Weighted limits in simplicial homotopy theory

By combining ideas of homotopical algebra and of enriched category theory, we explain how two classical formulas for homotopy colimits, one arising from the work of Quillen and one arising from the work of Bousfield and Kan, are instances of general formulas for the derived functor of the weighted colimit functor.     

Self-organized growth of nanoparticles on a surface patterned by a buried dislocation network

The self-organized growth of Co nanoparticles is achieved at room temperature on an inhomogenously strained Ag(001) surface arising from an underlying square misfit dislocation network of 10 nm periodicity buried at the interface between a 5 nm-thick Ag film and a MgO(001) substrate. This is revealed by in situ grazing-incidence small-angle x-ray scattering. Simulations of the data performed in the distorted wave Born approximation framework demonstrate that the Co clusters grow above the dislocation crossing lines. This is confirmed by molecular dynamic simulations indicating preferential Co adsorption on tensile sites.     

BN-Doped Metal–Organic Frameworks: Tailoring 2D and 3D Porous Architectures through Molecular Editing of Borazines

Building on the MOF approach to prepare porous materials, herein we report the engineering of porous BN-doped materials using tricarboxylic hexaarylborazine ligands, which are laterally decorated with functional groups at the full-carbon ‘inner shell’. Whilst an open porous 3D entangled structure could be obtained from the double interpenetration of two identical metal frameworks derived from the methyl substituted borazine, the chlorine-functionalised linker undergoes formation of a porous layered 2D honeycomb structure, as shown by single-crystal X-ray diffraction analysis. In this architecture, the borazine cores are rotated by 60° in alternating layers, thus generating large rhombohedral channels running perpendicular to the planes of the networks. An analogous unsubstituted full-carbon metal framework was synthesised for comparison. The resulting MOF revealed a crystalline 3D entangled porous structure, composed by three mutually interpenetrating networks, hence denser than those obtained from the borazine linkers. Their microporosity and CO₂ uptake were investigated, with the porous 3D BN-MOF entangled structure exhibiting a large apparent BET specific surface area (1091 m² g⁻¹) and significant CO₂ reversible adsorption (3.31 mmol g⁻¹) at 1 bar and 273 K.     

On the asymptotic stability properties of Runge-Kutta methods for delay differential equations

Summary. In this paper asymptotic stability properties of Runge-Kutta (R-K) methods for delay differential equations (DDEs) are considered with respect to the following test equation: where and is a continuous real-valued function. In the last few years, stability properties of R-K methods applied to DDEs have been studied by numerous authors who have considered regions of asymptotic stability for “any positive delay” (and thus independent of the specific value of ). In this work we direct attention at the dependence of stability regions on a fixed delay . In particular, natural Runge-Kutta methods for DDEs are extensively examined. Received April 15, 1996 / Revised version received August 8, 1996     

High-pressure spin-crossover in a dinuclear Fe(II) complex

The effect of pressure on the dinuclear spin crossover material [{Fe(bpp)(NCS)(2)}(2)(4,4'-bipy)]·2MeOH (where bpp = 2,6-bis(pyrazol-3-yl)pyridine and 4,4'-bipy = 4,4'-bipyridine, 1) has been investigated with single crystal X-ray diffraction and Raman spectroscopy using diamond anvil cell techniques. The very gradual pressure-induced spin crossover occurs between 7 and 25 kbar, and shows no evidence of crystallographic phase transitions. The pressure-induced spin transition leads to a complete LS state which is not thermally accessible. This structural evolution under pressure is in stark contrast to the previously reported thermal spin crossover behaviour, in which a symmetry-breaking, purely structural phase transition results in only partial conversion to the low spin state. This observation is attributed to the symmetry-breaking phase transition becoming unfavourable under pressure.     

Visible and near-infrared intense luminescence from water-soluble lanthanide [Tb(III), Eu(III), Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), Er(III)] complexes

The synthesis of a new ligand (1) containing a single phenanthroline (phen) chromophore and a flexibly connected diethylenetriamine tetracarboxylic acid unit (DTTA) as a lanthanide (Ln) coordination site is reported [1 is 4-[(9-methyl-1,10-phenantrol-2-yl)methyl]-1,4,7-triazaheptane-1,1,7,7-tetraacetic acid]. From 1, an extended series of water-soluble Ln.1 complexes was obtained, where Ln is Eu(III), Tb(III), Gd(III), Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), and Er(III). The stoichiometry for the association was found 1:1, with an association constant K(A) > or = 10(7) s(-1) as determined by employing luminescence spectroscopy. The luminescence and photophysical properties of the series of lanthanide complexes were investigated in both H2O and D2O solutions. High efficiencies for the sensitized emission, phi(se), in air-equilibrated water were observed for the Ln.1 complexes of Eu(III) and Tb(III) in the visible region (phi(se) = 0.24 and 0.15, respectively) and of Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), and Er(III) in the vis and/or near-infrared region [phi(se) = 2.5 x 10(-3), 5 x 10(-4), 3 x 10(-5), 2 x 10(-5), 2 x 10(-4), 4 x 10(-5), and (in D2O) 4 x 10(-5), respectively]. For Eu.1 and Tb.1, luminescence data for water and deuterated water allowed us to estimate that no solvent molecules (q) are bound to the ion centers (q = 0). Luminescence quenching by oxygen was investigated in selected cases.     

Enantioselective allylation of aldehydes with allyltrichlorosilane promoted by new chiral dipyridylmethane N-oxides

New chiral dipyridine N-monoxides and N,N′-dioxides, which possess an isopropylidene backbone between two pyridine rings, have been prepared from naturally occurring monoterpenes. Their utility as organocatalysts has been demonstrated in the enantioselective addition of allyltrichlorosilane to aldehydes. Enantioselectivities up to 85% ee have been obtained.     

From molecular to macroscopic engineering: shaping hydrogen-bonded organic nanomaterials

The self‐assembly and self‐organization behavior of chromophoric acetylenic scaffolds bearing 2,6‐bis(acetylamino)pyridine ( 1 , 2 ) or uracyl‐type ( 3 – 9 ) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2 , thanks to a combination of solvophilic/solvophobic forces and π–π stacking interactions, undergo self‐organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide‐field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2 , a more uniform size distribution is found (80–200 nm) compared to 1 (20–1000 nm). Temperature scans in the range 283–353 K show that the self‐organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen‐bonding units 3 – 9 . Depending on the specific geometrical structure of 3 – 9 , different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3 , which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis‐uracyl derivative 4 or 5 , a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3 , which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self‐assembly was studied in parallel with modules 8 and 9 , in which the uracyl recognition sites are protected with tert‐butyloxycarbonyl (BOC) groups. This strategy allows further control of the self‐assembly/self‐organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC‐protected ditopic modules 8 self‐assemble and self‐organize with 1 into ordered linear nanostructures, whereas BOC‐protected tritopic system 9 gives rise to extended domains of circular nano‐objects in combination with 1 .     

Surfactant-free nonaqueous synthesis of metal oxide nanostructures

Surfactant-free nonaqueous (and/or nonhydrolytic) sol-gel routes constitute one of the most versatile and powerful synthesis methodologies for nanocrystalline metal oxides with high compositional homogeneity and purity. Although the synthesis protocols are particularly simple, involving only metal oxide precursors and common organic solvents, the obtained uniform nanocrystals exhibit an immense variety of sizes and shapes. The small number of reactants in these routes enables the study of the chemical mechanisms involved in metal oxide formation. Nonhydrolytic routes to inorganic nanomaterials that used surfactants as size- and shape-controlling agents have been discussed recently. This Minireview supplements this topic by discussing surfactant-free processes, which have become a valuable alternative to surfactant-assisted as well as to traditional aqueous sol-gel chemistry routes.     

Radiative Effects on Mixed Convection in a Uniformly Heated Vertical Convergent Channel with an Unheated Moving Plate

Fluids engineering is extremely important in a wide variety of materials processing systems, such as soldering, welding, extrusion of plastics and other polymeric materials, Chemical Vapor Deposition (CVD), composite materials manufacturing. In particular, mixed convection due to moving surfaces is very important in these applications. Mixed convection in a channel, as a result of buoyancy and motion of one of its walls has received little research attention and few guidelines are available for choosing the best performing channel configuration, particularly when radiative effects are significant. In this study a numerical investigation of the effect of radiation on mixed convection in air due to the interaction between a buoyancy flow and an unheated moving plate induced flow in a uniformly heated convergent vertical channel is carried out. The moving plate has a constant velocity and moves in the buoyancy force direction. The principal walls of the channel are heated at uniform heat flux. The numerical analysis is accomplished by means of the commercial code Fluent. The effects of the wall emissivity, the minimum channel spacing, the converging angle and the moving plate velocity are investigated and results in terms of air velocity and temperature fields inside the channel and wall temperature profiles, both of the moving and the heated plates, are given. Nusselt numbers, both accounting and not for the radiative contribution to heat removal, are also presented.