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(¹ 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 ³ ππ* state. In the first one, the V(¹ ππ*) state decays to the V'(¹ ππ*) state. The resultant V'(¹ ππ*) state further jumps to the ¹ nπ* state by internal conversion at the ¹ ππ*/¹ nπ* conical intersection. Then, the ¹ nπ* state hops to the ³ ππ* state through an efficient ¹ nπ*→³ ππ* intersystem crossing process. In the second one, the V(¹ ππ*) state can diabatically relax along the photoisomerization reaction coordinate. In this process, a ¹ ππ*/³ nπ* crossing point helps the ¹ ππ* system decay to the ³ nπ* state, which further decays to the ³ ππ* state through internal conversion at the ³ nπ*/³ ππ* conical intersection. Once the ³ ππ* state is formed, a nearly barrierless relaxation path drives the ³ ππ* system to hop to the S₀ state via the ³ ππ*/S₀ crossing point. Our current work not only rationalizes recent experimental observations but also enriches our photophysical knowledge of UV filters.