chromophore

That part of a molecule giving rise to colour. Most chromophores in dyestuffs involve double bonds, especially conjugated ones. These lower the energy of radiation absorbed, so that visible radiation as well as ultraviolet radiation is absorbed by the compound.

A chromophore is part (or moiety) of a molecule responsible for its color.

When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum.

In biological molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change of the molecule when hit by light.

Chromophores almost always arise in one of two forms: conjugated pi systems and metal complexes.

In the former, the energy levels that the electrons jump between are extended pi orbitals created by a series of alternating single and double bonds, often in aromatic systems.

The metal complex chromophores arise from the splitting of d-orbitals by binding of a transition metal to ligands.

A common motif in biochemistry is chromophores consisting of four pyrrole rings.