Monoazo (Monohydrazone) pigments based on benzimidazolones

A series of azo pigments containing the benzimidazolone ring were introduced in the mid to late twentieth century as high-performance organic pigments in the yellow, orange, red, and brown shade areas. The structures of the commercial benzimidazolone azo pigments are derived from either the monoazoacetoacetanilide or monoazonaphtharylamide classical azo pigments systems and exist in the ketohydrazone tautomeric forms. The high-performance properties of the pigments have been attributed to a network of intermolecular hydrogen bonds involving the benzimidazolone group, and efficient molecular packing, as demonstrated by X-ray crystal structure determinations. The manufacturing processes leading to the pigments involve traditional diazotization and azo coupling reaction procedures, although they require special conditioning aftertreatments to optimize their performance. Although benzimidazolone azo pigments were initially developed for the coloration of plastics, they have probably had a greater impact on the paint industry. The application properties of the benzimidazolone azo pigments are discussed for individual products.