Chromophore
A chromophore is the part of a molecule responsible for its color. It is a group of atoms within a compound that absorbs light at specific wavelengths, often in the ultraviolet or visible regions of the electromagnetic spectrum. When a chromophore absorbs light, it undergoes an electronic transition, usually from a lower energy state (ground state) to a higher energy state (excited state). This absorbed light corresponds to a particular color, and the molecule appears to have this color as the unabsorbed light is reflected or transmitted.


Common chromophores include:

  • Double bonds (C=C, C=O, etc.)
  • Conjugated systems (like alternating double and single bonds)

Examples of chromophores:

  • Benzene rings
  • Carbonyl groups (C=O)
  • Nitro groups (NO₂)

Auxochrome
An auxochrome is a functional group that, when attached to a chromophore, modifies the ability of the chromophore to absorb light, typically by shifting the absorption to longer wavelengths (bathochromic shift) or by increasing the intensity of absorption (hyperchromic effect). Auxochromes do not produce color by themselves but enhance or alter the color of the chromophore to which they are attached.

Auxochromes typically have lone pairs or are capable of donating or withdrawing electrons, thus altering the electron distribution of the chromophore.

Common auxochromes include:

  • Hydroxyl group (-OH)
  • Amino group (-NH₂)
  • Carboxyl group (-COOH)
  • Alkoxy group (-OR)

The interaction between a chromophore and an auxochrome enhances the molecule’s overall light-absorbing capacity and alters the color characteristics, which is significant in dye chemistry and biological pigment studies.