An auxochrome is a functional group that, when attached to a chromophore, enhances the ability of the chromophore to absorb light, often shifting the wavelength of maximum absorption to a longer value.
Function:
Auxochromes generally contain lone pairs of electrons that can participate in resonance with the chromophore.
While they might not cause color by themselves, their presence, in conjunction with a chromophore, can intensify the color and shift the absorption maximum.
By doing so, they effectively extend the conjugated system of the chromophore.
Impact:
Typically, introducing an auxochrome to a chromophore results in a bathochromic shift (or redshift) in the absorption spectrum, meaning the absorption moves to a longer wavelength.
Examples:
–OH (hydroxyl group), –NH2 (amino group), and other groups possessing non-bonding electrons.