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Synthesis and Electron Transfer Properties of Metal Complex Oligomer Wires with an Inherent Potential Gradient on Gold Electrode



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-[Aazo-1CoLH-1FeTFc] ((Aazo)2 = bis((4-(4′-2,2′;6′,2″-terpyridyl)phenylazo)phenyl)disulfide, LH = 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-[Aazo-1FeLH-1CoTFc]. 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, (AH)2, or bis(4-(4′-5,5″-difluoro-2,2′;6′,2″-terpyridyl)phenyl)disulfide, (AF)2, as anchoring ligands and bis(terpyridyl)-p-phenylene, LH, or bis(terpyridyl)-tetrafluoro-p-phenylene, LF, as bridging ligands. Potential step chronoamperometry (PSCA) measurements of the redox reaction of Au-[AH-1FeL1-2FeLF] and Au-[AF-1FeLF-2FeLH] 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.