Azo dyes contain the azo chromophore:
They have the general formula:
Azo dyes are subgrouped according to the number of azo chromophores in the molecule.
Trisazo dyes contain three azo chromophores. These are not common in histotechnology.
Tetrakisazo dyes have four azo chromophores. An example is Sirius red F3B
Lysochromes, or fat dyes, are usually disazo dyes and frequently have a single hydroxyl group ortho to the azo nitrogen. Lillie, in Conn's Biological Stains, draws attention to the work of Michaelis in the early part of the 20th century saying that when this is so intramolecular reconfiguration occurs which results in the loss of hydrogen from the hydroxyl, gaining of a hydrogen by one of the azo nitrogens, and formation of an ortho quinoid ring. While still intensely coloured, the resulting compound is insoluble in water, but is soluble in non-polar solvents, including triglycerides.
R. D. Lillie.
Conn's Biological Stains
Williams & Wilkins, Baltimore, MD., U.S.A.
Virchow. Arch. Path. Anat., v.164, 263-270 (1901)