Yu. V. Fedorov

Ditopic complex formation of the crown-containing 2-styrylbenzothiazole

The complex formation of 2-styrylbenzothiazole containing a 15-crown-5 ether fragment with alkaline earth metal cations, proton, Ag+ and Hg2+ was studied by optical and X-ray diffraction methods. The compound is able to bind the metal cations through the participation of two centers: the crown ether moiety and the heterocyclic part. The alkaline earth metal cations form complexes with the macrocyclic part of the molecule. The formation of a strong sandwich complex was found in the case of Ba2+. The proton coordinates with the N atom of the heterocyclic fragment of the molecule. The Ag+ and Hg2+ cations bind with both centers of the molecule: the crown ether fragment and the heterocyclic residue.

MHEMT with a power-gain cut-off frequency of fmax = 0.63 THz on the basis of a In0.42Al0.58As/In0.42Ga0.58As/In0.42Al0.58As/GaAs nanoheterostructure

The method of molecular-beam epitaxy is used to grow a In0.42Al0.58As/In0.42Ga0.58As/In0.42Al0.58As nanoheterostructure with a step-graded metamorphic buffer on a GaAs substrate. The root-mean-square value of the surface roughness is 3.1 nm. A MHEMT (metamorphic high-electron-mobility transistor) with a zigzag-like gate of a length of 46 nm is fabricated on the basis of this nanoheterostructure; for this MHEMT, the cutoff frequencies for the current and power gain are fT = 0.13 THz and fmax = 0.63 THz, respectively.

Synthesis, complexation, and E—Z photoisomerization of azadithiacrown-containing styryl dyes as new optical sensors for mercury cations

New styryl dyes containing azadithia-15-crown-5 fragments were synthesized. The complexation of these compounds with Ag+, Pb2+, Cu2+, Hg2+, and H+ cations was studied by 1H NMR spectroscopy, steady-state, and time-resolved spectroscopy. The stability constants of the complexes were calculated from the spectrophotometric titration data. The photophysical properties and E—Z photoisomerization of styryl dyes and their complexes with mercury and copper(II) cations in acetonitrile were examined.

Supramolecular photochemical synthesis of an unsymmetrical cyclobutane

2-styrylbenzothiazole (1) and cinnamic acid (2) derivatives containing 15-crown-5 ether moieties form a supramolecular assembly in the presence of Ba(2+) cations in acetonitrile. The assembly is stabilized by hydrogen bonding between the heterocyclic N atom of 1 and the proton of the carboxylic group of 2, by sandwich Ba(2+) complex formation between the crown ether moieties of 1 and 2, and by pi-pi stacking interactions. Irradiation of solutions containing these supramolecular complexes leads to highly specific formation of an unsymmetrical cycloadduct. This investigation provides an interesting example of supramolecular control of [2 + 2]-photocyclization in solution.

AlGaN/GaN-HEMTs with a breakdown voltage higher than 100 V and maximum oscillation frequency fmax as high as 100 GHz

The N-Al0.27Ga0.73N/GaN High Electron Mobility Transistors (HEMTs) with different gate lengths Lg (ranging from 170 nm to 0.5 μm) and gate widths Ws (ranging from 100 to 1200 μm) have been studied. The S parameters have been measured; these parameters have been used to determine the current-gain cutoff frequency ft, the maximum oscillation frequency fmax, and the power gain MSG/MAG and Mason’s coefficients were investigated in the frequency range from 10 MHz to 67 GHz in relation to the gate length and gate width. It was found that the frequencies ft and fmax attain their maximum values of ft = 48 GHz and fmax = 100 GHz at Lg = 170 nm and Wg = 100 μm. The optimum values of Wg and output power P out of the basic transistors have been determined for different frequencies of operation. It has also been demonstrated that the 170 nm Al0.27Ga0.73N/GaN HEMT technology provides both good frequency characteristics and high breakdown voltages and is very promising for high-frequency applications (up to 40 GHz).

Promising materials for an electronic component base used to create terahertz frequency range (0.5–5.0 THz) generators and detectors

A THz transistor based on a metamorphic nanoheterostructure with generation frequency fmax = 0.63 THz and a zigzag-shaped gate Lg = 46 nm long is developed. A series of low-temperature GaAs structures are produced, and photoconductive antennas with generation frequencies above 1.5–2 THz are developed on their basis.

Cucurbituril as a new “host” of organic molecules in inclusion complexes

The latest data concerning the study of inclusion complexes based on cucurbiturils as a new class of molecular hosts are generalized in the review. The methods of synthesis, the structure, and physical and chemical properties of cucurbiturils are considered. Special attention is given to the discussion of the principles of construction of supramolecular complexes. The factors affecting stability, composition, and structure of inclusion complexes are considered. The potential applications of cucurbituril as molecular containers for a number of photochemical reactions are given. Advantages of using this class of compounds in comparison with other known cavitands are presented.

Photoluminescence and electron subband population in modulation doped AlGaAs/GaAs/AlGaAs heterostructures

The anomalies in photoluminescence spectra of modulation doped AlGaAs/GaAs/AlGaAs double barrier heterostructures with various GaAs quantum well (QW) widths are observed. The intensity of the photoluminescence peak excited by Ar laser and associated with electron optical transitions from the second QW subband to the valence band becomes higher than for the case of the first (lowest) subband in the GaAs QW width range L=22.5–30 nm. The calculations of electron-polar optical phonon scattering rate show that the well-width dependence of the intersubband electron scattering rate due to emission of confined and interface optical phonons is responsible for the photoexcited electron subband population redistribution and anomalies in the photoluminescence spectra. The transition rate for the second QW subband electron binding to the exciton state is evaluated as 25×1010 s−1 at L<22.5 nm and 4×1010 s −1 at 22.5

A novel highly efficient nanostructured organosilicon luminophore with unusually fast photoluminescence

Synthesis and theoretical and experimental investigations of a novel nanostructured organosilicon luminophore (NOL) containing six 2,2′-bithienyl donor units connected via silicon atoms to a 1,4-bis(5-phenylthienyl-2-yl)-benzene acceptor unit with efficient intramolecular Forster resonance energy transfer are reported. The NOL shows a unique combination of optical properties: a high photoluminescence (PL) quantum yield of up to 91%, a fast PL decay time of down to 800 ps, a large pseudo-Stokes shift of 101 nm and a huge molar extinction coefficient of 1.4 × 105 M−1 cm−1. These peculiarities caused by the specific arrangement of the donor and acceptor fragments at the nanoscale distance within the NOL were correlated with the molecular structure of the NOL using theoretical calculations, which for the first time allowed successful prediction of the oscillator strength, PL decay time and intramolecular energy transfer efficiency. A comparison of the photophysical properties of the NOL with the standard laser dye POPOP in THF and toluene solutions revealed its huge application potential in organic photonics and high energy physics.

Sound waves in a liquid with polydisperse vapor–gas bubbles

A mathematical model is presented for the propagation of plane, spherical, and cylindrical sound waves in a liquid containing polydisperse vapor–gas bubbles with allowance for phase transitions. A system of integro-differential equations is constructed to describe perturbed motion of a two-phase mixture, and a dispersion relation is derived. An expression for equilibrium sound velocity is obtained for a gas–liquid or vapor–liquid mixture. The theoretical results agree well with the known experimental data. The dispersion curves obtained for the phase velocity and the attenuation coefficient in a mixture of water with vapor–gas bubbles are compared for various values of vapor concentration in the bubbles and various bubble distributions in size. The evolution of pressure pulses of plane and cylindrical waves is demonstrated for different values of the initial vapor concentration in bubbles. The calculated frequency dependence of the phase sound velocity in a mixture of water with vapor bubbles is compared with experimental data.