Energy transfer in rhodium–ruthenium dimer-of-dimer assemblies

Abstract A synthetic route to linear pairs of dirhodium tetracarboxylate ‘paddlewheel’ dimers bridged by Ru(II) complexes of tridentate ‘terpyridine-like’ ligands is presented. Three dimer-of-dimer assemblies were synthesized as well as two monomer-of-dimer assemblies. A bis(4′-(4-carboxyphenyl)-terpyridine)Ru(II) complex spanning two dirhodium dimers displays a 26 A separation between the dimers in its X-ray crystal structure. Increased electronic interaction is found for the dimer of dimers without the phenyl groups using bis(4′-(4-carboxy)-terpyridine)Ru(II) as the bridging complex. The addition of one or two of the dirhodium dimers to the Ru photosensitizer leads to an increase in the molar absorptivity in an additive fashion. In the emission profile, the dirhodium tetracarboxylates act as energy transfer traps as the Ru based 3 MLCT (MLCT = metal-to-ligand charge-transfer) excited state is efficiently quenched by energy transfer from the Ru core to the dirhodium periphery, both at room temperature and low (77 K) temperature.