Heterocyclic Azo Dyes

The carbocyclic azo dyes are highly cost-effective and have good all-around fastness properties. However, they generally lack brightness and consequendy cannot compete with anthraquinone dyes in this respect. This shortcoming of carbocyclic azo dyes is overcome by heterocyclic azo dyes, as well as other dye classes such as triphenodioxazines and benzodifuranones. 

A long-term aim of dyes research has been to combine the brightness and fastness properties of anthraquinone dyes with the strength and economy of azo dyes. This aim is now being realized with heterocyclic azo dyes, which fall into two main groups those derived from heterocyclic coupling components and those derived from heterocyclic diazo components. 

All the heterocyclic coupling components that provide commercially important azo dyes contain only nitrogen as the heteroatom. They are indoles (85), pyrazolones (86), and especially pyridones (87) they provide yellow to orange dyes for various substrates. 

Many yellow dyes were of the azopyrazolone type, but these have now been largely superseded by azopyridone dyes. Azopyridone yellow dyes are brighter, stronger and generally have better fastness properties than azopyrazolone dyes. Both azopyrazolone and azopyridone dyes exist in the hydrazone tautomeric form. Typical dyes are the azopyrazolone C.I. Acid Yellow 23 (88) and the azopyridone (89). 

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Heterocoagulation Heterocyclic Heterocyclic amines Heterocyclic azo dyes Heterocyclic compounds Heterocyclic dyes Heterocyclic polymers Heterocyclic thiophenes Heteroepitaxy Heterogeneous catalysis Heterogemte Heteroglycan Heterojunction... 

Figure 2.11 Resonance forms that make a major contribution to the first excited state of heterocyclic azo dye 18...

Anthraquinone dyes are second only to azo dyes in importance as disperse dyes and are predominant in the red, violet, blue and blue-green sectors [14]. Because anthraquinone disperse dyes are relatively expensive to manufacture, successful attempts were made to replace some of them with technically equivalent and more economical products [15]. The replacement process has been most successful in the red region using, for example, heterocyclic azo dyes and novel chromogens. The brilliance of the anthraquinones with their narrow spectral absorption bands is difficult to attain with other structures, however, as is their high light fastness and chemical stability. The development of anthraquinone disperse dyes is included in a review by Dawson [16]. 

In contrast to the heterocyclic coupling components, virtually all the heterocyclic diazo components that provide commercially important azo dyes contain sulfur, either alone or in combination with nitrogen (the one notable exception is the triazole system). These S or S/N heterocyclic azo dyes provide bright, strong shades that range from red through blue to... 

The two overriding trends in traditional colorants research for many years have been improved cost-effectiveness and increased technical excellence. Improved cost-effectiveness usually means replacing tinctorially weak dyes such as anthraquinones, until recently the second largest class after the azo dyes, with tinctorially stronger dyes such as heterocyclic azo dyes, triphendioxazines, and benzodifuranones. This theme will be pursued throughout this chapter, in which dyes are discussed by chemical structure. 

The quatemization of heterocyclic azo dyes to form diazadimethine dyes can also be carried out in an acid medium if compounds containing vinyl groups are added to the protonated azo dyes. Acrylic acid [80], acrylates [81], and, in particular, acrylamide [82] are suitable for this purpose. The dye 32 colors polyacrylonitrile in greenish-blue shades. 

Brunner and co-workers and Bami and co-workers described examples of fused or heterocyclic oxazoles prepared using PPA (Scheme 1.127). Heating picolinic acid and 3-amino-2-naphthol in PPA afforded 2-(2-pyridyl)naphtho[2,3-d] oxazole 466 directly. Similarly, 2-(nitrophenyl)oxazolo[5,4-fe]pyridines 468, precursors to heterocyclic azo dyes, were prepared from 467. These authors... 

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