Photochemistry and Photochemical Reactions

Photochemistry is a branch of chemistry that deals with the effect of light on chemical reactions. Several chemical reactions are caused by the absorption of ultraviolet light (the light whose wavelength ranges from 100 to 400 nanometers), infrared light (the light whose wavelength ranges from 750 to 2500 nanometers), or visible light (light whose wavelength ranges from 400 to 750 nanometers). 

The chemical reactions that involve the absorption of energy in the form of light are commonly referred to as photochemical reactions. The first part of any photochemical process involves photoexcitation – a process through which a reactant absorbs energy from electromagnetic radiation (light) and jumps to an excited state. 

Fluorescence and Phosphorescence

When light is absorbed by an atom (or a molecule) which is in its ground state, an electron becomes excited and jumps to a higher orbital. This excitation can be from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). When the excited electron returns to its corresponding ground state, the radiative transition results in the emission of a photon. This emission of light caused by the de-excitation of electrons is commonly referred to as fluorescence. 

In some cases, the excited electron undergoes spin inversion and enters a triplet excited state in which two unpaired electrons have the same spin. When the electron in a triplet excited state undergoes de-excitation and returns to its corresponding ground state, it can emit the energy via a radiationless intersystem crossing or via the emission of electromagnetic radiation. The phenomenon in which an electron in a triplet excited state emits light while returning to its ground state is commonly referred to as phosphorescence. 

Several important organic reactions involve the π to π* transition of an electron which is induced by a photon. Examples of such reactions include:

  • The cycloaddition to different alkenes that are in the ground state (these reactions yield cyclobutane derivatives).
  • Cis-trans isomerization undergone by polyalkenes.
  • The DeMayo reaction – a photochemical reaction in which the enol of a 1,3-diketone undergoes a chemical reaction with an alkene to give a 1,5-diketone as the product. 
  • The di-pi-methane rearrangement – a photochemical reaction in which molecular entities containing two pi-systems (that are separated by a saturated carbon atom) participate to form a cyclopropane molecule with a substituted double bond.

An example of an industrially important photochemical reaction is the reaction between chlorine and toluene, both of which are present in the gaseous phase. Benzyl chloride is formed as the primary product in this reaction. The chlorine molecules absorb energy from photons in this photochemical reaction. 

It is important to note that even organometallic compounds and coordination compounds are susceptible to photochemical reactions. For example, the metal carbonyls that tend to resist thermal substitutions are subjected to decarbonylation with the help of ultraviolet light. 

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