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 (1)ππ*, (1)nπ*, (3)ππ*, and (3)nπ* states, we have proposed three energetically possible nonadiabatic relaxation pathways populating the lowest (3)ππ* triplet state from the initially populated excited (1)ππ* singlet state. The first is the direct (1)ππ*→ (3)ππ* intersystem crossing via the (1)ππ*/(3)ππ* crossing point. The second is an indirect (1)ππ* → (3)ππ* intersystem crossing relayed by the dark (1)nπ* singlet state. In this route, the (1)ππ* system first decays to the (1)nπ* state via the (1)ππ*/(1)nπ* conical intersection, followed by an (1)nπ*→ (3)ππ* intersystem crossing at the (1)nπ*/(3)ππ* crossing point to arrive at the final (3)ππ* state. The third is similar to the second one; but its intersystem crossing is relayed by the (3)nπ* triplet state. The (1)ππ* system first decays to the (3)nπ* state via the (1)ππ*/(3)nπ* crossing point; the generated (3)nπ* state is then de-excited to the (3)ππ* state through the (3)nπ*→ (3)ππ* internal conversion at the (3)nπ*/(3)ππ* conical intersection. According to the classical El-Sayed rule, we suggest the second and third paths play a much more important role than the first one in the formation of the lowest (3)ππ* state.