A theoretical study on the excited-state intramolecular proton transfer mechanism of 4-dimethylaminoflavonol chemosensor

In this work, density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods are used to explore the excited-state intramolecular proton transfer (ESIPT) mechanism of a novel system 4-dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S-0 and S-1 states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen-bond strengthening in the S-1 state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen-bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S-1 state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited-state behaviors of the DAF system but also successfully explains its spectral characteristics.