Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan and Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Sapporo 001-0021, Japan.
Phys Chem Chem Phys. 2021 Jan 21;23(2):834-845. doi: 10.1039/d0cp04402d.
Cinnamate derivatives are very useful as UV protectors in nature and as sunscreen reagents in daily life. They convert harmful UV energy to thermal energy through effective nonradiative decay (NRD) including trans → cis photoisomerization. However, the mechanism is not simple because different photoisomeirzation routes have been observed for different substituted cinnamates. Here, we theoretically examined the substitution effects at the phenyl ring of methylcinnamate (MC), a non-substituted cinnamate, on the electronic structure and the NRD route involving trans → cis isomerization based on time-dependent density functional theory. A systematic reaction pathway search using the single-component artificial force-induced reaction method shows that the very efficient photoisomerization route of MC can be essentially described as "1ππ* (trans) → 1nπ* → T1 (3ππ*) → S0 (trans or cis)". We found that for efficient 1ππ* (trans) → 1nπ* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied π orbital. Substitution at the para position of MC slightly lowers the 1ππ* state energy and photoisomerization occurs via a slightly less efficient "1ππ* (trans) → 3nπ* → T1 (3ππ*) → S0 (trans or cis)" pathway. Substitution at the meta or ortho positions of MC significantly lowers the 1ππ* state energy so that the energy barrier of IC (1ππ* → 1nπ*) becomes very high. This substitution leads to a much longer 1ππ* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to "1ππ* (trans) → C[double bond, length as m-dash]C bond twisting on 1ππ* → S0 (trans or cis)". As a whole, the "1ππ* → 1nπ*" IC observed in MC is the most important initial step for the rapid change of UV energy to thermal energy. We also found that the stabilization of the π orbital (i) minimizes the energy gap between 1ππ* and 1nπ* at the 1ππ* minimum and (ii) makes the 0-0 level of 1ππ* higher than 1nπ* as observed in MC. These MC-like relationships between the 1ππ* and 1nπ* energies should be ideal to maximize the "1ππ* → 1nπ*" IC rate constant according to Marcus theory.
肉桂酸衍生物在自然界中作为紫外线吸收剂非常有用，在日常生活中作为防晒霜试剂。它们通过有效的非辐射衰减（NRD）将有害的紫外线能量转化为热能，包括顺式-反式光异构化。然而，由于不同取代肉桂酸的光异构化途径不同，其机制并不简单。在这里，我们基于含时密度泛函理论，从理论上研究了取代肉桂酸（MC）中苯环上的取代基对电子结构和涉及顺式-反式异构化的 NRD 途径的影响。使用单组分人工力诱导反应方法进行系统的反应途径搜索表明，MC 非常有效的光异构化途径可以基本描述为“1ππ*（顺式）→1nπ*→T1（3ππ*）→S0（顺式或反式）”。我们发现，对于有效的 1ππ*（顺式）→1nπ内转换（IC），MC 应该在苯环的适当位置具有取代基，以稳定最高占据π轨道。MC 的对位取代略微降低了 1ππ态的能量，使得光异构化通过稍微不太有效的“1ππ*（顺式）→3nπ*→T1（3ππ*）→S0（顺式或反式）”途径发生。MC 的间位或邻位取代会显著降低 1ππ态的能量，使得 IC（1ππ→1nπ*）的能垒非常高。这一取代导致 IC （1ππ*→1nπ*）的半衰期比 MC 和对位取代 MC 长得多，并且主导光异构化途径发生变化，变为“1ππ*（顺式）→1ππ上的 C[双键，长度为破折号]C 键扭曲→S0（顺式或反式）”。总的来说，在 MC 中观察到的“1ππ→1nπ*”IC 是将紫外线能量快速转化为热能的最重要初始步骤。我们还发现，（i）π轨道的稳定化使 1ππ最低点的 1ππ和 1nπ之间的能隙最小，（ii）使 MC 中观察到的 1ππ的 0-0 能级高于 1nπ*。根据 Marcus 理论，这些 MC 型的 1ππ和 1nπ能量之间的关系应该是理想的，以最大化“1ππ*→1nπ*”IC 速率常数。
