Coupling of metal-based light-harvesting antennas and electron-donor subunits: trinuclear ruthenium(II) complexes containing tetrathiafulvalene-substituted polypyridine ligands.

Three new tetrathiafulvalene-substituted 2,2'-bipyridine ligands, cis-bpy-TTF(1), trans-bpy-TTF(1), and cis-bpy-TTF(2) have been prepared and characterized. X-ray analysis of trans-bpy-TTF(1) is also reported. Such ligands have been used to prepare two new trinuclear Ru(II) complexes, namely, (bpy)(2)Ru(micro-2,3-dpp)Ru(bpy-TTF(1))(6) (9; bpy=2,2'-bipyridine; 2,3-dpp=2,3-bis(2'-pyridyl)pyrazine) and (bpy)(2)Ru(micro-2,3-dpp)Ru(bpy-TTF(2))(6) (10). These compounds can be viewed as coupled antennas and charge-separation systems, in which the multichromophoric trinuclear metal subunits act as light-harvesting antennas and the tetrathiafulvalene electron donors can induce charge separation. The absorption spectra, redox behavior, and luminescence properties (both at room temperature in acetonitrile and at 77 K in a rigid matrix of butyronitrile) of the trinuclear metal complexes have been studied. For the sake of completeness, the mononuclear compounds (bpy)(2)Ru(bpy-TTF(1))(2) (7) and (bpy)(2)Ru(bpy-TTF(2))(2) (8) were also synthesized and studied. The properties of the tetrathiafulvalene-containing species were compared to those of the model compounds Ru(bpy)(2)(4,4'-Mebpy) (4,4'-Mebpy=4,4'-dimethyl-2,2'-bipyridine) and (bpy)(2)Ru(micro-2,3-dpp)Ru(bpy). The absorption spectra and redox behavior of all the new metal compounds can be interpreted by a multicomponent approach, in which specific absorption features and redox processes can be assigned to specific subunits of the structures. The luminescence properties of the complexes in rigid matrices at 77 K are very similar to those of the corresponding model compounds without TTF moieties, whereas the new species are nonluminescent, or exhibit very weak emissions relative to those of the model compounds in fluid solution at room temperature. Time-resolved transient absorption spectroscopy confirmed that the potentially luminescent MLCT states of 7-10 are significantly shorter lived than the corresponding states of the model species. Photoinduced electron-transfer processes from the TTF moieties to the (excited) MLCT chromophore(s) are held responsible for the quenching processes.