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Organic compounds having the functional group R-N=N-R' are called azo compounds. Aryl azo compounds are usually stable. In contrast, most alkyldiazo compounds are explosive. Unsymmetrical and symmetrical azo derivatives were prepared by diazotisation-coupling reactions. Aromatic azo compounds can be synthesized by azo coupling reactions. The oxidation of hydrazines (R-NH-NH-R') also gives azo compounds.
As a consequence of pi-delocalization, aryl azo compounds display intense colors, especially red, orange, and yellow colors. These compounds are used as dyes, and are commonly known as azo dyes. Azo dyes account for 60-70% of all dyes. Some of the azo compounds are used as acid-base indicators, owing to the ability of their acid and salt forms to display pH dependent changes in their colors (e.g., methyl orange). Azo compounds are also used as pigments in the textile industries. Azo pigments are important in a variety of paints as they have excellent coloring properties. With their ability of cis-trans isomerization in the presence of radiations, they are excellent candidates to function as Molecular Switches (See: Merino, E. Synthesis of Azobenzenes: the coloured pieces of molecular materials. Chem. Soc. Rev. 2011, 40, 3835-3853).