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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]

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]

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]

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: 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]

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]

We provide a brief overview of recent measurements and predictions of thermoelectric properties of single-molecules and porous nanoribbons and discuss some principles underpinning strategies for enhancing their thermoelectric performance. The latter include (a) taking advantage of steep slopes in the electron transmission coefficient T ( E ) , (b) creating structures with delta-function-like transmission coefficients and (c) utilising step-like features in T ( E ) . To achieve high performance, we suggest that the latter may be the most fruitful, since it is less susceptible to inhomogeneous broadening. For the purpose of extrapolating thermoelectric properties of single or few molecules to monolayer molecular films, we also discuss the relevance of the conductance-weighted average Seebeck coefficient. [ABSTRACT FROM AUTHOR]

We study the quantum interference (QI) effects in three-terminal Andreev interferometers based on polyaromatic hydrocarbons (PAHs) under non-equilibrium conditions. The Andreev interferometer consists of a PAH coupled to two superconducting and one normal conducting terminals. We calculate the current measured in the normal lead as well as the current between the superconducting terminals under non-equilibrium conditions. We show that both the QI arising in the PAH cores and the bias voltage applied to a normal contact have a fundamental effect on the charge distribution associated with the Andreev Bound States (ABSs). QI can lead to a peculiar dependence of the normal current on the superconducting phase difference that was not observed in earlier studies of mesoscopic Andreev interferometers. We explain our results by an induced asymmetry in the spatial distribution of the electron- and hole-like quasiparticles. The non-equilibrium charge occupation induced in the central PAH core can result in a π transition in the current-phase relation of the supercurrent for large enough applied bias voltage on the normal lead. The asymmetry in the spatial distribution of the electron- and hole-like quasiparticles might be used to split Cooper pairs and hence to produce entangled electrons in four terminal setups. [ABSTRACT FROM AUTHOR]

We report quantum interference effects in the electrical conductance of chemical vapor deposited graphene nanoconstrictions fabricated using feedback controlled electroburning. The observed multimode Fabry-Pérot interferences can be attributed to reflections at potential steps inside the channel. Sharp antiresonance features with a Fano line shape are observed. Theoretical modeling reveals that these Fano resonances are due to localized states inside the constriction, which couple to the delocalized states that also give rise to the Fabry-Pérot interference patterns. This study provides new insight into the interplay between two fundamental forms of quantum interference in graphene nanoconstrictions. [ABSTRACT FROM AUTHOR]

If quantum interference patterns in the hearts of polycyclic aromatic hydrocarbons could be isolated and manipulated, then a significant step toward realizing the potential of single-molecule elec tronics would be achieved. H e re we demonstrate experimentally and theoretically that a simple, parameter-free, analytic theory of interference patterns evaluated at the mid-point of the HOMO--LUMO gap (referred to as Mfunctions) correctly predicts conductance ratios of molecules with pyrene, naphthalene, anthracene, anthanthrene, or azulene hearts. M-functions provide new design strategies for identifying molecules with phase-coherent logic functions and enhancing the sensitivity of molecular-scale interferometers. [ABSTRACT FROM AUTHOR]