Controlling the Cycloreversion Reaction of a Diarylethene Derivative Using Sequential Two- Photon Excitation By Cassandra Lee Ward Submitted to the graduate degree program in the Department of Chemistry and the Graduate Faculty of the University of Kansas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. ________________________________ Chair- Christopher G. Elles ________________________________ Ward H. Thompson ________________________________ Carey K. Johnson ________________________________ Robert C. Dunn ________________________________ Christopher J. Fischer Date Defended: May 16 2014 The Dissertation Committee for Cassandra Lee Ward certifies that this is the approved version of the following dissertation: Controlling the Cycloreversion Reaction of a Diarylethene Derivative Using Sequential Two- Photon Excitation ________________________________ Chair- Christopher G. Elles Date approved: May 16 2014 ii Abstract Diarylethenes (DAE) are a class of photochromic molecular switches that convert between two structural isomers upon excitation with light. A great deal of research has been dedicated to elucidating the mechanisms of the reversible electrocyclic reactions to make optical memory devices with DAE compounds, but details of the fundamental reaction mechanism after one- or two-photons of light is still lacking. The primary DAE discussed in this dissertation is 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DMPT-PFCP), which is a model compound for studying the fundamental reaction dynamics using one- and two-photon excitation experiments. Pump-probe spectroscopy was used to study the low one-photon quantum yield cycloreversion reaction of DMPT-PFCP by changing the excitation wavelength, solvent, and temperature to describe the dynamics on the ground- and excited states. However, the primary goal of this work was to use sequential two-photon excitation with fs laser pulses to map out the cycloreversion reaction dynamics for DMPT-PFCP compound on the first and higher excited states. The cycloreversion quantum yield was selectively increased using sequential two-photon excitation, where after promotion to the S1 state, a second excitation pulse promotes the molecules to an even higher excited state. The mechanism of increasing the yield by promoting the molecules to a higher excited state was explored using pump-repump-probe (PReP) spectroscopy. The PReP experiments follow the excited-state dynamics as the molecules sample different regions of the S1 potential energy surface. The projection of the S1 dynamics onto the higher excited states showed that by changing the secondary excitation wavelength and the delay between excitation pulses, the cycloreversion quantum yield was selectively controlled. Future studies to obtain the specific modes involved in the ring-opening reaction coordinate on the excited-state would further improve our knowledge of the cycloreversion reaction and therefore iii improve the efficiency of the sequential two-photon excitation process to make very efficient optical memory devices using DAE compounds. iv I would like to dedicate this dissertation to my grandmother, Louise G. Ward, for setting the pathway to my success in math and science. v Acknowledgements I would first like to thank my advisor, Chris Elles, for devoting many hours working with me to get the lab up and running. Chris was able to patiently teach me about optics, electronics, and programming, so I am grateful for his time and dedication. I am also grateful for the many opportunities Chris has given me to attend conferences so I can present our work to other students and professors from other universities. Without a great advisor, great work could not have been accomplished. I would also like to thank the current and past Elles group members for their contributions to this research or for emotional support. Jenna Wasylenko, you have been especially helpful in listening to my research problems, proof reading papers, and just being there for a friend. I need to thank the office mates that I spent many many hours complaining to, consulting with, or just goofy around with. I will never forget Matt DeVore’s delightful “feed the children” song or his wild turkeys. I also need to thank Jen Settle for being around for support or to just waste time talking to me. I also need to thank Cassie Norton for getting me into KU basketball. I went to my first KU game with her and I promise to stay a Jayhawk fan when I am among the tar heels and blue devils. And finally, I need to thank the person I spent most of my time with, Tom Linz, for being a huge important part of my life. The time not spend in lab was spent with other great people I meet through KU. Carl, Megan, Dan, Phil, Maggie, Jess, Mary, Theresa, John, Racheal, Rachel, Kolbe, Andrew, Eric, and Will, thanks for the memorable times and for allowing me to be a part of the two-time champion coed C league softball team. Last, but not least, I need to thank my family for their support for the last several years while I’ve been striving to reach my educational goals. vi Table of Contents 1. Chapter One: Introduction to Photochromic Molecular Switches ........................................... 1 1.1 Overview of Dissertation ................................................................................................. 1 1.2 Motivation for Diarylethene Photochromic Molecular Switches .................................... 2 1.3 Woodward-Hoffmann Rules for Electrocyclic Reactions ................................................ 3 1.4 Structure and Physical Properties of Diarylethene Derivatives ....................................... 6 1.5 The Cycloreversion Reaction of 1,3-Cyclohexadiene .................................................... 10 1.6 Computational Studies for Diarylethenes ...................................................................... 10 1.7 Experimental Studies of the Cycloreversion Reaction for Diarylethenes ...................... 12 1.7.1 Dynamics of Fulgides ............................................................................................. 15 1.8 Dissertation Overview .................................................................................................... 17 1.9 References ...................................................................................................................... 19 2. Chapter Two: Experimental Approach ................................................................................. 30 2.1 Overview ........................................................................................................................ 30 2.2 Transient Absorption Techniques: Details of the Experimental Set-up ........................ 31 2.2.1 Pump-Probe Spectroscopy ...................................................................................... 31 2.2.1.1 Pump-Probe Using the Integrating Single Photodiode for Single-Wavelength Detection ..................................................................................................................... 32 2.2.1.2 Broadband PP with the Photodiode Array (PDA) ........................................ 34 2.2.2 Pump-Repump-Probe Spectroscopy ....................................................................... 36 2.2.2.1 Chopper Set-up for Pump-Repump-Probe Experiments .............................. 38 2.2.2.2 Broadband Pump-Repump-Probe with the Photodiode Array ..................... 42 2.3 Laser System .................................................................................................................. 42 2.3.1 Ti:Sapphire Lasers .................................................................................................. 42 2.3.2 Non-Linear Frequency Conversion ......................................................................... 47 2.3.2.1 Generation of White-Light Continuum ........................................................ 47 2.3.2.2 Optical Parametric Amplification ................................................................. 48 2.4 Sample Preparation and Delivery ................................................................................... 55 2.5 Detection Electronics ..................................................................................................... 56 2.5.1 Single Wavelength Detection with an Integrating Single Photodiode.................... 57 2.5.2 Broadband Detection with the Photodiode Array ................................................... 62 2.6 LabVIEW Programs ....................................................................................................... 66 2.6.1 Pump-Probe with the Photodiode Array ................................................................. 66 vii 2.6.2 Pump-Repump-Probe with the Photodiode Array .................................................. 79 2.6.3 Single Channel Detection ....................................................................................... 80 2.6.4 “Run Quantum Yield Experiment” VI .................................................................... 83 2.6.5 “Teraherz” VI.......................................................................................................... 84 2.7 Data Analysis ................................................................................................................. 84 2.7.1 Global and Target Analysis .................................................................................... 85 2.8 References .....................................................................................................................