Semiconductor-based interfacial electron-transfer reactivity: Decoupling kinetics from pH-dependent band energetics in a dye-sensitized titanium dioxide/aqueous solution system

Hexaphosphonation of Ru(bpy)₃²⁺ provides a basis for surface attachment to nanocrystalline TiO₂ in film (electrode) or colloidal form and for subsequent retention of the molecule over an extraordinarily wide pH range. Visible excitation of the surface-attached complex leads to rapid injection of an electron into the semiconductor. Return electron transfer, monitored by transient absorbance spectroscopy, is biphasic with a slow component that can be reversibly eliminated by adjusting the potential of the dark electrode to a value close to the conduction-band edge (E_CB). Evaluation of the fast component yields a back-electron-transfer rate constant of 5(±0.5) × 10⁷ s^-1 that is invariant between pH = 11 and H₀ = -8, despite a greater than 1 eV change in E_CB (i.e., the nominal free energy of the electron in the electrode). The observed insensitivity to large changes in band-edge energetics stands in marked contrast to the behavior expected from a straightforward application of conventional interfacial electron-transfer theory and calls into question the existing interpretation of these types of reactions as simple inverted region processes.