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. A 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 zero-point level of the S₁ 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 T₁ state. For m- and o-HMC, the lifetime at the zero-point energy level of S₁ 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 (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 CC double bond by 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.