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Development of theoretical and computational tools to study photochemistry involving multiple electronic excited states

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The development, implementation and benchmarking of the computational

tools to study photochemical processes is reported in this dissertation.

Fewest switches surface hopping (FSSH) in conjunction with density

functional theory (DFT) and time-dependent DFT (TDDFT) forms the

basis of the methodology. DFT and TDDFT was also used to explain

the unusual electronic structure of the first synthesized square

planar actinide complex. A $\pi$--donating ligand field theory

was proposed to explain the $d_{z^2}$ character of the

highest occupied molecular orbital of this complex. The theory

was supported by the agreement between the simulated and

the experimentally observed UV/Vis spectrum.

The excited state deactivation of 5-methoxyquinoline, a known

photobase, was studied using the developed methodology. Active

participation of the water molecules in quenching the excitation

of the chromophore was observed from the simulations. The

observed mechanism challenges the validity of F{\"o}rster cycle

to understand photoacidity and photobasicity. The simulations

were also able to explain the lack of kinetic isotope effect

observed from the experimental transient absorption spectroscopy.

With access to stable analytical derivative couplings between

excited states, it was possible to study the performance of

TDDFT/FSSH methodology for thymine. The simulated excited state

lifetimes agreed with experimental observations. Addition of decoherence

correction to the FSSH algorithm resulted in no decay to the

ground state. Analysis of the lowest excited potential energy

surface of thymine as generated from TDDFT showed a kinetic

barrier to a conical intersection, explaining the lack of

decay to the ground state. Two different flavors of decoherence

correction was implemented, and will be available in the next

release of Turbomole.

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