Phenylogous Hemicyanine Dyes

Zeromethine (Apocyanine) Dyes. Some representatives of the zeromethine dyes (apocyanine dyes) have been known for a long time, e.g., C.I. Basic Yellow 1, 49005 [2390-54-7] (19, Thioflavine T). 

This dye is obtained by heating p-toluidinc with sulfur and subsequent alkylation with methyl or ethyl sulfate. The dye colors silk and tanned cotton a greenish yellow shade. Thioflavine S [ 1326-12-1] is obtained by sulfonation [42], 

Apocyanine dyes of the acridine series are obtained by treating acridinium iodide [43] or acridinium chloride [44] with tertiaiy aromatic amines. The dyes can also be obtained by ipso substitution of acridinium iodide with 4-haloaryl-amines in dimethyl sulfoxide at room temperature [45], 

The most important method for preparing apocyanine dyes is the reaction of N heterocycles containing 2- or 4-carbonyl groups with secondary or tertiary aromatic amines in the presence of dehydrating agents (e.g., phosphorus oxychloride or tin tetrachloride). 

Naphtholactam is a particularly important condensation component for preparing apocyanine dyes. At first only dyes unsubstituted at the lactam nitrogen atom were prepared and recommended for dyeing tanned cotton and wool [46], However, it was later found that the /V-alkyl derivatives dye polyacrylonitrile in very lightfast violet and blue shades (e.g., 20). 

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