Effects of mixed-valent composition and bathing environment on solid-state electron self-exchanges in osmium bipyridine redox polymer films

Abstract

Rate constants for the electrical gradient driven, bimolecular electron-self-exchange reaction between Os(II) and Os(III) sites in dry, mixed-valent films of the undiluted, polymeric metal complexes poly[Os(bpy)₂(vpy)₂](BF₄)(subscript m) and poly[Os(vbpy)₃](BF₄)(subscript m) in interdigitated array electrodes and in sandwich electrodes are measured as a function of m. Linear potential sweep and ac impedance measurements show that the reaction follows the expected, but in solid-state materials, seldom evaluated, bimolecular rate law from C[subscript (Osⁱⁱⁱ)]/C[subscript (Osⁱⁱ)] = 6 to 0.1. Comparison of rate constants for poly[Os(bpy)₂(vpy)₂](BF₄)(subscript m) and poly[Os(vbpy)₃](BF₄)(subscript m) self-exchanges driven by electrical and concentration gradients, measured in a variety of bathing environments, shows that electron hopping rates decrease in the order liquid solvent > solvent vapor > dry N₂ bathing environment, and in each environment, the rate constant for the former complex is larger. It appears that the electron hopping rate is affected by the rigidity of the polymeric matrix; matrices that are more rigid by virtue of the absence of solvent or through enhanced cross-linking (as in the vbpy complex) exhibit slower rates. The results imply that electron hopping involves (short range) nuclear displacement of the pendant osmium site from its equilibrium location in solvent-wetted polymers and is slowed when polymer rigidity inhibits such displacement.