Energetics of electron transfer at the nanocrystalline titanium dioxide semiconductor/aqueous solution interface: pH invariance of the metal-based formal potential of a representative surface-attached dye couple

Mediator-based spectroelectrochemical assessment of the metal-centered formal potential (E_f^Ru) of a representative inorganic dye, Ru(4,4′-(CH₂PO₃)-2,2′-bipyridine)₃^10-, bound to a nanocrystalline titanium dioxide film shows that the potential is insensitive to changes in solution pH, despite significant shifts in the conduction band edge energy (E_CB) of the underlying semiconductor electrode in response to the same environmental perturbations. The observations are important in the context of recent work showing that back-electron reactivity for the same semiconductor/dye combination is pH independent over a 19 pH unit range, despite apparent changes in back-reaction driving force of greater than 1.2 eV over the same interval (Yan, S.; Hupp, J. T. J. Phys. Chem. 1996, 100, 6867). In particular, the spectroelectrochemical findings serve to rule out a suggested alternative interpretation of the unusual kinetic effects whereby the crucial energy difference quantity, E_CB - E_f^Ru, remains fixed because of compensating changes in the dye potential with pH.