Zhao Xi, Luo Jian, Liu Yan, Pandey Pramod, Yang Songqiu, Wei Donghui, Han Keli
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, China.
University of Chinese Academy of Sciences, Beijing 10049, China.
J Phys Chem Lett. 2019 Sep 5;10(17):5244-5249. doi: 10.1021/acs.jpclett.9b02175. Epub 2019 Aug 26.
An ultraviolet energy dissipation mechanism plays a critical role in the photoprotection effect 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 -OMeMS relax to the ground state nonradiatively along an ultrafast trans-cis photoisomerization in tens of picoseconds. However, for -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 -OMeMS. In contrast, the V'(ππ*) state decays to the ground state mainly by emitting photons, exhibiting a lifetime as long as 5 ns. It is the first time to definitely distinguish the dynamics between V(ππ*) and V'(ππ*) states in the study of sinapates and cinnamates. These results indicate the anticipation of the V'(ππ*) state should be avoided when designing sunscreens.
紫外线能量耗散机制在防晒霜的光保护作用中起着关键作用。在这项工作中，我们发现了模型植物防晒霜芥子酸甲酯（MS）的取代基依赖性紫外线能量耗散机制。我们发现，MS和-OMeMS最初占据的V(ππ*)态在几十皮秒内沿着超快的反式-顺式光异构化以非辐射方式弛豫到基态。然而，对于-HMS，V(ππ*)到相对暗的V'(ππ*)态的内转换在不到1皮秒内发生，导致激发态弛豫出现分支。V(ππ*)态仍如MS和-OMeMS那样以非辐射方式弛豫。相比之下，V'(ππ*)态主要通过发射光子衰减到基态，其寿命长达5纳秒。这是首次在芥子酸盐和肉桂酸盐的研究中明确区分V(ππ*)态和V'(ππ*)态之间的动力学。这些结果表明，在设计防晒霜时应避免对V'(ππ*)态的预期。
