Auxochromes

Typical auxochromes are hydroxyl, alkoxyl and aroxyl, amino, alkyl-amino and arylamino, all of which promote conjugation with lone pairs on oxygen or nitrogen atoms. 

A great number of monoaza or polyaza. either symmetrica] or unsym-metrical, mono trimethine thiazolocyainines have been synthesized in order to verify or to obtain semiempirical rules, more or less based on the resonance theory, concerning the relation between the color of a thiazolo dye and the number and place of nitrogen atoms in the chromophoric chain. For example. Forster s rule applies to ionic dyes and stipulates that the will increase with the decreasing tendency of chromophoric atoms lying between the two auxochromes to take up the characteristic charges (90). 

Knott s rule concerns the importance of the place of the nitrogen atom replacing a methine carbon in the conjugated chain when the atom is separated from the active auxochromic atoms by an odd number of conjugated atoms, the shift is bathochromic. It is hypsochromic when there is an even number, Tne importance of the shift could establish a measure of M effect of various heterocyclic nuclei (79. 124). Many papers have been published, and examples have been given to verify these rules (79-84). 

Xanthene dyes are those containing the xanthylhim [261-23-4] (la) or diben2o-y-pryan nucleus [92-83-1] (xanthene) (lb) as the chromophore with amino or hydroxy groups meta to the oxygen as the usual auxochromes. They are... 

Dyestuff organic chemistry is concerned with designing molecules that can selectively absorb visible electromagnetic radiation and have affinity for the specified fiber, and balancing these requirements to achieve optimum performance. To be colored the dyestuff molecule must contain unsaturated chromophore groups, such as a2o, nitro, nitroso, carbonyl, etc. In addition, the molecule can contain auxochromes, groups that supplement the chromophore. Typical auxochromes are amino, substituted amino, hydroxyl, sulfonic, and carboxyl groups. 

Reactive groups have minimal auxochrome effect on color intensity, and color yield per molecular weight decreases with increasing numbers of reactive groups. Increased dye fixation and reduced environmental impact of hydrolyzed dye more than compensate for color reduction of additional reactive groups. 

A large body of information is available on the UV spectra of pyrazine derivatives (B-61MI21400, B-66MI21400). Pyrazine in cyclohexane shows two maxima at 260 nm (log e 3.75) and 328 nm (log e 3.02), corresponding to ir->ir and n ir transitions respectively (72AHC(14)99). Auxochromes show similar hypsochromic and bathochromic shifts to those observed with the corresponding benzenoid derivatives. 

The ultraviolet spectra were also used for determination of the pyrroline structure (1,158-160). They exhibit a bathochromic shift to 225-235 m, caused by the auxochromic action of the nitrogen-free electron pair which is in conjugation with n electrons of the enamine double bond (161,162). 

With imines, salts formation is accompanied by characteristic spectral changes (153) (a) a bathochromic shift in the ultraviolet region by as much as 50 m/i, according to compound type and to properties of any auxochrome present, and (b) a high frequency shift of the... 

Note how auxochromes shift and usually intensify the peaks. ... 

Some simple azo compounds, because of restricted rotation about the ( N=N ) double bond, are capable of exhibiting geometrical isomerism. The geometrical isomerism of azobenzene, the simplest aromatic azo compound which may be considered as the parent system on which the structures of most azo colorants are based, is illustrated in Figure 3.1. The compound is only weakly coloured because it absorbs mainly in the UV region giving a 2m.lx value of 320 nm in solution in ethanol, a feature which may be attributed to the absence of auxochromes (see Chapter 2). 

Experimental work carried out in these laboratories during recent years has been based on the theory that insecticides owe their activity to a toxic nucleus—the toxophore—the properties of which may be modified by auxiliary radicals—the auxotoxes. This nomenclature is suggested by the names of analogous functions in dyestuffs, the chromophore and auxochrome groups. 

Azo dye molecules have color due to their azo bond, auxochromes, and system of conjugated double bonds. The azo bond, while resistant to aerobic degradation, can be cleaved under anaerobic or anoxic condition, resulting in decolorization and the production of aromatic amines. Anaerobic reduction of the azo dyes is relatively easy to achieve, but the products have been found to be biorecalcitrant... 

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