Absorption and emission properties of di- and trinuclear Ruthenium(II) rack-type complexes

The absorption spectra and the luminescence properties of three dinuclear Ru(II) complexes and one trinuclear Ru(II) complex have been investigated. All the complexes have rack-type structures. The dinuclear complexes 1, 2, and 3 incorporate a bis-tridentate bridging ligand made up of a pyrimidine and four pyridine moieties, as well as two 2,2':6',2''- terpyridyl (tpy) ligands. The trinuclear complex 4 incorporates a tris- tridentate bridging ligand made up of two pyrimidine and five pyridine moieties, as well as three tpy ligands. The absorption spectra of the complexes show a large number of ligand-centered (LC) and metal-to-ligand charge-transfer (MLCT) bands. All the complexes exhibit emission from a triplet MLCT state, with maxima in the spectral range 840-950 nm (lifetimes between 40 and 80 ns) at 298 K in fluid solution, and in the spectral range 760-810 nm (lifetimes between 2 and 3 μs) at 77 K in rigid matrices. A free tuning of the absorption and luminescence properties of complexes 1-3 can be achieved by changing the substituents on the pyrimidine ring of the bridging ligand. Efficient energy transfer within the rack structure 4 occurs from the (upper-lying) central metal-based chromophore to the (lower-lying) peripheral ones.