Solvent effects on redox properties of radical ions1

Abstract

The one-electron reduction potentials of the radical cations of 1,4-diazabicyclo[2.2.2]octane (DABCO) and N,N,N′,N′-tetramethylphenylene-1,4-diamine (TMPD) in propan-2-ol, ethanol, methanol, acetone, acetonitrile and dimethyl sulfoxide have been measured by cyclic voltammetry. Furthermore, the one-electron reduction potentials of 1,4-benzoquinone, 1,4-benzoquinone radical anion, methyl viologen dication and methyl viologen radical cation also have been measured in a number of solvents. The present results, together with previously published data on radical anions, have been used to evaluate solvent effects in view of the Kamlet–Taft relationship. The main factors affecting the magnitude of the solvent effects are the gas-phase redox properties of the corresponding neutral molecule (ionization potential and electron affinity) and the charge. In general, the magnitude of the solvent effects on solution redox properties of radical ions decreases with increasing redox stability of the radical ion, reflected by low ionization potential of the corresponding neutral molecule for radical cations and by high electron affinity of the corresponding molecule for radical anions.