Recent developments of single‐molecule conductance measurements allow us to understand fundamental conducting properties of molecular wires. While a wide variety of organic molecular wires have been ...studied so far, inorganic and organometallic molecular wires have received much less attention. However, molecular wires with transition‐metal atoms show interesting features and functions distinct from those of organic wires. These properties originate mainly from metal–ligand dπ–pπ interactions and metal–metal d–d interactions. Thanks to the rich combination of metal atoms and supporting ligands, frontier orbital energies of the molecular wires can be finely tuned to lead to highly conducting molecular wires. Moreover, the unique electronic structures of metal complexes are susceptible to subtle environmental changes, leading to potential functional molecular devices. This article reviews recent advances in the single‐molecule conductance study of inorganic and organometallic molecular wires.
Recent developments for single‐molecule conductance measurements allow to access fundamental properties of molecular wires in metal–molecule–metal junctions. This Concept article presents a review of advances in recent single molecule conductance studies with metal complexes.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A single molecular junction is a nanoscale structure prepared by bridging a single molecule between macroscopic metal electrodes. It has attracted significant attention due to its unique structure ...and potential applications in ultra-small single molecular electronic devices. It has two metal-molecule interfaces, and thus the electronic structure of the molecule can be significantly modulated from its original one. The single molecular junction can be regarded as a new material that includes metal electrodes, a so-called “double interface material”. Therefore, we can expect unconventional physical and chemical properties. To develop a better understanding of the properties and functionalities of single molecular junctions, their atomic and electronic structures should be characterized. In this review, we describe the development of these characterization techniques, such as inelastic electron tunneling spectroscopy, surface-enhanced Raman scattering, as well as shot noise and thermopower measurements. We have also described some unique properties and functionalities of single molecular junctions, such as switching and diode properties.
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FFLJ, NUK, ODKLJ, UL, UM, UPUK
Aromaticity is a fundamental concept in chemistry. It is described by Hückel's rule that states that a cyclic planar π-system is aromatic when it shares 4n+2 π-electrons and antiaromatic when it ...possesses 4n π-electrons. Antiaromatic compounds are predicted to exhibit remarkable charge transport properties and high redox activities. However, it has so far only been possible to measure compounds with reduced aromaticity but not antiaromatic species due to their energetic instability. Here, we address these issues by investigating the single-molecule charge transport properties of a genuinely antiaromatic compound, showing that antiaromaticity results in an order of magnitude increase in conductance compared with the aromatic counterpart. Single-molecule current-voltage measurements and ab initio transport calculations reveal that this results from a reduced energy gap and a frontier molecular resonance closer to the Fermi level in the antiaromatic species. The conductance of the antiaromatic complex is further modulated electrochemically, demonstrating its potential as a high-conductance transistor.
Abstract
The electrical properties of DNA have been extensively investigated within the field of molecular electronics. Previous studies on this topic primarily focused on the transport phenomena in ...the static structure at thermodynamic equilibria. Consequently, the properties of higher-order structures of DNA and their structural changes associated with the design of single-molecule electronic devices have not been fully studied so far. This stems from the limitation that only extremely short DNA is available for electrical measurements, since the single-molecule conductance decreases sharply with the increase in the molecular length. Here, we report a DNA zipper configuration to form a single-molecule junction. The duplex is accommodated in a nanogap between metal electrodes in a configuration where the duplex is perpendicular to the nanogap axis. Electrical measurements reveal that the single-molecule junction of the 90-mer DNA zipper exhibits high conductance due to the delocalized π system. Moreover, we find an attractive self-restoring capability that the single-molecule junction can be repeatedly formed without full structural breakdown even after electrical failure. The DNA zipping strategy presented here provides a basis for novel designs of single-molecule junctions.
Bowl-shaped π-conjugated compounds, or buckybowls, are a novel class of sp2-hybridized nanocarbon materials. In contrast to tubular carbon nanotubes and ball-shaped fullerenes, the buckybowls feature ...structural flexibility. Bowl-to-bowl structural inversion is one of the unique properties of the buckybowls in solutions. Bowl inversion on a surface modifies the metal–molecule interactions through bistable switching between bowl-up and bowl-down states on the surface, which makes surface-adsorbed buckybowls a relevant model system for elucidation of the mechano-electronic properties of nanocarbon materials. Here, we report a combination of scanning tunneling microscopy (STM) measurements and ab initio atomistic simulations to identify the adlayer structure of the sumanene buckybowl on Au(111) and reveal its unique bowl inversion behavior. We demonstrate that the bowl inversion can be induced by approaching the STM tip toward the molecule. By tuning the local metal–molecule interaction using the STM tip, the sumanene buckybowl exhibits structural bistability with a switching rate that is two orders of magnitude faster than that of the stochastic inversion process.
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IJS, KILJ, NUK, PNG, UL, UM
By use of the reversible trimerization of boronic acids, the series of boroxine cages 3-mer, 6-mer, and 12-mer were constructed from rationally designed diboronic acids whose bond angles between two ...C–B bonds are 60°, 84°, and 117°, respectively. Boroxine cages 6-mer and 12-mer have 1.5 and 2.5 nm sized cavities, respectively.
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IJS, KILJ, NUK, PNG, UL, UM
Structural and electronic detail at the metal–molecule interface has a significant impact on the charge transport across the molecular junctions, but its precise understanding and control still ...remain elusive. On the single-molecule scale, the metal–molecule interface structures and relevant charge transport properties are subject to fluctuation, which contain the fundamental science of single-molecule transport and implication for manipulability of the transport properties in electronic devices. Here, we present a comprehensive approach to investigate the fluctuation in the metal–molecule interface in single-molecule junctions, based on current–voltage (I–V) measurements in combination with first-principles simulation. Contrary to conventional molecular conductance studies, this I–V approach provides a correlated statistical description of both the degree of electronic coupling across the metal–molecule interface and the molecular orbital energy level. This statistical approach was employed to study fluctuation in single-molecule junctions of 1,4-butanediamine (DAB), pyrazine (PY), 4,4′-bipyridine (BPY), and fullerene (C60). We demonstrate that molecular-dependent fluctuation of σ-, π-, and π-plane-type interfaces can be captured by analyzing the molecular orbital (MO) energy level under mechanical perturbation. While the MO level of DAB with the σ-type interface shows weak distance dependence and fluctuation, the MO level of PY, BPY, and C60 features unique distance dependence and molecular-dependent fluctuation against the mechanical perturbation. The MO level of PY and BPY with the σ+π-type interface increases with the increase in the stretch distance. In contrast, the MO level of C60 with the π-plane-type interface decreases with the increase in the stretching perturbation. This study provides an approach to resolve the structural and electronic fluctuation in the single-molecule junctions and insight into the molecular-dependent fluctuation in the junctions.
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IJS, KILJ, NUK, PNG, UL, UM
Here, we report multinuclear organometallic molecular wires having (2,5-diethynylthiophene)diyl-Ru(dppe)
2
repeating units. Despite the molecular dimensions of 2-4 nm the multinuclear wires show high ...conductance (up to 10
−2
to 10
−3
G
0
) at the single-molecule level with small attenuation factors (
β
) as revealed by STM-break junction measurements. The high performance can be attributed to the efficient energy alignment between the Fermi level of the metal electrodes and the HOMO levels of the multinuclear molecular wires as revealed by DFT-NEGF calculations. Electrochemical and DFT studies reveal that the strong Ru-Ru interaction through the bridging ligands raises the HOMO levels to access the Fermi level, leading to high conductance and small
β
values.
Multinuclear organometallic molecular wires having (diethynylthiophene)diyl-Ru(dppe)
2
repeating units show high conductance with small attenuation factors. The strong Ru-Ru interaction is the key for the long-range carrier transport.
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