TEM characterization of organic nanocrystals grown in sol–gel thin films

The tetracene molecule (2,3-benzanthracene, C₈H₁₂) was used to synthesize nanocrystals grown in sol–gel thin films, ranging from 10 to 100 nm of diameter. This confined nucleation and growth was compared to microcrystallizations of the same molecule in free solution. Transmission electron microscopy (TEM) was used to characterize these two kinds of tetracene crystals. The observation was performed under low-dose illumination to avoid amorphization of the samples during electron irradiation. Spatial confinement and size distribution of micro- and nanocrystals were compared. Using electron microdiffraction and diffraction patterns simulations, we showed that free microcrystals and nanocrystals confined in gel glasses exhibit the same triclinic structure. In addition, time-resolved spectroscopy was used to record fluorescence decays, showing a monoexponential fluorescence decay for nanocrystals while microcrystals exhibit a multiexponential decay. The simple signature of nanocrystals luminescence is promising for the future development of chemical or biological sensors.     

Stabilized aqueous dispersion of multi-walled carbon nanotubes obtained by RF glow-discharge treatment

This article reports a method for surface modification of multi-walled carbon nanotubes (MWNTs) using a low-pressure capacitively coupled RF glow-discharge. Ar/C₂H₆ and Ar/C₂H₆/O₂ gaseous mixtures were used to produce non-polar (np-) and polar (p-) coatings, respectively, onto MWNTs. After 5 min of plasma treatment at 20 W and 20 torr, strongly hydrophobic and non-electrically conductive np-MWNTs were produced. The p-MWNTs were strongly hydrophilic and showed no measurable hydrophobic recovery 2 weeks after treatment. Aqueous suspensions of p-MWNTs remained stable and free of agglomerates after being boiled. The ζ-potential of p-MWNT nanofluids was −40.3 mV, indicating a highly stable dispersion.     

Optimal linear estimator of origin-destination flows with redundant data

Suppose given a network endowed with a multiflow. We want to estimate some quantities connected with this multiflow, for instance the value of an s–t flow for one of the sources–sinks pairs s–t, but only measures on some arcs are available, at least on one s–t cocycle (set of arcs having exactly one endpoint in a subset X of vertices with s∈X and t?X). These measures, supposed to be unbiased, are random variables whose variances are known. How can we combine them optimally in order to get the best estimator of the value of the s–t flow?This question arises in practical situations when the OD matrix of a transportation network must be estimated. We will give a complete answer for the case when we deal with linear combinations, not only for the value of an s–t flow but also for any quantity depending linearly from the multiflow. Interestingly, we will see that the Laplacian matrix of the network plays a central role.     

The importance of defects for carbon nanoribbon based electronics

The utilization of graphene nanoribbons for next generation nanoelectronics is commonly expected to depend on the controlled synthesis that yields a low density of defects. Edge roughness and vacancies have been shown to have a large impact on the performance of graphene nanoribbon transistors. In contrast, we show how certain defects can be used to enhance the electronic and magnetic properties of graphene nanoribbons. We explore the properties of hybrid graphene nanoribbons with armchair and zigzag features joined by an array of pentagon–heptagon structural defects. The graphene nanoribbons display an increased density of states at the Fermi level, and half metallicity in absence of external fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The strange eigenmode in Lagrangian coordinates

For a distribution advected by a simple chaotic map with diffusion, the "strange eigenmode" is investigated from the Lagrangian (material) viewpoint and compared to its Eulerian (spatial) counterpart. The eigenmode embodies the balance between diffusion and exponential stretching by a chaotic flow. It is not strictly an eigenmode in Lagrangian coordinates, because its spectrum is rescaled exponentially rapidly.     

Three-photon absorption in anthracene-porphyrin-anthracene triads: a quantum-chemical study

We have applied correlated quantum-chemical methods to investigate the three-photon absorption (3PA) response of a porphyrin triad derivative, where the central macrocycle is linked in mesopositions to two anthracene units via acetylenic bridges. The 3PA frequency-dependent spectrum of this derivative is dominated by a single resonance feature in the transparent region, associated with charge-transfer states between porphyrin and anthracene. The calculations indicate a two order of magnitude enhancement in the 3PA cross section in the triad molecule with respect to the individual entities, which is attributed to close one-, two-, and three-photon resonances together with strong electronic couplings among the units.     

Rapid analysis of nucleotide-activated sugars by high-performance liquid chromatography coupled with diode-array detection, electrospray ionization mass spectrometry and nuclear magnetic resonance

A generally applicable method for HPLC analysis of sugar nucleotides was established. Separation was achieved using ion-pair chromatography on a reversed-phase column. Ion-pair reagents were selected and various parameters optimized with respect to separation of 11 of the most important sugar nucleotides and compatibility with on-line detection by electrospray ionization MS and NMR. The method was applied to the on-line analysis of the GDP-D-mannose-4,6-dehydratase (Gmd) and GDP-4-keto-6-deoxy-D-mannose reductase (Rmd) catalyzed conversion of GDP-D-mannose to GDP-D-rhamnose. By LC-NMR, the intermediate product of the reaction was shown to be a mixture of GDP-4-keto-6-deoxy-D-mannose and GDP-3-keto-6-deoxy-D-mannose. Nucleotide co-factors of enzymatic reactions such as ATP and NADH did not interfere with the analysis of nucleotide-activated sugars.     

Electronic delocalization in discotic liquid crystals: a joint experimental and theoretical study

Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2',3'-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper pi-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the pi-electrons along the discotic stacks.     

New flavin and deazaflavin oligonucleotide conjugates for the amperometric detection of DNA hybridization

The functionalization of an oligonucleotide by flavin and deazaflavin derivatives allowed an amperometric determination of the hybridization process through the disappearance of the electroactivity of the free oligonucleotide and the appearance of a new electrochemical signal characteristic of the resulting duplex.     

11-Aminoundecanoic acid: a versatile unit for the generation of low molecular weight gelators for water and organic solvents

The use of 11-aminoundecanoic acid as a synthetic building-block allows the systematic preparation of (oligo)amide organogelators-including chiral ones-which display remarkable gelation properties in organic solvents and water.