UNIVERSITY OF CALIFORNIA RIVERSIDE Ultrafast Spectroscopy of Force- or Light-Induced Chemical and Electronic Changes in Condensed Phase Materials A Dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Chemistry by Chad Cruz June 2019 Dissertation Committee: Dr. Eric Chronister, Chairperson Dr. Gregory Beran Dr. Jingsong Zhang Copyright by Chad Cruz 2019 The Dissertation of Chad Cruz is approved by: ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ Committee Chairperson University of California, Riverside ACKNOWLEDGEMENTS “There are no shortcuts to any place worth going.” – Beverly Sills It seems like it took a lifetime but now, when asked what it is that I do, I’ll be able to say something other than, “I’m in school.” It was a long and often a trying journey but well worth it in the end. As I look back, I can’t help but think of those who were crucial in helping me achieve this goal. First, I would like to thank my advisor, Dr. Eric Chronister, you have been an encouragement to me and have allowed me to develop as an independent physical chemist. You have forever impacted the way that I communicate science as I now constantly ask myself, “Are all those words necessary?” I am very grateful for your support and patience as I went through grad school at a snail’s pace. To Dr. Chris Bardeen, grad school would have been rather lonely were it not for your inclusion of me in all your groups’ activities. More importantly, you have dedicated so much time and energy to helping further my research and career endeavors. Your, “They’re-full-of-shit” skepticism has certainly rubbed off on me and has taught me how to critically assess the literature. I am very grateful for your willingness to work with me. iv To Dr. Kerry Hanson, you have been both a friend and mentor to me. Your remarkable ability to be so positive all the time has helped me maintain a better attitude about my research when it wasn’t going so well. You have provided me with so much practical advice from parenting to work-life balance, and I’m thankful that I got to know you over these past few years. Thank you as well for inviting my family into your home for celebrations and making them feel welcomed. To my parents, thank you for your love and support throughout the years. Dad, your work ethic has been a shining example to me as I’ve tried to develop the grit to succeed. Mom, your words and text messages of love and encouragement have lifted my spirits during stressful times. You two have always believed in me, and your faith in me gave me hope when I doubted myself. I am so grateful that Colton has both of you in his life, and on that note, thank you for all the spoiling and all the love that you have shown him. To my sisters, Hailee and Shayne, On-ee-Nah-nee-NEENEENINE-MmMmMm. You two weirdos have helped probably more than you realize. I can’t count all the times you have joyously taken Colton off my hands so that I could focus on a project or just make it to class. Your silliness and endless supply of inside jokes brightens my day, and despite my usual grumpy demeanor, (this will be the sole admission of this) I enjoy the time we get to spend together. I am so proud of both of you as you pursue your own academic v goals, and I hope that in some way I can repay you for the way you have helped me succeed. To my son, Colton, you don’t know how much your loving distractions have kept me grounded throughout this journey. Among all the baseball, biking and video games that we’ve played you have been a constant source of joy and inspiration in my life. Thank you for your patience and understanding during all the times that we were unable to play together because I had something to work on. I also appreciate all the times that you weren’t so patient and kept pushing for us to play together. Many times, that was exactly what I needed. You are an incredibly bright, funny and athletic young man, and I’m so very proud of you. Finally, to my wife, Alysson, I don’t have the words to truly capture how grateful I am to you. That you were able to put up with me and my nonsense this whole time with patience, love, encouragement and without smacking me with a baseball bat is astounding. You have carried our family on your back while I’ve been distracted with school for far too long. You have been right there with me on long sleepless nights helping me proofread papers and proposals, making graphics and putting together PowerPoint presentations. You have been my constant and it is no exaggeration that I would not have endured without you. It is a shame that the degree will only have my name on it, because your fingerprints are all over every success that I’ve had, and you deserve much more credit than you are getting. vi Acknowledgement of Previously Published Material (1) Section 3.1 was rewritten and is based on the following published work: Tong, F.; Cruz, C.; Jezowski, S.; Zhou, X.; Zhu, L.; Al-Kaysi, R. O.; Chronister, E. L.; Bardeen, C. J. Pressure Dependence of the Forward and Backward Rates of 9-tert-Butylanthracene Dewar Isomerization. J. Phys. Chem. A 2014, 118 (27), 5122. (2) Section 4.2 was rewritten and is based on the following published work: 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 (39), 12552. (3) Chapter 5 was rewritten and is based on the following published work: Cruz, C. D.; Choi, H. H.; Podzorov, V.; Chronister, E. L.; Bardeen, C. J. Photon Upconversion in Crystalline Rubrene: Resonant Enhancement by an Interband State. J. Phys. Chem. C 2018, 122 (31), 17632. vii ABSTRACT OF THE DISSERTATION Ultrafast Spectroscopy of Force- or Light-Induced Chemical and Electronic Changes in Condensed Phase Materials by Chad Cruz Doctor of Philosophy, Graduate Program in Chemistry University of California, Riverside, June 2019 Professor Eric Chronister, Chairperson A mechanophore is a molecule which displays sensitivity toward some applied force. Mechanophores show promise for applications in smart materials and nanomachines. Identifying structural motifs in photoactive molecules which enhance pressure sensitivity has been one focus of this research. It was demonstrated that 9-tert-butylanthracene dissolved in Zeonex polymer shows increased rates of back reaction upon applying mild pressures (< 1.5 GPa) in a diamond anvil cell. While strained rings can enhance pressure sensitivity, it is not the most effective handle for tuning mechanophoric properties. This work highlighted that large structural changes in the photoproduct are also required to bring about the desired pressure sensitivity. Intermolecular interactions cause assemblies of conjugated chromophores to behave distinctly different from the monomer. Delocalized excited states which lead to novel photophysics are examined in systems of varying complexity from covalently linked chromophores to molecular crystals. In covalent dimers of simple organic molecules like terthiophene and anthracene, a sulfur atom is used to link the chromophores, and the viii oxidation state of the sulfur is found to modulate the electronic coupling leading to photophysics and photochemistry which are controlled by the electronic structure of the linker. An added advantage of using the sulfur linker is that the geometry between molecules remains constant regardless of the oxidation state, allowing purely electronic effects to be isolated, demonstrating that chemical modification of a single atom can dramatically alter the excited state behavior of a molecular assembly. In molecular crystals, delocalized excited states can lead to technologically important multiexciton processes such as fission and fusion. In crystalline rubrene NIR-to-visible upconversion is observed without the use of extrinsic sensitizers, a process which may be facilitated by low energy intermolecular states. The mechanism of singlet fission in crystalline tetracene is also explored particularly regarding the correlated triplet pair state. Evidence of this spin superposition state is manifest by temporal oscillations in the photoluminescence decay of tetracene. Based on the temperature dependence, the triplets diffuse independently throughout the crystal while maintaining spin coherence. These results could have important implications for strategies which seek to use triplet states to enhance device efficiencies. ix TABLE OF CONTENTS Acknowledgements………………………………………………………………………………….……………………iv Abstract…………………………………………………………………………………………………………..…………..viii List of Figures……………………………………………………………………………………………..………………..xiv Chapter 1: Introduction………………………………………………………………………………………………....1 1.1 Sunlight – the limitless energy source…………………………………………………………….1 1.2 Tuning the photoresponse of symmetrically tethered dimers…………………………4 1.3 Photon upconversion with low energy intermolecular states…………………………7 1.4 Singlet fission in crystalline tetracene………………………………………………………….10 1.5 Mechanochemistry………………………………………………………………………………..……14 1.6 Research Projects Preview………………………………………………………………………….18 1.7 References………………………………………………………………………………………………….20 Chapter 2: Experimental…………………………………………………………………………………………….31 2.1 Sample synthesis and preparation……………………………………………………………… 31 2.1.1 9-tert-butylanthracene…………………………………………………………………31