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1 UNDERSTANDING INTRA AND INTERMOLECULAR INTERACTIONS IN THIOPHENE CONTAINING SYSTEMS By DWANLEEN ERIC SHEN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREME NTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011
2 2011 Dwanleen Eric Shen
3 To all those who shared this journey with me their patience, their support their enduring love
4 ACKNOWLEDGMENTS In the long list of people I would like to thank for making this work possible, I must begin by thanking Professor Reynolds for being the best graduate advisor I could have had. Graduate school, it seems for a number of us on the polymer floor, was an unusually trying period in life in more ways than one. In our group, Professor Reynolds was not only a wonderful advisor on the academic side actively involved in the projects, the progress and the growth of my capabilities but he was equally dedicated to the people that we were a ctively concerned with our problems, and truly happy for our successes. It was the balance I needed throughout my many years here, and I thank him for his guidance, and his patience, his kindness and his understanding. I would like to thank the people tha t worked most closely with me on the PheDOT supercapacitor project David Liu and Laura Moody. David was actively involved in the synthesis, the device fabrication and the characterization aspects of the project, and was the first (and only) graduate stu dent I spent a great deal of time training when he first arrived. He has been a good friend over the years and especially during the dissertation writing period, of which we were both concurrently embroiled in the midst. He was also a wonderful friend to my pets for whom he was kind enough to take care of on a number of occasions, usually involving my oven mitts for handling purposes. Laura was active on the device testing side. Her cheery personality made the daily grind of PheDOT chemistry all the mor e bearable. One particular work related trip with flaming car on the side of the h ighway.
5 extend a special thanks (in alphabetical order) to Frank Arroyave, Andy Chilton, Aubrey Dyer, and Egle Puodziukynaite. Frank has been a wonderful labmate, especially duri ng those odd hours of the night and on weekends, when it seemed we were the only two we were listening to Bob Marley, sharing Colombian coffee candy, and overall makin g those crazy hours a little less crazy. Andy thank you for the last minute AFMs you helped gather, even if you and I will probably be the only ones to ever see those images. Aubrey and Egle have been the electrochemistry and photophysics gurus, respec tively, of this group whom I have turned to on numerous occasions for help as well as for good conversations. Without them, my spectra would be noisier and my life less colorful. All of you and all the Reynolds group members past and present have just be about 7 years ago. professors Alex Angerhofer, Ronald Castellano, Lisa McElwee White, Andrew Rinzler, and Adrian Roitberg for their tutelage and support through these many years. Much appreciation to Cheryl Googins and Sara Klossner who have not only been indispensible behind the scenes keeping the many cogs and gears of the labs running smoothly, but also for being wonderful sources of Larry Westra with whom I had a number of interactions as Sisler 309 was an old lab that fell apart at will, and they were more than kind enough to do what they could to prop up the old bones of the lab. Bob Johnson and Lisa Moscato down in the stock room
6 thanks for all your help, for the friendliness that you always greeted me with, and for putting up with my tendency to pay for my argon tanks after I had already ta ken them. And last but by no means least I must thank Lori Clark and Ben Smith for all their patience and understanding during my many trips to their offices. Thank you to everybody for all your support, without which I would never have been able to comp lete this journey.
7 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ .......... 10 LIST OF FIGURES ................................ ................................ ................................ ........ 11 LIST OF ABBREVIATIONS ................................ ................................ ........................... 15 ABSTRACT ................................ ................................ ................................ ................... 16 CHAPTER 1 THE IMPORTANCE OF INTRA AND I NTERMOLECULAR ORDERING AND CONTROL IN CONJUGATED SYSTEMS ................................ .............................. 19 1.1 Introduction to Organic Semiconducting Materials ................................ ............ 19 1.1.1 Modeling Int ermolecular Charge Transport Using Marcus Theory .......... 20 1.1.2 Modeling Charge Transport in Bulk Systems ................................ .......... 24 1.2 Effects of Reorganization E nergy and Electronic Coupling Matrix on Electronic Properties ................................ ................................ ........................... 24 1.2.1 Reorganization Energy ................................ ................................ ............ 24 1.2.2 Obtaining Reorganization Energy Values ................................ ................ 27 1.2.3 Electronic Coupling Matrix ................................ ................................ ....... 29 1.2.4 Obtaining Electronic Coupling Matrix and Bandwidths ............................ 35 1.2.5 Dependence on Oligomer Length ................................ ............................ 36 1.2.6 Experimental Proof of Theoretical Concepts ................................ ........... 38 1.3 Synth etic Strategies to Tune Intra and Intermolecular Interactions ................... 41 1.3.1 Synthetic Strategies Towards Increasing Intramolecular Order ............... 41 1.3.2 Synthetic Strategies Towards Decreasing Intramolecular Order ............. 42 1.3.3 Synthetic Strategies Towards Maximizing Electronic Coupling Matrix ..... 44 1.3.4 Strategies Towards Decreasing Intermolecular Order ............................. 47 1.4 Applications of Intra and Intermolecular Ordering in This Dissertation ............. 48 2 EXPERIMENTAL TECHNIQUES ................................ ................................ ............ 49 2.1 Molecular Characterization ................................ ................................ ............... 49 2.1.1 Reagents and General Synthetic Methods ................................ .............. 49 2.1.2 Molecular Characterization ................................ ................................ ...... 49 2.1.3 Thermal Characterization ................................ ................................ ........ 50 2.2 Elect rochemical Characterization ................................ ................................ ..... 50 2.2.1 Reagents and Instrumentation for Electrochemical Characterization ...... 50 2.2.2 Cyclic Voltammetry ................................ ................................ .................. 51 2.2.3 Differential Pulse Voltammetry ................................ ................................ 54 2.2.4 Evaluation of Film Capacitance ................................ ............................... 55
8 2.2.5 Potential Square Wave Chronoabsorptometry ................................ ........ 59 2.3 Absorbance and Emission Spectroscopy ................................ .......................... 61 2.3.1 Reagents and Instrumentati on for Spectroscopy ................................ ..... 61 2.3.2 Evaluation of Intramolecular Organization from Absorbance and Emission Spectroscopy ................................ ................................ ................. 61 2.3.3 Evaluation o f Intermolecular Organization from Absorbance and Emission Spectroscopy ................................ ................................ ................. 66 3 WELL ORDERED PHEDOT OLIGOMERS AND POLYMERS ............................... 70 3.1 Motivat ion to Develop PheDOTs ................................ ................................ ....... 70 3.2 PheDOT Monomers ................................ ................................ .......................... 73 3.2.1 Synthesis of PheDOT Monomers ................................ ............................ 73 3.2.2 The Electrochemical Polymerization of PheDOT Monomers ................... 78 3.3 BiPheDOT Dimers ................................ ................................ ............................ 91 3.3.1 Synthesis of BiPheDOTs ................................ ................................ ......... 91 3.3.2 Electrochemical Polymerization of BiPheDOT Dimers ............................ 93 3.4 Formation of Conducting Films of Electropolymerized PPheDOTs ................... 9 7 3.5 Charge Storage Properties of PheDOT Systems ................................ .............. 99 3.5.1 Systems Based on Electrochemically Polymerized PheDOT Bu 2 ........... 99 3.5.2 Systems Based on Electrochemically Polymerized BiPheDOTs ........... 104 3.5.3 Devices Based on Electrochemically Polymerized BiPheDOT .............. 107 3.6 Conclusion ................................ ................................ ................................ ...... 110 3.7 Synthetic Details ................................ ................................ ............................. 111 4 THIOPHENE PHEDOT OLIGOMERS AND POLYMERS ................................ ..... 122 4.1 Brief History of Thiophene Oligomers and Polymers ................................ ...... 122 4.2 Synthesis and Molecular Characterization of Thiophene PheDOT Oligomers 124 4.2.1 Synthesis of Thiophene PheDOT Oligomers ................................ ......... 124 4.2.2 Crystal Structures of Thiophene PheDOT Oligomers ............................ 127 4.3 Thermal Properties of Thiophene PheDOT Oligomers ................................ ... 131 4.3.1 TGA Thermograms of Thiophene PheDOT Oligomers .......................... 131 4.3.2 DSC Thermograms of Thiophene PheDOT Oligomers ......................... 132 4.4 Optical Properties of PheDOT Oligomers ................................ ....................... 135 4.4.1 Optical Propert ies of Exemplary Thiophene PheDOT Systems ............ 135 4.4.2 Solid State Emission of HxTTPTTHx ................................ ..................... 144 4.4.3 Electrochemical Properties of PheDOT O ligomers ................................ 147 4.4.4 Electropolymerization and Spectroelectrochemical Characterization of TPT and EPE ................................ ................................ .............................. 148 4.5 Thiophene PheDOT Polymers ................................ ................................ ........ 152 4.5.1 Synthesis of Thiophene PheDOT Polymers ................................ .......... 152 4.5.2 Optical Properties of Thiophene PheDOT Polymers ............................. 154 4.6 Synthetic Details ................................ ................................ ............................. 160 5 SYSTEMS INCORPORATING SPIROBIPRODOT FOR DECREASED INTERMOLECULAR ORDER ................................ ................................ ............... 171
9 5.1 Brief Survey of Spiro Compounds ................................ ................................ ... 171 5.2 Synthesis of SpirobiProDOT Oligomers ................................ .......................... 172 5.3 Optoelectronic Properties of SpirobiPr oDOT Compounds .............................. 176 5.3.1 Electrochemical Properties of SpirobiProDOT Oligomers ..................... 176 5.3.2 Electrochemical Properties of Electropolymeri zed SpirobiProDOT Films ................................ ................................ ................................ ........... 181 5.3.3 Spectroelectrochemical Properties of SpirobiProDOT Compounds ...... 184 5.3.4 Chronoabsorptometry Measureme nts of SpirobiProDOT Films ............ 188 5.4 Synthetic Details ................................ ................................ ............................. 191 6 PERSPECTIVE ON WORK ACCOMPLISHED IN THIS DISSERTATION AND PROPOSED DIRECTIO NS FOR CONTINUATION OF THIS WORK .................. 201 LIST OF REFERENCES ................................ ................................ ............................. 204 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 212
10 LIST OF TABLES Table page 3 1 Attempts at forming highly conducting films of PPheDOTs ................................ 99 4 1 DSC and TGA results of PheDOT oligomers ................................ .................... 135 4 2 Optical absorption data of PheDOT oligomers ................................ ................. 142 4 3 Optical emission data of PheDOT oligomers ................................ .................... 144 4 4 HOMO and LUMO energies of PheDOT oligomers and the ir corresponding HOMO LUMO gaps ................................ ................................ .......................... 149
11 LIST OF FIGURES Figure page 1 1 Cartoon and free energy curve of a charge transfer reaction ............................. 20 1 2 Cartoon illustrating the intramolecular and intermolecular rearrangements involved in char ge transfer between two species ................................ ............... 21 1 3 Cartoon and free ........ 23 1 4 Reorganization processes during charge transfer ................................ .............. 25 1 5 Computed free energies required to rotate from the energetic maxima to the energetic minima ................................ ................................ ................................ 27 1 6 Dimer HOMO and HOMO 1 energ y levels resulting from the interaction of the HOMO levels of two ethylene monomers ................................ ........................... 30 1 7 F ormation of a valence band from the theoretical interaction of an infi nite number of monomer orbitals ................................ ................................ ............... 30 1 8 Cartoon depiction of two conjugated units approach ing along the x y and z axes ................................ ................................ ................................ .................... 31 1 9 Commonly observed deviations from perfe ct cofacial z axis alignment .............. 33 1 10 Typical herringbone packing structure ................................ ................................ 33 1 11 The relationship between x axis and y axis translat ions on orbital interactions .. 35 1 12 1, and LUMO & LUMO+1 orbital splitting with increasing number of double bonds ................................ ............... 37 1 13 Family of perfluoroacene thiophene oligomers investigated in reference 28. ..... 39 1 14 The three groups of tetrathiafulvalenes investigated in reference 30 ................. 40 1 15 A sampling of the functional groups screened for their effects on the hole and electron coupling matrices ................................ ................................ .................. 41 1 16 B3LYP/6 31G** calculated hole r eorganization energies ................................ ... 42 1 17 Molecular structures of several fused systems ................................ ................... 45 1 18 Structures of TIPS pentacene and fluorine substit uted TIPS pentacenes .......... 46 1 19 Structure of iptycene ................................ ................................ ........................... 47
12 2 1 Electropolymerization of BiPheDOT ................................ ................................ ... 53 2 2 CV of a film of pBiPheDOT ................................ ................................ ................. 54 2 3 Current voltage plot of a hypothetical polymer film displaying near ideal capacitance behavio r ................................ ................................ .......................... 56 2 4 Typical current voltage plot of an electrodeposited polymer ............................... 57 2 5 Current voltage plots of PBiPheDOT at three different scan rates ..................... 59 2 6 Chronoabsorptometry of a polymer film ................................ .............................. 61 2 7 Energy level diagram illustrating transitions between the ground state and excited state to various vibrational levels ................................ ........................... 62 2 8 Energy level diagram illustrating the large number of possible vibrational transitions ................................ ................................ ................................ ........... 63 2 9 Emission and absorbance spectra of (2,5 bithienyl) PheDOT) in THF ....... 66 2 10 Absorbance spectra of polymer thin films dr op cast from different solvents ....... 69 3 1 X ra y structures of PheDOT Br 2 taken from ref 6 illustrating the planarity of the molecule ................................ ................................ ................................ ....... 71 3 2 P olyPheDOT (C12) 2 film morphology ................................ ................................ 73 3 3 Family of PheDOT derivatives synthesized for this dissertation ......................... 77 3 4 The 1 st and 10 th scan of the electropolymerization of a 10 mmol solution of PheDOT ................................ ................................ ................................ ............. 80 3 5 Voltammograms of PPheDOT ................................ ................................ ............ 81 3 6 Spectroelectrochemical results for PPheDOT ................................ .................... 83 3 7 Electrochemical behavior of model 1,2 substituted phenylenes ......................... 84 3 8 Cyclic voltammograms of PheDOT Et 2 ................................ ............................... 86 3 9 Cyclic voltammograms of PheDOT Bu 2 ................................ .............................. 88 3 10 Spectroelectrochemistry of a PPheDOT Bu 2 film ................................ ............... 89 3 11 1 st and 10 th scan of the electropolymerization of 10 mmol PheDOT (3MB) 2 ....... 90 3 12 1 st and 50 th scan of the electropolymerization of 10 mmol PheDOT (3,3 dMB) 2 ................................ ................................ ................................ .................. 91
13 3 13 Cyclic voltammograms of biPheDOT ................................ ................................ .. 94 3 14 Spectroelectrochemistry of a PbiPheDOT film ................................ .................... 95 3 15 Cyclic voltammograms of bi(PheDOT Bu 2 ) ................................ ......................... 97 3 16 Effects of different electrolyte salt and solution on electrochemical behavior ... 102 3 17 Cyclic voltammogram of PPheDOT Bu 2 ................................ ........................... 104 3 18 Cyclic voltammogram of PbiPheDOT at various scan rates ............................. 105 3 19 Areal capacitance as a function of scan rate of PbiPheDOT films .................... 106 3 20 Areal capacitance as a function of scan rate of Pbi(PheDOT Bu 2 ) films .......... 107 3 21 Schematic representation of charge storage device ................................ ......... 108 3 22 Device performance of PbiPheDOT on gold coated Kapton electrodes over several voltage windows to probe operational win dow of the device ................ 109 3 23 Five devices of varying film thic knesses ................................ ........................... 109 3 24 Current density as a function of scan rate for a PbiPheDOT device ................. 110 4 1 X ray crystal structures of TPT HxTPTHx ................................ ......................... 129 4 2 TGA thermograms for select PheDOT oligomers ................................ ............. 132 4 3 DSC thermograms of PheDOT containing systems ................................ .......... 133 4 4 Solution, thin film and emission spectra of TPT ................................ ................ 136 4 5 Solution, thin film and emission spectra of DPD ................................ ............... 139 4 6 Solution, thin film and emission spectra of TTPTT ................................ ........... 141 4 7 Emission spectra of a thin and a thicker film of HxTTPTTHx ............................ 145 4 8 Emission spectra of HxTTPTTHx ................................ ................................ ..... 14 6 4 9 ................... 147 4 10 Electropolymerizations of 7.5 mmol TPT and 5 mmol EPE .............................. 150 4 11 CVs of pTPT and pEPE ................................ ................................ .................... 150 4 12 Spectroelectrochemistry of PheDOT containing polymers ............................... 152 4 13 Temperature dependent absorption spectra of P heDOT containing polymers 155
14 4 14 PL spectra of poly(TP12) in chlorobenzene ................................ ...................... 156 4 15 One possible model explaining the magnitude of t hermochromic blue s hifting observed in Figure 4 13 ................................ ................................ .................... 158 4 16 Spectroelectrochemistries of a) poly(TP12) and b) poly(TTP12) ...................... 159 5 1 Cycl ic voltammetric scan of 3MTsBP ................................ ............................... 177 5 2 Cyclic voltammetric study of BTsBP ................................ ................................ 178 5 3 Cyclic voltammetric scan of tetraarylsBP compounds ................................ ...... 180 5 4 Cartoon illustrating the two mechanisms through which electropolymerization could proceed ................................ ................................ ................................ ... 181 5 5 A freshly electropolymerized film of p3MTsBP ................................ ................. 182 5 6 1st through 50 th scan of pProMsBP ................................ ................................ .. 183 5 7 Cyclic voltammetric scan of tetraarylsBP polymers ................................ .......... 184 5 8 Absorbance spectra of films of tetraarylsBP compounds ................................ 186 5 9 Absorbance spectra of a film of pProMsBP before and after breaking in ......... 187 5 10 Spectroelectrochemistry of electropolymerized tetraarylsBP films ................... 188 5 11 Chronoabsorptometry measurements of tetraarylsBP polymer films ................ 190
15 LIST OF ABBREVIATION S ACN acetonitrile CV cyclic voltammogram DCM dichloromethane DPV differential pulse voltammetry EDOT 3,4 ethylenedioxythiophene EMI BTI 1 ethyl 3 methylimidazolium bistrifluoromethanesulfonimide Fc ferrocene HOMO highest occupied molecular orbital ITO indium tin oxide LiBTI lithium bistrifluoromethanesulfonimide LUMO lowest unoccupied molecular orbital OFET organic field effect transistor OLED organic light emitting diode PheDOT 3,4 phenylenedioxyth iophene ProDOT 3,4 propylenedioxythiophene TBABF 4 tetrabutylammonium tetrafluoroborate TBAP tetrabutylammonium perchlorate TBAPF 6 tetrabutylammonium hexafluorophosphate
16 Abstract of Dissertation Presented to the Graduate School of the U niversity of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy UNDERSTANDING INTRA AND INTERMOLECULAR INTERACTIONS IN THIOPHENE CONTAINING SYSTEMS By Dwanleen Eric Shen December 2011 Chair: John R. Reynolds Major : Chemistry Intra and intermolecular organization and interactions in conjugated organic semiconductors play a critical role in the performance of devices fabricated from these materials. In organic field effect transistors (OFETs), for instance, intima stacking along with suppression of intramolecular disorder are critical strategies towards maximizing charge mobilities. In contrast, t he performance of organic light emiting diodes (OLEDs) is enhanced by preventing strong intermolecular interactions and aggregation, both of which would otherwise give rise to charge trapping and quenching and by extension poorer device performance. The ability to understand, evaluate, and control intra and intermolecular organization in a material is therefore inval uable to enhance and tune the electronic properties of a system. The desire to explore ways in which these interactions can be controlled is the motivation for the molecules synthes ized in this dissertation. In C hapter 3, 3,4 phenylenedioxythiophenes (Ph eDOTs) are introduced as attractive synthons due to their highly planar structures as elucidated from x ray crystal structures, which could stacking interactions. As this molecule has not been thoroughly investigated in the literature, the elect rochemical properties of a family of alkyl substituted PheDOTs
17 are explored and elucidated. Likely owing to the ability of this planar structure to form intimate intermolecular interactions, promising conductivities are measured for free standing films of electropolymerized pPheDOTs. Additionally, the charge storage abilities of the electropolymerized films are evaluated and also found to be promising. In C hapter 4, the effects on intra and intermolecular organization caused by the presence of PheDOT ar e more directly investigated. When incorporated into thiophene oligomers and polymers, an improvement in both intra and intermolecular organization are observed when compared to their all thiophene analogs. Sulfur oxygen interactions are believed to loc k the system into a more planar conformation; this planarity in turn promotes strong intermolecular interactions. X ray crystal structures again demonstrate that three ring systems incorporating PheDOT are highly planar structures with close packing dista nces. Throughout the chapter, we demonstrate how the absorbance and emission spectra can be powerful tools to elucidate and qualify the intra and intermolecular interactions present in the system. In C hapter 5, molecules are designed to deliberately fru strate intermolecular packing. This is accomplished by utilizing spirobiProDOT as a core synthon. Extension of the conjugation through the four active sites on spirobiProDOT followed by electropolymerization of these systems is predicted to give a highly branched, porous and rigid network through which ion diffusion can readily occur. A variety of electrochemical and spectroelectrochemical data are brought together to evaluate this model. While the theoretical groundwork informs the synthetic chemist of the ideal intra and intermolecular interactions an application requires, the ability to design molecules to
18 meet these requirements proves challenging. Nevertheless, this dissertation illustrates several design strategies as well as several readily acce ssible means to experimentally evaluate the intra and intermolecular interactions present in our systems.
19 CHAPTER 1 THE IMPORTANCE OF IN TRA AND INTERMOLECUL AR ORDERING AND CONTROL IN CONJUGATE D SYSTEMS 1.1 Introduction to Organic Semiconducting Materials Conjugated organic materials encompass a broad family of systems displaying a great deal of promise as electronic materials in a number of applications, such as light emitting diodes (LEDs), solar cells, field effect transistors (FETs), and electrochromic s to name a few. 1 6 Part of their attractiveness and success owes to the relatively facile and diverse ways in which novel structures can be synthesized and then functionalized or modified, typically with one of t wo broad objectives in mind. The first is the manipulation and fine tuning significant implications on how a material performs in a device. For example, the O values is important in solar cells to optimize several key parameters, such as the open circuit voltage which relates magnitude of the HOMO LUMO energy gap pla ys a large role in determining the color of a material. The second objective, and of equal importance to device performance and optimization, is the control of the intra molecular and intermolecular interactions present in a material. These interactions are critical not only to the physical morphology of a system, but to their electronic properties as well. While a certain proficiency has been achieved in being able to reliably design and tune the HOMO and LUMO levels of a material, the theoretical groun dwork for understanding the relationship between intra developed today. Here in Chapter 1, we will explore the theoretical groundwork
20 describing how material prope rties are impacted as intra and intermolecular interactions are manipulated. From there, design strategies for tuning these interactions will be discussed. 1.1.1 Modeling Intermolecular Charge Transport Using Marcus Theory Intermolecular charge transfer plays a large role in a number of semiconducting applications, and as such we begin by investigating how intra and intermolecular interactions affect this process utilizing Marcus theory as our framework. 7 To approach the nuances of this theory, we start with a simple model as shown in Figure 1 1 A where charge is transferred from a charged species A* to a neutral species B. Fig ure 1 1 B illustrates this process along a set of reaction coordinates where the x axis represents the progression from reactants to products and the y axis represents the free energy of the system. Figure 1 1. C artoon and free energy curve of a charge tr ansfer reaction a) The changes in sphere sizes are representative of changes in the nuclear lattices of the two species upon charge transfer. b) adiabatic free energy curve of a generic charge transfer reaction between A* and B. To understand what is oc curring in Figure 1 1, we first consider the relationship between the electron cloud and the nuclear lattice. In particular, as A* transfers its
21 charge and becomes A, the nuclear geometry that best stabilized the charged species is unlikely to be the best geometry to stabilize the neutral species, leading to intramolecular reorganization upon charge transfer. The same is of course true for B and B*. By the same token if we expand our model to encompass the environment surrounding A and B to include, fo r instance, surrounding solvent molecules then the environment that best stabilized A* is not expected to be the best environment to stabilize A, again with the same being true for B and B*. As a result, as shown in Figure 1 2, a large number of rearran gements and reorganizations both internally, as well as between the molecule and its environment must take place for the entire system over the course of a charge transfer process. The energy req uired is the activation energy Figure 1 1 B above. Figure 1 2. C artoon illustrating the intramolecular and intermolecular rearrangements involved in charge transfer between two species. The power of Marcus theory lies in its ability to relate this reorganization energy to the activation energy t hrough a relatively simple series of equations. 8 From here, the activation energy can be related to the overall charge transfer rate, k CT by modification of the Arrhenius equation to become: k CT = A [exp ( / k B T)] (1 1)
22 where A is a term related to the type of reaction (bimolecular, intramolecular, etc.), k B is G is the activation energy. Expanding G* leads to another important equation: = ( /4) 0 / )] 2 (1 2) where G 0 is the standard free energy of the reaction, and is the reorganization energy that occurs during the reaction. As described above, this reorganization consists of both intramolecular and intermolecular componen ts. As it was initial ly conceived Marcus theory served to describe the rate of charge transfer occurring in solution. Under such conditions, solvent molecule reorganization made up the bulk of the term; in the framework of solid state materials, howev er, solvent is absent. In such systems, is dominated by sources of intramolecular reorganization. Additionally, in the specialized case where charge transfer occurs between the charged form of a transf 3, the standard change in free energy of the reaction G 0 can be simplified to 0. In such systems the activation energy G is therefore solely determined by intramolecular reorganization: = /4 (1 3) Combining Equat ion s 1 1 and 1 3, the rate equation can be rewritten as: k CT = A exp ( / 4k B T) (1 4) classical formulation of charge transfer and incorporate the quantum mechanical c oncept of tunneling into the equation. In particular, we can relate the rate of charge
23 transfer to the orbital overlap of the two species to reach the form that will be most useful for the discussion to follow: Figure 1 3. C artoon and free energy curve a) between a charged species A* and a neutral species A. b) adiabatic free k CT = ( V 2 / B T )] 1/2 exp ( / 4k B T) (1 5) wh V describes the electronic coupling matrix, largely dependent on the extent of intermolecular orbital overlap between A* and A (it should be pointed out that V has alternatively been called the transfer integral and represent ed as b H 12 and t 12 ). Equation 1 5 provides a clear relationship between the rate of intermolecular charge transfer and the extent of reorganization occurring in both species upon charge transfer as well as the extent of orbital overlap between the two species. In Section 1.2, we will utilize a simplified dimeric system as a model to investigate more precisely how and V influence the electronic properties of materials, with the assumption that these observations on the dimeric level can then be extrap olated to an extended chain system with little loss of generality.
24 1.1.2 Modeling Charge Transport in Bulk Systems While Marcus theory provides an understanding of charge transport on an intermolecular scale, the process of charge transport throughout a b ulk material can be affected by macroscopic parameters not encompassed by Marcus theory formalism, namely the effects of long range Coulombic and dipolar interactions on the energetic manifold traversed by a charge, extrinsic factors such as grain boundari es, the effect of defects and traps, etc. A discussion into these topics is outside of the scope of this dissertation, however the reader is directed to several references which highlight some of the key models used to describe the energetics of the bulk. 9 13 1.2 Effects of Reorganization Energy and Electronic Coupling Matrix on Electronic Properties 1.2.1 Reorganization Energy As discussed above, a reorganization of the nuclear lattice occurs during charge trans fer to accommodate the change in the electronic environment as a system switches between its cha rged and neutral states. From E quation 1 5, the rate of charge transfer and the reorganization energy are inversely proportional to one another, that is, the g reater the reorganization energy accompanying a charge transfer process, the slower the rate of charge transfer, and vice versa. In solid state conjugated organic materials, reorganization derives primarily from two sources: changes in bond length and tor sional rotation between aromatic subunits. Figure 1 4 demonstrates the origins of this reorganization in a model bithiophene dimer. In its neutral form, the two thiophene rings are aromatic, and the bond between them has a predominantly single bond cha racter, allowing a high degree of rotational freedom between the two rings. The dihedral angle between the rings is unlikely to be
25 0 for steric reasons, and the energetic minimum is expected to exist in a state with some amount of twisting between the ri ngs. Upon oxidation however, a more quinoidal geometry is adopted each ring loses their aromaticity while the bond joining the two rings acquires more double bond character. This double bond character both forces the rings into coplanarity and suppress es free rotation about the bond in the charged state. This change in nuclear geometry stabilizes the acquired charge by localizing, or binding, it as a structural defect whose energy level lies within the HOMO LUMO gap of the neutral species, which in tur n lowers the energy of the system relative to the scenario where no reorganization occurs. The energy required for these changes in dihedral angle as well as bond lengths makes up the majority of the reorganization energy. Figure 1 4. Reorganization pro cesses during charge transfer. T he purple arrows depict the movement of electrons resulting in changes to the bond lengths; the green arrow shows the rotation that occurs. Computational analyses performed on a large family of heteroaromatic oligomers gave a clear picture of the role of torsional and bond length reorganization on the overall reorganization energy. 14 The relationship between reorganization energy and the magnitude of the ring twist (i.e. the c hange in the dihedral angle between two adjacent rings) observed upon charge transfer, as well as between reorganization energy and the
26 magnitude of the changes in bond length (i.e. the change in the lengths of the bonds) upon charge transfer, were assesse d and found to be poorly correlated. This observation captures a crucial point, that a large or small change in dihedral angle or bond length does not necessarily reflect a need to overcome a respectively large or small change in activation barrier. For example, Figure 1 5 A difluoro bipyrrole has a rotationally energetic minimum when the two rings are oriented 180 from one another and a rotational maximum at 0 with a calculated barrier to rotation of 5.33 kcal/mol. On the ot her hand, in Figure 1 5 B bisisoindole is found to have an energetic minimum at 144.2 and a maximum at 0 with a calculated barrier of 21.32 kcal/mol. Thus, even though the bipyrrole must rotate through a much larger angle than bisisoindole to go from its minimum to its maximum, the barrier is flatter than that of bisisoindole. The same is true for bond length alternations: simply measuring the changes in bond lengths upon going from the neutral to the charged state does not give any indication to the energetic cost of such an operation. The above point is highlighted by the fact that even when the magnitudes of both changes in dihedral angle and changes in bond length alte rnation are combined to estimate the reorganization energy, the estimated values and the calculated values are still surprisingly poorly correlated with a statistical R 2 value of 0.417. However, by also incorporating the effects of heteroatoms and substit uents on the reorganization energy a very strong correlation arises between the estimated and calculated values (R 2 = 0.943). In Figure 1 5 B b elow exist to make the 0 dihedral orientation of bisi soindole particularly unfavorable; in
27 bipyrrole in Figure 1 5 A those steric interactions are diminished as the fluorine at the 3 provides us with a model to rationall y predict the expected reorganization energy present in systems as we go about designing molecules. Figure 1 5. C omputed free energies required to rotate from the energetic maxima to the e difluoro 2,2 bipyrrole, and b) bisisoindole. 1.2.2 Obtaining Reorganization Energy Values Reorganization energies associated with charge transfer processes are often calculated using computational methods. However, the values ca n also be obtained experimentally using various spectroscopic methods, such as gas phase high resolution ultraviolet photoelectron spectroscopy, or Raman spectroscopy. 15 19 Reorganization energies associated with exciton formation however can be evaluated from absorbance and emission spectroscopy. In the same way that a change in the nuclear lattice accompanies charge transfer, the formation of an exciton also leads to an increase in the quinodal character and an overall planarization of the system. In Chapter 2 we will more thoroughly explore how these spectra can experimentally provide us with the reorganization energies.
28 The above discussion provides a backdrop for addressing one of the princip al complications in calculating reorganization energies. Because reorganization energy involves physical motion, any factors which would constrain the movement of molecules have a large effect on the anticipated reorganization energy. In particular, molecules in solid st ate phases are expected to have drastically different values than their gas phase analogs due to packing forces which are present in the former and absent in the latter. For example, isolated oligothiophenes posses an energetic minimum when there is a s light twist between thiophene rings to relieve steric pressure; oligothiophenes embedded in crystal structures however adopt a far more planar geometry. 20 Another telling case study reiterates this point while introducing another source of complication. Isolated dithienyltetrathiafulvalene (DT TTF) was investigated as its isolated single molecule conformation was known to be different than its cry stal packing conformation. 21 In the isolated state, DT TTF possesses a boat like arrangement, while in crystals, DT TTF has been found to adopt a more planar geometry. Computational models found tha t the calculated reorganization energy upon going from the neutral to the radical cation conformation was larger by over a factor of two for the isolated boat molecule (574 meV) than for the isolated planar molecule (238 meV). Even more surprising was tha t when the isolated molecule was placed in an embedded environment (simulating the crystalline environment) the geometry was further altered and the reorganization energy was calculated as being exceedingly low (42 meV). Up until now, we have only conside red how charge transfer affects intramolecular geometry i.e. changes in bond lengths and dihedral angles between the lattice and the charge; however in certain cases, as is clearly illustrated
29 above, intermolecular interact ions or nonlocal coupling can also have dramatic effects on molecular geometry. It is common in the literature to see the assumption made that nonlocal coupling effects are minimal, and indeed this is often true in nonpolar systems. In the same paper isolated pentacene is shown to have nearly the same reorganization energy in the isolated (98 meV) and in the embedded (80 meV) phases. However, in all other instances this assumption cannot be readily made; while still not readily recognized in the lit erature, the phenomenon of nonlocal coupling is beginning to receive more attention. 1.2.3 Electronic Coupling Matrix The electronic coupling matrix V describes the extent of orbital over lap between A* and A and, from E quation 1 5, is directly proportiona l to the charge transfer rate in general, the greater the orbital overlap, the more rapid the charge transfer. The electronic coupling matrix can be conceptualized by looking at a molecular orbital energy diagram and seeing how the monomer orbitals inte ract as they are brought together to form a dimer. In Figure 1 6, we use a simplified model where we only consider the HOMO levels of two ethylene molecules to illustrate this process. The HOMO energy levels of each monomer interact to form two hybridize d dimer energy levels the HOMO and the HOMO 1 levels. The latter is stabilized by the strong bonding interactions between the orbitals; the HOMO level is destabilized by the anti 12 be tween the HOMO and HOMO 1 of the dimer is proportional to the electronic coupling matrix; in particular it can be estimated by, 12 = 2 V (1 6)
30 Figure 1 6. Dimer HOMO and HOMO 1 energy levels resulting from the in teraction of the HOMO levels of two ethylene monomers. Thus, the greater the electronic coupling between the species, the greater the splitting of the energy levels. Building on the ethylene model, an infinite number of well ordered and interacting units would continue to hybridize, leading to the formation of a band (Figure 1 7). The total bandwidth, W is approximated as: W = 4 V (1 7) Figure 1 7. Formation of a valence band from the theoretical interaction of an infinite number of monomer or bitals. Inorganic semiconductors have bandwidths that are several factors and sometimes orders of magnitude greater than those of organic materials. From the relationship between bandwidth and electronic coupling matrix, we see this increased bandwidt h is directly derived from strong orbital overlap interactions, which leads to greater charge delocalization and higher charge transfer rates. Unfortunately, the
31 electronic coupling matrix is a parameter which is highly sensitive to the slightest perturba tions and depends heavily on packing forces in the solid state, a behavior which unlike reorganization energy is extremely difficult to predict and synthetically control a priori. Below, we will investigate the factors that contribute to as well as re duce the electronic coupling matrix. In a three dimensional lattice, a molecule may have several neighbors oriented along the x, y and z axes with which to interact. The effects of intermolecular separation along each of these axes has been investigated c omputationally. 22 Several results follow predictably, as illustrated in Figure 1 8: 1) the orbital overlap is enhanced as the intermolecular distance is decr eased and the orbitals are brought into closer contact; 2) this enhancement is least pronounced as two molecules approach along the x and the y axes in an edge to edge fashion; 3) conversely, this enhancement is most pronounced as two molecules approach a long the z axis cofacially. Figure 1 8. Cartoon depiction of two conjugated units approaching along the x y and z axes.
32 These results can be readily understood by noting that the orbitals of each individual system are located primarily above and bel ow the plane of the molecules. Thus, the increase in orbital overlap as two molecules approach along the x axis is minimal as overlap can only occur at the endpoints of the long axis of the molecule; this poor overlap is compounded by the fact that orbita l density is often very low at the ends of oligomers and polymers. Approach along the y axis allows for increased orbital overlap as interactions occur along entire lengths of each molecule. However, approach along z axis allows for the greatest orbital overlap as the entire surface area above and below the plane of the molecule where orbital density is greatest is involved in the overlap. In accordance with Marcus theory, we thus expect charge transfer to be slowest along the x and y axes and greate st along the z axis. Indeed, the anisotropic nature of mobility in a pentacene single crystal has been observed experimentally and found to vary significantly based on the direction in which the measurements were taken. 23 While the above results indicate the advantage of using materials which pack in a perfectly cofacial orientat ion along the z axis with minimal intermolecular spacing, in reality the vast majority of molecules pack in a far more complicated fashion. Figure 1 9 shows two of the most common distortions to this picture, which are 1) for one of the molecules to be rot ated out of parallel alignment, or 2) for one molecule translated along either the x or y axes to adopt a staggered conformation; combinations of both are also observed experimentally and have received significant theoretical consideration. 22,24,25 In fact, many systems which have displayed some of the highest mobilities such as pentacene and sexithiophenes pack in a herringbone structure as shown in Figure 1
33 10, which display a combination of translation and rotation. It is important to note, however, that despite the fact that the highest mobility materials pack in a herringbone fashion, there is no reason to believe the herringbone packing is the best packing structure to optimize the electronic coupling ma trix; indeed the perfectly cofacial arrangement would intuitively seem to be the arrangement which maximizes electronic coupling matrix and leads to the largest bandwidth materials. Figure 1 9. Commonly observed deviations from perfect cofacial z axis al ignment Figure 1 10. Typical herringbone packing structure viewed from two different perspectives. To understand the effects of orbital splitting upon such translations, computational work was performed on two cofacial sexithiophene (6T) dimers, wherein the z axis separation was kept constant while one 6T monomer was slipped along the x axis and the y axis (Figure 1 11). 22,24 When slipped along the y axis, the HOMO orbital splitting was observed to decrease minim ally; when slipped along the x axis however, large periodic oscillations in orbital splitting were observed that at times resulted in complete
34 attenuation. The opposite was observed in the LUMO splitting. To understand these trends, we take a closer look at the pertinent LCAO coefficients. In the case of the HOMO orbitals, when one 6T is slipped along the y axis, the center to center distance increases, leading to a gradual decrease in orbital overlap, but the interactions between the orbitals remain pred ominantly bonding in nature. However, when slipped along the x axis, antibonding interactions are introduced which greatly reduce the orbital overlap of the system. The reverse is true in the case of the LUMO orbitals. This case study demonstrates the s ensitive nature of orbital overlap upon the slightest fluctuations in molecular orientation; additionally, the relationship between orbital overlap and molecular orientation depends heavily on the precise nature of the coefficients describing the orbitals in question, which vary dramatically from one system to another. Thus, it is difficult to predict a priori without knowledge of how the molecules will order in materials, and without a careful model of the orbitals what sorts of electronic coupling ma trices can be expected for a given system. A final point of complication involves the implicit assumption embedded in modeling the HOMO and HOMO 1 splitting of the dimer as arising from the interactions of the HOMO levels of each monomer, and hence derivin g the electronic coupling matrix (and by extension the bandwidth) from this calculation. Rather, when the valence energy levels of the dimer draw from additional orbital levels (e.g. HOMO 1, HOMO 2, etc.) the calculations have been shown to deviate si gnificantly from expected values. 26 In spite of these complications which make it difficult to accurately predict coupling matrix values in advance, the fundamental understanding of what i mproves and attenuates the coupling matrix parameter is invaluable in rationalizing and
35 correlating measured mobility values to the observed crystal structure and spectroscopic evidence elucidating the packing and morphology of the material. Figure 1 1 1. The relationship between x axis and y axis translations on orbital interactions in cofacial sexithiophene dimers kept at a fixed separation along the z axis. Note, the size of the spheres are meant to be a qualitative not a precise measure of the m agnitude of the coefficient describing that atomic orbital. Additionally, the blue and red colors are meant to indicate the sign (i.e. positive or negative) associated with the coefficient on each atomic orbital. 1.2.4 Obtaining Electronic Coupling Matrix and Bandwidths As with reorganization energy, the electronic coupling matrix is often acquired through density functional theory computational modeling. Recent theoretical work has shown that in the special regime of weakly interacting aggregates, absor bance spectroscopy can be utilized to calculate the magnitude of the electronic coupling
36 photogenerated exciton. In strongly aggregated systems with large bandwidths, the exciton delocalizes over several molecules. In weakly aggregated systems, however, the exciton resides primarily on a single molecule. Conveniently, conjugated polymers typically fall in this latter category, which allows films and devices fabricated fr om this class of material to be readily evaluated for the strength of the intermolecular ordering. A more detailed discussion on how absorbance spectra can be utilized to calculate this parameter will be elucidated in Chapter 2. 1.2.5 Dependence on Oligom er Length Investigating how charge transfer rates depend on chain length reveals an interesting interplay between the reorganization energy and the electronic coupling matrix parameters. Specifically, as chain length increases, the reorganization energy d ecreases, however the electronic coupling matrix decreases as well. 14,27 We can understand the effects on reorganization energy by considering how charge lattice coupling evolves with chain length. When a charge is introduced onto a molecule, the conjugated backbone reorganizes to accommodate the charge and lower the overall energy of the system. It has been demonstrated computationally that this delocalization occurs over roughly 4 aromatic subunits. 27 In smaller systems, the charge is f orced to delocalize over the entire system and large reorganizations must occur to accommodate the charge. As the oligomer length approaches four units, the charge is allowed to delocalize over a larger area and less dramatic reorganizations are necessary For oligomer lengths beyond the delocalization length, the additional aromatic subunits do not participate in charge stabilization. We would thus expect a plot of reorganization energy with respect to oligomer length to decrease dramatically as short c hains grow
37 longer, then to plateau off as we pass the delocalization length. Indeed, this is the trend observed. Understanding the relationship between increasing oligomer length and the electronic coupling matrix requires an analysis of the LCAO coeffic ients of the interacting dimers. As a representative case study, we consider the evolution of the HOMO level in a series of ethylene oligomers of varying lengths. Figure 1 12 illustrates that, for a fixed separation between two monomers, as the number of double bonds in each monomer increases, the number of nodes in the HOMO levels increases in the system. As a result the number of antibonding interactions increases as well, resulting in an overall decrease in the orbital overlap strength between the HOM O orbitals of the dimer. This in turn leads to a decrease in orbital splitting with increasing oligomer length. A similar trend occurs in the evolution of the LUMO level. Figure 1 1, and LUMO & LUMO+1 orbital spli tting with increasing number of double bonds. Note only a few of the bonding and antibonding interactions present are shown above.
38 Combining our understanding of how reorganization energy and electronic coupling matrix evolve with oligomer length allows us to understand the overall effect these two offsetting trends have on the rate of charge transfer: the reorganization energy decreases at a far sharper rate than the electronic coupling matrix; this in turn suggests that the rate increase gained by the d rop in reorganization energy should vastly offset the rate decrease from the drop in transfer integral. 1.2.6 Experimental Proof of Theoretical C oncepts Several studies have attempted to test and correlate the dependence of charge transfer rates on reor ganization energy and electronic coupling matrix. In a series of perfluoroarene modified oligothiophenes shown in Figure 1 13, experimentally derived mobilities were investigated based on the expectations outlined above regarding computationally calculate d reorganization energies and electronic coupling matrices. 28 Computationally optimized stru ctures were correlated against X ray diffraction measurements and found to be in good agreement. Also, the reorganization energies were comparable in all species; as a result the electronic coupling matrix was predicte d to be the governing factor with regards the charge mobility. A couple notable exceptions stood out however: TFFFFT was calculated as having the largest electronic coupling matrix, and indeed the experimentally derived electron mobility was found to be q uite high (0.08 0.43 cm 2 V 1 s 1 depending on processing conditions); however no measureable hole mobility could be detected. Additionally, TTFFTT was found to have the lowest mobilities by several orders of magnitude, in spite of having comparable a reo rganization energy and coupling matrix to the other systems. Further modeling work showed some improvement, although it still could not satisfactorily account for the anomalous behavior of the two cases mentioned above. 29 The large dependence of
39 mobility values on processing temperatures was also noted; taking all this data into consideration, the authors concluded that while reorganization ene rgy and electron coupling matrix taken together can be a useful tool to estimate mobility values, ultimately macroscopic parameters such as film morphology, grain boundaries and defects played too large a role to be ignored. Figure 1 13. Family of perflu oroacene thiophene olig omers investigated in reference 28. An earlier study attempted to circumvent the intricacies of macroscopic effects by studying the evolution of mobility in single crystals of closely related tetrathiafulvalene and had far greater su ccess. 30 Figure 1 14 shows the molecules investigated. The various derivatives differed subtly but in ways that drastically altered the crystal packing of the material. As a result, any differences in mobility could be attributed to di fferences in reorganization energy and electronic coupling matrix. Indeed in the absence of significant macroscopic effects the mobility was found to correlate very strongly to the expected trends in reorganization energy and charge coupling matrix. In particular, the species in group 1 were found to have the largest values of and the smallest values of V with mobility values around 10 4 cm 2 / V s; systems in group 2 had smaller values and comparable V values with mobilities between 10 3 0.1 cm 2 / V s; finally, molecules
40 in group 3 had values comparable to those of group 2, however with significantly larger values of V leading to mobilities between 0.01 1 cm 2 / V s. Figure 1 14. The three groups of tetrathiafulvalenes investigated in ref erence 30; molecules are grouped based on the nature of their crystal packing, with a cartoon representation of the packing shown. While not directly related to investigations of reorganization energies or the electronic coupling matrix, recent work from t he Bao group showed experimentally over a large group of systems that device mobility correlated poorly with orbital energy levels, provided that the energy levels were sufficient to allow charge injection from electrodes. 31 While several references previously mentioned have suggested that mobility should be dominated by and V and not by molecular orbital energies, this was the first such study to lend strong support to this claim. As a final example, an interesti ng application of these parameters has been to rapidly screen the effects on charge transfer rates of substituting promising conjugated aromatic cores with solublizing groups. Perylenediimides (PDIs) are examples of high mobility charge transport material s, however they are also highly insoluble. As we have seen above, the addition of solublizing groups can have large effects on the packing of the materials and thus on the electronic coupling matrix. To synthesize a large family would be a time consuming process; recently, however, the expected electronic coupling matrices were calculated for a large family of substituted perylenediimides to
41 rapidly identify promising candidates to synthesize for further investigation (Figure 1 15). 32 Figure 1 15. A sampling of the functional groups screened for their effects on the hole and electron coupling matrices; note, in four instances above inequivalent stacks are predicted to form in the crystal structure, a nd the coupling matrix in both stacks are reported. 1.3 Synthetic Strategies to Tune Intra and Intermolecular Interactions 1.3.1 Synthetic Strategies Towards Increasing Intramolecular Order As we discussed above in Section 1.2.1 reorganization consists of bond length changes as well as dihedral angle changes. By minimizing either or both, we would expect the reorganization energy to decrease as well. An example of a system that minimizes the energy required for bond length reorganization is isothianaphth alene. This molecule is comprised of two moieties the upper aromatic thiophene which serves as the conjugated backbone, and the lower 1,3 cyclohexene that is responsible for minimizing the reorganization energy of the system. In its oxidized form, the lower moiety aromatizes to become a benzene ring. This aromatization provides a strong driving force that greatly reduces the energy required to induce reorganization.
42 Strategies to reduce undesired free rotation around the single bonds between aromatic subunits aim to fuse the two rings together in one way or another. This intuitive tactic has been extremely successful in Figure 1 16, a family of fused oligoacene thiophenes were found to have some of the smallest hole reorganization energies ever rep orted. 33 In comparison, most conjugated aromatic systems have reorganization energies on the order of several hundred meV. The drawback of these f used systems is that the solubility of the material is poor, given the lack of entropic options and free motion available. A similar strategy is adopted by molecules such as dithienosilole (DTS) and dithienogermole (DTG), where the bridging atom also serv es to restrict the rotational freedom of molecule. Systems based on DTS and DTG have achieved some of the highest solar cell efficiencies to date. 34,35 Figure 1 16. B3LYP/6 31G** calculated hole reorganization e nergies (electron reorganization energies in parentheses) for a series of fused benzene thiophene oligomers. 1.3.2 Synthetic Strategies Towards Decreasing Intramolecular Order While so far we have discussed the merits of increasing intramolecular order, th ere are instances where a decrease in order is in fact desirable. Increasing the
43 dihedral angle between aromatic units decreases the extent of conjugation along the backbone. This in turn can significantly increase the oxidation potential of the system, which improves the oxidative stability of the material. This strategy was employed to tune the ambient stability of poly 3 alkylthiophenes: whereas rrP3HT displayed poor stability after prolonged exposure to ambient conditions, deliberately introducing re gular head to material without compromising device performance. 36,37 This seeming contradiction rests on a point made earlier, that lar ge dihedral angles do not necessarily correlate with large reorganization energies. Thus, a property such as conjugation which is highly sensitive to the dihedral angle between aromatic rings can in theory be tuned at little expense to the charge transfer kinetics of the system. Additionally, increased rotational stacking tendencies in a material. This leads to improved solubility, as well as a tendency for materials to adopt an amorphous state, rather than a crystalline one In OFETs, the edges of crystalline domains form grain boundaries, and these defects severely reduce the mobilities of a material as charges are unable to propagate rapidly across grain boundaries. The precise size and properties of grain boundaries dep end heavily on processing conditions used to fabricate the device. In amorphous systems however, these crystalline domains, and by extension the grain boundaries, are absent. This can lead to improvements in the transport properties of the material, as w ell as a decreased dependence on processing conditions and by extension greater reproducibility in device performance. Experimentally amorphous materials have demonstrated that a lack of crystallinity has neither reduced device performance nor precluded t he ability for regions of a polymer to
44 stacking interactions while exhibiting fairly reliable batch to batch reproducibility. 38,39 1.3.3 Synthetic Strategies Towards Maximizing Electron ic Coupling Matrix As we have seen in Section 1.2.3, the electronic coupling matrix is a complex parameter to derive a priori as it is dependent on numerous subtle fluctuations in the packing structures of molecules. Furthermore, the ordering and morphol ogy of a material is often dependent on processing conditions as well. Synthetic attempts at maximizing the coupling matrix of oligomers often take an indirect approach by taking molecules with an already established packing arrangements and functionalizi ng them in such a way as to bring them closer into cofacial alignment to maximize orbital overlap. 6,13 bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) is a well studied example of such a phenomenon while unsubstituted pentacene packs in a herri ngbone fashion, the addition of bulky TIPS groups sterically prevent the substituted TIPS pentacene from being able to adopt the herringbone configuration, forcing it into a more favorable arrangement of stacked sheets with improved orbital overlap (Figure 1 18). 40 A similar trend was noticed upon substituting tetracene at the 5 and 11 positions with chlorines to alter the herringbone packing of tetracene to a similar slipped stacking configuration with a 30% improvement in mobility from 1.3 to 1.6 cm 2 /V s relative to tetracene. 41 While mobilities of TIPS pentacene have yet to exceed the highest reported values of pentacene, several points should be kept in mind: the highest mobilities in pentacene were reported on single crystals of the molecule; TIPS pentacene mobilities are typically reported from solution cast films and are by no means
45 low, with values of 1 cm 2 /V s reported in 2007 and values in excess of 2 cm 2 /V s recently reported. 42 Figure 1 17 Molecular structures of several fused systems and representations of their respective crystal packing. It has recently been discovered that halogenation offers an interestin g route towards fine tuning intermolecular interactions. 43 In one of the earliest such reports, it was discovered that by replacing the peripheral hydrogens by fluorines the stacking distance between TIPS pentacene units decreased (Figure 1 19). 44 Mobilities correlated well with expectations, where decreased stacking distance lead to increased mobilities (note it is important to mention that the crystal structures showed essentially no difference in the molecular orientations of TIPS pentacene and fluorinated TIPS pentacene aside from the sma ller packing distance). This strategy of utilizing fluorines to decrease stacking distance has been generalized to other systems as well with success. 45 In completely fluorinated tetr acene and pentacene (i.e. fluorine at all 12 available positions of tetracene and all 14 positions of pentacene) strong C F F and C H F interactions were observed to occur. 46 Strong changes in the electronic
46 properties were also noted, however, as the electron densities we re shown to be completely inverted relative to their unfluorinated counterparts: whereas tetracene and pentacene have electron rich cores, their perfluoro analogs now posses electron deficient cores with the majority of the electron density now located at the periphery along the fluorine atoms. As a result, different crystal packing compared to tetracene and pentacene were observed to accommodate this reorganization of electron density. This in turn resulted in significantly different electronic coupling matrices between adjacent oligomers as well as in the observed mobilities. Figure 1 18 Structures of TIPS pentacene and fluorine substituted TIPS pentacenes; representations of their crystal packing as well as their respective mobilities are also indi cated. Finally, the vibronic coupling in the fluorinated analogs is shown to be stronger, resulting in stronger polaron binding energies; coupled with the reduced coupling matrices, these two factors can accounted for the decreased mobility values observed in the fluorinated species. These effects on intra and intermolecular interactions are certainly not limited to fluorine atoms, and the effects of chlorination, bromination and
47 iodination on intermolecular interactions and materials properties have been recently reviewed. 43 1.3.4 Strategies Towards Decreasing Intermolecular Order There are many inst ances where strong intermolecular interactions are not desirable, and it is far easier to design structures with motifs which will discourage stacking leads to the fo rmation of low energy sites to which excitons readily diffuse and become trapped, resulting in reduced emission efficiencies and a self quenching effect. The use of large, bulky building blocks such as iptycene shown in Figure 1 20 has been a successful s trategy to inhibit the formation of aggregates. 47 Similarly, emission from aggregates is typically red shifted relative to emission from non aggregated regions, resulting in complications when emission color control is desired. The use of a large 48,49 Another application where intermolecular interactions are deliberately discouraged is supercapacitors, where both a high surface area and a highly porous morphology are required for charge storage a nd for rapid ion diffusion stacking would inhibit both properties and decrease the device performance. Figure 1 19 Structure of iptycene, and a cartoon illustrating suppressed intermolecular interactions in oligomers or polymers of iptycene.
48 1.4 Applications of Intra and Intermolecular Ordering in This Dissertation The design logic behind the molecules synthesized in this dissertation was motivat ed by a desire to see how and to what extent a conceptualization of molecules as physical building blocks coincided with the intra and intermolecular interactions displayed by the materials. In Chapter 3, we are motivated by the planarity of 3,4 phenylene dioxythiophene (PheDOT) to see whether this planarity translates into strong stacking, and by extension strong intermolecular electronic interactions in electropolymerized PheDOT systems. In Chapter 4 this investigation is extended to thiophene oligomer s containing PheDOT. In particular, we can better understand the impact PheDOT has by comparing these novel oligomers to their well studied oligothiophene analogs. Finally, in Chapter 5, a family of spirobiProDOT oligomers is synthesized with the intent to deliberately frustrate intermolecular interactions. We observe how these materials behave in systems where ion diffusion rates are important to see whether the spiro moiety is a strategy towards introducing a high degree of porosity into the material.
49 CHAPTER 2 EXPERIMENTAL TECHNIQ UES 2.1 Molecular Characterization 2.1.1 Reagents and General Synthetic Methods All reagents and starting materials used were purchased from commercial sources and used without further purification, unless otherwise noted. D iethyl ether, tetrahydrofuran, methylene chloride, toluene and heptanes were obtained from an anhydrous solvent system and were not further dried before use. Solvents used in Suzuki and Stille couplings and polymerizations were freeze pump thawed three ti mes to rigorously remove oxygen. Reactions were performed under an argon atmosphere unless otherwise noted, using standard Schlenk line techniques. 2.1.2 Molecular Characterization All new compounds were fully characterized using 1 H NMR, 13 C NMR, elemen tal analysis and mass spectroscopy, except in instances where material solubility precluded the use of one or more of those techniques. The structures of synthesized compounds which were previously reported in the literature and any intermediates were ver ified at least with 1 H NMR, and usually 13 C NMR as well. 1 H NMR (300 MHz) and 13 C NMR (75 MHz) were obtained from a Gemini 300, Mercury 300, or VXR 300 using CDCl 3 and values were referenced to the solvent residual peak (CDCl 3 : 1 7.24 ppm, 13 C NMR the services at the University of Florida. High resolution mass spectroscopy was also carried out with the services at the University of Florida using a Finnigan MAT 95Q mass spectrometer. X ray crystal structures were obtained at the University of Florida Center for X ray Crystallography.
50 2.1.3 Thermal Characterization Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC Q1000 under a helium atmosphere. Thermogravim etric analysis (TGA) was performed on a TA Instruments TGA Q5000 under a nitrogen atmosphere using a dynamic ramp rate. 2.2 Electrochemical Characterization 2.2.1 Reagents and Instrumentation for Electrochemical Characterization Acetonitrile (ACN), and pro pylene carbonate (PC) were distilled over CaH 2 and molecular sieves, respectively, prior to use. TBAP, TBABF 4 and TBAPF 6 were recrystallized and stored in a dessicator prior to use. LiBTI was received from Aldrich and used as is. A standard three elect rode cell was used to carry out electrochemical characterizations. The cell consists of a platinum button (surface area = 0.02 cm 2 ) or indium tin oxide (ITO) working electrode, a platinum flag counter electrode, and either a silver wire pseudo reference o r a Ag/Ag + reference electrode. All reference electrodes were calibrated to Fc/Fc + Electrochemical measurements were made using an EG&G PAR 273A potentiostat controlled using CorrWare software. Most electrochemical experiments were carried out under am bient conditions, however solvents and solutions were bubbled with argon for at least five minutes before use and the experiments blanketed with an argon stream when possible. HOMO and LUMO values were acquired using differential pulse voltammetry (DPV), with the measurements performed inside a glove box. The HOMO and LUMO values were calculated using a value of 5.1 eV relative to vacuum for the Fc/Fc + redox couple. A detailed discussing about the validity of this value can be found in the dissertation o f Barry C Thompson as well as in the literature. 50,51
51 2.2.2 Cyclic Voltammetry Cyclic voltammet ry (CV) is a technique used in numerous capacities throughout this dissertation, primarily for the electropolymerization and electrodeposition of materials onto the working electrode, and subsequently for the characterization of the electrodeposited materi als. In a CV experiment, the current across the working and counter electrodes is measured as a function of the applied voltage. The user controls the desired voltage scan rate as well as the voltage window to be scanned. A nice discussion of various el ectrochemical techniques specifically as it applies to conjugated polymers can be found in a recent review by Heinze. 52 Here, we will touch briefly on a few concepts. During an anodic CV electropolymerization, radical cations are generated in situ as the applied voltage sweep exceeds the oxidation potential of th e monomer. The oxidized molecule can now couple with other systems and electropolymerize. While the precise mechanism has been a subject of debate, a plausible route is illustrated in Scheme 2 1, beginning with the dimerization of two oxidized monomers, followed by the loss of two hydrogens. Because oxidation of this extended chain occurs at lower potentials than oxidation of the starting material, the system is already in a regime where oxidation of the dimer can occur. This oxidized dimer can now coup le with other oxidized systems to further the electropolymerization. In the first cyclic voltammetric scan during electropolymerization, the use of the onset of the current response as an estimate of the oxidation potential of the monomer (and by extensi common practice as shown in Figure 2 1 A However, the validity of this relationship is complicated by a number of factors, such as the effects of electrolyte and solvent,
52 image charge effects at the electrode surface, and the concurrence of Faradaic and non Faradaic charging. For reversible redox processes, a model has been formulated that establishes the validity of using solution CV to derive HOMO and LUMO values, however this model does not extend to electropolymerizable systems (which are irreversible), or thin films. 53,54 In S ection 2.2.3 we will briefly discuss differential pulse voltammetry, a technique which is able to deconvolute the Faradaic and non Faradaic processes. Scheme 2 1. Proposed mechanism for the anodic electropolymerization of thiophene. In spite of this limitation, CV is a useful tool to monitor the progress of electropolymerization. Oligomers a nd polymers formed should give rise to new peaks at lower potentials in the anodic sweep that were absent in the first scan, as demonstrated in Figure 2 1 B Another phenomenon is that as polymerization proceeds, material is electrodeposited onto the elect rode surface, effectively increasing the electrode surface area. As a result, the current passing through the electrode increases with each successive scan.
53 Figure 2 1. Electropolymerization of BiPheDOT a) First CV scan of a 5 mM solution of BiPheDOT in methylene chloride using TBAP as electrolyte. The monomer oxidation is estimated by the onset of the current response. A reduction peak far removed from the monomer redox process is identified as poly mer reduction. b) First through tenth CV scan of BiPheDOT electropolymerization, showing steady increases in current densities over ten cycles, as well as the emergence of a polymer oxidation peak. Once a film has been deposited, cyclic voltammetry can be used to characterize several properties of the material. Estimating the oxidation potential of a film at the onset of oxidation encounters the same problems that we discussed in the monomeric case above. However, when the electrodeposited film is a reve rsible or quasi reversible system, then the half wave potential E 1/2 is a thermodynamically meaningful quantity. If the peak oxidation potential E p,o and the peak reduction potential E p,r are known, then the E 1/2 can be estimated by, E 1/2 = (E p,o + E p,r ) / 2 (2 1) oxidation and reduction peaks, p,o E p,r (2 2) There is, in part, a physical component which give rise to this phenomenon, although a number of other factors m ay also play a role. 52,55 As the oxidation and
54 reduction of a film depends on the ability of ions to diffuse in and out of the material, structures and morphologies which allow for rapid ion diffusion should have a narrower hysteresis. The hysteresis can therefore provide a qualitative understanding of the fil m morphology. The concepts in E quations 2 1 and 2 2 are illustrated below in Figure 2 3. Figure 2 2 CV of a film of pBiPheDOT i n 0.1 M TBAP with ACN as solvent. The peak oxidation and potential values are denoted, as well as the E 1/2 and hysteresis of the polymer film. 2.2.3 Differential Pulse Voltammetry In a cyclic voltammagram, the current response measured is comprised of a F aradaic component and a non Faradaic component. The first refers to electron transfer processes where charge is removed from the conjugated system and injected into the electrode, or vice versa. For example, when evaluating processes such as oxidation or reduction potentials of a material, these are Faradaic processes. Non Faradaic processes are comprised of charges adsorbing or desorbing onto and off the electrode surface without charge physically crossing the plane of the electrode. Non Faradaic charg ing occurs as a potential is applied across an electrode, leading to
55 charge accumulation at the surface of the electrode. In order to satisfy charge balance, counterions in solution migrate towards the electrode and adsorb onto the surface. While for mos t systems involving conjugated materials the Faradaic component dominates the current voltage response, non Faradaic charging typically comprises the background current, making it challenging to deconvolute the two processes. Differential pulse voltammet ry (DPV) is a technique that subtracts the non Faradaic component and in essence enhancing the signal to noise ratio such that the Faradaic process is more adequately resolved. As such it is an excellent technique for determining precise onset values of r edox processes, and by extension for determining oxidation and reduction potentials of oligomers and polymers. A more thorough treatment on the matter is discussed in the dissertations of Chris Thomas and Emilie Galand. 56,57 2.2.4 Evaluation of Film Capacitance The charge storage ability of the electrodeposited film can also be determined using CV. A good discussion of supercapacitor fundamentals and background is described in the dissertation of M erve Ertas. Briefly, the capacitance C of a material is defined as the amount of charge Q passed over a given voltage window, C = Q/V (2 3) In an ideal capacitor, the capacitance is not a function of the applied voltage. Then, as the voltage in creases steadily with respect to time, the charge passed should also increase at a constant rate with respect to time. To put it another way, if we divide the numerator and denomin ator of the right hand side in E quation 2 3 by time t, we get, C = (Q/t) / (V/t) (2 4)
56 A steady change in charge with respect to time denotes a constant current i, while V/t is the scan rate s, whic h is a preset parameter. Then E quation 2 4 simplifies to, C = i / s (2 5) Under these conditions, the current vol tage plot would be rectangular in shape; a film displaying near ideal behavior is shown in Figure 2 4, and C can then be easily calculated from the current voltage plot, C = i / s (0.1 mA/cm 2 ) / (50mV/s) = 0.002 F/cm 2 = 2 mF/cm 2 Figure 2 3 Current voltage plot of a hypothetical polymer film displaying near ideal capacitance behavior. For instructive purposes, the current density is estimated at 0.1 mA/cm 2 and the film is scanned at 50 mV/s. In practice for conjugated systems however, the amount of charge passed depends on the applied voltage, and as a result the current voltage plot deviates from a rectangular shape. Figure 2 5 displays an actual current voltage plot of a polymeric material. Because of this dependence of Q o n V, the capacitance is no longer constant over the potential window. However, the average capacitance of a material for a given
57 potential window can be discussed instead. The current voltage plot can be converted to a current versus time plot by dividin g the x axis values of V by the scan rate. If we then integrate the current as a function of time, we can obtain the total charge passed. Then, by dividing Q by the voltage window, we can acquire an average capacitance over the voltage window, defined as : C avg = Q total / V (2 6) A faster estimate of C avg can be obtained if the average current over a voltage window can be reasonably estimated. In Figure 2 5 for example, the average current can be taken at the midpoint of the current voltage curve Then, C avg = i avg / s (2 7) Figure 2 4 Typical current voltage plot of an electrodeposite d polymer displaying a curve that deviates significantly from the rectangular plot in Figure 2 4. An average current density can nevertheless be estimated to acquire the average capacitance of the material over the given voltage window. It is worth emphasizing that the calculations above utilize current and charge density values (i.e. C/cm 2 and A/cm 2 respectively), and so the c apacitance obtained is the areal capacitance (F/cm 2 ). The specific capacitance (F/g) is another useful quantity,
58 however it can be difficult to accurately measure the weight of the electrodeposited material unless the surface area of the electrode is larg e enough to deposit an amount that can be measured. Then, the electrode mass is simply subtracted from the electrode + electrodeposited film to acquire the mass of electrodeposited film. When determining the efficacy of a material for use as an electroact ive charge storage material, it is also important to assess the rates at which charging and discharging occur. One strategy to evaluate this parameter is to observe how the current voltage plot evolves as a function of scan rate. The underlying philosoph y is related to the previous discussion about hysteresis, where the process of charging and discharging are, in part, kinetically constrained by the rate at which ions are able to diffuse in and out of the film. At low scan rates ( ca. < 50 mV/s), ions hav e sufficient time to move through the film. At large scan rates however ( ca. > 500 mV/s), ions are unable to diffuse rapidly enough, and the film is more resistive, leading to a decrease in capacitance. Figure 2 6 shows a characteristic set of current vo ltage plots taken at different scan rates. The capacitance of the film for a given scan rate has been determined by estimating an average current density and utilizing E quation 2 7 to obtain the series of plots. Qualitatively, the current voltage curves adopt a more resistive shape at increasing scan rates, accompanied by an increase in the hysteresis of the redox process. Quantitatively, this is also mirrored in the large drop in average ca pacitance. The electrochemical techniques outlined here provide a rapid and facile way to screen molecular systems as well as film growth conditions to identify candidates for use in supercapacitor device.
59 Figure 2 5 Current v oltage plots of PBiPheDOT at three different scan rates. The average currents have been estimated at the midpoint of the voltage window. 2.2.5 Potential Square Wave Chronoabsorptometry Related to the discussion about scan rate dependence in the previous s ection, the rate at which a film switches is in part kinetically determined by the rate at which ions can diffuse through the film, which can depend on morphological properties such as how strongly molecules within the film are packed, and conversely how p orous the film is. In a potential square wave chronoabsorptometry experiment, a film is switched between its neutral and doped states using a potential square wave, and the evolution max is ob served. For many materials with colorimetric applications, the rate at which a system switches between two states is an important parameter. Chronoabsorptometry allows for the max is an ap propriate estimation of the rate at which the film as a whole changes color across the whole visible spectrum.
60 In Figure 2 7 A we can see the general features of such an experiment. When the film is switched from 700 mV to 700 mV, for example, the bleach max is not instantaneous. Rather, a majority of regions on the film are doped shortly after the switch, followed by a tailing response that reflects a slower doping of the remaining regions. This delayed response may arise from deeply imbedded re gions that require more time for the diffusing ions to reach. Alternatively, in the case of polymer chains, more disordered segments will have deeper HOMO levels and be more difficult, and therefore slower, to oxidize. Computational modeling of the decay response may be a strategy to elucidate and disentangle the various phenomena responsible for this behavior. Another important parameter taken from chronoabsorptometry is the contrast max between the two applied potentials. For applications such as electrochromic windows, th e ability to switch rapidly is not in itself sufficient; rather, the need to switch between a colored state and a highly transmissive state is also crucial, i.e. a material with a high contrast ratio is necessary. The time it takes for the film to comple max is often used to human eye to the remaining 5% is negligible. In Figure 2 7 A the times to reach 90%, 95%, and 100% have been indicated. Depending on the application, it may be more useful to observe how much of the contrast ratio is retained at different switching speeds. In Figure 2 6 B we see that the contrast ratio decreases as the switching speed increases, reflecting the diminished t ime that ions have to diffuse through the film and fully dope or dedope a material.
61 Figure 2 6 Chronoabsorptometry of a polymer film on electrochemically polymerized onto ITO and switched in a solution of 0.1 M TBAPF 6 in ACN. In figure a) the times required to achieve 90%, 95% and 100% contrast are shown. In figure b) the evolution of the contrast ratio is shown as dependent on the time allotted for each switching event. 2.3 Absorbance and Emission Sp ectroscopy 2.3.1 Reagents and Instrumentation for Spectroscopy Absorbance measurements were taken with a Varian Cary 500 UV vis NIR spectrophotometer. Thermochromism was performed using a dual cell Peltier thermocouple. Emission spectra were ob tained on a Fluorolog 3. 9,10 D ex f = 0.9). 2.3.2 Evaluation of Intramolecular Organization from Absorbance and Emission Spectroscopy The shape of the absorbance profile can provide both a qua litative and quantitative understanding of the intramolecular organization of a material. As discussed more thoroughly in Chapter 1, when an electron absorbs a photon and is excited, a nuclear deformation occurs to compensate for the change in the electro nic well ordered system that is conformationally constrained, the excitation is coupled to a
62 small number of vibrational modes. When the molecule is excited, tr ansitions arise from those discrete vibrations to give an absorption profile characterized by a number of distinct peaks. This point is illustrated in Figure 2 8 where due to the presence of only a small number of vibrational modes in each energy levels, the transitions can be well resolved in the absorbance pro file as vibronic fine structure. Figure 2 7 Energy level diagram illustrating transitions between the ground state and excited state to various vibrational levels. Note that while absorption i s only shown as occurring from the lowest vibrational mode, that transitions from other levels are possible as well; emission, however, typically only occurs from the lowest S 1 fine structure are al so illustrated, and a hypothetical absorbance spectrum is shown at right. Conversely, the absorbance of an intramolecularly disordered molecule (i.e. a molecule with many degrees of freedom) will be broad and featureless. This phenomenon is observed beca use there are a number of conformations from which
63 excitation can occur, as shown in Figure 2 9, and the coupling of the electron to the nuclear lattice can be achieved through any number of lattice distortions. 58 In this example, free rotation between neighboring thiophenes gives rise to a number of ground state configur ations (and hence energies) from which excitation could occur, leading to a broad and featureless absorption profile. Figure 2 8 Energy level diagram illustrating the large number of possible vibrational transitions when a molecule possesses numerous degrees of rotational freedom, giving rise to a broad and poorly resolved absorbance spectrum. In well ordered conjugated systems, the exciton is coupled mainly to C = C stretching modes as the molecule shifts from an aromatic to a quinodal geometry. IR 59 and Raman 18 spectroscopy can be employed to confirm the presence, as well as the relative prominence and frequencies, of one or more different vibrational modes. This information can then be used to analyze the spacing between the vibronic peaks in the absorbance profile. The electronic excitation in a f ilm of poly (3 alkylthiophene) for 30 twist 0 twist other twist a ngles broad absorbance with poorly resolved features
64 example is coupled to a C = C stretch with a frequency of 1464 cm 1 (0.18 eV). 60 As a result, the vibronic peaks should also be separated by approximately this value. This allows for the rapid screening, identification and assigning of peaks in the absorbance profile. Any peak sepa rated by a significantly different value likely arises from a different source, such as from aggregate absorption, as described in more detail below. While the vibronic peaks are not always prominent in the absorbance spectra, the emission spectra typical ly features well resolved fine structure. This is due to two properties of the excited state first, the quinodal excited state geometry, having double bond character between the conjugated rings, is more conformationally constrained than the aromatic gr ound state, reducing the vibrational degrees of the lowest energy excited state, limiting the number of transitions we would expect to occur. In Figure 2 10, the spe ctra of a thiophene oligomer (2,5 bithienyl) PheDOT) show that the absorbance profile gives rise to a broad response with faintly resolved peaks, while the emission spectrum displays much more pronounced vibronic coupling. To more accurately quantify the reorganization occurring between the neutral and excited states, the Huang Rhys parameter described in Chapter 1 can be extracted from the progression of vibronic features in either the absorbance or emission spectra. Specifically, the intensities of the vibronic peaks is described by a Frank Condon progression, I 0 n / I 0 = S n e S /n! (2 8) where S is the Huang Rhys factor, n is the vibrational level, I 0 n is the area of the peak associated with the 0 n transition, and I 0 is the total area of the spectrum. There are
65 several assumptions being made in Equation 2 8. First and foremost is that the excitation is coupled predominantly to one vibrational mode. In the case of polythiophenes, it has been established that while other lattice distortio ns are also coupled to the exciton, the dominant vibration arises from the C = C stretching mode, and models utilizing this assumption have successfully reconstructed the spectra of polythiophenes. 18 Secondly, it is important when selecting peaks to fit to the above equation that the peaks are indeed vibronic in origin. This concept is illustrated in a nice study th at elucidated the absorbance and emission spectra of films of regioregular poly 3 hexylthiophene. 18 In the absorb ance profile of rrP3HT a low energy transition at ca. 2.04 eV had long been associated with the 0 0 transition; however the failure of the Frank Condon progression to accurately describe the observed peak intensities, as well as the success in predicting the intensities if this low energy transition were omitted, led to a reevaluation of this assignment as due to aggregate absorption, which was corroborated by further experiments. While the Huang Rhys factor can be calculated from both the absorbance and emission spectra, the latter is again often more convenient to use. The implicit assumption, then, if we wish to extract a single Huang Rhys factor to describe the entire system, is that the relaxation energies associated with a transition from the neutra l to the excited state, and from the excited state back to the neutral, are equivalent. Fortunately, in many conjugated systems, the relaxation energies of both processes are similar. Experimentally, then, we can take the emission spectrum in Figure 2 10 to generate a Huang Rhys factor for the system using Equation 2 8. Doing so results in a value of S 1.
66 Figure 2 9 Emission and absorbance spectra of (2,5 bithienyl) PheDOT) in THF. The vibronic coupling is calculated by the clearly resolved 0 0 and 0 1 peak separation in the emission spectrum. The Huang Rhys factor is also calculated from the relative emission intensities. 2.3.3 Evaluation of Intermolecular Organization from Absorbance and Emission Spectroscopy T he absorbance profile can also provide insight into the intermolecular organization of a system. This requires first establishing the absorbance of the unaggregated species for comparison. One strategy is to prepare as dilute a solution of the sample in question as possible. However, the dilution at which the material achieves an unaggregated state may be below the detection limits of the spectrophotometer. A supplementary strategy would be to employ thermochromism as a means to further break apart the aggregates. In a thermochromic experiment, a thermocouple is connected to the spectrophotometer and the absorbance profile is measured as a function of the temperature. At increasing temperatures, the nuclear motion (ring twisting, bond stretching, etc.) and the overall entropy of the system
67 increases and favors the separation of aggregate structures. At the same time however, an increase in intramolecular disorder such as increased free rotation between rings leads to a breaking of the conjugation l ength, which can lead to noticeable changes in the absorption profile. However, the effects of distorting the conjugation length are typically not difficult to identify and interpret. Once the absorption profile of the unaggregated system is determined, t he absorbance of the aggregated system can be analyzed. This can be accomplished in solvent typically a polar solvent such as methanol to induce the material to aggre gate. Alternatively, the solid state properties of the material can be measured from drop, spin or spray cast films. When comparing the absorbance of the solid state versus the dilute solution, a new red shifted peak is often present. As discussed in Ch apter 1, close intermolecular packing leads to orbital mixing between the two systems, generating a lower energy aggregate HOMO LUMO gap. In special cases where the exciton is coupled predominantly to a single lattice distortion, and the disorder of the sy stem is not significant, intermolecular interactions can be further quantified from the intensities and ratios of vibronic peaks. As discussed in Chapter 1, strongly interacting oligomers cannot be rigorously analyzed with this model, as the excitons are typically delocalized over larger correlation lengths. Most polymers, however, lend themselves readily to the analyses outlined below, as the orbital mixing between polymer chains is strongly attenuated, leading excitons to rest on one polymer chain. Bec
68 disorder, the analysis is further restricted to polymer films, where polymer solutions and blends are also unlikely to fit the model beyond a qualitative point. In the absorbance profile of the polymer film, the relationship between the ratio of the aggregate 0 0 and 0 1 transitions and the exciton bandwidth W is complicated. However, if we assume that the Huang Rhys factor S 1, then, A 0 0 /A 0 1 [(1 0.24 W / E p ) / (1 + 0.073 W / E p )] 2 (2 9) Where E p is the energy of the vibrational mode to which the exciton is coupled, a value typically estimated as 0.18 eV for thiophene based polymers. 61 This relationship is invaluable in the optimization of processing conditions, such as determining i deal annealing temperatures or solvents from which to cast the film, as it provides a rapid means of assessing exciton bandwidth, which is directly proportional to the strength of the intermolecular interactions present in the polymer film. We consider an example spectrum in Figure 2 10. Using Equation 2 9, the values of A 0 0 and A 0 1 shown in Figure 2 10 and 0.18 eV for E p we find that W = 17 meV for films drop cast from toluene, and W = 3 meV for films drop cast from chlorobenzene. Even if our estima te of E p is slightly off, the trend holds that the exciton bandwidth is smaller for the film cast from chlorobenzene than for the film cast from toluene. W e recall from Chapter 1 that the exciton coupling matrix decreases, for a fixed intermolecular separ ation distance, as chain length increases, then a smaller value of W here implies longer conjugation lengths. Specifically, this suggests that films cast from toluene give rise to shorter effective conjugation lengths (through disorder related to intramol ecular disorder, for example) which give rise to larger intermolecular interactions with neighboring polymer chains. Films cast from chlorobenzene on the other hand give rise to longer effective
69 conjugation lengths. In this way, the absorbance spectra ca n provide information about the intermolecular interactions in polymer films and provide a means to both rapidly screen processing conditions and also to correlate them to device performance. Figure 2 10 Absorbance spectra of po lymer thin films drop cast from different solvents. As a final note, other parameters can also provide information about the organization of the system, but are more difficult to evaluate. For example, the peak widths are directly related to the disorder of the system, and whether the lines are broadened homogeneously or inhomogeneously provide further information about the nature of the disorder present. However, these assessments can prove to be difficult and understanding such properties is greatly fac ilitated with computational software that can assess whether, for example, a broad peak is due to disorder or is in fact due to the superposition of multiple peaks.
70 CHAPTER 3 WELL ORDERED PHEDOT OLIGO MERS AND POLYMERS 3.1 Motivation to Develop PheDOTs Po ly(3,4 ethylenedioxythiophene) (PEDOT) is among the most widely utilized conjugated polymers today. Its attractiveness owes to high conductivities on the order of 1000 S/cm, a combination of low oxidation potential coupled with a moderate bandgap, its sta bility, and electrooptical properties such as its transmissiveness in the conducting state, to name a few of its desirable characteristics. 62 Processability of the polymer however is limited due to its insolubility, and a great deal of PEDOT literature is focused on finding ways around this limitation. When the polymer is doped with poly(styrenesulfonate) (PSS), for instance, a water p rocessable dispersion forms from which high quality films can then be cast. 62 Vapor phase polymerizations (VPP) which utilize EDOT vap ors to coat an oxidant treated surface have been fine tuned to yield some of the highest conducting PEDOT films to date. 63 65 Electrochemical polymerization has also yielded relatively high conductivity films, a lthough this process generally limits the types of substrates that can be coated to conductive surfaces, a constraint that is bypassed using PEDOT:PSS or VPP. The more traditional chemical polymerization yields insoluble material, as previously mentioned, however functionalizing the ethylene bridge with long alkyl chains improves the solubility to some extent. The primary disadvantage of this route is that the alkyl substituted EDOT cannot be coupled in a regioregular fashion, leading to disorder in the p olymer. Additionally, substitution at an sp 3 center leads to alkyl chains which protrude out of the
71 plane of the conjugated backbone, which frustrates packing and ordering of the material. 3,4 Phenylenedioxythiophene (PheDOT) was synthesized to address several of these issues. As we can see from crystal structures of 3,4 dibromoPheDOT in Figure 3 1, the mole stacking interactions. 66 Additionally, substitutions on the ph enyl ring occur at sp 2 centers, which allow them to protrude outwards yet still in the plane of the molecule preserving the planarity of the overall structure. Furthermore, di substitution on the 4 and 5 positions of the phenyl ring would allow polymerize d systems to retain regiosymmetry. Thus it was believed PheDOT could retain the desirable electroactive properties of EDOT while improving on the structural properties. Figure 3 1. X ray structures of PheDOT Br 2 taken from ref 6 illustrating th e planarity of the molecule. An initial route towards PheDOT was laid out in 2003, with an improvement on the electrochemical polymerization of PheDOT. 67,68 The results in that work directly compared the electropolymerization of EDOT with that of PheDOT, from which the authors concluded that PheDOT electropolymerizes relatively poorly. This was explained by the oretical calculations which demonstrated that the cation radical of PheDOT shows low electron density at the 2 and 5 positions of the thiophene moiety,
72 which are the sites through which electropolymerization would occur. As a result, the reactivity at the se sites is similarly low, leading to slow coupling of the cation radicals. In contrast, the cation radical of EDOT is shown to have a high electron density on the 2 and 5 position of the thiophene moiety, leading to a higher reactivity and faster electro polymerization. The Roncali group followed this work with the synthesis and electropolymerizations of biPheDOT and terPheDOT oligomers. 69 While theoretical calculations show that the electron d ensities of the cation radicals are much higher at the electropolymerization sites of both oligomers, the excruciatingly low solubitilies of biPheDOT and especially of terPheDOT allow for only the preparation of dilute samples for electropolymerization and make these oligomers unattractive from a practical standpoint. In the Reynolds group, the first chemically polymerized PPheDOTs were synthesized in 2007 by Grenier starting with PheDOT (C12) 2 monomer, dibrominating the thiophene ring, and then polymerizin g via Grignard metathesis to form PPheDOT (C12) 2 in high yields and relatively high molecular weights. 70 In spite of the long dodecyl solub i lizing chains appended onto the phenyl ring, the polymers were only soluble at high temperatures. Thermochromic absorption spectra and temperature dependent fluorescence measurements strongly suggested that the polymers existed in an aggregated state in solution at room temperature, whereas at higher temperatures the aggregates w ere at least in large part broken up and the polymer better solub i lized. In spite of the insolubility at room temperature, 2D WAXS of an extruded film, shown in Figure 3 2, verified several conjectures outlined above; in particular it demonstrated that th stack efficiently and that the alkyl chains were in fact
73 extended outwards in the plane of the molecule. Fully room temperature soluble PPheDOTs were finally achieved by Grenier with the synthesis of PPheDOT C12EtHx, which possesse d one linear dodecyl and one branched 2 ethylhexyl solublizing group on the phenyl ring. 71 polymerization to occur in a regioregular fashion more precisely, if the linear and r, then it is unclear that monomers couple in a consistently head to tail fashion. Additionally, a mixture of R and S stereochemistries are present at the stereocenter on the ethylhexyl chain. Given these findings, we decided instead to turn our attentio n to synthesizing a family of PheDOT monomers, from which a series of PPheDOTs could be electropolymerized to assess how its electroactivity compared with that of well established PEDOT. Figure 3 2. polyPheDOT (C12) 2 film morphology as imaged by a) 2D WA XS of extruded film, and b) the corresponding model derived from a) illustrating the intra and intermolecularly well ordered nature of the film, taken from ref 5. 3.2 PheDOT Monomers 3.2.1 Synthesis of PheDOT Monomers The synthesis of PheDOT occurs via a transetherification between commercially available dimethoxythiophene and catechol (1,2 benzenediol, 3 1 ) catalyzed by p TSA in toluene (step d in Scheme 3 1). Whereas transetherification of alkyldiols and dimethoxythiophene proceeds readily such as wit h ProDOTs where the reaction
74 proceeds in ca. 70% yields overnight transetherification of 1,2 benzenediol and dimethoxythiophene to form PheDOT occurs in typically 20% or less yields and requires 5 days to achieve. This is due to the significant reductio oxygen atoms compared with the oxygen atoms of alkyldiols. This reduction in delocalize into the benzene ring, which in turn reduces thei r reactivity. Additionally, methanol is released during the course of the reaction which could both compete with catechol during the transetherification process and could also react reversibly with the product to regenerate starting materials. Attempts t o remove methanol to drive the equilibrium towards the desired products were surprisingly unsuccessful at improving yields. Scheme 3 1. Route towards substituted PheDOT R 2 via Friedel Crafts acylation. a) AlCl 3 alkanoyl chloride, DCM; b) LiAlH 4 AlCl 3 diethyl ether; c) BBr 3 DCM; d) 3,4 dimethoxythiophene, p TSA, toluene, reflux, 5 days.
75 The functionalization of the benzene ring must be done prior to the transetherification as the thiophene ring is more electron rich tha n the benzene ring in PheDOT. Friedel Crafts acylation, for example, leads to functionalization of the thiophene moiety rather than the benzene ring. One means of functionalization can be accomplished by Friedel Crafts acylation of veratrole to give comp ound 3 2 followed by reduction of the carbonyl group to give monoalkylveratrole 3 3 A second acylation and reduction generates the disubstituted veratrole 3 5 which after deprotection with BBr 3 results in the disubstituted diol 3 6 which is then ready fo r transetherification. The benefits of this route are that on smaller scales the purification is not difficult at each step products are readily resolved from impurities via column chromatography. Yields at each step are ca 80%. The drawbacks of this route arise when the reaction is scaled up often on the 100 g scale in order to compensate for the low yields of the final transetherification step. Purification by column chromatography becomes less practical at this point; fortunately the products of each Friedel Crafts acylation and reduction can be dissolved in hot methanol or ethanol and will precipitate out of solution when left at room temperature or cooled in the refrigerator. Yields are worse using this purification technique however, typica lly around 60%. The biggest drawback of this route however is that it only allows for functionalization with alkyl chains. A second means of functionalization can be accomplished by halogenating the benzene ring first and then functionalizing the ring via metal mediated cross coupling reactions, in particular here via Sonogashira coupling. In Scheme 3 2, veratrole (1,2 dimethoxybenzene) is iodinated to compound 3 11 followed by Sonogashira coupling to give the dialkyne 3 12 The triple bond can be reduce d with palladium on carbon and
76 hydrogen gas to yield the disubstituted alkylveratrole 3 13 which upon deprotection with BBr 3 gives the disubstituted catechol 3 14. Compared to the previous route, the yields are comparable if not better, and the overall re action requires one less step. Additionally, careful purification is not necessary until after the hydrogenation reaction: the iodination product crashes out during the reaction and is merely collected on a Bchner funnel and washed with methanol; the pro duct of the Sonogashira reaction can be flushed rapidly through a silica gel plug. Even the product of the hydrogenation reaction only requires passing through a thin plug to remove the carbon. These relatively trivial purifications make scaling up the r eaction more practical. Finally, the Friedel Crafts route in Scheme 3 1 was unable to be utilized to attach two bulky groups to the benzene ring. After the addition of a first 2 ethylhexyl group, a second 2 ethylhexyl group did not react with the ring. 66 While this novel Sonogashira route in Scheme 3 2 was not put to this test directly, two bulky 3,3 dimethylbutyl chains were succ essfully attached using this route. The one synthetic drawback is the continued need for BBr 3 on a relatively large scale in the final step to make the bisalkylated catechol. Additionally, the current equipment used for the hydrogenation step in partic ular the various Parr reaction vessels does not allow for large scale reactions. Ideally, a wider variety of functional groups can be attached as well utilizing this route, compared with the Friedel Crafts route, however finding functional groups that r emain stable under BBr 3 deprotection continued to limit our scope to using alkyl chains. Theoretically, the deprotection can occur after the iodination and the oxygens reprotected with a group that can be cleaved under milder conditions. For our investig ation here however we have decided to limit our scope to the family alkyl
77 substituted molecules shown in Figure 3 3 in developing a better understanding of the effects of alkyl subs titution on the electro polymerization of PheDOTs. Scheme 3 2. Modified synthetic s cheme towards disubstituted PheDOT R 2 via Sonogashira coupling. a) H 5 IO 6 I 2 methanol, 70 C; b) 1 alkyne, Pd(PPh 3 ) 2 Cl 2 TEA, 70 C; c) H 2 Pd/C, methanol/ethyl acetate; d) BBr 3 DCM; e) 3,4 dimethoxythiophene, p T SA, toluene, reflux, 5 days. Figure 3 3. Family of PheDOT derivatives synthesized for this dissertation. While the transetherification step proceeds in low yields as previously discussed, the purification of the family of PheDOT molecules shown in Figure 3 3 is straightforward, requiring only a hexanes column for good separation and giving the desired PheDOT as either a white solid or colorless oil. Interestingly, PheDOT sublimes
78 at a fairly low temperature, ca. 60C, whic h makes vapor phase polymerization onto an investigated whether the functionalized PheDOTs sublime, it seems likely that the shorter chain derivatives should also be able to. All the PheDOTs exhibit robust stability under ambient conditions, with a slight yellowing being observable on occasions after over a year of storage with exposure to oxygen and room lighting. While the 2,5 dibrominated monomer is not mentioned in this dissertation until the following Chapter, it is worth stating here that 2,5 dibromoPheDOT is similarly very stable, in stark contrast to 2,5 dibromoEDOT, which spontaneously polymerizes. Curiously, however, the monobrominated 2 bromoPheDOT turns f rom colorless to dark crystalline solid within days and requires rapid handling if it is to be used in a reaction. The mechanism through which 2,5 dibromoEDOT reacts (and presumably 2 bromoPheDOT as well) is through the loss of two bromines on adjacent mo nomers to release bromine, followed by the coupling of the two monomers. Previous work in Wudl's group has demonstrated that the reactivity of the monomer is related to how closely two halogen atoms are located on adjacent monomers. 72 The bromine bromine distance in 2,5 dibromoEDOT their van der Waals radii. In contrast, Roncali reported that 2,5 dibromoPheDOT crystals have bromine bromine distances in excess bromoPheDOT they were not of sufficien t quality for analysis by X ray crystallography to confirm our hypothesis. 3.2.2 The Electrochemical Polymerization of PheDOT Monomers We begin by investigating the electrochemical polymerization of PheDOT. Previous studies by Roncali have demonstrated t hat electropolymerization proceeds
79 poorly; this has been shown computationally to be due to the low electron density on the 2 and 5 positions of the oxidized thiophene ring, which reduces the likelihood of coupling through these positions. In a well behav ed electropolymerization, each scan results in the electrodeposition of polymer onto the electrode. On the subsequent sweep this additional amount of polymer contributes to an increase in the current density for both polymer and monomer redox processes. This is due to the fact that the deposited layer increases the overall surface area of the effective electrode and also contributes its own redox process to the overall current density profile. Typically after 10 cycles, a significant amount of polymer ha s been deposited such that a sizeable current density is observed in the window where polymer oxidation and reduction take place. As Figure 3 4 A illustrates however, the electropolymerization of PheDOT from an acetonitrile solution results in hardly any i ncrease in current density after 10 scans, suggesting that very little material has been electrodeposited. Additionally, no redox processes are discernable, except for the peak corresponding to monomer oxidation at 950 mV. In contrast, Figure 3 4 B illustr ates that when the solvent is replaced with methylene chloride, a noticeable improvement in electropolymerization is observed. The current density after 10 scans is much larger and redox processes corresponding to oligomer or polymer chains can be disting uished. The inset to Figure 3 4 B shows the full progression and evolution of the current density over 10 scans, illustrating steady electrodeposition throughout. In Figure 3 5 we observe the electrochemistry of a film of PPheDOT electropolymerized via pote ntiodynamic cycling from a 10 mM monomer solution in methylene chloride. Two oxidation processes are readily apparent. Based on the redox
80 properties of high molecular weight PPheDOT (C12) 2 chemically synthesized in our group by Grenier, the peak at lower oxidation potential is close to that of high molecular weight PPheDOT, suggesting that this peak is due to the presence of much longer oligomers. The peak at higher oxidation centered at 480 mV is harder to assign at present, as it may correspond with a short oligomeric species or with bipolaron formation. Below we will utilize spectroelectrochemical data to assign this peak as an oxidation process associated with shorter oligomeric species. Figure 3 4. The 1 st and 10 th scan of the electropolymerization of a 10 mmol solution of PheDOT a) 0.1 M TBAPF 6 in ACN, and b) TBAPF 6 in DCM; the inset of b) shows all 10 scans of the electropolymerization. The film in Figure 3 5 was cycled 50 times, with every 1 0 th scan shown. The current density after 50 scans is nearly identical to the first scan, illustrating that the polymer film is quite stable towards repeated electrochemical switching. There are ling to too high a potential can overoxidize the polymer and lead to either side reactions or decomposition pathways. Additionally, during film oxidation and reduction, counterions continuously penetrate and leave the film in order to satisfy charge balan ce. This repeated swelling of the film can also degrade the film quality. The robustness of the PPheDOT film over
81 repeated switching is a promising indicator that makes the system, as well as the entire family of PheDOT monomers, good candidates for furt her study for electrochemical applications. Figure 3 5. Voltammograms of PPheDOT electropolymerized from a 10 mmol solution of PheDOT in 0.1 M TBAPF 6 in DCM. The polymer film was then switched in a solution of 0.1 M TBAPF 6 in AC N for 50 cycles, with every 10 th cycle shown above. The formation of electropolymerized PPheDOT films on ITO of sufficient quality for spectroelectrochemical analysis was difficult to achieve. Potentiostatic deposition from a 10 mM solution of PheDOT in m ethylene chloride for over 10 minutes was required for a film of modest thickness, and the appearance of the film was inconsistent and patchy. Increasing the concentration of monomer from 10 mM to 50 mM in methylene chloride however formed films of much h igher quality requiring less than a couple minutes of deposition. In particular, a much thicker film was grown in approximately an order of magnitude less time, and the appearance of the film was far more uniform in color and texture. This improvement is reflected in the spectroelectrochemistry of both films (Figure 3 6). In the spectra of the two neutralized films, the greater thickness of the PPheDOT grown from the 50 mM solution is apparent in the greater absorbance of the
82 film. Analyzing the neutral spectra in more detail, we can see more pronounced vibronic peaks in the film formed from the 50 mM solution than from the 10 mM solution, suggesting that films from the more concentrated solution form more ordered systems. A comparison of the optical ba ndgaps of both films is misleading: at first glance it appears the film formed from the less concentrated solution has a lower bandgap, suggesting the presence of longer conjugation length chains compared with the film formed from the more concentrated sol ution. It is also apparent however that the presence of doped regions within the film. A more telling indicator can be derived from the position of the vibronic peaks in t he visible region, where the peaks of the film formed from the 10 mM solution are blue shifted by 20 nm relative to the peaks of the film formed from 50 mM solution. This suggests that films from the more concentrated solution form chains of longer conjug ation lengths. As the films are oxidized, the formation of a polaron and (upon further oxidation) a bipolaron give rise to new signals whose wavelengths can also provide information about the conjugation lengths over which the charged species is delocaliz ed. In Figure 3 6 A the dominant polaron peak is centered near 750 nm while the dominant bipolaron peak is centered around 1400 nm. In Figure 3 6 B both these values are red shifted to ca. 800 nm and 1600 nm. This trend suggests that the polaron and bipol aron charges are also delocalized along longer chain segments in films grown from the 50 mM solutions. Additionally, the peak values of the neutral and oxidized film, as well as the optical band gap of the neutral film formed from the 50 mM solution corre spond closely with values obtained from high molecular weight pPheDOT (C12) 2 This makes it reasonable to conclude that films
83 from high concentration solutions of PheDOT lead to well ordered systems of near polymer length. In terms of the film color, the polymer switches from purple in the neutral state to a transmissive green in the oxidized state. In the polaronic absorption, two distinct transitions are observed: a dominant peak centered around 800 nm and a shoulder centered near 1000 nm that coalesce s into the main peak. This dominant peak tails sharply into the visible region and gives the oxidized state its green hue. In summary the films from more concentrated solutions give films of dramatically higher quality with more ordered and conjugated ch ain lengths that readily accommodate polaron and bipolaron formation. Figure 3 6. Spectroelectrochemical results for PPheDOT prepared from a solution of a) 10 mmol and b) 50 mmol PheDOT dissolved in 0.1 M TBAPF 6 in DCM and potentiostatically polymerized onto ITO slide; the film was switched in a solution of 0.1 M TBAPF 6 in ACN a) at 600 mV and from 200 to 1100 mV; b) at 770 mV and from 270 to 1030 mV. Often in electropolymerizations, it is difficult t o evaluate whether the reaction proceeds at the expected sites in this case through the 2 and 5 positions of the thiophene moiety and to what extent side reactions are occurring. In the case of PPheDOT, it has been speculated that the benzene moiety m ay also become incorporated into the conjugated backbone. Figure 3 7 illustrates that for several model
84 compounds ressembling the dioxybenzene portion of various PheDOT monomers, the oxidation potential lies close to the oxidation potential of the thiophe ne ring. Two pieces of evidence however suggest that electropolymerization through the benzene ring is likely to be minimal, if at all present. Firstly, if the dioxybenzene moiety were incorporated into the conjugated backbone, we wouldn't expect the spe ctroelectrochemistry of the electro poly merized PPheDOT to so closely ressemble that of the chemically polymerized PPheDOT (C12) 2 Later, we will also see that the spectroelectrochemistry of PPheDOT is essentially identical to that of PbiPheDOT, a system t hat electropolymerizes outside of the region where the benzene ring oxidizes. Secondly, a large surface area glassy carbon electrode was used for the electropolymerization of PheDOT from a 50 mM solution in methylene chloride, from which a sufficient amou nt of polymer could be obtained for FTIR analysis. The spectra of pressed KBr pellets of PPheDOT and PbiPheDOT had nearly identical signals in the fingerprint region, albeit the signals were somewhat distorted. Figure 3 7. Ele ctrochemical behavior of model 1,2 substituted phenylenes illustrating the proposed source of the reduction peak at high potentials observed in Figure 3 8.
85 The presence of even short ethyl chains on the benzene ring leads to pronounced differences in the e lectropolymerization of PheDOT Et 2 when compared to the electropolymerization of PheDOT. Looking at the first scan of the monomer solution in Figure 3 8 A we observe that both the onset and peak values of monomer oxidation are shifted to lower potentials relative to PheDOT. This may seem at first surprising given that the alkyl chains are located relatively far from the redox centers on the thiophene ring; however the slight electron donating nature of the alkyl chains can increase the electron density in the benzene ring, which in turn may lead the oxygens in the dioxane ring to donate less of its electron density into the phenyl moiety and more into the thiophene core, thereby lowering the oxidation potential. Another difference in the first scan of Phe DOT Et 2 is the presence of the monomer reduction peak in the cathodic scan, which was not observed in the PheDOT cyclic voltammogram. In the case of PheDOT, the absence of a monomer reduction peak in the cyclic voltammograms and the presence of new reduct ion peaks at lower potentials illustrate that nearly all of the oxidized PheDOTs coupled to form longer chain oligomers. These oligomers, because of their extended conjugation, reduce at a lower potential; meanwhile, little to no amount of monomer remains to exhibit a noticeable monomer reduction peak. In contrast, the presence of a significant monomer reduction peak in the PheDOT Et 2 voltammogram signifies that some of the oxidized monomers do not couple with other oxidized monomers, and instead simply r educe on the reverse scan. This suggests that the presence of even short ethyl chains provides sufficient steric interaction to hinder the rate at which oxidized monomers couple. Focusing now on the overlay of the first through the 10 th scan in the inset of Figure 3 8 A we can see from the progressive
86 increase in the current density, as well as from the emergence of a redox peak around 0.4 V, that some of the PheDOT Et 2 cations are in fact coupling and depositing as oligomers onto the electrode. Unlike Ph eDOT, PheDOT Et 2 electropolymerizes equally well in both acetonitrile as well as methylene chloride, with a gradual increase in current density over ten scans, and noticeable redox processes corresponding to oligomeric units present as well. Figure 3 8. Cyclic voltammograms of PheDOT Et 2 a) F irst cyclic voltammogram of a 10 mmol PheDOT Et 2 in a solution of 0.1 M TBAPF 6 in ACN; the inset displays the progression of 10 cyclic voltammograms of the electro polymerization of PheDOT Et 2 ; b) 50 cyclic voltammograms of PPheDOT Et 2 film switched in 0.1 M TBAPF 6 in ACN with every 10 th scan shown; the arrows illustrate a decrease in current density over the fifty scans. Looking in Figure 3 8 B at the cyclic voltammo grams of PPheDOT Et 2 films electropolymerized from acetonitrile solution, we only see a single redox process with an oxidation peak at 0.56 V and a reduction peak at 0.34 V. (Although not shown, the voltammograms of films grown from methylene chloride wer e very similar). These values are very close to the redox process in Figure 3 5 for PPheDOT films which was attributed to short oligomers. In contrast to PPheDOT, no redox peaks corresponding to longer chain oligomers are apparent in films of PPheDOT Et 2
87 The electropolymerization of PheDOT Bu 2 bears similarities to that of PheDOT Et 2 (Figure 3 9 A ). For a 10 mM solution of PheDOT Bu 2 in acetonitrile, the first electropolymerization scan again displays a monomer peak and an onset shifted to lower potentia ls relative to PheDOT. Also similar to PheDOT Et 2 the monomer reduction peak is again apparent certainly if we attributed this reversibility to steric encumbrance by the ethyl chains, then the larger butyl chains should again sterically decrease the ra te of coupling of two oxidized PheDOT Bu 2 monomers. In contrast to the previous systems however, the electropolymerization proceeds more readily in acetonitrile solvent compared to methylene chloride. To illustrate this point we will consider the electro chemical behavior of PPheDOT Bu 2 films electrodeposited from both solvents. For films grown from acetonitrile solvent, shown in Figure 3 9 B multiple redox processes are apparent. While the most prominent redox processes occur at potenials corresponding to short oligomer chains, a small redox process centered between 100 and 200 mV corresponds closely with the redox process of pPheDOT (C12) 2 suggesting this process arises from longer conjugated chains. In contrast, films electropolymerized from methyl ene chloride (not shown) have only a single redox process that ressembles what we saw earlier with PPheDOT Et 2 films Figure 3 8 and is believed to be composed mostly of short oligomers. These results lead us to conclude that the electropolymerization from acetonitrile leads to superior films compared with methylene chloride. The polymeric film on platinum button also shows good stability over repeated cycling in Figure 3 9 B While after 100 scans there is some change in the overall shape of the cyclic vol tammograms, the peak currents of the redox processes are essentially unchanged.
88 Figure 3 9. Cyclic voltammograms of PheDOT Bu 2 a) 1 st electropolymerization CV of a 10 mmol PheDOT Bu 2 in a solution of 0.1 M TBAPF 6 in ACN; the inset shows the electropolymerization over all 10 scans; b) 100 CVs of PPheDOT Bu 2 in 0.1 M TBAPF 6 in ACN showing every 10 th scan. In Figure 3 10, we observe that the spectroelectrochemistry of the film also suggests that PheDOT Bu 2 electropolymerizes to form long conjugated segments and well ordered films. The absorbance of the neutral film displays a level baseline absent of trapped polarons or bipolarons. In the visible region, the absorbance displays well defined vibronic c oupling and a bandgap comparable to the PPheDOT films from concentrated solution, attesting to the well ordered and highly conjugated nature of the system. As with PPheDOT, the polaron and bipolaron form readily with polaronic transitions centered near 80 0 and 1000 nm. In contrast however, the dominant transition here is the peak near 1000 nm, the consequence of which is that the absorption at 800 nm tails far less significantly into the visible region. As a result, the film switches from purple in the n eutral state to a colorless transmissive hue in the oxidized state. The prevalence of the peak at 1000 nm is a feature of all alkyl substituted PPheDOT films in this dissertation as well as the chemically polymerized
89 PPheDOT (C12) 2 while the prevalence o f the blue shifted peak at 800 nm is present in all unsubstituted PPheDOT films. This suggests that polarons are more delocalized in the alkyl substituted system. Figure 3 10. Spectroelectrochemistry of a PPheDOT Bu 2 film potent iostatically deposited onto ITO slide from a solution of 10 mmol PheDOT Bu 2 in 0.1 M TBAPF 6 in ACN. The film was switched in a solution of 0.1 M TBAPF 6 in ACN at 770 mV and from 170 to 1030 mV. To conclude our investigation into the electrochemistry of PheDOT monomers, we looked at the effects of placing branched alkyl substituents onto the phenyl ring. When two 3 methylbutyl chains were placed on the benzene ring, the electrochemical polymerization showed little increase in current density after 10 sc ans, indicating that electrodeposition is poor (Figure 3 11). Aside from the monomer redox process, only one other redox process is observable. This second process has an oxidation peak around 0.2 V and a reduction peak around 0 V that suggests that the coupling arrests predominantly at the level of short oligomers. The spectroelectrochemistry of the film
90 (not shown) has a neutral and oxidized absorption that is quite blueshifted compared to the absorption of the other pPheDOTs discussed so far. This c orroborates the assertation that electropolymerized films of PheDOT (3MB) 2 are comprised primarily of short oligomeric chains. Figure 3 11. 1 st and 10 th scan of the electropolymerization of 10 mmol PheDOT (3MB) 2 from a solution of 0.1 M TBAPF 6 in ACN. In Figure 3 12, the electrochemistry of monomers with bulky 3,3 dimethylbutyl chains on the phenyl ring shows even less increase in current density after repeated cycling. Additionally, after 50 scans no redox peak of any sort bes ides that of the monomer is observed in either acetonitrile or methylene chloride solution. Evidently, the bulkiness of the t butyl groups effectively hinders the coupling of two oxidized monomers. The behavior observed here once again attests to the rol e sterics play in the electropolymerization of PheDOTs even though these functional groups are located relatively far from the redox center.
91 Figure 3 12. 1 st and 50 th scan of the electropolymerization of 10 mmol PheDOT (3,3 dMB) 2 from a solution of 0.1 M TBAPF 6 in ACN, illustrating that at best a negligible quantity of material is electrodeposited onto the electrode. 3.3 BiPheDOT Dimers 3.3.1 Synthesis of BiPheDOTs In the previous section, we observed that electropolymerized film s of PPheDOT were of poor quality unless a high concentration monomer solution was used. Given the low transetherification yields, this is a chemically expensive way of forming quality films. Additionally, the possibility of overoxidation or side reactio ns occurring through the benzene ring was difficult to rule out. One strategy to combat these obstacles is to first chemically synthesize dimers which can then be electropolymerized. The rationale behind this strategy is that dimers are more readily oxid ized than their monomer counterpart their extended conjugation is better able to stabilize the radical cation that forms upon oxidation. Thus, electropolymerization can be carried out at lower potentials where the risks of side reactions and overoxidati on are greatly reduced. This in turn leads to the formation of films with fewer defects and overall higher quality. A well established route for the coupling of conjugated monomers to form dimers is laid out in
92 Scheme 3 3, and begins with deprotonation o f the conjugated system with n butyllithium followed by copper mediated coupling. Coupling yields are low, but the ease of synthesizing the dimer in one step from the monomer makes this route attractive. Scheme 3 3. Syn thesis of PheDOT R 2 dimers. a) 1. n BuLi, 2. CuCl 2 THF. Two dimers were synthesized in this fashion biPheDOT 3 19 and bi(PheDOT Bu 2 ) 3 20 Considering how readily PheDOT monomer dissolves in most organic solvents, biPheDOT is remarkably insoluble, wit h only a few milligrams dissolving in five milliliters of hot methylene chloride. The purification requires running a column in either hot hexanes or with a mixture of methylene chloride and hexanes. The use of just hexanes results in a product that is a white powder, while the presence of methylene chloride tends to result in an off white material. Both are pure however by elemental analysis and NMR. Bi(PheDOT Bu 2 ) is more soluble and a hexanes column is sufficient to yield the desired product. Its sy nthesis, however, requires a total of six steps from commercially available materials, while BiPheDOT requires only two, both in excruciatingly low yields. Bi(PheDOT Bu 2 ) is particularly precious due to the difficulties involved in appropriately scaling f or such a low overall yielding synthesis and the large
93 amount of solvent and silica required in the purification. We will return to this point towards the end of the chapter. 3.3.2 Electrochemical Polymerization of BiPheDOT Dimers Figure 3 13 A shows the f irst of ten scans in the electropolymerization of a 3 mM solution of biPheDOT in methylene chloride. As expected, the onset of oxidation of the dimer is lower than that of the monomer by more than 200 mV the more highly conjugated dimer is better able t o stabilize the radical cation that forms upon oxidation, facilitating removal of the electron. Previous computational calculations have predicted the electron densit y at the 2 and 5 positions of PheDOT, leading to enhanced electropolymerization of the former. A comparison of biPheDOT electropolymerization in the inset of Figure 3 13 A with PheDOT electropolymerization indeed shows that the 3 mM biPheDOT solution and t he 10 mM PheDOT solution achieve comparable current densities after 10 scans. As previously discussed, the increase in current density reflects an increase in the electrode surface area resulting from repeated electrodeposition of polymer. The similariti es in current density after 10 scans suggest that the lower concentration dimer solution is able to electrodeposit more efficiently than the higher concentration monomer solution. Additionally, the electrodeposited film of PbiPheDOT in Figure 3 13 B displa ys a redox process at significantly lower potentials than that of PPheDOT with an oxidation peak at 0.24 V and a reduction peak at 0.41 V corresponding closely to the redox process of high molecular weight PPheDOT (C12) 2 In both PPheDOT and PbiPheDOT films, a second redox process centered around 0.3 V is noticeable and was previously in Figure 3 5 attributed to the presence of shorter oligomers. In the PbiPheDOT films however this peak is suppressed, which
94 suggests that these shorter oligomer units ma ke up a smaller portion of the overall film, which is instead dominated by longer conjugated chains. Like the previous PPheDOT films, the polymer film displays good stability over 50 cycles, with little degradation in current density observable. Figure 3 13. Cyclic voltammograms of biPheDOT. a) F irst scan of 3 mmol solution of biPheDOT in 0.1 M TBAPF 6 in DCM displaying a markedly lower onset of oxidation compared to PheDOT; the inset shows the progre ssion over 10 scans of the electropolymerization; b) CVs of electropolymerized PbiPheDOT in 0.1 M TBAPF 6 ACN over 50 scans, with every 10 th scan displayed. Films of PbiPheDOT were potentiostatically grown on ITO in under a minute for spectroelectrochemical analysis. The spectroelectrochemistry of the film on ITO is shown in Figure 3 14 and displays a similar overall appearance to PPheDOT electropolymerized from high monomer concentration solution. The optical bandgaps are comparable, as are the peak value s of the neutral and oxidized system. The most pronounced difference is the better resolved vibronic peaks in the neutral film, which suggest an overall improvement in film order for the PbiPheDOT system. From the electrochemical and spectroelectrochemic al results, we conclude that electropolymerized biPheDOTs lead to more conjugated, better ordered, and overall higher quality films of PPheDOT than electropolymerized PheDOT monomer. The
95 biggest drawback lies in the solubility of the dimer, with concentra tions in excess of 3 mmol in methylene chloride difficult to achieve without application of heat. Even then, cloudy over time as the dimer slowly precipitates out of solut ion. Figure 3 14. Spectroelectrochemistry of a PbiPheDOT film potentiostatically deposited onto ITO slide from a solution of 3 mmol biPheDOT in 0.1 M TBAPF 6 in DCM. The film was switched in a solution of 0.1 M TBAPF 6 in ACN at 900 mV and from 500 to 700 mV. To remedy the solubility problems, bi(PheDOT Bu 2 ) was synthesized. Indeed, the presence of the butyl chains easily allowed 10 mM dimer solutions to be prepared in methylene chloride as well as less concentrated solutions in solvent mixtures with acetonitrile and propylene carbonate. Looking in Figure 3 15 A at the first scan of the electropolymerization from a 10 mM solution in methylene chloride, we note that the onset of oxidation occurs at 0.79 V, which is higher than t he onset of biPheDOT. This trend goes against what we had previously observed between PheDOT and dialkylPheDOT monomers, and we will explore a possible explanation of this trend momentarily. Additionally, the electropolymerization of bi(PheDOT Bu 2 ) displ ays the
96 presence of the so on the cathodic sweep crosses back over the anodic signal. While the name implies that this phenomenon describes something about the electrodeposition of the material, recent studies strongly suggest that this is something of a misnomer in conjugated systems. 73 Rather, the occurrence is often observed when formation of the oxidized biPheDOT is slow, as well as when the oxidation potential of biPheDOT occurs near potentials where the dication radical of the four ring system also exists. When these conditions are met, an autocatalytic process occurs whereby the tetrameric dication can assist in the oxidation of the dimer The precise mechanism has been proposed to start with the oxidation of two dimers, which then couple with one another. Upon loss of two protons the neutral tetramer is generated. The sufficiently high potential then generates the dication of the tetram er which reacts with neutral dimer to generate the cation tetramer as well as the cation dimer, effectively catalyzing the formation of additional cation dimer. This autocatalytic process results in the presence of an anodic current in the reverse sweep. One possible reason that the oxidation of biPheDOT Bu 2 may proceed slowly is due to the steric bulk of the butyl chains, which may force the neutral molecule to adopt a large torsional angle between the two PheDOT Bu 2 moieties. The oxidized molecule, on the other hand, adopts a planar geometry to better delocalize the cation radical. The rate of oxidation is thus hindered by the large change in molecular conformation upon going from the neutral to the oxidized species. This also could account for the hi gher oxidation potential of biPheDOT Bu 2 compared to biPheDOT. Additional activation energy may be required to overcome the barrier associated with planarization of the dimer.
97 Redox behavior of the electropolymerized film is more comparable in appearance to that of films of PPheDOT Bu 2 than films of PbiPheDOT. In Figure 3 15 B we again notice a redox process at low oxidation potential, however unlike PbiPheDOT the dominant redox processes occur at higher potentials corresponding to short oligomer oxidatio n and reduction. The results from work on biPheDOT and bi(PheDOT Bu 2 ) indicate that the electropolymerized dimers lead to highly conjugated systems with improved order and film stability at the expense of significantly decreased solubility. Figure 3 15. Cyclic voltammograms of bi(PheDOT Bu 2 ). a) Electrochemical polymerization of bi(PheDOT Bu 2 ) in 0.1 M TBAP DCM solution demonstrating the presence of a nucleation loop in the first cyclic voltammogram; the inset shows the electropolymerization over 10 scans; b) the redox behavior of Pbi(PheDOT Bu 2 ) in a solution of 0.1 M TBAP in ACN. 3.4 Formation of Conducting Films of Electropolymerized PPheDOTs As mentioned in the introduction, one of the initial mo tivations of synthesizing pPheDOTs was to observe whether the planarity of the system led to better ordered and, subsequently, more highly conducting polymers compared with their EDOT counterparts. Electropolymerized films of PEDOT in our group have produ ced glossy, free standing films and which have achieved conductivities on the order of 10 100 S/cm. The formation of high quality films depends on a variety of variables, including
98 but not limited to solvent and electrolyte choice, temperature of electr odeposition, and method of electrodeposition (potentiostatic, galvanostatic, potentiodynamic, etc.) The most conducting films of EDOT were acquired using propylene carbonate as solvent with TBAP as electrolyte and polymerized potentiodynamically at 0C wi th conductivities of 120 S/cm achieved. The formation of highly conducting PPheDOTs from PheDOT monomers was frustrated by a number of factors previously discussed, and the various attempts are listed in T able 3 1. The low electron density on the thiop hene ring redox centers inherently inhibits the electropolymerization process. Side reactions through the phenyl ring are possible due to the high oxidation potentials needed for electropolymerization. The presence of alkyl chains also sterically inhibit s the likelihood of electropolymerization. Attempts at film formation by electropolymerization of PheDOT and PheDOT Et 2 resulted in a powdery film upon the electrode that could only be analyzed by adhering the polymer to tape to obtain a continuous powder conductivity could then be measured. From these results the best conductivities that could be obtained were on the order of 10 4 S/cm. BiPheDOTs on the other hand were able to overcome several of these problems. In particular, higher elect ron density on the thiophene rings and lower oxidation potentials led to more efficient electropolymerizations and a minimization of side reactions. The main drawback was that the low solubility of the biPheDOTs precluded the use of high concentration dim er solutions for electropolymerization, as well as limited the choice of available solvent mixtures. As we saw earlier with PheDOT monomer, an increase in concentration can lead to dramatic improvements in film
99 properties. Nevertheless much improved film s were formed using BiPheDOT small intact films achieved conductivities of 10 S/cm. Utilizing bi(PheDOT Bu 2 ) the solubility was improved such that higher concentrations of dimer solutions could be used for electropolymerization. A free standing film wa s formed for the first time for PPheDOTs and an increase in conductivity to 40 S/cm was obtained. Table 3 1. Attempts at forming highly conducting films of PPheDOTs via potentiostatic deposition onto a glassy carbon electrode from a variety of solvents and electrolytes. Monomer Solvent Electrolyte Film quality PheDOT ACN TBAPF 6 TBABF 4 TBAP Powder None PheDOT PC TBAPF 6 TBABF 4 TBAP n/a n/a PheDOT DCM TBAP Powder 10 4 PheDOT Et 2 ACN TBAPF 6 TBABF 4 TBAP, LiBTI, EMIBTI Powder None Phe DOT Et 2 DCM TBAP Powder 10 4 BiPheDOT ACN TBAP Powder None BiPheDOT DCM TBAP Flakes 10 BiPheDOT DCM TBAPF 6 Flakes 10 4 Bi(PheDOT Bu 2 ) DCM TBAP Free standing 40 While still short of the values obtained for PEDOTs, these experiments demonstrate that PPh eDOTs have the potential to be highly conducting materials. By optimizing the conditions for electropolymerization substantial improvements in conductivities are expected to be procured. 3.5 Charge Storage Properties of PheDOT Systems 3.5.1 Systems Based on Electrochemically Polymerized PheDOT Bu 2 We have observed several properties of pPheDOTs that make them promising candidates for charge storage devices. As discussed in Chapter 2, charge storage devices need to be able to charge and discharge quickly and efficiently over a large number of cycles. We have seen from spectroelectrochemical data that PPheDOT films
100 readily accommodate polaron and bipolaron formation. Additionally, the films are stable in both neutral and oxidized state, owing to its relat ively low oxidation potential which gives it good stability under ambient conditions. From the cyclic voltammograms of PPheDOT, PPheDOT Bu 2 proven to be robust to repeated switching, with minimal ch ange in current densities after 100 scans. Finally, we have observed that the CVs of electropolymerized films are a composite overlay of the redox processes of oligomeric systems of various lengths. This was especially true in the case of PPheDOT Bu 2 and Pbi(PheDOT Bu 2 ). The effect of overlapping redox signals is that a relatively large current density is observed over a wide voltage window. This has the effect of imbuing the polymer with capacitive properties charge storage over a voltage range and is proposed to be the source of capacitive behavior in conjugated polymer systems. These properties taken together make PPheDOTs an excellent candidate for charge storage applications. Because a number of redox processes were observed in PPheDOT Bu 2 fil ms, this system was seen as a promising starting point for our investigation into the charge storage behavior of PPheDOTs. The performance of the polymer film depends heavily on the conditions used to grow the films as well as the conditions used to switc h the films. In particular, when investigating the efficacy of a material for use as a capacitor, we are interested in how rapidly and efficiently charges are formed or neutralized on the film, which is intimately related to how rapidly and efficiently el ectrolyte counterions can stabilize the charges to preserve charge balance. This in turn is related to the morphology of the film, as well as the size of the counterions a more porous morphology would allow for more rapid counterion diffusion, while sma ller counterions
101 would naturally be able to move through the film more readily. The dielectric constant of the medium is also important as it allows for more efficient charge formation and conduction, however solvent choice during electropolymerization is ostensibly limited by monomer or dimer solubility issues. Determining the best conditions is difficult to do a priori and rather requires methodical testing and optimizing of conditions. A number of conditions were tested to determine which were best s uited to give optimal PPheDOT Bu 2 films, a few of which are shown below in Figure 3 16. From these experiments, it can be readily seen that the choice of electrolyte and solvent have a dramatic impact on the electropolymerization. Films grown from TBAP i n acetonitrile are shown in Figure 3 16 A and gave materials with the highest current densities after 10 scans; a change in either electrolyte (Figure 3 16 B ) or solvent (Figure 3 16 C ) resulted in a drop in current density of over 50%. As discussed in Chapt er 2, the capacitance of a material is proportional to the average current density divided by the scan rate. Thus by identifying the film with the highest current density we can rapidly screen for optimal polymerization conditions. From these results as well as many other screened conditions not shown here optimal conditions for the electropolymerization of PheDOT Bu 2 were determined to be achieved in 0.1 TBAP in acetonitrile; taking the average current density to be roughly 0.4 mA/cm 2 the areal capa citance can be estimated at 8 mF/cm 2
102 Figure 3 16. Effects of different electrolyte salt and so lution on electrochemical behavior in 5 mmol PheDOT Bu 2 electropolymerization and polymer switching; electrolyte solutions were as follows: a) 0.1 M TBAP/ACN; b) 0.1 M TBABF 4 /ACN; c) 0.1 M TBAP/propylene carbonate. A more rigorous value can be obtained by plotting the amount of charge passing through the material as a function of time and integrating the area under the curve to obtain the total charge passed; this value can then be divided by the potential window to obtain a precise measurement of the capac itance, however for the screening purposes
103 here our estimation is more than sufficient. As we have seen in previous sections, a strategy to form electropolymerized films with higher current densities is to polymerize from a solution of higher monomer conc entration to form thicker films. We will see in a moment however that the added advantage of thicker film is offset by a reduction in the switching speed of the film. A second important parameter in charge storage devices is the rate of charging and disch arging. This is related to properties of both the polymer itself how readily the system is able to accommodate charge and of film morphology where the more porous a film the more rapidly it should allow counterions to diffuse through the film. As m entioned previously, a thicker film of higher current density can be grown from more concentrated monomer solutions; however a thicker film typically switches more slowly due to a decrease in counterion diffusivity as it simply takes longer for counterions to diffuse through the bulk of the material and stabilize embedded charges near the electrode surface. To gain a better understanding of the charging rate of a film, the dependence of the current density (and thus by extension the capacitance) on the sc an rate is assessed. The shape of the voltammogram can be utilized to quickly gain a qualitative estimation of charging rates. In particular, a similarly shaped voltammogram at low and high scan rates would suggest that the response is not being limited by ion diffusivity and that charging and discharging are occurring efficiently. Figure 3 17 shows the voltammogram of PPheDOT Bu 2 for a number of scan rates. We can se e that the shape of the voltammo gram at both low and high scan rates are similar, with an onset of oxidation at approximately 0 V, a subsequent plateauing of the current density, a clear
104 reduction peak between 0.6 V and 0.8 V and a reduction shoulder located between 0.2 and 0.4 V. Estimating the average current density of the charged state to be that at 0.6 V, we evaluate the areal capacitance to be 25.4, 24.7, and 24.1 mF/cm 2 at 50, 100 and 200 mV/s scan rates. As previously mentioned, the capacitance can be calculated more precisely, albeit with more difficulty, by first calculating total charge passed. However, even if these values above are imprecise, the qualitative trend suggests little change in capacitance over a large range of scan rates. This demonstrates that PPheDOT Bu 2 continues to charge and discharge efficiently even at rela tively rapid switching speeds. Figure 3 17. Cyclic voltammogram of PPheDOT Bu 2 in 0.1 M TBAP ACN as a function of scan rate. The current densities at 0.6 V are shown for scans at 50, 100 and 200 mV/s. 3.5.2 Systems Based on Elec trochemically Polymerized BiPheDOTs Based on the improvements in electropolymerization and overall film property observed in BiPheDOTs, these systems were also investigated for their charge storage properties. Similar to work done on PheDOT Bu2, a variet y of conditions were tested,
105 the two most illustrative of which are shown below. In Figure 3 18, a 5 mM solution of biPheDOT was electropolymerized from methylene chloride, and the effects of different electrolytes is apparent on the rates of charging and discharging. Figure 3 18 A shows a film electropolymerized and switched in LiBTI which retains a similar, almost rectangular shape typical of capacitive behavior over a range of switching speeds. In contrast, in Figure 3 18 B the film electropolymerized a nd switched in TBAP begins with a similar rectangular shape but adopts a heavily skewed ellipsoid shape at high scan rates which is characteristic of the film adopting a resistive behavior. This loss of capacitive behavior in the latter film has a kinetic basis where the electrolyte is no longer able to diffuse in and out of the film rapidly enough at high scan rates. From this we can conclude that films grown and switched using LiBTI as electrolyte result in higher rates of film switching. Figure 3 18. Cyclic voltammogram of PbiPheDOT at various scan rates. The films were electropolymerized from a 5 mmol solution of biPheDOT in a) 0.1 M LiBTI in DCM, and b) 0.1 M TBAP in DCM. Figure 3 19 illustrates how varying film thickness, via changing the number of potentiodynamic sweeps during electropolymerization, affects how capacitance depends on scan rate. At low scan rates, Figure 3 19 B illustrates that BiPheDOT
106 electropolymerized using TBAP gives films o f slightly higher capacitance than BiPheDOT electropolymerized from LiBTI (Figure 3 19 A ). The rates of charging and discharging, however, are significantly poorer for the former given by the dramatic decrease in capacitance with scan rate. Films grown fr om LiBTI of all thicknesses continue to charge and discharge rapidly up to scan rates of 100 mV/s, making LiBTI the better electrolyte for capacitive materials based on PbiPheDOT. Figure 3 19. Areal capacitance as a function of scan rate of PbiPheDOT films electropolymerized from various numbers of potentiodynamic cycles as well as from different electrolyte solutions, in particular a) from LiBTI in DCM, and b) from TBAP in DCM. The final molecule that was screened was bi(PheDOT Bu 2 ), given the robust electroactivity it demonstrated. The increased solubility afforded by the butyl chains allowed for a wider variety of conditions and concentrations from which the results of Figure 3 20 were compiled. Like PbiPheDOT, Pbi(PheDOT Bu 2 ) films grown and switched in a solution using TBAP as electrolyte in general gives films whose current densities are initially higher than films electropolymerized using LiBTI at low scan rates. At higher scan rates the current density drops rapidly howe ver for solutions using TBAP
107 as electrolyte. A decrease is still noticeable when the electrolyte is replaced with LiBTI, but it plateaus off more slowly. Additionally, we can see that increasing the ratio of acetonitrile or propylene carbonate to methyle ne chloride leads to increases in capacitance. Finally, increasing the concentration of dimer from 5 to 10 mmol leads to the expected increase in capacitance at the expense of switching rates. Films grown from a 5 mmol dimer solution in 2:1 DCM:ACN with LiBTI were determined to yield the best compromise between capacitance and switching speeds; capacitance at switching speeds of 500 mV/s were a little over 50 mF/cm 2 the largest values seen in any systems studied. Unfortunately the scarcity of the dimer led us to complete the remainder of our studies using the more readily synthesized biPheDOT dimers. Figure 3 20. Areal capacitance as a function of scan rate of Pbi(PheDOT Bu 2 ) films electropolymerized from a variety of electroly te solutions. 3.5.3 Devices Based on Electrochemically Polymerized BiPheDOT Based on the preliminary work from previous sections performed on platinum electrode s our group fabricated prototypical devices shown in Figure 3 21 to better gauge how PPheDOTs performed as capacitors. A 5 mmol solution of BiPheDOT was
108 potentiodynamically deposited from a solution of 0.1 M LiBTI and methylene chloride onto two gold coated flexible Kapton surfaces; a gel electrolyte was then sandwiched in between to construct a device. Figure 3 21. Schematic representation of charge storage device using potentiodynamically deposited PbiPheDOT on conducting gold coated Kapton surfaces. In Figure 3 22, the device was observed to store charge over a p otential window in excess of 1 V. Beyond approximately 1.1 V the device current drops and no longer retains charge. Films of various thicknesses were then grown by varying the number of potentiodynamic scans and its capacitance was monitored as a functio n of the amount of charge passed. As previously discussed, it was expected that thicker films would lead to slower charging and discharging and subsequently lower capacitance. As Figure 3 23 illustrates however, over the range of thicknesses measured, th ere is in fact a relatively linear trend between device performance and film thickness. Figure 3 24 shows the correlation between rates of charging and discharging and capacitance of a single device. At first glance it is impressive that the voltammogram s even at rates as large as 3000 mV/s retain a roughly rectangular shape. More precisely, we observe that when the scan rate is increased from 50 mV/s to 1000 mV/s there is less than a four percent decrease in capacitance; from 50 mV/s to 3000 mV/s nearly eighty eight percent of the device capacitance is retained. The performance of these prototypical devices demonstrate that PbiPheDOTs have great potential for use in charge storage Flexible Gold on Kapton PolyBiPheDOT Film (positive electrode) Separator Paper (Gel Electrolyte) PolyBiPheDOT Film (negative electrode)
109 applications the films are robust over a relatively large potential win dow and are able to store charge efficiently, as well as charge and discharge at rapid rates. Figure 3 22. Device performance of PbiPheDOT on gold coated Kapton electrodes over several voltage windows to probe operational wind ow of the device. Figure 3 23. Five devices of varying film thicknesses acquired by varying the number of potentiodynamic scans during electropolymerization were constructed and the capacitance as a function of charge passed was plotted. The red line shows a best fit of the five devices.
110 Figure 3 24. Current density as a function of scan rate for a PbiPheDOT device, with scan rates varying from 50 mV/s to 3000 mV/s. The areal capacitance for three scan rates 50 mV/s, 1000 mV/s, and 3000 mV/s are indicated. 3.6 Conclusion A family of PheDOT monomers and dimers have been synthesized and analyzed electrochemically and spectroelectrochemically. Some of the highlights of the film properties are its moderate bandgap, its low oxidation potential, and stability over repeated potentiodynamic scanning. The vibronic coupling in the spectra of neutral films illustrates the well ordered nature of the material, and initial results from conductivity measureme nts also support this model. Additionally, in the electrochemistry of the film, an anodic and cathodic current persist over a large voltage window, which is likely attributed to the overlapping redox processes of a number of oligomeric segments. Neverthe less, this phenomenon lends the film its capacitive behavior, and prototype devices fabricated in our group illustrate that PbiPheDOT films are able to store charge over a wide voltage range and also charge and discharge rapidly. One of the peculiarities of the PheDOT family is that substituents located on the
111 phenyl ring relatively far from the redox center have a large influence on the electrochemical process. Not only do alkyl chains tune the oxidation potential of the material, they can also steri cally inhibit the coupling. The insolubility of biPheDOT one of the most promising candidates in the PheDOT family also remains a problem. Coupled with the low synthetic yield in the transetherification step necessary to generate all PheDOT systems, it is difficult to foresee the development of all PheDOT systems in spite of the promise offered by several species unless the synthesis is first improved. In light of this c onclusion, we move in the next c hapter to explore how PheDOT can contribute in le 3.7 Synthetic Details General procedure for a Friedel Crafts acylation A flame dried 3 necked round bottom flask with a stirbar and a pressure equalizing addition funnel was placed under an argon atmosphere. AlCl 3 (110 mmol, 1.1 eq.) was added, followed by the gradual addition of dry methylene chloride (200 mL), taking care that the solution may heat up significantly in the presence of trace water. The reaction was cooled to 0C and a solution of veratrole (100 mmol, 1 eq.) i n dry methylene chloride (100 mL) was added dropwise. Afterwards, a solution of alkanoyl chloride (100 mmol, 1 eq.) in dry methylene chloride (100 mL) was added dropwise. The addition funnel was replaced with a reflux condenser, and the reaction was care fully and gradually warmed to reflux and run overnight. Care should be taken that a significant amount of precipitate not gather in the bottom of the flask and inhibit stirring prior to warming, or else upon application of heat the reaction may become exo thermic. After overnight reflux, the reaction was cooled to 0C and quenched with the gradual addition of 6M HCl (50 mL). The solution was poured into a separatory
112 funnel and extracted with methylene chloride (3 100 mL). The organic layer was dried wi th MgSO 4 filtered, and the solvent removed under rotary evaporation. The crude was purified by column chromatography using 3:1 methylene chloride:hexanes as eluent. General procedure for the reduction of a ketone A flame dried 3 necked round bottom flas k with a stirbar and a pressure equalizing addition funnel was placed under an argon atmosphere. LiAlH 4 (150 mmol, 1.5 eq.) was added followed by gradual addition of dry diethyl ether (100 mL) to form a gray suspension. In a separate flame dried flask un der argon, AlCl 3 (100 mmol, 1 eq.) was added followed by gradual addition of dry diethyl ether (100 mL). The 3 necked round bottom flask was cooled to 0C and the solution of AlCl 3 was added via cannula to the solution of LiAlH 4 After 15 minutes, a solu tion of the ketone (100 mmol, 1 eq.) in dry diethyl ether (100 mL) was added dropwise. The solubility of the ketone is often poor and may require gentle heating or additional solvent to dissolve sufficiently to allow for dropwise addition. After complete addition, the reaction was warmed to room temperature and run overnight. Afterwards, the reaction was cooled to 0C and quenched with the gradual addition of 6M HCl (50 mL). The solution was poured into a separatory funnel and extracted with diethyl eth er (3 100 mL). The organic layer was washed with brine, dried with MgSO 4 filtered and the solvent removed under rotary evaporation. The crude was purified by column chromatography using 1:1 methylene chloride:hexanes as eluent. General procedure for t he deprotection of a 1,2 dimethoxybenzene using BBr 3 A flame dried round bottom flask with a stirbar and a pressure equalizing addition funnel was placed under an argon atmosphere. The veratrole (100 mmol, 1 eq.) was
113 added followed by methylene chloride ( 200 mL). The reaction was cooled to 0C, and a solution of BBr 3 (300 mmol, 3 eq.) in methylene chloride (100 mL) was added dropwise to the reaction. Depending on the rate of addition, the solution either turns a slight pink or a dark red color. After co mplete addition, the reaction was run for 3 hours at room temperature. Afterwards, the reaction was cooled to 0C and quenched with the gradual addition of water (100 mL); care should be taken as the addition of water can result in a fairly exothermic rea ction. The solution was poured into a separatory funnel and extracted with methylene chloride (3 100 mL). The organic layer was washed with brine, dried with MgSO 4 filtered and the solvent removed under rotary evaporation. This affords the crude as a slightly colored to dark brown material which was used without further purification. General procedure for Sonogashira coupling To a 3 necked round bottom flask with stirbar was added 1,2 diiodo 4,5 dimethoxybenzene (3 10), Pd(PPh 3 ) 2 Cl 2 (5 mol%), and C uI (10 mol%). The flask was placed under vacuum and backfilled with argon, and this process was repeated three times. Degassed triethylamine was added via cannula, followed by the addition of degassed alkyne. The reaction was run for three days at room temperature, after which time the reaction was poured into diethyl ether and washed with ammonium chloride. The organic layer was dried with MgSO 4 filtered, and the solvent removed by rotary evaporation. The crude was purified by column chromatography u sing 1:1 methylene chloride:hexanes as eluent to afford the product. General procedure for the hydrogenation of a 1,2 dialkynyl 4,5 dimethoxybenzene
114 Under an argon blanket, to a low pressure glass reactor with a stirbar was added the dialkynylveratrole and 10% Pd/C. Degassed methanol and ethyl acetate were added in one portion and the reactor quickly transferred to a sealed Parr pressure vessel connected to a hydrogen gas tank. The vessel was purged three times with hydrogen gas and the reaction run at 30 0 psi overnight. Afterwards the reaction was passed through a short silica gel plug using 1:1 hexanes:methylene chloride as eluent. The solvent was removed under rotary evaporation and the crude purified by passing through a column using 1:1 hexanes:meth ylene chloride as eluent to afford the product as a colorless oil. General procedure for the transetherification of a benzenediol with 3,4 dimethoxythiophene A flame dried 3 necked round bottom flask with stirbar and condenser was placed under an argon atm osphere. 3,4 dimethoxythiophene (50 mmol, 1 eq.), the dialkylated catechol (50 mmol, 1 eq.), and dry toluene (200 mL) were added to the flask followed by p TSA (5 mmol, 0.1 eq). The reaction was warmed to 110C and run for 5 days. The solution turns dar k within hours. Afterwards, the reaction was cooled to room temperature and vacuum filtered through a pad of celite or silica gel; while this filtration is optional, failure to do so results in an emulsion during extraction that is difficult to break up. The solution is then poured into water (200 mL) and extracted with diethyl ether (3 100 mL). The organic layer was washed with brine, dried with MgSO 4 filtered, and the solvent removed by rotary evaporation to afford the crude as a dark product. The crude was purified by column chromatography using hexanes as eluent to afford the product.
115 PheDOT (3 7a): The general procedure for transetherification was followed, starting with 3,4 dimethoxythiophene (10.61 g, 73.59 mmol), catechol (11.62 g, 147.19 mmol 2 eq.), p TSA (1.40 g, 7.35 mmol, 0.1 eq.), and toluene (300 mL). The pure product was afforded as a white powder (3.5 g, 18.4 mmol, 25% yield). Synthesis of PheDOT Et 2 (3 7b) 1 (3,4 dimethoxyphenyl)ethanone (3 2b): The general procedure for a Friedel C rafts acylation was followed starting with acetyl chloride (6.25g, 79.9 mmol). The pure product was obtained as a slightly yellow oil (12 g, 83%). 1 7.5 (t, 2H), 6.7 6.8 (d, 1H), 3.92 (s, 6H), 2.42 2.45 (s, 3H). 4 ethyl 1,2 dimethoxybenzene (3 3b): The general procedure for the reduction of a ketone was followed starting with 3 2b (11.9g, 66 mmol). The pure product was obtained as a slightly yellow oil (8.7g, 80%). 1 6.82 (m, 3H), 3.92 (s, 6H), 2.5 2.7 (q, 2H), 1.2 1.3 (t, 3H). 1 (2 ethyl 4,5 dimethoxyphenyl)ethanone (3 4b): The general procedure for a Friedel Crafts acylation was followed starting with 3 3b (5.6g, 33.7 mmol). The pure product was obtained as a slightly yellow oil (5.5g, 79%) 1 (s, 1H) 3.92 (s, 6H), 2.55 2.7 (q, 2H), 2.4 (s, 3H), 1.2 1.3 (t, 3H). 1,2 diethyl 4,5 dimethoxybenzene (3 5b): The general procedure for the reduction of a ketone was followed starting with 3 4b (5.5g, 26.6 mmol). The pure product was obtained as a slightly yel low oil (3.98, 77%) 1 2.7 (q, 4H), 1.2 1.3 (t,6H). 4,5 diethylbenzene 1,2 diol (3 6b): The general procedure for deprotection using BBr 3 was followed starting with 3 5b (3.98g, 20.5 mmol). The crude product was
116 obt ained as a brown solid and used without further purification (3g, 88%). 1 6.68 (s, 2H), 4.9 (s, 2H), 2.48 2.57 (q, 4H), 1.1 1.2 (t, 6H) 13 115.71, 25.04, 15.66, 11.2. PheDOT Et 2 (3 7b): The general procedure for transetheri fication was followed starting with 3 6b (3 g, 18 mmol). The pure product was obtained as a colorless oil that turns to a translucent solid upon standing (1.06 g, 4.33 mmol, 24%). 1 2H), 6.4 (s, 2H), 2.43 2.6 (q, 4H), 1.13 1.2, (t, 6H); 13 25.13, 13.89, 11.72; HRMS m/z: calcd, 246.0715; found 246.0702. Anal. calcd for C 14 H 14 O 2 S: C, 68.26%; H, 5.73%. Found C, 68.645%; H, 6.049%. Synthesis of PheDOT Bu 2 (3 7c) 1 (3,4 dimethoxyphenyl)butanone (3 2c): The gene ral procedure for a Friedel Crafts acylation was followed starting with butanoyl chloride (24.1g, 145 mmol). The pure product was obtained as a slightly yellow oil (14 g, 49%). 1 7.6 (t, 2H), 6.85 6.88 (d, 1H), 3.92 (s, 6H), 2.86 2.91 (t, 2H) 1.73 1.76 (m, 2H); 13 (153.30, 144.11, 122.89, 110.14, 56.19, 40.28, 18.37, 14.18). 4 butyl 1,2 dimethoxybenzene (3 3c): The general procedure for the reduction of a ketone was followed starting with 3 2c (14g, 67 mmol). The pure product was obt ained as a slightly yellow oil (11.3g, 87%). 1 6.81 (m, 3H), 3.86 (s, 3H), 2.45 2.6 (t, 2H), 1.5 1.6 (m, 2H), 1.25 1.28 (m, 2H), 0.96 1.0 (t, 3H). 1 (2 butyl 4,5 dimethoxyphenyl)butanone (3 4c): The general procedure for a Friedel Crafts acyl ation was followed starting with 3 3c (11.3g, 58 mmol). The pure product was obtained as a slightly yellow oil (14g, 91%). 1
117 (s, 1H), 3.91 (s, 3H), 3.87 (s, 3H), 2.55 2.71 (q, 2H), 2.52 (s, 2H), 1.55 1.31 (m, 4H), 0.88 0.93 ( m, 6H). 1,2 dibutyl 4,5 dimethoxybenzene (3 5c):The general procedure for the reduction of a ketone was followed starting with 3 4c (14g, 52.9 mmol). The pure product was obtained as a slightly yellow oil (12.5g, 88%) 1 2.52 (t, 4H), 1.45 1.48 (m, 4H), 1.3 1.42 (m, 4H), 0.87 0.96 (t, 3H); 13C NMR 141.66, 134.07, 116.83, 34.24, 32.34, 23.33, 14.65. 4,5 diethylbenzene 1,2 diol (3 6c): The general procedure for deprotection with BBr 3 was followed starting with 3 5c (12.5g, 49.8 mmol ). The crude product was obtained as a brown solid and used without further purification (10.6g, 98%). 1 H NMR: 2.52 (t, 4H), 1.45 1.58 (m, 4H), 1.3 1.42 (m, 4H), 0.87 0.96 (t, 3H) 13 24, 32.34, 23.33, 14.65. PheDOT Bu 2 (3 7c): The general procedure for transetherification was followed starting with 3 6c (5 g, 22.49 mmol). The pure product was obtained as a colorless oil that turns to a translucent solid upon standing (1.5g, 4.96 mmol, 23% yield). 1 0.9 0.95 (t, 6H), 1.3 1.6 (m, 8H), 2.45 2.51 (t, 4H), 6.38 (s, 2H), 6.68 (s, 2H); 13 C NMR: calcd, 302.1341; found, 302.1333. Anal. calcd. for C 18 H 22 O 2 S : C, 71.48%; H, 7.33%. Found: C, 69.534%; H, 7.750%. Synthesis of PheDOT 3MB (3 7d) 1 (3,4 dimethoxyphenyl) 3 methylbutan 1 one (3 2d): The general procedure for a Friedel Crafts acylation was followed starting with 3 methylbutyryl chloride (25 g, 207
118 mmo l). The pure product was obtained as a slightly yellow oil (35.5g, 160 mmol, 77% yield). 4 isopentyl 1,2 dimethoxybenzene (3 3d): The general procedure for the reduction of a ketone was followed starting with 3 2d (35.5 g, 160 mmol). The pure product was obtained as a colorless oil (24g, 115 mmol, 72% yield). 4 isopentylbenzene 1,2 diol (3 6d): The general procedure for deprotection with BBr 3 was followed starting with 3 3d (8g, 38 mmol). The crude product was obtained as a dark oil (6.2 g, 34 mmol, 90% yield). PheDOT 3MB (3 7d): The general procedure for transetherification was followed starting with 3 6d (6.2 g, 34.4 mmol). The pure product was obtained as a colorless oil (1.52 g, 5.84 mmol, 17% yield). 1 6.31 (m, 3H), 2.44 (t, 2H), 1.58 (m, 1H), 1.41 (m, 2H), 0.92 (m, 6H); Anal. calcd. for C 15 H 16 O 2 S: C, 69.20%; H, 6.19%. Found: C, 69.33%; H, 6.40%. Synthesis of PheDOT 3MB 2 (3 7e) 1 (2 isopentyl 4,5 dimethoxyphen yl) 3 methylbutan 1 one (3 4e): The general procedure for a Friedel Crafts acylation was followed starting with 3 3d (15.3 g, 73 mmol) and 3 methylbutyryl chloride. The pure product was obtained as a slightly yellow oil (17.6 g, 60 mmol, 82% yield). 1,2 d iisopentyl 4,5 dimethoxybenzene (3 5e): The general procedure for the reduction of a ketone was followed starting with 3 4e (17.6 g, 60 mmol). The pure product was obtained as a colorless oil (13.4 g, 48 mmol, 80% yield). 1 (s, 2H), 3.83 (s, 6H), 2.45 (t, 4H), 1.52 (m, 2H), 1.26 (m, 4H), 0.87 (d, 12H).
119 4,5 diisopentylbenzene 1,2 diol (3 6e): The general procedure for deprotection with BBr 3 was followed starting with 3 5e (13.4 g, 48 mmol). The crude product was obtained as a dark oil (11.2 g, 45 mmol, 93% yield). PheDOT 3MB 2 (3 7e): The general procedure for transetherification was followed starting with 3 6d (11.2 g, 45 mmol). The pure product was obtained as a colorless oil (1.9 g, 5.85 mmol, 13% yield). 1 H NMR: 6.67 (s, 2H), 6.37 (s, 2H), 2.46 (m, 4H), 1.60 (m, 2H), 1.41 (m, 4H), 1.37 (d, 12H). 13 C NMR: 139.62, 136.41, 117.00, 100.70, 40.73, 30.04, 28.33, 22.76. Synthesis of PheDOT 3,3dMB 2 (3 7f) 1,2 bis(3,3 dimethylbut 1 yn 1 yl) 4,5 dimethoxybenzene (3 9f ): The general procedure for Sonogashira coupling was followed starting with 3,3 dimethylbut 1 yne ( 10 g, 121.74 mmol, 2.5 eq.). The crude was obtained as a slightly yellow oil (11.33 g, 38 mmol, 78% yield). 1 18H) 13 148.34, 119.16, 114.34, 100.40, 78.12, 56.01, 31.39, 28.26. 1,2 bis(3,3 dimethylbutyl) 4,5 dimethoxybenzene (3 5f): The general procedure for the hydrogenation of an alkyne was followed starting with 3 9f ( 11.33 g, 38 mmol ). The product w as obtained as a slightly yellow oil (10.59 g, 34.56 mmol, 91% yield). 4,5 bis(3,3 dimethylbutyl)benzene 1,2 diol (3 6f): The general procedure for deprotection with BBr 3 was followed starting with 3 5f ( 10.59 g, 34.56 mmol ). The product was obtained a d ark red oil (9.04 g, 32.49 mmol, 94% yield). PheDOT 3,3dMB 2 (3 7f): The general procedure for transetherification was followed starting with 3 6f (9.04 g, 32.49 mmol ). The product was obtained as a colorless oil (1.28 g, 3.57 mmol, 11% yield).
120 1,2 diiodo 4,5 dimethoxybenzene (3 8): To a 3 necked round bottom flask with stirbar and condenser was added periodic acid (2.92g, 12.8 mmol, 1 eq.) and methanol (50 mL). Iodine (6.38g, 25.1 mmol, 2 eq.) was added and the reaction stirred for 15 minutes at room temp erature, during which time the solution turns a dark color. Afterwards, veratrole (4.0 mL, 31.4 mmol, 2.5 eq) was added and the reaction stirred at 70C for 4 hours. Over the course of the reaction a white precipitate crashes out. After 4 hours, the rea ction was cooled to room temperature and quenched with gradual addition of 30% hydrogen peroxide (20 mL). The white precipitate was collected using a Buchner funnel and washed with methanol to afford the product which was used without further purification (4.3g, 86%). 1 BiPheDOT (3 12a): To a round bottom flask with stirbar under argon atmosphere was added PheDOT (1 g, 5.25 mmol, 1 eq.) and dry THF (30 mL) The reaction was cooled to 78C, at which point n BuLi ( 5.51 mmol, 1.05 eq.) was added dropwise. The reaction was allowed to warm to 0C and stirred for 1 hour, after which point the reaction was cooled back down to 78C. CuCl 2 (0.74 g, 5.51 mmol, 1.05 eq.) was added in 1 portion and the reaction allowed to gradually warm to roo m temperature and stirred overnight. Afterwards, the reaction was poured into water and added to a separatory funnel. The solution was extracted with methylene chloride (3 5 0 mL). The organic layer was dried with MgSO 4 filtered, and the solvent remov ed under rotary evaporation. The crude was purified by column chromatography using hexanes as eluent to afford the pure product as a white powder (0.21 g, 0.57 mmol, 22% yield). 1 H
121 Bi(PheDOT Bu 2 ) (3 12b): To a round bot tom flask with stirbar under argon atmosphere was added PheDOT Bu 2 and dry THF. The reaction was cooled to 78C, at which point n BuLi (1.05 eq.) was added dropwise. The reaction was allowed to warm to 0C and stirred for 1 hour, after which point the r eaction was cooled back down to 78C. CuCl 2 (1.05 eq.) was added in 1 portion and the reaction allowed to gradually warm to room temperature and stirred overnight. Afterwards, the reaction was poured into water and added to a separatory funnel. The sol ution was extracted with methylene chloride (3 100 mL). The organic layer was dried with MgSO 4 filtered, and the solvent removed under rotary evaporation. The crude was purified by column chromatography using hexanes as eluent to afford the pure produ ct as a white powder. 1 (s, 2H), 6.69 (s, 2H), 6.37 (s, 2H), 2.46 (t, 8H), 1.55 (m, 8H), 1.24 (m, 8H), 0.87 (s, 12H).
122 CHAPTER 4 THIOPHENE PHEDOT OLIGOMERS AND POLYMERS 4.1 Brief History of Thiophene Oligomers and Polymers Oligothiophenes and polythiophenes have be en an important class of molecules in the field of organic electronics performing respectably in a number of devices such as LEDs, chemical sensors, OFETs, and solar cells, to name a few. Much of the early work on oligothiophenes was pioneered by Gilles Horowitz and Francis Garnier in the 1990s. To name some examples, their work was responsible for the first oligothiophenes with hexyl substituents at the termini, 74 as well as a great deal of work elucidating the optoelectronic properties of oligothiophenes of various lengths 75,76 and optimizing the performance of oligothiop henes in OFETs as p type materials, achieving values on the order of 0.01 cm 2 /Vs for evaporated films and an order of magnitude higher for single crystals. 77 81 In the 2000s, the Marks group found that by attachin g perfluorohexyl chains or other electron withdrawing substituents, these traditionally p type materials could be readily converted to high mobility n type materials. 82 84 Values as high as 0.32 cm 2 /Vs were measur ed when quarterthiophenes were substituted at the 84 Towards the end of the 1990s and evolving into a popular and successful concept in the following decade, a number of fused ring systems based on oligothiophenes were synthesized to attain high performance materials. 34,85 92 In one of the earliest reports of such a system, polymers incorporating fused thienothiophenes achieved hole mobilities of 0.2 0.6 cm 2 /Vs, and mobilities exceeding 1 cm 2 /Vs have more recently been reported as well. 93,94 Additionally, solar cells incorporating fused thiophenes have consistently achieved
123 power conversion efficiencies on the order of 4 6% 94 96 Recently, dithienylsilole and dit hienylgermole based polymers have achieved efficiencies of over 7%. 97,98 Regarding polythiophenes, much of the synthetic work was carried out in the 1990s to develop highly regioregular poly(3 hexythiophene) (rr P3 HT), and it has since been shown in various poly(3 alkylthiophene) systems that the regioregular systems perform significantly better than their regioirregular analogs. 99,100 In an early study, for instance, it wa s shown that poly(3 dodecylthiophene) with 91% HT couplings demonstrated conductivities upon oxidative doping on the order of 600 S/cm, while those with only 54% HT couplings saw their conductivity decrease to only 10 S/cm. 101 Since this time, rr P3HT has been thoroughly studied as the active material in man y devices with respecTable properties. Solar cell efficiencies of 5% have been achieved by a number of groups. 102 Hole mobilities of 0.1 0.3 cm 2 /Vs for a variety of rr P3ATs were recently reported by the McCullough group. 103 While other systems have achieved higher conductivities, mobilities, and solar cell efficiencies, the relative ease in synthesizing the polymer both in high yields and large quantities has made rr P3HT the workhorse in elucidating the complex processes involved in device performance. To name a few, the effects on device performance of re gioregularity, 100 molecular weight, 104 108 crystallinity, film deposition conditions, and device annealing 109 113 have all been elucidated using P3HT as a model system. As mentioned above, the incorporation of fused thiophene rings into conjugated systems has led to the development of materials with promising properties. The theoretical basis was discussed more thoroughly in Chapter 1. In short, fusing the aromatic rings effectively decreases the conformational fre edom and lowers the
124 leads to more rapid charge transport. Additionally, fused systems provide a highly stacking occurs, leading to improved ordering and crystallinity. One primary drawback, however, remains the processability of these materials, as the fused systems are often highly insoluble. Thus, while the guiding principles underlying the fused systems are attractive, alternate means of restricting stacking should also be explored. Here, we investigate the incorporation of PheDOTs into thiophene systems. As we saw in Chapter 3, PheDOT is a h stacking and lead to strong intermolecular ordering. Additionally, it has previously been shown that neighboring thiophenes and dioxythiophenes (in particular, EDOT) interact via intramolecular sulfur oxyg en interactions that may have the effect of non covalently molecule 114 These properties make PheDOT an attractive synthon to incorporate into thiophene oligomers and polymers. 4.2 Synthesis and Molecular Characterization of Thiophene PheDOT Oligomers 4.2.1 Synthesis of Thiophene PheDOT Oligom ers A family of three ring and five ring oligomers, containing a central PheDOT moiety and either thiophene or dioxythiophene arms, were synthesized according to Schemes 4 1 and 4 2. Beginning with a description for the synthesis of the three ring oligome rs, thiophene, 2 hexylthiophene, 3,4 dimethoxythiophene and EDOT were all purchased from commercial sources. Stannylation of these thiophenes, followed by Stille coupling to 2,5 dibromoPheDOT, afforded the three ring oligomers TPT, HxTPTHx, DPD, and
125 EPE in low to moderate yields. In the case of the five ring oligomers, the bithiophene arms were first synthesized. Bithiophene 4 5 was synthesized from 2 bromothiophene by generating the Grignard reagent, followed by a Kumada coupling to 2 bromothiophene To synthesize 5 hexyl bithiophene 4 8 2 hexylthiophene was treated with NBS to form 2 hexyl 5 bromothiophene, which was then coupled with 2 trimethyltinthiophene using Stille conditions. These bithiophene arms were then stannylated and then coup led to either 2,5 dibromoPheDOT or 2,5 dibromoPheDOT butyl 2 to afford the five ring oligomers TTPTT, HxTTPTTHx, TTP4TT, and HxTTP4TTHx in low to moderate yields. Scheme 4 1. Synthesis of PheDOT oligomer precurors. a) 1. n BuLi, THF, 78C to 0C; 2. SnMe 3 Cl, rt, quantitative; b) 1. Mg (1.2 eq.), diethyl ether, reflux, 3 hours; 2. 2 bromothiophene, Ni(dppp)Cl 2 (1%), diethyl ether, reflux, overnight 80%; c) NBS, DMF, rt, 95%; d) 4 1a Pd(PPh 3 ) 2 Cl 2 2%, CuI 4%, DMF, 80 C, 3 days, ca. 80%. With the exception of the highly soluble HxTPTHx, the three and five ring oligomers can all be purified by passing the crude through a short basified silica gel plug
126 using 1:1 hexanes:methylene chloride as eluent to remove catalyst. Th e crude was then dissolved in hot heptanes and allowed to precipitate out at room temperature. Repeating this process leads to quite pure materials at the expense of some oligomer which remained in solution. Often a slight red tint may persist which was present in too low quantities to adequately characterize and identify. Extremely pure oligomers without the red tint may be obtained using a long basified silica gel column employing hot hexanes or heptanes as eluent, though due to the poor solubility of the oligomers this process typically takes hours to a full day or longer. The use of even a small amount of methylene chloride typically results in some of the red impurity ending up in the product. Scheme 4 2. Synthes is of PheDOT oligomers. a) Pd(PPh 3 ) 2 Cl 2 2%, CuI 4%, DMF, 80C, 3 days, 30 80%. The three ring oligomers possess a faint yellow tint with the exception of EPE, which has a darker yellow hue. These oligomers are quite soluble at room temperature in a wide v ariety of solvents, such as THF, toluene, and chlorinated solvents. Hot
127 hexanes and heptanes are also sufficient to dissolve the materials. The five ring oligomers all possess colors ranging from a robust yellow hue (TTPTT, TTP4TT) to an orange yellow (H xTTPTTHx, HxTTP4TTHx). The materials are less soluble than their three ring counterparts, as expected, although they all readily dissolve at room temperature in THF, toluene, and chlorinated solvents, as well as in hot heptanes. The solubilities of the o ligomers are improved by several factors when alkane groups are attached either on the thiophene ring or on the phenyl ring. While the five ring systems display a markedly diminished solubility compared with the three ring systems, all oligomers studied h ere are sufficiently soluble to fabricate films necessary for evaluating the full range of their properties. For instance, 15 mg/mL of HxTTPTTHx dissolves in warm chlorobenzene and remains in solution, an adequate solubility from which to fabricate device s, while the same concentration of TTPTT dissolves in hot chlorobenzene, but some precipitate is observed if left standing for about a day at room temperature. 4.2.2 Crystal Structures of Thiophene PheDOT Oligomers Crystals of TPT and HxTPTHx were prepare d by dissolving the oligomer in a solution of hot THF and acetone with a minimal amount of diethyl ether and left to stand at room temperature in a sealed vial. Looking at the crystal structures shown in Figure 4 1, several features coincide with our expe ctations of these systems. The edge perspective of both TPT and HxTPTHx shows that the oligomers are highly planar structures. The PheDOT moiety retains its planar configuration, while the three thiophene rings exhibit small torsional angles between them with the exception of TPT, where one thiophene ring is twisted 12.9 out of plane. While such twisting of the terminal thiophenes is a common feature in terthiophene oligomers, typical dihedral
128 values are about 7%. 82,115,116 In contrast, the thiophenes of HxTPTHx have much smaller torsional angles with the central PheDOT moiety. We will explore a possible explanation for this trend shortly. Looking at the bond lengths between the dioxane oxygens and its neighb oring carbons we observe that the bond to the thiophene has slightly more double bond character than the bond to the benzene. This suggests the oxygens donate more of their electron density into the thiophene ring, but that some density delocalizes into t he benzene ring as well. This helps explain the trend we observed in Section 3.2.2 in Chapter 3 where PheDOT was easier to oxidize than thiophene, but more difficult to oxidize than EDOT. Interestingly, sulfur oxygen interactions previously observed in EDOT thiophene oligomers are less pronounced here. In TPT an all syn conformation is adopted between thiophene rings, whereas in HxTPTHx one thiophene is anti to the PheDOT core. It has been proposed that sulfur oxygen interactions arise from a donation of oxygen electrons into the empty d orbital of sulfur. However, the ability of oxygen electrons to donate may be lessened in PheDOT relative to EDOT due to partial delocalization into the benzene ring. Nevertheless, in HxTPTHx the sulfur oxygen distance (2.95 ) is smaller than the sum of their van der Waals radii (3.35 ). This value is essentially the same as distances observed in EDOT thiophene oligomers (2.96 ) and biEDOT (2.92 ) suggesting that strong intramolecular interactions are still present 114,117 This discrepancy will also be addressed below in the context of packing driven geometry.
129 Figure 4 1. X ray crystal structures of a) TPT and b) HxTPTHx. Face on (top left), edge on (top right) an d packing arrangements (bottom) demonstrate the planarity of stack. Hydrogens omitted for clarity. The hexyl chains have also been omitted for clarity from the packing structures of HxTPTHx. = carbon, oxygen and sulfur atoms, respectively.
130 The crystal packing differs dramatically between the hexyl substituted and unsubstituted oligomers. In the latter, TPT adopts a herringbone ty pe packing arrangement typical of many thiophene oligomers. A number of intermolecular distances are smaller than or almost smaller than the sum of their van der Waals radii. In particular, we note that the thiophene ring that is twisted out of coplanari ty has close interactions with three neighboring oligomers in various face on and edge on interactions. These numerous interactions could provide multiple pathways for charge transport through the material. In contrast, the hexyl substituted HxTPTHx adop ts a slipped stack packing arrangement. Presumably, the bulky hexyl groups prevent the edge on interactions seen in TPT and force neighboring molecules into strictly face on interactions. As shown in the packing structures, each oligomer has fewer neighb ors than their herringbone TPT counterpart only one neighbor is present in a face on orientation. Additionally, intermolecular distances are slightly larger than the sum of their van der Waals radii. Nevertheless, the interactions that are present occu r across a larger surface cross section of the molecule. The different packing arrangements provide a plausible explanation for the differences in torsional angles in TPT and HxTPTHx, in particular the relatively large 12.9 twist observed in TPT. Becau se of the herringbone type packing adopted by TPT, in order to maximize nearest neighbor interactions, one of the thiophenes is twisted to bring it into closer contact with three other oligomers this twist maximizes the intermolecular interactions. In c ontrast, HxTPTHx maximizes its contact when two face on oligomers are as planar as possible. This argues that intermolecular forces are more responsible in this instance for determining molecular geometry than intramolecular
131 forces. Similarly, in the cas e of the anticipated sulfur oxygen interactions which are absent in both oligomers, the adoption of a syn conformation in TPT allows the carbon atoms of one thiophene ring to interact closely with the carbon atoms of neighboring thiophene rings at distance s smaller than their van der Waals radii. In particular, the 3.209 and 3.554 packing distances arise from one carbon center on the 12.9 twisted thiophene interacting with two neighboring molecules. Rotation of the thiophene into an anti configuratio n would rotate this carbon away from both interactions, replacing it instead with a sulfur atom, which may not be able to interact as strongly. In spite of the speculative nature of these arguments, we tentatively conclude that intramolecular geometry lar gely influenced by intermolecular packing forces. Additionally, from these two structures, it is apparent that through simple alkyl substitution, dramatically different molecular organizations are accessible to explore across our family of oligomers. 4.3 Thermal Properties of Thiophene PheDOT Oligomers 4.3.1 TGA Thermograms of Thiophene PheDOT Oligomers The thermal behavior of the compounds synthesized in Scheme 4 2 was investigated to assess their stability and also the possible presence of liquid crysta lline phases. The presence of such phases would allow us to explore the effects of various orientations and phases on film properties. The thermal stability of the materials was first probed using TGA, with the plots of TPT, DPD and TTPTT shown in Figure 4 2. Most of the molecules showed robust thermal stability TPT displayed the lowest temperature at which 95% weight retention is observed, at 270C, while TTPTT continued to maintain 95% weight retention up to 338C. Only EPE displayed multiple low te mperature processes over the course of the heating cycle. One promising feature is that several systems TPT, HxTPTHx, and TTPTT appear to sublime quite cleanly,
132 as evidenced by the drop in weight retention to 0%. This property could provide a channel for the preparation of high purity materials; additionally, highly organized films thermal properties is compiled in Table 4 1. Figure 4 2. TGA ther mograms for select PheDOT oligomers under a nitrogen atmosphere using a dynamic ramp rate. 4.3.2 DSC Thermograms of Thiophene PheDOT Oligomers In unsubstituted oligothiophenes, DSC thermograms do not display any transitions besides the melt; substitution with hexyl chains however has been observed to introduce new transitions in ter quarter and sexithiophenes. 82 In our thiophene PheDOT oligomers, a similar trend is observed. DSC results are shown for TPT, TTPTT, and HxTTPTTHx in Figure 4 3. In the cases of TPT and TTPTT in Figure 4 3 A and 4 3 B a single sharp transition is observed upon heating and cooling. Two cycles have been overlaid to demonstrate the reproducibility of the DSC thermograms. This reproducibility indicates a high de gree of thermal stability across the family of thiophene PheDOT molecules. The exceptions to this are the dioxythiophene substituted molecules DPD and EPE (not tested due to the multiple transitions observed in the TGA). On the other hand, the thermogram of HxTTPTTHx in Figure 4 3 C shows
133 the presence of a second, smaller transition as well. This phenomenon is also seen in TTP4TT and HxTTP4TTHx oligomers, while all other systems exhibit only a single transition. Thus, alkyl substitution at either the term ini or the phenyl ring offer the distinct possibility of introducing liquid crystalline phases to obtain long range ordering of the materials. Figure 4 3. DSC thermograms of PheDOT containing systems, a) TPT, b) TTPTT and c) HxTTPTTHx under helium atmosphere, and scanned at 5C/min. For a) and b), the second and third cycles are overlaid to demonstrate thermal stability. For c), only the second scan is shown for clarity. Th e inset of Figure c) is a magnification showing the presence of a much smaller second transition characteristic of alkyl substituted five ring thiophene PheDOT oligomers. Several trends are apparent within, and between, the families of substituted and unsu bstituted thiophene PheDOT compounds. Within each family, an increase in the
134 DSC melt temperature is observed with an increase in the number of rings comprising the oligomer. Any trend in TGA between decomposition onset and number of rings is harder to e stablish, since both TPT and HxTPTHx appear to sublime, while EPE displays multiple decomposition transitions. However, it can be said that the onset of sublimation of the five ring TTPTT appears at higher temperatures than the onset of sublimation of the three ring molecules TPT and HxTPTHx. These expected trends owe to the increase in molecular weight and intermolecular interactions with an increasing number of rings. For systems with the same number of rings, the introduction of hexyl substituents lea ds to a large decrease in melt temperature, and a large increase in decomposition or sublimation onset temperatures. The hexyl chains likely ser stacking and decrease intermolecular interactions, which in turn lowers the melt temperature, while the increase in molecular weight again increases the temperatures observed using TGA. Interestingly, dibutyl substitution on the phenyl rin g of PheDOT leads to little change in either DSC or TGA. This is in contrast to the analogous trend of thiophene substitution at the 3 and 4 positions seen in the literature. While such substitution in oligothiophenes has been a successful strategy towar ds improving solubility, it comes at the price of introducing large inter ring twists as a result of steric hindrance deriving from the bulkiness of the newly introduced solublizing groups at the 3 and 4 positions. This in turn translates to dramatically decreased intermolecular interactions and subsequently decreased melt transition temperatures relative to the unsubstituted oligothiophenes. In thiophene PheDOT systems however, the alkyl groups are located sufficiently far from the aromatic cores that we would not expect their presence to introduce any new sources of steric strain. This expectation is
135 mirrored in the similarities of their thermal properties with the properties of the unsubstituted phenyl ring analogs. The PheDOT moiety therefore offers attractive new sites to introduce solublizing substituents without compromising backbone conjugation. Table 4 1. DSC and TGA results of PheDOT oligomers. Compound DSC heating (C) a DSC cooling (C) a TGA degradation onset T d (C) c TPT 168 141 270 Hx TPTHx 56 4 309 DPD n/a n/a 310 EPE n/a n/a 224 TTPTT 233 206 338 TTP4TT 239 b 214 200 b 186 348 HxTTPTTHx 151 b 65 136 b 56 414 HxTTP4TTHx 146 b 109 127 b 108 397 a DSC heating and cooling were performed under helium gas at rates of 10C/min; values were taken from the thermogram from the second of three cycles. The three ring systems were heated to 250C while the five ring systems were heated to 300C. b Transitions with the largest enthalpies were assigned to the melt transition. c TGA was perform ed under a nitrogen atmosphere; values were recorded as the temperature at which 95% weight retention was observed. 4.4 Optical Properties of PheDOT Oligomers 4.4.1 Optical Properties of Exemplary Thiophene PheDOT Systems The solution and thin film absorba nce of the oligomers, as well as their emission spectra, provide much information regarding the intra and intermolecular interactions present in these systems. Figures 4 4, 4 5 and 4 6 selectively show the optical properties of TPT, DPD and TTPTT, respec tively, as the optical properties of all other oligomers in this chapter display similar features to one of these three systems. The optical properties of all eight oligomers are compiled in Tables 4 2 and 4 3 at the end of this section. In Figure 4 4, ab sorption and emission spectra are shown for TPT. The solution absorbance spectrum shows a dominant peak at 365 nm and the presence of shoulder peaks on either side. As described in Chapter 2, vibronic shoulders are illustrative of
136 the extent of intramole cular order present in the system. In particular, it suggests the molecule is limited in its vibrational degrees of freedom in solution, which correlates well with th e planar structures seen in the X ray crystal structures of TPT. Figure 4 4. Solution, thin film and emission spectra of TPT taken under ambient conditions. Solution absorbance (10 measured in THF solutions. Thin films were prepared by drop casting from THF solution onto ITO. T he dotted line is present to m ore clearly illustrate the bath ochromic shift of the spectra on going from the solution to the solid state. The thin film absorbance of a material, when compared with its solution absorbance, typically displays two trends a red shifting of the peaks due to a packing driven increase in the intramolecular ordering of the system, and the emergence of a new peak at longer wavelengths from aggregate absorption. Here a small red shift is seen in the peak values the peak at 381 n m in solution for example is bathochromically shifted to 385 nm in the film. This red shift reflects an improvement in ordering of the intramolecular geometry going from solution to the solid state, specifically an increase in the planarity of the molecul e. On the other hand, the small magnitude of the shift supports our claim in the previous paragraph that a planar
137 geometry is adopted in solution, with only a small increase in planarity occurring in the solid state. A new peak at 411 nm is also now pres ent in the thin film absorbance that was absent in the solution spectrum. The peak spacing between this peak and the spectrum maximum at 385 nm is too large (~ 1650 cm 1) to be attributable to a vibronic signal. This new peak is both strongly red shifted and intense, both suggestive that strong intermolecular interactions are present to give rise to this aggregate absorbance peak. Care should be taken not to read too much into the intensity of this absorbance, however, since the extent of intermolecular organization depends strongly on deposition conditions (solvent, temperature, etc.). A weak peak for example does not suggest that more likely instead that optimal depo sition conditions have yet to be found. More shift, which is directly proportional to the intimacy of the intermolecular interactions. While an unoptimized film may not have a large population of aggregated s pecies to give rise to an intense peak, the aggregates that are present would still express a similar extent of red shifting in the aggregate peak. It is worth noting that the vibronic shoulders present in both TPT and HxTPTHx are completely absent in the all thiophene oligomeric analogs for both the solution and solid state absorbances, and as such we conclude that the presence of the PheDOT moiety improves intramolecular organization over all thiophene trimers. As a final note, the spectra of TPT and Hx TPTHx are red shifted relative to terthiophene and dihexylterthiophene, owing to the electron richness of the PheDOT subunit. The solution emission spectrum of TPT in Figure 4 4 shows the presence of vibronic peaks which are much more prominent than in the solution absorbance
138 more quinodal and even more planar conformation upon excitation. This planarized geometry in turn is better ordered, leading to more distinct vib ronic signals. The emission spectra of terthiophenes display some vibronic splitting, although less prominently than the thiophene PheDOT oligomers. These trends in vibronic splitting present in both absorbance and emission spectra suggest an overall imp rovement in the intramolecular ordering of thiophene PheDOT systems over oligothiophenes. Moving on to DPD shown in Figure 4 5, the presence of the peripheral dioxythiophenes leads to far stronger vibronic splitting in the solution and thin film absorptio n spectra than what we observed for TPT in Figure 4 4. This strongly improved intramolecular ordering is likely a result of stronger sulfur oxygen interactions in particular, the additional oxygens on the dioxythiophene can interact with the sulfur of t he PheDOT core and further rigidify the structure. A red shifting of the peaks is again observed upon transitioning from the solution to the solid state. The shift is in fact more pronounced than that observed in TPT, suggesting that the overall differen ce in planarity between solution and solid states is greater for DPD than for TPT. A small, but strongly red shifted shoulder is present in both the solution spectrum and more prominently in the thin film spectrum. It is possible that this peak arises fr om an aggregate absorbance, however the presence of this peak in the solution spectrum makes this assignment more questionable, as we would expect the oligomer to be well dissolved and unaggregated in dilute solutions. An alternative explanation relates t o the possible presence of oxidized material, as the electron rich DPD oligomer is expected to possess a low oxidation potential and may oxidize under ambient conditions. In the
139 solid state, the oligomer remains yellow over extended periods of time. In s olution however, dark precipitate forms within an hour, and within days the initially yellow solution has turned a murky black color, suggesting that oxidation occurs readily in solution, and giving rise to this peak at longer wavelengths. Figure 4 5. S olution, thin film and emission spectra of DPD taken under ambient conditions. Solution absorbance (10 (concentration unknown) were measured in THF solutions. Thin films were prepared by drop castin g from THF solution onto ITO. The emission spectrum again displays the prominent vibronic splitting we saw previously in Figure 4 4. Interestingly, the absorbance and emission spectra are all blue shifted relative to the TPT family of oligomers. As we w ill see later when we look at the electrochemical properties of the oligomers in Section 4.2.5, while dioxythiophene PheDOT systems possess lower oxidation potentials, their LUMOs are raised relative to TPT and HxTPTHx. This leads to a family of larger HO MO LUMO gap materials which subsequently blue shifts their spectra. Altogether, the introduction of dioxythiophenes into the oligomer appears to strengthen the intramolecular interactions present, although its effect on intermolecular order is less clear at this time.
140 Turning to the spectra of TTPTT in Figure 4 6, we first note that the spectra of this five ring system are red shifted relative to the three ring systems in Figures 4 4 and 4 max of the three max of TTPTT is 426 nm. The expansion of the conjugation leads to an expected narrowing of the HOMO LUMO gap and a red shifting of all the peak values The solution spectrum exhibits only faint vibronic shoulders, suggesting a decrease in the intramolecular order present in this system. While sulfur oxygen interactions are capable of decreasing the free rotation between PheDOT and its neighboring thio phenes, the terminal thiophenes of the five member rings are not capable of engaging in such interactions and can therefore freely rotate, leading to a decrease in the ordering of the material. The thin film spectrum however again shows a noticeable progr ession of vibronic peaks. A red shifting of the peaks is again observed when the spectrum of the film is compared with the spectrum taken in solution, the magnitude of which is larger than the red shifting seen in the three ring systems. This supports ou r assertion that free rotation of the peripheral thiophenes in TTPTT is pronounced and responsible for the lack of fine structure in the solution spectrum. At the same time, this also suggests that the improvement in the intramolecular ordering of the sys tem upon going from solution to the solid state is likely due to a suppression of rotational freedom of the terminal thiophene rings. A pronounced shoulder at 480 nm also emerges, separated by approximately 1800 cm 1 ruling out the possibility of this be ing a vibronic peak. The magnitude of the red shift here is greater than those observed in previous systems, suggesting intermolecular interactions are more prominent in five ring systems than in three ring systems. Comparing the solution and solid state absorbance spectra of
141 TTPTT with the all thiophene pentameric analog, both systems show solution spectra void of vibronic fine structure. However, fine structure continues to be absent in the solid state films of quinquethiophene. Thus, in conjunction w ith the same comparison made previously with the three ring systems, we conclude that the addition of PheDOT into thiophene oligomers improves intramolecular ordering. Finally, the absorbance spectra of the five ring systems here are red shifted compared with their all thiophene analog, which we again attribute as we did with the three ring systems to the electron richness of the PheDOT moiety. Figure 4 6. S olution, thin film and emission spectra of TTPTT taken under ambient conditions. Solution absorbance (10 (concentration unknown) were measured in THF solutions. Thin films were prepared by drop casting from THF solution onto ITO. In Table 4 2, the solution and solid state absorbance of all eight oligomers is compiled. As previously mentioned, the spectrum of each oligomer bears some resemblance to one of the spectra in Figures 4 4, 4 5 and 4 6. TPT and HxTPTHx for instance closely ressemble one another in the magnitude and distribution of vibronic shoulders. DPD and EPE both demonstrate strong vibronic progressions in their
142 absorbance spectra. All four of the five ring systems also have faint vibronic shoulders in their solution spectra and more pronounced vibronic fine structure in the s olid state. Hexyl substitution at the thiophene rings of HxTPTHx, HxTTPTTHx, and HxTTP4TTHx, all lead to a red shifting of both the solution and thin film spectra, as we would expect the hexyl chains to increase the electron density of the systems, raisi ng the HOMO and decreasing the HOMO LUMO gap. Dibutyl substitution of the phenyl ring, however, does not result in any change in the solution absorbance spectra (i.e. in comparing TTPTT with TTP4TT, and HxTTPTTHx and HxTTP4TTHx). Slight variations in the solid state absorbance peak values are noted, however. Taken in conjunction with DSC and TGA data, we find support in our earlier assertion that dibutyl substitution of the phenyl ring leads to slight variations in intermolecular organization without alt ering the electronic properties of the system. Table 4 2. Optical absorption data of PheDOT oligomers. All observable vibronic peaks are noted with the maximum intensity denoted with an asterisk. Compound abs sol a (nm) abs film c (nm) E HOMO LUMO film b (eV) TPT 352, 365*, 381 367, 385*, 411 2.87 HxTPTHx 333, 359, 375*, 392 313, 329, 357*, 379, 406 2.88 DPD 347, 365*, 386 355, 372*, 397 3.11 EPE 351, 371*, 393 350, 369*, 390 3.06 TTPTT 426 396*, 414, 441, 480 2.36 TTP4TT 426 402, 417*, 447, 482 2.33 HxTTPTTHx 433 380, 404, 424*, 450, 486 2.36 2.29 HxTTP4TTHx 434 398*, 419, 448, 484 2.36 2.29 a Solution absorption spectra were taken in THF. b HOMO LUMO gaps were calculated by taking the intercept of the most red shifted edge of the absorbance profile with the baseline. c Films were prepared by drop casting the oligomer from a dilute THF solution (< 1 mg/mL) onto ITO. Absorption spectra were then measured either by immersing the ITO in acetonitrile o r propylene carbonate, or spectra were take of the native film in the absence of solvent. In Table 4 3 the emission spectra of the eight oligomers are compiled along with their fluorescence quantum yields. Several trends are readily explained by reasoning
143 identical to what was used to explain absorbance spectra trends, such as the red shifting of the emission spectra with an increase in the number of rings, the red shifting with the presence of the hexyl chains, and finally the lack of any significant chan ge in the spectra with addition of the dibutyl chains on the phenyl ring. The trends in the quantum yields are more challenging to explain. For instance, a large increase in quantum yield is observed upon expanding the number of rings from three to five. While this has also been observed in oligothiophenes and thoroughly understood, here without further experiments conclusions cannot be drawn as to the origin of this increase. It is more useful, rather, to note that the quantum efficiencies of TTPTT and TTP4TT (50%) are higher than quinquethiophene (39%). Because quantum efficiency is improved by a reduction in the degrees of motional freedom in a molecule (among other factors), we tentatively ascribe this increase in part to the improved intramolecular order in PheDOT containing oligomers that we previously established. The lack of change in quantum efficiency upon substitution of the phenyl ring again leads us to the conclusion that the energy levels of the molecule are minimally affected by this subs titution. Beyond these conclusions which rely heavily on conclusions drawn from various facets of information, it would be premature to make any further assertions regarding the trends in quantum yields without further experimentation. From these results, we have seen how the various optical spectra provide a qualitative understanding of the intra and intermolecular interactions present in our molecules. The solid state emission spectra of HxTTPTTHx will be discussed in more detail in the following parag raph to expand our understanding of intermolecular interactions present in that particular oligomeric system.
144 Table 4 3. Optical emission data of PheDOT oligomers. All readily observable vibronic peaks are noted with the maximum intensity denoted with a n asterisk. Compound em a (nm) f c TPT 401, 423*, 450 0 .08 HxTPTHx 415, 436*, 466 0 .08 DPD 398, 417*, 446 0 .09 EPE n/a n/a TTPTT 479*, 513, 550 0 .50 TTP4TT 480*, 513, 550 0 .50 HxTTPTTHx 489*, 523, 564 (493, 525*, 557) b ( 561*, 605 ) b 0 .54 HxTTP4TTHx 488*, 522, 562 0 .5 4 a Solution emission was measured in THF solutions under ambient conditions. b Films for solid state emission were prepared as described in the Figure 4 7 caption. Emission from thicker films is denoted in italics. c Quantum yields were measured using 9 ,10 diphenylanthracene in cyclohexane as a standard. 4.4.2 Solid State Emission of HxTTPTTHx One of the primary goals in developing the family of oligomers in this chapter was to assess their mobilities in field effect transistors. Towards this end, HxTT PTTHx was chosen as the first molecule to evaluate, however after initial experiments, mobilities were not observed. While mobility measurements were put on hold at this point, we wished to simultaneously probe the intermolecular interactions present in t he films of HxTTPTTHx fabricated for device testing to elucidate possible explanations for the lack of observed hole mobility. To this end we examined the solid state emission spectra of spun cast from a 5 mg/mL chlorobenzene s olution and a spun cast from a 20 mg/mL chlorobenzene solution were prepared and compared in Figure 4 7, along with the solution emission spectrum. In particular, if we look at the progression from solution, to thin film, to thick film, w e see a steady depletion of the short wavelength peak at 493 nm, and a strong attenuation of the peak at 525 nm. In dilute solution, the emission signals are dominated by those arising from unaggregated molecules. Thus, we see a steady depletion in the e xtent of emission
145 arising from unaggregated molecules as we progress from solution to thick film. Simultaneously, strong emission signals at 561 nm and 605 nm rapidly emerge as well and are likely due to aggregate emission. Morphologically, we can imagin e that when spinning from a dilute solution, regions of aggregated oligomers are interspersed with regions of unaggregated oligomers, resulting in a mixture of intermolecularly ordered and disordered regions, giving rise to an emission spectrum that resemb les a superposition of the solution spectrum and the thick film spectrum. When spinning from a concentrated solution however, a larger ratio of aggregated structures is present relative to the unaggregated molecules, leading to the depleted unaggregated m olecule emission and the intense aggregated emission. Figure 4 7. Emission spectra of a thin and a thicker film of HxTTPTTHx excited near the absorbance maxima at 420 nm. Films were prepared by spin coating from chlorobenzene solution (5 mgs/mL for thin films, 20 mgs/mL for thicker films) onto a glass slide at 2000 rpm for 30 seconds. Films were then dried overnight under vacuum. To verify the peak assignments, films were excited at two wavelengths at 420 nm, corresponding t o the solution absorbance maximum (i.e. the absorbance maximum
146 of the unaggregated system), and at 485 nm, corresponding to the absorbance peak assigned earlier as arising from aggregate absorption. Figure 4 8 illustrates that exciting at the aggregate ab sorption maximum depletes the short wavelength emission while intensifying the long wavelength emission, strengthening our above emission peak assignments. Figure 4 8. Emission spectra of HxTTPTTHx as a) a thin film, and b) a thick film prepared as described in the Figure 4 7 caption. The black curve shows the emission spectra of the film excited near the absorbance maxima at 420 nm. The red curve shows the emission spectra of the film excited at the peak corresponding to aggregate absorption at 485 nm. Finally, thick films were examined by AFM. In Figure 4 9, we see from the height image that large crystalline domains dominate the morphology of the film, while the mostly monochromatic phase image ver ifies that the observed features arise from one uniform composition. From the emission results and the AFM images, we conclude that our PheDOT oligomers, when spun from high concentration solutions, have a strong tendency to aggregate into large crystalli ne domains. Additionally, now that we understand the origin of the various emission peaks, the solid state emission spectra offers a rapid way to qualitatively evaluate the ratio of aggregated to unaggregated structures present in our film through the rat io of the long wavelength to short
147 wavelength emission peaks. This in turn allows us to quickly screen for deposition conditions which enhance intermolecular organization in our systems. Figure 4 coated from a 15 mg/mL chlorobenzene solution onto ITO. 4.4.3 Electrochemical Properties of PheDOT Oligomers HOMO and LUMO values of all oligomers, and their corresponding HOMO LUM O gap, were estimated using DPV. We begin by observing how these PheDOT containing oligomers compare to the all thiophene analogs. Across the board, the HOMO values of thiophene PheDOT oligomers are raised closer to vacuum compared with the analogous oli go thiophene s due to the presence of electron rich oxygens on PheDOT. The LUMO values of the PheDOT oligomers are slightly lower and more stable, presumably owing again to the electron rich oxygens reducing the electron accepting ability of the oligomer. As a result of the sizeable difference in HOMO values and the negligible difference in LUMO values, the HOMO LUMO gaps are smaller in the thiophene PheDOT oligomers compared with the all thiophene oligomers. Within the family of thiophene PheDOT compound s, several other trends can be established. Increasing the number of rings from 3 to 5 leads to t he expected raising of
148 the HOMO and lowering of the LUMO, due to the extended conjugation of the system. Substitution with the inductively electron donating hexyl chains raises the HOMO while also stabilizing the LUMO. The effects of substituting the phenyl ring with butyl chains, however, are minimal with no clear trend present, suggesting that the butyl chain has a negligible effect on the overall electroni c properties of the molecule. It is likel y that the inductively electron donating butyl chains donate only a small amount of electron density into the aromatic phenyl ring, which has little effect on the molecular orbitals of the conjugated system. Final ly, the incorporation of dioxythiophenes leads to a substantial increase in the electron richn ess of the system and an increased HOMO. The HOMO value of EPE however may be a little misleading as previously mentioned, the electron rich EPE seems especial ly prone to oxidation in solution. By the time the oligomer is drop cast, a small amount of oxidized material may be present, and the measured HOMO likely does not reflect the electronic properties of a fully neutralized material. All oligomers show robu st stability as a solid under ambient conditions, retaining their appearance for over a year when merely stored in a vial on the shelf. 4.4.4 Electropolymerization and Spectroelectrochemical Characterization of TPT and EPE In the family of oligomers syn thesized in this chapter, four of the oligomers TPT, DPD, EPE and TTPTT possessed unsubstituted sites at the 5 positions of the thiophene rings through which electropolymerization could occur. The optoelectronic properties of the two most abundant oli gomers on hand TPT and EPE were investigated further and their electropolymerization shown in Figure 4 10. In spite of TTPTT also having open sites for electropolymerization, its low solubility in solvents typically used for electropolymerization remo ved it from consideration. The oxidation
149 potentials of both oligomers, estimated from the onset of their oxidation peak, are in good agreement with the values obtained from DPV. EPE electropolymerizes readily, with a redox process emerging below the olig omer oxidation peak and the current density increasing steadily with each additional scan. TPT however does not display a prominent oxidation peak below the oligomer oxidation. Nevertheless, electrodeposition is evident from the steady increase in curren t density with successive scans. Table 4 4. HOMO and LUMO energies of PheDOT oligomers and their corresponding HOMO LUMO gaps. Compound HOMO (eV) LUMO (eV) E HOMO LUMO (eV) TPT 5.59 2.77 2.82 HxTPTHx 5.48 2.64 2.83 DPD 5.34 2.65 2.69 EPE 5.24 2.60 2.64 TTPTT 5.27 3.11 2.16 TTP4TT 5.24 3.12 2.12 HxTTPTTHx 5.19 2.82 2.37 HxTTP4TTHx 5.23 2.84 2.39 Compounds were drop cast from a THF solution onto platinum button. The button was transferred to a glove box, immersed in an electrolyte s olution of 0.1 M TBAPF 6 in propylene carbonate, and the energies measured by DPV using a Ag/Ag + reference electrode (140 mV relative to Fc/Fc + ). HOMO and LUMO values were calculated using a value of 5.1 eV for Fc/Fc + relative to vacuum. E HOMO LUMO = E HOM O E LUMO After film formation, the redox process of the monomer is present, indicating that some starting material remains trapped in the film. With successive scanning, however, this peak is observed to be depleted, most likely as the starting mater ial is consumed through coupling processes. The onset of oxidation of pTPT is considerably higher ( ca. 50 mV) than pEPE ( ca. 700 mV). Taken in conjunction with the electropolymerization results of Figure 4 10, this calls into question whether TPT oligom ers coupled to any appreciable extent to be considered a polymer, or if only short oligomers were generated. This question will be resolved below. The electrochemical behavior of the
150 stabilized film of pEPE shows a current response over a broad voltage w indow, making this material a potential candidate for charge storage applications. Figure 4 10. E lectropolymerizations of 7.5 mmol TPT (b) and 5 mmol EPE (c) in 0.1 M lithium perchlorate ACN:DCM solution cycled at 25 mV/s. The f irst scans of TPT and EPE are shown overlaid in (a). Figure 4 11. CV s of pTPT and pEPE. Figure a) shows the overlay of the first scan of both pTPT and pEPE. The arrows in figure a) denote peaks corresponding to the presence of monomeric species, which are subsequently shown to be absent in figures b) and c).
151 Films were electropolymerized onto ITO and their spectroelectrochemical properties are displayed in Figure 4 12. Both pTPT and pEPE are materials with moderate optical band bandgap than polythiophene, owing to the presence of the electron rich PheDOT moiety. The bandgap of pEPE is slightly higher than that of PEDOT, however care should be taken as the optical bandg ap of pEPE is difficult to determine due to the large near IR absorbances that make it difficult to accurately assess a neutral onset of absorption. It is safer to point out that the overall spectra of pEPE is red shifted relative to that of pTPT. In the neutral state, the significantly lower oxidation potential of pEPE gives rise to a smaller bandgap and a red shifted absorption maximum. In the oxidized state, the polaronic transition is also red shifted in pEPE. As both films are further oxidized, a c lear transition to a bipolaronic state can be observed in pEPE, but is absent in pTPT. This again suggests that pTPT may be composed of short segments that are unable to accommodate a bipolaronic charge, whereas pEPE benefits both from an extended conjuga tion as well as electron rich moieties to stabilize the bipolaron. Later we will see from polymeric studies more conclusive evidence that the electropolymerized pTPT should not be considered to be polymeric in nature. From these results, the oligomer tha t shows the most promising electrochemical properties is EPE, with particular potential as a material for supercapacitors displaying electroactivity over a broad voltage window.
152 Figure 4 12. Spectroele ctrochemistry of PheDOT containing polymers a) pTPT and b) pEPE electropolymerized from a 0.1M lithium perchlorate solution of a) 4:1 ACN:DCM and b) 1:1 ACN:DCM. For pTPT, measurements were taken in 100 mV intervals from (a) 0.26 V to (m) 0.94 V. For p EPE, measurements were taken at 100 mV intervals from (a) 0.75 V to (p) 0.75 V. 4.5 Thiophene PheDOT Polymers 4.5 .1 Synthesis of Thiophene PheDOT Polymers Having gained an understanding of how the presence of PheDOT in thiophene oligomers influences the intra and intermolecular ordering of the system, we wished to investigate the thiophene PheDOT motif in polymeric systems. The synthesis of thiophene PheDOT polymers is outlined in Scheme 4 3. First, the distannyl moiety was prepared and purified via re verse phase preparative HPLC using acetonitrile as eluent to afford high purity products. The preparation of highly pure starting materials for polymerization is necessary to generate high molecular weight polymers. The distannyl
153 compounds were then coup led with 2,5 dibromoPheDOT (C12) 2 via Stille polymerization to afford the polymer as a purple solid. To purify the polymer, the solid was dissolved in a minimal amount of hot chlorobenzene and stirred with a palladium scavenger to remove traces of the cat alyst. The polymer was then precipitated into methanol and filtered. The filtered material was then purified using Soxhlet extraction. Specifically, the crude was washed with methanol and hexanes. Afterwards, a chloroform fraction and a chlorobenzene f raction were collected. The chloroform and chlorobenzene fractions were redissolved in their respective solvents and treated with a palladium scavenger once more. Finally, the solutions were precipitated into methanol and filtered to afford the desired p roducts. Unfortunately, the dodecyl chains were not able to impart sufficient solubility to either fraction to assess the molecular weights using THF as the mobile phase. However, 1 H NMR, 13 C NMR, and elemental analysis were used to conclude that the des ired polymer was indeed synthesized. Scheme 4 3. Synthesis of thiophene PheDOT oligomers via Stille polymerization. a) 1. n BuLi (2.1 eq)., TMEDA (2.1 eq.), heptanes 78C to RT; 2. SnMe 3 Cl (2.1 eq.) 0C to RT, overnigh t; 40%. b) Pd 3 (dba) 2 2%, ( o tolyl) 3 P 4%, chlorobenzene, 90C, 3 days.
154 The solubility of these polymers is negligible without the application of sufficient heat. At reflux, the chlorobenzene fraction gives 2 mg/mL solutions in toluene, chloroform and chlo robenzene. When cooled to room temperature, the chlorobenzene solution remains dissolved for some time, whereas the polymer crashes out of toluene and chloroform relatively quickly. 4.5 .2 Optical Properties of Thiophene PheDOT Polymers The solution absor bance of both polymers was investigated to see whether intra and intermolecular interactions could be investigated as previously accomplished with the oligomers. As shown in Figure 4 13, both poly(TP12) and poly(TTP12) display sharp peaks around 600 nm a nd 560 nm, as well as a shoulder around 520 nm. In order to investigate whether these peaks are vibronic in nature or due to aggregation, we investigated the concentration dependence on the spectra and performed thermochromic experiments. Several fold di lutions were performed with no significant changes in the ratio of the peaks around 600 nm and 560 nm. As dilution increases, we would expect intermolecular interactions to become less prominent as the polymers become better diss olved. The similarity of the spectra over several fold dilution however would suggest that intermolecular interactions are not responsible for the appearance of these peaks. This appears to be contradicted in the thermochromic data however, where we see behavior consistent with the presence of aggregation effects. As temperatures increase, the ratio of the two peak magnitudes is inverted, where the peak near 600 nm has begun to deplete at the expense of the short wavelength region which has grown in inte nsity. This suggests that the peak around 600 nm is due to aggregation, where at higher temperatures these aggregates are largely broken up leading to signals associated with well dissolved systems becoming
155 more prominent. The literature values of rr P3H T and PPheDOT (C12) 2 also corroborate this assignment, where both have had aggregation peaks assigned around 600 nm and unaggregated peaks assigned from ca 470 nm to 554 nm. 118,119 Figure 4 13. Temperature dependent absorption spectra of PheDOT containing polymers; chlorobenzene, taken at 10 C increments. One model that would reconcile the concentration independent behavior and the thermochromic data would be to suggest that intramolecular aggregation due to chain
156 folding is responsible for the observed behavior. Such chain folding could be present at both high and low concentrations, leading to no appr eciable difference in the appearance of the spectra. However, chain folding would be expected to depend on solution temperature, where a less folded and more extended conformation would be expected at higher temperatures. This model is also supported by temperature dependence emission intensity. In Figure 4 14, it is shown that the intensity of the emission of poly(TP12) is dramatically enhanced as temperatures are increased (the trend is identical for poly(TTP12) which is not shown here). The low intensities at room temperature may be due to self quenching due to aggregation. Once this aggregation is broken up however, the emission is greatly enhanced Figure 4 14. PL spectra of poly(TP12) in chlorobenzene at ro om temperature and high temperature (~100C). Returning to the thermochromic data, we observe a strong blue shifting of the absorption spectra with increasing temperatures. The increase in temperature is expected to enhance the free rotation of the rings decreasing the conjugation of the
157 system and subsequently blue shifting the spectra. It is interesting however to note that the extent of this blue shift is larger in poly(TP12) than in poly(TTP12). We might expect that sulfur oxygen interactions may discourage free rotation in poly(TP12) where every thiophene has a neighboring PheDOT to engage in sulfur oxygen bonding interactions. In contrast, in poly(TTP12) free rotation around the thiophene thiophene bond should be allowed to occur unhindered, le ading to larger torsional motion. However, the opposite trend is observed. One possible explanation for this phenomena rests with the proximity of neighboring dodecyl chains in poly(TP12) compared with neighboring chains in poly(TTP12) as modeled in Fi gure 4 15. With only one thiophene spacer between two PheDOT (C12) 2 in poly(TP12), the steric repulsion between dodecyl chains will introduce twisting between the thiophene rings. At elevated temperatures, the free motion of the dodecyl chain increases, leading to an increase in the steric repulsion. In contrast, the dodecyl chain containing units of (TTP12) are inherently spaced further apart. The steric repulsion is still present at elevated temperatures, but minimized due to this spacing. The spectro electrochemistry of both polymers drop cast onto ITO is shown in Figure 4 16. Interestingly, the solid state spectra of the neutral films are nearly identical to the solution spectra. In particular, no new red shifted aggregation peak is observed in the solid state spectra. This strengthens our earlier assertion that in solution, the polymers already existed in an intramolecularly aggregated state. The spectra of both polymers are similar, with nearly identical peak values. Both films also readily acco mmodate polaronic and bipolaron ic transitions upon oxidation.
158 Figure 4 15 One possible model explaining the magnitude of thermochromic blue shifting observed in Figure 4 13. uctive to compare the chemically polymerized film of poly(TTP12) to the electrochemically polymerized pTPT from Figure 4 12, which have an identical repeating unit. The peak values of the neutral film of the electropolymerized film are all blue shifted re lative to the chemically polymerized film, suggesting a lower degree of conjugation in the former. Additionally, the peak near 600 nm is depleted in the electropolymerized film at the expense of the peak near 500 nm, where the opposite is true in the chem ically polymerized film, suggesting a larger degree of aggregation in the latter. The polaronic transition poly(TTP12) around 850 nm is significantly red shifted compared with the polaronic transition of pTPT located around 700 nm. This also indicates th at poly(TTP12) possesses a longer conjugation length, as the polaron is better able to delocalize. Finally, the bipolaronic transition, which was not accessible in
159 the electrochemically polymerized film, is present in the chemically polymerized film. The ability of the latter to accommodate the bipolaron again suggests the presence of longer conjugation lengths, and supports the assertion that the electropolymerized films Figure 4 16. Spectroelectrochemistries of a) poly(TP12) and b) poly(TTP12) drop cast from 2 mg/mL solutions in chlorobenzene onto ITO. Films were switched in 100 mV increments. The work outlined in this chapter illustrates that thiophene P heDOT oligomers and polymers are promising candidates for optoelectronic applications, where the tendency of the materials to strongly aggregate and readily crystallize can be a desirable quality for charge transporting materials. The processability of th e oligomers and its thermal robustness make these molecules particularly attractive starting points to fabricate some initial thiophene PheDOT oligomer based devices. The solubilities of the polymers remains a barrier, however the ability to handle the po lymers in promising.
160 4.6 Synthetic Details General procedure for the stannylation of thiophenes To a round bottom flask with stirbar and under an argon atmosphere was added the thiophene (1 eq.) and dry THF. The reaction was cooled to 78C and n BuLi (1.05 eq.) was added dropwise. After complete addition, the reaction was allowed to gradually warm to 0C and stirred at that temperature for 1 hour. Afterwards, the reaction wa s cooled back to 78C and trimethyltin chloride (1.05 eq.) was added in one portion. The reaction was allowed to gradually warm to room temperature and stirred overnight. Afterwards, the reaction was poured into water, added to a separatory funnel, and extracted with diethyl ether (3 100 mL). The organic layers were combined and washed with brine, dried with MgSO 4 filtered, and the solvent remove via rotary evaporation. The crude was dissolved in diethyl ether and passed through a basified silica ge l plug to remove salts. The solvent was removed via rotary evaporation to afford the crude. General procedure for Stille coupling To a Schlenk tube with stirbar was added Pd(PPh 3 )Cl 2 (2%), CuI (4%), and the appropriate 2,5 dibromoPheDOT (1 eq.). The vess el was evacuated and flushed with argon three times. Afterwards, degassed DMF (20 mL) was added, followed by the stannylated thiophene, and the reaction run at 80C for 3 days. Afterwards, the reaction was cooled and poured into water (60 mL), resulting in the formation of precipitate. The solid was filtered through a Buchner funnel and washed with water. The remainder of the purification depends on the oligomer in question and will discussed on a case by case basis.
161 Synthesis of 3 and 5 ring precursors 2 (trimethylstannyl)thiophene (4 1): The general procedure for the stannylation of thiophenes was followed starting with thiophene (10 g, 118.84 mmol). The crude was afforded as a yellow oil (30 g, quantitative yield). 1 H), 0.35 (s, 9H). 5 hexyl 2 (trimethylstannyl)thiophene (4 2): The general procedure for the stannylation of thiophenes was followed starting with 2 hexylthiophene (4.68 g, 27.80 mmol). The crude was afforded as a yellow oil (9.27 g, quantitative). 1 H NM (d, 1H), 6.88 (d, 1H), 2.84 (t, 2H), 1.68 (m, 2H), 1.36 1.26 (m, 10H), 0.90 0.85 (m, 3H), 0.32 (s, 9H). 13 22.81, 14.32, 8.07. 2 (trimethylstannyl) 3,4 dimethoxythiophene (4 3): Th e general procedure for the stannylation of thiophenes was followed starting with 3,4 dimethoxythiophene (2.5 g, 17.33 mmol). The product was obtained as a slightly yellow oil (5.29 g, 17.23 mmol, quantitative yield). 1 ; 3.79 (3H, s); 0.34 (9H, s). 2 (trimethylstannyl)EDOT (4 4): The general procedure for the stannylation of thiophenes was followed starting with EDOT(5 g, 35.16 mmol). The product was obtained as a slightly yellow oil (9.8 g, 32.13 mmol, 91% yield). bithiophene (4 5): To a 3 necked flame dried round bottom flask with stirbar and a condenser was added magnesium turnings (2.46 g, 101 mmol, 1.1 eq.). The reaction was evacuated and backfilled with argon three times, followed by addition of a single iodi ne crystal, and then dry diethyl ether via cannula (200 mL). The reaction was stirred vigorously for at least 30 minutes until the initial light brown solution turned
162 colorless, indicating the consumption of the iodine and the activation of the magnesium turnings. The flask was cooled to 0C and 2 bromothiophene (16.5 g, 100 mmol, 1.1 eq.) was added slowly. The reaction was warmed to a gentle reflux and stirred for 3 hours to form the Grignard. Afterwards, the reaction was cooled to room temperature. I n a second 3 necked flame dried round bottom flask with stirbar and a condenser was added 2 bromothiophene (15 g, 92 mmol, 1 eq.) and Ni(dppp)Cl 2 (0.49 g, 0.92 mmol, 1%). Dry diethyl ether (200 mL) was added via cannula and the flask cooled to 0C. The G rignard solution from the first flask was added to the second flask via cannula. The reaction was then warmed to a gentle reflux and stirred overnight. Afterwards, the reaction cooled to room temperature and poured into a dilute solution of ammonium chlo ride to quench the reaction. The solution was poured into a separatory funnel, extracted with diethyl ether (3 100 mL), washed with brine, dried with MgSO 4 filtered, and the solvent removed under rotary evaporation to afford the crude. The crude was p assed through a silica gel column using hexanes as eluent to afford the desired product as a white crystalline solid (12.2 g, 74 mmol, 80%). 5 (trimethylstannyl) bithiophene (4 6): The general procedure for the stannylation of thiophenes was followed starting with bithiophene (4 5) (1.08g, 6.49 mmol). The product was obtained as a yellow orange oil (2g, 6.07 mmol, 93% yield). 1 2 bromo 5 hexylthiophene (4 7): To a round bottom flask with stirbar was added 2 hexylthiophene (10 g, 59 mmol) and DMF (200 mL). The solution was bubbled through with argon for 1 hour, after which the reaction was covered with foil to exclude light and NBS (11 g, 62 mmol, 1.05 eq.) was added in one portion and the reaction stirred
163 overnight. Afterwards, the reaction was poured into water. The solution was poured into a sep aratory funnel, extracted with diethyl ether (3 100 mL), washed with brine (2 100 mL), dried with MgSO 4 filtered, and the solvent removed under rotary evaporation to afford the crude. The crude was passed through a silica gel plug using hexanes as el uent to afford the desired product as a colorless oil (14 g, 95% yield). 1 (s, 1H), 6.50 (s, 1H), 2.71 (t, 2H), 1.60 (m, 2H), 1.35 1.29 (m, 6H), 0.86 (t, 3H). 13 C .29. 5 hexyl bithiophene (4 8): The general procedure for Stille coupling was followed, starting with 2 bromo 5 hexylthiophene (4 7) (7.30 g, and 2 (trimethylstannyl)thiophene (4 1). The product was obtained as a colorless oil (80%). 1 (dd, 1H), 7.13 (dd, 1H), 6.98 (dd, 2H), 6.65 (d, 1H), 2.77 (t, 2H), 1.68 (m, 2H), 1.39 1.27 (m, 6H), 0.88 (t, 3H). 5 hexyl (trimethylstannyl) bithiophene (4 9): The general procedure for the stannylation of thiophenes was followed starting with b ithiophene (4 5). 1 (d, 1H), 7.04 (d, 1H), 6.95 (d, 1H), 6.65 (d, 1H), 2.76 (t, 3H), 1.68 (m, 2H), 1.29 (m, 6H), 0.88 (t, 3H), 0.36 (s, 9H) 2,5 dibromoPheDOT (4 10): To a round bottom flask with stirbar was added PheDOT (2 g, 10.51 mmol, 1 eq .) and DMF (100 mL). The solution was bubbled through with argon for 1 hour, after which the reaction was covered with foil to exclude light and NBS (4.11 g, 23.13 mmol, 2.2 eq.) was added in one portion and the reaction stirred overnight. Afterwards, th e reaction was poured into water. The solution was poured into a separatory funnel, extracted with diethyl ether (3 100 mL), washed with brine (2 100 mL), dried with MgSO 4 filtered, and the solvent removed under rotary
164 evaporation to afford the crude The crude was passed through a silica gel plug using hexanes as eluent to afford the desired product as a white powder. 1 4H). 2,5 dibromoPheDOT Bu 2 (4 11): To a round bottom flask with stirbar was added PheDOT Bu 2 (0.18 g, 0.59 mmol) and DMF (20 mL). The solution was bubbled through with argon for 1 hour, after which the reaction was covered with foil to exclude light and NBS (0.23 g, 1.30 mmol, 2.2 eq.) was added in one portion and the reaction stirred overnight. Afterwards, the rea ction was poured into water. The solution was poured into a separatory funnel, extracted with diethyl ether (3 100 mL), washed with brine (2 100 mL), dried with MgSO 4 filtered, and the solvent removed under rotary evaporation to afford the crude. Th e crude was passed through a silica gel plug using hexanes as eluent to afford the desired product as a white powder (0.25 g, 92%). 1 (s, 2H), 2.48 (t, 4H), 1.53 1.45 (m, 4H), 1.39 1.32 (m, 4H), 0.93 (t, 6H). Synthesis of PheDOT containin g 3 and 5 ring systems TPT (4 12): The general procedure for Stille coupling was followed, starting with 2,5 dibromoPheDOT (0.3 g, 0.86 mmol), 4 1 (0.53 g, 2.15 mmol, 2.5 eq.) and 20 mL of DMF. The crude was purified using a 3:1 methylene chloride:hexane s column to afford the product as a light yellow solid (60% yield). The general procedure for Stille coupling was followed, starting with 2,5 dibromoPheDOT (0.3 g, 0.86 mmol), 4 1 (0.53 g, 2.15 mmol, 2.5 eq.) and 20 mL of DMF. The crude was purified usin g a 3:1 methylene chloride:hexanes column to afford the product as a light yellow solid (60% yield). 1 H 7.21 (m, 6H); 13 CNMR (300 MHz, (CD 3 ) 2 CO)
165 129.12, 126.99, 126.02, 125.85, 118.59; Anal. calcd for C 18 H 10 O 2 S 3 : C, 60.99%; H, 2.84%. Found: C, 60.71%; H, 2.97%. HRMS m/z: calcd, 353.9843; found, 353.9840. HxTPTHx (4 13): The general procedure for Stille coupling was followed, starting with 2,5 dibromoPheDOT (0.5 g, 1.43 mmol) and 4 2 (1.42 g, 4.31 mmol, 3 eq.). The crude was purified using a basified silica gel column with hexanes as eluent to afford the product as a light yellow solid (0.52 g, 1 mmol, 70% yield). 1 7.02 (m, 4H), 6.95 (m, 2H), 6.70 (d, 2H), 2.80, (t, 4H), 1.68 (m, 4H), 1.40 1.29 (m, 10H), 0.88 (t, 6H). 13 117.32, 110.38, 31.84, 31.79, 30.35, 29.01, 22.80, 14.31. Anal. calcd for C 30 H 34 O 2 S 3 : C, 68.92; H, 6.56. Found: C, 68.720; H, 6.614. HRMS m/z: calcd, 523.1794; found, 523.1791. DPD (4 14): The general procedure for Stille coupling was followed, starting with 0.5 g (1.43 mmol) of dibromoPheDOT (4 10), 1 g (3.25 mmol, 2.2 eq.) of 4 3, and 40 mL of DMF. After filtration, the crude was purified by passing through a basified silica gel column using 2:1 hexanes:methylene chloride as eluent to afford the product as a bright yellow solid (180 mgs, 0.23 mmol, 27% yield). 1 H 6.92 (m, 4H), 6.18 (s, 2H), 3.89 (d, 12H). EPE (4 15): To a 3 necked round bottomed flask equipped with a condenser was added 2 trimethylstannylEDOT (1.92 g, 6.3 mmol), 2,5 dibromoPheDOT (0.57 g, 3.0 mmol), PdCl 2 (0.026 g, 0.15 mmol), ( t Bu) 3 PBF 4 (0.087 g, 0.3 mmol), CuI (0.057 g, 0.3 mmol), and CsF (0.95 g, 6.3 mmol). The flask was purged with argon and degassed dimethylformamide (30 mL) was added via syringe. The reaction was stirred at 80C overnight, after which it was poured into water and filtered. The solid was concentrated
166 with methylene chloride, dried with MgSO 4 filtered, and the solvent removed under vacuum to yield the crude as a brown solid. The crude was purified by recrystallizing from methylene chloride and methanol to yiel d the product as a yellow orange solid (90% yield); 1 H NMR (300 MHz, CDCl 3 ) 7.01 (m, 4H), 6.37 (s, 2H), 4.32 (m, 10H); HRMS m/z: calcd, 469.99; found, 470.14. Anal. calcd for C 22 H 14 O 6 S 3 : C, 56.16%; H, 3.00%. Found: C, 55.80%; H, 2.98%. TTPTT (4 16): The general procedure for Stille coupling was followed, starting with 0.5 g (1.43 mmol) of dibromoPheDOT, 1.4 g (4.31 mmol, 3 eq.) of 4 6, and 20 mL of DMF. After filtration, the brown crude was heated in refluxing heptanes; any undissolved material was rapid ly filtered off. The hot heptanes solution was cooled to room temperature, and precipitate was observed to form over time. The solution was cooled at 0C and filtered to afford an orange crude. This was repeated until TLC under long wavelength irradiati on showed only a single yellow fluorescent spot. The product was obtained as a robust yellow solid (100 mgs, 0.19 mmol, 14% yield). 1 7.22 7.15 (m, 6H), 7.10 7.06 (m, 4H), 7.03 9.69 (m, 4H). 13 140.80, 128.148, 125.013, 125.009, 124.865, 124.454, 124.344, 124.100, 117.489, 115.803, 109.983. HRMS m/z: calcd, 518.9670; found, 518.9693. TTP4TT (4 17): The gen eral procedure for Stille coupling was followed, starting with 0.25 g (0.54 mmol) of 4 11, 0.51 g (1.59 mmol) of 4 6, and 20 mL of DMF. After filtration, the crude was passed through a basified silica gel column using hot hexanes as eluent to afford the p ure as a yellow solid (100 mgs, 0.16 mmol, 29%). Anal. calcd for C 34 H 30 O 2 S 5 : C, 64.72; H, 4.79. Found: HRMS m/z: calcd, 631.0922; found, 631.0948.
167 HxTTPTTHx (4 18): The general procedure for Stille coupling was followed, starting with 0.745 g (2.41 mmol) of dibromoPheDOT, 2.5 g (6.05 mmol, 2.5 eq.) of 4 9, and 30 mL of DMF. After filtration, the brown crude was heated in refluxing heptanes; any undissolved material was rapidly filtered off. The hot heptanes solution was cooled to room temperature, and p recipitate was observed to form over time. The solution was cooled at 0C and filtered to afford an orange crude. This was repeated until TLC under long wavelength irradiation showed only a single yellow fluorescent spot. The product was obtained as a y ellow orange solid (500 mgs, 0.72 mmol, 30% yield). 1 7.14, (d, 2H), 7.07 (m, 2H), 6.99 (m, 6H), 6.68 (m, 2H), 2.78 (t, 4H), 1.66 (m, 4H), 1.30 (m, 12H), 0.90 (t, 6H). 13 123.71, 115.12, 109. 65, 33.91, 31.97, 30.33, 28.97, 22.80, 14.31 Anal. calcd for C 38 H 38 O 2 S 5 : C, 66.43; H 5.57. Found: C, 66.418; H, 5.471. HRMS m/z: calcd, 687.1548; found, 687.1553. HxTTP4TTHx (4 19): The general procedure of Stille coupling was followed, starting with 0.4 6 g (1 mmol) of 4 11, 1.03 g (2.5 mmol) of 4 9, and 50 mL of DMF. The crude was purified using a basified silica gel column with hot hexanes as eluent to afford the pure as an orange solid (93 mgs, 0.11 mmol, 11% yield). 1 2H), 7.00 (d, 4H), 6.83 (s, 2H), 6.67 (d, 2H), 2.78 (t, 4H), 2.51 (t, 4H), 1.67 (m, 4H), 1.52 (m, 4H), 1.38 1.25 (m, 16H), 0.95 (t, 6H), 0.88 (t, 6H). 13 137.37, 136.80, 134.73, 131.96, 125.04, 124.52, 123.56, 123.37, 117.33, 109.99, 33.55, 32 .11, 31.99, 31.81, 30.46, 29.26, 29.02, 22.99, 22.82, 14.35. Anal. calcd for C 46 H 54 O 2 S 5 : C, 69.13; H, 6.81; found: HRMS m/z: calcd, 798.27; found.
168 Synthesis of PheDOT containing polymers 2,5 bis(trimethylstannyl)thiophene (4 20a): To a 3 necked flame dr ied round bottom flask with stirbar was added thiophene (3.81 g, 45.28 mmol, 1 eq.) and dry heptanes (100 mL). The reaction was cooled to 78C and n BuLi (95.09 mmol, 2.1 eq.) followed by TMEDA (14.35 mL, 95.09 mmol, 2.1 eq.) were added. The reaction was warmed to 45C and stirred for two hours. Afterwards the reaction was cooled to 0C and trimethyltin chloride (18.94 g, 95.09 mmol, 2.1 eq.) was added. The reaction was warmed to room temperature and stirred overnight. Afterwards the reaction was poure d into water. The solution was poured into a separatory funnel, extracted with diethyl ether (3 100 mL), washed with brine, dried with MgSO 4 filtered and solvent removed under rotary evaporation to afford the crude. The pure product was achieved by re peated recrystallization from ethanol as a white powder (6.6 g, 16.1 mmol, 36% yield). 1 bis(trimethylstannyl) bithiophene (4 20b): To a 3 necked flame dried round bottom flask with stirbar was added bithiophene (4 g, 24.05 mmol, 1 eq.) and dry heptanes (100 mL). The reaction was cooled to 78C and n BuLi (50.52 mmol, 2.1 eq.) followed by TMEDA (7.62 mL, 50.52 mmol, 2.1 eq.) were added. The reaction was warmed to 45C and stirred for two hours. Afterwards the reaction was cooled to 0C and trimethyltin chloride (10.06 g, 50.52 mmol, 2.1 eq.) w as added. The reaction was warmed to room temperature and stirred overnight. Afterwards the reaction was poured into water. The solution was poured into a separatory funnel, extracted with diethyl ether (3 100 mL), washed with brine, dried with MgSO 4 filtered and solvent removed under rotary evaporation to afford the crude. The pure product was achieved by
169 passing through a preparative HPLC using acetonitrile as eluent; the pure product was obtained as a white powder (4.72 g, 9.60 mmol, 40% yield). 1 7.07 (d, 2H), 0.38 (s, 18H). 2,5 dibromoPheDOT (C12) 2 (4 21): To a round bottom flask with stirbar was added PheDOT (C12) 2 (1g, 1.89 mmol, 1 eq.) and DMF (50 mL). The solution was bubbled through with argon for 1 hour, after which t he reaction was covered with foil to exclude light and NBS (0.743 g, 4.17 mmol, 2.2 eq.) was added in one portion and the reaction stirred overnight. Afterwards, the reaction was poured into water. The solution was poured into a separatory funnel, extrac ted with diethyl ether (3 100 mL), washed with brine (2 100 mL), dried with MgSO 4 filtered, and the solvent removed under rotary evaporation to afford the crude. The crude was passed through a silica gel plug using hexanes as eluent to afford the des ired product as a white solid (1.14 g, 1.67 mmol, 88% yield). 1 0.86 (t, 6H). Poly(TP12) (4 22a): To a Schlenk flask with stirbar was added 4 20a (0.17 g, 0.43 mmol, 1.05 eq.), 4 21 (0.28 g, 0.42 mmol), Pd 2 (dba) 3 (0.0087 g, 0.0084 mmol, 2%), ( o tolyl) 3 P (0.01 g, 0.033 mmol, 8%). The flask was evacuated and backfilled with argon three times. Degassed toluene (20 mL) was added via syringe and the reaction was covered with foil to exclude ligh t, warmed to 80C and stirred for three days. Afterwards, the reaction was cooled to room temperature and slowly added into methanol while stirring, resulting in the precipitation of polymer. The mixture was passed through a 0.45 m Nylon filter. The so lid was collected and further purified via Soxhlet extraction by washing with methanol, hexanes, and then extracting with
170 chloroform and chlorobenzene. The chloroform and chlorobenzene fractions were again dissolved in a minimal amount of chlorobenzene an d slowly added into methanol while stirring. The precipitate was once again filtered to afford the polymer (230 mgs, 90% yield). Anal. calcd: C, 75.19%; H, 8.97%. Found: C, 74.64%; H, 9.47%. Poly(TTP12) (4 22b): To a Schlenk flask with stirbar was adde d 4 20b (0.24 g, 0.48 mmol), 4 21 (0.33 g, 0.48 mmol), Pd 2 (dba) 3 (0.009 g, 0.0098 mmol, 2%), ( o tolyl) 3 P (0.012 g, 0.039 mmol, 8%). The flask was evacuated and backfilled with argon three times. Degassed toluene (30 mL) was added via syringe and the reac tion was covered with foil to exclude light, warmed to 80C and stirred for three days. Afterwards, the reaction was cooled to room temperature and slowly added into methanol while stirring, resulting in the precipitation of polymer. The mixture was pass ed through a 0.45 m Nylon filter. The solid was collected and further purified via Soxhlet extraction by washing with methanol, hexanes, and then extracting with chloroform and chlorobenzene. The chloroform and chlorobenzene fractions were again dissolv ed in a minimal amount of chlorobenzene and slowly added into methanol while stirring. The precipitate was once again filtered to afford the polymer (183 mgs, chlorobenzene fraction, 54% yield; 100 mgs, chloroform fraction, 30% yield). Anal. calcd: C, 73 .21%; H, 8.19%. Found: C, 73.10%; H, 8.59%.
171 CHAPTER 5 SYSTEMS INCORPORATIN G SPIROBIPRODOT FOR DECREASED INTERMOLECULAR ORDER 5.1 Brief Survey of Spiro Compounds In contrast to the motifs of the previous chapters, where the goal was to improve intramolec ular order and increase intermolecular interactions, in this chapter molecules with large amounts of steric bulk are targeted to minimize intermolecular interactions. In this capacity, spiro compounds have received attention due to the twisting around the spiro moiety, which can turn the two halves of the molecule perpendicular to one another and lead to steric bulk. 120 This in turn leads to a decreased tendency of the molecule to aggregate and crystallize. One potentially advantageous consequence is an increa sed tendency to form amorphous films. 121,122 These amorphous films lack charge trapping defects such as grain boundaries associated with polycrystalline materials, and as a result properties from batch to batch ar e more reproducible. Additionally, the decreased tendency of these systems to form strong intermolecular interactions has prompted the investigation of these molecules as highly fluorescent materials. Indeed, relatively high solution and solid state fluo rescence quantum yields have been reported for molecules based on a spirobifluorene core. 123 126 Thirdly, films of polymers containing spiro motifs have been found to be porous with a high surface area. 127 Such materials have traditionally been of interest in catalysis and gas storage, and may also find use in electrochromic appl ications. In particular, a highly porous morphology should allow for the rapid diffusion of ions through the film, allowing for the fabrication of films which can rapidly switch between colored states. In a recent paper, tetrabrominated spirobiProDOT und erwent a Sonogashira polymerization with bis and tris ethynyl linkers, and the resulting films were found to have both a high surface area
172 and a high degree of porosity. 127 Unfortunately, the highly cross linked nature of the polymer rendered the material insoluble. The desire to overcome the limitations regarding processability, as well as to expand the family of spiro biProDOT compounds and investigate their optoelectronic properties, serves as the motivation for the work carried out here in Chapter 5. 5.2 Synthesis of SpirobiProDOT Oligomers Scheme 5 1 outlines the synthetic pathway towards spirobiProDOT (sBP) oligomer s. The synthesis of the central sbP core was first developed in our group 128 and starts with nucleophilic aromatic substitution between pentaerythritol and 2.1 equivalents of dimethoxythiophene to form spirobiProDOT ( 5 1 ). sBP is then treated with an excess of NBS to form Br 4 sBP ( 5 2 ), which can readily undergo Suzuki or Stille coupling to generate a wide array of systems. The synthesis of the aromatic units which couple to 5 2 varies. 3 meth ylthiophene 2 boronic acid pinacol ester ( 5 4 ) can be synthesized starting with bromination of commercially available 3 methylthiophene (slight excess) with NBS to form 2 bromo 3 methylthiophene ( 5 3 ). While this typically leads to a small amount of unrea cted starting material which is readily removed at low temperatures (boiling point of 3 methylthiophene is ca. 114C) under vacuum (e.g. under rotary evaporation) while the desired product has a much higher boiling point ( ca. 173C) and remains in the flas k. Lithium halogen exchange between 2 bromo 3 methylthiophene and n BuLi, followed by addition of the appropriate dioxaborolane affords the desired boronic acid pinacol ester ( 5 4 ). (Attempts to form 5 4 directly from 3 methylthiophene using n BuLi and t he dioxaborolane lead to formation of both the 5 substituted and 2 substituted pinacol ester which could not be separated readily.) Passing the crude rapidly through a silica gel column using 50:1 hexanes:ethyl acetate
173 affords the product as a white cryst dimethyl bithiophene ( 5 5 ) is readily synthesized from 2 bromo 3 methylthiophene. Formation of the Grignard 3 methylthiophene 2 magnesiumbromide followed by Kumada coupling to 5 3 leads to the bithienyl compound in high yields. T reatment with n BuLi followed by addition of dioxaborolane gives the desired boronic acid pinacol ester ( 5 6 ). The two ProDOT R 2 were synthesized by nucleophilic aromatic substitution between dimethyoxythiophene and the appropriate 2,2 dialkylpropane 1,3 diol. Stannylation proceeded by attacking ProDOT with n BuLi followed by addition of SnMe 3 Cl. The stannylated derivatives were flashed through a basified silica gel plug to remove salts and polar impurities. Scheme 5 1. Synthesis of precursors to spirobiProDOT containing compounds. a) p TSA (0.1 eq.), toluene, 90C, 24 hours; b) NBS (5 eq.), chloroform, 24 hours; c) NBS (0.95 eq.), DMF, 3 hours; d) 1. n BuLi (1.05 eq.), THF, 78C 50C, 1 hour; 2. 4,4,5,5 tetramethyl 1,3,2 dioxaborolane (1.05 eq.) 78C RT, overnight; e) 1. Mg (1.2 eq.), diethyl ether, reflux, 3 hours; 2. 2 bromo 3 methylthiophene, Ni(dppp)Cl 2 (1%), diethyl ether, reflux, overnight; f) 1. NaH (6 eq.), 2 ethylhexan 1 ol (3 eq.), DMF, 100C, overnight; 2. 5 8a reflux, overnight; g) 1. n BuLi (1.05 eq.), THF, 78C 0C, 1 hour; 2. SnMe 3 Cl (1.05 eq.), 78C RT, overnight.
174 The coupling between Br 4 sBP 5 2 and the various boronic esters and tin derivatives are outlined in Scheme 5 2 and proceeded with l ow yields ( ca. 20%); furthermore, the mono di and trisubstituted derivatives were also collected (though not isolated individually), demonstrating that complete conversion was difficult to accomplish. The reasons for this are not immediately clear. R unning the reaction for 2 days or 5 days did not lead to an improvement in the yield. Poisoning of the palladium catalyst with oxygen also does not seem to be the problem solvents were degassed via freeze pump thawing; also, no significant amount of pal ladium mirror was observed to deposit on the outside of the flask, a common sign that oxygen has entered the reaction. The addition of fresh catalyst after 2 days also did not lead to an improvement in the yields. Microwave reactions of the Suzuki coupli ngs were performed, with all solvents freeze pump thawed and all reagents assembled inside a glove box prior to being introduced into the microwave reactor. Yields remained about the same, with a distribution of mono di and trisubstituted products pres ent. However, the yields were achieved after only an hour in the microwave, although times were not optimized. The design logic of the various compounds had two guiding goals in mind. First, considering the low solubility of Br 4 sBP 5 2 flanking thiophen e moieties were synthesized based on their ability to improve the solubility of the molecule. Second, molecules were designed to maximize intermolecular repulsion. The two goals often found a similar solution. For compounds 5 11 and 5 12 solubility was induced by introducing methyl groups on the thiophene rings, which increases the steric bulk of the thiophene moiety and twists it out of planarity with the neighboring thiophenes. The free rotation of the thiophene simultaneously serves to decrease inte rmolecular interactions
175 and introduce solubility. Also, for compound 5 12 the presence of an extra flanking thiophene is intended to increase the spacer distance between spirobiProDOT moieties when electropolymerized to observe what effects this has on t he rigidity and porosity of the system. For compounds 5 13 and 5 14 solubilizing alkyl groups are present on the propylene moiety where they should extend normal to the plane of the molecule and frustrate intermolecular packing. Scheme 5 2. Synthesis of spirobiProDOT containing compounds. a) Pd 2 dba 3 (2%), ( o tolyl) 3 P (12%), aliquot 336, K 3 PO 4 (3M), toluene, water, 110C, 1 hour, microwave reactor; b) Pd(PPh 3 )Cl 2 (5%), CuI (10%), DMF, 80C, 3 days.
176 5.3 Optoelectronic Prop erties of SpirobiProDOT Compounds 5.3.1 Electrochemical Properties of SpirobiProDOT Oligomers The electropolymerizations of the oligomers proceeded readily via cyclic voltammetry, as indicated by the distinct emergence of a redox couple associated with for mation of a longer chain species, and also by the steady increase in current on each subsequent scan. For all systems, electropolymerization depend ed heavily on the electrolyte used, as illustrated in Figure 5 1 for the electropolymerization of 3MTsBP 5 1 1 When TBABF 4 is used as the electrolyte ( Figure 5 1 A ), electropolymerization proceeds smoothly with current densities in excess of 2 mA/cm 2 after 10 scans. A redox peak associated with longer chained species is observed with an oxidation peak around 0. 4 V and reduction peak at slightly lower potentials. Looking at Figure 5 1 B using TBAPF 6 as electrolyte results in the same overall profile as we saw in Figure 5 1 A however current densities are almost 40% lower, indicating that electrodeposition is not as efficient using TBAPF 6 as the electrolyte. When LiBTI is used as the electrolyte however, as shown in Figure 5 1 C polymerization is almost entirely suppressed. Next we investigate the electropolymerization of BTsBP, shown in Figure 5 2 A Numerous re dox peaks are observed in the first scan. It is postulated that the first cyclic voltammetric cycle, which displays two oxidation peaks, reflects oxidation of both a five ring and a three ring species in solution. The latter arises because of the high de gree of torsional strain introduced by the presence of the methyl groups on the thiophenes, which may twist the peripheral thiophenes out of coplanarity with the rest of the system. Comparing the peak oxidation potential of a three ring thiophene ProDOT t hiophene moiety (for example, in Figure 5 1) with the peak of the highest oxidation potential in
177 Figure 5 2 A shows that the two appear at roughly equal potentials, lending support to the idea that this process is due to the BTsBP system behaving as a three ring system. Figure 5 1. Cyclic voltammetric scan of 1 st through 10 th cycles in a solution of 5 mM (based on the number of aromatic rings) 3MTsBP in a 1:1 solution of ACN:DCM using a) TBABF 4 b) TBAPF 6 and c) LiBTI as electrolyte. Measurements were made using a platinum button as working electrode and a Ag/Ag + reference electrode. Scans were taken at 50 mV/s. The two reduction peaks at ca. 0.3 V and 0.2 V were found to be r elated to one another, as investigated in Figure 5 2 B First, the voltage window was truncated to a maximum of 0.4 V. Scanning in this region resulted in negligible electropolymerization evidenced by the lack of current density increase upon successive cycling such that we can safely assume that the reduction peaks are not arising from polymer reduction (note the near absence of electropolymerization does not contradict the assignment of the oxidation peak at 0.4 V to the five ring species; as discuss ed in Chapter 2, the rate
178 limiting step in electropolymerization is often proton elimination, which is suppressed with increasing chain lengths and requires a sufficient overpotential to drive the reaction. We postulate that limiting the voltage window to 0.4 V does not provide sufficient driving force for electropolymerization to occur). At high scan rates (50 mV/s or greater) the peak at 0.3 V is dominant while the peak at 0.2 V is diminished. As the scan rate is lowered however to 10 mV/s, the peaks a re reversed in prominence, with the peak at 0.3 V nearly entirely suppressed. One possibility is that the peak at 0.3 V reflects reduction from a kinetic species, while the peak at 0.2 V reflects reduction from a thermodynamic species. In particular, the peak at 0.2 V may arise from the reduction of a more planar system. Figure 5 2. Cyclic voltammetric study of BTsBP. a) F irst CV scan in a solution of 3:2 DCM:ACN and 0.1 M TBAPF 6 ; inset shows the c yclic voltammetric scan of 1 st through 10 th cycle. b) the same monomer solution as from part a) was cycled from 0.4 V to 0.4 V at varying scan rates. Measurements were made using a platinum button as working electrode and a Ag/Ag + reference electrode. T hus, at higher scan rates the oxidized compound has not had sufficient time to fully resolve the torsional strain of the methylthiophene moieties, leading to reduction from a less planar state. At low scan rates however the system is able to more fully pl anarize. Note, that as electrodeposition proceeds, the reduction waves at lower
179 potentials are no longer present and the peak at 0.3 V becomes dominant. One possible explanation is that as electropolymerization gives rise to a rigidified network and degr ees of motional freedom become suppressed, the system is no longer able to relax to lower energy conformations and as a result reduction becomes dominated by a conformational geometry, and likewise becomes characterized by a single reduction peak. The elec tropolymerizations of the ProDOT spirobiProDOT oligomers are shown in Figure 5 3. In these systems TBAPF 6 led to the highest current densities after 10 cyclic voltammetric cycles. The lower current densities observed in the electropolymerization of ProEH sBP in Figure 5 3 B (relative to the current densities in the electropolymerization of ProMsBP in Figure 5 3 A ; note the different y axes scales) are believed to be due to the soluble nature of the system. Specifically, during electropolymerization a purple stream can be observed diffusing away from the electrode surface, indicating that some of the electropolymerized species fails to electrodeposit, leading to less film on the electrode surface after 10 scans and subsequently lower current densities. The mechanism of electropolymerization could proceed in one of two fashions, with Figure 5 4 showing a cartoon of the two possibilities. In Figure 5 4 A if all four ends of the spiro compound were able for both electronic and steric reasons to continue to couple with other oligomers, then we could envision the formation of a highly crosslinked network, leading to the presence of porous voids throughout the scaffold. In Figure 5 4 B if coupling to the second half became sterically encumbered after coupling to the other half, or if coupling through the growing chain end grew electronically more
180 favorable with increasing chain length, then electropolymerization would be expected to occur in a one dimensional fashion through only one half of the oligomer. Figure 5 3. Cyclic voltammetric scan of tetraarylsBP compounds; 1 st through 10 th cycle of a) 5 mM ProMsBP in 3:1 DCM:ACN using TBAPF 6 as electrolyte; and b) 5 mM ProEHsBP in 1:1 DCM:ACN using TBAPF 6 as ele ctrolyte. Measurements were made using a platinum button as working electrode and a Ag/Ag + reference electrode. All scan were taken at a scan rate of 50 mV/s. Two points suggest that the first mechanism dominates the growth mechanism of the spiro oligome rs. First, it has been shown that the rate of electrochemical coupling between oxidized oligomers can decrease by orders of magnitude as the length of the oligomer increases. 129 This is attributed to the improved ability of longer chains to stabilize the dication which forms af ter coupling, leading to a decreased rate of proton elimination. This would discourage the mechanism shown in Figure 5 4 B as the rate of coupling should be lowest through the longer propagating chain length and highest through the shorter unreacted half o f the compound. Second, in the case of compound 5 14 (ProEHsBP), if electropolymerization occurred in accordance with the mechanism in Figure 5 4 B we would expect the ethylhexyloxy side chains to be capable of solubilizing the resulting polymer. However, the electropolymerized film is completely insoluble, even in boiling chlorobenzene. This suggests that a crosslinked matrix as
181 shown in Figure 5 4 A is present which renders the film insoluble. Further evidence favoring the mechanism in Figure 5 4 A is late r provided in the spectroelectrochemistry of the systems. Figure 5 4. Cartoon illustrating the two mechanisms through which electropolymerization could proceed; a) reaction occurs readily through both halves of the spiro compound; b) reaction occurs predominantly in a one dimensional fashion along one growing chain length. 5.3.2 Electrochemical Properties of Electropolymerized SpirobiProDOT Films Typically, once a film has been electropolymerized, the first few cyclic voltammetry scans of the film in a monomer free solution are noticeably different from subsequent intra and intermolecular reorganizations n ecessary for the film to acquire an equilibrium geometry. For all electropolymerized films here, however, the break in process is rapid and involves minimal change in the cyclic voltammetry wave. An exemplary system is shown in Figure 5 5 for electropoly merized p3MTsBP 5 11 where after one or two electropolymerization b) unreacted monomer electropolymerization a ) unreacted monomer
182 cycles the electrochemical behavior of the film remains constant. This suggests the film is comprised of a rigid network that does not allow for a great deal of reorganization. Similar break in processes were observed for all spirobiProDOT containing systems. Figure 5 5. A freshly electropolymerized film of p3MTsBP was placed in an ACN solution of 0.1 M TBABF 4 and broken in by scanning via cyclic voltammetry for 10 scans. All scan we re taken at a scan rate of 50 mV/s. Further support that the spiro oligomers electropolymerize to form a rigid, porous film can be acquired by assessing the separation between the oxidation and the reduction peaks or the hysteresis voltammetry. Hysteresis depends in part on the rates at which ions can diffuse through the film to stabilize charge formation, which in turn can reflect how porous the material is. Figure 5 6 shows the cyclic voltammetry of pProMsBP 5 13 electropolymeriz ed from a variety of electrolyte solutions. In Figure 5 1 we noted that the electropolymerization behavior depended on the solvent electrolyte system. Here, we note that this extends to the electrochemical properties of the film as well.
183 Figure 5 6. 1st through 50 th scan (with every 10 th scan shown) of pProMsBP in a) 0.1 M TBAPF 6 in ACN; b) 0.1 M TBABF 4 in ACN; and c) 0.1 M LiBTI in ACN. All scan were taken at a scan rate of 50 mV/s. In Figure 5 6 A we observe that films of 5 13 electropolymerized from TBAPF 6 show the narrowest hysteresis and peaks compared with films electropolymerized from other electrolytes. The ability of ions to diffuse through films often depends als o on the film thickness, with thicker films giving rise to larger barriers to diffusion and subsequently a larger hysteresis. In Figure 5 7 A however, the film electropolymerized from TBAPF 6 has a larger curr ent density (and by extension greater surface are a) compared with films electropolymerized from LiBTI ( Figure 5 7C ), and comparable current densities to films electropolymerized from TBABF 4 ( Figure 5 7 B ) however it
184 possesses the largest hysteresis in spite of the greater surface area. The results of Figur e 5 6 suggest that film porosity may be tunable thr ough electrolyte choice. Figure 5 7. Cyclic voltammetric scan of tetraarylsBP polymers. a) 1 st through 100 th scan (with ever y 10 th scan shown) of p3MTsBP in TBABF 4 in ACN; b) 1 st through 100 th scan (with every 10 th scan shown) of pBTsBP in TBAPF 6 in ACN c) 10 th through 50 th scan (with every 10 th scan shown) of pProEHsBP in TBAPF 6 in ACN. All scan were taken at a scan rate of 5 0 mV/s. 5.3.3 Spectroelectrochemical Properties of SpirobiProDOT Compounds The spectra of the monomers are instructive starting points for understanding how these compounds behave and organize themselves intra and intermolecularly. Looking at Figure 5 8, the solid state absorbance spectra of the four compounds can be compared. In the spectra of 3MTsBP 5 11 and BTsBP 5 12 ( Figure 5 8 A and 5 8 B respectively) the absorbance is broad and featureless, suggesting little intramolecular
185 order in these systems. T his is in line with our expectations of the molecules, as the methyl group on the thiophene rings is expected to provide sufficient steric bulk to force the rings out of coplanarity and promote free rotation. In contrast, the spectra of ProMsBP 5 13 and P roEHsBP 5 14 ( Figure 5 8 C and 5 8 D respectively) show distinct vibronic coupling and improved intramolecular ordering of the molecule. Here, the alkyl groups project normal to the plane of the molecule rather than in a way that would force a large ring twi st. Additionally, sulfur oxygen interactions may also lock the molecule into a rigid conformation and improve intramolecular ordering. Intermolecular order in all four systems seems entirely suppressed, due to the lack of any red shifted shoulders that w e would expect arising from aggregate absorbance. The spectroelectrochemistry of the electropolymerized films show several common features. We start by observing how the spectra of the freshly prepared film differ from that of a film that has first 9 shows spectra of electropolymerized pProMsBP 5 13 where there is little change in the cyclic voltammogram before and after the film has been cycled. In particular, there is no red shifting observed in the max and minimal red shifting of the overall profile. (It should be noted that the array of peaks in the absorbance spectra around 400 nm prior to breaking in the film is due to residual monomer; this peak becomes depleted if rinsed with methylene chlor ide however, so is not attributed to unreacted monomer halves.) The same trends are observed in all systems, suggesting little reorganization occurs during this period, which corroborates similar results seen in the electrochemistry of the films earlier i n Figure 5 5.
186 Figure 5 8. Absorbance spectra of films of tetraarylsBP compounds. a) 3MTsBP; b) BTsBP; c) ProMsBP; and d) ProEHsBP were drop cast f rom methylene chloride onto ITO. Turning now to the spectra of all the films in Figure 5 10, we observe that the fully neutral spectra are all broad and featureless. As we saw with the monomer absorbance spectra, the absence of any red shifted shoulders a gain suggests the absence of intermolecular interactions. Additionally, an absorbance arising from any unreacted monomer halves is largely absent, which relates back to the discussion of the dominant electropolymerzation mechanism for these systems. As s hown earlier in Figure 5 4 B the one dimensional model for electropolymerization would lead to the persistence of a large population of unreacted monomer, whereas a mechanism where growth extends from all points would have a reduced population of unreacted monomer. The absence
187 in the absorbance spectra of most signals arising from monomer absorbance lends further support for the latter model. Figure 5 9. Absorbance spectra of a film of pProMsBP before (black curve) and after (r ed curve) the film has been subjected to breaking in via 10 cyclic voltammetry scans. One final point regarding the neutral spectra, the breadth of the absorbance in the visible region likely suggests that polymers or materials of varying chain lengths con tribute to the spectra. We expect a broad distribution of chain lengths, as each coupling process unless proceeding in a ladder like fashion necessarily results in one half of the monomer extending its chain length while the second half is left unreac ted. In Figure 5 10, as the films are oxidized, the absence of a distinct isobestic point is also indicative of a distribution of chain lengths that are all switching at once.
188 Figure 5 10. Spectroelectrochemistry of electropolymerized tetraarylsBP films. a) p3MTsBP from a) 400 mV to t) 700 mV; b) BTsBP from a) 400 mV to t) 700 mV; c) ProMsBP from a) 400 mV to u) 800 mV; and d) ProEHsBP from a) 300 mV to 700 mV on ITO. Film a) was switched in a solution of 0.1 M TBABF 4 in ACN, while films b), c), and d) were switched in a solution of 0.1 M TBAPF 6 in ACN. 5.3.4 Chronoabsorptometry Measurements of SpirobiProDOT F ilms As mentioned at the outset o f this chapter, one of the guiding principles in the design of these spiro containing compounds was to observe whether they provided a strategy towards rapidly switching materials. So far we have shown evidence that the films are organized into a rigid an d porous network. This in turn would lead us to believe that ions could rapidly diffuse through the films during redox switching, leading directly to fast switching speeds. In Figure 5 11, chronoabsorptometry reveals that two
189 of the films switch rapidly between their fully neutral and fully oxidized states. As shown in Figure 5 11 A films of p3MTsBP display little deterioration in percent transmittance max as switching times are decreased from 5 seconds to 0.5 seconds. In other words, films of the p3MTsBP are able to achieve complete doping and dedoping throughout near ly the entire volume of the film at switching speeds of 5, 2 and 1 second. At 500 milliseconds, the contrast ratio deteriorates, likely suggesting that these switching speeds approach the limit at which ions can fully diffuse through the film to participat e in doping and dedoping processes. On the other hand, films of pProMsBP in Figure 5 11 B outperformed all others with hardly any change in contrast being observed at all switching speeds. The decrease in %T even at switching speeds of 500 milliseconds is negligible, suggesting that even at such rapid speeds the kinetic limits of ion diffusion throughout the entire volume of the material have yet to be realized. Films of pProEHsBP were generally of poor quality and the results of chronoabsorptometry studi es on this film are not shown; this was likely owing to the solubility of the material as mentioned previously, which may have prevented the formation of an even film. As a result, portions of the film were observed to switch rapidly, however other region s switched far more slowly. The regions that do switch rapidly speak to the promise this compound holds, however the difficulty in forming a uniform film may ultimately preclude its usefulness. Finally, in Figure 5 11 C films of pBTsBP display the larges t decreases in %T with shorter switching speeds. In the initial design of this molecule, we had expected the pore sizes generated by electropolymerized pBTsBP to be largest and thus most permissive towards rapid ion diffusion through the matrix. Instead, the opposite appears to be the case. One possibility is that the increase in pore volume
190 leads to a larger tendency of the electropolymerized material to become entangled. cha nnels for rapid diffusion. Figure 5 11. Chronoabsorptometry measurements of tetraarylsBP polymer films. a) p3MTsBP in 0.1 M TBABF 4 in ACN; b) pProMsBP in 0.1 M TBAPF 6 in ACN; and c) pBTsBP in 0.1 M TBAPF 6 in ACN. a) Transmissivity was measured at 453 nm; the absorbance of the neutral film at 453 nm was 0.35; b) transmissivity was measured at 513 nm; the absorbance of the neutral film at 513 nm was 0.68. c) transmissivity was m easured at 414 nm; the absorbance of the neutral film at 414 nm was 0.73. From the work in this chapter, we conclude that introduction of the spirobiProDOT moiety into conjugated systems serves as a novel method for engineering porosity and rigidity into e lectropolymerized films. While the colors of the fully neutral and fully
191 oxidized states are not particularly interesting, these experiments serve as a proof of concept to a new role which spirobiProDOT can play in optoelectronic applications, specificall y the ability to induce a rigid and porous matrix for rapid ion diffusion. In theory, because ProDOTs already comprise so many of the polymers used to complete the color palette of electrochromic polymers, the incorporation of spirobiProDOT into conjugate d systems may be a strategy to render fast switching times into any color material. 5.4 Synthetic Details Synthesis of precursors to spirobiProDOT containing compounds spirobiProDOT (5 1): To a round bottom flask with stirbar and condenser under an argon a tmosphere was added pentaerythritol (11 g, 81 mmol 1 eq.), 3,4 dimethoxythiophene (24.5 g, 170 mmol, 2.1 eq.), and toluene (500 mL). The reaction was heated to 90C and p TSA (2.25 g, 12 mmol, 0.15 eq.) was added in one portion. The reaction was stirred for 3 days. Afterwards the reaction was poured directly through a silica gel plug. The organic layer was further washed with brine (2 100 mL), dried with MgSO 4 filtered and solvent removed under rotary evaporation. The crude was washed with hot hexan es (until the yellow color was mostly gone) and then with diethyl ether to afford the product as a slightly yellow powder (7.20 g, 24 mmol, 30%). tetrabromospirobiProDOT (5 2): A vial filled with spirobiProDOT (5 1) ( 4.3g, 14.50 mmol, 1 eq.) and second vial filled with NBS (12.9 g, 72.54 mmol, 5 eq.) were evacuated and backfilled with argon three times. Next, to a round bottom flask with stirbar was added chloroform (500 mL). The solution was bubbled through with arg on for 1 hour. Unlike other brominations performed in this dissertation, degassing a
192 solution including the material to be brominated (in this case spirobiProDOT) is unsuccessful due to the low solubility of the starting material, which leads to the reage nt precipitating out of solution and clogging the bubbling needle. After 1 hour, the reaction was covered with foil to exclude light and 4 1 and NBS were added in quick succession and the reaction stirred until complete conversion was observed via TLC (ty pically 24 48 hours). Afterwards, the reaction was poured through a silica gel plug and eluted with 2:1 DCM:hexanes. The solvent was removed under rotary evaporation to afford the product as an off white solid. On occasion, a slight yellow color may per sist which can be removed by repeated washing with acetone to afford the pure as a white powder (6.7 g product, 10.94 mmol, 76% yield). 1 2 bromo 3 methylthiophene (5 3): To a round bottom flask with stirbar was added DMF (200 mL). T he solvent was degassed by bubbling through with argon for 1 hour. Afterwards 3 methylthiophene (8.91g, 90.76 mmol, 1 eq.) was added in one portion, followed by NBS (16.15g, 90.76 mmol) in one portion. The reaction was covered with foil to exclude light and the reaction was monitored via TLC for the consumption of starting material (approximately three hours). Afterwards the reaction was poured into 600 mL of water and the solution added to a separatory funnel. The aqueous layer was extracted with dieth yl ether (3 100 mL), washed with brine, dried with MgSO 4 filtered and the solvent removed under rotary evaporation to afford the crude as a light brown oil. The oil was passed through a silica gel plug using hexanes as eluent to afford the desired prod uct as a colorless oil (12.5 g, 70.59 mmol, 70%). 1 6.76 (d, 1H), 2.18 (s, 3H).
193 3 methylthiophene 2 boronic acid pinacol ester (5 4): To a round bottom flask with stirbar was added 5 3 (2.28 g, 12.92 mmol, 1eq.) The vessel was evacuated and backfilled with argon three times, and dry THF (50 mL) was added via cannula. The solution was cooled to 78C and n BuLi (13.57 mmol, 1.05 eq.) was added dropwise. The reaction was warmed to 50C over the course of an hour. Afterwards the reaction was cooled back down to 78C and 4,4,5,5 tetr amethyl 1,3,2 dioxaborolane was added in one portion. The reaction was gradually allowed to warm to room temperature and run overnight. Afterwards, the solvent was removed via rotary evaporation and the crude passed through a silica gel column using 45:1 hexanes:ethyl acetate as eluent to afford the pure product as a colorless oil which solidifies into a white crystalline solid (1.42 g, 6.33 mmol, 50% yield). 1 (s, 12H). 13 31.48, 83.75, 25.02, 16.24. dimethyl bithiophene (5 5): To a 3 necked flame dried round bottom flask with stirbar and a condenser was added magnesium turnings (0.68 g, 28.32 mmol, 1.2 eq.). The reaction was evacuated and backfilled with argon th ree times, followed by addition of a single iodine crystal, and then dry diethyl ether (50 mL) via cannula. The reaction was stirred vigorously for at least 30 minutes until the initial light brown solution turned colorless, indicating the consumption of the iodine and the activation of the magnesium turnings. The flask was cooled to 0C and 2 bromo 3 methylthiophene (4.4 g, 24.85 mmol, 1.05 eq.) dissolved in 50 mL of diethyl ether was added slowly via cannula. The reaction was warmed to a gentle reflux and stirred for 3 hours to form the Grignard. Afterwards, the reaction was cooled to room temperature. In a second 3 necked flame dried round bottom flask with stirbar and a condenser was added 2
194 bromo 3 methylthiophene (4.18 g, 23.60 mmol, 1 eq.) and Ni (dppp)Cl 2 (0.13 g, 0.24 mmol, 1%). Dry diethyl ether (50 mmol) was added via cannula and the flask cooled to 0C. The Grignard solution from the first flask was added to the second flask via cannula. The reaction was then warmed to a gentle reflux and stirred overnight. Afterwards, the reaction cooled to room temperature and poured into a dilute solution of ammonium chloride to quench the reaction. The solution was poured into a separatory funnel, extracted with diethyl ether (3 100 mL), washed with brine, dried with MgSO 4 filtered, and the solvent removed under rotary evaporation to afford the crude. The crude was passed through a silica gel column using hexanes as eluent to afford the desired product as a colorless oil (3.95 g, 20.32 mmol, 82%). 1 6.92 (d, 2H), 2.17 (s, 6H). 13 dimethyl bithiophene 5 boronic acid pinacol ester (5 6): To a round bottom flask with stirbar was added 5 5 (2.21 g, 11.37 mmol, 1eq.) The ve ssel was evacuated and backfilled with argon three times, and dry THF (50 mL) was added via cannula. The solution was cooled to 78C and n BuLi (11.94 mmol, 1.05 eq.) was added dropwise. The reaction was warmed to 0C over the course of an hour. Afterw ards, the reaction was cooled back down to 78C and 4,4,5,5 tetramethyl 1,3,2 dioxaborolane (2.11 g, 11.94 mmol, 1.05 eq.) was added in one portion. The reaction was gradually allowed to warm to room temperature and run overnight. Afterwards, the solven t was removed via rotary evaporation and the crude passed through a silica gel column using 45:1 hexanes:ethyl acetate as eluent to afford the pure product as an off white solid (1.82 g, 5.68 mmol, 50% yield). 1 (d 1H), 2.19 (s, 6H), 1.34 (s, 12H).
195 ProDOT Me 2 (5 7): To a round bottom flask with stirbar and condenser under an argon atmosphere was added 2,2 dimethyl 1,3 propanediol (6.24 g, 60 mmol 1.2 eq.), 3,4 dimethoxythiophene (7.20 g, 50 mmol, 1 eq.), and toluen e (200 mL). The reaction was heated to 90C and p TSA (0.95 g, 5 mmol, 0.1 eq.) was added in one portion. The reaction was stirred for 3 days. Afterwards the reaction was poured directly through a silica gel plug. The organic layer was further washed w ith brine, dried with MgSO 4 filtered and solvent removed under rotary evaporation. The crude was passed through a silica gel column using 1:1 hexanes:methylene chloride to afford the product as a colorless oil (5.68 g, 31 mmol, 62% yield). 1 6H). ProDOT Br 2 (5 8a): To a round bottom flask with stirbar and condenser under an argon atmosphere was added 2,2 bis(bromomethyl) 1,3 propanediol (31.43 g, 120 mmol 1.2 eq.), 3,4 dimethoxythiophene (14.41 g, 100 mmol, 1 eq.), and toluene (500 mL). The reaction was heated to 90C and p TSA (1.9 g, 10 mmol, 0.1 eq.) was added in one portion. The reaction was stirred for 3 days. Afterwards the reaction was poured directly through a silica gel plug. The organi c layer was further washed with brine 2, dried with MgSO 4 filtered and solvent removed under rotary evaporation. The crude was passed through a silica gel column using 1:1 hexanes:diethyl ether to afford the product as a white powder (23 g, 67 mmol, 70 % yield). 1 4H), 3.60 (s, 4H). ProDOT CH 2 OEtHx 2 (5 8b): To a 3 necked flame dried round bottom flask with stirbar and condenser was added DMF (200 mL) and sodium hydride (8.3 g, 200 mmol, 6 eq. 60% w/w). 2 ethyl 1 hexanol ( 13 g, 15.6 mL, 100 mmol, 3 eq.) was added and the
196 reaction was warmed to 100C and stirred overnight. In a second 3 necked flame dried round bottom flask with stirbar and condenser was added 5 8a (11.3 g, 33 mmol, 1 eq.) and DMF (500 mL). The first flask containing the alkoxide was cooled to room temperature and added via cannula to the second flask. The reaction was warmed to 90C and stirred overnight. Afterwards, the reaction was poured into 1 L of water. The solution was poured into a separatory fu nnel, extracted with diethyl ether (3 100 mL), washed with brine (2 200 mL), dried with MgSO 4 filtered, and the solvent removed under rotary evaporation to afford the crude. The crude was passed through a silica gel column using 1:1 hexanes:methylene chloride as eluent to afford the desired product as a colorless oil (13 g, 27 mmol, 85% yield). 1 4H), 3.25 (d, 4H), 1.46 (m, 2H), 1.34 1.24 (m, 16H), 0.89 0.82 (m, 12H). 2 (trimethylstannyl) ProDOT Me 2 (5 9): To a round bottom flask with stirbar and under an argon atmosphere was added ProDOT Me 2 (5 7) (4.11 g, 22.30 mmol, 1 eq.) and dry THF (50 mL). The reaction was cooled to 78C and n BuLi (24.53 mmol, 1.05 eq.) was added dropwise. After complete addition, the reaction was allowed to gradually warm to 0C and stirred at that temperature for 1 hour. Afterwards, the reaction was cooled back to 78C and trimethyltin chloride (4.88 g, 24.53 mmol, 1.05 eq.) was added in one portion. The reaction was allowed to gradually warm to room temperature and stirred overnight. Afterwards, the reaction was pour ed into water, added to a separatory funnel, and extracted with diethyl ether (3 100 mL). The organic layers were combined and washed with brine, dried with MgSO 4 filtered, and the solvent remove via rotary evaporation. The crude was dissolved in diet hyl ether and passed through a basified silica gel plug to remove salts. The solvent was removed via
197 rotary evaporation to afford the crude as a yellow oil (7.67 g, quantitative yield). 1 H 2 (trimethylstannyl) ProDOT (CH 2 OEtHx) 2 (5 10): To a round bottom flask with stirbar and under an argon atmosphere was added ProDOT (CH 2 OEtHx) 2 (5 8) (2.62 g, 5.94 mmol, 1 eq.) and dry THF (5 0 mL). The reaction was cooled to 78C and n BuLi (6.54 mmol, 1.1 eq.) was added dropwise. After complete addition, the reaction was allowed to gradually warm to 0C and stirred at that temperature for 1 hour. Afterwards, the reaction was cooled back t o 78C and trimethyltin chloride (1.30 g, 6.54 mmol, 1.05 eq.) was added in one portion. The reaction was allowed to gradually warm to room temperature and stirred overnight. Afterwards, the reaction was poured into water, added to a separatory funnel, and extracted with diethyl ether (3 100 mL). The organic layers were combined and washed with brine, dried with MgSO 4 filtered, and the solvent remove via rotary evaporation. The crude was dissolved in diethyl ether and passed through a basified silic a gel plug to remove salts. The solvent was removed via rotary evaporation to afford the crude (4.12 g, quantitative yield). 1 1H), 3.96 (s, 2H), 3.90 (s, 2H), 3.44 (s, 4H), 3.25 (d, 4H), 1.45 (m, 2H), 1.33 1.23 (m, 16H), 0.89 0.81 (m, 12H), 0.29 (s, 9H). Synthesis of spirobiProDOT containing oligomers 3MTsBP (5 11): To a microwave tube with stirbar was added tetrabromospirobiProDOT (5 2) (0.5 g, 0.81 mmol, 1 eq.), 5 4 (0.91 g, 4.08 mmol, 5 eq.), and three drops of Aliquot 336. In separate vials were placed Pd 2 dba 3 (0.014 g, 0.01 mmol, 2%), ( o tolyl) 3 P (0.029 g, 0.09 mmol, 12%), and K 3 PO 4 (3.18 g, 15 mmol, 3 M). 2 Schlenk tubes of triply freeze pump thawed toluene (15 mL) and distilled water (5
198 mL) were also prepared. All materials were brought into a glove box and assembled by first placing all solids into the microwave tube, followed by toluene and water. The microwave tube was sealed with an appropriate cap, removed from the glove box and rapidly transferred to the microwave reactor. The reaction was stirred at 110C for 45 minutes. Afterwards, the reaction was poured into 30 mL of water and extracted w ith methylene chloride (3 30 mL), washed with brine (2 50 mL), dried with MgSO 4 filtered, and the solvent removed under rotary evaporation. The crude was passed through a basified silica gel column using 3:1 hexanes:diethyl ether as eluent to afford the product as a bright yellow solid (68 mgs, 0.1 mmol, 13% yield). 1 4H), 6.85 (d, 4H), 4.20 (s, 8H), 2.33 (s, 12H). 13 127.57, 124.94, 114.57, 71.33, 51.05, 15.88. Anal. calcd for C 33 H 28 O 4 S 6 : C, 58.20; H, 4.14. Found: C, 58.19; H, 3.98. HRMS m/z: calcd, 681.03; found, BTsBP (5 12): To a microwave tube with stirbar was added tetrabromospirobiProDOT (5 2) (0.3 g, 0.5 mmol, 1 eq.), 5 6 (0.8 g, 2.49 mmol, 5 eq.), and three drops of Aliquot 336. In separa te vials were placed Pd 2 dba 3 (0.009 g, 0.009 mmol, 2%), ( o tolyl) 3 P (0.018 g, 0.06 mmol, 12%), and K 3 PO 4 (3.18 g, 15 mmol, 3 M). 2 Schlenk tubes of triply freeze pump thawed toluene (15 mL) and distilled water (5 mL) were also prepared. All materials wer e brought into a glove box and assembled by first placing all solids into the microwave tube, followed by toluene and water. The microwave tube was sealed with an appropriate cap, removed from the glove box and rapidly transferred to the microwave reactor The reaction was stirred at 110C for 45 minutes. Afterwards, the reaction was poured into 30 mL of water and extracted with methylene chloride (3 30 mL), washed with brine (2 50 mL), dried with MgSO 4
199 filtered, and the solvent removed under rotary evaporation. The crude was passed through a basified silica gel column using 3:1 hexanes:diethyl ether as eluent to afford 4H), 7.05 (s, 4H), 6.90 (d, 4H), 4.26 (s, 8H), 2 144.39, 136.85, 136.71, 133.53, 130.31, 129.28, 129.10, 126.36, 125.36, 113.99, 71.46, 51.10, 30.53, 15.06. Anal. calcd for C 53 H 44 O 4 S 10 : C, 59.74; H, 4.16. Found: HRMS m/z: calcd, 1065.0520; found, 1065.0516. P roMsBP (5 13): To a Schlenk tube with stirbar was added Pd(PPh 3 )Cl 2 (0.04 g, 0.06 mmol, 5%), CuI (0.023 g, 0.12 mmol, 10%), and tetrabromospirobiProDOT (5 2) (0.75 g, 1.22 mmol, 1 eq.). The vessel was evacuated and flushed with argon three times. Afterwa rds, degassed DMF (20 mL) was added, followed by 5 9 (2.55 g, 7.35 mmol, 6 eq.) and the reaction run at 80C for 3 days. Afterwards, the reaction was cooled and poured into water (60 mL), resulting in the formation of precipitate. The solid was filtered through a Buchner funnel and washed with water. The solid was then dissolved in methylene chloride and dispersed onto silica gel. Solvent was removed under rotary evaporation, and the dispersion was added to the top of a basified silica gel column. The product was eluted using 2:1 methylene chloride:hexanes as eluent to afford the product as a light yellow solid (180 mgs, 0.17 mmol, 14% yield). 1H NMR: 6.43 (s, 4H), 4.22 (s, 8H), 3.82 (s, 8H), 3.72 (s, 8H), 1.06 (s, 24H). Anal. calcd for C 49 H 52 O 12 S 6 : C 57.40; H, 5.11. Found: C, 57.05; H, 5.09 HRMS m/z: calcd, 1025.1856; found, 1025.1865. ProEHsBP (5 14): To a Schlenk tube with stirbar was added Pd(PPh 3 )Cl 2 (0.028 g, 0.04 mmol, 5%), CuI (0.015 g, 0.08 mmol, 10%), and tetrabromospirobiProDOT (5 2)
200 (0.5 g, 0.81 mmol, 1 eq.). The vessel was evacuated and flushed with argon three times. Afterwards, degassed DMF (20 mL) was added, followed by 5 10 (2.95 g, 4.90 mmol, 6 eq.) and the reaction run at 80C for 3 days. Afterwards, the reaction was cooled and p oured into water (60 mL), resulting in the formation of precipitate. The solid was filtered through a Buchner funnel and washed with water. The solid was then dissolved in methylene chloride and dispersed onto silica gel. Solvent was removed under rotar y evaporation, and the dispersion was added to the top of a basified silica gel column. The product was eluted using 2:1 hexanes:methylene chloride as eluent to afford the product as a bright orange solid (150 mgs, 0.07 mmol, 9% yield). 1 6.39 (s, 4H), 4.21 (s, 8H), 4.10 (s, 8H), 4.02 (s, 8H), 3.51 (s, 16H), 3.27 (d, 16H), 1.46 (m, 8H), 1.33 1.24 (m 64H), 0.88 0.82 (m, 48H). Anal. calcd for C 113 H 180 O 20 S 6 : C, 66.17; H, 8.85. Found: C, 66.18, H, 9.16. HRMS m/z: calcd, 2049.138 7; found, 2049.1329
201 CHAPTER 6 PERSPECTIVE ON WORK ACCOMPLISHED IN THIS DISSERTATION AND PROPOSED DI RECTIONS FOR CONTINU ATION OF THIS WORK Throughout this dissertation, we have explored synthetic strategies to tune intra and intermolecular interactions o f conjugated materials, and also discussed spectroscopic strategies to better understand the nature of the interactions. Here we briefly consider the directions in which these projects merit further investigation. First, while PheDOT has demonstrated an e nhancement in the intra and intermolecular organization when incorporated into conjugated systems, the widespread use of this molecule is hindered by several factors. Certainly, the synthetic yield of the transetherification must be increased from the lo w 20% range. Taking the entire synthetic scheme towards alkyl substituted PheDOTs for instance, 50 grams of the starting 1,2 dimethoxythiophene can amount to less than a gram of final product, which is a tremendous waste of material. Another drawback is the insolubility of polymers when PheDOT is incorporated into the conjugated chain. To gain widespread use, the processability of a polymer is practically crucial, and the use of chlorinated solvents and high heats to solubilize the polymer are impractica l. From the work conducted by Christophe Grenier in our research group, it is difficult to imagine that all PheDO T polymers can simultaneously achieve a high degree of solubility without compromising stacking interactions of the system. When the phenyl ring was substituted with a dodecyl and a 2 ethylhexyl chain, for example, room temperature solubility was ac hieved, however at the expense of a regiosymmetric polymer, which subsequently led to a decrease in the strength of the intermolecular interactions observed in this system. It seems that copolymers are therefore the most attractive system in which to
202 inco rporate PheDOTs. Pyrroles for example can be substituted with solubilizing groups not only along the 3 and 4 positions of the aromatic ring, but additionally at the nitrogen atom. Another alternative is the use of solubilizing groups which can be removed in a post processing step. In this way, intra and intermolecular interactions may be temporarily compromised during polymer casting only to recover the desired interactions when convenient. While the framework for which these interactions can be evaluat ed using a variety of spectroscopic techniques has been laid out in this dissertation, it remains to be seen whether our qualitative understanding of interactions translates to expected trends in device performance as well. The hole mobilities of the fami ly of PheDOT containing oligomers, for example, is a project worth exploring, especially considering the ready availability of the oligomers. Considering the mobilities of analogous all thiophene oligomers, values on the order of 10 4 10 3 should be ach ievable, and ultimately increasable with optimization of processing conditions. On a broader scope, the underlying philosophy of utilizing sulfur oxygen interactions to rigidify the intramolecular geometry is an intriguing idea. While rigid structures hav e been acquired through more covalent methods such as fusing rings together or introducing a bridging atom to prevent torsional free rotation these systems generally suffer from low solubilities. Non bonding interactions, on the other hand, offer a mo the polarity of the solvent could modulate the strength of the interaction and by extension the solubility of the system. Alternatively, substituent effects could also be utilized to a lter the strength of the interaction. Electron withdrawing groups could be
203 employed to reduce the electron orbital, for example. Similar ways to influence the electron donating strength of dimethoxythi Finally, while the spirobiProDOT moiety offers a route towards fast swi tching electropolymerized films currently, films do not switch between colors that have much applicability (red to transmissive light blue). There is a ne ed to find the appropriate synthons to attach to the spirobiProDOT core to produce films which switch between a colored neutral state and a transmissive as well as colorless oxidized state. Fortunately, Suzuki and Stille type couplings can be readily perf ormed to give rise to a large family of oligomers to electropolymerize. As a general theme of the work that should be carried out as an extension of what has been accomplished in this dissertation, there is a need to bridge the gap between the proof of con cept stage with the practical application end. However, it is believed that the work here illustrates promise for all systems developed in this dissertation that merit carrying these systems on to the next phase of development.
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212 BIOGRAPHICAL SKETCH Dwanleen Eric Shen has lived most of his life as a resident of Gainesville Florida. He began his laboratory work in the biochemistry lab of Arthur Edison at the McKnight Brain Institute at the University of Florida the summer of his senior year in high school where he utilized 2D N MR to elucidate the structure of proteins. In 2000 Eric left for Houston, Texas where he attended Rice University. He continued his focus on biochemistry labwork when a combination of taking an undergraduate organic chemistry course and spending an enti re semester measuring the distance hundreds of cells moved over the course of a minute under a microscope p ropelled him into the field of c hemistry. In his junior year Eric joined the research group of Jim Tour where he was involved in the synthesis of c onjugated oligomers for use in molecular electronics throughout the remainder of his undergraduate career. Thanks to the enormous enthusiasm of Professor Tour and the wonderful environment of the group, Eric quickly took a liking to c hemistry and never lo oked back. When the time came to consider graduate school, Eric was extremely fortunate to be directed by Professor Tour to look into the research group of John Reynolds at the University of Florida. There, he continued in the field of molecular electron ics, not only familiarizing himself with the synthetic nuances of the field but also acquiring electrochemical and spectroscopic skills, as well as a deeper understanding and appreciation of the devices in which these materials were applied. Eric defended his dissertation in the fall of 2011.