An Asymmetric Edge-Sharing Bioctahedral Complex: (η2-DAniF)MoIII(μ-DAniF)2(μ-O,Cl)MoVCl2 (DAniF=N,N′-di-p-anisylformamidinate)

A novel asymmetric, edge-sharing bioctahedral complex with formamidinato ligands ( ²-DAniF)Mo(-DAniF)₂(-O,Cl)MoCl₂, 1, (DAniF=N,N-di-p-anisylformamidinate) has been isolated as a byproduct from the preparation of the dimolybdenum(II,III) compound Mo₂(DAniF)₃Cl₂. An X-ray crystallographic study shows that the structure consists of edge-sharing bioctahedra, with the shared edge defined by the vector joining the: -O and -Cl ligands. Compound 1 is best formulated as a mixed-valent Mo^IIIMo^V compound, the Mo(V) ion being that with the two terminal Cl^– ligands. The cyclic voltammogram of 1 shows a reversible reduction at –0.472 V and a reversible oxidation at 1.028 V vs. the Ag/AgCl reference electrode, while the absorption spectrum of 1 reveals an intense low energy absorption at 523 nm ( _max=12 500) which is attributed to an intervalence charge transfer transition. Crystallographic data for 1 are as follows: a=13.387(1) Å, b=15.500(2) Å, c=21.855(2) Å, =98.786(2)°, V=4481.8(8) ų, P2₁/c, R1 (wR2)=0.082 (0.177).     

Intramolecular charge transfer in pyrromethene laser dyes: photophysical behaviour of PM650.

Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media.     

Role of structural flexibility in the fluorescence and photochromism of salicylideneaniline: the general scheme of the phototransformations

The mechanism of light-induced transformation in the salicylideneaniline molecule was studied by semiempirical PM3 calculations. The structures and energies of the minima and saddle points (transition states) on the S₀, S₁ and T₁ potential energy hypersurfaces (PESs) were obtained, together with the gradient lines on the PESs. The structure-energy scheme was compared with the experimental findings. According to the results obtained, the following principle processes are observed: fast S₁ excited state intramolecular proton transfer (ESIPT), followed by typical ESIPT fluorescence; the formation of two S₁ twisted intramolecular charge transfer (TICT) structures which quench the ESIPT fluorescence; the diabatic formation of two ground state metastable coloured “post-TICT” structures responsible for photochromism.     

The characterization of silica microparticles by electrophoretic mobility measurements

Summary Electrophoretic mobility measurements in the pH 2‐10 range are described for several commercial HPLC silica microparticles and a laboratory-produced product. The content of metal impurities for the silicas was also determined by AAS. An acidic/hydrothermal treatment was used to generate a more homogenous surface for some of the silicas. The zero points of charge (zpc) for both a native and a treated silica plus several commercial HPLC silicas were compared. The electrophoretic mobility method may be useful in predicting the utility of certain types of silica supports for chromatographic separations.     

Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes

A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial reverse osmosis and nanofiltration membranes. Zeta potentials of these membranes were analyzed for aqueous solutions of various chemical compositions over a pH range of 2 to 9. In the presence of an indifferent electrolyte (NaCl), the isoelectric points of these membranes range from 3.0 to 5.2. The curves of zeta potential versus solution pH for all membranes display a shape characteristic of amphoteric surfaces with acidic and basic functional groups. Results with salts containing divalent ions (CaCl₂, Na₂SO₄, and MgSO₄) indicate that divalent cations more readily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used to investigate the effect of dissolved natural organic matter on membrane surface charge. Other solution chemistries involved in this investigation include an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge.     

Short Te …︁ Te bonding contacts in a new layered ternary telluride: Synthesis and crystal structure of 2D Nb3GexTe6 (x ≃ 0.9)

The new layered ternary compound Nb₃Ge_xTe₆ (x ? 0.90) was prepared by direct combination of the elements taken in the stoichiometric proportions 3 : 1 : 6, heated at 1 000 °C for 10 days in silica tubes and quenched to room temperature. The phase crystallizes in the orthorhombic symmetry, space group Pnma (#62), with the following single crystal refined parameters: a = 643.18(5) pm, b = 1391.98(11)pm and c = 1 154.07(5) pm, with Z = 4. The structure was refined to an R of 3.4% (R_w = 4.6%), with 1969 independent reflexions and 49 parameters. The structure is based on the close stacking of trigonal prismatic (TP) slabs in the AA/BB mode. The slabs can be seen as built up from face sharing biprisms, which are filled either by one or by two niobium cations situated in the middle of the trigonal prisms. The germanium is located in the middle of the common face of two prisms, leading to a rather unusual anionic square coordination. The refinements showed that this latter cation does not fill completely its square site. No cation was found in the van der Waals gap between the slabs. The mean d_{Ge? Te} distance (276.5 pm) is in agreement with Ge^II cations, while some Te …? Te distances (from 333.84 to 361.65pm) are too short for anions in a simple contact. These bonding distances, already mentionned in some MTe₂ compounds, are to be ascribed to charge transfer in the structure, with a partial oxidation state for the tellurium anions. Short Nb? Nb and Nb? Ge distances (292.0 and 281.3 pm, respectively) imply intercationic bonding within the slabs.     

Phase Relations in the Ni–Mn–Ga Ternary System and Aging Effect on Shape Memory Properties of Ferromagnetic Ni<Subscript>2</Subscript>MnGa Sputtered Films

Summary. Isothermal sections of the Ni–Mn–Ga ternary phase diagram at 1073 and 1273 K were investigated over a wide range of alloy compositions. The range of the β-Ni₂MnGa phase, its equilibria with the γ-(Mn, Ni), α′-Ni₃Ga, and γ-Ni₃Ga₂ phases, and the liquidus and solidus lines were determined experimentally. The aging effect on the shape memory effect (SME) of Ni₂MnGa sputtered films was also investigated. The two-way SME of the constraint-aged films was confirmed by the temperature change.     

新型陶瓷材料的光激励发光性质研究

针对以往电子俘获光存储材料存在的问题，利用高温固相反应发制备了一种Eu^2 掺杂的氟氧化物玻璃陶瓷，研究表明在该材料中具有光激励发光(PSL)特性，并可用于电子俘获光存储。XRD分析表明该材料含有大量的BaF2微晶。根据其与BaF2相似的发光特性，推断玻璃陶瓷的PSL有可能来源于微晶中Eu^2 的5d-4f发射。但是决定光激励发光过程中的电子(空穴)陷阱的存在方式、电子迁移途径都有待进一步探讨的问题。与传统的BaFCl：Eu相比，这种材料的光激励发光衰减速度较慢。     

Markov processes in single molecule fluorescence

This article examines the current status of Markov processes in single molecule fluorescence. For molecular dynamics to be described by a Markov process, the Markov process must include all states involved in the dynamics and the first-passage time (FPT) distributions out of those states must be describable by a simple exponential law. The observation of non-exponential FPT distributions or other evidence of non-Markovian dynamics is common in single molecule studies and offers an opportunity to expand the Markov model to include new dynamics or states that improve understanding of the system.