Chemiluminescence

During a chemical reaction, the emission of light (of any wavelength) is normal. However, if the reaction may also give off a sufficient quantity of heat. We say this type of reaction is chemiluminescence.

The word “chemiluminescence” has two words, i.e.,  chemi and luminescence. Here, chemo means chemical reaction and luminescence means something that gives off light.

So, chemiluminescence is also known as chemiluminescence. This is the process of emission of light as a result of the chemical reaction. 

Therefore, given reactants A and B, with an exciting intermediate ◊, we have:

                                     [A] + [B] → [◊] → [Products] + light

We will understand what is chemiluminescence with illustrative chemiluminescence examples in detail.

Chemiluminescence Definition

We define chemiluminescence as the light emission as a result of the chemical reaction. 

During the product formation, light isn't necessarily the only form of energy released by a chemiluminescent reaction. Besides, heat may also release, making the reaction exothermic.

How Chemiluminescence Works?

During a chemical reaction, the reactant atoms/molecules/ions collide with each other. These particles interact with each other to form what we call a transition state. 

From the transition state, product formation occurs. 

The transition state is the state at which enthalpy stays maximum. However, the products usually have less energy than the reactants. 

Further, a chemical reaction occurs when there is an increase in the stability/decrease in the energy of the molecules. 

Thus chemical reactions that release energy as heat, the vibrational state of the product remains excited. The energy disperses through the product, makes it warmer, in short, the release of heat. The same happens in chemiluminescence. 

Now, we will understand what chemiluminescence is.

What is Chemiluminescence?

The process of chemiluminescence is the same we discussed in the “how chemiluminescence works,” section.

In this process, electrons become excited. The excited state is the transition/intermediate state. 

When excited electrons come back to the ground state, the energy is released in the form of chunks of energy called photons. 

The decay to the ground state can occur through a quick release of light, like fluorescence or a forbidden transition (likewise phosphorescence).

In theoretical terms, each molecule that participates in a reaction releases one photon of light. 

In reality, the production is much lower.  For instance, non-enzymatic reactions possess around 1% quantum efficiency. 

On adding a catalyst, a great increase in the brightness or luminescence can be seen in many reactions.

Chemiluminescence Examples

Example 1:

An H2 (hydrogen) atom in its ground state has a single electron. A single electron is in a shell, i.e.,  n = 1. Since each shell has its own energy level.

When the hydrogen atom absorbs a quantum (quantized) amount of energy, it reaches a higher energy level (shell n = 2).

When hydrogen reaches an excited state or a high-energy state. We make an asterisk (*) aside the molecule to indicate this.

The electron retraces to its original position, i.e., the ground state (shell n = 1). 

In the process, a packet of energy (a photon) releases in the form of electromagnetic radiation. 

The wavelength of the light emitted depends on the amount of energy. 

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Furthermore, if the wavelength is within the range of visible light, the electron transition is perceived as the light of a particular colour. Therefore, the wavelength of light determines the colour.

Example 2:

A reaction between hydrogen (H2) and oxygen (O2) to form water (H2O) is an example of a chemical change. In this reaction, the H-H bond in H2and the O-O bonds in O2 break.

Here, new H-O bonds form to make H2O. 

For the most part, when chemicals undergo a change in this manner, they either give off heat (exothermic) or absorb heat (endothermic). Hence H2 plus O2 reaction is exothermic.

We say that a few very interesting kinds of chemical reactions occur in which the energy produced is given off not as heat but as light. 

These reactions are what we term chemiluminescent. In living organisms, we call it bioluminescent.

Examples of Chemiluminescence

C8H7N3O2 (luminol) when reacts with  H2O2 (hydrogen peroxide)  → gives → 3-APA (vibronic excited state) → 3-APA (decays to a lower energy level) + light (release)

Here, 3-APA is 3-Aminopthalalate.

Point To Note:

No difference is there in the chemical formula of the transition state. Wherefore, only the energy level of the electrons. 

This happens because iron is one of the metal ions that catalyzes the reaction. Chemists use the luminol reaction to detect blood. 

Iron from hemoglobin results in the chemical mixture glows brightly.

Another good example of chemiluminescence is the reaction occurring in glow sticks. 

The color of the glow stick occurs from a fluorescent dye called a fluorophore, which absorbs the light from chemiluminescence and releases it as another color.

Point To Note:

Chemiluminescence not only occurs in liquids; however, in gases as well. 

For instance, a gas-phase reaction between vaporized phosphorus and oxygen results in the green glow of white phosphorus in the damp air.