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In this work I present excited state molecular dynamics (MD) calculations and improvements to the existing simulation procedures. The method employed is trajectory surface hopping (TSH) in conjunction with the semiempirical OM2/MRCI Hamiltonian. It has been applied to three molecules of recent scientifc interest. In TSH simulations a set of samples is generated in the ground state. For each sample an excited state MD simulation is run with the possibility to hop between electronic states during the relaxation to the ground state. The statistical analysis of all trajectories gives insight into competing deactivation processes, deactivation times, quantum yields, and product distributions. Such simulations require many trajectories that run independently of each other (typically several hundred) with many singlepoint energy and gradient evaluations per trajectory (typically several thousand). This vast number of calculations renders high-level ab initio methods impractical for medium-sized organic molecules. Therefore, we employed the semiempirical OM2/MRCI method which delivers realistic energies and structures for ground and excited states at greatly reduced computational costs.
We first investigated salicylideneaniline, which is a typical representative of aromatic Schiff bases that exhibit photochromic behavior. Our TSH simulations shed light on the competing deactivation channels, the time scale of the processes involved, and the product responsible for photochromism.
In a second study we addressed non-equilibrium isotope effects on ultrafast excited state intramolecular proton transfer (ESIPT) by performing TSH simulations for 7-(2-pyridyl)indole and its deuterated isotopologue. We were able to reproduce the experimental results on isotope shifts and show that the ESIPT is an essentially barrierless process.
In a third application we investigated the full photocycle of salicylidene methylamine, a possible photoswitching agent. We ran two sets of excited state dynamics simulations starting from the two relevant conformers. We characterized the main channels for photoswitching and quantified the effciency of the targeted pathways. Additionally, we identified a photoinactive isomer that breaks the photocycle.
In the course of this work I encountered problems in the existing methods and procedures arising from changes in the active configuration interaction space during the simulations. These problems were addressed and largely overcome in our latest publication, in which we introduced an adaptive time step in the dynamics code, in order to improve the precision and stability of the simulations in numerically diffcult areas of configuration space.
This thesis showed the significance and usefulness of TSH simulations despite their simple underlying model. It clarified the photocycles of three molecules of current scientific interest and enabled us to suggest a molecule for the application as a photoswitch. The use of adaptive time steps will help to improve TSH simulations in combination with incomplete active space configuration interaction methods.
