Photochemical Reactions of Free Radicals

Although we think of free radicals mostly as highly reactive transient species which are intermediates in the chemical transformations of closed-shell molecules, there are some stable free radicals of which the triphenylmethyl 

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Luminescence is seldom observed from free radicals and radical ions because of the low energy of the lowest excited states of open-shell species, the benzophenone ketyl radical being however a noteworthy exception. There are few reports of actual photochemical reactions of free radicals, but the situation is different with biradicals such as carbenes. These have two unpaired electrons and can exist in singlet or triplet states and they take part in addition and insertion reactions (Figure 4.90). 

Photochemical reactions of free radicals derived from S-heterocycles 87MI51. 

However, the mentioned reaction features in evacuated CTA films are displayed simultaneously with direct proportion of the naphthalene consiunption rate and accumulation of the product A, absorbing UV-light. This fact allows an exclusion of participation of twofold excited triplet states of N. In this case, according to the authors point of view, the attention should be paid to secondary photochemical reactions of free radicals formed at interaction with CTA macromolecules. 

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