Variations in Mechanism for Photoinduced Valence Isomerization of an Electron-Donor Nonconjugated Diene

Article abstract of DOI:10.1021/ja00343a032

Photoisomerization of 1,4,4a,5,8,8a-hexahydro-1,4,5,8-endo,endo-dimethanonaphthalene (3) has been studied in detail.Rearrangement to the cage photoisomer 4 occurs on direct irradiation or with triplet sensitizers with quantum efficiency as high as 1.0.Irradiation in the presence of electron acceptors whose fluorescence is quenched by 3 also results in cage formation.Quantum yields approach 1.0 for donor-acceptor sensitization in a nonpolar solvent.For this reaction, a mechanism is proposed that involves excitation transfer within an exciplex of sensitizer and 3 and partitioning of locally excited 3 at a biradicaloid geometry.Quantum efficiencies exceed unity for rearrangement sensitized by electron acceptors in polar media.A chain mechanism is proposed that involves radical-cation intermediates obtained through electron transfer from 3.Irradiation of ground-state (charge-transfer) complexes of 3 and fumaronitrile or diethyl 1,2-dicyanofumarate also results in valence isomerization with quantum yields that are dependent on solvent polarity and excitation wavelength.The options available for photosensitization of 3 and a related diene, norbornadiene, are discussed.