Crystal Structure Analysis of Azo Pigments Involving Beta-Naphthol: A Review

The crystal structures of six azo pigments involving beta-naphthol have been determined by single crystal X-ray diffraction techniques. A review of these crystal structures indicates the existence of certain common features. It is found that the molecules exist as keto-hydrazone tautomers with intramolecular hydrogen bonding which holds the molecule in the trans-form. If the phenyl group is ortho substituted, this hydrogen bond is shared so as to hold the two ring systems, and hence the molecule, in the same plane. In the case of pigments where there is an anilide side group on the naphthol, the amido group is hydrogen bonded to the naphthol, but this bond may also be shared. The molecules are linked by van der Waals forces. A relationship between molecular stacking and the crystal morphology is pointed out