Dioxazine Pigments

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(m) chloride or benzenesulfonyl... 

Pigment Violet 23 may serve as an example to illustrate the synthesis of dioxazine pigments in general. The technique has not changed much since it was first developed [4],... 

Dioxazine pigments are found in more than one crystal modification [6]. 

Commercially Available Dioxazine Pigments and Their Application 533... 

The universally applicable dioxazine pigment, especially Pigment Violet 23, which is distinguished by good color strength, is commercially important. It is produced by the reaction of 3-amino-N-ethylcarbazole and chloranil in o-dichloro-benzene at temperatures between 130 and 150 °C followed by oxidative ring closure to the dioxazine in the presence of benzenesulfonyl chloride. The 3-amino-N-ethylcarbazole required for the synthesis is produced from carbazole by N-alkyla-tion, e.g. with diethyl sulfate or ethyl chloride, and nitration followed by the reduction to the amine. 

Dioxazine pigments are available in violet and are unusually strong although expensive. They exhibit excellent stability in light and satisfactory heat stability. 

Used in acrylic sheet and extruded low-density polyethylene (LDPE), dioxazine pigments are also widely used for shading greenish phthalocyanine bine and as a toning pigment to blue white. 

As part of a general program to improve performance characteristics of the traditional carbazole dioxazine pigment in such areas as bleed resistance, solvent fastness and heat stability, and at the same time to extend the range of shades beyond violet, a new class of dioxazine pigment bearing quinoxalinedione and benzimidazo-lone moieties in place of carbazole residue has been developed at Clariant (Figure 20-14). By simple manipulation of the substituents on the heterocyclic... 

Preparation of the benzimidazolone-dioxazine pigments usually involves a multi-step synthesis followed by finishing. A typical reaction scheme depicting the synthesis of the unsubstituted derivative, but which can be generalized for all other substituents, is shown in Figure 20-15. 

Figure 20-15 Typical synthesis of benzimidazolone-dioxazine pigments.

The proof of this hypothesis, and particularly of the role of supramolecular conjugation in explaining the relationship between color and constitution of organic molecules, is provided by the absorption spectra of the carbazole-dioxazine pigment and the three benzimidazolone-dioxazine pigments shown in Figure 20-18. 

Figure 20-18 Absorption spectra (DMSO) of benzimidazolone-dioxazine pigment versus carbazole-dioxazine pigments.

Benzimidazolone-dioxazine pigments identified for commercialization thus far are red-violet to reddish-blue in shade. They are characterized by an exceptionally high tinting strength, and their heat stability and resistance to aggressive solvents and plasticizers is much superior to that of carbazole dioxazine pigments. 

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