Measurement of lateral charge propagation in [Os(bpy)2(PVP) nCl]Cl thin films: A scanning electrochemical microscopy approach

The use of scanning electrochemical microscopy (SECM) to measure charge transport diffusion constants (SCT) in metallopolymers of the type [Os(bpy)2(PVP)nCl]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)63-, in an initial potential step, via the oxidation of Ru(CN) 64-. 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 Os III along the film. After the first potential step, a waiting period is introduced in which Ru(CN)63- is converted back to Ru(CN)64- at the UME and OsII can recover in concentration by electron self-exchange between OsIII and Os II moieties. After a defined time, the potential of the UME is switched again to cause the generation of the solution-phase oxidant, Ru(CN)63-. 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 DCT 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)nCl]Cl, where n = 5 or 10, which indicate that DCT is affected significantly by redox site loading and film structure (as determined by atomic force microscopy).