The direct observation of the doorway 1nπ* state of methylcinnamate and hydrogen-bonding effects on the photochemistry of cinnamate-based sunscreens

Abstract

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 low-temperature 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.