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1 ORGANIC SEMICONDUCTI NG MOLECULES AND POL YMERS FOR SOLUTION PROCESSED ORGANIC EL ECTRONICS By ROMAIN STALDER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIRE MENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2012
2 2012 Romain Stalder
3 To my parents
4 ACKNOWLEDGMENTS I would like to thank my advisor Prof. John R Reynolds, for his guidance and support over the five years I have spe nt in his research group. His balanced leadership has created an environment where I was able to develop as a young scientist in way I had not imagined possible, in retrospect. He was able to maintain a great synergy within the group between group members and also with outside collaborators which I am thankful for. As a researcher, he trained me how to properly write a technical report, draft a cover letter and a manuscript, review a scientific paper and give a public presentation. He is also willing to spe nd time on more personal aspects of the graduate school process, even in late afternoons after a long work day. His dedication to the development of his students into independent researchers will remain a model to me. I would also like to thank the members of my committee, Prof. Aaron Aponick, Prof. Ken Wagener, Prof. Valeria Kleiman, and Prof. Franky So, for contributing their valuable time and expertise to my education at the University of Florida. I also wish to thank Prof. Ken Wagener for the thoughtful discussions and advice he has given over the years, and for his efforts to make the Butler Polymer Laboratory an excellent and friendly research environment. I am grateful to my collaborators who have contributed to the projects I have carried out at UF. Dr. Kirk Schanze, Dr. Jiangeng Xue, Dongping Xie and Renjia Zhou contributed significantly to the work carried out on hybrid systems. Prof. Franky So and Dr. Jegadesan Subbiah in the Department of Material Science and Engineering at UF have worked on polym er based devices with some of the materials I have synthesized. They grea tly contributed to increasing the significance of my work in the field of organic electronics.
5 I am certainly indebted to Jianguo Mei and Ken Graham, the initial members of the team. The work initiated by Jianguo on isoindigo has given me the opportunity to expand my capabilities as a synthetic chemist, and he has helped me towards some of the first work I was able to publish. His knowledge and great kindness is commendable. Ken interesting discussions linking the abilities of synthetic chemists and analytical chemists in a natural and fruitful way. I thank him for his patience and diligence with some of th e materials I delivered to him. After joining the Reynolds group, many people took the time to help me in my work in acquiring the proper techniques. I am grateful for the mentoring I received from Dr. Christian Nielsen, and the help from Pengjie Shi in my early days in the synthetic labs. I would also like to thank Dr. Stefan Ellinger, Dr. Timothy Steckler, Dr. Svetlana Vasilyeva Dr. Dan Patel, Dr. Mike Craig, Dr. Chad Amb, Dr. Pierre Beaujuge. Special thanks go to Dr. Leandro Estrada for his time and inv olved discu ssions on the isoindigo systems and to Dr. Aubrey Dyer for her assistance in many different aspects of the group life. The younger generations of students have also contributed to make this stay a pleasant and productive one. I especially thank Caroline Grand for her diligence in the solar cell work we collaborated on, and Natasha Teran for her patience and help in the last years. I would also like to show my appreciation to Cheryl Googins, Gena Borrero and Sara Klossner for their great service, which made everything easy and convenient. Additionally, I will take this opportunity to thank George and Josephine Butler. I have enormously benefited from the environment and facility brought by their generous gifts. I
6 am also grateful for the Butler Pol ymer Award funded by the Butler Foundation. Many thanks to Lori Clark and Dr. Ben Smith for all their assistance with the Graduate School process.
7 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ .......... 10 LIST OF FIGURES ................................ ................................ ................................ ........ 11 LIST OF SCHEMES ................................ ................................ ................................ ...... 15 LIST OF ABBREVIATIONS ................................ ................................ ........................... 16 ABSTRACT ................................ ................................ ................................ ................... 18 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 20 1.1 Semiconducting Materials ................................ ................................ ................. 20 1.2 Organic Semiconductors in the Solid State. ................................ ...................... 22 1.2.1 Band Analogy in Organic Semiconductors. ................................ ............. 22 1.2.2. Degree of Homogeneity in Solid State Organic Systems. ...................... 24 1.2.3 Nature of the Charge Carriers i n Organic Solids. ................................ .... 25 1.2.4 Valuable Charged Species. ................................ ................................ ..... 27 1.3 Organic Electronics: Which Parameters Can the Synthetic Chemist Opti mize? ................................ ................................ ................................ ............ 28 1.3.1 Organic Field Effect Transistors. ................................ ............................. 28 1.3.2 Electrochromics. ................................ ................................ ...................... 31 1.3.3 Organic Solar Cells. ................................ ................................ ................. 33 1.4 Energy Levels and Morphology: How to tailor these two Key Parameters? ...... 36 1.4.1 E nergy Levels Control. ................................ ................................ ............ 37 1.4.2 Morphology control in single component active layers. ........................... 41 1.5 Morphology Control in Organic Solar Cells: Successful Variations. .................. 42 1.5.1 Polymer/PCBM solar cells. ................................ ................................ ...... 42 1.5.2 Small molecule/PCBM solar cells. ................................ ........................... 44 1.5.3 Organic/inorganic hybrid solar cells. ................................ ........................ 45 1.5.4 Polymer/polymer solar cells. ................................ ................................ .... 47 1.6 Thesis of This Dissertation. ................................ ................................ ............... 48 2 EXPERIMENTAL METHODS AND CHARACTERIZATIONS ................................ 51 2.1 Structural and Polymer Characterizati on. ................................ ......................... 51 2.1.1 General Structural Characterizations. ................................ ...................... 51 2.1.2 X Ray Spectroscopy. ................................ ................................ ............... 51 2.1.2 Molecular Weight Characterizations. ................................ ....................... 54 2.1.3 Thermal Characterizations. ................................ ................................ ..... 54
8 2.1.4 Polymer Free Standing Film Preparation. ................................ ............... 55 2.2 Electrochemical Methods. ................................ ................................ ................. 55 2.3 Optical and Spectroscopic Characterizations. ................................ .................. 57 2.3.1. UV vis Spectroscopy. ................................ ................................ ............. 57 2.3.2. Photoluminescence Quenching Experiments. ................................ ........ 57 2.3.3 Polarized Optical Microscopy ................................ ................................ .. 58 2.4 Device Fabrication. ................................ ................................ ........................... 59 2.4.1 OFETs Fabrication. ................................ ................................ ................. 59 2.4.2. All Polymer Solar Cells. ................................ ................................ .......... 60 2.4.3. Polymer/PCBM Solar Cells. ................................ ................................ .... 61 3 CONJUGATED SMALL MOL ECULES FOR ACTIVE LAYER MORPHOL OGY CONTROL IN TRANSISTO RS AND SOLAR CELLS A PPLICATIONS .................. 63 3.1 Design of Symmetrical and Unsymmetrical Oligomers for Three Different Approaches to Morphology Control ................................ ................................ ..... 63 3.2 Synthesis of Functionalized Oligomers ................................ ............................. 65 3.2.1 Symmetrical Sexithiophene Bearing Two Terminal Alcohol Groups ........ 65 3.2.2 Unsymmetrical Oligomers Bearing one Phosphonic Acid Group ............. 67 3.2.3 Symmetrical and Unsymmetrical Functionality Free Donor Acceptor Donor Oligomers ................................ ................................ ........................... 73 3.3 Morphology Control via Telechelic Oligomer Polycondensation ....................... 76 3.3.1 Synthesis of T6PC from T6diol ................................ ................................ 76 3.3.2 Spectroscopy, Electrochemistry and Spectroelectrochemistry of T6PC .. 79 3.3.3 Liquid Crystallinity and Bulk Morphology ................................ ................. 85 3.4 Morphology Control via Monofunctional Oligomer/Inorganic Nanoparticle Hybrids ................................ ................................ ................................ ................ 92 3.4.2 Oligomer/CdSe NC PL Quenching Experiments ................................ ..... 97 3.4.3 Hybrids Synthesis and Characterization ................................ ................ 101 3.5 Morphology Control via BHJ Crystallinity Disruption. ................................ ...... 105 3.5.1 Electrochemical, Thermal and Optical Properties. ................................ 106 3.5.3 Crystallization Behavior and Influence on Solar Cell Performance ........ 113 3.6 Synthetic Details ................................ ................................ ............................. 117 4 ISOINDIGO, A VERSATI LE ELECTRON DEFICIENT UNIT FOR P TYPE AND N TYPE ORGANIC ELECTRO NIC APPLICATIONS ................................ ............ 142 4.1 The isoindigo molecule ................................ ................................ ................... 142 4.2 Isoindigo model compounds. ................................ ................................ .......... 147 4.3 Isoi ndigo Based Donor Acceptor Conjugated Polymers. ................................ 154 4.3.1 Polymer Synthesis and Characterization ................................ ............... 154 4.3.2 Electrochemistry and Optical Properties. ................................ ............... 157 4.4 All Acceptor Isoindigo Based Conjugated Polymers. ................................ ...... 163 4.4.1 Polymer Synthesis and Optical Properties ................................ ........... 164 4.4.2 Electrochemistry and Spectroelectrochemical measurements. ............. 167 4.4.3 All Polymer Solar Cells. ................................ ................................ ......... 173 4.5 Isoindigo Based D A Polymers for BHJ Polymer Solar Cells. ......................... 177
9 4.5.1 Isoindigo in Polymer Solar Cells. ................................ ........................... 177 4.5.2. Polymer Synthesis and Characterization. ................................ ............. 178 4.5.3 Polymer/PCBM Solar Cells. ................................ ................................ ... 182 4.6 Synthetic D etails. ................................ ................................ ............................ 184 5 CONCLUSIONS AND PERSPECTIVES ................................ ............................... 197 APPENDIX: CRYSTALLOGRAPHIC DAT A ................................ ................................ 199 LIST OF REFERENCES ................................ ................................ ............................. 202 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 214
10 LIST OF TABLES Table page 1 1 Processing method, p channel field effect mobility and on/off ratio for some of the classic OFET materials reported in the literature. ................................ ..... 42 2 1 Crystal growth methods employed for P iI P and T iI T ................................ .... 51 3 1 Absorption and fluorescence max optical HOMO LUMO gaps, extinction coefficients, FL quantum yields and FL lifetimes for each oligomer. ................... 94 3 2 Solution peak and onset absorptions, solution optical energy gap, and the co rresponding values or the as spun films and annealed films. ....................... 112 4 1 Effect of substituents on the longest wavelength absorption maxima of indigo. ................................ ................................ ................................ ............... 143 4 2 SEC results, optical properties and electrochemical data measured for the six polymers. ................................ ................................ ................................ .......... 162 4 3 Solar cell characteristics of the P(iI DTS) :PC 70 BM (1:4) blend. ........................ 183 A 1 Crystal data and structure refinement for T6 benzoate 3 9 ............................. 199 A 2 Crystal data and structure refinement for T iI T ................................ ............... 200 A 3 Crystal data and structure refinement for P iI P ................................ ............... 201
11 LIST OF FIGURES Figure page 1 1 Schematic electron population of allowed energy bands for a metal (left), a semiconductor and an insulator. ................................ ................................ ......... 21 1 2 Simplified molecular orbital (MO) diagram representing the effect of conjugat ion extention on the emergence of a band like structure. ...................... 23 1 3 Schematic description of polymer chains and illustration of the distribution o f conjugation units in the bulk. ................................ ................................ .............. 24 1 4 Schematic representations of typical OFET organic solar cell and electrochromic devices architectures. ................................ ................................ 27 1 5 Schematic representations o f a bottom gate/top contact OFET, and bottom gate/bottom contact OFET.. ................................ ................................ ................ 29 1 6 Schematic energy diagram illustrating the working principle of an OFET with respect to applied V G ................................ ................................ ......................... 31 1 7 Repeat unit structures and photographs of spray cast dioxythiophene based polymer films in the neutral colored, and oxidized transmissive states .............. 32 1 8 Schematic representation of the electronic processes involved i n a bilayer heterojunction cell ................................ ................................ .............................. 34 1 9 Example I V curves for a solar cell under illumination and in the dark, along with the two equations relating the solar cell parameters. ................................ .. 36 1 10 Illustrati on of the donor acceptor concept ................................ .......................... 38 1 11 St ructures of several acceptors from the literature, along with the LUMO energy level distribution of polymers incorporating them.. ................................ .. 40 1 12 Structure of the best performing solar polymers report ed to date, along with their energy levels, electrochemical band gaps, and PCE. ................................ 43 1 13 Structure of the two best performing solar small molecules reported to date, along with their energy lev els, electrochemical band gaps, and PCE. ................ 45 2 1 Pictorial description of the photoluminescence quenching experiments. ............ 58 2 2 Fre e standing film stretching setup and the stretched film set under the polarized light microscope objective. ................................ ................................ .. 59 3 1 Three synthetic approaches to control the active layer morphology of organic electronic devices ................................ ................................ ............................... 63
12 3 2 Crystal structure of T6 dibenzoate 3 9 ................................ ............................... 67 3 3 1H NMR of the polycarbonate T6PC and IR spectra of T6 di ol and T6PC .. ...... 78 3 4 UV vis spectra of T6 diol and T6PC in solution and solid state and spectra of the chemical doping process of T6PC ................................ ............................ 79 3 5 Tenth and 150 th cyclic voltammograms from 0 to 0.4 V and from 0 to 0.95 V of T6PC ................................ ................................ ................................ .............. 81 3 6 Cyclic voltammograms from 0.1 to 0.45 V and from 0.1 to 1.05 V and DPV of T6PC sprayed onto ITO ................................ ................................ .................. 82 3 7 Spectroelectrochemistry for a spray cast film of T6PC on ITO coated glass, from 0.23V to 0.54V versus Fc/Fc + 10mV potential increments. ........................ 84 3 8 Picture of a 7.0cm x 1.5cm free standing film of T6PC TGA and DSC thermograms of T6 diol and T6PC ................................ ................................ ..... 86 3 9 Polarized light optical microscope images of T6 diol and T6PC at crossed polarizer/analyzer ................................ ................................ ............................... 88 3 10 2D WAXS pattern of T6PC a s an extruded filament at 30 C and scattering intensity distribution as a function of the scattering vector ................................ .. 90 3 11 POM capture of the free standing film before and after stretching at 0 and 45 with respect to the analyzer at crossed polarizer/analyzer. .......................... 91 3 12 UV vis absorption and fluorescence spectra of the two oligomers and the CdSe NPs in chloroform solution ................................ ................................ ........ 93 3 13 CV and DPV of T6 PA and T4BTD PA in 0.1 M TBAPF 6 in dichloromet hane, at 50 mV/s scan rate ................................ ................................ ........................... 95 3 14 Energy levels diagram (absolute values) for the HOMO and LUMO levels of T6 PA T4BTD PA and NCs. ................................ ................................ .............. 96 3 15 Evolution of the fluorescence in chloroform of T6 PE and T6 PA and T4BTD PE and T4BTD PA upon addition of CdSe NPs into the solution .......... 99 3 16 Absorption spectra of the T6 based hy brid and the T4BTD based hybrid and that of free CdSe NC s in solution and their TGA thermograms ........................ 102 3 17 Absorption profiles of the hybrids compared to that of the free species in solution. ................................ ................................ ................................ ............ 104 3 18 Cyclic voltammograms of iIT 2 C6 2 iIT 2 C6Si and iIT 2 Si 2 and the corresponding differential pulse voltammograms ................................ ............. 107
13 3 19 DSC and TGA thermograms of iIT 2 C6 2 iIT 2 C6Si and iIT 2 Si 2 ....................... 108 3 20 UV vis absorption of iIT 2 C6 2 iIT 2 C6Si and iIT 2 Si 2 in solution and thin films before and after annealing ................................ ................................ ................ 110 3 21 Polarized light microscope images showing iIT 2 C6 2 crystals as a function of added iIT 2 C6Si in solution ................................ ................................ ............... 114 3 22 AFM height images o f [ iIT 2 C6 2 / iIT 2 C6Si ]:PC 61 BM (1:1 by weight) blend films with varying mole % of iIT 2 C6Si after 100 C thermal annealing ............. 115 4 1 Solution absorption spectra of the isoindigo model compounds, and solution electrochemistry of isoindigo, along with the ir reduction DPVs ........................ 149 4 2 DFT optimized structures and fronti er orbital density distributions for model compound T iI T ................................ ................................ .............................. 150 4 3 Pictures of T iI T crystals grown by slow evaporation of a chloroform solution and vapor diffusion between chloroform and acetonitrile. ................................ 151 4 4 Crystal packing of T iI T and P iI P ................................ ................................ 152 4 5 Crystal structures of T iI T and P iI P ................................ .............................. 153 4 6 Picture of 20 cm diame ter free standing film of P(iI F) and proton NMR spectrum of P(iI F) recorded in CDCl 3 ................................ ............................. 156 4 7 Cyclic voltammogram and differential pulse voltammogram of thin films of each polymer on Pt butt on electrode, recorded at a 50 mV/s scan rate ........... 158 4 8 Solution absorption spectra of P(iI T) EH and P(iI T) HD and the corresponding solid state absorption spectra. ................................ .................. 159 4 9 Normalized UV vis absorption spectra the five hi gh molecular weight polymers in chloroform solut ion and as thin films on ITO coated glass. ........... 161 4 10 TGA thermograms of Poly(iI) and Poly(iI BTD) under nitrogen flow, an d normalized absorption spectra in solution and in solid state ............................. 166 4 11 Cyclic and differential pulse voltammograms of Poly (iI) and Poly(iI BTD) recorded from thi n films on Pt button electrodes ................................ .............. 167 4 12 Overlaid reduction CVs of Poly(iI) at increasing scan rat es, and o verlayed ten first oxidation CVs of Poly(iI) ................................ ................................ ...... 168 4 13 Overlaid reduction CVs of Poly(iI BTD ) at increasing scan rates and o verlayed ten first oxidation CVs of Poly(iI BTD) ................................ ............. 169
14 4 14 Spectroelectrochemistry of Poly(iI) sprayed onto an ITO coated glass slide. Pictures of the neutral and reduced Poly(iI) film. ................................ ............. 170 4 15 Spectroelectrochemistryof Poly(iI BTD ) sprayed on to an ITO coated glass sl ide. ................................ ................................ ................................ ................. 172 4 16 Band structure diagram comparing the HOMO and LUMO levels of Poly(iI) PC 60 BM and P3HT. Solution electrochemistry of PC 60 BM ................................ 173 4 17 Schematic diagram of the all polymer solar cell in conventional device geometry, and thin film absorption spectra of the P3HT: Poly(iI) blends ......... 174 4 18 J V curves of the P3HT: Poly(iI) based solar cells with various blend ratios under AM1 .5 and e xternal quantum efficiency of the 1:1 blend ........................ 175 4 19 TGA thermogram, CV and DPV, solution and s olid state absorption of P(iI DTS) and film blend absorption of 1:4 blend of P(iI DTS) :PC70 BM ................. 180 4 20 P(iI DTS) :PC 70 BM (1:4) based BHJ solar cells and AFM images of the P(iI DTS) :PC 70 BM blend at 1:4 ratio ................................ .......... 182
15 LIST OF SCHEMES Scheme page 3 1 Synthesis of the thiophene end capping moiety bearing a protected terminal alcohol. ................................ ................................ ................................ ............... 65 3 2 Synthesis of T6 diol and T6 dibenzoate ( 3 9 ) for crystal growth. ....................... 66 3 3 Synthesis of thiophene end capp ed with a phosphonate group. ........................ 68 3 4 Synthesis of the regio regular T7 phosphonate rrT7 PE ................................ ... 69 3 5 Synthesis of the phosphonic acid functionalized oligomers T6 PA and T4BTD PA ................................ ................................ ................................ ......... 71 3 6 Synthesis of bithiophene end capp ed with a tr iisobutylsilyl group. ..................... 74 3 7 One pot synthesis (a) of iIT 2 C6Si and iIT 2 C6 2 and synthesis (b) of iIT 2 Si 2 .... 75 3 8 Polymerizatio n of T6 diol into T6PC using triphosgene. ................................ .... 77 3 9 Structure of T6 PA and T4BTD PA oligomers ................................ .................... 92 3 10 Structure of iIT 2 C6 2 iIT 2 C 6Si and iIT 2 Si 2 ................................ ..................... 105 4 1 Structures of indigo, diketopyrrolopyrrole and isoindigo. ................................ .. 142 4 2 Donor acceptor pattern, substituents positions and conjugation extent of indigo. ................................ ................................ ................................ ............... 143 4 3 Donor acceptor pattern, substituents positions and conjugation extent of DPP. ................................ ................................ ................................ ................. 144 4 4 Donor acceptor pattern, substituents positions and conjugation extent of isoindigo. ................................ ................................ ................................ .......... 145 4 5 Synthesis of the dibromo and diboron isoindigo precursors. ............................ 147 4 6 Synthesis of the bisphenyl, bisthiophene and bisEDOT isoindigo model compounds.. ................................ ................................ ................................ ..... 148 4 7 Synthesis of a family of iI based D A polymers.. ................................ .............. 155 4 8 Synthesis of the all acceptor polymers Poly(iI) and Poly(iI BTD) ................... 164 4 9 Structures of all the D A conjugated polymer reported in th e literature so far. 177 4 10 Synthesis of P(iI DTS) via Stille cross coupling. ................................ ............... 179
16 LIST OF ABBREVIATION S ACN Acetonitrile AFM Atomic force microscopy BHJ Bulk heterojunction BTD 2,1,3 benzothidiazole CT Charge transfer CV Cyclic voltammetry DCM Dichloromethane DIO Diiodooctane DMF Dimethylformamide DPP Diketopyrrolopyrrole D A D Donor Acceptor Donor dppf diphenylphosphino ferrocene DPV Differential pulse voltammetry Fc/Fc + Ferrocene/Ferrocenium FF Fill factor GPC Gel permeation chromatography J sc Short current density BTI B is(trifluoromethylsulfonyl)imide NBS N bromosucci ni mide OPVs Organic photovoltaics OFETs Organic field effect transistors Pd 2 (dba) 3 T ris (di benzylidene acetone ) dipalladium(0) PC Propylene carbonate PCE Power conversion efficiency
17 PC 60 BM [6,6] phenyl C61 butyric acid methyl ester fullerene PC 7 0 BM [6,6] phenyl C71 butyric acid methyl ester fullerene PDI Polydispersity index PL Photolumines cence POM Polarized optical microscopy P3HT Poly(3 hexylthiophene) TBAPF 6 T etrabutyammonium hexafluorophosphate THF Tetrahydrofuran THP Tetrahydropyran TLC Thin layer chromatography V oc Open circuit voltage
18 Abstract of Dissertation Presented to the Gradu ate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ORGANIC SEMICONDUCTI NG MOLECULES AND POL YMERS FOR SOLUTION PROCESSED ORGANIC EL ECTRONICS By Romain Stalder May 2012 Chair: John R. Reynolds Major: Chemistry In the f ield of organic electronics e ach material differs in its ability to balance hole (p type) and electron (n type) carrier creation and transport in devices, which depends on the energy level of its frontier molecular orbitals and its ability to adopt a suitable morphology for charge carrier transport. In this dissertation, both aspects of organic semiconductor design are presented. The first portion of the dissertation focuses on monodisperse conjugated olig omers, while the second portion describes a new electron deficient moiety and its u se in fully conjugated polymers. Three approaches to active layer morphology control are presented in the first portion. First the synthesis of a symmetrical oligothiophene which can further react via two terminal alcohol groups is pre sented, followed by i ts polymerization D espite the inherent conjugation break along the polymer main chain, the resulting polycarbonate remains electroactive, and liquid crystalline behavior i s identified by polarized optical microscopy and thermal analyses. Second monofunctional oligomers bearing phosphonic acid groups are synthesized as reactive molecules for hybrid organic/inor ganic photovoltaic applications, as they are designed to bind to inorganic nanocrystals. Third the synthesis of symmetrical and unsymmetrical oligomers is
19 presented, and the influence of the blend of unsymmetrical/symmetrical oligomers in the active layer is studied. In the second portion of this dissertation, the ele ctron deficient molecule isoindigo is presented as a valuable building block for conjugated materials applied to organic photovoltaics First, the synthesis of model compounds is described Fully conjugated donor acceptor polymers are then synthesized usin g electron donor co monomers of various donating strengths. T hese materials are of low band gaps thus absorb towards the near IR, and they have low HOMO and LUMO energy levels. This make s isoindigo based conjugated polymers good candidates as n type materi als. The synthesis of fully conjugated polymers composed exclusively of electron deficient units was thus targeted. In particular, t he homopolymer of isoindigo is use d in all polymer solar cells The last part of this dissertation presents the synthesis of the copolymer of isoindigo and dithienosilole, targeted as p type material for polymer/fullerenes solar cell applications. The photovoltaic characteristics of the blends are described, both in conventional and inverted solar cell architectures.
20 CHAPTER 1 INTRODUCTION 1.1 S emiconducting M aterials Technologies based on electronic processes rely, in one way or another, on the conduction of electrons. Depending on the nature of the atoms which constitute the electronically active material, the extent of ele ctrical conductivity varies greatly. The electrons involved in conductivity are found in the outermost shell of the atoms of the active material, or valence electrons, since these are the least tightly bound to the ionic core of the atoms. Metals have the highest conductivity, as they constitute the class of materials for which valence electrons are not localized around a particular atom, but rather can move freely about the lattice. Metals tend to crystallize in close packed structures, and the bonds forme d by the valence electrons are relatively weak, such that the latter can become conduction electrons. The model of a metal crystal is sometimes described as a sea of free electrons in which the positively charged ions are arranged, according to the particu lar crystal lattice the atoms pack in. 1 In the quantum theory formalism, an electron is described by a wavefunction which is a solution to the Schrdinger equation. The energy of an electron is then quantized, hence a distribution of energy levels which t he electrons can occupy. In the simple free electron model, the distribution of energy levels is continuous from zero to infinity. A less approximate model takes into account the effect of the crystal lattice on the distribution of states. A key feature of crystal lattices is that the propagation of waves within is influenced by Bragg reflections. This disturbs the continuous distribution of states, as Bragg reflections of electron waves in the crystal result in regions of energy in which the wavelike solut ions of the Schrdinger equation do not exist. This removes
21 some energy levels from the allowed distribution, resulting in allowed energy bands separated by forbidden energy gaps, or band gaps. 1 In a crystal according to this model, the position of the ban d gaps relative to the highest populated energy level determines the electrical conduction. Figure 1 1. Schematic electron population of allowed energy bands for a metal (left), a semiconductor (center) and an insulator (right). The dark grey regions r epresent filled states within the allowed bands. Solid state physics segregates materials typically inorganic for historical reasons in several classes depending on the population of electronic energy levels with respect to the band gaps, as depicted in F igure 1 1. A metal has high conductivity because an allowed energy band is partially filled, and so electrons respond readily to an applied electric field. In the contrary, for an insulator the highest occupied energy level corresponds with the beginning o f a band gap of too high energy for electrons to access the conduction band. The concept of semiconductor appear in materials which are insulators but for which the bang gaps is small enough so that external excitation may promote electrons from the valenc e band to the conduction band, turning the electrical conductivity on. 1 Regardless of the material employed, the conductivity of semiconductors will not surpass that of the highly conducting metals. Rather, the strength of semiconductors
22 resides in the act ual event of electron excitation from valence to conduction bands. Its advent upon various external stimuli at temperatures around 298 K has enabled a breadth of specific applications, some of which are treated in this dissertation as presented in Section 1.3 of this Chapter. In contrast with the inorganic materials for which the formalism of semiconductors was developed historically and briefly presented above, the active material herein is organic. There are significant differences between the charge tran sport characteristic of organic and inorganic materials, and thus the next section points out the key differences of organic semiconductors. This will lead into describing the important characteristics of the devices used in the applications relevant to th is dissertation. Finally the key parameters that the organic chemist can tune in order to improve device performance are highlighted. 1.2 Organic Semiconductors in the Solid State. 1.2.1 Band Analogy in Organic S emiconductors. Organic semiconductors are es sentially carbon based compounds. Carbon has the possibility of hybridizing its 2s and 2p orbitals in three different ways, resulting in sp, sp 2 and sp 3 hybridization. The four hybrid orbitals in a sp 3 hybridized C will bind covalently to other atoms into a molecular structure in which electrons are so tightly bound in the highly overlapping bonds that they cannot move freely outside of their respective hybrid orbital. When a carbon is sp 2 hybridized, one p z orbital remains unchanged, while the rest hybri dize. If two sp 2 carbons are brought together as in ethylene, the electrons in the bonding orbitals still form highly overlapping covalent bonds with other atoms, while the p z orbitals produce less strongly overlapping bonds, as illustrated in Figure 1 2. For ethylene (left), the highest occupied molecular orbital
23 (HOMO) and the lowest unoccupied molecular orbital (LUMO) correspond to the bonding and antibonding orbitals. Figure 1 2. Simplified molecular orbital (MO) diagram of a sp 2 hybridized e thylene type single unit (left) and representation of an MO diagram (center) for an ethylene type unit conjugated with several other ones: the unhybridized p z can overlap with a significant number of conjugated units, leading a buildup of energy band of co njugate polymer chain As more and more sp 2 carbons are covalently bound together, and provided there is sufficient overlap of each p z orbital with its neighbors, then the bonds become delocalized. In other words, and as illustrated in Figure 1 2 (center), in a fully conjugated chain of sp 2 carbons, the electrons in the hybridized orbitals overlap with a finite number of electrons, essentially only with their direct neigh bor in the bond they form, whereas the electrons in the p z orbitals can delocalize over a succession of other p z orbitals in the extended system. As the conjugation length increases, the energy gap between the HOMO and LUMO thus reduces as a result of electron delocalization. This leads to one dimensional bands with significant bandwidths, and a band gap still set between the HOMO and the LUMO (Figure 1 2, right). If the bandgap is small enough, then the fully conjugated system presents the electronic c haracteristic of a semiconductor. 2
24 1.2 2. Degree of Homogeneity in Solid State Organic Systems By analogy with inorganic crystals, covalent bo nding of sp 2 hybridized carbons via bonds results in an arrangement of tightly bound atoms, which constitutes a scaffold for the electrons to delocalize. Unlike inorganic crystals though, organic molecules are of finite sizes at the nanometer scale. Then, orbital overlap between successi ve molecules determines the extent of charge transport at the macroscopic scale. 2 This is a critical difference between the organic semiconductors studied in this dissertation and their inorganic counterparts. This means that for organic materials, the ana logy with inorganic semiconductors suffers a decreased level of order in the bulk. The different models accounting for charge motion in organic solids underline a more difficult charge transport than in inorganic crystals. In organic thin films, it is generally thought to take place through a hopping mechanism: the charge hops from one conjugated unit to the next. 3 Depending on the nature, the purity and the morphology of the organic material, the hopping process can occur between adjacent molecules, ad jacent polymer chains or parts of a same polymer chain, as illustrated schematically in Figure 1 3 Figure 1 3. Schematic description (a) of polymer chains, illustration (b) of the distribution of conjugation units in the bulk, and schematic depiction o f how transport is distributed both i n space (c) and in energy (d). [Adapted from Tessler, N and coworkers Adv. Mater. 2009 21 2741 2761 ] Hopping is favored between states of the material that are close in energy, which entails better charge transport i n more uniform and ordered materials. Organic
25 semiconductors are considered as disordered media relative to inorganic crystals, thus not as electronically homogeneous (Figure 1 3.d) as their inorganic counterparts, which results in a broad distribut ion of energy states at the macroscopic level. 3 Energy states far away from the band gap edges can be considered as trap states, which have a negative impact on the charge transport properties of organic semiconductors 1.2.3 Nature of the Charge Carriers in Orga nic S olids The characteristics of the charge carriers in organic semiconductors also differ from that found in inorganic semiconductors. Charged species (electron or hole) are created in the organic semiconductor i f enough energy is provided so that electrons may acquire this energy thereby transiting from the HOMO to the LUMO But because the dielectric constant of organic semiconductors is low, 4 the generated electron and hole remain bound together into an electrically neutral pair under electros tatic attraction, the exciton. In conjugated organics, the exciton is of the Frenkel type, with binding energies on the order of 0.5 eV. 4 It is localized on the molecule or segment of polymer chain where it was formed, and because it is susceptible to reco mbination, small diffusion lengths of 5 to 10 nm are typical in organic materials 5 For charge motion to occur under an electric field, the electrically neutral exciton needs to be further separated into the positive and negative charge carriers. In the ri gid inorganic crystal lattice, the generation of a charged species does not influence its surroundings organic solids, polarization of the electron clouds electronic environment. 6 The term polaron is used to designate a charged species accompanied by the local distortion it created. The electronic polarization, also
26 designated as electron electron correlation, is complimentary to a distortion involving nucl ei, known as the electron lattice correlation of lattice distortion. 7 The coupling between electronic and lattice evolution was illustrated simply on butadiene, by comparing the three bonds. 8 Similarly, polyaromatic conjugated chains will deviate from their twisted benzenoid like structure in the ground state to a more planar quinoid structure upon gene ration of charge species in the system. At high charge concentrations, two polarons combine to form a bipolaron, which is defined as a pair of same charges associated with one (increased) local distortion. 7 Chemically, polarons (spin of one half) and bip olarons (spinless) can be assimilated to radical cations and dications, re s pectively, although the concept of local lattice distortion then is lost. The polaronic nature of charged carriers in organic solids implies that their motion has more inertia since the localized distortion has to travel along with the charge. Qualitatively, this impedes efficient charge transport in organic semiconductors as compared to inorganic equivalents. This drawback is well balanced with the many advantages that organic mate rials can offer to the field of electronic technologies for which industrial applications are envisioned 9 An immediate yet critical one resides in their light weight and mechanical durability as compared to inorganic semiconductors. The production of low cost electronic devices is also envisioned thanks to (1) the low amount of energy required to synthesize the organic semiconductors, and (2) their room temperature solvent processing using readily available industrial techniques such as slot dye coating, 1 0,11 spray casting, 11,12 screen printing, 10 or inkjet printing. 13 More importantly in the frame of
27 the present dissertation, powerful synthetic tools are available to the chemist in order to tune the properties of organic compounds. 1. 2 .4 Valuable Charged Species. In the work presented in this dissertation, the targeted applications take advantage of the organic semiconductors ability to promote an electron from HOMO to LUMO upon (1) application of a potential across a dielectric, (2) application of a poten tial in an electrochemical cell or (3) absorption of photons. 14 The general device structures are shown in Figure 1 4, and each is detailed in the following section. Figure 1 4. Schematic representations of typical OFET (a), organic solar cell (b) and electrochromic (c) devices architectures. When an electric field is applied to an organic semiconductor film across a dielectric layer, charged species can be formed in the film. In a device architecture where a second, orthogonal electric field can be app lied, then these charges can flow in the direction of the second field resulting in field effect mobility of charge carriers. 2,1 5 ,1 6 A ty p ical bottom gate top contact organic field effect transistor architecture is shown in Figure 1 4.a. This will be deve loped is Section 1.3.1. When a potential is applied to an organic semiconductor film adhered to an electrically conducting surface plunged in a proper electrolyte solution, charges are generated in the film, which are balanced and stabilized by the electro lytic counter ions.
28 The formation of these charges is accompanied by changes in the absorption spectrum of the material which can be appreciated by the naked eye as the film changes color. 1 7 This is the concept behind electrochromic devices based on conju gated organic materials, for which a basic electrochemica l setup is shown in Figure 1 4.c This will be developed is Section 1.3.2 When the energy of photons is sufficient to be absorbed and form charged species in an organic semiconductor film, then prov ided the film is in contact with electrodes of proper work function (one of which should be transparent to the incident photons), then the charged species can migrate to the two elect r odes and result in photovoltaic current. 18 A classic architecture for a n organic solar cell is depicted in Figure 1 4.b and will be detailed in Section 1.3.3 1.3 Organic Electronic s : Which Parameters Can the Synthetic Chemist O ptimize? All three organic electronics applications described above can be optimized by influencing two parameters, which will be identified in the following as the applications are described in further details. 1.3 .1 Organic Field Effect Transistors. Figure 1 5 shows two different architectures for bottom gate organic thin film transistors. The active part of the device is constituted of an organic semiconductor film equipped with two electrodes, called the source (S) and the drain (D). In Figure 1 5.a, these are set above the semiconductor film (bottom gate, top contact FET), usually by thermal evapora tion of the metal on top of the spin coated film. In Figure 1 5.b, the source and drain are set under the semiconductor film directly onto the dielectric. The distance between the source and the drain is the channel length L, and the transverse dimension o f the device is the channel width W. A third electrode, the gate (set at the
29 bottom of the device is this case), is electrically isolated from the semiconductor film by a dielectric layer. The gate overlaps the whole channel length and width, such that whe n a potential V G is applied between source and gate across the dielectric, charges are generated in the semiconductor layer. The accumulation of charges in the active layer forms a conducting channel between the source and the drain. These charges are then driven across the channel from source to drain by applying an orthogonal potential between the latter two electrodes. OFETs act essentially as electronic valves, as the gate field tunes the amount of charge carriers in the channel while the source and the drain determine the flow of these charges. Figure 1 5 Schematic representations of a bottom gate/top contact OFET (a), and bottom gate/bottom contact OFET (b). L is the channel length, W is the channel width, V D is the potential bias between source (S ) and drain (D) electrodes and V G is the potential bias between gate and source. The response of the devices is measured as current voltage characteristics. These can be done by either varying the drain voltage while keeping the gate voltage constant, or b y varying the gate voltage at a fixed drain voltage. A linear regime and a saturation regime exist in the I V characteristic of FETs, for which the currents are gi ven by Equation 1 1 and Equation 1 2 respectively. 2
30 (1 1) (1 2) C is the capacitance of the dielectric, the charge mobility in the semiconductor and V T is the threshold voltage. The latter parameter can be understood as the lower limit for V G beyond which the channel becomes conducting. The latter two equations clearly show the dependence of the current ou tput on the value of the charge carrier mobility As explained in the previous section, charge transport in organic semiconductors is strongly dependent on the degree of uniformity and ordering in the bulk. An important technological measure of device pe rformance related to mobility is the ratio of the current intensity when the current is flowing to that of when the channel is off, also called on/off ratio. Obviously, the morphology of the semiconductor thin film in the channel between S and D is thus a key parameter for high performance OFETs. The nature of the charge carriers accumulated in the channel upon application of V G depends on the sign of the applied voltage. As illustrated in Figure 1 6, the application of a negative V G generates positive char ges in the organic semiconductors work function s to the HOMO level of the organic semiconductor, applying a potential between S and D lead s to extraction of positive charges, or holes. A semiconductor able to stabilize and carry such charges is designated as p type. Under positive gate bias, the opposite situation occurs, and provided the work function s of the electrodes is well chosen, then electrons can be extracted at the electrodes. Electron transporting
31 semiconductors are designated as n type. A material able to conduct both hole and electrons with comparable significant mobilities is considered to be ambipolar Figure 1 6. Schematic energy diagram illustrating the working principle of an OFET with respect to applied V G Depending on the work function of the metal used, once hole (b) or electrons (c) are created depending on the sign of V G then a flow of holes (electrons) can take place between the two metal el ectrodes. The position of the HOMO and LUMO energy levels of the semiconductor thus determines the propensity of the material for p or n type character, which will influence the nature of the charge carriers in an OFET. The accessibility of the HOMO (LUMO ) also determines the extent of the potential to be applied at the gate to generate holes (electrons). The lower V G is likely to be at significant current output, the lower V T will be also. 2 Lastly, the ambient stability of the device requires that it oper ates at potentials at which exposure to oxygen or water does not lead to chemical degradation of the active layer. 19 For the reasons stated above, the position of the HOMO and LUMO levels of the organic semiconductor is another key parameter in high perfor mance OFETs. 1.3.2 Electrochromics An electrochromic material by definition will change color upon doping (addition or removal of electrons) of the material. 17 For the electrochromism to be of interest in display type applications, the material should be of a particular color in one electrical state and transmissive in an electrically different state. For conjugated organic
32 materials, this has been best achieved with conjugated polymers spray cast onto transparent conducting electrodes based on indium tin oxide (ITO). 12,20 Figure 1 7. Repeat unit structures and photographs of spray cast dioxythiophene based polymer films in the neutral colored, and oxidized transmissive states and their corresponding normalized absorption spectra. [ A dapted from Dyer, A. L and coworkers ACS Appl. Mater. Interfaces 2011 3 1787 1795] This has been extensively reviewed by Beaujuge and Reynolds 17 and is a main aspect of the research conducted in the Reynolds group, although not the primary focus of this dissertation. In sho rt, the color depends on how far in the visible region (400 to 750 nm) of the spectrum the polymer absorbs, and what the relative intensity of the absorption profile is at each wavelength. 21 The best electrochromic polymers so far incorporate the dioxythio phene unit in their backbones, which has led to the full palette of primary colors available as soluble conjugated polymers, as displayed in Figure 1 7. Since absorption profile and energy gap are closely related, controlling color entails controlling the energy of the HOMO and LUMO levels. The polymers displayed in Figure 1 7 switch from colored to transmissive upon oxidation in an electrochemical
33 cell. In general, in a properly prepared electrochemical setup, the lower the potential at which the electroch emical process takes place, the more reversible, fast and durable the switching will be, since low potentials mean less energy stressing the polymer film. Hence, in the case of cathodically coloring polymers such as the poly(dioxythiophene)s family, a read ily accessible HOMO (low ionization potential) is an important parameter for high performance polymer electrochromic display applications based on oxidative processes. T he influence of the nanoscale morphology of the material has not yet been fully underst ood in the context of electrochromic applications as the operation of electrochromic devices rely on the contribution of external parameters such as electrolytes and counterions 1.3 .3 Organic Solar Cells. Solar cells are designed to absorb photons. Immed iately, a s described for electrochromics, the absorption profile of the material is important for solar cells. Specifically, the more extended the absorption towards the near IR, the more photons are susceptible to be absorbed, and this is achieved by orga nic semiconductors with small HOMO LUMO gaps. 22,23 The influence of the frontier molecular orbitals energy cannot be limited to extended absorption when it comes to organic solar cell performance. The following briefly describes the mechanisms at play for solar energy conversion to identify the parameters relevant to the work presented in this dissertation. Through absorption of light, excitons are created in the active layer. The electrically neutral electron hole pair has to be split in order to generate a photocurrent. Because the binding energy of the exciton, on the order of 0.5 eV, is too high for a spontaneous thermal separation and because the exciton diffusion length, on the order of 10 nm, is too small for a pair, on average, to be able to migrate though the film to the
34 electrodes where it might be separated, 3 another component has to be added into the active layer. The concept of active layer heterojunction, where two semiconductors with different HOMO and LUMO energies are in contact, was first a pplied in solid state organic photovoltaics in bilayer devices. 24 The goal of the heterojunction as schemat ically illustrated in Figure 1 8 is to create a local energy offset which can drive the exciton dissociation to the separated charged species. Figu re 1 8. Schematic representation of the electronic processes involved in a bilayer heterojunction cell: (a) formation of the exciton, (b) diffusion of the exciton to the heterojunction, (c) dissociation of the exciton into positive and negative charge car riers, and (d) migration of the charge carriers to their respective electrodes. Illustration of the energy offset between the HOMOs and LUMOs of the two components in the heterojunction. The heterojunction component with the higher HOMO and LUMO levels (lo wer ionization potential and electron affinities) is designated as the donor ( p type ) and the other component is the acceptor ( n type ). Briefly, as illustrated in Figure 1 8 absorption of light creates an exciton (a) in a semiconductor (here the donor), t he exciton diffuses (b) to the donor acceptor interface, undergoes charge (c) separation, and the charges
35 are then allowed to migrate (d) to their respective electrodes for charge collection and photocurrent. Whether the dissociation at the interface occur s by direct charge separation, through a charge transfer state or via an energy transfer followed by charge separation in the opposite direction is beyond the scope of this Chapter. 2 5 29 As the exciton is created, it acquires a certain energy related to th at of its parent photon. As the exciton then undergoes the different energetic steps described above, some energy loss has occurred from the initial generation of the exciton to the final extraction of the separated charges. For instance, a minimum of 0.3 eV is a commonly accepted value for the LUMO (HOMO) offset required to drive electron (hole) transfer at the D A interface. 30 At the end of the process the energy difference between the charges collected at the electrodes determines the amplitude of the de photocurrent on the other hand, is linked to the number of electrons and holes collected. The electrical power generated by the solar cell is the photovoltage times the photocurrent. A typical solar cell characteristic, or I V cur ve, is displayed in Figure 1 9. The most important parameters describing the performance of a solar cell are the open circuit voltage (V oc ), the short circuit current (J sc ), the fill factor (FF) and the power conversion efficiency (PCE) At any point on th e I V curve, the power is given by the product of the current and the voltage. The point of maximum power (P out ) is the point on the curve where the latter product is maximum. The power conversion efficiency, then, is the ratio of the maximum power output to the total power input in terms of incident phot ons, as described in Eq uation 1 3 For the latter value, 1000 W/m 2 is usually selected as solar simulator intensity
36 Figure 1 9. Example I V curves for a solar cell under illumina tion and in the dark al ong with the two equations relating the fill factor (FF) and the power conversion efficiency (PCE) to the solar cell parameters. The fill factor is defined in Equation 1 4 as the maximum power divided by the product of the open circuit voltage and the sho rt circuit current and is a measure of the deviation of the device response to the maximum power theoretically attainable. Equation 1 3 relates the power conversion efficiency to the V oc J sc and FF. Over time, a more suitable approach than bilayer hetero junctions appeared: the bulk heterojunction (BHJ). It is still widely used nowadays and will be discussed in Section 1.5.1 in more details. 1.4 Energy Levels and Morphology: How to tailor these two Key Parameters? From the description of the three applicat ions above, control over the morphology and control of the energy of the HOMO and LUMO are the two main materials properties that influence performance, and how the synthetic chemist can contribute is described in the following.
37 1.4 .1 Energy Levels Control As conjugated successions of aromatic rings (except for polyacetylene), most conjugated polymers have significant bond length alternations, which lead to non degenerate ground states between aromatic and quinoid forms. While the band gap of a conjugated polymer depends on several structural features which can be varied synthetically, such as planarity, substitution, aromaticity and interchain interaction, bond length alternation has the greatest effect on band gap. 31 The donor acceptor (D A) approach has proven to be a very powerful method to tune the energy of the HOMO and the LUMO of conjugated molecules and polymers. The donor acceptor (D A) approach is based on the conjugation of an electron rich aromatic unit (donor) and an electron deficient aromatic (or ethylenic) unit (acceptor). The resulting push pull driving forces favor electron delocalization and the formation of quinoid mesomeric structures (D A to D + =A ) over the conjugation length, reducing the extent of bond length alternation. When spectro scopy is used to evaluate the HOMO LUMO energy gap, intramolecular charge transfer can also account for the extended absorption, which is linked to the high lying HOMO of the donor unit and the low lying LUMO of the acceptor unit. A pictorial way to repres ent thi s concept is shown in Figure 1 10 (center). The strength of the D A approach resides in its versatility, since many aromatic variations are synthetically accessible to tune the push pull character along the conjugated backbone while providing sites for alkylation to retain solubility. 22, 23 Electron rich units are typically based on phenyl, thiophene or pyrrole rings substituted with inductive donating group such as alkyl, alkoxy or alkylamine groups. Variations of the latter have led to a library of donor moieties for D A conjugated systems. Examples are
38 shown in Figure 1 10 (left). Electron deficient units are mostly based on phenyl and thiophene rings which are substituted with electron withdrawing groups such as carbonyls, nitrile and imine fun c tio nalities. Examples of such are also depicted in Figure 1 10 (right). Figure 1 10. Illustration of the donor acceptor concept (center): mixing of the HOMOs and the LUMOs in the donor acceptor (D A) fragment result in a compressed band gap. Structures of some electron rich (donor, left) and electron deficient (acceptor, left) aromatic units used in organic electronics The simplest electron rich units are benzene, thiophene and pyrrole. These have been substituted with alkoxy groups to increase their elect ron donor character. In organic electronics, dioxypyrroles (DOP) are amongst the most electron rich single aromatic units. 32 Fused phenyls like fluorene 33 35 and carbazoles 36 3 9 first introduced the carbon bridged structural advantage of planarizing a two aromatic ring moiety while providing an alkylation site away from the backbone twisting points. This carried over to the bithiophene unit with the synthesis of cyclopentadithiophene (CPDT), 40 ,4 1 and later to the substitution of the carbon bridge for silico n (dithienosilole, DTS) 42 4 5 and recently germanium (dithienogermole, DTG). 4 6 4 8 It is thought that the bigger the bridging atom (Ge>Si>C), the farther the alkyl solubilizing group can branch out from the conjugated units, improving the planarization of th e whole backbone. Fused di or tri ring aromatics
39 also have spurred in the recent years, with the development of thieno[3,2 b]thiophene, 4 9 benzodithiophenes, 50 5 1 dithienopyrrole 5 2 ,5 3 and more. The most widely used electron accepting moieties were initiall y based on the cyanovinylene unit, 5 4 5 7 and then the benzothiadiazole unit (BTD) later on. 36 ,39,41, 42 ,59 The development of new electron acceptors in the recent years resulted in materials with considerably deeper LUMO energy levels (higher electron affin ities). Figure 1 11 shows the energetic distribution of some of the acceptors which are part of the best performing D A materials in organic solar cells and FETs. The energies in Figure 1 11 are that reported by the different authors from the polymer thin films onsets of reduction, which I have attempted to homogenize (when needed based on the electrochemical conditions reported) by correcting the calculation from reduction onset to energy level using 5.1 eV for Fc/Fc + vs vacuum. This discrepancy is best e xplained in Barry PhD dissertation, and the 5.1 eV value was recently highlighted by Bazan and coworkers. 60 The LUMO levels gathered for conjugated polymers based on BTD are in the 3.4 to 3.7 eV range. Those of polymers based on thieno[3,4 b] thiophene (TT) 51,61 are reported as slightly lower, between 3.5 and 3.8 eV, and so are that of thienopyrroledione (TPD). 43,46,48,62 When BTD and TT were substituted with fluorine, 50, 61, 63 their LUMO levels shifted downwards. Adding a nitrogen atom in the ring of BTD had a similar effect. 64 Imide based acceptors, such as di k etopyrrolopyrrole (D PP), 49, 65 70 naphthalene diimide (NDI) 71 74 and perylene diimide (PDI) 74 78 have LUMO levels which are generally lower than the previous acceptors, approaching 4.0 eV. The benzothiadiazole quinoxaline 58,79 and bisbenzothiadiazole 53 acceptors lower the LUMO even more.
40 Figure 1 11 Structures of several acceptors from the literature, along with the LUMO energy level distribution of polymers incorporating them LUMO energies are corrected to Fc/Fc + at 5.1 eV vs vacuum (when needed), to homogenize the values. Deep LUMOs have several implications for organic solar cells and field effect transistors. First, this brings the electrons in the doped semiconductor within th e range of stability against reaction with ambient a tmospheric contaminants. 19 Homo or co polymers of diketopyrrolopyrrole (DPP), 80,81 benzobisimidazobenzophenanthroline (BBL), 82 84 perylene diimide (PDI) or naphthalene diimide (NDI), 71 74 bithiophene imi de (BTI) 85,86 and bisindenofluorene 87 have been reported as high electron mobility materials, some exceeding 0.1 cm 2 /Vs in air stable OFETs. Second, the most prominently used n type material in OPVs heterojunctions are not conjug ated polymers (as discussed in S ection 1.5.4 in more detail), rather they are fullerene derivatives, with LUMO levels around 4.2 eV. 18,88 90 Because the value of the V oc in organic solar cells is linked to the offset between the HOMO of the p type material (conjugated polymer) and the LUMO of the n type material (fullerenes), deep LUMO levels for D A polymers combined with their low band gaps (1.2 1.6 eV) entail that they have deep HOMO levels
41 as well. Hence the propensity for high V oc in devices using deep LUMO, low band gap polyme rs. One concern to nuance the latter point is that should the LUMO be too deep, then there would not be enough LUMO (p type) LUMO (n type) offset at the heterojunction to efficiently drive exciton separation at the interface. 1. 4 .2 Morphology control in sing le component active layers. There are two levels of morphology control that relate to the field of organic electronics: 1) morphology control in a single component active layer to achieve highest degree of ordering and 2) morphology control in two componen ts blends to induce favorable phase segregation in the active layer. Only the first one is treated here, and the second one will be covered in Section 1.5 of this Chapter. H igh mobility devices often require processing techniques such as single crystal gr owth or vapor deposition, which are much more demanding than solution based techniques in terms of cost and reproducibility. 2 Table1 1 gathers some of the best performances with classic materials reported in the literature. At satisfying on/off ratios in p channel OFETs, devices that are solution processed only recently manage to overcome the 1 cm 2 V 1 s 1 threshold in hole mobility, whereas numerous devices made by vapor deposition or using single crystals have been reported with hole mobilities above unity. A comprehensive review was recently published by Zhu and coworkers. 91 The synthetic design of oligomers to achieve liquid crystallinity is one approach to induce long range ordering in solvent processible systems, and oligothiophenes are good candidates. 9 2 Liquid crystallinity has also been exploited to induce ordering in fully conjugated poly(alkylthiophenes) leading to high p type OFET performances as in the case of PQT 12 or PBTTT 9 3 ,9 4
42 Table 1 1. Processing method, p channel field effect mobility and on/off ratio for some of the classic OFET materials reported in the literature. Single Crystal Vapor Deposited Solution Processed Mobility (cm 2 V 2 s 1 ) On/Off ratio Mobility (cm 2 V 2 s 1 ) On/Off ratio Mobility (cm 2 V 2 s 1 ) On/Off ratio 1 1.3 10 6 4 1.0 10 4 7 0.14 2x10 7 2 15.4 10 6 5 6.0 10 6 8 0.63 10 7 3 1.0 10 4 6 0.2 10 6 9 1.4 10 5 Supramolecular assemblies of conjugated systems have been reviewed extensively, and the reader is directed to the relevant litera ture. 95,96 1.5 Morphology Control in Organic Sol ar Cells: Successful Variations. 1.5 .1 Polymer/PCBM solar cells. Probably the most efficient active layer morphology control in organic solar cells was the advent of the bulk heterojunction. 18 T his approach c onsists in intimately blending the two components (p type and n type) in the active layer, such that a greater interface area could be achieved. It results in an interpenetrated junction betw een electron donor and electron acceptor materials Bulk heteroju nctions can dissociate excitons efficiently over the thickness of the solar cell active layer and thus create separated electron hole pairs anywhere in the film. The main disadvantage is the increased disorder, as the reduced percolation pathways of the s eparated charges to
43 the contacts may result in spatially trapped charges, leading to undesired recombination. It is thus necessary to add a level of control over the BHJ, and considerable effort across the field was made in that direction. Because the BHJ is obtained after spin coating a blend from solution, depending on the solvent evaporation rate, the morphology is not necessarily the most thermodynamically stable one. This means that the choice of the casting solvent will have an i nfluence on the bulk m orphology. 97 For similar reasons, solvent vapor annealing treatments 98 can also impact the BHJ, as the blend exposed to the solvent vapor is allowed to rearrange. Thermally annealing the devices after spin casting the active layer has also become a popular and powerful method to increase the solar cell efficiency. 99 Figure 1 12. Structure of the best performing solar polymers reported to date, along with their energy levels, electrochemical band gaps, solar cell parameters and PCE. Both annealing methods usually yield higher phase segregation between the p and n type materials, with bigger domains which can also feature higher degrees of crystallinity. The latest lever for BHJ morphology control consists i n the use of solvent
44 additives. These small molec ules are added in a low volume percent (up to 8 %) to blend solution. During deposition of the active layer blend, the presence of the extra solvent molecules of different boiling points and polarity result in an altered BHJ morphology. This was shown to s ignificantly reduce the phase segregation size in some cases, using alkyl dithiols 100 ,101 or diiodoalkanes, 46,65,102 leading to an increased p /n type interface area and increased photocurrents. The highest efficiencies reported for po lymer solar cells now exceed 7%: the structure s of PBDTTT CF 61 PDTSTPD 43 DTG TPD 46 PBnDT DTffBT 64 and PBnDT XTAZ 50 are shown in Figure 1 12 1.5 .2 Small molecule/PCBM solar cells. Conjugated small molecules, which in the field of organic electronics are usually considered a s monodisperse elongated chromophores in the 1 to 5 kg/mol range, have shown some interest in molecular BHJ solar cells. The synthesis of discrete molecules requires less stringent stoichiometry than that of conjugated polymers, and the purification is mor e straightforward such that from a materials science perspective, there is less batch to batch variation. The technology and the device fabrication are essentially the same, with the molecules as p type and the fullerenes as n type materials. B ut because o f the low molecular weight of a conjugated small molecule compared to a polydispers e high molecular weight polymer the morphological behavior of the molecular active layer differs from that of a polymer based one Specifically, both components in the blen d have the ability to crystallize, which is advantageous for charge carrier mobility but can lead to ex cessive domain sizes. A most up to date review of molecular BHJ solar cells was recently published by Nguyen and coworkers, 103 which also reported the fi rst molecular devices exceeding 4% efficiency. 104 From the review of all molecular solar cell systems, the team observed that
45 carrier extraction and recombination in these systems appear more prevalent than in polymer based devices, which they suggest the finite size and the crystallinity of the small molecules may be responsible for. Figure 1 13. Structure of the two best performing solar small molecules reported to date, along with their energy levels, electrochemical b and gaps, solar cell parameters and PCE. Nevertheless, there is improvement to expect from molecular systems, as the regained interest in such is recent compared to polymer based devices. More detailed studies on device processing conditions designed speci fically for the more crystalline active layers can improve efficiency. New materials also can lead to improved devices, as the latest two reports of high performance molecular solar cells, based on the two new molecules shown in Figure 1 13, reached 5.4% 10 5 and 5.8% 106 in BHJ with fullerenes. 1. 5 .3 Organic/inorganic hybrid solar cells. Early reports by Alivisatos et al. of photovoltaic devices based on hybrid systems combining a conjugated polymer and cadmium selenide nanocrystals (NCs) in thin film blends have sparked considerable research efforts on organic semiconductor / chalcogenide NC hybrids. 107 Since NC do not disperse well within the unfunctionalized polymer matrix and tend to aggregate 108,109 a limiting factor to the latter type of hybrid solar varying the shape of the inorganic NCs, 110,111 inorganic chemists have offered solutions to this morphology issue: blends of three dimensional branched NCs with
46 unfunctionalize d polymers afforded power conversion efficiencies up to 2.2% with poly(3 hexylthiophene), 107,112 2.1% with poly(phenylene vinylene) 113,114 and up to 3.2% with polymers taking advantage of the donor acceptor approach. 115,116 Since NCs are coated with trial kylphosphine oxide or alkylcarboxylate surfactants depending on the colloidal NC synthesis method employed, they are inherently surrounded by an insulating layer of aliphatic molecules, which was early determined to be detrimental to the electronic interac tion between the organic and inorganic components of the hybrids. 117 Subjecting the NCs to a solvent treatment aimed at replacing the original surfactants also contributed to increased efficiencies of hybrid solar cells. 108, 118 121 As a means of controllin g both the morphology of the hybrid active layer and the NCs surfactants composition, conjugated polymers that bare functional groups such as amines, 122,123 phosphine oxides, 124 126 thiols 123,127,128 and carbodithioic acids 129 were introduced. Although they provided better control of the dispersion of the NCs in the polymer matrix, little enhancement of the overall power conversion efficiencies was observed. A related approach consists in using discrete conjugated oligomers in place of polymers, allowing for a greater molecular control of the hybrids formation due to the well defined structure of the oligomers. In most previous studies, the oligomers bare functional groups enabling their grafting onto the inorganic NPs: amongst others, 130,131 oligoaniline s with carbodithioic acid groups; 132,133 oligo(phenylene vinylene)s with phosphine oxide groups, 134,135 oligo(phenylene ethynylene)s with thiol groups 136 and oligothiophenes with thiol, 137 carbodithioic acid, 129 carboxylic acid, 138 140 phosphonate 141 and phosphonic acid 142 144 anchoring groups have been reported. Some report the further electropolymerization of the attached ligands, but most systems are treated as
47 discrete inorganic core / organic shell type entities to be characterized and processed as suc h into optoelectronic devices. 1.5 .4 Polymer/polymer solar cells. The majority of conjugated materials for all organic electronics developed up to date are p type, but l ow bandgap n type conjugated polymers with high electron affinities and high ionizatio n potentials (ambient stable) are also important in the related field of all polymer solar cells, because the commonly used fullerene derivatives typically have limited absorption in the visible region. Fullerene derivatives, such as PC 60 BM and PC 70 BM, are constant components in the highest efficiency cells due to their advantageous electron mobility and their ability to crystallize into charge percolation networks. 88 90 The main disadvantage of fullerenes for BHJ cells is the limited chemical modifications available to extend their light absorption to wavelengths longer than 600 nm, 145 147 explaining the extensive synthetic effort focusing rather on broadening the spectral absorption of their donor acceptor (D A) p type polymer ic counterparts. 22,23 Soluble n type polymers are attractive because of their versatile processability: their macromolecular nature yields high quality thin films as active layers, while variations in the side separation in the bulk Except for BBL based devices, 148,149 palladium catalyzed cross couplings are used to synthesize n type D A polymers for most all polymer solar cells incorporating cyanovinylenes, 150 152 PDIs 76 78,153 or BTD 154,155 acceptors in conjugation with various donors, yielding maximum efficiencies between 1.8 % and 2.3 % at AM 1.5. In all polymer solar cells, the n type material is a polymer which should fulfill specific energy levels requirements with respect to the p type polymer in the active layer. 154,156 The most common p type material used in all polymer OPVs are
48 derivatives of alkylated poly(thiophenes) and poly(phenylene vinylenes), which ha ve HOMO and LUMO levels in the 5.2 to 5.4 eV and 3.1 to 3.2 eV ranges respectively. 30 The n type polymer used in heterojunction with such p type polymers should thus be designed with HOMO and LUMO levels lower than 5.5 to 5.7 eV and 3.4 to 3.5 eV, respectively, to achieve energy levels offsets greater than 0.3 eV and drive the excitons to the charge separated state at the p /n type interface. To be able to compete with the current fullerene derivatives, the energy offsets for the n type polymer should be balanced with a bandgap below 1.8 eV to extend its absorption into the near IR. 1.6 Thesis of T his Dissertat ion As the field of organic electronics learns the mechanisms at work behind successful device operation, two parameters stand out as key to high performance: control over the morphology of the active layer, and control over the energy of the frontier mol ecular orbitals ( HOMO and LUMO ) of the conjugated organic semiconductors in the active layer. ompound itself, through synthetic design. Depending on the application, the active layer can be composed of a single component or of (at least) two c omponent s. There are thus two levels of morphology control relevant to organic electronics. Regarding speci fic applications in this dissertation, single component active layers relate to both organic FETs and solar cells while the latter level pertains mostly to heterojunction organic solar cells Well defined oligomers have the advantage of being monodisperse and can often readily crystallize into ordered domains. Therefore, they are ideal candidates to probe the efficacy of a new approach to morphology control. The third chapter of this
49 dissertation presents the use of synthetic chemistry to tailor well defin ed oligomers towards both levels of morphology. In a first part, telechelic oligomers are polymerized into higher molecular weight compounds with the goal of accessing the mechanical properties of polymeric materials while retaining some morphological free dom characteristic to the single oligomer, as desired for solution processed OFETS. The second part of Chapter 3 describes how unsymmetrical oligomers with variable energy gaps can be functionalized such that they may graft onto inorganic nanocrystals (NC) for hybrid solar cell applications. Such hybrid systems could become useful tools to control the phase segregation domain size in the active layer of hybrid solar cells, particularly since the NC can be of various shapes with controlled aspect ratios, an could eventually be anisotropically distributed within the active layer. The last part of Chapter 3 presents a synthetic strategy affording symmetrical and unsymmetrical oligomers, which can be mixed as part of the active layer in a molecular solar cell wi th an improved effect on its morpho logy and thus its efficiency. With the development of donor acceptor chemistry in the past decade, a wide variety of electron deficient moieties were incorporated in the backbones of conjugated molecules and polymers, res ulting in organic semiconductors with reduced bandgaps and tailored energy levels. In particular for heterojunction OPVs, researchers seek to adjust the position of the energy levels of a p type compound with respect to that of the n type component. Isoind igo is an electron deficient molecule introduced in 2010 by us as a new acceptor for organic electronics. A common property of isoindigo based conjugated molecules and polymers is their low lying LUMO (high electron affinity) between 3.8 and 4.0 eV, whic h is close to that of fullerene derivatives. The electron
50 accepting strength of isoindigo reduces the bandgap of the materials to 1.55 eV, extending their absorption to 800nm. This results in deep HOMO levels (high ionization potential) compared to other s mall bandgap systems, which is also an attractive feature of isoindigo based systems. The fourth chapter of this dissertation demonstrates the use of isoindigo as a n ew acceptor in solution processi ble donor acceptor conjugat ed polymers. The first two part s of Chapter 4 introduce the isoindigo molecule and some model oligomers with properties relevant to organic electronics. The third part of Chapter 4 illustrates the breadth of the absorption profiles depending on the design of the polymer repeat unit, whi ch is related to the position of the FMO energies. Taking advantage of the deep HOMO and LUMO energy levels, and yet extended absorption, the fourth part of C hapter 4 sheds a different light on isoindigo based conjugated polymers, now synthetically designe d as all acceptor for n type applications. Reductive electrochromics and all polymer solar cell results are presented to illustrate the use of all acceptor poly(isoindigos) as n type materials. The last part of Chapter 4 focuses on conjugated polymers desi gned specifically as p type for polymer solar cells in bulk heterojunctions with fullerene derivatives
51 CHAPTER 2 EXPERIMENTAL METHODS AND CHARACTERIZATION S Detailed synthetic methods are located at the end of Chapters 3 and 4 for the respective compounds described in this dissertation. 2.1 Structural and Polymer Characterization. 2.1.1 General Structural Characterization s All 1 H NMR (300 MHz) and 13 C NMR (75 MHz) spectra were recorded on a Varian Mercury 300 spectrometer. Chemical shifts for 1 H and 13 C NMR were referenced to residual signals from CDCl 3 ( 1 ppm and 13 spectrograms were recorded on a Finnigan MAT95Q Hybrid Sector mass spectrometer. Elemental analyses were carried out by Atlantic Microlab, Inc or by the CHN elemental analysis service in the Chemistry Department of the University of Florida 2.1.2 X Ray Spectroscopy. Crystals of T6 dibenzoate ( 3 9 ) were grown by slow evaporation from a 50:50 dichloromethane:pentane solution Crystal growth was attempted for compounds P iI P and T iI T (Section 4 2) using several methods which are summarized in Table 2 1. Chloroform, THF and toluene are good solvents, while acetonitrile is a poor solvent for the present compounds. Table 2 1. Crystal growth methods employ ed for P iI P and T iI T CHCl 3 :ACN (3:1) evaporation THF:ACN (3:1) evaporation CHCl 3 evaporation Toluene evaporation CHCl 3 :ACN vapor diffusion Toluene:ACN vapor diffusion P iI P No sub mm crystals small clustered No mm scale single small clustered T iI T mm scale single No small clustered No mm scale single No
52 The best crystals were obtained for both compounds by dissolving 10 mg of the material in 1 mL of chloroform in a small glass vial (12 x 35 mm) with a plastic cap. The solutions were gently heated to ensure full dissolution. The plastic cap was perforated with a needle (five holes) and tightened to the vial containing the solution. This was then inserted in a bigger glass vial (27.5 x 7.0 mm, screw cap) containing acetonitrile (3 mL), and the cap was tightened onto the big vial. This setup was allowed to stand for 4 days without disruption, affording mm scale single crystals. X ray data was obtained by the Center for X ray Crystallography, supervised by Dr. Khalil A. Abboud at the University of Florida, Department of Chemistry. For T6 dibenzoate ( 3 9 ), d ata were collected at 173 K on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined usi ng up to 8192 reflections. A full sphere of data (1850 frames) was collected using the scan method (0.3 frame width). The first 50 frames were re measured at the end of data collection to monitor instrument and crystal stability (maxim um correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL6, and refined u sing full matrix least squares. The non H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. The asymmetric unit consists of four half molecu les with no solvent molecules. One of the half molecules in the asymmetric unit has a significan t disord er from O9 till the aryl ring. The disorder is refined in two parts with the minor part constrained to maintain a geom etry similar to the major part. A total of 1280 parameters were refined in
53 the final cycle of refinement using 6973 reflections wi th I > 2 (I) to yield R 1 and wR 2 of 9.03% and 15.78%, respectively. Refinement was done using F 2 For P iI P and T iI T X Ray Intensity data were collected at 100 K on a Bruker SMART diffractometer using MoK radiation ( = 0.71073 ) a nd an APEXII CCD area detec tor. Raw data frames were read by program SAINT and integrated using 3D profiling algorithms. The resulting data were reduced to produce hkl reflections and their intensities and estimated standard deviations. The data were corrected for Lorentz and polari zation effects and numerical absorption corrections were applied based on indexed and measured faces. The structure was solved and refined in SHELXTL6.1 using full m atrix least squares refinement. The non H atoms were refined with anisotropic thermal para meters and all of the H atoms were calculated in idealized positions and refine d riding on their parent atoms. The molecules are located on inversion centers thus a half molecule exists in the asymmetric unit. For P iI P i n the final cycle of refinement, 3457 reflections (of which 3099 are observed with I > 2 (I)) were used to refine 200 parameters and the resulting R 1 wR 2 and S (goodness of fit) were 3.48 %, 9.15 % and 1.063 respectively. The refinement was carried out by minimizing the wR 2 function using F 2 rather than F values. R 1 is calculated to provide a reference to the conventional R value but its function is not minimized. For T iI T t he thiophene ring is disordered along a 180 rotation along the C4 C5 minor par t was possible to loc ate and refine isotropically. In the final cycle of refinement, 3400 reflections (of which 2694 are observed with I > 2 (I)) were used to refine 195
54 parameters and the resulting R 1 wR 2 and S (goodness of fit) were 3.53 %, 9.28 % and 1.077 respectively. The refinement was carried out by minimizing the wR 2 function using F 2 rather than F values. R 1 is calculated to provide a reference to the conventional R value but its function is not minimized. 2.1.2 Molecular Weight Characterizations. Gel permeation chromatography (GPC) was performed at 40 C using a Waters Associates GPCV2000 liquid chromatography system with an internal differential refractive index detector and two Waters Styragel HR 5E columns (10 m PD, 7.8 mm ID, 300 mm length) using HPLC grade THF as the mobile phase at a flow rate of 1.0 mL/min. Injections were made at 0.05 0.07 % w/v sample concentration using a 22 injection volume. Retention times were calibrated against narrow molecular weight polystyrene standards (Polymer Laboratories; Amherst, MA). 2.1.3 Thermal Characterizations. Thermogravimetric analysis (TGA) was pe rformed on TA Instruments TGA Q5 000 Series using dynamic scans under nitrogen. The heating rate for all samples used (on the 3 to 5 mg scale) was 10 C/min, starting from 25 C up to 700 C. Pt pans were used as sample holders. Differential scanning calorimetry (DSC) analysis was performed usi ng a TA Instruments Q1000 series equipped with a controlled liq uid nitrogen cooling system The samples were prepared by loading 1 3 mg of sample in an aluminum pan, and enclosing it with a hermetic aluminum lid. All DSC experiments consisted in an initial heating scan from 40 C to 200 C 250 C to erase inconsistencies because of the thermal history of the sample preparation, and then cooled at 10 C/min to a temperature between 50 and 100 C/min.
55 2.1.4 Polymer F ree Standing Film P reparation. Two different m ethods were used to created free standing films of T6PC. First, 20 mg of the polymer was dissolved in THF and cast it in a 7 x 1.5 cm Teflon mold. After the solvent evaporated, the film was peeled off of the mold easily. Alternatively, the polymer can be d issolved in toluene and the solution cast on top of a water layer in a vial. Toluene is less dense then water so the polymer solution stays above the water layer and as toluene evaporates, a nice film forms on top of the water layer, easy to pick up and dr y. To prepare the free standing film of P(iI F) the polymer (400 mg) was dissolved in 20 mL of toluene and cast in a 20 cm diameter glass Petri dish. An argon flow was set over the dish using a wide funnel. 2. 2 Electrochemical Methods. Electrochemistry was employed to evaluate the electroactivity of the materials synthesized as presented in this dissertation. In the case of small molecules, the material was dissolved in a 0.1 M t etra n butylammonium hexafluorophosphate (TBAPF 6 ) in dichloromethane solutio n. The background was current was recorded from 2.0 V to +1.5 V vs Fc/Fc + prior to any experiment to evaluate the electrochemical purity of the electrolyte. In the case of polymeric samples, thin films were used instead. For general electrochemical behavi or and energy levels determination, the polymer films were dropcast from toluene or chloroform solution (0.2 g/mL) onto platinum disk electrodes (0.02 cm 2 ) and switched 10 times at a scan rate of 50 mV s 1 in the corresponding range of potentials prior to characterization until a complete stabilization of the current responses was reached. For reduction spectroelectrochemical measurements, the polymer thin films were spray coated onto ITO coated glass slides ( Delta Technologies, Ltd. (7 x 50 x 0.7 mm,
56 sheet resistance, Rs 8 ) The films were sprayed from 2 mg/mL toluene solutions of the polymers, and contacts were made using copper tape. The coated ITO slides were transferred to an argon filled glovebox, in which the electrochemical cells were assembl ed. The cells consisted of a quartz cuvette, a Ag/Ag + reference electrode and platinum wire as a counter electrode fit together through a Teflon cap with proper holes. Each cell was filled with the proper electrolyte, sealed with Teflon tape and taken out of the glovebox to perform the initial electrochemical reduction break in cycles and the subsequent spectroelectrochemical experiments. The TBAPF 6 salt was purchased (98%, Acros) and recrystallized from ethanol. Tetraethylam monium tetrafluoroborate was pu r chased (Aldrich) and thoroughly dried under vacuum prior to utilization. The salts were transferred to an argon filled drybox (OmniLab model, Vacuum Atmospheres). Acetonitrile was distilled over calcium hydride freeze pump thawed, and kept under inert at mosphere (Ar) before being transferred to the drybox in which the electrolyte solutions were made. Dichloromethane was collected at the solvent drying system, freeze pump thawed, and used in the same way as acetonitrile. Cyclic voltammetry (CV) and differe ntial pulse voltammetry (DPV) studies were performed using an EG&G Princeton Applied Research model 273A potentiostat/galvanostat in the argon filled drybox. Experiments were carried out in a one compartment electrochemical cell using Ag/Ag + reference elec trode and platinum foil as a counter electrode; all potentials were reported vs Fc/Fc + redox couple. The following setup parameters were applied for the DPV studies: a step size of 1.4 mV, a step time of 0.035 s, and amplitude of 55 mV.
57 2.3 Optical and Spe ctroscopic Characterizations. 2.3.1. UV vis S pectroscopy. 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.). The oligomer films were spin coated o from 10 mg/mL chloroform solutions The polymer films in the solid state UV vis absorption experiments, including spectroelectrochemistry, were sprayed from toluene solutions unless solubility was an issue in which case chloroform was used. 2.3.2. Photoluminescence Quenching Experiments. The ground state absorption measurements were recorded on a Cary100 UV vis absorption spectrometer and corrected for background due to solvent (HPLC grade) absorption. Fluorescence emission spectra an d PL quenching data were collected on a Photon Technology International (PTI) photon counting fluorescence spectrometer. Fl uorescence lifetime measurement was conducted on a PicoQuant Picoharp 300 TCSPC instrument The concentration of the CdSe NCs stock s olution was determined to be 27 M by a reported method 157 and diluted to 2 0 M for experiment A or 1 M for experiment B. Experiment A (Figure 2 1) describes the PL quenching experiment of oligomer emission by addition of incremental amounts NC solution. To a solution of 2 mL oligomer in CHCl 3 (5 M) in 1 cm quartz FL cuvette CdSe NCs in CHCl 3 with known concentration were titrated so that the ratio of oligomer/ CdSe was controlled. M ore than 90% of photoluminescence of the oligomers were quenched when 1 0 0 nM of CdSe NPs were added ( Oligomer : CdSe 5 0:1)
58 Figure 2 1 Pictorial description of the photoluminescence quenching experiments. Experiment B describes the reverse experiment, where the evolution of the emission of the NCs in solution is monitored at various ratios of CdSe NC/oligomer. As depicted in Figure 2 1 (right), this was achieved by mixing 0.5 mL of a 1 M CdSe solution with 1.5 mL of oligomer solution of the following concentrations: 16.7 M (for 50:1 oligomer:CdSe ) ; 3 3 4 M (for 100:1 oligomer:CdSe ) and 6 6 8 M (for 200:1 oligomer:CdSe ) The solutions were stirred and irradiated with light at the CdSe p eak absorption wavelength (which does not overlap with the absorption of the all thiophene organic oligomer ). The resulting emission was recorded and plotted against that of other oligomer:CdSe ratio mixtures. 2.3.3 Polarized Optical Microscopy In combinat ion with DSC thermograms, p olarized light microscopy (POM) was used to identify possible crystalline phase transition and liquid crystallinity. The sample covered with a microscope glass cover slide and fitted in the hot stage. The temperature
59 would then be increased (180 C max) to reach that of sample flow and the cover slide would be pressed to spread the sample between the two slides and obtain a thin layer. In the iso tropic melt, no light was observed at crossed polarizer and analyzer. As the sample was cooled down, crystallization would be observed as structural patterns appeared close to the crystallization temperature identified by DSC. In the case of the polycarbon ate in Section 3 3, T6PC the structural features would be observed at room temperature but no higher than 65 C. The images of the stretched free standing films were captured on a film which was held in between two clips, and set in an oven, in which the temperature was increased from 40 C to 70 C. When the temperature was high enough (60 65 C), the film was stretched automatically from the spring of the paper clip. Figure 2 2 depicts the setup. Figure 2 2. Free standing film stretching setup (left) and the stretched film set under the polarized light microscope objective. The POM was performed on a Leica DMRXP polarizing microscope equipped with a Wild Leitz MPS46 camera. Samples were heated in a Linkam Scientific LTS350 hot stage controlled by a Linkam TP92 central processor. 2. 4 Device Fabrication. 2.4.1 OFETs Fabrication. For the devices fabricated and studied, highly doped silicon was used as the gate electrode, while the dielectric was a 200 nm thick SiO 2 film. The bottom contact FET
60 (channel widt hs 5 to and lengths 0.35 to 7.0 mm) was prepared by spin coa ting (2000 rpm, 60 s) a 10 mg/mL T6PC trichloromethane solution. According to the DSC results annealing steps (RT to 200C to RT at 20C/min) were performed leading to no transistor charact eristics. Initial attempts to mechanically orient drop cast films of T6PC onto a silicon substrate by rolling, using a 5 mm diameter rubber roll, a 15 mm diameter polypropylene roll and a 2 mm diameter steel roll were unsuccessful. Successful orientation (observed by POM) was achieved by stretching free standing films prior to deposition on the SiO 2 dielectric. T op contact FETs (channel widths 25 to 70 and lengths 0.5 to 1.5 mm) were prepared by man ually stretching a film (1 mg/mL T6PC TH F solution) onto untreated, hexamethyldisilazane ( HMDS ) treated, and octadecyltrichlorosilane ( OTS ) treated dielectrics. Solution processing and electrical measureme nts by using a Keithley 4200 machine were performed inside a nitrogen filled glovebox at room temperature. The film stretching was performed under ambient conditions. 2.4.2. All Polymer Solar Cells. Polymer solar cells were processed on pre patterned indiu m tin oxide (ITO) coated glass substrates with a sheet r thin layer (30 nm) of poly(3,4 ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS; Baytron AI 4083 fro m HC Starck) was spin coated on ultrasonically cleaned ITO substrates, followed b y backing on a hot plate at 180 C for 10 min. An active layer of the device consisting of the blend of polymer (P3HT) and Poly(iI) was then spin coated from chlorobenzene solvent with a thickness 95 nm. The device was subsequen tly heated on a hotplate at 150 C for 10 min. LiF (1 nm) and aluminum (100 nm ) were thermally
61 evaporated at a vacuum of ~0.10 n bar on top of active layer as a cathode. The area of the devices was 0.04 cm 2 The current density voltage measurements of the devices were carried out using a semiconductor parameter a nalyzer system. The photocurrents were measured under AM 1.5G illumination at 1000 W/m 2 from a solar simulator. Device fabrication was done under nitrogen atmosphere and characterizations were performed in an ambient environment without any encapsulation. 2.4.3. Polymer/PCBM Solar Cells. Conventional architecture b ulk heterojunction (BHJ) solar cells were fabricated by the spin coating of 30 nm thick layers of poly(3,4 ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS; Baytron AI 4083 from HC Starc k) on a ultrasonically cleaned, indium tin oxide (ITO) coated, patterned glass substrates, followed b y backing on a hot plate at 180 C for 10 min. An active layer of the device consisting of the blend of polymer P(iI DTS) and PC 70 BM (99% pure, Solenne BV) with a ratio of 1:4 (with and without 4% DIO) was then spin coated from chlorobenzene solvent with a thickness of 105 nm. The device was subsequen tly heated on a hotplate at 150 C for 10 min. LiF (1 nm) and aluminum (100 nm) were thermally evaporated at a vacuum of ~10 7 mbar on top of active layer as a cathode. For the inverted geometry, a t hin layer of sol gel ZnO (35 nm) was spin coated onto ITO coated glass. The ZnO sol gel films were then annealed in air for 30 min at 200C. The same process for the ac tive layer in the conventional architecture was used for the inverted devices. After annealing the active layer, a thin layer of MoO 3 (10 nm) was thermally evaporated and then Ag electrode was deposited to complete the inverted device structure. The area o f the devices was 0.04 cm 2 The current density voltage measurements of the devices were carried out using a 150 W Newport ozone
62 free xenon arc lamp as the light source in conjunction with a Keithley 4200 semiconductor parameter analyzer system. Solar meas urements were carried out under 1000 W/m 2 AM 1.5G illumination conditions. Device fabrication was done under nitrogen atmosphere and characterizations were performed in an ambient environment without any encapsulation
63 CHAPTER 3 CONJUGATED SMALL MOL ECUL ES FOR ACTIVE LAYER MORPHOLOGY CONTROL IN TRANSISTO RS AND SOLAR CELLS A PPLICATIONS 3.1 Design of Symmetrical and Unsymmetrical Oligomers for Three Different A pproaches to Morphology Control Driven by their ability to stack, discrete conjugated small mol ecules can arrange into very ordered crystalline phases. Solution processing requires non conjugated aliphatic chains to be covalently introduced onto the chromophore, disrupting the crystalli ne order otherwise adopted by un alkylated molecules. Still, solu tion processability is considered as one of the great advantages of organic electronics: solubilizing groups are often employed to the detriment of higher degrees of ordering. Figure 3 1. Three synthetic approaches to co ntrol the active layer morphology of organic electronic devices In some instances, suitable aliphatic/aromatic phase separation as well as ordering of the solubilizing groups themselves yield improved ordering in the solid state. In this Chapter, conjugat ed small molecules bearing some commonly used solubilizing
64 groups are also functionalized with different chemical functionalities tailored to enable Three approaches to morphology control are pre sented, as depicted in Figure 3 1. First, a symmetrical all thiophene oligomer bearing two reactive terminal alcohol groups (one on each end, Figure 3 1.a) is synthesized. This telechelic coil rod coil oligomer is designed to further polymerize with an app ropriate co equivalent. The ability of the afforded polymer to demonstrate electroactivity while retaining some of the ordering capabilities of the monomer suitable to OFETs is studied. Second, unsymmetrical oligo mers bearing a reactive phosphonic acid (PA) group (one PA on one end, Figure 3 1.b) are synthesized based on either all donor or donor acceptor donor (D A D) aromatic patterns. These unsymmetrical reactive molecules are designed to bind onto inorganic CdS e nanocrystals (NCs) as a means to control the active layer morphology of organic/inorganic hybrid solar cells. Third, symmetrical and unsymmetrical D A D oligomers bearing either common n hexyl or specific triisobutylsilyl end groups are synthesized (Figu re 3 dissertation, initial results on the symmetrical n h exyl derivative showed propensity for the conjugated molecule to crystallize, and this was developed extensively for molecular ork. 158,159 In particular in this Chapter, the influence of an unsymmetrical triisobutylsilyl end capped derivative on the crystallization of the symmetrical derivative is presented. The unsymmetrical molecule was treated as an additive to the composition of molecular BHJ solar cells based on the symmetrical derivative main component, influencing the morphology of the active layer.
65 3.2 Synthesis of Functionalized Oligomers The first part of this Chapter describes the synthesis of the small molecules descri b ed above, while each following sections focus on the approach each type of molecules was designed for. 3.2.1 Symmetrical Sexithiophene Bearing Two Terminal Alcohol Groups The synthesis of a telechelic sexithiophene bearing two terminal alcohol groups requi res the preparation of a proper end capping unit, as shown is Scheme 3 1. Scheme 3 1. Synthesis of the thiophene end capping moiety bearing a protected terminal alcohol. a) thiophene, n BuLi, THF, 78 C, 63% b) 1. n BuLi THF, 78 C; 2. trimethyltin chloride, >90% The alcohol functionality is attached as a THP protected alcohol to a thiophene ring via displacement by 2 lithiat ed thiophene of the bromine on 2 ((5 bromopentyl)oxy) tetrahydro 2H pyran starting ma terial 3 1 affording compound 3 2 which is subsequently stannylated at the 5 position. This yields the end capping moiety 3 3 to be used in a subsequent palladium catalyzed coupling reaction. The core of the sexithiophene is prepared as shown in Scheme 3 2, s tartin g with the bromination of 3 4 bromination of 3 hexylthiophene at the 2 position using NBS in cold DMF yields 2 bromo 3 hexylthiophene 3 5 which is subsequently converted to the correspon ding Grignard reagent. Two equivalents of the latter reagent are reacted with compound 3 4 in a nickel catalyzed Kumada cross coupling to afford the dihexylquaterthiophene 3 6 in good yields. This is dibrominated into 3 7 with little purification as the in soluble product
66 can be filtered and recrystallized from a hexanes/ethanol mixture ; it is subsequently reacted with two equivalents of the end capping moiety 3 3 described in Scheme 3 1, under Stille coupling conditions. S cheme 3 2. Synthesis of T6 diol and T6 dibenzoate ( 3 9 ) for crystal growth. a) NBS, DMF, r.t., 89%. b) NBS, DMF, 0 C, 93%. c) 1. Mg, ether; 2. 3 4 Ni(dppp)Cl 2 toluene:ether, 60 C, 86%. d) NBS, DMF, 0 C, 78%. e) 3 3 PdCl 2 P(Ph 3 ) 2 THF, reflux, 92%. f) HC l (conc.), DCM:MeOH, r.t., 95%. g) pyridine, THF, benzoyl chloride, r.t., 80% This affords the sexithiophene 3 8 a precursor to the telechelic T6 diol In order to recover the reactivity of the terminal alcohols, the THP protecting groups on 3 8 are clea ved off in dilute acidic conditions in a dichloromethane:methanol mixture at room temperature. Upon deprotection of the hydroxyl groups, an orange solid precipitates out of the reaction mixture. The filtered solids were identified by NMR as T6 diol with go od purity. Because the purity of monomers is key in achieving high molecular weight polymers, further purification of T6 diol was carried out. Several attempts to crystallize T6 diol either by solution slow evaporation or vapor diffusion between good and bad solvents were unsuccessful. Column chromatography using a mixture of hexanes and ethyl acetate proved to be the best purification method. Since attempts to crystallize the T6 oligomer failed for the diol form, one more synthetic step was carried out wh ere T6 diol was end capped with phenyl rings into its dibenzoate derivative 3 9 using benzoyl chloride as a reagent. Crystals of compound 3 9 were successfully grown by slow
67 evaporation from a 50:50 dichloromethane:pentane solution. The crystal structure i s displayed in Figure 3 2. Figure 3 2. Crystal structure of T6 dibenzoate 3 9 The inner bithiophene core is completely planar in the anti conformation, and it has a 34.9 dihedral angle with respect to the neighboring 3 h exylthiophene moieties. T he twisting of thiophene rings in conjugated backbones can have a significant effect on the band gap of the material. The hexyl side chains are opposite to one another in this crystal structure and exhibit an all trans conformation which is consistent with the dialkyl quaterthiophenes. 94,160 ,161 With T6 diol synthesized and of suitable purity for polymerization, this telechelic oligomer can be further reacted with a proper linker to afford mac romolecules with tertiary structures potentially offering advantageous morphology in the context of organic electronic applications. The polymerization of T6 diol the opto electronic characteristics of the afforded polymer and its morphologic al properties are described in S ection 3 3 of this Chapter. 3.2.2 Unsymmetrical Oligomers Bearing one Phosphonic Acid Group The synthesis of rod like unsymmetrical oligothiophenes bearing a phosphonic acid group at one end requires the functional group to be attached t o a thiophene ring
68 during the synthesis of the chromophore, as depicted in Scheme 3 3. The synthetic strategy employed here relies on the conversion of 2 bromothiophene into diethyl thiophen 2 ylphosphonate 3 11 under nickel mediated Arbuzov type conditio ns. This was followed by stannylation of 3 11 at the 5 position of the thiophene ring to afford the phosphonate bearing end capping moiety 3 12 to be used in subsequent palladium catalyzed coupling reactions. S c heme 3 3. Synthesis of the thiophene end capping moiety bearing a phosphonate group. a) NiCl 2 (anhydrous), P(OEt) 3 145 C, 55%. b) 1. LDA, THF, 78 C; 2. t r imethyltin chloride, >85% Head to tail (HT) fully regio regular oligomers based on 3 alkylthiophene offer th e benefit of a controlled degree of regio regularity compared to polymers that always exhibit some degree of region irregularity. Each step of the oligomer synthesis needs to be completely regio selective, since the separation of regio isomers is tedious. The oligomers are once again assembled by metal catalyzed cross couplings between substituted aromatic rings. Regio regularity allows for more uniform secondary and tertiary structures in the bulk 16 2 A similar regio regular oligomer has been previously re ported by Frchet et al. where five 3 hexylthiophene rings were connected in a head to tail fashion. 143 The synthetic approach was different though, as the oligomers were build in a centro symmetric fashion around a silyl core and separated in half after t he last coupling. The approach described in the following reduces the number of steps while giving access to a longer oligomer.
69 The regio selectivity is achieved by taking advantage of both the electronic and steric effects o f the alkyl side chain on the 3 position of the thiophene ring. Bromination with NBS at low temperatures in the absence of light affords the 2 brominat ed species alone (compound 3 5 ).Deprotonation using LDA accesses the 5 position of the ring selectively, where the anion can react with 2 isopropoxy 4,4,5,5 tetramethyl 1,3,2 dioxaborolane to afford the borylated thiophene 3 13 for subsequent Suzuki coupling. The synthesis of the regio regular heptathiophene phosphonate ester rrT7 PE is described in Scheme 3 4. Scheme 3 4. Synthesis of the regio regular T7 phosphonate rrT7 PE a) 1. LDA, THF, 78 C; 2. 2 isopropoxy 4,4,5,5 tetramethyl 1,3,2 dioxaborolane 86%. b) 3 5 Pd 2 (dba) 3 P( o tyl) 3 Et 4 NOH, toluene, 85 C, 72%, c) NBS, DMF, 0 C, 55%. d) 3 13 Pd 2 (dba ) 3 P( o tyl) 3 Et 4 NOH, toluene, 85 C, 80%. e) NBS, THF, 0 C, 71%. f) 1. LDA, THF, 78 C; 2. 2 isopropoxy 4,4,5,5 tetramethyl 1,3,2 dioxaborolane 52%. g) Pd 2 (dba) 3 P( o tyl) 3 Et 4 NOH, toluene, 85 C, 39%. h) NBS, THF, 0 C, 83%. i) 3 12 Pd 2 (dba) 3 P( o tyl) 3 toluene, 85 C, 28% The first Suzuki coupling reacting 3 5 and 3 13 to yield pure 3 14 was low yielding (~30%) mostly because of the troublesome separation of the dimer from its starting materials and a small amount of undesired head to head (HH) and tai l to tail (TT) coupling by product s (less than 5% by proton NMR). One way to overcome the tedious separation of the HT product from the HH and TT by product s was to limit the extent of purification on 3 14 and carry out the next bromination step on the mix ture of isomers.
70 The bromination being regio selective to the heterocyclic carbon between the alky l side chain and the sulfur atom, the desired HT isomer is mono brominated and the TT by product is dibrominated. The increased polarity of the three products then allowed the purification of 3 15 by column chromatograph y. This afforded pure 3 15 in 40 % yield over 2 steps from 3 13 A second Suzuki coupling between 3 13 and 3 15 afforded the regio regular terthiophene 3 16 in decent yield. Since by products in this step have different numbers of rings, the purification of 3 16 by column chromatography is possible. Subsequent bromination and borylation steps to afford 3 17 and 3 18 respectively and their Suzuki cross coupling eventually yields the regio regular s exithiophene 3 19 which is again brominated to 3 20 The last coupling reaction is the Stille coupling of the stannylated thiophene 3 12 bearing the phosphonate to the sexithiophene 3 20 to afford rrT7 PE Although this synthetic route enabled the synthes is of an extended strictly regio regular heptathiophene functionalized with a phosphonate group, the number of steps involved was still high a nd the overall yield rather low: t o extend the oligomer to six conjugated thiophene rings, seven steps were necess ary from the 3 hexylthiophene starting material with an overall yield of 5%. This is especially critical in the field of organic electronics as low cost is a pursued advantage of organic based materials. A second route was designed to still enable the synt hesis of functionalized unsymmetrical thiophene based oligomers. Symmetrical aromatic cores are first extended though high yielding symmetrical cross couplings, and then subjected to reaction with commercially available 2 (5 hexylthiophen 2 yl) 4,4,5,5 tet ramethyl 1,3,2 dioxaborolane ( 3 10 ) in a
71 stoichiometry such that the monocoupled product is isolated for further extension to the target asymmetric functional oligomer. The synthesis is described in Scheme 3 5. Scheme 3 5. Synthesis of the phosphonic acid functionalized T6 PA and T4BTD PA a) Pd 2 ( dba ) 3 P( o tyl) 3 Et 4 NOH, toluene, 85 C, 31%. b) 3 12 Pd 2 (dba) 3 P( o tyl) 3 toluene, 85 C, 30%. c) 1. TMS Br, DCM, r.t. 2. MeOH, r.t, 85%. d) 3 13 Pd 2 (dba) 3 P( o tyl) 3 Et 4 NOH, toluene, 85 C, 92%. e) NBS, AcOH, CHCl 3 0 C, 97%. f) 3 10, Pd 2 (dba) 3 P( o tyl) 3 Et 4 NOH, toluene, 85 C, 26%. g) 3 12 Pd 2 (dba) 3 P( o tyl) 3 toluene, 85 C, 64%. h) 1. TMS Br, DCM, r.t. 2. MeOH, r.t, 94% In the synthesis of the first thiophene based oligom er a sexithiophene (T6) bearing one phosphonic acid group ( T6 PA ) the olig othiophene core is extended to the dibromo 3,3''' bis(hexyl) 2,2':5',2'':5'',2''' quaterthiophene core ( 3 7 ) as described previously in Scheme 3 2 In the following step targe ting a monocoupled product, 3 7 is reacted with 1.5 equivalents of 3 10 in a Suzuki cross coupling reaction in toluene. We selected Pd 2 (dba) 3 and tri( o tolyl)phosphine as palladium and phosphine ligand respectively, and tetraethylammonium hydroxide as bor on activating base. Under
72 such conditions and after purification by column chromatography, yields of 31% for the targeted monocoupled product 3 21 and 23% for the dicoupled symmetrical sexithiophene by product were obtained. Moderate yields are expected in this unsymmetrical synthesis step because of the necessary stoichiometry and the purification process. Monobrominated pentathiophene 3 21 was reacted with the stannylated thiophene 3 12 under Stille coupling conditions, which yielded the phosphonate monof unctionalized sexithiophene T6 PE The increased polarity of the oligomer induced by the presence of the phosphonate group facilitated purification by column chromatography. The last step to the phosphonic acid T6 PA involves treatment of the phosphonate T 6 PE using trimethylsilyl bromide in DCM followed by hydrolysis with methanol. T he second thienylene oligomer a five ring oligomer consisting of one central benzothiadiazole ( BTD ) unit flanked by two thiophene rings on each side and bearing one phosphonic acid ( T4BTD PA ) was synthesized by reacting 4,7 dibromo benzothiadiazole with two equivalents of 3 13 under the same Suzuki cross coupling conditions than that used for the synthesis of the pentathiophene 3 21 to afford compound 3 22 in high yields. The l atter is then dibrominated using NBS in chloroform to afford the symmetrical precursor 3 23 Similarly to the conversion of 3 7 into 3 21 during the synthesis of the T6 PA oligomer, the stoichiometry of the reaction of 3 23 with borylated thiophene 3 10 ne eded to be adjusted in order to optimize the ratio of targeted monocoupled compound 3 24 to the unreacted and dicoupled by products. Unlike for the synthesis of the sexithiophene, for which 1.5 equivalents of the borolane were used, in this case 3 23 was r eacted with 0.80 equivalents of 3 10 S uch a
73 stoichiometry was expected to still afford the targeted monocoupled product in acceptable yields while being able to recover the valuable starting material 3 23 rather than the unreactive dicoupled by product. A fter purification of the reaction by flash chromatography, the unsymmetrical product 3 24 was obtained in 26% yield while the recovered starting compound 3 22 accounted for 53% of the material. Although slightly lower, the yield for the latter unsymmetrica l coupling was comparable to the one when 1.5 equivalents of the borolane were used in the synthesis of T6 PA Therefore, the stoichiometry for this kind of unsymmetrical cross coupling can be chosen depending on interest for specific by products. Compound 3 24 was then coupled to the stannylated thiophene 3 12 to install the phosphonate group onto the oligomer, affording T4BTD PE in 64% yield. The phosphonate was hydrolyzed as previously using trimethylsilyl bromide to yield the phosphonic acid monofunctio nalized T4BTD PA Once the two phosphonic acid functionalized oligomers T6 PA and T4BTD PA were synthesized, their interaction with CdSe nanocrystals was studied. Section 3 4 of this Chapter describes the evolution of the photoluminescence in mixtures of t he oligomers and their inorganic counterparts in solution, as well as the synthesis of the hybrid systems and their opto electronic properties. 3.2.3 Symmetrical and Unsymmetrical Functionality Free Donor Acceptor Donor Oligomers In the previous section, u nsymmetrical oligomers with one reactive functional group were synthesized in a step by step procedure, particularly involving the isolation of the unsymmetrical monobrominated precursor to the full oligomer. The following section describes the one pot syn thesis of an unsymmetrical oligomer by sequential addition of two different thienyl borolanes to a symmetrical dibrominated core under
74 Suzuki cross coupling conditions. This is an adaptation of a procedure which we first reported in 2010, to accommodate th e synthesis of unsymmetrical compounds. 158 The first thienyl borolane is the commercially available 2 (5' hexyl [2,2' bithiophen] 5 yl) 4,4,5,5 tetramethyl 1,3,2 dioxaborolane ( 3 25 ). The second thienyl borolane differs from 3 25 in that the n hexyl chain (referred to as C6) is replaced by a triisobutylsilyl group (referred to as Si) As shown in Scheme 3 6, 3 2 7 is synthesized in two steps by first lithiating 5,5' dibromo 2,2' bithiophene with on equivalent of n BuLi followed by quenching with the addition of triisobutylsilyl chloride. Separation of the monosilylated product 3 26 from starting material and disilylated by product by column chromatography was facilitated by the polarity and solubility difference conferred by the halogen atoms. This was then l ithiated again under the same conditions and quenched with 2 isopropoxy 4,4,5,5 tetramethyl 1,3,2 dioxaborolane to afford the borolane 3 27 Scheme 3 6. Synthesis of the bithiophene end capping moiety bearing a triisobut ylsilyl group. a) 1. n BuLi, THF, 78 C; 2. chlorotriisobutylsilane, 35%. b) n BuLi, THF, 78 C; 2. 2 isopropoxy 4,4,5,5 tetramethyl 1,3,2 dioxaborolane >70% Because the unsymmetrical oligomer differs from the parent symmetrical derivative by the bulkine ss of one side chain it is designed as a molecular additive to the main symmetrica l component of the active layer. A convenient one pot synthetic procedure affording both the main symmetrical component and the unsymmetrical additive alleviates concerns ov er the synthetic cost of such an approach to active layer
75 morphology control. As shown in Scheme 3 7.a, the dibromoisoindigo starting material is first reacted with 1.75 equivalents of 3 25 under Suzuki coupling conditions. Scheme 3 7 One pot synthesis (a) of iIT 2 C6Si and iIT 2 C6 2 and synthesis (b) of iIT 2 Si 2 a) Pd 2 (dba) 3 P( o tyl) 3 Et 4 NOH, toluene, 85 C, 29%, 40 % 64% for iIT 2 C6Si iIT 2 C6 2 and iIT 2 Si 2 respectively The stoichiometry was chosen so that ca. 65% of the starting material undergoing cross coupling with 3 25 would do so twice in the reaction mixture to yield the symmetrical iIT 2 C6 2 leaving 35% of the starting material coupled only on one side, thus still able to undergo further cross coupling. The rea ction was monitored by TLC using 2:1 hexanes:dichloromethane as eluent. After heating and stirring the reaction mixture for 3 hours, the red starting material spot for the dibromoisoindigo with a retention factor of 0.5 disappeared while two spots develope d at lower retention factors (0.25, blue and 0.35, purple) corresponding to the di coupled iIT 2 C6 2 and the monocoupled intermediate species, respectively. TLC showing almost complete conversion of the dibrominated starting material, the second borolane 3 27 was added to the reaction mixture, which was allowed to stir while heated for an additional 12hours. TLC of the crude showed two main blue spots with retention factors of 0.25 and 0.33 corresponding to the symmetrical iIT 2 C6 2 and unsymmetrical iIT 2 SiC6 products as confirmed by co spoting with the pure compounds. A very faint spot corresponding to the symmetrical disilyl iIT 2 Si 2 by product was observed, suggesting that a little amount of dibromoisoindigo
76 starting material remained in the mixture upon the addition of 3 27 despite the lack of evidence by TLC. After workup, the two targeted compounds iIT 2 SiC6 and iIT 2 C6 2 were successfully isolated by column chromatography in 29% and 40% yields re s pectively. Scheme 3 7.b shows the synthesis of the symmetric al disilyl oligomer iIT 2 Si 2 which was designed as main active layer component in control devices. In summary, this first S ection described the synthesis of three different types of oligomers designed to provide morphology control over the active layer of optoelectronic devices, each in their particular way. In the following, the ability of each oligomer to do so is described, the experiments carried out in each case being specific to the system studied. 3.3 Morphology Control via Telechelic Oligomer Polyco ndensation Half way between discrete oligomers and fully conjugated polymers is a class of polymers consisting of a non conjugated backbone where aliphatic segments either bear pendant oligothiophenes or are alternated with oligothiophenes in the main chai n, most frequently as polyesters. 163 169 In the following, the polymerization of T6 diol into T6PC is described. T he e lectrochemical and spectroscopic properties of T6PC were investigated. Wide angle X ray diffraction gave insight on the morphological feat ures suggested by thermal analysis and polarized optical microscopy of thin films and free standing films The performance of mechanically oriented polymer samples in OFETs was evaluated. 3.3.1 Synthesis of T6PC from T6diol With T6 diol in hand, a linker m olecule had to be selected, with which the terminal diols of the telechelic oligomer would react to form a macromolecule. The nature of the linkages would determine the nature of the alternating copolymer, and the polymer
77 repeat unit would be composed of 1 ) two hexyl aliphatic chains, 2) one sexithiophene conjugated rod and 3) the molecular structure of the chosen linker. The linker itself should not impair the electronic properties of the sexithiophene. It should also be of low molecular weight and size to electro optically ina c tive matrix. Phosgene was a good candidate because of its reactivity and its short length. Phosgene is essentially the smallest linker possible leading to a polycarbonate (PC) wh en reacted with diols. The only but major drawback is its toxicity, especially as phosgene is a gas at atmospheric conditions. In its trimeric form though, triphosgene is a solid which makes its handling much safer and better suited for the precise stoichi ometric balance required in polymerization reactions. Scheme 3 8 shows the polymerization of T6 diol using triphosgene, resulting in the polycarbonate T6PC The best polymerization conditions were a modification of that reported in the literature for the s ynthesis of polycarbonates from bisphenol A and triphosgene. 170 Scheme 3 8 Polymerization of T6 diol into T6PC using triphosgene. a) triphosgene, pyridine, THF, 79% The polymerization occurred smoothly in either anhydro us DCM or anhydrous THF, the choice of the solvent depending mostly on the initial solubility of the diol oligomer, which was not an issue for T6 diol Stoichiometric amounts of the diol and triphosgene were dissolved in the appropriate solvent at room tem perature under inert atmosphere, and stirred until complete dissolution of the reagents. Approximately 4 equivalents of anhydrous pyridine were added dropwise at room temperature. The
78 reaction mixture started gelling after an hour and half of stirring, and was allowed to stir at room temperature for an additional 12 hours. The extent of polymerization could easily be monitored by 1 H NMR as the triplet at 3.65 ppm corresponding to the two H h of the methylene next to the hydroxyl group moves downfield to 4.14 ppm after polymerization as a result of the withdrawing effect of the newly formed carbonate functionality. As can be seen on Figure 3 3.a, a small peak remains at 3.65ppm that could correspond to methylenes next to unreacted end groups (red arrow). The I R spectrum of T6PC displayed in Figure 3 3.b, shows the appearance of the carbonate carbon oxygen singl e and double bonds peaks, centered at 1260 cm 1 and 1742 cm 1 respectively (blue spectrum), compared to that of T6 diol (black spectrum). Figure 3 3 1 H NMR (a) of the polycarbonate T6PC and IR spectra (b) of T6 diol and T6PC The red arrow at 3.65 ppm indicates the protons on the carbon alpha to unreacted terminal alcohols. The reaction mixture was then diluted with chloroform, washed with water and fi nally the organic extracts were precipitated in methanol and purified by Soxhlet extraction using methanol, hexanes and chloroform. The remainder of this study is performed on the polymer sample from the chloroform Soxhlet fraction. A number
79 average molecu lar weight (M n ) and polydispersity index (PDI) of 22,700 kDa (PDI = 2.07) for T6PC from the latter fraction was measured by gel permeation chromatography (GPC) against polystyrene standards. The polymer is soluble in THF, toluene and chlorinated solvents s uch as dichloromethane, chloroform and chlorobenzene. 3.3.2 Spectroscopy, Electrochemistry and Spectroelectrochemistry of T6PC The UV vis absorption spectra of T6PC and T6 diol in THF solution are identical, as shown in Figure 3 4.a (black plain and dashed lines, respectively). The polymer was then sprayed onto ITO coated glass slides from THF solution (2 mg/mL) and the thin film absorption was recorded (blue line). Figure 3 4 UV vis spectra (a) of T6 diol and T6PC in solution (black lines) and T6PC thi n film sprayed onto ITO coated glass slide (blue line), and UV vis spectra (b) of the chemical doping process of T6PC with EPR signals of t he neutral and oxidiz ed species (inset). Orange lines are neutral and blue lines are oxidized species In solution, o ne symmetrical absorption band centered at max = 424 nm is observed for T6PC with an absorption onset at 500 nm. In the solid state, the absorption of T6PC is red shifted with the appearance of local maxima: the peak absorption is shifted to 479 nm, with a higher energy shoulder at 455 nm and a lo wer energy shoulder
80 at 515 nm. The transition from solution to thin film is thus characterized by a higher vibronic resolution, which could be accounted for by a better ordering of the material in the solid state. With a solid state absorption onset at 556 nm, the optical energy gap of T6PC is calculated to be 2.23 eV, which is about 0.2 eV higher than the bandgap typically reported for the fully conjugated polythiophene P3HT. 30 In order to test the electroactivity of the polymer in solution, chemical oxida tion experiments were carried out. The chemical oxidation of a dichloromethane solution of T6PC by the addition of s ilver hexafluorophosphate as oxidant was monitored in parallel by UV vis absorption spectroscopy and EPR spectroscopy, as shown in Figure 3 4.b. As the concentration of oxidant in solution is increased, the neutral state absorption band centered at 424 nm gradually decreases while two new bands centered at 630 and 1090 nm emerge. This translates into the yellow neutral solution switching to a blue color as the oxidant concentration is increased ( inset). While the neutral solution is EPR silent (orange line in the inset, Figure 3 4.b) as expected for a diamagnetic sample, the addition of silver hexafluorophosphate oxidant resulted in a broad EPR signal (blue line, inset) centered at g = 2.005 with a peak to peak width of 2.8 G The emergence of the two absorption bands upon chemical doping coupled with the appearance of an EPR signal supports the formation of radical cations in solution, and the results are consistent with previous reports of oligothienylene doping. 161 No additional band was observed when excess oxidant was added in solution. To investigate the redox properties of T6PC in the solid state, electrochemical measurements were conducte d on thin films of the polymer dropcast from THF solution onto Pt button electrodes. Figure 3 5.a shows the cyclic voltammograms of T6PC films
81 recorded in 0.1M lithium bis(trifluoromethylsulfonyl)imide (LiBTI) in acetonitrile (ACN) under inert atmosphere. All potentials are calibrated versus Fc/Fc + Figure 3 5 Tenth (solid lines) and 150 th (dashed lines) cyclic voltammograms (a) from 0 to 0.4 V (black lines) and from 0 to 0.95 V (blue lines) of T6PC drop cast onto Pt button electrodes in 0.1 M LiBTI/ ACN under inert atmosphere Differential pulse voltammogram (b) of T6PC drop cast onto Pt button electrodes under the same conditions Initial scans up to 1.0 V revealed two oxidation processes which were not stable to repeated scans. A first oxidation wave, displayed in black in Figure 3 5.a, was isolated by confining the CV potential window from 0 V to 0.40 V. With an anodic peak potential at 0.34 V and a cathodic peak potential at 0.21V, this first oxidation process centered at a half wave potential of 0.27 V was quasi reversible and stable to at least 150 scans from 0 to 0.40 V (black voltammograms, Figure 3 5.a) When the potential window was increased to from 0 to 0.95 V, a second oxidation process with anodic and cathodic peak potentials at 0.82 V and 0. 52 V was observed, but the current intensity decreased over repeated cycles, as shown in blue in Figure 3 5.a. The first oxidation process likely corresponds to the formation of the radical cation in the film of T6PC while the second recorded oxidation wo uld correspond to the formation of the dication species. This suggests that the electrochemically generated radical cation is a readily accessible and
82 stable species, while accessing the dication is only possible at potentials at which film degradation occ urs. Figure 3 5.b shows the DPV of the T6PC film on Pt button electrode, recorded in the same conditions as for the CV measurements. From the onset of oxidation at 0.20 V, a HOMO energy level of 5.30 eV is calculated, which is within 0.1 eV of that reporte d in the literature for P3HT. With an optical energy gap of 2.23 eV, the LUMO is calculated to be at 3.07 eV. The spectroelectrochemistry of T6PC thin films was conducted on films sprayed onto ITO coated glass slides. The electrolyte was switched to 0.1M L iBTI in PC. The CV and DPV of the polymer were first recorded to break in the films and identify the required potential window, as displayed in Figure 3 6.a. Figure 3 6. Cyclic voltammograms (a) from 0.1 to 0.45 V (black line) and from 0.1 to 1.05 V ( dashed line) and DPV (dash dot line) of T6PC sprayed onto ITO coated glass slides in 0.1 M LiBTI/ PC under inert atmosphere. Spectroelectrochemistry (b) for a spray cast film of T6PC on ITO coated glass, from 0.2 V to 1.05 V versus Fc/Fc + 0.1 V potential inc rements, re corded in LiBTI/PC solution Consistent with the Pt button electrochemistry, two oxidation waves are observed when the potential window was scanned from 0 to 1.05 V (CV2, dashed line). The current intensities decreased significantly, by half ove r thirty cycles (not shown) confirming the poor stability of the polymer film at such high potentials. Nevertheless,
83 spectroelectrochemical measurements were performed on the T6PC films, stepping the potential from 0.20 V to 1.05 V at 0.1 V increments, as shown in Figure 3 6.b. Initial oxidation leads to the decrease of the neutral absorption band centered at 480 nm, while two new bands emerge around 630 nm and 1050 nm, which then merge into a sin gle absorption band peaking at 7 30 nm, at potentials higher than 0.8 V. The overall absorption intensity at potentials higher than 0.5 V start s decreasing, which is another indication of thin film degradation. The appearance of the two bands at lower potentials corresponds with the formation of radical cation spec ies in the film, as identified from the solution chemical doping experiments. Although it leads to film degradation, the progressive fusion at higher potentials of these two bands into a single one at intermediate wavelengths corresponds to the formation o f dication species in the film, which was previously documented for similar oligothienylene systems. 171 Since the first oxidation process was significantly more stable to repeated cycles than the second one, spectroelectrochemical measurements focusing on a shorter potential window were conducted. A film of T6PC on ITO already subjected to at least 10 CV scans from 0.1 to 0.45 V was inserted in the spectrophotometer with the spectroscopy cuvette as electrochemical cell, and step potential s were applied fro m 0.23 V to 0.54 V in 10 mV increments, resulting in the spectra displayed in Figure 3 7.a. The neutral spectrum (yellow line) featuring the peak absorption around 480 nm and some vibronic resolution as described previously gradually decreases as two new b ands centered a 621 and 1036 nm appear. They eventually stabilize to ca. 4 0% of the neutral film peak absorption intensity. This is accompanied by a reversible color change from orange to blue as displayed in the inset of Figure 3 7.a. Consistent with the
84 solution chemical doping spectra an d the CV experiments, this dual color electrochromism can be attributed to the generation of radical cations with in the thin film of T6PC Figure 3 7 (a) Spectroelectrochemistry for a spray cast film of T6PC on ITO c oated glass, from 0.23 V to 0.5 4 V versus Fc/Fc + 10mV potential increments, re corded in LiBTI/PC solution (switching film pictures in inset) and (b) Square wave potential step absorptometry, from 10s to 0.5s switching times The contrast can be visually app reciated on the oxidized film in the inset as both neutral and oxidized areas coexist on either side of the electrolyte solution meniscus. A well defined isosbestic point can be seen at 533 nm. This is consistent with the polymer repeat unit structure, as the chromophore content in the backbone is monodisperse: spectral change arises from the removal of an electron from systems which are all of the same conjugation length. In the switching speed experiment ( Figure 3 7.b ), the contrast ( defined as the diff erence in % transmittance at 454 nm) was recorded as a function of the potential step time The contrast decreases from 27% at 10s to 16% at 0.5s. Although the maximum contrast does not compare to that of the best electrochromic poly mers because the design of T6PC does not allow it to become fully
85 transmissive in the first oxidation process, still it appears that the electrochromic switching is fast with little loss of contrast even at potential step times of 1s. The polycondensation of T6 diol in to its polycarbonate affords a polymer which by design contains conjugation breaks, yet the previous set of experiments shows that electroactivity, at least in an electrochemical sense, is maintained in thin films. The nature of the charged species was ide ntified and correlated to the observed electrochromism in the films. The next section focuses on the morphological characteristics induced by the covalent linking of the telechelic oligomers into macromolecules. 3.3.3 Liquid Cr ystallinity and Bulk Morpholo gy Upon polymerization, T6PC acquired film forming characteristics with sufficient mechanical strength that free standing films were easily obtained by simple evaporation of a THF solution. A 7.0cm x 1.5cm free standing film of T6PC was easily peeled off o f a rectangular Teflon mold, as displayed in Figure 3 8.a. At room temperature, the film does not stretch, but once heated at 65 C, the film can be stretched up to 300% of its initial length prior to mechanical failure. The thermal behavior of conjugated s ystems is an important property to investigate, as thermal treatments can have a significant T6 diol and T6PC in Figure 3 8.b show that both compounds are thermally stable up to 422 C and 370 C re spectively, setti ng a 5% weight loss as thermal stability threshold. The DSC thermogram of T6 diol (Figure 3 8.c, dashed line ) shows a sharp melting trans ition that occurs at 104 C during the second heating scan. Upon cooling, a crystallization peak appear s at 64 C. Compared to T6 diol the DSC thermogram of T6PC shows
86 broaden ed peaks which are shi fted to lower temperatures at 84 C. In particular, a second peak at 52 C appears in the heating scan of T6PC Figure 3 8 Picture (a) of a 7.0cm x 1.5cm fre e standing film of T6PC TGA thermograms (b) of T6 diol (solid line) and T6PC (dashed line) under nitrogen, DSC thermograms (c) of T6diol (dashed line) and T6PC (solid line), and evolution of the DSC thermogram (d) of T6PC with annealing time a room temper ature A glass transition temperature (T g ) is observed at 18C, but n o crystallization peak was recorded in the cooling scan. Annealing experiments were performed at various temperatures to identify possible phase transitions yet the only effective proced ure took place at room temperat ure, as detailed in Figure 3 8, and described in the following The polymer was subjected to one heating and cooling cycle (10 C/min) to erase its
87 thermal history. A second heating scan was recorded immediately afterwards (an nealing time t = 0), then cooled in similar conditions. The polymer was then allowed to rest in the DSC pan for one hour in the instrument sampler (which is kept a room temperature) and then heated again (t = 1h). This was repeated three more times with in creasing annealing time s of 3, 24 and 48 hours. This le d to identify one reproducible trend: as the polymer was left at room temperature (23C), the two endothermic transition peaks intensify with time These results suggest that room temperature (which is close to the T g ) offers enough energy for T6PC to undergo some phase transition, in an overall relatively slow process. This behavior was observed previously on samples of poly(3 decylthiophene) (P3DT) of 14.1 kDa weight average molecular weight (PDI = 1. 64). 172 In the repo r ted DSC thermograms of P3DT, after in i tial heating and cooling, reheating the sample immediately only displayed broad features; but after a day at room temperature, samples crystallized ( either from the melt or from the mesophase ) recov ering a thermal behavior closely similar to that of the pristine sample s. In the case of T6PC n o noticeable change was observed after 2 days of annealing at room temperature Polarized optical microscopy coupled to a heating stage to monitor microscopic m orphology changes upon thermal treatment was used to identify possible phase transitions. Figure 3 9 shows the POM images of T6 diol and T6PC When a sample of T6 diol is heated above its melting point, and then allowed to cool back to its crystallization temperature, a well ordered phase with strong birefringence under polarized light emerges (Figure 3 9, left).
88 Figure 3 9 Polarized light optical microscope images of T6 diol (left) and T6PC (center and right), at crossed polarizer/analyzer The observe d Maltese cross patterns are typical of a spherulitic arrangement of the crystallized domains. Specifically, the spherulites could be a result of needle s haped crystals that emerge from a common center and are radially oriented, which would explain the Mal tese cross pattern. Spherulites were also observed under similar conditions in reports by Pisula et al. of the self assembly of phenylene thienylene oligomers bearing linear alkyl and alcohol terminated chains, like T6 diol 173 The molecular arrangement co uld be explained in terms of amphiphilicity, as a result of competition between hydrogen bonding of the terminal hydroxyl groups and mutual exclusion of the alkyl and alcohol groups; with the added propensity of the conjugated cores to stack. Upon polym erization of the diol into T6PC the oligomers lose some degree of freedom as a consequence of the covalently f ormed carbonate functionalities. T he polarized light pattern changes accordingly to a less ordered structure, as shown in Figure 3 9. POM images of a T6PC film on ITO coated glass reveal a microstructure similar to a Schlieren type nematic texture but on a small scale Schlieren textures characteristic of nematic phases typically display features on the 100 microns scale. 174 In the case of T6PC as detailed in the right hand side of Figure 3 9, the birefringence
89 features are on the 1 to 5 microns scale. Optical micrographs showing a fine nematic texture identical to that in Figure 3 9 have been described previously by Windle et al on POM captions o f random copolyesters of ethylene terephthalate and hydrobenzoic acid. 175 Another example of such fine texture can be found in a report of liquid crystalline poly(phenylene ethynylene) by Bunz et al. 176 In the case of T6PC it was observed at room temperat ure after the sample was held at 140 C for one hour and allowed to cool down. When reheated, the texture holds up to the second melting temperature in the 50 to 55 C range, after which the sample becomes optically isotropic. Therefore, t he T6PC thin film s hows local optical anisotropy on a scale of a few microns at room temperature This could be explained by phase separation between the aromatic cores and the aliphatic segments as well as stacking between neighboring chromophores, which can lead to some degree of order in the polymer Quantitative insight on how well T6PC organizes when the polymer chains are aligned is provided by two dimensional wide angle X ray spectroscopy measurement s performed on extruded filaments of the polymer. The sample was prepared as a thin filament of 0.7 mm diameter by heating it up for extrusion to 65C at which it becomes plastically deformable. The diffractogram in Figure 3 10.a was obtained at 30C. The distance for the outer reflections is 3.7 This peak position corresponds to the intermolecular distance between two stacked chains. Being located on the equatorial axis in the wide angle region, this reflection indicates that the lamellae of stacked chains are aligned along the extrusion direction as depicted in Figure 3 10.b. Additionally, several pronounced reflec tions on the equatorial axis appeared related to the d spacings of 1.70 nm, 0.76 nm, 0.50 nm, which are attributed to the interlamellae
90 distance. Some of t he best polymeric OFETs reported in the literature so far are based stac king distance lies in the range of 3.9~3.6 42 Figure 3 10 2D WAXS pattern (a) of T6PC as an extruded filament at 30C (above) and scattering intensity distribution as a function of the scattering vector (below). (b) Model for the aligned polymer chai ns OFETs were fabricated at the MPI with T6PC as active layer. Highly doped silicon was used as the gate electrode, while the dielectric was a 200 nm thick SiO 2 film. A bottom contact FET (channel widths 5 prepared by spin coating a 10 mg/mL T6PC chloroform solution. The solution processing and electrical measurements were performed inside a nitrogen filled glovebox at room temperature. Unfortunately, little transistor behavior was obtained under such conditions, as hole mobilities of 10 7 cm 2 V 1 s 1 with an on/off ration of 10 2 were recorded. Annealing studies on the solution processed device did not improve the
91 performance. Since T6PC has mechanical properties such that it can be stretched up to 300% of its length without mechanical failure once heated to 65 C, it was proposed that polymer chains could be mechanically oriented by stretching a sample prior to device fabricatio n. Figure 3 11 .a shows the evolution of the birefringence of a T6PC free standing film with film orientation with respect to the crossed polarizer/analyzer direction (0 and 45 degrees) before (top) and after (bottom) stretching. Figure 3 11. POM captur e of the free standing film (a) before (top) and after (bottom) stretching at 0 (left) and 45 (right) with respect to the analyzer at crossed polarizer/analyzer POM capture of the stretched film transistor (b) at 0 (left) and 45 (right) with respect t o the analyzer at crossed polarizer/analyzer While there is no preferred orientation before stretching, the film becomes clearly anisotropic after it is stretched; suggesting that such a mechanical treatment efficiently aligns the polymer chains. This obs ervation made in our labs was communicated to our collaborators at the MPI, who applied it in OFET device fabrication. In this setup, top contact FETs (channel widths 25 manually stretching a film of T6PC onto the dielectric surface. Figure 3 11 .b shows the POM images of the stretched film transistor at 0 (left) and 45 (right) degrees with respect to the crossed polarizer/analyzer. Unfortunately, no transistor characteristics were obtained likely due to a poor interface between the film and the dielectric most probably caused by the clamping of the film.
92 In summary, the secondary structure designed through polycondensation of the terminal diols (a process likely applicable to many other electroactive oligomers) allows the material to acquire physical properties of macromolecules, while retaining electroac tivity and displaying micron scale ordering in thin films. Extruded polymer s amples show that chromophores stack with a distance of 3.7 which is within the range of high mobility materials reported in the literature. Unfortunately, the material did no t perform well in OFETs, even after attempts to mechanically align the polymer chains by taking advantage of the good mechanical properties of T6PC 3.4 Morphology Control via Monofunctional Oligomer/Inorganic Nanoparticle Hybrids With the phosphonic acid functionalized sexithiophene and bithiophene BTD bithiophene oligomers synthesized (Scheme 3 9), their design as electroactive ligands for inorganic CdSe nanocrystals was tested. First their optical and electrochemical properties were studied, and then the ir interaction with the nanocrystals was probed by solution photoluminescence evolution in mixtures. Finally, hybrids were obtained and their composition was analyzed. Scheme 3 9 Structure of T6 PA and T4BTD PA oligomers T he UV vis absorption and fluorescence spectra were obtained for T6 PA and T4BTD PA as shown in Figure 3 12 T6 PA has one absorption band centered at 426 nm, while the spectrum of T4BTD P A feature s two absorption bands peaking at 360 nm
93 and 505 nm. M ol ar absorptivities of 2 0,000 50,000 M 1 cm 1 were recorded in CHCl 3 solutions, as summarized in Table 3 1 From the absorption onset in solution, a relatively high energy gap of 2.4 eV is calculated for T6 PA as expected of an oligomers with homogeneous system. The BTD based oligomer, on the other hand, features a longer wavelength absorption onset corresponding to a lower HOMO LUMO gap of 2.0 eV. This is due to the DA interaction attributable to the mixing of the BTD acceptor unit with the flanking bith iophene donors. The CdSe NCs have a long wavelength absorption peak at 624 nm characteristic of the quantum confinement effect and the absorption increases steadily towards the UV region of the spectrum. Figure 3 12 UV vis absorption (a) and fluoresce nce (b) spectra of the two oligomers and the CdSe NPs in chloroform solution This is in accordance with the size of the NCs, and an optical energy gap of 1.9 eV is calculated. We measured the photoluminescence of each oligomer, in ester and acid form, in dilute chloroform solution. The oligomer solutions exhibit intense fluoresce nce with quantum efficiency near or above 50%, as summarized in Table 3 1. The peak emission wavelength of T4BTD PA is red shifted compared to T6 PA : the peak
94 fluorescence is at 56 5 nm for T6 PA and 676 nm for T4BTD PA which is consistent with the absorption results. Solution fluorescence lifetimes were determined for both acids and esters, and t here is little difference between the acid and the ester form for each oligomer, as exp ected for the dilute solutions used where little aggregation is expected. While the T6 oligomer show s short lifetimes of ca 0.9 1.0 ns, the BTD oligomers exhibit s significantly longer lifetimes of ca 5 ns The longer the exciton lifetime is, the better chance it has to reach a heterojunction at which it can be separated into a hole and an electron before recombination occurs. Table 3 1 Absorption and fluorescence max optical HOMO LUMO gaps, extinction coefficients FL quantum yields and FL lifetimes for each oligomer. max abs (nm) Optical E (eV) abs (M 1 cm 1 ) max Fl (nm) Fl Fl (ns) T6 P E 424 2.4 55600 537/564 0.54 0.86 T6 P A 426 2.4 48700 539/565 0.49 0.85 T4 BTD P E 504 2.1 30000 675 0.79 5.60 T4 BTD P A 508 2.0 21000 676 0.79 5.55 CdSe 624 1.9 632000 650 0.001 1.26 The redox properties of each oligomer were investigated using cyclic and differential pulse voltammetry in solution. For each oligomer, a small amount of material was dissolved in a dry and degassed dichloromethane based electro lyte containing 0.1M tetrabutylammonium hexafluorophosphate (TBAPF 6 ), so as to achieve a concentration of 1 mM in oligomer. All measurements were perfor med in an argon filled glovebox. All potentials are reported against the Fc/Fc + standard. For the T6 PA oligomer in solution, the oxidative CV ( Figure 3 13.a ) shows two quasi reversible processes centered at half wave pote ntials of 0.33 V and 0.56 V. No reduction was
95 observed when the potentials were scanned cathodically of 0 V up to 2.0 V. The absence of r eduction process for T6 PA is not surprising since it is an electron rich chromophore which would require even more negative potentials to accommodate the addition of an electron in its system. For T4BTD P A the oxidative CV show s one reversible oxidation process centered at a half wave potential of 0.50 V. In contrast to T6 PA the reductive CV of T4BTD P A recorded one reversible reduction process centered at a half wave potential of 1.66 V. This is consistent with the D A nature of the chromophore, which results in a lowered LUMO energy level ( higher electron affinity ) Figure 3 13 CV and DPV of (a) T6 PA and (b) T4BTD PA in 0.1 M TBAPF 6 in dichloromethane at 50 mV/s scan rate F rom the oxidative DPV, oxidation onsets for T6 PA and T4BTD PA were measured at 0.22 V and 0.40 V respectively. As accounted for above, only T4BTD PA showed a reduction process in reductive DPV experiments, with an onset of reduction at 1.5 5 V (see Figure 3 13, dashed lines ). C onvert ing the voltage values calibrated against the Fc/Fc + standard into energy values against vacuum, using a Fc/Fc + redox standard set at 5.1 eV HOMO energy level s were calculated at 5.32 eV for T6 P A and at 5.50 eV for T4BTD P A We could calculate the LUMO energy of T4BTD PA from the reductive DPV ons et to be at 3.55 eV, giving an electrochemical energy gap of 1.95 eV
96 which is close to its optical ener gy gap value of 2.0 eV measured spectroscopically The absence of a voltammo gram wave attributable to the reduction of the all thiophene oligomer prevented the electrochemical estimation of LUMO energy for the latter. Since the optical energy gap of the BTD based oligomer is only within 0.05 eV of its electrochemical energy gap, t he corresponding optical energy gap listed in Table 3 1 was used to deduce the energy of the LUMO for T6 PA which was 2.92 eV vs vacuum. Figure 3 14 depicts the position of the HOMO and LUMO energy levels with respect to the positions of the conduction a nd valence bands of the CdSe NCs used in this study, between 4. 3 and 4.5 eV, and between 6.2 and 6.3 eV respectively. 17 7 Figure 3 14. Energy levels diagram (absolute values) for the HOMO and LUMO levels of T6 PA T4BTD PA and NCs. T he LUMO levels o f the oligomers are more than 1 eV higher than the CB of the NCs and likewise the HOMO levels are more than 0.5 eV higher than the VB of the NCs. Such energetic offsets result in staggered energy gaps for each organic oligomer / CdSe NCs complex, which is analogous to that described as type II heterojunctions in solid state semiconductor physics. 1 In terms of the expected photoelectrochemical behavior, this type of heterojunctions suggests that photoexcitation of either the
97 oligomers or the NCs should lead to electron transfer from the oligomer (as an electron donor) to the CdSe NCs (as an electron acceptor ). The two phosphonic acid functionalized oligomers in this study are designed to undergo ligand exchange with native surfactants. In the following, w e mo nitored the evolution of the photoluminescence ( PL ) intensity of each oligomer upon addition of incremental amounts of CdSe NCs in solution, and compared the evolution for the phosphonate derivatives versus the phosphonic acid ones 3.4.2 Oligomer/ CdSe N C PL Quenching Experiments Photoluminescence quenching is a powerful tool to probe the electronic interaction between two different electroactive species. This technique was used in particular by Frechet et al to decipher between charge and electron transfer processes in a system composed of 4 nm CdSe NCs and phosphonic acid functionalized pentathiophenes. 143 emission upon addition of the pentamer. The PL quenching of shorter thiophene trimers was also quenched by CdSe NCs, but the emission of the NC in the reverse experiment increased. This was accounted for by the difference in staggered energy gaps between the NCs and the pentamer (type II heterojunction) compared to straddling energy gaps (type I heterojunctions) between the NCs and the wider energy gap trimer. Ruling out the possibility of energetic surface defect passivation by the phosphonic acid anchori ng group itself, the dual luminescence quenching was explained by an electron transfer mechanism from the thiophene pentamers to the NCs. Similar observations were reported by Advincula et al. for phosphonic acid functionalized thiophene dendrons, and this type of experiments was used by others as well. In the dilute solutions typically
98 used for fluorescence experiments, photoluminescence evolution upon the interaction of two different species requires them to be in close proximity of one another, regardles s of the quenching mechanism. 109 The inorganic synthesis of CdSe NCs involves the use of surfactant s usually composed of long alkyl chains and a polar functional group, such as oleic acid or trioctylphosphine oxide (TOPO), with which the NC surface is coat ed after the reaction is over. There is thus an inherent insulating layer of aliphatic surfactants coating each NC, which has been shown to be detrimental to their electronic interaction with conjugated polymers. 178 The exact nature of the aliphatic surfac tants coating the NCs is not straightforward, as it depends on the nature and purity of that used during NC synthesis, and the purification process that followed. Nevertheless, the use of functional groups such as phosphonic acids or carboxylates, which bi nd strongly to the NCs surface, have been shown to displace some of the aliphatic native surfactants, during a ligand exchange process which results in new molecules anchored to the NC surface 1 79 The two phosphonic acid functionalized oligomers in this st udy are designed to undergo ligand exchange with native surfactants. We monitored the evolution of the PL intensity of each oligomer upon addition of incremental amounts of CdSe NCs in solution (Experiment A, Figure 3 15) and compared the evolution for th e phosphonate derivatives versus the phosphonic acid ones. Figure 3 15 .a shows the PL evolution for the T6 phosphonate ( T6 PE left) and the T6 phosphonic acid ( T6 PA right) in dilute chloroform solution (5 M) as 2 0 M CdSe in chloroform was added in in crements The relative concentrations were such that only microliters of CdSe solution were added to the fixed volume of 2 mL of oligomer solution, thereby negating the effect of dilution on
99 the PL intensity For the phosphonate T6 PE t he addition of the CdSe leads to little quenching of the oligomer luminescence. This suggests that the ester form of T6 has, at best, a weak interaction and thus there is little binding of the oligomers to the NCs. The opposite is true for the acid form T6 PA where very str ong luminescence quenching was observed at substantially low CdSe concentration (nanomolar range). The fluorescence is essentially fully quenched at a concentration ratio of T6 PA :CdSe equals 50:1 in solution. Figure 3 15 Evolution of the fluorescence in chloroform of (a) T6 PE (left) and T6 PA (right) upon addition of CdSe NPs into the solution, (b) CdSe NPs upon addition of T6 PE (left) and T6 PA (right) solutions, (c) T4BTD PE (left) and T4BTD PA (right) upon addition of CdSe NPs into the solution, and figurative description of the two types of experiments (top right) When carefully monitoring the 610 to 630 nm range for any enhance ment of the emission from the CdSe in the oligomer/CdSe mixture no luminescence increase was
100 observed This was not su rprising considering the low concentration of CdSe NCs in solution, and was not sufficient to decipher between electron or energy transfer mechanisms. This was further tackled by studying the PL quenching of CdSe by T6 oligomers When solutions of CdSe NCs were selectively excited at 630 nm, their emission intensities were recorded when various amounts of T6 oligomer s were mixed in This was done according to the experimental procedure B described in Figure 3 15, keeping the concentration of CdSe constant i n each measurement. It is observed that the photoluminescence of CdSe decreased upon addition of T6 PA while by comparison, the same amount of T6 PE had no influence on the CdSe emiss ion. As the mechanism of photo induced charge transfer process is concer ned, either of the following scenario are to be considered: 1) direct excitation of the organic ligands (CdSe NCs) followed by the electron injection (hole migration) from the ligands (CdSe NCs) to the CdSe NCs (the ligands), or 2) Energy transfer happens from the excited state of the ligands to the CdSe NCs generating excitons in the NCs before the hole migration from NCs back to the ligands. In all events, charge separation between the components of t he hybrid materials is involved. Figure 3 15 .c shows th e same PL quenching experiment of oligomer emission by addition of CdSe conducted with the BTD based oligomers (phosphonate T4BTD PE left and phosphonic acid T4BTD PA right). The quenching intensity difference between the phosphonate and the phosphonic a cid was similar to that observed for the T6 oligomers: the luminescence of the acid form was very sensitive to the addition of CdSe, while the ester form remained fluorescent when the same amount of CdSe was added. This led to the same conclusion that the phosphonic acid functionalized oligomers have
101 a strong binding ability to the CdSe NCs. Unfortunately, the reverse experiment type B for the BTD based oligomer s was not possible since the absorption of both organic and inorganic species overlap significant ly. In summary, the intensity of the PL quenching suggests that the phosphonic acid group allo ws the oligomers to be in close contact with the CdSe NCs, to an extent where a 50:1 oligomer:NC ratio is sufficient to achieve complete transfer of the excited state from the oligomer to the NP. Because of the low concentrations employed any evolution in the PL intensity sh ould be due to an oligomer/NC complex formation, i.e. direct interaction between the two This seems to occur by a charge transfer process ra ther than energy transfer as the emission of both species is quenched in the case of T6 PA 3.4.3 Hybrids Synthesis and Characterization From the PL quenching experiments, a 50:1 ratio of T6 PA :CdSe or T4BTD PA :CdSe was found to be sufficient to completely quench the luminescence of the organic chromophore. We thus stipulated that such a ratio or higher would be suitable for the synthesis of the hybrids themselves. The hybrid preparation consists in the exchange of the superficial native ligands of the NCs with T6 PA or T4BTD PA by mixing in chloroform, followed by precipitation of the NC/oligomer hybrid in an appropriate solvent and centrifugation to remove the supernatant containing any unbound species. Experimentally, 10 mg of the oligomer was dissolved i n 5 mL of degassed chloroform, to which was added a solution of the NCs in chloroform at the appropriate concentration for an excess of 200:1 ratio in oligomer:NCs. The mixture was stirred vigorously in the absence of light at room temperature for 30 minut es, after which i t was precipitated in a poor solvent for the NCs/oligomer hybrid, but good solvent for
102 the unbound surfactants and excess oligomer. For the T6 PA /CdSe NC system, ethyl acetate was used to precipitate the hybrids, while methanol was suitabl e to precipitate the T4 BTD PA/CdSe NC system. After centrifugation of the suspension and removal of the supernatant containing unbound species, the precipitates were redissolved in chloroform and precipitated once again in the proper solvent. This was rep eated several times, while recording the UV vis absorption spectrum of the chloroform solutions in each step. As unbound oligomers remained in the supernatant which was removed after each precipitation, the overall absorption profile of the redissolved pre cipitates featured less absorption contribution from the oligomers. Once the relative absorption intensities of the NCs versus that of the oligomer stabilized, the chloroform solution containing the redissolved oligomer/NC hybrid was considered free of unb ound oligomers. T he UV vis absorption spectra of such washed hybrids s olutions are shown in Figure 3 16.a. Figure 3 16 Absorption spectra (a) of the T6 based hybrid (blue line) and the T4BTD based hybrid (red line) along with the spectrum of free CdSe NCs in solution. TGA thermograms (b) of the pristine CdSe NCs (dashed line) and the two hybrids, under nitrogen flow Compared to the pure NCs solution absorption displayed as a dashed line, the absorption profile of the T6 hybrid (blue line) has a broad a bsorption band centered at
103 426 nm from the contribution of the bound T6 PA oligomers. Likewise the absorption profile of the BTD based hybrid shows the contribution of the NCs bound T4BTD PA peaking at 360 nm and 508 nm, as well as that of the NCs themselv es as a shoulder around 625 nm in the red curve. Only weak fluorescence was observed in dilute solutions of the hybrids in chloroform, with quantum yields below 0.1% at 564 nm for the T6 hybrid solution and at 676 nm for the T4BTD hybrid solution. This alo ng with the absorption profiles of the hybrids supports the strong binding and interaction between the oligomers and the NCs. With the hybrids synthesized, and the presence of surface bound oligomers established, a more quantitative estimation of the avera ge number of oligomers at the NCs surface was attempted. Thermogravimetric analysis can be employed to determine a total weight loss difference between the pristine NCs and the ones functionalized with the electroactive oligomers. In principle, during the ligand exchange process, if a native surfactant such as TOPO (MW = 415 g/mol) is replaced by T6 PA (MW = 827 g/mol) or T4BTD PA (MW = 797 g/mol), then a NC/ T6 PA or NC/ T4BTD PA hybrid should have a higher organic content by weight than the pristine NC. One obvious limitation to this method is that it is in fact very difficult to determine the exact number of native surfactants before ligand exchange. The results from a TGA experiment on hybrids are thus at best qualitative. Figure 3 16.b shows the TGA therm ograms for a CdSe sample before ligand exchange (dashed line) and after ligand exchange with T6 PA (blue line) or T4BTD PA (red line). A 6% weight loss difference at 500 C was observed for the BTD based hybrid compared to the pristine CdSe sample, and an 8 % difference for the T6 based one. This confirms that the ligand exchange process did increase the organic
104 content in the hybrid, supporting the presence of higher molecular weight species bound to the surface of the NCs. A more quantitative way to estimat e the number of surface bound oligomers and the free oligomer. 109 T he absorption spectra of the latter t hree species in the case of T6 is shown in Figure 3 16.a. Fi gure 3 17 Absorption profiles (a) of the T6 PA /CdSe hybrid (blue line), the free T6 PA (dashed blue), the free CdSe (dashed black) and the sum of the latter two (black line). Absorption profiles (b) of the T4BTD PA /CdSe hybrid (red line), the free T 4BTD PA ( dashed red ), the free CdSe (dashed black) and the sum of the latter two (black line) The relative absorption intensities of the free oligomers (dashed blue line) and the free NCs (dashed black line) was adjusted such that the sum of their absorption s pectra (black solid line) resulted in a profile for which the intensities at the respective line). This was achieved for an absorbance of 0.912 at 426 nm for T6 PA and 0.087 at 18.7 M and 136 nM were calculated respectively, using the extinction coefficients listed in Table 3 1, resulting in an oligomer to NC ratio of 137. The same spectral analysis and calculations were
105 applied to the BTD based system (Figure 3 17.b) yielding conce ntrations of 185 nM and 25.9 M in NC and oligomer respectively, and a ratio of 140 oligomers per NC. These ratios are of course average values and remain an approximation of the number of oligomers bound to the NCs, but they suggest that a significant cov erage of the NCs was achieved using T6 PA and T4BTD PA 3.5 Morphology Control via BHJ Crystallinity Disruption. Contrary to the first two oligomeric systems studied in this chapter, the three molecules that are shown in Scheme 3 10 and are the focus of t his section do not bear any reactive functional group. This is a set of molecules which are all based on the same bis bithiophene (T 2 ) isoindigo (iI) aromatic core, but differ by the nature of their aliphatic end chains. As described in the synthesis part in Section 3.2.3, iIT 2 C6 2 is symmetrical and has two n hexyl end chains. Its unsymmetrical counterpart, iIT 2 C6Si has one n hexyl chain on one side and a triisobutylsilyl group on the other side. The third molecule is the symmetrical triisobutylsilyl sub stituted derivative. Scheme 3 10 Structure of iIT 2 C6 2 iIT 2 C6Si and iIT 2 Si 2 The first studies on isoindigo based molecular BHJ solar cells revealed the existence of crystalline domains in the active layer when iIT 2 C 6 2 and PC 60 BM were blended. 158 Two processing methods focusing on additives have been investigated to tune the morphology of the iIT 2 C6 2 /PC 60 BM bulk heterojunction. 159,1 80 These additives are electro optically inactive molecules that change the crystalliz ation behavior of the
106 blend components when added in small amounts to the solution used for device fabrication. The three molecules described in this section were synthetically designed to provide a similar level of BHJ morphology control without the use o f electro optically inactive additives in the blend solutions. Specifically, it was anticipated that by disrupting the crystallization process of the symmetrical iIT 2 C6 2 by adding some percent of unsymmetrical iIT 2 C6Si in the blend solution, the final si ze of the crystalline domains in the active layer could be tuned, influencing the overall solar cell efficiency. 3.5.1 Electrochemical, Thermal and Optical Properties Before studying the effect on the solar cell active layer morphology by varying the side chain nature of the oligomers, the electro optical characteristics of each molecule should be understood. Their electrochemistry was studied in solution as displayed in Figure 3 18, all potentials being referenced against Fc/Fc + The CVs and DPVs for each molecule dissolved at 1 mM in a DCM electrolyte containing 0.1 M TBAPF 6 were plotted the same potential scale to facilitate their comparison. The two dotted vertical lines overlapping the three graphs are set at 0.42 V and 1.16 V, which correspond to the onset of oxidation and reduction of the DPVs for iIT 2 C6 2 respectively. All three oligomers showed two quasi reversible reduction processes, centered at half wave potentials between 1.27 and 1.28V for the first one and between 1.67 V and 1.72 V for t he more cathodic one. In the positive potentials range, two overlapping oxidation waves could be distinguished for iIT 2 C6 2 and iIT 2 C6Si centered at 0.54/0.55 V and 0.66/0.68 V respectively. The oxidation of iIT 2 Si 2 only showed one wave centered at 0.62 V.
107 Figure 3 18. Cyclic voltammograms of iIT 2 C6 2 (top), iIT 2 C6Si (center) and iIT 2 Si 2 (bottom), and the corresponding differential pulse voltammograms (dashed lines) in 0.1 M TBAPF 6 in dichloromethane. Approximately 1mM concentration in oligomer Ove rall, the electrochemical processes as recorded by CV in solution occured at very similar potentials, which was further supported by the DPV measurements. The DPV results showed that the onsets of oxidation and reduction for all three molecules are within 0.08 V and 0.03 V of one another respectively, and likewise for the DPV peak currents. This sets the HOMO and LUMO levels of the three molecules around 5.50/ 5.60 eV and 3.90 eV respectively, with electrochemical energy gaps between 1.58 eV and 1.66 eV. These results support that the comparison of the molecular structure effect
108 on the solar cell performance in this study could be based mostly on morphological characteristics. Next, the thermal properties of the oligomers were investigated, employing TGA and DSC. The DSC results are shown in Figure 3 19, with the TGA thermograms displayed in the inset. From a 5% weight loss set as threshold for thermal decomposition, it appeared from the TGA (recorded under a flow of nitrogen) that all three oligomers are thermally stable up to at least 340 C. The DSCs were recorded for each oligomer separately all at 10 C/min from 50 C to 250 C. The thermograms shown in Figure 3 19 are the firs t cooling (a) and second heating (b) cycles for each oligomer. Figure 3 19 DSC and TGA (inset) thermograms of iIT 2 C6 2 iIT 2 C6Si and iIT 2 Si 2 (endo up) The thermogram for iIT 2 C6 2 showed a melting peak at 185 C upon heating and a sharp crystallizati on peak at 170 C (dash dot line). For iIT 2 C6Si (dashed line), a broad melting peak centered at 132 C appeared upon heating, and no crystallization peak was
109 observed. Rather, during the second heating scan after a featureless cooling scan, a cold crystalli zation broad peak starting at 90 C appeared before the melting peak. The thermogram for the symmetrical disilyl derivative iIT 2 Si 2 shows one melting peak at 145 C and a faint crystallization peak at 63 C. The differences in melting temperatures are consis tent with an increased ability of the n hexyl side chain oligomers to pack more tightly compared to the bulkier triisobutylsilyl side chain oligomers. More energy is required to separate molecules into a melt for iIT 2 C6 2 than for iIT 2 C6Si and even more so than for iIT 2 Si 2 which would explain the 185 C, 145 C and 132 C decrease in melting temperature, respectively. This is further supported by the cooling cycles, where iIT 2 C6 2 appears to crystallize well with a sharp peak and little hysteresis, while i IT 2 Si 2 barely crystallizes at the same cooling rate. iIT 2 C6Si does not even crystallize well enough for a peak to be observed during cooling at that rate. The material appears to reorganize upon reheating starting at 85 C. These results confirm a signifi cant difference between the crystallization behaviors of the three oligomers designed in this study. Specifically, iIT 2 C6 2 crystallizes more readily than iIT 2 Si 2 and even more so than iIT 2 C6Si 2 The absorption of the three oligomers was measured, in sol ution and in the solid state. Figure 3 20 gathers the UV vis spectra of iIT 2 C6 2 (a), iIT 2 C6 2 (b) and iIT 2 C6 2 (c) in chloroform solution (solid lines) and as thin films spin coated from chloroform solution ( ca. 10 mg/mL, 2000 rpm) onto glass slides. The solid state spectra were recorded for the films as spun prior to thermal annealing (dotted lines), and after thermal annealing (dash dot lines). Annealing was carried out by placing the films in an oven held at 90 C for 20 minutes.
110 Figure 3 20. UV vis absorption of iIT 2 C6 2 (a), iIT 2 C6Si (b) and iIT 2 Si 2 (c) in chloroform (solid lines), as thin films spun cast onto glass slides (dotted lines) and after thin film annealing (90 C, 20 min, dash dot lines). Comparison of (d) solution absorption, (e) as spu n thin film absorption and (f) annealed thin film absorption of the three oligomers
111 The solution and as spun spectra were normalized, while the annealed spectra were scaled to reflect the exact spectral changes observed from as spun to after annealing. As a means of comparison, Figure 3 20 also shows in the right hand side overlaid plots of the absorption profiles of the three oligomers in solution (d), as spun (e) and after thermal annealing (f). All spectra in the right hand side were normalized to ease comparison. The UV vis absorption results are summarized in Table 3 2. Focusing first on each oligomer, and starting with iIT 2 C6 2 (Figure 3 20.a), the solution absorption profile in chloroform features two absorption bands centered at 358 nm and 592 nm, w ith an absorption gap of approximately half the maximum intensities between 400 and 500 nm. The low energy onset of absorption in solution is at 702 nm. In the spun coated films, the absorption maximum is red shifted by 66 nm to 658 nm, with a low energy a bsorption onset at 745 nm, but the overall profile remains similar. Consequently, the deep purple blue color in solution matched the blue color of the thin films. Annealing the film as described above did not have any effect on the as spun absorption profi le aside from a slight decrease in intensity. This red shift in the solid state suggests that the iIT 2 C6 2 oligomers are able to aggregate well likely through stacking. This is consistent with the sharp peaks observed in the DSC thermogram. Since there is essentially no change upon annealing, the iIT 2 C6 2 molecules seem to acquire a rather thermodynamically stable packing phase in the short time of solvent eva poration during spin coating. In Figure 3 20.b, the solution absorption of the unsymmetrical iIT 2 C6Si is identical as that of iIT 2 C6 2 (see Figure 3 20.d for comparison), while the solid state absorption broadened slightly with an increased intensity at h igher energy compared to the solution
112 spectrum. A significant spectral change was observed upon annealing, whereby the contribution of the low energy band peaking at 598 nm increased by 30% compared to the high energy one. The as spun films were purple, an d a clear switch to blue was observed upon annealing. The low energy onset of absorption increased from 702 nm in solution to 710 nm as spun to 720 nm after annealing. The small red shift in absorption onset observed from solution to as spun, to be contras ted with the blue shift of the max by 13 nm, suggests that iIT 2 C6Si does not undergo significant aggregation upon spin coating. Rather, annealing at 90 C which was identified as within the cold crystallization temperature range in the DSC thermograms led to further red shifts of both the absorption onset and the max with an overall profile more alike the solution one. This suggests that while frozen in a less aggregated morphology during spin coating, thermal treatment can allow the molecules to rearrange in a more thermodynamically stable morphology, which is consistent with the thermal behavior observed by DSC. Table 3 2. Solution peak and onset absorptions, solution optical energy gap, and the corresponding values or the as spun films and annealed films. max sol. (nm) onset so l. (nm) E sol. (eV) max as spun (nm) onset as spun (nm) max ann. (nm) onset ann. (nm) E ann. (nm) iIT 2 C6 2 358, 592 702 1.77 602, 658 745 601, 658 745 1.66 iIT 2 C6Si 358, 592 702 1.77 579 710 564, 598 720 1.72 iIT 2 Si 2 358, 592 702 1.77 478, 540 7 00 460, 538 700 1.77
113 The solution absorption of the disilyl iIT 2 Si 2 derivative (Figure 3 20.c) is also identical to the two other derivatives (see Figure 3 20.d for comparison). The as spun films show no shift in the absorption onset, and a significant blue shift of the max from 592 nm to 540 nm, with most of the absorption between 350 nm and 600 nm. The color of the films was brown, and did not change upon annealing although a small blue shi ft of the absorption was observed spectroscopically as a result of the thermal t reatment Comparing the as spun and annealed solid state absorption profiles of the three oligomers, as plotted in Figures 3 20.e and 3 20.f, iIT 2 C6 2 has the most red shifted absorption, followed by iIT 2 C6Si and finally iIT 2 Si 2 with approximately 50 n m shifts from one another. This is an important parameter to consider when selecting the main component for p type material in the solar cell active layer. Essentially, and as already reported, the main component should be iIT 2 C6 2 since it is able to cry stallized best and has the most extended absorption. Then, iIT 2 C6Si or iIT 2 Si 2 should be chosen as molecular additive to investigate its effect on the active layer morphology. Electrochemistry shows little difference in the electronics of the two additiv es (Figure 3 18) which is also supported by their identical solution absorption, but thermal analysis and solid state absorption suggest that iIT 2 C6Si is a good candidate as additive, since it offers a more extended absorption balanced with a likely more effective crystal size disruption. 3.5.3 Crystallization Behavior and Influence on Solar Cell Performance It was hypothesized that the bulky triisobutylsilyl group would not insert as well as the n hexyl chain into the iIT 2 C6 2 crystal lattice as it devel ops, naturally creating a triisobutylsilyl bithiophene rich grain boundary. Monitoring the crystal sizes by optical
114 microscopy as a function of percent added iIT 2 C6Si to the main iIT 2 C6 2 component would provide a semi quantitative insight on this effect. This was performed by Danielle Salazar in the Reynolds group by recrystallizing small amounts of iIT 2 C6 2 and iIT 2 C6 2 / iIT 2 C6Si mixtures from hexanes at low concentrations. By dispersing 0.05 mg of solids per mL of hexanes and heating the suspension to 60 C until complete dissolution, crystals of either pure iIT 2 C6 2 or of the iIT 2 C6 2 / iIT 2 C6Si mixture were obtained upon cooling. Their sizes were recorded using an optical microscope under polarized light at crossed polarizer/analyzer to enhance the con trast. Figure 3 21 (left) shows representative pictures of the pure iIT 2 C6 2 crystals (0% iIT 2 C6Si added) and the crystals obtained when 2%, 5% and 10% of iIT 2 C6Si was added to the main iIT 2 C6 2 component. Figure 3 21 Polarized light microscope image s showing iIT 2 C6 2 crystals as a function of added iIT 2 C6Si in solution The graph in Figure 3 21 (right) shows the evolution of the average crystal size (population of 32 to 74 crystals depending on the ratio) as a function of the percent unsymmetrical o iIT 2 C6Si is increased from 0 to 10%.
115 These results confirmed the anticipation of a iIT 2 C6 2 crystal size reduction effect upon addition of sm all amounts of the unsymmetrical molecular additive iIT 2 C6Si To test the hypothesis in solar cells, devices based on [ iIT 2 C6 2 / iIT 2 C6Si ]:PC 61 BM (1:1 by weight) blend films were prepared by Dr. Ken Graham in a conventional architecture (ITO/PEDOT:PSS/[ i IT 2 C6 2 / iIT 2 C6Si ]:PC 61 BM/Al) and the active layer surface morphologies were imaged using AFM. Figure 3 22 shows the AFM images of devices made with varying iIT 2 C6Si to iIT 2 C6 2 ratios of 0% to 50%. At 0% additive, well defined crystalline features were visible with sizes at 200 nm scale. Figure 3 22. AFM height images of [ iIT 2 C6 2 / iIT 2 C6Si ]:PC 61 BM (1:1 by weight) blend films with varying mole % of iIT 2 C6Si after 100 C thermal annealing, 5 5 and 20 nm height scale (top); 1 1 s and 10 nm height scale (center). PCE of [ iIT 2 C6 2 / iIT 2 C6Si ]:PC 61 BM cells (bottom right) with varying mole % iIT 2 C6Si
116 As the concentration of iIT 2 C6Si was increased, the crystalline features remained sharp with decreasing sizes on the order of 20 to 50 nm for up to 30% additive. At 50% additive, the definition of features worsened, suggesting a transition to a more amorphous morphology. A more detailed morphological study was performed by Dr. Ken Graham involving top down and cross sectional TEM imagi ng to support the hypothesis that the asymmetric iIT 2 C6 Si oligomer disrupts crystallization and at high concentration leads to an amorphous morphology. The detailed solar cell characteristics were described and corroborated with the AFM and TEM imaging pe rformed by Dr. Ken Graham as part of his PhD dissertation. The general trend is summarized here in Figure 3 22 (bottom), as the power conversion efficiencies for each set of cells described above were recorded and their average value plotted against the ce in iIT 2 C6Si The short circuit currents, open circuit voltages and fill factors all increased in going from 0% to 30% additive, although the J sc started decreasing after 20% added iIT 2 C6Si This resulted in the trend in Figure 3 22, where the average efficiencies increased from 1.34% 0.41 to 2.24% 0.16 as 0% to 20% iIT 2 C6Si was added, stabilizing around 2% PCE from 20 to 30% additive, followed by a steady decrease to 0.7 1% 0.05 at 50% additive. In summary, substituting a linear side chain for a bulkier group at one end of a conjugated molecule significantly changes its solid state properties, as observed by DSC and solid state spectroscopy. This was used to alter the crystallization of the parent symmetrical molecule, which was observed for simple mixtures of the two. The hypothesis that reduced crystal size would translate into reduce crystalline domains in
117 the active layer of molecular BHJ solar cell was verified by AFM, with a positive influence on the overall solar cell perfo rmance. 3.6 Synthetic Details 2 ((6 (thiophen 2 yl)hexyl)oxy)tetrahydro 2H pyran (3 2). 161 In a dry flask was added thiophene (3.8 g, 45 mmol) which was then diluted with anhydrous tetrahydrofuran (200 mL). The mixture was stirred and cooled to 78 C under a flow of nitrogen. To the cooled mixture was then added a solution of n butyllithium in hexanes (30 mL, 39.3 mmol) dropwise over 30 minutes. Stirring was continued at low temperature for 30 minutes after the addition of n BuLi was complete, and then the flask was removed for the cooling bath to be stirred at room temperature of 1 hour. After cooling back to 78 C, compound 3 1 dissolved in 30 mL of tetrahydrofuran was added dropwise to the mixture. After the addition was complete, the mixture was allowed to warm up to room temperature and stirred for 12 hours. Water was then added to the flask, and the organics were extracted with diethyl ether and washed with water and brine. After drying the combined organics over magnesium sulfate, evaporation of the vo latiles yielded a yellow oil. This was purified using bulb to bulb distillation in a Kugelrohr apparatus (140 C, 0.05 mmHg) to afford the title compound as a colorless oil (5.0 g, 22 mmol, 63%). 1 H NMR (CDCl 3 ): 7.10 (dd, J = 5.1, 1.1 Hz, 1H), 6.91 (dd, J = 5.1, 3.4 Hz, 1H), 6.77 (dd, J = 3.4, 1.1 Hz, 1H), 4.57 (dd, J = 4.3, 2.8 Hz, 1H), 3.86 (m, 1H), 3.74 (dt, J = 9.6, 6.6 Hz, 1H), 3.50 (m, 1H), 3.38 (dt, J = 9.6, 6.6 Hz, 1H), 2.82 (t, J = 7.6 Hz, 2H), 1.84 1.2 5 (m, 14H). 13 C NMR (CDCl 3 ): 145.68, 126.58 123 .88, 122.69, 98.82, 67.52, 62.32 31. 70, 30.75, 29.80, 29.68 28.89, 25.97, 25. 45, 19.69
118 T rimethyl(5 (2 (tetrahydro 2H pyran 2 yloxy)hexyl)thiophen 2 yl)stannane (3 3) 161 To a solution of 1 (1.61 g, 6.0 mmol) in THF (20 mL) cooled at 0C was added n butyllithium (1.31 M in hexanes, 5.50 mL, 7.2 mmol). The cooling was removed for one hour, then recooled to 0C prior to the addition of trimethyltin chloride (1 M in hexanes, 7.2mL, 7.2 mmol) dropwise. The reaction mixture was allowed to warm up to room temperature overnight, after which the solvent was evaporated. The residue, a light brown slurry, was used with no further purification. 1 H NMR (CDCl 3 ): 7.01 (d, J = 3.2 Hz, 1H), 6.89 (d, J = 3.2 Hz, 1H), 4.57 (m 2H), 3.86 (m, 1H), 3.7 5 (m, 1H), 3.49 (m 1H), 3.38 (dt J = 9.6, 6.5 Hz, 1H), 2.86 (t, J = 7.5 Hz, 2H) 1.84 1.2 5 (m, 14H) 0.35 (s, 9 H). 5,5" d ibromo 2,2' bi hiophene (3 4). 94 In the absence of light, N bromosuccinimide ( 5.60 g, 31.5 mmol) was added portion wise to a solution of bithiophene (2 .50 g, 1 5 .0 mmol) in anhydrous dimethylformamide ( 80 mL). The reaction was left stirring at room temperature for three hours and subsequently poured onto ice whereupon a white precipitate forms. Filtration and recrystallization from ethanol affords the tit le compound (4.32 g, 13.30 mmol, 89%) as white crystals. 1 H NMR (CDCl 3 ): 6.96 (d, J = 3.8 Hz, 1H), 6.85 (d, J= 3.8 Hz, 1H). 13 C NMR (CDCl 3 ): 138.00, 130.88, 124.37, 111.74. General procedure for 2 brominat ion of 3 hexylthiophene species: 2 bromo 3 hexy lthiophene ( 3 5 ) 94 To a solution of 3 hexylthiophene (2.52 g, 15 mmol) in dry dimethylform amide (DMF, 130 mL) cooled to 0 C was added N bromosuccinimide (NBS, 3.20 g, 18 mmol) protionwise in the absence of light. The reaction mixture was stirred at 0C fo r 3 hours, then poured in water (150 mL) and extracted with diethyl ether
119 (3x100 mL). The combined organic extracts were washed with water, brine and dried over magnesium sulfate. After evaporation of the solvent, the residue was distilled on Kugelrohr to yield the pure title compound (3.44 g, 13.90 mmol, 93 %) as a colorless liquid. 1 H NMR (CDCl 3 ): 7.18 (d, J = 5.6 Hz,1H), 6.79 (d, J = 5.6 Hz, 1H), 2.57 (t, J = 7.5 Hz, 2H), 1.58 (quintet J = 7.7 Hz, 2H), 1.32 (m, 6H), 0.89 (t, J = 6.7 Hz, 3H). 13 C NMR ( CDCl 3 ): 142.20, 128.45, 125.34, 109.00, 31.84, 29.92, 29.61, 29.11, 22.82, 14.30. Note: Dry THF instead of dry DMF was used for brominations of the trimer and the seximer of 3 hexylthiophene. 3,3''' bis(hexyl) 2,2':5',2'':5'',2''' quaterthiophene (3 6). 1 8 1 To a suspension of magnesium turnings (319 mg, 13.1 mmol) in anhydrous diethyl ether (20mL) is added compound 7 (3.10 g, 12.54 mmol) dropwise while heating gently. After refluxing for 2 hours, the Grignard reagent is transferred dropwise to a solution o f compound 3 4 (1.62g, 5.02 mmol) and Ni(dppp)Cl 2 (35 mg, 0.07 mmol) in a mixture of toluene and diethyl ether (50mL, 3:2). The reaction mixture was refluxed at 55~60C overnight, then quenched with a saturated aqueous solution of ammonium chloride (100mL) and extracted twice with chloroform. The combined organic extracts were washed with saturated aqueous sodium hydrogen carbonate, brine and water, and then dried over magnesium sulfate. The solvent was evaporated and the residue, an orange brown oil, was p urified by column chromatography on silica gel with pure hexanes as eluent to yield the title compound (2.15g, 4.31 mmol, 86% ) as a yellow oil. 1 H NMR (CDCl 3 ): 7.18 (d, J = 5.2 Hz, 1H), 7.13 (d, J = 3.8 Hz, 1H), 7.03 (d, J = 3.8 Hz, 1H), 6.94 (d, J = 5. 2 Hz, 1H), 2.79 (t, J = 7.6 Hz, 2H), 1.66 (quintet J = 7.7 Hz, 2H), 1.36 (m, 6H), 0.89 (t, J = 6.7 Hz, 3H). 13 C NMR (CDCl 3 ): 140.08, 137.00, 135.53, 130.52, 130.30, 126.73,
120 124.06, 124.03, 31.88, 30.86, 29.50, 29.44, 22.84, 14.32. HRMS (ESI TOF): m/z ca lcd for C 28 H 34 S 4 (M H + ) 499.1616 fo und 499.1645 Anal. calcd for C 28 H 34 S 4 H : C 67.42, H 6.87 found C 67.51 H 6.85 d ibromo 3,3''' bis(hexyl) 2,2':5',2'':5'',2''' quaterthiophene (3 7). A solution of 3 6 (2.00 g, 4.01 mmol) in anhydrous dimethylformam ide (30mL) is stirred at 0C and N bromosuccinimide ( 1.50 mg 8.42 mmol) is added portion wise to the reaction mixture in the absence of light. After stirring for five hours while warming up to room temperature, a precipitate crashes out of the solution. F iltration and washing with methanol yields the title compound (2.32 g, 3.53 mmol) as a yellow powder, which is then recrystallized from a hexanes/ethanol mixture to afford yellow crystals (2.05 g 3.12 mmol, 78%) Mp 67 68C. 1 H NMR (CDCl 3 ): 7.11 (d, J = 3.8 Hz, 1H), 6.96 (d, J = 3.8 Hz, 1H), 6.90 (s, 1H), 2.71 (t, J = 7.5 Hz, 2H), 1.61 (m, 2H), 1.33 (m, 6H), 0.89 (t, J = 6.6 Hz, 3H). 13 C NMR (CDCl 3 ): 140.76, 137.25, 134.30, 132.92, 131.91, 127.19, 124.22, 110.88, 31.82, 30.71, 29.43, 29.31, 22.79, 14.30 HRMS (ESI TOF): m/z calcd for C 28 H 32 Br 2 S 4 H (M H + ) 656.9782 fo und 656.9806 Anal. calcd for C 28 H 32 Br 2 S 4 : C 51.22, H 4.91 found C 51.07 H 5,02 4',3'''' b is(hexyl) 5,5''''' bis(6 (tetrahydro 2H pyran 2 yloxy)hexyl) 2,2':5',2'':5'',2''':5''',2'''':5'''',2'' ''' hexathiophene (3 8). A solution of compound 3 7 ( 1.80g 2.74 mmol), compound 3 3 ( 38.4 mM in anhydrous THF, 200mL, 7.67 mmol ), and dichlorobis(triphenylphosphine)palladium (II) ( 77 mg, 0.1 mmol) was degassed and then stirred at reflux overnight. The r eaction mixture is subsequently poured in water and extracted with dichloromethane. The combined organic extracts are then washed with water and dried over magnesium sulfate. After filtration and concentration to a dark
121 oil, the residue is purified by flas h chromatography (hexanes:ethyl acetate, 9:1) to yield the title compound (2.61 g, 2.53 mmol, 92 %) as a red waxy oil. 1 H NMR (CDCl 3 ): 7.12 (d, J = 3.8 Hz, 1H), 7.02 (d, J = 3.8 Hz, 1H), 6.97 (d, 1H), 6,93 (s, J = 3.5 Hz, 1H), 6.68 (d, J = 3.5 Hz, 1H), 4 .57 ( m 1H), 3.88 (m, 1H), 3.75 (dt, J = 9.6, 6.7 Hz, 1H), 3.51 (m, 1H), 3.40 (dt, J = 9.6, 6.7 Hz, 1H), 2.80, (m, 2H), 2.76 (m 2H), 1.86 1.30 (m, 22H), 0.91 (m 3H). 13 C NMR (CDCl 3 ): 145.50, 140.50, 136.68, 135.91, 135.30, 134.68, 128.84, 126.29, 126.0 7, 125.04, 123.98, 123.47, 99.03, 67.71, 62.52, 31.84, 31.66, 30.95, 30.56, 30.29, 29.81, 29.70, 29.42, 29.04, 26.18, 25.68, 22.79, 19.88, 14.29. HRMS (ESI TOF): m/z calcd for C 58 H 78 O 4 S 6 (M + ) 1030.4219 fo und 1030.4221 Anal. calcd for C 58 H 78 O 4 S 6 : C 67.53, H 7.62 found C 67.47 H 7.71 6,6' (4',3'''' b is(hexyl) 2,2':5',2'':5'',2''':5''',2'''':5'''',2''''' hexathiophene 5,5''''' diyl) dihexan 1 ol ( T6 diol ). Compound 3 8 (800 mg, 0.78 mmol) is dissolved in a dichloromethane:methanol (3:2, 250mL) mixture and s tirred at room temperature, to which are then added 10 drops of hydrochloric acid (12 N). The reaction mixture is stirred at room temperature for five hours, concentrated and slowly poured in hexanes (400 mL). The resulting orange precipitate is filt ered t o yield the title compound (636 mg, 0.736 mmol, 95 %) as an orange solid. Further purification by flash chromatography (hexanes:ethyl acetate, 6:4) affords T6 diol (540 mg, 0.625 mmol, 80 %) with excellent purity. Mp. 103.5 104.5C. 1 H NMR (CDCl 3 ): 7.12 (d, J = 3.8 Hz, 1H), 7.02 (d, J = 3.8 Hz, 1H), 6.97 (d, J = 3.5 Hz, 1H), 6,93 (s, 1H), 6.68 (d, J = 3.5 Hz, 1H), 3.65 (t, J = 6.5, Hz 2H), 2.80 (m, 2H), 2.75 (m, 2H), 1.80 1 30 (m, 16H), 0.91 (m, 3H) 13 C NMR (CDCl 3 ): 145.45, 140.58, 136.74, 135.92, 135. 33, 134.75, 128.89, 126.36, 126.14, 125.10, 124.04, 123.52, 63.13, 32.84, 31.87, 31.70, 30.63, 30.30, 29.72, 29.44, 29.01, 25.69,
122 22.82, 14.32. HRMS (ESI TOF): m/z calcd for C 48 H 62 O 2 S 6 (M + ) 862.3054 fo und 862.3069 Anal. calcd for C 48 H 62 O 2 S 6 : C 66.77, H 7. 24 found C 66.63 H 7.21 6,6' (4',3'''' Bis(dodecyl) 2,2':5',2'':5'',2''':5''',2'''':5'''',2''''' hexathiophene 5,5''''' diyl)bis(hexane 6,1 diyl) dibenzoate (3 9). To as solution of T6 diol (129 mg, 0.15 mmol) and anhydrous pyridine (0.1 mL, 1.3mmol) in dry THF (5mL) is added a solution of benzoyl chloride (90 mg, 0.64 mmol) in dry THF (1mL) dropwise at room temperature. After two hours of stirring at room temperature, the reaction mixture is diluted with dichloromethane and poured in water. After three e xtractions with dichloromethane (3x15 mL), the combined organic extracts are washed with water and brine and dried over magnesium sulfate. After evaporation of the solvent, the red viscous residue is dissolved in 2 mL of dichloromethane and precipitated in methanol (40 mL) at 10C, to which hexanes (10 mL) is slowly added. Shiny crystals form after 10 minutes, which proved to be the title compound (124 mg, 0.12 mmol, 80%) after filtration and dryness, as a golden solid. Mp. 50 51C. 1 H NMR (CDCl 3 ): 8.05 (m, 2H), 7.56 (m, 1H), 7.44 (m, 2H), 7.12 (d, J = 3.8 Hz, 1H), 7.02 (d, J = 3.8 Hz, 1H), 6.07 (d, J = 3.5 Hz, 1H), 6.93 (s, 1H), 6.68 (d, J = 3.5 Hz, 1H), 4.33 (t, J = 6.5 Hz, 2H), 2.82 (m, 2H), 2.75 (m, 2H), 1.85 1.30 (m, 16H), 0.90 (m, 3H) 13 C NMR (CDCl 3 ): 166.86, 145.34, 140.60, 136.75, 135.93, 135.35, 134.79, 133.02, 130.67, 128.90, 126.39, 126.16, 125.15, 124.06, 123.54, 65.18, 31.86, 31.64, 30.64, 30.28, 29.73, 29.44, 28.87, 28.85, 26.04, 22.82, 14.32. HRMS (ESI TOF): m/z calcd for C 62 H 70 O 4 S 6 Na (M N a + ) 1093.3491 fo und 1093.3478 Anal. calcd for C 62 H 70 O 4 S 6 : C 69.49, H 6.58 found C 69.12 H 6.50
123 D iethyl thiophen 2 ylphosphonate (3 11). 18 2 2 bromothiophene (10 g, 62.5 mmol) and anhydrous nickel chloride (387 mg, 3.1 mmol) were added in a dry flask equi pped with a stir bar, an addition funnel and a short path distillation apparatus under a flow of argon. The mixture was heated to 145 C while stirring, at which point triethylphosphite was added dropwise from the addition funnel into the reaction mixture. The mixture alternates from deep blue to brown at each drop of triethylphosphite, with a concomitant evolution of ethylbromide which is distilled off. The reaction is left to stir at 145 C for 3 hours until the phosphite addition was complete and no more e thylbromide evolution was observed. The mixture is then allowed to cool back to room temperature, and the crude is distilled under reduced pressure (1 mtorr). After some of the starting material is collected off at 30 C under reduced pressure, the title co mpound is collected at 85 C as a pale yellow oil. (7.58 g, 34.4 mmol, 55 %). 1 H NMR (CDCl 3 ): 7.70 7.62 (m, 2H), 7.16 (quartet, J = 3.3 Hz, 1H), 4.20 4.00 m, 4H), 1.31 (t, J = 7.1 Hz, 6H). 13 C NMR (CDCl 3 ): 136.98, 136.81, 133.63, 133.53, 128.35, 128.10, 62.88, 62.81, 16.46, 16.37. 31 P NMR (CDCl 3 against H 3 PO 4 ): 13.07. D iethyl (5 (trimethylstannyl)thiophen 2 yl)phosphonate (3 12). 18 2 Thienylphosphonate 13 was diluted with anhydrous tetrahydofuran (40 mL) in a dry flask equipped with stir bar under argon flow, and cooled down to 78 C. Lithium diisopropylamine was prepared in a separate flask by adding n butyllithium (4.5 mmol, 1.4 M in hexanes) to a diisopropylamine (0.71 mL, 5 mmol) solution in anhydrous THF (7mL) cooled to 78 C and stirring at 78 C fo r 30 minutes. The LDA solution was then added slowly to the cooled thienylphosphonate reaction mixture over the course of five minutes, and left to stir at such temperature for 3 hours, after which trimethyltin chloride
124 (solid) was added to the reaction fl ask in one portion. The mixture was left stirring while slowly warming up to room temperature over 3 hours. The solvent was then evaporated, and the crude was redissolved in 3 mL of dichloromethane to which the addition of 20 mL of hexanes results in white precipitates. The salts were filtered off, and the clear yellow solution was evaporated to afford the title compound as a clear oil in 85 90 % purity (by 1 HNMR), which was used without further purification. 1 H NMR (CDCl 3 ): 7.74 ( d d, J = 4.2, 3.3 Hz, 1H ) 7.23 (t, J = 2.7, 1H), 4.20 4.00 (m, 4H), 1.33 (t, J = 7.1, 6H), 0.40 (s, 9H). General procedure for 5 borylat ion of 3 hexylthiophene species: 2 (4 hexylthiophen 2 yl) 4,4,5,5 tetramethyl 1,3,2 dioxaborolane (3 13). 18 3 To a solution of freshly distilled diisopropylamine (4.22 g, 41.8 mmol) in anhydrous THF (50 mL) cooled to 0C was added n butyllithium (2.08 M in hexanes, 19.1 mL, 39.7 mmol) dropwise. The reaction mixture was stirred at 0C for 30 minutes, after which the freshly made lithium diisopropyla mine (LDA) solution was transferred dropwise over 30 minutes to a solution of 3 hexylthiophene (6.68 g, 39.7 mmol) in anhydrous THF (300 mL) cooled to 78C. After stirring the reaction mixture at 78C for two hours, 2 isopropoxy 4,4,5,5 tetramethyl 1,3,2 dioxaborolane ( 2 ) (8.88 g, 47.7 mmol) was slowly added. The mixture was allowed to warm up to room temperature while stirring overnight, and subsequently poured in water/diethyl ether and extracted with diethyl ether (2x250 mL). After drying over magnesiu m sulfate, the combined organic extracts were concentrated and purified by flash column chromatography using pure hexanes as eluent, to afford the title compound ( 10.1 g, 34.3 mmol, 86 %) as a colorless oil. 1 H NMR (CDCl 3 ): 7.47 (s, 1H), 7.21 (s, 1H), 2 .62 (t, 2H), 1.61 (m, 2H), 1.34 (s, 12H), 1.29 (m, 6H), 0.88 (t,
125 3H). 13 C NMR (CDCl 3 ): 144.93, 138.70, 127.76, 84.20, 31.88, 30.84, 30.19, 29.20, 24.97, 22.80, 14.30 General procedure for Suzuki Miyaura coupling reactions of 3 hexylthiophene species: 3, 4' dihexyl 2,2' bithiophene ( 3 14 ). 18 4 Tris(dibenzylideneacetone)dipalladium(0) (Pd 2 (dba) 3 202 mg, 0.22 mmol) and tri(o tolyl)phosphine (P( o tyl) 3 269 mg, 0.88 mmol) were added in a dry flask and purged with argon/vacuum three times. A degassed solution of 3 13 (4.33 g, 14.71 mmol) and 3 5 (4.0 g, 16.18 mmol) in toluene (50 mL) was added to the flask containing the catalyst. A degassed 1M aqueous solution of tetraethylammonium hydroxide (22 mL, 22 mmol) was then added to the mixture, which was stirred vig orously and at 80~85C for 12 hours, after which it was diluted with hexanes and poured in water. The organic phase was washed with water and brine and dried over magnesium sulfate. The solvents were then evaporated and the crude residue was filtrated thou gh a short plug of silica using hexanes as eluent, then bulb to bulb distilled on Kugelrohr (60 C, 0.02 torr) to yield the title compound with a small amount of the head to head and tail to tail coupled isomers (3.93 g, 11.70 mmol, 72%) as a light yellow o il. No further purification was performed before the next step. 1 H NMR (CDCl 3 ): 7.14 (d, 1H), 6.94 (s, 1H), 6.92 (d, 1H), 6.88 (s, 1H), 2.74 (t, 2H), 2.61 (t, 2H), 1.63 (m, 4H), 1.40 1.20 (m, 12H), 0.89 (m, 6H). 5' bromo 3,4' dihexyl 2,2' bithiophene ( 3 15 ). 18 4 To a solution of 3 14 (3.93 g, 11.7 mmol) in DMF (80 mL) cooled to 0 C and covered from light was added NBS (2.40 g, 12.48 mmol) in one portion. The reaction mixture was allowed to stir at 0 C for 3 hours in the absence of light. The mixture was th en poured in water and extracted with
126 diethyl ether. The combined organic extract were washed with water and brine and dried over magnesium sulfate. After evaporation of the volatiles, the crude oil was purified using column chromatography with pure hexane s as eluent. The fractions containing the pure product were evaporated to afford the title compound (2.64 g, 6.38 mmol, 55 %) as a light yellow oil. 1 H NMR (CDCl 3 ): 7.16 (d, 1H), 6.91 (d, 1H), 6.80 (s, 1H), 2.71 (t, 2H), 2.57 (t, 2H), 1.62 (m, 4H), 1.34 (m, 12H), 0.90 (m, 6H) 13 C NMR (CDCl 3 ): 142.54, 140.15, 135.90, 130.23, 130.12, 127.07, 124.12, 108.68, 31.86, 31.85, 30.92, 29.87, 29.76, 29.39, 29.33, 29.13, 22.83, 14.31, 14.30. Note: Because of the presence in the starting material of some head to h ead and tail to tail isomers, a slight excess of NBS was added 3,4 ,4 '' Trihexyl 2,2 :5 ,2 '' terthiophene (3 16). 18 4 The general procedure for Suzuki Miyaura coupling using 3 13 was followed (see compound 3 14 ). Purification by flash column chromatograph y using hexanes a pure eluent afforded the title compound ( 2.42 g, 4.83 mmol, 80 %) as thick yellow oil. 1 H NMR (CDCl 3 ): 7.15 (d, 1H), 6.97 (s, 1H), 6.94 (s, 1H), 6.93 (d, 1H), 6.90 (s, 1H), 2.78 (t, 2H), 2.76 (t, 2H), 2.62 (t, 2H), 1.65 (m, 6H), 1.40 1 .20 (m, 18H), 0.90 (m, 9H). 13 C NMR (CDCl 3 ): 143.79, 139.68, 135.76, 134.07, 131.09, 130.80, 130.23, 128.80, 127.31, 123.66, 120.13, 31.89, 30.87, 30.78, 30.71, 30.61, 29.48, 29.44, 29.23, 22.85, 14.31 5 Bromo 4,3',3'' trihexyl 2,2';5',2'' terthiophene (3 17). 18 4 The general procedure for 2 bromination of 3 hexylthiophene was followed (see compound 3 5 ), using dry THF instead of dry DMF. After evaporation of the solvent, the crude residue was purified by flash column chromatography using pure hexanes as eluent, affording the title compound (160 mg, 0.28 mmol, 71 %) as a yellow oil. 1 H NMR (CDCl 3 ): 7.16
127 (d, 1H), 6.93 (d, 1H), 6.92 (s, 1H), 6.81 (s, 1H), 2.77 (1, 2H), 2.70 (t, 2H), 2.57 (t, 2H), 1.63 (m, 6H), 1.40 1.20 (m, 18H), 0.89 (m, 9H) 13 C NMR (CDC l 3 ): 142.63, 140.27, 139.91, 135.59, 134.65, 130.55, 130.28, 130.03, 128.78, 126.83, 123.89, 108.75, 31.87, 31.84, 30.86, 30.79, 29.87, 29.76, 29.47, 29.42, 29.41, 29.13, 22.84, 22.82, 14.31 2 (3,4',4'' trihexyl 2,2';5',2'' terthiophen 5 yl) 4,4,5,5 tet ramethyl 1,3,2 dioxaborolane (3 18). 183 The general procedure for 5 borylation was followed (see compound 3 13 ) to afford the title compound (980 mg, 1.56 mmol, 52 %) as a thick yellow oil. 1 H NMR (CDCl 3 ): 7.46 (s, 1H), 7.01 (s, 1H), 6.97 (s, 1H), 6.90 ( s, 1H), 2.79 (t, 2H), 2.75 (t, 2H), 2.62 (t, 2H), 1.66 (m, 6H), 1.36 (s, 12H), 1.40 1.20 (m, 18H), 0.90 (m, 9H). 3,4 ,4 '' 4 ''' 4 '''' 4 ''''' hexa hexyl 2,2 :5 ,2 '' :5 '' 2 ''' :5 ''' 2 '''' :5 '''' 2 ''''' sexi thiophene (3 19). The general procedure for Suzuki Miyaur a coupling was followed (see compound 3 14 ). Extraction was done with chloroform instead of diethyl ether. Purification by flash column chromatography using pure hexanes as eluent afforded the title compound (610 mg, 0.61 mmol, 39 %) as an orange solid. 1 H NMR (CDCl 3 ): 7.16 (d, 1H), 7.00 (s, 1H), 6.99 (s, 1H), 6.98 (s, 2H), 6.96 (s, 1H), 6.94 (d, 1H), 6.91 (s, 1H), 2.81 (m, 8H), 2.78 (t, 2H), 2.63 (t, 2H), 1.69 (m, 12H), 1.45 1.25 (m, 36 H), 0.92 (m, 18H). 13 C NMR (CDCl 3 ): 143.86, 140.03, 139.99, 139.84 139.83, 135.75, 134.32, 133.96, 133.86, 133.75, 131.24, 130.80, 130.77, 130.71, 130.64, 130.29, 129.00, 128.80, 128.77, 128.67, 127.39, 123.78, 120.22, 31.92, 30.87, 30.79, 30.74, 30.63, 29.67, 29.53, 29.46, 29.24, 22.86, 14.31 HRMS ( APCI TOF): m/z calc d for C 60 H 86 S 6 H (M H + ) 999.5127 fo und 999.5098 Anal. calcd for C 60 H 86 S 6 : C 72.08, H 8.67 found C 72.24 H 9.14
128 5''''' bromo 3,4',4'',4''',4'''',4''''' hexahexyl 2,2':5',2'':5'',2''':5''',2'''':5'''',2''''' sexithiophene ( 3 20 ). The general procedure for t he 2 bromination of 3 hexylthiophene (see compound 3 5 ), was followed, using dry THF instead of dry DMF. Extraction was done with chloroform instead of diethyl ether. Purification by flash column chromatography using pure hexanes as eluent afforded the tit le compound (360 mg, 0.34 mmol, 83 %) as an orange solid. Mp 50 51C. 1 H NMR (CDCl 3 ): 7.17 (d, 1H), 7.01 (s, 2H), 7.00 (s, 1H), 6.99 (s, 1H), 6.96 (d, 1H), 6.87 (s, 1H), 2.84 (m, 8H), 2.76 (t, 2H), 2.61 (t, 2H), 1.80 1.65 (m, 12H), 1.45 1.25 (m, 36H), 0. 96 (m, 18H). 13 C NMR (CDCl 3 ): 142.62, 140.29, 140.11, 140.02, 139.94, 139.75, 135.57, 134.31, 134.26, 134.05, 134.00, 130.72, 130.54, 130.46, 130.27, 130.11, 128.91, 128.71, 128.52, 126.75, 123.73, 108.82, 31.91, 31.88, 31.86, 31.82, 30.86, 30.79, 30.72, 29.94, 29.88, 29.76, 29.67, 29.52, 29.48, 29.44, 29.14, 22.87, 14.33 HRMS (ESI TOF): m/z calcd for C 60 H 85 BrS 6 H (M H + ) 1079.4222 fo und 1079.4207 Anal. calcd for C 60 H 85 BrS 6 : C 66.81, H 7.94 found C 67.27 H 8.53 D iethyl (3',3'',3''',3'''',3''''',3'''''' h exahexyl [2,2':5',2'':5'',2''':5''',2'''':5'''',2''''':5''''',2'''''' sepithiophen] 5 yl)phosphonate (rrT7 PE). In a dry Schlenk tube were added compound 3 20 (450 mg, 0.42 mmol), Pd 2 (dba) 3 (40 mg, 0.04 mmol, chloroform adduct) and P( o tyl) 3 (30 mg, 0.1 mm ol) and kept under vacuum during 30 minutes while being subjected to three vacuum/argon purge cycles, and finally refilled with argon. Compound 3 12 (400 mg, 1.05 mmol) was dissolved in degassed anhydrous toluene (5 mL) and transferred to the reaction flas k trough a septum using a syringe. The mixture was heated to 90 C and stirred for 12 hours. After cooling back to room temperature, it was diluted with dichloromethane and washed with
129 water and brine. The solvents were evaporated and the dark thick oil was diluted with a minimum of dichloromethane and purified by column chromatography using pure dichloromethane as eluent (Rf = 0.3). After evaporation of the combined pure fractions, the title compound (145 mg, 0.12 mmol, 28 %) was obtained as a thick orange oil, which solidified over a period of several days. 1 H NMR (CDCl 3 ): 7.61 (dd, J = 4.2 Hz, 1H), 7.20 (t, J = 3.6 Hz, 1H), 7.15 (d, J = 5.2 Hz, 1H), 7.02 (s, 1H), 7.00 (s, 1H), 6.99 (s, 1H), 6.97 (s, 1H), 6.95 (s, 1H), 6.93 (d, J = 5.2 Hz, 1H), 4.30 4.10 (m, 12H), 2.80 (m, 12H), 1.80 1.65 (m, 12H), 1.50 1.30 (m, 44H), 0.92 (m, 18H). 13 C NMR (CDCl 3 ): 144.54, 144.43 141 .43 140. 46, 140.15, 140.03, 139.93, 139.74, 137.26, 137.11, 135.36, 134.31, 134.26, 134.04, 133.97, 130.65, 130.47, 130.39, 130.35, 130.25, 130.06, 129.14, 129.11, 128.93, 128.89, 128.84, 128.82, 128.77, 128.69, 128.58, 128.53, 128.19, 126.34, 126.12, 125.40, 123.73, 62.93, 62.86, 31.85, 31.82, 30.81, 30.66, 30.56, 29.63, 29.47, 29.43 29.37 22.82, 22.78, 16.52, 16.43, 14.30, 14.25. Anal. ca lcd for C 68 H 97 O 3 PS 7 : C 67.06, H 8.03 found C 67.22 H 8.05 5 bromo 3,3''',5'''' trihexyl 2,2':5',2'':5'',2''':5''',2'''' quinquethiophene (3 21). In a dry Schlenk flask, compound 3 7 (3.35 g, 5.1 mmol), Pd 2 (dba) 3 (114 mg, 0.11 mmol, chloroform adduct) and P( o tyl) 3 (70 mg, 0.23 mmol) were loaded under a flux of argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. 2 (5 hexylthiophen 2 yl) 4,4,5,5 tetramethy l 1,3,2 dioxaborolane ( 3 10 2.25 g, 7.65 mmol) was dissolved in degassed toluene (50 mL), and the afforded solution was added to the reaction Schlenk flask through a septum, using a syringe. A degassed tetraethylammonium hydroxide aqueous solution (11.5 m L, 11.5 mmol) was then added to the reaction flask, and the
130 mixture was vigorously stirred under argon at 90 C for twelve hours. After the mixture had cooled down, it was poured in water and washed several times with water and finally with brine, then drie d over magnesium sulfate. The solvents were then evaporated to afford a dark red oil. Column chromatography on silica gel using pure hexanes as eluent (Rf = 0.45) yielded the title compound 12 as an orange oil (1.17 g, 1.6 mmol, 31 %). Note: With an Rf of 0.3 in pure hexanes, the dicoupled sexithiophene by product was isolated as an orange solid (0.98 g, 1.18 mmol, 23 %). 1 H NMR (CDCl 3 ): 7.12 (d, J = 3.8 Hz, 1H), 7.11 (d, J = 3.8 Hz, 1H), 7.02 (d, J = 3.8 Hz, 1H), 6.97 (d, J = 3.6, 1H), 6.96 (d, J = 3.8, 1H), 6.94 (s, 1H), 6.90 (s, 1H), 6.68 (d, J = 3.6, 1H), 2.80 (t, J = 7.8, 2H), 2.75 (t, J = 7.6, 2H), 2.72 (t, J = 7.8, 2H), 1.68 (m, 4H), 1.62 (m, 2H), 1.39 (m, 6H), 1.33 (m, 12H), 0.90 (m, 9H). 13 C NMR (CDCl 3 ): 145.82 140.68, 140.66, 137.56, 136.45, 1 36.11 135.66, 134.65, 134.00, 132.90, 132.02, 128.75, 127.18, 126.38, 126.12, 125.04, 124.28, 123.97, 123.57, 110.77, 31.88, 31.83, 31.79, 31.77, 30.72, 30.65, 30.43, 29.74, 29.45, 29.44, 29.32, 28.97, 22.83, 22.81, 22.80, 14.32, 14.30 HRMS (APCI): m/z c alcd for C 38 H 47 BrS 5 H (MH + ) 745.1510 found 745.1539. Anal. calcd for C 38 H 47 BrS 5 : C 61.34 ; H 6.37 found C 61.27; H 6.39. Diethyl (3'''',4',5''''' trihexyl [2,2':5',2'':5'',2''':5''',2'''':5'''',2''''' sexithiophen] 5 yl)phosphonate (T6 P E). In a dry Schlenk flask, compound 3 21 (1.82 g, 2.45 mmol), Pd 2 (dba) 3 (40 mg, 0.04 mmol, chloroform adduct) and P( o tyl) 3 (30 mg, 0.1 mmol) were loaded together and kept under vacuum during 30 minutes while being subjected to three vacuum/argon purge cycles, and finally ref illed with argon. Compound 3 12 (3 mmol) was dissolved in degassed anhydrous toluene (30 mL) and transferred to the reaction flask trough a septum using a syringe. The mixture was heated to 90 C and
131 stirred for 12 hours. After cooling back to room temperat ure, the mixture was diluted with ethyl acetate and washed with water and brine. After drying over magnesium sulfate, the solvent was evaporated and the crude was purified by column chromatography using 9:1 dichloromethane : ethyl acetate (Rf = 0.2) as elu ent. This afforded the title compound as thick red oil which eventually solidifies to a red orange solid over time (650 mg, 0.74 mmol, 30 %). 1 H NMR (CDCl 3 ): 7.55 (dd, J = 4.2, 3.8 Hz, 1H), 7.18 (t, J = 3.5 Hz, 1H), 7.14 (s, 1H), 7.13 (s, 1H), 7.10 (s, 1 H), 7.06 (d, J = 3.8 Hz, 1H), 7.03 (d, J = 3.8 Hz, 1H), 6.97 (d, J = 3.5 Hz, 1H), 6.93 (s, 1H), 6.68 (d, J = 3.6 Hz, 1H), 4.20 4.00 (m, 4H), 2.79 (t, J = 7.6 Hz, 2H), 2.77 (t, J = 7.9 Hz, 2H), 2.75 (t, 7.8 Hz, 2H), 1.68 (m, 6H), 1.40 (m, 6H), 1.36 (t, J = 7.1 Hz, 6H), 1.33 (m, 12H), 0.90 (m, 9H). 13 C NMR (CDCl 3 ): 145.75, 140.70, 140.58, 136.95 136.6 6, 136.02, 135.46, 135.18, 135.03, 134.68, 129.72, 128.86, 126.77, 126.46, 126.34, 126.11, 125.03, 124.09, 124.05, 123.53, 31.90, 31.80, 31.77, 30.65, 30.63 30.43 29.79, 29.76 29.49 29.48, 28.99, 22.85, 22.80, 14.34 14.31 31 P NMR (CDCl 3 against H 3 PO 4 ): 11.80. HRMS ( APCI ): m/z calcd for C 46 H 59 O 3 PS 6 H (M H + ) 883.2599 fo und 883.2526 Anal. calcd for C 46 H 59 O 3 PS 6 : C, 62.55; H, 6.73 found C 62.93 ; H 7.04. (3'''',4',5''''' trihexyl [2,2':5',2'':5'',2''':5''',2'''':5'''',2'''' sexithiophen] 5 yl)phosphonic acid ( T6 PA ). T6 PE (300 mg, 0.34 mmol) was added in a dry flask equipped with a stir bar and under argon flow, then dissolved in anhydrous dichloromethane (10 mL). To the orange solution was added trimethylsilylbromide (0.45 mL, 3.4 mmol) dropwise at room tem perature, over the course of five minutes. The mixture was stirred at room temperature for 5 hours, after which methanol (10 mL) was added to the flask and allowed to stir for an additional 3 hours at room temperature. The
132 precipitates that formed were fil tered to give an orange solid. Only one spot with Rf = 0 was observed by TLC on silica gel plates eluting with a 1:1 mixture of dichloromethane : ethyl acetate, suggesting complete conversion. The solids were dissolved in a minimum of dichloromethane and r eprecipitated in methanol then filtered, and repeating this twice afforded the title compound as an orange solid (240 mg, 0.29 mmol, 85 %). 1 H NMR ((CD 3 ) 2 SO): 7.40 7.36 (br, 5H), 7.23 (br, 1H), 7.18 (br, 1H), 7.15 (br, 1H), 7.14 (br, 1H), 6.82 (br, 1H), 2 .59 257 (br, 6H), 1.63 (br, 6H), 1.40 1.20 (br, 18H), 0.87 (br 9H). 31 P NMR ((CD 3 ) 2 SO against H 3 PO 4 ): 5.91. HRMS (ESI): m/z calcd for C 42 H 51 O 3 PS 6 H (M H ) 825.1827 found 825.1812. Anal. calcd for C 42 H 51 O 3 PS 6 : C, 60.98; H, 6.21 found C 60.15 ; H 6.41. 4, 7 bis(4 hexylthiophen 2 y l)benzo[c][1,2,5]thiadiazole (3 22 ). 18 5 In a dry Schlenk flask, 4,7 dibromobenzo[c][1,2,5]thiadiazole (3.0 g, 10.3 mmol), Pd 2 (dba) 3 (283 mg, 0.23 mmol, chloroform adduct) and P( o tyl) 3 (338 mg, 1.11 mmol) were loaded under a flux o f argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. 2 (4 hexylthiophen 2 yl) 4,4,5,5 tetramethyl 1,3,2 dioxaborolane ( 3 13 7.3 g, 24.8 mmol) was disso lved in degassed toluene (40 mL), and the afforded solution was added to the reaction Schlenk flask through a septum, using a syringe. A degassed tetraethylammonium hydroxide aqueous solution (36 mL, 36 mmol) was then added to the reaction flask, and the m ixture was vigorously stirred under argon at 90 C for twelve hours. After it cooled back to room temperature, the mixture was poured in 300 mL of cold methanol (0 C). The orange precipitates that formed were filtered and redissolved in 100 mL of hexanes an d pass through a short plug of silica gel using pure hexanes.
133 After evaporation of the solvent, the title compound was obtained as a bright orange solid (4.58 g, 9.77 mmol, 92 %). %). 1 H NMR (CDCl 3 ): 7.98 (d, J = 1.3 Hz, 1H ), 7.83 (s, 1H), 7.04 (s, 1H), 2.70 (t, J = 7.9 Hz, 2H), 1.71 (quintet, J = 7.7 Hz, 2 H), 1.35 (m, 4H), 0.90 (t, J = 6.9 Hz, 3H) 13 C NMR (CDCl 3 ): 152.81, 144.55, 139.20, 129.25, 126.20, 125.71, 121.72, 31.93, 30.87, 30.69, 29.28, 22.86, 14.34. 4,7 bis(5 bromo 4 hexylthiophen 2 y l)be nzo[c][1,2,5]thiadiazole (3 23 ). 18 5 Compound 3 22 (4.58 g, 9.77 mmol) was dissolved in chloroform (150 mL) and cooled to 0 C while protecting the solution from light exposure. N bromosuccinimide (3.97 g, 22.3 mmol) was then added to the mixture in one port ion, followed by two drops of acetic acid. The mixture was allowed to stir while warming up to room temperature for 12 hours. The mixture was then poured in water and washed once with water. The solvent was evaporated and the crude was purified by column c hromatography on silica gel using pure hexanes as eluent. Evaporating the collected fractions afforded the title compound as a thick red oil which eventually crystallized to a bright red solid (5.97 g, 9.53 mmol, 97 %). 1 H NMR (CDCl 3 ): 7.74 (s 1H ), 7.69 (s, 1H), 2.63 (t, J = 7.4 Hz, 2H), 1.67 (quintet, J = 3.9 Hz, 2H), 1.36 (m, 6H), 0.91 (t, J = 6.9 Hz, 3H) 13 C NMR (CDCl 3 ): 152.38, 143.26, 138.69, 128.29, 125.45, 124.98, 111.81, 31.87, 29.97, 29.90, 29.19, 22.85, 14.33. 4 (5 bromo 4 hexylthiophen 2 yl) 7 (3,5' dihexyl [2,2' bithiophen] 5 y l)benzo[c][1,2,5]thiadiazole (3 24 ). In a dry Schlenk flask, compound 3 23 (5.97 g, 9.53 mmol), Pd 2 (dba) 3 (250 mg, 0.20 mmol, chloroform adduct) and P( o tyl) 3 (275 mg, 0.90 mmol) were loaded under a flux of argon and t hen kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and
134 finally refilled with argon. 2 (5 hexylthiophen 2 yl) 4,4,5,5 tetramethyl 1,3,2 dioxaborolane ( 3 10 2.26 g, 7.69 mmol) was dissolved in dega ssed toluene (100 mL), and the afforded solution was added to the reaction Schlenk flask through a septum, using a syringe. A degassed tetraethylammonium hydroxide aqueous solution (11.5 mL, 11.5 mmol) was then added to the reaction flask, and the mixture was vigorously stirred under argon at 90 C for twelve hours. The mixture was allowed to cool down to room temperature, diluted with 100 mL of hexanes and poured in water. The organic phase was washed with water twice, dried over magnesium sulfate and evapo rated. The red oil was then purified by column chromatography using pure hexanes as eluent (Rf = 0.5) to afford the title compound as a red thick oil (1.75 mg, 2.45 mmol, 26 %). Note: With an Rf of 0.75 in pure hexanes, the starting material 16 was recover ed (3.16 g, 5.04 mmol). 1 H NMR (CDCl 3 ): 7.93 ( s 1H ), 7.72 (s, 1H), 7.70, 7.66 (ABq, J AB = 7.7 Hz, 2H), 7.02 (d, J = 3.6 Hz, 1H), 6.75 (d, J = 3.6 Hz, 1H), 2.84 (t, J = 8.1 Hz, 2H), 2.81 (t, J = 9.2 Hz, 2H), 2.63 (t, J = 7.5 Hz, 2H), 1.72 (m, 6H), 1.45 1 .30 (m, 12 H), 0.92 (m, 9H) 13 C NMR (CDCl 3 ): 152.52, 152.44, 146.72, 143.14, 140.01, 138.88, 136.29, 133.58, 133.39, 130.97, 127.93, 125.94, 125.88, 125.05, 125.00, 124.87, 124.69, 111.51, 31.92, 31.87, 31.79, 30.80 30. 39, 29.97, 29.90, 29.72, 29.55, 2 9.21, 29.05, 22.89, 22.86, 22.81, 14.34, 14.32. HRMS (APCI): m/z calcd for C 36 H 45 BrN 2 S 4 H (MH + ) 71 3 .1649 found 71 3 .1665. Anal. calcd for C 36 H 45 BrN 2 S 4 : C, 60.56; H, 6.35 ; N 3.92 found C, 60. 68 ; H, 6. 23; N 3.72 Diethyl (5' (7 (3,5' dihexyl [2,2' bithiophen] 5 yl)benzo[c][1,2,5]thiadiazol 4 yl) 3' hexyl [2,2' bithiophen] 5 yl)phosphonate ( T4BTD PE ). In a dry Schlenk flask, compound 3 24 (1.75 g, 2.45 mmol), Pd 2 (dba) 3 (60 mg, 0.05 mmol, chloroform adduct)
135 and P( o tyl) 3 (70 mg, 0.23 mmol) were loaded under a flux of argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. Compound 3 12 (9 mmol) was dissolved in degassed toluene (30 mL), and the afforded solution was a dded to the reaction Schlenk flask through a septum, using a syringe. The mixture was then heated to 90 C and stirred for 12 hours. After cooling down to room temperature, the mixture was diluted with chloroform (100 mL) and washed once with water. The sol vent was evaporated to a dark red oil. The crude was purified by column chromatography using 7:3 hexanes : ethyl acetate as eluent, followed by a second column using 9:1 dichloromethane : acetone as eluent, to afford the title compound as a dark red thick oil (1.34 g, 1.57 mmol, 64 %). 1 H NMR (CDCl 3 ): 7.90 (s, 1H), 7.88 (s, 1H), 7.69, 7.65 (ABq, J AB = 7.7 Hz, 2H), 7.62 (dd, J = 4.2, 3.7 Hz, 1H), 7.24 (d, J = 3.6 Hz, 1H), 7.01 (d, J = 3.6 Hz, 1H), 6.73 (d, J = 3.6 Hz, 1H), 4.21 (m, 4H), 2.85 2.75 (m, 6H), 1.71 (m, 6H), 1.50 1.30 (m, 24H), 0.90 (m, 9H) 13 C NMR (CDCl 3 ): 152.46, 152.44, 146.64, 144.65, 144.54, 141.85, 139.91, 138.33, 137.26, 137.12, 136.18 133.529, 133.418, 130.958, 130.837, 130.807, 130.438, 128.353, 12 6.542, 126.319, 125.975, 125.79, 125 .56, 125.50, 124.82, 124.67, 124.64, 62.94, 62.87, 31.86, 31.84, 31.75, 31.73, 30.73, 30.60, 30.33, 29.82, 29.68, 29.51, 29.46, 29.00, 22.84, 22.81, 22.76, 16.54, 16.45, 14.30, 14.27. 31 P NMR (CDCl 3 against H 3 PO 4 ): 12.00 HRMS (APCI): m/z calcd for C 44 H 57 N 2 O 3 PS 5 H (MH + ): 853.2783 found 853.2790. Anal. calcd for C 44 H 57 N 2 O 3 PS 5 : C, 61.94; H, 6.73; N, 3.28 found: C, 61. 6 0, N, 7. 1 0, H, 3.16.
136 (5' (7 (3,5' dihexyl [2,2' bithiophen] 5 yl)benzo[c][1,2,5]thiadiazol 4 yl) 3' hexyl [2,2' bithiophen] 5 yl)phosphonic acid ( T4BTD PA ). In a dry round bottom flask equipped with a stir bar, T4BTD PE (300 mg, 0.35 mmol) was added and dissolved in 10 mL of anhydrous dichloromethane. The mixture was stirred under a flow of argon, and tr imethylsilylbromide (0.46 mL, 0.35 mmol) was added dropwise at room temperature. The mixture was stirred at room temperature for five ho urs, after which methanol (10 mL ) was added and left to stir at room temperature for 3 hours. The mixture was then dilut ed with dichloromethane (50 mL) and washed with water (100 mL). After the emulsion decanted, the organic phase was collected and evaporated without drying, and kept under vacuum to afford the title compound as a dark purple solid (265 mg, 0.33 mmol, 94 %). 1 H NMR (CDCl 3 ): 7.70 7.55 (br, 3H), 7.15 7.00 (br, 3H), 6.90 (br, 1H), 6.68 (br, 1H), 2.80 (br, 2H), 2.75 2.55 (br, 4H), 1.80 1.50 (m, 6H), 1.50 1.35 (m, 18H), 1.00 0.85 (m, 9H) 31 P NMR (CDCl 3 against H 3 PO 4 ): 12.44. Anal. calcd for C 40 H 49 N 2 O 3 PS 5 : C, 60.27; H, 6.20; N, 3.51 found .60.17, 6.22, 3.31 (5' bromo [2,2' bithiophen] 5 yl)triisobutylsilane (3 26). 5,5' dibromo 2,2' bithiophene (3.24 mg, 10 mmol) was loaded in a 250 mL dry flask under an argon flow, an d dissolved in anhydrous tetrahydrofuran (100 mL). The solution was stirred and cooled to 78 C, after which as solution of n butyllithium (7.1 mL, 10 mmol) was added dropwise to the reaction mixture over the course of 30 minutes. The solution was then sti rred at 78 C for two hours, and then chloro triisobutylsilane (2.58 g, 11 mmol) was added using a syringe via a septum, in one portion. The mixture was allowed to warm up to room temperature while stirring for 12 hours. The solution was then concentrated a nd rediluted with hexanes (100 mL). The afforded white precipitate is filtered off, and
137 the remaining clear solution is concentrated to a brown slurry. This was purified by column chromatography using pure hexanes as eluent, to afford the title compound (1 .51 g, 3.5 mmol, 35 %) as a clear oil. 1 H NMR (CDCl 3 ): 7.15 (d, J = 3.5 Hz, 1H), 7.12 (d, J = 3.5 Hz, 1H), 6.96 (d, J= 3.8 Hz, 1H), 6.93 (d, J= 3.8 Hz, 1H), 1.83 (nonuplet, J = 6.6 Hz, 3H), 0.92 (d, J = 6.6 Hz, 18H), 0.86 (d, J = 6.9 Hz, 6H). 13 C NMR (CD Cl 3 ): 141.44, 139.32, 139.28, 135.59, 130.78, 125.22, 123.85 110.94, 26.69, 25.38, 25.07. T riisobutyl(5' (4,4,5,5 tetramethyl 1,3,2 dioxaborolan 2 yl) [2,2' bithiophen] 5 yl)silane (3 27). Compound 3 26 (650 mg, 1.47 mmol) was dissolved in a dry flask w ith anhydrous tetrahydrofuran (10 mL) and cooled to 78 C. A solution of n butyllithium in hexanes (1.25 mL, 1.75 mmol) was added dropwise to the reaction mixture and the solution was then stirred at 78 C for two hours. 2 isopropoxy 4,4,5,5 tetramethyl 1, 3,2 dioxaborolane (355 mg, 1.91 mmol) was added using a syringe via a septum, in one portion. The mixture was allowed to warm up to room temperature while stirring for 12 hours. The mixture was then diluted with hexanes, poured in water and extracted twice with hexanes. After washing the combined organic extracts with brine, the solution was dried over magnesium sulfate. The volatiles were then evaporated to afford a yellow oil. The crude contained 70 % of the title compound, the remaining being unreacted s tarting material as determined by NMR. This was used without further purification. 1 H NMR (CDCl 3 ): 7.51 (d, J = 3.6 Hz, 1H), 7.27 (d, J = 3.8 Hz, 1H), 7.19 (d, J= 4.3 Hz, 1H), 7.14 (d, J= 4.8 Hz, 1H), 1.83 (nonuplet, J = 6.6 Hz, 3H), 1.35 (s, 12H), 0.92 (d, J = 6.6 Hz, 18H), 0.86 (d, J = 6.9 Hz, 6H).
138 (E) 1,1' bis(2 ethylhexyl) 6,6' bis(5' hexyl [2,2' bithiophen] 5 yl) [3,3' biindolinylidene] 2,2' dione (iIT 2 C6 2 ) and (E) 1,1' bis(2 ethylhexyl) 6 (5' hexyl [2,2' bithiophen] 5 yl) 6' (5' (triisobutylsily l) [2,2' bithiophen] 5 yl) [3,3' biindolinylidene] 2,2' dione (iIT 2 C6Si) In a purged Schlenk flask, (E) 6,6' dibromo 1,1' bis(2 ethylhexyl) [3,3' biindolinylidene] 2,2' dione (291 mg, 0.45 mmol) and 2 (5 hexylthiophen 2 yl) 4,4,5,5 tetramethyl 1,3,2 dioxa borolane ( 3 10 296mg, 0.79 mmol), Pd 2 (dba) 3 (30 mg, chloroform adduct) and P( o tyl) 3 (20 mg) were loaded under a flux of argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. Degassed toluene (4 mL) was then added to flask, followed by a degassed aqueous solution of tetraethylammonium hydroxide (1.8 mL, 1.8 mmol). The mixture was stirred and heated up to 90 C. The progession of the reaction was monitored by TLC, using a 3:2 mixture of hexanes:dichloromethane as eluent. When complete consumption of the dibromoisoindigo (red spot, Rf = 0.5) starting material was confirm by TLC, a solution of compound 3 27 (166 mg, 0.34 mmol) in degassed toluene (2 mL) was adde d to the flask. The mixture was kept stirring at 90 C for 12 hours, then allowed to cool to room temperature and slowly precipitated in methanol (40 mL). The precipitates were filtered and collected in a 100 mL round bottom flask, to which a minimum amount of hot chloroform (~15mL) was added in order to dissolve the solids completely. Silica gel (15 mL) was then added to the flask and swirled to absorb the solution. Careful evaporation of the solvent adsorbed the crude on the dry silica gel, which was loade d onto a silica gel column packed with 2:1 hexanes:dichloromethane. Eluting with 2:1 and then 3:2 hexanes:dichloromethane separated and purified the two title compounds, affording
139 iIT 2 C6 2 (178 mg, 0.18 mmol, 40%) and iIT 2 C6 Si (144 mg, 0.13 mmol, 29%) as dark purple blue solids. iIT 2 C6 2 1 H NMR (CDCl 3 ) : 9.01 (d, J = 8.7, 2H), 7.25 7.18 (m, 4H), 7.05 (d, J = 3.6 Hz, 2H ) 7.02 (d, J = 3.6 Hz, 2H ), 6.78 (s, 2H), 6.70 (d, J = 3.3 Hz), 3.68 3.41 (m, 4H), 2.80 (t, J = 4.2 Hz, 4H), 1.82 1.65 (m, 6H), 1.44 1 .20 (m, 28H), 0.98 0.81 (m, 18H) 13 C NMR (CDCl 3 ) : 168.7 3 146.3 4 145.6 1 142.1 0 138.9 0 137.4 3 134.7 2 131.6 8 130.3 4 125.1 2 125.07, 124.1 9 123.9 0 121.1 8 118.8 0 104.5 3 44.2 4 38.0 5 31.79, 31.75, 31.1 2 30.4 1 29.2 6 29.0 3 24.5 4 23.3 5 22.8 1 14.4 5 14.3 6 11.0 3 HRMS (ESI TOF) Calculated for C 60 H 74 N 2 O 2 S 4 (M+H) + : 983.4706 Found: m/z 983.4741. Anal. Calcd for C 60 H 74 N 2 O 2 S 4 : C, 73.27; H, 7.58; N, 2.85. Found: C, 73.39; H,7.57; N, 2.80. iIT 2 C6Si 1 H NMR (CDCl 3 ) : 9.10 (dd, J = 3.0, 1.8 Hz, 1H) 9.08 (dd, J = 3.0, 1.8 Hz, 1H), 7.29 (d, J = 3.8 Hz, 1H), 7.28 (d, J = 3.5 Hz, 1H), 7.27 (d, J = 3.8 Hz, 1H), 7.25 (dd, J = 4.2, 1.6 Hz, 1H), 7.23 (dd, J = 4.0, 1.6 Hz, 1H), 7.17 (d, J = 3.8 Hz, 1H), 7.16 (d, J = 3.4 Hz, 1H), 3.71 3.53 (m, 4H), 2.80 (t, J = 7.4 Hz, 2H), 1.86 (nonuplet, J = 6.6 Hz, 3H), 1.86 1.76 (br, 2H), 1.70 (quintet, J = 7.2 Hz, 2H), 1.43 1.28 (m, 24H), 0.95 (d, 6.6 Hz, 18H), 0.88 (d, J = 6.8 Hz, 6H), 0.96 0.87 (m, 15H). 13 C NMR (CDCl 3 ) : 168.81 146.34, 145.79, 145.77, 142.73, 142.1 6, 142.14, 139.52, 138.97, 138.48, 137.56, 137.47, 135.78, 134.71, 131.87, 131.75, 130.38, 130.37, 125.35, 125.23 1 25.20, 125.18, 124.96, 124.22, 123.99, 121.30 121.19 118.97, 118.91, 104.69, 104.65, 44.27 38.03, 31.83 31.08, 30.48, 29.21, 29.18, 29.0 3, 26.75, 26.70, 25.44 25.13, 24.56, 23.35, 22.83, 14.42, 14.34, 11.07. Anal. calcd for C 66 H 88 N 2 O 2 S 4 Si: C, 72.21; H, 8.08; N, 2.55; found : C, 72.51; H, 8.56; N, 2.48 (E) 1,1' bis(2 ethylhexyl) 6,6' bis(5' (triisobutylsilyl) [2,2' bithiophen] 5 yl) [3,3' b iindolinylidene] 2,2' dione (iIT 2 Si 2 ) In a purged Schlenk flask, (E) 6,6' dibromo
140 1,1' bis(2 ethylhexyl) [3,3' biindolinylidene] 2,2' dione (220 mg, 0.34 mmol), compound 3 27 (470 mg, 0.89 mmol), Pd 2 (dba) 3 (15 mg, chloroform adduct) and P( o tyl) 3 (10 mg) were loaded under a flux of argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. Degassed toluene (5 mL) was then added to flask, followed by a degassed aqueous solution of tetraethylammonium hydroxide (1.4 mL, 1.4 mmol). The mixture was stirred and heated to 90 C for 12 hours. After cooling back to room temperature, the mixture was slowly poured in methanol (30 mL) and the precipitates were collected by f iltration. The solids had enough solubility that they were dissolved in a minimum of a 3:2 hexanes:dichloromethane, and purified by column chromatography using 3:2 hexanes:dichloromethane as eluent. This afforded the title compound (265 mg, 0.22 mmol, 64%) as a dark gray purple solid. 1 H NMR (CDCl 3 ): 9.13 (dd, J = 4.2, 1.6 Hz, 1H), 7.32 (d, J = 3.8 Hz, 1H), 7.29 (d, J= 3.5 Hz, 1H), 7.27 (dd, J = 4.2, 1.7 Hz, 1H), 7.20 (d, J = 3.8 Hz, 1H), 7.17 (d, J = 3.5 Hz, 1H), 6.92 (t, J = 1.7 Hz, 1H), 3.76 3.60 (m, 2 H), 1.86 (nonuplet, 6.6 Hz, 4H), 1.46 1.30 (m, 8H), 1.00 0.90 (m, 30H). 13 C NMR (CDCl 3 ): 168.86, 145.88, 142.71, 142.14, 139.55, 138.53, 137.60, 135.75, 131.92, 130.37, 125.34, 125.27, 124.95, 121.29, 119.06, 104.77, 38.01, 31.05, 29.16, 26.71, 25.41 2 5 .09, 24.51, 23.33, 14.38, 11.04. Anal. calcd for C 72 H 102 N 2 O 2 S 4 Si 2 : C, 71.35; H, 8.48; N, 2.31; found: C, 71.62; H, 9.04; N, 2.23 Poly( 6,6' (4',3'''' b is(hexyl) 2,2':5',2'':5'',2''':5''',2'''':5'''',2''''' hexathiophene 5,5''''' diyl) dihex yl) carbonate ( T 6PC ). To a solution of T6 diol (259 mg, 0.30 mmol) and triphosgene (32.8 mg, 0.11 mmol) in dry THF (5 mL) is added anhydrous pyridine (0.1 mL) diluted in dry THF (1 mL) dropwise at room temperature. Gelation occurs after
141 90 minutes, after which 5 mL of dry THF are added to the reaction mixture, which is then left stirring at room temperature overnight. The reaction mixture is subsequently diluted with chloroform and poured in water, extracted twice with chloroform. After drying over magnesium sulfate, the c ombined organic extracts are concentrated to a red solid, which is dissolved in a minimum amount of chloroform and precipitated in methanol. The filtered precipitate is then fractionated using methanol, hexanes and chloroform in a Soxhlet apparatus. Precip itation of the concentrated chloroform fraction in methanol affords T6PC (210 mg, 0.236 mmol, 79 %) as an orange solid after filtration. 1 H NMR (CDCl 3 ): 7.10 (d, J = 3.7 Hz, 1H), 7.01 (d, J = 3.8 Hz, 1H), 6.96 (d, J = 3.5 Hz, 1H), 6.93 (s, 1H), 6.67 (d, J = 3.5 Hz, 1H), 4.14 (t, J = 6.6 Hz, 2H), 2.85 2.65 (m, 4H), 1.68 (m, 6H), 1.43 (m, 6H), 1.34 (m, 4H), 0.90 (m, 3H) 13 C NMR (CDCl 3 ): 155.60, 145.30, 140.58, 136.75, 135.90, 135.34, 134.80, 128.93, 126.38, 126.17, 125.14, 124.04, 123.54, 68.11, 31.88, 3 1.59, 30.64, 30.27, 29.74, 29.46, 28.81, 25.70, 22.83, 14.33. Anal. calcd for C 49 H 60 O 3 S 6 : C 66.17, H 6.80 found C 66.08 H 6.73.
142 CHAPTER 4 ISOINDIGO, A VERSATI LE ELECTRON DEFICIENT UNIT FOR P TYPE AND N TYPE ORGANIC ELECTRONIC A PPLICATIONS 4.1 The isoindi go molecule In the indigoid family, the most prominent structural isomer of isoindigo (iI) is the well known and widely used indigo molecule. The latter is one of the oldest natural dyes, whose structure was first proposed by Adolf von Baeyer in the late 1 800s. 1 8 6 The structure of the indigo chromophore is shown in Scheme 4 1 (left). Another dye outside of the indigoid family, diketopyrrolopyrrole (DPP, Scheme 4 1, center), was introduced by Ciba in 1983, as a vibrant red pig ment in its bis phenyl N H form, 18 7 although its synthesis was first reported in 1974. 18 8 Soluble derivatives of the latter have become very popular in the field of organic electronics, mostly in the bis thiophene form for high mobility and photovoltaic applications. 65 70 Isoindigo itsel f, depicted in Scheme 4 1 (right), has not been widely employed as a dye nor pigment, probably because of the rather dull tone of the N H form Only since 2010, isoindigo was deemed a useful electron deficient moiety for organic electronic applications, as first reported by the Reynolds group. 158 Scheme 4 1. Structures of indigo, diketopyrrolopyrrole and isoindigo. As a most studied analog of isoindigo, a few characteristics of indigo are worth describing in order to unde rstand isoindigo itself. The chromogen in the indigo molecule
143 has been identified as the central double bond decorated with two electron donating (blue arrows) nitrogens and two electron accepting (red arrows) carbonyls (Scheme 4 2, left). Scheme 4 2. Donor acceptor pattern, substituents positions and conjugation extent of indigo. These electron donors (N) and acceptors (C=O) are arranged in a trans configuration, hence the so called cross conjugated or H chromophore. 1 8 9 Calculations showed that the outer benzene rings only play a secondary role in chromophore of indigo. Each donor (N) and acceptor (C=O) is also bonded to the outer benzene ring, for which the substituents pattern has a significant impact on the absorption of the derivatized indigo. 1 8 9 Table 4 1 Effect of substituent on the longest wavelength absorption maxima of indigo. Substituent X Absorption maxima (nm) None 606 OEt 645 570 NO 2 580 635 For instance, as depicted in Scheme 4 2 (center) and summarized in Table 4 1, the central chromogen is influenced by electron donating ethoxy substituents on the
144 ositions, with a 35 nm hypsochromic shift of the max compared to the unfunctionalized indigo. The reverse effects are observed for electron withdrawing nitro groups. This strong dependence on the nature of the substituent and its position is explained by the strengthening of the electron donor character of the chromophore nitrogen by a para the electron accepting character of the carbonyl by a para thus increasing the donor acceptor effect. The latter effect is decreas ed when the reverse substitution pattern occurs, explaining the observed hypsochromic shifts for an the strong substituents effect, the outer benzene ring itself onl y plays a secondary role in chromophore of indigo. As shown in Scheme 4 2 (right), the central double bond is not directly bonded to the benzene rings, hence the limited conjugation. Although the synthetic ability to tune the absorption of the chromophore is attractive, indigo is therefore not a valuable unit for fully conjugated systems a priori. The diketopyrroloyrrole unit is also based on a central donor acceptor chromo gen involving electron donating nitrogens and electron accepting carbonyls (Scheme 4 3). Scheme 4 3. Donor acceptor pattern, substituents positions and conjugation extent of DPP.
145 Whether to call it a cross conjugated chromophore is debatable, since each of the two central double bonds is only bonded to one N and one C=O; although when combined into the conjugated 1,3 butadiene core, a crossed D A pattern is visible. Unlike indigo, there is only one substitution position available at the 3 and 6 positions in the DPP unit (Scheme 4 3, center) and this is inherently aryl substituted because of the synthesis DPP itself. The electron donating strength of the aryl groups at the 3 and 6 position s importantly, there is an extended conjugation of the Pi system across the molecule, visible as a 1,4 diarylbuta 1,3 diene core depicte d in Scheme 4 3 (right). This extended conjugation is responsible for the extensive use of DPP as an acceptor unit in fully conjugated molecules and polymers. The subject of this Chapter, isoindigo, also has a cross conjugated chromophore as part of the indigoids family. Displayed in Scheme 4 4 (left), the double D A pattern across the central double bond can be understood as a direct electron withdrawing effect (red) of the trans c arbonyls on the double bond and an indirect donating effect (blue) of the nitrogens via conjugation trough the ortho positions of the benzene rings. Scheme 4 4. Donor acceptor pattern, substituents positions and conjugation extent of isoindigo.
146 Compared to the direct donating effect of the nitrogen in indigo (Scheme 4 2, left), one could stipulate that this indirect donating effect in isoindigo c ould be responsible for a reduced D A interaction, which is consisten t with the blue dihexyl isoindigo ( max = 496 nm, in CHCl 3 diethyl indigo ( max = 653 nm, in CHCl 3 ). Another contributing factor could be the poorer electron accepting character of the carbonyl in the amide of isoindigo compared to the ketone in indigo. As for indigo, si gnificant substituent position and strength effects are expected on the benzene ring of isoindigo (Scheme 4 4, center). The 4 and 6 positions are conjugated with the central double bond, while the 5 and 7 positions are ortho and para to the nitrogen. A det ailed study of the various substituent/position effects is underway, 1 90 but is not part of this Chapter, mainly because of the applications targeted herein. As for DPP, isoindigo has a fully conjugated system which is based on trans stilbene as shown in Scheme 4 4 (right). Because the organic electronic applications here are d out already as steric hindrance with the carbonyls is likely to impair efficient system extension ortho to the central double bond. The para in this Chapter. By c functionalized isoindigo can be viewed as a structural hybrid of the latter two molecules: it displays the cross conjugated chromophore and likely significant substituent/position effects on the benzene rings characteris tic of indigo; but also, as DPP, it has an extended system
147 4.2 Isoindigo model compounds. described in Scheme 4 5. Importantly, the two building blocks 6 bromooxind ole and 6 bromoisatin are commercially available. Their acid catalyzed condensation in refluxing dibromoisoindigo in quantitative yields, with little purification required as simple filtration and washing with water, ethanol and et hyl acetate provides the pure compound 4 1 This is readily alkylated in high yields under basic conditions, using potassium carbonate in refluxing anhydrous DMF in the presence of the proper alkyl bromide or using sodium hydride in anhydrous DMF at room t emperature followed dibromoisoindigo was alkylated with linear n hexyl chains ( 4 2 ) for model compounds, but with branched 2 ethylhexyl ( 4 3 ) and 2 hexyldecyl chains ( 4 4 ) in the case of m ore extended conjugated molecules and polymers. These dibrominated isoindigos are the first set of precursors Scheme 4 5. Synthesis of the dibromo and diboron isoindig o precursors. a) 6 bromooxindole, 6 bromoisatin, HCl conc., AcOH, 90 C, 95%. b) 1.NaH, DMF, r.t. 2. n hexyl bromide, 80 C, 95% for 4 2 or K 2 CO 3 DMF, alkyl bromide, 100 C, 85% for 4 3 and 70% for 4 4 c) Pinacolester diboron, PdCl 2 (dppf), KOAc, dioxane, 80 C, 75%. The second precursor involves converting the bromides into boron pinacol ester via the Miyaura borylation route using the pinacol ester of diboron in anhydrous dioxane
148 in the presence of potassium acetate and catalytic amounts of PdCl 2 (dppf) (Sche me 4 5). This affords compound 4 5 in high yields with little purification as simple precipitation in cold methanol and washing the filtered solids with methanol suffices. With precursor 4 2 in hand, model compounds were synthesized as described in Scheme 4 6. By reacting 4 2 with the pinacol ester of benzene under Suzuki cross diphenylisoindigo P iI P molecule was obtained. R eacting 4 2 with 2 trimethyltin thiophene or 2 tr imethyltin 3,4 ethylenedioxythiophene (EDOT) under St ille cross dithiophene isoindigo ( T iI T ) and diEDOT isoindigo ( E iI E ). Scheme 4 6. Synthesis of the bisphenyl, bisthiophene and bisEDOT isoindigo model compounds. a) Phenyl boroni c ester, Pd 2 (dba) 3 P(o tyl) 3 ,Et 4 NOH, toluene, 90 C, 74%. b) 2 tributyltin thiophene, Pd 2 (dba) 3 P(o tyl) 3 toluene, 90 C, 94%. c) 2 trimethyltin 3,4 ethylenedioxythiophene, Pd 2 (dba) 3 P(o tyl) 3 toluene, 100 C, 87%. The absorption spectra of all three mod el compounds were recorded in chloroform and are displayed in Figure 4 1.a (molar absorptivities vs wavelength) along with that of
149 dihexyl isoindigo ( H iI H ) recorded in the same conditions. All spectra have two absorption bands, the high energy ones being confined below 450 nm and the low energy ones above 500 nm. The molar absorptivities at the low energy absorption peaks increase from 3,700 M 1 cm 1 for H iI H to 26,800 M 1 cm 1 for E iI E Figure 4 1. Solution absorption spec tra (a) of the isoindigo model compounds, and solution electrochemistry (b) of isoindigo, along with the reduction DPVs of the isoindigo model compounds, recorded in 0.1M TBAPF 6 in DCM. The fact that the absorption of the unfunctionalized H iI H shows two bands typical of donor acceptor systems is consistent with the intrinsic D A character of isoindigo discussed in the previous section. As the electron donating strength of the aryl group linked to isoindigo increases from phenyl ( P iI P ) to thiophene ( T iI T ) to EDOT ( E iI E ), the charge transfer band increases in intensity and red shifts from 496 nm for H iI H to 567 nm for E iI E The electrochemistry of H iI H in solution (CV and reductive DPV) was recorded as shown in Figure 4 1.b; all potentials are c alibrated against Fc/Fc + The reductive CV shows two reversible reduction processes centered at half wave potentials of 1.38 V and 1.85 V, while the oxidative CV shows one irreversible oxidation process at potentials higher than 1V. The accessible reduct ion of H iI H is consistent with its
150 electron deficient nature. The reductive DPV of H iI H was recorded (black dashed line) in the same experimental conditions. With an onset of reduction in the DPV at 1.17 V, the LUMO energy level of isoindigo alone is calculated a 3.93 eV. Interestingly, when the reductive DPVs were measured for the phenyl, thiophene and EDOT model compounds (colored dashed lines, Figure 4 1.b), the onset of reduction for each compound was confined between 1.10 and 1.20 V. Therefore, the nature of the aryl model compounds. Because of the bathochromic shifts observed for increasingly electron rich substituents, electron rich aryl groups at the 6,6 seem to have a significant stabilizing effect on the HOMO energies of chromophore. Calculations performed by Dr. Leandro Estrada in the Reynolds group were aimed at evaluating the electron density in the system of such compounds. The unconstrained geometry of T iI T in gas phase was optimized by Density Functional Theory (DFT) using the B3LYP/6 31G* level of theory The solubilizing n hexyl groups on isoindigo were replaced for methyl to speed up the com putations. Figure 4 2 shows the optimized structures and frontier orbital isodensity distributions for T iI T Figure 4 2. DFT optimized structures and frontier orbital densi ty distributions for T iI T
151 It appears that t he HOMO is dominated by a stilben e like structure with electron delocalization along the whole molecule, while the LUMO is localized on the central bipyrrolidine) dione unit. This is consistent with the spectroscopy and tron rich substituents influence mostly the HOMO energy levels. The level of influence of the electron donating character on the stabilization of the HOMO energy should be balanced with the contribution of dihedral angle tw isting on the extent of system overlap. The higher the dihedral angle, the less stabilized the system becomes as a result of poor overlap. Geometry optimization in the DFT calculations already suggests that the thiophene and isoindigo units are quite coplanar. Attempts were made to gro w crystals of the model compounds P iI P and T iI T to evaluate the difference in dihedral angle between the two. The initial attempts to slowly evaporate chloroform solutions at room temperature did not yield suitable crystals. The material did look cryst alline, but the crystals were too small for X ray analysis. Figure 4 3. Pictures of T iI T crystals grown by (a) slow evaporation of a chloroform solution and (b) vapor diffusion between chloroform and acetonitrile. Millimeter scale single crystals wer e eventually obtained by the vapor diffusion method between a concentrated chloroform (good solvent) solution of either P iI P or T
152 iI T ( ca. 10 mg/mL) and acetonitrile (poor solvent). Figure 4 3 shows representative crystals from the slow solvent evaporat ion (a) and vapor diffusion (b) methods for T iI T samples, kept at room temperature without disturbance for 4 days. X ray analysis of the crystals resulted in the packing structure displayed in Figure 4 4.a for P iI P and 4 4.b for T iI T The two compoun ds pack differently in the crystal lattice. For P iI P (Figure 4 4.b), the stacking direction is the same for every molecule, and each molecule is slid half way along the chromophore axis as compared to its closest neighbors, so that each phenyl ring overlaps with a n isoindigo core. Figure 4 4. Crystal packing of T iI T (a) and P iI P (b). For T iI T the packing occurs in two orthogonal stacked sheet directions (Figure 4 5.a). For one molecule in one sheet, its stacked neighbor is slid two thirds of its length along the chromophore axis, such than the thiophene ring ove rlaps with the benzene ring of the neighboring isoindigo. Thus, only the benze ne and thiophene rings are in overlap, leaving a gap on each side of the isoindigo central double bond. This gap is filled by two hexyl chains from adjacent molecules of the or thogonal sheet of stacked molecules.
153 For both compounds, the isoindigo unit is essentially flat in a trans configuration consistent with the DFT calculations. In the crystal structure of P iI P the dihedral angle between the phenyl substituent and the isoindigo core is 36 much greater than the 4 dihedral extracted from the crystal structure of T iI T The steric interaction between the protons ortho to the biphenyl linkages in P iI P likely result is such a dihedral angle difference, as T iI T is alm ost flat. Figure 4 5. Crystal structures of T iI T (top) and P iI P (bottom). Going beyond these simple model molecules, extending the donor unit to two thiophene rings resulted in a red shifted absorption spectrum as described in the last part of Ch apter 3 : Figure 3 20d showed a peak absorption in solution at 592 nm for the bithiophene capped isoindigo iIT 2 C6 2 When the alkyl chains on the latter were linear n hexyl chains, the crystallinity of the compound resulted in an even greater bathochromic s hift in the solid state, up to 658 nm. This led to the first report of isoindigo as electron acceptor in efficient molecular BHJ solar cells. Since then, W rthner and coworkers
154 have shown that functionalizing T iI T with amines at the 5 position can increa se the solution peak absorption up to 696 nm. 1 9 1 Similar effects on the dependence of the HOMO energies on the nature of the electron donating substituent were reported. The following section describes the synthesis of donor acceptor conjugated polymers in corporating isoindigo as an electron deficient unit. 4.3 Isoindigo Based Donor Acceptor Conjugated Polymers. 4.3.1 Polymer Synthesis and Characterization Six representative isoindigo based conjugated polymers were synthesized by the palladium catalyzed p olycondensations of dibromoisoindigo compound s 4 3 and 4 4 with functionalized electron rich moieties, as depicted in Scheme 4 7 The 2 ethylhexyl derivative 4 3 was used in most cases, but solubility issues for one polymer required the use of the 2 h exyldecyl monomer 4 4 The borylations of 3,4 dioctylthiophene, 1,4 bis(hexyloxy)benzene and 9,9 dihexylfluorene were performed according to previously reported procedures. The di tin thiophene and ditin 3,4 propylene dioxythiophene (ProDOT) compounds were purchased or prepared by direct lithiation with n butyllithium followed by que nching with trimethyltin chloride. The polycondensations were carried out under Stille or Suzuki coupling conditions depending on the nature of the electron rich co monomer using Pd 2 (dba) 3 as palladium source and P( o tyl) 3 as phosphine ligand. A 20 wt% d egassed aqueous solution of Et 4 NOH was selected as an organic base for the Suzuki polycondensations. The polymerizations were carried out at 85 C for 36 hours in degassed toluene/water mixture, where both the activated boronate salts and the monomers are s oluble
155 Scheme 4 7. Synthesis of a family of iI based D A polymers. a) 2,7 diboron fluorene, Pd 2 (dba) 3 P(o tyl) 3 ,Et 4 NOH, toluene, 85 C, 93%. b) 2,5 diboron 2,4 dihexyloxybenzene, Pd 2 (dba) 3 P(o tyl) 3 ,Et 4 NOH, toluene, 85 C, 63%. c) 2,5 diboron 3,4 diocylthiophene, Pd 2 (dba) 3 P(o tyl) 3 ,Et 4 NOH, toluene, 85 C, 57%. d) 2,5 bis(trimethyltin)thiophene, Pd 2 (dba) 3 P(o tyl) 3 toluene, 85 C, 42% for P(iI T) EH and 74% for P(iI T) HD e) 2,5 bis ( trimethy l tin) ProDOT, Pd 2 (dba) 3 P (o tyl) 3 toluene, 85 C, 34%. The polymers were collected by filtration after precipitation into methanol and purified by Soxhlet extraction using methanol, hexanes and chloroform fractions. The fractionation yields vary depending on the solubility of the material: for P (iI OB) P (iI AT) and P (iI ProDOT) both the hexanes and the chloroform fractions contained significant amounts of polymer, while only the chloroform fraction of P (iI F) contained significant amounts of material. After evaporation of the chl oroform P1 shows considerable mechanical strength, and a 20 cm diameter free standing film was easily obtained as shown in Figure 4 6.a. The lack of alkyl chains on the thiophene unit of P(iI T) EH resulted in a poorly soluble polymer even in hot chlorin ated solvents and limited yields after extraction from
156 chloroform. This is why 4 4 was also polymerized with ditin thiophene to afford P(iI T) HD which is soluble in THF (> 10 mg/mL) and chlorinated solvents (>20 mg/mL). Figure 4 6. Picture (a) of 20 cm diameter free standing film of P(iI F) and proton NMR spectrum (b) of P(iI F) recorded in CDCl 3 The remainder of this study was carried out on samples from the chloroform fractions after Soxhlet extraction s, which contain the higher molecular weights. Estimation of these molecular weights against polystyrene standards using size exclusion chromatography (SEC) with THF as eluent gives number average molecular weights (M n ) within 10 to 22 kDa for the polymers bearing alkyl chains on the donor un it. T he re corded M n for P (iI T) EH suff ered from poor solubility in THF f or SEC analysis leading to a low M n of 2.4 Poor solubility of the growing polymer chain in the toluene reaction solvent could also explain the low molecular weight of P(iI T) EH The 2 hexylde cyl version P(iI T) HD was a higher molecular weight polymer of 19.9 kDa as measured by SEC in THF The proton NMR spectra of all polymers show broad peaks in the 6.5 to 9.4 ppm range corresponding to the aromatic protons, as well as broad multiplets in th e 0.70 to 1.80 ppm corresponding to the alkyl chain protons other than the ones on the tertiary carbon of the branched chains (around 1.90 ppm) and the ones on the carbon next to the isoindigo nitrogen (in the 3.50 to 3.90 ppm range). A
157 representative 1 H N MR spectrum of the polymer set is shown for P (iI F) in Figure 4 6.b Thermogravimetric analysis (TGA) under nitrogen flow was used to evaluate the thermal stability of the purified polymers. A mass loss of 5% is defined as the threshold for thermal decompo sition. All materials demonstrate good thermal stability with decomposition temperatures above 325C The thermal data and the SEC results are summarized in Table 4 2. 4.3.2 Electrochemistry and Optical Properties. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were carried out on thin films of each polymer to evaluate their redox properties The CV s and DPV s of each polymer are displayed in Figure 4 7, all potentials being versus Fc/Fc + As a representative polymer, P(iI T) sh ows a quasi reversible oxidation, with a half wave potential located at 0.91 V and two quasi reversible reductions centered 1.24 V and 1.62 V While the generation of the anion radical during the first reduction process at 1.24 V is stable to repeated c ycles, the second reduction process was unstable under the selected electrochemical conditions. The overall CV and DPV data for the six p olymers is summarized in Table 4 2 The HOMO and LUMO energy levels can be calculated from the onsets of oxidation and reduction, respectively, with Fc/Fc + set at 5.1 e V. T he onsets of oxidation and reduction from the solid state DPV displayed in Figure 4 7 (dashed lines) are also summarized in Table 4 2 The LUMO levels are comprised in a narrow range: the LUMO for P (iI T) HD is the deepest at 4.03 eV, while P (iI F) has the highest at 3.84 eV. The deepest HOMO level is at 5.83 eV for P(iI F) and the highest is at 5.68 eV for P(iI ProDOT) This is consistent with the ProDOT unit being more electron rich than the fluor ene in P(iI F) resulting in a stabilized HOMO for P(iI PrDOT)
158 Figure 4 7. Cyclic voltammogram and differential pulse voltammogram of thin film s of each polymer on Pt button electrode, recorded at a 50 mV/s scan rate in 0.1M TBAPF 6 acetonitrile soluti on. The electrochemistry of the polymers is consistent with the results observed for the model compounds is Section 4.2, as the energies of the LUMOs confined in a 0.2
159 eV range are only slightly influenced by the nature of the electron donor moiety. The HO MO energies are also within 0.2 eV. T he electrochemical band gaps w ere calculated to be between 1.70 and 2.0 eV (Table 4 2 ). Before analyzing the difference in the absorption spectra of polymers with different electron donor moieties, the absorption of the P(iI T) EH and P(iI T) HD should be compared. As shown in Figure 4 8, although the aromatic part of the repeat unit of the two polymers is the same, the solution spectrum P(iI T) HD is significantly red shifted by 40 nm to 687 nm as compared to the maximu m absorption of P(iI T) EH (dashed lines). In the solid state for P(iI T) HD there is little difference with the solution spectrum (a 7 nm red shift to 694 nm) while the solid state absorption of P(iI T) EH broadens towards the near IR compared to its sol ution spectrum with a shoulder appearing at 687 nm. Figure 4 8. Solution absorption spectra (dashed lines) of P(iI T) EH (blue) and P(iI T) HD (black) and the corresponding solid state (solid lines) absorption spectra.
160 Since the solution spectra for th e two polymers are different, the polymer bearing the shorter alkyl chains P(iI T) EH is likely to have a different conjugation length than its longer alkyl chains equivalent P(iI T) HD In particular, t h e solution UV vis spectroscopy suggest s that P(iI T) EH has not reached its conjugation length high limit, because the 2 ethylhexyl side chains on isoindigo are not sufficient solubilizing group to allow the growing polymer chain s to attain high enough molecular weights I t is likely that only oligomers of isoindigo and thiophene were obtained during the polymerization for P(iI T) EH which is consistent with the broadening of its absorpti on in the solid state as shorter polymer backbones will be able to reorganize and aggregate more freely. Therefore, when comparing the absorption spectra for the set of D A polymers synthesized here, P(iI T) EH should be set aside as it is not representative of a fully conjugated polymer with isoindigo co thiophene as repeat unit. P(iI T) HD will be used in the following. As depicted in Figure 4 9.a t he UV vis spectrum of the five high molecular weight polymer s in solution displays two absorption bands characteristic of donor acceptor systems: a high energy band located in the 420 to 490 nm range attributed to the trans ition, and a low energy band in the 520 to 800 nm range assigned to intramolecular charge transfer. P olymers P (iI F) and P (iI OB) have similar absorption spectra in solution with high energy peaks at 462 and 452 nm respectively, and low energy peaks at 564 and 555 nm respectively. For these two polymers the relative intensities of these two bands are comparable and are close to unity. With red shifted absorption maxima, the peaks for P (iI AT) are located at 472 and 609 nm, with a decreased intensity of the high energy band relative to the low energy band. The trend on going from P (iI F) or
161 P (iI OB) to P (iI AT) is accentuated for P (iI T) HD and even further for P (iI ProDOT) : bathochromic shifts of 35 nm and 89 nm for the low energy absorption maxima of P (iI T) HD and P (iI ProDOT) respectively compared to P(iI AT) The intensities of their high energy bands decrease further relative to their low energy band, to a point where little absorption remains at high energy (below 500 nm) for the thiophene and ProDOT c opolymers in solution. Figure 4 9. Normalized UV vis absorption spectra the five high molecular weight polymers (a) in chloroform s olution and (b) as thin films on ITO coated glass. T he trend for chromatic shifts in absorption maxima can be explained by the variations of the electron donating character of the different electron rich co m onomers to the overall system. For instance, t he electron donating character of t he fluorene spacer in P(iI F) is relatively weak Arguably, this moderately electron rich unit lie s out of plane relative to the isoindigo unit because of steric interaction of the benzene rings, as suggested by the X ray crystal structures on model compound P iI P For the dialkoxybenzene donor in P(iI OB) the donating effect of the alkoxy side chains into the polymer backbone could be impaired by an even greater dihedral twist of the alternatin g units, since the alkoxy chains are ortho to the backbone linkages. This could explain
162 why the absorption of P(iI F) and P(iI OB) are confined below 650 nm T he three thiophene based spacers of the other three copolymers are more electron rich than the be nzene spacers and X ray crystal structures show a low dihedral angle between thiophene and isoindigo, which is why the absorption of P(iI AT) P(iI T) HD and P(iI ProDOT) extend further towards the near IR. For P(iI AT) it is likely that steric hindrance arising from the presence of the dialkyl s ubstituents on the thiophene of would explain the blue shift in its UV vis spectrum compared to P (iI T) HD The ProDOT spacer of P (iI ProDOT) is the most electron rich co monomer, resulting in the greatest bathoch romic shift. The extinction coefficients of the polymers in solution are between 22,000 and 40,000 M 1 cm 1 Table 4 2. SEC results, optical properties and electrochemical data measured for the six polymers. UV vis NIR Electrochemistry c CV DPV M n (kDa) M w / M n max a (nm) max b (nm) E gap opt (eV) E 1/2 ox (V) E 1/2 red (V) E ons ox (V)/ HOMO(eV) E ons red (V)/ LUMO(eV) E gap (eV) P(iI F) 21.2 1.9 564 561 1.87 1.01 1.17 0.73/ 5.83 1.26/ 3.84 1.9 9 P(iI OB) 10.5 2.1 555 573 1.75 0.90 1.34 0.70/ 5.80 1.23/ 3.87 1.93 P(iI AT) 16.7 1.9 609 633 1.79 1.02 1.30 0.64/ 5.74 1.25/ 3.85 1.89 P(iI T) EH 2.4 1.1 647 637 1.60 0.91 1.24 0.71/ 5.81 1.15/ 3.95 1.86 P(iI T) HD 19.9 2.5 687 694 1.60 0.84 1.28 0.65/ 5.75 1.07/ 4.03 1.72 P(iI ProDOT) 19.3 1.7 701 710 1.55 0.75 1.21 0.58/ 5.68 1.19/ 3.91 1.77 a In chloroform solution. b Recorded for thin films spayed onto ITO coated glass. c Recorded for thin films drop cast from toluene onto Pt button electrodes. The solid state absorption spectra of the polymer s are displayed in Figure 4 9.b Thin films were prepared by spraying solutions of the polymers onto ITO coated glass. The trend delineated for solution absorption still holds in the s olid state, only red shifted
163 on the order of 10 nm compared to the solution spectra. The colors of the polymer thin films are consistent with their absorption profile: P (iI F) and P (iI OB) films are red purple due to the broad abs orption from 400 nm to ca. 675 nm and little absorption beyond 675 nm. Thin films of P (iI T) have a blue gray color due to their long wavelength absorption and the reduced intensity of the low wavelength absorption band below 550 nm. With long wavelength absorption even more red sh ifted and further reduced low wavelength absorption, P (iI T) HD and P(iI ProDOT) have a blue green hue. From the low energy onset of absorption in the solid state, optical band gaps in the 1.55 to 1.90 eV range are calculated (Table 4 2 ), consistent with t he measured electrochemical band gaps trend In summary, D A conjugated polymers based on the isoindigo acceptor are able to absorb light up to 800 nm when thiophene based electron rich co monomers are used. The LUMO energies of the polymers, much like th e model compounds studied in the beginning of this Chapter, are rather insensitive to the nature of the electron rich moiety and are confined between 3.8 0 and 4.0 0 eV, which are deep (high electron affinities) compared to other conjugated polymers. With bandgaps in the 1.55 to 2.00 eV range, the HOMO energy levels are also deep (high ionization potentials), in the 5.6 0 to 5.85 eV range. 4.4 All Acceptor Isoindigo Based Conjugated Polymers. Motivated by such low lying energy levels and extended light abs orption conjugated polymers were designed for n type applications as a substitute for the widely used fullerene derivatives in BHJ solar cells, since the latter n type materials have limited absorption. In the following, two new isoindigo based conjugated polymers with backbones exclusively composed of electron deficient units. In contrast to the more
164 common donor acceptor approach widely used to synthesize n type conjugated polymers, the versatile chemistry of isoindigo provided a synthetic route to two s oluble high molecular weight all acceptor polymers: a homopolymer of isoindigo Poly(iI) and a copolymer of isoindigo with 2,1,3 benzothiadiazole alternating in the repeat unit Poly(iI BTD) 4.4.1 Polymer Synthesis and Optical Properties. As shown in Sch eme 4 8, d iborylated isoindigo 4 5 was copolymerized with its dibrominated analogue 4 4 under Suzuki polycondensation cond itions in degassed toluene at 80 C, using Pd 2 (dba) 3 and tri( ortho tolyl)phosphine as catalytic system, and a degassed 1M tetraethylamm onium hydroxide aqueous solution as boron activating base. These efficient Suzuki cross coupling conditions afforded the homopolymer of isoindigo, Poly(iI), in excellent yield. The purification process involved precipitation into methanol, followed by Soxh let extraction with methanol and then hexanes to remove low molecular weight product. A soluble higher molecular weight fraction of Poly(iI) was then extracted using chloroform, precipitated into methanol and filtered giving Poly(iI) in an overall yield of 74 %. Scheme 4 8. Synthesis of the all acceptor polymers Poly(iI) and Poly(iI BTD) a) Pd 2 (dba) 3 P(o tyl) 3 ,Et 4 NOH, toluene, 85 C, 74%. b) Pd 2 (dba) 3 P(o tyl) 3 ,Et 4 NOH, toluene, 85 C, 95%.
165 The number average molecular we ight of the chloroform soluble fraction of Poly(iI) is 28.7 kDa with a polydispersity index (PDI) of 2.4 as measured by size exclusion chromatography in THF against polystyrene standards. Poly(iI) is soluble in a range of common organic solvents, including tetrahydrofuran, toluene, dichloromethane, chloroform and chlorinated benzenes. The 1 H NMR spectrum of the chloroform soluble fraction shows broadened peaks in the aromatic region, between 8.8 9.1 ppm and 6.8 7.4 ppm, and a wide peak in the 3.6 to 4.3 ppm region with consistent integration corresponding to the methylene proton on the tertiary carbon of the 2 hexyldecyl side chains These chemical shifts are consistent with the repeat unit structure of Poly(iI) further confirmed by elemental analysis. By copolymerizing the diborylated isoindigo 4 5 with 4,7 dibromo 2,1,3 benzothiadiazole (Scheme 4 8 ), an alternating copolymer Poly(iI BTD) was obtained. Similar high yielding polymerization and purification procedures afforded Poly(iI BTD) with an average mo lecular weight of 16.3 kDa and a PDI of 3.5 in 95 % yield for the chloroform fraction after Soxhlet extraction. The solubility of Poly(iI BTD) is similar to Poly(iI) using the same solvents. The remainder of the study was conducted on the materials extract ed from chloroform during Soxhlet purification to ensure the highest average molecular weights available. Setting a 5 % weight loss as the threshold for thermal decomposition, TGA under nitrogen flow to showed that the polymers were both thermally stable u p to 380 C as displayed in Figure 4 10 .a. We recorded the UV vis absorption spectra of Poly(iI) and Poly(iI BTD) in solution and in the solid state. As displayed in Figure 4 10 .b the UV vis spectra of the polymer in solution and in thin
166 films show little difference, suggesting little aggregation in the solid state without further film treatment after spray coating. Figure 4 10 TGA thermograms (a) of Poly(iI) and Poly(iI BTD) under nitrogen flow, and normalized absorption spectra (b) in solution (dashe d lines) and in solid state (solid lines) of the two polymers. In the solid state, Poly(iI) absorbs light at wavelengths longer than 700 nm, with max at 690 nm and a low energy onset of absorption at 731 nm. Of the two main absorption bands in the 400 73 0 nm region the low energy absorption band centered at 690 nm for Poly(iI) is more intense than its high energy absorption band with a local maximum at 460 nm. Films of Poly(iI) have a blue green color in the neutral state, as most of the red light is abs orbed by the polymer. Thin films of Poly(iI BTD) have a shorter max at 464 nm, with a low energy absorption onset at 700 nm. The polymer absorbs from 400 to 600 nm. The measured molar absorptivities for Poly(iI) and Poly(iI BTD) in toluene were 25,000 M 1 cm 1 and 22,300 M 1 cm 1 respectively at their max From the lo w energy onsets of the thin film absorption, solid state optical bandgaps of 1.70 eV and 1.77 eV were calculated for Poly(iI) and Poly(iI BTD) respectively.
167 4.4.2 Electrochemistry and Spectroelectrochemical measurements. In order to experimentally determi ne the energy levels of the polymers, and be able to compare them to that of soluble fullerenes, we investigated the electrochemistry of Poly(iI) and Poly(iI BTD) as thin films drop cast onto Pt button electrodes in a 0.1M TBAPF 6 acetonitrile solution unde r inert atmosphere. All potentials reported here are calibrated against Fc/Fc + Figure 4 11 .a shows the tenth CV cycles of the oxidation and reduction of Poly(iI) and the reductive DPV. The reductive CV of Poly(iI) thin films shows one reversible redox pr ocess with cathodic and anodic peak currents at 1.36 V and 1.24 V, respectively, and a half wave potential at 1.30 V. We used the onset of reductive DPV (dashed line) to calculate the energy of the LUMO level. With a Fc/Fc + redox standard set at 5.10 e V versus vacuum, the measured DPV reduction onset found at 1.26 V corresponds to a LUMO energy of 3.84 eV Figure 4 1 1 Cyclic (solid line) and differential pulse (dashed line) voltammograms of Poly(iI) (a) and Poly(iI BTD) (b) recorded from thin fil ms on Pt button electrodes, in 0.1M TBAPF 6 /acetonitrile electrolyte. The reductive CV experiments on thin films of Poly(iI BTD) performed under the same conditions (Figure 4 1 1 .b) show one cathodic peak at 1.47 V and two anodic peaks upon reduction center ed at 1.42 V and 1.21 V From the reductive DPV (dashed
168 line) an onset of reduction was measured at 1.20 V corresponding to a LUMO energy of 3.90 eV When the scan rates were increased from 10 to 200 mV/s in the reductive CV of Poly(iI) t he half wave potentials remained constant (Figure 4 1 2 .a) With an anodic peak to cathodic peak potential difference under 160 mV even at relatively high scan rates, these results indicate a stable and relatively reversible redox process. Figure 4 1 2 Overlaid red uction CVs (a) of Poly(iI) recorded in 0.1M TBAPF 6 /ACN, at increasing scan rates from 10mV/s to 200 mV /s with a 10mV/s rate increment, with scan rate dependence of peak currents in inset. Overlaye d ten first oxidation CVs of (b ) Poly(iI) recorded in 0.1M T BAPF 6 /ACN, at 50mV/s scan rate. Displayed in the inset, the peak currents dependence on scan rate is close to linear, suggesting that the doping of the well adhered film on the electrode surface is not diffusion limited at the chosen scan rates. Attempts t o electrochemically oxidize Poly(iI) resulted in an irreversible and unstable redox process w ith a peak potential at +1.40 V, as shown in Figure 4 12 .b Since the poor oxidation of the polymer prevents a viable electrochemical calculation of the HOMO energ y level, we deduced it from the optical bandgap of the thin films: for an optical bandgap of 1.70 eV, the corresponding HOMO energy level is at 5.54 eV.
169 Similar scan rate dependence experiments were performed on the reduction of thin films of P(iI BTD) shown in Figure 4 13 .a. While the half wave potentials remained constant at 1.31 V when the scan rate was increased from 10 to 200 mV/s, the peak to peak potential difference widened from 170 mV to 610 mV. In the inset, the peak currents dependence on sca n rate is also close to linear, dismissing concerns of film deterioration or electrolytic limitations at the scan rates employed. This indicates that the reduction of Poly(iI BTD) is less reversible than that of Poly(iI) In a similar way to Poly(iI) the oxidation process is irreversible and unstable with currents steadily decreasing with successive recording cycles ( Figure 4 1 3 .b ). The solid state optical bandgap of 1.77 eV is equivalent to a HOMO energy of 5.67 eV. Figure 4 13 Overlaid reduction CVs (a) of Poly(iI BTD ) recorded in 0.1M TBAPF 6 /ACN, at increasing scan rates from 10mV/s to 200 mV /s with a 10mV/s rate increment, with scan rate dependence of peak currents in inset. Overlaye d ten first oxidation CVs of (b ) Poly(iI BTD ) recorded in 0.1M TBA PF 6 /ACN, at 50mV/s scan rate. Spectroelectrochemistry provides insight into the nature of the charged species generated along the conjugated backbone during the solid state reduction process. To investigate the spectroelectrochemical behavior of Poly(iI) films of the polymer were sprayed from toluene solution s onto ITO coated glass slides, which serve as
170 transmissive working electrodes. T etraethylammonium tetrafluoroborate in propylene carbonate (TEABF 4 /PC) was selected as the supporting electrolyte for th e reduction of the polymer films on ITO, since the redox processes proved to be more stable than when the TBAPF 6 /ACN electrolyte was used. Figure 4 1 4 depicts the spectral changes upon application of successive step potentials from 1.26 V to 1.45 V, with 10 mV potential increments for a Poly(iI) film on ITO. This small voltage difference to attain full reduction from the neutral polymer suggests a narrow distribution of states and that each species being reduced is chemically similar. Figure 4 1 4 Spe ctroelectrochemistry (left) of Poly(iI) sprayed onto an ITO coated glass slide. The film was subjected to 20 mV potential increments (first five spectra) then 10mV increments (last nine spectra) from 1.26 V to 1.45 V vs Fc/Fc + in a 0.1M TEABF 4 /propylene carbonate electrolyte. Pictures (right) of the neutral and reduced Poly(iI) film. No spectral change was observed when the potential was swept negati ve of 0 V up to 1.20 V: the blue line in Figure 4 14 with peak absorption at 688 nm in the neutral film re further reduction from 1.26 V to 1.45 V, the absorption bands at 459 nm and 688 nm of the neutral film decreased steadily to an almost complete bleaching of the absorption in the visible region. Concomitantly, a well defined absorption band centered at 1522 nm
171 emerged stabilizing in intensity at 1.45 V ( thick black line ). While blue green in the neutral state the polymer film cathodically bleaches at 1.45 V, as displayed in F igure 4 14 with a well defined isosbestic point at 737 nm. We evaluated the color of the polymer films in the undoped and the reduced states as the human eye perceives them by measuring their L*a*b* values (CIE 1976 L*a*b* Color Space). The neutral film s hown in Figure 4 14 with a maximum absorbance of 0.4 at 688 nm has a low optical density with a* and b* values of 16 and 4, respectively. These values confirm a green to blue green color of the undoped polymer, which is consistent with the trough observe d in its neutral UV vis spectrum at 520 nm and the lower relative intensity of the band at 459 nm with respect to the one at 688 nm. In the reduced state, the a* and b* values of the polymer film are respectively 1 and 6, confirming a transmissive doped st ate with a slight yellow hue, as expected from the small remnant absorption band at 459 nm at 1.45 V. Aside from dioxythiophene cyanovinylene based copolymers, 19 2 this is the only example of a stable colored to transmissive electrochromic polymer upon n d oping (i.e. anodically coloring material). 17 The spectroelectrochemistry of Poly(iI BTD) was studied under the same conditions. Starting from a neutral film previously subjected to ten reduction CV cycles from 0.5 to 1.5 V at 50 mV/s, potential steps wit h a 10mV increment were then applied from 1.05 V to 1.46 V and the spectra were recorded after each increment. Figure 4 15 shows the progression of chromatic changes upon stepping the potential from 1.21 V to 1.41 V, as little spectral changes were obs erved outside this potential window. In the absorption of the neutral film (red line), the two absorption bands at 468 nm and 594
172 nm decrease in intensity as a high wavelength band appears in the near IR centered at 1134 nm. Figure 4 1 5 Spectroelectroc hemistryof Poly(iI BTD ) sprayed onto an ITO coated glass sl ide. The film was subjected to 1 0 mV potential increments from 1.21 V to 1.41 V vs Fc/Fc + in a 0.1M TEABF 4 /propylene carbonate electrolyte. Unlike for Poly(iI) the absorption of Poly(iI BTD) in the visible is not fully bleached, as a band remains between 450 and 550 nm even at more negative potentials. The electrochemical experiments show ed a pronounced difference between the stable, reversible reduction processes compared to the unstable, irreve rsible oxidation processes, under the same electrochemical setup. The spectroelectrochemistry shows that the reversible generation of stable negative charge s in Poly(iI) thin films takes place in less than 20 mV, from 1.26 V to 1.45 V. T he excellent reve rsibility and speed of the reduction and the well defined isosbestic point suggest a single electron process which yields a radical anion on the repeat unit of isoindigo T he absorption band in the near IR centered at 1522 nm likely corresponds to that of the radical anion T he spectroelectrochemistry of Poly(iI BTD ) shows a similarly low potential range (20 mV)
173 for generation of negatively charged species in the film although the process is not as reversible as for Poly(iI) This is an indication of the potential n type character of all acceptor polyisoindigos. 4.4.3 All Polymer Solar Cells. The electrochemistry results confirmed our expectations of high electron affinities (deep LUMOs) and high ionization potentials (deep HOMOs) for electron deficient po lyisoindigos. Figure 4 16 .a depicts where the energy levels of Poly(iI) lie with respect of that of P3HT (electron donor) and PC 60 BM (electron acceptor). Figure 4 16 .b shows the electrochemical reduction of a solution PC 60 BM carried out in 0.1M TBAPF6 in D CM under inert atmosphere. The onset of reduction in the DPV was measured at 1.0 V vs Fc/Fc + corresponding to a LUMO energy of 4.1 eV for PC 60 BM. T he electron affinity of Poly(iI) around 3.9 eV approaches that of the commonly used PC 60 BM, measured at 4.1 eV electrochemically and set at 4.2 eV in the literature. 30 As can be seen in Figure 4 16 .a, t he ionization potential of Poly(iI) around 5.6 eV is sufficiently high to drive exciton se paration in BHJ cells with an appropriately chosen donor material. Figure 4 1 6 Band structure diagram (a) comparing the HOMO and LUMO levels of Poly(iI) and PC 60 BM, and their offsets relative to the electron donor P3HT. Solution electrochemistry (b) o f PC 60 BM, recorded in 0.1M TBAPF 6 in DCM.
174 Having a bandgap below 1.8 eV allows for extended absorption throughout the visible spectrum validating the candidacy of Poly(iI) as electron accepting material for all polymer solar cells. We selected P3HT as the p type counterpart to Poly(iI) for all polymer solar cells, since P3HT is a well characterized polymer with HOMO and LUMO energy levels around 5.2 eV and 3.2 eV respectively. 30 T his not only enables energy offsets greater than 0.3 eV between the HOMO (LU MO) of P3HT and Poly(iI) but the latter has a complementary absorption to P3HT, extending the absorption of the resulting blend by almost 100 nm into the near IR in thin films shown in Figure 4 17 .b. Bulk heterojunction photovoltaic cells were fabricated using P3HT as the donor and Poly(iI) as the acceptor by Caroline Grand in the Reynolds group and Dr. The polymers were dissolved separately in chlorobenzene under inert atmosphere and from the stock so lutions, blends of 2:1, 1:1 and 1:2 P3HT: Poly(iI) were spin cast onto adequately prepared patterned ITO slides. Figure 4 17 Schematic diagram (a) of the all polymer solar cell wi th conventional device geometry, and thin film absorption spectra (b) of the P3HT: Poly(iI) blends at 2:1, 1:1 and 1:2 ratios.
175 A schematic diagram of an all polymer solar cell with conventional device architecture is shown in Figure 4 1 7 .a along with the blend films absorptions for each ratio (Figure 4 17 .b). T he level of contr ibution of Poly(iI) in the overall film absorption for the 2:1 P3HT: Poly(iI) is small, which is expected at such a ratio given the difference in molar absorptivities between the two polymers At 1:2 P3HT: Poly(iI) blend, the absorption band at 688 nm from t he contribution of Poly(iI) scales to approximately to 68 % of the maximum absorption of P3HT. The J V characteristics of the devices made with three different P3HT: Poly(iI) weight ratios of 2:1, 1:1 and 1:2 as the active layer are shown in Figure 4 1 8 .a. Figure 4 18 J V curves (a) of the P3HT: Poly(iI) based BHJ solar cells with various blend ratio s under AM 1.5 solar illumination, 100 mW cm 2 in c onventional solar cell architecture. Photovoltaic parameters are inset: J sc in mA.cm 2 ; V oc in V; FF and PCE in %. External quantum efficiency (b) of the 1:1 P3HT: Poly(iI) device. T he photovoltaic parameters (short circuit current (J sc ), open circuit voltage (V oc ), fill factor (FF) and power conversion efficiency (PCE)) of these devices are summarized in the ins et in Figure 4 1 8 .a The P3HT: Poly(iI) weight ratio of 1:1 showed the best
176 device performance with a V oc value of 0.62 V, a FF of 41 %, and a J sc value of 1.91 mA.cm 2 resulting in a PCE value of 0.47 %. We denote an increasing fill factor of the devices with increasing Poly(iI) content, which reaches a maximum value of 50 % when the blend ratio is 1:2. Meanwhile, increasing the Poly(iI) content in the blend ratio from 1:1 to 1:2 decreases the J sc lowering the overall device performance. W hen AFM was used to probe the surface morphology of the active laye r at different blend ratios, no significant difference in the feature sizes for the different blends was observed. Likewise, no significant information was obtained from TEM images of the blends. Unlike bl ends of polymers and fullerenes, the difficulties in monitoring the surface morphology of polymer blends by AFM and low contrast in TEM are expected, since the two components are of similar physical and electronic nature on the scale of the latter two meth The external quantum efficiency (EQE) of the P3HT: Poly(iI) device with the blend ratio of 1:1 is shown in Figure 4 1 8 .b The EQE measurements showed that the best polyisoindigo based device exhibited broad photo response ranging from 35 0 to 750 nm with a maximum EQE of 12 % at 520 nm, confirming the contribution of Poly(iI) to the photogenerated current. In summary, the maximum PCE of 0.47 % obtained for the polyisoindigo based all polymer solar cells is to put in perspective of the bes t all polymer solar cells efficiency of 2.2~2.3 % reported so far for BHJ devices. Better morphological control of the polymer blends could lead to increased efficiencies in the solar cells described above, although low electron mobilities in Poly(iI) coul d impair the performance to a greater extent. Indeed, electron mobilities of 3.7 10 7 cm 2 V 1 s 1 on average in pristine
177 films of Poly(iI) were measured in an electron only device based on a vertical architecture (Al/ Poly(iI) /LiF/Al), in the space charge limited current regime 4.5 Isoindigo Based D A Polymers for BHJ Polymer Solar Cells. 4.5.1 Isoindigo in Polymer Solar Cells. From the previous sections, using donor moieties based on the thiophene unit result in polymers with a more extended absorption t oward the near IR rather than phenyl based donors. The backbone is likely more planar for thiophene donor, at least as long as the thiophene ring itself is not alkylated in a way that would induce twisting ( cf. P(iI AT) ). Scheme 4 9. Structures of all the D A conjugated polymer reported in the literature so far. At the same time, non alkylated thiophenes worsen the solubility of the D A polymer to an extent that not only would hamper solvent processing, but also can lead
178 to oligomeric species rather than actual polymers ( cf. P(iI T) EH vs P(iI T) HD ). As a general rule, isoindigo should bear 2 ethylhexyl chains when the donor is alkylated; 2 hexyldecyl chains when the donor is not alkylated. Since the first report of mol ecular solar cells and conjugated polymers based on isoindigo by the Reynolds group, researchers in the field have followed with the synthesis of a variety of D A conjugated polymers, for which the structures are displayed in Scheme 4 9. 19 3 20 1 Remarkably, the LUMO energy levels of all iI based conjugated materials reported so far are confine d to the 3.7 to 3.9 eV range. With bandgaps between 1.9 and 1 .5 eV depending on the aromatic units conjugated with isoindigo, the HOMO energy levels are between 5.5 and 5.9 eV. Such high ionization potentials (deep HOMOs) for D A polymers led to devices with high open circuit voltages (V oc ) for BHJs with fullerene derivatives Some of these polymers performed very well, with 6.3% BHJ solar cell efficiency with PCBM r eported by Andersson and coworkers 200 for the copolymer of isoindigo and terthiophene and 0.79 cm 2 V 1 s 1 in air stable p type OFETs reported by Lei and coworkers 1 9 4 for the copolymer of isoindigo and unalkylated bithiophene. These remain the best performan ces reported for isoindigo based materials so far. 4.5.2. Polymer Synthesis and Characterization. In an effort to tai lor the structure of isoindigo based D A polymers for optimized solar cells the synthesis of the copolymer of isoindigo and dithieno[3,2 b d]silole, P(iI DTS) is described in the following The simplest electron rich moiety serving this purpose is one thiophene ring, which was used by Andersson and co workers in an i I based conjugated polymer similar to P(iI T) HD achieving 4.5 % solar cell efficiency. 1 9 5 Extending the donor unit length to two thiophene rings likely increases the delocalization
179 of positive charge carriers along the backbone thereby enhancing the p type character of the i I based polymer, as demonstrated by Lei and co wor kers 1 9 4 In copolymers based on different acceptors than isoindigo, the presence of a bridging atom such as carbon (cyclopentadithiophene, CPDT) or silicon (dithienosilole, DTS) between two thiophene rings has been shown to further planarize the electron r ich unit, 44 while providing an alkylation site for solubility purposes. The silicon bridge of DTS is advantageous as the alkyl chains stemming from silicon are able to remain in plane to a greater extent than in CPDT, resulting in a more planar backbone. T herefore, we suspec ted DTS to be an electron copolymers based on isoindigo as a conjugated acceptor. For solubility purposes, both monomers were functionalized with 2 ethylhexyl side chains. The DTS moiety was prepared and conver ted to its ditin derivative by Dr. Chad Amb following a previously reported procedure, 46 and was purified by preparative HPLC in order to guarantee proper functional group stoic hiometry during polymerization As shown in Scheme 4 10, the dibromo (2 ethylhexyl) isoindigo monomer 4 3 was then copolymerized bistrimethylstannyl bis (2 ethylhexyl) dithie no[3,2 b:2',3' d]silole under Stille coupling conditions to afford P(iI DTS) in 94% overall yield after purification. The copolymerizati on was carried out using Pd 2 (dba) 3 as Pd source and P( o ty l) 3 as ligand, in dry degassed toluene at 85 C. Scheme 4 10. Synthesis of P(iI DTS) via Stille cross coupling. a) Pd 2 (dba) 3 P(o tyl) 3 toluene, 85 C, 97%.
180 Before quenching the reaction 2 bromothiophene and 2 trimethyltin thiophene were added in succession in the reaction medium as an attempt to replace undesired backbone chain end groups with thiophene rings. After purification of the polymer in a Soxhlet extracto r using methanol and hexanes, the high molecular weight fraction of P(i I DTS) extracted with chloroform was analyzed using size exclusion chromatography in THF against polystyrene standards. Figure 4 19 TGA thermogram (a) of P(iI DTS) under nitrogen f low; CV and DPV (b) of the polymer film drop cast onto a Pt button electrode recorded in 0.1M TBAPF 6 in ACN; solution (dashed) and solid state (solid) absorption spectra (c) of P(iI DTS) and film absorption of a 1:4 blend of P(iI DTS) :PC 70 BM. The polymer from the chloroform fraction used in the following study has a number average molecular weight of 36.0 kDa and a polydispersity of 2.77, and is soluble in all chlorinated solvents and in THF and toluene. The analyzed elemental composition for
181 C, H and N is within 0.4% of the calculated elemental composition. From the thermog ravimetric analysis (Figure 4 19 .a) performed under nitrogen with a 5% weight loss set as decomposition threshold, the polymer was found to be thermally stable up to 410 C. The electroch emistry of P(iI DTS) is displayed in Figure 4 19 .b. The CV and DPV were performed on thin films drop cast onto Pt button electrodes, using 0.1M TBAPF 6 in acetonitrile as supporting electrolyte In the oxidative CV one reversible oxidation process was obse rved with a half wave potential at 0.69 V. In the oxidative DPV experiment, we recorded an onset of oxidation at 0.45 V. The co reduction processes, with half From the onset of reduction at 1.15 V in the DPV, the calculated LUMO energy level The ele ctrochemical bandgap of 1.60 eV is consistent with the optical bandgap of 1.54 eV. The UV vis absorption spectrum of P(i I DTS) in solution and in t hin films is shown in Figure 4 1 9 .c. With a peak absorption ( max ) at 720 nm and a low energy onset of absorp tion ( onset ) at 805 nm in the solid state, the polymer absorbs strongly in the visible towards the near IR with an optical bandgap of 1.54 eV as calculated from onset At wavelengths less than 550 nm, the absorption decreases peaking at 435 nm with 33% of the intensity of the max at 720 nm. Thin films of P(iI DTS) thus look blue green as they absorb mostly in the red region of the visible spectrum. This absorption gap at wavelengths lower than 550 nm is compensated by the absorption of PC 70 BM, and blend films of P(iI DTS) :PC 70 BM absorb broadly across the entire visible spectrum. The high value of the LUMO at 3.95 eV is closer to that of PC 70 BM than the ~0.3 eV offset recommended for efficient electron transfer, but the extended absorption of
182 the polyme r, and the high ionization potential (deep HOMO) of 5.55 eV were promising for devices with high V oc as the latter is closely related to the offset of the LUMO of the fullerene derivative and the HOMO of the p type polymer in BHJ solar cells. 4.5.3 Polym er /PCBM Solar Cells. conventional cell architecture was first investigated based on ITO/PEDOT:PSS/ P(iI DTS) :PCBM/LiF/Al, with PC 60 BM and PC 70 BM as electron acceptors. After initial device testing, PC 7 0 BM was deemed a better acceptor than PC 60 BM, because of the enhanced photon absorption of the cells under AM1.5 illumination, due to the more extended absorption of PC 70 BM in the visible. Figure 4 20 J V curves of the P(iI DTS) :PC 70 BM (1:4) based BHJ solar cells with and without DIO additive, under AM1.5 solar illumination, in conventional ( circle and triangle lines ) and inverted architecture (square line). AFM images of the P(iI DTS) :PC 70 BM blend at 1: 4 ratio, processed without (a) and with (b) 4% DIO additive (2 m side, 20 nm height scales). TEM images of the aforementioned blend, processed without (c) and with (d) 4% DIO additive (200 nm scale bars). The ratio of P(iI DTS) to PC 70 BM, solvent (chlorof orm (CF) and chlorobenzene (CB)), solution concentration, spin coating speed and annealing conditions were
183 optimized. A donor/acceptor weight ratio of 1:4 in CB at a concentration of 25 mg/mL spun cast at 1000 rpm and annealed at 150C before LiF/Al deposi tion gave the bes t mWcm ) are shown in Figure 4 20.a The use of solvent additives such as octanedithiol or diiodooctane (DIO) has previously been shown to decrease the domain sizes in the BHJ of PSCs. 46,65 Given the morphological limitations stated above, we monitored the effect of two solvent additives (1,8 diiodoo ctane (DIO), chloronaphthalene) on device performance. As shown in the A FM and TEM images in Figure 4 20 .b, the addition o f 4% in volume of DIO in the spin casting solution significantly reduces the features sizes in the active layer. Table 4 3 Solar cell characteristics of the P(iI DTS) :PC 70 BM (1:4) blend. Device processing J sc (mA/cm 2 ) V oc (V) FF (%) PCE (%) Conventional cell without DIO 2.82 0.86 60 1.45 Conventional cell with DIO (4%) 8.26 0.76 42 2.62 Inverted cell with DIO (4%) 10.49 0.77 50 4.01 While the AFM images show a smoother surface morphology, the TEM images reveal an intricate network of donor/acceptor ph ases in the bulk of the film, similar to that observed in previously reported studies, with segregation scales reduced from 0.5 micron without additives to tens of nanometers with 4% DIO. Because of the reduced domain size, excitons are more likely to reac h the P(iI DTS) /PC 70 BM interface and generate charge car riers. As can be seen in Table 4 3 4% DIO additive leads to a three fold increase of the J sc To further improve carrier extraction at the electrodes,
184 devices using the inverted architecture ITO/ZnO/ P(iI DTS) :PC 70 BM(1:4)/MoO 3 /Ag were fabricated, while keeping the processing conditions for the active layer the sam e. As can be seen in Figure 4 20 and Table 4 3 this architecture leads to increased device performance from 2.62% to 4.01%, since as it is l ikely to take better advantage of a vertical phase separation present in the BHJ film. In summary, the copolymer P(iI DTS) of isoindigo and dithienosilole is soluble at high molecular weight when functionalized with 2 ethylhexyl side chains on each conjuga ted unit, and absorbs light up to 800 nm in the solid state. With HOMO and bandgap of 1.60 eV as measured by DPV in the solid state, which correlates well with an optical bandgap of 1.54 eV. Despite its high elect ron affinity (deep LUMO) close to that of fullerene derivatives, P(iI DTS) still enables moderate PCE of 1.45% when blended with PC 70 BM in PSCs processed without solvent additives: the high open circuit voltage of 0.86V is undermined by a low short circuit current. When 4% diiodooctane was used as an additive, a PCE increase to 2.62% was measured, accountable to improved film morphology for charge separation. When the device architecture was modified to enhance carrier extraction at the electrodes, the P(iI DTS) :PC 70 BM cells reached 4% PCE, one of the highest reported for isoindigo based conjugated polymers. 4.6 Synthetic D etails. 6 dibromoisoindigo ( 4 1 ). 158 To a suspension of 6 bromooxindole (500 mg, 2.36 mmol) and 6 bromoisatin (533 mg, 2.36 mmol) in A cOH (15 mL), conc. HCl solution (0.1 mL ) was added and heated under reflux for 24. The mixture was allowed to cool and filtered. The solid material was wash ed with water, EtOH and AcOEt. After dibromoisoindigo (95 1 mg, 95%). 1 H NMR
185 ( (CD 3 ) 2 NCOD), DMF at 80 7.15 (m, 4H). 13 C NMR (CDCl 3 3 147.2 5 134.0 2 132.3 6 127.0 4 125.3 5 122.6 7 113.9 3 dibromo di hexyl isoindigo (4 2). Compound 4 1 (1.0 g, 2 .38 mmol) was add ed in a dry flask, to which was added anhydrous DMF (20 mL). To t he suspension was then added sodium hydride (0.24 g, 5.95 mmol) in one portion while stirring at room temperature. The reaction mixture turns blue green after 5 minutes. This was allowed to stir for 30 minutes at room temperature under a flow of argon, after which n hexylbromide (0.98 g, 5.95 mmol) was added to reaction using a syringe via a septum. The reaction mixture was then stirred at 80 C for 3 hours, upon which it turns bright red. It was then allowed to cool to room temperature, leading to the formation of bright red precipitates. This was poured in cold methanol (50 mL, 60 C), and the precipitates were filtered, to afford the title compound (1.33 g, 2.26 mmol, 95%) as bright red needles. 1 H NMR (CDCl 3 4 (d, J = 9 Hz, 2H), 7.1 4 (dd, J = 9 Hz 1.8 Hz, 2H), 6. 86 (d, J = 1.8 Hz, 2H), 3.71 (t, J = 7.2 Hz, 4H), 1.75 1.60 (m, 4H), 1.40 1.20 (m, 1 2 H), 0.87 (t, J = 6.6 Hz, 6H); 13 C NMR (CDCl 3 4 146.0 1 132.9 1 131.4 3 127.0 6 125.3 7 120.6 0 11 1.5 4 40.5 3 31.9 5 29.2 2 27.6 0 22.8 7 14.3 3 Anal. Calcd for C 28 H 32 B r 2 N 2 O 2 : C, 57.16; H, 5.48; N, 4. 76; Found: C, 57.07 ; H, 5.68 ; N, 4.65 dibromo di (2 ethylhexyl) isoindigo ( 4 3 ). 158 To a suspension of dibromoisoindigo (420 mg 1 mmol) a nd potassium carbonate (829 mg, 5 mmol) in dimethylformaldehyde (DMF) (20 mL), 1 bromo 2 ethylhexane (425 g, 2.2 mmol) was injected through a septum under nitrogen. The mixt ure was stirred for 15 h at 100 C and then poured into water (200 mL). The organic phase was extracted by CH 2 Cl 2 washed
186 with brine and dried over MgSO 4 After removal of the solvent under reduced pressure, the deep red solids were purified by silica chromatography, eluting with (CH 2 Cl 2 : dibromo (2 ethylhe xyl) isoindigo (548 mg, 85 %) 1 H NMR (CDCl 3 d, J = 8.7 Hz, 2H), 7.13 (dd, J = 8.7 1.5 Hz, 2H), 6.81 (d, J = 1.5 Hz, 2H), 3.60 3.48 (m, 4H), 1.90 1.72 (m, 2H), 1.43 1.20 (m, 16H), 0.95 0.82 (m, 12H). 13 C NMR (CDCl 3 3 146.2 4 132.6 6 131.2 2 126.8 9 125.2 0 120.5 6 111.6 6 44.5 6 37.6 3 30.7 5 28.7 2 24.1 5 23.2 1 14.2 5 10.8 7 HRMS (ESI TOF) Calculated for C 32 H 40 Br 2 N 2 O 2 (M+H) + : 645.1512, found: m/z 645.1510. Anal. Calcd for C 32 H 40 B r 2 N 2 O 2 : C, 59.64; H, 6.26; N, 4.35; Found: C, 59.79; H, 6.30; N, 4.26. dibromo di ( 2 hexyld ecyl) dibromoisoindigo (4 4) 201 To a suspension dibromoisoindigo (4.20 g, 10.0 mmol) and potassium carbonate (8.29 g, 60 mmol) in DMF (45 mL), 2 hexyl bromodecane (9.16 g, 30 mmol) was injected through a septum under nitrogen. The mixture was stir red for 15 h at 10 0 C and then poured into water (200 mL). The organic phase was extracted by CH 2 Cl 2 washed with brine and dried over MgSO 4. After removal of the solvent under reduced pressure, the dark red liquids were purified by silica chromatography, eluting with (CH 2 Cl 2: Hexane = 1:2) to dibromo (2 hexyldecyl) isoindigo (6.1 g, 70 %) as dark red solids. 1 H NMR (CDCl 3 = 9 Hz 1.8 Hz, 2H), 6.83 (d, J = 1.8 Hz, 2H), 3.56 (d, J = 7.5 Hz, 4H), 1.83 (bs, 2H), 1.40 1.24 (m, 48H), 0.88 0.84 (m, 12H); 13 C NMR (CDCl 3 ) : 168.2 5 146.3 3 132.6 5 131.3 6 126.9 2 125.2 3 120.6 4 112.7 5 44.9 1 36.3 9 32.1 0 32.0 7 31.7 5 30.2 7 29.87, 29.77, 29.5 9 26.6 3 22.89, 22.87, 14.34, 14.31
187 2 hexyldecyl) pinacoldiboronisoindigo (4 5 ). 201 2 hexyldecyl) dibromoisoindigo (4.35 g, 5.0 mmol), pinacol ester of diboron (3.05 g, 12 mmol ), PdCl 2 (dppf) (220 mg), and potassium acetate (2.95 g, 30 mmol) were mixed at room temperature under an argon atmosphere. Anhydrous 1,4 dioxane (2 mL) was injected with a syringe through a septum. The solution was heated at 80 C for 30 h and then cooled t o room temperature. The reaction mixture was filtered by passing through a short pad of silica gel, and washed by a mixture of methylene chloride and hexane (1:1). The collected filtration was concentrated and precipitated into cold methanol (100 mL). The precipitates was filtered and dried to give a dark red shiny powder (3.6 g, 75%). 1 H NMR (CDCl 3 ) : 9.15 (d, J = 7.2 Hz, 2H), 7.48 (d, J = 8.1 Hz 0.6 Hz, 2H), 7.15 (d, J = 0.6 Hz, 2H), 3.69 (d, J = 7.5 Hz, 2H), 1.95 (bs, 2H), 1.59 1.19 (m, 72H), 0.85 (t, J = 6.6 Hz, 6H); 13 C NMR (CDCl 3 ) : 168.3 5 144.7 4 134.5 1 129.0 0 128.9 6 124.4 7 113.7 3 84.2 5 44.6 2 36.3 1 32.1 6 32.0 7 31.8 3 31.2 6 29.8 4 29.78, 29.5 1 26.6 5 25.1 0 22.9 0 22.8 8 14.32, 14.30. HRMS (MALDI TOF) Calculated for C 60 H 96 B 2 N 2 O 6 (M+Na) + : 963.7548 Found: m/z 963.7583. Anal. Calcd for C 60 H 96 B 2 N 2 O 6 : C, 74.83; H,10.05; N,2.91. Found: C 74.91; H,10.75; N, 2.80. 6,6' diphenyl di hexyl isoindigo (P iI P). In a purged Schlenk flask, compound 4 2 (294 mg, 0.5 mmol), phenylboron pinacol ester (265 mg, 1.3 mmol), Pd 2 (dba) 3 (18 mg, chloroform adduct) and P( o tyl) 3 (15 mg) were loaded under a flux of argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. Degassed toluene (5 mL) was then added to flask, followed by a degassed aqueous solution o f t etraethylammonium hydroxide (1 mL, 1 mmol). The mixture was stirred and heated to
188 90 C for 12 hours. After cooling back to room temperature, the mixture was slowly poured in methanol (4 0 mL) and the precipitates were collected by filtration. The solids were purified by column chromatography using 2:1 hexanes:dichloromethane as eluent. This afforded the title compound (214 mg, 0. 37 mmol, 74 %) as a brown solid 1 H NMR (CDCl 3 ) : 9.26 (d, J = 8.3 Hz, 1H), 7.65 (d, J = 7.3 Hz, 2H), 7.49 (t, J = 7.4 Hz, 2H), 7.41 (t, J = 7.3 Hz, 1H), 7.28 (dd, J =4.2, 1.4 Hz, 1H), 6.98 (s, 1H), 3.83 (t, J = 7.4 Hz, 2H), 1.75 (quintet, J = 7.4 Hz, 2H), 1.44 (quintet, J = 7.4 Hz, 2H), 1.34 (m, 4H), 0.90 (t, J = 7.0 Hz, 3H). 13 C NMR (CDCl 3 ) : 168.41, 145.51, 145.30, 140.78, 132. 79, 130.50, 129.10, 128.37, 127.28, 121.14, 121.09, 106.59, 40.31, 31.74, 27.77, 26.96, 22.77, 14.24 Anal. Calcd for C 40 H 42 N 2 O 2 : C, 82.44 ; H, 7.26 ; N, 4. 81 Found: C, 81.97 ; H, 7.98 ; N, 4.72 6,6' di thiophene di hexyl isoindigo (T iI T) In a dry Schle nk flask, compound 4 2 (294 mg, 0.5 mmol), Pd 2 (dba) 3 (18 mg, chloro form adduct) and P( o tyl) 3 (15 mg) were loaded under a flux of argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. T ributyl(thiophen 2 yl)stannane (485 mg, 1.3 mmol) was then added to the flask via syringe through a septum. Degassed toluene (5 mL) was then added to flask. The mixture was stirred and heated to 90 C for 12 hours. After coolin g back to room temperature, the mixture was slowly poured in methanol (30 mL) and the precipitates were collected by filtration. The solids were dissolved in a minimum of a dichloromethane, and purified by column chromatography using 2:1 hexanes:dichlorome thane as eluent. This afforded the title compound (280 mg, 0.47 mmol, 94%) as a dark dark brown solid. 1 H NMR (CDCl 3 ) : 9.17 (d, J = 8.3 Hz, 1H),
189 7.42 (dd, J = 1.7, 0.9 Hz, 1H), 7.36 (dd, J = 2.5, 0.9 Hz, 1H), 7.28 (dd, J = 4.2, 1.4 Hz, 1H), 7.12 (dd, J = 3.2, 3.7 Hz, 1H), 6.95 (s, 1H), 3.80 (t, J = 7.3 Hz, 2H), 1.72 (quintet, J = 7.3 Hz, 2H), 1.42 (m, 2H), 1.34 (m, 4H), 0.89 (t, J = 7.0 Hz, 3H). 13 C NMR (CDCl 3 ) : 168.40, 145.50, 144.26, 138.06, 132.20, 130.64, 128.50, 126.29 1 24.53, 121.26 119.61, 105. 01, 53.61, 40.26, 31.69, 27.71, 26.92, 22.76, 14.24. Anal. Calcd for C 36 H 38 N 2 O 2 S 2 : C, 72.69 ; H, 6.44 ; N, 4.71 Found: C, 72.02 ; H, 6.96 ; N, 4.58 6 ,6' di (3,4 ethylenedioxythiophene) di hexyl isoindigo (E iI E) In a dry Schlenk flask, compound 4 2 ( 1 00 g, 1.7 mmol), Pd 2 (dba) 3 (0.07 g, 80 mol), P(o tolyl) 3 (0.05 g, 0.16 mmol), EDOT SnMe 3 (1.14 g, 3.7 mmol) were loaded under a flux of argon and then kept under vacuum for 30 minutes, during which the flask was subjected to three vacuum argon purge cycles, and finally refilled with argon. The homogeneous atmosphere. The formed suspension was then evaporated to dryness, redissolved in chloroform and dry packed over silica gel. This was purified by column chromatograph y Product containing fractions were combined and evaporated to dryness. The residual 50 mL). Aft er drying under vacuum in a dessicator overnight, brown powder was obtained (87% yield). m.p. > 200 C; 1 H NMR (CDCl 3 5 00 MHz) (ppm) 9. (d, J = 8 5 Hz, 2H), 7. 32 36 (dd, J 1 = 1 5 Hz, J 2 = 8 5 Hz, 2H), 7.21 7.22 (d, J = 1 5 Hz, 2H), 6.38 (s, 2H) 4.26 4.37 (m, 8H), 3. 78 83 (t, J = 7 5 Hz, 4H), 1. 70 7 (quintet, J = 7.5 Hz, 4H), 1. 30 5 ( m 12 H ), 0. (t, J = 7.0 Hz, 6H) ; 13 C NMR (CDCl 3 125 MHz) (ppm) 14.3 6 22.8 3 27.0 7 27.7 3 31.7 1 40.2 1 64.6 9 65.2 6 99.5 7 105.3 4
190 118.0 6 119. 4 3 120.5 6 130.2 2 132.0 7 137.0 9 139.8 0 140.0 3 142.7 1 145.3 6 168.7 3 HRMS (EI DIP) [M+H] + 711.2557, calcd for [C 40 H 43 N 2 O 6 S 2 ]: 711.2563 General procedure for Suzuki polycondensations : (P (iI F) P (iI OB) and P (iI AT) ). In a 100 mL flame dried Schlenk flask, the dibromoisoindigo monomer 4 3 (0.5 mmol, 1 equiv.), the bis(pinacolato)diboron comonomer (0.5 mmol, 1 equiv.), Pd 2 (dba) 3 (15 mg) and P( o tyl) 3 (10 mg) were subjected to three cycles of evacuation/argon purging, and then dissolved with 5 mL of deg assed toluene after which 1.5 mL (1M) of degassed aqueous solution of Et 4 NOH was added. The reaction mixture was stirred at 85 C for 36 hours under argon, and then cooled down to room temperature. The mixture was precipitated in 100 mL of methanol and filt ered through a 0.45 m nylon filter. The dark solids were purified using a Soxhlet apparatus with methanol until the extracts appeared colorless. The polymers were then fractionated in the Soxhlet apparatus using hexanes and chloroform fractions which contained varying amount s of polymer after complete extraction depending on the nature of the comonomer used. Concentration and reprecipitation in methanol allowed filtering the solids through a 0.45 m nylon filter to afford the targeted polymer after complete drying in vacuo. P dihexyl 2,7 fluorene alt (2 ethylhexyl) isoindigo] P(iI F ) : The general Suzuki polymerization procedure was followed using 322 mg of compound 4 3 and 293 mg of 2,2' (9,9 dihexyl 9H fluorene 2,7 diyl)bis(4,4,5,5 tetramethyl 1,3,2 dioxabo rolane) to afford a shiny brown solid. (466 mg, yield: 93%, chloroform fraction). 1 H NMR (300 MHz, CDCl 3 ppm): 9.40 9.30 (br, 2H), 7.90 7.80 (br, 2H), 7.70 7.60 (br, 4H), 7.45 7.35 (br, 2H), 7.10 7.05 (br, 2H), 4.00 3.50 (br, 4H), 2.15 1.90 (br, 2H), 1.65 1.30 (br, 16H), 1.10 1.05 (br, 20H), 1.05 0.95 (br, 6H), 0.95 0.80 (br, 6H), 0.80 0.70 (br,
191 6H). M n : 21.2 kDa ; PDI: 1.96. Anal. Calcd for C 57 H 72 N 2 O 2 : C, 83.78; H, 8.88; N, 3.43. Found: C, 82,72; H, 8.84; N, 3.42. Poly[2,5 dioctyloxy 1,4 phenyl alt (2 ethylhexyl) isoindigo] P(iI OB) : The general Suzuki polymerization procedure was followed using 322 mg of compound 4 3 and 265 mg of 2,2' (2,5 bis(octyloxy) 1,4 phenylene)bis(4,4,5,5 tetramethyl 1,3,2 dioxaborolane) to afford a dark brown fluffy sol id. (116 mg, yield: 31%, hexanes fraction); (241 mg, yield: 63%, chloroform fraction). 1 H NMR (300 MHz, CDCl 3 ppm): 9.30 9.20 (br, 2H), 7.35 7.25 (br, 2H), 7.20 7.15 (br, 2H), 7.10 7.05 (br, 2H), 4.05 3.90 (br, 4H), 3.85 3.65 (br, 4H), 1.95 1.85 (br, 2H), 1.85 1.65 (br, 4H), 1.40 1.20 (br, 32H), 1.00 0.75 (br, 18H). M n : 10.5 kDa ; PDI: 2.13. Anal. Calcd for C 50 H 68 N 2 O 4 : C, 78.91; H, 9.01; N, 3.68. Found: C, 78.97; H, 9.01; N, 3.57. Poly[3,4 dioctyl 2,5 thiophene alt (2 ethylhexyl) isoindigo] P(iI AT) : The general Suzuki polymerization procedure was followed using 322 mg of compound 4 3 and 280 mg of 2,2' (3,4 dioctylthiophene 2,5 diyl)bis(4,4,5,5 tetramethyl 1,3,2 dioxaborolane) to afford a dark brown fluffy powder. (109 mg, yield: 28%, hexanes fra ction); (224 mg, yield: 57%, chloroform fraction). 1 H NMR (300 MHz, CDCl 3 ppm): 9.35 8.90 (br, 2H), 7.40 7.10 (br, 2H), 7.10 6.75 (br, 2H), 3.90 3.45 (br, 4H), 3.05 2.50 (br, 4H), 2.05 1.80 (br, 2H), 1.80 1.15 (br, 40H), 1.15 0.75 (br, 12H), 0.75 0.45 (br 6H). M n : 16.7 kDa ; PDI: 1.94. Anal. Calcd for C 52 H 74 N 2 O 2 S: C, 78.94; H, 9.43; N, 3.54. Found: C, 79.26; H, 9.45; N, 3.58. General procedu re for Stille polycondensations: P(iI T) EH, P(iI T) HD and P(iI ProDOT)). The dibromoisoindigo monomer 4 3 (0.5 mmol 1 equiv.), Pd 2 (dba) 3 (15 mg) and P( o tyl) 3 (10 mg) were added to a flame dried Schlenk flask which was
192 evacuated and backfilled with argon three times. The bis(trimethylstannyl) comonomer (0.5 mmol, 1 equiv.) was dissolved in a dried se parate vial in 5 m L of toluene subsequently degassed with argon for one hour. The solution was then added to the Schlenk flask and the rea ction mixture was stirred at 85 C for 36 hours. The mixture was precipitated in 100 mL of methanol and filtered through a 0.45 m nylon filter. The dark solids were purified using a Soxhlet apparatus with methanol until the extracts appeared colorless. The polymers were then fractionated in the Soxhlet apparatus using hexanes and chloroform fractions which contained varying amounts of pol ymer after complete extraction depending on the the nature of comonomer used. Concentration and reprecipitation in methanol allowed filtering the solids through a 0.45 m nylon filter to afford the targeted polymer after complete drying in vacuo. Poly[2,5 thiophene alt (2 ethylhexyl) isoindigo] P(iI T) EH : The general procedure for Stille polymerization was followed using 322 mg of compound 4 3 and 205 mg of 2 ,5 bis(trimethylstannyl)thiophene to afford a deep blue solid after extraction. (120 mg, yield: 42%, chloroform fraction) 1 H NMR (300 MHz, CDCl 3 ppm): 9.20 8.90 (br, 2H), 7.30 7.15 (br, 2H), 7.15 6.90 (br, 2H), 6.75 6.60 (br, 2H), 3.75 3.45 (br, 4H), 1.95 1.70 (br, 2H), 1.55 1.20 (br, 16H), 1.20 0.80 (br, 12H). M n : 2.4 kDa ; PDI: 1.13. Anal. Calcd for C 36 H 42 N 2 O 2 S: C, 76.29; H, 7.47; N, 4.94. Found: C, 75.88; H, 7.31; N, 4.75. Poly[2,5 thiophene alt (2 ethylhexyl) isoindigo] P(iI T) HD : 198 The gen eral procedure for Stille polymerization was followed using 424 mg of compound 4 4 and 205 mg of 2,5 bis(trimethylstannyl)thiophene to afford a dark blue solid after extraction. ( 2 9 4 mg, yield: 74 %, chloroform fraction) 1 H NMR (300 MHz, CDCl 3 ppm):
193 9.20 8 .90 (br, 2H), 7.30 7.15 (br, 2H), 7.15 6.90 (br, 2H), 6.75 6.60 (br, 2H), 3.75 3.45 ( br, 4H), 1.95 1.70 (br, 2H), 1.60 0.65 (m, 56H) M n : 19.9 kDa ; PDI: 2.5 Poly[2,5 propylenedioxythiophene alt (2 ethylhexyl) isoindigo] P (iI ProDOT) : The gener al procedure for Stille polymerization was followed using 322 mg of compound 4 3 and 383 mg of (3,3 bis(((2 ethylhexyl)oxy)methyl) 3,4 dihydro 2H thieno[3,4 b][1,4]dioxepine 6,8 diyl)bis(trimethylstannane) to afford a dark blue solid. (290 mg, yield: 63%, hexanes fraction); (161 mg, yield: 34%, chloroform fraction). 1 H NMR (300 MHz, CDCl 3 ppm): 9.20 9.05 (br, 2H), 7.45 7.35 (br, 2H), 7.35 7.25 (br, 2H), 4.30 4.15 (br, 4H), 3.65 3.50 (br, 4H), 3.55 3.45 (br, 4H), 3.45 3.25 (br, 4H), 1.95 1.75 (br, 4H), 1.50 1.20 (br, 32H), 1.05 0.80 (br, 24H). M n : 19.3 kDa; PDI: 1.70. Anal. Calcd for C 57 H 82 N 2 O 6 S: C, 74.15; H, 8.95; N, 3.03. Found: C, 73.65; H, 8.65; N, 3.10. P oly[ (2 hexyldecyl ) isoindigo ], Poly(i I) In a flame dried Schlenk flask (50 mL), 4 4 (434. 45 mg, 0.5 mmol) and 4 5 (481.52 mg, 0.5 mmol), Pd 2 (dba) 3 CHCl 3 (18 mg) and P(o tyl) 3 (12 mg) were added. The flask was evacuated and back filled with argon three times, after which degassed toluene (15 mL) and tetraethylammonium hydroxide (3 mmol, 1M) was transferred to the mixture through a septum. The resulti ng solution was heated up to 85 C under argon and stirred for 36 h. The mixture was cooled to room temperature and poured slowly in methanol (300 mL). PTFE filter. The crude polymer (711 mg of dark solids) was purified with Soxhlet extraction with methanol, hexane to remove low molecular species and catalyst residues. From the hexanes fraction, 150 mg of lower molecular weight species were collected. La st, the higher molecular weight fraction was extracted with chloroform, to which diethylammonium
194 diethyldithiocarbamate (as palladium scavenger, ~30 mg) was subsequently added in one portion. The latter mixture was then stirred for two hours at room temper ature, and then poured slowly in methanol (300mL). The precipitates were filtered and dissolved in a minimum amount of chloroform, and precipitated a second time in methanol (100mL). TFE filter and dried, yielding dark blue solids (523 mg, 74%). 1 H NMR (CDCl 3 ) : 8.98 8.70 (M, 2H), 7.40 6.80 (m, 4H), 3.60 3.25 (m, 4H), 2.20 0.60 (m, 62H). GPC: M n = 28.7 kDa, PDI = 2.4. Anal. Calcd for C 48 H 72 N 2 O 2 : C, 81.30; H,10.23; N,3.95. Found: C, 80 .47; H, 10.52; N, 3.84. Poly[ 2,1,3 benzothiadiazole alt (2 ethylhexyl) isoindigo] Poly(iI BTD) In a flame dried Schlenk flask (50 mL), 4,7 dibromobenzothiadiazole (73.49 mg, 0.25 mmol) and 4 5 (240.76 mg 0.25 mmol), Pd 2 (dba) 3 CHCl 3 (9 mg) and P (o tyl) 3 (6 mg) were added. The flask was evacuated and back filled with argon three times, after which degassed toluene (15 mL) and tetraethylammonium hydroxide (2 mmol, 1M) was transferred to the mixture through a septum. The resulting solution was heated up to 85 C under argon and stirred for 36 h. The mixture was cooled to room temperatur e and PTFE filter. The crude polymer (220 mg of dark solids) was purified with Soxhlet extraction with methanol to remove catalyst residues. The polymer was extracted wit h chloroform, to which fraction diethylammonium diethyldithiocarbamate (as palladium scavenger, ~20 mg) was added in one portion. The mixture was stirred for two hours at room temperature, and then poured slowly in methanol (300mL). The precipitates were f iltered and dissolved in a minimum amount of chloroform, and precipitated a second
195 time in methanol (100mL). The precipitates were collected via vacuum filtration through 1 H NMR (CDC l 3 ) : 9.42 8.95 (m, 2H), 7.78 5.98 (m, 8H), 3.60 3.20 (m, 4H), 2.20 0.45 (m, 62H). GPC: M n = 16.3 kDa, PDI = 3.5. Calcd for C 54 H 74 N 4 O 2 S: C, 76.91; H,8.85; N,6.64. Found: C, 76.55; H, 8.86; N, 6.30. Poly[ bis (2 ethylhexyl) dithieno[3,2 b:2',3' d] silole) alt (2 ethylhexyl) isoindigo ] P(iI DTS). The faint yellow oil bistrimethylstannyl bis (2 ethylhexyl) dithieno[3,2 b:2',3' d]silole ref was added in a tared clean glass vial, which was kept under vacuum. The oil, weighing 381.6 m g (0.510 mmol) once dry, was then diluted in 1 mL of hexanes and transferred to a dry Schlenk flask equipped with a stir bar. The dilution/transfer of 2 was repeated four times so that the compound was completely transferred to the reaction Schlenk flask, as a total of 5 mL of hexanes solution. The hexanes were then evaporated carefully under vacuum for 2 hours at dibromo (2 ethylhexyl) isoindigo (monomer 4 3 330.4 mg, 0.510 mmol), Pd 2 (dba) 3 (chloroform adduct, 15 mg, 0.015 mmol) and P( o tyl) 3 (10 mg, 0.03 mmol) were added to the flask and purged with 3 vacuum/argon refill cycles. Degassed toluene (5 mL, 5 freeze pump thaw cycles prior to addition) was then added to the flask and the reaction medium was stirred and heated to 85 C under argon. The reaction medium viscosity had notably increased after 12 hours at 85 C, and was left to stir for an additional 2 days at 85 C after which the temperature was increased to 100 C for 2 hours. A fter the reaction medium was cooled back to 85 C, a solution of 2 bromothiophene (0.2 mmol) in degassed toluene (2 mL) was added to the flask along with a small amount of the
196 catalytic system and allowed to react for 6 hours. A solution of 2 (tributylstann yl) thiophene (0.2 mmol) in degassed toluene (2 mL) was subsequently added to the flask and allowed to react for 12 hours. At this point, the reaction medium was then cooled to 60 C and a spatula tip of diethylammonium diethyldithiocarbamate was added to t he flask. After 2 hours of stirring at 60 C, the reaction medium was precipitated into methanol (200 mL). The precipitates were filtered and collected into a cellulose thimble, then purified in a Soxhlet apparatus using methanol (1 day), hexanes (12 hours) and chloroform (2 hours). The methanol and hexanes fractions were discarded, while the chloroform fraction was precipitated into methanol (200 mL). The precipitates were collected and dried to afford 445 mg of dark purple solids (0.494, 97%). GPC: Mn = 36 ,000 kDa; Mw = 99,600; PDI = 2.77. 1 H NMR (CDCl 3 8.8 ppm (br, 2H), 7.8 7.0 (br, 4H), 6.8 6.2 (br, 2H), 4.0 3.6 (br, 4H), 2.0 1.0 (br, 40H), 1.0 0.6 (br, 24H). Elemental Analysis Calc. for C 56 H 76 O 2 S 2 Si: C, 74.61; H, 8.50; N, 3.11. Found: C, 74.22; H, 8.61; N, 3.02.
197 CHAPTER 5 CONCLUSIONS AND PER SPECTIVES Over the past 30 years of research on conjugated systems for organic electronics, several parameters have stood out as key to making a conjugated organic material part of efficient opto electronic devices. This dissertation has illustrated how a synthetic chemist might contribute to the field, first by designing compounds which provide some degree of morphological control in the devices. Chapter 3 introduced three of such approaches. In the first one, a linear alternating aliphatic/chromophoric p olymer was shown to retain electroactivity despite the inherent conjugation break in its backbone. It was observed to adopt some degree of ordering in the bulk, which could be further induced by mechanical stress. Another approach consisted in covalently b inding organic conjugated oligomers onto inorganic nanocrystals to create electro active hybrid materials This was achieved thanks to the design of unsymmetrical oligomers bearing one reactive end group. Their interaction with the nanocrystals was demonst rated, which led to the synthesis of the organic/inorganic complexes where a significant amount of oligomers were bound to the inorganic nanocrystals surface. One last yer. This was enabled via the synthesis of symmetrical and unsymmetrical oligomers based on the same chromophore, which were shown to have different solid state optical properties and morphological behavior, depending on the molecular structure. The use of an unsymmetrical oligomer as an additive to molecular solar cells was beneficial to the morphology of the active layer, leading to increased efficiencies. With the development of isoindigo based materials, access to conjugated oligomers and polymers with deep energy levels and low bandgaps was presented in
198 Chapter 4. The synthesis of donor acceptor polymers showed the ability of isoindigo as an acceptor to extend the absorption of the materials towards the near IR. Conjugated polymers with all acceptor bac kbones were also presented, and spectroelectrochemical measurements gave insight on their n type character. The homopolymer of isoindigo was used as an n type material in all polymer solar cells, with encouraging results. Efficient polymer/PCBM solar cells were also shown to be possible with isoindigo based donor acceptor polymers : the copolymer of isoindigo and dithienosilole demonstrated solar cell efficiencies up 4% when used with heterojunction with PC 70 BM. As a chemist, I believe that organic electroni cs have a bright future. The wide variety of synthetic tools available to tune the properties of the organic materials it relies on is far from being depleted. It is also a research field which brings together chemists, materials scientists and physicists in such an enthused and competitive way that it can only move forward faster and faster. Such collaborations can help to pinpoint the critical enhancement to be carried out, which sometimes leads to great success. 46 They also lead to new, simple ideas. And sometimes, simple works. 203 One should not forget the main strength of organic electronics: their low cost. Parallel to the understanding of the materials and devices fundamental properties, the field is now also moving toward taking the technology to the production scale, 9 which is a good indication of its success.
199 APPENDIX CRYSTALLOGRAPHIC DAT A Table A 1. Crystal data and structure refinement for T6 benzoate 3 9 Empirical formula C62 H70 O4 S6 Formula weight 1071.54 Temperature 173(2) K Wavelength 0.71073 Crystal system Triclinic Space group P 1 Unit cell dimensions a = 17.059(3) = 98.546(4). b = 17.200(3) = 95.279(3). c = 22.169(4) = 116.120(4). Volume 5684.1(15) 3 Z 4 Density (calculated) 1.252 Mg/m 3 Absorption coefficient 0.2 87 mm 1 F(000) 2280 Crystal size 0.17 x 0.11 x 0.05 mm 3 Theta range for data collection 1.35 to 22.50. Index ranges Reflections collected 26182 Independent reflections 14855 [R(int) = 0.0846] Completeness to theta = 22.50 100 .0 % Absorption correction Integration Max. and min. transmission 0.9858 and 0.9528 Refinement method Full matrix least squares on F 2 Data / restraints / parameters 14855 / 15 / 1280 Goodness of fit on F 2 1.025 Final R indices [I>2sigma(I)] R1 = 0.0903, w R2 = 0.1578  R indices (all data) R1 = 0.1928, wR2 = 0.1970 Largest diff. peak and hole 0.392 and 0.474 e. 3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p) 2 +n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants.
200 Table A 2. Crystal data and structure refinement for T iI T Empirical formula C36 H38 N2 O2 S2 Formula weight 594.80 Temperature 100(2) K Wavelength 0.71073 Crystal system Monoclinic Space group C2/c Unit cell dimensions a = 23.7346(5) = 90. b = 9.4569(2) = 123.740(1). c = 15.8426(4) = 90. Volume 2957.02(12) 3 Z 4 Density (calculated) 1.336 Mg/m 3 Absorption coefficient 0.217 mm 1 F(000) 1264 Crystal size 0.31 x 0.16 x 0.04 mm 3 Theta range for data collection 2.06 to 27.50. Index ranges Reflections collected 16335 Independent refle ctions 3400 [R(int) = 0.0514] Completeness to theta = 27.50 100.0 % Absorption correction Numerical Max. and min. transmission 0.9918 and 0.9349 Refinement method Full matrix least squares on F 2 Data / restraints / parameters 3400 / 0 / 195 Goodness of f it on F 2 1.077 Final R indices [I>2sigma(I)] R1 = 0.0353, wR2 = 0.0928  R indices (all data) R1 = 0.0481, wR2 = 0.0987 Largest diff. peak and hole 0.366 and 0.271 e. 3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p) 2 +n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants
201 Table A 3. Crystal data and structure refinement for P iI P. Empirical formula C40 H42 N2 O2 Formula weight 582.76 Temperature 100(2) K Wavelength 0.71073 Crystal system Triclinic Space group P 1 Unit cell dimensions a = 7.9552(3) = 69.237(2). b = 10.0864(4) = 77.021(2). c = 10.7629(4) = 74.620(2). Volume 770. 39(5) 3 Z 1 Density (calculated) 1.256 Mg/m 3 Absorption coefficient 0.077 mm 1 F(000) 312 Crystal size 0.27 x 0.25 x 0.19 mm 3 Theta range for data collection 2.05 to 27.50. Index ranges Reflections collected 18073 Independent reflections 3547 [R(int) = 0.0438] Completeness to theta = 27.50 100.0 % Absorption correction Numerical Max. and min. transmission 0.9853 and 0.9796 Refinement method Full matrix least squares on F 2 Data / restraints / parameters 3547 / 0 / 200 Goodnes s of fit on F 2 1.063 Final R indices [I>2sigma(I)] R1 = 0.0348, wR2 = 0.0915  R indices (all data) R1 = 0.0403, wR2 = 0.0956 Largest diff. peak and hole 0.345 and 0.186 e. 3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p) 2 +n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants.
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214 BIOGRAPHI CAL SKETCH Romain Stalder was born in April of 1984, in Saint Avold, France. His parents, both teachers, were hired by the Michelin tire company in 1986 to become expatriate teachers for be came an expatriate at the age of two, living for two years in Campo Grande, Brazil; followed by two years in Pusan, South Korea; five years in Pattaya, Thailand; and four years in Ashikaga, Japan. When he was fourteen years old, his family settled back to France, where he was able to (re)discover his native country and begin high school in the Lycee Berthollet of Annecy. After obtaining his baccalaureat in science, Romain remained in Annecy to study advanced topics in maths, physics and chemistry in the cla sses preparatoires of the Lycee Berthollet. The Councours aux Grandes Ecoles gave him access to the Graduate School of Physics and Chemistry of Bordeaux (ENSCBP), France. He spent two years in Bordeaux learning the principles of chemical engineering, for w hich he obtained his Masters in Chemical Engineering. He then joined the chemistry graduate school at the University of Florida (UF) in Gainesville, Florida. At UF Romain joined the group of Professor John Reynolds, working in the area of organic chemistry synthesizing conjugated molecules and polymers for charge transport and organic solar cell applications, which resulted in the present dissertation. Romain received his Ph D from the University of Florida in the spring of 2012.