Effect of temperature on the luminescence processes of SrAl2O4:Eu

The UV excited and persistent luminescence properties as well as thermoluminescence (TL) of Eu²⁺ doped strontium aluminates, SrAl₂O₄:Eu²⁺ were studied at different temperatures. Two luminescence bands peaking at 445 and 520 nm were observed at 20 K but only the latter at 295 K. Both Sr-sites in the lattice are thus occupied by Eu²⁺ but at room temperature efficient energy transfer occurs between the two sites. The UV excited and persistent luminescence spectra were similar at 295 K but the excitation spectra were different. Thus the luminescent centre is the same in both phenomena but excitation processes are different. Two TL peaks were observed between 50 and 250 °C in the glow curve. Multiple traps were, however, observed by preheating and initial rise methods. With longer delay times only the high temperature TL peak was observed. The persistent luminescence is mainly due to slow fading of the low temperature TL peak but the step-wise feeding process from high temperature traps is also probable.     

Microscopically structural studies of lithium niobate powders

On the basis of the crystallographic characteristics of lithium niobate (LN) crystals, Law of Bravais and Pauling's third rule (i.e. Polyhedral Sharing Rule) are employed with the aim to find the relationship between the crystal structure and morphological faces of LN powders. In order to validate our analytical results, we have successfully synthesized LN powders and measured the corresponding X-ray powder diffraction. Our results show that the structural analysis is consistent with the experimental data and is helpful and effective for us to control the single-crystal growth and to design superstructures at the specific plane, starting from the viewpoint of the microscopic behaviors of constituent chemical bonds and polyhedra in the crystallographic frame.     

Influence of temperature-pressure treatment on heavily hydrogenated silicon surface

Microstructure and related properties of hydrogenated silicon samples, Si:H, treated at high-temperature (HT) up to 1270 K under hydrostatic argon pressure (HP) up to 1.1 GPa are investigated. To prepare Si:H, Czochralski grown 0 0 1 oriented single crystalline Si wafer with 50 nm thick surface SiO₂ layer was heavily implanted with hydrogen using the immersion plasma source of hydrogen ions with energy 24 keV.The surface of HT-HP treated Si:H was characterised by scanning electron microscopy. Reflectivity pattern measurements in the wavelength range of 350-2000 nm have been performed to analyse their surface and bulk properties. The volume averaging method for a model of layer-like structure has been used to simulate the HT-HP treated Si:H. The analysis of Si:H samples suggests the multi-layer structure composed of Si, Si:H, SiO, SiO₂, and of porous Si layers in the sub-surface region. The porous Si:H samples model is in good consistency with experimental data from reflectance measurements.     

Structure and vibrational spectra of the bis(betaine)-selenic acid molecular crystal

The crystal structure of bis(betaine)-selenic acid has been determined by X-ray diffraction as orthorhombic, space group Pbca, with a = 11.591(2), b = 22.930(5), c = 12.045(2) Å and Z = 8. The crystal comprises hydrogen selenate ions, HSeO⁻₄, and two distinct betaine molecules, which are held together into a complex by short hydrogen bonds. One of the betaine molecules is present as the zwitterion form (CH₃)₃N⁺---CH₂---COO⁻ and the second occurs as the protonated form (CH₃)₃N⁺---CH₂---COOH. Powder FTIR and Raman spectra were measured. An assignment of the observed bands to vibrations of the hydrogen bonds and internal vibrations of the hydrogen selenate ion and the betaine molecules is proposed.     

9-acetoxyanthracene derivatives. Part VIII. The mechanism of degradation of 9-acetoxy-10-arylanthracenes to anthraquinone

The mechanism of degradation of 9-acetoxy-10-arylanthracenes to anthraquinose with the use of HNO₃ in AcOH has been reported. This was proved by way of example on 9-acetoxy-10-(4′-acetoxyphenyl )anthracene (1) by means of separation and identification of intermediate products $\text{2}$, $\text{3}$ and $\text{4}$.     

Vibrational spectra and stereochemistry of mono- and polysaccharides. III. β-methyl-d-glucoside, α-methyl-d-glucoside,and β-methyl-d-xyloside

The IR and Raman spectra of β- and α-methyl-D-glucosides and β-methyl-D-xyloside are compared. Experimental data are correlated with theoretical calculations of the frequencies of normal vibrations. Predominant contributions (20% and more) of particular CiOi and CiC(i+1) bonds to the potential energy distribution of normal vibrations (i.e., localization of vibrations on these bonds, which form skeletons of monosaccharide molecules) are estimated. This approach is used to interpret the main distinctions between the IR and Raman spectra of the test compounds. The use of vibrational spectra in selective analysis of certain functional groups is discussed. It is shown that replacing the hydroxyl group at C1 has a specific effect on the predominant localization of vibrational modes in particular skeletal bonds (mainly in C1O1) of the monosaccharide molecules. B. I. Stepanov Institute of Physics, Belarus Academy of Sciences. V. Tshebyatovski Institute of Low Temperatures and Structural Studies, Polish Academy of Sciences. Translated fromZhurmal Strukturnoi Khimii, Vol. 36, No. 3, pp. 443–455. May–June, 1995. Translated by I. Izvekova.     

The synthesis and the solvent and substituent effect on the spectroscopic characteristic of 3-ethyl-2-(p-substitued styryl)benzothiazolium iodides

The photophysical and photochemical properties of p-substitued 2-styryl-ethylbenzothiazolium iodides, possessing different electron-withdrawing or electron-donating groups are described. The dyes were prepared by the condensation of 3-ethyl-2-methylbenzothiazole salts with p-substituted benzaldehydes. The synthesis of suitable substrates is presented as well. We describe here the absorption, emission spectra and the luminescence quantum yield of hemicyanine dyes (SH) measured in 11 different organic solvents of varying polarity. Molecular structure of the synthesized dyes was established by (1)H NMR, electronic absorption and fluorescence spectrometry. The spectral data confirmed that all the compounds exist in E-configuration of their styryl residues. The planar molecular conformation is typical for the compounds with five-membered side aromatic moieties (for example benzothiazole). The compounds possessing N-alkyl substituent in phenyl ring, in contrast to the compounds with other substituents, exhibit low fluorescence quantum yield in THF solution. This indicates that for N-alkyl derivatives the non-radiative processes are much more effective than the radiative ones. The electronic absorption and fluorescence emission spectra of tested dyes demonstrate high sensitivity to the nature of substituent introduced into the aromatic ring.     

Implications of active site constraints on varied DNA polymerase selectivity

DNA polymerase selectivity often varies significantly depending on the DNA polymerase. The origin of this varying error propensity is elusive. It is assumed that DNA polymerases form nucleotide binding pockets that differ in properties such as shape and tightness. We tested this prediction and studied HIV-1 RT by employment of size-augmented nucleotides and site-directed mutagenesis of the enzyme. New valuable insights into the mechanism of DNA polymerase fidelity were obtained. The presented study provides experimental evidence that variations of steric constraints within the nucleotide binding pocket of at least two DNA polymerases cause variations in nucleotide incorporation selectivity. Thus, our results support the concept of active site tightness as a causative in differential fidelity among DNA polymerases.     

Malatoplatinum(II) complexes – carboplatin analogs

A series of new neutral dicarboxylatoplatinum(II) complexes, containing D(+), L(−) or DL-malate dianion and ethylenediamine, 1-ethylimidazole or 1-propylimidazole as amine ligands, have been synthesized and characterized by elemental analysis, t.l.c., conductance, i.r., n.m.r., mass spectra (m/z) and electronic spectra. The new cytotoxic malatoplatinum(II) complexes are very soluble in water and resist hydrolysis. Their reactivity against glutathione is comparable with the reactivity of carboplatin, which suggests that these new complexes may be promising candidates in the search for low toxicity platinum cytostatics. This revised version was published online in June 2006 with corrections to the Cover Date.     

Bicyclic [b]-heteroannulated pyridazine derivatives. 4. Cyclization reactions of 4-aryltetrahydropyridazine-3,6-dione 3-hydrazones with some keto esters

Ethoxycarbonylalkylidene derivatives 2 and 6 of the title hydrazones were obtained in the reaction with ethyl pyruvate or ethyl aroylformate and ethyl acetoacetate, respectively, in methanol. Both compounds were mixtures of geometric isomers with high predominance of one of them. Nmr spectroscopy revealed an unexpected magnetic non-equivalence of the CH₂ protons in the ester ethyl group of the major isomer of 6 . On heating (?200°) in an inert medium or on refluxing in ethanolic sodium ethoxide 2 cyclized to the corresponding pyridazino[6,1-c]-triazines 4 , whereas 6 formed pyrazolylpyridazines 7 . The structure of the latter was unambigously established by X-ray analysis. Alkylation of 4a with benzyl bromide in the presence of tetrabutylammonium bromide occurred selectively on the pyridazine N atom.