Two‐Dimensional Oligo(phenylene‐ethynylene‐butadiynylene)s: All‐Covalent Nanoscale Spoked Wheels

Round and round : Covalently bound spokes induce an efficient template‐directed cyclization towards a rigid molecular wheel (see figure) and afford dramatically increased shape‐persistence properties compared with non‐strutted macrocycles.

     

In silico studies of 1,3(R):2,4(S)‐dibenzylidene‐D‐sorbitol as a gelator for polypropylene

The organic gelator 1,3(R):2,4(S)‐dibenzylidene‐D‐sorbitol (DBS) self‐organizes to form a 3‐D network at relatively low concentrations in a variety of nonpolar organic solvents and polymer melt. In this work, we have investigated the interactions between DBS molecules in polypropylene (PP) by molecular modeling. We have used quantum mechanics to elucidate the preferred geometry of one molecule and a dimer of DBS, and molecular mechanics and molecular dynamics to simulate pure DBS, pure PP, and mixture of DBS and PP as condensed phases, at various temperatures. It was found that inter‐ and intramolecular H‐bonds between DBS molecules are formed in PP in a much more pronounced manner than those formed in pure DBS. The most significant intermolecular H‐bonds are formed between the terminal hydroxyl groups. The most significant intramolecular H‐bonds are formed between O5 /H‐O6 groups. Due to the H‐bonds, DBS molecules form a rigid structure similar to liquid crystal forming molecules, which might explain their tendency to create nanofibrils. It seems that the aromatic rings do not contribute significantly to the intermolecular interactions. Their main role is probably to stiff the molecular structure. Temperature dependences of inter‐ and intramolecular interactions are different. Whereas intermolecular interactions peak heights decrease when temperature increases for pure DBS, the intramolecular interaction almost does not change. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental Evidence for a Triboluminescent Antiperovskite Ferroelectric: Tris(trimethylammonium) catena‐Tri‐μ‐chloro‐manganate(II) Tetrachloromanganate(II)

The perovskite structure is rich in ferroelectricity. In contrast, ferroelectric antiperovskites have been scarcely confirmed experimentally since the discovery of M₃AB‐type antiperovskites in the 1930s. Ferroelectricity is now revealed in an organic–inorganic hybrid X₃AB antiperovskite structure, which exhibits a clear ferroelectric phase transition 6/mmmF6mm with a high Curie point of 480 K. The physical properties across the phase transition are obviously changed along with the symmetry requirements, providing solid experimental evidence for the ferroelectric phase transition. More interestingly, the discovered antiperovskite shows intense photoluminescence and triboluminescence properties. The confirmation of the triboluminescent ferroelectric antiperovskite will open new avenues to explore excellent optoelectronic properties in the antiperovskite family.     

Mechanical properties of molecular composites. I. Poly (p‐phenylene terephthalamide) anion molecules dispersed in poly(4‐vinylpyridine)

Molecular composites have been prepared by dispersing rigid‐rod molecules of ionically‐modified poly(p‐phenylene terephthalamide) (PPTA anion) in a polar poly(4‐vinylpyridine) (PVP) matrix. For concentrations up to 5 wt % of the rigid‐rod reinforcement, the resulting composites are transparent and possess a single glass transition temperature that increases with concentration of the PPTA anion. The mechanical properties of the molecular composites are found to increase with concentration and to attain maximum values at about 5 wt % of the PPTA anion. The enhancement in properties, and the miscibility induced between the two component polymers, is attributed to the development of specific interactions between the ionic groups of the PPTA anion and the polar units of the PVP matrix. When such interactions are not present, as in composites reinforced with non‐ionic PPTA, the samples are opaque and their properties are significantly reduced compared to those of the PPTA anion/PVP composites. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2201–2209, 1999     

Inside Cover: A Synthetic Lectin for O‐Linked β‐N‐Acetylglucosamine (Angew. Chem. Int. Ed. 10/2009)

Carbohydrates are slippery customers in water. Camouflaged by solvent‐mimicking hydroxy groups, they make challenging targets, even for natural receptors. In their Communication on page 1775 ff., A. P. Davis and co‐workers describe a synthetic receptor that is remarkably effective for the important β‐N‐acetylglucosaminyl (β‐GlcNAC) unit. The affinities of the receptor are good and its selectivities are excellent, even by the standards of natural carbohydrate‐binding proteins.

     

Molecular composites made of ionic poly(p‐phenylene terephthalamide) and poly(4‐vinylpyridine): Deformation modes

Deformation modes were examined on strained thin films of a series of molecular composites containing ionically modified rodlike molecules of poly(p‐phenylene terephthalamide) (PPTA) dispersed in a polar polymer matrix. The rigid molecules were a modified form of PPTA in which the H atom of the amide group was replaced, on 30 mol % of the monomer units, by an ionic propane sulfonate group. The polar polymer matrix of these composites was the flexible‐coil polymer, poly(4‐vinylpyridine). Ionic interactions between the two components increased the effective entanglement strand density and produced changes in the deformation modes. The observed changes were dependent on the relative concentration of the two components and on the nature of the counterion. With K⁺ as the counterion, the induced deformation mode changed from pure crazing, as in the matrix polymer, to combined crazing and shear deformation at 5 wt % of the ionic polymer and to essentially pure shear deformation as the concentration increased to 15 wt %. However, when Ca²⁺ was the counterion, pure shear deformation developed at a concentration of only 5 wt %. This effect was attributed to a greater ionic interaction and to a higher effective strand density of the composites, when monovalent K⁺ was replaced by divalent Ca²⁺. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 429–436, 2003