Dye molecules are coloured because they are selectively able to absorb and reflect some of the wavelengths of the incident light.
When all the visible light rays are reflected, it appears black.
When one or more rays are reflected the viewer senses the colour produced by the specific reflected ray or combination of rays.
Organic molecules become coloured, and thus useful dye molecules, if they contain at least one of each of the radicals called chromophores and auxochromes.
In general, chromophores give the dye molecule its particular colour, while the auxochromes intensify the hue of the dye molecules colour, and make the dye molecule water soluble.
Chromophores:
The name is derived from the Greek word chrome=colour and phore from pherein = to bear.
chromophores are unsaturated organic radicals
Their specific state of unsaturation enables them to absorb and reflect incident electromagnetic radiation within the very narrow band of visible light.
Chromophores are made up of some organic molecular arrangements as the azo group, quinonoid group, nitro group, nitrose group, carboxyl group.
To produce colour visible to the human eye, the dye stuff must have at least two chromophore groups.
A molecule possessing no chromophore would be colour less.
Auxochromes:
The intensity or the brilliance of the colour depends on the presence of one or more substance called auxochromes.
The name is derived from Greek auxein= to increase, and chroma=colour.
Auxochromes are organic radicals which include carboxyl group, sulphonic group, amino group, methyl amino, and hydroxyl group.
A dye molecule without auxochromes would lack intensity of colour. This is because the incident light waves would not be absorbed and reflected as selectively as occurs in the presence of auxochromes.
Auxochromes tend to be polar. This increases the overall polarity of the dye molecule and makes it more readily soluble in water.
As a result of physical interaction between chromophores and auxochromes, textile dyes have some of the highest colour intensities of all colourants in common use.
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