Chromophore:
The term chromophore is applied to the structural grouping which imports the color of the dye molecule. Usually more than one chromophoric group is needed to give rise to color in a substance and rather large numbers may be present in ordinary dye molecules .
Basic chromophore . the basic chromophore include
- azo group
- Azine group
- The indamines group
- Azo group :
It is found in all azo dyes. In this compounds a benzene ring is attached to each nitrogen atom. The dyes in this group may be considered as derivatives of azobenzene. Examples of biological stains containing this chromophore are bismarck brown, methyl red and methyl orange
- Azine group:
It is found in the phenazines. Neutral red and the saffranines are examples of azines stains.
- The indamines group:
It is found in the indamines, the thiazines, and other dyes. Many of the dyes have two benzene rings attached to nitrogen atoms. One of the rings shows the quinonoid structure. The thiazines have the two benzene rings further joined together by an atom of sulfur. The simplest thiazine nucleus has the following structure. The e best example of thiazine biological stains is methylene blue
Acid chromophore:
The acid chromophore includes following major groups of dyes:
- The nitro group is found in many compounds , an example of it is picric acid.
- The quinonoid ring:
It occurs in many dyes such as the indamines, the xanthenes, and the diphenyl methane and triphenylmethane. Some of the famous examples of stains in this group are rosolic acid, fuchsin, the methyl violets, methyl green , crystal violet and pararosaniline.
Leuco compounds:
The chromophores all have unsatisfied affinities and are easily reduced by combining with hydrogen at the double bonds
The nitro group may be reduces to amino radical; the bond between the nitrogen atoms of the azine group may break and be replaced by two atoms of hydrogen,the double bonds of quinonoid ring may break and one atom of hydrogen be taken up by each valence set free. A reduction of the chromophore group results in a loss of color. These decolorized stains are known as leuco compounds. Stains often are used as indicators of oxidation and reduction . the decolorization of pararosaniline and methylene blue can be represented by the following equation.