The structures of dye molecules are complex in
comparison with those of most common organic compounds. Compare the molecular formula
of acetic acid, C2H4O2, with that of even a simple dye such as Orange II, C16H11N2SO4Na. Despite their
complexity, dye structures have a number of common features. Most dye molecules contain a number of
aromatic rings, such as those of benzene or naphthalene, linked in a fully conjugated
system. This means that there is a long sequence of alternating single and
double bonds between the carbon and other atoms throughout most of the formal
written structure. This type
of arrangement is often called the chromophore or
colour-donating unit. The conjugated system allows extensive delocalisation of the electrons from the double bonds and results in smaller differences in
energy between the occupied and unoccupied molecular orbitals for these electrons.
At least five or six conjugated double bonds are required in the molecular
structure for a compound to be coloured. The wavelength of the absorbed light
able to excite an electron
from an occupied orbital to the first unoccupied
molecular orbital then corresponds to visible light.