(Time-dependent) Kohn-Sham density functional theory and a combined density functional/ multi-reference configuration interaction method (DFT/MRCI) were employed to explore the ground and low-lying electronically excited states of alloxazine, a flavin related molecule. Spin-orbit coupling was taken into account using an efficient, nonempirical mean-field Hamiltonian. Intersystem crossing (ISC) rate constants for S -> T transitions were computed, employing both direct and vibronic spin-orbit coupling. Solvent effects were mimicked by a conductor-like screening model and micro-hydration with up to six explicit water molecules. Multiple minima were found on the first excited singlet (S,) potential energy hypersurface (PEH) with electronic structures ~1(nπ*) and ~1(ππ*), corresponding to the dark 1 ~1A" (S1) state and the nearly degenerate, optically bright 2 ~1A' (S2) state in the vertical absorption spectrum, respectively. In the vacuum the minimum of the ~1(rat*) electronic structure is clearly found below that of the ~1(ππ*) electronic structure. Population transfer from ~1(ππ*) to ~1(nπ*) may proceed along an almost barrierless pathway. Hence, in the vacuum, internal conversion (IC) between the 2 ~1A' and the 1 ~1A" state is expected to be ultrafast and fluorescence should be quenched completely. The depletion of the ~1(nπ*) state is anticipated to occur via competing IC and direct ISC processes. In aqueous solution this changes, due to the blue shift of the ~1(nπ*) state and the red shift of the ~1(ππ*) state. However, the minimum of the ~1(nπ*) state still is expected to be found on the S1 PEH. For vibrationally relaxed alloxazines pronounced fluorescence and ISC by a vibronic spin-orbit coupling mechanism is expected. At elevated temperatures or excess energy of the excitation laser, the ~1(nπ*) state is anticipated to participate in the deactivation process and to partially quench the fluorescence.
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