One of the most important transformations in radical cations is bond cleavage. This process is much easier than in neutral species as a consequence of bond weakening, due to removal of an electron from a fully occupied orbital of the neutral substrate. The importance of bond fragmentation processes lies on their involvement in biological processes such as DNA repair or sulfide oxidation by peroxidases and biomimetic models. We have recently reported that the regioselectivity of the oxidative and reductive electron transfer cycloreversion (CR) of 2,3-diaryloxetanes depends on the substitution of the aryl groups and on the nature of the photosensitizer.

In the present communication, new insights into the CR reactions of thietanes 1 are presented. Direct photolysis and thiapyrylium salt-photoinduced electron transfer are used as tools for the evaluation of the splitting mode. Direct photolysis renders products arising from the fragmentation of C3-C4 and C2-S bonds. Conversely, sensitized radical cation formation triggers the CR reaction of 1 in a different fashion: under these conditions C4-S and C2-C3 bonds are broken, with formation of a neutral molecule and a radical cation. The latter is finally neutralized by back electron transfer from the reduced form of the sensitizer. The nature of the intermediates has been studied using transient absorption spectroscopy, which reveals that thiobenzophenone or olefin radical cations are formed in the sensitized reaction, depending on the employed thietane. This observation points to generation of the thietane radical cation as a primary photoproduct. The issue of concerted versus stepwise fragmentation reaction of 1·+ is also discussed.