ELECTRON-TRANSFER PHOTOISOMERIZATION OF NORBORNADIENE TO QUADRICYCLANE COSENSITIZED BY DIBENZOYLMETHANATOBORON DIFLUORIDE AND AROMATIC-HYDROCARBONS

Chemically induced dynamic nuclear polarization (CIDNP) was used to st udy the valence isomerization of norbornadiene (NBD) to quadricyclane (QC) sensitized by dibenzoylmethanatoboron difluoride (DBMBF(2)). Whil e DBMBF(2) readily sensitized QC to NBD via an electron transfer mecha nism, it did not promote the reverse isomerization. In contrast, in th e presence of an aromatic co-sensitizer, such as toluene, ethylbenzene , biphenyl or durene, DBMBF(2) sensitized NBD isomerization to QC. The novel result is rationalized by invoking a photoinduced electron tran sfer mechanism, involving triplexes as the key intermediate which faci litates triplet recombination of the ion radical pair.