Excited-State Relaxation Mechanism of 5-Hydroxyflavone: A CASPT2 Investigation in the Microsolvated Aqueous Surrounding.

5-Hydroxyflavone (5HF) is a naturally occurring flavonol with a hydroxyl group at the C5 position and shows unusual proton-transfer properties with a very low fluorescence quantum yield, which justifies its role as a natural UV filter. Using the CASPT2//CASSCF method to study the mechanistic photophysics of its two-water hydrogen-bonded complex 5HF-2H2O (referred to as 5HF-2W), we have identified four competitive S2(ππ*) radiationless relaxation channels from the Franck-Condon (FC) point. The first is barrierless excited-state intramolecular proton transfer (ESIPT) to generate the 1ππ*-T tautomer, which further evolves toward the nearby 1ππ*/S0-T conical intersection and then deactivates back to the S0 state, followed by favorable reverse ground-state proton transfer. The second is indirect 1ππ*→3ππ* intersystem crossing (ISC) mediated by the dark 1nπ* state. In this route, the 1ππ* state first hops to the 1nπ* state via the 1ππ*/1nπ*-N conical intersection, followed by 1nπ*→3ππ* ISC at the 1nπ*/3ππ*/3nπ*-N intersection structure to reach the 3ππ* state, enhanced by the CASPT2-computed large 1nπ*/3ππ* spin-orbit coupling (SOC) of 30.2 cm-1. The generated 3ππ* state undergoes ESIPT by overcoming a 3.5 kcal/mol energy barrier to yield the 3ππ*-T tautomer, which subsequently runs into the nearby 3ππ*/S0-T crossing point and hops to the S0 state. The third is similar to the second one, but its ISC is relayed by the 3nπ* state. At the 1nπ*/3ππ*/3nπ*-N intersection structure, it first transfers to the 3nπ* state (1nπ*/3nπ* SOC: 17.4 cm-1) and then hops to the 3ππ* state through 3nπ*→3ππ* internal conversion (IC) at the 3nπ*/3ππ*-N conical intersection, which is followed by direct ISC from T1 to S0 via the 3ππ*/S0-N crossing point. The last one is direct 1ππ*→S0 IC from the FC region through the 1ππ*/S0-N conical intersection. This work contributes to the understanding of the photophysics of 5HF-based flavonoids and their analogues.