Difference Between Auxochrome and Chromophore

The key difference between auxochrome and chromophore is that an auxochrome is a group of atoms that modify the structure of a chromophore, whereas a chromophore is a molecular moiety that gives the colour of the molecule.

Chromophores are able to display a colour when it is exposed to visible light. This is because chromophores can absorb wavelengths from the visible wavelength range of light. An auxochrome is a modifier of the chromophore structure.

1. Overview and Key Difference
2. What is an Auxochrome
3. What is a Chromophore
4. Side by Side Comparison – Auxochrome vs Chromophore in Tabular Form
5. Summary

What is an Auxochrome?

An auxochrome is a group of atoms that can get attached to a chromophore, thereby increasing the colourfulness of the chromophore. Therefore, it is a modifier of a chromophore. An auxochrome itself cannot cause the development of colour. It can increase the ability of chromophore to absorb the wavelengths from visible range of light. Some examples for auxochrome groups include the followings:

Hydroxyl group (-OH)
Amine group (-NH2)
Aldehyde group (-CHO)
Methyl mercaptan group (SCH3)

Therefore, an auxochrome can be defined as a functional group in a molecule. These functional groups contain one or more lone electron pairs. These lone electrons cause the alteration of wavelength and the intensity of absorption when attached to a chromophore. This is done via resonance; the lone electron pairs undergo delocalization with the pi-electron system in the chromophore.

An auxochrome can increase the colour of any organic compound. E.g. benzene is a colourless compound, but nitrobenzene is a yellow-coloured compound (nitrobenzene contains a nitro group attached to benzene). Here, the nitro group is a chromophore for benzene molecule. When a hydroxyl group gets attached to the para position of nitrobenzene, it appears in a dark yellow colour (the intensity of nitrobenzene is increased due to the auxochrome group).

What is a Chromophore?

A chromophore is a part of a molecule that is responsible for the colour of that molecule. This region of molecules has an energy difference between two separate molecular orbitals which falls within the wavelength range of the visible spectrum. Then, when visible light hits this region, it absorbs the light. This causes the excitations of electrons from a ground state to an excited state. Therefore, the colour that we see is the colour that is not absorbed by the chromophore.

Difference Between Auxochrome and Chromophore

Figure 1: Conjugated double bonds that form the chromophore of the β-carotene molecule (in red)

In biological molecules, a chromophore is a region which undergoes conformational changes of the molecule when hit by light. Conjugated pi systems often serve as chromophores. A conjugated pi system has single bonds and double bonds in an alternating pattern. These systems often occur in aromatic compounds.

What is the Difference Between Auxochrome and Chromophore?

The key difference between auxochrome and chromophore is that an auxochrome is a group of atoms that modify the structure of a chromophore, whereas a chromophore is a molecular moiety that gives the colour of the molecule. Auxochromes can get attached to chromophores and increase the colour appearance of the chromophore.

The following table summarizes the difference between auxochrome and chromophore.

Summary – Auxochrome vs Chromophore

Auxochromes can get attached to chromophores and increase the colour appearance of the chromophore. The key difference between auxochrome and chromophore is that an auxochrome is a group of atoms that modify the structure of a chromophore, whereas a chromophore is a molecular moiety that gives the colour of the molecule.

Reference:

1. “Chromophore.” An Overview | ScienceDirect Topics, Available here.
2. “Chromophore.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 6 Dec. 2011, Available here.