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Dense monolayers of [Os(bpy) 2 (p3p) 2 ] 2+ , where bpy is 2,2′-bipyridyl and p3p is 4,4′-trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and the area occupied per molecule suggests that only one of the p3p ligands binds to the electrode surface, the other being available for protonation. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron transfer rate constant k for the Os 2+/3+ redox reaction. For electrolyte concentrations above 0.1 M, heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s -1 ). Tafel plots of the dependence of ln k on the overpotential η show curvature, and larger cathodic than anodic rate constants are observed for a given absolute value of η. This response is consistent with electron transfer occurring via a through-space tunneling mechanism. Plots of k vs pH are sigmoidal, and the standard heterogeneous rate constant k°decreases from (6.1 ( 0.2) × 10 4 to (1.6 ( 0.1) × 10 4 s -1 as the pH of the contacting solution is decreased from 5.05 to 1.07. When in contact with pH 5.05 electrolyte, the electrochemical enthalpy ∆H q is 37.5 ( 2.1 kJ mol -1 , which decreases to 24.6 ( 1.5 kJ mol -1 at a pH of 1.07. The reaction entropy ∆S rc°i s independent of the pH over this range, maintaining a value of 82 ( 7 J mol -1 K -1 . In contrast to the behavior expected from the decrease of k with decreasing pH, the free energy of activation ∆G q decreases with decreasing pH. The electronic transmission coefficient κ el , describing the probability of electron transfer once the nuclear transition state has been reached, is considerably less than unity for all pH's investigated. κ el decreases with decreasing solution pH, suggesting an increasingly weaker electronic interaction between the metallic states of the electrode and the orbitals of the redox center as the monolayer becomes protonated. These results suggest that monolayer protonation modulates the heterogeneous electron transfer rate by changing the through-space electron transfer distance. This may be caused either by a change in the tilt angle between the adsorbate and the electrode or by the methylene spacer units within the bridging ligand becoming extended, when the monolayer is protonated.

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The mechanism for the formation of monolayers of [Os(2,2Ј-bipyridyl) 2 (4,4Ј-trimethylenedipyridine) 2 ] 2ϩ [Os(bpy) 2 (p3p) 2 ] 2ϩ on gold and platinum microelectrodes has been investigated by monitoring the time evolution of the surface coverage. The monolayers were formed in 2:1 H 2 O:dimethylformamide solution containing between 10 and 100 M of the complex. The voltammetric response of these films corresponding to the Os 2ϩ/3ϩ redox reaction is nearly ideal. Cyclic voltammetry was used to determine the surface coverage vs. time profile following transfer of the modified electrode into aqueous perchlorate electrolyte. Monolayer formation occurs on the tens of minutes time scale and follows a second order rate law for both gold and platinum electrodes. For gold, the rate constant is 4.6 Ϯ 0.5 ϫ 10 2 M Ϫ1 s Ϫ1 for bulk concentrations between 10 and 100 M. In contrast, more complex behavior is observed on platinum with multilayers being formed where C B exceeds approximately 20 M. The apparent rate constants for binding to platinum are at least a factor of 2 smaller than for gold. For both electrode materials, desorption follows first order rate kinetics indicating that the adsorbates do not interact strongly with one another. The time constants for desorption are comparable with those found for adsorption. The first order desorption rate constants are 4.1 Ϯ 0.3 and 1.8 Ϯ 0.2 ϫ 10 Ϫ5 s Ϫ1 for modified gold and platinum electrodes, respectively.

Monolayers of anthraquinone-2,7-disulfonic acid, 2,7-AQDS, have been formed by equilibrium adsorption onto mercury electrodes. In low pH electrolytes, cyclic voltammetry is nearly ideal with a peak-to-peak splitting of 8 ± 2 mV, and a full width at half maximum of 57 ± 1 mV, being observed for scan rates less than 3 V s 21 . The dependence of the surface coverage of 2,7-AQDS as measured using voltammetry on its bulk concentration is described by the Langmuir isotherm over the concentration range 0.06-9 mM. A saturation surface coverage, G s , of (8.7 ± 0.5) 3 10 211 mol cm 22 and an adsorption coefficient, b, of (5.6 ± 0.5) 3 10 5 M 21 are observed where the supporting electrolyte is 1.0 M HClO 4 . Consistent with the formation of an organic film, the double layer capacitance, C dl , as measured at 20.700 V, decreases from approximately 40 mF cm 22 in the absence of dissolved 2,7-AQDS to 10 mF cm 22 when the bulk concentration exceeds 10 mM. By measuring the potential dependence of C dl as the bulk concentration of 2,7-AQDS is varied systematically, an insight into the potential dependence of the free energy of adsorption, DG ‡ ads , has been obtained. These data reveal that DG ‡ ads is similar for both fully oxidised and fully reduced monolayers. However, these quinonoid monolayers are least strongly bound (DG ‡ ads = 228.5 kJ mol 21 ) when the film exists in the quinhydrone form.