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 ( n π*) and 1 (ππ*), 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 ( n π*) electronic structure is clearly found below that of the 1 (ππ*) electronic structure. Population transfer from 1 (ππ*) to 1 ( n π*) 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 ( n π*) 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 ( n π*) state and the red shift of the 1 (ππ*) state. However, the minimum of the 1 ( n π*) 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 ( n π*) state is anticipated to participate in the deactivation process and to partially quench the fluorescence.