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 structure).

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.