Absorbing Light with Organic Molecules

Representations of Organic Molecules

In 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.

  • A line in a structure will represent a carbon-carbon bond unless it is labeled with another atom.

  • All carbon atoms will make 4 bonds (some combination of single, double, and triple bonds). Any bonds not specified will be to hydrogen atoms.

For information on names of organic molecules, click here.

Chlorophyll and Other Light-Absorbing Molecules

Chlorophyll 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.

chlorophyll, green

hemoglobin, red

melanins, yellow and brown

beta-carotene, orange

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.

The lowest energy light (in the red region) that chlorophyll absorbs corresponds to the energy gap between the highest filled pi orbital and the lowest empty pi orbital.

The next band absorbed by the chlorophyll molecule (in the blue region) corresponds to light with the same energy as the gap between the highest filled pi orbital and the second lowest empty pi orbital.

Chlorophyll can absorb higher energy light, too, but that light is in the UV.

There are chlorophyll molecules with slightly different structures that absorb light that has slightly lower and slightly higher bands. Taken together, the chlorophyll molecules do a good job of absorbing all the red and blue light, leaving only the middle region of the spectrum to be reflected to our eyes.

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.

Professor Patricia Shapley, University of Illinois, 2012