Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate.

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S, S, and T states, in a methanol solution. Based on the optimized geometric and electronic structures, conical intersections and crossing points, and minimum-energy paths combined with the computed linearly interpolated Cartesian coordinate paths, the photophysical mechanism of MS has been proposed. The S state is a spectroscopically bright ππ* state in the Franck-Condon region. From the initially populated S state, there exist three nonradiative relaxation paths to repopulate the S state. In the first one, the S system (i.e., ketoB form) first undergoes an ESIPT path to generate an S tautomer (i.e., enol form) that exhibits a large Stokes shift in experiments. The generated S enol tautomer further evolves toward the nearby S/S conical intersection and then hops to the S state, followed by the backward ground-state intramolecular proton transfer (GSIPT) to the initial ketoB form S state. In the second one, the S system first hops through the S → T intersystem crossing (ISC) to the T state, which then further decays to the S state via T → S ISC at the T/S crossing point. In the third path, the T system that stems from the S → T ISC process via the S/T crossing point first takes place a T ESIPT to generate a T enol tautomer, which can further decay to the S state via T-to-S ISC. Finally, the GSIPT occurs to back the system to the initial ketoB form S state. Our present work could contribute to understanding the photophysics of MS and its derivatives.