Different photoisomerization routes found in the structural isomers of hydroxy methylcinnamate

作者：Kinoshita Shin nosuke; Miyazaki Yasunori; Sumida Masataka; Onitsuka Yuuki; Kohguchi Hiroshi; Inokuchi Yoshiya; Akai Nobuyuki; Shiraogawa Takafumi; Ehara Masahiro^*; Yamazaki Kaoru^*; Harabuchi Yu; Maeda Satoshi; Taketsugu Tetsuya; Ebata Takayuki^*

来源：Physical Chemistry Chemical Physics, 2018, 20(26): 17583-17598.

DOI：10.1039/c8cp00414e

An experimental and theoretical study has been carried out to elucidate the nonradiative decay (NRD) and trans(E) -> cis(Z) isomerization from the S-1 ((1)pi pi*) state of structural isomers of hydroxy methylcinnamate (HMC); ortho-, meta- and para-HMC (o-, m- and p-HMC). A low temperature matrix-isolation Fourier Transform Infrared (FTIR) spectroscopic study revealed that all the HMCs are cis-isomerized upon UV irradiation. A variety of laser spectroscopic methods have been utilized for jet-cooled gas phase molecules to investigate the vibronic structure and lifetimes of the S-1 state, and to detect the transient state appearing in the NRD process. In p-HMC, the zero-point level of the S-1 state decays as quickly as 9 ps. A transient electronic state reported by Tan et al. (Faraday Discuss. 2013, 163, 321-340) was reinvestigated by nanosecond UV-tunable deep UV pump-probe spectroscopy and was assigned to the T-1 state. For m- and o-HMC, the lifetime at the zero-point energy level of S-1 is 10 ns and 6 ns, respectively, but it becomes substantially shorter at an excess energy higher than 1000 cm(-1) and 600 cm(-1), respectively, indicating the onset of NRD. Different from p-HMC, no transient state (T-1) was observed in m- nor o-HMC. These experimental results are interpreted with the aid of TDDFT calculations by considering the excited-state reaction pathways and the radiative/nonradiative rate constants. It is concluded that in p-HMC, the trans -> cis isomerization proceeds via a [trans-S-1 -> (1)n pi*-> T-1 -> cis-S-0] scheme. On the other hand, in o- and m-HMC, the isomerization proceeds via a [trans-S-1 -> twisting along the C=C double bond by 90 degrees on S-1 -> cis-S-0] scheme. The calculated barrier height along the twisting coordinate agrees well with the observed onset of the NRD channel for both o- and m-HMC.

• 出版日期2018-7-14