Electrochemistry of spontaneously adsorbed monolayers. Equilibrium properties and fundamental electron transfer characteristics

Article abstract of DOI:10.1021/ja00091a057

The effects of solvent, electrolyte, and temperature on the electrochemical response of spontaneously adsorbed monolayers of [Os(bpy)₂Cl(pNp)]⁺, where bpy is 2,2′-bipyridyl and pNp is 4,4′-bipyridyl, 1,2-bis(4-pyridyl)ethane, or 4,4′-trimethylenedipyridine, are examined. In tetrahydrofuran oxidation of the redox centers causes association of an extra anion, while two extra anions are bound to the oxidized centers in aqueous media. In aqueous solutions containing hydrophilic anions, such as nitrate or chloride, little ion pairing is observed. The temperature dependence of the formal potential over the range -5 to 40 °C shows that the reaction entropy (ΔS_rc^o) is positive, indicating increased solvent ordering in the higher oxidation state. A linear correlation is observed between the experimental reaction entropies and those predicted by the Born dielectric continuum model in a range of solvents. A non-adsorbing model compound, [Os(bpy)₂(pyridine)Cl]PF₆, typically exhibits considerably smaller ΔS_rc^o values, suggesting that the formation of a dense monolayer significantly affects the reaction entropy. The heterogeneous rate constant k for the O_s^2+/3+ reaction has been evaluated using nanosecond time scale chronoamperometry. Importantly, k is approximately independent of the supporting electrolyte concentration over the range 0.1 to 1.0 M. This may suggest that ion pairing is an equilibrium reaction that either precedes or follows electron transfer. Tafel plots of the dependence of In k on overpotential show curvature, indicating that the transfer coefficient is potential dependent. For sufficiently large overpotentials k tends to become independent of the free energy driving force, which is consistent with Marcus theory. The response is asymmetric with respect to overpotential, with the slope for the oxidation process tending toward zero more rapidly than that for the reduction process. This response can be modeled as a tunneling process between electronic manifolds on the two sides of the interface.