Homogeneous and heterogeneous electron transfer dynamics of osmium-containing monolayers at the air/water interface

Langmuir monolayers have been formed at the air/water interface using [Os(dpp)(2)Qbpy](ClO4)(2), where dpp is 4,7-diphenyl-1,10-phenanthroline and Qbpy is 2,2':4,4":4'4"-quarterpyridyl. The mean molecular area decreases in a sigmoidal manner from 128 +/- 1.25 to 119 +/- 1.6 Angstrom(2) as the pH of the subphase is increased from 3.0 to 7.0. Over the range 3.0 less than or equal to pH less than or equal to 7.0 a single pK(a), of 4.3 +/- 0.3 is observed for the unbound Qbpy pyridine nitrogens when confined within a Langmuir monolayer. In solution, over the range 1.6 I pH I 7.0 two pK(a)s of 3.2 +/- 0.2 and 4.3 +/- 0.2 are observed. Horizontal touch voltammetry, performed using microelectrodes, has been used to determine the apparent charge transport diffusion coefficients, D-app, as the mean molecular area is systematically varied. For surface concentrations, Gamma, between approximately 1.0 x 10(-10) and 1.4 x 10(-10) mol cm(-2), the monolayer is in a liquid state and D-app increases linearly with increasing surface concentration. Although surface concentration dependent physical diffusion of the complexes prevents an accurate determination of the self-exchange rate constant, k(ex), analyzing these data in accordance with a 2-D Dahms-Ruff approach provides a lower bound of 1.0 x 10(8) M-1 s(-1) for k(ex). When the monolayers are further compressed so that the molecular area is less than approximately 100 Angstrom(2), charge transport proceeds via a percolation mechanism and the maximum value of the apparent diffusion coefficient, D-app,D-max, yields a rate constant for electron self-exchange between Os2+ and Os3+ of 1.6 x 10(9) M-1 s(-1). D-app,D-max depends on the solution pH in a sigmoidal manner, increasing from approximately 6.6 +/- 0.2 x 10(-6) to 8,9 +/- 0.4 x 10(-6) cm(2) s(-1) as the subphase pH is increased from 3.0 to 7.0. To probe the effect of molecular orientation on interfacial electron transfer rates, we have used chronoamperometry conducted on a microsecond time scale to measure the heterogeneous electron transfer rate, k, for both Langmuir and spontaneously adsorbed monolayers. In this way, the effect of making electrical contact through the Qbpy or dpp ligands on k can be probed. Although both rates are large, the rate measured at an overpotential of 50 mV for Langmuir monolayers (1.7 +/- 0.2 x 10(5) s(-1)) is approximately an order of magnitude smaller than that found for spontaneously absorbed systems (1.2 +/- 0.3 x 10(6) s(-1)), suggesting that the local microenvironment, e.g., packing density and molecular orientation, influence the rate of heterogeneous electron transfer in these systems.