Solvent Polarity-Regulated Excited-State Intramolecular Proton Transfer of a Novel Flavonol Derivative

This paper systematically elucidates the solvent polarity-regulated excited-state intramolecular proton transfer (ESIPT) behavior of a novel flavonol-type ESIPT fluorophore HMF. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to optimize the geometric structures of HMF in solvents with different polarities. Analysis of hydrogen bond-related structural parameters, infrared vibrational frequencies, the core-valence bifurcation (CVB), and bond critical point (BCP) parameters reveals that intramolecular hydrogen bonding is markedly strengthened in the first excited state. This enhancement shows a clear dependence on solvent polarity. Further calculations on the reaction transition state energy barrier demonstrate that reduced solvent polarity reduces the reaction energy barrier, thus facilitating the ESIPT process. Our findings provide a theoretical basis for designing new luminescent materials that respond to solvent polarity.