The 'belle epoque' of photochemistry

In 1908 the pioneer of photochemistry, Giacomo Ciamician, presented a comprehensive account on his ongoing work in molecular photochemistry. He reported a number of reactions caused by light and ascertained that these were caused by light alone, not heat. These included geometrical isomerization of C=C, C=N and N=N bonds, 2 + 2 cycloaddition involving olefins as well as alpha,beta-unsaturated ketones and esters, reactions of ketones such as alpha-cleavage and intermolecular hydrogen abstraction from alcohols leading to reduction or coupling, inter- and intramolecular reactions of nitro compounds, photooxygenation reactions: in fact a large fraction of the presently known reaction types. The rich perspective that photochemistry offered to synthesis was revealed by this talk. Actually, Ciamician thought that this would become a serious alternative to thermal methods and provide a less polluting way for synthesis, imitating the mild conditions of biochemical reactions. This hope did not materialize, however, and the actual development of molecular photochemistry had to wait for several decades.