Photochemistry of Dyes and Pigments

A dye is a compound which is added to give colour to a normally colourless material, for instance natural or artificial fibres. By their very nature dyes must absorb light in the VIS region of the spectrum, and this can lead to two separate problems. 

Although there are many hundreds of man-made dyes at the present time, it will be seen presently that most of these fall into a few large groups of structurally related molecules, the general structural features explaining their photostability. 

The two major classes of industrial dyes that we shall consider here are the anthraquinone dyes and the azo dyes. The molecules which form the very extensive range of anthraquinone dyes are all based on the 2-amino- or 

2-hydroxyanthraquinone structures, with the proximal electron acceptor carbonyl group and the electron donor, NH2 or OH group held together by an intramolecular hydrogen bond. 

It is noteworthy that the proximal ortho position of the electron donor and acceptor groups is essential for the photostability of the dye molecule. Thus 

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