Variation of Raman spectrum after optical excitation in amorphous As2Se3 at 1.6°K

Several experiments, involving electronic states have shown that amorphous As₂Se₃ is modified at low temperature by the action of not very strongly absorbed light (α～10²cm^?1). This paper presents Raman scattering results before and after illumination. The shape of the Raman spectrum is changed by illumination: its high energy side has grown near 260 cm^?1. This light induced modification of the vibrational states is discussed within the framework of a previous qualitative model involving changes in local atomic configuration.     

A convenient protocol for the synthesis of ligands from a 4-methyl-3,5-diacylaminophenyl platform

The synthesis of stable and highly organized phenanthroline, terpyridine, and pyridino-oxazoline ligands bearing one or two 4-methyl-3,5-diacylaminophenyl modules equipped with two lateral dialkoxyphenyl groups has been performed using EDC.HCl and DMAP reagents in the final coupling reaction. Evidently, in the final ligands and in the solid state intermolecular hydrogen bonding maintains the coherence of the tridimensional structure as clearly evidenced by FT-IR and X-ray diffraction spectroscopy in the cases of the methoxy ligands. The supramolecular packing is also maintained by additional pi-pi stacking interactions.     

Constituents from leaves of Quillaja brasiliensis

A new abietane diterpene, the 19-O-beta-D-glucopyranoside of 16-hydroxylambertic acid was isolated from the leaves of Quillaja brasiliensis together with a known prosapogenin (3-O-beta-D-glucuronopyranosyl-quillaic acid), quercetin, and rutin. The new compound was identified by chemical and spectroscopic methods.     

Luminescence lineshape of free excitons and electron-hole drops in Si

We study experimentally and theoretically the lineshape of the TO-LO and TA phonon assisted luminescence of free excitons and electron-hole drops in Si. We show that, in the case of the TA replica, the appropriate electron-phonon matrix element should be taken k-dependent, contrary to previous investigations.     

Electron-hole droplets size at equilibrium

A finite lifetime τ implies that the liquid phase can only form droplets of finite size. Introducing this finite τ in the usual nucleation theory, one can obtain analytical expressions for the electron-hole droplets size, number and exciton density at steady state, in the limit of large τ. This equilibrium size results, in fact, from a Maxwell construction on an appropriate chemical potential.     

Time of flight of excitonic polaritons in CdSe

Time of flight of polaritons is studied in the forbidden, mixed and allowed modes of propagation. With only one adjustable parameter, we obtain a very good agreement between all the experimental data and the usual oscillator model. In the calculation, the excited states of the A exciton and the n = 1 state of the B exciton are taken into account. The longitudinal-transverse splitting energy ?ω_LT is found to be equal to 1.3 meV. We show that very close to resonances, it is not possible to describe the temporal evolution of the pulse by the group velocity of the wave packet. We give an example which shows we have to use the Fourier transform method.     

Tuning organogels and mesophases with phenanthroline ligands and their copper complexes by inter- to intramolecular hydrogen bonds

A novel family of highly functionalized molecules consisting of a central 4-methyl-3,5-diacylaminobenzene platform linked in close proximity to the methyl group by two lateral aromatic rings each equipped with two long alkoxy chains has been rationally designed. The presence of amide tethers and a chelating phenanthroline fragment connected via an ester dipole formed a new class of gelating reagents and mesomorphic materials. A few of these compounds have the tendency to form macromolecule-like aggregates through noncovalent interactions in hydrocarbon solvents and were found to exhibit thermotropic cubic mesophases. In light of the X-ray molecular structure of the methoxy ligand, an infinite network maintained by intermolecular hydrogen bonds as well as by pi-pi stacking of the phenyl subunits was evidenced. FT-IR studies confirm that the common driving force for aggregation in the organogels and microsegregation in the mesophase is the occurrence of a tight intermolecular H-bonded network that does not persist in diluted solution. This situation is switched when the ligands are interlocked by a copper(I) cation. A strong intramolecular H-bond confirmed by X-ray diffraction of a single crystal for the methoxy case provides very stable complexes but inhibits the gelation of the solvents. Heating the complexes bearing long paraffin chains (n = 12 and 16) in the dried state leads to a self-organization into a columnar liquid-crystalline phase in which the columns are arranged along a 2D oblique symmetry as deduced from powder XRD experiments. In this case, the complexes with the appended counteranions self-assemble in a specific way to form columns. A striking observation is that the intramolecular hydrogen bond persists in the mesophase as it does in solution without any evidence of an extended network. As far as we are aware, these ligands and complexes are rare examples in which organogelation and thermotropic mesomorphic behavior could be observed in parallel with molecules bearing a chelating platform. Due to the synthetic availability of the 4-methyl-3,5-diacylaminobenzene core and the simplicity by which the chelating platforms can be graphed, this methodology represents a practical alternative to the production of functionalized organogelators and mesomorphic materials.     

Large structural modification with conserved conformation: analysis of delta(3)-fused aryl prolines in model beta-turns

For the first time, the influence of a fused Delta3-arylproline on peptide conformation has been studied by the synthesis and comparison of the conformations of peptides containing proline and pyrrolo-proline, 3 (PyPro). Pyrrolo-proline was demonstrated to be a conservative replacement for Pro in model beta-turns, 4 and 5, as shown by their similar DMSO titration curves, cis/trans-isomer populations, and NOESY spectral data. Pyrrolo-proline may thus be used for studying the structure activity relationships of Pro-containing peptides with minimal modification of secondary structures.     

Thermodynamic approach for studying both the retention and complexation mechanisms with hydroxy-propyl-beta-cyclodextrin of a phenoxy-propionic acid herbicide series

The retention and complexation mechanisms of a herbicide series were studied from a chromatographic approach using a novel column called “Nautilus^®”. The effects of water fraction and the hydroxy-propyl-β-cyclodextrin (HP-β-CD) concentration in the mobile phase were analysed in relation to the column temperature. Two retention models of phenoxy-propionic acid (PPA) derivatives were investigated. It was shown that the retention mechanism was led by free PPA herbicide for low HP-β-CD concentrations and by the PPA/HP-β-CD complex for the highest ones. In addition, an enthalpy-entropy compensation study revealed that both the solute retention and complexation mechanisms were independent of the number of chlorine atoms in the structure. Also the thermodynamic results showed that (1) the retention process depended on the water fraction (X) in the mobile phase and (2) the PPA/HP-β-CD complexation mechanism was shown to be entropically controlled.     

Phase Turbulence in the Complex Ginzburg-Landau Equation via Kuramoto–Sivashinsky Phase Dynamics

We study the Complex Ginzburg-Landau initial value problem for a complex field uC, with ,R. We consider the Benjamin–Feir linear instability region We show that for all and for all initial data u₀ sufficiently close to 1 (up to a global phase factor eⁱ₀,₀R) in the appropriate space, there exists a unique (spatially) periodic solution of space period L₀. These solutions are small even perturbations of the traveling wave solution, and s, have bounded norms in various L^p and Sobolev spaces. We prove that apart from corrections whenever the initial data satisfy this condition, and that in the linear instability range the dynamics is essentially determined by the motion of the phase alone, and so exhibits phase turbulence. Indeed, we prove that the phase satisfies the Kuramoto–Sivashinsky equation for times while the amplitude 1+² s is essentially constant.Supported in part by the Fonds National Suisse.