TDDFT study on intermolecular hydrogen bond of photoexcited hydroxy coumarin/methyl imidazole

• Intermolecular hydrogen bond is strengthened upon ICT photoexcitation.
• The experimental UV absorption peak at 346 nm is the S1 with ππ* electron transfer.
• The internal conversion rate of HCMI can be tuned by the hydrogen bonding.
• Intramolecular electron transfer occurs from OH to C8CF3 upon ICT excitation.

The excited hydrogen bonding between hydroxy coumarin (HC) and hydroxy coumarin/methyl imidazole (HCMI) has been studied using TDDFT method. The intermolecular hydrogen bond OH⋯NH is strengthened upon intramolecular electron transfer (ICT) excitation. The calculated ultraviolet absorption spectrum is in good agreement with the experimental data, and the experimental peak at 310 nm is predicted to be the excited S1 state. From the frontier molecular orbitals of HCMI, the HOMO and the LUMO are of π and π* character and the transition from the ground state to the S1 state is a local exciting process. The intramolecular electron transfer of π electron from the OH group to an π* orbital of C8CF3 group not to CO group is found upon ICT excitation. The calculated gap between HOMO and LUMO shows that HCMI can be more easily excited than free HC. The hydrogen bonding can modulate the internal conversion rate of HCMI.

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