Azo-hydrazone tautomerism of an o,o'-dihydroxy azo dye-a spectroscopic study

Abstract The aggregation behavior and tautomerism of an o,o'-dihydroxy azo dye, namely 4-(3',5'-dinitro-2'-hydroxyphenylazo)-3-hydroxy-2-naphthanilide ( A ) and its ionized counterpart B were studied by UV-VIS spectroscopy. The dyes showed similar absorption behaviors in all the solvents investigated and in the solid state. Concentration dependent spectroscopic changes with the formation of isobestic points were observed in DMF and DMSO, indicating dimer-monomer equilibria. Deconvolution of representative absorption profiles of A and B gave four Gaussian peaks at about 450, 550, 580 and 620 nm. Whilst these peak positions remained relatively constant, the peak areas varied with respect to the change of the dye concentration. By comparing the deconvoluted absorption maxima to those estimated by PPP-CI calculations, the conclusion was reached that the spectroscopic changes were due to a shift of the hydrazone-azo equilibria caused by the shifting of the dimer-monomer equilibrium, i.e., the aggregated dye molecules exist mainly in hydrazoneform while the monomeric dye molecules prefer the azo form. A theory is presented to explain the observation that the tautomeric mixtures of A and B were closely related, that is, they had similar UV-VIS behavior and they showed almost identical IR, Roman and NMR spectra. An intermediate which involves a completely delocalized structure shown above is proposed. This structure may be regarded as the transitional species that bridges the three possible tautomers, namely azo, naphthoquinone-hydrazone and benzoquinone-hydrazone of A or B . The X (X= H or Na) may undergo rapid intramolecular transfer between the azo nitrogens and the phenoxy oxygens and the tautomers of A may be as ionic as the tautomers of B .