Photochemical Generation, Isomerization, and Oxygenation of Stilbene Cation Radicals

The cation radicals of cis- and trans-stilbene and several of their ring-substituted derivatives have been generated in solution directly by means of pulsed-laser-induced electron transfer to singlet cyanoanthracenes or indirectly via electron transfer from biphenyl to the singlet cyanoanthracene followed by secondary electron transfer from the stilbenes to the biphenyl cation radical. Transient absorption spectra of the cis- and trans-stilbene cation radicals generated by secondary electron transfer are similar to those previously obtained in 77 K matrices. Quantum yields for radical ion-pair cage escape have been measured for direct electron transfer from the stilbenes to three neutral and one charged singlet acceptor. These values increase as the ion-pair energy increases due to decreased rate constants for radical ion-pair return electron transfer, in accord with the predictions of Marcus theory for highly exergonic electron transfer. Cage-escape efficiencies are larger for trans- vs cis-stilbene cation radicals, possibly due to the greater extent of charge delocalization in the planar trans vs nonpolar cis cation radicals. Cage-escaped stilbene cation radicals can initiate a concentration-dependent one way cis- → trans-stilbene isomerization reaction. In the presence of oxygen, isomerization is inhibited and oxidative cleavage of either trans- or cis-stilbene to benzaldehyde occurs. Correlation of the measured quantum yields for oxygenation and isomerization with quantum yields for cage escape establishes that oxygenation is a nonchain process whose efficiency shows little variation with stilbene substituent or configuration, whereas isomerization is a chain process whose chain length is dependent upon substituent.