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

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.

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).

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