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Molecular junctions consisting of a Ru(bpy) oligomer between conducting carbon contacts exhibit an exponential dependence of junction current on molecular layer thickness (d) similar to that observed for other aromatic devices when d < 4 nm. However, when d > 4 nm, a change in transport mechanism occurs which coincides with light emission in the range of 600-900 nm. Unlike light emission from electrochemical cells or solid-state films containing Ru(bpy), emission is bipolar, occurs in vacuum, has rapid rise time (<5 ms), and persists for >10 h. Light emission directly indicates simultaneous hole and electron injection and transport, possibly resonant due to the high electric field present (>3 MV/cm). Transport differs fundamentally from previous tunneling and hopping mechanisms and is a clear "molecular signature" relating molecular structure to electronic behavior.

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

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Thin layers of oligomers with thickness between 7 and 9 nm were deposited on flat gold electrode surfaces by electrochemical reduction of diazonium reagents, then a Ti(2 nm)/Au top contact was applied to complete a solid-state molecular junction. The molecular layers investigated included donor molecules with relatively high energy HOMO, molecules with high HOMO-LUMO gaps and acceptor molecules with low energy LUMO and terminal alkyl chain. Using an oligo(bisthienylbenzene) based layer, a molecule whose HOMO energy level in a vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, i.e., tetrafluorobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of naphtalene diimides having low LUMO energy level. Rectification persisted at low temperature (7 K), and was activationless between 7 and 100 K. The results show that rectification is induced by the asymmetric contact but is also directly affected by orbital energies of the molecular layer. A "molecular signature" on transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed, and the rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

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Thin layers of viologen-based oligomers with thicknesses between 3 and 14 nm were deposited on gold electrodes by electrochemical reduction of a diazonium salt, and then a Ti/Au top contact was applied to complete a solid-state molecular junction (MJ). MJs show symmetric J- V curves and highly efficient long-range transport, with an attenuation factor as small as 0.25 nm. This is attributed both to the fact that the viologen LUMO energy lies between the energies of the Fermi levels of the two contacts and to strong electronic coupling between molecules and contacts. As a consequence, resonant tunneling is likely to be the dominant transport mechanism within these MJs, but the temperature dependence of the transport properties suggests that activated redox hopping plays a role at high temperature.

Cobalt terpyridine oligomers are compared with π‐conjugated and ruthenium‐centered layers in molecular junctions (MJs) with identical contacts. A wide range of layer thickness is investigated, and attenuation plots are obtained. Strong dependence of charge transport on molecular layers is found with a variation of four orders of magnitude of current density (  J) for different molecules and d = 7 nm. For a Ru(bpy)3 complex and bis‐thienylbenzene MJs, the attenuation plot shows two different regions corresponding to two different dominant transport mechanisms. On the contrary Co(tpy)2 and viologen‐based MJs show no transition thickness in the attenuation plot, indicating a possible change of mechanism with film thickness, and very low attenuation factors (β of 0.17 and 0.25 nm−1 from 2 to 14 nm, respectively). These β values indicate highly efficient long‐range transport. This is attributed to the fact that the energy levels of the frontier orbital involved in transport are between, and thus almost in resonance with, the Fermi levels of the electrodes. Temperature‐dependence measurements suggest that field ionization followed by multistep hopping and redox events can occur above 100 K, while the activationless region at low T indicates incoherent tunneling between redox sites with reorganization concerted with charge transfer.