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Substitution Dependent Ultrafast Ultraviolet Energy Dissipation Mechanisms of Plant Sunscreens.

Ultravioletenergy dissipation mechanism plays a critical role in the photoprotectioneffect of sunscreens. In this work, we discovered substitution dependent UV energy dissipation mechanisms of model plant sunscreen methyl sinapate(MS). We found that the initially populated V(ππ*) states of MS and p-OMeMS relax to the ground state nonradiatively along an ultrafast trans-cis photoisomerization in tens of picoseconds. However, for p-HMS, an internal conversion from V(ππ*) to a relative dark V'(ππ*) state occurs in less than 1 ps, leading to a branching of the excited-state relaxations. The V (ππ*) state still relaxes nonradiatively as in the case of MS and p-OMeMS. In the contrary, the V'(ππ*) state decays to the ground state mainly by emitting a photon, exhibiting a lifetime as long as 5 nanoseconds. It is the first time to definitely distinguish the dynamics between V(ππ*) and V'(ππ*) statesin the study of sinapates and cinnamates. These results indicate the anticipation of the V'(ππ*) state should be avoided when designing sunscreens.