Hyperfine interaction of polyacetylene intercalated with iron halide anions: 57Fe Mössbauer and EPR spectroscopy studies

⁵⁷Fe Mössbauer spectroscopy and EPR spectroscopy were applied to study the chemical nature of the intercalant in polyacetylene oxidized (doped) chemically with FeCl₃ and FeBr₃. For the (CH) _x/FeCl₃ system the Mossbauer lattice temperature (θ_M = 89 K) was estimated from the temperature dependence of the recoil-free fraction. The strong influence of the high Fe^III state of the dopant is clearly evidenced through EPR spectra of both systems: while the chloride system shows a superposition of a narrow and a broad lines, the bromide one exhibits only a narrow line, whose intensity decreases with increasing dopant concentration.     

Reaction of tetraoxopyrrolo [2,3-d] pyrimidines,isatin analogues,with o-phenylenediamine

In the course of general studies on new antitumor tetracyclic intercalating agents, the condensation of o-phenylenediamine with two new tetraoxopyrrolo[2,3-d]pyrimidine derivatives has been carried out. The structure of the reaction products as arylketimines has been elucidated and mass spectral fragmentations are discussed.     

Improved procedure for the synthesis of enamine N-oxides

An improved procedure for the preparation of enamine N-oxides involving aminolysis of epoxides, chlorination, N-oxidation, and dehydrochlorination is described. Although isolated beta-chloroamine N-oxides are prone to rearrangements when isolated, these side reactions can be slowed by the presence of stabilizing organic acids. The scope and limitations of this strategy are discussed.     

EPR study of copper(II) complexes of hydroxysalen derivatives in order to be used in the DNA cleavage

Two series of functionalized hydroxy-salen-copper(II) complexes with various side chain lengths have been synthesized. The first one is characterised by amino side chain protected by the tert-butyloxycarbonyl group (Boc) whereas, the second series is obtained by removal of the Boc-protecting group under acidic conditions and formation instead of it an ammonium salt. EPR studies were carried out on the copper(II) complexes. EPR signals attributed to monomers and dimers of Cu2+ species were evidenced. Determination of the copper(II) environment in each complex was attempted using all the experimental results. Square planar and tetrahedral symmetries were found for the copper(II) monomers. From the fine structure observed for the pair signal, the distance between the Cu2+ ions in the pair has been calculated (3.9-4.3A). From these values, it seems that the formation of pairs is obtained by a face-to-face bimolecular association.     

Monolithic fluoride-fiber laser at 1480 nm using fiber Bragg gratings

We report what we believe is the first monolithic fluoride-fiber laser making use of fiber Bragg gratings written directly in the doped fluoride-fiber core. The Tm(3+):ZBLAN fiber laser is upconversion pumped at 1070 nm and emits at 1480 nm. Using two different all-fiber cavities, we observed a threshold as low as 75 mW and a conversion efficiency of up to 40% with respect to launched pump power.     

trans-Methylpyridine cyclen versus cross-bridged trans-methylpyridine cyclen. Synthesis, acid-base and metal complexation studies (metal = Co2+, Cu2+, and Zn2+)

The synthesis of the cross-bridged cyclen CRpy(2) {4,10-bis((pyridin-2-yl)methyl)-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane}, a constrained analogue of the previously described trans-methylpyridine cyclen Cpy(2) is reported. The additional ethylene bridge confers to CRpy(2) proton-sponge type behaviour which was explored by NMR and potentiometric studies. Transition metal complexes have been synthesized (by complexation of both ligands with Co(2+), Cu(2+) and Zn(2+)) and characterized in solution and in the solid state. The single crystal X-ray structures of [CoCpy(2)](2+), [CuCpy(2)](2+) and [ZnCpy(2)](2+) complexes were determined. Stability constants of the complexes, including those of the cross-bridged derivative, were determined using potentiometric titration data and the kinetic inertness of the [CuCRpy(2)](2+) complex in an acidic medium (half-life values) was evaluated by spectrophotometry. The pre-organized structure of the cross-bridged ligand imposes an additional strain for the complexation leading to complexes with smaller thermodynamic stability in comparison with the related non-bridged ligand. The electrochemical study involving cyclic voltammetry underlines the importance of the ethylene cross-bridge on the redox properties of the transition metal complexes.     

Heteroditopic receptor based on crown ether and cyclen units for the recognition of zwitterionic amino acids

The molecular recognition of five targeted amino acids differing in the nature of the side (R)- group and in the size of the aliphatic chain, glycine (Hgly), phenylalanine (Hphe), glutamic acid (Hglu^?), 4-aminobutyric acid (Hgaba), and 6-aminohexanoic acid (Heahx), has been studied with a new heteroditopic receptor based in two distinct macrocycles, a cyclen and a crown ether moiety. The bismacrocycle L was synthesized via the bis-aminal route allowing to obtain the designed compound in gram scale and in good yield. Protonation constants of L and its binding constants with amino acids were determined by potentiometry in H₂O/MeOH (1:1 v/v) solutions at 298.2 K and I=0.10 mol dm^?3 in NMe₄NO₃. Stronger binding ability of the H_nLⁿ⁺ receptor for α-amino acids, Hgly and Hphen, than for the other studied substrates were found. Structural data derived from NMR studies showed that the binding of α-amino acids result from the cooperative participation of hydrogen bonds between the carboxylate group of amino acids and the polyammonium sites of cyclen, and the ion-dipole interactions between the ammonium group of the amino acids and the oxygen atoms of the crown ether.     

In Situ Raman and Epr Investigations of A Cis-(Ch)X Electrode

Abstract

In situ Raman and EPR experiments have been performed on a cis-(CH)_x electrode in an electrochemical cell {(CH)_x/1M L1C10₄ in THF/Li}. After a n-type doping with Li, the Raman spectra exhibit new features at = 1600 cm^?1 and 1270 cm^?1, comparable to those obtained with [CH)_X films Li doped via a chemical treatment. The evolution of the EPR signal is followed in a doping-dedoping cycle, showing both the reversibility of the process as well as the evidence for a metallic behavior at the maximum doping concentration.     

Modeling the influence of the porosity of laser-ablated silicon films on their photoluminescence properties

Nanostructured porous Si-based films produced by pulsed laser ablation (PLA) from a silicon target in residual helium gas can exhibit both size-dependent (1.6-3.2 eV) and fixed photoluminescent (PL) bands (1.6 and 2.2 eV) with their relative contributions depending on the film porosity. We study the influence of prolonged oxidation in ambient air on properties of the fixed PL bands, associated with oxidation phenomena, and their correlation with structural properties of the films. In addition, we propose a model describing the appearance of surface radiation states for oxidized films of various porosities. Our experiments and numerical simulations led to a conclusion that the 1.6 eV PL is due to a mechanism involving a recombination through the interfacial layer between Si core and an upper oxide of nanocrystals. This mechanism gives the optimal porosity of 73% for the most efficient production of 1.6 eV PL centers that is in excellent agreement with our experimental results.     

A methodology to optimize the quantification of sp carbon fraction from K edge EELS spectra

A methodology is presented to select a consistent method, using electron energy loss spectroscopy (EELS), to extract the fraction of sp²-bonded carbon atoms in carbonaceous materials. According to this methodology, a reliable method has to conjointly fulfill two criteria. The first one consists in verifying, on a perfect graphite sample, that the experimental evolution of R = I_π*/(I_π* + I_σ*)-ratios is in good agreement with the one theoretically predicted as a function of experimental settings. The second criterion consists in measuring sp² fractions in amorphous carbon samples with a minimum of fluctuation. We test three commonly used R-extraction techniques, and we show that they exhibit some failures. We thus implement a more accurate R-extraction process that accounts for the predicted graphite R-evolution and exhibits a low intrinsic 4%-noise, as determined from the sp² fraction fluctuation. Moreover, we check the transferability of our method on a wide range of EELS spectra, acquired with different experimental resolutions on samples exhibiting various sp² contents.