Investigation of the azo-hydrazone tautomeric equilibrium in an azo dye involving the naphthalene moiety by UV–vis spectroscopy and quantum chemistry

Photophysical properties of the azo-hydrazone tautomerism of Eriochrome Blue Black B (1-(1-hydroxy-2-naphthylazo)-2-naphthol-4-sulphonic acid) in DMF, MeCN and water were investigated using UV–visible spectroscopy and quantum chemical calculations. The optimized molecular structure parameters, relative energies, mole fractions, electronic absorption spectra and HOMO–LUMO energies for possible stable tautomeric forms of EBB were theoretically calculated by using hybrid density functional theory, (B3LYP) methods with 6-31G(d) basis set level and polarizable continuum model (PCM) for solvation effect. The effects of varying pH-, dye concentration-, solvent-, temperature-, and time-dependences on the UV–vis spectra of Eriochrome Blue Black B were also investigated experimentally. The calculations showed that the dye exhibited acid–base, azo-hydrazone and aggregate equilibria in DMF solution, while the most probably preferred form in MeCN solution was azo form. Thermodynamic parameters of dimerization reaction in DMF solution proved that entropy was the driving force of this reaction.