Azo Dyes and Pigments

Abstract

Azo dyes and pigments constitute by far the most important chemical class of commercial organic colorant, accounting for around 60–70% of the dyes used in traditional textile applications. Azo colorants contain as their common structural feature the azo (–NN–) linkage which is attached at either side to two sp2 carbon atoms. In terms of their colour properties, azo colorants are capable of providing virtually a complete range of hues. They are significantly more important commercially in yellow, orange and red colours (i.e., absorbing at shorter wavelengths), than in blues and greens. However, in more recent times the range of longer wavelength absorbing azo dyes has been extended, leading to the emergence of significant numbers of commercially important blue azo dyes. Perhaps the prime reason for the commercial importance of azo colorants is that they are the most cost-effective of all the chemical classes of organic dyes and pigments. It is conceivable that azo dyes may assume even greater importance in the future as some of the other chemical types, notably anthraquinones, become progressively less economic. This chapter contains a discussion of the fundamental structural chemistry of azo colorants, including a description of the types of isomerism that they can exhibit, the principles of their synthesis, and a discussion on the ability of azo dyes to form metal complexes.