As we shall see in Chapter 23, the carotenes are assistants to
chlorophyll in photosynthesis. They absorb light of wavelengths
that chlorophyll does not, and transfer the energy in the form of
excited electronic states to chlorophyll molecules.
They are called antenna molecules, and the different numbers of
electrons in their delocalized systems represent a "tuning"
of the antenna.
Conjugated double bonds and delocalization also are found in cyclic
molecules, of which benzene is a familiar example. Naphthalene has
two fused six-member rings (right), anthracene and phenanthrene
are structural isomers with different arrangements of three rings,
coronene has seven rings, and the limit is one sheet of graphite,
which is an infinite array of carbon hexagons (click no pencil for
In all of these molecules, the doublebond electrons are delocalized
over the entire set of carbon atoms (over the entire sheet for graphite).
Delocalization sometimes is represented by dotted circles within
the rings, as shown right for benzene and naphthalene.
The Kekule'-like structures with localized double bonds are easier
to draw, however, and cause no confusion as long as one remembers
that the double-bond electrons really are delocalized.