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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]

We used a range of scanning tunnelling microscopy (STM)-based methods to conduct a detailed study of single molecule junction conductance enhancement upon charge transfer complex formation, using bis(thiaalkyl)arene molecular wires as electron donors and tetracyanoethylene (TCNE) as an electron acceptor. Using the "hard" STM break junction (STM-BJ) method, in which a Au STM tip is pushed into a Au substrate and then withdrawn in the presence of molecules, we see a single, very broad, peak in the resulting conductance histogram when all data are used; the conductance enhancement is 25-fold for a terthiophene donor and 15-fold for a phenyl group. After rational data selection, in which only current-distance curves that contain a current plateau >0.2 nm long are used in the conductance histogram, three sharper peaks are resolved in the histograms for the charge transfer complexes; two substantially lower-conductance peaks are resolved for the uncomplexed molecules. Using the "soft" STM I(s) technique, in which initial contact between tip and substrate is avoided and the current limit is about an order of magnitude lower, we were able to resolve two peaks for the uncomplexed molecules depending upon the initial set point current (i.e., tip height), one at the same value as the lower of the two data-selected STM-BJ histogram peaks and an additional peak beyond the low-current limit for the STM-BJ experiment. For the terthiophene, the low, medium, and high conductance peaks for the TCNE complex are, respectively, ca. 70, 70, and 46 times higher in conductance than the corresponding peaks for the free molecule. [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]

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]

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]

Host−guest interactions are of central importance in many biological and chemical processes. However, the investigation of the formation and decomplexation of host−guest systems at the single-molecule level has been a challenging task. Here we show that the single-molecule conductance of organoplatinum(II) metallocycle hosts can be enhanced by an order of magnitude by the incorporation of a C60 guest molecule. Mechanically stretching the metallocycle-C60 junction with a scanning tunneling microscopy break junction technique causes the release of the C60 guest from the metallocycle, and consequently the conductance switches back to the free-host level. Metallocycle hosts with different shapes and cavity sizes show different degrees of flexibility to accommodate the C60 guest in response to mechanical stretching. DFT calculations provide further insights into the electronic structures and charge transport properties of the molecular junctions based on metallocycles and the metallocycle-C60 complexes. Studying the single-molecule behavior of host-guest complexes can provide fundamental insights into their supramolecular interactions. Here, the authors use the scanning tunneling microscopy break junction technique to show that encapsulation of a C60 molecule significantly enhances the conductance of an organoplatinum metallocycle; mechanical stretching of the junction releases the guest, returning the conductance to free-host level. [ABSTRACT FROM AUTHOR]

A convenient two-step, one-pot synthesis of oligo(arylene–ethynylene) (OAE) type molecular wires in yields of up to 70% via in situ desilylation of protected bis(alkynes) Me3SiC≡CArC≡CSiMe3 (Ar = 2,5-thienyl, 1,4-naphthylene, 9,10-anthrylene) and subsequent Sonogashira cross-coupling with S -(4-iodophenyl) ethanethiolate, 4-iodothioanisole, or 5-bromo-3,3-dimethyl-2,3-dihydrobenzo[ b ]thiophene is described. The in situ desilylation avoids the manipulation of the sensitive terminal dialkynes (HC≡CArC≡CH), whilst the general approach presented has some advantages over alternative synthetic strategies based on coupling of aryl dihalides (XArX) by avoiding the multi-step preparation and purification of the terminal alkynes S -(4-ethynylphenyl) ethanethiolate, 4-ethynylthioanisole and 5-ethynyl 3,3-dimethyl-2,3-dihydrobenzo[ b ]thiophene. The molecular conductance of the resulting thiolate or thioether functionalised OAE molecular wires has been determined using scanning tunneling microscope break junction (STM-BJ) methods. The trends in molecular conductance do not track simply with the degree of aromaticity of the molecular core despite the rather similar molecular lengths. Rather, the STM-BJ data are better correlated with the nature of the anchor group, highlighting the important role of electrode–molecule coupling on electron transport in a molecular junction. The experimental conductance data are in good agreement with recently described quantum circuit rules, further highlighting the potential for these relationships to be used as predictive tools in molecular electronics research. A two-step, one-pot synthesis of oligo(arylene–ethynylene) molecular wires that avoids manipulation of sensitive terminal bis(alkynes) is described. Trends in molecular conductance do not track with the molecular core aromaticity, but correlate with the recently described quantum circuit rules and associated bridge and anchor parameters. [ABSTRACT FROM AUTHOR]

A series of diarylacetylene (tolane) derivatives functionalised at the 4- and 4'-positions by thiolate, thioether, or amine groups capable of serving as anchor groups to secure the molecules within a molecular junction have been prepared and characterised. The series of compounds have a general form X-B-X, Y-B-Y, and X-B-Y where X and Y represent anchor groups and B the molecular bridge. The single-molecule conductance values determined by the scanning tunnelling microscope break-junction method are found to be in excellent agreement with the predictions made on the basis of a recently proposed 'molecular circuit law', which states 'the conductance GXBX of an asymmetric molecule X-B-Y is the geometric mean šGXBXGYBY of the conductance of the two symmetric molecules derived from it, GXBX and GYBY.' The experimental verification of the circuit law, which holds for systems in which the constituent moieties X, B, and Y are weakly coupled and whose conductance takes place via off-resonance tunnelling, gives further confidence in the use of this relationship in the design of future compounds for use in molecular electronics research. [ABSTRACT FROM AUTHOR]

We have employed the scanning tunneling microscope break-junction technique to investigate the single-molecule conductance of a family of 5,15-diaryl porphyrins bearing thioacetyl (SAc) or methylsulfide (SMe) binding groups at the ortho position of the phenyl rings (S2 compounds). These ortho substituents lead to two atropisomers, cis and trans, for each compound, which do not interconvert in solution under ambient conditions; even at high temperatures, isomerization takes several hours (half-life 15 h at 140 °C for SAc in C2Cl4D2). All the S2 compounds exhibit two conductance groups, and comparison with a monothiolated (Si) compound shows the higher group arises from a direct Au-porphyrin interaction. The lower conductance group is associated with the S-to-S pathway. When the binding group is SMe, the difference in junction length distribution reflects the difference in S-S distance (0.3 nm) between the two isomers. In the case of SAc, there are no significant differences between the plateau length distributions of the two isomers, and both show maximal stretching distances well exceeding their calculated junction lengths. Contact deformation accounts for part of the extra length, but the results indicate that cis-to-trans conversion takes place in the junction for the cis isomer. The barrier to atropisomerization is lower than the strength of the thiolate Au-S and Au-Au bonds, but higher than that of the Au-SMe bond, which explains why the strain in the junction only induces isomerization in the SAc compound. [ABSTRACT FROM AUTHOR]

We report the measurement of conductanceand thermopower of C60molecular junctions using a scanningtunneling microscope(STM). In contrast to previous measurements, we use the imaging capabilityof the STM to determine precisely the number of molecules in the junctionand measure thermopower and conductance continuously and simultaneouslyduring formation and breaking of the molecular junction, achievinga complete characterization at the single-molecule level. We findthat the thermopower of C60dimers formed by trapping aC60on the tip and contacting an isolated C60almost doubles with respect to that of a single C60andis among the highest values measured to date for organic materials.Density functional theory calculations show that the thermopower andthe figure of merit continue increasing with the number of C60molecules, demonstrating the enhancement of thermoelectric preformanceby manipulation of intermolecular interactions. [ABSTRACT FROM AUTHOR]

Employing a scanning tunneling microscopy based beak junction technique and mechanically controlled break junction experiments, we investigated tolane (diphenylacetylene)-type single molecular junctions having four different anchoring groups (SH, pyridyl (PY), NH2, and CN) at a solid/liquid interface. The combination of current-distance and current-voltage measurements and their quantitative statistical analysis revealed the following sequence for junction formation probability and stability: PY > SH > NH2 > CN. For all single molecular junctions investigated, we observed the evolution through multiple junction configurations, with a particularly well-defined binding geometry for PY. The comparison of density functional theory type model calculations and molecular dynamics simulations with the experimental results revealed structure and mechanistic details of the evolution of the different types of (single) molecular junctions upon stretching quantitatively. [ABSTRACT FROM AUTHOR]

We report the electrical conductance at the single molecule level of the oligoyne molecular wires Py-(C≡C)n-Py (n = 1, 2 and 4; Py = 4-pyridyl) using STM-molecular break junction techniques in AulmoleculelAu configurations. The conductance histograms reveal multiple series of peaks attributed to differing contact geometries between the pyridyl head groups and the gold electrodes. Both experimental and theoretical evidence point to the higher conduction groups being related to adsorption of the pyridyl group at more highly coordinated sites such as step edges or alongside gold adatoms. All three conduction groups in the oligoyne series show a remarkably low β value of (0.06 ± 0.03) Å-1, that is, the conductance is almost independent of molecular length. 4,4'-Bipyridyl studied under the same conditions does not follow this exponential decay series. Theoretical calculations using a combination of density functional theory and nonequilibrium Green's function formalism support the experimental results. We conclude that oligoynes and polyynes are a very promising class of molecular wires for integration into electronic circuitry. [ABSTRACT FROM AUTHOR]