A photochemical reaction is a chemical reaction that is initiated by the absorption of light energy, usually in the form of ultraviolet UV or visible light. In organic compounds, photochemical reactions occur when a molecule absorbs a photon, which leads to the promotion of an electron from a lower energy level ground state to a higher energy level excited state . This change in the electronic state of the molecule can cause a rearrangement of its chemical bonds, leading to the formation of new products.The mechanism of photochemical reactions in organic compounds typically involves three main steps:1. Absorption of light: The organic molecule absorbs a photon, which excites an electron from the ground state to an excited state. This process is called electronic excitation.2. Excited state chemistry: In the excited state, the molecule undergoes various chemical transformations, such as bond cleavage, bond formation, or rearrangement of atoms. These transformations can lead to the formation of reactive intermediates, such as radicals, ions, or excited states of other molecules.3. Product formation: The reactive intermediates formed in the excited state can react with other molecules or undergo further transformations to form the final products of the photochemical reaction.Photochemical reactions differ from thermal reactions in several ways:1. Energy source: Photochemical reactions are initiated by the absorption of light energy, while thermal reactions are driven by heat energy.2. Reaction rate: Photochemical reactions can occur at much faster rates than thermal reactions because the energy required for the reaction is directly provided by the absorbed photon.3. Selectivity: Photochemical reactions can be more selective than thermal reactions because specific wavelengths of light can be used to selectively excite certain molecules or functional groups, leading to specific reaction pathways.Examples of photochemical reactions in organic compounds include:1. Photoisomerization: The conversion of cis-trans isomers, such as the cis-trans isomerization of azobenzene, which has applications in molecular switches and data storage.2. Photodissociation: The cleavage of a chemical bond upon absorption of light, such as the photodissociation of halogenated organic compounds, which can generate reactive radicals that participate in ozone depletion.3. Photosensitization: A process in which a photosensitizer molecule absorbs light and transfers the energy to another molecule, leading to a chemical reaction. An example is the use of photosensitizers in photodynamic therapy for cancer treatment.Significance of photochemical reactions in various applications:1. Medicine: Photochemical reactions play a crucial role in photodynamic therapy, where a photosensitizer is used to generate reactive oxygen species upon light irradiation, leading to the destruction of cancer cells.2. Materials science: Photochemical reactions are used in the synthesis of new materials, such as light-responsive polymers and molecular switches, which have applications in smart materials and data storage.3. Environmental science: Photochemical reactions are involved in the degradation of pollutants in the atmosphere, such as the photodissociation of halogenated organic compounds that contribute to ozone depletion. Additionally, photochemical reactions play a role in the conversion of solar energy to chemical energy in natural processes like photosynthesis.