ADSORPTION DYNAMICS OF ELECTROACTIVE SELF-ASSEMBLING MOLECULES

The kinetics of adsorption of redox active self-assembling molecules of the type [Os(bpy)2LCl]+ (bpy = 2,2'-bipyridine and L = 4,4'-bipyridine (pypy), trans-1,2-bis(4-pyridyl)ethylene (py=py), 1,3-bis(4-pyridyl)propane (dipy), or 1,2-bis(4-pyridyl)ethane (dipyH-2)) have been investigated as a function of concentration, applied potential, and solvent. For all the complexes studied, the adsorption process appears to be under kinetic rather than diffusion control. Whereas the rate constant appears to be invariant with concentration at values below 1 muM, at high concentrations the kinetics are much faster. The electrode potential appears to affect the equilibrium coverage (likely due to electrostatic repulsions) but not the kinetics of adsorption. At low concentrations, there appears to be an induction period that we have tentatively ascribed to variations in local concentration in order to achieve a critical value after which adsorption is induced. In nonaqueous solvents, the solution concentration needed to achieve a saturation coverage is at least an order of magnitude larger than that in aqueous media pointing to the importance of adsorption strength and solubility differences. Whereas the rate of adsorption was not very sensitive to the nature of the solvent, the equilibrium value of the surface coverage was.