Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions

Self-assembled bilayers on nanocrystalline metal oxide films are an increasingly popular strategy for modulating electron and energy transfer at dye-semiconductor interfaces. A majority of the work to date has relied on Zr-II and Zn-IV linking ions to assemble the films. In this report, we demonstrate that several different cations (Cu-II, Cu-II, Fe-II, La-III, Mn-II, and Sn-IV) are not only effective in generating the bilayer assemblies but also have a profound influence on the stability and photophysical properties of the films. Bilayer films with Zr-IV ions exhibited the highest photostability on both TiO2 and ZrO2. Despite the metal ions having a minimal influence on the absorption/emission energies and oxidation potentials of the dye, bilayers composed of Cu-II, Fe-II, and Mn-II exhibit significant excited-state quenching. The excited-state quenching decreases the electron injection yield but also, for Cu-II and Mn-II bilayers, significantly slows the back electron transfer kinetics.