chromophore

A chromophore is the part of a molecule responsible for its color, by absorbing visible or other wavelengths of light. It contains a conjugated system of pi electrons or non-bonding electrons capable of electronic transitions between molecular orbitals, such as pi to pi* or n to pi* transitions.

Absorbed light is removed from the spectrum, and the remaining transmitted or reflected light determines the observed color. The specific wavelengths absorbed depend on the extent of conjugation, the presence of heteroatoms, and the local environment. Substituents called auxochromes (such as amino or hydroxyl groups) can attach to chromophores and shift absorption toward longer wavelengths (bathochromic shift) and increase molar absorptivity.

Common chromophores include azo groups (−N=N−), carbonyl-containing systems (C=O in conjugation), polyenes, and heterocycles such as porphyrins and quinones. Dye molecules and natural pigments derive their color from chromophores, and many chromophores extend absorption into the visible region to produce vivid colors.

In biology, chromophores are components of visual pigments and photosynthetic molecules. The porphyrin ring in chlorophyll and in heme, and retinal in opsins, are chromophores that absorb light to trigger biological processes such as vision and photosynthesis.

Chromophores are central to spectroscopy, as their absorption spectra reveal their presence and concentration. They also inform the design of dyes, pigments, indicators, and organic electronic materials, and underpin understanding of color in chemistry and biology.