Chemiluminescence
Chemiluminescence is the process of light emission resulting from a chemical reaction. If the reaction occurs in living organism such as butterfly, the phenomenon is called bioluminscence. Enzymes play an important role in catalyzing bioluminescent reactions, primarily through the action of a specific enzyme known as luciferase.
Chemiluminescence occurs when reactants combine to form an excited intermediate, which subsequently releases energy in the form of light (photons) as it transitions to a more stable state. Such a reaction is the reverse of a photochemical reaction which proceeds by absorption of light. The light emitted in a chemiluminescent reaction is also called cold light because it is produced at ordinary temperature. The light emitted have various wavelengths. Visible light is the most commonly observed and utilized form.
Examples:
1. The glow of fireflies due to the aerial oxidation of luciferin (a protein) in the presence of enzyme luciferase.
2. The oxidation of 5-aminophthalic cyclic hydrazide (luminol) by hydrogen peroxide in alkaline solution, producing bright green light.
Quantum efficiency of chemiluminiscent reaction
A + B → C* → D
is defined as-
Quantum efficience ΦCL = No. of photons emmited/Molecules of A or B consumed
ΦCL = Φr* Φf
ΦCL depends on the chemical efficienc Φr* of the formation of excited product molecules and on the quantum yield of emission Φf from this excited molecule.
Mechanism of Chemiluminescence
The general mechanism of chemiluminescence may be written as-
Reactants Combine: Two or more chemical species (A and B) react to form an intermediate (C).
A + B → C
Formation of Excited State: The intermediate (C) then transfor into a new compound (D) in an excited state (D∗).
C → D∗
Photon Emission: The excited compound (D∗) returns to its ground state (D), releasing energy in the form of light.
D∗ → D + h𝜈
