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

Abstract

(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 ST 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 (S₁) 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(nπ*) 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.