Anke Hoffmann

Crystal structures and solid-state CPMAS 13C NMR correlations in luminescent zinc(II) and cadmium(II) mixed-ligand coordination polymers constructed from 1,2-bis(1,2,4-triazol-4-yl)ethane and benzenedicarboxylate

The hydrothermal reaction of M(NO(3))(2).4H(2)O (M = Zn and Cd) with benzene-1,4-dicarboxylic acid (H(2)bdc) or benzene-1,3-dicarboxylic acid (H(2)ip) and 1,2-bis(1,2,4-triazol-4-yl)ethane (btre) produced the mixed-ligand coordination polymers (MOFs) {[Zn(2)(micro(2)-bdc)(2)(micro(4)-btre)]} (), {[Cd(2)(micro(4)-bdc)(micro(4)-btre)(2)](NO(3))(2).H(2)O} and {[Zn(2)(micro(3)-ip)(2)(micro(2)-btre)(H(2)O)(2)].2H(2)O} (). The compounds, characterized by single-crystal X-ray diffraction, X-ray powder diffraction, solid-state cross-polarization (CP) magic-angle-spinning (MAS) (13)C NMR and thermoanalysis, feature 3D metal-organic frameworks for and and 2D double layers which are connected through hydrogen bonds from the aqua ligands for 3. The CPMAS (13)C NMR spectra picture the symmetry-independent (unique) C atoms and the bdc/ip-to-btre ligand ratio in agreement with the crystal structures. The zinc and cadmium coordination polymers show a strong bluish fluorescence upon excitation with UV light (the free btre ligand is non-luminescent).

Phase Biaxility in Smectic-A Side-Chain Liquid Crystalline Elastomers

(2) H NMR investigations on the biaxial phase behavior of smectic-A liquid crystalline side-chain elastomers are presented. Biaxiality parameters were determined by measuring the quadrupolar splitting of two spin probes, namely benzene-d(6) and hexamethylbenzene-d(18) , at various angles between the principal director and the external magnetic field: while for a uniaxial sample the angular dependence can be described by the second Legendre polynomial, an additional asymmetric term needs to be included to fit the data of the two investigated biaxial systems. Two elastomers synthesized from mesogens that differ in the molecular geometry in order to study the molecular origin of biaxiality were compared. Biaxiality is observed for both elastomers when approaching the glass transition, suggesting that the network dynamics dominate the formation of the biaxial phase.

Structure–solid-state CPMAS 13C NMR correlation in palladacycle solvates (pseudo-polymorphs) with a transformation from Z′ = 1 to Z′ = 2

The dinuclear µ-acetato-µ-benzophenone iminato palladium complex [{(C∧N)Pd}2(µ-OAc)(µ-NCPh2)] (1) [C∧N = N,N-dimethylbenzylamine-κN,κC)] is prepared by reaction of [{(C∧N)Pd(µ-OAc)}2] with benzophenone imine and [NBu4]OH in ethanol. The dinuclear palladacycle 1 can crystallize with different solvents molecules as 1·1.5C6H5CH3, 1·0.25C6H5CH3 (Z′ = 2), 1·1.5C6H6 and 1·C6H14 (n-hexane) upon hexane diffusion into a toluene, benzene or CH2Cl2 solution, respectively. The structure of 1·0.25C6H5CH3 with two palladacycle molecules and two partly occupied toluene molecules in the asymmetric unit (Z′ = 2) is a consequence of partial toluene solvent loss from 1·1.5C6H5CH3 (Z′ = 1) as was followed by solid-state CPMAS 13C NMR. The transformation from Z′ = 1 to Z′ = 2 (crystal “on the way”?) toluene solvate can proceed in a solid-state single-crystal-to-crystal transition as evidenced from multiple single-crystal X-ray diffraction studies, also when the crystals are still in their mother liquor. During this transformation the remaining toluene crystal solvent becomes “locked in” (immobile from static 2H (D) NMR, only lost above 80° from TGA) and the crystals of 1·0.25C6H5CH3 (Z′ = 2) remain crystalline in air in the absence of mother liquor or toluene, different from the other solvates. A rotational disorder of one of the benzene molecules in 1·1.5C6H6 (Z′ = 1) around its pseudo-six-fold axis is supported by the line-shape analysis of the static 2H (D) spectrum.

Size matters! On the way to ionic liquid systems without ion pairing.

Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium-sized [NTf2 ](-) (0.230 nm(3) ; Tf=CF3 SO3 (-) ) and the large [Al(hfip)4 ](-) (0.581 nm(3) ; hfip=OC(H)(CF3 )2 ) anions were synthesized and characterized. Their temperature-dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel-Fulcher-Tammann (VFT) behavior. Ion-specific self-diffusion constants were measured at room temperature by pulsed-gradient stimulated-echo (PGSTE) NMR experiments. In general, self-diffusion constants of both cations and anions in [Al(hfip)4 ](-) -based ILs were higher than in [NTf2 ](-) -based ILs. Ionicities were calculated from self-diffusion constants and measured bulk conductivities, and showed that [Al(hfip)4 ](-) -based ILs yield higher ionicities than their [NTf2 ](-) analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)4 ](-) -based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)4 ](-) . Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions.

Molecular dynamics and biaxiality of nematic polymers and elastomers

To gain insight into the origins of phase biaxiality in nematic polymers and elastomers the relation of molecular dynamics and biaxiality is studied. 2H-NMR investigations of various nematic polymers differing in the attachment geometry of the mesogens to the polymer (side-chain side-on, end-on and main-chain attachment) are presented. The mesogens are 2H-labelled at specific positions of their spacers and in the aromatic core. The NMR experiments are supplemented by dielectric relaxation measurements of polymers labelled with dipole groups. Only minor differences are found for the molecular dynamics of polymers and elastomers and comparing the various labelling positions suggests that the mesogenic moiety rotates as a whole. We find broad biaxial nematic phases for all investigated systems and a slowing down of the rotational diffusion around the mesogens′ long axes upon approaching the glass transition. It seems that in nematic polymers and elastomers phase biaxiality sets in when the rotational diffusion around the long axis gets hindered enough to allow biaxial nematic order. Comparing different attachment geometries we find that the side-on attachment of the mesogens seems to hinder the rotation of the molecules around their long axes considerably more than the other geometries.

Anisotropic Ionic Mobility of Lithium Salts in Lamellar Liquid Crystalline Polymer Networks

New mesogens presenting smectic A (SmA) phases and capable of hosting lithium salts are designed. The mesogens comprise a vinyl-functionalized spacer to allow further reaction to the polymer backbone, an aromatic core and ethylene oxide chains, able to coordinate lithium ions. Copolymerizing these monomers with a suitable crosslinker yields the first lithium containing liquid crystalline elastomers (LCEs). The SmA structure where the ethylene oxide chains are microphase separated in layers is fixed by the crosslinking and permanent macroscopic orientation is obtained. Diffusion and conductivity measurements of the monomer sample show a large anisotropy of the ion mobility (100 for the cation and 400 for the anion). In the elastomer the anisotropy of the lithium mobility is comparable to that in the monomers.

Inhibiting Polysulfide Shuttle in Lithium–Sulfur Batteries through Low-Ion-Pairing Salts and a Triflamide Solvent

The step-change in gravimetric energy density needed for electrochemical energy storage devices to power unmanned autonomous vehicles, electric vehicles, and enable low-cost clean grid storage is unlikely to be provided by conventional lithium ion batteries. Lithium-sulfur batteries comprising lightweight elements provide a promising alternative, but the associated polysulfide shuttle in typical ether-based electrolytes generates loss in capacity and low coulombic efficiency. The first new electrolyte based on a unique combination of a relatively hydrophobic sulfonamide solvent and a low ion-pairing salt, which inhibits the polysulfide shuttle, is presented. This system behaves as a sparingly solvating electrolyte at slightly elevated temperatures, where it sustains reversible capacities as high as 1200-1500 mAh g-1 over a wide range of current density (2C-C/5, respectively) when paired with a lithium metal anode, with a coulombic efficiency of >99.7 % in the absence of LiNO3 additive.

Influence of the mesogenic shape on the molecular dynamics and phase-biaxiality of liquid crystal main-chain polymers

Ever since the first theoretical phase diagram that was published by Straley in 1974, extensive theoretical efforts have been made on predicting the molecular shape and properties of mesogens that can form a biaxial-nematic phase. The theories suggest that mesogens, whose form is board-like rather than rod-like, are more likely to form biaxial phases. In this study we show that the onset of phase-biaxiality for nematic main-chain polymers can indeed, without significantly changing the glass transition temperature, be shifted to considerably higher temperatures by laterally broadening the mesogenic moieties. The investigations were performed by means of 2H-NMR spectroscopy on directly deuterated polymers and yield results that differ considerably from what could hitherto be observed in spin-probe measurements on side-chain polymers with similarly broadened mesogens.

Orientation behaviour of the minor director of homeotropically oriented nematic elastomers in mechanical fields

We report the synthesis of a thin nematic elastomer film where the major director of the mesogenic moieties (i.e. the average orientation of the main molecular axes) is permanently oriented parallel to the films normal. This allows us to directly observe the orientation of the minor director (average orientation of the minor molecular axes) using optical techniques. We find that the elastomer exhibits spontaneous biaxiality at room temperature and the minor director is oriented in large domains. When a mechanical field is applied perpendicular to the major director, the domains of the minor director reorient into the direction of strain above a characteristic threshold, forming a monodomain with 3D orientational long-range order. This reorientation process is accompanied by a change in the slope of the stress–strain curve and X-ray scattering experiments reveal that the reorientation of the major director only sets in after the reorientation of the minor director is complete.

Synthesis, Crystal Structure, and Properties of Bi3TeBO9 or Bi3(TeO6)(BO3): A Non-Centrosymmetric Borate-Tellurate(VI) of Bismuth

Pale-yellow single crystals of the new borate tellurate(VI) Bi3 TeBO9 were obtained by reaction of stoichiometric amounts of Bi2 O3 , B2 O3 , and Te(OH)6 at 780 °C. The non-centrosymmetric crystal structure (P63 , Z=2, a=8.7454(16), c=5.8911(11) Å, 738 refl., 43 param, R1=0.037, wR2=0.093) contains isolated trigonal-planar BO3 units and nearly undistorted TeO6 octahedra. The Bi3+ cations are located in between in octahedral voids. The BiO6 octahedra are significantly distorted to a [3+3] pattern (2.25/2.50 Å) due to the ns2 configuration. According to the structural features, the formula can be written as Bi3 (TeO6 )(BO3 ). Alternatively, the structure can also be described as hcp of oxygen with TeVI and BiIII in octahedral voids and BIII in trigonal- planar voids. The vibrational spectra show the typical features of BO3 and TeO6 units with a significant 10 B/11 B isotopic splitting of the IR-active B-O valence mode (1248 and 1282 cm-1 ). The UV/Vis spectrum shows an optical band edge with an onset around 480 nm (2.6 eV). MAS-NMR spectra of 11 B show an anisotropic signal with a quadrupole coupling constant of CQ =2.55 MHz. and a very small deviation from rotational symmetry (η=0.2). The isotropic chemical shift is 20.1 ppm. The second harmonic generation (SHG) test was positive with an activity comparable to potassium dihydrogen phosphate (KDP). Bi3 TeBO9 decomposes in air at 825 °C to Bi2 TeO5 .