Hole superexchange across a triazole bridged osmium monolayer/electrode interface†

Abstract

The effects of electrolyte and temperature on the electrochemical response of spontaneously adsorbed monolayers of [Os(bpy)₂bptCl], where bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4-triazole, on clean platinum microelectrodes are reported. While cyclic voltammetry of the Os^2+/3+ redox reaction is nearly ideally reversible, the bipyridyl based reductions are reversible only for scan rates greater than approximately 5 V s^–1, suggesting that the highly reduced species undergoes a subsequent chemical reaction. The electrolyte concentration dependence of the double layer capacitance, C_dl, was measured for monolayers containing only Os²⁺ centres at potentials on either side of the potential of zero charge, p.z.c. While the limiting interfacial capacitance observed at high electrolyte concentrations is approximately 25 µF cm^–2 for potentials negative of the p.z.c., it decreases to only 12 µF cm^–2 for potentials positive of the p.z.c. This observation suggests that at positive potentials the monolayers are relatively more perfect and contain less solvent and electrolyte ions. Oxidation of the monolayer to Os³⁺ causes C_dl to increase by less than 15%. These observations suggest that the physical location of charges within monolayers (bridge versus remote redox site) has a profound effect on the double layer structure. Chronoamperometry, conducted on a microsecond time-scale, was used to measure the heterogeneous electron transfer rate constant, k, for the Os^2+/3+ redox reaction. For electrolyte concentrations above 0.1 M, redox switching is characterised by a single unimolecular rate constant (k/s^–1). Tafel plots of the dependence of ln k on overpotential show that the rate constants for reduction of the Os³⁺ centres are approximately four times larger than those for oxidation of Os²⁺ sites within the monolayer for a wide range of overpotentials. This observation is interpreted in terms of a bridge mediated hole superexchange mechanism.

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