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Nenov, Artur (2012): Relation between molecular structure and ultrafast photoreactivity with application to molecular switches. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Photoinduced ultrafast isomerizations are fundamental reactions in photochemistry and photobiology. This thesis aims for an understanding of the generic forces driving these reactions and a theoretical approach is set up, able to handle realistic systems, whose fast relaxation is mediated by conical intersections. The main focus is on the development of strategies for the prediction and accelerated optimization of conical intersections and their application to artificial compounds with promising physicochemical properties for technical applications as molecular switches. All calculations are based on advanced quantum chemical methods and mixed quantum-classical dynamics. In the first part of this thesis the two-electron two-orbital theory by Michl and Bonacic-Koutecky used in its original formulation to rationalize the structure of conical intersections in charged polyene systems is extended by including the interactions of the active pair of electrons with the remaining closed-shell electrons that are present in any realistic system. A set of conditions, called resonance and heterosymmetry conditions, for the formation of conical intersections in multielectronic systems are derived and verified by calculations on the basic units ethylene, cis-butadiene and 1,3-cyclohexadiene at various geometries and functionalizational patterns. The quantitative results help to understand the role of geometrical deformations and substituent effects for the formation of conical intersections and to derive rules of thumb for their qualitative prediction in arbitrary polyenes. An extension of the rules of thumb to conical intersection seams is formulated. The strategy pursued is to divide the molecular system into basic units and into functional groups. Each unit and its intersection space are treated independently, thereby reducing the dimensionality of the search space compared to the complete molecule. Subsequently, the interconnectivity of the intersection spaces of the different units is determined and an initial guess for the complete seam is constructed. This guess is then fed into a quantum chemistry package to finalize the optimization. The strategy is demonstrated for two multi-functionalized systems, hemithioindigo-hemistilbene and trifluoromethyl-pyrrolylfulgide. In the second part of this thesis state-of-the-art quantum chemical calculations and time-resolved transient and infrared spectroscopy are used to reconstruct the complex multi-channel isomerization mechanisms of hemithioindigo-hemistilbene and trifluoromethyl-indolylfulgide. Both the cis-trans isomerization in hemithioindigo-hemistilbene and the electrocyclic ring closure/opening in indolylfulgide are characterized by a charge transfer in the excited state. The ability of each system to stabilize this charge transfer is essential for the returning to the ground state. The relaxation to the ground state through extended regions of the seam is found to be the decisive step determining the reaction speed and the quantum yield. In the last part of this thesis mixed quantum-classical dynamics simulations at multi-configurational perturbation theory (MS-CASPT2) level, using Tully's fewest switches surface hopping approach, are performed to study the ultrafast photoreactivity of 1,3-cyclohexadiene in the gas-phase. For this purpose a numerical routine for the efficient calculation of non-adiabatic couplings at MS-CASPT2 level is presented. The major part of the excited molecules are found to circumvent the 1B2/2A1 conical intersection and reach the conical intersection seam between the excited state and the ground state instantaneuosly. Time constants for the evolution of the wavepacket on the bright 1B2-state, the relaxation into the 2A1-state and the return to the ground state are extracted. It is demonstrated that the accessibility of the conical intersection seam depends on its energetic and spatial relation to the minimum energy path, as well as on the momentum which is accumulated during relaxation on the excited state potential energy surface.