Enantioselective PCCP Brønsted acid-catalyzed aza-Piancatelli rearrangement.

An enantioselective aza-Piancatelli rearrangement has been developed using a chiral Brønsted acid based on pentacarboxycyclopentadiene (PCCP). This reaction provides rapid access to valuable chiral 4-amino-2-cyclopentenone building blocks from readily available starting material and is operationally simple

A comparative study for the pair-creation contact process using series expansions

A comparative study between two distinct perturbative series expansions for the pair-creation contact process is presented. In contrast to the ordinary contact process, whose supercritical series expansions provide accurate estimates for its critical behavior, the supercritical approach does not work properly when applied to the pair-creation process. To circumvent this problem a procedure is introduced in which one-site creation is added to the pair-creation. An alternative method is the generation of subcritical series expansions which works even for the case of the pure pair-creation process. Differently from the supercritical case, the subcritical series yields estimates that are compatible with numerical simulations

Local chain ordering in amorphous polymer melts: Influence of chain stiffness

Molecular dynamics simulation of a generic polymer model is applied to study melts of polymers with different types of intrinsic stiffness. Important static observables of the single chain such as gyration radius or persistence length are determined. Additionally we investigate the overall static melt structure including pair correlation function, structure function and orientational correlation function.Comment: 13 pages, 15 figures, PCCP accepte

Mutual energy transfer luminescent properties in novel CsGd(MoO4)2:Yb3+,Er3+/Ho3+ phosphors for solid-state lighting and solar cells

In this work, we prepared a novel kind of Yb3+, Er3+/Ho3+ co-doped CsGd(MoO4)(2) phosphors with a different structure from the reported ALn(MoO4)(2) (A = Li, Na or K; Ln = La, Gd or Y) compounds using a high-temperature solid-state reaction method. X-ray diffraction showed that the as-prepared samples had a pure phase. Based on the efficient energy transfer from Yb3+ to Er3+/Ho3+, the up-conversion (UC) luminescence of the optimal CsGd(MoO4)(2): 0.30Yb(3+), 0.02Er(3+) sample showed intensely green light with dominant emission peaks at 528 and 550 nm corresponding to Er3+ transitions H-2(11/2)-I-4(15/2) and S-4(3/2)-> I-4(15/2), respectively, as well as a weak emission peak originating from F-4(9/2)-I-4(15/2) at 671 nm, under 975 nm laser excitation. The CsGd(MoO4)(2): Yb3+, Ho3+ samples mainly displayed two emission bands around 540 and 660 nm together with a negligible one at 755 nm, which corresponded to Ho3+ transitions F-4(4),F-5(2)-> I-5(8), F-5(5)-> I-5(8) and F-4(4),F-5(2)-> I-5(7), respectively, under 975 nm laser excitation. With increasing Yb3+ concentration in CsGd(MoO4)(2): Yb3+, Ho3+ phosphors, the emission color could be tuned from orange red to light yellow due to the large energy gap between levels F-4(4),F-5(2) and F-5(5). In addition, the CsGd(MoO4)(2): Yb3+, Er3+ showed green light under 376 nm UV irradiation similar to that upon 975 nm laser excitation. However, the emissions for CsGd(MoO4)(2): Yb3+, Ho3+ samples under 358 nm UV or 449 nm blue excitation showed dominant emission peaks at 540 nm and weak 660 nm and 752 nm peaks, which were a bit different from those under 975 nm excitation. Interestingly, we observed efficient energy transfer phenomena (possible quantum cutting) from Er3+/Ho3+ to Yb3+ and a Yb3+-O2- charge transfer (CT) transition in the molybdates, which was deduced from the visible and near-infrared emission spectra and the decrease of the Er3+/Ho3+ luminescent lifetimes with increasing Yb3+ concentration in the CsGd(MoO (4))(2): Yb3+, Er3+/Ho3+ samples. The luminescence properties of these phosphors suggest their potential possibility for applications in solid-state lighting and displays as well as in c-Si solar energy conversion systems

Lattice and thermodynamic characteristics of N-stearoyl-allo-threonine monolayers

The effect of the second chiral center of diastereomeric N-alkanoyl-allo-threonine on the main monolayer characteristics has been investigated. The characteristic features of the enantiomeric and racemic forms of N-stearoyl-allo-threonine monolayers are studied on a thermodynamic basis and molecular scale. The π–A curves of the enantiomeric and racemic allo-forms show similar features to those of N-stearoyl-threonine. The compression curves are always located above the corresponding decompression curves and the decompression curves can be used as equilibrium isotherms for both the enantiomeric and racemic N-stearoyl-allo-threonine. The absolute T0-values (disappearance of the LE/LC-transition) are 4–5 K larger compared with the corresponding N-stearoyl-threonines,} but the ΔT0 between the enantiomeric (d) and the racemic (dl) forms is only slightly larger than that of N-stearoyl-threonine. The difference in the critical temperatures Tc{,} above which the monolayer cannot be compressed into the condensed state{,} between the enantiomeric and the racemic forms{,} is quite small (ΔTc = 0.8 K) and is smaller compared to that of the corresponding threonines (ΔTc = 1.8 K). This is consistent with the dominance of the van der Waals interactions between the alkyl chains reducing the influence of chirality on the thermodynamic parameters. GIXD studies of N-stearoyl-allo-threonine monolayers provide information about the lattice structure of condensed monolayer phases on the Angstrom scale and stipulate the homochiral or heterochiral preference in the condensed phases. Comparable to N-stearoyl-threonine{,} the enantiomers exhibit an oblique lattice structure{,} whereas the racemates form a NNN tilted orthorhombic structure demonstrating the dominance of heterochiral interactions in the racemates independent of the diasteomeric structure change of the polar head group. The A0 values are characteristic for rotator phases. The smaller A0 value obtained for the racemic monolayers indicates their tighter packing caused by heterochiral interactions. The program Hardpack was used to predict the geometric parameters of possible 2-dimensional packings. For comparison with the experimental GIXD data{, the two-dimensional lattice parameters and characteristic features of the enantiomeric and racemic diastereomeric stearoyl-threonine monolayers were calculated and are in reasonable agreement with the experimental GIXD data

Practical applications of small-angle neutron scattering.

Recent improvements in beam-line accessibility and technology have led to small-angle neutron scattering (SANS) becoming more frequently applied to materials problems. SANS has been used to study the assembly, dispersion, alignment and mixing of nanoscale condensed matter, as well as to characterise the internal structure of organic thin films, porous structures and inclusions within steel. Using time-resolved SANS, growth mechanisms in materials systems and soft matter phase transitions can also be explored. This review is intended for newcomers to SANS as well as experts. Therefore, the basic knowledge required for its use is first summarised. After this introduction, various examples are given of the types of soft and hard matter that have been studied by SANS. The information that can be extracted from the data is highlighted, alongside the methods used to obtain it. In addition to presenting the findings, explanations are provided on how the SANS measurements were optimised, such as the use of contrast variation to highlight specific parts of a structure. Emphasis is placed on the use of complementary techniques to improve data quality (e.g. using other scattering methods) and the accuracy of data analysis (e.g. using microscopy to separately determine shape and size). This is done with a view to providing guidance on how best to design and analyse future SANS measurements on materials not listed below

Optical activity in the scattering of structured light

We observe that optical activity in light scattering can be probed using types of illuminating light other than single plane (or quasi plane) waves and that this introduces new possibilities for the study of molecules and atoms. We demonstrate this explicitly for natural Rayleigh optical activity which, we suggest, could be exploited as a new form of spectroscopy for chiral molecules through the use of illuminating light comprised of two plane waves that are counter propagating

Practical applications of small-angle neutron scattering.

Recent improvements in beam-line accessibility and technology have led to small-angle neutron scattering (SANS) becoming more frequently applied to materials problems. SANS has been used to study the assembly, dispersion, alignment and mixing of nanoscale condensed matter, as well as to characterise the internal structure of organic thin films, porous structures and inclusions within steel. Using time-resolved SANS, growth mechanisms in materials systems and soft matter phase transitions can also be explored. This review is intended for newcomers to SANS as well as experts. Therefore, the basic knowledge required for its use is first summarised. After this introduction, various examples are given of the types of soft and hard matter that have been studied by SANS. The information that can be extracted from the data is highlighted, alongside the methods used to obtain it. In addition to presenting the findings, explanations are provided on how the SANS measurements were optimised, such as the use of contrast variation to highlight specific parts of a structure. Emphasis is placed on the use of complementary techniques to improve data quality (e.g. using other scattering methods) and the accuracy of data analysis (e.g. using microscopy to separately determine shape and size). This is done with a view to providing guidance on how best to design and analyse future SANS measurements on materials not listed below