Organic photochemistry encompasses organic reactions that are induced by the action of light. The absorption of ultraviolet light by organic molecules often leads to reactions. In the earliest days, sunlight was employed, while in more modern times ultraviolet lamps are employed. Organic photochemistry has proven to be a very useful synthetic tool. Complex organic products can be obtained simply. History Early examples were often uncovered by the observation of precipitates or color changes from samples that were exposed to sunlights. The first reported case was by Ciamician that sunlight converted santonin to a yellow photoproduct: An early example of a precipitate was the photodimerization of anthracene, characterized by Yulii Fedorovich Fritzsche and confirmed by Elbs. Similar observations focused on the dimerization of cinnamic acid to truxillic acid. Many photodimers are now recognized, e.g. pyrimidine dimer, thiophosgene, diamantane. Another example was uncovered by Egbert Havinga in 1956. The curious result was activation on photolysis by a meta nitro group in contrast to the usual activation by ortho and para groups. Organic photochemistry advanced with the development of the Woodward-Hoffmann rules. Illustrative, these rules help rationalize the photochemically driven electrocyclic ring-closure of hexa-2,4-diene, which proceeds in a disrotatory fashion. Organic reactions that obey these rules are said to be symmetry allowed. Reactions that take the opposite course are symmetry forbidden and require substantially more energy to take place if they take place at all. Key reactions Organic photochemical reactions are explained in the context of the relevant excited states. Parallel to the structural studies described above, the role of spin multiplicity - singlet vs triplet - on reactivity was evaluated. The importance of triplet excited species was emphasized. Triplets tend to be longer-lived than singlets and of lower energy than the singlet of the same configuration. Triplets may arise from (A) conversion of the initially formed singlets or by (B) interaction with a higher energy triplet (sensitization). It is possible to quench a triplet reactions. Common organic photochemical reactions include: Norrish Type I, the Norrish Type II, the racemization of optically active biphenyls, the type A cyclohexadienone rearrangement, the type B cyclohexenone rearrangement, the di-π-methane rearrangement, the type B bicyclo[3.1.0]hexanone rearrangement to phenols, photochemical electrocyclic processes, the rearrangement of epoxyketones to beta-diketones, ring opening of cyclopropyl ketones, heterolysis of 3,5-dimethoxylbenzylic derivatives, and photochemical cyclizations of dienes. Practical considerations Reactants of the photoreactions can be both gaseous and liquids. In general, it is