Simplistically, light is one mechanism for providing the activation energy required for many reactions. If laser light is employed, it is possible to selectively excite a molecule so as to produce a desired electronic and vibrational state.
If the chemical system is at low pressure, this enables scientists to observe the energy distribution of the products of a chemical reaction before the differences in energy have been smeared out and averaged by repeated collisions. The absorption of a photon of light by a reactant molecule may also permit a reaction to occur not just by bringing the molecule to the necessary activation energy, but also by changing the symmetry of the molecule's electronic configuration, enabling an otherwise inaccessible reaction path, as described by the Woodward—Hoffmann selection rules.
The photon can be absorbed directly by the reactant or by a photosensitizer , which absorbs the photon and transfers the energy to the reactant. The opposite process is called quenching when a photoexited state is deactivated by a chemical reagent. Most photochemical transformations occur through a series of simple steps known as primary photochemical processes. One common example of these processes is the excited state proton transfer. Examples of photochemical organic reactions are electrocyclic reactions , radical reactions , photoisomerization and Norrish reactions.
These reactions include cis-trans isomerization, cycloaddition to other ground state alkene to give cyclobutane derivatives.
The cis-trans isomerization of a poly alkene is involved in retinal , a component of the machinery of vision. Such dimers interfere with transcription.
The beneficial effects of sunlight are associated with the photochemically induced retro-cyclization decyclization reaction of ergosterol to give vitamin D. In the DeMayo reaction , an alkene reacts with a 1,3-diketone reacts via its enol to yield a 1,5-diketone. Still another common photochemical reaction is Zimmerman's Di-pi-methane rearrangement.
In an industrial application, about , tonnes of benzyl chloride are prepared annually by the gas-phase photochemical reaction of toluene with chlorine. The photon induces homolysis of the Cl-Cl bond, and the resulting chlorine radical converts toluene to the benzyl radical:. Mercaptans can be produced by photochemical addition of hydrogen sulfide H 2 S to alpha olefins.
Coordination complexes and organometallic compounds are also photoreactive. These reactions can entail cis-trans isomerization. More commonly photoreactions result in dissociation of ligands, since the photon excites an electron on the metal to an orbital that is antibonding with respect to the ligands.
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Thus, metal carbonyls that resist thermal substitution undergo decarbonylation upon irradiation with UV light. In a related reaction, photolysis of iron pentacarbonyl affords diiron nonacarbonyl see figure :. Select photoreactive coordination complexes can undergo oxidation-reduction processes via single electron transfer. This electron transfer can occur within the inner or outer coordination sphere of the metal.
Although bleaching has long been practiced, the first photochemical reaction was described by Trommsdorff in In a study the reaction was described as a succession of three steps taking place within a single crystal. The bursting effect is attributed to a large change in crystal volume on dimerization. From Wikipedia, the free encyclopedia. Jablonski diagram. Radiative paths are represented by straight arrows and non-radiative paths by curly lines. Main article: Mechanistic organic photochemistry. Photochemistry , 1st ed. Journal of the American Chemical Society.
Photochemical and Photobiological Sciences , 8, — Chemistry World. Nature , , , — Turro, V. Ramamurthy, J.
Rossberg et al. Photochemistry of Coordination Compounds.
4 Photochemistry of Organic Compounds From Concepts to Practice Klán P and Wirz
Tsai, Saeed I. Khan, Patrick McCarren, K. Houk, and Miguel A. Branches of chemistry.
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Photochemistry of Organic Compounds