Abstract Intramolecular [2+ 2] cycloaddition of 2, 6-diaryl-1, 6-heptadienes 1a–c to bicycloheptanes 2 can be induced by electron-transfer photosensitization with 9, 10- dicyanoanthracene. A stepwise mechanism involving cyclic, 1, 4-cation radical 3 has been proposed based on the trapping experiments.