Thermodynamics of supramolecular naphthalenediimide nanotube formation: the influence of solvents, side chains, and guest templates

Amino-acid functionalized naphthalenediimides self-assemble into hydrogen-bonded supramolecular helical nanotubes via a noncooperative, isodesmic process; the self-assembly of ordered helical systems is usually realized through a cooperative process. This unexpected behavior was rationalized as a manifestation of entropy-enthalpy compensation. Fundamental insights into the thermodynamics governing this self-assembly were obtained through the fitting of the isodesmic model to (1)H NMR spectrometry and circular dichroism spectroscopy measurements. Furthermore, we have extended the application of this mathematical model, for the first time, to quantitatively estimate the effect of guests, solvents, and side chains on the stability of the supramolecular nanotube; most significantly, we demonstrate that C(60) acts as a template to stabilize the nanotube assembly and thereby substantially increase the degree of polymerization.     

Star-shaped oligo(p-phenylenevinylene) substituted hexaarylbenzene: purity, stability, and chiral self-assembly

An oligo(p-phenylenevinylene) (OPV)-substituted hexaarylbenzene has been synthesized and fully characterized. Recycling gel permeation chromatography appeared to be a powerful technique to obtain the OPV molecules in a very pure form. X-ray analysis and polarization optical microscopy revealed that the OPV molecule is plastic crystalline at room temperature with an ordered columnar superstructure. In apolar solvents, the molecules self-assemble via a highly cooperative fashion into right-handed chiral superstructures, which are stable even at high temperatures and low concentration. Atomic force microscopy revealed right-handed fibers with a diameter of 6 nm, indicating pi-stacked aggregates; on a silicon oxide substrate, supercoiled chiral structures were observed. STM studies on a liquid-solid interface showed that the star-shaped OPV molecule forms an organized monolayer having a chiral hexagonal lattice.     

The Role of Synthetic Pharmaceutical Polymer Excipients in Oral Dosage Forms – Poly(ethylene oxide)-graft-poly(vinyl alcohol) Copolymers in Tablet Coatings

A number of tablet coatings are available to provide a controlled release of pharmaceutically active compounds in the stomach or intestine using both instant or sustained released systems. The preferred properties of these tablet coatings are a low solution viscosity (preferable in water) combined with a phase separated morphology, showing good mechanical properties. PEO-g-PVAl copolymers have been developed as an instant-release tablet coating, and were obtained via a conventional radical polymerisation of VAc in the presence of PEO. No free PEO was observed in the PEO-g-PVAl copolymers 1f and 1i using 2D LCCC-SEC and MALDI-TOF analysis. Next to the requirement of being PEO free, the PEO-g-PVAl copolymers show a good combination of film forming properties, a fast dissolution and a low solution viscosity in water. The phase separated morphology, as demonstrated by TEM, DSC, DMTA, and WAXS experiments, should provide the PEO-g-PVAl copolymers with relatively constant mechanical properties. A model reaction, using 2-methoxyethyl-ether and 1,4,7,10-tetraoxacyclododecane, has been developed to imitate the grafting reaction of VAc on PEO. Using this model reaction and using the same reaction conditions (temperature, initiator, concentration, VAc:“PEO” ratio, etc.) as applied in the PEO-g-PVAl polymerisation procedure, a degree of grafting for PEO of 13±3% was found. Comparing this figure with the results of LCCC-SEC and MALDI-TOF measurements, this figure seems a few percent too high, pointing to a slightly increased reactivity of the two model compounds compared to the PEO used.     

Characterisation of propagation in 60 GHz radio channels

Narrowband as well as wideband measurements have been performed in various indoor and outdoor environments in order to enable the development of reliable prediction models for 60 GHz radio channels. In addition, results of deterministic modelling on the basis of geometric ray-tracing have been compared with measurement results, showing that simple ray-tracing can be used to estimate both the narrowband and wideband characteristics of a 60 GHz radio channel. This paper reviews the measurement and modelling activities performed by various research institutes     

Lifetimes and branching ratios of excited states of Na

The decay of ²⁴Na levels below 4.3 MeV excitation was studied by means of the ²³Na(d, pγ)²⁴Na reaction at E_d = 2.45 MeV. Gamma-ray spectra were measured at three angles, in coincidence with proton groups detected around 180°. Excitation energies, branching ratios and Doppler shifts were determined. Mean lives were obtained for the levels at 1341 keV (62±15 fs), and 1846 keV (200±50 fs). The 1347 keV level has τ >3 ps. For other levels above 1 MeV upper limits of ≈ 60 fs are set. In some cases spin restrictions follow. In particular J = 2 is assigned to the 1341 keV level.     

Mm-wave indoor radio channel modelling vs. measurements

A propagation model for the prediction of indoor radio channel characteristics at millimetre-wave frequencies, based on geometrical optics is described. This model has been shown to agree well with wide-band measurements carried out at Eindhoven University of Technology [8]. Simulation results for 4th and 6th order reflection are compared with corresponding measured data obtained in small and medium size rooms. The influence of objects located inside the room superstructure, on the indoor radio channel characteristics, is analysed. The agreement between measured and simulated results, confirms the validity of the proposed simulation method.J.J.G. Fernandes would like to thank JNICT — Portugal, for sponsorship.     

Electromagnetic transition strengths in 24Na

Mean lifetimes, excitation energies and branching ratios of ²⁴Na states, populated in the ²³Na(d, p)²⁴Na reaction have been measured. Gamma-ray spectra were measured at three angles in coincidence with proton groups at θ_p = 169°. Mean lifetimes obtained from DSA are (excitation energy in keV, lifetime in fs): 563, > 1000; 1341, 95 ± 30; 1344, 38 ± 11; 1346, > 1500; 1512, 38 ± 11; 1846, 260 ± 50; 1885, 36 ± 9; 2513, 15 ± 7; 2563, < 25; 2904, 50 ± 15; 2978, < 25; 3216, 22 ± 8; 3372, 19 ± 5. For higher levels up to 4207 keV upper limits of 30 fs were set. In combination with earlier work the following unique spin(-parity) assignments could be made: J^π(1846) = 2⁺, J(2513) = 3, J^π(3745) = 3^?. A new level at E_x = 3681.7 ± 0.6 keV is reported.