Effect of electrode density of states on the heterogeneous electron-transfer dynamics of osmium-containing monolayers

Dense monolayers of [Os(OMe-bpy)(2)(p3p)Cl](I+), where OMe-bpy is 4,4'-dime thoxy-2,2'-bipyridyl and p3p is 4,4'-trimethylenedipyridine, have been form ed by spontaneous adsorption onto clean platinum, mercury, gold, silver, ca rbon, and copper microelectrodes. These systems have been used to probe the influence of the electrode density of states on the rate of electron trans fer across the electrode/monolayer interface. Monolayers on each material e xhibit well-defined voltammetry for the Os2+/3+ redox reaction where the su pporting electrolyte is aqueous 1.0 M NaClO4. The high scan rate (> 2000 V s(-1)) voltammetric response has been modeled using a nonadiabatic electron -transfer model. The standard heterogeneous electron-transfer rate constant , k(o), depends on the identity of the electrode material, e.g., k(o) is 6 x 10(4) and 4 x 10(3) s(-1) for platinum and carbon electrodes, respectivel y. Chronoamperometry, conducted on a microsecond rime scale, has been used to probe the potential dependence of the heterogeneous electron-transfer ra te constant. These values range from (4.0 +/- 0.2) x 10(4) to (3.0 +/- 0.3) x 10(3) s(-1) on going from platinum to carbon electrodes. Temperature-res olved chronoamperometry and cyclic voltammetry reveal that the electrochemi cal activation enthalpy, DeltaH(double dagger), and the reaction entropy, D eltaS(RC)(double dagger), are both independent of the electrode material ha ving values of 11.1 +/- 0.5 kJ mol(-1) and 29.6 +/- 2.4 J mol(-1) K-1, resp ectively. The effect of electrode material on the preexponential factors is discussed in terms of the electrode density of states. These experimental data indicate that the heterogeneous electron-transfer rate for a nonadiaba tic process is not simply proportional to the density of states but is modu lated by the electronic coupling efficiency. Moreover, the matrix coupling elements, H-AB, are between 0.1 and 0.5 J mol(-1), which is approximately 4 orders of magnitude smaller than those found from studies of intervalence charge-transfer intensities within comparable dimeric complexes.