473 research outputs found
Halogen bonding stabilizes a cis-azobenzene derivative in the solid state: A crystallographic study
Crystals of trans- and cis-isomers of a fluorinated azobenzene derivative have been prepared and characterized by single-crystal X-ray diffraction. The presence of F atoms on the aromatic core of the azobenzene increases the lifetime of the metastable cis-isomer, allowing single crystals of the cis-azobenzene to be grown. Structural analysis on the cis-azobenzene, complemented with density functional theory calculations, highlights the active role of the halogen-bond contact (N...I synthon) in promoting the stabilization of the cis-isomer. The presence of a long aliphatic chain on the azobenzene unit induces a phase segregation that stabilizes the molecular arrangement for both the trans- and cis-isomers. Due to the rarity of cis-azobenzene crystal structures in the literature, our paper makes a step towards understanding the role of non-covalent interactions in driving the packing of metastable azobenzene isomers. This is expected to be important in the future rational design of solid-state, photoresponsive materials based on halogen bonding. We show by single-crystal X-ray diffraction studies and computational analysis that halogen bonding can stabilize a metastable cis-azobenzene derivative in the solid state
Coordination networks incorporating halogen-bond donor sites and azobenzene groups
Two Zn coordination networks, [Zn(1)(Py)2]2(2-propanol)n (3) and [Zn(1)2(Bipy)2](DMF)2n (4), incorporating halogen-bond (XB) donor sites and azobenzene groups have been synthesized and fully characterized. Obtaining 3 and 4 confirms that it is possible to use a ligand wherein its coordination bond acceptor sites and XB donor sites are on the same molecular scaffold (i.e., an aromatic ring) without interfering with each other. We demonstrate that XBs play a fundamental role in the architectures and properties of the obtained coordination networks. In 3, XBs promote the formation of 2D supramolecular layers, which, by overlapping each other, allow the incorporation of 2-propanol as a guest molecule. In 4, XBs support the connection of the layers and are essential to firmly pin DMF solvent molecules through Iâ‹ŻO contacts, thus increasing the stability of the solvated systems
Efficient light-induced phase transitions in halogen-bonded liquid crystals
Here, we present a new family of light-responsive, fluorinated supramolecular liquid crystals (LCs) showing efficient and reversible light-induced LC-to-isotropic phase transitions. Our materials design is based on fluorinated azobenzenes, where the fluorination serves to strengthen the noncovalent interaction with bond-accepting stilbazole molecules, and increase the lifetime of the cis-form of the azobenzene units. The halogen-bonded LCs were characterized by means of X-ray diffraction, hot-stage polarized optical microscopy, and differential scanning calorimetry. Simultaneous analysis of light-induced changes in birefringence, absorption, and optical scattering allowed us to estimate that <4% of the mesogenic units in the cis-form suffices to trigger the full LC-to-isotropic phase transition. We also report a light-induced and reversible crystal-to-isotropic phase transition, which has not been previously observed in supramolecular complexes. In addition to fundamental understanding of light-responsive supramolecular complexes, we foresee this study to be important in the development of bistable photonic devices and supramolecular actuators
Suitability of high-pressure xenon as scintillator for gamma ray spectroscopy
In this paper we report the experimental study of high-pressure xenon used as
a scintillator, in the context of developing a gamma ray detector. We measure a
light yield near 2 photoelectrons per keV for xenon at 40 bar. Together with
the light yield, we also measured an energy resolution of ~9% (FWHM) at 662
keV, dominated by the statistical fluctuations in the number of photoelectrons.Comment: 15 pages, 11 figure
First operation and performance of a 200 lt double phase LAr LEM-TPC with a 40x76 cm^2 readout
In this paper we describe the design, construction, and operation of a first
large area double-phase liquid argon Large Electron Multiplier Time Projection
Chamber (LAr LEM-TPC). The detector has a maximum drift length of 60 cm and the
readout consists of a cm LEM and 2D projective anode to
multiply and collect drifting charges. Scintillation light is detected by means
of cryogenic PMTs positioned below the cathode. To record both charge and light
signals, we have developed a compact acquisition system, which is scalable up
to ton-scale detectors with thousands of charge readout channels. The
acquisition system, as well as the design and the performance of custom-made
charge sensitive preamplifiers, are described. The complete experimental setup
has been operated for a first time during a period of four weeks at CERN in the
cryostat of the ArDM experiment, which was equipped with liquid and gas argon
purification systems. The detector, exposed to cosmic rays, recorded events
with a single-channel signal-to-noise ratio in excess of 30 for minimum
ionising particles. Cosmic muon tracks and their -rays were used to
assess the performance of the detector, and to estimate the liquid argon purity
and the gain at different amplification fields.Comment: 23 pages, 21 figure
Effects of High Charge Densities in Multi-GEM Detectors
A comprehensive study, supported by systematic measurements and numerical
computations, of the intrinsic limits of multi-GEM detectors when exposed to
very high particle fluxes or operated at very large gains is presented. The
observed variations of the gain, of the ion back-flow, and of the pulse height
spectra are explained in terms of the effects of the spatial distribution of
positive ions and their movement throughout the amplification structure. The
intrinsic dynamic character of the processes involved imposes the use of a
non-standard simulation tool for the interpretation of the measurements.
Computations done with a Finite Element Analysis software reproduce the
observed behaviour of the detector. The impact of this detailed description of
the detector in extreme conditions is multiple: it clarifies some detector
behaviours already observed, it helps in defining intrinsic limits of the GEM
technology, and it suggests ways to extend them.Comment: 5 pages, 6 figures, 2015 IEEE Nuclear Science Symposiu
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