Absorbing Light with Organic Molecules
Representations of Organic MoleculesIn this unit, we'll be concerned with many complex organic molecules. Organic molecules consist mainly of carbon and hydrogen, but can also contain other elements. Because the molecules contain many atoms, we need a way to represent the structures without having to draw all the atoms.
For information on names of organic molecules, click here.
Chlorophyll and Other Light-Absorbing MoleculesChlorophyll is the pigment in green plants. You should remember from the discussion on color and absorption spectroscopy that this pigment is green because it reflects green light and absorbs light in the red and blue regions of the spectrum.
What is it about this molecule that causes it to absorb visible light? Let's look at the structure of chlorophyll and several other light-absorbing molecules.
melanins, yellow and brown
These are very different molecules but they have something in common. What is the common structural motif? Each has conjugated multiple bonds, that is alternating single bonds and double bonds. There is a long string of connected atoms (mostly carbon atoms) that, according to the hybrid orbital bonding scheme, make sigma bonds with sp2 hybrid orbitals.
Each one of these atoms has one remaining p orbital and 1 electron. These p orbitals combine over the whole molecule. When they combine, there is 1 pi orbital formed for every atomic p orbital. As the number of p orbitals increases, the spacing between the orbitals decreases.
As you can see in the figure above, as the number of p orbitals that combine to form pi bonding and pi antibonding orbitals increases, the spacing between them decreases. Chlorophyll has 24 p orbitals in conjugation from 23 carbon atoms and 1 oxygen atom. Hemoglobin, the red pigment in blood, has 26 p orbitals in conjugation. Beta-carotene, the colored molecule in carrots, has 22 p orbitals in conjugation. One of the melanin molecules, the pigment in human skin, has 32 p orbitals in conjugation.
When chlorophyll absorbs light an electron is promoted to a high energy state. The resulting excited state molecule can transfer its energy and its electron to other molecules.