Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the 1 ππ*, 1 nπ*, 3 ππ*, 3 nπ*, and S 0 states, we have identified several feasible excited-state relaxation pathways for the initially populated S 2 ( 1 ππ*) state to decay to the initial enol isomer' S 0 state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless 1 ππ* ESIPT process to generate the 1 ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S 1 → S 0 internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S 2 ( 1 ππ*) minimum, an S 2 ( 1 ππ*)/S 1 ( 1 nπ*)/T 2 ( 3 nπ*) three-state intersection region helps the S 2 system branch into the T 1 state through a S 2 → S 1 → T 1 or S 2 → T 2 → T 1 process. Once it has reached the T 1 state, the system may relax to the S 0 state via direct ISC or via subsequent nearly barrierless 3 ππ* ESIPT to yield the T 1 keto tautomer and ISC. The resultant S 0 keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T 1 /S 0 crossing points, the ISC to S 0 state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature. We report a QM(CASPT2//CASSCF)/MM study on the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution.