A Molecular Platinum Cluster Junction: A Single-Molecule Switch

We present a theoretical study of electron transport through single-molecule junctions incorporating a Pt₆ metal cluster bound within an organic framework. The insertion of this molecule between a pair of electrodes leads to a fully atomically engineered nanometallic device with high conductance at the Fermi level and two sequential high on/off switching states. The origin of this property can be traced back to the existence of a degenerate HOMO consisting of two asymmetric orbitals with energies close to the Fermi level of the metal leads. The degeneracy is broken when the molecule is contacted to the leads, giving rise to two resonances that become pinned to the Fermi level and display destructive interference

The Role of Oligomeric Gold–Thiolate Units in Single-Molecule Junctions of Thiol-Anchored Molecules

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(<i>G</i>/<i>G</i>₀) = −1.7 (±0.4)

Bioengineering a Single-Protein Junction

Bioelectronics moves toward designing nanoscale electronic platforms that allow <i>in vivo</i> determinations. Such devices require interfacing complex biomolecular moieties as the sensing units to an electronic platform for signal transduction. Inevitably, a systematic design goes through a bottom-up understanding of the structurally related electrical signatures of the biomolecular circuit, which will ultimately lead us to tailor its electrical properties. Toward this aim, we show here the first example of bioengineered charge transport in a single-protein electrical contact. The results reveal that a single point-site mutation at the docking hydrophobic patch of a Cu-azurin causes minor structural distortion of the protein blue Cu site and a dramatic change in the charge transport regime of the single-protein contact, which goes from the classical Cu-mediated two-step transport in this system to a direct coherent tunneling. Our extensive spectroscopic studies and molecular-dynamics simulations show that the proteins’ folding structures are preserved in the single-protein junction. The DFT-computed frontier orbital of the relevant protein segments suggests that the Cu center participation in each protein variant accounts for the different observed charge transport behavior. This work is a direct evidence of charge transport control in a protein backbone through external mutagenesis and a unique nanoscale platform to study structurally related biological electron transfer