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Different Photoisomerization Routes Found in the Structural Isomers of Hydroxy Methylcinnamate


An experimental and theoretical study has been carried out to elucidate the nonradiative decay (NRD) and trans (E) → cis (Z) isomerization from the S1 (1ππ*) 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. Variety of laser spectroscopic methods have been utilized for jet-cooled gas phase molecules to investigate the vibronic structure and lifetimes of the S1 state, and to detect the transient state appearing in the NRD process. In p-HMC, the S1 state at v=0 decays as short 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 T1 state. For m- and o-HMC, the S1 lifetime at v=0 is 10 ns and 6 ns, respectively, but 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 (T1) 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-S1 → 1nπ* → T1 → cis-S0] scheme. On the other hand, in o- and m-HMC, the isomerization proceeds via a [trans-S1 → twisting along the C=C double bond to 90° on S1 → cis-S0] 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.

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Publication details

The article was received on 19 Jan 2018, accepted on 06 Apr 2018 and first published on 06 Apr 2018

Article type: Paper
DOI: 10.1039/C8CP00414E
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Different Photoisomerization Routes Found in the Structural Isomers of Hydroxy Methylcinnamate

    S. Kinoshita, Y. Miyazaki, M. Sumida, Y. Onitsuka, H. Kohguchi, Y. Inokuchi, N. Akai, T. Shiraogawa, M. Ehara, K. Yamazaki, Y. Harabuchi, S. Maeda, T. Taketsugu and T. Ebata, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP00414E

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