Hopping Conductance in Molecular Wires Exhibits a Large Heavy-Atom Kinetic Isotope Effect

We report a large kinetic isotope effect (KIE) for intramolecular charge transport in π-conjugated
oligophenyleneimine (OPI) molecules connected to Au electrodes. 13C and 15N substitution on the imine bonds produces a
conductance KIE of ∼2.7 per labeled atom in long OPI wires >4 nm in length, far larger than typical heavy-atom KIEs for chemical
reactions. In contrast, isotopic labeling in shorter OPI wires <4 nm does not produce a conductance KIE, consistent with a direct
tunneling mechanism. Temperature-dependent measurements reveal that conductance for a long 15N-substituted OPI wire is
activated, and we propose that the exceptionally large conductance KIEs imply a thermally assisted, through-barrier polaron
tunneling mechanism. In general, observation of large conductance KIEs opens up considerable opportunities for understanding
microscopic conduction mechanisms in π-conjugated molecules.