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)(3)(2+) 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) x 10(7) s(-1) that is invariant between pH = 11 and H-0 = -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.