Science, Faculty of
Chemistry, Department of
DSpace
UBCV
Hean, Duane
Doctor of Philosophy - PhD
University of British Columbia
Novel solid state photofunctional materials have been designed, and their structure-properties relationships are investigated. This thesis focuses on two distinct photofunctional properties: generating photomechanical motion in photoswitch-based crystals and ‘afterglow’ emission from organic alloy crystals and novel organic emitters. A novel hemithioindigo (HTI) photoswitch is functionalized with an anthracene group, and its photophysical properties are investigated in the solution and solid state. When irradiated with light, the photoswitch exclusively undergoes photoisomerization. In the crystalline state, E- isomer crystals isomerize and undergo a photosalient effect upon exposure to green light, while the Z- isomer undergoes a thermosalient event when heated. [2+2] Photodimerization is also employed to generate photomechanical motion in hemithioindigo photoswitch analogs. Hemithioindigo can undergo solid state reactions generating photoproducts not observable in solution. Three hemithioindigo photoswitch analogs are designed and used to elicit photomechanical motion in the crystalline state. Additionally, further mechanical motion is observed when previously irradiated crystals are heated. Crystallography and 1H NMR spectroscopy were used to provide insights into the mechanism of photomechanical motion. These proof-of-concept photomechanical responses in crystalline hemithioindigo photoswitches demonstrate their potential use as light-driven microactuators. The facile synthesis of HTIs enables tuning the photomechanical system based on the desired application. Solid state host-guest chemistry is employed to produce a series of seven organic alloys of two organic emitters with contrasting photophysical properties. One emitter has a long lifetime with ‘afterglow’ emission, while the other has a high photoluminescence efficiency. By carefully tuning the composition of the components between 30% - 100%, interesting photophysical properties emerge in addition to the fine-tunability of the system, which are probed using steady-state and time-resolved photoluminescence spectroscopy at variable temperatures. This provides a new perspective of the rules established for developing organic long-lived emissive materials for host-guest doped systems. Finally, novel organic carbazole-thiophene-based organic emitters are designed, synthesized, and characterized. The repositioning of the carbazole around the thiophene as well as introducing a secondary carbazole moiety results in solid state thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP).
PHOTOFUNCTIONAL MOLECULAR CRYSTALS by Duane Hean M.Sc., The University of the Witwatersrand, 2015 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) November 2021 © Duane Hean, 2021 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Photofunctional Molecular Crystals submitted by Duane Hean in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry Examining Committee: Dr. Michael O. Wolf, Chemistry, UBC Supervisor Dr. Mark M. MacLachlan, Chemistry, UBC Supervisory Committee Member Dr. David M. Perrin, Chemistry, UBC University Examiner Dr. Gregory R. Dake, Chemistry, UBC University Examiner Additional Supervisory Committee Members: Dr. Mark Thachuk, Chemistry, UBC Supervisory Committee Member Dr. Parisa Mehrkhodavandi, Chemistry, UBC Supervisory Committee Member iii Abstract Novel solid state photofunctional materials have been designed, and their structure-properties relationships are investigated. This thesis focuses on two distinct photofunctional properties: generating photomechanical motion in photoswitch-based crystals and ‘afterglow’ emission from organic alloy crystals and novel organic emitters. A novel hemithioindigo (HTI) photoswitch is functionalized with an anthracene group, and its photophysical properties are investigated in the solution and solid state. When irradiated with light, the photoswitch exclusively undergoes photoisomerization. In the crystalline state, E- isomer crystals isomerize and undergo a photosalient effect upon exposure to green light, while the Z- isomer undergoes a thermosalient event when heated. [2+2] Photodimerization is also employed to generate photomechanical motion in hemithioindigo photoswitch analogs. Hemithioindigo can undergo solid state reactions generating photoproducts not observable in solution. Three hemithioindigo photoswitch analogs are designed and used to elicit photomechanical motion in the crystalline state. Additionally, further mechanical motion is observed when previously irradiated crystals are heated. Crystallography and 1H NMR spectroscopy were used to provide insights into the mechanism of photomechanical motion. These proof-of-concept photomechanical responses in crystalline hemithioindigo photoswitches demonstrate their potential use as light-driven microactuators. The facile synthesis of HTIs enables tuning the photomechanical system based on the desired application. Solid state host-guest chemistry is employed to produce a series of seven organic alloys of two organic emitters with contrasting photophysical properties. One emitter has a long lifetime with ‘afterglow’ emission, while the other has a high photoluminescence efficiency. By carefully tuning the composition of the components between 30% - 100%, interesting photophysical iv properties emerge in addition to the fine-tunability of the system, which are probed using steady-state and time-resolved photoluminescence spectroscopy at variable temperatures. This provides a new perspective of the rules established for developing organic long-lived emissive materials for host-guest doped systems. Finally, novel organic carbazole-thiophene-based organic emitters are designed, synthesized, and characterized. The repositioning of the carbazole around the thiophene as well as introducing a secondary carbazole moiety results in solid state thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). v Lay Summary Technological progression is essential for the human race, and one of the ways this can be achieved is by controlling molecules inside crystals. The molecules can be powered by light to perform functions at an ever-decreasing size. In this work, photoswitches (molecules that change shape when irradiated with light) are used to make crystals perform mechanical actions such as bending and jumping. The intended application is to make tiny light-powered switches where wiring may not be possible. The other half of the work in this thesis revolves around molecules that emit light when hit with a different wavelength of light. The light being emitted can be finely tuned by controlling the compositions within the crystal entities. New molecules have also been made and studied. vi Preface In all chapters, I am the principal author of the work reported in this thesis, carried out under the supervision of Prof. Michael O. Wolf. I performed the majority of all the experiments with additional contributions and collaborations below. A version of Chapter 2 has been previously published with the contributions from M. O. Wolf.* Compound (Z)-HTI-An were synthesized by Luis G. Alde. The contents of Chapter 3 are included in a manuscript in preparation, written by me with contributions from M. O. Wolf. Anita Lam collected X-ray powder diffraction data. A version of Chapter 4 has been submitted for publication with Dr. Zhen Xu and me as principal authors with contributions from M. O. Wolf. Dr. Zhen Xu collected the time-resolved solid state photophysical data. Jennifer Yuan collected solid state photoluminescence quantum yield data. The contents of Chapter 5 are included in a manuscript in preparation, written by me with contributions from M. O. Wolf. Dr. Zhen Xu collected solid state photophysical data. *Hean, D.; Alde, L.G.; Wolf, M.O. J. Mater. Chem. C., 2021, 9, 6789-6795. vii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Tables .............................................................................................................................. xiii List of Figures ............................................................................................................................. xiv List of Schemes ......................................................................................................................... xxiii List of Abbreviations and Symbols ........................................................................................ xxiv Acknowledgements .................................................................................................................. xxix Dedication ...................................................................................................................................xxx Chapter 1: Introduction ................................................................................................................1 1.1 Overview ..........................................................................................................................1 1.2 General Principles of Photomechanical Motion ..............................................................2 1.3 Modes of Mechanical Action ...........................................................................................5 1.3.1 Photosalient Phenomenon ............................................................................................5 1.3.2 Thermosalient Phenomenon ........................................................................................8 1.4 Photoswitches as Stimuli-Responsive Building Blocks ..................................................9 1.4.1 Ring-opening/closing ...................................................................................................9 1.4.2 Isomerization .............................................................................................................11 1.4.3 Dimerization ..............................................................................................................12 1.5 The Hemithioindigo Photoswitch ..................................................................................14 viii 1.6 Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence . .......................................................................................................................................16 1.6.1 General Photophysical Principles ..............................................................................16 1.6.2 Thermally Activated Delayed Fluorescence (TADF) ................................................17 1.6.3 Room Temperature Phosphorescence (RTP) .............................................................19 1.7 Crystallization Induced Phosphorescence (CIP) ...........................................................20 1.7.1 Single Component Systems .......................................................................................21 1.7.2 Co-crystal Systems ....................................................................................................22 1.7.3 Host-guest Systems ....................................................................................................23 1.8 Goals of the Thesis ........................................................................................................24 Chapter 2: Mechanical Responses of Hemithioindigo-Anthracene Based Molecular Crystals to Heat and Light ..........................................................................................................26 2.1 Introduction ....................................................................................................................26 2.2 Synthesis and Characterization ......................................................................................27 2.3 Absorption Spectroscopy of HTI-An ............................................................................30 2.4 Photoresponses of Crystalline HTI-An .........................................................................33 2.4.1 Photochromism (Z)-HTI-An Crystals .......................................................................33 2.4.2 Photosalient Effect of (E)-HTI-An Crystals .............................................................34 2.5 Thermoresponses of Crystalline HTI-An .....................................................................36 2.5.1 Thermosalient Effect of (Z)-HTI-An Crystals ..........................................................36 2.5.2 Polymorphism of (Z)-HTI-An ..................................................................................40 2.5.3 Solid State Thermal Reversion of (E)-HTI-An ........................................................42 2.6 Conclusions ....................................................................................................................44 ix 2.7 Experimental ..................................................................................................................45 2.7.1 General .......................................................................................................................45 2.7.2 1H NMR Spectroscopy ..............................................................................................46 2.7.3 UV-Vis Spectroscopy ................................................................................................46 2.7.4 X-ray Diffraction .......................................................................................................46 2.7.5 Hot-Stage POM Microscopy .....................................................................................47 2.7.6 Differential Scanning Calorimetry (DSC) .................................................................47 2.7.7 High-Performance Liquid Chromatography (HPLC) ................................................47 2.7.8 Synthesis ....................................................................................................................47 Chapter 3: Crystal Photochromism and Photomechanical Responses of Hemithioindigo Photoswitch Analogs ....................................................................................................................50 3.1 Introduction ....................................................................................................................50 3.2 Synthesis and Characterization ......................................................................................51 3.3 Solution Photochromic Properties .................................................................................54 3.4 Solid State Photochromic Properties and [2+2] Dimerization ......................................56 3.5 Photomechanical Effects and Photochromism in Crystalline HTI Photoswitches ........63 3.6 Conclusions ....................................................................................................................66 3.7 Experimental ..................................................................................................................67 3.7.1 General .......................................................................................................................67 3.7.2 NMR Spectroscopy ....................................................................................................67 3.7.3 UV-Vis Spectroscopy ................................................................................................68 3.7.4 X-ray Diffraction .......................................................................................................68 3.7.5 Hot-Stage POM Microscopy .....................................................................................69 x 3.7.6 Differential Scanning Calorimetry (DSC) .................................................................69 3.7.7 High-Performance Liquid Chromatography (HPLC) ................................................69 3.7.8 Synthesis ....................................................................................................................69 Chapter 4: Fine Tuning ‘Afterglow’ Emission of Carbazole Based Luminogens with Organic Alloying ..........................................................................................................................74 4.1 Introduction ....................................................................................................................74 4.2 Synthesis and Characterization ......................................................................................75 4.2.1 Molecular Synthesis ...................................................................................................75 4.2.2 Organic Alloy Synthesis ............................................................................................77 4.2.3 Structural Characterization ........................................................................................79 4.3 Solid State Photophysical Properties .............................................................................82 4.3.1 Photophysical Properties of Parent Compounds ........................................................82 4.3.2 Photophysical Properties of Organic Alloys ..............................................................85 4.3.3 Origin of Emission .....................................................................................................90 4.4 Conclusions ....................................................................................................................94 4.5 Experimental ..................................................................................................................95 4.5.1 General .......................................................................................................................95 4.5.2 X-ray Diffraction .......................................................................................................95 4.5.3 POM Microscopy .......................................................................................................96 4.5.4 Spectroscopy ..............................................................................................................96 4.5.5 Quantum Yields .........................................................................................................96 4.5.6 Synthesis ....................................................................................................................97 xi Chapter 5: RTP and TADF from Mono- and Di- Substituted Carbazole-Thiophene Geometrical Isomers ....................................................................................................................99 5.1 Introduction ....................................................................................................................99 5.2 Synthesis and Characterization ....................................................................................101 5.3 Structural Characterization ..........................................................................................102 5.4 Solution State Photophysical Properties ......................................................................106 5.5 Solid State Photophysical Properties ...........................................................................106 5.5.1 Room Temperature Phosphorescence ......................................................................108 5.5.2 Thermally Activated Delayed Fluorescence ............................................................109 5.6 Conclusions ..................................................................................................................111 5.7 Experimental ................................................................................................................112 5.7.1 General .....................................................................................................................112 5.7.2 1H NMR Spectroscopy ............................................................................................113 5.7.3 UV-Vis Spectroscopy ..............................................................................................113 5.7.4 Solid State Spectroscopy .........................................................................................113 5.7.5 X-ray Diffraction .....................................................................................................113 5.7.6 Synthesis ..................................................................................................................114 Chapter 6: Conclusion and Future Work ................................................................................117 References ...................................................................................................................................120 Appendices ..................................................................................................................................132 Appendix A ..............................................................................................................................132 Appendix B ..............................................................................................................................136 Appendix C ..............................................................................................................................138 xii Appendix D ..............................................................................................................................141 xiii List of Tables Table 2-1 Change in unit cell parameters of (Z)-HTI-An at 298 K and 400 K ...........................38 Table 4-1 Literature unit cell parameters of CBZ-Br (RTP), CBZ-Br and CBZ-Me ...............77 Table 4-2 Unit cell parameters for organic alloy MexBr10-x where x = 3 to 9 ..............................78 Table 4-4 Mole ratio and site occupancy factors for organic alloy MexBr10-x where x = 3 to 9,and the corresponding photophysical parameters. ............................................................................... 89 xiv List of Figures Figure 1-1 Representative examples of photoswitches. Ring opening/closing of (a) diarylethenes and (b) furylfulgides.30 Cis-trans isomerization of (c) azobenzenes and proton transfer followed by cis-trans isomerization (d) salicylideneanilines. ........................................................................ 3 Figure 1-2 Molecular arrangements in solution and crystalline state. ........................................... 4 Figure 1-3 Photodimerization of trans-cinnamic acid and its photodimer products. ..................... 5 Figure 1-4 Elastic photomechanical responses such as bending, twisting, or coiling when plate and rod-like crystals are exposed to light. ...................................................................................... 6 Figure 1-5 (a) Bimorph formation when irradiating attenuated light produces a gradient of reactant and photoproduct molecules. (b) Reconstruction of the crystal entity by complete photoreaction resulting in rearrangement of the photoproducts in the lattice. ............................... 7 Figure 1-6 Chemical structure of oxitropium bromide and its molecular conformation in the low- and high-temperature phases........................................................................................................... 9 Figure 1-7 Chemical structure of the diarylethene derivatives and the rod-like mixed crystal of 7a and 8a (63:37) undergoing reversible bending when irradiated with UV and visible light respectively. .................................................................................................................................. 10 Figure 1-8 Reversible ring-opening/closing of the furylfulgide and the microplate-plate crystal undergoing a bending motion toward the light source.................................................................. 11 Figure 1-9 Photoisomerization of the perhalogenated azobenzene derivative and the cis-crystal irreversibly bending when exposed to 457 nm light. The photoisomerization results in the transformation to a polycrystal containing the trans- isomer. ...................................................... 12 xv Figure 1-10 Intramolecular proton transfer followed by cis-trans isomerization of the salicylideneaniline derivative and the reversible bending motion of the plate-like crystal. ......... 13 Figure 1-11 [2+2] Photodimerization of 12a and the resulting photomechanical effects. .......... 14 Figure 1-12 Hemithioindigo (HTI) photoswitch architecture and (a) the photoisomerization between the Z- and E- isomers. (b) The absorption profiles are red-shifted into the visible region with the E- form bathochromically shifted by 20-30 nm .............................................................. 15 Figure 1-13 (a) Photoisomerization of 14 and (b) associated color change between the Z- (14a) and E- (14b) isomer. ..................................................................................................................... 16 Figure 1-14 Jablonski diagram showing various photophysical processes. ................................. 17 Figure 1-1 Examples of structures displaying the TADF phenomenon……………………….……18 Figure 1-16 (a) Examples of compounds displaying the RTP phenomenon. (b) “Afterglow” effect from a crystalline powder sample of 19.............................................................................. 20 Figure 1-17 Examples of compounds displaying crystallization induced phosphorescence (CIP)........................................................................................................................................................ 21 Figure 1-18 (a) Carbazole-based RTP emitters. (b) Different packing morphologies of CPM impact “afterglow” emission efficiency........................................................................................ 22 Figure 1-19 (a) Chemical structure of co-crystal components. (b) Energy transfer to different aggregation states results in green and yellow RTP. .................................................................... 23 Figure 1-20 (a) Excimer formation in chromophore resulting in reduced phosphorescence efficiency. (b) Substitution into the host prevents excimer formation and promotes high phosphorescence efficiency. (c) Structures used to tune emission color in (d) crystals. ............. 24 Figure 2-1 Comparison of the calculated (from SCXRD data at 100 K) and experimental (298 K) powder X-ray diffractograms of (Z)-HTI-An.. ....................................................................... 28 xvi Figure 2-2 Photoisomerization of HTI-An and 1H NMR (400 MHz, CD2Cl2) spectra of (Z)-HTI-An and (E)-HTI-An (~77% isomeric purity) at room temperature ..................................... 29 Figure 2-3 ORTEP diagrams of (a) (Z)-HTI-An and (b) (E)-HTI-An where ellipsoids are drawn at 50% probability and only heteroatoms atoms are labelled for clarity. ..................................... 30 Figure 2-4 UV-Vis absorption spectra of (Z)-HTI-An in various solvents. ............................... 31 Figure 2-5 UV-Vis absorption spectra of solution (solid) and solid state (dashed) of (Z)-HTI-An (red) and (E)-HTI-An (black). ..................................................................................................... 32 Figure 2-6 (a) Absorption spectra of HTI-An showing the reversible Z – E photoisomerization in dichloromethane. (b) Isomer yields from photoisomerization of (Z)-HTI-An (black) and (E)-HTI-An (red) by cycling 405 nm and 515 nm irradiation in a dilute dichloromethane solution. 32 Figure 2-7 (a) Microcrystal of (Z)-HTI-An crystal under ambient light. (b) Microcrystal of (Z)-HTI-An irradiated with 365 nm over 10 minutes produces a more pronounced red color in the crystal. (c) As assessed by HPLC the percentage of HTI-An isomers before and after 365 nm irradiation shows a decrease of (Z)-HTI-An by ~5%. Scale bars are 100 µm. ............................ 33 Figure 2-8 (a) – (b) Microscopy images demonstrating the photosalient effect of (E)-HTI-An upon exposure to 515 nm light. (c) Composition change of isomers as determined by site occupancy of the thioindigo fragment. (d) Asymmetric unit and molecular packing of (E)-HTI-An mixed crystal system viewed along the b-axis........................................................................ 35 Figure 2-9 Short contact interactions of (a) (E)-HTI-An before irradiation and (b) (E)-HTI-An post photosalient event. ................................................................................................................. 35 Figure 2-10 (a) – (c) Thermosalient effect of (Z)-HTI-An at 130 °C. (d) Zig-zag lattice configuration viewed along the c-axis. (e) – (f) Angle between the thioindigo and anthracene xvii planes of (Z)- and (Z’)-HTI-An. (g) – (h) Torsional configuration between thioindigo and anthracene fragment for (Z)- and (Z’)-HTI-An. .......................................................................... 36 Figure 2-11 Slow heating of single crystal of (Z)-HTI-An induces a phase transition resulting in cracking along (001) plane. ........................................................................................................... 37 Figure 2-12 Short contact interactions and carbonyl oxygen distances of (a) (Z)-HTI-An at room temperature and (b) (Z’)-HTI-An at 400 K. ....................................................................... 38 Figure 2-13 DSC heating trace of (Z)-HTI-An with thermosalient ‘hopping’ response at 130 °C followed by cooling to room temperature. The heating rate is 5 °C/min. .................................... 39 Figure 2-14 2D Hirshfeld fingerprint plots of (a) (Z)-HTI-An and (b) (Z’)-HTI-An. ............... 40 Figure 2-15 (a) Polarized light hot-stage photomicrographs of a thin film of (Z)-HTI-An II and III crystallized from the melt. (b) At 170 ° C the first phase transition of III to II occurs (red arrows mark the transition boundary). (c) At 180 °C the second phase transition occurs from II to I followed by (d) the melting of (Z)-HTI-An at 209 °C. (e) DSC traces of three successive heating/cooling runs beginning with (Z)-HTI-An I ..................................................................... 42 Figure 2-16 DSC heating/cooling runs beginning with crystalline (Z)-HTI-An. Run 1 shows the melting point of (Z)-HTI-An I. Run 2 shows both crystallization and melting of two distinct new phases. Run 3 shows the recrystallization and remelting of phase (Z)-HTI-An I. ...................... 43 Figure 2-17 Crystal size reduction during thermal reversion of (E)-HTI-An at 180 °C for 2 hours. Scale bars are 200 µm. ....................................................................................................... 44 Figure 2-18 (a) – (c) Single crystal of (E)-HTI-An undergoing thermal isomerization reversion at 180 °C. (d) Thermal reversion resulting in 18% production of Z- isomer occurring in the (E)-HTI-An crystal. (e) Isomer concentration of amorphous sample of (E)-HTI-An before and after heating determined by HPLC. Scale bars are 200 µm. ................................................................. 44 xviii Figure 3-1 ORTEP of (a) HTI, (b) HTI-Nap, and (c) HTI-Mes drawn at 50% probability with only heteroatoms labelled for clarity. ........................................................................................... 53 Figure 3-2 1H NMR (400 MHz, CD2Cl2) spectra of HTI, HTI-Nap, and HTI-Mes at room temperature. The peak at 5.3 ppm is from the residual solvent. ................................................... 53 Figure 3-3 Representative 1H NMR (400 MHz, CD2Cl2) spectra of (Z)-HTI before and (E)-HTI in CD2Cl2 after irradiation at 405 nm irradiation for 18 h at room temperature. .......................... 54 Figure 3-4 UV-Vis spectra changes of (a) HTI, (b) HTI-Nap, and (c) HTI-Mes, in ~2.5 ×10-5 M CH2Cl2 upon reversible photoisomerization with 405 nm irradiation, followed by 515 nm irradiation. ..................................................................................................................................... 55 Figure 3-5 (a) Solid state photoirradiation reaction, (b) Formation of solid state HTI-SSD (orange spot) upon 405 nm LED irradiation of HTI thin film. (c) 1H NMR (400 MHz, CD2Cl2) spectra of HTI (orange) before irradiation and HTI-SSD (black) after irradiation at room temperature. .................................................................................................................................. 56 Figure 3-6 (a) HTI bond distance and angle of dimer pair. (b) Crystal structure of HTI-SSD with bond distance and angle of cyclobutane ring between the dimer pair shown. Packing diagrams viewed along the a-axis of (c) HTI and c-axis of (d) HTI-SSD recrystallized from i-propanol. ....................................................................................................................................... 58 Figure 3-7 (a) Experimental (red) and calculated (blue) powder diffractograms of crystalline HTI. (b) Two differential scanning calorimetry heating (red and black) and cooling (blue colors) runs show two single melting and recrystallization temperatures. ............................................... 59 Figure 3-8 (a) HTI-Nap bond distance and angle of dimer pair. (b) Crystal structure of HTI-Nap SSD with bond distance and angle of cyclobutane ring between the dimer pair shown. xix Packing diagrams viewed along the a-axis of (c) HTI-Nap and (d) HTI-Nap recrystallized from dichloromethane. ........................................................................................................................... 60 Figure 3-9 (a) 1H NMR (400 MHz, CD2Cl2) spectra of HTI-Nap before irradiation (orange) and (b) HTI-Nap SSD after irradiation (black) at room temperature. ................................................ 61 Figure 3-10 1H NMR (400 MHz, CD2Cl2) spectrum of (top) HTI-Mes before irradiation (blue) and (middle) HTI-Mes after solution irradiation for 18 h (orange) and (bottom) HTI-Mes after crystal irradiation. ......................................................................................................................... 62 Figure 3-11 Crystal structure of HTI-Mes with only heteroatoms and H1 labelled for clarity and packing diagram showing the orientations of the double bonds. .................................................. 62 Figure 3-12 (a) Thin needle-like crystal of HTI at the start of 405 nm laser irradiation. (b) – (c) The thin needle of HTI undergoing photomechanical bending in response to irradiation. Scale bars are 400 µm. ............................................................................................................................ 63 Figure 3-13 (a) Localized deformation of HTI-Nap crystals in the highlighted area. Scale bars are 400 µm. (b) Long plate-like crystal of HTI-Nap undergoing a photosalient hopping by twisting to 540°. * Is a crystal used as a reference point during motion of the larger crystal and the arrows highlight nondestructive twisting motion. ................................................................... 64 Figure 3-14 Single crystals of HTI-Nap before (a) and after melting (b) by heating to 141 °C. Scale bars are 1 mm (c) Irradiated microcrystal of HTI-Nap undergoing further bending in response to heating to 130 °C. Scale bars are 100 µm. ................................................................. 65 Figure 3-15 (a) Polarized optical microscopy image of a single plate-like crystal of HTI-Mes before irradiation. (b) Upon 405 nm irradiation, focused on the corner of the crystal, solid state isomerization occurs in a wave-front like manner. (c) – (d) Phase transformation of the crystal results in a color change from green to orange under the polarizer. Scale bars are 400 µm. ....... 66 xx Figure 4-1 Typical crystal morphology of (a) CBZ-Br (RTP) (b) CBZ-Br (TADF) and (c) CBZ-Me. Scale bars are 200 µm. ................................................................................................. 76 Figure 4-2 Molecular crystals of CBZ-Br and organic alloy MexBr10-x where x = 3 to 9, grown from slow evaporation of acetonitrile crystallization experiments. .............................................. 78 Figure 4-3 1H NMR (400 MHz, CD2Cl2) spectral analysis of CBZ-Me, CBZ-Br and the organic alloys MexBr10-x where x is 3 to 9 at room temperature. .............................................................. 79 Figure 4-4 (a) Unit cell dimension variation of CBZ-Me and MexBr10-x where x is 3 to 9. (b) Decreasing β angle with increasing CBZ-Me composition (c) Decreasing unit cell volume with increasing CBZ-Me composition. ................................................................................................ 80 Figure 4-5 Representative diffraction pattern of Me5Br5 showing Bragg peaks and noise from the detector as view in MAX3D (Britten/Guan). .......................................................................... 81 Figure 4-6 (a) Representative ORTEP of substitutional disorder for organic alloy (MexBr10-x, x = 3 to 9) system drawn at 50% probability, hydrogens are plotted as spheres. Only heteroatoms and the methyl group are labelled for clarity. (b) DSC traces of organic alloys (MexBr10-x, x = 3 to 9) where endotherms are labeled with their corresponding composition ratio, Me4Br6, Me6Br4, Me9Br1 labels are excluded for clarity. (Inset: Melting point temperatures as a function of CBZ-Me percentage.) ............................................................................................................................ 82 Figure 4-7 Steady-state (blue line) and 1 ms delay (blue line) photoluminescence spectrum of CBZ-Me (a) under air at room temperature and (e) under helium at 77 K. (b) Phosphorescence decay of CBZ-Me measured under air at room temperature at 450 nm, 530 nm and above 550 nm. Variable-temperature (c) steady-state photoluminescence spectrum and (d) phosphorescence decay of CBZ-Me under helium atmosphere. .............................................................................. 84 xxi Figure 4-8 (a) Steady-state (blue line) and 1 ms delay (orange line) photoluminescence spectrum of CBZ-Br under air at room temperature and (c) under helium at 77 K. (b) Microsecond photoluminescence decay of CBZ-Br under air at room temperature. (d) Millisecond photoluminescence decay of CBZ-Br above 500 nm under air at room temperature. ................. 85 Figure 4-9 Steady-state (solid line) and delayed (1ms) (dashed line) photoluminescence spectrum of organic alloys. ........................................................................................................... 86 Figure 4-10 Phosphorescence decay of organic alloys under air at (a) room temperature and at (b) 77K. ......................................................................................................................................... 87 Figure 4-11 (a) Photos of CBZ-Br, Me3Br7, Me5Br5, Me9Br1 and CBZ-Me upon the removal of UV excitation. (b) Plots of PLQYs and delayed lifetimes against CBZ-Me composition. ..... 88 Figure 4-12 Variable-temperature steady-state photoluminescence spectrum of organic alloys. 92 Figure 4-13 Variable-temperature phosphorescence decay of organic alloys under helium atmosphere. ................................................................................................................................... 93 Figure 4-14 Pull-out drawer for Edinburgh Instruments SC-30 sample module with solid sample holder (PTFE tray) and quartz cap.. .............................................................................................. 97 Figure 5-1 Structural isomers of various RTP and TADF organic emitters. ............................. 100 Figure 5-2 1H NMR (400 MHz, CD2Cl2 ) spectra of CBZ-2Th, CBZ-3Th, DCBZ-2,5Th, and DCBZ-3,4Th at room temperature. ............................................................................................ 102 Figure 5-3 ORTEP of (a) CBZ-2Th, (b) CBZ-3Th, (c) DCBZ-3,4Th, and (d) DCBZ-2,5Th drawn at 50% probability with only heteroatoms labeled for clarity.......................................... 104 Figure 5-4 Packing modes and packing interactions of (a) CBZ-2Th, (b) CBZ-3Th, (c) DCBZ-2,5Th, and (d) DCBZ-3,4Th. ..................................................................................................... 105 xxii Figure 5-5 Absorption (solid lines) and photoluminescence spectra (dashed lines) of CBZ-2Th, CBZ-3Th, DCBZ-2,5Th, and DCBZ-3,4Th. ............................................................................ 106 Figure 5-6 Emission and lifetime decay profiles of CBZ-2Th, CBZ-3Th, and DCBZ-3,4Th....................................................................................................................................................... 108 Figure 5-7 Photophysical data of DCBZ-2,5Th.. ...................................................................... 111 xxiii List of Schemes Scheme 2-1 Relationship between corresponding isomers of HTI-An upon thermal and light stimulation..................................................................................................................................... 27 Scheme 2-2 Synthesis of the hemithioindigo photoswitch (Z)-HTI-An. ................................... 28 Scheme 3-1 Synthesis of hemithioindigo (HTI) analogs ............................................................. 52 Scheme 4-1 Synthesis of CBZ-Me and CBZ-Br......................................................................... 75 Scheme 5-1 Synthesis of amide-linked carbazole-thiophene compounds. ................................ 101 xxiv List of Abbreviations and Symbols 1H proton 13C carbon-13 Å Angström (10–10 m) ºC degrees Celsius ACN acetonitrile ACQ aggregation-caused quenching AIE aggregation-induced emission An anthracene CCD charge coupled device CIP crystallization induced phosphorescence cm centimeter cm–1 wavenumber CT charge transfer Δ difference d doublet d spatial resolution DAE diarylethene DCM dichloromethane DFT density functional theory DMSO dimethylsulfoxide DSC differential scanning calorimetry ΔEST energy gap between S1 and T1 states xxv EQE external quantum efficiency ESI electrospray ionization equiv. equivalent eV electron volts FT-IR Fourier transform infrared spectroscopy g grams h hours Hex cyclohexane HOMO highest occupied molecular orbital HPLC high performance liquid chromatography HRMS high resolution mass spectrometry HSM hot-stage microscopy HTI hemithioindigo Hz hertz IC internal conversion IR infrared ISC intersystem crossing K Kelvin kB Boltzmann constant kHz kilohertz λ wavelength λem wavelength of emission λex wavelength of excitation xxvi λmax wavelength at peak maximum LED light emitting diode LUMO lowest unoccupied molecular orbital μL microliter μs microsecond m multiplet m/z mass-to-charge ratio M molar MeOH methanol meV millielectron volt MHz megahertz mL milliliter mmol millimole MO molecular orbital mol mole MS mass spectrometry Nap naphthalene nm nanometer NMR nuclear magnetic resonance ns nanosecond (10–9 s) OFET organic field effect transistor OLED organic light-emitting diode ORTEP Oak Ridge Thermal Ellipsoid Plot xxvii OSC organic solar cell ΦF fluorescence quantum yield PLQY photoluminescence quantum yield POM polarized optical microscopy ps picosecond (10–12 s) PS photosalient PSS photostationary state PTFE polytetrafluoroethylene PTI Photon Technology International PXRD powder X-ray diffraction QY quantum yield RISC reverse intersystem crossing RTP room temperature phosphorescence S0 ground-state S1 first excited singlet state S1* intermediate excited singlet state S2 second excited singlet state s seconds SCXRD single-crystal X-ray diffraction SOC spin-orbit coupling SRO short range order SSD solid-state dimer t triplet xxviii T temperature T1 first excited triplet state TADF thermally activated delayed fluorescence TCSPC time-correlated single photon counting THF tetrahydrofuran TLC thin layer chromatography UV ultraviolet Vis visible XRD X-ray diffraction xxix Acknowledgements First, I would like to thank Dr. Michael Wolf for the life-changing experience. I will be eternally grateful for the opportunity to come to do a PhD in your lab. Thank you for the freedom to explore different research directions and the constant support and guidance thereafter. A special thanks to Dr. Mark MacLachlan for reading my thesis. The PhD journey would not have been possible with the support of the great staff and facilities. In particular, Ben Herring, for being so helpful and maintaining a well-run facility; Brian Patrick and Anita Lam for being amazing crystallography mentors. Thank you for everything you have taught me. I benefited greatly from your guidance. I have been fortunate enough to work with friend and colleague Dr. Zhen Xu, thanks for the collaborations, assistance, and answers to my endless questions. It was indeed a highlight of the PhD to work with you. For all the high-quality spectroscopy measurements, I would like to thank Dr. Saied Kamel at LASIR. I had the pleasure of work with summer students Julia and Luigi whose hard work contributed to a publication. I’d like to thank all members of the Wolf group. I would like to give special thanks to Chris Brown, Janet, Elise, Yang, Peter, Allen, Nicole, Jenn, Olga, Tobias and Urmi for all the good times in the lab. I would also like to thank Hessam, Veronica, Mo, Lev, Andy, Chris, Miguel, Francesco, Yitao, and the MacLachlan group for always being there during the ups and downs of a PhD journey. To all the other friends I have made along the way: Elan, Damon, Nirmal, Jason, Sean, and Al, thanks for exploring British Columbia and all the memories that will be remembered for many years to come. My significant other Yoshika has colored my life in beautiful ways. I look forward to the many years ahead of us. Finally, a massive thank you to my parents for all the love and support throughout the long distance separation. The distance was challenging at times, but everything is looking much brighter onwards. xxx Dedication To Yoshi and the Family1 Chapter 1: Introduction 1.1 Overview Over the centuries, humans have become ever more technologically advanced, with increasingly sophisticated control over matter and the environment. One of the newest frontiers of controlling the world around us is the ability to control the movement of atoms on the nanoscale to produce macroscale responses.1 This may be achieved by stimuli-responsive materials which produce a measurable response to external stimuli such as light, heat, electrical potential, or chemical agents. The responses to external stimulation usually manifest as the emission of photons, a change in color, viscosity, rigidity, or mechanical motion.2 Recently, stimuli-responsive materials that display mechanical motion upon external stimulation have been utilized as smart materials in flexible electronics, artificial muscles, soft robotics, and as actuators.3 Light is an ideal source of external stimulation as this enables actuators to work in a non-contact mode to the controlling apparatus. Furthermore, photons can penetrate a wide variety of media and simultaneously transmit both information and energy.4 Photons can be absorbed by molecules enabling chemical transformations such as bond formation, breaking, and geometrical rearrangements. Under suitable conditions, these transformations facilitate mechanical motion in smart materials, and in many cases, can be reversed by absorption of a different wavelength of light or by heating, enabling repeatable mechanical motion.5 Efficient light-generating materials generally contain heavy metals. These are usually toxic and expensive. Recently, organic compounds have emerged as a suitable alternative for efficient emissive materials. However, achieving highly efficient photoluminescence in organic compounds at room temperature remains a challenge. By packing these organic compounds in a crystalline 2 lattice, drastic improvements are made, which enable highly efficient photoluminescent materials for biological imaging and anti-counterfeiting applications.6,7 This thesis focuses on the structure-property relationship of an emerging organic photoswitch and several analogs for use as an actuator, developing novel carbazole-based chromophores, and using crystal lattices to tune photophysical properties in the solid state. Section 1.2 – 1.4 provides an overview of mechanistic effects and photoswitches used to generate a photomechanical response in crystals. Section 1.5 briefly introduces a photoswitch with unexplored solid state properties. The following Sections 1.6 – 1.7 detail the underlying photophysical properties underlying efficient room-temperature emission in crystalline materials. Finally, Section 1.8 describes the goals of the thesis. 1.2 General Principles of Photomechanical Motion A photoactive molecule that transforms light energy into mechanical motion is required to generate a response in photomechanical materials.8 Photoswitches are photochromic molecules that undergo a reversible transformation between two isomers when irradiated, with different absorption spectra, and typically show a color change between the species.9 Representative photoswitches are shown in Figure 1-1. When irradiated, diarylethenes (DAEs) 1a, and furylfulgides 2a, undergo an intramolecular ring-opening/closing reaction between isomers which is thermally irreversible. Azobenzenes 3a are well-known photoswitches that undergo cis-trans photoisomerization, salicylideneanilines 4a, also incorporate cis-trans photoisomerization, including photoinduced proton transfer. Both photoproducts 3b and 4b revert to the stable isomer when left in the dark. 3 In most cases, the conversion between isomers of these photoswitches occurs in solution, resulting in random molecular orientations (Figure 1-2), preventing any practical mechanical motion generation due to the uncoordinated geometrical changes between the individual molecules. Therefore, to generate a mechanical response on any measurable length scale, the photoswitches must be well organized such that “cooperative” motion occurs.10 A common strategy is to orient photoswitches in a host material such as a polymer matrix.11 Alternatively, the “self-assembly” approach can be used where photoswitch molecules nucleate together in the solid state and form well-ordered three-dimensional lattices such as in single crystals.12 Figure 1-1 Representative examples of photoswitches. Ring opening/closing of (a) diarylethenes and (b) furylfulgides.30 Cis-trans isomerization of (c) azobenzenes and proton transfer followed by cis-trans isomerization (d) salicylideneanilines. 4 Single crystals have been used in organic solid state photochemistry for over 100 years, with pioneering work on cinnamic acid photodimerization.13 Schmidt and coworkers showed that the lattice within a crystal provides a well-defined reaction geometry for chemical reactions. Furthermore, these photochemical reactions yield specific products unobtainable in the solution state (the seminal work on the [2+2] photodimerization of cinnamic acid (Figure 1-3) delineated how this process of molecular packing within crystal lattices can predetermine the outcome of a photochemical reaction and coined the term “topochemistry”).14 UV light irradiation of the three polymorphs of trans-cinnamic acid (5a) produce photoproducts based on the different molecular orientations within the crystals. The α-polymorph produces the α-truxillic acid dimer (5b), the β-polymorph produces the β-truxinic acid dimer (5c), and no photoreaction occurs for the γ-polymorph.15 Figure 1-2 Molecular arrangements in solution and crystalline state. In solution the random distribution of the photoswitches results in disordered displacement, while in well-ordered arrangements in crystals, ordered displacement of the photoswitches occurs, resulting in observable motion. Image reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA © 2014.10 5 The photoproducts observed in 5a contributed to the development of the term “crystal engineering” which is used to describe structure-property relationships and predict potential lattice packing environments and intermolecular interactions observed in the solid state.16,17 This is still an active area of research, including areas such as “green chemistry” solvent-free approaches in organic synthesis.18,19 The first instance of using a solid state reaction to generate a photomechanical response in single crystals was reported by Abakumov and Nevodchikov in 1982.20 Crystals were grown from a semiquinone complex of platinum group metals, that generated a free-radical complex and when exposed to visible or near-infrared light.21 The change in molecular geometry resulted in a buildup of internal strain, culminating in reversible bending of the crystal. The crystal returned to its original shape when no longer irradiated. 1.3 Modes of Mechanical Action 1.3.1 Photosalient Phenomenon Photomechanical motion in crystals can be divided into three consecutive processes: light absorption, an induction period, and the following macroscopic motion.22 After absorption, the induction period occurs where a series of structural transformations take place within the lattice. In crystals where elastic strain occurs, there is no induction period as the structural transformations Figure 1-3 Photodimerization of trans-cinnamic acid 5a and its photodimer products. 6 coincide. The elastic strain results in continuous stress relaxation and mechanical reconfiguration while maintaining structural integrity, as shown in Figure 1-4.22 In other crystal systems, energy accumulation occurs during the induction period, resulting in a metastable state. The crystal then relaxes to a lower energy state by the rapid mechanical transformation, often resulting in fracturing, jumping, and creeping.23,24 Additionally, light direction, intensity, and duration of exposure are also crucial factors in generating photomechanical motion.25 In larger crystals, the photochemical reaction is localized on only the irradiated side. A bimorphic structure is formed with two distinct phases of reactant and photoproduct, which generates a strain gradient at the phase interface (Figure 1-5(a)).26 The generation of photomechanical motion occurs as the strain at the phase interface is released. Furthermore, the type of mechanical motion generated is dependent on crystal shape and size.27 Examples of photomechanical motion observed in photoreactive crystals will be discussed further Figure 1-4 Elastic photomechanical responses such as bending, twisting, or coiling when plate and rod-like crystals are exposed to light. These crystals maintain structural integrity while undergoing mechanical deformations. Image reproduced with permission from the American Chemical Society, Copyright © 2015.22 7 in Section 1.4. Hence, by carefully controlling the crystallization conditions and judicious light source selection, controlling the direction and magnitude of the photomechanical motion in larger crystals is relatively straightforward. Conversely, if the photochemical reaction goes to completion within the crystal, mechanical motion can be generated by reconfiguring the crystal (Figure 1-5(b)). The photoproduct would ideally have an absorption spectrum that does not overlap with the absorption spectrum of the reactant to drive the reaction to completion. These high-yielding photochemical reactions often lead to complete reconstruction of the entire lattice. As a result, the mechanical motion is intrinsically related to the crystal shape and molecular packing, not the irradiation conditions.28,29 Mechanical motion driven by a complete transformation of this type is analogous to thermally driven motion in single crystals (thermosalient effect). (a) (b) Figure 1-5 (a) Bimorph formation when irradiating attenuated light produces a gradient of reactant and photoproduct molecules. (b) Reconstruction of the crystal entity by complete photoreaction resulting in rearrangement of the photoproducts in the lattice. Image reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA © 2014.10 8 1.3.2 Thermosalient Phenomenon Thermosalient crystals generate a mechanical motion such as jumping or exploding when heated or cooled.30 The thermosalient response is accompanied by sharp phase transitions with anomalous thermal expansion or contraction of the unit cell. While the associated martensitic-type phase transitions (a coordinated shift of atoms without any long-range diffusion) have closely related crystal structures with the exact molecular symmetry and space group, minor adjustments of the crystal packing and lattice parameters can result in crystals jumping up to 5 cm in the air.31 Bernstein, Naumov, and coworkers delineated mechanistic insights of the thermosalient phenomenon with the anticholinergic drug oxitropium bromide (Figure 1-6).32 Crystals of compound 6 undergo a reversible phase transition when heated to 45 °C and cooled to 27 °C. This results in the anisotropic expansion of the unit cell (Δa = 4%; Δb = 11%; Δc = -7%) with a volume increase of 4%. While the two phases maintain the same crystal symmetry, slight phenyl and tricyclic ring conformation adjustments occur. The rapid anisotropic expansion and conformational adjustments result in the crystal undergoing macroscopic motion such as hopping, expanding, or disintegrating at the phase transition point. As the high-temperature phase is metastable, cooling results in reversion to the room temperature phase. 9 1.4 Photoswitches as Stimuli-Responsive Building Blocks In Section 1.3, the mechanistic effects to produce macroscopic photomechanical motion in crystals were discussed. The following section describes the photomechanical effects generated from irradiating photoswitch molecules in the crystalline state. 1.4.1 Ring-opening/closing Irie and coworkers demonstrated rapid and reversible bending of rod-like crystals of diarylethene derivatives (Figure 1-7).4,5 Mixed crystals of 7a and 8a in a ratio of 63:37 promoted rod-like growth of microcrystals that, when irradiated with 365 nm light, undergo a bending motion towards the light source. When irradiated with visible light (>500 nm), the microcrystal returns to its original state, allowing this process to be cycled over 1000 times without any visible fatigue on the crystal. Low temperature phase High temperature phase Figure 1-6 Chemical structure of oxitropium bromide and its molecular conformation in the low- and high-temperature phases. Image reproduced with permission from the American Chemical Society, Copyright © 2010.32 6 (a) (b)) 10 Koshima and coworkers also used intramolecular ring-opening/closing reactions to cause photomechanical bending of micro plate-like crystals of a furylfulgide, 9a.33 When irradiated with 365 nm light, the crystal displays photochromism from yellow to red accompanied by a bending motion of 9° towards the light source (Figure 1-8). The bending motion and color change result from the ring-closing reaction from the E- form to the C- form. The crystal can return to its original shape with > 390 nm light exposure with irreversible surface modifications. Figure 1-7 Chemical structure of the diarylethene derivatives and the rod-like mixed crystal of 7a and 8a (63:37) undergoing reversible bending when irradiated with UV and visible light respectively. Image reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA © 2012.4 (a) (b)) 11 1.4.2 Isomerization The first report of crystal-to-crystal cis-trans azobenzene isomerization was by Barrett and coworkers. In the cis perhalogenated azobenzene derivatives 10a, the rod-like crystals irreversibly bend (up to 180°) when irradiated with 457 nm light (Figure 1-9).34 In addition to the mechanical response, the crystal would transform into a polycrystalline aggregate of the trans isomer. Ojima and coworkers also used the cis-trans photoisomerization reaction to generate the photomechanical motion of a crystalline salicylideneaniline derivative 11a. In this case, the cis-trans photoisomerization occurs with an intramolecular proton transfer (Figure 1-10).35 When irradiated with 365 nm, light plate-like crystals of 11a bend up to 45 ° away from the light source within 1s. The original crystal shape can be recovered within 10 s when irradiated with 530 nm, and this process was cycled over 200 times, demonstrating this crystal system’s versatility. Figure 1-8 Reversible ring-opening/closing of the furylfulgide and the microplate-plate crystal undergoing a bending motion toward the light source. Image reproduced with permission from CSJ Journnals © 2012.33 (a) (b)) 12 1.4.3 Dimerization Naumov and coworkers reported various photomechanical motions of 12a crystals based on a [2+2] photodimerization reaction (Figure 1-11).36 The observed photomechanical motions included cracking, hopping, splitting, and rolling depended on the crystal aspect ratio. Splitting and rolling motions occur due to decreasing surface adhesion force while the surface energy increases, which results in the shrinkage of the surface layer and a decreasing intermolecular distance culminating in delamination of the surface. The crystal bending resulted from macroscopic shrinking by deeper light penetration of one side of the crystal. Larger block-like crystals undergo splintering and explosion effects. Figure 1-9 Photoisomerization of the perhalogenated azobenzene derivative and the cis-crystal irreversibly bending when exposed to 457 nm light. The photoisomerization results in the transformation to a polycrystal containing the trans- isomer. Image reproduced with permission from the American Chemical Society, Copyright © 2013.34 (a) (b)) 13 Figure 1-10 Intramolecular proton transfer followed by cis-trans isomerization of the salicylideneaniline derivative and the reversible bending motion of the plate-like crystal. The length of the scale bar is 10 μm. Image reproduced with permission from the Royal Society of Chemistry, Copyright © 2011.35 (a) (b)) 14 1.5 The Hemithioindigo Photoswitch Hemithioindigos (HTIs) are an emerging class of photoswitches with underexplored solid state properties (Figure 1-12).37–41 HTIs originate from the stilbene photoswitch archetype, with modifications to create unsymmetrical molecules consisting of a thioindigo and stilbene moiety connected by a double bond. Photoisomerization occurs between the thermodynamically stable Z- and metastable E- forms. The metastable form is thermally active and slowly converts back to the thermodynamically stable form. However, the barrier for thermal E-Z isomerization is > 27 kcal/mol, which is notably higher than the thermal barrier for azobenzene photoswitches (< 25 Figure 1-11 [2+2] Photodimerization of 12a and the resulting photomechanical effects. (a,b) Before and after photographs of the photosalient effect in crystals of 12a irradiated with UV light. (c–f) Photographs of other photomechanical motions such as cracking, hopping, splitting, and bending. (g) Distribution of photomechanical effects of 12a and the crystal aspect ratio. Image reproduced with permission from the American Chemical Society, Copyright © 2015.37 15 kcal/mol). This high thermal barrier inherently results in greater stability of both forms of HTI, creating a bistable switching system. One significant advantage of HTIs over other photoswitching systems is that due to the acceptor/donor and the charge transfer nature of the thioindigo and stilbene fragments, the absorption profile is redshifted, enabling excitation of photoisomerization in both directions with visible light (Figure 1-12(b)). HTIs are also photochromic, with the E- isomer generally bathochromically shifted around 20 – 30 nm from the Z- isomer absorption maxima.42 To date, the only reported example of solid state photoswitching of HTIs was by Dube and coworkers, where 14a isomerizes between the Z- and E- form in the amorphous state (Figure 1-13).43 However are no reports of photomechanical effects in crystalline HTIs in the literature. Figure 1-12 Hemithioindigo (HTI) photoswitch architecture and (a) the photoisomerization between the Z- and E- isomers. (b) The absorption profiles are red-shifted into the visible region with the E- form bathochromically shifted by 20-30 nm. Image reproduced with permission from Elsevier, Copyright © 2015.38 (a) (b)) 16 1.6 Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence In addition to photomechanical effects, molecular crystals also serve as a structural framework for emissive organic compounds. The following section outlines general photophysical properties and how carefully manipulating compounds and lattice structures can realize highly efficient photoluminescent materials. 1.6.1 General Photophysical Principles Photoexcitation of a molecule occurs by absorbing a photon of commensurate energy, promoting an electron from the ground state (S0) to a higher energy singlet excited state (S1, S2, Sn) (Figure 1-14).44 Absorption of a photon occurs on the femtosecond time scale (10-15 s), followed by immediate internal conversion (IC) from unstable higher singlet states to the first excited state. The electron decays back to the ground state, releasing the absorbed energy either radiatively as light (fluorescence) or dissipated by non-radiative decay in the form of heat. Alternatively, the excited electron can undergo intersystem crossing (ISC), whereby a spin-flip occurs, resulting in the lower energy excited triplet state. ISC is a spin forbidden process (as shown by the selection rules) and occurs relatively slowly and inefficiently.45 However, the efficiency Figure 1-13 (a) Photoisomerization of 14 and (b) associated color change between the Z- (14a) and E- (14b) isomer. Image reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA © 2020.44 (a) (b)) 14a 14b 17 improves with spin-orbit coupling (SOC) with the presence of a transition metal or heavy atoms such as bromine.46 After triplet internal conversion and vibrational relaxation to the lowest triplet excited state, the electron decays to the ground state releasing its energy via radiative decay (phosphorescence) or non-radiative decay as heat.47 1.6.2 Thermally Activated Delayed Fluorescence (TADF) Spin statistics show that the singlet state contributes 25%, and the triplet state contributes 75% to the exciton population. Therefore, to improve emission efficiency, harvesting both singlet and triplet excited states is necessary.48 One approach is to repopulate the first singlet excited by reverse intersystem crossing (RISC) utilizing the triplet excited state as a reservoir of energized electrons. After repopulation, delayed fluorescence occurs with the identical emission spectrum as the initial fluorescence. For efficient RISC, the singlet-triplet energy difference (ΔEST) must be < 0.12 eV. Since the excited singlet state is higher in energy than the excited triplet state, RISC is an endothermic process and requires thermal activation.49 At very low temperatures, phosphorescence is more prevalent due to the excitons decaying primarily from the triplet excited state. As the Figure 1-14 Jablonski diagram showing various photophysical processes. 18 system’s temperature increases, more fluorescence occurs as repopulation and subsequent decay of the singlet excited state occurs. Since RISC is a forbidden process, the delayed fluorescence emission has a longer lifetime (µs) compared to the primary fluorescence emission (ns).50 The first report of TADF in organic molecules was in 1961 with red fluorescent Eosin 15.51 Only in 2010, with the groundbreaking work by Adachi and coworkers, was it shown that the TADF phenomenon could be effectively utilized in OLED applications.52 Quantum mechanical analysis found that the reduction in the overlap of the molecule’s orbitals, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), can realize small ΔEST for efficient TADF. Following this result, a molecular structure design strategy was presented, which involved constructing molecules with a twisted electron donor-acceptor (D–A) structure stabilized by steric hindrance from bulky substituents (Figure 1-15). OLEDs created with compound 16 had a ΔEST as small as 0.11 eV and a high EQE (5.3%). In 2012 Adachi and coworkers reported a very high EQE of 19.3% with carbazolyl dicyanobenzene (17) based emitters, far above the theoretical limit for organic fluorescent emitters.53 Figure 1-15 Examples of structures displaying the TADF phenomenon. 19 1.6.3 Room Temperature Phosphorescence (RTP) Another approach to improving emission efficiency is utilizing room-temperature phosphorescence from otherwise wasted excitons in the excited triplet state. Achieving room temperature phosphorescence in organic emitters is challenging as inefficient SOC and forbidden ISC processes are only observed in metal complexes. In organic emitters with only (π – π*) excited singlet and triplet electronic transitions, the ISC rate is very low, and the ΔEST gaps are significant. For organic emitters containing nitrogen and sulfur atoms or carbonyl groups, both (π, π*) and (n, π*) electronic configurations can occur. Following El-Sayed’s rule, if the transition from the singlet to triplet excited state involves different electronic configurations (1(π, π*) to 3(n, π*) or from 1(n, π*) to 3(π, π*)), efficient SOC and high ISC rate can be realized in organic emitters.54,55 Furthermore, a larger (π, π*) component produces a longer lifetime of > 10 ms while a larger (n, π*) component increases efficiency but reduces the lifetime to < 10 ms. Hence, tuning the phosphorescence emission can be performed by mixing appropriate proportions of the hybrid (n, π*) and (π, π*) configurations.56 Huang and coworkers showed that by carefully manipulating molecular structure, intermolecular space, and the heavy atom effect, the phosphorescence lifetime and quantum yield improves to 217 ms and 38.1% in 18 (Figure 1-16).57 Tang and coworkers utilized the synergistic effects of aromatic carbonyls to enhance phosphorescence (19–23).58 Compounds displaying RTP have seen use in bioimaging,59,60 cancer therapy,61 and antibacterial agents.62 20 1.7 Crystallization Induced Phosphorescence (CIP) Organic emitters’ ultra-long lived triplet excitons are highly susceptible to environmental quenchers such as heat induced non-radiative decay and oxygen, promoting non-radiative transitions. Organic emitters embedded into rigid matrices help stabilize triplet excitons and enhance phosphorescence. The rigidification of the matrix can occur in polymers or crystal lattices, with the latter consisting of a more facile approach to vibrational stabilization and prevention of oxygen quenching.63–65 Additionally, crystal lattices can be used to tune the molecular packing and noncovalent intermolecular interactions to modulate solid state emission properties. By modifying the molecular interactions such as hydrogen bonding, π–π and C–H⋯π, and halogen-halogen Figure 1-16 (a) Examples of compounds displaying the RTP phenomenon. (b) “Afterglow” effect from a crystalline powder sample of 19. Image reproduced with permission from the American Chemical Society, Copyright © 2019.58 (a) (b)) 21 interactions, intramolecular motions can be further restricted, promoting efficient phosphorescence with a long lifetime at room temperature.66,67 1.7.1 Single Component Systems Tang and coworkers popularized the concept of CIP in 2010 with a systematic study of phosphorescent emission of halogenated benzophenone-based organic emitter under normal conditions (Figure 1-17).68 In solution and amorphous films, these organic emitters were all non-emissive while highly phosphorescent in the crystalline state as the intramolecular motions were restricted, locking the conformations by intermolecular interactions (C−H···O, N−H···O, C−H···X (X = F, Cl, Br), C−Br···Br−C, and C−H···π hydrogen bonding). Xie and coworkers showed that the phosphorescence emission could be strongly impacted by packing arrangements.6 Compounds 32 – 34 display similar emission properties in solution at 77 K and r.t; however, when crystallized, the emission properties changed with packing efficiency. Compounds 32 – 34 demonstrate that weaker packing arrangements gradually decreased phosphorescence efficiency and lifetime. Figure 1-17 Examples of compounds displaying crystallization induced phosphorescence (CIP). 22 From SCXRD and density functional theory (DFT) studies, the close face-to-face packing promoted intermolecular coupling to stabilize the triplet exciton and stabilize the single excited state. Compound 32 was polymorphic, and the emission properties varied between forms CPM and CPM-A (Figure 1-18(b)). The second form had drastically decreased phosphorescence efficiency and lifetime (63 ms) due to the similar but reduced interplanar distance of 3.49 vs. 3.45 Å. 1.7.2 Co-crystal Systems Yan and coworkers showed phosphorescence emission color tuning by co-crystallization of two components consisting of hydrogen bonding networks and (π,π) and (n,π*) interactions.69 Two co-crystals were designed and synthesized consisting of melamine and 1,2,4- benzene tricarboxylic acid in molar ratios of 1:1 and 2:1. DFT calculations showed that energy transfer occurred from 35 to 36 (Figure 1-19). By altering the molar ratio, two different stacking modes, Figure 1-18 (a) Carbazole-based RTP emitters. (b) Different packing morphologies of CPM impact “afterglow” emission efficiency. Image reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA © 2017.6 (a) (b)) 23 H-aggregation and J-aggregation, cause green and yellow emission colors. The system was then further shown to have potential applications in information encryption and data storage. 1.7.3 Host-guest Systems Kim and coworkers demonstrated that guest-host crystalline systems could be effective room temperature phosphorescent emitters (Figure 1-20(c)).70 By incorporating triplet promoting aromatic aldehydes and halogenated aromatic chromophores, halogen bonding in the crystal lattice enables highly efficient phosphorescence in the solid state (Figure 1-20(a)). Doping dilute concentrations of the aromatic aldehyde chromophore into the halogenated aromatic crystalline host allowed the quantum yield to reach up to 55% at room temperature (Figure 1-20(b)). Furthermore, this approach can be utilized to color tune other organic emitters to produce blue, green, yellow, and orange emission colors (Figure 1-20(d)). Figure 1-19 (a) Chemical structure of co-crystal components. (b) Energy transfer to different aggregation states results in green and yellow RTP. Image reproduced with permission from the Royal Society of Chemistry, Copyright © 2020.70 (a) (b)) 24 1.8 Goals of the Thesis The central focus of this thesis is the study of photofunctional molecules in the crystalline state. Chapter 2 and Chapter 3 focus on photomechanical crystals that undergo deformation or move when stimulated with heat and light, potentially useful as non-contact microactuators. Photoswitches are the main compounds to induce photomechanical responses in crystals, and the goal of this part of the thesis is to develop novel photoswitches for use as photomechanical materials. Using crystals as host matrices (Chapter 4) for luminescent emitters can dramatically Figure 1-20 (a) Excimer formation in chromophore resulting in reduced phosphorescence efficiency. (b) Substitution into the host prevents excimer formation and promotes high phosphorescence efficiency. (c) Structures used to tune emission color in (d) crystals. Image reproduced with permission from Springer Nature, Copyright © 2011.71 (a) (b) (c) (d) 25 improve the efficiency and duration of emission afterglow properties used in anti-counterfeiting and biological applications. This part of the thesis aims to tune the afterglow emission properties by solid solution substitution. Chapter 5 explores the photophysical properties of novel, structural isomers of organic luminescent compounds in the crystalline state. Chapter 2 aims to investigate a novel hemithioindigo-anthracene photoswitch in which both of its isomers are photoactive for photomechanical motion in the crystalline state. Chapter 3 explores photomechanical motion by a [2+2] photodimerization. By crystallizing compounds that contain unsaturated bonds in the correct alignment, a [2+2] photoreaction can occur in the solid state. A series of hemithioindigo photoswitch analogs are synthesized and their solid state properties explored. Chapter 4 examines solid substitution in crystalline materials to enhance emission in the solid state at room temperature. Incorporating host-guest emitters with similar structural properties can enable adequate mixing in a continuous molar ratio. When using emitter materials with contrasting photophysical properties, the substitution enables fine-tuning of the emission produced. Finally, structural isomerism of the same compound is well known to influence the lattice packing arrangements, which strongly impact the physical properties. Chapter 5 investigates the photophysical properties and influence of isomerism of novel room temperature organic emitters in the crystalline state. 26 Chapter 2: Mechanical Responses of Hemithioindigo-Anthracene Based Molecular Crystals to Heat and Light 2.1 Introduction Stimuli-driven mechanical responses in molecular crystalline solids have attracted much attention in recent years.10,22,30,71 The salient effect as a mechanical response in molecular crystals is described as a ‘hopping’ phenomenon following the application of an external stimulus and is being actively investigated for application in microactuators.72–74 The manifestation of macroscopic mechanical effects from the stress exerted on the lattice should be highly reconstructive to preserve the single crystalline phase. Therefore, any substantial differences in molecular packing between the two phases would irreversibly change the crystal lattice.75–77 The hemithioindigo (HTI) photoswitch can undergo reversible Z–E photoisomerization reactions in solution with the Z- configuration thermodynamically stable and the E- configuration metastable.37 In the solid state, HTI crystallizes in planar structural arrangements with the molecules aligned so they may undergo [2+2] cycloaddition reactions.38,41,78 HTIs are asymmetrical with an electron-withdrawing thioindigo fragment bound to an electron donor stilbene fragment through a double bond.79 As a consequence of the electron donor/acceptor nature of HTIs, the wavelengths required for Z–E isomerization reactions are red-shifted into the visible region.40 To build HTIs with new photophysical and photochemical properties, it is intriguing to consider incorporating a secondary photoactive compound into the HTI framework. This chapter shows that replacing the stilbene fragment with an anthracene unit enables reversible isomerization in both solution and the solid state as the rigid and bulky anthracene fragment disrupts the inherent planar configuration, promoting the reversible Z–E isomerization and minimizing [2+2] cycloaddition side reactions.80–83 Structural and thermal analysis and the photochemistry for both 27 (Z)- and (E)-HTI-An, are characterized in solution and the crystalline state. The Z–E isomerization results in solid state photochromism for (Z)-HTI-An, and a photosalient effect and thermal isomerization reversion for (E)-HTI-An (Scheme 2-1). Furthermore, thermal treatment of (Z)-HTI-An crystals induces a phase transition prompting a thermosalient response while thin films reveal multiple polymorphs. 2.2 Synthesis and Characterization The synthesis of the HTI-An photoswitch involves two steps, first an aldol condensation between 2-nitroacetophenone and 9-anthraldehyde in i-propanol at 0 °C, which results in the formation of the 2-nitrochalcone ((E)-3-(anthracen-9-yl)-1-(2-nitrophenyl)prop-2-en-1-one) (Scheme 2-2).84 The 2-nitrochalcone is then reacted with elemental sulfur in the presence of triethylamine and dimethyl sulfoxide at 50 °C under a nitrogen atmosphere. This reaction results in the elimination of the nitro group followed by ring closure to produce the five-membered sulfur heterocycle, yielding the phase pure cis isomer (Z)-HTI-An (Figure 2-1 and Figure 2-2), with conjugation to the anthracene moiety about the Z–E isomerizable double bond. Scheme 2-1 Relationship between corresponding isomers of HTI-An upon thermal and light stimulation. 28 The synthesis of the trans- isomer (E)-HTI-An was achieved by irradiating a ~3 mM solution of (Z)-HTI-An in dichloromethane with 405 nm for 24 hours (Figure 2-2). Both (Z)- and (E)-HTI-An isomers were characterized by 1H NMR and 13C{1H} NMR spectroscopies and high-resolution mass spectrometry. Both isomers show aromatic proton peaks with the diagnostic alkene proton (Ha) peak at 8.6 ppm. Figure 2-1 Comparison of the calculated (from SCXRD data at 100 K) and experimental (298 K) powder X-ray diffractograms of (Z)-HTI-An. The inset is a photograph of the bulk crystalline material. Scheme 2-2 Synthesis of the hemithioindigo photoswitch (Z)-HTI-An. 29 When the Z- isomer is irradiated with 405 nm in solution, the photostationary state can be achieved with ~77% yield of the E- isomer, which shows a slight upfield shift of the alkene proton (Hb) to 8.75 ppm. X-ray quality single crystals of (Z)-HTI-An were obtained from recrystallization from i-propanol. Furthermore, high isomeric purity single crystals of (E)-HTI-An for X-ray structure analysis were obtained by irradiating a solution of (Z)-HTI-An with 405 nm to achieve the photostationary state. (Z)-HTI-An (E)-HTI-An Ha Hb Figure 2-2 Photoisomerization of HTI-An and 1H NMR (400 MHz, CD2Cl2) spectra of (Z)-HTI-An and (E)-HTI-An (~77% isomeric purity) at room temperature. 30 The solution was then evaporated in vacuo to obtain a red powder and purified by preparative TLC followed by recrystallization from i-propanol. The prism-shaped crystals of (E/Z)-HTI-An were obtained as a mixed crystal system in a ratio of 92.3 : 7.7 E : Z (Figure 2-3). 2.3 Absorption Spectroscopy of HTI-An The absorption spectra of both isomers in dichloromethane show higher energy absorption bands at 355, 365, and 385 nm, which correspond to the π–π* excitation localized on the anthracene moiety.85 The broad absorption band with maximum absorption at 455 nm and extending into the visible region, tailing off at 550 nm is assigned as a charge transfer (CT) transition.41 The energy of this charge transfer band does not correlate with solvent polarity as observed for other hemithioindigo compounds;39 however, this is consistent with the behavior of other anthracene-acceptor-based compounds (Figure 2-4).85 In the excited state, a transfer of electron density from the anthracene donor to the thioindigo acceptor likely occurs. The absorption spectra for microcrystalline films of both isomers maintain the same overall features in the solid state. However, considerable broadening of the charge transfer band, tailing off at 750 nm, is observed compared to the solution spectra (Figure 2-5). Figure 2-3 ORTEP diagrams of (a) (Z)-HTI-An and (b) (E)-HTI-An where ellipsoids are drawn at 50% probability and only heteroatoms atoms are labelled for clarity. The site occupancy was modelled as disordered thioindigo fragments of (E)-HTI-An, was determined to be 92.3: 7.7 E : Z. 31 When dilute (2.95 × 10−5 M) dichloromethane solutions are irradiated with 405 nm light, (Z)-HTI-An undergoes a Z–E isomerization leading to the formation of (E)-HTI-An, which shows a sharp increase in the higher energy bands and a decreased, red-shifted charge transfer band (Figure 2-6(a)). The isosbestic points observed at 480 nm and 390 nm suggest that photoisomerization is the primary reaction pathway, with only the two isomers present and without the formation of other potential photodimer by-products as confirmed by HPLC analysis.86,87 The photoisomerization reaction yields ∼77% (E)-HTI-An, and upon subsequent 515 nm exposure, reversion to (Z)-HTI-An occurs and can be recovered to ∼89%. Multiple cycles of this process were possible without significant fatigue (Figure 2-6(b)). Figure 2-4 UV-Vis absorption spectra of (Z)-HTI-An in various solvents. 00.250.50.751300 400 500 600Normalized AbsorbanceWavelength (nm)HexanesDCMTHFACN32 Figure 2-5 UV-Vis absorption spectra of solution (solid) and solid state (dashed) of (Z)-HTI-An (red) and (E)-HTI-An (black). 00.250.50.751300 400 500 600 700 800Normalized AbsorbanceWavelength (nm)Z-HTI-An solutionZ-HTI-An solid stateE-HTI-An solutionE-HTI-An solid stateFigure 2-6 (a) Absorption spectra of HTI-An showing the reversible Z – E photoisomerization in dichloromethane. (b) Isomer yields from photoisomerization of (Z)-HTI-An (black) and (E)-HTI-An (red) by cycling 405 nm and 515 nm irradiation in a dilute dichloromethane solution. 33 2.4 Photoresponses of Crystalline HTI-An 2.4.1 Photochromism (Z)-HTI-An Crystals When microplate crystals of (Z)-HTI-An are irradiated with 365 nm light for 10 minutes, the crystal shifts to a red/orange color while maintaining crystal structural integrity (Figure 2-7(b)). Irradiation results in Z–E isomerization as observed in solution, with the conversion to (E)-HTI-An determined to be 5% by HPLC analysis (Figure 2-7(c)). While prolonged exposure retained the structural integrity and no other by-products were detected, further conversion was not achieved. This is possibly due to the broadening of the absorption spectra in the solid state, leading to a greater degree of overlapping absorption profiles between the two isomers and a reduction in specific wavelength excitation to shift the photostationary state toward (E)-HTI-An. Figure 2-7 (a) Microcrystal of (Z)-HTI-An crystal under ambient light. (b) Microcrystal of (Z)-HTI-An irradiated with 365 nm over 10 minutes produces a more pronounced red color in the crystal. (c) As assessed by HPLC the percentage of HTI-An isomers before and after 365 nm irradiation shows a decrease of (Z)-HTI-An by ~5%. Scale bars are 100 µm. 34 2.4.2 Photosalient Effect of (E)-HTI-An Crystals Upon exposure with light of wavelength 515 nm, the mixed crystals of (E/Z)-HTI-An produce a photosalient effect where initial rocking followed by ‘hopping’ out of the microscope field of view was observed (Figure 2-8(a-b)). Surprisingly, the structural integrity of the (E/Z)-HTI-An crystal remained intact following the ‘hopping’ motion. (E/Z)-HTI-An crystallizes in the P21/c space group with one molecule in the asymmetric unit where the thioindigo and anthracene fragments are twisted at 67.2°. When viewed along the b-axis, the molecules stack in alternating orientations with the anthracene fragment building a zig-zag formation in the lattice (Figure 2-8(3d)). The ‘hopping’ motion is accompanied by an anisotropic contraction of 2% of the cell and a minor decrease in the cell dimensions (Δa = −0.88%, Δb = −0.64%, Δc = −0.55%). While the crystal retains its original packing configuration, the site occupancy of the thioindigo fragment changes from 92.3 : 7.7 to 83.8 : 16.2 E : Z suggesting the ‘hopping’ motion is a result of strain release from the rearrangement of the thioindigo fragment upon 8.5% photoisomerization in the crystal lattice (Figure 2-8(c) and Figure 2-9).43 The ‘hopping’ motion results in ∼1 cm displacement of the crystal across the stage, which has been observed with other photosalient materials with relatively small anisotropic unit cell expansion/contraction parameters.30,36,88–92 Other photosalient effects, including the splitting of the crystal, were also observed. 35 Figure 2-8 (a) – (b) Microscopy images demonstrating the photosalient effect of (E)-HTI-An upon exposure to 515 nm light. (c) Composition change of isomers as determined by site occupancy of the thioindigo fragment. (d) Asymmetric unit and molecular packing of (E)-HTI-An mixed crystal system viewed along the b-axis. Figure 2-9 Short contact interactions of (a) (E)-HTI-An before irradiation and (b) (E)-HTI-An post photosalient event. 36 2.5 Thermoresponses of Crystalline HTI-An 2.5.1 Thermosalient Effect of (Z)-HTI-An Crystals Upon heating, single crystals of (Z)-HTI-An were found to undergo a phase transition at 130 °C to produce the high-temperature phase (Z′)-HTI-An (Figure 2-10(a–c) and Appendix A). Applying a higher heating rate (40 °C min−1) induces a thermosalient response where single crystals ‘hop’ across the hot stage. Figure 2-10 (a) – (c) Thermosalient effect of (Z)-HTI-An at 130 °C. (d) Zig-zag lattice configuration viewed along the c-axis. (e) – (f) Angle between the thioindigo and anthracene planes of (Z)- and (Z’)-HTI-An. (g) – (h) Torsional configuration between thioindigo and anthracene fragment for (Z)- and (Z’)-HTI-An. 37 When a lower heating rate (5 °C min−1) is used, a pronounced crack along the (001) plane and no thermosalient effects are observed during the phase transition (Figure 2-11). Single crystal analyses performed at room temperature and 127 °C reveal that while the lattice retains the P21/c space group, the phase transition is accompanied by minor anisotropic expansions along all crystallographic cell axes (Δa = +1.82%, Δb = +1.54%, Δc = +0.77%) and a volume expansion of ΔV = 4.24% (Table 2-1). These minor thermal anisotropic expansions correlate with the lower intensity of the mechanical effect of (Z)-HTI-An, whereas other thermosalient crystals with large anomalous thermal expansion display more substantial mechanical effects.93,94 While cell dimension changes are small, the volume expansion of this system is comparable to other thermosalient crystals reported in the literature that typically exhibit volume changes between 0.5–5%.95–100 Structurally, both (Z)- and (Z′)-HTI-An phases have very similar packing arrangements and show a slight difference in the conformation of (Z)-HTI-An for the thioindigo and anthracene fragment (65.87° and 66.11°; −119.40° and −118.48°, respectively) (Figure 2-10(d–h) and Figure 2-12). Figure 2-11 Slow heating of single crystal of (Z)-HTI-An induces a phase transition resulting in cracking along (001) plane. 38 Table 2-1 Change in unit cell parameters of (Z)-HTI-An at 298 K and 400 K Unit Cell (Z)-HTI-An at 298K (Z’)-HTI-An at 400K Expansion (%) a (Å) 12.9474(18) 13.183(11) 1.82 b (Å) 11.1005(17) 11.272(8) 1.54 c (Å) 11.7019(17) 11.792(8) 0.77 Volume (Å3) 1668.3(4) 1737(2) 4.24 Further investigations into the observed thermosalient phenomena were carried out using differential scanning calorimetry experiments at 5 °C min−1 heating/cooling rates. Upon heating (Z)-HTI-An single crystals, an exothermic peak at ∼130 °C was observed, corresponding to the phase transition and accompanying thermosalient response (Figure 2-13). The exothermic peak suggests that the phase transition between (Z)- and (Z′)-HTI-An is a monotropic phase transition indicating this process is irreversible. This is further confirmed upon cooling, where no other thermal effects were observed.101–103 Figure 2-12 Short contact interactions and carbonyl oxygen distances of (a) (Z)-HTI-An at room temperature and (b) (Z’)-HTI-An at 400 K. 39 Additionally, to confirm that the observed minor unit cell changes between the (Z)- and (Z′)-HTI-An forms are from a phase transition and not regular thermal expansion of the same phase, Hirshfeld surface analysis was performed to elucidate changes in the intermolecular interactions. The Hirshfeld fingerprint plots distinguish the minor changes in the weak interactions between the two phases. Such small changes have also been observed for other thermosalient systems.96,104–107 The fingerprint plot is the 2D representation of a 3D molecular surface created by binning (di, de) pairs.108 These pairs represent the distance the Hirshfeld surface to the nearest nucleus inside the surface (di) and Hirshfeld surface to the nearest nucleus outside the surface (de)) in intervals of 0.01 Å and coloring each pixel based on the fraction of surface points in that bin, where blue represents a small number of points and green to red represents many points (Figure 2-14). Therefore, it can be speculated that (Z)-HTI-An crystals undergo the thermosalient effect in response to the structural strain induced by the lattice packing, while the crystal packing retains a very similar molecular conformations between the (Z)- and (Z′)-HTI-An phases. Figure 2-13 DSC heating trace of (Z)-HTI-An with thermosalient ‘hopping’ response at 130 °C followed by cooling to room temperature. The heating rate is 5 °C/min. 40 2.5.2 Polymorphism of (Z)-HTI-An Further phases and phase transitions were observed in crystalline thin films of (Z)-HTI-An (Figure 2-15). Upon cooling to room temperature, a super-cooled melt is obtained with crystallization of (Z)-HTI-An II and III, proceeding at 100 °C (Figure 2-15(a)). Further heating to 170 °C induces the first phase transition of (Z)-HTI-An III to (Z)-HTI-An II (Figure 2-15(b)) followed by the second phase transition of (Z)-HTI-An II to (Z)-HTI-An I at 180 °C (Figure 2-15(c)) and subsequent melting of (Z)-HTI-An I at 209 °C (Figure 2-15(d)). The concomitant phases and transitions observed on the hot-stage can be confirmed by complimentary DSC heating/cooling runs. Figure 2-14 2D Hirshfeld fingerprint plots of (a) (Z)-HTI-An and (b) (Z’)-HTI-An. Distance from the Hirshfeld surface to the nearest nucleus inside the surface (di) and Hirshfeld surface to the nearest nucleus outside the surface (de) show intermolecular interactions in the lattice. The color gradient represents different intermolecular interaction strength between molecules. 41 Additionally, the DSC runs reveal the thermal behavior of (Z)-HTI-An in two distinct reproducible cycles (Figure 2-16). For the first cycle, initial heating of bulk crystalline samples of (Z)-HTI-An shows the melting of (Z)-HTI-An I and confirmation of the super-cooled melt in the first run (Figure 2-15(e)). Heating/cooling runs 2 and 3 show reproducible recrystallizations from the melt of (Z)-HTI-An II and III, the phase transition of III to II, and the melting of II. For the second cycle, heating/cooling Run 1 proceeds in the same manner as Run 1 in the first cycle, but in Run 2 the melting of (Z)-HTI-An II and (Z)-HTI-An III are observed. In Run 3, the recrystallization of (Z)-HTI-An I exclusively is observed. The thermosalient effect exotherm is not observed in the DSC traces and the second phase transition on the hot-stage. It is speculated that the different environments presented in the DSC and hot-stage experiments may influence different observable thermal phenomena in this system, but in-depth explorations over this influence are beyond the scope of this work. 42 2.5.3 Solid State Thermal Reversion of (E)-HTI-An Single crystals of (E/Z)-HTI-An were also found to be thermally active, where thermal treatment induced an isomerization reversion to increase the concentration of the Z- isomer while maintaining the crystalline structural integrity (Figure 2-17(a–c)). The site occupancies of (E/Z)-HTI-An of the carbonyl and sulfur atoms in the E- thioindigo fragment decreased from 95% to 82% over two hours (Figure 2-17(d) and Appendix A). As a result of the high thermal isomerization temperature, sublimation of the (E/Z)-HTI-An crystal occurs as a competing process, and heating over extended periods prevents further XRD analysis (Figure 2-18). Figure 2-15 (a) Polarized light hot-stage photomicrographs of a thin film of (Z)-HTI-An II and III crystallized from the melt. (b) At 170 ° C the first phase transition of III to II occurs (red arrows mark the transition boundary). (c) At 180 °C the second phase transition occurs from II to I followed by (d) the melting of (Z)-HTI-An at 209 °C. (e) DSC traces of three successive heating/cooling runs beginning with (Z)-HTI-An I. The initial heating trace shows the melting of (Z)-HTI-An I. The second and third heating traces show the recrystallization of (Z)-HTI-An II and (Z)-HTI-An III and phase transition of (Z)-HTI-An III to (Z)-HTI-An II and subsequent melting of (Z)-HTI-An II. No endo/exotherms were observed during the cooling cycles. The heating rate is 10 °C/min. 43 However, when amorphous powders of (E/Z)-HTI-An are thermally treated at 180 °C for 30 minutes, the concentration of the Z- isomer can be dramatically increased to ∼94% (as determined by HPLC) as the lack of structural integrity allows for increased molecular rotation. This high conversion from the E- to Z- isomer shows this photoswitch system’s high thermal stability and potential reusability (Figure 2-17(e)). Figure 2-16 DSC heating/cooling runs beginning with crystalline (Z)-HTI-An. Run 1 shows the melting point of (Z)-HTI-An I. Run 2 shows both crystallization and melting of two distinct new phases. Run 3 shows the recrystallization and remelting of phase (Z)-HTI-An I. 44 2.6 Conclusions This work investigated a novel hemithioindigo-anthracene (HTI-An) based derivative where the installation of the bulky anthracene moiety promoted the reversible photoisomerization Figure 2-18 (a) – (c) Single crystal of (E)-HTI-An undergoing thermal isomerization reversion at 180 °C. (d) Thermal reversion resulting in 18% production of Z- isomer occurring in the (E)-HTI-An crystal. (e) Isomer concentration of amorphous sample of (E)-HTI-An before and after heating determined by HPLC. Scale bars are 200 µm. Figure 2-17 Crystal size reduction during thermal reversion of (E)-HTI-An at 180 °C for 2 hours. Scale bars are 200 µm. 45 reaction in both solution and crystalline states while preventing side [2+2] cycloaddition reactions from occurring. High-quality single crystals of each Z- and E-isomer were obtained, enabling monitoring of the microscopic changes responsible for the mechanical effects. When (Z)-HTI-An crystals are exposed to 365 nm light, a photoisomerization reaction is observed while heating these crystals to 130 °C induces a thermosalient response, accompanied by an irreversible phase transition. Recrystallization from the melt revealed two additional phases, which further undergo phase transitions between the phases at 170 °C and 180 °C. When (E/Z)-HTI-An crystals are exposed to 515 nm light, a photosalient response is observed where the crystal either ‘hops’ across the stage or ‘splits’ in two while heating (E/Z)-HTI-An crystals induce thermal isomerization reversion towards (Z)-HTI-An. Future work entails tailoring the crystal growth conditions to produce variable crystal morphology and polymorph selection, enabling mechanical responses. The facile synthesis and stimuli responsiveness of each isomer could further promote the development of actuators based on the HTI class of compounds. 2.7 Experimental 2.7.1 General Dichloromethane (Sigma-Aldrich), i-propanol (Sigma-Aldrich), sodium hydroxide (Sigma-Aldrich), 2-nitroacetophenone (Alfa Aesar), 9-anthraldehyde (Alfa Aesar), elemental sulfur (Fischer Scientific), dimethylsulfoxide (Sigma-Aldrich) and triethylamine (Sigma-Aldrich) were used as obtained from commercial sources and without further purification. 46 2.7.2 1H NMR Spectroscopy The NMR spectra were recorded at 25 °C on an Avance 400 spectrometer (Bruker, 400 MHz). Chemical shifts (in ppm) are reported relative to the non-deuterated residual solvent signals of CD2Cl2 (δ = 5.33 ppm). 2.7.3 UV-Vis Spectroscopy A Perkin Elmer UV-vis spectrometer was used to record the spectra in solution (Lambda 35) and solid state (Lambda 365). E-Z Reactions were monitored using UV-visible absorption spectroscopy where spectra of the compounds were recorded in dichloromethane solution with concentration 1.574 × 10‒5 M. The solution was irradiated with a 405 nm LED for 30 seconds to photoisomerize (Z)-HTI-An to (E)-HTI-An. After no further transformations were observed, the sample was irradiated with 515 nm LED light to isomerize (E)-HTI-An to (Z)-HTI-An. 2.7.4 X-ray Diffraction Single crystal X-ray data were collected using a Bruker APEX DUO diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 100 K. Raw frame data were processed using APEX2.109 The program SAINT+, v. 7.68110 was used to reduce the data and the program SADABS111 was used to make corrections to the empirical absorptions. Space group assignments were made using XPREP 110 on all compounds. In all cases, the structures were solved in the Olex2112 suite of programs using Intrinsic Phasing and refined using full-matrix least-squares/difference Fourier techniques on F2 using SHELXL.113 All non-hydrogen atoms were refined anisotropically. Diagrams and publication material were generated using CrystalMaker. 47 Powder X-ray diffraction was collected using a Bruker D8 Advance Powder Diffractometer, and data was analyzed using Bruker DiffracPlus EVA software. 2.7.5 Hot-Stage POM Microscopy Microscopy images were collected on an Olympus BX53M polarizing microscope equipped with an Instec hot stage. Thermal and photosalient effects were recorded with an Olympus UC90 camera equipped with a λ = 530 nm filter. 2.7.6 Differential Scanning Calorimetry (DSC) The differential scanning calorimetry experiments were performed on NETZSCH DSC 214 Polyma instrument with aluminum sample pans. The heating was carried out in the temperature range from 25 °C to 230 °C at a rate of 5 °C min‒1. 2.7.7 High-Performance Liquid Chromatography (HPLC) The HPLC was performed using an Agilent Technologies 1260 Infinity and an Agilent ZORBAX Extend-C18 column with 4.6 x 150 mm dimensions. The isocratic mobile phase was composed of 90% acetonitrile, 10% water with a flow rate of 1 mL min-1 at 35 ℃. The detector wavelength was set to 260 nm. 2.7.8 Synthesis (Z)-HTI-An The hemithioindigo photoswitch (Z)-HTI-An was prepared using the reported literature procedure.84 2-Nitroacetophenone (100 mg, 0.61 mmol) and NaOH (26.6 mg, 0.67 mmol, 1.1 eq.) 48 were dissolved in i-propanol (5 mL) and stirred for 30 minutes at 0 °C. 9-Anthraldehyde (124.9 mg 0.67 mmol, 1 eq.) dissolved in 5 mL of i-propanol was added dropwise to the reaction mixture and further stirred at 0 °C for 30 minutes. The reaction mixture was allowed to return to r.t. and stirred overnight (18 h). The crude product was collected via vacuum filtration and gently washed with cold i-propanol (10 mL) and recrystallized from hot i-propanol. The bright yellow product 2-nitrochalcone, (E)-3-(anthracen-9-yl)-1-(2-nitrophenyl)prop-2-en-1-one was used without any further purification. Yield 128 mg, 60%. The 2-nitrochalcone (100 mg, 10 mmol) was added to elemental sulfur (45.4 mg, 50 mmol, 5 eq.), dimethyl sulfoxide (110 mg, 50 mmol, 5 eq.), and triethylamine (143 mg, 50 mmol, 5 eq.) and stirred and heated to 50 °C under nitrogen for 18 h. The reaction mixture was cooled to room temperature, diluted with dichloromethane (20 mL) and filtered through a silica plug. The filtrate was dried with Na2SO4, filtered, and the solvent evaporated in vacuo. The crude mixture was purified using silica gel column chromatography (using a gradient starting with pure hexanes, ending with 80% hexanes/20% ethyl acetate) to yield 2-[(anthracen-9-yl)methylidene]-2,3-dihydro-1-benzothiophen-3-one (Z)-HTI-An as an orange solid (47.9 mg, 50%) (mp 209–210 °C). 1H NMR (400 MHz, CD2Cl2) δ (ppm) 8.85 (d, 1H), 8.61 (s, 1H), 8.18–8.06 (m, 4H), 8.04–7.97 (m, 1H), 7.65–7.53 (m, 5H), 7.41–7.33 (m, 2H). HR-ESI MS: m/z calcd. for C23H13OS: 337.0759; found: 338.1038 [M+H]+. (E)-HTI-An The preparation of (E)-HTI-An was performed by irradiating a 3 mM solution of (Z)-HTI-An in dichloromethane with a 405 nm 150 mW LED. The reaction was monitored using HPLC until the photostationary state maximum ratio of 78 : 22 E : Z was obtained, and the solvent was 49 evaporated in vacuo to obtain a red powder. Further purification was achieved by preparative TLC, yielding over 90% isomer purity of (E/Z)-HTI-An. 1H NMR (400 MHz, CD2Cl2) δ (ppm) 8.86 (s, 1H), 8.57 (s, 1H), 8.18–8.04 (m, 4H), 7.71–7.43 (m, 6H), 7.26 (t, J = 7.4 Hz, 2H). 50 Chapter 3: Crystal Photochromism and Photomechanical Responses of Hemithioindigo Photoswitch Analogs 3.1 Introduction In recent years, photomechanical molecular crystals have attracted much attention for applications in actuators, smart medical devices, artificial muscles, sensors, flexible electronics, and probes.114–117 The photoinduced mechanical deformations include bending, twisting, rotation, crawling, peeling, and hopping.118–122 Mechanical effects resulting in jumping or violently exploding are termed photosalient (PS) effects.89 Using light to power the photoreactions within the crystals is particularly advantageous as photons can transport energy and information into a wide variety of media in a non-contact mode. Molecular crystals with highly ordered structures exhibit shorter response times and have a higher Young's modulus when compared to polymer-based materials.22 Furthermore, topochemical reactions can be finely controlled in molecular crystals to achieve high precision mechanical motion.4,5 Photoswitches are photochromic molecules that are ideal for generating photoinduced mechanical motion as the change in their absorption spectra is accompanied by a geometrical change.123 When these molecular geometrical changes occur on the nanoscale in crystals in a cooperative fashion, visible effects such as motion occurs. The photomechanical motion is powered by: the isomerization of azobenzenes,34 ring-opening/closing cyclization in diarylethenes,23 [4+4] dimerization of anthracenes,29 and [2+2] photodimerization of parallel olefins.80 Schmidt's topochemical criteria must be satisfied in [2+2] photodimerization reactions that occur in the solid state to generate photomechanical motion. Such systems include organic molecules, metal complexes, and coordination polymers.13 51 Hemithioindigo photoswitches are an emerging class of compounds based on the stilbene moiety modified with a thioindigo fragment, creating a charge transfer effect across the molecule.37 This is advantageous as the absorption spectrum is shifted into the visible region, negating the need for UV light.39 HTIs can undergo photoisomerization in solution and the solid state.124 Modifications to HTI's can strongly impact the packing arrangements in the crystal lattice, where bulky modifications that prevent planar geometries disrupt [2+2] photodimerization and promote isomerization.43 Whereas modifications that enhance the planar and close packing nature can enhance photomechanical effects by increasing [2+2] photodimerization efficiency within the lattice. Naphthyl groups were also used to increase π stacking to fabricate highly efficient photomechanical crystals. In this chapter, the synthesis and characterization of three HTI photoswitches are described. Molecular crystals of HTI, HTI-Nap, and HTI-Mes displayed photosalient effects and color changes, crystal bending, and localized deformations. This work presents the first example of photomechanical motion generated from [2+2] photodimerization reactions in HTI molecular crystals. 3.2 Synthesis and Characterization The hemithioindigo (HTI) analogs were synthesized using two different synthetic procedures (Scheme 3-1).39,84 Photoswitches HTI and HTI-Nap were prepared by an aldol condensation reaction between 2-nitroacetophenone and benzaldehyde or 1-naphthaldehyde in a basic ethanol solution. The corresponding 2-nitrochalcone undergoes ring cyclization with sulfur in the presence of dimethylsulfoxide (DMSO) and triethylamine in 60% and 65% yield. The synthesis for the photoswitch HTI-Mes was unsuccessful during the aldol condensation step, and 52 hence a Friedel Crafts reaction was employed with (phenylthio)acetyl chloride to yield benzo[b]thiophen-3(2H)-one. This was further reacted with mesitaldehyde to give the product in 44% yield. The synthesis yielded the thermodynamically stable Z- isomer for all the photoswitches. All compounds were characterized by 1H NMR and 13C{1H} NMR spectroscopies and high-resolution mass spectrometry. The alkene proton Ha was identified as a singlet that shifted upfield or downfield depending on the donor substituent (Figure 3-1). High-quality crystals for each photoswitch were obtained by recrystallization from i-propanol. The crystal structures of HTI, HTI-Nap, and HTI-Mes confirm the cis configuration (Figure 3-2, Appendix B). Scheme 3-1 Synthesis of hemithioindigo (HTI) analogs. HTI and HTI-Nap was synthesized beginning with 2-nitroacetophenone, and HTI-Mes was synthesized starting with (phenylthio)acetyl chloride. 53 Figure 3-1 1H NMR (400 MHz, CD2Cl2) spectra of HTI, HTI-Nap, and HTI-Mes at room temperature. The peak at 5.3 ppm is from the residual solvent. HTI HTI-Nap HTI-Mes Ha Ha Ha Figure 3-2 ORTEP of (a) HTI, (b) HTI-Nap, and (c) HTI-Mes drawn at 50% probability with only heteroatoms labelled for clarity. 54 3.3 Solution Photochromic Properties The photoisomerization of HTIs in solution can be monitored by 1H NMR spectroscopy. Taking HTI as an example, solutions of ~2.95 × 10-4 M in dichloromethane were irradiated with 405 nm light for 18 h to achieve the photostationary state (PSS) (Figure 3-3). The trans E- isomer formation results in a general downfield shift of the aromatic protons while the diagnostic alkene hydrogen peak (Ha at 8.0 ppm) is shifted upfield to 7.3 ppm (Hb). This shift can be attributed to the local environment of the proton. In the Z- isomer, the proton is closer to the carbonyl moiety, causing deshielding, while in the E- isomer, the proton is closer to the less electronegative sulfur atom. The PSS ratio of the Z: E isomer was determined to be 17: 83. Figure 3-3 Representative 1H NMR (400 MHz, CD2Cl2) spectra of (Z)-HTI before and (E)-HTI in CD2Cl2 after irradiation at 405 nm irradiation for 18 h at room temperature. Ha Hb Before irradiation After irradiation 55 The photoisomerization in solution can be also be monitored by UV-Vis absorption spectroscopy. The absorption profiles for HTI, HTI-Nap, and HTI-Mes show similar features with two major absorption peaks (Figure 3-4). The higher energy peak around 300 – 350 nm can be assigned as the π – π* transitions, while the lower energy band around 430 nm is due to the charge transfer character of the photoswitch. Upon 405 nm irradiation, the formation of the E- isomer results in reduced intensity of the charge transfer band accompanied by a slight bathochromic shift resulting in the yellow solution turning to an orange color. The formation of an isosbestic point indicates that the photoisomerization is the primary pathway without any side photoproducts. When re-irradiated with 515 nm light, reversion to the Z- isomer occurs, resulting in the initial absorption spectrum. Figure 3-4 UV-Vis spectra changes of (a) HTI, (b) HTI-Nap, and (c) HTI-Mes, in ~2.5 ×10-5 M CH2Cl2 upon reversible photoisomerization with 405 nm irradiation, followed by 515 nm irradiation. 56 3.4 Solid State Photochromic Properties and [2+2] Dimerization The HTI photoswitches exhibit different behavior in the solid state than what was observed in solution (Figure 3-5(a)). Upon localized 405 nm irradiation, crystalline thin films of HTI turn yellow to orange (Figure 3-5(b)). After the solid state reaction was completed, the sample was dissolved in deuterated dichloromethane, and 1H NMR spectroscopy was used to analyze the reaction products. Figure 3-5 (a) Solid state photoirradiation reaction, (b) Formation of solid state HTI-SSD (orange spot) upon 405 nm LED irradiation of HTI thin film. (c) 1H NMR (400 MHz, CD2Cl2) spectra of HTI (orange) before irradiation and HTI-SSD (black) after irradiation at room temperature. HTI HTI-SSD (a) (b) (c) Ha 57 The peak at 5.08 ppm is assigned to the cyclobutane hydrogen protons in the dimer pair formed during the solid state reaction. Additionally, the loss of the alkene proton peak at 8.0 ppm suggests that the formation of the solid state dimer pair occurs preferentially over isomerization. Examining the crystal structure of the HTI, photodimerization is expected as the distance between the alkene moieties in the monomer pairs is 3.882 Å, and they are in the correct geometrical orientation to meet Schmidt's topochemical requirements (Figure 3-6). HTI crystallizes in the monoclinic crystal system and forms head-to-head monomer pairs with alternating orientations, forming carbonyl-facing channels throughout the structure. The structure also contains other intermolecular interactions, namely, C-H···S (2.922 Å), C-H···O (2.664 Å), and C-H···C (2.853 Å). The dihedral angle between the thioindigo and phenyl ring is 8.01°, and the angle between the alkenes in the monomer pairs is 113.9°. Crystals of HTI were suspended in water and degassed with nitrogen. The solution was exposed to 405 nm light for 18 hours to produce HTI-SSD. The crystal structure of HTI-SSD reveals the dimer pair fused by the formation of the strained cyclobutane dimer (Figure 3-6(b)). The bond distance (1.574 Å) and angle (88.97°) between the dimer pair are consistent with other photodimers.80 Intriguingly, the photodimer has the thioindigo and phenyl fragments on opposite sides of the cyclobutane ring. This is unexpected since the packing modes of HTI would result in a photodimer with the fragments on the same side of the cyclobutane ring (Figure 3-6(d)). The presence of other polymorphs of HTI cannot be ruled out, however, no other phases were detected. Powder diffraction measurements show that the HTI crystalline sample is phase pure with differential scanning calorimetry producing a single sharp melting point of 131 – 132 °C (Figure 3-7(b)). The difference in intensity of the experimental and calculated powder diffraction patterns can be accounted for by temperature differences. At room temperature molecular motions can lead 58 to higher symmetries eliminating some diffraction peaks. The presence of HTI-SSD confirms that [2+2] photodimerization occurs in HTI crystalline samples, but further experiments are required to elucidate the origins of the photodimer HTI-SSD. Molecules of HTI-Nap also forms almost planar (dihedral angle between ring systems = 2.44°) monomer pairs, however these pairs stack in alternating head-to-tail orientations resulting in the carbonyl of the thioindigo fragment forming stabilizing π···π interactions (3.375 Å and 3.392 Å) with the neighboring naphthyl ring system (Figure 3-8(a)). These π···π interactions increase the stability of the monomer pairs, reducing the intermolecular distance between the alkene moieties by 8.5% with respect to HTI and the angle between the alkene double bonds in the monomer pairs is 95.64°. Figure 3-6 (a) HTI bond distance and angle of dimer pair. (b) Crystal structure of HTI-SSD with bond distance and angle of cyclobutane ring between the dimer pair shown. Packing diagrams viewed along the a-axis of (c) HTI and c-axis of (d) HTI-SSD recrystallized from i-propanol. 59 These factors further increase the propensity for the system to undergo [2+2] photodimerization. Other stabilizing intermolecular interactions include C-H···O (2.514 Å) and C-H···C (2.879 Å). When irradiated at 405 nm, photodimerization also occurs in HTI-Nap crystals. The crystal structure of HTI-Nap SSD shows the formation of the cyclobutane ring with the dimer pair with bond distances and angles consistent with the literature (Figure 3-8(b)). Figure 3-7 (a) Experimental (red) and calculated (blue) powder diffractograms of crystalline HTI. (b) Two differential scanning calorimetry heating (red and black) and cooling (blue colors) runs show two single melting and recrystallization temperatures. 60 Interestingly, the photodimer is disordered over two major positions, with the thioindigo fragments occurring between its E- and Z- isomers in a ratio of 53.1: 46.9. This suggests that photoisomerization occurs before photodimerization in the solid state. 1H NMR spectroscopy on the dissolved crystal further corroborates the occurrence of photoisomerization as two signals appear at 6.05 and 6.40 ppm corresponding to the cyclobutane hydrogen peak in two orientations (Figure 3-9). The crystal structure of HTI-Mes reveals that the packing arrangements of the alkenes in the monomer pairs are not appropriately aligned, with alkene distances at 4.473 Å and oriented at 155.49°. Therefore, isomerization is the more dominant process to occur during solid state irradiation (Figure 3-10). Figure 3-8 (a) HTI-Nap bond distance and angle of dimer pair. (b) Crystal structure of HTI-Nap SSD with bond distance and angle of cyclobutane ring between the dimer pair shown. Packing diagrams viewed along the a-axis of (c) HTI-Nap and (d) HTI-Nap recrystallized from dichloromethane. 61 The structure is further stabilized by C-H···O (2.696 Å), and C-H···O (2.478 Å), C-H···O (2.676 Å), and C-H···C (2.834 Å) intermolecular interactions. The photoisomerization is confirmed by 1H NMR spectroscopy as two peaks (8.06 and 7.42 ppm) are assigned to the alkene hydrogen in the Z- and E- isomer. Both solution and crystalline samples show the alkene peaks without any diagnostic cyclobutane hydrogen signals, further confirming that upon 405 nm irradiation, photoisomerization occurs within HTI-Mes (Figure 3-11). Figure 3-9 (a) 1H NMR (400 MHz, CD2Cl2) spectra of HTI-Nap before irradiation (orange) and (b) HTI-Nap SSD after irradiation (black) at room temperature. (a) (b) HTI-Nap HTI-Nap SSD 62 Figure 3-10 1H NMR (400 MHz, CD2Cl2) spectrum of (top) HTI-Mes before irradiation (blue) and (middle) HTI-Mes after solution irradiation for 18 h (orange) and (bottom) HTI-Mes after crystal irradiation. Solution irradiation Crystal irradiation Before irradiation Figure 3-11 Crystal structure of HTI-Mes with only heteroatoms and H1 labelled for clarity and packing diagram showing the orientations of the double bonds. 63 3.5 Photomechanical Effects and Photochromism in Crystalline HTI Photoswitches Interestingly, molecular crystals of HTI, HTI-Nap, and HTI-Mes display macroscopic mechanical motion when exposed to light with a wavelength of 405 nm. Long, thin needles of HTI bend towards the light source upon irradiation and continue to bend for ~40 seconds, after which no further motion is observed (Figure 3-12). For molecular crystals of HTI-Nap, multiple photomechanical effects were observed, such as bending, flipping, and photosalient effects. Upon 405 nm irradiation, thin plate-like crystals bend and deform at the localized area of irradiation (Figure 3-13). The irradiation results in localized fracturing of the crystal, disrupting the structural integrity. Other photosalient effects occur without any apparent destruction in the crystal. In this case, the crystal flips 90° onto its side face. This process can be repeated where the crystal performs a total of 540° rotation about its longest axis. These rotations are accompanied by twisting along the crystal, releasing strain buildup as motion. These temporary twisting events do not destroy the crystal and demonstrate the inherent plastic nature of crystalline HTI-Nap. Photoisomerization as the driving motion accompanying these mechanical events cannot be ruled out since the crystal structure contains disordered thioindigo and naphthyl fragments resulting from solid state photoisomerization. Figure 3-12 (a) Thin needle-like crystal of HTI at the start of 405 nm laser irradiation. (b) – (c) The thin needle of HTI undergoing photomechanical bending in response to irradiation. Scale bars are 400 µm. 64 Further investigations are required to fully elucidate the driving mechanism behind the photomechanical motion in HTI-Nap crystals. Prior to irradiation, HTI-Nap crystals, show no thermal events other than melting at 141 °C (Figure 3-14(a–b)). In contrast, other mechanical motion occurs when irradiated crystals are heated. Previously irradiated HTI-Nap crystals produced a second mechanical effect when heated to 90 °C where the needle-like crystal continues to bend over 90°, ultimately curving into a semi-circle before further heating induces the melting event (Figure 3-14(c-e)). It is speculated that the secondary motion results from a phase transition where the irradiated crystals are forced into a metastable state. Alternatively, the strained cyclobutane dimer becomes unstable, and dimer pairs reform, or the E- isomer present within the crystal thermally reverts to the Z- isomer, as described in Chapter 2. Figure 3-13 (a) Localized deformation of HTI-Nap crystals in the highlighted area. Scale bars are 400 µm. (b) Long plate-like crystal of HTI-Nap undergoing a photosalient hopping by twisting to 540°. * Is a crystal used as a reference point during motion of the larger crystal and the arrows highlight nondestructive twisting motion. 65 Lastly, molecular crystals of HTI-Mes undergo photoisomerization when irradiated with 405 nm light (Figure 3-15). Under the polarized optical microscope, the natural yellow color of the crystal is shown as green. Upon irradiation, the crystal changes color to orange in a wavefront-like manner similar to what is observed in thermal phase transitions. The transformation is completed within 30 seconds. This birefringence color change results from photoisomerization and the rearrangement of the molecules in the lattice upon formation of the E- isomer. Figure 3-14 Single crystals of HTI-Nap before (a) and after melting (b) by heating to 141 °C. Scale bars are 1 mm (c) Irradiated microcrystal of HTI-Nap undergoing further bending in response to heating to 130 °C. Scale bars are 100 µm. 66 3.6 Conclusions In conclusion, hemithioindigo and three analogs were synthesized and characterized. All of the hemithioindigo photoswitches display photochromic properties and undergo reversible photoisomerization upon irradiation with light. Crystal structures of the analogs reveal that HTI and HTI-Nap adopt an almost planar geometry and form monomer pairs that may undergo [2+2] photodimerization upon irradiation with light, displaying photochromism in the solid state. In molecular crystals, the [2+2] photodimerization can be employed as a topochemical reaction to drive photomechanical motion such as bending, flipping, rocking, and photosalient effects. Further mechanical motion can be realized in molecular crystals of HTI-Nap by applying thermal energy to induce further structural changes leading to bending of linear needle-shaped crystals to form semi-circle-shaped needles. In the case of HTI-Mes, the crystal structures reveal that the bulky Figure 3-15 (a) Polarized optical microscopy image of a single plate-like crystal of HTI-Mes before irradiation. (b) Upon 405 nm irradiation, focused on the corner of the crystal, solid state isomerization occurs in a wave-front like manner. (c) – (d) Phase transformation of the crystal results in a color change from green to orange under the polarizer. Scale bars are 400 µm. 67 methyl groups in the mesityl moiety disrupt the inherent planar nature and monomer pair formation within the lattice, with this disrupting [2+2] photodimerization and enabling photoisomerization as the dominant photoreaction instead, which can also be used to drive photomechanical motion and photochromism in molecular crystals. While HTI has been well studied in solution and solid state photoisomerization in amorphous samples has been reported for HTI-Mes, the work reported in this chapter is the first example of the application of topochemical [2+2] photoisomerization to drive photomechanical motion in molecular crystal entities based on hemithioindigo photoswitches. This work establishes hemithioindigo as a viable photoswitch for photomechanical motion in two discrete pathways based on facile chemical modification: [2+2] photodimerization and photoisomerization, which is uncommon for other classical, well-established photoswitches. 3.7 Experimental 3.7.1 General Solvents were of reagent grade and were used without any further purification. HPLC grade solvents were used for analyses. All purchased chemicals were used without further purification. HTI was synthesized following the literature procedure. 3.7.2 NMR Spectroscopy The NMR spectra were recorded at 25 °C on an Avance 400 spectrometer (Bruker, 400 MHz). Chemical shifts (in ppm) are reported relative to the non-deuterated residual solvent signals of CD2Cl2 (δ = 5.33 ppm). 68 3.7.3 UV-Vis Spectroscopy A Perkin Elmer UV-vis spectrometer was used to record the spectra in solution (Lambda 35) and in solid state (Lambda 365). E–Z Reactions were monitored using UV-visible absorption spectroscopy where spectra of the compounds were recorded in dichloromethane solution with concentration 2.5 × 10‒5 M. The solution was irradiated with a 405 nm LED for 30 seconds to photoisomerize (Z)-HTI to (E)-HTI. After no further transformations were observed, the sample was irradiated with 515 nm LED light to isomerize (E)-HTI to (Z)-HTI. 3.7.4 X-ray Diffraction Single crystal X-ray data were collected using a Bruker APEX DUO diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 100 K. Raw frame data were processed using APEX2.109 The program SAINT+, v. 7.68110 was used to reduce the data, and the program SADABS111 was used to make corrections to the empirical absorptions. Space group assignments were made using XPREP 110 on all compounds. In all cases, the structures were solved in the Olex2112 suite of programs using Intrinsic Phasing and refined using full-matrix least-squares/difference Fourier techniques on F2 using SHELXL.113 All non-hydrogen atoms were refined anisotropically. Diagrams and publication material were generated using CrystalMaker. Powder X-ray diffraction was collected using a Bruker D8 Advance Powder Diffractometer, and data was analyzed using Bruker DiffracPlus EVA software. 69 3.7.5 Hot-Stage POM Microscopy Microscopy images were collected on an Olympus BX53M polarizing microscope equipped with an Instec hot stage. Thermal and photosalient effects were recorded with an Olympus UC90 camera equipped with a λ = 530 nm filter. 3.7.6 Differential Scanning Calorimetry (DSC) The differential scanning calorimetry experiments were performed on NETZSCH DSC 214 Polyma instrument with aluminum sample pans. Heating was carried out in the temperature range from 25 °C to 230 °C at a rate of 5 °C min‒1. 3.7.7 High-Performance Liquid Chromatography (HPLC) The HPLC was performed using an Agilent Technologies 1260 Infinity and an Agilent ZORBAX Extend-C18 column with 4.6 x 150 mm dimensions. The isocratic mobile phase was composed of 90% acetonitrile, 10% water with a flow rate of 1 mL min-1 at 35 ℃. The detector wavelength was set to 260 nm. 3.7.8 Synthesis HTI 2-Nitroacetophenone (100 mg, 0.61 mmol) and NaOH (26.6 mg, 0.67 mmol, 1.1 eq.) were dissolved in EtOH (5 mL) and stirred for 30 minutes at 0 °C. Benzaldehyde (64.7 mg 0.61 mmol, 70 1 eq.) dissolved in 5 mL of EtOH was added dropwise to the reaction mixture and further stirred at 0 °C for 30 minutes. The reaction mixture was allowed to return to r.t and stirred overnight (18 h). The crude product was collected via vacuum filtration and gently washed with cold EtOH (10 mL) and recrystallized from hot EtOH. The white product 2-nitrochalcone, (E)-1-(2-nitrophenyl)-3-phenylprop-2-en-1-one was used without any further purification. Yield 105 mg, 68%. The 2-nitrochalcone (100 mg, 0.4 mmol) was added to elemental sulfur (64 mg, 2 mmol, 5 eq.), dimethylsulfoxide (156 mg, 2 mmol, 5 eq.), and triethylamine (202 mg, 2 mmol, 5 eq.) and stirred at r.t under nitrogen for 18 h. The reaction mixture was diluted with dichloromethane (20 mL) and filtered through a silica plug. The filtrate was dried with Na2SO4, filtered and solvent evaporated in vacuo. The crude mixture was purified using silica gel column chromatography (hexanes/ethyl acetate 10:0 – 8:2) to yield (Z)-2-benzylidenebenzo[b]thiophen-3(2H)-one, HTI as a yellow solid (60 mg, 63.8%) (mp 131 °C). 1H NMR (400 MHz, CD2Cl2) δ 7.98 (s, 1H), 7.95 (ddd, J = 7.7, 1.4, 0.7 Hz, 1H), 7.81 – 7.74 (m, 2H), 7.66 (ddd, J = 8.4, 7.1, 1.3 Hz, 1H), 7.62 – 7.51 (m, 3H), 7.52 – 7.44 (m, 1H), 7.37 (ddd, J = 7.7, 7.1, 1.1 Hz, 1H). 13C{1H} NMR (101 MHz, CD2Cl2) δ 188.3, 146.0, 135.3, 134.3, 133.1, 130.9, 130.4, 130.3, 130.1, 129.1, 126.8, 125.7, 124.0. HR-ESI MS: m/z calcd. for C15H10OS: 238.0571; found: 239.2008 [M+H]+. HTI-Nap 2-Nitroacetophenone (100 mg, 0.61 mmol) and NaOH (26.6 mg, 0.67 mmol, 1.1 eq.) were dissolved in EtOH (5 mL) and stirred for 30 minutes at 0 °C. 1-Naphthaldehyde (64.7 mg 0.61 71 mmol, 1 eq.) dissolved in 5 mL of EtOH was added dropwise to the reaction mixture and further stirred at 0 °C for 30 minutes. The reaction mixture was allowed to return to r.t and stirred overnight (18 h). The crude product was collected via vacuum filtration and gently washed with cold EtOH (10 mL), and recrystallized from hot EtOH. The yellow product 2-nitrochalcone (E)-3-(naphthalen-1-yl)-1-(2-nitrophenyl)prop-2-en-1-one was used without any further purification. Yield 120 mg, 65%. The 2-nitrochalcone (100 mg, 0.33 mmol) was added to sulfur (52.5 mg, 1.64 mmol, 5 eq.), dimethylsulfoxide (128 mg, 1.64 mmol, 5 eq.), and triethylamine (166 mg, 1.64 mmol, 5 eq.) and stirred at r.t under nitrogen for 18 h. The reaction mixture was diluted with dichloromethane (20 mL) and filtered through a silica plug. The filtrate was dried with Na2SO4, filtered and solvent evaporated in vacuo. The crude mixture was purified using silica gel column chromatography (hexanes/ethyl acetate 10:0 – 8:2) to yield (Z)-2-(naphthalen-1-ylmethylene)benzo[b]thiophen-3(2H)-one, HTI-Nap as an orange solid (58 mg, 61%) (mp 141.5 °C). 1H NMR (400 MHz, CD2Cl2) δ 8.75 (s, 1H), 8.39 – 8.12 (m, 1H), 8.09 – 7.84 (m, 4H), 7.78 – 7.53 (m, 5H), 7.38 (ddd, J = 7.9, 7.1, 1.0 Hz, 1H). 13C{1H} NMR (101 MHz, CD2Cl2) δ 187.9, 146.5, 135.3, 133.8, 133.1, 132.3, 131.4, 130.9, 130.7, 129.6, 128.9, 127.7, 127.1, 126.9, 126.5, 125.6, 125.4, 124.1, 123.6. HR-ESI MS: m/z calcd. for C15H10OS: 288.0623; found: 289.1515 [M+H]+. HTI-Mes 72 (Phenylthio)acetyl chloride (150 mg, 0.8 mmol) was dissolved in 1,2-dichloroethane (10 mL) and cooled to 0 °C. AlCl3 (128 mg, 0.96 mmol, 1.2 eq.) was added in portions over 10 mins, and the reaction mixture was stirred for an additional 30 mins at 0 °C then for 2 h at r.t. The reaction mixture was poured over ice, and the aqueous phase was extracted with dichloromethane (3x 20 mL). The organic phases were collected and dried over Na2SO4, and solvent removed in vacuo to yield the crude benzo[b]thiophen-3(2H)-one (103 mg, 68%) as a red oil which was used without further purification. Benzo[b]thiophen-3(2H)-one (100 mg, 0.67 mmol), mesitaldehyde (119 mg, 0.8 mmol, 1.2 eq.) and 1 drop of piperidine was dissolved in toluene (20 mL) and the reaction mixture was heated to reflux for 2 h. After cooling to r.t the reaction mixture was quenched with sat. ammonium chloride solution and the aqueous phase was extracted with ethyl acetate (3x 25 mL). The organic phases were combined and dried over Na2SO4 and solvent removed in vacuo. The crude product was purified using column chromatography (SiO2, hexanes/ethyl acetate 95:5) and recrystallized from i-propanol to yield pale yellow needles of HTI-Mes (88 mg, 44%) (mp 104 °C). 1H NMR (400 MHz, CD2Cl2) δ 8.04 (s, 1H), 7.92 (ddd, J = 7.7, 1.4, 0.7 Hz, 1H), 7.61 (ddd, J = 7.9, 7.2, 1.4 Hz, 1H), 7.46 (dt, J = 7.9, 0.9 Hz, 1H), 7.41 – 7.22 (m, 1H), 6.97 (s, 2H), 2.35 (s, 3H), 2.27 (s, 6H). 13C{1H} NMR (101 MHz, CD2Cl2) δ 187.2, 146.4, 138.4, 136.6, 135.7, 135.4, 134.0, 131.5, 131.3, 128.5, 126.8, 125.3, 124.0, 20.8, 19.9. HR-ESI MS: m/z calcd. for C15H9ClOS: 272.0132; found: 273.1235 [M+H]+. 73 HTI-SSD HTI (10 mg) was suspended in 10 mL H2O degassed solution and with strong stirring. The suspension was then irradiated with 405 nm LED light for 18 h until the suspension turned orange. The reaction products were extracted using ethyl acetate (3x 15 mL), the organic layers were collected and dried over Na2SO4, and the solvent evaporated in vacuo. The crude material was further purified by column chromatography (SiO2, hexanes/ethyl acetate 95:5) to yield an orange solid HTI-SSD, 30% yield, recrystallized from i-propanol to yield crystals suitable for X-ray crystallography. 1H NMR (400 MHz, CD2Cl2) δ 8.05 (dd, J = 7.8, 1.4 Hz, 2H), 7.77 – 7.70 (m, 2H), 7.59 – 7.53 (m, 2H), 7.46 – 7.38 (m, 3H), 7.20 – 7.07 (m, 6H), 6.57 (dq, J = 6.8, 1.3 Hz, 3H), 5.08 (s, 2H). 74 Chapter 4: Fine Tuning ‘Afterglow’ Emission of Carbazole Based Luminogens with Organic Alloying 4.1 Introduction Organic luminescent materials are actively being investigated for use in chemical sensors, bioimaging agents, and organic light-emitting diode (OLED) based displays.7,125,126 To enhance OLED efficiencies, triplet excitons can be harvested and effectively utilized by employing processes such as thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). Section 1.6 details the background on TADF and RTP processes. Briefly, by reducing the singlet-triplet energy gap (ΔEST) to < 0.12 eV, efficient intersystem crossing can be achieved, enabling TADF and RTP processes with high efficiency.127–129 Additionally, according to El Sayed’s rule, RTP can be further realized by utilizing mixed (n, π*) and (π, π*) transitions to enhance intersystem crossing (ISC) and extend phosphorescence lifetimes.54,130 Further suppression of non-radiative processes and protection from singlet oxygen quenching occurs in the crystalline state, enabling higher emission efficiencies for both TADF and RTP processes at room temperature. Host compounds are typically non-emissive and serve as rigid matrices for triplet emission. Co-crystallization of a small percentage of a phosphorescent guest compound with a structurally similar non-emissive host resulted in a 20-fold phosphorescence enhancement compared to the pure guest crystal alone.70 Zhang and coworkers developed an efficient TADF emitter with long lifetimes based on a doped host-guest crystalline system. However, host-guest materials suffer from wide-range tunability of either lifetime or efficiency due to the limited compositional ratio range, as guest materials usually comprise a small fraction within the host matrix.70 On the other hand, organic alloys offer an opportunity to fine-tune the 75 photoluminescent properties of organic emitters over a larger compositional ratio. Organic alloys consist of binary or ternary organic compounds homogenized within a lattice. Isostructural guests and host molecules occupy sites randomly, and substitution can occur over the entire compositional range within the structure.131–133134 Furthermore, organic alloys have been used to tune optoelectronic behavior and energy levels of organic semiconductors for applications such as organic solar cells (OSCs) and organic field-effect transistors (OFETs),135–138 In this Chapter, crystalline organic alloys are used to tune contrasting photoluminescent properties of two structurally similar organic emitters for the first time. In this proof-of-concept study, one component has a longer emission lifetime, with the other component displaying a higher photoluminescence quantum yield. By crystallizing the two components in different ratios, TADF and RTP processes can be finely tuned as well as avoid commonly seen self-quenching in higher concentrations of doped host-guest systems. 4.2 Synthesis and Characterization 4.2.1 Molecular Synthesis Scheme 4-1 Synthesis of CBZ-Me and CBZ-Br 76 The two components used in the organic alloys are CBZ-Me ((9H-carbazol-9-yl)(p-tolyl)methanone) and CBZ-Br (4-bromophenyl)(9H-carbazol-9-yl)methanone (Scheme 4-1). The components were synthesized using a reported procedure where the 4-methylbenzoic acid or 4-bromobenzoic acid is converted into its acid chloride form and reacted with deprotonated carbazole in an inert environment. Both compounds have been previously reported, where CBZ-Me is an RTP emitter with 21 ms (@ 453 nm) and 340 ms (@ 530 nm) lifetime and a PLQY of 0.02.6 CBZ-Br crystallized into two polymorphs with different photophysical properties, CBZ-Br and CBZ-Br-RTP (Figure 4-1, Table 4-1).60,139 CBZ-Br is isostructural with CBZ-Me and shows TADF, with a delayed lifetime of 3.4 µs and a PLQY of 0.18. The contrasting difference in lifetime and PLQY between CBZ-Me and CBZ-Br is speculated to result from the methyl group enabling efficient intermolecular interactions and the bromine atom contributing to the heavy atom effect and effectively facilitating RISC.140 Figure 4-1 Typical crystal morphology of (a) CBZ-Br (RTP) (b) CBZ-Br (TADF) and (c) CBZ-Me. Scale bars are 200 µm. 77 Table 4-1 Literature unit cell parameters of CBZ-Br (RTP), CBZ-Br and CBZ-Me CBZ-Br (RTP) CBZ-Br CBZ-Me CCDC No. 2016853 1533213 1503835 a (Å) 19.509(4) 11.5066(15) 11.6578(3) b (Å) 5.1417(10) 10.2017(13) 10.0346(3) c (Å) 15.023(3) 12.349(16) 12.3140(4) α (°) 90 90 90 β (°) 103.54(3) 95.332(4) 94.619(3) γ (°) 90 90 90 Volume (Å3) 1465.07 1443.42 1435.83 Space group P21/c P21/n P21/n 4.2.2 Organic Alloy Synthesis CBZ-Me and CBZ-Br are soluble in acetonitrile across the molar ratio range, allowing the growth of large (~1 mm × ~1 mm × ~1 mm) high-quality alloy crystals (MexBr10-x, where x = 3 to 9) by slow evaporation (Figure 4-2, Table 4-2, Appendix C). Acetonitrile also promoted the crystallization of the CBZ-Br polymorph over the CBZ-Br-RTP polymorph allowing further access across the molar range by isostructural substitution. Attempts to produce organic alloy crystals with compositions containing CBZ-Br percentages higher than 70% were unsuccessful due to CBZ-Br crystallizing as a mixture of the CBZ-Br-RTP polymorph and alloy crystals with CBZ-Me compositions of approximately 30%. 78 Table 4-2 Unit cell parameters for organic alloy MexBr10-x where x = 3 to 9 Me3Br7 Me4Br6 Me5Br5 Me6Br4 Me7Br3 Me8Br2 Me9Br1 a (Å) 11.5265(6) 11.5680(7) 11.5657(6) 11.5850(7) 11.5964(1) 11.6030(8) 11.6141(6) b (Å) 10.1543(5) 10.1429(5) 10.1316(5) 10.1100(6) 10.0911(8) 10.0711(6) 10.0515(5) c (Å) 12.3229(7) 12.3305(7) 12.3187(6) 12.3005(7) 12.3080(1) 12.3038(8) 12.2961(6) β (°) 95.063(2) 95.097(3) 95.031(2) 94.882(4) 94.854(2) 94.846(2) 94.745(1) Volume (Å3) 1436.69(1) 1441.05(1) 1437.92(12) 1435.46(4) 1435.1(2) 1432.62(16) 1430.52(12) Figure 4-2 Molecular crystals of CBZ-Br and organic alloy MexBr10-x where x = 3 to 9, grown from slow evaporation of acetonitrile crystallization experiments. 79 4.2.3 Structural Characterization Similar to the crystals of the pure compounds CBZ-Me and CBZ-Br, the organic alloys are isostructural and crystallize in the P21/n space group with one molecule in the asymmetric unit. Crystallographically, the organic alloys are observed as a substitutional disorder where the molecules occupy the exact fractional coordinates in the lattice and refining the Me- and Br- groups using a two-part disorder model (Figure 4-6(a)). Figure 4-3 1H NMR (400 MHz, CD2Cl2) spectral analysis of CBZ-Me, CBZ-Br and the organic alloys MexBr10-x where x is 3 to 9 at room temperature. Peaks marked with stars and triangles were used in integration analysis to determine the ratio of CBZ-Me : CBZ-Br in each organic alloy sample. Peaks at 5.3 ppm and 1.5 ppm correspond to residual solvent and water respectively. Peaks between 1.0 ppm and 2.0 ppm are impurity peaks resulting from paratone oil used to mount crystals on a loop for X-ray diffraction. 80 The site occupancy factors obtained from the crystal structures match the stoichiometric ratios determined by solution 1H NMR spectroscopy upon dissolution of the alloy crystals (Figure 4-3, Table 4-3). With an increasing percentage of CBZ-Me in the alloy, the unit cell axes remain essentially unchanged with a slight decrease of 0.32° in the β angle tending toward the pure CBZ-Me structure (Figure 4-4). The unit cell volume decreases with increasing CBZ-Me percentage as the van der Waals radii of the Me- group occupies 1.53 Å3 less volume than the Br- group at the para position.141 Furthermore, short-range order (SRO) defects usually occur in disordered materials giving rise to diffuse scattering and Bragg diffraction peaks.142,143 These organic alloys produce very low-Figure 4-4 (a) Unit cell dimension variation of CBZ-Me and MexBr10-x where x is 3 to 9. (b) Decreasing β angle with increasing CBZ-Me composition (c) Decreasing unit cell volume with increasing CBZ-Me composition. 81 intensity diffuse scattering, indicating that the SRO populations are very low and that the two components are homogenous within the lattice (Figure 4-5). The homogeneity of the organic alloys was probed by differential scanning calorimetry (DSC) to assess the melting points of the parent compound and crystal alloys (Figure 4-6(b)). CBZ-Br (TADF polymorph) melts at 156 °C, and with increasing CBZ-Me composition, the melting point of the crystal alloys increases, with pure CBZ-Me melting at 177 °C. The linearity of the melting point range is consistent with the alloys being comprised of a single-phase system with homogenous distribution of the components in the crystals.134 Figure 4-5 Representative diffraction pattern of Me5Br5 showing Bragg peaks and noise from the detector as view in MAX3D (Britten/Guan). The absence of diffuse scattering peaks or streaks indicates that the amount of short range order in the disordered regions is very low. Bragg peaks are the major yellow spots with background noise between the Bragg peaks. 82 4.3 Solid State Photophysical Properties 4.3.1 Photophysical Properties of Parent Compounds The photophysical properties of the parent compounds CBZ-Me and CBZ-Br were reexamined in the single-crystal state. The fluorescence emission spectrum of CBZ-Me is consistent with the reports by Liu and coworkers; however, the phosphorescence emission spectrum only shows a broad featureless delayed spectrum consistent with the literature (Figure 4-7(a)). 144 This emission feature has a long-lived TADF feature at 450 nm with a 38 ms lifetime (Figure 4-7(b)). Variable temperature emission experiments reveal that the emission feature becomes further redshifted with a decrease in temperature while the delayed lifetime feature increases with a rise in temperature (Figure 4-7(c)). A longer lifetime (>300 ms) for emission Figure 4-6 (a) Representative ORTEP of substitutional disorder for organic alloy (MexBr10-x, x = 3 to 9) system drawn at 50% probability, hydrogens are plotted as spheres. Only heteroatoms and the methyl group are labelled for clarity. (b) DSC traces of organic alloys (MexBr10-x, x = 3 to 9) where endotherms are labeled with their corresponding composition ratio, Me4Br6, Me6Br4, Me9Br1 labels are excluded for clarity. (Inset: Melting point temperatures as a function of CBZ-Me percentage.) 83 >500 nm is present, which is assigned as RTP; however, this feature is relatively low in amplitude compared to the prompt fluorescence. CBZ-Br, on the other hand, shows a delayed lifetime (µs) consistent with previous reports for this compound.138 This compound shows the spectral overlap between the steady-state and delayed emission spectrum (1 ms delay) assigned as TADF. This TADF also has a long-lived emission at 76.3 ms (Figure 4-8(a-b)). In the lower energy region, the emission is extended to 143 ms, preventing accurate measurements of the delayed photoluminescence spectrum. This extended lifetime is tentatively assigned as RTP, and this energy state could serve as a reservoir of triplet excitons to power the TADF emission.145 Lastly, the PLQY values of both parent compounds (44% for CBZ-Br and 11.8% for CBZ-Me) are considerably higher than reported in the literature, possibly due to sample crystallinity or instrumental differences. 84 Figure 4-7 Steady-state (blue line) and 1 ms delay (blue line) photoluminescence spectrum of CBZ-Me (a) under air at room temperature and (e) under helium at 77 K. (b) Phosphorescence decay of CBZ-Me measured under air at room temperature at 450 nm, 530 nm and above 550 nm. Variable-temperature (c) steady-state photoluminescence spectrum and (d) phosphorescence decay of CBZ-Me under helium atmosphere. 85 4.3.2 Photophysical Properties of Organic Alloys Steady-state and time-gated photoluminescence measurements were carried out on the alloy crystals, MexBr10-x, where x = 3 to 9. The steady-state and delayed emission spectra of the alloys show significant overlap throughout the compositional range. However, the steady-state spectra show a blue shift toward 453 nm with an increasing CBZ-Me component (Figure 4-9). While the alloys’ delayed emission spectra (1 ms delay) remain identical with a broad emission Figure 4-8 (a) Steady-state (blue line) and 1 ms delay (orange line) photoluminescence spectrum of CBZ-Br under air at room temperature and (c) under helium at 77 K. (b) Microsecond photoluminescence decay of CBZ-Br under air at room temperature. (d) Millisecond photoluminescence decay of CBZ-Br above 500 nm under air at room temperature. 86 peak with a maximum of 475 nm across the compositional range. The photoluminescence decay of the alloy crystals consists of both a prompt and delayed component. The alloy crystals show an increasing delayed lifetime decay component with an increase in CBZ-Me composition. Alloy crystal Me3Br7 has the lowest lifetime (107 ms) in the alloy series but has a greater amplitude and is a 1.5-fold to 3-fold improvement to the parent compounds delayed lifetimes (CBZ-Br at 76.3 ms and CBZ-Me at 37.8 ms, respectively) where alloy crystal Me9Br1 shows the most extended delayed lifetime of a 197 ms which is a 2-fold increase to alloy crystal Me3Br7 (Figure 4-10(a), Table 4-3). By lowering the temperature to 77 K, non-radiative pathways are further suppressed, greatly enhancing the lifetimes with a similar trend observed at room temperature. By varying the compositional ratio of the parent compounds in organic alloys, the emission lifetime is strongly enhanced and can be finely tuned. Figure 4-9 Steady-state (solid line) and delayed (1ms) (dashed line) photoluminescence spectrum of organic alloys. 87 The photoluminescence quantum yields of the alloys show an opposite trend compared to the lifetimes, where a higher CBZ-Br composition results in increased luminescence efficiency. Organic alloy Me3Br7 has the highest efficiency at 26.7%, while Me9Br1 only has an efficiency of 14% (Figure 4-11 (b)). The afterglow emission can be observed by eye after the cessation of UV light excitation. Figure 4-11(a) shows the afterglow effect of the alloy crystals captured using a digital single-lens camera. The alloy crystals display the most prolonged afterglow emission compared to the parent compounds despite CBZ-Br having the highest overall luminescence and Figure 4-10 Phosphorescence decay of organic alloys under air at (a) room temperature and at (b) 77K. 88 the most extended RTP lifetime of CBZ-Me. The short, delayed lifetime of CBZ-Br and weak luminescence of CBZ-Me resulted in a poor afterglow for the parent compounds, whereas the organic alloys provide an opportunity to blend both components and achieve enhanced photoluminescent properties. Figure 4-11 (a) Photos of CBZ-Br, Me3Br7, Me5Br5, Me9Br1 and CBZ-Me upon the removal of UV excitation. (b) Plots of PLQYs and delayed lifetimes against CBZ-Me composition. 89 Table 4-3 Mole ratio and site occupancy factors for organic alloy MexBr10-x where x = 3 to 9, and the corresponding photophysical parameters. CBZ-Me: CBZ-Br Ratio Lifetime 77K Phosphorescence PLQY (%) Crystallization Mole Ratio Site Occupancy Factor 1H NMR Integration Ratio Long lived (ms) Fluorescence (ns) Lifetime (ms)[b] Me3Br7 30:70 35.91(15) : 64.09(15) 29.4 : 70.6 107 1.38 29.3 (0.03), 271 (0.421), 558 (0.549) 14.0 Me4Br6 40:60 43.99(12) :56.01(12) 40.1 : 59.9 119 1.38 291. (0.036), 302 (0.479), 570 (0.585) 10.2 Me5Br5 50:50 52.76(12) : 47.24(12) 50.9 : 49.1 126 1.26 37.1 (0.01), 353 (0.431), 621 (0.559) 19.7 Me6Br4 60:40 55.70(2) : 44.30(2) 61.4 : 38.6 134 1.25 36.6 (0.012) 370 (0.4), 657 (0.588) 10.8 Me7Br3 70:30 71.32(11) : 28.68(11) 72.2 : 27.8 140 1.19 44.2 (0.01), 387 (0.349), 693 (0.641) 26.9 Me8Br2 80:20 81.25(9) : 18.75(9) 75.9 : 24.1 170 1.02 670 24.1 Me9Br1 90:10 91.21(8) : 8.790(8) 89.9 : 10.1 197 0.9 548 (0.456), 785 (0.544) 26.7 a Relative amplitudes in parentheses. 90 4.3.3 Origin of Emission Variable-temperature emission experiments were carried out on the organic alloys to elucidate the source of the delayed emission. The organic alloys show structured emission peaks at 485 and 517 nm when below 200 K. As the temperature increases to 250 K, the vibronic character of the bands become structureless, confirming the temperature-dependent phosphorescence arises from a locally excited state and not from carbazole isomers at low temperatures (Figure 4-12).6,146,147 From 265 K to 295 K, a broad blue-shifted emission band with a maximum of 460 nm occurs. The photoluminescent decay lifetimes are also sensitive to increasing temperature as the delayed features with shortened lifetimes grow in amplitude (Figure 4-13). Triplet-triplet annihilation as an energy transfer mechanism cannot be ruled out and excitation power dependence studies would be required. However, these studies were not conducted due to instrumental limitations. These results suggest TADF is the primary room temperature process with the prolonged decay tail at lower energy assigned as an RTP process. Additionally, the blue-shifted steady-state spectrum of the organic alloys with higher CBZ-Me composition may result from the prompt emission of parent CBZ-Me, consistent with observations in TADF doped films.148,149 The organic alloys’ lifetime enhancement and PLQY tunability can be tentatively explained using the cluster exciton model.150 The hidden RTP states of the parent compounds generate so-called cluster excitons, which ‘trap‘ and help stabilize the triplet excitons and undergo RISC and emit TADF.145 Furthermore, the presence of the bromine atom may promote RISC over RTP emission, as seen in parent CBZ-Br where TADF is the primary emission process. Further 91 investigations are needed to fully understand the underlying mechanism behind the TADF lifetime enhancement and PLQY tunability. 92 Figure 4-12 Variable-temperature steady-state photoluminescence spectrum of organic alloys. 93 Figure 4-13 Variable-temperature phosphorescence decay of organic alloys under helium atmosphere. 94 4.4 Conclusions In conclusion, seven crystalline organic alloys (MexBr10-x, where x = 3 to 9) were employed to demonstrate the ability to tune the photoluminescent lifetime and efficiency of long-lived emissive materials. The composition of the organic alloys was assessed by X-ray crystallography, 1H NMR spectroscopy, and differential scanning calorimetry. The compositions of the organic alloys were determined by the site occupancies of the methyl and bromine atoms in the crystal structures and further confirmed by 1H NMR spectroscopy experiments. The organic alloys were composed of a single phase and exhibited a linear increase in melting points with increasing CBZ-Me composition. The diffuse scattering show very low short range order populations further indicating phase homogeneity throughout the structure. The organic alloys display afterglow emission observable by the human eye, which is assigned as long-lived TADF with lifetimes 1.5- to 6-fold enhanced over the parent compounds. Furthermore, the organic alloys solve the trade-off between lifetime and PLQYs by tuning the composition. Over the last few years, intense interest in organic RTP and TADF materials has resulted in many new emitters being developed. Furthermore, the use of host-guest design principles can be employed to enhance long-lived luminescent properties. On the other hand, organic alloys can be employed to minimize self-quenching by using crystalline triplet emitters and over a wide range of tunability and further enhancing the photoluminescence performance of these commonly host-guest organic emitters. In addition, organic alloys could be potentially expanded into a ternary component alloy system or combined with host-guest systems to offer numerous opportunities in both material design and practical applications. 95 4.5 Experimental 4.5.1 General Solvents used were reagent grade and used without any further purification. HPLC grade solvents were used for analyses. 9-H carbazole was purchased from Sigma-Aldrich (China), and purity was confirmed by HPLC. Other purchased chemicals were used without further purification. CBZ-Br and CBZ-Me were synthesized according to the literature procedure. Afterglow photos were taken using a Nikon D750 camera equipped with Sigma 35 mm F1.4 DG HSM lens (F:1.4, Exposure time: 0.1 s and ISO: 1600). HPLC was performed using an Agilent Technologies 1260 Infinity and an Agilent ZORBAX Extend-C18 column with 4.6 x 150 mm dimensions. The isocratic mobile phase was composed of 25% acetonitrile, 75% water with a flow rate of 3 mL/min. The detector wavelength was set to 292 and 360 nm. 4.5.2 X-ray Diffraction Single crystal X-ray data were collected using a Bruker APEX DUO diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 100 K. Raw frame data were processed using APEX2.109 The program SAINT+, v. 7.68110 was used to reduce the data, and the program SADABS111 was used to make corrections to the empirical absorptions. Space group assignments were made using XPREP110 on all compounds. In all cases, the structures were solved in the Olex2112 suite of programs using Intrinsic Phasing5 and refined using full-matrix least-squares/difference Fourier techniques on F2 using SHELXL.113 All non-hydrogen atoms were refined anisotropically. Diagrams and publication material were generated using CrystalMaker. 96 4.5.3 POM Microscopy Microscopy images were collected on an Olympus BX53M polarizing microscope with an Olympus UC90 camera equipped with a λ = 530 nm filter. 4.5.4 Spectroscopy 1H NMR data were collected using a Bruker AV-400 spectrometer and referenced first to the residual protonated solvent peak. NMR solvents (Sigma Aldrich) were used as received. Steady-state and time-resolved photoluminescence spectra were collected using a Photon Technology International (PTI) QuantaMaster 400 fluorimeter utilizing a 75 W Xe arc lamp and a Horiba 359 nm Spectral LED as the sources, respectively. Fluorescence lifetime data were collected using QuantaMaster 400 with a 375 nm pulsed laser. Phosphorescence lifetime data were collected using Horiba Yvon Fluorocube with a 359 nm Horiba spectral LED with band-pass and long-pass filters. The photoluminescence lifetime data were fitted using the DAS6 Data Analysis software package. Samples for low-temperature spectroscopy and lifetime measurements were cooled using an Oxford Instruments Optistat DN. 4.5.5 Quantum Yields Quantum yields (QY) were measured using an Edinburgh Instruments barium sulfate coated integrating sphere (diameter, SC-30 sample module) fitted on an FS5 spectrofluorometer. Single crystal samples were placed in the middle of a 10 mm diameter PTFE sample holder (tray), enclosed by a quartz cap (Figure 4-14). The typical size of crystals was approximately 0.5 × 0.5 × 0.5 mm. The excitation wavelength was set to 380 nm. The sample was excited by direct illumination. The settings were set to maximize the emission signal before saturating the detector 97 with excitation light (excitation bandwidth = 12 nm, emission bandwidth 0.55-0.60 nm). The reference sample was measured with just the tray and cap (no single crystal). Each sample was run in triplicate, with each quantum yield measurement coming from a new crystal (if available). Measurements proved reproducible, and errors are given as standard deviations. Figure 4-14 Pull-out drawer for Edinburgh Instruments SC-30 sample module with solid sample holder (PTFE tray) and quartz cap. The crystal is illuminated with 365 nm excitation under ambient lighting. 4.5.6 Synthesis CBZ-Me and CBZ-Br were synthesized using the same procedure.6 The respective substituted benzoic acid (10 mmol) was dissolved in thionyl chloride (50 mmol, 5 eq.) and heated to reflux for 4 h. Excess thionyl chloride was diluted with hexanes (3 × 15 mL) and the solvent evaporated in vacuo. The crude benzoic acid chloride was used without further purification. Carbazole (20 mmol, 2 eq.) was dissolved in dry THF (15 mL) and cooled to 0 °C under an N2 atmosphere. NaH (22 mmol, 2.2 eq) was added to the solution and stirred for a further 30 minutes at 0 °C. The crude acid chloride was dissolved in dry THF (10 mL) and added dropwise to the 98 carbazole reaction mixture. The reaction was stirred at 0 °C for 2 h and r.t for 18 h. The reaction mixture was quenched with sat. ammonium chloride solution and extracted into ethyl acetate (3 × 20 mL), the organic layers were combined and dried over Na2SO4 and solvent evaporated in vacuo. The crude product was purified using column chromatography (SiO2, hexanes/ethyl acetate 80:20). X-ray quality crystals were grown from slow vapor diffusion of hexanes in dichloromethane solutions. (9H-carbazol-9-yl)(p-tolyl)methanone, CBZ-Me White solid, yield 1.03 g, 36%. 1H NMR (400 MHz, CD2Cl2) δ 8.05-8.02 (d, 2H), 7.56-7.53 (d, 2H), 7.46-7.43 (m, 2H), 7.24 (m, 6H), 2.43 (s, 3H). HR-ESI MS: m/z calcd. for C20H15NO: 285.1207; found: 286.1045 [M+H]+. (4-Bromophenyl)(9H-carbazol-9-yl)methanone, CBZ-Br White solid, yield 1.37 g, 39%. 1H NMR (400 MHz, CD2Cl2) δ 8.01 (dd, 2H), 7.58 (d, 2H), 7.51 (d, 2H), 7.42 (dd, 2H), 7.22-7.29 (m, 4H). HR-ESI MS: m/z calcd. for C19H12BrNO: 349.0166; found: 350.1257 [M+H]+. 99 Chapter 5: RTP and TADF from Mono- and Di- Substituted Carbazole-Thiophene Geometrical Isomers 5.1 Introduction Organic compounds are emerging as viable candidates for OLED-based displays.145,148,151 Conventionally, phosphorescence has been limited to heavy materials, which increase cost and pose environmental hazards.55,57,70 However, it has been shown that organic emitters can display fluorescence and room-temperature phosphorescence (RTP) and the emission properties can be finely tuned by modifying chemical structures with an array of synthetic routes. The primary design strategy is to incorporate (n, π*) and (π, π*) transitions to promote intersystem crossing (ISC) and extend the photoluminescent decay lifetime.54,128,152 Additionally, structural modifications such as incorporating donor/acceptor moieties to induce frontier orbital separation in organic emitters are employed to achieve highly efficient TADF.65,153–156 Alternatively, RTP can be also be effectively utilized by crystallization of the organic emitters can assist in stabilizing the triplet excitons through rigid packing nodes within the lattice as well as induce emission processes not observed in solutions or amorphous environments,67 as crystallization suppresses non-radiative emission and prevents singlet oxygen quenching of the excited triplet state. Intermolecular interactions within the lattice are crucial to the emission process and can be tuned by careful synthetic design. Yuan and coworkers designed a series of geometrical isomers of amide-linked carbazole-phenyl systems to realize impressive RTP properties in these materials, including biomedical imaging applications.7 While Tang and coworkers showed that varying the steric and conjugation effect imposed by the ester substituent 100 at different positions result in small ΔEST and pure (π, π*) states enabling an ultralong lifetime of 795 ms in the meta isomer (Figure 5-1).153 In Chapter 4, two TADF and RTP active organic emitters were crystallized in non-stoichiometric ratios to realize a novel approach to tune the photophysical properties in the crystalline state. This chapter's work explores the role of structural isomerism in chemically similar organic emitters to achieve RTP and TADF in the crystalline state. This is achieved by linking carbazole units through an amide bond to the 2- and 3- position of a thiophene ring to create two mono-carbazole substituted and two di-carbazole substituted organic emitters. It is proposed that the five-membered heterocycle may introduce different angles between the carbazole moieties and the thiophenes to generate novel photophysical properties. The presence of the sulfur atom may also enable more efficient ISC. The four organic emitters were synthesized and characterized as Figure 5-1 Structural isomers of various RTP and TADF organic emitters. 101 shown in Scheme 5-1. While the monosubstituted carbazole-thiophene compounds display similar properties, adding a second carbazole unit and shifting from 3,4- to the 2,5- positions on the thiophene drastically alters the photophysical properties, presenting an additional route to tune and control emission properties based on geometrical isomerism. 5.2 Synthesis and Characterization The carbazole-thiophene compounds were synthesized using the same procedure; thiophene carboxylic acid was converted into the acid chloride using thionyl chloride, then added to carbazole previously treated with sodium hydride in THF. All compounds were characterized by NMR spectroscopy and mass spectrometry. The 1H NMR spectra show only aromatic peaks with Ha (the thiophene proton) tentatively assigned (Figure 5-2). Due to the symmetrical nature of the disubstituted carbazole DCBZ-2,5Th, and DCBZ-3,4Th, the 1H NMR spectra only show four distinct peaks. When in the ortho position the thiophene protons are shifted downfield when due to increasing electronegativity of the sulfur atom. Scheme 5-1 Synthesis of amide-linked carbazole-thiophene compounds. 102 5.3 Structural Characterization High-quality crystals for structural analysis for all compounds were grown from slow evaporation of acetonitrile solutions. The crystal structure, as well as the intermolecular packing interactions, are described. These interactions play a crucial role in understanding the solid state photoluminescence properties. The structural isomerism of the compounds, results in different crystal systems with different unit cell parameters. The crystal data and refinement parameters can be found in Appendix D. For the monosubstituted carbazole compounds, CBZ-2Th and CBZ-3Th crystallize in the Cc monoclinic space group and P212121 orthorhombic space group, respectively. Both compounds Figure 5-2 1H NMR (400 MHz, CD2Cl2 ) spectra of CBZ-2Th, CBZ-3Th, DCBZ-2,5Th, and DCBZ-3,4Th at room temperature. CBZ-2Th CBZ-3Th DCBZ-2,5Th DCBZ-3,4Th Ha Ha Ha Ha 103 contain one molecule in the asymmetric unit (Figure 5-3(a-b)), and the thiophene fragment is disordered over two positions with an occupancy of CBZ-2Th – 81.0:19.0 and CBZ-3Th - 84.1:15.9. This disorder is likely due to the free rotation of the thiophene carbonyl C-C bond, resulting in two main packing orientations of the thiophene ring during the crystallization process. The torsional angle between the carbazole and thiophene ring systems for CBZ-2Th and CBZ-3Th compounds is ~59.79°. While the geometry of both structures is similar, the packing orientations and stabilizing intermolecular interactions are divergent. For CBZ-2Th, in addition to the π···π interactions (3.355 Å) between the carbazole ring systems, there are also C-O···π interactions with a distance of 3.308 Å and carbonyl···carbazole hydrogen (C-O···H-Ar 2.629 Å) interactions. The carbonyl also forms intermolecular interactions with the sulfur and hydrogen in the neighboring thiophene ring (C-O···H 2.555 Å and C-O···S 3.190 Å) (Figure 5-4(a)). For compound CBZ-3Th similar intermolecular interactions are present throughout the lattice. However, the thiophene fragment forms dimer interactions between the hydrogen and neighboring thiophene sulfur (C-H···S 2.969 Å) along with carbazole π···π (3.361 Å) interactions and the carbonyl and carbazole π (C-O···π 3.393 Å) and hydrogen interactions (C-O···H 2.707 Å) (Figure 5-4(b)). Compounds DCBZ-2,5Th and DCBZ-3,4Th crystallize with one molecule in the asymmetric unit; however, DCBZ-2,5Th crystallizes in the P-1 triclinic crystal system and DCBZ-3,4Th in the P21/n monoclinic crystal system (Figure 5-5(c-d)). The most considerable difference in packing modes in the lattice is the carbazole orientation and the resultant intermolecular interactions. Compound DCBZ-2,5Th has extended carbazole moieties forming π···π (3.391 Å) interactions with the neighboring carbazole ring system, while the carbazole rings of DCBZ-3,4Th overlap with each other to form intramolecular π···π (3.361 Å) interactions with 104 the carbonyl oxygen atoms facing away from the overlapping ring systems (Figure 5-4(c-d)). Both structures primarily comprise of carbazole based intermolecular interactions such as C-O···π (~3.296 Å) and C-O···H (2.473 Å), and only DCBZ-3,4Th has thiophene-based interactions S···π (~3.447 Å) present within the lattice. Figure 5-3 ORTEP of (a) CBZ-2Th, (b) CBZ-3Th, (c) DCBZ-3,4Th, and (d) DCBZ-2,5Th drawn at 50% probability with only heteroatoms labeled for clarity. 105 Figure 5-4 Packing modes and packing interactions of (a) CBZ-2Th, (b) CBZ-3Th, (c) DCBZ-2,5Th, and (d) DCBZ-3,4Th. 106 5.4 Solution State Photophysical Properties The absorption profile and photoluminescence spectra of CBZ-2Th, CBZ-3Th, DCBZ-2,5Th, and DCBZ-3,4Th were measured in dilute (~2 × 10-5 M) acetonitrile solutions (Figure 5-5). The absorption spectra of all the compounds are very similar, with peaks between 300 nm and 310 nm corresponding to π – π* transitions and the 270 nm peak corresponding to n – π* transitions.157 DCBZ-2,5Th has a broad feature from 330 nm – 430 nm, which is attributed to the charge transfer nature of the compound.158 In the solution state, photoluminescence spectra of all the compounds show finely structured emission bands attributed to the locally excited state of carbazole.157 The broad emission band around 530 nm for CBZ-2Th, CBZ-3Th, DCBZ-3,4Th is attributed to the intramolecular charge transfer state (ICT). 5.5 Solid State Photophysical Properties In the crystalline state, the carbazole-thiophene compounds display different photophysical properties. Compounds CBZ-2Th, CBZ-3Th, and DCBZ-3,4Th display room temperature Figure 5-5 Absorption (solid lines) and photoluminescence spectra (dashed lines) of CBZ-2Th, CBZ-3Th, DCBZ-2,5Th, and DCBZ-3,4Th. 107 phosphorescence (RTP), while DCBZ-2,5Th shows thermally activated delayed fluorescence (TADF). The emission spectra and corresponding lifetime decay data are summarized in Table 5-1. Table 5-1 Summary of crystalline state photophysical data Fluorescence Delayed Emission RT Phosphorescence (77 K) λem (nm)a τ (ns) λem (nm)a τ (ms)b λem (nm)a τ (ms)b CBZ-2Th 440 2.49 535 97.5 483 252 (21.8) 435 (71.5) 32.5 (6.70) CBZ-3Th 425 3.87 545 78.4 480 71.8 (51.3) 38.3 (11.6) 384 (37.1) DCBZ-2,5Th 500 0.670 (22) 8.46 (78) 500 0.0166 (82.1) 0.0388 (17.9) 500 153 (31.4) 28.6 (30.2) 1680 (38.3) DCBZ-3,4Th 485 3.65 610 4.17 (26.0) 18.9 (74.0) 524 42.5 (3) 253 (63.2) 466 (33.8) a Excitation wavelength = 359 nm. b Relative amplitudes in parentheses. 108 5.5.1 Room Temperature Phosphorescence The room temperature steady-state emission spectra of crystalline CBZ-2Th, CBZ-3Th, and DCBZ-3,4Th display a broad featureless peak with wavelength maxima at 440 nm, 425 nm, and 485 nm, respectively (Figure 5-6(a)). The maxima shift between the compounds can be attributed to the difference in intermolecular interactions of the carbazole and carbonyl-thiophene moieties within the lattice. The significant shift of DCBZ-3,4Th compared to CBZ-2Th and CBZ-3Th can be attributed to the through-space intermolecular interactions within the dimer pair. Figure 5-6 Emission and lifetime decay profiles of CBZ-2Th, CBZ-3Th, and DCBZ-3,4Th (a) Steady-state (solid lines) and time-resolved (0.15 – 3 ms delay) (dashed lines) photoluminescence spectra under ambient conditions. (b) Fluorescence decay profile and (c) phosphorescence decay profile at room temperature. (d) Steady-state (solid lines) and time-resolved (0.15 – 3 ms delay) (dashed lines) photoluminescence spectra at 77 K. (e) Phosphorescence decay profile at 77 K. 109 The room temperature fluorescence decay profiles show short-lived monoexponential decay (Figure 5-6(b)). Room temperature time-resolved photoluminescence measurements further reveal additional emission processes for crystalline CBZ-2Th, CBZ-3Th, and DCBZ-3,4Th. The time-resolved spectra in the millisecond time range (0.15 ms – 3 ms) show red-shifted emission spectra from the steady-state maxima, at 535 nm, 545 nm, and 610 nm, respectively (Figure 5-6(a)). The corresponding lifetime decay associated with this emission event is considered ultralong lifetimes, a crucial property of RTP-based emitters. The lifetime decay CBZ-2Th (97.5 ms) occurs as a monoexponential process while CBZ-3Th (78.4 ms) and DCBZ-3,4Th are bi-exponential with two components: 4.17 ms and 18.9 ms with relative amplitudes of 26% and 74% respectively (Figure 5-6(c), Table 5-1). Low-temperature emission and lifetime decay measurements of the crystalline state reveal that the steady-state emission maxima are all blue-shifted compared to the emission maxima at room temperature, attributed to more compact packing in the lattice. The well-defined features of the emission maxima originate from the excited triplet states of the carbazole moiety. The photoluminescence lifetime shows a 4-25 fold increase in emission duration as non-radiative deactivation pathways are further suppressed at 77 K. 5.5.2 Thermally Activated Delayed Fluorescence To assess the emission properties of compound DCBZ-2,5Th, similar photoluminescence emission and lifetime measurements were carried out. Interestingly, the steady-state emission spectra of DCBZ-2,5Th show a broad, featureless peak with the most significant redshift (λmax = 500 nm) amongst the series of carbazole-thiophene compounds (Figure 5-7(a)). The room temperature prompt fluorescence decay profile consists of two components, a short 0.667 ns and 110 an 8.46 ns lifetime with respective amplitudes of 22% and 78% (Figure 5-7(b)). Compound DCBZ-2,5Th also shows a delayed fluorescence decay profile with bi-exponential decay of 16.6 µs and 38.8 µs with amplitudes 82.1% and 17.9% (Figure 5-7(c)). Time-resolved emission spectra (120 µs – 1 ms delay) further confirm the presence of the delayed fluorescence component as the emission spectrum overlaps very well with the steady-state emission spectrum, indicating the delayed emission originates from the same excited singlet state. The origin of the delayed emission was further probed at 77 K. Both steady-state and the delayed emission spectra show well-structured emission bands with the same maximum (500 nm) as in the room temperature emission spectra, indicating that at 77 K, the TADF emission feature is still the dominant emission feature. The emission decay time is greatly extended at 77 K and undergoes a tri-exponential emission decay of 42.5 ms (3%), 253 ms (63.2%), and 466 ms (33.8%), respectively (Figure 5-7(d)). This can be attributed to the packing modes of the dicarbazole isomers. 111 5.6 Conclusions In summary, four amide-linked carbazole-thiophene structural isomers were reported where structural isomerism affects the photophysical properties. Two of the carbazole-thiophene-based isomers were mono-carbazole substituted, and the other two isomers were di-carbazole substituted. All compounds were characterized by NMR spectroscopy and mass spectrometry, with their crystal structures revealing structural insights that are crucial to understanding the emissive nature of these organic emitters in the crystalline state. The structural isomers all crystallized in Figure 5-7 Photophysical data of DCBZ-2,5Th. (a) Steady-state (solid lines) and time-resolved (0.15 – 3 ms delay) (dashed lines) photoluminescence spectra under ambient conditions and 77 K.(b) Fluorescence decay profile and (c) delayed fluorescence decay profile at room temperature. (d) Phosphorescence decay profile at 77 K. 112 different crystal systems and produced different packing modes, dominated by intermolecular interactions with the carbazole π system, carbonyl lone pair, and thiophene ring system. The compounds are weakly emissive in solution but showed enhanced emission in the solid state by triplet exciton stabilization. Compounds CBZ-2Th, CBZ-3Th, and DCBZ-3,4Th show prompt emission and a red-shifted delayed emission which was tentatively assigned as RTP with ultra-long decay lifetimes measured in milliseconds. Compound DCBZ-2,5Th also showed interesting solid state emission properties with prompt and delayed fluorescence, tentatively assigned as TADF with emission lifetimes measured in microseconds. All the compounds showed extended emission decay lifetimes at lower temperatures as non-radiative pathways were further suppressed. As for compound DCBZ-2,5Th, at 77 K, the delayed fluorescence feature remains dominant with a minor phosphorescence feature. To fully elucidate this compound's 'thermally activated' reverse intersystem crossing ability, further cooling to 4 K would be required. By facile structural modifications of these carbazole-thiophene compounds, the photophysical properties can be effectively tuned for selective emission processes between TADF and RTP for use in its appropriate application. 5.7 Experimental 5.7.1 General Solvents were of reagent grade and were used without any further purification. HPLC grade solvents were used for analyses. All purchased chemicals were used without further purification. 113 5.7.2 1H NMR Spectroscopy The NMR spectra were recorded at 25 °C on an Avance 400 spectrometer (Bruker, 400 MHz). Chemical shifts (in ppm) are reported relative to the non-deuterated residual solvent signals of CD2Cl2 (δ = 5.33 ppm). 5.7.3 UV-Vis Spectroscopy A Perkin Elmer UV-vis spectrometer was used to record the spectra in solution (Lambda 35) and the solid state (Lambda 365). 5.7.4 Solid State Spectroscopy Steady-state data were collected on a Photon Technology International (PTI) QuantaMaster 50 fluorimeter fitted with an integrating sphere, double excitation monochromator, and utilizing a 75 W Xe arc lamp as the source. Fluorescence and phosphorescence lifetime data were collected using a Horiba Yvon Fluorocube TCSPC apparatus with a 370 nm NanoLED and a 359 nm Horiba spectral LED, respectively. The lifetime data were fitted using the DAS6 Data Analysis software package. Time-resolved spectra with millisecond delay were measured using a Photon Technology International (PTI) QuantaMaster 400. 5.7.5 X-ray Diffraction Single crystal X-ray data were collected using a Bruker APEX DUO diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 100 K. Raw frame data were processed using APEX2.109 The program SAINT+, v. 7.68110 was used to reduce the data and the program SADABS111 was used to make corrections to the empirical absorptions. Space group 114 assignments were made using XPREP 110 on all compounds. In all cases, the structures were solved in the Olex2112 suite of programs using Intrinsic Phasing and refined using full-matrix least-squares/difference Fourier techniques on F2 using SHELXL.113 All non-hydrogen atoms were refined anisotropically. Diagrams and publication material were generated using CrystalMaker. 5.7.6 Synthesis All of the carbazole-thiophene compounds were synthesized using the same procedure. The respective thiophene carboxylic acid (10 mmol) was dissolved in thionyl chloride (50 mmol, 5 eq.) and heated to reflux for 4 h. Excess thionyl chloride was diluted with hexanes (3 × 15 mL) and the solvent evaporated in vacuo. The crude thiophene acid chloride was used without further purification. Carbazole (20 mmol, 2 eq.) was dissolved in dry THF (15 mL) and cooled to 0 °C under an N2 atmosphere. NaH (22 mmol, 2.2 eq) was added to the solution and stirred for a further 30 minutes at 0 °C. The crude thiophene acid chloride was dissolved in dry THF (10 mL) and added dropwise to the carbazole reaction mixture. The reaction was stirred at 0 °C for 2 h and r.t for 18 h. The reaction mixture was quenched with sat. ammonium chloride solution and extracted into ethyl acetate (3 × 20 mL), the organic layers were combined and dried over Na2SO4 and solvent evaporated in vacuo. The crude product was purified using column chromatography (SiO2, hexanes/ethyl acetate 80:20). X-ray quality crystals were grown from slow vapor diffusion of hexanes in dichloromethane solutions. 115 (9H-carbazol-9-yl)(thiophen-2-yl)methanone, CBZ-2Th White solid, 998 mg, 36 % yield. 1H NMR (300 MHz, CD2Cl2) δ 8.13 – 7.98 (m, 2H), 7.80 (dd, J = 5.0, 1.2 Hz, 1H), 7.76 – 7.63 (m, 2H), 7.60 (dd, J = 3.8, 1.2 Hz, 1H), 7.45 – 7.31 (m, 4H), 7.18 (dd, J = 5.0, 3.8 Hz, 1H). 13C{1H} NMR (101 MHz, CDCl3) δ 162.8, 139.2, 137.8, 133.8, 133.2, 127.7, 126.64, 125.8, 123.3, 119.9, 115.3. HR-ESI MS: m/z calcd. for C17H11NOS: 277.0601; found: 278.1127 [M+H]+. (9H-carbazol-9-yl)(thiophen-2-yl)methanone, CBZ-3Th White solid, 1.10 g, 40 % yield. 1H NMR (300 MHz, CD2Cl2) δ 8.13 – 8.00 (m, 2H), 7.97 (dd, J = 3.0, 1.3 Hz, 1H), 7.67 – 7.55 (m, 2H), 7.50 (dd, J = 5.1, 3.0 Hz, 1H), 7.44 – 7.30 (m, 5H). 13C{1H} NMR (101 MHz, CDCl3) δ 164.3, 139.1, 137.4, 131.9, 127.9, 126.7, 126.6, 125.9, 123.34 119.9, 115.5. HR-ESI MS: m/z calcd. for C17H11NOS: 277.0601; found: 278.1098 [M+H]+. 116 Thiophene-2,5-diylbis((9H-carbazol-9-yl)methanone), DCBZ-2,5Th Pink solid, 1.18 g, 25 % yield. 1H NMR (300 MHz, CD2Cl2) δ 8.12 – 8.01 (m, 4H), 7.84 – 7.72 (m, 4H), 7.56 (s, 2H), 7.50 – 7.35 (m, 8H). 13C{1H} NMR (101 MHz, CDCl3) δ 161.8, 143.2, 138.8, 132.5, 126.9, 126.2, 123.9, 120.1, 115.5. HR-ESI MS: m/z calcd. for C30H18N2O2S: 470.1124; found: 471.1211 [M+H]+. Thiophene-3,4-diylbis((9H-carbazol-9-yl)methanone), DCBZ-3,4Th Pale yellow solid, 940 mg, 20 % yield. 1H NMR (400 MHz, CD2Cl2) δ 7.99 (s, 2H), 7.85 – 7.75 (m, 4H), 7.40 – 7.31 (m, 4H), 7.33 – 7.23 (m, 8H). 13C{1H} NMR (101 MHz, CDCl3) δ 162.2, 138.2, 137.7, 131.9, 126.7, 126.0, 123.6, 119.7, 115.2. HR-ESI MS: m/z calcd. for C30H18N2O2S: 470.1121; found: 471.1231 [M+H]+. 117 Chapter 6: Conclusion and Future Work This thesis explores photofunctional molecules in the crystalline state with crystals containing either photoswitches or organic emissive chromophores. The crystals containing photoswitches were shown to undergo various photophysical processes such as photochromism and photomechanical motion. The crystalline state was also used to mix two isostructural chromophores enabling fine-tuning of emission properties. Lastly, novel isomeric organic emissive chromophores were explored for their solid state emission properties. In Chapter 2, the Z- and E- isomers of hemithioindigo-anthracene photoswitch (HTI-An) were shown to be stimuli-responsive in both solution and the crystalline state. In solution, HTI-An can be switched between its isomers with the appropriate wavelength of light. In the crystalline state, crystal structures were obtained for both isomers. When crystals of (Z)-HTI-An are heated, they produce a mechanical response in the form of ‘hopping’ (thermosalient effect), and concomitant recrystallization from the melt indicates multiple phases are present. These additional phases may present interesting future explorations of thermo- and photo-salient phenomena in this compound. Crystals of (E)-HTI-An ‘jump’ across the microscope when irradiated with 515 nm light demonstrating the photosalient effect. The driving force is photoisomerization towards the Z- isomer and the release of strain in the crystal during the rearrangement. Heating solid (E)-HTI-An induces thermal reversion with yields up to 90% Z- isomer, indicating that the system can be used over multiple cycles. In Chapter 3, the photomechanical properties of HTI and three analogs HTI-Nap, HTI-2Cl, and HTI-Mes, were explored. All four photoswitches undergo reversible photoisomerization in solution when irradiated with light. In the solid state, when exposed to 405 nm, all four 118 photoswitches displayed photochromism with HTI, HTI-Nap, and HTI-2Cl undergoing [2+2] photodimerization and HTI-Mes undergoing photoisomerization. Furthermore, these photoreactions induce mechanical motion in single crystals. As such, needle-like crystals of HTI underwent a bending motion, plate-like crystals of HTI-Nap produced impressive photosalient effects with single crystals twisting before rotation upon its face. The rotations can be observed up to 540° without any noticeable loss of structural integrity. Other crystals underwent bending with localized structural deformation upon irradiation. Other mechanical effects can be achieved by heating light-treated crystals of HTI-Nap, as they continue to bend over 90°, demonstrating versatility in these materials. HTI-2Cl and HTI-Mes crystals displayed photochromism when irradiated with 405 nm light. In Chapter 4, photophysical properties of two isostructural carbazole-based organic emitters (CBZ-Me and CBZ-Br) were finely tuned by crystallization together in a continuous ratio from 30% – 90%. This is the first report of employing organic alloys to develop long-lived emissive materials. The seven organic alloy crystals (MexBr10-x where x = 3 to 9) had both components well homogenized and displayed a linear increase in melting point with increasing CBZ-Me composition. The organic alloys display strong afterglow emission from long-lived TADF processes. The photoluminescent quantum yield of the alloys can also be tuned along the compositional range allowing tunability of efficiency and lifetime. This work suggests that organic alloys can provide a framework for fine tunability of luminescent materials and can incorporate additional components to produce ternary systems. In Chapter 5, four novel amide-linked carbazole-thiophene isomers were investigated for RTP and TADF properties in the solid state. All four compounds were poor emitters in solution; however, the compounds in the crystalline state displayed long-lived emission. The two 119 monosubstituted compounds (CBZ-2Th and CBZ-3Th) and disubstituted compounds (DCBZ-3,4Th) displayed RTP properties while DCBZ-2,5Th exhibited TADF. The positioning on the thiophene ring plays an essential role in the solid state intermolecular interactions that arise, which plays a vital role in the emission process. This work adds to the library of solid state organic emitters, and future work involves bromination of the compounds to make use of the heavy atom effect to enhance emission properties. In summary, the work in this thesis puts forward the first photomechanical observation for hemithioindigo photoswitches using both photoisomerization and [2+2] photodimerization as the driving force in the crystals. These proof-of-concept photomechanical responses in crystalline hemithioindigo photoswitches demonstrate their potential use as light-driven microactuators. The facile synthesis of HTIs enables tuning the photomechanical system based on the desired application. This thesis also puts forward the first example of solid state substitution to fine-tune and control the afterglow emission properties of two carbazole-based organic emitters. This provides a new perspective of the rules established for developing organic long-lived emissive materials for host-guest doped systems. It is hoped that the work will provide a framework for future research entailing the fine-tuning of emission properties in organic compounds in the solid state. Finally, four novel organic emitters were synthesized and characterized, and photophysical properties elucidated, and it is hoped that the work would provide insight for future scientists in the design of solid state organic emitters. 120 References 1 M. R. 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Chem., 2002, 84, 2135–2141. 132 Appendices Appendix A Listing A.1 Crystal data and refinement parameters of (E)-HTI-An before irradiation, post photosalient event, and after 2-hour heating. (E)-HTI-An before irradiation (E)-HTI-An post photosalient event (E)-HTI-An after 2-hour heating CCDC No. 2068879 2068880 2068881 Empirical formula C23H14OS C23H14OS C23H14OS Formula weight 338.40 338.40 338.40 Temperature/K 120 133 100 Crystal system monoclinic monoclinic monoclinic Space group P21/c P21/c P21/c a/Å 10.4430(4) 10.351(12) 10.453(3) b/Å 13.9757(5) 13.886(17) 13.935(4) c/Å 11.2410(4) 11.185(13) 11.229(3) α/° 90 90 90 β/° 102.9160(10) 102.99(3) 102.990(8) γ/° 90 90 90 Volume/Å3 1599.09(10) 1567(3) 1593.9(8) Z 4 4 4 ρcalcg/cm3 1.406 1.435 1.410 μ/mm-1 0.210 0.214 0.210 F(000) 704.0 704.0 704.0 Crystal size/mm3 0.398 × 0.3 × 0.254 0.398 × 0.3 × 0.254 0.178 × 0.123 × 0.09 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2Θ range for data collection/° 4.002 to 61.1 4.038 to 53.126 3.998 to 45.03 133 Index ranges -12 ≤ h ≤ 14, -19 ≤ k ≤ 16, -16 ≤ l ≤ 16 -13 ≤ h ≤ 13, -17 ≤ k ≤ 17, -14 ≤ l ≤ 14 -11 ≤ h ≤ 11, -14 ≤ k ≤ 15, -11 ≤ l ≤ 12 Reflections collected 24994 17973 17901 Independent reflections 4866 [Rint = 0.0324, Rsigma = 0.0266] 3260 [Rint = 0.0398, Rsigma = 0.0290] 2085 [Rint = 0.0780, Rsigma = 0.0429] Data/restraints/parameters 4866/486/254 3260/485/254 2085/485/254 Goodness-of-fit on F2 1.027 1.091 1.062 Final R indexes [I>=2σ (I)] R1 = 0.0361, wR2 = 0.0862 R1 = 0.0359, wR2 = 0.0783 R1 = 0.0358, wR2 = 0.0709 Final R indexes [all data] R1 = 0.0475, wR2 = 0.0924 R1 = 0.0465, wR2 = 0.0825 R1 = 0.0611, wR2 = 0.0795 Largest diff. peak/hole / e Å-3 0.39/-0.27 0.22/-0.21 0.17/-0.18 Listing A.2 Crystal data and refinement parameters of (Z)-HTI-An at 298K, (Z’)-HTI-An at 400K (Z)-HTI-An (Z’)-HTI-An CCDC No. 2068877 2068878 Empirical formula C23H14OS C23H14OS Formula weight 338.40 338.40 Temperature/K 298 400 Crystal system monoclinic monoclinic Space group P21/c P21/c a/Å 12.9474(18) 13.183(11) b/Å 11.1005(17) 11.272(8) c/Å 11.7019(17) 11.792(8) α/° 90 90 134 β/° 97.279(5) 97.60(3) γ/° 90 90 Volume/Å3 1668.3(4) 1737(2) Z 4 4 ρcalcg/cm3 1.347 1.294 μ/mm-1 0.201 0.193 F(000) 704.0 704.0 Crystal size/mm3 0.399 × 0.211 × 0.111 0.399 × 0.211 × 0.111 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2Θ range for data collection/° 3.172 to 52.824 3.116 to 53.81 Index ranges -14 ≤ h ≤ 16, -13 ≤ k ≤ 11, -14 ≤ l ≤ 14 -16 ≤ h ≤ 16, -14 ≤ k ≤ 13, -14 ≤ l ≤ 14 Reflections collected 22309 18361 Independent reflections 3417 [Rint = 0.0323, Rsigma = 0.0255] 3576 [Rint = 0.0512, Rsigma = 0.0482] Data/restraints/parameters 3417/0/226 3576/0/226 Goodness-of-fit on F2 1.027 1.010 Final R indexes [I>=2σ (I)] R1 = 0.0365, wR2 = 0.0840 R1 = 0.0437, wR2 = 0.0934 Final R indexes [all data] R1 = 0.0593, wR2 = 0.0967 R1 = 0.1038, wR2 = 0.1183 Largest diff. peak/hole / e Å-3 0.19/-0.22 0.14/-0.18 135 Listing A.3 Face indexing of (Z)-HTI-An. Listing A.4 Face indexing of (E)-HTI-An. 136 Appendix B Listing B.1 Crystal data and refinement parameters for HTI, HTI-Nap, and HTI-Mes HTI HTI-Nap HTI-Mes Empirical formula C15H10OS C19H12OS C18H16OS Formula weight 238.29 288.35 280.37 Temperature/K 100.15 120 100 Crystal system monoclinic monoclinic monoclinic Space group P21/n P21/n P21/n a/Å 13.982(7) 6.889(2) 13.5107(4) b/Å 3.8818(18) 14.063(4) 8.4433(2) c/Å 20.264(10) 13.708(4) 13.5676(4) α/° 90 90 90 β/° 91.885(14) 100.170(5) 109.3110(10) γ/° 90 90 90 Volume/Å3 1099.2(9) 1307.1(7) 1460.64(7) Z 4 4 4 ρcalcg/cm3 1.440 1.465 1.275 μ/mm-1 0.270 0.242 0.214 F(000) 496.0 600.0 592.0 Crystal size/mm3 0.445 × 0.09 × 0.05 0.09 × 0.07 × 0.01 0.1 × 0.09 × 0.09 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2Θ range for data collection/° 3.486 to 52.886 4.184 to 61.184 6.364 to 54.97 Index ranges -16 ≤ h ≤ 17, -4 ≤ k ≤ 2, -21 ≤ l ≤ 25 -9 ≤ h ≤ 9, 0 ≤ k ≤ 20, 0 ≤ l ≤ 19 -17 ≤ h ≤ 17, -10 ≤ k ≤ 10, -17 ≤ l ≤ 15 Reflections collected 8268 4022 23832 137 Independent reflections 2211 [Rint = 0.0355, Rsigma = 0.0375] 4022 [Rint = ?, Rsigma = 0.0734] 3344 [Rint = 0.0468, Rsigma = 0.0306] Data/restraints/parameters 2211/0/158 4022/0/191 3344/0/184 Goodness-of-fit on F2 1.055 1.056 1.066 Final R indexes [I>=2σ (I)] R1 = 0.0460, wR2 = 0.1133 R1 = 0.0546, wR2 = 0.1164 R1 = 0.0404, wR2 = 0.0933 Final R indexes [all data] R1 = 0.0646, wR2 = 0.1245 R1 = 0.0942, wR2 = 0.1343 R1 = 0.0605, wR2 = 0.1070 Largest diff. peak/hole / e Å-3 0.52/-0.26 0.38/-0.47 0.33/-0.34 Flack parameter 138 Appendix C C.1 Listing C.1 Crystal data and refinement parameters for organic alloy MexBr10-x where x = 3 to 9 Compound Me3Br7 Me4Br6 Me5Br5 Me6Br4 Me7Br3 Me8Br2 Me9Br1 Empirical formula C19.36H13.07Br0.64NO C19.44H13.32Br0.56NO C19.47H13.41Br0.53NO C19.56H13.67Br0.44NO C19.71H14.14Br0.29NO C19.81H14.44Br0.19NO C19.91H14.74Br0.09NO Formula weight 327.01 321.66 319.55 314.04 303.98 297.49 291.01 Temperature/K 100 120.0 100 100.0 100.0 100.15 100 Crystal system monoclinic monoclinic monoclinic monoclinic monoclinic monoclinic monoclinic Space group P21/n P21/n P21/n P21/n P21/n P21/n P21/n a/Å 11.5265(6) 11.5680(7) 11.5657(6) 11.5850(7) 11.5964(10) 11.6030(8) 11.6141(6) b/Å 10.1543(5) 10.1429(5) 10.1316(5) 10.1100(6) 10.0911(8) 10.0711(6) 10.0515(5) c/Å 12.3229(7) 12.3305(7) 12.3187(6) 12.3005(7) 12.3080(1) 12.3038(8) 12.2961(6) α/° 90 90 90 90 90 90 90 β/° 95.063(2) 95.097(2) 95.031(2) 94.882(4) 94.854(2) 94.846(2) 94.7450(10) 139 γ/° 90 90 90 90 90 90 90 Volume/Å3 1436.69(13) 1441.05(14) 1437.93(12) 1435.46(15) 1435.1(2) 1432.62(16) 1430.52(12) Z 4 4 4 4 4 4 4 ρcalcg/cm3 1.512 1.483 1.476 1.453 1.407 1.379 1.351 μ/mm-1 1.868 1.633 1.546 1.313 0.881 0.604 0.326 F(000) 667.0 658.0 655.0 646.0 630.0 620.0 609.0 Crystal size/mm3 0.399 × 0.389 × 0.379 0.381 × 0.38 × 0.379 0.398 × 0.378 × 0.222 0.399 × 0.345 × 0.322 0.389 × 0.365 × 0.312 0.387 × 0.345 × 0.312 0.399 × 0.389 × 0.378 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2Θ range for data collection/° 4.638 to 61.126 4.628 to 61.08 4.632 to 61.112 4.638 to 61.402 4.634 to 61.074 4.634 to 61.134 4.636 to 61.232 Index ranges -16 ≤ h ≤ 16, -9 ≤ k ≤ 14, -17 ≤ l ≤ 17 -16 ≤ h ≤ 16, -14 ≤ k ≤ 11, -17 ≤ l ≤ 17 -16 ≤ h ≤ 16, -14 ≤ k ≤ 13, -17 ≤ l ≤ 17 -16 ≤ h ≤ 15, -14 ≤ k ≤ 14, -17 ≤ l ≤ 17 -16 ≤ h ≤ 16, -14 ≤ k ≤ 9, -17 ≤ l ≤ 17 -16 ≤ h ≤ 16, -14 ≤ k ≤ 13, -17 ≤ l ≤ 17 -16 ≤ h ≤ 16, -14 ≤ k ≤ 14, -17 ≤ l ≤ 17 140 Reflections collected 24000 27288 25444 25359 23841 25717 25534 Independent reflections 4369 [Rint = 0.0369, Rsigma = 0.0292] 4402 [Rint = 0.0225, Rsigma = 0.0166] 4414 [Rint = 0.0266, Rsigma = 0.0194] 4401 [Rint = 0.0981, Rsigma = 0.0902] 4369 [Rint = 0.0234, Rsigma = 0.0182] 4386 [Rint = 0.0336, Rsigma = 0.0270] 4395 [Rint = 0.0198, Rsigma = 0.0144] Data/restraints/parameters 4369/354/204 4402/351/210 4414/0/210 4401/351/192 4369/351/210 4386/0/210 4395/0/210 Goodness-of-fit on F2 1.043 1.024 1.027 1.046 1.066 1.041 1.040 Final R indexes [I>=2σ (I)] R1 = 0.0339, wR2 = 0.0775 R1 = 0.0284, wR2 = 0.0667 R1 = 0.0300, wR2 = 0.0716 R1 = 0.0682, wR2 = 0.1966 R1 = 0.0370, wR2 = 0.0893 R1 = 0.0367, wR2 = 0.0980 R1 = 0.0388, wR2 = 0.1028 Final R indexes [all data] R1 = 0.0455, wR2 = 0.0815 R1 = 0.0355, wR2 = 0.0696 R1 = 0.0388, wR2 = 0.0752 R1 = 0.0919, wR2 = 0.2147 R1 = 0.0458, wR2 = 0.0931 R1 = 0.0448, wR2 = 0.1020 R1 = 0.0449, wR2 = 0.1071 Largest diff. peak/hole / e Å-3 0.59/-0.41 0.38/-0.25 0.37/-0.24 1.19/-1.15 0.38/-0.25 0.42/-0.23 0.48/-0.23 141 Appendix D Listing D.1 Crystal data and refinement parameters for CBZ-2Th, CBZ-3Th, DCBZ-2,5Th, and DCBZ-3,4Th CBZ-2Th CBZ-3Th DCBZ-2,5Th DCBZ-2,5Th Empirical formula C17H11NOS C17H11NOS C30H18N2O2S C30H18N2O2S Formula weight 277.33 277.33 470.52 470.52 Temperature/K 100.15 100.15 100.15 100.15 Crystal system monoclinic orthorhombic triclinic monoclinic Space group C2 P212121 P-1 P21/n a/Å 18.780(5) 6.7167(4) 7.7972(12) 8.6526(3) b/Å 5.2878(16) 13.9239(8) 11.3897(19) 21.2499(6) c/Å 13.029(4) 14.0566(8) 12.736(2) 11.9734(3) α/° 90 90 101.140(9) 90 β/° 99.191(18) 90 93.398(9) 92.6751(10) γ/° 90 90 93.531(9) 90 Volume/Å3 1277.2(6) 1314.61(13) 1104.7(3) 2199.11(11) Z 4 4 2 4 ρcalcg/cm3 1.442 1.401 1.415 1.421 μ/mm-1 0.246 0.239 0.180 0.181 F(000) 576.0 576.0 488.0 976.0 Crystal size/mm3 0.39 × 0.08 × 0.04 0.3 × 0.09 × 0.05 0.35 × 0.27 × 0.18 0.41 × 0.14 × 0.13 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2Θ range for data collection/° 3.166 to 61.202 4.118 to 61.048 6.382 to 61.17 3.834 to 61.072 142 Index ranges -24 ≤ h ≤ 26, -7 ≤ k ≤ 7, -18 ≤ l ≤ 18 -9 ≤ h ≤ 9, -19 ≤ k ≤ 19, -20 ≤ l ≤ 20 -11 ≤ h ≤ 11, -16 ≤ k ≤ 14, 0 ≤ l ≤ 18 -10 ≤ h ≤ 12, -30 ≤ k ≤ 30, -17 ≤ l ≤ 17 Reflections collected 12728 22848 6476 32550 Independent reflections 3902 [Rint = 0.0320, Rsigma = 0.0335] 4017 [Rint = 0.0359, Rsigma = 0.0260] 6476 [Rint = 0.0412, Rsigma = 0.0458] 6720 [Rint = 0.0396, Rsigma = 0.0316] Data/restraints/parameters 3902/433/206 4017/5/179 6476/0/316 6720/0/316 Goodness-of-fit on F2 1.036 1.059 1.051 1.032 Final R indexes [I>=2σ (I)] R1 = 0.0296, wR2 = 0.0716 R1 = 0.0338, wR2 = 0.0834 R1 = 0.0600, wR2 = 0.1573 R1 = 0.0390, wR2 = 0.0978 Final R indexes [all data] R1 = 0.0324, wR2 = 0.0735 R1 = 0.0372, wR2 = 0.0861 R1 = 0.0716, wR2 = 0.1658 R1 = 0.0492, wR2 = 0.1039 Largest diff. peak/hole / e Å-3 0.27/-0.19 0.38/-0.25 1.52/-0.45 0.40/-0.29 Flack parameter -0.03(3) -0.02(3)
Thesis/Dissertation
eng
Chemistry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Wolf, Michael O.
Photofunctional molecular crystals
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