um at = 355 nm which is about three times more intense th an that of the bithienylene segment. Thus with 355 nm laser excitation the platinum acetylide chromophore is excited to its singlet excited state followed by rapid intersystem crossin g to decay to its triplet state. Then photoinduced electron transfer from the triplet state of the platinum acetylide segment to the NDI acceptor create s the first charge separated state followed by hole shift to the bithienylene segment to create the sec ond charge separated state. The processes are similar to the PtnT1A series (Figure 2 14A) Note that direct electron transfer from the bithienylene segment to the NDI acceptor is unlikely to happen considering the deficient electronic coupling between the donor and the acceptor, in particular for the longer chain oligomers, such as Pt4T2A and Pt6T2A Second, the non radiative decay is the major pathway for the relaxation of the triplet state of the bithienylene segment in the PtnT2A series oligomers. The ph otophysical process es in the PtnT 2 A series are illustrated in Figure 2 1 5 ( blue lines) For the PtnT2A series oligomers an important feature is that absorption of the bithienylene chromophore is separated from that of the platinum acetylide chromophore in the red region. The bithienylene chromophore absorbs at above 400 nm, while the platinum acetylide chromophore

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66 does not. This feature allows us to selectively excite the bithienylene chromophore without interfere nce from the excitation of the platinum ace tylide chromophore. A B Figure 2 1 5 P hotophysical processes of PtnT2A series following 355 and 420 nm laser excitatio n A) Photoinduced electron transfer with 420 nm laser excitation; B) E nergy level diagram e nergies of charge separated states were estimated from electrochemistry measurements

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67 A ccording to our previous study, 33 triplet exciton is more likely to localize in a chromophore consisting of one repeat unit in platinum acet ylide oligomers. As shown in Figure 2 1 6 the electron ic coupling between the triplet state of the bithienyl ene segment and the NDI acceptors in Pt2T2 A is sufficient to initiate electron transf er when the bithienyl ene segment is directly excited. C ompared with Pt2T2A there are one and two platinum acetylide spacer s between the bithienylene segment and the NDI acceptor in Pt4T2A and Pt6T2A respectively. The increasing platinum acetylide spacers interrupt the electronic coupling and consequently electron transfer does not occur in these oligomers when the bithienylene donor is direct ly photoexcited. T he photophysical processes of the PtnT2A series following 420 nm laser excitation are shown in Fi gure 2 15A and B (red lines) Electron transfer in both series oligomers is strongly exergonic ( G < 0.6 eV). Figure 2 1 6 E lectronic coupling in Pt2T2 A Conclusion In this investigation, a series of p latinum acetylide oligome rs that feature a dono r space acceptor architecture were synthesized. Nanosecond transient absorption spectroscopy was ap plied to study intrachain electron transfer properties in th e oligomers. The formation of the charge separated states was directly obse rved in the transient absorption spectra. The radical anion absorption of the NDI acceptor appears as an intense band centered at 480 nm and the radical cation absorption of the [ Pt T n Pt ] donor was also observed i n the transient absorption spectra ( the absorption band between 500 and 600 nm for the PtnT1 A series and the absorption band centred at = 650 nm for the PtnT2 A series ). The most important finding of

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68 this work is that it demonstrates the ability to control a photoinduced charge separation p rocess by photoselection. It also provides a model for investigating the wavelength dependent behavior of the platinum acetylide systems. Experimental Electrochemistry Electrochemical measurements were recorded in dry dichloromethane solutions containing 0.1 M tetra n butylammonium hexafluorophosphate (TBAH, Aldrich) as the supporting electrolyte. T he setup consisted of a platinum microdisk (2 mm 2 ) working electrode, a platinum wire auxiliary electrode and a silver wire quasi reference electrode. Solutions were degassed with bubbling nitrogen for ca. 5 min before measurements and a positive pressure of nitrogen was maintained during the measurements T he concentrations of the oligomers were ca. 0. 15 m M. All potentials obtained were internally calibrated aga inst the ferrocene/ferricinium couple (E = 0.43 V vs SCE in dichloromethane 59 ). Photophysical M easurements Steady state absorption measurements 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 such that the solutions were o ptical ly dilute (A max < 0.20). Photoluminescence quantum yields were determined according to the ly method described by Demas and Crosby, with the quan tum yield being computed according to eq. 14 in their paper. 6 0 Solutions of Ru(bpy) 3 2+ were used as a reference ( = 0.0379 in air saturated water ).

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69 Transient absorption measurements were conducted on a home built apparatus, 61 which used a Nd: YAG laser for excitation and PI Max intensified CCD camera coupled with spectrograph as a detector. Sample concentration were adjusted so that A 0.8. Synthesis General Solvents and chemicals used for synthesis were of reagent grade and used without further purification unless noted. Reac tions were carried out under an argon atmosphere. NMR spectra were recorded on Varian VXR, Gemini or Mercury 300 MHz spectrometer s Cis dich loro bis (tri n butylphosphine) platinum(II) 62 and 1,4 diethy nylbenzene 33 were prepared according to literature methods. 5,5 D iiodo 2,2 bithiophene ( 1 ) 2 ,2 B ithiophene (0.5 g, 3 mmol) and N iodosuccinimide (1.67 g, 7.4 mmol) were dissolved in methanol (45 m L ). To this solution, acetic acid (0.5 m L ) was added. After stirring for 2 h, a precipitate formed and the flask was placed in a freezer overnight to e nsure complete precipitation of the product. The white solid was then filtered by suction filtration and washed with cold methanol. After drying under vacuum, a white solid was obtained as the product (0.95 g, 76%). 1 H NMR (CDCl 3 300 MHz) 7.1 (d, 2H), 6. 7 (d, 2H). 2,5 Bis [(trimethylsilyl) ethynyl] thiophene ( 2 a) To a solution of 2,5 dibromothiophene (1.41 g, 5.83 mmol) and diisopropylamine (50 m L ) in a flask equipped with a magnetic stirrer and a refluxing conde nser was added copper iodide (11 mg, 0.058 mmol) and palladium(II) chloride (211 mg, 0.3 mmol) under an argon atmosphere. The solution was degassed for 30 min under a ice bath. Then trimethylsilylacetylene (2.3 g, 23.40 mmol) was added via syringe. T he solution was stirred for 1 h at 0 0 C, then ra ised to room temperature and stirred for another 1 h. Then the reaction mixture was allowed to warm to 75 0 C for 20 h until TLC analysis indicated

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70 that all starting material had disappeared T he solution was allowed to cool to room temperature. Diethyl e th er ( 200 m L ) was added to the solution and the produced precipitation was removed by vacuum filtration. T he filtrate wa s washed with 10% HCl solution followed by water twice. T he organic phase was dried with anhydrous magnesium sulfate and filtered. T he sol vent was removed by evaporation under reduced pressure. T he crude product was dissolved in hexanes and was purified by column chromatography on silica u s ing hexane as the eluent. T he product 2 a was obtained (1.13 g, 70%). 1 H NMR (CDCl 3 300 MHz) 7.02 (s, 2H), 0.22 (s, 18H). 5, 5 Bi s [(trimethylsilyl ethynyl)] 2, 2 bithiophene ( 2 b). 5, 5 D iiodo 2, 2 bithophene ( 1 ) (0.2 g, 0.48 mmol) and trimethylsilylacetylene (98 mg, 1 mmol) were dissolved in diethylamine (8 m L ) and the solution was degassed with argon f or 30 min. Then, dichloro bis (triphenylphosphine) palladium (II) (0.05 eq, 17 mg, 0.024 mmol) and CuI (0.1 eq, 9 mg, 0.048 mmol) were added and the mixture was stirred at room temperature for 4 h. Methylene chloride ( 50 m L) was added and the solution was w ashed with 10% NH 4 OH (100 m L ) and DI water (100 m L ), dried with anhydrous sodium sulfate. Chromatography on silica (hexane) gave the des ired product 2 b as a yellow solid (0.12 g, 70%). 1 H NMR (CDCl 3 300 MHz) 7.1 (d, 2H), 7.0 (d, 2H), 0.22 (s, 18H). 2,5 D iethynyl thiophene ( 3 a). 2,5 Bis [(trimet hylsilyl) ethynyl] thiophene ( 2 a ) (0.32 g, 1.16 mmol) was dissolved in methanol (20 m L ) and degassed for 15 min with argon. To this solution potassium hydroxide ( 0.1 m L, 0.5 M in water ) was added and the mixture w as stirred at room temperature for 4 h. Then water (50 m L ) was added and the mix ture was extracted with pentane T he organic phase was dried with anhydrous sodium sulfate The solvent was removed under reduced pressure at room temperature. A light yellow oil was obtained as the product (100 mg, 65%). 1 H NMR (CDCl 3 300 MHz) 7.22 (s, 2H), 3.5 (s, 2H).

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71 5,5 D iethynyl 2,2 bithiophene ( 3 b). 5,5 Bis [(trimethylsilyl ethynyl)] 2,2 bithiophene ( 2 b ) (115 mg, 0.32 mmol) was dissolved in THF (6 m L ) and the s olution was degassed for 15 min with argon. Then, TBAF (1.28 m L of a 1 M solution, 1.28 mmol) was added via syringe and the mixture was stirred at room temperature protected from light for 3 h. Then, the solvent was removed and chromatography on silica ( 9 : 1 h exane /CH 2 Cl 2 ) gave a yellow solid as the product (65 mg, 95%). 1 H NMR (CDCl 3 300 MHz) 7.15 (d, 2H), 7.0 (d, 2H), 3.4 (s, 2H). Compound 4 a. 2,5 D iethynyl thiophene ( 3 a ) (43 mg, 0.325 mmol) and cis dich loro bis (tri n butylphosphine) platinum(II) (0.46 g, 0.686 mmol) were dissolved in diethylamine (15 m L ) and the resulting solution was degassed for 15 min with argon. The m ixture was stirred under reflux overnight The s olvent was removed and chromatography on silica (7:3 hexane/CH 2 Cl 2 then 1:1) gave a yellow solid as the product (0.4 g, 88%). 1 H NMR (CDCl 3 300 MHz) 6.6 (s, 2H), 1.9 2.0 (m, 24H), 1.4 1.6 (m, 48H), 0.9 (t, 36H); 31 P NMR (CDCl 3 121 MHz) 8.05 (J Pt P = 2398.3 Hz). Compound 4 b. This compound was synthesized according to the same proced ure used for compound 4 a except 5,5 diethynyl 2,2 bithiophene ( 3 b ) (65 mg, 0.3 mmol) and cis dich loro bis (tri n butylphosphine) platinum(II) (423 mg, 0.63 mmol) were used. Chromatography on silica (hexane first, then 9:1, 4:1, 7:3, 3:2 hexane/CH 2 Cl 2 ) ga ve a yellow green solid as the product (325 mg, 73%). 1 H NMR (CDCl 3 300 MHz) 6.8 (d, 2H), 6.65 (d, 2H), 1.9 2.0 (m, 24H), 1.4 1.6 (m, 48H), 0.9 (t, 36H); 31 P NMR (CDCl 3 121 MHz) 8.2 (J Pt P = 2353.7 Hz). Compound 5 a. This compound was synthesized acco rding to the same procedure used for compound 4 a except compound 4 a (540 mg, 0.386 mmol) and 1 ethynyl 4 (trimethylsilylethynyl) benzene (191 mg, 0.963 mmol) were used. Chromatography on silica (7:3

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72 hexane/CH 2 Cl 2 ) gave a yellow green solid as the product ( 500 mg, 75%). 1 H NMR (CDCl 3 300 MHz) 7.28 (d, 4H), 7.15 (d, 4 H), 6.6 (s, 2H), 1.9 2.0 (m, 24H), 1.4 1.6 (m, 48H), 0.9 (t, 36H), 0.2 (s, 18H); 31 P NMR (CDCl 3 121 MHz) 4.21 (J Pt P = 2333.2 Hz). Compound 5 b. This compound was synthesized according to the same procedure used for compound 4 a except compound 4 b (400 mg, 0.27 mmol) and 1 ethynyl 4 (trimethylsilylethynyl) benzene (117 mg, 0.59 mmol) were used. Chromatography on silica (7:3 hexane/CH 2 Cl 2 ) gave a yellow green solid as the product (390 mg, 80% ). 1 H NMR (CDCl 3 300 MHz) 7.3 (d, 4H), 7.15 (d, 4H), 6.8 (d, 2H), 6.6 (d, 2H), 1.9 2.0 (m, 24H), 1.4 1.6 (m, 48H), 0.9 (t, 36H), 0.2 (s, 18H); 31 P NM R (CDCl 3 121 MHz) 4.33 (J Pt P = 2340.1 Hz). Compound 6 a. Compound 5 a (545 mg, 0.316 mmol) was dissol ved in methanol (10 m L ) and THF (10 m L ). The yellow solution was degassed for 15 min with argon Then potassium hydroxide ( 1.6 m L, 2M in water) was added via syringe. The mixture was stirred for 4 h at room temperature. The solvent was removed by vacuum ev aporation and the residue was diluted with methylene chloride. The solution was washed with DI water, dried over anhydrous sodium sulfate. After remov al of the solvent, a yellow solid was obtained as the product (386 mg, 77%). 1 H NMR (CDCl 3 300 MHz) 7.2 8 (d, 4H), 7.15 (d, 4 H), 6.6 (s, 2H), 3.1 (s, 2H), 1.9 2.0 (m, 24H), 1.4 1.6 (m, 48H), 0.9 (t, 36H); 31 P NMR (CDCl 3 121 MHz) 4.25 (J Pt P = 2344.5 Hz). Compound 6 b. This compound was synthesized according to the same procedure used for compound 6 a exc ept compound 5 b (186 mg, 0.1 mmol) and potassium hydroxide (0.5 m L, 2M in water) were used. A y ellow green solid was obtained as the product (147 mg, 88%). 1 H NMR (CDCl 3 300 MHz) 7.3 (d, 4H), 7.15 (d, 4H), 6.8 (d, 2H), 6.6 (d, 2H), 3.1 (s, 2H), 1.9 2.0 (m, 24H), 1.4 1.6 (m, 48H), 0.9 (t, 36H); 31 P NMR (CDCl 3 121 MHz) 4.36 (J Pt P = 2338.9 Hz).

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73 1 Ethynyl 4 (trimethylsilylethynyl)benzene ( 7 ) A solution of 1,4 diethynylbenzene (2 30 m g, 1.82 mmol) in dried THF ( 30 mL ) was cooled to 78 0 C. To this solutio n was added LDA ( 1.09 mL 2.73 mmol, 2.5 M in hexanes) via syringe over a period of 1 h. The mixture was stirred at 78 0 C for 2 h, and then trimethylsilyl chloride ( 0.35 mL 2.73 mmol) was added. The mixture was warmed to room temperature and stirred over night. The solvent was removed under reduced pressure, and the remaining residue was dissolved in diethyl ether. The ether solution was washed with water, dried over anhydrous sodium sulfate and the solvent was removed under reduced pressure. The crude pro duct was purified by chromatography using hexanes as an eluent. The product was exposed to high vacuum (25 mtorr) for 12 16 h at room temperature to remove unreacted starting material. The desired product 7 was obtained ( 280 m g 7 8 % ). 1 H NMR (CDCl 3 300 MH z) 7.4 (s, 4H), 3.1 (s, 1H), 0.2 (s, 9H). Compound 8 C is d ichloro bis (tri n butylphosphine)platinum(II) ( 372 mg, 0. 55 mmol) and 1 ethy nyl 4 (trimethylsilylethynyl)benzene ( 7 ) ( 110 mg, 0. 55 mmol ) were dissolved in diethylamine ( 10 mL ) and the solution was degassed for 15 min with argon The mixture was stirred under reflux for 4 h. Chromatography on silica (7:3 hexane/CH 2 Cl 2 ) gave a yellow solid as the product (330 mg, 72%). 1 H NMR (CDCl 3 300 MHz) 7.3 (d, 2H), 7.1 (d, 2H), 1.9 2.0 (m, 12H), 1.4 1.6 (m, 24H), 0.9 (t, 18H), 0.2 (s, 9H); 31 P NMR (CDCl 3 121 MHz) 8.04 (J Pt P = 2365.0 Hz). Compound 9 a. Compound 6 a ( 100 mg, 0.06 mmol) was dissolved in diethylamine (10 mL) and the solution was degassed for 15 min with argon. Then, compound 8 ( 126 mg, 0.1 5 mmol) and CuI ( 1.1 mg, 0.0 06 mmol) were added and the mixture was stirred at room temperature overnight T he solvent w as removed and the crude product was purified by chromatography on silica (3: 2 hexane/CH 2 Cl 2 ) to give a yellow solid as the product ( 143 mg,

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74 75% ) 1 H NMR (300 MHz, CDCl 3 ) 7.28 (d, 4H), 7.15 (d, 4H), 7.07 (s, 8H), 6.6 (s, 2H), 1.9 2.0 (m, 48H), 1.4 1.6 (m, 96H), 0.9 (t, 72H), 0.2 (s, 18H); 31 P NMR (CDCl 3 121 MHz) 4.12, 4.09 (J Pt P = 2352.5 Hz). Compound 9 b. This compound was synthesiz ed according to the same procedure used for compound 9 a except compound 6 b (90 mg, 0.054 mmol), compound 8 (100 mg, 0.12 mmol) and CuI (1.0 mg, 0.005 mmol) were used. Chromatography on silica (7:3, then 3:2 hexane/CH 2 Cl 2 ) gave a yellow solid as the produc t (144 mg, 82%). 1 NMR (300 MHz, CDCl 3 ) 7.28 (d, 4H), 7.15 (d, 4H), 7.07 (s, 8H), 6.8 (d, 2H), 6.6 (d, 2H), 1.9 2.0 (m, 48H), 1.4 1.6 (m, 96H), 0.9 (t, 72H), 0.2 (s, 18H); 31 P NMR (CDCl 3 121 MHz) 4.24, 4.18 (J Pt P = 2351.8 Hz). Compound 10 a. This compo und was synthesized according to the same procedure used for compound 6 a except compound 9 a (94 mg, 0.03 mmol) and potassium hydroxide ( 0.15 m L 2 M in water ) were used. A y ellow solid was obtained as the product (86 mg, 95%). 1 H NMR (300 MHz, CDCl 3 ) 7. 28 (d, 4H), 7.15 (d, 4H), 7.07 (s, 8H), 6.6 (s, 2H), 3.1 (s, 2H), 1.9 2.0 (m, 48H), 1.4 1.6 (m, 96H), 0.9 (t, 72H); 31 P NMR (CDCl 3 121 MHz) 4.12, 4.09 (J Pt P = 2352.5 Hz). Compound 1 0 b. This compound was synthesized according to the same procedure used for compound 6 a except compound 9 b (134 mg, 0.04 mmol) and potassium hydroxide ( 0.2 m L 2 M in water ) were used. A y ellow solid was obtained as the product (124 mg, 99%). 1 H NMR ( CDCl 3 300 MHz ) 7.28 (d, 4H), 7.15 (d, 4H), 7.07 (s, 8H), 6.8 (d, 2H), 6.6 (d, 2H), 3.1 (s, 2H), 1.9 2.0 (m, 48H), 1.4 1.6 (m, 96H), 0.9 (t, 72H); 31 P NMR (CDCl 3 121 MHz) 4.24, 4.18 (J Pt P = 2351.8 Hz). N (n octyl) naphthalene 1,8 dicarbo xyanhydride 4,5 dicarboximide (11 ) 43 1,4,5,8 N aphthalenetetracarboxydianhydride (10.0 g, 0.0373 mol) was refluxed under argon with stirring

PAGE 75

75 in DMF (100 m L ). 1 Aminooctane (4.82 g, 0.0373 mol) was added dropwise down a condenser over 5 min. The m ixture was refluxed for 15 h, and then cooled in a refrigerator for 2 h. The white precipitate was suction filtered. The filtrate was stripped to near dryness on a rotary evaporator, taken up in p xylene, and the solvent was evaporated again. The residue w as purified by column chromatography on silica ( CH 2 Cl 2 ) A light yellow solid was obtained as the product (5.1 g, 36%). 1 H NMR (CDCl 3 300 MHz) 8.82 (s, 4H), 4.2 (t, 2H), 1.28 (s, 12H), 0.88 (t, 3H). N (n octyl) N (4 iodo phenyl) naphthalen e 1,8:4,5 tet racarboxydiimide (12 ) N (n octyl) naphthalene 1,8 dicarbo xyanhydride 4,5 dicarboximide ( 11 ) (0.4 g, 1.05 mmol) was dissolved in DMF. The solution was degassed for 15 min with argon and heated to 80 0 C. Then 4 iodoaniline (0.276 g, 1.26 mmol) and CaO (15 m g, 0.266 mmol) were added under a n argon atmosphere The solution was refluxed for 18 h at 150 0 C. The hot solution was filtered to remove CaO and the filtrate was cooled in an ice bath The residue yellow precipitate was filtered again and washed with hot hexane to afford a pale yellow powder as the product (0.4 g, 66%). 1 H NMR (CDCl 3 300 MHz) 8.8 (s, 4H), 7.9 (d, 2H), 7.0 (d, 2H), 4.2 (t, 2H), 1.28 (s, 12H), 0.85 (t, 3H). N (n octyl) N (4 triisopropylsilylethynylphenyl) naphthalene 1,8:4,5 tetraca rbox ydiimide ( 13 ) N (n octyl) N (4 iodo phenyl) naphthalene 1,8:4,5 tetracarboxydiim ide ( 12 ) (130 mg, 0.224 mmol) was dissolved in a mixed solvent system ( 1:1: 1, THF / i Pr 2 NH / CH 3 CN 20 m L) and the mixture was degassed for 30 min with argon. Pd(PPh 3 ) 4 (0.02 eq 0.0045 mmol, 5.2 mg) and CuI (0.04 eq, 0.009 mmol, 1.71 mg) were added under an argon atmosphere. Then triisopropylsilyl acetylene (1.5 eq, 0.334 mmol, 61 mg) was added dropwise and the reaction solution was heated to 40 0 C for 20 h. The solution was coo led to room temperature and diluted with chloroform (75 m L ), washed with saturated NH 4 Cl (50 m L ),

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76 DI water (50 m L ) and saturated NaCl (50 m L ), and dried over anhydrous sodium sulfate. After remov al of the solvent s under a rotary evaporator, a yellow solid was obtained as the product (97 mg, 68%). 1 H NMR (CDCl 3 300 MHz) 8.8 (s, 4H), 7.6 (d, 2H), 7.25 (d, 2H), 4.2 (t, 2H), 1.26 (s, 12H), 1.1 (s, 21H), 0.86 (t, 3H). N (n octyl) N (4 ethynyl phenyl) naphthale ne 1,8:4,5 tetracarboxydiimide ( 14 ) N (n octyl) N (4 triisopropylsilyl acetylenyl phenyl) naphthalene 1,8:4,5 tetracarboxydii mide ( 13 ) (232 mg, 0.365 mmol) was dissolved in THF and the solution was degassed for 15 min with argon. Then TBAF (3 eq, 1. 1 mmol, 1. 1 m L 1 M in THF) was added After stirr ing at room temperature for 3 h t he mixture was diluted with methylene chloride (100 m L ) washed with saturated NaCl (100 m L ) and DI water (100 m L ), and dried over anhydrous sodium sulfate. The solvent was removed under a rotary evaporator and the residue was pu rified by chromatography on silica ( CH 2 Cl 2 ) A d ark brown solid was obtained as the product (140 mg, 80%). 1 H NMR (CDCl 3 300 MHz) 8.8 (s, 4H), 7.6 (d, 2H), 7.25 (d, 2H), 4.2 (t, 2H), 3.15 (s, 1H), 1.26 (s, 12H), 0.86 (t, 3H). Compound ( 15 ) N (n octyl) N (4 acetylenyl phenyl) naphthalen e 1,8:4,5 tetracarb oxydiimide ( 14 ) (0.2 g, 0.418 mmol) and cis dich loro bis (tri n butylphosphine) platinum(II) (0.34 g, 0.507 mmol) were dissolved in THF (12 m L ) and diethylamine (15 m L ) and the solution was degassed for 15 min with argon. Then the mixture was stirred under reflux overnight. The s olvents were removed and chromatography on silica ( 4:1 CH 2 Cl 2 / hexane ) gave a brown solid as the product (0.3 g, 65%). 1 H NMR (CDCl 3 300 MHz) 8.8 (s, 4H), 7.4 (d, 2H), 7.1 (d, 2H), 4.2 (t, 2H), 2.09 (m, 12H), 1.55 (m, 12H), 1.42(m, 12H), 1.26 (s, 12H), 0.87 (m, 21H); 31 P NMR (CDCl 3 121 MHz) 8.17 (J Pt P = 2362.0 Hz).

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77 Pt2T1 A Compound 1 4 (38 mg, 0.079 mmol) and compound 4 a (50 mg, 0.036 mmol) were added to diethylamine ( 10 m L) to form a yellow suspension. T he s uspension was degassed for 15 min with argon Then, t he m ixture was stirred under reflux overnight The s olvent was removed and chromatography on silica ( 1:4 then 1:9 hexane/CH 2 Cl 2 ,) g ave a green solid as the product (60 mg, 73%). 1 H NMR (CDCl 3 300 MHz) 8.8 (s, 8H), 7.4(d, 4H), 7.1 (d, 4H), 6.6 (s, 2H), 4.2 (t, 4H), 2.1 (broad singlet, 24H), 1.26 1.75 (m, 72H), 0.8 0.9 (m, 42H); 31 P NMR (CDCl 3 121MHz) 4.38 (J Pt P = 2345.1 Hz) ; Elemental anal. c alc d C: 60.98, H: 7.0 6, N: 2.45; Found C: 60.79, H: 7.33, N: 2.39. Pt2T2 A This compound was synthesized according to the same procedure used for Pt2T1 A except compound 14 (57 mg, 0.12 mmol) and compound 4 b (80 mg, 0.054 mmol) were used Chromatography on silica (3:7, then 1: 4 hexane/CH 2 Cl 2 ) gave a green solid as the prod uct (77 mg, 60%). 1 H NMR (300 MHz, CDCl 3 ) 8.8 (s, 8H), 7.4(d, 4H), 7.1 (d, 4H), 6.8 (d, 2H), 6.6 (d, 2H), 4.2 (t, 4H), 2.1 (broad singlet, 24H), 1.26 1.75 (m, 72H), 0.8 0.9 (m, 42H); 31 P NMR (CDCl 3 121 MHz) 4.50 (J Pt P = 2339.6 Hz) ; Elemental anal. c alc d C: 60.90, H: 6.90, N: 2.37; Found C: 60.80, H: 6.72, N: 2.33. Pt4T1 A Compound 6 a ( 30 mg, 0.019 mmol) was dissolved in THF (10 m L ) and diethylamine (10 mL) and the solution was degassed for 15 min with argon. Then, compound 15 ( 50 mg, 0. 045 mmol) and CuI ( 0.36 mg, 0.0 019 mmol) were added and the mixture stirred at room temperature overnight T he solvent s w ere removed and the crude product was purified by chromatography on silica ( 2:3, 3:7, 1 : 4, then 1:9 hexane/CH 2 Cl 2 ) to give a gray dark solid as the product ( 43 mg, 61% ) 1 H NMR ( CDCl 3 300 MHz) 8.8 (s, 8H), 7.4(d, 4H), 7.1 (m, 12H), 6.6 (s, 2H), 4.2 (t, 4H), 2.1 (broad singlet, 48H), 1.26 1.75 (m, 120H), 0.8 0.9 (m, 78H); 31 P NMR

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78 (CDCl 3 121 MHz) 4.2 6, 4.10 (J Pt P = 2356.7 Hz) ; Elemental anal. c alc d C: 59.21, H: 7.45, N: 1.5 0; Found C: 58.95, H: 7.19, N: 1 .40 Pt4T2 A This compound was synthesized according to the same procedure used for Pt4T1 A except compound 6 b (106 mg, 0.064 mmol), compound 15 ( 145 mg, 0.130 mmol) and CuI (1.2 mg, 0.006 3 mmol) were used. Chromatography on silica (2:3, then 1:4 hexane/CH 2 Cl 2 ) gave a gray dark solid as the product (146 mg, 60%). 1 H NMR ( CDCl 3 300 MHz) 8.8 (s, 8H), 7.4(d, 4H), 7.1 (m, 12H), 6.8 (d, 2H), 6.6 (d, 2 H), 4.2 (t, 4H), 2.1 (broad singlet, 48H), 1.26 1.75 (m, 120H), 0.8 0.9 (m, 78H); 31 P NMR (CDCl 3 121 MHz) 4.26, 4.22 (J Pt P = 2351.6 Hz) ; Elemental anal. c alc d C: 59.20 H: 7.35 N: 1.47 ; Found C: 59.34 H: 7. 10 N: 1.40 Pt6T1 A This compound was syn thesized according to the same procedure used for Pt4T1 A except compound 10 a (82 mg, 0.027 mmol ), compound 15 (62 mg, 0. 056 mmol ) and CuI (0.52 mg, 0.0 027 mmol ) were used. Chromatography on silica (1:4, then 1:9 hexane/CH 2 Cl 2 ) gave a gray dark solid as th e product (67 mg, 48%). 1 H NMR ( CDCl 3 300 MHz) 8.8 (s, 8H), 7.4 (d, 4H), 7.1 (m, 20H), 6.6 (s, 2H), 4.2 (t, 4H), 2.1 (broad singlet, 72H), 1.26 1.75 (m, 168H), 0.8 0.9 (m, 114H); 31 P NMR (CDCl 3 121 MHz) 4.26, 4.10, 4.00 (J Pt P = 2355.0 Hz) ; Elemental anal. c alc d C: 58.43 H: 7. 63 N: 1.08 ; Found C: 58.61 H: 7.3 8 N: 1.06 Pt6T2 A This compound was synthesized according to the same procedure used f or Pt4T1 A except compound 1 0 b (114 mg, 0.037 mmol ), compound 15 (82 mg, 0. 0735 mmol ) and CuI (0.67 mg, 0.0 035 mmol ) were used. Chromatography on silica (3:7, hexane/CH 2 Cl 2 then 100% CH 2 Cl 2 ) gave a gray dark solid as the product (160 mg, 82%). 1 H NMR ( CDCl 3 300 MHz) 8.8 (s, 8H), 7.4(d, 4H), 7.1 (m, 20H), 6.8 (d, 2H), 6.6 (d, 2H), 4.2 (t, 4H), 2.1 (broa d singlet, 72H), 1.26 1.75 (m, 168H), 0.8 0.9 (m, 114H); 31 P NMR (CDCl 3 121 MHz) 4.29,

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79 4.25, 4.04 (J Pt P = 2351.2 Hz) ; Elemental anal. c alc d C: 58.43, H: 7.55, N: 1.06; Found C: 58.29, H: 7.27, N: 1.09.

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80 CHAPTER 3 PHOTOINDUCED ENERGY TRANSFER IN BIB LOCK PLATINUM ACETYL IDE OLIGOMERS Introduction T riplet triplet energy transfer is the most common and most important type of energy transfer in chemical and biochemical processes. 2 The mechanism for triplet energy transfer is usually described by Dexter electron exchange int eraction and may be visualized in terms of two electron transfer processes. Triplet triplet energy transfer has been studied in many donor acceptor systems. 63 75 Eng and coworkers 73 designed a series of porphyrin based donor bridge acceptor systems with varying bridge length to investigate triplet excitation energy transfer. T hey found that triplet triplet energy transfer rates decrease ex ponentially with increasing the distance between the donor and the acceptor. Both their experimental and theoretical results indicate that energy transfer adopts an el ectron superexchange mechanism. In the other study Danilov and coworkers 69 reported ultrafa st energy migration in platinum (II) diimine complexes bearing pyrenylacetylide chrompophores. They found that triplet triplet energy transfer (from 3 MLCT [metal to ligand charge transfer] to ligand 3 ( *)) occ urs with a few hundred femtosec onds because of highly electronically coupled structures. In our group, we recently reported intrachain triplet energy transfer i n platinum acetylide copolymers. 70 T hese copolymers consist of major repeat units of the type [ tran s Pt(PBu 3 ) 2 ( C C P h C C )] where P h = 1,4 phenylene, with randomly incorporating repeat units of the typ e [ trans Pt(PBu 3 ) 2 ( C C T C C )], where T = 2,5 thienylene. Photoluminescence and transient absorption spectroscopy indicate that at room temperature intrachain triplet transfer from 1,4 phenylene to the 2,5 thienlyene repeats occurs rapidly and efficientl y in the copolymer with a low content of 2, 5 thienyl ene repeats T he rate constant for energy transfer is greater than 10 8 s 1 At low temperature, triplet energy transfer is

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81 much less efficient and a fraction of the triplet excitations is trapped on the h igh energy 1,4 phenylene units. In continuation of our investigation of intr a chain triplet energy transfer in conjugated platinum acetylide systems, we designed and synthesized donor acceptor systems by incorporating an anthracene moiety i nto the platin um acetylide backbone The anthracene based conjugated systems have been extensively studied due to their high yield fluorescence 76 87 Kashiwagi and coworker 85 recently reported fluorine containing biethynylanthracene derivatives, which they utilized to fabricate organic field effect transistors. Zhao and coworkers 87 reported synthesis and characterization of a series of conjugated anthracene/ flu orene oligomers. T hey found that t hese oligomers emit a high yield ye llow fluorescence due to th e 9, 10 biethynylanthracene incorporated into the oligomer backbone. However, little work has been done to explore phosphorescence and triplet excited state s of anthracene based derivatives due to low intersystem crossing yield in these systems Triplet tr iplet energy transfer provides a good way to sensitize a triplet excited state that is not efficiently populated by intersystem crossing. It requires that the lowest triplet energy level of the donor must lie above that of the acceptor. A ccording to litera ture reports, 88 the first sin glet excited state energy of 9, 10 b is(phenyl ethynyl ) anthracene ( BPEA ) lies at ~2 .4 eV, which is lower than that of the platinum acetylide chrom o phore (~3.0 eV). Our studies i n platin um acetylide oligomers indicate that the first triplet excited state energy of the platinum acety lide chromophore is ~2.38 eV. 33 The first tri plet excited state energy of BPEA or its derivatives has not been reported to our best knowledge However, according to the DFT calculation s i n our newly designed compounds (refer to the discussion section), the first triplet excited state energy

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82 of the anthracene moiety lies at ~1.31 e V which is much lower than that of the platinu m acetylide chromophore. Because of the difference in energy level s between the anthracene moiety and the platinum acetylide moiety, it allows us to study the influences of the low energy trap on the triplet exciton in the platinum acetylide systems The structures of the focus of the current study are shown in Figure 3 1 Pt2An and Pt4An consist of the core segment of anthracene and one and two repeat units of the type [ C C trans Pt(PBu 3 ) 2 C C P h1 ], where P h1 = 1 phenylene, respectively. Our previously studied platinum complexes Pt2 and Pt4 are also shown as references. Figure 3 1. Chemical structures of anthracene based platinum acetylide oligomers and reference complexes Synthesis The oligomer Pt2An was synthesized accordin g to Figure 3 2. It start ed from the Sonogashira coupling reaction between 9, 10 dibromoanthracene and trimethylsilyl acetylene T he resulting TMS protected compound 1 was deprotected with potassium hydroxide in a

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83 THF/ methanol solvent system to give 9, 10 bi s( ethynyl ) anthracene ( 2 ). T he Ha gihara coupling reaction between compound 2 and cis dichloro bis (tri n butylphosphine)platinum (II) gave the platinum complex 3 in 77% yield. The c omp lex 3 was further reacted with phenyl acet yl ene with CuI as catalyst in die thylamine to give the oligomer Pt2An in 80% yield. Figure 3 2. Synthesis of Pt2An The oligomer Pt4An was prepared following steps listed in Figure 3 3. First, the coupling reaction between phenylacetylene and cis dichloro bis (tri n butylphosphine)platinum (II) gave the platinum complex 4 that was f urther reacted with 1 ethynyl 4 (trimethylsilylethynyl)benzene to give the TMS protected platinum complex 5 After deprotection of the complex 5 under basic conditions, the resulting complex 6 was coupled with the complex 3 to give the final product Pt4An in moderate yield. T he oligomers were characterized with 1 H and 31 P NMR and elemental analysis The coupling constant (J P Pt ) in 31 P NMR is below 2500 Hz, indicating that platinum ce nters adopt trans configuration in the oligomers

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84 Figure 3 3. Synthesis of Pt4An Results and Discussion UV V is Absorption The absorption spectra of Pt2An and Pt4An were recorde d in dilute THF solution s ( Figure 3 4) Both spec tra feature two primary transitions. The low energy band s with two absorption maxima ( max = 4 50 and 480 nm), which are attribut ed to transition of the anthracene segment T his is co nsistent with a literature report i n an anthracene based platinum containing Figure 3 4 Absorption spectra in THF solution The spectra were normalized at 480 nm for Pt2An and Pt4An and 450 nm for BPEA

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85 compound that is structurally similar to Pt2An 89 A ccording to a polarized electronic spectroscopy stud y of B PEA by Levitus and coworkers 88 the low er energy absorption band ( max = 480 nm) is attributed to the long axis transition (the short axis of anthracene) and the high er energy absorption band ( max = 4 50 nm) arises from the overlap of the long and short axis transition of the anthracene segment Compared with the absorption spectrum of B PEA two features can be seen with respect to the anthracene absorption in Pt2An and Pt4An First, both anthracene absorption bands in Pt2An and Pt4An red shift ca. 30 nm, which is caused by the longer conjugation length in Pt2A n and Pt4An H owever, the shift from Pt2An to Pt4An is small (~3 nm), indicating that the effective oligomer chain length is confined within one repeat unit Second, the two absorption bands ( = 450 and 480 nm) are better resolved in Pt2An and Pt4An likel y due to the rigid confirmation in these molecules that freezes the free rotation of the anthracen e segment The absorption band s near UV region are very different for the two c omplexe s. For Pt2An a narrow absorption band centered at = 3 17 nm arises from the platinum a cetylide segment that features the type of [ C C Pt C C P h1 ]. F or Pt4An a broad and intense band concentrated at = 3 55 nm is assigned to transition of the platinum acetylide segment that features the type of [ Pt C C Ph C C Pt ] Due to the longer conjugation length, the absorption band of the platinum acetylide seg ment in Pt4An red shift s ca. 25 nm relative to the absorption band in Pt2An To explore influence s of the conjugation length of the platinum acetylide segment on the absorpt ion spectra we use d our previous ly studied platinum acetylide oligomers Pt2 and Pt4 as references for Pt2An and Pt4An respectively ( see Figure 3 1 for structures ). The a bsorption spectra of the complexes Pt2 and Pt4 exhibit the dominant bands at UV regio n ( 355 nm for Pt2 and 367 nm for Pt4 ) according to our previous report. 33 The absorption bands of the platinum acetylide segments in both Pt2An and Pt4An blue shift

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86 compared with their corresponding references, indicating that the effective conjugation length is confined within the platinum acetylide segment in particul ar, the unit of the type [P h1 C C Pt C C ] for Pt2An and the unit of the type [ Pt C C P h C C Pt ] for Pt4An Steady State Photoluminescence T he photoluminescence spectr a of Pt 2An and Pt4An were record ed in dilute THF solution s with several different exci tation wavelengths ( Figure 3 5 ) The photoluminescence spectra of Pt2An exhibit a n intense emission band centered at max = 5 05 nm along with a vibronic band concentrated at = 550 nm T he band intensit y increase s with increasing excitation wavelength and reaches the maximum with ex = 480 nm. We assigned these bands to the fluorescence emission that arises from the anthracene segment Compared with BPEA the emission band red shifts ca. 40 nm due to the increased conjugation length in Pt2An (Figure 3 6). In addition, the vibronic band becomes broader and less resolved, indicating that different conformers of Pt2An may exist in its excited state. P hosphorescence emission originat ing from the platinum acetylide segment was not observed. The fluorescence qua ntum yield measured with BPEA as standard is ~0.96 for Pt2An For Pt4An the intense fluorescence bands originating from the anthracene segment are also shown when the anthracene segment was directly excited ( ex = 450 and 480 nm). With direct excitation o f the platinum acetylide segment at = 355 nm the spectrum features the dominant fluorescence arising from the anthracene segment, as well as a weak band centered at 400 nm that is assigned to fluorescence originating from the platinum acetylide segm ent. The fluorescence quantum yield of Pt4An measured with Ru(bpy) 3 2+ in air saturated water as standard is ~0.014. Compared with Pt2An fluorescence in Pt4An is dramatically quenched. P hotophysical data of both complexes are shown in Table 3 1.

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87 Figure 3 5 Photoluminescence spectra of A ) Pt2An and B ) Pt4An in THF solution Figure 3 6 Normalized photoluminescence spectra of Pt2An and Pt4An in THF solution with ex = 450 nm. For BPEA ex = 440 nm

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88 Tabl e 3 1. Photophysical data of Pt2An and Pt4An Absorption max /nm Emission max /nm Quantum yield (F) ( ) Pt2An 317, 450, 480 504 0.96 Pt4An 355, 453, 483 504 0.014 To further confirm the orig in of the bands in the photoluminescence spectra, the excitation spectra of Pt2An and Pt4An were measured in THF solutions (Figure 3 7 ). The absorption Figure 3 7 Excitation spectra of A ) Pt2An and B ) Pt4An monitored at 536 nm Absorption spectra are also shown for comparison purposes spectra are also shown for comparison purposes. T he fluorescence band at = 536 nm was monitored when scanning for excitation. The resulting excitation spectra are in the same shape with their co rresponding absorption spectra. However, the bands at 450 and 480 nm are almost

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89 twice as strong as their absorption bands, indica ting that the emission bands arise from the anthracene segment. To explore effect s of temperature on the spectroscopy of Pt4An variable temperature photoluminescence were carried out in 4 m e thyltetr a hydrofuran (Me THF) solution with the excitation wavelength at = 450 nm ( Figure 3 8 ). Below the solvent glass temperature the vibr onic band is more resolved This is believed due t o the rigid conformation of Pt4An in the frozen glass. In this case, only one conformation exists compared with the coexistence of several conformations in the solution state, which is consistent with literature reports. 88 Low temperature emission spectra were also recorded with ex = 355 nm. The spectra are similar to the one that was obtained with ex = 450 nm. Note that the weak peak at ca. 610 nm is a scanning peak Figure 3 8 Low temperature emission spectra of Pt4An in Me THF solution According to our DFT calculations (see later section for detail ) the energy level of the first triplet state of the anthracene segment in Pt2An and Pt4An is ~ 1.31eV (946 nm) To detect the corresponding phosphorescence emission, the near IR photoluminescence was carried out in deoxygenated THF solution. We monitored the spectra ranging from 850 to 1 400 nm. The phosphorescence emission was not observed with excitation at = 355, 450 and 480 nm.

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90 Time R esolved Photoluminescence To f urther gain insight into the dynamics and decays of singlet and triplet excited states, t ime resolved emission measur ements were carried out in deoxygenated THF solutions for Pt2An and Pt4An The spectra were obtained following 355 nm laser excitation ( Figure 3 9 ) For Pt2An the fluorescence decay is fast and the lifetime recovered from time resolved spect ra is Figure 3 9 Time resolved emission spectra of A ) Pt4An ; delay increment: 1 s and B ) Pt2An ; delay increment : 10 ns in THF solution following 355 nm laser excitation less than 10 ns. For Pt4An in the early time (less than 20 ns) following laser pulse s the fluorescence emission of the anthracen e segment appears and it decays rapidly A slow decay band centred at = 517 nm along with a vibronic band shows up at ~60 ns after laser pulse s which is assigned to the phosphorescence decay of th e platinum acetylide segment consistent with our previous study. 33 The lifetime of this decay band is ~1.6 s which is much shorter

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91 compared with the lifetime of its corresponding reference compound Pt4 ( = 18.6 s ), indicating that the phosphorescence emission of the platinum acetylide segment in Pt4An is dramatically quenched. Transient Absorption In order to gain insight into the dynamics of the triplet excited states, t ransient absorption spectra of Pt2An and Pt4An were recorde d in deoxygenated THF solution s following 355 nm laser excitation ( Figure 3 1 0 ) Both c o mplexe s exhibit intense excited state absorption bands centered at max = 530 nm along with the ground state bleaching bands (450 500 nm). In order to assign the origin of these bands, we compare the spectra with the triplet triplet absorption spectra of the reference compound Pt4 The triplet absorption band of Pt4 is ce ntered at = 660 nm, which arises from the platinum acetylide segment according to our previous study 33 A ca. 130 nm blue shift and the completely different band shape in Pt4An transient absorption spectra indicate that the excited state absorption band of Pt4An does not arise from the platinum acetylide segment. I t is therefore reasonable to assign this band to the triplet triplet absorption originat ing from the anthracene segment. It may also contain some contribution s of triplet triplet absorption of t he platinum acetylide segment, which is shown as the broad tail between 600 and 700 nm T he triplet lifetime s recovered from transient absorption spectr a are 3.7 s for Pt2An and 4.2 s for Pt4An Compared with their reference compounds the lifetimes are much shorter in Pt2An and Pt4An suggesting that the triplet decay is fast likely due to the low triplet energy level in Pt2An and Pt4An that result s in rapid non radi ative decay. T h is is consistent with the energy gap law which states that non radiative decay rates decrease expotentially with decreasing triplet energy.

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92 Figure 3 1 0 Transient absorption spectra of A ) Pt2An ; delay incre ment : 1 s B ) Pt4An ; delay increment : 0.7 s ex = 355 nm Density Functional Theory Calculations Density functional theory (DFT) calculations were applied to provide insight concerning the energies of the singlet and triplet states of Pt2An and Pt4An B oth calcul ation and spectroscopic energies of the singlet and triplet states are shown in Table 3 2 It is important to note that the calculation results are very consistent with spectroscopic study. The calculations indicate that two different conformers exist in t he triplet stat e of the platinum acetylide segment in Pt4An : one is defined twisted (t) as the conformation in which the planes defined by the square planar trans Pt(PBu 3 ) 2 (C) 2 and phenylene units are perpendicular and the other one is defined planar ( p) as the conformation in which these two units are coplanar (Figure 3 11 ).

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93 The twisted conformer in the triplet state lies ca. 0.2 eV lower in energy (Table 3 2 ). This is consistent with our previous DFT calculations i n platinum acetylide oligomers. 34 DFT calculations indicate that t he energy level of the first triplet excited state of the anthracene segment is ~ 1.31 eV. Khan a nd coworkers observe d a weak phosphorescence shoulder at the energy of ~1.5 0 eV in a platinum( II) di yne co mplex that consists of an anthracene spacer insert ed into a platinum acetylide backbone that is structurally similar to Pt2An Considering the calcul ation errors, our DFT calculations are in good agreement with the literature report 89 The orbital diagram of DFT calculations is shown in Figure 3 11 Figure 3 1 1 Energies of Pt2An and Pt4An by DFT calculations

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94 Table 3 2 Energies of Pt2An and Pt4An S 1 (An) /eV /Ca. /Sp. T 1 (Pt) /eV /Ca. /Sp. T 1 (An) /eV /Ca. /Sp. Pt2An 2.54 2.45 1.31 Pt4An 2.52 2.45 2.61(p) 2.40 2.42 (t) 1.31 Energy Transfer Dynamics Prior to further discussion of the energy transfer dynamics in the anthracene based platinum acetylide systems, w e first summarize the pho tophysical results of Pt4An based on the above spectroscopic and DFT studies : (1) The f luorescence emission originat ing from the anthracene segment is present and the p hosphorescence emission arising from the platinum acetylide segment is strongly quenched in the photoluminescence spectra; (2) i n contrary with Pt2An whose fluorescence quantum yield is close to unity, the fluorescence quantum yield of Pt4An is very low (0.014); (3) t he phosphorescence lifetime of the platinum acetylide segment (1.6 s) recov ered from time resolved emission is much shorter compared with its reference compound Pt4 ( = 18.6 s) ; (4) the phosphorescence emission of the anthracene segment is not present in near IR photoluminescence spectra; (5) DFT calculations indicate that the triplet energy gap between the platinum acetylide segment and the anthra cene segment is large (~1.1 eV); (6) t riplet triplet absorption of the anthracene segment dominates the transient absorption spectra along with small contribution s of triplet triplet a bsorption of the platinum acetylide segment. In order to explain the above observations, we propose d an energy transfer mechanism as shown in Figure 3 1 2 Two important energy transfer processes are involved in this mechanism: singlet to triplet energy tr ansfer from the anthracene segment to the platinum acetylide segment; triplet to triplet energy transfer from the platinum acetylide segment to the anthracene segment.

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95 Singlet to triplet energy transfer is rare because it is a spin forbidden process. But i t can occur in certain systems if a number of conditions are fulfilled. 90 First, the singlet excited state of the acceptor must lie above the singlet state of the donor to a void singlet singlet energy transfer. In Pt4An the singlet excited state of the platinum acetylide segment (3.1 eV) is ~ 0.65 eV higher than that of the anthracene segment. Second, the donor triplet level must be considerable lower than the acceptor triple t level. The larger the energy difference between the donor and acceptor triplet states, the higher the probability of appearance of acceptor molecules in the triplet state at the expense of S T energy transfer relative to the probability of T T energy tra nsfer from the donor to acceptor. Some studies indicate that the triplet triplet energy gap between the donor and acceptor must be more than 0.12 eV. In Pt4An the triplet state of the anthracene segment lies ~1.1 eV lower than the triplet state of platinu m acetylide segment. The dramatic fluorescence quenching in Pt4An demonstrates that singlet to triplet energy transfer is very efficient in this system. F i gure 3 12 Energy diagram of photophysical processes in Pt4An An = t he anthracene segment; Pt = the plati num acetylide segment; NR = non radiative decay

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96 Following singlet triplet energy transfer, triplet triplet energy transfer from the platinum acetylide segment to the anthracene segment occurs. The significant phosphores cence quenching of the platinum acetylide segment and the dominant triplet triplet absorption of the anthracene segment provide experimental evidence for efficient triplet triplet energy transfer. The phosphorescence emission originat ing from the anthracen e segment was not observed in the near IR spectroscopy indicat ing that the decay of its triplet state adopts non radiative pathway because of the energy gap law, which states that non radiative decay rate increases exponentially with decreasing triplet en ergy level. This is consistent with previous studies whi ch have been concluded that non radiative decay rate of platinum containing oligomers and polymers obeys the energy gap law. 91 In addition, from our DFT calculations the first triplet state level of the anthracene segment lies at ~1.31 eV, which is considera bly lower than th at of most conjugated platinum oligomers, indicating that non radiative decay rate must be extremely rapid in this system. T he other pathway of triplet triplet energy transfer called direct energy transfer is also involved in particular, when the platinum a cetylide segment is directly excited ( = 355 nm). First, the singlet excited state of the platinum acetylide segment is created followed by rapid intersystem crossing to give corresponding triplet excited state, then energy transfer to the triplet state o f the anthracene segment. These two pathways do not occur separately and they both contribute to the triplet excited state of the anthracene segment. For Pt2An it lacks the triplet state of the platinum acetylide segment according to both spectroscopic st udy and DFT calculations. The triplet triplet absorption of the anthracene segment may be explained by the increasing intersystem crossing when platinum metal is incorporated into the anthra cene segment. The singlet excited state of the platinum acetylide

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97 segment is well mixed with that of the anthracene segment in Pt2An A singlet singlet energy transfer equilibrium therefore exists, which enhances intersystem crossing. A diagram concerning photophysical processes in Pt2An is shown in Figure 3 13. F i gure 3 13 Energy diagram of photophysical processes in Pt 2 An An = the anthracene segment; Pt = the plati num acetylide segment; NR = non radiative decay Conclusion We designed and synthesized two platinum acetylide oligomers that in corpora te an anthracene moiety into a platinum acetylide backbone to study energy transfer dynamics. Both spectroscopic study and DFT calculations support an indirect energy transfer mechanism a rare singlet triplet energy transfer process followed by a triplet triplet energy transfer process. T his system provides a unique platform for studying singlet triplet energy transfer because of its fulfillment of energies required. Future studies of t hese systems with fast timescale spectroscopic techniques are suggested.

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98 Experimental Photophysical Measurements Steady state absorption spectra were recorded on a Varian Cary 100 dual beam spectrophotometer. Corrected steady state emission measurement s were conducted on a SPEX F 112 fluorescence spectrometer. Samples were degassed by argon purging for 30 min and concentrations were adjusted such that the solutions were optically dilute (A max < 0.20). Low temperature emission measurements were conducted in 1 cm diameter borosilicate glass tubes in a liquid nitrogen cooled Oxford Instruments DN 1704 optical cryostat connected to an Omega CYC3200 autotuning temperature controller. Samples were degassed by three consecutive freeze pump thaw cycles on a high vacuum (10 5 Torr) line. Photoluminescence quantum yields were determined by relative actinometry. BPEA F = 1 in THF) and Ru(bpy) 3 2+ ( F = 0. 0379 in air saturated H 2 O ) were used as actinometer s for Pt2An and Pt4An respectively Time resolved emission measurements were conducte d on a previously described home built apparatus 92 Optically dilute solutions were used. Transient absorption measurements wer e carried out on a home built apparatus consisting of a Continuum Surelite series Nd:YAG laser as the excitation source ( = 355 nm, 10 ns fwhm). T ypical excitation energies were 5 mJ/pulse. T he source for monitoring optical transients was a Hamamatsu Supe r Quiet series xenon flashlamp and the monitoring light was detected by a Princeton Instruments PI MAX intensified CCD camera detector coupled to an Acton SpectroPro 150 spectrograph. Samples were contained in a cell with a total volume of 10 mL and the c ontents were continuously circulated through the pump probe region of the cell. Solutions were degassed by argon purging for 30 m i n. Sample concentration s were adjusted so that A 355 0.8.

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99 DFT Calculations DFT calculations were carried out by Dr. Erkan M. Kose in National Renewable Energy Laboratory using the Gaussian 03 program. Synthesis General Solvents and chemicals used for synthesis were of reagent grade and used without puri fication unless noted. Reactions were carried out under an argon atmosphere. NMR spectra were recorded on Varian VXR, Gemini or Mercury 300 MHz spectrometer s Cis dich loro bis (tri n butylphosphine) platinum(II) 62 and 1 e thynyl 4 (trimethylsilylethynyl)benzene 33 were prepared by literature metho ds. 9,10 B is (trimethylsilylethynyl)anthracene ( 1 ) 9, 10 D i bromoanthracene (0.5 g, 1.5 mmol), Pd(II) ( 53 mg, 0.075 mmol) and CuI (29 mg, 0.15 mmol) were added to THF/i Pr 2 NH and the solution was degassed for 15 min. Then the temperature was increased to 50 0 C, at which time TMSA (0.59 g, 6 mmol) was added. T he mixture was stirred at 80 0 C overnight. S olvents were removed under vacuum and the crude product was further purified by column chromatography with hexane as an eluent A deep red solid was obtai ned as the product (0.55 g, 98%). 1 H NMR (CDCl 3 300 MHz) 8.55 (q, 4H), 7.6 (q, 4H), 0.4 (t, 18 H). 9,10 B is( ethynyl ) anthracene ( 2 ) Compound 1 (50 mg, 0.135 mmol) was dissolved in THF and the solution was degassed for 15 min. Then KOH (15 mg, 0.27 mmol, in methanol) was added via syringe A fter TLC analysis showed that all starting material was consumed, MeCl 2 ( 50 m L) was added. T he solution was washed with water and dried over anhydrous Na 2 SO 4 A b rown solid was obtained as the product after remov a l of the solvents under vacuum (30 mg, 98%). 1 H NMR (CDCl 3 300 MHz) 8.60 (q, 4H), 7.60 (q, 4H), 4.05 (s, 2H).

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100 Compound 3 9,10 B is( ethynyl ) anthracene ( 2 ) (45 mg, 0.2 mmol) and cis dichloro bis (tri n butylphosphine)platinum (II) (0.27 g, 0.4 mmol) were dissolved in diethylamine (10 m L ) and the s olution was degassed for 15 min t hen refluxed overnight. T he solvent was removed under vacuum. Column chromatography on silica ( 3:2 hexane/MeCl 2 ) gave a orange solid as the product (0.23 g, 77%). 1 H NMR (CD Cl 3 300 MHz) 8.60 (q, 4H), 7.38 (q, 4H), 1.90 (m, 24H), 1.60 (m, 24H), 1.40 (m, 24H), 0.90 (t, 36 H). 31 P NMR (CDCl 3 121 MHz) 8.50 (J Pt P = 2370.1 Hz). Pt2An Compound 3 (42 mg, 0.028 mmol), phenylacetylene (6 mg, 0.06 mmol) and CuI (3 mg) were added to diethylamine. T he solution was degassed for 15 min and stirred at room temperature overnight. T he solvent was removed under vacuum. C olumn chromatography on silica ( 4:1,then 3:2 hexane/MeCl 2 ) gave a orange solid as the product (3 6 mg, 80%). 1 H NM R (CDCl 3 300 MHz) 8.70 (bs, 4H), 7.1 7.3 (m, 14H), 2.10 (m, 24H), 1.65 (m, 24H), 1.40 (m, 24H), 0.90 (t, 36 H). 31 P NMR (CDCl 3 121 MHz) 4.75 (J Pt P =2356.7 Hz) ; Elemental anal. calc d C 60.57, H 7.81, found C 60.29, H 8.09. Compound 4 This c o mpound was synthesized according to the same procedure used for compound 3 except phenylacetylene (36 mg, 0.35 mmol) and cis dichloro bis (tri n butylphosphine)platinum (II) (0.23 g, 0.34 mmol) were used Column chromatography on silica ( 7:3 hexane/MeCl 2 ) gave a light yellow solid as the product (0.24 g, 96%). 1 H NMR (CDCl 3 300 MHz) 7.20 (m, 5H), 2.10 (m, 12H), 1.61(m, 12H), 1.45 (m, 12H), 0.90 (t, 18 H). 31 P NMR (CDCl 3 121 MHz) 7.94 (J Pt P = 2365.0 Hz). Compound 5 This compound was synthesize d according to the same procedure used for compound 3 except compound 4 (0.35 g, 0.475 mmol) and 1 ethynyl 4 ( trimethylsilyl ethynyl ) benzene (0.1 g, 0.5 mmol) were used Column chromatography on silica

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101 ( 4:1 hexane/MeCl 2 ) gave a yellow solid as the product (0.26 g, 61%). 1 H NMR (CDCl 3 300 MHz) 7.20 (m, 5H), 6.9 (m, 4H), 2.10 (m, 12H), 1.61(m, 12H), 1.45 (m, 12H), 0.90 (t, 18 H), 0.24 (t, 9H). 31 P NMR (CDCl 3 121 MHz) 4.21 (J Pt P = 2347.0 Hz). Compound 6 This compound was synthesized according to the same procedure used for compound 2 except compound 5 (0.25 g, 0.278 mmol) and KOH (0.16 g, 2. 78 mmol, in methanol) were used. A yellow solid was obtained as the product (0.2 g, 87%). 1 H NMR (CDCl 3 300 MHz) 7.20 (m, 5H), 6.9 (m, 4H) 3.10 (s, 1H), 2. 10 (m, 12H), 1.61(m, 12H), 1.45 (m, 12H), 0.90 (t, 18 H). 31 P NMR (CDCl 3 121 MHz) 4.21 (J Pt P = 2347.0 Hz). Pt4An. This compound was synthesized according to the same procedure used for Pt2An except compound 6 (0.2 g, 0.24 mmol) compound 3 (0.17 g, 0.114 mmol) and CuI (5 mg) were used Column chromatography on silica ( 1:1 hexane/MeCl 2 ) gave a red solid as the product (0.2 g, 57%). 1 H NMR (CDCl 3 300 MHz) 8.71 (bs, 4H), 7.36 (bs, 4H), 7.10 7.3 (m, 18 H), 2.10 (m, 48H), 1.61(m, 48H), 1.45 (m, 48H ), 0.90 (t, 72H). 31 P NMR (CDCl 3 121 MHz) 4.90 4.12 (J Pt P = 2363.0 Hz) Elemental anal. calc d C 58.61, H 7.94, found C 58.38, H 8.22.

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102 CHAPTER 4 PHOTOPHYSICS OF PLAT INUM TETRAYNE OLIGOM ERS: DELOCALIZATION OF TRIPLET EXCITION Introduction Organic an d o rganometallic oligomers that feature extended sp carbon chains, e.g., R (C C) n R and L y M (C C) n ML y have attracted recent interest due to their potential use as electroni c and optoelectronic materials 93 99 Oligoynes feature rigid rod structures and extended electron delocalization, which make s them attractive building blocks for the construction of linear carbon rich materials which may possess potential applications as molecular electronic wire s for studying exciton or charge transport on the nanoscale 35 To increase the stability of linear carbon chain compounds, transition metals such as rhenium, iron, ruthenium, platinum, manganese and gold, have been incorporated as e nd caps to the oligoyne chains. 94,97,98,100 104 M ost studies of these tr ansition metal containing carbon chain compounds have focused on synthesis, structural properties and optical absorption spectroscopy. However, an important feature of these compounds is that their triplet excited states are produced in relatively high yie l d due to the strong spin orbit coupling induced by the transition metals. There are o nly a few reports concerning this aspect Yam and coworkers 104 reported a spectroscopic st udy of a series of platinum ( II) terpyridyl capped carbon chain s They found that low temperature photoluminescence of these complexes features the phosphorescence emission originat ing f ro m carbon chain 3 ( ) transition with a vi brational progressional spacing of ca. 2052 cm 1 Che and coworkers 99 investigated a series of gold end capped carbon chain compounds. T heir spectroscopic results indicate that the lowest energy electronic excited states are dominate d by the acetyl enic 3 ( *) transition and a well defined vibronic progression with spacing of ca. 2000 cm 1 corresponds to the (C C) stretch in the 3 ( *) excited state.

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103 W e recently reported a photophysical study of a series of plati num end capped polyyne oli g o mers that fe atures increasing sp carbon chain length (Figure 4 1). 92 T he results of this study indicate that low temperature p hotoluminescence spectra exhibit moderately efficient phosphorescence appearing as a series of narrow vibronic band s separated by ca. 2100 cm 1 T he emission originates from the sp carbon c hain 3 ( *) transition and the vibronic progression arises from coupling of the excitation to the (C C) stretch. W ith increasing sp carbon chain length, the 0 0 energy of the phosphorescence decreases across the series. Moreover, a quantitative energy g ap law correlation has been revealed with the analysis of the triplet non radiative decay rates i n these compounds. Figure 4 1. Platinum end capped polyynes studied by Farley 92 W hile there have been a number of studies of transition metal end capped linear carbon chain mol ecules, little work has been done on metal carbon chain alternant constructed compounds, e.g. M [(C C) n M] m In particular, the type of the molecule with n 2 and m 1 is unknown according to our best knowledge. However, the molecule with this type of s tructure is very important as it provide s a unique platform to study triplet exciton delocaliz a tion. To extend our photophysical study o f this type of molecule o ne of our groups have recently synthesized and structural ly characteriz ed a series of platinu m containing tetrayne oligomers of the type Cl Pt(P 2 ) [(C C) 4 Pt(P 2 ) ] n Cl where P = a phosphine ligand and n = 1 3. 105

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104 We now report a detailed study of the photophysics of the serie s of platinum tetrayne oligomers, (PtC 8 ) n (n =1, 2, 3; Figure 4 2). The properties of the triplet excited states have been probed by variable temperature luminescence and transient absorption spectroscopy. Figure 4 2. Platinu m tetrayne oligomers of the current study Results U V V is Absorption T he absorption spectra of the series of (PtC 8 ) n oligomers were recorded in Me THF solutions As shown in Figure 4 3, a ll of three oligomers feature two electronic primary transitions which each appear as a manifold of vibronic bands. In general, the high energy transition occurs between 300 and 3 8 0 nm and the low energy transition occurs between 3 8 0 and 450 nm. B oth transitions red shift with increasing carbon chain length. However, the red shift is small from (PtC 8 ) 2 to (PtC 8 ) 3 indicating that the effective conjugation length for absorption is ~2 repeat units. T he low energy transition is relatively weak, whereas the higher energy transition is very intense. For (PtC 8 ) 2 and (PtC 8 ) 3 a well defined vibronic progression appears in both transitions. For (PtC 8 ) 1 the high energy transition features a broad absorption band along

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105 with a weak vibronic shoulder. According to our previous study, 92 both of these bands originate from the (C C) 4 carbon chain transitions. Figure 4 3 Absorption spectra of (PtC 8 ) n complex es in Me THF solution Steady State Photoluminescence P hotoluminescence spectra for all of the (PtC 8 ) n complexes were recorded in Me T HF solution at temperature ranging from 300 to 80 K. Several features can be seen from the spectra at 100 K in the Me THF solvent glass (Figure 4 4). First, all of complexes exhibit an intense and narrow 0 0 emission band followed by two vibronic progression sub bands. T h e 0 0 band s are assigned to phosphorescence from 3 excited states and the vibronic sub bands in each spectrum arise due to coupling of the triplet excitation to the C C stretch of the carbon chain. Second, the 0 0 phosphorescence bands red shifts ca. 20 nm from (PtC 8 ) 1 to (PtC 8 ) 2 whereas there is lit tle or no shift from (PtC 8 ) 2 to (PtC 8 ) 3 This indicates that the triplet exciton is localized which is consistent with our previous reports 33,34 Third, the ratio of the intensity of 0 1 to 0 0 bands (I 0 1 /I 0 0 ) decrease s with in creasing chain length. Interestingly, the ratio drops sharply from 0.5 to 0.25 between (PtC 8 ) 1 and (PtC 8 ) 2 but it is then approximately the same for (PtC 8 ) 2 and

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106 (PtC 8 ) 3 T his observation is consistent with the trends in the absorption and phosphorescence maxima, which imply that the triplet state is localized over ~2 repeat units. Figure 4 4. Photoluminescence spectra of (PtC 8 ) n complexes in Me THF solvent glass at 100 K. A ) (PtC 8 ) 1 ex = 327 nm. B ) (PtC 8 ) 2 ex = 368 nm. C ) ( PtC 8 ) 3 ex = 371 nm

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107 With increasing temperature, the phosphorescence intensity steadily decreases, whereas the spectra red shift s only ca. 4 nm (80 300 K) and the band shape are not change d Temperature dependent spectra of (PtC 8 ) 2 are shown in Figure 4 5 and the spectra of the other two complexes indicate similar trend. The quantum yield s measured at ambient temperature are in the range of 0.001 to 0.004, and increase with increasing chain length (Table 4 2 ). Excitation spectra were obtained for all of the complexes monitored at 0 0 emission peak at 100 K in Me THF glass. As shown in Figure 4 6, t he spectra are sim ilar to the absorption spectra, except that the bands are better resolved. This is likely due to the frozen glass matrix. Figure 4 5. Variable temperature photoluminescence spectra of (PtC 8 ) 2 in Me THF T he photoluminescence spectra o f all of three complexes appear as a narrow 0 0 bands with two vibronic sub bands separated by approximately 2100 cm 1 T he vibronic pro gression originates from the stretching mode of the (C C) 4 chains. In order to gain further insight into the nature of the triplet state, u tilizing methods described in previous papers, 92,106 the photoluminescence spectra of (PtC 8 ) 1 (PtC 8 ) 2 and (PtC 8 ) 3 at 100 K were analyzed by a singl e mode Franck Condon expression as shown in Equation 4 1.

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108 Figure 4 6. Excitation spectra of A ) (PtC 8 ) 1 B ) (PtC 8 ) 2 and C ) ( PtC 8 ) 3 The corresponding absorption spectra are also shown for comparison Excitation spectra were monitored at 0 0 emission peaks in Me THF solvent glass at 100 K

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109 = 00 m m 00 3 ( m ) m m exp 4 ln 2 E 00 + m m 0 1 / 2 2 5 m = 0 ( 4 1) where I ( ) is the relative emission intensity at energy E 00 is the en ergy of the 0 0 transition, m is the quantum number of the average medium frequency vibrational mode, m is the average medium frequency acceptor modes coupled to the triplet excited state to ground state transition, S m is the Huang Rhys factor, and 0, 1/2 is the half width of the individual vibronic bands. T he experimental emission spectra were fitted using a Visual Basic macro in Microsoft Excel. The fitted spectra are shown in the appendix B as Figure B 1. A summary of the parameters recovered from th e spectra fitting is provided in Table 4 1. Table 4 1. Emission s pectra f itting p arameters for (PtC 8 ) n complexes at 100 K max,em / nm E 00 /cm 1 /cm 1 0,1/2 /cm 1 S m E ST /eV E ST /eV a (PtC 8 ) 1 584 17 123 2100 230 0.9 0.64 0.92(Pt 1) (PtC 8 ) 2 595 16 778 2045 250 0.5 0.50 0.82(Pt 2) (PtC 8 ) 3 595 16 806 2085 235 0.4 0.50 0.79(Pt 3) Several interesting features emerge from Table 4 1 with respect to the parameters recovered from the spectra l fits. First, while the 0 0 emission energy for (PtC 8 ) 1 is ca. 350 cm 1 higher than that for (PtC 8 ) 2 the energy diffe rence between (PtC 8 ) 2 and (PtC 8 ) 3 is only ca. 30 cm 1 Second, the Huang Rhys parameter for (PtC 8 ) 2 is noticeably smaller than that for (PtC 8 ) 1 but is close to that for (PtC 8 ) 3 Both of these features indicate that the triplet exciton delocalization is res tricted within two to three repeat units. Transient Absorption In order to provide additional information regarding the properties of the triplet state of the oligomers, nanosecond transient ab s orption spectra were recorded at room temperature in deoxygena ted THF solution. As shown in Figure 4 7 all three oligomers feature strong triplet triplet absorption bands following near UV ground state bleachings. I nterestingly, the triplet

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110 absorption for each oligomer consists of two distinct bands: the first stro nger band which appears in the near UV region and the second relatively weaker band that is concentrated on middle and red of the visible. The first band is believed to be the overlap of the triplet absorption and the ground state bleaching s, and the secon d band is b roa d. T he ratio of the second and first bands Figure 4 7 Triplet absorption spectra of (PtC 8 ) n complexes following 355 nm laser excitation: A ) (PtC 8 ) 1 80 ns delay increment; B ) (PtC 8 ) 2 160 ns delay increment; C ) (PtC 8 ) 3 160 ns delay increment

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111 ( A 2 / A 1 ) increases about one times from (PtC 8 ) 1 ( 0 .3) to (PtC 8 ) 2 ( 0 .6) and then stay constant for (PtC 8 ) 3 ( 0 .6). Both absorption bands red shift with increasing oligomer chain length. However, no noticeable shift was observ ed from (PtC 8 ) 2 to (PtC 8 ) 3 (Figure 4 8 A ) Lifetimes recovered from transient absorption spectra are in a time scale of a few hundred nanoseconds and increase with the oligomer chain length (the decay profile s are shown in Figure 4 8 B and lifetime s are show n in Table 4 2). Figure 4 8. A ) Transient absorption spectra at t = 0 s; B ) Decay profiles While assignment of the triplet absorption bands will require further investigation several conclusions can be drawn from the transient absorption study. First, intersystem crossing for all

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112 of three oligomers is very efficient whic h is consistent with strong phosphorescence in photoluminescence spectra Second, the triplet triplet absorption of the carbon chains is strongly allowed which is indicated by the large A value ( 0.1). Third, the triplet exciton is localized and restric ted to two repeat units. Phosphorescence Decay Kinetics I n order to gain insight concerning phosphorescence decay kinetics for the (PtC 8 ) n series, temperature dependent time resolved emission were carried out in deoxygenated Me THF solution (glass) over t he 100 300 K temperature range. T he lifetimes recovered from time resolved emission by spectra decay fitting are plotted versus temperature as shown in Figure 4 9 Several interesting features emerge from Figure 4 9 First, for all three oligomers, the emi ssion lifetimes decrease with increasing temperature. In particular, the lifetimes de crease slightly more rapidly in the temperature region corresponding to the glass to fluid transition of Me THF (120 140 K). Second, the emission lifetimes increase with t he oligomer chain length in the whole temperature region. Figure 4 9 Temperature dependence of photoluminescence lifetimes for (PtC 8 ) n complexes in Me THF solution (glass)

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113 T he radiative and non radiative decay rates ( k r and k nr ) can be calculated using Equation s 4 4 and 4 5 which were obtained by rearrangement of Equation s 4 2 and 4 3. T = 1 + ( 4 2 ) p = isc T ( 4 3 ) = 1 p isc 1 T ( 4 4 ) = p isc 1 T ( 4 5 ) where T is the triplet lifetime, p is pho sphorescence quantum yield, isc is intersystem crossing efficiency. T he values of T p and isc are needed in order to compute k r and k nr Here T p are known and isc is unknown. However, according to previous studies on platinum containing compoun ds and the fact that the fluorescence is very weak for the compounds of the current investigation, it is safe to conclude that intersystem crossing effi ciency ( isc ) is close to unity ( isc 1 ). Furthermore, since p is small, p isc and under these conditions Equation s 4 4 and 4 5 can be further rearranged to Equation s 4 6 and 4 7 1 T ( 4 6 ) p T ( 4 7 ) T he values of k r and k nr computed from the experimental data are listed in Table 4 2. S everal features are shown regardi ng these data. F irst, for all of three oligomers, the values of k nr exceed k r by about 1000 fold, indicating that non radiative decay is the dominant pathway for these oligomers. Also note that non radiative decay rates decrease by approximately a factor of 10 at 100 K. Second, there is a slightly decrease in k nr with increasing the oligomer chain length, which is consistent with our previous study. 33

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114 Table 4 2. Photophysical parameters for (PtC 8 ) n complexes T / 10 6 s a p a k nr /10 6 s 1 a k r / 10 3 s 1 a T / 10 6 s b k nr / 10 5 s b / s c (PtC 8 ) 1 0.26 0.0011 3.8 4. 1 1.25 8.0 0.22 ( PtC 8 ) 2 0.49 0.0025 2. 1 5.1 1.81 5.5 0.45 (PtC 8 ) 3 0.70 0.0031 1.4 4. 4 2.50 4.0 0.62 Discussion Effect of Spacer on Delocali zation of Triplet E xciton These results provide evidence that triplet exciton in the platinum tetr a yne oligomers is localized to two repeat units. F irst, the phosphorescence energy shifts ca. 20 nm from (PtC 8 ) 1 to (PtC 8 ) 2 and then stays constant for (PtC 8 ) 3 Second, the transient absorption spectroscopy presented in this study further supports the notion of triplet localization. A number of recent studies have explored the variation of the singlet and triplet energies with oligomer length for conjugated p latinum containing oligomers or polymers. T he initial work was done by Beljonne and coworkers. They investigated the spatial extent of the singlet and triplet exciton in platinum acetylide monomers and polymers. T he key finding of their study is that while the singlet exciton is delocalized over a few repeat units, the triplet exciton is strongly local ized on a single phenylene ring. 27 A more systematic study of the delocaliza tion of singlet and triplet excitons in platinum acetylide oligomers was conducted by our group 33,34 Here we carry out a detailed comparison between our former and current studies. The structures of the oligomers from our previo us study chosen for comparison are shown in Figure 4 10 Note that the main structural difference between the two series oligomers is that the phe nyl ene units (spacers) are replaced by the butadiy nyl segment in the oligomers of the current study. According to our previous study, 34 the phenylene unit adopt s two different configurations with respect to the plane defined by the squar e planar platinum center : twist ed (t) and planar (p) conformation. DFT calculations indicate that in the ground state the all t

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115 conformation is energetically preferred over conformers in which terminal or internal phenylenes are rotated into the p confor mation, while in the triplet state the lowest energy conformation is p where the central phenylene ring is rotated planar. This investigation shows that the geometry of the triplet state differs from that of the ground state in phenyl based platinum oli gomers The geometry distortion is confined within only one phenylene unit and thus the triplet state in these series oligomers is confined to one repeat unit. T he oligomers of the current study consist of all ethynylenic units, which appear to be slight r igid with respect to photoexcitation. It is reasonable to conclude that there is little geometry distortion between the excited state and the ground state. As a result, triplet exciton s in these oligomers are a little more delocalized (two repeat units acc ording to the spectroscopic study). Figure 4 10. Platinum acetylide oligomers studied by Liu 33 T he delocalization of the triplet exciton also depends on the binding energy which is defined as the singlet triplet energy difference; the larger the binding energy, the stronger the confinement of the excited state wavefunction. 27 Since significant fluorescence was not observed for the current studied oligomers, we estimate the singlet energy based on the onset of the absorption bands. T he values of computed estimated singlet triplet splitting ( E S T ) for the (PtC 8 ) n series are listed in Table 4 1. T he E S T values of the corresponding reference compounds are also listed in Table 4 1. I t is evident that the binding energy in Pt n oligomers is approximate 0.3 eV larger than that in (PtC 8 ) n series. T his further supports the idea that the triplet exciton is more confined in Pt n oligomers.

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116 Effect of Platinum on Delocalization of Triplet Exciton It is well known that incorporation of transition metals, such as platinum, into conjugated organic sy stem s enhances intersystem crossing due to spin orbi t coupling induced by the heavy metal The triplet exciton delocaliz a tion is also affected by the platinum metal in conjugated metal organic systems. S tudies of both platinum phenyl based and platinum t etra yne based oligomers conclude that the triplet exciton is more localized and confined to one or two repeat units in these systems. W e recently reported a photophysical study of platinum end capped oligoynes (see Figure 4 1). 92 O ne key result of this study is that the triplet exciton is delocalized ac ross the entire conjugated system defined by the carbon chain for all oligomers. T he effective conjugation length for the triplet state has not been reached in the longest carbon chain (C C) 6 oligomer. I n contrast with the current study, the platinum m etal has no significant influence on triplet excitons of conjug ated organic systems. Rogers and coworkers 32 also reported photophysical studies of a series of platinum containing pheny ethynyl oligmoers with varying the ligand chain length. T he results indicate that the triplet exciton is delocalized with respect to the ligand chain length. With increasing the ligand chain length, the spin orbit coupling effect of platinum on the ground and excited state properties is reduced. B y comparison these studies with our current study, it is safe to conclude that the triplet exciton in conjugated platinum containing oligomers is more likely affected by the platinum metal if conjugated organic units are more proximate to the platinum metal. W ith increasing conjugated organic chain l ength, influence s by the platinum metal is less effective and thus the triplet exciton delocalization shows conjugated organic system features.

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117 Conclusion A detailed photophysical investigation of a series of platinum tetrayne oligomers has been carrie d out. T he photophysics of these oligomers is dominated by a 3 state that is concentrated on the (C C) 4 chain The low temperature emission spectrum of each oligomer shows an intense and narrow phosphorescence band followed by a vibronic progression of sub bands separated by ca. 2100 cm 1 which correspond s to the stretch of the (C C) 4 chain. B oth absorption and emission bands red shift from (PtC 8 ) 1 to (PtC 8 ) 2 and then are not change d for (PtC 8 ) 3 indicating that the triplet exciton is localized on t wo repeat units in the oligomers. Room temperature transient absorption measurement s also support the notion of triplet localization. T he slightly rigid nature of the (C C) 4 units more likely induces little geometry distortion upon photoexcitation, whic h causes the triplet exciton to be slightly delocalized (two repeat units) compared with platinum phenyl based oligomers (one repeat unit). The e ffect of the platinum centers on triplet exciton delocalization is less effective when the conjugated organic u nits are not close to platinum centers and as a consequence, the corresponding triplet state features the properties of conjugated organic systems. Experimental Materials The platinum tetrayne oligomers were provided by Dr. John A. Gladysz s lab in the Texas A&M University. The synthesis and characterization of the oligomers were described in the literature. 105 Photophys ical Measurements Steady state absorption, and low temperature and time resolved photoluminescence measurements were carried out by procedure s similar to those described in Chapter 3.

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118 Photoluminescence quantum yield s were determined by relative actionmetry with Ru(bpy) 3 2+ as an actinometer ( p = 0.03 7 9 in air saturated H 2 O ). Transient absorption measurement s were carried out by procedures similar to those described in Chapter 2. Photoluminescence Spectra l Fitting T he spectr al fitting was carried out according to the method described by Farley 92

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119 CHAPTER 5 PHOSPHORESCENT ORGAN OGELATORS Introduction he c onjugated oligomers and polymers have been widely used in fabrication of organic electronic devices such as light emitting diodes, photovoltaic cells and field effect transistors. 107,108 T he performance of these devices depends on the intrinsic optical an d electronic properties as well as intermolecular interaction s and morphology of the constituent materials. W hile molecular modification and functionalization in order to improve their optical and electronic properties have attracted the most attention, in termolecular interactions and material morphology are among the most important factors in determining the performance of an organic material in device applications. Supramolecular architectures or molecular aggregates have been achieved through controllin g self assembly of conjugated polymers or oligomers. 109 Three non bonding intermolecular interactions are responsible for self assembly of conjugated systems: interaction, hydrogen bonding and van der Waals interaction s 110 113 Self assembly of suitab ly functionalized derivatives of oligo(phenylene vinylene) and oligo(phenylene ethynylene) in solution has produced helical or lamellar supramolecular structures 114 120 Ajayaghosh and coworkers 121,122 recently discovered that well designed oligo(phe nylene vinylene) based derivatives have the ability to form organogels in hydrocarbon solvents such as hexane, cyclohexane and benzene, which is induced by intermolecular hydrogen bonding interaction. While considerable studies have been carried out on su pramolecular structures consisting of organic congjugated systems, relatively little work ha s been done on self assembling properties comprised of transition metal organic or organometallic congjugated oligomers or polymers. 123 128 H owever, these systems are of interest for several reasons First, introduction of

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120 transition metal centers into conjugated systems has a profound influence on their optical and electronic properties. Second, these systems provide a unique platform for the study of triplet excited state in aggregates or gels due to high intersystem crossing yield induced by a trans ition metal center. Inspired by the discovery of gelation of oligo(phenylene vinylene) derivatives by Ajayagosh and coworkers, 121 we designed and synthesized a series of platinum acetylide oligomers decorated with long alkyl chains and their gelation and photophysical properties were recently reported. 129 Several important findings with respect to this study are as follows. F irst, these platinum acetylide oligomers gel hydrocarbon solvents and form fiber structures as shown in TEM images Second, the b lue shift of the absorption spectr um of the oligomer in its aggregate d state relative to its molecular ly dissolved state indicates that self assembly of these oligomers form s H aggregates Third, the aggregated oligomers emit stron g phosphorescence and the triplet state is not strongly perturbed by close packing of the chromophores in the aggregates. Fourth, efficient triplet triplet energy transfer occurs in mixed aggregates consisting of a donor and a n acceptor. After these excit ing results, we continue to explore triplet exciton dynamics in phosphorescent aggregates consisting of platinum acetylide chromphores. In the current study, we designed platinum acetylide systems consisting of an electron donor and acceptor to investigate photoinduced electron transfer properties in their aggregated states. As shown in Figure 5 1, Pt2M and Pt2MT were u sed as electron donors and their photophysics have been previously studied. 129 Both oligomers gel hydrocarbon solvents such as hexane, cyclohexane and dodecane. A compound consisting of the core un it of the NDI and tridodecyloxy phenyl end groups were u sed as an electron a cceptor ( NDI 1 ).

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121 Figure 5 1. Pt2M/NDI 1 and Pt2MT / NDI 1 donor acceptor systems In solution, c harge dissociation into free and solvated radical ions is more favorable in p olar solvents where the energy barrier for ion separa tion is not as high as in nonpolar solvents where the ions are bound more tightly together by Columbic attraction In addition, a polar solvent can reduce Coulombic attraction by shielding the electrostatic attraction which exists between the positive an d negative ions. However, i n gel or aggregated state, solvent polarity may play a different role in photoinduced intermolecular electron transfer process due to the close packing of the aggregated donor and acceptor molecules. A ccording to our previous stu dy, 129 Pt2M and Pt2MT only self assemble in nonpolar solvents but are soluble in polar solvents. In order to explore effect of polar solvent s on photoinduced electron transfer in molecular aggregates, we designed and synthesized a new platinum acetylide oligomer ( Pt2MAM ) that is

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122 structurally similar to Pt2M but incorporat es amide functional groups as shown in Figure 5 2 Its counterpar t electron acceptor was also synthesized ( NDI 2 ). Several studies 111,130 134 have shown that molecular systems that incorporat e amide functional groups form organogel s in both nonpolar and polar solvents due to strong intermolecular hydrogen bonding interaction s T he platinum acetylide oligomer Pt2MAM was designed to include a combination of all of three major driving forces (hydrogen bonding, van der Waals and interactions) that induce s molecular self assembling which makes it a very p romising organogelator. We anticipated that it w ould gel in polar solvents such as acetonitrile and DMSO. Figure 5 2. Pt2MAM / NDI 2 donor acceptor system In our previous report, 42 we applied the pulse radioly s is technique for the study of the p roperties of ion radical state i n an extended series of platinum acetylide oligomers. T he st udy was carried out in dichloro ethane and THF solutions in which the oligomers are present in their molecularly dissolved state. To extend this study into molecularly aggregated state we used the pulse radiolysis method to investigate ion radical properti es in platinum acetylide organogelators. According to our best knowledge, t his is the first time that the pulse radiolysis technique is applied to conjugated organogel systems.

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123 I n this chapter, we first describe the synthesis of new platinum acetylide olig omers and then we investigate photoinduce d electron transfer dynamics i n first set s of donor acceptor systems. Second, photophysics and molecular morphology of Pt2MAM have been characterized. Finally we report the pulse radiolysis study in Pt2M anion radi cal properties in its pure and mixed gels. Synthesis Pt2M and Pt2MT were synthesized according to procedures described before. 129 P t2MAM was prepared according to Figure 5 3. Methyl 3,4, 5 trihydroxybenzoate was reacted with 1 bromododecane in N, N dimethylformamide (DMF) in the presence of potassium carbonate and a small amount of tetra n butylammounium bromide (TBAB) to give methyl 3 ,4, 5 tridodecyloxy benzoate 1 After hydrolysis under basic condition s the resulting acid 2 was activated by using thionyl chloride to give the acid chloride 3 in 97% yield. Condensati on between ethylene diamine and the acid chloride 3 in dichloromethane p roduced the monosubstituted amine 4 The yield of this step is only 25% due to the di substituted side product. T he a mine 4 was further c oupled with 4 iodo benzoyl chloride in dichloromethane and triethyl amine to yield the iodide 5 T he Sonogashira coupling between the iodide 5 and trimethylsilylacetylene (TMS A) produced the TMS protected acetylene 6 After deprotection with TBAF in THF solution, the resulting acetylene 7 was reacted with cis dichloro bis (tri methyl phosphine)platinum (II) in diethylamine to give the platinum complex 8 in 70% yield. The final step is a Ha gihara coupling between the platinum complex 8 and 1, 4 diethynylbenzene in diethylamine and copper iodide as catalyst to yield the product Pt2MAM in 82% yield. NDI 2 was prepared in a one step condensation reaction between the amine 4 and 1, 4 ,5, 8 tetracarboxylic naphthalene dianhydride in DMF at 140 0 C in 95% yield (Figure 5 3)

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124 Figure 5 3. Synthesis of Pt2MAM and NDI 2 NDI 1 was synthesized according to Figure 5 4 1,2,3 T rihydroxy benzene was reacted with 1 bromododecane in DMF with potassium carbonate and TBAB as catalyst s to yield 1,2, 3 tridodecyloxy benzene 9 in 92% yield. Nitration of 9 with 90% nitric acid absorbed in silica

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125 produced compound 10 in 74% yield fol lowed by reduction with hydrazine hydrate in ethanol and 10% Pd/C catalys t to give 3,4, 5 tridodecyloxy aniline 11 in high yield. The final step is the condensation reaction between 11 and 1,4,5, 8 tetracarboxylic naphthalene dianhydride in qu i noline with the addition of Zn(OAc) 2 to yield the product NDI 1 in moderate yield. Figure 5 4. Synthesis of NDI 1 Results and Discussion Photoinduced Electron Transfer in Pt2M/NDI 1 and Pt2MT/NDI 1 M ixed G els To explore photoinduced electron transfer properties in aggregated donor acceptor systems photoluminescence spectra were measured in Pt2M and Pt2MT dodecane gels with doping the acceptor NDI 1 The doping level s of NDI 1 were kept below 5 mol% to avoid disrupt ing the gel s. According to o ur previous study, 129 the photoluminescence of a Pt2M gel features two distinct bands: the band centered at = 495 nm originat es fr om its aggregated state and the band concentrated at = 515 nm is attributed to its molecularly dissolved state. W ith 5 mol% doping of the acceptor NDI 1 the band at = 495 nm is greatly quenched (~70%), whereas the band at = 51 5 nm shows almost no n oticeable decrease (Figure 5 5 A ) This is a clear evidence that quenching / electron transfer is efficient in aggregates due to the close proximity of

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126 donor / acceptor chromophores in the ordered supramolecular architectures, which is consi stent with our prev ious study of energy transfer in the mixed gel. 129 The fact that quenching/electron transfer only occurs in the aggregated state su ggests that exciton diffusion among Pt2M units may be involved. For Pt2MT / NDI 1 mixed gel, phosphorescence is quenched greater than 99% with 5 mol% doping once again indicating that efficient electron transfer occurs in this system (Figure 5 5 B ) Figure 5 5. Photoluminenscence spectra of pure and mixed dodecane gels. The instrument was kept in the same conditions during the measurement. Both pure and mixed gels were excited at the same wavelength. A ) ex = 300 nm; B ) ex = 340 nm. Mix = Pt2M / Pt2MT + 5mol% NDI 1

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127 In order to gain insight into electron transfer dynamics, transient absorption spectra were carried on Pt2M / Pt2MT pure and mixed dodecane gels. However, we only observed that lifetimes of T 1 T n absorption decrease in doped gels and the characteristic absorption of the NDI anion and corresponding cations were not detected in nanosecond transient absorption spectra. This is probably due to the very rapid charge recombination in non polar solvent (dodecane). To study elec tron transfer properties in aggregated molecules in polar solvent s we designed and synthesized a new platinum acetylide based organogelator Pt2MAM Morphology and Photophysics of Pt2MAM Gelator Gel f ormation T he structure of Pt2MAM features the core uni t of conjugated phenyl ring and long alkyl chain end groups connected by amide functional groups. T he feature combines all three driving forces that initiate molecular self assembling in one molecule. The gelator Pt2MAM was subjected to gelation test s wi th a variety of solvents. A screw capped sample vial containing the gelator and the solvent was heated until the solid was dissolve d. Then the sample vial was cooled to room temperature. The formation of the gels was evaluated by the stable to inversion o f a test tube method. T he testing results are listed in Table 5 1. Pt2MAM gels in both nonpolar and Table 5 1. Gelation tests of Pt2MAM S olvent C oncentration (mg/mL) R esult A cetonitrile 12 I nsoluble B enzene 12 S oluble DMSO 10 G el DMF 12 S oluble C yc lohexane 8 G el 2 P ropanol 12 S oluble THF 12 S oluble *The maximum concentrations used for gelation tests polar solvents. T he critical gelatio n concentration (CGC) is ~ 1 wt% in cyclohexane and DMSO. Figure 5 6 shows sol gel transition s upon heating and c ooling. Pt2MAM also indicates bright

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1 28 phosphorescence under UV light illumination. T he cyclohexane gel is transparent whereas the DMSO gel is slightly opaque. Figure 5 6. S ol gel transition s of Pt2MAM Morpholog ical c haracteriza tion T he m orphological feature of Pt2MAM cyclohexane gel was revealed by transmission electron microscopy (TEM) and atomic force microscopy (AFM) (Figure 5 7) TEM images of a dilute solution of Pt2MAM (1 10 5 M) evaporated from cyclohexane on a copper gri d show the formation of a complex network of fibers consisting of the aggregated Pt2MAM molecules. The average widths of the individual fibe rs range from 30 to 150 nm, suggest ing that the individual fibers are likely composed of many subfibers that consist of arrays of stacked Pt2MAM molecules. AFM studies of dried Pt2MAM cyclohexane solution on a mica surface support the morphological features observed in TEM images.

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129 A B Figure 5 7. Morphological features of Pt2MAM xero gels: A ) TEM image of a cyclohexane solution (1 10 5 M) evaporated on a copper grid; B ) AFM image of a cyclohexane solution (1 10 5 M) evaporated on a mica surface UV V is a bsorption C oncentration a nd temperature dependent UV Vis absorption spectra were measured in Pt2MAM dimethyl sulfoxide (DMSO) solution/gel. For concentrations above 0.1 mM, the absorbance was maintained below a value of 1.0 by using short path length (1 mm or 0.1 mm) cells as nece ssary. Figure 5 8 A shows normalized absorption spectra of Pt2MAM in DMSO at three concentrations (1.6 10 4 8 10 4 and 4 10 3 M). Prior to each measurement, the solution was subjected to a heating cooling cycle, but Pt2MAM only gels at the 4 10 3 an d 8 10 4 M concentration s T he absorption spectra of Pt2M AM in its gel state are dominated by a strong absorption band with max = 345 nm along with a shoulder at = 362 nm. With further decreasing concentration, the absorption spectrum ( c = 1.6 10 4 M) shows a strong light scattering band above 370 nm due to insolubility of Pt2MAM in DMSO. Nevertheless, it is evident that the absorption band centered at = 345 nm decreases with decreasing concentration. By comparison with the absorption spectrum in dilute THF solution (Figure 5 8C), we are able to

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130 Figure 5 8. Absorption spectra of Pt2MAM in solution/gel. A ) DMSO gel/solution at r oom temperature; the spectra are normalized at = 362 nm. B ) DMSO gel/solution at c = 4 10 3 M. Arrow indicates change in spectra with increasing temperature from 45 to 87 o C. The heating rate was approximately 1 o C/min, and spectra were acquired within 1 min after the temperature stabilized C ) THF dilute solution

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131 assign ed the absorption shoulder ( = 362 nm) to the long axis polarized transition of the molecularly dissolved (monomeric) chromophore. In contrast, the primary absorption band in Pt2MAM aggregate state blue shifts ca. 17 nm and contains some residual absorption in the re gion where monomer absorbs ( = 362 nm), indicating that the Pt2MAM DMSO gel forms H aggregates. Figure 5 8 B illustrate s the temperature dependent absorption spectra of the Pt2MAM gel/solution with c = 4 10 3 M in DMSO. W ith increasing temperature the absorption band concentrated at = 345 nm decreases which indicates the transition from gel to solution state. Note that the spectrum at 87 o C still shows strong absorption from the aggregated state, suggesting that the temperature range of the gel to sol ution transition has not been reached. Steady s tate p hotoluminescence P hotoluminescence spectra of Pt2MAM were measured in argon degassed solutions (or gels). For the gels, the samples were heated to the solution state to prevent gel formation while dega ssing. After degassing, the samples were sealed and cooled to room temperature. All of the spectra obtained with varying concentration, excitation wavelength and solvent exhibit a strong emission band at = 512 nm followed by a vibronic band at 550 nm These bands are assigned to phosphorescence from 3 state of Pt2MAM consistent with our previous studies of platinum acetylide oligomers. 33 Figure 5 9 illustrates the emission spectra of Pt2MAM gels in DMSO with the excitation at ex = 345 and 362 nm and the emission spectrum recorded in dilute THF solution is also shown for comparison purposes. Note that n o noticeable shifts were observed in the transition from molecular dissolved state to the gel state indicating that the phosphorescence band is not strong ly perturbed by close packing of the chromophores in the a ggregates. T he fact that the aggregated state of Pt2MAM exhibits almost no shift in the

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132 phosphorescence energy is consistent with the notion that in the triplet excited state the molecules interact weakly with other chromophores in the aggregate s A ccordi ng to our previous study, 129 the photoluminescence spectrum of the Pt2M gel blue shifts ca. 15 nm relative to its molecular dissolve d state. The shift of Pt2M aggregates in its excited state is because a change of its conformations is hindered by tightly packed aggregates However, the packing in Pt2MAM structure is probably less tight due to the hydrogen bonding between molecules, so molecules can undergo rapid conformation change in its excited state. Figure 5 10 shows a schematic representation of possible packing mode s in Pt2M and Pt2MAM aggregates. Figure 5 9. Photoluminescence spectra of Pt2MAM Black line: DMSO gel (4 10 3 M) with ex = 362 nm; green line: dilute THF solution with ex = 362 nm; red line: DMSO gel (4 10 3 M) with ex = 345 nm Figure 5 10 P ossible packing modes in Pt2M and Pt2MAM aggregates

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133 The photolumi nescence spectra of Pt2MAM with doping NDI 2 at 3 and 5 mol% were also recorded in DMSO gel s As shown in F igure 5 11 at 3 mol% doping of NDI 2 phosphorescence is dramatically quenched (~80 %) and with increasing doping level, the phosphorescence intensit y decrease s further (90% quenching with 5 mol%) which clearly indicates that electron transfer occurs in this system. In addition, t he electron t ransfer reaction is strongly exergonic ( G < 0.5 eV). In contrast, the same concentration Pt2MAM in THF solution with doping NDI 2 ( 5 mol% ) indicates that only ca. 40 % of phosphorescence is quenched. This suggests that electron transfer is more efficient in the Pt2MAM aggregated state where chr omophores are packed in close proximity. Taken together, the results of the mixed DMSO gels (and THF solution s ) clearly indicate that the Pt2MAM phosphorescence quenching by the doping of NDI 2 arises via a mechanism involving electron transfer from the tr iplet state of Pt2MAM to the NDI 2 acceptor. In addition, the process is more efficient when Pt2MAM molecules are aggregated suggesting that triplet exciton diffusion among Pt2MAM units may be involved. Figure 5 11 Photolumin escence spectra of Pt2MAM / NDI 2 mixed gel s in deoxygenated DMSO. [ Pt2MAM ] = 4 10 3 M and different mol % of NDI 2 ex = 345 nm

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134 Transient a bsorption I n order to gain insight into electron transfer dynamics, transient absorption spectra were measured in P t2MAM pure and mixed gel s in DMSO (Figure 5 12 ) A cell with 1 m m path Figure 5 12. Transient absorption spectra of Pt2MAM and mixed DMSO gels. A ) Pt2MAM gel (4 10 3 M). Delay increment: 0.8 s; B ) Pt2MAM (4 10 3 M) containing 5 mol% NDI 2 gel. Delay increment: 0.32 s. Both spectra were recorded with 355 nm laser excitation length and positioned at 45 o ang le relative to the laser beam was use d. Since the degassing for the gels in this cell is difficult, the spectra were recorded in non degassed DMSO gels. While the delta absorbance is low and the spectra are therefore noisy, the results are evident. T he spectra show triplet triplet absorption band centered at = 700 nm and the decay is muc h faster in the

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135 doped gel (Figure 5 1 3 ) Unfortunately we did n o t observe the characteristic absorption bands of the radical anion and the radical cation presumably due to very rapid charge recombination even in the polar solvent. Figure 5 1 3 Decay profiles of Pt2MAM in DMSO pure and mixed gels (containing 5 mol% NDI 2 ) at = 700 nm Pulse Radiolysis in Pt2M Pure and Pt2M/NDI 1 Mixed Gels Pulse radiolysis experiments were carried out at the Laser Electron Accelerator Facility (LEAF) at Brookhaven National Laboratory The facility has been previously des cribed in detail, 135,136 so we only p rovide a brief description here. A schematic representation of the LEAF is shown in Figure 5 1 4 T he LEAF uses a laser pulsed photocathode, radio frequency electron gun to generate 5 ps pulse of 9 MeV electrons for pulse radiolysis experiments. The elect ron pulse is produced by laser light impinging on a photocathode inside a resonant cavity, radio frequency (RF) gun. The emitted electrons are accelerated to ~9 MeV within the length of the gun. Then the laser pulse is synchronized with the RF power to pro duce the electron pulse as short as 5 ps

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136 Figure 5 1 4 T he LEAF facility in Brookhaven National Laboratory I n our previous study, 42 we have applied the pulse radiolysis technique for investigating charge transport and delocalization in a n extended series of platinum acetylide oligomers (PAOs) in solutions. T he generation of PAO bas ed ion radicals involves a series of reactions in which high energy electron pulses produce very reactive solvent cation radicals or holes (h + ) or electrons (e ), which are then transfer to the PAO molecules The reactions strongly depend on the solvent. F or example, in 1,2 dichloroethane ( DCE ) the electrons are immediately captured by the solvent to form solvent radical cations and Cl ions In tetrahydrofuran (THF), the high energy electron pulses produce THF radical cations and solvated electrons Then the solvent radical cations decompose to radicals and solv a ted protons. As a result, the production of ther malized solvated radical cations (h + ) in DCE and thermalized solvated electrons (e ) in THF are the net effect of the electron pulses The formation of PAO radical anion and cation species is a bimolecular process as the attachment of holes in DCE and electrons in THF occurs only after a diffusion reaction of the primary holes or electrons with the substrate molecules Because the substrates (PAOs) ar e present in excess in these experiments, these reactions are pseudo first order and the observed rates of h + or e transfer to substrate depends on the substrate concentration and the bimolecular rate constant. Laser RF Feed 5 ps Time delay 5 ps 9 MeV e h Electron Gun

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137 I n the current study, we applied the pulse radiolysis method to the p latinum ace t ylide organogelator Pt2M This method allows us to gain insight into the spectroscopy of the radical ion states as well as electron transfer dynamics in Pt2M aggregates. The morphological study of the Pt2M gel indic ates that Pt2M in its gel state form a fiber aggregate structure. In addition, the blue shift in its absorption spectrum suggests that self ass em ble of Pt2M produces H aggregates With doping of the electron acceptor NDI 1 phosphorescence arising from Pt2 M aggregate ( the band at = 495 nm in the photoluminescence spectrum) is efficient ly quenched via electron transfer to the NDI 1 acceptor. By applying the pulse radiolysis method to the Pt2M gel, we anticipated that solvated anion radicals would be first produced via high energy electron pulses followed by electron transfer to the Pt2M aggregates. Upon doping a small amount of the electron acceptor NDI 1 electrons would be finally captured by the NDI 1 acceptor. A schematic representation of electron tra nsfer processes is shown in Figure 5 1 5 Figure 5 1 5 P ossible electron transfer processes in the Pt2M / NDI 1 mixed gel

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138 A screw cappe d cell with 5 mm path length was used for pulse radiolysis experiments T he samples were pre pared in a glove box with isooctane as solvent. Isooctane is a good solvent for pulse radiolysis as it easily captures electrons and the electrons are weakly solvated and consequently they diffuse very rapidly I n addition, both Pt2M and NDI 1 gel in isoo ctane with critical gelation concentration (CGC) of 1 mM. Prior to each measurement, the sealed sample was subjected to a heating cooling cycle to form stable gel For comparison purposes, pulse radiolysis was first carried out in Pt2M THF solution with 1 mM concentration ( Figure 5 1 6 A ) The spectr um feat ure s a dominant band centered at = 540 nm and a high energy vibronic band concentrated at = 490 nm. This band is assigned to the absorption of the anion radical Pt2M and the result is consistent with our previous study. 42 The anion radical of Pt2M decays within several microseconds. Then the Pt2M isooctane gels were subjected to pulse radiolysis at three concentrations (5 10 3 1.5 10 2 and 3 10 2 M). With the concentration at 5 10 3 M, the absorption bands are extremely weak and non identifiable. When the concentration is increased to 1.5 10 2 M, w e began to see a weak absorption band below 500 nm and this band becomes stronger when the concentration reaches to 3 10 2 M (Figure 5 1 6 B ). Note that the absorption band show s a growing trend at the blue edge of the detector limit (440 nm) Due to the fa ct that the gel sample was somewhat opaque, little light pass ed through it at short er wavelength s making detection below 440 nm impossible Nevertheless, the s pectrum of Pt2M in the isooctane gel suggest s that electrons are captured by Pt2M aggregates and the absorption of Pt2M is below 440 nm and blue shift s compared with its ab s orption in THF solution. Note that an intense band centered at = 460 nm appears at 3 ns after electron pulses. W hile further measurements are needed in order to confirm if th is band arises from absorption of Pt2M this

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139 band decays quickly (within several nanoseconds). The decay profiles of Pt2M in isooctane gel and THF solution are shown in Figure 5 1 6 C Figure 5 1 6 Pulse radiolysis t ransient a bsorption spectra and decay profiles for radical anions of Pt2M A ) in 1 mM THF solution; B ) in 30 mM isooctane gel ; C ) decay profiles at = 490 nm for THF solution (1 mM) and at = 460 nm for isooctance gel (30 mM)

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140 In order to gain insight concerning e lectron transfer dynamics in the aggregated state the Pt2M isooctane gel (15 mM) was doped a small amount of NDI 1 (5 mol%) and the resulting mixed gel was subjected to pulse radiolysis experiments At room temperature, the spectrum exhibit s a narrow abso rption band centered at = 470 nm (Figure 5 1 7 A ). In order to assign this band, pulse radiolysis of the pure NDI 1 isooctane gel (5 mM) was carried out at high temperature (88 o C). Note that NDI gels in isooctane at 5 mM concentration but the gel is so o paque that the probe light is completely blocked. Thus we were not able to carry out pulse radiolysis stud y on the pure NDI 1 isooctane gel at room temperature. As shown in Figure 5 1 7 B the spectrum features a dominant absorption band centered at = 470 nm, which apparently arises from the anion radical NDI 1 consistent with literature reports of anion radicals spectroscopy of NDI derivatives. 46,52 Thus, the band in Pt2M / NDI 1 mixed gel is assigne d to the absorption arising from NDI 1 The decay profiles of NDI 1 at 470 nm in pure and mixed gels (or solution) are similar, which further supports the assignment of the absorption band in the mixed gel (Figure 5 1 7 C ). There are two possible mechan isms for the formation of NDI 1 in the Pt2M / NDI 1 mixed gel: (1) NDI 1 directly captures electrons; (2) electrons are first attached to Pt2M and then transferred to NDI 1 I n order to determine which mechanism applies in this system, pulse radiolysis mea surements were carried out for both the pure Pt2M and Pt2M / NDI 1 mixed gels at 88 o C, which is above the sol gel transition temperature. If electrons were directly captured by NDI 1 the spectrum is expected to show the characteristic absorption of NDI 1 ( = 470 nm) as we observed in the pure NDI 1 at high temperature. In addition, a t this temperature, both pure and mixed gels are less aggregated due to gel sol transition. As shown in Figure 5 18 A and B both spectra indicate similar absorption bands at 88 o C. By comparing with the spectrum of

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141 Figure 5 1 7 Pulse radiolysis t ransient absorption spectra and decay profiles of A ) Pt2M (15 mM) + 5 mol% NDI 1 isooctane gel at RT ; B ) Pure NDI 1 (5 mM) in isooctane at 88 o C ; C ) Deca y profiles at = 470 nm

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142 Pt2M in THF solution, these bands mainly arise from the molecularly dissolved state of Pt2M with possibly some contribution s of its aggregated state absorption. The NDI absorption band ( = 470 nm) does not appear in the Pt2M / N DI 1 mixture at 88 o C, suggesting that it is not likely that electrons are directly attached on NDI 1 The Pt2M decay at high temperature is faster than the decay in THF solution (Figure 5 18 C ). Figure 5 1 8 Pulse radiolysi s t ransient absorption spectra and decay profiles of A ) Pt2M (5 mM) in isooctane at 88 o C; B ) Pt2M + 5 mol% NDI 1 in isooctane at 88 o C ; C ) Decay profiles of Pt2M anion radicals at = 490 nm

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143 Taken together, several points can be drawn from pulse radiolysis measurement s in Pt2M pure and Pt2M / NDI 1 mixed gels First, Pt2M gels capture electrons and the absorption band of Pt2M is located in the blue region (below 440 nm) Compared with Pt2M absorption in THF solution, the band in Pt2M gel state blue shift s Second, Pt2M / NDI 1 mixed gels c apture electrons and the spectr um is dominated by the absorption of NDI This result suggests that formation of NDI 1 in the Pt2M / NDI 1 mixed gel occurs via a n indirect mechanism. The electrons are initially attached to Pt2M aggregates (Equation 5 1). Then electron transfer from Pt2M to the NDI 1 electron acceptor to produces NDI 1 (Equation 5 2). The reaction 5 2 is strongly ex erg on ic ( G < 0.6 eV), suggesting that the electron transfer process is very rapid. Pt2M + e (solvated) Pt2M ( 5 1 ) Pt2M + NDI 1 Pt2M + NDI 1 ( 5 2 ) Conclusion The p hotoluminescence of a donor acc eptor system consisting of Pt2M (or Pt2MT ) (donor) and NDI 1 (acceptor) in its gel state is efficiently quenched due to electron transfer from the don or to the acceptor In order to investigate electron transfer properties in polar solvent, a new electron donor ( Pt2MAM ) and its corresponding acceptor ( NDI 2 ) were synthesized. The organogelator Pt2MAM with the structure that features all three driving fo rces for molecular self assembly gels both nonpolar and polar solvents. The morphology of Pt2MAM gels has fiber like network s as shown in the TEM and AFM images. Blue shift of Pt2MAM absorption bands in its gel state relative to its molecular dissolved sta te indicate s that self assembly of Pt2MAM molecules produces H aggregates. No significant change is shown in the photoluminescence spectra of Pt2MAM gels suggesting that the triplet exciton is not strongly perturbed by close packing of the aggregated chro mophores. Photoluminescence spectra of

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144 Pt2MAM / NDI 2 mixed gels shows efficient phosphorescence quenching, indicating that electron transfer occurs in this system. Further evidence regarding electron transfer in this system is given by transient absorption measurement s which illustrate by much faster triplet decay in the mixed gel. The absorption of the radical anion or cation was not observed in the transient spectra, suggesting that charge recombination is very rapid even in polar solvent s The p ulse radio lysis technique was applied to study radical ion states of conjugated organogels ( Pt2M and Pt2M / NDI 1 gels) The results indicate that the Pt2M isooctane gel captures electron s and its radical anion absorption blue shifts relative to the absorption in TH F solution. T h e Pt2M / NDI 1 mixed gel also capture s electrons and the corresponding spectrum show s the NDI 1 a bsorption band. T he analysis of the detailed electron transfer dynamics indicates that electron transfer in the aggregated state adopts an indire ct mechanism which electrons are initially attached to the Pt2M aggregates followed by electron transfer to the acceptor NDI 1 to form NDI 1 Experimental Photophysical Measurements Steady state absorption spectra were recorded on a Varian 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. Corrected 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 solutions. 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

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145 Transient absorption measurements were conduct ed on a home built apparatus, which has been described elsewhere. Samples were contained in a cell with 1 mm path length and were subjected to a heating cooling cycle before measurements The cell was positioned at a n angle of 45 o C relative to the laser be am. Pulse Radiolysis This work was carried out at the Brookhaven National Laboratory Laser Electron Accelerator Facility (LEAF). The LEAF facility and the methods used ha ve been described elsewhere. 135,136 optical path length of 5 mm containing the solution /gel of interest. The monitoring light source was a 75 W Osram xenon arc lamp pulsed to a few hundred times its normal intensit y. Wavelengths were selected using either 40 or 10 mm band pass interference filters. Transient absorption signals were detected with either FND 500L InGas 680B oscillosc ope. Synthesis General. The same procedures were used as described in Chapter 2. cis D ichloro bis (trimethylphosphine) platinum(II) was prepared according to the literature methods 137,138 4 I odobenzoyl chloride was prepared according to the literature procedure 139 Methyl 3,4,5 tri s(n dodec an 1 yloxy ) benzoate (1). 140 To a solution of methyl 3 ,4,5 trihydroxy benzoate (4 g, 21.7 mmol) in DMF (50 m L ) was added pot assium carbonate (14.4 g, 104.2 mmol) while stirring at room temperature. The reaction mixture was stirred at 60 o C for 1 h. Then 1 bromododecane (17.3 g, 69.4 mmol) was added dropwise wi thin 10 min followed by the addition of tetra n butylammonium bromide ( TBAB) (0.31 g, 1 mmol). T he resulting mixture was stirred at 70 o C for 20 h. After cooling to room temperature the mixture was poured into 250 m L of ice/water with vigorous stirring. T he suspension was filtered and the resulting light yellow

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146 solid was subjected to column chromatography on silica (4:1 hexane/ethyl acetate) to g ive a pale solid as the product ( 13 g, 87%) after removing the solvents. 1 H NMR (CDCl 3 300 MHz ): 7.24 (s, 2H) 4.0 (tt, overlap, 6H), 3.88 (s, 3H), 1.83 1.73 (m, 6H), 1.53 1.26 (m, 54 H), 0.9 (t, 9H). 3,4,5 T ri s(n dodec an 1 yloxy ) benzoic acid (2) 140 Potassium hydroxide (0.5 g, 8. 9 mmol) was added to a suspension of m ethyl 3,4,5 tris(n dodecan 1 yloxy) benzoate ( 1 ) (3 g, 4.35 mmol) in 95% ethanol (40 m L ) at room temperature. Then the mixture (clear solution) was stirred at 78 o C for 4 h. After cooling to room temperature, the solution was poured into 250 m L DI water and acidified with 5% hydrochloric acid to pH 1. T he white precipitate was filtered as the product (2.79 g, 95%). 1 H NMR (CDCl 3 300 MHz ): 7.3 (s, 2H), 4.0 (tt, overlap, 6H), 1.84 1.77 (m, 6H), 1.49 1.27 (m, 54 H), 0.88 (t, 9H). 3,4,5 T ri s(n dodec an 1 yloxy ) be nzoyl chloride ( 3 ) 140 T o a solution of 3,4 ,5 tris(n dodecan 1 yloxy) benzoic acid ( 2 ) ( 1.01 g, 1.5 mmol ) in methylene chloride was added a catalytic amount of DMF (~0.5 mL) The reaction flas k was cooled in an ice bath and thionyl chloride (0.16 m L 2.2 mmol) was added dropwise. A white suspension was form ed immediately. Then the ice bath was removed and the mixture was stirred at room temperature for 4 h. The solvent was evaporated under vacuum and the crude product was used for the next step without further purification. Compound 4 134 A solution of ethylene diamine ( 5 m L ) and tri ethylamine (2 m L ) in methylene chloride (10 m L ) was placed in an ice bath and degassed for 10 min. Then, the solution of 3,4,5 tris(n dodecan 1 yloxy) benzoyl chloride ( 3 ) (1.2 g, 1.73 mmol) in methylene chloride (10 mL ) w as added dropwise to the ethylene diamine solution. The resulting yellow suspension was brought to room temperature overnight. M ethylene chloride (50 m L ) was added to the suspension and t he mixture was washed with 1 M HCl (100 m L twice). The aqueous layer

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147 was extracted with methylene chloride. The combined organic solution was dried over anhydrous sodium sulfate. After the solvent s w ere evaporated under vacuum, the resulting light yellow solid was recrystal l ized from ethanol. Further purification by column chromatography on silica (10:1 5:1 chloroform/methanol) gave an off white solid as the product (0.3 g, 25%) after removing solvents. 1 H NMR (CDCl 3 300 MHz ): 7.01 (s, 2H), 6.61 (t, 1H), 4.00 (m, 6H), 3.49 (q, 2H), 2.95 (t, 2H), 1.7 0 1. 82 (m, 6 H), 1.45 1 .49 (m, 6H), 1.26 1.33 (m, 48H), 0.84 0.88 (m, 9H). Compound 5. This compound was synthesized according to the same procedure used for compound 4 except compound 4 (0.2 g, 0.28 mmol) and 4 iodobenzoyl chloride (82 mg, 0.31 mmol) were used. T he desired pr oduct 5 was obtained as an off white solid ( 0.2 g, 76% ) 1 H NMR (CDCl 3 300 MHz ): 7.75 (d, 2H), 7.55 (d, 2H), 7.44 (b roa d singlet, 1H), 7.11 (b roa d singlet, 1H), 6.98 (s, 2H), 4.00 (m, 6H), 3.67 (b roa d singlet, 4H), 1.70 1.82 (m, 6H), 1.45 1.49 (m, 6H), 1.26 1.33 (m, 48H), 0.84 0.90 (m, 9H). Compound 6. To a solution of compound 5 (0.28 g, 0.296 mmol) in diethylamine/THF (7 m L /5 m L ) was added Pd(II) (20.8 mg, 0.0296 mmol) and CuI (2.8 mg, 0.0148 mmol). T he mixture was degassed for 15 min and then trimethy lsily l acetylene (44 mg, 0.4 5 mmol) was added via syringe The reaction mixture was stirred at room temperature for 5 h at which time TLC analysis showed that no starting material remained T he solution was diluted with 50 m L methylene chloride, washed wit h water. The combined organic solution was dried over sodium sulfate. After removing solvents under vacuum, the red solid was recrystallized from ethanol to give an off white solid as the product (0.21 g, 77%). 1 H NMR (CDCl 3 300 MHz ): 7.76 (d, 2H), 7.50 (d, 2H), 7.26 (b road singlet, 1H), 7.12 (broa d singlet, 1H), 6.98 (s, 2H), 4.00 (m, 6H), 3.67

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148 (b roa d singlet, 4H), 1.70 1.82 (m, 6H), 1.45 1.49 (m, 6H), 1.26 1.33 (m, 48H), 0.84 0.90 (m, 9H). Compound 7. To a solution of compound 6 (0.21 g, 0.23 mmol) in THF (8 m L ) was added TBAF (0.92 m L ) via syringe. T he resulting solution was stirred at room temperature for 0.5 h at which time TLC analysis indicated that no starting material remained Then methylene chloride (50 m L ) was added to di lute the solution. Aft er washing with water, the combined organic solution s were dried over sodium sulfate. A red solid was obtained after removing the solvents under vacuum The solid was dissolved in a few milliliter s of hot eth anol. An off white precipitate was collected as the product (0.18 g, 93%). 1 H NMR (CDCl 3 300 MHz ): 7.83 (d, 2H), 7.67 (b roa d singlet, 1H), 7.50 (d, 2H), 7.42 (b roa d singlet, 1H), 6.98 ( s, 2H), 4.00 (m, 6H), 3.67 (broad singlet, 4H), 3.17 (broa d singlet, 1H), 1.70 1.82 (m, 6H), 1.45 1.49 (m, 6H), 1.26 1.33 (m, 48H), 0.84 0.90 (m, 9H). Compound 8. To a solution of compound 7 (0.18 g, 0.21 mmol) in diethylamine (3 m L ) and THF (3 m L ) was added cis di chloro bis (trimethylphosphine) platinum(II) (0.106 g, 0.25 mmol). The resulting solution was stirred at room temperatu re for 20 h at which time methylene chloride (100 m L ) was added to dilute the solution. After washed with water, the combined organic solution was dried over sodium sulfate. A yellow solid was obtained after removing the solvents The solid was p recipitate d from ethanol to give a light yellow solid as the product (0.18 g, 70%). 1 H NMR (CDCl 3 300 MHz ): 7.65 (d, 2H), 7.32 (d, 2H), 7.18 (b roa d singlet, 1H), 7.08 (b roa d singlet, 1H), 6.98 (s, 2H), 4.00 (m, 6H), 3.67 (b roa d singlet, 4H), 1. 60 1.82 ( m, 24 H), 1.45 1.49 (m, 6H), 1.26 1.33 (m, 48 H), 0.84 0.90 (m, 9H). 31 P NMR (CDCl 3 121 MHz) 13.54 (J Pt P = 23 21 7 Hz).

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149 Pt2MAM. To a solution of compound 8 (0.16 g, 0.129 mmol) in diethylamine/THF (5 m L /5 m L ) was added 1,4 diethynylbenzene (7.9 mg, 0.063 mmol) and CuI (1.2 mg, 0.0063 mmol). The reaction mixture was stirred at room temperature for 3 h. At which time, TLC analysis showed that no 1,4 diethynylbenzene remained T he solution was diluted with 100 m L of methylene chloride, washed with water. The combined organic solution was dried over sodium sulfate. After removing solvents under vacuum, the red solid was precipitated from ethanol to give a yellow solid as the product (0.13 g, 82%). 1 H NMR (CDCl 3 300 MHz ): 7.66 (d, 4H), 7.34 (d, 4H), 7.25 (s, 2H) 7.17 (s, 2H), 7.00 (s, 4 H), 4.00 (m, 12H), 3.67 (b roa d singlet, 8 H), 1. 64 1.84 (m, 48 H), 1. 40 1.48 (m, 12 H), 1 .26 1.33 (m, 96 H), 0.84 0.90 (m, 18 H). 31 P NMR (CDCl 3 121 MHz) 19.39 (J Pt P = 2 292 6 Hz). NDI 2. 134 To a solution of compound 4 (0.59 g 0.82 mmol) in DMF (20 m L ) was added 1,4,5,8 tetracarboxylic naphthalene dianhydride (74 mg, 0.27 mmol) at room temperature. The reaction mixture was stirred at 140 o C for 7 h. After cooled to room temperature, the reaction mixture was placed in a freezer overnight. T he resulting precipitate was filtered out and dissolved in hot ethanol. A yellow solid was obtained as the product after filtration (0.43 g, 95%). 1 H NMR (CDCl 3 300 MHz ): 8.73 (s, 4H), 6.98 (s, 4H), 6.72 (t, 2H), 4.54 (m, 4H), 4.00 (m, 12H) 3.87 (m, 4H), 1.75 1.82 (m, 12H), 1.47(m, 12 H), 1.26 1. 33 (m, 96H), 0.84 0.90 (m, 18H); Elemental anal. calc d C: 74 96 H: 10 16 N: 3.36 ; Found C: 74 91 H: 11 05 N: 3.34 1,2,3 T ri s(n dodec an 1 yloxy ) benzene ( 9 ) 141 To a solution of 1,2,3 trihydroxyb enzene (4 g, 31.76 mmol) in DMF was added potassium carbonate (21.04 g, 152.45 mmol) at room temperature while stirring. The resulting colorless solution was heated to 60 o C. 1 bromododecane (25.33 g, 101.63 mmol) was added dropwise within 10 min followed by the addition of TBAB (0.31 g). The reaction mixture was stirred at 65 o C for 4 h and then poured into

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150 250 m L ice/water with vigorous stirring. The creamy, granular solid was filtered out and passed through a short plug of silica with ethyl acetate/hexan e (19:1) as the eluent An off white solid was obtained as the product after removing solvents under vacuum (18.44 g, 92%). 1 H NMR (CDCl 3 300 MHz ): 6.9 ( t 1H), 6.55 (d, 2H), 3.9 (t, 6H), 1.78 (m, 6H), 1.47 (m, 6H), 1.26 (m, 48H), 0.88 (t, 9H). 3,4,5 T ri s(n dodec an 1 yloxy ) 1 nitrobenzene ( 10 ). 141 111 T o a stirred suspension of nitric acid (0.15 g, 2.38 mmol, 25% on SiO 2 ) in methylen e chloride ( 4 m L ) was rapidly added 1,2,3 tridodecyloxybenzene ( 9 ) (0.3 g, 0.475 mmol) in methylene chloride (2 m L ). The r esulting red solution was stirred at room temperature for 20 min, after which time the SiO 2 was filtered and washed several times with methylene chloride. After removing solvent under vacuum, the resultant orange oil was dissolved in hexane (3 m L ). Upon ad dition of methanol (20 m L ) with vigorous shaking, the product was separated as a white solid. The solid was filtered, washed with cold methanol and dried in air (0.238 g, 74%). 1 H NMR (CDCl 3 300 MHz ): 7.47 (s, 2H), 4.00 (m, 6H), 1.78 (m, 6H), 1.47 (m, 6 H), 1.26 (m, 48 H), 0.88 (t, 9H). 3 ,4,5 T ri s(n dodec an 1 yloxy ) 1 aminobenzene (1 1 ). T o a suspension of 3,4,5 tridodecyloxy 1 nitrobenzene ( 10 ) (1 g, 1.48 mmol) and Pd/C (10%) (32 mg, 0.3 mmol) in ethanol (15 m L ) was added hydrazine monohydrate (0.71 g, 2 2.2 mmol). The reaction mixture was re f luxed for 20 h under an argon atmosphere. T he cooled mixture was diluted with methylene chloride (50 m L ). Carbon was filtered and washed several times with methylene chloride. T he solution was concentrated in a rotary evaporator. A n off white solid was obtained as the product (0.918 g, 96%). 1 H NMR (CDCl 3 300 MHz ): 5.91 (s, 2H), 3.91 (t, 4H), 3.84 (t, 2H), 3.46 (bs, 2H), 1.7 6 (m, 6H), 1.47 (m, 6H), 1.26 (m, 48 H), 0.88 (t, 9H).

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151 NDI 1. 142 3, 4,5 T ridodecyloxy 1 aminob enzene ( 1 1 ) (0.541 g, 0.84 mmol) 1,4,5,8 tetracarboxylic naphthalene dianhydride (94 mg, 0.35 mmol) and zinc acetate (76 mg, 0.35 mmol) was added to quinoline (10 m L ). The mixture was stirred at 180 o C for 3 h. The c ooled solution was poured into 1 M HCl ( 100 m L ) and the precipitate was filtered and washed with water and methanol to give a yellow solid. After column chromatography on silica (3:7 hexane/methylene chloride), the resulting solid was dissolved in methylene chloride and precipitate d by adding me thanol. After filtration, a light red solid was obtained as the product (0.33 g, 62%). 1 H NMR (CDCl 3 300 MHz ): 8.84 (s, 4H), 6.5 (s, 4H), 4.1 (t, 4H), 3.97 (t, 8H), 1.8 (m, 12H), 1.47 (m, 12H), 1.26 (m, 96 H), 0.88 (t, 18H).

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152 CHAPTER 6 CONCLUSION In t he previous chapters, the photophysical properties of several different types of conjugated platinum containing oligomers have been studied. The studies examined the basic photophysi cs of the oligomers, as well as intra and inter molecular interactions, such as photoinduced electron transfer and energy transfer and the effects of mo lecular self assembly on the photophysical properties T hese studies provide a fundamental guidance for the application of these materials or their analog u es in electronic or optoelectronic devices. The studies began with a comprehensive investigation of photophysics and photoinduced electron transfer in a series of platinum a cetylide oligomers that feature a donor spacer acceptor architecture T he phosphorescence quenching and the characteristic absorption bands of radical anions and radical cations obser ved in the transient absorption spectra provide evidence that intramolecular electron transfer occurs from the triplet state of the oligomers to the electron acceptors. O ne of the key findings of this study is that photoinduced electron transfer in PtnT2 A series of oligomers is excitation wavelength dependent W hen the spacer chromophore is excited, an indirect electron transfer mechanism occurs. I nitial charge separation arises from the triplet state of the spacer to the acceptor followed by hole shift fro m the spacer to the donor. This gives rise to the formation of a charge separated state that has a lifetime in excess of 100 ns. However, when the donor chromophore is directly excited, a direct electron transfer mechanism which occurs from the triplet sta te of the donor to the acceptor is applied. Interestingly, w hile electron transfer occurs in the short chain oligomer in the long chain oligomer s only the triplet state of the donor is present in the transient absorption spectra, indicating that electron transfer does not occur due to the absence of sufficient electronic coupling between the donor and the acceptor.

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153 I n a second study, two anthracene based platinum acetylide oligomers were prepared to investigate intramolecular energy transfer dynamics. Ou r previous studies and current DFT calculations indicate that the triplet state of the anthracene segment lies about 1.1 eV lower than that of the platinum acetylide segment in the Pt4An oligomer T he large energy gap provides an ideal environment for the occurrence of singlet triplet energy transfer. I ndeed, the S T energy transfer process is proved by the efficient quenching of the fluorescence emission in Pt4An and the strong triplet absorption of the anthracene segment in its transient absorption spect ra. Following S T energy transfer, T T energy transfer from the platinum acetylide segment to the anthracene segment takes place, which is confirmed by the rapid quenching of the phosphorescence emission in time resolved emission spectra. The anthracene ba sed platinum acetylide systems provide a unique platform for study ing rarely occurred singlet triplet energy transfer dynamics. To continue our studies on triplet ex citon delocalization in conjugated organometallic systems, we carried out a detailed phot ophysical investigation in a series of platinum tetrayne oligomers. The p hotoluminescence spectra of the oligomers feature a narrow and intense phosphorescence band followed by a vibronic progression of sub bands separated by ca. 2100 cm 1 Both absorption and low temperature emission spectra indicate that triplet excited state s of the series of oligomer s are localized and restricted within ~2 repeat units. In addition, the spectra are well fitted a Frank Condon expression. The parameters recovered from the spectra l fitting are also consistent with the notion of the localization of the triplet excited state. According to our previous studies in an extended series of platinum acetylide oligomers, the triplet excited state is confined with in ~1 repeat unit. In the current study, the triplet excited state of platinum tetrayne

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154 oligomers is restricted within ~ 2 repeat units likely due to the slightly rigid geometry of ethynylenic groups and the low S T splitting energy. T he final study was devoted to photophysics and electron transfer properties of phosphorescent pl atinum acetylide or ga nogelators. W e doped a small amount of the electron acceptor into aggregated platinum acetylide oligomers. T he effective phosphorescence quenching in the doped gels indicates that e lectron transfer occurs from the triplet state of the donor to the acceptor. I nterestingly, the quenching is more efficient in the aggregated state than in the molecularly dissolved state, which suggests that triplet exciton diffusion may be involved To e xplore electron transfer properties in the aggregated platinum acetylide oligomer in polar solvent s a new organogelator ( Pt2MAM ) that structurally combines all three driving forces that are responsible for molecular self assembl y was synthesized. The olig omer gels both nonpolar and polar solvents. Its morphology features fiber networks as shown in the TEM and AFM images. The bule shift in its absorption spectrum sugges ts that self assembly of Pt2MAM produces H aggregates With a small amount doping of an a cceptor, photoluminescence and transient absorption spectra confirm that electron transfer occurs in this system. To further gain insight into ion radical state s of the aggregated platinum acetylide oligomers, the pulse radiolysis technique was applied for this study. The results indicate that electrons are captured by the organog e la tor. The corresponding radical anion absorption blue shifts in the aggregated state relative to its molecularly dissolved state. With a small amount doping of an electron accept or, the radical ion spectrum features the absorption of the acceptor anion radical suggesting that electron transfer occurs from the surrounded host molecules to the electron acceptor molecules.

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155 APPENDIX A SUPPORTING INFORMATI ON FOR CHAPTER 2 Figure A 1. Chemical structure of the NDI model compound Figure A 2. Emission spectra of platinum acetylide oligomers

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156 Figure A 3. Emission spectrum of the NDI model compound Figure A 4. Chemical structure and absorption spectra of Pt2T2

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157 APPENDIX B SUPPORTING INFORMATI ON FOR CHAPTER 4 Figure B 1. Fitting spectra of (PtC 8 ) 1 (PtC 8 ) 2 and (PtC 8 ) 3 (100 K in Me THF glass ) 0 0.2 0.4 0.6 0.8 1 1.2 10000 12000 14000 16000 18000 20000 Intensity Energy / cm 1 E 0 = 17123 cm 1 h = 2100 cm 1 S m = 0.9, D 1/2 = 230cm 1 (PtC 8 ) 1 0 0.2 0.4 0.6 0.8 1 1.2 10000 12000 14000 16000 18000 20000 Intensity Energy / cm 1 E 0 = 16778 cm 1 h = 2045 cm 1 S m = 0.5, 1/2 = 250cm 1 (PtC 8 ) 2 0 0.2 0.4 0.6 0.8 1 1.2 10000 12000 14000 16000 18000 20000 Intensity Energy / cm 1 E 0 = 16806 cm 1 h = 2085 cm 1 S m = 0.4, D 1/2 = 235cm 1 (PtC 8 ) 3

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158 APPENDIX C NMR SPECTR A Figure C 1. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2T1 A Figure C 2. The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2T1 A

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159 Figure C 3. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2T 2 A Figure C 4. The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt2T 2 A

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160 Figure C 5. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt 4 T 1 A Figure C 6 The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt 4 T 1 A

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161 Figure C 7. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt 4 T 2 A Figure C 8 The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt 4 T 2 A

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162 Figure C 9. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt 6 T 1 A Figure C 10 The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt 6 T 1 A

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163 Figure C 1 1. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt 6 T 2 A Fig ure C 12. The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt 6 T 2 A

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164 Figure C 13. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2An Figure C 14. The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt 2An

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165 Figure C 15. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt4An Figure C 16. The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt 4An

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166 Figure C 17. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of Pt2MAM Figure C 18. The 3 1 P NMR (121 MHz, CDCl 3 ) spectrum of Pt 2MAM

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167 Figure C 19. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of NDI 2 Figure C 20. The 1 H NMR (300 MHz, CDCl 3 ) spectrum of NDI 1

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176 BIOGRAPHICAL SKETCH Yongjun Li was born in Chifeng, Inner Mongolia province, China. He received his B.E. in chemistry from Nanjing University of Science and Technology in July 1998. After graduation, he started his career as a junior rese archer in Xi an Modern Chemistry Research Institute Xi an, China. He worked there for five years and at the same time, he received his m aster s degree in organic chemistry. I n 2004, Yongjun had a chance to come to the University of Florida to continue his education pursuing a Ph.D in chemistry He joined Dr. Schanze s group and focused his study o n photochemistry. After his Ph. D Yongjun will work as a postdoctoral associate in Dr. Turro s group at Columbia University New York City, NY