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

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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 C[double bond, length as m-dash]C is twisted by 90°, suggesting the transcis 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 C[double bond, length as m-dash]O 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.

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

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Publication details

The article was received on 23 May 2019, accepted on 19 Jun 2019 and first published on 19 Jun 2019


Article type: Paper
DOI: 10.1039/C9CP02914A
Phys. Chem. Chem. Phys., 2019, Advance Article

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

    S. Kinoshita, Y. Inokuchi, Y. Onitsuka, H. Kohguchi, N. Akai, T. Shiraogawa, M. Ehara, K. Yamazaki, Y. Harabuchi, S. Maeda and T. Ebata, Phys. Chem. Chem. Phys., 2019, Advance Article , DOI: 10.1039/C9CP02914A

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