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

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, pKa 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.