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 so lution pH, despite significant shifts in the conduction band edge ener gy (E(CB)) Of the underlying semiconductor electrode in response to th e same environmental perturbations. The observations are important in the context of recent work showing that back-electron reactivity for t he same semiconductor/dye combination is pH independent over a 19 pH u nit 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. Ph ys. Chem. 1996, 100, 6867). In particular, the spectroelectrochemical findings serve to rule out a suggested alternative interpretation of t he unusual kinetic effects whereby the crucial energy difference quant ity, E(CB) - E(f)(Ru), remains fixed because of compensating changes i n the dye potential with pH.