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MO: Fachverband Molekülphysik

MO 3: Theory: Quantum Chemistry and Molecular Dynamics

MO 3.1: Hauptvortrag

Montag, 8. März 2010, 16:30–17:00, F 102

Mechanisms of Triplet Generation and Depletion in Organic Molecules — •Christel M. Marian — Theoretical and Computational Chemistry, HHU Düsseldorf, Germany

Inter-system crossing (ISC) plays a central role in photochemistry, photobiology, and photodynamic therapy. While a high triplet quantum yield is desirable for photosensitizers, rapid quenching of the T1 state is preferable for photoprotectors. A prerequisite for the understanding of the underlying mechanisms is the knowledge of the excited states and their interactions.

State-of-the-art quantum chemical methods can help to shine light on the cascade of relaxation processes following the primary photo excitation. The combined density functional and configuration interaction method (DFT/MRCI) has proven to yield high-quality electronic excitation spectra. Electronic spin-orbit matrix elements are calculated using the SPOCK program package while vibrational contributions to ISC rates are determined using the SNF and VIBES programs.

The mechanisms of triplet formation and depletion have been investigated for a variety of organic chromophores. Here, only a few model cases are presented: Flavone where the ISC mechanism follows the well-known El-Sayed rules, porphyrin where vibronic spin-orbit coupling between the S1 and T1 states is responsible for the high triplet quantum yield, and flavin where a solvent or protein environment have a significant impact on the ISC mechanism. Efficient triplet quenchers such as cyclooctatetraenes or diphenylhexatrienes are found to possess fast relaxation pathways to the electronic ground state.