A Memristive Element Based on an Electrically Controlled Single‐Molecule Reaction

The exponential proliferation of data during the information age has required the continuous exploration of novel storage paradigms, materials, and devices with increasing data density. As a step toward the ultimate limits in data density, the development of an electrically controllable single‐molecule memristive element is reported. In this device, digital information is encoded through switching between two isomer states by applying a voltage signal to the molecular junction, and the information is read out by monitoring the electrical conductance of each isomer. The two states are cycled using an electrically controllable local‐heating mechanism for the forward reaction and catalyzed by a single charge‐transfer process for the reverse switching. This single‐molecule device can be modulated in situ, is fully reversible, and does not display stochastic switching. The I–V curves of this single‐molecule system also exhibit memristive character. These features suggest a new approach for the development of molecular switching systems and storage‐class memories.     

Anisotropic Conductivity at the Single‐Molecule Scale

In most junctions built by wiring a single molecule between two electrodes, the electrons flow along only one axis: between the two anchoring groups. However, molecules can be anisotropic, and an orientation‐dependent conductance is expected. Here, we fabricated single‐molecule junctions by using the electrode potential to control the molecular orientation and access individual elements of the conductivity tensor. We measured the conductance in two directions, along the molecular plane as the benzene ring bridges two electrodes using anchoring groups (upright) and orthogonal to the molecular plane with the molecule lying flat on the substrate (planar). The perpendicular (planar) conductance is about 400 times higher than that along the molecular plane (upright). This offers a new method for designing a reversible room‐temperature single‐molecule electromechanical switch that controllably employs the electrode potential to orient the molecule in the junction in either “ON” or “OFF” conductance states.     

Ruthenium Tris‐bipyridine Single‐Molecule Junctions with Multiple Joint Configurations

Single‐molecule junctions are of particular interest in molecular electronics. To realize molecular electronic devices, it is crucial that functional single‐molecule junctions are connected to each other by using joint units on the atomic scale. However, good joint units have not been reported because controlling the charge transport directions through the junctions is not trivial. Here, we report a joint unit that controls and changes the charge transport directions through the junctions, by using a ruthenium–tris‐bipyridine (RuBpy) complex. The RuBpy single‐molecule junction was fabricated with scanning tunnelling microscopy‐based break junction techniques. The RuBpy single‐molecule junction showed two distinct high and low conductance states. The two states were characterized by the conductance measurement, the correlation analysis, and the comparative experiment of bipyridine (Bpy), which is the ligand unit of RuBpy. We demonstrate that the Ru complex has multiple charge transport paths, where the charge is carried vertically and horizontally through the complex depending on the path.     

Radical‐Enhanced Charge Transport in Single‐Molecule Phenothiazine Electrical Junctions

We studied the single‐molecule conductance through an acid oxidant triggered phenothiazine (PTZ‐) based radical junction using the mechanically controllable break junction technique. The electrical conductance of the radical state was enhanced by up to 200 times compared to the neutral state, with high stability lasting for at least two months and high junction formation probability at room‐temperature. Theoretical studies revealed that the conductance increase is due to a significant decrease of the HOMO–LUMO gap and also the enhanced transmission close to the HOMO orbital when the radical forms. The large conductance enhancement induced by the formation of the stable PTZ radical molecule will lead to promising applications in single‐molecule electronics and spintronics.     

Three‐State Single‐Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning

We studied charge transport through core‐substituted naphthalenediimide (NDI) single‐molecule junctions using the electrochemical STM‐based break‐junction technique in combination with DFT calculations. Conductance switching among three well‐defined states was demonstrated by electrochemically controlling the redox state of the pendent diimide unit of the molecule in an ionic liquid. The electrical conductances of the dianion and neutral states differ by more than one order of magnitude. The potential‐dependence of the charge‐transport characteristics of the NDI molecules was confirmed by DFT calculations, which account for electrochemical double‐layer effects on the conductance of the NDI junctions. This study suggests that integration of a pendant redox unit with strong coupling to a molecular backbone enables the tuning of charge transport through single‐molecule devices by controlling their redox states.     

Franck–Condon Blockade and Aggregation‐Modulated Conductance in Molecular Devices Using Aggregation‐Induced Emission‐Active Molecules

We report an effective modulation of the quantum transport in molecular junctions consisting of aggregation‐induced‐emission(AIE)‐active molecules. Theoretical simulations based on combined density functional theory and rate‐equation method calculations show that the low‐bias conductance of the junction with a single tetraphenylethylene (TPE) molecule can be completely suppressed by strong electron–vibration couplings, that is, the Franck‐Condon blockade effect. It is mainly associated with the low‐energy vibration modes, which is also the origin of the fluorescence quenching of the AIE molecule in solution. We further found that the conductance of the junction can be lifted by restraining the internal motion of the TPE molecule by either methyl substitution on the phenyl group or by aggregation, a mechanism similar to the AIE process. The present work demonstrates the correlation between optical processes of molecules and quantum transport in their junction, and thus opens up a new avenue for the application of AIE‐type molecules in molecular electronics and functional devices.     

Electron Transport through Single π‐Conjugated Molecules Bridging between Metal Electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron‐transport properties of single π‐conjugated molecule junctions, which include benzene, fullerene, and π‐stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single‐molecule junctions. The effect of the metal–molecule contact on the conductance of the single π‐conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π‐stacked system using π‐stacked aromatic molecules enclosed within self‐assembled coordination cages is reviewed. The electron transport in the π‐stacked systems is found to be efficient at the single‐molecule level, thus providing insight into the design of conductive materials.     

Understanding the Role of Parallel Pathways via In‐Situ Switching of Quantum Interference in Molecular Tunneling Junctions

This study describes the modulation of tunneling probabilities in molecular junctions by switching one of two parallel intramolecular pathways. A linearly conjugated molecular wire provides a rigid framework that allows a second, cross‐conjugated pathway to be effectively switched on and off by protonation, affecting the total conductance of the junction. This approach works because a traversing electron interacts with the entire quantum‐mechanical circuit simultaneously; Kirchhoff's rules do not apply. We confirm this concept by comparing the conductances of a series of compounds with single or parallel pathways in large‐area junctions using EGaIn contacts and single‐molecule break junctions using gold contacts. We affect switching selectively in one of two parallel pathways by converting a cross‐conjugated carbonyl carbon into a trivalent carbocation, which replaces destructive quantum interference with a symmetrical resonance, causing an increase in transmission in the bias window.     

Direct Measurement of Single‐Molecule DNA Hybridization Dynamics with Single‐Base Resolution

Herein, we report label‐free detection of single‐molecule DNA hybridization dynamics with single‐base resolution. By using an electronic circuit based on point‐decorated silicon nanowires as electrical probes, we directly record the folding/unfolding process of individual hairpin DNAs with sufficiently high signal‐to‐noise ratio and bandwidth. These measurements reveal two‐level current oscillations with strong temperature dependence, enabling us to determine the thermodynamic and kinetic properties of hairpin DNA hybridization. More importantly, successive, stepwise increases and decreases in device conductance at low temperature on a microsecond timescale are successfully observed, indicating a base‐by‐base unfolding/folding process. The process demonstrates a kinetic zipper model for DNA hybridization/dehybridization at the single base‐pair level. This measurement capability promises a label‐free single‐molecule approach to probe biomolecular interactions with fast dynamics.     

Studying single molecule electrochemistry with scanning tunneling microscope break-junction technique

The fundamental principle of molecular electronics is to comprehend electrical properties of single molecules connected between two probe electrodes. In recent years, substantial advances in this field have been made to underpin experimental and theoretical understanding of single molecule electrochemistry. By using scanning tunneling microscope (STM) break-junction technique, the switching events of electrical current from single molecule bridge tuning by electrochemical gating are investigated to uncover the relationship between electrochemical electron transfer and charge transport processes in chemical and biological molecule junctions. In this short review, we outline the latest works of single molecule electrochemistry studied with STM break-junction technique from Nongjian Tao's group, and share the insights on the opportunities and challenges for future research.     

A Novel Bat‐Shaped Dicyanomethylene‐4H‐pyran‐Functionalized Naphthalimide for Highly Efficient Solution‐Processed Multilevel Memory Devices

Small‐molecule‐based multilevel memory devices have attracted increasing attention because of their advantages, such as super‐high storage density, fast reading speed, light weight, low energy consumption, and shock resistance. However, the fabrication of small‐molecule‐based devices always requires expensive vacuum‐deposition techniques or high temperatures for spin‐coating. Herein, through rational tailoring of a previous molecule, DPCNCANA (4,4′‐(6,6′‐bis(2‐octyl‐1,3‐dioxo‐2,3‐dihydro‐1H‐benzo[de]isoquinolin‐6‐yl)‐9H,9′H‐[3,3′‐bicarbazole]‐9,9′‐diyl)dibenzonitrile), a novel bat‐shaped A‐D‐A‐type (A‐D‐A=acceptor–donor–acceptor) symmetric framework has been successfully synthesized and can be dissolved in common solvents at room temperature. Additionally, it has a low‐energy bandgap and dense intramolecular stacking in the film state. The solution‐processed memory devices exhibited high‐performance nonvolatile multilevel data‐storage properties with low switching threshold voltages of about −1.3 and −2.7 V, which is beneficial for low power consumption. Our result should prompt the study of highly efficient solution‐processed multilevel memory devices in the field of organic electronics.     

Influence of the Number of Anchoring Groups on the Electronic and Mechanical Properties of Benzene‐, Anthracene‐ and Pentacene‐Based Molecular Devices

One of the central issues of molecular electronics (ME) is the study of the molecule–metal electrode contacts, and their implications for the conductivity, charge‐transport mechanism, and mechanical stability. In fact, stochastic on/off switching (blinking) reported in STM experiments is a major problem of single‐molecule devices, and challenges the stability and reliability of these systems. Surprisingly, the ambiguous STM results all originate from devices that bind to the metallic electrode through a one‐atom connection. In the present work, DFT is employed to study and compare the properties of a set of simple acenes that bind to metallic electrodes with an increasing number of connections, in order to determine whether the increasing numbers of anchoring groups have a direct repercussion on the stability of these systems. The conductivities of the three polycyclic aromatic hydrocarbons are calculated, as well as their transmission spectra and current profiles. The thermal and mechanical stability of these systems is studied by pulling and pushing the metal–molecule connection. The results show that molecules with more than one connection per electrode exhibit greater electrical efficiency and current stability.     

Achieving Efficient Multichannel Conductance in Through‐Space Conjugated Single‐Molecule Parallel Circuits

Constructing single‐molecule parallel circuits with multiple conduction channels is an effective strategy to improve the conductance of a single molecular junction, but rarely reported. We present a novel through‐space conjugated single‐molecule parallel circuit (f‐4Ph‐4SMe) comprised of a pair of closely parallelly aligned p‐quaterphenyl chains tethered by a vinyl bridge and end‐capped with four SMe anchoring groups. Scanning‐tunneling‐microscopy‐based break junction (STM‐BJ) and transmission calculations demonstrate that f‐4Ph‐4SMe holds multiple conductance states owing to different contact configurations. When four SMe groups are in contact with two electrodes at the same time, the through‐bond and through‐space conduction channels work synergistically, resulting in a conductance much larger than those of analogous molecules with two SMe groups or the sum of two p‐quaterphenyl chains. The system is an ideal model for understanding electron transport through parallel π‐stacked molecular systems and may serve as a key component for integrated molecular circuits with controllable conductance.     

Modulation of a Molecular π‐Electron System in a Purely Organic Conductor that Shows Hydrogen‐Bond‐Dynamics‐Based Switching of Conductivity and Magnetism

New important aspects of the hydrogen‐bond (H‐bond)‐dynamics‐based switching of electrical conductivity and magnetism in an H‐bonded, purely organic conductor crystal have been discovered by modulating its tetrathiafulvalene (TTF)‐based molecular π‐electron system by means of partial sulfur/selenium substitution. The prepared selenium analogue also showed a similar type of phase transition, induced by H‐bonded deuterium transfer followed by electron transfer between the H‐bonded TTF skeletons, and the resulting switching of the physical properties; however, subtle but critical differences due to sulfur/selenium substitution were detected in the electronic structure, phase transition nature, and switching function. A molecular‐level discussion based on the crystal structures shows that this chemical modification of the TTF skeleton influences not only its own π‐electronic structure and π–π interactions within the conducting layer, but also the H‐bond dynamics between the TTF π skeletons in the neighboring layers, which enables modulation of the interplay between the H‐bond and π electrons to cause such differences.     

N‐Arylation of Diketopyrrolopyrroles with Aryl Triflates

A new methodology for the double N‐arylation of diketopyrrolopyrroles with aryl triflates has been developed. It is now possible to prepare diketopyrrolopyrroles bearing N‐substituents derived from naphthalene, anthracene and coumarin in two steps from commercially available phenols. This represents the first time arenes lacking strong electron‐withdrawing groups were inserted onto lactamic nitrogen atoms via arylation. The ability to incorporate heretofore unprecedented substituents translates to increased modulation of the resulting photophysical properties such as switching‐on/off solvatofluorochromism. TD‐DFT calculations have been performed to explore the nature of the relevant excited states. This new synthetic method made it possible to elucidate the influence of such substituents on the absorption and emission properties of tetraaryl substituted diketopyrrolopyrroles.     

Highly Reproducible Formation of a Polymer Single‐Molecule Junction for a Well‐Defined Current Signal

Single‐molecule devices attract much interest in the development of nanoscale electronics. Although a variety of functional single molecules for single‐molecule electronics have been developed, there still remains the need to implement sophisticated functionalization toward practical applications. Given its superior functionality encountered in macroscopic materials, a polymer could be a useful building block in the single‐molecule devices. Therefore, a molecular junction composed of polymer has now been created. Furthermore, an automated algorithm was developed to quantitatively analyze the tunneling current through the junction. Quantitative analysis revealed that the polymer junction exhibits a higher formation probability and longer lifetime than its monomer counterpart. These results suggest that the polymer provides a unique opportunity to design both stable and highly functional molecular devices for nanoelectronics.     

Energy‐Level Alignment for Single‐Molecule Conductance of Extended Metal‐Atom Chains

The use of single‐molecule junctions for various functions constitutes a central goal of molecular electronics. The functional features and the efficiency of electron transport are dictated by the degree of energy‐level alignment (ELA), that is, the offset potential between the electrode Fermi level and the frontier molecular orbitals. Examples manifesting ELA are rare owing to experimental challenges and the large energy barriers of typical model compounds. In this work, single‐molecule junctions of organometallic compounds with five metal centers joined in a collinear fashion were analyzed. The single‐molecule i–V scans could be conducted in a reliable manner, and the E_FMO levels were electrochemically accessible. When the electrode Fermi level (E_F) is close to the frontier orbitals (E_FMO) of the bridging molecule, larger conductance was observed. The smaller |E_F?E_FMO| gap was also derived quantitatively, unambiguously confirming the ELA. The mechanism is described in terms of a two‐level model involving co‐tunneling and sequential tunneling processes.     

Multichannel Conductance of Folded Single‐Molecule Wires Aided by Through‐Space Conjugation

Deciphering charge transport through multichannel pathways in single‐molecule junctions is of high importance to construct nanoscale electronic devices and deepen insight into biological redox processes. Herein, we report two tailor‐made folded single‐molecule wires featuring intramolecular π–π stacking interactions. The scanning tunneling microscope (STM) based break‐junction technique and theoretical calculations show that through‐bond and through‐space conjugations are integrated into one single‐molecule wire, allowing for two simultaneous conducting channels in a single‐molecule junction. These folded molecules with stable π–π stacking interaction offer conceptual advances in single‐molecule multichannel conductance, and are perfect models for conductance studies in biological systems, organic thin films, and π‐stacked columnar aggregates.     

Controlling Electrical Conductance through a π‐Conjugated Cruciform Molecule by Selective Anchoring to Gold Electrodes

Tuning charge transport at the single‐molecule level plays a crucial role in the construction of molecular electronic devices. Introduced herein is a promising and operationally simple approach to tune two distinct charge‐transport pathways through a cruciform molecule. Upon in situ cleavage of triisopropylsilyl groups, complete conversion from one junction type to another is achieved with a conductance increase by more than one order of magnitude, and it is consistent with predictions from ab initio transport calculations. Although molecules are well known to conduct through different orbitals (either HOMO or LUMO), the present study represents the first experimental realization of switching between HOMO‐ and LUMO‐dominated transport within the same molecule.     

The Correlation of Electrochemical Measurements and Molecular Junction Conductance Simulations in β‐Strand Peptides

Understanding the electronic properties of single peptides is not only of fundamental importance, but it is also paramount to the realization of peptide‐based molecular electronic components. Electrochemical and theoretical studies are reported on two β‐strand‐based peptides, one with its backbone constrained with a triazole‐containing tether introduced by Huisgen cycloaddition (peptide 1 ) and the other a direct linear analogue (peptide 2 ). Density functional theory (DFT) and non‐equilibrium Green’s function were used to investigate conductance in molecular junctions containing peptides 3 and 4 (analogues of 1 and 2 ). Although the peptides share a common β‐strand conformation, they display vastly different electronic transport properties due to the presence (or absence) of the side‐bridge constraint and the associated effect on backbone rigidity. These studies reveal that the electron transfer rate constants of 1 and 2 , and the conductance calculated for 3 and 4 , differ by approximately one order of magnitude, thus providing two distinctly different conductance states and what is essentially a molecular switch. A definitive correlation of electrochemical measurements and molecular junction conductance simulations is demonstrated using two different charge transfer techniques. This study furthers our understanding of the electronic properties of peptides at the molecular level, which provides an opportunity to fine‐tune their molecular orbital energies through suitable structural manipulation.