Since it was first suggested1 that a single molecule might function as an active electronic component, a number of techniques have been developed to measure the charge transport properties of single molecules2,3,4,5,6,7,8,9,10,11,12. Although scanning tunnelling microscopy observations under high vacuum conditions can allow stable measurements of electron transport, most measurements of a single molecule bonded in a metal–molecule–metal junction exhibit relatively large variations in conductance. As a result, even simple predictions about how molecules behave in such junctions have still not been rigorously tested. For instance, it is well known13,14 that the tunnelling current passing through a molecule depends on its conformation; but although some experiments have verified this effect15,16,17,18, a comprehensive mapping of how junction conductance changes with molecular conformation is not yet available. In the simple case of a biphenyl—a molecule with two phenyl rings linked by a single C–C bond—conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. Here we use amine link groups to form single-molecule junctions with more reproducible current–voltage characteristics19. This allows us to extract average conductance values from thousands of individual measurements on a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through π-conjugated biphenyl systems13.

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Dependence of single-molecule junction conductance on molecular conformation

Metal-metal interactions in binuclear complexes exhibiting mixed valency; molecular wires and switches

Organometallic molecular wires and other nanoscale-sized devices: An approach using the organoiron (dppe)Cp*Fe building block

Single molecule electronic devices in which individual molecules are utilized as active electronic components constitute a promising approach for the ultimate miniaturization and integration of electronic devices in nanotechnology through the bottom-up strategy. Thus, the ability to understand, control, and exploit charge transport at the level of single molecules has become a long-standing desire of scientists and engineers from different disciplines for various potential device applications. Indeed, a study on charge transport through single molecules attached to metallic electrodes is a very challenging task, but rapid advances have been made in recent years. This review article focuses on experimental aspects of electronic devices made with single molecules, with a primary focus on the characterization and manipulation of charge transport in this regime.

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Single Molecule Electronic Devices

Publisher Summary This chapter discusses the theory of mixed-valence systems and theory of electron exchange. In the case of strong coupling, for which it is no longer appropriate to use perturbation theory, the interaction of states is so great that bonding and antibonding potential energy curves result. Because the ground state is delocalized between metal ions, it is not strictly appropriate to describe the intervalence transition band of a class III complex as a metal-to-metal charge transfer transition. The metal-to-metal charge transfer (MMCT) designation is used for the sake of simplicity. The dependence of intervalence transition energy on the activation barrier to thermal electron transfer allows the solvent dependence of intervalence transitions to be understood in terms of relationships developed for the Marcus Theory of electron transfer. The Hush model is the preferred method of analysis of mixed valence complexes for the experimentalist because of its readily understandable derivation, its overlap with the Marcus theory of electron transfer, and the facility of its application. However, it is applicable only to weakly coupled class II complexes. A quantitative theory that is applicable to all mixed valence complexes is desirable.

Intervalence Charge Transfer and Electron Exchange Studies of Dinuclear Ruthenium Complexes

The possibility of molecular switching is investigated theoretically on a model system in which two redox active sites are connected by 4,4′-bipyridine. The electronic coupling between these sites is determined by two methods: the “dimer splitting” method and the effective Hamiltonian method. The pathways for electronic coupling are analyzed and computed for all dihedral angles between the pyridine cycles. It is found that the π-π coupling dominates the electronic interaction for most angle values. However, in strictly perpendicular conformation, the electronic interaction does not vanish, due to the existence of crossed σ-π interactions.

The possibility of molecular switching: Theoretical study of [(NH3)5Ru-4,4′-bipy-Ru(NH3)5]5+

Experimental results on the decay of the effective coupling parameter in mixed valence ruthenium complexes bridged by bipyridylpolyenes are discussed. Absolute values of the couplings are satisfactorily reproduced by extended Huckel calculations. An exponential decrease of the effective coupling is found, with a particularly small exponent, 0.078 A−1, corresponding to a decrease by a factor of 2 for each 9 A in the metal-metal distance. The results are also discussed in the frame of a simplified valence bond taking into account the main structural features of the system. It is concluded that, in the present compounds, the transmission of electronic couplings over large distances is limited by the Peierls distortion of the polyene chain.

S. Woitellier

Papers

The possibility of molecular switching: Theoretical study of [(NH3)5Ru-4,4′-bipy-Ru(NH3)5]5+

Distance dependence of the effective coupling parameters through conjugated ligands of the polyene type