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-2H 2 O (referred to as 5HF-2W), we have identified four competitive S 2 (ππ*) radiationless relaxation channels from the Franck–Condon (FC) point. The first is barrierless excited-state intramolecular proton transfer (ESIPT) to generate the 1 ππ*-T tautomer, which further evolves toward the nearby 1 ππ*/S 0 -T conical intersection and then deactivates back to the S 0 state, followed by favorable reverse ground-state proton transfer. The second is indirect 1 ππ*→ 3 ππ* intersystem crossing (ISC) mediated by the dark 1 nπ* state. In this route, the 1 ππ* state first hops to the 1 nπ* state via the 1 ππ*/ 1 nπ*-N conical intersection, followed by 1 nπ*→ 3 ππ* ISC at the 1 nπ*/ 3 ππ*/ 3 nπ*-N intersection structure to reach the 3 ππ* state, enhanced by the CASPT2-computed large 1 nπ*/ 3 ππ* spin–orbit coupling (SOC) of 30.2 cm –1 . The generated 3 ππ* state undergoes ESIPT by overcoming a 3.5 kcal/mol energy barrier to yield the 3 ππ*-T tautomer, which subsequently runs into the nearby 3 ππ*/S 0 -T crossing point and hops to the S 0 state. The third is similar to the second one, but its ISC is relayed by the 3 nπ* state. At the 1 nπ*/ 3 ππ*/ 3 nπ*-N intersection structure, it first transfers to the 3 nπ* state ( 1 nπ*/ 3 nπ* SOC: 17.4 cm –1 ) and then hops to the 3 ππ* state through 3 nπ*→ 3 ππ* internal conversion (IC) at the 3 nπ*/ 3 ππ*-N conical intersection, which is followed by direct ISC from T 1 to S 0 via the 3 ππ*/S 0 -N crossing point. The last one is direct 1 ππ*→S 0 IC from the FC region through the 1 ππ*/S 0 -N conical intersection. This work contributes to the understanding of the photophysics of 5HF-based flavonoids and their analogues.