Intramolecular photocyclizations of o,o′-bis-(2-arylvinyl)biphenyls

Abstract

In a series of o,o′-bis-(2-arylvinyl)biphenyls (1a–f) the quantum yields of the photocycloaddition to give trans,trans,trans,-1,2-diaryl-1,2,2a,10b-tetrahydrocyclobuta[cis]phenanthrene (2a–f)(ϕ_cb), of their photocyclization to 4,9-diaryl-4,5,9,10-tetrahydropyrene (3a–f)(ϕ_thp), of trans–cis photoisomerization, ϕ_ic, and of fluorescence (ϕ_f) have been determined. An explanation, based on the different properties of the aryl groups, is given for the differences in product formation, which arise on irradiation of compounds (1a–f). For the p-methoxyphenyl (1e) and p-dimethylaminophenyl (1f) derivatives an enhanced internal conversion from the singlet n→π* state to the ground state is responsible for the low values of ϕ_cb. o,o′-Divinylbiphenyl (1g) cannot be converted into the cyclobutane derivative (2g). This is caused by the absorption characteristics of (1g) and (2g). The stereoselective formation of the cyclobutane derivatives (2a–f) takes place in a concerted [_π2_s+_π2_s] photocycloaddition via the S₁ state of the trans,anti,trans,anti-conformers of (1). The photocyclization to the tetrahydropyrenes (3a–f) also arises from the S₁ state.