Intra- and intermolecular proton transfer reactions in 2-phenyl substituted benzazoles

Abstract

The present review describes the salient features of inter- and intramolecular proton transfer reactions of 2-(2′-aminophenyl)-, 2-(3′-aminophenyl)-, 2-(4′-aminophenyl)-, 2-(2′-hydroxyphenyl)-, 2-(3′-hydroxyphenyl)- and 2-(4′-hydroxyphenyl)-benzimidazoles, benzoxazoles and benzothiazoles. Fluorescence quantum yield of the phototautomer produced by the intramolecular hydrogen bonding decreases on going from benzimidazole to benzoxazole to benzothiazole. This indicates that the rate of internal conversion increases in the order of compounds as mentioned above. The biprotonic phototautomerism or the presence of intermolecular proton transfer has led to the formation of (i) nonfluorescent zwitterions in case of hydroxyphenyl derivatives and the ground state precursor of this species in neutral molecules, (ii) nonfluorescent monoanions from fluorescent monoanions and (iii) nonfluorescent monocations from monocations in case of aminophenyl derivatives. In the case of 2-(4′-aminophenyl)-substituted compounds, the first protonation has always led to the formation of two types of monocations; one by protonating the amino group and the other by protonating the tertiary nitrogen atom. The former is more stable in aqueous media and the latter in non-polar media.