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 $S{\rightsquigarrow}T$ transitions were computed, employing both direct and vibroni 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 $1^1A"(S_1)$ state and the nearly degenerate, optically bright $2^1A"(S_2)$ 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 $2^1A'$ and the $1^1A"$ 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.