Near field and far field power distributions in a multimode graded fibre

It is shown in this paper that the near field and far field power distributions corresponding to any distribution of rays in an arbitrary graded fibre, reach a steady state along the fibre. This steady state is usually a function of the input distribution of rays and of the refractive index profile of the fibre. The case of parallel beam injection is considered and an analytical expression for the near field power distributions for any arbitrary profile is found. This suggests a new method of refractive index profile measurement which is free of leaky ray corrections and can be very sensitive near the optimum profile for time dispersion.     

On the Flexibility of Thioantimonate Networks: Solvothermal Syntheses and Crystal Structures of Six New Thioantimonates(III) with the [Sb4S7]2− Anion

Six new thioantimonates(III) with the [Sb₄S₇]²⁻ anion were obtained under solvothermal conditions with in‐situ formed transition metal complexes as structure directors. In the two isostructural compounds [Fe(dien)₂]Sb₄S₇·H₂O ( 1 ) and [Co(dien)₂]Sb₄S₇·0.5 H₂O ( 2 ) (dien = diethylenetriamine; space group: P2₁/c) the layered [Sb₄S₇]²⁻ anion is characterized by Sb₈S₈ rings with a diameter of about 9.6·7.6Å. The cation complexes are located above and below the pores of the rings. Despite the larger size of the cation complex the network topology of the third thioantimonate [Ni(dien)(tren)]Sb₄S₇ ( 3 ) (tren = tris(2‐aminoethyl)amine; space group: P2₁/n) is similar to that of the first two compounds. In the isostructural thioantimonates [M(trien)]Sb₄S₇ (M = Zn ( 4 ); M = Mn ( 5 ); trien = triethylenetetramine; space group: ) the M²⁺ ions are fivefold coordinated by four N atoms of the amine molecule and by one S atom of the thioantimonate anion forming a MN₄S trigonal bipyramid. Sb₈S₁₆ building blocks are the central structural motifs of the anion. Two of the terminal S atoms at the periphery of the Sb₈S₁₆ units are bound to M²⁺ ions and the four remaining terminal S atoms connect adjacent Sb₈S₁₆ groups into the final [Sb₄S₇]²⁻ chain. [Ni(tren)]Sb₄S₇ ( 6 ) (space group: ) contains a one‐dimensional anionic chain. The Ni²⁺ ion has two bonds to the [Sb₄S₇]²⁻ anion which is a unique feature in the thioantimonate(III) chemistry. The NiN₄S₂ octahedron is severly distorted with one very long Ni‐S bond of 2.782(2) Å. In all compounds several short S···H distances indicate hydrogen bonding interactions.     

The synthesis and natural distribution of the major ketone constituents in Echinacea pallida

The first synthesis of a series of ketones naturally occurring in E. pallida is described. The natural distribution of these ketones among different Echinacea species is also reported.     

Colossal magnetoresistance in Mn2+ oxypnictides NdMnAsO(1-x)F(x)

Colossal magnetoresistance is a rare phenomenon in which the electronic resistivity of a material can be decreased by orders of magnitude upon application of a magnetic field. Such an effect could be the basis of the next generation of memory devices. Here we report CMR in the antiferromagnetic oxypnictide NdMnAsO(1-x)F(x) as a result of competition between an antiferromagnetic insulating phase and a paramagnetic semiconductor upon application of a magnetic field. Mn(2+) oxypnictides are relatively unexplored, and tailored synthesis of novel compounds could result in an array of materials for further investigation and optimization.     

Comparison of excited-state energy transfer in arrays of hydroporphyrins (chlorins, oxochlorins) versus porphyrins: rates, mechanisms, and design criteria

A set of chlorin-chlorin and oxochlorin-oxochlorin dyads has been prepared with components in the same or different metalation states. In each case a 4,4'-diphenylethyne linker spans the respective 10-position of each macrocycle. The dyads have been studied using static and time-resolved absorption and emission spectroscopy, resonance Raman spectroscopy, and electrochemical techniques. Excited-state energy transfer from a zinc chlorin to a free-base (Fb) chlorin occurs with a rate constant of (110 ps)(-1) and an efficiency of 93%; similar values of (140 ps)(-1) and 83% are found for the corresponding oxochlorin dyad. Energy transfer in both dyads is slower and less efficient than found previously for the analogous porphyrin dyad, which displays a rate of (24 ps)(-1) and a yield of 99%. The slower rates and diminished efficiencies in the ZnFb chlorin and oxochlorin dyads versus the ZnFb porphyrin dyad are attributed to substantially weaker linker-mediated through-bond (TB) electron-exchange coupling (as indicated by resonance Raman data). Although the through-space (TS, i.e., dipole-dipole) coupling in the ZnFb-chlorin and -oxochlorin dyads is enhanced relative to the ZnFb porphyrin dyad (as indicated by F?rster calculations), this enhancement is insufficient to compensate for the greatly diminished TB coupling. Taken together, the chlorin and oxochlorin dyads examined herein serve as benchmarks for elucidating the energy-transfer, electrochemical, and other properties of light-harvesting arrays containing multiple chlorins or oxochlorins.     

The thermal dehydration and decomposition of Ba[Cu(C2O4)2(H2O)]·5H2O

The thermal behaviour of Ba[Cu(C₂O₄)₂(H₂O)]·5H₂O in N₂ and in O₂ has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (about 80°C), but continues until the onset of the decomposition (about 280°C). The decomposition takes place in two major stages (onsets 280 and 390°C). The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311±30 kJ mol^–1 (or 52±5 kJ (mol of H₂O)^–1). The overall enthalpy change for the decomposition of Ba[Cu(C₂O₄)₂] in N₂ was estimated from the combined area of the peaks of the DSC curve as –347 kJ mol^–1. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the -time curves could be described by the three dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125±4 kJ mol^–1 and (1.38±0.08)×10¹⁵ min^–1, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model withn=5. The other process was slow and could also be described by the contracting-geometry model, but withn=2.The values ofE _a andA were 206±23 kJ mol^–1 and (2.2±0.5)×10¹⁹ min^–1, respectively, for the fast process, and 259±37 kJ mol^–1 and (6.3±1.8)×10²³ min^–1, respectively, for the slow process.Dedicated to Prof. Menachem Steinberg on the occasion of his 65th birthday     

Norm-preserving extensions of functionals and denting points of convex sets

Let W and Z be Banach spaces, and let and be closed subspaces. Let be a subspace of , the Banach space of bounded linear operators from W* to Z**, containing . We describe, for and , all norm-preserving extensions of to the space in terms of convergence of convex combinations. We also characterize denting points of bounded convex subsets of Banach spaces in similar terms. Various applications are presented. Supported by Estonian Science Foundation Grant 5704.