text: 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.