Photochemical studies. Irradiation induced transformations of reversibly dissociating cyclopentadienone-dimers and their monomers

Irradiation of dissociating cyclopentadienone-dimers (2 b, c, e) results in formation of two types of cage-products (5 and 8) in very low quantum yields and good, but wavelength and phase dependent chemical yields. Both processes are intramolecular, for 2b this was shown using isotopic labeling and massspectrometric techniques. Sensitization and quenching experiments indicate that triplet states are involved The mechanistic pathways are discussed. The cyclopentadienones(1 b,c,e) were isolated for the first time and irradiation of 1b in the solid gave exclusively the symmetric cage (8b).