Abstract:

The development, implementation, and benchmarking of computational methods for studying photochemical processes and computing associated spectroscopic observables are reported in this dissertation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used for the underlying electronic structure calculations. Vibronic perturbation theory (VPT) and fewest switches surface hopping (FSSH) were applied to simulate and explain the gas-phase fluorescence of pyrene. Together, VPT and FSSH were established as a two-step approach for investigating nonadiabatic effects in the absorption and emission spectra of molecular systems.

A methodology was established to compute time-resolved spectroscopic observables, such as the transient absorption (TA) spectrum, from multi-state FSSH simulations. The computed TA spectrum of pyrene was found to agree with experiment.

DFT and TDDFT were further employed to simulate the absorption spectra of novel 2.2.2-cryptand complexes of neptunium(III) and plutonium(III), and to determine the minimum-energy conformers of nicotinamide cofactor biomimetics. The results are consistent with experimental observations.

Finally, an integrated framework combining weighted-ensemble ab initio molecular dynamics with nonadiabatic molecular dynamics was developed to study the bioluminescence of dinoflagellate luciferin. The computed bioluminescence spectra and mechanistic details show agreement with experiment and previous studies. This framework provides a generalizable strategy for simulating long-timescale processes and excited-state dynamics in complex photochemical systems