Binuclear Wirelike Dimers Based on Ruthenium(II)−Bipyridine Units Linked by Ethynylene−Oligothiophene−Ethynylene Bridges

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Abstract
The syntheses, structural characteristics, electrochemical behavior, ground-state spectra, photophysical properties, and transient absorption (TA) spectra in CH3CN solvent are reported for binuclear [(bpy)2Ru(bpy-E(T)nE-bpy)Ru(bpy)2]4+ complexes, Ru(bpyTnbpy)Ru, where the Ru-based units are connected by alternating 3,4-dibutylthiophene (DBT‘)/thiophene (T‘) fragments linked via ethynyl groups (E) to bpy ligands at the 5-position (bpy is 2,2‘-bipyridine). The ligand bpyT3bpy represents a module containing DBT‘/T‘/DBT‘ subunits, and bpyT5bpy accounts for a DBT‘/T‘/DBT‘/T‘/DBT‘ pattern. The syntheses and electrochemical and spectroscopic (emission and TA) properties in CH2Cl2 solvent of the bpyTnbpy ligands are likewise reported. The behavior of the Ru(bpyTnbpy)Ru dimers has been compared to that of the bpyTnbpy ligands and to that of a related mononuclear complex, [(bpy)2Ru(bpy-E-DBT‘)]2+, Ru(bpyDBT‘). For the dimeric complexes, the electrochemical results show that the first reduction step takes place at the bpy ligand(s) bearing an ethynylene group, the first oxidation step is thiophene-centered, and further oxidation involves the metal centers, which are only weakly interacting. The photophysical and TA results for the Ru(bpyTnbpy)Ru dimers account for the presence of low-lying oligothiophene-centered 3π,π* levels, while higher-lying metal−ligand charge transfer (3MLCT) levels are thermally accessible only for the case of Ru(bpyT3bpy)Ru; the possible role of charge separation (CS) levels (from oxidation at the thiophene bridge and reduction at one of the coordinated bpy's) is also discussed.