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Kölle, Patrick (2016): Light-induced molecular processes in organic-based energy conversion and biomimetic synthesis of natural products. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



Processes initiated by sunlight are fundamental steps in photovoltaic devices as well as in biosyntheses. The present work investigates the photoinduced processes in organic-based energy conversion materials and biomimetic synthesis of natural products by quantum chemical calculations. The work is performed in close collaboration with experimental groups and enables a deeper understanding of the observations. The detailed knowledge allows to predict the optimal conditions to initiate the photochemical syntheses and the chemical substitution to achieve the desired properties. In the first and second part of the thesis, two classes of molecules commonly used in organic-based optoelectronic devices are considered and potential factors influencing the performance of the optical devices are revealed. In the third part, the photochemical and biomimetic syntheses of two natural products and the details of the complex reaction mechanisms are elucidated. In the first part of the present work the deactivation pathways from the first excited singlet state S1 of thiophene and of small oligothiophenes containing up to four rings are investigated by state-of-the-art quantum chemical methods. For thiophene a low-lying S1/S0 conical intersection seam is easily accessible and drives the fast internal conversion. In the oligothiophenes barriers in combination with fast intersystem crossing channels inhibit this passage. The calculated spin-orbit coupling strength together with the singlet-triplet energy gaps can explain the decreasing triplet and increasing fluorescence quantum yields for growing chain length. The present theoretical results allow a deeper understanding of the deactivation pathways of thiophene and small oligothiophenes and are of potential interest for the photophysics of longer oligothiophenes and polythiophenes used in optoelectronic devices. In the second part the photoinduced dynamics of perylene diimide dyads based on a donor-spacer-acceptor motif are considered. The dyads based on pyridine spacer undergo energy transfer from the donor to the acceptor with near-unity quantum efficiency. In contrast in the dyads with phenyl spacers the energy transfer decreases below 50%, suggesting the presence of a competing electron transfer from the spacer to the donor. However, the measurements indicate that the spacer itself mediates the energy transfer dynamics. Ab initio calculations reveal the existence of bright charge transfer states which enable the energy transfer. This new energy transfer represents a first example that show how electron transfer can be connected to energy transfer for the use in novel photovoltaic devices. Additional experiments and calculations of subsystems demonstrate that the solvation time and not the polarity of the solvent is surprisingly the crucial property of the solvent for the charge and energy transfer dynamics. In the last part the photochemical syntheses of the two natural products intricarene and aplydactone are studied. Intricarene was isolated from a Carribbean coral and according to its proposed biosynthesis it arises from an oxidopyrylium intermediate via an intramolecular 1,3-dipolar cycloaddition. By a combination of experiments and theory it is shown that oxidopyrylium indeed forms under biomimetic and photochemical conditions and that it represents the key intermediate in the complex reaction cascade leading to intricarene. Triplet states as well as conical intersections enable the formation of intricarene and of an intriguing by-product which may constitute a new natural product. In the second part of the last chapter a quantum chemical study of the [2+2] photocycloaddition of dactylone to aplydactone is performed. Both compounds were isolated from a Madagascan sea hare and especially aplydactone exhibits an unprecedented molecular structure. However, for both compounds no total syntheses have been reported yet. According to the proposed biosynthesis, aplydactone is formed by a photochemical [2+2] cycloaddition out of dactylone but attempts to synthesize aplydactone through irradiation of dactylone failed. In the present work quantum chemical calculations elucidate the optimal biomimetic conditions to initiate the photochemical reaction and the different reaction pathways on the excited state potential energy surface are revealed. Overall, the last chapter highlights the importance of weak absorption bands and long-lived triplet states for the photochemical synthesis of natural products.