Concept of Chromophore, types of Chromophore and Auxochrome in UV-Visible spectroscopy

Otto Witt, a German chemist, proposed the chromophore and auxochrome theory to explain how a substance's color and composition are related. Chromophores and auxochromes are the two major categories of molecular elements in charge of colour in molecules.

This hypothesis states that:

The presence of specific unsaturated groups, or groups with numerous bonds, in an organic substance, causes it to seem colored. These multi-bonded groups are known as chromophores.

Chromogens are substances that include the chromophore group. The number of chromophores or the degree of conjugation raises the intensity of the color.

For instance, while the molecule CH₃ - (CH = CH)₆ - CH₃ is yellow, ethene (CH2 = CH2) is colorless.

The existence of certain groups that, although not chromophores as such themselves, deepen the colour of the chromogen. These auxiliary groups are referred to as auxochromes.

Concept of Chromophore

In UV-Visible spectroscopy, we have discussed different types of transitions that occur due to the movement of electrons from one level to another when molecules absorb light. Those compounds that show absorbance in the 400-800 mu (visible) region appeared colored to our eyes. So, Chromophores are the groups that impart color to the compounds. The exact color depends upon the wavelength type that absorbs by the molecule. Generally, the Nitro group is yellowish in color. When in any compound if a nitro group is present it cause a yellow color to the overall molecule, so the Nitro group is a chromophore because it gives characteristics yellow color to the compound.

“Chromophore is defined as any isolated covalently bonded group that shows a characteristics absorption in the ultra-violet and visible region.” 

Important chromophore includes ethylenic, carbonyls, acetylenic, nitriles, esters, acids group, etc. Among all, Carbonyl is the most important chromophore.

Types of chromophore

There are two types

i)  Those chromophores that contain pi electrons and undergo pi to pi* transitions, these include acetylenes, ethylene’s, etc.

ii) Chromophores that contain pi electrons as well as non-bonding (n) electrons. Such type of chromophores shows two types of transitions л to л* and n to л*. Examples include nitriles, carbonyls, azo/ nitro compounds, etc.

Not any specific rule is present that provide the identification of chromophore. Changing the position and intensity of absorption contribute to various factors. Following are the keys that help to determine chromophore

Points to identify chromophore

a) In the spectrum absorption band near 270 to 350 mu with low intensity and _ᵋmax at 10-100 due to the n to pi* carbonyl transitions.

b) A band near to 300 mu may consist of two or three conjugated units (chromophores).

c) Aromatic system is shown by the absorption with _ᵋmax between 1000-10,000.

d) The high value of _ᵋmax from 10,000 to 20,000 shows the presence of simple conjugated chromophores like alpha-beta-unsaturated ketones.

Substitution of an aromatic nucleus with other groups can extend chromophore, and the absorption possibly occurs at a higher extinction coefficient (_ᵋmax).

From the above discussion, it is concluded that the presence of functional groups is possible by UV-Visible spectroscopy but when the conjugation (alternate double and single bond) is absent in molecules and when different compounds with the same functional groups show absorption at the same extinction co-efficient, it is not possible to detect functional groups in compounds. So, we say that functional groups or compounds can be determined by other spectroscopic techniques.

The following figure shows the different chromophores, transitions, and absorption values.

Types of Chromophore

Auxochrome

The Greek term auxochrome means "to increase the color."

It is a group that, by itself, does not function as a chromophore, but when bound to a chromophore, it shifts adsorption towards a wavelength that is longer while also intensifying it.

When combined with the chromophores in an organic molecule, they do not produce the color on their own but rather enhance the color of the chromogen (it is the substance that bears chromophore group).

Auxochromes can act as electron donors or acceptors. Both varieties of auxochromes can be found in nitro phenylenediamine compounds. Auxochrome is a collection of atoms that is joined to a chromophore and alters how well it can absorb light. Auxochromes all include one or more electron pairs that are not bonded.

Auxochrome examples include -OH, -OR, -NH2, −NH2, −NHR, −NR2 are auxochromes. etc. The absorption maxima in this kind shift to longer wavelengths.

 

Auxochrome Example