Chemiluminescence: Definition and Examples

Chemiluminescence occurs when chemical reactions release energy in the form of light.
Chemiluminescence occurs when chemical reactions release energy in the form of light. Charles O'Rear / Getty Images

What Is Chemiluminescence?

Chemiluminescence is defined as light emitted as the result of a chemical reaction. It's also known, less commonly, as chemoluminescence. Light isn't necessarily the only form of energy released by a chemiluminescent reaction. Heat may also be produced, making the reaction exothermic.

How Chemiluminescence Works

In any chemical reaction, the reactant atoms, molecules, or ions collide with each other, interacting to form what is called a transition state. From the transition state, the products are formed. The transition state is where enthalpy is at its maximum, with the products generally having less energy than the reactants. In other words, a chemical reaction occurs because it increases the stability/decreases the energy of the molecules. In chemical reactions that release energy as heat, the vibrational state of the product is excited. The energy disperses through the product, making it warmer. A similar process occurs in chemiluminescence, except it's the electrons that become excited. The excited state is the transition state or intermediate state. When excited electrons return to the ground state, the energy is released as a photon. The decay to the ground state can occur through an allowed transition (quick release of light, like fluorescence) or a forbidden transition (more like phosphorescence).

Theoretically, each molecule participating in a reaction releases one photon of light. In reality, the yield is much lower. Non-enzymatic reactions have about 1% quantum efficiency. Adding a catalyst can greatly increase the brightness of many reactions.

How Chemiluminescence Differs From Other Luminescence

In chemiluminescence, the energy that leads to electronic excitation comes from a chemical reaction. In fluorescence or phosphorescence, the energy comes from outside, like from an energetic light source (e.g., a black light).

Some sources define a photochemical reaction as any chemical reaction associated with light. Under this definition, chemiluminescence is a form of photochemistry. However, the strict definition is that a photochemical reaction is a chemical reaction that requires absorption of light to proceed. Some photochemical reactions are luminescent, as lower frequency light is released.

Examples of Chemiluminescent Reactions

Glowsticks are an excellent example of chemiluminescence.
Glowsticks are an excellent example of chemiluminescence. James McQuillan / Getty Images

The luminol reaction is a classic chemistry demonstration of chemiluminescence. In this reaction, luminol reacts with hydrogen peroxide to release blue light. The amount of light released by the reaction is low unless a small amount of suitable catalyst is added. Typically, the catalyst is a small amount of iron or copper.

The reaction is:

C8H7N3O2 (luminol) + H2O2 (hydrogen peroxide) → 3-APA (vibronic excited state) → 3-APA (decayed to a lower energy level) + light

Where 3-APA is 3-aminopthalalate

Note there is no difference in the chemical formula of the transition state, only the energy level of the electrons. Because iron is one of the metal ions that catalyzes the reaction, the luminol reaction can be used to detect blood. Iron from hemoglobin causes the chemical mixture to glow brightly.

Another good example of chemical luminescence is the reaction that occurs in glow sticks. The color of the glow stick results from a fluorescent dye (a fluorophor), which absorbs the light from chemiluminescence and releases it as another color.

Chemiluminescence doesn't only occur in liquids. For example, the green glow of white phosphorus in damp air is a gas-phase reaction between vaporized phosphorus and oxygen.

Factors That Affect Chemiluminescence

Chemiluminescence is affected by the same factors that affect other chemical reactions. Increasing the temperature of the reaction speeds it up, causing it to release more light. However, the light doesn't last as long. The effect can be easily seen using glow sticks. Placing a glow stick in hot water makes it glow more brightly. If a glow stick is placed in a freezer, its glow weakens but last much longer.

Bioluminescence

Decaying fish are bioluminescent.
Decaying fish are bioluminescent. Paul Taylor / Getty Images

Bioluminescence is a form of chemiluminescence that occurs in living organisms, such as fireflies, some fungi, many marine animals, and some bacterial. It does not naturally occur in plants, unless they are associated with bioluminescent bacteria. Many animals glow because of a symbiotic relationship with Vibrio bacteria.

Most bioluminescence is a result of a chemical reaction between the enzyme luciferase and the luminescent pigment luciferin. Other proteins (e.g., aequorin) may assist the reaction, and cofactors (e.g., calcium or magnesium ions) may be present. The reaction often requires energy input, usually from adenosine triphosphate (ATP). While there is little difference between luciferins from different species, the luciferase enzyme varies dramatically between phyla.

Green and blue bioluminescence is most common, although there are species that emit a red glow.

Organisms use bioluminescent reactions for a variety of purposes, including prey luring, warning, mate attraction, camouflage, and illuminating their environment.

Interesting Bioluminescence Fact

Rotting meat and fish is bioluminescent just prior to putrefaction. It isn't the meat itself that glows, but bioluminescent bacteria. Coal miners in Europe and Britain would use dried fish skins for weak illumination. Although the skins smelled horrible, they were much safer to use than candles, which could spark explosions. Although most modern people are unaware dead flesh glows, it was mentioned by Aristotle and was a well-known fact in earlier times. In case you're curious (but aren't up for experimentation), rotting meat glows green.

Reference

Smiles, Samuel (1862). Lives of the Engineers. Volume III (George and Robert Stephenson). London: John Murray. p. 107.