Structure-Independent Conductance of Thiophene-Based Single-Stacking Junctions

The experimental investigation of intermolecular charge transport in π-conjugated materials is challenging. Herein, we describe the investigation of charge transport through intermolecular and intramolecular paths in single-molecule and single-stacking thiophene junctions by the mechanically controllable break junction (MCBJ) technique. We found that the ability for intermolecular charge transport through different single-stacking junctions was approximately independent of the molecular structure, which contrasts with the strong length dependence of conductance in single-molecule junctions with the same building blocks, and the dominant charge-transport path of molecules with two anchors transited from an intramolecular to an intermolecular path when the degree of conjugation increased. An increase in conjugation further led to higher binding probability owing to the variation in binding energies, as supported by DFT calculations.     

Supramolecular Enhancement of Charge Transport through Pillar[5]arene-Based Self-Assembled Monolayers

Intermolecular charge transport is one of the essential modes for modulating charge transport in molecular electronic devices. Supermolecules are highly promising candidates for molecular devices because of their abundant structures and easy functionalization. Herein, we report an efficient strategy to enhance charge transport through pillar[5]arene self-assembled monolayers (SAMs) by introducing cationic guests. The current density of pillar[5]arene SAMs can be raised up to about 2.1 orders of magnitude by inserting cationic molecules into the cavity of pillar[5]arenes in SAMs. Importantly, we have also observed a positive correlation between the charge transport of pillar[5]arene-based complex SAMs and the binding affinities of the pillar[5]arene-based complexation. Such an enhancement of charge transport is attributed to the efficient host–guest interactions that stabilize the supramolecular complexes and lower the energy gaps for charge transport. This work provides a predictive pattern for the regulation of intermolecular charge transport in guiding the design of next generation switches and functional sensors in supramolecular electronics.     

基于裂结技术的单分子尺度化学反应研究进展

分子电子学是研究单分子器件的构筑、性质以及功能调控的一门新兴学科。其中,金属/分子/金属结的构筑和表征是现阶段分子电子学的主要研究内容。裂结技术是当前分子电子学研究的主要实验方法,主要包括机械可控裂结技术和扫描隧道显微镜裂结技术。本文对裂结技术进行了介绍,并对近年来利用这些技术,在单分子尺度化学反应的检测和动力学研究,以及将这些技术与溶液环境、静电场、电化学门控等方法相结合,调控单分子器件的电输运性质等方面所取得的进展进行了概述。     

A chiral SrSi2 (srs) superstructure constructed by a dual interaction system showing isotropic electrical conductivity

Most porous conductive frameworks are highly anisotropic in their structures thus leading to anisotropic charge transport. Here we report a supramolecular self-assembly which is constructed by intermolecular hydrogen bonding and π ···π interactions. This material features a chiral, porous, cubic framework structure with π-stacked helical columns along all of the three Cartesian coordinates. As a result, isotropic charge transport with an electrical conductivity(σ) of 2.1 × 10^–7S/cm is...     

Towards single-molecule optoelectronic devices

Benefiting from the development of molecular electronics and molecular plasmonics, the interplay of light and electronic transport in molecular junctions has attracted growing interest among researchers in both fields, leading to a new research direction of “single-molecule optoelectronics”. Here, we review the latest developments of photo-modulated charge transport, electroluminescence and Raman spectroscopy from single-molecule junctions, and suggest future directions for single-molecule optoelectronics.     

A core-shell strategy for constructing a single-molecule junction

Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions.     

Supramolecular electronics; nanowires from self-assembled pi-conjugated systems

The conditions required for supramolecular electronics, e.g. nano-sized optoelectronic devices, will be illustrated on the basis of the programmed self-assembly of pi-conjugated systems into individual nanosized wires. Using the supramolecular design rules nanowires can be created from almost any polymeric and oligomeric pi-conjugated system. In the case of oligomers it is even possible to construct individual wires having a uniform diameter of one molecule thickness. The construction of wires on a substrate is possible by self-assembly in solution or during the deposition. The transfer of the supramolecular stacks from solution to a solid support is a very delicate process. A comprehensive knowledge of all intermolecular interactions gives rise to controlled transfer of pi-conjugated assemblies to specific surfaces. There are a large number of very appealing targets that should be reached before supramolecular electronics can serve as an attractive alternative in between single molecule electronics and bulk devices. Nevertheless, the combination of exciting scientific results and intriguing technological challenges creates an interesting future for supramolecular electronics.     

Mechanically activated molecular switch through single-molecule pulling

We investigate a prototypical single-molecule switch marrying force spectroscopy and molecular electronics far from the thermodynamic limit. We use molecular dynamics to simulate a conducting atomic force microscope mechanically manipulating a molecule bound to a surface between a folded state and an unfolded state while monitoring the conductance. Both the complexity and the unique phenomenology of single-molecule experiments are evident in this system. As the molecule unfolds/refolds, the average conductance reversibly changes over 3 orders of magnitude; however, throughout the simulation the transmission fluctuates considerably, illustrating the need for statistical sampling in these systems. We predict that emergent single-molecule signatures will still be evident with conductance blinking, correlated with force blinking, being observable in a region of dynamic bistability. Finally, we illustrate some of the structure-function relationships in this system, mapping the dominant interactions in the molecule for mediating charge transport throughout the pulling simulation.     

Charge transport through single-molecule bilayer-graphene junctions with atomic thickness

The van der Waals interactions (vdW) between π-conjugated molecules offer new opportunities for fabricating heterojunction-based devices and investigating charge transport in heterojunctions with atomic thickness. In this work, we fabricate sandwiched single-molecule bilayer-graphene junctions via vdW interactions and characterize their electrical transport properties by employing the cross-plane break junction (XPBJ) technique. The experimental results show that the cross-plane charge transport through single-molecule junctions is determined by the size and layer number of molecular graphene in these junctions. Density functional theory (DFT) calculations reveal that the charge transport through molecular graphene in these molecular junctions is sensitive to the angles between the graphene flake and peripheral mesityl groups, and those rotated groups can be used to tune the electrical conductance. This study provides new insight into cross-plane charge transport in atomically thin junctions and highlights the role of through-space interactions in vdW heterojunctions at the molecular scale.

Charge transport through single-molecule bilayer-graphene junctions fabricated by a cross-plane break junction technique can be tuned at the atomic level.     

Long-Lived Photocharges in Supramolecular Polymers of Low-Band-Gap Chromophores

Photoinduced charge separation in supramolecular aggregates of π-conjugated molecules is a fundamental photophysical process and a key criterion for the development of advanced organic electronics materials. Herein, the self-assembly of low-band-gap chromophores into helical one-dimensional aggregates, due to intermolecular hydrogen bonding, is reported. Chromophores confined in these supramolecular polymers show strong excitonic coupling interactions and give rise to charge-separated states with unusually long lifetimes of several hours and charge densities of up to 5 mol % after illumination with white light. Two-contact devices exhibit increased photoconductivity and can even show Ohmic behavior. These findings demonstrate that the confinement of organic semiconductors into one-dimensional aggregates results in a considerable stabilization of charge carriers for a variety of π-conjugated systems, which may have implications for the design of future organic electronic materials.     

Arylthio-substituted coronenes as tailored building blocks for molecular electronics

The electron transport through molecules in molecular devices is typically influenced by the nature of the interfaces with the contacting electrodes and by the interactions between neighbouring molecules. It is a major goal of molecular electronics to adjust the electronic function of a molecular device by tailoring the intrinsic molecular properties and the interfacial and intermolecular interactions. Here, we report on the tunability of the electronic properties of coronene derivatives, namely dodecakis(arylthio)coronenes (DATCs), which are found to exhibit a three-dimensional aromatic system. Scanning tunnelling microscopy (STM), spectroscopy (STS) and simulations based on the density functional theory (DFT) are employed to characterize the structural and electronic properties of these molecules deposited on Au(111) surfaces. It is shown that modifications of the peripheral aryl-groups allow us to specifically affect the self-assembly and the charge transport characteristics of the molecules. Molecular assemblies like supramolecular wires with highly delocalized orbitals and single molecules with molecular "quantum dot" characteristics are obtained in this way.     

Charge transport at the protein–electrode interface in the emerging field of BioMolecular Electronics

The emerging field of BioMolecular Electronics aims to unveil the charge transport characteristics of biomolecules with two primary outcomes envisioned. The first is to use nature's efficient charge transport mechanisms as an inspiration to build the next generation of hybrid bioelectronic devices towards a more sustainable, biocompatible and efficient technology. The second is to understand this ubiquitous physicochemical process in life, exploited in many fundamental biological processes such as cell signalling, respiration, photosynthesis or enzymatic catalysis, leading us to a better understanding of disease mechanisms connected to charge diffusion. Extracting electrical signatures from a protein requires optimised methods for tethering the molecules to an electrode surface, where it is advantageous to have precise electrochemical control over the energy levels of the hybrid protein–electrode interface. Here, we review recent progress towards understanding the charge transport mechanisms through protein–electrode–protein junctions, which has led to the rapid development of the new BioMolecular Electronics field. The field has brought a new vision into the molecular electronics realm, wherein complex supramolecular structures such as proteins can efficiently transport charge over long distances when placed in a hybrid bioelectronic device. Such anomalous long-range charge transport mechanisms acutely depend on specific chemical modifications of the supramolecular protein structure and on the precisely engineered protein–electrode chemical interactions. Key areas to explore in more detail are parameters such as protein stiffness (dynamics) and intrinsic electrostatic charge and how these influence the transport pathways and mechanisms in such hybrid devices.     

Supramolecular chemistry — Scope and perspectives: Molecules — Supermolecules — Molecular devices

Supramolecular chemistry is the chemistry of the intermolecular bond, covering the structures and functions of the entities formed by association of two or more chemical species. Molecular recognition in the supermolecules formed by receptor-substrate binding rests on the principles of molecular complementarity, as found in spherical and tetrahedral recognition, linear recognition by co-receptors, metallo-receptors, amphilic receptors and anion coordination. Supramolecular catalysis by receptors bearing reactive groups effects bond cleavage reactions as well as synthetic bond formation via co-catalysis. Lipophilic receptor molecules act as selective carriers for various substrates and allow the setting up of coupled transport processes linked to electron and proton gradients or to light. Whereas endo-receptors bind substrates in molecular cavities by convergent interactions, exo-receptors rely on interactions between the surfaces of the receptor and the substrate; thus new types of receptors such as the metallonucleates may be designed. In combination with polymolecular assemblies, receptors, carriers and catalysts may lead to molecular and supramolecular devices, defined as structurally organized and functionally integrated chemical systems built on supramolecular architectures. Their recognition, transfer and transformation features are analyzed specifically from the point of view of molecular devices that would operate via photons, electrons or ions, thus defining the fields of molecular photonics, electronics and ionics. Introduction of photosensitive groups yields photoactive receptors for the design of light conversion and charge separation centres. Redox active polyolefinic chains represent molecular wires for electron transfer through membranes. Tubular mesophases formed by stacking of suitable macrocyclic receptors may lead to ion channels. Molecular self-assembling occurs with acyclic ligands that form complexes with a double helical structure. Such developments in molecular and supramolecular design and engineering open perspectives towards the realization of molecular photonic, electronic and ionic devices, that would perform highly selective recognition, reaction and transfer operations for signal and information processing at the molecular level.     

肽基超分子胶体

肽基超分子胶体是基于肽分子间超分子作用,自发形成且具有有序分子排布及规整结构,兼具传统胶体及超分子特性的组装体系。利用超分子弱相互作用构筑功能性胶体,不仅是人们对生命组装进程深入理解的有效手段,也是实现优异的超分子材料的重要途径。肽分子具有组成明确、性能可调、生物安全性高及可降解等优势,是超分子化学、胶体与界面化学领域重要的组装基元。基于肽的超分子自组装,能够实现多尺度、多功能的生物胶体的构筑,被广泛应用于医药、催化、能源等领域。如何通过对肽序列的设计及分子间作用力的调控,实现对胶体结构和功能的精确控制,是近年来研究的重要课题之一。从分子尺度研究和揭示超分子胶体的组装过程及物理化学机制,探究胶体结构与功能的关系,是实现超分子结构和功能化的重要内容。本文基于"分子间作用的调控"及"结构与功能的关系"两个基本科学问题,系统地综述了肽基超分子胶体的组装机制、结构与功能,以及研究现状。     

Macrocycle-Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge-Transfer Interactions

Synthetic macrocycles have served as principal tools for supramolecular chemistry, have greatly extended the scope of organic charge transfer (CT) complexes, and have proved to be of great practical value in the solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycle-based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycle-based crystalline CT assemblies, MCCAs for short), which are classified by their donor–acceptor (D-A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts, and host–guest D-A pairs. Particular attention will be focused on their diverse functions and applications, as well as the underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.     

Application of electrochemistry to single-molecule junctions: from construction to modulation

State-of-the-art molecular electronics focus on the measurement of electrical properties of materials at the single-molecule level.Experimentally, molecular electronics face two primary challenges. One challenge is the reliable construction of single-molecule junctions, and the second challenge is the arbitrary modulation of electron transport through these junctions. Over the past decades, electrochemistry has been widely adopted to meet these challenges, leading to a wealth of novel findings. This review starts from the application of electrochemical methods to the fabrication of nanogaps, which is an essential platform for the construction of single-molecule junctions. The utilization of electrochemistry for the modification of molecular junctions,including terminal groups and structural backbones, is introduced, and finally, recent progress in the electrochemical modulation of single-molecule electron transport is reviewed.     

Tuning Single-Molecule Conductance in Metalloporphyrin-Based Wires via Supramolecular Interactions

Nature has developed supramolecular constructs to deliver outstanding charge-transport capabilities using metalloporphyrin-based supramolecular arrays. Herein we incorporate simple, naturally inspired supramolecular interactions via the axial complexation of metalloporphyrins into the formation of a single-molecule wire in a nanoscale gap. Small structural changes in the axial coordinating linkers result in dramatic changes in the transport properties of the metalloporphyrin-based wire. The increased flexibility of a pyridine-4-yl-methanethiol ligand due to an extra methyl group, as compared to a more rigid 4-pyridinethiol linker, allows the pyridine-4-yl-methanethiol ligand to adopt an unexpected highly conductive stacked structure between the two junction electrodes and the metalloporphyrin ring. DFT calculations reveal a molecular junction structure composed of a shifted stack of the two pyridinic linkers and the metalloporphyrin ring. In contrast, the more rigid 4-mercaptopyridine ligand presents a more classical lifted octahedral coordination of the metalloporphyrin metal center, leading to a longer electron pathway of lower conductance. This works opens to supramolecular electronics, a concept already exploited in natural organisms.     

Stabilizing Liquids Using Interfacial Supramolecular Assemblies

Stabilizing liquids based on supramolecular assembly (non-covalent intermolecular interactions) has attracted significant interest, due to the increasing demand for soft, liquid-based devices where the shape of the liquid is far from the equilibrium spherical shape. The components comprising these interfacial assemblies must have sufficient binding energies to the interface to prevent their ejection from the interface when the assemblies are compressed. Here, we highlight recent advances in structuring liquids based on non-covalent intermolecular interactions. We describe some of the progress made that reveals structure–property relationships. In addition to treating advances, we discuss some of the limitations and provide a perspective on future directions to inspire further studies on structured liquids based on supramolecular assembly.     

High pressure and supramolecular systems

The present review summarizes the results of studies of supramolecular systems under high hydrostatic pressure. The possibilities of using high pressure for the preparation of new supramolecular structures, investigation of intermolecular interactions in these structures, and control over the physical and chemical properties of supramolecular assemblies are considered.     

Supramolecular compounds of azo dyes derived from 1-phenylazo-2-naphthol and their nickel and copper complexes

In this study, experimental techniques including Raman, infrared and X-ray crystal diffraction, as well as quantum chemistry calculations, are used to investigate two new azo dyes supramolecular complexes: 1-phenylazo-2-naphthol (Sudan I) with nickel(II) ion and 1-(2,4-xylylazo)-2-naphthol (Sudan II) with copper(II) ion. The crystallographic structures have been solved for bis-1-(pheny-l-azo)-2-naphtholatenickel(II) and bis-1-(2,4-xylylazo)-2-naphtholatecopper(II) complexes, revealing the presence of supramolecular interactions, such as the centroid–centroid π-stacking interactions and CH…π hydrogen interactions. These weak intermolecular interactions appear to play an important role on the crystal structure stability for both compounds. Density functional theory calculations at B3LYP/6-311++G(d,p) level were performed in order to help understanding such molecular interactions and to assign the vibrational spectra. The experimental and theoretical data have allowed the analysis of the packing forces, revealing charge accumulation at key molecular regions.