Synthesis and structure of <Emphasis Type="Italic">N</Emphasis>-(2-silatranylethyl)imidazoles

Organosilicon imidazole derivatives containing a 2-silatranylethyl [N(CH₂CH₂O)₃Si(CH₂)₂] group on the nitrogen atom were synthesized, and their steric and electronic structures, including the nature of interaction between the imidazole and silatrane fragments, were studied by X-ray analysis and ¹H, ¹³C, and ²⁹Si NMR spectroscopy.     

Intra- and intermolecular coordination reactions in solution involving pentacoordinated fluorosilanes based on proline

NMR spectroscopy was used to study a series of C,O-monochelate N′-(fluorodimethyl-silylmethyl)-N′-methyl-N-(organosulfonyl)proline amides in the solid state and in solutions in chloroform, pyridine, and acetone. The structure of one of the complexes was established by X-ray crystallography. Quantum chemical calculations were performed to interpret the spectral data obtained. The molecular conformation observed in crystal was found to be retained in solutions. The coordination bonds of silicon atom with acetone and pyridine are very weak and cannot stabilize the acyclic form (without the Si-O coordination bond). The ²⁹Si chemical shift was found to be directly dependent on the Si-O bond distance. The mutual influence of organosulfonyl group and chelate ring was described in terms of polarization effects.     

Phase separation in a manganite La<Subscript>0.95</Subscript>Ba<Subscript>0.05</Subscript>MnO<Subscript>3</Subscript> crystal

The structure and magnetic states of a crystal of lightly doped manganite La_0.95Ba_0.05MnO₃ were studied using thermal-neutron diffraction, magnetic measurements, and electrical resistance data in a wide temperature range. It is shown that, in terms of its magnetic properties, the orthorhombic crystal is characterized by two order parameters, namely, antiferromagnetic (T _N = 123.6 K) and ferromagnetic (T _C = 136.7 K). The results obtained differ in detail from known information on the manganites La_0.95Ca_0.05MnO₃ and La_0.94Sr_0.06MnO₃. Two models of the magnetic state of the La_0.95Ba_0.05MnO₃ crystal are discussed, one of which is a model of a canted antiferromagnetic spin system and another is associated with the phase separation of the manganite. Arguments are advanced in favor of the coexistence in this crystal of the antiferromagnetic phase (about 87%) with a Mn⁴⁺ ion concentration of 0.048 and the 1/16-type charge-ordered ferromagnetic phase (about 13%) with a Mn⁴⁺ ion concentration of 0.0625. The specific features of the manganite studied are due to self-organization of the La_0.95Ba_0.05MnO₃ crystal lattice caused by the relatively large barium ion size.     

Formation of Heterogeneous Powder Coatings with a Two-Level Micro-and Nanocomposite Structure under Gas-Dynamic Spraying Conditions

Heterogeneous coatings have been deposited by the cold gas-dynamic spraying of mechanically synthesized AMg2/graphite + Al₂O₃ powders. A specific feature of the coatings formed is the existence of a two-level micro-and nanocomposite structure. It has been established that an increase in the content of microsized Al₂O₃ particles in the mixture from 10 to 30 wt % produces a twofold increase in the thickness of the coating deposited for the same time period from 140 to 310 μm. A further growth in the content of microsized Al₂O₃ particles in the mixture up to 50 wt % leads to a decrease in the thickness of the coating formed to 40 μm. The manufactured coatings have a high microhardness ranging from 1.7 to 3.2 GPa depending on their composition. The high microhardness of these coatings is caused by an increase in the hardness of the matrix material due to the creation of a nanocomposite structure, which strengthens the immobilization of microsized Al₂O₃ particles in it, thus improving the properties of the heterogeneous coating as a whole.     

On the Complementary Photoluminescent and Electroluminescent Images of Poly(phenylenevinylene)‐Based Light‐Emitting Diodes with a Thin Active Layer

A clear complementary relationship between photoluminescent (PL) and electroluminescent (EL) images was observed for organic light‐emitting diodes (OLEDs) based on poly(phenylenevinylene) (PPV) and dye‐doped PPV. So‐called ‘black spots' (dark circular regions observed on the active area of running OLEDs) become bright ones, when the photoluminescence of the same area is excited. A very small thickness of the active layer (ca. 10 nm) was the crucial point to observe this anticorrelation between EL and PL. A substantial increase of the PL yield (‘anti‐burning' effect) was observed after strong light exposure (ca. 10 mJ/cm²) of the polymer covered by an aluminium layer. The same light exposure without aluminium protection resulted in complete photobleaching of the polymer. The presence of a thin insulating layer between the polymer and aluminium was proposed to be responsible for these effects. This layer prevents electron injection and PL quenching due to exciton dissociation at the metal‐polymer interface. The former effect leads to black spots in the EL image, the latter one gives rise to bright spots on the PL image situated on the same places. The intermediate layer can be also induced by light exposure. A very efficient energy transfer from the polymer to the dye and to the J‐aggregates of the dye was demonstrated in PPV/dye composite films.     

Exciton dissociation in conjugated polymers

In conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field-assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field-independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.