The Fundamentals of Photochemistry: Understanding the Basics

Overview

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• Source: Microbioz India


• Date: 19 Nov,2023

Photochemistry, a branch of chemistry that deals with the study of chemical reactions initiated or influenced by light. Understanding the Fundamentals of Photochemistry involves grasping the basic concepts and processes governing these reactions caused by light.

Here are some key fundamentals of photochemistry:

Light Absorption

Each photochemical reaction starts when photons (individual units of light) are absorbed by molecules. The photonic energies should be able to shift electrons from a ground state to an excited state in the molecule.

Electronic States:

1. Molecules have different electronic energy states including ground state (lowest energy) and excited state (higher energy).
2. When a molecule absorbs a photon, it moves from ground state to excited state.

Jablonski Diagram:

1. The Jablonski diagram is a graphical representation that illustrates energy levels as well as transitions involved in photochemistry.
2. Ground state, singlet excited states (S1, S2, etc.) and triplet excited states (T1, T2, etc.) in typical cases.

Singlet and Triplet States:

1. Singlet states have paired electrons in their molecular orbitals so they are usually more reactive in photochemical reactions.
2. Triplet states have unpaired electrons so they tend to be longer lived but typically less reactive than singlet states.

Photophysical Processes:

1. Molecules in the excited state after absorption can undergo various photophysical processes such as internal conversion, intersystem crossing and fluorescence emission.
2. Rapid relaxation of an excited molecule to a lower energy excited state or the ground state through non-radiative processes is called internal conversion.
3. Intersystem crossing involves transition from singlet to triplet or vice versa.
4. Fluorescence is simply the emission of light as a molecule comes back down from an elevated singlet level back into its ground state.

Photchemical Reactions:

1. Chemical transformations of molecules in the excited state can take place during photochemical reactions which are unlikely to occur under thermal (non-photolytic) conditions.
2. Such reactions often involve breaking and making of chemical bonds
3. Photochemical reactions include photodissociation, photoisomerization, and photoreduction.

Quantum Yield:

It is a measure of efficiency quantifying the number of molecules that undergo a desired photochemical transformation divided by the number of photons absorbed per quantum yield.

Factors Affecting Photochemical Reactions:

Factors such as wavelength and intensity of the incident light, reactant concentration, sensitizers or catalyst presence determine efficiency and outcome of photochemical reactions.

Applications of Photochemistry:

There are many applications for photochemistry in photography, photovoltaics (solar cells), environmental chemistry (e.g., degradation of pollutants), and organic synthesis (e.g., photochemical reactions in the synthesis of pharmaceuticals and natural products).

Understanding the fundamentals of photochemistry is essential for scientists and industrial chemists working across different scientific disciplines because it provides insights on how to utilize light for driving chemical reactions leading to desired products.