The E166 experiment: Development of an undulator-based polarized positron source for the international linear collider

A longitudinal polarized positron beam is foreseen for the international linear collider (ILC). A proof-of-principle experiment has been performed in the final focus test beam at SLAC to demonstrate the production of polarized positrons for implementation at the ILC. The E166 experiment uses a 1 m long helical undulator in a 46.6 GeV electron beam to produce a few MeV photons with a high degree of circular polarization. These photons are then converted in a thin target to generate longitudinally polarized e ⁺ and e ⁻. The positron polarization is measured using a Compton transmission polarimeter. The data analysis has shown asymmetries in the expected vicinity of 3.4% and ∼1% for photons and positrons respectively and the expected positron longitudinal polarization is covering a range from 50% to 90%.      

Supramolecular helical columns from the self-assembly of chiral rods

Chiral-bridged rod molecules (CBRs) that consisted of bis(penta-p-phenylene) conjugated to an opened or closed chiral bridging group as a rigid segment and oligoether dendrons as flexible segments were synthesized and characterized. In the bulk state, both molecules self-assemble into a hexagonal columnar structure, as confirmed by X-ray scatterings and transmission electron microscopy (TEM) observations. Interestingly, these structures display opposite Cotton effects in the chromophore of the aromatic unit in spite of the same chirality (R,R) of the chiral bridging groups. The molecules were observed to self-assemble into cylindrical micellar aggregates in aqueous solution, as confirmed by light scattering and TEM investigations, and exhibit intense signals in the circular dichroism (CD) spectra, which are indicative of one-handed helical conformations. The CD spectra of each molecule showed opposite signals to each other, which were similar to those in the bulk. Notably, when the opened CBR was added to a solution of closed CBRs up to a certain concentration, the CD signal of the closed CBR was amplified. This implies that both molecules co-assemble into a one-handed helical structure because the opened chiral bridge is conformationally flexible, which is inverted to co-assemble with the closed CBR. These results demonstrate that small structural modifications of the chiral moiety can transfer the chiral information to a supramolecular assembly in the opposite way.     

Using noncovalent intra-strand and inter-strand interactions to prescribe helix formation within a metallo-supramolecular system

The effect of inter-strand and intra-strand interactions is explored in a metallo-supramolecular system in which the metal-ligand coordination requirements may be satisfied by more than one different supramolecular architecture. This is achieved by introducing alkyl substituents onto the spacers of readily prepared bis(pyridylimine) ligands. The alkyl substituents induce twisting within the ligand strand and this intra-strand effect favours formation of helical architectures. The alkyl substituents also introduce inter-strand CH.pi interactions into the system. For the smaller methyl group these are most effectively accommodated in a trinuclear circular helicate architecture. A solution mixture of dinuclear double-helicate and trinuclear circular helicate results from which, for copper(I), the trinuclear circular helicate crystallises. The CH.pi interactions endow the circular helicate with a bowl-shaped conformation and the triangular unit aggregates into a tetrahedral ball-shaped array. Low-temperature NMR studies indicate that the CH.pi interactions also confer a bowl-shaped conformation on the triangle in solution. The larger ethyl groups can sustain intra-strand CH.pi interactions in the lower nuclearity double-helical system and this is the unique architecture for that ligand system in both solution and the solid state. Crystal structures are described for both the copper(I) and silver(I) complexes. Thus we show that intra-strand interactions may be used to induce helicity within this system, while the nuclearity of the array can be prescribed by the inter-strand interactions.     

Relationship between the helical structure and optical activity of some chiral cyclic esters

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Modulated phases and upsilon points in a spin model with helical ordering

The ground state of a one-dimensional, classicalXY model with competing nearest- and next-nearest-neighbor interactions, a sixfold anisotropy, and an external field is studied. The model shows various modulated phases, depending on the values of the model parameters. Evidence is found that the ground-state phase diagram of this model containsupsilon points, multicritical points of a new class recently discussed by Bassler, Sasaki, and Griffiths. The phase diagram has a self-similar structure near these points, filled with multitudes of mixed phases whose spin configurations consist of segments of helical and fan structures separated by interfaces between them.     

A versatile ditopic ligand system for sensitizing the luminescence of bimetallic lanthanide bio-imaging probes

The homoditopic ligand 6,6'-[methylenebis(1-methyl-1H-benzimidazole-5,2-diyl)]bis(4-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}pyridine-2-carboxylic acid) (H(2)L(C2)) has been tailored to self-assemble with lanthanide ions (Ln(III)), which results in the formation of neutral bimetallic helicates with the overall composition [Ln(2)(L(C2))(3)] and also provides a versatile platform for further derivatization. The grafting of poly(oxyethylene) groups onto the pyridine units ensures water solubility, while maintaining sizeable thermodynamic stability and adequate antenna effects for the excitation of both visible- and NIR-emitting Ln(III) ions. The conditional stability constants (log beta(23)) are close to 25 at physiological pH and under stoichiometric conditions. The ligand triplet state features adequate energy (0-phonon transition at approximately 21 900 cm(-1)) to sensitize the luminescence of Eu(III) (Q=21 %) and Tb(III) (11 %) in aerated water at pH 7.4. The emission of several other VIS- and NIR-emitting ions, such as Sm(III) (Q=0.38 %) or Yb(III) (0.15 %), for which in cellulo luminescence is evidenced for the first time, is also sensitized. The Eu(III) emission spectrum arises from a main species with pseudo-D(3) symmetry and without coordinated water. The cell viability of several cancerous cell lines (MCF-7, HeLa, Jurkat and 5D10) is unaffected if incubated with up to 500 microM [Eu(2)(L(C2))(3)] during 24 h. Bright Eu(III) emission is seen for incubation concentrations above 10 microM and after a 15-minute loading time; similar images are obtained with Tb(III) and Sm(III). The helicates probably permeate into the cytoplasm of HeLa cells by endocytosis. The described luminescent helical stains are robust chemical species which remain undissociated in the cell medium and in presence of other complexing agents, such as edta, dtpa, citrate or L-ascorbate. Their derivatization, which would open the way to the sensing of targeted in cellulo phenomena, is currently under investigation.     

一维螺旋链状配位聚合物[Cu(p-BDOA)(2,2′-bipy)·H_2O]_n的合成与表征(英文)

0IntroductionIn recent years many interests have been focusedon the self-assembly of supramolecular architectures,especially on the coordination polymers with rigid or-ganic aromatic carboxylic acid ligands such as tereph-thalate and benzene tetracarboxyl…     

A new coordination polymer exhibiting unique 2D hydrogen-bonded (H2O)16 ring formation and water-dependent luminescence properties

A new coordination polymer, [Zn(dpe)(bdc)]·4H(2)O (ZndB; dpe=1,2-bis(4-pyridyl)ethane, bdc(2-)=dianion of benzenedicarboxylic acid), which possesses a 3D metal-organic framework (MOF) has been synthesized and structurally characterized. This 3D MOF is constructed by the assembly of helical channels filled with guest water molecules in both inner and outer regions of the channel. The resulting network also creates a 2D water layer containing hydrogen-bonded (H(2)O)(16) rings as the basic building units. Thermogravimetric and powder X-ray diffraction measurements of ZndB revealed a two-step weight loss of water molecules with a reversible water adsorption/desorption process in the inner channel for the first stage, and irreversible water desorption in the outer channel for the second stage. This spongelike property is manifested by the excimer emission originating from interaction between dpe (π*) and the other dpe (π) of the proximal helical channel, which is highly sensitive to the environmental perturbation. Powder X-ray analyses reveal that the dehydration process induces the readjustment of dpe π-π stacking distance/orientation, which results in dramatic luminescence changes from dim pale blue (λ(em)≈470 nm) upon hydration to bright white-light generation (broad, λ(em)≈500-550 nm) upon water depletion, accompanied by a ≈100-fold increase in the emission intensity.