A dye has three parts in its structure – chromophore, chromogen and auxochrome.
Dye = Chromophore + Chromogen + Auxochrome
Chromophore
Chromophore is an unsaturated group that absorbs light and reflects it at specific angle to give the hue, e.g., azo, keto, nitro, nitroso, thio, ethylene etc. So, the color bearing unsaturated group of a dye molecule is called chromophore group.
The name is derived from greek word “chroma” meaning “color‟ & “phore” from protein meaning “to bear”
Their specific state of unsaturation enables them to absorb & reflect incident electromagnetic radiation within the very narrow band of visible light. It gives dye molecule its particular color. A molecule without any chromophore would be colorless.
For example,
Chromogen
Chromogen retains chromophore and plays a crucial role to determine the final hue and its affinity for fibre, fastness, stability etc.
So, a compound containing chromophore is called chromogen. Or, The dye structure which contains both chromophore & auxochrome is called chromogen.
Auxochromes
Auxochromes are substituted acidic or basic groups in dye structure to intensify depth of shade, e.g. –OH, –COOH, SO3H, –NH2, –NH(CH3) etc. The auxochrome (color helping group) by itself does not produce any color but deepens the color of a chromogen.
The name is derived from greek word “auxein” meaning “to increase” & “chroma” meaning “color”.
They intensify & deepen the hue of the dye molecules color. They also make the dye more soluble in water & also improve the color fastness properties.
For example: –OH, -NH2, -COOH, -SO3, NHR etc.
Further addition of substituents to dye structure deepens the shade and the extent of deepening varies with increase in molecular weight of dye.
Following are the examples of various parts of a dye.
Chrysophenine G, C I Direct Yellow 12, C I 24895, is a direct dye, in which chromophores are –N=N– and –CH=CH–, auxochrome –OC2H5 on both sides and chromogen is the part containing three chromophores but excluding the auxochromes as depicted in Figure.
