Mainz 2017 – wissenschaftliches Programm
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MO: Fachverband Molekülphysik
MO 18: Biomolecules and Electron Transfer
MO 18.8: Vortrag
Freitag, 10. März 2017, 12:45–13:00, N 25
Individual Control of Lifetime and Triplet Yield in Halogen-Substituted Coumarin Derivatives — •Katrin Oberhofer1, Martin Wörle1, Mikayel Musheghyan1, Sebastian Wegscheider1, Rositsa Nikolova2, Reinhard Kienberger1, and Hristo Iglev1 — 1Physik-Department, Technische Universität München, James-Franck-Str., 85748 Garching — 2Faculty of Chemistry and Pharmacy, Sofia University, 1 J.Bourchier Blvd., 1126 Sofia, Bulgaria
Broadband transient absorption spectroscopy is utilized to examine lifetime and triplet yield in novel medicinally relevant Cl- and Br-substituted coumarin derivatives. Contradictory to the heavy-atom effect (HAE) absorption coefficient and fluorescent quantum yield for Cl-substituted compounds exhibit a strong enhancement. As DFT-calculations confirm several closely lying ππ* and nπ* states, we explain this behavior with an increased ππ* character of the lowest emitting state, which overcompensates the HAE. Femtosecond pump-probe spectroscopy reveals lifetimes of 100 ps for Cl-, 20 ps for Br- and 40 ps for unsubstituted compounds in acetonitrile. In order to get a quantity for the intersystem-crossing rate we determined the relative triplet yield by the ratio of triplet-triplet and excited singlet absorption after thermalization. While the lifetimes correlate qualitatively with fluorescence quantum yields, attributed to the ππ* increase, the triplet yield is significantly increasing with atomic weight of the substituted atom according to the HAE. Hence these two competing effects enable an independent control of lifetime and triplet yield.