Correlated Electronic and Vibrational Motion: A Direct Perspective Through Multidimensional Electronic-Vibrational Spectroscopy
Gaynor, James Daniel
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Developing a thorough mechanistic understanding of photoexcited charge transfer and proton transfer reactions at the molecular-level requires a deep knowledge of coupled electronic and nuclear motion. Two-Dimensional Electronic-Vibrational (2D EV) spectroscopy and Two-Dimensional Vibrational-Electronic (2D VE) spectroscopy are recently developed techniques that are directly sensitive to molecular vibronic couplings. This dissertation advances 2D EV spectroscopy by implementing new broadband femtosecond light sources in the UV (370 nm – 440 nm) and the mid-IR (1600 cm-1-3200 cm-1) frequency domains. A theoretical interpretation of 2D EV and 2D VE spectral signals is constructed using a vibronic Hamiltonian to systematically simulate spectra of a single anharmonic vibration with linear (equilibrium displacement) and quadratic (frequency shifting) vibronic coupling in the first excited electronic state. Selection rules are established from this analysis which demonstrate these techniques’ sensitivities to vibronic couplings, non-Condon effects, and electronic-state-dependent vibrational dephasing. This treatment is extended to a system of two coupled anharmonic vibrations with consideration of the orientational response. These simulations show that excited state vibrational (Duschinsky) mixing and non-Condon effects may be directly measured through polarization-dependent 2D EV spectroscopy. Finally, three photochemical reactions are studied using 2D EV spectroscopy: the ligand-to-metal charge transfer (LMCT) of ferricyanide ([Fe(CN)6]3-), the metal-to-ligand charge transfer (MLCT) of the solar cell dye molecule [Ru(dcbpy)2(NCS)2]4- (dcbpy = 4,4’-dicarboxy-2,2’-bipyridine) known as N34-, and the excited state intramolecular proton transfer (ESIPT) of 10-hydroxy[h]benzoquinoline (HBQ). Notably, specific MLCT excited states in N34- are revealed to vibronically couple the intramolecular charge donor and acceptor during the MLCT reaction. Their electronic distributions suggest that excited states with electron density in the charge donating molecular plane aid efficient charge transfer through these vibronically coupled motions. Three-dimensional (3D) EV spectroscopy is then demonstrated for the first time in a time-dependent study of N34-. This 3D EV experiment directly monitors the temporal evolution of vibrational and vibronic coherences involving two MLCT states, high frequency vibrations, and different low frequency vibrations. These coupled motions appear to facilitate the charge transfer and electronic delocalization in photoexcited N34- through an initial electronically-localized wavepacket and a consequent vibronic coherence participating in non-adiabatic internal conversion lasting for 1 picosecond. Analysis of the LMCT dynamics in ferricyanide combines 2D EV spectra and 2D VE spectra on the same system for the first time and iteratively fits the spectra using the one mode vibronic Hamiltonian. The LMCT in ferricyanide may be loosely approximated by this one mode picture; however, more subtle structural details become apparent from considering 2D EV and 2D VE spectra simultaneously. Early results toward a new direction for 2D EV spectroscopy concludes this dissertation: the study of coupled vibronic motions in the case of ESIPT on the benchmark system HBQ. Anisotropic analysis of the transient-IR spectroscopy on HBQ and its deuterated analogue, DBQ, suggest that non-Born-Oppenheimer dynamics play a role in the ESIPT mechanism of HBQ. Polarization-selective 2D EV spectra report a constant electronic excitation dependent anisotropy, indicating that the equilibration of the excited state keto structure does not involve a ~350 fs internal conversion process.
- Chemistry