Single-molecule conductance of pyridine-terminated dithienylethene switch molecules

Cooperative and Orthogonal Switching in the Solid State Enabled by Metal-Organic Framework Confinement Leading to a Thermo-Photochromic Platform

Cooperative behavior and orthogonal responses of two classes of coordinatively integrated photochromic molecules towards distinct external stimuli were demonstrated on the first example of a photo-thermo-responsive hierarchical platform. Synergetic and orthogonal responses to temperature and excitation wavelength are achieved by confining the stimuli-responsive moieties within a metal-organic framework (MOF), leading to the preparation of a novel photo-thermo-responsive spiropyran-diarylethene based material. Synergistic behavior of two photoswitches enables the study of stimuli-responsive resonance energy transfer as well as control of the photoinduced charge transfer processes, milestones required to advance optoelectronics development. Spectroscopic studies in combination with theoretical modeling revealed a nonlinear effect on the material electronic structure arising from the coordinative integration of photoresponsive molecules with distinct photoisomerization mechanisms. Thus, the reported work covers multivariable facets of not only fundamental aspects of photoswitch cooperativity, but also provides a pathway to modulate photophysics and electronics of multidimensional functional materials exhibiting thermo-photochromism.

Recent Advances in Single-Molecule Sensors Based on STM Break Junction Measurements

Single-molecule recognition and detection with the highest resolution measurement has been one of the ultimate goals in science and engineering. Break junction techniques, originally developed to measure single-molecule conductance, recently have also been proven to have the capacity for the label-free exploration of single-molecule physics and chemistry, which paves a new way for single-molecule detection with high temporal resolution. In this review, we outline the primary advances and potential of the STM break junction technique for qualitative identification and quantitative detection at a single-molecule level. The principles of operation of these single-molecule electrical sensing mainly in three regimes, ion, environmental pH and genetic material detection, are summarized. It clearly proves that the single-molecule electrical measurements with break junction techniques show a promising perspective for designing a simple, label-free and nondestructive electrical sensor with ultrahigh sensitivity and excellent selectivity.

Single-Molecule Junction: A Reliable Platform for Monitoring Molecular Physical and Chemical Processes

Monitoring and manipulating the physical and chemical behavior of single molecules is an important development direction of molecular electronics that aids in understanding the molecular world at the single-molecule level. The electrical detection platform based on single-molecule junctions can monitor physical and chemical processes at the single-molecule level with a high temporal resolution, stability, and signal-to-noise ratio. Recently, the combination of single-molecule junctions with different multimodal control systems has been widely used to explore significant physical and chemical phenomena because of its powerful monitoring and control capabilities. In this review, we focus on the applications of single-molecule junctions in monitoring molecular physical and chemical processes. The methods developed for characterizing single-molecule charge transfer and spin characteristics as well as revealing the corresponding intrinsic mechanisms are introduced. Dynamic detection and regulation of single-molecule conformational isomerization, intermolecular interactions, and chemical reactions are also discussed in detail. In addition to these dynamic investigations, this review discusses the open challenges of single-molecule detection in the fields of physics and chemistry and proposes some potential applications in this field.

Stimuli-Modulated Metal Oxidation States in Photochromic MOFs

Tuning metal oxidation states in metal-organic framework (MOF) nodes by switching between two discrete linker photoisomers via an external stimulus was probed for the first time. On the examples of three novel photochromic copper-based frameworks, we demonstrated the capability of switching between +2 and +1 oxidation states, on demand. In addition to crystallographic methods used for material characterization, the role of the photochromic moieties for tuning the oxidation state was probed via conductivity measurements, cyclic voltammetry, and electron paramagnetic resonance, X-ray photoelectron, and diffuse reflectance spectroscopies. We confirmed the reversible photoswitching activity including photoisomerization rate determination of spiropyran- and diarylethene-containing linkers in extended frameworks, resulting in changes in metal oxidation states as a function of alternating excitation wavelengths. To elucidate the switching process between two states, the photoisomerization quantum yield of photochromic MOFs was determined for the first time. Overall, the introduced noninvasive concept of metal oxidation state modulation on the examples of stimuli-responsive MOFs foreshadows a new pathway for alternation of material properties toward targeted applications.

Electronic transport through single-molecule oligophenyl-diethynyl junctions with direct gold-carbon bonds formed at low temperature

We report on the first systematic transport study of alkynyl-ended oligophenyl-diethynyl (OPA) single-molecule junctions with direct Au-C anchoring scheme at low temperature using the mechanically controlled break junction technique. Through quantitative statistical analysis of opening traces, conductance histograms and density functional theory studies, we identified different types of junctions, classified by their conductance and stretching behavior, for OPA molecules between Au electrodes with two to four phenyl rings. We performed inelastic electron tunneling spectroscopy and observed the excitation of Au-C vibrational modes confirming the existence of Au-C bonds at low temperature and compared the stability of molecule junctions upon mechanical stretching. Our findings reveal the huge potential for future functional molecule transport studies at low temperature using alkynyl endgroups.

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.

Electron-Vibration Entanglement of Resonating Dimers in Quantum Transport

Electron transport in a single molecule resulting from the superposition of its vibronic states depends on the coupling strength with the metallic leads. However, dynamical coherence and Fermionic correlation in molecule-molecule and molecule-lead coupling necessitates a critical approach to treat the current and its noise level, especially in the presence of a variable external bias for temperature-dependent conduction. Primarily, this work is a generalization of the theoretical approach of the atomic dimers to incorporate the effect of vibrational modes in current and conductance characteristics. The variation of current and differential conductance due to the external bias reveals a vibrational Coulomb blockade structure corresponding to the functioning vibrational mode in the system. The numerical demonstration for a diverse class of molecules generically shows that electron-vibration interaction can quantitatively predict the nature of coherent electron transport and current noise. Secondly, an attempt has been made to illustrate the effect of magnitude of coherence-induced noise suppression of current as a signature of electron-vibration entanglement. Finally, temperature-dependent conductance of the molecular junction in dimer structure has been estimated along with the peak shifts due to the applied gate voltage.

Voltage-Controlled Binary Conductance Switching in Gold-4,4'-Bipyridine-Gold Single-Molecule Nanowires

We investigate gold-4,4'-bipyridine-gold single-molecule junctions with the mechanically controllable break junction technique at cryogenic temperature (T = 4.2 K). We observe bistable probabilistic conductance switching between the two molecular binding configurations, influenced both by the mechanical actuation and by the applied voltage. We demonstrate that the relative dominance of the two conductance states is tunable by the electrode displacement, whereas the voltage manipulation induces an exponential speedup of both switching times. The detailed investigation of the voltage-tunable switching rates provides an insight into the possible switching mechanisms.

Coordinative-to-covalent transformation, isomerization dynamics, and logic gate application of dithienylethene based photochromic cages

Photochromic coordinative cages containing dynamic C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N imine bonds are assembled from a dithienylethene-based aldehyde and tris-amine precursors via metallo-component self-assembly. The resulting metal-templated cages are then reduced and demetalated into pure covalent-organic cages (COCs), which are otherwise difficult to prepare via de novo organic synthesis. Both the obtained coordinative and covalent cages can be readily interconverted between the ring-open (o-isomer) and one-lateral ring-closed (c-isomer) forms by UV/vis light irradiation, demonstrating distinct absorption, luminescence and photoisomerization dynamics. Specifically, the ring-closed c-COCs show a blue-shifted absorption band compared with analogous metal-templated cages, which can be applied in photoluminescence (PL) color-tuning of upconversion materials in different ways, showing potential for constructing multi-readout logic gate systems.

Metal-templated component self-assembly and then demetalation affords photochromic covalent organic cages applicable for upconversion PL-color tuning for logic gates.

Reversible Photo-Switching of Dual-Color Fluorescent Mn-Doped CdS-ZnS Quantum Dots Modulated by Diarylethene Molecules

Dynamic materials have been given an increased amount of attention in recent years with an expectation that they may exhibit properties on demand. Especially, the combination of fluorescent quantum dots (QDs) and light-responsive organic switches can generate novel photo-switchable materials for diverse applications. In this work, a highly reversible dynamic hybrid system is established by mixing dual-color emitting Mn-doped CdS-ZnS quantum dots (QDs) with photo-switchable diarylethene molecules. We show that the diarylethene 1,2-bis(5-(3,5-bis(trifluoromethyl)phenyl)-2-methylthiophen-3-yl)cyclopent-1-ene (switch molecule 1) performs fabulous photo-switching property (between its open, 1o and closed, 1c forms), and high fatigue resistance in this hybrid system. The emission color switching between blue and pink of the system can be induced mainly by selective quenching/recovering of the Mn- photoluminescence (PL) of the QDs due to the switchable absorbance of the molecule 1. Mechanistic studies show that quenching of QD emission following UV illumination was caused by both Förster resonance energy transfer (FRET) and reabsorption by surrounding 1c molecules in the case of the Mn-PL, and solely by reabsorption in the case of badngap- (BG-)PL. This photo-switchable system could be potentially used in applications ranging from self-erasing paper to super-resolution fluorescence imaging.

Probing Lewis acid-base interactions in single-molecule junctions

A novel strategy to regulate the tunneling mechanism for charge transport through an organoborane wire via Lewis acid-base interactions has been developed. A change from LUMO- to HOMO-dominated charge transport upon the addition of the fluoride is verified both experimentally and theoretically.

Molecular Photoswitching Aided by Excited-State Aromaticity

Central to the development of optoelectronic devices is the availability of efficient synthetic molecular photoswitches, the design of which is an arena where the evolving concept of excited-state aromaticity (ESA) is yet to make a big impact. The aim of this minireview is to illustrate the potential of this concept to become a key tool for the future design of photoswitches. The paper starts with a discussion of challenges facing the use of photoswitches for applications and continues with an account of how the ESA concept has progressed since its inception. Then, following some brief remarks on computational modeling of photoswitches and ESA, the paper describes two different approaches to improve the quantum yields and response times of switches driven by E/Z photoisomerization or photoinduced H-atom/proton transfer reactions through simple ESA considerations. It is our hope that these approaches, verified by quantum chemical calculations and molecular dynamics simulations, will help stimulate the application of the ESA concept as a general tool for designing more efficient photoswitches and other functional molecules used in optoelectronic devices.

Single-Molecule Photocurrent at a Metal-Molecule-Semiconductor Junction

We demonstrate here a new concept for a metal-molecule-semiconductor nanodevice employing Au and GaAs contacts that acts as a photodiode. Current-voltage traces for such junctions are recorded using a STM, and the "blinking" or "I(t)" method is used to record electrical behavior at the single-molecule level in the dark and under illumination, with both low and highly doped GaAs samples and with two different types of molecular bridge: nonconjugated pentanedithiol and the more conjugated 1,4-phenylene(dimethanethiol). Junctions with highly doped GaAs show poor rectification in the dark and a low photocurrent, while junctions with low doped GaAs show particularly high rectification ratios in the dark (>10³ for a 1.5 V bias potential) and a high photocurrent in reverse bias. In low doped GaAs, the greater thickness of the depletion layer not only reduces the reverse bias leakage current, but also increases the volume that contributes to the photocurrent, an effect amplified by the point contact geometry of the junction. Furthermore, since photogenerated holes tunnel to the metal electrode assisted by the HOMO of the molecular bridge, the choice of the latter has a strong influence on both the steady state and transient metal-molecule-semiconductor photodiode response. The control of junction current via photogenerated charge carriers adds new functionality to single-molecule nanodevices.

Thermally Activated Tunneling Transition in a Photoswitchable Single-Molecule Electrical Junction

Exploring the charge transport process in molecular junctions is essential to the development of molecular electronics. Here, we investigate the temperature-dependent charge transport mechanism of carbon electrode-diarylethene single-molecule junctions, which possess photocontrollable molecular orbital energy levels due to reversible photoisomerization of individual diarylethenes between open and closed conformations. Both the experimental results and theoretical calculations consistently demonstrate that the vibronic coupling (thermally activated at the proper temperature) drives the transition of charge transport in the junctions from coherent tunneling to incoherent transport. Due to the subtle electron-phonon coupling effect, incoherent transport in the junctions proves to have different activation energies, depending on the photoswitchable molecular energy levels of two different conformations. These results improve fundamental understanding of charge transport mechanisms in molecular junctions and should lead to the rapid development of functional molecular devices toward practical applications.

Electrochemical control of the single molecule conductance of a conjugated bis(pyrrolo)tetrathiafulvalene based molecular switch

The single molecule conductance of a conjugated molecular wire is electrochemically switched upon oxidising or reducing a central bispyrrolotetrathiafulvalene unit.

As the field of unimolecular electronics develops, there is growing interest in the development of functionalised molecular wires, such as switches, which will allow for more complex molecular-scale circuits. To this end, a three redox state single molecule switch, 1, based on bis(pyrrolo)tetrathiafulvalene (BPTTF) has been designed, synthesised and investigated using scanning tunnelling microscopy break junction (STM-BJ) studies and quantum transport calculations. Oxidising the BPTTF unit increases its conjugation, which was anticipated to increase the molecular conductance of 1. By changing the redox state of 1 electrochemically it was possible to vary the single molecule conductance by more than an order of magnitude (from 10^–5.2G₀ to 10^–3.8G₀). Simulations afforded a qualitatively similar trend. An additional, higher conductance feature is present in most traces at junction sizes of around 2.0 nm – further extension affords the switchable lower conductance feature at junction sizes closer to the molecular length (ca. 3.0 nm). Analysis of the conductance traces shows that these two conductance features occur sequentially in nearly all junctions. This behaviour is attributed to an alternative initial junction conformation in which one or more of the BPTTF sulfur atoms acts as an anchoring group. This hypothesis is supported by a computational study of binding conformations and STM-BJ studies on a model compound, 2, with only one thiol anchor. Our results indicate that the redox properties of BPTTF make it an excellent candidate for use in single molecule switches.

Single-Molecule Transport at a Rectifying GaAs Contact

In most single- or few-molecule devices, the contact electrodes are simple ohmic resistors. Here we describe a new type of single-molecule device in which metal and semiconductor contact electrodes impart a function, namely, current rectification, which is then modified by a molecule bridging the gap. We study junctions with the structure Au STM tip/X/n-GaAs substrate, where "X" is either a simple alkanedithiol or a conjugated unit bearing thiol/methylthiol contacts, and we detect current jumps corresponding to the attachment and detachment of single molecules. From the magnitudes of the current jumps we can deduce values for the conductance decay constant with molecule length that agree well with values determined from Au/molecule/Au junctions. The ability to impart functionality to a single-molecule device through the properties of the contacts as well as through the properties of the molecule represents a significant extension of the single-molecule electronics "tool-box".

Photonics and spectroscopy in nanojunctions: a theoretical insight

Green function methods for photonics and spectroscopy in nanojunctions.

Stretching-Induced Conductance Increase in a Spin-Crossover Molecule

We investigate transport through mechanically triggered single-molecule switches that are based on the coordination sphere-dependent spin state of Fe(II)-species. In these molecules, in certain junction configurations the relative arrangement of two terpyridine ligands within homoleptic Fe(II)-complexes can be mechanically controlled. Mechanical pulling may thus distort the Fe(II) coordination sphere and eventually modify their spin state. Using the movable nanoelectrodes in a mechanically controlled break-junction at low temperature, current-voltage measurements at cryogenic temperatures support the hypothesized switching mechanism based on the spin-crossover behavior. A large fraction of molecular junctions formed with the spin-crossover-active Fe(II)-complex displays a conductance increase for increasing electrode separation and this increase can reach 1-2 orders of magnitude. Theoretical calculations predict a stretching-induced spin transition in the Fe(II)-complex and a larger transmission for the high-spin configuration.

Light-Modulation of the Charge Injection in a Polymer Thin-Film Transistor by Functionalizing the Electrodes with Bistable Photochromic Self-Assembled Monolayers

High fatigue resistance, bistability, and drastic property changes among isomers allow efficient modulation of the current output of organic thin-film transistors (OTFTs) to be obtained by a photogating of the charge-injection mechanism.

Photoswitching of Conductance of Diarylethene-Gold Nanoparticle Network Based on the Alteration of π-Conjugation

Diarylethenes, which have same core structures but have different positions of thiol groups that are bound to gold nanoparticles, were prepared. In one diarylethene, which has two thiol groups at the positions equivalent to 5,5'-positions of di(3-thienyl)ethene, the π-connectivity between two thiol groups increases upon photocyclization, but in the other diarylethene, which has two thiol groups at 2- and 5-positions of one of the 3-thenyl group, the π-connectivity decreases upon photocyclization. The gold nanoparticle networks of these diarylethenes were prepared and the change in conductance was measured upon alternate irradiation with UV and visible light. For two diarylethenes, the direction of the photoswitching was opposite, reflecting the difference in the π-connectivity. The result suggests that the topology of π-conjugation between electrodes is the decisive factor in the conductance of gold nanoparticle network.

Rigid multipodal platforms for metal surfaces

In this review the recent progress in molecular platforms that form rigid and well-defined contact to a metal surface are discussed. Most of the presented examples have at least three anchoring units in order to control the spatial arrangement of the protruding molecular subunit. Another interesting feature is the lateral orientation of these foot structures which, depending on the particular application, is equally important as the spatial arrangement of the molecules. The numerous approaches towards assembling and organizing functional molecules into specific architectures on metal substrates are reviewed here. Particular attention is paid to variations of both, the core structures and the anchoring groups. Furthermore, the analytical methods enabling the investigation of individual molecules as well as monomolecular layers of ordered platform structures are summarized. The presented multipodal platforms bearing several anchoring groups form considerably more stable molecule-metal contacts than corresponding monopodal analogues and exhibit an enlarged separation of the functional molecules due to the increased footprint, as well as restrict tilting of the functional termini with respect to the metal surface. These platforms are thus ideally suited to tune important properties of the molecule-metal interface. On a single-molecule level, several of these platforms enable the control over the arrangement of the protruding rod-type molecular structures (e.g., molecular wires, switches, rotors, sensors) with respect to the surface of the substrate.

Cross-Conjugated Systems Based On An (E)-Hexa-3-en-1,5-diyne-3,4-diyl Skeleton: Spectroscopic and Spectroelectrochemical Investigations

A series of cross-conjugated compounds based on an (E)-4,4'-(hexa-3-en-1,5-diyne-3,4-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) skeleton (1-6) have been synthesized. The linear optical absorption properties can be tuned by modification of the substituents at the 1 and 5 positions of the hexa-3-en-1,5-diynyl backbone (1: Si(CH(CH3)2)3, 2: C6H4C≡CSi(CH3)3, 3: C6H4COOCH3, 4: C6H4CF3, 5: C6H4C≡N, 6: C6H4C≡CC5H4N), although attempts to introduce electron-donating (C6H4CH3, C6H4OCH3, C6H4Si(CH3)3) substituents at these positions were hampered by the ensuing decreased stability of the compounds. Spectroelectrochemical investigations of selected examples, supported by DFT-based computational studies, have shown that one- and two-electron oxidation of the 1,2-bis(triarylamine)ethene fragment also results in electronic changes to the perpendicular π-system in the hexa-3-en-1,5-diynyl branch of the molecule. These properties suggest that (E)-hexa-3-en-1,5-diynyl-based compounds could have applications in molecular sensing and molecular electronics.

Electronic transport in benzodifuran single-molecule transistors

Benzodifuran (BDF) single-molecule transistors have been fabricated in electromigration break junctions for electronic measurements. The inelastic electron tunneling spectrum validates that the BDF molecule is the pathway of charge transport. The gating effect is analyzed in the framework of a single-level tunneling model combined with transition voltage spectroscopy (TVS). The analysis reveals that the highest occupied molecular orbital (HOMO) of the thiol-terminated BDF molecule dominates the charge transport through Au-BDF-Au junctions. Moreover, the energy shift of the HOMO caused by the gate voltage is the main reason for conductance modulation. In contrast, the electronic coupling between the BDF molecule and the gold electrodes, which significantly affects the low-bias junction conductance, is only influenced slightly by the applied gate voltage. These findings will help in the design of future molecular electronic devices.

Light-Induced Switching of Tunable Single-Molecule Junctions

A major goal of molecular electronics is the development and implementation of devices such as single-molecular switches. Here, measurements are presented that show the controlled in situ switching of diarylethene molecules from their nonconductive to conductive state in contact to gold nanoelectrodes via controlled light irradiation. Both the conductance and the quantum yield for switching of these molecules are within a range making the molecules suitable for actual devices. The conductance of the molecular junctions in the opened and closed states is characterized and the molecular level E0, which dominates the current transport in the closed state, and its level broadening Γ are identified. The obtained results show a clear light-induced ring forming isomerization of the single-molecule junctions. Electron withdrawing side-groups lead to a reduction of conductance, but do not influence the efficiency of the switching mechanism. Quantum chemical calculations of the light-induced switching processes correlate these observations with the fundamentally different low-lying electronic states of the opened and closed forms and their comparably small modification by electron-withdrawing substituents. This full characterization of a molecular switch operated in a molecular junction is an important step toward the development of real molecular electronics devices.

Precursor configurations and post-rupture evolution of Ag-CO-Ag single-molecule junctions

Experimental correlation analysis and first-principles theory are used to probe the structure and evolution of Ag-CO-Ag single-molecule junctions both before the formation and after the rupture of the junctions. Two dimensional correlation histograms and conditional histograms demonstrate that prior to the single-molecule bridge configuration the CO molecule is already bound parallel to the Ag single-atom contact. This molecular precursor configuration is accompanied by the opening of additional conductance channels compared to the single-channel transport in pure Ag monoatomic junctions. To investigate the post-rupture evolution of the junction we introduce a cross-correlation analysis between the opening and the subsequent closing conductance traces. This analysis implies that the molecule is bound rigidly to the apex of one electrode, and so the same single-molecule configuration is re-established as the junction is closed. The experimental results are confirmed by ab initio simulations of the evolution of contact geometries, transmission eigenvalues and scattering wavefunctions.

Hapticity-dependent charge transport through carbodithioate-terminated [5,15-bis(phenylethynyl)porphinato]zinc(II) complexes in metal-molecule-metal junctions

Single molecule break junction experiments and nonequilibrium Green's function calculations using density functional theory (NEGF-DFT) of carbodithioate- and thiol-terminated [5,15-bis(phenylethynyl)-10,20-diarylporphinato]zinc(II) complexes reveal the impact of the electrode-linker coordination mode on charge transport at the single-molecule level. Replacement of thiolate (-S(-)) by the carbodithioate (-CS2(-)) anchoring motif leads to an order of magnitude increase of single molecule conductance. In contrast to thiolate-terminated structures, metal-molecule-metal junctions that exploit the carbodithioate linker manifest three distinct conductance values. We hypothesize that the magnitudes of these conductances depend upon carbodithoate linker hapticity with measured conductances across Au-[5,15-bis(4'-(dithiocarboxylate)phenylethynyl)-10,20-diarylporphinato]zinc(II)-Au junctions the greatest when both anchoring groups attach to the metal surface in a bidentate fashion. We support this hypothesis with NEGF-DFT calculations, which consider the electron transport properties for specific binding geometries. These results provide new insights into the origin of molecule-to-molecule conductance heterogeneity in molecular charge transport measurements and the factors that optimize electrode-molecule-electrode electronic coupling and maximize the conductance for charge transport.

Regulating a benzodifuran single molecule redox switch via electrochemical gating and optimization of molecule/electrode coupling

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (-CS2(-)) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.

Electron transport characteristics of the dimeric 1,4-benzenedithiol junction

Understanding the electron transport between single molecules connected through weak interaction is of great importance for molecular electronics. In this paper, we report measurements of the conductivity of the dimeric 1,4-benzenedithiol (BDT) junction using the scanning tunneling microscopy (STM)-based current-displacement I(s) method. The conductance was measured to be 6.14×10(-6)  G0 , a value almost two orders of magnitude lower than that of the monomer BDT junction. In control experiments, the probability of junction formation decreased with the presence of tris(2-chloroethyl) phosphate (TCEP), a reducing reagent for the disulfide bond. According to theoretical computations, the dihedral angle of the SS bond tends to take a perpendicular conformation. This non-conjugated structure localizes the electron distribution and accounts for the low conductivity of the disulfide linkage.

Single-molecule electronics: from chemical design to functional devices

The use of single molecules in electronics represents the next limit of miniaturisation of electronic devices, which would enable to continue the trend of aggressive downscaling of silicon-based electronic devices.

Orthogonally modulated molecular transport junctions for resettable electronic logic gates

Individual molecules have been demonstrated to exhibit promising applications as functional components in the fabrication of computing nanocircuits. Based on their advantage in chemical tailorability, many molecular devices with advanced electronic functions have been developed, which can be further modulated by the introduction of external stimuli. Here, orthogonally modulated molecular transport junctions are achieved via chemically fabricated nanogaps functionalized with dithienylethene units bearing organometallic ruthenium fragments. The addressable and stepwise control of molecular isomerization can be repeatedly and reversibly completed with a judicious use of the orthogonal optical and electrochemical stimuli to reach the controllable switching of conductivity between two distinct states. These photo-/electro-cooperative nanodevices can be applied as resettable electronic logic gates for Boolean computing, such as a two-input OR and a three-input AND-OR. The proof-of-concept of such logic gates demonstrates the possibility to develop multifunctional molecular devices by rational chemical design.

Two-dimensional supramolecular spring: coordination driven reversible extension and contraction of bridged half rings

A tetraethylene glycol ether bridged derivative 9 has been designed and synthesized, and its two-dimensional (2D) self-assembled behavior has been investigated at the single-molecule level.

Conductance measurement of pyridyl-based single molecule junctions with Cu and Au contacts

We studied the conductance of pyridyl-based single molecule junctions with Cu contacts by using an electrochemical jump-to-contact scanning tunneling microscopy break junction (ECSTM-BJ) approach. The single molecule junctions of 4,4'-bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE) and 1,2-di(pyridin-4-yl)ethane (BPY-EA) bridged with Cu clusters show three sets of conductance values. These values are smaller than the conductance values of single molecule junctions with Au electrodes measured by the traditional scanning tunneling microscopy break junction in acidic or neutral solutions, which can be attributed to the different electronic coupling efficiencies between molecules and electrodes. The consistent conductance of pyridyl-based molecules in acidic and neutral solutions may show that the protonated pyridyl group contacts to the electrode through the deprotonated form.