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This article completes our comprehensive understanding of the electron transport properties of our original π-conjugated redox-active molecular wires comprising Fe bridged by p-phenylene linkers (tpy=2,2′:6′,2′′-terpyridine). The Fe(tpy)2 oligomer wires comprise three types of tpy ligands: the anchor tpy ligand ( A series) makes a junction between the wire and electrode, the bridging bis-tpy ligand ( L series) connects the Fe(tpy)2 units, and the terminal tpy ligand ( T series) possesses a redox site as a probe for the long-range electron transport ability. Taking advantage of the precise tunability of the composition of the Fe(tpy)2 oligomer wires, thus far we investigated how A and L impacted on the electron-transport ability. The excellent long-range electron transport ability with ultrasmall attenuation constants ( βd, 0.002 Å−1 as the minimum) depends on L significantly [ Chem. Asian J. 2009, 4, 1361], whereas A is unrelated to the βd value, but influences the zero-distance electron-transfer rate constant, ket0 [ J. Am. Chem. Soc. 2010, 132, 4524]. Herein we study the influence of terminal ligand T x ( x=1-3). βd is independent of T, however, T3, with a cyclometallated Ru complex as the redox site, gives rise to a ket0 value greater than T1 and T2 with ferrocene. This series of simple but definitive conclusions indicates that we have reached the stage of being able to precisely design molecular wires to attain desirable single-molecule electron conduction. [ABSTRACT FROM AUTHOR]

π-Conjugated bis(terpyridine)metal (M(tpy)2) oligomer wires were prepared by surface coordination programming such that they displayed an inherent potential gradient along the wire through the chain of metals and bridging ligands. The electron transfer properties of these wires were examined. Heterometal complex wires prepared in the sequence Au(111)-Co(tpy)2-Fe(tpy)2-ferrocene (Fc); that is, Au-[ A azo -1CoL H -1FeT Fc] (( A azo)2 = bis((4-(4′-2,2′;6′,2″-terpyridyl)phenylazo)phenyl)disulfide, L H = bis(terpyridyl)- p-phenylene), displayed a rate constant, ket, for electron transfer at the terminal Fc moiety that was twice that obtained from wires prepared in the sequence electrode-Fe(tpy)2-Co(tpy)2-Fc, Au-[ A azo -1FeL H -1CoT Fc]. This suggested that the oxidation reaction at the terminal Fc moiety was mediated by electron transfer via Co(tpy)2 more efficiently than via Fe(tpy)2. Metal complex wires with an inherent potential gradient based on the heteroligand combination were synthesized using bis(4-(4′-2,2′;6′,2″-terpyridyl) phenyl)disulfide, ( A H)2, or bis(4-(4′-5,5″-difluoro-2,2′;6′,2″-terpyridyl)phenyl)disulfide, ( A F)2, as anchoring ligands and bis(terpyridyl)- p-phenylene, LH, or bis(terpyridyl)-tetrafluoro- p-phenylene, L F, as bridging ligands. Potential step chronoamperometry (PSCA) measurements of the redox reaction of Au-[ A H -1FeL 1 -2FeL F] and Au-[ A F -1FeL F -2FeL H] revealed only small differences in the current-time curves, indicating that the electron self-exchange between the Fe(tpy)2 units in a wire was faster than electron transfer between an electrode surface and the adjacent Fe(tpy)2 units. Fast electron self-exchange suppressed the expected rectification behavior of the wire, despite the presence of a potential staircase within the wire. [ABSTRACT FROM AUTHOR]

Abstract: We previously contributed a review of bis(terpyridine) metal complex wires (Coord. Chem. Rev. 251 (2007) 2674) in which we described the bottom-up fabrication of complex wires on Au/mica electrodes, and intra-wire redox conduction by successive electron hopping between neighboring redox sites. The present review considers recent progress on bis(terpyridine) metal complex wire chemistry, with a focus on the long-range electron transport abilities of such wires. Potential step chronoamperometry (PSCA) is used to quantify electron transport between the terminal redox site of the wires and the Au/mica electrode, revealing an extremely small attenuation constant, β d . The second topic examined in this study is the fabrication of bis(terpyridine) metal complex wires on semiconducting silicon electrodes, which show dopant-dependent and photo-switchable intra-wire redox conduction. [Copyright &y& Elsevier]