Abstract: Irradiation of 2-phenyl-l, 2-dihydronaphthalene (4) at 300 nm in hexane leads to three primary photoproducts: endo-and exo-6-phenylbenzobicyclo [3.1. O] hex-2-ene (endo- 5 and exc-5) and 7, 12-dihydrodibenzo [a, dlcyclooctatetraene (6). Irradiation of labeled 4 (4- 3, 4-d2) showed that 5 arises via a di-n-methane rearrangement as well as via electrocyclic ring opening, single bond rotation, and photocycloaddition. The di-n-methane ...