Science, Faculty of
Chemistry, Department of
DSpace
UBCV
Yunyaeva, Olga
Master of Science - MSc
University of British Columbia
Organic π-conjugated materials are growing in popularity as they can be employed in flexible devices such as OLED displays, used in bioimaging, and for sensing applications. The incorporation of heteroatoms, in particular nitrogen, provides a simple synthetic handle that can be used to modify organic emitters through reactions such as protonation, oxidation, alkylation, and chelation, leading to a new class of molecules with different photophysical properties. This thesis explores a series of pyridine-containing luminophores arranged in donor-acceptor architectures and the changes in the photophysical properties when functionalized at the nitrogen center. First, a spectroscopic study examines the effect of substituent size at the nitrogen on the conformational and rotational properties of the molecule. Modification of the nitrogen by protonation, oxidation and alkylation are investigated and differences in photophysical properties are observed. Substitutions with groups larger than a hydrogen induce restricted rotation in specific molecules resulting in atropisomerism. NMR, UV-Vis, and emission spectroscopies are used to explain the impact of different substitutions on fluorescence quantum yield. Next, a new photoreaction is demonstrated in which a C–N bond is formed between pyridine and different polyaromatic hydrocarbons (PAHs). Protonation and chelation of the pyridine enables compounds to react under UV irradiation. The mechanism of this photoreaction is explored using kinetic experiments, revealing a photochemically driven single electron transfer (SET) reaction. The scope of this reaction is tested with different chromophores, electron-withdrawing/donating groups and under various reaction conditions. The formation of the C–N bond leads to planarization and rigidification of the photoproduct, leading to a redshift in emission wavelength and an increase in fluorescence quantum yield.
PHOTOPHYSICS AND PHOTOCHEMISTRY OF PYRIDINE-BRIDGED CHROMOPHORES by Olga Yunyaeva B.Sc., The University of Western Ontario, 2018 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) September 2021 © Olga Yunyaeva, 2021 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Photophysics and Photochemistry of Pyridine-Bridged Chromophores submitted by Olga Yunyaeva in partial fulfillment of the requirements for the degree of Master of Science in Chemistry Examining Committee: Michael Wolf, Professor, Department of Chemistry, UBC Supervisor Glenn Sammis, Professor, Department of Chemistry, UBC Supervisory Committee Member Derek Gates, Professor, Department of Chemistry, UBC Supervisory Committee Member Lawrence McIntosh, Professor Emeritus, Department of Chemistry and Department of Biochemistry and Molecular Biology/ The Michael Smith Laboratories, UBC Additional Examiner iii Abstract Organic π-conjugated materials are growing in popularity as they can be employed in flexible devices such as OLED displays, used in bioimaging, and for sensing applications. The incorporation of heteroatoms, in particular nitrogen, provides a simple synthetic handle that can be used to modify organic emitters through reactions such as protonation, oxidation, alkylation, and chelation, leading to a new class of molecules with different photophysical properties. This thesis explores a series of pyridine-containing luminophores arranged in donor-acceptor architectures and the changes in the photophysical properties when functionalized at the nitrogen center. First, a spectroscopic study examines the effect of substituent size at the nitrogen on the conformational and rotational properties of the molecule. Modification of the nitrogen by protonation, oxidation and alkylation are investigated and differences in photophysical properties are observed. Substitutions with groups larger than a hydrogen induce restricted rotation in specific molecules resulting in atropisomerism. NMR, UV-Vis, and emission spectroscopies are used to explain the impact of different substitutions on fluorescence quantum yield. Next, a new photoreaction is demonstrated in which a C–N bond is formed between pyridine and different polyaromatic hydrocarbons (PAHs). Protonation and chelation of the pyridine enables compounds to react under UV irradiation. The mechanism of this photoreaction is explored using kinetic experiments, revealing a photochemically driven single electron transfer (SET) reaction. The scope of this reaction is tested with different chromophores, electron-withdrawing/donating groups and under various reaction conditions. The formation of the C–N bond leads to planarization and rigidification of the photoproduct, leading to a redshift in emission wavelength and an increase in fluorescence quantum yield. iv Lay Summary Materials that emit light are found in many everyday uses, ranging from medicine to television screens. These applications require molecules that are efficient at converting energy into light. A common issue for these materials is that absorbed energy is lost through the production of heat, generated from molecular vibrations and rotations. This thesis examines ways to reduce molecular motion through structural modifications. Methods such as attaching bulky molecular groups (to restrict motion) or locking the molecule in a specific orientation (through bond formation) are explored. Various light-emitting molecules are prepared and modified using chemical or light-based methods, and their optical properties characterized. The impact of reducing molecular motion in these molecules is studied and correlated to their efficiency at producing light. This research aims provide new insights and strategies for the preparation of bright and efficient light-emitting materials. v Preface I am the principal author of the work reported in this thesis, under the supervision of Prof. Michael Wolf at the University of British Columbia. I performed all experiments and data interpretation except contributions noted below. In Chapter 2 and Chapter 3, single-crystal X-ray data was collected and solved by Duane Hean. vi Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface .............................................................................................................................................v Table of Contents ......................................................................................................................... vi List of Tables ................................................................................................................................ ix List of Figures .................................................................................................................................x List of Schemes .............................................................................................................................xv List of Symbols .......................................................................................................................... xvii List of Abbreviations ............................................................................................................... xviii Acknowledgements .................................................................................................................... xxi Dedication ................................................................................................................................. xxiii Chapter 1: Introduction ................................................................................................................1 1.1 Light and Luminescence ................................................................................................. 1 1.2 π-Conjugated Materials ................................................................................................... 2 1.3 Photophysical Principles ................................................................................................. 4 1.3.1 Fundamentals .............................................................................................................. 4 1.3.2 Photoluminescent Quantum Yield (PLQY) ................................................................ 7 1.4 Methods to Increase Photoluminescent Quantum Yields ............................................... 8 1.5 Heteroatoms in Emissive Molecules ............................................................................. 10 1.6 Nitrogen-Containing Luminescent Materials ............................................................... 13 1.7 Thesis Scope ................................................................................................................. 17 vii Chapter 2: Photophysics of Structurally Tunable Pyridine-Bridged Chromophores ..........19 2.1 Introduction ................................................................................................................... 19 2.2 Results and Discussion ................................................................................................. 21 2.2.1 Synthesis ................................................................................................................... 21 2.2.2 Absorbance and Fluorescence Spectroscopy of DNP Compounds .......................... 22 2.2.3 Fluorescence of Mono and Disubstituted Compounds ............................................. 24 2.2.4 NMR Spectroscopy of DNP-Me ............................................................................... 25 2.2.5 PLQY of DNP-Me and NP-Me................................................................................. 32 2.2.6 Larger Functionalization: DNP-Et ............................................................................ 33 2.2.7 Absorbance and Fluorescence of DNP-Et ................................................................ 36 2.3 Conclusion .................................................................................................................... 37 2.4 Experimental ................................................................................................................. 38 2.4.1 General Considerations ............................................................................................. 38 2.4.2 Spectroscopy ............................................................................................................. 38 2.4.3 X-Ray Diffraction ..................................................................................................... 39 2.4.4 Synthetic Procedures and Characterization .............................................................. 40 2.4.4.1 Synthesis of Disubstituted Compounds ............................................................ 40 2.4.4.2 Synthesis of Monosubstituted Compounds ....................................................... 43 Chapter 3: Photochemical C-N Annulation of Pyridine-Containing Chromophores ...........46 3.1 Introduction ................................................................................................................... 46 3.2 Results and Discussion ................................................................................................. 48 3.2.1 Photochemical Synthesis of Naphthyl-Indolium Complexes ................................... 48 3.2.2 Naphthyl-Indolium Cyclization Conditions and Mechanism ................................... 51 viii 3.2.3 Photophysics of Naphthyl Indolium Complexes ...................................................... 54 3.2.4 Expansion of Chromophore Type in Photocyclization Reaction .............................. 56 3.2.5 Phenyl Naphthalene Derivatives ............................................................................... 61 3.2.6 Double Cyclization Mechanism ................................................................................ 64 3.2.7 Photophysics of Asymmetric Naphthyl-Phenyl-Pyridine Complexes ...................... 65 3.3 Conclusions ................................................................................................................... 66 3.4 Experimental ................................................................................................................. 67 3.4.1 General ...................................................................................................................... 67 3.4.2 Spectroscopy ............................................................................................................. 67 3.4.3 X-Ray Diffraction ..................................................................................................... 68 3.4.4 Synthetic Procedures and Characterization .............................................................. 69 Chapter 4: Conclusion and Future Work ..................................................................................79 4.1 Conclusions ................................................................................................................... 79 4.2 Future Work .................................................................................................................. 80 Bibliography .................................................................................................................................83 Appendices ....................................................................................................................................99 Appendix A : Additional Data for Chapter 2 ............................................................................ 99 A.1 NMR Spectroscopy ................................................................................................... 99 A.2 Crystallography Data .............................................................................................. 110 Appendix B : Additional Data for Chapter 3 .......................................................................... 111 B.1 UV-Vis and Emission Spectroscopy ....................................................................... 111 B.2 NMR Spectroscopy ................................................................................................. 114 B.3 Crystallography Data .............................................................................................. 128 ix List of Tables Table 2.1 Photophysical properties of DNP and NP compounds measured in DCM................... 25 Table 3.1 Reaction conditions of Scheme 3.1 with various deuterated solvents. Experiments were conducted in an NMR tube at a concentration of 0.014 M, irradiated with 365 nm light for 3 h. 52 Table 3.3 Scope of pyridine-chromophore systems photochemical cyclization. .......................... 57 Table 3.4. Asymmetric naphthyl-pyridyl chromophores prepared to probe scope of photochemical cyclization. ........................................................................................................... 58 x List of Figures Figure 1.1 Use of natural light in (a) the Pantheon through an oculus and in (b) stained-glass windows in Sagrada Familia Cathedral. ......................................................................................... 1 Figure 1.2 Examples of π-conjugated chromophores. Chlorophyll A is found in plants and -Carotene is found in many vegetables such as carrots, pumpkin, and sweet potato. ...................... 2 Figure 1.3 (a) Increasing conjugation length decreases the energy gap between frontier orbitals (b) Increasing conjugation length in PAHs increases their absorption wavelength. ...................... 3 Figure 1.4 Schematic representation illustrating the absorption of energy (depicted as light energy) and emission of light in a luminescent material. ............................................................... 5 Figure 1.5 Simplified Jablonski diagram illustrating different radiative and non-radiative transitions involved in luminescence. ............................................................................................. 6 Figure 1.6 Equations describing photoluminescence quantum yield (ΦPL) .................................... 7 Figure 1.7 In solution TPE rotates dynamically resulting in weak emission, upon aggregation rotational pathways are reduced and intense emission is produced. Image is adapted from the original reference.22......................................................................................................................... 8 Figure 1.8 A systematic increase in PLQY is seen in BODIPY dyes with an increase in steric hindrance. Image adapted from reference.23 ................................................................................... 9 Figure 1.9 The physical constraint achieved in 9-phenylanthracene by annulation leads to rigidification and planarity yielding enhanced photophysical properties.28 ................................. 10 Figure 1.10 Chemical structures of three DPP derivatives with different ending heterocycles. Figure adapted from reference.33,31 ............................................................................................... 11 xi Figure 1.11 Intramolecular B-N Lewis pair formation resulting in photophysical changes in diborylated-thiophene-benzothiadiazole thin films. Image adapted form reference.36 ................. 12 Figure 1.12 Oxidation of sulfur-bridged of napthyl/pyrene chromophore systems result in a PLQY increase. ............................................................................................................................. 13 Figure 1.13 A small portion of nitrogen’s reactivity scope. Some reactions can occur in situ (protonation, chelation, Lewis pairing), while others require longer reaction conditions (oxidation, alkylation, arylation)................................................................................................... 14 Figure 1.14 Proposed binding mode for a naphthyl-pyrimidine emitter and Al3+. Before chelation, free rotation results in low emission, after chelation, bright fluorescence is seen. Image adapted from reference.45 .............................................................................................................. 15 Figure 1.15 Structure of a piperazine-linked naphthalimide fluorescent chemosensor and the proposed mechanism for pH detection in mitochondria.50 ........................................................... 16 Figure 1.16 Protonation of a pyridyl heterohelicene caused red-shifted emission and CPL sign reversal. Image adapted from reference.51 .................................................................................... 16 Figure 2.1 A simple core framework can be easily modified at a tunable site (X) with substituents of different composition, size, oxidation state etc. ................................................... 19 Figure 2.2 Structures of N-functionalized naphthyl pyridine compounds. ................................... 22 Figure 2.3 (a) Normalized UV-Vis spectra of 1.0 x 10-5 M solutions of DNP compounds in CH2Cl2 (b) relative emission of DNP compounds in CH2Cl2 collected at an absorbance of 0.1, excited at λmax. Inset: Emission of DNP-Me under 365 nm irradiation. ....................................... 23 Figure 2.4 Relative emission spectra of DNP (solid line) and NP (dashed line) compounds taken in CH2Cl2 collected at an absorbance of 0.1, excited at λmax. ....................................................... 24 xii Figure 2.5 (a) Proposed conformations of two DNP-Me atropisomers (b) Two triplet peaks present in the 1H NMR spectra of DNP-Me in CD2Cl2 (400 MHz) at 25 °C at 𝛿 8.88 and 8.74 ppm and two singlet peaks at 𝛿 3.70 and 3.68 ppm. ..................................................................... 26 Figure 2.6 Structures of DNP-Me illustrating (a) free rotation at the blue bond of a naphthyl moiety (b) steric clash at the blue bond between the naphthyl and methyl groups. ..................... 27 Figure 2.7 1H NMR spectrum of DNP-Me (400 MHz, DMSO-d6) at 25°C. Atropisomer A/B is assigned. ........................................................................................................................................ 28 Figure 2.8 Crystal structure of Me-DNP with I- as the counterion solvated with dichloromethane (DCM). Ellipsoids are plotted at the 50% probability level. ........................................................ 29 Figure 2.9 VT NMR of DNP-Me (400 MHz, DMSO-d6) from 25-105 °C. The two red dots correspond to the two distinguishable protons of both DNP-Me atropisomers, no coalescence is seen and peaks stay separate and distinct from 25-105 °C. .......................................................... 30 Figure 2.10 2D NOESY spectra of DNP-Me in DMSO-d6 at 75°C. Atropisomers A and B are distinguishable at δ = 8.05, 7.97. A cross peak is present at elevated temperatures indicating chemical exchange is occurring between A and B. ...................................................................... 31 Figure 2.11. (a) Two atropisomers of DNP-Et and their conformations that lead to equivalent and diastereotopic peaks of the methylene protons in 1H NMR. (b) Close up of the three methylene peaks present in the 1H NMR spectrum of DNP-Et in CD2Cl2 (400 MHz at 25 °C) from 𝛿 4.05-4.70, integrations relative to Hb and Hc, doublet of quartets. ........................................................ 34 Figure 2.12 (a) Atropisomer ratios of DNP-Me overtime in ambient light in DCM and ACN (b) and in DMSO at 100°C. (c) Atropisomer ratios of DNP-Et overtime in ambient light in DCM and ACN (d) and in DMSO at 100°C. ................................................................................................. 35 xiii Figure 2.13. Relative (a) absorbance and (b) emission of methyl and ethyl DNP (solid line) and NP (dashed line) compounds in CH2Cl2 at 1×10-5 M collected at an absorbance of 0.1, excited at λmax. ............................................................................................................................................... 36 Figure 3.1 Crystal structure of indolium complex 1b with PF6 serving as the counterion. Ellipsoids are plotted at the 50% probability level. ...................................................................... 49 Figure 3.2 1H NMR spectrum of 1b (400 MHz, CD3CN) at 25°C. * = DCM, water ................... 49 Figure 3.3 Different concentrations of 1a (0.01M, 0.02M, 0.04M) in ACN irradiated with a 365 nm 5 W hand lamp for 24 h. Mole percentages for product 1b calculated using approximate NMR integrations.......................................................................................................................... 50 Figure 3.4 (a) Normalized absorbance and (b) emission spectra of compounds 1 and 2 in ACN at approximately 5.0  10-6 M for 1a and 2a and 5.0  10-5 M for 1b and 2b. Emission intensity for 1a, 2a and 1b has been increased (5) for clarity. (b) Inset: photographs of solutions of 1b and 2b respectively under 365 nm irradiation in ACN. ............................................................................ 55 Figure 3.5 Normalized emission spectra of 1b and 2b in DCM (solid line) and ACN (dashed line) at 5.0 × 10-5 M. .............................................................................................................................. 56 Figure 3.6 (a) Absorbance (dashed lines) and photoluminescence (solid lines) spectra of cyclized compounds 7b and 8b in ACN at 1.0  10-5 M. Inset: photographs of solutions of 7b (left) and 8b (right) under 365 nm irradiation in ACN. (b) Absorbance (dashed lines) and photoluminescence (solid lines) of cyclized asymmetric pyridyl-naphthyl compounds with different phenyl substituents (11b-13b) in ACN approximately 1.0  10-5 M. ....................................................... 60 Figure 3.7 Irradiation at 365 nm of (a) 11a and (b) 13a in ACN with H+ for 48 h at 0.015 M. % abundance calculated from approximate 1H NMR integrations. .................................................. 63 xiv Figure 3.8 (a) Absorbance and (b) emission spectra of compound(s) 11 at about 1.0  10-6 M (11a, 11a*(H+)) and 1.0  10-5 M (11b, 11c) in ACN. Inset. Emission of samples 11b (left) and 11c (right) under 365 nm irradiation. ............................................................................................ 65 xv List of Schemes Scheme 2.1 Synthetic route to disubstituted parent compound DNP and its N-functionalized derivatives. .................................................................................................................................... 40 Scheme 2.2 Synthetic route to monosubstituted parent compound NP and its N-functionalized derivatives. .................................................................................................................................... 43 Scheme 3.1 Photocyclization of pyridyl-naphthalene 1a/2a to yield iodolium complexes 1b and 2b................................................................................................................................................... 48 Scheme 3.2 An overcrowded 8-tert-butyl-(2-pyridyl) naphthalene is anodically oxidized to give a new C-N bond formation between the pyridine and naphthalene. Figure adapted from reference.102 ................................................................................................................................... 51 Scheme 3.3 Proposed mechanism of the photo-cyclization of 1a with the addition of acid to produce compound 1b. .................................................................................................................. 54 Scheme 3.4 Photocyclization of 11a results into multiple products: 11b, 11c and 11d. .............. 62 Scheme 3.5 Photochemical ring closure of 11b to form double cyclized 11c in ACN at 0.018 M using 365 nm or 395 nm light. ...................................................................................................... 65 Scheme 3.6 Reaction conditions for the synthesis of the pyridyl-naphthalene precursors 1a and 2a ................................................................................................................................................... 69 Scheme 3.7 Reaction conditions for the synthesis of symmetric or asymmetric chromophore substituted pyridine precursors with anthracene, pyrene and phenanthrene. ................................ 69 Scheme 3.8 Reaction conditions for the synthesis for asymmetric naphthyl-pyridyl chromophores. ............................................................................................................................... 71 Scheme 3.9 Reaction conditions for the synthesis of charged indolium species. ......................... 73 xvi Scheme 4.1 Thiophene and napthyl-pyridyl (NP) copolymers with NP doped in at 2-10 weight %. Multiple reactions at the nitrogen such as protonation, chelation and alkylation can occur (1) and can be followed by irradiation to yield annulated NP (2). ..................................................... 81 Scheme 4.2 8-bromoquinoline can be used to achieve novel architectures such as ionic N-perylenes or anthryl-quinolinium. ................................................................................................. 81 xvii List of Symbols Å Angstrom (10–10 m) 13C carbon °C degrees Celsius cm-1 wavenumber ΔE HOMO-LUMO energy gap 1H proton h Planck’s constant kf rate of fluorescence knr rate of non-radiative decay m/z mass-to-charge ratio W watt 𝛿 chemical shift (parts per million) ε molar absorptivity λ wavelength (nm) λmax wavelength at peak maximum υ frequency Φ quantum yield τ lifetime xviii List of Abbreviations ACN acetonitrile ACQ aggregation-caused quenching AIE aggregation-induced emission ATR attenuated total reflection COSY correlation spectroscopy CPL circularly polarized luminescence CT charge transfer d doublet DCM dichloromethane DMSO dimethyl sulfoxide EDG electron-donating group equiv equivalents ESI electrospray ionization EtOAc ethyl acetate eV electron volts EWG electron-withdrawing group FT-IR Fourier transform infrared spectroscopy g grams h hours HMBS heteronuclear multiple-bond correlation spectroscopy HOMO highest occupied molecular orbital xix HRMS high resolution mass spectrometry Hz hertz IC internal conversion IR infrared ISC intersystem crossing LED light emitting diode LUMO lowest unoccupied molecular orbital m multiplet m-CPBA meta-chloroperoxybenzoic acid Me methyl MeOH methanol mL milliliter mmol millimole MS mass spectrometry NaHCO3 sodium carbonate NIR near infrared nm nanometer NMR nuclear magnetic resonance NOESY nuclear Overhauser effect spectroscopy OFET organic field-effect transistor OLED organic light emitting diode OPV organic photovoltaic PCE power conversion efficiency xx PET photoinduced electron transfer PLQY photoluminescent quantum yield ppm parts per million RIR restricted intramolecular rotation RISC reverse intersystem crossing s singlet S0 ground state S1 lowest singlet state SET single electron transfer T temperature t triplet T1 lowest triplet state TFA trifluoroacetic acid TLC thin-layer chromatography TM transition metal TPE tetraphenylethene UV ultraviolet Vis visible VT variable temperature XRD X-ray diffraction xxi Acknowledgements This thesis would not have been possible without the support and encouragement from many individuals. I would first like to thank my supervisor, Mike Wolf, for his guidance and enthusiasm over the past three years. Your patience, vast knowledge, and willingness to sit and discuss unusual chemical reactions for hours made working with you an absolute pleasure (not to mention your willingness to buy us copious pizza, ice cream and beer!). I couldn’t have asked for a better supervisor and mentor. I would also like to offer a huge thanks to my fellow lab mates, past and present, that helped my experience be unforgettable. To my elders: Chris, Allen, Nicole, Duane, Jenn and Taylor, thank you for all your expertise and mentorship, in chemistry and in life. I would be lost without you! To my junior colleagues, Tobias and Urmi, thank you for bringing in lots of energy to the group, it is never a dull moment with you guys around! A special shout out to Taylor for always being available for a cup of coffee and to discuss project ideas, chemistry struggles and life crises. Also, to Jenn for being my bachelor viewing gal pal and vital support system through it all. An additional thanks to our honorary lab-mates; the MacLachlan’s, for their company and their chemicals. A huge thanks to Jeanette and Charlotte for their friendship and to Miguel and Mo for being bottomless pits of chemistry knowledge. xxii A big thank you to Maria, Paul, Saeid, and Brian and all the faculty and staff in the chemistry department - you are amazing at what you do! Thank you to the electronics, glass and mech shops (especially Ken) for knowing how to fix anything (even if it looks beyond repair). Thank you to everyone I have TA’d with over the last three years, especially to José and Christina who made my time teaching 211 rewarding and invaluable! Most importantly, I would like to thank my friends and family for their unconditional love and support over the last three years. I wouldn’t have made it without their constant phone calls, video chats, pictures and visits, always reassuring me that I can do it! Last but not least, a big thank you to Gabriel for coming on this wild adventure to Vancouver with me and for being the most patient and loving partner I could ask for. I am forever grateful for the thousands of meals you cooked for me and the (at least) one time you washed the dishes. I couldn’t have done it without you! xxiii Dedication To my wonderful parents, Anna Yunyaeva and Sergey Yunyaev 1 Chapter 1: Introduction 1.1 Light and Luminescence Light has played a defining role in human existence since the beginning of time. The oxygen humans breathe is generated by photosynthesis, a light-driven process which occurs in plants, algae, and cyanobacteria. The ability for humans to see is the result of visible light reflecting off of objects and entering our eyes. Even cultural aspects of society such as architecture and art use light to create awe-inspiring effects such as the Pantheon in Rome or Gaudi’s cathedral in Barcelona (Figure 1.1). Figure 1.1 Use of natural light in (a) the Pantheon through an oculus and in (b) stained-glass windows in Sagrada Familia Cathedral. Humans rely on light for almost all their daily activities and for thousands of years relied solely on natural sunlight or from combustion (i.e. fire). Nowadays, with modern technologies, light can be generated in many ways. Luminescence is the spontaneous light generation (emission of photons) derived from energy sources that are not heat based (incandescence).1 Luminescent materials or organisms can convert “invisible” energy sources such as electricity, mechanical 2 motion, electromagnetic radiation, chemical reactions and radiation into “visible” energy in the form of light. Some notable examples include televisions and smartphone screens (electroluminescence), bioimaging dyes (fluorescence), glow-in-the-dark watch dials (phosphorescence) and deep-sea creatures that generate light as a result of biological reactions (chemiluminescence).2–5 Luminescence is often produced in the visible (vis) region (400–750 nm), but can also be generated in regions that are not detected by the human eye such as ultraviolet (UV) (200–400 nm) or near infrared (NIR) (750–1500 nm). Due to the vast number of applications for luminescent molecules, the development of new materials with improved optical properties (specific absorption and emission wavelengths, high quantum yields) specifically tailored to meet the individual needs of current and future applications is an important field of research. Additionally, materials with low toxicity to biological and environmental systems, increased stability, and lower production costs are desired for practical applications. It is through constant research and innovative design that scientific boundaries are pushed and methods and materials are developed that will not only enhance the lives of humans but of the whole planet. 1.2 π-Conjugated Materials Figure 1.2 Examples of π-conjugated chromophores. Chlorophyll A is found in plants and -Carotene is found in many vegetables such as carrots, pumpkin, and sweet potato. 3 Compounds that absorb light are referred to as chromophores and are responsible for the intense colours observed in nature such as green in plants (chlorophyll A) and orange in carrots (β-carotene) (Figure 1.2). A common structural feature of chromophores is a conjugated -system, comprising of three or more adjacent p-orbitals. For a molecule with limited π-bonds (one or two), the energy difference between the HOMO and LUMO is typically too large for an electron to be promoted using visible light. However, as conjugation length increases, so does the contribution from a greater number of molecular orbitals, thereby narrowing the energy gap (ΔE) (Figure 1.3a). Figure 1.3 (a) Increasing conjugation length decreases the energy gap between frontier orbitals (b) Increasing conjugation length in PAHs increases their absorption wavelength. 4 This allows for lower energy light (such as long wave UV or visible light) to be used for promotion of an electron into an excited state. A molecule with fewer than eight π-bonds will usually absorb in the UV region, such as polyaromatic hydrocarbons (PAHs) naphthalene and anthracene. However, as the conjugation length increases, such as in tetracene, the absorption of light will shift into the visible region. (Figure 1.3b). Recently, interest has increased in using pure organic π-conjugated scaffolds for applications such as optoelectronics (OLEDS, OPV, OFETS).6–13 Organic materials have numerous advantages over their inorganic analogs such as lower toxicity and solution processibility which allows for lower manufacturing costs.14,15 Additionally, organic molecules can be synthesized using well developed methods and easily tuned to obtain specific optical and electronic properties.16 These materials are easier to integrate into devices and can be made to be transparent, lightweight, and flexible. The rapid growth of this field makes the development of novel organic materials an important task which requires a detailed understanding of the fundamental pathways occurring upon light absorption. 1.3 Photophysical Principles 1.3.1 Fundamentals When a molecule is exposed to an energy source, the energy can be absorbed and result in promotion of an electron from the highest energy filled orbital, the HOMO, into the LUMO. The excited state is unstable, and the excess energy dissipated as light and/or heat, returning the molecule to the ground state (Figure 1.4). Most commonly, there are at least two different electronic excited states: a singlet (S) state, where the two electrons have opposite spin (paired) 5 and a multiplicity of one, or a triplet (T) state, where the electrons have the same spin (unpaired) and multiplicity of three. Figure 1.4 Schematic representation illustrating the absorption of energy (depicted as light energy) and emission of light in a luminescent material. The various luminescence processes possible can be shown using a Jablonski diagram, first depicted by Aleksander Jablonski in 1933 (Figure 1.5).17 The first event that occurs is the absorption of light energy (hυ) leading to the promotion of an electron to a higher energy state. This can only occur when the incoming photon has energy equal to or greater than the energy between the two electronic states. The S0→Sn process happens quickly on a time scale of femtoseconds (10-15 s). The initial singlet excited state is unstable and will relax back down to the ground state through several different relaxation pathways. The fastest subsequent process to occur is vibrational relaxation or internal conversion (IC) which returns the electron to the lowest excited vibration level or electronic state. This is a nonradiative event and occurs on time scales of 10-14-10-11 s. From the lowest excited singlet state (S1), two main processes compete for the S1→S0 transition. The energy can be dissipated non-radiatively as heat, occurring from the vibrational and rotational motion of the molecule, or radiatively through the emission of a photon. The latter 6 process is termed fluorescence and occurs on timescales of 10-9-10-7 s. In some molecules, intersystem crossing (ISC) can occur, which involves an electron spin flip yielding two electrons with parallel spin (triplet state, S1→T1) (Figure 1.5). From the T1 state, two processes can occur, non-radiative relaxation or the radiative transition known as phosphorescence, which involves another spin flip to return the electron back down to the ground state with an anti-parallel configuration (T1→S0). The second spin flip prolongs the excited state lifetime of the triplet state to 10-6-10-1s. Figure 1.5 Simplified Jablonski diagram illustrating different radiative and non-radiative transitions involved in luminescence. It is important to note that all processes discussed above are in competition, radiative decay (fluorescence and phosphorescence) compete with one another as well as with non-radiative decay. Since practical applications require materials with high photoluminescence quantum yields 7 (PLQY), many methods to increase PLQY have been explored such as fine-tuning energy levels, reverse intersystem crossing (RISC) and suppression of non-radiative decay. This thesis will study molecules that suppress non-radiative decay and promote photoluminescence, involving the synthesis of specific molecular structures where vibrational and rotational modes are reduced. 1.3.2 Photoluminescent Quantum Yield (PLQY) For practical applications, materials with high photoluminescence quantum yields (PL) are highly desired. Quantum yield represents the efficiency of a photophysical process and is defined by Eq 1.1 (Figure 1.6) Figure 1.6 Equations describing photoluminescence quantum yield (ΦPL) . Competition between different relaxation processes such as fluorescence, phosphorescence and non-radiative decay are governed by rate constants (Eq 1.2) Where the rate of non-radiative decay (knr) processes include IC and ISC. For organic molecules, ISC is usually not a major contributor. However, relaxation by internal conversion via non-radiative decay (i.e. heat) is the main process that lowers PLQY. Non-radiative decay typically occurs from the vibrational and rotational motion of molecules, arising in freely rotating, non-rigid molecules. Therefore, there is a great interest in blocking these non-radiative pathways, so photoluminescence is promoted and non-radiative transitions are reduced. 8 1.4 Methods to Increase Photoluminescent Quantum Yields One technique to improve PLQY is aggregation induced emission (AIE).18–20 AIE mechanisms usually employ molecules with freely rotating aryl groups, an archetypical example being tetraphenylethene (TPE), (Figure 1.7). In dilute solution TPE demonstrates low PLQY due to excited state deactivation by the dynamic intramolecular rotation of the phenyl rotors.21 Upon aggregation (in concentrated solutions or in the solid state), TPE molecules experience physical constraints which deactivates radiationless decay channels (through rotation) and allows radiative relaxation (emission) to occur. AIE molecules have been investigated for use in biological sensors, chemical probes and in optoelectronic devices.20 A limitation of AIE is the requirement for specific solvents, high concentrations and low PLQY when not aggregated. Figure 1.7 In solution TPE rotates dynamically resulting in weak emission, upon aggregation rotational pathways are reduced and intense emission is produced. Image is adapted from the original reference.22 A method that is less solvent dependent and can be used at low concentrations is the introduction of substituents to sterically crowd molecules. By conformationally restricting the molecule and reducing nonradiative decay, aggregation is not necessary. For example, Prasad and coworkers demonstrated several BODIPY dyes with which modifications at the meso position induce steric crowding in the molecule (Figure 1.8). 9 Figure 1.8 A systematic increase in PLQY is seen in BODIPY dyes with an increase in steric hindrance. Image adapted from reference.23 Upon adding a methyl group to the meso position of the dye, steric hindrance between the methyl and phenyl group increases the PLQY 3-fold. Extending the phenyl group into a naphthalene, further crowds the molecule and restricts the conformation; a quantum yield near unity is achieved in this compound.23,24 Benefits of this method include wide solvent compatibility, and easy rationalization of further synthetic steps increase PLQY. In this thesis, this method will be explored in Chapter 2. Another method to decrease non-radiative decay rates and increase PLQY in organic chromophores is the use of permanent structural constraint to create planar and rigid molecules.25–27 For example, Yamaguchi et al. present a freely rotating chromophore (9-phenylanthracene) (Figure 1.9).28 Upon annulation of the ring system, 9-phenylanthracene is constrained in a coplanar fashion by two methylene tethers yielding a π-conjugated rigid and planar structure. An increase in molar absorptivity, bathochromic shifts and fluorescence intensity are observed, a decrease in the rate of non-radiative decay by an order of magnitude is seen. 10 Figure 1.9 The physical constraint achieved in 9-phenylanthracene by annulation leads to rigidification and planarity yielding enhanced photophysical properties.28 This is popular method especially useful in photovoltaic materials and transistors as planar architectures are preferred due to their increased conjugation lengths which yields lower bandgaps and increases their ability to foster bulk long range intermolecular interactions.29 Their use in emissive materials is also beneficial but a fine balance is needed between rigidness (to prevent non-radiative decay) and disorder (to prevent aggregation issues, ACQ). This method will be explored in Chapter 3. 1.5 Heteroatoms in Emissive Molecules Incorporation of heteroatoms provides a straightforward and powerful way to modify molecular structure and alter properties. Heteroatoms such as boron, silicon, pnictogens, chalcogens and halogens can add functionality and tunability to purely organic-based compounds. Heteroatoms can change electronic properties by altering molecular geometry and packing, shifting the frontier energy levels leading to altered photophysical properties.35–38 For example, Nguyen and coworkers investigated the effect of changing the heteroatom in diketopyrrolopyrroles (DPP) (Figure 1.10). 11 Figure 1.10 Chemical structures of three DPP derivatives with different ending heterocycles. Figure adapted from reference.33,31 These authors noted that while optical properties such HOMO-LUMO gap and absorption wavelengths remained relatively similar, physical properties such as solubility, thermal stability, morphology, charge carrier ability and photovoltaic performance were altered significantly, attributed to changes in molecular packing seen in crystal structures. Grubbs and co-workers examined the effects of chalcogen (S, Se and Te) substitution with a similar DPP molecule (Figure 1.10).31 They found that the optical bandgap could be finely tuned by modulating the electron donating ability of the chalcogen (S>Se>Te) and obtained different efficiencies when incorporated into OPV devices with Se derivatives performing best (PCE = 5.1 %). Photophysical properties such as absorption/ emission wavelengths and PLQY can also be altered upon heteroatom modulation.34,35 These studies highlight the importance of developing and understanding structure-property relationships in relation to heteroatom effects and are key to developing efficient organic electronics as well as luminescent emitters and dyes. A limitation of this approach is these heteroatom substitutions cannot be carried out in situ, and compounds must be synthesized to obtain different properties. Heteroatoms with efficient and in situ reactivity provide a valuable scaffold for stimuli-responsive materials. For example, nitrogen-boron-containing emitters, are susceptible to 12 fluorescence changes upon complexation (Lewis pair formation). Murata et al. report a diborylated-thiophene-benzothiadiazole derivative with thermochromic absorbance properties based on reversible Lewis pair formation (Figure 1.11).36 Figure 1.11 Intramolecular B-N Lewis pair formation resulting in photophysical changes in diborylated-thiophene-benzothiadiazole thin films. Image adapted form reference.36 In the open form, the boryl derivative exists as a bright yellow compound in the solid state, while in the closed state it is a deep blue colour arising from the intramolecular Lewis-pair formation between the boron and nitrogen. In solution, reversal of open/closed stated states results in thermochromism. Lower temperatures (-40°C) favour the Lewis-pair while higher temperatures favour the freely rotating open form (100°C). Many studies exist on similar borylated compounds that show not only temperature dependent fluorescence but also induced photophysical changes based on other stimuli.37–39 Our group has shown that incorporation of sulfur bridges in organic chromophore allows for control of photoluminescent quantum yield by sulfur oxidation. This was demonstrated in molecules containing bithiophene, terthiophene, naphthalene and pyrene chromophores (Figure 1.12).40 The sulfide molecule can be sequentially oxidized to the sulfoxide, then sulfone with an increase in PLQY observed at each step. Computational studies reveal that different electronic 13 effects are responsible for increase PLQY between sulfoxide and sulfone such as reduced screening in sulfur lone pairs and stabilized charge transfer states. Figure 1.12 Oxidation of sulfur-bridged of naphthyl/pyrene chromophore systems result in a PLQY increase. Building on this motif, this thesis explores a similar bridged species, where modifications are performed on a heteroatom that bridges two chromophores. Here, a focus on steric changes and their impact on PLQY are investigated. 1.6 Nitrogen-Containing Luminescent Materials Nitrogen-containing compounds are ubiquitous, they are found in natural products and synthetic materials, find applications in pharmaceuticals, agricultural and in material science, and play an important role in biologically relevant pathways.41 The range of structural diversity in nitrogen-containing molecules is vast and can range from simple functional groups to complex heterocyclic networks. In particular, nitrogen-containing heterocycles such pyridine, pyrimidine, quinoline, pyrrole and carbazole are common building blocks found in luminescent materials. The lone pair on nitrogen is Lewis basic, which makes the nitrogen atom susceptible to substitution and oxidation reactions (Figure 1.13). In situ reactions where the nitrogen atom is protonated, 14 chelated to metals or reacted with Lewis acidic moieties can result in changes in the photophysical properties making them ideal candidates for stimuli-responsive materials. Figure 1.13 A small portion of nitrogen’s reactivity scope. Some reactions can occur in situ (protonation, chelation, Lewis pairing), while others require longer reaction conditions (oxidation, alkylation, arylation). The strong chelation ability of nitrogen-containing heterocycles make it a commonly used motif in fluorescent probes for the detection of metals. The large photophysical response in nitrogen containing emitters upon chelation such as turn-on/turn-off fluorescence or emission red-shifts makes it an ideal moiety in sensing applications.42–44 For example, a naphthyl-pyrimidine derivative was prepared as a selective probe for an Al3+ ion (Figure 1.14).45 In its free ligand form, weak emission at 470 nm is observed, low yields are attributed to the cis-trans interconversion between the C-N bond. Upon binding to Al3+ an intense green fluorescence can immediately be visualized at 505 and 538 nm. Fluorescence enhancement of the ligand is observed due to inhibition of the isomerization and the production of a rigid host-guest framework.46 15 Figure 1.14 Proposed binding mode for a naphthyl-pyrimidine emitter and Al3+. Before chelation, free rotation results in low emission, after chelation, bright fluorescence is seen. Image adapted from reference.45 The ability of nitrogen to be protonated has also been exploited in numerous applications such as fluorescence enhancement, absorption/emission modification and yielding multi-colour fluorescent materials.47–49 It has even been demonstrated as an effective probe for measuring specific pH values inside living cells. For instance, Sessler and coworkers developed a mitochondria-immobilized fluorescent chemosensor (Figure 1.15) to measure pH.50 The system consists of a piperazine-linked naphthalimide fluorescent unit attached to mitochondrial targeting groups. At neutral mitochondrial levels (pH = 8), photoinduced electron transfer (PET) from nitrogen’s lone pair quenches the fluorescent naphthalimide unit. Upon mitochondrial stress (i.e. nutrient starvation), the pH levels decrease within the mitochondrial matrix, protonating the piperazine ring, thereby quenching the PET effect and enhancing fluorescence emission at 525 nm. This biocompatible probe affords the direct and reliable measurement of mitochondrial pH as well as real-time pH monitoring. 16 Figure 1.15 Structure of a piperazine-linked naphthalimide fluorescent chemosensor and the proposed mechanism for pH detection in mitochondria.50 Electronic changes at the nitrogen can give rise to conformational changes in the whole structure and lead to interesting emissive properties. For example, Audisio and coworkers developed a series heterohelicene molecules with a pyridyl nitrogen site.51 The initial unprotonated molecule displayed blue circularly polarized luminescence (CPL). In the presence of acid, red-shifted emission (to orange) was observed, and the sign of CPL was switched (Figure 1.16). Figure 1.16 Protonation of a pyridyl heterohelicene caused red-shifted emission and CPL sign reversal. Image adapted from reference.51 17 This was attributed to the change in direction of intramolecular charge transfer upon protonation. Similar CPL sign reversal upon protonation of pyridyl chromophores but with no fluorescence colour changes were also observed in binaphthyl dyads.52 The breadth of nitrogen-containing emitters is wide and continues to expand with the discovery and development of new N-containing molecules. This thesis will explore the simplest of nitrogen-containing heterocycles: pyridine. Pyridine provides a simple tunable site that can be protonated, oxidized, alkylated, chelated and annulated to suppress molecular motion, with the impact of substitution on PLQY studied. 1.7 Thesis Scope The work described in this thesis focuses on tuning the photophysical properties such as emission colour and PLQY of pyridine-containing fluorophores using chemical and photochemical modifications. Chapter 2 describes the design and synthesis of pyridyl naphthyl compounds and the changes in photophysical properties observed when functionalized through protonation, oxidation, and alkylation with different sized substituents. Compounds with one naphthyl group (D-A) and two naphthyl groups (D-A-D) are prepared and the impact of a second chromophore is studied. Changes in PLQY are studied for molecules with substituents that cause significant restriction to molecular motion seen as atropisomers. This effect is studied by NMR spectroscopy as well as with absorption/emission spectroscopy. Chapter 3 explores a new photochemical reaction involving an intramolecular C-N bond formation with a pyridyl-bridged di-naphthyl chromophore. This reaction yields a new cyclized and rigid product with drastically different photophysics than its starting material. Derivatives containing anthracene, pyrene and phenanthrene are prepared and studied. The mechanism of this new light-driven, photocatalyst free 18 reaction is explored through kinetic experiments as well as derivatives. A second subsequent photochemical C-C bond formation in explored in molecules with electron deficient functional groups. The photophysics are studied and compared for non, single, and double annulated products. 19 Chapter 2: Photophysics of Structurally Tunable Pyridine-Bridged Chromophores 2.1 Introduction Emissive organic materials are currently attracting increasing attention due to their uses in applications such as electronics, biological and environmental sensors, bioimaging dyes, and lasers.53–62 The ease with which such materials can be chemically functionalized to alter properties such as color and brightness makes them ideal candidates for the development of low cost and efficient materials. In addition, organic chromophores are more easily processed and integrated into devices and substrates.63 It is especially interesting to develop emissive molecules based on simple architectures, incorporating molecular building blocks with a tunable site.64 Using a versatile core, molecules can be modified post-synthesis with various substituents to give desired photophysical properties such as emission color, intensity and lifetime.65,66 This can give rise to a series of structurally similar compounds with distinct properties but using fewer synthetic steps and reagents (Figure 2.1). Figure 2.1 A cartoon depicting a simple core framework that can be easily modified at tunable site (X) with substituents of different composition, size, oxidation state etc. 20 One of the most desired properties in an emissive molecule is high photoluminescence quantum yield (PLQY). High PLQY is an important consideration in applications such as OLED displays (brightness) or biological dyes and sensors (high signal to noise ratios).67–69 Common strategies for achieving high PLQY include incorporating rigid and large π-conjugated aromatic systems and twisted donor-acceptor (D-A) structures. These structural motifs can give rise to molecules with intense emission while suppressing non-radiative decay pathways that may arise from molecular rotation and vibrations. An example of a method to reduce molecular motion is aggregation induced emission (AIE).18 This technique utilizes aggregation in solutions of high concentration or poor solubility to restrict intramolecular rotation (RIR) of molecules. Molecular rotations can also be controlled by external factors such as decreased temperature and increased solvent viscosity, or intrinsically through built-in rigid molecular components, inducing a high barrier to rotation. This can be done by introducing sterically large substituents or via non-covalent interactions (ionic complexes, H-bonding, metal chelation).70 However, this may involve the design of complex architectures that can introduce synthetic difficulty and increase costs. Nitrogen containing heterocycles possess a reactive nucleophilic site that is well suited for further functionalization. These heterocycles are ubiquitous in natural products, pharmaceuticals, agrochemicals as well as many luminescent chromophores and dyes.41 Our group has previously studied the photophysical effects of protonation, oxidation and alkylation in nitrogen containing compounds.71 In this chapter, a system is presented that incorporates a simple pyridine-containing building block and uses the heterocyclic nitrogen as a tunable site for a variety of structural modifications. A donor-acceptor-donor (D-A-D) template with pyridine as the modifiable bridging 21 acceptor and naphthalene as the donor is used. The role of various small functional groups are correlated with the photophysical properties in a series of related chromophores. The bridged binapthyl system (D-A-D) is studied alongside a monosubstituted naphthyl-pyridine system (D-A) allowing a probe of whether photophysical changes are arising from chemical modification at the nitrogen or induced steric hindrance/electronic coupling from the presence of both chromophores. In addition, the correlation between the size of a substituent and its ability to influence and restrict free rotation within a molecule and its role in enhancing PLQY is investigated. These results reveal a promising method to increase photoluminescent quantum yield of chromophores by inducing permanent restricted rotation to previously unhindered systems, providing useful insights into the design and development of simple and functional fluorescent molecules. 2.2 Results and Discussion 2.2.1 Synthesis To investigate the effects of nitrogen functionalization in pyridine bridged chromophore systems, a parent molecule with a modifiable core flanked by two chromophores was designed. Naphthalene was selected as the pendant chromophore because of its well-studied photophysical properties and small size. Bi-naphthyl dimers have also been extensively researched due to their axial chirality as a result of restricted rotation about the 1,1' bond.72,73 The disubstituted parent compound 2,6-di(naphthalen-1-yl) pyridine (DNP) was synthesized in 94% yield by Suzuki-Miyaura coupling between 2,6-dibromopyridine and 1-naphthaleneboronic acid. Next, a series of substitutions were performed on DNP at the nitrogen. Protonation of DNP yields the pyridinium complex, DNP-H, which was obtained in situ by adding 22 trifluoroacetic acid (TFA) to DNP in dichloromethane (DCM). Oxidation of DNP with m-CPBA gives the pyridine N-oxide (DNP-O) in 79% yield. Lastly, methylation of DNP affords the methylated analog DNP-Me in 60% yield. The monosubstituted compound, 2-(naphthalen-1-yl) pyridine (NP) was also prepared to probe whether photophysical changes are a result of modification at the pyridine or steric hindrance between chromophore arms. All previously described reactions, excluding oxidation, were also performed on NP. A summary of the synthesized compounds is shown in Figure 2.2. Figure 2.2 Structures of N-functionalized naphthyl pyridine compounds. 2.2.2 Absorbance and Fluorescence Spectroscopy of DNP Compounds The absorbance of the bichromophoric DNP compounds was studied to probe the effects of nitrogen modification on the photophysics of the di-naphthyl pyridyl core. All compounds show a featureless broad absorbance peak from 260-300 nm corresponding to a local excited (LE) state. DNP-Me and DNP-H show an additional broad peak at 356 and 370 nm respectively, assigned to a charge transfer (CT) transition (Figure 2.3a).74 The molar absorptivity of all DNP and NP compounds increases with the size of the molecule (i.e. DNP > NP > naphthalene) due to the increased conjugation (Table 2.1). 23 Figure 2.3 (a) Normalized UV-Vis spectra of 1.0 × 10-5 M solutions of DNP compounds in CH2Cl2 (b) relative emission of DNP compounds in CH2Cl2 collected at an absorbance of 0.1, excited at λmax (DNP: 299 nm, DNP-H: 370 nm, DNP-O: 289 nm, DNP-Me: 356 nm). Inset: Emission of DNP-Me under 365 nm irradiation. Excitation of DNP using 300 nm light results in emission in the UV region, centered at 370 nm and a ΦF of 0.05 in DCM. (Figure 2.3b). The low quantum yield is attributed to quenching of the emission via photoinduced electron transfer (PET) involving the lone pair of electrons on the pyridine.75 This has been previously reported for numerous nitrogen-containing chemosensors.59 DNP-O shows no observable fluorescence, attributed to PET with the oxygen lone pairs.43 DNP-Me and DNP-H both show enhanced emission relative to DNP in the visible region, emitting at 485 nm and 457 nm respectively. DNP-Me has almost a 9-fold increase in ΦF compared to DNP at 0.46, while DNP-H shows a 16-fold increase with a ΦF of 0.80. These results indicate the need to bind the pyridine lone pair to suppress PET as an important factor to consider in the enhancement of PLQY. 24 2.2.3 Fluorescence of Mono and Disubstituted Compounds Figure 2.4 Relative emission spectra of DNP (solid line) and NP (dashed line) compounds taken in CH2Cl2 collected at an absorbance of 0.1, excited at λmax (DNP: 299 nm, NP: 293 nm, DNP-Me: 356 nm, NP-Me: 336 nm, DNP-H: 370 nm, NP-H: 341 nm). It is interesting to compare the mono-substituted, single chromophore NP compounds with the disubstituted DNP compounds to determine the impact of the second chromophore. The absorbance spectra of DNP and NP closely match one another in shape and λmax. The same trend is seen in methylated and protonated pairs respectively with slight redshifts for the bichromophore molecules (DNP-H and DNP-Me), likely as a result of the extended π conjugation (Table 2.1). The fluorescence of monosubstituted and di-substituted naphthyl compounds was also studied to examine any differences arising from the presence of the extra chromophore. (Figure 2.4). Comparing DNP and NP, both show similar emission spectra and have a PLQY of 0.05. Protonated compounds DNP-H and NP-H both have a large PLQY of 0.80 and 0.86 and emit at 457 nm and 459 nm respectively. Their similar ΦF demonstrates that protonation of the pyridine center has a 25 larger effect on PLQY than the presence of a second chromophore. Interestingly, the methylated compounds DNP-Me and NP-Me do not show the same trend for PLQY values between mono and di-substituted species. NP-Me has a PLQY of 0.15, a 3-fold increase from its parent compound NP, while DNP-Me has a QY of 0.46, an 8-fold increase from DNP and approximately three times that of NP-Me. The large ΦF increase in DNP-Me relative to NP-Me indicates that the presence of a second chromophore along with methylation of the pyridine has a strong contribution to the increased PLQY. A potential rationale for this phenomenon is explored in the next section. All photophysical data are summarized in Table 2.1. Table 2.1 Photophysical properties of DNP and NP compounds measured in DCM Compound λabs/nm λem/nm ε/M-1 cm-1 (x 103) Φf a Τf/ns kf/s-1 (x 107) knr/s-1 (x 107) DNP 299 373 13.9 0.05 N/A N/A N/A DNP-O 289 367 15.4 0.00 N/A N/A N/A DNP-H 269, 370 457 9.26, 5.56 0.79 4.203 18.9 4.88 DNP-Me 273, 356 485 16.1, 5.59 0.46 4.977 9.28 10.8 DNP-Et 275, 355 498 14.7, 3.35 0.55 5.452 10.1 8.25 NP 293 369 8.54 0.05 N/A N/A N/A NP-H 257, 341 439 6.98, 3.66 0.86 7.644 11.3 1.82 NP-Me 285, 335 468 6.95, 2.87 0.16 7.018 2.24 12.0 NP-Et 285, 335 472 6.48, 1.90 0.20 7.910 2.53 9.86 a Quantum yields were measured in triplicate with an approximate error of ± 0.03. 2.2.4 NMR spectroscopy of DNP-Me 1H NMR spectra for all mono-substituted compounds (NP, NP-H, NP-Me) and the di-substituted species DNP and DNP-H showed sharp peaks attributed to a single structure in 26 solution. However, the 1H NMR spectrum of DNP-Me reveals the presence of two distinct species. This was observed as a doubling of the expected number of peaks in the 1H and 13C NMR spectra (Figure A.4/A.5). The 1H NMR spectrum of DNP-Me in CD2Cl2 (Figure 2.5b) shows two clear triplets at 𝛿 8.7-8.9, assigned to the para protons of the pyridine ring (Figure 2.5a). At 𝛿 3.68 and 3.70, two distinct singlets, assigned to the methyl protons in DNP-Me are observed integrating in the same ratios as the aromatic para-position triplets (Figure 2.5b). All other proton peaks in CD2Cl2 overlap and separate integrations could not be obtained. This data is consistent with two isomeric structures of DNP as a result of the methylation. Figure 2.5 (a) Proposed conformations of two DNP-Me atropisomers (b) Two triplet peaks present in the 1H NMR spectra of DNP-Me in CD2Cl2 (400 MHz) at 25 °C at 𝛿 8.88 and 8.74 ppm and two singlet peaks at 𝛿 3.70 and 3.68 ppm. 27 As a result of the symmetrical structure of DNP-Me, the two species were identified as atropisomers, a form of rotational isomers (rotamers), present due to hindered rotation about the single bond between the pyridine and naphthalene rings. Many examples exist of such axially chiral molecules and they are ubiquitous in nature and biological systems.76 Although separation of these atropisomers is possible, it is challenging due to the dynamic nature of molecular rotation.77 We were not successful in separating the atropisomers of DNP-Me in bulk and worked with the compound as a mixture in solution. In the parent compound DNP, the naphthyl groups rotate freely. Upon methylation, the methyl group sterically hinders free rotation of the naphthyls, thus, resulting in two distinct atropisomers in solution, resolvable by NMR spectroscopy. These are proposed to be anti and syn forms (Figure 2.5a). The syn-isomer adopts a conformation where the naphthyl rings face the same direction (cis) whereas in the anti-isomer the naphthyl rings point in opposite directions (trans). Figure 2.6 Structures of DNP-Me illustrating (a) free rotation at the blue bond of a naphthyl moiety (b) steric clash at the blue bond between the naphthyl and methyl groups. 28 Rotation of one of the naphthyl groups about the bond with pyridine from a perpendicular to parallel position with respect to the pyridine can occur due to the staggered position of the methyl hydrogens (Figure 2.6a). Conversely, full rotation of the naphthyl on the opposite side of the molecule is blocked by the methyl hydrogen, (eclipsed with the naphthyl-pyridine bond) (Figure 2.6b). The naphthyls can therefore interconvert between anti and syn forms, but there is a high barrier to rotation. Using 2D NMR (NOESY, COSY, HMBC, HSQC) spectroscopies in DMSO-d6 (Figure A.13-16) all protons in the molecule can be assigned and the atropisomer they belong to, although which of the two isomers (syn/anti) each is cannot be determined from the available NMR data and thus they are referred to as A and B (Figure 2.7). Figure 2.7 1H NMR spectrum of DNP-Me (400 MHz, DMSO-d6) at 25°C. Atropisomer A/B is assigned. 29 Single crystals of DNP-Me were grown from slow evaporation in a DCM solution (Figure 2.8). X-ray diffraction results reveal one molecule in the asymmetric unit, solvated with DCM. Both naphthyl groups in the structure face the same direction and are almost perpendicular to the plane of the pyridine with dihedral angles of 75.75° and 87.35° respectively, adopting the syn conformation. Figure 2.8 Crystal structure of Me-DNP with I- as the counterion solvated with dichloromethane (DCM). Ellipsoids are plotted at the 50% probability level. To probe the energy difference between the two atropisomers, variable temperature (VT) NMR experiments were performed in DMSO-d6 from 25 to 105°C. The distinguishable isomer peaks at 𝛿 7.94 and 8.04 in DMSO-d6 (Figure 2.9) did not show any coalescence over this temperature range, indicating that there is a high rotational barrier between the two atropisomers. 30 Figure 2.9 VT NMR of DNP-Me (400 MHz, DMSO-d6) from 25-105 °C. The two red dots correspond to the two distinguishable protons of both DNP-Me atropisomers, no coalescence is seen and peaks stay separate and distinct from 25-105 °C. 2D VT NOESY experiments in DMSO-d6 at 75 °C reveals a cross peak at two distinguishable locations on the 2-position of the naphthyl group (𝛿 7.94 and 8.04) indicating that chemical exchange between rotamers occurs at elevated temperatures (Figure 2.10). 31 Figure 2.10 2D NOESY spectra of DNP-Me in DMSO-d6 at 75°C. Atropisomers A and B are distinguishable at δ = 8.05, 7.97. A cross peak is present at elevated temperatures indicating chemical exchange is occurring between A and B. The ratio of isomers in the 1H NMR experiments was observed to change as a function of time. Initially, in all solvents tested (CD2Cl2, CD3CN, DMSO-d6), a ratio of approximately 20:80, A: B is observed. This is attributed to one of the atropisomers being more kinetically accessible during the coupling reaction and therefore formed in greater amounts compared to the other. The NMR peaks cannot be unambiguously assigned to a specific isomer. When left in ambient light at room temperature (RT), the atropisomers reach an approximate 50:50 ratio within 5 hours at RT or 1 hour of heating at 100°C (Figure 2.12a/b). This demonstrates that the barrier to rotation can 32 be overcome slowly at RT or faster with heating. From the data collected, unhindered rotation is not possible (or possible only at extremely high temperatures) as coalescence isn’t observed in the NMR spectra, only interconversion between syn and anti-forms. A control experiment was also completed in darkness to rule out photochemical conversion with similar results obtained. These NMR experiments show that substantial steric hindrance can be induced with only the addition of a single N-methyl group in a pyridine-bridged bi-naphthyl compound. While DNP-H did not exhibit rotamers, DNP-O did show evidence of atropisomerism by peak doubling in its 13C NMR spectra, however this compound was not studied further due to its non-emissive nature. This demonstrates that N-substituent size is an important consideration and there is a steric threshold for restricted intramolecular motion (RIR) effects to be observed. 2.2.5 PLQY of DNP-Me and NP-Me The significant PLQY increase from NP-Me (0.15) to DNP-Me (0.46), which was not observed in the non-methylated analogs, correlates with the observed NMR data (Figure 2.7). The presence of the two atropisomers in the 1H NMR spectrum of DNP-Me demonstrate that the compound has restricted intramolecular rotation (RIR). Monosubstituted NP-Me does not show this effect and only one structure is observed in the 1H NMR spectrum (Figure A.9). Restricted intramolecular motion through steric hindrance has been shown to cause increases in PLQY.23 The same principle can be applied to the presence of atropisomers. In non-sterically hindered molecules such as DNP, DNP-H, NP and NP-Me, the naphthyl moieties have free rotation around the pyridine and can relax from the excited state through non-radiative decay (i.e. rotation). However, DNP-Me has restricted intramolecular rotation and therefore a much faster rate of fluorescence (kf) than NP-Me (9.28 × 107 s-1 vs. 2.24 × 107 s-1) (Table 2.1) thus resulting in brighter emission. 33 2.2.6 Larger Functionalization: DNP-Et Given the interesting atropisomerism observed for DNP-Me, a second compound with a larger alkyl modification on the pyridine (an ethyl group), DNP-Et, was synthesized to further probe the effect of substituent size and steric hindrance on PLQY. As with DNP-Me, the 1H NMR spectrum of DNP-Et exhibits the presence of two distinct rotational isomers (Figure A.4/A.5) resulting in two sets of peaks for every proton in the molecule. Only one set of peaks is observed for the mono-substituted ethyl compound (NP-Et) (Figure A.11/A.12), confirming the requirement for two chromophores to induce restricted rotation in these pyridine-bridged compounds. The 1H NMR spectrum of DNP-Et also contains peaks due to the methylene protons (adjacent to the nitrogen) that provide valuable information when assigning atropisomers (syn/anti). There are two splitting patterns that are observed for the methylene protons, a quartet (Ha) and two doublets of quartets (Hb and Hc) (Figure 2.11). The quartet (J = 7 Hz) is observed when the naphthyl rings are in a syn conformation and both methylene protons are equivalent. However, when the naphthyl rings are in an anti-conformation, the methylene protons are diastereotopic, giving rise to two separate peaks and a doublet of quartet multiplicity (J = 7 Hz (q), J = 14 Hz (d)) (Figure 2.11a). 34 Figure 2.11. (a) Two atropisomers of DNP-Et and their conformations that lead to equivalent and diastereotopic peaks of the methylene protons in 1H NMR. (b) Close up of the three methylene peaks present in the 1H NMR spectrum of DNP-Et in CD2Cl2 (400 MHz at 25 °C) from 𝛿 4.05-4.70, integrations relative to Hb and Hc, doublet of quartets. The two equivalent protons (Ha) have an integration of 0.6 relative to diastereotopic protons (Hb and Hc) each integrating to 1.0, indicating three times more anti form is present than syn form. This 70:30 ratio is similar to the isomer ratio found in DNP-Me samples (80:20, B:A). Assuming similar energetic factors, this helps assign the identity of which atropisomer is present in greater quantities in DNP-Me before equilibrium is reached. Given the integration values shown in Figure 2.11b, the anti-form makes up approximately 70-80% of the initial population of the isomers, consistent with this form being kinetically favoured during synthesis, likely due to decreased steric hindrance between the two naphthyl rings. Since unequal atropisomer ratios were 35 observed in DNP-Et, room temperature and thermal equilibrium studies were also performed in different solvents (CD2Cl2, CD3CN, DMSO-d6). At a starting ratio of approximately 70:30, it takes one week (168 hours) to reach a ratio of 60:40 at room temperature and 3 h at 100 °C to reach a 50:50 ratio. (Figure 2.12c/d). Longer equilibration times reveal that with a bulkier alkyl group, interconversion between syn and anti forms is much slower compared to DNP-Me, demonstrating that rotation is even more hindered in DNP-Et, likely the result of the larger pyridine alkyl group. Figure 2.12 (a) Atropisomer ratios of DNP-Me overtime in ambient light in DCM and ACN (b) and in DMSO at 100°C. (c) Atropisomer ratios of DNP-Et overtime in ambient light in DCM and ACN (d) and in DMSO at 100°C. 36 2.2.7 Absorbance and Fluorescence of DNP-Et The observation of restricted intramolecular rotation in DNP-Me and DNP-Et provides an interesting avenue to explore via photophysical studies. Both di-substituted molecules as well as their single chromophore versions (NP-Me and NP-Et) were studied by absorbance and emission spectroscopy. The absorbance spectrum of the ethyl compounds is almost identical to those of the methyl analogs with a λmax at 355 nm for disubstituted compounds and 335 nm for monosubstituted (Figure 2.13a). These spectra suggest that no major electronic change occurs with the presence of a larger alkyl group. Emission from NP-Et occurs at 472 nm with a PLQY of 0.20, a small increase from NP-Me (0.15) (Figure 2.13b). DNP-Et emits at 498 nm and has a ΦF of 0.55, a small increase from DNP-Me (0.46). Rates of fluorescence in ethyl compounds also see slight increases (kf = 10.1 and 2.53 × 107 s-1 for DNP-Et and NP-Et) compared to their methyl analogs. Figure 2.13. Relative (a) absorbance and (b) emission of methyl and ethyl DNP (solid line) and NP (dashed line) compounds in CH2Cl2 at 1×10-5 M collected at an absorbance of 0.1, excited at λmax (DNP-Me: 365 nm, NP-Me: 335 nm, DNP-Et: 355 nm, NP-Et: 335 nm). It is proposed that the small increase in PLQY can be attributed to increased steric hindrance in DNP-Et (bulkier alkyl group on the pyridine), but no major change is seen likely because the ethyl 37 group is only slightly bigger than methyl, therefore the molecules are generally similar. The similarity of a 3-fold quantum yield increase in bridged DNP-Me and DNP-Et molecules over their mono-chromophore counterparts NP-Me and NP-Et further supports the proposal that restricted intramolecular rotation is occurring in solution and leads to an enhanced quantum yield. Further PLQY increases could potentially be obtained in these compounds with even bulkier N-functionalization such as isopropyl groups or phenyl substituents. 2.3 Conclusion Here, a simple dinaphthyl pyridine system is presented in which electronic properties at the nitrogen center can be tuned through protonation, oxidation, and alkylation. Photoluminescence quantum yield can be increased by binding the pyridine lone pair with either a proton or alkyl group and thereby reducing PET. Restricted rotation was observed for the alkylated DNP-Me, as evidence by the presence of atropisomers in the 1H NMR spectrum. Steric hindrance in DNP-Me molecules causes the isomers to adopt anti/syn conformations where the atropisomers interconvert, but coalescence is not observed in the NMR spectra demonstrating a high barrier to rotation. Compared to the mono-substituted counterpart (NP-Me), a 3-fold quantum yield increase is seen in DNP-Me, correlated to the restriction of molecular motion. This study demonstrates a straightforward approach to use RIR to yield enhanced photophysical properties of molecules in solution. This method may be used as a powerful tool when designing high quantum yielding chromophores for applications in sensing and organic electronics. 38 2.4 Experimental 2.4.1 General Considerations Reactions were conducted either under air atmosphere or using standard Schlenk techniques. All solvents used were reagent grade and used without further purification. HPLC grade solvents were used for spectroscopic analyses. All reagents (Alfa Aesar, Sigma-Aldrich, Tokoyo Chemical Industry) were used as received without further purification unless otherwise indicated. Compounds DNP and NP were prepared according to literature procedures.78 All other compounds were prepared using modified literature procedures. 2.4.2 Spectroscopy NMR solvents were purchased from Cambridge Isotope Laboratories (CDCl3, CD2Cl2). Solution-phase NMR spectroscopy was conducted on a Bruker AV III HD 400MHz spectrometer, Bruker Avance 300 MHz spectrometer or Bruker Avance 600 MHz spectrometer at 298 K. Variable temperature (VT) NMR was conducted on a Bruker Avance 400 MHz spectrometer from 298 K – 378 K. The chemical shifts (δ, ppm) in all 1H NMR spectra were referenced first to TMS and then to the residual CHCl3 or CH2Cl2 peak (1H δ = 7.26 or δ = 5.32). Multiplicities are described as s (singlet), d (doublet), dd (doublet of doublets), dq (doublet of quartets), t (triplet), td (triplet of doublets), or m (multiplet). Electrospray ionization high-resolution mass spectrometry (HRMS) was obtained using an Agilent 6545 QTOF-LCMS mass spectrometer. Fourier transform infrared (FTIR) spectroscopy was conducted on a PerkinElmer FTIR spectrometer with a universal attenuated total reflectance (ATR) attachment using a diamond crystal. For photophysical characterization, solution samples in a quartz cell with a 1 cm path. 39 length was used. UV-Vis spectroscopy was conducted on a Cary Varian 5000 UV-Vis-NIR spectrophotometer. Steady state photoluminescence (PL) spectroscopy was conducted on Photon Technology International (PTI) QuantaMaster 50 fluorimeter fitted with double excitation monochromator and 75 W Xe arc lamp as the source. For solution samples, the slit width for excitation was 0.50 mm and the slit width for emission was 0.50 mm. The integration time was 1 s. Absolute quantum yields were measured on an Edinburgh Instruments FS5 spectrofluorometer using a SC-30 integrating sphere module. Samples were measured in triplicate. Emission lifetime experiments were carried out on a Horiba Yvon Fluorocube TCSPC apparatus, using a 370 nm NanoLED excitation source with a pulse width of 1.2 ns, pulsing at a repetition rate of 1 MHz, and using a 399 long-pass filter between sample and detector. Lifetime data were fitted against the NanoLED source background using the DAS6 Data Analysis software package. 2.4.3 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 APEX2.79 The program SAINT+, v. 7.6880 was used to reduce the data and the program SADABS81 was used to make corrections to the empirical absorptions. Space group assignments were made using XPREP80 on all compounds. In all cases, the structures were solved in the Olex282 suite of programs using Intrinsic Phasing and refined using full-matrix least-squares/difference Fourier techniques on F2 using SHELXL.83 All non-hydrogen atoms were refined anisotropically. Diagrams and publication material were generated using CrystalMaker. 40 2.4.4 Synthetic Procedures and Characterization 2.4.4.1 Synthesis of Disubstituted Compounds Scheme 2.1 Synthetic route to disubstituted parent compound DNP and its N-functionalized derivatives. 2,6-di(naphthalen-1-yl) pyridine (DNP) The synthesis of DNP was adapted from known literature procedures.78 A mixture of naphthalene-1-boronic acid (300 mg, 1.74 mmol, 2.25 equiv), 2,6-dibromopyridine (182 mg, 0.77 mmol, 1 equiv), Pd(OAc)2 (2.5 mg, 1.5 mol%) and K3PO4 (407 mg, 1.92 mmol, 2.5 equiv) and 50:50 isopropanol: water (12 mL) was heated to reflux in air for 1.5 hours. The mixture was added to 41 brine (15 mL) and extracted 4 times with ethyl acetate (4 × 15 mL). The organic layer was dried over MgSO4 and filtered; the solvent removed in vacuo. The product was isolated using column chromatography (silica, 20:1 hexanes/ethyl acetate), yielding a white crystalline powder (240 mg, yield 94%). NMR spectroscopic data agree with known literature values.84 2,6-di(naphthalen-1-yl) pyridine 1-oxide (DNP-O) The synthesis of DNP-O was adapted from known literature procedures.85 DNP (97 mg, 0.29 mmol, 1 equiv) was dissolved in DCM (5 mL), to which m-CPBA (meta-chloroperoxybenzoic acid) (60 mg, 0.35 mmol, 1.2 equiv) was slowly added at 0 °C. The reaction was stirred at room temperature for 12 h and monitored by TLC. The reaction was quenched with NaHCO3 and was isolated using silica column chromatography (100:1 ~ 50:1 gradient DCM/MeOH). The product was a beige-orange powder (96 mg, yield 79%). 1H NMR (600 MHz, CD2Cl2) δ 8.04 – 7.99 (m, 2H), 7.97 (dd, J = 9.4, 7.5 Hz, 2H), 7.74 (dd, J = 8.1, 6.7 Hz, 1H), 7.70 (dd, J = 7.0, 1.2 Hz, 1H), 7.68 – 7.49 (m, 10H), 7.44 (dd, J = 15.4, 7.4 Hz, 1H).13C NMR{1H} (101 MHz, CD2Cl2) δ 150.5, 150.4, 133.8, 132.7, 132.6, 131.6, 130.3, 130.2, 129.0, 128.9, 128.7, 128.6, 128.4, 128.3, 127.0, 126.9, 126.7, 126.6, 126.3, 126.0, 125.9, 125.8, 124.6, 124.5. ESI-HR MS Calcd. for C25H17NO +H: 348.1388; Found: 348.1386. IR (neat): ṽ (σ(NO)) 1256 cm-1, 1372 cm-1 42 1-methyl 2,6-di(naphthalen-1-yl) pyridinium (DNP-Me) DNP (200 mg, 0.75 mmol, 1 equiv) was dissolved in ethyl acetate (8 mL), to which a large excess (~1 mL) of methyl iodide was added. The solution was heated at reflux in air for 72 h. The reaction was monitored by NMR. The crude product was filtered with a fine frit, rinsing with ethyl acetate to dissolve any residual starting material. No further purification was required. The product was isolated as a yellow powder (180 mg, yield 60%). 1H NMR (300 MHz, CD2Cl2) δ 8.88 (t, J = 7.9 Hz, 1H), 8.74 (t, J = 7.9 Hz, 1H), 8.51 (d, J = 7.1 Hz, 2H), 8.23 (d, J = 7.9 Hz, 2H), 8.18 (dd, J = 7.9, 6.3 Hz, 6H), 8.06 (dd, J = 10.3, 4.0 Hz, 6H), 7.81 – 7.71 (m, 4H), 7.71 – 7.62 (m, 10H), 7.38 – 7.31 (m, 2H), 3.69 (s, 3H), 3.68 (s, 3H). 13C NMR{1H} (101 MHz, CD2Cl2) δ 157.3, 146.5, 145.7, 134.1, 134.0, 132.7, 132.3, 131.7, 131.5, 130.6, 130.5, 130.5, 123.0, 129.7, 129.5, 129.4, 129.4, 129.0, 128.0, 127.6, 126.5, 126.0, 124.8, 123.3, 45.2, 45.1. *NMR spectra is a mixture of two atropisomers, therefore number of peaks is doubled. ESI-HR MS: Calcd. for C26H20N+: 346.1596; Found: 346.1596. 1-ethyl 2,6-di(naphthalen-1-yl) pyridinium (DNP-Et) DNP (300mg, 0.75 mmol, 1 equiv) was dissolved in ethyl acetate (10 mL), to which a large excess (~1 mL) of ethyl iodide was added. The solution was heated at reflux in air for 96 h. The reaction was monitored by NMR. The crude product was filtered with a fine frit, rinsing with ethyl acetate 43 to dissolve any residual starting material. No further purification was required. The product was isolated as a yellow powder (118 mg, yield 44%). 1H NMR (400 MHz, CD2Cl2) δ 8.96 – 8.87 (m, 4H), 8.27 (d, J = 7.1 Hz, 2H), 8.19 (d, J = 6.2 Hz, 17H), 8.10 – 8.04 (m, 8H), 8.01 (d, J = 6.5 Hz, 7H), 7.80 – 7.72 (m, 9H), 7.72 – 7.61 (m, 24H), 7.35 (d, J = 7.3 Hz, 2H), 4.56 (dq, J = 14.3, 7.2 Hz, 3H), 4.17 (q, J = 7.3 Hz, 2H), 4.08 (dq, J = 14.5, 7.2 Hz, 3H) 1.02 – 0.95 (m, 3H), 0.91 – 0.83 (m, 15H). *NMR spectrum contains an unequal mixture of two atropisomers, therefore integrations do not reflect the actual number of protons.13C NMR{1H} (101 MHz, CD2Cl2) δ 156.5, 156.2, 146.7, 134.0, 133.9, 132.5, 132.5, 132.4, 130.9, 130.7, 129.8, 129.8, 129.5, 129.2, 129.2, 129.1, 127.9, 127.7, 126.0, 125.8, 124.9, 123.6, 52.8, 52.6, 16.8, 16.6. ESI-HR MS: Calcd. for C27H22N+: 360.1752; Found: 360.1750. 2.4.4.2 Synthesis of Monosubstituted Compounds Scheme 2.2 Synthetic route to monosubstituted parent compound NP and its N-functionalized derivatives. 44 2-(naphthalen-1-yl) pyridine (NP) The synthesis of NP was adapted from known literature procedures.78 A mixture of naphthalene-1-boronic acid (226 mg, 1.31 mmol, 1.25 equiv), 2-bromopyridine (166 mg, 1.05 mmol, 1 equiv), Pd(OAc)2 (2.9 mg, 1.5 mol%) and K3PO4 (488 mg, 2.30 mmol, 2.5 equiv) and 50:50 isopropanol: water (7 mL) was heated to reflux in air for 1.5 h. The mixture was added to brine (10 mL) and extracted four times with ethyl acetate (4  10 mL). The organic layer was dried over MgSO4 and filtered, the solvent was removed in vacuo. The product was purified using column chromatography (silica, 10:1 hexanes/ethyl acetate), yielding a colourless oil (101 mg, yield 47%). NMR spectroscopic data agree with literature values.86 1-methyl 2-(naphthalen-1-yl) pyridinium (NP-Me) NP (250 mg, 1.22 mmol, 1 equiv) was dissolved in ethyl acetate (8 mL), to which a large excess (~1 mL) of methyl iodide was added. The solution was heated to reflux in air for 48 h. The reaction was monitored by NMR. The crude product was filtered with a fine frit, rinsing with ethyl acetate to dissolve any residual starting material. The product did not require further purification and was isolated as an orange powder (217 mg, yield 81 %) 1H NMR (400 MHz, CD2Cl2) δ 9.83 (d, J = 6.2 Hz, 1H), 8.59 (t, J = 7.8 Hz, 1H), 8.27 (t, J = 7.0 Hz, 1H), 8.23 – 8.15 (m, 1H), 8.07 (d, J = 7.9 Hz, 1H), 7.99 (d, J = 9.1 Hz, 1H), 7.74 – 7.68 (m, 2H), 7.68 – 7.58 (m, 2H), 7.28 (d, J = 7.6 Hz, 1H), 4.21 (s, 3H). 13C NMR{1H} (101 MHz, CD2Cl2) δ 155.5, 148.6, 146.1, 134.0, 132.7, 131.2, 130.4, 45 129.7, 129.2, 129.0, 128.6, 128.3, 128.0, 126.0, 124.2, 48.0. ESI-HR MS: Calcd. for C16H14N+: 220.1126; Found: 220.1129 1-ethyl 2-(naphthalen-1-yl) pyridinium (NP-Et) NP (150 mg, 0.73 mmol, 1 equiv) was dissolved in ethyl acetate (6 mL), to which a large excess (~1 mL) of ethyl iodide was added. The solution was heated to reflux under air for 48 h. The reaction was monitored by NMR. The crude precipitate was filtered using a fine frit, rinsing with ethyl acetate to dissolve any residual starting material. The product did not require further purification and was isolated as an orange powder (128 mg, yield 75%). 1H NMR (400 MHz, CD2Cl2) δ 9.91 (dd, J = 6.3, 1.0 Hz, 1H), 8.70 (td, J = 7.8, 1.4 Hz, 1H), 8.36 (t, J = 7.0 Hz, 1H), 8.21 – 8.16 (m, 1H), 8.06 (d, J = 7.9 Hz, 1H), 7.97 (d, J = 7.9 Hz, 1H), 7.70 (dd, J = 6.2, 2.1 Hz, 2H), 7.68 – 7.56 (m, 2H), 7.28 (d, J = 8.5 Hz, 1H), 4.74 (dq, J = 14.6, 7.3 Hz, 1H), 4.43 (dq, J = 14.5, 7.3 Hz, 1H), 1.42 (t, J = 7.3 Hz, 3H). 13C NMR{1H} (101 MHz, CD2Cl2) δ 154.9, 147.5, 146.0, 145.9, 134.0, 132.6, 131.5, 130.8, 129.6, 129.2, 129.0, 128.8, 128.4, 128.0, 125.8, 124.4, 55.3, 17.6. ESI-HR MS: Calcd. for C17H16N+: 234.1283; Found: 234.1277 46 Chapter 3: Photochemical C-N Annulation of Pyridine-Containing Chromophores 3.1 Introduction Emissive materials have gained widespread popularity in recent decades due to their applicability in technologies such as optoelectronics, sensors, and biological imaging.59,87,88 Desirable properties such as narrow emission spectra, high photoluminescence quantum yield (PLQY) and increased thermal stability help to bridge the gap in luminescent emitters between initial discovery and implementation into everyday devices and practices. Inducing structural rigidity is one design technique used in π-conjugated molecules to achieve these properties.26,89 Rigidity results in the suppression of non-radiative decay pathways through reduction of vibrational motion within the molecule and is most easily achieved by deliberate and directed structural design. For example, aggregation induced emission (AIE) uses bulky substituents to induce restricted rotation when molecules are aggregated either in concentrated solutions or poor solvents (i.e. water).18 An alternate approach is to synthetically incorporate rigid segments within molecules, removing solvent or concentrations dependencies. This method has been effectively used for small organic molecules, polymers, metal complexes, nanomaterials, and MOFs.90–94 Our research group is interested in employing heterocycles as highly tunable components of emissive molecules.95–97 Pyridines are heterocycles which can be easily functionalized by reaction at the electronegative nitrogen center.41 Quaternizing the pyridine nitrogen results in significant changes to the excited state and photophysical properties of molecules containing this moiety.71 Here, we demonstrate a donor-acceptor-donor (D-A-D) system, with pyridine as the acceptor, that undergoes a novel C-N cyclization reaction at the pyridine nitrogen when irradiated 47 with light. This causes rigidification and planarization across the fused D-A system, leading to increased PLQY and redshifted luminescence. Aryl C-N coupling reactions are typically transition metal-mediated such as Ullman and Goldberg couplings, or Buchwald-Hartwig reactions using Cu and Pd catalysts, respectively.98 These methods require pre-functionalized substrates and high temperatures in many cases, limiting their applicability. More recently, significant progress has been made in photochemical and electrochemical C-N bond formation using milder reagents and reaction conditions.99 Most of these reactions, however, still involve the use of transition metal (TM) photocatalysts but studies demonstrating the use of TM-free conditions with organic photocatalysts are becoming more common.100 Building on the work in Chapter 2, a photochemical reaction is explored here that occurs for the pyridine-bridged chromophores DNP and NP. We demonstrate a photocatalyst free intramolecular C-N bond formation reaction that can be used to construct rigid emissive heteroatom-containing conjugated molecules. We explore the mechanism of the photochemical reaction and propose a light-induced single electron transfer (SET) followed by radical cation bond formation and aromatization. The photocyclization of naphthalene, anthracene, pyrene, and phenanthrene derivatives are also investigated and provide insight into the mechanism. A second subsequent C-C photoreaction is also demonstrated which occurs between donor molecules for systems bearing an EWG or neutral substituent. This strategy provides a new technique for tuning emission by introducing planarity and rigidity in luminescent materials. 48 3.2 Results and Discussion 3.2.1 Photochemical Synthesis of Naphthyl-Indolium Complexes Scheme 3.1 Photocyclization of pyridyl-naphthalene 1a/2a to yield iodolium complexes 1b and 2b. Compound 1a (DNP in Chapter 2), a di-substituted 2,6-di(naphthalen-1-yl) pyridine was synthesized in high yield by a Suzuki-Miyaura coupling between 2,6-dibromopyridine and 1-naphthaleneboronic acid. Irradiation of 1a in ACN using 365 nm light does not result in any reaction. However, when the pyridine site of 1a is protonated, irradiation of the resulting compound with 365 nm light results in the initially colourless solution turning yellow, accompanying the formation of a new compound 1b. We have identified this novel transformation as a photochemical intramolecular cyclization reaction of 1a in the presence of acid resulting in the formation of a new C-N bond between the pyridine nitrogen and the 8-position of the naphthalene ring (Scheme 3.1). The structure of 1b is confirmed by single crystal x-ray diffraction (XRD) experiments. Yellow single crystals were obtained from slow evaporation of the naphthyl-indolium salt in ACN with PF6 as the counterion. 1b crystallizes in monoclinic crystal system adopting a P21/n space group with two molecules in the asymmetric unit (Figure 3.1). The newly fused indolium ring and free naphthyl group have a dihedral angle of 63.89° and 70.97° respectively. New C-N bond lengths (1.485 and 1.454 A) match reported literature values.101 49 Figure 3.1 Crystal structure of indolium complex 1b with PF6 serving as the counterion. Ellipsoids are plotted at the 50% probability level. The structure of 1b was also identified using 1H NMR spectroscopy, the spectrum of the new molecule contains 14 peaks (Figure 3.2). Figure 3.2 1H NMR spectrum of 1b (400 MHz, CD3CN) at 25°C. * = DCM, water 50 In addition, a new through-space interaction between the 7-position of the planarized naphthyl and 2-position of the second naphthalene group is observed in 2D NOESY NMR (Figure B.10). Mass spectrometry data also agrees with a new structure with molecular ion peak m/z = 330. The photochemical ring closing occurs with one naphthalene group to form the planar indolium complex, while the other naphthalene unit remains unaltered. Full conversion of 1a to 1b occurs in approximately 24 h using a handlamp with a max wavelength output of 365 nm (5 W) at concentrations between 0.01 M - 0.02 M. Longer irradiation times are necessary at higher concentrations (> 0.02 M), attributed to inadequate light penetration through the sample (Figure 3.3). The reaction was found to proceed at reasonable rates at dilute concentrations ranging from 10-6 M – 10-2 M, and is only limited by saturation at concentrations >10-2 M. Figure 3.3 Different concentrations of 1a (0.01M, 0.02M, 0.04M) in ACN irradiated with a 365 nm 5 W hand lamp for 24 h. Mole percentages for product 1b calculated using approximate NMR integrations. Next, an analog containing only one naphthyl and one pyridine group (2a) was prepared, to probe the need for a second aromatic group for the photochemical reaction to proceed. 51 Irradiation of 2a under the same conditions as 1a resulted in the cyclized photoproduct 2b at similar reaction rates (Scheme 3.1). The structure of 2b was confirmed by 1H NMR and MS data (Figure B.13). Successful cyclization of 2b confirms that the second naphthyl group in 1a is a spectator and is not necessary for the reaction to proceed. 3.2.2 Naphthyl-Indolium Cyclization Conditions and Mechanism Similar C-N bond formation between pyridine and different polyaromatic hydrocarbons (PAHs) have been observed under electrochemical conditions. Popp and coworkers demonstrated an intramolecular ring closing between a pyridine and tert-butyl substituted naphthyl group via the formation of a new C-N bond (Scheme 3.2).102 Scheme 3.2 An overcrowded 8-tert-butyl-(2-pyridyl) naphthalene is anodically oxidized to give a new C-N bond formation between the pyridine and naphthalene. Figure adapted from reference.102 The starting material, a 8-tert-butyl-(2-pyridyl) naphthalene was first electrochemically oxidized to produce a radical species. This was followed by a nucleophilic substitution at the naphthyl group, releasing tert-butyl as a leaving group and finally aromatization of the final complex. Iyoda et al. reported a similar reaction where pyridine was directly coupled to PAHs such as, pyrene, perylene and thiophene to form pyridinium coupled assemblies.103,104 While these examples utilize 52 electrochemically generated radical cations, Gilbert et al. demonstrated the formation of a π-radical cation on naphthalene using photoionization.105 Given these precedents in the literature, a similar mechanism for the formation of cyclized product 1b is hypothesized. In this case, the initial radical cation is generated photochemically. Compound 1a was heated at 70°C to probe whether the reaction proceeds thermally, with no product being observed. Irradiation of 1a in the absence of acid also did not result in any product formation, indicating the need for quaternization of the nitrogen to a pyridinium in order for the reaction to proceed. The reaction to synthesize 1b can also be facilitated by addition of certain metal salts such as Zn2+, Ni2+ and Pd2+. However, addition of Fe2+ did not result in the formation of the indolium species possibly owing to potential excited state deactivation pathways. Experiments carried out in the presence of sub-stoichiometric amounts of H+ or M+ did not result in complete conversion to 1b, demonstrating that these additives are not catalytic, and are required in stoichiometric amounts. The formation of 1b was also observed under inert atmospheres (N2 and Ar), suggesting oxygen does not play a major role in this photoreaction. Photoconversion of 1a to 1b was also probed in different solvents (Table 3.1). Table 3.1 Reaction conditions of Scheme 3.1 with various deuterated solvents. Experiments were conducted in an NMR tube at a concentration of 0.014 M, irradiated with 365 nm light for 3 h. Solvent Outcome % Conversion to 1b after 3 h Acetonitrile-d3 Cyclization Reaction 21 Dichloromethane-d2 Cyclization Reaction 5.5 Chloroform-d3 Cyclization Reaction Trace Methanol-d3 Multiple Reactions Trace DMSO-d6 Side-Reaction N/A 53 The reaction proceeded the fastest in ACN, followed by DCM. However, in chloroform and methanol, only trace amounts of 1b were observed and in DMSO no reaction was observed. The solvent dependence of this reaction suggests the solvent plays an important role in the formation of 1b. Solvent polarity is known to have a strong influence on electron transfer (ET) processes by affecting the reorganization energy and influencing the rate of reactions, which may be a determining factor for this mechanism.106,107 Pyridinium salts are known to be strong electrophiles capable of participating in ET processes due to the electron deficiency at the charged base.108,109 This suggests the first step of the reaction may involve a single electron transfer (SET) event between the electron rich naphthyl to the electron poor pyridinium group. SET in organic reactions can be induced using strong electron transfer reagents (metals, organic donors/acceptors), via electrochemical or photochemical radical generation.110, 111 The following mechanism is proposed for transformation of 1a to 1b (Scheme 3.3). First, a naphthyl π-radical species is generated after light absorption. Due to the high electrophilicity of the protonated pyridinium, the naphthyl moiety in 1a undergoes a SET to the pyridinium. This results in a radical cation on the naphthalene, making the nitrogen nucleophilic. The second step involves a nucleophilic attack/substitution on the radical cation and generation of a new intramolecular C-N bond. Rearomatization of the bi-radical species yields the final cyclized indolium complex 1b. 54 Scheme 3.3 Proposed mechanism of the photo-cyclization of 1a with the addition of acid to produce compound 1b. 3.2.3 Photophysics of Naphthyl Indolium Complexes The photophysical properties of compounds 1 and 2 were examined in ACN, before and after cyclization. Compounds 1a and 2a show similar absorption spectra with a single peak centering at 300 nm (Figure 3.4a). Both 1a and 2a emit in the UV region at 370 nm with quantum yields of 0.03 and 0.04, respectively (Figure 3.4b). Upon cyclization, the absorbance spectra are both redshifted relative to their starting material analogues. The λmax for 1b and 2b are 397 nm and 386 nm with two distinctive vibronic peaks around 315 and 330 nm. This is attributed to the planarization of the cyclized molecules that reduces the vibrational modes in these compounds. Despite the similarity in absorption spectra, 1b and 2b have large differences in emission profiles. The emission of 1b is highly redshifted with a λmax of 587 nm (deep orange in ACN), more than a 200 nm shift from the precursor 1a, with a slight increase in PLQY to 0.11 55 Figure 3.4 (a) Normalized absorbance and (b) emission spectra of compounds 1 and 2 in ACN at approximately 5.0  10-6 M for 1a and 2a and 5.0  10-5 M for 1b and 2b, excited at λmax. Emission intensity for 1a, 2a and 1b has been increased (5) for clarity. (b) Inset: photographs of solutions of 1b and 2b respectively under 365 nm irradiation in ACN. In contrast, 2b shows blue emission centering at 424 nm and a PLQY near 0.80 which is 8 times greater than for 1b and almost 25 times greater than 2a. The large emission difference between 1b and 2b may be due to the absence of the second naphthyl ring. 2b is highly planar and rigid with fewer available modes of molecular rotations and vibrations, reducing nonradiative decay pathways and resulting in increased PLQY. Differences in emission between 1b and 2b are attributed to charge transfer in 1b occurring from the free naphthyl ring to the planar indole naphthyl-moiety, which 2b lacks. In DCM, the emission of 1b is blueshifted to 558 nm (yellow emission) further supporting the assignment of this as a CT emission. No emission shift is observed for 2b in DCM relative to ACN (Figure 3.5). 56 Figure 3.5 Normalized emission spectra of 1b and 2b in DCM (solid line) and ACN (dashed line) at 5.0 × 10-5 M. 3.2.4 Expansion of Chromophore Type in Photocyclization Reaction To further explore the scope of the photocyclization reaction, pyridine containing chromophores using anthracene (3a, 4a), pyrene (5a, 6a) and phenanthrene (7a, 8a) substituents, (both the singly and doubly substituted compounds) were prepared (Table 3.3). Asymmetric structures with two different aromatic substituents were also prepared (9a-13a) (Table 3.4). Compounds 11a-13a contain a naphthyl group and phenyl ring that is either unsubstituted or contains an electron donating (EDG) or electron withdrawing (EWG) substituent to further explore the scope of the photocyclization and to investigate the effects of push-pull scaffolds. All compounds were reacted under the same acidic conditions in ACN as for 1a and 2a, adjusting the irradiation wavelength to λmax after protonation (365 nm or 395 nm). 57 Table 3.2 Scope of pyridine-chromophore systems photochemical cyclization. ID R1 R2 λem (nm), PLQY (%)d Cyclization Wavelength (nm) Product Name Major Product(s) λem (nm), PLQY (%)d 1a 375, 0.03 365 1b 587 0.11 2a H 369, 0.04 365 2b 424, 0.80 3a 421, N/A 395 3b Multiple Oxidation Products 478 (Broad), N/A 4a H 417, N/A 395 4b 450 (Broad),N/A 5a 567, N/A 365, 395 N/A No Reaction N/A 6a H 550, N/A 365, 395 N/A No Reaction N/A 7a 375, 0.07 365 7ba,b 573, N/A 8a H 375, 0.05 365 8b 470, 0.53 a Formation of other minor products, b Double cyclization occurs, d Quantum yields were measured in triplicate with an error of ±0.03. 58 Table 3.3. Asymmetric naphthyl-pyridyl chromophores prepared to probe scope of photochemical cyclization. ID R1 λem (nm), PLQY (%)d Cyclization Wavelength (nm) Product Name Major Product(s) λem (nm), PLQY (%)d 9a 418, N/A 395 9ba N/A 10a 396, N/A 365, 395 N/A No Reaction N/A 11a 371, 0.11 365 11ba,b 487, 0.92 12a 378, 0.20 365 12b 578, 0.19 13a 385, 0.09 365 13bb 469, 0.74 14a Br N/A 365 14bc N/A a Formation of other minor products, b Double cyclization occurs, c Only minor product formation d Quantum yields were measured in triplicate with an error of ±0.03. The anthracene-pyridine derivatives (3a, 4a and 9a) were irradiated with 395 nm light for 4 h resulting in a variety of oxidized compounds. The single anthryl-pyridine compound (4a) undergoes the annulation reaction at the 1-position of the anthracene as well oxidation at the 5- 59 position to yield an anthraquinone product. A new peak in the IR spectrum at 1671 cm-1 confirms the presence of a new carbonyl group belonging to anthraquinone. NMR data indicates the loss of a proton and symmetry across anthracene, consistent with the annulation reaction (Figure B.14). Conversely, the asymmetric anthryl-naphthyl compound (9a) is oxidized to anthraquinone with no annulation occurring. The same was observed for the double anthryl analog (3a). These results are consistent with anthracene being oxidized upon irradiation.112–114 The mixture of oxidation products along with the irreproducibility of the cyclization reaction makes these anthracene-derivatives unsuitable for further study. The pyrene-pyridine derivatives (5a, 6a and 10a) did not result in photoreaction after 8 h of irradiation using 365 nm or 395 nm light. The non-reactivity of these pyrene-containing compounds may be the result of the low energy triplet state of pyrene (2.10 eV), which is significantly lower in energy than the triplet state of naphthalene (2.62 eV).115 Upon irradiation, energy is transferred from the initially excited singlet state into the triplet state, which relaxes to the ground state via non-radiative decay pathways before cyclization can occur. Another possibility is that the pericyclic nature of pyrene results in greater delocalization of the radical cation preventing the reaction from occurring at the 10-position with the pyridine nitrogen site. It is interesting to note that 10a, which contains both a pyrene and naphthalene group was unreactive. Phenanthrene-pyridine compounds (7a, 8a) successfully photocyclize at the pyridine nitrogen and 8-position on the phenanthrene, confirmed by both 1H NMR spectroscopy and MS data. Phenanthrene is less susceptible to oxidation than anthracene and also possesses a higher lying triplet state (2.70 eV) than pyrene.115–117 The photophysical properties of phenanthrene compounds are similar with the naphthalene analogs with an absorption λmax at 300 nm and emission at 365 nm (PLQY= 0.05) in the pre-cyclized compounds (7a, 8a) (Figure B.3/B.4). Upon 60 cyclization absorbance is redshifted to 400 nm (Figure 3.6a) and emission differences arise between the double (7b) and single (8b) phenanthrene species. While 8b exhibits bright blue emission at 470 nm with a quantum yield of 0.53, 7b emits at a redshifted yellow emission at 578 nm with a lower quantum yield (Figure 3.6a). Changes in emission properties between phenanthrene-containing molecules are similar to the naphthalene derivatives and can be attributed to CT in 7b and the highly rigid and planar nature of 8b. Successful photocyclization of phenanthrene analogs with notable PLQY increases show promise in the utility of this system with substituents other than naphthalene but requiring careful consideration of chromophore electronics. Figure 3.6 (a) Absorbance (dashed lines) and photoluminescence (solid lines) spectra of cyclized compounds 7b and 8b in ACN at 1.0  10-5 M excited at λmax. Inset: photographs of solutions of 7b (left) and 8b (right) under 365 nm irradiation in ACN. (b) Absorbance (dashed lines) and photoluminescence (solid lines) of cyclized asymmetric pyridyl-naphthyl compounds with different phenyl substituents (11b-13b) in ACN approximately 1.0  10-5 M. Compounds 11-13 are naphthalene-pyridine-phenyl derivatives with different substituents on the 4 position of the phenyl unit. Compound 11a has an unsubstituted phenyl ring, 12a contains methoxy (OMe), an EDG, and 13a contains trifluoromethyl, (CF3), an EWG. All three compounds, 61 11a-13a successfully photocyclize (Table 3.4). The absorbance spectra of the cyclized analogs (11b-13b) closely resemble that of compounds 1b and 2b with a single peak near 400 nm and additional vibronic peaks at 315 and 330 nm (Figure 3.6b). Relatively broad emission peaks are seen at 469, 494 and 578 nm for 13b, 11b and 12b respectively. These emission shifts are consistent with the push-pull functionality that these molecules contain. Initially, in their pre-cyclized states, the PLQY values are 0.10 for both 11a and 13a, and 0.20 for 12a. After cyclization, a substantial 7 to 9-fold increase in PLQY is observed due to the rigidity and planarity of the cyclized products (0.92 and 0.75 for 11b and 13b, respectively). Methoxy-substituted 12b had the largest redshift in the series but no increase in PLQY relative to 12a which remained at around 0.20, potentially as a result of the energy gap law, which states the rate of radiationless transitions increases with lower energy gaps.118 The successful photocyclization of 11a-13a demonstrates that this reaction proceeds with some functional group diversity (OMe, CF3) present, and that emission can be tuned with different substituents present. 3.2.5 Phenyl Naphthalene Derivatives Irradiation of the unsubstituted phenyl naphthyl compound (11a) resulted in two additional photoproducts observed in addition to the singly cyclized 11b (Scheme 3.4). These products were purified and identified and their photophysical properties were measured. Cyclization occurring between the pyridine and the 8-position on the naphthalene results in compound 11b. An additional C-C coupling between the 2-position on the phenyl and 7-position on the naphthyl ring forms doubly cyclized 11c. An unusual cleavage of the naphthalene ring results in symmetrical phenyl substituted pyridine compound 11d. 62 Scheme 3.4 Photocyclization of 11a results in multiple products: 11b, 11c and 11d. Irradiation of 11a over 48 h was studied to examine the formation of different compounds (Figure 3.7). Integration of the 1H NMR spectrum was used to determine the percentage of each compound in solution over time. After 12 h of irradiation, 90% of the starting material 11a was depleted, and the presence of three additional compounds can be seen in the 1H NMR spectrum. The single cyclized product 11b is 20% of the mixture, while double cyclized product 11c is at 25%, and double phenyl product 11d at 45%. After 48 h, only two compounds were left, 11c and 11d, in equal amounts. This suggests that 11d forms directly from 11a as it is no longer formed when 11a is depleted, while 11c appears to form via the intermediate single cyclized compound 11b. The mixture of products (11c and 11d) was separated and purified through a series of extractions. 63 Figure 3.7 Irradiation at 365 nm of (a) 11a and (b) 13a in ACN with H+ for 48 h at 0.018 M. % abundance calculated from approximate 1H NMR integrations. Compound 11d was determined to be 2,6-diphenylpyridine, confirmed by comparison of the 1H and 13C NMR spectra to literature values.119 This suggests that 11d is a direct photodecomposition product of 11a. PAHs including naphthalene, are known to react with oxygen when irradiated with light, resulting in derivatives containing functional groups such as alcohols, aldehydes and quinones.120 Benzene is not a known photoproduct from the decomposition of naphthalene. To the best of our knowledge, only one example of naphthalene decomposing to form benzene is known in the literature, achieved under extreme conditions via pressurized single pulse shock tube at a temperature of 1575 K under argon atmosphere.121 How this is achieved by mild photoreaction conditions is not clear and requires further studies. No other compounds studied in this thesis suggests the formation of decomposition products analogous to 11d. Compound 11c, the double cyclized product of 11a, arises from an additional cyclization step between the 2-position on the phenyl group and 7-position on the naphthyl group to form the fully cyclized and planar structure. This was confirmed by the loss of three protons in the MS data 64 and NMR integrations. The same double cyclization also occurred after irradiation of CF3-phenyl naphthyl derivative 13a to form 13c. The formation of 13c (Figure 3.7b) was found to occur more slowly than for 11c. Synthesis of 11c and 13c occurs faster using 395 nm light, due to the increased molar absorptivity at 395 vs 365 nm. Additionally, small amounts of double cyclized product 7c were detected after the irradiation of the double phenanthrene compound 7a. Compound 7c was formed in minor quantities, only observable in the MS spectrum, and was not isolated. 3.2.6 Double Cyclization Mechanism Annulation of phenyl-pyridyl compounds 11c and 13c is believed to proceed through a photochemical 6π-electrocyclic ring closure of single cyclized 11b and 13b, followed by oxidation to restore aromaticity (Scheme 3.5). This is analogous to the well-studied intramolecular ring closing of cis-stilbene to form phenanthrene.122,123 It is interesting to note that compounds with electron donating groups on the pre-cyclized ring, such as methoxy (12a), or the double naphthyl substituted compound 1a, do not undergo a second ring closing reaction, only the initial cyclization with the pyridine. This implies the need for electron deficiency in the uncyclized ring for the formation of the fully annulated compound. Further studies are needed to explain this phenomenon. 65 Scheme 3.5 Photochemical ring closure of 11b to form double cyclized 11c in ACN at 0.018 M using 365 nm or 395 nm light. 3.2.7 Photophysics of Asymmetric Naphthyl-Phenyl-Pyridine Complexes Figure 3.8 (a) Absorbance and (b) emission spectra of compound(s) 11 at about 1.0  10-6 M (11a, 11a*(H+)) and 1.0  10-5 M (11b, 11c) in ACN and excited at 11a: 294 nm, 11a*(H+): 337 nm, 11b: 417 nm, 11c: 394 nm. Inset. Emission of samples 11b (left) and 11c (right) under 365 nm irradiation. The double cyclization of 11c and 13c provides an additional avenue for further enhancement of the PLQY and to tune the emission properties of these pyridine-containing chromophores. Compound 11c shows a large increase in fine vibronic peaks at 281, 322 and 392 nm in the absorption spectrum (Figure 3.8a) This suggests that the structure is more rigid and rotationally locked than 11b, supporting the notion that 11c adopts a fully planar conformation. 66 The emission profile of 11c is slightly more structured with a λmax of 420 nm (indigo in ACN) and a PLQY of 0.44 (Figure 3.8b). This is a 4-fold increase in PLQY when compared to precursor 11a (0.11) and a 2-fold decrease in PLQY compared to 11b (0.92). Interestingly, the emission for 11c is blueshifted by 70 nm when compared to single cyclized 11b. This may be due to the formation of a new localized π-π* excited state, as a result of the second annulation of the ring system. The excited CT state in compound 11b is replaced by a new lower energy π-π* excited state resulting in blueshifted emission. This trend is also observed for the CF3-phenyl derivative (13c), showing a highly structured absorption spectrum, with violet emission from a local excited state with a λmax at 405 nm (Figure B.7). The PLQY for 13c is 0.60, a significant 6 times increase compared to starting material 13a (0.10) but slight decrease from single cyclized 13b (0.75). The additional photocyclization observed for molecules 11 and 13, both bearing an EWG or neutral group, provides an extra avenue with which the photophysics of these asymmetric compounds can be tuned. Double cyclization gives these compounds more planar rigidified structures, while increasing PLQY but blueshifting emission, inherently different from their single cyclized counterparts. 3.3 Conclusions In conclusion, a novel photoreaction in pyridine-containing chromophores is demonstrated that leads to highly rigid and planar structures. Photoexcitation of a protonated naphthyl-pyridyl compound induces SET and results in carbocation formation. This is followed by nucleophilic attack by the nitrogen center and results in a new C-N bond. This reaction proceeds with naphthalene and phenanthrene, with pyrene being unreactive. Anthracene analogs primarily exhibit oxidation products along with annulation. Annulation results in up to a 20-fold increase in 67 PLQY and emission redshifts that are attributed to induced structural constraint and increases in conjugation. Compounds with electron poor substituents undergo an additional subsequent C-C bond formation that fully planarizes the molecule and causes blueshifts in emission (compared to single annulated products). This photoreaction may be useful in planarizing chromophores and increasing their PLQY for use photoluminescent bioimaging and sensing applications. 3.4 Experimental 3.4.1 General Reactions were conducted under atmospheric conditions or using standard Schlenk techniques. All solvents used were reagent grade and used without further purification. HPLC grade solvents were used for spectroscopic analyses. All reagents (Alfa Aesar, Sigma-Aldrich, Tokyo Chemical Industry) were used as received without further purification unless otherwise indicated. 3.4.2 Spectroscopy NMR solvents were purchased from Cambridge Isotope Laboratories (CDCl3, CD3CN) or Sigma-Aldrich (CD3CN). Solution-phase NMR spectroscopy was conducted on a Bruker AV III HD 400 or Bruker Avance 300 MHz spectrometer at 298 K. The chemical shifts (δ, ppm) in all 1H NMR spectra were referenced first to TMS and then to the residual CHCl3 or CH3CN peak (1H δ = 7.26 or δ = 1.94) Multiplicities are described as s (singlet), d (doublet), dd (doublet of doublets), t (triplet), or m (multiplet). Electrospray ionization high-resolution mass spectrometry (HRMS) was obtained using an Agilent 6545 QTOF-LCMS mass spectrometer. Fourier transform infrared 68 (FTIR) spectroscopy was conducted on a PerkinElmer FTIR spectrometer with a universal attenuated total reflectance (ATR) attachment using a diamond crystal. For photophysical characterization, solution samples in a quartz cell with a 1 cm pathlength was used. UV-vis spectroscopy was conducted on a Cary Varian 5000 UV-vis-NIR spectrophotometer. Steady state photoluminescence (PL) spectroscopy was conducted on Photon Technology International (PTI) QuantaMaster 50 fluorimeter fitted with double excitation monochromator and 75 W Xe arc lamp as the source. For solution samples, the slit width for excitation was 0.75 mm and the slit width for emission was 0.75 mm. The integration time was 1.0 s. Absolute quantum yields were measured on an Edinburgh Instruments FS5 spectrofluorometer using a SC-30 integrating sphere module. Samples were measured in triplicate. 3.4.3 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 APEX2.79 The program SAINT+, v. 7.6880 was used to reduce the data and the program SADABS81 was used to make corrections to the empirical absorptions. Space group assignments were made using XPREP80 on all compounds. In all cases, the structures were solved in the Olex282 suite of programs using Intrinsic Phasing and refined using full-matrix least-squares/difference Fourier techniques on F2 using SHELXL.83 All non-hydrogen atoms were refined anisotropically. Diagrams and publication material were generated using CrystalMaker. 69 3.4.4 Synthetic Procedures and Characterization Synthetic Procedure (A) Scheme 3.6 Reaction conditions for the synthesis of the pyridyl-naphthalene precursors 1a and 2a Molecules 1a and 2a were prepared using a reported literature procedure for an oxygen promoted Suzuki-Miyaura coupling between 2,6-dibromopyridine or 2-dibromopyrdine and 1-naphthalene boronic acid (Scheme 3.6).78 NMR spectroscopic data agree with known literature values.84,124 Synthetic Procedure (B) Scheme 3.7 Reaction conditions for the synthesis of symmetric or asymmetric chromophore substituted pyridine precursors with anthracene, pyrene and phenanthrene. 70 Molecules 3a-8a have been previously reported and are prepared using a modified literature procedure for an air-free Suzuki-Miyaura coupling between 2,6-dibromopyridine or 2-bromopyrdine and their respective boronic acids (Scheme 3.7). In general, to a 3-neck round bottom flask, a mixture of boronic acid (300 mg, 1.22 mmol, 1.05 equiv (or 2.10 equiv for disubstituted compounds)), 2(,6)-bromopyridine (183 mg, 1.16, 1 equiv) and K2CO3 (733 mg, 5.3 mmol, 4.5 equiv) was dissolved in 16 mL of solvent (2:1:1, toluene: H2O: ethanol). The solution was degassed under nitrogen for 30 mins. Pd(PPh3)4 (53.6 mg, 0.04 mmol, 4 mol %) was quickly added to the degassed solution and heated to reflux for 24 h. The product was then cooled to room temperature and extracted 3 times with DCM. The organic layer was dried over MgSO4 and filtered; the solvent removed in vacuo. The product was isolated using column chromatography (silica, 1:50-1:10 gradient, hexanes: ethyl acetate). NMR spectroscopic data agree with known literature values 3a125, 4a126, 5a127, 6a128, 7a129, 8a.90 71 Synthetic Procedure (C) Scheme 3.8 Reaction conditions for the synthesis for asymmetric naphthyl-pyridyl chromophores. Molecules 9a-13a were prepared using a two-part Suzuki-Miyaura coupling using conditions described in synthetic procedure (B). Intermediate 14a, a bromo-naphthyl-pyridyl compound was prepared and isolated using 2,6-dibromopyridine coupled with one equivalent of 1-naphthalene boronic acid.129 To compound 14a, 1 equivalent of another boronic acid was added (anthracene, pyrene, phenyl, 4-methoxy phenyl and 4-trifluoromethyl-phenyl) under identical reaction conditions. Compounds 11a and 12a match literature values.130,129 2-(anthracen-9-yl)-6-(naphthalen-1-yl)pyridine (9a) 72 Compound 9a was prepared using synthetic procedure (C) and purified using column chromatography (silica, 1:50, ethyl acetate: hexanes) to afford a white solid in 91% yield. 1H NMR (400 MHz, CDCl3) δ 8.51 (s, 1H), 8.31 – 8.23 (m, 1H), 8.09 – 7.98 (m, 3H), 7.89 – 7.83 (m, 2H), 7.78 – 7.68 (m, 4H), 7.57 – 7.49 (m, J = 15.4, 7.9, 4.1 Hz, 2H), 7.48 – 7.36 (m, 6H). 13C NMR{1H} (75 MHz, CDCl3) δ 159.6, 158.2, 138.5, 136.7, 135.4, 134.1, 131.6, 131.4, 130.3, 129.0, 128.6, 128.5, 128.0, 127.6, 126.63, 126.3, 125.9, 125.8, 125.4, 125.2, 123.9. ESI-HR MS: Calcd. for C29H19N + H: 382.1595; Found: 382.1595. 2-(naphthalen-1-yl)-6-(pyren-1-yl)pyridine (10a) Compound 10a was prepared using synthetic procedure (C) and purified using a hexane rinse and column chromatography (silica, 1:25, ethyl acetate, hexanes) to afford an orange-brown compound in 73% yield. 1H NMR (400 MHz, CDCl3) δ 8.58 (d, J = 9.3 Hz, 1H), 8.37 (d, J = 7.8 Hz, 1H), 8.31 (q, J = 7.9 Hz, 2H), 8.24 – 8.17 (m, 2H), 8.14 – 8.10 (m, J = 8.5 Hz, 3H), 8.06 – 7.94 (m, 4H), 7.82 (dd, J = 7.0, 0.9 Hz, 1H), 7.78 (d, J = 7.3 Hz, 1H), 7.69 (d, J = 7.6 Hz, 1H), 7.65 – 7.53 (m, 3H). 13C NMR{1H} (75 MHz, CDCl3) δ 159.0, 138.4, 136.3, 135.6, 133.7, 131.1, 131.0, 130.6, 128.6, 128.4, 128.1, 127.7, 127.6, 127.6, 127.5, 127.2, 126.2, 125.7, 125.6, 125.5, 125.1, 125.0, 124.7, 124.7, 124.6, 123.8, 123.1. ESI-HR MS: Calcd. for C31H19N + H: 406.1595; Found: 406.1595. 73 2-(naphthalen-1-yl)-6-(4-(trifluoromethyl)phenyl)pyridine (13a) Compound 13a was prepared using synthetic procedure (C) and purified using column chromatography (silica, 1:20, ethyl acetate hexanes) to afford a white solid in 79% yield. 1H NMR (400 MHz, CDCl3) δ 8.27 – 8.20 (m, 3H), 7.98 – 7.90 (m, J = 10.4, 5.0 Hz, 3H), 7.83 (dd, J = 7.9, 0.8 Hz, 1H), 7.73 (d, J = 8.2 Hz, 2H), 7.70 (dd, J = 7.1, 1.2 Hz, 1H), 7.60 (dd, J = 10.7, 4.0 Hz, 2H), 7.56 – 7.47 (m, 2H). 13C NMR{1H} (101 MHz, CDCl3) δ 159.4, 155.3, 142.8, 138.5, 137.5, 134.2, 131.3, 129.2, 128.6, 127.8, 127.4, 126.6, 126.0, 125.8, 125.7, 125.4, 124.4, 118.9. ESI-HR MS: Calcd. for C22H14F3N + H: 350.1156 Found: 350.1157. Synthetic Procedure (D) for Single-Cyclized Photochemical Reactions Scheme 3.9 Reaction conditions for the synthesis of charged indolium species. In an NMR tube, a 0.01-0.02 M solution of precursor (uncyclized) compound (denoted with postscript a) was made in ACN with 5 µM of trifluoracetic acid (TFA) added. The solution was irradiated with long wave UV-light (365 nm, 5 W, 10 cm distance). Purification was performed with a water-ethyl acetate extraction, the aqueous layer collected. To the aqueous layer, an excess about of ammonium hexafluorophosphate was added to perform a salt metathesis. A 74 second extraction was completed 3 times with DCM, the organic layer collected. The organic layer was dried over MgSO4 and filtered; the solvent was removed in vacuo to yield the resulting indolium salt. Quantitative yields were obtained for all compounds unless otherwise noted. Due to an inadequate photochemical setup, reactions were done on small scales and only 1H NMR spectra were obtained, except for 1b where a 13C NMR spectrum was also obtained. 8-(naphthalen-1-yl)benzo[cd]pyrido[1,2-a]indol-7-ium (1b) Compound 1b was obtained as bright yellow powder.1H NMR (400 MHz, CD3CN) δ 8.90 (dd, J = 8.2, 1.2 Hz, 1H, H7), 8.82 (d, J = 7.2 Hz, 1H, H6), 8.68 (t, J = 9.9, 6.0 Hz, 1H, H8), 8.47 (d, J = 8.2 Hz, 1H, H4), 8.41 (dd, J = 7.4, 1.8 Hz, 1H, H14), 8.20 (d, J = 8.3 Hz, 2H, H3,H13), 8.12 (dd, J = 8.1, 7.4 Hz, 1H, H5), 8.01 (dd, J = 7.7, 1.2 Hz, 1H, H9), 7.93 – 7.84 (m, 2H, H15,H16), 7.67 (t, J = 8.1, 6.8, 1.1 Hz, 1H, H12), 7.53 (d, J = 8.4 Hz, 1H, H10), 7.44 (dd, J = 12.2, 6.7, 3.3 Hz, 1H, H11), 7.38 (t, J = 8.0 Hz, 1H, H2), 6.26 (d, J = 7.8 Hz, 1H, H1).13C NMR{1H} (101 MHz, CD3CN) δ 144.2, 134.0, 133.9, 132.5, 131.1, 130.8, 129.8, 129.7, 129.4, 129.3, 128.9, 128.4, 128.4, 128.3, 127.7, 127.5, 126.4, 126.1, 124.4, 123.6, 121.7, 121.00 117.3 ESI-HR MS: Calcd. for C25H16N: 330.1282; Found: 330.1278 benzo[cd]pyrido[1,2-a]indol-7-ium (2b) 75 A yellow powder was obtained for compound 2b. 1H NMR (400 MHz, CD3CN) δ 9.52 (d, J = 6.3 Hz, 1H), 8.72 (t, J = 7.0 Hz, 2H), 8.60 – 8.54 (m, 2H), 8.51 (d, J = 8.3 Hz, 1H), 8.42 (d, J = 8.3 Hz, 1H), 8.13 – 8.00 (m, 3H). ESI-HR MS: Calcd. for C15H10N: 204.0813; Found: 204.0807. 12-(phenanthren-9-yl)naphtho[3,2,1-cd]pyrido[1,2-a]indol-13-ium (7b) Compound 7b was obtained as an orange powder. 1H NMR (400 MHz, CD2Cl2) δ 9.96 (s, 1H), 9.84 (d, J = 8.0 Hz, 1H), 9.05 – 8.90 (m, 3H), 8.80 (d, J = 8.4 Hz, 1H), 8.70 (d, J = 8.2 Hz, 1H), 8.57 (d, J = 7.9 Hz, 1H), 8.25 (s, 1H), 8.12 (dd, J = 6.9, 4.7 Hz, 2H), 8.07 – 7.91 (m, 3H), 7.85 (t, J = 7.6 Hz, 2H), 7.52 (t, J = 7.1 Hz, 1H), 7.47 – 7.37 (m, 2H), 6.59 (d, J = 8.0 Hz, 1H). ESI-HR MS: Calcd. for C33H20N: 430.1596; Found: 430.1593. naphtho[3,2,1-cd]pyrido[1,2-a]indol-13-ium (8b) A bright yellow powder was obtained for 8b.1H NMR (400 MHz, CD3CN) δ 9.52 (d, J = 6.3 Hz, 1H), 9.11 (s, 1H), 8.93 – 8.86 (m, J = 12.0, 8.2 Hz, 2H), 8.78 (d, J = 8.3 Hz, 1H), 8.61 (dd, J = 8.3, 4.0 Hz, 1H), 8.50 (d, J = 7.7 Hz, 1H), 8.35 (d, J = 8.2 Hz, 1H), 8.21 – 8.00 (m, 3H), 7.91 (dd, J = 16.5, 4.7 Hz, 1H). ESI-HR MS: Calcd. for C19H12N: 254.0970; Found: 254.0966. 76 8-phenylbenzo[cd]pyrido[1,2-a]indol-7-ium (11b) The synthesis of 11b follows procedure D with a reaction time of 12 h. 11b was obtained as a mixture with compound 11c and purified using column chromatography (alumina, 20:1, DCM: MeOH) and isolated as a yellow solid. 1H NMR (300 MHz, CD3CN) δ 8.81 – 8.71 (m, 2H), 8.57 (t, J = 8.0 Hz, 1H), 8.48 (d, J = 8.2 Hz, 1H), 8.28 (d, J = 8.3 Hz, 1H), 8.10 (dd, J = 8.2, 7.3 Hz, 1H), 7.88 – 7.71 (m, 6H), 7.61 (t, J = 8.0 Hz, 1H), 6.78 (d, J = 7.8 Hz, 1H). ESI-HR MS: Calcd. for C21H14N: 280.1126; Found: 280.1124. 2,6-diphenyl pyridine (11d) The synthesis of 11d follows procedure D for a reaction time 12 hours. 11d was obtained as a mixture with compounds 11b and 11c and purified through a water-ethyl acetate extraction, the organic layer is collected. 11d is isolated as a white powder. 1H NMR (400 MHz, CDCl3) δ 8.20 – 8.09 (m, 4H), 7.86 (dd, J = 8.2, 7.4 Hz, 1H), 7.71 (d, J = 7.8 Hz, 2H), 7.54 – 7.48 (m, 4H), 7.47 – 7.42 (m, 2H). ESI-HR MS: Calcd. for C17H13N + H: 232.1126; Found: 232.1126. 77 8-(4-methoxyphenyl)benzo[cd]pyrido[1,2-a]indol-7-ium (12b) Compound 8b is collected as a yellow powder.1H NMR (400 MHz, CD3CN) δ 8.71 (dd, J = 11.3, 4.3 Hz, 2H), 8.52 (t, J = 7.9 Hz, 1H), 8.46 (d, J = 8.2 Hz, 1H), 8.28 (d, J = 8.2 Hz, 1H), 8.08 (dd, J = 8.2, 7.3 Hz, 1H), 7.82 (dd, J = 7.7, 1.2 Hz, 1H), 7.71 – 7.60 (m, 3H), 7.33 – 7.26 (m, 2H), 6.98 (d, J = 7.8 Hz, 1H, 3.97 (s, 3H). ESI-HR MS: Calcd. for C22H16NO: 310.1232; Found: 310.1231 8-(4-(trifluoromethyl)phenyl)benzo[cd]pyrido[1,2-a]indol-7-ium (13b) The synthesis of 13b follows procedure D for a reaction time 12 hours. 13b was obtained as a mixture with compound 13c and purified using column chromatography (alumina, 20:1, DCM: MeOH) and isolated as a yellow compound. 1H NMR (400 MHz, CD3CN) δ 8.83 (dd, J = 12.6, 4.3 Hz, 2H), 8.62 (t, J = 8.0 Hz, 1H), 8.52 (d, J = 8.2 Hz, 1H), 8.33 (d, J = 8.2 Hz, 1H), 8.17 – 8.10 (m, 3H), 7.98 (d, J = 8.1 Hz, 2H), 7.88 (dd, J = 7.7, 1.1 Hz, 1H), 7.66 (t, J = 8.0 Hz, 1H), 6.88 (d, J = 7.8 Hz, 1H). ESI-HR MS: Calcd. for C22H13F3N: 348.1000; Found: 348.0999. Synthetic Procedure (E) for Double-Cyclized Photochemical Reactions In an NMR tube, a 0.01-0.02 M solution of precursor (uncyclized) compound (denoted with postscript a) was made in ACN with 5 µM of trifluoracetic acid (TFA) added. The solution was irradiated with long wave UV-light (365 nm, 5 W, 10 cm distance) for 12h. The solution was 78 then further irradiated for 4 h with a 395 nm (60W Equivalent, 10 cm distance) LED lamp. To purify the resulting double cyclized iodolium salt (denoted with postscript c) column chromatography (alumina, 20:1 DCM: MeOH) was performed, the last fraction collected. benzo[a]benzo[3,4]indolo[7,1,2-cde]quinolizin-13-ium (11c) Compound 11c was synthesized following synthetic procedure (E), an orange powder was obtained. 1H NMR (400 MHz, CD3CN) δ 9.48 (d, J = 8.5 Hz, 1H), 9.37 (d, J = 8.2 Hz, 1H), 9.26 (d, J = 7.9 Hz, 1H), 9.14 (d, J = 8.2 Hz, 1H), 9.09 (d, J = 7.3 Hz, 1H), 9.02 (t, 1H), 8.96 (d, J = 8.8 Hz, 1H), 8.78 (d, J = 8.7 Hz, 2H), 8.46 – 8.37 (m, 2H), 8.30 (t, J = 8.4, 7.3, 1.2 Hz, 1H). ESI-HR MS: Calcd. for C21H12N: 278.0970; Found: 278.0968. 9-(trifluoromethyl)benzo[a]benzo[3,4]indolo[7,1,2-cde]quinolizin-13-ium (13c) Compound 11c was synthesized following synthetic procedure (E), an orange powder was collected. 1H NMR (400 MHz, CD3CN) δ 9.60 (dd, J = 12.1, 5.1 Hz, 2H), 9.49 (s, 1H), 9.39 (d, J = 8.0 Hz, 1H), 9.17 (d, J = 7.3 Hz, 1H), 9.13 – 9.06 (m, 2H), 8.88 – 8.80 (m, 2H), 8.54 (d, J = 8.7 Hz, 1H), 8.47 (t, 1H). ESI-HR MS: Calcd. for C22H11F3N: 346.0844; Found: 346.0839. 79 Chapter 4: Conclusion and Future Work 4.1 Conclusions The development of π-conjugated luminescent emitters with easily tunable photophysical properties is of great interest for a multitude of applications. This thesis demonstrates the use of nitrogen modification on pyridine containing chromophores as a strategy to tune optical properties. Chapter 2 demonstrates how different modifications at the nitrogen unit of pyridyl naphthyl chromophores affects photophysical properties. Compounds with lone pairs exhibit low or non-emissive properties attributed to quenching by PET mechanisms. Quaternized nitrogen analogs with no lone pairs such as protonated and alkylated derivatives show CT emission and enhanced photoluminescent quantum yields ranging from 10-40 fold increases. Alkylated compounds containing methyl and ethyl groups demonstrate increases in PLQY which are only present in di-substituted species, attributed to the presence of permanent rotational isomers (atropisomerism). These atropisomers can interconvert between syn and anti forms but never into one compound indicating greatly restricted rotation. These results demonstrate that alkylation of sterically bulky nitrogen-containing molecules can be used as a technique to reduce PET quenching and simultaneously increase PLQY by reducing intramolecular motion. Chapter 3 explores a new photochemical reaction that occurs upon UV light irradiation of protonated or metal-chelated molecules of pyridyl containing chromophores. A photo-mediated ring annulation occurs between pyridine nitrogen and a carbon position on naphthalene or phenanthrene derivatives. Anthracene and pyrene exhibit oxidation side reactions or unreactivity upon irradiation. The mechanism was determined to be a photochemical SET and nucleophilic substitution reaction, with a second subsequent electrocyclic C-C bond formation occurring for molecules with reduced electron donating ability (i.e phenyl or CF3 groups). The new C-N bond 80 planarizes and rigidifies the chromophores resulting in increases in PLQY up to 20× and luminescence redshifts with emission ranging from 405 nm to 587 nm. Compounds that undergo a second C-C cyclization showed blue-shifted emission (compared to their single cyclized analogs) as a result of the loss of a CT state upon full annulation. Derivatives bearing electron-donating and electron-withdrawing substituents show functional-group tolerance and an additional method to tune emission colour. This thesis demonstrates a simple method of tuning the photophysics of pyridine chromophore systems by nitrogen modulation as a powerful technique to modify organic π- conjugated emitters. 4.2 Future Work Polymer morphology in the solid state is an important consideration when designing materials for applications such as high-performance optoelectronics.131–133 Small scale structural changes to monomer units has large scale consequences in the arrangement of polymers and affects properties such as stability, strength, and conductivity. An interesting extension to the molecules studied in Chapters 2 and 3 would be to create statistical co-polymers with a well-known conducting polymer such as thiophene (Scheme 4.1). Preparation of such thiophene-NP random statistical polymers would enable the nitrogen group to be reactive to a variety of substrates. Functionalization with protonation, chelation or alkylation can occur resulting in ionic polymers with altered emission and packing properties arising from the new substituents. A further reaction may occur upon irradiation with UV light extending conjugation and drastically altering the structure with planarization. Spin coating the copolymers into thin films and studying properties such as crystallinity, strength, and conductivity before and after modification would be an exciting new avenue to explore. 81 Scheme 4.1 Thiophene and naphthyl-pyridyl (NP) copolymers with NP doped in at 2-10 weight %. Multiple reactions at the nitrogen such as protonation, chelation and alkylation can occur (1) and can be followed by irradiation to yield annulated NP (2). The novel photoreaction described in Chapter 3 was exclusively studied with pyridine containing molecules with different PAH groups. A future direction for this annulation reaction would be to expand the nitrogen substrate scope past pyridine and into other N-heterocycles such as quinolines to achieve new architectures (Scheme 4.2). Scheme 4.2 8-bromoquinoline can be used to achieve novel architectures such as ionic N-perylenes or anthryl-quinolinium. 82 For example, large PAH’s such as N-perylene could be synthesized using a straightforward two step reaction. More complicated structures such as anthryl-quinoliniums could also be achieved with this relatively simple reaction creating architectures that almost resemble graphene nanoribbons. An interesting aspect to explore in the anthryl-quinolinium structures is if the reaction proceeds to double C-N bond formation or will stop at one. Large and rigid PAH’s have exceptional PLQYs and increased conjugation could lead these chromophores to absorb and emit light in the visible region. Incorporating ionic character into these molecules could increase their solubility in polar solvents making them more practical dyes to be used in bioimaging and sensing applications.134 83 Bibliography (1) Gundermann, K. D. Luminesence https://www.britannica.com/science/luminescence/Luminescence-excitation. (2) Costa, R. D.; Ortí, E.; Bolink, H. J.; Monti, F.; Accorsi, G.; Armaroli, N. Luminescent Ionic Transition-Metal Complexes for Light-Emitting Electrochemical Cells. Angew. Chem. Int. Ed. 2012, 51, 8178–8211. (3) Zhang, F.; Tang, B. Z. Near-Infrared Luminescent Probes for Bioimaging and Biosensing. Chem. Sci. 2021, 12, 3377–3378. (4) Wu, S.; Pan, Z.; Chen, R.; Liu, X. Long Afterglow Phosphorescent Materials; 2017. (5) Martini, S.; Haddock, S. H. D. Quantification of Bioluminescence from the Surface to the Deep Sea Demonstrates Its Predominance as an Ecological Trait. Sci. Rep. 2017, 7, 1–11. (6) Zou, S. J.; Shen, Y.; Xie, F. M.; Chen, J. De; Li, Y. Q.; Tang, J. X. Recent Advances in Organic Light-Emitting Diodes: Toward Smart Lighting and Displays. Mater. Chem. Front. 2020, 4, 788–820. (7) Song, J.; Lee, H.; Jeong, E. G.; Choi, K. C.; Yoo, S. Organic Light-Emitting Diodes: Pushing Toward the Limits and Beyond. Adv. Mater. 2020, 32, 1–17. (8) Shajing, S. S.; Serdar, S. N. Organic Photovoltaics: Mechanisms, Materials and Devices, 1st ed.; 2017. (9) Lizin, S.; Van Passel, S.; De Schepper, E.; Maes, W.; Lutsen, L.; Manca, J.; Vanderzande, D. Life Cycle Analyses of Organic Photovoltaics: A Review. Energy Environ. Sci. 2013, 6, 3136–3149. (10) Cheng, P.; Li, G.; Zhan, X.; Yang, Y. Next-Generation Organic Photovoltaics Based on Non-Fullerene Acceptors. Nat. Photonics 2018, 12, 131–142. 84 (11) Kola, S.; Sinha, J.; Katz, H. E. Organic Transistors in the New Decade : Toward n-Channel, Printed and Stabilized Devices. J. Polym. Sci. 2012, 50, 1090–1120. (12) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Organic Semiconductors for Solution-Processable Field-Effect Transistors (OFETs). Angew. Chem. Int. Ed. 2008, 47, 4070–4098. (13) Di, C. A.; Zhang, F.; Zhu, D. Multi-Functional Integration of Organic Field-Effect Transistors (OFETs): Advances and Perspectives. Adv. Mater. 2013, 25, 313–330. (14) Ulansaki, J.; Luszczynska, B.; Matyjaszewski, K. Solution‐Processable Components for Organic Electronic Devices; 2019. (15) Critchey, L. New Solution Processing Methods for Organic Electronic Devices https://www.electropages.com/blog/2020/04/new-solution-processing-methods-organic-electronic-devices. (16) Scharber, M. C.; Sariciftci, N. S. Low Band Gap Conjugated Semiconducting Polymers. Adv. Mater. Technol. 2021, 6, 2000857. (17) Jablonski, A. Efficiency of Anti-Stokes Fluorescence in Dyes. Nature 1933, 131, 839–840. (18) Luo, J.; Xie, Z.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Aggregation-Induced Emission of 1-Methyl-1,2,3,4,5-Pentaphenylsilole. Chem. Commun. 2001, 18, 1740–1741. (19) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission. Chem. Soc. Rev. 2011, 40, 5361–5388. (20) Wang, H.; Zhao, E.; Lam, J. W. Y.; Tang, B. Z. AIE Luminogens: Emission Brightened by Aggregation. Mater. Today 2015, 18, 365–377. 85 (21) Yang, Z.; Qin, W.; Leung, N. L. C.; Arseneault, M.; Lam, J. W. Y.; Liang, G.; Sung, H. H. Y.; Williams, I. D.; Tang, B. Z. A Mechanistic Study of AIE Processes of TPE Luminogens: Intramolecular Rotation vs. Configurational Isomerization. J. Mater. Chem. C 2015, 4, 99–107. (22) Chen, S.; Wang, H.; Hong, Y.; Tang, B. Z. Fabrication of Fluorescent Nanoparticles Based on AIE Luminogens (AIE Dots) and Their Applications in Bioimaging. Mater. Horiz. 2016, 3, 283–293. (23) Zheng, Q.; Xu, G.; Prasad, P. N. Conformationally Restricted Dipyrromethene Boron Difluoride (BODIPY) Dyes: Highly Fluorescent, Multicolored Probes for Cellular Imaging. Chem. A Eur. J. 2008, 14, 5812–5819. (24) Kee, H. L.; Kirmaiery, C.; Yu, L.; Thamyongkit, P.; Youngblood, W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheldt, W. R.; Birge, R. R.; Lindsey, J. S.; Holten, D. Structural Control of the Photodynamics of Boron-Dipyrrin Complexes. J. Phys. Chem. B 2005, 109, 20433–20443. (25) Jin, T.; Zhao, J.; Asao, N.; Yamamoto, Y. Metal-Catalyzed Annulation Reactions for π-Conjugated Polycycles. Chem. Eur. J. 2014, 20, 3554–3576. (26) Hirai, M.; Tanaka, N.; Sakai, M.; Yamaguchi, S. Structurally Constrained Boron, Nitrogen, Silicon and Phosphorus Centered Polycyclic π ‑ Conjugated Systems. Chem. Rev 2019, 119, 8291–8331. (27) Fukazawa, A.; Yamaguchi, S. Ladder π-Conjugated Materials Containing Main-Group Elements. Chem. Asian J. 2009, 4, 1386–1400. (28) Iwahara, H.; Kushida, T.; Yamaguchi, S. A Planarized 9-Phenylanthracene: A Simple Electron-Donating Building Block for Fluorescent Materials. Chem. Commun. 2016, 52, 86 1124–1127. (29) Conboy, G.; Spencer, H. J.; Angioni, E.; Kanibolotsky, A. L.; Findlay, N. J.; Coles, S. J.; Wilson, C.; Pitak, M. B.; Risko, C. To Bend or Not to Bend – Are Heteroatom in Dictating Conformation and Planarity ? Mater. Horiz. 2016, 3, 333–339. (30) Zhang, W.; Liu, Y.; Yu, G. Heteroatom Substituted Organic / Polymeric Semiconductors and Their Applications in Field-Effect Transistors. Adv. Mater. 2014, 26, 6898–6904. (31) Park, Y. S.; Kale, T. S.; Nam, C.; Choi, D.; Grubbs, R. B. Effects of Heteroatom Substitution in Conjugated Heterocyclic Compounds on Photovoltaic Performance : From Sulfur to Tellurium. Chem. Commun. 2014, 50, 7964–7967. (32) Monkman, A. P.; Pålsson, L.; Higgins, R. W. T.; Wang, C.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K. Protonation and Subsequent Intramolecular Hydrogen Bonding as a Method to Control Chain Structure and Tune Luminescence in Heteroatomic Conjugated Polymers. J. Am. Chem. Soc. 2002, 124, 6049–6055. (33) Liu, J.; Walker, B.; Tamayo, A.; Zhang, Y.; Nguyen, T. Q. Effects of Heteroatom Substitutions on the Crystal Structure, Film Formation, and Optoelectronic Properties of Diketopyrrolopyrrole-Based Materials. Adv. Funct. Mater. 2013, 23, 47–56. (34) Ma, M.; Usea, L.; Ferao, A. E.; Montenegro, F.; Ram, C.; Ta, A.; Rodr, J. N. Single Heteroatom Fine-Tuning of the Emissive Properties in Organoboron Complexes with 7 ‑ (Azaheteroaryl)Indole Systems. J. Org. Chem. 2016, 81, 3296–3302. (35) Christensen, P. R.; Wolf, M. O. Photopatterned Multidimensional Fluorescent Images. Adv. Funct. Mater. 2016, 26, 8471–8477. (36) Shimogawa, H.; Yoshikawa, O.; Aramaki, Y.; Murata, M.; Wakamiya, A.; Murata, Y. 4,7-Bis[3-(Dimesitylboryl)Thien-2-Yl]Benzothiadiazole: Solvato-, Thermo-, and 87 Mechanochromism Based on the Reversible Formation of an Intramolecular B−N Bond. Chem. Eur. J. 2017, 23, 3784–3791. (37) Pais, V. F.; Lassaletta, J. M.; Fernández, R.; El-Sheshtawy, H. S.; Ros, A.; Pischel, U. Organic Fluorescent Thermometers Based on Borylated Arylisoquinoline Dyes. Chem. Eur. J. 2014, 20, 7638–7645. (38) Watanabe, H.; Kawano, Y.; Tanaka, K.; Chujo, Y. Enhancing Light-Absorption and Luminescent Properties of Non-Emissive 1,3,4,6,9b-Pentaazaphenalene through Perturbation of Forbidden Electronic Transition by Boron Complexation. Asian J. Org. Chem. 2020, 9, 259–266. (39) Mellerup, S. K.; Wang, S. Boron-Based Stimuli Responsive Materials. Chem. Soc. Rev. 2019, 48, 3537–3549. (40) Christensen, P. R.; Nagle, K.; Bhatti, A.; Wolf, M. O. Enhanced Photoluminescence of Sulfur-Bridged Organic Chromophores. J. Am. Chem. Soc 2013, 135, 8109–8112. (41) Katritzky, A. R.; Ramsden, C. A.; Joule, J. A.; Zhdankin, V. V. Handbook of Heterocyclic Chemistry, 3rd ed.; Elsevier, 2010. (42) Chowdhury, S.; Rooj, B.; Dutta, A.; Mandal, U. Review on Recent Advances in Metal Ions Sensing Using Different Fluorescent Probes. J. Fluoresc. 2018, 28, 999–1021. (43) Hirayama, T.; Okuda, K.; Nagasawa, H. A Highly Selective Turn-on Fluorescent Probe for Iron to Visualize Labile Iron in Living Cells. Chem. Sci. 2013, 4, 1250–1256. (44) Beneto, A. J.; Siva, A. A Pyridine – Containing Fluorescent Probe for the Detection of Trivalent Cations in Aqueous Medium and in the Solid State. ChemistrySelect 2016, 1, 3548–3554. (45) Karak, D.; Lohar, S.; Sahana, A.; Guha, S.; Banerjee, A.; Das, D. An Al 3+ Induced Green 88 Luminescent Fluorescent Probe for Cell Imaging and Naked Eye Detection. Anal. Methods 2012, 4, 1906–1908. (46) Wu, J. S.; Liu, W. M.; Zhuang, X. Q.; Wang, F.; Wang, P. F.; Tao, S. L.; Zhang, X. H.; Wu, S. K.; Lee, S. T. Fluorescence Turn on of Coumarin Derivatives by Metal Cations: A New Signaling Mechanism Based on C=N Isomerization. Org. Lett. 2007, 9, 33–36. (47) Ward, J. S.; Priimagi, A. Fluorescence Enhancement of Quinolines by Protonation. RSC Adv. 2020, 10, 29385–29393. (48) Li, M.; Yuan, Y.; Chen, Y. Acid-Induced Multicolor Fluorescence of Pyridazine Derivative. ACS Appl. Mater. Interfaces 2018, 10, 1237–1243. (49) Dong, Y.; Zhang, J.; Tan, X.; Wang, L.; Chen, J.; Li, B.; Ye, L.; Xu, B.; Zou, B.; Tian, W. Multi-Stimuli Responsive Fluorescence Switching: The Reversible Piezochromism and Protonation Effect of a Divinylanthracene Derivative. J. Mater. Chem. C 2013, 1, 7554–7559. (50) Lee, M. H.; Park, N.; Yi, C.; Han, J. H.; Hong, J. H.; Kim, K. P.; Kang, D. H.; Sessler, J. L.; Kang, C.; Kim, J. S. Mitochondria-Immobilized PH-Sensitive Off − On Fluorescent Probe. J. Am. Chem. Soc 2014, 136, 14136–14142. (51) Yen-Pon, E.; Buttard, F.; Frédéric, L.; Thuéry, P.; Taran, F.; Pieters, G.; Champagne, P. A.; Audisio, D. Heterohelicenes through 1,3-Dipolar Cycloaddition of Sydnones with Arynes: Synthesis, Origins of Selectivity, and Application to PH-Triggered Chiroptical Switch with CPL Sign Reversal. JACS Au 2021, 1, 807–818. (52) Takaishi, K.; Yasui, M.; Ema, T. Binaphthyl − Bipyridyl Cyclic Dyads as a Chiroptical Switch. J. Am. Chem. Soc 2018, 140, 5334–5338. (53) Forrest, S. R.; Thompson, M. E. Introduction : Organic Electronics and Optoelectronics. 89 Chem. Rev. 2007, 107, 1–3. (54) Klauk, H. Organic Electronics: Materials Manufacturing, and Applications; Wiley-VCH, 2006. (55) Stennett, E. M. S.; Ciuba, M. A.; Levitus, M. Photophysical Processes in Single Molecule Organic Fluorescent Probes. Chem. Soc. Rev. 2014, 43, 1057–1075. (56) Hou, J. T.; Ren, W. X.; Li, K.; Seo, J.; Sharma, A.; Yu, X. Q.; Kim, J. S. Fluorescent Bioimaging of PH: From Design to Applications. Chem. Soc. Rev. 2017, 46, 2076–2090. (57) Pan, D.; Hu, Z.; Qiu, F.; Huang, Z. L.; Ma, Y.; Wang, Y.; Qin, L.; Zhang, Z.; Zeng, S.; Zhang, Y. H. A General Strategy for Developing Cell-Permeable Photo-Modulatable Organic Fluorescent Probes for Live-Cell Super-Resolution Imaging. Nat. Commun. 2014, 5, 1–8. (58) Yang, Q.; Ma, Z.; Wang, H.; Zhou, B.; Zhu, S.; Zhong, Y.; Wang, J.; Wan, H.; Antaris, A.; Ma, R.; Zhang, X.; Yang, J.; Zhang, X.; Sun, H.; Liu, W.; Liang, Y.; Dai, H. Rational Design of Molecular Fluorophores for Biological Imaging in the NIR-II Window. Adv. Mater. 2017, 29, 1–9. (59) Wu, D.; Sedgwick, A. C.; Gunnlaugsson, T.; Akkaya, E. U.; Yoon, J.; James, T. D. Fluorescent Chemosensors: The Past, Present and Future. Chem. Soc. Rev. 2017, 46, 7105–7123. (60) Li, X.; Gao, X.; Shi, W.; Ma, H. Design Strategies for Water-Soluble Small Molecular Chromogenic and Fluorogenic Probes. Chem. Rev. 2014, 114, 590–659. (61) Li, S.; Yu, Z.; Xiao, X.; Geng, H.; Wang, K.; Jin, X.; Liao, Q.; Liao, Y.; Wu, Y.; Yao, J.; Fu, H. Modulation of Amplified Spontaneous Emissions between Singlet Fluorescence and Triplet Phosphorescence Channels in Organic Dye Lasers. Laser Photonics Rev. 2019, 90 13, 1–8. (62) Gierschner, J.; Varghese, S.; Park, S. Y. Organic Single Crystal Lasers: A Materials View. Adv. Opt. Mater. 2016, 4, 348–364. (63) Bera, M. K.; Pal, P.; Malik, S. Solid-State Emissive Organic Chromophores: Design, Strategy and Building Blocks. J. Mater. Chem. C 2020, 8, 788–802. (64) Zhang, J. N.; Kang, H.; Li, N.; Zhou, S. M.; Sun, H. M.; Yin, S. W.; Zhao, N.; Tang, B. Z. Organic Solid Fluorophores Regulated by Subtle Structure Modification: Color-Tunable and Aggregation-Induced Emission. Chem. Sci. 2016, 8, 577–582. (65) Wakamiya, A.; Mori, K.; Yamaguchi, S. 3-Boryl-2,2′-Bithiophene as a Versatile Core Skeleton for Full-Color Highly Emissive Organic Solids. Angew. Chem. Int. Ed. 2007, 46, 4273–4276. (66) Ishow, E.; Brosseau, A.; Clavier, G.; Nakatani, K.; Tauc, P.; Fiorini-Debuisschert, C.; Neveu, S.; Sandre, O.; Léaustic, A. Multicolor Emission of Small Molecule-Based Amorphous Thin Films and Nanoparticles with a Single Excitation Wavelength. Chem. Mater. 2008, 20, 6597–6599. (67) Lee, J.; Chen, H. F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.; Forrest, S. R. Deep Blue Phosphorescent Organic Light-Emitting Diodes with Very High Brightness and Efficiency. Nat. Mater. 2016, 15, 92–98. (68) Antaris, A. L.; Chen, H.; Diao, S.; Ma, Z.; Zhang, Z.; Zhu, S.; Wang, J.; Lozano, A. X.; Fan, Q.; Chew, L.; Zhu, M.; Cheng, K.; Hong, X.; Dai, H.; Cheng, Z. A High Quantum Yield Molecule-Protein Complex Fluorophore for near-Infrared II Imaging. Nat. Commun. 2017, 8, 1–11. (69) Domaille, D. W.; Que, E. L.; Chang, C. J. Synthetic Fluorescent Sensors for Studying the 91 Cell Biology of Metals. Nat. Chem. Biol. 2008, 4, 168–175. (70) Tang, B. Z.; Qin, A. Aggregation-Induced Emission: Fundamentals; 2013. (71) Xu, Z.; Liu, Q. T.; Wang, X.; Liu, Q.; Hean, D.; Chou, K. C.; Wolf, M. O. Quinoline-Containing Diarylethenes: Bridging between Turn-on Fluorescence, RGB Switching and Room Temperature Phosphorescence. Chem. Sci. 2020, 11, 2729–2734. (72) Lin, P. Introduction About 1,1’-Binaphthyls. In 1,1′-Binaphthyl-Based Chiral Materials; Imperial College Press, 2009. (73) Pu, L. 1 , 1 ′ -Binaphthyl Dimers, Oligomers , and Polymers : Molecular Recognition, Asymmetric Catalysis and New Materials. Chem. Rev 1998, 98, 2405–2494. (74) Wang, Z. W.; Cao, Q. Y.; Lin, S.; Zhuo, L.; Li, Z. H. 2,6-Diphenylpyridine-Based Fluorophores: Synthesis, Photophysical Properties and Effects of Protonation. J. Photochem. Photobiol. A Chem. 2013, 251, 106–112. (75) Wardle, B. Principles and Applications of Photochemistry, 1st ed.; Wiley, 2009. (76) Smyth, J. E.; Butler, N. M.; Keller, P. A. A Twist of Nature – The Significance of Atropisomers in Biological Systems. Nat. Prod. Reports 2015, 32, 1562–1583. (77) Zhang, L.; Hu, Y.; Galella, E.; Tomasella, F. P.; Fish, W. P. Separation of Atropisomers by Chiral Liquid Chromatography and Thermodynamic Analysis of Separation Mechanism. J. Pharm. Anal. 2017, 7, 156–162. (78) Liu, C.; Yang, W. A Fast and Oxygen-Promoted Protocol for the Ligand-Free Suzuki Reaction of 2-Halogenated Pyridines in Aqueous Media. Chem. Commun. 2009, No. 41, 6267–6269. (79) APEX2 (Version 11.0). Bruker AXS Inc: Madison, Wisconsin, USA 2014. (80) SAINT+ (Version 8.43A). Bruker AXS Inc.: Madison, Wisconsin, USA 2013. 92 (81) SADABS (Version 2014/5). Bruker AXS Inc.: Madison, Wisconsin, USA 2014. (82) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. No Title. J. Appl. Cryst. 2009, 42, 339. (83) Sheldrick, G. M. No Title. Acta Cryst. 2015, 71, 3–8. (84) Ma, Z.; Liu, H.; Zhang, C.; Zheng, X.; Yuan, M.; Fu, H.; Li, R.; Chen, H. One-Pot Synthesis of Symmetrical 2,6-Diarylpyridines via Palladium/Copper-Catalyzed Sequential Decarboxylative and Direct C-H Arylation. Adv. Synth. Catal. 2015, 357, 1143–1148. (85) Puthanveedu, M.; Polychronidou, V.; Antonchick, A. P. Catalytic Selective Metal-Free Cross-Coupling of Heteroaromatic N-Oxides with Organosilanes. Org. Lett. 2019, 21, 3407–3411. (86) Zinser, C. M.; Warren, K. G.; Meadows, R. E.; Nahra, F. Towards Environmentally Friendlier Suzuki – Miyaura Reactions with Precursors of Pd-NHC. Green Chem. 2018, 20, 3246–3252. (87) Guo, X.; Baumgarten, M.; Müllen, K. Progress in Polymer Science Designing ␲ -Conjugated Polymers for Organic Electronics. Prog. Polym. Sci. 2013, 38, 1832–1908. (88) Martino, M. Di; Marrafino, F.; Diana, R.; Iannelli, P.; Simona Concilio. Fluorescent Probes for Applications in Bioimaging. In Advances in Bionanomaterials II; 2019; pp 243–258. (89) Roncali, J.; Jousselme, B.; Blanchard, P.; Fre, P. Enhancement of the P-Electron Delocalization and Covalent Rigidification. Tetrahedron Lett. 2000, 41, 5057–5061. (90) Xiao, T.; Dong, X.; Tang, Y.; Zhou, L. Phenanthrene Synthesis by Eosin Y-Catalyzed, Visible Light- Induced [4+2] Benzannulation of Biaryldiazonium Salts with Alkynes. Adv. Synth. Catal. 2012, 354, 3195–3199. 93 (91) Wang, T.; Li, K.; Yao, B.; Chen, Y.; Zhan, H.; Xie, Z.; Xie, G.; Yi, X.; Cheng, Y. Rigidity and Polymerization Amplified Red Thermally Activated Delayed Fluorescence Polymers for Constructing Red and Single-Emissive-Layer White OLEDs. Adv. Funct. Mater. 2020, 30, 1–10. (92) Rausch, A. F.; Murphy, L.; Williams, J. A. G.; Yersin, H. Improving the Performance of Pt ( II ) Complexes for Blue Light Emission by Enhancing the Molecular Rigidity. Inorg. Chem. 2012, No. 51, 312–319. (93) Pyo, K.; Thanthirige, V. D.; Kwak, K.; Pandurangan, P.; Ramakrishna, G.; Lee, D. Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)-Thiolate Shell. J. Am. Chem. Soc. 2015, 137, 8244–8250. (94) Wei, Z.; Gu, Z. Y.; Arvapally, R. K.; Chen, Y. P.; McDougald, R. N.; Ivy, J. F.; Yakovenko, A. A.; Feng, D.; Omary, M. A.; Zhou, H. C. Rigidifying Fluorescent Linkers by Metal-Organic Framework Formation for Fluorescence Blue Shift and Quantum Yield Enhancement. J. Am. Chem. Soc. 2014, 136, 8269–8276. (95) Xu, Z.; Liu, Q. T.; Wang, X.; Liu, Q.; Hean, D.; Chou, K. C.; Wolf, M. O. Quinoline-Containing Diarylethenes: Bridging between Turn-on Fluorescence, RGB Switching and Room Temperature Phosphorescence. Chem. Sci. 2020, 2729–2734. (96) Cruz, C. D.; Christensen, P. R.; Chronister, E. L.; Casanova, D.; Wolf, M. O.; Bardeen, C. J. Sulfur-Bridged Terthiophene Dimers : How Sulfur Oxidation State Controls Interchromophore Electronic Coupling. J. Am. Chem. Soc. 2015, 137, 12552–12564. (97) Caron, E.; Wolf, M. O. Soluble Oligo- and Polythienyl Sulfides and Sulfones: Synthesis and Photophysics. Macromolecules 2017, No. 50, 7543–7549. (98) Beletskaya, I. P.; Cheprakov, A. V. The Complementary Competitors: Palladium and 94 Copper in C − N Cross-Coupling Reactions. Organometallics 2012, No. 31, 7753–7808. (99) Zhao, Y.; Xia, W. Recent Advances in Radical-Based C-N Bond Formation: Via Photo-/Electrochemistry. Chem. Soc. Rev. 2018, 47, 2591–2608. (100) Kang, Q. Q.; Wu, W.; Li, Q.; Wei, W. T. Photochemical Strategies for C-N Bond Formation: Via Metal Catalyst-Free (Hetero) Aryl C(Sp2)-H Functionalization. Green Chem. 2020, 22, 3060–3068. (101) Brown; LeMay; Bursten. Chemistry: The Central Science, 7th Ed; Prentice Hall: Upper Saddle River, NJ, 1997. (102) Popp, G. Nucleophilic Substitutions Initiated by Electrochemical Oxidation. I. Intramolecular Nucleophilic Substitutions. J. Org. Chem. 1972, 37, 3058–3062. (103) Li, Y.; Asaoka, S.; Yamagishi, T.; Iyoda, T. Electrochemical Synthesis of Pyridinium-Conjugated Assembly Based on Nucleophilic Substitution of Pyrene/Perylene π-Radical Cation. Electrochemistry 2004, 72, 171–174. (104) Li, Y.; Kamata, K.; Kawai, T.; Abe, J.; Iyoda, T. Electrochemical Synthesis of a Pyridinium-Conjugated Assembly Based on Nucleophilic Substitution of Oligothiophene -Radical Cation. J. Chem. Soc. Perkin Trans. 2002, 1, 1135–1140. (105) Steenken, S.; Warren, C. J.; Gilbert, B. C. Generation of Radical-Cations from Naphthalene and Some Derivatives, Both by Photoionization and Reaction with So4-̇: Formation and Reactions Studied by Laser Flash Photolysis. J. Chem. Soc. Perkin Trans. 2 1990, No. 2, 335–342. (106) Marcus, R. A.; Sutin, N. Electron Transfers in Chemistry and Biology. Nature 1936, 138, 206. (107) Biasin, E.; Fox, Z. W.; Andersen, A.; Ledbetter, K.; Kjær, K. S.; Alonso-Mori, R.; 95 Carlstad, J. M.; Chollet, M.; Gaynor, J. D.; Glownia, J. M.; Hong, K.; Kroll, T.; Lee, J. H.; Liekhus-Schmaltz, C.; Reinhard, M.; Sokaras, D.; Zhang, Y.; Doumy, G.; March, A. M.; Southworth, S. H.; Mukamel, S.; Gaffney, K. J.; Schoenlein, R. W.; Govind, N.; Cordones, A. A.; Khalil, M. Direct Observation of Coherent Femtosecond Solvent Reorganization Coupled to Intramolecular Electron Transfer. Nat. Chem. 2021, 13, 343–349. (108) Grossi, B. L.; Minisci, F. Electron -Transfer Process with Pyridinium, Quinolinium, and Pyrazinium Salts; an Electron Spin Resonanance Study. J.C.S Perkin 1976, 948–952. (109) Rössler, S. L.; Jelier, B. J.; Magnier, E.; Dagousset, G.; Carreira, E. M.; Togni, A. Pyridinium Salts as Redox-Active Functional Group Transfer Reagents. Angew. Chem. Int. Ed. 2020, 59, 9264–9280. (110) Zhang, N.; Samanta, S. R.; Rosen, B. M.; Percec, V. Single Electron Transfer in Radical Ion and Radical-Mediated Organic, Materials and Polymer Synthesis. Chem. Rev. 2014, 114, 5848–5958. (111) Mariano, P. S.; Stavinoha, J. L. Synthetic Aspects of Photochemical Electron Transfer Reactions. In Synthetic Organic Photochemistry; 1984; pp 145–257. (112) Fox, M. A.; Olive, S. Photooxidation of Anthracene on Atmospheric Particulate Matter. Science 1979, 205, 582–583. (113) Čeppan, M.; Lapčík, L.; Liška, M.; Pelikán, P. Molecular Orbital Study of Anthracene Photo-Oxidation. Eur. Polym. J. 1980, 16, 607–609. (114) Debestani, R.; Ellis, K. J.; Sigman, M. E. Photodecomposition of Anthracene on Dry Surfaces: Products and Mechanism. J. Photochem. Photobiol. A Chem. 1995, 86, 231–239. 96 (115) Prodi, L.; Gandolfi, M. T.; Group, F. Handbook of Photochemistry, 3rd ed.; 2006. (116) Pysh, E. .; Yang, N. . Polarographic Oxidation Potentials of Aromatic Compounds. J. Am. Chem. Soc 1963, 85, 2124–2130. (117) Majcherczyk, A.; Johannes, C.; Hüttermann, A. Oxidation of Polycyclic Aromatic Hydrocarbons (PAH) by Laccase of Trametes Versicolor. Enzyme Microb. Technol. 1998, 22, 335–341. (118) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, Third Edit.; Springer, Ed.; 2006. (119) Kergreis, A.; Lord, R. M.; Pike, S. J. Influence of Ligand and Nuclearity on the Cytotoxicity of Cyclometallated C^N^C Platinum(II) Complexes. Chem. Eur. J. 2020, 26, 14938–14946. (120) Lane, D. A.; Lee, J. Detection of Known Photochemical Decomposition Products of PAH in Particulate Matter from Pollution Episodes in Seoul, Korea. Polycycl. Aromat. Compd. 2010, 30, 309–320. (121) Laskin, A.; Tamburu, C.; Dubnikova, F.; Lifshitz, A. Production of Major Reaction Products in the Initial Steps of the Thermal Decomposition of Naphthalene. Experimental Shock-Tube Results and Computer Simulation. Proceedings of the Combustion Institute. 2015, pp 299–307. (122) Mallory, F. B.; Wood, C. S.; Gordon, J. T. Photochemistry of Stilbenes. III. Some Aspects of the Mechanism of Photocyclization to Phenanthrenes. J. Am. Chem. Soc 1964, 86, 3094–3102. (123) Jørgensen, K. B. Photochemical Oxidative Cyclisation of Stilbenes and Stilbenoids-the Mallory-Reaction. Molecules 2010, 15, 4334–4358. 97 (124) Zinser, C. M.; Warren, K. G.; Meadows, R. E.; Nahra, F.; Al-Majid, A. M.; Barakat, A.; Islam, M. S.; Nolan, S. P.; Cazin, C. S. J. Towards Environmentally Friendlier Suzuki-Miyaura Reactions with Precursors of Pd-NHC (NHC = N-Heterocyclic Carbene) Complexes. Green Chem. 2018, 20, 3246–3252. (125) Marcinkowski, D.; Wałęsa-Chorab, M.; Kubicki, M.; Hoffmann, M.; Kądziołka, G.; Michalkiewicz, B.; Patroniak, V. A New 2,6-Di(Anthracen-9-Yl)Pyridine Ligand and Its Complexes with Ag(I) Ions: Synthesis, Structure and Photocatalytic Activity. Polyhedron 2015, 90, 91–98. (126) Ackermann, L.; Potukuchi, H. K.; Kapdi, A. R.; Schulzke, C. Kumada–Corriu Cross-Couplings with 2-Pyridyl Grignard Reagents. Chem. Eur. J. 2010, 16, 3300–3303. (127) Pu, Y. J.; Yoshizaki, M.; Akiniwa, T.; Nakayama, K. ichi; Kido, J. Dipyrenylpyridines for Electron-Transporting Materials in Organic Light Emitting Devices and Their Structural Effect on Electron Injection from LiF/Al Cathode. Org. Electron. 2009, 10, 877–882. (128) Wu, W.; Wu, W.; Ji, S.; Guo, H.; Zhao, J. Observation of Room-Temperature Deep-Red / Near-IR Phosphorescence of Pyrene with Cycloplatinated Complexes : An Experimental and Theoretical. Chem. Eur. J. 2010, 4470–4482. (129) Prajapati, D.; Schulzke, C.; Kindermann, M. K.; Kapdi, A. R. Selective Palladium-Catalysed Arylation of 2,6-Dibromopyridine Using N-Heterocyclic Carbene Ligands. RSC Adv. 2015, 5, 53073–53085. (130) Yin, C.; Zhong, K.; Li, W.; Yang, X.; Sun, R.; Zhang, C.; Zheng, X.; Yuan, M.; Li, R.; Lan, Y.; Fu, H.; Chen, H. C6-Selective Direct Arylation of 2-Phenylpyridine via an Activated N-Methylpyridinium Salt: A Combined Experimental and Theoretical Study. Adv. Synth. Catal. 2018, 360, 3990–3998. 98 (131) Xie, L.; Zhu, Y. Tune the Phase Morphology to Design Conductive Polymer Composites: A Review. Polym. Compos. 2018, 39, 2985–2996. (132) He, Z.; Chen, J.; Li, D. Polymer Additive Controlled Morphology for High Performance Organic Thin Film Transistors. Soft Matter 2019, 15, 5790–5803. (133) Agbolaghi, S.; Abbaspoor, S.; Abbasi, F. A Comprehensive Review on Polymer Single Crystals—From Fundamental Concepts to Applications. Prog. Polym. Sci. 2018, 81, 22–79. (134) Johansson, L. B. A.; Molotkovsky, J. G.; Bergelson, L. D. Fluorescence and Absorption Properties of Perylenyl and Perylenoyl Probe Molecules in Solvents and Liquid Crystals. J. Am. Chem. Soc. 1987, 109, 7374–7381. 99 Appendices Appendix A : Additional Data for Chapter 2 A.1 NMR Spectroscopy Figure A.1 1H NMR spectrum of DNP (300 MHz, CD2Cl2) at 25°C. * = water. 100 Figure A.2 1H NMR spectrum of DNP-O (600 MHz, CD2Cl2)at 25°C, * = H grease Figure A.3 13C NMR spectrum of DNP-O (101 MHz, CD2Cl2) at 25°C, peak doubling can be observed. 101 Figure A.4 1H NMR spectrum of DNP-Me (300 MHz, CD2Cl2) at 25°C, doubling of the peaks can be seen. Figure A.5 13C NMR spectrum of DNP-Me (101 MHz, CD2Cl2) at 25°C, doubling of the peaks can be seen. 102 Figure A.6 1H NMR spectrum of DNP-Et (400 MHz, CD2Cl2) at 25°C, doubling of the peaks can be seen, atropisomers are not in a 1:1 ratio, therefore integration does not accurately represent number of protons. * = water, H grease. Figure A.7 13C NMR spectrum of DNP-Et (101 MHz, CD2Cl2) at 25°C, doubling of the peaks can be seen. 103 Figure A.8 1H NMR spectrum of NP (300 MHz, CD2Cl2) at 25°C. * = water, hexanes, H grease. 104 Figure A.9 1H NMR spectrum of NP-Me (400 MHz, CD2Cl2) at 25°C. * = water. Figure A.10 13C NMR spectrum of NP-Me (101 MHz, CD2Cl2) at 25°C. 105 Figure A.11 1H NMR spectrum of NP-Et (400 MHz, CD2Cl2) at 25°C. Figure A.12 13C NMR spectrum of NP-Et (101 MHz, CD2Cl2) at 25°C. 106 Figure A.13 COSY spectrum of DNP-Me in DMSO-d6 at 25°C. 107 Figure A.14 HSQC spectrum of DNP-Me in DMSO-d6 at 25°C. 108 Figure A.15 HMBC spectrum of DNP-Me in DMSO-d6 at 25°C. 109 f Figure A.16 NOESY spectrum of DNP-Me in DMSO-d6 at 25°C. 110 A.2 Crystallography Data Table A.1 Crystal data and structure refinement for DNP-Me Empirical formula C27H22Cl2IN Formula weight 558.25 Temperature/K 296.15 Crystal system triclinic Space group P-1 a/Å 7.4594(5) b/Å 11.1518(7) c/Å 14.7331(9) α/° 94.669(4) β/° 101.035(4) γ/° 99.434(4) Volume/Å3 1178.58(13) Z 2 ρcalcg/cm3 1.573 μ/mm-1 1.600 F(000) 556.0 Crystal size/mm3 0.4 × 0.38 × 0.228 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 2.836 to 61.154 Index ranges -10 ≤ h ≤ 10, -15 ≤ k ≤ 15, -21 ≤ l ≤ 21 Reflections collected 25789 Independent reflections 7074 [Rint = 0.0349, Rsigma = 0.0351] Data/restraints/parameters 7074/0/281 Goodness-of-fit on F2 1.047 Final R indexes [I>=2σ (I)] R1 = 0.0381, wR2 = 0.0973 Final R indexes [all data] R1 = 0.0439, wR2 = 0.1009 Largest diff. peak/hole / e Å-3 1.72/-1.71 111 Appendix B : Additional Data for Chapter 3 B.1 UV-Vis and Emission Spectroscopy Figure B.1 Normalized absorbance and emission spectra of starting material 1a, in situ acid product 1a*, and iodolium complex 1b in ACN at 5.0 × 10-6 M and 5.0 × 10-5 M respectively in ACN. Figure B.2 Normalized absorbance and emission spectra of starting material 2a, in situ acid product 2a* and iodolium complex 2b in ACN at 5.0 × 10-6 M and 5.0 × 10-5 M respectively. 112 Figure B.3 Normalized absorbance and emission spectra of starting material 7a, in situ acid product 7a* and iodolium complex 7b in ACN at 1.0 × 10-5 M. Figure B.4 Normalized absorbance and emission spectra of starting material 8a, in situ acid product 8a* and iodolium complex 8b in ACN at 1.0 × 10-5 M respectively. 113 Figure B.5 Absorbance and normalized emission spectra of starting material 11a, in situ acid product 11a*, single cyclized iodolium complex 11b and double cyclized compound 11c in ACN at 1.0 × 10-6 M (11a, 11a*) and 1.0 × 10-5 M (11b and 11c). Figure B.6 Normalized absorbance and emission spectra of starting material 12a, in situ acid product 12a* and iodolium complex 12b in ACN at 1.0 × 10-6 M (12a, 12a*) and 1.0 × 10-5 M (12b). 114 Figure B.7 Absorbance and normalized emission spectra of starting material 13a, in situ acid product 13a*, single cyclized iodolium complex 13b and double cyclized compound 13c in ACN at 1.0 × 10-6 (13a, 13a*) and 1.0 ×10-5 M (13b and 13c). B.2 NMR Spectroscopy Figure B.8 1H NMR spectrum of 1a (300 MHz, CDCl3) at 25°C. * = water, grease 115 Figure B.9 COSY NMR spectrum of 1b (400 MHz, CD3CN) at 25°C. Figure B.10 NOESY NMR spectrum of 1b (400 MHz, CD3CN) at 25°C. 116 Figure B.11 1H NMR spectrum of 1b (400 MHz, CD3CN) at 25°C. Figure B.12 1H NMR spectrum of 2a (400 MHz, CDCl3) at 25°C. * = water, hexanes 117 Figure B.13 1H NMR spectrum of 2b (400 MHz, CD3CN) at 25°C. * = water, hexanes, grease Figure B.14 1H NMR spectrum of 4b, cyclized anthraquinone photoproduct (400 MHz, CD3CN) at 25°C. * = TFA, water 118 Figure B.15 1H NMR spectrum of 7a (400 MHz, CDCl3) at 25°C. * = DCM Figure B.16 1H NMR spectrum of 7b (400 MHz, CD3CN) at 25°C. * = DCM 119 Figure B.17 1H NMR spectrum of 8a (400 MHz, CDCl3) at 25°C. * = ethyl acetate Figure B.18 1H NMR spectrum of 8b (400 MHz, CD3CN) at 25°C. * = DCM, methanol, water, hexanes 120 Figure B.19 1H NMR spectrum of 9a (400 MHz, CDCl3) at 25°C. * = DCM, ethyl acetate Figure B.20 13C NMR spectrum of 9a (75 MHz, CDCl3) at 25°C. * = grease 121 Figure B.21 1H NMR spectrum of 9b, anthraquinone photoproduct (400 MHz, CD3CN) at 25°C. * = DCM, water, hexanes. 122 Figure B.22 1H NMR spectrum of 10a (400 MHz, CDCl3) at 25°C. * = DCM, ethyl acetate, hexanes, grease Figure B.23 13C NMR spectrum of 10a (75 MHz, CDCl3) at 25°C. * = grease 123 Figure B.24 1H NMR spectrum of 11a (400 MHz, CDCl3) at 25°C. * = DCM, ethyl acetate, grease Figure B.25 1H NMR spectrum of 11b (300 MHz, CD3CB) at 25°C. * = DCM, water, hexanes 124 Figure B.26 1H NMR spectrum of 11c (400 MHz, CD3CN) at 25°C. * = DCM, water, acetone hexanes Figure B.27 1H NMR spectrum of 11d (400 MHz, CDCl3) at 25°C. * = grease 125 Figure B.28 1H NMR spectrum of 12a (400 MHz, CDCl3) at 25°C. * = DCM, ethyl acetate Figure B.29 1H NMR spectrum of 12b (400 MHz, CD3CN) at 25°C. * =water, hexanes 126 Figure B.30 1H NMR spectrum of 13a (400 MHz, CDCl3) at 25°C. * = DCM, ethyl acetate, hexanes, grease Figure B.31 13C NMR spectrum of 13a (101 MHz, CDCl3) at 25°C. * = grease 127 Figure B.32 1H NMR spectrum of 13b (400 MHz, CD3CN) at 25°C. * = DCM, water, hexanes Figure B.33 1H NMR spectrum of 13c (400 MHz, CD3CN) at 25°C. * = DCM, water, acetone, hexanes 128 B.3 Crystallography Data Table B.1 Crystal Data for compound 1b Empirical formula C25H16F6NP0.98 Formula weight 474.66 Temperature/K 273.15 Crystal system monoclinic Space group P21/n a/Å 23.826(4) b/Å 6.9766(11) c/Å 24.425(4) α/° 90 β/° 101.591(3) γ/° 90 Volume/Å3 3977.3(11) Z 8 ρcalcg/cm3 1.585 μ/mm-1 0.207 F(000) 1933.0 Crystal size/mm3 0.31 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 2.672 to 56.046 Index ranges -31 ≤ h ≤ 30, -9 ≤ k ≤ 9, -31 ≤ l ≤ 32 Reflections collected 9484 Independent reflections 9484 [Rint = ?, Rsigma = 0.0444] Data/restraints/parameters 9484/0/597 Goodness-of-fit on F2 1.118 Final R indexes [I>=2σ (I)] R1 = 0.0860, wR2 = 0.2164 Final R indexes [all data] R1 = 0.1091, wR2 = 0.2257 Largest diff. peak/hole / e Å-3 0.62/-0.59
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Photophysics and photochemistry of pyridine-bridged chromophores
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