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 1 ( 1 ππ*) state and the lower-lying T 1 and T 2 states. In the major relaxation pathway, the photoexcited S 1 keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S 1 enol minimum, near which a favorable S 1 /S 0 conical intersection decays the system to the S 0 state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S 0 keto species. In the minor one, an S 1 /T 2 /T 1 three-state intersection in the keto region makes the T 1 state populated via direct and T 2 -mediated intersystem crossing (ISC) processes. In the T 1 state, an ESIPT occurs, which is followed by ISC near a T 1 /S 0 crossing point in the enol region to the S 0 state and finally back to the S 0 keto species. In addition, a T 1 /S 0 crossing point near the T 1 keto minimum can also help the system decay to the S 0 keto species. However, small spin-orbit couplings between T 1 and S 0 at these T 1 /S 0 crossing points make ISC to the S 0 state very slow and make the system trapped in the T 1 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.