Structure-Property Relationships in Phosphonate-Derivatized, RuII Polypyridyl Dyes on Metal Oxide Surfaces in an Aqueous Environment

The performance of dye-sensitized solar and photo-electrochemical cells is strongly dependent on the light absorption and electron transfer events at the semiconductor-small molecule interface. These processes as well as photo/electrochemical stability are dictated not only by the properties of the chromophore and metal oxide but also by the structure of the dye molecule, the number of surface binding groups, and their mode of binding to the surface. In this article, we report the photophysical and electrochemical properties of a series of six phosphonate-derivatized [Ru(bpy)(3)](2+) complexes in aqueous solution and bound to ZrO2 and TiO2 surfaces. A decrease in injection yield and cross surface electron-transfer rate with increased number of diphosphonated ligands was observed. Additional phosphonate groups for surface binding did impart increased electrochemical and photostability. All complexes exhibit similar back-electron-transfer kinetics, suggesting an electron-transfer process rate-limited by electron transport through the interior of TiO2 to the interface. With all results considered, the ruthenium polypyridyl derivatives with one or two 4,4'-(PO3H2)(2)bpy ligands provide the best balance of electron injection efficiency and stability for application in solar energy conversion devices.