chromophore

Why is a carrot orange, a leaf green, or a dye brilliant red? Inside each of these is a chromophore — the particular part of a molecule that absorbs visible light and so gives the substance its colour. The word literally means 'colour-bearer.' Most chromophores in organic chemistry are extended conjugated pi systems: long runs of alternating single and double bonds.

The connection between conjugation and colour comes from molecular orbitals. When pi electrons absorb light, they jump from the highest filled orbital (the HOMO) to the lowest empty one (the LUMO); the colour absorbed corresponds to the energy of that gap. A single double bond has a large HOMO-LUMO gap, so it absorbs only high-energy ultraviolet light and looks colourless. But the more you conjugate — the longer the chain of alternating bonds — the more the orbitals spread and split, and the smaller the gap becomes. Conjugate enough, and the gap shrinks until the molecule starts absorbing visible light; whatever wavelengths it absorbs, we see the leftover complementary colour.

Chromophores are why UV-Vis spectroscopy is so useful: the wavelength a compound absorbs is a direct readout of how extended its conjugation is, so it tells you about the pi system without ever isolating it. The same principle runs through biology and industry — beta-carotene's eleven conjugated double bonds make it orange, retinal in your eye changes shape when its chromophore absorbs a photon (the first step of vision), and synthetic dyes are engineered chromophores tuned to absorb exactly the right colours.