Auxochrome
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An auxochrome is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. Examples include the hydroxyl group (-OH), the amino group (-NH2), and an aldehyde group (-CHO).
An auxochrome is a functional group of atoms with nonbonded electrons which, when attached to a chromophore, alters both the wavelength and intensity of absorption. If these groups are in direct conjugation with the pi-system of the chromophore, they may increase the wavelength at which the light is absorbed and as a result intensify the absorption. A feature of these auxochromes is the presence of at least one lone pair of electrons which can be viewed as extending the conjugated system by resonance.
It increases the color of any organic compound. For example, benzene does not display color as it does not have a chromophore; but nitrobenzene is pale yellow color because of the presence of a nitro group (-NO2). Para-hydroxynitrobenzene exhibits a deep yellow color. Here an auxochrome (-OH) is conjugated with the chromophore -NO2. Similar behavior is seen in azobenzene which has a red color, but para-hydroxy azobenzene is dark red in color.
There are mainly two types of auxochromes:
The presence of an auxochrome in the chromogen molecule is essential to make a dye. However, if an auxochrome is present in the meta position to the chromophore, it does not affect the color.
An auxochrome is known as a compound that produces a red shift because it increases the wavelength of absorption, therefore moving closer to infrared light. Woodward-Fieser rules estimate the shift in wavelength of maximum absorption for several auxochromes attached to a conjugated system in an organic molecule.
An auxochrome helps a dye to bind to the object that is to be colored. Electrolytic dissociation of the auxochrome group helps in binding and it is due to this reason a basic substance takes an acidic dye.
The presence of certain groups which are not chromophores themselves, but deepen the colour of chromogens.such supporting groups are calld auxochromes.auxochromes may be acidic(phenolic)or basic.some important auxochromes are -OH,NH2,-NHR,NR2.
The presence of an auxochrome in the chromogen molecule is essential to make it a dye.however, if an auxochrome is present in the metaposition to the chromophore,it doesnot affect the colour.for example,in the compound p-Hydroxyazobenzene(a bright red eye).
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