Photophysics of a UV-B Filter 4-Methylbenzylidene Camphor: Intersystem Crossing Plays an Important Role.

4-Methylbenzylidene camphor (4MBC) is a frequently used ultraviolet (UV) filter in commercial sunscreens, which is experimentally found to undergo efficient intersystem crossing to triplet manifolds followed by predominant radiationless decay to the ground state. However, its photophysical mechanism is unclear. Herein, we have employed combined CASPT2 and CASSCF methods to study the spectroscopic properties, geometric and electronic structures, conical intersections and crossing points, and excited-state deactivation channels of 4MBC. We have found that the V( ππ*) state is populated with large probability in the Franck-Condon region. Starting from this state, there are two efficient nonradiative relaxation processes to populate the ππ* state. In the first one, the V( ππ*) state decays to the V'( ππ*) state. The resultant V'( ππ*) state further jumps to the nπ* state by internal conversion at the ππ*/ nπ* conical intersection. Then, the nπ* state hops to the ππ* state through an efficient nπ*→ ππ* intersystem crossing process. In the second one, the V( ππ*) state can diabatically relax along the photoisomerization reaction coordinate. In this process, a ππ*/ nπ* crossing point helps the ππ* system decay to the nπ* state, which further decays to the ππ* state through internal conversion at the nπ*/ ππ* conical intersection. Once the ππ* state is formed, a nearly barrierless relaxation path drives the ππ* system to hop to the S state via the ππ*/S crossing point. Our current work not only rationalizes recent experimental observations but also enriches our photophysical knowledge of UV filters.