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 ππ*, nπ*, ππ*, nπ*, and S states, we have identified several feasible excited-state relaxation pathways for the initially populated S(ππ*) state to decay to the initial enol isomer' S state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless ππ* ESIPT process to generate the ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S → S 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(ππ*) minimum, an S(ππ*)/S(nπ*)/T(nπ*) three-state intersection region helps the S system branch into the T state through a S → S → T or S → T → T process. Once it has reached the T state, the system may relax to the S state direct ISC or subsequent nearly barrierless ππ* ESIPT to yield the T keto tautomer and ISC. The resultant S 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/S crossing points, the ISC to S 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.