Self-Assembled Light-Harvesting Systems: Ru(II) Complexes Assembled about Rh−Rh Cores

Ru(II) polypyridine species have been assembled about dirhodium(II, II) tetracarboxylate cores.
The complexes prepared have general formulas [{(terpy)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (a-type
compounds:  terpy = 2,2‘:6‘,2‘ ‘-terpyridine; La = 4‘-(p-carboxyphenyl)-2,2‘:6‘,2‘ ‘-terpyridine; n = 1, 1a; n
= 2, cis-2a and trans-2a-cis and trans refer to the arrangement of the Ru(II) species around the dirhodium
core; n = 3, 3a), [{(Lb)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (b-type compounds:  Lb = 6-phenyl-2,4-di(2-pyridyl)-s-triazine; n = 1, 1b; n = 2, an inseparable mixture of cis-2b and trans-2b; n = 3, 3b; n = 4,
4b), and [{(terpy)Ru(Lc)}{Rh2(CH3COO)3(CH3CN)2}]2+ (1c; Lc = 6-(p-carboxyphenyl)-2,4-di(2-pyridyl)-s-triazine). As model species, also the mononuclear [(terpy)Ru(La)]2+ (5a), [(La)Ru(Lb)]2+ (5b), and [(terpy)Ru(Lc)]2+ (5c) have been prepared. All of the complexes have been characterized by several techniques,
including NMR and mass spectra, and the stability of the various species is discussed. The absorption
spectra of all of the compounds are dominated by the Ru(II) polypyridine moieties, showing intense ligand-centered (LC) bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the
visible. The compounds exhibit several metal-centered oxidation and ligand-centered reduction processes,
which have been assigned to specific subunits. Both absorption and redox data indicate a supramolecular
nature of the assembled systems. Efficient energy transfer from the MLCT triplet state of the Ru-based
components to the lowest-energy excited state of the dirhodium core takes place for the a-type compounds
at 298 K in acetonitrile solution, whereas such a process is inefficient for the b-type and c-type species,
which exhibit the typical MLCT emission. At 77 K in butyronitrile matrix, Ru-to-Rh2 energy transfer is partly
efficient for both the a-type and the b-type compounds and is inefficient for 1c. The reasons for such behavior
are discussed by taking into account arguments concerning the driving force and reorganization energy of
the complexes.