A. A. Knyazev

Tris(β-diketonates) lanthanum nematic adducts

A series of new liquid-crystalline adducts of lanthanum tris(β-diketonates) displaying nematic and smectic mesomorphism was synthesised and investigated. The temperatures and thermodynamic parameters of the phase transitions were determined. Even–odd alternation of the nematic–isotropic transition was observed in the series of synthesised complexes. It was found that films of the obtained liquid-crystal complexes are vitrified when cooling, and packings of molecules, formed in the mesophase, are retained. Interlayer distances were determined and a model of packing of molecules in the mesophase was suggested based on data obtained by a small-angle X-ray scattering method.

Mesomorphic behaviour and luminescent properties of mesogenic -diketonate lanthanide adducts with 5,5′-di(heptadecyl)-2,2′-bipyridine

A series of aryl-substituted β-diketonate adducts with 5,5′-di(heptadecyl)-2,2′-bipyridine has been synthesised. All the compounds exhibit a mesomorphic (smectic) behaviour. It has been shown that the metal ion does not drastically influence the mesophase type and the transition temperatures. A thin film of Eu(III) complex shows more effective luminescence in the aligned state.

Anisotropy of the Magnetic Susceptibility of Mesogeneous Lanthanide Complexes

Liquid crystalline adducts of lanthanide tris(β-diketonates) with 5,5′-diheptadecyl-2,2′-bipiridine forming a nematic mesophase were obtained. Molar constants of magnetic birefringence and the magnetic susceptibility anisotropy of synthesized complexes were determined according to the magnetic birefringence in solution. The values of magnetic susceptibility anisotropy are two orders of magnitude higher than those of organic liquid crystals.

Influence of Structural Anisotropy on Mesogenity of Eu(III) Adducts and Optical Properties of Vitrified Films Formed on their Base

A new series of europium adducts with the general formula Eu(CPDk3-CnH2n+1)3Phen, where CPDk3-CnH2n+1 denotes β-diketones and Phen is 1,10-phenanthroline, was synthesized. The obtained mesogenic complexes were heated to the temperatures of the isotropic liquid state and then cooled. The complexes having short CH3 and C2H5 substituents crystallized upon cooling, and the complexes with longer substituents from C3H7 to C6H13 underwent a glass transition with the formation of optically transparent amorphous films. Inside the series the complexes with C7H15 and C8H17 substituents exhibited a unusual smectic C mesomorphism for lanthanidomesogens. On the basis of quantum-chemical simulations and the results of small-angle X-ray scattering the dependence between the anisotropy of Eu(III) complexes with various ligand environments and their supramolecular organization was found. The synthesized Eu(III) complexes in the solid state show intense red photoluminescence upon irradiation by ultraviolet light (λmax - 337 nm).

Optical anisotropy of liquid-crystal lanthanide complexes

Refractive and optical anisotropy indices have been measured for a number of liquid-crystal lanthanide complexes with different counter ions, ligands, and complexing agents. The optical anisotropy of the complexes is found to range from 0.002 to 0.09, i.e., to be one to two orders of magnitude smaller than that of classical calamitic liquid crystals. Analysis of the chemical structure of mesogenic complexes showed that the complexes containing counter ions with sizes much smaller than the ligand sizes and mixed-ligand complexes containing less than five ligands exhibit the largest optical anisotropy. The optical anisotropy of other mixed-ligand complexes with a large number of ligands is much smaller. The reason is the different degrees of anisometry of complexes and, as a consequence, the difference in the polarizability anisotropy and, correspondingly, the optical anisotropy of the mesophase.

Dielectric properties of an ytterbium-based nematic liquid-crystal complex

The dielectric properties of an yttrium-based paramagnetic thermotropic nematic liquid-crystal complex have been studied. The conditions for the measurement of the magnitude and sign of the dielectric anisotropy of the mesophase have been determined. The relaxation of the components of the relative permittivity of a macroscopically homogeneously oriented sample has been observed. A high polarity of the metallomesogenic complex has been revealed.

Luminescence of nematic lanthanide-containing mixtures

The possibility of obtaining eutectic binary nematic liquid crystal systems containing lanthanide mesogens was studied. As lanthanide mesogens, we used lanthanide(III) tris-(β-diketonate) adducts exhibiting nematic mesomorphism. The luminescence of the compositions containing Eu(III) adducts was investigated.

Luminescence and energy transfer in poly(N-vinylcarbazole) blends doped by a highly anisometric Eu(III) complex

Abstract Highly fluorescent blends based on PVK (poly(N-vinylcarbazole)) doped by a new greatly anisometric Eu(III) mesogenic complex were described. The structure of the ligands in the complex was selected in such a way that its absorption maximum was close to the emission maximum of the polymer. The full-energy transfer conditions in the conjugated polymer–lanthanide complex blends were revealed. Improving luminescence efficiency of the blend occurs due to an increase in the threshold concentration of the emitting ions to prevent self-quenching phenomena. The resulting relative luminescence quantum yield of the blend increases more than twice in comparison with the individual complex. The optimized blends may be promising for application as red emitters for OLED, etc.

The influence of a complexing agent ion on the magnitude of the optical anisotropy of lanthanidomesogens

Measurements of the refractive indices and optical anisotropy of a series of liquid crystalline coordination compounds based on lanthanides with an identical ligand environment have been carried out. It was found that the magnitude of the optical anisotropy of the investigated complexes was several times smaller than the anisotropy of organic liquid crystals. Analysis of the results showed that the variation of the ion complexing agent had little effect on the magnitude of the optical anisotropy in the mesogenic complexes containing the same ligands. The even-odd alternation of the optical anisotropy of lanthanidomesogens at the increasing the number of protons in the ions of the lanthanides has been observed.

Mechanisms of dielectric polarization in thermotropic liquid-crystalline complexes based on lanthanides

The components of the dielectric constant of a terbium-based liquid-crystalline complex have been measured in the frequency range of 350–5 × 106 Hz. The magnitude and sign of the dielectric anisotropy of the complex have been determined. Dispersion of the dielectric constants in the liquid-crystalline and isotropic phases has been found. The mechanisms responsible for the relaxation phenomena that appear in the studied sample have been determined. The time of dielectric relaxation, the activation energy, and the dipole moment of the complex have been obtained.