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There are several binding groups used within molecular electronics for anchoring molecules to metal electrodes (e.g., R−SMe, R−NH2, R−CS2−, R−S−). However, some anchoring groups that bind strongly to electrodes have poor/unknown stability, some have weak electrode coupling, while for some their binding motifs are not well defined. Further binding groups are required to aid molecular design and to achieve a suitable balance in performance across a range of properties. We present an in‐depth investigation into the use of carbodithioate esters as contact groups for single‐molecule conductance measurements, using scanning tunnelling microscopy break junction measurements (STM‐BJ) and detailed surface spectroscopic analysis. We demonstrate that the methyl carbodithioate ester acts as an effective contact for gold electrodes in STM‐BJ measurements. Surface enhanced Raman measurements demonstrate that the C=S functionality remains intact when adsorbed on to gold nanoparticles. A gold(I) complex was also synthesised showing a stable C=S→AuI interaction from the ester. Comparison with a benzyl thiomethyl ether demonstrates that the C=S moiety significantly contributes to charge transport in single‐molecule junctions. The overall performance of the CS2Me group demonstrates it should be used more extensively and has strong potential for the fabrication of larger area devices with long‐term stability. [ABSTRACT FROM AUTHOR]

Quantum interference (QI) plays a crucial role in determining the charge transport in molecular devices. In this work, the efficient modulation of QI in meta‐phenylene ethylene oligomer molecular devices by a double N‐substitution strategy is demonstrated. By altering the positions of two N atoms in the central ring with respect to the connecting sites, the molecular conductance can be tuned by more than one order of magnitude. Theoretical analysis, including magic ratio theory, orbital rule, and transmission simulations, reveals how the two N atoms synergistically modulate the molecule conductance between destructive QI and constructive QI states. Remarkably, addition of a second N atom does not simple reinforce the effect of the first; in contrast, it may completely cancel the effect of the first. Understanding the complex electronic interplay between the two N atoms in double N‐substituted molecules paves a path toward utilization of heterocyclic aromatic hydrocarbons in molecular electronics. [ABSTRACT FROM AUTHOR]

Heteroatom substitution into the cores of alternant, aromatic hydrocarbons containing only even‐membered rings is attracting increasing interest as a method of tuning their electrical conductance. Here, the effect of heteroatom substitution into molecular cores of non‐alternant hydrocarbons, containing odd‐membered rings, is examined. Benzodichalcogenophene (BDC) compounds are rigid, planar π‐conjugated structures, with molecular cores containing five‐membered rings fused to a six‐membered aryl ring. To probe the sensitivity or resilience of constructive quantum interference (CQI) in these non‐bipartite molecular cores, two C2‐symmetric molecules (I and II) and one asymmetric molecule (III) were investigated. I (II) contains S (O) heteroatoms in each of the five‐membered rings, while III contains an S in one five‐membered ring and an O in the other. Differences in their conductances arise primarily from the longer S−C and shorter O−C bond lengths compared with the C−C bond and the associated changes in their resonance integrals. Although the conductance of III is significantly lower than the conductances of the others, CQI was found to be resilient and persist in all molecules. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As the field of molecular‐scale electronics matures and the prospect of devices incorporating molecular wires becomes more feasible, it is necessary to progress from the simple anchor groups used in fundamental conductance studies to more elaborate anchors designed with device stability in mind. This study presents a series of oligo(phenylene‐ethynylene) wires with one tetrapodal anchor and a phenyl or pyridyl head group. The new anchors are designed to bind strongly to gold surfaces without disrupting the conductance pathway of the wires. Conductive probe atomic force microscopy (cAFM) was used to determine the conductance of self‐assembled monolayers (SAMs) of the wires in Au–SAM–Pt and Au–SAM–graphene junctions, from which the conductance per molecule was derived. For tolane‐type wires, mean conductances per molecule of up to 10−4.37 G0 (Pt) and 10−3.78 G0 (graphene) were measured, despite limited electronic coupling to the Au electrode, demonstrating the potential of this approach. Computational studies of the surface binding geometry and transport properties rationalise and support the experimental results. [ABSTRACT FROM AUTHOR]

Together with the more intuitive and commonly recognized conductance mechanisms of charge‐hopping and tunneling, quantum‐interference (QI) phenomena have been identified as important factors affecting charge transport through molecules. Consequently, establishing simple and flexible molecular‐design strategies to understand, control, and exploit QI in molecular junctions poses an exciting challenge. Here we demonstrate that destructive quantum interference (DQI) in meta‐substituted phenylene ethylene‐type oligomers (m‐OPE) can be tuned by changing the position and conformation of methoxy (OMe) substituents at the central phenylene ring. These substituents play the role of molecular‐scale taps, which can be switched on or off to control the current flow through a molecule. Our experimental results conclusively verify recently postulated magic‐ratio and orbital‐product rules, and highlight a novel chemical design strategy for tuning and gating DQI features to create single‐molecule devices with desirable electronic functions. [ABSTRACT FROM AUTHOR]

Single‐molecule junctions that are sensitive to compression or elongation are an emerging class of nanoelectromechanical systems (NEMS). Although the molecule–electrode interface can be engineered to impart such functionality, most studies to date rely on poorly defined interactions. We focused on this issue by synthesizing molecular wires designed to have chemically defined hemilabile contacts based on (methylthio)thiophene moieties. We measured their conductance as a function of junction size and observed conductance changes of up to two orders of magnitude as junctions were compressed and stretched. Localised interactions between weakly coordinating thienyl sulfurs and the electrodes are responsible for the observed effect and allow reversible monodentate⇄bidentate contact transitions as the junction is modulated in size. We observed an up to ≈100‐fold sensitivity boost of the (methylthio)thiophene‐terminated molecular wire compared with its non‐hemilabile (methylthio)benzene counterpart and demonstrate a previously unexplored application of hemilabile ligands to molecular electronics. [ABSTRACT FROM AUTHOR]

Cumulenes are sometimes described as "metallic" because an infinitely long cumulene would have the band structure of a metal. Herein, we report the single‐molecule conductance of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n=1, 2, 3, and 5. The [n]cumulenes with n=3 and 5 have almost the same conductance, and they are both more conductive than the alkene (n=1). This is remarkable because molecular conductance normally falls exponentially with length. The conductance of the allene (n=2) is much lower, because of its twisted geometry. Computational simulations predict a similar trend to the experimental results and indicate that the low conductance of the allene is a general feature of [n]cumulenes where n is even. The lack of length dependence in the conductance of [3] and [5]cumulenes is attributed to the strong decrease in the HOMO–LUMO gap with increasing length. [ABSTRACT FROM AUTHOR]

Understanding and controlling quantum interference (QI) in single molecules is fundamental to the development of QI‐based single‐molecule electronics. Simple rules such as counting rules, curly arrow rules (CARs), circuit rules, and more recently magic ratio rules have been developed to predict QI patterns in polycyclic aromatic hydrocarbons. CARs are widely used to predict destructive QI. Here we examine the validity of CARs in fully conjugated anthracene and dihydroxyanthracene, cross‐conjugated anthraquinone, and broken conjugated dihydroanthracene attached to either graphene or gold electrodes through π‐π stacking or thiol and Au−C anchors. For the first time, we demonstrate that CARs break down in molecular junctions formed by cross‐conjugated anthraquinone. In contrast with the destructive QI predicted by CARs for a meta‐connected anthraquinone core, we demonstrate that QI is constructive. This behavior is independent of the choice of electrode material or anchor groups. This is significant, because by changing the redox state of meta‐connected dihydroxyanthracene to form meta‐connected anthraquinone, the conductance of the junction increases by a couple of orders of magnitude due to the crossover from constructive to destructive QI. This opens new avenues for realization of QI‐based single‐molecule switches. Short and curly: Curly arrow rules break down in molecular junctions formed by cross‐conjugated anthraquinone. In contrast with the destructive quantum interference (QI) predicted by curly arrow rules for a meta‐connected anthraquinone core, it is demonstrated here that QI is constructive. [ABSTRACT FROM AUTHOR]

Abstract: The design of thermoelectric materials for the efficient conversion of waste heat into electricity requires simultaneous tuning of their electrical and thermal conductance. A comparative theoretical study of electron and phonon transport in thiophene and ethylenedioxythiophene (EDOT) based molecular wires is performed. It is shown that modifying thiophene by substituting ethylenedioxy enhances the thermoelectric figure of merit ZT for molecules of the same length. Furthermore, it is demonstrated that the electrical conductance of EDOT‐based wires decays more slowly with length than that of thiophene‐based wires and that their thermal conductance is lower. The room‐temperature ZT of undoped EDOT is found to be rather low. However, doping of EDOT by the electron acceptor tolunenesulfunate increases the Seebeck coefficient and electrical conductance, while decreasing the thermal conductance, leading to a thermoelectric figure of merit as high as ZT = 2.4. [ABSTRACT FROM AUTHOR]

Abstract: This Concept article will give a glimpse into chemical design principles for exploiting quantum interference (QI) effects in molecular‐scale devices. Direct observation of room temperature QI in single‐molecule junctions has stimulated growing interest in fabrication of tailor‐made molecular electronic devices. Herein, we outline a new conceptual advance in the scientific understanding and technological know‐how necessary to control QI effects in single molecules by chemical modification. We start by discussing QI from a chemical viewpoint and then describe a new magic ratio rule (MRR), which captures a minimal description of connectivity‐driven charge transport and provides a useful starting point for chemists to design appropriate molecules for molecular electronics with desired functions. The MRR predicts conductance ratios, which are solely determined by QI within the core of polycyclic aromatic hydrocarbons (PAHs). The manifestations of QI and related quantum circuit rules for materials discovery are direct consequences of the key concepts of weak coupling, locality, connectivity, mid‐gap transport and phase coherence in single‐molecule junctions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The compounds and complexes 1,4-C6H4(C≡C- cyclo-3-C4H3S)2 ( 2), trans-[Pt(C≡C- cyclo-3-C4H3S)2(PEt3)2] ( 3), trans-[Ru(C≡C- cyclo-3-C4H3S)2(dppe)2] ( 4; dppe=1,2-bis(diphenylphosphino)ethane) and trans-[Ru(C≡C- cyclo-3-C4H3S)2{P(OEt)3}4] ( 5) featuring the 3-thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3- 5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break-junction (STM-BJ) and STM- I( s) methods (measuring the tunnelling current ( I) as a function of distance ( s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM- I( s) methods, and a higher feature by the STM-BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi-parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non-resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2- 5. [ABSTRACT FROM AUTHOR]

To guide the choice of future synthetic targets for single-molecule electronics, qualitative design rules are needed, which describe the effect of modifying chemical structure. Here the effect of heteroatom substitution on destructive quantum interference (QI) in single-molecule junctions is, for the first time experimentally addressed by investigating the conductance change when a 'parent' meta-phenylene ethylene-type oligomer (m-OPE) is modified to yield a 'daughter' by inserting one nitrogen atom into the m-OPE core. We find that if the substituted nitrogen is in a meta position relative to both acetylene linkers, the daughter conductance remains as low as the parent. However, if the substituted nitrogen is in an ortho position relative to one acetylene linker and a para position relative to the other, destructive QI is alleviated and the daughter conductance is high. This behavior contrasts with that of a para-connected parent, whose conductance is unaffected by heteroatom substitution. These experimental findings are rationalized by transport calculations and also agree with recent 'magic ratio rules', which capture the role of connectivity in determining the electrical conductance of such parents and daughters. [ABSTRACT FROM AUTHOR]

The ancillary ligands 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine and 4′-(2,3-dihydrobenzo[ b]thiophene)-2,2′-6′,2'-terpyridine were used to synthesize two series of mono- and dinuclear ruthenium complexes differing in their lengths and anchoring groups. The electrochemical and single-molecular conductance properties of these two series of ruthenium complexes were studied experimentally by means of cyclic voltammetry and the scanning tunneling microscopy-break junction technique (STM-BJ) and theoretically by means of density functional theory (DFT). Cyclic voltammetry data showed clear redox peaks corresponding to both the metal- and ligand-related redox reactions. Single-molecular conductance demonstrated an exponential decay of the molecular conductance with the increase in molecular length for both the series of ruthenium complexes, with decay constants of βPY=2.07±0.1 nm−1 and βBT=2.16±0.1 nm−1, respectively. The contact resistance of complexes with 2,3-dihydrobenzo[ b]thiophene (BT) anchoring groups is found to be smaller than the contact resistance of ruthenium complexes with pyridine (PY) anchors. DFT calculations support the experimental results and provided additional information on the electronic structure and charge transport properties in those metal|ruthenium complex|metal junctions. [ABSTRACT FROM AUTHOR]

Easily reversible aqueous dispersion/precipitation of multiwalled carbon nanotubes (MWNTs) has been demonstrated using small-molecule non-ionic pyrene-based surfactants, which exhibit lower critical solution temperature (LCST) phase behaviour. The MWNTs are dispersed by means of non-covalent interactions. The dispersibility can be switched "off" (i.e., MWNTs precipitated) upon heating and switched "on" (i.e., MWNTs re-dispersed) upon cooling and merely swirling the sample at room temperature, that is, under very mild conditions. This effect is also observed under high ionic strength conditions with NaCl in the aqueous phase. [ABSTRACT FROM AUTHOR]

In the second column of this table, the single-molecule conductances for compounds B 3 b and B 5 b were incorrect (but the values plotted in Figure 3 g were correct). The authors of this Communication have noticed a mistake in Table 1. [Extracted from the article]

The authors of this Communication have noticed a mistake in Table 1. In the second column of this table, the single-molecule conductances for compounds B 3 b and B 5 b were incorrect (but the values plotted in Figure 3 g were correct). [Extracted from the article]

Innenrücktitelbild: Hemilabile Ligands as Mechanosensitive Electrode Contacts for Molecular Electronics (Angew. Durch die Anwendung eines Konzepts aus der homogenen Katalyse - der Hemilabilität - erhalten mit (2-Methylthio)thiophen terminierte Moleküle eine Leitfähigkeit (grüne Linie), die exponentiell auf mechanische Modulationen der Position einer der Elektroden anspricht (rote Linie). Dichtefunktionalrechnungen, Hemilabile Liganden, Molekulare Elektronik, Molekulare Schaltkreise, Schwefel-Liganden. [Extracted from the article]

The effectiveness of five different anchor groups for non‐covalent interfacing to graphene electrodes are compared. A family of six molecules is tested in single‐molecule junctions: five consist of the same porphyrin core with different anchor groups, and the sixth is a reference molecule without anchor groups. The junction formation probability (JFP) has a strong dependence on the anchor group. Larger anchors give higher binding energies to the graphene surface, correlating with higher JFPs. The best anchor groups tested are 1,3,8‐tridodecyloxypyrene and 2,5,8,11,14‐pentadodecylhexa‐peri‐hexabenzocoronene, with JFPs of 36% and 38%, respectively. Many junctions are tested at 77 K for each molecule by measuring source‐drain current as a function of bias and gate voltages. For each compound, there is wide variation in the strength of the electronic coupling to the electrodes and in the location of Coulomb peaks. In most cases, this device‐to‐device variability makes it impossible to observe trends between the anchor and the charge‐transport characteristics. Tetrabenzofluorene anchors, which are not π‐conjugated with the porphyrin, exhibit different charge transport behavior to the other anchors tested, and they show multiple Coulomb peaks with characteristically small molecular electron‐addition energies of 0.3–0.7 eV, whereas the other compounds give single Coulomb peaks. Molecules can be interfaced with graphene electrodes using non‐covalent interactions to generate single‐molecule transistors. A family of π‐stacking anchor groups is synthesized and the yield of transistors depends strongly on the nature of the anchor group. Whilst coupling to electrodes varies between devices, it is possible to identify junctions functionalized with single molecules if they exhibit characteristic addition energies. [ABSTRACT FROM AUTHOR]