Photoinduced Electron Transfer in the p-Phenylenediamine-Paraquat Molecular Complex

Article abstract of DOI:10.1021/j150607a019

Flash photolysis of the charge-transfer band of the p-phenylenediamine-paraquat complex yields the paraquat radical cation (λ_max=610 nm) and the p-phenylenediamine radical cation (λ_max=475 nm).Since the photoreaction is energetically uphill, back-electron transfer from the paraquat radical to the p-phenylenediamine radical occurs spontaneously, with a rate constant (25 deg C) of 1.0E9 M^-1s^-1.The excited state from which electron transfer occurs is shown not to be S₁ of paraquat (E_S0-S1=92 kcal/mol), T₁ of paraquat (E_S0-S1=71.5 kcal/mol), S₁ of p-phenylenediamine (E_S0-S1=87 kcal/mol), or T₁ of p-phenylenediamine (E_S0-T1>44 kcal/mol).Failure of a cyanine dye (E_S0-T1=39.2 kcal/mol) and oxygen (E_3Σg-1Δ=22.5 kcal/mol) to decrease the radical yield implies that, if the reaction involves a triplet state, it lies lower than 22,5 kcal/mol; otherwise electron transfer originates from a charge-transfer singlet state.