The role of central bond torsional motions in the direct cis⇌trans-photoisomerization of conjugated dienes

The photochemistry of the E,E- and E,Z-geometric isomers of 1,2-bis(ethylidene)-cyclopentane and -cyclobutane, and 2,3-bis(ethylidene)norbornane has been studied in hydrocarbon solution at room temperature; the results are inconsistent with the classical theoretical models for diene cis⇌trans photoisomerization and ring closure, which assume that both processes involve only simple torsional motions about (one or both of) the termini of buta-1,3-diene.