1,720,986 research outputs found
Calixarene-based porous 3D polymers and copolymers with high capacity and binding energy for CO2, CH4 and Xe capture
No full textThe supramolecular capacity of calixarenes towards guests is largely consolidated; in contrast, the synthesis of porous calixarene-based frameworks by covalent bond formation is still a challenge. Our target was to yield 3D polymers and copolymers based on calixarenes for selective gas-capture, endowed with easy pore accessibility and specific sites, and built via a straightforward synthetic route. The covalent calixarene frameworks (CXFs) were prepared by the Yamamoto coupling reaction starting from tetrabromo calixarene propoxy- and methoxy-monomers of three stable calixarene (partial cone, effective cone, and 1,3-alternate) conformers and complete post-synthetic deprotection to achieve polar phenolic calixarene derivatives. Moreover, the copolymer of calixarene-based monomers with tetrabromo-tetraphenylmethane exhibited remarkable surface area up to about 3000 m2 g−1. Smart architectures endowed with hierarchical porosity from micro- to meso-porosity showed notable sponge-like swellability by CO2, which was captured effectively at room temperature, even in competition with N2, yielding CO2 removal in column breakthrough experiments. Indeed, CXFs displayed excellent CO2 and CH4 energy binding of 35 and 24 kJ mol−1, respectively. Ultramicropore sites were highlighted by Xe capture and in situ detection after a xenon diffusion time of a few milliseconds, by laser-assisted hyperpolarized 129Xe NMR, revealing the accessibility of calixarene capsules and the available space. This synthetic route demonstrated the possibility to modulate at will the pore capacity and selectivity, displaying porous frameworks with two distinct pore families, wherein calixarene moieties play the role of small and selective sites. A contractile behavior of the frameworks was observed upon deprotection which produced more polar sites, due to the formation of hydrogen bond networks
Porous molecular crystals by macrocyclic coordination supramolecules
No full textIn this study, we show how the combination of metal ions, counter-anions and opportunely functionalized and preorganized ligands gives rise to two distinct supramolecular isomers, coordination polymeric chains and hexameric macrocycles. The hexamers then aggregate to form a cubic structure exhibiting permanent microporosity. The supramolecular assemblies are formed with Ag(+), thioether functionalized bis(pirazolyl)methane ligands and CF3SO3(-)/PF6(-) as the counter-anions. Five different ligands were prepared by modifying the peripheral thioether moiety with naphthyl, methoxy, m-Me, p-Me and F groups (L(SNf), L(SPhOMe), L(SPhm-Me), L(SPhp-Me), and L(SPhF)). Helicoidal coordination polymeric chains are formed with CF3SO3(-) (general formula [Ag(L)]n(CF3SO3)n), whereas macrocyclic hexamers are formed with PF6(-) (general formula [Ag(L)]6(PF6)6). The macrocycles self-assemble into ordered capsules with the shape of a tetrahedron, and the overall framework is sustained by Ag(+)···(PF6(-))···Ag(+) contacts. The capsules generate a highly symmetric structural arrangement, which is characterized by permanent microporosity arising from two distinct types of microporous chambers in the structure. The gas absorption isotherms show that the materials can selectively adsorb CO2 and N2O over CH4 and N2. The modulation of the microporosity of the materials is achieved by the different thioether functionalization of the ligands L(SNf), L(SPhOMe), L(SPhm-Me), and L(SPhF). The diffusion and localization of the gas molecules within the cavities were investigated by 2D (1)H-(13)C solid state NMR on samples loaded with enriched (13)CO2, showing that both types of cavities are accessible to guest molecules from the gas phase
When long bis(pyrazolates) meet late transition metals: Structure, stability and adsorption of metal-organic frameworks featuring large parallel channels
No full textA family of bis(pyrazolato)-based metal-organic frameworks (MOFs) was isolated by reacting 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB) with a number of transition metal ions. Special attention was dedicated to their structural features, their thermal and chemical stability, as well as their spectroscopic and adsorption properties. The rod-like ligands, connecting Zn(II), Ni(II) and Fe(III) nodes, fabricate 3-D networks containing 1-D pervious channels. The combination of thermal analysis and variable-temperature XRPD demonstrated the remarkable thermal robustness of the three materials, which are stable in air up to at least 410 °C, and showed their structural response to increasing temperature. Specific experiments permitted us to test the chemical stability of the three species toward water as well as moderately acidic and basic solutions, the Ni(II) derivative being stable and hydrophobic in all the conditions assayed. The electronic transitions of both the ligand and the MOFs were investigated by solid-state UV-Vis absorption as well as by steady-state and time-resolved fluorescence analysis, which showed that the high fluorescence of the linker is perturbed in the three MOFs, suggesting high sensitivity to environmental changes. N2 adsorption measurements at 77 K allowed to estimate promising Langmuir specific surface areas, peaking at 2378 m2 g^-1 in the case of the Ni(II) derivative. The best CO2 and CH4 uptake performances were achieved with the Fe(III)-based MOF. Indeed, adsorption experiments with CO2 revealed that a considerable amount, up to 40% wt, is adsorbed by the Fe(III) derivative under the mild conditions of 298 K and 10 bar
Porous dipeptide crystals as selective CO2 adsorbents: experimental isotherms vs. grand canonical Monte Carlo simulations and MAS NMR spectroscopy
No full textMolecular crystals of dipeptides containing open channels can selectively absorb CO2 over N-2 and CH4, as shown by experimental and simulated isotherms. The efficient CO2 capture enabled separation from methane and allowed the direct observation by 2D MAS NMR of CO2 sequestered in the peptide-based biozeolites
Molecular rotors in a metal–organic framework: muons on a hyper-fast carousel
Full text linkUsing muon-spin spectroscopy, we study the exceptional dynamical properties of rotating molecular struts engineered within a Zn-based metal–organic framework at cryogenic temperatures, where the internal motions of almost any other organic substance are quenched. Muon-spin spectroscopy is particularly suited for this aim, as the experimental evidence suggests several implantation sites for the muons, among which at least one directly onto the rotating moiety. The dynamics of the molecular rotors are characterized by the exceptionally low activation energy EA ∼ 30 cal mol–1. At the same time, we evidence a highly unusual temperature dependence of the dipolar interaction of muons with nuclear magnetic moments on the rotors, suggesting a complex influence of the rotations on the muon implantation and diffusion
Atomistic Model of Realistic Crystalline Mesoporous Organosilica Materials Including Nanochannels
No full textThe
new class of periodic mesoporous organosilica materials (PMOs),
due to the peculiar features, has attracted growing interest from
several research areas. We present an atomistic model of a p-phenylenesilica crystalline mesoporous structure with
a hexagonal framework, explicitly including channels on nanoscale.
OPLS-AA force-field optimization, to get a suitable PMO structure
compared with the experimental one, is described. In particular, DFT calculations have been performed to calculate torsional energy barrier of phenyl rings connected to the silicon atoms belonging to inorganic layers and to improve the OPLS-AA force field performances for these materials. Finally, inclusion of small molecules and their interactions with PMO walls have been investigated for CO2 and H2O
Organic semiconductor frameworks integrating bay-substituted perylene bisimides as screw dislocation units in onion-like π–π stacked architectures
No full textImine-linked covalent organic frameworks (COFs) are built with precision chemistry control, where the tetra-p-CHO-aryloxy bay-substituted PBI-1 serves as a rylene chromophore subunit encoding (i) a broad visible light cross-section (ελ552 nm = 42 000 L mol−1 cm−1); (ii) an excited state redox manifold (E(PBI-1*/·—) = 1.82 V vs. NHE, E(PBI-1·+/*) = −0.43 V vs. NHE); (iii) a dynamic chiral distortion of the aromatic core with a dihedral angle of up to 33°; and (iv) two tertiary amines as N-imide terminals favoring proton-coupled charge transfer mechanisms in aqueous media. PBI-1-COFs are designed herein following an isoreticular expansion strategy based on the elongation of polyaryldiamine linkers (n = 1–3 as Ph, bPh, and tPh), where convergent FT-IR, ssNMR, PXRD, TEM and SEM evidence points to a slip-stacked arrangement of the 2D-COF layers, likely induced by the conformational distortion of the PBI-1 cores, resulting in a prevalent J-type coupling scheme and a distinct red-shift of the material absorption (up to 700 nm), optical bandgaps of ∼1.9 eV, and a morphological progression from onion-like, curved π–π stacked domains to fully folded spheroidal structures (quasi-monodispersed particles with D = 700 ± 100 nm). In situ polymerization of high surface area 3D-tungsten oxide nanosheets (WO3 3D-NS) affords robust photoanodes integrating the n-type COF semiconductor layer capable of record photocurrent outputs (up to 590 ± 50 μA cm−2) under green-light irradiation (1 sun, λ > 490 nm), probed with anionic hydroquinone shunts (an applied bias of 0.8 V vs. RHE) and favored by a preferential host–guest response due to complementary charge interactions mapped by NMR-DOSY and FT-IR spectroscopy
Multifunctional Organosulfonate Anions Self-Assembled with Organic Cations by Charge-Assisted Hydrogen Bonds and the Cooperation of Water
Full text linkThe
present study focuses on the assembly of organo-cations with
organo-anions in water. The anions, characterized by symmetric moieties
(carbon-, adamantane-, or calixarene-based) functionalized with directional
hydrogen bond (HB) acceptor functions (tetra-sulfonate moieties),
are combined with planar guanidinium or terephtalimidamide cations
as hydrogen bond donors, the purpose being to integrate water molecules
into the lattice. The imbalance between the charge on the two components,
and the considerable number of HB donor and acceptor sites, promotes
the insertion of water into the structures. In the reported structures,
a part of the water molecules serves as a structural linker between
the anions and cations, while the remaining molecules cluster into
channels and cavities in a loose association with the supramolecular
matrix framework
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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