SPECTROSCOPIC CHARACTERIZATION OF THE STRUCTURAL DYNAMICS OF PHOTOEXCITED METALLOPORPHYRINS by Baxter Abraham A dissertation submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry and Biochemistry Summer 2018 c 2018 Baxter Abraham All Rights Reserved SPECTROSCOPIC CHARACTERIZATION OF THE STRUCTURAL DYNAMICS OF PHOTOEXCITED METALLOPORPHYRINS by Baxter Abraham Approved: Brian J. Bahnson, Ph.D. Chair of the Department of Chemistry & Biochemistry Approved: George H. Watson, Ph.D. Dean of the College of Arts & Sciences Approved: Ann L. Ardis, Ph.D. Senior Vice Provost for Graduate & Professional Education I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy. Signed: Lars Gundlach, Ph.D. Professor in charge of dissertation I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy. Signed: Andrew V. Teplyakov, Ph.D. Member of dissertation committee I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy. Signed: Karl S. Booksh, Ph.D. Member of dissertation committee I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy. Signed: Matthew F. DeCamp, Ph.D. Member of dissertation committee ACKNOWLEDGEMENTS I would like to acknowledge and thank all those who have contributed to this work in one way or another. Foremost is Professor Lars Gundlach, for his mentorship and guidance. He introduced me to many of the topics in this thesis, and was a constant source of wisdom and motivation. I also thank the members of my dissertation committee, Dr. Andrew Teplyakov, Dr. Karl Booksh, and Dr. Matthew DeCamp, for their collective supervision of this work. I am thankful to Dr. Cecil Dybowski for extended conversations on teaching and research. Thank you to the past members of the Gundlach group, most notably Dr. Jesus Nieto-Pescador and Dr. Jolie Blake for making lab experiments easier and more fun. Thank you to current members as well, especially Zhengxin Li for keeping the lab running smoothly. Thank you to Susan Cheadle for help navigating any conceivable administrative task in the department. I would also like to express considerable gratitude to Professor Lin X. Chen. Her support and mentorship helped to me to develop new scientific and professional directions. Similarly, I am thankful to Dr. Ryan Hadt, Dr. Dugan Hayes, and the other members of the solar energy conversion group for their hospitality and assistance during my time at Argonne lab. Additional thanks go to Chen group members at Northwestern: Dr. Denis Leschev, Dolev Rimmerman, Darren Hsu, and Jiyun Hong. Their acceptance and guidance over multiple beamtime experiments was both highly educational and enjoyable. I am appreciative of too many members of the chemistry department to name, from whom I received varying forms of assistance. Thanks to those in the LDL basement for continuous unpredictable conversations and entertainment, particularly Chris Goodwin, Zachary Voras, Marcie Wiggins, and frequent special guest Macken- zie Williams. The establishment of the SAF caf´ewas very much appreciated. Thank iv you to Mingzhang Wang and Dr. Yichen Duan for numerous fun discussions, travels, and activities over the years. Thank you to Dr. Scott Shuler for help with organic chemistry and many fun evenings. Finally, I am eternally grateful to my parents. Their continuous love and support made this work possible. v TABLE OF CONTENTS LIST OF TABLES :::::::::::::::::::::::::::::::: ix LIST OF FIGURES ::::::::::::::::::::::::::::::: x ABSTRACT ::::::::::::::::::::::::::::::::::: xx Chapter 1 INTRODUCTION :::::::::::::::::::::::::::::: 1 1.1 Overview :::::::::::::::::::::::::::::::::: 1 1.2 Molecular Excited States and Dynamics :::::::::::::::: 3 1.3 Nuclear Effects in Dynamics ::::::::::::::::::::::: 5 1.4 Objectives ::::::::::::::::::::::::::::::::: 8 2 EXPERIMENTAL METHODS :::::::::::::::::::::: 10 2.1 Abstract :::::::::::::::::::::::::::::::::: 10 2.2 Raman Spectroscopy ::::::::::::::::::::::::::: 10 2.3 Ultashort Pulse Generation and Characterization ::::::::::: 13 2.3.1 Noncollinear Optical Parametric Amplification ::::::::: 14 2.3.2 Cross-Correlation ::::::::::::::::::::::::: 17 2.4 Transient Absorption Spectroscopy ::::::::::::::::::: 18 2.5 Pump-Degenerate Four-Wave-Mixing Spectroscopy :::::::::: 23 2.6 X-Ray Transient Absorption Spectroscopy ::::::::::::::: 31 2.7 Materials ::::::::::::::::::::::::::::::::: 35 2.7.1 Metalloporphyrins :::::::::::::::::::::::: 35 2.7.2 TiO2 :::::::::::::::::::::::::::::::: 37 3 ZINC PORPHYRIN VIBRATIONAL DYNAMICS IN SOLUTION :::::::::::::::::::::::::::::::::: 40 3.1 Abstract :::::::::::::::::::::::::::::::::: 40 vi 3.2 Background and Motivation ::::::::::::::::::::::: 40 3.3 Steady State Results ::::::::::::::::::::::::::: 42 3.4 Transient Absorption Results :::::::::::::::::::::: 46 3.5 Pump-DFWM Results :::::::::::::::::::::::::: 55 3.6 Discussion ::::::::::::::::::::::::::::::::: 63 3.7 Conclusion ::::::::::::::::::::::::::::::::: 64 4 VIBRATIONAL DYNAMICS IN A HETEROGENEOUS ELECTRON TRANSFER SYSTEM :::::::::::::::::: 66 4.1 Abstract :::::::::::::::::::::::::::::::::: 66 4.2 Background and Motivation ::::::::::::::::::::::: 66 4.3 Transient Absorption Results :::::::::::::::::::::: 71 4.4 DFWM Results :::::::::::::::::::::::::::::: 74 4.5 Pump-DFWM Results :::::::::::::::::::::::::: 76 4.6 Discussion ::::::::::::::::::::::::::::::::: 80 4.7 Conclusion ::::::::::::::::::::::::::::::::: 80 5 NICKEL PORPHYRIN EXCITED-STATE LIGATION :::::: 82 5.1 Abstract :::::::::::::::::::::::::::::::::: 82 5.2 Background and Motivation ::::::::::::::::::::::: 82 5.3 Optical Results :::::::::::::::::::::::::::::: 86 5.4 X-Ray Transient Absorption Results :::::::::::::::::: 91 5.5 Discussion ::::::::::::::::::::::::::::::::: 99 5.6 Conclusions :::::::::::::::::::::::::::::::: 100 6 CHARACTERIZATION OF AN INHERENT TRANSIENT ABSORPTION ARTIFACT :::::::::::::::::::::::: 102 6.1 Abstract :::::::::::::::::::::::::::::::::: 102 6.2 Background and Motivation ::::::::::::::::::::::: 102 6.3 Experimental Design ::::::::::::::::::::::::::: 105 6.4 Time-Domain Results ::::::::::::::::::::::::::: 106 6.5 Frequency-Domain Results :::::::::::::::::::::::: 109 6.6 Discussion ::::::::::::::::::::::::::::::::: 112 6.7 Conclusions :::::::::::::::::::::::::::::::: 114 7 SUMMARY :::::::::::::::::::::::::::::::::: 115 BIBLIOGRAPHY :::::::::::::::::::::::::::::::: 118 vii Appendix A COPYRIGHT PERMISSIONS :::::::::::::::::::::: 137 viii LIST OF TABLES 4.1 Table of Vibrational Mode Assignments for Raman-Active −1 Frequencies (cm ) Observed in Zn-PE-(COOH)2 and + Zn-PE-(COOH)2 Bound to TiO2 :::::::::::::::: 78 6.1 FWHM of Cross-Correlations and Transient Absorption Rise Times at Different Concentrations. :::::::::::: 112 ix LIST OF FIGURES 1.1 The general relaxation processes possible after photoexcitation are summarized with their typical corresponding timescales. Adapted with permission from [1]. Copyright 2005 Springer. ::::::::: 4 1.2 Nonadiabtic (left) and adiabatic (right) heterogeneous electron transfer is shown through energy diagrams along the reaction coordinate. The coupling strength between the donor and acceptor states dictates the degree of adiabaticity. Franck-Condon factors can modulate vibrational coherence in the nonadiabatic case, but not for adiabatic reactions. Vibrationally hot products may be produced in either case. This process is investigated in Chapter4. :::::::: 6 2.1 Raman Stokes, anti-Stokes, and Rayleigh scattering are the result of vibrational energy level transitions. :::::::::::::::::: 11 2.2 The NOPA contains a beamsplitter (BS), concave spherical mirrors (SM), and a lens pair that acts as a telescope. A variable delay stage allows for matching the timing of the pump and supercontinuum pulses in the amplification BBO. The amplification of the green portion of the white light is depicted. ::::::::::::::::: 15 2.3 At the correct alignment conditions, the NOPA outputs a very broadband amplified spectrum. The measured auto-correlation produces a pulse duration below 10 fs. :::::::::::::::: 16 2.4 Transient absorption spectroscopy is performed using a chopped pump beam on a variable delay path, and a supercontinuum probe beam on an all-reflective path. The probe pulse is compressed with a dispersion compensation mirror (DCM) before its focus on the sample by an off-axis parabolic mirror (OAPM). ::::::::::::::: 20 2.5 The detected spectrum of the supercontinuum probe pulse is spread across the visible region from 450 nm to 750 nm. :::::::::: 21 x 2.6 The time ordering of the pump-DFWM sequence consists of an initial actinic pulse to promote the system, to an electronically excited state, followed by a set of pulses that generate a DFWM signal on the excited state. Evolution of the excited state is tracked by varying `T', and wavepacket dynamics are followed by scanning `t'. The DFWM pump and Stokes pulses are kept coincident in time. ::::::::: 24 2.7 A vibrational wavepacket is prepared by impulsive excitation