AUXOCHROME

a group of atoms and electrons forming part of an organic molecule and modifying the absorption of light by an adjacent chromophore so as to intensify and alter the colour of the substance. The most potent auxochromes are amino groups, halogen atoms, and hydroxyl and alkoxyl groups, although none of these substituents alone confers colour on a compound. The essential component of an auxochrome is at least one pair of electrons not involved in covalent bond formation; such unshared electrons are usually provided by an electronegative atom such as nitrogen, oxygen, sulfur, or one of the halogens. Combination of an auxochrome with a chromophore creates an electron system that absorbs a greater amount of light than the chromophore alone, and the light absorbed is of longer wavelength. As a result, the colour is more intense and has a deeper shade. In older usage, the term auxochrome included groups such as sulfonic acid (-SO3H) and carboxyl (-CO2H) that are now recognized as influencing the solubility of a dye or its affinity for textile fibres but not its colour.