With the electron-deficient alkene S18 and the electron-rich alkene 4-methoxystyrene, the yield of the corresponding cyclobutane products (8, 47%; and 9, 20% respectively) was significantly lower than when the alkene S21 was used (81%). This observation contrasts with a reported [2+2] dimerisation of (only) electron-rich alkenes. Whilst a singleelectron transfer mechanism has been proposed for photocatalysed [2+2] cycloadditions of electron-rich alkenes, it is likely that the catalyst functions as a triplet sensitiser, at least with more electron-deficient substrates.