Study of fast switching processes due to electric and magnetic fields-an NMR approach

Solid state NMR techniques have been developed to investigate dynamic molecular effects (e.g., molecular reorientations) due to simultaneously applied external electric fields on electrically sensitive materials such as liquid crystals (LC), liquid crystalline polymers (LCP) and polymeric electrets. Such effects can be observed only on relatively thin systems (10-200 μm). That means that many scans are necessary to achieve a sufficiently high signal-to-noise-ratio in the spectra (500-1000 scans). If the material is also magnetically sensitive, the electric field can be used to orient molecules in a starting orientational state and by switching-off the voltage to access fast reorientation processes in the magnetic field B₀. Until now, the behaviour of orientable molecular systems under the influence of electric fields has been investigated by means of a more or less quasistatic approach (LCP: 100 V, electrets: 1 kV) in equilibrium states. The achievable time resolution depends on the desired signal-to-noise-ratio. For the case of proton NMR this means a time resolution of about 10 min. However, very often switching processes occur on a much shorter time scale. Using conventional techniques it is impossible to observe fast (ca. 100 μs) electrically or magnetically induced reorientation processes. In this work, we present a concept to overcome the problems outlined above and to extend the area of our current in situ NMR investigations on thin electrically-switched or poled polymeric layers. The basic idea is to include synchronized electric pulses during the NMR experiment using the preparation and/or mixing periods of a 1D or 2D pulse sequence for the application of an orienting field (electric or magnetic) and to use the reversibility of the molecular switching phenomenon to achieve a sufficient signal-to-noise-ratio. The techniques extend the range of possible investigations from about 100 μs to approximately T₁ for correlated spectra (and to longer times of applied fields for uncorrelated spectra). Results are shown for a nematic LC and a nematic polymer having a similar side chain.     

Update on ignition studies at CEA

This article sums up the theoretical and experimental studies about ignition. Three experiments are salient this year on the Omega laser in collaboration with DOE laboratories (1) 3 cones of beams allow to mimic the LMJ configuration and to get symmetry measurements. (2) We measured perturbations due to hydro-instability in CHGe planar samples with face-on and side-on radiographs. (3) We improved our nuclear diagnostics, particularly the neutron image system tested on direct drive implosions. As far as LMJ target design is concerned, we defined a preliminary domain corresponding to the possible operation at 2ω. At 3ω we studied the low mode instability effects on the DT deformation (due to the laser or to the target) and on the yield. The stability is clearly improved with graded doped CH for our nominal capsule L1215.     

Engineering Soluble Diketopyrrolopyrrole Chromophore Stacks from a Series of Pd(II)-Based Ravels**

A strategy to engineer the stacking of diketopyrrolopyrrole (DPP) dyes based on non-statistical metallosupramolecular self-assembly is introduced. For this, the DPP backbone is equipped with nitrogen-based donors that allow for different discrete assemblies to be formed upon the addition of Pd(II), distinguished by the number of π-stacked chromophores. A Pd₃L₆ three-ring, a heteroleptic Pd₂L₂L′₂ ravel composed of two crossing DPPs (flanked by two carbazoles), and two unprecedented self-penetrated motifs (a Pd₂L₃ triple and a Pd₂L₄ quadruple stack), were obtained and systematically investigated. With increasing counts of stacked chromophores, UV/Vis absorptions red-shift and emission intensities decrease, except for compound Pd₂L₂L′₂, which stands out with an exceptional photoluminescence quantum yield of 51 %. This is extraordinary for open-shell metal containing assemblies and explainable by an intra-assembly FRET process. The modular design and synthesis of soluble multi-chromophore building blocks offers the potential for the preparation of nanodevices and materials with applications in sensing, photo-redox catalysis and optics.     

Application of electric fields in NMR

Anisotropic orientation states in materials such as liquid crystals, liquid crystalline polymers or polymeric electrets can be induced by the application of electric fields. This behaviour is exploited extensively in displays or sensors. Anisotropic optical properties induced by relatively high electric field strengths are used in the well known displays made from liquid crystalline materials. The anisotropy of electric properties in polymeric materials gives rise to properties such as the high piezoelectricity of fluorine containing electrets. Attempts have been made to investigate the orientational state induced by electric fields after poling using NMR techniques. However, there is only access to the irreversible part of the electrically induced orientation. In-situ application of (high) voltages during NMR experiments enables the investigation of the influence of the electric field on molecular orientational states or electrically-induced morphological changes. The NMR methods have been modified to investigate the dynamics of reorientation processes in electric fields and to extend the possible time range towards below1ms. The application of high voltages in NMR experiments requires experimental specifications. Some of our experiences are outlined in this paper.     

NMR investigations of polymer electrets

1H NMR investigations were performed on thin films of poly(vinylidene fluoride) (PVDF) and polyamide 11 (PA 11). The variation of the angle between the static magnetic B_o-field and preferred directions given by drawing and poling leads to characteristic dependences of T_20ff, which can be discussed in terms of Legendre polynomials of the orientational distribution functions of crystallographic c- and b-axis. A good c-axis alignment can be realized by uniaxial drawing. The tendency for b-axis to orient towards the direction of the applied electric field via the orientation of the electric dipoles is evident. The b-axis alignment can be “switched” periodically.