Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Electron Paramagnetic Resonance Study of Radical Pairs and Isolated Radicals Trapped in Irradiated Long-Chain Hydrocarbons at Low Temperatures

The intrinsic characteristics of radical pairs produced in squalane and in cetane receiving high gamma-dose are extensively studied with the EPR technique at temperatures from 77°K up to 150°K. The spectra of the paired radicals occur at g=4 with a very low transition probability in contrast to that of isolated radicals which appear at g=2 A well-resolved hyperfine spectrum corresponding to the species (CH₃CH₂.CH₂CH₃) is observed in cetane. The isothermal decay rates of radical pairs in cetane below 100°K are significantly slow; however, the decay kinetics at 150°K is first order with rate constant=1.86 min^?1. A relatively slower decay rate is obtained for isolated radicals suggesting that the decay mechanism of paired radicals is through geminate recombination. The relative inter-radical distance in radical pairs is known from a decay curve as a function of temperature. The yields of radical pairs are low in both matrices, only few percents of those of isolated radicals. The formation mechanisms of paired radicals with direct radiolytic bond scission process are discussed in connection with the experimental observations.     

Synthesis, structure, and transformation of novel osmium azine and ylide complexes

Three novel triosmium complexes with unusual coordination characteristics are reported. Treatment of the hydridotriosmium cluster (mu-H)2Os3(CO)10 with CNNPPh3 in CH2Cl2 gave complexes (mu-H)Os3(CO)(10)(mu2-eta2-C(H)NNPPh3) (1) and (mu-H)Os3(CO)10(mu2-eta1-CHPPh3) (2). Complex 1 represents the first example of the existence of a coordinated phosphinazine ligand. An in-situ 1H NMR study showed that the reaction of (mu-H)2Os3(CO)10 with CNNPPh3 produced complex 1 as the initial product in 100% conversion. The latter is not stable in solution and slowly eliminates nitrogen to form an unusual ylide complex 2 in quantitative yield. The thermolysis of 2 in refluxing toluene afforded (mu-H)3Os3(CO)9(mu3-eta1-CCO2CH2Ph) (3) as a colorless compound. Complexes 1-3 were characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. The interesting feature of structure 3 is the presence of a mu3-alkylidyne ligand where the symmetrically triply bridged CCO2CH2Ph fragment lies perpendicular to and above the triosmium triangle.     

Synthesis and Antiproliferative Activity of Cyano and Amidino Substituted 2-Phenylbenzothiazoles

Series of cyano, dicyano, amidino, and diamidino substituted 2-phenylbenzothiazoles were prepared. Mono- and dicyano substituted benzothiazoles were obtained by condensation of appropriate substituted benzaldehydes with 2-aminothiophenol or 4-amino-3-mercaptobenzonitrile. The appropriate amidines or diamidines were prepared by Pinner reaction. The compounds were tested against breast, prostate, and lung cancer cell lines in a 72 h cytotoxicity assay. Many of the compounds had at 10 μM activity equivalent to 2-(4-aminophenyl)benzothiazole, while four compounds had significantly better activity, particularly in the breast cancer model.     

Reversing a rotaxane recognition motif: threading oligoethylene glycol derivatives through a dicationic cyclophane

An already well-established recognition motif-namely one in which the NH2+ centers in the rod sections of the dumbbell components of rotaxanes are encircled by macrocyclic polyether components-has been turned simultaneously outside-in and inside-out, a fact that has been proved beyond any doubt by the stoppering of both ends of a [2]pseudorotaxane to give a stable [2]rotaxane. The [2]pseudorotaxane is formed in nitromethane when a benzylic dibromide, obtained after reacting an excess of 1,4-bis(bromomethyl)benzene with hexaethylene glycol, is added to an equimolar amount of a dicationic cyclophane in which two -CH2OCH2- chains link a pair of dibenzylammonium ions through the para positions on their phenyl rings. When the [2]pseudorotaxane is reacted in nitromethane with triphenylphosphine, a [2]rotaxane and the corresponding free dumbbell compound are isolated in 58 and 31% yields, respectively. The structure of the [2]rotaxane is established by using mass spectrometry (FABMS and ESMS) and NMR (1H and 13C) spectroscopy in nitromethane-d3. The [2]rotaxane exhibits quite dramatic changes in the 1H chemical shifts of the signals for its CH2N+ and CH2O protons compared with those in the free dumbbell compound. The 1H NMR spectrum of the [2]pseudorotaxane shows many similar features. Titration experiments with three of the six different CH2O probes give an average Ka value of 2900 +/- 750 M-1 in nitromethane-d3. The new recognition motif for the template-directed synthesis of rotaxanes can now be exploited at both the molecular and macromolecular levels of structure with numerous potential applications in sight.     

Synthesis of an elusive,stable 2-azaallyl radical guided by electrochemical and reactivity studies of 2-azaallyl anions

The super electron donor (SED) ability of 2-azaallyl anions has recently been discovered and applied to diverse reactivity, including transition metal-free cross-coupling and dehydrogenative cross-coupling processes. Surprisingly, the redox properties of 2-azaallyl anions and radicals have been rarely studied. Understanding the chemistry of elusive species is the key to further development. Electrochemical analysis of phenyl substituted 2-azaallyl anions revealed an oxidation wave at E_1/2 or E_pa = −1.6 V versus Fc/Fc⁺, which is ∼800 mV less than the reduction potential predicted (E_pa = −2.4 V vs. Fc/Fc⁺) based on reactivity studies. Investigation of the kinetics of electron transfer revealed reorganization energies an order of magnitude lower than commonly employed SEDs. The electrochemical study enabled the synthetic design of the first stable, acyclic 2-azaallyl radical. These results indicate that the reorganization energy should be an important design consideration for the development of more potent organic reductants.

The super electron donor (SED) capabilities of 2-azaallyl anions has recently been discovered and applied to diverse reactivity; their structures and electron transfer characteristics are reported herein.     

Translational isomerism in a [3]catenane and a [3]rotaxane

[structure: see text] Post-assembly covalent modification using Wittig chemistry of [2]rotaxane ylides, wherein NH(2)(+) centers in the dumbbell-shaped components are recognized by dibenzo[24]crown-8 (DB24C8) rings, has afforded a [3]catenane and a [3]rotaxane with a precise and synthetically prescribed shortage of DB24C8 rings. The nondegenerate pairs of translational isomers present in both of these interlocked molecular compounds provide the fundamental platform on which to construct sensory devices and nanochemomechanical systems.     

Intramolecular 1,3-dipolar cycloaddition of cyclo-1,3-diene-tethered nitrile oxides

Intramolecular 1,3-dipolar cycloaddition of cyclo-1,3-diene- and -1,3,5-triene-tethered nitrile oxides gave tricyclic isoxazolines as a single stereoisomer in most cases. The relative stereochemistry of tricycle-fused isoxazolines resulting from 1,3-dipolar cycloaddition of cyclo-1,3-diene-tethered nitrile oxides is cis-cis, whereas from cyclohepta-1,3,5-triene-tethered nitrile oxides the cis-trans isomer predominates.     

ICP-MS for elemental speciation studies

The use of inductively coupled plasma mass spectrometry (ICP-MS) coupled with separation techniques for the purpose of elemental speciation has recently gained a lot of attention. Much of this is due to ever improving separation capabilities of Chromatographic techniques, the high sensitivity of ICP-MS, and the continuing development of better interface techniques. Additionally, there is a growing awareness of the need to monitor various species of an analyte, rather than just total analyte concentrations, due to their often varying natures. For the sake of learning from different elemental speciation approaches, this review brings together some selected types of elemental speciation which have been recently seen in literature. These include separations using various forms of liquid chromatography, such as reversed phase, reversed phase ion pairing, micelle, ion exchange, and size exclusion. Elemental speciation employing gas Chromatographie separations and supercritical fluid separations are discussed as well as elemental speciation using capillary electrophoresis.     

Palladium complexes with ethylene-bridged bis(N-heterocyclic carbene) for C-C coupling reactions

A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)₂ produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs.