Full-Dimensional Excited-State Intramolecular Proton Transfer Dynamics of Salicylic Acid.

Salicylic acid (SAc) and its excited-state intramolecular proton transfer (ESIPT) capabilities have been studied both experimentally and theoretically by static calculations. However, to our knowledge, no radiationless pathway has been proposed so far. Instead, excited-state deactivation was only investigated via fluorescence. Therefore, we will present full-dimensional photodynamics of SAc using the floating-occupation configuration-interaction (FOCI) treatment with single and paired double excitations based on the semiempirical RM1 Hamiltonian. To further clarify mechanistic details, the potential energy surface (PES) is scanned along the proton transfer coordinates in one and two dimensions. The time-evolution of relevant degrees of freedom (DOF), quantum yields and isomer populations are evaluated from 200 surface-hopping trajectories. It was found that a deactivation pathway from the excited state to the ground state is indeed accessible through a conical intersection, via rotation of the carboxyl group. Together with the ESIPT process, this rotation can also interchange the protons of the two (formal) OH groups, which makes the overall dynamics still more complex. Our full-dimensional photodynamics study provides a comprehensive overview of all these entangled steps.