DIFFUSION AND HETEROGENEOUS ELECTRON-TRANSFER RATES IN ACETONITRILE AND IN POLYETHER POLYMER MELTS BY ALTERNATING-CURRENT VOLTAMMETRY AT MICRODISK ELECTRODES

Alternating current voltammetry at microdisk electrodes is used to measure diffusion coefficients and heterogeneous electron-transfer rate constants in a fluid solvent, acetonitrile, and in two polymer electrolyte solvents (400 and 1000 MW poly(ethylene oxide)/LiClO4). The dependence of ac current-potential phase angle on frequency for the one-electron oxidations of [CpFeCpCH2N(CH3)3]+ and of [Co(bpy)3]2+ in the polyether solvents follows that expected for quasi-reversible reactions. The heterogeneous electron-transfer rate constants (k(s)) for the [CpFeCpCH2N(CH3)3]2+/+ couple are 4.9 x 10(-2) cm/s and 2.8 x 10(-2) cm/s in MW 400 (41-degrees-C) and 1000 (68-degrees-C) polyether solvents, respectively, and for the [Co(bpy)3]3+/2+ couple are 1.6 x 10(-2) cm/s and 7.0 x 10(-3) cm/s in MW 400 and 1000 (both 68-degrees-C) polyether solvents, respectively. k(s) for the [Co(bpy)3]3+/2+ couple in acetonitrile is 2.0 x 10(-1) cm/s at 25-degrees-C. The k(s) values for the ferrocene and cobalt complexes in the warm polymer solvent melts are almost-equal-to 200-fold and greater-than-or-equal-to 13-fold smaller, respectively, than the corresponding room-temperature rate constants in fluid acetonitrile solution. It is proposed that the polymer solvent electron-transfer dynamics are affected by the relaxation rates of the ether dipole sites on the polymer chains, which are in turn constrained by the rates of polymer chain segment or local structure relaxations.