Synthesis, spectroscopy, and electrochemical studies of binuclear tris-bipyridine ruthenium(II) complexes with oligothiophene bridges

A new series of bipyridine-capped oligothiophene ligands and their binuclear Ru(II) complexes of the general formula [(bpy)(2)Ru-bpy(th)(x)bpy-Ru(bpy)(2)](4+) (where bpy = 2,2'-bipyridyl, th = 2,5-thienyl, and x = 1, 3, or 6) has been synthesized. Comparison of the bipyridine-capped oligothiophenes to the corresponding uncapped oligomers shows that there is coupling of the bipyridine to the oligothiophene-conjugated pi -system. Each of the bipyridine-capped ligands reduces at a bipyridine cap at potentials less negative than those of free bipyridine. The lambda (max) values of the lowest pi-pi* transition increase and the E-pa values for the first oxidation decrease with an increase in the number of thiophene rings in the bridge. The increase in the number of thiophene rings also leads to more accessible oxidation states, up to four for the x = 6 compound. Upon complexation of the ligands with the (bpy)(2)Ru2+ moiety the electronic spectra of the complexes show a significant redshift in comparison to the spectra of Ru(bpy)(3)(2+), the oligothiophene-based oxidation(s) shifts to more positive potentials, and new oxidation and reduction processes are also observed. A single concurrent Ru(II/III) oxidation process is observed in all cases at about the same potential (E-ave(0) = 1.32 V (acetonitrile solution)) as is observed in the parent complex Ru(bpy)(3)(2+), suggesting that the couples are not strongly coupled. The series of complexes shows a unique range of ligand-based oxidation processes with respect to the position of the Ru(II/III) redox process. In the case of x = 1, no ligand-based oxidation process is observed; for x = 3, the first oligothiophene oxidation occurs at potentials less positive than those of the Ru(II/III) redox process, and for x = 6, two oligothiophene oxidations are less positive than the Ru(II/III) redox process. A series of bpy reduction processes that are similar to those observed for Ru(bpy)(3)(2+) also occurs. The x = 1 compound shows two, one-electron reductions and then two, two-electron reductions; the x = 3 and x = 6 compounds show three, two-electron reduction processes. The first reductions occur at the capping bipyridines of the bridging ligand in all three complexes, and subsequent reductions occur at the ancillary bipyridine ligands in a stepwise fashion.