Electron transfer photoisomerization of norbornadiene to quadricyclane cosensitized by dibenzoylmethanatoboron difluoride and aromatic hydrocarbons

Chemically induced dynamic nuclear polarization (CIDNP) was used to study the valence isomerization of norbornadiene (NBD) to quadricyclane (QC) sensitized by dibenzoylmethanatoboron difluoride (DBMBF2). While DBMBF2 readily sensitized QC to NBD via an electron transfer mechanism, it did not promote the reverse isomerization. In contrast, in the presence of an aromatic co-sensitizer, such as toluene, ethylbenzene, biphenyl or durene, DBMBF2 sensitized NBD isomerization to QC. The novel result is rationalized by invoking a photoinduced electron transfer mechanism, involving triplexes as the key intermediate which facilitates triplet recombination of the ion radical pair.