Evidence for a hopping mechanism in metal|single molecule|metal junctions involving conjugated metal-terpyridyl complexes; potential-dependent conductances of complexes [M(pyterpy)2]2+ (M = Co and Fe; pyterpy = 4'-(pyridin-4-yl)-2,2':6',2''-terpyridine) in ionic liquid.

Extensive studies of various families of conjugated molecules in metal|molecule|metal junctions suggest that the mechanism of conductance is usually tunnelling for molecular lengths < ca. 4 nm, and that for longer molecules, coherence is lost as a hopping element becomes more significant. In this work we present evidence that, for a family of conjugated, redox-active metal complexes, hopping may be a significant factor for even the shortest molecule studied (ca. 1 nm between contact atoms). The length dependence of conductance for two series of such complexes which differ essentially in the number of conjugated 1,4-C6H4- rings in the structures has been studied, and it is found that the junction conductances vary linearly with molecular length, consistent with a hopping mechanism, whereas there is significant deviation from linearity in plots of log(conductance) vs. length that would be characteristic of tunnelling, and the slopes of the log(conductance)-length plots are much smaller than expected for an oligophenyl system. Moreover, the conductances of molecular junctions involving the redox-active molecules, [M(pyterpy)2]2+/3+ (M = Co, Fe) have been studied as a function of electrochemical potential in ionic liquid electrolyte, and the conductance-overpotential relationship is found to fit well with the Kuznetsov-Ulstrup relationship, which is essentially a hopping description.