Auxochrome

In organic chemistry, an auxochrome (from Ancient Greek αὐξάνω (auxanō) '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 instead intensify the colour of the chromogen when present along with the chromophores in an organic compound. Examples include the hydroxyl (−OH), amino (−NH2), aldehyde (−CHO), and methyl mercaptan groups (−SCH3). An auxochrome is a functional group of atoms with one or more lone pairs of 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.