Redox switching in solid deposits:: triazole bridged osmium dimers

Solid deposits of the dimeric complex [Os(bpy)2Cl 4-bpt Os(bpy)2Cl]PF6, where bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4,-triazole have been deposited onto platinum microelectrodes. These layers exhibit unusually ideal electrochemical responses over a wide range of electrolyte compositions and pH values. Scanning electron microscopy reveals that repeated switching of the redox composition of these layers does not induce any significant structural change within the deposits. Cyclic voltammetry (CV) has been used to determine the apparent charge transport diffusion coefficient, DCT, describing homogeneous charge transport through the deposit. DCT is independent of the electrolyte concentration suggesting that electron self-exchange between adjacent redox centres limits the overall rate of charge transport through the solid. In 1.0 M LiClO4 and 1.0 M HClO4, DCT values of 2.0±0.1×10−10 and 1.7±0.4×10−10 cm2 s−1 are observed, corresponding to second order electron transfer rate constants of 1.8×107 and 3.0×107 M−1 s−1, respectively. The rate of heterogeneous electron transfer across the electrode ∣ deposit interface has been determined using fast scan CV. The standard heterogeneous electron transfer rate constant, k°, is 1.08±0.05 cm s−1 irrespective of the electrolyte pH. Significantly, this value is less than one order of magnitude smaller than that determined for a monomeric complex containing the same bridging ligand and redox active metal centre.