Chromophores hydroxyl group auxochrome

Thus the dye alizarin (red) contains two benzene rings, two carbonyl groups (chromophores) and two hydroxyl groups (auxochromes) ... 

The chromophore of phenylephrine is not extended but its structure includes a phenolic hydroxyl group. The phenolic group functions as an auxochrome under both acidic and alkaline conditions. Under acidic conditions it has two lone pairs of electrons, which can interact with the benzene ring and under basic conditions it has three. Figure 4.11 shows the bathochromic and hyperchromic shift in the spectrum of phenylephrine, which occurs when 0.1 M NaOH is used as a solvent instead of 0.1 M HCl. Under acidic conditions the X max is at 273 and has an A (1 %, 1 cm) value of 110 and under alkaline conditions the X max is a 292 nm and has an A (1%, 1 cm) value of 182. 

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. 

Substituents that alter the wavelength or absorptivity of a chromophore significantly are called auxochromes, and tables of the effect of substituents plus rules for their application in particular structures are to be found in specialist texts. For example, an unsubstituted, unsaturated ketone would have a peak maximum at about 215 nm. Substitution of a hydroxyl group on the carbon next to the carbonyl (a) raises the peak to 250 nm, and two alkyl groups on the next (j8) carbons would raise it to 274 nm. 

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. 

Dyes of this class are the ortho- or para-nitrophenols, nitronaphthols, and nitroamines. The chromophore is the nitro (—NO2) group, and the auxochromes are hydroxyl and amino groups. 

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