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)pi pi*, (1)n pi*, (3)pi pi*, (3)n pi*, and S-0 states, we have identified several feasible excited-state relaxation pathways for the initially populated S-2((1)pi pi*) 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)pi pi* ESIPT process to generate the (1)pi pi* 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)pi pi*) minimum, an S-2((1)pi pi*)/S-1((1)n pi*)/T-2((3)n pi*) 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)pi pi* 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.