MONOMER AND POLYMER SOLVENT DYNAMIC CONTROL OF AN ELECTRON-TRANSFER CROSS-REACTION RATE AT A REDOX POLYMER-SOLUTION INTERFACE

The electron-transfer cross-reaction rate constant k12 for the thermodynamically disfavored oxidation of the metal complex solute [Fe(4,7-dimethylphenanthroline)3]2+ by a poly([Os(bpy)2(vpy)2]3+) redox polymer surface has been measured with a microelectrode-based voltammetric method as a function of the solvent in which the iron complex is dissolved. In the monomeric solvents acetonitrile, acetone, methylene chloride, dimethyl sulfoxide, pyridine, dimethoxyethane, and propylene carbonate, variations in k12 follow a theoretical model incorporating the monomer solvent relaxation time, dielectric constant, and reaction free energy. The osmium metal complexes at the redox polymer/solvent interface are shown to be well solvated without significant constraints on solvent properties. k12 is much smaller when the [Fe(Me2Phen)3]2+ complex is dissolved in the poly(ether) solvent CH3O(CH2CH2O)8CH3, Me2PEG-400. In the context of solvent dynamics theory, the results indicate that the relaxation time for the polymer chain segment's dipole reorientation that influences the barrier-crossing frequency is ca. 22 ps. This measurement is a new approach to polymer solvent dynamics and is the first example of an outer sphere electron-transfer cross-reaction rate in a polymeric solvent.