ADIABATIC PROTON-COUPLED CHARGE-TRANSFER REACTIONS AND PHOTOCHEMISTRY OF N,N-DIMETHYL-3-ARYLPROPAN-1-AMMONIUM SALTS

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ADIABATIC PROTON-COUPLED CHARGE-TRANSFER REACTIONS AND PHOTOCHEMISTRY OF N,N-DIMETHYL-3-ARYLPROPAN-1-AMMONIUM SALTS A Dissertation submitted to the Faculty of the Graduate School of Arts and Sciences of
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ADIABATIC PROTON-COUPLED CHARGE-TRANSFER REACTIONS AND PHOTOCHEMISTRY OF N,N-DIMETHYL-3-ARYLPROPAN-1-AMMONIUM SALTS A Dissertation submitted to the Faculty of the Graduate School of Arts and Sciences of Georgetown University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry By Trevor M. Safko, M.S. Washington, DC December 14 th, 2016 Copyright 2016 by Trevor M. Safko All Rights Reserved ii ADIABATIC PROTON-COUPLED CHARGE-TRANSFER REACTIONS AND PHOTOCHEMISTRY OF N,N-DIMETHYL-3-ARYLPROPAN-1-AMMONIUM SALTS Trevor M. Safko, M.S. Thesis Advisor: Richard G. Weiss, Ph.D. ABSTRACT The coupling of proton and electron transfers in concerted or sequential processes is of central importance to many biochemical and catalytic reactions. In this context, proton-coupled electron transfer reactions are typically described electrochemically, whereby the transfer of a proton is coordinated to a change in the oxidation state of the constituent donor/acceptor pairs. In the excited-state, intermolecular proton transfers are often facilitated through a redistribution of electron-density (charge-transfer) along the proton-transfer reaction coordinate without a corresponding change in the oxidation state of the donor/acceptor pair. Distinguishing charge-transfer from full electron-transfer reactions along the excited-state potential energy surface has received increased attention as advancements in engineering allow for the interrogation of the fastest molecular events. This dissertation seeks primarily to examine the confluence of charge-transfer, electron-transfer, and proton-transfer reactions which occur adiabatically in the excited-states of N,N-dimethyl-3-arylpropan-1-ammonium salts in solution. For these compounds, an excited-state intermolecular proton-transfer to the solvent is accompanied by intramolecular charge-transfer and the formation of either an emissive exciplex or a transient solventseparated ion pair. The various electronic configurations have been interrogated through an array of spectroscopic techniques in order to more thoroughly understand the convergence of thermodynamic and kinetic factors affecting the proposed mechanism. In this regard, a range of temperatures, solvents, counterions, and lumophores have been explored. In addition, the ground-state equilibrium has been iii investigated through targeted theoretical calculations and experiments. The summation of these experiments provide unique insights into a class of novel exciplex-mediated proton-coupled chargetransfer reactions. iv ACKNOWLEDGEMENTS The research presented in this dissertation would not have been possible without the continued professional and personal support of many people in my life. I feel very fortunate to have been advised and mentored by Professor Richard Weiss throughout the last five years. In 2011 I could not have foreseen how much personal growth was possible under his guidance. Every group meeting presentation, individual meeting, bimonthly report, and manuscript revision improved my understanding of molecular photochemistry, technical writing, and the scientific process generally. Professor Weiss has continually pushed me to work harder, think more critically, and to not accept superficial explanations of natural phenomena. I am truly humbled to have been able to learn from such a brilliant and thorough scientist. Of course I must also thank Professor Miklos Kertesz, Professor Christian Wolf, and Professor Edward Van Keuren for serving on my thesis committee. The insights gained from my phase exams provided new insights into my research and opened my eyes to new perspectives. In particular, I must thank Professor Kertesz for all of the time spent meeting with me in the first few years of my research. In many ways, Professor Kertesz was a co-advisor for my first two years. The computational chemistry course and our individual meetings truly changed the way I think about molecular systems and indeed much of my work relies on the insights gained from these early discussions. In addition, I would like to thank Professor Milena Shahu, whom I worked with almost exclusively in teaching upper-level undergraduate courses during my first two years. Professor Shahu not only helped to develop my teaching ability, but has been a continued source of support and encouragement throughout this program. I will absolutely miss our hallway conversations. I also want to take this opportunity to thank Dave Zapple, Dr. Ron Davis, and Dr. Ercheng Li. It would have taken me over a decade to receive my degree without Dave. He has worked tirelessly (on his own time!) to repair, maintain, and improve many of our instruments. I do not believe that I have ever v met someone so obviously overqualified in every type of engineering. This university would not function without people like Dave. To Dr. Davis and Dr. Li, thank you for training me on multiple instruments and for interrupting your busy schedules to troubleshoot the most basic instrumental problems with me. I have been very fortunate during my time here to work with many incredible visiting scientists from all over the world including Dr. Ni Yan, Dr. Ajay Mallia, Dr. Amrita Pal, Dr. Flor Mitre, Dr. Divya Sachin Nair, Dr. Yong He, Dr. Jingjing Li, Dr. Yan Zhang, Dr. Claudio Resta, Dr. Caterina Matarrese, Dr. Suman Samai, Dr. Carla Rizzo, Dr. Magdalena Cid, Dr. Eduardo Troche, Hui Zhao, Dr. Fabio Mercado, and Theresa Mekelburg. You all shared valuable insights into both chemistry and your perspective cultures that have truly changed the way I view the world. In particular, I must thank Dr. Divya Sachin Nair who effectively managed our laboratory for two years and was in many ways a role model for me. In addition, through the support of the National Science Foundation and the International Center for Materials Research I have been afforded the opportunity to travel to both Campinas, Brazil and Hefei, China to conduct important experiments for my research. This would not have been possible without the sponsorship and support of Professor Teresa Atvars, Professor Gaolin Liang, and Professor Qun Zhang. The researchers within their perspective groups including Dr. Marcelo Faleiros, Kaka Germino, Dr. Raquel Domingues, Shenlong Jiang, Lei Zhang, Zhen Zheng, and Xiaomei Liu. Thank you all for welcoming me and making me feel at home so far away from Washington, DC. I especially would like to thank the many graduate students I have worked and studied with at Georgetown University including Daniel Choi, Mandy Ley, Sonia Singh, Kelly Tran, Ves Dobrev, Ana Ivanova, Haydee Dalafu, Nick Rosa, Natasha Khatri, and Susette Ingram. To the Weiss Research Group specifically, I am completely indebted to Dr. Lora Angelova, Louis Poon, Michael Bertocchi, Teresa Duncan, and Mohan Zhang. Over the last five years you all have supported me through the ups and the downs of graduate school. I must apologize for all of the productivity I sapped from the group through our countless arguments and discussions about politics, music, culture, current events, and food. I expect productivity to increase dramatically in my absence. It vi has been amazing to grow and develop with all of you and I look forward to celebrating your upcoming professional success. Finally, I need to thank my family and friends who are my personal foundation. I could accomplish nothing without your continued love and support. Thank you to my DC family at 3819 including Tom Murphy, Alex Martin, Andrea Schrauben, Dan Beck, Kara Thomas, Marion Madsen, Mick Kowaleski, and Abby Tran. To my actual family, I will never be able to fully describe my appreciation for the sacrifices you have made for me. This degree is the culmination of decades of love from my parents and grandparents, from my brother, Melissa, and Zoë, from my extended family, and from my girlfriend Christine Tran. You all have supported me intellectually, emotionally, financially, and spiritually. Any personal accomplishment I achieve is as much yours as it is mine. I love you all. Trevor vii TABLE OF CONTENTS 1 INTRODUCTION TO PROTON-COUPLED ELECTRON-TRANSFER REACTIONS Ground-State Proton-Coupled Electron Transfer Reactions Excited-State Proton-Coupled Electron Transfer Reactions Statement of the Problem GENERAL EXPERIMENTAL METHODS Experimental Instrumentation and Procedures List of Materials Syntheses of the ArS Preparation of N,N-Dimethyl-3-arylpropan-1-amines (ArA) Synthesis of 4-(1-Pyrenyl)butan-1-amide Synthesis of Methyl-3-(1-pyrenyl)propyl Carbamate Synthesis of N-Methyl-3-(1-pyrenyl)propyl Carbamate Synthesis of N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium Chloride (PyCl) Synthesis of N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium Bromide (PyBr) Synthesis of N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium Trifluoroacetate (PyTFA) Synthesis of N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium Nitrate (PyNO3) Synthesis of N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium p-toluenesulfonate (PyTSA) Synthesis of N,N-Dimethyl-3-(9-anthrhyl)propan-1-ammonium Chloride (AnCl) Synthesis of N,N-Dimethyl-3-(9-anthryl)propan-1-amide Synthesis of 3-(9-Anthryl)propanoic Acid Synthesis of 3-(9-Anthryl)propenoic Acid Synthesis of N,N-Dimethyl-3-(2-naphthyl)propan-1-ammonium Chloride (NapCl) Synthesis of N,N-Dimethyl-3-(2-naphthyl)propan-1-amide 23 viii Synthesis of 3-(2-Naphthyl)propanoic Acid Synthesis of 3-(2-Naphthyl)propenoic Acid Synthesis of N,N-Dimethyl-3-(1-indolyl)propan-1-ammonium Chloride (InCl) Synthesis of N,N,N-Trimethyl-3-(1-indolyl)propan-1-ammonium Iodide (MeInI) Synthesis of N,N,N-Trimethyl-3-(1-indolyl)propan-1-ammonium Chloride (MeInCl) 26 3 THE ROLE OF THE GROUND-STATE Introduction Ground-State Conformational Equilibria of the ArS Approximating the Ground-State Potential Energy Surfaces of the ArS Ground-State Aggregation of ArS in Solution Conclusions on Ground-State Dynamics of ArS Salts THERMODYNAMIC RELATIONSHIPS ALONG THE EXCITED STATE POTENTIAL ENERGY SURFACE Introduction Steady-State Fluorescence of ArS in Solution Transient Absorption Spectra of ArS in Solution Comparing the Excited-States of ArS to ArA in Solution Role of the Counterion in the PCCT Mechanism Exhibited by PyS in Solution Conclusions Concerning Excited-State Thermodynamic Relationships PCCT DYNAMICS EXHIBITED BY SOLUTIONS OF ARS Introduction Temperature Dependent Steady-State and Dynamic Fluorescence of ArS in Solution Kinetic Relationships among the Locally-Excited State, the Exciplex, and the Ion Pair Conclusions Regarding PCCT Dynamics PHOTOACIDITY..78 ix 6.1 Introduction to Photoacidity Förster Treatment of ArS Photoacidity Short-Term Reversibility of the PCCT Reaction Conclusions Concerning ArS Photoacidity.85 7 PHOTOPHYSICS OF N,N-DIMETHYL-3-(1-INDOLYL)PROPAN-1-AMMONIUM CHLORIDE Introduction Photophysics of InCl Conclusions on the Photophysics of InCl.98 APPENDIX A 1 H NMR, UV-VIS ABSORPTION, AND FLUORESCENCE SPECTRA OF ARS 100 APPENDIX B TIME-CORRELATED SINGLE PHOTON COUNTING DECAYS AND EXPONENTIAL RECONVOLUTION FITS OF ARS IN SOLUTION 107 REFERENCES 114 x LIST OF FIGURES Figure 3.1 Ground-State Equilibrium between O and C for N,N-Dimethyl-3-(1-pyrenyl)propan-1- ammonium Salts (PyS) where the Anion X - is Chloride (PyCl), Bromide (PyBr), Trifluoroacetate (PyTFA), Nitrate (PyNO3), or p-toluenesulfonate (PyTSA).30 Figure 3.2 Optimized Geometries (M06/6-31G(d,p)) of N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium Chloride (PyCl), N,N-Dimethyl-3-(9-anthryl)propan-1-ammonium Chloride (AnCl), N,N-Dimethyl-3-(2- naphthyl)propan-1-ammonium Chloride (NapCl), and N,N-Dimethyl-3-(1-indolyl)propan-1-ammonium Chloride (InCl) Cations in their Open (O) and Closed (C) Conformations..30 Figure 3.3 Ground-State Geometry Optimization, Using the M06/6-31G(d,p) Model Chemistry, of PyCl in the Closed Conformation with a Lone THF Molecule in a SCRF (ε = 7)...34 Figure 3.4 Calculated Potential Energy Surfaces for S 0 and S 1 States of a PyS Cation in the Gas-Phase with Rotation about the C-N bond (θ) as Calculated Using M06/6-31G(d,p).36 Figure 3.5 Calculated Potential Energy Surfaces for S 0 and S 1 States of the PyCl Cation and a THF Molecule in a SCRF (ε = 7) with Translation of the Proton from D P to A P as Calculated Using M06/6-31G(d,p)..37 Figure 3.6 Normalized Emission Spectra (λ excitation = 344 nm) of PyCl in 1,4-Dioxane at 293 K at Concentrations of 10-3 M (Black), 10-4 M (Red), and 10-5 M (Blue) Figure 3.7 Contin Analyses from DLS Experiments on 10-3 M PyCl in 1,4-Dioxane.41 Figure 4.1 Normalized Steady-State Fluorescence Spectra of 10-5 M ArS at 293 K in 1,4-Dioxane (Red), Tetrahydrofuran (Magenta), Acetonitrile (Blue), and Dichloromethane (Black): a) PyCl (λ excitation = 344 nm), b) AnCl (λ excitation = 369 nm), and c) NapCl (λ excitation = 305 nm)..47 Figure 4.2 Transient Absorption Spectra of 10-5 M a) NapCl, b) AnCl, and c) PyCl in 1,4-Dioxane (Black), Tetrahydrofuran (Red), and Acetonitrile (Blue) Collected at 1.4 ns after Excitation at 320 nm..49 Figure 4.3 Normalized Steady-State Fluorescence Spectra of a) 10-6 M PyCl (Black) and 10-4 M PyA (Red) (λ excitation = 344 nm), b) 10-5 M AnCl (Black) and 10-4 M AnA (Red) (λ excitation = 369 nm), and c) 10-5 M NapCl (Black) and 10-4 M NapA (Red) (λ excitation = 305 nm) at 293 K in Dichloromethane.50 Figure 4.4 Steady-State, Intensity-Normalized (at 377 nm) Emission Spectra (λ ex = 344 nm) of Solutions of PyCl ( ), PyBr (- - -), PyTFA ( ), PyNO3 (- - ), and PyTSA (- ): (a) 10-5 M in Tetrahydrofuran, (b) 10-5 M in 1,4-Dioxane, (c) 10-5 M in Ethyl Acetate, and (d) 10-6 M in Dichloromethane..53 Figure 5.1 Original Rehm-Weller Plot (Reproduced with Permission) of Quenching Rate Constants as a Function of the Free Energy of Electron-Transfer...57 xi Figure 5.2 Steady-State, Intensity-Normalized (at 377 nm) Fluorescence Spectra of 10-5 M a) PyCl, b) PyBr, c) PyTFA, d) PyNO3, and e) PyTSA in THF at K in 20 C Increments. f) Adjusted Relative Exciplex Emission (I Ex/I LE), for 10-5 M PyCl, PyBr, PyNO3, PyTFA, and PyTSA in THF, from K..60 Figure 5.3 Arrhenius Plots for 10-5 M PyS and PyA in THF Using the Time-Correlated Single Photon Counting Decay Components, a) τ Ex 1 and b) τ Ex 2) at 515 nm as a Function of Inverse Temperature..65 Figure 5.4 Eyring Plots for 10-5 M PyS and PyA in THF Using Time-Correlated Single Photon Counting Decay Components a) τ Ex 1 and b) τ Ex 2 at 515 nm as a Function of Inverse Temperature.66 Figure 5.5 Transient Absorption Spectra of 10-5 M PyCl in Acetonitrile (λ Ex = 350 nm) Obtained over 200 ns..74 Figure 5.6 Decay of Transient Absorbance (ΔA) of 10-5 M PyCl in Acetonitrile as a Function of Probe-Delay Time: the ΔA Decay at 470 nm was Fitted with a Monoexponential Decay Function (Left); the ΔA Decay at 415 nm was Fitted with a Monoexponential Growth Function and Monoexponential Decay Function (Right) 74 Figure 7.1 Structures and Abbreviations of Studied Indole Derivatives...87 Figure 7.2 Image Depicting Excited-State Potential Energy Surfaces and Conical Intersections (CI1 CI5) of Indole under Two Different Relaxation Coordinates: a) Cleavage of the N-H Bond Through Population of the πσ* state and b) Internal Conversion through Ring Deformation..89 Figure 7.3 UV-Vis Absorption Spectra of 10-5 M InCl (Black), MeI (Red), and MeInCl (Blue) in Acetonitrile under Air.93 Figure 7.4 Normalized Excitation (λ emission = 330 nm) and Emission (λ excitation = 295 nm) Spectra of 10-4 M MeI (Black) and InCl (Red) and MeInCl (Blue) in Acetonitrile (Degassed).93 Figure 7.5 Steady-State Excitation and Emission Spectra of 10-4 M MeI (Left), InCl (Middle), and MeInCl (Right) in Acetonitrile (Degassed)..94 Figure 7.6 TRES of 10-4 M InCl (λ ex = 295 nm): (Left) in 1,4-Dioxane, Collected from ns at 292 K over 1024 Channels at a Peak Threshold of 6000 Counts and Intensity Normalized to the Steady-State Emission Spectrum; (Right) in Acetonitrile Intensity Normalized Time-Slices of TRES Spectra 97 Figure 7.7 Transient Absorption Spectra of 10-4 M InCl in 1,4-Dioxane (Black), Tetrahydrofuran (Red), and Acetonitrile (Blue) Collected 1.4 ns after Excitation at 310 nm..98 Figure A.1 1 H NMR Spectrum of PyCl.100 Figure A.2 1 H NMR Spectrum of PyBr.100 Figure A.3 1 H NMR Spectrum of PyTFA 101 xii Figure A.4 1 H NMR Spectrum of PyNO Figure A.5 1 H NMR Spectrum of PyTSA.102 Figure A.6 1 H NMR Spectrum of AnCl 102 Figure A.7 1 H NMR Spectrum of NapCl.103 Figure A.8 1 H NMR Spectrum of InCl..103 Figure A.9 1 H NMR Spectrum of MeInCl..104 Figure A.10 Normalized UV-Vis Absorption (Black), Excitation (Red, λ emission = 377 nm), and Emission (Blue, λ excitation = 344 nm) Spectra of 10-5 M PyCl in THF at Room Temperature.104 Figure A.11 Normalized UV-Vis Absorption (Black), Excitation (Red, λ emission = 415 nm), and Emission (Blue, λ excitation = 369 nm) Spectra of 10-5 M AnCl in THF at Room Temperature 105 Figure A.12 Normalized UV-Vis Absorption (Black), Excitation (Red, λ emission = 350 nm), and Emission (Blue, λ excitation = 305 nm) Spectra of 10-5 M NapCl in THF at Room Temperature..105 Figure A.13 Normalized UV-Vis Absorption (Black), Excitation (Red, λ emission = 350 nm), and Emission (Blue, λ excitation = 295 nm) Spectra of 10-5 M InCl in THF at Room Temperature 106 Figure B.1 Fluorescence Decay Histograms of 377 nm Emission (λ ex = 325 nm for THF and Dichloromethane Solutions and λ ex = 344 nm for Acetonitrile and 1,4-Dioxane Solutions) for 10-5 M PyCl in Dichloromethane (Black), 1,4-Dioxane (Red), THF (Magenta), and Acetonitrile (Blue).107 Figure B.2 Fluorescence Decay Histograms of 393 nm Emission (λ ex = 369 nm) for 10-5 M AnCl in Dichloromethane (Black), 1,4-Dioxane (Red), THF (Magenta), and Acetonitrile (Blue)..108 Figure B.3 Fluorescence Decay Histograms of 350 nm Emission (λ ex = 305 nm) for 10-5 M NapCl in Dichloromethane (Black), 1,4-Dioxane (Red), THF (Magenta), and Acetonitrile (Blue)..108 Figure B.4 Fluorescence Decay Histograms of 515 nm Emission (λ ex = 325 nm) for 10-5 M PyCl in THF from K..109 Figure B.5 Fluorescence Decay Histograms of 515 nm Emission (λ ex = 325 nm) for 10-5 M PyBr in THF from K..109 Figure B.6 Fluorescence Decay Histograms of 377 nm Emission (λ ex = 325 nm) for 10-5 M PyTFA in THF from K..110 Figure B.7 Fluorescence Decay Histograms of 377 nm Emission (λ ex = 325 nm) for 10-5 M PyNO3 in THF from K.110 Figure B.8 Fluorescence Decay Histograms of 377 nm Emission (λ ex = 325 nm) for 10-5 M PyTSA in THF from K..111 xiii Figure B.9 Fluorescence Decay Histograms of 377 nm Emission (λ ex = 344 nm) for 10-5 M PyA in THF from K..111 Figure B.10 Fluorescence Decay Histograms of 500 nm Emission (λ ex = 344 nm) and for 10-5 M PyA in THF from K. 112 Figure B.11 Fluorescence Decay Histogram of 330 nm Emission (λ ex = 295 nm) for 10-5 M InCl in Acetonitrile.112 Figure B.12 Fluorescence Decay Histogram of 330 nm Emission (λ ex = 295 nm) for 10-5 M MeI in Acetonitrile.113 xiv LIST OF SCHEMES Scheme 1.1 Proposed mechanism for the Artificial Leaf Co-catalyst. Electrochemical oxidation of the cobalt metal systems facilitates water oxidation through proton-coupled electron-transfer equilibrium which precedes the rate-determining O-O bond formation... 3 Scheme 1.2 Simplified Intermolecular Excited-State Proton Transfer Exhibited by the 4- Hydroxybenzylidene-1,2-dimethylimidazolinone Chromophore Contained in the Barrel of the Green Fluorescent Protein.6 Scheme 1.3 Simplified Mechanism of PCCT Reaction Exhibited by N,N-Dimethyl-3-arylpropan-1- ammonium Salts in Proton-Accepting Solvents of Varied Polarities.. 8 Scheme 3.1 Proposed Mechanism 31 for Deprotonation of and Exciplex Formation by PyS in Solutions of Intermediate Polarity..29 Scheme 4.1 Proposed PCCT Mechanism Exhibited by Irradiated Solutions of N,N-Dimethyl-3-arylpropan- 1-ammonium Salts (ArS)..46 Scheme 5.1 Proposed PCCT Mechanism Exhibited by Irradiated Solutions of N,N-Dimethyl-3-arylpropan- 1-ammonium Salts (ArS)..58 Scheme 6.1 Förster-Cycle for the PCCT Processes of ArS..81 Scheme 7.1 H
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