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 binuclea r 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 bipyri dine-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 increa se 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 ligand s 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 concurren t Ru(II/III) oxidation process is observed in all cases at about the same p otential (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 oxid ation processes with respect to the position of the Ru(II/III) redox proces s. In the case of x = 1, no ligand-based oxidation process is observed; for x = 3, the first oligothiophene oxidation occurs at potentials less positi ve than those of the Ru(II/III) redox process, and for x = 6, two oligothio phene oxidations are less positive than the Ru(II/III) redox process. A ser ies of bpy reduction processes that are similar to those observed for Ru(bp y)(3)(2+) also occurs. The x = 1 compound shows two, one-electron reduction s 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 sub sequent reductions occur at the ancillary bipyridine ligands in a stepwise fashion.