Methine group 352 INDEX

Porphyrins - Natural pigments containing a fundamental skeleton of four pyrrole nuclei united through the a-positions by four methine groups to form a macrocyclic structure (porphyrin is designated porphine in Chemical Abstracts indexes). [5]... 

The name of this structural class ( quinoline ) in the Colour Index is not ideal because quinoline derivatives feature in other related classes, such as the methine basic dyes with a quinolinium cationic group. The class is more precisely associated with quinophthalone (1.15), the characteristic chromogen derived by condensation of quinoline derivatives with phthalic anhydride. This small class of yellow compounds contributes to the disperse, acid, basic and solvent ranges of dyes. 

Dyes in this category, classified as methines (1.21) and polymethines (1.22) in the Colour Index, characteristically contain a conjugated system through one or more methine (-CH=) groups terminating with heterocyclic atoms, usually nitrogen as in the quinoline (1.21) and... 

Most of these products are azo or anthraquinone types, often with a localised quaternary ammonium group isolated from the chromogen by a saturated alkyl chain, as in Cl Basic Red 18 (1.52). Such products often exhibit higher light fastness than the traditional delocalised types. Improved azomethine, methine and polymethine basic dyes of good light fastness are also available. In contrast to the more specialised traditional classes, the azo and methine dyes have contributed to the basic dye range across the entire spectrum of hues (see Table 1.6) and now account for a clear majority of all basic dyes listed in the Colour Index. 

Quinophthalone (6.229) and its derivatives [86] also fall into the methine category, although they appear in the Colour Index under quinoline colouring matters. The parent compound was discovered in 1882 by Jacobsen, who condensed 2-methylquinoline (quinaldine) with phthalic anhydride. The product, quinoline yellow, is used as a solvent dye (Cl Solvent Yellow 33). The light fastness is improved by the presence of a hydroxy group in the quinoline ring system. Derivatives of this type provide greenish yellow disperse dyes for polyester. The moderate sublimation fastness of Cl Disperse Yellow 54 (6.230 R = H) is improved by the introduction of an adjacent bromine atom in Cl Disperse Yellow 64 (6.230 R = Br). 

Dye Classification Based on chemical structure or chromophore, 20-30 different groups of dyes can be discerned. Azo (monoazo, disazo, triazo, polyazo), anthraquinone, phtha-locyanine and triarylmethane dyes are quantitatively the most important groups. Other groups are diarylmethane, indigoid, azine, oxazine, thiazine, xanthene, nitro, nitroso, methine, thiazole, indamine, indophenol, lactone, aminoketone and hydroxyketone dyes and dyes of undetermined structure (stilbene and sulphur dyes). The vast array of commercial colourants is classified in terms of colour, structure and application method in the Colour Index (C.I.), which has been edited since 1924 (and revised every three months) by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. Each dye is given a C.I. generic name determined by... 

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