Miyazaki Yasunori, Yamamoto Kanji, Aoki Jun, Ikeda Toshiaki, Inokuchi Yoshiya, Ehara Masahiro, Ebata Takayuki
Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
Institute for Molecular Science and Research Center for Computational Science, 38 Myodaiji, Okazaki 444-8585, Japan.
J Chem Phys. 2014 Dec 28;141(24):244313. doi: 10.1063/1.4904268.
The S1 state dynamics of methoxy methylcinnamate (MMC) has been investigated under supersonic jet-cooled conditions. The vibrationally resolved S1-S0 absorption spectrum was recorded by laser induced fluorescence and mass-resolved resonant two-photon ionization spectroscopy and separated into conformers by UV-UV hole-burning (UV-UV HB) spectroscopy. The S1 lifetime measurements revealed different dynamics of para-methoxy methylcinnamate from ortho-methoxy methylcinnamate and meta-methoxy methylcinnamate (hereafter, abbreviated as p-, o-, and m-MMCs, respectively). The lifetimes of o-MMC and m-MMC are on the nanosecond time scale and exhibit little tendency of excess energy dependence. On the other hand, p-MMC decays much faster and its lifetime is conformer and excess energy dependent. In addition, the p-MMC-H2O complex was studied to explore the effect of hydration on the S1 state dynamics of p-MMC, and it was found that the hydration significantly accelerates the nonradiative decay. Quantum chemical calculation was employed to search the major decay route from S1(ππ(∗)) for three MMCs and p-MMC-H2O in terms of (i) trans → cis isomerization and (ii) internal conversion to the (1)nπ(∗) state. In o-MMC and m-MMC, the large energy barrier is created for the nonradiative decay along (i) the double-bond twisting coordinate (∼1000 cm(-1)) in S1 as well as (ii) the linear interpolating internal coordinate (∼1000 cm(-1)) from S1 to (1)nπ(∗) states. The calculation on p-MMC decay dynamics suggests that both (i) and (ii) are available due to small energy barrier, i.e., 160 cm(-1) by the double-bond twisting and 390 cm(-1) by the potential energy crossing. The hydration of p-MMC raises the energy barrier of the IC route to the S1/(1)nπ(∗) conical intersection, convincing that the direct isomerization is more likely to occur.
在超声速喷射冷却条件下研究了甲氧基肉桂酸甲酯（MMC）的S1态动力学。通过激光诱导荧光和质量分辨共振双光子电离光谱记录了振动分辨的S1 - S0吸收光谱，并通过紫外 - 紫外空穴烧蚀（UV - UV HB）光谱将其分离为构象异构体。S1寿命测量结果表明，对甲氧基肉桂酸甲酯与邻甲氧基肉桂酸甲酯和间甲氧基肉桂酸甲酯（以下分别简称为p -、o - 和m - MMCs）的动力学不同。o - MMC和m - MMC的寿命在纳秒时间尺度上，并且几乎没有过剩能量依赖性的趋势。另一方面，p - MMC衰变快得多，其寿命取决于构象异构体和过剩能量。此外，研究了p - MMC - H2O复合物以探索水合作用对p - MMC的S1态动力学的影响，发现水合作用显著加速了非辐射衰变。采用量子化学计算从S1(ππ(∗))出发，针对三种MMC和p - MMC - H2O，依据（i）反式→顺式异构化和（ii）内转换至(1)nπ(∗)态来寻找主要衰变途径。在o - MMC和m - MMC中，对于沿（i）S1中双键扭曲坐标（约1000 cm(-1)）以及（ii）从S1到(1)nπ(∗)态的线性内插内坐标（约1000 cm(-1)）的非辐射衰变产生了较大的能垒。对p - MMC衰变动力学的计算表明，由于能垒较小，即通过双键扭曲为160 cm(-1)，通过势能交叉为390 cm(-1)，（i）和（ii）两种途径都是可行的。p - MMC的水合作用提高了IC途径至S1/(1)nπ(∗)锥形交叉点的能垒，这表明直接异构化更有可能发生。
