Stereoprogrammed direct synthesis of calixarene-based [3]rotaxanes

Directional calixarene wheels were threaded along a bis(benzylalkylammonium) axle in a stereoprogrammed way to obtain the first examples of calixarene-based [3]rotaxanes. The base/acid treatment demonstrated that these systems act as nanosized molecular shuttles. An unprecedented switching between the tail-to-tail and head-to-head relative orientation of the calix-wheels was observed.     

Recent advances in processing and applications of microwave ferrites

Next generation magnetic microwave devices will be planar, smaller, weigh less, and perform well beyond the present state-of-the-art. For this to become a reality advances in ferrite materials must first be realized. These advances include self-bias magnetization, tunability of the magnetic anisotropy, low microwave loss, and volumetric and weight reduction. To achieve these goals one must turn to novel materials processing methods. Here, we review recent advances in the processing of microwave ferrites. Attention is paid to the processing of ferrite films by pulsed laser deposition, liquid phase epitaxy, spin spray ferrite plating, screen printing, and compaction of quasi-single crystals. Conventional and novel applications of ferrite materials, including microwave non-reciprocal passive devices, microwave signal processing, negative index metamaterial-based electronics, and electromagnetic interference suppression are discussed.     

Olive Trees By-Products as Sources of Bioactive and Other Industrially Useful Compounds: A Systematic Review

The need to produce an ever-increasing quantity of material products and food resulting from the planet globalization process has contributed to the spread of modern agriculture based on a linear production resulting in the generation of tons of waste. This huge amount of waste is generally accumulated in landfills, causing different environmental problems. Hence, researchers moved on to study the processes used to recover agro-industrial by-products within a circular and sustainable bio-economy concept. A systematic quest on Scopus and PubMed databases was performed to identify the data available to date on recycling agro-industrial by-products of Olea europaea L. This systematic review summarizes the knowledge regarding the use of olive trees by-products for producing animal feed, biocomposites, bioethanol, cellulose pulp, activated carbon, and as a fuel source for energy production. Furthermore, the data regarding the potential biological activity of extracts from olive roots, wood, bark, and pruning were analyzed. Olive trees by-products are, indeed, rich in molecules with antioxidant, antimicrobial, cardioprotective, and anticancer activity, representing a promising candidate for treat several human diseases.     

Quantum interference effects in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> topological insulator nanowires with variable cross-section lengths

Stimulated by the recent realization of three dimensional topological insulator nanowireinterferometers, a theoretical analysis of quantum interference effects on the low energyspectrum of Bi₂Se₃ nanowires is presented. The electronic properties areanalyzed in nanowires with circular, square and rectangular cross-sections starting from acontinuum three dimensional model with particular emphasis on magnetic and geometricaleffects. The theoretical study is based on numerically exact diagonalizations of thediscretized model for all the geometries. In the case of the cylindrical wire, anapproximate analytical solution of the continuum model is also discussed. Although amagnetic field corresponding to half quantum flux is expected to close the band gapinduced by Berry phase, in all the studied geometries with finite area cross-sections, thegap closes for magnetic fields typically larger than those expected. Furthermore,unexpectedly, due to geometrical quantum interference effects, for a rectangular wire witha sufficiently large aspect ratio and smaller side ranging from 50 Å and 100 Å, the gapcloses for a specific finite area cross-section without the application of a magneticfield.     

The calixarene p-bromodienone route: from a chemical curiosity to an useful synthetic tool

This review gives an account on the development of the calixarene “p-bromodienone route” which include the beginning of the story, the tested and potential applications, and a mention to a different strictly related procedure. The development of this route is due to the serendipitous discovery of calixarene derivatives containing the 4-tert-butyl-4-bromo-2,5-cyclohexadienone moiety (shortly, the p-bromodienone moiety). These derivatives undergo a silver-mediated nucleophilic substitution and a subsequent rearomatization to give products in which a nucleophilic agent is directly linked to the para-position of a calixarene aromatic ring in a sort of “aromatic ring umpolung”. The nucleophiles so far tested are O-nucleophiles (alcohols, phenols, carboxylates) or C-nucleophiles (activated aromatic rings, acetylides) which has led to the corresponding p-substituted derivatives or to unusual m-substituted calixarenes through an intermediate dienone-phenol rearrangement. In this way, a range of additional chemical functionalities can be attached to the calixarene scaffold which include interalia alkyno, carboxy, peptido and glyco substituents. On the basis of literature data, it can be expected that several additional nucleophiles, including S-, N-, and P-nucleophiles, could be used in the calixarene p-bromodienone route thus expanding its scope.     

NMR observations of fluorine ion-implanted into polyacetylene and graphite

¹⁹FNMR spectroscopy has been used to characterize fluorine implanted into polyacetylene and highly orientated pyrolytic graphite. The intensities of the ¹⁹F NMR spectra at 4.5 K show that essentially all the fluorine is retained in these materials after the implantation process is over. The Une width of the spectrum of the graphite sample indicates that the fluorine remains where it was deposited.For the polyacetylene sample the ratio of the NMR solid echo height to the spin echo height, as calibrated herein, shows that the predominant nuclear diploar interaction of the implanted fluorine is heteronuclear.     

Regioselective O-substitution of p-tert-butylcalix[7]arene

[structure: see text]. The first examples of selectively functionalized calix[7]arenes have been obtained by weak-base promoted O-alkylation or O-benzoylation of p-tert-butylcalix[7]arene. Mono-, 1,3- and 1,4-disubstituted calix[7]arenes have been obtained in workable yields, while the 1,2,4,6-tetrasubstitution was achieved with surprisingly high selectivity (50-88% yield) by using K2CO3 as base. A rationale for this finding is proposed.     

Regioselective intramolecular bridging of p-tert-butylcalix[7]arene

The first examples of singly bridged calix[7]arenes 2–4 have been obtained by base-promoted direct O-alkylation of p-tert-butylcalix[7]arene with a variety of bis-electrophiles including BrCH₂Cl, oligoethylene glycol ditosylates, and 1,4-bis(bromomethyl)benzene. 1,2-Bridging was favored with ‘short bite’ spanning elements, while the 1,4-isomer predominated with the others (yields up to 72% in the presence of Cs₂CO₃). Assignment of bridging pattern was mainly based on chemical shift of OH groups, in some cases confirmed by 2D NMR experiments. A hampered conformational mobility, depending on the position and nature of the bridge, was observed for compounds 2–4.     

Synthesis of the first examples of p-bromodienone and transannular spirodienone calixarene derivatives

The first examples of p-bromodienone calixarene derivatives (6-7 and 9-10) have been obtained by treatment of 1,5-dihydroxy-hexaalkoxycalix[8]arenes 5 or tripropoxycalix[4]arene 8 with trimethylphenylammonium tribromide and a saturated solution of NaHCO₃. The first transannular spirodienone derivative 11 was only obtained in the presence of NaOH or using the KOH/I₂/PEG-200 oxidizing system.