Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Physical Chemistry → Chemical kinetics ↓


Photochemical reactions


Photochemical reactions are a fascinating subset of chemical reactions that are initiated by the absorption of light energy. Unlike thermal reactions, which are caused by heat, photochemical reactions are driven by photons, or light particles, which provide the energy needed to break chemical bonds or facilitate their formation. Photochemistry, the study of these reactions, plays a vital role in fields ranging from biology and environmental science to industrial applications.

Understanding the basics

To understand how photochemical reactions work, let's take a deeper look at the process at the molecular level. When a molecule absorbs light, it usually transitions from a lower energy state, known as the ground state, to an excited state. This transition is only possible when the energy of the light matches the energy difference between these two states.

Ground state (S0) + photon (hν) → Excited state (S1)

The symbol hν represents the energy of the photon, where h is the Planck constant and ν is the frequency of light.

Principles of photochemical reactions

Light absorption

The first step in any photochemical reaction is the absorption of light. This absorption is governed by the Beer-Lambert law, which relates the absorption of light to the properties of the substance through which the light is traveling.

A = εlc

In this equation, A is the absorbance, ε is the molar absorbance, l is the path length of the sample, and c is the concentration of the absorbing species.

Excited states and reactivity

Once excited, the molecule can undergo a variety of processes, such as fluorescence, phosphorescence, or photochemical reactions. In photochemical reactions, the molecule being excited undergoes a chemical change. This change can result from bond dissociation, electron transfer, or isomerization.

For example, when a benzene molecule absorbs ultraviolet light, it changes from its stable planar structure to an excited state, where it can rearrange or react with other chemicals.

Examples of photochemical reactions

Photosynthesis

One of the most important photochemical reactions in nature is photosynthesis. In this process, plants use sunlight to convert carbon dioxide and water into glucose and oxygen. The light-dependent reactions of photosynthesis involve the absorption of light by chlorophyll, which triggers a series of redox reactions.

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Ozone formation

Another important example of photochemistry is the formation and disintegration of ozone in the Earth's stratosphere. This reaction begins with the absorption of ultraviolet (UV) light by diatomic oxygen (O₂), which splits it into two oxygen radicals.

O₂ + hν → 2 O•

These radicals then react with another O₂ molecule to form ozone (O₃).

O• + O₂ → O₃

Photodegradation of polymers

In industrial applications, photochemical reactions have practical implications, such as the degradation of polymers. Polymers exposed to sunlight can undergo photodegradation, where their chemical structure changes, weakening the material.

Mechanism of photochemical reactions

Photochemical reactions usually follow a sequence of light absorption, excited state lifetime, and chemical transformation. The mechanism can be better understood through kinetic and kinetic terms.

Jablonski diagram

A valuable tool for visualizing photochemical processes is the Jablonski diagram. It shows the possible energy levels of a molecule and the transitions between these levels. The diagram shows:

  • Excitation arises from the absorption of a photon.
  • Non-radiative transitions, such as internal conversion and inter-system crossing.
  • Radiation processes such as fluorescence and phosphorescence.

Here is a simplified representation:

S0 → S1 (excitation)  
S1 → S0 (fluorescence)  
S1 → T1 (inter-system crossing)
T1 → S0 (phosphorescence)

Factors affecting photochemical reactions

Several factors can affect the efficiency and outcome of photochemical reactions:

Wavelength of light

The wavelength of light affects which photons are absorbed by the molecule. Generally, specific wavelengths correspond to different electronic transitions in molecules, which affect the types of chemical reactions that occur.

Nature of the molecule

The structure of the molecule, including its functional groups and electronic configuration, affects its photochemical behavior. For example, conjugated systems often have a small energy gap between the ground and excited states, which facilitates reactions under visible light.

Environment

Solvents, temperature, and the presence of catalysts also play important roles in photochemical reactions. Solvents can stabilize or destabilize certain excited states, while temperature can affect the kinetics of the reaction. Catalysts can lower the energy barrier for particular photochemical processes, increasing the reaction rate.

Applications of photochemical reactions

Photochemical reactions have numerous applications in various fields:

Synthetic chemistry

In synthetic chemistry, photochemical reactions are used to create specific compounds that are challenging to make via thermochemical methods. For example, photochemical reactions can facilitate isomerization or the creation of strained ring compounds.

Environmental science

Photochemical reactions significantly impact environmental science, particularly in the study of atmospheric chemistry and the degradation of pollutants. Understanding these reactions helps scientists develop strategies to manage air quality and combat climate change.

Medicine and technology

Photodynamic therapy, a treatment for cancer, involves the use of light-sensitive compounds that produce reactive oxygen species upon light absorption, selectively killing cancer cells. Photochemical technologies also include solar energy conversion and light-emitting devices such as OLEDs.

Challenges in photochemistry

One of the main challenges in photochemistry is controlling reaction selectivity and efficiency. Many photochemical reactions lead to multiple products or unwanted side reactions. Researchers continually work to design photoreaction systems with improved specificity.

Future prospects

The future of photochemical reactions looks promising, especially with advances in materials science and nanotechnology. For example, designing nanostructures that efficiently use sunlight for chemical reactions could lead to more sustainable energy solutions and innovative photochemical processes.

Conclusion

Photochemical reactions are the cornerstone of both natural and artificial processes, enabling countless scientific and technological advancements. By understanding the principles and applications of photochemistry, scientists can use these reactions for beneficial purposes, from sustaining life through photosynthesis to pioneering new technologies in renewable energy and medicine.


Graduate → 1.3.6


U
username
0%
completed in Graduate

← Prev (1.3.5)
Chain Reactions and Polymerization
Next (1.4) →
Statistical mechanics

Comments 



Photochemical reactions

https://www.chemistryelite.com/g/1.3.6?ceal=en