Unstable Diradicals Produced Photochemically

While it is in the triplet state a molecule may undergo typical diradical reactions. This provides a plausible mechanism for radical-like reactions of substances that are largely diamagnetic. They are partly converted to the triplet state by light, or in the case of low lying triplet states, by heat. Probable examples of this mechanism are the photooxidation of rubrene and the photooxidation and dimerization of anthracene and higher members of the acene series.76 

76 In the case of most organic diradicals, it is possible to convert a classical diradical formula into a classical non-radical formula by a process of opening and closing double bonds. This is true of porphyrindine only if the unpaired electrons are on nitrogen. 

Etienne, Traiti de Chimie Organique, Volume XVII, Masson et Cie, Paris, 1949, p. 1299. 

On heating, rubrene peroxide gives up most of its oxygen to regenerate rubrene. 

The explanation for these diradical reactions can not be quite so simple as the one hinted at above however, since the quantum yield of rubrene peroxide does not appear to depend only on the concentration of photoactivated rubrene triplet molecules but also on the concentrations of rubrene itself and of oxygen.77 

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Non-stereospecific photochemical [2+2]-cycloadditions occur in the dimerization of phenyl cyclohexene 23 in the presence of a sensitizer to produce 24 and 25 [17], and in reactions of Z/E-2 butene with cyclohexenone 26 to give 27 and 28 [18] through the formation of intermediate diradicals. The photoaddition of cyclohexene to an enolised form of 1,3 diketone 29 gives 30 in a concerted process via the formation of an unstable cycloadduct [18]. 

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