Azo or Hydrazone Structure in Some Hydrogen-Bonded Azo Pigments

Azo dyes and pigments are typical of the classical colorants as characterized by the azo group. However, there is still a controversial issue on the azo (-N=N-) or hydrazone (=N-NH-) form in some hydrogen-bonded azo pigments: beta-naphthol type [for example, pigment red 3 (PR3)], acetoacetanilide type [for example, pigment yellow 3 (PY3)], and pyrazolone type [for example, pigment yellow 60 (PY60)]. This is essentially a problem which resonance form should be designated as the chemical scheme. The authors have tackled this problem from the standpoint of the x-ray structure analysis and the chromophoric theory together with semiempirical molecular orbital calculations using methyl orange derivative (MOH) as the model substance. Our close examination of the MOH structure confirmed that the protonated-azo structure (-N=N(+)H-) is still the case rather than the hydrazone form. This motivated us to apply this form to PR3, PY3, and PY60. The result suggested that the protonated-azo structure could be more reasonable rather than the hydrazone one as far as the azo colorants are concerned. In other words, the more azolike structure is the case rather than the hydrozonelike form. (C) 2011 Society for Imaging Science and Technology. [DOI: 10.2352/J.ImagingSci.Technol.2011.55.3.030508]