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.