Synthesis and characterization of silicon-doped gallium oxide nanowires for optoelectronic UV applications

Silicon-doped gallium oxide nanowires have been synthesized by thermal methods using either a mixture of gallium oxide and silicon powders or metallic gallium with silicon powder as precursor materials. The growth mechanism has been found to be a vapour–liquid–solid (VLS) or vapour–solid (VS) process, respectively, depending on the precursor used. In the former case, silicon oxide droplets at the end of the nanowires have been observed. Their possible role during the growth of the nanostructures is discussed. Structural and morphological characterization of the doped nanowires has been performed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show a high crystalline quality and a uniform distribution of silicon along the nanowires. Room temperature cathodoluminescence (CL) in the SEM shows that slight variations in the composed UV–blue emission band appear due to the influence of Si impurities in the oxygen vacancy defect structure.     

Linear and nonlinear optical properties of carbon nanotube-coated single-mode optical fiber gratings

Single-wall carbon nanotube deposition on the cladding of optical fibers has been carried out to fabricate an all-fiber nonlinear device. Two different nanotube deposition techniques were studied. The first consisted of repeatedly immersing the optical fiber into a nanotube supension, increasing the thickness of the coating in each step. The second deposition involved wrapping a thin film of nanotubes around the optical fiber. For both cases, interaction of transmitted light through the fiber core with the external coating was assisted by the cladding mode resonances of a tilted fiber Bragg grating. Ultrafast nonlinear effects of the nanotube-coated fiber were measured by means of a pump-probe pulses experiment.     

Ionic Liquid Crystals Formed by Self‐Assembly around an Anionic Anthracene Core

We have designed and synthesised a series of modular, mesogenic complexes based on anthracene‐2,6‐disulfonate and trialkoxybenzyl‐functionalised imidazolium cations. Each complex contains a central, rigid, dianionic anthracene core and two flexible monocations bearing paraffin chains anchored on imidazolium rings. Anthracene‐2,6‐disulfonate can be crystallised with various simple alkylammonium ions and, in the case of ⁺N(CH₃)₂(C₁₆H₃₃)₂, a crystal structure determination has shown that the long paraffinic chains are intercalated between the anthracene moieties. The dianion forms columnar mesophases with trialkoxybenzylimidazolium cations, as identified by polarising optical microscopy and X‐ray scattering measurements. Differential scanning calorimetry studies confirmed mesomorphic behaviour from room temperature to about 200 °C for alkyl chains containing 8, 12 and 16 carbon atoms. The strong luminescence of anthracene is maintained in the mesophase and fluorescence measurements confirmed the presence of J aggregates in all cases. The new functional materials described herein provide an easy access to stable and luminescent mesomorphic materials engineered by an ionic self‐assembly process.     

The effect of the "inert" counteranions in the deprotonation of the dihydrogen complex trans-[FeH(eta 2-H2)(dppe)2]+: kinetic and theoretical studies

Kinetic studies indicate that trans-[FeH(H2)(dppe)2]+ reacts with an excess of NEt3 to form cis-[FeH2(dppe)2] in a single kinetic step. The second-order rate constant is strongly affected by the presence of added salts, an acceleration being observed with BF4- and PF6- salts and a deceleration with BPh4-. Theoretical calculations indicate that the role of the accelerating anions consists of the formation of ion pairs that provide a more effective reaction pathway for deprotonation. However, for the ion pair with the bulky BPh4- anion, steric crowding in the proximities of the dihydrogen ligand hinders the approach of the base, and the reaction is decelerated.     

Domino metathesis of 2-azanorbornenones: a new strategy for the enatioselective synthesis of 1-azabicyclic compounds

Domino metathesis of N-alkylated derivatives of (1S)-2-azanorborn-5-en-3-one allowed for the enantioselective synthesis of pyrrolizidine, quinolizidine, pyrrolidinoazepine, and pyrrolidinoazocine derivatives in a straightforward process.     

Designing liquid-crystalline dendronised hexa-adducts of [60]fullerene via click chemistry

ABSTRACT

Liquid-crystalline [60]fullerodendrimers were constructed via click chemistry based on the reaction between hexa-adducts of [60]fullerene (C₆₀) bearing 12 azide groups and alkyne-terminated cyanobiphenyl dendrons of first- and second-generation. The structure of all the new compounds was confirmed by IR, UV, ¹H and ¹³C NMR spectroscopies and mass spectrometry. The mesomorphic properties were studied by polarised optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. The hexa-adduct of C₆₀ functionalised with the first-generation dendrons gave rise to the formation of a smectic A phase and a rectangular columnar phase (c2mm symmetry) while the hexa-adduct of C₆₀ decorated with the second-generation dendrons displayed only a rectangular columnar phase (c2mm symmetry). Our results show that the hexa-adduct of C₆₀ is a unique synthetic platform for the design of fullerodendrimers and dendronised materials.     

Characterization and study of microwave activation effects on the polystyrene tacticity

¹³C Nuclear magnetic resonance proved to be an advantageous tool to determine the stereoregularity of polystyrene polymers. The latter was achieved through the analysis of the signal of the quaternary carbon and that of the carbon-p in the aromatic ring too. Styrene was polymerized through microwaves and conventional heating activation using two different polymerization techniques: emulsion and bulk. Microwave activation was performed in a mono-modal type device under the following experimental conditions: various initiator concentrations, an average irradiation power of 50?W, temperature of 70°C, and using a batch reactor for emulsion and bulk experiments. The results obtained in these experiments were compared with those obtained by conventional heating activation polymerization under the same initiator concentration and temperature conditions. Microwave-activated reactions resulted in shorter reaction times and higher yields. The tacticity of the polymer samples was not significantly altered, which lead to the conclusion that, in this case, the stereoregularity of polystyrene was not influenced by microwave irradiation.     

Multivalent binding motifs for the noncovalent functionalization of graphene

Single-layer graphene is a newly available conductive material ideally suited for forming well-defined interfaces with electroactive compounds. Aromatic moieties typically interact with the graphene surface to maximize van der Waals interactions, predisposing most compounds to lie flat on its basal plane. Here we describe a tripodal motif that binds multivalently to graphene through three pyrene moieties and projects easily varied functionality away from the surface. The thermodynamic and kinetic binding parameters of a tripod bearing a redox-active Co(II) bis-terpyridyl complex were investigated electrochemically. The complex binds strongly to graphene and forms monolayers with a molecular footprint of 2.3 nm(2) and a ΔG(ads) = -38.8 ± 0.2 kJ mol(-1). Its monolayers are stable in fresh electrolyte for more than 12 h and desorb from graphene 1000 times more slowly than model compounds bearing a single aromatic binding group. Differences in the heterogeneous rate constants of electron transfer between the two compounds suggest that the tripod projects its redox couple away from the graphene surface.