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