AbstractThe regular photocyclisation of α‐(9‐phenanthryl)stilbene (1) to 9‐phenylbenzo[g]chrysene (2) is accompanied by formation of 4,5‐diphenylacephenanthrylene (3). A similar photocyclisation, providing a five‐membered ring, occurs on irradiation of 1‐(9‐phenanthryl)‐1‐phenylethylene (4) which yields a small amount of 5‐phenylacephenanthrylene (5). The structures of3and5were confirmed by comparison of their spectral data with those of acephenanthrylene (10). This was synthesized by photolysis of 1‐(o‐iodobenzylidene)indane (8), followed by dehydrogenation, however,10cannot be obtained by irradiation of 1‐(o‐iodobenzylidene)indene (11). The different photoreactivity