Conformer-specific photoisomerizaton of some 2-vinylbiphenyls

Abstract

The ground-state conformations, spectroscopy, and photochemistry of 2-vinylbiphenyl and three of its vinyl-substituted derivatives have been investigated. The ground state exists as a mixture of syn and anti conformers in which the vinyl group is directed either toward or away from the unsubstituted phenyl ring. Both conformers are highly non-planar, having large phenyl–phenyl and phenyl–vinyl dihedral angles. The vinylbiphenyls are found to undergo conformer-specific photochemistry. The syn rotamers are non-fluorescent and undergo rapid and efficient cyclization to 8a,9-dihydrophenanthrene intermediates, which undergo rapid thermal sigmatropic rearrangements to yield 9,10-dihydrophenanthrenes. The intermediates have been generated at 77 K and detected by absorption spectroscopy. The singlet states of the anti rotamers are fluorescent, and undergo competing intersystem crossing and activated CC torsion. The torsional barriers are of the order of those for styrenes with similar vinyl substituents.