Excited state proton transfer and internal conversion in o-hydroxybenzaldehyde: new insights from non-adiabatic ab initio molecular dynamics

A recently developed non-adiabatic ab initio molecular dynamics method coupling the S₁ excited electronic state to the ground state by a surface hopping technique has been applied to study photoexcited o-hydroxybenzaldehyde. Vertical excitation of the enol tautomer to the π π* state leads to spontaneous proton transfer (PT) and thus formation of the keto tautomer. The PT process has been found to be entirely driven by the breathing mode of the H-chelate ring. In fact, there exists no barrier for PT in the S₁ state. Non-radiative decay of the π π* excited keto tautomer through internal conversion (IC) is observed on a picosecond time scale. The probability for a non-adiabatic surface hop from the S₁ state back to the ground state is seen to be correlated to the chelate ring breathing mode and to the temporal changes in the methoxy OH bond length. There are indications of an increased IC rate associated with the initial PT event.