Cationic Methine Dyes

Due to the great varietity of different types of cationic methine dyes the syntheses of the numerous groups of these dyes are described under each specific group in Section 3.8.2. Any further division would have led to an unreasonably large expansion of this book. 

Most of the qualitative relationships between color and structure of methine dyes based on the resonance theory were established independently during the 1940 s by Brooker and coworkers (16, 72-74) and by Kiprianov (75-78), and specific application to thiazolo dyes appeared later with the studies of Knott (79) and Rout (80-84). In this approach, the absorptions of dyes belonging to amidinium ionic system are conveyed by a group of contributing structures resulting from the different ways of localization of the 2n rr electrons on the 2n l atoms of the chromophoric cationic chain, rather than by a single formula ... 

Cationic functionality is found in various types of dyes, mainly in cationic azo dyes (Section 3.7) and methine dyes (Section 3.8), but also in anthraquinone (Section 3.4), di- and triarylcarbenium (Section 2.6), phthalocyanine dyes (Section 2.7), and in various polycarbocyclic and solvent dyes (Section 3.10). 

Cationic dyes with delocalized charge are classified with the methine dyes (see Section 2.6). They may be viewed as vinylogous amidinium salts 3 [1] ... 

These dyes possess two independent chromophoric chains of even methine (neutro) and uneven methine (cyanine) fixed on a central ketometbylene nucleus. The methylene reactive group is first used for the neutrocyanine synthesis in position 5. the, quaternization of which can ensure a subsequent polymethine synthesis in position 2 of a cationic dye by ordinary means (Scheme 58). As indicated, this quaternized neutrocyanine (37) may as well give another neutrocyanine. 

Many methine cationic dyes, styrylic (141), pyrrolic. or amino-substituted (142) derivatives of thiazolium, possess interesting anthelmintic properties (143). This last class has been used as accelerators of the catabolism and activators of cellular exchanges (144). 

Reactions with Parting of Radicals. The one-electron oxidation of cationic dyes yields a corresponding radical dication. The stabihty of the radicals depends on the molecular stmcture and concentration of the radical particles. They are susceptible to radical—radical dimerization at unsubstituted, even-membered methine carbon atoms (77) (Fig. 6). 

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

Yagi S, Fujie Y, Hyodo Y, Nakazumi H (2002) Synthesis, structure, and complexation properties with transition metal cations of a novel methine-bridged bisquarylium dye. Dyes Pigm 52 245-252... 

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. 

An example of an N/C conjugated system involving an exocyclic carbon is the enamine conjugation in cyanine dyes, in which, the j3-carbon atom constitutes the methine bridge ([164], n = 0), or part of the bridge ([164], n = 1). Protonation of these cations occurs in... 

Many cationic dyes for acrylic fibres, especially methines, such as Cl Basic Red 13, Cl Basic Violet 7 and Cl Basic Red 74 are very bright and fluorescent and hence this fibre has a place in fluorescent textiles. 

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