Methine Dyes

Hamer s book in this series (1). which reviewed the synthesis of sensitizing dyes, their physical characteristics, and general photographic properties up to 1958, remains the basic encyclopedic source for the study of methine dyes. 

Another objective is to discuss briefly recent and major trends in the field of methine dyes color. Indeed, because of its relatively simple structure, the thiazole ring has been chosen in the past for studying color-structure relations. Using Brooker s basicity concepts (5), numerous valuable attempts in different countries succeeded in establishing semiempirical rules for explaining the effects of structural changes on color. 

Merocyanines belong to the class of nonionic methine dyes combining two nuclei, one of which is a ketomethylene of acidic nature such as pyrazolone, rhodanine, oxazolone, thiohydantoin,. 

In 1923, Mills introduced thiazole for the first time in the synthesis of methine dyes through a somewhat indirect route. In order to demonstrate the 2,4 -cyanine mechanism of formation by quinoline and quinaldine quaternary salts reacting together, Mills used other pairs of quaternary salts as 2-methylthiazolium with either quinolinium or benzothiazolium (42, 43). 

The methine chain is obtained by reacting ethyl o-formate (method A ) or ethylisoformanilide (method B) with a bis quaternary salt of bis-(2-thiazolyllbutane. Concerning dyes with fused thiazolo rings pyrrolo[2. lb]thiazoIe. thiazolo[2.3a]indole. thiazolo[2.3c]1.4-benzox-azine. the a carbon directly linked to the carbon 2 of the thiazoJe ring is also responsible for the classical syntheses giving trimethine or penta-methine dyes. 

Among the authors whose contribution has been important in the past, either by their experimental work or by their theoretical approach, the names of Koenig. Hamer, Fischer. Brooker, Kiprianov, Sklar, Schwarzen-bach, Eistert, Dimroth, and Platt are well familiar to scientists having an interest in the field of methine dyes, as well as more recently those of Kuhn, Wizinger, Scheibe, Hiinig, Van Dormael, Dahne, and Fabian. 

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 ... 

An alkyl group in the chain of a monoraethine thiazolocyanine prevents the molecule from being planar. It gives a bathochromic shift of 40 nm and a decrease of the oscillator strength (26), as is the case for other methine dyes. 

The use of sensitizing dyes in photography has been the subject of many studies and constitutes. still now. one of the most studied areas in specialized periodic publications (125, 126) or in textbooks (88. 127). It can be ascertained that one hundred years after Vogel s discovery of spectral sensitization, the basic mechanisms of action of dyes on their silver halide support still remain not fully understood. However, the theoretical reasons explaining why among many other dye families practically only cyanine methine dyes appear to be spectral sensitizers (128) are better known. 

Styryl derivatives have been patented as sensitizers (76) and other methine dyes for sensitization in direct photothermoGraphic material (134). 

Another compound of interest is adenine [73-24-5] or 6-aminopurine (53) derived from pheny1a 2oma1ononitri1e (92). The introduction of the dicyanostyryl moiety has led to the industriali2ation of several methine dyes such as the Cl Disperse Yellow [6684-20-4] (54) (93). The Cl Disperse Blue 354 [74239-96-6] (55) also represents a new class of anTinoarylneutrocyariine dyes with a brilliant blue shade (94). The dimer of malononitrile is also used for the synthesis of new dyes (95). 

Methicillin — see Penicillin, 2,6-dimethoxyphenyl-Methidathion insecticidal activity, 6, 576 as insecticide, 1, 196 Methine, dipteridyl-synthesis, 3, 303 Methine dyes, 1, 323-325, 332 L-Methionine, S-adenosyl-in metabolic iV-methylation, 1, 236 Methionine, dehydro- C NMR, 6, 139 X-ray crystallography, 6, 136 Methiothepin... 

Quantum chemical calculations, for poly-methine dyes, 73... 

Mesoionic thiazol-4-one, see 4-Hydroxy-thiazolium hydroxide inner salts Methanol, action on thiazolium salts, 32 Methine dyes, nonionic, nomenclature of, 27... 

Azoic soluble dyes, 7 373t Azo initiators, 14 293, 294-295t handling, 14 296 worldwide producers of, 14 303 Azolla, in nitrogen fixation, 17 299 Azo-methine dyes, 9 503 Azomethine ylides, 17 51, 56 Azonitriles, 14 293... 

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. 

There is as yet no agreed international list of permitted food colours. Thus a food dye that is permitted in one country may be considered unacceptable in another. The synthetic food colorants permitted in the European Union are listed in Table 1.8 [60]. All were originally introduced as acid dyes for wool many years ago. Furthermore, more than thirty colorants of natural origin are permitted in most countries. The natural carotenoid dyes are of outstanding importance for colouring edible fats and oils. These yellow to red methine dye structures occur in many families of plants and animals, including vegetables, berries,... 

Rosenbaum VM, Dekhtyar ML (1993) A simple model for formation of aggregate of poly-methine dye molecules. Ukr J Phys 38 1296-1299... 

Beersmans, j. (Gevaert) Methine dyes for sensitizing photographic emulsions. British patent 626470 (July 15, 1949). 

Azo dyes containing indolizines have been extensively studied. Indolizines have been incorporated into mono- and poly-methine dyes, which have been used as photographic sensitizing and fabric brightening agents. 

The bis(triazolopyrimidyl)methanes 39 were quaternized with methyl p-toluenesulfonate to give 254, whose treatment with base gave the mesoionic methine dyes 255 (82UKZ79). The methylene group of 39 was active to be condensed with aldehydes (Scheme 48). 

Polymethine and Related Dyes. Cyanine dyes are the best known poly methine dyes. Nowadays, their commercial use is limited to sensitizing dyes for silver halide photography. However, derivatives of cyanine dyes provide important dyes for polyacrylonitrile. They include azacarbocyanines, hcmicyanines. and diazahemicyanines. 

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