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Synthesis and characterization of photochromic platinum-coordinated dithienylethenes Roberts, Matthew 2011

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SYNTHESIS AND CHARACTERIZATION OF PHOTOCHROMIC PLATINUM-COORDINATED DITHIENYLETHENES by MATTHEW ROBERTS  A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in The Faculty of Graduate Studies (CHEMISTRY)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) MARCH 2011 ! Matthew Roberts, 2011  Abstract The syntheses and characterization of photochromic platinum-coordinated dithienylethenes (DTEs) are reported.  Platinum acetylide complexes were investigated  as potentially useful chromphores to integrate with photoresponsive DTEs. Acetylides were observed to be successful at promoting strong electronic interaction between the Pt atom and the DTE’s thienyl scaffold. As a result of platinum’s large spin-orbit coupling, these metal complexes exhibit a high propensity for generating excited states with triplet character upon photoexcitation. Consequently, platinum acetylide complexes were found to be effective sensitizers for population of DTE-localized triplet states via energy transfer between the two chromophores. Platinum terpyridine complexes were initially targeted because they exhibit longlived excited states, whose lifetime could be potentially modulated by photoswitching of the appended DTE. In fact, selective irradiation of the Pt complex with visible light resulted in energy transfer from a metal-based excited state to the DTE chromophore, in either the ring-open or ring-closed state. The triplet excited state of the ring-open DTE was observed to be photoactive and underwent cyclization to the ring-closed form. The effect of the alkynyl linkage on energy transfer was studied by incorporating a longer, non-conjugated alkynyl linkage between the two chromophores. Lengthening the linker reduced excited-state interaction between the metal-based and DTE-localized states, but did not completely eliminate energy transfer. The high efficiency of metal-sensitized ring-closing in the Pt terpyridine system led to subsequent exploration of multifunctional systems, those containing more than one DTE.  Symmetrical Pt(II)-bis(phosphine)bis(acetylide) complexes were used for the  preparation of discrete model systems and extended oligomeric species. The Pt-acetylide ii  linkage was successful in allowing photoswitching of multiple adjacent DTE chromophores while maintaining a conjugated pathway between DTE units. Optical and electrochemical characterization supported that adjacent DTEs are capable of electronic communication through the Pt center. Extended electronic delocalization observed when multiple adjacent DTEs are ring-closed makes this system suitable for applications utilizing "-conjugated materials.  iii  Preface In all chapters, Prof. Michael Wolf acted in a supervisory role. Chapters 2-5 involved collaboration with the research group of Prof. Neil Branda within the Department of Chemistry at Simon Fraser University, in Burnaby, Canada. Portions of Chapters 2 and 3 have been published previously. I am the principal author of this work and carried out all experiments, except where noted. Compounds 4 and 34, were prepared by Dr. Jeremy Finden at Simon Fraser University. All DFT and TDDFT calculations were performed by Dr. Jeffrey Nagle at Bowdoin University. A version of Chapter 4 has been published previously. I am the principal author of this work and carried out all experiments, except where noted. Compound 4, also used for this work, was prepared by Carl-Johan Carling at Simon Fraser University. All DFT calculations were performed by Dr. Jeffrey Nagle at Bowdoin University. I am the principal author of Chapter 5 and carried out all of the experiments. Compound 39, was prepared by Carl-Johan Carling at Simon Fraser University.  List of Publications Arising From This Work Roberts, M.N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.; Wolf, M. O. Inorg. Chem. 2009, 48, 19 – 21. Roberts, M.N.; Carling, C.-J.; Nagle, J. K.; Branda, N. R.; Wolf, M. O. J. Am. Chem. Soc. 2009, 161, 16644 – 16645.  iv  Table of Contents Abstract .........................................................................................................................ii Preface .......................................................................................................................... iv Table of Contents .......................................................................................................... v List of Tables ..............................................................................................................viii List of Figures............................................................................................................... ix List of Schemes............................................................................................................ xv List of Charts.............................................................................................................. xvi List of Symbols and Abbreviations........................................................................... xvii Acknowledgements..................................................................................................... xxi Dedication................................................................................................................. xxiii Chapter 1  Introduction ............................................................................................ 1  1.1 Overview............................................................................................................ 1 1.2 Organic "-Conjugated Materials ......................................................................... 2 1.2.1 Structure of "-Conjugated Materials ............................................................ 3 1.2.2 Electronic and Optical Properties of Poly- and Oligothiophenes................... 5 1.3 Metal-Containing "-Conjugated Hybrid Materials .............................................. 8 1.3.1 Types of Metal-Containing "-Conjugated Materials..................................... 9 1.3.2 Optical Properties of Metal-Containing "-Conjugated Materials ................ 11 1.4 Dithienylethene Photoswitches ......................................................................... 16 1.4.1 Photochromism .......................................................................................... 16 1.4.2 Structure and Photoswitching of Dithienylethenes...................................... 18 1.5 Transition Metal-Coordinated DTE Hybrid Materials ....................................... 22 1.5.1 Effect of Coordination Geometry on Photobehavior ................................... 23 1.5.2 Modulation of Metal-Based and Ligand-Based Redox Behavior ................ 25 1.5.3 Interactions of Metal-Based and DTE-Localized Excited States ................. 30 1.6 Goals and Scope of Thesis................................................................................ 37 Chapter 2  Platinum-Sensitized Photocyclization of Dithienylethenes.................. 39  2.1 Introduction...................................................................................................... 39 2.2 Experimental .................................................................................................... 41 2.2.1 General ...................................................................................................... 41 2.2.2 Synthesis.................................................................................................... 42 2.2.3 Spectroscopic Measurements ..................................................................... 44 2.2.4 TDDFT Calculations.................................................................................. 44 2.3 Results and Discussion ..................................................................................... 45 2.3.1 Synthesis.................................................................................................... 45 v  2.3.2 UV-vis Absorption Spectroscopy ............................................................... 46 2.3.3 Low Temperature Photobehavior ............................................................... 50 2.3.4 Quenching of the Sensitized Photocyclization of Dithienylethenes............. 52 2.3.5 Intermolecular versus Intramolecular Photosensitization ............................ 55 2.4 Conclusions ...................................................................................................... 58 Chapter 3  Linker-Dependent Photophysics of Platinum-Coordinated Dithienylethnes..................................................................................... 60  3.1 Introduction...................................................................................................... 60 3.2 Experimental .................................................................................................... 62 3.2.1 General ...................................................................................................... 62 3.2.2 Synthesis.................................................................................................... 62 3.2.3 Spectroscopic Measurements ..................................................................... 65 3.2.4 TDDFT Calculations.................................................................................. 67 3.3 Results and Discussion ..................................................................................... 68 3.3.1 Synthesis.................................................................................................... 68 3.3.2 UV-vis Absorption Spectroscopy ............................................................... 71 3.3.3 Luminescence Spectroscopy ...................................................................... 76 3.3.4 Ultrafast Laser Spectroscopy...................................................................... 78 3.3.5 DFT Calculations....................................................................................... 87 3.4 Conclusions ...................................................................................................... 90 Chapter 4  Bifunctional Photoswitching of Platinum Acetylide Bridged Dithienylethenes ................................................................................... 93  4.1 Introduction...................................................................................................... 93 4.2 Experimental .................................................................................................... 97 4.2.1 General ...................................................................................................... 97 4.2.2 Synthesis.................................................................................................... 97 4.2.3 Determination of Photocyclization Wavelengths ........................................ 98 4.2.4 DFT Calculations....................................................................................... 99 4.2.5 Chemical Oxidation of 38oo/38oc/38cc...................................................... 99 4.3 Results and Discussion ................................................................................... 100 4.3.1 Synthesis.................................................................................................. 100 4.3.2 UV-vis Absorption Spectroscopy ............................................................. 101 4.3.3 NMR Spectroscopy.................................................................................. 104 4.3.4 Electrochemical Characterization............................................................. 107 4.3.5 Chemical Oxidation of 38oo/38oc/38cc.................................................... 112 4.3.6 Proposed Excited State Pathway for Bifunctional Photoswitching............ 116 4.4 Conclusions .................................................................................................... 117 Chapter 5  Photoresponsive Conjugated Platinum Acetylide Oligomers............ 118  5.1 Introduction.................................................................................................... 118 5.2 Experimental .................................................................................................. 120 5.2.1 General .................................................................................................... 120 vi  5.2.2 Synthesis.................................................................................................. 120 5.2.3 Chemical Oxidation of [40o/c]n ................................................................ 122 5.3 Results and Discussion ................................................................................... 123 5.3.1 Synthesis.................................................................................................. 123 5.3.2 NMR Spectroscopy.................................................................................. 127 5.3.3 UV-vis Absorption Spectroscopy ............................................................. 131 5.3.4 Electrochemical Characterization............................................................. 135 5.3.5 Chemical Oxidation of [40o/c]n ................................................................ 139 5.4 Conclusions .................................................................................................... 145 Chapter 6 6.1 6.2  Conclusions and Future Work .......................................................... 146  Conclusions .................................................................................................... 146 Future Work ................................................................................................... 152  References.................................................................................................................. 154 Appendix.................................................................................................................... 169  vii  List of Tables Table 3-1  Emission lifetimes and quantum yields for CH2Cl2 solutions of complexes 31–33. .................................................................................. 78  Table 4-1  Relevant DFT-calculated bond lengths (pm) are given for geometry-optimized structures. The Pt–P (o) and Pt–P (c) distances refer to the phosphines that are cis to the open and closed DTEs, respectively. .............................................................................. 112  Table 4-2  Relevant DFT-calculated atomic charges using the Hirschfeld method are given for the geometry-optimized structures. The Pt–P (o) and Pt–P (c) charges refer to the phosphines that are cis to the open and closed DTEs, respectively. The C atom charges refer to the Pt-coordinated C atoms................................................................... 112  Table 4-3  IVCT parameters and calculated electronic coupling for 38oc+/38cc+.......................................................................................... 115  Table 5-1  Electrochemical oxidation potentials of model compounds and oligomers. ............................................................................................ 138  Table 5-2  IVCT parameters for [40o/c]n+ and 38oc+/38cc+.................................... 142  viii  List of Figures Figure 1-1 Figure 1-2 Figure 1-3 Figure 1-4  Figure 1-5 Figure 1-6 Figure 1-7  Figure 1-8 Figure 1-9  Figure 1-10  Figure 2-1 Figure 2-2  Simplified electronic band diagram showing changes resulting from increasing chain length of oligothiophenes to polythiophene.  4  Jablonski diagram illustrating possible decay pathways upon photoexcitation.  6  Three general types of transition metal-containing "-conjugated materials.  9  The relative energies of metal-based d states and ligand-based " states can result in d-d transitions, MLCT transitions, or LMCT transitions as the lowest energy transitions.  12  Jablonski diagram for a protypical bichromophoric metal complex with 3MLCT and 3IL states nearly the same in energy.  14  Potential energy diagram for symmetrical, Class II mixed-valence complex.  15  Reversible photoisomerization of DTEs occurs between two isomers with different structural rigidity, color, and pathways of conjugation (shown in bold).  19  Representative absorption spectra for ring-open and ring-closed DTE isomers exemplified by 4o and 4c.  20  CV trace for (a) 7 and (b) the ring-closed version of 7 generated by irradiation with UV light. [7] = ~ 10-3 M, 0.1 M NBu4PF6 in CH2Cl2. (Permission for reproduction has been granted by the Royal Society of Chemistry.)  27  Generalized types of excited-state interactions for metalcoordinated DTEs. (1) Photoregulation of metal complex’s emission. (2) Metal-sensitized photoswitching of DTE. (3) Quenching of DTE photoactivity by energy transfer to the metal complex.  31  -5  Absorption spectra of CH3CN solutions (1.5 # 10 M) of complexes 27o (black) and 28 (blue) at room temperature.  47  -5  Absorption spectra of complex 27o (~ 10 M) in CH3CN, acetone, CH2Cl2, and toluene at room temperature. Dielectric constants ($) of each solvent are given in parenthesis.  48  Changes in absorption spectra when a CH3CN solution of (a) DTE proligand 4o and (b) complex 27o are irradiated with 302 nm light at room temperature.  49  Figure 2-4  Contour plots of the HOMO energy level for 27o and 27c.  50  Figure 2-5  UV-vis absorption spectra (a) and changes in emission spectra (b) when an EtOH:CH3OH (4:1) glass of complex 27o is warmed from 85 K to 125 K. Excitation = 470 nm.  51  Figure 2-3  ix  Figure 2-6 Figure 2-7  Figure 2-8  Figure 2-9  Figure 2-10  Figure 3-1  Proposed Jablonski diagrams rationalizing the low-temperature emission of complex 27o.  52  Changes in absorbance at % = 600 nm of CH3CN solutions of 27o (1.4 # 10-5 M) without quencher (!) or with varying concentrations of DABCO, 1.0 # 10-3 M ("), 1.0 # 10-2 M (#), and 0.10 M ($), upon irradiation with light >420 nm at room temperature.  53  Changes in absorbance at % = 600 nm when an EtOH:CH3OH solution of 27o is irradiated with light >420 nm at 150 K (#), 170 K ("), 200 K (!), and 295 K ($).  55  TCSPC lifetime measurements of phosphorescence from CH2Cl2 solutions of 29 (2 # 10-5 M) at room temperature, with increasing concentrations of 4c as the quencher (Q).  57  Proposed Jablonski diagram summarizing the photophysical behavior of complex 27o. Wavy lines represent a nonradiative process.  59  1  Changes in the 400 MHz H NMR spectra of 30o, in d6-acetone at room temperature, when irradiated with 365 nm light. (a) 30o, (b) 30o/30c, (c) 30c.  71  UV-vis absorption spectra of CH3CN solutions of complexes 30o, 32, and 33 at room temperature.  73  Changes in the UV-vis absorption spectra of a CH3CN solution of (a) 34o is irradiated with 254 nm light and (b) complex 30o is irradiated with 302 nm light at room temperature.  74  Percent conversion to the PSS as a function of time for CH3CN solutions of 27o and 30o irradiated at 415 < ! < 425 nm. Linear fits are shown.  75  Corrected emission (solid line) and excitation (dashed line) spectra for CH2Cl2 solutions of complexes 31 (black), 32 (red), and 33 (blue) at room temperature. Emission spectra were collected by exciting the solutions at 425 nm. Excitation spectra were collected by monitoring emission at 550 nm for complex 32 and 560 nm for complexes 31 and 33.  77  Transient absorption difference spectra of CH3CN solutions of complexes 32 (red) and 28 (black) at 800 ps after excitation at 355 nm at room temperature.  80  Figure 3-7 Transient absorption difference spectra of CH3CN solutions of complexes 31 (black) and 32 (red) at 800 ps after excitation at 355 nm at room temperature.  81  Figure 3-8 Transient absorption difference spectra of CH3CN solutions of complexes 32 (red) and 33 (blue) at 800 ps after excitation at 355 nm at room temperature.  82  Figure 3-2 Figure 3-3  Figure 3-4  Figure 3-5  Figure 3-6  x  Figure 3-9  Figure 3-10  Figure 3-11 Figure 3-12  Figure 3-13  Figure 4-1  Figure 4-2  Figure 4-3  Figure 4-4 Figure 4-5  Figure 4-6  Figure 4-7  Transient absorption difference spectra of CH3CN solutions of complexes 30o (black) and 33 (blue) at 800 ps after excitation at 355 nm at room temperature.  85  Transient absorption difference spectra of CH3CN solutions of (a) complexes 27o (black) and 30o (red) or (b) complexes 27c (black) and 30c (red) at 800 ps after excitation at 355 nm at room temperature.  86  Contour plots of the LUMO, HOMO, and HOMO-1 for complexes 32 and 33.  88  Proposed Jablonski diagram comparing the excited-state interaction of the 3CT state and varying 3IL states for complexes 31 – 33 after excitation of the CT transition. The shaded box represents limits on the energy of the 3IL state.  90  Proposed Jablonski diagram comparing the excited-state interaction of the 3CT state and 3IL state for complexes 30o after excitation of the CT transition. The shaded box represents limits on the energy of the 3IL state.  92  Generalized Jablonski diagram showing the energy transfer pathway responsible for preventing photoswitching of multiple interacting photochromes.  94  UV-vis absorption spectra of CH2Cl2 solutions of 38oo and 4o at room temperature, and the photostationary states generated when solutions of 38oo and 4o are irradiated with 365 and 254 nm light, respectively.  101  Changes in the UV-vis absorption spectra of CH2Cl2 solution of 38oo (2.3 # 10-5 M) when it is irradiated with 365 nm light at room temperature. Arrows indicate the isobestic points present after the first two 10-sec irradiations.  102  DFT calculated contour plots of the HOMO and LUMO of 38oo, 38oc, and 38cc.  104  Concentrations of 38oo ($), 38oc ("), and 38cc (#) as determined by 125 MHz 31P NMR spectroscopy when a CD2Cl2 solution of 38oo is irradiated with 365 nm light at room temperature.  105  125 MHz 31P NMR spectra at room temperature showing the conversion of 38oo & 38oc & 38cc in a CD2Cl2 solution upon irradiation with 365 nm light.  106  400 MHz 1H NMR spectra of (a) 38oo, (b) 38oo/38oc/38cc, and (c) 38oo/38oc/38cc in CD2Cl2 solution at room temperature.  106  xi  Figure 4-8  Figure 4-9 Figure 4-10  Figure 4-11 Figure 4-12  Figure 4-13 Figure 4-14 Figure 5-1  Figure 5-2 Figure 5-3  Figure 5-4 Figure 5-5  Changes in the DPVs of a CH2Cl2 solution of 38oo, at room temperature, as it is irradiated with 365 nm light. Electrolyte = 0.1 M [(n-Bu)4N]PF6, working electrode = Pt disk, counter electrode = Pt mesh, reference electrode = Ag wire. Referenced to SCE using the one-electron oxidation of decamethylferrocene as an internal standard.  108  Illustration of changes in electronic delocalization interconversion amongst 38oo, 38oc, and 38cc.  110  upon  Changes in the IR spectra for CH2Cl2 solutions at room temperature of (a) 4o and (b) 38oo upon irradiation with 254 and 365 nm light, respectively.  111  Schematic drawing illustrating the differences in delocalization between 38oc+ and 38cc+.  114  vis-NIR absorption spectra of a CH2Cl2 solution, at room temperature, of 38oo/38oc (black) and 38oc/38cc (blue) before oxidation and after 1 equivalent of oxidant is added to generate 38oo+/38oc+ (dash) and 38oc+/38cc+ (red).  114  +  +  vis-near-IR absorption spectrum of 38oc /38cc with deconvolution by Gaussian bands and the calculated fit (red).  115  Energy diagram showing the proposed excited state pathway for the ring-closing reaction from 38oc & 38cc.  117  MALDI-TOF mass spectrum of [40o]n. The primary series of ions, identified to have an (AB)n+ repeat pattern, is shown with squares. The secondary series of ions is shown by circles.  126  Changes in the IR spectrum of a CH2Cl2 solution of [40o]n at room temperature upon irradiation with 365 nm light.  127  Changes in the 400 MHz 1H NMR spectrum for a CDCl3 solution of [40o]n, at room temperature, upon conversion of the initially ring-open system to the PSS using UV light (365 nm). Circles indicate the resonances assigned to the two thienyl protons and squares correspond to the resonances assigned to the six methyl protons of the DTE.  128  Changes in the 125 MHz 31P NMR spectra at room temperature upon UV irradiation (365 nm) of a CDCl3 solution of [40o]n.  129  31  Changes in the integrated peak intensity for the P resonances appearing at 3.99 (! ), 4.14 (" ), and 4.28 ppm (!). For each 31P NMR spectrum collected at a given time interval, the most intense signal was integrated with a reference value of 1. The other signals were integrated relative to the most intense peak. The integrated values were totaled, and the relative percentage of each integrated peak intensity was calculated.  130  xii  Figure 5-6 Figure 5-7  Figure 5-8  Figure 5-9  Figure 5-10  Figure 5-11 Figure 5-12 Figure 5-13  Figure 5-14  Figure 6-1 Figure 6-2 Figure A-1  Changes in the 125 MHz 31P NMR spectrum, at room temperature, upon irradiation of a CDCl3 solution of 41o with 365 nm light.  131  Changes in the UV-vis absorption spectra of CH2Cl2 solutions of (a) 38oo or (b) [40o]n at room temperature upon irradiation with 365 nm light.  132  Changes in the UV-vis absorption spectra of CH2Cl2 solutions of (a) 41o or (b) [40o]n at room temperature when irradiated with 365 nm light.  134  -1  Cyclic voltammograms of 41o at 0.2 V s (0.1 M [(n-Bu)4N]PF6 in CH2Cl2) with (a) two consecutive scans (first scan = solid black line, second scan = dashed red line) and (b) after irradiation with 365 nm light until the PSS was reached (blue line). Electrolyte = 0.1 M [(n-Bu)4N]PF6, working electrode = Pt disk, counter electrode = Pt mesh, reference electrode = Ag wire. Referenced to SCE using the one-electron oxidation of decamethylferrocene as an internal standard.  136  Changes in the DPVs of a CH2Cl2 solution of [40o]n as it is irradiated with 365 nm light. (a) Changes observed up to 10 min of UV irradiation. (b) Changes observed between 12 and 45 min of UV irradiation. Electrolyte = 0.1 M [(n-Bu)4N]PF6, working electrode = Pt disk, counter electrode = Pt mesh, reference electrode = Ag wire. Referenced to SCE using the one-electron oxidation of decamethylferrocene as an internal standard.  137  +  +  Absorption spectra of CH2Cl2 solutions of 38oc /38cc [40o/c]n+ at their respective PSSs, at room temperature.  and 140  UV-vis absorption spectrum of [40o/c]n+ at the PSS (black) with the deconvolution by Gaussian bands and the calculated fit (red).  141  Schematic drawing illustrating how the possible extent of delocalization involved in IVCT transitions might occur in [40o/c]n depending on whether (a) several ring-closed DTEs are adjacent to each other or (b) only two ring-closed DTEs are adjacent to each other.  143  vis-NIR absorption spectra of CH2Cl2 solutions of [40o/c]n at the PSS (black dash) and partly converted to the PSS (red dash) before oxidation and after 1 equivalent of oxidant is added to generate [40o/c]n+ at the PSS (solid blue) and [40o/c]n+ partly converted to the PSS (solid green).  144  Types of photochromic Pt-alkynyl complexes discussed in this thesis.  146  Optical band gaps measured by the onset of absorption in CH2Cl2 solution.207,208  152  Decay of emission from complexes 31 (green), 32 (orange), and 33 (blue) in CH2Cl2 solution (% ex c = 453 nm). Red lines indicate monoexponential fits.  169 xiii  Figure A-2  Figure A-3  Figure A-4  Figure A-5  Figure A-6  Figure A-7  Figure A-8  Decays of transient signal averaged at wavelength regions 410 – 440 nm (black) or 650 – 680 nm (blue) for complex 32 upon excitation at 355 nm in CH3CN solution. Monoexponential fits of the data are shown in red.  169  Decays of transient signal averaged at wavelength regions 410 – 440 nm (black) or 650 – 680 nm (blue) for complex 31 upon excitation at 355 nm in CH3CN solution. Monoexponential fits of the data are shown in red.  170  Decays of transient signal averaged at wavelength regions 425 – 445 nm (black) or 680 – 700 nm (blue) for complex 33 upon excitation at 355 nm in CH3CN solution. Monoexponential fits of the data are shown in red.  170  Decays of transient signal averaged at wavelength region 435 – 455 nm for complex 30o upon excitation at 355 nm in CH3CN solution. A monoexponential fit of the data is shown in red.  171  Decays of transient signal averaged at wavelength regions 435 – 455 nm (black) or 600 – 620 nm (blue) for a CH3CN solution of complex 27c at the PSS upon excitation at 355 nm. Monoexponential fits of the data are shown in red.  171  Decays of transient signal averaged at wavelength regions 435 – 455 nm (black) or 600 – 620 nm (blue) for a CH3CN solution of complex 30c at the PSS upon excitation at 355 nm. Monoexponential fits of the data are shown in red.  172  Decays of transient signal averaged at wavelength regions 390 – 410 nm (black) or 720 – 740 nm (blue) for complex 28 upon excitation at 355 nm in CH3CN solution. Monoexponential fits of the data are shown in red.  172  xiv  List of Schemes Scheme 1-1……………………………………………………………………………… 17 Scheme 1-2……………………………………………………………………………… 21 Scheme 2-1……………………………………………………………………………… 41 Scheme 2-2……………………………………………………………………………… 46 Scheme 2-3……………………………………………………………………………… 57 Scheme 3-1……………………………………………………………………………… 69 Scheme 3-2……………………………………………………………………………… 83 Scheme 4-1……………………………………………………………………………… 95 Scheme 4-2……………………………………………………………………………… 96 Scheme 4-3…………………………………………………………………………….. 101 Scheme 5-1…………………………………………………………………………….. 124 Scheme 5-2…………………………………………………………………………….. 125  xv  List of Charts Chart 1-1…………………………………………………………………………………. 3 Chart 1-2………………………………………………………………...……………… 10 Chart 3-1……………………………………………………………………………...… 69  xvi  List of Symbols and Abbreviations °C  degrees Celsius  Å  Angstrom  '()1/2  bandwidth at half-maximum (in wavenumbers)  *  chemical shift  !  molar absorptivity  %  wavelength  +  lifetime  ,  quantum yield  )  frequency  ()max  energy at absorption maximum (in wavenumbers)  ADF  Amsterdam Density Functional  Bu  butyl  bpy  bipyridine  CCD  charge-coupled device  cm-1  wavenumber  CS  charge-separated  CT  charge transfer  CV  cyclic voltammetry/voltammogram  d  doublet  DABCO  1,4-diazabicyclo-[2.2.2]octane  DFT  density functional theory  DMF  dimethylformamide xvii  DPV  differential pulse voltammetry/voltammogram  DTE  dithienylethene  Eg  energy gap/electronic bandgap  EI-HRMS  electron impact high-resolution mass spectrometry  ESI-MS  electrospray ionization mass spectrometry  ET  energy transfer  EtOH  ethanol  FT  fourier transform  fwhm  full width half maximum  GS  ground state  HOMO  highest occupied molecular orbital  hrs  hours  IL  intraligand  IR  infrared  IVCT  intervalence charge transfer  KC  conproportionation constant  LED  light emitting diode  LLCT  ligand-to-ligand charge transfer  LMCT  ligand-to-metal charge transfer  LUMO  lowest unoccupied molecular orbital  m  multiplet  m/z  mass-to-charge ratio  min  minute(s)  MALDI  matrix-assisted laser desorption ionization  MLCT  metal-to-ligand charge transfer xviii  MO  molecular orbital  nm  nanometers  NMR  nuclear magnetic resonance  NLO  nonlinear optical  O.D.  optical density  OFET  organic field effect transistor  OLED  organic light emitting diode  OPV  organic photovoltaic  Ph  phenyl  ppm  parts per million  PSS  photostationary state  PTFE  polytetrafluoroethylene  R  donor-to-acceptor distance  RET  resonance energy transfer  S  Siemens  s  singlet  SCE  standard calomel electrode  t  triplet  TA  transient absorption  TCSPC  time-correlated single photon counting  TDDFT  time dependent density functional theory  THF  tetrahydrofuran  TMS  trimethylsilane  TOF  time-of-flight  trpy  terpyridine xix  TLC  thin layer chromotography  UV  ultraviolet  Vab  electronic coupling  vis  visible  xx  Acknowledgements The completion of this thesis would not have been possible without the unwavering support of my friends, family, and co-workers. First and foremost, the biggest thank you goes to my supervisor, Mike Wolf. I am eternally indebted to you for all of the knowledge, the advice, and the opportunities you have given me. You have made the whole experience as enjoyable as possible, and I will miss our discussions, over a cup of coffee of course. I was extremely fortunate to be surrounded by an amazing group of co-workers in the Wolf and MacLachlan groups during my whole time here. There are of course too many to list, but I feel particularly grateful to the “Boyz in Da Hood” – Tim Kelly, Ago Pietrangelo, and Bryan Sih, who combined taught me most of what I know about working in a lab. I would like to express my appreciation to Pierre Kennepohl for reading this thesis in its entirety (something even I have not done) and for all the useful discussions during the past five years. I would like to acknowledge and thank Neil Branda, who first sparked my interest in photochromic materials. Our marathon discussions and brainstorming sessions were an indelible part of the experience, and I am grateful for being a more skeptical and thorough researcher because of it. Along with Neil, I would like to thank Dr. Jeremy Finden and C.-J. Carling, who provided me with reliably pure starting materials.  Many of the experiments would not have been possible without various  support staff in the chemistry department.  Many thanks go to Saeid Kamal, who  introduced me to lasers and without whom I would never have been able to collect a large portion of data for this thesis. And I appreciate all of the help from the guys in the Mechanical shop – who were always eager to help out. xxi  Finally, and most importantly, thank you to my family. You provided me with all of the tools and support I could possibly have to make it to this point. You have always encouraged me to pursue my dreams and go wherever they might take me. Without that, I would probably still be in New Jersey!  xxii  Dedication  For my grandparents  xxiii  CHAPTER 1  Introduction  1.1  Overview At the present time it is nearly impossible for most people to live a day without  having at least one interaction with an electronic device. Even in remote areas of Africa where there is no electricity, it is common for people to communicate via cell phones. The ubiquity of microelectronics in the modern era is a credit to the industry’s unfettered push to make integrated circuits smaller, faster, and cheaper. Since the discovery of the transistor in 1947,1 the dramatic increase in computing power and speed is primarily attributed to the miniaturization of the individual transistors.  The success of the  computing revolution is rooted in the development of techniques for the massively parallel patterning of silicon. Despite silicon’s many useful properties, the increasingly smaller feature size and spacing of the individual transistors is pushing the material to its physical limits.2 The entire process of growing electronics-grade single crystal silicon and then patterning the wafers is incredibly energy intensive and costly.3 Moreover, future applications such as photovoltaic fabrics and flexible displays are not amenable to current single crystal-based platforms, although recent breakthroughs in nanowire4,5 and thin-film6,7 technologies are beginning to reverse that dogma. One strategy to circumvent the drawbacks of conventional silicon-based technology is the development of synthetic, organic-based materials that can be tailored to exhibit desired properties.  Organic-based materials offer the biggest benefits in 1  versatile processing such as spray-coating,8 inkjet printing,9 and stamping10 – all of which are amenable to high-throughput, low-cost roll-to-roll processing.11  These methods  generate significantly less waste and use less energy than silicon fabrication processes resulting in huge savings in cost and reduced environmental impact. The contents of this thesis are a contribution to the development of synthetic "conjugated materials, an area of research that has been intensively studied for the past thirty years.12 This introductory chapter serves to form the conceptual basis for this work and provide perspective on how this research contributes to the advancement of the field. First, a general description of organic "-conjugated materials is given. The discussion covers their structure and the origin of their optical and electronic properties. This is followed by an introduction to transition metal-containing "-conjugated materials. The second half of this Chapter is devoted to photochromic materials. An overview of existing photochromes is given with emphasis on the dithienylethene (DTE) family. This is followed by a literature review of metal-coordinated DTEs, with a focus on the intramolecular electronic interactions present in such systems. Finally, the goals and scope of this thesis are stated.  1.2  Organic "-Conjugated Materials In the late 1970s, metal-like conductivity was observed for the first time in  organic polymers.13 This work, for which the Nobel Prize was awarded in 2000, began a period of intensive effort working towards the implementation of these materials in electronic devices. To date, the number of applications investigated for the use of "conjugated polymers has reached staggering proportions.12,14  The wide-ranging list  includes laser dyes, light-emitting diodes (LEDs), solar cells, electrode interconnects, 2  artificial muscles, electrochromic windows, and numerous sensing applications. Currently, the state-of-the-art materials have reached the commercialization stage for organic light emitting diode (OLED)15 and organic photovoltaic (OPV)16 applications. 1.2.1 Structure of "-Conjugated Materials One of the benefits of organic materials is their synthetic versatility. Rational synthetic design can be utilized to manipulate the chemical structure in order to tune various properties of the material. Since the advent of polyacetylene, several families of conjugated polymers have emerged that demonstrate marked improvement in optoelectronic properties and processability (Chart 1-1). Although only the most widely studied polymeric systems are shown here, the number of conceivable oligomeric and polymeric architectures with extended "-conjugated structure gives credence to the scope of achievable properties offered by these materials by purposeful synthetic tuning. Chart 1-1  n  n  polyacetylene  n  poly-p-phenylene H N  S n  polythiophene  poly-p-phenylenevinylene H N n  n  polypyrrole  polyaniline  The common structural scaffold of "-conjugated materials is a backbone consisting of alternating single and double bonds between sp2-hybridized carbon atoms. The critical element of this structure is that all of the adjacent carbon atoms possess one non-bonding electron in the pz orbital.  Additionally, some derivatives have been  prepared that incorporate heteroatoms, such as sulfur or nitrogen, which also contribute 3  electrons to the " system. Compared to the other --bonding electrons in the polymer backbone, the " electrons are loosely bound enough to migrate along the polymer chain or to other nearby polymer chains. Since the carbon atoms of the conjugated backbone are closely spaced together, the wavefunction of each pz electron overlaps with the wavefunctions of the adjacent pz electrons.  As increasingly more electrons are  incorporated into the conjugated system, the discrete energy levels that define these electrons broaden into a band of continuous energies. This model is illustrated in Figure 1-1 using polythiophene as an example. To remain within the scope of this thesis, the subsequent discussion of "-conjugated materials will focus on thiophene-based systems.  Figure 1-1 Simplified electronic band diagram showing changes resulting from increasing chain length of oligothiophenes to polythiophene. The monomer, one thiophene ring, contributes six electrons to the "-system, one from each carbon and two from sulfur. Addition of subsequent thiophene rings results in an increase in energy of the highest-energy occupied molecular orbital (HOMO) and decrease in energy of the lowest unoccupied molecular orbital (LUMO). Finally, past a given length of conjugation, the electronic structure of the whole system can be viewed 4  as band-like. Analogous to the semiconductor band model, the occupied " orbitals form the valence band, and the unoccupied "* orbitals form the conduction band. These bands are separated by an energy gap, Eg, a range of energies for which no states exist. 1.2.2 Electronic and Optical Properties of Poly- and Oligothiophenes The electronic and optical properties of "-conjugated materials are dominated by the filled " orbitals and unoccupied "* orbitals. The size of Eg, is related to both the color of the material and its conductivity. Provided that Eg is small enough, "-conjugated polymers are generally semiconducting when prepared as the pristine material in the neutral state. The dramatic increase in conductivity of conducting polymers originates from oxidizing, or doping, the material.  For instance, polythiophene undergoes an  increase in conductivity from 10-8 S/cm to 103 S/cm upon doping with iodine.12 Doping increases the conductivity of the material by the formation of states between the HOMO and LUMO energies. Population of these states leads to a partially-filled valence band, the origin of the enhanced conductivity. The HOMO and LUMO energies are largely governed by the geometric conformation of the "-conjugated pathway. Since polythiophenes lack a rigidly defined structure, inter-ring twisting breaks up the overlap of pz orbitals forming the conjugated pathway thereby affecting their electronic and optical properties.17 Therefore, strategies such as rigidification of the "-conjugated backbone by ring-fused monomers18 or substitution of the thiophene rings19,20 have been employed to tune the length of "conjugation. The consequences of these structural changes give rise to the observable chromism of these materials.21  5  Photoexcitation of oligothiophenes excites an electron from an occupied " orbital, to an unoccupied "* orbital.  These transitions are therefore referred to as "&"*  transitions. As shown in Figure 1-1, lengthening the conjugated backbone reduces the energy gap between these states. This change is reflected in the absorption and emission spectra of oligothiophenes, for which both the absorption and emission energies shift to lower energy with increasing chain length. A Jablonski diagram showing the typical excited-state dynamics of a photoexcited, conjugated system is shown in Figure 1-2. The notation used here is consistent throughout this thesis. Absorption and emission are indicated by a straight line, which is representative of their instantaneous nature, occurring in about 10-15 s. Non-radiative transitions, such as internal conversion and intersystem crossing, are indicated by wavy lines and occur on a slower timescale on the order of 10-12 s. The singlet ground state is depicted as S0, and the higher energy excited states are denoted S1, S2, and so forth. Within each of the singlet states, vibrational states exist that are separated by some energy spacing.  Figure 1-2 Jablonski diagram illustrating possible decay pathways upon photoexcitation. Absorption usually occurs to one of the higher vibrational states, so immediately following absorption, the excited molecule relaxes to the lowest possible vibrational state 6  in S1. This process is referred to as internal conversion and is non-radiative. Internal conversion is responsible for what is called the Stokes shift, the difference in energy between an absorbed photon and an emitted photon. For thiophene-based systems, the different vibrational states reflect differences in the lengths of conjugated segments along a given chain. The torsion angle between adjacent thiophene rings governs the extent of orbital overlap and can limit the effective conjugation length of a long chain to shorter segments.  Population of multiple vibrational states results in broad, structureless  absorption bands.  Consequently, absorption bands of polythiophene are typically  significantly broadened compared to those of short oligomers. Emission from the excited singlet state is called fluorescence and often occurs to various vibrational states in S0, which then all undergo internal conversion to the lowest vibrational state. The decay to multiple vibrational states in S0 from the same S1 state is the origin of band structure in the emission spectrum. The structured fluorescence spectra observed for many thienylbased systems is attributed to a well-defined excited-state structure with co-planar thienyl rings.22  Another potential route for decay back to the ground state is population of the  triplet manifold. If sufficient spin-orbit coupling is present, the spin of the excited electron in the S1 state can flip, a process referred to as intersystem crossing. Decay from the T1 state to the S0 state is formally spin-forbidden, so this process is very slow, on the order of microseconds up to seconds.  Emission from the T1 state is termed  phosphorescence and is characterized by a large Stokes shift and long excited state lifetime. One electronic property of oligo- and polythiophenes that has enormous potential for applications is the ability to be reversibly oxidized and reduced by electrochemical means.  Analogous to how the absorbance of oligothiophenes shifts to lower energy as  the chain lengthens, a similar relationship exists between the oxidation potential and 7  chain length. The rise in the HOMO energy with each successive addition of another thiophene ring results in a decrease in oxidation potential.23 Longer oligothiophenes are capable of stabilizing charge because it can be delocalized over the entire conjugated system.24  Cyclic voltammetry (CV) has been used extensively to characterize the  electronic properties of poly- and oligothiophenes. For polymers and long oligomers, voltamograms typically show broad redox waves that reflect the varying conjugation lengths present due to defects in the chain, the same origin of the broadness of absorption bands in the UV-vis spectrum.  Short oligomers tend to exhibit more resolved and  reversible redox waves. Overall, the capability of thiophenes to exhibit well-behaved redox processes and their propensity to delocalize charge makes thiophene-based materials appealing for a wide variety of electronics-based applications.  1.3  Metal-Containing "-Conjugated Hybrid Materials Organic "-conjugated materials that incorporate transition metal complexes make  up a class of materials that continues to garner significant interest.25,26 As discussed in the previous section, the electronic structure of a "-conjugated backbone gives rise to functional properties such as conductivity and luminescence.  On the other hand,  transition metal complexes offer a wide range of optical, electronic, magnetic, catalytic, and electrochemical properties and can also be used as structural elements. The general aim for these hybrid materials is to combine the important properties of the organic and inorganic components in a way that either generates new properties or enhances the optical or electronic properties of the bulk material.  8  1.3.1 Types of Metal-Containing "-Conjugated Materials Transition metal-containing "-conjugated materials are classified into three structural types.25  The categorization of the material gives some indication of the  strength of interaction, or coupling, between the metal center and conjugated backbone. Type I materials are characterized by attachment of the metal center to the conjugated backbone via a saturated linkage, such as an alkyl chain. For these systems, the metal complex has little or no coupling to the conjugated backbone and therefore the properties of each component remain relatively the same. Complex 1, shown in Chart 1-2, is a Type I material that insulates the Ru center from the terthienyl moiety. Mirkin and coworkers show that localization of charge on the thienyl backbone does not allow tuning of the electronic properties of the appended metal center.27  Figure 1-3 Three general types of transition metal-containing "-conjugated materials.  9  Chart 1-2  Cl  Ru  S  S  PPh2  S  S  S  S N  C8H17  C8H17 S  C8H17  C8H17  CO  M  N  n  M = [Ru(bpy)2][PF6]2 [Os(bpy)2][PF6]2  2  1 P(C4H6)3  S  Pt P(C4H6)3  S  S n  3  When the metal center and conjugated backbone are electronically coupled, the material is classified as Type II. In these systems, the metal coordinates in a manner that directly affects the "-conjugation pathway.  One possibility is that the coordination  geometry perturbs the planarity of the conjugated backbone, affecting the "-orbital delocalization. These effects can be manifested in changes in the absorption spectrum, luminescence, and redox behavior of either the metal and/or polymer. Schanze and coworkers synthesized polymer 2 with a thiophene backbone that incorporates a bipyridyl moiety, in order to chelate Ru and Os atoms directly to the conjugated backbone.28 They demonstrate that strong interaction between metal-based and thienyl-based excited states allows for energy transfer to occur between them, depending on the relative alignment of their respective energy levels, which can be tuned by changing the metal atom. The final category, Type III, consists of materials in which the metal center is part of the "-conjugated pathway.  These materials demonstrate the strongest coupling  10  between the metal center and conjugated backbone.  A good example of Type III  materials, and of utmost relevance to the work in this thesis, is the family based on the metal alkynyl framework, complex 3 for example. These systems feature a wide range of metals (e.g. Fe, Au, Ru, Rh, Ni, Pd, and Pt) and are known to form rigid, linear systems.29 Bond formation between the metal d orbital and alkynyl " orbital permits the conjugated pathway to extend through the metal center between the organic segments.30 Depending on the electronic nature of the organic segment linking the metal centers, the HOMO and LUMO represent various mixtures of d orbital and " orbital character. The structure and electronic delocalization observed in these systems has led to discoveries of materials exhibiting large nonlinear optical (NLO) responses31 and electrical conductivity.32 Additionally, these properties can be combined with other useful physical properties of these materials such as liquid crystallinity.33,34 1.3.2 Optical Properties of Metal-Containing "-Conjugated Materials The optical properties of hybrid metal-containing "-conjugated systems often derive from combinations of metal-based, ligand-based, and mixed metal-ligand absorptions. Once coordinated to the metal, the electronic transitions localized to the "conjugated ligand without participation of the metal are referred to as intraligand (IL) transitions. Whereas photoexcitation of organic "-conjugated systems results exclusively in electronic transitions between " MOs, electronic transitions involving d orbital electrons are commonplace for transition metal complexes. There are three types of common optical transitions that occur involving d states. One possibility is a d&d transition, which is essentially an excitation within the metal atom itself. These transitions are Laporte forbidden, but the loss the symmetry in a compound, due to vibrational modes for instance, allows these transitions to occur, albeit at a far lower 11  oscillator strength than IL, MLCT, or LMCT transitions.  The d&d transitions are  widely regarded to undergo radiationless decay or result in ligand dissociation because of geometrical deformation experienced in the excited state.35,36 The energy gap between the d states is a tunable property controlled by the ligand field and nature of the metal, characterized by its oxidation state and spin. When ligands have low-lying "* orbitals, below the lowest accessible d state, d electrons can be promoted to the ligand "* orbital. These d&"* transitions are referred to as metal-to-ligand charge transfer (MLCT) transitions. The converse scenario is also possible. Electronic "&d transitions are referred to as ligand-to-metal charge transfer (LMCT) transitions. Although widely used in the literature, the descriptions used here represent simplified cases that ignore realistic considerations for systems in which electrons occupy orbitals with both metal and ligand character. Therefore in some cases, transitions might be more appropriately referred to as mixed transitions, e.g. MLCT/LLCT transitions.  Figure 1-4 The relative energies of metal-based d states and ligand-based " states can result in d-d transitions, MLCT transitions, or LMCT transitions as the lowest energy transitions.  12  The presence of the metal ion can have a significant effect on the excited state dynamics. When a heavy metal atom is incorporated into the organic "-conjugated system, the rate of intersystem crossing is increased due to the enhanced spin-orbit coupling effect.37-39  This behavior has become the salient feature of complexes  incorporating metals such as Ru, Os, Re, Pt, or Ir. Due to the spin-forbidden relaxation of the triplet state to the singlet ground state, the excited state species is typically longlived enough to undergo intra- or intermolecular energy transfer or electron transfer. Predicting the photophysics of transition metal-containing conjugated materials is often complicated by the fact that there are many states close in energy that are either thermally accessible or accessible by energy transfer processes such as Förster or Dexter energy transfer.40 The Jablonski diagram shown in Figure 1-5 illustrates the potential decay pathways of a metal complex possessing nearly isoenergetic 3MLCT and 3IL states. For metals with large spin-orbit coupling, it is widely accepted that after selective excitation of a MLCT transition, the populated 1MLCT state undergoes intersystem crossing to the 3  MLCT state with a quantum yield of unity.41 Provided that a 3IL state is close enough in  energy with enough coupling to the 3MLCT state, both states can be populated simultaneously in equilibrium with each other.42 The decay of the excited-state species ultimately depends on the magnitude of rate constants for the energy transfer between the two states, k1 and k–1, and the rates of decay from each respective state, k2 and k3. The forward and backward rates of energy transfer between the 3MLCT and 3IL are primarily governed by the magnitude of the energy gap between the two. As the 3IL state falls below the 3MLCT state in energy, k–1 decreases until the point at which excited-state decay exclusively proceeds through the 3IL state.  13  Figure 1-5 Jablonski diagram for a protypical bichromophoric metal complex with 3 MLCT and 3IL states nearly the same in energy. Another factor affecting the optical properties of metal-containing "-conjugated materials is the extent of charge delocalization throughout the metal centers and organic fragments. One method for evaluating the extent of delocalization is Hush theory,43,44 which was established to quantitatively describe the delocalization present in mixedvalence complexes by analysis of charge transfer bands appearing in their absorption spectra. The term mixed-valence describes those complexes in which two, interacting nuclei possess either different oxidation states or non-integral oxidation states, depending on the exact nature of the system. The archetypical mixed-valence compound is the dye Prussian blue, KFeIII[FeII(CN)6], whose deep blue color is the result of a charge transfer transition between the FeII and FeIII ions.45 Robin and Day have classified mixed-valence systems based on the extent of delocalization.46 Systems in which the charges are completely localized are categorized as Class I. No charge transfer transitions are observed in Class I systems. If there is some delocalization of charge with weak coupling, the system is categorized as Class II. The remaining systems that demonstrate complete delocalization of charge make up Class III. The case of a symmetrical Class II system is illustrated by a potential energy 14  diagram in Figure 1-6. The initial and final states, or donor and acceptor states, are coupled via a single harmonic oscillator. For Classes II and III, sufficient coupling exists for an intervalence charge transfer (IVCT), an optically induced electron transfer between states, to occur. The absorption bands originating from IVCTs typically appear in the visible or near-IR region of the spectrum.  Figure 1-6 Potential energy diagram for symmetrical, Class II mixed-valence complex.  Equation 1-1 Equation to calculate the effective electronic coupling present in a Class II mixed-valence system. According to the Hush model, the effective electronic coupling for a Class II system is calculated by using Equation 1-1, where Vab is the electronic coupling (in cm-1), !max is the maximum extinction coefficient, ()max is the energy of the band (in cm-1) at its maximum absorbance, '()1/2 is the full-width of the band at half-maximum (in cm-1), and R is the distance between redox centers in Ångstroms. The Vab value reflects the degree 15  of overlap of electronic wave functions of the donor and acceptor and is the definitive measure of delocalization in the system. Evaluating the degree of delocalization is important because it is a measure of other physical properties, such as conductivity and non-linear optical response, which are both enhanced by electron delocalization.  1.4  Dithienylethene Photoswitches  1.4.1 Photochromism Photochromism is defined as the light-induced interconversion of a chemical species between two isomers with different absorption spectra.47  Essentially, a  photochrome undergoes a color change in response to the absorption of light. Since each isomer has a unique absorption spectrum, it is possible to cycle between isomers by selectively using different wavelengths of light. Photochromic lenses for eyeglasses are one the most commercially successful and recognizable applications of this phenomenon. Upon exposure to the sun’s UV light outdoors, photochromes isomerize from a transparent isomer to a colored isomer in order to tint the lens. Photochromic molecules are often referred to as “molecular switches” or “photoswitches” due to their reversible switching between two molecular species. Throughout the rest of this thesis, the term “photoswitching” will be in the context of the light-induced transformation between isomers. On a fundamental level, the observable change in color is indicative of a change in the electronic structure of the chromophore. As discussed in Section 1.2, extension of the "-conjugated pathway reduces the HOMO-LUMO gap, giving rise to lower energy absorption and thereby appears as a color change of the sample. Since the discovery of 16  spiropyrans48 in 1952, many photochromes have been studied.49 The three most common families of photochromes – azobenzenes, spiropyrans, and DTEs – are shown in Scheme 1-1. Generally, these chromophores switch between two isomers that exhibit changes in the length or pathway of conjugation. Molecular switches are an interesting class of "conjugated materials because they incorporate a dynamic switching element that is absent in conventional, static conjugated systems. Scheme 1-1 R  azobenzenes  UV  N N  N N  vis or ! R  R  R spiropyrans  UV N O  N  O  vis or ! dithienylethenes  UV R  S  S  R  vis  R  S  S  R  Photochromes that are responsive to light and temperature are T-type, and those that are only light sensitive are P-type.50  Azobenzenes and spiropyrans are both  examples of T-type chromophores. Azobenzenes exhibit cis-trans isomerism around the dinitrogen double bond. Spiropyrans undergo a bond formation/cleavage of a carbonoxygen bond to interconvert between a netural isomer and zwitterionic species exhibiting  17  extended conjugation. For azobenzenes and spiropyrans, the isomer exhibiting the longer conjugated pathway is thermally unstable because the transition state leading to isomerization is thermally accessible. DTEs are P-type chromophores and rely solely on a photoinitiated 6" electrocyclization to interconvert between isomers that are both thermally stable. Recent work in photochromism has been devoted to utilizing photochromic materials for advanced applications that exploit the change in electronic structure in ways other than observing a color change. By functionalizing the photochromic scaffold with various substituents, reversible changes in the electronic structure of the photochrome can affect properties such as luminescence,51,52 magnetism,53,54 refractive index,55,56 oxidation/reduction potentials,57 and conformational flexibility.58 Modulation of these properties can be applied for use in data storage,57,59 molecular logic systems,60,61 drugdelivery,62 and light-gated catalysis,58 for example.  Considerable interest has been  focused on the DTEs because their properties are amenable to applications in devices that conceivably would need to be cycled thousands of times in devices at elevated operating temperatures. The low frequency of undesirable side-reactions during isomerization, thermal stability of both isomers, and the ability to maintain photoactivity in the solid state makes DTEs suitable for rigors of commercial application.63 1.4.2 Structure and Photoswitching of Dithienylethenes The DTE photochromic system was developed by Irie in the late 1980s and is based on the well-established photochemistry of 1,3,5-hexatrienes.64 Upon exposure to UV light, 1,3,5-hexatrienes undergo a 6" electrocyclization. This ring-formation occurs in a conrotatory fashion, abiding by Woodward-Hoffman rules.65 The controtatory ringclosure produces two new stereogenic centers that result in two stereoisomers, either R,R 18  or S,S. The cycloreversion, or ring-opening reaction, is accomplished by selectively irradiating the low energy absorption of the ring-closed isomer with visible light. X  R  X X  X  X X  S  S  ring-open flexible transparent  X X  UV R  vis X = H or F  R  S  X X  X X S  R  ring-closed rigid colored  Figure 1-7 Reversible photoisomerization of DTEs occurs between two isomers with different structural rigidity, color, and pathways of conjugation (shown in bold). To create the hexatriene scaffold, two thiophenes are linked by an ethene bridge. Thiophenes were found to be the most suitable aromatic rings, compared to phenyl, furan, and pyrrole, because their low aromatic stabilization energy enhances thermal stability.66 Typically, the ethene bridge is part of a cyclic system to prevent cis to trans isomerization around the ethene bond. This also drastically enhances photochromic performance by red-shifting the absorption of the ring-closed isomer, a consequence of improved planarity of the "-conjugated pathway in the ring-closed isomer. The inclusion of a perfluorinated cyclopentyl ring, instead of the perhydro derivative, enhances fatigue resistance and increases the difference in absorption maxima of the ring-open and ringclosed isomers.63 The significant difference in the absorption of the ring-open and ringclosed isomers is largely attributed to the change in the "-conjugated pathway (Figure 17).  Typical absorption spectra that characterize the ring-open and ring-closed DTE  isomers are shown in Figure 1-8.  19  Figure 1-8 Representative absorption spectra for ring-open and ring-closed DTE isomers exemplified by 4o and 4c. Since the ring-open isomer is a flexible system, it is able to interconvert between the so-called parallel and anti-parallel isomers (Scheme 1-2).67 This nomenclature refers to the relative geometric arrangment of the thienyl rings to each other. Photocyclization of the ring-open isomer can only occur from the anti-parallel conformation, which exhibits C2 symmetry.68 It has been demonstrated that derivatives favoring the antiparallel conformation exhibit higher quantum yields.69-71 Strategies such as increasing the steric bulk of the thiophenes or inclusion of the DTE in a linear framework, such as a polymer backbone, have proven successful in this vein.  20  Scheme 1-2  R  F F  F F F F  S  S  R  R  F F  F F F F  S  S  R  ring-open parallel  ring-open anti-parallel  X  UV  UV  vis F F R  S  F F  F F S  R  ring-closed  The photostationary state (PSS) is defined as the percentage of conversion from the ring-open to ring-closed isomer for a given wavelength of irradiation. Since the ringopen and ring-closed isomers both exhibit absorbance in the UV region, irradiation with light in this spectral region simultaneously initiates ring-closing and ring-opening reactions that generate an equilibrium composition of ring-open and ring-closed isomers that is dependent on the relative rates of each photoreaction. The rate is proportional to the isomer’s absorption cross-section at the particular wavelength(s) of irradiation and the quantum yield of the photoreaction. Once the PSS is reached further irradiation does not result in any additional conversion. Throughout this thesis, the term PSS is determined to be reached when there is no observable change in the absorption spectrum, NMR spectrum, IR spectrum, or voltammogram. It is calculated using Equation 1-2.  21  PSS =  # of molecules isomerized " 100% total # of molecules  Equation 1-2 Equation to calculate the PSS for a mixture of photochromic isomers.  ! 1.5  Transition Metal-Coordinated DTE Hybrid Materials As discussed in section 1.3, hybrid, metal-containing "-conjugated systems  combine the properties unique to the inorganic and organic components to generate materials with new properties.  Despite a plethora of studies directed towards the  development of DTEs over the past two decades, relatively few have investigated the intramolecular interactions present in metal-coordinated DTEs. In addition to offering a wide spectrum of tunable light-absorbing and emitting behavior, metal complexes can be interesting structural elements utilizing different coordination modes that are capable of affecting intra- and intermolecular electronic interactions. The flurry of recent work in this area is beginning to elucidate the interactive roles of the metallic and photochromic components, and the interest in this area is evidenced by the publication of a few recent review articles that cover the general area of hybrid photochromic systems.72-74 For the remaining part of this chapter, I will review the interactions that exist between metal centers and DTEs and how these interactions result in the observed behavior of these dynamic systems. The interactions between various metal complexes and the DTE moiety can be generally classified as either ground state interactions or excited state interactions. Examples of ground state interactions are those in which properties of the DTE and metal center are altered simply as a result of coordination to each other. For instance, the metal center can coordinate to the DTE in such a way that it forces the photochrome into a 22  geometrical  conformation  unattainable  as  a  proligand  in  solution.  Changes in the redox potentials of the metal center or DTE are also considered to occur due to ground state electronic interactions. Excited state interactions are those that typically involve energy transfer processes between metal-based and DTE ligand-based excited states. Since isomerization of the DTE is a photoinitiated excited-state process, the strength of coupling and the relative energies of the different excited states present in the hybrid systems are of utmost importance to understand. 1.5.1 Effect of Coordination Geometry on Photobehavior One useful approach to designing hybrid photochromes is to utilize the metal center as a structural element. Metal complexes offer a variety of coordination modes and geometries which can be carefully chosen to elicit a certain behavioral response of the photochrome.  Furthermore, once the photochrome is coordinated, it adopts a  geometry dictated by the metal center. The adoption of a given geometry can affect the racemization of the ring-open isomer between parallel and anti-parallel conformers. Since metal complexes are bulky, steric effects often force exclusive formation of the anti-parallel conformation and thereby increase the quantum yield.71 On the contrary, coordination can restrict the conformation to the non-photoactive parallel conformer75 or to influence solid-state packing in such a way that photocyclization is suppressed.76  N  O O Ir N S  S  N  2  5  N  N  S  Re(CO)3Cl  S  6  23  The absorption of DTEs is governed by the extent of " conjugation present in each isomer. Being able to tune the spectral response of DTEs to different wavelengths is an important goal in the development of photochromic materials for specific applications. Some groups have taken an approach to modifying the length of " conjugation based on coordination of the DTE to a metal center. In these cases, the metal centers are acting as structural elements that rigidify the structure of the DTE. Tian and co-workers prepared complex 5, featuring an Ir complex coordinated to one of the thiophenes and an appended pyridine.77 When the DTE is uncomplexed, free rotation of the pyridine ring restricts the extent of conjugation over the two aromatic rings.  By forming a bidentate binding  pocket with the two aryl rings, complexation to the metal locks the two aromatic rings into a planar conformation and effectively lengthens the conjugated pathway. This effect is most clearly manifested in differences in the absorption spectra of the Ir complex relative to the uncomplexed DTE precursor. Although the open form only displays a modest 10-nm red shift, the lowest energy absorption of the ring-closed isomer undergoes approximately a 100-nm red shift when complexed. These changes in absorption reflect a strategy for extending the sensitivity of cycloreversion reaction of DTEs to the near-IR region. The same strategy for using metal coordination to achieve near-IR photochromic behavior was utilized by metal coordination of the bridging unit between the thiophenes. Complex 6 incorporates an imidazole as the bridging unit between the thiophenes, instead of the more common perfluorocyclopentyl group.78 The N-containing imidazole acts as part of the binding site involving the central ring. Forcing the imidazole and appended 2pyridyl group to be planar, upon coordination to Re, extends the "-conjugated system. This is particularly evident from large red shifts in the absorption spectra of the open and closed forms. Compared to the uncoordinated ligand, the ring-closed isomer has its 24  absorption maxima red-shifted 130 nm into the near-IR, from 580 nm to 710 nm, when it is complexed to the metal. 1.5.2 Modulation of Metal-Based and Ligand-Based Redox Behavior As discussed so far, the reversible isomerism of DTEs is a light-induced reaction, taking place via excited state pathways. In the early 2000s, groups began investigating the redox activity of DTE systems, which owe their favorable electrochemical behavior to the electron-rich thiophenes.79-82  It was discovered that substitution of the DTE  scaffold with redox-active substituents could result in electrochromic behavior, i.e. electrochemical oxidation or reduction caused isomerization of the DTE.  This  significant discovery paved the way to develop DTE-based systems with more sophisticated switching components that respond to multiple external stimuli with the ability to trigger the reversible switching of DTEs. With regards to developing electrochromic materials, coordination of DTEs to transition metal complexes is a very appealing strategy. First, a large body of work already exists studying the electrochemical behavior of redox-active metal complexes. Many transition metals are stable in multiple oxidation states. Varying the metal center can make various redox states accessible offering another potential switching component to compliment the ring-opening and ring-closing of the DTE. The oxidation potential is also tunable by choice of metal and ancillary ligands, which is convenient for tuning the oxidation potential using the same DTE core unit.  Finally, metal complexes are  appealing due to their propensity to form mixed-valence systems. The reversible changes in electronic structure of the DTE are perfectly amenable to modulation of the interaction and delocalization of charge between two metal centers.  25  Although the exact mechanism of reactivity remains to be defined for most of these complexes, a few general similarities amongst the complexes are evident that likely are related to reactivity. To date, metal centers with well-defined, reversible redox behavior have been targeted as candidates for imparting electrochemical reactivity of the DTE. The bulk of work has used Fe or Ru complexes, which reflects the ubiquity of these metals in the electrochemical literature.  Also of note is the symmetrical  substitution of the DTE with the metal centers. This architecture likely stems from the fact that a two-electron oxidation is required for the electrochromic process to occur. Additionally, the strength of coupling between the metal center and DTE core also affects the reactivity. When the DTE core and metal center are closely coupled, changes in the spin density due to oxidation of the metal are experienced by the DTE. If the metal center is far away from the DTE core, changes in spin density are localized around the metal and are isolated from the DTE. The linkage between the metal atom and DTE is obviously of critical importance in the design of these systems because ultimately the electronic interactions of redox active fragments will determine the reactivity. Beginning with one extreme, the case in which no linker exists, Ru and Fe atoms were directly --bonded to the DTE scaffold.83 The direct coupling results in the strongest possible interaction between the metal and DTE fragments. The electrochemical behavior of complexes 7 and 8 was studied by CV (Figure 1-9). Scanning positive potentials first revealed a single redox wave, assigned as a two-electron oxidation of the complex. The reverse scan yielded two resolved redox waves, each attributed to a one-electron reduction. These two reversible redox waves were found to occur at the same potentials as in the ring-closed isomers of 7 and 8 when generated by UV irradiation. This result led to the conclusion that the oxidized complex must rapidly undergo cyclization before diffusing away from the electrode. This is 26  followed by a two-electron reduction of that species to form the neutral ring-closed complex. This behavior was general for all of the Fe and Ru complexes that were studied.  The significant change in electronic structure of the bridging DTE ligand  dramatically affected the extent of interaction between the terminal metal complexes, as indicated by the measured conproportionation constant (KC). The conjugated backbone of the ring-closed isomer resulted in KC values on the order of 104 times higher when the DTE was ring-closed compared to ring-open.  F F M  Ph3P  F F  S OC M = Fe Ru  F F M  S CO  PPh3  7 8  Figure 1-9 CV trace for (a) 7 and (b) the ring-closed version of 7 generated by irradiation with UV light. [7] = ~ 10-3 M, 0.1 M NBu4PF6 in CH2Cl2. (Permission for reproduction has been granted by the Royal Society of Chemistry.) Metal-alkynyl complexes are appealing candidates to couple with DTEs because of their widely documented electronic interactions between metal atoms and conjugated, alkynyl ligands. To date, several Ru and Fe adducts have been investigated.  Similar  oxidation-induced ring-closing reactions were observed for these complexes as well. The half-sandwich complexes 9 and 10 exhibit much lower KC values than the --bonded complexes 7 and 8.84 This demonstrates how the acetylide linkage changes where the spin density of the oxidized species is localized, and thereby diminishes coupling of the radicals. Akita and co-workers also show that the nature of the metal atom, whether it is 27  Ru or Fe, affects the localization of spin density and results in different thermal reactivity and photoactivty of the radical species. Rigault and co-workers investigated Ru-alkynyl complex 11 which exhibited the same general behavior as complexes 7–10.85 Interestingly, the cationic species 11c2+ is photochemically inert and requires reduction to the neutral form to become photoactive again. This type of gated behavior would be useful for memory applications for which certain states could be “locked” to prevent overwriting the data. F F S  M Ph2P  F F  PPh2  F F  F F  S Ph2P  M = Fe Ru  Ph2P  M  9 10  PPh2  F F  F F Ph2P  PPh2  Ru Cl Ph2P PPh2  S  S  Ru Ph2P  PPh2  Cl  PPh2  11  Coudret and Launay studied the bisferrocenyl derivative and found that a thermal ring-opening process could be initiated by oxidation of the ferrocene.86 This discovery was significant because it showed that DTE isomerization could occur without direct oxidation of the DTE. Oxidation of the ferrocenes resulted in a strong electronwithdrawing effect to destabilize the ring-closed species. The salient difference in the structure of 12, which might be related to its different reactivity compared to complexes 7–11, is that there is no metal alkyne bond directly coupling the DTE to the metal. Presumably, this has consequences in terms of the delocalization of charge in the oxidized species.  28  F F  F  F  S Fe  PPh2  Cl  S Fe  12  F F  Ph2P  F F  F F  F F S  S  Ph2P Ru  Ru  Ph2P  PPh2  13  Ph2P  PPh2  Cl  PPh2  The development of molecule-based memories and logic gates requires systems that can access multiple stable states, making metal complexes appealing for this application. Preservation of the metal complex’s electrochemical behavior when coupled to the DTE expands the photoswitchable DTE from a two-state system to a multi-state, photoactive and electrochemically active system. Humphrey and co-workers successfully demonstrated this concept using the DTE-bridged, binuclear Ru-alkynyl complex 13.87 The photochromic Ru complex can switch between six stable states, each containing unique cubic NLO properties, using photo-induced, electrochemically-induced, and chemically-induced switching. In terms of the reactivity of this system, it is notable that oxidation of the metal complex does result in isomerization of the DTE. Due to the longer linker separating the DTE and Ru fragments, spin density of the Ru cations is localized far enough away from the DTE to render the oxidized state stable.  29  1.5.3 Interactions of Metal-Based and DTE-Localized Excited States Predicting the interactions of metal-based and DTE-localized excited states is a very complex and challenging task. The means by which the two components interact can be viewed two ways: how changes in the electronic structure of the DTE ligand affect the metal-based photophysics, and how metal-based states affect the photoactivity of DTEs. The most common interactions are shown schematically in Figure 1-10. One avenue that has been heavily pursued is the engineering of systems in such a way that emission of a metal complex can be turned on and off by reversibly toggling the state of the appended DTE (Pathway 1). This typically occurs by energy transfer from the metal complex to only one of the DTE isomers. On the other hand, metal complexes have been shown to provide access to new photoactive pathways for the DTE via metal-sensitized population of DTE states (Pathway 2). Alternatively, metal complexes can completely quench photoactivity of the DTE (Pathway 3). The origin of these interactions will be discussed in this section.  30  Figure 1-10 Generalized types of excited-state interactions for metal-coordinated DTEs. (1) Photoregulation of metal complex’s emission. (2) Metal-sensitized photoswitching of DTE. (3) Quenching of DTE photoactivity by energy transfer to the metal complex. One property of many metal complexes is their propensity to exhibit emission in the visible spectrum. There is significant interest in the regulation of emission for applications in optoelectronic devices and optical memory.59 Since the ring-closed DTE isomer has appreciable absorption in the visible region, and the ring-open isomer is transparent (Figure 1-8), systems that incorporate emissive metal complexes and DTEs are appealing for the photoregulation of emission. pathway 1 in Figure 1-11.  This is shown schematically by  Lehn and co-workers first demonstrated that pyridines  appended to the DTE scaffold could be used to bind emissive W, Ru, and Re complexes.88 The dinuclear Re-carbonyl complex 14 exhibited photoregulated emission with an ON/OFF ratio of 2.7, the ratio of the fluorescence intensity comparing the ring31  open and ring-closed states. The main drawback of this system is that overlap between DTE absorption and the excitation wavelengths required for Re-complex emission results in some photoswitching of the DTE during excitation of the fluorophore. Branda and coworkers improved upon this system by axially coordinating Ru-porphyrin complexes instead of the Re-carbonyl complexes.89 The Ru-porphyrin complex is excited and emits at wavelengths in the spectral region in which the DTE isomers are transparent. This allows for a non-destructive readout mechanism for optical memory. Photoregulation of metal complex emission by resonance energy transfer (RET) to covalently linked DTEs has now been established for several systems, including Ru,90 Ir,91 and Pt92 centers. Tian and co-workers have also investigated a family of photochromic phthalocyanines incorporating Zn and Mg centers.93,94  F F  (CO)3(bpy)Re  N  F F  (CF3SO3)2  F F S  S  N  Re(bpy)(CO)3  14  F F F F F F  S  O O  O S O  Eu  O O  CF3  CF3  3  15  Non-destructive monitoring of emission was also accomplished by utilizing a modified DTE with sulfonyl groups, which shifts the absorptions of both DTE isomers to higher energy. The Eu(III) complex 15 was formed by coordination of the Eu center to the sulfonyl moieties.95 Alternation of UV and visible light irradiation demonstrated 32  reversible changes in the emission intensity of the Eu(III) emission. Since the DTE isomers absorb at higher energy than the emission of the Eu(III) complex, RET to the DTE is ruled out at a possible mechanism for luminescence quenching. Instead, the authors conclude that reversible changes in the ligand field, between ring-open and ringclosed DTE isomers, affect the radiative rate constant for emission from the Eu(III) center, and therefore the observed emission intensity can be modulated. This strategy was also applied to photoregulation of another Eu(III) complex incorporating a related terarylene photochrome.96 One unique feature of metal complexes is their potential to access electronic states that either do not exist or are not accessible in most organic materials. In particular, triplet excited states are readily accessible with the help of suitable metals, as opposed to organic systems for which triplet states are seldom populated.  The presence of  substantial excited state interaction between metal-centered states and ligand-localized states is a well-established concept for multi-chromophoric complexes. In the early 1970s, Whitten97 and Wrighton98 were amongst the first to observe systems with equilibrated 3MLCT and 3IL states in which a photoreaction occurred from the ligandlocalized 3IL state after selective excitation of an appended metal complex. They both reported cis-trans isomerization of a stilbene derivative to occur through a lower-lying triplet manifold, rather than the conventional excited singlet state. This work spawned a new area of research focusing on synthesizing metal complexes featuring isoenergetic 3  MLCT and 3IL states, whose interactions were found to generate some new, useful  properties.  33  M N  N  M = Re (CO)3Cl Pt (C  C C6H5)2  Zn (S C6H5)2 S  16 17 18  S  In 2004, Yam and co-workers extended this concept when they reported that a 3IL state localized on the DTE scaffold could be populated by energy transfer from a chelated Re carbonyl complex.99,100  Population of this excited triplet state resulted in  photocyclization, analogous to the reaction pathway of its excited singlet state. This mechanism is shown schematically by pathway 2 in Figure 1-10. It was determined that the phenanthroline-modified DTE strongly couples the metal-based 3MLCT state with the DTE 3IL state. Photocyclization through the triplet manifold requires significantly lower energy light than triggering the photoreaction in the uncomplexed DTE. Subsequent work with the same DTE proligand demonstrated that the DTE is also sensitized by complexing Pt, another heavy metal with large spin-orbit coupling to promote formation of a triplet excited-state species.92 Coordination of aryl acetylides to the remaining sites in Pt’s coordination sphere incorporates a long-lived, low-lying 3MLCT state from which the energy can be transferred to the 3IL state. The notion that a metal with large spinorbit coupling is necessary for the population of the 3IL state was supported by the preparation of a Zn complex, which does not have appreciable spin-orbit coupling and did not result in metal-sensitized photocyclization of the DTE.  34  F F  M  N  F F S  S  N N  F F  N  N  N  N  N  M  N  N N  N M = Ru Os  19 20  Metal-sensitized ring-closing was also observed to occur through other polypyridyl functionalized ligands. DeCola and co-workers studied the energy transfer dynamics for a series of Ru(bpy)3 and Os(bpy)3 complexes (19 and 20).90,101 Metalsensitized ring-closing was concluded to occur via the same energy transfer pathway, from the Ru(bpy)-based 3MLCT state to the DTE-localized 3IL state. The importance of the relative energies for 3MLCT and 3IL states was demonstrated by simply changing the metal from Ru to Os, which lowers the energy of the 3MLCT excited state below that of the 3IL state. This exclusively results in emission from the 3MLCT state rather than sensitized ring-closing from the 3IL state. Considering the known triplet energies of the metal complexes from their respective phosphorescence spectra, estimation of the energy of the photoactive 3IL state was made to lie between 1.7 eV – 2.0 eV. Only one previous study has directly addressed the effect of the linkage on the metal-sensitized ring-closing reaction.  Scandola and co-workers studied the energy  transfer processes present in a Ru-coordinated, phenanthroline-modified DTE system for which the DTE was either coupled directly to the phenanthroline (21) or through a nonconjugated methylene spacer (22).102 The effect of the methylene spacer is notable, slowing energy transfer between the 3MLCT and 3IL from 1.5 ns to 40 ns. 35  N N  N N  Ru  N  N  N  Ru  N  N N  N N  O R  N  S  CH2 O  S  21  N  M  N  R  R  R =CH2CH2COOH  S  N  N  O  S  O R  S  22  S  N  N  N  N  N  N  M  N N  N M = Ru Os Co Fe  23 24 25 26  An interesting comparison to the bipyridyl-containing complexes 19 and 20 is the behavior of analogous terpyridyl complexes reported by Abruña and co-workers.103 Complexes 23 – 26 were prepared with various metals in the terpyridyl binding pocket. Despite  the  uncoordinated,  terpyridyl-substituted  DTE  demonstrating  normal  photoactivity, coordination of the metal drastically changes the photobehavior. The Ru, Os, and Fe-containing complexes were found to be inert to both UV and visible irradiation.  The Co-containing complex underwent photocyclization upon UV  irradiation, but subsequent irradiation with visible light did not trigger the ring-opening reaction. Compared to bipyridine, the weaker ligand field of terpyridine results in lower36  lying d&d states, which rapidly undergo radiationless decay to the ground state.104,105 Strong coupling between a d&d state and the photoactive IL state is a likely reason for the lack of photoactivity in these complexes. Overall, this study highlights how the ligand field should be considered in the choice of linkage to the metal center since it affects the relative energies of the ligand-based and metal-based excited states, ultimately affecting the exited-state dynamics. Considering all of these examples, it is obvious that metal-containing DTE systems are positioned to offer new optical properties compared to the individual components. The versatility and tunability of emission from metal complexes is appealing for developing systems requiring photoregulated luminescence. Additionally, a unique feature of incorporating metal centers compared to the all-organic congeners is the accessibility of the photoactive 3"* state. The wavelengths required for ring-closing are much lower in energy, up to approximately 1.65 eV less energy in some cases compared to the uncomplexed DTE. Based on these systems, complexation of a metal center has emerged as an effective approach for tuning the photoactive wavelengths required for photoswitching of the DTE.  1.6  Goals and Scope of Thesis The premise of this thesis is the preparation and characterization of hybrid metal-  organic photochromic materials.  Specifically, I have focused my research on Pt-  coordinated DTEs. These materials are appealing to develop because many Pt complexes exhibit rich photophysics,106,107 so their implementation within a photochromic material has the potential to generate novel multichromphoric systems. In Chapters 2 and 3, I focus on how Pt terpyridyl acetylide complexes influence the photobehavior of a 37  coordinated DTE. Two bichromophoric systems were studied which differed by how closely coupled the Pt complex and DTE fragment were. Chapter 2 details how excitedstate energy transfer from the Pt complex to the DTE sensitizes the ring-closing isomerization to lower energy photoexcitation. Chapter 3 explores how the linkage between the Pt complex and DTE affects the energy transfer processes between the two moieties. The work in these chapters forms the basis for how incorporation of a Pt complex can provide access to the photoactive, excited-state triplet manifold of DTEs. Chapters 4 and 5 examine systems in which more than one DTE is linked via Ptbis(acetylide) linkages. Multifunctional systems have garnered interest because they offer potential for applications requiring more complex and versatile switching functionality.  These systems, however, have proved challenging to design because  interactions of closely coupled chromophores typically results in unexpected, and often undesirable, photobehavior. First, a bifunctional system, incorporating two DTEs, is studied as a model system for an extended multifunctional system. Chapter 4 focuses on how the coordinated Pt atom affects the photoswitching of DTEs and the electronic interactions present between the two DTEs. The multifunctional system is introduced and its behavior is discussed in Chapter 5. Characterization of these materials supports their promise as photoresponsive materials capable of exhibiting significant changes in optical and electronic properties to be utilized in molecular-based memory and logic applications.  38  CHAPTER 2  Platinum-Sensitized Photocyclization of Dithienylethenes*  2.1  Introduction Molecular  systems  that  integrate  metal  coordination  complexes  with  photoswitchable DTEs are appealing because they have the potential to create new optical and electronic properties by combining those unique to each component.72-74,108 It is of interest to utilize this synergistic effect to modulate properties of the metal complex via the innate light sensitivity of the photochrome. In particular, the excited-state lifetime of the metal complex is an attractive target for modulation. Long-lived charge-separated species resulting from photoexcitation have been shown to be useful for photocatalysis by initiating intermolecular redox reactions in solution.109-111 The initial breakthrough in this field was reported in a series of publications in the late 1970s that used Ru(bpy)32+ as the visible-light absorbing chromophore in a multicomponent system capable of generating H2 from water.112-114 One of the specific interests in the Wolf group, in this area, is to control the fate of a platinum(II) terpyridyl complex’s excited state to influence how it acts as a photosensitizer for water reduction115,116 and in the photo-induced oxidation of alkenes.117 Since the metal complex plays the role of a photosensitizer in both applications, reversibly regulating the lifetime of the metal-ligand’s excited state by *  A version of this chapter has been published. Reproduced with permission from Roberts, M. N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.; Wolf, M. O. Inorg. Chem. 2009, 48, 19–21. 39  toggling the DTE component between its two isomers within a hybrid system can potentially provide a means to regulate the catalytic processes on command. This concept is possible assuming the two DTE isomers have unique optical and electronic properties, an assumption that is supported by a wide body of published examples.63,118 Some related work has been done in this area with limited success. Gust119 and Effenberger120 successfully demonstrated modulation of the excited-state lifetime for two organic systems, but the picosecond lifetime for the charge-separated species in these systems is too short to be utilized in an intermolecular, diffusion-controlled electron transfer process.  Another system, studied by Castellano, incorporated a Ru(phen)32+  chromophore that exhibits an excited-state lifetime in the microsecond range, where quenching by the DTE was accomplished intermolecularly.121 There would be obvious benefits to the modulation efficiency if the metal complex and photochromic unit were covalently linked together rather than relying on diffusion-controlled bimolecular processes. Platinum-DTE hybrid complex 27o was targeted to achieve these goals (Scheme 2-1). The molecule was designed to closely link the two components (the platinum terpyridine and the photoresponsive DTE) through an acetylide linker in order to maximize intramolecular energy transfer122 and consequently reduce the excited state lifetime of the metal complex, thus preventing it from being able to participate in the photocatalytic reduction of water.  40  Scheme 2-1 F F  F F F F  S  S  N N Pt N  27o  F F  PF6  h!1 h!2  F F  F F  PF6  N N  Pt  N  S  S  27c  Considering the known photophysical characteristics of the individual components, the intention was that the Pt complex and DTE could be independently addressed with deliberately chosen wavelengths of light. In this chapter, I describe how complex 27o undergoes photochemical ring-closing when irradiated with light of wavelengths absorbed only by the metal chromophore, and not directly by the DTE photoswitch. This phenomenon has been observed for a select few of the previously reported metal-DTE complexes where ring-closing occurs by energy transfer from a triplet charge transfer (3CT) state involving the metal to a localized intraligand (3IL) state on the photoswitch.90,92,99,100,102 Investigations regarding the excited state interactions present in this system will be discussed in detail in this chapter.  2.2  Experimental  2.2.1 General All solvents and reagents including those for NMR analysis (Cambridge Isotope Laboratories) were obtained from commercial sources and used as received except where noted. 1H NMR spectra were recorded on a Bruker AV400-Direct (400 MHz) or Bruker AV400-Indirect (400 MHz) spectrometer at room temperature. All chemical shifts are referenced to residual solvent signals which were previously referenced to 41  tetramethylsilane and splitting patterns are designated as s (singlet), d (doublet), t (triplet), m (multiplet), or br (broad). IR Spectra were obtained on a Thermo Nicolet 6700 FT-IR.  ESI-MS (Bruker Esquire), EI-HRMS (Concept IIHQ), and elemental  analysis were obtained at the UBC Microanalysis facility. For UV irradiations, an unfiltered 302 nm Hg lamp (18 mW/cm2) was used, unless otherwise noted.  For broadband visible irradiation, a handheld lamp with a  tungsten bulb (65 mW/cm2) fitted with the appropriate low-pass filter was used. To determine the active wavelengths for photocyclization, a 75-W arc lamp with a double monochromator was used to irradiate the sample. The bandwidth in these experiments was tuned by reducing or increasing the slit widths. 2.2.2 Synthesis [Pt(trpy)Cl]Cl123 and complex 29124  were prepared according to literature  procedures and their identity confirmed via 1H NMR and mass spectral analysis. A procedure for Sonogashira couplings was adapted to synthesize 2-trimethylsilyl ethynyl thiophene.125 DTE 4o was prepared by Dr. Jeremy Finden (Dept. of Chemistry, Simon Fraser University) according to the published procedure.126  [Pt(4’-tolyl-trpy)C. C-DTE]PF6  (27o)  Compound 4o (0.050 g, 9.25 # 10-5 mol) and KOH (0.015 g, 2.67 # 10-4 mol) were dissolved in N2 purged CH3OH (50 mL). After stirring for 30 minutes at room temperature, [Pt(trpy)Cl]Cl (0.054 g, 9.25 # 10-5 mol) and CuI (0.006 g, 3.15 # 10-5 mol) were added. The reaction mixture was stirred for 72 hours under N2 in the absence of light.  CH3OH was then removed under reduced pressure and the remaining solid  42  dissolved in DMF (3 mL). The black solution was added drop-wise to an aqueous solution of NH4PF6 (0.150 g in 125 mL H2O) and stirred for 1 hour at room temperature. The resulting precipitate was filtered and washed sequentially with copious amounts of H2O and diethyl ether. The product was purified by slow evaporation of diethyl ether into an CH3CN solution yielding 27o (70.1 mg, 67%) as a maroon solid. 1H NMR {400 MHz, d7-DMF}: $ 2.24 (s, 3H), 2.30 (s, 3H), 2.64 (s, 3H), 7.45 (s, 1H), 7.50 (m, 5H), 7.79 (s, 1H), 7.90 (d, J = 7.23 Hz, 2H), 8.34 (d, J = 6.80 Hz, 2H), 8.81 (t, J = 7.37 Hz, 2H), 9.18 (d, J = 7.14, 2H), 9.32 (s, 2H), 9.43 (d, J = 5.06 Hz, 2H). FT-IR (neat, cm-1): 2097 (m, )C.C). ESI-MS m/z: 986.0 [M%PF6]+. Anal. Calcd for C45H30F12N3PS2Pt: C, 47.79; H, 2.67; N, 3.72. Found: C, 47.45; H, 3.01; N, 4.00.  [Pt(4’-tolyl-trpy)C. C-2’-C4H3S]PF6  (28)  2-trimethylsilyl ethynyl thiophene (0.035 g, 1.94 # 10-4 mol) and KOH (0.006 g, 1.07 # 10-4 mol) were dissolved in N2 purged CH3OH (45 mL). After stirring for 45 minutes at room temperature, [Pt(trpy)Cl]Cl (0.098 g, 1.66 # 10-4 mol) was added followed by CuI (0.007 g, 3.68 # 10-5 mol). The reaction mixture was stirred for 40 hours under N2. CH3OH was removed under reduced pressure and the remaining precipitate was dissolved in DMF (5 mL) to give a dark red solution. The solution was added drop-wise to an aqueous solution of NH4PF6 (0.270 g in 125 mL H2O) and stirred for 1 hour at room temperature. The resulting precipitate was filtered and subsequently washed with copious amounts of H2O and diethyl ether. Slow evaporation of diethyl ether into an CH3CN solution yielded 2 (100 mg, 78%) as a black solid. 1H NMR {400 MHz, d6-DMSO}: $ 2.44 (s, 3H), 7.01 (t, J = 4.48 Hz, 1H), 7.07 (s, 1H), 7.39 (d, J = 4.96 Hz, 1H), 7.43 (d, J = 7.40 Hz, 2H), 7.74 (t, J = 5.03 Hz, 2H), 7.98 (d, J = 6.94 Hz, 2H), 8.36 (t, J = 6.83 Hz, 2H), 8.66 (d, J = 6.76 Hz, 2H), 8.73 (br d, 2H), 8.80 (s, 2H). FT-IR 43  (neat, cm-1): 2109 (s, )C.C).  ESI-MS m/z: 626.0 [M%PF6]+. Anal. Calc’d for  C28H20F6N3PSPt: C, 43.64; H, 2.62; N, 5.46. Found: C, 44.00; H, 2.80; N, 5.68. 2.2.3 Spectroscopic Measurements Samples for all spectroscopic measurements were prepared using HPLC-grade Fisher solvents.  Absorption spectra were obtained with a Varian Cary 5000  spectrometer. Fluorescence spectra were collected on a Photon Technology International fluorimeter using a 75 W arc lamp as the source. All measurements were recorded at room temperature unless noted otherwise. Low temperature spectroscopy was completed using an Oxford Instruments Optistat DN cryostat. Emission lifetimes were measured by the TCSPC method using a Horiba Fluorocube Lifetime Spectrofluometer equipped with a 453-nm NanoLED source. All samples for luminescence studies were prepared in 1 cm2 anaerobic quartz cells (NSG PCI cells) and deoxygenated prior to measurement by purging with N2 for a minimum of 15 minutes. 2.2.4 TDDFT Calculations ADF 2007.01127,128 all-electron calculations were performed by Dr. Jeff Nagle with TZ2P basis sets, scalar relativistic effects included through the ZORA129-131 formalism and solvation effects (CH2Cl2) through the COSMO132 formalism. Geometry optimizations were carried out with the Becke-Perdew GGA potential and TDDFT133,134 calculations of electronic transitions with the SAOP model potential.  44  2.3  Results and Discussion  2.3.1 Synthesis Preparation of the platinum(II) terpyridyl complexes was accomplished by a modified procedure adapted from Yam and co-workers.135 Initially, the TMS-protected acetylides were stirred with KOH in CH3OH until completion of the deprotection was evident by TLC analysis. Then one equivalent of [Pt(trpy)Cl]Cl and a catalytic amount of CuI were added to the reaction mixture. The reactions were stirred under nitrogen at room temperature, typically overnight to allow significant accumulation of the final complex. In the case of coupling to the photochromic DTE (4o), reactions were carried out in the absence of light to avoid mixtures of ring-open and closed DTE isomers in the product. Immediately after the reaction, a counter-ion exchange of the chloride for the more soluble PF6 anion was completed by precipitating a DMF solution of the crude product into an aqueous solution of NH4PF6. The filtered precipitates were washed with copious amounts of water to remove ionic impurities, and diethyl ether to remove unreacted terminal acetylide proligand. Pure Pt-alkynyl complexes were isolated by recrystallization of complexes by slowly diffusing diethyl ether into concentrated CH3CN solutions of the product.  45  Scheme 2-2 1) KOH, MeOH F F  TMS  F F  F F  2) CuI,  Cl  S  S  4o  N Pt N  Cl  F F F F  S  S  Pt N  27o  PF6  or 3) NH4PF6 (aq.) TMS  PF6  N N  N  F F  S  N N  Pt  S  N  28  2.3.2 UV-vis Absorption Spectroscopy A CH3CN solution of complex 27o exhibits strong absorption in the UV region (!max = 285 nm, & = 52800 L•mol–1•cm–1) attributable to a combination of intraligand terpyridyl, thienyl, and acetylide "'"* transitions (Figure 2-1). Weaker absorption in the visible region from 400–520 nm (!max = 465 nm, & = 6700 L•mol–1•cm–1) includes overlapping transitions assigned to a mixture of MLCT and LLCT states, with the latter predominating at lower energy.123,136 These CT states, which are salient features of the Pt terpyridyl complexes, involve a redistribution of charge, upon light excitation, from the Pt atom and aryl acetylide framework towards the terpyridine ligand.  46  Figure 2-1 Absorption spectra of CH3CN solutions (1.5 # 10-5 M) of complexes 27o (black) and 28 (blue) at room temperature. The origin of these absorption bands as combinations of MLCT/LLCT states, as opposed to metal-perturbed localized transitions on the DTE component, is confirmed by TDDFT calculations and by comparing the spectrum to model compound 28, which only contains a thiophene ring attached to the acetylide. The absorption spectrum of 28 is virtually identical to that of 27o (Figure 2-1). Further support for the CT assignment of these transitions is provided by the fact that the absorption bands for 27o demonstrate the negative solvatochromic effect (Figure 2-2) previously observed for other platinum terpyridines that undergo similar charge transfer processes. The negative solvatochromic effect is indicative that the excited, charge-separated state is less polar than the ground state.123  47  Figure 2-2 Absorption spectra of complex 27o (~ 10-5 M) in CH3CN, acetone, CH2Cl2, and toluene at room temperature. Dielectric constants ($) of each solvent are given in parenthesis. Ring-closing of 27o can either be directly triggered by irradiation with UV light (! = 302 nm), which is absorbed by the DTE chromophore, or indirectly by selectively exciting the MLCT/LLCT absorption bands (!max = 465 nm) of the metal complex component with visible light. The latter observation contrasts with the behavior of DTE 4o, which lacks the metal component and requires <340 nm light for ring-closing to occur. Cyclization of 27o & 27c can be monitored by the appearance of a new low energy band (!max = 600 nm) in the UV-vis absorption spectrum (Figure 2-3b) characteristic of the "'"* transition of the ring-closed DTE isomer. Simultaneously, the MLCT/LLCT absorption band red shifts overlapping with the low energy band, largely due to the higher HOMO energy of the ring-closed isomer. The increased HOMO energy is attributed to both a decrease in the Pt contribution to the HOMO in 27c compared to 27o (DFT calculated values of 0% and 9% for 27c and 27o, respectively) as well as the antibonding character of the HOMO present in 27c (Figure 2-4).  The extended 48  conjugation in 27c results in orbital contributions that extend the full length of the acetylide-DTE unit instead of being confined only to the acetylide and proximate thiophene moiety as in 27o. When the Pt contributes to the HOMO, as in 27o, the hybridization of the 5d and alkynyl " orbitals serves to stabilize or lower the energy of the HOMO, resulting in a higher-energy CT transition. Therefore, electronic transitions with more LLCT character than MLCT character appear at lower energy. The observed red shift of the charge transfer band further supports its assignment as a mixture of MLCT and LLCT states and provides evidence for orbital interaction between LLCT and DTE-localized IL states.  Figure 2-3 Changes in absorption spectra when a CH3CN solution of (a) DTE proligand 4o and (b) complex 27o are irradiated with 302 nm light at room temperature.  49  Figure 2-4 Contour plots of the HOMO energy level for 27o and 27c. By irradiation with UV light, conversion of 27o & 27c takes place nearly quantitatively, reaching a photostationary state >95% of 27c present in solution (by 1H NMR spectroscopy). Selective irradiation of the CT transitions with visible light (450 nm < % < 490 nm) also generates the same photostationary state in solution. As the wavelengths of irradiation go further into the red, however, a lower photostationary state is observed and is attributed to competition between the ring-closure and reverse ringopening reactions, of 27o and 27c respectively, which are both initiated by excitation in that spectral region.  The photoinduced cycloreversion of 27c & 27o occurs by  broadband irradiation of light with wavelengths >540 nm. Complete conversion back to 27o is accomplished at this wavelength because there is no competing back reaction by irradiation into the CT bands of 27o. 2.3.3 Low Temperature Photobehavior When cooled to 85 K in an EtOH:CH3OH (4:1) glass, strong emission was observed from complex 27o (Figure 2-5b). The spectral profile of the orange emission is characterized by multiple peaks with well-defined vibronic spacing of approximately 1200 cm-1 signifying the ring breathing of the terpyridyl scaffold. Comparison of the energy and vibronic spacing to other Pt terpyridyl acetylide complexes strongly supports that the emission originates from a state on the Pt terpyridine moiety.124,137 As the temperature warms towards the melting range of the glass, the emission profile becomes 50  less structured and intense until it completely disappears above 125 K. It is important to note that excitation of the CT transitions at these low temperatures (<140 K) does not result in formation of 27c. This observation will be discussed in more detail in Section 2.3.4 regarding the quenching of sensitized photocyclization. The ring-closed isomer 27c is non-emissive presumably due to rapid energy transfer to the ring-closed DTE resulting in non-radiative decay.  Figure 2-5 UV-vis absorption spectra (a) and changes in emission spectra (b) when an EtOH:CH3OH (4:1) glass of complex 27o is warmed from 85 K to 125 K. Excitation = 470 nm. There are two plausible, interrelated reasons why emission is observed at low temperature. Efficiency of energy transfer is governed by the energy gap between donor and acceptor energy levels, and by the height of the energy barrier that exists between them.42 It is reasonable to consider that at low temperatures, there is not enough thermal energy available to overcome the energy barrier between the 3CT and 3IL states. Although the 3IL excited state might still lie at an energy near the 3CT state, the large energy barrier forces deactivation from the 3CT state, which is characterized by orange phosphorescence (Figure 2-6a). Also, examination of the absorption spectrum at low temperature reveals CT transitions at lower energies than at room temperature (Figure 251  5a). Red shifting of these transitions could possibly occur due to the freezing out of lower-energy geometrical conformations, particularly those in which the " system exhibits planarity. This suggests that the 3CT state might drop in energy below the 3IL state, making radiative decay from the 3CT state the primary pathway of excited-state deactivation rather than energy transfer to the ligand (Figure 2-6b). The most likely scenario is that contributions from both the adjustments in the relative energies of the excited states and the increased barrier height play a role in the photophysical behavior at low temperature.  Figure 2-6 Proposed Jablonski diagrams rationalizing the low-temperature emission of complex 27o.  2.3.4 Quenching of the Sensitized Photocyclization of Dithienylethenes As a means of elucidating the excited state processes and the nature of the states involved in photosensitization, we sought to probe the photoreaction in a variety of ways. Recently, Castellano and co-workers have demonstrated for various Pt terpyridyl complexes that 1,4-diazabicylco-[2.2.2]octane (DABCO) efficiently reduces the transient hole localized on the Pt atom and aryl acetylide ligand in the charge-separated excited state.138 DABCO is a suitable quencher for this experiment since it does not have any absorption in the spectral region where the Pt complex is excited. We attempted to utilize 52  this bimolecular quenching reaction as a means to provide evidence for the existence of a charge-separated state involved in the photosensitized ring closure of 27o. Building on the hypothesis that a charge-separated state is formed initially after excitation and then transfers its energy to the DTE, efficient quenching of the charge-separated state should prohibit cyclization of the DTE. The rate of photocyclization of 27o & 27c, by irradiation with light >420 nm, was monitored over time with various concentrations of DABCO in solution (Figure 2-7).  Formation of 27c in solution was evidenced by  increasing absorbance at 600 nm, a wavelength at which only 27c absorbs.  Figure 2-7 Changes in absorbance at % = 600 nm of CH3CN solutions of 27o (1.4 # 10-5 M) without quencher (!) or with varying concentrations of DABCO, 1.0 # 10-3 M ("), 1.0 # 10-2 M (#), and 0.10 M ($), upon irradiation with light >420 nm at room temperature. At sufficiently high concentrations of DABCO, diffusion-controlled quenching of the excited 27o is possible when irradiating with visible light. This is evidenced by suppression of the cyclization of 27o & 27c. In this case, the rate of intermolecular reductive quenching is faster than the rate of energy transfer to the DTE.  At 53  approximately 7000 times the concentration of 27o, quenching nearly suppresses all photocyclization of the DTE when selectively exciting the CT transitions with visible light. Significantly, irradiation of this solution with UV light, invoking direct excitation of the DTE moiety, photocyclizes 27o at a comparable rate to the rate when quencher is not present in solution. This result highlights the difference in the rate of photoswitching with respect to the excited state pathways following excitation with visible light or UV light. Quenching of energy transfer was also accomplished by cooling to low temperature. A EtOH:CH3OH (4:1) solution of 27o was slowly cooled to 85 K to form a glass. Photocyclization of 27o & 27c was monitored by measuring the absorption at 600 nm, an absorption wavelength unique to 27c. When complex 27o was irradiated with light >420 nm, there was not an appreciable amount of 27c generated after 20 minutes of irradiation (Figure 2-8). As the glass was warmed through its melting point (130 – 150 K), cyclization of 27o & 27c was observed by an increase in the absorbance at 600 nm. It is important to note that 27o is photoactive at temperatures down to 90 K when irradiated with UV light. The temperature dependence of photoactivity further supports the conclusion that there are at least two separate excited state pathways resulting in the photocyclization of complex 27o. Irradiation with UV light directly populates the photoactive excited singlet state of the DTE. Despite being geometrically locked in the frozen glass, since the DTE moiety in 27o predominantly adopts the anti-parallel conformation, a comparatively small amount of reorganizational energy is required to cyclize the DTE. This accounts for its photoactivity at fairly low temperatures when irradiated with UV light. On the other hand, photocyclization of 27o with visible light, by means of triplet-triplet energy 54  transfer, occurs by a more complicated pathway with additional energy barriers and competing deactivation pathways, such as phosphorescence. Energy transfer from the 3  MLCT/LLCT state to the 3IL seemingly requires enough thermal energy for energy  transfer to occur.  Figure 2-8 Changes in absorbance at % = 600 nm when an EtOH:CH3OH solution of 27o is irradiated with light >420 nm at 150 K (#), 170 K ("), 200 K (!), and 295 K ($).  2.3.5 Intermolecular versus Intramolecular Photosensitization Mixtures of DTE 4o and model complex 29 were studied to examine the importance of a linkage between the two chromophores regarding the energy transfer from the metal complex to the photochrome. Although the thienyl-substituted model complex 28 might seem more appropriate for these experiments, the phenyl-substituted complex 29 was purposefully chosen for reasons related to differences in the photophysics of their respective excited states. Complex 29, unlike complex 28, is emissive upon excitation of its MLCT/LLCT state and has a much longer excited state 55  lifetime in solution.  The phosphorescence from complex 29 provides a convenient  handle to monitor bimolecular excited-state quenching by TCSPC lifetime measurements. Furthermore, it is reasonable to assume that the MLCT/LLCT state of complex 29 lies at a higher energy than complex 28 due to the nature of the aryl-alkynyl ligand. This ensures that the metal complex has enough energy to populate the excited 3IL state of the DTE if energy transfer is possible. In order to investigate the feasibility of intermolecular energy transfer, a CH2Cl2 solution of complex 29 (2 # 10-5 M) and DTE 4o (5.3 # 10-4 M) was irradiated with 453 nm light, a wavelength at which only the metal complex absorbs. Upon irradiation there was no observed reduction of either the emission intensity or the lifetime of Pt complex 29, compared to when 4o is not present in solution. Significantly, intermolecular energy transfer resulting in photosensitized cyclization of 4o is not observed when 29 is excited in the presence of 4o. This is true even at concentrations where, in a separate experiment, quenching of emission from complex 29 by 3c occurs (Figure 2-9). This highlights the necessity for the two components to be linked for triplet-triplet energy transfer to occur from the 3CT state to the 3IL state.  56  Scheme 2-3 no energy transfer F F PF6  TMS  F F  S  F F  4o  S  N N  Pt N  F F  29 TMS  S  F F  F F  4c  S  energy transfer  Figure 2-9 TCSPC lifetime measurements of phosphorescence from CH2Cl2 solutions of 29 (2 # 10-5 M) at room temperature, with increasing concentrations of 4c as the quencher (Q). Direct linkage of the DTE to the Pt moiety in 27o allows for significant orbital overlap between the two components, a key requirement for Dexter energy transfer. The orbital overlap between the Pt and DTE moiety in 27o is supported by considerable 57  TDDFT-calculated oscillator strengths for transitions between them (e.g., >0.1 for 27o). Colinear orientation of the orbitals and electronic delocalization through the bridge promotes the significant electronic coupling required for energy transfer to occur between adjacent 3MLCT/LLCT and 3IL states. The absence of bimolecular energy transfer does not eliminate the possibility of an electron transfer mechanism also playing a role in the sensitized ring-closing of the DTE. Recently, the Castellano group has proposed that excited 3MLCT/LLCT states might be capable of transiently oxidizing aromatic groups on the alkynyl ligand that are not involved with the initial CT excitation.139 With this work in mind and no data that definitively eliminates or confirms either mechanism, it is not possible at this point to conclude that sensitization occurs by exclusively an energy transfer or electron transfer mechanism.  2.4  Conclusions In summary, the excited state interactions between a Pt terpyridyl complex  covalently linked to a photochromic DTE chromophores were examined in this chapter. Complexation to the metal extends the photosensitivity for cyclization to lower energy by about 175 nm. The use of a Pt terpyridyl complex as a triplet sensitizer to populate the 3  IL state localized on the DTE via the metal complex’s 3CT state was demonstrated. The  energy transfer between the two chromophores is a thermally allowed process and requires a covalent linkage with strong enough coupling between the two excited states. The overall photophysical behavior of complex 27o is summarized below as a Jablonski diagram (Figure 2-10).  58  Figure 2-10 Proposed Jablonski diagram summarizing the photophysical behavior of complex 27o. Wavy lines represent a nonradiative process. In terms of achieving the initial goal of modulating the excited-state behavior of the Pt complex by toggling the DTE photoswitch, this system is not suitable. The strategy utilized relies on selective excitation of the Pt complex without affecting the state of the photoswitch. The alkynyl linkage in 27o results in too much coupling between the 3MLCT/LLCT and 3IL states for this to occur.  For the purposes of  modulating the excited state lifetime of the Pt complex, efforts to circumvent the undesirable photosensitization are discussed in the following chapter.  59  CHAPTER 3  Linker-Dependent Photophysics of Platinum-Coordinated Dithienylethenes*  3.1  Introduction The structural design of multichromophoric systems is of the utmost importance  in determining the electronic interactions present amongst the constituent chromophores in the ground and/or excited states. Often, the linkage between two chromophores is the primary factor in governing these interactions.140,141 The electronic character of the linkage, whether it is conjugated or non-conjugated, and the geometric constraints imposed upon the chromophores at either end of the linkage can promote or diminish interactions between the individual chromophores. One strategy for building multichromophoric systems is to chelate metal centers with chromophoric ligands. This combines the photophysical and structural elements of the metal center with the chromophoric elements of the ligands. A significant amount of work has been directed towards understanding the interactions present in metal complexes between delocalized CT states and ligand-localized IL states and how those interactions manifest themselves in the observed photobehavior.42,142  *  Sections of this chapter have been published or will be submitted for publication. Reproduced with permission from Roberts, M. N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.; Wolf, M. O. Inorg. Chem. 2009, 48, 19 – 21. 60  In Chapter 2, I discussed the metal-sensitized photocyclization of a DTE when it is closely linked to a Pt(trpy) complex through a short, conjugated alkynyl linker. In those complexes, a photoactive 3IL state localized on the DTE is populated by energy transfer from the 3CT state. In order to achieve the goal of toggling the excited-state lifetime of the Pt complex, interaction between the two chromophores is required for RET to occur; however, the two chromophores must remain individually addressable, which means excitation of the Pt complex should have no influence on the photoswitching of the DTE. Therefore, efforts were directed towards studying how the linkage between the Pt complex and DTE affects energy transfer between the two moieties. Short, Conjugated Linker DTE F  N  27o  N  PF6  F F F F F  Pt complex  S  Pt  S  N  F F F F F  PF6  F  N  30o  N Pt N  O  S  S  DTE  Pt complex Long, Non-Conjugated Linker  Now, I introduce the photochromic Pt complex, 30o/30c, which incorporates a non-conjugated linker that is longer than the short, alkynyl linker in complex 27o/27c. In 61  this chapter, the interactions of excited states involved in energy transfer between the two chromophores and how the linkage affects the excited-state dynamics will be discussed.  3.2  Experimental  3.2.1 General All solvents and reagents including those for NMR analysis (Cambridge Isotope Laboratories) were obtained from commercial sources and used as received except where noted. 1H NMR spectra were recorded on a Bruker AV400-Direct (400 MHz) or Bruker AV400-Indirect (400 MHz) spectrometer at room temperature. All chemical shifts are referenced to residual solvent signals that were previously referenced to tetramethylsilane and splitting patterns are designated as s (singlet), d (doublet), t (triplet), m (multiplet), or br (broad). IR Spectra were obtained on a Thermo Nicolet 6700 FT-IR. ESI-MS (Bruker Esquire), EI-HRMS (Concept IIHQ), and elemental analysis were obtained at the UBC Microanalysis facility. 3.2.2 Synthesis [Pt(trpy)Cl]Cl123 and complex 31124  were prepared according to literature  procedures and their identity confirmed via 1H NMR and mass spectral analysis. 4-(2thienyl)phenol and propargyl phenyl ether143 were prepared according to adapted literature procedures. DTE 34o was prepared by Dr. Jeremy Finden (Dept. of Chemistry, Simon Fraser University) according to the published procedure.144  62  [Pt(4/-tolyl-trpy)C. C-CH2O-p-C6H5-DTE]PF6  (30o).  Compound 34o (0.039 g, 6.8 # 10-5 mol) and KOH (0.011 g, 2.0 # 10-4 mol) were dissolved in N2-sparged CH3OH (60 mL).  After stirring for 30 minutes at room  temperature, [Pt(trpy)Cl]Cl (0.032 g, 5.4 # 10-5 mol) was added followed by CuI (0.003 g, 1.6 # 10-5 mol). The reaction mixture was stirred at room temperature for 48 hours under nitrogen and in the absence of light. CH3OH was removed under reduced pressure and the remaining solid dissolved in DMF (5 mL). The black solution was added drop-wise to an aqueous solution of ammonium hexafluorophosphate (0.089 g in 125 mL H2O) and stirred for 1 hour at room temperature. Filtration of the resulting red precipitate and subsequent washing with copious amounts of H2O, CH3OH, and diethyl ether yielded 30o (30.4 mg, 44%) as an orange powder. 1H NMR {400 MHz, d6-acetone}: $ 2.46 (s, 3H), 5.11 (s, 2H), 7.22 (d, J = 8.54 Hz, 2H), 7.33 (t, J = 7.05 Hz, 1H), 7.46 (m, 5H), 7.65 (t, J = 8.55 Hz, 3H), 7.76 (t, J = 6.50 Hz, 2H), 8.04 (d, J = 7.98 Hz, 2H), 8.51 (t, J = 7.43 Hz, 2H), 8.78 (d, J = 7.62 Hz, 2H), 8.94 (s, 2H), 8.99 (d, J = 5.39 Hz, 1H). FT-IR (neat, cm-1): 2133 (s, )C.C). ESI-MS m/z: 1091.9 [M%PF6]+. Anal. Calcd for C52H36F12N3OPS2Pt: C, 50.48; H, 2.93; N, 3.40. Found: C, 50.62; H, 2.94; N, 3.10.  [Pt(4/-tolyl-trpy)C. C-CH2OC6H5]PF6  (32)  Propargyl phenyl ether (37) (0.058 g, 4.38 # 10-4 mol) and KOH (0.036 g, 6.4 # 10-4 mol) were dissolved in N2-sparged CH3OH (75 mL). After stirring for 30 minutes at room temperature, [Pt(trpy)Cl]Cl (0.200 g, 3.4 # 10-4 mol) was added followed by a catalytic amount of CuI (0.007 g, 3.7 # 10-5 mol). The reaction mixture was stirred for 40 hours under nitrogen. CH3OH was removed under reduced pressure and the remaining 63  precipitate dissolved in DMF (5 mL). The black solution was added drop-wise to an aqueous solution of ammonium hexafluorophosphate (0.500 g in 125 mL H2O) and stirred for 1 hour at room temperature. An orange precipitate was isolated and washed with H2O and diethyl ether. The solid was further purified by column chromatography on silica using a CH2Cl2:CH3OH (10:1) mixture as eluant. Pure 32 (150 mg, 56%) was obtained as an orange powder.  1  H NMR {400 MHz, d6-acetone}: $ 2.46 (s, 3H), 4.98  (s, 2H), 7.01 (t, J = 7.44 Hz, 1H), 7.13 (d, J = 7.85 Hz, 2H), 7.37 (t, J = 7.50 Hz, 2H), 7.46 (d, J = 7.85 Hz, 2H), 7.67 (t, J = 6.20 Hz, 2H), 8.04 (d, J = 8.02 Hz, 2H), 8.43 (t, J = 7.83 Hz, 2H), 8.71 (d, J = 7.54 Hz, 2H), 8.78 (d, J = 5.26 Hz, 2H), 8.84 (s, 2H). FT-IR (neat, cm-1): 2141 (s, )C.C).  ESI-MS m/z: 649.2 [M%PF6]+.  Anal. Calcd for  C31H24N3F6OPPt: C, 43.70; H, 2.30; N, 4.78. Found: C, 44.08; H, 2.45; N, 4.63.  Propargyl 4-(2-thienyl)phenyl ether  (36)  4-(2-thienyl)phenol (35) (0.138 mg, 0.789 mmol), potassium bicarbonate (0.16 g, 0.011 mol), and propargyl bromide (0.15 mL, 1.0 mmol) were dissolved in anhydrous DMF. The reaction mixture was stirred at room temperature for 20 hours. The product was extracted into diethyl ether and subsequently washed with H2O and brine. The organic layer was isolated, dried with MgSO4, and the solvent removed in vacuo. Purification by column chromatography (SiO2, hexanes/ethyl acetate 19:1) afforded 0.147 g (87%) of the title compound as a white powder. 1H NMR (400 MHz, CDCl3) $ 2.54 (t, J = 2.03 Hz, 1H), 4.73 (d, J = 2.23 Hz, 2H), 7.00 (d, J = 8.53 Hz, 2H), 7.06 (m, 2H), 7.22 (m, 2H), 7.56 (d, J = 8.73 Hz, 2H).  64  [Pt(4/-tolyl-trpy)C. C-CH2O-p-C6H5-2’-C4H3S]PF6  (33)  Compound 36 (0.056 g, 0.263 mmol) and KOH (0.044 g, 0.79 mmol) were added to 50 mL of N2-sparged CH3OH. The reaction was stirred at room temperature for 30 minutes after which the [(4/-tolyl-trpy)PtCl]Cl complex (0.135 g, 0.24 mmol) and CuI (0.007 g, 0.036 mmol) were added.  The reaction mixture was stirred under N2  atmosphere for 48 hours at room temperature. The CH3OH was removed in vacuo leaving an orange precipitate. The remaining solid was dissolved in a minimal amount of DMF and added dropwise to an aqueous solution of NH4PF6 (0.300 g in 150 mL H2O). After stirring for one hour, the resulting precipitate was filtered off and washed with H2O, CH3OH, and diethyl ether.  The filtered orange solid was added to a solution of  EtOH:CH3CN (20:1) that was heated to reflux. The undissolved solid was filtered off, yielding 0.100 g (60%) of product.  The solid powder was recrystallized by slow  diffusion of diethyl ether into CH3CN. 1H NMR (400 MHz, acetone-d6) $ 2.46 (s, 3H), 5.08 (s, 2H), 7.11 (dd, J = 5.11 Hz, 1H), 7.20 (d, J = 8.72 Hz, 2H), 7.35 (d, J = 2.83 Hz, 1H), 7.41 (d, J = 5.23 Hz, 1H), 7.49 (d, J = 8.09 Hz, 2H), 7.66 (d, J = 8.72 Hz, 2H), 7.73 (t, 2H), 8.04 (d, J = 8.06 Hz, 2H), 8.49 (t, 2H), 8.77 (d, J = 8.28 Hz, 2H), 8.91 (s, 2H), 8.94 (d, J = 5.88 Hz, 2H). ESI-MS m/z 731.3 [M+ 0 PF6]. FT-IR (neat, cm-1): 2135 (s, )C.C).  Anal. Calcd for C35H26F6N3OPSPt: C, 47.95; H, 2.97; N, 4.79. Found: C,  47.58; H, 3.10; N, 4.44. 3.2.3 Spectroscopic Measurements For UV irradiations, either an unfiltered 302 nm Hg lamp (18 mW/cm2) or filtered TLC hand-lamp (UVP Model:UVGL-58) was used, unless otherwise noted.  For  broadband visible irradiation, a handheld lamp with a tungsten bulb (65 mW/cm2) fitted with the appropriate low-pass filter was used. To determine the active wavelengths for 65  photocyclization, a 75-W arc lamp with a double monochromator was used to irradiate the sample. Samples for all spectroscopic measurements were prepared using HPLC-grade Fisher solvents.  Absorption spectra were obtained with a Varian Cary 5000  spectrometer. Fluorescence spectra were collected on a Photon Technology International fluorimeter using a 75-W arc lamp as the source. Quantum yields were measured using an integrating sphere. The reported quantum yields are averages of two measurements. Time-resolved emission data was collected using a Horiba Yvon Fluorocube TCSPC apparatus. A 470 nm NanoLED source pulsing at a repetition rate of 50 kHz was used for excitation. Broadband emission was monitored by a CCD detector at % > 500 nm, using a low pass filter. Data was fit using the DAS6 Data Analysis software package. The reported lifetimes are averages of three measurements. All samples for luminescence studies were prepared in CH2Cl2 that had been previously dried by passing through an alumina column and deaerated by no less than three freeze-pump-thaw cycles.  All  measurements were recorded at room temperature. Sample solutions were maintained under N2 atmosphere for the duration of the experiment in 1 cm2 anaerobic quartz cells (NSG PCI cells) fitted with a PTFE valve. Transient absorption experiments were performed using a Nd:YAG laser (EKSPLA PL2241) that generated pulses of 35 ps (fwhm) in duration.  The third  harmonic output (355 nm) was employed as the pump beam. The fundamental, 1064-nm output pumped a Xe-filled quartz cell to generate the white light continuum used as the probe beam. The pump and probe beams were aligned to pass through the sample at a 90° angle and focused so that the probe beam was completely enclosed by the pump beam within the cuvette. The probe beam was detected by first passing through a 66  monochromator (Princeton Instruments SpectraPro 2300i) equipped with a 150 g/mm diffraction grating. The grating was centered at 470 nm, 540 nm, or 640 nm in order to collect data over the 350 – 800 nm spectral region. This was coupled to a streak camera (Hamamatsu C7700) and a CCD detector (Hamamatsu C8484), which digitized the images from the streakscope. The trigger delay between the streak camera and pulse firing was controlled using a passive delay unit (Hamamatsu C1097-01). Computer control of data acquisition was via HPD-TA software (ver. 8.3) from Hamamatsu. The data consists of a 200-shot average. Samples for transient absorption were prepared in HPLC grade CH3CN (Fisher) that had been previously deaerated by three freeze-pumpthaw cycles and were maintained under N2 atmosphere for the duration of the experiment. Steady-state UV-vis absorption spectra were collected before and after laser photolysis to confirm sample degradation was not significant during the course of the experiment. The samples were prepared such that the absorbance of the solution was 0.8 – 1 at 355 nm. 3.2.4 TDDFT Calculations ADF 2007.01127,128 all-electron calculations were performed by Dr. Jeffrey Nagle with TZ2P basis sets, scalar relativistic effects included through the ZORA129-131 formalism and solvation effects (CH2Cl2) through the COSMO132 formalism. Geometry optimizations were carried out with the Becke-Perdew GGA potential and TDDFT133,134 calculations of electronic transitions with the SAOP model potential.  67  3.3  Results and Discussion  3.3.1 Synthesis The structural design for this family of complexes is based on the objective of reducing excited-state interaction between the chromophores, which resulted in the photosensitized ring-closing of complex 27o. Elimination of energy transfer to the ringopen DTE 3IL state should allow for the two chromophores to be selectively addressed independently. Using an acetylide for coordination to the Pt atom is still preferred because of the favorable photophysical characteristics it instills in the Pt complex, but lengthening it to include an ether fragment is a sensible choice for two reasons. First, Tung and co-workers had previously reported the Pt terpyridyl propargyl alcohol complex, 31, to exhibit strong 3MLCT-based phosphorescence with a long-lived excited state lifetime.124  Therefore, it was reasonable to predict that changing the terminal  alcohol to an ether linkage might afford the same desirable photophysics. The other motivation for this linkage choice is that the methylene spacer adjacent to the acetylide breaks the conjugation between the acetylide and the remaining portion of the ligand. This is an attempt to eliminate the orbital overlap that promotes the energy transfer process leading to the photosensitized ring-closing, yet spatially keep the two chromophores close enough that RET could potentially still take place to the DTE when it is ring-closed.  68  Chart 3-1 PF6  F F F F F F  N  N  <460 nm N  S  S  O  N Pt  >470 nm  PF6  N  PF6  F F F F F  S  S  O  30c  F  S  N  N Pt N  30o  R=  F F F F F F  27o  F F F F F F  S  28  S  34o O  S  S  R Pt  31  OH  F F F F F F  N  32  O  O  34c O  S  S  S  33  Model complexes were prepared without the photochromic DTE component in order to elucidate the Pt complex’s contribution to the behavior of the photochromic system, complex 30. Additionally, it is important to understand how the linkage between the Pt center and DTE affects the excited-state dynamics upon excitation of the Pt complex. Therefore, a series of model compounds was targeted to incrementally append segments of the linkage starting by termination with an alcohol, complex 31, then a phenyl ring, complex 32, and finally ending with a thiophene, complex 33. The same strategy to synthesize the Pt(II) terpyridyl acetylide complexes in Chapter 2 was also employed here (Scheme 2-2). The precursor complex [Pt(trpy)Cl]Cl was dehalogenated using KOH and then coupled to the terminal acetylide using CuI as a catalyst. Propargyl alcohol, used to form 31, is commercially available and was used without further purification. The preparation of acetylide-terminated proligands 36 and 37 is outlined in Scheme 3-1. Compound 35 was prepared by first coupling 2-bromothiophene with 69  p-iodoanisole, using a Suzuki coupling, to form p-(2-thienyl)anisole.  The methoxy  substituent was deprotected with BBr3 to form the phenolic precursor required for the subsequent Mitsunobu reaction. Proligands 36 and 37 were synthesized by deprotonating either 35 or phenol, respectively, with K2CO3 and reacting it with propargyl bromide. These were purified by silica gel chromatography before complexation to the Pt-chloro starting material. Scheme 3-1 O B  S  +  (a)  I  67%  O  O  (b) S  O  71%  S  HO  35 (c) S  HO Br  +  87%  35  S  O  36 (c)  HO  70%  O  37  (a) K2CO3, Pd(PPh3)4, CuI, THF, H2O, reflux, 18 hrs; (b) BBr3, CH2Cl2, -78 °C& r.t., 16 hrs; (c) K2CO3, DMF, r.t., 20 – 24 hrs.  The DTE precursor, 34o, underwent photocyclization by irradiation with light % < 365 nm. The relative concentrations of 34o:34c at the PSS were determined by integrating the resonances of the two methylene protons which shift downfield from 4.84 to 4.91 ppm, in d6-acetone, upon ring-closing of the DTE. The PSS consists of 4% to 96% mixture of 34o:34c. Once coordinated to the Pt complex, photocyclization was accomplished by irradiation with light % < 460 nm. In this case, the PSS was also determined to be a mixture of 4% 30o and 96% 30c. Compared to the uncomplexed DTE, 34o/34c, the resonances for the methylene protons of 30o and 30c are slightly 70  shifted downfield signifying a minor electronic effect of coordination. The resonances assigned to the thienyl and phenyl protons that make up the DTE core are largely unchanged upon coordination, suggesting that there is no long-range electronic effect of coordination to the Pt complex. Selective irradiation coincident with the CT absorption band (440 < % < 460 nm) or UV irradiation (% < 365 nm) of complex 30o resulted in the same photostationary state.  Figure 3-1 Changes in the 400 MHz 1H NMR spectra of 30o, in d6-acetone at room temperature, when irradiated with 365 nm light. (a) 30o, (b) 30o/30c, (c) 30c.  3.3.2 UV-vis Absorption Spectroscopy The importance of the nature of the linker connecting the metal and DTE components is obvious when the absorption spectrum of compound 27o is compared to that of 30o, in which a longer, non-"-conjugated linker connects the two chromophores. The ether-functionalized acetylides present in 30 – 33 exhibit absorption originating from CT transitions at higher energy than complexes with an aromatic group directly coupled 71  to the acetylide, such as in complex 28.  This is attributed to a higher degree of  participation of the Pt atom in the HOMO and therefore, these transitions can be viewed as predominantly MLCT transitions rather than LLCT transitions. However, the HOMO of these complexes is still considered to have mixed Pt-alkynyl character. Comparison of the absorption spectrum of 30o to model compounds 32 and 33 (Figure 3-2) suggests that the HOMO involved in the lowest energy CT transition is mostly limited to the Pt atom and acetylide unit, the segment that the complexes share in common. The longer linker appears to eliminate the participation of the thienyl-based molecular orbitals of the DTE chromophore in the CT transition, unlike when the DTE is directly linked in complex 27o. The CT absorptions for all three of the complexes exhibit nearly identical molar absorptivities and band structure. In the UV region the absorption spectra share similar band structure originating from localized "&"* transitions of the terpyridine, acetylide, and the aromatic groups terminating the alkynyl ligand. The increasing incorporation of aromatic rings from 32 to 33 to 30o is responsible for the increase in the molar absorptivity observed in this region.  72  Figure 3-2 UV-vis absorption spectra of CH3CN solutions of complexes 30o, 32, and 33 at room temperature. Irradiation of CH3CN solutions of either DTE 34o or complex 30o with UV light generates a colored solution indicative of photocyclization of the DTE (Figure 3-3). The purple color is the result of an increase in absorption, centered at 592 nm, attributed to a "&"* transition involving the extended "-conjugated backbone of the ring-closed DTE isomer.  For complex 30o, changes in the visible region upon ring-closing can be  correlated to the appearance of the same transitions (%max = 592 nm) for the uncomplexed DTE 34c. There is no bathochromic shift of this "&"* band in 30c compared to 34c. This result supports that the metal complex is not perturbing the localized DTE states. Furthermore, as ring-closing proceeds from 30o & 30c, there is no change in the absorbance of the CT band. This is in stark contrast to the observed red-shift of the CT band during the conversion of 27o & 27c (Figure 2-3b). The lack of any shift in the CT absorption is consistent with the assignment of these transitions as mostly MLCT in character with diminished ligand involvement.  73  Figure 3-3 Changes in the UV-vis absorption spectra of a CH3CN solution of (a) 34o is irradiated with 254 nm light and (b) complex 30o is irradiated with 302 nm light at room temperature. In addition to conventional ring-closing of the DTE component in 30o using UV light, similar cyclization is also triggered with selective irradiation into the MLCT band. Although this photobehavior is similar to that of complex 27o, it is clear that there is a significant difference in the efficiency of photocyclization using visible light. In order to compare the relative efficiencies of ring-closing via excitation with visible light, solutions of 27o and 30o were irradiated under the same conditions with light between wavelengths 415 and 425 nm.  74  Figure 3-4 Percent conversion to the PSS as a function of time for CH3CN solutions of 27o and 30o irradiated at 415 < ! < 425 nm. Linear fits are shown. The progress of the photoreactions was monitored by UV-vis spectroscopy. To ensure each sample absorbed the same amount of photons, the solutions were prepared with absorbances > 2.5 at the irradiation wavelength so that the majority of light was absorbed by both samples. Conversion to the ring-closed DTE was kept below 10% for both samples so that absorbance by the ring-closed isomers marginally interfered with the ring-closing photoreaction.  The amount converted to the ring-closed form was  determined by calculating the ratio of absorbance at either 602 nm (27o) or 592 nm (30o) relative to the absorbance at 286 nm after each time interval of irradiation. The ratios at various time intervals were then divided by the same ratio representative of the total change in the sample to give the percent photocyclized of the total amount achievable at the PSS. The relative rates of the photoreactions 27o & 27c and 30o & 30c are illustrated in Figure 3-4. Clearly, the directly-linked system, 27o, demonstrates more efficient photocyclization using visible light. The remaining part of this chapter will focus on characterizing the excited state dynamics present in the ether-linked system to 75  fully understand the nature of interactions between the two chromophores and how the linker is involved in excited-state decay. 3.3.3 Luminescence Spectroscopy At room temperature, complexes 31 – 33 are emissive in CH2Cl2 solution. All of the complexes generally exhibit the same structured emission, for which the wavelength and band shape of emission are characteristic of Pt terpyridyl phosphorescence (Figure 3-5).  3  MLCT-based  Since there is virtually no effect on the emission,  regardless of how the alkynyl ligand is terminated, it appears that the CT state for all of these complexes involves transfer of an electron from a mixed Pt-alkynyl orbital to a lowlying terpyridyl "* orbital.  This interpretation is also supported by the identical  excitation spectra, which each correspond to the respective absorption spectrum for each complex. Observation of emission from either complex 30o or 30c was unsuccessful. Despite characterization of 30o indicating the synthesized sample is a pure product, emission from the [Pt(trpy)Cl]PF6 starting material was detected for CH2Cl2 solutions of complex 30o/30c.  The emission was determined to at least partly originate from  [Pt(trpy)Cl]PF6 by comparison of its excitation spectrum with that observed for 30o. The only conclusion that can be drawn from this result is that complex 30o must be very weakly emissive.  76  Figure 3-5 Corrected emission (solid line) and excitation (dashed line) spectra for CH2Cl2 solutions of complexes 31 (black), 32 (red), and 33 (blue) at room temperature. Emission spectra were collected by exciting the solutions at 425 nm. Excitation spectra were collected by monitoring emission at 550 nm for complex 32 and 560 nm for complexes 31 and 33. The excited-state lifetimes and quantum yields for complexes 31, 32, and 33 are listed in Table 3-1. The emission decay profiles for all three complexes were fit to monoexponential models (Figure A-1), supporting that only one excited state was responsible for the observed emission. The excited-state lifetimes for all three complexes are on the order of a few microseconds, a result consistent with the notion that the same emitting state is responsible for emission. The complexes also exhibit approximately the same emission efficiency. There is no evident trend comparing the quantum yields for the three complexes. Complex 31 exhibits emission with a quantum yield of 0.055, followed by a small increase for complex 32, and then a decrease in quantum yield to 0.048 for complex 33.  77  Table 3-1 Emission lifetimes and quantum yields for CH2Cl2 solutions of complexes 31–33 at room temperature. +  ,  31  5.22 µs ± 25 ns  0.055 ± 0.005  32  2.92 µs ± 6 ns  0.076 ± 0.003  33  3.50 µs ± 12 ns  0.048 ± 0.003  Given only slight differences in the lifetimes and quantum yields, there are several possible interpretations of the data. Minor differences might appear because varying the substitution on the alkynyl ligand might perturb the relaxation processes of the 3MLCT state differently. Alternatively, additional states might be present, depending on how the alkynyl ligand is substituted, that could be accessible or interact with the emitting 3MLCT state. The different terminal functionalities of complexes 31 – 33 have 3  IL states of different energy. The aromatic groups have low-lying 3IL states that could  be populated by energy transfer from the 3MLCT state. If a 3IL state were populated, the observed decay of the excited-state species would be dependent on the relative rates of relaxation from the 3MLCT and 3IL states and the rates of energy transfer between them. The differences in rates would account for some fluctuation in the excited-state lifetime and quantum efficiency. This would explain the appearance of emission from the same 3  MLCT state, with slightly perturbed dynamics affected by the substitution of the alkynyl  ligand. 3.3.4 Ultrafast Laser Spectroscopy Transient absorption of the excited state species of these complexes was measured by picosecond pump-probe laser spectroscopy to further elucidate the complexes’ excited state dynamics. The absorption difference spectrum of the charge-separated state is 78  characterized by two positive absorptions, one attributed to the terpyridyl-based radical anion, and another absorption at higher energy is indicative of the radical cation.138 The two positive absorptions are typically accompanied by a decrease in absorption, or bleach, in the spectral region in which the ground state CT absorption occurs. Upon excitation of complex 32 at 355 nm, the resulting transient species has a similar spectral profile to the directly-linked thienyl model complex 28 (Figure 3-6). The difference spectrum of 32 is characterized by a positive absorption centered at 365 nm, a bleach from 390 – 450 nm, and a broad, intense positive absorption throughout the visible region greater than 450 nm. The appearance of the bleach coinciding with ground state CT absorption, and its similarity to spectra observed for other Pt terpyridyl complexes,137 provides strong supporting evidence that this spectrum originates from the 3MLCT state. The transient signal decays monoexponentially at all wavelengths with + = 6.3 ns (Figure A-2).  79  Figure 3-6 Transient absorption difference spectra of CH3CN solutions of complexes 32 (red) and 28 (black) at 800 ps after excitation at 355 nm at room temperature. The transient spectra for complexes 31 and 32 indicate formation of a similar excited-state species upon excitation at 355 nm. By comparison to previous work, the spectra are consistent with formation of a charge-separated state with the anion localized on the terpyridyl segment and the hole localized in a mixed Pt-alkynyl orbital.138 This is indicated by the sharp absorption at high energy and the intense broad absorption at lower energy, separated by a bleach in the region coinciding with the ground state CT absorption. The features of the transient spectrum for 31 decay monoexponentially with a lifetime, + = 11 ns (Figure A-3). The longer lifetime of 31 compared to 32, + = 6.3 ns, is consistent with its longer lifetime observed for emission in CH2Cl2 solution.  80  Figure 3-7 Transient absorption difference spectra of CH3CN solutions of complexes 31 (black) and 32 (red) at 800 ps after excitation at 355 nm at room temperature. Despite having nearly identical ground state absorption spectra and similar emission profiles, the TA difference spectra for complexes 32 and 33 indicate different excited-state behavior.  The difference spectra measured at 800 ps after 355-nm  excitation for CH3CN solutions are shown in Figure 3-8. For complex 33, an intense, positive absorption centered at 440 nm appears in the region coinciding with the bleach observed for complex 32. The positive absorption that appears at lower energy for 33 is consistent in band shape and energy with that appearing in 32. Comparing the decay of absorption from 650 - 680 nm, complex 33 exhibits a shorter lifetime than 32, + = 2.0 ns (Figure A-4) compared to + = 6.3 ns, respectively.  81  Figure 3-8 Transient absorption difference spectra of CH3CN solutions of complexes 32 (red) and 33 (blue) at 800 ps after excitation at 355 nm at room temperature. The assignment of the transient signal for 33 is complicated by the nature of a difference spectrum, for which the data may be a combination a signals of different signs. As such, concurrent overlapping positive and negative changes can mask the appearance of one or the other based on their relative intensities. With this in mind, there are two possible states to which the observed signal could be attributed (Scheme 3-2). The spectrum could be assigned as a charge-separated state (3CS) with the cation localized over the phenyl and thienyl rings instead of the Pt-acetylide unit (the 3CT state). This would account for the bathochromic shift and change in band shape of the absorption band attributed to the cation relative to complex 32. Since this band coincidentally overlaps with the ground state CT absorption, a bleach does not appear as in 31 and 32 because the concurrent positive absorption is more intense. Despite the alteration of the spectrum attributed to the transient cation, the appearance of the lower energy band corresponding to the terpyridyl-localized anion remains unchanged compared to 31 and 82  32. If direct excitation resulted in ligand-to-ligand charge transfer, one would expect this difference to manifest itself in the ground-state absorption spectra, for which the CT absorptions of 31 – 33 are identical. Therefore, the formation of this state would likely be a two-step process similar to the behavior of Eisenberg’s Pt chromophores designed to undergo photoinitiated electron transfer cascades.137,145 Within the limitations of our picosecond TA experimental setup, however, the excited-state species responsible for this transient spectrum shown in Figure 3-8 is the only one observed. Scheme 3-2 3  PF6 N O  N Pt  *  S  N  3CT  state  1  2  3  PF6 N N  O  Pt  S  3  *  *  N N  O  Pt  PF6  S  N  N  3CS  state  3IL  state  The other explanation is that the signal arises from a 3IL state localized on the phenyl-thiophene subunit, shown as pathway #2 in Scheme 3-2.  DFT calculations  estimate that with the addition of the thiophene, in complex 33, the 3IL state falls considerably in energy relative to 31 and 32. The lowering of the 3IL state could make it accessible from the 3MLCT state. If these two excited states were in equilibrium that 83  would explain the appearance of 3MLCT-based emission in 33. The nearly identical ground state absorption spectra and similar phosphorescence of complexes 31 – 33 support that the same excited state is responsible for radiative decay in all of those complexes. The appearance of the 3IL state in the transient spectrum and emission from the 3MLCT state is consistent with that observed for other metal complexes with approximately isoenergetic 3MLCT and 3IL states.146 Although, the excited-state lifetime is not observed to be significantly longer as one might expect if the two states were in equilibrium.147-149 It is possible that the rate of decay from the 3IL state is comparable to that of 3MLCT state and therefore there is no enhancement of excited-state lifetime. The ability to populate the 3IL state in complex 33 would be supporting evidence that a lowlying 3IL state exists within the linkage and could act as a conduit to a 3IL state responsible for DTE photocyclization. In order to acquire a TA spectrum for the excited state species of 30o, fresh sample solution was continuously pumped through a flow cell to prevent buildup of the photoproduct 30c in the probe beam. Although 355-nm excitation addresses both the CT transition and the DTE directly, the photoactive 1"* DTE state, which is only accessible by direct UV excitation, is assumed to have completely decayed within tens of picoseconds, an assumption supported by other work.150 Therefore, the transient signal observed on the nanosecond time scale is interpreted as being directly related to the excited-state dynamics resulting from excitation of the CT transition.  84  Figure 3-9 Transient absorption difference spectra of CH3CN solutions of complexes 30o (black) and 33 (blue) at 800 ps after excitation at 355 nm at room temperature. A comparison of the TA spectra for complexes 33 and 30o shows the similarity of the two spectra (Figure 3-9).  A positive absorption appears centered at 450 nm, red-  shifted 15 nm from the similar absorption observed for 33. Fitting the decay of the absorption, between 425 – 445 nm, to a monoexponential model indicates similar lifetimes, + = 2.4 ns for 30o (Figure A-5) and + = 1.4 ns for 33 (Figure A-4), for the two excited-state species. The very shallow bleach centered at 600 nm is attributed to the accumulation of some 30c (Figure 3-10b) in the sample beam despite being replenished continually in the flow cell. The bleach appears to diminish the intensity of a coincident positive absorption that would be consistent with the spectrum appearing for 33. Therefore, my interpretation is that this excited-state absorption is attributed to the same state in both 33 and 30o. Evidence that these spectra might be attributed to a 3IL state is found by comparison to the TA signal observed for 27o (Figure 3-10a). Although the positive absorption exhibited by 30o is slightly red-shifted to that observed for 27o, the similar band shape and spectral features throughout the visible region suggest that these 85  features originate from the same state. For 27o, the intense bleach centered at about 600 nm is indicative of accumulation of 27c. Assignment of this signal as a 3IL state is consistent with the observed photoswitching behavior of 30o, which undergoes ringclosing with lower energy light when coordinated to the Pt complex than when not coordinated.  Figure 3-10 Transient absorption difference spectra of CH3CN solutions of (a) complexes 27o (black) and 30o (red) or (b) complexes 27c (black) and 30c (red) at 800 ps after excitation at 355 nm at room temperature. A comparison of the TA spectra for CH3CN solutions of 27c and 30c is shown in Figure 3-10b. The essentially identical spectra support that the same state is populated by excitation of either complex. The only excited state in common between 27c and 30c that should generate the same spectral signature is the DTE-localized 3IL state. Additionally, 86  the comparable excited-state lifetimes, + = 10 ns for 27c (Figure A-6) and + = 8.8 ns for 30c (Figure A-7), support the assignment of these spectra as originating from the same excited state. The TA spectra of the DTE-localized 3IL state is characterized by an intense bleach coinciding with the ground state "&"* absorption of the ring-closed DTE. This is accompanied by a positive absorption appearing at higher energy. The traces shown in Figure 3-10b represent the only spectral signature observed within the time resolution of our experimental setup. Considering that the ring-closed DTE species exhibits appreciable absorption at 355 nm (Figures 2-3 and 3-3a), one possibility is that direct excitation of the 1IL state is followed by rapid intersystem crossing to the 3IL state, a process facilitated by the spin-orbit coupling of the Pt atom.  Although it is not  observed, it cannot be discounted that the 3CT state could be populated first, have a short lifetime, and subsequently lead to population of the 3IL state. 3.3.5 DFT Calculations DFT and TDDFT calculations were carried out to aid in understanding of the distribution of electron density and the orbitals involved in the lowest energy CT transitions. Contour plots of the LUMO, HOMO, and HOMO-1 for 32 and 33 are shown in Figure 3-11.  87  PF6  PF6 N  N O  N Pt  N Pt  S  O  N  N  32  33  Figure 3-11 Contour plots of the LUMO, HOMO, and HOMO-1 for complexes 32 and 33. Since the lowest energy transitions that appear in the absorption spectra of complexes 31 - 33 are identical, the orbitals involved in those transitions must be similar. The contour plots show the orbitals relevant to the lowest energy transition. Since the majority of the electron density of the HOMO is located on the aromatic rings, which are kinked out of the Pt terpyridyl plane, the oscillator strength of the HOMO to LUMO transition is diminished. The next lowest allowed transition from HOMO-1 to LUMO is 88  more favorable and occurs more efficiently. The similarity of the HOMO-1 and LUMO of complexes 32 and 33 supports the nearly identical CT absorption bands observed for these complexes. Since the lowest 3MLCT/LLCT energy of 30o is comparable to that of 27o, there should be sufficient energy to populate the 3IL state if energy transfer is possible. DFT calculations of the MLCT/LLCT energies, together with observed absorption spectra of 27o and 30o, indicate MLCT/LLCT energies to be ( 2.0 eV, in agreement with experimental and DFT-calculated values for related compounds.151 The triplet energy of the DTE photoswitch state that leads to ring-closing is estimated from DFT calculations to be about 1.9 eV for 27o and 1.8 eV for 30o, values that are in good agreement with an experimental value of 1.85 ± 0.16 eV for DTE in a related Ru(II) complex.90 For the model complexes 31 – 33, the trend of DFT-calculated energies for their alkynyl-ligand-based 3IL states is consistent with the differences observed in their luminescence behavior and TA spectra. These empirical differences have been attributed to the changes in the relative energies of the 3CT and 3IL states, which interact with each other if they are close enough in energy. Whereas the energy of the 3CT state remains relatively constant for the series of the models, the energy of the 3IL state varies considerably based on the functionality of the alkynyl ligand. The estimated values of the 3IL state energy for complexes 31, 32, and 33 are 4.1 eV, 3.2 eV, and 2.5 eV, respectively. If the 3IL state is accessible from the 3CT state, it might act as an alternate pathway for decay. Otherwise, emission from the 3CT state is the predominant decay pathway. This model is summarized in the Jablonski diagram shown in Figure 3-12.  89  Figure 3-12 Proposed Jablonski diagram comparing the excited-state interaction of the 3 CT state and varying 3IL states for complexes 31 – 33 after excitation of the CT transition. The shaded box represents limits on the energy of the 3IL state.  3.4  Conclusions In this chapter, I presented and discussed experimental data that probes the ground  and excited state interactions present in a bichromophoric Pt-terpyridyl DTE system that features a non-conjugated linkage separating the two chromophores. The photobehavior of this complex was compared to that observed for another system that incorporated a short, conjugated linker between the two chromophores.  Studying these two  architectures helped to evaluate the effect of the linkage between the two chromophores. The experimental evidence put forth in this chapter demonstrates how the linkage can affect ground state and excited state interactions differently, and that the nature of the linker undoubtedly influences the observed photobehavior of the overall system. Unlike the closely-linked system, 27o, discussed in Chapter 2, the non-conjugated, ether linkage seems to largely remove interactions between the two chromophores in the 90  ground state. This is supported by the lack of any significant shifts of absorption bands in 30o compared to the model complexes of its constituents, 32 and 33.  The  luminescence and transient absorption data, however, show that interactions between the two chromophores exist in the excited state. The comparable emission profiles and lifetimes for the model complexes 31 – 33 suggest that the three complexes have the same emissive state in common, which is assigned to 3MLCT parentage. Observation of the 3MLCT-based emission is consistent with the reduced interaction between the chromophores, as opposed to efficient energy transfer to a 3IL state as observed in 27o. The absorption of the excited-state species shows that substitution of the alkynyl ligand definitely affects the excited-state decay pathway. DFT calculations support a model in which lowering of the 3IL state permits its population from the 3MLCT state. Population of a 3IL state would also explain the ring-closing observed for 30o when selectively exciting its MLCT absorption. The absence of comparably intense emission in 30o compared to 31 – 33 suggests a few possible scenarios. First, the energy of the 3IL state may have fallen far enough below the 3MLCT state that reverse energy transfer is slower than the rate of non-radiative decay from the 3IL state. Another possibility is that the rate of non-radiative decay from the 3IL state is faster in 30o than 33. An energy diagram for 30o illustrating the proposed model is shown in Figure 3-13.  91  Figure 3-13 Proposed Jablonski diagram comparing the excited-state interaction of the 3 CT state and 3IL state for complexes 30o after excitation of the CT transition. The shaded box represents limits on the energy of the 3IL state. The photobehavior  of 30o  demonstrates the challenges in  designing  multichromophoric systems. Pt-terpyridyl acetylide complexes are undoubtedly excellent triplet sensitizers of ligand-based triplet states.123,149,152,153 Considering the behavior of just the model complexes discussed in this chapter, in order to effectively eliminate energy transfer processes between the two chromophores, the linkage must have inaccessible 3IL states higher in energy than the 3MLCT state. Future design of these PtDTE systems should move in the direction of incorporating DTE derivatives in which the thiophenes are not substituted with other conjugated or aryl moieties. This only serves to lower the 3IL state energy of the DTE, and thereby increasing the chances of energy transfer to occur from the 3MLCT state.  92  CHAPTER 4  Bifunctional Photoswitching of Platinum Acetylide Bridged Dithienylethenes*  4.1  Introduction Over the past two decades there has been a surge of interest in the development of  systems incorporating covalently connected photoswitchable components.  When the  individual components act synergistically, they have the potential to act as multistate switches for guiding energy or electron transfer within the system.154 In particular, the addition of molecular switching elements into conjugated polymers would provide a means to modulate the optoelectronic properties these versatile materials offer to molecular electronic,155 sensing,156 and logic157 technologies. Despite  efforts  directed  towards  developing  photochromic  conjugated  materials,71,158-172 systems that incorporate multiple chromophores into a single conjugated backbone while retaining the photoswitching behavior of all components have been elusive.  The major problem here stems from rapid energy transfer along the  delocalized singlet excited state energy surface, a consequence of the extended conjugation present in these materials. Implicit to the concept of photochromism, by which a compound interconverts between two metastable states with different absorption spectra,63 one of the two isomers must have an excited state lower in energy than the *  A version of this chapter has been published. Reproduced with permission from Roberts, M. N.; Carling, C.-J.; Nagle, J. K.; Branda, N. R.; Wolf, M. O. J. Am. Chem. Soc. 2009, 131, 16644 - 16645. Copyright 1 2009 American Chemical Society. 93  other. The lower energy isomer acts as a sink for the energy rapidly diffused along the conjugated pathway after photoexcitation. For photochromes that rely on population of an excited singlet state for interconversion, rapid energy transfer from the higher energy isomer through a conjugated linkage to the lower energy isomer takes precedence over photoisomerization (Figure 4-1).  The end result is that photoswitching of all the  individual chromophores in the material is incomplete. This behavior has been observed for several families of photochromes including DTEs,71,160,168,169,171,172 napthopyrans,165 spiropyrans,170 spirooxazines,164,166,167 and azobenzenes.163 Some success has also been met in preparing conjugated materials with dihydropyrenes173 and napthopyrans.174,175  Figure 4-1 Generalized Jablonski diagram showing the energy transfer pathway responsible for preventing photoswitching of multiple interacting photochromes. Photoresponsive molecular systems based on the DTE architecture are particularly suitable choices as ‘on/off’ modulators of conjugation due to the reversible ring-closing reactions between two isomers with markedly different optical and electronic properties.63,118 Additionally, their thiophene-based architecture provides the suitable optoelectronic properties that have led to the success of the versatile polythiophenes.176 Incomplete photoswitching of multiple DTEs linked by conjugated pathways has been observed for both molecular71,160,168,169,171,172 and polymeric158,159,161,177 systems (Scheme 94  4-1). Irie and co-workers found that by diluting the DTE content in the conjugated polymer chain, essentially creating longer distances between adjacent DTEs, a higher percentage of DTEs photocyclize.162 This strategy was successful at achieving a high PSS, however, at the expense of the on/off ratio, which could potentially be larger if a higher content of DTEs was present in the material. Another approach is to introduce steric hindrance between adjacent DTEs, effectively limiting the orbital overlap and coupling between localized states.178,179 The few examples that have multiple, adjacent DTEs undergoing photoinduced ring-closing tend to have more localized excited states and, therefore, properties similar to those of a single DTE unit.162,180 Therefore, little benefit has been gained from integrating multiple DTEs into a single polymer and nontraditional approaches to photochromic switching must be exercised to eliminate this paradox. Scheme 4-1 energy transfer  (a)  F F S  F F  F F  F F  S  S  F F  F F  UV  S  F F  F F  F F S  F F  F F  F F  F F  F FF F  S  S  S  R R  F F  F F  F F  UV  X  F F  F F  F F  S  S  S  F F  F F  F F S  R R  (b) S  S  S  S  n  energy transfer  One possible solution to this problem is to access more localized excited states that result in a higher probability of ring closing rather than intrachain energy transfer. In Chapter 2 it was demonstrated that photoswitching of DTEs is possible from triplet states 95  that are populated by triplet energy transfer from CT states on an attached Pt center. The goal of this chapter is to show how metal-sensitized population of the triplet manifold can be used to ring-close more than one DTE via the organic chromophores’ triplet states. It will be shown how the interesting optoelectronic properties typical for conjugated oligomers can be reversibly modulated by photoswitching multiple DTEs. Compound 38oo (Scheme 4-2) is used to illustrate this unprecedented approach, based on the fact that triplet states in Pt-acetylide-based conjugated oligomers are localized on only one conjugated ligand rather than delocalized over the whole oligomer as in the singlet state.181,182  The Pt-bis(acetylide) moiety makes an excellent sensitizer in 38oo since  ligand-localized triplet states can be populated by excitation with visible light to trigger the cyclization of both DTE photoswitches without the ring-open DTE transferring its excited state energy to the adjacent ring-closed DTE in 38oc. Cyclization of the two ring-open isomers produces a fully "-conjugated pathway that extends through the Pt and over the full length of both DTEs.183 Scheme 4-2 F F S  F F  F F  F F  P Pt  S  S  P  F F  F F S  < 415 nm > 470 nm  38oo  F F  F F S  F F  < 415 nm  38cc F F S  P Pt P  F F S  F F  F F  S  F F  38oc F F S  P Pt P  F F S  F F  F F S  > 470 nm  S  96  4.2  Experimental  4.2.1 General All solvents and reagents including those for NMR analysis (Cambridge Isotope Laboratories) were obtained from commercial sources and used as received except where noted. 1H and 31P NMR spectra were recorded on a Bruker AV400-Direct (400 MHz) or Bruker AV400-Indirect (400 MHz) spectrometer. All chemical shifts are referenced to residual solvent signals, which were previously referenced to tetramethylsilane (1H) or H3PO4 (31P). Splitting patterns are designated as s (singlet), d (doublet), t (triplet), and m (multiplet). IR Spectra were obtained on a Thermo Nicolet 6700 FT-IR spectrometer in transmission mode. MALDI-TOF MS (Bruker Biflex IV) and elemental analysis were obtained at the UBC Microanalysis facility. UV-vis absorption spectra were obtained on a Varian Cary 5000 using Fisher (HPLC Grade) solvents. For UV irradiations, a filtered 254/365 nm 6-Watt handheld lamp (UVP) was used, unless otherwise noted.  For  broadband visible irradiation, a handheld lamp with a tungsten bulb fitted with the appropriate low-pass filter was used. 4.2.2 Synthesis trans-Pt(PBu3)Cl2 was prepared according to literature procedure.184 DTE 4o was prepared by Carl-Johan Carling (Dept. of Chemistry, Simon Fraser University) according to the published procedure.126  97  Pt(PBu3)2(C. C-DTE)2  (38oo)  DTE precursor 4o (0.051 g, 9.41 # 10–5 mol) and trans-Pt(PBu3)Cl2 (0.030 g, 4.48 # 10–5 mol) were dissolved in a degassed mixture of CH2Cl2 and diisoproylamine (5:1, 36 mL). Tetrabutylammonium fluoride (0.1 mL, 1 M in THF) and CuI (2 mg) were then added, and the reaction mixture was stirred in the dark at room temperature, under N2, for 30 h. The reaction mixture was extracted with H2 O (3 # 15 mL) and sat. NaCl solution (1 # 15 mL). The CH2Cl2 layer was dried over MgSO4 and the solvent removed in vacuo to afford 35 mg (68%) of 38oo. 1H NMR (400 MHz, CD2Cl2): ! 7.55 (d, J = 7.52, 4H), 7.39 (t, J = 7.96, 4H), 7.30 (t, J = 7.15, 2H), 7.29 (s, 2H), 6.80 (s, 2H), 2.10–2.02 (m, 12H), 1.96 (s, 3H), 1.86 (s, 3H), 1.63–1.53 (m, 12H), 1.50–1.40 (m, 12H), 0.93 (t, J = 7.37, 18H).  31  P NMR (125 MHz, CD2Cl2): ! 6.05 (JPtP = 2320 2z). FT-IR (CH2Cl2, cm-1):  2092 (s, )C.C). MALDI-TOF MS m/z: 1534.3 [M]+. Anal Calcd. C70H80F12P2S4Pt [+H2O]: C 54.15, H 5.32; Found: C 53.90, H 5.66. 4.2.3 Determination of Photocyclization Wavelengths Solutions of complex 38oo (1.8 # 10–5 M) and 4o (2.6 # 10–5 M) were prepared in CH2Cl2. UV-vis Absorption spectra of each solution were collected and confirmed that only the open DTE isomer was initially present. The solutions were then irradiated using the 75 W arc lamp source from a Photon Technology International fluorimeter. The excitation beam was first passed through a QuadraScopic monochromator and a 4mm slit resulting in an excitation beam with a ~10 nm spectral width. Starting at low energy, each sample was irradiated for 5 min at a particular wavelength. After each irradiation, an absorbance spectrum was collected to monitor the formation of photocyclized DTE. This cycle was repeated, incrementally reducing the excitation wavelength by 10 nm each time, until evidence of photocyclization was observed, indicated by an increased in the 98  absorbance at 600 nm. The lower energy limit for onset of photocyclization is reported as the blue edge of the excitation beam. 4.2.4 DFT Calculations ADF 2008.01127,128 all-electron calculations were performed by Dr. Jeffrey Nagle with TZ2P basis sets, scalar relativistic effects included through the ZORA129-131 formalism and solvation effects (CH2Cl2) through the COSMO132 formalism. Geometry optimizations were carried out with the Becke-Perdew GGA potential. Both C2 and Ci geometry constraints gave nearly identical results to unconstrained C1 calculations. 4.2.5 Chemical Oxidation of 38oo/38oc/38cc Separate stock solutions of complex 38oo (8.6 # 10–5 M) and [(4BrC6H4)3N][SbCl6] (1.43 # 10–4 M) were prepared in CH2Cl2 (dried by passing the solvent through an alumina column followed by degassing it with 3 freeze-pump-thaw cycles). A solution of 38oo (4.3 # 10–5 M, 8.6 # 10–8 mol) was prepared by diluting 1 mL of the stock solution to 2 mL. This solution was irradiated with 365 nm light until the photostationary state (PSS) was reached. This solution was treated with one equivalent (600 µL, 8.6 # 10–8 mol) of the [(4-BrC6H4)3N][SbCl6] stock solution. An absorption spectrum of the resulting solution was collected immediately. A second equivalent (600 µL, 8.6 # 10–8 mol) of the [(4-BrC6H4)3N][SbCl6] stock solution was added to the same sample and another spectrum collected. A second, fresh sample solution of 38oo (4.3 # 10–5 M, 2 mL) was prepared, and irradiated for approximately 5 s to generate some 38oc in a solution containing predominantly 38oo. The presence of isobestic points in the UVvis absorption spectrum during this irradiation indicated that 38cc had not yet formed (see Figure 4-3). This mixture of 38oo/38oc was treated with one equivalent (600 µL, 8.6 99  # 10–8 mol) of the [(4-BrC6H4)3N][SbCl6] stock solution, and an absorption spectrum of the resulting solution collected immediately. The resulting absorption spectra were fit to Gaussian-shaped peaks by a leastsquares regression analysis using the PeakFit software package (v4.11).  The Hush  equation (Equation 1-1) was evaluated using the peak parameters determined by the calculated fit and R = 17.8Å, the donor-acceptor distance estimated for a similar Pt(bis)acetylide complex.185  4.3  Results and Discussion  4.3.1 Synthesis The preparation of Pt dialkynyl complexes and polymers was developed primarily by Hagihara in the late 1970s.186,187 Complex 38oo was prepared by a slight modification of this method by incorporating an in situ deprotection of the terminal acetylide. The cleavage of the trimethylsilyl protecting group on 4o was accomplished by stirring with n-Bu4NF for 30 minutes before the addition of trans-Pt(PBu3)2Cl2 and a catalytic amount of CuI resulting in the coupling of two acetylene-terminated DTE photoswitches to transPt(PBu3)2Cl2. The reaction was stirred under N2 atmosphere and in the absence of light. After an aqueous wash of the reaction mixture and removal of the solvent in vacuo, a blue solid was isolated. The blue color is consistent with the initial NMR spectrum that indicates a small percentage of ring-closed DTE isomer was present. Pure 38oo, a yellow solid, was isolated by dissolving the blue solid in CH2Cl2, irradiating the solution with light >470 nm, and subsequently removing the solvent in vacuo while in the absence of light. 100  Scheme 4-3  P Cl  Pt P  Cl  + TMS  F F  F F  F F  NBu4F (1M in THF)  F F  F F  F F  CuI, iPr2NH, CH2Cl2 S  S  S  S  P Pt P  F F S  F F  F F S  4o 38oo  4.3.2 UV-vis Absorption Spectroscopy The absorption spectrum of 38oo in CH2Cl2 (Figure 4-2) contains several overlapping transitions in the UV region attributable to localized mixed metal-ligand "'"* transitions and "'"* thienyl transitions found in DTE 4o. A second absorption band appearing at 350 nm is assigned to a long-axis "&"* transition with some CT character involving both the metal and alkynyl ligands.183  Figure 4-2 UV-vis absorption spectra of CH2Cl2 solutions of 38oo and 4o at room temperature, and the photostationary states generated when solutions of 38oo and 4o are irradiated with 365 and 254 nm light, respectively.  101  Whereas photocyclization of DTE 4o is accomplished by irradiation with light of wavelengths shorter than 340 nm, complex 38oo also photocyclizes by irradiation into the MLCT at wavelengths as long as 415 nm. In both cases, lower energy absorptions (590–630 nm) corresponding to the ring-closed isomers appear (Figure 4-2). In the case of 38oo, the MLCT and the ring-closed DTEs’ "'"* bands gradually red-shift by 16–20 nm (Figure 4-3), resulting from the conversion of the ring-open isomer to a changing mixture of two ring-closed isomers (38oc and 38cc). The ring-closing of only one DTE unit (38oo ' 38oc) can be observed in the early stages of the irradiation, where isobestic points at 307, 327 and 364 nm exist. These isobestic points disappear with further irradiation once the more complex equilibrium of isomers is established (38oo " 38oc " 38cc).  Ring-opening of all compounds is achieved by irradiation with visible light  (typically at wavelengths greater than 470 nm).  Figure 4-3 Changes in the UV-vis absorption spectra of CH2Cl2 solution of 38oo (2.3 # 10-5 M) when it is irradiated with 365 nm light at room temperature. Arrows indicate the isobestic points present after the first two 10-sec irradiations.  102  The progressive 20 nm red shift of the low energy "'"* band that appears in the absorption spectra when 38oo is converted to the PSS is attributed to increased delocalization of the singlet state over both DTE units in 38cc relative to 38oc. This is in agreement with other reports that show singlet state delocalization in Pt-bis(acetylide)s spanning the entire system, including several repeat units in oligomeric and polymeric analogues.188 DFT-calculated orbital plots of 38oo, 38oc and 38cc suggest that electron density in the HOMO is limited to the metal and its two proximate alkynyl thiophene rings in 38oo, but extends over both DTE thiophenes when ring-closed (Figure 4-4). In 38cc, the HOMO is delocalized over the entire molecule with significant orbital density on both DTEs and at the metal.  103  Figure 4-4 DFT calculated contour plots of the HOMO and LUMO of 38oo, 38oc, and 38cc.  4.3.3 NMR Spectroscopy The sequential ring-closing (38oo ' 38oc ' 38cc) is supported by 1H and  31  P  NMR spectroscopy, which both show the formation of significant amounts of 38oc prior to 38cc, ultimately producing a PSS consisting of 80% 38cc and 20% 38oc. A plot of the relative concentrations of each component (determined by  31  P NMR spectroscopy)  against time is consistent, qualitatively, with consecutive pseudo-first order kinetics 104  (Figure 4-5). The NMR studies also show that the chemical shifts for the phosphorous and the thienyl proton closest to the metal move downfield as 38oo is converted to 38oc and then to 38cc signifying increased donation of electron density from the phosphine to Pt in the case of the  31  P signals (6.02, 6.24, and 6.41 ppm), and decreased electronic  shielding from the alkyne due to a withdrawing of electron density by the adjacent ringclosed DTE for the 1H signals (Figures 4-6 and 4-7).  These effects will become  important in rationalizing the electrochemical properties later in this chapter.  Figure 4-5 Concentrations of 38oo ($), 38oc ("), and 38cc (#) as determined by 125 MHz 31P NMR spectroscopy when a CD2Cl2 solution of 38oo is irradiated with 365 nm light at room temperature.  105  Figure 4-6 125 MHz 31P NMR spectra at room temperature showing the conversion of 38oo & 38oc & 38cc in a CD2Cl2 solution upon irradiation with 365 nm light.  Figure 4-7 400 MHz 1H NMR spectra of (a) 38oo, (b) 38oo/38oc/38cc, and (c) 38oo/38oc/38cc in CD2Cl2 solution at room temperature.  106  Upon cyclization of a DTE chromophore, two chiral centers are formed at the adjacent, reactive carbon atoms involved in the ring-forming reaction. In solution, the conrotatory ring-closing reaction of a single DTE affords the R,R and S,S isomer in a 1:1 ratio.  As such, compound 38cc is expected to be generated as a mixture of four  stereoisomers: 1) R,R-R’,R’ , 2) S,S-S’,S’ , 3) R,R-S’,S’ , and 4) S,S-R’R’. The NMR data obtained for 38cc did not indicate the presence of distinct stereoisomers, and therefore, the influence of possible stereoisomers of 38cc on its electronic and spectroscopic properties was presumed to be minor.  4.3.4 Electrochemical Characterization In order to further elucidate the changing electronic interactions present in the bifunctional system, the three isomers of complex 38 were characterized by electrochemical methods. Differential pulse voltammetry (DPV) of 38oo shows a single oxidation wave at 1.00 V (vs. SCE) corresponding to oxidation of the ring-open DTE (Figure 4-8), more positive than that of 4o as a consequence of Pt coordination. A cyclic voltammogram of 38oo reveals this oxidation wave is irreversible, common behavior for the electrochemical oxidation of a ring-open DTE isomer. When 38oo is converted to 38oc, two new waves (0.65, 0.90 V) assigned only to the ring-closed DTE of 38oc appear. This assignment is based on the reversible nature of these oxidations and by comparison to 4c which also exhibits two sequential oxidations. Oxidation of the remaining ring-open DTE in 38oc is likely positive of the solvent limit. Again, this electrochemical behavior is supported by the DFT calculations that indicate a localization of electron density on the ring-closed isomer of 38oc (Figure 4-4). The fully ring-closed isomer (38cc) exhibits a single oxidation wave at 0.76 V.  Unlike the 107  localization of electron density experienced for 38oc, the HOMO of 38cc is proposed to be delocalized over the entire conjugated backbone. The change in the HOMO might be why a single oxidation wave was observed for 38cc as opposed to the two successive waves observed for 38oc or 4c. The shifting redox potentials for the three isomers are supporting evidence that this bifunctional system behaves as a single molecular unit because clearly, each ring closing or opening event has an effect by the adjacent photochrome.  Figure 4-8 Changes in the DPVs of a CH2Cl2 solution of 38oo, at room temperature, as it is irradiated with 365 nm light. Electrolyte = 0.1 M [(n-Bu)4N]PF6, working electrode = Pt disk, counter electrode = Pt mesh, reference electrode = Ag wire. Referenced to SCE using the one-electron oxidation of decamethylferrocene as an internal standard. It is proposed that the large difference in "-acidity of the ring-open and ringclosed isomers is largely responsible for generating the unique electronic character for each of the open/open, open/closed, and closed/closed states. Because the ring-closed DTE isomer is a stronger "-acid than its ring-open counterpart, it accepts more electron density via backbonding from the Pt center. This difference in bond character of the acetylide of the ring-open and ring-closed DTE influences the adjacent DTE via the trans 108  effect. As a result, the first oxidation potential of 38oc is shifted cathodically relative to 38cc, despite 38cc having a more delocalized conjugated system than 38oc.  A  rationalization of this behavior using the "-acidity argument is that a closed DTE adjacent to a ring-open DTE, as in 38oc, is more electron-rich than a closed DTE adjacent to another ring-closed DTE, as in 38cc, since each closed isomer in 38cc will compete equally for electron density from the metal center. This competition leaves each DTE with a smaller share of electron density relative to the ring-closed isomer in 38oc.  109  e- delocalization F F  F F  S  F F S  P Pt P  F F  F F  F F  38oo  S  S  e- delocalization F F  F F  S  F F S  P Pt P  F F  F F  F F  38oc  S  S  e- delocalization F F S  F F  F F S  P Pt P  F F S  F F  F F  38cc  S  Figure 4-9 Illustration of changes in electronic delocalization upon interconversion amongst 38oo, 38oc, and 38cc. The changes in electron density between the three isomers are supported by IR spectroscopy, which shows a shift of the acetylide stretch upon photoisomerization (Figure 4-9), and by DFT-calculations, which estimate bond lengths and atomic charges that indicate greater backbonding to the ring-closed isomer (Table 4-1). Since greater backbonding from the Pt center to the ring-closed DTE increases the bond order of the acetylide bond, a red shift of the alkynyl stretch in the IR spectrum is expected to occur. 110  Indeed, the first reaction (38oo ' 38oc) produces a large red shift (~24 cm–1) due to a combination "-backbonding and the "-acidity of the adjacent ring-closed DTE in 38oc. The influence of the backbonding is supported by the larger magnitude of red shift observed for 38oo ' 38oc compared to 4o ' 4c. In 38cc, both ring-closed DTEs compete for electron density through a conjugated system that includes the metal center resulting in a smaller red shift (~4 cm–1) for the second reaction (38oc'38cc).  Figure 4-10 Changes in the IR spectra for CH2Cl2 solutions at room temperature of (a) 4o and (b) 38oo upon irradiation with 254 and 365 nm light, respectively. The relevant bond lengths and atomic charges indicating the bonding character of the metal center and the acetylide are listed in Table 4-1 and Table 4-2, respectively. With each successive ring-closing event, the Pt and P atoms become more electropositive. This trend is supported experimentally by the downfield shifts of  31  P  NMR resonances (Section 4.3.3) and implies that the Pt-phosphine scaffold acts as a source of electron density that is allocated around the complex depending on the state of each attached DTE. The shifting electron density is manifested in changes of the lengths of bonds involving these atoms. Upon ring-closing, the Pt-C bonds shorten and C-C alkynyl bonds lengthen relative to those of a ring-open DTE. An important distinction 111  here is the difference in the acetylide bond lengths of 38oc and 38cc. The alkynyl bond linking Pt to the ring-closed DTE in 38oc is longer than the symmetric alkynyl bonds in 38cc. The changes in these calculated bond lengths are consistent with the proposed explanation that a ring-closed DTE will withdraw more electron density from the metal center when adjacent to a ring-open chromophore instead of another ring-closed DTE. Table 4-1 Relevant DFT-calculated bond lengths (pm) are given for geometry-optimized structures. The Pt–P (o) and Pt–P (c) distances refer to the phosphines that are cis to the open and closed DTEs, respectively. 38oo (Ci)  38oc (C1)  38cc (Ci)  Pt – P  235.9  241.5 (o) / 242.6 (c)  246.0  Pt – C  199.2  200.5 (o) / 198.5 (c)  197.8  C. C  123.5  123.7 (o) / 124.2 (c)  123.8  Table 4-2 Relevant DFT-calculated atomic charges using the Hirschfeld method are given for the geometry-optimized structures. The Pt–P (o) and Pt–P (c) charges refer to the phosphines that are cis to the open and closed DTEs, respectively. The C atom charges refer to the Pt-coordinated C atoms. 38oo (Ci)  38oc (C1)  38cc (Ci)  Pt  0.077  0.080  0.083  P  0.272  0.277 (o) / 0.288 (c)  0.282  C  -0.197  -0.196 (o) / -0.168 (c)  -0.175  4.3.5 Chemical Oxidation of 38oo/38oc/38cc Ground state electronic communication in 38cc is illustrated by comparing the vis-near-IR spectra of each system after it is chemically oxidized with one equivalent of [(4-Br-C6H5)3N][SbCl6], which is capable of only removing an electron from a ringclosed DTE. Oxidation of a solution containing only 38oo and 38oc results in the 112  disappearance of the "'"* band of 38oc (Figure 4-12).  The loss of this band is  attributed to oxidation of the ring-closed DTE’s conjugated backbone responsible for the low energy "'"* absorption. Despite the evidence for successful formation of 38oc+, no additional absorption bands were observed for this species. On the other hand, the oxidation of a solution at the PSS containing only 38oc and 38cc generates several new bands, notably two in the near-IR region assigned as an IVCT transitions, the result of electronic coupling between the DTEs in 38cc+ to give optically induced exchange of the electron-hole pair. This assignment is partly based on the behavior of an analogous triarylamine-substituted Pt-(bis)acetylide reported in the literature that has shown similar charge transfer absorption bands.185 The oxidized species was expected to potentially exhibit several types of electronic transitions, including IL, MLCT, and IVCT transitions. The intensity of bands attributed to IL and MLCT transitions should increase for the dication relative to the cation. No near-IR absorption bands are observed for any of the species when two equivalents of oxidant are added to generate dications. For this reason, both overlapping absorption bands in the near-IR region are assigned to be IVCT transitions. The lack of an analogous low energy band in the spectrum of 38oc+ indicates that the cation is more localized in this species than in 38cc+.  113  Figure 4-11 Schematic drawing illustrating the differences in delocalization between 38oc+ and 38cc+.  Figure 4-12 vis-NIR absorption spectra of a CH2Cl2 solution, at room temperature, of 38oo/38oc (black) and 38oc/38cc (blue) before oxidation and after 1 equivalent of oxidant is added to generate 38oo+/38oc+ (dash) and 38oc+/38cc+ (red). The absorption of 38oc+/38cc+ in the near-IR region is a combination of two overlapping bands. In order to estimate the magnitude of electronic coupling by analysis of the IVCT bands, the complex absorption spectrum was deconvoluted by fitting the 114  spectrum to a combination of Gaussians (Figure 4-13). The number of Gaussians used in the fit was chosen to be the lowest number of peaks that gave a reasonable fit to the spectrum (defined as having a minimum r2 = 0.97). It should be noted that with multiGaussian fits other solutions are possible, and the electronic coupling parameters derived from the specific fit offered here are dependent on the number of Gaussians used in the fit.  The parameters defining the Gaussians centered at 7369 and 8763 cm-1 are listed in  Table 4-3. The electronic coupling, Vab, was estimated from these values by using the Hush equation (Equation  1-1). The calculated electronic coupling present in 38cc+ is  close to that determined for another Pt-bis(acetylide) complex185 and categorizes 38cc+ as a Class II mixed-valence system.  Figure 4-13 vis-near-IR absorption spectrum of 38oc+/38cc+ with deconvolution by Gaussian bands and the calculated fit (red). Table 4-3 IVCT parameters and calculated electronic coupling for 38oc+/38cc+. ()max (cm-1)  !max (M-1 cm-1)  '()1/2 (cm-1)  Vab (cm-1)  7369  3606  1521  234  8763  5181  2605  400  115  4.3.6 Proposed Excited State Pathway for Bifunctional Photoswitching Both the IVCT absorption bands in 38cc+ and the shifts in electrochemical redox potentials support ground state electronic interaction between the two linked DTEs. The shift in the "&"* transition of 38cc relative to 38oc supports a delocalized singlet excited state exists in 38cc. Ring closing of the second DTE in 38oc is unexpected since rapid energy transfer to the adjacent closed DTE should quench the excited state before ring closing occurs. The results discussed here are rationalized by involvement of the excited state triplet manifold in the photocyclization of the DTEs. In Chapter 2 it was demonstrated that ring closing can occur from a triplet state populated by energy transfer from a Pt-based moiety. Recent reports of similar thienyl-substituted Pt-(bis)acetylide compounds show that rapid intersystem crossing to the triplet manifold occurs on the femtosecond timescale.189 Complex 38oo is expected to behave similarly, with ring closing occurring from a populated triplet state. The localized exciton model, supported by published experimental and computational evidence, is consistent with this observation.181,182,189-193 Although the triplet state of the ring-closed DTE is lower in energy than the ring-open form in 38oc, an energy barrier between two potential wells, representing a localized exciton on either side of the Pt linkage, would prevent rapid energy transfer from occurring to the lowest energy excited state (Figure 4-14). In other words, ring-closing from the excited triplet state proceeds at a faster rate than energy transfer to the adjacent ring-closed DTE. Consequently, ring-closing of the ring-open DTE in 38oc generates the fully conjugated 38cc.  116  Figure 4-14 Energy diagram showing the proposed excited state pathway for the ringclosing reaction from 38oc & 38cc.  4.4  Conclusions In summary, it was demonstrated in this chapter that rapid intersystem crossing  induced by large spin-orbit coupling from the heavy Pt atom enables population of the triplet manifold from which both DTEs are independently photoactive via localized triplet states. When DTEs are linked only through an alkyne bond,160 population of the triplet state is not rapid and delocalization in the singlet state prevents ring-closure of the two adjacent DTEs. The platinum acetylide linkage in 38cc maintains a conjugated pathway between DTEs resulting in ground state electronic communication.  117  CHAPTER 5  Photoresponsive Conjugated Platinum Acetylide Oligomers  5.1  Introduction "-Conjugated conducting polymers are positioned to replace conventional  inorganic semiconductors in the next generation of versatile thin-film, flexible devices. Their ability to efficiently migrate charge along the "-conjugated backbone is critical to their function as the active material in devices such as OLEDs,194,195 organic field effect transistors (OFETs),9 and OPV devices.196,197 Charge carriers migrate through these materials by thermally activated interchain and intrachain charge hopping processes.198 Although many extrinsic factors can affect the measured conductivity of a material, ideally, the conductivity of a material reflects its charge mobility. A considerable body of research has identified several families of "-conjugated structures whose properties are amenable to use as organic semiconductors.199 Although these static systems represent a major advance in the field, dynamic responsive organic semiconductors, which experience a change in conductivity due to an external stimulus, are being now investigated for sensors and memory applications.200 Photochromic materials have an interesting role in the development of this broader class of conducting materials because photochromes undergo reversible structural changes that alter the "-conjugated structure. In an OFET, for example, a photochromic semiconductor could switch between a more conductive and less conductive state, delineated as the so-called “on” and “off” states,  118  respectively.  The photomodulation of conductivity could compliment the switching  conventionally induced by the gate of the OFET. In the previous chapter, the bifunctional photoswitching system 38oo was introduced, and its photoswitching behavior and electronic character were discussed with respect to how two neighboring photochromic units interact with each other. This model system serves as a basis for the development and understanding of the extended multifunctional system that is discussed in this chapter. One primary motivation for targeting oligomeric and polymeric materials was based on the initial results obtained on the smaller, discrete molecular model system. The basis for inducing large changes in optical and electronic properties of these materials is the dramatic changes in electronic delocalization of the "-conjugated pathway as a result of photoswitching. Incorporating more photochromic material could potentially allow for the creation of extended states exhibiting further delocalization compared to the bifunctional molecular system.  The  larger change in electronic delocalization, as a result of photoswitching, is manifested by a larger magnitude of change in properties, such as conductivity or non-linear optical response.  The major challenge associated with this approach is that increasing the  density of photochromic material often results in incomplete photoswitching as a result of energy transfer processes among the closely spaced chromophores.161 On the other hand, diffusing the photochromic material throughout the bulk semiconductor material, in order to minimize energy transfer between chromophores, does not result in a large modulation of the bulk material’s properties.162 This chapter explores the viability of multifunctional systems using the Pt-bridged DTE architecture that are capable of a high degree of photoswitching.  119  5.2  Experimental  5.2.1 General All solvents and reagents including those for NMR analysis (Cambridge Isotope Laboratories) were obtained from commercial sources and used as received except where noted. 1H and 31P NMR spectra were recorded on a Bruker AV400-Direct (400 MHz) or Bruker AV400-Indirect (400 MHz) spectrometer. All chemical shifts are referenced to residual solvent signals, which were previously referenced to tetramethylsilane (1H) or H3PO4 (31P). Splitting patterns are designated as s (singlet), d (doublet), t (triplet), and m (multiplet). IR spectra were obtained on a Thermo Nicolet 6700 FT-IR spectrometer in transmission mode. MALDI-TOF MS (Bruker Biflex IV) was obtained at the UBC Microanalysis facility. UV-vis absorption spectra were obtained on a Varian Cary 5000 using Fisher (HPLC Grade) solvents. For UV irradiations, a filtered 254/365 nm 6-Watt handheld lamp (UVP) was used, unless otherwise noted.  For broadband visible  irradiation, a handheld lamp with a tungsten bulb fitted with the appropriate low-pass filter was used. 5.2.2 Synthesis trans-Pt(PBu3)Cl2184  and  trans-phenylethynylchlorobis(tri-n-  butylphosphine)platinum(II)183 were prepared according to literature procedure. DTE 39o was prepared by Carl-Johan Carling (Dept. of Chemistry, Simon Fraser University) according to the published procedure.201  120  [Pt(PBu3)2(C. C-DTE-C. C)]n  ([40o]n)  DTE precursor 39o (0.056 g, 1.0 # 10–4 mol) and trans-Pt(PBu3)Cl2 (0.067 g, 1.0 # 10–4 mol) were dissolved in a N2-sparged mixture of CH2Cl2 and diisopropylamine (5:1, 36 mL). Two equivalents of n-Bu4NF (0.2 mL, 1 M in THF) and CuI (3 mg) were then added, and the reaction mixture was stirred in the dark, at room temperature, under N2 atmosphere for 19 h. The majority of the solvent was removed in vacuo leaving a few milliliters of a viscous blue-green liquid in the flask. The remaining reaction mixture was added dropwise to 75 mL of CH3OH, which was stirred for 30 minutes. A blue-green precipitate was filtered from the CH3OH, then reprecipitated twice more from THF/CH3OH, and finally dried under vacuum to yield 80 mg of blue solid. The solid was dissolved in CHCl3, and the solution irradiated with filtered visible light (% > 500 nm) until the blue color dissipated, indicating complete cycloreversion to the ring-open form. The solvent was removed in vacuo to isolate a waxy yellow-green solid.  1  H NMR (400  MHz, CDCl3): 6.84 (s, 2H), 2.09 (m, 16H), 1.77 (s, 6H), 1.46 (m, 16H), 1.26 (s, 12H), 0.93 (m, 24H).  31  P NMR (125 MHz, CDCl3): ! 3.99 (JPtP = 2320 2z). FT-IR (CH2Cl2,  cm-1): 2092 (s, )C.C).  Pt(PBu3)2(C. C-C6H5)(C. C-DTE-C. C-)Pt(PBu3)2(C. C-C6H5) (41o) DTE precursor 39o (0.022g, 3.9 # 10–5 mol) and n-Bu4NF (80 µL, 1 M in THF) were dissolved in 10 mL of CH2Cl2. The reaction mixture was stirred for 20 minutes at room temperature and then transferred to a prepared CH2Cl2 solution (20 mL) of transphenylethynylchlorobis(tri-n-butylphosphine)platinum(II) (0.069 g, 8.1 # 10–5 mol) and diisopropylamine (6 mL). The reaction mixture was sparged with N2 for 15 minutes and a catalytic amount of CuI (3 mg) was added. The reaction mixture was stirred in the 121  dark, at room temperature, under N2 atmosphere for 20 h. The solvent was removed in vacuo yielding a blue solid as the crude product. The crude product was purified by preparative TLC (silica, hexanes:CH2Cl2, 8:3). While dissolving the complex in CHCl3 to desorb it from the silica, the solution was irradiated with filtered visible light (% > 500 nm) until the blue color dissipated, indicating complete cycloreversion to the ring-open form.  1  H NMR (400 MHz, CDCl3): 7.27 (s, 1H), 7.25 (s, 2H), 7.21 (t, 4H), 7.12 (t, 2H),  6.85 (s, 2H), 2.12 (m, 24H) 1.78 (s, 6H), 1.61 (m, 24H), 1.47 (m, 24H), 0.93 (m, 36H). 31  P NMR (125 MHz, CDCl3): ! 3.32 (JPtP = 2340 2z).  5.2.3 Chemical Oxidation of [40o/c]n All of the samples for chemical oxidation studies were prepared in CH2Cl2 that had been previously dried by passing through an alumina column and deaerated by no less than three freeze-pump-thaw cycles. Sample solutions were maintained under N2 atmosphere for the duration of the experiment in 1 cm2 anaerobic quartz cells (NSG PCI cells) fitted with a PTFE valve. Separate stock solutions of complex [40o]n (1.5 # 10–4 M) and [(4-BrC6H4)3N][SbCl6] (2.7 # 10–4 M) were prepared in CH2Cl2. A solution of [40o]n (5.9 # 10–5 M, 1.5 # 10–7 mol) was prepared by diluting 1 mL of the stock solution to 2.5 mL. This solution was irradiated with 365 nm light until the PSS was reached according to its absorption spectrum. This solution was treated with 600 µL (1.6 # 10–7 mol) of the [(4-BrC6H4)3N][SbCl6] stock solution. An absorption spectrum of the resulting solution was collected immediately. A second, fresh sample solution of [40o]n (5.9 # 10–5 M, 2.5 mL) was prepared, and irradiated with 365 nm light until the absorbance centered at 630 nm, indicative of the formation of [40o/c]n, was roughly half the intensity of the sample at the PSS concentration. This solution of [40o/c]n was treated  122  with 600 µL (1.6 # 10–7 mol) of the [(4-BrC6H4)3N][SbCl6] stock solution, and an absorption spectrum of the resulting solution was collected immediately. The resulting absorption spectra were fit to Gaussian-shaped peaks by a leastsquares regression analysis using the PeakFit software package (v4.11).  The Hush  equation (Equation 1-1) was evaluated using the peak parameters determined by the calculated fit and R = 17.8Å, the donor-acceptor distance estimated for a similar Pt(bis)acetylide complex.185  5.3  Results and Discussion  5.3.1 Synthesis Synthesis of the polymeric analogue of complex 38oo was achieved using a symmetrically-terminated dialkynyl DTE, 39o, to create the repeating structural unit of alternating DTE chromphores and Pt centers (Scheme 5-1).  The polymerization  proceeded by the reaction of equimolar quantities of 39o and trans-Pt(PBu3)Cl2 in the presence of diispropylamine and a catalytic amount of CuI.  The TMS-protected  acetylides of 39o were first deprotected by n-Bu4NF in situ and then coupled to the Ptbisphosphine unit. The product was isolated by removing the solvent from the reaction mixture and was purified by precipitation of a blue solid from THF by CH3OH. Due to photoactivity of the product within a small region of visible light spectrum, limited photocyclization probably occurred during the work up of the reaction as a result of ambient light. Dissolving the blue solid in CHCl3 and irradiating the solution with filtered visible light > 500 nm succeeded in the complete cycloreversion of any ringclosed DTEs to the ring-open form to generate [40o]n. For the remainder of this chapter, 123  the nomenclature [40o/c]n is used to denote a state of the sample in which various relative compositions of the ring-open and ring-closed forms exist within the same chains. Scheme 5-1  F F  P Cl  Pt P  Cl  F  F  F F  + TMS  S  S  TMS  Bu4NF (1M in THF) CuI, iPr2NH, CH2Cl2  39o  P Pt P  F F  F F  S  F F S  n  [40o]n  A model complex, 41o, was envisaged to elucidate the effect of the symmetrical coordination of two Pt centers to each photochromic unit. It is of interest to observe the extent to which Pt coordination has an electronic effect on a single DTE considering that previous studies have shown that metal alkynyl complexes strongly influence the behavior of the DTE relative to its noncoordinated form.84,85 The two models, 38oo and 41o, compliment each other in the sense that a multifunctional system should be a hybrid of the behavior observed for each model complex.  Complex 41o was prepared by  coupling the dialkyne 39o with two equivalents of the asymmetrically substituted Pt starting material (Scheme 5-2). Before coordination to the DTE, the Pt complexes were mono-substituted with a phenyl ethynyl group to prevent polymerization.  The  disubstituted DTE 41o was purified by preparative TLC.  124  Scheme 5-2  F F  P  2  Pt P  Cl  +  F  F  S  TMS  F F S  TMS  CuI, iPr2NH, CH2Cl2  39o  P Pt  > 500 nm  P Pt  S  F  F  F F S  P  F F S  F  F  F F S  P  < 415 nm  F F  Bu4NF(1M in THF)  P Pt P  41o  P Pt P  41c [40o]n was characterized by matrix-assisted laser desorption-ionization (MALDI) mass spectrometry to assess the length of oligomers present in the sample. Two series of equally spaced peaks, approximately 1030 amu apart, appear in the spectrum (Figure 5-1). This spacing is consistent with the mass of one repeat unit. The ion at 7223 m/z corresponds to an oligomer with 7 repeat units, the longest species detected. These masses correspond to an (AB)n+ structural repeat pattern with termination by a DTE and a Pt complex. The secondary series of ions present in the spectrum corresponds to a different repeating pattern. These ions are also separated by approximately 1030 amu, but the corresponding structural repeat pattern could not be identified.  The major  conclusion that can be drawn from the MALDI-TOF spectrum is that the sample contains  125  a mixture of short oligomers. It is possible that longer oligomers or polymers do exist in the sample, but their large masses are not detected by MALDI mass spectrometry.  Figure 5-1 MALDI-TOF mass spectrum of [40o]n. The primary series of ions, identified to have an (AB)n+ repeat pattern, is shown with squares. The secondary series of ions is shown by circles. The IR spectrum of [40o]n in CH2Cl2 solution shows one alkynyl stretch centered at 2092 cm-1 (Figure 5-2). Irradiation of this solution with 365 nm light resulted in similar changes to the spectra observed for 38oo (Figure 4-9b). A second alkynyl stretch grows in at lower energy and progressively red shifts until the PSS is reached. At the PSS, the similar energies of the alkynyl stretch for both 38cc and [40o/c]n suggests that any further electronic delocalization that might exist in the oligomers does not affect the structure of the conjugated backbone.  126  Figure 5-2 Changes in the IR spectrum of a CH2Cl2 solution of [40o]n at room temperature upon irradiation with 365 nm light. 5.3.2 NMR Spectroscopy The photocyclization reaction was monitored by 1H and  31  P NMR spectroscopy.  The photoswitching behavior of the oligomeric system was similar to that observed for the bifunctional model 38oo. Conversion of the ring-open DTEs to the ring-closed form results in a downfield shift of the  31  P NMR resonance and an upfield shift of thienyl  protons’ resonance in the 1H NMR spectrum. For [40o]n, only one signal, at 6.84 ppm, appears in the aromatic region in the 1H NMR spectrum and is attributed to the two equivalent thienyl protons of the ring-open DTE. This signal is shifted slightly upfield from the proligand, 39o, indicating increased electron density on the thiophene resulting from Pt coordination. Upon cyclization, the resonance at 6.84 ppm decreases in intensity and another upfield resonance at 6.00 ppm intensifies.  Simultaneously, the signal  corresponding to the DTE’s six methyl protons shifts downfield from 1.77 to 2.08 ppm. The other upfield resonances are assigned to the remaining protons on the phosphine ligands and do not shift upon photoswitching. 127  Figure 5-3 Changes in the 400 MHz 1H NMR spectrum for a CDCl3 solution of [40o]n, at room temperature, upon conversion of the initially ring-open system to the PSS using UV light (365 nm). Circles indicate the resonances assigned to the two thienyl protons and squares correspond to the resonances assigned to the six methyl protons of the DTE.  The changes observed in the  31  P NMR spectrum upon photoswitching are more  illustrative of how photoswitching proceeds in the oligomeric system (Figure 5-4). Initially, when all of the DTEs are in the ring-open form, the phosphorus signal at 3.99 ppm is indicative of a phosphine with two ring-open DTEs on either side of the Pt center.  Once one of those DTEs converts to the ring-closed form, the  resonance shifts downfield to 4.14 ppm.  31  P NMR  This resonance then undergoes a further  downfield shift to 4.28 ppm once both DTEs coordinated to the Pt center are converted to the ring-closed isomer.  128  Figure 5-4 Changes in the 125 MHz 31P NMR spectra at room temperature upon UV irradiation (365 nm) of a CDCl3 solution of [40o]n.  For the  31  P NMR spectra collected over the course of the photoreaction  proceeding to the PSS, the resonances for each type of phosphine were integrated relative to each other and the percentage of each phosphine present was plotted over time (Figure 5-5). Eventually, the signal at 3.99 ppm completely disappears, indicating that every Pt center is adjacent to at least one ring-closed DTE. The 31P NMR spectrum indicates that approximately 70% of phosphines are coordinated to Pt centers that are sandwiched between two ring-closed DTEs. The remaining 30% of Pt centers are attached to only one ring-closed DTE. Qualitatively, this plot illustrates the same type of photoswitching behavior that was observed for the bifunctional system 38oo & 38oc & 38cc (Figure 4-5).  A  significant amount of the Pt centers are adjacent to one ring-closed DTE before an appreciable amount of Pt centers with two adjacent ring-closed DTEs are generated. This suggests that in the longer oligomers, the quantum yield of photocyclization for a DTE 129  adjacent to a ring-closed DTE is less than if it is adjacent to another ring-open DTE. This is a reasonable conclusion because energy transfer to adjacent ring-closed DTEs is likely to occur and would account for suppressed photoactivity.  Figure 5-5 Changes in the integrated peak intensity for the 31P resonances appearing at 3.99 (!), 4.14 ("), and 4.28 ppm (!). For each 31P NMR spectrum collected at a given time interval, the most intense signal was integrated with a reference value of 1. The other signals were integrated relative to the most intense peak. The integrated values were totaled, and the relative percentage of each integrated peak intensity was calculated. The changes observed in the  31  P NMR spectrum of 41o upon UV irradiation are  shown in Figure 5-6. Unlike the multifunctional systems 38oo and [40o]n, a single downfield shift, from 3.32 to 3.52 ppm, of the 31P resonance is observed upon conversion of 41o to 41c. This observation is consistent with the conclusion that photoswitching on either side of the Pt complex affects the electronic environment of the phosphine appended to the metal center. The magnitude of the downfield shift between the  31  P  resonances of 41o and 41c is greater than the first shift observed for [40o]n but less than the change experienced when two adjacent DTEs photocyclize. This is indicative that there are small, but discernable differences in the electronic character of the Pt center 130  when it is symmetrically substituted with two thiophenes, as in [40o/c]n, compared to the asymmetric substitution in 41o/41c.  Figure 5-6 Changes in the 125 MHz 31P NMR spectrum, at room temperature, upon irradiation of a CDCl3 solution of 41o with 365 nm light.  5.3.3 UV-vis Absorption Spectroscopy The oligomers [40o]n exhibit similar absorption characteristics to the model complex, 38oo (Figure 5-7). When the DTEs are in the ring-open form, the absorption bands appear at nearly identical wavelengths.  This suggests that these electronic  transitions are between orbitals that are relatively localized, extending no further than over both proximal thiophenes adjacent to the Pt centers. The other notable comparison of the absorption spectra for [40o]n and 38oo is the absorptivity of the band centered at 349 nm. This band is far more intense in the [40o]n than in 38oo. The difference in absorptivity supports the assignment of this band as involving the Pt atom and its adjacent thiophenes. Since each DTE in the polymer is coordinated to a Pt atom at both ends, there are twice as many absorbing moieties present in the polymer. This hypothesis is supported by comparison of the absorption spectra for [40o]n and 41o (Figure 5-8). 131  Both [40o]n and 41o exhibit an absorption band at 349 nm with a higher extinction coefficient relative to the absorption bands at higher energy.  This supports the  assignment of this transition as involving mixed Pt-ethynyl-thiophene molecular orbitals.191  Figure 5-7 Changes in the UV-vis absorption spectra of CH2Cl2 solutions of (a) 38oo or (b) [40o]n at room temperature upon irradiation with 365 nm light. As supported by the NMR studies, the photoswitching behavior of [40o]n is akin to that of the model complex 38oo. Irradiation of [40o]n with light % < 415 nm also results in the photocyclization of the DTEs. The photoreaction is observable in solution as a color change from yellow to deep blue. Identical to the changes in absorption observed when 38oo & 38cc, the band at 349 nm in [40o]n red shifts to 367 nm when converted to [40o/c]n. The identical shift suggests that although the DTE is clearly involved in this transition, the orbitals involved in this transition are not delocalized over 132  multiple repeat units in the polymer. The lowest energy "&"* band, indicative of the extended conjugation of the ring-closed DTE backbone, initially appears centered at 640 nm. As photocyclization of the DTE units proceeds, this band undergoes a 35 nm red shift. The red shift observed during the conversion of [40o]n & [40c]n is attributed to a higher percentage of adjacent DTEs cyclizing, thereby extending the conjugated pathway and delocalizing the " MOs involved in these electronic transitions. Additionally, the band shape of this low energy band at the PSS is remarkably similar to the absorption of a related polyyne incorporating an electron-deficient thieno[3,4-b]pyrazine spacer between Pt centers.202 The close similarity of the two absorption spectra is suggestive that strong donor-acceptor interactions between the Pt centers and perfluorinated cyclopentyl rings are present when the DTEs are ring-closed. A systematic study of the conjugation of Pt-acetylide oligomers found that MOs could be delocalized over up to seven repeat units when incorporating a simple phenyl spacer between the Pt units.183 Although no definitive conclusions can be made regarding the effective conjugation length of [40o]n, the fact that a larger red shift is observed for the polymer, 35 nm compared to 20 nm for 38oo, is suggestive of electronic delocalization over more than two repeat units.  133  Figure 5-8 Changes in the UV-vis absorption spectra of CH2Cl2 solutions of (a) 41o or (b) [40o]n at room temperature when irradiated with 365 nm light. The conclusions drawn from the comparisons in the absorption spectra of 38oo and [40o]n are consistent with the changes observed in the absorption spectra during the cyclization of 41o & 41c. Unlike the changes observed for 38oo and [40o]n, the lowest energy "&"* band centered at 625 nm does not red shift as 41o is converted to 41c (Figure 5-8a). The lack of a bathochromic shift in this band strongly supports the notion that the red shifts exhibited by 38cc and [40o/c]n are attributed to increasing delocalization of " MOs, over multiple DTEs, as adjacent photoswitches are cyclized. Generation of the PSS is also marked by a red shift in the absorption band located at approximately 350 nm. Although slightly more structured in 41c, there is no significant difference in the energy of this band compared to [40o/c]n. Comparison of changes in the absorption spectra for 41c and 39c reveals that this band is a combination of two 134  overlapping bands, one of which is a Pt-perturbed DTE-localized transition and the other is a mixed metal-ligand transition. After a brief period of UV irradiation of [40o]n, it can be assumed that no two adjacent DTEs have been cyclized.  This is supported by the observed changes in  isosbestic points appearing in the absorption spectra. Isobestic points are present initially during photoconversion but shift as the photoreaction proceeds to the PSS. Significantly, these isobestic points are maintained throughout the photoreaction for 41o & 41c. It is noteworthy to compare the low energy "&"* band of the polymer sample at early irradiation times with the spectrum of 41c. The band shape of the "&"* band for the first scan of [40o/c]n after UV irradiation appears the same as the "&"* band for 41c. Overall, these comparisons support the conclusion that complex 41o/c models the optical properties of the individual DTEs within the oligomers well. 5.3.4 Electrochemical Characterization Cyclic voltammograms of CH2Cl2 solutions of 41o collected at 0.2 V s-1 are shown in Figure 5-9a. On the first cathodic scan, an irreversible oxidation appears at approximately +1.1 V (vs SCE) and is followed by the appearance of two new waves on the reverse scan. The second cathodic scan shows the reversibility of the two new redox waves located at E° = +0.58 and +0.80 V. A separate solution of 41o was irradiated with UV light until the PSS was reached, as indicated by no post-irradiation changes in the voltammogram. The cyclic voltammogram of the solution at the PSS is shown in Figure 5-9b. This is characterized by the two reversible waves at +0.80 and +0.58 V and the diminished intensity of the irreversible wave at +1.1 V. The electrochemical behavior observed for 41o is strikingly similar to that observed for the dialkynyl DTE when substituted with other redox-active metal complexes.84,85 Based on the results of these 135  previous studies, the irreversible oxidation is assigned as a two-electron oxidation of 41o. The appearance of new redox waves in the CVs is attributed to the cation 41o2+ cyclizing on the electrode to 41c2+. This cation, 41c2+, then undergoes two one-electron reductions to generate 41c. This behavior was observed at scan rates up to 0.5 V s-1.  Figure 5-9 Cyclic voltammograms of 41o at 0.2 V s-1 (0.1 M [(n-Bu)4N]PF6 in CH2Cl2) with (a) two consecutive scans (first scan = solid black line, second scan = dashed red line) and (b) after irradiation with 365 nm light until the PSS was reached (blue line). Electrolyte = 0.1 M [(n-Bu)4N]PF6, working electrode = Pt disk, counter electrode = Pt mesh, reference electrode = Ag wire. Referenced to SCE using the one-electron oxidation of decamethylferrocene as an internal standard.  Since cyclic voltammograms of [40o/c]n were characterized by broad, unresolvable redox waves, DPV was used for electrochemical characterization. Initially, when all of the DTEs are in the ring-open state, there is a single oxidation wave centered at 1.04 V (Figure 5-10a).  Irradiation of [40o]n with 365-nm light resulted in the  appearance of two oxidation waves at lower potential, centered at 0.55 and 0.76 V. Longer irradiation times resulted in the simultaneous increase in intensity of these oxidation waves and decrease of the oxidation wave at 1.04 V. Successive scans at 136  irradiation times longer than 12 min resulted in an overall decrease in the current (Figure 5-10b). Additionally, the more cathodic oxidation at 0.76 V diminished in intensity relative to the oxidation at 0.55 V. The final scan shown in Figure 10b was determined to be the PSS since their was no observable change in the voltammograms after prolonged irradiation.  Figure 5-10 Changes in the DPVs of a CH2Cl2 solution of [40o]n as it is irradiated with 365 nm light. (a) Changes observed up to 10 min of UV irradiation. (b) Changes observed between 12 and 45 min of UV irradiation. Electrolyte = 0.1 M [(n-Bu)4N]PF6, working electrode = Pt disk, counter electrode = Pt mesh, reference electrode = Ag wire. Referenced to SCE using the one-electron oxidation of decamethylferrocene as an internal standard. The electrochemical characterization of 38oo/38oc/38cc and 41o/41c helps to understand the changes observed in the voltammograms for [40o/c]n.  The single  oxidation wave for [40o]n is consistent with that observed for the models 38oo and 41o. The potential of this wave is shifted about 60 mV less positive than in 41o and 50 mV 137  more positive than 38oo.  Consistent with the assignments in 38oc and 41c, the  appearance of the two oxidation waves after UV irradiation is credited to oxidation of single ring-closed DTE units. These two waves in [40o/c]n are shifted by 110 mV and 140 mV to lower potential relative to the same oxidations in 38oc. This cathodic shift is attributed to the electron density donation from the Pt centers on both sides of the ringclosed DTE in the oligomers. This is confirmed by comparison to the disubstituted model 41c, whose oxidations are similarly shifted with respect to 38oc.  Table 5-1 Electrochemical oxidation potentials of model compounds and oligomers.a E° (V) 38oo  +1.00  38oc +0.65,+ 0.90 38cc  +0.76  E° (V) 41o  +1.10  41c +0.58, +0.80  E° (V) [40o]n  +1.04  [40o/c]nb  +0.55, +0.76  [40o/c]nc  +0.55  a  Measured by DPV in CH2Cl2 with 0.1 M [(n-Bu)4N]PF6 as supporting electrolyte. Potentials are referenced to a decamethylferrocene standard and are reported in volts vs SCE. b Composition of ring-open to ring-closed DTEs in [40o/c]n is such that ring-closed isomers are adjacent only to ring-open isomers. c Composition of ring-open to ringclosed DTEs in [40o/c]n is such that ring-closed isomers are adjacent to at least one other ring-closed DTE.  The sudden decrease in current observed for the scans shown in Figure 5-10b is attributed to precipitation of insoluble oligomers that form because the rigidity of the ring-closed isomer results in rigid rod oligomeric chains that are less soluble in organic solvents than oligomers that have flexible ring-open DTEs.162 In addition to the overall decrease in current, the intensity of the oxidation wave at +0.76 V diminishes relative to the wave at +0.55 V. These changes in the voltammogram of [40o/c]n are rationalized by considering the changes in electrochemical behavior when 38oc & 38cc.  Whereas 138  complex 38oc exhibits two resolved oxidation waves, complex 38cc exhibits a single oxidation wave shifted to slightly more positive potential that the first oxidation of 38oc. These changes are discussed in detail Chapter 4, but briefly, a change in the delocalization of the HOMO upon conversion of 38oc & 38cc causes the differences in electrochemical behavior. This behavior is also exhibited by [40o/c]n, but not as clearly as in the model complex. The oxidation potential of the discrete model complex, 38cc, is shifted positive relative to that of 38oc, making its growth in intensity easily identifiable. In contrast, the oxidation potentials of [40o/c]n when two ring-closed DTEs are adjacent, or when ring-closed DTEs are isolated along the chain, are coincident with each other. This assignment is based on the assumption that since 38cc exhibits a single oxidation wave, the oligomeric system should also exhibit a single oxidation wave when multiple ring-closed DTEs are adjacent to each other. This accounts for the relative changes in peak intensity observed for the oxidation waves centered at 0.55 and 0.76 V. The wave at higher potential diminishes at longer UV irradiation times because a higher degree of photocyclization results in a higher percentage of adjacent ring-closed photoswitches in each oligomer chain. 5.3.5 Chemical Oxidation of [40o/c]n Ground state electronic communication along the conjugated backbone of [40o/c]n was probed by chemically oxidizing the polymer and observing its vis-near-IR absorption spectrum. One equivalent of [(4-Br-C6H5)3N][SbCl6], a one electron oxidant, was added to CH2Cl2 solutions of [40o]n and [40o/c]n. The resulting behavior was in agreement with the differences in behavior observed for the various isomers 38oo, 38oc, and 38cc. Oxidation of the polymer at its PSS resulted in the appearance of several new overlapping bands, notably with the same band shape as those for 38cc+ (Figure 5-11). 139  The bands in the near-IR, which have been assigned as IVCT transitions, are slightly redshifted in [40o/c]n+ compared to 38oc+/38cc+. The band centered around 15300 cm-1 remains significantly more intense relative to the other bands, for [40o/c]n+, compared to the relative intensity of the same absorption in 38oc/38cc+.  This is attributed to a  considerable amount of ring-closed DTEs that are not oxidized in [40o/c]n. As oxidation of the oligomers proceeds, charge builds up on each chain. As a result of intrachain electronic interactions, the oxidation potentials of the remaining neutral ring-closed DTEs likely increase above the reduction potential of the oxidant. Adding two equivalents of oxidant resulted in changes in the absorption spectrum that were interpreted to be related to decomposition. Several spectra collected upon consecutive scans after the oxidant was added, were not consistent with the presence of a stable radical species in solution.  Figure 5-11 Absorption spectra of CH2Cl2 solutions of 38oc+/38cc+ and [40o/c]n+ at their respective PSSs, at room temperature. The IVCT absorption bands appearing in the near-IR region were evaluated using the Hush model, defined by Equation 1-1. As with the bifunctional system 38cc+, the absorption spectrum of [40o/c]n+ is a complex combination of overlapping bands, 140  reflected by the calculated multi-Gaussian fit (Figure 5-12). The number of Gaussians used in the fit was chosen to be the lowest number of peaks that gave a reasonable fit to the spectrum (defined as having a minimum r2 = 0.97). It should be noted that with multi-Gaussian fits other solutions are possible, and the electronic coupling parameters derived from the specific fit offered here are dependent on the number of Gaussians used in the fit. For the bands assigned as IVCT transitions, the parameters defining the Gaussians and the calculated electronic coupling, Vab, for each, are shown in Table 5-2. The analysis of the bands centered at 6929 and 7369 cm-1 determines the electronic coupling to be 275 and 527 cm-1, respectively, relatively similar to that of observed in 38cc. These are reasonable estimates since Vab was estimated to be 350 cm-1 in a related Pt bisacetylide complex.185  Figure 5-12 UV-vis absorption spectrum of [40o/c]n+ at the PSS (black) with the deconvolution by Gaussian bands and the calculated fit (red).  141  Table 5-2 IVCT parameters for [40o/c]n+ and 38oc+/38cc+.  [40o/c]n+ 38oc+/38cc+  ()max (cm-1)  $max (M-1 cm-1)  '()1/2 (cm-1)  Vab (cm-1)  6929  5576  1442  275  8248  8412  2959  527  7369  3606  1521  234  8763  5181  2605  400  The origin of the red-shift of the IVCT in [40o/c]n compared to the discrete bifunctional system 38cc is open to interpretation. There are two prominent differences between the two types of systems. First, the DTEs themselves are structurally different. Compound 4o/4c is asymmetric, whereas compound 39o/39c is symmetric. The cations formed in 38cc+ or [40o/c]n+ would be in different electronic environments because the radical can be influenced by more than one adjacent Pt center in [40o/c]n. The second primary difference is that "-MOs can be delocalized over multiple repeat units in the oligomer. The extended delocalization present in [40o/c]n is supported by its red-shifted absorption compared to 38cc at the PSS (Figure 5-7). One possible explanation for the red shift is that the IVCT transitions occur between more delocalized states that exist over several repeat units.  142  Figure 5-13 Schematic drawing illustrating how the possible extent of delocalization involved in IVCT transitions might occur in [40o/c]n depending on whether (a) several ring-closed DTEs are adjacent to each other or (b) only two ring-closed DTEs are adjacent to each other. These two interpretations were probed by oxidizing [40o/c]n at various stages of ring-closure. It was determined that a minimum amount of ring-closing was required for the appearance of the IVCT band. The 31P NMR spectra indicate that this is related to the fact that the initial ring-closing events do not occur on adjacent DTEs in the oligomer backbone. Oxidizing the oligomers before the PSS was reached ensured that a significant number of ring-open DTEs remained along the oligomer chain to limit the extent of delocalization. At the PSS, the molecular orbitals are at their limit of delocalization. These two scenarios are shown schematically in Figure 5-13.  143  Figure 5-14 vis-NIR absorption spectra of CH2Cl2 solutions of [40o/c]n at the PSS (black dash) and partly converted to the PSS (red dash) before oxidation and after 1 equivalent of oxidant is added to generate [40o/c]n+ at the PSS (solid blue) and [40o/c]n+ partly converted to the PSS (solid green). The absorption spectra of [40o/c]n+ when it is partially converted to the PSS and at the PSS are very similar (Figure 5-14). The same set of bands appears throughout the visible and near-IR regions of the spectrum. For the sample at the PSS, the intense absorption centered at about 15300 cm-1 corresponds with the "&"* absorption of the neutral oligomer and suggests that a significant amount of ring-closed DTEs are not oxidized. The intensity of this same peak is much less in the sample partially-converted to the PSS.  Both oxidized samples generate the same two overlapping near-IR  absorptions. This result does not support the notion that the extent of conjugation present in the oligomer backbone affects the optically-induced charge transfer. Although the radical species exhibits some delocalization in this system, supported by the presence of IVCT transitions, modulating the delocalization of MOs involved in these transitions by 144  structural changes of the DTEs does not appear to occur. The behavior of this system is perhaps best modeled by the scenario shown in Figure 5-13b, even when more than two ring-closed DTEs are adjacent to each other.  5.4  Conclusions A photochromic Pt-containing oligomeric system was presented in this chapter.  This system exhibits successful multifunctional photoswitching of DTEs that are within close spatial proximity of each other.  Moreover, the photochromes were shown to  electronically interact with each other, resulting in significant changes in optical properties and electrochemical redox potentials. A 1.6 eV decrease in the optical band gap substantiates the extensive "-conjugation that results from photoswitching of DTEs to the ring-closed form. Comparing the absorption spectra of the model complexes and the oligomers demonstrates that further delocalization of the "-conjugated backbone in an extended oligomeric or polymeric system with respect to small, discrete complexes might be possible.  Upon chemical oxidation of the oligomers at different degrees of  photoswitching, IVCT bands appear once a minimum amount of DTEs are photoswitched to the ring-closed form. But since there is no difference in the IVCT bands when only a couple ring-closed DTEs are adjacent or when more than two are adjacent, these experiments do not provide any evidence that extended delocalization would affect the mechanism of charge carrier migration in this material. Still, the oliogmeric system [40o/c]n unequivocally demonstrates that a large change in electronic structure is made possible by photoswitching of the DTE chromophores.  145  CHAPTER 6  Conclusions and Future Work  6.1  Conclusions The work in this thesis supports the emerging field of research focused on the  development of hybrid transition metal-containing photochromic materials. Integrating metals with conventional organic photochromes to create hybrid metal-organic systems is an important facet in exploring the range of properties these materials can offer. This thesis describes the synthesis and characterization of two types of photochromic Pt(II)coordinated DTE systems (Figure 6-1). Generally, this work establishes the use of Ptalkynyl complexes as an effective strategy for accessing the excited-state triplet manifold, a secondary photoactive pathway, of DTEs.  Figure 6-1 Types of photochromic Pt-alkynyl complexes discussed in this thesis.  146  The first type of system, discussed in Chapters 2 and 3, contains a DTE covalently bound to a Pt terpyridyl complex. Many research groups are interested in studying the interactions between DTEs and polypyridyl metal complexes because these metal complexes often display rich photophysics amenable to a number of applications. The initial motivation for this work was to create a photochromic system that was capable of modulating the excited-state lifetime of a metal complex by toggling the state of the photoswitch. It was previously established that Pt terpyridine acetylide complexes are capable of forming charge-separated excited states with lifetimes on the order of microseconds.115 Considering the prior work in the field, the chosen approach was to bind the DTE to the Pt complex via an alkynyl linkage. To my knowledge, complexes 27o/27c and 30o/30c are the first examples of DTE-containing polypyridyl metal complexes that coordinate the photochrome to the metal by a means other than substitution on the polypyridyl scaffold.  This approach is important for exploring  potential differences in reactivity resulting from structural differences in how the photochrome is incorporated. Chapter 2 describes the investigation of complexes in which the Pt terpyridine acetylide and DTE are simply linked by the acetylide, a short conjugated linker. Selective irradiation of the Pt complex with visible light resulted in photocyclization of the DTE chromophore. This metal-sensitized photoswitching mechanism was supported by intermolecular quenching experiments, variable temperature photoswitching studies, and picosecond laser spectroscopy. The capability of metal-sensitized photoswitching in complex 27o is a direct result of the covalent bonding of the two chromophores. Regarding the goal of this photoswitchable system, the efficient energy transfer from the Pt terpyridyl complex to the DTE drastically shortens the lifetime of the charge-separated  147  state and prevents the Pt complex from being selectively addressed, a requirement for achieving the goal of excited-state lifetime modulation. Based on these initial results, a longer, non-conjugated linkage was developed to electronically isolate the two chromophores. The linkage includes a methylene unit to disrupt conjugation between the chromophores and allow for movement of the DTE out of the plane of the square planar Pt-terypyridyl acetylide moiety. This approach is successful in the sense that these complexes exhibit 3MLCT-based emission. Absorption spectroscopy shows that the two chromophores are electronically isolated in the ground state. A study of the photophysics for a series of model complexes showed that 3IL states localized on the linkage affect the excited-state dynamics of this system. Although the presence of emission in these complexes does suggest that the 3MLCT and 3IL state interactions are reduced compared to the directly linked compounds, inefficient population of the DTE-localized 3IL state in 30o resulted in some metal-sensitized photoswitching, albeit at a far lower efficiency than observed for complex 27o. There is also room for improvement in the excited-state lifetime of the CT state in this class of complexes.  Initial experiments using 30o as a sensitizer with methyl viologen and  triethanolamine in solution were not successful in demonstrating intermolecular electron transfer amongst the components. Successful sensitization of electron relays utilized in processes such as water reduction require long excited-state lifetimes (i.e. microsecond timescale) of the sensitizer. This might be achievable with the design of a new alkynyl linkage. The behavior of this family of compounds demonstrates the challenge in creating multichromophoric molecular systems in which the individual chromophores can function both independently and in tandem simultaneously.  With respect to the 148  photochromic Pt complexes discussed in this thesis, the two chromophores must be independently addressable, i.e. metal-sensitized photoswitching cannot take place. On the other hand, some interaction must be present, in the ground state or excited state, for the DTE to actually perturb the photophysics of the Pt terpyridyl complex.  The  photoactivity of the ring-open form’s 3IL state dramatically increases the difficulty of achieving such a system because it acts as such a good energy acceptor. This work contributes to a growing body of work that shows Pt-polypyridyl complexes are indeed excellent sensitizers for populating 3IL states of ligands.122,153,203,204 Metal-sensitized photoswitching is a drawback in the systems just described, but it can be utilized in other ways. The second type of photochromic Pt-containing system described in this thesis, covered in Chapters 4 and 5, features Pt(II)-bis(phosphine)bis(acetylide) centers linking multiple DTEs together. Most previous attempts at creating conjugated systems incorporating multiple photochromes have resulted in incomplete photoswitching of all the chromophores.161,171  Rapid energy transfer between  chromophores via delocalized excited singlet states is attributed as the reason for the incomplete photoswitching.162 Triplet states, on the other hand, are regarded to be more localized than singlet states and therefore, long-range energy transfer in the triplet manifold is unlikely.205  Systems that incorporate a high content of photochromic  material are desirable for inducing large changes in non-linear optical properties and conductivity. The Pt-bis(phosphine)-bis(acetylide) framework was targeted because a large body of work exists that supports the Pt center’s ability to maintain a "-conjugated pathway through the metal center connecting the alkynyl ligands.29 Additionally, as evidenced by complex 27o, the photoactive 3IL state of DTEs can be accessed by closely linking the photochromes to Pt centers through alkynes.  149  In Chapter 4, a bifunctional system consisting of two DTEs linked by a Ptbisacetylide unit is discussed. This discrete system serves as a model for understanding how photoswitching proceeds and how electronic properties change in longer oligomeric and polymeric samples that contain many photochromic units.  Characterization of  complex 38oo by NMR and absorption spectroscopy confirms that adjacent DTEs remain photoactive after coordination to the Pt center. This result is significant because the individual DTEs exhibit electronic interaction. Electronic communication between the two DTEs is supported by electrochemical characterization, which shows that the oxidation potentials of the DTEs change depending on the whether the adjacent DTEs are in the same state or different states. A previously studied system that linked DTEs by alkynyl bonds, without the Pt center, resulted in a system in which two adjacent DTEs could not cyclize.160 Likewise, incorporating two DTEs in between two Pt centers also suppresses photoactivity of one of the DTEs.206 The inclusion of the Pt center between the two photoswitches clearly influences the photobehavior observed in complex 38oo. Systems that are designed using this framework are promising for application as a photoswitchable conducting material because chemical oxidation of 38cc results in IVCT transitions, observable by absorption spectroscopy. The IVCT transitions are analogous to the hopping of charge carriers, the accepted model for carrier migration in organic "conjugated materials.  The IVCT absorptions only appear when 38cc is present in  solution, thus indicating that the degree of coupling between two adjacent ring-closed DTEs is stronger than when ring-open and ring-closed DTEs are adjacent to each other. The results with the bifunctional model complex were sufficiently encouraging to pursue a multifunctional system. Chapter 5 discusses the behavior for photochromic Ptcontaining oligomers. Analogous to the optical properties of complex 38oo/38oc/38cc, 150  the oligomers exhibit delocalized states that likely extend over several repeat units. The photoswitching of the multifunctional system proceeds similarly to that of the model system. Irradiation with UV light generates a PSS in which about 70% of the Pt centers are sandwiched between two ring-closed DTEs. The remaining 30% of Pt centers are adjacent to at least one ring-closed DTE. This remarkable degree of photocyclization is essential to creating systems that exhibit large changes in electronic character upon photoswitching between the extremes of the completely ring-open state and the PSS. Electrochemical characterization of [40o/c]n shows that a large drop in ionization potential takes place upon photoswitching.  For "-conjugated semiconductors, the  ionization potential of the material is important because it contributes to governing the efficiency of hole injection from the electrode into the semiconductor.  Additionally, the  large drop in the optical band gap when [40o]n is converted to [40o/c]n indicates a drastic change in electronic structure occurs. The optical band gaps, estimated by the red edge of absorption, for a series of Pt polyynes are shown in Figure 6-2. Comparison of these values to those observed for [40o]n and [40o/c]n illustrates the significant change in band gap achieved by photoswitching, relative to the other static systems.  151  Figure 6-2 Optical band gaps measured by the onset of absorption in CH2Cl2 solution.207,208  6.2  Future Work There are several potential directions for future work in this area based on the work  presented in this thesis. Although the systems studied here are far from exhaustive, the Pt-acetylide functionality could be generally effective in photoswitching of DTEs. As multichromophoric systems are being designed with higher complexity, engineering the interactions between chromophoric subunits that exhibit delocalized states becomes increasingly more challenging. Unlike other metal complexes which have demonstrated quenching of the DTE’s photoactivity,103 the Pt-acetylide unit appears to be relatively innocuous, based on the findings in this thesis. Functionalizing a DTE with a covalentlylinked Pt-acetylide complex might be able to result in full photoactivity of the DTE in a 152  system in which photoactivity is suppressed without the metal. This hypothesis is based on the fact that the appended Pt complex can sensitize photoswitching of the DTE in an excited triplet state, which tend to be more localized and less susceptible to long-range energy transfer than excited singlet states.182,205 The most logical progression of this work would be to implement these types of materials, particularly the multifunctional system [40o/c]n, into molecular electronic devices. Previous work has shown that Pt polyynes demonstrate the field-effect and can be used as the semiconductor material in OFET devices.208,209 Implementation of a photoresponsive material as the semiconductor would be useful because light could be used to modulate the conductivity of the transistor in addition to the gate voltage. 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Monoexponential fits of the data are shown in red. 169  Figure A-3 Decays of transient signal averaged at wavelength regions 410 – 440 nm (black) or 650 – 680 nm (blue) for complex 31 upon excitation at 355 nm in CH3CN solution. Monoexponential fits of the data are shown in red.  Figure A-4 Decays of transient signal averaged at wavelength regions 425 – 445 nm (black) or 680 – 700 nm (blue) for complex 33 upon excitation at 355 nm in CH3CN solution. Monoexponential fits of the data are shown in red. 170  Figure A-5 Decays of transient signal averaged at wavelength region 435 – 455 nm for complex 30o upon excitation at 355 nm in CH3CN solution. A monoexponential fit of the data is shown in red.  Figure A-6 Decays of transient signal averaged at wavelength regions 435 – 455 nm (black) or 600 – 620 nm (blue) for a CH3CN solution of complex 27c at the PSS upon excitation at 355 nm. Monoexponential fits of the data are shown in red.  171  Figure A-7 Decays of transient signal averaged at wavelength regions 435 – 455 nm (black) or 600 – 620 nm (blue) for a CH3CN solution of complex 30c at the PSS upon excitation at 355 nm. Monoexponential fits of the data are shown in red.  Figure A-8 Decays of transient signal averaged at wavelength regions 390 – 410 nm (black) or 720 – 740 nm (blue) for complex 28 upon excitation at 355 nm in CH3CN solution. Monoexponential fits of the data are shown in red.  172  

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