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.     

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.     

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.     

A Chemically Soldered Polyoxometalate Single‐Molecule Transistor

Polyoxometalates have been proposed in the literature as nanoelectronic components, where they could offer key advantages with their structural versatility and rich electrochemistry. Apart from a few studies on their ensemble behaviour (as monolayers or thin films), this potential remains largely unexplored. We synthesised a pyridyl‐capped Anderson–Evans polyoxometalate and used it to fabricate single‐molecule junctions, using the organic termini to chemically “solder” a single cluster to two nanoelectrodes. Operating the device in an electrochemical environment allowed us to probe charge transport through different oxidation states of the polyoxometalate, and we report here an efficient three‐state transistor behaviour. Conductance data fits a quantum tunnelling mechanism with different charge‐transport probabilities through different charge states. Our results show the promise of polyoxometalates in nanoelectronics and give an insight on their single‐entity electrochemical behaviour.     

Ferrocene‐Substituted Naphthalenediimide with Broad Absorption and Electron‐Transport Properties in the Segregated‐Stack Structure

A new naphthalenediimide (NDI) molecule, where two ferrocene (Fc) units were directly attached to both imide nitrogens ( Fc‐NDI‐Fc ), was synthesized. The Fc units provide high crystallinity to Fc‐NDI‐Fc with good solubility to conventional organic solvents. The Fc units also work as electron‐donating substituents, in contrast to the electron‐deficient NDI unit, resulting in broad charge‐transfer absorption of Fc‐NDI‐Fc from the UV region to 1500 nm in the solid state. The crystal structure analysis revealed that Fc‐NDI‐Fc formed a segregated‐stack structure. The DFT calculation based on the crystal structure showed that the NDI π‐orbitals extended over two axes. The extended π‐network of the NDI units led to the electron‐transport properties of Fc‐NDI‐Fc , which was confirmed using a flash‐photolysis time‐resolved microwave conductivity technique.     

Side‐Group‐Mediated Mechanical Conductance Switching in Molecular Junctions

A key target in molecular electronics has been molecules having switchable electrical properties. Switching between two electrical states has been demonstrated using such stimuli as light, electrochemical voltage, complexation and mechanical modulation. A classic example of the latter is the switching of 4,4′‐bipyridine, leading to conductance modulation of around 1 order of magnitude. Here, we describe the use of side‐group chemistry to control the properties of a single‐molecule electromechanical switch, which can be cycled between two conductance states by repeated compression and elongation. While bulky alkyl substituents inhibit the switching behavior, π‐conjugated side‐groups reinstate it. DFT calculations show that weak interactions between aryl moieties and the metallic electrodes are responsible for the observed phenomenon. This represents a significant expansion of the single‐molecule electronics “tool‐box” for the design of junctions with electromechanical properties.     

Electron Transport through Conjugated Molecules: When the π System Only Tells Part of the Story

In molecular transport junctions, current is monitored as a function of the applied voltage for a single molecule assembled between two leads. The transport is modulated by the electronic states of the molecule. For the prototypical delocalized systems, namely, π‐conjugated aromatics, the π system usually dominates the transport. Herein, we investigate situations where model calculations including only the π system do not capture all of the subtleties of the transport properties. Including both the σ and π contributions to charge transport allows us to demonstrate that while there is generally good agreement, there are discrepancies between the methods. We find that model calculations with only the π system are insufficient where the transport is dominated by quantum interference and cases where geometric changes modulate the coupling between different regions of the π system. We examine two specific molecular test cases to model these geometric changes: the angle dependence of coupling in (firstly) a biphenyl and (secondly) a nitro substituent of a cross‐conjugated unit.     

Gated Electron Sharing Within Dynamic Naphthalene Diimide‐Based Oligorotaxanes

The controlled self‐assembly of well‐defined and spatially ordered π‐systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template‐directed methods, provide a scaffold for directing the construction of monodisperse one‐dimensional assemblies in which the functional units communicate electronically through‐space by way of π‐orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox‐active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.     

A Molecular Platform for Multistate Near‐Infrared Electrochromism and Flip‐Flop,Flip‐Flap‐Flop,and Ternary Memory

A diruthenium complex with a redox‐active amine bridge has been designed, synthesized, and studied by single‐crystal X‐ray analysis and DFT and TDDFT calculations. It shows three well‐separated redox processes with exclusive near‐infrared (NIR) absorbance at each redox state. The electropolymerized film of a related vinyl‐functionalized complex displays multistate NIR electrochromism with low operational potential, good contrast ratio, and long retention time. Flip‐flop, flip‐flap‐flop, and ternary memories have been realized by using the obtained film (ca. 15–20 nm thick) with three electrochemical inputs and three NIR optical outputs that each displays three levels of signal intensity.     

A Molecular Platform for Multistate Near‐Infrared Electrochromism and Flip‐Flop,Flip‐Flap‐Flop,and Ternary Memory

A diruthenium complex with a redox‐active amine bridge has been designed, synthesized, and studied by single‐crystal X‐ray analysis and DFT and TDDFT calculations. It shows three well‐separated redox processes with exclusive near‐infrared (NIR) absorbance at each redox state. The electropolymerized film of a related vinyl‐functionalized complex displays multistate NIR electrochromism with low operational potential, good contrast ratio, and long retention time. Flip‐flop, flip‐flap‐flop, and ternary memories have been realized by using the obtained film (ca. 15–20 nm thick) with three electrochemical inputs and three NIR optical outputs that each displays three levels of signal intensity.     

Light‐Driven Reversible Intermolecular Proton Transfer at Single‐Molecule Junctions

Photoresponsive molecular systems are essential for molecular optoelectronic devices, but most molecular building blocks are non‐photoresponsive. Employed here is a photoinduced proton transfer (PIPT) strategy to control charge transport through single‐molecule azulene junctions with visible light under ambient conditions, which leads to a reversible and controllable photoresponsive molecular device based on non‐photoresponsive molecules and a photoacid. Also demonstrated is the application of PIPT in two single‐molecule AND gate and OR gate devices with electrical signal as outputs.     

[Re(CO)3Cl]‐Chelation‐Mediated Electronic Coupling between Two Amine Redox Sites through the 5,5′‐Positions of 2,2′‐Bipyridine

The electronic coupling between two amine redox sites bridged through the 5,5′‐positions of the [Re(CO)₃Cl]‐chelated 2,2′‐bipyridine was studied by the electrochemical, spectroscopic, and EPR analysis. Interestingly, multiple near‐infrared bands were observed in this new organic mixed‐valent system. The results are interpreted with the aid of DFT and TDDFT calculations.     

Single‐Molecule Sensing of Environmental pH—an STM Break Junction and NEGF‐DFT Approach

Sensors play a significant role in the detection of toxic species and explosives, and in the remote control of chemical processes. In this work, we report a single‐molecule‐based pH switch/sensor that exploits the sensitivity of dye molecules to environmental pH to build metal–molecule–metal (m‐M‐m) devices using the scanning tunneling microscopy (STM) break junction technique. Dyes undergo pH‐induced electronic modulation due to reversible structural transformation between a conjugated and a nonconjugated form, resulting in a change in the HOMO–LUMO gap. The dye‐mediated m‐M‐m devices react to environmental pH with a high on/off ratio (≈100:1) of device conductivity. Density functional theory (DFT) calculations, carried out under the non‐equilibrium Green’s function (NEGF) framework, model charge transport through these molecules in the two possible forms and confirm that the HOMO–LUMO gap of dyes is nearly twice as large in the nonconjugated form as in the conjugated form.     

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.     

Tetrathiafulvalene‐Based Mixed‐Valence Acceptor–Donor–Acceptor Triads: A Joint Theoretical and Experimental Approach

This work presents a joint theoretical and experimental characterisation of the structural and electronic properties of two tetrathiafulvalene (TTF)‐based acceptor–donor–acceptor triads (BQ–TTF–BQ and BTCNQ–TTF—BTCNQ; BQ is naphthoquinone and BTCNQ is benzotetracyano‐p‐quinodimethane) in their neutral and reduced states. The study is performed with the use of electrochemical, electron paramagnetic resonance (EPR), and UV/Vis/NIR spectroelectrochemical techniques guided by quantum‐chemical calculations. Emphasis is placed on the mixed‐valence properties of both triads in their radical anion states. The electrochemical and EPR results reveal that both BQ–TTF–BQ and BTCNQ–TTF–BTCNQ triads in their radical anion states behave as class‐II mixed‐valence compounds with significant electronic communication between the acceptor moieties. Density functional theory calculations (BLYP35/cc‐pVTZ), taking into account the solvent effects, predict charge‐localised species (BQ ^{. ?}–TTF–BQ and BTCNQ ^{. ?}–TTF–BTCNQ) as the most stable structures for the radical anion states of both triads. A stronger localisation is found both experimentally and theoretically for the BTCNQ–TTF–BTCNQ anion, in accordance with the more electron‐withdrawing character of the BTCNQ acceptor. CASSCF/CASPT2 calculations suggest that the low‐energy, broad absorption bands observed experimentally for the BQ–TTF–BQ and BTCNQ–TTF–BTCNQ radical anions are associated with the intervalence charge transfer (IV‐CT) electronic transition and two nearby donor‐to‐acceptor CT excitations. The study highlights the molecular efficiency of the electron‐donor TTF unit as a molecular wire connecting two acceptor redox centres.     

Multicenter‐Bond‐Based Quantum Interference in Charge Transport Through Single‐Molecule Carborane Junctions

Molecular components are vital to introduce and manipulate quantum interference (QI) in charge transport through molecular electronic devices. Up to now, the functional molecular units that show QI are mostly found in conventional π‐ and σ‐bond‐based systems; it is thus intriguing to study QI in multicenter bonding systems without both π‐ and σ‐conjugations. Now the presence of QI in multicenter‐bond‐based systems is demonstrated for the first time, through the single‐molecule conductance investigation of carborane junctions. We find that all the three connectivities in carborane frameworks show different levels of destructive QI, which leads to highly suppressed single‐molecule conductance in para‐ and meta‐connected carboranes. The investigation of QI into carboranes provides a promising platform to fabricate molecular electronic devices based on multicenter bonds.     

Charge Transport and Conductance Switching of Redox‐Active Azulene Derivatives

Azulene (Az) is a non‐alternating, aromatic hydrocarbon composed of a five‐membered, electron‐rich and a seven‐membered, electron‐poor ring; an electron distribution that provides intrinsic redox activity. By varying the attachment points of the two electrode‐bridging substituents to the Az center, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3 Az derivative is at least one order of magnitude lower than those of the 2,6 Az and 4,7 Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3 Az exhibits pronounced nonlinear current–voltage characteristics with hysteresis, indicating a bias‐dependent conductance switching. DFT identifies the LUMO to be nearest to the Fermi energy of the electrodes, but to be an active transport channel only in the case of the 2,6 and the 4,7 Az derivatives, whereas the 1,3 Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3 Az creates a slow electron‐hopping channel responsible for the voltage‐induced switching due to the occupation of a single molecular orbital (MO).     

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.     

Cation‐Dependent Stabilization of Electrogenerated Naphthalene Diimide Dianions in Porous Polymer Thin Films and Their Application to Electrical Energy Storage

Porous polymer networks (PPNs) are attractive materials for capacitive energy storage because they offer high surface areas for increased double‐layer capacitance, open structures for rapid ion transport, and redox‐active moieties that enable faradaic (pseudocapacitive) energy storage. Here we demonstrate a new attractive feature of PPNs—the ability of their reduced forms (radical anions and dianions) to interact with small radii cations through synergistic interactions arising from densely packed redox‐active groups, only when prepared as thin films. When naphthalene diimides (NDIs) are incorporated into PPN films, the carbonyl groups of adjacent, electrochemically generated, NDI radical anions and dianions bind strongly to K⁺, Li⁺, and Mg²⁺, shifting the formal potentials of NDI’s second reduction by 120 and 460 mV for K⁺ and Li⁺‐based electrolytes, respectively. In the case of Mg²⁺, NDI’s two redox waves coalesce into a single two‐electron process with shifts of 240 and 710 mV, for the first and second reductions, respectively, increasing the energy density by over 20 % without changing the polymer backbone. In contrast, the formal reduction potentials of NDI derivatives in solution are identical for each electrolyte, and this effect has not been reported for NDI previously. This study illustrates the profound influence of the solid‐state structure of a polymer on its electrochemical response, which does not simply reflect the solution‐phase redox behavior of its monomers.     

Role of the electronic properties of azurin active site in the electron‐transfer process

Electron transfer proteins, such as azurin (a blue copper protein), are promising candidates for the implementation of biomolecular nanoelectronic devices. To understand the details of electron transfer in redox active azurin molecules, we performed plane‐wave pseudo‐potential density functional theory (DFT) calculations of the protein active site in the two possible oxidation states Cu(I) and Cu(II). The ab initio results are used to discuss how the electronic spectrum and wavefunctions may mediate the shuttling of electrons through the copper ion. We find that the Cu‐ligand hybridization is very similar in the two charge states of the metal center, but the energy spectrum changes substantially. This result might indicate important effects of electronic correlations in the redox activity and consequent electron transfer through the Cu site. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005