Effect of self-fields on the electron cyclotron maser instability in a dielectric loaded waveguide

The influence of self-fields on the cyclotron maser instability in a hollow electron beam propagating parallel to a uniform axial magnetic field B ₀ ê _z in a dielectric loaded waveguide is investigated. The theoretical analysis is carried out within the framework of linearized Vlasov-Maxwell equations. It is assumed that the beam is thin with the radial thickness much smaller than the beam radius. A new dispersion relation for azimuthally symmetric electromagnetic perturbation is derived and analyzed numerically. The influence of self-fields on the cyclotron maser instability in a dielectric loaded waveguide for different dielectric medium is studied. It is found that unlike the hollow waveguide the growth rate is increased by increasing self-fields. The instability band width decreases due to the increasing self-fields. The maximum growth rate increases gradually as self-fields increase as regards a different dielectric medium.     

Coulomb screening in graphene with topological defects

We analyze the screening of an external Coulomb charge in gapless graphene cone, which is taken as a prototype of a topological defect. In the subcritical regime, the induced charge is calculated using both the Green’s function and the Friedel sum rule. The dependence of the polarization charge on the Coulomb strength obtained from the Green’s function clearly shows the effect of the conical defect and indicates that the critical charge itself depends on the sample topology. Similar analysis using the Friedel sum rule indicates that the two results agree for low values of the Coulomb charge but differ for the higher strengths, especially in the presence of the conical defect. For a given subcritical charge, the transport cross-section has a higher value in the presence of the conical defect. In the supercritical regime we show that the coefficient of the power law tail of polarization charge density can be expressed as a summation of functions which vary log periodically with the distance from the Coulomb impurity. The period of variation depends on the conical defect. In the presence of the conical defect, the Fano resonances begin to appear in the transport cross-section for a lower value of the Coulomb charge. For both sub and supercritical regime we derive the dependence of LDOS on the conical defect. The effects of generalized boundary condition on the physical observables are also discussed.     

Hexameric MnII Dendrimer as MRI Contrast Agent

A Mn^II chelating dendrimer was prepared as a contrast agent for MRI applications. The dendrimer comprises six tyrosine‐derived [Mn(EDTA)(H₂O)]^2? moieties coupled to a cyclotriphosphazene core. Variable temperature ¹⁷O NMR spectroscopy revealed a single water co‐ligand per Mn^II that undergoes fast water exchange (k_ex=(3.0±0.1)×10⁸ s^?1 at 37 °C). The 37 °C per Mn^II relaxivity ranged from 8.2 to 3.8 mM ^?1 s^?1 from 0.47 to 11.7 T, and is sixfold higher on a per molecule basis. From this field dependence a rotational correlation time was estimated as 0.45(±0.02) ns. The imaging and pharmacokinetic properties of the dendrimer were compared to clinically used [Gd(DTPA)(H₂O)]^2? in mice at 4.7 T. On first pass, the higher per ion relaxivity of the dendrimer resulted in twofold greater blood signal than for [Gd(DTPA)(H₂O)]^2?. Blood clearance was fast and elimination occurred through both the renal and hepatobiliary routes. This Mn^II containing dendrimer represents a potential alternative to Gd‐based contrast agents, especially in patients with chronic kidney disease where the use of current Gd‐based agents may be contraindicated.     

Ultrafast Photoswitching in a Copper‐Nitroxide‐Based Molecular Magnet

Molecular compounds with photoswitchable magnetic properties have been intensively investigated over the last decades due to their prospective applications in nanoelectronics, sensing and magnetic data storage. The family of copper‐nitroxide‐based molecular magnets represents a new promising type of photoswitchable compounds. We report the first study of these appealing systems using femtosecond optical spectroscopy. We unveil the mechanism of ultrafast (<50 fs) spin state photoswitching and establish its principal differences compared to other photoswitchable magnets. On this basis, we propose potential advantages of copper‐nitroxide‐based molecular magnets for the future design of ultrafast magnetic materials.     

Stress-Induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn³⁺ compound [Mn(3,5-diBr-sal₂(323))]BPh₄, 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress-induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc-P1 transition     

Photoswitchable Sol–Gel Transitions and Catalysis Mediated by Polymer Networks with Coumarin-Decorated Cu24L24 Metal–Organic Cages as Junctions

Photoresponsive materials that change in response to light have been studied for a range of applications. These materials are often metastable during irradiation, returning to their pre-irradiated state after removal of the light source. Herein, we report a polymer gel comprising poly(ethylene glycol) star polymers linked by Cu₂₄L₂₄ metal–organic cages/polyhedra (MOCs) with coumarin ligands. In the presence of UV light, a photosensitizer, and a hydrogen donor, this “polyMOC” material can be reversibly switched between Cu^II, Cu^I, and Cu⁰. The instability of the MOC junctions in the Cu^I and Cu⁰ states leads to network disassembly, forming Cu^I/Cu⁰ solutions, respectively, that are stable until re-oxidation to Cu^II and supramolecular gelation. This reversible disassembly of the polyMOC network can occur in the presence of a fixed covalent second network generated in situ by copper-catalyzed azide-alkyne cycloaddition (CuAAC), providing interpenetrating supramolecular and covalent networks.     

Metalla‐Assembled Electron‐Rich Tweezers: Redox‐Controlled Guest Release Through Supramolecular Dimerization

Developing methodologies for on‐demand control of the release of a molecular guest requires the rational design of stimuli‐responsive hosts with functional cavities. While a substantial number of responsive metallacages have already been described, the case of coordination‐tweezers has been less explored. Herein, we report the first example of a redox‐triggered guest release from a metalla‐assembled tweezer. This tweezer incorporates two redox‐active panels constructed from the electron‐rich 9‐(1,3‐dithiol‐2‐ylidene)fluorene unit that are facing each other. It dimerizes spontaneously in solution and the resulting interpenetrated supramolecular structure can dissociate in the presence of an electron‐poor planar unit, forming a 1:1 host–guest complex. This complex dissociates upon tweezer oxidation/dimerization, offering an original redox‐triggered molecular delivery pathway.     

2D Oligosilyl Metal–Organic Frameworks as Multi‐state Switchable Materials

We report the synthesis of a set of 2D metal–organic frameworks (MOFs) constructed with organosilicon‐based linkers. These oligosilyl MOFs feature linear Si_nMe_2n(C₆H₄CO₂H)₂ ligands (lin‐Si_n, n=2, 4) connected by Cu paddlewheels. The stacking arrangement of the 2D sheets is dictated by van der Waals interactions and is tunable by solvent exchange, leading to reversible structural transformations between many crystalline and amorphous phases.     

Atroposelective Synthesis of Axially Chiral N‐Arylpyrroles by Chiral‐at‐Rhodium Catalysis

A transformation of fluxional into configurationally stable axially chiral N‐arylpyrroles was achieved with a highly atroposelective electrophilic aromatic substitution catalyzed by a chiral‐at‐metal rhodium Lewis acid. Specifically, N‐arylpyrroles were alkylated with N‐acryloyl‐1H‐pyrazole electrophiles in up to 93 % yield and with up to >99.5 % ee, and follow‐up conversions reveal the synthetic utility of this new method. DFT calculations elucidate the origins of the observed excellent atroposelectivity.     

Rate constants for the gas-phase reaction of ozone with 1, 1-disubstituted alkenes

The gas-phase reaction of ozone with eight alkenes including six 1,1-disubstituted alkenes has been investigated at ambient T (285–298 K) and p = 1 atm. of air. The reaction rate constants are, in units of 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹, 9.50 ± 1.23 for 3-methyl-1-butane, 13.1. ± 1.8 for 2-methyl-1-pentene, 11.3 ± 3.2 for 2-methyl-1,3-butadiene (isoprene), 7.75 ± 1.08 for 2,3,3-trimethyl-1-butene, 3.02 ± 0.52 for 3-methyl-2-isopropyl-1-butene, 3.98 ± 0.43 for 3,4-diethyl-2-hexene, 1.39 ± 17 for 2,4,4-trimethyl-2-pentene, and >370 for (cis + trans)-3,4-dimethyl-3-hexene. For isoprene, results from this study and earlier literature data are consistent with: k (cm³ molecule⁻¹ s⁻¹) = 5.59 (+ 3.51, &minus 2.16) × 10⁻¹⁵ e^{(−3606±279/RT)}, n = 28, and R = 0.930. The reactivity of the other alkenes, six of which have not been studied before, is discussed in terms of alkyl substituent inductive and steric effects. For alkenes (except 1,1-disubstituted alkenes) that bear H, CH₃, and C₂H₅ substituents, reactivity towards ozone is related to the alkene ionization potential: In k<(10⁻¹⁸ cm³ molecule⁻¹ s⁻¹) = (32.89 ± 1.84) − (3.09 ± 0.20) IP (eV), n = 12, and R = 0.979. This relationship overpredicts the reactivity of C_≥3 1-alkenes, of 1,1-disubstituted alkenes, and of alkenes with bulky substituents, for which reactivity towards ozone is lower due to substituent steric effects. The atmospheric persistence of the alkenes studied is briefly discussed. © 1996 John Wiley & Sons, Inc.