Excited state processes in ruthenium(II) polypyridyl complexes and cerium oxide nanoparticles

Abstract

Solar driven hydrogen production from water is a sustainable alternative to fossil fuels, but suffers greatly from the large energy cost associated with splitting water. This report uses ultrafast transient absorption and other spectroscopic techniques to analyze several components that show potential for this photocatalysis, in particular observing the excited state dynamics of electron separation and recombination. In ruthenium(II) polypyridyl systems, the rate of interligand electron transfer (ILET) was found to change with time, initially behaving as an ultrafast barrierless process, but transforming into a much slower activated process as excess energy is vibrationally released over 100 ps following excitation. The change in ILET rates lead to changes in the population of localized 3 MLCT states distributed among each ligand, which are initially randomized, but favor the lower energy bipyridine ligands at longer times. Three analogous ruthenium complexes were then linked via a triazole bridge to a cobalt(II) polypyridyl center known to catalyze the formation of H 2, observing the electron transfer from ruthenium to cobalt using emission decay signals of the ruthenium complex. The electron transfer decay pathway was slower and relatively minor compared to similar ruthenium(II)-cobalt(II) systems; however, this reduced efficiency can potentially be explained by localizations on peripheral ligands, as well as a possible energy barrier on the 5-position of phenanthroline. Finally, citrate coated CeO 2 nanoparticles displayed ultrafast trapping of holes upon excitation with UV light, forming significantly deeper traps than has been observed in other metal oxides. Transient absorption signals of the excited holes decayed over hundreds of picoseconds, with lifetimes dependent on the pH of the solution, indicating that the trapping sites are influenced by the surface of the nanoparticle. The corresponding electrons appear to form long lived Ce 3+ sites, observable on timescales of minutes. The fate of these Ce 3+ sites is also pH dependent, indicating that CeO 2 may be an effective water-splitting photocatalyst under basic conditions.