Electron transfer from aromatic compounds to phenyliodinium and diphenylsulfinium radical cations

Abstract

Phenyliodinium (I^˙+) and diphenylsulfinium radical cations (II^˙+) have been generated by flash photolysis (λ_inc= 347 nm) of diphenyliodonium ions (I⁺) and diphenyl (4-phenylthiophenyl) sulfonium ions (II⁺) in acetonitrile solutions at room temperature. I^˙+ and II^˙+ were found to undergo electron-transfer reactions with benzene derivatives resulting in the formation of radical cations of the aromatic compounds. A study involving 25 compounds including various methyl- and methoxy-benzenes, biphenyl, phenol and cresols revealed that electron transfer is independent of the ionization energy E_i provided that the rates are encounter-controlled. This applies to cases where E_i does not exceed a critical value: E_{i, crit}≈ 820 kJ mol^–1(I^˙+) and 780 kJ mol^–1(II^˙+). Bimolecular rate constants decrease with increasing E_i in the case of aromatic compounds having ionization energies exceeding the critical values. A Marcus-inverted region was not detected.