Electronic conductivity of molecular wires is a critical fundamental issue in molecular electronics. pi-Conjugated redox molecular wires with the superior long-range electron-transport ability could be constructed on a gold surface through the stepwise ligand-metal coordination method. The beta(d) value, indicating the degree of decrease in the electron-transfer rate constant with distance along the molecular wire between the electrode and the redox active species at the terminal of the wire, were 0.008-0.07 A(-1) and 0.002-0.004 A(-1) for molecular wires of bis(terpyridine)iron and bis(terpyridine)cobalt complex oligomers, respectively. The influences on beta(d) by the chemical structure of molecular wires and the terminal redox units, temperature, electric field, and electrolyte concentration were clarified. The results indicate that facile sequential electron hopping between neighboring metal-complex units within the wire is responsible for the high electron-transport ability.

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)2C6H4 (3) and 1,3,5-(C[triple bond]C-tpy)3C6H3 (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, k1 (s(-1)), as well as that for intrawire electron transfer between neighboring redox-complex moieties, k2 (cm2 mol(-1) s(-1)). The strong effect of the electrolyte concentration on both k1 and k2 indicates that the counterion motion limits the electron-hopping rate at lower electrolyte concentrations. Analysis of the dependence of k1 and k2 on the potential gave intrinsic kinetic parameters without overpotential effects: (k1(0) = 110 s(-1), k2(0) = 2.6x10(12) cm2 mol(-1) s(-1) for [n Fe3], and k1(0) = 100 s(-1), k2(0) = 4.1x10(11) cm2 mol(-1) s(-1) for [n Fe4] (n = number of complexation cycles).

Surface junction effects on the electron conduction of p-phenylene-bridged bis(terpyridine)iron oligomers terminated with a ferrocene moiety were quantitatively analyzed by employing three different surface-anchoring terpyridine ligands. The dependence of the electron-transfer rate constant for oxidation of the ferrocene moiety, k(et), on the distance between the electrode surface and the ferrocene moiety, x, showed that the attenuation factor, beta(d), which indicates the degree of reduction of k(et) with x, was approximately 0.018 in all cases. However, the absolute k(et) value depended strongly on both electronic and steric factors of the surface-anchoring ligand.

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