Chemistry Reference
In-Depth Information
structure contains alternating multiple and single bonds. The longer the run
of conjugated double or triple bonds in the molecule, the more easily the
molecule will absorb light. Aromatic compounds, which contain a benzene
ring, will absorb ultraviolet light of wavelength 254 nm and this property is
exploited in many spectroscopic analyses and in detectors for chromato-
graphic systems. If the chromophore is more extensive, then the molecule
will be excitable by light of lower energy, until, if the chromophore is very
large, visible light will have sufficient energy to excite the electrons of the
chromophore and the compound will absorb visible light.
O
O
Anthraquinone
Benzene
O
CH
3
HO
NN
N
S
CH
3
O
Methyl orange
Figure 7.4
Examples of chromophores.
A molecule of this type, which absorbs light in the visible part of the
electromagnetic spectrum, is said to be
coloured
because our eyes will
detect the light reflected back from the compound, which will be the
complementary colour to the light absorbed. White light, remember, is
made up of all the colours of the rainbow, and can be split into its
constituent colours by a prism or droplet of water. For example, if a dye
molecule absorbs light of red, orange and yellow wavelengths, our eyes will
detect the reflected blue, green and purple light and we will see the material
as coloured blue. Similarly, a red dye will absorb the short-wavelength blue
light and reflect the reds and oranges back to our eyes. This property is
utilised in the use of indicators for titrations (see Chapter 6), where the
absorption spectrum (and hence the colour) of the indicator changes with
the pH of the solution.
