Nonadiabatic excited-state intramolecular proton transfer in 3-hydroxyflavone: S₂ state involvement via multi-mode effect

The excited-state intramolecular proton transfer process in 3-hydroxyflavone is investigated based on the computed structural parameters and energetics of stationary points of vibronically coupled S₁-S₂ potential energy surfaces. A conical intersection close to the Franck-Condon point on S₁ is identified. The minimum energy of the conical intersection is found to be near-degenerate with the equilibrium minimum of S₂. Quantum nuclear wavepacket simulations revealed a small population transfer from the "bright" S₁ to "dark" S₂ on a time scale shorter than the O - H stretching vibrational period.