Abstract
Azo dyes and pigments are typical of the classical pigments as characterized by the azo group (−N=N−). However, there are still controversial discussions on the azo form whether it exists as the azo form in the solid state or in the form of hydrazone structure (=N−NH−). We have tackled this problem from the aspects of X-ray structure analysis and the chromophoric theory in an attempt to construct a model that satisfies these requirements, using methyl orange derivative (MOH), Pigment Red 3 (PR3), and Pigment Yellow 3 (PY3). Then, we found a protonated azo structure (−N=N+H−) in these compounds as caused by NH…O “inter”- molecular hydrogen bonds in MOH, as well as by “intra”-molecular hydrogen bonds in PR3 and PY3. The present protonated azo model is borne out by the crystallographic azo bond-length as well as the chromophoric theory. These results lead us to conclude that the protonated azo structure is the right form in the solid state of hydrogen-bonded azo pigments as described above.
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