Regulating ESIPT Behavior and Fluorescent Properties by Introducing -CN Group at Different Positions of BACN: A TD-DFT Study.

A new fluorophore BACN, which is composed of 2-(2'-hydroxyphenyl)benzothiazole and α-cyanostilbene unit was obtained experimentally (Spectrochim. Acta A 281 (2022) 121601), but the direct information of excited state intramolecular proton transfer (ESIPT) process in it cannot be provided in the experiment. In this work, the ESIPT behaviors and photophysical features as well as the influence of substitution position of -CN group have been explored in detail via density functional theory (DFT) and time-dependent DFT. The experimental absorption and emission wavelengths of BACN are reproduced at the TD-PBE38/6-311 + G(d, p) level. The theoretical results of structural parameters, infrared frequencies, topological parameters and reduced density gradient (RDG) analysis showed that the intramolecular hydrogen bonds (IHBs) are enhanced in the excited state (S₁). The proton transfer is more likely to occur in S₁ state. The introduction of -CN group at different positions weakens the IHB strength, increases the ESIPT barrier, red-shifts the fluorescence wavelength of the tautomer and enlarge the Stokes shifts of the tautomer.