Prototropic reactions of 2(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine in aqueous and organic solvents

Abstract

The effect of acid–base concentration on the spectral characteristics of 2-(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (2-HPIP) has been studied in both organic and aqueous solvents. Due to the poor fluorescent quantum yield of 2-HPIP in pure water, prototropic reactions were carried out in a 25% ethanol–water mixture. One kind of dication, three kinds of monocations, one kind of monoanion and one dianion are observed. In pure water, p K a values for the dication–monocation, the monocation–neutral, the neutral–monoanion and the monoanion–dianion equilibria have been determined. In protic solvents, one of the cation dissociates to yield the phototautomer on excitation. Semi-empirical quantum mechanical calculations have been carried out to assign the formation of species and site of protonation.