Experimental and theoretical studies of lattice distortions caused by charged 3d impurities in II–IV semiconductors

The results of studies of photoinduced lattice vibration modes associated with charged 3d impurities (in particular, Ni) in II–IV semiconductors (ZnSe, ZnO) and solid solutions ZnSSe and ZnCdSe are presented. Experimental optical exciton-vibration spectroscopy studies revealed that the local vibration modes are coupled, which is due to strong anharmonicity of the local vibrations. Analysis of phonon replicas of the zerophonon line in the spectra allows one to elucidate the nature of the anharmonicity. Model calculations performed for ZnSe: Ni and ZnO: Ni crystals revealed that the nearest neighbor environment of a charged Ni impurity is highly distorted. A change in the charged state of Ni leads to shifts in lines of calculated and experimentally measured x-ray emission spectra and, therefore, should be taken into account.     

Thermally induced spin transitions in nitroxide-copper(II)-nitroxide spin triads studied by EPR

Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in exchange-coupled spin triads of nitroxide-copper(II)-nitroxide. The values of exchange interactions in spin triads of studied systems are typically on the order of tens to hundreds of inverse centimeters. The large magnitude of exchange interaction determines the specific and very informative peculiarities in EPR spectra due to the predominant population of the ground state of a spin triad and spin exchange processes. The variety of these manifestations depending on structure and magnetic properties of spin triads are described. EPR is demonstrated as an efficient tool for the characterization of spin transitions and for obtaining information on the temperature-dependent sign and value of the exchange interaction in strongly coupled spin triads.     

Structures of the racemate and (S)-enantiomer of 7,8-difluoro-3-methyl-2,3-dihydro-4H-[1,4]benzoxazine

The comparative X-ray diffraction study of racemic and (S)-enantiomer of 7,8-difluoro-3-methyl-2,3-dihydro-4H-[1,4]benzoxazine was carried out. The both forms of benzoxazine are crystallized in the orthorhombic crystal system, the (S)-enantiomer is crystallized in the chiral space group P2₁2₁2₁, while the racemate is crystallized in the centrosymmetric Pbca space group. The bond lengths and bond and torsional angles in the both molecules are almost equal. The packing of the racemate is characterized by a closer interaction of polar NH…O groups.     

Thermodynamic and NMR studies of mixed-ligand complex formation of cadmium ethylenediaminetetraacetate with diamines in an aqueous solution

The mixed-ligand complex formation in the systems Cd2 + Edta4–(CH₂) _n (NH₂)₂, n = 2 (En), 6 (L) has been NMR and calorimetrically studied in aqueous solution at 298.15 K and the ionic strength of I = 0.5 (KNO₃). The thermodynamic parameters of formation of the CdEdtaL²⁻, CdEdtaHL⁻, (CdEdta)₂L⁴⁻, and (CdEdta)₂En⁴⁻ complexes have been determined. The most probable coordination mode for the complexone and the diamine ligand in the mixed-ligand complexes was discussed.     

Proton-conducting gel electrolytes based on poly(methyl methacrylate) doped with phosphoric acid in <Emphasis Type="Italic">N,N</Emphasis>-dimethylformamide

Time-stable proton-conducting gel electrolytes based on poly(methyl methacrylate) doped by o-phosphoric acid solutions of different concentrations in DMF are prepared. The dependences of the electrical conductivity of the gels on the concentrations of the acid and polymer and the molecular mass of PMMA at 25–65°C are studied. The thermal stability of electrolytes is estimated. It is found that an increase in the content of the polymer in gels changes the thermal behavior of the samples.     

Self-trapping of the <Emphasis Type="Italic">d</Emphasis>-<Emphasis Type="Italic">d</Emphasis> charge transfer exciton in bulk NiO evidenced by X-ray excited luminescence

Soft X-ray (XUV) excitation did make it possible to avoid the predominant role of the surface effects in luminescence of NiO and revealed a bulk luminescence with a puzzling well isolated doublet of very narrow lines with close energies near 3.3 eV which is assigned to recombination transitions in self-trapped d-d charge transfer (CT) excitons formed by coupled Jahn-Teller Ni⁺ and Ni³⁺ centers. The conclusion is supported both by a comparative analysis of the CT luminescence spectra for NiO and solid solutions Ni _x Zn_{1 − x} O, and by a comprehensive cluster model assignment of different p-d and d-d CT transitions, their relaxation channels. To the best of our knowledge, it is the first observation of the luminescence due to self-trapped d-d CT excitons.     

Excitons and photoluminescence in ZnO and Zn0.99Mn0.01O nanocrystals

The photoluminescence and photoluminescence excitation spectra for Zn_1?x Mn _x O nanocrystals are presented. After annealing of powders in air, the intensity of the bands attributable to manganese decreases noticeably. This suggests that the oxygen vacancies affect the Zhang-Rice-like states appearing due to strong d-p-hybridization, which is confirmed by an increase in the band gap of Zn_1?x Mn _x O for low x. The origin of the 2.9-eV peak and the shape of its excitation spectrum are discussed qualitatively. For Zn_1?x Mn _x O nanocrystals, the shape of the excitation spectrum is as unusual as the intense absorption in the range (2.2–3.0) eV.     

Stability of ZIF-8 Nanoparticles in Most Common Cell Culture Media

Zeolite imidazolate framework-8 (ZIF-8) is a promising platform for drug delivery, and information regarding the stability of ZIF-8 nanoparticles in cell culture media is essential for proper interpretation of in vitro experimental results. In this work, we report a quantitative investigation of the ZIF-8 nanoparticle’s stability in most common cell culture media. To this purpose, ZIF-8 nanoparticles containing sterically shielded nitroxide probes with high resistance to reduction were synthesized and studied using electron paramagnetic resonance (EPR). The degradation of ZIF-8 in cell media was monitored by tracking the cargo leakage. It was shown that nanoparticles degrade at least partially in all studied media, although the degree of cargo leakage varies widely. We found a strong correlation between the amount of escaped cargo and total concentration of amino acids in the environment. We also established the role of individual amino acids in ZIF-8 degradation. Finally, 2-methylimidazole preliminary dissolved in cell culture media partially inhibits the degradation of ZIF-8 nanoparticles. The guidelines for choosing the proper cell culture medium for the in vitro study of ZIF-8 nanoparticles have been formulated.