The Irreversible Photochemistry of Bianthrone

A 4a,4b-dihydrophenanthrene-type cyclic photoisomer, the C isomer, is the major primary photoproduct of bianthrone in protic and aprotic polar solvents, and undergoes solvent-dependent secondary reactions, including the formation of dihydrohelianthrone in protic solvents. The C isomer was shown to be formed through the singlet excited state while the B isomer is formed via the triplet manifold.