Theoretical study of solvent polarity on regulating the excited-state intramolecular proton transfer process of 2-(2′-hydroxyphenyl)benzimidazole derivative

In this work, the solvent effect on the excited-state intramolecular proton transfer (ESIPT) mechanism of the derivative (BTEP) based on 2-(2′-hydroxyphenyl)benzimidazole (HBI) modification in cyclohexane, dichloromethane and acetonitrile solvents has been investigated by utilizing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The detailed study of the main geometrical parameters, infrared (IR) vibration spectra, and reduced density gradient (RDG) versus Sign(λ₂)ρ(r) scatter plots related to intramolecular hydrogen bond (IHB) reveals that the IHB strength of BTEP is enhanced in the excited state comparing with that in the ground state and that the magnitude of the enhancement decreases as solvent polarity increased. In addition, analysis of the potential energy curves (PECs) at the S₀ and S₁ states revealed that the energy barrier of the ESIPT reaction increases with increasing solvent polarity. These analyses have shown that increasing solvent polarity makes the ESIPT reaction increasingly difficult, and we hope that our study can provide guidance for subsequent research into further application of BTEP to fluorescent probes.