Solvent-polarity-dependent excited-state behaviors for 2-(2-hydroxyphenyl) benzothiazole-5-(9H-carbazol-9-yl)phenol fluorophore: A theoretical study

In this work, we mainly focus on elaborating the effects of solvent polarity on excited state interactions and intramolecular proton transfer (ESIPT) behavior for the novel dye 2-(2-hydroxyphenyl) benzothiazole-5-(9H-carbazol-9-yl)phenol (HBT-Cz). Six aprotic solvents with different polarities are considered. Via comparing geometrical parameters, infrared (IR) vibrational spectra, and atomic charge distributions, we confirm that the hydrogen bond of HBT-Cz is enhanced in S-1 state. The enhancement of hydrogen bonding with low polarity is particularly important. Via exploring molecular orbitals, we find that intramolecular charge transfer happens and charge redistribution facilitates ESIPT reaction. Results show that low solvent polarity is more conducive to ESIPT process for HBT-Cz. In order to clarify detailed reaction mechanism, we construct potential energy curves and determine that the ESIPT process of HBT-Cz fluorophores could be controlled by solvent polarity. We not only elucidate the excited state behavior of HBT-Cz systems but also present the polarity-dependent ESIPT mechanism for HBT-Cz fluorophore.