The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion angle was measured using the scanning tunneling microscopy (STM)-break-junction method. We found that it depends on the torsion angle phi between two phenyl rings; twisting the biphenyl system from flat (phi = 0 degrees ) to perpendicular (phi = 90 degrees ) decreased the conductance by a factor of 30. Detailed calculations of transport based on density functional theory and a two level model (TLM) support the experimentally obtained cos(2) phi correlation between the junction conductance G and the torsion angle phi. The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the pi-pi coupling dominates the transport under "off-resonance" conditions where the HOMO levels are well separated from the Femi energy.

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.

Electrochemical double-layer capacitors exhibit high power and long cycle life but have low specific energy compared with batteries, limiting applications. Redox-enhanced capacitors increase specific energy by using redox-active electrolytes that are oxidized at the positive electrode and reduced at the negative electrode during charging. Here we report characteristics of several redox electrolytes to illustrate operational/self-discharge mechanisms and the design rules for high performance. We discover a methyl viologen (MV)/bromide electrolyte that delivers a high specific energy of ∼14 Wh kg−1 based on the mass of electrodes and electrolyte, without the use of an ion-selective membrane separator. Substituting heptyl viologen for MV increases stability, with no degradation over 20,000 cycles. Self-discharge is low, due to adsorption of the redox couples in the charged state to the activated carbon, and comparable to cells with inert electrolyte. An electrochemical model reproduces experiments and predicts that 30–50 Wh kg−1 is possible with optimization.

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A Polyaniline-Supercapacitor with quinone electrolytes remains stable over 50 000 galvanostatic charge-discharge cycles. The quinones provide superior stability by preventing the conversion of porous polyaniline to a highly reactive state. Our work shows that highly stable polymer-supercapacitors can be engineered by combining electrochemically active polymers and redox-active electrolytes with concerted electrochemical properties.

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 [D. Vonlanthen et al., Angew. Chem., Int. Ed. 48, 8886 (2009); A. Mishchenko et al., Nano Lett. 10, 156 (2010)]. 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 which vary by one order of magnitude for the same molecule. Irrespective of the coordination geometries, the charge transport calculations predict a cos 2 ϕ 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.

In situ gap-mode Raman spectra were acquired in an electrochemical environment on a single-crystal gold electrode employing a Au(100)|4,4'-biphenyldithiol (BPDT)|Au-NP(55 nm) sandwich assembly. This geometry enabled an investigation of the influence of an applied electrochemical gate field on the conformational changes in nanojunctions, such as the torsion angle (φ) of molecules. A linear correlation between the intensity ratio I(C═C)/I(C(ring)-S) and cos(2) φ in 4,4'-BPDT-type molecular junctions was established and subsequently utilized to estimate the potential dependence of the torsion angle of the "flexible" molecule M1 at different potentials. The latter decreases as the potential (charge) becomes more negative, resulting in better π-π coupling, which correlates with enhanced junction conductance. The demonstrated spectroelectrochemical strategy and the direct correlation of the spectroscopic results with (single) molecular conductance studies may guide the selection and elucidation of functional molecules for potential applications in novel nanodevices.

In Europe, the compound 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy, Adam), in addition to cannabis, is the most abused illicit drug at all-night "techno" parties. Methods for the determination of MDMA and its metabolites, 4-hydroxy-3-methoxymethamphetamine (HMMA), 3,4-dihydroxy-methamphetamine (HHMA), 3,4-methylenedioxyamphetamine (MDA), 4-hydroxy-3-methoxyamphetamine (HMA), and 3,4-dihydroxyamphetamine (HHA), in biological fluids were established. Plasma and urine samples were collected from two patients in a controlled clinical study over periods of 9 and 22 h, respectively. MDMA and MDA were determined in plasma and urine by reversed-phase high-performance liquid chromatography with diode array detection (HPLC-DAD) after solid-phase extraction on cation-exchange columns. Acidic or enzymatic hydrolysis was necessary to detect HMMA, HMA, HHMA, and HHA, which are mainly excreted as glucuronides. Gas chromatography-mass spectrometry (GC-MS) was used for confirmation. Sample extraction and on-disc derivatization with heptafluorobutyric anhydride (HFBA) were performed on Toxi-Lab SPEC solid-phase extraction concentrators. After administration of a single oral dose of 1.5 mg/kg body weight MDMA, peak plasma levels of 331 ng/ml MDMA and 15 ng/mL MDA were measured after 2 h and 6.3 h, respectively. Peak concentrations of 28.1 micrograms/mL MDMA in urine appeared after 21.5 h. Up to 2.3 micrograms/mL MDA, 35.1 micrograms/mL HMMA, and 2.1 micrograms/mL HMA were measured within 16-21.5 h. Conjugated HMMA and HHMA are the main urinary metabolites of MDMA.