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Oesterling, Sven (2017): Ab initio studies on photorelaxation: exploring, altering and crossing the excited state landscape. Dissertation, LMU München: Fakultät für Chemie und Pharmazie
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Abstract

This work addresses relaxation mechanisms of photoexcited organic molecules of small and medium size, up to 62 atoms. For most systems it is investigated theoretically, how modifications, often in the form of substituents, influence the decay processes. The research in large parts is done in close collaboration with groups providing experimental data, which allows to formulate robust hypotheses and models. Four systems are discussed in this context. We find the formation of the dewar lesion in deoxyribonucleic acid (DNA) to only occur, when the nucleobase is embedded in the DNA backbone, which sterically hinders accessing alternative channels. Substituting hydroxy groups at certain points of thioindigo is shown to open up an efficient deactivation channel via excited state intramolecular proton transfer, and greatly enhance the photostability of the molecule. By substituting electron donating groups to the stilbene moiety of the hemithioindigo photoswitch and correlating their effect to their Hammett parameters, the isomerization speed of hemithioindigo is optimized. And lastly, when adding an aldehyde group to furan, an additional pathway is found for its derivatives furfural and β-furfural. Their relaxation is slowed down regardless. The effects on the excited state potential energy surfaces are described as general means, by which the surfaces can be influenced, and likely can be translated to other molecules as well. This eventually allows to predict properties and tailor molecules to yield desired behavior. In this context, for example for furan, furfural and β-furfural the structural implications of the aldehyde substituent on one conical intersection are deducted from the extended two-electron two-orbital model prior to any calculations or experiments. Alongside the system specific investigations, an interface for the on-the-fly dynamics package NewtonX to the quantum chemistry package Molpro was programmed. Non-adiabatic semiclassical on-the-fly dynamics are a powerful tool to simulate complete relaxation processes without constraints in the dimensionality. For the interface, which in its primary setup uses complete active space self consistent field theory calculations, a number of features has been implemented. Most notably, it enables non-adiabtatic dynamics on complete active space perturbation and ONIOM level of theory.