Spectroscopic Identification of Disordered Molecular Cations in Defect Perovskite‐like ALnLn(HCO2_2)(C2_2O4_4)1.5_{1.5} (LnLn = Tb‐Er) Phases

This work reports a new series of A$Ln$(HCO$_2$)(C$_2$O$_4$)$_{1.5}$ (A = [(CH$_3$)$_2$NH$_2$]$^+$ and Ln$^{3+}$ = Tb$^{3+}$-Er$^{3+}$) compounds made solvothermally. These ${Cmce}$ phases combine monovalent and divalent ligands, which enables a scarce combination of A$^+$ and B$^{3+}$ cations in a hybrid perovskite-like compound. The ratio of ligands leads to ordered anion vacancies, which alternate with oxalate linkers along the $c$-axis. The A-site cations are disordered and cannot be identified crystallographically, likely a result of the larger pores of these frameworks compared to the recently reported AEr(HCO$_2$)$_2$(C$_2$O$_4$) phases. Neutron and infrared spectroscopy, supported by elemental composition, enables these cations to be identified as [(CH$_3$)$_2$NH$_2$]$^+$ molecules. Magnetic property measurements suggest these materials have weak antiferromagnetic interactions but remain paramagnetic to 1.8 K

Intercalation and confinement of poly(ethylene oxide) in porous carbon nanoparticles with controlled morphologies

Polymers confined at the nanometer scale often exhibit a distinct structural and dynamical response compared to their bulk counterparts. In this study, we observe that the confinement of poly(ethylene oxide) (PEO) in the nanopores of carbon nanoparticles (CNPs) leads to the suppression of crystallization and to a significant reduction of the Cp at the glass transition. We ask whether these changes are dominated by interfacial interactions (van der Waals type) or by geometrical constraints. For pore diameters below 2 nm (micropores following IUPAC nomenclature), we find that the larger the pore surface, the higher the amount of PEO intercalated in the micropores and, consequently, the larger the reduction of the Cp at the glass transition (up to 50%). For pore diameters in the range 2-50 nm (mesopores), larger pore surfaces lead to a higher amount of PEO adsorbed on the mesopore walls and the smaller the reduction of the Cp at the glass transition. Under these conditions of spatial confinement at the nanoscale, PEO chains cannot arrange themselves into large crystalline domains, as evidenced by a negligible degree of crystallization of at most 1.8%. High-resolution inelastic neutron scattering data show that the PEO chains confined in the pores of CNP adopt a planar zigzag conformation, which is distinctly different from those characteristic of the 7/2 helical structure of the bulk crystal.The authors gratefully acknowledge the support of the Spanish MEC (MAT2012-39199-C02-02 and MAT2012-31088), the Basque Government (IT-654-13), and the UK Science and Technology Facilities Council for the provision of beam time on the TOSCA spectrometer. P.P. acknowledges a PhD research contract from UCM (BE45/10). F.F.A. and S.R. acknowledge financial support from the UK Science and Technology Facilities Council.Peer Reviewe

Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based Metal-Organic Framework

We show clear experimental evidence of co-operative terahertz (THz) dynamics observed below 3 THz (~100 cm-1), for a low-symmetry Zr-based metal-organic framework (MOF) structure, termed MIL-140A [ZrO(O2C-C6H4-CO2)]. Utilizing a combination of high-resolution inelastic neutron scattering and synchrotron radiation far-infrared spectroscopy, we measured low-energy vibrations originating from the hindered rotations of organic linkers, whose energy barriers and detailed dynamics have been elucidated via ab initio density functional theory (DFT) calculations. For completeness, we obtained Raman spectra and characterized the alterations to the complex pore architecture caused by the THz rotations. We discovered an array of soft modes with trampoline-like motions, which could potentially be the source of anomalous mechanical phenomena, such as negative linear compressibility and negative thermal expansion. Our results also demonstrate coordinated shear dynamics (~2.5 THz), a mechanism which we have shown to destabilize MOF crystals, in the exact crystallographic direction of the minimum shear modulus (Gmin).Comment: 10 pages, 6 figure

Influence of solvent on poly(2-(dimethylamino)ethyl methacrylate) dynamics in polymer-concentrated mixtures: a combined neutron scattering, dielectric spectroscopy and calorimetric study

We have investigated the dynamical processes-α-relaxation, local motions of the side-groups, and methyl group rotations-in poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) in the dry state and in mixtures (at 70 wt% polymer concentration) with tetrahydrofuran (THF) and water, to address the question as to how these polymer motions are affected by plasticizers interacting in different ways with the polymer. Differential scanning calorimetry, dielectric spectroscopy, and neutron scattering techniques on labeled samples (with deuterated solvents to isolate the signal of the polymer component) have been combined. The α-relaxation is drastically affected, with similar shifts of the glass-transition temperature for both solvents. Effects of compositional heterogeneities and reduction of the fragility are also observed. On the contrary, methyl-group dynamics are unaffected by the presence of solvent. Regarding side-group local motions (β-relaxation), two kinds of components-a slow and a fast one-could be identified in the dry state. On the basis of the spatial information provided by neutron scattering, a model for the geometry of the motions involved in the fast component has been proposed. Adding solvent, this process would remain essentially unaltered, but the population involved in the slower one would be reduced. With THF as solvent, this reduction would be complete, but with water it would be only partial. This could be attributed to rather heterogeneous distribution of water molecules in the polymer likely associated with the presence of water clusters. Such a scenario would also explain the much more pronounced broadening of the glass-transition region observed for the polymer in the aqueous mixture with respect to that induced by THF.Financial support from the Projects MAT2012-31088 (Spanish MINECO and EU) and IT-654-13 (Basque Government) is acknowledged. This work is based on experiments performed at FOCUS (SINQ, Paul Scherrer Institute, Villigen, Switzerland), and at TOFTOF and SPHERES (Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany), and has been supported by the European Commission under the seventh Framework Programme through the “Research Infrastructures” action of the “Capacities” Programme, NMI3-II Grant Number 283883.Peer Reviewe

Structural dynamics of a metal-organic framework induced by CO2 migration in its non-uniform porous structure

Stimuli-responsive behaviors of flexible metal-organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO -adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility

Structural dynamics of a metal-organic framework induced by CO2 migration in its non-uniform porous structure.

Stimuli-responsive behaviors of flexible metal-organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO2-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO2 migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility

From PEF to PBF: what difference does the longer alkyl chain make a computational spectroscopy study of poly (butylene 2,5-furandicarboxylate)

This work explores the conformational preferences and the structureproperty correlations of poly(butylene 2,5-furandicarboxylate) (PBF), a longer chain analogue of the most well-known biobased polyester from the furan family, poly(ethylene 2,5-furandicarboxylate) (PEF). A thorough computational spectroscopic study–including infrared, Raman and inelastic neutron scattering spectroscopy, combined with discrete and periodic density functional theory calculations–allowed the identification of dominant structural motifs in the amorphous and crystalline regions. Discrete calculations and vibrational spectroscopy of semi-crystalline and amorphous samples strongly support the predominance of gauche, trans, gauche conformations of the butylene glycol fragment in both the crystalline and amorphous domains. In what concerns the furandicarboxylate fragment, amorphous domains are dominated by syn,syn conformations, while in the crystalline domains the anti,anti forms prevail. A possible crystalline structure–built from these conformational preferences and including a network of C-H···O hydrogen bond contacts—was optimized using periodic density functional theory. This proposed crystal structure avoids the unrealistic structural features of the previously proposed X-ray structure, provides an excellent description of the inelastic neutron scattering spectrum of the semi-crystalline form, and allows the correlation between microscopic structure and macroscopic properties of the polymer.publishe