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International audience; Natural radioactive gases and anthropogenic radionuclides such as radon, xenon, hydrogen and krypton isotopes, need to be carefully monitored to be properly managed as pathogenic agents, radioactive diagnostic agents or indicators of nuclear activity. State-of-the-art gas detectors based on liquid scintillators suffer from many drawbacks such as lengthy sample preparation procedures and limited solubility of gaseous radionuclides, which produces a detrimental effect on measurement sensitivity. A potential breakthrough solution to this problem is the use of solid porous scintillators that act as gas concentrators and therefore could increase detection sensitivity. Highly porous scintillating metal-organic frameworks (MOFs) stand out as relevant materials for the realization of these devices. We demonstrate the capability of porous hafnium-based MOF nanocrystals exploiting dicarboxy-9,10-diphenylanthracene (DPA) as a scintillating conjugated linker to detect gas radionuclides. The nanocrystals show fast scintillation properties in the nanosecond domain, a fluorescence quantum yield of ~40% and an accessible porosity suitable to host noble gas atoms and ions. The MOFs have been tested for the adsorption and detection of the radionuclides 85Kr, 222Rn and 3H in a customized newly developed device based on a time coincidence technique. For all of them, MOF nanocrystals demonstrate an improved sensitivity with respect to a reference detector, showing an excellent linear response down to an activity value lower than 1 kBq·m-3 that outperforms that of commercial devices. The results obtained strongly support the possible use of scintillating porous MOF nanocrystals as the building block of ultrasensitive sensors for the detection of natural and anthropogenic radioactive gases.

Achieving sophisticated juxtaposition of geared molecular rotors with negligible energy-requirements in solids enables fast yet controllable and correlated rotary motion to construct switches and motors. Our endeavor was to realize multiple rotors operating in a MOF architecture capable of supporting fast motional regimes, even at extremely cold temperatures. Two distinct ligands, 4,4′-bipyridine (bipy) and bicyclo[1.1.1]pentanedicarboxylate (BCP), coordinated to Zn clusters fabricated a pillar-and-layer 3D array of orthogonal rotors. Variable temperature XRD, 2H solid-echo, and 1H T1 relaxation NMR, collected down to a temperature of 2 K revealed the hyperfast mobility of BCP and an unprecedented cascade mechanism modulated by distinct energy barriers starting from values as low as 100 J mol–1 (24 cal mol–1), a real benchmark for complex arrays of rotors. These rotors explored multiple configurations of conrotary and disrotary relationships, switched on and off by thermal energy, a scenario supported by DFT modeling. Furthermore, the collective bipy-ring rotation was concerted with the framework, which underwent controllable swinging between two arrangements in a dynamical structure. A second way to manipulate rotors by external stimuli was the use of CO2, which diffused through the open pores, dramatically changing the global rotation mechanism. Collectively, the intriguing gymnastics of multiple rotors, devised cooperatively and integrated into the same framework, gave the opportunity to engineer hypermobile rotors (107 Hz at 4 K) in machine-like double ligand MOF crystals.

A high efficiency emission with a massive Stokes shift is obtained by fluorescent conjugated acene building blocks arranged in nanocrystals. The two ligands of equal molecular length and connectivity, yet complementary electronic properties, are co-assembled by zirconium oxy-hydroxy clusters, generating highly crystalline hetero-MOF nanoparticles The fast diffusion of singlet molecular excitons in the framework, coupled with the fine matching of ligands absorption and emission properties, enables to achieve an ultrafast activation of the low energy emission by diffusion-mediated non-radiative energy transfer in the 100 ps time scale, by using a low amount of co-ligands. This allow to obtain MOF nanocrystals with a fluorescence quantum efficiency of ̴ 70% and an actual Stokes shift as large as 750 meV. This large Stokes shift suppresses the reabsorption of fast emission issues in bulk devices, pivotal for a plethora of applications in photonics and photon managing spacing from solar technologies, imaging, and detection of high energy radiation. These features allowed to realize a prototypal fast nanocomposite scintillator that shows an enhanced performance with respect to the homo-ligand nanocrystals, achieving benchmark. values which compete with those of some inorganic and organic commercial systems.

The 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.

In 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.

The structure of a synthetic fluoromica has been investigated and elucidated by complementary 29Si, 1H, 19F, 23Na, and 27Al single and double resonance experiments. The phyllosilicate possesses a charge heterogeneity arising from the presence of different charged sites in the octahedral sheets of the T-O-T layers. Part of the magnesium atoms in the octahedral sheets is missing, creating vacancies. Charge balancing proceeds mostly by incorporation of sodium ions into the interlayer space and substitution of Mg by sodium in the lattice. The latter process produces a nonexchangeable fraction of sodium ions (∼20% of the total inventory) and creates multiple sites for the fluoride ions, as evidenced from 19F NMR spectroscopy. Detailed quantitative information about the sodium-fluoride distance geometries were obtained from 19F/ 23Na double rotational echo double resonance (REDOR) experiments; furthermore, different types of fluoride sites could be discriminated on the basis of 19F/23Na double resonance experiments. The chemical reactivity of this system was modified by ball milling, cation exchange with octadecylammonium ions, and grafting with an organosilane (γ-aminopropyltriethoxysilane), and the corresponding structural consequences were probed by multinuclear single and double resonance NMR techniques. © 2011 American Chemical Society.

Linear conjugated guest molecules with high aspect ratios form inclusion compounds with guanidinium organodisulfonate (GDS) host frameworks in which organodisulfonate "pillars" connect opposing GS sheets to generate lamellar architectures that reflect templating by the guest. Through judicious selection of pillars having adjustable lengths (l(S-S), as measured by the separation between distal sulfur atoms) and guests of various lengths (l(g)), the framework architecture can be controlled systematically in a manner that enables regulation of the guest orientation and aggregation in the host framework. Inclusion compounds for which l(g)/l(S-S) ≤ 0.9 exhibit a bilayer architecture with 1-D channels containing guests oriented parallel to the long axis of the pillar. Guests with values of l(g) comparable to l(S-S), however, promote the formation of a brick architecture in which the guests and the pillar are arranged in a herringbone motif. Surprisingly, longer guests (l(g) = 1.25l(S-S)) favor the formation of the bilayer architecture despite their larger volume because the guests are forced to align end-to-end as single-file arrays due to the vertical constraints of the 1-D channels. Bithiophene and biphenyl guests (l(g)l(S-S)) are exceptional, promoting bilayer structures in which turnstile rotations of the pillars afford an unusual motif in which the guests are isolated from one another. The ability to synthesize a large family of compounds based on a common supramolecular building block (the GS sheet) permits construction of a structural "phase diagram" based on two simple molecular parameters, l(g) and l(S-S), that can be used to sort the inclusion compounds according to their framework architectures and enable prediction of crystal structures for new host-guest combinations. The effects of these different framework architectures and packing motifs is manifested as bathochromic shifts in the absorption and emission spectra of the guests compared with their spectra in methanol solutions. This behavior is supported by ab initio TDDFT calculations that reproduce the bathochromic shifts associated with the effects of guest-guest and guest-host interactions, combined with conformational constraints imposed on the guest molecules by the rigid host framework.

The combination of 2D 1H-13C and 1H-29Si solid state NMR, hyperpolarized 129Xe NMR, synchrotron X-ray diffraction, together with adsorption measurements of vapors and gases for environmental and energetic relevance, was used to investigate the structure and the properties of periodic mesoporous hybrid p-phenylenesilica endowed with crystalline order in the walls. The interplay of 1H, 13C, and 29Si in the 2D heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, revealed the spatial relationships (5 angstroms) among various spin-active nuclei of the framework. Indeed, the through-space correlations in the 2D experiments evidenced, for the first time, the interfaces of the matrix walls with guest molecules confined in the nanochannels. Organic-inorganic and organic-organic heterogeneous interfaces between the matrix and the guests were identified. The open-pore structure and the easy accessibility of the nanochannels to the gas phase have been demonstrated by highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution. Two-dimensional exchange experiments showed the exchange time to be as short as 2 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the nanochannel space and surface, a physisorption energy of 13.9 kJ mol-1, and the chemical shift value of xenon probing the internal surfaces. These results prompted us to measure the high storage capacity of the matrix towards benzene, hexafluorobenzene, ethanol, and carbon dioxide. Both host-guest, CH...pi, and OH...pi interactions contribute to the stabilization of the aromatic guests (benzene and hexafluorobenzene) on the extended surfaces. The full carbon dioxide loading in the channels could be detected by synchrotron radiation X-ray diffraction experiments. The selective adsorption of carbon dioxide (ca. 90 wt %) vs that of oxygen and hydrogen, together with the permanent porosity, high thermal stability, and high degree of order, makes this a suitable matrix for purifying hydrogen in clean-energy generation.

Powder neutron diffraction data at high resolution (HRPD instrument, ISIS Facility, U.K.) were collected on the lithium ion conductor α-LiZr2(PO4)3 at 673 K (a = 8.84448(3) A, c = 22.2875(1) A), and 873 K (a = 8.83667(4) A, c = 22.4177(2) A). Previous data measured at 423 K were included in the analysis. By Fourier maps and Rietveld refinement of the rhombohedral R3c crystal structure, it was shown that most lithium occupies the 6-fold disordered primary Li1 site, located off the symmetry center, at all temperatures. However, a 3-fold disordered secondary Li2 site was discovered, between two adjacent ZrO6 octahedra along the z axis, where a minor part of lithium is located. The fraction of lithium on Li2 increases with temperature from 10% (423 K) to 22% (873 K), and this may affect the mechanism of Li+ ion mobility. Li2 was not taken into account in the previous analysis of the 423 K data. Small but significant configurational changes of the Li1 and Li2 coordination environments occur between 423 and 87...

Novel channel-like inclusion compounds were formed by tris(o-phenylenedioxy)spirocyclotriphosphazene (TPP) with linear hydrocarbons. The molecular characterization was performed by 13C and 31P solid-state NMR under magic angle spinning conditions. The hexagonal symmetry of the host was verified in the 31P and 13C MAS spectra and by X-ray powder diffraction patterns. The new inclusion compounds present congruent meltings, and the temperatures increase with chain length. These compounds provide the opportunity to observe hydrocarbons confined as isolated chains in the solid state at temperatures 200 K above their melting points. Furthermore, the aliphatic chains are in the unusual state of being surrounded by aromatic rings displaced parallel to the channel. This geometry was proved by a through-space 1.4 ppm upfield shift on the 13C resonances of the guests, due to the aromatic ring current. The mobility at the chain ends and the propagation of the conformational defects within the guest molecules is discu...

The polymer−silica interaction in elastomeric materials composed of 1,4-cis-polyisoprene rubber and silica was studied by magic angle spinning NMR. The relaxation times in the hydrogen domain are informative about phases at the nanometer scale when a spin propagation phenomenon, due to strong dipolar couplings in the solid, prevails. However, magnetization diffusion is quenched in a rubbery phase where spin−spin relaxation rates are slow. The problem was overcome by promoting spin diffusion below the glass transition at low temperature. The interface was then observed under high-resolution conditions by the reciprocal points of view of 13C and 29Si nuclei. In particular, a fast transfer of magnetization from the polymer to the silica surface was observed by cross-polarization dynamics following the nonprotonated silicon atoms. Bidentate coupling agents and vulcanization promote a better interaction with the silica surface. The morphological modifications of the silica particles after processing could also...

The inclusion compounds of tris(2,3-naphthalenedioxy)cyclotriphosphazene (TNP) with several small molecules have been studied, for the first time, by 13C magic-angle spinning (MAS) NMR. The channel-type structure of TNP inclusion compounds (ICs) with benzene (and tetrahydrofuran, THF), and the cage-type structure of TNP ICs with p-xylene has been associated with the multiplicity of the 13C signals due to each carbon atom in the unit cell. Bending of the large paddles of the TNP matrix was established on the basis of the chemical shifts. The release of the guest molecules has been followed by differential scanning calorimetry and by solid-state NMR. The 13C cross-polarization (CP) MAS spectrum of guest-free TNP structure has been interpreted in accordance with the formation of the monoclinic cell. 31P CP MAS spectra reflect the symmetry of the crystal cell. A residual dipolar coupling with 14N is also observed. The dynamic behavior of the guest and host molecules has been studied by the measument of 13C T1...

Spin-diffusion NMR techniques were applied to the description of a heterogeneous interphase of composite materials formed by the hydration of calcium orthosilicate and poly(vinyl alcohol) (MDF cements). Hydrogen relaxation times were measured by magnetization transfer to 29Si and 13C atoms and indicate the degree of interaction between the organic and inorganic phases. Magnetization diffusion through the interfaces, measured by T1H and T1ρH relaxation times, seems effective, showing that the phases are intimately mixed and restricted defective regions are interposed between the bulk organic and the inorganic phases. On the other hand, some components of the hydrogen relaxation times, observed in both the 13C and the 29Si domain, identify an extended interphase. The distances travelled by the magnetization in homogeneous domains of the polymer are limited, being confined to the 4−90 nm range.

Poly(diethylphosphazene) (PDEP), contrary to other polyphosphazenes, is hardly soluble in standard organic solvents and presents a melting point 80 o C higher than that of the dimethyl analog. Stimulated by this specificity, we have investigated the polymorphic behavior of PDEP and its cyclic trimer by X-ray diffraction, MAS NMR, and differential scanning calorimetry. The crystal structure of the native room temperature form of the cyclic trimer has been solved. The resulting arrangement of the ethyl groups has provided the basis to interpret the MAS NMR spectra of the trimer itself and of the polymer in the native modification, which present remarkable similarities. The MAS NMR data show analogous averaging of the ethyl side-chain conformations in the modifications obtained heating samples above the transition temperature of both the trimer and the polymer. X-ray and thermal data indicate that extensive tridimensional order must be maintained in these forms. Both modifications of PDEP obtained after thermal treatment present very significant side-chain mobility, coupled, in one of them, with only minor perturbations of the main-chain conformation and of the packing as compared to the native form. These unusual features imply for the disordered phases of PDEP a model substantially differing from the hitherto reported polyphosphazenic mesophases, which may account for the specificity of crystalline PDEP

The MNDO potential energy surface for the reaction of acetaldehyde lithium enolate with acrolein was studied. Two water molecules were used to mimic ether solvent. Relative energies of stationary points were computed at the 3-21G//MNDO level. Four cyclic eight-membered transition structures 8-11 could be located, which allowed a qualitative rationalization of the observed stereoselectivity trends in the title reaction. The effects of enolate geometry and 3-enone substituent were evaluated at this level. Transition structure geometries were also optimized by ab initio molecular orbital calculations (3-21G) on the unsolvated structures 20-23. The ab initio optimized geometries were found to be very similar to the MNDO-optimized geometries. The relative energies of 20-23 were computed up to the MP2/6-31G*//3-21G level, which helped to explain the unrealistically high stereoselectivity predicted at the 3-21G//MNDO level.