Incorporation of the trimethoxyphenyl group at position 7 of flavin can drastically change the photophysical properties of flavin. We show unique fast singlet 1(π,π*) excited state deactivation pathway through nonadiabatic transition to the 1(n,π*) excited- state, and subsequent deactivation to the ground electronic state (S0), closing the photocycle. This mechanism explains the exceptionally weak fluorescence and the short excited-state lifetime for the flavin trimethoxyphenyl derivative and the lack of excited triplet T1 state formation. Full recovery of flavin in its ground state takes place within a 15 ps time window after photoexcitation in a polar solvent such as acetonitrile. According to quantum chemical calculations, the C(2)-O distance elongates by 0.16 Å in the 1(n,π*) state, with respect to the ground state. Intermediate-state structures are predicted by theoretical ab initio calculations and their dynamics are investigated using broadband vis-NIR time-resolved transient absorption and fluorescence up-conversion techniques.

Department of Biomolecular Mechanisms Max Planck Institute for Medical Research Jahnstrasse 29 69120 Heidelberg Germany

Department of Organic Chemistry University of Chemistry and Technology Prague 16628 Prague Czech Republic

Faculty of Chemistry Adam Mickiewicz University Uniwersytetu Poznanskiego 8 61 614 Poznan Poland

Faculty of Physics and Astronomy Adam Mickiewicz University Uniwersytetu Poznanskiego 2 61 614 Poznan Poland

Institute of Physics Polish Academy of Sciences Aleja Lotników 32 46 02 668 Warsaw Poland

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