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Difference between auxochrome and chromophore

Basic auxochromes

Auxochrome vs chromophore

Auxochrome is a Greek word arising from two word roots; ‘auxo’ meaning “to increase” and ‘chrome’ meaning “color”. Auxochrome is a group of atoms which will impart a particular color when attached to a chromophore but when present alone, will fail to produce that color. Chromophore is that part of the molecule which when exposed to visible light will absorb and reflect a certain color.

Auxochrome is a group of atoms which is functional and has the capability to alter the capacity of the chromophore to reflect colors. Azobenzene is an example of a dye which contains a chromophore. All substances like dyes produce colors by absorption of visible light owing to the various constituent compounds. The electromagnetic spectrum has a very wide variation in wavelengths but the human eye visualizes only short wavelength radiation. Chromophores do not absorb light without the requisite contents but with the presence of an auxochrome there is a shift in the absorption of these chromogens. Auxochrome increases the color of any organic substance. For instance, benzene does not have any color of its own, but when it is combined with the -nitro group which acts as a chromophore; it imparts a pale yellow color.

Auxochromes are commonly called as ‘color helpers’ or ‘color intensifiers’. Dyes containing auxochromes are basically aromatic compounds and include the presence of aryl rings which have delocalized electron systems. These are responsible for the absorption of different radiations with different wavelengths based on the energy of the electron. If an auxochrome is present in the meta position of the chromophore, then there is no change in the color. Electrons present in the chromophore get excited from the ground level to the excited state when visible light falls on them. Chromophores also alter the energy in the delocalized systems. A chromophore gives the dye the property of absorbing different radiations whereas auxochrome imparts it with the property of being colored.

We have an understanding that chromophores are atomic configurations with presence of delocalized electrons. Chromophores are represented as nitrogen, carbon, oxygen and sulphur which usually have single or double bonds. Chromophores with double covalent bond appear to be colored as a result of the heightened state of the electrons. The electrons which were in the resting state are elevated into the excited state because of the energy incorporated into it. If the energy incorporated is changed, automatically the wavelength of the radiation it absorbs will also change and the compound will appear colored.

Auxochromes are molecules which are attached to non ionizing compounds yet retain their ability to ionize and affect the ability of absorbing light when attached to a chromophore. Hence, they are also called as “color helpers”. Auxochromes are classified as either positively charged or negatively charged. Amino groups are examples of positively charged whereas carboxyl, hydroxyl and sulphonic groups are examples of negatively charged auxochromes. To convert basic dyes to acidic dyes, the negatively charged sulphonic groups are often used.

Summary: In order to prepare dyes, auxochromes are attached to chromophores so as to achieve the deep color intended for the product. Auxochromes are a bunch of atoms that when combined with an appropriate chromophore heighten or enhance the color. Chromophores are constituents of molecules that absorb or reflect certain colors when light fall on them. They are used together to make dyes.
Image Credit : http://commons.wikimedia.org/wiki/File:Auxochromes002.png

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