Regulation of ESIPT process in (E)-N′-(5-bromo-2-hydroxybenzylidene)-4-hydroxybenzohydrazide molecules by different solvent polarity environments

Recently, a novel probe was synthesized experimentally and found to exhibit different photophysical behaviors in various solvents. Our work focus on elucidating the mechanism underlying this phenomena by density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods. The strength of hydrogen bonds increased in all four solvents after photoexcitation, with a gradual decrease from n-Heptane, Trichloromethane (CHCl₃), Methanol (MeOH) to water. This trend aligns with the energy barrier changes obtained by scanning the potential energy surface(PES), which indicates that the polarity of the solvent can affect the strength of hydrogen bonds, thereby further influencing the ease of proton transfer. Additionally, the simulation of electron spectra validates the reliability of the theoretical method. The electron-hole analysis confirms that higher solvent polarity negatively affects the ESIPT process at the microscopic level. Our work provides some ideas for designing and synthesizing fluorescent probes through excited state intramolecular proton transfer (ESIPT) operation.