see 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 and not 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 a 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.

Photochemistry | organic reactions