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
Test Your Knowledge
1. Which statement is correct regarding the mechanism of chemiluminescence?
A. Chemiluminescence requires absorption of light to excite the product.
B. Light is emitted when an excited intermediate returns to its ground state.
C. The reaction temperature must be above 100°C for photon emission.
D. All chemiluminescent reactions are catalyzed by luciferase.
View Answer
Light is emitted when an excited intermediate (D*) returns to its ground state (D), releasing energy as photons during a chemiluminescent reaction.
2. What is the role of luciferase in bioluminescent chemiluminescence?
A. It serves as a light receptor.
B. It acts as a catalyst, accelerating the chemical reaction.
C. It absorbs photons from the environment.
D. It only participates in non-living chemical reactions.
View Answer
Luciferase acts as an enzyme catalyst, accelerating the bioluminescent chemical reaction in living organisms.
3. Which description best defines the quantum efficiency of a chemiluminescent reaction?
A. The fraction of energy released as heat.
B. The number of photons emitted per molecule of reactant consumed.
C. The amount of product formed per second.
D. The absorption coefficient of the reactant.
View Answer
Quantum efficiency is defined as the number of photons emitted per molecule of reactant (A or B) consumed in the chemiluminescent reaction.
4. Which chemical is responsible for the glow in fireflies?
A. Luminol
B. Luciferin
C. Hydrogen peroxide
D. Phthalhydrazide
View Answer
The glow in fireflies is due to the aerial oxidation of luciferin (a protein) in the presence of luciferase enzyme.
5. How does chemiluminescence differ from photochemical reactions?
A. Chemiluminescence absorbs light to produce excited states.
B. Photochemical reactions are always exothermic.
C. Chemiluminescence emits light as excited intermediates relax, while photochemical reactions absorb light to achieve excitation.
D. Both processes require the presence of enzymes.
View Answer
Chemiluminescence emits light as excited intermediates relax, whereas photochemical reactions proceed by the absorption of light to achieve an excited state.