Excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)-oxazole and -thiazole

The azoles 2-(2'-hydroxyphenyl) oxazole (HPO) and 2-(2'-hydroxyphenyl) -4-methylthiazole (HPT) have been synthesised and studied in order to compare their photophysical characteristics. Their absorption and emission properties are reported in non-polar, alcoholic and aqueous media. Ground and excited state pK data were determined by spectroscopy and a model is proposed to explain the behaviour of HPT and HPO as a function of the pH. Excitation spectra and quantum chemical calculations suggest an equilibrium of ground state conformers. The calculations also predict a small energy barrier for rotation in the first excited singlet state for the proton transferred tautomers. The resulting twisted structure of the tautomer form possesses a biradicaloid nature, and is near-degenerate in energy with the first excited triplet state. (C) 2000 Elsevier Science S.A. All rights reserved.