Quantum mechanics/molecular mechanics investigation on the mechanistic photophysics of plant pigment 5,7-dihydroxyflavone in acetonitrile solution

The photoinduced excited-state intramolecular proton transfer (ESIPT) and dual fluorescence emission of flavonoids have been the subject of extensive experimental studies from femtosecond to microsecond time scales. Recent studies have suggested that the ultrafast intersystem crossing (ISC) of 5-hydroxyflavones under direct UV excitation can take place from the Franck-Condon region and compete with the well-known ESIPT reaction, thereby leading to the formation of triplet states. However, mechanistic insights into the excited state nonradiative deactivation and triplet state generation remain elusive. Herein, we present an XDW-CASPT2//CASSCF theoretical investigation to comprehensively elucidate the relevant intersection structures, the potential energy surfaces of the proton transfer paths and the nonradiative decay channels of natural plant pigment 5,7-dihydroxyflavone (5,7-DHF) in acetonitrile with solvent effects treated by explicit QM/MM model. Computational results show that the mixed of 1 nπ* and 1 ππ* characteristics and nearly degenerate energies of the S 1 and S 2 states in the Franck-Condon region enables fast internal conversion (IC) from 1 ππ* to 1 nπ*, and further triggers ISC from 1 nπ* to 3 ππ*, leading to the formation of the triplet state. Moreover, the involved 1 nπ*→ 3 ππ* and 1 nπ*→ 3 nπ* ISCs are further enhanced by the predicted large spin-orbit couplings of 24.5 and 20.3 cm −1 , respectively, at the QM(XDW-CASPT2)/MM level. Additionally, in the ESIPT paths, the potential energy surface is essentially barrierless for the 1 ππ* state, while it needs to overcome a small energy barrier for the 3 ππ* state but very large barriers for the 1 nπ* and 3 nπ* states. Interestingly, the optimized intersection structures between the 1 ππ*, 3 ππ*, and S 0 states show distinct puckering of the C2-phenyl group and the H atom of the C3-H moiety out of the benzo-γ-pyrone plane, indicating the IC and ISC to the S 0 state occur by the way of out-of-plane, which is different from the in-plane ESIPT reaction. The present computational results could provide useful mechanistic insights into the photoinduced tautomerization and excited state nonradiative decay of 5,7-DHF and its derivatives.