Substituent effect on the photoinduced electron-transfer reaction of para-substituted triphenylphosphines sensitised by 9,10-dicyanoanthracene

The photoinduced electron-transfer reaction of para-substituted triphenylphosphines sensitised by 9,10-dicyanoanthracene (DCA) occurred in acetonitrile containing 2 vol% water to form the corresponding triphenylphosphine oxide. Transient absorption spectral measurements were carried out during 355 nm laser flash photolysis of a mixture of the phosphine and DCA. The electron transfer from the phosphine to singlet excited DCA initiated the reaction. A para substituent on the benzene ring affects the quantum yields of the phosphine radical cation and phosphine oxide. The back electron transfer from the DCA radical anion to the phosphine radical cation governed the quantum yield of the phosphine radical cation. The quantum yield of the phosphine oxide was dependent on the conjugation between the pi-electron of the benzene ring and the n-electron of the phosphorus atom in the phosphine radical cation. The phosphine oxide forms through nucleophilic attack of H2O toward the phosphorus atom of the phosphine radical cation, producing the phosphoranyl radical.