Spectral characteristics of 2-(2′-hydroxy-3′-pyridyl)benzimidazole: effects of solvents and acid or base concentrations

Absorption, fluorescence and fluorescence excitation spectroscopy and time dependent fluorimetry have been used to study the spectral characteristics of 2-(2′-hydroxy-3′-pyridyl)benzimidazole (2-H3PyBI) in different solvents and at different acid-base concentrations. Observation of only one small Stokes shifted fluorescence band reveals the absence of excited state intramolecular proton transfer (ESIPT). 2-H3PyBI is mainly present as the keto form even in cyclohexane. First site of protonation is imidazole nitrogen atom in both ground (S0) and first excited singlet state (S1). Dication in S0 state is formed by protonating exo carbonyl group and pyridine nitrogen in S1 state. Deprotonation occurs from >N-H moiety of pyridone, followed by structural reorganization to yield enolate ion. AM1 semi-empirical quantum mechanical and density functional theory (DFT) calculations have been used to assign the structure of neutral species and the site of protonation and deprotonation of 2-H3PyBI. Dissociation constants of various prototropic equilibria were determined and discussed. Theoretical calculations have been carried out to find the cause for the absence of ESIPT.