Effect of Deposition Time on the Orientation of [Ru(bpy)2Qbpy]2+ Adsorbed on Platinum

The mechanism for the formation of monolayers of [Ru(bpy)2 Qbpy]2+ on platinum microelectrodes has been investigated by monitoring the time evolution of the surface coverage using in-situ voltammetry; bpy is 2,2‘-bipyridyl and Qbpy is 2,2‘:4,4‘ ‘:4‘4‘ ‘-quarterpyridyl. The monolayers were formed in 2:1 H2O:DMF solution containing between 0.01 and 150 μM of the complex. The dependence of the surface coverage on the bulk concentration is described by the Frumkin adsorption isotherm in which the free energy of adsorption, ΔGads, is −33.8 ± 1.2 kJ mol-1 and the interaction parameter is 1.6 indicating destabilizing, surface coverage dependent, lateral interactions. Monolayer formation occurs on the 10−20 h time scale and is controlled by the dynamics of adsorption rather than bulk diffusion. The first-order rate constant for adsorption is 5.1 ± 0.2 × 10-6 s-1. Significantly, the heterogeneous electron-transfer rate constant measured at an overpotential of 50 mV, k50mV, is independent of surface coverage but in...