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Synthetic Control of Light Absorption in Conjugated Polymers

Permanent Link: http://ufdc.ufl.edu/UFE0043623/00001

Material Information

Title: Synthetic Control of Light Absorption in Conjugated Polymers
Physical Description: 1 online resource (188 p.)
Language: english
Creator: Shi, Pengjie
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: conjugated -- electrochromism -- photovoltaics -- polymers
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: Conjugated polymers with alternating single and double carbon-carbon bonds along polymer backbones have become a unique branch in the family of polymeric materials. Researchers are especially interested in the optoelectronic properties of conjugated polymers, due to their capabilities in light weight and inexpensive organic electronic devices, such as photovoltaics, light emitting diodes, field-effect transistors and electrochromic devices. This dissertation focuses on the use of chemistry as a basis for optimization of these properties. In particular, the investigations involve development of new synthetic routes and methods to produce conjugated polymers with controlled absorption profiles, energy level distributions, band gaps and solubility, as well as evaluation of their use in electrochromic and photovoltaic applications. In a first instance, a synthetic methodology is devised for the synthesis of black-to-transmissive switching electrochromic polymers. By introducing the random Stille polymerization method, which can be used to combine multiple monomers, adjust their relative ratios and control the monomer sequence distribution, a random black-to-transmissive electrochromic conjugated polymer (ECP) has been synthesized with a broad absorption across the entire visible region, and the reproducibility of the polymerization was examined in detail. Black-to-transmissive ECPs were reproduced in batches with highly repeatable absorption spectra, number average molecular weights and polydispersity index. Moreover, successful characterization and device work enhance the probability that this polymer may be used for commercial electrochromic windows and displays. In a second project, the synthesis of a blue-to-transmissive ECP and a black-to-transimissive ECP, which are functionalized with carboxylate ester groups, was accomplished. Basic hydrolysis of the carboxylate ester side chains affords the polymer salts with water solubility, allowing thin films to be formed by spray-casting from the polymer/water solutions. Upon subsequent neutralization of the thin-films, the resulting polymer acid films are ready to be redox switched in a KNO3/water electrolyte solution and show a dramatic improvement in the switching speed compared with their ester derivatives at the sub-second switching time scale. A third project extended the random polymerization to improve the light-harvesting efficiency of conjugated polymers by broadening their absorption spectra. Diketopyrrolopyrrole-based conjugated copolymers were synthesized with a broad absorption from 350 to 800 nm, except for a small absorption gap between 550 to 650 nm. Further exploring the external quantum efficiency, photoluminescence quenching effect with fullerene, photovoltaic prosperities and polymer:fullerene film morphology of these random copolymers has provided a fundamental understanding of how polymer light absorption, side chains, stacking ability, and energy levels influence the performance of solar cell materials with thiophene and benzodithiophene as donors and diketopyrrolopyrrole as an acceptor.
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 Pengjie Shi.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Reynolds, John R.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2013-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043623:00001

Permanent Link: http://ufdc.ufl.edu/UFE0043623/00001

Material Information

Title: Synthetic Control of Light Absorption in Conjugated Polymers
Physical Description: 1 online resource (188 p.)
Language: english
Creator: Shi, Pengjie
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: conjugated -- electrochromism -- photovoltaics -- polymers
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: Conjugated polymers with alternating single and double carbon-carbon bonds along polymer backbones have become a unique branch in the family of polymeric materials. Researchers are especially interested in the optoelectronic properties of conjugated polymers, due to their capabilities in light weight and inexpensive organic electronic devices, such as photovoltaics, light emitting diodes, field-effect transistors and electrochromic devices. This dissertation focuses on the use of chemistry as a basis for optimization of these properties. In particular, the investigations involve development of new synthetic routes and methods to produce conjugated polymers with controlled absorption profiles, energy level distributions, band gaps and solubility, as well as evaluation of their use in electrochromic and photovoltaic applications. In a first instance, a synthetic methodology is devised for the synthesis of black-to-transmissive switching electrochromic polymers. By introducing the random Stille polymerization method, which can be used to combine multiple monomers, adjust their relative ratios and control the monomer sequence distribution, a random black-to-transmissive electrochromic conjugated polymer (ECP) has been synthesized with a broad absorption across the entire visible region, and the reproducibility of the polymerization was examined in detail. Black-to-transmissive ECPs were reproduced in batches with highly repeatable absorption spectra, number average molecular weights and polydispersity index. Moreover, successful characterization and device work enhance the probability that this polymer may be used for commercial electrochromic windows and displays. In a second project, the synthesis of a blue-to-transmissive ECP and a black-to-transimissive ECP, which are functionalized with carboxylate ester groups, was accomplished. Basic hydrolysis of the carboxylate ester side chains affords the polymer salts with water solubility, allowing thin films to be formed by spray-casting from the polymer/water solutions. Upon subsequent neutralization of the thin-films, the resulting polymer acid films are ready to be redox switched in a KNO3/water electrolyte solution and show a dramatic improvement in the switching speed compared with their ester derivatives at the sub-second switching time scale. A third project extended the random polymerization to improve the light-harvesting efficiency of conjugated polymers by broadening their absorption spectra. Diketopyrrolopyrrole-based conjugated copolymers were synthesized with a broad absorption from 350 to 800 nm, except for a small absorption gap between 550 to 650 nm. Further exploring the external quantum efficiency, photoluminescence quenching effect with fullerene, photovoltaic prosperities and polymer:fullerene film morphology of these random copolymers has provided a fundamental understanding of how polymer light absorption, side chains, stacking ability, and energy levels influence the performance of solar cell materials with thiophene and benzodithiophene as donors and diketopyrrolopyrrole as an acceptor.
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 Pengjie Shi.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Reynolds, John R.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2013-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043623:00001


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1 SYNTHETIC CONTROL OF LIGHT ABSORPTION IN CONJUGATED POLYMERS By PENGJIE SHI 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 2011

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2 2011 Pengjie Shi

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3 To my parents

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4 ACKNOWLEDGMENTS I would like to take this opportunity to express my appreciation to those who helped me in one way or another on conducting my research and the w riting of this dissertation. I would like to first thank my advisor Dr. John R. Reynolds for his continuous and generous guidance throughout my six year study. I have to admit that it was a highly challenging and difficult time for me to grow from an inex perienced student into a successful polymer chemist. On many occa sions when I was about to give up my synthesis and project, it was his wise insight and creative inspiration which saved my research. The work presented in this dissertation would not have been possible without his support an d advice. Moreover, he dedicates himself to creating an excellent environment for our study, handle s the research group as a warm family, and educates us well for our future. I am very grateful for the time he put in re viewing my research report s publications, oral presentations, and dissertation, and also for his consideration for my well being. I extend these thanks to the professors who have made great contributions to my study at the University of Florida, especial ly my supervisory committee for their support and invaluable discussions: Prof. Kenneth B. Wagener, Prof. Kirk S, Schanze, Prof. Michael J. Scott and Prof. W. David Wei in the Chemistry Department, and Prof. Jiangeng Xue in the Material Science and Enginee ring Department. I gratefully acknowledge the help and advice from many members in the Christian B. Nielsen Dr. Stefan Ellinger, Dr. Timothy Steckler, Dr. Genay Jones, Dr. Robert Brookins, Dr. Ryan Walzack, Dr. Aub rey Dyer, Dr. Svetlana Vasilyeva, Dr. Chad Amb, Dr. Dan Patel, Dr. Jianguo Mei, Dr. Pierre

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5 Beaujuge, Dr. Mike Craig, Dr. Andy Spring, Dr. Fengjun Deng, Dr. Leandro Estrada, Dr. Eric Shen and Dr. David Liu I can still remember the moment when Dr. Galand ta ught me how to perform column chromatography, and Dr. Nielsen showed me the titration of n BuLi. Special thanks to Dr. Mei, who helped me not only scientifically but also in my everyday life throughout my unpleasa nt chemistry routine. I also owe thanks to those who are in the group and still working on their PhDs: Romain Stalder, Frank Arroyave, Unsal Koldemir, Egle Puodziukynaite, Coralie Richard, Justin Kerszulis, Caroline Grand, Natasha Teran and Jimmy Deininger. My life in Florida could not have been so happy and pleasant without their support and friendship. I would also like to thank my collaborators who have contributed to the projects I have carried out during my study at the University of Florida. The work on broadly absorbing black to transmissiv e switching electrochromic polymers was conducted in collaboration with Dr. Jianguo Mei, Dr. Chad Amb, Dr. Aubrey Dyer, Dr. Svetlana Vasilyeva, Dr. David Liu, Eric Knott, and Emily Thompson. I wish to point out that the repeatability study in this work wou ld no t be so perfect without Dr. Dyer being so demanding of my synthetic capabilities The project on fast switching water soluble electrochromic polymers was conducted in collaboration with Dr. Chad Amb, Dr. Aubrey Dyer, Dr. Mike Craig and Justin Kerszuli s. This project would never have been receive his phone call s from Michigan. I am looking forward to visiting his house and going fishing with him on his little boat. Cha pter 5 on diketopyrrolopyrrole based copolymers for organic solar cells was conducted in collaboration with Prof. Franky So, Dr. Jianguo Mei, Dr. Jegadesan Subbiah and Tzung Han Lai. Here, Prof. Franky So

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6 deserves a lot of credit for his wise advice and ge nerous hospitality. Moreover, I still feel guilty to ask Tzung Han to walk from the Material Science and Engineering Department to the Chemistry Department under the Florida sunshine in the summer time and discuss the project with me again and again. I rea lly appreciate his patie nce and professionalism. I also have to thank my undergraduate advisor Prof. Caiyuan Pan at the University of Science and Technology of China, who built my enthusiasm in polymer science and engineering, as well as those colleagues who helped and supported me during my stay in the lab: Dr. Yugang Li, Dr. Baoqing Zhang, Dr. Yezi You, Dr. Chunyan Hong, Dr. Xiang Yu and Dr. Bin Luan. I give my special thanks to my friends Prof. Pierre Audebert, Dr. Xian Chen, Dr. Seoung Ho Lee Dr. Ka i Lang and his wife Yidan Jiang, Dr. Ruowen Wang, Dr. Yan Chen, and Dr. Huaizhi Kang. Without their support, I would have never made it through these six years. who are alwa ys supporting and encouraging me all the time. I know that they are willing to sacrifice everything for my growing up and being successful.

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7 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ .......... 10 LIST OF FIGURES ................................ ................................ ................................ ........ 11 LIST OF ABBREVIATIONS ................................ ................................ ........................... 15 ABSTRACT ................................ ................................ ................................ ................... 17 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 19 1.1 Conjugated Polymers ................................ ................................ .................... 19 1.1.1 Conjugated Polymers Electronic Structures ................................ ............ 20 1.1.2 Energy Level Distribution ................................ ................................ ......... 23 1.1.3 Solubility of Conjugated Polymers ................................ ........................... 26 1.2 Band Gap Engineering ................................ ................................ ...................... 27 1.2.1 Meth ods of Band Gap Control in Conjugated Polymers .......................... 28 1.2.2 The Donor Acceptor Approach in Band Gap Control ............................... 30 1.3 Polymer Synthesis ................................ ................................ ............................ 32 1.3.1 Oxidative Polymerizations ................................ ................................ ....... 32 1.3.2 Transition Metal Mediated Polymerizations ................................ ............. 33 1.3.3 Other Polymerization Methods in the Synthesis of Conjugated Polymers ................................ ................................ ................................ ....... 39 1.3.4 Random Polymerization ................................ ................................ .......... 41 1.4 Synthetic Control of Polymer Spectral Absorption ................................ ............ 46 1.5 Processing and Patterning of Conjugated Polymers ................................ ......... 51 1.6 Selec ted Applications of Conjugated Polymers ................................ ................ 53 1.6.1 Electrochromic Devices ................................ ................................ ........... 53 1.6.2 Photovoltaic Cells ................................ ................................ .................... 56 1.7 Thesis of This Dissertation ................................ ................................ ................ 60 2 EXPERIMENTAL METHODS AND CHARACTERIZATIONS ................................ 63 2.1 Ge neral Synthetic Methods ................................ ................................ ............... 63 2.2 Purification of Polymeric Materials ................................ ................................ .... 63 2.3 A Brief Discussion of Polymerization Repeatability and Reproducibility ........... 68 2.4 Materials Characterization ................................ ................................ ................ 70 2.4.1 Structural Characterization ................................ ................................ ...... 70 2.4.2 Polymer Molecular Weight Characterization ................................ ............ 70 2.4.3 Thermal Characterization ................................ ................................ ........ 70 2.4.4 Electroche mical Characterization ................................ ............................ 71

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8 2.4.5 Optical Spectra Characterization ................................ ............................. 72 2.4.6 Spectroelectrochemistry ................................ ................................ .......... 72 2.4.7 Colorimetry ................................ ................................ .............................. 72 2.4.8 Morphology Characterization ................................ ................................ ... 74 2.5 Electrochromic Devices ................................ ................................ .................... 74 2.5.1 Transmissive/Window Type Devices ................................ ....................... 74 2.5.2 Reflective/Display Type Devices ................................ ............................. 75 2.6 Photovoltaic Devices ................................ ................................ ......................... 75 3 BROADLY ABSORBING BLACK TO TRANSMISSVIE SWITCHING ELECTROCHROMIC POLYMERS USING HIGHLY REPRODUCIBLE STILLE METHODS ................................ ................................ ................................ .............. 77 3.1 Electrochromic Polymers ................................ ................................ .................. 77 3.2 Polymer Synthesis and Characterization ................................ .......................... 82 3.2.1 Monomer S yntheses and Purification ................................ ...................... 82 3.2.2 Random Stille Polymerization with Multi Monomers ................................ 86 3.2.3 Absorption Control of Polymers by V arying the Monomer Ratios ............ 88 3.2.4 Polymerization Repeatability ................................ ................................ ... 90 3.2.5 Polymer Thermal Analysis ................................ ................................ ....... 92 3.3 Polymer Electrochemistry and Spectroelectrochemistry ................................ ... 93 3.3.1 Electrochemistry Studies ................................ ................................ ......... 93 3.3.2 Spectroelectrochemistry Studies ................................ ............................. 94 3.4 Polymer Colorimetric Analysis ................................ ................................ .......... 96 3.5 Polymer Switching Study ................................ ................................ .................. 98 3.5.1 Polymer Switching Rate ................................ ................................ .......... 98 3.5.2 Polymer Switching Stability ................................ ................................ ..... 99 3.6 Electro chromic Devices ................................ ................................ .................. 100 3.6.1 Black to Transmissive Electrochromic Window ................................ ..... 100 3.6.2 Black to Transmissive Electrochromic Display ................................ ...... 102 3.7 Chapter Summary ................................ ................................ ........................... 102 3.8 Experimental Details ................................ ................................ ....................... 104 4 WATER SO LUBLE BLUE AND BLACK TO TRANSMISSIVE SWITCHING ELECTROCHROMIC POLYMERS ................................ ................................ ....... 109 4.1 Motivations for Water Soluble Electrochromic Polymers ................................ 109 4.2 Concept and Design of Carboxylic Acid Functionalized CPEs from Chemically Cleavable Esters ................................ ................................ ............. 112 4.3 Polymer Synthesis and Characterization ................................ ........................ 114 4.3.1 Monomer Synthesis ................................ ................................ ............... 114 4.3.2 Polymer Synthesis: A Pendant Group Modification ............................... 115 4.3.3 Polymer Thermal Analysis ................................ ................................ ..... 118 4.3.4 Polymer Optical Characterization ................................ .......................... 119 4.4 Polymer Electrochemistry and Spectroelectrochemistry ................................ 120 4.4.1 Electrochemistry Studies ................................ ................................ ....... 120 4.4.2 Spectroelectrochemistry Studies ................................ ........................... 122

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9 4.5 Polymer Switching Study: A Comparison of Organic Soluble Polymers and Water Soluble Polymers ................................ ................................ .................... 124 4.5.1 Polymer Colorimetric Measurements ................................ ..................... 124 4.5.2 Polymer Switching Rate ................................ ................................ ........ 127 4.6 Chapter Summary ................................ ................................ ........................... 134 4.7 Experimental Details ................................ ................................ ....................... 136 5 FINE ABSORPTION TUNING IN DIKETOPYRROLOPYRROLE BASED CONJUGATED POLYMERS FOR PHOTOVOLTAIC APPLICATIONS ................ 141 5.1 Improvement of the L ight H arve sting E fficiency ................................ .............. 141 5.2 Concept and Design of Broadly Absorbing Diketopyrrolopyrrole Based Low Band Gap Polymers ................................ ................................ .......................... 145 5.3 Polyme r Synthesis and Characterization ................................ ........................ 147 5.3.1 Monomer Synthesis ................................ ................................ ............... 147 5.3.2 Polymer Synthesis: Expanding the Absorption Spectra of DP P based Donor Acceptor Polymers ................................ ................................ ........... 149 5.3.3 Polymer Electrochemistry and Energy Level Estimation ....................... 153 5.3.4 Polymer Thermal Analysi s ................................ ................................ ..... 156 5.4 Photovoltaic Devices ................................ ................................ ....................... 156 5.5 Polymer Morphology Studies ................................ ................................ .......... 160 5.6 Chapter Summary ................................ ................................ ........................... 162 5.7 Experimental Details ................................ ................................ ....................... 163 6 CONCLUSIONS AND PERSPECTIVES ................................ ............................... 169 LIST OF REFERENCES ................................ ................................ ............................. 172 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 188

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10 LIST OF TABLES Table page 2 1 GPC results of polymer samples before and after the Soxhlet extraction. .......... 66 3 1 GPC estimated molecular weights in THF and elemental analysis of the polymers ................................ ................................ ................................ ............. 88 3 2 GPC estimated molecular weights of reproduced polymers. .............................. 91 4 1 GPC estimated molecular weights in THF and elemental analysis of the p olymers. ................................ ................................ ................................ .......... 117 5 1 GPC estimated molecular weights in THF and elemental analysis of the polymers. ................................ ................................ ................................ .......... 151 5 2 Electrochemically determ ined HOMO energy levels, LUMO energy levels and bandgaps for PDPP3T, P3 and P4 (by CV and DPV). ................................ ..... 155 5 3 Photovoltaic Properties of PDPP3T, P3 and P4 with PC 71 BM as an acceptor. 158

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11 LIST OF FIGURES Figure page 1 1 Band structures of polyacetylene ................................ ................................ ........ 22 1 2 Energy levels of PTh, PPy, PEDOT, PProDOT, P3HT ................................ ................................ ...... 25 1 3 Structural factors in the origin of band gap ................................ ......................... 29 1 4 Illustration of the donor acceptor concept. Hybridization of the HOMOs and the LUMOs of the donor acceptor fragment yields a compressed band gap ...... 31 1 5 General reaction scheme and mechanism of the Stille Reaction ........................ 35 1 6 Examples of the synthesis of aromatic tin compounds ................................ ....... 36 1 7 Deboronation of boron monome rs ................................ ................................ ...... 39 1 8 Examples of typical Wittig and Knoevenagel polycondensations. ...................... 40 1 9 Schematic illustration of random polymerization w ith monomer alternation sequence via Stille polymerization. ................................ ................................ ..... 42 1 10 Synthesis of random copolymer for white organic light emitting diodes ............. 43 1 11 Synthesis of random copolymer for OPVs ................................ .......................... 44 1 12 Synthesis and structure of PDPP3Ts with distinct molecular weights by Suzuki polymerization ................................ ................................ ......................... 45 1 13 Structures of poly{3,4 di(2 ethylhexyloxy)thiophene} and a random copolymer of di(2 ethylhexyloxy)thiophene and dimethoxythiophene ................. 48 1 14 Electrochemical polymerization of DDTP and BDT ................................ ............ 49 1 15 Spectral engineering of green and black to transmissive conjugated polymers ................................ ................................ ................................ ............. 50 1 16 Sche matic demonstration of a typical absorptive/transmissive window type electrochromic device and a n absorptive/reflective electrochromic device ......... 55 1 17 Schematic demonstration of a typical poly m er/PCBM BHJ solar cell ................. 57 1 18 Representative conjugated polymers with high power conversion efficiency and fullerene derivatives. ................................ ................................ .................... 59 2 1 Demonstration of polymer purification methods ................................ ................. 65

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12 2 2 Solution absorption change of polymer BASF Black 2266 70 after soxhlet extraction in chloroform. ................................ ................................ ..................... 67 2 3 Solution absorption of polymers BASF Black 2266 70 and ECP 3 in chloroform. ................................ ................................ ................................ .......... 69 2 4 Demonstration of spectroelectrochemical series for a blue to transmiss ive electrochromic polymer film ................................ ................................ ................ 73 3 1 Repeat unit structures and photographs of spray cast polymer fi lms in the neutral colored, a nd oxidized transmissive states ................................ .............. 80 3 2 Reaction scheme for the Stille polymerization of three monomers. .................... 81 3 3 Reaction scheme for the synthesis of ditin compound 4 and dibromo compounds 5 and 6 for random Stille polymerization. ................................ ........ 83 3 4 Proposed reaction scheme for the impurities in the synthesis of compound 4 using THF as a solvent. ................................ ................................ ...................... 84 3 5 Demonstration of purifying compound 6 by recycling the eluent hexane ............ 86 3 6 Uv vis absorption spectra of random copolymers ECP 1 to ECP 5 .................... 89 3 7 Normalized solution absorption of polymers EPC 3, EPC 3 s1, EPC 3 s2 EPC 3 s3, ECP 4, ECP 4 s1 ECP 4 s1, ECP 6 and ECP 7 ............................. 92 3 8 Thermogravimetric an alysis of polymer ECP 3 and ECP 4 ................................ 93 3 9 Differential pulse voltammetry of ECP 3 drop cast from toluene solution onto platinum button electrode (A= 0.02 cm 2 ) ................................ ............................ 94 3 10 Spectroelectrochemical behavior of an ECP 3 thin film ................................ ...... 95 3 11 Colormetric analysis of ECP 3 thin films ................................ ............................. 97 3 12 Switching speed study of polymer ECP 3 ................................ ........................... 99 3 13 Long term stability study via square wave potential stepping while monitoring electrochromic switching of ECP 3 at 540 nm ................................ .................. 100 3 14 Window type and display type electrochromic devices using ECP 3 as an active layer ................................ ................................ ................................ ....... 101 3 15 Synthesis of copolymer via el ectrochemical polymerization ............................. 10 3 4 1 Structures of ProDOT monomer, organic soluble precursors ECP Blue and ECP Black. ................................ ................................ ................................ ....... 112

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13 4 2 Sy nthesis of ECP Magenta and s ide chain defunctionalization of the polymer 113 4 3 Reaction scheme for the synthesis of monomers. ................................ ............ 115 4 4 Synthesis of polymer ECP Blue and side chain defunctionalization of the polymer ................................ ................................ ................................ ............. 116 4 5 Synthesis of monomer compound 7 with reduced side chain size and random copolymer ECP Black. ................................ ................................ ...................... 117 4 6 Thermogravimetric analysis of polymers ECP Blue and ECP Black. ............... 118 4 7 Normalized UV vis NIR absorption spectra of polymers ................................ ... 120 4 8 Differential puls e voltammetry of polymer esters ................................ .............. 121 4 9 Spec troelectrochemistry of polymers ECP Blue, ECP Black, WS EC P Blue acid and WS ECP Black acid ................................ ................................ ........... 123 4 10 Lightness ( L* ) as a function of applied pote ntial for spray coated polymers ..... 126 4 11 Square wave potential step chronoabsorptom etry of polymers ........................ 129 4 12 Transmittance change (contrast) as a function of switching frequency for ECP Blue spray coated on ITO ................................ ................................ ........ 130 4 13 Rapid full spect rum measurement of polymers WS ECP Blue acid and WS ECP Black acid film ................................ ................................ .......................... 132 4 14 Time dependence of polymers WS ECP Blue acid at 635 nm and WS ECP Black acid at 555 nm ................................ ................................ ........................ 134 5 1 Illustration of a tandem bulk heterojunction solar cell ................................ ....... 143 5 2 Bulk heteroju nction solar cell with SiPc as an additional component ............... 145 5 3 Chemical structure of PDPP3T and P1 P4. ................................ ...................... 146 5 4 P3HTT DPP copolym ers for solar cells ................................ ............................ 147 5 5 Synthesis of PDPP T DTT copolymer ................................ .............................. 147 5 6 Synthesis of monomers 3, 4, 6a and 6b. ................................ .......................... 149 5 7 Synthesis of control polymer PDPP3T and random copolymer P1 P4 via Stille polymerization ................................ ................................ .......................... 150 5 8 GPC trace of DPP based random copolymers P DPP3T P3 and P4 in THF solution at 40C using polystyrene as a standard ................................ ............. 151

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14 5 9 Normalized UV vis NIR absorption spectra of DPP based random copolymers ................................ ................................ ................................ ....... 153 5 10 Electrochemistry results of polymer PDPP3T ................................ ................... 154 5 11 Differential pulse vo ltammetry of random copolymers P3 and P4 drop cast from chlorobenzene solution onto p latinum bu tton electrode ........................... 155 5 12 Thermogravimetric analysis of polymer P3 and P4. ................................ ......... 156 5 13 Illuminated J V characteristics of P DPP3T:PC 71 BM solar cells using ITO/ZnO/Polymer:PC 71 BM/MoO 3 /Ag device architecture with DIO .................. 157 5 14 Illuminated J V characteristics of polymer:PC 71 BM solar cells with and witho ut DIO as a proc essing additive ................................ ............................... 158 5 15 Absorption and EQE of polymer:PC 71 BM blends ................................ .............. 159 5 16 Photoluminescence spectra of P4 and P4:P 71 CBM (1:1 ) chlorobenzene solution ................................ ................................ ................................ ............. 160 5 17 Atomic force microscope height images of P3 :PC 71 BM (1:1) based PV cells with 5% DIO spin coated from dichlorobenzene. ................................ .............. 161

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15 LI ST OF ABBREVIATIONS AFM Atomic force microscopy BTD 2,1,3 Benzothiadiazole CV Cyclic voltammetry DOT 3,4 Dioxythiophene DPV Differential pulse voltammetry DMF Dimethylformamide EC Electrochromic ECD Electrochromic device ECP Electrochromic polymer E tHx 2 Ethylhexyl Fc/Fc + Ferrocene/Ferrocen ium FET Field effect transistor GPC Gel permeation chromatography HOMO Highest occupied molecular orbital ICBA Indene C 60 bisadduct ITO Indium tin oxide IR Infrared Spectrum J sc Short current density LED Light emitting diode LUMO Lowest occupied molecular orbital M n Number average molecular weight M w Weight average molecular weight NMR Nuclear magnetic resonance OPVs Organic photovoltaics

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16 OFET Organic field effect transistor PCE Power conversion efficiency PCBM [6,6] P henyl C61 butyric acid methyl ester fullerene PDI Polydispersity index ProDOT Propylenedioxythiophene PSC Polymer solar cell p TSA p Toluenesulfonic acid PV Photovoltaic SCE Saturated calomel electrode TGA Ther mogravimetric analysis TLC Thin layer chromatography UV Ultraviolet spectrum Vis Visible spectrum V oc Open circuit voltage

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17 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SYNTHETIC CONTROL OF LIGHT ABSORPTION IN CONJUGATED POLYMERS By Pengjie Shi December 2011 Chair: John R. Reynolds Major: Chemistry Conjugated polymers with alternating single and double carbon carbon bonds along polymer backbones have become a unique branch in the family of polymeric materials. Researchers are especially interested in the optoelectronic properties of conjugated polymers, due to their capabilities in light weight and inexpensive organic electronic devices, such as photovoltaics, light emitting diodes, field effect transistors and electrochromic devices. This dissertation focuses on the use of chemistry as a basis for optimization of these properties. In particular, the investigati ons involve development of new synthetic routes and methods to produce conjugated polymers with controlled absorption profiles, energy level distributions, band gaps and solubility, as well as evaluation of their use in electrochromic and photovoltaic appl ications. In a first instance, a synthetic methodology is devised for the synthesis of black to transmissive switching electrochromic polymers. By introducing the random Stille polymerization method, which can be used to combine multiple monomers, adjust their relative ratios and control the monomer sequence distribution a random black to transmissive electrochromic conjugated polymer (ECP) has been synthesized with a broad absorption across the entire visible region, and the reproducibility of the

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18 polyme rization was examined in detail. Black to transmissive ECPs were reproduced in batches with highly repeatable absorption spectra, number average molecular weights and polydispersity index Moreover, successful characterization and device work enhance the p robability that this polymer may be used for commercial electrochromic windows and displays. In a second project, the synthesis of a blue to transmissive ECP and a black to transimissive ECP, which are functionalized with carboxylate ester groups, was acco mplished. Basic hydrolysis of the carboxylate ester side chains affords the polymer salts with water solubility, allowing thin films to be formed by spray casting from the polymer/water solutions. Upon subsequent neutralization of the thin films, the resul ting polymer acid films are ready to be redox switched in a KNO 3 /water electrolyte solution and show a dramatic improvement in the switching speed compared with their ester derivatives at the sub second switching time scale. A third project extended the r andom polymerization to improve the light harvesting efficiency of conjugated polymers by broadening their absorption spectra Diketopyrrolopyrrole based conjugated copolymers were synthesized with a broad absorption from 350 to 800 nm, except for a small absorption gap between 550 to 650 nm. Further exploring the external quantum efficiency, photoluminescence quenching effect with fullerene, photovoltaic prosperities and polymer:fullerene film morphology of these random copolymers has provided a fundamenta l understanding of how polymer light absorption, side chains, stacking ability, and energy levels influence the performance of solar cell materials with thiophene and benzodithiophene as donors and diketopyrrolopyrrole as an acceptor.

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19 CHAPTER 1 INTRODUCTI ON 1.1 Conjugated Polymers Conjugated organic polymers have received considerable interest due to the combination of physical properties of polymers or macromolecules (low density, processibility, solubility, mechanical properties, flexibility etc. ) with the optoelectronic properties of semiconductors. These properties qualify conjugated organic polymers as a special category of unique and novel materials with numerous attractive applications, such as photovoltaic devices (OPVs), 1,2 organic field effect t ransistors (OFETs), 3,4 organic light emitting diodes (OLEDs), 5,6 electrochromic devices (ECDs) 7,8 and chemical sensors. 9,10 Besides their interesting applications, one of the most desirable aspects of conjugated organic polymers is the easy fine tuning and optimization of their physical and optoelectronic properties via modification of their chemical structures, such as polymer backbones and pendant groups. Moreover, the potential of being manufacture d into large area and flexible devices by low cost printing and patterning techniques (spray casting, inkjet printing and roll to roll printing) has dramatically stimulated the development of these organic semiconducting materials. 11 15 To a chemist, espec ially a polymer synthesis chemist in this field, the necessary skills include not only his/her synthetic capabilities in looking for efficient, scalable and consistent methods to produce polymer batches with highly reproducible properties, but also the abi lity to understand the structure property relationships of the materials, as well as the working principles of the optoelectronic devices. Therefore, this chapter will introduce the unique electronic structures of organic conducting polymers, followed by a detailed discussion of the energy level variation with changes in the chemical structures.

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20 After that, band gap control, as an important aspect in this field, will be addressed. In this synthesis oriented dissertation, several polymerization methods to ac hieve the final target materials will also be described. Finally, the processing and patterning of conjugated polymers, as well as the selected applications which lead the eventual goal of polymer synthesis, will be discussed. 1.1.1 Conjugated Polymers Ele ctronic Structures Attempts to understand the electronic behavior of conjugated polymers started with the synthesis of the simplest form of conjugated polymer -polyacetylene (PA) -in the late 1950s. However, not until two decades later was the key break through in the synthesis of iodine doped high conductivity PA (~ 10 3 1 ) disclosed by Hideki Shirakawa, Alan J. Heeger and Alan G. MacDiarmid. 16 19 Subsequent discussions of the theory of soliton excitations and doping effects in polyacetylene initiate d the development of modern applications in organic conjugated materials, and the field of conducting polymers was born. 20 22 Here, the analysis of the electronic structure of conjugated systems starts with polyacetylene. Considering a linear PA with an infinite chain length at the Hckel molecular orbital (HMO) level, there is a slight split of the energy levels as the number of repeat units increases, the overall spread in e nergy distribution for each orbital set becomes larger. As more and more repeat units are joined together, the overlapping of the orbitals spans a much wider range of energy level, and the energy gap (band gap, E g ) between the highest occupied molecular orbital (often referred to as the HOMO level) and the lowest unoccupied molecular orbital (referred to as the LUMO level) becomes smaller and smaller. For an infinite conjugation chain length, each of the

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21 orbital sets contains an infinite number of nearly continuous energy levels, and thus is called a band. At the theory level, the band structures of PA consist of a valence band (VB) filled with electrons and an empty conduction band (CB). With all equal bond lengths in the HMO model, the two bands are dege nerate at the zone edge and produce a zero band gap. However, in contrast, the structure of PA shows a long and a short bond alternation (1.45 and 1.35 respectively) with a band gap about 1.5 eV. The geometrical distortion and the opening of a band gap are due to the electron electron repulsions, which cause a Jahn Teller distortion, also recognized as a Peierls distortion in condensed matter physics. Considering the distortion effect, the two degenerate ground states of PA, which are caused by the deloc alization of electrons along the polymer backbone, will not interconvert, and strictly speaking they are not resonance structures ( Figure 1 1a). In fact, the more symmetrical intermediate form with a zero band gap does not exist in the real world. 23 Figu re 1 1b shows a simplified model with the buildup of energy bands in a conjugated polymer as described above. It is worth mentioning that this simple approach can be applied to many other conjugated polymer systems. More detailed and accurate theoretical c alculations of energy levels for polyacetylene, polycyclopentadiene, polypyrrole, polyfuran and polythiophene have been reported by Salzner et al 24 One of the well accepted guidelines to distinguish metals, semiconductors and insulators is the size of the band gap. More specifically, when the E g is about 0 eV, the material is considered to be a metal. Due to the disappearance of the energy barrier between VB and CB, there is a high probability of finding electrons in the conduction

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22 band of this type materi al. On the other hand, when the E g rises to the range between 0 and 3 eV, the material is identified as a semiconductor. In this case, the electrons are tightly bonded in the valence band, and very few of them can be found in the conduction band. A direct influence over this phenomenon is the conductivity of semiconductors, which falls in the range of 10 8 to 10 2 1 compared to > 10 3 1 for metals. Based on the E g and conductivity values, conjugated polymers are classified as semiconductors. Event ually, a further increase of the band gap to higher than 3 eV, affords insulators, which have no electrons in the CB. a) b) Figure 1 1. B and structures of polyacetylene a) The degenerate PA b) Simplified band structur es illustrating the buildup of energy bands in a conjugated polymer.

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23 1.1.2 Energy Level Distribution Band gap is the most widely used terminology in this field, and it has a great influence on the optoelectronic properties of the conjugated polymers inclu ding the conductivity and the spectral absorption and emission. The energy level distributions, on the other hand, have a large influence on the redox properties and the stability of the polymers, and thus the final applications. Here, the change of polyme r properties based on the change of polymer energy levels will be discussed using several common homopolymers, for example, polythiophene (PTh), polypyrrole (PPy), poly(3,4 ethylenedioxythiophene) (PEDOT), poly(3,4 propylenedioxythiophene) (PProDOT), and p oly(3 hexylthiophene) ( P3HT ). As shown in Figure 1 2, polythiophene shows a HOMO level at 5.3 eV. The relative high HOMO level means that PTh can be easily oxidized (p doping) chemically and electrochemically. However, considering the air stability thres hold at 5.2 eV, PTh actually shows an excellent environmental stability (stable toward oxygen and moisture in the air) in both its neutral and doped states, and the polymer is an extensively studied material for applications such as transistors, conductor s, sensors and solar cells. 25 27 By replacing the sulfur atom with a nitrogen atom in the aromatic ring, polypyrrole shows much higher HOMO and LUMO energy levels at 4.6 eV and 2.2 eV, respectively. Clearly, the high energy levels cause the polymer to ha ve a low resistance to oxidation and a high resistance to reduction. In other words, PPy is a good material for p type doping; however, it cannot be n type doped. With a band gap of about 2.4 eV, PPy films are yellow/green color in their neutral form and a re sensitive to air and oxygen. 28 30 Moreover, the HOMO level of PEDOT is observed at 4.6 eV, which is 0.7 eV higher than that of PTh. The raised energy level can be attributed to the electron

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24 density donating effect of the oxygen atoms in the 3 and 4 positions of the thiophene ring. With the ethylenedioxy bridge, EDOT monomer has become one of the most electron rich aromatic rings in the thiophene family, and it is widely used as a repeat unit in conjugated polymers. In fact, EDOT can be easily elect ropolymerized, whereas thiophene is more difficult to polymerize by this route. Compared with PTh, PEDOT can be oxidized easily, but is more difficult to n dope than PTh. 31 33 More interestingly, with a band gap of 1.6 eV, polymer PEDOT shows a saturated b lue color in its neutral state and becomes highly transparent in the fully oxidized state. Similar to PEDOT, the family of poly(3,4 propylene dioxythiophene) derivatives, PProDOT, has been shown to exhibit varied properties relative to PEDOT due to a slig ht change in the substituent bridge. PProDOT shares a similar oxidation potential (HOMO level) to that of the PEDOT, but a larger band gap (1.9 eV) compared with PEDOT at 1.6 eV. In its neutral state, the polymer thin film shows a purple color, and can be converted into a highly transmissive state upon full oxidation. 34 It is worth mentioning that the alkyl chain on the propylenedioxy bridge has afforded the polymer with considerable processability in organic solvents such as toluene, THF and chloroform. In this dissertation, several donor monomers based on the ProDOT unit have been synthesized and utilized in the synthesis of electrochromic pol ymers, as will be described in C hapter 3 and 4. Another interesting case in this discussion is regioregular poly(3 hexylthiophene), which was first reported by Rieke, and then by the McCullough group. 35 39 The HOMO energy level of P3HT ( 5.4 eV) is 0.1 eV lower than that of PTh, and the band gap is around 2.1 eV. 40 The lowered HOMO energy level has increased the stabil ity of the

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25 polymer thin films, while also opening a larger potential window between the HOMO level of P3HT and the LUMO level of PCBM (a potential increase of the open circuit voltage) in the bulk heterojunction (BHJ) solar cell. Actually, P3HT has become the most investigated conjugated polymer in the field of BHJ solar cells. Figure 1 2. Energy levels of PTh, PPy, PEDOT, PProDOT, P3HT and cells employing PCBM as an acceptor). Black dashed lines indicate the thresholds for air stability (5.2 eV). 57 In all cases orbital energies are given based on the assumption that the energy of SCE is 4.7 eV vs vacuum 41 and Fc/Fc + is +0.38 V vs SCE (i.e. 5.1 eV relative to vac uum). Reported energy levels can be different depending on the method of determination. Based on the discussion above, it is obvious that the HOMO energy level needs to be relatively high in order to achieve a good p type doping character in conjugated pol ymers. In the application of electrochromic devices, it is important that this type of material be oxidized easily by electrochemical methods and achieve a stable full

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26 oxidized state in the potential windows of the electrolyte solutions. On the other hand, the ideal donor polymers for BHJ solar cells should have HOMO levels as low as possible for a larger V oc and thus a higher power conversion efficiency (more detailed discussion will be presented in S ection 1.6.2). 1.1.3 Solubility of Conjugated Polymers Due to the nature of their intrinsic chemical structure, conjugated polymers were earlier stages of the field in the early 1980s, since none of these conducting polymer s (for example, PA, PPy, polyfuran, PTh, polyaniline and PEDOT) is soluble in common organic solvents. Moreover, from the polymerization to the purification, characterization and processing of the polymers, the solubility of conjugated polymers is the only issue that orients the entire preparation procedure. Thus, improving their solubility and the processability in organic or even aqueous solutions has become an important factor in the synthesis of conjugated polymers. The first effort was the synthesis o f poly(3 alkylthiophene). When prepared by chemical or electrochemical methods, polythiophene cannot be dissolved or processed in solutions. Although iodine doped PTh has shown a high conducting property (0.1 1 ), the lack of processability and environ mental stability has dramatically limited the further usage of this material. 42 44 To solve that problem, Kaeriyama reported the synthesis of soluble conducting PThs by electrochemical polymerization of thiophenes having a long alkyl group (hexyl, octyl, d odecyl and octadecyl). 45 As expected, the electrochemically polymerized poly(3 alkylthiophene) can be prepared in organic solutions, which were then used to determine their molecular weight and to cast films. In the meantime, Elsenbaumer reported the synth esis and characterization of a series of

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27 irregular P3ATs which can form highly conducting, environmentally stable complexes with electron acceptor dopants. 46,47 Their synthesis approach was achieved by nickel catalysed Grignard coupling of the 2,5 diiodo 3 alkylthiophene using THF and 2 methyltetrahydrofuran as the reaction solvents. One of the important finding of that study was that homopolymers of 3 alkylthiophenes incorporating alkyl groups equal to or greater than butyl in size are readily soluble at r oom temperature in common organic solvents such as THF, toluene, xylene and methylene chloride. From the chemistry stand point, fine tuning of the solubility of conjugated polymers can be easily achieved by introducing solubilizing side chains onto the po lymer backbone. Generally, there are two types of functional groups to be utilized alkyl groups (linear or branched), which improve the polymer solubility in organic solvents; and ionic groups (sulfonate, carboxylate, phosphonate and ammonium), which make the polymers water soluble. One special case is poly(3,4 ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), which is a mixture of positively charged PEDOT and negatively charged PSS. These polymer salts can be dispersed in water and processed int o thin films. In terms of the chemical control of the molecule structure, the selection of the side groups is mostly guided by the processability and applications of the polymers. 1.2 Band Gap Engineering Band gap control has always been an essential synt hetic consideration in the field of conducting polymers. In the early stages, conducting polymers and their doped forms were considered as potential alternatives for metals. Preparation of conjugated polymers with low or zero band gaps and thus high intrin sic conductivity was the primary goal. 48 50 After that, the discovery of the fact that conjugated polymers can be

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28 applied in a series of electronic and photonic devices such as OPVs, OLEDs and FETs as semiconductors, has dramatically changed the situation. Band gap engineering has focused on a precise control in the size of the energy gap and has always been involved in the control of optical properties, energy level distribution, and charge transport properties. 51 1.2.1 Methods of Band Gap Control in Conju gated Polymers In the previous discussion, polyacetylene was demonstrated to have a band gap of 1.5 eV due to the bond length alternation (BLA), which results from Peierls distortion ( S ection 1.1.1). As a matter of fact, this effect has been the major cont ribution to the opening of an energy gap between VB and CB for all the conjugated polymers. Without doubt, band gap control can be achieved by tuning the bond length alternations on the molecule level via chemical modifications. Moreover, conjugated polym ers based on aromatic systems show a non degenerate ground state. In Figure 1 3a, the aromatic and quinoid forms of PTh are not energetically equivalent. In this case, the aromatic form is more energetically favored than the quinoid form, and electrons are more localized in the aromatic rings. As a result, the single bond character of the C C bond between two aromatic rings is actually increased, leading to a large bond length alternation. This resonance effect is recognized as E Res in the ba nd gap contribution ( Figure 1 3b). Introduction of a fused ring system is a direct way to reduce the aromatic stabilization energy. For example, when polyisothionaphthene adopts its quinoid form ( Figure 1 3a), the loss of aromaticity on the thiophene ring is retrieved by the formation of benzene ring. This transition of aromaticity dramatically decreases the energy barrier between the aromatic form and

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29 the quinoid form, and reduces the band gap of the polymer (1.0 eV compared with 2 eV of PTh). 52 a) b) Figure 1 3. S tructural factors in the origin of band gap. a) Aromatic and quinoid structure of polythiophene and polyisothionaphthene b) Schematic illustration of structural factors in the band gap control of conjugated p olymers using regioregular poly(3 alkylthiophene) (P3AT) as an example. (Adapted and modified with permission from Ref. 51)

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30 Another aspect in band gap tuning comes from the substituents on the polymer backbone (E Sub ). Although most of the side chains are i ntentionally introduced to increase the solubility of the conjugated polymers, they still have the ability to influence the electron density of the conjugated systems, especially for substituents with non neglectable electron donating and electron withdra wing effects. Furthermore, the torsion angle ( evaluating the conjugation length and thus the band gap. The delocalization of the electrons requires a continual overlapping of the orbitals. A planar molecular geometry favors the delocalization of electrons and thus decreases the band gap. The last contribution comes from the intermolecular interactions (E Int ). Clearly, strong interactions between two adjacent polymer chains will help the electron distribution and reduce the band gap. In fact, all the factors in the band gap control will interact with each other, resulting in a complicated overall contribution to the band gap and the energy levels of the conjugated polymers. Changing one of the parameters may cause a series of changes to the rest. For example, increasing the size of the substituents (R group in Figure 1 3b) can potentially cause more steric hindrance between the adjacent rings as well as between adjacent polymer chains. The overall effect will include an incr ease in the rotational disorder and a decrease in the intermolecular interaction. 1.2.2 The Donor Acceptor Approach in Band Gap Control As a powerful synthetic tool in controlling the band gaps of conjugated polymers, s first introduced by Havinga et al ., 53 55 and utilized to achieve narrow and low band gap polymers. The donor represents an electron rich unit and the acceptor shows a strong electron affinity. The interaction

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31 between the donors and the acceptors in the p olymer backbone will potentially increase the delocalization of the electrons, reducing the bond length alternation and thus the band gap. Figure 1 4 shows the hybridization process of the HOMOs and LUMOs of a D A system. The formed complex has broader val ence and conduction bands and a small HOMO LUMO separation in between This concept has served as a basis for structure modification of many conjugated systems. By carefully selecting the structures of the donors and acceptors and their respective electron donating and withdrawing strengths, conjugated polymers with controlled energy levels and band gaps have been synthesized and applied in applications such as OPVs 56 59 O FETs, 60,61 and OLEDs. 62 Another area is electrochromics, which will be discussed in d etail later in this dissertation. Figure 1 4. Illustration of the donor acceptor concept. Hybridization of the HOMOs and the LUMOs of the donor acceptor fragment yields a compressed band gap. (Adapted with permission from Ref. 55)

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32 1.3 Polymer Synthesis During the past three and a half decades, considerable knowledge ( theoretical and experimental) has been gained in the field of organic conjugated polymers. This section will place the role of synthesis in perspective, with a general introduction to oxidat ive polymerization, metal mediated polymerization and Knoevenagel condensation, with a particular focus on random polymerization. 1.3.1 Oxidative Polymerizations Oxidative polymerization was one of the earliest polymerization methods for producing conjugated polymers. The polymerization usually proceeds by either chemical or electrochemical oxidation. Both methods aim to produce the radical cation intermediate by oxidizing the monomers and the polymer fragments. The radical cation then couples wit h another radical cation to form a dication, or with a neutral species to form a new radical cation, which can be oxidized into a dication. The rearomatization into a neutral form involves the the loss of two protons. Eventually, polymers are formed after repeated oxidation coupling cycles. Compared with other polymerization methods, oxidative polymerizations are rather inexpensive, convenient, and able to produce high quality conjugated polymers under certain conditions. In an electrochemical oxidative po lymerization, the polymer films are grown directly on the anode. The quality of the polymerization and thus the polymer films depends on the applied potential, the monomer structure, the electrode material, current density, temperature, solvent and many ot her factors. 63 One major drawback of this polymerization is that the formed polymers can be irreversibly over oxidized and can decompose on the electrode during the electrochemical switches, since the polymers usually have lower oxidation potentials than t he monomers. In some cases, they also

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33 produce undesirable crosslinks and prevent control of the regioregularity of the unsymmetrical monomers. Unlike electrochemical polymerization, chemical oxidative polymerization is usually accomplished with a chemical oxidant (such as FeCl 3 or SbCl 5 ) in organic solutions. 64 66 It is worth mentioning that the polymerization produces the oxidized polymers, and an extra reduction process with hydrazine or ammonium hydroxide is necessary in order to obtain the neutral polym ers. On the other hand, the propagation of the polymerization can be limited by the solubility of the oxidized polymers, which are less soluble than the neutral polymers and tend to precipitate out of the solution. 67 There are also other disadvantages for this polymerization. For example, the metal ions can be easily trapped in the polymer network; 68 and the polymerization process is difficult to repeat, leading to low reproducibility of the final products. Oxidative polymerization, ether electrochemical o r chemical, requires monomers with low oxidation potentials, and this dramatically limits application in more complicated conjugated systems. More efficient, reproducible and controllable polymerizations are indeed needed. 1.3.2 Transition Metal Mediated P olymerizations The development of organometallic chemistry, especially in the area of transition metal mediated cross coupling reactions, has stimulated the growth of conjugated polymer research for 20 years. With the ability to form carbon carbon single b onds 2 carbons, many cross coupling reactions provide an opportunity to build polymer backbones with an extended single double bond alternation, which is the key structure of conjugated polymers. Most of these reactions are coup ling reactions of aromatic halides with aromatic organotin, organozinc,

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34 organoboron, organomagnesium and organolithium derivatives, namely Stille, Negishi, Suzuki Miyaura, Kumada Corriu coupling, respectively. 69 However, they are not just limited to aromat ic systems. Interestingly, these reactions share a similar catalytic cycle, which contains a transition metal catalyzed oxidative addition, a transmetallation with an organometallic nucleophile, and a reductive elimination of the transition metal. Other us eful reactions may include the Heck and the Sonogashira reactions, which are specifically used to build double bond and triple bond linkages. The Stille coupling reaction 70,71 is perhaps the most effective cross coupling reaction in the synthesis of conjug ated polymers due to its mild reaction conditions and high resistance to the surrounding chemical environment. 72,73 This coupling reaction involves two types of starting materials: organic tin compounds and aryl halides ( Figure 1 5). In the case of a polym erization, the starting materials are usually difunctionalized aromatic rings. Since Stille polymerization is the dominant polymerization method used in this dissertation, the discussion here will be more complete and detailed. Figure 1 5 shows a catalytic cycle of a typical Stille coupling reaction. Except for the similar three step mechanism (oxidative addition, transmetalllation and reductive elimination), an important consideration is the change to Pd(0) from Pd(II) catalysts, which consumes two equival ent organostannane monomers, and changes the stoichiometry of the reactive functional groups. Given the fact that the high molecular weight polymers are restricted by the precise stoichiometric control of the functional groups (or the monomers) in a step g rowth polycondensation, this imbalance of the monomers needs to be corrected at the beginning of the reaction or by using Pd(0) catalysts.

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35 Figure 1 5. General reaction scheme and mechanism of the Stille Reaction. General ly, the monomers in a Stille polymerization are electron deficient halides (bromide or iodide) and electron rich organotin compounds. It is believed that an aromatic system with better electron affinity favors the oxidative addition of the Pd(0) catalyst; and a system, which is strongly electron donating, activates the C Sn bond in the transmetalation step. 71 Between the two types of halide monomers, it is also well known that diiodo monomers always show a higher reactivity than dibromo monomers. 72 Moreover the organotin compounds are usually made by lithiation of the aromatic rings,

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36 followed by addition of trimethyltin chloride or tributyltin chloride ( Figure 1 6). For some cases, the lithiation reagents ( e.g. n BuLi) may react with the original functional groups on the aromatic rings, changing the reaction to another pathway. 74 To solve this problem, a new method of introducing the trialkyltin groups onto the aromatic system based on a Stille reaction of hexamethylditin and aromatic bromide is established in this dissertation ( C hapter 4). One other problem in the synthesis of ditin monomers is the purification. While some of the ditin monomers can be purified by vacuum distillation and recrystallization, most of them are high boiling oils. Purification with triethylamine treated silica gel can reduce the decomposition of the tin compounds only to a relatively low point, but cannot totally eliminate it. This lack of purification options can become a major issue in applying Stille polymerizations, especially i n large scale production, and thus needs more investigation in the future. Figure 1 6. Examples of the synthesis of aromatic tin compounds. Except for the reactivity of the monomers, the choice of catalysts and ligands a lso plays an important role in the Stille polymerization. Here, PdCl 2 (PPh 3 ) 2 Pd(PPh 3 ) 4 and

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37 Pd 2 (dba) 3 are commonly used catalysts in the polymerization. 75,76 Clearly, the first two catalysts are not stable in air; and the use of these two catalysts does n ot require extra ligands. Compared with the first two Pd catalysts, Pd 2 (dba) 3 shows much better stability in air, however, it becomes very unstable when mixed with the ligands. The amount of Pd used in the reaction is usually 2 mol % equivalent relative to the tin monomer. For a typical reaction using Pd 2 (dba) 3 as a catalyst, 4 equivalent of the ligand (for each Pd) should be used. These ligands can be triphenylphosphine (PPh 3 ), tri(o tolyl)phosphine ( P ( o tolyl ) 3 ) triphenylarsine (AsPh 3 ) and tri(2 furyl)ph osphine (TFP). Interestingly, the 3 and TFP have shown a large rate acceleration in the Stille reactions. 72,77 The reactivity of the ligand can be roughly evaluated as AsPh 3 > TFP > P ( o tolyl ) 3 > PPh 3 Despite the reactivity, TFP associated Pd 2 (dba) 3 catalyst is surprisingly stable, and faster rates are obtained with 2 equivalent of the ligand for each Pd. In some cases, additives are used together with the catalysts and the ligands to improve the reaction time and the molecular w eight. For example, when 1 equivalent of CuO was added to the Stille polymerization of distannyl alkyl dithienopyrroles and 4,7 bis(5 bromo 4 alkylthiophen 2 yl)benzothiadiazole, the molecular weight of the polymers was observed to increase 4 5 times. 78 Rep orted reactions based on CuO additive can also be fo und in other works. 79,80 CuI is also an optional choice for the additive. 81 83 When the salt is added, it reacts with organostannanes to form organocopper intermediates, which yield a much faster transmet alation with the Pd(II) species. 84 The effect of these additives is believed to highly depend on the solvent. In highly polar solvents like DMF and N methyl 2 pyrrolidone ( NMP), which can dissolve

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38 the salts very well, the effect can be dramatic. However, t hese highly polar solvents can also make the polymers precipitate out before they reach a high molecular weight. Clearly, the solvent or solvent mixture needs to be carefully selected to achieve a satisfactory polymerization result. The ideal solvent for a Stille polymerization should be a good solvent for the monomers, the catalysts/ligands, the additives and the final polymers. It is even better if they can stabilize the catalysts and fulfill the reaction temperature requirements. The most commonly used s olvents in this type of polymerization are toluene, xylene, chlorobenzene, THF, DMF, NMP, dioxane, and chloroform. A high boiling point solvent, such as toluene, xylene, and chlorobenzene, can be easily applied to high temperature Stille polymerizations an d used to dissolve a large variety of conjugated polymers. THF and dioxane have shown some interesting results in stabilizing the Pd catalysts. DMF and NMP are needed to dissolve the Cu salt additives. They can also act as coordinating ligands to the Pd ce nter to accelerate the polymerizations. 85 Mixed solvent systems, such as toluene/DMF and THF/DMF, have also been investigated and have shown some promising results. 86 88 Eventually, it is important to remind scientists that the organotin compounds have be en demonstrated to be highly toxic, and will cause serious health and environmental problems. 89 Tin compounds need to be handled with extreme caution and tin waste need to be kept separate for disposal. The Suzuki polymerization, which uses boronic acid o r esters as the organometallic species to react with the aryl halides, uses much less toxic materials than Stille polymerization. Generally, Suzuki coupling requires a base to activate the

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39 boron containing reagents for transmetalation. 90 The choice of the base and the solvent mixture has immediately increased the complexity of the polymerization; in other words, optimization of the reaction conditions is usually more time consuming than Stille coupling. Compared with organotin compounds, o rganoboron reagen ts for Suzuki coupling can be synthesized by versatile chemical reactions. 91 93 However, these monomers usually present low stabilities in the presence of acid and base ( Figure 1 7). This problem becomes even worse during polymerizations, because the const ant decomposition of boronic monomers changes the stoichiometric balance of the monomers and lowers the molecular weights of the polymers. 94 More detailed 95 Figure 1 7. Deboronation of boron monomers. (Adopted and modified with permission from Ref. 95) 1.3.3 Other Polymerization Methods in the Synthesis of Conjugated Polymers Except for the synthesis of conjugated polymers via the formation of C C single bonds from sp 2 carbons, there are other useful methods, such as Wittig and Knoevenagel coupling, which are able to build carbon carbon double bonds (vinylene linkage) by a polycondensation. Figure 1 8 shows an example of Wittig polycondensat ion in the synthesis of poly( p phenylene vinylene) ( PPV ) based

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40 conjugated polymers. The reaction can be processed in mild conditions. Although several different conditions have been reported in order to favor one of the geometries ( cis or trans ), there is still no absolute control of the double bond configuration. 96 98 Figure 1 8. Examples of typical Wittig and Knoevenagel polycondensations. The Knoevenagel condensation was first used to produce CN PPV derivatives from xy lylene dinitrile and an aromatic dialdehyde. The reaction involves nucleophilic addition of a carbanion to the aldehyde under basic conditions, and then the formation of a double bond between two aromatic rings by the elimination of water ( Figure 1 8). 99 B y changing the structures of the aromatic systems, the Knoevenagel polycondensation can be applied to the synthesis of various cyanovinylene linked conjugated polymers. Previous work reported by our group involved a detailed study of the synthesis and

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41 char acterization of donor acceptor cyanovinylene polymers with narrow band gaps based on thiophene, EDOT, ProDOT and alkyloxy benzene. 100 103 1.3.4 Random Polymerization Random polymerization, as a useful methodology in polymerization, is designed to incorpo rate two or more monomer units in the synthesis of statistical copolymers with irregular monomer sequences. In the field of conjugated polymers, this method is broadly used to combine multiple monomers, adjust their relative ratio and control the monomer s equence distribution in order to achieve the desired energy level distribution, band gaps, optical properties and interchain interactions. Some useful random polymerization constructions, which use Stille polymerization as a model and can be applied to ot her transition metal mediated cross coupling reactions or to a variety of polymerization methods by one skilled in the art will now be introduced. In order to reduce the complexity of the discussion, the functional group reactivities are assumed to be ind ependent of their positions. In Figure 1 9, the Stille polymerization of single monomer Bu 3 Sn A Br with two different functionalities will produce a homopolymer. Furthermore, the polymerization of two monomers Bu 3 Sn A Br and Bu 3 Sn B Br will produce a truly random copolymer, in which the probability of finding A or B at a particular point in the polymer backbone is equal to the mole fraction of that monomer residue in the chain. More complicated systems can be easily achieved by adding a third, fourth and mo re monomers with the same type of functionality. There is no absolute formula for the amounts of added monomers. On the other hand, in the polymerization of difunctional monomers with the same functionality, the sum of the monomers with organotin function al groups must be equal to the sum of the monomers with bromo groups. A direct polymerization of Bu 3 Sn A

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42 SnBu 3 and Br B Br will produce a polymer with strict alternation of A and B repeat units. When a third monomer Br C Br is incorporated in the polymeriz ation, the reaction produces a random copolymer, in which any A unit must be bonded to ether a B unit or a C unit, while B and C cannot be next to each other or themselves. Looking at a bigger picture of this polymer, it is a truly random copolymer with ( AB ) and ( AC ) as repeat units. Moreover, the blending of two distinct types of monomers will generate a random polymer with a more complicated sequence. In here, any A or B unit can be connected to each other or to a C unit, but cannot be next to itself; only C units are able to be linked to each other (Figure 1 9). It should be noted that the polymer structure of this polymerization is not correlated to the polymer sequence due to its complexity. Figure 1 9. Schematic il lustration of random polymerization with monomer alternation sequence via Stille polymerization.

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43 In this field, many interesting polyme rs have been produced by random polymerization. Wang et al. 104 reported the synthesis of a white light emitting copolymer via a random Suzuki polymerization. By incorporating 4,7 bis(4 ( N phenyl N (4 methylphenyl) amino)phenyl) 2,1,3 benzothiadiazole (OMC) as an orange dopant unit in the main chain of polyfluorene (PF) as the blue host ( Figure 1 10), the copolymer emitted wh ite light with simultaneous blue ( max = 421 nm/445 nm) and orange emission ( max = 564 nm) in a single layer device. Figure 1 10. Synthesis of random copolymer for white organic light emitting diodes. (Adopted and modified with permission from Ref. 104) Yu et al. 105 prepared a series of copolymers based on thieno [3,4 b]thiophene, thiophene and 3 hexylthiophene units via a random Stille polymerization ( Figure 1 11). By controlling the ratio of the monomers in the copolymer composition, the optoelectronic properties of the copolymers could be fine tuned. For example, by increasing the thienothiophene composition, the HOMO levels of the copolymers increased with a decrease of the LUMO levels, leading to smaller band gaps (from polymer D to polymer A). Polym er C showed a number average molecular weight as high as 132 kDa, and a polymer/PCBM solar cell PCE about 1.93%, which is higher

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44 than that of PTTD and P3HT OPVs prepared using similar conditions (0.73% and 1.39 % respectively). Figure 1 11. Synthesis of random copolymer for OPVs (Adopted and modified with permission from Ref. 105). Besides these well established polymerization methods, an important feature in the synthesis of conjugated polymers is the ability to prepare t he polymers in batches with a reproducible molecular weight (MW) and PDI range, the same purity level after purification, and thus the same optoelectronic properties and device performance. In one case, McGehee and Frchet et al. 106 pointed out that the mo lecular weight of regioregular P3HT had a profound effect on both the morphology and the charge transport property of the polymer thin films. They showed that with an increase in MW from 3.2 to 36.5 kDa, the corresponding field effect mobility of the trans istor increased from 1.7 10 6 to 9.4 10 3 cm 2 V 1 s 1 an increase of four orders of magnitude. A further study on the morphology of the films confirmed that the improvement in the mobility occurred because the longer polymer chains brought neighboring gra ins together, thereby minimizing the grain boundaries and favoring the charge transport. 107,108

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45 In another report, Janssen et al. 109 demonstrated the synthesis of two PDPP3T polymers with distinct MW and PDI via Suzuki polymerization under different cond itions ( Figure 1 12). Surprisingly, both the hole and electron mobilities of the two polymers remained about the same ( h = 0.04, e = 0.01 cm 2 V 1 s 1 for high MW polymer and h = 0.05, e = 0.008 cm 2 V 1 s 1 for low MW polymer, respectively), and were independent of the polymer molecular weight. In spite of the similarity in the mobilities, the two polymers showed a dram atic difference in their OPV performance. When blended with PC 71 BM, the high molecular weight PDPP3T ( M n = 54 kDa) provided a PCE up to 4.7% with V oc of 0.65 V, J sc of 11.8 mA cm 2 and FF of 0.6, while the low MW polymer ( M n = 10 kDa) only showed a PCE of 2.7% with V oc of 0.68 V, J sc of 6.3 mA cm 2 and FF of 0.63. The reduced short circuit current and efficiency in the low MW polymer were mainly due to the more corrugated morphology of the blending film, which limited the interaction between the polymer a nd PCBM. 110 112 Figure 1 12. Synthesis and structure of PDPP3Ts with distinct molecular weights by Suzuki polymerization. (Adopted and modified with permission from Ref. 109).

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46 1.4 Synthetic Control of Polymer Spectral Abs orption Combining theory with practical experience in chemical reactions, this section will address how the polymer spectral absorption (color) is controlled by synthetic modifications in real cases. It is worth mentioning that the colors of these materia ls are not driven only by the wavelength of the absorption (energy), but also by the intensity of the absorption band. This is especially true in the case of donor acceptor conjugated polymers, which will mostly present dual optical transitions with distin ct absorption intensities. This discussion will begin with polymers with one absorption band in the visible region, and then move to the D A copolymers with complex absorption profiles. As one of the simplest conjugated polymers, the electrochemically poly merized polythiophene film, which can reflect or transmit visible light, shows a red color due to its neutral state absorption transition from 620 nm to below 350 nm, which essentially covers the entire visible spectrum except for the red region. 113 1 15 In the case of PEDOT, combining the electron donating effect of the oxygen atoms directly linked to the 3 and 4 positions of the thiophene rings, with a sulfur oxygen interaction of adjacent EDOT rings, which dramatically reduces torsion angles and forc es the polymer backbones to adopt a more planar conformation than that of PT, the band gap is reduced. 116 The neutral PEDOT film has a single absorption band with max at 610 nm and is blue in color due to the lack of absorption in the range of 350 500 nm. 117, 118 Moreover, by replacing the ethylene bridge of EDOT with a propylene bridge and adding substituents (for example, CH 2 OEtHx) on the central carbon, a thin f ilm of the new polymer P ProDOT(CH 2 OEtHx) 2 showed a blue shift of the absorption band ( max = 581 nm) compared with that of PEDOT, and an increase of the band gap, probably due to a limited effective conjugation length and reduced interchain interactions ind uced by the

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47 bulky substituents. A red purple color was observed, since the polymer film transmits part of the blue (400 450 nm) and most of the red regions (650 700 nm). 119 On the other hand, compared with PT, the bulky acyclic side chains ( e.g. OEtHx) o f the 3,4 dialkyloxythiophenes will directly generate strong steric repulsions to the adjacent rings, resulting in a decrease of the effective conjugation length and an increase of the band gap. A thin film of polymer poly{3,4 di(2 ethylhexyloxy)thiophene} shows an orange color with absorption maximum at 483 nm ( Figure 1 13) 120,121 Randomly incorporating the dimethoxythiophene into the polymer backbone reduces the steric effect and lowers the band gap. The reduced band gap leads to a red shift of the absor ption, which affords the copolymer a red color ( max = 525 nm). Furthermore, conjugated polymers with dual optical transitions can be obtained via the donor acceptor approach 122 124,130 which provides an elegant method of modifying the spectral characteristics of the polymers by controlling the strength of the electron rich and electron deficient moieties, as well as their feed ratios. One particular example is the synthesis of conjugated polymers with the color green a color state that requires an efficient absorption in the blue and red regions with a window of transmission in the green region of the visible spectrum. In 2004, the Wudl group reported the synthesis of bithiophene thienopyrazine copolymer PDDTP ( Figure 1 14a), which gave the necessary two band absorption with a transmission window at 550 nm. By absorbing strongly in the blue (below 500 nm) and red (above 600 nm) region the polymer film was able to reflect a saturated green color. 125 In a similar approach, Toppare et al. 126 reported another green colored copolymer based on 4,7 di(2,3 dih ydro thieno[3,4 b][1,4]dioxin 5 yl)benzo[1,2,5]thiadiazole (BDT)

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48 ( Figure 1 14b). The electropolymerized donor acceptor polymer PBDT was investigated as a possible candidate for electrochromic applications. The insoluble polymer film was revealed to be a ne utral state green electrochromic material, which had a highly transmissive sky blue oxidized state. Figure 1 13. Structures of poly{3,4 di(2 ethylhexyloxy)thiophene} and a random copolymer of di(2 ethylhexyloxy)thiophene and dimethoxythiophene. Steric repulsions between side chains in the first polymer decrease the effective conjugation length, and shift the absorption band to the blue region Photographs are of the polymers in their neutral states. (Adopted and modified w ith permission from Ref. 121).

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49 a) b) Figure 1 14. Electrochemical polymerization of a) DDTP and b) BDT. Photograph are of the polymers in its neutral state. (Adopted and mod ified with permission from Ref. 125 and 126, respectively). In the mean time, our group developed a series of solution processable green to transmissive conjugated polymers using ProDOP, thiophene or EDOT as donors and BTD as an acceptor ( Figure 1 15a and b). 127,128 In this systematic study, it was shown that the polymer absorption spectra (both absorption wavelength and intensity) can be controlled by tuning the feed ratios of the donors and acceptors. With more and more donor moieties in the polymer main chain, the high energy absorption band shows a red shift as well as an increase in the intensity, while the low energy absorption moves to shorter wavelength with lower intensity ( Figure 1 effect of the two optical transit ions and provided an opportunity to make conjugated copolymers that absorb broadly and evenly over the entire spectrum. In one instance, a polymer was prepared by copolymerization of a ProDOT BTD ProDOT trimer with ProDOT in a 1:4 molar ratio. The polymer film showed an inklike black color in its neutral state and was highly transmissive in the oxidized state ( Figure 1 15c). More

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50 discussion of the D A effect in the polymer absorption spectral engineering can be 129 and subsequent publication. 130 a) b) c) Figure 1 15. Spectral engineering of green and black to transmissive conjugated polymers a) Molecular structures for the DA copolymers P1 and P2 Photographs are of the polymers in their neutral states (Adopted and modified with permission from Ref. 127. Copyright 2008 Wiley VCH.) b) Molecular structures for the copolymers P3 P6 The solution optical absorption spectra of polymers in tolu dual optical transition with increasing the donor ratio. c) Synthesis of inky black copolymer. Photographs are of the polymers switched from their neutral colored state to oxidized transmissive state. (Adopted and modifie d with permission from Ref. 128. Copyright 2008 Nature Publishing Group)

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51 1.5 Processing and Patterning of Conjugated Polymers high throughput material processing and patte rning techniques, are critical to enable 131 Reviewing the history of conjugated polymers, the interest in these materials has changed from simply pursuing high conductivity of soli d polymers to manufacturing complex organic electronic devices, 132,133 which usually require processing of the materials from their solutions into solid films. Consequently, this change has involved two important aspects of the field: the synthesis and cha racterization of soluble conjugated polymers and the processing of these materials into devices. While the synthesis and characterization of these soluble polymers is essential in understanding fundamental structure property relationships, only the incorpo ration of these materials in device structures and subsequent testing will demonstrate the practical usage and the commercial applicability of materials. This section briefly introduces some common processing and patterning techniques available for soluble polymers. Spin coating 134 is a procedure to produce uniform thin films on hard flat substrates. Specifically, by rotating the substrate, most of the polymer fluid on top of the substrate will spin off the edges of the substrate by centrifugal force and r emain as a thin layer of the polymer (0.01 1 m). The thickness of the polymer films can be controlled by the rotation speed and time, the polymer concentration and the solvent. This method has been broadly utilized in the processing of films for polymer light emitting devices and photovoltaics in r esearch laboratories and academia, but is rarely used in high volume manufacturing because of its low cost efficiency and waste considerations.

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52 Spray casting 135 is used to cast materials with homogeneous microstructures via the deposition of polymer soluti on droplets onto a substrate. The process can be easily achieved by utilizing a common pressurized airbrush sprayer. Moreover, this technique can be applied to a variety of substrates with all kinds of shapes or even soft and flexible substrates. Although the spray cast films are relatively rough (average roughness on the order of several tens to hundreds of nanometers) compared with the spin coat films, spray casting is still a simple and rapid processing method with few special requirements to obtain film s with thicknesses in the range of 50 nm to several micrometers. Spray coating has become one of the most straightforward approaches for processing soluble conjugated polymers in ECDs applications. 136,137 On the other hand, one disadvantage of spray castin g is the relatively low proc ess yield (70%) due to S creen printing 138 is a more realistic method for processing and patterning organic electronic devices and has been utilized for the fabrication of sol ar cells, LEDs and OFETs. 139 141 In screen printing, the films are sequentially deposited through a mask on a large area substrate to obtain a pattern with a resolution on the 100 m scale This versatile and comparatively simple technique offers obvious a dvantages in terms of materials and production costs, and is appropriate for fabricating organic electronics on an industrial scale Inkjet printing 142 is suitable for printing with low vis cosity soluble organic semiconductors. 143,144 This technique uses printheads, which use piezoelectric or thermal inkjet technology to deposit materials on substr ates. Since the printheads do not make direct contact with the substrates, it is considered a non contact method and

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53 can be utilized on a varity of substrates Moreover, a high er resolution is achievable by this method (35 to 40 m wide), compared with screen printing (> 100 m wide). As with the screen printing method, inkjet printing has been commonly used to prepare active layers for organic electronic devices. For example, Aernouts et al. 145 has reported the use of inkjet printing for depositing P3HT:fullerene blends t hin films. The r esulting organic films are used as the active layer for solar cells with power conversion efficiency of 1.4%. Slot die 146 is a coating technique for applying material solutions on a substrate to form thin films. Generally, material solutio ns are forced out from a reservoir through a slot by pressure, and transferred to a moving substrate. Since all of the coating fluid is applied to the substrate via the control of a positive displacement pump and the slot in the die exit, slot die coating is more efficient and cleaner than spin coating. This process has been successful ly applied to many types of conjugated polymer solutions such as water based PEDOT:PSS inks and organic solvent based P3HT:fullerene blends. 147,148 1.6 Selected Applications of Conjugated Polymers 1.6.1 Electrochromic Devices Electrochromic devices (ECDs) are electrochemical cells, in which the electrochromic material exhibits a reversible change in its absorption/trans mission or reflection upon an electrochemical redox reaction. Although many types of materials have been studied and utilized in the ECDs (metal oxides, organic molecules and inorganic complexes), conjugated polymers have gained more and more attention due to their better processability, faster switching speed and greater color variability. 149 Two types of dual polymer ECDs, which are used i n the characterization of black to transmissive ECP in Chapter 3, will be introduced.

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54 An absorptive/transmissive (wind ow type) ECD switches reversibly between a highly absorptive (colored) state and a highly transmissive (bleached) state. As shown in Figure 1 16a, films of two complementary electrochromic materials are deposited on the electrodes with a layer of electroly te sandwiched between. More specifically, the substrates can be glass or flexible plastic materials. The working and counter electrodes, as well as the electrolyte, must be transparent to allow light to pass through the polymers. In this instance, the anod ically coloring material is a minimally color changing polymer (N alkyl substituted poly(3,4 propylenedioxypyrrole), MCCP). 150 When a positive potential is applied to the electrode with cathodically coloring electrochromic polymer, the device is switched f rom its colored state to a highly transmissive state. An absorptive/reflective (display type) ECD is operated in a way that the light cannot pass through the device, but is reflected back. Depending on the reflective materials used in the device construct ion, this type of ECD can either be a specular reflective device 151 153 or a diffuse reflective device. 154 157 It is worth noting that the reflected light passes through the active layer (the working electrode with ECPs) twice before it reaches the human e ye. As shown in Figure 1 16b, the diffuse reflective device is constructed similarly to the window type device. In Figure 1 16b, TiO 2 particles are dispersed in the electrolyte and used as the diffuse reflective material. When one side of the electrochromi c polymer is oxidized and appears to be highly transmissive, the device shows the white appearance of TiO 2 on the transmissive side, but a colored appearance of the ECP on the other side.

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55 a) b) Figure 1 16. Schemat ic demonstration of a typical a) absorptive/transmissive window type electrochromic device. Light can pass through the device after the front ECP is oxidized and a minimally color changing polymer (MCCP) is used as a counter electrode b) An absorptive/re flective electrochromic device using TiO 2 particles as diffuse reflective materials. Light is reflected upon oxidation of the front ECP layer.

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56 1.6.2 Photovoltaic Cells Organic photovoltaics (OPVs), as potential replacements for silicon based solar cells, have gained considerable attention over the past decade due to their low cost, light weight and flexibility. Along with the improvement of the efficiency from less than 1% up to 11% (PCE of 11.5% in a DSSC with the ruthenium based sensitizer ), 158 several d ifferent device configurations have been developed, including dye sensitized solar cells (DSSCs), 159,160 bulk heterojunction solar cells (BHJ) 161 165 and organic inorganic hybrid solar cells. 166 This section will focus on polymer/fullerene BHJ solar cells, since they are the only conformation used in characterizing the DPP based copolymers in Chapter 5. Moreover, without an extensive summary of the principles and all of the factors that affect the power conversion efficiency (PCE), the discussion below will address only the most basic components in the synthesis of conjugated polymers for solar cell application. First reported by Heeger and coworkers, 167 the polymer/fullerene BHJ solar cell has become one of the most investigated branches in the OPV family o wing to the distributions and processabilities. Figure 1 17 demonstrates a typical polymer/fullerene BHJ solar cell construction. The active layer, which contains interpene trating networks of conjugated polymers and fullerene derivatives ( e.g. [6,6] phenyl C 61 butyric acid methyl ester, PC 61 BM), is sandwiched between two electrodes. The cathode Al and the anode ITO are used to collect the dissociated charges, and PEDOT:PSS i s spin coated from aqueous solution on ITO to facilitate hole extraction. Figure 1 18 shows the structures of the conjugated polymers and PCBM derivatives that will be discussed below.

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57 Figure 1 17. Schematic demonstration of a typical polymer/PCBM BHJ s olar cell. Photoactive layer based on polymer/PCBM blend comprises interconnected morphology, which is enriched by either the donor polymers or the acceptor PCBM. Interpenetrating interfaces of polymer/PCBM favor carrier generation and carrier transport ef ficiency. Among all the homo conjugated polymers, regioregular poly(3 hexylthiophene) (RR P3HT) is perhaps the most successful and well investigated polymer in BHJ solar cells. As discussed in S ection 1.1.2, RR P3HT shows HOMO LUMO levels at 5.2 and 3.3 eV, respectively, with an optical band gap of 1.9 e V. Nowadays, the BHJ solar cell with the structure of ITO/PEDOT:PSS/P3HT:indene C 60 bisadduct (ICBA) (1:1, w/w)/Ca/Al, has shown a PCE as high as 6.48% with V oc of 0.84 V, J sc of 10.61 mA cm 2 and FF of 0. 727. 168 In this report, the key factors for achieving high V oc and FF, and thus high performance are the use of indene C 60 bisadduct ( Figure 1 18 ) instead of PC 61 BM, the optimization of the donor (P3HT)/acceptor (ICBA) ratio, the solvent annealing, and the thermal annealing. More specifically, the raised LUMO energy level of ICBA (0.17 eV higher than that of PCBM) contributes significantly to the expansion of energy difference between the HOMO of the donor polymer P3HT and the LUMO of the acceptor fullerene thereby causing an increase in the V oc The annealing treatment

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58 favors the formation of a more uniform distribution of P3HT/ICBA and a better interpenetrating network. Actually, the processing techniques for optimizing the film morphology in the study of P3HT solar cells have become guidelines in all polymer/PCBM BHJ solar cell designs. However, despite the respectable V oc and excellent FF, the overall performance has been limited by the rather small photocurrent due to the relatively wide band gap of P3H T, which restricts the absorption below 650 nm, while an essential part of solar energy is located in the red and infrared region. Consequently, this concern leads to a new direction: pr eparation of low band gap donor acceptor conjugated polymers with effi cient light harvesting property for organic solar cells. The poly( thieno[3,4 b]thiophene co benzodithiophene) (PTB) family is a series of D A copolymers with benzodithiophene derivatives as donors and thienothiophene derivatives as acceptors (Figure 1 18) 161,169 171 As two members in this low band gap family, PTBF0 and PTBF1 show absorption onsets at 780 and 737 nm with optical band gaps at 1.59 and 1.68 eV, respectively. While PTBF0 gives a PCE of 5.1% with J sc of 14.1 mA cm 2 V oc of 0.58 V, and FF of 0. 624, the fluorinated PTBF1 shows a dramatic increase of the V oc (0.74 V) and FF (0.689) and thus a PCE (7.2%) due to the lowered HOMO level and better nanoscale morphology induced by the fluorine atom. 171 A further optimization of the solar cell processing conditions improves the PCE up to 7.4%. 170 Compared with P3HT solar cells above, it is obvious that the dramatic increase in PCE comes from the increase of J sc which is related to the polymer absorption in the visible and near IR region. It is worth ment ioning that 1,8 diiodooctane (DIO) is used to improve the miscibility between the polymer and PCBM and thus the film morphology.

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59 Figure 1 18. Representative conjugated polymers with high power conversion efficiency and fu llerene derivatives. In another case, polymer PBnDT DTffBT (Figure 1 18) 172 which is a D A copolymer using benzodithiophene and 5,6 difluoro 4,7 dithien 2 yl 2,1,3 benzothiadiazole (DTffBT) as repeat units, shows a PEC of 7.2% with J sc of 12.9 mA cm 2 V oc of 0.91 V, and FF of 0.61. Clearly, the introduction of electron withdrawing fluorine atoms into the polymer backbone has lowered both the LUMO ( 3.33 eV) and HOMO level s ( 5.54 eV) of the polymer and afforded a high V oc Moreover, an optimized film mor phology, which was possibly facilitated by the F atoms was observed and caused a noticeable increase of the J sc and FF.

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60 Furthermore, our group has reported a comparison result of two polymers, PDTS TPD (a copolymer of dithienosilole and N octyl thienopyrr olodione) and PDTG TPD (a copolymer of dithienogermole and N octyl thienopyrrolodione). 173 By simply changing the Si atom into a Ge atom, while keeping all the other groups the same, the inverted solar cell of PDTG TPD (glass/ITO/ZnO/Polymer:PCBM/MoO 3 /Ag) showed a PCE of 7.3%, compared with 6.6% for PDTS TPD. Although the V oc of the DTG based polymer solar cell decreased to 0.85 V (0.89 V for PDTS TPD), the increase of J sc (12.6 vs. 11.5 mA cm 2 for PDTS TPD) and FF (0.68 vs. 0.65 for PDTS TPD) has raised t he PCE to a significant level. This is possibly due to the slightly red shifted absorbance of the DTG analogue and the di ff erence in intermolecular packing. Clearly, recent developments in the synthesis of new materials for OPVs have brought solar power c onversion efficiencies above 7% or even higher than 8% with a few classified structures. The understanding of the structure property relationships of the conjugated polymers, as well as the optimization of the operating conditions of solar cells, is still the key to further improve the PCEs. Especially, fine tuning of the light absorption, the HOMO LUMO levels and the film morphologies as a function of polymer structures, as well as polymer structural details, purity levels, molecular weights, ending groups as a function of chemistry control, are the most important points for a synthetic chemist. 1.7 Thesis of T his Dissertation As discussed in this chapter, during the past four decades the synthesis and characterization of conjugated polymers has generated great attention and success. The applications of these organic semiconducting materials extends from energy producing devices (solar cells), to low energy consumption devices (light emitting

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61 diodes and field effect trans istors), to bistable devices (electrochromics). However, most of these materials and devices are currently limited to the research laboratory; the commercialization of these polymeric materials is not yet developed. This is largely due to synthetic difficu lties and the lack of deep understanding in the structure property relationship of conjugated polymer. These issues lead to the themes of this work: and electronic prope rties; utilizing basic organic and polymer chemistry knowledge in the search for new synthetic routes and methods to prepare conjugated polymers, in which band gap, energy levels and light absorption are finely controlled for different applications; and fi nally, accessing water soluble and processable conjugated polymers, which minimize environmental impact and processing costs. Chapter 2 describes briefly the techniques employed for the work presented in this dissertation. The purification of the polymers as well as the importance of the polymerization repeatability and reproducibility are also discussed in Chapter 2. In C hapter 3, a new synthetic methodology is devised for the synthesis of black to transmissive switching electrochromic polymers. By introdu cing the random Stille polymerization method, which has prove to be an efficient, scalable and more consistent polymerization process in producing conjugated polymers, random black to transmissive electrochromic polymers has been synthesized in batches wit h a highly repeatable absorption spectra, number average molecular weight ( M n ) and PDI Moreover, successful characterization and device work enhance the potential of this polymer to be used for electrochromic devices. Based on the work in C hapter 3, the s tudy in C hapter 4 is an extension of the random methodology to the synthesis and

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62 characterization of water soluble and processable conjugated electrochromic polymers, WS ECP Blue and WS ECP Black, with the goal of improving the polymer electrochromic switc hing rate and the ability of polymers to be utilized in large area spraying in an environmentally friendly aqueous solution. Along with the synthetic details and structure characterizations, a complete comparison of the electrochromic behaviors of organic and water soluble polymers is given in C hapter 4. Chapter 5 targets understanding the structural influences in the performance of solar cell materials with diketopyrrolopyrrole as a conjugated core. Although the same synthetic methodology has been applied to expand the absorption spectra of DPP based conjugated polymers, a fundamental understanding in the balance of polymer solubility, stacking ability, light absorption, and energy levels is the key to this chapter. Finally, Chapter 6 gives a summary of the work presented in this dissertation and provides suggestions for the future research.

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63 CHAPTER 2 EXPERIMENTAL METHODS AND CHARACTERIZATION S 2.1 General Synthetic Methods All chemical reagents and dehydrated solvents were commercially available and used w ithout further purification unless otherwise noted. All reactions were carried out with oven dried glassware and dry solvents (the purification of solvents can be found in reference 174) in an argon atmosphere unless otherwise stated. For column chromatogr aphy, Whatman Purasil silica gel (230 400 Mesh, pore diameter 6 nm) was used. Detailed synthesis and purification procedure for specific compounds is available in the experimental section of C hapter 3 5. 2.2 Purification of Polymeric Materials The purifica tion of polymeric materials has become an essential part of the synthesis procedure. In order to achieve high performance in applications such as OPVs, OLEDs and ECDs, the impurities arising from a polymerization must be thoroughly eliminated; otherwise th ey will act as defect sites in devices. In a Stille polymerization used in the synthesis of mostly the polymers in this dissertation, the impurities may include: 1) the toxic tin halides produced during the transmetallation step in the polymerization; 2) p alladium catalysts as well as ligands; 3) low molecular weight small molecules, oligomers and polymers due to the decomposition and oxidative homocouplings of organotin compounds. A step by step process to eliminate all these impurities will be introduced in this section. It is worth mentioning that this process can be applied not only to the polymers from the Stille polymerization, but also other types of polymerizations such as Suzuki, Negishi, GRIM and oxidative polymerization. For each different polymer ization, the procedure may change slightly.

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64 A general procedure for the purification may include the addition of a palladium scavenger, precipitation of the polymers, filtration and Soxhlet extraction Following the completion of the polymerization, diethy lammonium diethyldithiocarbamate (D DDC) is added directly into the polymer solution, and then the solution mixture is stirred for 1 h at 50C under an argon atmosphere (experimental details are availab le for DPP based copolymers in C hapter 5). D DDC is us ed as a scavenger to coordinate with the palladium catalyst, allowing the palladium to dissolve into the methanol solution during precipitation of the polymers in the following step. The amount of the D DDC added to the polymer reaction solution depends on how much of the catalyst is used (usually 10 times the catalyst amount). After cooling to room temperature, the reaction mixture is then pipetted into methanol, causing precipitation of the polymer. The amount of the methanol used here should be 20 times the polymerization solution volume. By this process, most of the palladium catalyst, tin halides and small molecules should remain in solution and be removed by a subsequent filtration process. However, there is still a chance of small amounts of these imp urities being trapped in the polymer network. In light of this, the following Soxhlet extraction is crucial. Soxhlet extraction is a continuous solid liquid extraction, generally used to collect the desired compounds from a solid material by recycling a sm all volume of warm solvent. In this case, the process is reversed, as the desired compounds are the pure polymers, and the low molecular weight impurities are extracted ( Figure 2 1a). Thus, the extraction solvents and the extraction sequence must be carefu lly designed to fulfill this purification purpose. Usually, the Soxhlet extraction of a conjugated polymer starts with methanol, which is a poor solvent for polymers but a good solvent for the residual

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65 catalysts, tin halides and other small molecules. Then hexane is used to remove low molecular weight oligomers and polymers. Sometimes, extraction with acetone is also applied in between due to its higher solubility for the catalysts. Finally, extraction with chloroform will dissolve the pure polymer into th e solution while filtering the polymer solution through the extraction thimble. The extract is concentrated and precipitated into methanol again to yield the solid polymer. Sometimes, chloroform is not able to dissolve the final polymer and so becomes an i ntermediate extraction solvent, with chlorobenzene employed as the final solvent. a) b) Figure 2 1. Demonstration of polymer purification methods. a) Soxhlet extraction and b)

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66 Table 2 1 shows comparative results of samples before and after Soxhlet extraction. These samples were received having been precipitated once without further polymerization, and sam Grignard Metathesis (GRIM) polymerization. As we can see, due to the loss of low molecular weight fractions (20 30 wt% of the samples), the polymers show higher number average molecular weights with lower PDIs after the extraction. Moreover, the optical properties of the polymers are also changed. For example, polymer BASF Black 2266 70 shows a reduced absorption intensity in the short wavelength region after the extraction ( Figure 2 2). It is worth noting that these samples were synthesized by our collaborators in BASF. More importantly, the polymerization scale of these samples is tremendously large compared to our laboratory production (usually less than 300 mg each time). This is especially true for sam ple BASF Black JB 1 which was prepared at the 28 g level. The success of these scale up reactions has brightened the prospect that conjugated polymers will become commercially available and be applied in realistic optoelectronic devices. Table 2 1. GPC re sults of polymer samples before and after the Soxhlet extraction. S ample ID M n (kDa) M w (kDa) PDI Amount BASF Black JB 1 ( ) 7.0 12.0 1.7 27.5 g BASF Black JB 1 ( ** ) 10.2 15.0 1.5 21 g BASF Black 2266 70 ( ) 8.0 14.0 1.7 11.5 g BASF Black 2266 70 ( ** ) 10.0 15.5 1.6 9.5 g BASF Magenta 2181 100 ( ) 20.0 35.0 1.8 10 g BASF Magenta 2181 100 ( ** ) 23.0 39.0 1.7 7.3 g Samples before Soxhlet extraction. ** Samples after Soxhlet extraction. Although the difference in absorbance is too small to be perceiv ed by the naked eyes, other indicators are present to suggest the loss of low molecular weight material.

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67 One example is disappearance of the strong smell of tin compounds after the extraction, and the decrease in the tendency to delaminate under repeated e lectrochemical switching. Delamination is usually caused by the presence of defects associated with a large amount of small molecules in the polymer film or at the polymer/substrate interface. As these small molecules are removed by the solvent during the switching, voids are created. This change of the material packing density makes the polymer matrix collapse and separate from the substrate. After eliminating the small molecules using Soxhlet extraction, the pure polymer will tend to form homogenous films with fewer defects, avoiding the phenomena described above. Figure 2 2. Solution absorption change of polymer BASF Black 2266 70 after soxhlet extraction in chloroform In spite of the positive effects in the purification of conjugated polymers, the So xhlet extraction is a slow process. Depending on the quality and quantity of the sample, it usually takes 2 10 days to purify one sample, as the process relies on

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68 was introduced to the group by Dr. Mike Craig. The extraction time can be reduced to less than a day using this technique. As shown in Figure 2 1b, by stirring the polymer solid dispersion with extraction solvents (the same solvents used in the Soxhlet extract ion) in a Millipore stirred cell, the extraction process is accelerated. Furthermore, a high pressure filtration of the mixture combined with concurrent stirring is also believed to reduce the risk of filter papers being blocked by the polymer particles, w hich is also a concern when using Soxhlet extraction thimbles. However, this process involves the use of more solvents than a typical Soxhlet extraction since it is not a recycling process. The recovery of organic solvents needs to be considered in the pra ctical use of this purification process in the future. A typical procedure of using C hapter 3, S ection 3.8. 2.3 A Brief Discussion of Polymerization Repeatability and Reproducibility The repeatability and reproducibility of a polymeri zation are important parameters. Repeatability refers to replication of results by an individual researcher, whereas reproducibility is replication of results by other operators. A reproducible process with a low batch to batch variability is extremely imp ortant for transferring the synthesis from the academic lab to industrial production. Clearly, it is meaningless to continue scientific research, which cannot be repeated or reproduced by other groups. A detailed repeatability study of the Stille random p olymerization is demonstrated in S ection 3.2.4. Considering the particularity of the polymerization, which follows a step growth mechanism, the stoichiometric control of the bifunctional monomers becomes one of the most crucial factors. For instance, facto rs such as the impurities in the monomers, human errors in the weighing step and unsuccessful transferring of the chemicals into the reaction vessel, resulting in inaccurate monomer ratios, will reduce

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69 the repeatability of the polymerization. According to the results in S ection 3.2.4, the Stille polymerization shows a great tolerance to various batches of monomers, catalysts and solvents. Polymers, which were produced with the same monomer ratio, exhibited consistent molecular weights and optical properties Moreover, the reproduction of ECP 3 (see Chapter 3 for ECP 3 details) on a large scale was accomplished by the BASF team. Sample BASF Black JB 1 and BASF Black 2266 70 have number average molecular weights similar to that of ECP 3 after the Soxhlet ext raction. However, there is a detectable deviation of the solution absorption spectra between the samples ( Figure 2 3). For example, BASF Black 2266 70 shows a lower absorption intensity in the range of 450 600 nm compared with EPC 3 Although the reason fo r the variability of the polymerization is still unknown, the reproducibility of the polymers is considered to be successful since this small change in the absorption spectra cannot be distinguished by the human eye. Figure 2 3. Solution absorption of p olymers BASF Black 2266 70 and ECP 3 in chloroform

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70 2.4 Materials Characterization 2.4.1 Structural Characterization Proton NMR (300 MHz) and 13 C NMR (75 MHz) spectra were recorded on a Mercury 300 spectrometer using deuterated chloroform as solvent and te tramethylsilane as internal reference. High resolution mass spectrometry was performed by the Spectroscopic Services at the Department of Chemistry of the University of Florida with a Finnigan MAT 96Q mass spectrometer. Elemental analyses were performed by Robertson Microlit Laboratories, Inc., Atlantic Microlab, Inc. or the University of Florida, Department of Chemistry spectroscopic services. 2.4.2 Polymer Molecular Weight Characterization Gel permeation chromatography (GPC) was performed using a Waters Associates GPCV2000 liquid chromatography system with its internal differential refractive index detector (DRI) at 40C, using two Waters Styragel HR i.d., 300 mm length) with HPLC grade THF as the mobile phase at a flow rate o f 1.0 mL min 1 Injections were made at 0.05 injection volume. Retention times were calibrated against narrow molecular weight polystyrene standards (Polymer Laboratories; Amherst, MA). 2.4.3 Thermal Charact erization Polymer thermal stability was assessed by thermogravimetric analysis (TGA) on a TA Instruments TGA Q1000 Series using high resolution dynamic scans under nitrogen. The TGA samples (3 4 mg) were typically heated to 50C to equilibrate to a constan t mass, and then heated at a heating rate of 50C/min with resolution number of 4.00 and sensitivity value of 1.00. The final temperature is set to be 650C.

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71 2.4.4 Electrochemical Characterization Electrochemistry is a powerful tool to evaluate the electro nic properties of conjugated polymers, providing information such as the onsets of oxidation and reduction (HOMO and LUMO energy levels), half wave potentials and electrochemical bandgaps. E lectrochemistry was performed using a three electrode cell with a platinum wire or a platinum flag as the counter electrode, a silver wire quasi reference electrode or Ag/Ag + reference electrode calibrated using a 5 mM solution of Fc/Fc + in 0.1 M electrolyte solution, and a platinum button or ITO coated glass slide (7 50 0.7 mm, 2 ) as the working electrode. The ITO electrodes were purchased from Delta Technologies, Ltd. All potentials were reported vs Fc/Fc + An EG&G Princeton Applied Research model 273 potentiostat was used under the control of CorrWare II s oftware from Scribner and Associates. The electrolyte solutions were prepared from dry lithium bistrifluoromethanesulfonimidate (LiBTI) electrolyte dissolved in freshly distilled propylene carbonate (PC) or KNO 3 in water, and bubbled with argon for 20 minu tes before the experiments. Two different electrochemistry methods were used in this dissertation cyclic voltammetry (CV) and differential pulse voltammetry (DPV). 175,176 Detailed discussions of these two methods can be found in dissertations from Jennif er A. Irvin, Christopher A. Thomas and Emilie M. Galand. 177 179 Since all electrochemical measurements reported in this dissertation will be referenced versus the Fc/Fc + redox couple, the conversion of the energy levels to the vacuum level were recalibrat ed by using 5.1 eV as the oxidation potential of ferrocene vs. vacuum It is worth mentioning that 4.8 eV is also commonly used as the Fc oxidation potential. The debate of which number is the true value can be found in the dissertation of Barry C. Thom pson 180 and reference 181.

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72 2.4.5 Optical Spectra Characterization Absorption spectra were obtained using a Varian Cary 500 Scan UV vis/NIR spectrophotometer and quartz crystal cells (1 cm x 1 cm x 5.5 cm, Starna Cells, Inc.). ATR IR measurements were perf ormed on a Perkin Elmer Spectrum One FTIR outfitted with a LiTaO 3 detector. 2.4.6 Spectroelectrochemistry As one of the most basic tools to evaluate the electrochromic behavior of ECPs, spectroelectrochemical measurements were performed to obtain abso rption or transmittance changes upon oxidation and reduction of polymer thin films. By progressively applying an electrical bias to a polymer film, the formation of polarons and bipolarons in the polymer chains will generate a series of continuous changes in the film absorption spectra. Figure 2 4a gives the result of a typical spectroelectrochemical experiment. In this case, the blue colored polymer film was gradually oxidized (p doping) in a 0.1 M LiBTI/PC solution. The depletion of the dual absorption ba nd in the visible region (350 400 and 500 800 nm, respectively), as well as the rise of electronic max = 1000 nm) and bipolaron max > 1600 nm), afford a highly transmissive oxidized state of the p olymer film 2.4.7 Colorimetry To evaluate the color changes of the ECPs occurring on electrochemical switching L a b color standards), in situ colorimetric measurements were perfo rmed using a Minolta CS100 Colorimeter in transmission mode with a GraphicLite LiteGuard II standard D50 light source. The light source and the sample to be measured were placed in a color viewing booth. The interior of the light booth is coated with a sta ndard gray neutral 8 (GTI

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73 a) b) c) Figure 2 4. Demonstration of s pectroelectrochemical series for a blue to transmissive electrochromic polym er film a) A typical spectroelectrochemistry data plot The films were spray cast onto ITO coated glass from toluene (1 mg/mL). Electrochemical oxidation of the films was carried out in 0.1 M LiBTI/PC supporting electrolyte using a silver wire as a refere nce electrode, and a platinum wire as the counter electrode. All potentials given are referenced vs. Fc/Fc + b) Photographs of the film held at 0.22 V (left) and 1.02 V (right). c) Structure of the polymer. Graphic Technology, Inc.) matte latex enamel (eq uivalent to Munsell N8) to allow for accurate assessment of color of the sample during measurement. Photography was performed in the same light booth using ether a Nikon D90 at a exposure time of 1/40~1/25 sec, f stop of f/5.3, ISO sensitivity of 200 or 40 0, and focal length of 75~80 mm, or a Canon EOS REBEL XSi at a exposure time of 1/125~1/60 sec, f stop of f/4~f/5.6, ISO sensitivity of 200, and focal length of 18~55 mm. The photograph file type

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74 was JPEG and the files were cropped to contain only the poly mer film to exclude extraneous background using Adobe Photoshop. No additional alterations to the photography files were performed. 2.4.8 Morphology Characterization Atomic force microscopy was used to characteri ze donor acceptor blend surface morphologies in bu lk heterojunction solar cells (C hapter 5). Data were acquired as tapping mode height image on a Veeco Innova scanning probe microscope (SPM) with a Nanodrive controller and MikroMasch NSC 15 AFM tips. The i nstrument was a sample scanning instrument, whereby the tip remains stationary while the sample was scanned. In tapping mode, the tip was oscillated at ~300 MHz (resonant frequency of tips used), and the tip oscillation amplitude was damped by the surface being probed until the oscillation amplitude reached a specified set point. The sample was then scanned and the height of the sample constantly adjusted by the piezoelectric scanner to maintain the specified set point. 182 2.5 Electrochromic Devices 2.5.1 T ransmissive/Window Type Devices The window type device was constructed using ITO/gl ass (25 x 37.5 x 1.1 mm, 8 and the minimally color changing polymer, PProDOP N C18, at the other. The polymer was spray cast from methylene chloride or toluene. The gel electrolyte was comprised of LiBTI dissolved in propylene carbonate to a concentration of 0.5 M followed by the addition of 1.1 g of poly(methyl methacrylate) (PMMA, M n ~1, 000 kDa) per 10 mL of electrolyte. A layer of the gel was spread onto one of the po lymer coated ITO electrodes

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75 and the device was assembled using 3M tape as a spacer and epoxy as a sealant on the outer edges of the glass. The device schematic is shown in Chapter 1. 2.5.2 Reflective/Display Type Devices The reflective device was construc ted using ITO/glass (25 x 37.5 x 1.1 mm, 8 12 spray max was ~ 0.8 a.u.. The gel electrolyte was compri sed of LiBTI dissolved in propylene carbonate to a concentration of 0.5 M, followed by the addition of 1.1 g of PMMA per 10 mL of electrolyte. Titanium dioxide (Acros) was added to the gel and stirred until the gel was an opaque white. A layer of the gel w as spread onto one of the polymer coated ITO electrodes and the device was assembled using double sided tape as a spacer and epoxy as a sealant on the outer edges of the glass. The device schematic is shown in Chapter 1. 2.6 Photovoltaic Devices Before dev ice fabrication, the ITO coated glass substrates were cleaned with acetone, isopropyl alcohol and DI water. The ZnO PVP composite was spin coated onto the cleaned ITO substrate at 3000 RPM, and then annealed at 200C for 40 mins. The PDPP3T :PC 71 BM (1:1) m ixture was dissolved in chlorobenzene with 5% v/v 1,8 diiodoctane (DIO) (C hapter 5). The polymer P3 and P4 :PC 71 BM solution was prepared in the same way at a concentration of 20 mg/mL. The mixture solution was spin coated onto ZnO films to an optimum thickn ess of 105 nm. Then the film was annealed at 70C for 40 min. A 5 nm layer of molybdenum oxide (MoO 3 ) and a 100nm layer of silver were thermal evaporated at 110 6 Torr. Devices were encapsulated before measurements.

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76 Photocurrent voltage ( J V ) characteriz ation was performed with a Keithley 4200 semiconductor characterization system and a Newport Thermal Oriel 94021 1000W solar simulator (4 in. by 4 in. beam size). The light intensity was determined by an ORIEL 91150V monosilicon reference cell calibrated b y Newport Corporation. EQE measurement was conducted using a Xe DC arc lamp as light source, an ORIEL 74125 monochromator, a Keithley 428 current amplifier, an SR 540 chopper system and an SR830 DSP lock in amplifier from SRS. The generated EQE spectrum wa s integrated with an A.M. 1.5 G spectrum to compare with the measured short circuit current values. A PerkinElmer LS55 fluorescence spectrometer and Lambda 750 UV/VIS spectrometer were used for photoluminescence and absorption experiments.

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77 CHAPTER 3 BROAD LY ABSORBING BLACK TO TRANSMISSVIE SWITCHI NG ELECTROCHROMIC POLYM ERS USING HIGHLY REP ROD UCIBLE STILLE METHODS 3.1 Electrochromic Polymers Compared to inorganic metal oxides ( e.g. tungsten oxide), 183 185 small molecule organic electrochromes ( e.g. viologen derivatives) 149,186 and electrodeposited conjugated polymers films ( e.g. polythiophene and polypyrrole), solution processable conjugated polymers have offered great advantages in the field of non emissive electrochromic devices (ECDs). More specifically, these advantages may include good proc essability, mechanical flexibility, high optical contrast, rapid redox switching and long term stability. 13,187,188 In addition, the utilization of conjugated polymers as electrochromic materials has significantly improved the development of applications such as low cost organic electronic displays, smart windows, electronic paper and rear view mirrors. 7,189,190 focus on the synthesis and characterization of soluble conjugated polymers, whi ch can be easily oxidized or reduced and show two or more distinct colored states based on redox activities. Generally, all of the co njugated electroactive polymers possess electrochromic behavior. For example, electrochemically polymerized polythiophen e undergoes a red to blue switch upon the oxidation of the polymer films, which corresponds to the bleaching of the transition in the visible region with simultaneous emergence of polaron and bipolaron infrared optical transitions tailing into the red region. 114,115,191 On the other hand, a red to black green color switch is also observed by reducing the polymer film from its neutral state to its reduced form. 192 In addition, unsubstituted

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78 polypyrrole yields a yellow to brown black switch on successful p doping, 193 and the switching colors of its N alkyl and N phenyl derivatives are not significantly affected by successive structural modifications at the nitrogen position. 194 Although various interesting switching colors have been attained by these conj ugated polymers and their derivatives (mainly based on side chain modifications), and the structure property relationships have been successfully investigated, it is almost impossible to reach the practical utility of ECDs by using these types of electroch romic materials, which can only be switched between two or more distinct and highly colored states. This triggers new questions for the materials scientists: what kind of polymeric electrochromic materials do we really want and how can we achieve them? On the other hand, electrochromic materials that can be switched between a strongly colored state and a highly transmissive or near transparent state would find much broader applicability, such as in transmissive window type devices and reflective display ty pe devices. Moreover, considering the existing print and spray techniques, the target polymers must have considerable solubility in either regular organic solvents (toluene, THF and chlorinated solvents) or water. Other important requirements may include: 1) stable chemical structures (backbones and side chains), which cannot be decomposed easily by a serious of chemical treatments, as well as by UV and visible during pro cessing and provide high device stabilities; on the other hand, high HOMO values allow for low oxidation potentials and stable oxidized states (no or low degradation after thousands of redox switches); 3) high molecular weights (usually higher than 10,000

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79 has been dedicated to fulfill this extensive program by the synthesis and characterization of conjugated polymers for electrochromic applications. With the invention of a neutral yellow to cathodic transmissive conjugated polymer (ECP yellow, Figure 3 1), the field of electrochromic polymers has now reached an important milestone. 136,195 During the past few years, a serie s of polymeric electrochromes which are colored (yellow, 195 red and orange, 120 magenta/purple, 119,196 blue, 197 green and cyan, 127,128 ) in the neutral state and highly transmissive in the oxidized state, have been investigated by our group (Figure 3 1). Bas ed on this successful work, the full color palette is now complete, allowing a large variety of colors for transmissive and re fl ective electrochromic applications. However, there are only two reports of black to transmissive electrochromic polymers. 128,198 This is due to the complexity in producing a polymer absorption spectrum which absorbs evenly over the entire visible spectrum (400 750 nm) in the fully neutralized state, while effectively bleaching out over the same region in the fully oxidized state. R ecently, we presented our first black to transmissive switching ECP based on the donor acceptor approach. 128 By varying the relative contribution of donor and acceptor moieties in the polymer backbone, the two band absorption in the visible spectrum could be controlled. In addition, using a particular feed ratio of two different monomers, the discrete absorption bands merged into a broad absorption across the entire visible region. The spray cast polymer thin film exhibited a deep black neutral state and at tained a highly transmissive state when fully oxidized. Due to the synthetic complexity of the polymer, which involved the synthesis and chromatographic separation of donor

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80 acceptor donor trimers, and which may limit reproducible large scale production of the material, we wanted to develop a method that would lead to polymer batches with highly reproducible absorption spectra and an efficient, scalable and consistent polymerization process. Figure 3 1. Repeat unit structures and photographs of spray cast polymer fi lms in the neutral colored, a nd oxidized transmissive states and n ormalized absorption spectra of polymer fi lms of P1 P7 (Adapted with permission from Ref.121 and Ref. 136 Copyright 2011 American Chemical Society) In order to achieve this goal, a transition metal mediated random coupling polymerization was chosen. In the specific instance demonstrated here, we used a Stille polymerization; however, the method can also be applied to many other transition metal mediated reactions, including Suzuki Negishi, and Kumada couplings. Under the most

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81 common Stille polymerization conditions, two monomers (typically an aryl distannane and an aryl dihalide) are cross coupled using a palladium catalyst to produce polymers with alternating Ar Ar couplings. 1 9 9 201 In this approach, we desired to incorporate a random mixture of donor and acceptor heterocycles in order achieve a black ECP by utilizing more than two monomers in the polymerizations. In order to simulate the ratio of electron rich to electron poor h eterocycles determined in our previous work, we chose to randomly polymerize both 2,5 dibromo and 2,5 tributylstannyl 2 ethylhexyloxy substituted 3,4 propylenedioxythiophene (ProDOT (CH 2 OEtHx) 2 ) with 4,7 dibromo 2,1,3 benzothiadiazole (BTD) in different f eed ratios (Figure 3 2). 202 Figure 3 2. Reaction scheme for the Stille polymerization of three monomers. In this chapter, several random broadly absorbing copolymers were produced with number average molecular weights be tween 10 to 18 kDa and polydispersities ranging from 1.3 to 1.6 after Soxhlet extraction. Polymers from successive repeated experiments showed highly reproducible absorption spectra as well as matched M n and PDI. The polymer ECP 3 was utilized as an exampl e to demonstrate the electrochromic properties of these polymers. The thin film of ECP 3 showed a transmittance change T ) as high as 42% (% T from 36% to 78%) at the 1 s switch time as well as a high continuous switching stability (18,000 cycles, 1.5 s s witch time). Successful

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82 characterization and application study indicate that these polymers have high potential for use in electrochromic devices. 3.2 Polymer Synthesis and Characterization 3.2.1 Monomer Syntheses and Purification Figure 3 3 s hows the synthesis of the monomers: 6,8 b is(tributylstannyl) 3,3 bis((2 ethylhexyloxy)methyl) 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine (compound 4 ), 6,8 d ibromo 3,3 bis((2 ethylhexyloxy)methyl) 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine (compound 5 ) and 4,7 d ibromobenzo[c][1,2,5]thiadiazole (compound 6 ). Starting from commercially available 3,4 dimethoxythiophene 1 ProDOT intermediate compound 2 with a propylenedioxy bridge was synthesized via transetherification with 2,2 bis(bromomethyl)propane 1,3 diol in 80% yield after column chromatography. Occasionally, in order to achieve a white crystalline solid of compound 2 an additional recrystallization step using a hexane/THF (5:1) mixture as the solvent was applied. The subsequent reaction of compound 2 wi th 2 ethylhexan 1 ol under basic conditions in DMF afforded ProDOT (CH 2 OEtHx) 2 (compound 3 ) as a colorless oil in 78% yield. Compared with the previously reported procedure for making compound 3 119 which required formation of the 2 ethylhexan 1 olate sodi um salt precursor before adding the ProDOT intermediate, the procedure used here was modified into one in situ reaction by adding all the reagents at once ( S ection 3.8 for experimental details). Moreover, the reaction time was reduced to one day instead of two days by monitoring the progress of the reaction using TLC and the reaction yield was improved from 70% to 78% after purification. A high purity level of compound 3 is crucial for the next step in the synthesis of di tributyltin substituted ProDOT (CH 2 OEtHx) 2 (compound 4 ) due to the complexity of the purification of 4

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83 Figure 3 3. Reaction scheme for the synthesis of ditin compound 4 and dibromo compounds 5 and 6 for random Stille polymerization. The first attempt at making compound 4 was carried out in THF, and n BuLi was used as a deprotonation agent. This process involved the formation of an aromatic dilithium salt at room temperature. However, at such a high temperature (25 C) the solvent THF was decomposed by n BuL i 204 and the resulting impurities could not be separated from the crude product mixture. The proposed reaction is shown in Figure 3 4. Ethylene and the acetaldehyde lithium enolate were formed during the decomposition of THF via a reverse [3 + 2] cycloadditi on 205 where the later enolate ion was a much

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84 weaker nucleophile than n BuLi, and could not react with the protons on the 2 and 5 position of thiophene rings. 206 208 After quenching with tributyltin chloride, the enolate was converted to an organotin compou nd, which could not be separated from compound 4 Figure 3 4. Proposed reaction scheme for the impurities in the synthesis of compound 4 using THF as a solvent. In order to avoid this situation and improve the reaction y ield, diethyl ether was used instead of THF, and fresh LDA, which was made by reacting diisopropylamine with n BuLi, was used as an alternative deprotonation agent. Using the conditions described in S ection 3.8, no decomposition of the solvent or the starti ng material was observed. After flash column chromatography with hexane as an eluent on treated silica gel (washed with neat triethylamine, then hexane), compound 4 was achieved in 95% yield. It is worth mentioning that compound 4 decomposed in the column, even though the silica gel was treated. The monotin compound could be observed under 1 H NMR after silica gel purification, and was estimated to comprise 2% of the total composition of the product; on the other hand, the crude NMR showed no trace of monoti n compound or unreacted starting material compound 3 This observation provided a motivation for searching for new purification methods such as neutral or basic alumina filtration

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85 instead of flash column, since most of the impurities in this reaction was s tannanamine, which should be readily absorbed by alumina. The preparation of compounds 5 and 6 was performed under traditional bromination conditions using NBS and bromine, respectively. The difficulty of purifying compound 5 was reduced by using highly pu re compound 3 and recrystallized NBS as starting materials, affording a high reaction yield of about 90%. However, the purification of compound 6 is more challenging as the recrystallization of the crude product does not eliminate the light yellow impuriti es. Column chromatography was always applied in order to achieve a white crystalline compound 6 but this process consumed a large amount of solvent because of the low solubility of compound 6 in hexane, which is the only proper eluent for this separation (more polar eluent will extract the impurities and wash them out). A more efficient and low cost chromatography technique solved this problem. As shown in Figure 3 5, the impure solid mixture was dispersed on top of silica gel in an addition funnel. Then h exane was refluxed, condensed and dripped on the crude product. The extracted materials were passed through the silica gel, and the impurities were absorbed by stationary phase (silica gel). Eventually, pure compound 6 was recovered in the round bottom fla sk. By recycling the solvent hexane, a large scale purification of 6 can be achieved in a cost effective manner. It is necessary to emphasize the importance of obtaining a high purity level of monomer compounds 4 5 and 6 Because the Stille polymerizatio n is a step growth polymerization, a high conversion is required in order to achieve high molecular weight polymers. Without considering the reaction time, which is assumed to be infinite in this case, the stoichiometric control of the bifunctional monomer s becomes the only factor

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86 that can affect the number average degree of polymerization (X n ) In other words, the existing non functional and monofunctional monomers, the impurities, and inaccurate weighing of the starting materials may cause a deviation fro m stoichiometric balance of the functional groups, thereby lowering the polymer molecular weight and expanding the polydispersity index. Figure 3 5. Demonstration of purifying compound 6 by recycling the eluent hexane. Th e condensed hexane will drip on the impure compound 6 ; extract the pure compound, which passes through the silica gel. The impurities are absorbed by the silica gel. 3.2.2 Random Stille Polymerization with Multi Monomers As describe d in the general int roduction (C hapter 1, S ection 1.3.4), to produce a random step growth polymerization, a minimum of three monomers is required when using bifunctional monomers bearing the same reaction functionality. In this case, we

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87 chose ditin compound 4 and two dibromo compounds 5 and 6 to simulate the ratios of donors and acceptors. As long as the number of Br functionality equals that of the SnBu 3 functionality, the Stille polymerization will proceed without any difficulty. Moreover, judging by the reactive sites as well as the chromophore cores of the monomers, where monomer 4 and 5 are sharing the same core unit structure (ProDOT (CH 2 OEtHx) 2 Figure 3 2), there will always be an odd number of ProDOT units between any two BTD units in the same polymer backbone. The polymerization was accomplished using Stille polymerization with Pd 2 (dba) 3 /P(o tolyl) 3 as the catalyst system and toluene as the solvent. Five polymerizations with different donor acceptor ratios were carried out in order to determine optimal ratio to prod uce a polymer absorbing broadly and evenly across the entire visible region. Before being characterized, all the polymers were purified via a Soxhlet extraction process to remove any unreacted monomer, low molecular weight oligomers and polymers as well as palladium catalyst T he polymers are highly soluble in common organic solvents such as chloroform, DCM, THF and toluene (> 10 mg/mL). Table 3 1 shows the GPC results and elemental analysis results for these polymers. All the polymers had number average mo lecular weights ( M n ) from 10.5 to 14.4 kDa, except for ECP 1 at around 9.7 kDa. Due to the successful elimination of low molecular weight materials by Soxhlet extraction, the PDI of the polymers ranges from 1.3 to 1.6, which is lower than the PDI of a typi cal step growth polymerization. The low PDI also indicates that all the chains in the same polymer have similar conjugation lengths and thus the same optical and electronic properties. Furthermore, a good agreement of the

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88 elemental analysis results with th e calculated values confirmed that the monomers were polymerized according to the ratios predetermined. Table 3 1. GPC estimated molecular weights in THF and elemental analysis of the polymers (Adapted with permission from Ref. 202 Copyright 2010 WILEY V CH Verlag GmbH & Co. KGaA, Weinheim) Monomer 4 ratio Monomer 5 ratio (x) Monomer 6 ratio (y) M n (kDa) M w (kDa) PDI EA (Calcd/Found) C H N ECP 1 1 0.6 0.4 9.7 15.5 1.6 67.08/66.79 9.51/9.22 1.48/1.39 ECP 2 1 0.7 0.3 10.9 15.2 1.4 67.70/66.99 9.23 /9.10 1.07/1.12 ECP 3 1 0.75 0.25 10.5 14.3 1.4 67.83/64.01 9.31/9.10 0.87/0.84 ECP 4 1 0.76 0.24 11.0 14.2 1.3 67.86/66.88 9.32/9.24 0.84/0.96 ECP 5 1 0.8 0.2 14.4 19.0 1.3 67.97/63.44 9.38/8.87 0.69/0.75 3.2.3 Absorption Control of Polymers by Varyi ng the Monomer Ratios The UV visible absorption spectra of the polymers in chloroform are shown in Figure 3 6a. A broad spectral absorption is evident ranging from approximately 400 nm to greater than 700 nm for each polymer. Unlike the spectra of typical donor acceptor polymers, which generally have two distinct absorption bands, the spectra of polymers ECP 1 to ECP 5 and long wavelength optical transitions, and no obvious peak to peak window is observed. As expected, ECP 1 exhibits the lowest intensity of the short wavelength absorption at 461 nm due to its relatively low concentration of electron rich moiety. Considering the reduced absorption of blue and green light (400 580 nm), as well as an absence of far red light abso rption from 700 to 750 nm, it is reasonable that the solution of ECP 1 gives a midnight blue color. By increasing the relative amount of ProDOT (CH 2 OEtHx) 2 repeat unit, the difference in intensity between the two absorption bands is reduced. At a specific point where the donor/acceptor ratio is approximately 7 to 1, the intensities of the two bands are balanced, and homogenous absorptions across most of the visible spectrum (450 650 nm) are observed for ECP 3 and ECP 4 Not surprisingly, solutions of ECP 3 and

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89 ECP 4 show a similar dark purple black color due to the lack of absorption in the far blue and red regions. Accordingly, the absorption of ECP 5 shows a higher intensity of the high energy transition (530 nm) than the low energy transition (572 nm) as the increase of the donor concentration, and the solution of ECP 5 exhibits a bright purple color due to the increased reflection/transmission of red light a) b) Figure 3 6. Uv vis absorption spectra o f polymer s. a) Solution absorption of Polymer ECP 1 (monomers ratio: x=0.6, y=0.4) ECP 2 (x=0.7, y=0.3) ECP 3 (x=0.75, y=0.25) ECP 4 (x=0.76, y=0.24) ECP 5 (x=0.8, y=0.2) in chloroform The inset shows the various colors obtained across the polymer seri es. b) Normalized film absorption of Polymer ECP 1 ECP 2 ECP 3 ECP 4 and ECP 5 (Adapted with permission from Ref. 202 Copyright 2010 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim)

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90 Thin films of each polymer were then spray cast onto indium tin oxide (ITO ) coated glass slides from toluene solution (2 mg mL 1 ). The visible absorption spectra of the films shown in Figure 3 6b exhibit a broadening of the long wavelength absorption with little to no change at the higher energy end in the solid state compared t o solutions for all five polymers. The significant extension of absorption into the far red region is important, as it leads to a more pure black color of the films. As determined from the onset of their neutral state lower energy optical transitions, the polymers have relatively low band gaps ranging from 1.6 1.7 eV. From these results, we observe that the absorptions of the polymers are sensitive to small changes in the donor acceptor ratios. For example, by lowering the acceptor ratio from 0.25 to 0.24, ECP 4 shows a slightly narrowed absorption with an observable increase in the intensity of the high energy transition compared to ECP 3 in both the polymer solution and the solid film. However, this change can be detected only by a UV Vis spectrophotomete r and it is not distinguishable by the human naked eye. 3.2.4 Polymerization Repeatability The importance of polymerization repeatability has been discussed in Chapter 2. A detailed study on the repeatability of the synthesis of the black to transmissive ECPs was carried out. The GPC results in Table 3 2 show that all the polymers had similar molecular weights, a crucial parameter related to the polymer solution viscosity. Considering the applications of these polymers, which require the polymers to be pro cessed from a solution into solid films by printing or spray casting, it is important to reproduce polymers in batches with minimal deviations in molecular weights and with similar solution viscosities. Furthermore, it is worth mentioning that the polymeri zation is also scalable. For example, 3.3 g of ECP 4 s2 was prepared successfully in our lab,

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91 and our collaborators in BASF were able to reproduce polymers with the same monomer ratio as ECP 3 in a 30 g scale (C hapter 2, S ection 2.2 for more details). Tab le 3 2. GPC estimated molecular weights of reproduced polymers. Monomer 4 ratio Monomer 5 ratio (x) Monomer 6 ratio (y) M n (kDa) M w (kDa) PDI Yield (%) ECP 3 1 0.75 0.25 10.5 14.3 1.4 50 ECP 3 s1 1 0.75 0.25 11.1 14.4 1.3 65 ECP 3 s2 1 0.75 0.2 5 10.2 13.7 1.3 67 ECP 3 s3 1 0.75 0.25 10.5 15.0 1.4 83 ECP 4 1 0.76 0.24 11.0 14.2 1.3 80 ECP 4 s1 1 0.76 0.24 12.7 18.2 1.4 72 ECP 4 s2 1 0.76 0.24 12.0 19.0 1.6 83 ECP 6 1 0.735 0.265 18.0 29.0 1.6 83 ECP 7 1 0.74 0.26 12.0 17.1 1.5 89 Polymers ECP 3 s1 ECP 3 s2 and ECP 3 s3 which share the same monomer feed ratio with ECP 3 are consistent with ECP 3 in terms of number average molecular weight ( M n ) and PDI. More importantly, no obvious difference was detected for the polym er solution absorption by either the spectrophotometer or the human eye ( Figure 3 7a). In another example, polymers ECP 4 s1 and ECP 4 s2 were synthesized utilizing the same monomer ratio as polymer ECP 4 As expected, the three polymers showed the same so lution absorption in the visible region (Figure 3 7b). Moreover, two additional random copolymers ECP 6 and ECP 7 were prepared with a small variation in the monomer feed ratio (Table 3 2). In Figure 3 7c, the absorption of ECP 6 which has a lower concen tration of donor moieties, shows a lower intensity for the high energy transition (530 nm) compared the low energy transition (600 nm). By increasing the donor/acceptor ratio from 1.735:0.265 to 1.74:0.26 (the donor ratio here is the sum of monomer 4 and m onomer 5 ratios), polymer ECP 7 showed relatively higher absorption intensity in the high energy transition. The results

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92 here confirmed the observation that the polymer absorption feature is really sensitive to a small change in the polymer structure ( S ect ion 3.2.3). a) b) c) Figure 3 7. Normalized solution absorption of polymers in chloroform a) EPC 3 EPC 3 s1 EPC 3 s2 and EPC 3 s3 ( x=0.75, y=0.25). b) ECP 4 ECP 4 s1 and ECP 4 s1 (x=0.76, y=0.24) c) ECP 6 (x=0.735, y=0.265) and ECP 7 (x=0.74, y=0.26). 3.2.5 Polymer Thermal Analysis The thermal stability of ECP 3 and ECP 4 was studied by TGA in a nitrogen atmosphere. The thermogram s displayed in Figure 3 8 show that the polymers exhibit a high thermal stability until the temperature reaches 330C, and then a drastic

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93 degradation process occurs from this temperature up to around 600C. Moreover, the two polymers processed the same the rmal profile, indicating that the thermal stability is not affected by changing the polymer donor acceptor ratio. Figure 3 8. Thermogravimetric analysis of polymer ECP 3 and ECP 4 The experiments were carried out using high resolution dynamic scans un der nitrogen atmosphere. 3.3 Polymer Electrochemistry and Spectroelectrochemistry 3.3.1 Electrochemistry Studies The redox properties of EPC 3 were studied by electrochemistry. The polymer was deposited on a Pt button electrode by drop casting from a dilu te toluene solution. Differential pulse voltammetry (DPV) was recorded in 0.1 M TBAPF 6 /propylene carbonate (PC) electrolyte solution with a Pt foil counter electrode and a Ag/Ag + reference electrode. An onset of oxidation (E onset,ox ) of 0.04 V and reducti on (E onset,red ) of 1.64 V vs. Fc/Fc + were determined (Figure 3 9). According to the results, the polymer has a HOMO energy level of about 5.1 eV, a LUMO energy level of 3.5 eV and a band gap ( E g ) of 1.6 eV, which agrees well with the optical band gap (1 .6 eV) determined from the onset of the polymer film neutral state lower energy optical transition ( S ection 3.2.3)

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94 a) b) Figure 3 9. Differential pulse voltammetry of ECP 3 dr op cast from toluene solution onto platinum button electrode (A= 0.02 cm 2 ). Measurements were performed in 0.1 M TBAPF 6 /propylene carbonate (PC) with a Pt foil counter electrode and a Ag/Ag + reference electrode calibrated vs. Fc/Fc + (E Fc/Fc+ = 0.087 V vs. Ag/Ag + ). a) Oxidation. b) Reduction. (Adapted with permission from Ref. 202 Copyright 2010 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim) 3.3.2 Spectroelectrochemistry Studies Spectroelectrochemical behavior of an ECP 3 thin film is shown in Figure 3 10a. The spray cast film was redox cycled to a stable and reproducible switch prior to the analysis, and then electrochemically oxidized from 0.25 to +0.35 V vs. Fc/Fc + (in 25 mV steps) using a silver wire as a quasi reference electrode (calibrated against Fc/Fc + ) and a platinum wire as the counter electrode. Upon oxidization of the polymer, the broad absorption in the visible region is depleted, and a polaronic transition in the near IR (800 1200 nm) arises, and then falls and merges into a bipolaronic transition which appears further into the NIR region. When fully oxidized, the generated bipolaronic absorption peaks beyond 1600 nm, which allows effective bleaching of the visible absorption and a remarkably high level of transmissivity to the human eye. In orde r to further accentuate the optical changes that occur upon oxidation, the electrochromic response is shown in terms of transmissivity across only the visible region in Figure 3 10b from 350 nm to 750 nm. For a film of ECP 3 in its neutral state, a

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95 nearly flat transmittance profile at 20% from 450 nm to 700 nm is observed. A small range of visible blue light (400 450 nm) and red light (700 750 nm) are not absorbed as strongly as the other parts of the visible region. On the other hand, when the film is ful ly oxidized, the transmittance increases to above 60% through most of the visible region. T ) is as high as 48% at 555 nm, the wavelength at which the human eye has greatest sensitivity. Howeve r, the bleaching of the film leaves an uneven transmittance in the visible region; more blue and green light (400 500 nm) is transmitted compared to the remainder of the visible region, giving the film a faint residual color. a) b) Figure 3 10. Spectroelectrochemical behavior of an ECP 3 thin film. a) Spectroelectrochemistry of ECP 3 The films were spray cast onto ITO coated glass from toluene (2 mg mL 1 ). Electrochemical oxidati on of the films was carried out in 0.1 M LiBTI/PC, supporting electrolyte using a silver wire as a quasi reference electrode (calibrated against Fc/Fc + ), and a platinum wire as the counter electrode. The applied potential was increased in 25 mV steps from 0.25 to + 0.35 V vs. Fc/Fc + b) Electrochromic response in terms of transmissivity in the visible region (replotted from Figure 3 10a). (Adapted with permission from Ref. 202 Copyright 2010 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim)

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96 3.4 Polymer Colorim etric Analysis To evaluate the color changes of the ECPs occurring on electrochemical switching L a b color standards), three ECP 3 films with varying thicknesses were subjected to color imetric analysis. Here, L* represents the lightness of the color (0= black, 100= diffuse white), a* represents how much red versus green (red for + a* values and green for a* values), and b* represents how much yellow versus blue (yellow for + b* values and blue for b* values). Figure 3 11a shows the determined CIE 1976 L values as a function of applied voltage. This gives an indication of relative brightness of the film as it is oxidized while illuminated from behind (transmission mode) with a standard D50 simulated daytime light source. In their neutral state, the polymer films exhibit L values from 46 for the thickest film to 75 for the thinnest film. Importantly, the film with absorption maximum of 1.1 a.u. displays a deep black color with a and b val ues as low as 3 and 11. This observation is consistent with the visible absorption spectra of the ECP 3 thin film as a small amount of red and slightly more blue light is transmitted by the polymer. In comparison, the fully oxidized polymer films exhibit high L values from 82 to 92 with smaller a and b values, demonstrating that this polymer is able to reach a highly transmissive near colorless state as defined by the L a b color coordinates. Moreover, based on the trace of the L change, all the films start to be oxidized at around 0.1 V vs. Fc/Fc + which agrees well with the polymer onset of oxidation ( S ection 3.3.1). Also, the films reach their high transmissive state in a 0.6 V window.

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97 a) b) c) Figure 3 11. Colormetric analysis of ECP 3 thin films. a) Lightness ( L* ) as a function of applied potential for spray coated ECP 3 L*a*b* values of fully neutral and oxidized states are reported for the films. Photographs are of the fully neutral (left) and fully oxidized films (right). (Adapted with permission from Ref. 202 Copyright 2010 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim) b) Plot of CIE 1976 a b color coordinates showing calculated values of ECP 3 as a function of applied potential. c) CIE 1976 a b color coordinates showing calculated values of the seven vibrantly colored ECPs (orange triangle, ECP orange; red square, ECP red; purple circle, ECP magenta; blue right facing triangle, ECP bl ue; teal left facing triangle, ECP cyan; green, diamond, ECP green; and yellow downward facing triangle, ECP yellow) as a function of applied potential. The polymer neutral states are furthest from the origin and the values track toward the origin as the p olymer is oxidized to the bleached state. (Adapted with permission from Ref. 136 Copyright 2011 American Chemical Society)

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98 Figure 3 11b shows a typical a and b color track of ECP 3 film during oxidation (A= 0.7 a.u.). As we can see, the color of the neut ral polymer film falls into a red blue region, which presents a dark purple color ( a *= 4, b *= 13). As the polymer is oxidized, the a and b values decrease, indicating a loss of color, with the fully oxidized state having a highly transmissive faint blue green tint ( a *= 3, b *= 5). It is worth mentioning that a b values of ECP 3 are changed in a rather small range, which is close to the achromatic point ( a *= 0, b *= 0), while a b values of electrochromic polymer s ECP orange, ECP red, ECP magenta ECP bl ue ECP cyan, ECP green and ECP yellow are reduced in magnitude as the saturation of the color is switched to a bleached state (Figure 3 11c). 136 3.5 Polymer Switching Study 3.5.1 Polymer Switching Rate Given that the speed at which electrochromic materi als change color states is important in display type devices, the film switching rate was examined by monitorin g T ) as a function of time by applying square wave potential steps for periods of 10, 2 and 1 s. As shown in Figure 3 12a, a transmittance change (monitored at 540 nm) as high as 47% is record ed at the longer switch time (10 s). By decreasing the switch time to 1 s, the transmittance change is reduced to 42% with a 5% contrast lost. Moreover, the switching rate was limited by the diffusion and migration of charge balancing counter ions within the polymer films because the electronic and ionic effects are strongly coupled. Thus the largest fraction of the redox process usually occurs within the first portion of the voltage pulse. Considering this, the switching times to reach anywhere from 90 to 95 or 98% of the full optical contrast, which can be

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99 considerably shorter than that of the full optical switch, are also reported in many display characterizations. In this case, by taking a close look at the feature of a 10 s switching time cycle, it tak T = 45%) when the polymer is switched from its colored state to bleached state. Interestingly, the time for a 95% switching contrast from fully bleached state to colored state is much less than the time for the rever se process (0.8 s compared to 1.6 s), which is possibly due to the repulsion from the polymer network on the counter ions. Clearly, it is much easier to eliminate the counter ions than to absorb them. a) b) Figure 3 12. Switching speed study of polymer ECP 3 a) Square wave potential step chronoabsorptometry of ECP 3 spray coated on ITO (monitored at 540 nm, 0.72 to + 0.48 V vs. Fc/Fc + in 0.1 M LiBTI/PC electrolyte solution). Th e step times (10 s, 2 s and 1 s) are indicated on the figure. b) Percent transmittance and time to reach 95% of the full optical contrast. (Adapted with permission from Ref. 202 Copyright 2010 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim) 3.5.2 Polymer Switc hing Stability The long term stability to repeated redox switching is essential to the practic al T at 540 nm) of a film was monitored while repeated square wave potential steps of 1.5 s (complete

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100 cycle is 3 s, switching between 0.72 to + 0.48 V vs. Fc/Fc + ) for 18,000 cycles were applied. As shown in Figure 3 13, polymer ECP 3 exhibits a continuous switching stability with a decrease of only 8% in electrochromic contrast over this time period, most of which happening in the beginning of the experiment. Moreover, it is necessa ry to point out that the stability study was performed in a LiBTI/PC solution and the system was not properly sealed. The stability of the polymer will possibly improve by keeping oxygen and water out of the system. Figure 3 13. Long term stability stud y via square wave potential stepping while monitoring electrochromic switching of ECP 3 at 540 nm in 0.1 M LiBTI/propylene carbonate solution switching between 0.72 to + 0.48 V ( vs. Fc/Fc + ), and with a switch time of 18,000 cycles (1.5 s step). In blue: % T of ox T as a function of the number of cycles. (Adapted with permission from Ref. 202 Copyright 2010 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim) 3.6 Electrochromic Devices 3.6.1 Black to Transmissive Electrochromic Windo w To further reinforce the utility of these polymers, we have demonstrated black to transmissive switching in an absorptive/transmissive window type electrochromic device. Using the d evice structure as detailed in C hapter 1 ( S ection 1.6.1, Figure 1

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101 16a), t he polymer ECP 3 was utilized as the active switching material at the working electrode, and a minimally color changing polymer (N alkyl substituted poly(3,4 propylenedioxypyrrole)), which is similar to that previously described by our group, 150 was used a s the charge balancing material at the counter electrode. As shown in Figure 3 14a, the transmittance spectra of the device in both the dark state and transmissive state were measured across the visible region from 400 to 750 nm. Higher transmittance in th e blue region is observed when the polymer is in the oxidized state, giving a highly transmissive light blue color. Conversely, when the polymer is neutralized at a device cell potential of 0.6 V, the transmittance decreases to a near featureless spectral profile. The device contrast is about 40% at 555 nm. a) b) Figure 3 14. Window type and display type electrochromic devices using ECP 3 as an active layer. a) Transmittance spectra of ECD in extreme states of highly absorptive (at an applied cell voltage of 0.6 V) and highly transmissive (at an applied cell voltage of +1.6 V). The insets show photographs of the same device in both states. b) Reflectance spectra of ECD in extreme sta tes of highly absorptive (at an applied cell voltage of 2.0 V) and highly reflective (at an applied cell voltage of 1.5 V). The insets show photographs of the same device in both states. (Adapted with permission from Ref. 202 Copyright 2010 WILEY VCH Verl ag GmbH & Co. KGaA, Weinheim)

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102 3.6.2 Black to Transmissive Electrochromic Display Additionally, an absorptive/reflective display type device was constructed utilizing polymer ECP 3 as the active electrochrome (Figure 3 14b). When oxidizing potentials are ap plied across the device, the reflectance in the visible region begins to increase as the polymer becomes faded, which allows more light to pass through the film and be reflected by the TiO 2 layer. A reflectance change of 25% was observed at 555 nm. 3.7 Ch apter Summary The incorporation of the donor acceptor theory with the random Stille polymerization method allows precise control of the absorption spectra of conjugated polymers. By varying monomer feed ratios, black to transmissive ECPs with broad and uni form visible absorptions were synthesized and characterized. More importantly, this type of polymerization was proved to be an efficient way to produce ECPs with highly reproducible M n optical and electronic properties between different runs. The resultin g polymer showed a high optical contrast (> 40%), rapid redox switching (1 s for % T = 42%) and long term redox stability (>18,000 cycles). The performance of the polymers was further evaluated through the construction of a series of ECDs and showed promising results. While the project described in this chapter was underway, the synthes is of a new black to transmissive ECP was reported by nal and coworkers. 198 The polymer was afforded via electrochemical polymerization of two donor acceptor systems, 2 decyl 4,7 bis(3,3 didecyl 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepin 6 yl) 2H benzotria zole and 2 decyl 4,7 bis(3,3 didecyl 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepin 6 yl) 2,1,3 benzoselenadiazole in a 1:4 feed ratio (Figure 3 15). Surprisingly, the polymer did not show encouraging EC properties. Although the polymer could be switched from a dark

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103 black neutral state to a dark gray oxidized state, the transmittance change was only 15.3% ( T % from 7.8% to 23.1%) when fully oxidized. From a chemistry point of view, it is difficult to achieve homogenous polymers via electropolymerization. For ins tance, there are always unreacted monomers, small oligomers as well as electrolytes and solvents, trapped in the polymer networks. Moreover, control of the polymerization process is difficult, and thus conjugated polymers with defects, cross linked sites a nd large PDIs are usually generated. All these drawbacks may be the main reason that the material prepared by nal et al. did not perform well in the switching study. 198 Figure 3 15. Synthesis of copolymer via electroche mical polymerization. Photographs are of the fully neutral (left) and fully oxidized films (right). (Adapted and modified with permission from Ref.198) However, this report also leads to a new idea for making perfect black to transmissive ECPs. Considering the absorption of ECP 3 which failed to cover part of

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104 the blue light (380 450 nm) and the red light (690 750 nm), it was logical that the polymer films showed a noticeable black purple hue. How to fill the absorption gaps has become an issue for the nex polymer structure demonstrated in the report discussed above, the authors coordinated two different acceptors (benzotriazole and benzoselenadiazole) and one donor (ProDOT) in the polymer backbone t o achieve a broad absorption. According to the expected to expand the low energy absorption band to a much higher wavelength, while an extra weaker donor (for example d ioxythiophene) will lower the high energy absorption band wavelength. The random Stille polymerization introduced in this chapter is expected to be an optimal route towards achieving such a system. With regard to tuning the absorption band, the modificatio n of polymer side chains is another interesting direction. By putting different side chains on polymers, the polymer solubility properties are changed, as well as its optoelectronic properties. One particular example is affording polymers with water solubi lity by grafting water soluble functional groups such as sulfonate, carboxylate, phosphonate and ammonium This strategy will be investigated more thoroughly in Chapter 4. 3.8 Experimental Details 3,3 Bis(bromomethyl) 3,4 dihydro 2H thieno[3,4 b][1,4]dioxe pine ( ProDOT(CH 2 Br) 2 2) 3,4 dimethoxythiophene (9.52 g, 66 mmol), 2,2 bis(bromomethyl)propane 1,3 diol (35.5 g, 136 mmol), p toluenesulfonic acid (1.26g, 6.6 mmol), and 300 mL toluene were combined in a 500 mL flask equipped with a soxhlet extractor with molecular sieves in a cellulose thimble. The solution was refluxed for 1 day. The reaction mixture was cooled, and a fter dilution with ether (100 mL), the

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105 organic phase was washed with water and dried ov er anhydrous magnesium sulfate. The solvents were re moved under vacuum, and the crude product was purified by column chromatography with hexane / methylene chloride (4:1) as an eluent to obtain compound 2 as a white crystalline solid ( 22.5 g, yield: 8 0%) 1 H NMR (CDCl 3 ) 6.49 (s, 2H), 4.10 (s, 4H), 3.6 1 (s, 4H). 13 C NMR (CDCl 3 ) 148.84, 105.95, 74.33, 46.37, 34.62. HRMS: m/z calcd for C 9 H 10 Br 2 O 2 S (M + ): 339.8768 found: 339.8796. Anal. calcd for C 9 H 10 Br 2 O 2 S: C 31.60, H 2.95, found C 31.78, H 2.90. 3,3 Bis((2 ethylhexyloxy)methyl) 3,4 dihydro 2H thieno[3, 4 b][1,4]dioxepine ( ProDOT(CH 2 OEtHex) 2 3) To a 250 mL flame dried round bottom flask filled with 120 mL of DMF, 2 ethylhexan 1 ol (15.3 g, 117 mmol), and compound 2 (10 g, 29.2 mmol), NaH (5.3 g, 131 mmol) was added in portion. The reaction mixture w as t hen heated at 110 C for overnight. After completion, the mixture was cooled and added into ice water slowly and extracted three times with ethyl ether. The organic layer was then washed three times with water and dried over magnesium sulfate, and the solve nt was removed by rotary evaporation under reduced pressure. The resulting crude product was purified by flash column chromatography with hexane as an eluent to obtain compound 3 as colorless oil ( 10 g, yield: 78 %) 1 H NMR (CDCl 3 ): 6.43 (s, 2H), 4.02 (s, 4H), 3.50 (s, 4H), 3.31 (d, 4H, J = 5.7 Hz) 1.5 8 0.86 (m, 30H) 13 C NMR (CDCl 3 ): 149.95, 105.06, 74.46, 73.91, 70.10, 48.11, 39.90, 30.97, 29.40, 24.30, 23.36, 14.35, 11.40. HRMS: m/z calcd for C 25 H 45 O 4 S (M+H + ): 441.3033 found 441.3049. Anal. C alcd for C 25 H 44 O 4 S: C 68.14, H 10.06, found C 68.50, H 10.17. 6,8 Bis(tributylstannyl) 3,3 bis((2 ethylhexyloxy)methyl) 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine (4) A solution of n butyllithium (16.0 mL, 20.4 mmol, 1.28

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106 M in hexane) was added slowly to d iisopropylamine (2.06 g, 20.4 mmol) in ether (50.0 mL) at 78 C. After stirring for 1 h at 78 C the solution was warmed to room temperature and then cooled to 0 C in an ice bath. Compound 3 (3 g, 6.81 mmol) was added, and the mixture was warmed to room t emperature and stirred overnight. After the reaction mixture was cooled to 0 C again, tributylchlorostannane (6.64 g, 20.4 mmol) was added. The mixture was stirred for 3 h at 0 C After dilution with ether (100 mL), the organic phase was washed with water and dried over anhydrous magnesium sulfate. The solvent was evaporated and the residue was yellow oil and purified by flash chromatography with hexane as an eluent on treated silica gel (washed the silica gel with neat triethylamine, then hexane) to give 6 .56 g (95%) of the title compound as colorless oil. 1 H NMR (CDCl 3 ): 3.89 (s, 4H), 3.46 (s, 4H), 3.30 (d, 4H, J = 5.4 Hz), 1.60 0.87 (m, 84H). 13 C NMR (CDCl 3 ): 156.59, 122.64, 74.42, 73.56, 70.40, 47.91, 39.84, 30.91, 29.37, 29.28, 27.48, 24.23, 23 .33, 14.34, 13.94, 11.33, 10.80. Anal. calcd for C 49 H 96 O 4 SSn 2 : C 57.77, H 9.50, found C 58.04, H 9.62. 6,8 Dibromo 3,3 bis((2 ethylhexyloxy)methyl) 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine (5) Compound 3 (2.0 g, 4.54 mmol) and NBS (2.02 g, 11.35 mmol) we re added into a round bottom flash and then degassed and refilled with argon 3 times. DMF (20 mL) was then added. The reaction mixture was stirred at room temperature for overnight, diluted with ethyl acetate (50 mL), and washed with water (2 times with 20 0 mL). The organic layer was dried with MgSO 4 After the solvent was evaporated, the residue was purified by column chromatography with hexane as an eluent to obtain compound 5 as colorless oil (2.44 g, yield: 90%). 1 H NMR (CDCl 3 ): 4.08 (s, 4H), 3.48 ( s, 4H), 3.29 (d, 4H, J = 5.7 Hz), 1.56 0.86 (m, 30H). 13 C NMR

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107 (CDCl 3 ): 147.23, 91.03, 74.55, 74.50, 69.94, 48.21, 39.82, 30.86, 29.34, 24.20, 23.31, 14.33, 11.37. HRMS (ESI FTICR): m/z calcd for C 25 H 42 Br 2 KO 4 S (M+K + ) 635.0802 found 635.0798. Anal. calc d for C 25 H 42 Br 2 O 4 S: C 50.17, H 7.07, found C 50.47, H 7.20. 4,7 D ibromobenzo[c][1,2,5]thiadiazole ( 6 ) 203 In a 500 mL three neck round bottom flask with benzothiadiazole (19.6 g, 144 mmol) and 150 mL of HBr (47%), a 100 mL of HBr solution containing Br 2 (68 .9 g, 432 mmol) was added dropwise. After total addition of the Br 2 the solution was refluxed for 6 h. Precipitation of an orange solid was noted. After cooling to room temperature, the mixture was filtered under vacuum and washed with saturated solution of NaHSO 3 for 3 times (100 mL solution per time) and then water for 5 times. The solid was then dried under reduced pressure and purified by column chromatography with hexane as an eluent affording compound 6 as a white crystalline solid ( 31.5 g, yield: 75 %) 1 H NMR (CDCl 3 ): 7.73 (s, 1 H) General polymerization procedure Polymer ECP 1 : A solution of compound 4 (0.509 g, 0.5 mmol), compound 5 (0.180 g, 0.3 mmol), compound 6 (0.059 g, 0.2 mmol), tris(dibenzylideneacetone)dipalladium (0) (9 mg, 0.01 mmol) a nd tri(o tolyl)phosphine (12 mg, 0.04 mmol) in toluene (20 mL) was degassed three times by successive freeze pump thaw cycles and heated at 100 C for 36 h in an oil bath The solution was then precipitated into methanol (300 mL). The precipitate was filter ed through a cellulose thimble and purified via Soxhlet extraction for 24 hours with methanol and then 48 hours with hexane. The polymer was extracted with chloroform, concentrated by evaporation, and then precipitated into methanol again (300 mL). The col lected polymer was a black solid (0.29 g, 76 %). 1 H NMR (300 MHz, CDCl 3 8.20 (m, 2H), 4.17 4.00 (m, 16H), 3.55 (bs, 16H), 3.27 (bs, 16H), 1.45 1.05 (m, 72H), 0.84 (bs, 48H). GPC analysis:

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10 8 M n = 9,700, M w = 15,500, PDI = 1.6. Anal. Calcd. for ECP 1 : C 67.08, H 9.51, N 1.48, found C 66.79, H 9.22, N 1.39. General proced : first, unscrew the locks, disassemble the device by removing the stainless steel top, the glass cylinder wall and the o ring on the bottom. Second, put a right size Nylon f ilter paper on the bottom and reasse mble the device. The polymer solid as well as the solvents can be added through the hole with a cap on the steel top. After stirring the polymer suspension for 1 2 hours, filter the mixture under high pressure gas (do not exceed the pressure limit). Then a dd more solvent for the next cycle.

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109 CHAPTER 4 WATER SOLUBLE BLUE AND B LACK TO TRANSMISSIVE SWITCHI NG ELECTROCHROMIC POLYM ERS 4.1 Motivations for Water Soluble Electrochromic Polymers The utilization of conjugated polymers is significantly influenced by their optoelectronic properties as well as their solution processability. In this regard, the fine tuning of the desired properties by synthetic modification of the polymer backbones and the nature of the pendant groups is perhaps the most widely used tool. 15 Despite there being a large number of conjugated polymers with diverse material properties, which have been synthesized and evaluated for applications such as OLEDs, 209 211 OPVs, 167,171,212 FETs 213,214 and ECDs 7,15,202,215 only poly(3,4 ethylenedi oxythiophene): poly(styrene sulfonate) (PEDOT:PSS) has been particularly commercially successful. This is largely due to its highly processable nature, which affords it the ability to be processed from an aqueous solution yielding thin films with a high co nductivity. 117,216,217 In addition, the high contrast between a deep blue neutral state and a highly transmissive oxidized state makes PEDOT:PSS an excellent candidate for electrochromic applications. 151,218 Combination of the water processable features of PEDOT/PSS with the optoelectronic properties of the electrochromic polymers (ECPs) is necessary to produce water soluble ECPs for fabrication of ECDs with minimal processing costs and environment impact. Water soluble conjugated polymers (WS CPs), wh ich are also referred to as conjugated polyelectrolytes (CPEs), were first introduced by Wudl and Heeger et al. 219 in the synthesis of polythiophene with sulfonate derivatives, and also by our group for polypyrroles. 220 Considering the unique hydrophilicit y of these materials, a combination of a fully conjugated polymer backbone and ionic pendant groups such as sulfonate (

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110 SO 3 ), carboxylate ( CO 2 ), phosphonate ( PO 3 ) and ammonium ( NR 3 + ) must be achieved. In parallel, like most of the organic soluble CPs WS CPs can be made via a variety of existing polymerization reactions, which, including electrochemical polymerization, 221,222 oxidative polymerization 223,224 and transition metal mediated polymerization such as Suzuki and Sonogashira polymerizations. 225 227 Although a large number of WS CPs have been prepared, the investigation of such polymers has been mainly focused on applications in chemical and biological sensors due to the high water solubility and high optical sensitivity. 228 230 Unfortunately, th e use of WS CPs in optoelectronic devices such as PLEDs and ECDs, has gained limited success. Within these contributions, several polymers have been proved as excellent materials for electron injection layers (EILs) in making multi layer PLEDs. 231 233 For example, conjugated polyelectrolyte PF PEO CO 2 Na was synthesized and used as an EIL in PLEDs by Nguyen et al 234 A luminance response time of microseconds was achieved by a combination of thermal and voltage treatments, aiming to control the ion motion at th e diphenylene vinylene) and its rotaxinated derivatives have been shown to attain a high degree of alignment in stretched PVA films, and to emit highly polarized light when they were excited by a pulsed d iode laser. 235 In this contribution, we focus on the preparation of water processable conjugated polymers which transmit or reflect between two or more different color states upon redox activity. More specifically, we are in pursuit of polymer electrochro mes with the following properties: 1) can be processed from an aqueous solution into solid films by existing print or spray technologies; 2) can be switched form a strongly colored neutral

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111 state to a highly transmissive oxidized state; 3) exhibit short swi tching time (sub second) without losing contrast when employed into electrochromic displays. To fulfill these three requirements, several approaches have been proposed. In a previous paper, our group presented the layer by layer (LbL) deposition of alkoxy sulfonated poly( p phenylene) (PPP OPSO 3 ) in conjunction with multiple types of conjugated or non conjugated polyelectrolytes. 236 Combining the optoelectronic properties and the advantages of LbL deposition, which promotes uniform thin films with a molecula r level thickness control, the polymer has been demonstrated to be a good candidate for PLEDs. Subsequently, a detailed study of alkoxy sulfonated PEDOT polyelectrolyte was reported by our group. 188,237 Multilayer films based on the polymer and poly(allyl amine hydrochloride) (PAH) were prepared by the LbL method and the electrochromic properties of the films were extensively characterized. Furthermore, a regioregular water soluble polymer based on 3,4 propylenedioxythiophene (ProDOT) was synthesized by Kum ar et al 238 It is worth mentioning that the solid state electrochromic devices consisting of PProDOT sultone/PAH bilayer films demonstrated fast switching times due to the rapid movement of ions in and out of the films, in agreement with the same observat This chapter describes the synthesis of a new ProDOT monomer bearin g four carboxylate ester groups ( Figure 4 1). Incorporation of the ProDOT monomer and BTD units has successfully yielded two electrochromic po lymers: ECP Blue (a donor acceptor alternating copolymer) and ECP Black (a donor acceptor random copolymer). Subsequent deprotection of the ester group afforded polymer carboxylate salts with high

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112 water solubility. The electrochromic properties of the neut ralized films were evaluated and showed promising sub second switching times. Figure 4 1. Structures of ProDOT monomer, organic soluble precursors ECP Blue and ECP Black 4.2 Concept and Design of Carboxylic Acid Functio nalized CPEs from Chemically Cleavable Esters Recently, our group reported the synthesis of a new polyProDOT (ECP Magenta) bearing cleavable carboxylate ester functional groups ( Figure 4 2). 239 In this study, the use of a side chain defunctionalization app roach yielded a PProDOT salt homopolymer (WS ECP Magenta) that can be dissolved and processed in water. Upon neutralization of the spray cast thin films, the formed polymer acid became insoluble in both organic solvents and water. Importantly, the resultin g film, which was switched in a KNO 3 /water supporting electrolyte, showed a very small loss in contrast (less than 3%) on going from a switching time of 0.5 s to 0.25 s, as compared with a contrast loss of around 13% from the toluene sprayed PProDOT ester film switched in a LiBTI/PC electrolyte. It is worth mentioning that the synthesis of the organic soluble polymer ester is necessary in this approach, since the polymer can be easily purified (precipitation and Soxhlet extraction) and characterized (NMR, G PC et al. ) in its organic form.

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113 Figure 4 2. Synthesis of ECP Magenta. Side chain defunctionalization yields the polymer carboxylate salt, which can be dissolved in water and spray cast onto ITO coated glass slides. The de posited films are then neutralized by immersion in a p toluenesulfonic acid/MeOH solution to afford the polymer acid. (Adapted and modified with permission from Ref.239 Copyright 2010 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim) However, we also found that it was difficult to extend this synthetic procedure to the preparation of other colored ECPs, which need a secondary repeat unit for fine tuning of the absorption spectra. 121,126,128,197,240 Clearly, the water solubilizing groups on the ProDOT monomer wer e not sufficient to make a copolymer dissolve in water. Moreover, the synthesis of the repeat unit (ProDOT diester), which involved four steps of organic synthesis, was rather complicated. In particular, the second reaction,

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114 preparation of the nitrile comp ound of ProDOT, required a 10 day reaction and the use of the highly toxic compound sodium cyanide. Therefore, it was necessary to find a new ProDOT repeat unit with modified pendant groups, which can be synthesized easily and provide strong water solubili ty in the copolymer chain after being defunctionalized. 4.3 Polymer Synthesis and Characterization 4.3.1 Monomer Synthesis The synthesis of the monomers is shown in Figure 4 3. In order to achieve organic processability of the monomers and copolymers befor e the defunctionalization process, 2 ethylhexyl alkyl chain was used as protecting groups in the ProDOT unit. Compounds 1 and 2 were synthesized according to the previously reported methods. 119,241 The nucleophilic substitution of 1 with 2 under basic cond itions afforded compound 3 in a good yield. It is worth noting that this S N 2 reaction is much slower using traditional oil bath heating The crude reaction mixture was predominantly the monosubstituted byproduct and the starting material (compound 1 ) after at 120 C ; only a small amount of the final product was found. Fortunately, by using a microwave reactor and raising the reaction mixture to 140 C the reaction was accomplished with a dramatically reduced reaction time (2 hours) and an im proved reaction yield (74%) after the purification. The subsequent bromination of 3 using NBS afforded compound 4 in a 90% yield, which was originally designed as one of the monomer units in the synthesis of ECP Black On the other hand, the common method in making ditin compound 5 by reacting compound 3 with n BuLi first and then adding Me 3 SnCl also failed, because BuLi reacted with the ester groups as a strong base. Finally, ditin compound 5 was synthesized by coupling 4 with hexamethyldistannane in toluen e at 115 C using Pd(PPh 3 ) 4 as the catalyst.

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115 Figure 4 3. Reaction scheme for the synthesis of monomers. 4.3.2 Polymer Synthesis: A Pendant Group Modification Alternating copolymer ECP Blue (the ester precursor of WS ECP Blue ) was successfully prepared by a Stille polymerization of ditin compound 5 and dibromo benzothiadiazole 8 ( Figure 4 4). After the Soxhlet extraction and re precipitation, the organic soluble polymer ECP Blue presented a satisfactory number average mol ecular weight ( M n ) of 25.8 kDa and PDI of 1.5 (estimated by GPC in THF), and there was good agreement of the elemental analysis results with the calculated numbers ( Table 4 1). The dark blue polymer solid can be dissolved in THF, toluene and chlorinated s olvents with good solubility (> 10 mg/mL).

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116 Figure 4 4. Synthesis of polymer ECP Blue Side chain defunctionalization yields polymer carboxylate salt ( WS ECP Blue ), which can be dissolved in water. Further treatment of th e spray cast thin films affords WS ECP Blue acid that is insoluble in both organic solvents and water. However, the original plan of making ECP Black by coupling compounds 4 5 and dibromo benzothiadiazole in a fixed monomer feed ratio failed. The steric h indrance generated from the bulky side chains on the ProDOT unit prevented a high extent of reaction in this step growth polymerization. As a result, the polymerizations according to this methodology resulted in low reaction yields, as well as polymers wit h low M n at around 7 kDa. In order to achieve high molecular weight conjugated polymers with maximized optical properties, it was necessary to reduce the size of one of the monomers. To pursure this an alternative way was to replace monomer compound 4 by

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117 another ProDOT unit with much smaller side chains. And thus, compound 7 was utilized in the synthesis of ECP Black ( Figure 4 5). In this approach, the polymer M n was increased to 14 kDa. The polymer molecular weights and elemental analysis are summarized i n Table 4 1. Figure 4 5. Synthesis of monomer compound 7 with reduced side chain size and random copolymer ECP Black Table 4 1. GPC estimated molecular weights in THF and elemental analysis of the polymers. Monomer 5 ratio Monomer 7 ratio (x) Monomer 8 ratio (y) M n (kDa) M w (kDa) PDI EA (Calcd/Found) C H N ECP Blue 1 / 1 25.8 38.5 1.5 67.23/66.94 7.52/7.64 2.49/2.42 ECP Black 1 0.7 0.3 13.9 23.3 1.7 67.46/67.11 7.79/7.89 0.73/0.68

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118 To obtain the WS ECPs, th e organic soluble ester precursors ECP Blue and ECP Black were suspended separately in 1 M KOH/methanol, and the mixture was then refluxed overnight under argon. The successful deprotection of the ester group afforded WS ECP blue and WS ECP Black as fine p owdery polymer salts, which were highly soluble in water at room temperature but insoluble in organic solvents such as toluene, THF and chloroform. Furthermore, the water solutions of the polymers (4 mg mL 1 ) were spray cast onto ITO coated glasses to achi eve homogeneous polymer thin films with varying thicknesses. By immersing the films in a p TSA/MeOH solution (1 mg/mL 1 ) for 2 minutes, the polymers were finally converted into their acid forms, which cannot be dissolved in water or organic solvents. 4.3.3 Polymer Thermal Analysis The thermal stability of the polymers was studied by TGA in a nitrogen atmosphere using high resolution dynamic scans from 50C to 600C. As shown in Figure 4 6 the polymers exhibited high thermal stabilities with negligible weig ht loss below 340C. And then a drastic degradation process occurred for ECP Black at this temperature. Interestingly, ECP Blue with a clear sign of degradation starting at 350C, showed a slightly higher thermal stability than ECP Black Figure 4 6 The rmogravimetric analysis of polymers ECP Blue and ECP Black

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119 4.3. 4 Polymer Optical Characterization The optical properties of the polymers were investigated by UV vis NIR absorption spectroscopy in dilute THF solutions and as spray casted films on ITO glas ses. As shown in Figure 4 7 a, ECP Blue and its salt derivative WS ECP Blue exhibit a typical two band absorption in the UV vis spectra with a gap in the blue and bluish green region (380 550 nm). The absorption maxima of the polymers in solutions as well a s thin films fall into the wavelength range of 600 650 nm. Clearly, the deprotection of the ester group, as well as the use of a highly polar solvent, has affected the o ptical absorption of the chromo ph o res in solution. The generated polymer salt WS ECP Bl ue demonstrates a 30 nm red shift of its long wavelength absorption peak in the water solution compared with that of ECP Blue in THF. Moreover, due to a better intermolecular interaction in the solid state, the long wavelength optical transition of ECP Blu e thin film shows a 50 nm red shift as well as a broadening of the peak compared to its solution absorption. Interestingly, there is no dramatic change observed between the solution and film absorption of water soluble polymer WS ECP Blue As determined fr om the onsets of their neutral state lower energy optical transitions (polymer film absorption), the optical band gaps of ECP Blue and WS ECP Blue are 1.56 and 1.59 eV, respectively, in good agreement with the band gaps evaluated by differential pulse volt ammetry ( S ection 4.4.1). As expected, ECP Black and low energy transitions, and no obvious peak to peak window is observed in the visible region (Figure 4 7b). However, judging by the absorption profile of ECP Black which is unable to cover the entire visible region evenly, it is logical that the solution and the film of the polymer show a dark purple black color due to the absorption missing in part of the blue

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120 (350 470 nm) and the red (650 700 nm) regions. Furthermore, the broad absorption of ECP Black in THF (from 450 to 650 nm) shifts to a higher wavelength after deprotection of the ester group (see absorption spectrum of WS ECP Black in water), which is consistent with the similar observation from ECP Blue Although ther e is only a small max change between WS ECP Black water solution and its film absorption, the polymer thin film does show a broadening of the absorption in the range of 500 560 nm. The optical bandgaps of ECP Black and WS ECP Black are estimated to be 1.5 8 and 1.60 eV, which are slightly higher than the band gaps of the corresponding blue polymers, due to the reduced donor accepter effect upon decreasing the number of electron deficient heterocycles along the polymer backbone. a) b) Figure 4 7 Normalized UV vis NIR absorption spectra of polymers. a) ECP Blue and WS ECP Blue and (b) ECP Black and WS ECP Black in dilute THF and water s olution and thin films on ITO coated glasses. 4.4 Polymer Electrochemistry and Spectroelectrochemistry 4.4.1 Electrochemistry Studies For electrochromic applications it is necessary to have a good understanding of the redox properties of the polymers and to be able to estimate the oxidation and

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121 reduction onset levels. Differential pulse voltammetry (DPV) was performed in order to characterize the accessible redox states of both ECP Blue and ECP Black Figure 4 8 shows the DPV results obtained for the polym ers. ECP Blue exhibits onsets of oxidation and reduction at 0.24 and 1.38 V vs Fc/Fc + respectively ( Figure 4 8a). The difference between the oxidation and reduction potentials yields electrochemical band gap of 1.62 eV, which is slightly higher than the optical band gap (1.56 eV, S ection 4.3.3). According to the results, the polymer has a HOMO energy level at about 5.34 eV and a LUMO energy level at 3.72 eV. The relatively low HOMO value allows the polymer to be easily handled in air without encounteri ng undesired oxidation. a) b) Figure 4 8. Differential pulse voltammetry of polymer esters. a) ECP Blue and b) ECP Black drop cast from toluene solution onto platinum button electrod e (A= 0.02 cm 2 ). Measurements were performed in 0.2 M LiBTI /propylene carbonate (PC) with a Pt foil counter electrode and a Ag/Ag + reference electrode calibrated vs Fc/Fc + (E Fc/Fc+ = 0.085 V vs Ag/Ag + ). On the other hand, ECP Black shows much lower ons ets of oxidation and reduction at 0.02 and 1.68 V vs Fc/Fc + than those of ECP Blue which leads to less negative HOMO ( 5.08 eV) and LUMO ( 3.42 eV) energy levels. Clearly, as the relative amount of the electron rich units (ProDOP in this case) increase s, ECP Black can be oxidized more easily than the D A alternating copolymer ECP Blue Interesting, both

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122 polymers have almost the same energy gap, even though their energy levels and colors are different. 4.4.2 Spectroelectrochemistry Studies The full spect roelectrochemical behavior of the polymers was evaluated by monitoring the absorption changes of the polymer thin films upon a simultaneous change of the applied external bias across the films. A film of ECP Blue max = 655 nm, A= 0.87 a.u.) and a film of ECP Black max = 609 nm, A= 0.76 a.u.) were spray cast onto ITO coated glass from their 3 mg mL 1 toluene solution, respectively Electrochemical oxidation of the polymers was carried out in 0.2 M lithium bis(trifluoromethylsulfonyl)imide (LiBTI)/propylene carbonate (PC) supporting electrolyte using a Ag/Ag + reference electrode ( calibrated against Fc/Fc + ) and a platinum wire as the counter electrode. As shown in Figure 4 9a and b when the potential increased, the two band absorption of ECP Blue (350 400 nm and 450 700 nm) and the broad absorption of ECP Black (400 800 nm) in the visible region are depleted with a clear appearance of the polaronic transitions in the near IR region from 800 to 1200 nm. As expected, the development of the polaronic transitions stops at a particular point, and then the absorption falls back and merges into a much broader bipolaronic transition due to the further increase of the oxidation potentials applied to the films. When fully oxidized, both of the polymer films show a drama tic loss of absorption in the visible region and present a remarkably high level of transmissivity to the human eye. It is worth mentioning that a slight blue hue is observed for the transmissive oxidized films since both of the low energy bipolaronic tra nsitions tail into in the 600 nm to NIR region, even though they peak beyond 1500 nm.

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123 a) c ) b ) d) Figure 4 9. Sp ectroelectrochemistry of polymers. a) ECP Blue b ) ECP Black c) WS ECP Blue acid and d) WS ECP Black acid The films were spray cast onto ITO coated glass from toluene (a and b 3 mg mL 1 ) or from water ( c and d, 4 mg mL 1 ). Electrochemical oxidation of the films was carried out in 0.2 M LiBTI/PC supporting electrolyte using a Ag/Ag + reference electrode (a and b, calibrated against Fc/Fc + ); or in 0.2 M KNO 3 /water supporting electrolyte using a Ag/AgCl reference electrode (c and d). A platinum wire was use d as the counter electrode. The applied potential was increased in 25 mV steps from (a) 0.25 to +0.35 V vs. Fc/Fc + ( b ) 0.36 to +0.47 V vs. Fc/Fc + (c) 0 to +0.8 V vs. Ag/AgCl and (d) 0.2 to +0.5 V vs. Ag/AgCl. On the other hand, spectroelectrochemist ry of WS ECP Blue acid and WS ECP Black acid (Figure 4 9 c and d) was carried out in 0.2 M KNO 3 /water supporting electrolyte using a Ag/AgCl reference electrode. The spray cast polymer films were prepared from their aqueous solutions and neutralized as prev iously described.

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124 Different from their ester derivatives, the absorption measurements of the polymer films are cut off at 1300 nm due to the extensive absorption of water beyond that point. In the neutral state, the film absorptions of the neutralized poly mer acids show a good agreement with these of their salts, respectively, which means no further optical change upon the treatment of the films with p TSA/MeOH solution. Moreover, a transmittance change ( % T ) of 46% is estimated by examining the depletion o f the long wavelength absorption maximum at 630 nm for WS ECP Blue acid indicating a well defined electrochromic performance. However, WS ECP Black acid shows a transmittance change of 51% at 555 nm, the wavelength at which the human eye has greatest sens itivity. Here, it is obvious that the thin film of WS ECP Black acid starts oxidizing at a much lower potential ( 0.2 V vs. Ag/AgCl ) than that of WS ECP Blue acid ( 0 V vs. Ag/AgCl ), which is also observed in ECP Black and ECP Blue ( 0.36 and 0.25 V vs. Fc /Fc + respectively ) This is due to the increase of donor ratio in the random ECP Black polymer, thus raising the HOMO level. 4.5 Polymer Switching Study: A Comparison of Organic Soluble Polymers and Water Soluble Polymers 4.5.1 Polymer Colorimetric Measur ements Colorimetric analysis of the polymer films with varying thickness was performed in order to evaluate the color changes of the ECPs occurring on electrochemical oxidation L a b c olor standards). Before the test, films were spray cast and redox cycled several times using the same conditions for each polymer, as previously discussed in Section 4.4.2. Figure 4 10 shows the determined L values as a function of applied voltage. This g ives an indication of relative transmission of the films as they are oxidized while illuminated from

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125 behind ( in essence a measure of the atten ) with a standard D50 simulated daytime light source. Here, in its neutral state, fi lms of ECP Blue exhibit L values from 47 for the thickest film (A= 1.3 a.u.) to 78 for the thinnest film (A= 0.4 a.u.) ( Figure 4 10a). Moreover, a saturated blue color at all film thicknesses, which is supported by the large negative b values, is also ob served. As expected, a small amount of green light, as indicated by the relative small negative a values, also passed through the polymer films due to the partial absorption missing in the range between 500 and 600 nm. By increasing the applied potentials the polymer films begin to increase in transmission dramatically at about 0.2 V vs. Fc/Fc + Upon full oxidation, ECP Blue demonstrates L values ranging from 86 to 95 depending on the optical density of the thin films. In the mean time, the decreased a* and b* potential shift corresponding to the lightness change, the potential window of the polymer full switch is calculated to be 0.6 V. Compared with ECP Blue ECP Black illustrates an obvious drop of L value in its neutral state. For example, an ECP Black film with an optical density A= 1.1 a.u. shows a L* value at about 33 ( F igure 4 10 b ), which is much lower than that of an ECP Blue film with a even higher optical density (A=1.3 a.u., L* = 47). Clear ly, this is due to the broad absorption nature of the polymer. However, as discussed previously, the broad absorption of ECP Black does not evenly cover the entire visible spectrum, and thus allows part of the red and blue light to pass through the films, as indicated by the small positive a* values and the relative large negative b values. For instance, the color of the neutral polymer film ( A= 0.9 a.u.) falls into a red blue color region, which presents a

PAGE 126

126 dark black purple color ( a *= 10, b *= 27). When f ully oxidized, the polymer film shows a highly transmissive faint blue green tint with a *= 2 and b *= 3. a) c ) b ) d) Figure 4 10. Lightness ( L* ) as a function of applied potential for spray coated polymers. a) ECP Blue b) ECP Black c ) WS ECP Blue acid and d) WS ECP Black acid L*a*b* values of fully neutral and oxidized states are reported for the films. Photog raphs are of the fully neutral (left) and fully oxidized films (right). In spite of changing to a KNO 3 /water supporting electrolyte with Ag/AgCl as the reference electrode, WS ECP Blue acid and WS ECP Black acid show features in the EC performance similar to these in the corresponding polymer esters. The prominent increase of L* values, as well as the decrease in a* and b values, is a direct evidence of polymers being switched into a highly transmissive state upon oxidation of the films. It

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127 is worth menti oning that the measurements were stopped when the potential was about 1 V vs. Ag/AgCl to avoid over oxidation of the polymer films. Under these conditions, the experimental data was unable to show the track of its fully oxidized state for WS ECP Blue acid (Figure 4 10 c ). Fortunately, the polymer films still present satisfactory transmissive state as defined by the L a b color coordinate. For example, an WS ECP Blue acid film with an optical density A= 0.5 a.u. shows a L* value of 90 with a *= 2 and b *= 3 in its oxidized state, which are comparable to these values of an ECP Blue film with the same optical density (A=0.5 a.u., L *= 93, a *= 1 and b *= 1 Figure 4 10a ). T he potential window of WS ECP Blue acid in a near full switch is 0.7 V, which is about th e same as that of ECP Blue (0.6 V). Due to the high HOMO level of WS ECP Black acid which makes it easier to be oxidized, the polymer reaches its fully oxidized state at 0.9 V vs. Ag/AgCl. For a thick film (A= 1.6 a.u.), WS ECP Black acid presents an inte nse black purple state before being oxidized ( L* = 28) and a faint blue transmissive state when fully oxidized ( L* = 80), a lightness change higher than 50%. 4.5.2 Polymer Switching Rate To evaluate the switching rate of the polymers, the transmittance ch ange (EC T ) at a single wavelength of the polymer films was monitored as a function of time by applying square wave potential steps for periods of 10, 5, 2, 1, 0.5, 0.25 and 0.2 s. In Figure 4 11a, ECP Blue shows a transmittance change as high as 48% at the longer switch time (10 s), which is only minimally reduced to 45% for the 2 s switch. Further decrease of the switch time causes a significant loss of contrast. For example, T = 20%) remains at the 0.5 s switch and this number T = 6%) at the 0.2 s switch, where the switch of the polymer can barely be detected. For a n ECP Black film with an optical density of

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128 A= 1.02 a.u. ( Figure 4 11b), the contrast drops from 54% to 40 % when the switch time is reduced from 10 s to 2 s. This represents a n earlier stage contrast loss, which is possibly due to the longer diffusion time of the counter balancing ions penetrating the thick polymer film. At the 0.2 second switch, the polymer f ilm shows a transmittance change of 6%, corresponding a total contrast loss of around 90%. In order to compare the switching performance, the polymer acid films were obtained with nearly identical thickness as their corresponding ester films (taking the o ptical density as representative of the thickness). As shown in Figure 4 11c, the WS ECP Blue acid film, which is switched in a 0. 2 M KNO 3 /water electrolyte solution exhibits a much faster EC switching speed than that of ECP Blue From 10 s to 0.5 s switc h time, there is only a 5% loss of the full contrast observed compared with 60% for an ECP Blue film with the same optical density Unfortunately, the WS ECP Blue acid film show s a bit lower contrast compared to its ester film even at the 10 s switch time, since the film is not fully oxidized as discussed in Section 4.5.1 On the other hand, a WS ECP Black acid film with an optical density of A=1.04 a.u. (Figure 4 11d), has a comparable contrast (52%, 10 s switch time) as its ester film, and is still able to maintain the contrast at 48% when the switch time is reduced to 1 s. The faster switching speed is possibly due to a better affinity to the electrolyte of the defunctionalized polymers, as well as the reduction of the counter ion size, and thus a faste r ion diffusion process. 239 Although, the contrasts of both polymer films drop to a relative low level (20 30%) at much higher switching rates, the results still demonstrate a significant possibility of these polymers using in fast switching ECDs.

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129 a) b) c) d) Figure 4 11. Square wave potential step chronoabsorptometry of polymers. a) ECP Blue (monitored at 655 nm, switc hing potential range: 0.31 to + 0.74 V vs. Fc/Fc + ); b) ECP Black (monitored at 555 nm, switching potential range: 0.49 to + 0.56 V vs. Fc/Fc + ) in 0.2 M L iBTI/PC electrolyte solution; c) WS ECP Blue acid (monitored at 630 nm, switching potential range: 0. 1 to + 0.95 V vs. Ag/AgCl); d) WS ECP Black acid (monitored at 555 nm, switching potential range: 0.2 to + 0.95 V vs. Ag/AgCl) in 0.2 M KNO 3 /water electrolyte solution. The switch times (10 s, 5 s, 2 s, 1 s, 0.5 s, 0.25 s and 0.2 s) are indicated on the fi gure. In order to achieve an idea about how the switching performance is affected by the thickness of the film, the transmittance change (contrast) and the switching time of several films of polymer ECP Blue with varied thickness were compared ( Figure 4 1 2).

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130 In the figure, the switching time is represented by its frequency for a better view, for example, the frequency at 0.1 Hz corresponds to a switching time of 10 s. Moreover, the film thickness is represented by its optical density monitored at 655 nm. I n all cases, the contrast drops as the polymer films are switched faster (higher frequency). For instance, the contrast of the film with absorption at 1.25 a.u. drops from 45% at a frequency of 0.1 Hz to 2% at a frequency of 5 Hz. Interestingly, the contra sts of different polymer films at a fixed frequency are also related to the their thicknesses. For example, the film with the absorption at 0.54 a.u. shows the highest contrast among all the films at all switching frequencies. Figure 4 12. Transmittanc e change (contrast) as a function of switching frequency for ECP Blue spray coated on ITO ( switching potential range: 0. 31 to + 0. 74 V vs. Fc/Fc + in 0. 2 M L iBTI/PC electrolyte solution). Film absorption was monitored at 655 nm, with absorption intensity of A= 0.45, 0.54, 0.80 and 1.25 a. u. Instead of monitoring the transmittance change at one single wavelength, fast switching polymer films of WS ECP Blue acid and WS ECP Black acid were subjected to a novel technique which evaluates electrochromic properti es by

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131 associating a time parameter with a specific full visible spectrum during the electrochromic transition. 242 Using a fiber optic light source and a spectrometer containing a photodiode array detector, this measurement is capable of rapid data acquisit ion to track the electrochromic change in the polymer films It is important to note that this absorption/transmission profile is not a typical spectroelectrochemical experiment in which the electrochromic film is monitored at a steady state ( i.e. constant applied potential). Rather, it is an in situ measurement of the dynamic change in the film optical transitions. In this measurement, polymer films were stepped between the neutralized and the oxidized state ( 0. 2 and + 0.8 V vs. Ag/AgCl ) using a 2 s switch time, and the optical spectra were captured every 6 ms during the electrochromic transition period. The selected transmittance spectra in Figure 4 13 are shown at intervals of 50 ms for clarity. (A total of 41 spectra are abstracted from about 320 spectra in a 2 s switch.) For WS ECP Blue acid the polymer film in the colored neutral state shows a low transmittance of 10% at max (635 nm). Upon applying a positive potential (+0.8 V vs. Ag/AgCl) to the polymer film, the transmittance steadily increases to a higher level ( Figure 4 13a). When the bleached state is reached, the film shows a transmittance of 46% with a transmittance contrast of 36% ( % T ). Although the polymer shows a dual max at 370 nm will not be discussed in this case, since the human eyes are not sensitive at that region. Moreover, the polymer film of WS ECP Black acid demonstrates a tran smittance of 12% in the neutral state and 68% in the fully bleached state at 555 nm with a transmittance contrast of 56% ( Figure 4 13b).

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132 a) b) Figure 4 13. Rapid full spectrum measurement of polymers. a) WS ECP Bl ue acid a nd b) WS ECP Black acid film. Switching from 0. 2 V to + 0.8 V vs. Ag/AgCl with the full potential step in 0. 2 M KNO 3 /water electrolyte solution. Spectra are showed at 50 ms intervals for a total of 41 spectra in 2000 ms. Another interesting aspec t in this new methodology is that an estimate of the switching speed profile can be roughly observed in the absorption/transmission spectra, because each spectrum shown has a 50 ms time separation in Figure 4 13a and b. And this forms a time line in the w hole spectra. In the case of WS ECP Blue acid the

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133 transmission increase in the long wavelength region (500 700 nm) is relatively steady and slow, and the first 20 spectra can be clearly observed in the figure. However, the WS ECP Blue acid shows a more pr onounced transmission increase, so that the spectra after the first 10 transitions are compressed at a transmission level close to the fully bleached state, indicating a faster switching speed compared to the blue polymer. In order to quantitatively evalua te how fast the polymers are switched, the transmittance values at a single wavelength were extracted from the collected data and plotted vs. their acquisition times. Figure 4 14a shows the results for WS ECP Blue acid Judging by the transmittance profile which is still increasing after 2 seconds, the polymer film is unable to reach a fully transimissive oxidized state in this time frame using a potential of + 0.8 V vs. Ag/AgCl. Although the low potential will protect the polymer film from being overoxidiz ed, it could lower the switching speed because of insufficient driving force for ion motion, and lower the contrast as well since the polymers are not fully oxidized. In this case, 95% of the full transmittance contrast (t 95% ) for the bleaching process is achieved after 1060 ms (from transmittance of T= 9.6% to T= 45.0%) whereas the neutralization process is achieved in 190 ms (from T= 46.9% to T= 11.5%) On the other hand, WS ECP Black acid presents a fully bleached state in the 2 seconds time frame Th level), which results in a large potential difference compared to + 0.8 V vs. Ag/AgCl, and thus a stronger driving force for ion motion. A t 95% (from T= 12.9% to T= 65.4%) of 440 ms is observed f or the bleaching process, which is faster than that of WS ECP Blue

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134 acid ( t 95% = 1060 ms). However, a slightly slower neutralization process is observed with t 95% of 390 ms (from T= 68.2%% to T= 15.7%). a) b) Figure 4 14. Time dependence of polymers. a) WS ECP Blue acid at 635 nm and b) WS ECP Black acid at 555 nm with an applied 2 second potential square wave from 0. 2 V to + 0.8 V vs. Ag/AgCl in 0. 2 M KNO 3 /water electrolyte solution. 4 .6 Chapter Summary The design of conjugated electrochromic polymers (ECPs) has been driven by their excellent solution processability and rapid switching properties, as well as the growing interest in their application in non emissive electrochromic devic es (ECDs). In this chapter, we have demonstrated the synthesis of two new ECPs by Stille polymerization: a blue to transmissive conjugated polymer ( ECP Blue ) and a black to transimissive conjugated polymer ( ECP Black ). The structural, optical, electrochemi cal and electrochromic (EC) properties of both resulting polymers were characterized. Importantly, the defunctionalization of the carboxylate ester side chains affords the polymer salts with great water solubility, which can be processed from polymer/water solutions into thin films by spray casting onto ITO glasses. Upon the subsequent neutralization of the thin films, the resulting polymer acid films are ready to be switched

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135 in a KNO 3 /water electrolyte solution and show a dramatic improvement in the EC swi tching speed performance compared with their ester derivatives at the sub second switching time scale. Moreover, a rough estimate of the effect of film thicknesses on switching performance was obtained using ECP Blue as an example. The results of the elect rochromic properties study indicate that these water soluble electro chromic conjugated polymers are promising candidates for rapid switching electrochromic devices. Regardless of all the inspiring results shown in this chapter, the side chain defunctionali zation approach, as well as the following neutralization process, provides a new perspective in making multilayer devices. More specifically, a second polymer layer can be easily processed on top of the first lay with polymers that cannot be dissolve d in o rganic solvents or water. One major concern in this work is that the ECP Black is not really black due to its bulky side chain could be used on monomer 7 ( Figure 4 5) wi thout significantly affecting its reactivity in Stille polymerization. The bulky side chain on monomer 7 will interact with the massive functional groups on the monomer 5 repeat unit, and afford a more twisted plane conformation of the backbone, resulting in an increase in the band gap and blue shift of the optical transition. Future work can also be directed toward applying this methodology to the synthesis of water soluble conjugated polymers with other saturated colors (yellow, cyan, and red etc .), as we ll as tuning the switching conditions for rapid EC switches.

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136 4.7 Experimental Details Bis(2 ethylhexyl) 5 hydroxyisophthalate (2) To a stirred suspension of 5 hydroxy isophthalic acid (30 g, 165 mmol) and 2 ethylhexan 1 ol (51.5 g, 395 mmol) in 250 mL tol uene, concentrated H 2 SO 4 (5 drops) was added and the reaction mixture was refluxed with a Dean Stark apparatus for 48 h. After removal of unreacted 5 hydroxy isophthalic acid by filtration, the residue was washed with water for three times and dried over a nhydrous MgSO 4 The solvent was evaporated and the residue was purified by flash chromatography using 8:1:1 hexane: ethyl acetate: acetone to give compound 2 as a yellow oil (50 g, 75%). 1 H NMR (CDCl 3 ): 8.22 (t, 1H, J = 1.2 Hz), 7.85 (d, 2H, J = 1.2 Hz), 7.06 (br, 1H), 4.27 (d, 4H, J = 5.7 Hz), 1.72 (m, 2H), 1.50 1.31 (m, 16H), 0.96 0.87 (m, 12H). 13 C NMR (CDCl 3 ): 166.52, 156.82, 132.32, 122.75, 121.23, 68.17, 39.10, 30.73, 29.18, 24.17, 23.16, 14.23, 11.28. HRMS (ESI FTICR): m/z calcd for C 24 H 38 NaO 5 (M+Na + ) 429.2611 found 429.2618. Anal. calcd for C 24 H 38 O 5 : C 70.90, H 9.42, found C 70.53, H 9.78. Tetrakis(2 ethylhexyl) 5,5' (3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine 3,3 diyl)bis(methylene)bis(oxy)diisophthalate (3) Compound 1 (5 g, 14.6 mmol), compound 2 (17.8 g, 43.8 mmol), K 2 CO 3 (12.1 g, 87.7 mmol), KI (0.97 g, 5.9 mmol) and DMF (60 mL) were added to a 100 mL round bottom flask. And the solution was irradiated with microwaves under reflux at 150 C (maximum power 200 W) for 1 h. After removal of the extra salt by filtration, the reaction mixture was diluted with methylene chloride and washed with water. The organic phase was dried over MgSO 4 and the solvent was removed. The crude product was purified by column chromatography on silica gel (hexa ne: ethyl acetate, 9:1) to yield the desired product as a yellow oil (10.8 g,

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137 74%). 1 H NMR (CDCl 3 ): 8.26 (t, 2H, J = 1.2 Hz), 7.75 (d, 4H, J = 1.5 Hz), 6.52 (s, 2H), 4.31 (s, 4H), 4.30 (s, 4H), 4.24 (d, 8H, J = 6.0 Hz), 1.72 (m, 4H), 1.48 1.26 (m, 32H), 0.97 0.86 (m, 24H). 13 C NMR (CDCl 3 ): 165.85, 158.75, 149.34, 132.50, 123.54, 119.82, 105.91, 72.88, 68.05, 67.12, 47.61, 39.08, 30.74, 29.18, 24.18, 23.16, 14.26, 11.27. HRMS (ESI FTICR): m/z calcd for C 57 H 85 O 12 S (M+H + ) 993.5756 found 993.57 40. Anal. calcd for C 57 H 84 O 12 S: C 68.92, H 8.52, found C 68.68, H 8.74. Tetrakis(2 ethylhexyl) 5,5' (6,8 dibromo 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine 3,3 diyl)bis(methylene)bis(oxy)diisophthalate (4) A solution of NBS (1.1 g, 6.3 mmol, in 15 mL aceto nitrile) was added slowly to a compound 3 (3.0 g, 3.0 mmol) chloroform (15.0 mL) solution at 0 C The mixture was stirred at the same temperature for 2 h before allowing it to warm to room temperature. After stirring at room temperature for overnight, the solution was diluted with methylene chloride and the organic phase was washed with water and dried over anhydrous MgSO 4 The solvent was evaporated and the residue was purified by column chromatography on silica gel (hexane: ethyl acetate, 9:1) to yield th e desired product as a light yellow oil (3.1 g, 90%). 1 H NMR (CDCl 3 ): 8.27 (t, 2H, J = 1.2 Hz), 7.74 (d, 4H, J = 1.2 Hz), 4.38 4.25 (m, 16H), 1.72 (m, 4H), 1.51 1.26 (m, 32H), 0.97 0.85 (m, 24H). 13 C NMR (CDCl 3 ): 165.78, 158.53, 146.67, 132.54, 123.67, 119.75, 91.88, 73.25, 68.07, 66.80, 47.75, 39.07, 30.72, 29.17, 24.17, 23.16, 14.26, 11.27. HRMS (ESI FTICR): m/z calcd for C 57 H 83 Br 2 O 12 S (M+H + ) 1151.3955 found 1151.3917. Anal. calcd for C 57 H 82 Br 2 O 12 S: C 59.47, H 7.18, found C 59.68, H 7.49. Tetr akis(2 ethylhexyl) 5,5' (6,8 bis(trimethylstannyl) 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine 3,3 diyl)bis(methylene)bis(oxy)diisophthalate (5) A

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138 solution of compound 4 (5 g, 4.3 mmol), hexamethylditin (4.3 g, 13.1 mmol) and Pd(PPh 3 ) 4 (1 g, 0.9 mmol) in to luene (50 mL) was degassed three times by successive freeze pump thaw cycles and heated at 115 C for 3 h during which time TLC analysis indicated completion of the reaction. The reaction mixture was then cooled to room temperature and water was added. Afte r extraction with methylene chloride, the organic phase was dried with anhydrous MgSO 4 and solvent was removed under vacuum. The remaining residue was then taken up in hexanes and filtered over a pad of silica gel (the gel was pre treated with neat triethy lamine and washed with hexanes), and the solvent was then evaporated to give a colorless oil (4.1 g, 72%). 1 H NMR (CDCl 3 ): 8.27 (t, 2H, J = 1.2 Hz), 7.77 (d, 4H, J = 1.2 Hz), 4.33 4.23 (m, 16H), 1.75 (m, 4H), 1.50 1.35 (m, 32H), 0.98 0.88 (m, 24H ), 0.35 (s, 18H). 13 C NMR (CDCl 3 ): 165.86, 158.84, 156.28, 132.42, 124.10, 123.35, 119.79, 72.94, 67.98, 67.42, 47.34, 39.05, 30.71, 29.15, 24.16, 23.12, 14.22, 11.25, 8.35. Anal. calcd for C 63 H 100 O 12 SSn 2 : C 57.37, H 7.64, found C 57.65, H 7.73. 3,3 Dimethyl 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine ( 6 ) 3,4 dimethoxythiophene (10 g. 69 mmol), 2,2 dimethylpropane 1,3 diol (14.4 g, 139 mmol), p toluenesulfonic acid (1.3 g, 6.9 mmol), and 300 mL toluene were combined in a 500 mL flask equipped with a s oxhlet extractor with molecular sieves in a cellulose thimble. The solution was refluxed for 1 day. The reaction mixture was cooled, and a fter dilution with ether (100 mL), the organic phase was washed with water and dried ov er anhydrous magnesium sulfate. The solvents were removed under vacuum, and the crude product was purified by column chromatography with hexane as an eluent to obtain compound 6 as a white crystalline solid ( 9.7 g, yield: 76 %) 1 H NMR (CDCl 3 )

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139 6.47 (s, 2H), 3.73 (s, 4H), 1.03 (s, 6H). 13 C NMR (CDCl 3 ) 150.23, 105.74, 80.33, 39.11, 21.91. 6,8 Dibromo 3,3 dimethyl 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine ( 7 ) A solution of NBS (7.9 g, 44.4 mmol, in 50 mL acetonitrile) was added slowly to a compound 6 (3.9 g, 21.2 mmol) chloroform (50 mL) solution at 0 C The mixture was stirred at the same temperature for 2 h before allowing it to warm to room temperature. After stirring at room temperature for overnight, the solution was diluted with methylene chloride and the organic phase was wa shed with water and dried over anhydrous MgSO 4 The solvent was evaporated and the residue was purified by column chromatography on silica gel (hexane) to yield the desired product as a white solid (6.85 g, 95%). 1 H NMR (CDCl 3 ): 3.80 (s, 4H), 1.04 (s, 6H). 13 C NMR (CDCl 3 ): 147.46, 91.70, 80.53, 39.26, 21.80. HRMS (ESI FTICR): m/z calcd for C 9 H 10 Br 2 O 2 S (M + ) 341.8748 found 341.8689. Anal. calcd for C 9 H 10 Br 2 O 2 S: C 31.60, H 2.95, found C 31.77, H 2.81. General Polymerization Procedure. Polymer ECP Blue: A solution of compound 5 (0.9 g, 0.68 mmol), compound 8 (0.201 g, 0.68 mmol), tris(dibenzylideneacetone)dipalladium (0) (13.8 mg, 0.015 mmol) and tri(o tolyl)phosphine (18.3 mg, 0.06 mmol) in toluene (20 mL) was degassed three times by successive freeze pum p thaw cycles and heated at 115 C for 36 h in an oil bath. The solution was then precipitated into methanol (400 mL). The precipitate was filtered through a cellulose thimble and purified via Soxhlet extraction for 24 hours with methanol and then 48 hours with hexane. The polymer was extracted with chloroform, concentrated by evaporation, and then precipitated into methanol again (400 mL). The

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140 collected polymer was a blue solid (0.7 g, 91 %). 1 H NMR (300 MHz, CDCl 3 2H), 8.27 (s, 2H), 7.81 (d, 4H J = 0.9 Hz ), 4.67 (s, 4H), 4.50 (s, 4H), 4.30 4.20 (m, 8H), 1.74 1.29 (m, 36H), 0.95 0.85 (m, 24H) GPC analysis: M n = 25.8 kDa, M w = 38.5 kDa, PDI = 1.5. Anal. Calcd. for C 63 H 84 N 2 O 12 S 2 : C 67.23, H 7.52, N 2.49, found C 66.94, H 7.64, N 2.42. ECP Black : Compound 5 (0.9 g, 0.682 mmol), compound 7 (0.163 g, 0.478 mmol), compound 8 (0.060 g, 0.205 mmol), tris(dibenzylideneacetone)dipalladium (0) (13.8 mg, 0.015 mmol) and tri(o tolyl)phosphine (18.3 mg, 0.06 mmol), toluene (20 mL). The collected polymer was a black solid (0.66 g, 84 %). 1 H NMR (300 MHz, CDCl 3 8.25 (m, 13H), 7.79 (s, 20H), 4.48 4.21 (m, 80H), 4.50 (s, 4H), 3.85 (s, 14H), 1.69 1.11 (m, 180H), 0.95 0.85 (m, 141H) GPC analysis: M n = 13.9 kDa, M w = 23.3 kDa, PDI = 1.7. Anal. Calcd. for C 651 H 896 N 6 O 134 S 20 : C 67.46, H 7.79, N 0.73, found C 67.11, H 7.89, N 0.68. WS ECP Blue : A solution of 1 M KOH in methanol (50 mL) was refluxed and simultaneously sparged with argon for two hours, and ECP Blue (360 mg) was added as a solid. This suspension was refluxed for 24 hours, during which time the polymer dispersed into fine particles. The suspension was filtered on a nylon filter paper and washed with 100 mL methanol followed by 100 mL diethyl ether, and dried under vacuum to yield 260 mg of a dark soli d (90%). WS ECP Black : The same reaction and purification procedure as described for WS ECP Blue was followed. ECP Black (395 mg) was used. Got polymer WS ECP Black 300 mg as a dark black powder, y ield 92%. WS ECP Black acid and WS ECP Blue acid: Refer to manuscript text.

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141 CHAPTER 5 FINE ABSORPTION TUNI NG IN DIKETOPYRROLOP YRROLE BASED CONJUGATED POLYMERS FOR PHOTOVOLTAIC APP LICATIONS 5.1 Improve ment of the L ight H arvesting E fficiency In C hapter 3 and 4, two polymer systems for electrochromic applications w ere developed. Although these two types of polymeric materials are distinct in their structures, they share the same chemistry approach, the random Stille polymerization method. In this chapter, we will focus on extending this methodology to the synthesis of conjugated polymers with efficient light harvesting properties (light absorption) as well as well defined energy levels and band gaps ; and explore their potential application in photovoltaic devices. As mentioned in C hapter 1, due to the development of donor acceptor conjugated polymers with low band gaps and reasonable solution processabilities, significant progress has been made in the field of OPVs. Power conversion e ffi ciencies (PCEs) of polymer/fullerene BHJ solar cells have reached 7% in academia, 1 61 164 or even above 8%, a psychological barrier, in industry. 243 245 This dramatic improvement in OPV performance, along with the capability of low cost fabrication into light weight, large area and flexible devices, 138,142,146 has made them a strong comp etitor to the silicon based solar cells. However, the improvement of PCEs does not occur spontaneously. It is based on how for synthesis and characterization of conjugated polymers as efficient p type ma terials for OPVs. 212 Of present a broad absorption (400 850 nm) across the visible and near IR region with a high absorption coefficient, since the short circuit current density ( J sc ) of the solar cell is

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142 proportional to the spectral absorption breadth and absorption probability of the polymer/PCBM active layer. 246 Second, the material should have a low lying HOMO energy level to assure a large energy difference compared to the LUMO of PCBM, and thus a high open circuit voltage ( V oc ). 57,212,247 249 Third, a proper offset between the LUMO energy levels of the polymer and the fullerene derivative is necessary in order to provide sufficient driving force for charge generation Fourth, the materials should present high hole mobilities 250,251 and a balanced charge carrier mobility in the polymer/PCBM blends, which favor s charge extraction and transport, leading to an increase in fill factors. 252 Finally, the polymers must be abl e to mix with fullerene effectively in order to generate a nanoscale bicontinuous morphology with favored phase separation and interpenetrating network. 253 255 For the first parameter, the improvement of the light harvesting efficiency is usually achieved via the donor acceptor approach, 53 55 which reduces the polymer band gap by incorporating electron rich and electron deficient units together. However, instead of broadening the polymer absorption profile, this approach mostly shifts the polymer absorptio n into the solar radiation region that has a larger fraction of solar photons; on the other hand, it diminishes the absorption in the region of 400 600 nm, which hinders the desired increase of J sc and efficiency. 59,256 259 Consequently, several methods ha ve been devised in order to fulfill the purpose of broadening the polymer absorption. One of the most important methods here is the use of tandem bulk heterojunction solar cells to minimize the loss of absorption. More specifically, two BHJ solar cells wi th different absorption characteristics are stacked in series to generate a wider range of absorption in the solar spectrum. The first tandem BHJ solar cell was reported by

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143 Heeger and coworkers. 260 In th is report, a D A polymer PCPDTBT/PC 61 BM blend, which absorbs mainly in the range of 600 850 nm ( Figure 5 1b), is used as the active layer in the front cell ( Figure 5 1a), while P3HT/PC 71 BM blend, which absorbs from 450 to 650 nm in the visible region, is used as complementary light absorbing active layer in the back cell. For comparison, the PCPDTBT/PC 61 BM single cell yields a PEC of 3.0% with J sc of 9.2 mA cm 2 V oc of 0.66 V and FF of 0.5; the P3HT/PC 71 BM single cell yields a PEC of 4.7% with J sc of 10.8 mA cm 2 V oc of 0.63 V and FF of 0.69; and the tandem cell shows a PEC of 6.5% with J sc of 7.8 mA cm 2 V oc of 1.24 V and FF of 0.67. Clearly, the major contribution to the improvement of the tandem cell is the large open circuit voltage, which is the sum of the individual subcell voltages. The current, whic h is extracted from the tandem cell, is limited by the current generated from the subcell with smaller J sc a) b) Figure 5 1. Illus tration of a tandem bulk heterojunction solar cell. a) Device structure (right) and TEM cross sectional image (left) of the polymer tandem solar cell. Scale bars, 100 nm (lower image) and 20 nm (upper image). Structure of PCPDTBT (top right). b) Absorption spectra of a PCPDTBT:PC 71 BM bulk heterojunction composite film, a P3HT:PC 71 BM bulk heterojunction composite film, and a bilayer of the two, as relevant to the tandem device structure (O.D., optical density). (Adopted and modified with permission from Ref. 260).

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144 Although a similar strategy has been successfully applied to other polymer systems and shows promising results, 261 264 it is also worth noting that this technique requires considerable work to optimize and balance the current in each subcell, as wel l as the complicated processing procedure to deposit a large number of different layers, thereby increasing the cost of production and limiting the commercial use. An alternative way to increase the light harvesting efficiency is by incoperating small mol ecule sensitizers for near infrared absorption 265 Generally, dye molecules, which can absorb at longer wavelengths of the solar spectrum (compared to the donor polymer P3HT), are simply blended into the polymer/PCBM mixture as additional components. These dye molecules located at the interface can contribute not only to the photocurrent generation by direct photoexcitation, but also they can harvest excitons efficiently from the donor polymer to the dye molecules through long range energy transfer. For exa mple, Ito et al. demonstrated the use of the near infrared dye silicon phthalocyanine bis(trihexylsilyl oxide) ( Figure 5 2a for structure of SiPc) in a P3HT/PCBM/SiPc ternary blend device, which showed an increased of J sc from 6.5 to 7.9 mA cm 2 while kee ping the V oc and FF unchanged, and thus a 20% increase of the PCE from 2.2% to 2.7% ( Figure 5 2b). 266 However, this method also suffers from the formation of dye aggregates in blend films, which reduce the absorption efficiency and the charge mobility of t he active layer. 267,268 In this chapter, we discuss our efforts in the improvement of the light harvesting efficiency of conjugated polymers by broadening their absorption spectra on the molecular level. More specifically, we use diketopyrrolopyrrole (DPP) as an acceptor, incorporate it into a random donor acceptor polymer system by Stille polymerization,

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145 and control the polymer absorption by tuning the monomer feed ratio. We further explore the photovoltaic prosperities of these DPP based random copolymers by applying them in solar cells. a) b) Figure 5 2. Bulk heterojunction solar cell with SiPc as an additional component. a) Structure of SiPc. b) J V characteristics of P3HT/PCBM (br oken lines) and P3HT/PCBM/SiPc blend films (solid lines), both before (thin lines) and after (thick lines) annealing. (Adopted and modified with permission from Ref. 266). 5.2 Concept and Design of Broadly Absorbing Diketopyrrolopyrrole Based Low Band Gap Polymers Derivatives of 2,5 diketopyrrolo[3,4 c]pyrrole (DPP) have been widely used as high performance pigments for fibers, plastics, prints and inks during the past three decades, due to their excellent photochemical and thermal stability, various color choices and high luminescence. 269 The electron withdrawing effect of the DPP units causes the chromophore to have a high electron affinity, and DPP has become a popular acceptor used in the conjugated polymer systems for FETs, OLEDs and OPVs. 58,270 275 For example, Janssen et al. demonstrated the synthesis of PDPP3T for high performance OPV s with a PCE of 4.7% (also see C hapter 1). 109 As an extension of

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146 that work, Frchet and coworkers reported the synthesis of the copolymer PDPP2FT (a furan derivative of P DPP3T), with which the polymer/PC 71 BM BHJ solar cell achieved a PCE of 5%. 276 In this work, a series of copolymers were synthesized using DPP as an acceptor, and thiophene and dialkyloxybenzodithiophene (BDT) as donor repeat units ( Figure 5 3). Fundamental understanding of the balance of polymer solubility, stacking ability, light absorption, and energy levels is key to this chapter. Figure 5 3. Chemical structure of PDPP3T and P1 P4 During the course of our studies, two reports by Thompson 277 and Chen 278 appeared, respectively, describing a strategy similar to ours for the synthesis of DPP based random copolymers with broad absorption spectra for OPVs. In the first report, three novel semi random P3HT based donor acceptor copolymers containing 5 15% of the acceptor DPP were synthesized by Stille polymerization ( Figure 5 4a). For copolymers P3HTT DPP 10% and P3HTT DPP 15%, broad intense absorption spectra covering the range of 350 to 850 nm with absorption maxima at 685 and 703 nm were observed ( Figure 5 4b). A BHJ solar cell using P3HTT DPP 10%/PC 61 BM blend (1:1.3) as an active layer showed an efficiency of 4.94% with J sc of 13.87 mA cm 2 V oc of 0.57 V and FF of 0.63. In the second report, random donor acceptor copolymer PDPP T DTT based on DPP as the acceptor unit, dithienothiophene (DTT) and thiophene as the donor units,

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147 was synthesized by random Stille polymerization (Figure 5 5) BHJ solar cells using PDPP T DTT as a donor polymer and PC 71 BM as an acceptor (1:2) exhibited J sc of 12.76 mA cm 2 V oc of 0.58 V and FF of 0.67 with a PCE of 5.02%. a) b) Figure 5 4. P3HTT DPP copolymers for solar cells. a) Synthesis of P3HTT DPP copolymers b) Thin fi lm (spin coated from o DCB) where (i) is P3HT (annealed at 150 C for 30 min for the thin fi lms), (ii) is P3HTT DPP 5%, (iii) is P3HTT DPP 10% and (iv) is P3HTT DPP 15% (thin fi lm as cast). (Adopted and modified with permission from Ref. 277) Figure 5 5. Synthesis of PDPP T DTT copolymer (Adopted and modified with permission from Ref. 278) 5.3 Polymer Synthesis and Characterization 5.3.1 Monomer Synthesis Referring to Figure 5 6, commercially available thiophene 2 carbonitrile was used to syn thesize compound 1 which was used without further purification due to the low

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148 solubility in organic solvents caused by inter and intramolecular H bonding. N alkylation of compound 1 with 7 (bromomethyl)pentadecane in the presence of potassium carbonate u sing DMF as solvent afforded compound 2 with a yield of 40%. Subsequent bromination of compound 2 with N bromosuccimide in chloroform gave DPP based monomer compound 3 in 90% yield. Instead of column chromatography, compound 3 was purified via a modified p recipitation procedure by slowly adding the bad solvent (methanol) into the crude product THF solution. This slow precipitation process could extensively eliminate the impurities and afford a good yield of the reaction. It is worth mentioning that precipit ation by adding the crude product THF solution into methanol (a reversed procedure) was not helpful in the purification process. Moreover, fresh lithium diisopropylamide (LDA), which was made by reacting diisopropylamine with n BuLi, was used as a base in the synthesis of d onor unit compound 4 The ditin monomer is a white crystalline solid, which is believed to favor the formation of high molecular weight polymers, since solid starting material can be weighed more easily and accurately than the liquid one, affording better stoichiometric control The synthesis procedure for making compound 5a and 5b was also modified in our case. Dialkyloxybenzodithiophene i s commonly synthesized in aqueous solution using tetrabutylammonium bromide as a phase transfer agent and zinc powder as a reducing agent. 279,280 When this procedure was repeated in our lab, low reaction yield was observed (around 20 30%). By simply replacing water with DMF, the reaction yields were improved to 84% for 5a and 76% for 5b Finally, monomer 6a and 6b were synthesized by bromination using NBS in a solvent mixture of chloroform and acetonitrile. Reaction yields of 74% and 69% were achieved, respectively.

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149 Figure 5 6. Synthesis of monomers 3 4 6a and 6b 5.3.2 Polymer Synthesis: Expanding the Absorption Spectra of DPP based Donor Acceptor Polymers As a control polymer, alternating copolymer PDPP3T was successfully prepared by a Stille polymerization of dibromo compound 3 and ditin compound 4 ( Figure 5 7). After Soxhlet extraction and re precipitation, PDPP3T exhibited a number average molecular weight ( M n ) of 124.9 kDa and PDI of 1.3 ( Figure 5 8a). It is worth noting that the M n here is much higher than that of the same polymer prepared by Suzuki

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150 polymerization ( M n = 54.0 kDa, PDI= 3.15, estimated by GPC in ODCB at 80 C ). 109 Good agreement of the elemental analysis results with the calculated numbers confirmed that the monomers were polymerized into the desired structure and the formed polymer was pure ( Table 5 1) The dark green polymeric solid showed good solubility in chloroform and fair solubility in toluene and THF. Figure 5 7. Synthesis of control polymer PDPP3T and random copolymer P1 P4 via Stille polymerization The synth esis of random copolymers P1 and P2 were carried out using the same polymerization condition as the control polymer PDPP3T ( Figure 5 7). Copolymerization of 2,5 bis(trimethylstannyl)thiophene ( 4 ) 2,6 dibromo 4,8 diethylhexyloxybenzo[1,2 b:4,5 ne ( 6a ) and dibromo bisthiophenediketopyrrolopyrrole ( 3 ) in a ratio of

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151 1:0.5:0.5 and 1:0.75:0.25 yielded polymers P1 and P2 with M n as high as 130.8 and 54.6 kDa, respectively ( Table 5 1). Table 5 1. GPC estimated molecular weights in THF and elemental an alysis of the polymers. Monomer 4 ratio Monomer 6 ratio Monomer 3 ratio M n (kDa) M w (kDa) PDI EA (Calcd/Found) C H N PDPP3T 1 / 1 124.9 158.3 1.3 72.41/71.97 8.75/9.16 3.38/3.05 P1 1 0.50 ( 6a ) 0.5 130.8 158.3 1.2 70.85/70.43 8.18/8.98 2.07/1.76 P2 1 0.75 ( 6a ) 0.25 54.6 96.4 1.8 69.78/70.73 7.78/8.46 1.16/0.94 P3 1 0.65 ( 6b ) 0.35 59.4 114.3 1.9 73.12/74.06 9.15/9.94 1.26/1.17 P4 1 0.75 ( 6b ) 0.25 89.3 139.7 1.6 73.24/74.23 9.22/10.19 0.91/0.83 a) b) c) Figure 5 8. GPC trace of DPP based random copolymers. a) PDPP3T b) P3 and c) P4 in THF solution at 40 C using polystyrene as a standard.

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152 Poly mer solution absorption spectra are shown in Figure 5 9a. Compared with PDPP3T P1 and P2 show a blue shift of the low energy optical transition in the range of 600 to 850 nm. By increasing the relative amount of donor monomer 6a the difference in intensi ty between the two absorption bands is reduced. In the case of P2 the intensities of the two bands, which peak at 532 and 712 nm, are balanced. However, the broad absorption of P2 from 350 to 850 nm is unable to cover the entire visible NIR spectrum and l eaves an absorption gap between 550 and 650 nm. Unfortunately, the solubility of P1 and P2 in organic solvents such as chlorobenzene, chloroform and THF is rather low, which prohibits the further utilization of these polymers in OPV application. In order t o improve the solubility of the random copolymers, monomer 6b with 2 hexyldecyl side chains was synthesized and used in the copolymerization of polymers P3 and P4 With the increased solubility, the polymerization yields were also improved to 67% for P3 an d 77% for P4 Importantly, the two polymers showed high molecular weights at 59.4 and 89.3 kDa with PDI of 1.9 and 1.6, respectively, which agreed well with the first three polymers. The GPC traces of P3 and P4 are shown in Figure s 5 8b and 8c, and the res ults are summarized in Table 5 1. Polymer P4 showed an absorption profile similar to P2 in chlorobenzene solution ( Figure 5 9b), which is as expected, since they shared the same monomer ratio, and thus the same polymer backbone. Compared with P1 P3 with a higher benzodithiophene (BDT) ratio exhibited a relatively high intensity of the low wavelength absorption peak with an absorption maximum at 528 nm. A definite blue shift of the low energy optical transition in the range of 600 to 850 nm was also observe d. Due to their good solubility in organic solvents, P3 and P4 were used in the subsequent characterizations.

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153 a) b) Figure 5 9. Normalized UV vis NIR absorption spectra of DPP based ran dom copolymers. a) PDPP3T, P1 and P2 (b) PDPP3T, P3 and P4 in dilute chlorobenzene solution 5.3.3 Polymer Electrochemistry and Energy Level Estimation The HOMO and LUMO energy levels and band gaps for PDPP3T P3 and P4 were measured by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using Ag/Ag + as a reference electrode calibrated vs. Fc/Fc + and converted to the vacuum scale using the approximation that the ferrocene redox couple is 5.1 eV relative to vacuum level. Figure 5 10 shows resp ectively the CV and DPV results obtained for the PDPP3T The polymer exhibits onsets of oxidation at 0.45 and 0.18 V vs. Fc/Fc + and onsets of reduction at 1.44 and 1.38 V vs. Fc/Fc + according to the CV and DPV, respectively. Electrochemical band gaps, wh ich were determined by the difference between the oxidation and reduction potentials, were calculated to be 1.89 and 1.56 eV ( Table 5 2). Not surprisingly, there were differences between the CV and DPV results, since DPV measures a current difference, in w hich the major component of that difference is the faradaic current, and largely eliminates the capacitive component due to the charging of the electrode double layer. The polymer HOMO and LUMO energies

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154 are estimated from the onsets of oxidation and reduct ion potential, respectively, and the results are shown in Table 5 2. The low lying HOMO level of PDPP3T (5.55 or 5.28 eV determined by CV or DPV) indicates that the polymer can be easily handled in air (threshold of air stability is at 5.2 eV). The LUMO le vel at around 3.6 3.7 eV provides a strong driving force for charge transfer from the polymer to PCBM with a LUMO at around 4.2 eV. a) b) Figure 5 10. Electrochemistry results of pol ymer PDPP3T a) Cyclic voltammograms (scan rate of 50 mV/s) and b) differential pulse voltammograms (step time of 0.1 s) of PDPP3T drop cast from chlorobenzene solution onto a platinum button electrode (A= 0.02 cm 2 ) Measurements were performed in 0.2 M Li BTI /propylene carbonate (PC) with a Pt foil counter electrode and a Ag/Ag + reference electrode calibrated vs. Fc/Fc + Incorporating the BDT unit into the polymer backbone slightly raises the oxidation and reduction potentials of the random copolymers P3 and P4 For example, polymer P3 shows onsets of oxidation and reduction at 0.22 and 1.35 V vs. Fc/Fc + (measured by DPV, Figure 5 11a) compared to those of 0.18 and 1.38 V vs. Fc/Fc + for PDPP3T As expected, the potential onsets increase in proportion to the relative number of BDT units in the polymer main chain (decrease in the number of DPP units). More

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155 specifically, polymer P4 with a monomer ratio of 1:0.75:0.25 (monomer 4 : 6b : 3 as indicated in Figure 5 7) shows increased oxidation and reduction onsets o f 0.36 and 1.26 V vs. Fc/Fc + compared to these of P3 with a monomer ratio of 1:0.65:0.35 (monomer 4 : 6b : 3 ) ( Figure 5 11b). a) b) Figure 5 11. Differential pulse voltammetry of random copolymers. a) P3 ; b) P4 drop cast from chlorobenzene solution onto platinum button electrode (A= 0.02 cm 2 ). Measurements were performed in 0.2 M LiBTI /propylene carbonate (PC) with a Pt foil counter electrode and a Ag/Ag + reference electrode. Table 5 2. Electrochemically determined HOMO energy levels, LUMO energy levels and bandgaps for PDPP3T P3 and P4 (by CV and DPV). CV DPV E ox (V) HOMO (eV) E red (V) LUMO (eV) E gap (eV) E ox (V) HOMO (eV) E red (V) LUMO (eV) E gap (eV) PDPP3T 0.45 5.55 1.44 3 .66 1.89 0.18 5.28 1.38 3.72 1.56 P3 0.49 5.59 1.27 3.83 1.76 0.22 5.32 1.35 3.75 1.57 P4 0.57 5.67 1.34 3.76 1.91 0.36 5.46 1.26 3.84 1.62 Note: Oxidation ( E ox ) and reduction ( E red ) onset potentials are reported vs. Fc/Fc + Energy lev els are given based on the assumption that the energy of SCE is 4.7 eV vs vacuum, 41 and Fc/Fc + is +0.38 V vs SCE (i.e. 5.1 eV relative to vacuum). Furthermore, the calculated HOMO and LUMO energy levels of P3 and P4 are shown in Table 5 2. Clearly, P4 w ith the lowest DPP ratio has the lowest HOMO and LUMO levels at 5.46 and 3.84 eV according to the DPV measurements. Interestingly, this observation that the HOMO energy levels change according to the change in

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156 relative donor acceptor ratios is not consiste work, 277 which showed that the HOMO levels were independent of the DPP content. This is possibly due to the use of benzodithiophene (BDT) unit in our case, which lowers the HOMO levels of the donor acceptor copolymer. 279 The band gaps of P3 and P4 were also increased to 1.57 and 1.62 eV, respectively, which agrees well with the blue shift of the low energy optical transition in the absorption spectra ( Figure 5 9b). 5.3.4 Polymer Thermal Analysis The thermal stabilities of P3 and P4 were studied by TGA in a nitrogen atmosphere. The thermograms displayed in Figure 5 12 show that the polymers exhibit a high thermal stability until the temperature reaches 315C, and then a drastic degradation process occurs from this temperatu re up to around 600C. Figure 5 12. Thermogravimetric analysis of polymer P3 and P4 5.4 Photovoltaic Devices (Experimental work was achieved and results kindly supplied by Tzung Han Lai in Prof. Franky So ) Solution processed polymer bulk heteroj unction solar cells were fabricated using PDPP3T P3 and P4 as electron donor materials and PC 71 BM as an electron acceptor

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157 material in a device configuration ITO/ZnO/polymer:PCBM/MoO 3 /Ag. Here, due to the mobility difference between electrons and holes of PDPP3T ( h = 0.05 cm 2 V 1 s 1 e = 0.008 cm 2 V 1 s 1 ), 109 a higher ratio of PC 71 BM was needed. PDPP3T solution was made with PC 71 BM in a 1:3 weight ratio with 5% 1,8 diiodoctane (DIO) by volume. The device shows a PCE of 4.01% with short circuit current ( J sc ) of 11.2 m A cm 2 open circuit voltage ( V oc ) of 0.68 V and fill factor ( FF ) of 0.525 ( Figure 5 13). The enhanced absorption in longer wavelength provides a high short circuit current. Figure 5 13. Illuminated J V characteristics of PDPP3T :PC 71 BM solar cells usin g ITO/ZnO/Polymer:PC 71 BM/MoO 3 /Ag device architecture with DIO as a processing additive. Random copolymer P3 has enhanced absorption at 400 600 nm compared to PDPP3T but also shows a blue shift of the onset of long wavelength absorption from 900 to 850 nm. Higher PCE along with higher short circuit current is expected for this polymer. However, the polymer:PCBM solar cell (with 5% DIO) shows a low PCE of 2.41 % with J sc of 4.81 mA cm 2 V oc of 0. 72 V and FF of 0.63 (Figure 5 14a). The results become even wor se without DIO additive ( Figure 5 14a). Moreover, copolymer P4 which has greater absorption in the range of 400 600 nm, shows an even lower J sc

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158 (2.69 mA cm 2 ) than P3 With a V oc of 0.71 V and FF of 0.62, the efficiency of the P4 :PC 71 BM solar cell (with 5 % DIO) is about 1.2%. Clearly, the low short circuit currents have become the limiting factor in the performance of OPVs based on P3 and P4 The solar cell results of all the polymers are summarized in Table 5 3. a) b) Figure 5 14. Illuminated J V characteristics of polymer:PC 71 BM solar cells using ITO/ZnO/Polymer:PC 71 BM/MoO 3 /Ag device architecture with and without DIO as a processing additive. a) P3 :PC 71 BM (1:2) as the active layer. b) P4 :PC 71 BM (1:1) as the active layer. Table 5 3. Photovoltaic Properties of PDPP3T P3 and P4 with PC 71 BM as an acceptor. Polymer:PC 71 BM (ratio) Additive J sc (mA cm 2 ) V oc (V) FF (%) PDPP3T (1:3) 5% DIO 11.22 0.68 0.53 4.01 P3 (1:2) 5% DIO 4.81 0.72 0. 70 2.41 P3 (1:2) 3.18 0.72 0.64 1.47 P4 (1:1) 5% DIO 2.69 0.71 0.62 1.19 P4 (1:1) 2.37 0.73 0.64 1.11 To understand why the enhanced absorption of P3 and P4 does not give a higher short circuit current, further characterizations were made using PD TG TPD as a standard (see C hapter 1 for more information about this polymer). 173 Polymer P4 was characterized and the results were compared with these of PDTG TPD Figure 5 15a shows the absorption spectra of polymer:PC 71 BM blend films. Clearly, the P4 :PC 7 1 BM

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159 film demonstrates a stronger absorption in the range of 350 600 nm and an extension of the low energy optical transition to 800 nm. However, the external quantum efficiency (EQE) measurement tells another story ( Figure 5 15b). From the EQE data, we can see that while the PDTG TPD :PC 71 BM blend shows almost 70% of EQE at its highest, P4 :PC 71 BM blend shows less than 20% in the polymer absorption region. The missing EQE value in the range of 600 to 800 nm, where the polymer also absorbs light, is direct evi dence that the short circuit currents were mainly coming from PC 71 BM. a) b) Figure 5 15. Absorption and EQE of polymer:PC 71 BM blends. a) Thin fi lm UV visible absorption spectra of P4 : PC 71 BM blend and PDTG TPD :PC 71 BM blend. b) EQE spectra of solar cell devices using P4 and PDTG TPD According to these results, it is obvious that polymer P4 absorbs the sunlight, but there is only a small number of electrons coming out of the device, lea ding to low short circuit currents. In order to further understand this result, the photoluminescence (PL) quenching experiment was carried out. Figure 5 16 shows the quenching results of polymer P4 As we can see, the photoluminescence quenching efficienc y is rather low when PC 71 BM is added, which presents low photo induced charge transfer

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160 efficiency. 281 284 This is possibly because most of the generated excitons did not dissociate on the polymer:PCBM interface. Figure 5 16. Photoluminescence spectra o f P4 and P4 :P 71 CBM (1:1) chlorobenzene solution shows a limited PL quenching of the P4 :P 71 CBM blend, compared to the PL emission from a pure P4 solution. From the energy level point of view, the LUMO levels of the polymers are decreased from PDPP3T at 3.7 2 eV to P3 at 3.75 eV to P4 at 3.84 eV as the number of DPP units is decreased. The reduced energy offsets between the LUMO energy levels of the polymers and the LUMO level of PCBM dramatically lower the driving force for exciton dissociation and charge generation, and thus decrease the photon currents. Another possible reason for the low OPV performance is the unfavorable film morphology, which will be discussed in the next section. 5.5 Polymer Morphology Studies As with many examples found in this fiel d, the device performance is highly dependent on the active layer thin film morphology. A typical device of P3 :PC 71 BM (1:1) with 5% DIO was characterized by atomic force microscope (AFM). Figure 5 17 shows

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161 the height images of the active layer. Clearly, ev en with the addition of 5% DIO, phase separation is observed to be on the order of a couple of hundred nanometers, with large aggregates of PC 71 BM or polymer observed. The coarse morphology revealed here could result from a rather pronounced phase segregat ion (demixing) between PCBM and the polymer, as described previously. 285,286 The lack of an interpenetrating network and the existence of large dimensions would decrease exciton harvesting, and thus the OPV performance. The unfavorable morphology could be due to the increased bulky side (2 hexyldecyl) density of the copolymers, which prevents PCBM interpenetration into the polymer frame. Clearly, the use of side chains not only affects the solubility of the conjugated polymers, but also the molecular packin g and the interaction between the polymer and acceptor molecules. Figure 5 17. Atomic force microscope height images of P3 :PC 71 BM (1:1) based PV cells with 5% DIO spin coated from dichlorobenzene.

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162 5.6 Chapter Summary In this chapter, several DPP based copolymers differing by the donor acceptor ratios have been synthesized and characterized. Although polymers P1 and P2 with smaller alkyloxy side chains on the benzodithiophene repeat unit showed limited solubility in organic solvents, they provided insigh t into the control of polymer absorption by tuning the relative concentrations of electron donating and withdrawing substituents in the polymer backbone. With a better solubilizing group (2 hexyldecyl) on the BDT unit, random copolymers P3 and P4 showed c onsiderable solubility in chlorobenzene, chloroform and THF. More importantly, P4 with a monomer ratio of 1:0.75:0.25 (monomer 4 : 6a : 3 ) showed a broad absorption across 350 to 800 nm, except for a small absorption gap between 550 to 650 nm. This enhanced ab sorption profile was expected to lead to more photon induced current and improved performance of the solar cells. However the PCEs of P3 and P4 were dramatically lower than that of the control polymer PDPP3T The low J sc (3.87 and 2.69 mA cm 2 for P3 and P4 respectively) is the main reason that the solar cells showed undesirable low performances. In order to understand why enhanced absorption of P3 and P4 did not give expected higher short circuit currents, external quantum efficiency (EQE) and photolumi nescence quenching measurements were performed using P4 as an example. Less than 25% of EQE at all wavelengths revealed a low ratio of extracted free charge carriers to incident photons. Moreover, low charge transfer efficiency was demonstrated by the inef ficient quenching of the polymer luminescence using PCBM as an acceptor. The unfavorable polymer:PCBM film morphology caused by the demixing of polymer and PCBM domains led to a lack of dissociation of excitons, and thus the low J sc The bulky side chains could be the predominant factor involved in demixing of the polymers

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163 and PCBM. To provide further evidence for the conclusion, smaller side chains, which can provide sufficient solubility to the copolymers without blocking the interpenetration of PCBM, sho uld be examined. 5.7 Experimental Details 3,6 Di(thiophen 2 yl)pyrrolo[3,4 c]pyrrole 1,4(2H,5H) dione (1) Thiophene 2 carbonitrile (6.54 g, 60 mmol), t BuOK (11.2 g, 100 mmol) and t amyl alcohol (50 mL) were combined in a 500 mL round bottom flask and hea ted to 110 C under nitrogen. Then a solution of dimethyl succinate (2.92 g, 20 mmol) in t amyl alcohol (16 ml) was added using an addition funnel. When the addition was completed, the mixture was kept at 110 C for another 2 h, and then cooled to 50 C an d diluting with 100 ml methanol. After the mixture was carefully neutralized with glacial acetic acid, the suspension was filtered and washed with methanol and water. The product was obtained as a dark purple solid and used directly without further purific ation. 2,5 Bis(2 hexyldecyl) 3,6 di(thiophen 2 yl)pyrrolo[3,4 c]pyrrole 1,4(2H,5H) dione (2) Compound 1 (4.5 g, 15 mmol), potassium carbonate (10.4 g, 75 mmol) and 7 (bromomethyl)pentadecane ( 18.3 g, 60 mmol) were dissolved in DMF (200 mL). The mixture wa s heated to 120 C and stirred for 40 h. Then the solvent was removed under vacuum. The crude material was purified by flash chromatography (CHCl 3 ) to yield purple solid of compound 2 (4.5 g, 40%). 1 H NMR (CDCl 3 ): 8.69 (d, 1H), 7.56 (d, 1H), 7.17 (d, 1H) 3.93 (d, 2H), 1.85 (m, 1H), 1.30 1.25 (m, 24H), 0.85 (m, 6H). 3,6 Bis(5 bromo 2 thienyl) 2,5 dihydro 2,5 di(2 hexyldecyl) pyrrolo[3,4 c]pyrrolo 1,4 dione (3) Compound 2 (2.0 g, 2.67 mmol) and NBS (1 g, 5.6 mmol) were added into a round bottom flash and t hen degassed and refilled with argon 3 times

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164 Chloroform (50 mL) was then added The reaction mixture was stirred at room temperature for overnight, diluted with chloroform (100 mL), and washed with water The organic layer was dried with MgSO 4 After the solvent was evaporated, the residue was dissolved in 30 mL THF. And then MeOH was added into the THF solution until the crude product was precipitated. After filtration, the crude product was dissolved in 30 mL THF again, and two more precipitation cycles were applied. Then the product was obtained as a dark red solid (2.18 g, 90%). 1 H NMR (CDCl 3 ): 8.62 (d, 1H), 7.21 (d, 1H), 3.91 (d, 2H), 1.86 (m, 1H), 1.29 1.22 (m, 24H), 0.86 (m, 6H). 13 C NMR (CDCl 3 ): 161.57, 139.58, 135.52, 131.62, 131.37, 119.17, 10 8.19, 46.52, 37.94, 32.09, 31.96, 31.36, 30.18, 29.85, 29.71, 29.50, 26.34, 22.88, 22.84, 14.33, 14.30. HRMS (ESI FTICR): m/z calcd for C 46 H 71 Br 2 N 2 O 2 S 2 (M+H + ) 907.3303 found 907.3325 Anal. calcd for C 46 H 70 Br 2 N 2 O 2 S 2 : C 60.91, H 7.78, N 3.09 found C 61.32, H 8.05, N 3.00. 2,5 Bis(trimethylstannyl)thiophene (4) A solution of n butyllithium (16.5 mL, 35.65 mmol, 2.165 M in hexane) was added slowly to diisopropylamine (3.61 g, 35.65 mmol) in ether (70.0 mL) at 78 C After stirring for 1 h at 78 C the solutio n was warmed to room temperature and then cooled to 0 C in an ice bath. Thiophene (1 g, 11.88 mmol) was added, and the mixture was warmed to room temperature and stirred overnight After the reaction mixture was cooled to 0 C again, trimethylchlorostannane (7.1 g, 35.65 mmol) was added The mixture was stirred for 3 h at 0 C After dilution with ether (100 mL), the organic phase was washed with water and dried over anhydrous magnesium sulfate The solvent was evaporated and the residue was purified by flash chromatography with hexane as an eluent on treated silica gel (washed the silica gel with neat triethylamine, then hexane) to give 4.6 g (95%) of compound 4

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165 as a white solid 1 H NMR (CDCl 3 ): 7.39 (s, 1H), 0.38 (s, 9H) 13 C NMR (CDCl 3 ): 143.25, 136.0 3, 7.96 Anal. calcd for C 10 H 20 SSn 2 : C 29.31, H 4.92, found C 29.04, H 5.11. 4,8 Diethylhexyloxybenzo[1,2 b:4,5 Benzo[1,2 b:4,5 b']dithiophene 4,8 dione (4.4 g, 20 mmol), zinc dust (3.92 g, 60 mmol), NaOH (16 g, 400 mmol) and 2 ethylhe xyl bromide (15.5 g, 80 mmol) were added into a 250 mL round bottom flask and degassed. After DMF 150 mL was added, the reaction mixture was bubbled with argon and stirred at room temperature for 20 min, then heated at 90 C for overnight. At room temperatu re, the mixture was quenched with MeOH and diluted with ether. The organic phase was washed with water and dried over anhydrous magnesium sulfate The crude product was purified by chromatography with hexane to obtain compound 5a as colorless oil (7.5 g, y ield: 84%) 1 H NMR (CDCl 3 ): 7.50 (d, 2H, J = 5.4 Hz ), 7.38 (d, 2H, J = 5.4 Hz ), 4.20 (d, 4H, J = 5.4 Hz ), 1.82 1.38 (m, 18H), 1.05 0.92 (m, 12H). 13 C NMR (CDCl 3 ): 144.87, 131.71, 130.16, 126.14, 120.47, 76.26, 40.89, 30.69, 29.43, 24.08, 23.33, 14.36, 11.53 HRMS (ESI FTICR): m/z calcd for C 26 H 38 O 2 S 2 (M + ) 446.2308 found 446.2328 Anal. calcd for C 26 H 38 O 2 S 2 : C 69.91, H 8.57, found C 70.07, H 8.85. 4,8 dihexyldecyloxybenzo[1,2 b:4,5 The same reaction and purification procedure as described for 5a was followed. Benzo[1,2 b:4,5 b']dithiophene 4,8 dione (1.5 g, 6.81 mmol), zinc dust (1.34 g, 20.5 mmol), NaOH (5.45 g, 136.2 mmol), 2 ethylhexyl bromide (8.32 g, 27.24 mmol) and DMF 50 mL. Compound 5b was collected as light yellow oil (3.5 g, 76%). 1 H NMR (CDCl 3 ): 7.50 (d, 1H, J = 5.4 Hz ), 7.38 (d, 1H, J = 5.4 Hz ), 4.19 (d, 2H, J = 5.1 Hz ), 1.88 1.31 (m, 25H), 0.92 0.87

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166 (m, 6H). 13 C NMR (CDCl 3 ): 144.89, 131.71, 130.15, 126.09, 120.48, 76.60, 39.45, 32.14, 31.57, 30.31, 29.98 29.87, 29.60, 27.22, 27.20, 22.93, 14.36 HRMS (ESI FTICR): m/z calcd for C 42 H 71 O 2 S 2 (M+H + ) 671.4890 found 671.4882 Anal. calcd for C 42 H 70 O 2 S 2 : C 75.16, H 10.51, found C 75.44, H 11.07. 2,6 Dibromo 4,8 diethylhexyloxybenzo[1,2 b:4,5 A solution of NBS (1.67 g, 9.4 mmol, in 15 mL acetonitrile) was added slowly to a compound 5a (2.0 g, 4.48 mmol) chloroform (15.0 mL) solution at 0 C The mixture was stirred at the same temperature for 2 h before allowing it to warm to room temperature. A fter stirring at room temperature for overnight, the solution was diluted with methylene chloride and the organic phase was washed with water and dried over anhydrous MgSO 4 The solvent was evaporated and the residue was purified by column chromatography o n silica gel (hexane) to yield the desired product as a light yellow oil (2 g, 74%). 1 H NMR (CDCl 3 ): 7.41 (s, 1H), 4.10 (d, 2H, J = 5.4 Hz ), 1.79 1.38 (m, 9H), 1.03 0.93 (m, 6H). 13 C NMR (CDCl 3 ): 142.90, 131.11, 130.95, 123.31, 115.11, 76.52, 40. 82, 30.56, 29.38, 24.00, 23.30, 14.35, 11.50 HRMS (ESI FTICR): m/z calcd for C 26 H 37 Br 2 O 2 S 2 (M+H + ) 605.0577 found 605.0589 Anal. calcd for C 26 H 36 Br 2 O 2 S 2 : C 51.66, H 6.00, found C 51.27, H 6.02. 2,6 Dibromo 4,8 dihexyldecyloxybenzo[1,2 b:4,5 (6b) The same reaction and purification procedure as described for 6a was followed. NBS (1.12 g, 6.26 mmol), acetonitrile (15 mL), compound 5b (2.0 g, 2.98 mmol) and chloroform (15.0 mL). Compound 6b was collected as light yellow oil (1.7 g, 69%). 1 H NMR (CDCl 3 ): 7.41 (s, 1H), 4.09 (d, 2H, J = 5.4 Hz ), 1.82 1.26 (m, 25H), 0.91 0.83 (m, 6H). 13 C NMR (Benzene d6): 143.76, 131.86, 131.70, 124.03, 115.86, 76.99, 39.92,

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167 32.74, 32.69, 32.01, 30.87, 30.53, 30.48, 30.21, 27.77, 27.73, 23.53, 14.79 HRMS (ESI FTICR): m/z calcd for C 42 H 71 O 2 S 2 (M 2Br+3H + ) 671.4890 found 671.4870 Anal. calcd for C 42 H 68 Br 2 O 2 S 2 : C 60.86, H 8.27, found C 61.19, H 8.55. General Polymerization Procedure. Polymer PDPP3T: A solution of compound 4 (0.1024 g, 0.25 mmol), compound 3 ( 0.2267 g, 0.25 mmol), tris(dibenzylideneacetone)dipalladium (0) (Pd 2 (dba) 3 ) (4.6 mg, 0.005 mmol) and tri(o tolyl)phosphine (P(o tolyl) 3 ) (6.1 mg, 0.02 mmol) in toluene (10 mL) was degassed three times by successive freeze pump thaw cycles and heated at 115 C for 72 h in an oil bath. After the reaction was cooled to 50C, large spatula tip of diethylammonium diethyldithiocarbamate (D DDC) was added into the polymer solution, and then the solution mixture was stirred for 1 h at 50C under argon atmosphere. Th e mixture was then precipitated into methanol (200 mL). The precipitate was filtered through a cellulose thimble and purified via Soxhlet extraction with methanol (1 day), acetone (1 day) hexanes (1 day). The polymer was extracted with chloroform, concentr ated by evaporation, and then precipitated into methanol again (200 mL). The collected polymer was a dark green solid (157 mg, 76 %). 1 H NMR (CDCl 3 ): 8.89 (b, 1H), 7.36 (b, 1H), 7.07 (b, 1H), 4.10 (b, 2H), 1.80 1.00 (b, 25H), 0.80 (b, 6H). GPC analysis: M n = 124.9 kDa, M w = 158.3 kDa, PDI = 1.3. Anal. Calcd. for C 50 H 72 N 2 O 2 S 3 : C 72.41, H 8.75, N 3.38, found C 71.97, H 9.16, N 3.05. P1: The same polymerization and purification procedure as described for PDPP3T was followed. Compound 4 (0.1024 g, 0.25 mmol) compound 6a (0.0756 g, 0.125 mmol), compound 3 (0.1134 g, 0.125 mmol), Pd 2 (dba) 3 (4.6 mg, 0.005 mmol) and P(o tolyl) 3 (6.1 mg, 0.02 mmol) and toluene (10 mL). The collected polymer was a dark

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168 green solid (45 mg, 27 %). GPC analysis: M n = 130.8 kDa, M w = 158.3 kDa, PDI = 1.2. Anal. Calcd. for C 80 H 110 N 2 O 4 S 6 : C 70.85, H 8.18, N 2.07, found C 70.43, H 8.98, N 1.76. P2: Compound 4 (0.1024 g, 0.25 mmol), compound 6a (0.1133 g, 0.1875 mmol), compound 3 (0.0567 g, 0.0625 mmol), Pd 2 (dba) 3 (4.6 mg, 0.005 mmol) and P(o tolyl) 3 (6.1 mg, 0.02 mmol) and toluene (10 mL). The collected polymer was a dark green solid (60 mg, 40 %). GPC analysis: M n = 54.6 kDa, M w = 96.4 kDa, PDI = 1.8. Anal. Calcd. for C 140 H 186 N 2 O 8 S 12 : C 69.78, H 7.78, N 1.16, found C 70.73, H 8.46, N 0.9 4. P3: Compound 4 (0.1024 g, 0.25 mmol), compound 6b (0.0756 g, 0.125 mmol), compound 3 (0.1134 g, 0.125 mmol), Pd 2 (dba) 3 (4.6 mg, 0.005 mmol) and P(o tolyl) 3 (6.1 mg, 0.02 mmol) and toluene (10 mL). The collected polymer was a dark green solid (130 mg, 6 7 %). GPC analysis: M n = 59.4 kDa, M w = 114.3 kDa, PDI = 1.9. Anal. Calcd. for C 474 H 707 N 7 O 20 S 30 : C 73.12, H 9.15, N 1.26, found C 74.06, H 9.94, N 1.17. P4: Compound 4 (0.1318 g, 0.322 mmol), compound 6b (0.2 g, 0.241 mmol), compound 3 (0.0729 g, 0.0804 m mol), Pd 2 (dba) 3 (5.9 mg, 0.0064 mmol) and P(o tolyl) 3 (7.9 mg, 0.026 mmol) and toluene (15 mL). The collected polymer was a dark green solid (189 mg, 77 %). 1 H NMR (500 MHz, CDCl 3 ) 16H), 1.82 1.00 (b, 248H) GPC analysis: M n = 89.3 kDa, M w = 139.7 kDa, PDI = 1.6. Anal. Calcd. for C 188 H 282 N 2 O 8 S 12 : C 73.24, H 9.22, N 0.91, found C 74.23, H 10.19, N 0.83.

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169 CHAPTER 6 CONCLUSIONS AND PERS PECTIVES T his dissertation has focused on the utilization of random Stille polymerization as a tool for the synthesis of donor acceptor conjugated copolymers, and an understanding of structural parameters in controlling the optoelectronic properties of these materia ls. A variety of methods was used to characterize the resulting conjugated polymers and investigate their structure property relationships. As described in Chapters 2 and 3, other significant accomplishments of this work were refinement of the synthesis an d purification procedures and reproducible preparation of polymers with consistent optoelectronic properties. Chapter 3 presented the synthesis and characterization of random black to transmissive conjugated polymers for electrochromic applications. A seri es of broadly absorbing copolymers was produced with number average molecular weights ranging from 10 to 18 kDa and polydispersities ranging from 1.3 to 1.6 after Soxhlate extraction. Polymer ECP 3 was utilized as an example for the subsequent electrochrom ic property study and showed a transmittance contrast as high as 42% at the 1 s switch time and a high continuous switching stability (18,000 cycles). However, this polymer was not ideally black, because it failed to cover part of the blue (380 450 nm) and red (690 750 nm) regions. According to the structure of this polymer, an extra stronger acceptor (for example thienopyrazine) is expected to extend the low energy absorption band to a much longer wavelength, while an extra weaker donor (for example dioxyt hiophene) will enhance absorption at the shot wavelength end. The random Stille polymerization introduced in this chapter is expected to be an optimal route towards achieving such systems.

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170 Chapter 4 demonstrated a facile approach to synthesize two electroc hromic polymers ECP Blue and ECP Black bearing alkyl ester side chains, which were converted into water soluble and processable polymer salts after subsequent defunctionalization of the ester groups. Further neutralization of the polymer salt films (spray cast from water solution) yielded WS ECP Blue acid and WS ECP Black acid films with a dramatic improvement in the electrochromic switching speed performance compared to their ester derivatives. Future work can be directed toward applying this chemistry in the synthesis of water soluble conjugated polymers with other saturation colors (yellow, cyan, and red et al. ) Another aspect from this side chain defunctionalization approach, as well as the following neutralization process, is that multilayer electroch romic devices can be easily achieved by depositing a second polymer layer on top of the first lay, which cannot be dissolved in organic solvents or water. T he investigation of the utilities of these water soluble conjugated polymers in other applications s uch as OLEDs, c hemical and biochemical sensors could also be another interesting direction. In Chapter 5, attention was turned to making a new type of diketopyrrolopyrrole based random conjugated copolymer for organic solar cells based on the success in ex tending polymer absorption spectra for efficient light harvesting. The resulting polymers were expected to improve the photocurrents and power conversion efficiencies. However, reduced performance was observed for the new polymers compared with the control polymer. Subsequent experiments (EQE, photoluminescence quenching and film morphology studies) revealed that the decreased performance could be attributed to a lack of dissociation of excitons caused by the demixing of polymer and

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171 y levels and band gaps are important, morphology is still key Over the past 40 years, significant research effort has been conducted in the field of conjugated polymers. By now the success in applying these materials in optoelectron ic devices has gained great attention not only in academia but also in industry. In this work, the understanding of structure property relationship is an ultimate goal in this field. However, it should also be pointed out that chemistry is the foundation of the development of this course This dissertation demonstrates a fundamental chemical approach random polymerization, which is used to combine multiple monomers, adjust their relative ratio and fine tune the optoelectronic properties of polymers. The essentials of this method are now established. With further development, this method can be extended to a much broad er family of conjugated polymers.

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188 BIOGRAPHICAL SKETCH Pengjie Shi was born in Changchun, China, in 1980. He graduated with his B.S. in chemistry from the University of Science and Technology of China in 2004, where he worked in the research group of Prof. Caiyuan Pan in the field of living free radical polymerization and block copolymers design. In 2005, Pengjie moved to Gainesville, Florida to pursue his Ph.D. under the guidance of Prof. John R. Reynolds. His research has been focus ed on the synthesis and characterization of discrete conjugated oligomers and polymers for optoelectronic devices. Pengjie earned his doctoral degree from the University of Florida in the fall 2011. He will work as a R&D scientist (polymer synthesis) at Sigma Aldrich.