Fast singlet excited-state deactivation pathway of flavin with a trimethoxyphenyl derivative

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.