pH-dependent redox potential induced in a sensitizing dye by adsorption onto TiO₂

The oxidation potential of a ruthenium(II)-polypyridine sensitizing dye is independent of pH when the dye is dissolved in solution. However, when the dye is adsorbed onto nanocrystalline TiO₂, its oxidation potential becomes pH-dependent, decreasing 53 mV per unit pH. This is approximately the same pH-dependence as followed by the flatband potential of the TiO₂. Therefore, the driving forces for both forward and back electron transfer reactions are practically independent of pH. This may explain the pH-independent rate of back electron transfer recently observed by Yan and Hupp (J. Phys. Chem. 1996, 100, 6867). To our knowledge, the induction of a pH-dependent redox potential upon adsorption of an otherwise pH-independent dye has not been previously reported.