Conductance Of Molecular Wires Research Articles

Front Cover: Large Enhancement of the Single‐Molecular Conductance of a Molecular Wire through a Radical Substituent (Chem. Eur. J. 27/2022)

The molecular conductance of radical-substituted molecular wire was evaluated by STM apparent-height measurement compared with a non-radical-substituted wire. Statistical analysis of the STM images revealed that the radical-substituted wire has a 3.2-fold larger conductance. Although only 17 % of the SOMO is calculated to exist on the wire moiety, the effect was significant. This study presents the potential of radical substituents to achieve high conductivity in molecular wires. More information can be found in the Research Article by K. Matsuda and co-workers (DOI: 10.1002/chem.202104242).

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Giant Conductance Enhancement of Intramolecular Circuits through Interchannel Gating

Summary For neutral intramolecular circuits with two constitutionally identical branches, a maximum 4-fold increase in total conductance can be obtained according to constructive quantum interference (CQI). For charged intramolecular circuits, however, the strong electrostatic interactions entangle the quantum states of these two parallel pathways, thus introducing complicated transport behavior that warrants experimental investigation of the intramolecular circuit rules. Here, we report that a tetracationic cyclophane with parallel channels exhibits a 50-fold conductance enhancement compared with that of a single-channel control, an observation that supplements intramolecular circuit law in systems with strong Coulombic interactions. Flicker noise measurements and theoretical calculations show that strong electrostatic interactions between charged parallel channels—serving as the chemical gate to promote the effective conductance of each channel—and CQI boosts the total conductance of the two-channel circuit. The molecular design presented herein constitutes a proof-of-principle approach to charged intramolecular circuits that are desirable for quantum circuits and devices.

STM apparent height measurements of molecular wires with different physical length attached on 2-D phase separated templates for evaluation of single molecular conductance.

Single molecular conductance of molecular wires is effectively evaluated by the combination of STM apparent height measurement and a 2-D phase separation technique. Previously the method was only applied to a set of molecular wires with the same physical length, but herein we applied the method to thienylene-based and phenylene-based molecular wires with different physical lengths. By considering the difference in physical molecular height including thermal contribution of conformational isomers, the conductance ratio was determined to be 1.3 ± 0.7, which is in agreement with the reported value determined by a break-junction method.

Features of superexchange nonresonant tunneling conductance in anchored molecular wires

A modified superexchange model is used to clarify the physical mechanisms for the formation of nonresonant tunneling conductance in terminated molecular wires. Due to the specific relationship between its key parameters, this model has wider areas of applicability compared to the flat-barrier model and the standard superexchange model, which are widely involved for the physical interpretation of experimental results. Moreover, the results obtained in the two latest models appear in the modified model as characteristic limiting cases. Our estimates show that the exponential decay of conductance, characterized by an attenuation factor β (per repeating unit), is limited by the conditions β ≤ 1.2 and β ≥ 3.7 for the flat-barrier and standard models, respectively. At the same time, the modified superexchange model yields β > 0, which, thus, allows us to analyze the tunneling conductance in molecular wires containing both saturated and conjugated bonds. We also show that for a small number of N repeating wire units (about 3–6 depending on the value of β), the exponential dependence of conductance on N is violated and, accordingly, contact conductance is not identical to conductance at N = 0. Formulas are found which, on the basis of experimental data, make it possible to establish the values of superexchange parameters as well as indicate the conditions of possible hybridization between the orbitals of the anchor groups and the adjacent end units belonging to the interior wire region. One example is the establishment of features in the tunneling conductance of terminated alkane chains caused by the nature of their anchor groups.

Assessing the Reversed Exponential Decay of the Electrical Conductance in Molecular Wires: The Undeniable Effect of Static Electron Correlation.

An extraordinary new family of molecular junctions, inaccurately referred to as "anti-Ohmic" wires in the recent literature, has been proposed based on theoretical predictions. The unusual electron transport observed for these systems, characterized by a reversed exponential decay of their electrical conductance, might revolutionize the design of molecular electronic devices. This behavior, which has been associated with intrinsic diradical nature, is reexamined in this work. Since the diradical character arises from a near-degeneracy of the frontier orbitals, the employment of a multireference approach is mandatory. CASSCF calculations on a set of nanowires based on polycyclic aromatic hydrocarbons (PAHs) demonstrate that, in the frame of an appropriate multireference treatment, the ground state of these systems shows the expected exponential decay of the conductance. Interestingly, these calculations do evidence a reversed exponential decay of the conductance, although now in several excited states. Similar results have been obtained for other recently proposed candidates to "anti-Ohmic" wires. These findings open new horizons for possible applications in molecular electronics of these promising systems.

Modulation of the conductance in platinum(ii) bis(acetylide) molecules through “gating” metal ions

The conductance of molecular wires was precisely regulated by “gating” metals installed on the side of the conductive backbone.

The length and hydrogenation effects on electronic transport properties of carbon-based molecular wires

Abstract It is known that the conductance of a wire decreases linearly with the increase of its length in the macro circuit. But things are different in mesoscopic field, especially in molecular dimensional circuits due to the quantum interferences in molecular size. In this work, first principles is used to investigate how wire length and hydrogenation degree influence the conductance of molecular wires. For comparison, two types of carbon-based molecular wires, single carbon atomic wire and phenyl ring wire with single hydrogenation or dihydrogenation, are chosen and coupled to two zigzag graphene nanoribbon electrodes, respectively. We show that the conductance of zigzag carbon atomic wire with single hydrogenation exhibits an evident even-odd oscillatory behavior with the increase of wire length, while that of phenyl ring wire starts to increase before decreasing. However, when the above types of wires are edged with dihydrogenation, their conductance decreases exponentially with the increasing wire length. In addition, fine current rectification is found in asymmetric phenyl wires induced by hydrogenation degree, which can be used as diode in future molecular devices. Analyses via orbital hybridization, transmission path, HOMO-LUMO position and so on reveal the microscopic mechanism of the above interesting results.

Charge transport in molecular junctions: From tunneling to hopping with the probe technique.

We demonstrate that a simple phenomenological approach can be used to simulate electronic conduction in molecular wires under thermal effects induced by the surrounding environment. This "Landauer-Büttiker's probe technique" can properly replicate different transport mechanisms, phase coherent nonresonant tunneling, ballistic behavior, and hopping conduction. Specifically, our simulations with the probe method recover the following central characteristics of charge transfer in molecular wires: (i) the electrical conductance of short wires falls off exponentially with molecular length, a manifestation of the tunneling (superexchange) mechanism. Hopping dynamics overtakes superexchange in long wires demonstrating an ohmic-like behavior. (ii) In off-resonance situations, weak dephasing effects facilitate charge transfer, but under large dephasing, the electrical conductance is suppressed. (iii) At high enough temperatures, kBT/ϵB > 1/25, with ϵB as the molecular-barrier height, the current is enhanced by a thermal activation (Arrhenius) factor. However, this enhancement takes place for both coherent and incoherent electrons and it does not readily indicate on the underlying mechanism. (iv) At finite-bias, dephasing effects may impede conduction in resonant situations. We further show that memory (non-Markovian) effects can be implemented within the Landauer-Büttiker's probe technique to model the interaction of electrons with a structured environment. Finally, we examine experimental results of electron transfer in conjugated molecular wires and show that our computational approach can reasonably reproduce reported values to provide mechanistic information.

Fine-tuning of single-molecule conductance by tweaking both electronic structure and conformation of side substituents.

Herein, we describe a method to fine-tune the conductivity of single-molecule wires by employing a combination of chemical composition and geometrical modifications of multiple phenyl side groups as conductance modulators embedded along the main axis of the electronic pathway. We have measured the single-molecule conductivity of a novel series of phenyl-substituted carotenoid wires whose conductivity can be tuned with high precision over an order of magnitude range by modulating both the electron-donating character of the phenyl substituent and its dihedral angle. It is demonstrated that the electronic communication between the phenyl side groups and the molecular wire is maximized when the phenyl groups are twisted closer to the plane of the conjugated molecular wire. These findings can be refined to a general technique for precisely tuning the conductivity of molecular wires.

Effect of metal complexation on the conductance of single-molecular wires measured at room temperature.

The present work aims to give insight into the effect that metal coordination has on the room-temperature conductance of molecular wires. For that purpose, we have designed a family of rigid, highly conductive ligands functionalized with different terminations (acetylthiols, pyridines, and ethynyl groups), in which the conformational changes induced by metal coordination are negligible. The single-molecule conductance features of this series of molecular wires and their corresponding Cu(I) complexes have been measured in break-junction setups at room temperature. Experimental and theoretical data show that no matter the anchoring group, in all cases metal coordination leads to a shift toward lower energies of the ligand energy levels and a reduction of the HOMO-LUMO gap. However, electron-transport measurements carried out at room temperature revealed a variable metal coordination effect depending on the anchoring group: upon metal coordination, the molecular conductance of thiol and ethynyl derivatives decreased, whereas that of pyridine derivatives increased. These differences reside on the molecular levels implied in the conduction. According to quantum-mechanical calculations based on density functional theory methods, the ligand frontier orbital lying closer to the Fermi energy of the leads differs depending on the anchoring group. Thereby, the effect of metal coordination on molecular conductance observed for each anchoring could be explained in terms of the different energy alignments of the molecular orbitals within the gold Fermi level.

Electronic coupling for charge transfer in donor–bridge–acceptor systems. Performance of the two-state FCD model

Electronic coupling is a key parameter that determines the rate of electron transfer reactions and electrical conductivity of molecular wires. To examine the performance of a two-state approach based on the orthogonal transformation of adiabatic states to diabatic states, we compare the effective donor-acceptor coupling V(DA) computed with three different approaches in model donor-bridge-acceptor (D-B-A) systems. It is found that V(DA) derived with the two-state method accounts properly for both the direct and superexchange interactions. The approach becomes, however, less accurate with the increasing energy difference of the donor and acceptor states. We suggest a simple diagnostic to identify the situation when the estimated coupling might be inaccurate and consider how to improve the performance of the two-state scheme in such a case.

Designing molecular devices by altering bond lengths

The work focuses on a theoretical approach to investigating the electric field (EF) dependence of bond-length alternation, the geometric and electronic structures of molecular wires used in the design of molecular electronic devices, the EF dependence of SCF energy, and the spatial distribution of the frontier orbitals of the molecular wires. Just as the bond length is an important influence on the conductance of the molecular wire, the dependence of the conductance on the chain length was also studied. We have also investigated how the current-voltage (I-V) characteristics change with bond length, as the bond length plays an important role in determining the conductance of molecular wires.

Self-assembled multilayers of CdSe nanocrystals and carboxylate-handled phenylene-based molecules: optical, electrochemical and photoconductive properties

A series of linear phenylene-based molecules, bearing carboxylic acid or carboxylate functionalities at both ends, were reacted with (hexadecylamine/stearate)-capped CdSe nanocrystals (7.5 nm diameter) dispersed in CHCl3 to form multilayers on ITO-glass surfaces via layer-by-layer (LBL) alternation. The new multilayered materials were investigated by UV-Vis spectroscopy, cyclic voltammetry, photoelectrochemistry and photoconductivity. The conjugated linkers, with reversible reduction potentials in the range −1.7 to −2.8 V vs. Ag/Ag+, ease charge transport between dots (LUMO at −1.4 V). Photoconduction is dominated by interdot distance and linker characteristics rather than by LUMO energy levels. The logarithm of photoconductivity and linker length are linearly related, as in conduction of molecular wires, and the tunnelling attenuation factor β value is 2.4 nm−1. Comparison of n-type aromatics and p-type oligothiophenes is discussed.

Controlling the conductance of molecular wires by defect engineering

Understanding the charge transport mechanisms in nanoscale structures is essential for the development of molecular electronic devices. Charge transport through one-dimensional (1D) molecular systems connected between two contacts is influenced by several parameters, such as the electronic structure of the molecule and the presence of disorder and defects. In this work, we have modeled 1D molecular wires connected between electrodes and systematically investigated the influence of both soliton formation and the presence of defects on properties such as conductance and the density of states. Our numerical calculations have shown that the transport properties are highly sensitive to the positions of both the solitons and the defects. Interestingly, the introduction of a single defect in the molecular wire that divides it into two fragments, both consisting of an odd number of sites, creates a new conduction channel at the center of the band gap, resulting in higher zero-bias conductance than for defect-free systems. This phenomenon suggests alternative routes for the engineering of molecular wires with enhanced conductance.

Surface Junction Effects on the Electron Conduction of Molecular Wires

Surface junction effects on the electron conduction of p-phenylene-bridged bis(terpyridine)iron oligomers terminated with a ferrocene moiety were quantitatively analyzed by employing three different surface-anchoring terpyridine ligands. The dependence of the electron-transfer rate constant for oxidation of the ferrocene moiety, k(et), on the distance between the electrode surface and the ferrocene moiety, x, showed that the attenuation factor, beta(d), which indicates the degree of reduction of k(et) with x, was approximately 0.018 in all cases. However, the absolute k(et) value depended strongly on both electronic and steric factors of the surface-anchoring ligand.

Superior Electron‐Transport Ability of π‐Conjugated Redox Molecular Wires Prepared by the Stepwise Coordination Method on a Surface

Electronic conductivity of molecular wires is a critical fundamental issue in molecular electronics. pi-Conjugated redox molecular wires with the superior long-range electron-transport ability could be constructed on a gold surface through the stepwise ligand-metal coordination method. The beta(d) value, indicating the degree of decrease in the electron-transfer rate constant with distance along the molecular wire between the electrode and the redox active species at the terminal of the wire, were 0.008-0.07 A(-1) and 0.002-0.004 A(-1) for molecular wires of bis(terpyridine)iron and bis(terpyridine)cobalt complex oligomers, respectively. The influences on beta(d) by the chemical structure of molecular wires and the terminal redox units, temperature, electric field, and electrolyte concentration were clarified. The results indicate that facile sequential electron hopping between neighboring metal-complex units within the wire is responsible for the high electron-transport ability.

Conduction Pathway of π-Stacked Ethylbenzene Molecular Wires on Si(100)

One of the most important challenges of molecular electronics is to enable systematic fabrication of molecular functional components on well-characterized solid-state substrates in a controlled manner. Recently, experimental techniques were developed to achieve such fabrication where lines of pi-stacked ethylbenzene molecules are induced to self-assemble on an H-terminated Si(100) surface at precise locations and along precise directions. In this work, we theoretically analyze charge transport properties of these ethylbenzene wires using a state-of-the-art first-principles technique where density functional theory (DFT) is used within the nonequilibrium Green's function formalism (NEGF). Our device model consists of ethylbenzene stacks bonded to an H-terminated Si(100) surface and bridging two metal leads. The electron transmission spectrum and its associated scattering states as well as the resistance of the molecular wire are determined by the self-consistent NEGF-DFT formalism. The transmission spectrum has a resonance nature for energies around the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the ethylbenzene wires. However, near the Fermi level of the device, which sits inside the HOMO-LUMO gap, the Si substrate is found to play an important role in providing additional pathways for conduction. It has emerged that, within our model system, the transmission peak nearest to the Fermi level corresponds to transport through the Si substrate and not the pi-stacked molecular line. The low-bias resistance R is found to increase exponentially with the length of the molecular line n, as R approximately e(betan), indicating a tunneling behavior in conduction. We further found that the exponential scaling has two regimes characterized by two different scaling parameters beta: a high value for conduction through the molecular stack in short lines and a lower value for conduction through the substrate in longer lines. Our results suggest that when the conduction of molecular wires bonded to semiconductor substrates is theoretically analyzed, conduction pathways through the substrate need to be taken into account.

Tailoring the Fermi level of the leads in molecular-electronic devices

The dependence of the transport properties on the specific location of the Fermi level in molecular electronics devices is studied by using electrodes of different materials. The zero-bias transport properties are shown to depend dramatically on the elemental composition of the electrodes even though the shape of the transmission coefficients is very similar. By using alkaline materials it is possible to move the Fermi level from the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap to the LUMO resonance and change dramatically the length dependence of the conductance of molecular wires, which opens the possibility of using molecules with different lengths and very similar conductances in nanoscale circuits. This method shows how to dramatically increase the conductance of molecular devices and alter qualitatively and quantitatively their electronic and transport properties.

Length‐Dependent Conductance of Molecular Wires and Contact Resistance in Metal–Molecule–Metal Junctions

Molecular wires are covalently bonded to gold electrodes--to form metal-molecule-metal junctions--by functionalizing each end with a -SH group. The conductance of a wide variety of molecular junctions is studied theoretically by using first-principles density functional theory (DFT) combined with the nonequilibrium Green's function (NEGF) formalism. Based on the chain-length-dependent conductance of the series of molecular wires, the attenuation factor beta is obtained and compared with the experimental data. The beta value is quantitatively correlated to the molecular HOMO-LUMO gap. Coupling between the metallic electrode and the molecular bridge plays an important role in electron transport. A contact resistance of 6.0+/-2.0 Kohms is obtained by extrapolating the molecular-bridge length to zero. This value is of the same magnitude as the quantum resistance.