Auxochromic groups

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. 

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. 

The NO-group is the most active colour-producing (chromophoric) group known. With a radical such as isobutyl which is of no account for the absorption of light, it produces a blue nitrosohydrocarbon. In spite of their intense coloration the nitroso-compounds are not dyes, since they lack the auxochromic groups (e.g. NH2 or OH) necessary for combination with textile fibres. 

Wool and silk are protein-like substances and hence are amphoteric. Accordingly, they can combine with acids as well as with bases. For this reason wool and silk can be dyed directly by dyes in virtue of their auxochromic groups. 

The tetrazole ring exhibits only weak end absorptions at 200-220 nm in the ultraviolet and only tetrazoles with conjugated auxochromic groups give normal ultraviolet spectra. Absorption maxima are controlled by the conjugation which is usually more extensive in 2,5-disubstituted derivatives relative to the 1,5-disubstituted isomers. Thus 2,5-disubstituted tetrazoles have higher as seen from the following examples 1-Me,5-Ph, 232 nm, 2-Me,5-Ph, A ax 240 nm 1-Ph,5-H, A ax 236... 

The fluorescence of coumarin compounds has been well known since 1911, when it was observed that the absorption band of coumarin at 311 nm was bathochromically shifted by one of several auxochromic groups, such as hydroxyl or amino, in the 3- or 7-position. A comparison of the stilbene molecule (88) with 3-phenylcoumarin (93) indicates the similarity of structure, and basically similar substitution patterns in the latter molecule have produced similarly useful FBAs (69FRP1568007). 

The data of Table 22-3 show the effect on the benzene chromophore of this type of substituent —the substituent often being called an auxochrome.2 This term means that, although the substituent itself is not responsible for the absorption band, it shifts the absorption of the chromophoric group, in this case the benzene ring, toward longer wavelengths. The auxochromic groups usually increase the intensity of the absorption also. 

Determining chromophores and auxochromic groups in kraft lignin. 

The different types of chromophores, auxochromic groups, and originating reactions which will be discussed are ... 

Groups which do not themselves produce explosive properties, but may influence them in the same way that auxochromic groups vary the colour intensity and shade of a dye, are called auxoplosives by these authors. We may quote hydro-XY, carboxyl, chlorine, sulphur, ether, oxygen, amine, etc. as examples of such groups. 

As we have already seen these lone pairs can form part of the system of n electrons. The difference between chromo-phoric and auxochromic groups is in this way of secondary importance. Also the much discussed question whether a ben-zoid (benzene-like) or a quinonoid (quinone-like) structure should be attributed to dyestuffs becomes, in the light of the resonance theory, an incorrectly chosen alternative. It is the possibility of resonance which is reflected in the multiplicity of the valence structure that forms the true basis for light absorption. An isolated benzoid configuration is just as little a colouring matter as a quinonoid structure compare the uncoloured hydroquinone and the very weakly coloured quinone. 

Mention should be made of oxazine dyes, used also as biological stains, which are oxidized phenoxazine derivatives containing suitable auxochromic groups. A detailed treatment of these dyes, however, is beyond the scope of this review. Most of the industrial phenoxazine dyes are derived from benzophenoxazines (e.g., Meldola s blue) or from more complex ring systems containing the phenoxazine residue (triphendioxazine dyes).3,118 The long-known dyestuffs orcein and litmus which are prepared by the action of ammonia on certain lichens, and may also occur accidentally in nature, are both based on the oxidized phenoxazine ring system as shown by Musso and co-workers.119... 

The first modification is peripheral substitution to reduce the oxidation potential of the carbonyl group—a method which often switches the reactivity because of hole localization at positions remote with respect to the C=0 group (e.g. acetophenone or 4-methoxyacetophenone). Actually, the electrophore is extended by additional conjugation which reduces the oxidation potential and the HOMO-LUMO energy gap (i.e. the effect of auxochromic groups on the chromophore skeleton). 

With the aid of this theory it is explained that just as in coloured substances the introduction of some auxochromic groups changes the colour of the substances, so in the war gases the presence of certain autotoxes can alter the type of biological action. Thus, for example, halogen introduced into the hydrocyanic acid molecule reduces the toxicity of the toxophor, —CN, and confers on the product lachrymatory properties. 

In some cases there are actually strongly marked differences. Thus in the coloured substances the auxochrome group brings out the latent colour-potentiality of the chromophore group and so makes colouration possible, while in the war gases the auxotox group merely develops the characteristic property of the toxophor... 

Derivatives of perylene 141, have, indeed, found practical applications and it appears, that the addition of auxochromic groups, such as in perylene-3,4 9,10-tetracarboxdiimides (PTCDI) 171 gives rise to a bathochromic shift and also to a dramatic increase in photostability (chart 28) [243]. 

Sunlight, especially a small portion of UV light, is the principal instigator of weathering reactions. The immediate consequence of the interaction of wood with light is the generation of free radicals at the exposed surface (7, 19). As these labile free radicals terminate and stabilize, chromophoric and auxochromic groups are formed and discoloration and deterioration occur. 

From this it may be conceived that the simultaneous presence of a basic auxochromic group and an acid-forming chromophor, or vice versa, gives rise to a weak dyestuff. The nitranilincs form a case in point they are weak dyestufis, while on the other hanu the nitrophenols have tinctorial properties much more fully eveloped. 

The quinones belong to the most powerful class of chromogens, and this is equally true of both the para- and orthoquinones. They yield actual dyestuffs by introduction of auxochromic groups. The oxyquinones possess a specially marked dyestuff-character, as the quinone group belongs to the acid-forming chromophors, and the hydroxyl group introduced develops powerful acid properties. 

The features of the absorption spectra change if the so-called auxochromes (e.g., -NH2, -NR2, -SH, -OH, -OR) are introduced into the molecules. The presence of free electron pairs in the auxochromic group, that interact with electrons of the chromophoric group (e.g., the free electron pair at nitrogen in the -NH2 group) leads to a state of conjugation which may result in formation of a new absorption band in the spectrum. 

Chromophore and auxochrome groups are listed early in the Connections essay. Look for these along with extensive conjugation in the structures of dyes presented. 

Dyes generally contain two or more cyclic rings that may or may not be aromatic and condensed. From a chemical point of view, a dye molecule can be characterised, on the one hand, by the basic structure, which is related to a dye family and contains chromophores (conjugated double bonds, aromatic rings), which induce the dye solution coloration, and, on the other hand, by the substituents or auxochromic groups, which infer aqueous solubility by ionisation (NH2, OH, COOH, SO3H, etc.) and can enhance conjugation in the dye molecule. 

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