Substituent and solvent effects on excited state intramolecular proton transfer in novel 2-(2′-hydroxyphenyl)benzothiazole derivatives

A group of novel 2-(2′-hydroxyphenyl)benzothiazole derivatives 1–5 were synthesized with electron-donating or -withdrawing substituent introduced in para position of N atom in benzothiazolyl ring. The excited state intramolecular proton transfer (ESIPT) in 1–5 along with non-substituted 2-(2′-hydroxyphenyl)benzothiazole 6 was studied by means of UV–vis absorption and steady-state fluorescence in solutions. Compounds 1–6 exhibit dual fluorescences including purple normal emission and green tautomer emission. Systematical comparison of the fluorescence of any analogue in a series of solvents ranging from protic ethanol to non-polar hexane demonstrated that polar solvents favor the normal emission while non-polar solvents facilitate ESIPT process and tautomer formation and emission. In either protic or non-polar solvent the tautomer emission intensity of 1–6 decreases consecutively in the order of decreasing electron-donating ability or increasing electron-withdrawing ability of the substituents, on the premise of identical normal emission intensity. This indicates that electron-donating substituents in these derivatives favor ESIPT process and tautomer emission. Competition of intra- and intermolecular hydrogen bonding was studied in dioxane-water binary solvent. It is demonstrated that intermolecular hydrogen bonding with protic solvent impedes ESIPT and tautomer emission. The fluorescent behaviors of 1–6 were interpreted in terms of the population of ground-state rotamers responsible for normal and tautomer emission respectively.