Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution.

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S(ππ*) state and the lower-lying T and T states. In the major relaxation pathway, the photoexcited S keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S enol minimum, near which a favorable S/S conical intersection decays the system to the S state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S keto species. In the minor one, an S/T/T three-state intersection in the keto region makes the T state populated via direct and T-mediated intersystem crossing (ISC) processes. In the T state, an ESIPT occurs, which is followed by ISC near a T/S crossing point in the enol region to the S state and finally back to the S keto species. In addition, a T/S crossing point near the T keto minimum can also help the system decay to the S keto species. However, small spin-orbit couplings between T and S at these T/S crossing points make ISC to the S state very slow and make the system trapped in the T state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.