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Thiophene-containing photofunctional molecules : Pacman complexes, metallacycles and flexible Lewis pairs Cao, Yang 2016

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THIOPHENE-CONTAINING PHOTOFUNCTIONAL MOLECULES: PACMAN COMPLEXES, METALLACYCLES AND FLEXIBLE LEWIS PAIRS  by  Yang Cao B. SC., PEKING UNIVERSITY, 2010  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY  in  THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (CHEMISTRY)  THE UNIVERSITY OF BRITISH COLUMBIA (VANCOUVER) June 2016 © Yang Cao, 2016 ii  Abstract  Three types of photofunctional molecules based on thiophene-containing conjugated backbones have been designed, synthesized and characterized. It is shown that their light absorption and emission properties can be manipulated and applied for applications such as fluorescent imaging.  Binuclear Pt(II) terpyridine Pacman complexes with flexible thiophene-containing bisacetylide ligands are shown to exhibit structural folding/unfolding controllable by temperature change and different solvent environments. The structural folding and unfolding give rise to structural changes of the thiophene-containing backbone and different interactions between two metal-containing moieties, which are analyzed by DFT calculations and evidenced by UV-Vis and NMR spectroscopy.  Metallacycles with a cis-diphosphino Pt(II) metal center and different thiophene-containing bisacetylides are designed to show room-temperature fluorescence and phosphorescence dual emission with different intensity ratios. Their absorption and emission properties are explained by DFT and TD-DFT analysis of ground state and singlet and triplet state energies and geometries.  An intramolecular Lewis pair system between a Lewis acidic -BMes2 group and a Lewis basic phosphine oxide group based on a flexible bithiophene backbone is reported. Evidenced by NMR and IR spectra, the system is found to exhibit fast equilibrium between an open structure with unbound Lewis acid and a closed structure with Lewis adduct. The equilibrium is manipulated iii  by lowering the temperature to favor the closed structures, or adding strong hydrogen bond donors that favor the open forms. The difference in coordination state of the boron center in these two states gives rise to an interesting single-component-two-state system with drastically different emission colors between states. The scope of such system was explored and different emission colors of the open and closed forms of the Lewis pairs are achieved by changing the backbone conjugation or strength of electron-donating groups. Furthermore, this system has been used as a two-color fluorescent dye system for fluorescence imaging of hydrophobic/hydrophilic environments in biological or medical applications.    iv  Preface  In all chapters, Prof. Michael Wolf acted as the supervisor. I am the principal author of the work reported in this thesis, and I carried out all the experiments except the ones noted otherwise. All X-ray crystallography structures were collected by Dr. Brian O. Patrick. DFT and TD-DFT calculations were performed by Dr. Jeffery Nagle at Bowdoin University in Chapter 4 and by myself in other chapters. Portions of Chapter 2 have been previously published1 except the synthesis and photophysical properties of compound 3 and all the computational results. A version of Chapter 4 has been previously published.2 I am the principal author of both works. All the fluorescence imaging experiments in Chapter 5 were performed by Xiaozhu (Hunter) Wang in the laboratory of Prof. Chris Orvig at UBC. The specimens of human-brain slices were provided by the laboratory of Prof. Emeritus Patrick L. McGeer at Faculty of Medicine, UBC.  A list of publications arising from work presented in the thesis: (1) Cao, Y.; Wolf, M. O.; Patrick, B. O. Inorg. Chem. 2013, 52, 5636. (2) Cao, Y.; Nagle, J. K.; Wolf, M. O.; Patrick, B. O. J. Am. Chem. Soc. 2015, 137, 4888. v  Table of Contents THIOPHNE-CONTAINING PHOTOFUNCTIONAL .............................................................. i Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents ...........................................................................................................................v List of Tables ................................................................................................................................ ix List of Figures ............................................................................................................................... xi List of Schemes ......................................................................................................................... xviii List of Symbols and Abbreviations .......................................................................................... xxi Acknowledgements ....................................................................................................................xxv Dedication ................................................................................................................................. xxvi Chapter 1 Introduction................................................................................................................. 1 1.1 Overview ..................................................................................................................... 1 1.2 Conjugated Materials .................................................................................................. 3 1.2.1 Structures and Molecular Orbitals .......................................................................... 3 1.2.2 Photophysical Properties ......................................................................................... 5 1.3 Thiophene-Containing Conjugated Materials ........................................................... 10 1.4 Photofunctional Conjugated Materials with Pt(II) Moieties ..................................... 13 1.4.1 Photofunctional Pt(II) Acetylides with Phosphine Ligands.................................. 16 1.4.2 Photofunctional Pt(II) Acetylides with Polypyridyl Ligands ............................... 18 1.5 Photofunctional Conjugated Materials Containing Lewis Acidic Boron Groups .... 24 vi  1.5.1 Small Conjugated Molecules ................................................................................ 25 1.5.2 Conjugated Oligomers and Polymers ................................................................... 27 1.5.3 Reactions with External Lewis Bases ................................................................... 30 1.5.4 Interactions with Internal Lewis Bases ................................................................. 33 1.6 Goals and Scope ........................................................................................................ 34 Chapter 2 Platinum(II) Terpyridine Pacman Complexes with Thiophene-Containing Alkynyl Ligands .......................................................................................................................... 37 2.1 Introduction ............................................................................................................... 37 2.2 Results and Discussion ............................................................................................. 39 2.2.1 Synthesis and Solid-state Structures ..................................................................... 39 2.2.2 Electronic Absorption and Emission Spectra ....................................................... 45 2.2.3 NMR Spectra ........................................................................................................ 50 2.2.4 Density Functional Theory (DFT) Simulations .................................................... 53 2.3 Experimental ............................................................................................................. 57 2.3.1 General .................................................................................................................. 57 2.3.2 Synthesis ............................................................................................................... 57 2.3.3 DFT Simulations ................................................................................................... 59 2.4 Conclusions ............................................................................................................... 64 Chapter 3 Dual Emissive Platinum(II) Metallacycles ............................................................. 65 3.1 Introduction ............................................................................................................... 65 3.2 Result and Discussion ............................................................................................... 70 3.2.1 Synthesis ............................................................................................................... 70 vii  3.2.2 Solid-State Structures............................................................................................ 71 3.2.3 Electronic Absorption and Emission Spectra ....................................................... 73 3.2.4 DFT and TD-DFT Simulations ............................................................................. 79 3.3 Experimental ............................................................................................................. 86 3.3.1 General .................................................................................................................. 86 3.3.2 Synthesis ............................................................................................................... 87 3.3.3 X-ray Crystallography .......................................................................................... 91 3.3.4 DFT Simulations ................................................................................................... 93 3.4 Conclusions ............................................................................................................... 95 Chapter 4 Bithiophene-Based Flexible Lewis Pairs ................................................................. 96 4.1 Introduction ............................................................................................................... 96 4.2 Result and Discussion ............................................................................................... 99 4.2.1 Synthesis and Solid-State Structures .................................................................... 99 4.2.2 Infrared and NMR Spectroscopic Studies ........................................................... 101 4.2.3 Electronic Absorption and Emission Spectra ..................................................... 109 4.2.4 DFT and TD-DFT Simulations ........................................................................... 113 4.3 Experimental ........................................................................................................... 115 4.3.1 General ................................................................................................................ 115 4.3.2 Synthesis ............................................................................................................. 116 4.3.3 DFT Simulations ................................................................................................. 120 4.4 Conclusions ............................................................................................................. 121 Chapter 5 Bithiophene-Based Flexible Lewis Pairs: Scope and Applications .................... 123 viii  5.1 Introduction ............................................................................................................. 123 5.1.1 Fluorescence Microscopy and Fluorescent Dyes ................................................ 124 5.1.2 Targets................................................................................................................. 127 5.2 Results and Discussion ........................................................................................... 128 5.2.1 Scope of the Lewis Pair System.......................................................................... 128 5.2.2 Emission Color Tunability .................................................................................. 134 5.2.3 Fluorescence Imaging ......................................................................................... 141 5.2.4 DFT Simulations ................................................................................................. 146 5.3 Experimental Section .............................................................................................. 150 5.3.1 General ................................................................................................................ 150 5.3.2 Synthesis ............................................................................................................. 151 5.3.3 Fluorescence Imaging ......................................................................................... 173 5.3.4 DFT Simulations ................................................................................................. 173 5.4 Conclusions ............................................................................................................. 174 Chapter 6 Conclusions .............................................................................................................. 175 References ...................................................................................................................................179 Appendix .....................................................................................................................................194  ix  List of Tables Table 2-1. Molecular orbitals and energies of complex 2 involving dz2(Pt) orbitals................... 60 Table 2-2. Molecular orbitals and energies of complex 3 involving dz2(Pt) orbitals. ................. 61 Table 2-3. Molecular orbitals and energies of complex 3 orbitals in CHCl3. .............................. 62 Table 2-4. Molecular orbitals and energies of complex 3 orbitals in CH3CN. ............................ 63 Table 3-1. Electronic absorption and emission of metallacycles in CH2Cl2 solution. ................. 78 Table 3-2. Calculated geometry of 4, 5 and 6 at S0, S1 and T1 electronic states. ......................... 80 Table 3-3. Frontier orbitals and their energies of ground state metallacycles. ............................ 81 Table 3-4. Bond lengths (Å) of 4, 5 and 6 in the ligand at different electronic states. ................ 82 Table 3-5. Relative energies of optimized S0 and T1 states. ......................................................... 83 Table 3-6. Relative energies of optimized S0 and S1 states. ......................................................... 84 Table 3-7. Vertical excitation energies of lowest singlet and triplet excited states. .................... 85 Table 3-8. Calculated absorption and emission properties. ......................................................... 85 Table 3-9. Selected bond lengths (Å) and angles (°) for crystal structures of 4 and 5. ............... 92 Table 3-10. Selected bond lengths (Å) and angles (°) for the two crystallographic independent structures in 6•CH2Cl2 crystals. ..................................................................................................... 92 Table 3-11. Calculated and experimental vibrational frequencies of C≡C bonds. ..................... 94 Table 4-1. Calculated molecular orbitals of 24aclosed and 24aopen in different solvents. ............ 113 Table 4-2. TD-DFT simulated vertical excitation energy and oscillator strengths of 24 in different solvents. ...................................................................................................................................... 114 Table 5-1. Summary of low energy absorption maxima (abs), emission maxima (em) and x  emission quantum yields (em) of 33 and 36-43 in MeOH and THF. ........................................ 140 Table 5-2. HOMO-1, HOMO and LUMO of the open structure of the compounds in Series B...................................................................................................................................................... 147 Table 5-3. HOMO and LUMO of the closed structure of the compounds in Series B.............. 148 Table 5-4. HOMO and LUMO energies and HOMO-LUMO gaps (EHOMO, ELUMO and E, respectively, in eV) of the closed and open structures of the compounds in Series B. .............. 149 Table 5-5. TD-DFT calculated low-energy absorption transitions of the closed and open structures of the compounds in Series B. .................................................................................................... 150     xi  List of Figures  Figure 1-1. Simplified diagram showing the narrowed HOMO-LUMO energy gap and the formation of band structure with increasing -conjugation from ethylene to polyacetylene. ........ 3 Figure 1-2. Examples of hybrid conjugated polymers (top) and schematics of the orbital overlap of corresponding conjugated systems. ............................................................................................ 5 Figure 1-3. HOMO and LUMO of different electronic states (left) and a Jablonski diagram showing the vibronic states of each electronic states (right, bold lines indicate the lowest vibronic states). ............................................................................................................................................. 6 Figure 1-4. Schematics of HOMO of * and CT excited states of thiophene (left) and thiophene with an sp2 boron CT acceptor at the -position (right). Red solid and hollow circles are representative of the contributions of atomic p orbitals to the HOMO. ......................................... 8 Figure 1-5. Schematics of solvatochromism of CT transitions. ..................................................... 9 Figure 1-6. Thiophene-thiophene s-trans and s-cis coplanar conformations (left) and conformation changes as the result of weak interactions. ................................................................................... 12 Figure 1-7. Transition metal-containing conjugated materials (blue circles represent metal complexes while grey tiles indicate the conjugated backbone).22 ................................................ 13 Figure 1-8. Two scenarios for light absorption in transition metal-containing -systems. ......... 14 Figure 1-9. Cis- and trans-substituted Pt(II) bisacetylides with phosphine ancillary ligands, and an example of a trans-substituted Pt(II) bisacetylide transforming to its cis-isomer.26 ................ 16 Figure 1-10. Schematics of S1 and T1 states of polymer 15 and origin of its fluorescence and phosphorescence. .......................................................................................................................... 17 Figure 1-11. MLCT and LLCT using a terpyridyl Pt(II) acetylide complex as an example (left) and energetics of singlet and triplet MLCT excited states and LC excited states. The terpyridine xii  ligand is highlighted in blue while the acetylide ligand is in red.................................................. 19 Figure 1-12. Schematics of MLCT and MMLCT transitions. ..................................................... 21 Figure 1-14. Synthesis and emission colors of boron-containing bithiophene derivatives.46 ...... 26 Figure 1-15. Reaction of a -conjugated Lewis acid with external Lewis bases. ........................ 30 Figure 2-1. (a) Structure of one of the “folded” 2 molecules in the crystal obtained from CH3CN. (b) Structure of “unfolded” 2 obtained from CHCl3 (hydrogen atoms, tert-butyl groups and solvent molecules have been omitted for clarity in both cases, thermal ellipsoids drawn at 50% probability). ................................................................................................................................... 40 Figure 2-2. ORTEP structure of 2·3CH3CN. Anions, hydrogen atoms, CH3CN molecules and minor disordered fragments have been removed for clarity. ........................................................ 41 Figure 2-3. ORTEP structure of 2·5CHCl3. Anions, hydrogen atoms, CHCl3 molecules have been omitted for clarity. ........................................................................................................................ 42 Figure 2-4. ORTEP structure showing (a) C-H∙∙∙O and C-H∙∙∙Cl hydrogen bonds in the crystal structure of 2·5CHCl3, and (b) Cl-interactions in the same structure........................................ 42 Figure 2-5. (a) Helixes in the structure of 2·3CH3CN. (b) Top view of chiral columns formed by a single stereoisomer of the binuclear complex. (c) Zigzag chains in the structure of 2·5CHCl3. (d) Top view of the chains in 2·5CHCl3. Hydrogen atoms, tert-butyl groups, solvent molecules and counter-ions have been omitted for clarity in all cases. ................................................................ 44 Figure 2-6. UV-vis absorption spectra of 1 in CHCl3 with increasing percentage of CH3CN added (from 0 % to 100 %, 5×10-5 M). The inset shows the shift in the low energy MLCT/LLCT absorbance band. ........................................................................................................................... 45 Figure 2-7. UV-vis absorption spectra of 2 in CHCl3 with increasing percentage of CH3CN added (from 0 % to 100 %, 2.0×10-5 M). The inset shows the change in the low energy absorbance band with an isosbestic point at ~550 nm. ............................................................................................. 46 Figure 2-8. Subtle negative solvatochromism of 2 when CH3CN is greater than 50 vol % (2.0×10-xiii  5 M). .............................................................................................................................................. 47 Figure 2-9. UV-Vis absorbance spectra of 2 in CH3CN, from 1.14 to 100 M. Inset: Plot of the absorbance at 530 nm as a function of concentration. .................................................................. 48 Figure 2-10. UV-vis absorption spectra of 3 in CHCl3 with increasing percentage of CH3CN added (from 0 % to 100 %, 1.5×10-5 M). ................................................................................................ 49 Figure 2-11. Solid state emission spectra of 3 on glass slides (background subtracted). ............ 49 Figure 2-12. Partial 1H-NMR spectra (aromatic region) of 1 in (a) CDCl3 and (b) CD3CN, 2 in (c) CDCl3 and (d) CD3CN. Pyridine protons are labeled. .................................................................. 50 Figure 2-13. Variable temperature 1H NMR spectra of complex 2 in CDCl3. ............................ 51 Figure 2-14. Variable temperature 1H NMR spectra of complex 2 in CD3CN. ........................... 51 Figure 2-15. Partial NOESY spectrum of 2 in CD3CN. ............................................................... 52 Figure 2-16. Partial NOESY spectrum of 2 in CDCl3. ................................................................ 52 Figure 2-17. Partial NOESY spectrum of 1 in CD3CN. ............................................................... 52 Figure 2-18. Optimized structure of folded and unfolded 2 in CH3CN and CHCl3. ................... 53 Figure 2-19. Optimized structure of folded and unfolded 3 in CH3CN and CHCl3. ................... 53 Figure 2-20. Interactions of dz2(Pt) orbitals in 2 (up) and 3 (down). .......................................... 55 Figure 2-21. Frontier MOs of geometry optimized 3 in CHCl3 and CH3CN............................... 56 Figure 3-1. Energy diagrams of electronic transitions involving GS and excited states (kISC, kNR, kFL and kPH are rates of ISC, NR decay, fluorescence and phosphorescence, respectively).......... 65 Figure 3-2. ORTEP presentations of metallacycle (a) 4 and (b) 5 showing CH2Cl2 solvent molecules hydrogen-bonding with each metallacycle in different ways. Ellipsoids at the 50% probability level, hydrogens on metallacycles omitted for clarity. ............................................... 71 Figure 3-3. (a) ORTEP presentations two different crystallographic structures of 6 connected by xiv  hydrogen bonding interactions. (b) Top-view of one of the structures. Ellipsoids at the 50% probability level, hydrogens on metallacycles omitted for clarity. ............................................... 72 Figure 3-4. (a) UV-Vis spectra of ligands 8, 9 and 10 in CH2Cl2 (1.0×10-5 M) and excitation and emission spectra of 8 (b), 9 (c) and 10 (d) in CH2Cl2 (1.0×10-5 M). ............................................. 73 Figure 3-5. UV-Vis spectra of 4 (a), 5 (b) and 6 (c) (1.0×10-5 M in CH2Cl2). ............................. 74 Figure 3-6. Excitation spectrum of 4 and its emission spectra under different conditions. ......... 75 Figure 3-7. (a) Variable temperature fluorescence spectra of 5 in a MeOH/EtOH medium, and (b) excitation and emission spectra of metallacycle 5 in air-equilibrated solution. No phosphorescence was observed under argon (1.0 × 10-5 M in CH2Cl2). ................................................................... 76 Figure 3-8. (a) Emission spectra of 6 under argon show two bands (500 nm and 620 nm) while excitation spectra of the two bands diverge at higher energy. (b) Emission of 6 in air, and dual emission under argon. ................................................................................................................... 77 Figure 3-9. Summarized energy diagrams and electronic transitions of metallacycles 4, 5 and 6. Energies shown in the above diagrams are in eV (Ev = vertical transition energy, E00 = 0-0 transition energy). ......................................................................................................................... 84 Figure 4-1. Solid-state molecular structure of 24a. The thermal ellipsoids are drawn at 50% probability and H atoms are omitted for clarity. ......................................................................... 100 Figure 4-2. Solid-state molecular structure of 24c. The thermal ellipsoids are drawn at 50% probability and H atoms are omitted for clarity. ......................................................................... 101 Figure 4-3. Stacked 31P{1H} NMR spectra of 24a in various solvents. .................................... 102 Figure 4-4. Stacked 31P{1H} NMR spectra of 25 in various solvents. ...................................... 102 Figure 4-5. FTIR spectra of 24a in CD2Cl2 solution (CD2Cl2 was used instead of CH2Cl2 to avoid overlapping C-H bending absorptions). ...................................................................................... 103 Figure 4-6. Stacked 31P{1H} NMR of 24a and 24a+F– in various NMR solvents..................... 104 xv  Figure 4-7. Stacked 31P{1H} NMR of 24b in various NMR solvents (very broad peak in hexanes due to restricted rotation induced by bulky isopropyl groups). .................................................. 104 Figure 4-8. Assignment of the 1H NMR spectrum of 24bopen in methanol-d4 (25 °C). ............. 105 Figure 4-9. 1H NOESY spectrum of 24bopen in methanol-d4 (25 °C, mixing time = 800 ms). No NOE is observed between H(6)/H(7) and H(13). ....................................................................... 106 Figure 4-10. Assignment of the 1H-NMR spectrum of 24bclosed in benzene-d6 (25 °C) Signals from H(6) and H(13) are very broad, which leads to low sensitivity for NOESY experiments at this temperature. ................................................................................................................................ 106 Figure 4-11. Assignment of 1H-NMR spectrum of 24bclosed in benzene-d6 (50 °C, mixing time = 800 ms). Peak ~ 0.6 ppm arises from residue H2O. .................................................................... 107 Figure 4-12. 1H NOESY spectrum of 24bclosed in benzene-d6 (50 °C, mixing time = 800 ms). NOE interactions between H(6)/H(7) and H(13) are indicated with red dashed boxes. Even though H(4) and H(13) overlapped in the 1H NMR spectrum, the labeled NOE signal should only be attributed to H(13) since intramolecular through space interactions between H(4) and H(6)/H(7) are not geometrically allowed. ................................................................................................................ 107 Figure 4-13. Stacked 31P{1H} NMR spectra of 24c in different NMR solvents (the spectrum of 24c in MeOH-d4 is not shown due to poor solubility). ............................................................... 108 Figure 4-14. Left: UV-Vis spectra of 24a in various solvents; right: UV-Vis spectra of 24b in various solvents. .......................................................................................................................... 109 Figure 4-15. Emission from 24a, ex = 366 nm using an 18.4 W UV lamp for excitation. ....... 110 Figure 4-16. (a) Normalized emission and excitation spectra of 24aclosed in hexanes (black trace) and 24aopen in methanol (red trace). (b) Excitation energy dependent mixed emission from 24aopen and 24aclosed in CHCl3. ................................................................................................................ 110 Figure 4-17. (a) Emission and excitation spectra of 24a in methanol, n-BuOH, s-BuOH and t-BuOH. (b) Emission from 24a irradiated at 366 nm using an 18.4 W UV lamp. ...................... 111 xvi  Figure 4-18. (a) Emission spectra of 24a at decreasing temperatures in a MeOH/EtOH (v/v =1:4) solution excited at 330 nm. (b) Variable temperature 31P NMR spectra of 24a and 25 in CD3OD (-65 °C to 25 °C at 10 °C intervals). .............................................................................................. 112 Figure 4-19. Optimized ground state geometries of (a) 24aclosed and (b) 24aopen rendered by PyMol software. ...................................................................................................................................... 113 Figure 5-1. Schematics of an inverted optical fluorescence microscope. .................................. 124 Figure 5-2. UV-Vis spectra of 33 at various concentrations in (a) MeOH and (b) THF. .......... 131 Figure 5-3. (a) Normalized excitation (em = 520 nm) and emission (ex = 320 nm) spectra of 33 in MeOH. (b) Normalized excitation ((em = 420 nm) and emission (ex = 350 nm) spectra of 33 THF. ............................................................................................................................................ 131 Figure 5-4. (a) Normalized excitation (em = 320 nm) and emission (ex = 520 nm) spectra of 24a-d in MeOH. (b) Normalized excitation (em = individual maxima) and emission (ex = 350 nm) spectra of 24a-d in THF. ............................................................................................................. 132 Figure 5-5. UV-Vis spectra of 35b and 35c in methanol and THF (left). Excitation and emission spectra of 35b and 35c in methanol and THF (right).................................................................. 133 Figure 5-6. UV-Vis spectra of 1.0  10-5 M solutions of Series A compounds (a) in MeOH with open structures and (b) in THF with closed structures. .............................................................. 136 Figure 5-7. Normalized excitation (em = individual emission maxima) and emission spectra (ex = individual excitation maxima) of 1.0  10-5 M solutions of the Series A compounds (a) in MeOH with open structures and (b) in THF with closed structures. ...................................................... 137 Figure 5-8. UV-Vis spectra of 1.0  10-5 M solutions of Series B compounds (a) in MeOH with open structures and (b) in THF with closed structures. .............................................................. 138 Figure 5-9. Normalized excitation and emission spectra of 1.0  10-5 M solutions of the Series B compounds (a) in MeOH with open structures and (b) in THF with closed structures. ............. 139 xvii  Figure 5-10. (a) Schematic of adipocytes and (b) blue emission of molecule 24a inside the lipid droplets of adipocytes (scale bar = 100 m, 485 nm long pass filter was used). ....................... 142 Figure 5-11. (a) Yellow-greenish emission of molecule 46 in the membrane of adipocytes (scale bar 100 m, 520 nm long pass filter was used) and (b) blue emission on the cell membrane (scale bar 100 m, 410-460 band-pass filter was used). ....................................................................... 142 Figure 5-12. (a) Yellow emission of molecule 46 in the periphery and blue emission of 24a in the lipid droplet of adipocytes (scale bar = 100 m). (b) Yellow emission of molecule 46 in the periphery and yellow emission of 41 in the lipid droplet of adipocytes (scale bar = 100 m). A 485 nm long pass filter was used in both cases. ................................................................................ 143 Figure 5-13. (a) Schematics of neurons and their structures.153 (b) Parallel nerve fibers can be seen in dashed box with stronger emission from dye 46 (scale bar 100 m). .................................... 144 Figure 5-14. Fluorescent images of brain slices showing cross-section of axons using 46. Scale bars are (a)100 m and (b) 20 m. ............................................................................................. 145 Figure 5-15. Fluorescent images of brain slices showing cross-section of axons using 41. Scale bars are (a)100 m and (b) 20 m. ............................................................................................. 146 Figure 6-1. Modulating the fluoride anion affinity at the boron center by deriving the phosphine groups. ......................................................................................................................................... 177 Figure A-1. Predicted isotopic distribution of the [M-2OTf]2+ ion of 2 using ISO PRO3.1.6 ... 194 Figure A-2. Measured isotopic distribution of [M-2OTf]2+ of 2 using MALDI-TOF MS. ....... 194  xviii  List of Schemes Scheme 1-1. Examples of conjugated polymers. ............................................................................ 4 Scheme 1-2. Nomenclature of the thiophene ring and general synthetic routes to oligothiophenes and polythiophenes. ...................................................................................................................... 11 Scheme 1-3. Examples of thiophene-containing groups (left) and thiophene-containing polymers (right) with red color indicating electron deficient groups. .......................................................... 11 Scheme 1-4. Two major types of photofunctional Pt(II) acetylides. ............................................ 15 Scheme 1-5. Examples of common polypyridyl ligands. ............................................................. 18 Scheme 1-6. Terpyridyl Pt(II) acetylides for water reduction36 and DSC applications.37 ........... 20 Scheme 1-7. Terpyridyl Pt(II) acetylide 89 that is able to form emissive metallogels.39............. 22 Scheme 1-8. Binuclear Pt(II) containing foldamer by Che42 (left) and Yam43 (right). ................ 23 Scheme 1-10. Lewis acidic compounds 10249 and 10450 binding with fluoride anion. ............... 25 Scheme 1-11. Boron containing heterocycles.51 ........................................................................... 25 Scheme 1-12. Preparation of main-chain boron-containing polymers via hydroboration53 and halo-phenylboration.54 ........................................................................................................................... 28 Scheme 1-13. Preparation of main-chain boron-containing polymers via tin-boron exchange.55 28 Scheme 1-14. Preparation of main-chain boron-containing polymers via coupling reactions.56 . 29 Scheme 1-15. Compounds 132-136. ............................................................................................ 31 Scheme 1-16. Molecular switches 138 and 139.59 ....................................................................... 32 Scheme 1-17. Photochemistry of Lewis adduct 14160 and 143.61 ................................................ 32 Scheme 1-18. Lewis adducts 145,62 14663 and 147.63 .................................................................. 33 xix  Scheme 1-19. Compounds 3165 and 32.66 ..................................................................................... 33 Scheme 1-20. Reaction of intramolecular Lewis pair 148.67 ........................................................ 34 Scheme 1-21. Bithiophene based interacting Lewis acid-base pair. ............................................ 35 Scheme 2-1. Pt(II) terpyridine complexes synthesized in this chapter. ....................................... 38 Scheme 2-2. Synthesis of complexes PtHa2T3-OTf (1) and Pt2a2T3-OTf (2). .............................. 39 Scheme 2-3. Synthesis of complexes Pt2a2TCCT-OTf (3). .......................................................... 39 Scheme 3-1. Pt(II) bisacetylides 11,99 12,100 13101, 14102 and 15.102 ............................................ 67 Scheme 3-2. Polymers and oligomers containing [trans-Pt(PR3)2(‒C≡C‒Ar‒C≡C‒)] units. .. 68 Scheme 3-3. Pt(II) bisacetylide metallacycles. ............................................................................. 70 Scheme 3-4. Synthesis of metallacycles 8-10. ............................................................................. 71 Scheme 3-5. Synthesis of ligands. ................................................................................................ 87 Scheme 4-1. FLP activation of dihydrogen and catalyzed hydrogenation of imines. .................. 96 Scheme 4-2. Examples of flexible Lewis acid-base pairs.120 ....................................................... 97 Scheme 4-3. A naphthalene-based Lewis adduct 3165 and an acenaphthene-based FLP 32.66 .... 98 Scheme 4-4. Bithiophene-based Lewis pair between -BMes2 and -P(O)R2 groups. .................... 98 Scheme 4-5. Synthesis of 24a-c and 25. ....................................................................................... 99 Scheme 5-1. Work in Chapter 4 and Chapter 5. ...................................................................... 123 Scheme 5-2. Examples of fluorescent dyes. ............................................................................... 125 Scheme 5-3. “Fluorescent flippers”149 and work in this chapter. ............................................... 126 Scheme 5-4. Lewis pairs discussed in this chapter..................................................................... 128 Scheme 5-5. The 3,3'- and 5,5'-substituted Lewis pairs 33 and 34. ........................................... 129 xx  Scheme 5-6. Lewis pairs containing different Lewis bases. ...................................................... 129 Scheme 5-7. Variations of the conjugation lengths of the bithiophene backbone (Series A).... 134 Scheme 5-8. Electron-donating and withdrawing groups on the backbone (Series B).............. 135 Scheme 5-9. General synthesis of flexible phosphine oxide-borane Lewis pairs. ..................... 135 Scheme 5-10. Preparation of water-soluble flexible Lewis pair 20. .......................................... 135 Scheme 5-11. Molecules used for fluorescence imaging trials. ................................................. 141 Scheme 5-12. Phosphine oxide-borane Lewis pairs and their precursors in parentheses. ......... 152 Scheme 5-13. Synthesis of dibromo-bithiophene precursors 50-55. .......................................... 166 Scheme 5-14. Synthesis of dibromo precursors 56-58. .............................................................. 169 Scheme 5-15. Synthesis of 58. ................................................................................................... 171 Scheme 5-16. Synthesis of 49. ................................................................................................... 172  xxi  List of Symbols and Abbreviations ACN acetonitrile ATR attenuated total reflectance BMes2 bis(2,4,6-trimethylphenyl)boryl group, or dimesitylboryl group bpy 2,2'-bipyridine br broad calcd. calculated cat. catalyst or catalytical amount CB conduction band CT charge transfer Cy cyclohexyl group  chemical shift (ppm)  difference Eg band gap D-A donor-acceptor dbbpy 4,4'-di-tert-butyl-2,2'-bipyridine DCM dichloromethane DFT density functional theory dppp 1,3-bis(diphenylphosphino)propane DSCs dye-sensitised solar cells DTE dithienylethylene E00 0-0 transition energy EM emission ESI electrospray ionization ET electron transfer eV electronvolt Ev vertical transition energy EX excitation xxii  f oscillator strength em emission quantum yield FL fluorescence FLP frustrated Lewis pair FTIR Fourier transform infrared spectroscopy GGA generalized gradient approximation GS ground state HB hydrogen bond HBA hydrogen bond acceptor HOMO highest occupied molecular orbital HRMS high resolution mass spectrometry Hz hertz IC internal conversion IR infrared ISC intersystem crossing K kelvin kFL rate of fluorescence kISC rate of intersystem crossing kNR rate of nonradiative decay kPH rate of phosphorescence LANL2DZ Los Alamos National Laboratory effective core and valence basis set of double-zeta quality LASER light amplification by stimulated emission of radiation LC ligand-centered or liquid chromatography LDA local density approximation LEDs light emitting diodes em emission wavelength ex excitation wavelength LLCT ligand-to-ligand charge transfer xxiii  LMCT ligand-to-metal charge transfer LUMO lowest unoccupied molecular orbital  micro m/Z mass-to-charge ratio MC metal-centered m-CBPA meta-chloroperoxybenzoic acid MeOH methanol Mes mesityl group MHz megahertz mL milliliter MLCT metal-to-ligand charge transfer MMLCT metal-metal-to-ligand charge transfer mmol millimole Mn number average molecular weight MO molecular orbital mol mole MS mass spectrometry MV2+ methyl viologen cation n-BuLi n-butyllithium NIR near infrared NMR nuclear magnetic resonance NOESY nuclear overhauser effect spectroscopy NR nonradiative decay OFET organic field effect transistors OLEDs organic light emitting diodes ORTEP Oak Ridge Thermal Ellipsoid Plot OTf trifluoromethylsulfonate P3HT poly-3-hexylthiophene PBE0 Perdew–Burke–Ernzerhof exchange-correlation functional xxiv  PBS buffer phosphate-buffered saline PCM polarizable continuum model PDI perylene diimide PEDOT poly-3,4-ethylenedioxythiophene PH phosphorescence ppm parts per million PVs photovoltaics Ref reference RISC reversed intersystem crossing RNA ribonucleic acid SEM scanning electron microscope SOC spin-orbit coupling  lifetime TADF thermal-activated delayed fluorescence tbtpy 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine tBu tert-butyl group TD-DFT time-dependent density functional theory em emission lifetime TEM transmission electron microscopy THF tetrahydrofuran TIP 2,4,6-tri(i-propyl)phenyl group TMS trimethylsilyl group tpy 2,2':6',2''-terpyridine  frequency UV ultraviolet VB valence band Vis visible  spin-orbit coupling constant for atoms Z charge or atomic numbers of the element xxv  Acknowledgements  I would like to express my deepest gratitude to my supervisor Prof. Michael Wolf for his support and guidance throughout the years. Thank you for giving me the space and transforming my random ideas into purposeful pursuits. Nothing would have come together without your help and encouragement. Thank you to all the lovely Wolf group members for making the past few years in the lab so enjoyable and memorable. Special thanks to Angela, Matt, Stephanie and Renee for getting me started. Thanks to Ashlee for teaching me how to use the UV-Vis spectrometer and the cryostat, and Peter for training me on the fluorimeter.  I would like to thank all the faculty members on my supervisory committee, especially Prof. Sammis and Prof. Gates for being my readers. I’d also like to express my appreciation to Prof. Jeffrey Nagle (Bowdoin College) for the computational collaboration and DFT trainings during his sabbatical leave in 2014. Big thanks to Dr. Brian Patrick for solving all the crystal structures in this thesis without which many conclusions cannot been made. Thank you Hunter for all the late-night imaging experiments and showing me the beautiful things under the fluorescence microscope. Thanks to Maria and Paul at the NMR facilities for the trainings and timely helps on 1D, 2D and VT NMR experiments. Dr. Saied Kamal at LASIR and the microanalysis staff members are also thanked for their help on my projects. Finally, but most importantly, I’d like to thank my father Qishan Cao and my mother Jiping Yang for their unconditional love and support. Thank you Cindy Yang, the light of my life, for always being there and making my life so wonderful. xxvi  Dedication     To my parents Qishan and Jiping and my wife Cindy  1  Chapter 1 Introduction  1.1 Overview Light is deeply involved in many important theories of modern physics including relativity, quantum theory and the theory of gravity and electromagnetism. Many applications of light such as laser technology, X-ray and optical materials not only contribute greatly to advances in modern sciences but also improve the quality of human lives in many different ways. To acknowledge and promote of the achievements of light science and its applications, 2015 was proclaimed by the UN General Assembly as the International Year of Light and Light-based Technologies (IYL).3  Light has become an important workhorse for human society with the coming of the Information Age. Other than widely applied uses in illumination, light is also widely used in modern electronics to generate graphical signals, such as on television screens and the displays of almost all digital devices. Far beneath the oceans, light is used to carry heavy intercontinental Internet traffic through optical fibers in communication cables.4 Solar energy in the form of light is considered to be a sustainable alternative to fossil fuels, the combustion of which causes global environmental issues such as air pollution and climate change. Moreover, the generation, manipulation, and detection of light are also believed to be the key to possible future technologies5 such as quantum computing6 and quantum communication.7 The development of materials with a variety of different photofunctionalities is therefore highly important. Photovoltaics (PVs) and light emitting diodes (LEDs) are two types of devices employing photofunctional materials. Both types of devices have been successfully commercialized with 2  multi-billion global markets. In PVs charge separation occurs after which absorbed light energy is converted to electricity, whereas in LEDs charges are combined using electric power to emit light. Both applications are moving towards using more flexible, light-weight materials with high efficiency and lower economic and environmental costs. Organic semiconductors are becoming more and more advantageous compared to conventional inorganic counterparts for these applications. Organic photofunctional materials can be easily applied to flexible substrates using low-cost and high-throughput processing techniques such as roll-coating8 and inkjet printing9, etc. For example, SolarWindow Technologies, Inc. in Maryland has developed an organic photovoltaic (OPV) material that can be coated on any window surface to generate on-site electricity for buildings.10 Samsung, LG, and Apple have recently released wearable devices such as smartwatches integrated with highly energy-efficient organic LED (OLED) displays.  Conjugated materials with delocalized -electrons have been promising photofunctional materials since their discovery, due to the unique optical and electrical properties of organic semiconductors.11 Moreover, they can be synthetically tailored to have desired properties such as strong light absorption or high charge carrier mobility for certain applications such as in OPVs, OLEDs and organic field effect transistors (OFETs). It has also been shown that conjugated materials can also incorporate other photofunctionalities such as metal-containing moieties, or donor-acceptor pairs. In this thesis, three different thiophene-containing photofunctional conjugated materials that show environment-dependent light absorption and emission properties are demonstrated. 3  1.2 Conjugated Materials 1.2.1 Structures and Molecular Orbitals The metal-like conductivity of polyacetylene doped with halogen vapour was first observed in the late 1970s by Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa,12 who in 2000, shared the Nobel prize in Chemistry "for the discovery and development of conductive polymers".11 This finding challenged the commonly held perception of organic materials as being poor conductors and started decades of extensive scientific research on different types of conjugated materials.   Figure 1-1. Simplified diagram showing the narrowed HOMO-LUMO energy gap and the formation of band structure with increasing -conjugation from ethylene to polyacetylene. The properties of polyacetylene can be explained by its molecular orbital (MO) diagram. As shown in Figure 1-1, with additional conjugation length from ethylene to 1,3-butadiene and 1,3,5-4  hexatriene, the highest occupied molecular orbital (HOMO) increases in energy while the energy of the lowest unoccupied molecular orbital (LUMO) decreases. The HOMO-LUMO energy difference reduces as a result. In polyacetylene where the conjugation is extended enough, the occupied and unoccupied MOs merge into continuous band-like structures with a valence band (VB) which is filled with paired electrons, and an empty conduction band (CB). The band gap Eg in polyacetylene is small enough for the material to be semiconducting. In pristine polyacetylene no MO is present between the VB and CB, however, additional interband states can be introduced by doping the material with iodine, resulting in metal-like electric conductivity. Scheme 1-1. Examples of conjugated polymers.  Over the years, many variants of different conjugated materials have been developed such as polythiophene, polypyrrole, poly(p-phenyleneacetylene) and polyfluorene, etc (Scheme 1-1). A common feature of these systems is alternating double and single carbon-carbon bonds with overlapped -bonding orbitals along the length of the structures. Different building blocks with different steric and electronic properties can be used alone or combined to engineer the conformation and electronics of the resulting conjugated material. Side-groups such as alkyl chains can also be added to improve the solubility and processability of conjugated materials which were not available in the original synthesis of polyacetylene.  Conjugated materials with atoms other than carbon in the backbone can also be made 5  (Figure 1-2). For instance, the filled p orbital of sp2 hybridized nitrogen atoms or the empty p orbital of sp2 hybridized boron atoms can also overlap effectively with the carbon p orbitals of the conjugated  system to form either electron-rich or electron-deficient materials, respectively. Similarly, the d-orbitals of some transition metals with suitable symmetry and energy level (such as the dxz orbital of Pt(II) in Figure 1-2) can also result in a less degree of conjugation of the conjugated  system. These materials can potentially exhibit the photophysical properties of both the heteroatoms and the carbon-containing counterparts, and in some cases, new properties such as light-induced charge transfer (CT) can also arise between them.  Figure 1-2. Examples of hybrid conjugated polymers (top) and schematics of the orbital overlap of corresponding conjugated systems.  1.2.2 Photophysical Properties Conjugated materials with different band structures or frontier molecular orbitals show 6  different photophysical properties when interacting with light. They may absorb light at different wavelengths and give rise to different emission.  For instance, in Figure 1-3, a photon is absorbed by a ground state (GS) molecule which is in the lowest singlet state (total electron spin, S = 0) and promotes an electron from the HOMO to LUMO (Figure 1-3, left) on a femtosecond (10-15 s) time scale. This process of light-induced electron excitation is extremely fast and can be approximated without changing the position of nucleus in the molecule, as known as the vertical excitation. Immediately after excitation, molecules are usually at higher vibronic states of the resulting excited state, and always undergo rapidly relaxation to the lowest vibronic states prior to emission (Figure 1-3, right).  Figure 1-3. HOMO and LUMO of different electronic states (left) and a Jablonski diagram showing the vibronic states of each electronic states (right, bold lines indicate the lowest vibronic states). The excited molecule will be in a singlet state (e.g. S1 state) if the total spin of electrons is conserved, or can be converted into a triplet state (e.g. T1 state) in a dark process called intersystem 7  crossing (ISC) in which one of the electrons undergoes a spin flips without interacting with additional photons. Conversion between states with different electron spin multiplicity is forbidden by selection rules. Therefore, ISC in organic molecules is usually very slow unless facilitated by spin-orbit coupling (SOC) of heavy atoms.13 Another dark process is internal conversion (IC), which is the fast conversion ( ~10-12 s) of excited states to lower energy states with the same multiplicity, such as S2→S1, T2→T1 or S1→S0.  An excited state molecule can go back to GS via radiationless decay, or by emitting a photon. Emission from the singlet and triplet excited states are called fluorescence and phosphorescence, respectively (Figure 1-3, right). Again due to selection rules, the former is usually many orders of magnitude faster than the latter (kFL >>kPH). Phosphorescent triplet excited states are sensitive to O2, whereas fluorescence is usually not. The O2 sensitivity of phosphorescent materials has been widely used in applications such as O2 sensors and hypoxia imaging in tumor tissues.  The fluorescence and phosphorescence intensity and emission lifetimes () of -conjugated materials can be attenuated by the presence of heavy atoms and engineering the energies and MOs of corresponding singlet and triplet excited states. This has become especially important for OLED applications in that phosphorescent materials are considered to have much higher theoretical electroluminescence quantum efficiencies than fluorescent ones.14  1.1.1 Charge Transfer Processes Light absorption by pristine -conjugated materials usually does not result in excited state electron density that is significantly shifted from the GS. Such excitation processes are called * 8  excitations which are typically highly probable due to good overlap of MOs. By contrast, processes that involve large displacement of electron density are called charge transfer (CT) transitions, these are less probable compared to* transitions due to reduced orbital overlap. Charge transfer transitions with absolutely no overlap between frontier MOs of the corresponding states are forbidden. However, most frontier MOs of CT excited states share some degree of overlap with the GS and can be populated with low probability..   Figure 1-4. Schematics of HOMO of * and CT excited states of thiophene (left) and thiophene with an sp2 boron CT acceptor at the -position (right). Red solid and hollow circles are representative of the contributions of atomic p orbitals to the HOMO. In general, in a given molecule the CT absorption bands are often weaker than the * bands in the UV-Vis spectra. Unfortunately, radiationless decay (or nonradiative decay, NR) of an excited state with * character can also be very rapid for similar reasons. This is one of the major limitations to high emission quantum efficiency of -conjugated materials. However, NR can be minimized by enabling lower energy CT excited states so that NR from the * excited state can be suppressed (Figure 1-4).  9  Some molecules with charge transfer S1 and T1 excited state have very small S1-T1 energy gaps due to weak interaction between unpaired electrons, which is beneficial for reversed intersystem crossing (RISC) from T1→S1. This principle has been used in thermally-activated delayed fluorescence (TADF) applications to harness triplet excited states without the presence of transition metals, achieving high emission efficiency in fluorescent materials. Some compounds with CT excited state can also undergo photo-induced electron transfer (ET) with neighboring molecules inducing charge separation between molecules, this is an important process in both dye-sensitized solar cells (DSSCs) and photo-redox catalysts.  Figure 1-5. Schematics of solvatochromism of CT transitions. Light absorption and emission of CT transitions usually shows some degree of a spectroscopic solvent effect depending on the polarities of the states involved, called solvatochromism (illustrated in Figure 1-5). If the excited CT state is more polar than the ground state, it will be stabilized more in more polar solvents (Figure 1-5, left). As a result, the absorption and emission colors are red-shifted with a higher solvent polarity, known as positive 10  solvatochromism. By contrast, a molecule with a more polar ground state will give rise to negative solvatochromism, thus showing blue-shifted features in more polar solvents (Figure 1-5, right). In rare cases, the polarities of the initial and final states in a CT process are very similar. Therefore, solvatochromism can be used to identify a CT transition, however, a lack of solvent effect cannot be used to exclude CT processes. In the context of photofunctional conjugated materials, charge transfer transitions can be achieved by adding -donating groups (e.g. aniline derivatives) and/or -accepting groups (e.g. nitro- and cyanide groups) to the conjugated backbone. The combination of these two types of groups results in materials containing donor-acceptor (D-A) pairs. In the following sections of this chapter,-accepting Lewis acidic groups with unquenched reactivity (e.g. 3-coordinated boranes) and transition metals (e.g. Pt(II) containing moieties), which can be both -donating and-accepting, will be further discussed in the context of thiophene-containing conjugated materials.   1.3 Thiophene-Containing Conjugated Materials Conjugated oligomers and polymers can be constructed using thiophene-containing building blocks via palladium-catalyzed Suzuki, Negishi, and Stille coupling reactions (Scheme 1-2). In recent years, direct C-H activation methods15 that avoid usage of organometallic reagents have become increasingly popular as more step-economic alternatives. It is also common practice to add side-chains such as hexyl groups to enhance the solubility of the final product so that higher molecular weight can be achieved when making polymers. 11  Scheme 1-2. Nomenclature of the thiophene ring and general synthetic routes to oligothiophenes and polythiophenes.  Scheme 1-3. Examples of thiophene-containing groups (left) and thiophene-containing polymers (right) with red color indicating electron deficient groups.  As one of the common building blocks for conjugated materials, thiophene is more electron-rich than benzene.16 As shown in Scheme 1-3, materials with only thiophene rings on the conjugated backbone, such as poly-3-hexylthiophene (P3HT, 68) and poly-3,4-ethylenedioxythiophene (PEDOT, 69), are electron-rich and can be P-doped with oxidants. 12  Thiophene derivatives with electron-donating groups (e.g. 65 and 66) are incorporated in donor-acceptor (D-A) materials (polymer 70)17, serving as electron donors to electron-deficient groups such as perylene diimide (PDI). Some thiophene derivatives such as compound 67 in Scheme 1-3 with strong electron-withdrawing groups can also become electron acceptors.17    Figure 1-6. Thiophene-thiophene s-trans and s-cis coplanar conformations (left) and conformation changes as the result of weak interactions.  Pairs of adjacent non-substituted thiophene rings have two energy minima: the s-trans and s-cis conformations (Figure 1-6, left). In 2,2'-bithiophene, the former is slightly more stable, about 0.8 kcal/mol lower in energy compared to the latter with a energy barrier between the two of 20±8 kcal/mol.18 Substitutions on thiophene rings may affect the planarity of the backbone (Figure 1-6, right) which can be important for the performance of the material. Torsion between thiophene rings usually results in less effective conjugation of the backbone and consequently weaker light absorption. 13  Regioregular head-to-tail poly-3-hexylthiophene (HT-P3HT, 71 in Figure 1-6) shows better conductivity and a lower bandgap compared to isomers with head-to-head (HH, 72) coupling patterns, due to a higher degree of coplanity between thiophene rings.19 In solar cell applications, on the other hand, the torsion between thiophene rings by repulsion of alkyl groups has been used to increase the open-circuit voltage.20 Recently, it was shown that the weak attractions between thiophene donors and imide acceptors can also contribute to the planarity of the D-A conjugated backbone.17,21 These weak interactions are an important factor to be considered when engineering the molecular conformation and intermolecular interaction in thiophene-containing conjugated materials, especially for applications that require reversible tuning of backbone conjugation.  1.4 Photofunctional Conjugated Materials with Pt(II) Moieties  Figure 1-7. Transition metal-containing conjugated materials (blue circles represent metal complexes while grey tiles indicate the conjugated backbone).22  Additional photophysical properties can be introduced to thiophene-containing conjugated 14  materials by incorporating transition metals. There are three basic types of transition metal-containing conjugated materials as summarized in Figure 1-7.22 In Type I systems, the metal-containing moieties are tethered to the backbone by non-conjugated linkers such as alkyl chains. Such systems show relatively separate behavior of the metal and the -system. Type II and III systems involve the electronic coupling of the metal to the -system, where in the latter case the metal becomes part of the main-chain -conjugation.  Figure 1-8. Two scenarios for light absorption in transition metal-containing -systems. As shown in Figure 1-8, ligand-centered (LC) processes are typically  transitions with little or no participation of the metal. Forbidden by selection rules, metal centered (MC) d-d transitions are usually very weak and give rise to NR decay which can be minimized by increasing the d-d energy gap with strong-donating ligands.23 Photo-induced electronic transitions between metal and -system will result in CT processes. Ligand-to-metal charge transfer (LMCT) indicate CT from the -system to the metal (Figure 1-8, left), which is common between metal centers with low-energy empty d-orbitals and ligands with high-lying HOMO levels. CT in the opposite direction, namely metal-to-ligand charge transfer 15  (MLCT) (Figure 1-8, right), is common between late transition metals with high-energy filled d-orbitals and ligands with low-lying  orbitals. Ligand-to-ligand charge transfer (LLCT) between multiple ligands on the same metal center is also possible and sometimes can become predominant. Charge transfer transitions with mixed origins in large and complex -systems can be described with their major components such as MLCT/LLCT, or simply CT in general for convenience. Scheme 1-4. Two major types of photofunctional Pt(II) acetylides.  Square planar Pt(II) metal centers with d8 electronic configurations are important photofunctional entities that can be incorporated into conjugated materials. In most Pt(II) photofunctional materials, strong -donating conjugated ligands such as acetylides are used to avoid NR decay through d-d excited states and increase conjugation and electronic coupling between the metal and its ligand(s).  Two major types of photofunctional Pt(II) acetylides include complexes with phosphine ancillary ligands, and complexes containing polypyridyl ligands (Scheme 1-4). Pt(II) acetylides with phosphine ligands generally do not show a strong tendency for CT unless the acetylide ligands are highly electron rich or deficient.24 By contrast, Pt(II) acetylides with electron-deficient polypyridyl ligands feature planar structures and strong CT 16  transitions.  1.4.1 Photofunctional Pt(II) Acetylides with Phosphine Ligands Pt(II) acetylides with phosphine ancillary ligands have two possible geometries: the cis- isomer with two acetylides adjacent to one another, and the trans- isomer with acetylides on opposite sides of the metal. Due to the trans effect,25 in the process of installing the two acetylides, they prefer to be trans to each other on the Pt(II) center unless the cis- geometry is enforced by chelating ligands such as 1,3-diphenylphosphinopropane (dppp). The trans-substituted complexes can also be converted to the cis-isomers by simply reacting with diphosphine chelating ligands (Figure 1-9, right).26  Figure 1-9. Cis- and trans-substituted Pt(II) bisacetylides with phosphine ancillary ligands, and an example of a trans-substituted Pt(II) bisacetylide transforming to its cis-isomer.26 17   The cis-substituted Pt(II) centers have been used in supramolecular assembly and synthesis of metal-containing macrocycles as templates for 90° geometries.27 It is also known that cis-substituted Pt(II) acetylides with phosphine ancillary ligands can undergo oxidation (by I2) inducing reductive elimination to form coupled alkynes while the trans-substituted isomers cannot (Figure 1-9).28 This reaction is useful in making conjugated C-C bonds between alkynes that are otherwise difficult to obtain due to structural tension or steric repulsion,29 when the corresponding cis-substituted Pt (II) bisacetylides precursor is easier to synthesize.  Figure 1-10. Schematics of S1 and T1 states of polymer 15 and origin of its fluorescence and phosphorescence. The photophysical properties of Pt(II) bisacetylides with phosphine ancillary ligands relies heavily on the properties of the conjugated ligand. For instance, absorption and emission properties of trans-substituted Pt(II) bisacetylide polymers indicates that S1 singlet excited states are delocalized between a few repeating units while the T1 excited states are only localized on one of 18  the conjugated ligands (Figure 1-10). As a result, conjugated polymers of trans-Pt(II) bisacetylides with phosphine ancillary ligands shows phosphorescence that is not dependent on the conjugation length. In rare cases fluorescence from the S1 state is also observed, result in dual fluorescence and phosphorescence. This phenomenon can be enabled by low-lying triplet excited states of thiophene-containing conjugated ligands, which will be further discussed and explored in Chapter 3.  To sum up, in Pt (II) acetylides with phosphine ligands, the role of the metal atom is to facilitate ISC to triplet excited states, to provide communication between acetylide ligands in the singlet manifolds and to segregate ligand based triplet states.  1.4.2 Photofunctional Pt(II) Acetylides with Polypyridyl Ligands Pt (II) acetylides with polypyridyl ligands exhibit extensive electron delocalization over the entire -system giving rise to intense charge transfer absorption and emission. The orbitals on the metal and the electron poor polypyridyl ligand are highly mixed, resulting in more complicated photophysics. Scheme 1-5. Examples of common polypyridyl ligands.  19   The first syntheses of Pt(II) acetylide photofunctional materials with bipyridine and terpyridine ligands (Scheme 1-5) were first reported by Yam30 and Eisenberg,31 respectively. Their light absorption and emission properties have been further investigated by Schanze32 and Castellano33 using nanosecond spectroscopy and ultrafast transient absorption spectroscopy. In general, with light excitation, these compounds undergo CT to the electron deficient polypyridyl ligand from Pt(II) (MLCT) or from the acetylide ligands (LLCT) (Figure 1-11). In cases where aromatic acetylides are used, MLCT/LLCT mixture is observed and LLCT contributes more when the acetylides are more electron-rich. The initial excited state after light absorption is the singlet state (e.g. 1MLCT) which rapidly undergoes ISC to a triplet state (3MLCT) slightly lower in energy.  Figure 1-11. MLCT and LLCT using a terpyridyl Pt(II) acetylide complex as an example (left) and energetics of singlet and triplet MLCT excited states and LC excited states. The terpyridine ligand is highlighted in blue while the acetylide ligand is in red. In some cases, the triplet CT state can further relax by transferring its energy to a low-lying 3LC excited state localized on the acetylide ligand. In these cases, 3LC of the acetylide is 20  “sensitized” with relatively low energy photon compared to direct excitation to its singlet * excited state 1LC. Such processes can be very useful to encourage ligand based phosphorescence from 3LC without undesired fluorescence from the 1LC state,34 and has also been used to encourage up-conversion using triplet-triplet annihilation.35 The 3LC state may also encourage fast NR from 3LC state depending on the nature of the acetylide ligand.  Scheme 1-6. Terpyridyl Pt(II) acetylides for water reduction36 and DSC applications.37  The charge transfer excited state of complex 86 was found to reduce water and generate H2 under visible light with the assistance of the methyl viologen cation MV2+ and Pt colloidal catalyst.36 Yam et al. also developed a series of terpyridyl Pt(II) acetylide dyes including 87 and 88 with thiophene based ligands for DSC applications.37 Another outstanding feature of polypyridyl Pt(II) acetylides is their tendency to form aggregates.38 The large planar conjugated system in polypyridyl Pt(II) acetylides allows tight stacking between molecules driven by  dispersion attractions and metallophilic Pt-Pt interactions (Figure 1-12). The aggregation of Pt(II) polypyridyl complexes can be tuned by changing the substituents on the bipyridine and terpyridine ligand (R1 in Figure 1-12). The ligands 83 and 85 in (Scheme 1-5) with bulky t-Bu groups can effectively prevent intermolecular 21  aggregation in both solution and solid state whereas non-substituted ligands 82 and 84 can give rise to significant  stacking and Pt-Pt interactions with the addition of poor solvents.   Figure 1-12. Schematics of MLCT and MMLCT transitions.   Aggregation of a terpyridyl Pt(II) acetylides can give rise to a new type of electronic transition called metal-metal-to-ligand charge transfer (MMLCT). In Figure 1-12, the formation of aggregated dimer leads to interaction and splitting of the MOs of each individual molecule. The two Pt dz2 orbitals can interact strongly and produce one “bonding” and one “antibonding” orbital with a larger energy gap than the -orbitals. The “antibonding” orbital can be elevated to become 22  the HOMO which requires less energy to be promoted to the LUMO compared to monomeric complexes.   Therefore, the aggregation of terpyridyl Pt(II) acetylides can be identified in UV-Vis spectra with red-shifted, low energy MMLCT absorption bands. Similarly, MMLCT phosphorescence that is redshifted from monomeric MLCT emission is expected, arising from the corresponding 3MMLCT triplet state relaxed from the initial 1MMLCT state. Additionally, in 1H NMR spectra, aggregation may also lead to peak broadening due to restricted rotation of substitutions, as well as up-field shifts of pyridyl proton signals due to the shielding effect of the folded aromatic rings.   Scheme 1-7. Terpyridyl Pt(II) acetylide 89 that is able to form emissive metallogels.39  Positively charged terpyridyl Pt(II) acetylides are stable as monomers in good solvents due to the static repulsion between cations. The intermolecular aggregation of these compounds can be encouraged by the addition of a poor solvent (a solvent that the compounds have very low solubility in) such as diethyl ether40 to the solution of the compound, or using static attraction of negatively charged macromolecules such as RNA to overcome the static repulsion.41 The 23  aggregation can also be used in metallophilic gelation of molecules containing these “sticky ends” and multiple long alkyl chains, such as 89.39 The metallogel formed by 89 in DMSO has a coil-like superstructures under SEM, and give rise to strong MMLCT emission in the NIR region. A wide variety of applications in sensing have also been reported, using MMLCT absorption or emission feature as spectroscopic handles for negatively charged analytes. Scheme 1-8. Binuclear Pt(II) containing foldamer by Che42 (left) and Yam43 (right).  Such Pt-Pt and π–π interaction between Pt(II) “sticky ends” can also be used to encourage conformation changes of the conjugated backbone. Che42 et al. reported foldable dimers 90-06 with an ortho-phenylene ethynylene backbone with 60° angles (Scheme 1-8, left). By adding water into an acetonitrile (CH3CN) solution of these compounds, the emission is red-shifted which is attributed to inter- and/or intramolecular aggregation of terpyridyl Pt(II) moieties. A TEM image of 94 shows that the aggregate forms nanoparticles in a H2O/CH3CN mixture.  A similar system of binuclear complexes 91- 101 is reported by Yam43 et al.  featuring a meta-phenylene ethynylene backbone with 120° angles (Scheme 1-8, right). Only 101 can offer a full 360° turn to form effective intramolecular Pt-Pt interactions, evidenced by red-shifted 24  MMLCT features in UV-Vis and phosphorescence spectra, upon changing the solvent from CH3CN to dichloromethane (CH2Cl2). This work was followed by the preparation of chiral binuclear Pt(II) helical structures enabled by enantiomeric pure binaphthalene containing bisacetylide ligands.44 The aggregation results in changed circular dichroism. These systems are considered to be a synthetic mimic of the chiral helical structure in biological systems and foldamers.44  1.5 Photofunctional Conjugated Materials Containing Lewis Acidic Boron Groups A Lewis acid is a molecule with empty orbitals that can accept a pair of electrons from a donor while a Lewis base is a molecule that can provide such a pair of electrons. Lewis acids tend to bind with Lewis bases to form Lewis adducts.  Three-coordinated boron is especially suitable for photofunctional materials when integrated with conjugated materials due to the good orbital overlap of the empty p orbital with the -system.45 The electron-demanding nature can give rise to CT excited states with suppressed NR decay.46 These conjugated materials containing Lewis acidic boron centers are usually highly fluorescent in contrast to their transition metal-containing counterparts, which are usually phosphorescent due to fast ISC. Additionally, the Lewis acid has unquenched chemical reactivity towards binding Lewis bases, which can be applied in sensing47 applications or anionic supermolecular recognition.48  25  1.5.1 Small Conjugated Molecules In 2000, Yamaguchi et al. reported an anthracene-based molecule 102 which allows the conjugation of three conjugated aromatic groups to delocalize through the empty p orbital of boron (Scheme 1-10, left).49 It also gives rise to a CT excited state that localizes on the boron atom. Compound 102 is found to react with fluoride ion in a 1:1 molar ratio to give 103, accompanied by reduced conjugation and disappearance of CT absorption in the UV-Vis spectrum. This group also reported 104 with boron “side-on” (Scheme 1-10, right),50 which allows change of the LUMO from boron-based in 104 to  based in 105, giving rise to either CT emission or emission. Scheme 1-10. Lewis acidic compounds 10249 and 10450 binding with fluoride anion.  Scheme 1-11. Boron containing heterocycles.51  Conjugated systems with Lewis acidic boron can be built based on two important type of boron containing heterocycles shown in Scheme 1-11. Type I is an anti-aromatic five-membered 26  borole ring which is isoelectronic to the 1,3-cyclopentadiene cation as described by Yamaguchi51a et al., while Type II is the aromatic seven-membered borole compounds isoelectronic to tropylium cation as described by Piers et al.51b Both types can be synthesized from the corresponding organotin precursor. For instance, the synthesis of the seven-membered borole 108 involves the transformation of 106 to 107 followed by installation of a bulky R group such as the mesityl group (Mes, 2,4,6-trimethylphenyl) to protect the empty p-orbital of boron.51b It is shown that the aromatic Type II heterocycles can undergo similar reaction as the benzene analogs, such as bromination by electrophilic aromatic substitution and palladium-catalyzed coupling reactions, which allows construction of larger conjugated systems.52  Figure 1-14. Synthesis and emission colors of boron-containing bithiophene derivatives.46 A bithiophene-based conjugated system with a 3-coordinate boron group in the 3-position of one of the thiophene rings has also been developed by the Yamaguchi group (Figure 1-14).46 Lithiation of 109 with n-BuLi followed by installation of the boron group using-BMes2 gives 110 27  which can be brominated under mild condition to afford 111. Molecules 112-116 with various substitutions can be synthesized by reacting 111 with corresponding aryl boronic acids under Suzuki coupling conditions. All these compounds are stable in coordinating solvents and stable to oxidation in air in that the empty p orbital of the boron is “blocked” by the ortho- methyl groups of the mesityl groups on boron.  Compounds 110 and 112-116 are highly emissive compounds in both solution and the solid state. The photophysical process can be described as shown in Figure 1-4: Upon excitation, the electron-rich bithiophene backbone undergoes CT to the -BMes2 group resulting in an excited state that is heavily localized on the boron atom. Such CT excited states give rise to very efficient fluorescence due to weak NR decay. The emission color can be red-shifted with added conjugation and the presence of more electron-donating groups on the bithiophene backbone as they elevate the energy of the -bonding HOMO without notably changing the LUMO energy. Additionally, the bithiophene backbone is significantly twisted out of the plane (e.g. 56.0° torsion angle in 110) resulting in poor intermolecular  interaction and high emission quantum yield in the solid state. An interesting feature of this system is that the synthetic method can be easily adapted to make larger conjugated systems, either oligomeric or polymeric.  1.5.2 Conjugated Oligomers and Polymers Polymeric conjugated materials containing multiple Lewis acidic boron groups can be made by either coupling the monomers via boron atoms using boration reactions, or via carbon atoms using Pd catalyzed coupling reactions.  28  Scheme 1-12. Preparation of main-chain boron-containing polymers via hydroboration53 and halo-phenylboration.54  The first well-characterized polymers were obtained by hydroboration of bisacetylides using arylboranes MesBH2 (Scheme 1-12, left), reported by the Chujo group.53 The resulting polymers are oxygen stable and strongly blue-emissive. The molecular weight of polymer 119 are as high as 16000 (Mn). Boron-containing polymers can also be obtained using halo-phenylboration reactions between bisacetylides and Ph2BBr, also reported by the same group (Scheme 1-12, right)54 with a lower degree of polymerization. Scheme 1-13. Preparation of main-chain boron-containing polymers via tin-boron exchange.55   29  Scheme 1-14. Preparation of main-chain boron-containing polymers via coupling reactions.56   The Jäkle group has explored another approach via highly selective tin-boron exchange reactions (Scheme 1-13, right).55 Compound 124 can be made from 123, which then reacts in a 1:1 molar ratio with 123 to give polymer 125 containing reactive B-Br bonds. Compound 125 can then react with aryl Cu(I) reagents to allow installation of a bulky aryl group such as a mesityl or 2,4,6-tri(i-propyl)phenyl (TIP) group, and obtain polymers 126 and 127, respectively. Recently, the Jäkle group reported the synthesis of thiophene containing conjugated polymers analogs to 126 and 127, using a C-C coupling approach.56 Both the monomer 129 and monomer 130 can be prepared from a non-substituted precursor 128 via either deprotonation-stannylation or iodination reactions at the  position of the thiophene rings.   30  1.5.3 Reactions with External Lewis Bases  Figure 1-15. Reaction of a -conjugated Lewis acid with external Lewis bases. Reactions with Lewis bases will give rise to major photophysical changes to a conjugated system with Lewis acidic boron centers installed (Figure 1-15). The empty p orbital on boron can be filled and transformed into a  bonding orbital with the Lewis base. This results in quenched Lewis acidity of the material and disappearance of corresponding CT absorption and emission bands. Additionally, the conjugation between the aromatic groups on the boron center will decrease, resulting in more localized * transitions. Structure-wise, the geometry of the boron atom changes from trigonal planar to tetrahedral with the C-B-C bond angles going from ~ 120 ° to ~108 °. The C-B bonds will also lengthen due to decreased bond orders.  Most photofunctional materials with Lewis acidic boron centers have bulky aromatic groups on the boron atom to increase their stability. Therefore, many such materials can only react with a limited scope of small Lewis bases such as fluoride anions, cyanide anions and pyridine.  For instance, in sensory applications for fluoride anions, reactions with the analyte results in CT fluorescence turn-off. This process can be reversed to recover the unbound free Lewis base 31  with addition of water which strongly hydrogen bonds to fluoride anion with much higher affinity. However, for the same reason, most boron-based fluoride-sensing molecules do not work in aqueous medium unless their affinity towards fluoride anions are enhanced by a second interaction. A few fluoride binding molecules are shown in Scheme 1-15. Compound 132 has strong affinity for fluoride because the resulting Lewis adduct is stabilized by cooperative binding with the adjacent boron center.57 Compound 135 binds to fluoride anion stronger than 133 and 134 due to the favourable Coulombic attraction of the positively charged phosphonium cation as well as “chelation” effect of the interaction between the fluoride and the P atom in the adduct 136.58 Many boron-based fluorescent sensor molecules with different colors have been developed based on these principles. Scheme 1-15. Compounds 132-136.  Yam et al showed that reaction of the Lewis acidic center can be used to modulate photo-isomerization of the -conjugated system (Scheme 1-16).59 The photo-induced ring closing of the dithienylethene (DTE) moiety involves the 6 conrotatory cyclization of the * orbitals. Molecule 72 with a Lewis acidic center has a boron-centered CT excited state that is lower in energy than 32  the * orbital, thus the latter cannot be effectively populated. Therefore, 72 can’t undergo light induced ring-closing while 73, with a favorable * transition, can.  Scheme 1-16. Molecular switches 138 and 139.59  Scheme 1-17. Photochemistry of Lewis adduct 14160 and 143.61  In some cases, the Lewis base can only bind weakly to the boron center and is, therefore, “hemilabile”. These systems can exhibit interesting photo-induced changes. For example, the 2,6-lutidine moiety in Lewis adduct 141 can migrate to the 2-position of the four-membered borole ring under irradiation at low temperature. The resulting isomer 142 can revert to 141 at elevated temperatures.60 Strained Lewis adduct 143 can undergo photo-induced dissociation of the Lewis base, releasing the free Lewis acid 144 that emits at higher wavelengths.61 33  1.5.4 Interactions with Internal Lewis Bases The 3-coordinate boron center can also interact with internal Lewis bases to either form Lewis adducts or remain unreacted pairs. The latter case is often due to steric hindrance or low reactivity of the Lewis base.  The Wang group has studied 4-coordinate N,C-chelated borates which are intramolecular adducts between the Lewis basic pyridine-ring and the Lewis acidic -BMes2 group.62 While similar N,N-chelates such as 145 based on the bipyridine backbone are stable OLED materials,63 these molecules exhibit interesting photo-reactivity (Scheme 1-18). Under UV-light, 146 is transformed to another adduct 147 with a boron containing 3-membered ring. The mechanism involves photo-reactive triplet excited states as the isomerization can be inhibited by triplet acceptors.64 Scheme 1-18. Lewis adducts 145,62 14663 and 147.63  Scheme 1-19. Compounds 3165 and 32.66   34  The Wang group has investigated a Lewis acid-base containing naphthalene-based system 31 with a phosphine-borane interaction (Scheme 1-19, left).65 The P-B interaction is evidenced by chemical shifts in the 31P NMR spectra as well as 11B NMR spectra. Mebs et al. showed that such interaction can be interrupted or “frustrated” by using a rigid acenaphthene bridge which enforces a longer P-B distance.66 The resulting molecule 32 shows 31P NMR and 11B NMR chemical shifts typical for free phosphine and boranes.  Scheme 1-20. Reaction of intramolecular Lewis pair 148.67  Frustrated Lewis pairs (FLPs) are interesting systems where free Lewis acid and base coexist without forming adduct due to steric reasons. They often exhibit special cooperative reactivity of the Lewis acid and base. An interesting example of a FLP was reported in the context of conjugated materials by Yamaguchi et al. (Scheme 1-20).67  The frustration of the Lewis acid and Lewis base in 148 can add to the acetylene bridge in a 5-endo-dig fashion to form ladder-like structure 149.  1.6 Goals and Scope The goal of this thesis is to synthesize and characterize photo-functional molecules based on thiophene-containing conjugated molecules. Specifically, I’ve focused my research on tunable 35  absorption and emission properties of conjugated molecules with two types of functionalities: square planar Pt(II) acetylides (Chapters 2 and Chapter 3), and 3-coordinated Lewis acidic boron groups (Chapter 4 and Chapter 5). Chapter 2 explores thiophene-containing complexes with two identical square planar terpyridyl Pt(II) metal centers. Flexible thiophene-containing conjugated ligands are designed to facilitate Pt-Pt and  interaction between metal centers, which give rise to MMLCT-based absorption and emission. Inspired by the work by Che and Yam, modulation of the folding/unfolding of the structure by changing the solvent environment, monitored by UV-Vis and photoluminescence spectra is explored in Chapter 2.  Chapter 3 examines the emission properties of three Pt(II) bisacetylide metallacycles with thiophene-containing ligands. The ligands are modified systematically to change the S1 and T1 excited state energies of the corresponding metallacycles, resulting in room-temperature dual emissive materials with tunable fluorescence/phosphorescence (FL/PH) ratios. Scheme 1-21. Bithiophene based interacting Lewis acid-base pair.  Chapter 4 sets out to explore intramolecular interactions between a boron-containing Lewis acid and a phosphine oxide Lewis base, based on a flexible bithiophene skeleton (Scheme 1-21). The bound/unbound states of the Lewis acid-base pair exhibit different emission properties, and 36  are shown to be in equilibrium with each other as evidenced by UV-Vis, fluorescence, infrared (IR) and 31P NMR spectra. Chapter 4 also contains an exploration of the impact of solvent and temperature on this equilibrium. More intramolecular Lewis bases are explored based on the findings in Chapter 5. Additionally, the effect of the conjugation lengths and electron densities on the backbone are systematically explored using a series of molecules with different emission colors in two states. The use of such a binary equilibrium as a fluorescent probe to indicate hydrophilic/hydrophobic environments in biological samples is also attempted.  37  Chapter 2  Platinum(II) Terpyridine Pacman Complexes with Thiophene-Containing Alkynyl Ligands  2.1  Introduction Functional oligothiophenes have attracted attention for their application in organic light emitting devices (OLEDs), organic photovoltaic cells (OPVs) and in chemical/biological sensors.68 Conformational control of the conjugated backbone is critical to the functionality of these molecules since the effective -conjugation length is determined by torsion along the backbone.69 Stimuli responsive torsion of oligothiophene backbones is interesting as it allows access to variable conjugation lengths in the backbone, thus providing control over electronic and photophysical properties. Photoisomerizable moieties,70 cation binding sites,71 and switchable transition metal centers72 have all been introduced to oligothiophene backbones to alter conformation in different physical or chemical environments. Approaches that utilize supramolecular interactions to achieve this, however, have scarcely been explored, with only a few reported cases to date.73 Differing molecular packing modes as a result of altered oligothiophene backbone conformation have rarely been studied, even though packing can have a profound impact on the performance of bulk optoelectronic molecular materials.74 Recently, square planar alkynylplatinum terpyridine complexes have been explored as tools to encourage reversible aggregation via their  stacking propensities. The aggregate-dependent spectroscopic (NMR, UV-Vis and luminescence) properties of these complexes can be used to 38  report aggregation.75 Attaching alkynylplatinum terpyridine end groups to flexible organic ligands such as meta-/ortho-phenyleneethynylene oligomers, Yam43 and Che42 were able to obtain solvent-dependent and temperature-responsive materials which form folded/helical structures under certain conditions (Scheme 1-8). Other metal groups have been previously used to template the folding of oligoamides76 and other systems.77 Scheme 2-1. Pt(II) terpyridine complexes synthesized in this chapter.  In this Chapter, two foldable Pt(II) terpyridine complexes 2 and 3 using thiophene-containing ligands that tethers two alkynylplatinum terpyridine moieties are reported. Solid state crystal structures of 2 with the coplanar alkynylplatinum terpyridine moieties folded and unfolded, leading to torsion of the flexible terthiophene linker, are obtained. The “Pacman-like” behavior of these complexes is studied in solution by 1H NMR, NOESY NMR, and UV-Vis absorption spectroscopy. 39  2.2 Results and Discussion 2.2.1 Synthesis and Solid-state Structures Scheme 2-2. Synthesis of complexes PtHa2T3-OTf (1) and Pt2a2T3-OTf (2).  Scheme 2-3. Synthesis of complexes Pt2a2TCCT-OTf (3).  The bimetallic and monometallic complexes shown in Scheme 2-2 and Scheme 2-3 were 40  synthesized in good yields from precursor [(tBu3tpy)PtCl](OTf) (4) and terthiophene-based ligand H2a2T3 (5) or H2a2TCCT (6). All reactions were carried out under a N2 atmosphere using standard Schlenk techniques. The synthesis of the ligand 578, 679 and precursor 480 has been previously reported. Mono-metalation of 5 by 4 gave PtHa2T3-OTf (1), and bimetalation of 5 and 6 gave complexes Pt2a2T3-OTf (2) and Pt2a2TCCT-OTf (3). Complexes with perchlorate counter ions were also prepared and showed similar photophysical properties to the complexes with PF6- counter ions.  (a)       (b)    Figure 2-1. (a) Structure of one of the “folded” 2 molecules in the crystal obtained from CH3CN. (b) Structure of “unfolded” 2 obtained from CHCl3 (hydrogen atoms, tert-butyl groups and solvent molecules have been omitted for clarity in both cases, thermal ellipsoids drawn at 50% probability). Two crystal structures of the bimetallic complex 2 were obtained: one “folded” structure from CH3CN (2·3CH3CN, Figure 2-2), and an “open” structure from a chloroform (CHCl3) solution (2·5CHCl3, Figure 2-3). In the structure of the folded complex crystallized from CH3CN, the thiophene rings adopt S-C-C-S torsion angles of 172.35o and 153.70o, slightly twisted from an all-trans conformation of terthiophene. The spatial proximity of the two terminal alkynes allows the two alkynylplatinum terpyridine moieties to be almost parallel to each other with a small 41  dihedral angle of only 1.89o between planes (Figure 2-1a). The bulkiness of the peripheral tert-butyl groups on the pyridine rings keeps the two intramolecular terpyridine planes at interplane distances of 3.62 to 4.06 Å from one another, while the pyridine rings interact more strongly with the closest alkynyl groups (distance = 3.34 and 3.52 Å). The side view of this structure resembles a closed Pacman figure (Figure 2-1a). By contrast, the structure obtained from CHCl3 is similar to an open Pacman (Figure 2-1b). The thiophene rings in the unfolded 2·5CHCl3 structure are further twisted from the all-trans conformation with S-C-C-S torsion angles of 144.82o and 143.76o. The two terpyridine planes in the molecule open up in the unfolded structure, resulting in a dihedral angle of 58.98o. As a result, no intramolecular  interaction is observed in this case, and an open cavity is created between the terpyridine planes (Figure 2-1b).   Figure 2-2. ORTEP structure of 2·3CH3CN. Anions, hydrogen atoms, CH3CN molecules and minor disordered fragments have been removed for clarity. 42   Figure 2-3. ORTEP structure of 2·5CHCl3. Anions, hydrogen atoms, CHCl3 molecules have been omitted for clarity. (a) (b)  Figure 2-4. ORTEP structure showing (a) C-H∙∙∙O and C-H∙∙∙Cl hydrogen bonds in the crystal structure of 2·5CHCl3, and (b) Cl-interactions in the same structure. Interestingly, one of the CHCl3 molecules in the 2·5CHCl3 structure is captured in the open cavity of the Pacman structure and anchored to the cation by the triflate counter ion with C-H∙∙∙O hydrogen bonds (Figure 2-4a). The remaining four solvent molecules form a network of weak C-H∙∙∙Cl hydrogen bonds,81 with two of them connected to the triflate counter-ion by C-H∙∙∙O 43  hydrogen bonds.82 Two different modes of Cl-interactions are present in the structure,83 an edge-on mode with Cl∙∙∙C contacts and a face-on mode with a short Cl to centroid distance (Figure 2-4b).84 Some disorder is found in the 2·3CH3CN structure in both solvent molecules and the tert-butyl groups. By contrast, in the 2·5CHCl3 structure, there are five CHCl3 molecules surrounding each bimetallic cation, and all the solvent molecules, cations and anions are highly ordered (Figure 2-4a). The open Pacman structure appears to be stabilized by these weak intermolecular C-H∙∙∙O hydrogen bonds, C-H∙∙∙Cl hydrogen bonds and Cl- interactions. Together, these weak interactions are sufficient to overwhelm the tendency of the conjugated backbone to maintain coplanarity between thiophene rings and the interactions that keep the structure closed in the 2·3CH3CN crystals. Recently, Cl interactions have been recognized as important in biology,83,85 however here a case is presented where this interaction is shown to be involved in the conformation of a synthetic foldable structure. The two different conformations of the Pacman molecules also result in changes in molecular packing. In the 2·3CH3CN structure (Figure 2-5a, b), identical molecules form chiral helical columns, in which each molecule is rotated 120o about the c-axis with respect to the one above it. Moreover, columns with the same chirality organize into a hexagonal packing arrangement, in which only one of the stereoisomers of the bimetallic complex is found. In the 2·5CHCl3 structure, however, the bimetallic complex forms infinite zigzag chains with racemic molecules (Figure 2-5c, d). This may be driven by intermolecular interactions between head-to-tail stacked alkynylplatinum pyridine moieties (shortest interplane distance ~3.35 Å), which are weaker in the helical structure.   44                        (a)                 (b)                    (c)  (d)      Figure 2-5. (a) Helixes in the structure of 2·3CH3CN. (b) Top view of chiral columns formed by a single stereoisomer of the binuclear complex. (c) Zigzag chains in the structure of 2·5CHCl3. (d) Top view of the chains in 2·5CHCl3. Hydrogen atoms, tert-butyl groups, solvent molecules and counter-ions have been omitted for clarity in all cases.   45  2.2.2 Electronic Absorption and Emission Spectra  300 400 500 600 7000.00.30.60.9500 6000.00.10.20.3Wavelength (nm)AbsorbanceWavelength (nm) Figure 2-6. UV-vis absorption spectra of 1 in CHCl3 with increasing percentage of CH3CN added (from 0 % to 100 %, 5×10-5 M). The inset shows the shift in the low energy MLCT/LLCT absorbance band. The monometallic complex 1 is bright orange in CH3CN solution. The UV-vis absorption spectra in CHCl3 to which an increasing amount of CH3CN is added (Figure 2-6), show an intense absorption feature at 300-350 nm assigned to intraligand (IL) π→π* transitions of the terpyridyl and alkynyl moieties. Features between 350 nm and 450 nm are attributed to π→π* transitions of terthiophene. The low-energy band between 450-600 nm is assigned as a MLCT/LLCT (mixed metal-to-ligand charge-transfer and alkynyl-to-terpyridine ligand-to-ligand charge transfer) transition. This band blue-shifts as increasing amounts of CH3CN are added to the CHCl3 solution. 46  This negative solvatochromism is typically observed in alkynylplatinum (II) terpyridine complexes since the excited state is less polar than the ground state (Figure 1-5).42-43,86   300 400 500 600 7000.00.20.40.6500 600 7000.00.10.2Wavelength (nm)   10%   20%   30%   50%AbsorbanceWavelength (nm)CH3CN in CHCl3 (vol%) Figure 2-7. UV-vis absorption spectra of 2 in CHCl3 with increasing percentage of CH3CN added (from 0 % to 100 %, 2.0×10-5 M). The inset shows the change in the low energy absorbance band with an isosbestic point at ~550 nm.  The absorption spectra of the bimetallic complex 2 show significantly different behavior as a function of solvent composition. With increasing amounts of CH3CN added to CHCl3, the MLCT/LLCT absorption at 510 nm decreases while a new feature grows in at 530 nm, showing a clear isosbestic point (inset, Figure 2-7). Interestingly, this feature still shows subtle negative solvatochromism when the percentage of CH3CN present is greater than 50% (Figure 2-8), indicating that this new feature still maintains its MLCT/LLCT character. Intermolecular aggregation can be ruled out as an explanation for this behavior since the extinction coefficient of 47  this absorbance remains constant from 1.14 to 100 M (Figure 2-9). Similar phenomena have also been reported in other di-platinum complexes with flexible linkages,42-43 and this behavior is consistent with the formation of a folded structure in solution. The absorbance of the π→π* (alkynyl) shoulder at 340 nm, and the π→π* (terthiophene) transition at 380 nm, decreases with the addition of increasing amounts of CH3CN to CHCl3. Both of these changes are not observed in the monometallic complex, and are consistent with conformational changes of the ligand resulting from the formation of the closed Pacman structure in solution.   300 400 500 600 7000.00.20.40.6500 600 7000.0000.0250.0500.075AbsorbanceWavelength (nm)AbsorbanceWavelength (nm)   50%   60%   70%   80%   90% 100% Figure 2-8. Subtle negative solvatochromism of 2 when CH3CN is greater than 50 vol % (2.0×10-5 M).   48  400 500 600 7000.00.51.01.52.02.53.00 20 40 60 80 1000.00.10.20.30.40.50.6Absorbance @ 530 nmconcentration (M)AbsorbanceWavelength (nm)  Figure 2-9. UV-Vis absorbance spectra of 2 in CH3CN, from 1.14 to 100 M. Inset: Plot of the absorbance at 530 nm as a function of concentration. A similar bidentate ligand 6 was employed to make complex 3. UV-vis spectra of 3 in CHCl3 solution with increasing amount of CH3CN content are shown in Figure 2-10. In CHCl3, the MLCT/LLCT low energy absorption of 3 shows a maximum at 470 nm higher in energy than that of 2 (510 nm). Similar to 2, with higher CH3CN content, the ligand based π→π* absorption of 3 between 350-390 nm increases, and a low energy tail grows in while MLCT/LLCT band at 470 nm decreases in intensity with a isosbestic point at 510 nm. These features show that excited states involved in the MLCT absorption processes are higher in energy in 3 compared to 2, and that 3 is also capable of folding/unfolding conformational changes in similar conditions. No emission was observed in solution from 1, 2 or 3, possibly due to energy transfer quenching from low-lying, ligand-based triplet states on the terthiophene containing ligand moiety. Yet 3 is emissive in the solid state where hindered molecular motions lead to less favorable thermal non-radiative 49  processes (spectra shown in Figure 2-11). The two broad emission features observed at 610 nm and 700 nm are unassigned. 400 500 6000.00.10.20.30.4CH3CN in CHCl3 (vol%)AbsorptionWavelength (nm) 100 %   90 %   80 %   60 %   50 %   40 %   30 %   20 %   10 %     0 % Figure 2-10. UV-vis absorption spectra of 3 in CHCl3 with increasing percentage of CH3CN added (from 0 % to 100 %, 1.5×10-5 M). 550 600 650 700 750 80001000020000300004000050000IntensityWavelength (nm)Emission of Solid 3 EX@420 nm EX@400 nm Figure 2-11. Solid state emission spectra of 3 on glass slides (background subtracted). 50  2.2.3 NMR Spectra 1H NMR experiments were carried out to further investigate the molecular behavior in solution. The pyridine protons on the bimetallic complex appear at a lower chemical shift compared to those of the monometallic complex. They are also shifted upfield in CD3CN relative to the signals in CDCl3 (Figure 2-12). This is typically observed in systems with π-π interactions.43,87 Furthermore, variable temperature NMR experiments with the bimetallic complex showed no significant shift or broadening in CDCl3 (Figure 2-13). In CD3CN, however, the pyridine protons are broadened and shifted upfield at lower temperatures, while the thiophene proton signals are only slightly altered (Figure 2-14). These changes are consistent with restricted rotation of the alkynylplatinum terpyridine moieties and stronger π-π stacking.  Figure 2-12. Partial 1H-NMR spectra (aromatic region) of 1 in (a) CDCl3 and (b) CD3CN, 2 in (c) CDCl3 and (d) CD3CN. Pyridine protons are labeled. 51  VT CDCL3 45 25 15 05 -05 -15 -25.ESP9.0 8.5 8.0 7.5 7.0Chemical Shift (ppm)2 in CDCl345 oC25 oC15 oC5 oC- 5 oC- 15 oC- 25 oC Figure 2-13. Variable temperature 1H NMR spectra of complex 2 in CDCl3. VT ACN 65 45 25 15 5 -5 -15 -25.ESP9.0 8.5 8.0 7.5 7.0Chemical Shift (ppm)65 oC45 oC25 oC15 oC5 oC- 5 oC- 15 oC- 25 oC 2 in CD3CN Figure 2-14. Variable temperature 1H NMR spectra of complex 2 in CD3CN. NOESY experiments of 2 (mixing time = 0.8 s) show cross peaks between the tert-butyl protons and the thiophene protons in CD3CN (Figure 2-15), indicating spatial proximity of these protons, that is absent in CDCl3 (Figure 2-16). An intermolecular origin of these cross peaks can be excluded since they are absent in 1 (Figure 2-17). These results are consistent with the unfolding of the molecule in CDCl3 and folding in CD3CN, analogous to the conformations observed in the solid state structures obtained from the respective solvents. 52                                                 Figure 2-15. Partial NOESY spectrum of 2 in CD3CN.                                                Figure 2-16. Partial NOESY spectrum of 2 in CDCl3.                                                       Figure 2-17. Partial NOESY spectrum of 1 in CD3CN. 53  2.2.4 Density Functional Theory (DFT) Simulations Density functional theory (DFT) calculations were carried out to find the optimized geometry of folded and unfolded cations of molecule 2 and 3 in solutions with PCM88 solvation model without interaction with their triflate anions.         Figure 2-18. Optimized structure of folded and unfolded 2 in CH3CN and CHCl3.      Figure 2-19. Optimized structure of folded and unfolded 3 in CH3CN and CHCl3. 54  The geometry optimized structures of 2 (Figure 2-18) show angles of 9.6° and 57.1° between two platinum terpyridine moieties in CH3CN and CHCl3 respectively, in line with 1.9° for the folded structure and 59.0° for the unfolded structure in the solid state. Similarly, by DFT calculation, 3 (Figure 2-19) shows angles of 7.5° and 86.5° between two platinum terpyridine moieties in CH3CN and CHCl3 respectively. It is intriguing that with only difference in dielectric constant, we are already seeing folding and unfolding of the cationic structure. Even though the Pt-Pt distance in folded structures of 2 and 3 are too long (4.32 Å and 3.97 Å, respectively) for effective Pt-Pt bonds, the two Pt atoms still interact with each other through dz2(Pt) orbitals. As depicted in Figure 2-20 and detailed in Table 2-1 and Table 2-2, the two dz2(Pt) orbitals still interact in folded 2 and 3 structures to give a bonding MO and antibonding MO. By contrast in unfolded 2 and 3, the MOs resulting from two dz2(Pt) orbitals are mostly degenerate (with very small energy differences of 0.008 and 0.005 eV, respectively) with no effect on Pt-Pt bonding. The intramolecular interaction between two dz2(Pt) orbitals can be evaluated by the splitting (energy difference) of the resulting bonding and antibonding MOs (Figure 2-20), which is smaller in folded 2 (0.373 eV) than that in folded 3 (0.601 eV). However, both bonding and antibonding orbitals are lower in energy than HOMO thus populated with a pair of electrons, so the overall effect is still non-bonding. MMLCT absorption is not the lowest energy transition as in Che’s and Yam’s systems (Scheme 1-8, page 23) and depicted in Figure 1-12 (page 21) since the Pt-Pt anti-bonding MO is still lower in energy compared to Pt +  MOs.  55   Figure 2-20. Interactions of dz2(Pt) orbitals in 2 (up) and 3 (down). 56   Figure 2-21. Frontier MOs of geometry optimized 3 in CHCl3 and CH3CN. The frontier MOs of 3 are shown in Figure 2-21, and detailed in Table 2-3 and Table 2-4. The lowest energy absorptions of 3 are attributed to MLCT/LLCT transitions, since HOMO and HOMO-1 orbitals are localized mostly on the conjugated bisacetylide ligands and partially on the metal centers while the LUMO and LUMO-1 orbitals are mostly on the Pt(II) polypyridyl moieties. The MLCT/LLCT absorption is lower in energy in the folded structures of 3 due to different degrees of energy splitting in HOMO and LUMO.  57  2.3 Experimental 2.3.1 General 1H NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer. All syntheses were carried out under a dry N2 atmosphere using standard Schlenk techniques. Dry toluene and tetrahydrofuran (THF) were obtained after treatment with solvent purification system. All other chemicals were purchased from commercial sources, and used without further purification. 1H NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer, and chemical shifts are referenced to the residual protio-solvent signals. MALDI-TOF mass spectra were measured using a Bruker Autoflex MALDI-TOF mass spectrometer. UV-Vis absorption spectra were obtained on a Varian Cary 5000 UV-Vis-NIR spectrophotometer. Excitation and emission spectra were obtained on a Photon Technology International (PTI) QuantaMaster Fluorimeter and were uncorrected for lamp intensity.  2.3.2 Synthesis Copper(I) iodide (purum, ≥ 99.5%), diisopropylamine (≥ 99.5%), 4,4’,4’’-tri-tert-butyl-2,2’:6’,2’’-terpyridine (tbtpy, 95%) and lithium trifluoromethanesulfonate (96 %) were purchased from Sigma-Aldrich and potassium tetrachloroplatinate(II) (K2PtCl4, 99.9 %) were purchased from Strem Chemicals, Inc. All chemicals were used without purification. [(tBu3tpy)PtCl](OTf) (4)42 and ligand 589 and 6 were synthesized according to literature.   58   Pt2a2T3-OTf (2): A mixture of CH2Cl2 (50 mL) and diisopropylamine (15 mL) was sparged with nitrogen gas for 15 minutes, then added to a N2 filled reaction vessel containing H2a2T3 (5) (60 mg, 0.20 mmol) and 4 (312 mg, 0.40 mmol). The vessel was quickly sealed after introduction of CuI (1 mg), and the reaction mixture was stirred overnight at room temperature. The solvent was then removed under vacuum and the crude product purified by column chromatography on silica gel using a CH2Cl2-acetone mixture (5:1 v/v) as eluent, giving a dark red powder (231 mg, 65 %). 1H NMR (400 MHz, CD3CN): 9.00 (d with Pt satellites, 3J = 6.0 Hz, 4H, terpyridyl CH), 7.93 (d, 4J = 2.0 Hz, 4H, terpyridyl CH), 7.88 (s, 4H, terpyridyl CH), 7.50 (dd, 3J = 6.0 Hz, 4J = 2.0 Hz, 4H, terpyridyl CH), 7.35 (s, 2H, thienyl CH), 7.31 (d, 3J = 6.0 Hz, 2H, thienyl CH), 7.06 (d, 3J = 6.0 Hz, 2H, thienyl CH), 1.44 (s, 18H, CH3), 1.41 (s, 36H, CH3). Positive MALDI-TOF: m/z = 743.7 [M-2OTf]2+, 1487.4 [M-2OTf]+ and 1636.4 [M-OTf]+. For experimental and calculated isotopic pattern analysis of [M-2OTf]2+ see Figure A-1 and Figure A-2. Elemental analysis calc. for C72H76F6N6O6Pt2S5: C, 48.42; H, 4.29; N, 4.71. found: C, 48.02; H, 4.61; N, 4.36. PtHa2T3-OTf (1): The same procedure used for 2 was followed except 5 (148 mg, 0.50 mmol) and 4 (196 mg, 0.25 mmol) were used. The crude product was purified by silica chromatography using a CH2Cl2-acetone mixture (10:1 v/v) as eluent, giving a red powder (130 mg, 51 %). 1H-NMR (400 MHz, CD3CN): 9.10 (d with Pt satellites, 3J = 6.0 Hz, 2H, terpyridyl CH), 8.34 (s, 2H, terpyridyl CH), 8.26 (d, 3J = 2.0 Hz, 2H, terpyridyl CH), 7.57 (dd, 3J = 6.0 Hz, 4J = 2.0 Hz, 2H, terpyridyl CH), 7.50 (d, 3J = 6.0 Hz, 1H, thienyl CH), 7.36 (d, 3J = 6.0 Hz, 1H, thienyl CH), 7.27 (d, 3J = 6.0 Hz, 1H, thienyl CH), 7.18 (d, 3J = 6.0 Hz, 1H, thienyl CH), 7.08 (s, 1H, thienyl CH), 6.96 (d, 3J = 6.0 Hz, 1H, thienyl CH), 3.59 (s, 1H, alkynyl CH), 1.56 (s, 9H, 59  CH3), 1.40 (s, 18H, CH3). Positive MALDI-TOF: m/z = 445.6 [M-OTf]+. Elemental analysis calc. for C44H42F3N3O3PtS4: C, 50.76; H, 4.07; N, 4.04. found: C, 51.06; H, 4.24; N, 3.87. Pt2a2TCCT-OTf (3): The same procedure used for 2 was followed except that ligand H2a2TCCT (6) (48 mg, 0.20 mmol) and 4 (312 mg, 0.40 mmol) were used. The crude product was purified by column chromatography on silica gel using a CH2Cl2-acetone mixture (5:1 v/v) as eluent, giving a red powder (207 mg, 60 %). 1H-NMR (400 MHz, CD3CN): 9.00 (d with Pt satellites, 3J = 5.9 Hz, 4H, terpyridyl CH), 7.93 (d, 4J = 1.6 Hz, 4H, terpyridyl CH), 7.88 (s, 4H, terpyridyl CH), 7.46 (d, 3J = 5.2 Hz, 2H, thienyl CH), 7.50 (d, 3J = 4.7 Hz, 4H, terpyridyl CH), 6.99 (d, 3J = 5.2 Hz, 2H, thienyl CH), 1.42 (s, 18H, CH3), 1.41 (s, 36H, CH3). Positive MALDI-TOF [M-OTf]+ : m/z = 1575.4190 calc. for C68H74N6O3F3S3194Pt2, found: 1575.4213.   2.3.3 DFT Simulations Calculations were carried out using the Gaussian 09 Rev.D01 suite of Programs.90 The PBE0 hybrid functional91 with 6-31G* basis set (for C, H, S and P atoms) and the LANL2DZ effective-core pseudopotential92 (for Pt) was employed to simulate ground state S0 structure of 2 and 3. Structural coordinates from X-ray crystallography have been used as starting points of geometry optimizations for 2. Optimized structures were confirmed to be the minimum on the potential energy surface by vibrational frequency calculations. Vertical absorptions are simulated using time-dependent density functional theory (TD-DFT) by looking at the first 20 low-energy electronic transitions. For all simulations, solvation effect of CHCl3 (ε = 4.7113) or CH3CN (ε = 35.688) was implemented using the polarizable continuum model88 (PCM) on Gaussian 09. 60   Table 2-1. Molecular orbitals and relative energies of complex 2 involving dz2(Pt) orbitals.   Molecular Orbital Relative Energy* (eV) 2 in CH3CN (folded) HOMO-6 Anti-bonding   0.84 HOMO-10 Bonding   0.47 2 in CHCl3 (unfolded)  HOMO-9 Pt(1) dz2  0.01 HOMO-10 Pt(2) dz2  0*      * Energies are referenced to the MO with the lowest energy in the table.   61   Table 2-2. Molecular orbitals and relative energies of complex 3 involving dz2(Pt) orbitals.   Molecular Orbital Relative Energy* (eV) 3 in CH3CN (folded) HOMO-5 Anti-bonding   0.98 HOMO-10 (Bonding)   0.38 3 in CHCl3 (unfolded)  HOMO-8 Pt(1) dz2  0.01 HOMO-9 Pt(2) dz2  0      * Energies are referenced to the MO with the lowest energy in the table.  62  Table 2-3. Molecular orbitals and energies of complex 3 orbitals in CHCl3. MOs eV Top-view Side-view LUMO+1 (297) -3.58   LUMO (296) -3.59   HOMO (295) -6.60   HOMO-1 (294) -6.62   TD-DFT simulated absorptions: Band A: 2.38 eV, 521 nm, oscillator strength f=0.2176.      HOMO-1 → LUMO (45 %)            HOMO → LUMO+1 (51 %) Band B: 2.41 eV, 514 nm, oscillator strength f=0.1407.    HOMO-1 → LUMO+1 (33 %)       HOMO → LUMO (63 %) 63  Table 2-4. Molecular orbitals and energies of complex 3 orbitals in CH3CN. MOs eV Top-view Side-view LUMO+1 (297) -2.90   LUMO (296) -3.00   HOMO (295) -5.65   HOMO-1 (294) -6.32   TD-DFT simulated absorptions: Band A: 2.05 eV, 606 nm, oscillator strength f=0.0183.      HOMO → LUMO  Band B: 2.19 eV, 565 nm, oscillator strength f=0.0801.        HOMO → LUMO+1  64  2.4 Conclusions In conclusion, a novel terthiophene based alkynylplatinum terpyridine Pacman complex 2 has been made, which exhibits reversible folding in both solution and solid state. Analogue 3 was synthesized from ligand 6 showing higher MLCT absorption energies, and also shows reversible folding in solution. In the cases of both 2 and 3, reversible torsion of the flexible thiophene-containing backbone in response to changes in the solvent environment have been successfully achieved via introduction of alkynylplatinum terpyridine moieties.  It is also revealed by DFT simulations that the two Pt atoms in folded 2 and 3 are able to interact even though there is no net bonding between them. Finally, as revealed by solid state structures in 2, Cl- interactions, along with weak C-H hydrogen bonding play a role in the folding/unfolding behavior of this synthetic foldable structure, one of the first observations of this behavior in a non-protein structure. 65  Chapter 3 Dual Emissive Platinum(II) Metallacycles  3.1 Introduction Heavy atom-induced spin-orbit coupling (SOC) has been widely used to encourage intersystem crossing (ISC) in organic molecules from singlet exited-states to triplet manifolds (Figure 3-1). For instance, iridium containing dyes allows highly efficient ISC and emission from both the triplet and singlet excited states, resulting in up to 100% internal efficiency in organic light emitting devices (OLEDs) using these materials. This is significantly higher than the 25%  efficiency limit of most fluorescent compounds that only emit from their singlet excited states.93    Figure 3-1. Energy diagrams of electronic transitions involving GS and excited states (kISC, kNR, kFL and kPH are rates of ISC, NR decay, fluorescence and phosphorescence, respectively). According to the selection rules, any electronic transitions that change the total spin quantum number are “spin forbidden” due to conservation of the spin angular momentum of electrons (with 66  operator ?̂?). This is why ISC is very slow compared to “spin-allowed” transitions in systems containing only atoms in the first two rows on the periodic table. In heavy atoms, the interaction between the magnetic moment of the electron spin and orbiting nuclei (in the reference frame of the electron) is no longer negligible, the spin will couple with the orbital angular momentum of the electron (with operator ?̂?). The coupling strength can be defined using Hamiltonian ?̂?𝑆𝑂 in Equation 3-1, where 𝜁 represent the SOC constant for atoms. The value 𝜉 is found to rapidly increase with higher atomic numbers of the element (𝜉 ∝ 𝑍4).94 ?̂?𝑆𝑂 = ℎ𝑐𝜉?̂? ∙ ?̂?  …………………… (Equation 3.1) For instance, while 𝜁 for Na is only 0.1 kcal/mol (0.004 eV), 𝜁5𝑑 of Pt(II) in the 5d10 configuration is 4129 cm-1 (0.51 eV),95 comparable with the energy gaps between electronic states. As the result of stronger SOC, the “spin flip” of the electron on heavier atoms becomes more favourable in heavy atoms.  In molecules, SOC leads to the mixing of singlet and triplet wave functions and the coupling coefficient 𝜆 is proportional to the spin-orbit coupling energy 𝐸𝑠𝑜 in Equation 3.2. 𝜆 = 𝐸𝑠𝑜/(𝐸𝑆 − 𝐸𝑇)  …………………… (Equation 3.2) It is reported that ISC is much faster when corresponding frontier MOs of singlet and triplet states have a stronger contribution from heavy atoms since there is more involvement of atomic orbitals with higher angular momentum.24,96 Therefore, in molecules with heavy metal centers such as platinum(II), it is possible to attenuate ISC by variating the relative contribution of the metal atom in related MOs, or by tuning the singlet-triplet (𝐸𝑆 − 𝐸𝑇) energy gap (Equation 3.2), so that S1 and T1 states can both be populated upon excitation of the molecules. This also allows control 67  over fluorescence/phosphorescence (FL/PH) ratios which is potentially useful for various important applications such as white-emitting OLEDs97, ratiometric oxygen sensors98 and hypoxia imaging in biological systems.99 It is worth mentioning that limiting the non-radiative decay rates (kNR) in Figure 3-1 is also crucial for these applications since such decay usually result in degraded performance and device overheating. Scheme 3-1. Pt(II) bisacetylides 11,99 12,100 13101, 14102 and 15.102  Oligomers and polymers containing trans-substituted Pt(II) bisacetylide complexes (Scheme 3-1) have broad applications in non-linear optics materials, OLEDs, polymer solar cells and other molecular electronics applications,100 and have been extensively studied. They typically exhibit high triplet yields and long-lived ligand-localized triplet excited states.24,103 The strong 68  donating ligands of trans-substituted Pt(II) bisacetylides reduces the non-radiative decay from excited metal 5d states by elevating their energy. However, they still suffer from insufficient photo-stability and relatively low emission quantum efficiency for many applications due to relatively weak Pt-C bonds and thermal deactivation through rotations and vibrations in the molecule. Metallacycles containing cis-Pt bisacetylide metal centers were first reported almost two decades ago,101-102 and were also found to give rise to very efficient ISC, thus have been considered good candidates as triplet sensitizers, and for non-linear optical applications.104 Castellano et al. showed that metallacycles show enhanced light absorption compared to their non-cyclic analogues.104b,104c Additionally, the more rigid cyclic structure gives rise to suppressed thermal deactivation and thus higher emission quantum efficiency. Therefore, cis-Pt bisacetylide metallacycles with higher stability and room temperature tunable florescence and phosphorescence dual emission would be interesting for the applications discussed above, and remain unknown to date.  Scheme 3-2. Polymers and oligomers containing [trans-Pt(PR3)2(‒C≡C‒Ar‒C≡C‒)] units.  69  It is reported in molecules containing [trans-Pt(PR3)2(‒C≡C‒Ar‒C≡C‒)] units (Scheme 3-2, left), replacing the phenylene aryl groups in 11 with a thiophene rings results in lower T1 states in molecule 16 with relatively unchanged energy gap between S1 and T1 states (term 𝐸𝑆 − 𝐸𝑇 in Equation 3-2). A similar effect was observed both in monomeric and polymeric 11 and 16, except the 𝐸𝑆 − 𝐸𝑇 gap is smaller in polymers.24 Furthermore, when polymeric 11 was randomly blended with small percentage of repeating unit of 16, the resulting polymer shows preferential formation of triplet excited states on the thiophene containing segments due to localized lower T1 states, known as “triplet traps”.105 In polymer 17, 18 and 19, the T1 excited state is lowered even more when the ligand is extended in length from thiophene, bithiophene to terthiophene (Scheme 3-2, right), while the energy gap between S1 and T1 (term 𝐸𝑆 − 𝐸𝑇 in Equation 3-2) remains relatively constant.106  From 17 to 19, the increasing number of thiophene rings reduces the influence of the heavy metal center which is mainly responsible for the intersystem crossing, making the T1 state less accessible. Additionally, a larger conjugated system gives rise to a lower energy T1 state which makes ISC less efficient from higher singlet excited states (Sn). Therefore, S1 states are relatively more efficiently populated than T1 via fast IC processes resulting in stronger florescence.106 These observations provide guidance to use similar strategies in phosphorescent platinum metallacycles to encourage stronger fluorescence, allowing room temperature dual fluorescence (FL) and phosphorescence (P) to be simultaneously observed with various FL/PH ratios.  In this chapter, three new cis-Pt(II) bisacetylides metallacycles 4, 5 and 6 are discussed, and compared with complex 7104c previously reported by Castellano et al (Scheme 3-3). Complex 7 provides a good starting point for these studies since it only shows green ligand-based 70  phosphorescence at 497 nm in deaerated solutions. The effect of introducing thiophene rings at different positions in the ligand system while maintaining the cyclic structure is systematically explored, first by replacing the two phenyl rings in 7 with thiophene rings to give complex 4. Similarly, by introducing one additional thiophene ring to 4 metallacycle 5 is formed. Finally, by changing the bridging acetylene in 7 to a thiophene ring 6 is obtained. Complex 5 is found to fluoresce while 6 and 7 show dual emission with different FL/PH ratios. Scheme 3-3. Pt(II) bisacetylide metallacycles.  3.2 Result and Discussion 3.2.1 Synthesis  Metallacycles 4, 5 and 6 were synthesized using a modification of Hagihara’s method107 using TMS-protected bidentate pro-ligands 8, 9 and 10 respectively, as shown in Scheme 3-4. Alkynes were deprotected in situ and reacted with Pt(dppp)Cl2 to incorporate the metal center with phosphine ancillary ligands. Pro-ligand 9 was previously reported by our group,89 and 8 and 10 71  were conveniently synthesized via Sonogashira reactions between trimethylsilylacetylene and the corresponding dibromo precursors in reasonable yields (see Scheme 3-5, Page 87 for details). Scheme 3-4. Synthesis of metallacycles 8-10.   3.2.2 Solid-State Structures (a)     (b)  Figure 3-2. ORTEP presentations of metallacycle (a) 4 and (b) 5 showing CH2Cl2 solvent molecules hydrogen-bonding with each metallacycle in different ways. Ellipsoids at the 50% probability level, hydrogens on metallacycles omitted for clarity. 72  (a)      (b)    Figure 3-3. (a) ORTEP presentations two different crystallographic structures of 6 connected by hydrogen bonding interactions. (b) Top-view of one of the structures. Ellipsoids at the 50% probability level, hydrogens on metallacycles omitted for clarity. Crystal structures of 4, 5 and 6 (Figure 3-2 and Figure 3-3) were obtained. Important bond lengths and angles are summarized in Table 3-9 and Table 3-10 on page 92. Complex 6 has two crystallographically distinct structures (A and B) in the unit cell with different ligand conformations as shown in Figure 3-3, with multiple hydrogen bonds between them. The metal centers in both structures A and B have almost identical C-Pt-C bond angles, unaffected by conformational differences in the relatively remote bisacetylide ligands. All the metallacycles have square-planar geometries at the platinum centers, in line with other similar cis-platinum bisacetylides in the literature.101-102,108 Compared with 5 and 6, 4 has a smaller C-Pt-C bond angle of 84.8 °, which indicates some degree of ring strain in this structure. While the bisacetylide ligands in 4 and 5 are only slightly twisted (S1-C4-C7-S2 torsion angle of 14.8 ° in 4, dihedral angles between adjacent rings of 9.9 ° and 4.9 ° in 5), the central thiophene ring in 6 is significantly out of the plane (dihedral angles between adjacent rings of 35.2 ° and 30.6 ° in structure A; 26.0 °and 25.5 ° in structure B). Interestingly, crystals of 4, 5 and 6 all have CH2Cl2 solvent molecules in the crystal lattice. The CH2Cl2 molecules form hydrogen bonds with only one side of the ligand in 4, 73  while in 5 they bound to both sides of the ligand as well as with the sulfur atom on the central thiophene ring (with H44A). The CH2Cl2 molecules interact with 6 in a similar fashion to how they do with 5, however, two protons are involved (Figure 3-2).   3.2.3 Electronic Absorption and Emission Spectra            Figure 3-4. (a) UV-Vis spectra of ligands 8, 9 and 10 in CH2Cl2 (1.0×10-5 M) and excitation and emission spectra of 8 (b), 9 (c) and 10 (d) in CH2Cl2 (1.0×10-5 M). The absorption spectra of 8, 9 and 10 are shown in Figure 3-4 (a). The lowest energy absorption maxima of 8 and 10 are at 380 nm and 343 nm, respectively, in CH2Cl2. 9 absorbs at 74  higher wavelength into the visible region showing a maximum at 405 nm and a shoulder at ~430 nm. The resulting yellow color is attributed to the relatively smaller HOMO-LUMO gap of the * transition on the more conjugated terthiophene moiety. The excitation spectra of 8, 9 and 10 are shown in Figure 3-4 (b, c, and d). All the emission spectra show some vibrational fine structure. Emission maxima are at 392 nm, 475 nm and 412 nm for 8, 9 and 10, respectively. Complex 8 has the smallest Stokes shift indicating a smaller degree of geometrical relaxation in the excited state.                 Figure 3-5. UV-Vis spectra of 4 (a), 5 (b) and 6 (c) (1.0×10-5 M in CH2Cl2). 75  The absorption spectra of metallacycles 4, 5 and 6 in CH2Cl2 are shown in Figure 3-5. All the complexes absorb strongly below wavelength of 350 nm, attributed to high energy localized * transitions of the acetylene moieties. Complex 4 shows a lowest-energy peak at 398 nm, similar to the absorption features of 8, however red shifted approximately 20 nm. The low-energy absorption shoulder of 5 is shifted to 500 nm, from 430 nm in 9, and 6 shows only a very weak feature tailing to 460 nm without a clear maximum indicating a low oscillator strength between S0 and S1 states. In all cases, the metallacycle absorption is somewhat red-shifted from the absorption of the proligands. This can be rationalized by restricted rotation and planarization of the structure upon formation of a bidentate chelate, and the resulting more extended-conjugation narrowing the HOMO-LUMO gap.  Figure 3-6. Excitation spectrum of 4 and its emission spectra under different conditions. The excitation and emission spectra of metallacycle 4 in CH2Cl2 are shown in Figure 3-6. Compared with the purely phosphorescent analogue 7 in the literature, which emits at 497 nm, metallacycle 4 with two thiophene rings in the ligand instead of two o-phenylene groups shows 76  significantly red-shifted phosphorescence at 610 nm and additionally a broad weak band between 400 nm and 550 nm. This relatively higher energy emission is considered to be fluorescence as assessed by its sensitivity to oxygen. According to Figure 3-1, this is suggestive of kISC to T1 being on the same time-scale as kFL, slow enough so that both S1 and T1 states are effectively populated and emitting.     Figure 3-7. (a) Variable temperature fluorescence spectra of 5 in a MeOH/EtOH medium, and (b) excitation and emission spectra of metallacycle 5 in air-equilibrated solution. No phosphorescence was observed under argon (1.0 × 10-5 M in CH2Cl2). By contrast, metallacycle 5 with three thiophene rings shows no phosphorescence up to 800 nm (even at lower temperatures, Figure 3-7a) but instead bright yellow fluorescence is observed in solution (Figure 3-7b). Some terthiophene-containing structures are known to have low-lying non-emissive triplet states. The non-radiative decay rate of triplet excited states is known to increase exponentially as the triplet energy (T1-S0 gap) decreases while non-radiative decay of S1 is relatively energetically insensitive.1,103a,109 Therefore, the absence of phosphorescence of 5 in the visible region may be explained by a terthiophene-centered T1 excited state that is very low in 77  energy and decays rapidly through non-radiative pathways or near-IR emissions. Metallacycle 6 was designed by introducing only one thiophene ring by replacing the central acetylene bridge in complex 7, with anticipation to lower the ligand based triplet state and allow both phosphorescence and fluorescence. Complex 6 can also be considered as an analogue of 5, but with two thiophene rings replacing the phenyl rings. This change indeed gives rise to dual emission with a higher FL/PH ratio close to 1 (Figure 3-8). Interestingly, phosphorescence of 6 is stronger with shorter excitation wavelengths compared to fluorescence. After ruling out the possibility of contamination by trace amounts of emissive residue ligand, it is considered possible that higher singlet excited states (Sn states in Figure 3-1) of 6 may have stronger coupling with the T1 state. Therefore, when excited with higher-energy photons, these states are populated giving rise to faster ISC to T1 thus stronger phosphorescence. Similar effects are known in some intrachain platinum conjugated polymers110.     Figure 3-8. (a) Emission spectra of 6 under argon show two bands (500 nm and 620 nm) while excitation spectra of the two bands diverge at higher energy. (b) Emission of 6 in air, and dual emission under argon. 78  Emission of both 4 and 6 are sensitive to oxygen to some extent, yet still remain slightly phosphorescent in air-equilibrated solutions (Figure 3-6 and Figure 3-8b). The phosphorescence can only be completely quenched under 100% O2, which is beneficial for high concentration ratiometric O2 sensing using unaffected fluorescent intensity as a reference. Moreover, 4 and 6 have similar T1-S0 energy gaps in that they phosphoresce at similar wavelengths. However, 6 fluoresces at a higher wavelength than 4 at 490 nm indicating a lower-lying first single excited state (S1) state in 6. This is consistent with the computed energy diagrams in Figure 3-9, showing that the higher FL/PH ratio in 6 than 4 can be achieved without a significant change in T1 energies relative to S0.  Table 3-1. Electronic absorption and emission of metallacycles in CH2Cl2 solution. Complex Absorption Fluorescence Phosphorescence abs (nm) em (nm) em em (ns) em (nm) em em (s) 4 398 405 0.011 0.70 610 0.053 2.23 5 500 536 0.15 0.45 - - - 6 385 490 0.0062 0.61 620 0.0054 1.97  Electronic absorption and emission data including emission lifetimes (em) and quantum yields (em) of the three metallacycles are summarized in Table 3-1. All complexes show short fluorescence life time of less than 1 ns, while 4 and 6 show relatively long phosphorescent lifetimes ~ 2 s, indicating that kPH is 3 orders of magnitude slower than kFL. The fluorescence quantum efficiency of 5 is moderate while 4 shows much weaker dual emission with a FL/PH ratio around 0.2. The FL/PH ratio of 6 is approximately 0.9 higher than that of 4, however, the overall emission 79  quantum efficiency is barely 1 percent.  By comparing 4 with 7, and 5 with 6 (Scheme 3-3), it is noticeable that replacing phenyl groups with thiophene rings in metallacycles can induce or enhance fluorescence without adding extra conjugation. The difference between 4 and 5 demonstrates that adding conjugation length to the ligand leads to drastically increased fluorescence, however, the phosphorescence is diminished in the visible region. It should be pointed out that although complex 4 and 6 show many similarities, they are not directly comparable in an empirical way in which only one type of group (C-C triple bond, benzene or thiophene ring) is replaced with another.  3.2.4 DFT and TD-DFT Simulations DFT and TD-DFT calculations were carried out to help understand the photophysics of the metallacycles and simulate their ground and excited state properties. Optimized ground-state (S0) geometries of 4, 5 and 6 are shown in Table 3-2 (left column). All structures show no symmetry (C1) despite their symmetric bisacetylide ligands. Complexes 4 and 5 show almost completely planar ligand geometries, however, the central thiophene ring on 6 still lies out of the plane by 44.5 °.  DFT calculated geometry optimization of the lowest triplet state (T1) of the metallacycles in CH2Cl2 solutions are shown in Table 3-2 (right column). TD-DFT calculated optimized geometries of the first singlet excited state (S1) of metallacycles in CH2Cl2 solutions are presented in Table 3-2 (central column). All three metallacycles exhibit highly planar T1 and S1 excited states structures. 80   Table 3-2. Calculated geometry of 4, 5 and 6 at S0, S1 and T1 electronic states. Compound S0 S1 T1 4    5    6     Frontier orbitals of the ground-state metallacycles are presented in Tables 3-3. All three complexes have character localized on the ligand in the HOMO and character in the LUMOs. The HOMO and LUMO of 4 are uniformly distributed with some contributions from 5dxz and 5dyz orbitals on platinum, yet mostly comprised of  or * orbitals from the bisacetylide ligand. In metallacycles 5 and 6, however, the LUMOs are strongly metal-centered with contributions from empty 6pz orbitals on platinum while the HOMOs are delocalized. This is indicative of metal-to-ligand charge transfer (MLCT) states in 5 and 6, which have previously been observed in conjugated Pt polymers with electron-rich ligands.110-111  81  Table 3-3. Frontier orbitals and their energies of ground state metallacycles. * According to TD-DFT calculations, only the HOMO and LUMO are involved in the lowest energy S0→S1 electronic transitions for all three complexes Compound MOs E (eV) Top-down Side View 4 HOMO (178) -5.29    LUMO (179) -1.53   5 HOMO (193) -4.93    LUMO (194) -1.68   6 HOMO (191) -5.61    LUMO (192) -1.55   82  Table 3-4. Bond lengths (Å) of 4, 5 and 6 in the ligand at different electronic states.  Selected bond lengths in the optimized geometries of 4, 5 and 6 in different electronic states are presented in Table 3-4. In the ground state, all metallacycles show bond averaging between single and double bonds on the innermost ring. In 4 and 5, the thiophene double bond is notably longer (1.40 Å) on the inside circle compared to the ones outside (1.36 Å). These observations along with a down field shift of the proton signal on the central thiophene ring of 5 ( = 6.72 ppm, while  = 7.06 ppm in the proligand 9, both in CDCl3), indicate the presence of “ring currents” on the metallacycle.   S0 S1 T1 4    5    6    83  At the S1 and T1 excited states, however, all metallacycles have almost perfectly planar ligand structures. This indicates that 6 undergoes significant planarization going from GS to S1 or T1 excited states, resulting in a poor overlap between GS and excited state MOs. It’s also consistent with its very weak low energy absorption features, and may also lead to very low fluorescence and phosphorescence quantum efficiencies of 6 since NR processes with low energy barrier becomes more favorable. Additionally, in Table 3-4, all metallacycles in the excited state specially in T1, show “quinoid-like” structure with reversed bond length of single and double bonds in the GS geometry. It’s also clear that the geometry of metallacycles have much more in common between the excited states S1 and T1 than between GS and T1/S1.  Relative energies of metallacycles 4, 5 and 6 in T1 and S0 electronic states with T1/S0 geometries are summarized in Table 3-5. Their relative energies in S1 and S0 electronic states with S1/S0 geometries are summarized in Table 3-6. These energies are used to construct energy diagrams of electronic states of metallacycles 4, 5 and 6 in Figure 3-9. Table 3-5. Relative energies of optimized S0 and T1 states. Complex Electronic State S0 Geometry T1 Geometry Relative Energy (eV) Relative Energy (eV) 4 S0 0.00 0.34  T1 2.25 1.89 5 S0 0.00 0.37  T1 1.92 1.54 6 S0 0.00 0.45  T1 2.80 2.06   84  Table 3-6. Relative energies of optimized S0 and S1 states. Complex Electronic State S0 Geometry S1 Geometry Relative Energy (eV) Relative Energy (eV) 4 S0 0.00 0.20  S1 3.10 2.90 5 S0 0.00 0.29  S1 2.44 2.23 6 S0 0.00 0.35  S1 3.22 2.63             Figure 3-9. Summarized energy diagrams and electronic transitions of metallacycles 4, 5 and 6. Energies shown in the above diagrams are in eV (Ev = vertical transition energy, E00 = 0-0 transition energy). Metallacycle 5 has the lowest calculated T1 state. The T1-S0 transition energies of 1.54 eV 85  (0-0) and 1.17 eV (vertical) corresponds to phosphorescence in the near-IR region (808 nm and 1060 nm), where fast thermal-deactivation may render the T1 state non-emissive at room temperature. Similarly, metallacycle 4 and 6 are calculated to exhibit phosphorescence with 0-0 transition energies of 1.89 eV and 2.06 eV (Table 3-5), respectively, close to the experimental values at maximum intensity (2.02 eV and 1.99 eV).  Table 3-7. Vertical excitation energies of lowest singlet and triplet excited states. Metallacycle Electronic State Major transition Vertical Transition Energy (eV) oscillator strength 4 S1 HOMO-LUMO 3.10 0.169  T1 HOMO-LUMO 2.25 - 5 S1 HOMO-LUMO 2.44 0.100  T1 HOMO-LUMO 1.92 - 6 S1 HOMO-LUMO 3.22 0.001  T1 HOMO-LUMO 2.80 -   Table 3-8. Calculated absorption and emission properties.  Absorption wavelength (nm) Emission wavelength (nm) Metallacycles Vertical S0 → S1 Vertical S1 → S0 0-0 S1 →S0 0-0 T1 →S0 4 400 (398)a 459 428 (410) 655 (610) 5 508 (500) 639  557 (536) - 6 385 (no maxima) 544 471(485) 602 (620) 7b 329 (355) - - 497 (497)       a Experimental values given in parentheses. b Data of 7 adopted from literature.104c 86  Vertical transition energies and oscillator strengths have also been calculated and are summarized in Table 3-7. From 4 to 6, the decreasing oscillator strength of the vertical S0-S1 excitation is in line with the incrementally weaker molar extinction coefficients of the lowest energy absorption bands (Figure 3-5). The T1 state in 6 has a much higher vertical excitation energy, but suffers more energy loss during relaxation to its lowest vibronic state: 0.74 eV is lost vs. 0.35 eV in 4 and 0.39 eV in 5 (Figure 3-9). This may be related to the rotation of the thiophene ring between the ground state and excited states. Calculated absorption and emission wavelengths are summarized in Table 3-8. Calculated absorption and triplet emission (phosphorescence) energies are in good agreement with the experimental values. Singlet emission energies were calculated using non-equilibrium solvation of the excited states, assuming vertical emission from relaxed S1 states decaying with frozen geometry (i.e. vertical relaxation), yet large discrepancies with experimental data were observed. Interestingly calculated emission by assuming decay from relaxed S1 state to relaxed GS (i.e. 0-0 relaxation) agrees very well with the experimental values. Indeed, the vibronic fine structure in the fluorescence spectra of 5 at both room temperature and lower temperatures (Figure 3-7b) shows that first peak is always strongest in intensity. Therefore, 0-0 relaxation is the more reasonable assumption to predict fluorescence of 4, 5 and 6.  3.3 Experimental 3.3.1 General 1H NMR, 13C{1H} NMR, and 31P{1H} NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer. 31P NMR chemical shifts are referenced to external 85% H3PO4, and other chemical shifts are referenced to the residual solvent signals. ESI mass spectra were 87  measured in-house using a Waters LC-MS ESI mass spectrometer. Infrared spectroscopy (IR) was performed on an attenuated total reflection (ATR) crystal using a Perkin-Elmer Frontier FTIR spectrometer. Absorption spectra were obtained on a Varian Cary 5000 UV-Vis-NIR spectrophotometer, and emission measurements were performed on a PTI QuantaMaster 50 fluorimeter. Quantum yields were measured using a Labsphere general purpose integrating sphere.   All syntheses were carried out under a dry N2 atmosphere using standard Schlenk techniques. Dry toluene and tetrahydrofuran (THF) were obtained after treatment with solvent purification system. All other chemicals were purchased from commercial sources and used without further purification. All syntheses involving air sensitive compounds were carried out using standard Schlenk-type procedures under an atmosphere of N2.   3.3.2 Synthesis Scheme 3-5. Synthesis of ligands.  (a) CaC2, cat. Pd(PPh3)4, cat. CuI, CH3CN/NEt3, room temperature.  (b) 2,5-diiodothiophene, cat. Pd(PPh3)4, THF/H2O, 70 °C. (c) Trimethylsilylacetylene, cat. Pd(PPh3)4, (i-Pr)2NH/toluene, reflux. 88  Proligand TMS2T3 (9)89 and Pt(dppp)Cl2107 were synthesized using reported methods. Br2TCCT (20)79 and Br2BTB (21)112 were synthesized using different methods from literature. Br2TCCT (20). CaC2 (3.85 g, 60 mmol), Pd(PPh3)4 (1.16 g, 1.0 mmol) and CuI (380 mg, 2.0 mmol) were added to a round-bottom flask with a side-arm under N2. N2 -sparged CH3CN (60 mL) was then added to the flask, followed by triethylamine (8.4 mL, 60 mmol) and 3-bromo-2-iodothiophene (3.70 g, 12.8 mmol). H2O (1.1 mL, 60 mmol) was added slowly with stirring over 30 minutes while the reaction temperature was maintained below 50 °C. The mixture was then kept at 50 °C using an oil bath and stirred overnight. Black insoluble solids were filtered off and the solvent removed from the filtrate under vacuum to obtain a brown-yellowish crude product. The pure product was obtained using flash column chromatography on silica (hexanes) as a white solid. Yield 1.70 g, 76 %. The 1H NMR and mass spectra matched with literature values.  Br2BTB (21). 2-bromo-phenylboronic acid (1.69 g, 8.4 mmol), 2,5-diiodothiophene (1.34 g, 4.0 mmol), K2CO3 (2.32 g, 16.8 mmol) and Pd(PPh3)4 (462 mg, 0.4 mmol) were added to a round-bottom flask with a stir-bar under N2. N2 -sparged mixed solvent consisting of 20 mL THF and 20 mL de-ionized water was added to the flask with stirring. The mixture was slowly warmed to 70 °C and stirred for 18 hours. The mixture was concentrated in vacuo to remove THF and extracted with diethyl ether (3×20 mL). The organic phases were combined, washed with water and brine, and dried over magnesium sulfate. The crude product was obtained after filtration and removal of the solvent under vacuum. The pure product was obtained using flash column chromatography on silica (hexanes) as an off-white solid. Yield 1.08 mg, 68 %. 1H NMR (CDCl3, 400 MHz):  7.20 (td, J1 = 10.4 Hz, J2 = 2.2 Hz, 2H, 2 CH), 7.33-7.39 (m, 4H, 4 CH), 7.59 (dd, J1 = 10.3 Hz, J2 = 2.2 Hz, 2H, 2 CH), 7.72 (dd, J1 = 10.6 Hz, J2 = 1.3 Hz, 2H, CH). 13C{1H} NMR (CDCl3, 100 89  MHz):  122.6 (C4, C-Br), 127.6 (CH, thienyl), 127.8 (CH, phenyl), 129.1 (CH, phenyl), 131.9 (CH, phenyl), 133.9 (CH, phenyl), 135.0 (CH, phenyl), 142.4 (CH, thienyl). Positive HRMS (APCI/ESI): m/Z = 392.8944, [M+H]+. TMS2TCCT (8). 20 (696 mg, 2.0 mmol), CuI (80 mg, 0.4 mmol) and Pd(PPh3)4 (280 mg, 0.4 mmol) were added to a round-bottom flask with a stir-bar under N2. N2 -sparged mixed solvent of 15 mL diisopropylamine and 30 mL toluene was added to the flask with stirring, followed bv trimethylsilylacetylene (471.5 mg, 4.8 mmol). The mixture was slowly warmed to 75 °C and stirred for 12 hours. The reaction mixture was added to 30 mL saturated NH4Cl solution and extracted with ethyl acetate (4×20 mL). The organic phases were combined, washed with water and brine, and dried over magnesium sulfate. The brown crude product was obtained after filtration and removal of the solvent under vacuum. The pure product was obtained using flash column chromatography on silica (hexanes) as a light-yellow solid. Yield 580 mg, 76 %. 1H NMR (CDCl3, 400 MHz):  0.28 (s, 18H, 6 CH3), 7.03 (d, J = 5.3 Hz, 2H, CH), 7.19 (d, J = 5.2 Hz, 2H, CH). 13C{1H} NMR (CDCl3, 100 MHz):  0.1 (CH3), 89.8 (C4, C≡C), 98.8(C4, C≡C), 99.7 (C4, C≡C), 126.7 (CH, thienyl), 127.0 (C4, thienyl), 127.1 (C4, thienyl), 129.8 (CH, thienyl). Positive HRMS (APCI/ESI): m/Z = 383.0774, [M+H]+. FTIR (ATR) = 2152 cm-1 ( C≡C). TMS2BTB (10). The same reaction conditions, work-up procedure and purification method were used for 8 except that 21 was used as the starting material. A pale-yellow solid was obtained after column purification in 52 % yield. 1H NMR (CDCl3, 400 MHz):  0.29 (s, 18H, 6 CH3), 7.23 (td, J1 = 7.6 Hz, J2 = 1.2 Hz, 2H, CH), 7.35 (td, J1 = 7.6 Hz, J2 = 1.4 Hz, 2H, CH), 7.59-7.64 (m, 4H, 2 CH), 7.79 (s, 2H, CH). 13C{1H} NMR (CDCl3, 100 MHz):  -0.1 (CH3), 99.7 (C4, C≡C), 90  105.2 (C4, C≡C), 120.2 (C4, phenyl), 127.0 (CH, phenyl), 127.4 (CH, phenyl), 128.6 (CH, phenyl), 129.0 (CH, phenyl), 134.6 (CH, thienyl), 135.9 (C4, phenyl), 142.2 (C4, thienyl). Positive HRMS (APCI/ESI): m/Z = 429.1522, [M+H]+. FTIR (ATR) = 2150 cm-1 ( C≡C). Pt(dppp)TCCT (4). Proligand 8 (193 mg, 0.505 mmol), Pt(dppp)Cl2 (339 mg, 0.500 mmol) and CuI (10 mg, 0.05 mmol) were added a round-bottom flask with a stir-bar under N2. Then 20 mL N2 -sparged mixed solvent of 15 mL diisopropylamine and 15 mL CH2Cl2 was added to the flask with stirring. A THF solution of 1.0 M tetrabutylammonium fluoride (TBAF, 2.1 mL, 2.1 mmol) was slowly added to the mixture over 30 minutes, and the resulting solution is stirred for 18 hours. Solvents were removed under vacuum to give the crude mixture, which was then purified using flash column chromatography on silica (20% CHCl3 in hexanes, v/v) to give the pure product as a yellow powder. Yield 282 mg, 67 %. 1H NMR (CD2Cl2, 400 MHz):  2.00-2.11 (m, 2H, CH2), 2.62 (br, 4H, 2CH2), 6.51 (d, J = 5.2 Hz, 2H, 2 CH), 7.10 (d, J = 5.2 Hz, 2H, 2 CH), 7.39-7.44 (m, 12H, 12 CH), 7.77-7.82 (m, 8H, 8 CH). 31P{1H} NMR (CD2Cl2, 162 MHz):  -6.92 (JPt-P = 2215 Hz). Positive ESI-TOF: m/Z = 843.0982, [C41H31S2P2194Pt+H]+. FTIR (ATR) = 2095 cm-1 ( C≡C). Pt(dppp)T3 (5). The same procedure used for 4 was used employed except proligand 9 was used instead. An orange powder was obtained as the pure product. Yield 288 mg, 64 %. 1H NMR (CD2Cl2, 400 MHz):  1.93-2.06 (m, 2H, CH2), 2.56-2.60 (m, 4H, 2CH2), 5.95 (d, J = 5.2 Hz, 2H, 2 CH), 6.72 (s, 2H, 2 CH), 6.74 (d, J = 5.2 Hz, 2H, 2 CH), 7.41-7.44 (m, 12H, 12 CH), 7.67-7.72 (m, 8H, 8 CH). 31P{1H} NMR (CD2Cl2, 162 MHz):  -5.99 (JPt-P = 2177 Hz). Positive ESI-TOF: m/Z = 923.0652, [C43H32S3P2194Pt+Na]+. FTIR (ATR) = 2098 cm-1 ( C≡C). Pt(dppp)BTB (6). The same procedure used for 4 was used employed except proligand 10 91  was used instead. A yellow powder was obtained as the pure product. Yield 249 mg, 56 %. 1H NMR (CDCl3, 400 MHz):  2.02-2.17 (m, 2H, CH2), 2.52 (br, 4H, 2CH2), 6.23 (dd, J1= 7.7 Hz, J2= 0.9 Hz, 2H, 2 CH), 6.78 (td, J1= 7.6 Hz, J2= 1.2 Hz, 2H, 2 CH), 6.98-7.02 (m, 4H, 4 CH), 7.35-7.42 (m, 12H, 12 CH), 7.70-7.72 (m, 8H, 8 CH). 31P{1H} NMR (CDCl3, 162 MHz):  -5.58 (JPt-P = 2183 Hz). Positive ESI-TOF: m/Z = 889.1721, [C47H37SP2194Pt+H]+. FTIR (ATR) = 2105 cm-1 ( C≡C).   3.3.3 X-ray Crystallography Crystals of 4, 5 and 6 suitable for single crystal X-ray diffraction were obtained by diffusion of pentane into solutions of the complexes in CH2Cl2 or CH2Cl2/CH3CN mixed solvent. X-ray diffraction data were collected with a Bruker X8 APEX II diffractometer with graphite monochromated Mo-K radiation. Data were collected and integrated using the Bruker SAINT70 software package. Data were corrected for absorption effects using the multi-scan technique (SADABS).113 The data were corrected for Lorentz and polarization effects. The structures were solved by direct methods.114 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions. Selected bond lengths and bond angles for 4 and 5 (Table 3-9) and two crystallographically independent structures of 6 (Table 3-9) are summarized.     92  Table 3-9. Selected bond lengths (Å) and angles (°) for crystal structures of 4 and 5. 4•CH2Cl2 5•1.2[CH2Cl2]•0.8CH3CN Pt(1)–C(12) 2.0061(16) Pt(1)–C(14) 2.026(2) Pt(1)–C(14) 1.9927(16) Pt(1)–C(16) 2.012(2) Pt(1)–P(1) 2.2916(5) Pt(1)–P(1) 2.3019(6) Pt(1)–P(2) 2.2843(4) Pt(1)–P(2) 2.2928(5) C(11)–C(12) 1.214(2) C(13)–C(14) 1.217(3) C(13)–C(14) 1.215(2) C(15)–C(16) 1.216(3) C(12)–Pt(1)–C(14) 84.80(6) C(14)–Pt(1)–C(16) 88.95(8) P(1)–Pt(1)–P(2) 94.875(16) P(1)–Pt(1)–P(2) 92.91(2) Pt(1)–C(12)–C(11) 170.61(14) Pt(1)–C(14)–C(13) 172.85(19) Pt(1)–C(14)–C(13) 175.30(14) Pt(1)–C(16)–C(15) 174.52(19) Table 3-10. Selected bond lengths (Å) and angles (°) for the two crystallographic independent structures in 6•CH2Cl2 crystals. Structure A Structure B Pt(1)–C(1) 2.016(2) Pt(2)–C(48) 2.007(2) Pt(1)–C(20) 2.010(2) Pt(2)–C(67) 2.008(2) Pt(1)–P(1) 2.2846(6) Pt(2)–P(3) 2.2927(6) Pt(1)–P(2) 2.2834(6) Pt(2)–P(4) 2.2827(6) C(1)–C(2) 1.206(3) C(48)–C(49) 1.209(3) C(19)–C(20) 1.204(3) C(66)–C(67) 1.206(3) C(1)–Pt(1)–C(20) 88.95(8) C(48)–Pt(1)–C(67) 88.94(9) P(1)–Pt(1)–P(2) 96.99(2) P(3)–Pt(2)–P(4) 93.79(2) Pt(1)–C(1)–C(2) 172.5(2) Pt(2)–C(48)–C(49) 175.0(2) Pt(1)–C(20)–C(19) 177.3(2) Pt(2)–C(67)–C(66) 177.1(2)  93  4•CH2Cl2 crystallizes with one disordered CH2Cl2 solvent molecule in the asymmetric unit. The structure is in P 21/n space group, with a primitive monoclinic unit cell containing four 4 molecules. 5•1.2[CH2Cl2]•0.8CH3CN crystallizes with one complete CH2Cl2 molecule as well as one site that is partially occupied by CH2Cl2 and also by CH3CN (~1:4) in the asymmetric unit. The structure is in P -1 space group, with a primitive triclinic unit cell containing two 5 molecules. 6•CH2Cl2 crystallizes with two crystallographically independent molecules in the asymmetric unit. Additionally, there are two molecules of CH2Cl2 in the asymmetric unit. The structure is in P 21/n space group with a primitive monoclinic unit cell containing eight 6 molecules.  3.3.4 DFT Simulations Density functional theory (DFT) calculations were carried out using the Gaussian 09 Rev.D01 suite of Programs.90 The PBE0 hybrid functional91 with 6-31G* basis set (for C, H, S and P atoms) and the LANL2DZ effective-core pseudopotential92 (for Pt) was employed to simulate ground state (S0) and T1 structure of all three metallacycles. Triplet states are treated using unrestricted formalism. Structural coordinates from X-ray crystallography were used as starting points of geometry optimizations. Optimized structures were confirmed to be the minimum on the potential energy surface by vibrational frequency calculations.  Time-dependent DFT (TD-DFT) calculations were also performed to understand S1 excited state electronics and absorption and emission behaviors of these complexes. For all simulations, solvation effect of CH2Cl2 ( = 8.93) was implemented using the polarizable continuum model88 (PCM) on Gaussian 09. TD-DFT method is also employed to calculate optimized structures of the 94  metallacycles in their S1 excited states which are confirmed to be the minimum on the potential energy surface by excited state vibrational frequency calculations. Table 3-11. Calculated and experimental vibrational frequencies of C≡C bonds. Complex Calculated Frequency Experimental Symmetric Mode (strong) Antisymmetric Mode (weak) 4 2067 cm-1 2014 cm-1 2095 cm-1 5 2112 cm-1 2096 cm-1 2098 cm-1 6 2125 cm-1 2116 cm-1 2105 cm-1  Frequency calculations on 4, 5 and 6 in their optimized geometry confirmed that they are on the global minimum on the potential energy surface. Frequency calculations also give us some insight into their IR absorptions. They are predicted to show symmetric and antisymmetric stretching modes where vibrations of the two C≡C bonds are in phase or out of phase respectively. The antisymmetric mode is calculated to have much weaker IR absorption than the symmetric mode. Experimentally, only one peak is observed in 4, 5 and 6. The same goes for compound 7 in literature.104c This may be due to constrained geometry in cyclic structures as similar acyclic cis-Pt(II) bisacetylides show two peaks.115 After applying an empirical scaling factor of 0.95 (for PBE0 hybrid functional and 6-31G* basis set),116 the predicted IR frequencies are reasonably close to experimental values (Table 3-11). Small discrepancies may be attributed to the PCM solvation assumptions used in DFT calculations while IR spectra were actually recorded in the solid state using powder samples.   95  3.4 Conclusions In summary, we have prepared and characterized three different thiophene-containing Pt metallacycles 4, 5 and 6 with dppp ancillary ligands. Crystal structures show that they all possess square planar metal centers with similar metallacycle ring size. In solution, both phosphorescence and fluorescence are attributed to ligand-based excited states. It is confirmed that replacing phenyl groups with thiophene rings in metallacycles can induce or enhance fluorescence without adding extra conjugation to the ligand, thus obtaining room temperature fluorescence and phosphorescent dual emission from with different FL/PH ratios. It is also learned that adding thiophene rings to extend conjugation length in metallacycles (from 4 to 5) will likely shut down phosphorescence in the visible region due to ligand-based low lying T1 states. Excited state transient absorption spectroscopy could be used in the future to gain more detailed experimental understanding of the photophysics of these metallacycles. DFT and TD-DFT calculations show that upon excitation from S0 to S1, 5 and 6 undergo charge transfer from the ligand to the empty 6pz orbital of Pt atom while 4 shows only * transitions. It is also revealed that all three metallacycles show very planar quinoid-like structure in the S1 and T1 excited state with contracted single bonds and lengthened double/triple bonds in the ligand. Complex 6 undergoes significant planarization going from the GS to the S1 or T1 excited states, resulting in weaker absorbance and much larger energy loss in IC processes compared to 4 and 5. Both fluorescence and phosphorescence maxima of these metallacycles are attributed to 0-0 relaxation from corresponding relaxed S1 and T1 excited states, and match up with calculated values. We anticipate the above findings may be useful in future applications in single-component white-emitting OLEDs, self-referencing oxygen sensors, and hypoxia contrast agents. 96  Chapter 4 Bithiophene-Based Flexible Lewis Pairs  4.1 Introduction Lewis acids and bases are important reactive species with applications in catalytic transformations,117 molecular recognition and chemical sensing.47,50,118 Molecules containing both Lewis acid and base functionalities (Lewis pairs) are intriguing systems where either Lewis adducts, or sterically hindered, unbound species with unusual reactivity, such as frustrated Lewis pairs (FLPs), can form (Scheme 4-1).119  Scheme 4-1. FLP activation of dihydrogen and catalyzed hydrogenation of imines.   Although most Lewis pairs exist either as unreactive adducts, or reactive unbound pairs, the development of systems where these forms exist in equilibrium with each other is intriguing (Scheme 4-2).120 Such “flexible” systems may allow control of chemical behavior by tuning of the solvent, temperature or other environmental factors to favor either the unbound form or Lewis 97  adduct.  The ability to use absorption or emission color to instantaneously assess the state of such flexible systems would be useful, yet the photophysical properties of unbound Lewis pairs have rarely been explored to date.121 It is known that the presence of charge-accepting Lewis acidic boron centers gives rise to intense light absorbance or photoluminescence,46,52,122 which is absent when 4-coordinate boron is formed upon reaction with fluoride or cyanide anions.47,118b Carefully designed flexible Lewis pairs may, therefore, enable reversible tuning of photophysical behavior, based on whether the boron is free or engaged in a Lewis adduct. Absorption or emission changes can then be used to conveniently assess catalytic activity in situ, and may allow applications in sensors.  Scheme 4-2. Examples of flexible Lewis acid-base pairs.120  There are ways to reduce or even prohibit the interaction between Lewis acids and base, such as introducing strong steric hindrance to make them inaccessible to one another, lowering Lewis 98  acidity/basicity so that the adduct is less thermodynamically favored, or in rare cases by using rigid acenaphthene bridges between the Lewis acid and base moieties to make the adduct geometrically unfavored (Scheme 4-3).66 However, it is still difficult to design a robust system with a tunable and reversible equilibrium between the Lewis adduct and unbound Lewis pairs. Scheme 4-3. A naphthalene-based Lewis adduct 3165 and an acenaphthene-based FLP 32.66  Scheme 4-4. Bithiophene-based Lewis pair between -BMes2 and -P(O)R2 groups.  In this chapter, a flexible boron-phosphine oxide Lewis pair system will be disclosed in which manipulation of the equilibrium between unbound pair and Lewis adducts results in a fundamental alteration in the photophysical properties of the system. A key feature of this system is the flexibility of the bithiophene backbone which allows rotation about the interannular C-C bond to form the closed structure as a Lewis adduct or the open structure as an unbound Lewis pair (Scheme 4-4). The phosphine oxide functionality as the Lewis basic component of the system has special advantages in that it is air stable, easy to synthesize, and has dual functionality as both 99  Lewis base123 and strong hydrogen bond acceptor (HBA) capable of forming hydrogen bonds (HBs) with amines,124 alcohols,125 H2O, H2O2,126 and even weak hydrogen donors such as CHCl3.127 Other intramolecular borane-phosphine oxide pairs are known however none of them are able to switch between three- and four-coordinated boron centers.120i,128    4.2 Result and Discussion 4.2.1 Synthesis and Solid-State Structures Scheme 4-5. Synthesis of 24a-c and 25.  The boron-phosphine oxide (B-PO) Lewis acid-base pairs 24a-24c were synthesized from 3,3´-dibromo-5,5´-dibutylbithiophene (22) by sequentially introducing the phosphine and Lewis acidic boron centers (Scheme 4-5). Oxidation gave air-stable 24a-24c as light yellow solids. Compound 25 was synthesized in a similar fashion for comparison except that a proton was introduced instead of the dimesitylboryl group. 100  The solid-state structure of 24a (Figure 4-1) obtained by recrystallizing the compound from acetone shows that the phosphine oxide moiety forms a Lewis adduct with the boron center (24aclosed), despite the presence of the bulky mesityl groups. The B-O bond length in 24aclosed is comparable to that in a related Lewis adduct supported on a 1,8-naphthalene linker;65 however in that case the structure is highly congested and rigid whereas in 24aclosed there is sufficient flexibility for repulsions to be minimized. In 24aclosed the two thiophene rings are slightly twisted relative to each other with a small S(1)-C(4)-C(5)-S(2) dihedral angle of 20.94(15)°. The P-O bond length of 1.5194(10) Å in 24aclosed is longer than that in a similar, but free, phosphine oxide123 due to weakened oxygen-to-phosphorus bonding. By contrast, 24c crystallizes as an unbound Lewis pair 24copen (Figure 4-2) due to substantial steric hindrance in this compound. In 24copen the S-C-C-S dihedral angle is large (130.10(7)°), and the P=O bond length (1.4846(8) Å).  Figure 4-1. Solid-state molecular structure of 24a. The thermal ellipsoids are drawn at 50% probability and H atoms are omitted for clarity. 101   Figure 4-2. Solid-state molecular structure of 24c. The thermal ellipsoids are drawn at 50% probability and H atoms are omitted for clarity.  4.2.2 Infrared and NMR Spectroscopic Studies In the solid state, the infrared (IR) spectrum of 24a shows a stretching band (P=O) at 1116 cm-1, close to values obtained for similar Lewis acid-base adducts containing -P(O)Ph2 group,129 and consistent with the bonding found in the crystal structure. However, the IR stretching band of 24c in the solid state (P=O = 1185 cm-1) is typical of free phosphine oxides. The solution structure of compound 24a was found to vary in different solvents, as evidenced by both IR and NMR spectra. In hexanes, a non-HB donating solvent, the P=O stretching band is at 1120 cm-1, similar to P=O in the solid state and consistent with a Lewis adduct (24aclosed) also being present in this solvent. In addition, the 31P NMR spectrum in hexanes shows a single peak at  = 34.5 ppm, significantly downfield from the peak in MeOH-d4 or CDCl3 (Figure 4-3) and broadened (peak width 28.5 Hz) relative to the narrower peak in MeOH-d4 (peak width = 3.5 Hz). 102  These spectral features are consistent with the Lewis adduct 24aclosed being present in hexanes, with the broadening attributed to the presence of the quadrupolar 10B and 11B centers bonded to the oxygen center.  Figure 4-3. Stacked 31P{1H} NMR spectra of 24a in various solvents.  Figure 4-4. Stacked 31P{1H} NMR spectra of 25 in various solvents. In the strong HB solvent MeOH, P=O of 24a is at 1187 cm-1, similar to the value typically observed for free P=O bonds.130 The sharp upfield resonance in the 31P NMR spectrum in MeOH-d4 is consistent with the presence of mostly 24aopen in this solvent. In contrast to 24a, the 31P NMR resonance in boron-free 25 shifts downfield with progressively more HB donating solvents131 (Figure 4-4) due to the increasingly deshielded 31P nucleus. 11B NMR spectra of 24a also show a downfield shift in stronger HB donor solvents, consistent with the presence of 3-coordinate boron. 103   Figure 4-5. FTIR spectra of 24a in CD2Cl2 solution (CD2Cl2 was used instead of CH2Cl2 to avoid overlapping C-H bending absorptions). Interestingly, in CD2Cl2 the IR spectrum of 24a shows two P=O stretching peaks at 1196 cm-1 and 1120 cm-1 (Figure 4-5). The former is close to P=O of 25 at 1187 cm-1, corresponding to the presence of a free P=O group in 24aopen, while the latter is due to the presence of 24aclosed. This is indicative of the coexistence of both structures in this weak HB donating solvent. In CDCl3, another weak HB donating solvent, only a single broadened 31P peak ( = 26.2 ppm) is observed (Figure 4-3) and the peak width of 12.6 Hz lies between the 28.5 Hz observed for 24aclosed in hexanes and the 3.5 Hz of 24aopen in methanol, due to rapid equilibration in CDCl3. The interconverting two forms of 24a cannot be distinguished due to a much faster exchange rate than the chemical shift difference in 31P NMR (kex >> ).132 The equilibrium responds to the addition of an external Lewis base such as fluoride anion. Reaction of 24a in either CDCl3 or acetone-d6 solution with tetrabutylammonium fluoride (TBAF) results in an upfield shift, and sharpening, of the 31P resonance (from  = 26.1 to 21.7 ppm in 104  CDCl3 and from  = 33.4 to 19.6 ppm in acetone-d6). This is attributed to cleavage of the B-O bond and formation of a fluoride adduct at boron (Figure 4-6).  Figure 4-6. Stacked 31P{1H} NMR of 24a and 24a+F– in various NMR solvents.  Figure 4-7. Stacked 31P{1H} NMR of 24b in various NMR solvents (very broad peak in hexanes due to restricted rotation induced by bulky isopropyl groups). Compound 24b behaves similarly to 24a in different solvents (Figure 4-7). The 31P NMR peak of 24bclosed is very broad in hexanes due to restricted rotations of both mesityl groups on boron and isopropyl groups on phosphine in a steric crowded environment. The broadening is less 105  evident in CDCl3 or MeOH-d4 in that rotation of isopropyl groups in 24bopen is faster. This can be evidenced by 1H NMR spectra: all peaks are well resolved at 25 °C in methanol (Figure 4-8), however in non-HB solvent benzene-d6 the isopropyl CH signals and one of the mesityl CH3 signals are very broad at 25 °C (Figure 4-10) and much sharper at 50 °C (Figure 4-11).  Moreover, the phosphine oxide moiety on 24b has alkyl 1H NMR signals due to the isopropyl substituents, well separated from aromatic peaks, allowing the intramolecular through-space interaction between the Lewis acid and base groups to be probed with 2D-NOESY experiments. In methanol-d4, a NOE was not detected between the CH and CH3 groups on the isopropyl moiety and the methyl groups on the Mes2B unit (Figure 4-8 and Figure 4-9), consistent with the spatial separation of these groups in 24bopen. In contrast, in the non-HB solvent benzene-d6, NOE interactions were observed (Figure 4-11 and Figure 4-12), indicating proximity of these groups in agreement with the closed structure 24bclosed.   Figure 4-8. Assignment of the 1H NMR spectrum of 24bopen in methanol-d4 (25 °C). 106   Figure 4-9. 1H NOESY spectrum of 24bopen in methanol-d4 (25 °C, mixing time = 800 ms). No NOE is observed between H(6)/H(7) and H(13).  Figure 4-10. Assignment of the 1H-NMR spectrum of 24bclosed in benzene-d6 (25 °C) Signals from H(6) and H(13) are very broad, which leads to low sensitivity for NOESY experiments at this temperature. 107   Figure 4-11. Assignment of 1H-NMR spectrum of 24bclosed in benzene-d6 (50 °C, mixing time = 800 ms). Peak ~ 0.6 ppm arises from residue H2O.  Figure 4-12. 1H NOESY spectrum of 24bclosed in benzene-d6 (50 °C, mixing time = 800 ms). NOE interactions between H(6)/H(7) and H(13) are indicated with red dashed boxes. Even though H(4) and H(13) overlapped in the 1H NMR spectrum, the labeled NOE signal should 108  only be attributed to H(13) since intramolecular through space interactions between H(4) and H(6)/H(7) are not geometrically allowed.  Figure 4-13. Stacked 31P{1H} NMR spectra of 24c in different NMR solvents (the spectrum of 24c in MeOH-d4 is not shown due to poor solubility). The similarities between the behavior of 24a and 24b shows that bithiophene-based Lewis pair systems with either aryl or alkyl phosphine oxide substituents show flexible switching between Lewis adducts and unbound Lewis pairs in response to HB donating solvents. By introducing excessively bulky mesityl groups on both Lewis acid and base centers (24c) the Lewis adduct can be prevented from forming. Similarly to B-free phosphine oxide 25, the 31P NMR resonance of 24c shifts downfield with more HB donating solvents (22.07 ppm in MeOH-d4, 20.52 ppm in CDCl3 and 19.50 ppm in hexanes), with no indication of Lewis adduct formation in any solvent (Figure 4-13). Attempts have been made to determine the equilibrium constant between 24aopen and 24aclosed from 31P NMR chemical shifts in different solvents. However, this is not practical since the inherent chemical shifts of 24aopen and 24aclosed unrelated to the equilibrium may also be solvent dependent and are unknown. 109   4.2.3 Electronic Absorption and Emission Spectra       Figure 4-14. Left: UV-Vis spectra of 24a in various solvents; right: UV-Vis spectra of 24b in various solvents. The absorption spectrum of 24a depends on the HB donor ability of the solvent (Figure 4-14a). In very weak or non-HB donor solvents such as hexanes, the spectra show a dominant band at ~370 nm, however, with increasing HB donor strength (CH2Cl2 < CHCl3 < MeOH),14 this absorption band becomes weaker, and the bands between 260-350 nm strengthen. In MeOH, the absorption spectrum is similar to that observed previously for a boryl-substituted bithiophene,46 where the absorption is assigned as a charge-transfer band involving the 3-coordinate Lewis acidic boron center. Although the low energy absorptions of 24aclosed and 24aopen are very close in wavelength, they have quite different origins (see Table 4-2 in the DFT calculation section below). The same trend is observed in the absorption spectra of 24b (Figure 4-14b), while those of 24c shows no significant solvent dependence due to its persistent unbound nature.  110     Figure 4-15. Emission from 24a, ex = 366 nm using an 18.4 W UV lamp for excitation.       Figure 4-16. (a) Normalized emission and excitation spectra of 24aclosed in hexanes (black trace) and 24aopen in methanol (red trace). (b) Excitation energy dependent mixed emission from 24aopen and 24aclosed in CHCl3. The emission of 24a is also solvent dependent (Figure 4-15). In MeOH, 24a shows strong emission at 540 nm (Figure 4-16a) with a quantum yield of 0.60. The relatively large Stokes shift (13400 cm-1) is consistent with a CT excited state in 24aopen as the source of the emission. In hexanes, 24a shows bright blue emission with a smaller Stokes shift (5050 cm-1) assigned to decay of a bithiophene-localized excited state. In weak HB donating solvents such as CH2Cl2 and CHCl3, 111  compound 24a luminesces with mixed color. Emission spectra of 24a in CHCl3 show excitation-dependent luminescence (Figure 4-16b) with higher energy excitation resulting in lower energy emission. This unusual behavior is the result of simultaneous emission from both 24aclosed and 24aopen; at different excitation wavelengths the two compounds contribute differently to the total output. In contrast, dissolving 24c in any of the solvents used here results only in intense CT emission coincident with that of 24aopen, since the Lewis adduct is not able to form in this case.  (b)  Figure 4-17. (a) Emission and excitation spectra of 24a in methanol, n-BuOH, s-BuOH and t-BuOH. (b) Emission from 24a irradiated at 366 nm using an 18.4 W UV lamp. The emission spectra of 24a in n-butanol, s-butanol and t-butanol show an increase in the blue region near 440 nm concomitant with increased steric bulk of the solvent molecule and higher electron density on the -OH groups (Figure 4-17). This is due to a decrease in the strength of the HB donating ability of the solvent131 resulting in weaker interaction with the phosphine oxide thus more of the closed structure 24aclosed being present. 112         Figure 4-18. (a) Emission spectra of 24a at decreasing temperatures in a MeOH/EtOH (v/v =1:4) solution excited at 330 nm. (b) Variable temperature 31P NMR spectra of 24a and 25 in CD3OD (-65 °C to 25 °C at 10 °C intervals).   The emission of 24a is highly temperature-dependent in alcohol solvent. At room temperature, where 24aopen is the major species present in strong HB donor solvents, the dominant emission from 24a is at ~540 nm. However, with cooling, the emission band at 440 nm (assigned to 24aclosed) increases in intensity with a concomitant decrease in the band at 540 nm (Figure 4-18). The variable temperature 31P NMR spectra of 24a in methanol-d4, show significant downfield shifts and broadening as more 24aclosed forms at lower temperatures while the 31P NMR spectrum of boron-free compound 25 shows only slight broadening and upfield shifts over the same temperature range (Figure 4-18b). The observed temperature dependence of the 24aclosed/24aopen equilibrium is possibly due to the decrease in entropy resulting from the increased rigidity of 24aclosed relative to 24aopen.  113  4.2.4 DFT and TD-DFT Simulations (a) (b)  Figure 4-19. Optimized ground state geometries of (a) 24aclosed and (b) 24aopen rendered by PyMol software. Table 4-1. Calculated molecular orbitals of 24aclosed and 24aopen in different solvents. Structures Molecular Orbitals HOMO-1 HOMO LUMO 24aopen  (MeOH)    24aclosed  (THF)     DFT computational (all-electron TZ2P basis sets at the BP86 level using ADF2013 software suite) studies by Prof. Jeffery Nagle confirmed that 24aopen and 24aclosed both correspond to minima on the ground state potential energy surface (Figure 4-19). The calculated B-O distance increases 114  from 1.64 Å in 24aclosed to 3.67 Å in 24aopen, and the S-C-C-S dihedral angle also significantly increases from 23.4° in 24aclosed to 65.0° in 24aopen. Single-point calculations of the ground-state structures confirmed that the LUMO of 24aclosed is mostly bithiophene-localized, while the LUMO of 24aopen is strongly boron-centered (Table 4-1). Table 4-2. TD-DFT simulated vertical excitation energy and oscillator strengths of 24 in different solvents. Structure State Transition Type Energy Oscillator Strength 24aclosed 1A 71.5% HOMO – LUMO 368 nm 0.281 23.7 % HOMO-1 - LUMO CT 24aopen 1A HOMO - LUMO CT 366 nm 0.075 2A HOMO-1 - LUMO CT 340 nm 0.345  TD-DFT calculations (using BPE0 hybrid functional and all-electron TZ2P basis sets) were used to calculate absorption transitions for both 24aclosed and 24aopen by simulating vertical electronic excitation with frozen nuclear coordinates. The results are presented in Table 4-2, absorption wavelengths and oscillator strengths (indicative of relative absorption intensity) are in good agreement with experimental values in Figure 4-14 (page 109). These show that the lowest energy absorption of 24aclosed involves a bithiophene-localized π−π* transition mixed with a charge transfer transition from a mesityl π−orbital to the LUMO. The lowest energy absorption in 24aopen arises from charge transfer (CT) processes from a mesityl-localized π-orbital and a bithiophene-localized π-orbital to the boron-centered LUMO. This again confirmed the predominant existence of 24aopen and 24aclosed in MeOH and THF respectively.  115  4.3 Experimental 4.3.1 General 1H NMR, 13C{1H} NMR, and 31P{1H} NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer at 25 °C. 11B NMR spectroscopic data were recorded at 25 °C on a 600 MHz Varian INOVA spectrometer equipped with a B-free 5 mm dual broadband gradient probe (Nalorac, Varian Inc., Martinez, CA) using boron-free quartz NMR tubes. Residual protio-solvent peaks were used in the calibration of 1H NMR spectra. Peaks of the deuterated solvents were used to calibrate 13C{1H} NMR spectra. All 31P{1H} NMR spectra are referenced to 85% H3PO4 as an external standard. Absorption spectra were obtained on a Varian Cary 5000 UV-Vis-NIR spectrophotometer, and emission measurements were performed on a PTI QuantaMaster 50 fluorimeter. Infrared spectroscopy (IR) was performed on an attenuated total reflection (ATR) crystal using a Perkin-Elmer Frontier FTIR spectrometer. X-Ray crystallography data were collected on a Bruker X8 APEX II diffractometer with graphite monochromated Mo-Kα radiation. CHN elemental analyses were performed using an EA1108 elemental analyzer, using a suitable certified organic standard (OAS) of a known elemental composition for calibration. All syntheses involving air sensitive compounds were carried out using standard Schlenk-type procedures under an atmosphere of N2. Tetrahydrofuran (THF) was distilled from Na and stored under a N2 atmosphere. Chlorodiphenylphosphine (ClPPh2, 98%), bis(2,4,6-trimethylphenyl)phosphorus chloride (Mes2PCl, 95%), chlorodiisopropylphosphine (i-Pr2PCl, 96%), dimesitylboron fluoride (Mes2BF, 90%), di and hydrogen peroxide solution (H2O2, 30 wt. % in H2O) were purchased from Sigma-Aldrich and used without further purification. 3,3 -´116  Dibromo-5,5 -´dibutyl-2,2 -´bithiophene (22) was synthesized according to literature procedure.133  4.3.2 Synthesis 3-Bromo-3 -´diphenylphosphino-5,5 -´dibutyl-2,2 -´bithiophene, (23a): Compound 22 (1.4 g, 3.2 mmol) was placed in a dry round bottom flask with a side-arm under N2, and anhydrous THF (40 mL) was added to the flask. The solution was cooled to -78 °C and n-BuLi (1.6 M in hexanes, 2.1 mL, 3.4 mmol) was added drop-wise, after which the solution was stirred for another 1 hour at -78 °C. Ph2PCl (0.77 g, 3.5 mmol) was then added and the mixture was slowly warmed up to room temperature and stirred overnight. The reaction was then quenched with 1 mL deionized water, and THF was removed using a rotary evaporator. The mixture was extracted with diethyl ether (4×10 mL) and the organic phase washed with first water and then brine before drying the organic layer over anhydrous MgSO4. The mixture was filtered and the solvent removed under vacuum. Purification by flash column chromatography (hexanes: CHCl3 = 3:1) gave a pale yellow viscous liquid. Yield: 1.25 g, 72%. 1H NMR (CDCl3, 400 MHz):  0.95-1.01 (m, 6H, 2CH3), 1.37-1.48 (m, 4H, CH2), 1.64-1.72 (m, 4H, CH2), 2.76-2.80 (m, 4H, 2CH2), 6.49 (s, 1H, thienyl CH), 6.76 (s, 1H, thienyl CH), 7.35-7.44 (m, 10H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  13.8 (CH3), 13.8 (CH3), 22.2 (CH2), 22.3 (CH2), 30.0 (overlapping 2CH2), 33.1 (CH2), 33.6 (CH2), 111.1 (thienyl quaternary C-Br), 127.5 (thienyl CH), 127.9 (d, 3JPC = 3.6 Hz, thienyl C4), 128.3 (d, 3JPC = 6.5 Hz, phenyl CH), 128.4 (phenyl CH), 128.9 (d, 2JPC =1.5 Hz, thienyl C4), 133.2 (d, 2JPC = 19.2 Hz, phenyl CH), 136.7 (d, 2JPC = 16.5 Hz, thienyl C4), 137.7 (d, 1JPC = 37.1 Hz, thienyl C4), 138.3 (d, 1JPC = 11.0 Hz, phenyl C4), 147.2 (d, 5JPC = 1.7 Hz, thienyl C4), 147.5 (d, 3JPC = 2.3 Hz, thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz):  -23.62. Elemental Analysis calc. 62.10 117  % C, 5.58 % H. found. 62.28 % C, 5.58 % H.  3-dimesitylboryl-3 -´diphenylphosphoryl-5,5 -´dibutyl-2,2 -´bithiophene, (24a): Compound 23a (1.5 g, 2.7 mmol) was placed in a dry round bottom flask with a side-arm under N2, and anhydrous THF (40 mL) was added to the flask. The solution was cooled to -78 °C and n-BuLi (1.6 M in hexanes, 1.8 mL, 2.9 mmol) was added drop-wise and stirred for 1 hour at -78 °C. Mes2BF (90 %, 912 mg, 3.4 mmol) was added at -78 °C and the solution was slowly warmed up to room temperature and stirred overnight. The reaction was then quenched by addition of H2O and an excess amount of H2O2 (10 mL, 30 wt. % in H2O) was added. The THF was removed from the resulting mixture on a rotary evaporator, and the mixture was then extracted with diethyl ether (4×10 mL) and the organic phase washed with first water and then brine, before drying over anhydrous MgSO4. The mixture was filtered and the solvent was removed from the filtrate under vacuum. Purification by flash column chromatography (hexanes: acetone=10:1) gave a yellow solid. Yield: 1.0 g, 52%. 1H NMR (CDCl3, 400 MHz):  0.88-0.92 (m, 6H, 2CH3), 1.22-1.25 (m, 2H, CH2), 1.29-1.34 (m, 2H, CH2), 1.42-1.47 (m, 2H, CH2), 1.52-1.57 (m, 2H, CH2), 2.05 (s, 12H, 4CH3), 2.23 (s, 6H, 2CH3) 2.56 (t, J = 7.4 Hz, 2H, CH2), 2.67 (t, J = 7.6 Hz, 2H, CH2), 6.10 (d, 3JPH = 4.1 Hz, 1H, thienyl CH), 6.49 (s, 1H, thienyl CH), 6.61 (s, 4H, mesityl phenyl CH), 7.39-7.44 (m, 4H, phenyl CH), 7.49-7.55 (m, 6H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  13.9 (CH3), 13.9 (CH3), 21.2 (CH3), 22.0 (CH2), 22.2 (CH2), 23.9 (CH3), 29.4 (CH2), 29.6 (CH2), 33.6 (CH2), 33.9 (CH2), 123.8 (d, 2JPC = 9.9 Hz, thienyl C4), 128.3 (d, 3JPC = 12.2 Hz, phenyl CH), 128.3 (mesityl CH), 128.8 (d, 2JPC = 17.5 Hz, thienyl C4), 131.8 (d, 4JPC = 2.3 Hz, phenyl CH), 132.2 (d, 2JPC = 9.9 Hz, phenyl CH), 132.5 (thienyl CH), 133.3 (d, 1JPC = 107.4 Hz, phenyl C4), 136.9 (mesityl C4), 138.3 (d, 3JPC = 2.3 Hz, thienyl C4), 141.4 (mesityl C4), 144.5 (br, mesityl 118  C4), 145.1 (d, 3JPC = 16.0 Hz, thienyl C4), 147.1 (thienyl C4), 147.9 (d, 1JPC = 9.9 Hz, thienyl C4), 154.7 (br, thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz):  26.3. 11B NMR (CDCl3, 193 MHz):  55.8. Elemental Analysis calc. 76.02 % C, 7.21 % H. found. 76.24 % C, 7.23 % H. 3-Dimesitylboryl-3 -´diisopropylphosphoryl-5,5 -´dibutyl-2,2 -´bithiophene, (24b): compound 24b was synthesized from 1 without isolation of intermediate 23b. Compound 1 (1.09 g, 2.5 mmol) was placed in a dry round bottom flask with a side-arm under N2, and anhydrous THF (40 mL) was added to the flask. The solution was cooled to -78 °C and n-BuLi (1.6 M in hexanes, 1.6 mL, 2.5 mmol) was added drop-wise, after which the solution was stirred for another 1 hour at -78 °C. i-Pr2PCl (0.40 g, 2.6 mmol) was then added and the mixture was slowly warmed up to room temperature and stirred overnight. After the formation of 23b, the solvent and unreacted iPr2PCl was removed under vacuum, and the resulting residue dissolved using 40 mL fresh anhydrous THF. The solution was cooled to -78 °C and n-BuLi (1.6 M in hexanes, 1.7 mL, 2.7 mmol) was added. The mixture was stirred at -78 °C for 2 hours and reacted with Mes2BF (90 %, 834 mg, 2.8 mmol) followed by the same work-up procedure as used in the preparation of 3. Compound 24b was obtained as a yellow solid. Yield: 542 mg, 33%. 1H NMR (CD3OD, 400 MHz): 0.88-0.94 (m, 6H, 2CH3), 1.01 (q, 3JPH = 16 Hz, 3JHH = 7.2 Hz, 6H, 2CH3), 1.20-1.26 (q, 3JPH = 16 Hz, 3JHH = 7.1 Hz , 6H, 2CH3), 1.31-1.40 (m, 2H, CH2), 1.43-1.51 (m, 2H, CH2), 1.56-1.64 (m, 2H, CH2), 2.04 (s, 12H, 4CH3), 2.16 (s, 6H, 2CH3), 2.23-2.33 (m, 2H, 2CH), 2.65 (t, 3JHH = 7.3 Hz, 2H, CH2), 2.72 (t, 3JHH = 7.3 Hz, 2H, CH2), 6.44 (s, 1H, thienyl CH), 6.55 (d, 3JPH = 2.7 Hz, 1H, thienyl CH), 6.61 (s, 4H, phenyl CHs). 13C{1H} NMR (CD2Cl2, 100 MHz): overlapped 2CH3), 15.8 (d, 2JPC = 62.9 Hz, CH3), 20.0 (2CH3), 22.5 (CH2), 22.5 (CH2), 24.5 (4CH3), 27.1 (d, 1JPC =70.0 Hz, 2CH), 29.8 (CH2), 30.0 (CH2), 33.9 (CH2), 34.5 (CH2), 118.8 (d, 1JPC =85.9 Hz, thienyl C4), 126.2 119  (d, 2JPC =15.5 Hz, thienyl CH), 129.2 (phenyl CHs), 135.1 (phenyl C4),135.1 (thienyl CH), 136.0 (thienyl C4), 141.5 (br, phenyl C4), 145.7 (d, 1JPC =13.5 Hz, thienyl C4), 146.4 (thienyl C4), 147.9 (phenyl C4), 151.5 (thienyl C4), 157.9 (thienyl C4). 31P{1H} NMR (CD3OD, 162 MHz): 53.3 Hz. Elemental Analysis calc. 72.93 % C, 8.57 % H. found. 72.95 % C, 8.57 % H. 3-dimesitylboryl-3 -´dimesitylphosphoryl-5,5 -´dibutyl-2,2 -´bithiophene, (24c): Compound 24c was synthesized from 22 without isolation of intermediate 23c, following a similar synthetic procedure to that used for 24b, except that Mes2PCl (0.80 g, 2.6 mmol) was used instead of i-Pr2PCl. Compound 24c was obtained as a yellow solid. Yield: 1.0 g, 50%. 1H NMR (CD3OD, 400 MHz): 0.83-0.87 (m, 6H, 2CH3), 1.08-1.17 (m, 2H, CH2), 1.23-1.37 (m, 4H, 2CH2), 1.43-1.51 (m, 2H, CH2), 2.14 (br, 12H, 4CH3), 2.19 (s, 12H, 4CH3), 2.23 (br, 6H, 2CH3), 2.31 (s, 6H, 2CH3), 2.47 (t, 3JHH = 7.2 Hz, 2H, CH2), 2.61 (t, 3JHH = 7.5 Hz, 2H, CH2), 6.09 (d, 3JPH = 5.2 Hz, 1H, thienyl CH), 6.47 (s, 1H, thienyl CH), 6.66 (br, 4H, phenyl CH), 6.86 (d, 3JPH = 3.3 Hz, 4H, phenyl CHs). 13C{1H} NMR (CDCl3, 100 MHz): 13.8 (CH3), 13.9 (CH3), 21.1 (d, 5JPC =1.1 Hz, CH3), 21.3 (CH3), 21.8 (CH2), 22.2 (CH2), 23.5 (d, 3JPC =8.3 Hz, CH3), 23.9 (br, CH3), 29.31 (CH2), 29.4 (CH2), 33.4 (CH2), 33.8 (CH2), 128.2 (br, phenyl CHs), 130.0 (d, 2JPC =18.5 Hz, thienyl CH), 130.8 (d, 1JPC=98.9 Hz, thienyl C4), 131.0 (thienyl CH), 131.0 (d, 3JPC =11.0 Hz, phenyl CH), 138.0 (phenyl C4), 140.6 (d, 2JPC=2.7 Hz, phenyl C4), 141.1 (br, phenyl C4), 142.1 (d, 4JPC=2.2 Hz, phenyl C4), 142.5 (d, 1JPC=10.1 Hz, phenyl C4), 144.3 (d, 2JPC =9.4 Hz, thienyl C4), 145.5 (d, 3JPC =15.4 Hz, thienyl C4), 147.4 (thienyl C4), 151.3 (thienyl C4), signals from the two carbons linked to boron were not observed. 31P{1H} NMR (CD3OD, 162 MHz): 20.52 Hz; Elemental Analysis calc. 77.01 % C, 7.95 % H. found. 77.37 % C, 7.87 % H. 3 -´Diphenylphosphoryl-5,5 -´diphenyl-2,2 -´bithiophene, (25): Compound 23a (1.08 g, 2.0 120  mmol) was added to a dry round bottom flask under N2, and anhydrous THF (40 mL) added to the flask. The solution was cooled to -78 °C and n-BuLi (1.6 M in hexanes, 1.3 mL, 2.1 mmol) was added drop-wise.  The cold solution was then stirred for 1 hour while maintaining the temperature at -78 °C. The reaction was first quenched with H2O, and H2O2 (10 mL, 30 wt. % in H2O) was then added to oxidize the phosphine. The THF was removed from the resulting mixture on a rotary evaporator. The mixture was then extracted with diethyl ether (4×10 mL) and the organic phase washed with water and brine and then dried over anhydrous MgSO4. The mixture was filtered after which the solvent was removed under vacuum. Recrystallization from methanol gave white needle-like crystals of pure 25. Yield: 840 mg, 88%. 1H NMR (CDCl3, 400 MHz): 0.85-0.89 (m, 6H, 2 CH3 triplets), 1.19-1.37 (m, 4H, 2 CH2), 1.45-1.59 (m, 4H, 2 CH2), 2.43-2.67 (m, 4H, 2 CH2), 6.36 (s, 1H, thienyl CH), 6.37 (s, 1H, thienyl CH), 7.17 (d, 3JPC = 3.6 Hz, thienyl CH), 7.31-7.42 (m, 10H, phenyl CH), 7.63-7.68 (m, 4H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz): 13.8 (CH3), 13.8 (CH3), 21.9 (CH2), 22.1 (CH2), 29.5 (CH2), 29.6 (CH2), 33.6 (CH2), 33.6 (CH2), 124.9 (thienyl CH), 128.2 (d, 3JPC = 12.2 Hz, phenyl CH), 128.5 (d, 1JPC = 105 Hz, phenyl C4), 129.7 (thienyl CH), 130.0 (d, 2JPC = 16.0 Hz, thienyl CH), 131.4 (d, 4JPC = 2.7 Hz, phenyl CH), 131.5 (thienyl C4), 131.6 (d, 2JPC = 9.9 Hz, phenyl CH), 133.4 (d, 1JPC =106 Hz, phenyl C4), 144.4 (d, 2JPC = 10.8 Hz, thienyl C4), 144.9 (d, 3JPC = 15.3 Hz, thienyl C4), 147.8 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 21.37. Elemental Analysis calc. 70.26 % C, 6.53 % H. found. 70.45 % C, 6.43 % H.  4.3.3 DFT Simulations The 2013.01 version of the Amsterdam Density Functional (ADF) program was used for all 121  calculations.134 All electrons were included in the variational treatment (i.e., no frozen-core approximation was applied). C1 (NOSYM) symmetry-unrestricted ground state geometry optimizations were performed with experimental X-ray crystallographic geometries, where available, as starting points. The ground state optimized geometries were used as starting points for the TD-DFT excited state geometry optimizations. Energies, geometries, and the orbital electronic structure were calculated using the generalized gradient approximation (GGA) of density functional theory (DFT) at the BP86 level. The GGA proceeds from the local density approximation (LDA), where exchange is described by Slater’s X potential135 and correlation is treated in the VWN parameterization,136 and is augmented with nonlocal corrections to exchange due to Becke137 and correlation due to Perdew added self-consistently.138 Relativistic effects were taken into account in all cases using the zeroth-order relativistic approximation (ZORA)139 and all-electron TZ2P basis sets from the ADF ZORA basis sets library were used for all atoms.140 The BPE0 hybrid functional141 was used for TD-DFT142 calculations of excitation energies.143 All other parameters were the same as for the BP86 ground state geometry optimizations.  4.4 Conclusions This chapter reports the design and synthesis of flexible Lewis pairs 24a and 24b that undergo a temperature or solvent dependent structural change. 31P and 11B NMR shifts, NOESY effects and P=O stretching frequencies are used to track the structural changes. An unusual reversible system where various interactions between Lewis acidic and basic moieties on a -122  conjugated bithiophene backbone give rise to different light absorbing and luminescence properties is demonstrated. The formation of the open structure is driven by hydrogen bond formation with the solvent, which stabilizes the free P=O bond.          123  Chapter 5 Bithiophene-Based Flexible Lewis Pairs: Scope and Applications  5.1 Introduction In Chapter 4, a bithiophene-based system with tunable interaction between pendant dimesitylboryl group (-BMes2) Lewis acid and phosphoryl group (-P(O)R2, where R = phenyl or isopropyl) Lewis base moieties was described. This system exhibits a dynamic equilibrium of competitive binding of the Lewis base, to either the internal Lewis acid or external HB donors. Such equilibrium is found to be temperature dependent and can be shifted by changing the solvent’s HB donating strength. Scheme 5-1. Work in Chapter 4 and Chapter 5.  In this Chapter, the bithiophene based Lewis acid-base pair system is further explored with 124  different Lewis bases, as well as variations on the bithiophene backbone. Some preliminary results of fluorescence microscopy imaging of these flexible Lewis pairs in hydrophobic/hydrophilic biological environments will also be discussed.  5.1.1 Fluorescence Microscopy and Fluorescent Dyes            Figure 5-1. Schematics of an inverted optical fluorescence microscope. Fluorescence microscope allows scientists to detect microscopic fluorescent entities which may not be visible under a regular light microscope (Figure 5-1). Since its invention in 1910, fluorescence microscopy has become one of the most important and frequently used imaging methods in biology and medicinal applications.144 For instance, in 2008, the Nobel Prize in Chemistry was awarded jointly to Osamu Shimomura, Martin Chalfie and Roger Y. Tsien "for the discovery and development of the green fluorescent protein, GFP".145 And in 2014, the Nobel 125  Prize in Chemistry was given to Erik Betzig, Stefan W. Hell and W. E. Moerner “for the development of super-resolution fluorescence microscopy”,146 solving the long standing resolution issue known as the Abbe diffraction limit147 for a microscope, making subwavelength resolution possible. Modern fluorescence microscopy has truly entered the era of single molecule detection and imaging in the nanometer regime.148 Scheme 5-2. Examples of fluorescent dyes.  Intrinsic fluorescence of biological samples is usually weak and non-specific. Therefore, it is necessary to introduce fluorescent entities which can be used to selectively label targets of interest. Chemists can contribute greatly to this area by developing a wide variety of fluorescent dyes that are available for routine imaging using optical fluorescence microscopes, as well as more detailed measurements using advanced methods such as confocal laser fluorescence microscopy. Even though most fluorescent dyes suffer from photo-bleaching with long irradiation time, they are still preferred in many applications due to advantages such as ease of use, relatively small size compared to fluorescent proteins and the ability to be synthetically tailored for specific purposes. There are many commercialized fluorescent dyes such as the members of the fluorescein, rhodamine and BODIPY families (Scheme 5-2). Functionalities can be added such as the 126  isothiocyanate group in 61 for bio-conjugation and aromatic groups for tuning of HOMO-LOMO energy levels for a desired emission color. A common feature of these dyes is that the core structure of the molecule (Scheme 5-2, highlighted with blue color) only exhibits one type of emission in different chemical environments. However, fluorescent dyes with reversible and stimuli responsive emission changes are scarce. A rare example was reported recently149 where a flexible thiophene-based push-pull “fluorescent flippers” system (Scheme 5-3, left) exhibits shifted charge-transfer fluorescence under stress. The mechanosensitive emission of 64 is due to planarization of the backbone under pressure which gives rise to increased conjugation and decreased HOMO-LUMO energy gap. These molecules are able to indicate tension on membranes.73b,150  Scheme 5-3. “Fluorescent flippers”149 and work in this chapter.  The bithiophene-based flexible Lewis acid-base pair system reported in the last chapter is found to exhibit two emission modes with different photophysical origins in different environments: strong HB donating solvents and non HB-donating solvents give rise to bright 127  yellow and blue emissions of compound 24a, respectively (Scheme 5-3, right). These chemical environments coincide with the hydrophilic (such as in lipid droplets and between lipid bilayers, etc.) and hydrophobic environments (in aqueous medium and ionic channels, etc.) in biological systems. Therefore, it would be interesting to use this type of flexible Lewis pair molecule as a two-color-channel fluorescent indicator for hydrophobic/hydrophilic environments., This would be potentially applicable to important research topics such as lipid metabolism, membrane crossing processes and even the study of diseases such as multiple sclerosis (MS) etc. Other benefits of this system include generally high emission quantum efficiency, high stability (long shelf-life) and generally low cytotoxicity of boron-containing compounds. Other fluorescent dyes that that can provide fluorescence contrast for hydrophobic and hydrophilic environments are not known to date.  5.1.2 Targets The compounds shown in Scheme 5-4 were synthesized. Compound 33 and 34 were made by arranging the Lewis acid and base differently on the bithiophene backbone. Variations of the Lewis base moiety give rise to compounds 24a-d and 35a-c. The color tunability in terms of conjugation length was also systematically explored by comparing 33, 36, 37, 42 and 43, and the electron density on the backbone by comparing compound 37-41. Bi-cationic Lewis pair 46 was made through intermediates 44 and 45, for better water solubility. Preliminary fluorescence imaging was carried out by Xiaozhu (Hunter) Wang using compounds 24a, 41 and 46 on pig adipocytes (fat cells) and human brain slices (provided by Dr. Pat McGeer) as a proof of concept. 128  Scheme 5-4. Lewis pairs discussed in this chapter.    5.2 Results and Discussion 5.2.1 Scope of the Lewis Pair System 5.2.1.1 Synthesis Lewis pair 33 was prepared from 47, and 34 was synthesized from 48 (Scheme 5-5). The synthetic strategy is the same as described in Chapter 4, however diethyl ether was used as the solvent for the synthesis of 33 to avoid deprotonation at the 5 and 5' positions of bithiophene. 129  During purification by flash column chromatography, 34 showed much higher polarity than 33 in the hexanes-THF eluent, indicative of the presence of free phosphine oxide instead of the less polar Lewis adduct. Scheme 5-5. The 3,3'- and 5,5'-substituted Lewis pairs 33 and 34.   Scheme 5-6. Lewis pairs containing different Lewis bases.  Lewis pairs 24a-c were reported in Chapter 4, and compound 24d was made using a similar approach except that chlorodicyclohexylphosphine (Cy2PCl) was used instead of Ph2PCl. Compound 24d has a similar electronic structure to 24c, but is expected to be more congested 130  between the Lewis acid and base centers. Compound 35a was made by introduction of -SMe and -BMes2 groups in a stepwise fashion. Selective oxidation of 35a with one or two equivalent of m-CPBA (3-chloroperbenzoic acid) at 0 °C gave 35b and 35c (Scheme 5-6). Please refer to Section 5.3.2.2 for synthetic details.  5.2.1.2 IR and Photophysics IR resonances of polar X=O groups (X = C, N, S and P, etc.) are considered to be good spectroscopic handles in that they usually give rise to strong absorptions and are less strongly coupled with other IR transitions. Combined with UV-Vis and fluorescence spectra of these compounds, solid state IR spectra of compounds 24a-d, 33, 34, 35a-c were measured to provide insight into the chemical environment of the S=O or P=O groups. A red-shift in IR absorption of the X=O group is expected in the closed structure due to decreased bond strength when the group is bond to a Lewis acid center, while the open structure with 3-coordinated boron should give rise to charge transfer emission with larger Stokes shifts.  For bithiophene-based Lewis pairs with phosphine oxides as the Lewis base component, adducts can only be effectively formed intramolecularly: 33 and 34 show IR absorption at 1199 cm-1 and 1117 cm-1 in the solid state, respectively, indicative of Lewis adduct and free phosphine oxide. As discussed in Chapter 4, compound 24a also shows a strong resonance at 1116 cm-1 consistent with a Lewis adduct. Similarly, 24c and 24d do not show IR absorption expected of free alkyl phosphine oxides consistent with the presence of a Lewis adduct in the solid state. However, 24b shows strong absorption at 1185 cm-1 indicative of free phosphine oxide. The closed structure of 24b containing a Lewis adduct cannot be formed due to bulky mesityl groups on both Lewis 131  acid and base centers. Concentration dependent (0.5 × 10-5 M to 1 × 10-4 M) UV-Vis spectra of 33 are shown in Figure 5-2, in both methanol and THF solution, with no indication of intermolecular Lewis adduct formation. This confirms that the Lewis adduct cannot be effectively formed intermolecularly in solution at these concentrations.  (a)       (b)  Figure 5-2. UV-Vis spectra of 33 at various concentrations in (a) MeOH and (b) THF. (a)      (b)  Figure 5-3. (a) Normalized excitation (em = 520 nm) and emission (ex = 320 nm) spectra of 33 in MeOH. (b) Normalized excitation ((em = 420 nm) and emission (ex = 350 nm) spectra of 33 THF. 132     Figure 5-4. (a) Normalized excitation (em = 520 nm) and emission (ex = 320 nm) spectra of 24a-d in MeOH. (b) Normalized excitation (em = individual maxima) and emission (ex = 350 nm) spectra of 24a-d in THF. Excitation and emission spectra of 33 in THF and MeOH are shown in Figure 5-3. Similar to compound 24a discussed in Chapter 4, 33 shows a much larger Stokes shift in MeOH than in THF consistent with a mostly open structure with 3-coordinated boron.  Excitation and emission spectra of Lewis pairs 24a-d with different phosphine oxide Lewis bases are shown in Figure 5-4. Only 24b remained open regardless of the solvent with CT emission maxima at 530 nm due to steric hindrance. By comparison, in methanol, 24c and 24d with dialkyl phosphine oxide moieties show significant amount of emission from the closed structure near 425 nm due to incomplete opening of Lewis adducts. Compound 24a shows only a very small amount of emission from the closed form (emits at 450 nm in THF) in MeOH. This can be attributed to more polarized P=O bonds in the dialkyl phosphine oxide group compared to in the diaryl ones, resulting in stronger B-O bonds that are harder for methanol molecules to disrupt. Even though HB between solvent and P=O bonds may also be stronger in these cases, the overall effect still favors the closed structure. Compared to 24c with the (i-Pr)2P(O)- group, 24d with 133  Cy2P(O)- group shows slightly less emission from the closed form in methanol. It is suggestive of a less favorable closed form of 24d due to greater steric bulk of the Lewis base, considering they have similar emission quantum yields in both solvents (Table 5-1).       Figure 5-5. UV-Vis spectra of 35b and 35c in methanol and THF (left). Excitation and emission spectra of 35b and 35c in methanol and THF (right). With Lewis pairs containing S=O groups as the Lewis base, 35b shows a typical strong S=O stretching resonance of a free sulfoxide group at 1048 cm-1, and 35c shows both symmetric and asymmetric stretching peaks of free S=O bonds at 1138 cm-1 and 1311 cm-1, respectively. These findings indicate no significant binding of sulfoxide or sulfone groups to the Lewis acidic dimesitylboryl group in the solid state. UV-Vis spectra of 35b and 35c (Figure 5-5, left) are almost identical in both solvents, showing only slightly weaker low energy absorbance near 370 nm in MeOH. Even though 35c emits at slightly higher wavelength than 35b (Figure 5-5, right), these compounds show similar excitation and emission features in different solvents, with large Stokes shifts typical for open structures. Therefore, it can be concluded that S=O bonds in sulfoxides or 134  sulfones do not form closed Lewis adducts with the dimesitylboryl Lewis acid center. This may be attributed to a weaker polarization of the S=O bond compared to the P=O bond, making the partially negatively charged oxygen atom a less potent Lewis base to form a Lewis adduct with the -BMes2 group.  5.2.2 Emission Color Tunability  5.2.2.1 Synthesis Two series of Lewis pairs were synthesized (a) Series A with different backbone conjugation lengths as shown in Scheme 5-7 and (b) Series B with different electron donating or electron withdrawing groups as shown in Scheme 5-8. The purpose of introducing various substitutions was to access more emission colors of the Lewis pair in both open and closed structures, so that the system has wider scope in applications such as fluorescence imaging. Scheme 5-7. Variations of the conjugation lengths of the bithiophene backbone (Series A).  135  Scheme 5-8. Electron-donating and withdrawing groups on the backbone (Series B).  Scheme 5-9. General synthesis of flexible phosphine oxide-borane Lewis pairs.  Scheme 5-10. Preparation of water-soluble flexible Lewis pair 20.  Synthesis of Lewis pairs 36-44 was successfully carried out following a general one-pot method as shown in Scheme 5-9, from corresponding dibromo-starting materials (please refer to 136  Section 5.6.2.2 for details). Compound 45 with reactive alkyiodide groups was synthesized by heating 44 to reflux with excess sodium iodide in acetone for two days (Scheme 4-10). Compound 46 was conveniently obtained by treating 45 with trimethylamine. 46 have the same fluorophore as 33, 44 and 45, however, it is specifically designed for higher water solubility while 33, 44 and 45 is only soluble in organic solvents.  5.2.2.2 Electronic Absorption and Emission Spectra    Figure 5-6. UV-Vis spectra of 1.0  10-5 M solutions of Series A compounds (a) in MeOH with open structures and (b) in THF with closed structures. UV-Vis spectra of the Series A compounds (33, 36, 37, 42 and 43) in MeOH are shown in Figure 5-6a. All compounds show weaker CT absorption at higher wavelengths relative to their high energy  absorptions. This shoulder-like feature which is typical for oligothiophene derivatives with pendant 3-coordinate -BMes2 groups,46 red-shifts from 33 (~350 nm) to 36 (~370 nm), 37 (~390 nm) to 43 (~420 nm). This is consistent with increasing backbone conjugation since 36 with 137  the mesityl group out of the plane is less conjugated than 37, and the absorbance of 43 is red-shifted relative to 37 because thiophene rings offer more effective conjugation than phenyl rings in this context.151 The CT absorption of 42 is only slightly lower in energy than that of 37, indicating that alkynyl groups do not contribute significantly to the increase in effective conjugation length. However, the molar absorption coefficient of 42 is much higher than 37 as a result of the presence of two more bridging alkynyl groups. UV-Vis spectra of Series A in THF are shown in Figure 5-6b. The lowest energy bands of the Series A molecules in THF show much stronger absorption and red-shifted compared to corresponding open structures observed in MeOH. This feature is attributed to predominantly * absorptions of the more planar conjugated backbone in the closed Lewis adduct in THF. The same trend is seen here, molecules with longer conjugation length shows more red-shifted absorptions.     Figure 5-7. Normalized excitation (em = individual emission maxima) and emission spectra (ex = individual excitation maxima) of 1.0  10-5 M solutions of the Series A compounds (a) in MeOH with open structures and (b) in THF with closed structures. 138  Electronic excitation and emission spectra of the Series A compounds in MeOH and THF are shown in Figure 5-7a and Figure 5-7b, respectively. They all show large Stokes shifts in MeOH which is in line with energy loss in CT processes of the open structures. However, in THF where the closed structure is dominant, a much smaller Stokes shift is observed due to less energy loss between absorption and emission in the * transitions. The emission colors of the Series A compounds range from blue to orange in the open structures and violet to green in the closed ones. (a)      (b)  Figure 5-8. UV-Vis spectra of 1.0  10-5 M solutions of Series B compounds (a) in MeOH with open structures and (b) in THF with closed structures.  The UV-Vis spectra of the Series B compounds (37-41) are shown in Figure 5-8. In MeOH, all these compounds show a low energy weak CT absorption shoulder typical for open structures. This feature red-shifts from 37 to 41 with relatively consistent intensity. This is consistent with increasing electron density in the -conjugated backbone resulting in a lower energy requirement for charge transfer to the electron deficient boron center. The absorptions of the closed forms of Series B in THF (Figure 5-8, right) show a similar trend where the more electron-rich substitution on the backbone gives rise to a lower energy absorption, except in 38 with -CF3 groups. In both 139  solvents, the absorption energy differences between the Series B compounds are much smaller compared to that of the Series A compounds. The only exception is compound 41 with a strongly electron donating -NMe2 group which behaves very differently from 37-40. In fact, 41 also shows a strong effect of added conjugation as seen in Series A in that the -NMe2 group is coplanar with the backbone with a notable contribution to its conjugation length, consistent with DFT calculated frontier molecular orbitals (entry 41, Table 5-2 and Table 5-3).     Figure 5-9. Normalized excitation and emission spectra of 1.0  10-5 M solutions of the Series B compounds (a) in MeOH with open structures and (b) in THF with closed structures. Electronic excitation and emission spectra of the Series B compounds in MeOH and THF are shown in Figure 5-9. Again, compound 41 shows a significant red-shift from 37-40, due to the reasons discussed above. The emission colors of Series B range from green to red in the open structures and cyan to yellow in the closed ones. Emission of 41 in methanol shows a shoulder feature at around 540 nm possibly due to a small amount of closed structures which is much more 140  emissive than the open ones (41 shows a much higher emission quantum efficiency in THF than in MeOH, see Table 5-1). It is also possible that the electron-rich compound gets oxidized by oxygen under excitation and forms highly emissive impurities. Table 5-1. Summary of low energy absorption maxima (abs), emission maxima (em) and emission quantum yields (em) of 33 and 36-43 in MeOH and THF. Compound Open Structure (in MeOH) Closed Structure (in THF) abs (nm) em (nm) em abs (nm) em (nm) em 33 350* 512 0.52 345 429 0.23 36 370* 536 0.84 366 472 0.55 37 392 558 0.75 402 488 0.58 38 389 548 0.41 405 468 0.089 39 396 572 0.54 409 498 0.16 40 402 586 0.55 415 513 0.52 41 435 645 0.18 445 552 0.49 42 400 560 0.61 417 498 0.063 43 418 605 0.17 431 536 0.060 * Maxima wavelength of some shoulder-like features are estimated. The electronic properties of flexible Lewis pairs in Series A and Series B are summarized in Table 5-1. Interestingly for the open and closed structures of the same compound, there is almost no overlap between the absorption of one structure and the emission spectra of the other structure. This effectively prevents energy transfer between adjacent closed and open structures in the case of an equilibrium, so that the existence of one structure does not decrease/quench the emission of the other. This can be important for imaging applications where both forms are present and may allow quantification of the open/closed ratio via measurement of emission intensities.  141  5.2.3 Fluorescence Imaging Initial trials of staining pig adipocytes (fat cells) with culture medium were performed by Hunter (Xiaozhu) Wang in the Orvig research group at UBC. Pig adipocytes were selected because their large size (~100 m) and the hydrophobic environment in their large internal lipid droplets (Figure 5-10a). Three molecules (Scheme 5-10) were used as dyes, compounds 24a and 46 have the same fluorophore and were measured to show the same emission pattern of yellow/blue colors in hydrophilic/hydrophobic environments. By contrast, compound 41 with a longer conjugation length and electron donating groups show red-shifted emission in hydrophilic/hydrophobic media (orange/green). These experiments were carried out as proof-of-concept of the application of the reversible two-color fluorescent dye for hydrophobic/hydrophilic environments.  Scheme 5-11. Molecules used for fluorescence imaging trials.  The sample slides were immersed in the dye-containing PBS buffer (phosphate-buffered saline) with 1 vol % DMSO and 10g/mL dyes for one hour, and was then washed extensively 142  with fresh buffer to remove any dye that wasn’t absorbed by the sample. The sample was then placed under UV (385 nm) excitation and observed using an inverted fluorescence microscope (Figure 5-1). The emission was filtered by long pass or band-pass lenses to cut off undesired light.  (a)     (b)  Figure 5-10. (a) Schematic of adipocytes and (b) blue emission of molecule 24a inside the lipid droplets of adipocytes (scale bar = 100 m, 485 nm long pass filter was used). (a)     (b)  Figure 5-11. (a) Yellow-greenish emission of molecule 46 in the membrane of adipocytes (scale bar 100 m, 520 nm long pass filter was used) and (b) blue emission on the cell membrane (scale bar 100 m, 410-460 band-pass filter was used). Preliminary results showed that 24a preferentially stayed in the lipid droplets (Figure 5-10) 143  due to its low water solubility while 46 was only attracted to the peripheral of the cells (Figure 5-11) showing both blue and yellow emission. This can be explained by part of the 46 molecules inserting into the hydrophobic lipid bilayer in a similar fashion to some membrane probes,152 while the rest of the molecule dissolved in the very thin layer of water medium around the lipid droplet (the cytoplasm). It is also possible that part of the molecules of 46 are attracted to either/both sides of the cell membrane by the slightly negatively charged membrane surface. (a)   (b)  Figure 5-12. (a) Yellow emission of molecule 46 in the periphery and blue emission of 24a in the lipid droplet of adipocytes (scale bar = 100 m). (b) Yellow emission of molecule 46 in the periphery and yellow emission of 41 in the lipid droplet of adipocytes (scale bar = 100 m). A 485 nm long pass filter was used in both cases. The different distribution of 24a and 46 in adipocytes allows us to use a mixture of the two molecules to simultaneously light up both environments (Figure 5-12a). Similarly, 41 can also be used in combination with 46 (Figure 5-11, right) in that 41 is also much more soluble in lipid than in water. The green emission color inside the adipocytes comes from closed 41 while the peripheral green color is attributed mostly to the open 46 structures. An advantage of these dyes is that the 144  distribution and color can be tuned separately by changing the side chain and fluorophore, respectively. Moreover, many more functionalities such as targeting vectors can be easily introduced by nucleophilic substitution of the SN2 reactive iodine atoms on 45 (Scheme 5-10). (a)  (b)  Figure 5-13. (a) Schematics of neurons and their structures.153 (b) Parallel nerve fibers can be seen in dashed box with stronger emission from dye 46 (scale bar 100 m). These results suggested it would be useful to look for other important biological structures with rich lipid content or membranes. Neural system samples such as those from the brain are of interest since they contain numerus neurons with rich lipid contents and complicated intra-cellular and inter-cellular membrane structures. For instance, the insulating myelin sheath of axons contains many layers of membranes that help with the fast propagation of out-going neural signals. Some diseases such as multiple sclerosis (MS) and Guillain-Barré syndrome are related to malfunctioning or degrading myelin sheath structures. It may be interesting to apply the dyes investigated here for fluorescence imaging membrane structures in brain samples such as myelin sheath, since myelin sheathes is conventionally imaged under TEM spectroscope using heavy metal stained samples.154  145  Staining of human brain slices was performed by Xiaozhu (Hunter) Wang using bi-cationic dye 46 and dye 41 following similar protocols to those described above. The fluorescence image of nerve fibers parallel to axons (Figure 5-13, right) shows moderate contrast in that only emission from the outside membrane is detected. By comparison, fluorescence images of cross sections of axons are much more illustrative. The images obtained using dye 46 (Figure 5-14) shows myelin sheath structures as bright circles since this dye is attracted to the negative charges on membranes. There is more blue emission from the background compared to areas inside the circular myelin sheath structures where emission is visibly more yellow. This is due to richer lipid content outside axons and thus more emission is observed from the closed Lewis pair structure. However, inside axons is a more aqueous environment which give rise to the open structure that emits yellow.  (a)   (b)  Figure 5-14. Fluorescent images of brain slices showing cross-section of axons using 46. Scale bars are (a)100 m and (b) 20 m. The images obtained using dye 41 (Figure 5-15) show stronger green emission from the lipid-rich matter surrounding the axons due to the much higher emission quantum yield of the closed form compared to the open form (Table 5-1). The open structure emission inside the axons 146  is much weaker by comparison. However, 41 does clearly outline the multiple layer of membranes in the myelin sheath (Figure 5-15b). (a)   (b)  Figure 5-15. Fluorescent images of brain slices showing cross-section of axons using 41. Scale bars are (a)100 m and (b) 20 m.  5.2.4 DFT Simulations The purposes of DFT calculations are to have insight into the different emission colors of the flexible Lewis pairs with different substitutions and to understand the unusual photophysical trends of the Series B compounds. These include red-shifted excitation and emission features of compound 41 in THF and MeOH, as well as slightly red-shifted low-energy absorption of 38 in THF compared to 37.  DFT calculations were carried out to simulate GS structure of Lewis pairs 37-41 in Series B. For all simulations, the solvation effect of either methanol or THF was implemented using the polarizable continuum model (PCM) solvation method. DFT calculated frontier orbitals of the optimized GS structures of 37-41 are summarized in Table 5-2 (open structure in MeOH) and 147  Table 5-3 (closed structure in THF). The HOMO and LUMO energy levels of the compounds in Series B are summarized in Table 5-4 for qualitative comparisons. TD-DFT simulated low-energy absorption transitions of the closed and open structures of Series B are presented in Table 5-5. Table 5-2. HOMO-1, HOMO and LUMO of the open structure of the compounds in Series B. Entry Molecular Orbitals HOMO-1 HOMO LUMO 37    38    39    40    41    * The colors for atoms are C (grey), O (red), S (yellow), P (orange), B (pink) and N(blue).  148  Table 5-3. HOMO and LUMO of the closed structure of the compounds in Series B. Entry Molecular Orbitals HOMO LUMO 37   38   39   40   41   * In the ball-and-stick models the colors for atoms are C (grey), O (red), S (yellow), P (orange), B (pink) and N(blue). Hydrogens are omitted. As for the open structures modeled with MeOH solvation, the LUMOs of 37, 39-41 are heavily boron-centered, while the LUMO of 38 also receives a strong contribution from the  orbital of the relatively electron poor backbone (Table 5-2, LUMO entry 38) due to the presence of the electron withdrawing -CF3 groups. This leads to a relatively consistent LUMO energy levels 149  of 37, 39 to 40 and 41, while the LUMO of 38 is slightly higher in energy (Table 5-4, left). The HOMO energies of the compounds in Series B consistently increases with more electron density on the backbone in the order of 38 < 37 < 39 < 40 < 41. Therefore, the HOMO-LUMO energy gap (EOPEN) in methanol gets larger in the same order as the result of more electron donating groups.  Frontier orbitals of the closed structures the compounds in Series B modeled with THF solvation show very consistent  character. Their HOMO-LUMO energy gaps also do not show significant differences from one another except for compound 41 (Table 5-4, right), indicating that the electron donating and withdrawing groups of the closed structure have similar effects on the orbitalsand  orbitals (LUMO). Compound 41 shows different behavior since there are some degree of charge transfer from HOMO to LUMO, and the sp2 nitrogen atoms also contribute significantly to extended conjugation of the backbone.  Table 5-4. HOMO and LUMO energies and HOMO-LUMO gaps (EHOMO, ELUMO and E, respectively, in eV) of the closed and open structures of the compounds in Series B. Compound Open Structure (in MeOH) Closed Structure (in THF) EHOMO ELUMO EOPEN EHOMO ELUMO ECLOSED 37 -6.05 -1.87 4.17 -5.75 -2.13 3.62 38 -6.20 -2.01 4.19 -5.55 -1.87 3.67 39 -5.94 -1.86 4.08 -5.46 -1.83 3.63 40 -5.71 -1.84 3.87 -5.30 -1.75 3.55 41 -5.15 -1.76 3.39 -4.86 -1.61 3.25  150  Table 5-5. TD-DFT calculated low-energy absorption transitions of the closed and open structures of the compounds in Series B. Compound Open Structure (in MeOH) Closed Structure (in THF) Wavelength Major Transitions Oscillator Strength Wavelength Transitions Oscillator Strength 37 372 nm  HOMO-LUMO 0.089 410 nm HOMO-LUMO 0.89 345 nm  HOMO-1-LUMO 0.32 38 372 nm  HOMO-LUMO 0.072 415 nm HOMO-LUMO 0.93 347 nm  HOMO-1-LUMO 0.36 39 379 nm  HOMO-LUMO 0.16 416 nm HOMO-LUMO 1.05 348 nm  HOMO-1-LUMO 0.32 40 396 nm  HOMO-LUMO 0.26 425 nm HOMO-LUMO 1.00 354 nm  HOMO-1-LUMO 0.15 41 446 nm  HOMO-LUMO 0.28 466 nm HOMO-LUMO 1.17 347 nm  HOMO-1-LUMO 0.22   5.3 Experimental Section 5.3.1 General 1H NMR, 13C{1H} NMR, and 31P{1H} NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectromete. Residual protio-solvent peaks were used in the calibration of the 1H NMR spectra. Peaks of the deuterated solvents were used to calibrate the 13C{1H} NMR spectra. All 31P{1H} NMR spectra were referenced to 85% H3PO4 as an external standard. Absorption spectra were obtained on a Varian Cary 5000 UV-Vis-NIR spectrophotometer, and emission 151  measurements were performed on a PTI QuantaMaster 50 fluorimeter. Infrared spectroscopy (IR) was performed on an attenuated total reflection (ATR) crystal using a Perkin-Elmer Frontier FTIR spectrometer. All syntheses involving air sensitive compounds were carried out using standard Schlenk-type procedures under an atmosphere of N2. Anhydrous tetrahydrofuran (THF) and diethyl ether (Et2O) was obtained using solvent purification system (SPS). Chlorodiphenylphosphine (ClPPh2, 98 %), chlorodicyclohexylphosphine (Cy2PCl, 97 %), dimethyl disulfide (CH3SSCH3, 99 %), 5-bromo-2-hexylthiophene (97 %), 3-chloroperbenzoic acid (m-CPBA, ≤ 77 %), hydrogen peroxide solution (H2O2, 30 wt. % in H2O), NMe3 solution (~45 wt.% in H2O), phenylboronic acid (95 %), 4-trifluoromethyl-phenylboronic acid (95 %), 4-tert-butylphenylboronic acid (95 %), 4-methoxyphenylboronic acid (95 %), 4-(dimethylamino)phenylboronic acid (95 %) were purchased from Sigma-Aldrich. Dimesitylfluoroborane (Mes2BF, 98%) was ordered from TCI America. All purchased chemicals were used without further purification.   5.3.2 Synthesis The phosphine oxide-borane Lewis pairs were synthesized using a general method described in Scheme 5-9 from corresponding substituted dibromo precursors (Scheme 5-12 in parentheses) which were made from either 3,3',5,5'-tetrabromo-2,2'-bithiophene (33)155 or 3,3'-dibromo-5,5'-diiodo-2,2'-bithiophene (34)156 via Suzuki, Sonogashira or Negishi cross-coupling reactions (Scheme 5-13 and Scheme 5-14). 22133, 47157, 48158, 54159 are known in literature. The sulfur-borane Lewis pair 35a-c were synthesized according method described in Scheme 5-6, from 152  an intermediate 49 which was made from 22. Scheme 5-12. Phosphine oxide-borane Lewis pairs and their precursors in parentheses.  5.3.2.1 Synthesis of Lewis Pairs 3'-Dimesitylboryl-3-diphenylphosphinyl-2,2'-bithiophene (33): 47 (745 mg, 2.30 mmol) was dissolved in 40 mL of dry diethyl ether and cooled to -78 °C. n-BuLi (1.6M in hexanes, 1.58 mL, 2.53 mmol) was added and the mixture was stirred for 1 hour at the same temperature. Ph2PCl (560 mg, 2.53 mmol) was then added and the mixture was slowly warmed up to room temperature and stirred for another 1 hour. The mixture was cooled to -78 °C again and n-BuLi (1.6M in 153  hexanes, 1.73 mL, 2.76 mmol) was added followed by one hour of stirring. Solid Mes2BF (800 mg, 3.04 mmol) was directly added under a high N2 flow, and the reaction was stirred for 1 hour before it was warmed up to room temperature and stirred for an additional 18 hours. The reaction was then quenched by 5 mL H2O and an excess amount of H2O2 (1 mL, 30 wt. % in H2O) was added. The mixture was stirred for 4 more hours and then extracted with diethyl ether (4×10 mL). The organic phases were combined and washed with water and brine, before drying over anhydrous MgSO4. The mixture was filtered and the solvent was removed from the filtrate under vacuum. Purification by flash column chromatography (hexanes: THF = 5:1) gave a white solid. Yield: 1.06 g, 76 %. 1H NMR (CDCl3, 400 MHz): 2.04 (s, 12H, CH3), 2.23 (s, 6H, CH3), 6.49 (dd, 3JHH = 5.3 Hz, 3JPH = 3.7 Hz, 1H, thienyl CH), 6.61 (s, 4H, phenyl CH), 6.82 (d, 3JHH = 5.0 Hz, 1H, thienyl CH), 7.05 (dd, 3JHH = 5.4 Hz, 4JPH = 2.4 Hz, 1H, thienyl CH), 7.18 (d, 3JHH = 5.0 Hz, 1H, thienyl CH), 7.39-7.55 (m, 10H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.1 (CH3), 24.0 (CH3), 123.9 (d, 1JPC = 105.5 Hz, thienyl C4), 125.1 (d, 2JPC = 16.4 Hz, thienyl CH), 126.4 (thienyl CH), 128.4 (d, 3JPC = 13.6 Hz, phenyl CH), 128.5 (phenyl CH), 131.9 (d, 3JPC = 17.8 Hz, thienyl CH), 132.0 (d, 4JPC = 2.8 Hz, phenyl CH), 132.0 (d, 1JPC = 109.3 Hz, thienyl C4), 132.2 (d, 2JPC = 10.5 Hz, phenyl CH), 136.0 (thienyl CH), 136.5 (phenyl C4), 138.5 (d, 3JPC = 2.2 Hz, thienyl C4), 141.3 (phenyl C4), 144.7 (phenyl C4), 150.5 (d, 3JPC = 10.4 Hz, thienyl C4), 156.1 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): ESI-TOF: m/Z [M+H]+ 614.2153 calc. for C38H37OPS210B, found: 614.2143. 5'-Dimesitylboryl-5-diphenylphosphinyl-2,2'-bithiophene (34): A similar procedure as for 1 was followed using 48 (650 mg, 2.00 mmol) as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=2:1) gave an off-white solid. Yield: 154  882 mg, 72 %. 1H NMR (CDCl3, 400 MHz):  2.14 (s, 12H, CH3), 2.31 (s, 6H, CH3), 6.84 (s, 4H, phenyl CH), 7.31 (dd, 3JHH = 3.7 Hz, 4JPH = 1.8 Hz, 1H, thienyl CH), 7.35 (m, 2H, thienyl CH), 7.40 (dd, 3JHH = 3.8 Hz, 3JPH = 7.5 Hz, 1H, thienyl CH), 7.47-7.51 (m, 4H, phenyl CH), 7.55-7.57 (m, 2H, phenyl CH), 7.73-7.79 (m, 4H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.4 (CH3), 23.1 (CH3), 125.8 (d, 3JPC = 12.7 Hz, thienyl CH), 127.3 (thienyl CH), 128.4 (thienyl CH), 128.8 (d, 3JPC = 12.6 Hz, thienyl CH), 132.0 (d, 2JPC = 10.3 Hz, thienyl CH), 132.5 (d, 4JPC = 2.7 Hz, thienyl CH), 132.7 (d, 1JPC = 109.4 Hz, thienyl C4), 133.6 (d, 1JPC = 109.6 Hz, thienyl C4), 137.9 (d, 2JPC = 8.7 Hz, thienyl C4), 139.0 (phenyl C4), 141.0 (phenyl C4), 141.0 (br, phenyl C4), 141.4 (thienyl CH), 145.9 (d, 3JPC = 5.3 Hz, thienyl C4),  147.9 (thienyl C4), 150.8 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 21.74. ESI-TOF: m/Z [M+H]+ 615.2117 calcd. for C38H37OPS211B, found: 615.2127. 5,5'-Dimesityl-3'-dimesitylboryl-3-diphenylphosphinyl-2,2'-bithiophene (36): A similar as method for 33 was followed except 50 (1.12 g, 2.00 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=5:1) gave a light-yellow solid. Yield 1.14 g, 67 %. 1H NMR (CD2Cl2, 400 MHz):  2.05 (s, 6H, 2CH3), 2.09 (s, 12H, 4CH3), 2.17 (s, 6H, 2CH3), 2.24 (s, 6H, 2CH3), 2.29 (s, 6H, 2 overlapped CH3), 6.27 (d, 3JPH = 4.0 Hz, 1H, thienyl CH), 6.44 (s, 1H, thienyl CH), 6.64 (s, 4H, phenyl CH), 6.90 (2H, phenyl CH), 6.91 (2H, phenyl CH), 7.43-7.45 (m, 4H, phenyl CH), 7.52-7.56 (m, 6H, phenyl CH). 13C{1H} NMR (CD2Cl2, 100 MHz):  20.9 (CH3), 21.0 (CH3), 21.2 (2 overlapped CH3), 21.2 (CH3), 24.2 (CH3), 124.3 (d, 1JPC = 103.3 Hz, thienyl C4), 128.3 (phenyl CH), 128.5 (phenyl CH), 128.7 (d, 3JPC = 12.6 Hz, phenyl CH), 129.0 (phenyl CH), 129.4 (phenyl C4), 131.1 (phenyl C4), 132.0 (d, 2JPC = 17.1 Hz, thienyl CH), 132.4 (d, 4JPC = 2.5 Hz, phenyl CH), 132.5 (d, 2JPC = 10.6 Hz, phenyl 155  CH), 132.5 (d, 1JPC = 109.2 Hz, phenyl C4), 136.4 (thienyl CH), 136.8 (phenyl C4), 138.1 (phenyl C4), 138.2 (phenyl C4), 138.4 (phenyl C4), 138.9 (phenyl C4), 138.9 (thienyl C4), 141.7 (d, 3JPC = 16.1 Hz, thienyl C4), 141.8 (phenyl C4), 142.8 (thienyl C4), 145.4 (phenyl C4), 150.1 (d, 2JPC = 9.7 Hz, thienyl C4), 157.4 (thienyl C4). 31P{1H} NMR (CD2Cl2, 162 MHz): 30.2. ESI-TOF: m/Z [M+H]+ 850.3718 calcd. for C56H57OPS210B, found: 850.3723. 5,5'-Dibutyl-3-dicyclohexylphosphinyl-3'-dimesitylboryl-2,2'-bithiophene (24d): A similar method as for 33 was followed except 22 (870 mg, 2.00 mmol) was used as starting material. Also THF was used as the solvent and chlorodicyclohexylphosphine was used instead of chlorodiphenylphosphine. Purification by flash column chromatography (hexanes: THF=4:1) gave a yellow powder. Yield 1.20 g, 81 %. 1H NMR (CDCl3, 400 MHz):  0.88-0.95 (m, 6H, 2CH3), 1.21-1.76 (m, 30H, 14CH2 and 2CH), 2.05 (s, 12H, 4CH3), 2.20 (s, 6H, 2CH3), 2.66-2.72 (m, 4H, 2CH2), 6.30 (d, 3JPH = 2.4 Hz, 1H, thienyl CH), 6.44 (s, 1H, thienyl CH), 6.62 (s, 4H, phenyl CH). 13C NMR (CD2Cl2, 100 MHz): 13.9 (CH3), 13.9 (CH3), 21.1 (CH3), 21.9 (CH2), 22.2 (CH2), 24.1 (CH3), 25.6 (d, 3JPC = 3.1 Hz, CH2), 25.6 (CH2), 25.9 (CH2), 26.5 (d, 2JPC = 12.8 Hz, CH2), 26.7 (d, 2JPC = 13.5 Hz, CH2), 29.4 (CH2), 29.6 (CH2), 33.5 (CH2), 34.0 (CH2), 37.3 (d, 1JPC = 68.8 Hz, CH2), 121.4 (d, 1JPC = 111.3 Hz, thienyl C4), 125.7 (d, 2JPC = 13.5 Hz, thienyl CH), 128.5 (phenyl CHs), 132.9 (thienyl CH), 136.2 (phenyl C4), 136.2 (thienyl C4), 141.1 (mesityl C4), 145.3 (d, 2JPC =17.6 Hz, thienyl C4), 145.5 (phenyl C4), 146.5 (thienyl C4), 148.7 (phenyl C4), 154.8 (thienyl C4). 31P NMR (CDCl3, 162 MHz49.1. ESI-TOF: m/Z [M+H]+ 739.4308 calcd. for C46H65OPS211B, found: 739.4323. 3'-Dimesitylboryl-3-diphenylphosphinyl-5,5'-diphenyl-2,2'-bithiophene (37): A similar method as for 33 was followed except 51 (943 mg, 2.00 mmol) was used as starting material and 156  THF as the solvent. Purification by flash column chromatography (hexanes: THF=4:1) gave a yellow solid. Yield 859 mg, 56 %. 1H NMR (CDCl3, 400 MHz):  2.10 (s, 12H, CH3), 2.27 (s, 6H, CH3), 6.63 (s, 4H, phenyl CH), 6.74 (d, 3JPH = 4.4 Hz, 1H, thienyl CH), 7.10 (s, 1H, thienyl CH), 7.25-7.58 (m, 20H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.1 (CH3), 24.4 (CH3), 123.7 (d, 1JPC = 104.6 Hz, thienyl C4), 125.9 (phenyl CH), 126.2 (phenyl CH), 127.0 (d, 2JPC = 17.8 Hz, thienyl CH), 127.5 (phenyl CH), 128.3 (phenyl CH), 128.5 (d, 3JPC = 12.8 Hz, phenyl CH), 128.7 (phenyl CH), 128.8 (phenyl CH), 129.1 (phenyl CH), 131.1 (d, 1JPC = 110.6 Hz, phenyl C4), 132.3 (d, 4JPC = 2.5 Hz, phenyl CH), 132.5 (d, 2JPC = 10.6 Hz, phenyl CH), 132.7 (thienyl CH), 132.9 (phenyl C4), 134.2 (phenyl C4), 136.1 (phenyl C4), 137.0 (thienyl C4), 141.6 (phenyl C4), 143.1 (d, 3JPC = 16.0 Hz, thienyl C4), 144.9 (thienyl C4), 145.4 (phenyl C4), 150.4 (d, 2JPC = 10.0 Hz, thienyl C4), 159.7 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 30.2. ESI-TOF: m/Z [M+Na]+ 789.2562 calcd. for C50H44OPS211BNa, found: 789.2573. 3'-Dimesitylboryl-3-diphenylphosphinyl-5,5'-bis(4-trifluoromethylphenyl)-2,2'-bithiophene (38): A similar method as for 33 was followed except 52 (1.22 g, 2.00 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=5:1) gave a light-yellow solid. Yield 1.04 g, 58 %.1H NMR (CDCl3, 400 MHz):  2.02 (s, 12H, CH3), 2.19 (s, 6H, CH3), 6.57 (s, 4H, phenyl CH), 6.74 (d, 3JPH = 4.4 Hz, 1H, thienyl CH), 7.10 (s, 1H, thienyl CH), 7.44-7.60 (m, 18H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.1 (CH3), 24.4 (CH3), 124.0 (q, 1JFC = 270.3 Hz, C4 on -CF3), 124.3 (q, 1JFC = 270.0 Hz, C4 on -CF3), 125.0 (d, 1JPC = 104.7 Hz, thienyl C4), 125.0 (q, 3JFC = 3.9 Hz, phenyl CH), 125.7 (q, 3JFC = 3.7 Hz, phenyl CH), 126.0 (phenyl CH), 126.3 (phenyl CH), 128.4 (d, 2JPC = 17.6 Hz, thienyl CH), 128.7 (d, 3JPC = 12.7 Hz, phenyl CH), 128.9 (phenyl CH), 129.2 (q, 2JFC = 32.5 Hz, phenyl 157  CH), 130.1 (q, 2JFC = 32.6 Hz, phenyl CH), 130.8 (d, 1JPC = 110.6 Hz, phenyl C4), 132.4 (d, 2JPC = 10.8 Hz, phenyl CH), 132.6 (d, 4JPC = 2.7 Hz, phenyl CH), 134.1 (thienyl CH), 136.2 (phenyl C4), 136.5 (phenyl C4), 137.4 (phenyl C4), 138.0 (d, 3JPC = 2.3 Hz, thienyl C4), 141.6 (phenyl C4), 141.6 (d, 3JPC = 15.7 Hz, thienyl C4), 143.3 (thienyl C4), 145.0 (phenyl C4), 150.8 (d, 2JPC = 9.5 Hz, thienyl C4), 160.1 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 30.5. 19F{1H} NMR (CDCl3, 376 MHz): -62.2, -62.3. ESI-TOF: m/Z [M+Na]+ 924.2346 calcd. for C52H42OF6PS210BNa, found: 924.2341. 3'-Dimesitylboryl-3-diphenylphosphinyl-5,5'-bis(4-tert-butylphenyl)-2,2'-bithiophene (39): A similar method as for 33 was followed except 53 (950 mg, 2.00 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=5:1) gave a light-yellow solid. Yield 894 mg, 51 %. 1H NMR (CDCl3, 400 MHz):  1.32 (s, 9H, CH3), 1.34 (s, 9H, CH3), 2.03(s, 12H, CH3), 2.22 (s, 6H, CH3), 6.57 (s, 4H, phenyl CH), 6.65 (d, 3JPH = 4.4 Hz, 1H, thienyl CH), 7.00 (s, 1H, thienyl CH), 7.26-7.45 (m, 12H, phenyl CH), 7.50-7.56 (m, 6H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.1 (CH3), 24.4 (CH3), 31.3 (t-butyl CH3), 31.4 (t-butyl CH3), 34.7 (t-butyl C4), 34.8 (t-butyl C4), 123.1 (d, 1JPC = 105.4 Hz, thienyl C4), 125.6 (phenyl CH), 125.7 (phenyl CH), 125.9 (phenyl CH), 126.0 (phenyl CH), 126.7 (d, 2JPC = 17.8 Hz, thienyl CH), 128.5 (d, 3JPC = 12.7 Hz, phenyl CH), 128.8 (phenyl CH), 130.2 (phenyl C4), 131.0 (d, 1JPC = 110.5 Hz, phenyl C4), 131.5 (phenyl C4), 132.3 (d, 4JPC = 2.7 Hz, phenyl CH), 132.5 (d, 2JPC = 10.6 Hz, phenyl CH), 132.6 (thienyl CH), 135.9 (phenyl C4), 136.4 (thienyl C4), 141.6 (phenyl C4), 143.0 (d, 3JPC = 15.8 Hz, thienyl C4), 144.7 (thienyl C4), 145.6 (phenyl C4), 150.4 (d, 2JPC = 9.6 Hz, thienyl C4), 150.6 (phenyl C4), 151.6 (phenyl C4), 159.9 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 31.2. ESI-TOF: m/Z [M+Na]+ 900.3850 calcd. 158  for C58H60OPS210BNa, found: 900.3850. 3'-Dimesitylboryl-5,5'-bis(4-methoxyphenyl)-3-diphenylphosphinyl-2,2'-bithiophene (40): A similar method as for 33 was followed except 54 (803 mg, 1.50 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=2:1) gave an orange-colored solid. Yield 533 mg, 43 %. 1H NMR (CDCl3, 400 MHz):  2.06 (s, 12H, CH3), 2.22 (s, 6H, CH3), 3.80 (s, 6H, CH3), 3.82 (s, 6H, CH3), 6.57 (d, 3JPH = 4.4 Hz, 1H, thienyl CH), 6.59 (s, 4H, phenyl CH), 6.84 (d, 3JHH = 8.8 Hz, 2H, phenyl CH), 6.89 (d, 3JHH = 8.8 Hz, 2H, phenyl CH), 6.95 (s, 1H, thienyl CH), 7.33 (d, 3JHH = 8.8 Hz, 2H, phenyl CH), 7.43 (d, 3JHH = 8.8 Hz, 2H, phenyl CH), 7.41-7.45 (m, 4H, phenyl CH), 7.52-7.57 (m, 6H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.1 (CH3), 24.4 (CH3), 55.4 (s, -OCH3), 55.5 (s, -OCH3), 114.1 (phenyl CH), 114.5 (phenyl CH), 122.9 (d, 1JPC = 105.7 Hz, thienyl C4), 125.7 (thienyl C4), 125.8 (d, 2JPC = 18.0 Hz, thienyl CH), 127.10 (thienyl C4), 127.2 (phenyl CH), 127.4 (phenyl CH), 128.5 (d, 3JPC = 12.6 Hz, phenyl CH),128.7 (phenyl CH), 131.0 (d, 1JPC = 110.5 Hz, phenyl C4),131.8 (thienyl CH), 132.2 (d, 4JPC = 2.7 Hz, phenyl CH), 132.5 (d, 2JPC = 10.7 Hz, phenyl CH), 135.8 (phenyl C4), 136.0 (d, 3JPC = 2.4 Hz, phenyl C4), 141.6 (phenyl C4), 142.8 (d, 3JPC = 16.1 Hz, thienyl C4), 144.5 (thienyl C4), 145.6 (phenyl C4), 149.8(d, 2JPC = 9.8 Hz, thienyl C4), 159.2 (phenyl C4), 159.7 (thienyl C4), 159.7 (phenyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 31.0. ESI-TOF: m/Z [M+H]+ 827.2954 calcd. for C52H49O3PS210B, found: 827.2955. 3'-Dimesitylboryl-5,5'-bis(4-dimethylaminophenyl)-3-diphenylphosphinyl-2,2'-bithiophene (15): A similar method as for 33 was followed except 55 (840 mg, 1.50 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=2:1) gave an orange-red solid. Yield 410 mg, 33 %. 1H NMR (CDCl3, 400 MHz):  2.27 (s, 159  12H, CH3), 2.45 (s, 6H, CH3), 3.19 (s, 6H, CH3), 3.22 (s, 6H, CH3), 6.75 (d, 3JPH = 4.3 Hz, 1H, thienyl CH), 6.79 (s, 4H, phenyl CH), 6.89 (d, 3JHH = 8.7 Hz, 2H, phenyl CH), 6.91 (d, 3JHH = 8.7 Hz, 2H, phenyl CH), 7.10 (s, 1H, thienyl CH), 7.52 (d, 3JHH = 9.0 Hz, 2H, phenyl CH), 7.61 (d, 3JHH = 8.8 Hz, 2H, phenyl CH), 7.64-7.66 (m, 4H, phenyl CH), 7.73-7.78 (m, 6H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.1 (CH3), 24.4 (CH3), 40.5 (s, -N(CH3)2), 40.6 (s, -N(CH3)2), 112.4 (phenyl CH), 112.5 (phenyl CH), 121.2 (phenyl C4), 121.8 (d, 1JPC = 107.3 Hz, thienyl C4), 122.9 (phenyl C4), 124.6 (d, 2JPC = 18.2 Hz, thienyl CH), 126.8 (s, phenyl CH), 127.1 (s, phenyl CH), 128.4 (d, 3JPC = 12.6 Hz, phenyl CH), 128.7 (s, phenyl CH), 130.5 (s, thienyl CH), 131.0 (d, 1JPC = 111.2 Hz, phenyl C4), 132.1 (d, 4JPC = 2.5 Hz, phenyl CH), 132.6 (d, 2JPC = 10.7 Hz, phenyl CH), 135.0 (d, 3JPC = 2.3 Hz, thienyl C4), 135.5 (phenyl C4), 141.7 (phenyl C4), 143.4 (d, 3JPC = 16.2 Hz, thienyl C4), 145.2 (thienyl C4), 146.0 (phenyl C4), 149.4 (d, 2JPC = 9.9 Hz, thienyl C4) 150.0 (phenyl C4), 150.3 (phenyl C4), 159.8 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 31.7. ESI-TOF: m/Z [M+Na]+ 852.3623 calcd. for C54H55OPS210B, found: 852.3646. 3'-Dimesitylboryl-5,5'-bis(2-phenylethynyl)-3-diphenylphosphinyl-2,2'-bithiophene (42): A similar method as for 33 was followed except 56 (1.12 g, 2.0 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=4:1) gave a yellow powder. Yield 944 mg, 58 %. 1H NMR (CDCl3, 400 MHz):  1.98 (s, 12H, CH3), 2.25 (s, 6H, CH3), 6.58 (s, 4H, phenyl CH), 6.74 (d, 3JPH= 8.7 Hz, 1H, thienyl CH), 7.02 (s, thienyl CH), 7.32-7.59 (m, 20H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  20.9 (CH3), 24.4 (CH3), 80.9 (alkynyl C4), 83.2 (alkynyl C4), 94.8 (alkynyl C4), 95.6 (alkynyl C4), 121.7 (d, 1JPC = 105.8 Hz, phenyl C4), 122.1 (phenyl C4), 122.6 (d, 3JPC = 18.9 Hz, thienyl C4), 122.9 (phenyl C4), 124.0 (thienyl C4), 128.3 (phenyl CH), 128.3 (phenyl CH), 128.5 (d, 3JPC = 13.0 Hz, phenyl CH), 128.5 160  (phenyl CH), 128.9 (phenyl CH), 129.0 (phenyl CH), 129.1 (d, 1JPC = 112.7 Hz, thienyl C4), 131.3 (phenyl CH), 131.4 (phenyl CH), 132.5 (d, 2JPC = 10.1 Hz, phenyl CH), 136.4 (d, 4JPC = 2.8 Hz, phenyl CH), 135.3 (phenyl C4), 135.5 (d, 2JPC = 18.8 Hz, thienyl CH), 136.4 (d, 3JPC = 2.2 Hz, thienyl C4), 141.6 (phenyl C4), 142.5 (thienyl CH), 145.8 (phenyl C4), 152.1 (d, 2JPC = 9.2 Hz, thienyl C4), 161.3 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 33.7. ESI-TOF: m/Z [M+Na]+ 836.2598 calcd. for C54H44OPS210BNa, found: 836.2593. 3'-Dimesitylboryl-5,5'-bis(5-hexylthienyl)-3-diphenylphosphinyl-2,2'-bithiophene (43): A similar method as for 33 was followed except 57 (1.12 g, 2.0 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=5:1) gave a red powder. Yield 572 mg, 31 %. 1H NMR (CDCl3, 400 MHz):  0.89-0.93 (m, 6H, CH3), 1.33-1.39 (m, 12H, CH2), 1.62-1.69 (m, 4H, CH2), 1.96 (s, 12H, CH3), 2.20 (s, 6H, CH3), 2.75-2.80 (m, CH2), 6.51 (d, 3JPH = 4.2 Hz, 1H, thienyl CH), 6.53 (s, 4H, phenyl CH), 6.65 (d, 3JHH = 3.6 Hz, 1H, thienyl CH), 6.69 (d, 3JHH = 3.6 Hz, 1H, thienyl CH), 6.71 (s, thienyl CH), 6.90 (d, 3JHH = 3.6 Hz, 1H, thienyl CH), 6.92 (d, 3JHH = 3.6 Hz, 1H, thienyl CH), 7.40-.61 (m, 10H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  14.3 (CH3), 14.3 (CH3), 21.0 (CH3), 23.0 (CH2), 23.0 (CH2), 24.6 (CH3), 29.1 (CH2), 29.1 (CH2), 30.5 (CH2), 30.5 (CH2), 32.0 (CH2), 32.0 (overlapped 2CH2), 32.0 (CH2), 122.0 (d, 1JPC = 106.3 Hz, thienyl C4), 124.1 (thienyl CH), 124.9 (thienyl CH), 125.3 (thienyl CH), 125.5 (thienyl CH), 126.4 (d, 2JPC = 18.2 Hz, thienyl CH), 129.0 (d, 3JPC = 13.0 Hz, phenyl CH), 129.3 (phenyl CH), 129.8 (d, 1JPC = 110.7 Hz, phenyl C4), 132.9 (thienyl C4), 132.9 (d, 4JPC = 2.9 Hz, phenyl CH), 133.0 (d, 2JPC = 10.7 Hz, phenyl CH), 133.4 (thienyl CH), 134.4 (d, 3JPC = 2.0 Hz, thienyl C4), 134.8 (thienyl C4), 135.6 (phenyl C4), 137.0 (d, 3JPC = 17.0 Hz, phenyl C4), 138.3 (thienyl C4), 141.9 (phenyl C4), 146.3 (thienyl C4), 146.5 (phenyl C4), 147.3 (thienyl 161  C4), 150.3 (d, 2JPC = 9.8 Hz, phenyl C4), 161.9 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 33.6. ESI-TOF: m/Z [M+Na]+ 947.3749 calcd. for C58H65OPS411B, found: 947.3755. 5,5'-Bis(6-chlorohexyl)-3'-dimesitylboryl-3-diphenylphosphinyl-2,2'-bithiophene (44): A similar method as for 33 was followed except 58 (1.62 g, 2.5 mmol) was used as starting material and THF as the solvent. Purification by flash column chromatography (hexanes: THF=5:1) gave a pale-yellow powder. Yield 1.54 g, 73 %. 1H NMR (CDCl3, 400 MHz):  1.22-1.60 (m, 12H, CH2), 1.72-1.76 (m, 4H, CH2), 2.04 (s, 12H, CH3), 2.23 (s, 6H, CH3), 2.57 (t, J = 7.4 Hz, 2H, CH2), 2.68 (t, J = 7.4 Hz, 2H, CH2), 3.48-3.53 (m, 4H, CH2), 6.10 (d, JPH = 4.1 Hz, 1H, thienyl CH), 6.48 (s, thienyl CH), 6.60 (s, 4H, phenyl CH), 7.40-7.55 (m, 10H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  21.2 (CH3), 23.9 (CH3), 26.4 (CH2), 26.5 (CH2), 28.0 (CH2), 28.0 (CH2), 29.3 (CH2), 29.5 (CH2), 31.1 (CH2), 31.3 (CH2), 32.5 (CH2), 32.6 (CH2), 45.0 (CH2), 45.1 (CH2), 123.4 (d, 1JPC = 105.7 Hz, thienyl C4), 128.3 (phenyl CH), 128.3 (d, 3JPC = 12.3 Hz, phenyl CH), 128.8 (d, 2JPC = 17.6 Hz, thienyl CH), 131.8 (d, 4JPC = 2.6 Hz, phenyl CH), 132.1 (d, 2JPC = 10.3 Hz, phenyl CH), 132.9 (thienyl CH), 132.9 (d, 1JPC = 108.0 Hz, phenyl C4), 136.7 (phenyl C4), 137.9 (d, 3JPC = 2.5 Hz, thienyl C4), 141.3 (phenyl C4), 144.5 (thienyl C4), 144.7 (d, 3JPC = 15.6 Hz, phenyl C4), 146.5 (thienyl C4), 148.2 (d, 2JPC = 9.8 Hz, phenyl C4), 155.2 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 27.1. ESI-TOF: m/Z [M+H]+ 851.3215 calcd. for C50H59OPS2Cl211B, found: 851.3239. 5,5'-Bis(6-iodohexyl)-3'-dimesitylboryl-3-diphenylphosphinyl-2,2'-bithiophene (45): 44 (850 mg, 1.0 mmol) was heated to reflux with stirring in 20 mL of acetone with excess amount of NaI (3.0g, 20 mmol) in dark for 2 days. Then 100 mL deionized water was added and the aqueous phase was extracted with diethyl ether (4×5 mL). Organic phases were combined and dried over 162  anhydrous MgSO4. The mixture was filtered and the solvent was removed under vacuum. The resulting crude product was then purified by flash column chromatography (hexanes: THF = 5:1) giving a yellow powder. Yield 960 mg, 93 %. 1H NMR (CDCl3, 400 MHz):  1.22-1.59 (m, 12H, CH2), 1.75-1.81 (m, 4H, CH2), 2.04 (s, 12H, CH3), 2.23 (s, 6H, CH3), 2.57 (t, 3JHH = 7.4 Hz, 2H, CH2), 2.68 (t, J = 7.3 Hz, 2H, CH2), 3.14-3.18 (m, 4H, CH2), 6.11 (d, 3JPH = 4.0 Hz, 1H, thienyl CH), 6.48 (s, 1H, thienyl CH), 6.60 (s, 4H, phenyl CH), 7.40-7.55 (m, 10H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  7.0 (I-CH2), 7.3(I-CH2), 21.2 (CH3), 23.9 (CH3), 27.6 (CH2), 27.7 (CH2), 29.3 (CH2), 29.5 (CH2), 30.0 (CH2), 30.2 (CH2), 31.1 (CH2), 30.3 (CH2), 33.4 (CH2), 33.5 (CH2), 123.4 (d, 1JPC = 105.7 Hz, thienyl C4), 128.3 (phenyl CH), 128.3 (d, 3JPC = 12.4 Hz, phenyl CH), 128.7 (d, 2JPC = 17.4 Hz, thienyl CH), 131.8 (d, 4JPC = 2.7 Hz, phenyl CH), 132.1 (d, 2JPC = 10.3 Hz, phenyl CH), 132.8 (thienyl CH), 132.8 (d, 1JPC = 108.0 Hz, phenyl C4), 136.7 (phenyl C4), 137.8 (d, 3JPC = 2.6 Hz, thienyl C4), 141.3 (phenyl C4), 144.6 (thienyl C4), 144.6 (d, 3JPC = 15.7 Hz, phenyl C4), 146.4 (thienyl C4), 148.2 (d, 2JPC = 9.8 Hz, phenyl C4), 155.2 (thienyl C4). 31P{1H} NMR (CDCl3, 162 MHz): 27.2. ESI-TOF: m/Z [M+Na]+ 1057.1747 calcd. for C50H58OPS2I211BNa, found: 1057.1744. 6,6'-(3'-Dimesitylboryl-3-diphenylphosphinyl-2,2'-bithienyl-5,5'-diyl)-bis(N,N,N-trimethyl-1-hexylammonium) diiodide. (46): 45 (516 mg, 0.50 mmol) was dissolved in 5.0 mL acetone and added to excess amount of aqueous solution of trimethylamine (~45 wt. %, 1.0 mL) with stirring at room temperature in dark. The mixture was stirred for 12 hours and solvent was evaporated under vacuum. Yield 575 mg, 99 %. 1H NMR (methanol-d4, 400 MHz):  1.36-1.62 (m, 12H, CH2), 1.74-1.78 (m, 4H, CH2), 2.07 (s, 12H, CH3), 2.22 (s, 6H, CH3), 2.59 (t, J = 7.4 Hz, 2H, CH2), 2.72 (t, J = 7.4 Hz, 2H, CH2), 3.13 (s, 9H, CH3), 3.15 (s, 9H, CH3), 3.32-3.40 (m, 4H, CH2), 6.21 163  (d, JPH = 4.3 Hz, 1H, thienyl CH), 6.50 (s, thienyl CH), 6.65 (s, 4H, phenyl CH), 7.53-7.64 (m, 10H, phenyl CH). 13C{1H} NMR (methanol-d4, 100 MHz):  21.4 (CH3), 23.8 (CH2), 23.8 (CH2), 24.1 (CH3), 26.9 (overlapped 2CH2), 29.3 (CH2), 29.4 (CH2), 30.1 (CH2), 30.3 (CH2), 32.3 (CH2), 32.5 (CH2), 53.6-53.7 (m, 2CH3 on -NMe3), 67.8 (CH2), 67.8 (CH2), 126.9 (d, 1JPC = 106.9 Hz, thienyl C4), 129.3 (phenyl CH), 129.8 (d, 3JPC = 12.3 Hz, phenyl CH), 130.3 (d, 2JPC = 17.9 Hz, thienyl CH), 132.6 (thienyl CH), 132.9 (d, 2JPC = 9.3 Hz, phenyl CH), 133.4 (d, 4JPC = 2.6 Hz, phenyl CH), 134.7 (d, 1JPC = 106.8 Hz, phenyl C4), 139.4 (phenyl CH), 141.4 (d, 3JPC = 2.6 Hz, thienyl C4), 142.0 (phenyl C4), 144.4 (phenyl C4), 147.1 (d, 2JPC = 9.5 Hz, phenyl C4), 147.6 (d, 3JPC = 15.8 Hz, phenyl C4), 148.3 (thienyl C4) 153.8 (thienyl C4). 31P{1H} NMR (methanol-d4, 162 MHz): 26.0. ESI-TOF: m/Z [M-I]+ 1025.4275 calcd. for C56H76N2OPS211BI, found: 1025.4264.  5,5'-Dibutyl-3'-dimesitylboryl-3-thiomethoxy-2,2'-bithiophene (35a): 49 (1.30 g, 2.21 mmol) was dissolved in 30 mL of dry diethyl ether and cooled to -78 °C. n-BuLi (1.6M in hexanes, 1.52 mL, 2.43 mmol) was added and the mixture was stirred for 1 hour at the same temperature. Solid Mes2BF (710 mg, 2.65 mmol) was then added under a high N2 flow and the reaction was stirred for 1 hour before it was warmed up to room temperature and stirred for an additional 18 hours. The reaction was then quenched by H2O. The organic phases were combined and washed with water and brine, before drying over anhydrous MgSO4. The mixture was filtered and the solvent was removed from the filtrate under vacuum. Purification by flash column chromatography in hexanes gave a yellow solid. Yield: 920 mg, 73 %. 1H NMR (CD2Cl2, 400 MHz):  0.94-1.00 (m, 6H, CH3), 1.29-1.30 (m, 2H, CH2), 1.33-1.49 (m, 2H, CH2), 1.51-1.54 (m, 2H, CH2), 1.64-1.71 (m, 2H, CH2), 2.11 (s, 12H, CH3), 2.24 (s, 6H, CH3), 2.61 (t, 3JHH = 7.6 Hz, 2H, CH2), 2.81 164  (t, 3JHH = 7.4 Hz, 2H, CH2), 6.47 (s, 1H, thienyl CH), 6.62 (s, 1H, thienyl CH), 6.70 (s, 4H, phenyl CH). 13C{1H} NMR (CD2Cl2, 100 MHz):  14.1 (CH3), 14.2 (CH3), 19.1 (CH3), 21.4 (CH3), 22.5 (CH2), 22.7 (CH2), 23.6 (CH3), 30.0 (CH2), 30.2 (CH2), 34.1 (CH2), 34.5 (CH2), 126.4 (thienyl CH), 128.4 (phenyl CH), 130.8 (thienyl C4), 131.5 (thienyl C4), 131.7 (thienyl CH), 138.7 (phenyl C4), 141.1 (phenyl C4), 142.9 (thienyl C4), 143.0 (phenyl C4), 146.4 (thienyl C4), 146.8 (thienyl C4), 150.5 (thienyl C4). ESI-TOF: m/Z [M+Na]+ 595.2674 calcd. for C35H45S311BNa, found: 595.2678. 5,5'-Dibutyl-3'-dimesitylboryl-2,2'-bithiophene-3-methylsulfoxide (35b): 35a (570 mg, 1.00 mmol) was dissolved in 10 mL CH2Cl2 and solid m-CPBA (<77%, 224 mg, 1.30 mmol) was slowly added with stirring at 0 oC. The mixture was stirred for 8 hours and then washed with 5 mL 10 wt% aqueous solution of NaHCO3 and 5 mL brine. The organic phases were then dried over MgSO4 and filtered. The mixture was filtered and the solvent was removed from the filtrate under vacuum. Purification by flash column chromatography (hexanes:THF = 5:1) gave a yellow solid. Yield: 522 mg, 89 %. 1H NMR (CD2Cl2, 400 MHz):  0.93 (t, 3JHH = 7.4 Hz, 2H, CH2), 0.96 (t, 3JHH = 7.3 Hz, 2H, CH2), 1.26-1.33 (m, 2H, CH2), 1.35-1.43 (m, 2H, CH2), 1.47-1.54 (m, 2H, CH2), 1.62-1.69 (m, 2H, CH2), 2.08 (s, 12H, CH3), 2.23 (s, 6H, CH3), 2.61 (t, 3JHH = 7.4 Hz, 2H, CH2), 2.80 (t, 3JHH = 7.5 Hz, 2H, CH2), 3.01 (s, 3H, CH3), 6.64 (s, 1H, thienyl CH), 6.70 (s, 1H, thienyl CH), 6.80 (s, 4H, phenyl CH). 13C{1H} NMR (CD2Cl2, 100 MHz):  14.0 (CH3), 14.0 (CH3), 21.3 (CH3), 22.4 (CH2), 22.6 (CH2), 23.6 (CH3), 29.7 (CH2), 29.8 (CH2), 33.7 (CH2), 34.2 (CH2), 45.7 (CH3), 125.1 (thienyl CH), 128.5 (phenyl CH), 131.8 (thienyl CH), 134.8 (thienyl C4), 137.8 (thienyl C4), 139.2 (phenyl C4), 139.8 (thienyl C4), 141.1 (phenyl C4), 143.0 (phenyl C4), 147.3 (thienyl C4), 148.8 (thienyl C4), 153.2 (thienyl C4). ESI-TOF: m/Z [M+Na]+ 610.2679 calcd. for 165  C35H45OS311BNa, found: 610.2660. 5,5'-Dibutyl-3'-dimesitylboryl-2,2'-bithiophene-3-methylsulfone (35c): Similar to 35b, 35c was prepared by oxidation of 35a with two equivalents of m-CPBA. 35a (320 mg, 0.56 mmol) was dissolved in 10 mL CH2Cl2 and solid m-CPBA (<77%, 250 mg, 1.45 mmol) was slowly added with stirring at 0 oC. The mixture was stirred for 8 hours and then washed with 5 mL 10 wt% aqueous solution of NaHCO3 and 5 mL brine. The organic phases were then dried over MgSO4 and filtered. The mixture was filtered and the solvent was removed from the filtrate under vacuum. Purification by flash column chromatography (hexanes:THF = 10:1) gave a yellow solid. Yield: 287 mg, 85 %. 1H NMR (CD2Cl2, 400 MHz):  0.92 (t, 3JHH = 7.4 Hz, 2H, CH2), 0.95 (t, 3JHH = 7.3 Hz, 2H, CH2), 1.25-1.32 (m, 2H, CH2), 1.35-1.42 (m, 2H, CH2), 1.46-1.54 (m, 2H, CH2), 1.60-1.68 (m, 2H, CH2), 2.04 (s, 12H, CH3), 2.21 (s, 6H, CH3), 2.64 (t, 3JHH = 7.7 Hz, 2H, CH2), 2.69 (s, 3H, CH3), 2.80 (t, 3JHH = 7.5 Hz, 2H, CH2), 6.63 (s, 1H, thienyl CH), 6.68 (s, 1H, thienyl CH), 6.90 (s, 4H, phenyl CH). 13C{1H} NMR (CD2Cl2, 100 MHz):  14.0 (CH3), 14.0 (CH3), 21.3 (CH3), 22.4 (CH2), 22.5 (CH2), 23.4 (CH3), 29.8 (CH2), 30.1 (CH2), 33.9 (CH2), 34.3 (CH2), 42.6 (CH3), 120.2 (thienyl CH), 128.6 (phenyl CH), 132.2 (thienyl CH), 135.8 (thienyl C4), 138.6 (thienyl C4), 139.1 (phenyl C4), 141.0 (phenyl C4), 141.3 (thienyl C4), 142.7 (phenyl C4), 148.3 (thienyl C4), 149.1 (thienyl C4), 151.7 (thienyl C4). ESI-TOF: m/Z [M+Na]+ 626.2625 calcd. for C35H45O2S311BNa, found: 626.2609.  166  5.3.2.2 Synthesis of Precursors Scheme 5-13. Synthesis of dibromo-bithiophene precursors 50-55.  3,3'-Dibromo-5,5'-dimesityl-2,2'-bithiophene (50): With stirring, 50 mL of deaerated H2O-THF (vol/vol=1:1) mixed solvent was added under N2 into a three-necked round-bottom flask equipped with a condenser containing 59 (964 mg, 2.00 mmol), K2CO3 (2.21 g, 16.0 mmol) and 2,4,6-trimethylphenylboronic acid (688 mg, 4.20 mmol). Then Pd(PPh3)4 (230 mg, 0.20 mmol) was added against high N2 flow. The mixture was slowly warmed to 70 °C, and stirred at the same temperature for 24 hours. THF was removed under reduced pressure and remaining mixture was extracted with diethyl ether (4×20 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered and the solvent was removed from the filtrate under reduced pressure. Purification by flash column chromatography (hexanes) gave a white solid. Yield 680 mg, 61 %. 1H NMR (CDCl3, 400 MHz):  2.24 (s, 12H, CH3), 2.35 (s, 6H, CH3), 6.82 (s, 2H, thienyl CH), 6.98 (s, 4H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  20.9 (CH3), 21.3 (CH3), 111.5 (thienyl C4, C-Br), 128.4 (phenyl CH), 129.2 (thienyl C4), 129.5 (phenyl C4), 130.0 (thienyl CH), 138.3 (phenyl C4), 138.7 (phenyl C4), 143.5 (thienyl C4). EI-MS: m/Z [M]+ 557.96862 calcd. for C26H24Br2S2, found: 557.96842. 167  3,3'-Dibromo-5,5'-diphenyl-2,2'-bithiophene (51): A similar method to 50 was followed except phenylboronic acid (513 mg, 4.20 mmol) was used. The crude product can be recrystallized in hot ethanol to give pure product as yellow needles (626 mg, 66 %). 1H NMR (CDCl3, 400 MHz):  7.30 (s, 2H, thienyl CH), 7.36-7.62 (m, phenyl CH).13C{1H} NMR (CDCl3, 100 MHz):  112.7 (thienyl C4, C-Br), 125.7 (phenyl CH), 126.6 (thienyl CH), 128.0 (thienyl C4), 128.6 (phenyl CH), 129.1 (phenyl CH), 132.8 (phenyl C4), 145.5 (thienyl C4). EI-MS: m/Z [M]+ 473.87472 calcd. for C20H1279Br2S2, found: 473.87449. 3,3'-Dibromo-5,5'-bis(4-trifluoromethylphenyl)-2,2'-bithiophene (52): A similar method was followed except 4-trifluoromethyl-phenylboronic acid (797 mg, 4.2 mmol) was used. Purification by flash column chromatography (hexanes) gave an off-white solid. Yield 707 mg, 58 %. 1H NMR (CDCl3, 400 MHz):  7.38 (s, 2H, thienyl CH), 7.66-7.71 (m, 8H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  113.3 (C-Br, C4), 124.1 (q, 1JFC = 270.3 Hz, phenyl C4), 126.0 (phenyl CH), 126.3 (q, 3JFC = 3.8 Hz, phenyl CH), 128.1 (thienyl CH), 129.2 (phenyl C4), 130.5 (q, 2JFC = 32.5 Hz, phenyl C4), 136.1 (thienyl C4), 143.9 (thienyl C4). 19F{1H} NMR (CDCl3, 376 MHz): -62.4. EI-MS: m/Z [M]+ 609.84949 calcd. for C22H1079Br2F6S2, found: 609.84913. 3,3'-Dibromo-5,5'-bis(4-tert-butylphenyl)-2,2'-bithiophene (53): A similar method was followed except 4-tert-butylphenylboronic acid (748 mg, 4.20 mmol) was used. Purification by flash column chromatography (hexanes) gave a yellow solid. Yield 762 mg, 65 %. 1H NMR (CDCl3, 400 MHz):  1.36 (s, 18H, CH3), 7.25 (s, 2H, thienyl CH), 7.45 (d, J = 8.4 Hz, 4H, phenyl CH), 7.53 (d, J = 8.4 Hz, 4H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  31.4 (CH3), 34.9 (tert-butyl, C4), 112.6 (thienyl C4, C-Br), 125.6 (phenyl CH), 126.2 (phenyl CH), 126.3 (thienyl 168  CH), 127.8 (thienyl C4), 130.2 (phenyl C4), 145.7 (thienyl C4), 152.0 (phenyl C4). EI-MS: m/Z [M]+ 473.87472 calcd. for C20H2879Br2S2, found: 585.99981. 3,3'-Dibromo-5,5'-bis(4-methoxyphenyl)-2,2'-bithiophene (54): A similar method was followed except 4-methoxyphenylboronic acid (638 mg, 4.20 mmol) was used. Purification by column chromatography (hexanes) gave a yellow solid. Yield 588 mg, 55 %. 1H NMR (CDCl3, 400 MHz):  3.85 (s, 6H, CH3), 6.94 (d, J = 8.8 Hz, 4H, phenyl CH), 7.17 (s, 2H, thienyl CH), 7.52 (d, J = 8.8 Hz, 4H, phenyl CH). EI-MS: m/Z [M]+ 533.89585 calcd. for C22H16O279Br2S2, found: 533.89540. This compound is known in literature.159 3,3'-Dibromo-5,5'-bis(4-dimethylaminophenyl)-2,2'-bithiophene (55): A similar method was followed except 4-(dimethylamino)phenylboronic acid (693 mg, 4.20 mmol) was used and 60 (1.15 g, 2.00 mmol) was added instead of 59. Purification by column chromatography (hexanes:THF=5:1) gave an orange powder. Yield 559 mg, 49 %. 1H NMR (CD2Cl2, 400 MHz):  3.00 (s, 12H, CH3), 6.73 (d, J = 8.9 Hz, 4H, phenyl CH), 7.13 (s, 2H, thienyl CH), 7.47 (d, J = 8.9 Hz, 4H, phenyl CH). 13C{1H} NMR (CD2Cl2, 100 MHz):  40.5 (s, -N(CH3)2), 112.6 (phenyl CH), 112.7 (thienyl C4), 121.0 (phenyl C4), 124.2 (thienyl CH), 126.1 (thienyl C4), 126.9 (phenyl CH), 147.0 (thienyl C4), 151.2 (phenyl C4). EI-MS: m/Z [M]+ 559.95911 calcd. for C24H2279Br2N2S2, found: 559.95942.   169  Scheme 5-14. Synthesis of dibromo precursors 56-58.  3,3'-Dibromo-5,5'-bis(2-phenylethynyl)-2,2'-bithiophene (56): With stirring, 60 mL of deaerated diisopropylamine-THF (vol/vol=1:3) mixed solvent was added under N2 into a three-necked round-bottom flask containing 60 (1.15 g, 2.00 mmol), CuI (34.4 mg, 0.18 mmol) and Pd(PPh3)2Cl2 (112 mg, 0.16 mmol). Phenylacetylene (430 mg, 4.2 mmol) was added through syringe, and the mixture was stirred at room temperature for 24 hours. THF was removed under reduced pressure and remaining mixture was extracted with diethyl ether (4×20 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered and the solvent was removed from the filtrate under reduced pressure. Purification by column chromatography (hexanes:CH2Cl2=10:1) gave a yellow powder solid. Yield 960 mg, 92 %. 1H NMR (CDCl3, 400 MHz):  7.23 (s, 2H, thienyl CH), 7.36-7.38 (m, 6H, phenyl CH), 7.51-7.54 (m, 4H, phenyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  81.0 (C4, alkynyl), 96.2 (C4, alkynyl), 111.9 (thienyl C4, C-Br), 122.3 (phenyl C4), 125.6 (thienyl C4), 128.6 (phenyl CH), 129.2 (thienyl CH), 129.6 (thienyl C4), 131.7 (phenyl CH), 134.9 (phenyl CH). EI-MS: m/Z [M]+ 521.87472 170  calcd. for C24H1279Br2S2, found: 521.87506.  3,3'-Dibromo-5,5'-bis(5-hexylthienyl)-2,2'-bithiophene (57): An oven-dried 100 mL round-bottom flask under N2 was added 5-bromo-2-hexylthiophene (1.04 g, 4.2 mmol) and 40 mL dry THF. The solution was cooled to -78 °C, and n-BuLi (1.6 M in hexanes, 2.90 mL, 4.6 mmol) was added slowly. The mixture was stirred at -78 °C for an hour after which ZnBr2 (1.14 g, 4.6 mmol) was added against high N2 flow. The mixture was warmed up to room temperature and stirred for another one hour. Pd(PPh3)4 (230 mg, 0.20 mmol) and 59 (964 mg, 2.00 mmol) was added against high N2 flow. The mixture was slowly warmed up to 50 °C and stirred for 24 hours. Solvent was removed under reduced pressure and remaining mixture was added 50 mL H2O and 5 mL 1M HCl solution and extracted with diethyl ether (4×20 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered and the solvent was removed from the filtrate under reduced pressure. Purification by column chromatography (hexanes) gave a yellow waxy solid. Yield 1.03 mg, 79 %. 1H NMR (CDCl3, 400 MHz):  0.91 (t, J = 6.0 Hz, 6H, CH3), 1.32-1.41 (m, 12H, CH2), 1.65-1.73 (m, 4H, CH2), 2.81(t, J = 7.5 Hz, 4H, CH2), 6.70 (d, J = 3.4 Hz, 2H, thienyl CH), 7.02 (d, J = 3.4 Hz, 2H, thienyl CH), 7.05 (s, 2H, thienyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  14.2 (CH3), 22.7 (CH2), 28.9 (CH2), 30.3 (CH2), 31.6 (CH2), 31.7 (CH2), 112.3 (thienyl C4, C-Br), 124.5 (thienyl CH), 125.2 (thienyl CH), 126.1 (thienyl CH), 126.6 (thienyl C4), 133.0 (thienyl C4), 139.4 (thienyl C4), 147.1 (thienyl C4). EI-MS: m/Z [M]+ 653.97536 calcd. for C28H3279Br2S4 found: 653.97500.    171  Scheme 5-15. Synthesis of 58.  3,3'-Dibromo-5,5'-bis(6-chlorohexyl)-2,2'-bithiophene (58): Under N2, 59 (1.93 g, 4.00 mmol) was dissolved in 100 mL dry THF and cooled to -78 °C. With stirring, n-BuLi (1.6 M in hexanes, 5.25 mL, 8.40 mmol) was added to the solution dropwise, and the mixture was stirred at the same temperature for 1 hour. 6-Chloro-1-iodohexane (2.20 g, 8.8 mmol) was added via syringe, and the mixture was slowly warmed up to room temperature and stirred for 12 hours. The reaction was quenched with 1 mL deionized water and solvent was removed under reduced pressure. Remaining mixture was added 20 mL H2O and 5 mL 1M HCl solution and extracted with hexanes (4×10 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered and the solvent was removed from the filtrate under reduced pressure. Purification by column chromatography (hexanes) gave white waxy solid. Yield 1.90 g, 74 %. 1H NMR (CDCl3, 400 MHz):  1.40-1.51 (m, 8H, CH2), 1.69-1.73 (m, 4H, CH2), 1.76-1.81 (m, 4H, CH2), 2.79 (t, J = 7.5 Hz, 4H, CH2), 3.55 (t, J = 6.7 Hz, 4H, CH2), 6.75 (s, 2H, thienyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  26.6 (CH2), 28.4 (CH2), 30.2 (CH2), 31.0 (CH2), 32.5 (CH2), 45.1 (CH2), 111.1 (thienyl C4, C-Br), 126.7 (thienyl C4), 127.8 (thienyl CH), 147.2 (thienyl C4). EI-MS: m/Z [M]+ 653.97536 calcd. for C28H3279Br2S4, found: 653.97500.    172  Scheme 5-16. Synthesis of 49.   3-Bromo-5,5'-dibutyl-3-thiomethoxy-2,2'-bithiophene (49): Under N2, 22 (2.10 g, 4.80 mmol) was dissolved in 100 mL dry THF and cooled to -78 °C. With stirring, n-BuLi (1.6 M in hexanes, 3.30 mL, 5.28 mmol) was added to the solution dropwise, and the mixture was stirred at the same temperature for 1 hour. Dimethyl disulfide (543 mg, 5.76 mmol) was added via syringe, and the mixture was slowly warmed up to room temperature and stirred for 12 hours. The reaction was quenched with water and solvent was removed under reduced pressure. Remaining mixture was added 20 mL H2O and extracted with hexanes (4×15 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered and the solvent was removed from the filtrate under reduced pressure. Purification by column chromatography in hexanes gave a colorless oil. Yield 1.74 g, 90 %. 1H NMR (CDCl3, 400 MHz):  0.94-0.99 (m, 6H, CH3), 1.40-1.47 (m, 4H, CH2), 1.64-1.73 (m, 4H, CH2), 2.40 (s, 3H, CH3), 2.77-2.82 (m, 4H, CH2), 6.74 (s, 1H, thienyl CH), 6.77 (s, 1H, thienyl CH). 13C{1H} NMR (CDCl3, 100 MHz):  13.8 (CH3), 13.9 (CH3), 18.6(CH3), 22.2 (CH2), 22.3 (CH2), 29.9 (CH2), 30.0 (CH2), 33.2 (CH2), 33.4 (CH2), 110.4 (thienyl C4, C-Br), 126.1 (thienyl CH), 127.0 (thienyl C4), 127.5 (thienyl CH), 127.6 (thienyl C4), 133.0 (thienyl C4), 146.6 (thienyl C4), 147.0 (thienyl C4). ESI-TOF: m/Z [M+H]+ 403.0224 calcd. for C17H2479BrS3, found: 403.0228. 173   5.3.3 Fluorescence Imaging Preliminary fluorescent images were obtained by Hunter (Xiaozhu) Wang in the Orvig research group at UBC, using facilities at the Biological Services Laboratory in the Chemistry Department, UBC. Olympus IX70 Inverted Fluorescence Microscope with a DC-330 3CCD MTI color camera and Image-Pro Plus analysis software were used to obtain the fluorescence images. A 11000v3 - UV filter cube purchased from Chroma was used with its emission filter changed for specific emission colors.   5.3.4 DFT Simulations DFT calculations were carried out using the Gaussian 09 Rev.D01 suite of Programs.90 The PBE0 hybrid functional91 with 6-31G* basis set was employed to simulate GS structure of Lewis pairs 37-41. Optimized structures were confirmed to be the minimum on the potential energy surface by vibrational frequency calculations. Vertical absorptions are simulated using TD-DFT methods by looking at the first 20 low-energy electronic transitions. For all simulations, the solvation effect of either methanol ( = 32.613, for the open structures) or THF ( = 7.4257, for the closed structures) was implemented using the polarizable continuum model88 (PCM) on Gaussian 09.   174  5.4 Conclusions In this chapter, the bithiophene-based Lewis pair system was systematically studied by changing the relative position of the Lewis acid and base on the backbone, and altering the Lewis base from different alkyl and aryl phosphine oxide to sulfoxide and sulfone groups. The Lewis adduct can only be formed intramolecularly with the 3,3'- substitution of Lewis acid and base centers on bithiophene. Only phosphine oxides can effectively form the B-O bond with -BMes2 moiety, and the equilibrium of the open and closed structure is more favorable to the closed structures when the R2P(O)- group is more polarized on oxygen atom and less sterically hindered. It is also demonstrated that the emission colors of open and closed forms of the Lewis pairs can be red-shifted by increasing the backbone conjugation or adding electron-donating groups.   Additionally, preliminary bio-imaging experiments were carried out using pig adipocytes and human brain slices as proof-of-concept of a two-color fluorescent dye system for fluorescence imaging of hydrophobic/hydrophilic environments in biological or medical applications. It is also shown that the hydrophobic/hydrophilic distribution and emission colors can be modified separately, and other functionalities can be conveniently introduced by substituting the reactive iodine atoms in one of the precursors.    175  Chapter 6 Conclusions and Future Work   This thesis sets out to explore photo-functional molecules by incorporating either Pt(II)-containing functionalities or 3-coordinated Lewis acidic boron groups with thiophene-containing conjugated backbones. Molecules and materials that can give rise to controllable/tunable light absorption and emission properties are obtained, and attempts have been made to utilize these properties in corresponding applications.  In Chapter 2, two binuclear Pt(II) terpyridyl Pacman complexes 2 and 3 with flexible conjugated bisacetylide ligands are shown to exhibit structural folding/unfolding controllable by temperature changes and different solvent environments. For the first time, crystal structures of both the folded and unfolded structures of a foldable molecule containing Pt(II) terpyridyl groups are obtained. It is revealed that intramolecular  interactions between pedant Pt(II) terpyridyl moieties can be used to encourage structural folding and planarization of the conjugated backbone while weak interactions such as Cl- interactions, and weak C-H hydrogen bonds can be used to stabilize unfolded structures with reduced conjugation on the backbone. This strategy can be used for designing stimuli responsive materials with different light absorption or electrical conductivity in different states. It is also shown by DFT simulations that the two metal centers in unfolded structures of 2 and 3 are weakly or non-interacting, which give rise to almost degenerate MOs. By contrast, in folded structures, these MOs are coupled and split in energy. Such energy splitting can be used to modulate redox potentials and optical properties of the metal centers which may be useful in 176  electrochemistry and photochemistry applications. Additionally, the results indicate that the flexible thiophene-containing ligands are good foldable platforms to encourage and investigate tunable chromophore-chromophore interactions and allow convenient assessment by changes in light absorption and emission properties.  In Chapter 3, three Pt(II) metallacycles 4, 5 and 6 with thiophene-containing bisacetylide ligands were synthesized and their fluorescence and phosphorescence properties examined. These complexes can be fluorescent when emitting from their S1 states and phosphorescent when emitting from their T1 states after intersystem crossing. These results show that by replacing the phenylene or ethynylene groups with thiophene rings on the bisacetylide ligand of cis-diphosphine Pt(II) metallacycles, the S1 and T1 states can be modulated to show relatively stronger fluorescence and weaker phosphorescence. It is also shown that more rigid and planar metallacycles suffer less from energy loss due to smaller degrees of structure reorganization and thermal deactivation. These are important factors to consider when designing similar single component fluorescence and phosphorescence dual emissive materials for applications such as white-emitting OLEDs, self-referencing oxygen sensors, and hypoxia contrast agents.  Chapter 4 discussed an intramolecular Lewis pair system between a Lewis acidic -BMes2 group and a Lewis basic phosphine oxide group based on a flexible bithiophene backbone. The system is able to establish a fast equilibrium between open structures with unbound Lewis acids and bases and closed structures of Lewis adducts. Such equilibrium can be manipulated by changing the temperature or HB donating strength of the solvent, and conveniently probed according to the different emission feature of these two structures. This gives rise to an interesting single-component-two-state system with stimuli responsive emission colors. It is also shown that 177  the flexible bithiophene backbone can serve as a good platform for tunable and easily detectable interactions between Lewis acids and bases.  The scope of such system was further explored in Chapter 5 showing that a flexible Lewis pair with a -BMes2 Lewis acidic group can only be achieved by using Lewis basic groups of phosphine oxides. The equilibrium between the open and closed structure can be affected by the steric and electronic properties of the phosphine oxide groups. A wide variety of emission colors of the open and closed forms of the Lewis pairs can also be achieved by changing the backbone conjugation or electron density of the -system on the backbone, as shown in Series A and Series B (page 134 to 135), respectively. Such flexible Lewis pair molecules can be used as two-color fluorescent dyes for fluorescence imaging of hydrophobic/hydrophilic environments in the context of biological or medical applications, with benefits of high-emission quantum yield, many choices of emission colors and potentially low toxicity. The hydrophobic/hydrophilic distribution of the molecules and their emission colors can be modified separately, and more functionalities can be conveniently installed by substituting the reactive iodine atoms in precursor 45.   Figure 6-1. Modulating the fluoride anion affinity at the boron center by deriving the phosphine groups. 178  One future direction is to compare the fluoride anion affinity of the phosphine oxide-borane system with the corresponding phosphine-borane and phosphonium-borane systems (Figure 6-1). It is expected that molecule 145 with a phosphine-oxide group has the lowest affinity due to competitive binding of the Lewis base while molecule 147 with a phosphonium group has the highest affinity due to a more stabilized adduct with fluoride anion. By deriving the phosphine groups in 146, the fluoride anion affinity at the boron center can be easily modified for fluoride sensors that requires different sensitivities.  In summary, three types of thiophene-containing photofunctional molecules with (a) two interacting terpyridyl Pt(II) acetylide groups and (b) a cyclometalated cis-diphosphino Pt(II) center and (c) a pair of flexible Lewis acid-base pair have been designed, synthesized and characterized. 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