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

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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)(ϕcb), of their photocyclization to 4,9-diaryl-4,5,9,10-tetrahydropyrene (3a–f)(ϕthp), of trans–cis photoisomerization, ϕic, and of fluorescence (ϕf) 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 ϕcb. 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 [π2s+π2s] 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.

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