Conjugated Systems

We now know that pigments absorb certain wavelengths of light and reflect others, but why are some molecules colorful and others are not?

Most organic molecules only absorb high energy UV light because the energy gap between the molecular orbitals is quite large.

In pigments, the energy gap is small enough for visible light to be absorbed. Pigment molecules often contain vast conjugated systems. These systems of connected molecular orbitals with delocalized electrons lower the overall energy of the molecules and decrease the energy gaps between molecular orbitals. Conjugated systems are identified by the presence of alternating single and multiple bonds and lone pairs of electrons (i.e. in oxygen or nitrogen).

This image shows a very simple conjugated system. The alternating double bonds can be drawn in either position. In reality, the electrons are delocalized and are spread all over the molecule. The colorful heat map pictured on the right symbolizes the distribution of the electrons over the molecule. Red refers to the maximum electron density.