Mononuclear and Binuclear Wirelike Ruthenium(II) Complexes with Oligo-diethynyl-thiophene Bridged Back-to-Back Terpyridine Ligands:  Synthesis and Electrochemical and Photophysical Properties

The syntheses, structural features, electrochemical behavior, absorption spectra, and photophysical properties of five mononuclear complexes [(terpy)Ru(terpy-DEDBT_n-terpy)]²⁺, RuT_n, and five binuclear complexes [(terpy)Ru(terpy-DEDBT_n-terpy)Ru(terpy)]⁴⁺, RuT_nRu, are reported, where n varies from 1 to 5 so that the metal−metal distance is estimated to be 42 Å for the largest binuclear complex, RuT₅Ru (terpy is 2,2‘:6‘,2‘ ‘-terpyridine and DEDBT is 2,5-diethynyl-3,4-dibutylthiophene). The metal-centered oxidation potentials for the mononuclear and binuclear species are slightly more positive than for the reference [Ru(terpy)₂]²⁺ complex, owing to the withdrawing nature of the back-to-back terpyridine ligands incorporating the repeat diethynyl-thiophene units. Comparison of the reduction potentials for the mononuclear and binuclear complexes reveals that the reduction steps are localized either at the terpy fragments of the T_n ligands or at the terpy peripheral ligands. The spectroscopic results (absorption spectra at room temperature, luminescence spectra and lifetimes at room temperature and at 77 K) in acetonitrile solvent are consistent with the establishment of electronic delocalization within the oligomeric diethynyl-thiophene fragments (DEDBT_n) of the T_n ligands; however, the results also indicate that the terpy units of these ligands and the DEDBT_n fragments are not strongly coupled. Both at room temperature and at 77 K, the ³metal-to-ligand charge-transfer luminescence of RuT_n and RuT_nRu complexes is strongly depressed in the larger species with respect to what happens for n ≤ 2 (where the luminescence quantum yield is φ ≈ 10^-4); this is discussed in terms of the possible intervention of triplet levels localized at the oligothiophene DEDBT_n fragments.