[reaction: see text] Cycloreversion of oxetane 1 is achieved using (thia)pyrylium salts as photosensitizers. RadicaI cation intermediates involved in the electron-transfer process have been detected using laser flash photolysis. The experimental results are consistent with the reaction taking place from the triplet excited state of the sensitizer.

Cycloreversion of 1,2,3,4-tetraphenylcyclobutanes 1a,b and oxetane 2 is achieved using (thia)pyrylium salts as electron-transfer photosensitizers. Radical cation intermediates involved in the electron-transfer process have been detected using laser flash photolysis. The experimental results are consistent with the reaction taking place from the triplet excited state of the sensitizer.

The thiapyrylium salt 1b is an efficient electron-transfer photosensitizer in the preparative irradiation of trans,trans-2,3-diphenyl-4-methyloxetane (2a). Previously, the reaction has been assumed to occur through the triplet excited state of 1b, which has a very high intersystem crossing yield (φ ISC ) 0.97). In the present report, direct evidence for triplet quenching in the oxidative cycloreversion of the 2,3-diaryloxetanes is provided. The rate constants k q (T 1 ) and the free energy changes of the electron transfer ∆G ET (T 1 ) have been determined for the reaction between 1b and oxetanes 2a-d and 2a′. The process has been found to be exergonic in all cases except for 2d; this agrees with the fact that no photoproduct was obtained in the preparative irradiation of 1b in the presence of 2d. Good correlation between k q (T 1 ) and ∆G ET (T 1 ) was observed, confirming the involvement of the excited triplet state in the reaction mechanism.