O-H vibrational motions promote sub-50 fs nonadiabatic dynamics in 3-hydroxypyran-4-one: interplay between internal conversion and ESIPT.

A theoretical study is used to explore the involvement of O-H vibrational motions in the S-0 -> S-2 photoinduced dynamics of 3-hydroxypyran-4-one (3-HOX). Two transitions, S-0 -> S-1 and S-0 -> S-2, are attributed to the experimentally observed electronic absorption spectral features in the range of 3.5-5.5 eV. We compute model potential energy surfaces of vibronically coupled S-1 (n pi*) and S-2 (pi pi*) states with the aid of extensive electronic structure calculations. The S-1-S-2 conical intersection is characterized in the O-H bend and O-H stretch vibrational coordinate space. Quantum wavepacket dynamics simulations reveal an ultrafast S-2 -> S-1 internal conversion decay, where about 90% of the S-2 population disappears within the first 50 fs of the propagation time. The participation of O-H vibrational motions in the early events of nonadiabatic dynamics is analyzed based on the time evolution of nuclear densities on S-2. We discuss the implications of these observations to provide fundamental insights into the nonadiabatic excited-state intramolecular proton transfer in 3-HOX and its derivatives.