Systematic Study of Modifications to Ruthenium(II) Polypyridine Dyads for Electron Injection Enhancement

Spatial localization of excited-state electrons in transition-metal complexes used as photocatalysts or dye sensitizers in solar cells is important for efficient electron injection into the metal oxide nanoparticles. We use density functional theory to investigate the excited states in a prototype catalyst-chromophore assembly [(bpy)(H2O)Ru(tpy-tpy)Ru(tpy)](4+) ([Ru(tpy)-(bpy)(H2O)](2+) = catalyst, [Ru(tpy)(2)](2+) = chromophore, tpy = 2,2':6',2 ''-terpyridine, and bpy = 2,2'-bipyridine) and a series of related compounds. We explore several bridging ligand and terminal tpy ligand modifications of the prototype assembly, with the aim of inducing electronic excitations into the terminal tpy ligand upon irradiation with visible light. The excitations into the terminal ligand (i.e., ligand covalently attached to the semiconductor in the photocatalytic synthetic cell) should, in turn, enhance electron injection into the semiconductor. Our results suggest that both introduction of a spacer group (such as phenylene or alkane) into the tpy-tpy bridge and replacement of the terminal tpy group by a more extended pi-conjugated ligand are necessary to shift the electronic excitations from the bridging ligand into the terminal ligand. These results have implications for the design of photocatalysts and dye-sensitizer assemblies based on ruthenium(II) terpyridine compounds.