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 (S(0)), closing the photocycle. This mechanism explains the exceptionally weak fluorescence and the short excited-state lifetime for the flavin trimethoxyphenyl derivative …