Comprehensive Studies on Excited-State Proton Transfer of a Series of 2-(2′-Hydroxyphenyl)benzothiazole Derivatives: Synthesis, Optical Properties, and Theoretical Calculations

A group of novel 2-(2'-hydroxyphenyl)benzothiazole derivatives (1-4) with excited-state intramolecular proton transfer (ESIPT) character were synthesized. Their photophysical properties were studied by means of steady-state absorption and fluorescence spectra and time-resolved emission method as well as theoretical calculation in a variety of solvents. All of these compounds can yield single fluorescence at the green region in nonpolar solvents such as n-hexane, while dual fluorescence consisting of the blue and green bands was captured in strong polar solvents like acetonitrile. In addition, a third emission band between the former two bands was detected for these molecules simultaneously with the blue and green ones generating the well-structured triple fluorescence in protic solvent like ethanol. Systematical comparison of the fluorescence of these compounds in a series of solvents demonstrated that nonpolar solvents would facilitate ESIPT process and the green emission from the keto format, while the strong polar solvents impede the ESIPT process and favor the blue normal emission from enol. Protic solvents facilitate deprotonation and make the phenolic anion coexist with keto and enol and consequently lead to triple fluorescence. On the premise of identical keto emission intensity, the normal emission intensity of these compound increases consecutively in the order of increasing electron-withdrawing ability of the substituents regardless of solvents. The results of quantum-chemical calculations are well in line with the experimental spectra.