The direct observation of the doorway 1nπ* state of methylcinnamate and hydrogen-bonding effects on the photochemistry of cinnamate-based sunscreens
The direct observation of the doorway 1nπ* state of methylcinnamate and hydrogen-bonding effects on the photochemistry of cinnamate-based sunscreens
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DOI：
10.1039/c9cp02914a
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发表时间：
2019-09-28
影响因子：
3.3
通讯作者：
Ebata, Takayuki
中科院分区：
文献类型：
--
作者：
Kinoshita, Shin-nosuke;Inokuchi, Yoshiya;Ebata, Takayuki
在喷射冷却条件下，研究了肉桂酸甲酯（MC）及其与甲醇的氢键复合物的电子态和光化学，包括非辐射衰变（NRD）和光化学.在MC中直接观察到S-1（（1）n pi*）和S-2（（1）pi pi*）。这是在肉桂酸酯衍生物中首次直接观察到S-1（1 n pi*）。令人惊讶的是，n π * 和π π * 状态之间的能量顺序与取代的肉桂酸酯相反。TD-DFT和SAC-CI计算支持了观察到的结果，并表明苯环上的取代大大降低了（1）π π * 能量，而对（1）n π * 的影响相当小。测定了MC的S-2（pi pi*）态寿命等于或小于10 ps，并观察到瞬态T-1态的产生。T-1（pi pi*）态被计算为具有丙烯基C=C扭曲90度的结构，表明反式->顺式异构化通过T-1进行。顺式异构体的产生通过低温基质隔离FTIR光谱法确认。氢键的影响进行检查的MC-甲醇复合物。MC-甲醇的S-2寿命被确定为180 ps，表明与C=O基团的H-键合在很大程度上阻止了（1）pi pi* ->（1）n pi* 内部转化。在甲醇络合物中的这种寿命延长也很好地描述了MC在甲醇溶液中比在环己烷中更高的荧光量子产率，而在对位取代的MC中没有观察到这种溶剂依赖性。确定MC和MC-甲醇的光化学反应途径有助于我们设计光功能肉桂酸酯衍生物。
The electronic states and photochemistry including nonradiative decay (NRD) and trans(E) -> cis(Z) isomerization of methylcinnamate (MC) and its hydrogen-bonded complex with methanol have been investigated under jet-cooled conditions. S-1((1)n pi*) and S-2((1)pi pi*) are directly observed in MC. This is the first direct observation of S-1(1n pi*) in cinnamate derivatives. Surprisingly, the order of the energies between the n pi* and pi pi* states is opposite to substituted cinnamates. TD-DFT and SAC-CI calculations support the observed result and show that the substitution to the benzene ring largely lowers the (1)pi pi* energy while the effect on (1)n pi* is rather small. The S-2(pi pi*) state lifetime of MC is determined to be equal to or shorter than 10 ps, and the production of the transient T-1 state is observed. The T-1(pi pi*) state is calculated to have a structure in which propenyl C=C is twisted by 90 degrees, suggesting the trans -> cis isomerization proceeds via T-1. The production of the cis isomer is confirmed by low-temperature matrix-isolated FTIR spectroscopy. The effect of H-bonding is examined for the MC-methanol complex. The S-2 lifetime of MC-methanol is determined to be 180 ps, indicating that the H-bonding to the C=O group largely prohibits the (1)pi pi* -> (1)n pi* internal conversion. This lifetime elongation in the methanol complex also describes well a higher fluorescence quantum yield of MC in methanol solution than in cyclohexane, while such a solvent dependence is not observed in para-substituted MC. Determination of the photochemical reaction pathways of MC and MC-methanol will help us to design photofunctional cinnamate derivatives.