We have joined two fundamental concepts of organic chemistry to provide a deep, yet intuitive, understanding of how side groups influence destructive quantum interference (DQI) in the transport ...through conjugated molecules. Using density functional theory combined with Green's function techniques, and employing tight-binding models in which all the π-systems are considered, we elucidate the separate roles of bond-resonance and induction in tuning DQI. We show that the position of the anti-resonances produced by DQI is sensitive to the number of side groups, but not in a simple additive way. Instead, addition of multiple groups results in a weaker overall contribution per group, and this can be understood using a straight forward graphical analysis. Furthermore, we show that additional fine tuning of DQI is possible
via
attachment of a chain of atoms to a second site around the ring. DQI is controlled by modifying the length of the chain, thus providing exquisite control over the anti-resonance position. This insight provides chemists with a large number of options to tune DQI for unprecedented device optimization.
Chemical bond induction and mesomerism/resonance are theoretically demonstrated to control quantum interference in single molecule junctions.
The effects of antiaromaticity and destructive quantum interference (DQI) are investigated on the charge transport through dibenzo-
a
,
e
pentalene (DBP). 5,10-Connectivity gives high single-molecule ...conductance whereas 2,7 gives low conductance due to DQI. Comparison of the 5,10-DBP with phenyl and anthracene analogues yields the trend
G
DBP
G
Anth
>
G
Ph
, despite the aromatic anthracene having a larger HOMO-LUMO gap than 5,10-DBP. This is explained by unfavourable level alignment for 5,10-DBP.
Single molecule junctions of antiaromatic dibenzo
a
,
e
pentalene are studied. 5,10 connection gives high conductance, whereas 2,7 gives low conductance due to destructive quantum interference. Comparison with aromatic analogues is made.
A combined experimental and theoretical study is presented revealing the influence of metal–molecule coupling on electronic transport through single‐molecule junctions. Transport experiments through ...tolane molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups are performed. By fitting the experimental current–voltage characteristics to a single‐level tunneling model, we extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal–molecule coupling. The values found for these parameters are rationalized with the help of density‐functional‐theory‐based transport calculations. In particular, these calculations show that the anchoring groups determine the junction conductance by controlling not only the strength of the coupling to the metal but also the position of the relevant molecular energy levels.
Attaching molecules to metallic electrodes in order to establish a reliable electrical contact is still an open task in the field of molecular electronics. By inspection of the current–voltage characteristics and comparison to theoretical model descriptions, insight is gained into the role of the anchoring groups in the electrical properties of the molecules.
The finding that electronic conductance across ultrathin protein films between metallic electrodes remains nearly constant from room temperature to just a few degrees Kelvin has posed a challenge. We ...show that a model based on a generalized Landauer formula explains the nearly constant conductance and predicts an Arrhenius-like dependence for low temperatures. A critical aspect of the model is that the relevant activation energy for conductance is either the difference between the HOMO and HOMO–1 or the LUMO+1 and LUMO energies instead of the HOMO–LUMO gap of the proteins. Analysis of experimental data confirms the Arrhenius-like law and allows us to extract the activation energies. We then calculate the energy differences with advanced DFT methods for proteins used in the experiments. Our main result is that the experimental and theoretical activation energies for these three different proteins and three differently prepared solid-state junctions match nearly perfectly, implying the mechanism’s validity.
Antiaromaticity is a fundamental concept in chemistry, but the study of molecular wires incorporating antiaromatic units is limited. Despite initial predictions, very few studies show that ...antiaromaticity has a beneficial effect on electron transport. Dibenzoa,epentalene (DBP) is a stable structure that displays appreciable antiaromaticity within the five-membered rings of the pentalene core. We have investigated derivatives of DBP furnished with pyridyl (Py) and F4-pyridyl (PyF4) anchor groups, and compared the conductance with purely aromatic phenyl and anthracene analogues. We find that the low-bias conductance of DBP-Py is approximately 60% larger than that of the anthracene analogue Anth-Py and 250% larger compared to the phenyl derivative Ph-Py. This is due to a better alignment of the LUMO with the gold Fermi level, which we confirm by conductance-voltage spectroscopy where the conductance of DBP-Py shows the greatest voltage-dependence. The F4-pyridyl compounds, which have lower LUMO energies compared to the pyridyl analogues, did not, however, form detectable molecular junctions. The strongly electron-withdrawing fluorine atoms reduce the donor capability of the nitrogen lone-pair to the point where stable N-Au bonds no longer form.
We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling ...microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N−Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure−conductance relationship.
We carried out first-principles density-functional theory calculations to study the work of separation for five different metal-metal interfaces, each of them comprising thin layers of selected ...metals (Cr, W, Ta, Al or Ti) lying on top of Au surfaces. We found that the highest work of separation is obtained for one-atom-thick layers. Increasing the number of atomic layers leads the work of separation to oscillate with the thickness, and ultimately tend to a limiting value for a large number of layers. Interestingly, for most cases the lowest work of separation is obtained for two-atom layers. We find that this behaviour is mirrored by the quantity of charge transferred between the two metals on the one hand, and their spatial distance on the other.
Using the break-junction technique, we show that “Au(RS)2” units play a significant role in thiol-terminated molecular junctions formed on gold. We have studied a range of thiol-terminated ...compounds, with the sulfur atoms either in direct conjugation with a phenyl core or bonded to saturated methylene groups. For all molecules we observe at least two distinct groups of conductance plateaus. By a careful analysis of the length behavior of these plateaus, comparing the behavior across the different cores and with methyl sulfide anchor groups, we demonstrate that the lower conductance groups correspond to the incorporation of Au(RS)2 oligomeric units at the contacts. These structural motifs are found on the surface of gold nanoparticles, but they have not before been shown to exist in molecular-break junctions. The results, while exemplifying the complex nature of thiol chemistry on gold, moreover clarify the conductance of 1,4-benzenedithiol on gold. We show that true Au–S–Ph–S–Au junctions have a relatively narrow conductance distribution, centered at a conductance of log(G/G 0) = −1.7 (±0.4).
Understanding charge transport in DNA molecules is a long-standing problem of fundamental importance across disciplines
. It is also of great technological interest due to DNA's ability to form ...versatile and complex programmable structures. Charge transport in DNA-based junctions has been reported using a wide variety of set-ups
, but experiments so far have yielded seemingly contradictory results that range from insulating
or semiconducting
to metallic-like behaviour
. As a result, the intrinsic charge transport mechanism in molecular junction set-ups is not well understood, which is mainly due to the lack of techniques to form reproducible and stable contacts with individual long DNA molecules. Here we report charge-transport measurements through single 30-nm-long double-stranded DNA (dsDNA) molecules with an experimental set-up that enables us to address individual molecules repeatedly and to measure the current-voltage characteristics from 5 K up to room temperature. Strikingly, we observed very high currents of tens of nanoamperes, which flowed through both homogeneous and non-homogeneous base-pair sequences. The currents are fairly temperature independent in the range 5-60 K and show a power-law decrease with temperature above 60 K, which is reminiscent of charge transport in organic crystals. Moreover, we show that the presence of even a single discontinuity ('nick') in both strands that compose the dsDNA leads to complete suppression of the current, which suggests that the backbones mediate the long-distance conduction in dsDNA, contrary to the common wisdom in DNA electronics
.
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