chromophore
/ KROH-muh-for /
Why is a carrot orange, a rose red, your blood crimson? Each owes its colour not to the whole molecule but to a particular working part inside it — a special arrangement of bonds that catches certain colours of light. That light-catching part is the chromophore. It is the engine of colour: the rest of the molecule may be large, but the colour lives in this one region.
A chromophore is the part of a molecule responsible for absorbing visible or ultraviolet light, and therefore for the substance's colour. It is usually a system of alternating double and single bonds (a conjugated system) or certain metal-containing groups, where electrons are loosely held and easily lifted to a higher energy level by a photon. The colours absorbed are removed from white light, and the colours we see are what is left over.
Chromophores matter because they connect a molecule's structure to its colour and to its UV–visible spectrum, which is the basis of dyes, pigments, sunscreens, and many colour-based chemical tests. A useful refinement is that nearby groups called auxochromes, though not coloured on their own, can shift or deepen a chromophore's absorption — which is how chemists fine-tune a dye to land on exactly the shade they want.
Beta-carotene, the pigment of carrots, has a long chain of eleven alternating double bonds. This chromophore is so extended that it absorbs in the blue and green, leaving the orange we see. Snip the chain shorter and the colour shifts; that is exactly how the body uses pieces of it to make colourless vitamin A.
Colour lives in one working part of the molecule — the chromophore.
The colour you see is the complement of what the chromophore absorbs, not the colour absorbed. A substance that soaks up blue light looks orange; one that soaks up green looks red — the eye registers what is left, not what was taken.