The photophysics of alloxazine: a quantum chemical investigation in vacuum and solution.

(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(1)) potential energy hypersurface (PEH) with electronic structures (1)(npi*) and (1)(pipi*), corresponding to the dark 1 (1)A'' (S(1)) state and the nearly degenerate, optically bright 2 (1)A' (S(2)) state in the vertical absorption spectrum, respectively. In the vacuum the minimum of the (1)(npi*) electronic structure is clearly found below that of the (1)(pipi*) electronic structure. Population transfer from (1)(pipi*) to (1)(npi*) may proceed along an almost barrierless pathway. Hence, in the vacuum, internal conversion (IC) between the 2 (1)A' and the 1 (1)A'' state is expected to be ultrafast and fluorescence should be quenched completely. The depletion of the (1)(npi*) 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)(npi*) state and the red shift of the (1)(pipi*) state. However, the minimum of the (1)(npi*) state still is expected to be found on the S(1) 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)(npi*) state is anticipated to participate in the deactivation process and to partially quench the fluorescence.