- Photochemical reaction
A photochemical reaction is a
chemical reaction which is induced by light. Examples of photochemicalorganic reactions areelectrocyclic reaction s,photoisomerization andNorrish reaction s.The basic requirements for photochemical reactions are:
* the energy of the light source must correspond to an electronic transition between orbitals
* the emitted light must be able to reach the targetedfunctional group without being blocked by the reactor, medium or other functional groups present.Photoexcitation is the first step in a photochemical process where the reactant is elevated to an excited state of higher energy.Photosensitizer s absorb radiation and transfer energy to the reactant. The opposite process is called quenching when a photoexited state is deactivated by a chemical reagent.The first ever photochemical reaction was described by Trommsdorf in 1834. [Trommsdorf, Ann. Chem. Pharm. 1834, 11] He observed that
crystal s of the compound α-santonin when exposed tosunlight turned yellow and burst. In a 2007 study the reaction was described as a succession of three steps taking place within a single crystal. ["The Photoarrangement of -Santonin is a Single-Crystal-to-Single-Crystal Reaction: A Long Kept Secret in Solid-State Organic Chemistry Revealed" Arunkumar Natarajan, C. K. Tsai, Saeed I. Khan, Patrick McCarren, K. N. Houk, and Miguel A. Garcia-GaribayJ. Am. Chem. Soc. , 129 (32), 9846 -9847, 2007. DOI|10.1021/ja073189o ]:
The first step is a
rearrangement reaction to acyclopentadienone intermediate 2, the second one adimerization in aDiels-Alder reaction (3) and the third one aintramolecular [2+2]cycloaddition (4). The bursting effect is attributed to a large change in crystal volume on dimerization.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 (ESPT) .References
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