Nitro dyes contain the nitro chromophore: NO2
They have the general formula:
The empirical way of designating the nitro group does not indicate that its electrons are delocalised. The bonds to the two oxygen atoms are of equal strength, and are shown as a single and a double bond only for comvenience. The electrons from both bonds are shared, and these delocalised electrons are integrated with the delocalised electrons of the aryl rings of dyes, causing absorption in the visible range. The dyes so formed are invariably acid dyes. Picric acid and martius yellow are the usual examples.
R. D. Lillie.
Conn's Biological Stains
Williams & Wilkins, Baltimore, MD., U.S.A.