4-amino- and 4-nitrodipicolinatovanadium(V) complexes and their hydroxylamido derivatives: synthesis, aqueous, and solid-state properties

A number of 4-substituted, dipicolinatodioxovanadium(V) complexes and their hydroxylamido derivatives were synthesized to characterize the solid state and solution properties of five- and seven-coordinate vanadium(V) complexes. The X-ray crystal structures of Na[VO2dipic-NH2].2H2O (2) and K[VO2dipic-NO2] (3) show the vanadium adopting a distorted, trigonal-bipyramidal coordination environment similar to the parent coordination complex, [VO2dipic]- (1), reported previously as the Cs+ salt. The observed differences in the chemical shifts of the complexes both in the 1H (ca. 0.7-1.4 ppm) and 51V (ca. 1-11 ppm) NMR spectra were consistent with the electron-donating or electron-withdrawing properties of the substituent groups, respectively. Stoichiometric addition of a series of hydroxylamine ligands (H2NOH, MeHNOH, Me2NOH, and Et2NOH) to complexes 1-3 led to the formation of seven-coordinate vanadium(V) complexes. The X-ray crystal structure of [VO(dipic)(Me2NO)(H2O)].0.5H2O (1c) was found to be similar to the previously characterized complexes [VO(dipic)(H2NO)(H2O)] (1a) and [VO(dipic)(OO-tBu)(H2O)]. While only slight differences in the 1H NMR spectra were observed upon addition of the hydroxylamido ligand, the signals in the 51V NMR spectra change by up to 100 ppm. The addition of the hydroxylamido ligand increased the complex stability of complexes 2 and 3. Evidence for a nonstoichiometric redox reaction was found for the monoalkyl hydroxylamine ligand. The reaction of an unsaturated five-coordinate species with a hydroxylamine to form a seven-coordinate vanadium complex will, in general, dramatically increase the amounts of the vanadium compound that remain intact at pH values near neutral.     

Synthesis and spectroscopic characterisation of BODIPY based fluorescent off-on indicators with low affinity for calcium

Two fluorescent off-on Ca2+ indicators based on APTRA (o-aminophenol-N,N,O-triacetic acid) as low-affinity ligand for Ca2+ and BODIPY(4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) as a fluorophore were synthesized. The new BODIPY-APTRA compounds absorb in the visible spectrum, with absorption maxima from 505 nm to 570 nm, and have fluorescence spectra that span the visible spectrum, with emission maxima ranging from 525 nm to 625 nm dependent on the substituents at the alpha-positions to the nitrogen atoms. The indicators show a large increase of the fluorescence quantum yield upon increasing Ca2+ concentration. The ground-state dissociation constants Kd estimated at 20 degrees C in 100 mM KCl aqueous buffered solution, pH 7.20, for the two complexes with Ca2+ were found to be around 100 microM.     

Two new mononuclear cobalt(II) complexes of pyrazole-based ligands: synthesis,structures and magnetic studies

We have synthesized two mononuclear cobalt(II) complexes (1 and 2) of pyrazole-based bidentate (NN) and tridentate (NNN) tripodal ligands. X-ray crystal structure determination reveals that complex 1 has a tetrahedral geometry, while complex 2 has a trigonal–bipyramidal geometry. Both the complexes have been characterized by variable-temperature magnetic measurements between 2 and 300 K. A weak ferromagnetic exchange interaction (J = +1.5 cm^?1) is observed for complex 2. Due to the presence of supramolecular CH···Cl and π···π interactions, a good magnetostructural correlation was found between the D parameter and angular distortion (δ) for complex 1 and related complexes reported in the literature.     

On some of Ramanujan's Schläfli-type “mixed” modular equations

We give elementary proofs of seven Schläfli-type “mixed” modular equations recorded by Ramanujan on p. 86 of his first notebook. Previously, these equations were proved by Berndt by using the theory of modular forms. In the process, we also found three new Schläfli-type mixed modular equations of the same nature.     

Density functional theory of polymer-polymer phase separation behavior

Polyatomic density functional theory is applied to a binary polymer blend. The polymer reference interaction site model (PRISM) liquid state theory provides the homogeneous state correlation functions necessary for the application of density functional theory. An effective chi parameter can be recognized from the density functional expression; however, the phase separation criteria does not depend solely upon the chi parameter, rather it depends upon various combinations of the species-dependent direct correlation functions of the blend. The Flory-Huggins chi parameter along with the associated phase diagram is obtained when the monomer volumes of the blend species are equal and for a range of monomer-monomer attractive interactions. Calculations are performed both with and without the assumption of incompressibility. The density functional theory along with the PRISM determined “input” predict that an isotopic polymer blend shows an upper critical solution temperature (UCST) phenomena. © 1995 John Wiley & Sons, Inc.     

Q-Switched Fiber Laser with a Hafnium-Bismuth-Erbium Codoped Fiber as Gain Medium and Sb2Te3 as Saturable Absorber

Journal of Russian Laser Research - We successfully demonstrate Q-switched fiber laser with a 20 cm long Hafnium-Bismuth-Erbium codoped fiber (HBEDF) as an active medium in conjunction with...     

Evolution and defect analysis of vertical graphene nanosheets

We report catalyst‐free direct synthesis of vertical graphene nanosheets (VGNs) on SiO₂/Si and quartz substrates using microwave electron cyclotron resonance – plasma enhanced chemical vapor deposition. The evolution of VGNs is studied systematically at different growth stages. Raman analysis as a function of growth time reveals that two different disorder‐induced competing mechanisms contributing to the defect band intensity. The VGNs grown on SiO₂/Si substrates predominantly consists of both vacancy‐like and hopping defects. On the other hand, the VGNs grown on quartz substrates contain mainly boundary‐like defects. X‐ray photoemission spectroscopy studies also corroborate Raman analysis in terms of defect density and vacancy‐like defects for the VGNs grown on SiO₂/Si substrates. Moreover, the grown VGNs exhibit a high optical transmittance from 95% to 78% at 550 nm and the sheet resistance varies from 30 to 2.17 kΩ/sq. depending on growth time. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamics of solid-melt front migration from a reaction–diffusion model

Using physical experiments we investigated the evolution of thermally driven melt patterns in a semi-infinite solid crystalline phase subjected to uniform heating from one side, maintaining melting temperature. We treat the melt initiation phenomenon theoretically in the perspective of two-phase interactions on the microscopic level, and propose a new reaction–diffusion model based on the prey–predator dynamics. This model predicts the fractal behavior of melt fronts observed in the experiments.