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. The metal complex
chromophores arise from the splitting of d-orbitals by binding of a transition metal to ligands.
Examples of such chromophores can be seen in chlorophyll (used by plants for photosynthesis),
hemoglobin, hemocyanin, and colorful minerals such as malachite and amethyst.
(i) The color of the organic compounds is due to the presence of certain multiple bonded groups
called chromophores. Important chromophores are,
O
||
. .
..
..
–N = N –, – N —> O, –N = N –, C = O, C = S
Azo
Nitro
Carbonyl Thio carbonyl
[Chromophore-Greek word, Chroma = color, Phorein = to bear].
The presence of chromophore is not necessarily sufficient for color. To make a substance
colored, the chromophore has to be conjugated with an extensive system of alternate single and
double bonds as exists in aromatic compounds.
The chromophore part of the colored substance (dye) absorbs some wavelengths from white light
and reflects back the complementary color. A colored compound having a chromophore is
known aschromogen.
(ii) Certain groups, while not producing color themselves, when present along with a
chromophore in an organic substance, intensify the color. Such color assisting groups are
called auxochromes(Greek word, Auxanien = to increase; Chrome = color), i.e. they make the
color deep and fast and fix the dye to the fabric. The auxochromes are acidic or basic functional
groups. The important auxochromes are,
Acidic : –OH,
–SO3H,
–COOH
Hydroxy Sulphonic Carboxylic
Basic :
–NH2,
–NHR,
Amino
–NHR2
Alkylamino Dialyliamino
Example :
HSO3 > N = N > N(CH3)2