Influence of conformation on conductance of biphenyl-dithiol single-molecule contacts

Effective Coupling Model to Treat the Odd-Even Effect on the Current-Voltage Response of Saturated Linear Carbon Chains Single-Molecule Junctions

The calculation of the electrical charge transport properties of alkanes C n H 2n S 2 with (n = 4-11) was performed to understand the odd-even effect on its current-voltage response. The extended molecule and broadband limit models were used to describe the molecular junction and covalent coupling with the electrodes. It was shown that among the participating molecular orbitals, HOMO and HOMO-1 are the ones with the most charge transport contribution. Moreover, the odd-even effect is caused by the alternation of the eigenvalues of some frontier orbitals as a function of the number of carbons, especially the HOMO that dominates the electrical transport. It could also be noted that when the current is analyzed outside the resonance, the relationship with the number of carbons exponentially decays, confirming the reports in the literature. To the best of our knowledge, a first principle study of the odd-even effect in symmetric systems composed by linear saturated carbon chains covalently coupled to electrodes has not been reported yet.

A Systematic Study of Methyl Carbodithioate Esters as Effective Gold Contact Groups for Single-Molecule Electronics

There are several binding groups used within molecular electronics for anchoring molecules to metal electrodes (e.g., R-SMe, R-NH2, R-CS2 -, R-S-). However, some anchoring groups that bind strongly to electrodes have poor/unknown stability, some have weak electrode coupling, while for some their binding motifs are not well defined. Further binding groups are required to aid molecular design and to achieve a suitable balance in performance across a range of properties. We present an in-depth investigation into the use of carbodithioate esters as contact groups for single-molecule conductance measurements, using scanning tunnelling microscopy break junction measurements (STM-BJ) and detailed surface spectroscopic analysis. We demonstrate that the methyl carbodithioate ester acts as an effective contact for gold electrodes in STM-BJ measurements. Surface enhanced Raman measurements demonstrate that the C=S functionality remains intact when adsorbed on to gold nanoparticles. A gold(I) complex was also synthesised showing a stable C=S→AuI interaction from the ester. Comparison with a benzyl thiomethyl ether demonstrates that the C=S moiety significantly contributes to charge transport in single-molecule junctions. The overall performance of the CS2Me group demonstrates it should be used more extensively and has strong potential for the fabrication of larger area devices with long-term stability.

Light Emission and Conductance Fluctuations in Electrically Driven and Plasmonically Enhanced Molecular Junctions

Electrically connected and plasmonically enhanced molecular junctions combine the optical functionalities of high field confinement and enhancement (cavity function), and of high radiative efficiency (antenna function) with the electrical functionalities of molecular transport. Such combined optical and electrical probes have proven useful for the fundamental understanding of metal-molecule contacts and contribute to the development of nanoscale optoelectronic devices including ultrafast electronics and nanosensors. Here, we employ a self-assembled metal-molecule-metal junction with a nanoparticle bridge to investigate correlated fluctuations in conductance and tunneling-induced light emission at room temperature. Despite the presence of hundreds of molecules in the junction, the electrical conductance and light emission are both highly sensitive to atomic-scale fluctuations-a phenomenology reminiscent of picocavities observed in Raman scattering and of luminescence blinking from photoexcited plasmonic junctions. Discrete steps in conductance associated with fluctuating emission intensities through the multiple plasmonic modes of the junction are consistent with a finite number of randomly localized, point-like sources dominating the optoelectronic response. Contrasting with these microscopic fluctuations, the overall plasmonic and electronic functionalities of our devices feature long-term survival at room temperature and under an electrical bias of a few volts, allowing for measurements over several months.

An Orthogonal Conductance Pathway in Spiropyrans for Well-Defined Electrosteric Switching Single-Molecule Junctions

While a multitude of studies have appeared touting the use of molecules as electronic components, the design of molecular switches is crucial for the next steps in molecular electronics. In this work, single-molecule devices incorporating spiropyrans, made using break junction techniques, are described. Linear spiropyrans with electrode-contacting groups linked by alkynyl spacers to both the indoline and chromenone moieties have previously provided very low conductance values, and removing the alkynyl spacer has resulted in a total loss of conductance. An orthogonal T-shaped approach to single-molecule junctions incorporating spiropyran moieties in which the conducting pathway lies orthogonal to the molecule backbone is described and characterized. This approach has provided singlemolecule conductance features with good correlation to molecular length. Additional higher conducting states are accessible using switching induced by UV light or protonation. Theoretical modeling demonstrates that upon (photo)chemical isomerization to the merocyanine, two cooperating phenomena increase conductance: release of steric hindrance allows the conductance pathway to become more planar (raising the mid-bandgap transmission) and a bound state introduces sharp interference near the Fermi level of the electrodes similarly responding to the change in state. This design step paves the way for future use of spiropyrans in single-molecule devices and electrosteric switches.

Phenol is a pH-activated linker to gold: a single molecule conductance study

Single molecule conductance measurements typically rely on functional linker groups to anchor the molecule to the conductive electrodes through a donor-acceptor or covalent bond. While many linking moieties, such as thiols, amines, thiothers and phosphines have been used among others, very few involve oxygen binding directly to gold electrodes. Here, we report successful single molecule conductance measurements using hydroxy (OH)-containing phenol linkers and show that the molecule-gold attachment and electron transport are mediated by a direct O-Au bond. We find that deprotonation of the hydroxy moiety is necessary for metal-molecule binding to proceed, so that junction formation can be activated through pH control. Electronic structure and DFT+Σ transport calculations confirm our experimental findings that phenolate-terminated alkanes can anchor on the gold and show charge transport trends consistent with prior observations of alkane conductance with other linker groups. Critically, the deprotonated O--Au binding shows features similar to the thiolate-Au bond, but without the junction disruption caused by intercalation of sulfur into electrode tips often observed with thiol-terminated molecules. By comparing the conductance and binding features of O-Au and S-Au bonds, this study provides insight into the aspects of Au-linker bonding that promote reproducible and robust single molecule junction measurements.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

The Geometry of Nanoparticle-on-Mirror Plasmonic Nanocavities Impacts Surface-Enhanced Raman Scattering Backgrounds

Surface-enhanced Raman scattering (SERS) has garnered substantial attention due to its ability to achieve single-molecule sensitivity by utilizing metallic nanostructures to amplify the exceedingly weak Raman scattering process. However, the introduction of metal nanostructures can induce a background continuum which can reduce the ultimate sensitivity of SERS in ways that are not yet well understood. Here, we investigate the impact of laser irradiation on both Raman scattering and backgrounds from self-assembled monolayers within nanoparticle-on-mirror plasmonic nanocavities with variable geometry. We find that laser irradiation can reduce the height of the monolayer by inducing an irreversible change in molecular conformation. The resulting increased plasmon confinement in the nanocavities not only enhances the SERS signal, but also provides momentum conservation in the inelastic light scattering of electrons, contributing to the enhancement of the background continuum. The plasmon confinement can be modified by changing the size and the geometry of nanoparticles, resulting in a nanoparticle geometry-dependent background continuum in SERS. Our work provides new routes for further modifying the geometry of plasmonic nanostructures to improve SERS sensitivity.

Microscopic theory, analysis, and interpretation of conductance histograms in molecular junctions

Molecular electronics break-junction experiments are widely used to investigate fundamental physics and chemistry at the nanoscale. Reproducibility in these experiments relies on measuring conductance on thousands of freshly formed molecular junctions, yielding a broad histogram of conductance events. Experiments typically focus on the most probable conductance, while the information content of the conductance histogram has remained unclear. Here we develop a microscopic theory for the conductance histogram by merging the theory of force-spectroscopy with molecular conductance. The procedure yields analytical equations that accurately fit the conductance histogram of a wide range of molecular junctions and augments the information content that can be extracted from them. Our formulation captures contributions to the conductance dispersion due to conductance changes during the mechanical elongation inherent to the experiments. In turn, the histogram shape is determined by the non-equilibrium stochastic features of junction rupture and formation. The microscopic parameters in the theory capture the junction's electromechanical properties and can be isolated from separate conductance and rupture force (or junction-lifetime) measurements. The predicted behavior can be used to test the range of validity of the theory, understand the conductance histograms, design molecular junction experiments with enhanced resolution and molecular devices with more reproducible conductance properties.

Stereoelectronic Switches of Single-Molecule Junctions through Conformation-Modulated Intramolecular Coupling Approaches

Stereoelectronic effects in single-molecule junctions have been widely utilized to achieve a molecular switch, but high-efficiency and reproducible switching remain challenging. Here, we demonstrate that there are three stable intramolecular conformations in the 9,10-diphenyl-9,10-methanoanthracen-11-one (DPMAO) systems due to steric effect. Interestingly, different electronic coupling approaches including weak coupling (through-space), decoupling, and strong coupling (through-bond) between two terminal benzene rings are accomplished in the three stable conformations, respectively. Theoretical calculations show that the molecular conductance of three stable conformations differs by more than 1 order of magnitude. Furthermore, the populations of the three stable conformations are highly dependent on the solvent effect and the external electric field. Therefore, an excellent molecular switch can be achieved using the DPMAO molecule junctions and external stimuli. Our findings reveal that modulating intramolecular electronic coupling approaches may be a useful manner to enable molecular switches with high switching ratios. This opens up a new route for building high-efficiency molecular switches in single-molecular junctions.

Stretch Evolution of Electronic Coupling of the Thiophenyl Anchoring Group with Gold in Mechanically Controllable Break Junctions

The current-voltage characteristics of a single-molecule junction are determined by the electronic coupling Γ between the electronic states of the electrodes and the dominant transport channel(s) of the molecule. Γ is profoundly affected by the choice of the anchoring groups and their binding positions on the tip facets and the tip-tip separation. In this work, mechanically controllable break junction experiments on the N,N'-bis(5-ethynylbenzenethiol-salicylidene)ethylenediamine are presented, in particular, the stretch evolution of Γ with increasing tip-tip separation. The stretch evolution of Γ is characterized by recurring local maxima and can be related to the deformation of the molecule and sliding of the anchoring groups above the tip facets and along the tip edges. A dynamic simulation approach is implemented to model the stretch evolution of Γ, which captures the experimentally observed features remarkably well and establishes a link to the microscopic structure of the single-molecule junction.

Torsional Wave-Packet Dynamics in 2-Fluorobiphenyl Investigated by State-Selective Ionization-Detected Impulsive Stimulated Raman Spectroscopy

We report the creation and observation of vibrational wave packets pertinent to torsional motion in a biphenyl derivative in its electronic ground-state manifold. Adiabatically cooled molecular samples of 2-fluorobiphenyl were irradiated by intense nonresonant ultrashort laser pulses to drive impulsive stimulated Raman excitation of torsional motion. Spectral change due to the nonadiabatic vibrational excitation is probed in a state-selective manner using resonance-enhanced two-photon ionization through the S₁ ← S₀ electronic transition. The coherent nature of the excitation was exemplified by adopting irradiation with a pair of pump pulses: observed signals for excited torsional levels exhibit oscillatory variations against the mutual delay between the pump pulses due to wave-packet interference. By taking the Fourier transform of the time course of the signals, energy intervals among torsional levels with v = 0-3 were determined and utilized to calibrate a density functional theory (DFT)-calculated torsional potential-energy function. Time variation of populations in the excited torsional levels was assessed experimentally by measuring integrated intensities of the corresponding transitions while scanning the delay. Early time enhancement of the population (up to ∼2 ps) and gradual degradation of coherence (within ∼20 ps) appears. To explain the observed distinctive features, we developed a four-dimensional (4D) dynamical calculation in which one-dimensional (1D) quantum-mechanical propagation of the torsional motion was followed by solving the time-dependent Schrödinger equation, whereas three-dimensional (3D) molecular rotation was tracked by classical trajectory calculations. This hybrid approach enabled us to reproduce experimental results at a reasonable computational cost and provided a deeper insight into rotational effects on vibrational wave-packet dynamics.

Single-molecule conductance studies on quasi- and metallaaromatic dibenzoylmethane coordination compounds and their aromatic analogs

The ability to predict the conductive behaviour of molecules, connected to macroscopic electrodes, represents a crucial prerequisite for the design of nanoscale electronic devices. In this work, we investigate whether the notion of a negative relation between conductance and aromaticity (the so-called NRCA rule) also pertains to quasi-aromatic and metallaaromatic chelates derived from dibenzoylmethane (DBM) and Lewis acids (LAs) that either do or do not contribute two extra d_π electrons to the central resonance-stabilised β-ketoenolate binding pocket. We therefore synthesised a family of methylthio-functionalised DBM coordination compounds and subjected them, along with their truly aromatic terphenyl and 4,6-diphenylpyrimidine congeners, to scanning tunneling microscope break-junction (STM-BJ) experiments on gold nanoelectrodes. All molecules share the common motif of three π-conjugated, six-membered, planar rings with a meta-configuration at the central ring. According to our results, their molecular conductances fall within a factor of ca. 9 in an ordering aromatic < metallaaromatic < quasi-aromatic. The experimental trends are rationalised by quantum transport calculations based on density functional theory (DFT).

Chemical Conformation Induced Transport Carrier Switching in Molecular Junction based on Carboxylic-Terminated Thiol Molecules

The paper demonstrates the effect of the chemical conformation of the -COOH group on the transport characteristic including conductance, rectification, and length effect in molecular junctions (MJs) formed by self-assembled monolayers of carboxylic-terminated thiol molecules. For an alkyl chain shorter than C₁₁, the transport mechanism was attributed to a direct off-resonant tunneling of a hole carrier, located at the Au-S interface, whereas a hopping mechanism was assigned to the alkyl chain longer than the C₁₁ chain located at the -COOH group. The hopping mechanism may be operated by electron transport associated with the breaking of the -OH bonding likely driven by a voltage. Importantly, at the C₁₁ alkyl chain, we observed that the transport carrier operating in MJs could change from a hole carrier into an electron carrier. The result strongly proves that the chemical conformation should be considered in analyzing molecular electronics and provides a basis for the rational design of molecular electronic devices.

Engineering Transport Orbitals in Single-Molecule Junctions

Controlling charge transport through molecules is challenging because it requires engineering of the energy of molecular orbitals involved in the transport process. While side groups are central to maintaining solubility in many molecular materials, their role in modulating charge transport through single-molecule junctions has received less attention. Here, using two break-junction techniques and computational modeling, we investigate systematically the effect of electron-donating and -withdrawing side groups on the charge transport through single molecules. By characterizing the conductance and thermopower, we demonstrate that side groups can be used to manipulate energy levels of the transport orbitals. Furthermore, we develop a novel statistical approach to model quantum transport through molecular junctions. The proposed method does not treat the electrodes' chemical potential as a free parameter and leads to more robust prediction of electrical conductance as confirmed by our experiment. The new method is generic and can be used to predict the conductance of molecules.

2,7- and 4,9-Dialkynyldihydropyrene Molecular Switches: Syntheses, Properties, and Charge Transport in Single-Molecule Junctions

This paper describes the syntheses of several functionalized dihydropyrene (DHP) molecular switches with different substitution patterns. Regioselective nucleophilic alkylation of a 5-substituted dimethyl isophthalate allowed the development of a workable synthetic protocol for the preparation of 2,7-alkyne-functionalized DHPs. Synthesis of DHPs with surface-anchoring groups in the 2,7- and 4,9-positions is described. The molecular structures of several intermediates and DHPs were elucidated by X-ray single-crystal diffraction. Molecular properties and switching capabilities of both types of DHPs were assessed by light irradiation experiments, spectroelectrochemistry, and cyclic voltammetry. Spectroelectrochemistry, in combination with density functional theory (DFT) calculations, shows reversible electrochemical switching from the DHP forms to the cyclophanediene (CPD) forms. Charge-transport behavior was assessed in single-molecule scanning tunneling microscope (STM) break junctions, combined with density functional theory-based quantum transport calculations. All DHPs with surface-contacting groups form stable molecular junctions. Experiments show that the molecular conductance depends on the substitution pattern of the DHP motif. The conductance was found to decrease with increasing applied bias.

Increased Molecular Conductance in Oligo[n]phenylene Wires by Thermally Enhanced Dihedral Planarization

Coherent tunneling electron transport through molecular wires has been theoretically established as a temperature-independent process. Although several experimental studies have shown counter examples, robust models to describe this temperature dependence have not been thoroughly developed. Here, we demonstrate that dynamic molecular structures lead to temperature-dependent conductance within coherent tunneling regime. Using a custom-built variable-temperature scanning tunneling microscopy break-junction instrument, we find that oligo[n]phenylenes exhibit clear temperature-dependent conductance. Our calculations reveal that thermally activated dihedral rotations allow these molecular wires to have a higher probability of being in a planar conformation. As the tunneling occurs primarily through π-orbitals, enhanced coplanarization substantially increases the time-averaged tunneling probability. These calculations are consistent with the observation that more rotational pivot points in longer molecular wires leads to larger temperature-dependence on conductance. These findings reveal that molecular conductance within coherent and off-resonant electron transport regimes can be controlled by manipulating dynamic molecular structure.

Analysis of molecular ligand functionalization process in nano-molecular electronic devices containing densely packed nano-particle functionalization shells

Molecular electronic devices based on few and single-molecules have the advantage that the electronic signature of the device is directly dependent on the electronic structure of the molecules as well as of the electrode-molecule junction. In this work, we use a two-step approach to synthesise functionalized nanomolecular electronic devices (nanoMoED). In first step we apply an organic solvent-based gold nanoparticle (AuNP) synthesis method to form either a 1-dodecanethiol or a mixed 1-dodecanethiol/ω-tetraphenyl ether substituted 1-dodecanethiol ligand shell. The functionalization of these AuNPs is tuned in a second step by a ligand functionalization process where biphenyldithiol (BPDT) molecules are introduced as bridging ligands into the shell of the AuNPs. From subsequent structural analysis and electrical measurements, we could observe a successful molecular functionalization in nanoMoED devices as well as we could deduce that differences in electrical properties between two different device types are related to the differences in the molecular functionalization process for the two different AuNPs synthesized in first step. The same devices yielded successful NO₂gas sensing. This opens the pathway for a simplified synthesis/fabrication of molecular electronic devices with application potential.

Coherent and incoherent contributions to molecular electron transport

We numerically isolate the limits of validity of the Landauer approximation to describe charge transport along molecular junctions in condensed phase environments. To do so, we contrast Landauer with exact time-dependent non-equilibrium Green’s function quantum transport computations in a two-site molecular junction subject to exponentially correlated noise. Under resonant transport conditions, we find Landauer accuracy to critically depend on intramolecular interactions. By contrast, under nonresonant conditions, the emergence of incoherent transport routes that go beyond Landauer depends on charging and discharging processes at the electrode–molecule interface. In both cases, decreasing the rate of charge exchange between the electrodes and molecule and increasing the interaction strength with the thermal environment cause Landauer to become less accurate. The results are interpreted from a time-dependent perspective where the noise prevents the junction from achieving steady-state and from a fully quantum perspective where the environment introduces dephasing in the dynamics. Using these results, we analyze why the Landauer approach is so useful to understand experiments, isolate regimes where it fails, and propose schemes to chemically manipulate the degree of transport coherence.

Mechanoresistive single-molecule junctions

Single-molecule junctions - devices fabricated by electrically connecting a single molecule to two electrodes - can respond to a variety of stimuli, that include electrostatic/electrochemical gating, light, other chemical species, and mechanical forces. When the latter is used, the device becomes mechanoresistive which means that its electrical resistance/conductance changes upon application of a mechanical stress. The mechanoresistive phenomenon can arise at the metal-molecule interface or it can be embedded in the molecular backbone, and several strategies to attain high reproducibility, high sensitivity and reversible behaviour have been developed over the years. These devices offer a unique insight on the process of charge transfer/transport at the metal/molecule interface, and have potential for applications as nanoelectromechanical systems, integrating electrical and mechanical functionality at the nanoscale. In this review, the status of the field is presented, with a focus on those systems that proved to have reversible behaviour, along with a discussion on the techniques used to fabricate and characterise mechanoresistive devices.

A simple model to engineer single-molecule conductance of acenes by chemical disubstitution

Understanding and controlling electrical conductivity at the single-molecule level is of fundamental importance for the development of new molecular electronic devices. This ideally requires considering the many different options offered by the molecular structure, the nature of the electrodes, and all possible molecule-electrode anchoring configurations, which is experimentally tedious and theoretically very expensive. Here we present a systematic theoretical study of the conductance of di-amino, di-methylthio and di-(4-methylthio)phenyl acenes, from benzene to pentacene, and for all possible distributions of two identical linkers symmetrically placed on opposite sides of the same ring. We show that, for all investigated compounds, the relative variation of the conductance is well explained by the variations of the HOMO energies as predicted by a simple extended-Hückel approach, i.e., without the need for further input from more elaborate calculations. The model predicts quite nicely that diamino acenes are better conductors than their corresponding dimethylthio analogues, and both much better than the di-(4-methylthio)phenyl counterparts, irrespective of the linkers' relative positions. It also predicts, for a given pair of linkers, the variations in the conductance resulting from changing the acene size and/or the relative position of the linkers. These variations can be as large as an order of magnitude, and therefore can be used to engineer molecular conductance. Finally, we show that a similar approach should be useful to predict trends in the relative conductance of a large variety of disubstituted acene isomers, including various linkers.

Control of Molecular Conformation and Crystal Packing of Biphenyl Derivatives

This Review presents a discussion of the conformation of biphenyl derivatives in different chemical environments. The interplay between aromatic stabilization and steric repulsion, normally considered to explain the conformation of the molecule, is contrasted with the interpretation provided by models not based on molecular orbitals. The electronic control of conformation by means of appropriate hydrogen substitution is discussed by examples taken from chemistry and molecular electronics. Supramolecular synthons involving biphenyl are critically analyzed in terms of the molecular conformation, crystal packing and intermolecular forces. Some directions for future research on the control of the conformation of biphenyls are also presented.

A single-molecule blueprint for synthesis

Chemical reactions that occur at nanostructured electrodes have garnered widespread interest because of their potential applications in fields including nanotechnology, green chemistry and fundamental physical organic chemistry. Much of our present understanding of these reactions comes from probes that interrogate ensembles of molecules undergoing various stages of the transformation concurrently. Exquisite control over single-molecule reactivity lets us construct new molecules and further our understanding of nanoscale chemical phenomena. We can study single molecules using instruments such as the scanning tunnelling microscope, which can additionally be part of a mechanically controlled break junction. These are unique tools that can offer a high level of detail. They probe the electronic conductance of individual molecules and catalyse chemical reactions by establishing environments with reactive metal sites on nanoscale electrodes. This Review describes how chemical reactions involving bond cleavage and formation can be triggered at nanoscale electrodes and studied one molecule at a time.

Multidimensional Characterization of Single-Molecule Dynamics in a Plasmonic Nanocavity

Nanoscale manipulation and characterization of individual molecules is necessary to understand the intricacies of molecular structure, which governs phenomena such as reaction mechanisms, catalysis, local effective temperatures, surface interactions, and charge transport. Here we utilize Raman enhancement between two nanostructured electrodes in combination with direct charge transport measurements to allow for simultaneous characterization of the electrical, optical, and mechanical properties of a single molecule. This multi-dimensional information yields repeatable, self-consistent, verification of single-molecule resolution, and allows for detailed analysis of structural and configurational changes of the molecule in situ. These experimental results are supported by a machine-learning based statistical analysis of the spectral information and calculations to provide insight into the correlation between structural changes in a single-molecule and its charge-transport properties.

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.

Structural Order of the Molecular Adlayer Impacts the Stability of Nanoparticle-on-Mirror Plasmonic Cavities

Immense field enhancement and nanoscale confinement of light are possible within nanoparticle-on-mirror (NPoM) plasmonic resonators, which enable novel optically activated physical and chemical phenomena and render these nanocavities greatly sensitive to minute structural changes, down to the atomic scale. Although a few of these structural parameters, primarily linked to the nanoparticle and the mirror morphology, have been identified, the impact of molecular assembly and organization of the spacer layer between them has often been left uncharacterized. Here, we experimentally investigate how the complex and reconfigurable nature of a thiol-based self-assembled monolayer (SAM) adsorbed on the mirror surface impacts the optical properties of the NPoMs. We fabricate NPoMs with distinct molecular organizations by controlling the incubation time of the mirror in the thiol solution. Afterward, we investigate the structural changes that occur under laser irradiation by tracking the bonding dipole plasmon mode, while also monitoring Stokes and anti-Stokes Raman scattering from the molecules as a probe of their integrity. First, we find an effective decrease in the SAM height as the laser power increases, compatible with an irreversible change of molecule orientation caused by heating. Second, we observe that the nanocavities prepared with a densely packed and more ordered monolayer of molecules are more prone to changes in their resonance compared to samples with sparser and more disordered SAMs. Our measurements indicate that molecular orientation and packing on the mirror surface play a key role in determining the stability of NPoM structures and hence highlight the under-recognized significance of SAM characterization in the development of NPoM-based applications.

Fine-tuning the DNA conductance by intercalation of drug molecules

In this work we study the structure-transport property relationships of small ligand intercalated DNA molecules using a multiscale modeling approach where extensive ab initio calculations are performed on numerous MD-simulated configurations of dsDNA and dsDNA intercalated with two different intercalators, ethidium and daunomycin. DNA conductance is found to increase by one order of magnitude upon drug intercalation due to the local unwinding of the DNA base pairs adjacent to the intercalated sites, which leads to modifications of the density of states in the near-Fermi-energy region of the ligand-DNA complex. Our study suggests that the intercalators can be used to enhance or tune the DNA conductance, which opens new possibilities for their potential applications in nanoelectronics.

Environmental Control of Single-Molecule Junction Evolution and Conductance: A Case Study of Expanded Pyridinium Wiring

Environmental control of single-molecule junction evolution and conductance was demonstrated for expanded pyridinium molecules by scanning tunneling microscopy break junction method and interpreted by quantum transport calculations including solvent molecules explicitly. Fully extended and highly conducting molecular junctions prevail in water environment as opposed to short and less conducting junctions formed in non-solvating mesitylene. A theoretical approach correctly models single-molecule conductance values considering the experimental junction length. Most pronounced difference in the molecular junction formation and conductance was identified for a molecule with the highest stabilization energy on the gold substrate confirming the importance of molecule-electrode interactions. Presented concept of tuning conductance through molecule-electrode interactions in the solvent-driven junctions can be used in the development of new molecular electronic devices.

Modulation of charge transport through single-molecule bilactam junctions by tuning hydrogen bonds

Bilactam derivatives with different side groups were synthesized and the twisting angle tuning effect induced by the intramolecular hydrogen bond on the charge transport through their single-molecule junctions was investigated. Molecules with strong intramolecular hydrogen bonds exhibited twice higher conductance because of the reduced dihedral twisting, which was reversible with the addition of hydrogen bond destroying solvent. Our findings reveal that the presence of intramolecular hydrogen bonds promotes the planarization of the molecular structure without additional transmission channels, offering a new strategy for controlling molecular switches via tuning the molecular twisting.

Single-Molecule Conductance of 1,4-Azaborine Derivatives as Models of BN-doped PAHs

The single-molecule conductance of a series of BN-acene-like derivatives has been measured by using scanning tunneling break-junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN-doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all-carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene-like compound, which is found to have a similar conductance, thus evidencing that B-N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state-of-the-art transport calculations.

Optical probes of molecules as nano-mechanical switches

Molecular electronics promises a new generation of ultralow-energy information technologies, based around functional molecular junctions. Here, we report optical probing that exploits a gold nanoparticle in a plasmonic nanocavity geometry used as one terminal of a well-defined molecular junction, deposited as a self-assembled molecular monolayer on flat gold. A conductive transparent cantilever electrically contacts individual nanoparticles while maintaining optical access to the molecular junction. Optical readout of molecular structure in the junction reveals ultralow-energy switching of ∼50 zJ, from a nano-electromechanical torsion spring at the single molecule level. Real-time Raman measurements show these electronic device characteristics are directly affected by this molecular torsion, which can be explained using a simple circuit model based on junction capacitances, confirmed by density functional theory calculations. This nanomechanical degree of freedom is normally invisible and ignored in electrical transport measurements but is vital to the design and exploitation of molecules as quantum-coherent electronic nanodevices.

The development of molecular electronics at single molecule level calls for new tools beyond electrical characterisation. Kos et al. show an optical probe of molecular junctions in a plasmonic nanocavity geometry, which supports in situ interrogation of molecular configurations.

Photoswitching Molecular Junctions: Platforms and Electrical Properties

Remarkable advances in technology have enabled the manipulation of individual molecules and the creation of molecular electronic devices utilizing single and ensemble molecules. Maturing the field of molecular electronics has led to the development of functional molecular devices, especially photoswitching or photochromic molecular junctions, which switch electronic properties under external light irradiation. This review introduces and summarizes the platforms for investigating the charge transport in single and ensemble photoswitching molecular junctions as well as the electronic properties of diverse photoswitching molecules such as diarylethene, azobenzene, dihydropyrene, and spiropyran. Furthermore, the article discusses the remaining challenges and the direction for moving forward in this area for future photoswitching molecular devices.

Simultaneous Suppression of π- and σ-Transmission in π-Conjugated Molecules

Molecular dielectric materials require ostensibly conflicting requirements of high polarizability and low conductivity. As previous efforts toward molecular insulators focused on saturated molecules, it remains an open question whether π- and σ-transport can be simultaneously suppressed in conjugated systems. Here, we demonstrate that there are conjugated molecules where the σ-transmission is suppressed by destructive σ-interference, while the π-transmission can be suppressed by a localized disruption of conjugation. Using density functional theory, we study the Landauer transmission and ballistic current density, which allow us to determine how the transmission is affected by various structural changes in the molecule. We find that in para-linked oligophenyl rings the σ-transmission can be suppressed by changing the remaining hydrogens to methyl groups due to the inherent gauche-like structure of the carbon backbone within a benzene ring, similar to what was previously seen in saturated systems. At the same time, the methyl groups fulfill a dual purpose as they modulate the twist angle between neighboring phenyl rings. When neighboring rings are orthogonal to each other, the transmission through both π- and σ-systems is effectively suppressed. Alternatively, breaking conjugation in a single phenyl ring by saturating two carbons atoms with two methyl substituents on each carbon, results in suppressed π- and σ-transport independent of dihedral angle. These two strategies demonstrate that methyl-substituted oligophenyls are promising candidates for the development of molecular dielectric materials.

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.

Single molecular conductance of molecular wires is effectively evaluated by the combination of STM apparent height measurement and a 2-D phase separation technique.

Nanomolecular electronic devices based on AuNP molecule nanoelectrodes using molecular place-exchange process

The implementation of electronics applications based on molecular electronics devices is hampered by the difficulty of placing a single or a few molecules with application-specific electronic properties in between metallic nanocontacts. Here, we present a novel method to fabricate 20 nm sized nanomolecular electronic devices (nanoMoED) using a molecular place-exchange process of nonconductive short alkyl thiolates with various short chain conductive oligomers. After the successful place-exchange with short-chain conjugated oligomers in the nanoMoED devices, a change in device resistance of up to four orders of magnitude for 4,4'-biphenyldithiol (BPDT), and up to three orders of magnitude for oligo phenylene-ethynylene (OPE), were observed. The place-exchange process in nanoMoEDs are verified by measuring changes in device resistance during repetitive place-exchange processes between conductive and nonconductive molecules and surface-enhanced Raman spectroscopy. This opens vast possibilities for the fabrication and application of nanoMoED devices with a large variety of molecules.

Nanoscale junctions for single molecule electronics fabricated using bilayer nanoimprint lithography combined with feedback controlled electromigration

Nanoimprint lithography (NIL) is a fast, simple and high throughput technique that allows fabrication of structures with nanometre precision features at low cost. We present an advanced bilayer nanoimprint lithography approach to fabricate four terminal nanojunction devices for use in single molecule electronic studies. In the first part of this work, we demonstrate a NIL lift-off process using a bilayer resist technique that negates problems associated with metal side-wall tearing during lift-off. In addition to precise nanoscale feature replication, we show that it is possible to imprint micron-sized features while still maintaining a bilayer structure enabling an undercut resist structure to be formed. This is accomplished by choosing suitable imprint parameters as well as residual layer etching depth and development time. We then use a feedback controlled electromigration procedure, to produce room-temperature stable nanogap electrodes with sizes below 2 nm. This approach facilitates the integration of molecules in stable, solid-state molecular electronic devices as demonstrated by incorporating benzenethiol as molecular bridges between the electrodes and characterizing its electronics properties through current-voltage measurements. The observation of molecular transport signatures, showing current suppression in the I-V behaviour at low voltage, which is then lifted at high voltage, signifying on- and off-resonant transport through molecular levels as a function of voltage, is confirmed in repeated I-V sweeps. The large conductance, symmetry of the I-V sweep and small value of the voltage minimum in transition voltage spectroscopy indicates the bridging of the two benzenethiol molecules is by π-stacking.

Molecular Wires with Controllable π-Delocalization Incorporating Redox-Triggered π-Conjugated Switching Units

A perfluorobiphenyl-2,2'-diyl dication and its corresponding dihydrophenanthrene-type electron donor are interconvertible upon two-electron transfer. Redox-triggered C-C bond-formation/cleavage caused a drastic change in the torsion angle of the biphenyl unit. Thus, π-delocalization ON/OFF switching was observed as a change in the UV absorption upon electrolysis of the linearly extended analogue with two phenylethynyl groups. A further extended π-system with a molecular length of ca. 3.5 nm, which has two switching units, was synthesized. Spectroelectrograms as well as voltammetric analyses showed that the two units act nearly simultaneously because of the very small inter-unit electrostatic repulsion in the tetracationic state. Thus, the present pair is a promising candidate as a switching unit for "molecular wires" with controllable π-delocalization, in which a higher ON/OFF ratio of delocalization could be realized by incorporating multiple switching units.

Single-molecule functionality in electronic components based on orbital resonances

A gateable single-molecule diode and resonant tunneling diode are realized using molecular orbital engineering in multi-site molecules.

Exploring antiaromaticity in single-molecule junctions formed from biphenylene derivatives

We report the synthesis of a series of oligophenylene-ethynylene (OPE) derivatives with biphenylene core units, designed to assess the effects of biphenylene antiaromaticity on charge transport in molecular junctions. Analogues with naphthalene, anthracene, fluorene and biphenyl cores are studied for comparison. The molecules are terminated with pyridyl or methylthio units. Single-molecule conductance data were obtained using the mechanically controllable break junction (MCBJ) technique. It is found that when electrons pass from one electrode to the other via a phenylene ring, the electrical conductance is almost independent of the nature of the pendant π-systems attached to the phenylene ring and is rather insensitive to antiaromaticity. When electrons pass through the cyclobutadiene core of the biphenylene unit, transport is sensitive to the presence of the relatively weak single bonds connecting the two phenylene rings of biphenylene, which arise from partial antiaromaticity within the cyclobutadiene core. This leads to a negligible difference in the molecular conductance compared to the fluorene or biphenyl analogues which have standard single bonds. This ability to tune the conductance of molecular cores has no analogue in junctions formed from artificial quantum dots and reflects the quantum nature of electron transport in molecular junctions, even at room temperature.

Chemical self-assembly strategies for designing molecular electronic circuits

Design principles are demonstrated for fabricating molecular electronic circuits using the inherently self-limiting growth of molecular wires between gold nanoparticles from the oligomerization of 1,4-phenylene diisocyanide.

In Operando Characterization and Control over Intermittent Light Emission from Molecular Tunnel Junctions via Molecular Backbone Rigidity

In principle, excitation of surface plasmons by molecular tunnel junctions can be controlled at the molecular level. Stable electrical excitation sources of surface plasmons are therefore desirable. Herein, molecular junctions are reported where tunneling charge carriers excite surface plasmons in the gold bottom electrodes via inelastic tunneling and it is shown that the intermittent light emission (blinking) originates from conformational dynamics of the molecules. The blinking rates, in turn, are controlled by changing the rigidity of the molecular backbone. Power spectral density analysis shows that molecular junctions with flexible aliphatic molecules blink, while junctions with rigid aromatic molecules do not.

Modularized Tuning of Charge Transport through Highly Twisted and Localized Single-Molecule Junctions

Although most molecular electronic devices and materials consist of a backbone with a planar structure, twisted molecular wires with reduced inter-ring π-orbital overlap offer a unique opportunity for the modularized fabrication of molecular electronic devices. Herein we investigate the modularized tuning of the charge transport through the localized molecules by designing highly twisted molecules and investigating their single-molecule charge transport using the scanning tunneling microscopy break junction technique. We find that the anthracenediyl-core molecule with a 90° inter-ring twist angle shows an unexpectedly high conductance value, which is five times higher than that of the phenylene-core molecule with a similar configuration, whereas the conductance of the delocalized planar molecule with an anthracenediyl core or a phenylene core is almost the same. Theoretical calculations revealed that highly twisted angles result in weak interactions between molecular building blocks, for which molecule orbitals are separated into localized blocks, which offers the chance for the modularized tuning of every single block. Our findings offer a new strategy for the design of future molecular devices with a localized electronic structure.