Electron transfer through molecules is an ubiquitous process underlying the function of biological systems and synthetic devices. The electronic coupling between components varies with the structure of the molecular bridge, often in classically unintuitive ways, as determined by its quantum electronic structure. Considerable efforts in electron-transfer theory have yielded models that are useful conceptually and provide quantitative means to understand transfer rates in terms of local contributions. Here we show how a description of the local currents within a bridging molecule bound to metallic electrodes can provide chemical insight into current flow. In particular, we show that through-space, as opposed to through-bond, terms dominate in a surprising number of instances, and that interference effects can be characterized by the reversal of ring currents. Together these ideas have implications for the design of molecular electronic devices, in particular for the ways in which substituent effects may be used for maximum impact.

The tunnelling of electrons through molecules (and through any nanoscale insulating and dielectric material ) shows exponential attenuation with increasing length , a length dependence that is reflected in the ability of the electrons to carry an electrical current. It was recently demonstrated that coherent tunnelling through a molecular junction can also be suppressed by destructive quantum interference , a mechanism that is not length-dependent. For the carbon-based molecules studied previously, cancelling all transmission channels would involve the suppression of contributions to the current from both the π-orbital and σ-orbital systems. Previous reports of destructive interference have demonstrated a decrease in transmission only through the π-channel. Here we report a saturated silicon-based molecule with a functionalized bicyclo[2.2.2]octasilane moiety that exhibits destructive quantum interference in its σ-system. Although molecular silicon typically forms conducting wires , we use a combination of conductance measurements and ab initio calculations to show that destructive σ-interference, achieved here by locking the silicon-silicon bonds into eclipsed conformations within a bicyclic molecular framework, can yield extremely insulating molecules less than a nanometre in length. Our molecules also exhibit an unusually high thermopower (0.97 millivolts per kelvin), which is a further experimental signature of the suppression of all tunnelling paths by destructive interference: calculations indicate that the central bicyclo[2.2.2]octasilane unit is rendered less conductive than the empty space it occupies. The molecular design presented here provides a proof-of-concept for a quantum-interference-based approach to single-molecule insulators.

Theory and experiment examining electron transfer through molecules bound to electrodes are increasingly focused on quantities that are conceptually far removed from current chemical understanding. This presents challenges both for the design of interesting molecules for these devices and for the interpretation of experimental data by traditional chemical mechanisms. Here, the concept of electronic coupling from theories of intramolecular electron transfer is extended and applied in the scattering theory (Landauer) formalism. This yields a simple sum over independent channels, that is then used to interpret and explain the unusual features of junction transport through cross-conjugated molecules and the differences among benzene rings substituted at the ortho, meta, or para positions.

We calculate that significant quantum interference effects can be observed in elastic electron transport through acyclic molecules. Interference features are evident in the transmission characteristics calculated for cross-conjugated molecules; significantly, these effects dominate the experimentally observable conduction range. The unusual transport characteristics of these molecules are highlighted through comparison with linearly conjugated and nonconjugated systems. The cross-conjugated molecules presented here show a large dynamic range in conductance. These findings represent a new motif for electron transfer through molecules that exhibit both very high and very low tunneling conductance states accessible at low bias without nuclear motion. In designing single molecule electronic components, a large dynamic range allows a high on/off ratio, a parameter of fundamental importance for switches, transistors, and sensors.

This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga(2)O(3) as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10(-1)A/cm(2) at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 < |V| < 0.4 V. The calculations predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calculated precisely. In this sense, the theoretical predictions and experimental conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theoretical predictions on single molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.

As brought to the attention of the community by Hendon et al. and noted by previous workers, the π orbitals of the equilibrium geometry odd-carbon (even number of double bonds = n) [n]cumulenes may be written in either rectilinear or helical form. We trace the origins and detailed composition of the helical orbitals of cumulenes, which emerge in the simplest Hückel model and are not much modified in advanced computations. For the α,ω-disubstituted even [n]cumulenes, the helical representation is obligatory as the symmetry is reduced from D2d to C2. A relationship is apparent between these helical orbitals of the even [n]cumulenes, seen as a Herges coarctate system, and the corresponding Möbius cyclic polyene orbitals. The twist of the orbitals varies in interesting ways along the helix, and so does the contribution of the component atomic orbitals. Though the electronic structures of even [n]cumulenes and Möbius cyclopolyenes are closely related, they differ for higher n in intriguing ways; these are linked to the constrained rotation of the basis orbitals along the helical twist itinerary. Relations are constructed between the level patterns of the π-systems of even [n]cumulenes and ideas of Hückel and Möbius aromaticity.

The mechanism for off-resonant electron transport through small organic molecules in metallic junctions is predominantly coherent tunneling. Thus, new device functionalities based on quantum interference could be developed in the field of molecular electronics. We invoke a partitioning technique to give an analytical treatment of quantum interference in a benzene ring. We interpret the antiresonances in the transmission as either multipath zeroes resulting from interfering spatial pathways or resonance zeroes analogous to zeroes induced by sidechains.

Molecular spintronics has received extensive interest in recent years. Due to their favorable properties such as long spin coherence lengths and an amenability to fine-tuning via chemical substituents, organic materials play a prominent role in this field. Here we discuss how organic radicals may act as spin filters in the coherent tunneling regime and how they may be tuned to filter either majority- or minority-spin electrons by adding electron-donating or -withdrawing substituents. For a set of benzene-based model systems, we identify dips in the spin-resolved transmission, which may be caused by destructive interference, as a desirable feature when aiming for efficient spin filtering. Furthermore, the qualitative predictions made for our model systems are shown to be transferable to larger stable radicals.