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, oil 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. (C) 2009 Elsevier B.V. All rights reserved.