Stilbenes photoreduction

Because of the rapidity with which the first-formed excited states are usually converted to the Sx or Tx state, most photochemical reactions start from these states. There are exceptions. An obvious one is when an upper dissociative excited state, in which the molecule immediately fragments, is populated.49 Other exceptions occur when a molecule contains two different chromophores. For example, 20 reacts analogously to franj-stilbene (21) from its state but also undergoes photoreduction, a process typical of an n,n state (see p. 719).60... 

Amines, too, undergo photoaddition to alkynes. From diphenylacetylene and a secondary amine are obtained an enamine (which is hydrolysed during work-up) and products which arise by further reaction of stilbene, the photoreduction product of diphenylacetylene (equation 28). The products can be rationalized in terms of an initial hydrogen transfer to the excited state of the alkyne the fact that the N—H... 

Pseudo-stilbene type azobenzenes are not as stable under UV irradiation as the azobenzenes. Irick and Pacifici found that in de-aererated alcoholic solvents, photoreduction gives the hydrazo compound with a yield of ca. 10 at 254 nm irradiation.Albini et aF found yields of one order higher, but at 313 nm and longer wavelength irradiation, the decomposition is virtually absent. Some caveat is necessary when triplet-sensitizing additives are present, because irradiation under air does not lead to photoreduction. Benzophenone-sensitized reduction in benzene proceeds with a yield of... 

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