Synthesis of molecular wires of linear and branched bis(terpyridine)-complex oligomers and electrochemical observation of through-bond redox conduction

Films of linear and branched oligomer wires of Fe(tpy)(2) (tpy = 2 2':6' 2-terpyridine) were constructed on a gold-electrode surface by the interfacial stepwise coordination method, in which a surface-anchoring ligand, (tpy-C6H4N=NC6H4-S)(2) (1), two bridging ligands, 1,4-(tpy)(2)C6H4 (3) and 1,3,5-(C=C-tpy)(3)C6H3 (4), and metal ions were used. The quantitative complexation of the ligands and Fe-II ions was monitored by electrochemical measurements in up to eight complexation cycles for linear oligomers of 3 and in up to four cycles for branched oligomers of 4. STM observation of branched oligomers at low surface coverage showed an even distribution of nanodots of uniform size and shape, which suggests the quantitative formation of dendritic structures. The electron-transport mechanism and kinetics for the redox reaction of the films of linear and branched oligomer wires were analyzed by potential-step chronoamperometry (PSCA). The unique current-versus-time behavior observed under all conditions indicates that electron conduction occurs not by diffusional motion but by successive electron hopping between neighboring redox sites within a molecular wire. Redox conduction in a single molecular wire in a redox-polymer film has not been reported previously. The analysis provided the rate constant for electron transfer between the electrode and the nearest redox-complex moiety, k(1) (s(-1)), as well as that for intrawire electron transfer between neighboring redox-complex moieties, k(2) (cm(2) mol(-1) s(-1)). The strong effect of the electrolyte concentration on both k(1) and k(2) indicates that the counterion motion limits the electron-hopping rate at lower electrolyte concentrations. Analysis of the dependence of k(1) and k(2) on the potential gave intrinsic kinetic parameters without overpotential effects: k(1)(0)= 110 s(-1), k(2)(0)=2.6x10(12) cm(2) mol(-1) s(-1) for [nFe3], and k(1)(0)=100 s(-1), k(2)(0)=4.1 x 10(11) cm(2) mol(-1) s(-1) for [n Fe4] (n = number of complexation cycles).