Involvement of triplet excited states and olefin radical cations in electron-transfer cycloreversion of four-membered ring compounds photosensitized by (thia)pyrylium salts.

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.