Chromophore

Chromophore

In Greek, chroma means color, and phoron means bearer that means it is a color producing group.
A chromophore is usually a group of atoms that are having electron-withdrawing nature, possess unsaturation, and when present in conjugation imparts color to the compound by absorption of visible light.
When an organic compound absorbs the radiation by exciting an electron from its ground state into an excited state in the visible part of the electromagnetic spectrum, it appears to be colored. So, the colored properties associated with the organic compounds are due to the presence of few groups of atoms known as chromophores which absorb visible light photons.
Chromophoric group decreases the energy of electronic transitions and shift the absorption bands from UV to visible region and we observe batho(or Red) shift i.e. benzene is colorless but nitrobenzene is yellow due to the presence of chromophoric nitro group.
Some of the important chromophores are ethylene, acetylene, carbonyls, acids, esters and nitrile groups etc. A carbonyl group is an important chromophore, although the absorption of light by an isolated group does not give rise to any colour in the ultra-violet spectroscopy.

Types of Chromophores

There are two types of chromophores-
1. Chromophores in which the groups have π electrons undergo π → π* transitions. For examples:-ethylenes, acetylenes etc.
2. Chromophores having both π- electrons and n (non-bonding) electrons undergo two types of transitions. i.e., π → π* and n → π*, for examples: - carbonyls, nitriles, azo compounds and nitro compounds etc.

Identification of Chromophores

There is no rule for the identification of a chromophore. The change in position as well as the intensity of the absorption depends upon a large number of factors. Following points may be useful to identify the chromophores-
1. Spectrum having a band near 300 mµ may possess two or three conjugated units.
2. Absorption bands near 270-350 mµ with very low intensity ɛmax 10-100 are because of n-π* transitions of carbonyl group.

3.Simple conjugated chromophores likes dienesor,α β –unsaturated ketones have ɛ_max values, i.e., from 10,000 to 20,000.
4.The absorption with ɛ_max value between1,000-10,000 reveals the presence of an aromatic system. If aromatic nucleus is substituted with groups which can extends the chromophore, the absorption take place at still higher value of extinction coefficients.

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