Auxochrome substituents

Furan exhibits a single band of medium intensity at 208 nm. Alkylation tends to effect a bathochromic shift of this band to ca. 220 nm as in the many natural products, e.g. difurospinulosin (7), that are /3-alkylfurans. The slight bathochromic shift in the spectrum of [8](2,5)furanocyclophane (69T(25)5357), as compared to 2,5-dimethylfuran, has been ascribed to strain. From the limited data available auxochromic substituents produce little effect on the furan spectrum. 

Effect of Auxochromic Substituents on Electronic Absorption by the Benzene Chromophore... 

Band group IV appears at 330 nm if no auxochromic substituent is attached to the molecule. 6-Methoxy-substitution displaces the maximum to 318 nm, while substitution in position 6 displaces the band bathochro-mically. This band is also characteristic for 1,3-diaryl-substituted salts, and it appears at —320-325 nm in acidic alcoholic solutions (87RRC417). 

Even more impressive is the effect of the auxochromic substituents on the longest-wavelength maximum in the case of the fused-ring color salts (329) and their vinylogs (330) the dimethylamino group... 

Auxochrome substituent which leads to shift of the absorption maximum. 

Unsaturated groups, known as chromophores, are responsible for — tz, and k — 7t absorption mainly in the near UV and visible regions and are of most value for diagnostic purposes and for quantitative analysis. The mx and e values for some typical chromophores are given in Table 9.2. The positions and intensities of the absorption bands are sensitive to substituents close to the chromophore, to conjugation with other chromophores, and to solvent effects. Saturated groups containing heteroatoms which modify the absorption due to a chromophore are called auxochromes and include -OH, -Cl, -OR and -NRr... 

Benzene derivatives exhibit medium to strong absorption in the UV region. Bands usually have characteristic fine structure and the intensity of the absorption is strongly influenced by substituents. Examples listed in Table 2.3 include weak auxochromes (-CH3, -Cl, -OCH3), groups which increase conjugation (-CH=CH2, -C(=0)-R, -NOj) and auxochromes whose absorption is pH dependent (-NH2 and -OH). 

Some substituents tend to enhance fluorescence. They are known as fluorochromes in the same sense as auxochromes. In general, these are electron donors such as — OH, NH2, etc. which enhance the transition probability or intensity of colour, e.g. acridine and acridine orange. 

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. 

The introduction of auxochromes into the almost colorless anthraquinone permits the tailoring of compounds to cover nearly all shades of dye colors types and positions of the substituents in the molecule determine the hue. As a rule the bathochromic shift in simple anthraquinones increases with increasing basicity of the substituents. This generalization is clearly shown by the wavelengths of the longest wavelength absorption maximum of anthraquinones mono substituted in the 1-position [1] (Table 3.5) ... 

The simple anthraquinone dyes, containing hydroxy or amino functions as their major auxochromes, are used for brilliant red to blue shades. Yellow and orange shades are obtained by other dye classes. Choosing appropriate substituents allows the best dyes for various fibers and colors to be obtained. 

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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