THIENOTHIOPHENE-BASED LIQUID CRYSTALS: SYNTHESIS AND COMPARATIVE EVALUATION OF MESOPHASE PROPERTIES A thesis submitted to Kent State University in partial fulfillment of the requirements for the degree of Master of Science by Jonathan I. Tietz August, 2012 Thesis written by Jonathan I. Tietz B.S., Kent State University, 2011 M.S., Kent State University, 2012 Approved by ______________________________, Chair, Thesis Committee Dr. Paul Sampson ______________________________, Chair, Thesis Committee Dr. Alexander J. Seed ______________________________, Members, Thesis Committee Dr. Robert J. Twieg ______________________________ Dr. Scott D. Bunge Accepted by ______________________________, Chair, Department of Chemistry Dr. Michael Tubergen ______________________________, Dean, College of Arts and Sciences Dr. Raymond Craig ii TABLE OF CONTENTS LIST OF FIGURES ........................................................................................................... ix LIST OF SCHEMES......................................................................................................... xv LIST OF TABLES .......................................................................................................... xxii LIST OF ABBREVIATIONS ......................................................................................... xxv ACKNOWLEDGEMENTS ......................................................................................... xxxiv CHAPTER 1. INTRODUCTION ....................................................................................... 1 1.1 Introduction to liquid crystals ........................................................................ 1 1.2 Applications of liquid crystals in displays ..................................................... 5 1.3 General structure of liquid crystals .............................................................. 15 1.4 Sulfur-based heterocycles as liquid crystals ................................................ 21 1.5 Thienothiophenes in the literature ............................................................... 23 1.6 Previous studies of thienothiophene liquid crystals ..................................... 39 1.7 Rationale for selected LC targets ................................................................. 42 1.8 Overview of thesis goals .............................................................................. 44 CHAPTER 2. SYNTHESIS OF THIENO[3,2-b]THIOPHENE-2-CARBOXYLATE ESTER-BASED LIQUID CRYSTALS ................................................................ 47 iii 2.1 Rationale for targeting thieno[3,2-b]thiophene-2-carboxylate ester-based liquid crystals ............................................................................................... 47 2.2.1 Literature routes to thieno[3,2-b]thiophenes ................................................ 49 2.2.2 Synthetic plan for target materials ............................................................... 54 2.2.3 Synthesis of alkyl 2-mercaptoacetates ......................................................... 57 2.3 Preparation of alkyl thieno[3,2-b]thiophene-2-carboxylate esters under mild conditions via tandem nucleophilic substitution and aldol condensation .... 62 2.4.1 Literature approaches to halogenation of thieno[3,2-b]thiophenes ............. 67 2.4.2 Regioselective C-5 halogenation of thieno[3,2-b]thiophene-2-carboxylate esters 2.3 ...................................................................................................... 76 2.5.1 Thienothiophenes as substrates for Pd(0)-catalyzed cross-coupling in the literature ....................................................................................................... 82 2.5.2 Synthesis of alkyl 5-(4-alkoxyphenyl)thieno[3,2-b]thiophene-2-carboxylate esters via Suzuki coupling ........................................................................... 86 CHAPTER 3. SYNTHESIS OF THIENO[2,3-b]THIOPHENE-2-CARBOXYLATE ESTER-BASED LIQUID CRYSTALS ................................................................ 93 3.1 Rationale for targeting thieno[2,3-b]thiophene-2-carboxylate ester-based liquid crystals ............................................................................................... 93 3.2.1 Literature routes to thieno[2,3-b]thiophenes ................................................ 94 3.2.2 Synthetic plan for target materials ............................................................. 113 iv 3.3 Preparation of alkyl 5-bromothieno[2,3-b]thiophene-2-carboxylate esters via mild tandem nucleophilic substitution and aldol condensation ........... 114 3.4.1. Use of thieno[2,3-b]thiophene substrates in Pd(0)-catalyzed cross-coupling in the literature ........................................................................................... 127 3.4.2 Synthesis of alkyl 5-(4-alkoxyphenyl)thieno[2,3-b]thiophene-2-carboxylate esters via Suzuki coupling ......................................................................... 133 CHAPTER 4. 5-ALKOXYTHIENO[3,2-b]THIOPHENE-2-CARBOXYLATE LIQUID CRYSTALS ........................................................................................................ 135 4.1 Rationale for targeting 2-alkoxythieno[3,2-b]thiophenes .......................... 135 4.2 Literature routes to 2-alkoxythiophenes and 2-alkoxythienothiophenes ... 137 4.3 Initially attempted entries into 2-alkoxythienothiophenes ......................... 143 4.4.1 Synthetic plan for brominated 2-alkoxythiophenes ................................... 154 4.4.2 Preparation of potassium thiophen-2-yltrifluoroborate precursors ............ 155 4.4.3 Oxidation of trifluoroborate salts 4.65 with aqueous Oxone ..................... 162 4.4.4 Preparation of brominated 2-alkoxythiophenes by Mitsunobu etherification of thiophenones 4.9 .................................................................................... 167 4.5.1 Synthesis of 5-alkoxythieno[3,2-b]thiophene-2-carboxylate-based mesogens 4.2 and 4.3 .................................................................................................. 172 4.5.3 Preparation of phenyl analogue of 2-alkoxythieno[3,2-b]thiophene-2- carboxylate-based mesogen 4.23 ............................................................... 175 v CHAPTER 5. TRANSITION TEMPERATURES AND COMPARISON OF TARGETED LIQUID CRYSTALS ................................................................... 178 5.1.1 Transition temperatures of primary alkyl 5-(4-alkoxyphenyl)thieno[3,2- b]thiophene-2-carboxylate esters ............................................................... 178 5.1.2 Comparison of primary alkyl thieno[3,2-b]thiophene ester-based liquid crystals ....................................................................................................... 183 5.1.3 Comparison of primary alkyl thieno[3,2-b]thiophene ester-based liquid crystals to analogous mesogens in the literature ........................................ 186 5.2.1 Transition temperatures of primary alkyl 5-(4-alkoxyphenyl)thieno[2,3- b]thiophene-2-carboxylate esters ............................................................... 187 5.2.2 Comparison of primary alkyl thieno[2,3-b]thiophene ester-based liquid crystals 5.4a-d ............................................................................................ 189 5.2.3 Comparison of primary alkyl thieno[2,3-b]thiophene ester-based liquid crystals to analogous mesogens in the literature ........................................ 189 5.3.1 Transition temperatures of secondary alkyl 5-(4-alkoxyphenyl)thieno[3,2- b]- and [2,3-b]-thiophene-2-carboxylate esters.......................................... 190 5.3.2 Comparison of primary and secondary thieno[3,2-b]- and [2,3-b]thiophene ester- based liquid crystals ......................................................................... 191 5.3.3 Comparison of secondary thieno[3,2-b]- and [2,3-b]thiophene ester-based liquid crystals to analogous mesogens in the literature ............................. 199 vi 5.4.1 Transition temperatures of novel 2-alkoxythieno[3,2-b]thiophene-2- carboxylate mesogens 5.8 and 5.10 ........................................................... 201 5.4.2 Comparison of novel 2-alkoxythieno[3,2-b]thiophene-2-carboxylate-based liquid crystals 5.8 and 5.10 to analogous mesogens in the literature ......... 202 CHAPTER 6. CONCLUSIONS ..................................................................................... 207 6.1.1 Conclusions regarding alkyl 5-(4-alkoxyphenyl)thienothiophene-2- carboxylate liquid crystals ......................................................................... 207 6.1.2 Future directions for alkyl 5-(4-alkoxyphenyl)thienothiophene-based LCs .................................................................................................................... 208 6.2 Conclusions regarding the synthesis of brominated 2-alkoxythiophenes .. 209 6.2.2 Future directions for brominated 2-alkoxythiophenes ............................... 209 6.3 Conclusions regarding alkyl 5-alkoxythieno[3,2-b]thiophene-2-carboxylate liquid crystals ............................................................................................. 210 6.3.2 Future directions for alkoxythienothiophenes ........................................... 211 CHAPTER 7. EXPERIMENTAL ................................................................................... 213 7.1 General considerations ............................................................................... 213 7.2 Numbering of compounds.........................................................................