Photochemistry: Principles – Reactions

Photochemistry is the branch of chemistry concerned with the chemical reactions caused by the absorption of light, mainly ultraviolet (100 to 400 nm), visible light (400–750 nm), or infrared (750–2500 nm).

In particular, it focuses on the phenomena of absorption, excitation, and emission of photons by atoms, ions, and molecules. The altered rates and mechanisms of reactions resulting from exposure of reactants to different types of radiations can be regarded as applications or effects of photochemistry.

Principles of Photochemistry

Grothuss-Draper Law

This fundamental law dictates that for a chemical reaction to occur, a molecule must absorb light.

Stark-Einstein Law

This law illuminates the relationship between photons and activated molecules, stating that each photon of light absorbed by a chemical system activates only one molecule for a photochemical reaction. The energy absorbed by one mole of reacting molecules is defined as one Einstein, denoted as;

E = Nhν

where,

 

• N is Avogadro’s number
• h is Planck’s constant
• ν is the frequency of radiation (c/λ)

Quantum yield

Quantum yield is the measure of the efficiency of a given photochemical process. It is defined as the number of molecules reacting per quantum energy absorbed or the number of molecules produced per quantum energy absorbed.

Since many photochemical reactions are complex and may compete with unproductive energy loss, the quantum yield is usually specified for a particular event.

Photochemical Reactions

Photo-addition

Bimolecular photochemical process in which a single product is formed by electronically excited molecules.

Photo-substitution

A substitution reaction is performed in the presence of sunlight.

Photo-dissociation

Also called photodissociation or photo fragmentation, is a type of chemical reaction in which photons interact and break down a single molecule.

Photo-induced rearrangements, and isomerization

Known as photoisomerization, is a cis-trans isomerization induced by photoexcitation. Usually, it involves the isomerization of organic molecules with a double bond in their structure.

Photo-redox reactions 

These reactions feature single electron transfers and find extensive use in organic synthesis.

Example of photochemical reaction

1. Photosynthesis

In plants, photochemical reaction occurs by the chlorophyll pigments that take energy (hν) from the sun and water (H₂O), to convert carbon dioxide (CO₂) into glucose (C₆H₁₂O₆) and oxygen (O₂).

2. Formation of vitamin D by exposure of skin to sunlight

When UV light is absorbed by the skin, it can initiate the synthesis of vitamin D from cholesterol.

3. Ozone formation

In the upper atmosphere, the formation of ozone results from the action of sunlight on oxygen.

Application of photochemical reactions

Photochemistry finds its applications in numerous practical fields, including:

1. Synthesis of vitamins, drugs, and fragrances
2. Production of hydrogen fuel
3. Food processing
4. Detection and identification of pollutants in water
5. Drug Delivery
6. Art Conservation
7. Photolithography
8. Development of new (photoluminescent and photorefractive) materials
9. Photovoltaics
10. Photodynamic Therapy (PDT)

Concepts Berg

What are the main properties of a good photocatalyst?

The most important properties of a good photocatalyst are:

• The ability to absorb radiation from a wide spectral range of light.
• Must be photochemically, thermodynamically, and electrochemically stable.

What is the name of the laws of photochemistry?

The two laws in photochemistry are; the Grothuss-Draper Law and the Stark-Einstein Law.

What are the types of photochemical reactions?

The types of photochemical reactions are:

• Photo-dissociation
• Photo-addition
• Photo-induced rearrangements
• Photo-substitution.
• photo-redox reaction

What is the difference between photochemistry and spectroscopy?

Spectroscopy is the study of the interaction of substances with electromagnetic radiation mainly (radio waves to gamma waves).

Photochemistry is the study of the chemical reactions that take place during the interaction of molecules with electromagnetic radiation.

Reference links

• Photochemistry (www.chemistryleraner.com)
• Law of photochemistry (www.kau.edu.sa)
• Photosensitization (www.britannica.com)
• Application of photochemical reaction (www.themasterchemistry.com)
• Photochemical reaction as a key step in organic synthesis; [Norbert Hofmann, Laboratoire des Réactions Sélectives et Applications, UMR 6519 CNRS et Université de Reims Champagne-Ardenne], (doi.org/10.1021/cr0680336)