Auxochrome

An auxochrome is a substituent attached to a chromophore that alters the color of a compound by changing its electronic absorption properties. Unlike a chromophore, which directly absorbs visible light, an auxochrome does not by itself produce color; instead it modulates the chromophore's ability to absorb light through resonance donation or withdrawal and by changing the extent of conjugation.

Through resonance and inductive effects, auxochromes can stabilize the excited state more than the ground state, lowering the energy gap and leading to a bathochromic (red) shift. They can also increase the molar absorptivity, producing a hyperchromic effect. The magnitude and direction of the shift depend on the nature of the auxochrome, its electronic properties, and its position (para or ortho relative to the chromophore is usually most effective).

Common auxochromes include electron-donating groups such as amino (-NH2), hydroxyl (-OH), and alkoxy (-OR) groups, as well as other substituents that can participate in resonance with the chromophore. Electron-withdrawing groups can also act as auxochromes in some systems, depending on the specific chromophore and conjugation pattern. In dye chemistry, auxochromes are used to tune color properties and absorption intensity of azo, quinone, and polyene chromophores.