Photochemical dimerization of acephenanthrylene and the heavy atom effect

The solution-phase photodimerization of acephenanthrylene (1) in cyclohexan e, chloroform, dibromomethane, and ethyl iodide with visible light (lambda = 419 nm) occurs to form cis (2) and trans (3) photodimers in ratios that v ary with the solvent and with the presence or absence of oxygen. The photod imerization exhibits a significant heavy atom effect and also is sensitive to selective quenching by dissolved oxygen, in which only cis stereoisomers are produced. Irradiation of the dimers (lambda = 254 nm) shows that the t rans dimers cleave more rapidly than the cis dimers. This is attributed to stabilization of the excited state in the cis dimers.