Chromophore
Definition and meaning of Chromophore in chemistry.
A chromophore is the part of a molecule responsible for absorbing visible or ultraviolet light, giving the compound its characteristic color. It is typically a system of conjugated (alternating) double bonds or a group containing nonbonding electrons.
In more detail
Absorption occurs when photons of the right energy promote electrons between molecular orbitals, most commonly a π-to-π* transition within the conjugated system, or an n-to-π* transition involving a lone pair on a heteroatom. Extending the conjugation adds more overlapping p-orbitals, narrowing the HOMO-LUMO energy gap so the molecule absorbs longer wavelengths, shifting color toward the red end of the spectrum (a bathochromic shift). Substituents called auxochromes, such as -OH or -NH2, attach to a chromophore and further modify the absorbed wavelength and intensity without themselves absorbing light.
Key facts
| Field | Organic Chemistry |
|---|---|
| Typical structure | Conjugated pi-electron system |
| Common transitions | pi to pi*, n to pi* |
| Related term | Auxochrome (color-modifying substituent) |
Beta-carotene, the pigment in carrots, contains a long polyene chain of 11 conjugated carbon-carbon double bonds; this extended chromophore absorbs strongly in the blue region (around 450 nm), so the molecule appears orange.
Frequently asked questions
What is the difference between a chromophore and an auxochrome?
A chromophore is the light-absorbing unit itself, usually a conjugated pi system; an auxochrome is an attached functional group, such as -OH or -NH2, that does not absorb light on its own but shifts or intensifies the chromophore's absorption.
Why does more conjugation change a compound's color?
Adding conjugated double bonds increases orbital overlap and lowers the energy gap between the highest occupied and lowest unoccupied molecular orbitals, so the compound absorbs lower-energy, longer-wavelength light.