Radical ion/triplet mechanism for sensitized valence photoisomerization of a norbornadiene

Quenching of the fluorescence of a series of electron donor aromatic hydrocarbons in acetonitrile by dimethyl bicyclo(2.2.1)hepta-2,5-diene-2,3-dicarboxylate results in isomerization of the norbornadiene with varying efficiency. Quantum yield, CIDNP, and flash photolysis results are consistent with a rearrangement mechanism involving electron transfer from the sensitizer, intersystem crossing of resultant radical-ion pairs, and ion recombination to give norbornadiene triplets followed by rearrangement to the valence isomer. Where energetic factors are favorable, the isomerization quantum yield is as high as 0.39 and the efficiency of formation of norbornadiene triplets via ion recombination as high as 65%.