David Vonlanthen

Single-molecule junctions based on nitrile-terminated biphenyls : a promising new anchoring group

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.

Chemically controlled conductivity: torsion-angle dependence in a single-molecule biphenyldithiol junction.

Stepwise regulation of the molecular conductance was observed in a series of eight biphenyldithiols with fixed torsion angles between the phenyl rings. These compounds were synthesized and their single-molecule conductance was investigated in an STM junction. A cos2 dependence was found between the interplane torsion angle and the single-molecule conductivity (see plot).

Conduction mechanisms in biphenyl dithiol single-molecule junctions

Based on density-functional theory calculations, we report a detailed study of the single-molecule charge-transport properties for a series of recently synthesized biphenyl-dithiol molecules. The torsion angle ϕ between the two phenyl rings, and hence the degree of π conjugation, is controlled by alkyl chains and methyl side groups. We consider three different coordination geometries, namely, top-top, bridge-bridge, and hollow-hollow, with the terminal sulfur atoms bound to one, two, and three gold surface atoms, respectively. Our calculations show that different coordination geometries give rise to conductances that vary by one order of magnitude for the same molecule. Irrespective of the coordination geometries, the charge transport calculations predict a cos2ϕ dependence of the conductance, which is confirmed by our experimental measurements. We demonstrate that the calculated transmission through biphenyl dithiols is typically dominated by a single transmission eigenchannel formed from π electrons. For perpendicular orientation of the rings a residual conductance arises from σ-π couplings. But only for a single molecule with a completely broken conjugation we find a nearly perfect degeneracy of the σ-π eigenchannels for the hollow-hollow-type contact in our theory.

Pharmacodynamics and pharmacokinetics of intravenously, orally and rectally administered diacetylmorphine in opioid dependents, a two-patient pilot study within a heroin-assisted treatment program.

OBJECTIVE The pharmacokinetics and pharmacodynamics of high-dose intravenous (i.v.), oral and rectal diacetylmorphine (diamorphine, heroin, DAM) preparations were compared. METHOD Two heroin-dependent patients participating in a heroin-assisted treatment program received single or repeated doses of 200 - 690 mg DAM i.v., orally (capsules, controlled-release tablets) and rectally. Plasma and urine profiles of DAM and metabolites were monitored by high-performance liquid chromatography and gas chromatography mass spectrometry, flash and high effects by visual analog scaling (VAS). RESULTS DAM was only detectable in plasma after i.v. administration. With a t 1/2 beta of 1.3 - 2.2 min it was rapidly desacetylated to 6-acetylmorphine which was further metabolized to morphine and its 3- and 6-O-glucuronide. Morphine-3-glucuronide was the dominating metabolite in plasma and urine independent of the administration route. Oral and rectal doses and dosage intervals were adequate to produce flash and high effects without any cardiovascular and respiratory side-effects nor withdrawal symptoms. CONCLUSIONS Oral and rectal DAM should further be tested and validated on a wider patient group for the non-invasive, long-term application of high-dose DAM within heroin-assisted treatment programs as alternative to the harmful i.v. application.

Conformationally Controlled Electron Delocalization in n-Type Rods: Synthesis, Structure, and Optical, Electrochemical, and Spectroelectrochemical Properties of Dicyanocyclophanes

A series of dicyanobiphenylcyclophanes 1-6 with various π-backbone conformations and characteristic n-type semiconductor properties is presented. Their synthesis, optical, structural, electrochemical, spectroelectrochemical, and packing properties are investigated. The X-ray crystal structures of all n-type rods allow the systematic correlation of structural features with physical properties. In addition, the results are supported by quantum mechanical calculations based on density functional theory. A two-step reduction process is observed for all n-type rods, in which the first step is reversible. The potential gap between the reduction processes depends linearly on the cos(2) value of the torsion angle φ between the π-systems. Similarly, optical absorption spectroscopy shows that the vertical excitation energy of the conjugation band correlates with the cos(2) value of the torsion angle φ. These correlations demonstrate that the fixed intramolecular torsion angle φ is the dominant factor determining the extent of electron delocalization in these model compounds, and that the angle φ measured in the solid-state structure is a good proxy for the molecular conformation in solution. Spectroelectrochemical investigations demonstrate that conformational rigidity is maintained even in the radical anion form. In particular, the absorption bands corresponding to the SOMO-LUMO+i transitions are shifted bathochromically, whereas the absorption bands corresponding to the HOMO-SOMO transition are shifted hypsochromically with increasing torsion angle φ.

Ab initio study of the thermopower of biphenyl-based single-molecule junctions

By employing ab initio electronic-structure calculations combined with the nonequilibrium Green’s function technique, we study the dependence of the thermopower Q on the conformation in biphenyl-based single-molecule junctions. For the series of experimentally available biphenyl molecules, alkyl side chains allow us to gradually adjust the torsion angle ϕ between the two phenyl rings from 0 ◦ to 90 ◦ and to control in this way the degree of π -electron conjugation. Studying different anchoring groups and binding positions, our theory predicts that the absolute values of the thermopower decrease slightly towards larger torsion angles, following an a + b cos 2 ϕ dependence. The anchoring group determines the sign of Q and a,b simultaneously. Sulfur and amine groups give rise to Q,a,b > 0, while for cyano, Q,a,b < 0. The different binding positions can lead to substantial variations of the thermopower mostly due to changes in the alignment of the frontier molecular orbital levels and the Fermi energy. We explain our ab initio results in terms of a π -orbital tight-binding model and a minimal two-level model, which describes the pair of hybridizing frontier orbital states on the two phenyl rings. The variations of the thermopower with ϕ seem to be within experimental resolution.