pH effects on the rate of heterogeneous electron transfer across a fluorine doped tin oxide/monolayer interface

Spontaneously adsorbed monolayers of [Ru(bpy)2PIC](PF6)2 have been formed on fluorine doped tin oxide macro- and microelectrodes, bpy is 2,2′-bipyridyl and PIC is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]phenanthroline. These monolayers exhibit well-defined, almost ideal electrochemical responses over a wide range of voltammetric scan rates. The formal potential of the Ru2+/3+ process shifts by less than 30 mV upon immobilization suggesting that the monolayers are well solvated. Significantly, chronoamperometry, conducted on a microsecond timescale, reveals that protonation of the PIC bridging ligand modulates the rate of interfacial electron transfer. The heterogeneous electron transfer rate constant, measured at an overpotential of +50 mV, decreases from 7.0 ± 1.1 × 105 to 0.7 ± 0.1 × 105 s−1 as the pH of the supporting electrolyte is increased from 1.7 to 9.3. These observations are consistent with the redox mechanism occurring via a heterogeneous electron transfer process, the rate of PIC which depends on the energy difference between the metal dπ-orbitals and the lowest unoccupied molecular orbital (LUMO) of the bridge. Protonation of the bridging ligand decreases this energy gap, resulting in an overall increase in the rate of the redox reaction. Significantly, despite the close proximity of the luminophores to a conducting surface, the monolayers remain luminescent suggesting that the electronically excited state is only weakly coupled to the electrode surface. This is consistent with bipyridyl as the site of the excited state in the metal complex.