Phase stability in the systems AeAl(2-x)Mgx (Ae = Ca, Sr, Ba): electron concentration and size controlled variations on the laves phase structural theme

The systems AeAl(2-x)Mgx (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu2 --> MgNi2 --> MgZn2 with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl(2-x)Mgx (0 < x < or = 2) and BaAl(2-x)Mgx (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu2 --> MgNi2 --> MgZn2 occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu2 structure at low x. For Ae = Ba a binary compound BaAl2 does not exist, but more Ba-rich Ba7Al13 forms. The reinvestigation of the crystal structure of Ba7Al13 by selected area and convergent beam electron diffraction in a transmission electron microscope revealed a superstructure, which subsequently could be refined from single X-ray diffraction data. The formula unit of the superstructure is Ba21Al40 (space group P31m, Z = 1, a = 10.568(1) angstroms, c = 17.205(6) angstroms). In Ba21Al40 a size match problem between Ba and Al present in Ba7Al13 is resolved. The structure of Ba7Al13 (Ba21Al40) can be considered as a Ba excess variant of the hexagonal MgNi2 Laves phase type structure. An incommensurately modulated variant of the MgNi2 structure is obtained for phases BaAl(2-x)Mgx with x = 0.8-1. At even higher Mg concentrations a structural change to the proper MgZn2 type structure takes place.     

Triphenylene silanes for direct surface anchoring in binary mixed self-assembled monolayers

New triphenylene-based silanes 2-(ω-(chlorodimethylsilyl)-n-alkyl)-3,6,7,10,11-penta-m-alkoxytriphenylene 4 (Tm-Cn) with n = 8 or 9 and m = 7, 8, 9, 10, or 11 were synthesized, and their self-assembly behavior in the liquid state and at glass and silicon oxide surfaces was investigated. The mesomorphic properties of triphenylene silanes 4 (Tm-Cn) and their precursors 3 (Tm-Cn) were determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. From the small-angle X-ray scattering (SAXS) regime, a preferential discotic lamellar mesophase can be deduced, and wide-angle X-ray scattering (WAXS) highlights the liquid-like characteristics of the alkyl side chains. To transfer these bulk structural properties to thin films, self-assembled monolayers (SAMs) were obtained by adsorption from solution and characterized by water contact angle measurements, null ellipsometry, and atomic force microscopy (AFM). Employing the concentration as an additional degree of freedom, binary SAMs of 2-(ω-(chlorodimethylsilyl)-undecyl)-3,6,7,10,11-penta-decyloxytriphenylene 4 (T10-C11) were coassembled with chlorodecyldimethylsilane or chlorodimethyloctadecylsilane, and their capability as model systems for organic templating was evaluated. The structure of the resulting binary mixed SAMs was analyzed by water contact angle measurements, null ellipsometry, and X-ray reflectivity (XRR) in combination with theoretical modeling by a multidimensional Parratt algorithm and AFM. The composition dependence of film thickness and roughness can be explained by a microscopic model including the steric hindrance of the respective molecular constituents.     

Electroclinic properties of chiral smectic a liquid crystalline polymers and block copolymers

The scope for the synthesis and investigation of chiral smectic A liquid crystalline (LC) polymers and block copolymers is discussed. LC block copolymers can combine the molecular order of liquid crystals and the supramolecular order typical of block copolymers, thus allowing to attain information on mesomorphic responses in restricted geometries. We present a general overview on several aspects of the syntheses and properties of LC block copolymers, specifically those obtained by starting from azomacroinitiators. These materials can exhibit an electroclinic effect, that is an electrically induced molecular tilt, which is characterized by a linear dependence on the applied field and a very fast response time in the paraelectric smectic A phase. The current progress in their potential application in electrooptics is outlined.     

Quantitative analysis of complex amino acids and RGD peptides by X‐ray photoelectron spectroscopy (XPS)

The C 1 s, N 1 s, and O 1 s core level binding energies (BEs) of the functional groups in amino acids (glycine, aspartic acid, glutamic acid, arginine, and histidine) with varied side‐chains and cell‐binding RGD‐based peptides have been determined and characterized by X‐ray photoelectron spectroscopy with a monochromatic Al K_α source. The zwitterionic nature of the amino acids in the solid state is unequivocally evident from the N 1 s signals of the protonated amine groups and the C 1 s signature of carboxylate groups. Significant adventitious carbon contamination is evident for all samples but can be quantitatively accounted for. No intrinsic differences in the XP spectra are evident between two polymorphs (α and γ) of glycine, indicating that the crystallographic differences have a minor influence on the core level BEs for this system. The two nitrogen centers in the imidazole group of histidine exhibit an N 1 s BE shift that is in line with previously reported data for theophylline and aqueous imidazole solutions, while the nitrogen and carbon chemical shifts reflect the unusual guanidinium chemical environment in arginine. It is shown that the complex envelopes of C 1 s and O 1 s photoemission spectra for short‐chain peptides can be analyzed quantitatively by reference to the less complex XP spectra of the constituent amino acids, provided the peptides are of high enough purity. The distinctive N 1 s photoemission from the amide linkages provides an indicator of peptide formation even in the presence of common impurities, and variations in the relative intensities of N 1 s were found to be diagnostic for each of the three peptides investigated (RGD, RGDS, and RGDSC). Copyright © 2013 John Wiley & Sons, Ltd.     

Ready-made matrix-assisted laser desorption/ionization target plates coated with thin matrix layer for automated sample deposition in high-density array format.

The methodology for ready-made matrix-assisted laser desorption/ionization (MALDI) target plates covered with an optimized thin layer consisting of matrix and nitrocellulose has been developed. Piezoelectric microdispensing enabled sample depositions in a high-density array format of 2000 sample depositions on a conventionally sized target plate (45 x 47 mm). The sample depositions were made reproducibly in a fully automated mode by using an in-house developed computer-controlled piezoelectric flow-through microdispenser. Additionally, the piezoelectric technique facilitated significant analyte enrichment that increased the detection sensitivity. The MS signal was obtained rapidly, generally within ten laser pulses. An airbrush device was used to generate a fine spray of matrix and nitrocellulose dissolved in acetone. The acetone evaporated instantly when reaching the target plate leaving the entire surface with a thin and uniform matrix/nitrocellulose coating consisting of very small crystals of matrix embedded in the nitrocellulose. These crystals acted as a seed-layer on subsequent analyte depositions, rendering homogeneous sample spots when using alpha-cyano-4-hydroxycinnamic acid (CHCA) as matrix. The relative standard deviation of the signal intensity between spots was (20-30)% (n = 30). The detection sensitivity was improved by restricting the sample spot diameter to 300 microm. The spot size was affected by the deposition rate and the evaporation rate of the dispensed sample volume. Mass spectra of a 25-amol peptide mixture deposition were successfully recorded.     

Macrocyclic polymerization - a new approach to molecular engineering

2,2-Dibutyl-2-stanna-1,3-dioxacycloalkanes were used as cyclic initiators for the ring-opening polymerization of various lactons. This method exclusively yielded series of macrocyclic polylactones without any competition with linear polymers. Under optimized reaction conditions these macrocyclic polymerizations obey the pattern of “living polymerizations”. The living chain ends allow the syntheses of macrocyclic blockcopolymers. The macrocyclic polylactones react with carboxylic acid chlorides by ring-opening yielding telechelic oligo or polylactones. Furthermore, the tin containing macrocyclic polylactones can be used as difunctional “monomers” for polycondensations with dicarboxylic acid dichlorides.