Conformer-specific photoisomerizaton of some 2-vinylbiphenyls.

The ground-state conformations, spectroscopy, and photochemistry of 2-vinylbiphenyl and three of its vinylsubstituted 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-planarn 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 C=C torsion. The torsional barriers are of the order of those for styrenes with similar vinyl substituents.