Insight into solvent-polarity-regulated photoinduced excited state behaviors for E-HBT fluorophore: a theoretical investigation

Inspired by the remarkable photochemical and photophysical properties of novel 2-(2′-hydroxyphenyl)benzothiazole (HBT) derivatives that could be potentially applied across various disciplines, in this work, effects of solvent polarity on excited state hydrogen bond effects and excited state intramolecular proton transfer (ESIPT) reaction of 5-{2-[2-(4-amino-phenyl)-2,3-dihydro-benzofuran-6-yl]-vinyl}-2-benzothiazol-2-yl-phenol (E-HBT) are focused. By comparing the structural changes and infrared (IR) vibrational spectra of the E-HBT fluorophore in polar acetonitrile, moderate polar dichloromethane and nonpolar cyclohexane solvents, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that the hydrogen bond could be strengthened in S₁ state, which is favorable for the occurrence of ESIPT reactions. The charge recombination behavior of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimization and searching transition state (TS) form, we confirm change of surrounding solvent polarity has a regulatory effect on the ESIPT behavior for E-HBT, that is, the higher the polarity of the solvent, the more favorable it is for the ESIPT reaction.