Interfacial Dynamics within an Organic Chromophore-Based Water Oxidation Molecular Assembly

Photoinduced electron injection, intra-assembly electron transfer, and back-electron transfer are investigated in a single-site molecular assembly formed by covalently linking a phosphonated terthiophene (T₃) chromophore to a Ru(terpyridine)(bipyridine)(L)²⁺ (L = MeCN or H₂O) water oxidation catalyst adsorbed onto a mesoporous metal-oxide (MO_x) film. Density functional theory calculations of the T₃-trpy-Ru-L assembly indicate that the molecular components are strongly coupled with enhanced low-energy absorptions owing to the presence of an intraligand charge transfer (ILCT) transition between the T₃ and trpy moieties. Ultrafast spectroscopy of the MO_x//T₃-trpy-Ru-L assemblies reveals that excitation of the surface-bound T₃ chromophore results in ps-ns electron injection into the metal-oxide conduction band. Electron injection is followed by rapid (<35 ps) intra-assembly electron transfer from the Ru^II catalyst to regenerate the T₃ chromophore with subsequent back-electron transfer on the microsecond time scale.