20 октября, 2015 
Pokraskin Dioxazine colorants, as the name implies, contain two oxazine ring systems as the chromophoric grouping. They are relatively few in number and generally restricted to violet to blue shades. Probably the most important dioxazine colorant is C. I. Pigment Violet 23, 136. This product is usually referred to as Carbazole Violet and is the most significant violet pigment for high-performance applications (Chapter 9). Compound 136 has been shown to have an angular structure as illustrated in Scheme 6.4 rather than the linear structure that is shown in most older texts. The pigment is characterised by a brilliant intense reddish-violet colour, very good lightfastness and resistance to heat and solvents. Its synthesis is illustrated in Scheme 6.4.
In the synthetic scheme, 3-amino-9-ethylcarbazole 138 (2 mol) is condensed with chloranil 137 (1 mol) to form the intermediate 139. This intermediate is then converted into the dioxazine pigment 136 by oxidative cyclisation at around 180 °C in an aromatic solvent and in the presence of a catalyst such as aluminium(iii) chloride or benzenesulfonyl chloride. Sulfonation of this pigment gives rise to water-soluble dyes which may be used as direct dyes for cotton. An example is C. I. Direct Blue 108 which contains 3 or 4 sulfonic acid groups. The dioxazine system is also used as the chromophoric group in some reactive dyes (Chapter 8).
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