QM/MM Study on Mechanistic Photophysics of Alloxazine Chromophore in Aqueous Solution

Compared with isoalloxazine, the core chromophore of biologically important flavins, alloxazine exhibits much lower fluorescence quantum yield and larger intersystem-crossing quantum yield. However, its efficient radiationless relaxation pathways are still elusive. In this work, we have used the QM(MS-CASPT2//CASSCF)/MM method to explore the mechanistic photophysics of alloxazine chromophore in aqueous solution. On the basis of the optimized minima, conical intersections, and crossing points in the lowest 1ππ*, 1nπ*, 3ππ*, and 3nπ* states, we have proposed three energetically possible nonadiabatic relaxation pathways populating the lowest 3ππ* triplet state from the initially populated excited 1ππ* singlet state. The first is the direct 1ππ*→ 3ππ* intersystem crossing via the 1ππ*/3ππ* crossing point. The second is an indirect 1ππ* → 3ππ* intersystem crossing relayed by the dark 1nπ* singlet state. In this route, the 1ππ* system first decays to the 1nπ* state via the 1ππ*/1nπ* conical intersection, follow...