This week, we build on the concept of molecular energy levels and the ground and excited states of molecules.

Fireflies

Firefly abdomens contain a chemical called luciferin, the structure of which is below:

There are many different types of luciferin. The structure above is sometimes called firefly luciferin to distinguish it from luciferins found in other organisms.

Luciferin reacts with adenosine triphosphase (ATP) - a chemical all cells use for energy - including ours. The reaction occurs in the presence Mg²⁺ ions and an enzyme known as luciferase. Luciferase acts as a catalyst. The reacting species bind to the enzyme, where the reaction takes place. This produces a species known as adenylluciferin (also called luciferyl adenylate) and pyrophosphate.

Adenylluciferin combines with oxygen to form oxyluciferin and adenosine monophosphate (AMP).

Oxyluciferin, shown below, is produced in an excited state.

That is, the electrons in oxyluciferin are not all in their lowest possible energy levels. When this molecule returns to the ground state, energy is released in the form of light. This is in the visible range due to the highly conjugated structure of oxyluciferin.

Some helpfull websites are below:

• The Firefly Files from the Museum of Biological Diversity at Ohio State University.
• The Fireflier Companion and Newsletter by Professor James Lloyd at the University of Florida.
• How do fireflies light up? from HowStuffWorks.com.

More on Electronic States

This figure shows three possible excited electronic states, though we could imagine many more. As the electron energy contributes to the total energy, we would expect these three states to have different total energies, all of which would be greater than the energy of the ground state. We can show this in an energy state diagram (below) in which each line represents one of the four states.

However, the arrangement of the electrons is not the only thing contributing to the molecules energy. Another contribution comes from molecular vibrations. Bonds in molecules behave like a spring connecting two masses. Molecules are always vibrating. During the course of a vibration, bonds are compressed and stretched around a midpoint. A vibration in a molecule only takes on the order of 10^-13-10^-10 seconds!

What this leads to is a series of possible vibrational energies that the molecule can possess. We can think of these as additional energy levels within a particular electronic state. On an energy diagram, these might look like this:

The two electronic levels are far apart in energy, the lowest is the ground electronic state, and the higher level is an excited state. Within each electronic level, there are closely spaced vibrational levels. At room temperature, most molecules are in the lowest of these vibrational levels in the electronic ground state.

Fluorescence

This excited state has a limited lifetime. The molecule will quickly lose some of its vibrational energy through "radiationless decay". This is a fancy way of saying it loses energy without involving light in the process. This excess vibrational energy is usually lost rapidly through collisions with other molecules.

Once the molecule reaches the lowest vibrational level, the energy required to drop down to the ground electronic state is too great to achieve by collisions with other molecules. Instead, the molecule releases this energy in the form of light. This is called fluorescence. This happens very quickly after the initial absorption - on the order of 10^-5-10^-9 seconds. That is 1/100,000th to 1/1,000,000,000th of a second! Fast, in other words. Very fast.

Because the absorption step can involve transitions to different vibrational levels, there is a range of possible wavelengths that can be absorbed. The same is true for fluorescent emission. Emission can occur to a number of possible vibrational levels in the ground state. Thus, fluorescent emission occurs over a range of possible wavelengths.

The resulting absorption and emission spectra may look something like this:

Note that the wavelength of the fluoresced light is longer than the wavelength of the absorbed light. Thus, a photon of fluoresced light has less energy than the photon that was absorbed. This rest of the energy is lost in those collision with other molecules.

This is how fluorescent dyes work. Why do some of the fluorescent dyes seem so bright? In many cases, the higher energy light that is absorbed is ultraviolet light, which we cannot see. Because fluorescence occurs at longer wavelengths, this light can fall in the visible region, making the dye or dyed article seem exceptionally bright.

Like the other dyes we have seen, fluorescent dyes usually are highly conjugated organic molecules.

Phosphorescence

The unique thing here is that the characteristics of this other excited state are such that transitions to the ground electronic state are very infrequent. Thus, the molecule sort of "hangs out" in this excited state, rather that immediately emitting a photon, as is the case with fluorescence.

Eventually, the molecule will emit a photon. This process is called phosphorescence.

Glow-in-the-dark products make use of phosphorescence. When exposed to light, the molecules in these products store energy in their excited state. This stored energy is then emitted over a period of minutes or hours. Eventually, a glow-in-the-dark substance will emit all of its stored energy and will no longer glow. This can be remedied by further exposure to light.