Synthetic Routes to Novel Conjugated Polymers Based on Fluorene and Synthetic Analogues of Fluorene

Permanent Link: http://ufdc.ufl.edu/UFE0022372/00001

Material Information

Title: Synthetic Routes to Novel Conjugated Polymers Based on Fluorene and Synthetic Analogues of Fluorene
Physical Description: 1 online resource (210 p.)
Language: english
Creator: Brookins, Robert
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008


Subjects / Keywords: aromatic, conjugated, fluorene, polyelectrolyte, polymer, suzuki
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation


Abstract: This work presents the synthesis and characterization of conjugated polymers and polyelectrolytes based on fluorene derivatives for use in light emitting, electrochromic, and energy transfer applications. Fluorene-based homo- and copolymers have been integral to the development of organic-based light-emitters, photovoltaics, and sensors, due to their facile synthesis and unique material/electronic properties; however, this progress has been limited by the narrow range of accessible polymers. The goal of this research is to develop new synthetic routes and methods towards fluorene-based polymers, as well as to assess the resulting material/electronic properties. One goal of this work is to develop a family of fluorene-based copolymers with the emissive properties of a polyfluorene homopolymer ? but with improved electrochemical and electrochromic properties. To this end, carbazole and 1-methyl pyrrole monomers functionalized with boronate esters have been polymerized with fluorene to give polymers with Mn = 10,000 and 16,000 Da, respectively. The latter polymer was synthesized via novel Suzuki conditions that give the highest degree of polymerization reported for a pyrrole-based conjugated polymer. Both polymers have HOMO-LUMO gaps within 0.2 eV of polyfluorene, giving them a similarly blue fluorescence. Electrochemically, the carbazole-fluorene and pyrrole-fluorene polymers are significantly different from a polyfluorene with HOMOs that are 0.5 eV and 0.8 eV less, respectively. A second project studies a series of conjugated polyelectrolytes that are derived from fluorenes functionalized with esters and are converted to carboxylic acids post-polymerization. Over this series, the polymers absorb different ranges of visible light. To polymerize monomers with base-sensitive moieties, a base-free Suzuki polymerization was developed and optimized by replacing the traditional base with a fluoride salt. Ester-functionalized polyfluorene was synthesized by this method, yielding polymers with molecular weights ranging 4,000 to 22,000 Da. Subsequent hydrolysis of this polymer afforded the first carboxylic acid functionalized polyfluorene. Applying this method to other monomers proved problematic, ultimately requiring a biphasic system, and under these conditions, phenylene, bithiophene, and benzothiadiazole comonomers were polymerized with Xn of ~25-50. The spectral properties of the polymers? carboxylic acid derivatives were studied in solution as their carboxylic acid and carboxylate forms. The carboxylates exhibited behavior expected of a conjugated polyelectrolyte (CPE); however, the carboxylic acid derivatives planarize the polymer backbone in more polar solutions. For low bandgap carboxylic acid functionalized CPEs, a new post-polymerization method was required due to the polymers' decomposition under base hydrolysis. To circumvent this problem, fluorenes functionalized with thermally cleavable esters were designed and synthesized to afford low bandgap CPEs. By heating the resulting polymers to ~185 oC, the esters are converted to carboxylic acids without altering the polymer backbone. A third project studied poly(benzodithiophenes) as synthetic analogues of polyfluorenes. While these monomers share a similar fused, tricyclic aromatic structure as fluorene, thienyl moieties raise the HOMO energy to a more accessible value and the solubilizing chains are oriented in the plane of the monomer, facilitating interchain interactions in the solid state and in solution. By changing the molecular weight and the steric hinderance of the solubilizing groups, the electrochemical properties are dramatically effected, with the onset of oxidation varying over a 500 mV range.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Robert Brookins.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Reynolds, John R.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022372:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022372/00001

Material Information

Title: Synthetic Routes to Novel Conjugated Polymers Based on Fluorene and Synthetic Analogues of Fluorene
Physical Description: 1 online resource (210 p.)
Language: english
Creator: Brookins, Robert
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008


Subjects / Keywords: aromatic, conjugated, fluorene, polyelectrolyte, polymer, suzuki
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation


Abstract: This work presents the synthesis and characterization of conjugated polymers and polyelectrolytes based on fluorene derivatives for use in light emitting, electrochromic, and energy transfer applications. Fluorene-based homo- and copolymers have been integral to the development of organic-based light-emitters, photovoltaics, and sensors, due to their facile synthesis and unique material/electronic properties; however, this progress has been limited by the narrow range of accessible polymers. The goal of this research is to develop new synthetic routes and methods towards fluorene-based polymers, as well as to assess the resulting material/electronic properties. One goal of this work is to develop a family of fluorene-based copolymers with the emissive properties of a polyfluorene homopolymer ? but with improved electrochemical and electrochromic properties. To this end, carbazole and 1-methyl pyrrole monomers functionalized with boronate esters have been polymerized with fluorene to give polymers with Mn = 10,000 and 16,000 Da, respectively. The latter polymer was synthesized via novel Suzuki conditions that give the highest degree of polymerization reported for a pyrrole-based conjugated polymer. Both polymers have HOMO-LUMO gaps within 0.2 eV of polyfluorene, giving them a similarly blue fluorescence. Electrochemically, the carbazole-fluorene and pyrrole-fluorene polymers are significantly different from a polyfluorene with HOMOs that are 0.5 eV and 0.8 eV less, respectively. A second project studies a series of conjugated polyelectrolytes that are derived from fluorenes functionalized with esters and are converted to carboxylic acids post-polymerization. Over this series, the polymers absorb different ranges of visible light. To polymerize monomers with base-sensitive moieties, a base-free Suzuki polymerization was developed and optimized by replacing the traditional base with a fluoride salt. Ester-functionalized polyfluorene was synthesized by this method, yielding polymers with molecular weights ranging 4,000 to 22,000 Da. Subsequent hydrolysis of this polymer afforded the first carboxylic acid functionalized polyfluorene. Applying this method to other monomers proved problematic, ultimately requiring a biphasic system, and under these conditions, phenylene, bithiophene, and benzothiadiazole comonomers were polymerized with Xn of ~25-50. The spectral properties of the polymers? carboxylic acid derivatives were studied in solution as their carboxylic acid and carboxylate forms. The carboxylates exhibited behavior expected of a conjugated polyelectrolyte (CPE); however, the carboxylic acid derivatives planarize the polymer backbone in more polar solutions. For low bandgap carboxylic acid functionalized CPEs, a new post-polymerization method was required due to the polymers' decomposition under base hydrolysis. To circumvent this problem, fluorenes functionalized with thermally cleavable esters were designed and synthesized to afford low bandgap CPEs. By heating the resulting polymers to ~185 oC, the esters are converted to carboxylic acids without altering the polymer backbone. A third project studied poly(benzodithiophenes) as synthetic analogues of polyfluorenes. While these monomers share a similar fused, tricyclic aromatic structure as fluorene, thienyl moieties raise the HOMO energy to a more accessible value and the solubilizing chains are oriented in the plane of the monomer, facilitating interchain interactions in the solid state and in solution. By changing the molecular weight and the steric hinderance of the solubilizing groups, the electrochemical properties are dramatically effected, with the onset of oxidation varying over a 500 mV range.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Robert Brookins.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Reynolds, John R.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022372:00001

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2008 Robert N. Brookins 2


To Paige and Katie Your love and patience sustain me 3


ACKNOWLEDGMENTS I would like to thank my advisor Dr. John Reynolds who has been an extraordinary mentor over the years. In addition to research trai ning, he provided opportuni ties to teach organic chemistry to a 200-student class, work on grant/pa tent/manuscript writing, and present research to a DARPA review meeting. These unique experien ces for a graduate stud ent always carried a great deal of fear, but I always went in feeling he had prepared me. He has generously shared his experiences and insights in a life of science and has shaped my ow n direction to an incalculable extent. I would also like to acknowle dge the professors who have contributed greatly to my education at the University of Florida. I th ank the members of my committee: Dr. Ronald Castellano, Dr. David Tanner, Dr. Eliot Douglas, Dr. Aaron Aponick, and Dr. Ken Wagener. Our interactions over my time in graduate scool ha ve always been both challenging and rewarding. I also wish to thank Dr. Ken Wagener for the va luable experiences while working in the Butler Polymer Laboratories and for our many discussions about careers in chemistry his insights were invaluable and determinant in the direction I have gone. I also thank Dr. Kirk Schanze with whom I have worked on the DOE/conjugated polyel ectrolyte program. He has been an integral part of this work and helped to shap e it into what I am proud it has become. Numerous people have contribute d to the work that has gone into my dissertation. I would like to thank Dr. Khalil Abboud fo r solving x-ray crystal struct ures; James Leonard for work with x-ray structures and GPC ch aracterization; Erik Berda and Kate Opper for all of their work with thermal analysis; Katsu Ogawa for hi s assistance and insight with the MCCL instrumentation; Ece Unur for working with my materials over the years. When I arrived at UF, I quickly realized how much I needed to learn and how helpful members of the polymer floor could be. I thank Tr avis Baughman and Ben Reeves for all of their 4


help in running columns, setting up reactions, di stillations the list goes on. Additionally, I thank them for assuring me I have no future as a riverboat gambler. I also thank Ben, Emilie Galand, and Tim Steckler for teaching me how to be a part-time electrochemist. One of the greatest changes in the Reynolds group over my tim e has been the collaborative spirit of the synthetic chemists In addition to creating intersec tions between our projects, the numerous discussions have had a tremendous influe nce on this dissertatio n. For that, I would like to thank Stefan Ellinger, Eric Shen, Pierre Beaujuge, Jianguo Mei, Ryan Walczak, and Prasad Taranakar. My respect for them is boundless. I would like to give a special thanks to Tim Steckler. Although our collaborative work is approximately 0-11 at this point, we learned a great de al in the process. Through all of this research, his discussions and a dvice as The Wolf shaped the direction of these projects. I always appreciated his help. More than that, he has been a great friend to con into one more lunch at the Monkey and joke around with in the lab. I also would like to thank my family for th eir wonderful support ove r this time. When I decided to leave a teaching job in Birmingham to go to graduate school in chemistry, everyone provided great encouragement. My father (H erman Brookins), mother (Nina Brookins), and brothers (Scott and Brian Brookins) have offered great advice and discus sions through every step of graduate school. Finally, I want to thank my wife, Paige, for her constant support a nd love through all of this time at UF, and my daughter, Katie, for th e love she fills everything with. At times my research has demanded more of them than I intended, but they always stood by me. From todays perspective, I believe it has surely been worth it It is a tremendous feeling to look forward to our future together. 5


TABLE OF CONTENTS page ACKNOWLEDGMENTS ............................................................................................................... 4LIST OF TABLES ...........................................................................................................................9LIST OF FIGURES .......................................................................................................................10LIST OF ABBREVIATIONS ........................................................................................................1 6ABSTRACT ...................................................................................................................... .............181 ADVANCES IN ARYL-ARYL BOND FORM ATION VIA SUZUKI COUPLINGS IN ORGANIC SYNTHESIS AN D POLYMERIZATION ..........................................................21Introduction .................................................................................................................. ...........21Background and History of Palladium Catalysis ....................................................................22Suzuki Coupling .....................................................................................................................25Overview of organoboron compounds ............................................................................26Synthesis of organoboron compounds .....................................................................27Reactivity of organoboron reagents .........................................................................30Ligand andCatalyst Design for Suzuki Couplings ..........................................................31Role of Base and Solvent in Suzuki Couplings ...............................................................34Unique Concerns for Macromolecule Synthesis ....................................................................36Soluble Conjugated Polymers and the Optimization/Development of Suzuki Polymerizations ............................................................................................................... ....38Early History and Properties of Conjugated Polymers ....................................................38Suzuki Polymerizations for Conjugated Po lymers: Optimization and Developments ...........42Optimization of Suzuki Polymerization for High Molecular Weight Conjugated Polymers ...................................................................................................................... 42Controlling Conjugated Polymer Molecu lar Structure via Modified Suzuki Polymerizations............................................................................................................45Thesis ........................................................................................................................ ..............492 EXPERIMENTAL METHODS .............................................................................................51Molecular Characterization .................................................................................................... 51Polymer Characterization ...................................................................................................... .51Structural Characterization ..............................................................................................51Thermal Characterization ................................................................................................52Electrochemical Characterization ....................................................................................53Spectral Characterization .................................................................................................55 6


Synthetic Methods ..................................................................................................................57Suppliers ..................................................................................................................... .....57Purification of Solvents /Reagents/Catalysts ....................................................................57Handling of Catalysts ......................................................................................................57General set-up of Suzuki polymerizati ons and Pd-catalyzed borylations .......................59Conversion of ester-functionalized polymers to carboxylic acids ..................................613 HIGH BANDGAP POLYMERS FOR DUAL PROPERTIES IN ELECTROCHROMIC AND LIGHT EMITTING APPLICATIONS .........................................................................62Introduction .................................................................................................................. ...........62Electrochromic Materials and Devices ............................................................................64Light-emitting Materials and Devices .............................................................................66Design of Fluorene-Based Copolymers ..................................................................................67Monomer and Polymer Synthesis ...........................................................................................69Characterization of Thermal, Electro chemical, and Spectral Properties ................................75Conclusion .................................................................................................................... ..........81Experimental .................................................................................................................. .........834 BASE-FREE SUZUKI POLYMERI ZATION FOR THE SYNTHESIS OF CARBOXYLIC ACID FUNCTIONALI ZED CONJUGATED POLYMERS ......................90Introduction .................................................................................................................. ...........90Properties and Applications of Conjugated Polyelectrolytes ..........................................90Synthesis of Conjugated Polyelectrolytes .......................................................................93Fluorene-Based Polymers Functionalized with Carboxylic Acids .........................................96Synthesis of Highand Mi d-Bandgap Fluorene-Based Conjugated Polyelectrolytes ............98Optimization of Base-Free Suzuki Polymerization I: Polyfluorene ................................98Optimization of Base-Free Suzuki Polymeri zation II: Fluorene-based Copolymers ....101Characterization of Highand MidBandgap Fluorene-Based Conjugated Polyelectrolyte...................................................................................................................111Poly(Fl-Ph)DA : Spectral Properties and Solvatochromic behavior .............................112Poly(Fl-Btd)DA : Spectral Properties and Solvatochromic behavior ...........................117Poly(Fl-BTh)DA : Spectral Properties and Solvatochromic behavior ..........................119Poly(Fl)-DA : Spectral Properties and Solvatochromic behavior ...................................122Conclusion .................................................................................................................... ........126Experimental .................................................................................................................. .......1275 SYNTHESIS OF LOW BANDGAP C ONJUGATED POLYELECTROLYTES VIA SUZUKI POLYMERIZATION OF MONOMERS WITH THERMALLY CLEAVABLE ESTERS .......................................................................................................133Introduction .................................................................................................................. .........133Concept and Design of Carboxylic-Acid F unctionalized CPEs from Thermally Cleavable Esters .............................................................................................................. ..134 7


CPEs from Thermally Cleavable Pendan t Groups: A Study with poly(Fl-Ph) ...................135Synthesis of Poly(Fl-Ph)-EE and Related Monomers ...................................................135Synthesis of Poly(Fl-Ph)-TB and related monomer ......................................................137Characterization of Monomers and Polymer with Thermally Cleavable Esters ...........138Donor-Acceptor-Donor (DAD) Monomers for CPEs ..........................................................140Synthesis of DAD Monomers .......................................................................................142Characterization of DADs .............................................................................................144Synthesis of Low Bandgap CPEs Functionalized With Carboxylic Acids via Suzuki Polymerization ................................................................................................................ ..147Synthesis of 9,9-(2-methylpentyl)alkanoate fluorene ...................................................147Synthesis of low bandgap CPEs ....................................................................................149Poly(Fl-BTBtd)DA : Spectral properties and Solvatochromic Behavior ......................154Conclusion .................................................................................................................... ........157Experimental .................................................................................................................. .......1606 POLY(BENZO[1,2-B:4,3-B]DIT HIOPHENES: SYNTHESIS OF A POLYFLUORENE ANALOGUE WITH IMPROVED ELECTROCHEMICAL PROPERTIES AND INTERCHAIN INTERACTIONS......................................................167Introduction .................................................................................................................. .........167Monomer and Polymer Synthesis .........................................................................................169Conclusion .................................................................................................................... ........184Experimental .................................................................................................................. .......1847 FINAL CONCLUSIONS and PERSPECTIVES ..................................................................188APPENDIX A X-RAY CRYSTALLOGRAPHIC DATA ...........................................................................192LIST OF REFERENCES .............................................................................................................198BIOGRAPHICAL SKETCH .......................................................................................................210 8


LIST OF TABLES Table page 2-1. Useful solvent mixtures for studying solution solvatochromism ...........................................564-1. Summary of polymerization methods for conjugated polyelectrolytes ..................................944-2. Conditions for base-free Suzuki polymerization shown in Figure 4-5 .................................100A-1. Crystal data and structure refinement for 7 (Chapter 3). ....................................................192A-2. Crystal data and structure refinement for 9 (Chapter 5). ....................................................194A-3. Crystal data and structure refinement for 4 (Chapter 6). ....................................................196 9


LIST OF FIGURES Figure page 1-1. Repeat unit structure of common conjugated polymers .........................................................211-2: Model Mechanism of Palladium Catalyst ..............................................................................231-3. Equilibration of tetrak is(triphenyl phosphine) palladi um (0) to active species. .....................241-4. Activation of boronic acid for transmetallation with palladium (II) ......................................251-5. Recent examples of Suzuki reactions in the literature. ...........................................................261-6. Classes of organoboron compounds ......................................................................................271-7. Synthesis of boranes/boronic acids /boronate esters/trifluorborate .........................................281-8. Protodeboronation of phenylboronic acid. .............................................................................321-9. Effect of ligand structure on Suzuki coupling to 4-methylchlorobenzene. ............................321-10. Exemplary ligands for Suzuki couplings ..............................................................................341-11. Transmetalation of arylboronic acid with activation by thallium hydroxide. ......................361-12. Schematic of A2/B2 and AB monomers for Suzuki polymerization .....................................371-13. Carothers equation ................................................................................................................371-14. Evolution of HOMO-LUMO gap with the ex tent of conjugation for poly(thiophene).. ......391-15. Absorption/emission spectra for conjuga ted molecule with schematic of the photophysical processes.. ...................................................................................................401-16. Poly(2,5-dihexylphenylene) the first application of a Suzuki polymerization. .................431-17. Effect of base and catalyst on the Suzuki polymerization of 1,5-dibromo-2,4dinitrobenzene. ............................................................................................................... ....441-18. Suzuki polymerization of thienyl boronic species with 1,4-diiodo-2,5dialkoxybenzene. .............................................................................................................. .451-19. Incorporation of phosphorous into th e polymer backbone by aryl exchange. ......................461-20. Diffusion-limited oxidative addition of Pd/P(t-Bu)3 ............................................................471-21. Regioregularity of oxidative polymeri zation versus a Suzuki polymerization. ...................48 10


2-1. Preparation of polymer solutions for solvatochromism experiments. ...................................563-1. Family of fluorene-based, hi gh bandgap conjugated polymers ..............................................633-2. Oxidation of an oligo(3,4-dialko xythiophene) to the dication state. ......................................653-3. Spectroelectrochemistry of a PEDOT deriva tive on ITO glass at applied potentials from -0.7 to 0.6 V (vs. Ag/Ag+). .................................................................................................653-4. Conformation of 9,9-dibutyl fluorene by MM2 calculation. ...................................................673-5. Synthesis of carbazole monomers. ......................................................................................... 703-6. Synthesis of fluorene monomers ........................................................................................... .703-7. Synthesis of pyrrole monomer ............................................................................................. ...713-8. Suzuki polymerization of Poly(Fl) and Poly(Fl-Cbz) ............................................................723-9. Crystal structure of 2,5-bis(boro nic acid pinacol ester)-pyrrole ( 7). ......................................733-10. Suzuki polymerization of Poly(Fl-Pyr) ................................................................................743-11. GPC chromatogram of poly(Fl-P yr) from Figure 3-10, Entry 3. .........................................743-12. TGA (10 C/min, N2) of poly(Fl)[dash -dot],poly(Fl-Cbz)[dash],and poly(Fl-Pyr) [solid] ....................................................................................................................... ..........763-13. DSC endotherms (left) and exotherms (right) for poly(Fl) [dash-dot], poly(Fl-Cbz) [dash], and poly(Fl-Pyr) [solid ] with arbitrary offsets. ......................................................763-14. UV-Vis absorbance of poly(Fl) [solid], poly(Fl-Cbz) [dash], and poly(Fl-Pyr) [broken dash] in 10 M solutions in chloroform. ...........................................................................783-15. PL of poly(Fl) [solid], poly(Fl-Cbz) [dash], and poly(Fl-Pyr) [broken dash] with 10 M solutions in THF. Excitations are at the max of each polymer. ..................................783-16. CVs of poly(Fl-Pyr) [left] and poly(Fl-Cbz) [right] films on ITO. Inset: Isolation of 1st oxidation process for poly(Fl-Pyr) .....................................................................................803-17. Solution oxidation of 10 M poly(F l-Pyr) in dichloromethane with antimony pentachloride as an oxidant. Arrows show change of absorbance with oxidation. ...........823-18. Solution oxidation of 10 M poly(FlCbz) in dichloromethane with antimony pentachloride as an oxidant. Arrows show change of absorbance with oxidation. ...........824-1. Examples of conjugated polyelectrolytes (CPEs) ..................................................................914-2. Solution behavior of CPEs in the present of a divalent cation. ..............................................92 11


4-3. General structure of fluorene-based conjugated polymers functionalized with carboxylic acids, and the family of comonomers employed. .............................................984-4. Synthesis of fluorene mono mers for polyfluorene poly(Fl)DE ............................................994-5. Base-free Suzuki polymerization of polyfluorene homopolymer diester derivative (poly[Fl]DE ) and diacid derivative (poly[Fl]-DA ). ..........................................................994-7. Three component polymerization of a fl uorene and bis-thienyl benzothiadiazole containing polymer. .........................................................................................................1014-8. Synthesis of 2,7-bis(pinacol borona te ester)-9,9-dibut ylesterfluorene ( 11) .......................1024-9. Synthesis fluorene-phenyl ene and fluorene-thiophene copolymers by base-free Suzuki polymerization ................................................................................................................ .1034-10. Optimization of base-free Suzuki polymerization with Pd/P(t-Bu)3 catalyt system ...........1044-11. Synthesis of poly(Fl-Ph)DE poly(Fl-BTD)DE and poly(Fl-BTh)DE ..........................1064-12. Poly(Fl-Ph)DE 1H NMR and GPC chromatogram ........................................................1074-13. Poly(Fl-Btd)-DE 1H NMR and GPC chromatogram .......................................................1084-14. Poly(Fl-Bth)-DE 1H NMR and GPC chromatogram .......................................................1094-15. IR spectra of a) poly(Fl-Ph), b) poly(Fl-Btd), and c) poly(Fl-BTh). ..................................1104-16. Base hydrolysis of highand midbandgap polymers to their diacid form ........................1114-17. UV-Vis absorbance solvatochromism of 10M solutions of Poly(Fl-Ph)DA in THF/water mixtures. ........................................................................................................1134-18. UV-Vis absorbane solvatochromism of 10M solutions of Poly(Fl-Ph)-DA in THF/basic water(pH 9) mixtures. ...................................................................................1134-19. PL solvatochromism of 10M solutions of Poly(Fl-Ph)-DA in THF/water mixtures. Excited at max, abs. ............................................................................................................1154-20. PL solvatochromism of 10M solutions of Poly(Fl-Ph)-DA in THF/basic water(pH 9) mixtures. Excited at max, abs. ............................................................................................1154-21. UV-Vis absorbance(black) an d PL intensities (blue) at max for the diacid (solid line) and dicarboxylate (dashed line) forms of Poly(Fl-Ph)DA ..............................................1164-22. UV-Vis absorbane solvatochromism of 10M solutions of Poly(Fl-Btd)DA in THF/basic water(pH 9) mixtures. ...................................................................................118 12


4-23. PL solvatochromism of 10M solutions of Poly(Fl-Btd)DA in THF/basic water(pH 9) mixtures. Excited at max, abs. ......................................................................1184-24. UV-Vis absorbance solvatochromism of 10M solutions of Poly(Fl-BTh)DA in THF/water mixtures. ........................................................................................................1204-25. UV-Vis absorbane solvatochromism of 10M solutions of Poly(Fl-BTh)DA in THF/basic water(pH 9) mixtures. ...................................................................................1204-26. UV-Vis absorbances of poly(Fl-BTh)-DA in 60% THF/40% water (solid line) and 60%THF/basic aq. Solution (dashed line). ......................................................................1214-27. PL solvatochromism of 10M solutions of Poly(Fl-BTh)DA in THF/water mixtures. Excited at max, abs. ............................................................................................................1234-28. PL solvatochromism of 10M solutions of Poly(Fl-BTh)DA in THF/basic water(pH 9) mixtures. Excited at max, abs. ......................................................................1234-29. UV-Vis absorbance solvatochromism of 10M solutions of Poly(Fl)DA in methanol/water mixtures. .................................................................................................1254-30. UV-Vis absorbance solvatochromism of 10M solutions of Poly(Fl)DA in methanol/basic aqueous solution (pH=9). .......................................................................1254-31. PL spectra of poly(Fl)DA (10 M) in 100% methanol (solid) and 60% methanol/40% water (dash). Excited at max, abs. ......................................................................................1265-1. Examples of donor-acceptor-donors. ....................................................................................1335-2. Mechanism of retro-Ene in the synthesis of polyacrylic acid from the corresponding thermally cleavable ester..................................................................................................1345-3. Molecular structure of Poly(Fl-Ph)EE and Poly(Fl-Ph)-TB ................................................1355-4. Synthesis of 2,7-dibromo-9,9-(1-et hoxyethyl ester propionate)fluorene .............................1365-5. Suzuki conditions for the synthesis of poly(Fl-Ph)EE ........................................................1375-6. Synthesis of 2,7-dibromo-9,9-( tbutyl propionate)fluorene .................................................1375-7. Synthesis of Poly(Fl-Ph)TB .................................................................................................1385-8. TGA data (scan 10 C/min, N2) for poly(Fl-Ph)EE ............................................................1395-9.TGA data (scan 10 C/min, N2) for poly(Fl-Ph)TB .............................................................1395-10. Suzuki polymerization and thermal treat ment of low bandgap CPE based on 4,7-bis(1H pyrrole) benzothiadiazole. ............................................................................................142 13


5-11. Synthesis of 4,7-bis(1-Boc pyrrole) benzothiadiazole .......................................................1435-12. Synthesis of pyrrole-based DAD derivatives. ....................................................................1445-13. UV-Vis absorbance (normalized) of ~10 M THF solutions of 4,7bis(thienyl)benzothiadiazole ( 1 : solid), 4,7-bis(1-Boc pyrro le) benzothiadiazole ( 9: dash), 4,7-bis(1-H pyrrole) benzothiadiazole ( 3: dot). ....................................................1455-14. Crystal structure of 4,7-bis(1Boc pyrrole) benzothiadiazole ( 9 ) .......................................1455-15. Solvatochromic effect on UV-Vis absorb ance of ~10 M solutions of 4,7-bis(1-H pyrrole) benzothiadiazole ( 3). Inset: Magnification of region around max .....................1465-16. PL data of ~10 M solutions of 4,7-bis(1-Boc pyrrole) benzothiadiazole ( 9 : dash), 4,7bis(1-H pyrrole) benzothiadiazole ( 3: solid). ...................................................................1475-17. Synthetic routes towards t-butyl propionate ester derivatives of fluorene .........................1485-18. Synthesis of 2,7-bis(boronic acid pina col ester)-9,9-(2-methylpentyl )butanoate fluorene (14 ).....................................................................................................................1495-19. Synthesis of poly(Fl-BPBtd)DE and poly(Fl-BTBtd)DE ................................................1505-20. TGA (scan 10 C/min, N2) of poly(Fl-BTBtd)DE ............................................................1515-21. TGA (isothermal, N2) of poly(Fl-BTBtd)DE at 185 C. ...................................................1515-22. TGA (scan 10 C/min, N2) of poly(Fl-BPBtd)DE ............................................................1525-23. TGA (isothermal, N2) of poly(Fl-BPBtd)DE at 185 C. ...................................................1525-24. Photos of poly(Fl-BPBtd) before and after heati ng at 185 C. ...........................................1545-25. IR spectra of poly(Fl-BTBtd) [Top] a nd poly(Fl-BPBTD) [bottom] on salt plates (NaCl). ....................................................................................................................... ......1555-26. UV-Vis absorbance solvatochromism of 10M solutions of Poly(Fl-BTBtd)DA in THF/water mixtures. ........................................................................................................1565-27. UV-Vis absorbane solvatochromism of 10M solutions of Poly(Fl-BTBtd)DA in THF/basic water(pH 9) mixtures. ...................................................................................1565-28. PL solvatochromism of 10M solutions of Poly(Fl-BTBtd)DA in THF/water mixtures. Excited at max, abs. ............................................................................................1585-29. PL solvatochromism of 10M solutions of Poly(Fl-BTBtd)DA in THF/basic water(pH 9) mixtures. Excited at max, abs. ......................................................................158 14


5-30. PL intensity of poly(Fl-BTBtd)DA in THF/water ( ) and THF/basic aqueous solutions ( ) .....................................................................................................................1596-1. Structure of poly(benzo[1,2-b:4,3-b]dithiophenes) [PBDTs] .............................................1696-2. Synthesis of BDT monomers. ............................................................................................... 169 6-3. X-ray crystal structure of 5,6-BDT diacetate (3)......171 6-4. Polymerization of BDT monomers. ......................................................................................1726-5. Left : Change of UV-Vis absorbance spectra of PBDT-EtHex with lower molecular weight fractions from GPC Right : Relationship of max to Mn for GPC eluents. The first eight data points represent the eight spectra shown to the left. ................................1736-6. Normalized solution ab sorption spectra of ( ) EtHex (---) Oct(low), (--) Oct(high). All solutions are ~ 1 mM in xylenes. .....................................................................................1746-7. Solution thermochromism of PBDT-Oct (low) [top] and PBDT-O ct(high) [bottom] from 25-95 C. Both solutions are 0.1 mM in xylenes. ...................................................1756-8. Normalized solution absorbances of ( ) PBDT-EtHex room temperature in xylenes and (---) PBDT-Oct(low) at 95 oC in xylenes ..................................................................1766-9. Left: PL spectra of PBDT-Oct(low) [red] and PBDT-EtHec [black] in 10 M THF solutions. Right: PL spectra of PBDT-Oct(low) [re d] and PBDT-EtHec [black] in solid state. ........................................................................................................................1776-10. DSC overlay of PBDT-Oct(low) and PBDT-Oct(high) .....................................................1786-11. MDSC of PBDT-Oct(low) and PBDT-Oct(high) ...............................................................1796-12. Cyclic voltammetry (CV) of PBDT-EtHex [solid] and PBDT-Oct(low) [dash] with 0.1 M TBAP in acetonitrile. ...................................................................................................1806-13. Differential pulse voltammetry (DPV) of PBDT-EtHex [solid] and PBDT-Oct(low) [dash] with 0.1 M TBAP in acetonitrile ...........................................................................1816-14. Cyclic voltammetry (CV) [top] and differe ntial pulse voltammetry (DPV) [bottom] of PBDT-Octyl(high) with 0.1 M TBAP in acetonitrile. .....................................................1826-15. Spectroelectrochemistry of PBDT-Oct(l ow) on ITO. Voltage is changed from 400, 550, 600, 650, 700, 750, 800 mV (vs Fc/Fc+). ................................................................183A-1. Crystal structure for compound 7 (Chapter 3) .....................................................................192A-2. Crystal structure for compound 9 (Chapter 5) .....................................................................194A-3. Crystal structure for compound 4 (Chapter 6) .....................................................................196 15


LIST OF ABBREVIATIONS BTh 2,2-bithiophene Bpy 2,2-bipyridine Btd 2,1,3-benzothiadiazole CPE Conjugated polyelectrolyte DA Di(carboxylic acid) derivative DAD Donor-acceptor-donor molecules DE Diester derivative EE (1-ethoxy)ethyl derivative DMSO Dimethyl sulfoxide DME 1,2-dimethoxyethane DMF N,N,-dimethylformamide dppf 1,1-diphenylphosphino-ferrocene dtbpy 4,4-dit -butyl-2,2-bipyridine LED Light-emitting diode Mn Number average molecular weight Mw Weight average molecular weight NBS N-bromosuccinimide PDI polydispersity Pd2dba3 Trisbenzylidene acetone dipalladium (0) Pd(PPh3)4 Tetrakis(triphenylphosphine) palladium (0) PL Photoluminescence QE Quantum efficiency (as measured by PL) TB t -Butyl ester derivative THF Tetrahydrofuran 16


TEBA Triethylbenzylammonium Chloride Xn Degree of polymerization 17


Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SYNTHETIC ROUTES TO NOVEL CONJ UGATED POLYMERS BASED ON FLUORENE AND SYNTHETIC ANALOGUES OF FLUORENE By Robert N. Brookins August 2008 Chair: John R. Reynolds Major: Chemistry This work presents the synthesis and ch aracterization of conjugated polymers and polyelectrolytes based on fluorene derivatives for use in light emitting, electrochromic, and energy transfer applications. Fluorene-based homoand copolymers have been integral to the development of organic-based light-emitters, phot ovoltaics, and sensors, due to their facile synthesis and unique material/el ectronic properties; how ever, this progress has been limited by the narrow range of accessible polymers. The goal of this research is to develop new synthetic routes and methods towards fluorene-based po lymers, as well as to assess the resulting material/electronic properties. One goal of this work is to develop a fa mily of fluorene-based copolymers with the emissive properties of a polyf luorene homopolymer but with improved electrochemical and electrochromic properties. To th is end, carbazole and 1-methyl pyrrole monomers functionalized with boronate esters have b een polymerized with fluoren e to give polymers with Mn = 10,000 and 16,000 Da, respectively. The latter polymer wa s synthesized via novel Suzuki conditions that give the highest degree of polymerization reporte d for a pyrrole-based conjugated polymer. Both polymers have HOMO-LUMO gaps within 0.2 eV of polyfluorene, giving them a similarly blue fluorescence. Electrochemically, the carbazole -fluorene and pyrrole-f luorene polymers are 18


significantly different from a polyfluorene w ith HOMOs that are 0.5 eV and 0.8 eV less, respectively. A second project studies a series of conjugated polyelectrol ytes that are derived from fluorenes functionalized with es ters and are converted to carb oxylic acids post-polymerization. Over this series, the polymers absorb different ranges of visible light. To polymerize monomers with base-sensitive moieties, a base-free Suzuki polymerization was developed and optimized by replacing the traditional base with a fluoride salt. Ester-f unctionalized polyfluorene was synthesized by this method, yielding polymers with molecular weights ranging 4,000 to 22,000 Da. Subsequent hydrolysis of this polymer affo rded the first carboxylic acid functionalized polyfluorene. Applying this method to other m onomers proved problematic, ultimately requiring a biphasic system, and under these conditions, ph enylene, bithiophene, and benzothiadiazole comonomers were polymerized with Xn of ~25-50. The spectral properties of the polymers carboxylic acid derivatives were studied in solu tion as their carboxylic acid and carboxylate forms. The carboxylates exhibite d behavior expected of a conj ugated polyelectrolyte (CPE); however, the carboxylic acid derivatives planarize the polymer backbone in more polar solutions. For low bandgap carboxylic acid functionaliz ed CPEs, a new post-polymerization method was required due to the polymers decompositi on under base hydrolysis. To circumvent this problem, fluorenes functionalized with thermally cleavable esters were designed and synthesized to afford low bandgap CPEs. By heating the resulting polymers to ~185 oC, the esters are converted to carboxylic acids wit hout altering the polymer backbone. A third project studied poly(benzodithiophene s) as synthetic anal ogues of polyfluorenes. While these monomers share a similar fused, tric yclic aromatic structur e as fluorene, thienyl moieties raise the HOMO energy to a more accessi ble value and the solubilizing chains are 19


20 oriented in the plane of the mono mer, facilitating interchain intera ctions in the solid state and in solution. By changing the molecular weight and th e steric hinderance of the solubilizing groups, the electrochemical properties are dramatically e ffected, with the onset of oxidation varying over a 500 mV range.


CHAPTER 1 ADVANCES IN ARYL-ARYL BOND FORMAT ION VIA SUZUKI COUPLINGS IN ORGANIC SYNTHESIS AN D POLYMERIZATION Introduction Since the discovery of the high conductivity of polyacetylene doped with halogens, research on conjugated polymers has been extensive.1 Though polyacetylene itself has problems with poor stability under ambient conditions, st udies on other conjugated systems such as polythiophene, polyfluorene, and poly(p-phenylene vinylene) [Figure 1-1] have demonstrated greater promise due to improved stability, novel el ectronic properties, a nd conventional polymer processability.2 These materials have been studied for numerous applications including lightemitting diodes (LEDs),3 photovoltaics,4 electrochromic devices,5 and antistatic coatings.6 From the basic structure of alterna ting double and single bon ds, an array of semiconductive properties arise t hough the underlying mechanisms are distinctly different from traditional inorganic semiconductors. For this reason, conjugated polymers have a host of parameters differentiating them from their in organic counterparts. In terms of molecular structure, a conjugated polymer ch ain has lower symmetry than the crystalline structures of an inorganic semiconductor. This distinction is significant for charge mobility, because delocalization of a charge along a one-dimensional chain (opposed to a three dimensional lattice) limits the charges mobility. For this reason, understanding the perfor mance of a conjugated polymer in an application where film thicknesses can range from nanoto micrometer scales Figure 1-1. Repeat unit structur e of common conjugated polymers (conjugated bonds highlighted in red) 21


requires an understanding of the films morphology. And again, th e polymeric nature of the material is paramount. As with all macromolecu les, the morphology of a conjugated polymer can vary from completely amorphous to some fractional crystallinity. The nature of these interchain interactions impacts the electroactive properties of a material for all potential applications. Working to realize these applications, great a dvances have been made over the past thirty years, though only with an intricate interp lay of synthesis, materials processing, and characterization. This introduction will place the role of synthesis in perspective, with a particular focus on Suzuki polymerizations. C ontinued insight into the mechanism of this reaction has increasingly influenced the work on Suzuki polymerizations; however, the issues concerning a polymer chemist do not necessarily coincide with those of an organic chemist. To develop the synthetic background that is the basis of a Suz uki polymerization, studies on palladium catalysis will be reviewed, outlining the role of substrate, solvent, base, and catalyst on these reactions. Due to the breadth of this topi c, aryl-aryl couplings via Suzuki conditions will be the focus. Building from this, the synthesis of soluble, conjugated polymers will be studied, focusing on research that works to optimize these polymerizations a nd even to direct them to the synthesis of new polymer structur es. Additional details related to the polymers designed for this dissertation will be discussed in subsequent chapters. Background and History of Palladium Catalysis While organometallic chemistry has played a key role in synthesis for over a century, palladium catalysis ushered in a new era. Work by Kumada and Corriu on reactions of organomagnesium reagents with palladium catalysts provided the key discovery, where aryl or alkenyl derivatives were coupled to aryl or alkenyl halides.7 This ability to bond two sp2 carbons directly without the conc omitant synthesis of new -bonds (i.e., McMurry and Wittig 22


polymerizations) provided a more direct route to compounds important to medicinal chemistry, natural product synthesis, and materials chem istry. Fortunately, this phenomenon was not limited to organomagnesium reagents but could be a pplied to many compounds including organozinc, organosilicon, organoboron, and organotin deriva tives each with their own reactivity, selectivity, and functi onal group compatability.8 Interestingly, all of these methods are subtle variations of the same basic mechanism, as shown in Figure 1-2. Each reacti on begins by the oxidative additi on of palladium (0) to a carbon(pseudo)halide bond. While alkynl, alkenyl, and aryl substrates have found the greatest utility, alkyl halides have been increasi ngly viable in the past decade.9 A substrates reactivity towards oxidative addition is largely determined by the (pseudo)halide according to the following trend: iodide > triflate > bromide >> chloride.10 The structure of the active palladium complex can vary due to the dissociation/association of ligands. For example, the prototypical catalyst in Suzuki Figure 1-2: Model Mechanism of Palladium Catalyst 23


couplings is Pd(PPh3)4 due to its ready availability, reactivity with a broad range of substrates,and its thermal stability; however, this complex is coordinatively saturated and is not the active species. The phosphines equilibrate between complex and solution (Figure 1-3), with the triand diphoshpine complexes being catalytic ally active. Because oxidative addition is generally the rate limiting step of a reaction, gr eater reactivity of coordinatively unsaturated palladium species is a guiding principle in catalyst design. Based on this, researchers have devised numerous catalysts including ligandfree palladium nanoparticles for enhanced reactivity.11 After generating the palladium (II) species from oxidative addition, the catalyst then undergoes transmetallation with another organometal lic species. In this step, a second organic substrate is transferred to palladium and a metal by-product is eliminated. The mechanistic details of this step are the least understood of th e three; however, one general rule maintains that the organometallic species undergoing transmetalla tion is more electropositive than palladium. The standard example of this is in the Suz uki reaction as shown in Figure 1-4. Organoboron compounds are generally unreactive to pa lladium complexes due to borons low electropositivity. When a Lewis base binds with boron, its electropositivit y increases, allowing it to undergo transmetallation.12 The final step of the catalytic cycle is reductive elimination, which is mechanistically the reverse of oxidative addition. In this step, the two organic groups are eliminated from palladium with a newly formed bond between them. Additiona lly, the palladium is reduced from palladium Figure 1-3. Equilibration of tetrakis(triphenyl phosphine) palladium (0) to active species. 24


t Figure 1-4. Activation of boronic acid for transmetallation with palladium (II) (II) back to palladium (0). The driving force for this step is generally ascribed to relieving steric hinderance, though there ar e indications that the -bonds or aryl groups weakly interact cofacially at the C1 position, favoring bond formation.12 While this outline of the palladium catalytic cycle presents the major steps and trends, this is a simplified account in a number of ways. First, any given step above may be composed of a sequence of steps, so this catalytic cycle eff ectively describes a sequence of key intermediates and transformations. Second, the role of the ligand is not describe d though it is integral to the electronic and steric properties of the catalysts. The ligands role is largely reaction specific and will be addressed more fully in subsequent sections. Third, the solvent system is not simply a medium with a given dielectric constant, but can bind with the metal catalyst as a ligand.8 Thus, the solvent can lead to significant changes in a reactions success. Finally, the palladium complex undergoes isomerization during this cycle, and the rate of this isomerization can be significant for catalyst turnover. Suzuki Coupling A number of organometallic compounds are am enable to the mechanistic route described, but the Suzuki coupling has risen to prominence as the most widely applied and studied. In this method, a boron containing compound is used as the organometallic species, and added base is used to activate the boron for transmetalation w ith palladium, as previously discussed. Research on Suzuki couplings over the past 15 years has made enormous strides to where sterically 25


encumbered and highly functionalized substrates have become feasible for these reactions. Exemplary Suzuki couplings are shown in Figur e 1-5 as examples of the state of the art.9, 13, 14 In this section, the details of how substrate, base, and ligand can be modified to optimize a Suzuki reaction will be discussed as well as the current understanding of the mechanisms driving these reactions. Overview of organoboron compounds A significant advantage of Suzuki coupling is the wide range of potential organoboron compounds as well as the methods to synthe size them. Examples of these organoboron compounds are shown in Figure 1-6. While these r eagents can be interc hangeable under some conditions, for many they are not. Additionally, the reactions used to synthesize these reagents differ significantly. Selecting a specific route is largely a matter of precedence and/or trial and error in determining which works be st. This subsection first presents an overview of the different methods available for synthesizing organoboron co mpounds followed by a brief discussion of the relative reactivity of these reagents. Figure 1-5. Recent examples of Suzuki reactions in th e literature. 26


Figure 1-6. Classes of organoboron compounds Synthesis of organoboron compounds The versatility of Suzuki coupling has been well-served by the synthetic methods for making organoboron compounds, allowing for elegant s ynthetic routes as we ll as ready access to otherwise difficult structures. Boranes were am ong the first widely synthesized organoboron compounds, based on the monumental work by H. C. Brown. While a number of variations are available, the textbook example of such a reac tion is the addition of a borohydride to a double bond in an anti-Markovnikov orientation as show n in Figure 1-7. The resulting compound can be used in a Suzuki coupling. Although boranes are now rarely used due to the relatively poor stability compared to other organoboron compounds the unique chemoa nd regioselectivity of this reaction is significant.10 Boronic acids are more stable organoboron compounds that react r eadily via a Suzuki cross-coupling. The most common method for th e synthesis of a boronic acid begins by generating an organolithium or organomagnesium reagent. Addition of a trialkyl borate (generally trimethyl or triisopropyl borate) followed by a mild acidic workup generates the boronic acid. Once isolated, boronic acids are stable under ambient conditions, though unstable to acidic/basic solutions and polar chromatogra phic media (e.g., silica, alumina). Additionally, 27


Figure 1-7. Synthesis of boranes/boronic acids/boronate esters/trifluorborate 28


boronic acids spontaneously dehydrate to form boroxines. Boroxines frustrate the characterization of boronic acid s although they have proven as reactive as boronic acids and do not inhibit Suzuki cross coupling. A closely related derivative to boronic aci ds is a boronate ester, where the hydroxyl groups of the boronic acid are repl aced by alkoxy groups. Boronate es ters are frequently used for two reasons. First, they are more stable than bo ronic acids, with many derivatives able to be purified by column chromotography. Second, a wide r range of synthetic methods are available for synthesizing boronate esters. Earlier methods relied on the same lithiation/boronate reaction used in the synthesis of boronic acids (Figure 1-7). In 1995, research by Miyaura et al found that aryl halides could be converted to aryl boronates by the reaction w ith bispinacolatodiboron in the presence of PdCl2(dppf) and potassium acetate.12 This salt and catalyst were key to this transformation, as other conditions resulted in Su zuki cross coupling between the aryl halide and generated aryl boronate. This reac tion is important, because it avoi ds the harsh conditions needed for lithiation, providing a direct route for aryl boronates functionalized with esters, ketones, amides, and nitriles. A wonderfully compleme ntary borylation reaction was found in 1999 with the iridium-catalyzed borylation of arenes.15 Whereas the Pd-catalyzed route requires an aryl halide, iridium operates by the ac tivation of C-H bonds (Figure 17). Interestingly, the activation is foremost directed by sterics and does not re act with halides. Taki ng the reaction shown in Figure 1-7 as an example, one can see that the bo rylation is directed to the meta position to the halide for this reason. The one substrate class for which electronics become important is heterocycles for which borylati on is directed to the position to the heteroatom (unless sterically hindered). With the new routes made possibl e by these palladium and iridium catalyzed borylations, the breadth of the Suzuki reaction has been significantly expanded. 29


The last class of organoboron compounds now employed in Suzuki reactions is organotrifluoroborates. These compounds have been studied for many years,16 but have received increased interest due to work by Gary Molander who found that treatment of boronic acids/boronate esters with a potassium hydr ogen fluoride solution yi elds a precipitated trifluoroborate salt.17 When employed with Suzuki reaction under polar conditions, the trifluoroborane undergoes cross coupling. These Suzuki reagents have garner ed great attention in a short time and offer one more route for Suzuki coupling. Reactivity of organoboron reagents A key factor in the efficiency of a Suzuki coupling is the reactiv ity of the organoboron reagent. While the propensity to undergo transmet allation is clearly important, equally so is the stability of an organoboron reagent under the reacti on conditions. Strictly in terms of reactivity, coupling an alkylboron reagent to an aryl or alkenyl halide has proven more effective with boranes than alkyl bor onic acids and esters.8 Outside of this application, boranes are less frequently used due to their poor air stability and lower reactivity than boronic acids. Boronic acids are generally recognized as the most reactive organoboron for Suzuki couplings. Boronate esters have proven comparably, but still less re active analogues of boronic acids. The basis of this difference has been ascribed to steric hinderance or Lewis acidity. Though organotrifluoroborates have only received great attention in re cent years, work thus far has shown their reactivity to be akin to boronate es ters, albeit under distinct ly different conditions.18, 19 A necessary concession regarding the hi gh reactivity of boroni c acids is their decomposition under many Suzuki conditions. While numerous minor side reactions have been cited,8 the hydrolysis of organoborons via protode boronation is a signific ant issue. Solvent 30


and electrolytes can lead to protodeborona tion of organoboron compounds under acidic/basic conditions or simply protic solvents, as shown schematically in Figure 1-8.20 Because organoborons require base to transmetallate wi th palladium (II) species, protodeboronation is always a problem. To circumvent this limitation, effective synthetic routes most often make the boronic acid the least valued component and then use it in excess (1.2-1.5 equiv).10 Notably, protodeboronation increases for substrates with ortho -substituents and with -heteroatoms.21 When the organoboron compound is more precious, boronate esters offer a more stable alternative. Boronate esters have the same thermodynamic stability of boronic acids; however, when functionalized with bulky groups such as a pinacol, steric hinde rance can significantly slow down the rate of oxidation, hydrolysis, and protodeboronation.22 Ligand & Catalyst Design for Suzuki Couplings Optimization of ligand structure and catalysts ha s led to enormous strides in the scope of Suzuki couplings, and has become the focus of most research in this field. Optimization has been directed towards three goals. One significant lim itation of the early work in Suzuki couplings was the poor reactivity of aryl chlorides. The su ccessful coupling of these has long been a goal due to high costs for bromide and iodide de rivatives. Additionally, steric hindrance of organoboron and organohalide substr ates can inhibit a Suzuki coupling, thus researchers have worked to minimize this effect. Finally, resear chers are perennially working towards the most cost efficient route either by using less expensive catalysts/liga nds or by using catalysts with high turnover numbers (TONs) in low concentrations. Interestingly, the goals for using aryl chloride s and sterically encumbered substrates were both met with the application of bulky, trialkyl phosphines. The traditional ligand for Suzuki catalyst is triphenyl phosphine;10 however, in comparison to bul ky, trialkyl phosphines such as 31


Figure 1-8. Protodeboronation of phenylboronic acid. tri(cyclohexyl)phosphine [PCy3] or tri(t-butyl)bu tyl phosphine [P(t-Bu)3], triarylphosphines are much less reactive with aryl chlorides. Wo rk by Wang Shen of Abbott Laboratories first highlighted this difference with the first high yield for an aryl-aryl coupling via an aryl chloride,23 with further research by Gregory Fu fully testing the scope of this catalyst, as demonstrated in Figure 1-9.24, 25 The exceptional performance of th ese ligands is attributed to their steric bulk, because palladium can only accommodate two P(t-Bu)3. Fu et al studied the effect of the Pd:P ratio and found that ~1: 1.5 was optimal, suggesting that the monophosphine complex is the active species. Assuming no pse udoligands, this complex would give palladium only 12 electrons, making it coordi nate unsaturated with an el ectron rich trialkyl phoshpine increasing its nucleophilicity, both of which activat e the palladium for fac ile oxidative addition with an aryl chloride. Additionally, this monophosphine complex opens coordination sites on palladium for oxidative addition to sterically bulky substrat es, as demonstrated by the 2,6dimethyl-bromobenzene shown in Figure 1-5. One difficulty in working with trialkyl phosphines Ligand (3.6%)NonePPh3P( o -Tol)3P(Cy)3P(t-Bu)3Yield0%0%10%75%86% Figure 1-9. Effect of ligand structure on Su zuki coupling to 4-methylchlorobenzene. 32


is their poor stability in air; however, more recent work by Fus research group has found that trialkyl phosphines can be used as air stable phosphonium salts that are converted to the active form in situ .26 Over the past ten years, numerous ligands have been designed and synthesized to determine what electronic and steric affects provi de the most effective catalyst. Phosphine-based ligands have received the greatest atte ntion. Coincident with the work by Fu et al were the (dialkyl)arylphosphines ( 1, Figure 1-10) developed by Stephen Buchwald at MIT which have been termed Phos ligands. Like P(t-Bu)3, these ligands work well w ith aryl chlorides and bulky substrates, with ligand 1 coupling an aryl bromide with two ortho isopropyl groups in greater than 93% yields.27 In difference to the trialkylphosphi nes, however, the Phos ligands are significantly more air stable.28 Great strides have also been ma de for coupling heterocycles, with (dialkyl)arylphosphines such as 1 and 2 working very effectively.29, 30 One final phosphine-based ligand is 3 that uses the principle of an openfaced palladium complex by tethering four cofacial, (diaryl)alkylphosphines to a cyclopentyl ring. This ligand generates a very stable and active catalyst with a TON of 97,000,000.31 An extremely promising alternative to th e phosphine-based ligands are N-heterocyclic carbene (NHC) ligands. Like the previously di scussed phoshines, NHC ligands show significant enhancements in Suzuki coupling by incorporatin g sterically crowded structures although they operate by different mechanisms. NHC ligands do not dissociate from palladium, but bind to it strongly with a Pd-carbene bond, making th e exact function of the ligand unclear.11 However, one can clearly see correlations between the be st phosphines and NHC liga nds in terms of being isoelectronic and showing improved performance with bulky ligands.9 33


Ph2P Ph2P PPh2 PPh2 1 2 3 4N N Pd N Cl Cl Cl 5 t-Bu t-Bu O Pd P O O O Cl 2 PCy2 OCH3 OCH3 Pd Cl Cl P P N N Cy Cy Cy Cy Figure 1-10. Exemplary ligands for Suzuki couplings Two final classes of palladium catalysts ha ve shown that with free palladium atoms one can generate some of the most active catalysts reported. While generating palladium (0) can easily lead to the creation of unreactive pallad ium black, many researchers have shown that by carefully controlling the conditi ons, palladium atoms and/or nanopa rticles can carry out coupling reactions with high TONs and at exceptionally low costs.11 An interesting class of catalysts which effectively operate by such a mechanism is palladacycles, which are complexes with a true Pd-C covalent bond. While the mechanism of palladacycles in coupling reactions has been debatable, the general consensu s is that they operate by deco mposing in a controlled manner, gradually releasing ligand-less pallad ium atoms to the reaction mixture.32 Complexes like 5 have shown very high reactivities with the example shown having a TON of nearly 2 million for Suzuki couplings.33 Role of Base and Solvent in Suzuki Couplings Palladium catalysis of organoboron and organosilicon compounds are unique among coupling reactions because of the need for a base to promote transmetallation. For Suzuki 34


couplings the role of base is complex due its role in activating the boron for transmetalation, promoting protodeboronation, and undergoing ion exchange w ith the palladium complex.8 Considering the role in transmetalation, the best base for a given reaction largely depends on the substrate and solvent employed. Whil e the influence of the base is not resolved, the central point is thought to be how well the anion binds with boron. This is rationaliz ed in terms of bond strengths of the B-X bond and sterics. The most fre quently used bases are alkali carbonates, with and without water, though phosphates, hydroxides, a nd fluorides have been used increasingly in the past decade. In terms of bond strength, B-O and B-F are the strongest reported, accounting for the effectiveness of the bases listed.34 However, sterics can play a key role; for example, t butoxides are effective for boronic ac ids but not pinacol bo ronate esters, presum ably due to steric hinderance of the base in bonding to the boronate ester. The counterion impacts the effectiveness of a base due to its role in promoting transmetalation as well as with the cation-base d by-product of transmetallation. First, how well the base binds with boron is affected by how we ll the anion is dissociated from the cation. This association/dissocation of ca tion and anion is termed the pairs st ability constant. In general, hard bases such as the oxygen and fluoride anions used in Suzuki reactions are more easily dissociated from soft acids (cations such as Cs+, Tl+, and Ba2+) than from hard acids (such as Na+, K+, and boronic acids). For this reason, cesium bases are generally mo re reactive than sodium or potassium bases.8 An additional role played by the cation is in its stability constant with the boronate ani on eliminated after transmetalation. When working with hindered boronic acids such as a mesityl boronic acid, research has shown that alkali hydroxides and carbonates were ineffective at Suzuki reactions run at room temperature while th allium hydroxide gave a 65% yield.35 The authors suggest that the boronate salt produced in the transmetallation step 35


Figure 1-11. Transmetalation of arylboronic acid with activation by thallium hydroxide. illustrated in Figure 1-11 is insoluble, helping to drive the catalytic cycle forward. A similar effect has been ascribed to an exchange of hydroxide/halide from the oxidative addition product (ArPdII[Ln]X, where X is a halide). Silver and thal lium cations bind strongly with halides, rendering them insoluble; this st ep also drives the reaction forw ard by preventing the return of the oxidative addition product to starting materials.36, 37 Unique Concerns for Macromolecule Synthesis The efficiency and broad applicability of Su zuki coupling has made it one of the most significant synthetic methods developed over the pa st thirty years, impacting researchers in natural products synthesis, medicinal chemis try, and organic electroni cs. When applied to polymers for organic electronics, new issues ar ise that are largely insignificant for organic chemist, yet are crucial for the polymer chemist. Improving the yield, reacti on time, and cost of a reaction is valuable for both small molecule and polymer syntheses, but these do not necessarily relate to the properties of the polymer. Determ ining the molecular weight, polydispersity, and molecular structure of a polymer allows one to tailor the propert ies of the polymer, but this requires one to address different aspe cts of the polymerization reaction. A Suzuki coupling of an organoboron with an organohalide follows a step-growth polymerization mechanism. One can desi gn the polymerization as a diboronate (A2) reacting with a second dihalide (B2) to yield a polymer or an AB monomer as shown in Figure 1-12. To achieve a polymer with a hi gh degree of polymerization (Xn) via a step growth mechanism 36


Figure 1-12. Schematic of A2/B2 and AB monomers for Suzuki polymerization entails certain restrictions. The basic statistics for this can be explained by Carothers equation (Figure 1-13). Carothers equation expresses how Xn depends on the extent of reaction (p), where No is the initial number of monomers presen t and N is the number of unreacted monomer. Effectively, p is the yield of the reac tion. Because of the relationship of Xn to p, high molecular weight is only achieved with very high yielding reactions.This necessitates the following criteria for an equilibrium stepgrowth polymerization: The monomers for an A2/B2 are precisely weighed out for stoichiometric balance Monomers are rigorously purified to assure stoichiometric balance The reaction is optimized for highest functional group conversion Side reactions are avoided that create a stoichiometric imbalance between functional groups. Figure 1-13. Carothers equation 37


The second half of this introduction analyzes the history of conjugated polymers and how the need for aryl-aryl coupling developed. To understand the vari ous routes pursued over the years, the basic principles of conjugated polymers as conducting polymers will be briefly reviewed. Optimization of electronic propertie s requires still another level of concern in conjugated polymer synthesis, introducing a number of unique issues for this class of macromolecules. From this, one can see how insight s into palladium catalysis have been gleaned from work on Suzuki polymerization and how th is polymerization is increasingly influenced by the more traditional organometallic studies. Soluble Conjugated Polymers and the Optimization/Development of Suzuki Polymerizations Early History and Properties of Conjugated Polymers Conjugated polymers couple the material prope rties of a rigid-rod polymer with the electroactive properties of a semiconductor. The basis of these electroa ctive properties can be understood in terms of the orbital mixing of p-or bitals aligned by the conjugated backbone. As shown in Figure 1-14 for polythiophene, increasing the conjugation length of the polymer lowers the LUMO of the polymer and raises the HOM O. With sufficient orbital mixing, the HOMOLUMO gap (or band gap [Eg] of a solid state material) reaches the range of a semiconductor (Eg < 4 eV). The energy level of the HOMO determines the oxidation potential, where increasing the energy of the HOMO makes the po lymer easier to oxidize. Convers ely, the energy level of the LUMO determines the reduction potential, wher e lowering the energy of the LUMO makes the polymer 38


Figure 1-14. Evolution of HOMO-LUMO ga p with the extent of conjugation for poly(thiophene). Reprinted with permission fr om Salzner,U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A. Synth. Met 1998 96, 177-189. easier to reduce. The energy levels of the HOMO and LUMO affects a polymers performance in electrochromic, capacitive, a nd conductive applications. Practical issues regarding polyacetylenes stab ility led researchers to extend this research to polyaromatics. Early research into polym ers such as polypyrroles and polythiophenes primarily focused on electropolymerization38 (electrochemical deposition of polymer on a working electrode via monomer oxi dation) and chemical oxidation39 affording intractable polymers. Variations of the monomer structure, such as heterocyclic analogues pyrrole and thiophene,40 electron donating/withdrawing functional groups,41 and polymer planarity42 revealed dramatic differences in electroactive properties. In addition to electrochemical phenomena, these structural modifications influence the HOMO-LUMO gap a significant propert y of conjugated polymers, for this plays a strong role in the optoelectronic proper ties. Figure 1-15 diagrams the photophysics for absorption and emission from a conj ugated material using molecules composed 39


Figure 1-15. Absorption/emission spectra for conjugated molecule with schematic of the photophysical processes. Reprin ted with permission from Gierschner, J.; Cornil, J.; Egelhaaf, H. Adv. Mater. 2007, 19, 173-91. of triphenyl moieties with different conformational fr eedom. When a photons energy corresponds to the energy diffe rence between the HOMO and LU MO levels, an electron is excited from the ground state (So) to the excited state (S1) with an energy of Evert(abs). Radiative decay of the excited electron leads to emission of a photon with an energy of Evert(em). With a rigid molecule, this transition occurs over a narrow energy band (Figure 1-15, right); however, with greater rotational freedom, th e vibrational energies are less defined leading to a broadened absorbance and emission. Differences betw een the equilibrium geometries for So and S1 lead to a relaxation of the mo lecule by a value of Eeq(abs). This difference in energy means that the emission spectrum [Evert(em)] is always red-shifted relative to the absorption spectrum [Evert(abs)].43 This difference is termed the Stokes shift. While these transitions are fundamental to all organic compounds, the extended conj ugation of a conducting po lymer brings these transitions into the visible/near IR range whic h makes them relevant fo r electrochromic, lightemitting, and photovoltaic applications. Research on conjugated polymers up to the mid-1980s made advances on the fundamental theory underlying th eir electroactive pr operties, but the fi eld discovered a 2nd 40


generation of materials with the advent of solubilized conjugated polymers. The first publication came from Sato et al who studied the influence of subs tituent size for electropolymerized 3alkylthiophenes.44 The authors found that with a substituent of hexyl or longer, the electropolymerized material was freely soluble, allowing traditional polymer processing and characterization such as casting films and GPC an alysis of the molecular weights. These first polymers were low MW (< 1 kDa); however, th is and subsequent research found that the electronic properties of a conjugated polymer saturated at much lower degree of polymerization than the material properties of tradi tional polymers. Later that year, Jen et al realized the potential of solubilized conjuga ted polymers by using Kumada c oupling of diiodinated, alkylated thiophnes with nickel as a catalyst.45 Much higher molecular weight s (3-35 kDa) were attained via this route, while still affording relatively high conductivities when doped. This research was a bellwether for the ne xt generation conjugated polymers where the electronic properties of a semic onductor are now coupled to the ma terial properties of a true polymer. In terms of applications, this opene d the way for new research into photovoltaics46 and light-emission47 which relied on charge mobility rather than conductivity. More subtly, by making soluble conjugated polymers, the synthetic work in conjugated polymers also changed in two major ways. First, these polymers could be characterized much more fully; NMR, GPC, and MALDI could now be used to understand the effect of molecular weight and molecular structure on polymer properties. This is most clearly evidenced by research on poly(3-alkylthiophenes) where well-defined polymers with 99% regioreg ularity defines the state-of-the-art for many applications.48 Second, the numerous synthetic routes to soluble conjugated polymers allow much better control of the polymer molecular structure as well as the ability to incorporate more intricate structures. 41


Suzuki Polymerizations for Conjugated Polymers: Optimization and Developments Suzuki polymerizations we re first studied by Rehahn et al in 1989 for the synthesis of poly( p-phenylenes),49 and since then have become one of the most widely used polymerization methods for conjugated polymers. There are a number of reasons fo r its widespread use. First, Suzuki polymerizations have ge nerally afforded higher molecular weights than parallel methods (e.g. Kumada, Yamamoto). 49, 50 Additionally, a traditional Suzuki coupling is tolerant of many functional groups, including ethers sulfonic esters, nitr iles, nitros, aldehydes, amides, amines, and protected alcohols.51 With modified conditions, es ters can also be employed.52, 53 Finally, Suzuki polymerization conditions ar e readily modified for atypical structures, such as conjugated polyelectrolytes.54 What follows is a review of Suzuki polymerizations that have optimized polymerization conditions, elucidated relavant mechanisms, and/or developed it for novel polymer structures. Optimization of Suzuki Polymerization for High Molecular Weight Conjugated Polymers The first Suzuki polymerization reported in the literature ach ieved a degree polymerization of approximately 30.49 While a degree of polymerization ra nging from 8-15 attains a limiting extent of conjugation, the material properties have shown a molecular weight dependence over a much larger range.55 For this reason research throughout th e 1990s studied variations of the traditional Suzuki coupling to improve the molecular weight. To this end, polymer chemists have studied the influence of boronic species, type of base, and catalyst. With few exceptions research in organic synthe sis generally uses boroni c acids due to their greater activity.10 With this, however, boronic acids bring difficulties due to self-condensation to form boroxines, residual water always associated with boronic acids, and loss of boronic acids due to deboronation during the reaction. To avoi d these problems, one adds 0.2-0.5 equivalents excess of boronic acid. Because stoichiometric balance is important for Suzuki polymerizations, 42


adding extra diboronic acid is not an option. Thus, polymer chemists have devised other routes to successfully use a Suzuki polymerization. In the initial work by Schluter and Wegner, traditional Suzuki conditions were used with a diboronic acid and dibromid e as shown in Figure 1-16.49 To attain stoichiometric balance, the authors used 1H NMR to calculate the amount of wa ter mixed with the diboronic acid. Additionally, to accoun t for other causes of stoi chiometric imbalance, th e authors later found that trial polymerizations should be run to determine the best ratio of comonomers.51 In later work, Wegners group developed a highly effective though tedious method for improving polymer molecular weight by slowly addi ng the boronic acid for 8-10 days.55 Under these conditions, polymers with Mw=137 kDa were synthesized from the dibor onic acid. In this and later work, Wegners group also modified the reaction by us ing a 1,3-propanediol bo ronate ester in a polymerization.56 While boronate esters are generally cons idered less reactive than boronic acids, boronate esters have the advantage of easie r purification, no self-condensation, and reduced hygroscopicity. The authors found that prolonged reaction times could somewhat account for the lost reactivity. With boronat e esters, polymers with Mw>100 kDa were formed. Using boronate esters rather than boronic aci ds is now the standard method for Suzuki polymerizations. Much like work by Anderson et al ,35 research by Kowitz et al found that the type of base and catalyst used has a significant eff ect on a polymers molecular weight.57 Figure 1-17 summarizes a portion of their data. With Pd(PPh3)4 as a catalyst at high temperatures, the data Figure 1-16.Poly(2,5-dihexylphenylene) the first application of a Suzuki polymerization. 43


B B C6H13 C6H13 O O O O + B r Br NO2 O2N C6H13 C6H13 NO2 O2N n Figure 1-17. Effect of base and catalyst on the Suzuki polymerization of 1,5-dibromo-2,4dinitrobenzene. follows that found by Anderson et al where the strongest base im proves the reaction efficiency; however, with PdCl2(dppf) as a catalyst at low temperatur es, this trend does not hold and the weakest base gives the highest molecular weight. Different reaction mechanisms are apparently at work, due in part to the decomposition of 1,5-dibromo-2,4-dinitrobe nzene under the reaction conditions. This is especially evident from the authors comment that even a few drops of toluene in the polymerizations with PdCl2(dppf) only formed oligomer. When using heterocycles, boronic acids and es ters have a well-documented problem with protodeboronation. Work by Rene Janssens gr oup has extensively studied the Suzuki polymerization of diboronic thiophenes in order to increase their molecular weights. In 2001, Janssen et al synthesized a thiophene -phenylene copolymer as shown in Figure 1-18.58 The particular focus of this work was how the st ructure of the boronic species influenced the molecular weight. The authors found that in every case, Pd(OAc)2 yields polymers with twice the molecular weight of polym erizations with Pd(PPh3)4. By changing the R group on the boronic species, they also found that molecular wei ght increased in the following manner: H < C3H6 < 44


C6H12. This trend was attributed to the greater stability of boronate esters toward deboronation; the pinacolato was said to be more stable than the propyl bridge, because the steric bulk of the former torques the vacant boron orbital out of plane with the lone pair of electrons of thiophenes sulfur. This explanation is speculative because no reference was given for this as the cause of the deboronation and the crystal structure of the pi nacolato derivative is not known. Other work by Janssens group optimized the Suzuki polymerization of a monobromo/monoboronic functionalized 3-hexylthiophene.59 The authors found that traditional triarylphosphines produced very low molecular weights (Mw: 2-11 kDa), but that PdCl2(dppf) and Pd(OAc)2 under ligandless conditions produced relatively hi gh molecular weights with Mw up to 85 kDa. Figure 1-18. Suzuki polymerization of thienyl boronic species with 1,4-diiodo-2,5dialkoxybenzene. Controlling Conjugated Polymer Molecu lar Structure via Modified Suzuki Polymerizations Shortly after the first reported Suzuki polymerization, researchers began using its versatility and broad functional group compatability to better control the polymers molecular structure. Some of this work has optimized the polymerization to decrease defects and polydispersities while others have been directed towards novel materials. 45


A significant contribution to the understanding of palladium cat alysis comes from research by Bruce Novak on the aryl exchange between triarylphosphines and th e monomer when both are bound to palladium. 60 This effect had essentially gone unnoticed by organic chemists, because it affects less than 1% of the monomer units; however such side reactions are incorporated into the main chain of the polymer product, making this effect more easily observed. A number of possible r outes were postulated by Novak, but the most interesting defect is the incorporation of phosphorous into the polymer chain, as s hown in Figure 1-19. While this type of defect was calculated to occur in 1 out of 4 polymer ch ains, applications sensitive to impurities such as light emission and photovolatic s could be deleteriously affected by such defects. The most recent discovery related to Suzuki polymerizations is by Yokozawa et al on the chain growth behavior of certain catalysts.61 The basis for this polymerization is a JACS communication62 that found the tri( tbutyl)phosphine ligand led to difunctionalization of a dihaloarene as opposed to monofunctionalization (a s shown in Figure 1-20). The authors explain this behavior by the high reactivity of the bulky, alkyl phosphine which makes the reaction a diffusion controlled processs. Yokozawa et al employed this type of sy stem for the first chaingrowth, Suzuki polymerization of a m onobromo/monoboronate ester of a 9,9-dioctyl Figure 1-19. Incorporation of phosphorous into the polymer backbone by aryl exchange. 46


fluorene.The authors found that the tri( tbutyl)phosphine ligand led to a polymerization whose kinetics showed a linear increase of molecula r weight with monomer conversion as well as a polydispersity less than two (P DI:1.3-1.4), both of which are indicative of a chain growth mechanism. Because oxidative additi on of the catalyst is diffusion limited, the catalyst is always associated with the polymer chain end, which acc ounts for the chain-growth kinetics. While this gives the same molecular structure as a pol yfluorene synthesized by traditional Suzuki conditions, this work provides a unique mech anism for possibly synthesizing conjugated block copolymers. With the discovery of high regioregular ity in the GRIM synthesis of poly(3alkylthiophenes), a number of researchers saw the potential for th e Suzuki polymerization as a viable route to 100% regioreg ular poly(3-alkylthiophene). While Kumada and oxidative polymerizations suffer from problems with head-h ead (H-H) and tail-tail (T-T) coupling creating defects in the polymer chain, the mechanism of the Suzuki polymerization theoretically can only give head-tail (H-T) coupling, as shown in Figure 1-21. Janssens group studied a wide range of catalysts and bases to determine which conditions led to the best regiore gularity for a 2-bromo-3hexyl-5Br Br + B(OH)2 2.5%Pd2(dba)3/10%ligand K3PO4,THF,RT Br + L i g a n d PPh3P(t-Bu)392% 8% 2% 98% Figure 1-20. Diffusion-limited oxidative addition of Pd/P( t-Bu)3 47


pinacoloatoboronate-thiophene. Th e authors found that with pot assium carbonate as base and Pd2(dba)3/P( o-tolyl)3 as the catalyst, they could attain high molecular weight (Mw= 31,500) with a regioregularity of 96% after fractionation. While this regioregularity is high, the source of 4% defects is curious considering the mechanism. One explanation offered by the authors is aryl exchange with phosphines. Another likely possibili ty not discussed by the authors is oxidative addition of palladium to the C-B bond, 8 from which self-coupling of boronic species could occur.63 A number of researchers have also used modified versions of the Suzuki polymerization to synthesize directly conjugate d polyelectrolytes. Due to the f unctional group compatability of a Suzuki coupling, monomers with polar groups such as carboxylates and sulfonates are readily coupled. Bruce Novaks group firs t reported the in aqua synthe sis of a water-soluble poly(pphenylene) functionalized with carboxylic acids.64 The authors synthesized this polymer by running the polymerization in a basic aqueous solution with a wate r soluble palladium catalyst. A rough estimation of the molecular weight via poly(acrylamide) gel electrophoresis gave an approximate measurement of Mw=50 kDa. Subsequent work by the Reynolds group extended this route to the synthesis of sulf onatoalkoxy-substituted poly(p-phenylene).65 S Oct S Oct S S S Oct Oct Oct T-T H-T H-HS Oct PdoBr B O O nS Oct n FeCl3base Figure 1-21. Regioregularity of oxidative poly merization versus a Suzuki polymerization. 48


This selected set of studies of Suzuki pol ymerizations highlights the extent to which synthetic polymer chemists have made advances in controlling the molecular structure and molecular weight of conjugated polymers. For all of these insights, however, the vast majority of conjugated polymers published in th e literature still use the synt hetic design of A. Suzukis original work,10 although now using boronate esters opposed to boronic acids. This is likely due to the broad applicability of these conditions and how they generally afford reasonably high molecular weights (Mn> 10 kDa) Thirty years of resear ch on Suzuki couplings have proven, however, that no one set of conditions is universally optimal. Thesis This dissertation focuses on the synthesis of fluorene-containing conjugated polymers via Suzuki polymerization and the characterization of these conjugated materials for use in light emitting, electrochromic, and energy transfer applications. Fluorene-based homoand copolymers have been integral to the developmen t of organic-based light-emitters, photovoltaics, and sensors, due to their facile synthesis and uni que material/electronic properties; however, this progress has been limited by the narrow range of accessible polymers with varied functionality. The goal of this research has been to devel op new synthetic routes and methods for fluorenebased polymers with new functionalities, as well as assess the resulting material/electronic properties. Chapter 3 has a brief introduction to polyfluorenes and presents a family of fluorenebased copolymers with the emissive propertie s of a polyfluorene homopolymer but with improved electrochemical and electrochromic prope rties. Chapters 4 a nd 5 introduce conjugated polyelectrolytes and studies fluor ene-based copolymers as polyele ctrolytes functionalized with carboxylic acids. This research required the de velopment of a number of new chemistries including a base-free Suzuki pol ymerization and a family of carboxylic acid functionalized 49


50 polymers derived from thermally cleavable es ters. Chapter 6 presents further work on poly(benzodithiophenes) as synthetic analogues of polyfluorenes. The design and synthesis of these materials is founded on the interplay of organic synthesis and lessons learned from the characterization of fluorene-based polymers, and by utilizing the research in transition metal catalysis over the past 15 years, these methods have been adapted to polymer synthesis, allowing the syntheses of previously unattainable polymers.


CHAPTER 2 EXPERIMENTAL METHODS This chapter provides an account of the inst rumentation used to synthesize, purify, and characterize the materials studied in this disser tation. Additionally, the methods I have developed for handling reagents and catalysts, setting up sp ecific reactions, and ch aracterizing monomers and polymers are discussed. Details for the synthe ses of specific compounds are provided in the respective chapters. Molecular Characterization Compounds were characterized by 1H NMR and 13C NMR using a VXR 300 FT-NMR, Gemini 300 FT-NMR, Mercury 300 FT-NMR, and Mercury 300-BB FT-NMR. Additional characterization was provided by high resolu tion mass spectrometry with a Finnigan MAT 96Q mass spectrometer, measured by services at the De partment of Chemistry at the University of Florida. Purity of small molecules was determined by melting point and elemental analysis (carbon, hydrogen, and nitrogen) meas ured by services at the Department of Chemistry at the University of Florida, Robertson Microl it Laboratories, and Atlantic Microlab. Polymer Characterization Structural Characterization All polymers were characterized by 1H NMR and 13C NMR using a VXR 300 FT-NMR, Gemini 300 FT-NMR, Mercury 300 FT-NMR, and Mercury 300-BB FT-NMR. For neutral polymers, deuteratred chloroform or benzene were used for room temperature measurements, and deuterated DMSO or 1,2-dichloroethane were used for high temperature measurements. For nonionized polyelectrolytes, deuterated metha nol, THF, or DMSO were used. Elemental analyses of polymers (carbon, hydr ogen, and nitrogen) were m easured by services at the Department of Chemistry at the University of Florida, Robertson Microlit Laboratories, and 51


Atlantic Microlab. Fourier transform infrared (FTIR) measurements were made on a Bruker Vector 22 infrared spectrophotometer. Samples we re prepared as polymer films cast from THF onto KBr plates. Polymer molecular weights were measured on two gel-permeation chromotgraphs (GPC). Measurements for polymers from Chapters 3 and 6 were performed on two 300 x 7.5 mm Polymer Laboratories PL Gel 5 M mixed-C columns with a 2996 photodiode array detector, measuring in the ultraviolet/visibl e light/near infrared range. A constant flow rate of 1 mL/min was used. Molecular weights were obtained relative to polystyrene standards (Polymer Laboratories, Amherst, MA). Polymer soluti ons (0.5 mg/ml) were prepared in THF or chloroform. Measurements for polymers from Ch apter 4 and 5 were determined by a Waters Associates GPCV2000 liquid chromatography system with an internal differential refractive index detector (DRI) and tw o Waters Styragel HR-5E co lumns (10 m PD, 7.8 mm i.d., 300 mmlength) at 40 C. HPLC grade tetrahydrofuran was used as the mobile phase (flow rate ) 1.0 mL/min). Retention times were calibrated agains t polystyrene standards (Polymer Laboratories, Amherst, MA). Thermal Characterization Polymer thermal transitions were analyzed by differential scanning calorimetry (DSC), temperature modulated different scanning calorimetry (MDSC), a nd thermogravimetric analysis (TGA). DSC and MDSC measure the same phe nomenon, though the former uses a constant heating rate while the latter superimposes on this rate a sinusoi dal temperature modulation; this modulated method allows one to improve the endotherms baseline, to measure the sample with greater sensitivity, and to extr act the thermodynamic and kinetic aspects of the endoand exotherms.66 DSC and MDSC were performed on a TA Instruments Q1000 equipped 52


with a liquid nitrogen cooling accessory calibrate d using sapphire and high-purity indium metal. All samples were prepared in hermetically sealed pans (4 7 mg/sample) and were referenced to an empty pan. A scan rate of 10 C/min was used unless otherwise specified. Modulated experiments were scanned with a 3 C/min linear heating rate with a modulation amplitude of 0.4 C and period of 80 s. TGA data was obtained with a TA series thermal analysis system. The TGA samples (2 5 mg) were typically heated to 80 0C to equilibrate to a constant mass, then heated at a rate of 10 0C/min up to 600 oC. Additionally, isothermal gravimetric analysis was used to determine the material stability and to study thermolytic reactions in the solid state. These experiments were performed by heati ng samples for 8 hours at a temperature 10-50 oC below the onset for decomposition. Electrochemical Characterization Electrochemistry provides a means of estimating the half-wave potential (E1/2), HOMO level, and LUMO level of a polymer, as well as offering potential electrochromic applications. All polymers studied in this dissertation that do not have previously reported electrochemical data have been characterized by electrochemical measurements. Cyclic voltammetry has been used to determine the polymers half-wave potential, and differential pulse voltammetry has been used to determine the onset of oxidation when the onset is poorly defined by cyclic voltammetry. A three electrode cell was used in all electrochemical measurements. Platinum button or ITO slide was used as the working electrode a nd silver wire was used as a pseudoreference electrode. A platinum flag was used as a counter-electrode when making CV or DPV measurements, and a platinum wire was used wh en performing spectroelectrochemistry. An 53


EG&G Princeton Applied Research Model 273 pote ntiostat was used under the control of Corrware II software from Scribner and Associates. All potentials are calibrated relative to the ferrocene redox couple. To determine the oxidation potential of a material relative to vacuum, the oxidation potential of ferrocene was taken as 4.8 eV vs. vacuum. In recent years this value has been a point of contention, with some re searchers claiming the true value as 5.1 eV.67 While this correction appears justified, the vast majo rity of the conjugated polymer field continues using 4.8 eV. To maintain a valid comparison between polymers reported herein and polymers reported in the literature, this uncorrected valu e was used. Characterization of the polymer films was performed in 0.1 M electrolyte solution. Th e electrolyte solutions were prepared from tetrabutylammonium perchlorate (TBAP) or tetrabutylammonium hexafluorophosphate (TBAPF6) electrolytes dissolved in freshly disti lled acetonitrile (ACN), benzonitrile, or propylene carbonate (PC). For performing elect rochemistry of polymers in solution, a 0.1 M electrolyte solution was used with a 10 mM con centration of polymer (re lative to repeat unit). Degassed and strictly anhydrous solvents were used when the oxidation of water interfered with the electrochemi cal experiments. To assess this, a background scan of a bare electrode with the electrolyt ic solution is run across a br oad potential window (-0.5 2.0 V relative to silver wire). Extraneous peaks are noted, and the presence/inte rference of these peaks is watched for during polymer characterization. A dditionally, if reductive or oxidative processes appear irreversible, the experiment was repeated with degassed a nd strictly anhydrous solvents to ensure the irreversibility is not due to water or oxygen. Meth ods for purifying solvents and electrolytes are discussed under th e section for synthetic methods. 54


Spectral Characterization The optical absorbance and fluorescence of a conjugated polymer is an important property for many applications (e.g., displa ys, photovoltaics, sensors); additi onally, these data can reveal other important properties about a material, su ch as the conformation of the backbone, the polymers aggregation in the solid st ate and solution, and orbital energies.42, 68 In particular, solution thermochromism (change in polymer optical absorbance and fluorescence with temperature) and solution solvatochromism (change in polymer optical absorbance and fluorescence with solvent polarity) were used to study the effect of polymer conformation and aggregation on the spectral properties. These measurements (and the interpretations thereof) have significantly informed this research. Instrumentation. UV-Vis-NIR absorption spectra were measured on a Varian Cary 500 scan UV-vis-NIR spectrophotometer in scan mode using baseline correction. Solution spectroscopy was measured with quartz crystal cells (1 cm x 1 cm x 5.5 cm, Fisher). A SPV 1x1 Varian Cary dual cell peltier accessory was used to perform the solution thermochromism experiments. Photoluminescence measurements we re made with a Fluoromax spectrofluorimeter in right-angle mode. Experimental Details. Solution thermochromism is a method for examining how the dissolution and conformation of a conjugated polymer affects the spectral properties. Using the Peltier accessory, one can va ry the temperature from 5-95 oC. To avoid solvent evaporation at elevated temperatures, xylenes were used as a solvent. These measurements were made by progressing from the lowest to th e highest temperatures, and test ed for reproducibility on cooling back to room temperature. Solution solvatochromism is another met hod for examining how the dissolution and conformation of a conjugated polymer affects the spectral properties. For these experiments, 55


Table 2-1. Useful solvent mixtures for studying solution solvatochromism Hydrophobic Conjugated Polymers H ydrophilic Conjugated Polymers Good solvent Poor Solvent Good Solvent Poor Solvent Chloroform Hexane THF Water Benzene Methanol THF Water (pH>9) THF Hexane THF Hexane mixtures of a good and poor solvent are used for solutions of a given polymer, and the gradient of solvent quality is used to assess the effect s of aggregation and polymer conformation. Table 21 gives a list of solvent mixtures that have shown solvatochromic effects for conjugated polymers. Assuring the reproducib ility of these experiments is paramount, because thought provoking but erroneous data is easily acquired. To ensure reliable data is obtained, dilution from a stock solution of precise concentration is used. This dilution is demonstrated in Figure 2-1. An Eppindorf pipetter was used for these dilutions, b ecause cumulative errors from reading menisci on graduated glassware easily l eads to irreprodu cible results. Figure 2-1. Preparation of polymer solu tions for solvatochromism experiments. 56


Suppliers All solvents were purchased from Aldrich a nd Acros. All reagents and starting materials were purchased from Aldrich, Acros, and Alfa Ae sar. All catalysts and ligands were purchased from Strem, except palla dium (II) chloride, 4,4-ditert -butylbipyridine, and triphenyl phosphine which were purchas ed from Aldrich. Purification of Solven ts/Reagents/Catalysts All solvents were used as received unle ss otherwise specified. Reactions cited as anhydrous used anhydrous solvents and reagents purchased from Aldrich or Acros with containers fitted with septa, except THF, toluene, methylene chloride, heptane, and diethyl ether which were taken from a MBraun Solvent Delivery System, and triethyl amine and benzonitrile which were distilled according to literature procedures.71 All reagents and catalysts were used as received unless otherwise specified. Other methods were also used to characteri ze the spectral properties of the conjugated polymers reported in this disse rtation. Solution doping of conjugated polymer solutions was carried out with methylene chloride as a solven t and antimony (V) chloride as the oxidant. The concentration of the polymer solutions were generally 10 M. Quantum efficiency was also measured for select polymers. The quantum effi ciency is a measure of the efficiency of a polymers photoluminescence, and is an impor tant property of a polymer. The quantum efficiency was measured according to the proc edures outlined in the following references.69, 70 Synthetic Methods These last sections briefly discuss the sour ce, purification, and handling of the solvents, reagents, and catalysts used in this work. Additionally, basic procedures for handling anaerobic and/or anhydrous reactions are discussed. 57


Handling of Catalysts The procedures for handling comm on organic reagents such as n-butyl lithium or thionyl chloride are well-documented in the literature;72, 73 however, the proper methods for handling catalysts are not. While this is to some degr ee a matter of cost and savings, there are also scientific impacts in terms of reproducibility of experiments and the statements made about them. For this need, the following is a brief outli ne of how to best handle the palladium and iridium based catalysts used in the synthesis of conjugated polymers. A general rule of thumb regarding palladium catalysts is that palladium (II) species are effectively air stable and require no special handling. PdCl2, Pd(OAc)2, and PdCl2(dppf) are not hygroscopi c or a reducing agent for oxygen. Palladium (0) species do require speci al handling and precautions, because they are generally easily oxidized in air. Due to genera l thermal and oxidative instabilities of Pd(PPh3)4, this compound is most effective when freshly prep ared according to literature procedures. (This was prepared by dissolving PdCl2 in DMSO at ~125 C with 8 equivalents of triphenyl phosphine. Reduction by slow addi tion of hydrazine gives a bri ght yellow precipitate. After cooling, ethanol was added to more fully precip itate the catalyst and the yellow catalyst was recovered by filtration. Benzene and methanol (5 :1) were used to recrystallize the catalyst, giving a yellow crystalline solid.) Pd2dba3 is more stable, but still requires the flask to be backfilled with argon. A simple procedure for doing this is momentarily replacing the containers cap with a septum, flushing the container with one needle acting as an argon inlet and a second needle venting argon and residua l oxygen. After purging the cont ainer with argon for fifteen seconds, the septum is replaced with the cap and tightly sealed with parafilm. The same procedure is used with iridium, though it is even more significant due to its greater instability under atmospheric conditions. 58


General set-up of Suzuki polymeriza tions and Pd-catalyzed borylations Numerous procedures for runni ng a Suzuki polymerization are reported in th e literature with very few guidelines for how to set it up. Wh ile these procedures all contain the same basic components, the nuances of any given polymerization are unique. Organic chemists working with small molecules use an excess of base a nd boronic acid, allowing great leeway in setting up the reaction; however, the stoi chiometric balance required for a Suzuki polymerization necessitates special procedures at times. The following guidelines address various issues in running a Suzuki polymerization. General set-up. All polymerizations are run in a single neck RBF with a volume approximately full after all components are added to avoid polymer precipitation on the glassware. (With too little solvent, the polymer tends to precipitate onto the top of the flask; with too much solvent, the polymer tends to precip itate onto the condenser.)The diboronate ester and dihalide are weighed out to th e best precision possible yet prac tical, aiming for at most 1% variance from stoichiometric balance for a balances precision. The base is weighed out to the given amount, opting for excess if any varian ce from the calculated amount. These three components are placed in the RBF and pur ged with argon at least three times. To this flask, catalyst with ligand is added, a nd the flask is again pur ged at least two more times. The inlet adapter is quick ly replaced with a septum, and solvents are added via syringe. While transferring solvent, a conde nser is connected to the inle t adapter and th e condenser is flushed with argon and the joint greased. After a dding solvent, the septum is quickly removed, and the condenser is placed on the flask. The flask is then heated to reflux with stirring for 40-72 hours. Degassing solvents. There are two options for degassing solvents: 1) freeze-pump-thaw 2) bubbling argon for 30+ minutes. While the former is considered more thorough and assured, the 59


latter works well for nonpolar organic solvents; ho wever, I have noticed that polar solvents (DMF, DMAC, DMSO, water) are difficult to de gas by simply bubbling argon for a short time. To address this, I have used three different options: 1) freeze-pump-thaw 2) bubbling argon for 1.5 hours while refluxing the solvent 3) pulling v acuum on uncondensed solvent for 15 minutes. The first option is universal. The second option ha s been used with water very effectively, and would presumably work with other solvents (tho ugh they have much higher boiling points). The third option works well for hi gh-boiling solvents (> 150 oC) where the reaction flask with all components except catalyst is fitted with an in let adapter, and the solution is held under full vacuum for 15 minutes with stirri ng. A trap can be used to a void pulling solvent into the pump. After purging with argon one additional time, th e catalyst is added and the reaction is run. The vigorous bubbling noted for the first 30 seconds is not solvent, it is solubilized gasses; these are the most volatile components of the solution. It is worth noting for the insightful, that this is essentially a variation of freeze-pump-thaw w ithout the tediousness of pumping and thawing. Hygroscopic salts. Most of the bases and pseudobases used in this research for Suzuki couplings and Pd-catalyzed boryla tions are very hygroscopic, and in the case of the latter, the presence of water can have detrimental effects. Fo r this reason, these salts can be stored in an oven and weighed out while still hot. These are then quickly transferred to a flask to be stored under argon. Additionally, one can use a dry box to add these components to the flask. For tetrabutylammonium fluoride, one cannot remove water from them and they are sold with 1520% water.74 Oily/liquid monomers. These monomers have two diffi culties. First, these compounds cannot be freed of oxygen by simply cycling be tween vacuum and argon, especially when the viscosity of these compounds is high. Second, they are extremely difficult to weigh out reliably 60


61 to ensure stoichiometric balance. To avoid th ese problems, the following procedures were used. First, the monomer is placed unde r vacuum with heating at ~60 oC overnight prior to running the polymerization. The flask is cooled to room te mperature, and the react ion flask is tared on a balance. The oily/liquid monomer is added direct ly to this flask, and the amount added is then used to determine the amount needed for the co monomer. With both monomers in the flask, the organic solvent is added, and the reaction solution is purged with argon for 30+ minutes. It is worth noting that the base is not added at this point to avoid protodeboronation of the diboronate ester. The reaction is then asse mbled as previously described. Conversion of ester-functionalized polymers to carboxylic acids Two different ester structures were used in this research, each differing in the conversion to carboxylic acids. Esters with n-alkyl substituents are converted to carboxylic acids via base hydrolysis, and esters with tert -alkyl substituents are converted to carboylxic acids via thermolysis. This section provides representati ve procedures for conduc ting these reactions. Base Hydroylsis. 4 mL of a ~0.3 M solution of polyme r in 1,4-dioxane is added to 2.5 mL of 1.5 M KOH. This solution was heated at 95 oC overnight. The dioxane was removed by rotary evaporation, and the pH of the a queous solution was adjusted to pH 4. The precipitate was collected by filtration, washed with copious amount of water, and dried under vacuum. Thermolysis. A solution of the polymers ester derivative was dissolved in a low-boiling solvent, and the solvent removed by rotary evaportation. This shoul d leave a film of the polymer on the walls of the flask. This flask is placed under vacuum and purged three times with argon. The flask is then heated under vacuum to a temp erature and for a time determined by isothermal TGA studies. The flask is cooled to room te mperature under argon, and the polymer recovered from the flask.


CHAPTER 3 HIGH BANDGAP POLYMERS FOR DUAL PROPERTIES IN ELECTROCHROMIC AND LIGHT EMITTING APPLICATIONS Introduction As discussed in the Introduction, the synthetic methods no w available for aryl-aryl bond formation open up new directions for design ing conjugated polymer s for optoelectronic applications. By coupling the electroactive properties of inorganic semiconductors with traditional synthetic and processing methods of polymer, these materials are thought to proffer the next generation of electronics. To this e nd, research on conjugated polymers is generally directed towards understanding fundamental propert ies and/or optimizing material performance for a given application with the ultimate goal of replacing inorganic semiconductors; however, this overlooks the more accessible and unique role conjugated polymers could play. This point was recently addressed in an editorial segment by M. Berggren et al for Nature Materials that asserts the commercial viability of organic electr onics does not lie in optimized performance in a specific device, but acceptable performance in multifunctional devices.75 Let us define science and technology approaches to develop [organic electronic materials that work in multiple applications (e.g., data processing, remote input/output transfer of data, information display) and that require a minimum number of processing materials and steps]. It is in this context that organic el ectronic materials disti nguish themselves from their inorganic counterparts. Typically, in in organic electronic technology, each individual type of device requires its own set of dedicated materials, doping compounds, conductors, encapsulation technology and so on. This approach is acceptable in a batch-based production flow in which labour and manufactur ing technology are the major costs in the resulting product. In the printing industry, th e cost structure is completely different; the primary costs are the inks, paper substrate a nd transportation. This contrast in cost structure will certainly have a significant imp act on what we expect from the materials. In the case of organic electronics th e materials simply must do more. Recent work by the Reynolds group embodies this principle with a new device model that serves as both an electrochromic and light-e mitting display that can operate under different lighting conditions.76 As the active material in an electroc hromic device, the conjugated polymer 62


absorbs different wavelengths of light depending on the oxidation state of the polymer.41 With strong ambient lighting, the electrochromic device provides a good display. As the active material in a light emitting device, the polymer emits light, providing a good display under poor lighting. By coupling the properties of an elect rochromic and light emitting device into a single unit called an EC/LEC, the display can be used in any setting; however, such a device requires much more of the active material, requiring two different properties to be optimized. Tailoring both the absorptive and emissive properties is a challenge in developing materials for an EC/LEC device. Because the colors of an electrochromic display are a subtractive color system and a LED is an additive color system,77 one conjugated polymer cannot easily provide the same color for both functions, t hus this hybrid device forces one to reconsider the design of the molecular structure. Buildi ng from previously published theoretical studies,78 the approach of this project is to copolymerize monomers (fluorene, carbazole, pyrrole) whose homopolymers have the same bandg aps but different oxidation potentials. According to these computational studies, copolymerization should e ffectively give an averaging of the HOMO and LUMO levels of the comonomers. This will give a family of fluorene-based high bandgap polymers (Figure 3-1) that maintain the dist inguishing blue fluore scence of a fluorene Figure 3-1. Family of fluorene-base d, high bandgap conjugated polymers 63


homopolymer while modifying the (spectro)elect rochemical properties. To provide a background for how the polymers were designed, the basic pr inciples of conjugated-polymer based LEDs and electrochromic devices will be discussed, followe d by the synthesis and characterization of these fluorene-based copolymers. Electrochromic Materials and Devices The band gaps of conjugated polymers ar e generally on the order of 0.8 3.0 eV, characterizing them as semiconducting materials. Charged states may be introduced by chemical reduction/oxidation (via alkali me tal or antimony pentachloride, re sepctively) or electrochemical reduction/oxidation. Focusing on oxidative processes, the sequential loss of electrons can be described as the formation of radical cations a nd dications. This is shown in Figure 3-2. Because polyarylenes have a non-degenerate ground stat e, oxidation of the polymer chain induces significant conformational changes in the polymer chain due to loss of ar omaticity within the charged segment.79 While there is a significant energy gain in this benzoidal to quinodal change, the accompanying relaxation energy of the charged state favors this transition. The coupling of the charged state to the lattice/structural defo rmation defines the radica l cation and dication as polarons and bipolarons, respectively. Formation of polarons and bipolarons introduces states in the forbi dden region between the valence and conduction bands, giving rise to lowe r energy transitions in a polymers absorbance spectra.2 A good example of this is illustrated in Figure 3-3 fo r a an electrochromic device incorporating two dioxyt hiophene derivatives.80 The onset of the to transition of the neutral state is at 1.6 eV. As the polymer is oxidized electrochemically, this absorbance decreases and a lower energy transition at 1.0 eV grows in. This tr ansition is due to the polaronic state. Further 64


S S S S O O O O O O O O S S S S O O O O O O O O S S S S O O O O O O O O S S S S O O O O O O O O S S S S O O O O O O O O S S S S O O O O O O O O [ox] [ox] Figure 3-2. Oxidation of an oligo(3,4-d ialkoxythiophene) to the dication state. Figure 3-3. Spectroelectrochemistry of a PEDOT de rivative on ITO glass at applied potentials from -0.7 to 0.6 V (vs. Ag/Ag+). Reprinted with permission from Schwendeman, I.; Gaupp, C. L.; Hancock, J. M.; Groenendaal, L.; Reynolds J. R. Adv. Func. Mat. 2003 (13) 541-547. 65


oxidation of the polymer shows a lower transiti on into the IR region which is due to the bipolaronic state. By incorporating such a materi al into a device that can change the polymers oxidation state, the device can be used as an spectral-absorption based display. Light-emitting Materials and Devices As described in Chapter 1, fluorescence results from the radiative decay of an electron from an excited state to a ground state. This de scription is of photolum inescence, the excitation of an electron by absorption of a photon. In a light-emitting device, light is generated by electroluminescence, where the excited state is produced by injecting positive (holes) and negative charges (electrons) into the active ma terial. Applying a potential across the active material leads to charge injec tion and mobility through the film.81, 82 These charges couple and form an exciton. Radiative decay of the exc itons results in light emission. For optimal performance, a light-emitting conjugated polymer should have limited non-radiative transitions and limited interchain interactions that can quench the fluorescence.75 The most basic device structure is a light-emitting diode (LED) which sandwiches the active material between a cathode and anode, where the latter is generally a transparent electrode. As described above, a pplying a potential (greater than the polymers bandgap) across the active material leads to light emission. A mo dification of this structure is a light emitting electrochemical cell (LEC), which is used in the EC/LEC hybrid device.76 While the basic structure described above is used for an LEC, the active material is blended with an electrolyte and a second polymer to improve the electrolytes miscibility.83 The electrolyte is thought to stabilize the injected charges, particularly at the conjugated polyme r/electrode interface, providing an ohmic contact.84 66


Design of Fluorene-Based Copolymers One of the most widely studied polymers for light-emitting applications is polyfluorene, and poly(9,9-dioctylfluorene) in particular. Polyfluorenes emit light with a blue color ( max=420425) and an absorbance bridging the UV and visible ranges, givi ng the polymers a faint yellow color.85 While polyfluorenes are impo rtant for providing a blue fluor escence (one of the three primary colors), the efficiency of its emission in solution and the solid st ate distinguish it from other conjugated polymers. The solubilizing chains connected at the 9 position extend above and below the plane of the monomer as shown in Fi gure 3-4; these chains sterically hinder the stacking between polymers whic h can quench excited state.86 This has led to fluorenes usage in a wide number of copolymers where coupling with different heterocyc lic units shifts the polymers absorbance and fluorescence bathochromically. For light emitting applications, polyfluorenes work well; howev er, for applications relying on electrochemistry, they are impractical due to their high oxidation potential.87 Additionally, polyfluorenes frequently have irreversib le electrochemical behavior making their characterization difficult.88 These factors make polyfluorenes impractical for the EC/LEC device. Other homopolymers have the same approxi mate bandgap and similar fluorescence of a polyfluorene while providing more accessible ox idation potentials. As an electroluminescent material, carbazoles have been widely studied both as homopolymers and copolymers. A great Figure 3-4. Conformation of 9,9-dibut ylfluorene by MM2 calculation. 67


part of this interest is the structural similar ity of carbazoles to fluor enes. In 1996, the first LED using a polycarbazole was made, giving a rela tively low EL intensity, with later studies improving the device performance and improving the quantum yield to 0.2.89, 90 Random copolymers with carbazoles have also been synthesized. 3, 6-Dibromo-carbazole and 2,7dibromo-fluorene have been polymerized by Ya mamoto coupling, giving a stable blue light under ambient conditions. In relation to the EC/LEC device, carbazoles have also been used with interesting results as an elec trochromic material. Early work on polycarbazole homopolymers found that they show three distinct colored states a colorless neut ral state, a green intermediate, and a blue fully oxidized state.91 This effect is most clearly seen in the work on 3,6-bisEDOTcarbazole derivatives synt hesized by the Reynolds group.92 As with the carbazole homopolymers, two oxidation states were obser ved for these polymers. Interestingly, when BiEDOT was electrochemically copolymerized with a bisEDOT-carbazole derivative, the absorbance spectra of the neutral polymers showed that as the proportion of carbazole in the polymers increased, max shifted to shorter wavelengths, c overing a range from 420-570 nm. This effect can be attributed to the effect of the carbazole acting as a conjugation break along the polymer backbone. A second conjugated polymer with a high ba ndgap (~3 eV) and low oxidation potential is polypyrrole. Polypyrrole has been widely studied since the mid-80s though primarily as an electropolymerized material. This limitation is due to the great difficulty with Pd-catalyzed couplings of pyrrole at the 2/ 5 positions. Work by G. Wegner93-96 and L. Groenendaal97, 98 have independently developed the methods to synt hesize oligoand polypyr roles via chemical methods but only with limited success. For Suzuki -based routes, the greatest difficulty was monomer instabilities for 2,5-dihalid es and 2,5-diboronates. Stille ro utes have been more widely 68


used, but these polymerize poorl y, yielding polymers with Xn<10.99-102 Despite these limitations, oxidative and electrochemical polymerization have been widely used generally affording poly(1H-pyrroles) with a light yellow neutral state (Eg 2.7 eV), and a colored oxidized state.103 Based on work by Heeger and Bredas, copolymer ization of electron ri ch/neutral monomers leads to an effective averaging of the HOMO and LUMO levels,78 thus copolymerizing these monomers should provide a genera l route to high bandgap, blue em issive materials with a range of oxidation potentials. This concept is the basis behind the materials described herein. Fluorenes have been polymerized via a Suzuki polymeri zation to synthesize a fluorene-carbazole and fluorene-pyrrole copolymer. With these systems, both polymers be nefit from the lower oxidation potential of the heterocyclic unit while maintaining the hi gh bandgap and blue fluorescence characteristic of a polyfluorene homopolymer. To simplify the fabrication of the LEC device, oligoethoxy pendant groups have been used w ith said monomers to improve the polymers miscibility with electrolytes and to obviat e the need for polymer blends with PEO. Monomer and Polymer Synthesis The fluorene and carbazole monomers were functionalized with oligoethoxy chains for solubility and improved miscibility with electrolytes for application in LECs. As shown in Figure 3-5, the carbazole monomer was synthesized by deprotonating the 3,6-dibromocarbazole ( 1) followed by addition of bisethoxymethyl tosylate, which affords compound 2. The diboronate derivative ( 3 ) was synthesized via lithium-halogen exch ange followed by addition of a trialkyl borate. One can see that a large excess of trimethyl borate was used. On going from 2 to 4 to 8 equivalents of trimethyl borate, the yield of the final product increased. The fluorene monomers were synthesized vi a a route like that described above. 2,7Dibromofluorene ( 4 ) was functionalized with bisethoxym ethyl by deprotonating the 9 position with potassium hydroxide to give compound 5 (Figure 3-6). Borylation via lithiation followed by 69


treatment with 2-isop ropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane gives 6 in a 65% yield. Compound 5 was used for coupling to the comonomers ( 3 and 7) because Pd-catalyzed couplings are more facile when coupling with a dihalide of the less electron rich species. Compound 6 was used to synthesize polyfluorene as a comparison fo r spectroscopic data as will be described later. Figure 3-5. Synthesis of carbazole monomers. Figure 3-6. Synthesis of fluorene monomers 70


Figure 3-7. Synthesis of pyrrole monomer The final monomer synthesized is the 2,5-pyrro lediboronic acid bis(pinacol) ester which was used for the fluorene-pyrrole copolymer. Very few examples of such structures are found in the literature, though a recent method based on Ir-catalyzed borylation provides a facile route.104 Previous research employing this reaction used the 1-H pyrrole and 1-isopropyl pyrrole, with each borylating at different positions. The former borylates the 2-position, because the heteroatom directs C-H activation to the -position; the latter borylat es the 3-position, because the -position is sterically hindered by th e N-iPr group, directi ng borylation to the -position. 1Methylpyrrole has a steric hinderance which is intermediate to these; however, treatment under the conditions shown in Figure 3-7 afforded the 2,5-diboronic acid bis(pin acol) ester-1-methyl pyrrole (7) in 70% yield. The three polymers used in this study were all synthesized via Suz uki conditions. Poly(Fl) and poly(Fl-Cbz) were both polymerized via traditional conditions with palladium (0) tetrakis(triphenyl)phosphine [Pd(PPh3)4] in a biphasic reaction of basic aqueous solution and toluene. For these two sets of monomers (Fig ure 3-8), these conditions yielded polymers with Mn 10 kDa. Poly(Fl) likely has a higher molecula r weight because the carbazole is more likely to undergo deboronation. Suzuki polymerizations of the 2,5-di boronatepyrrole proved much more difficult than its fluorene and carbazole analogues. The traditional Suzuki conditions with 7 and 5 yielded no polymer that could be recovere d, though the expected blue fluorescence was noted. These results mirror Wegners attempts to polymerize pyrrole via Suzuki polymerization.95, 96 71


Polymer Mn (g/mol) PDI Xn Poly(Fl) 21,200 2.2 29 Poly(Fl-Cbz) 10,000 1.9 16 Figure 3-8. Suzuki polymerization of Poly(Fl) and Poly(Fl-Cbz) Although previous work with 2-borylpyrroles was not successful, the failure of this reaction was not expected. In contrast to Wegne rs observations regarding the 2,5-diboronic acid and methyl ester of pyrrole, compound 7 was stable at room temperature in the air for weeks. Additionally, Janssen et al accounted for the improved molecular weight of polymers from 2(boronic acid pinacol ester) thiophe ne on the nonplanarity of the sulfur and boron atom; if true for thiophene, a similar effect would be expected with 7. For insight into this stability and the monomers reactivity, the crystal structure of 7 was obtained. As shown in Figure 3-9, the position of the ester oxygen and the pyrrole nitrog en only deviate from planarity by 7 dihedral angle. This dihedral angle c oupled with the stability of 7 disagrees with claims from Janssen et al The more likely basis for the stabililty of 7 is the steric hinderance of the methyl groups. Due to the conformation of the pinaco l bridge, the vacant orital on bor on is sterically encumbered, 72


Figure 3-9. Crystal structur e of 2,5-bis(boronic acid pinacol ester)-pyrrole ( 7). affording it greater stability towards nucleophiles. This steric hinderance also accounts for the decreased reactivity of boronate pinacol ester relative to boronic acid. If the basis for the stability of 7 is steric hinderance and deboronation is a significant side reaction, the molecular weight could be improved by lowering the reaction temperature. Based on this, the polymerization 7 and 5 was attempted with a catalyst /ligand system not requiring the high temperatures or harsh base of the traditional Suzuki conditions. First, a si mple translation of Fu conditions14 was used (Figure 3-10, Entry 2), yi elding relatively low molecular weight polymer though still of reasonable molecular wei ght (Xn = 9). Based on insights discussed in Chapter 4, further modifications to the Fu conditions were made (Figur e 3-10, Entry 3), yielding a polymer with nearly quadrupl e the molecular weight and a Xn=38. Figure 3-11 gives the GPC chromatogram that shows the monomodal distribu tion for poly(Fl-Pyr). This is the highest Xn reported for a pyrrole-based, Pd-catalyzed polym erization and the only po lymerization of an Nalkyl pyrrole. 73


Entry Reaction Conditions Mn (g/mol) PDI Xn 1 Pd(PPh3)4, 2 M Na2CO3, Toluene (reflux), PTC oligomers 2 Pd2dba3, HP( t -Bu3)BF4, CsF (6.6 eq.), THF (reflux) 4,200 1.6 9 3 Pd2dba3, HP( t -Bu3)BF4, 6 M CsF, THF (reflux) 16,000 2.1 38 Figure 3-10. Suzuki polymer ization of Poly(Fl-Pyr) Figure 3-11. GPC chromatogram of po ly(Fl-Pyr) from Figure 3-10, Entry 3. 74


Characterization of Thermal, Electr ochemical, and Spectral Properties The basis for evaluating these polymers is f ounded on two basic requirements for the dual EC/LEC device. First, the electrochemical prope rties of the copolymers will be measured to determine if the polymers can be oxidized revers ibly. Second, the ultimate goal is to have a polymer with many of the same a ttributes of a polyfluorene such as blue fluorescence, high QE, and transmissive neutral state. These goals can evaluate the design principle of averaging HOMO-LUMO values in the optimizati on of materials for EC/LEC devices. One distinguishing characteristic of polyfluorenes is their ther mal properties. In terms of thermal stabilities, pol yfluorenes (like many polyphenylen es) are thermally robust up to 400 oC. Previous measurements of the crystalline and liquid-crystalline properties show that polyfluorenes frequently have a highly or dered morphology with a melting point around 100 oC and nematic phase transition at higher temperatures.85, 86 This morphology has made polyfluorene a frequently utilized poly mer in creating polarized light emission.105 The thermal stability of all three polymers was measured by thermogravimetric analysis (TGA) under nitrogen. As shown in Figure 3-12, all three polymers show the same basic decomposition profile with an onset of decomposition at 400 oC. The percent mass loss between 450-500 C roughly corresponds to the percent mass of pendant groups. This suggests that there is no loss in thermal stability by in corporating the he terocyclic units. To study the order and morphology of these polymers, differential scanning calorimetry was measured. Figure 3-13 shows the heating and cooling cycles of the three polymers after annealing at 250 oC and cooling at 10 oC/min to -150 oC. On heating, all polymers show an endothermic peak at ~100 oC which corresponds to the polyme r melt previously reported for many polyfluorenes in the literature. For poly(Fl-Cbz ) this is the only endothermic peak, and it is very broad over an ~80 oC range. This broad endotherm may be due to transitions from more 75


Figure 3-12. TGA (10 C/min, N2) of poly(Fl)[dash-dot],poly(F l-Cbz)[dash],and poly(Fl-Pyr) [solid] Figure 3-13. DSC endotherms (lef t) and exotherms (right) for pol y(Fl) [dash-dot], poly(Fl-Cbz) [dash], and poly(Fl-Pyr) [solid ] with arbitrary offsets. than one crystalline phase. This is supported by the exotherms shown on the cooling cycle which show a number of small peaks correlated to th is broad endotherm. Poly(Fl) has a low enthalpy melting peak at ~100C and a complex 2nd peak from 150-200 C. Because of the supercooling for crystallization of these tran sitions on the cooling cycle, both peaks are attributed to two distinct crystalline phases.106 The same rationale also applie s to poly(Fl-Pyr) which has two 76


transitions with a high degree of supercooling over the cooling cycle.Interestingly, poly(Fl-Pyr) and poly(Fl) have transitions in the same temper ature ranges. One possibl e explanation of this similar behavior is the higher mass percent of fluorene in poly (Fl-Pyr) [82% fluorene monomer by mass] versus poly(Fl-Cbz) [ 58% fluorene monomer by mass]. The spectral properties of these fluorene-base d polymers are fundamental for evaluating the potential application of th ese polymers in the EC/LEC de vice. Figure 3-14 shows the absorbance spectra for all three polymers in solution. Poly(Fl-Cbz) has the most hypsochromically shifted absorbance which is in the UV-region and has a HOMO-LUMO gap of 3.1 eV. This high energy absorbance is due to th e carbazole nitrogen that acts as a conjugation break, limiting the extent of conjugation. Poly(Fl) and poly(Fl-Pyr) have more similar peaks. max for these peaks differ by only 2 nm, and the peak width at half-height for poly(Fl) is 51 nm and for poly(Fl-Pyr) is 77 nm. This difference in peak width leads to a difference in the HOMOLUMO gaps for these polymers of 2.9 eV and 2.7 eV, respectively. Due to this lower HOMOLUMO gap, poly(Fl-Pyr) does not have a colorless neutral state as poly(Fl) and poly(Fl-Cbz) do. As films, the absorbance spectra will see a bathochromic shift in the onset of the absorbance and max; however, the dual device ope rates under solvent swollen c onditions which would inhibit these effects. The fluorescence of these polymers was meas ured by photoluminescence (PL) in solution. As shown in Figure 3-15, the PL of these polymers all have emissions in the blue spectral region as was desired. Using poly(Fl ) as the standard for QE,107 poly(Fl-Cbz) has an efficiency of 0.68 and poly(Fl-Pyr) has an efficiency of 0.56. These values are both less than the 0.77 PL quantum efficiency of poly(Fl). Intere stingly, all polymers have the same vibronic coupling that is characteristic of polyfluorenes, with the st rength of these bands following the order of 77


300 350 400 450 0.0 0.2 0.4 0.6 0.8 1.0 Figure 3-14. UV-Vis absorbance of poly(Fl) [solid], poly(Fl-Cbz) [dash], and poly(Fl-Pyr) [broken dash] in 10 M solutions in chloroform. 400 450 500 550 0.0 0.2 0.4 0.6 0.8 1.0 Figure 3-15. PL of poly(Fl) [solid], poly(Fl-Cbz ) [dash], and poly(Fl-Pyr ) [broken dash] with 10 M solutions in THF. Excitations are at the max of each polymer. 78


poly(Fl) > poly(Fl-Cbz) > poly(Fl-P yr). The reverse order is seen for the magnitude of the Stokes shift for these polymers. Taken together, one can reasonably say that thes e properties reflect the relative rigidity of the polymer backbones with the ranking of rigidity being poly(Fl) > poly(FlCbz) >poly(Fl-Pyr). All polymers have a fair ly strong fluorescence wh ich reflects the strong influence the fluorenyl monomers have on the fluorescent properties. The electrochemical properties of the copolymers were characterized to determine whether these polymers give the stable and relatively low oxidation potentials needed for the EC/LEC device. Films of these polymers were drop cast onto platinum electrode s and spraycast onto ITO for electrochemical characterzation. The e xperiments proved problematic, because film characterization requires that the polymer be insoluble in the electrochemical solvent. The oligoethoxy pendant groups solubiliz e these polymers in a broad ra nge of solvents (very soluble in THF, chloroform, toluene; partially solubl e in acetone, ethyl acetate, hexane), causing the polymer to delaminate from the electrode when performing electrochemical experiments. With polar solvents such as acetonitrile, the polymer is effectively insoluble; however, after cycling the potential a number of times, the polymer film de laminates, presumably due to electrolyte flux through the film. A number of c onditions were attempted includi ngReproducible data could be gathered in propylene carbonate solutions with ITO as the working electrode though the range of electrochemical experi ments was limited. Cyclic voltammetry was used to characterize the basic electrochemical properties of the copolymers to determine if they are more st able than the electrochemical behavior of polyfluorene.87 A number of conditions were explored to obtain a reproducible cyclic voltammogram (CV) including aqueous and non-aqueous systems on platiunum, gold, and indium tin oxide (ITO). Though all aqueous system s (basic/acidic/lithium triflate) lead to 79

PAGE 80 -4 -2 0 2 4 6 8 -0.02 -0.01 0.00 0.01 0.02 0.03 Current Density (mA/cm2)Potential (vs. Fc/Fc+) -20 -10 0 10 20 30 Current Density (mA/cm2)Potential (vs. Fc/Fc+) Figure 3-16. CVs of poly(Fl-Pyr) [left] and poly(Fl -Cbz) [right] films on IT O. Inset: Isolation of 1st oxidation process for poly(Fl-Pyr) overoxidation with a single sca n, all non-aqueous systems provide a stable redox couple for both poly(Fl-Pyr) and Poly(Fl-Cbz); however, in non-aqueous systems, the polymer gradually delaminates from the electrode. The most stab le system found was with the polymers on ITO with 0.1 M TBAP in propylene carbonate. Figu re 3-16 show the CVs of poly(Fl-Cbz) and poly(Fl-Pyr) as films on ITO. The films were cycled more than 20 times with only minimal changes due to polymer delamination. Poly(F l-Pyr) has two redox processes with an E1/2=0.28 V for the first oxidation and E1/2 0.51 V for the second oxidation. Cy cling poly(Fl-Pyr) to higher potentials leads to overoxidation, as evidenced by no cathodic peak following polymer oxidation. The onset for the first oxidation is 0.16 V; base d on this value, the HOMO of the polymer is 4.96 eV vs. vacuum. Poly(Fl-Cbz) has a slight ly higher oxidation potential with an E1/2=0.59 V and an onset of oxidation of 0.53 V. Ba sed on the latter value, the HOMO of the polymer is 5.33 eV vs. vacuum. The difference in oxidati on potential between poly(Fl-Pyr) and poly(Fl-Cbz) is distinct but not dramatic, though these HOMO levels are significantly lower than a polyfluorenes and are lower than the oxidati on potential of water. 80


Due to gradual dissolution of these polym ers during electrochemical experiments, spectroelectrochemistry cannot be used with th ese polymers. To evaluate the color of these polymers on oxidation, however, solution oxidatio n with a chemical dopant was used. While solution doping does not account for interchain effects on oxidation and spect ral features, it does provide a first-order approximation. Figure 317 and 3-18 show the solution oxidation of poly(Fl-Pyr) and Poly(Fl-Cbz), respectively. Poly(F l-Pyr) has broad absorbances in the visible (450-700 nm) and IR (700-1300 nm) bands upon polymer oxidation. These bands change the polymer absorbance from yellow to a gray color. Poly(Fl-Cbz) has more well-defined bands with the visible (425-600 nm) and IR (700-1300 nm) bands separated by a 100 nm gap. These welldefined bands give poly(Fl-Cbz) purer colors on oxidation with the clear solution first turning pink then orange upon oxidation. Conclusion For conjugated polymers, copolymerizations ha ve essentially been used as a means of tweaking the electroactive proper ties of some known polymer. For example, fluorene-based have most often been used as a means of bathochrom ically shifting the absorb ance/fluorescence to a desired region. In this chapter, a new appr oach was used that treated two properties independently fluorescence and electrochemistry for possible application in a hybrid EC/LEC device. The design of these materials were based on monomers whose respective homopolymers have bandgaps of ~ 3 eV, but have different oxidation potentials. Because an alternating copolymer gives an averaging of the HOMO-LU MO levels, the bandgaps of these polymers would be expected to be ~ 3 eV though with a range of oxidation potentials. This design principle allows one to tailor th e (spectro)electrochemical prope rties while still retaining the characteristic blue fluorescence of a polyfluorene. 81


82 3004005006007008009001000110012001300 0.00 0.05 0.10 0.15 0.20 0.25 absWavelength (nm) Figure 3-17. Solution oxidation of 10 M poly(Fl-Pyr) in dichloromethane with antimony pentachloride as an oxidant. Arrows show change of absorbance with oxidation. 3004005006007008009001000110012001300 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 absWavelength (nm) Figure 3-18. Solution oxidation of 10 M poly(Fl-Cbz) in dichloromethane with antimony pentachloride as an oxidant. Arrows show change of absorbance with oxidation.


The polymers synthesized in this project have shown that this design works very well, yielding polymers whose HOMO-LUMO gaps differ by 50 nm and max,em varying by 70 nm. In contrast, the HOMO energies vary by 0.8 eV. In terestingly, the morphologies of these polymer also retained the characteristic properties of a polyfluorene, making thes e materials potential candidates for polarized emitters. The spectral ch anges on oxidation have also shown that this design provides a facile means of tailoring the electrochromic and light-emitting properties with some degree of independence. In terms of applicability in EC/LEC devi ces, further work remains. Some of these materials have been used in devices, but this has all been towards optimization of the EC/LEC hybrid device which does not allow for meaningful statements about this family of polymers. One potential problem is the ener gy of the polymers LUMO levels. By raising the energy of the HOMO level while maintaining a constant HOMO-LUMO gap may raise the LUMO to an impractical level. This difficulty may be circum vented by blending one of these polymers with an electron accepting material. Additionally, further changes to the solubilizing groups are likely needed as these hampered the characterization of these materials. Modifica tions such as placing PEO chains as telechelic groups to the conjugated polymer is one possibility, as is employing a lower mass percent of pendant groups in th e polymer. These limitations are functional limitations, however, that can be re medied with further optimization. Experimental Compounds bisethoxymethyl tosylate,108 1,109 and 2110 were synthesized according to previously published procedures. 3,6-dibromo-N-bisethoxymethyl-carbazole (2): A 500 mL three neck round bottom flask was fitted with a septum, stopper, and vacuum adap ter and was charged with a magnetic stir bar. This flask was flame dried twice under argon. 0.809 g of sodium hydride is added quickly to the 83


flask and ~125 mL of anhydrous DMF is transfer red to the flask by cannula. 7.3 g (22.5 mmol) of 3,6-dibromocarbazole is added in small por tions to allow for evol ution of hydrogen. A high argon flow is used during this addition. Once this has been added, 6.16 g (22.5 mmol) of bisethoxy methyl tosylate is adde d in one portion, and the solution allowed to stir overnight. 200 mL of 2N HCl was poured into the flask and st irred for an hour. The mixture was poured into a separatory funnel and extracted three times with diethyl ether. Organic fractions were combined and washed twice with brine. The organic solu tion was then dried with magnesium sulfate, filtered, and dried by rotary evaporation. This was recrystallized from 3:1 ethanol/water mixture. 7.41 g (77%) of a white powder. mp: 79-82 oC. 1H NMR (300 MHz, CDCl3) 3.25 (s, 3H), 3.38 (m, 2H), 3.42 (m, 2H), 3.82 (t, 2H), 4.43 (t, 2H ), 7.37 (d, 2H), 7.56 (dd, 2H), 8.13 (d, 2H); 13C NMR (75 MHz, CDCl3) 40.08, 43.24, 69.76, 71.08, 72.34, 108.81, 122.83, 123.61, 128.13, 132.50, 143.49; HRMS (M+Cl)calcd. for C17H17NO2ClBr2 461.9287 found 461.9300. Anal. Calcd for C17H17Br2NO4: C, 47.80; H, 4.01; Br, 37.41; N, 3.28; O, 7.49. Found: C, 47.478; H, 3.943; N, 3.190. 3,6-bis(ethylene boronate ester)-N -bisethoxy methyl-carbazole (3): A three neck 250 mL round bottom flask was charged with a magnetic stir bar and was fitted with a stopper, septum, and argon inlet. This was flame drie d under vacuum twice and backfilled with argon. 100 mL of anhydrous THF was added by cannula. 10.904 g (25.5 mmol) of 3,6-dibromo-Nbisethoxy methyl carbazole was added to the fl ask quickly under high argon flow. The flask was cooled to -78 oC and stirred for 15 minutes. To this fl ask, 21.5 mL of 2.5 M n-butyl lithium in hexane was added by syringe. The solution turns a bright yellow color. This solution is allowed to stir for one hour. 23 mL (204 mmol, 8 eq.) of trimethyl borate is added to the flask in one batch. This is stirred for 15 minutes at -78 oC, then allowed to stir and warm to room temperature 84


overnight. Flask is cooled with an ice bath, and 270 mL of 2 N HCl is a dded. This is stirred for one hour. The solution is extracted three times with diethyl ether. The organic fractions are collected and washed twice with brine. This is then dried with magnesium sulfate, filtered, and dried by rotary evaporation. This material is dissolved in a minimal amount of THF and precipitated into cold hexanes. The precipitate is immediately removed by filtration, yielding a white solid. This was carried on to the next step without further purification or any characterization. The diboronic acid carbazole was dissolved in 175 mL of toluene and 2.4 mL of ethylene glycol. This was refluxed for 3 hours with a Dean -Stark half-filled with 4A molecular sieves. Material allowed to cool to room temperature then the toluene was removed by rotary evaporation. This material was then dissolved in a minimal amount of ethyl acetate and allowed to recrystallize in the freezer. Crystals removed by filtration, the filtrate was concentrated again, and again recrystallized from et hyl acetate. 6.36 g (61%, 2 steps) of a white crystalline material. mp: 170-171 oC. 1H NMR (300 MHz, CDCl3) 3.29 (s, 3H), 3.39 (t, 2H), 3.50 (t, 2H), 3.88 (t, 2H), 4.42 (s, 8H), 4.53 (t, 2H), 7.47 (d, 2H), 7.81 (d, 2H), 8.63 (s, 2H); 13C NMR (75 MHz, CDCl3) 40.05, 43.21, 66.25, 69.72, 71.03, 72.30, 108.79, 122.80, 123.58, 128.10, 132.48, 143.47; HRMS calcd. for C21H25NB2O6 409.1868 found 409.1868. Anal. Calcd for C21H25B2NO6: C, 61.66; H, 6.16; B, 5.29; N, 3.42; O, 23.47. Found: C, 61.406; H, 6.012; N, 3.325. 2,7-dibromo-9-di(bisethoxy methyl)-fluorene (5): DMSO (15 mL), 50% NaOH solution (4 mL), and TEBA (107 mg) were added to a round bottom flask. 2,7-dibromofluorene (3.04 g, 9.2 mmol) was added to flask, turning the soluti on red. Bisethoxy methyl to sylate was added in one portion. The reaction was allowed to stir for two days. 50 mL of ethyl acetate was added and 85


the reaction stirred for 15 minutes The layers were separated a nd the organic layer was washed twice with dilute HCl, and then washed twice with water. The organi c phase was dried with magnesium sulfate, filtered, and the solvent removed by rotary evaporation. A silica gel column of 1:3 pentane/ether was run with the spot at Rf=0.55 collected. 3.1 g (63% ) of off white product recovered. mp: 52-54 oC. 1H NMR (300 MHz, CDCl3) 2.38 (t, 4H), 2.75 (t, 4H), 3.1-3.4 (m, 14H), 7.4-7.6 (m, 6H); 13C NMR (75 MHz, CDCl3) 39.54, 51.81, 59.01, 66.72, 69.93, 71.68, 121.137, 121.621, 126.69, 130.61, 138.35, 150.77 ; HRMS (M+H)+ calcd. for C23H28O4Br2 528.0334, found 528.0239. 2,7-bis(boronic acid pinacol ester)9,9-di(bisethoxy methyl)-fluorene (6) : Compound 5 (3 g, 5.68 mmol) was added to a dry three neck flask with 40 mL of anhydrous THF and strir bar. This flask was cooled to -78 C for 15 minutes. n-Butyl lithim (2.33 M in hexanes, 5.4 mL, 12.5 mmol) was added dropwise. After 10 minutes of stirring, a precipitate formed, at which time the flask was warmed to 0 C and stirred for 45 minutes. This flask was cooled to -78 C for 15 minutes when 2-isopropoxy-4,4,5,5-tetramet hyl-1,3,2-dioxaborolane (4.5 mL, ~ 20 mmol) was added in one portion.The reaction allowed to warm to room temperature and run overnight. The reaction was poured into a mixture of 50 mL of water and 100 mL of ether. The aqueous layer extract two more times with ether. The collected organic fractions were washed once with water and once with brine. Organic ex tracts dried with magnesium sulfate, filtered, and rotovapped filtrate to dryness. Off white-green solid recrystallized two times from hexanes, and washed with methanol. Dried under vacuum to give white crystalline solid. mp: 121-124oC. 1H NMR (300 MHz,CDCl3) 1.38 (s, 24H), 2.42 (t, 4H), 2.68 (t, 4H), 3.1-3.2 (m, 4H), 3.2-3.4 (m, 10H), 7.6-7.9 (m, 6H), 13C NMR (75 MHz,CDCl3) 24.9, 38.5, 51.0, 58.9, 66.9, 69.8, 71.7, 83.8, 119.4, 129.3, 133.9, 143.1, 148.5. HRMS calcd for C35H52B2O8 (M+), 645.3748; found 86


645.3773. Anal. Calcd for C35H52B2O8: C, 67.54; H, 8.42; B, 3.47; O, 20.56. Found: C, 67.45; H, 8.53. 2,5-bis(boronic acid pinacol ester)-1-methylpyrrole (7) : 1-methylpyrrole (2.03 g, 2.2 mL, 25 mmol), bis(pinacolato)d iboron (7 g, 27.5 mmol) and 4,4'-dit-butyl-2,2'-dipyridyl (201.3 mg, 0.75 mmol) was placed in a dry 250 mL flask and 175 mL of anhydrous THF. The solution was purged with argon for 45 minutes. Di-mu-me thoxobis(1,5-cyclooctadiene)diiridium(I) (126 mg, 0.375 mmol) was added quickly and the flask f itted with an oven dried condenser. The flask was refluxed overnight. Reaction was rototvappe d to dryness. The dark red-tan residue was washed with excess hot hexane that was hot filte red. The filtrate was then rotovapped to give a light tan solid. Recrysta llization from hexanes gives a white crystalline solid. 6.4 g (77%) mp: 155-160 oC. 1H NMR (300 MHz,CDCl3) 6.78 (s, 2H), 3.98 (s, 3H), 1.35 (s, 24H) 13C NMR (75 MHz,CDCl3) 24.9, 39.5, 51.0, 58.9, 66.9, 69.8, 71.7, 83.8, 119.4, 129.3, 134.0, 143.1, 148.5. HRMS calcd for NaC17H29B2NO4 (M+Na), 356.2180; found 356.2170. Anal. Calcd for C17H29B2NO4: C, 61.31; H, 8.78; B, 6.49; N, 4.21; O, 19.22. Found: C, 61.02; H, 8.72; N, 4.08. General procedure for Suzuki polymeri zation with basic aqueous solution: 1.22 mmol of aromatic diboronate ester and 1.22 mmol of aromatic dihalide were added to a flask with 0.25 mL of Aliquat 336, and 20 mL of toluene. Ar gon was bubble through this solution for 1 hour and stored under argon for the remainder of the reaction. A 2M sodium carbonate solution was refluxed in a separate flask with argon bubbling through it for 1.5 hours. 10 mL of this aqueous solution was added to the organic solution. Fr eshly prepared palladium tetrakis(triphenyl phosphine) [9 mg] was added to the reaction flask, and the flask was refluxed at ~105 oC for three days. The reaction mixture wa s allowed to cool to room temp erature. The organic layer was separated and then concentrated on a rotary evapor ator. This material was dissolved in a minimal 87


amount of dichloromethane and prec ipitated into methanol. This wa s allowed to stir overnight. The material was collected by filtration and the precipitation was repeated. General procedure for Suzuki polymerization with CsF with THF: Aromatic diboronate ester (1 eq.), aromatic dihalide (1 eq.), cesium fluoride (6.6 eq.), and tri( tbutyl)phosphonium tetrafluorobor ate (0.06 eq.) were added to a 25 mL dry flask. Flask was purged with argon four times. To this flask, Pd2dba3 (0.02 eq.) was added to the flask along with anhydrous/degassed THF (12 mL per mmol of mono mer). Reflux reaction for at least 40 hours. Polymer solution is precipitated into methanol. Redissolved in chloroform and precipitated into methanol again. Polymer is dried under vacuum. General procedure for Suzuki polymerization with 6 M CsF with THF: Aromatic diboronate ester (1 eq.), aromatic dihalide (1 eq.), cesium fluoride (30 eq.), and tri( tbutyl)phosphonium tetrafluoroborate (0.06 eq.) were added to a 25 mL (for 1 mmol of each monomer) fdry flask. Flask was purged with argon four times. To this flask, Pd2dba3 (0.02 eq.) was added to the flask along with degassed TH F (12 mL per mmol of monomer) and degassed water (5 mL) were transferred to the flask by cannulation. Reflux reaction for at least 40 hours. Organic phase is precipitated into methanol. Re dissolved in chloroform and precipitated into methanol again. Polymer is dried under vacuum. Poly(Fl-Cbz): 1H NMR (300 MHz, CDCl3) 2.63 (t, 4H), 2.95 (t, 4H), 3.2-3.4 (m, 17H), 3.45-3.65 (m, 4H), 3.97 (t, 2H), 4.61 (s, 2H), 7. 61 (d, 2H) 7.82 (m, 8H), 8.36 (s, 2H); Elemental analysis calc. for C40H45NO6: C, 75.56; H, 7.13; N, 2.20; O, 15.10; found C, 75.11; H,7.47; N, 2.09; O, 15.33. Poly(Fl): 1H NMR (300 MHz, CDCl3) 2.63 (t, 4H), 2.95 (t, 4H), 3.2-3.4 (m, 17H), 3.453.65 (m, 4H), 3.97 (t, 2H), 4.61 (s, 2H), 7.61 (d, 2H) 7.82 (m, 8H), 8.36 (s, 2H); Elemental 88


89 analysis calc. for C40H45NO6: C, 75.56; H, 7.13; N, 2.20; O, 15.10; found C, 75.11; H,7.47; N, 2.09; O, 15.33. Poly(Fl-Pyr): 1H NMR (300 MHz, CDCl3) 2.4-2.6 (m, 4H), 2.93 (p, 4H), 3.2-3.4 (m, 13H), 3.7-3.9 (m, 3H), 4.61 (s, 2H ), 6.42 (s, 2H), 7.5-7.9 (m, 6H). Elemental analysis calc. for C28H33NO4: C, 75.14; H, 7.43; N, 3.13; O, 14.30. found C, 65.259; H,7.660; N, 2.281.


CHAPTER 4 BASE-FREE SUZUKI POLYMERIZATION FOR THE SYNTHESIS OF CARBOXYLIC ACID FUNCTIONALIZED CONJUGATED POLYMERS Introduction The properties of a conjugated polymer are in tegrally determined by two basic parts. The conjugated backbone of the polymer consists of a sequence of -bonds that are the basis of the polymers optical and electroactive properties. Substituents or pendant groups on the polymer backbone also play key roles in the properties. For chemically polymerized polymers, pendant groups are most frequently used to solubilize the polymer, improving its processability. As discussed in Chapter 3, these solubilizing groups can also perform additional functions such as inhibiting interchain interactions, influencing polymer morphology, and determining the polymers miscibility with other components. One extreme example of how pendant groups influence the properties and pot ential applications of a conjugated polymer is conjugated polyeletrolytes (CPEs). CPEs ar e conjugated polymers that bear ionic pendant groups. These ionic groups change the solution and solid-state properties of these materials affording them unique applications relative to traditional conjugated polymers. Properties and Applications of Conjugated Polyelectrolytes The first work on CPE-type structures was for self-doped polymers. This concept was pioneered by Wudls group and Reynolds group who designed polymers such as 1 and 2 (Figure 4-1) so the ionic pendant group could act as th e counter ion to the oxi dized polymer. Polymer 1 was formed by the Wessling method that conver ts to its final conj ugated structure postpolymerization. 111 The first polymers from the Reynolds group were electropolymerized monomers ( 2) as this work predated the methods for soluble polymers.112 This research was 90


directed towards increasing the c onductivity of a given material as well as using them as ionexchange membranes.65, 113-116 Shortly following the initial work on soluble conjugated polymers, CPEs were studied for their solubility and conformation in polar solvents. Work by Novak64 and Wegner117 studied poly( p-phenylenes) with ionic groups such as 3, examining their liquid cr ystalline properties and distinctive character as rigid-rod polyelectrolytes; the latter is important, because in contrast to traditional polyelectrolytes which have flexible backbones, rigid-rod polyelectrolytes show a decreased influence of added electrolyte on the polymers c ontour length and the solutions viscosity.117, 118 The work cited above was directed to f undamental studies on CPEs, developing the synthetic methods to attain such materials and characterizing the polymers novel properties. The focus of recent research on CPEs is the eff ect of aggregation on the polymers spectral properties. Research by Swager119 and Whitten120 found that an ion or molecule can induce a polymers aggregation in solution, causing drama tic changes in the polymers fluorescence. This phenomenon is termed amplified fluorescence quenching 111 The mechanism of this is shown schematically in Figure 4-2. As a freely solubl e polymer, CPEs exhibit absorbance and Figure 4-1. Examples of conjugated polyelectrolytes (CPEs) 91


Figure 4-2. Solution behavior of CPEs in the present of a divalent cation. photoluminescence (PL) typical of th e parent polymer; however, in th e presence of a species that induces polymer aggregation (such as the polyvalent cation shown above), the polymer chains aggregate, causing the absorbance to shift ba thochromically and PL to be quenched via nonradiative processes. This phenomenon has b een applied with great success to potential applications as sensors for DNA ( 4)121-123 and divalent cations.124, 125 An additional application of CPEs is for incorporation in optoelectronic devices via novel methods. A number of researchers have used CPEs processed by layer-by-layer assembly, a method for creating thin films via sequential deposition of positively and negatively charged polyelectrolytes onto a substr ate. Reynolds and Schanze et al synthesized films of poly(arylene ethynlene)s for photovoltaic device s, achieving 5% efficiency at the polymers absorption maximum.126 Because CPEs are soluble in polar solv ents and insoluble in nonpolar solvents, researchers have also used CPEs for devel oping multilayer devices where a second conjugated polymer can be deposited from a nonpolar solvent. Bazan and coworkers have used positively charged CPEs as electron conducting layers in a LED. 127, 128 A number of reviews about CPEs have been written in recent years. The history and fundamental properties of CPEs ar e reviewed by Pinto and Schanze.129 The same authors have 92


published a more recent review that disc usses the literature in the interim.111 Reviews pertaining to work by specific researchers al so offer updated reviews of CPEs.130-133 Synthesis of Conjugated Polyelectrolytes The synthesis of CPEs involves more co mplications than f ound with traditional conjugated polymers. Many ionic groups can under go side reactions with a given polymerization route as well as affect the polymers solubility over the course of the reaction. Table 4-1 gives a comprehensive overview of the various routes th at have been used to synthesize CPEs. (This table does not contain ionic polyacetylenes,134 polyimines,135 or polyanilines.136) In this table, entries are also distinguished by the method used to incorporate the ionic group. Some polymerizations are called direct methods, becau se the monomer is polymerized with the ionic group present. Indirect met hods use monomers whose non-ionic functional groups can be converted to ionic groups post-poly merization. This distinction is not ed in the substituent column of Table 4-1 where an arrow indicates a func tional group has been converted to an ionic derivative post-polymerization. The direct and indirect met hods both have advantages and disadvantages. Direct routes were pioneered by Novak et al who had the key insight of using a sulfonated triaryl phosphine which allows one to conduct a Suzuki polymerization in water.64 As shown in Table 4-1, the polymers by this and analogous routes afford pol ymers with high degrees of polymerization; however, characterizing these polymers via size-exclusion chromatography or NMR is problematic, because CPEs aggregate in solutio n, readily absorb water, and may exchange counter ions while handling the polymer. For this reason, many researchers use indirect methods where the polymerization yields a precursor that can be converted to th e polyelectrolyte with high efficiency (>95%) post-polymerization. 93


Table 4-1. Summary of polymerization me thods for conjugated polyelectrolytes Comonomer Substituent Polymerization Method Solvent Catalyst Mn (kDa) Refs Poly( p-phenylenes) -NR2 -NR3 + Suzuki DMF/water Pd(OAc)2 10-15 137 -CO2 Suzuki DMF/water Pd with sulfonated phosphine 50 64 -Br CO2 Suzuki Toluene/water Pd(PPh3)4 138 -CO2CH3 -CO2 Colin-Kelsey DMF NiBr2 6-19 139 -SO3(Ph)CH3 -SO3 Suzuki Toluene/water Pd(PPh3)4 36 117 -SO3 Suzuki DMF/water Pd with sulfonated phosphine Mw = 5 65 -OPh -NR3 + Suzuki 140, 141 -NR2 -NR3 + Suzuki DMF/water Pd(OAc)2 4-12 142 Poly(Phenylene-comonomer) Thiophene -NR2 -NR3 + Stille DMF Pd(OAc)2 3-5 137 NPh3 -SO3 Suzuki DMF/water Pd(OAc)2 36 143 Polythiophene -CO2H Oxidative (FeCl3) Chloroform NA 144 -Br NH3 +, NHR2 +, -CO2 Negishi THF Ni(dppp) 6 145 -NH(CO) NH3 + Suzuki DMF Pd(OAc)2 1.5 146 Polyfluorene -NR2 -NR3 + Suzuki Toluene/water Pd(PPh3)4 14 147 -Br -NR3 + Suzuki Toluene/water Pd(PPh3)4 25 127 Poly(Fluorene-phenylene) -NR2 -NR3 + Suzuki Toluene/water Pd(PPh3)4 17-29 148 -NR2 -NR3 + Suzuki THF/water PdCl2(dppf) 14 147 -N(CH2CH2CO2 -)2 Suzuki THF/water PdCl2(dppf) >7 129 94


Comonomer Substituent Polymerization Method Solvent Catalyst Mn (kDa) Refs Poly(fluorene-comonomer) BTD -NR2 -NR3 + Suzuki Toluene/water Pd(PPh3)4 10-27 149 BTD -CO2R -CO2 Suzuki Toluene/water Pd(PPh3)4 26-56 150 Poly(Phenylene-ethynlene) -SO3 Sonagashira Water Pd with sulfonated phosphine 60 151 -CO2R -CO2 Sonagashira Toluene/water Pd(PPh3)4 96 152 -SO3 Sonagashira DMF Pd(PPh3)4 40 153 -CO2R -CO2 Sonagashira THF/TEA Pd(PPh3)4 127 124 -NR3 + Sonagashira DMF/water Pd(PPh3)4 67 154 -SO3 Sonagashira DMF/water Pd(PPh3)4 100 125 -P(O)(OC4H9) -PO3 2Sonagashira DMF/water Pd(PPh3)4 18 155 Poly(Arylene-ethynlene) Various -SO3Na or -NR3 + Sonagashira DMF/water Pd(PPh3)4 156 Fluorene -I -NR3 + Sonagashira Toluene Pd(PPh3)4 10 157 Poly(Phenylene-vinylene) -SO2Cl -SO3 Wessling DMF/water 1120 158 -CO2 CH2CH3 -CO2 Heck DMF Pd(OAc)2 P( o-tolyl)3 91 159 -CO2 CH2CH3 -CO2 Gilch THF 160 NR2 -NR3 + Gilch THF 11 161 -NR2 -NR3 + Heck Acetone Pd(OAc)2 P( o-tolyl)3 9 162 Poly(Arylene-vinylene) Fluorene -NR2 -NR3 + Heck DMF Pd(OAc)2 P( o-tolyl)3 9 163 95


The indirect route allows a grea ter variety of pendant groups a nd molecular structures to be incorporated into a CPE as well as allowing fu ll characterization of th e precursor polymer via traditional methods (e.g., GPC, NMR) Fluorene-Based Polymers Functionalized with Carboxylic Acids The materials described in this chapter and the next describe the synt hesis of a family of CPEs containg fluorenes functionalized with carb oxylic acids. These polymers were synthesized as the ester derivative and were converted to the acidic fo rm post-polymerization, thus allowing the full characterization of the CPE precursor. Thes e materials are of inte rest for a number of projects in the Reynolds group such as amplified fluorescence quenching by polyvalent analytes111 and as photoactive materials in dye-sensitized solar cell s where the carboxylic acid tethers the polymer to a metal oxide substrate.164 The fluorenyl structure was used for three reason s. First, a difficulty with all CPEs is the balance between the ionic groups hydrophilicity and the polymer backbones hydrophobicity. Because increasing the number of ionic groups impr oves a CPEs processibility in polar solvents, fluorenes are advantageous for their easy difunctionalization at the 9-position. Second, because both ionic groups are tethered at the 9-position, they extend above and be low the plane of the monomer (as described in Chapter 3). This structure is different from the PPP and PPE derivatives most widely studied in this field which have the ionic groups extended within the plane of the monomer ( 3, Figure 4-1). Properties influenced by polymer aggregation should show effects from these structures. A final reason for synthesizing this family of fluorene-based copolymers is for the strong fluorescence typically seen with fluorenes, which is important for amplified quenching experiments. 96


While fluorene-based CPEs have been studied by Bazan121, 123, 127, 150 and Cao127, 147, 149, the range of structures have been narrow. Their work has been on amm onium-functionalized CPEs ( 4, Figure 4-1) except reference 148 which repor ted a carboxylic acid derivative during the course of this authors work. While this narrow set of derivatives may be explained by limited interest in such compounds, this seems unlikel y given the widespread study of both fluorenes and CPEs. The more probable reason for this is the limitation of the synthetic methods. Perusing the routes shown in Table 4-1 s hows little variation in the methods employed all of which rely on aqueous basic conditions for a Suzuki pol ymerization. As described in Chapter 1, the traditional Suzuki coupling uses basic reagents to activate the boronic acid/ester to undergo the catalytic cycle and to form the new aryl-ary l bond. Unfortunately, an ester functionalized conjugated polymer is subject to hydr olysis under comparable conditions. To circumvent this problem, this project presents the first synthesi s of a carboxylic-acid functionalized CPE via a fluoride mediated Suzuki coupling. Building on previous work with Suzuki coupling for small molecule synthesis, 52 fluoride salts such as cesium fluoride and tetrabutylammonium fluoride were used to ac tivate boronate esters for Suzuki polymerization. The virture of these conditions is that esters are unaffected by these reagents. Because this method does not lead to ester hydrolysis, this ha s been termed a base-free Suzuki polymerization. This method is then extended and further optim ized for polymerization with other aromatic structures (Figure 4-3). Charact erization of the polymers absorption and photoluminescence in solution was studied for these polymers carboxylic aci ds and carboxylates to highlight the extent to which interchain interactions and intrachain ordering are affected by these polar functionalities. 97


Figure 4-3. General structure of fluorene-based conjugated polymers functionalized with carboxylic acids, and the family of comonomers employed. Synthesis of Highand Mid-Bandgap Fluo rene-Based Conjugated Polyelectrolytes Optimization of Base-Free Suzuki Polymerization I: Polyfluorene To optimize the conditions for the base-free Suzuki polymerization, fluorenyl monomers were used due to their facile synthesis (Figure 4-4), excellent performance in Suzuki couplings (as seen in Chapter 3), and th e opportunity to synthesize the first reported carboxylic acid functionalized polyfluorene homopolymer (Poly[Fl]DE Figure 4-5). As shown in Figure 4-4, the diboronate ester was synthesized from 2,7-di bromo-9,9-diethylfluorene (5). The 2 and 7 positions are converted to boronate esters by lithiation at -78 oC followed by the addition of 2isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxa borolane to give monomer 6 in 71% yield. The ester functionalized, dibromide monomer ( 7) was synthesized from 2,7-dibromofluorene with the 9 position functionalized via Michael Addition of the bridging car bon with two equivalents of butyl acrylate. Polymerization of 6 and 7 was optimized with 1 mol% Pd(PPh3)4 and the solvent, concentration, and salt varied (Table 4-2). Cesium fluoride (CsF) and tetrabutylammonium 98


fluoride (TBAF) were used as fluoride sources. A comparison of Entries 1 and 2 shows that excess fluoride was required to give a polymer with molecular weight >104 g mol-1. With TBAF as the fluoride source, the polymerization shows no significant influence of the solvent system on the molecular weight (Entries 24), providing polymers with Mn within a range of 1,500 Da. In every reaction, polymer precipitation occurr ed, likely limiting the molecular weight achieved. Cesium fluoride showed a more dr amatic effect of the solvent system on the molecular weight. For more traditional Suzuki conditions, biphasic sy stems with phase transfer agents (Entries 6 and 7) were used, giving similar results to th e conditions for TBAF. DMAC and DME (Entries 5 and 8) were also used, and though neither solven t fully dissolves cesium fluoride at the applied concentrations, both gave high polymers with the latter giving a notably larger number average molecular weight. Figure 4-4. Synthesis of fluorene mo nomers for polyfluorene poly(Fl)DE Figure 4-5. Base-free Suzuki polymerization of polyfluorene homopolym er diester derivative (poly[Fl]DE ) and diacid derivative (poly[Fl]-DA ). 99

PAGE 100

Table 4-2. Conditions for base-free Suz uki polymerization shown in Figure 4-5 Entry Fluoride Source Eq. of Salta Solvent System Mn(Da) DP PDI 1 TBAF 2 DME 4,100 6 2 TBAF 4 DME 12,700 20 1.76 3b TBAF 4 DME 13,200 21 1.89 4 TBAF 4 Toluene 14,000 22 1.92 5 CsF 4 DMAC 10,900 17 2.10 6 CsF 4 Toluene/WaterC9,900 15 1.64 7 CsF 4 DME/WaterC12,600 20 2.87 8 CsF 4 DME 22,000 34 2.57 a Equivalents relative to monomer 3 b Reaction diluted during the reaction, varying the monomer concentration from 0.1 M to 0.03 M c Tetrabutylammonium bromide was used as a phase transfer catalyst. One can see that the base-f ree conditions effectively polym erize these monomers with a range of potential solvents. The effectiveness of any given solvent appears less significant for TBAF than for CsF, with the latter affording th e polymer of highest mol ecular weight. The basis for these results is related to two separate factor s. First, the counter ion plays a significant role. Between entries 2 and 8, the molecular weight is nearly doubled for the latter presumably due to cesium as a counterion. This effect is readily appa rent in the comparison of entries 7 and 8. With the addition of tetrabutylammonium bromide as a phase transfer catalyst, CsF (entry 7) shows similar behavior to entry 2. Thus, the tetra butylammonium counterion appears to be less effective in promoting Suzuki coupling for these c onditions. The other factor that is at work in these polymerizations is the solubility of th e components. Based on the explanation above, one would expect similar performance for entries 5 a nd 8; however, DMAC is a polar solvent that poorly solubilizes polyfluorene wh ereas DME is a high boiling ether eal solvent that solublizes polyfluorene well. For entry 6, CsF is poorly solubl e in toluene, which will limit its effectiveness 100

PAGE 101

in activating the boronate ester for coupli ng. Thus, DME provides a balance of polymer solubility and salt solubility to prom ote an effective Suzuki polymerization. Optimization of Base-Free Suzuki Polyme rization II: Fluorene-based Copolymers With some understanding of the base-free Suzuki polymerization conditions, this method was then applied to a number of comonomers. The CPEs generated thereof will provide spectral absorbances across the visible region. The first copolymerization attempted with the base-free Suzuki polymerization was a copolymerization of the three monomers shown in Figure 4-7. For this unique structure, each comono mer plays a different role. Compound 8 (whose synthesis was described in Chapter 3) provides the diboronate ester to carry out the polymerization as well as an oligoethoxy pendant group that make the final CPE derivative more soluble in polar solvents. Compound 9 (which is a methyl ester variation of 7) is a dihalide monomer that gives the latent carboxylic acid functionality fo r the final polymer. Compound 10 is a second dihalide monomer that bathochromically shifts the spectra l absorbance and fluorescence of the polymer. Br Br O O O O B B O O O O O O O O S S N S N Br Br Pd(PPh3)4CsF O O O O O O O O S S N S N l m n O O O O 8910 Poly(0.66Fl-0.33BTBTD) Figure 4-7. Three component polymerization of a fluorene and bis-thienyl benzothiadiazole containing polymer. 101

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From this polymerization, two key points were discovered that affected the direction of this research. First, the number average molecu lar weight of this polymer was 3,900 Da a value that is much lower than the fl uorene homopolymers. The second disc overy was the instability of the bis-thienyl benzothiadiazole towards the base hydrolysis needed to convert the esters to carboxylic acids. To address the first point, the base-free polymeri zation required further optimization to attain polymers with number average molecular weights 104 Da. Regarding the second point, a new type of ester was used and will be described in the following chapter. For this reason, due to unique problems associated wi th the lower band gap materials, this chapter will focus on materials with bandgaps greater than 2 eV. The first optimization to the polymerization was to simplify the system by reducing the number of components. As discussed in Chapte r 1, a Suzuki polymerization is a step-growth polymerization, requiring an exact stoichiometr ic balance between the boronate and bromide functionalities. Polymerizing two rather than three comonomers reduces the amount of error in weighing out these materials. For this, the need for 8 was obviated by synthesizing a diester derivative that also carries the diboronate es ters. To this end, a pall adium catalyzed borylation12, 165 was used (Figure 4-8). Using this method compound 7 was converted to the diboronate ester ( 11) in 73% yield. A significan t aspect of this reaction was running it under anhydrous conditions. If any of the components had pr olonged exposure to ambient conditions, Figure 4-8. Synthesis of 2,7-bis(pinacol bor onate ester)-9,9-di butylesterfluorene (11) 102

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oligofluorenes from Suzuki-coupling side re actions occurred, maki ng monomer purification excessively difficult. Following this method, 11 and related monomers were synthesized with good yields and high purity. With 11 in hand, initial work was done with 1,4-dibromobenzene and 2,5dibromothiophene. Using the conditions described in entry 8 of Table 4-2, both polymers yielded lower molecular weights than expected (Entries 1 and 2, Figures 4-9). The suspected cause for the low molecular weights was the solvent, beca use both polymerizations formed an insoluble mass in as little as four hours for the phenyl derivative and overni ght for the thienyl derivative. To circumvent this problem, the solvent was changed to THF. Becau se the reactivity of Pd(PPh3)4 is temperature dependent,57 the catalyst was changed to a Pd/P( tBu)3 complex that has shown high activity at lowe r temperatures than Pd(PPh3)4.14 Using a Pd/P ratio of 1/1.5, Entry Ar Reaction Conditions Mn (Da) PDI Xn 1 A 4,300 1.41 9 2 A 4,500 1.41 9 3 B 7,000 2.41 14 4 B 4,600 2.41 9 Conditions: A) Pd(PPh3)4, 7 eq. CsF, DME, 100 oC B) Pd2dba3, HP( tBu)3BF4, 7 eq. CsF, THF (reflux) Figure 4-9. Synthesis fluorene-phenylene and fluorene-thiophene copolymers by base-free Suzuki polymerization 103

PAGE 104

these monomers were polymerized under these new c onditions. The results are listed as Entries 3 and 4 in Figures 4-9, showing a modest increase in molecular weight for the phenyl derivative with Xn increasing by five. For the thienyl derivative, the polydispersity increased for the new conditions but the other values remained unchanged. Because the Pd/P(t-Bu)3 complex showed greater potenti al and there was no precedence for these conditions, the catalyst was studied w ith a model system of 2,7-bis(pinacol boronate ester)-9,9-dioctylfluorene and 4,7-dibromo-benz othiadiazole (Figure 4-10). The fluorenyl monomer was used based on its facile synthesis and similarity to 11; benzothiadiazole was used because it is the most easily synthesized monomer to be included in this family of CPEs. With this system, the effect of temperature, Pd/P rati o, and salt were examined. Entries 1-3 show that the metal-to-ligand ratio is significant, and is in perfect agreement with the results previously cited in the literature.14 Potassium fluoride (KF) has been used for analogous conditions with success, but as shown in Entry 4, this polymerization failed with KF as the salt, leaving only Entry Salt Pd / P ratio Temp. Mn (Da) Xn 1 CsF 1 / 1.0 reflux 6700 13 2 CsF 1 / 1.5 reflux 11500 22 3 CsF 1 / 2.0 reflux 8800 17 4 KF 1 / 1.5 reflux 5 CsF 1 / 1.5 RT 6500 12 Figure 4-10. Optimization of base-fr ee Suzuki polymerization with Pd/P( t-Bu)3 catalyt system 104

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starting materials. Entry 5 additionally shows th at the reflux temperature of THF is optimal though not required to carry out the polymerization. In conclusi on these studies validated the conditions used in Entries 3 and 4 for Figure 49 as being the optimal conditions for the Pd/P( tBu)3 complex. Although the fluorene-benzothiadiazole polymer izations did not realize a new set of conditions, a key insight was made while running these reactions. Fo r every reaction that formed polymer, the end result was the precipitation of polymer and a wa ter soluble by-product. This behavior is apparently unique to these base-free Suzuki conditi ons (as it has not been noted for other Suzuki polymerizations), and this precipitation occurs with both Pd(PPh3)4 and Pd/P( tBu3). Because these polymers should be freely sol uble in THF at these molecular weights, the water soluble by-product appears to be the proble m; the precipitation of this by-product could induce the polymers precipitation. To circumve nt this problem, a biphasic variation of the Pd/P( t-Bu3) system was used with THF and 6 M CsF a queous solution as the solvents in a 5:1 ratio; under these conditions the water solubl e by-product and organic soluble polymer will not interact and no precip itation should occur. Figure 4-11 shows the results of the THF/water solvent mixture for the phenylene, bithiophene, and benzothiadiazo le derivatives. The molecular we ights of these polymers show a significant increase with the degrees of pol ymerization improved by two to three times. Additionally, these poly merizations show no precipitation, supporting the anal ysis described above. These polymer structures were confirmed by 1H NMR, IR, and GPC as is shown in Figure 4-12 4-15. To convert these polymers to their diacid fo rms, each polymer was dissolved in dioxane and treated with 1.5 M KOH (Fi gure 4-16). The solution was refluxed for 24 hours and then 105

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Figure 4-11. Synthesis of poly(Fl-Ph)-DE poly(Fl-BTD)DE and poly(Fl-BTh)DE recovered by acidification of the aqeous phase The final polymer structure was confirmed by 1H NMR, elemental analysis, and IR. The IR spectra of the diester and diacid derivatives of all three polymers are shown in Figure 4-15. Fo r each polymer, a broad peak above 3000 cm-1 is seen which corresponds to the conv ersion of the esters to carboxylic acids. Based on this data, the final structures of these high molecular we ight CPEs have been confirmed. The following characterization employs the high molecular weig ht products of each polymer as its carboxylic derivative to determine the eff ect of these functional groups on the polymers spectral and solution properties. 106

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Figure 4-12. Poly(Fl-Ph)DE 1H NMR and GPC chromatogram 107

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Figure 4-13. Poly(Fl-Btd)DE 1H NMR and GPC chromatogram 108

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109 Figure 4-14. Poly(Fl-Bth)DE 1H NMR and GPC chromatogram

PAGE 110

4000350030002500200015001000500 96 97 98 99 OR RO O O nWavenumber (cm-1)Transmittance (%) -Diacid40 50 60 70 80 90 100 Transmittance (%) -Diestera) 4000350030002500200015001000500 50 60 70 80 90 100 OR RO O O N S N nWavenumber (cm-1)Transmittance (%) -Diacid70 80 90 100 b)Transmittance (%) -Diester 4000350030002500200015001000500 40 50 60 70 80 90 100 110 c) OR RO O O S S nWavenumber (cm-1)Transmittance (%) -Diacid60 70 80 90 100 Transmittance (%) -Diester 110Figure 4-15. IR spectra of a) poly(Fl-Ph) b) poly(Fl-Btd), and c) poly(Fl-BTh).

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Figure 4-16. Base hydrolysis of highand mid-bandgap polymers to their diacid form Characterization of Highand Mid-Bandgap Fluorene-Based Conjugated Polyelectrolyte The interest in these polymers pertains to thei r behavior in solution and how this affects the polymersspectral properties, because subsequent work will study these po lymers for amplified fluorescence quenching. As seen in Table 4-1, carboxylates have b een studied as pendant groups to PPPs, PAEs, and polyfluorenes; however, thes e polymers have not been studied as carboxylic acids. This distinction is intere sting, because the former gives a repulsive interaction between the ionic groups and the latter gives an attract ive interaction by hydr ogen bonding. Based on the carboxylic acid/carboxylate groups, hydrophobic polym er backbone, and solvent, the polymer chains conformations and their aggregati on change based on the polymer/polymer and polymer/solvent interactions To study this effect, the UV-Vis absorbance and photoluminescence of the diacid derivatives was m easured in mixtures of organic and aqueous solutions to study solvatochromic effects. For th ese experiments, a neutral aqueous solution (pH: 6-7) was used for the polymers carboxylic acid form and a basic aqueous solution (pH 9) was 111

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used to generate the polymers carboxylate form. Fo r ease of reference, the PL data has blue axes and UV-Vis absorbance data has black axes. Poly(Fl-Ph)-DA: Spectral Properties and So lvatochromic behavior A fluorene-phenylene copolymer is a high bandga p material with a blue emissivity and UV absorbance which is slightly blue shifted relative to polyfluorene.166 The diacid derivative [Poly(Fl-Ph)DA, Mn = ~ 18,500 Da] has the same general sp ectral properties. Poly(Fl-Ph)DA is moderately soluble in THF, 1,4-dioxane, and DMSO ; slightly soluble in basic aqeous solutions; and insoluble in water, diet hyl ether, and chloroform. Figures 4-17 and 4-18 show the UV-Vis absorbance of Poly(Fl-Ph)DA in THF/water and THF/basic aqueous solutions, respectively. Focusi ng on the former, one can see the polymer is aggregated in the THF solution compared to the 60%THF/40% water solution. With the THF solution, poly(Fl-Ph)DA has a weak primary absorbance from 300-425 nm, and a low energy band at ~475 nm. The former apparently arises from the reduced oscillator strength of the aggregated form while the latter is a weaker bathochromically shifted absorbance due to polymer aggregation. With the addition of wa ter (20-40% water), this absorbance due to aggregation disappears and the pr imary absorbance grows in intens ity and narrows, indicative of a more solubilized polymer. As the amount of water increases beyond 50%, the polymer shows minor effects due to aggregation with increasi ng amounts of water, thoug h notably not to the degree as the 100 % THF solution. Based on this da ta, one can see that hydrophilic acid groups solubilize the polymer well, though with only THF present, the carboxylic acids hydrogen bond and induce polymer aggregation. Figure 4-18 shows the change of absorbance in THF/basic aqueous solutions, and here one sees a decreased solvatochromic effect. With the 80% THF solution, the UV-Vis absorbance 112

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300350400450500550 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 20% THF/ 80% water 40% THF/ 60% water 60% THF/ 40% water 80% THF/ 20% water 100% THFAbsWavelength (nm) Figure 4-17. UV-Vis absorbance solvatochrom ism of 10M solutions of Poly(Fl-Ph)DA in THF/water mixtures. 300325350375400425450475500 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 20% THF/80% aq. base 40% THF/60% aq. base 60% THF/40% aq. base 80% THF/20% aq. baseAbsWavelength (nm) Figure 4-18. UV-Vis absorbane solvatochrom ism of 10M solutions of Poly(Fl-Ph)DA in THF/basic water(pH 9) mixtures. 113

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decreases in intensity and broade ns, following the expected behavior of a CPE in a poor solvent. Only with 20% THF does one see effects due to aggr egation. For more polar solutions, this data supports the hypothesis posed earlier where the carboxylates experience io nic repulsion which inhibits the polymer aggregation re lative to the carboxylic acid form. A more sensitive measure of the polymers aggregation in solution is the photoluminescence (PL). Figures 4-19 and 420 shows the PL data for poly(Fl-Ph)DA in the same set of solutions used above In the THF/water mixtures, one sees the same effect observed for the UV-Vis absorbance though with more pr onounced results. The THF solution has lower intensity fluorescence than the 80% and 60% THF solutions, suggesting that the formers fluorescence is being quenched by aggregation. With 40% and 20% THF, the PL intensity decreases, indicating that th e hydrophobic segments are poorly solubilized which leads to quenching. Additionally, the peaks shift bathochrom ically, suggesting that the polymer chains are adopting a more extended conjug ation due to planarization of the backbone in the aggregated units. As the carboxylate form (Figure 4-20), the fl uorescence shows the expected effect of ionic repulsion between the polymer chains. In the 80% THF solution, poly(Fl-Ph)DA has a broad, lower intensity fluorescence than the 60% and 40% THF solutions due to quenching. Interestingly, the 20% THF solution fluorescence is shifted bathochromically by 10 nm, indicating that the polymer a dopts a more extended conjugati on through planarization. This effect could be due to interchain inter actions or solvent-pol ymer interactions. Based on the spectral properties of these solutions, one can see that this polymer is solubilized well in nonpolar and polar solutions, adopting extended conformations in the more 114

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400420440460480500520540 0 1x1062x1063x1064x106 20% THF/ 80% water 40% THF/ 60% water 60% THF/ 40% water 80% THF/ 20% water 100% THFPL (cps)Wavelength (nm) Figure 4-19. PL solvatochromism of 10M solutions of Poly(Fl-Ph)DA in THF/water mixtures. Excited at max, abs. 400420440460480500 0 1x1062x1063x1064x1065x106 20% THF/80% aq. base 40% THF/60% aq. base 60% THF/40% aq. base 80% THF/20% aq. basePL (cps)Wavelength (nm) Figure 4-20. PL solvatochromism of 10M solutions of Poly(Fl-Ph)DA in THF/basic water(pH 9) mixtures. Excited at max, abs. 115

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polar solvent mixtures. Additiona lly, one can see that the dicar boxylate form aggregates less in more polar solutions than the carboxylic acid form. Figure 4-21 plots the change of absorbance and PL intensity as a function of solvent mixture for poly(Fl-Ph)DA which highlights the balance between backbone hydrophobicity and i onic group hydrophillicity. For the diacid form, the polymer aggregates in THF, likely due to hydrogen bonding interactio ns between polymer 20 40 60 80100 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Abs% THF 2030405060708090100 0 1x1062x1063x1064x1065x106 PL (cps)% THF Figure 4-21. UV-Vis absorbance(black) and PL intensities (blue) at max for the diacid (solid line) and dicarboxylate (dashed line) forms of Poly(Fl-Ph)DA 116

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chains; however, with 20-40% water, these inte ractions are moderated and the polymer is wellsolubilized. At greater water concentrati ons, the hydrophobicity of the polymer backbone becomes the determining factor. For the dicarboxyl ate form, the same trend is observed with the dicarboxylate more soluble in high water concentrations than the diacid form, due to the ionic repulsion inhibiting inte rchain interactions. Poly(Fl-Btd)-DA: Spectral Properties and Solvatochromic behavior A fluorene-benzothiadiazole copolymer is a mid-bandgap mate rial with a yellow-orange fluorescence.167 The diacid derivative [Poly(Fl-BTD)DA, Mn = ~21,500 Da) has the same general spectral properties. Poly(Fl-BTD)DA is slightly soluble in THF, 1,4-dioxane, and DMSO; moderately soluble in basi c aqeous solutions; and insoluble in water, diethyl ether, and chloroform. This polymer is notably less soluble than poly(Fl-Ph)DA in the diacid form, but is more soluble in polar solvents as the dicarboxylate. Because of the poor sol ubility of the diacid form, only data for the dicarboxylate form is shown. Figures 4-22 and 4-23 show the UV-Vis absorbance and photoluminescence, respectively, of poly(Fl-Btd)DA in the carboxylate form. From th e absorbance data one sees that the intensity varies very little in the different solutions. This da ta suggests that the polymer is well-solubilized in th e carboxylate form. The fluorescence data shows a more complicated behavior in solution where the 60% THF solution shows the great est photoluminescence, as was seen for poly(Fl-Ph)-DA For both the 80% and 20% THF solutions, the fluorescence is notably quenched to a greater degree, which again fo llows the trend observed for the phenylene copolymer. Interestingly, max for absorbance is characteristic for the freely solubilized polymer in solution whereas max for fluorescence is not. The latter resembles the fluorescence from the 117

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420 490 560 0.00 0.06 0.12 absWavelength (nm) 80 % THF/ 20% aq. base 60 % THF/ 40% aq. base 40 % THF/ 60% aq. base 20 % THF/ 80% aq. base Figure 4-22. UV-Vis absorbane solvatochrom ism of 10M solutions of Poly(Fl-Btd)DA in THF/basic water(pH 9) mixtures. 500525550575600625650 0 1x1052x1053x1054x1055x1056x1057x105 80% THF/ 20% aq. base 60% THF/ 40% aq. base 40% THF/ 60% aq. base 20% THF/ 80% aq. basePL (cps)Wavelength (nm) Figure 4-23. PL solvatochromism of 10M solutions of Poly(Fl-Btd)DA in THF/basic water(pH 9) mixtures. Excited at max, abs. 118

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polymer film.167 This data suggests the polymer is aggregated to some extent in all solvent mixtures. As a whole, the solubility, absorbance and fluorescence data of poly(Fl-Btd)DA is similar to the data for poly(Fl-Ph)DA The basis for this is likely the similarity between the molecular structures. These similarities highlight the role of backbone hydr ophobicity and ionic group hydrophilicity on the spectral properties of CPEs. Poly(Fl-BTh)DA: Spectral Properties and Solvatochromic behavior A fluorene-bithiophene copolymer is a mid bandgap material with a yellow-orange fluorescence.168 These polymers have been studied fo r their liquid crystallinity and charge transport.169 The diacid derivative [Poly(Fl-BTh)DA, Mn = 12,500 Da] has the same general spectral properties. Poly(Fl-BTh)-DA is very soluble in THF, 1,4-dioxane, and DMSO and insoluble in basic aqeous solutions, wa ter, diethyl ether, and chloroform. As shown in Figures 4-24 and 4-25, Poly(Fl-Bth)-DA has distinct changes in the absorptive bands across this range of solvent mixtures. For the diacid derivative in THF, the absorbance closely resembles that reported in the literature,168 though the shoulder at 475 nm is more prominent here. With increased water concen tration, the absorbance undergoes a number of changes. First, from 80-40% THF, the absorbance shifts bathochromically with no significant changes in the oscillator strength, indicating the polymer backbone planarizes and increases the effective conjugation length. This effect may arise from an interaction of the solvent with the side chains which influences the backbone confor mation, or it could be due to the effect of interchain aggregation on th e conformation of the polymer backbone. Additionally, the resolution of the two peaks in the absorbance b ecome more refined with increasing amounts of water, indicative of a more constrained geometry in the more polar solutions. 119

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400 450 500 550 600 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 100% THF 80% THF/ 20% water 60% THF/ 40% water 40% THF/ 60% water 20% THF/ 80% waterAbsWavelength (nm) Figure 4-24. UV-Vis absorbance solvatochrom ism of 10M solutions of Poly(Fl-BTh)DA in THF/water mixtures. 400 450 500 550 600 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 80% THF/ 20% aq. base 60% THF/ 40% aq. base 40% THF/ 60% aq. base 20% THF/ 80% aq. baseAbsWavelength (nm) Figure 4-25. UV-Vis absorbane solvatochrom ism of 10M solutions of Poly(Fl-BTh)DA in THF/basic water(pH 9) mixtures. 120

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For the dicarboxylate form, the changes in th e polymer conformation are ameliorated by the ionic charges, as was seen for poly(Fl-Ph)DA A comparison of the 60% and 80% THF solutions in Figure 4-25 show the freely solubi lized polymer and the aggregated polymer, respectively, where the latter ha s a reduced intensity, red-shifted and broadened absorbance. As stated for the diacid form, th e dicarboxylate 40% and 60% THF so lution absorbances are similar to that reported for organic soluble polymers though with the lower energy band having increased intensity. An additional note regarding the diacid and dicarboxylate forms is the difference in the intensity and position of these peaks in a given solution. Figure 4-26 gives the absorbance of the diacid/dicarboxylate form in 40%THF solutions, where they both have two peaks, but the diacid form is red-shited with reduced intensity. Hence, both polymers undergo the more ordered conformation in polar so lvents, though the diacid derivative undergoes aggregation, likely due to hydrogen bonding interactions. 360380400420440460480500520540 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 AbsWavelength (nm) Figure 4-26. UV-Vis absorb ances of poly(Fl-BTh)DA in 60% THF/40% water (solid line) and 60%THF/basic aq. Solution (dashed line). 121

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From the photoluminescence data, one sees th e effect of polymer aggregation playing a more significant role. As the water concentratio n increases for solutions containing the diacid form (Figure 4-27), the PL inte nsity decreases which is indi cative of polymer aggregation. Interestingly, the aggregation effect on PL intens ity appears continuous with the change of water concentration, whereas the bathochr omic shift and the peak resolu tion occurs only at the 80% to 60% interval. This difference suggests that the latter effects are rela ted to solvent-polymer interactions, rather than interchain interacti ons. For the dicarboxylate form, the PL in solution follows the same trend observed for the previous polymers where the polymer is solubilized best in the 40% and 60% solutions. Poly(Fl)DA: Spectral Properties and So lvatochromic behavior As described in Chapter 3, polyfluorene is a high bandgap polymer that has a blue fluorescence. Numerous fluorene homopolymer s have been studied for light-emitting applications, all of which share the same spectral properties as poly(Fl)DA (Mn = ~18,000 Da). This polymer is very soluble in THF, methanol and DMSO; slightly so luble in basic aqueous solutions; and insoluble in water, chloroform, and ether. Poly(Fl)-DA has moderately improved solubilities in polar and nonpolar solvents than the other polymers. When solvatochromism of poly(Fl)DA was measured in THF/water mixtures, the spectral properties were similar to poly(Fl-Ph)-DA Because this polymer is soluble in alcohol, the spectral solvatochromism was also st udied in methanol/water solutions. Shown in Figure 4-29 is the UV-Vis absorbance of the diac id form, which is relatively soluble across the range of solvents. As was seen with poly(Fl -BTh), for 80% methanol and 60% methanol 122

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500 550 600 650 700 0.0 5.0x1051.0x1061.5x1062.0x1062.5x1063.0x106 100% THF 80% THF/ 20% water 60% THF/ 40% water 40% THF/ 60% water 20% THF/ 80% waterPL (cps)Wavelength (nm) Figure 4-27. PL solvatochromism of 10M solutions of Poly(Fl-BTh)DA in THF/water mixtures. Excited at max, abs. 500 550 600 650 0.0 5.0x1051.0x1061.5x1062.0x1062.5x1063.0x106 80% THF/ 20% aq. base 60% THF/ 40% aq. base 40% THF/ 60% aq. base 20% THF/ 80% aq. basePL (cps)Wavelength (nm) Figure 4-28. PL solvatochromism of 10M solutions of Poly(Fl-BTh)DA in THF/basic water(pH 9) mixtures. Excited at max, abs. 123

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solutions, a low energy shoulder ap pears on the absorption band with max = 400 nm. Concomitant with the addition of more water to the solvent (e.g., 40% methanol and 20% methanol), the absorbance band decreases in osci llator strength. These solvent-induced changes in spectral bandshape and intens ity are attributed to several ef fects that occur simultaneously. First, the red-shift that is seen for the 80% and 60% methanol solutions signals that the conformation of the polyfluorene backbone is altered so as to increase the effective conjugation length. This effect may arise solely from interact ion of the solvent with the side chains which influences the backbone conformati on, or it could be due to the eff ect of interchain aggregation on the conformation of the polymer backbone. Second, the pronounced decrease in absorption band oscillator strength that is seen for the waterrich solutions clearly ar ises due to interchain interactions (i.e., polymer aggr egation). For the dicarboxylate fo rm (Figure 4-31), the spectrum shows only a slight red shift in more polar so lutions due to polymer aggregation. The same properties are seen seen for the photoluminescence of the dicarboxylate form. In the photoluminescence of the diacid form, an abrupt change is seen in the 60% methanol solution. In both 100% and 60% methanol soluti ons, the fluorescence ap pears as a broad band with a distinct vibronic progression. Interestingly, the vibrational progressi on is the same in both solvents; however, the entire fluorescence band is red-shifted by ~ 10 nm in the 60% methanol solution. This red-shift is cons istent with the suggestion above that the conjugation length of the polymer increases with increasing water content in the solvent. 124

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350 400 0.0 0.1 0.2 0.3 0.4 0.5 100 % Methanol 80% Methanol/ 20% water 60% Methanol/ 40% water 40% Methanol/ 60% water 20% Methanol/ 80% waterAbsorbanceWavelength (nm) Figure 4-29. UV-Vis absorbance solvatochr omism of 10M solutions of Poly(Fl)DA in methanol/water mixtures. 350 375 400 425 0.0 0.1 0.2 0.3 0.4 0.5 80% methanol/ 20% aq. base 60% methanol/ 40% aq. base 40% methanol/ 60% aq. base 20% methanol/ 80% aq. baseabsWavelength (nm) Figure 4-30. UV-Vis absorbance solvatochr omism of 10M solutions of Poly(Fl)DA in methanol/basic aqueous solution (pH=9). 125

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400420440460480500520540 0.0 2.0x1054.0x1056.0x1058.0x1051.0x1061.2x1061.4x106 PL (cps)Wavelength (nm) Figure 4-31. PL spectra of poly(Fl)DA (10 M) in 100% methanol (solid) and 60% methanol/40% water (dash). Excited at max, abs. Conclusion A new family of CPEs have been synthesized that bear carboxylic acid pendant groups. These polymers show a greater reduction of aggr egation in their carboxylate form as expected from their functionalization at the 9-position of fluorene. Interestingly, the diacid form (which has not been previously studied) has a re markable tendency to undergo more ordered conformations in polar solvents for some polymers. This behavior is unique and warrants further study. The base-free Suzuki polymerization offers a new route to synthesize conjugated polymers functionalized with base-senstive groups. In additi on to this benefit, th e optimization of this route has highlighted some significant aspects of Su zuki polymerizations in general. First, there appears to be a significant difference in fluor ides and hydroxides tendenc y to deboronate the 126

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Suzuki monomer. Because the traditional Suzuki pol ymerization always uses aqueous base, this difference has gone unrecognized. Additionally, the need for a biphasic reaction in the Suzuki polymerizations appears significan t. Standard conditions employ an aqueous and organic phase, so this restriction has been unheeded and unneeded; however as the need for further modifications to the Suzuki conditions continue, the biphasic nature of a reaction appears to be key. Experimental Potassium acetate was dried in an oven (>150 C) for 24 hours before using. 1,4dibromobenzene and 2,5-dibromothiophene were purchased from Aldrich. 2,7-Bis(boronic acid pinacol ester)-9,9-dioctyl fluor ene, 4,7-dibromobenzothiadiazole, and 2,2-dibromodithiophene were synthesized according to previously published procedures. 170-173 Compound 8 and 9 was synthesized in Chapter 3 and 5 of this dissertation, respectively. Compound 10 was synthesized according to previously published procedures. 2,7-dibromo-9,9-diethyl-fluorene (5). 2,7-dibromofluorene (5.00 mmol, 1.62 g), ethyl bromide (12.5 mmol, 1.36 g), and potassium iodide (0.6 mmol, 0.11 g) were mixed with 15 mL of DMSO and stirred vigorously. Potassium hydr oxide (22.5 mmol, 1.26 g) was added slowly. After 3 hours, an extra 5 mL of DMSO was added and the solution stirre d an additional 2 hours. Reaction poured into water and fi ltered. The precipitate was recrys tallized from methanol/ethyl acetate to give 1.628 g (86%) of white crystalline solid. mp: 152-154 oC. (lit 144-153174 oC) 1H NMR (300 MHz,CDCl3) 0.33 (t, 5H), 1.98 (q, 4H), 7.6-7.4 (m, 6H). 13C NMR (75 MHz,CDCl3) 8.4, 23.6, 56.7, 121.0, 121.5, 126.2, 130.1, 139.4, 151.7. HRMS calcd for C27H32O4Br2 (M+), 474.3244; found 474.6269. Anal. Calcd for C17H16Br2: C, 53.72; H, 4.24; Br, 42.04. Found: C, 53.75; H, 4.17. 127

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2,7-Bis(boronic acid pinacol es ter)-9,9-diethylfluorene (6). A 250 mL three neck round bottom flask was fitted with a st ir bar, stopper, septum, and argon inlet. This flask was flamedried, and ~100 mL of anhydrous THF was added by cannula. To this flask, 2 (14.5 mmol, 5.5 g) was added, and the flask cooled to -78 oC. N-Butyl lithium (2.5 M, 30.45 mmol, 12.2 mL) was added by syringe and the solution stirred for 1 hour. A precipitate forms after 5 minutes. 2Isopropoxy-4,4,5,5-tetramethyl-1,3,2-di oxaborolane (50.6 mmol, 10.3 mL) was added by syringe and stirred for 15 minutes. The solution was then allowed to warm to room temperature and stirred overnight. So lution diluted with ~100 mL of diethyl ether, and ex tracted three times with water, once with brine. The organic fraction was then dried with magnesium sulfate, filtered, and rotovapped to dryness. The off-white crude product was then recrystallized from ethyl acetate and then heptane to give 4.88 g (71%) of a white crystalline solid. mp: >220 oC 1H NMR (300 MHz,CDCl3) 0.25 (t, 6H), 1.39 (s 24H), 2.15 (q, 4H), 7.9-7.7 (m, 6H). 13C NMR (75 MHz,CDCl3) 8.5, 24.9, 32.5, 56.2,119.3, 128.9, 133.7, 136.9, 144.3, 149.5. HRMS calcd for C27H32O4Br2 (M+), 474.3244; found 474.6269. Anal. Calcd for C29H40O4B2: C, 73.44; H, 8.50; B, 4.56; O, 13.49. Found: C, 73.11; H, 8.91. (2,7-dibromo)9H-Fluorene-9,9-dipr opanoic acid-dibutylester (7). 2,7dibromofluorene (24.68 mmol, 8.02 g) and TEBA (2.46 mmol, 0.56 g) was added to 30 mL of benzene. 50% NaOH (98.72 mmol, 8 mL) was adde d and stirred for 15 min. N-butyl acrylate (98.72 mmol, 12.65 g) was added slowly by addition funnel, and the reacti on was stirred for 5 hours. Reaction diluted with ~ 200 mL of ethyl acetate. This mixture was washed three times with water, once with brine, and the organic layer dr ied with magnesium sulfate, filtered, and rotovapped to dryness. Column run with dichloromethane and the product subsequently recrystallized from hexanes. Recovered 7.87 g (55%) of whit e crystalline solid. mp: 61-63 oC. 128

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1H NMR (300 MHz,CDCl3) 0.88 (t, 5H), 1.25 (s, 4), 1.4-1.6 (m 8H), 2.37 (t, 4H), 3.88 (t, 4H), 7.6-7.4 (m, 6H). 13C NMR (75 MHz,CDCl3) 13.7, 19.0, 28.9, 30.5, 34.4, 54.2, 64.4, 122.0, 126.4, 128.3, 131.2, 139.2, 149.5, 172.90. HRMS calcd for C27H32O4Br2 (M+), 578.0704; found 578.0709. Anal. Calcd for C27H32O4Br2: C, 55.88; H, 5.56; Br, 27.54; O, 11.03. Found: C, 56.00; H, 5.56. 2,7-bis(boronic acid pinacol ester)-9,9di(butyl propionate ester) (11): Compound 7 (2.053 g, 3.53 mmol), bis(pinacolatodiboron) (2.872 g, 11.30 mmol), and potassium acetate (2.080 g, 21.20 mmol) placed in a dry 100 mL fl ask. DMF (anhydrous, 40 mL) added to the flask. Vacuum (~0.1 mm Hg) pulled on the soluti on for 15 minutes, backfilled with argon, and then repeated once more. PdCl2(dppf) (145 mg, 0.18 mmol) was added, turning the catalyst from a red to a yellow color. Reacti on was heated overnight under argon at 70-75 C. The reaction was poured into water and extracte d with diethyl ether three time s. The organic extracts were then washed two times with water and once with brine. Organic phase was dried with magnesium sulfate, filtered, and the filtrate rotovapped to dryness. Crude product passed through 1 plug of silica with 2:1 DCM/hexanes used to flush off residual bis(pinacolatodiboron) which was monitored by TLC with iodine as an indi cator. After bis(pinacolatodiboron) has been removed, product is flushed off with 1:1 diethyl ether/ hexanes. Recovered 1.75 g (73%) of white crystalline solid. mp: 172-176 oC. 1H NMR (300 MHz,CDCl3) 0.86 (t, 6H), 1.2-1.6 (m, 36H), 2.45 (t, 4H), 3.88 (t, 4H), 7.7-7.9 (6H). 13C NMR (75 MHz,CDCl3) 13.6, 19.0, 24.9, 25.0, 29.0, 30.5, 34.5, 53.5, 64.1, 83.9, 119.8, 128.9, 134.5, 143.8, 147.5, 173.5. HRMS calcd for C39H56B2NaO8 (M+Na+), 697.4059; found 697.4066. Anal. Calcd for C39H56B2O8: C, 69.45; H, 8.37; B, 3.21; O, 18.98 Found: C, 69.6; H, 8.568. 129

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General procedure for Suzuki polymerization of Poly(Fl)DE [Table 4-2]: 6 (1 eq.), 7 (1 eq.), and fluoride source (2-4 eq.) were a dded to a 25 mL dry flask. Flask was purged with argon four times. To this flask, degassed so lvent was added (12 mL per mmol of monomer). Freshly prepared Pd(PPh3)4 (1 mol%) was added and the reaction refluxed for 48 hours. Polymer solution is precipitated into metha nol. Polymer is dried under vacuum. 1H NMR (300 MHz,CDCl3) 0.88 (t, 5H), 1.25 (s, 4), 1.4-1.6 (m 8H), 2.37 (t, 4H), 3.88 (t, 4H), 7.6-7.4 (m, 6H). Anal. Calcd for C44H48O4: C, 82.46; H, 7.55. Found: C, 79.15; H, 7.45. GPC (vs. polystyrene standard): 4,400 22,000 Da. Poly(0.66Fl-0.33BTBTD): 8 (1 eq.), 9 (0.66 eq.), 10 (0.33 eq.), and CsF (4 eq.) were added to a round bottom flask and purged with argon four times. Degassed DME (12 mL per mmol of 8) was added followed by freshly prepared Pd(PPh3)4 (2 mol%). Reaction was refluxed for 48 hours. Polymer solution is precipitated into methanol. Polymer is dried under vacuum. 1H NMR (300 MHz,CDCl3) 1.08 (t, 12H), 1.41 (s, 7), 2.58 (m 6H), 2.8-3.0 (m, 4H), 3.2-3.6 (t,20H), 7.4-8.2 (m, 18H). GPC (vs. polystyrene standard): 3,900Da. General procedure for Suzuki polymeri zation with CsF and ethereal solvent: Aromatic diboronate ester (1 eq.), aromatic dibromide (1 eq.), and cesium fluoride (4-7eq.) were added to a 25 mL dry flask. Flask was purged with argon four times. To this flask, degassed solvent was added (12 mL per mmol of monomer). Freshly prepared Pd(PPh3)4 (3 mol%) or Pd2dba3 (2 mol%) and tri( t-butyl)phosphonium tetr afluoroborate (4-8 mo l%) was added and the reaction refluxed for 48 hours. Polymer solution is precipitated into methanol. Polymer is dried under vacuum. General procedure for Suzuki polymerization with 6 M CsF with THF: Aromatic diboronate ester (1 eq.), aromatic dihalide (1 eq.), cesium fluoride (30 eq.), and tri( t130

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butyl)phosphonium tetrafluoroborate (0.06 eq.) were added to a 25 mL (for 1 mmol of each monomer) fdry flask. Flask was purged with argon four times. To this flask, Pd2dba3 (0.02 eq.) was added to the flask along with degassed TH F (12 mL per mmol of monomer) and degassed water (5 mL) were transferred to the flask by cannulation. Reflux reaction for at least 40 hours. Organic phase is precipitated into methanol. Re dissolved in chloroform and precipitated into methanol again. Polymer is dried under vacuum. Poly(Fl-Ph)-DE. 1H NMR (300 MHz,CDCl3) 0.88 (t, 6H), 1.22 (m, 4H), 1.4-2 (m, 6H), 2.48 (m, 2H), 3.83 (m, 4H), 7.4-8.0 (m, 10H). Anal. Calcd for C33H36O4: C, 79.81; H, 7.31; O, 12.89Found: C, 76.21; H, 6.84. GPC (vs. polys tyrene standard): 4,300 23,900 Da. Poly(Fl-Bth)-DE. 1H NMR (300 MHz,CDCl3) 0.84 (t, 6H), 1.22 (sextet, 4H), 1.43 (p, 4H), 1.55-1.80 (m, 3H), 2.45 (m, 3H), 3.88 (t, 4H), 7.0-7.8 (m, 10H). Anal. Calcd for C35H36O4S2: C, 71.88; H, 6.20; O, 10.94; S, 10.97. Found: C, 70.54; H, 6.04. GPC (vs. polystyrene standard): 15,200 Da. Poly(Fl-BTD)DE. 1H NMR (300 MHz,CDCl3) 0.84 (t, 6H), 1.22 (sextet, 4H), 1.43 (p, 4H), 1.7-2.0 (m, 4H), 2.6 (m, 3H), 3.88 (t, 4H), 7.8-8.3 (m, 8H). Anal. Calcd for C33H34N2O4S: C, 71.45; H, 6.18; N, 5.05; O, 11.54; S, 5.78. Found: C, 69.80; H, 6.07; N, 5.04. GPC (vs. polystyrene standard): 27,400 Da. Poly(Fl-Ph)-DA. 1H NMR (300 MHz,DMSOd6) 0.6-0.8 (m, 4H), 1.4-1.6 (m, 4H) 7.88.2 (m, 4H), 11.9 (s, 2H). Anal. Calcd for C25H20O4: C, 78.11; H, 5.24; O, 16.65. Found: C, 75.02 ; H, 5.40 Poly(Fl-BTh)DA. 1H NMR (300 MHz, DMSOd6) 0.8-2.0 (m, 8H), 7.2-8.2 (m, 10H), 11.8-12.0 (s, 2H). Anal. Calcd for C27H20O4S2: C, 68.62; H, 4.27; O, 13.54; S, 13.57. Found: C, 67.12; H, 4.46. 131

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Poly(Fl-BTD)DA. 1H NMR (300 MHz,DMSOd6) 1.4-2.0 (m, 4H), 2.6-3.0 (4H), 7.6-8.8 (8H), 11.9 (s, 2H). Anal. Calcd forC25H18N2O4S: C, 67.86; H, 4.10; N, 6.33; O, 14.46; S, 7.25. Found: C, 63.79; H, 4.50; N, 6.23 132

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CHAPTER 5 SYNTHESIS OF LOW BANDGAP CONJUGA TED POLYELECTROLYTES VIA SUZUKI POLYMERIZATION OF MONOMERS WITH THERMALLY CLEAVABLE ESTERS Introduction Chapter 4 described the optimization of the base-free Suzuki polymerization with which four new CPEs were made. Over the course of that work, polymers based on compound 1 (Figure 5-1) could not be used due to polymer decomposition during base hydrolysis of the pendant esters to carboxylic acids. This instability was attributed to 1, as this problem was not found with other polymers. Incor porating monomeric units compos ed of electron rich arenes (donors) and electron poor aren es (acceptors) such as th e donor-accepto r-donor units ( 13) shown in Figure 5-1 allows one to develop materials with small HOMO-LUMO gaps.175 These materials are of great interest because the absorbance and fluorescence can extend to wavelengths from the visibl e to the near-IR region.176 To incorporate this species into a CPE, this chapter describes the design and synthesis of novel polymer structures that use thermolysis opposed to base hydrolysis to make low bandgap CPEs functionalized with carboxylic acids. The first two sections describe model systems of th e thermolytically cleavable polymers and the low bandgap comonomers. The last s ection describes the synthesis and properties of low bandgap CPEs functionalized with carboxylic acids. Figure 5-1. Examples of donor-acceptor-donors. 133

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Concept and Design of Carboxylic-Acid Functi onalized CPEs from Thermally Cleavable Esters The most frequently used route to convert an ester to a carboxylic acid is acid or base hydrolysis.72 The efficiency of this reaction is generall y very high but requires the substrate to be compatible with the reaction conditions. With polymers based on compounds shown in Figure 51, such conditions result in pol ymer decomposition. For this reason, these polymers require novel routes to synthesize their correspond ing carboxylic-acid functionalized CPEs. To circumvent this problem, the polymers incorporating low bandgap monomers were functionalized with esters that are converted to carboxylic acids via thermolysis. Such esters have been used for poly(meth)acrylic acids sy nthesized via ATRP and RAFT techniques, where the polymer is converted to its acidic form by heating to ~200 oC post-polymerization.177-179 As shown in Figure 5-2, this reac tion is attributed to a retr o-Ene reaction. While such high temperatures can be detrimental to many orga nic species, conjugated polymers are notable for their thermal robustness, frequently being su fficiently stable for short term handling at temperatures greater than 300 oC. These low bandgap conjugated polymers are thus well-suited for use with thermally cleavable esters opposed to hydrolytically cleavable esters. Figure 5-2. Mechanism of retro-Ene in the synt hesis of polyacrylic acid from the corresponding thermally cleavable ester. Because this chapter introduces a number of new monomer structures, the experiments have been divided into three main sections. The first section describes th e synthesis and study of 134

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two model polymers that incorporate different type s of thermally cleavable esters. Based on this work, the synthetic utility of th e monomer is evaluated as well as the concept of thermally cleavable esters for CPEs. The second section discusses the donor-acceptor-donor monomers that are used in this project. Finally, a fluorene functionalized with thermally cleavable esters is then used in the synthesis of two new low bandgap polymers. The efficacy of this new route to carboxylic-acid functionalized CPEs was asse ssed as well as the polymer properties. CPEs from Thermally Cleavable Pendant Groups: A Study with poly(Fl-Ph) As discussed above, thermally cleavable esters have been used in the synthesis of poly (meth)acrylic acids, and two types of thermally cl eavable esters have freq uently been used. One is a 1-ethoxyethyl ester ( Poly[Fl-Ph]EE, Figure 5-3) which is repo rted to cleave at ~ 150 C.179 The second is a t -butyl ester (Poly[Fl-Ph]TB Figure 5-3) which is reported to cleave at 180-200 C.180 To evaluate their utility, model systems ba sed on the fluorene-phenylene copolymers were synthesized and characterized. Figure 5-3. Molecular stru cture of Poly(Fl-Ph)EE and Poly(Fl-Ph)-TB Synthesis of Poly(Fl-Ph)-EE and Related Monomers Poly(Fl-Ph)EE was synthesized from 1,4-benzenedibor onic acid bis(pin acol) ester and a 2,7-dibromofluorene bearing the 1-ethoxylethyl ester. The latter was synthesized from 2,7135

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Br Br Br Br O O O O Br Br OH O OH O O O O O O O p-TSA,EVE Diethylether 0OCRT THF,EtOH 5MKOH H3CO O 50%KOH,DMSOBr Br 4 5 6 Figure 5-4. Synthesis of 2,7-dibromo-9,9-(1 -ethoxyethyl ester propionate)fluorene dibromofluorene that was functi onalized with methyl propionate at the 9 position via Michael addition, giving 4 as shown in Figure 5-4. Hydrolysis of this compound gives 5 in nearly quantitative yield. Final synthesi s of the fluorenyl monomer is accomplished by esterification of 5 with ethyl vinyl ether (EVE) and a catalytic amount of p-TSA. These conditions differ from previously published conditions,179 where a yellow impurity is a voided by the use of diethyl ether at low temperatures. This modification was needed because 6 decomposes with column chromatography, and these new conditions yield a crude product which is easily purified via extractions. Compound 6 was polymerized with 1,4-benzenedibor onic acid bis(pinacol) ester via the base free Suzuki polymerization.53 Attempts to use either PPh3 or P( t -Bu)3 with DME at elevated temperatures did not yield recoverable pr oduct (Entries 1 & 2, Figure 5-5). While the characteristic blue fluorescence of the fluoren e-phenylene polymer was observed, no polymer precipitated into methanol. Based on NMR from the methanol filtra te, this problem was ascribed to conversion of the esters to carboxylic acids during the reaction. To avoid ester cleavage, the 136

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O O O O O O n Poly[Fl-Ph]EE Br O O O O O O Br B B O O O O + Suzuki Polymerization Entry Reaction Conditions Mn (Da) PDI 1 Pd(PPh3)4, 7 eq. CsF, DME (reflux) 2 Pd2dba3, HP( t-Bu)3BF4, 7 eq. CsF, DME, 85 C 3 Pd2dba3, HP( t-Bu)3BF4, 7 eq. CsF, THF (reflux) 11,100 2.2 Figure 5-5. Suzuki conditions for the synthesis of poly(Fl-Ph)EE reaction was run at lower temperat ures in refluxing THF. This re action was successful (Entry 3) yielding a polymer with a degr ee of polymerization of ~20. Synthesis of Poly(Fl-Ph)-TB and related monomer The second fluorene bearing a ther mally cleavable ester carries a t -butyl ester. This monomer is synthesized directly from 2,7-dibromofluorene via a Michael Addition with t-butyl acrylate (Figure 5-6). In contrast to the (1-ethoxyethyl) derivative, compound 7 is much easier to work with and purify. The former is a tacky oil th at readily decomposes in polar media, while the latter is a stable, white crystalline solid. Br Br O O O O Br Br Toluene 50%KOH,TEBA t-butylacrylate 7 Figure 5-6. Synthesis of 2,7-dibromo-9,9-( t-butyl propionate)fluorene 137

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Figure 5-7. Synthesis of Poly(Fl-Ph)TB Polymerization of 7 and 1,4-benzenediboronic acid bis(pi nacol) ester was carried out with the same conditions that were successful for Poly(Fl-Ph)EE. This is shown in Figure 5-7. The number average molecular weight of this polymer is 9,800 Da (Xn 20) with a PDI of 2.2, Because the GPC for characterization uses THF, this is an unrepresentative low value due to the polymers poor solubility in THF. This allows only the THF soluble fraction to be analyzed, though the polymer is fully soluble in other organic solvents (e.g., chloroform, toluene). Characterization of Monomers and Poly mer with Thermally Cleavable Esters In terms of the polymerization of the 1-ethoxyethyl ester ( 4 ) and t -butyl ester (7 ) derivatives, the former is significantly more di fficult to work with. All attempts to purify 4 by column chromatography (silica and alumina) resulted in decomposition of the monomer. Compound 4 was also very sensitive to the reaction c onditions. Finally, this monomer is a tacky oil while 7 is a crystalline material. This makes 7 easier to weigh, allowing stoichiometric balance for the step-growth polymerizati on. On these accounts, monomers based on t-butyl esters are advantageous. To evaluate the thermal properties, Figures 5-8 and 5-9 show the TGA data for poly(FlPh)-EE and poly(Fl-Ph)TB, respectively. Poly(Fl-Ph)EE shows a transiti on at 150 C with mass loss corresponding to the conv ersion of the esters to carboxylic acids. At 300 C, a second transition occurs which is consiste nt with decarboxyl ation. Poly(Fl-Ph)-TB is converted to the 138

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Figure 5-8. TGA data (scan 10 C/min, N2) for poly(Fl-Ph)EE Figure 5-9.TGA data (scan 10 C/min, N2) for poly(Fl-Ph)TB 139

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dicarboxylic acid at higher temperatures than the EE-derivative. This polymer shows a transition at 200 C with mass loss that agr ees with the conversion of the es ters to carboxylic acids. As seen with the previous polymer, a second transition at 300 C occurs with a mass loss corresponding with decarboxyla tion. Based on this data, the EE -derivative benefits from ester-tocarboxylic acid conversion at temperatures 50 degrees lower than the TB derivative, although both polymers could be cleanly converted due to the high ther mal stability of the backbone. In summary, the tbutyl ester has advantages regard ing synthetic utility while the 1ethoxyethyl ester can be converted to a carboxylic acid at lower temperatures. Considering both sides of this, the tbutyl ester is used in all future wor k. While the 1-ethoxyethyl ester allows lower temperatures to be used, the temperature required for the t-butyl ester is still well below the thermal instabilities of the carboxylic acids or the aromatic units of the polymer backbone. Additionally, the freedom to modify the molecular structure of the t-butyl ester derivatives without concerns for their st abilities is significant. Donor-Acceptor-Donor (DAD) Monomers for CPEs The primary method for synthesizing orga nic molecules/polymers with small HOMOLUMO gaps (< 2 eV) is to join electron rich se gments with electron poor segments. Examples of such materials are shown in Figure 5-1 for a series of trimers. The electron rich segment (donor) contributes strongly to the HOMO while the electron poor segment (acceptor) contributes strongly to the LUMO, with some degree of or bital mixing through the conjugated system. This concept allows one to tune the HOMO-LUMO gap, bathochromically shifting a polymers absorbance/fluorescence from the visible to th e near-IR. Structures su ch as the donor-acceptordonor (DAD) trimers shown in Figure 5-1 are most frequently incorporated into conjugated polymers based on their symmetr y and facile halogenation. 140

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For this project, two DAD monomers were in corporated into the fluorene-based CPEs using thermally cleavable esters. One mono mer is bisthienyl benzothiadiazole ( 1, Figure 5-1). This monomer is a frequently used DAD monomer for conjugated polymers, affording polymers with a purple to blue colo r and a strong red fluorescence.99, 100, 175, 181 For incorporation into CPEs, one notable aspect of this monomer is its hydrophobicity. As shown in Chapter 4, the hydrophobicity of the polymer backbone has a str ong influence on the CPEs solution properties; by incorporating a series of four aromatic units (fluorene + DAD) with only two carboxylic acids, this polymer will likely aggregate in polar solutions. With this in mind, a second DAD monomer was also synthesized based on a pyrrole derivative 3 (Figures 5-1). While 1 has been widely used in recent work on conjugated polymers, 3 has only been studied by one group.182, 183 Outside of its novelty, 3 is an interesting DAD monomer for three reasons. First, pyrrole is a more electron rich (higher HOMO) arene than thiophene which would lower the HOMO-LUMO gap of 3 relative to 1. The 1-H pyrrole used in 3 also has intramolecular bonding with the benzothi adiazole nitrogen; this will planarize this monomer and again lower the HOMO-LUMO gap.182 Finally, the 1-H pyrrole has a polar hydrogen that will improve the monomers solub ility in polar solvents an advantageous property for a low bandgap CPE. A consideration for 3 is the need to protect the 1-positi on of pyrrole due to potential side reactions during a Suzuki coupling.184, 185 The most frequently used protecting group on pyrrole is a t -butyl carbamate (BOC) that can be removed under acidic conditions or by heating to ~185 C. Under these conditions, the carbamate on the py rrole is converted to the carbamic acid which subsequently loses CO2 to form the 1-H pyrrole. As s hown in Figure 5-10, coupling the thermally cleavable esters from the previous se ction with the BOC-protect ed pyrrole allows both 141

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the ester-to-acid conversion and pyrrole deprotec tion to be carried out simultaneously by heating to higher temperatures following the Suzuki polymerization. Figure 5-10. Suzuki polymerization and ther mal treatment of low bandgap CPE based on 4,7bis(1-H pyrrole) benzothiadiazole. Synthesis of DAD Monomers Due to the limited information on 3 or any comparison of compounds 1 and 3, this section will briefly discuss the synthesis of these compounds and characte rize their spectral properties. Compound 1 was synthesized according to literature procedures which involves the Stille coupling of a 2-stannyl thiophene with 4,7-dibromobenzothiadiazole.186 Compound 3 is synthesized by a similar route though as the Bocprotected derivative. As shown in Figure 5-11, 142

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1-Boc pyrrole is stannylated in the 2 position by lithiation with LDA followed by the addition of trimethyltin chloride, giving 8 in a 90% yield. Stille couplings w ith 2-stannyl pyrrole have been reported but with generally poor yields. A number of conditions were attempted to synthesize 9. Using standard Stille conditions (Entry 1), no product was formed. Using recent work from Mee et al (Entry 2),187 the yield of 9 improved though it was still low. In the column purification of this reaction, many colored by-products were obs erved. Given the amount of starting material recovered from the previous reaction, thes e by-products are likely a result of unstable intermediate(s) in the reaction. Because the copper sa lt is used to activate the stannyl species, the reaction was carried out without this salt and in a less polar solven t (Entry 3). As seen in this entry, the yield was improved for these conditions. With the DAD unit formed, compound 9 was brominated according to procedures for brominating pyrroles with NBS,188 giving the desired dibromide (10) in quantitative yield (Figure 5-12). This monomer was synthesized to undergo Suzuki polymerization. To study the spectral properties of the bis(pyrrole) benzothiadiazole, the Boc groups were removed by heating N O O 1)1.1eq.LDA,-78oC 2)ClSnMe3N O O N S N Br Br N S N N N O O O O Sn Entries1-38 9 Entry Reaction Conditions Yield (%) 1 PdCl2(PPh)2, THF 0 2 PdCl2, HP( tBu)3BF4, CuI, CsF, DMF 27 3 Pd2dba3, HP( tBu)3BF4, CsF, Dioxane 52 Figure 5-11. Synthesis of 4,7-bis(1-Boc pyrrole) benzothiadiazole 143

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to 200 C in the solid state. From this reaction, compound 3 was synthesized in quantitative yield from 9. Figure 5-12. Synthesis of pyrrole-based DAD derivatives. Characterization of DADs The properties of the 4,7-bispyrrole-benzothiadiaz ole have only been briefly reported in the literature by Meijer et al .182 Given the potential utility of this system, this section presents data on the DAD unit itself. Figure 5-13 shows the normalized UV-Vis absorbance of compounds 1, 9, and 3 in THF solutions. Interestingly, the absorbance of the pyrrole derivatives are shifted in opposite directions relative to 1. This difference is likely due to the large dihedral angle between the pyrroles and benzothiadiazole for 9. Figure 5-14 shows the crystal structure of 9 where one can see the large dihedral angl es (40.4 and 52.4). Compound 1 does not have a substituent to cause such steric hinderance, lik ely giving it a planar structure.189 Between 1 and 3, the difference in donor strength of pyrrole and thi ophene is evident by the 100 nm difference for onset of absorbance. Additionally, as discussed in Meijers work, compound 3 also forms a more planar (and thus more fully conjugated) st ructure due to intram olecular hydrogen bonding. Multiple attempts to verify this interaction by acquiring a crystal structure were made, but all were unsuccessful. 144

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350400450500550600650 0.0 0.5 1.0 1.5 N S N N N O O O O N S N H N H N 9 11 N S N S S 1 abs (normalized)Wavelength (nm)19 3 3 Figure 5-13. UV-Vis absorbance (normali zed) of ~10 M THF solutions of 4,7bis(thienyl)benzothiadiazole ( 1 : solid), 4,7-bis(1-Boc pyrro le) benzothiadiazole ( 9: dash), 4,7-bis(1-H pyrrole) benzothiadiazole ( 3: dot). Figure 5-14. Crystal structure of 4,7-bi s(1-Boc pyrrole) benzothiadiazole ( 9) 145

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The role of hydrogen bonding in the UV-Vis absorbance of 3 is evident with the influence of solvent polarity. Figure 5-15 shows the absorbance of ~10M solutions of 3 in various solvents. As the solvent polarity a nd hydrogen-bonding ability increases, max shifts hypsochromically over a 50 nm range. This change suggests that the solvent interferes with the intramolecular hydrogen bonding, increasing the tors ional freedom of the trimer and thus reducing conjugation within the DAD unit. 300350400450500550600650700 0.0 0.6 1.2 1.8 2.4 3.0 3.6 460480500520540560580 0.5 1.0 Abs (normalized)Wavelength (nm) Hexanes Chloroform Ethyl acetate MethanolAbs (normalized)Wavelength (nm) Figure 5-15. Solvatochromic ef fect on UV-Vis absorbance of ~ 10 M solutions of 4,7-bis(1-H pyrrole) benzothiadiazole ( 3). Inset: Magnification of region around max From this data one can see that the bispyrro le benzothiadiazole doe s provide two important advantages. As predicted, compound 3 has a smaller HOMO-LUMO gap than 1 while the Boc protected derivative 9 has a larger gap. Additionally, from the solvatochromic effects on 3, one can see that this DAD does interact with polar solvents through hydrogen bonding interactions. 146

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A final note regarding these DADs is their fluorescent properties. Both 1 and 9 are strongly fluorescent compounds while 3 has an extremely weak fluorescence under UV illumination (Figure 5-15). This difference is likel y due to intramolecu lar hydrogen bonding of 3 which gives rise to non-radiative transitions from its excited state. 450500550600650700750800 0.0 0.2 0.3 0.5 0.7 0.8 1.0 PL (a.u.)Wavelength (nm) PL x 100 Figure 5-16. PL data of ~10 M solutions of 4,7-bis(1-Boc pyrrole) benzothiadiazole ( 9: dash), 4,7-bis(1-H pyrrole) benzothiadiazole ( 3: solid). Synthesis of Low Bandgap CPEs Functionalized With Carboxylic Acids via Suzuki Polymerization Synthesis of 9,9-(2-methylp entyl)alkanoate fluorene Building from the previous two sections, two low bandgap CPEs can be synthesized, using the thiophene and pyrrole based DADs and fluoren es functionalized with thermally cleavable esters. Because the t -butyl ester used in Poly(Fl-Ph)TB afforded little solubility, subsequent work with this type of ester requires a modified derivative wh ere one methyl group is exchanged for a longer alkyl chain. Initial model reactions to use the propionate ester were unsuccessful for 147

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Figure 5-17. Synthetic routes towards t-butyl propionate ester derivatives of fluorene a variety of reasons. As shown in Figure 517, Michael addition was attempted based on the chemistry used in Chapter 4 (Route A); however, the acrylate that was synthesized was difficult to purifiy and handle due to its spontaneous pol ymerization. Route B uses a substitution reaction which is generally very successful for alkylating fluorenes; howeve r, multiple by-products were formed, possibly related to E1cb/E2 side reactio ns which are possible given the acidity of the esters position. Route C used esterification of an aci d chloride as the final step; however, the monoester/monoacid derivative was the predom inant product amid other side products. 148

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To circumvent these problems, the pendant group was changed from a propionate to butanoate ester and was formed via Route B (a lkylation by substitution reaction). By placing a methylene space between the esters position and alkyl halide, e limination reactions are less significant with a negligible influence on the pr operties of the final polymer and CPE. Figure 518 shows the synthetic route used to synthesize this monomer. 2-Methyl-pent-2-ol was reacted with 4-bromobutryl chloride to give 12. Using this tertiary alcohol wi ll improve the solublility of the CPE precursor relative to the t-butyl ester. Compound 12 is then used in a simple alkylation reaction with 2,7-dibromofl uorene, as was used for 7 of Chapter 3. This reaction gives 13 in a 90% yield. Pd-catalyzed bor ylation was then used to give 2,7 -bis(boronic acid pinacol ester)-9,9(2-methylpentyl )butanoate fluorene ( 14) Figure 5-18. Synthesis of 2,7-bis(boronic acid pinacol ester)-9,9-(2-methylpentyl )butanoate fluorene (14 ). Synthesis of low bandgap CPEs Compound 10 and dibromoderivative of 1 ( 15) were both polymerized with 14 by the same base-free Suzuki conditions used in Chapte r 4 (Figure 5-19). The thie nyl derivative yielded 149

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B B O O O O C3H7 C3H7 O O O O 14 N S N S S Br Br 15 S S N S N N N N S N O O O O O O O O O O O O n n B B O O O O C3H7 C3H7 O O O O 14 N S N N N O O O O + + Pd2dba3,HP(t-Bu)3BF46MCsF,THF(reflux) Pd2dba3,HP(t-Bu)3BF46MCsF,THF(reflux)Br Br 10poly(Fl-BTBtd)DE (Mn=14,100,Xn=18,PDI=2.86) poly(Fl-BPBtd)DE (Mn=6,900,Xn=9,PDI=1.20) Figure 5-19. Synthesis of poly(Fl-BPBtd)-DE and poly(Fl-BTBtd)DE a higher molecular weight polymer than the pyrrole derivative. This differe nce is likely due to the increased steric hinderance of the Boc group at the 1-position for 10. While this molecular weight is significantly lower than the other polymers of this family, the degree of polymerization is high enough to be assured of the maximal conjugation length and to test the polymers properties before and after thermal deprotection.182 Both polymers were characterized by 1H NMR, GPC, and elemental analysis. To test the thermal cleavage of polymers esters, thermogravimetric experiments were performed. Figures 5-20 and 5-21 sh ow TGA data for poly(Fl-BTBtd)DE The former is under scanning conditions, giving a transition at 200 C co rresponding to cleavage of the esters. When held at 185 C, the esters are c onverted to carboxylic acids in approximately one hour (Figure 521). For the scanning TGA data (Figure 5-22) for the pyrrole derivative poly(Fl-BPBtd)DE the profile for thermolytic cleavage of the esters is si milar to the previous polymer. The isothermal 150

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Figure 5-20. TGA (scan 10 C/min, N2) of poly(Fl-BTBtd)DE Figure 5-21. TGA (isothermal, N2) of poly(Fl-BTBtd)DE at 185 C. 151

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Figure 5-22. TGA (scan 10 C/min, N2) of poly(Fl-BPBtd)DE Figure 5-23. TGA (isothermal, N2) of poly(Fl-BPBtd)DE at 185 C. 152

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experiment (Figure 5-23) at 185 C shows that th e reaction is effectively completed in less than an hour. Based on these experiments, the polymers we re converted to th eir diacid form by rotovapping a solution of the polymers in round botto m flasks, forming a thin film on the interior surface.The flasks were then heated at 185 C under argon for 1.25 hours. Photos of the flask containing poly(Fl-BPBtd) were taken before an d after the thermal treatment and are shown in Figure 5-24. The loss of the BOC groups is clearly seen in the color change, which is correlated very well with the color changes seen in Figure 513. As expected, no change in color is seen for poly(Fl-BTBtd) after thermal treatment, because the conformation of the polymer backbone is not changing. The diester form of poly(Fl-BTBtd) is so luble in nonpolar orga nic solvents (THF, chloroform, toluene) while the thermally treated derivative is insoluble in chloroform, toluene, and water and soluble in THF, DMSO, and wa ter/polar organic solvent mixtures. These solubilities are consistent with the CPEs synthesized in Chapter 4. Unfortunately, poly(FlBPBtd) was insoluble in all solvents after the thermal treatment, including basic aqueous solutions, refluxing chloroform, and refluxing DMSO. Based on the observed color changes and elemental analysis of the diester and diacid fo rms, the desired transformation did occur. A possible explanation for the polym ers insolubility is a side re action between carboxylic acid and 1-H pyrrole forming amide crosslinks. Because of this problem, the spectral properties and solvatochromic effects will only be studied for poly(Fl-BTBtd). Conversion of the esters to car boxylic acids was determined by 1H NMR, IR, and elemental analysis for poly(Fl-BTBtd)DA and poly(BPBtd)DA was characterized by IR and 153

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Figure 5-24. Photos of poly(Fl-BPBtd) before and after heating at 185 C. elemental analysis. For the former, the IR sp ectrum was obtained by casting a film from THF onto a salt plate; because poly(Fl-BPBtd) is insol uble in its acidic form, th e polymers ester form was drop cast onto a salt plate, and the film/pla te placed in a vacuum oven at 185 C for one hour. Figure 5-25 shows the IR spectra of the ester and acid derivatives of both polymers. For both polymers, the carbonyl stretch shifts to lo wer wavenumbers as expect ed, with the region at 3300 cm-1 showing a broad but notably weak peak for the carboxylic acid stretch. Additionally, for poly(BPBtd) one can see a peak at 3400 cm-1 that corresponds to the N-H stretch of pyrrole. Poly(Fl-BTBtd)DA: Spectral properties and So lvatochromic Behavior At the beginning of this chapter, the influence of the DAD monomer on the CPE properties were briefly addressed, suggesting that having a sequence of f our aryl groups per repeat unit would greatly increase its hydrophobicity. This effect is clearly seen in the UV-Vis absorbance and PL data for this pol ymer. Figure 5-26 shows poly(Fl-BTBtd)DA in THF/water solutions. Here one can see that as the water co ntent increases from zero to 40%, the peaks red shift slightly (5-10 nm). At 60% and 80% water, the peaks oscillator strength decreases due to aggregation. While these changes are evident, the interesting characteristic of this data is how minimal and insignificant the differen ces are. Considering poly(Fl-BTh)DA in the same set of solutions, the onset of the abso rbance varied over a 50 nm range. The reason for the different 154

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4000350030002500200015001000500 94 96 98 100 102 104 Wavenumber (cm-1)Transmittance (%) -Diacid78 80 82 84 86 88 90 92 94 96 98 S S N S N OR RO O O nTransmittance (%) -Diester 4000350030002500200015001000 70 80 90 H N HN N S N OH HO O O nWavenumber (cm-1)Transmittance (%) Diacid40 50 60 70 80 90 100 Transmitance (%) Diester Figure 5-25. IR spectra of poly(Fl-BTBtd) [Top] and poly(Fl-BPBTD) [bottom] on salt plates (NaCl). 155

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350400450500550600650700750800 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 100% THF 80% THF/ 20% water 60% THF/ 40% water 40% THF/ 60% water 20% THF/ 80% waterAbs.Wavelength (nm) Figure 5-26. UV-Vis absorbance solvatochromis m of 10M solutions of Poly(Fl-BTBtd)DA in THF/water mixtures. 350400450500550600650700750800 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 80% THF/ 20% aq. base 60% THF/ 40% aq. base 40% THF/ 60% aq. base 20% THF/ 80% aq. baseAbsWavelength (nm) Figure 5-27. UV-Vis absorbane solvatochromis m of 10M solutions of Poly(Fl-BTBtd)DA in THF/basic water(pH 9) mixtures. 156

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DAD systems are significantly more localized th an an all donor system like poly(Fl-BTh)DA Thus, the bithiophene-based polymer has dramatic differences in spectral properties when the conjugation length is changed, wh ereas the bisthienyl benzothi adiazole does not.Figure 5-27 displays the UV-Vis absorbance of poly(Fl-BTBtd)DA in THF/basic solutions, and here one sees even fewer differences between the solutions. There is clearly a transition in polymer conformation and spectral absorbance at the transi tion of 50:50 THF/basic solution, but this represents only a change of oscillator strengt h due to polymer aggregation. While the spectral absorbance shows minimal solvatochromic effects, the PL data displays more dramatic effects. Figures 528 and 5-29 are the PL data for poly(Fl-BTBtd)DA in THF/water and THF/basic solution, respect ively. For both the carboxylic acid and carboxylate form, one sees the same profile wher e increasing water content decreases the PL intensity. This is clearly a f unction of the polymer chains hyd rophobicity. In Figure 5-30, the PL intensity of these solutions is plotted relative to the amount of THF in solution. Here one can see that the carboxylate derivatives inhibit interchain interact ions in the 60% and 80% THF solutions, but not in the lower THF concentratio ns. The reason for this effect at lower THF concentrations may be that the polymer is so poo rly solubilized that the ionic groups have little effect. Conclusion This chapter presents a new design for CPE precursors whereby thermolysis is used to convert an ester to a carboxylic acid. This route is well-suited for conjugated polymers due to their thermal robustness, and a llows base-sensitive moieties such as DAD monomers to be used in CPEs. Two different esters were studied for this system, but the t-butyl derivatives proved far superior due to their greater synthetic utilility; howe ver, given the facile and direct route to the 1157

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600 650 700 750 800 0.0 2.0x1054.0x1056.0x1058.0x1051.0x106 PL (cps)Wavelength (nm) 100% THF 80% THF/ 20% water 60% THF/ 40% water 40% THF/ 60% water 20% THF/ 80% water Figure 5-28 PL solvatochromism of 10M solutions of Poly(Fl-BTBtd)DA in THF/water mixtures. Excited at max, abs. 600 650 700 750 800 0 1x1052x1053x1054x1055x105 80% THF/20% aq. base 60% THF/40% aq. base 40% THF/60% aq. base 20% THF/80% aq. basePL (cps)Wavelength (nm) Figure 5-29. PL solvatochromism of 10M solutions of Poly(Fl-BTBtd)DA in THF/basic water(pH 9) mixtures. Excited at max, abs. 158

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20 40 60 80 0 1x1052x1053x1054x1055x105 PL (cps)% THF Figure 5-30. PL intensity of poly(Fl-BTBtd)DA in THF/water ( ) and THF/basic aqueous solutions ( ) ethoxyethyl ester, the lower temper atures for cleaving this ester co uld be advantageous for other polymers. Of equal importance in this work is th e pyrrole chemistry that was studied. The bispyrrole benzothiadiazole system (BPBtd) o ffers more than a novel variation of the often studied bis-thienyl benzothiadiazole. The sp ectral properties of BPBtd shows a strong dependence on the functionalizati on of pyrroles nitrogen, sh ifting the UV-Vis absorbance by 150 nm and the PL intensity by more than two orders of magnitude. Reasonable explanations have been given for these dramatic effects, but in a sense, only two points were used to make a line, and other parameters such as the electron donating/withdrawing strength of the substituent could play a role. Additionally, although the final CPE proved insoluble, the basis for this insolubility is not known and could be avoided with optimization of the reaction conditions. 159

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Experimental Compound 3182, 895, 11182 and 15190 were synthesized accordi ng to previously reported procedures. All column chromatography was performed with silica by gravity 2,7-dibromo-9,9-di(methyl propionate)fluorene (4). 2,7-dibromofluorene (5 .00 g, 15.4 mmol), methyl acrylate (12 mL, 11.4 g, 154 mmol), and TE BA (~500 mg, 10 wt%) were dissolved in 50 mL toluene. 50 % KOH solution (11 mL) was added slowly and the reaction stirred overnight. Extracted reac tion between ethyl acetate and water. Organic extracts were dried with magnesium sulfate and rotovappe d to dryness. The yellow-white solid was recrystallized two times from heptanes and once from methanol. mp: 121-124oC 1H NMR (300 MHz,CDCl3) 1.58 (t, 4H), 2.39 (t, 4H), 3.48 (s, 6H), 7.2-7.4 (m, 6H) 13C NMR (75 MHz,CDCl3) 28.8, 34.4, 51.5, 54.2, 121.2, 128.3, 130.1, 131.2, 139.2, 149.4, 173.15. HRMS calcd for C21H20O4Br2 (M+): 493.9723, found: 493.9714. Anal. Calcd for C21H20O4Br2: C, 50.83; H, 4.06; Br, 32.21; O, 12.90. Found: C, 50.985; H, 4.106. 2,7-dibromo-9,9-di(propionic acid)-fluorene ( 5 ). 4 (1.99 g, 4 mmol) was dissolved in ~15 mL of THF. This organic solution was added to a 3 M KOH solution (20 mL) and mixed with 50 mL of an ethanol/water solution (1:1). This solution was refluxed for 24 hours. The reaction was allowed to cool a nd all organic solvents were removed by rota ry evaporation. The basic aqeous solution was extrac ted two times with ether. 2 N HCl were added to the basic aqeous solution to adjust the pH<6. A white precipitate formed. This precipitate was filtered and washed with copious amounts of water. The wh ite solid was dried under vacuum overnight and used without further purification. Yield: 1.96 g (97%) of white solid. 1H NMR (300 MHz,DMSO) 1.32 (t, 4H), 2.27 (t, 4H), 7.54 (d, 2H), 7.7-7.9 (m, 4H). 13C NMR (300 MHz,DMSO) 34.2, 38.8, 59.3, 126.8, 127.6, 131.8, 136.3, 144.2, 155.5, 179.3. 160

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2,7-dibromo-9,9-(1-ethoxyethyl)propionate-fluorene ( 6). 5 (2.6 g, 5.5 mmol) and p-TSA (1 mg) was placed in 10 mL of diethyl ether, in which 6 is insoluble. This solution is cooled to 0 C. Ethyl vinyl ether (6 mL) was added and the solution stirred for 5 min at 0 C. The solution was then stirred unti all of 6 has dissolved after which it was stirred for an additional 15 minutes. Organic solution was then washed with a satu rated sodium bicarbonate solution, followed by a brine solution. Organic phase dried with magnesium sulfate, filtered and rotovapped to yield a viscous yellow oil. Oil redissolv ed in diethyl ether and treated with activated carbon. Solution filtered through Celite, the solvent was removed by rotary evaporation and the product dried under vacuum at 50 C. Yield: 2.41 g (71%) of a clear viscous oil. 1H NMR (300 MHz,CDCl3) 1.16 (t, 6H), 1.28 (d, 6H), 1.5-1.6 (m, 4H), 2.3-2.4 (4H), 3.3-3.6 (4H), 5.78 (q, 2H), 7.4-7.6 (m, 6H). 13C NMR (75 MHz,CDCl3) 14.9, 20.7, 28.9, 34.0, 54.0, 64.6, 96.2, 121.5, 122.1, 126.4, 131.2, 139.1, 149.4, 172.4. HRMS calcd for C27H32Br2O6H (M+H)+: 635.0440, found: 635.0442. 2,7-dibromo-9,9-di( t-butyl propionate)-fluorene ( 7) 2,7-dibromofluorene (2.001 g, 6.17 mmol), t-butyl acrylate (7.9 g, 61.7 mmol), and TEBA (200 mg, 10 wt%) were added to 20 mL of toluene. 50% KOH solution (4 mL) was added slowly with vi gorous stirring. Reaction was run for 30 minutes. Reaction mixture was extr acted between ethyl acetate and water. The organic phase was dried with magnesium sulfate, filtered, and the organic solution rotovapped to dryness. The crude yellow product was pur ified by column chromatography (SiO2, hexanes/ether [5:1]). Yield: 2.9 g (84% yield) of white crystalline solid. mp:178-180 C. 1H NMR (300 MHz,CDCl3) 1.35 (s, 18H), 1.50 (t, 4H), 2.27 (t, 4H), 7.2-7.4 (m, 6H). 13C NMR (75 MHz,CDCl3) 27.9, 29.7, 34.3, 54.0. HRMS calcd for C27H32Br2NaO4 (M+): 603.3384, found: 603.0541. Anal. Calcd for C27H32Br2O4: C, 55.88; H, 5.56; Br, 27.54; O, 11.03. Found: C, 55.821; H, 5.576. 161

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4,7-Bis(N-Butoxycarbonylpyrrol-2-yl)-2,1,3-benzothiadiazole ( 9). Entry 1:4,7dibromobenzothiadiazole (352.8 mg, 1.2 mmol) and 8 (807 mg, 2.66 mmol) were placed in a flame dried 50 mL flask with ~30 mL of anhydrous THF. Solution was bubbled with argon for 45 minutes. PdCl2(PPh3)2 (42 mg, 0.06 mmol) was added and the solution refluxed for 48 hours. Solvent was removed by rotary evaporation. A ttempted purification by column chromatography with chloroform/hexane (1:1) as the eluent, recovering none of the product. Entry 2 : 4,7dibromobenzothiadiazole (760 mg, 2.61 mmol), 8 (1.896 g, 5.7 mmol), and cesium fluoride (1.586 g, 10.44 mmol) were added to a dry 50 mL fl ask and 5 mL of anhydrous DMF was added. Full vacuum (1 mm Hg) was pulled on the solution for 10 minutes, and the flask was backfilled with argon. Copper (I) iodide (19.9 mg, 0.1044 mmol), tri( tbutyl) phosphonium tetrafluoroborate (30.3 mg, 0.1044 mmol), and palladium (II) ch loride (9.3 mg, 0.0572 mmol) were added and the flask was heated at 45 C for 48 hours. Reaction poured into water and extracted with ether two times, then once with chloroform. Organic phases dried with magnesium sulfate, filtered, and the solvent removed by rotary evaporation. Red-brown crude product was purified by column chromatography w ith chloroform/hexanes (1:1). Recovered 325 mg of yellow-orange product (27%). Entry 3 : 4,7-dibromobenzothiadiazole (0.649 g, 2.20 mmol), 8 (1.6 g, 4.85 mmol), and cesium fluoride (1.34 g, 8.8 mmol) were added to a dry 50 mL flask and 5 mL of anhydrous 1,4-dioxane wa s added. Argon was bubbled through this mixture for 40 min. Tri( t-butyl) phosphonium tetrafluorobor ate (38 mg, 0.132 mmol), and Pd2dba3 (30 mg, 0.033 mmol) were added and the flask was h eated at 80 C for 48 hours. Reaction poured into water and extracted with ether two times. Organic phases dried with magnesium sulfate, filtered, and the solvent removed by rotary ev aporation. Red-brown crude product was purified by column chromatography with chloroform/hexanes (1:1). Recovered 535 mg of yellow-orange 162

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product (52%).1H NMR (300 MHz,CDCl3) 1.20 (s, 24H), 6.34 (t, 2H), 6.43 (q, 2H), 7.50 (q, 2H), 7.58 (s, 2H). 13C NMR (75 MHz,CDCl3) 27.4, 32.2, 83.4, 110.7, 115.6, 123.3, 127.4, 130.2, 149.0, 154.3. HRMS calcd for C24H26N4O4SNa (M+Na)+: 489.1572, found: 489.1567. Anal. Calcd for C24H26N4O4S: C, 61.78; H, 5.62; N, 12.01; O, 13.72; S, 6.87. Found: C, 61.048; H, 5.577. 4,7-Bis(2,2-dibromo-N-Butoxycarbonylpy rrol-5,5-yl)-2,1,3-benzothiadiazole ( 10 ). 9 (298 mg, 0.639 mmol) was added to a dry 50 mL flask with ~20 mL of anhydrous THF. The solution was cooled to -78 C and freshl y recrystallized/vacuum dried NBS (228 mg, 1.277 mmol) was added. The reaction flask was covered with aluminum foil.This solution was stirred overnight and allowed to slowly warm to r oom temp. Solution was poured into water and extracted with ether. The organic phase was dried with magnesium sulfate, filtered, and the solution rotovapped to remove so lvent. Recovered 398 mg (100%) of a yellow orange solid. mp: 150 C (dec). 1H NMR (300 MHz,CDCl3) 1.23 (s, 24H), 6.41 (m, 4H), 7.58 (s, 2H). 13C NMR (75 MHz,CDCl3) 27.4, 85.0, 103.6, 114.8, 115.6, 127.0, 132.4, 148.3, 153.6, 188.28. HRMS calcd for C24H24N4 Br2O4SNa (M+Na)+: 644.9783, found: 644.9771. Anal. Calcd for C24H24N4 Br2O4S: C, 46.17; H, 3.87; Br, 25.60; N, 8.97; O, 10.25; S, 5.14. Found: C, 46.38; H, 4.04: N, 8.69. 2-methylpentan-2-yl 4-bromobutanoate ( 12). 2-methyl-pent-2ol (2.75 g, 26.96 mmol) was placed in a dry 100 mL flask with 22 mL of anhydrous DCM and anhydrous pyridine (2.2 mL, 26.96 g). Flask was cooled to 0 C. 4-brom obutryl chloride (5 g, 26.96 mmol) was added slowly via syringe. Flask was allowed to slowly warm to room temperature and stirred overnight. Reaction was poured into water and extracted with DCM. The organic phase was washed with a basic aq. solution, water, then brine. The organic extracts we re then dried with magnesium 163

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sulfate, filtered, and rotovapped to dryness. Compound was used without further purification. Yield 5.21 g (77%) of clear oil. 1H NMR (300 MHz,CDCl3) 0.90 (t, 3H), 1.2-1.4 (m, 2H), 1.41 (s, 6H), 1.6-2.0 (m, 6H), 2.22 (t, 2H), 3.39 (t, 2H). 13C NMR (75 MHz,CDCl3) 14.3, 17.1, 23.6, 25.9, 31.9, 33.0, 34.4, 43.0, 82.5, 172.3. HRMS calcd for C10H19BrO2(M+H)+: 251.0641, found: 251.0622. Anal. Calcd for C10H19BrO2: C, 47.82; H, 7.62; Br, 31.81; O, 12.74. Found: C, 47.713; H, 7.787. 2,7-dibromo-9,9-(2-methylpentan-2-y l 4-butanoate) fluorene (13 ). 2,7-dibromofluorene (2.268 g, 7 mmol), 12 (4.403 g, 17.5 mmol) and potassium iodide (185 mg, 0.7 mmol) were added to 30 mL of DMSO and heated to solub ilize the components. The reaction was cooled to room temperature. Powdered KOH (2 g, 35 mm ol) was added slowly and the reaction run overnight. Solution was extracted between wa ter and ether. The or ganic fractions were extracted two times with water, and two times with brine. Organic phase was dried with magnesium sulfate, filtered, and rotovapped to dryness. Purified by passing through a 2 plug of silica with chloroform/hexanes (4:1) as the eluent discarding the yellow band that first elutes, and collecting the strong spot that followed. Yi eld 4.2 g (90%) of a off-white powder. mp: 67-69 C. 1H NMR (300 MHz,CDCl3) 0.5-0.6 (m, 4H), 0.81 (t, 6h) 1.1-1.4 (m, 20H), 1.5-1.6 (m, 4H) 1.8-2.0 (m, 8H) 7.4-7.8 (m, 6H). 13C NMR (75 MHz,CDCl3) 14.3, 16.9, 25.2, 25.7, 35.1, 39.7, 43.0, 55.2, 81.9, 121.1, 121.5, 125.8, 130.2, 138.8, 151.7, 172.4. Anal. Calcd for C33H44Br2O4: C, 59.65; H, 6.67; Br, 24.05; O, 9.63. Found: C, 59.58; H, 6.81. 2,7-bis(boronic acid pinacol ester)-9,9-(2 -methylpentan-2-yl 4-butanoate) fluorene (14 ) 13 (1. 001 g, 1.51 mmol), bispinacolatodiboron (1.23 g, 4.84 mmol), and potassium acetate (890 mg, 9.06 mmol) were mixed with 20 mL of anhydrous DMF. Full vacuum was placed on this flask for 15 minutes, and the flask was backfilled with argon. PdCl2(dppf) (42 mg, 0.0735 164

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mmol) was added and the flask was heated at 70 C overnight. Solution was extracted between water and ether. The organic fractions were extr acted two times with water, and two times with brine. Organic phase was dried with magnesium su lfate, filtered, and rotovapped to dryness. The solid residue was recrystallized two times fr om acetonitrile. Yield 357 mg (47%) of a white crystalline solid. mp: 167-169. 1H NMR (300 MHz,CDCl3) 0.5-0.6 (m, 4H), 0.82 (t, 6h) 1.1-1.5 (m, 56H), 1.5-1.6 (m, 4H) 1.8-2.1 (m, 8H) 7.6-7.9 (m, 6H). 13C NMR (75 MHz,CDCl3) 14.3, 17.0, 19.5, 24.9, 26.0, 35.8, 39.4, 43.0, 54.6, 82.0, 83.6, 119.5, 128.7, 134.0, 143.8, 149.2, 172.6. HRMS calcd for C45H68B2O8Na(M+Na)+: 781.5008, found: 781.5028. Anal. Calcd for C45H68B2O8: C, 71.24; H, 9.03; B, 2.85; O, 16.87. Found: C, 71.51; H, 9.08. General procedure for Suzuki polymerization: Aromatic diboronate ester (1 eq.), aromatic dihalide (1 eq.), cesiu m fluoride (30 eq.), and tri( tbutyl)phosphonium tetrafluoroborate (0.06 eq.) were added to a 25 mL (for 1 mmol of each monomer) fdry flask. Flask was purged with argon four times. To this flask, Pd2dba3 (0.02 eq.) was added to the flask along with degassed THF (12 mL per mmol of monomer) and degassed water (5 mL) were transferred to the flask by cannulation. Reflux r eaction for at least 40 hours. Organic phase is precipitated into methanol. Redissolved in chlo roform and precipitated into methanol again. Polymer is dried under vacuum. Poly(Fl-BTBtd)DE: 1H NMR (300 MHz,CDCl3) 0.8-1.4 (m, 26H), 1.5-1.7 (m, 4H), 2.0-2.2 (m, 8H), 7.4-8.2 (m, 12H). Anal. Calcd for C47H50N2O4S3: C, 70.29; H, 6.28; N, 3.49; O, 7.97; S, 11.98. Found: C, 69.31; H, 6.22; N, 3.49. Poly(Fl-BPBtd)DE: 1H NMR (300 MHz,CDCl3) 0.8-1.4 (m, 44H), 1.4-1.8 (m, 4H), 1.8-2.2 (8H), 6.4-6.8 (m, 4H), 7.3-7.9 (m, 8H). Anal. Calcd for C57H68N4O8S: C, 70.63; H, 7.07; N, 5.78; O, 13.21; S, 3.31. Found: C, 68.28; H, 6.83, N, 5.66. 165

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Poly(Fl-BTBtd)DA: 1H NMR (300 MHz,DMSO) 0.6-2.4 (m, 24H), 7.2-8.4 (m, 12H), 11.9 (s, 2H). Anal. Calcd for C35H26N2O4S3: C, 66.22; H, 4.13; N, 4.41; O, 10.08; S, 15.15. Found: C, 66.64; H, 5.23; N, 3.92. Poly(Fl-BPBtd)DA: Anal. Calcd for C35H28N4O4S: C, 69.98; H, 4.70; N, 9.33; O, 10.65; S, 5.34. Found: C, 65.76; H, 4.67; N, 8.57. 166

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CHAPTER 6 POLY(BENZO[1,2-B:4,3-B]D ITHIOPHENES: SYNTHESI S OF A POLYFLUORENE ANALOGUE WITH IMPROVED ELECTROCHE MICAL PROPERTIES AND INTERCHAIN INTERACTIONS Introduction Chapters 3-5 present families of conjugated po lymers that incorporate fluorene derivatives. Modifying the structure of the polymer backbone and fluorenes pendant groups have allowed the spectral and electrochemical properties of these polymers to be designed and modified effectively. A reoccurring property among these polymers is their propensity to form ordered structures. This was observed in the DSC da ta of the high bandgap polymers presented in Chapter 3 as well as the spectral properties of some fluorene-containing CPEs from Chapter 4. These sections (in addition to further discussions in Chapter 7) address how this tendency to order can be useful. Increasing the order between polymer chains has been an active area of research in the past decade. The hallmark of such work is research on poly(3-hexylthiophene) where the synthesis of the highly regioregular (>90%)48, 191 polymer gives contro l of nanoscale morphology,192, 193 due to the trans conformation of the adjacent monomers favoring interactions between polymer chains. This optimized morphology has led to exce llent performance in fi eld-effect transistors and photovoltaics. Additionally, a no table direction over the past de cade has been the synthesis of solvent processable polymers based on fused heterocyclic units.194-197 For structures such as thieno[a,b]thiophenes, these fused heterocy cles provide a means of controlling the electrochemical properties and bandgap.198 Fused heterocyclic units have also been used to promote the polymers aggregation or crystallinity. Recent work from Merck has made great strides with highly conductive poly mers based on thieno[3,2-b]thiophene with a thiophene-based copolymer for high charge mobility in field-effect transitors .193 167

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With this work as inspiration, the design and synthesis of benzo[1,2-b:4,3-b']dithiophene based polymers (PBDT) [Figure 6-1] were consid ered. Like fluorene, BDT is a three ring, fused aromatic system with a notable difference. Fluorene and carbazole (two widely studied monomers for conjugated polymers) both share the same structural motif of two six membered rings (benzenes) that are joined together by a bridging atom (carbon a nd nitrogen, respectively). This structure can be described as a 6-5-6 aromatic molecule. Interestingly, the inversion of this a 5-6-5 structure is not common, and while th e core BDT-structure has been studied in the past,199-201 a chemically polymerized, soluble homopolymer has not been studied. The interest in these polymers is due to three notable design characterist ics. First, as shown with polyfluorene, such fused aromatic mono mers give improved conjugation along the polymer backbone due to forced planarity within the monomer.85 Additionally, by placi ng the solubilizing groups of these polymers in the 4/5 positions, they will not induce torque between monomers nor inhibit interchain interactions. Finally, the cate nation angle is akin to a poly(m-phenylene) [ = 120]; though in difference, PBDT is fully conjugat ed. This kinked linkage can play a significant role in the conformational freedom of these polymers and thus its ability to order.123, 202 This chapter presents two poly(BDTs) which have been synthesized via a Colin/Kelsey or Suzuki polymerization. Substitution with linear versus branched substituents influences the spectral and electrochemical properties of these polymers, where the former shows effects due to increased polymer aggregation. From the spectra l absorbance one sees bathochromic bands associated with the aggregated species. Intere stingly, the electrochemical data also shows the influence of aggregation where the aggregate of the linear derivative has an oxidation potential that is 0.5 V less than th e branched derivatiive. 168

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Figure 6-1. Structure of poly(benz o[1,2-b:4,3-b]dithiophenes) [PBDTs] Monomer and Polymer Synthesis To synthesize most fused heterocycles, th e cyclic units must be formed directly. Fortunately the synthesis of BD T was refined by the work of Turro and coworkers, providing a facile and high yielding r oute to such materials.203 Based on this work, compound 3 was synthesized in 45% yield star ting from 3-bromothiophene (Fi gure 6-2). Hydrolysis of the acetates while stirring with the respective alkyl halide gives the primary monomer structure 4 in 93-97% yield. The dioctyl (BDT-Oct) and di[(2ethyl)hexyl] (BDT-EtHex) derivatives were synthesized via this method. These materials were brominated with NBS in DMF in 60-67% yield. S S O O S Br S S 1)n-BuLi 2)MgBr23)Ni(dppp)Cl2 S S O O O O 2 3 S S RO OR 1,2-dichlorethane(reflux) 10days Cl Cl O O Zn,Ac2O NEt3,DCM(anhydr) CsCO3,ACN RX NBS DMF S S RO OR Br Br R= 1 4a : 4b : R= 5a,5b Figure 6-2. Synthesis of BDT monomers. 169

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Figure 6-3. X-ray crystal stru cture of 5,6-BDT diacetate ( 3 ). The BDT structure was designed for impr oved interchain interaction from the corresponding polymer. To assess the potential -stacking of this structure, the crystal structure of 3 was obtained. While this structure lacks the al koxy substituents of the polymer repeat unit, monomers 4 and 5 are oils, and not amenable to this characterization. As shown in Figure 6-3, the BDT structure is planar as expected. From the unit cell of 3 one sees that the molecules are aligned in a herringbone structure. This latt ice places pairs of molecules in a cofacial orientation, as shown to the left of Figure 6-3. The perspe ctive of this pair of BDTs has the plane of the aromatic rings lying in the plane of th e page. The distance between these molecules is 170

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3.496 at their closest points. With this proxim ity, these molecules must have orbital overlap between their -systems. Such interactions are relevant to electroactive properties dependent on charge mobility,81 suggesting the potential of BDT in photovoltaic and charge transport applications. To form the respective polymers, both dihalide monomers 5a and 5b were polymerized via a modified Yamamoto coupling developed by Colin and Kelsey. This method provides a nickel-catalyzed aryl-aryl coupl ing by regenerating the nickel ca talyst with a stoichiometric amount of zinc. Various conditions were studied (Figure 6-4). For entries 1 and 2, BDT-EtHex 5b was polymerized under the same conditions w ith the monomer concentration varied. Using more concentrated monomer solutions, higher molecular weights were achieved, which agrees with findings by Sheares et al .204 The same set of conditions was applied to BDT-Oct ( 5a) (Figure 6-4, Entries 3 and 4) w ith effectively the same results: higher monomer concentrations yield higher molecular weights. The effectiveness of these polymerizations appear to be due to a balance of improved reactivity at high concentrat ions against the reduced polymer solubility in such a polar solvent. To improve the polymer solubility, BDTs functionalized with oligoethoxy chains were employed but these afforded only small improvements (Mn = 8600 g/mol). Due to the significantly lower Mn of PBDT-Oct via polymeriza tion with nickel, BDT-Oct was also polymerized by a Suzuki polymeriza tion. Whereas the Colin/Kelsey requires polar solvents such as DMAC, Suzuki couplings can be performed in better solvents for solubilizing conjugated polymers (e.g., THF, toluene). To use the Suzuki conditions, a 2-bromo-2pinacolatoboronate ester was synthesi zed. As shown in Figure 6-4, monomer 6 was obtained by the lithiation of 5 with l equivalent of n-butyl lithiu m followed by the addition of 2-isopropoxy4,4,5,5-tetramethyl-1,3,2-dioxaborolan e in 33% yield. This low yield is thought to be 171

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S S OR RO n S S Br Br OR RO NiCl2,bipy,PPh3Zn,DMAC S S Br B OR RO O O O B O O 1)1eq.n-BuLi THF,-78oC 1.5hrs 2) Pd2dba,HP(t-Bu)3BF4CsF,dioxane 80oC S S OR RO n R= (2-ethyl)hexyl octyl R=octyl 5 6 Entry Monomer Method Conc. (M) Mn (Da) PDI DP 1 EtHex (5b) Yamamoto 0.5 4800 1.2 11 2 EtHex (5b) Yamamoto 1.0 11200 1.4 25 3 Oct (5a) Yamamoto 0.5 4200 1.2 9 4 Oct (5a) Yamamoto 1.0 6600 1.2 15 5 Oct (6) Suzuki 0.1 12700 1.7 29 Figure 6-4. Polymerizati on of BDT monomers. due to deboronation during column puri fication. Suzuki polymerization of 6 via Fu conditions61 was performed (Figure 6-4, Entr y 5), yielding a polymer with twice the number average molecular weight achieved with the Colin and Kels ey route. For the remainder of this paper, all characterization refers to the highest molecular weight deri vatives unless otherwise noted. To determine the influence of molecular weig ht on PBDTs UV-Vis absorbance, data from the GPC photodiode array (GPC-PDA) was studied to see the evolution of these absorbances with degree of polymerization. Figur e 6-5 shows the relationship between max and Mn for the lower molecular weight fractions from GPC analys is as well as the UV-Vis absorbance spectra of these fractions. One can see that the spetral features at Mn 5,000 Da are similar, and the absorbances bathochromically shift up to Mn 7,000 Da. From this, one can conclude that PBDT-Oct and PBDT-EtHex have effectively th e same spectral properties under the dilute 172

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20004000600080001000012000 380 390 400 410 420 430 maxMn(Da) Figure 6-5. Left: Change of UV-Vis absorbance spectra of PBDT-EtHex with lower molecular weight fractions from GPC Right : Relationship of max to Mn for GPC eluents. The first eight data points represent th e eight spectra shown to the left. conditions of GPC eluents, and that valid comparisons of electroactive properties can be made of polymers with molecular weights greater than 5,000 Da. In contrast to GPC-PDA data, examining the UV-Vis spectra of 1 mM solutions of polymers from entries 2, 4, and 5 from Figure 64 show three distinctly different spectra. As shown in Figure Figure 6-6, PBDT-EtHex has poor ly defined features and a blue-shifted absorbance relative to the octyl derivatives. Entry 4 (hereafter called PBDT-Oct[low]) has a similar absorbance to the former though with shoulders at 455 and 490 nm. These shoulders are well-defined peaks for entry 5 (hereafter called PBDT-Oct[high]). Notably, the peak at 490 nm for PBDT-Oct derivatives is absent from the GPC-PDA spectra. Because the solubilizing groups are not positioned to induce twisting between monomer units these low energy peaks are attributed to the aggreg ation of the polymers. To corroborate the influence of aggregati on on the spectral properties of these PBDTs, solution thermochromism provides meaningful data.205, 206 Because aggregation in solution is a 173

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300 350 400 450 500 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Normalized AbsorbanceWavelength (nm) Figure 6-6. Normalized soluti on absorption spectra of () EtHex (---) Oct(low), (--) Oct(high). All solutions are ~ 1 mM in xylenes. thermodynamic process, one can shift the equilibr ium by changing the temperature. For a freely soluble polymer, higher temperatures induce greater rotational movement along the polymer backbone, simply causing a narrowing and hypsochromic shift of the spectrum. For aggregated species, an isobestic point arises from the spectra, signifying a shift in the equilibrium for the polymer between solvated and aggregated species. Solution thermochromism was studied for th e octyl derivatives. Figure 6-7 shows the absorbance spectra of PBDTOct(low) and PBDT -Oct(high) in xylenes over a temperature range of 25-95 oC. At higher temperatures, the low ener gy peaks at 455/490 nm are greatly reduced; this reduced intensity corresponds to dissoluti on of the polymer aggreg ates. Additionally, both Oct(low) and Oct(high) show is obestic points at ~375 nm an d 420 nm, respectively. As shown with work by Leclerc et al this isobestic point indicates a shifting equilibrium of the polymer between two distinct phases: the solubilized and aggregated polymer.205 After cooling back to 174

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350 400 450 500 0.00 0.02 0.04 0.06 0.08 AbsorbanceWavelength (nm) 350 400 450 500 0.00 0.02 0.04 0.06 0.08 AbsorbanceWavelength (nm) Figure 6-7. Solution thermochromism of PBDT-O ct(low) [top] and PBDT-Oct(high) [bottom] from 25-95 C. Both solutions are 0.1 mM in xylenes. 175

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RT, the polymers retain their original UV-Vis ab sorbances, indicating thes e changes are not due to decomposition of the polymer backbone. Furthe r evidence to support this is shown in Figure 6-8, which shows the spectra of PBDT-Oct[low] at 95 oC amd PBDT-EtHex at room temperature. One can see that these spectra overlap well, suggesting that these spectra represent a solubilized PBDT. The thermochromic behavior of PBDT as well as the strong influence of molecular weight on their spectral behavior is notable These characteristics have precedence in the structure-property relationships seen with PPE derivatives.207-209 Most significantly, Swager et al characterized PPE derivatives with linear and/ or branched substituents on a Langmuir-Blodgett trough.210 This work showed how branched substitu ents inhibit polymer aggregation whereas linear groups did not. As a result, the latter has low energy absorptive bands and a bathochromic photoluminescence due to these aggregated chai ns. Whether these spec tral properties are 300350400450500550 0.0 0.2 0.4 0.6 0.8 Normalized AbsorbanceWavelength Figure 6-8. Normalized solution absorbances of ( ) PBDT-EtHex room temperature in xylenes and (---) PBDT-Oct(low) at 95 oC in xylenes 176

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properties of the aggregate unit or induction of planarity by these aggregates has not been determined. To further support the similarity between PBDT and the PPE derivatives, the photoluminescence of PBDT-EtHex and PBDT-Oct(l ow) was studied (Figur e 6-9). In solution, the linear derivative shows more pronounced peak features, but is not shifted bathochromically relative to the branched. The defined spectral feat ures represent a more rigid geometry in the excited state for PBDT-Oct(low); these features agree with the effect of polymer aggregation. The polymer films have different PL spectra with the linear derivatives max bathochromically shifted by ~40 nm as well as different peak sh ape. This again points to the effect of the substituents and the role they play in inhibiting or allowing polymer aggregation. Thermal analysis provides a means of dir ectly testing the presence of interchain interactions in a polymeric materials. Notably, the effect of molecular weight on the bulk 450500550600650 0 1 2 cps (normalized)Wavelength (nm) 475500525550575600625 0.0 0.5 1.0 cps (normalized)Wavelength (nm) Figure 6-9. Left: PL spectra of PBDT-Oct(low) [red] and PBDT-EtHec [black] in 10 M THF solutions. Right: PL spectra of PBDT-Oct(low) [red] and PBDT-EtHec [black] in solid state. 177

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Figure 6-10. DSC overlay of PBDT -Oct(low) and PBDT-Oct(high) behavior of this material is evident by its thermal behavior. Both differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) were employed to study these effects. In the standard DSC plot (Figure 6-10) the differen ce in thermal response of PBDTOctsamples is clear. PBDT-Oct(low) exhibits a flat thermal profile, devoid of detectable transitions, on both heating and cool ing. This is in sharp contrast to the complex thermal profile witnessed for PBDT-Oct(high), which shows a number of transitions on both heating and cooling. The heating trace is particularly complicated and the assignment of individual transitions difficult as the ba seline is poorly resolved. Because of its increased sensitivity and abil ity to resolve overlapping transitions, MDSC was used (Figure 6-11).211-214 The MDSC heating trace for the PBDT-Oct(high) displays a cold crystallization event at -100 C s een in both the total and nonreversi ng heat flow signals. This is followed by a bimodal melting endotherm at -50 C and -38 C in the total and reversing heat flow signals; the bimodal nature a result of a melting and crystalline perfection mechanism as noted by the exothermic, followed by endothermic tran sitions occurring at th e same temperatures 178

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Figure 6-11. MDSC of PBDT-O ct(low) and PBDT-Oct(high) in the nonreversing heat flow signa l. There is a small transition obser vable in all three signals at 19 C; however it is unclear if this relates to the aforementioned crystalline perfection or a separate thermal event. The most noticeable tran sition present is a sharp endotherm at 58 C in the reversing and total h eat flow signals, but ab sent in the nonreversing. While it is possible that this transition is related to the disruption of molecular aggregates, the complexity of this thermogram prevents the absolute assignmen t of individual molecular motions to these transitions without further character ization using additional methods. Electrochemical studies of th ese polymers highlight the exte nt to which the ordering of PBDT-Oct affects the electronic properties. Figure 6-12 shows the cyclic voltammograms of PBDT-EtHex and PBDT-Oct(low). PBDT-EtHex has a broad cyclic voltammogram with an estimated E1/2=0.77 V (vs. Fc/Fc+). PBDT-Oct(low) has a more well-defined cyclic voltammogram with an E1/2= 0.56 V (vs Fc/Fc+). This difference of ~200 mV is significant for 179

PAGE 180 -0.5 0.0 0.5 1.0 1.5 Current Density (mA/cm2)Potential (vs. Fc/Fc+) Figure 6-12. Cyclic voltammetry (CV) of PBDT-E tHex [solid] and PBDT-O ct(low) [dash] with 0.1 M TBAP in acetonitrile. polymers whose molecular structures only differ in the branching of the solubilizing groups. Additionally, this difference seems incongruous gi ven that the polymers have approximately the same bandgap. Because bandgap relates to the HOMO of the polymers, differential pulse voltammetry (DPV) was used to resolve th e onset for PBDT-EtHex. [The onset of electrochemical oxidation is one method of approximating the value for the HOMO energy.] As shown in Figure 6-13, the onsets of oxidation fo r these two polymers overlap more than the cyclic voltammograms. This clar ifies why these polymers have a pproximately the same band gap yet different half-wave potentials; PBDT-Oct readily aggregates while the branched pendant groups of PBDT-EtHex significantly impede this aggregation, causing only a relatively small fraction (relative to the linear derivative) to aggregate. Electrochemistry of PBDT-Oct (high) shows a more dramatic change relative to EtHex. The CV shown in Figure 6-14(top) is much broa der with current passing at potentials 500 mV less than EtHex. Unfortunately, the CV for Oct(hi gh) is poorly defined, a nd does not allow an 180

PAGE 181 0 1 2 3 4 5 Current Density (mA/cm2)Potential (vs. Fc/Fc+) Figure 6-13. Differential pulse voltammetry (DPV ) of PBDT-EtHex [solid] and PBDT-Oct(low) [dash] with 0.1 M TBAP in acetonitrile E1/2 to be assigned. The reason for this can be seen in the DPV of Oct(high) [Figure 6-14 (bottom)] where two oxidation processes can be cl early distinguished. The first oxidation begins at 0 V and is very broad; a s econd oxidation begins at ~0.4 V. Thus, the CV actually shows two convoluted oxidation processes o ccurring. A reasonable assumption is that the first oxidation is related to the aggregated state. Spectroelectrochemistry allo ws one to follow the change in spectral properties as a function of potential. In Figure 6-15 the electrochemical oxidati on of PBDT-Oct(low) shows the formation of two peaks between 550 nm and 1100 nm, and a much broader peak extending into the infrared (not shown). The ne utral state is yellow, and on oxidation, the film is grey. Looking 181

PAGE 182 -0.4 -0.2 0.0 0.2 0.4 Current Density(mA/cm2)Potential (vs Fc/Fc+) - -4 -3 -2 -1 0 1 2 3 4 5 Current Density (mA/cm2)Voltage (vs. Fc/Fc+) Figure 6-14. Cyclic voltammetry (CV) [top] and differential pulse voltammetry (DPV) [bottom] of PBDT-Octyl(high) with 0.1 M TBAP in acetonitrile. 182

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at the spectroelectrochemistry of Oct(low) as the change in absorbance (inset), one sees that the peak at 480 nm changes to a disproportionate ex tent relative to the 420 nm and 450 nm peaks, and that this peak stops changing after 650 mV Based on solvatochromism experiments, the peak at 480 nm was ascribed to an effect of polymer aggregation. From the electrochemisty of PBDT-Oct, the oxidation from 500-750 mV was at tributed to polymer aggregation. Bringing these two points together, the spectroelectrochemis try correlates these data by showing the peak at 480 nm changes over the potential window of the aggregate oxidation. Spectral changes at higher potentials appear to be due to oxidati on of the non-aggregated segments. In contrast, EtHex shows only the initial grow th of the peak at 650 nm and a broad peak in the near-IR, though not in a reversible manner. This data is not shown, because the EtHex oxidation potential is too high and holding this polymer at these potentials results in overoxidation. 400600800100012001 0.0 0.2 0.4 0.6 ox. ox. 400420440460480 -0.30 -0.15 0.00 Wavelength (nm)Figure 6-15. Spectroelectrochemistry of PBDT-Oct(low) on ITO. Voltage is changed from 400, 550, 600, 650, 700, 750, 800 mV (vs Fc/Fc+). Inset show the differential absorbance spectra of the oxidized states relative to the neutral. 183

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Conclusion A new class of fused thiophene polymers have been synthesized based on benzo[1,2-b:4,3b']dithiophene (BDT). The routes used for monome r synthesis utilized very efficient and facile methods, though the ultimate polymerizations pres ent problems related to optimization of the polymerization conditions. Although notably high number averag e molecular weights were not achieved, successful polymerizatio ns were developed that provided important steps towards studying these materials. Characterization of the linear and branched de rivatives of PBDT continually returned to issues related to polymer aggregation. While th e aggregation of conjugated polymers is certainly a frequently cited phenomenon in the literature, the influence of aggregation on the spectral and electrochemical properties of PBDTs is unique These results were supported by the crystal structure of a BDT derivative, which showed significant interactions between molecules. What differentiates the PBDT family is the e ffect on the polymers electrochemical behavior. Based on the influence of aggregation, future work with PBDT could be directed towards different applications. One area for future work is to study the charge moibility for these polymers. The rationale behind this statement is PBDTs unique electrochemical behavior. Although the relationship between el ectrochemical characterization and charge mobility is not obvious, the fundamental relationship between the two is. Additionally, conjugated polymer aggregation has most frequently been used for sensors, which warrants the consideration of the PBDTs for these applications. Experimental Compounds 13 were synthesized according to previously published procedures.203 General procedure for Compound 4 Dry flask with condenser was charged with 3 (1 eq.), cesium carbonate (5 eq.), and alkyl (pseudo)halide (5 eq.). Anhydrous acetonitrile (100 184

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mL/2 g of 3) was added and the reaction was sttired for three days at reflux. Cooled reaction to room temperature and removed acetonitrile by ro tary evaporation. Residue was partitioned between water and dichloromethane. Organic ph ase was washed with 1 M HCl and then with water.Organic phase dried with sodium sulfate, filtered, and the solution rotovapped to yield a brown oil. Compound was purified by passing through a short plug of silica with pentane as the eluent. BDT-Oct ( 4a ): Yield 4.2 g (90%) of a clear oil. 1H NMR (300 MHz,CDCl3) 0.92 (t, 6H), 1.2-1.5 (m, 16H), 1.58 (p, 4H), 1.85 (p, 4H), 4.24 (t, 4H), 7.43(d, 2H), 7.62 (d, 2H). 13C NMR (75 MHz,CDCl3) 14.1, 22.7, 26.1, 29.3, 30.4, 31.8, 73.7, 122.1, 125.5, 131.2, 133.6, 142.2. HRMS calcd for C26H38O2S2H(M+H)+: 447.2386, found: 447.2346.Anal. Calcd for C26H38O2S2: C, 69.91; H, 8.57; O, 7.16; S, 14.36. Found: C, 69.61; H, 8.97. BDT-EtHex ( 4b ): Yield 2.01 g (74%) of a clear oil. 1H NMR (300 MHz,CDCl3) 0.9-1.1 (m, 12H), 1.3-1.9 (m, 18H), 4.17 (t, 4H), 7.46 (d, 2H), 7.63 (d, 2H). 13C NMR (75 MHz,CDCl3) 11.4, 14.4, 23.4, 24.0, 29.4, 30.6, 40.8, 76.5, 122.3, 125.7, 131.4, 133.6, 142.8. HRMS calcd for C26H38O2S2H(M+H)+: 447.2386, found: 447.2383.Anal. Calcd for C26H38O2S2: C, 69.91; H, 8.57; O, 7.16; S, 14.36. Found: C, 70.286; H, 8.845. General procedure for Compound 5 Compound 4 (1 eq.) was added to DMF (20 ml/ g of 4) and the solution was degassed by bubbling ar gon for 30 min. NBS (2 eq.) was added, the reaction covered with aluminum foil, and the reaction was run overnight. The reaction poured into water and stirred for 30 minutes. The reactio n was extracted with ether, and the organic extracts were washed with brine three times The organic phase was dried with magnesium sulfate, filtered, and rotovapped. The cr ude product was then purified by column chromotography. 185

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( 5a): Yield 1.4 g (65%) of a clear oil using hexanes/chlorofo rm (15:1) as th eluent. 1H NMR (300 MHz,CDCl3) 0.92 (t, 6H), 1.2-1.4 (m, 16H), 1.58 (p, 4H), 1.85 (p, 4H), 4.28 (t, 4H), 7.51 (s, 2H). HRMS calcd for C26H36Br2O2S2H(M+H)+: 625.0415, found: 625.0344. Anal. Calcd for C26H36Br2O2S2: C, 51.66; H, 6.00; Br, 26.44; O, 5.29; S, 10.61. Found: C, 51.95; H, 6.07. ( 5b ) Yield 2.2 g (71%) of a clear oil with pet. ether as the eluent. 1H NMR (300 MHz,CDCl3) 0.95 (m, 12H), 1.2-2.0 (m, 18H), 4.15 (t, 4H), 7.56 (s, 2H). HRMS calcd for C26H36Br2O2S2Na(M+Na)+: 625.0415, found: 625.0477.Anal. Calcd for C26H36Br2O2S2: C, 58.99; H, 7.43; B, 1.66; Br, 12.26; O, 9.82; S, 9.84Found: C, 52.02; H, 6.11. 2-bromo-2-boronic acid pinacol ester-BDT ( 6). 5a (0.774 g) dissolved in 25 mL of anhydrous THF and cooled to -78 C. N-BuLi (1.1 eq.) was added and the solution stirred for 2 hours. 2-Isopropoxy-4,4,5,5-tetramet hyl-1,3,2-dioxaborolane (4 eq.) was added and the reaction stirred overnight. Reactio n extracted between water and ethe r, then the organic extract was washed three times with water. The organic ph ase was then dried with magnesium sulfate, filtered, and rotovapped to give an oil. A 12 silica column was run with cyclohexane/ethyl acetate (4:1) and the top spot collected. R ecovered 277.4 mg (33%) of a faint blue oil. 1H NMR (300 MHz,CDCl3) 0.92 (t, 6H), 1.2-1.5 (m, 28H), 1.54 (p, 4H), 1.82 (h, 4H), 4.24 (m, 4H), 7.59(s, 1H), 8.2 (s, 1H). 13C NMR (75 MHz,CDCl3) 14.1, 22.6, 24.7, 26.0, 29.2, 29.4, 29.7, 30.4, 31.8, 73.5, 73.7, 84.3, 114.2, 125.0, 131.1, 131.3, 131.9, 134.3, 138.4, 142.0, 142.2. HRMS calcd for C32H48BBrO4S2Na(M+Na)+: 675.2152, found: 675.2184. Anal. Calcd for C32H48BBrO4S2: C, 58.99; H, 7.43; B, 1.66; Br, 12.26; O, 9.82; S, 9.84. Found: C, 60.662; H, 7.809. General Procedure for Colin/Kelsey Polymerization. Compound 5 (1 eq.) zinc powder (3.1 eq.), triphenyl phosphine (1 eq.), bipyridine (0.075 eq.), and nickely (II ) chloride (0.075 eq.) 186

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were all added to a dry round bottom flask inside of a dry box. The flask was sealed with a septum, and anhydrous DMAC was added via ca nnulation. The reaction was heated to 85 C. After five minutes, the reaction has a green-yellow color, with the yellow growing in intensity overtime. Reaction run for 24 hours. The polymer is precipitated into methanol, the precipitate collected, redissolved in chloroform, and precip itate again into methanol. Polymer dried under vacuum. Procedure for Suzuki polymerization: Aromatic monohalide/monoboronate (1 eq.) and cesium fluoride (7eq.) were added to a dry flask. Flask was purged with argon four times. To this flask, degassed solvent was added (12 mL per mmol of monomer). Pd2dba3 (2 mol%) and tri( tbutyl)phosphonium tetrafluoroborate (4-8 mol %) was added and the reaction refluxed for 48 hours. Polymer solution is precipitated into methanol, redissolved in chlo roform, and precipitated into methanol again. Polymer is dried under vacuum. PBDT-Oct : 1H NMR (300 MHz,CDCl3) 00.8-2.2 (m, 30H), 4.2-4.4 (m, 4H), 7.2-8.0 (m, 2H). Anal. Calcd for C26H36O2S2: C, 70.22; H, 8.16; O, 7.20; S, 14.42. Found: C, 69.99; H, 7.91. GPC (vs. polystyrene): 4,200 12,700 Da. PBDT-EtHex : 1H NMR (300 MHz,CDCl3) 0.8-2.2 (m, 30H), 4.2-4.4 (m, 4H), 7.2-8.0 (m, 2H). Anal. Calcd for C26H36O2S2: C, 70.22; H, 8.16; O, 7.20; S, 14.42. Found: C, 70.25; H, 8.17. GPC (vs. polystyrene): 4,800 11,200 Da. 187

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CHAPTER 7 FINAL CONCLUSIONS AND PERSPECTIVES This dissertation presents work on the synthe sis of polymers based on fluorene as a guide for desired properties, as a mono meric unit, and as a structural model for new molecules. While structure-property relationships were always the motivation for these materials, refining monomer syntheses and polymeri zation methods was the ever-renewing challenge in the lab. The goal in every case was material purity and polymers with Xn 10, which was rarely the meager goal it seemed. The basis for these problems was incorporating new moieties, whether a rarely used aromatic monomer or a pendant group. Some of these Pyrrihic victor ies yielded promising materials while others resulted in new synthetic routes and polymerization methods with broader implications. Chapter 3 presented a family of fluorene-ba sed polymers for a dual electrochromic and light emitting display. The desired electrochemi cal properties were found and the materials behaved as one would predict. This is significant, because modify ing the oxidation potential of a polymer while keeping the same HOMO-LUMO ga p is a valuable tool for designing such materials. One basic problem related to these polymers that this project has not addressed is the concomitant effect this structural modificat ion has on the polymers LUMO. Raising the HOMO on these polymers raises the LUMO to an equal degree. When using such a polymer in the dual display, a more accessible LUMO will be required. Researchers such as Samson Jenekhe have pursued research in this area where blending a material with a low LUMO have improved the performance of LEDs.215 Incorporating such electron-transporting materials into this material either as a pendant group or as a blend will be necessary. 188

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The most significant result from this work on high bandgap polymers is the effective polymerization of a pyrrole derivative via Suzuki coupling. Since metal mediated couplings grew to prominence in conjugated polymer research, phenyl and thienyl moieties have been most widely used due to their facile synthetic routes. Attempts to in corporate pyrrole derivatives in soluble, conjugated polymers have given generally poor results. On average, the degree of polymerization of these materials has been ni ne. This new set of conditions for a Suzuki polymerization of a N-alkyl pyrrole (with Xn=38, without fractionation) could easily be extended to areas where its structural analogue thiophene has been employed. In comparison to thiophene, a pyrrole has a higher lying HOMO energy level, the potential for func tionalization at the 1position, and a unique, deep history of organic synthesis to faci litate monomer design. These distinctions could be useful for developing new conjugated materials Chapter 4 discussed the synthe sis of four new CPEs functionalized with carboxylic acids. The potential applications of these materials ar e ample. For traditional CPE applications, these polymers are unique in that the carboxylates are bonded to the polym er chain at the 9-position of fluorene which extends the charged groups above and below the plane of the polymer backbone. This structural distinction fr om poly(phenylene)s and poly(arylen e ethynlene) derivatives would be expected to effect its performance in amp lified fluorescence quenching experiments. Based on their conformational changes in their acidic forms, these polymer s could also be interesting active materials in light em itting diodes. Taking poly(Fl)DA as an example, the extended conjugation of the polymer in 60% me thanol/40% water behaves as the -phase of traditional polyfluorenes, which researchers have develope d methods to induce. These polymers could be processed by spin coating from the appropriate solvent mixture, dried, and studied to determine if the polymers maintain this conformation in the solid state. 189

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In terms of synthetic work in Chapter 4, this project led to devel opment of the base-free Suzuki polymerization. While th is method was developed for base sensitive monomers, these conditions appear to have broade r applicability. In Chapters 3 and 6, this method was applied to heterocyclic monomers with excellent results. Based on th is work, these milder conditions appear to avoid potential side reactions such as deboronation, which are recognized problems with heterocyclic substrates. A larger study of the how these conditions work for compounds that perform poorly under traditional Su zuki conditions is needed. Chapter 5 presented the new i ndirect method for carboxylic-aci d functionalized conjugated polymers via thermally cleavable esters. The fund amentals of this method are now established and can be extended to a broader family of pol ymers. In regard to the failed bispyrryl benzothiadiazole polymer, the final insolubility of the polymer was unfortunate, but need not be the final result. Solid-state thermolysis may not be the best method, and high boiling solvents could be used which could inhib it the final crosslinking reaction. One last possibility regarding these polymers is regarding futu re applications. These modified esters were developed as a new indirect method for CPEs; however, such polymer s could also be used as a novel processing method. One could cast a film of the ester-derivatized polymers fr om a nonpolar organic solvent, anneal the film to its acidic form, and then a second polymer in a nonpolar organic solvent could be processed on top of the first. The most significant materials discovered in this dissertation ar e the polymers based on benzo[1,2-b:4,3-b']dithiophenes. They were design ed for improved interchain interactions and demonstrated this to an extraordinary degree. Developing these polymers would be of great interest for applications dependent on charge mobility, such as field effect transistors and photovoltaics. That the interchain interactions stabilize charges to such an extent clearly speaks 190

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191 to the delocalization of the charge between chains. The key questions to be determined now are the properties of these regions what are their dimensions? how ordered are the polymer chains in these regions? can these properties be controlled by processing parameters? With some answers to these questions, the applicability of these polymers can be determined. Further synthetic work is also warranted to determin e if altering the substituents may introduce other properties as well as to determ ine what properties arise when BDT is used as a comonomer.

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APPENDIX A X-RAY CRYSTALLOGRAPHIC DATA Figure A-1. Crystal structure for compound 7 (Chapter 3) Table A-1. Crystal data a nd structure refinement for 7 (Chapter 3). Identification code bb02 Empirical formula C17 H29 B2 N O4 Formula weight 333.03 Temperature 173(2) K Wavelength 0.71073 Crystal system Monoclinic Space group P2(1)/n Unit cell dimensions a = 13.4157(13) = 90. b = 10.5795(10) = 94.160(2). c = 13.6251(13) = 90. Volume 1928.7(3) 3 Z 4 Density (calculated) 1.147 Mg/m3 Absorption coefficient 0.078 mm-1 F(000) 720 Crystal size 0.29 x 0.16 x 0.09 mm3 Theta range for data coll ection 2.06 to 27.50. Index ranges -17 h 16, -13 k 13, -14 l 17 Reflections collected 12873 192

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Independent reflections 4429 [R(int) = 0.0809] Completeness to theta = 27.50 99.9 % Absorption correction None Max. and min. transmission 0.9930 and 0.9777 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 4429 / 0 / 246 Goodness-of-fit on F2 1.051 Final R indices [I>2sigma(I)] R1 = 0.0594, wR2 = 0.1462 [2590] R indices (all data) R1 = 0.1071, wR2 = 0.1664 Largest diff. peak and hole 0.376 and -0.385 e.-3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n-p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p)2+n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants. 193

PAGE 194

Figure A-2. Crystal structure for compound 9 (Chapter 5) Table A-2. Crystal data a nd structure refinement for 9 (Chapter 5). Identification code bb01 Empirical formula C24 H26 N4 O4 S Formula weight 466.55 Temperature 173(2) K Wavelength 0.71073 Crystal system Orthorhombic Space group Pna2(1) Unit cell dimensions a = 12.0764(7) = 90. b = 25.2689(15) = 90. c = 7.7893(5) = 90. Volume 2377.0(2) 3 Z 4 Density (calculated) 1.304 Mg/m3 Absorption coefficient 0.174 mm-1 F(000) 984 Crystal size 0.20 x 0.14 x 0.12 mm3 Theta range for data coll ection 1.61 to 27.50. Index ranges -15 h 15, -28 k 32, -10 l 9 194

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Reflections collected 15608 Independent reflections 5405 [R(int) = 0.0543] Completeness to theta = 27.50 99.9 % Absorption correction Integration Max. and min. transmission 0.9808 and 0.9681 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 5405 / 1 / 298 Goodness-of-fit on F2 0.992 Final R indices [I>2sigma(I)] R1 = 0.0352, wR2 = 0.0794 [4685] R indices (all data) R1 = 0.0424, wR2 = 0.0818 Absolute structure parameter 0.24(6) Largest diff. peak and hole 0.206 and -0.218 e.-3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n-p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p)2+n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants. 195

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Figure A-3. Crystal structure for compound 4 (Chapter 6) Table A-3. Crystal data a nd structure refinement for 4 (Chapter 6). Identification code bb03 Empirical formula C14 H10 O4 S2 Formula weight 306.34 Temperature 173(2) K Wavelength 0.71073 Crystal system Monoclinic Space group P2(1)/c Unit cell dimensions a = 10.4235(8) = 90. b = 15.9480(12) = 106.4020(10). c = 8.3570(6) = 90. Volume 1332.68(17) 3 Z 4 Density (calculated) 1.527 Mg/m3 Absorption coefficient 0.409 mm-1 F(000) 632 Crystal size 0.30 x 0.20 x 0.19 mm3 196

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197 Theta range for data coll ection 2.04 to 27.50. Index ranges -12 h 13, -20 k 20, -9 l 10 Reflections collected 8778 Independent reflections 3041 [R(int) = 0.0796] Completeness to theta = 27.50 99.2 % Absorption correction Integration Max. and min. transmission 0.9264 and 0.8872 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 3041 / 0 / 181 Goodness-of-fit on F2 1.076 Final R indices [I>2sigma(I)] R1 = 0.0471, wR2 = 0.1237 [2731] R indices (all data) R1 = 0.0508, wR2 = 0.1268 Largest diff. peak and hole 0.389 and -0.757 e.-3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n-p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p)2+n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constant

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210 BIOGRAPHICAL SKETCH Robert N. Brookins is the son of Herman a nd Nina Brookins of Donalsonville, Georgia. Outside of a few years living in Ohio, he was raised in Georgia for all of his childhood. He received a B.S. in chemistry from Georgia S outhwestern College (Americus, Georgia) and a B.A. in English from Oglethorpe University (Atlanta, Georgia). He then taught students gifted in math and sciences at the Alabama School of Fine Arts (Birmingham, Alabama). In May 2000, Robert married Paige Jones, al so of Donalsonville, Georgia. During this time, he met Dr. Rigoberto Advincula, a former graduate stude nt of the Butler Polymer Laboratories, who encourgaged him to work part time in Advincula s lab. With this experience, Robert decided to go to graduate school at the University of Flor ida with John R. Reynolds as an advisor. During his time here, Robert has worked as a synthe tic chemist of conjugated polymers and brought Kathryn Katie Belle Brookins into the world.

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