chromophore
/ KROH-moh-for /
Why is a carrot orange and a leaf green? The colour does not come from the whole molecule but from a special part of it that happens to soak up certain colours of light. That light-grabbing part is called a chromophore — literally the 'colour bearer'.
A chromophore is a group of atoms within a molecule, typically containing double bonds or alternating single-double bonds, whose electrons absorb visible or ultraviolet light. It is this group that is responsible for the molecule's absorption band, and therefore for the colour we see or the UV signal an instrument reads.
Chromophores explain why UV-Vis spectroscopy works at all: a substance is only visible to the technique if it carries one. Colourless compounds that lack a useful chromophore are often reacted with a reagent to attach one — a step called derivatization — before they can be measured.
The long chain of alternating double bonds in beta-carotene is the chromophore that absorbs blue-green light, leaving carrots their familiar orange colour.
The chromophore, not the whole molecule, decides the colour.
Longer chains of conjugated double bonds absorb at longer wavelengths, which is why extending a chromophore shifts a compound's colour from yellow toward red.