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Probing the Ultrafast Dynamics of Higher-Lying Excited States of Molecular Switches with Femtosecond Stimulated Raman Spectroscopy
Burns, Kristen Hope
Burns, Kristen Hope
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Abstract
Highly-excited states have shown to play a critical role in the optical control of reaction dynamics and product yield of many photoactive molecular systems. However, due to their short lifetimes and high density of states, the initial dynamics and molecular motions within the upper states is less understood. In this dissertation, the higher-lying excited states of photochemical systems are measured by resonance transient Raman spectroscopy and provide information on the dynamics of the system following two-photon sequential excitation through mode-specific enhancements. Sequential excitation allows for optical control of the system by accessing more efficient reaction pathways. Allowing the molecule to evolve along the upper potential energy surface in between excitation events enables access to new regions of the higher-lying state potential surface that are now away from the ground-state equilibrium geometry. In the resonance Raman measurement, the vibrational frequencies report on the molecular structure of the lower electronic state, while the relative intensities of the Raman transitions reflect the initial motions out of the Franck-Condon region of the upper state in the first tens of femtoseconds after excitation due to the resonance condition. This provides a direct probe of the higher-lying state with increased structural specificity. The resonance condition can also be changed to access different higher-lying states which exhibit different mode-specific resonance enhancements. By comparing our results using transient Raman spectroscopy with previous work using transient electronic absorption spectroscopy, a more detailed picture of the reaction mechanism and excited-state landscape is achieved in order to further our understanding of optical control in photoactive systems. This dissertation reports the Raman spectra of diarylethene (DAE) photochromic molecular switches in the ground and excited states under various resonance conditions. The cycloreversion reaction of DAE-based systems is known to be inefficient after one-photon excitation; however, sequential two-photon excitation into higher-lying excited states greatly increases the yield. Resonance Raman spectroscopy is used to explore the role higher-lying excited states play in the optical control of the reaction dynamics of DAE. Using stimulated Raman scattering (SRS), we first measured accurate cross sections for common solvents to be used as reference values for two-photon absorption and other spectroscopy techniques. Our next experiments explored the mode-specific enhancement of DAE in the ground state using resonance Raman scattering. Here, the resonance condition is tuned over a broad wavelength range corresponding to multiple electronic transitions in the DAE spectrum. We observe different mode-specific resonance enhancements and, by comparison with calculations, determine the vibrational assignments for those modes and interpret how they contribute to the reaction mechanism of DAE after a one-photon excitation. Next, we investigated the higher-lying excited states of DAE through the wavelength and time-dependence of the two-photon sequential excitation using resonant femtosecond stimulated Raman spectroscopy (FSRS). We observed mode-specific enhancements for the two different resonance conditions that allowed us to characterize the initial motions on the upper states at early times. Through the time dependence, we monitored how the enhanced vibrational modes decay as the molecule evolves along the S1 excited state and found how those modes contribute to the different parts of the cycloreversion reaction. A second photoswitch was also studied to explore the effect of structural rigidity on the excited-state dynamics through resonance Raman methods. Studying these photoswitches with various spectroscopic methods allows us to obtain a more detailed understanding of their optical control through higher-lying excited states.
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Date
2022-12-31
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University of Kansas
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Keywords
Physical chemistry, Photoswitch, Raman scattering, Spectroscopy, Ultrafast