auxochromen

Auxochromen is a term used in chemical spectroscopy to refer to a substituent that, when attached to a chromophore, does not itself produce strong coloration but modulates the optical properties of the chromophore through electronic interaction. In modern usage the preferred term is auxochrome, but auxiliary forms such as auxochromen appear in historical literature and some older texts.

The primary function of an auxochromen is to alter the absorption characteristics of a conjugated system. This can lead to bathochromic shifts (moves absorption to longer wavelengths, a red shift) or, less commonly, hypsochromic shifts (moves absorption to shorter wavelengths, a blue shift). The effect arises from resonance donation or withdrawal and from inductive interactions that extend or reorganize the electron delocalization in the excited state of the chromophore. The magnitude and direction of the shift depend on the entire molecular framework, including the nature of the chromophore, the substituent, and its position relative to the chromophore’s conjugated path, as well as solvent and, for some systems, pH.

Common auxochromen include electron-donating groups such as hydroxyl (-OH), alkoxy (-OR), amino (-NH2, -NR2), and related substituents, which tend to produce bathochromic shifts in many systems. Halogens can also act as auxochromes but often yield smaller or system-dependent effects. The concept is widely used in dye chemistry, pigment design, and organic electronics to tailor color and light-absorption properties.

Terminology note: while auxochromen appears in some sources, the standard term in contemporary literature is auxochrome. The two terms are sometimes used interchangeably in historical contexts.