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 attac hment to nanocrystalline TiO2 in film (electrode) or colloidal form an d for subsequent retention of the molecule over an extraordinarily wid e pH range. Visible excitation of the surface-attached complex leads t o rapid injection of an electron into the semiconductor. Return electr on transfer, monitored by transient absorbance spectroscopy, is biphas ic with a slow component that can be reversibly eliminated by adjustin g the potential of the dark electrode to a value close to the conducti on-band edge (E(CB)) Evaluation of the fast component yields a back-el ectron-transfer rate constant of 5(+/-0.5) x 10(7) s(-1) that is invar iant 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 electro de). The observed insensitivity to large changes in band-edge energeti cs stands in marked contrast to the behavior expected from a straightf orward application of conventional interfacial electron-transfer theor y and calls into question the existing interpretation of these types o f reactions as simple inverted region processes.