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Assembling and prototyping multiple circuits on a common breadboard scaffold is critical for developing functional single molecule electronic devices. However, at present, controlling and combining the electronic properties of multiple circuits within a single-molecule junction remains an unresolved challenge. Here, we describe the molecular conductance distributions for five single terminal circuits each within three constitutional isomers of a bis-terpyridine-based molecular breadboard junction through a rigorous computational framework which accounts for molecular conformational flexibility and the relative electrode accessibility (REA) of anchoring groups. The isomers, termed TPo, TPm, and TPp, differ in the relative placement of the linking nitrogen (N) atoms at ortho, meta, and para positions, respectively, of the peripheral pyridyl rings. We demonstrate that quantum interference effects (QIE) and REA of the anchoring N atoms can be exploited to alter the relative conductance of the five single terminal circuits within the molecular breadboards by ~ 4–32 times. We introduce a phase-plot-analysis to highlight the interdependence of QIE- and REA-induced changes in the conductance states of the basis circuits across breadboard pairs. Our studies predict that REA should not impact the QIE-induced boost in circuit conductance for TPp relative to that in TPm. In contrast, REA suppresses the QIE boost for circuit conductance in TPo relative to that in TPm. Our results showcase the possibility of accessing the combined effect of QIE and REA within an experimental break-junction setup to develop diverse molecular electronic breadboards with multiple embedded circuits and distinct electronic properties.

In this minireview, we discuss important aspects of the various quantum phenomena (such as quantum interference, spin-dependent charge transport, and thermoelectric effects) relevant in single-molecule charge transport and list some of the basic circuit rules devised for different molecular systems. These quantum phenomena, in conjunction with the existing empirical circuit rules, can help in predicting some of the structure-property relationships in molecular circuits. However, a universal circuit law that predicts the charge transport properties of a molecular circuit has not been derived yet. Having such law(s) will help to design and build a complex molecular device leading to exciting unique applications that are not possible with the traditional silicon-based technologies. Based on the existing knowledge in the literature, here we open the discussion on the possible future research directions for deriving unified circuit law(s) to predict the charge transport in complex single-molecule circuits.

In this letter, we study the structure-transport property relationships of small ligand intercalated DNA molecules using a multiscale modelling approach where extensive ab-initio calculations are performed on numerous MD-simulated configurations of dsDNA and dsDNA intercalated with two different intercalators, ethidium and daunomycin. DNA conductance is found to increase by one order of magnitude upon drug intercalation due to the local unwinding of the DNA base pairs adjacent to the intercalated sites which leads to modifications of the density-of-states in the near-Fermi energy region of the ligand-DNA complex. Our study suggests that the intercalators can be used to enhance/tune the DNA conductance which opens new possibilities for their potential applications in nanoelectronics., This work is under review at Physical Review Letters

In this paper we combine two operando methods, Raman spectroscopy and X-ray absorption spectroscopy (XAS), in order to probe reduced graphene-oxide supported tinIV oxide nanoparticles ( SnO 2 NPs @ rGO ) as they are being used to catalyse CO2 electroreduction. To achieve high reaction rates it is necessary to apply sufficiently cathodic electrode potentials. Under such conditions, however, not only CO2 is reduced electrochemically, but also the catalyst particles may be transformed from the initial SnIV state to SnII or, in an extreme case, to metallic Sn. While SnII species still favour CO2 electroreduction, yielding formate as a primary product, on metallic Sn CO2 reduction is disfavoured with respect to the competing hydrogen evolution reaction (HER). We show that operando XAS, a robust technique yielding information averaged over a large surface area and a relatively large thickness of the catalyst layer, is a very expedient method able to detect the reduction of SnO 2 NPs @ rGO to metallic Sn. XAS can thus be used to establish an optimum potential for the electroreduction in practical electrolysing cells. It takes, however, a complementary method offered by operando Raman spectroscopy, having greater sensitivity at the catalyst/electrolyte solution interface, to probe reduction intermediates such as the SnII state, which remain undetectable for ex situ methods. As it is shown in the paper, Raman spectroscopy may also find further use when investigating the recovery of catalyst particles following exposure to extreme reducing conditions.

This review introduces the concept of electrochemical gating, in conjunction with its working principle, and demonstrates its applicability and prospects towards fabricating functional molecular devices. In this vein, various redox-active and -inactive molecular systems of relevance have been discussed. In terms of the electrochemical environment for gating, the significance of ionic liquids over aqueous electrolytes has been highlighted in multiple examples. As a means of investigating the electrochemically gated molecular junctions, experimental testbeds based on scanning probe microscopy and break junction techniques—in terms of their setups and advantages—have been described. The manifestation of quantum interference effects and the approach for tuning these via electrochemical gating have been stressed. The relevance of electrochemically gated charge transport in biological systems has also been covered. A few miscellaneous systems—deploying indirect approaches involving chemical and environmental gating—have been exemplified. The review finally concludes by outlining the future research directions of the field. In essence, this review is aimed towards conveying the status of the conceptual understanding of electrochemical gating—which would further the development of emerging molecular device technologies.

Although molecular rectifiers were proposed over four decades ago 1,2 , until recently reported rectification ratios (RR) were rather moderate 2–11 (RR ~ 101). This ceiling was convincingly broken using a eutectic GaIn top contact 12 to probe molecular monolayers of coupled ferrocene groups (RR ~ 105), as well as using scanning tunnelling microscopy-break junctions 13–16 and mechanically controlled break junctions 17 to probe single molecules (RR ~ 102–103). Here, we demonstrate a device based on a molecular monolayer in which the RR can be switched by more than three orders of magnitude (between RR ~ 100 and RR ≥ 103) in response to humidity. As the relative humidity is toggled between 5% and 60%, the current–voltage (I–V) characteristics of a monolayer of di-nuclear Ru-complex molecules reversibly change from symmetric to strongly asymmetric (diode-like). Key to this behaviour is the presence of two localized molecular orbitals in series, which are nearly degenerate in dry circumstances but become misaligned under high humidity conditions, due to the displacement of counter ions (PF6 –). This asymmetric gating of the two relevant localized molecular orbital levels results in humidity-controlled diode-like behaviour. The rectification ratio of a molecular junction made of a self-assembled monolayer of di-nuclear ruthenium-complex molecules can be varied by more than three orders of magnitude by controlling relative humidity.

The modulation of excess electron transfer (EET) within DNA containing a dimethylaminopyrene (C-AP) as an electron donor and 5-bromouracil (BrdU) as an electron acceptor through phenanthrenyl pairs (phen-R) could be achieved by modifying the phenanthrenyl base surrogates with electron withdrawing and donating groups. Arranging the phenanthrenyl units to form a descending LUMO gradient increased the EET efficiency compared to the electron transfer through uniform LUMOs or an ascending LUMO gradient.

Here we report the first study on the electrochemical energy storage application of a surface-immobilized ruthenium complex multilayer thin film with anion storage capability. We employed a novel dinuclear ruthenium complex with tetrapodal anchoring groups to build well-ordered redox-active multilayer coatings on an indium tin oxide (ITO) surface using a layer-by-layer self-assembly process. Cyclic voltammetry (CV), UV-Visible (UV-Vis) and Raman spectroscopy showed a linear increase of peak current, absorbance and Raman intensities, respectively with the number of layers. These results indicate the formation of well-ordered multilayers of the ruthenium complex on ITO, which is further supported by the X-ray photoelectron spectroscopy analysis. The thickness of the layers can be controlled with nanometer precision. In particular, the thickest layer studied (65 molecular layers and approx. 120 nm thick) demonstrated fast electrochemical oxidation/reduction, indicating a very low attenuation of the charge transfer within the multilayer. In situ-UV-Vis and resonance Raman spectroscopy results demonstrated the reversible electrochromic/redox behavior of the ruthenium complex multilayered films on ITO with respect to the electrode potential, which is an ideal prerequisite for e.g. smart electrochemical energy storage applications. Galvanostatic charge-discharge experiments demonstrated a pseudocapacitor behavior of the multilayer film with a good specific capacitance of 92.2 F g(-1) at a current density of 10 μA cm(-2) and an excellent cycling stability. As demonstrated in our prototypical experiments, the fine control of physicochemical properties at nanometer scale, relatively good stability of layers under ambient conditions makes the multilayer coatings of this type an excellent material for e.g. electrochemical energy storage, as interlayers in inverted bulk heterojunction solar cell applications and as functional components in molecular electronics applications.

The modulation of excess electron transfer (EET) within DNA containing a dimethylaminopyrene (C-AP) as an electron donor and 5-bromouracil (BrdU) as an electron acceptor through phenanthrenyl pairs (phen-R) could be achieved by modifying the phenanthrenyl base surrogates with electron withdrawing and donating groups. Arranging the phenanthrenyl units to form a descending LUMO gradient increased the EET efficiency compared to the electron transfer through uniform LUMOs or an ascending LUMO gradient.

Controlling charge flow in single molecule circuits with multiple electrical contacts and conductance pathways is a much sought after goal in molecular electronics. In this joint experimental and theoretical study, we advance the possibility of creating single molecule breadboard circuits through an analysis of the conductance of a bis-terpyridine based molecule (TP1). The TP1 molecule can adopt multiple conformations through relative rotations of 7 aromatic rings and can attach to electrodes in 61 possible single and multi-terminal configurations through 6 pyridyl groups. Despite this complexity, we show that it is possible to extract well defined conductance features for the TP1 breadboard and assign them rigorously to the underlying constituent circuits. Mechanically controllable break-junction (MCBJ) experiments on the TP1 molecular breadboard show an unprecedented 4 conductance states spanning a range 10 −2G0 to 10 −7G0. Quantitative theoretical examination of the conductance of TP1 reveals that combinations of 5 types of single terminal 2–5 ring subcircuits are accessed as a function of electrode separation to produce the distinct conductance steps observed in the MCBJ experiments. We estimate the absolute conductance for each single terminal subcircuit and its percentage contribution to the 4 experimentally observed conductance states. We also provide a detailed analysis of the role of quantum interference and thermal fluctuations in modulating conductance within the subcircuits of the TP1 molecular breadboard. Finally, we discuss the possible development of molecular circuit theory and experimental advances necessary for mapping conductance through complex single molecular breadboard circuits in terms of their constituent subcircuits.

We report an electrochemical gating approach with ∼100% efficiency to tune the conductance of single-molecule 4,4'-bipyridine junctions using scanning-tunnelling-microscopy break junction technique. Density functional theory calculation suggests that electrochemical gating aligns molecular frontier orbitals relative to the electrode Fermi-level, switching the molecule from an off resonance state to "partial" resonance.

The understanding of the charge transport through single molecule junctions is a prerequisite for the design and building of electronic circuits based on single molecule junctions. However, reliable and robust formation of such junctions is a challenging task to achieve. In this topical review, we present a systematic investigation of the anchoring group effect on single molecule junction conductance by employing two complementary techniques, namely scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques, based on the studies published in the literature and important results from our own work. We compared conductance studies for conventional anchoring groups described earlier with the molecular junctions formed through π-interactions with the electrode surface (Au, Pt, Ag) and we also summarized recent developments in the formation of highly conducting covalent Au-C σ-bonds using oligophenyleneethynylene (OPE) and an alkane molecular backbone. Specifically, we focus on the electron transport properties of diaryloligoyne, oligophenyleneethynylene (OPE) and/or alkane molecular junctions composed of several traditional anchoring groups, (dihydrobenzo[b]thiophene (BT), 5-benzothienyl analogue (BTh), thiol (SH), pyridyl (PY), amine (NH2), cyano (CN), methyl sulphide (SMe), nitro (NO2)) and other anchoring groups at the solid/liquid interface. The qualitative and quantitative comparison of the results obtained with different anchoring groups reveals structural and mechanistic details of the different types of single molecular junctions. The results reported in this prospective may serve as a guideline for the design and synthesis of molecular systems to be used in molecule-based electronic devices.

The charge transport properties of a catechol-type dithiol-terminated oligo-phenylene-ethynylene was investigated by cyclic voltammetry (CV) and by the scanning tunnelling microscopy break junction technique (STM-BJ). Single molecule charge transport experiments demonstrated the existence of high and low conductance regions. The junction conductance is rather weakly dependent on the redox state of the bridging molecule. However, a distinct dependence of junction formation probability and of relative stretching distances of the catechol- and quinone-type molecular junctions is observed. Substitution of the central catechol ring with alkoxy-moieties and the combination with a topological analysis of possible π-electron pathways through the respective molecular skeletons lead to a working hypothesis, which could rationalize the experimentally observed conductance characteristics of the redox-active nanojunctions.

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., We acknowledge the financial support from the Swiss National Science Foundation (Grant No. 200020‐144471); SCCER Heat and Electricity storage; M.H. acknowledges the financial support from the Institute of Science and Engineering at Chuo University. H.O. is grateful to Tokuyama Science Foundation. C.J.L. and O.A.A. acknowledge financial support from the UK EPSRC (grant no. EP/M014452/1 and EP/N017188/1), the European Commission (EC) FP7 ITN “MOLESCO” (project no. 606728), and the Ministry of Higher Education and Scientific Research of Iraq. V.M.G.‐S. and J.F. thank the Spanish MICINN for funding (Grant No. FIS2012‐34858). V.M.G.‐S. also thanks the Spanish MICINN for a Ramón y Cajal Fellowship (RYC‐2010‐06053).

The electrochemical reduction of CO(2) has been extensively studied over the past decades. Nevertheless, this topic has been tackled so far only by using a very fundamental approach and mostly by trying to improve kinetics and selectivities toward specific products in half-cell configurations and liquid-based electrolytes. The main drawback of this approach is that, due to the low solubility of CO(2) in water, the maximum CO(2) reduction current which could be drawn falls in the range of 0.01-0.02 A cm(-2). This is at least an order of magnitude lower current density than the requirement to make CO(2)-electrolysis a technically and economically feasible option for transformation of CO(2) into chemical feedstock or fuel thereby closing the CO(2) cycle. This work attempts to give a short overview on the status of electrochemical CO(2) reduction with respect to challenges at the electrolysis cell as well as at the catalyst level. We will critically discuss possible pathways to increase both operating current density and conversion efficiency in order to close the gap with established energy conversion technologies.

Rate coefficients for the reaction of the hydroxyl radical with CH3OCH2F (HFE-161) were computed using transition state theory coupled with ab initio methods, viz., MP2, G3MP2, and G3B3 theories in the temperature range of 200-400 K. Structures of the reactants and transition states (TSs) were optimized at MP2(FULL) and B3LYP level of theories with 6-31G* and 6-311þþG** basis sets. The potential energy surface was scanned at both the level of theories. Five different TSs were identified for each rotamer. Calculations of Intrinsic reaction coordinates were performed to confirm the existence of all the TSs. The kinetic parameters due to all different TSs are reported in this article. The rate coefficients for the title reaction were computed to be k ¼ (9 6 1.08) � 10 � 13 exp (� (1,713 6 33)/T) cm 3 molecule � 1 s � 1 at MP2, k ¼ (7.36 6 0.42) � 10 � 13 exp (� (198 6 16)/T) cm 3 molecule � 1 s � 1 at G3MP2 and k ¼ (5.36 6 1.57) � 10 � 13 exp (� (412 6 81)/T) cm 3 molecule � 1 s � 1 at G3B3 theories. The atmospheric lifetimes of CH3OCH2F at MP2, G3MP2, and G3B3 level of theories were estimated to be 20, 0.1, and 0.3 years, respectively. V C 2011 Wiley Periodicals, Inc. Int J Quantum Chem 112: 1066-1077, 2012

Controlling charge transport through a single molecule connected to metallic electrodes remains one of the most fundamental challenges of nanoelectronics. Here we use electrochemical gating to reversibly tune the conductance of two different organic molecules, both containing anthraquinone (AQ) centers, over >1 order of magnitude. For electrode potentials outside the redox-active region, the effect of the gate is simply to shift the molecular energy levels relative to the metal Fermi level. At the redox potential, the conductance changes abruptly as the AQ unit is oxidized/reduced with an accompanying change in the conjugation pattern between linear and cross conjugation. The most significant change in conductance is observed when the electron pathway connecting the two electrodes is via the AQ unit. This is consistent with the expected occurrence of destructive quantum interference in that case. The experimental results are supported by an excellent agreement with ab initio transport calculations.

The conductance properties of a photoswitchable dimethyldihydropyrene (DHP) derivative have been investigated for the first time in single-molecule junctions using the mechanically controllable break junction technique. We demonstrate that the reversible structure changes induced by isomerization of a single bispyridine-substituted DHP molecule are correlated with a large drop of the conductance value. We found a very high ON/OFF ratio (10(4)) and an excellent reversibility of conductance switching.

et al., The charge transport characteristics of 11 tailor-made dithiol-terminated oligo(phenylene-ethynylene) (OPE)-type molecules attached to two gold electrodes were studied at a solid/liquid interface in a combined approach using an STM break junction (STM-BJ) and a mechanically controlled break junction (MCBJ) setup. We designed and characterized 11 structurally distinct dithiol-terminated OPE-type molecules with varied length and HOMO/LUMO energy. Increase of the molecular length and/or of the HOMO-LUMO gap leads to a decrease of the single-junction conductance of the linearly conjugate acenes. The experimental data and simulations suggest a nonresonant tunneling mechanism involving hole transport through the molecular HOMO, with a decay constant β = 3.4 ± 0.1 nm -1 and a contact resistance R c = 40 k per Au-S bond. The introduction of a cross-conjugated anthraquinone or a dihydroanthracene central unit results in lower conductance values, which are attributed to a destructive quantum interference phenomenon for the former and a broken π-conjugation for the latter. The statistical analysis of conductance-distance and current-voltage traces revealed details of evolution and breaking of molecular junctions. In particular, we explored the effect of stretching rate and junction stability. We compare our experimental results with DFT calculations using the ab initio code SMEAGOL and discuss how the structure of the molecular wires affects the conductance values. © 2012 American Chemical Society.

Interface engineering to attain a uniform and compact self-assembled monolayer at atomically flat surfaces plays a crucial role in the bottom-up fabrication of organic molecular devices. Here we report a promising and operationally simple approach for modification/functionalization not only at ultraflat single-crystal metal surfaces, M(111) (M=Au, Pt, Pd, Rh and Ir) but also at the highly oriented pyrolytic graphite surface, upon efficient in situ cleavage of trimethylsilyl end groups of the molecules. The obtained self-assembled monolayers are ultrastable within a wide potential window. The carbon–surface bonding on various substrates is confirmed by shell-isolated nanoparticle-enhanced Raman spectroscopy. Application of this strategy in tuning surface wettability is also demonstrated. The most valuable finding is that a combination of the desilylation with the click chemistry represents an efficient method for covalent and tailor-made functionalization of diverse surfaces., Formation of stable and uniform self-assembled monolayers on surfaces is a prerequisite for bottom-up fabrication of many organic molecular devices. Here, the authors present a fabrication approach based on desilylation chemistry for modification and functionalization on various metal and carbon surfaces.

A quantum circuit rule for combining quantum interference effects in the conductive properties of oligo(phenyleneethynylene) (OPE)-type molecules possessing three aromatic rings was investigated both experimentally and theoretically. Molecules were of the type X-Y-X, where X represents pyridyl anchors with para (p), meta (m) or ortho (o) connectivities and Y represents a phenyl ring with p and m connectivities. The conductances GXmX (GXpX) of molecules of the form X-m-X (X-p-X), with meta (para) connections in the central ring, were predominantly lower (higher), irrespective of the meta, para or ortho nature of the anchor groups X, demonstrating that conductance is dominated by the nature of quantum interference in the central ring Y. The single-molecule conductances were found to satisfy the quantum circuit rule Gppp/Gpmp=Gmpm/Gmmm. This demonstrates that the contribution to the conductance from the central ring is independent of the para versus meta nature of the anchor groups.

Room temperature molecular electronics get one step closer to reality by exploiting chemical contacts between a single molecule and graphite., An open challenge for single-molecule electronics is to find stable contacts at room temperature with a well-defined conductance. Common coinage metal electrodes pose fabrication and operational problems due to the high mobility of the surface atoms. We demonstrate how molecules covalently grafted onto mechanically robust graphite/graphene substrates overcome these limitations. To this aim, we explore the effect of the anchoring group chemistry on the charge transport properties of graphite-molecule contacts by means of the scanning tunneling microscopy break-junction technique and ab initio simulations. Molecules adsorbed on graphite only via van der Waals interactions have a conductance that decreases exponentially upon stretching the junctions, whereas the molecules bonded covalently to graphite have a single well-defined conductance and yield contacts of unprecedented stability at room temperature. Our results demonstrate a strong bias dependence of the single-molecule conductance, which varies over more than one order of magnitude even at low bias voltages, and show an opposite rectification behavior for covalent and noncovalent contacts. We demonstrate that this bias-dependent conductance and opposite rectification behavior is due to a novel effect caused by the nonconstant, highly dispersive density of states of graphite around the Fermi energy and that the direction of rectification is governed by the detailed nature of the molecule/graphite contact. Combined with the prospect of new functionalities due to a strongly bias-dependent conductance, these covalent contacts are ideal candidates for next-generation molecular electronic devices.