Solvent and pH effects on the electronic spectra of some azo benzimidazoles

The spectral behaviour of some new azobenzimidazoles in pure and mixed organic solvents and buffer solutions of varying pH have been studied. The observed bands are assigned to the possible electronic transition. The shoulder appearing in the visible region in the spectra of P-NO2 derivatives is ascribed to the existence of these compounds in azo-hydrazone tautomeric equilibrium. The possibility of the formation of an intermolecular H-bonded solvated complex between the molecules of p-NO2 derivatives and the proton acceptor solvents DMSO and DMF were studied. The pK values of these compounds were determined and explained on the basis of the relative contribution of acidic and basic character of respective species.