What is "auxochrome"

n. (context obsolete chemistry English) any atom or group which, when added to a chromophore, causes a bathochromic shift in its spectrum

An auxochrome (Greek αὐξάνειν auxánein: "to increase" and χρῶμα chrōma: "colour") is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. They themselves fail to produce the colour; but when present along with the chromophores in an organic compound intensifies the colour of the chromogen. Examples include the hydroxyl group (-OH), the amino group (-NH), the aldehyde group (-CHO), and the methyl mercaptan group (-SMe).

An auxochrome is a functional group of atoms with nonbonded electrons when attached to a chromophore, alters both the wavelength and intensity of absorption. If these groups are in direct conjugation with the pi-system of the chromophore, they may increase the wavelength at which the light is absorbed and as a result intensify the absorption. A feature of these auxochromes is the presence of at least one lone pair of electrons which can be viewed as extending the conjugated system by resonance.