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. S1(1nπ∗) and S2(1ππ∗) are directly observed in MC. This is the first direct observation of S1(1nπ∗) in cinnamate derivatives. Surprisingly, the order of the energies between the nπ∗ and ππ∗ 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 1ππ∗ energy while the effect on 1nπ∗ is rather small. The S2(ππ∗) state lifetime of MC is determined to be equal to or shorter than 10 ps, and the production of the transient T1 state is observed. The T1(ππ∗) state is calculated to have a structure in which propenyl CC is twisted by 90°, suggesting the trans → cis isomerization proceeds via T1. The production of the cis isomer is confirmed by lowerature matrix-isolated FTIR spectroscopy. The effect of H-bonding is examined for the MC-methanol complex. The S2 lifetime of MC-methanol is determined to be 180 ps, indicating that the H-bonding to the CO group largely prohibits the 1ππ∗ → 1nπ∗ 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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry