CIDNP Studies of Photoinitated Electron-Transfer Reactions. Sensitized Isomerization of an Electron Acceptor Norbornadiene

Abstract

Chemically induced dynamic nuclear polarization (CIDNP) has been observed on irradiation of dimethyl-bicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (3) in the presence of aromatic electron-donor sensitizers in acetonitrile. Nuclear polarization is detected for selected protons of 3 (emission) and its quadricyclene valence isomer 4 (enhanced absorption). The complementary CIDNP effects are rationalized in terms of the quenching of hydrocarbon singlests via electron transfer, intersystem crossing between resultant singlet and triplet radical-ion pairs, and competition between singlet and triplet ion recombination. The energetics of recombination allow that triplets of either 3 or the sensitizers may be populated. A distinction is made between pairs in which the sensitizer triplet level is above or below that of 3. The CIDNP effects are discussed in terms of other photochemical results regarding the electron-transfer photosensitized isomerization, 3 → 4. © 1982, American Chemical Society. All rights reserved.