Measurement of Lateral Charge Propagation in [Os(bpy)2(PVP) n Cl]Cl Thin Films: A Scanning Electrochemical Microscopy Approach

The use of scanning electrochemical microscopy (SECM) to measure charge transport diffusion constants (D CT) in metallopolymers of the type [Os(bpy)2(PVP) n Cl]Cl, bpy = 2,2‘-bipyridyl and PVP = poly(4-vinylpyridine), is described. In this approach, a triple potential step technique is employed in which the ultramicroelectrode (UME) tip of the SECM is used to electrogenerate a solution phase oxidant, Ru(CN)6 3-, in an initial potential step, via the oxidation of Ru(CN)6 4-. This moiety diffuses from the tip to the underlying polymer film where electron transfer occurs, causing the local oxidation of the polymer-bound complex of OsII to OsIII. The form of the current−time characteristic in this step provides information on the kinetics of the ET process between the solution species and the polymer-bound moiety, as well as the concentration of redox-active species in the polymer film. This process creates lateral concentration gradients of OsII and OsIII along the film. After the first potential step, a waiting period is introduced in which Ru(CN)6 3- is converted back to Ru(CN)6 4- at the UME and OsII can recover in concentration by electron self-exchange between OsIII and OsII moieties. After a defined time, the potential of the UME is switched again to cause the generation of the solution-phase oxidant, Ru(CN)6 3-. The current−time behavior associated with this step is influenced significantly by the extent of lateral electron hopping in the waiting period. It is shown that SECM is capable of measuring D CT values as low as 10-10 cm2 s-1 with good precision. We report experimental measurements on spin-coated films of [Os(bpy)2(PVP) n Cl]Cl, where n = 5 or 10, which indicate that D CT is affected significantly by redox site loading and film structure (as determined by atomic force microscopy).