Chromophores and Auxophores

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CHROMOPHORES

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.

​•For a molecule to absorb radiation in the right wavelength, it needs to have specific covalent groups which contain p orbitals (these are called unsaturated)

  C=C, C=C, C=O, C=N, N=N, NO2, etc.

•These groups are called chromophores

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CHROMOPHORES

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:

1. Hydroxyl group (-OH)

2. Amine group (-NH2)

3. Aldehyde group (-CHO)

4. Methyl mercaptan group (SCH3)​

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Auxophores