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

Reversible electric switching of NDI molecular wires by orthogonal stimuli

The charge transport of 1,4,5,8-naphthalene diimide (NDI)-based molecules is explored. Experimental results show that the conductance of the TH-NDI molecular junction can be reversibly tuned by bias voltage and solvent, while the conductance of the PH-NDI junction is almost independent of the bias voltage and solvent. Based on these orthogonal stimuli, an AND logic gate of TH-NDI junction with an electric signal as the output is constructed. These results will advance the development of functional molecular devices.

Supramolecular Transistors with Quantum Interference Effect

The charge transport through supramolecular junctions exhibits unique quantum interference (QI) effects, which provide an opportunity for the design of supramolecular transistors. Benefiting from the configuration dependence of QI, configuration control of the supramolecular assemblies to demonstrate the QI features is a key but challenging step. In this work, we fabricated the supramolecular transistors and investigated the charge transport through the conducting channel of the individual π-stacked thiophene/phenylene co-oligomers (TPCOs) using the electrochemically gated scanning tunneling microscope break junction technique. We controlled the configuration of the supramolecular channel and switched the QI features between the anti-resonance and resonance states of the supramolecular channels. We observed the supramolecular transistor with its on/off ratio above 103 (∼1300), a high gating efficiency of ∼165 mV/dec, a low off-state leakage current of ∼30 pA, and the channel length scaled down to <2.0 nm. Density functional theory calculations suggested that the QI features in π-stacked TPCOs vary depending on the supramolecular architecture and can be manipulated efficiently by fine-tuning the supramolecular configurations. This work reveals the potential of the supramolecular channels for molecular electronics and provides a fundamental understanding of intermolecular charge transport.

Radical Single-Molecule Junctions

Radicals are unique molecular systems for applications in electronic devices due to their open-shell electronic structures. Radicals can function as good electrical conductors and switches in molecular circuits while also holding great promise in the field of molecular spintronics. However, it is both challenging to create stable, persistent radicals and to understand their properties in molecular junctions. The goal of this Perspective is to address this dual challenge by providing design principles for the synthesis of stable radicals relevant to molecular junctions, as well as offering current insight into the electronic properties of radicals in single-molecule devices. By exploring both the chemical and physical properties of established radical systems, we will facilitate increased exploration and development of radical-based molecular systems.

Local cation-tuned reversible single-molecule switch in electric double layer

The nature of molecule-electrode interface is critical for the integration of atomically precise molecules as functional components into circuits. Herein, we demonstrate that the electric field localized metal cations in outer Helmholtz plane can modulate interfacial Au-carboxyl contacts, realizing a reversible single-molecule switch. STM break junction and I-V measurements show the electrochemical gating of aliphatic and aromatic carboxylic acids have a conductance ON/OFF behavior in electrolyte solution containing metal cations (i.e., Na⁺, K⁺, Mg²⁺ and Ca²⁺), compared to almost no change in conductance without metal cations. In situ Raman spectra reveal strong molecular carboxyl-metal cation coordination at the negatively charged electrode surface, hindering the formation of molecular junctions for electron tunnelling. This work validates the critical role of localized cations in the electric double layer to regulate electron transport at the single-molecule level.

Conductance Growth of Single-Cluster Junctions with Increasing Sizes

Quantum-tunneling-based nanoelectronics has the potential for the miniaturization of electronics toward the sub-5 nm scale. However, the nature of phase-coherent quantum tunneling leads to the rapid decays of the electrical conductance with tunneling transport distance, especially in organic molecule-based nanodevices. In this work, we investigated the conductance of the single-cluster junctions of a series of atomically well-defined silver nanoclusters, with varying sizes from 0.9 to 3.0 nm, using the mechanically controllable break junction (MCBJ) technique combined with quantum transport theory. Our charge transport investigations of these single-cluster junctions revealed that the conductance grows with increasing cluster size. The conductance decay constant was determined to be ∼-0.4 nm^-1, which is of opposite sign to that of organic molecules. Comparison between experiment and theory reveals that although charge transport through the silver single-cluster junctions occurs via phase-coherent tunneling, this is compensated by a rapid decrease in the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap) with size and the increase in the electrode-cluster coupling, which results in their conductance increase up to lengths of ∼3.0 nm. These results demonstrate that such families of nanoclusters provide unique bottom-up building blocks for the fabrication of nanodevices in the sub-5 nm size range.

The Rise of 1,4-BN-Heteroarenes: Synthesis, Properties, and Applications

BN-heteroarenes, which employ both boron and nitrogen in aromatic hydrocarbons, have gained great attention in the fields of organic chemistry and materials science. Nevertheless, the extensive studies on BN-heteroarenes are largely limited to 1,2-azaborine-based compounds with B-N covalent bonds, whereas 1,3- and 1,4-BN-heteroarenes are relatively rare due to their greater challenge in the synthesis. Recently, significant progresses have been achieved in the synthesis and applications of BN-heteroarenes featuring 1,4-azaborines, especially driven by their significant potential as multiresonant thermally activated delayed fluorescence (MR-TADF) materials. Therefore, it is timely to review these advances from the chemistry perspective. This review summarizes the synthetic methods and recent achievements of 1,4-azaborine-based BN-heteroarenes and discusses their unique properties and potential applications of this emerging class of materials, highlighting the value of 1,4-BN-heteroarenes beyond MR-TADF materials. It is hoped that this review would stimulate the conversation and cooperation between chemists who are interested in azaborine chemistry and materials scientists working in the fields of organic optoelectronics, metal catalysis, and carbon-based nanoscience etc.

Tetrathiafulvalenes as anchors for building highly conductive and mechanically tunable molecular junctions

The interface between molecules and electrodes has great impact on charge transport of molecular devices. Precisely manipulating the structure and electronic coupling of electrode-molecule interface at a molecular level is very challenging. Here, we develop new molecular junctions based on tetrathiafulvalene (TTF)-fused naphthalene diimide (NDI) molecules which are anchored to gold electrodes through direct TTF-Au contacts formed via Au-S bonding. These contacts enable highly efficient orbital hybridization of gold electrodes and the conducting π-channels, yielding strong electrode-molecule coupling and remarkably high conductivity in the junctions. By further introducing additional thiohexyl (SHe) anchors to the TTF units, we develop molecular wires with multiple binding sites and demonstrate reversibly switchable electrode-molecule contacts and junction conductance through mechanical control. These findings show a superb electrode-molecule interface and provide a new strategy for precisely tunning the conductance of molecular devices towards new functions.

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

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

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.

Probing the Electrostatic Barrier of Tetrathiafulvalene Dications using a Tetra-stable Donor-Acceptor [2]Rotaxane

A tetra-stable donor-acceptor [2]rotaxane 1⋅4PF₆ has been synthesized. The dumbbell component is comprised of an oxyphenylene (OP), a tetrathiafulvalene (TTF), a monopyrrolo-TTF (MPTTF), and a hydroquinone (HQ) unit, which can act as recognition sites (stations) for the tetra-cationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ ). The TTF and the MPTTF stations are located in the middle of the dumbbell component and are connected by a triethylene glycol (TEG) chain in such a way that the pyrrole moiety of the MPTTF station points toward the TTF station, while the TTF and MPTTF stations are flanked by the OP and HQ stations on their left hand side and right hand side, respectively. The [2]rotaxane was characterized in solution by ¹ H NMR spectroscopy and cyclic voltammetry. The spectroscopic data revealed that the majority (77 %) of the tetra-stable [2]rotaxane 1⁴⁺ exist as the translational isomer 1⋅MPTTF⁴⁺ in which the CBPQT⁴⁺ ring encircles the MPTTF station. The electrochemical studies showed that CBPQT⁴⁺ in 1⋅MPTTF⁴⁺ undergoes ring translation as result of electrostatic repulsion from the oxidized MPTTF unit. Following tetra-oxidation of 1⋅MPTTF⁴⁺ , a high-energy state of 1⁸⁺ was obtained (i.e., 1⋅TEG⁸⁺ ) in which the CBPQT⁴⁺ ring was located on the TEG linker connecting the di-oxidized TTF²⁺ and MPTTF²⁺ units. ¹ H NMR spectroscopy carried out in CD₃ CN at 298 K on a chemically oxidized sample of 1⋅MPTTF⁴⁺ revealed that the metastable state 1⋅TEG⁸⁺ is only short-lived with a lifetime of a few minutes and it was found that 70 % of the positively charged CBPQT⁴⁺ ring moved from 1⋅TEG⁸⁺ to the HQ station, while 30 % moved to the much weaker OP station. These results clearly demonstrate that the CBPQT⁴⁺ ring can cross both an MPTTF²⁺ and a TTF²⁺ electrostatic barrier and that the free energy of activation required to cross MPTTF²⁺ is ca. 0.5 kcal mol^-1 smaller as compared to TTF²⁺ .

Synthesis, Characterization, and Crystal Structures of Imides Condensed with p-Phenylamino(Phenyl) Amine and Fluorescence Property

A series of aromatic diimide and monoimide compounds condensed with p-phenylamino(phenyl)amine were synthesized and confirmed by Proton Nuclear Magnetic Resonance (1H NMR), Carbon-13 Nuclear Magnetic Resonance (13C NMR), Fourier Transform Infrared Spectroscopy (FT-IR), Elemental Analysis (EA), and High Resolution Mass Spectroscopy (HRMS). Meanwhile, single crystal X-ray diffraction showed the existence of intermolecular N···O hydrogen bonds, which affected the thermal stabilities of corresponding compounds by the support of Thermalgravimetric Analysis (TGA) curves. The steady-state UV-vis absorption peaks of synthetic compounds 1-6 appeared in the range of 220-380 nm. Fluorescence emission spectra showed peaks in the range of 290-420 nm. Meanwhile, deep-blue or violet-blue emissions for 2, 4, and 5 in THF under excitations of 254 nm and 365 nm, respectively, were observed at room temperature in air. Furthermore, Differential pulse voltammetry (DPV) and cyclic voltammogram CV were conducted within -1.5-+1.5 V to show quasi-reversible behavior for conjugated compounds and irreversible behavior for less conjugated ones.

Salts accelerate the switching kinetics of a cyclobis(paraquat-p-phenylene) [2]rotaxane

The rate at which the macrocyclic cyclobis(paraquat-p-phenylene) ring of a bistable [2]rotaxane moves from a tetrathiafulvalene station to an oxyphenylene station upon oxidation of the tetrathiafulvalene station is found to be increased in the presence of added salts. Compared to the salt-free case, 0.1 M solutions of a series of tetraalkylammonium hexafluorophosphate salts (R4N·PF6, R = H, Me, Et or n-Bu) and of tetrabutylammonium perchlorate (n-Bu4N·ClO4) all afford an increased switching rate, which is largest in the case of n-Bu4N·ClO4 with smaller anions. Variation in the size of the ammonium cation has no significant effect. These results indicate that the addition of excess ions can be used as an accelerator to speed up shuttling processes in rotaxanes and catenanes based on the mobile cyclobis(paraquat-p-phenylene) ring, and that the choice of anion offers a convenient means of controlling the extent of this effect.