Photochemical reaction

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Main article: Photochemistry.

A photochemical reaction is a chemical reaction which is induced by light. Examples of photochemical organic reactions are electrocyclic reactions, photoisomerization and Norrish reactions.

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 targeted functional 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. Photosensitizers 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.[1] He observed that crystals of the compound α-santonin when exposed to sunlight turned yellow and burst. In a 2007 study the reaction was described as a succession of three steps taking place within a single crystal.[2]

Santonin Photochemical reaction

The first step is a rearrangement reaction to a cyclopentadienone intermediate 2, the second one a dimerization in a Diels-Alder reaction (3) and the third one a intramolecular [2+2]cycloaddition (4). The bursting effect is attributed to a large change in crystal volume on dimerization.


  1. Trommsdorf, Ann. Chem. Pharm. 1834, 11
  2. 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-Garibay J. Am. Chem. Soc., 129 (32), 9846 -9847, 2007. doi:10.1021/ja073189o