Extended curly arrow rules to rationalise and predict structural effects on quantum interference in molecular junctions

Electron Transport through Linear-, Broken-, and Cross-Conjugated Polycyclic Compounds

Quantum interference (QI) effects offer unique opportunities to modulate charge transport through single molecules. In recent years, several transmission selection rules have been developed to determine constructive and destructive QIs in an intuitive and simple manner, although some of these rules fail for cross-conjugated systems. In this work, we evaluate the performance of distinct transmission rules on a broad series of anthracene and fluorene derivatives with distinctive structural features including linear-, broken-, and cross-conjugation, heteroatoms, and five-membered rings as such species affords a predictive challenge for the qualitative selection rules for QI effects. The electron transport properties and local transmission plots are first evaluated by combining DFT and the nonequilibrium Green function method allowing for an equal-footing comparison of the conductance of the different polycyclic compounds. Our findings are in line with experimental observations on the influence of the type of conjugation and the connectivity to the metallic electrodes on the transport properties. Thus, cross-conjugated systems exhibit reduced conductance values as compared to the linear-conjugated ones, although the transmission is enhanced in the meta-connected junctions. Remarkably, our study reveals that aromatic cores exhibit generally larger zero-bias conductance for a given connectivity, in contrast to the negative aromaticity-conductance relationship found in literature.

Mechanical Manipulation of Quantum Interference in Single-Molecule Junctions

Mechanosensitive molecular junctions, where conductance is sensitive to an applied stress such as force or displacement, are a class of nanoelectromechanical systems unique for their ability to exploit quantum mechanical phenomena. Most studies so far relied on reconfiguration of the molecule-electrode interface to impart mechanosensitivity, but this approach is limited and, generally, poorly reproducible. Alternatively, devices that exploit conformational flexibility of molecular wires have been recently proposed. The mechanosensitive properties of molecular wires containing the 1,1'-dinaphthyl moiety are presented here. Rotation along the chemical bond between the two naphthyl units is possible, giving rise to two conformers (transoid and cisoid) that have distinctive transport properties. When assembled as single-molecule junctions, it is possible to mechanically trigger the transoid to cisoid transition, resulting in an exquisitely sensitive mechanical switch with high switching ratio (> 102). Theoretical modeling shows that charge reconfiguration upon transoid to cisoid transition is responsible for the observed behavior, with generation and subsequent lifting of quantum interference features. These findings expand the experimental toolbox of molecular electronics with a novel chemical structure with outstanding electromechanical properties, further demonstrating the importance of subtle changes in charge delocalization on the transport properties of single-molecule devices.

Nuclear Magnetic Resonance Chemical Shift as a Probe for Single-Molecule Charge Transport

Existing modelling tools, developed to aid the design of efficient molecular wires and to better understand their charge-transport behaviour and mechanism, have limitations in accuracy and computational cost. Further research is required to develop faster and more precise methods that can yield information on how charge transport properties are impacted by changes in the chemical structure of a molecular wire. In this study, we report a clear semilogarithmic correlation between charge transport efficiency and nuclear magnetic resonance chemical shifts in multiple series of molecular wires, also accounting for the presence of chemical substituents. The NMR data was used to inform a simple tight-binding model that accurately captures the experimental single-molecule conductance values, especially useful in this case as more sophisticated density functional theory calculations fail due to inherent limitations. Our study demonstrates the potential of NMR spectroscopy as a valuable tool for characterising, rationalising, and gaining additional insights on the charge transport properties of single-molecule junctions.

Thermoelectric Properties of Porphyrin Nano Rings: A Theoretical and Modelling Investigation

Propagation of De Broglie waves through nanomolecular junctions is greatly affected by molecular topology changes, which in turn plays a key role in determining the electronic and thermoelectric properties of source|molecule|drain junctions. The probing and realization of the constructive quantum interference (CQI) and a destructive quantum interference (DQI) are well established in this work. The critical role of quantum interference (QI) in governing and enhancing the transmission coefficient T(E), thermopower (S), power factor (P) and electronic figure of merit (ZelT) of porphyrin nanorings has been investigated using a combination of density functional theory (DFT) methods, a tight binding (Hückel) modelling (TBHM) and quantum transport theory (QTT). Remarkably, DQI not only dominates the asymmetric molecular pathways and lowering T(E), but also improves the thermoelectric properties.

Carbon Nanohoops: Multiple Molecular Templates for Exploring Spectroscopic, Electronic, and Thermoelectric Properties

A combination of density functional theory (DFT) methods and quantum transport theory (QTT) has been used to investigate the spectroscopic, electronic, and thermoelectric properties of carbon nanohoop molecules with different molecular templates. The connectivity type, along with inherent strain, impacts the transport behavior and creates a destructive quantum interference (DQI), which proves itself to be a powerful strategy to enhance the thermoelectric properties of these molecules, making them promising candidates for thermoelectric applications.

Electronic Conductance and Thermopower of Cross-Conjugated and Skipped-Conjugated Molecules in Single-Molecule Junctions

We report a combined experimental and theoretical study of a series of thiomethyl (SMe) anchored cross-conjugated molecules featuring an acyclic central bridging ketone and their analogous skipped-conjugated alcohol derivatives. Studies of these molecules in a gold|single-molecule|gold junction using scanning tunneling microscopy-break junction techniques reveal a similar conductance (G) value for both the cross-conjugated molecules and their skipped-conjugated partners. Theoretical studies based on density functional theory of the molecules in their optimum geometries in the junction reveal the reason for this similarity in conductance, as the predicted conductance for the alcohol series of compounds varies more with the tilt angle. Thermopower measurements reveal a higher Seebeck coefficient (S) for the cross-conjugated ketone molecules relative to the alcohol derivatives, with a particularly high S for the biphenyl derivative 3a (-15.6 μV/K), an increase of threefold compared to its alcohol analog. The predicted behavior of the quantum interference (QI) in this series of cross-conjugated molecules is found to be constructive, though the appearance of a destructive QI feature for 3a is due to the degeneracy of the HOMO orbital and may explain the enhancement of the value of S for this molecule.

Planar aromatic anchors control the electrical conductance of gold|molecule|graphene junctions

The synthesis of a family of alkanethiol molecules with planar aromatic head groups, designed to anchor molecules effectively to graphene electrodes, is reported. Characterisation of self-assembled monolayers of these molecules on a gold surface via conductive atomic force microscopy shows that when an aromatic head group is present, the conductance G graphene obtained using a graphene coated probe is higher than the conductance G Pt obtained using a platinum (Pt) probe. For Pt probe and graphene probe junctions, the tunnelling decay constant of benzyl ether derivatives with an alkanethiol molecular backbone is determined as β = 5.6 nm-1 and 3.5 nm-1, respectively. The conductance ratio G graphene/G Pt increases as the number of rings present in the aromatic head unit, n, increases. However, as the number of rings increases, the conductance path length increases because the planar head groups lie at an angle to the plane of the electrodes. This means that overall conductance decreases as n increases. Density functional theory-based charge transport calculations support these experimental findings. This study confirms that planar aromatic head groups can function as effective anchoring units for graphene electrodes in large area molecular junctions. However, the results also indicate that the size and geometry of these head groups must be considered in order to produce effective molecular designs.

Visualizing and comparing quantum interference in the π-system and σ-system of organic molecules

Quantum interference effects in conjugated molecules have been well-explored, with benzene frequently invoked as a pedagogical example. These interference effects have been understood through a quantum interference map in which the electronic transmission is separated into interfering and non-interfering terms, with a focus on the π-orbitals for conjugated molecules. Recently, saturated molecules have also been reported to exhibit destructive quantum interference effects; however, the very different σ-orbital character in these molecules means that it is not clear how orbital contributions manifest. Herein, we demonstrate that the quantum interference effects in conjugated molecules are quite different from those observed in saturated molecules, as demonstrated by the quantum interference map. While destructive interference at the Fermi energy in the π-system of benzene arises from interference terms between paired occupied and virtual orbitals, this is not the case at the Fermi energy in saturated systems. Instead, destructive interference is evident when contributions from a larger number of non-paired orbitals cancel, leading to more subtle and varied manifestations of destructive interference in saturated systems.

Regio- and Steric Effects on Single Molecule Conductance of Phenanthrenes

Here, six phenanthrene (the smallest arm-chair graphene nanoribbon) derivatives with dithiomethyl substitutions at different positions as the anchoring groups were synthesized. Scanning tunneling microscopy break junction technique was used to measure their single molecule conductances between gold electrodes, which showed a difference as much as 20-fold in the range of ∼10^-2.82 G₀ to ∼10^-4.09 G₀ following the trend of G_2,7 > G_3,6 > G_2,6 > G_1,7 > G_1,6 > G_1,8. DFT calculations agree well with this measured trend and indicate that the single molecule conductances are a combination of energy alignment, electronic coupling, and quantum effects. This significant regio- and steric effect on the single molecule conductance of phenanthrene model molecules shows the complexity in the practice of graphene nanoribbons as building blocks for future carbon-based electronics in one hand but also provides good conductance tunability on the other hand.

Electrode effects on the observability of destructive quantum interference in single-molecule junctions

Destructive quantum interference (QI) has been a source of interest as a new paradigm for molecular electronics as the electronic conductance is widely dependent on the occurrence or absence of destructive QI effects. In order to interpret experimentally observed transmission features, it is necessary to understand the effects of all components of the junction on electron transport. We perform non-equilibrium Green's function calculations within the framework of density functional theory to assess the structure-function relationship of transport through pyrene molecular junctions with distinct QI properties. The chemical nature of the anchor groups and the electrodes controls the Fermi level alignment, which determines the observability of destructive QI. A thorough analysis allows to disentangle the transmission features arising from the molecule and the electrodes. Interestingly, graphene electrodes introduce features in the low-bias regime, which can either mask or be misinterpreted as QI effects, while instead originating from the topological properties of the edges. Thus, this first principles analysis provides clear indications to guide the interpretation of experimental studies, which cannot be obtained from simple Hückel model calculations.

Heteroatom Effects on Quantum Interference in Molecular Junctions: Modulating Antiresonances by Molecular Design

Controlling charge transport through molecular wires by utilizing quantum interference (QI) is a growing topic in single-molecular electronics. In this article, scanning tunneling microscopy-break junction techniques and density functional theory calculations are employed to investigate the single-molecule conductance properties of four molecules that have been specifically designed to test extended curly arrow rules (ECARs) for predicting QI in molecular junctions. Specifically, for two new isomeric 1-phenylpyrrole derivatives, the conductance pathway between the gold electrodes must pass through a nitrogen atom: this novel feature is designed to maximize the influence of the heteroatom on conductance properties and has not been the subject of prior investigations of QI. It is shown, experimentally and computationally, that the presence of a nitrogen atom in the conductance pathway increases the effect of changing the position of the anchoring group on the phenyl ring from para to meta, in comparison with biphenyl analogues. This effect is explained in terms of destructive QI (DQI) for the meta-connected pyrrole and shifted DQI for the para-connected isomer. These results demonstrate modulation of antiresonances by molecular design and verify the validity of ECARs as a simple "pen-and-paper" method for predicting QI behavior. The principles offer new fundamental insights into structure-property relationships in molecular junctions and can now be exploited in a range of different heterocycles for molecular electronic applications, such as switches based on external gating, or in thermoelectric devices.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

Oligo(arylene ethynylene) (OAE) derivatives are the "workhorse" molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure-property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.