Effect of the thickness of a dicarbocyanine dye layer on the conformational composition and spatial orientation of layer components

The conformational and aggregate structure and the spatial orientation of polyatomic organic molecules on solid substrates are investigated by the example of molecular layers of symmetric and asymmetric dicarbocyanine dyes. It is shown that the structure of a dicarbocyanine dye layer is determined by two mechanisms: (i) the change in the symmetry of the intramolecular charge distribution in the monomeric components of the layer as a result of the intermolecular interaction with the substrate, which leads to closer values of the equilibrium concentrations of two monomers with different spatial structure (rotational stereoisomers), and (ii) the association of monomer molecules with formation of dimers and J aggregates. The relative position of the spectra of the layer components, the spatial orientation of the components, and the concentration ratio of different monomeric forms depend on the structure and the electron-donating ability of terminal groups. In going from a monomolecular layer to multilayered structures, the asymmetry of the intramolecular charge distribution induced by the substrate and the ratio of different stereoisomeric forms first change and then become stabilized.     

The Influence of the Molecular Structure of Cyanine Dye on the Component Composition of Molecular Layers

The formation of the component composition of symmetric cationic cyanine dyes on glass is studied. The absorption spectra of layers of three homologous series of dyes with end heterocyclic groups of different spatial and chemical compositions are measured, and the absorption spectra of monomer components and aggregates are separated. The component compositions of layers of different thicknesses are compared. It is shown that the widening of the absorption spectra of molecular layers against the spectra of ethanol solutions of these compounds is caused mainly by the formation of various monomer stereoisomers and molecular aggregates and their interaction with the substrate surface and the neighborhood. The number of isomer forms and their relative concentrations depend on the layer thickness, the electron donor ability and spatial structure of end groups, and the cation conjugation chain length. The influence of the anion manifests itself only in the concentration ratio of the formed monomers and a small shift of the maxima of their absorption bands. The increase in the number of monomer forms produced in the layer corresponds to the increase in the conjugation chain length. Spatial obstacles created by heterocyclic groups inhibit the formation of definite stereoisomers, which reduces the number of components of the layer.     

Optical properties and component composition of layers of cyanine dyes on dielectric supports: influence of asymmetry of the molecular electron density distribution

The component composition of molecular layers of dicarbocyanine dyes of cationic type on glass is determined by the absorption spectroscopy methods. For symmetric and asymmetric molecules the component composition depends on the chemical structure of a molecule and its interaction with the substrate, which are responsible for the electron density distribution in a molecule. The interaction with the substrate depends on the thickness of a layer and the orientation of a layer molecule on the substrate. The increase in the separation between the molecule and the substrate in thicker layers decreases the interaction with the substrate and the asymmetry of a molecule. In symmetric and slightly asymmetric dyes the value of the charge difference on the end groups decreases with increasing layer thickness, whereas in dyes with a large asymmetry, both the value and the sign of the charge difference change. The above results are confirmed by data on the computer simulation of the charge distribution in a dicarbocyanine cation upon its interaction with the oxygen anion on the substrate.     

Spin-transfer torque in tunnel junctions with ferromagnetic layer of finite thickness

Two components of the spin torque exerted on a free ferromagnetic layer of finite thickness and a half-infinite ferromagnetic electrode in single tunnel junctions have been calculated in the spin-polarized free-electron-like one-band model. It has been found that the torque oscillates with the thickness of ferromagnetic layer and can be enhanced in the junction with the special layer thickness. The bias dependence of torque components also significantly changes with layer thickness. It is non-symmetric for the normal torque, in contrast to the symmetric junctions with two identical half-infinite ferromagnetic electrodes. The asymmetry of the bias dependence of the normal component of the torque can be also observed in the junctions with different spin splitting of the electron bands in the ferromagnetic electrodes.     

Forbidden character in electronic spectra of mono-halonaphthalenes

The singlet-singlet absorption and emission spectra of α-fluoro and the absorption spectra of α-chloro-, α-iodo-, and β-bromo-naphthalenes have been photographed and analyzed. The absorption spectra of all the four molecules show the presence of two discrete systems of bands, and the emission spectrum of α-fluoronaphthalene corresponds to the longer-wavelength system of absorption. With the α-chloro- and α-iodo-naphthalenes, the longer-wavelength system shows the appearance of pure electronic transition with weak bands, while the corresponding transitions in case of α-fluoro- and β-bromo-naphthalenes appear with the most intense bands of the system, as expected theoretically. The appearance of strong vibronic and weak electronic bands in the longer-wavelength system of the former two molecules has been interpreted as the interaction of totally symmetric vibrations, a rare example in the literature.     

Current-induced excitations in single cobalt ferromagnetic layer nanopillars

Current-induced excitations in Cu/Co/Cu single ferromagnetic layer nanopillars ( approximately 50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current-induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin-wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.     

Spectroscopic study of the association of 6Zh rhodamine in mixtures of carbon tetrachloride with various polar solvents

The concentration-dependent changes in the absorption spectrum of 6Zh rhodamine dye in binary mixtures of CCl₄ with eleven polar solvents has been investigated. The degrees of association of the dye in the solvents have been computed and the absorption bands of its monomers and associates determined. It is shown that all the polar solvents hinder the association of 6Zh rhodamine. The magnitude of this effect, however, depends on the structure of the solvent molecules. It is determined by the strength of their interaction with the dye molecules. In most cases this interaction is of the donor-acceptor type.The authors are grateful to D. N. Shigorin for his useful comments on the results obtained.     

Some features of the polarization characteristics of strongly absorbing helical periodic media

Results are presented on the polarization characteristics (rotation of the plane of polarization and polarization ellipticity) as a function of the layer thickness and the absorption anisotropy in strongly absorbing media having a helical structure. A strong resonancelike change in the polarization ellipticity is found as a function of the anisotropy of the absorption at frequencies of diffractional interaction of the light with the medium. A change in the sign of rotation of the plane of polarization of the light is observed as the layer thickness is varied. It is established that sign of the rotation also changes as the absorption anisotropy varies. These effects are studied under conditions of interaction of light with a half space and with a layer of medium of finite thickness. Some new features are identified in the previously observed effect wherein the absorption of radiation in media having a periodic structure decreases as the layer thickness increases. Zh. Tekh. Fiz. 69, 72–78 (August 1999)     

晶体X射线劳厄衍射分束特性研究

本文基于X射线衍射动力学分析了劳厄晶体的分束特性,模拟了晶体吸收和入射光角发散对于透射光和衍射光摇摆曲线的影响,定量给出晶体衍射面内角调节范围和晶体加工厚度对于劳厄衍射分束比的调制.在实验中,采用分析晶体和分束晶体的消色散配置限制入射光角发散的影响,实现300μm厚Si(220)晶体面内角调节劳厄衍射分束的精确测量,并得到300μm, 400μm和500μm厚度Si晶体分束比的调节范围,实现了透射光和衍射光强度的定量调制.     

The effect of an electrostatic field on the photoluminescence and photoconductivity of polymer films doped by cationic polymethine dyes with end groups with different electron-donating properties

The effect of an external electrostatic field on the photoluminescence spectra and photoconductivity of symmetric and asymmetric cationic polymethine dyes with end groups possessing different electron-donating properties (basicity) in polymer films based on photoconductive poly-N-epoxypropylcarbazole and nonphotoconductive polyvinylethylal is studied. It is found that application of an electric field to films can both decrease and increase their luminescence intensity. The strongest decrease in the luminescence intensity is observed for symmetric dyes in poly-N-epoxypropylcarbazole films with either a decrease in the electron-donating ability of the end groups or with a decrease in the length of the polymethine chain. Colored polyvinylethylal films exhibit both a negative and a positive effect of the field on the photoluminescence intensity. The character of changes in the luminescence spectra does not depend on the electron-donating ability of the end groups. The spectral effects are explained by a change in the probability of vibronic transitions due to the redistribution of the electron density in the chromophore of the dye in the ground and excited states in an external electric field.     

Intense laser field effect on the interband absorption in differently shaped near-surface quantum wells

The 1S-exciton properties and interband absorption spectra in differently shaped near-surface quantum wells (NSQWs) with symmetrical/asymmetrical barriers, under intense laser field, are investigated taking into account the correct dressing effect for the confinement potential and electrostatic interaction between carriers and their image-charges. We found that: i) the 1S-exciton binding energy is significantly reduced by the laser intensity in InGaAs NSQWs of different asymmetrical shape; ii) the red-shift of the absorption peak induced by the asymmetry diminution or by increasing cap layer thickness can be effectively compensated using the blue-shift caused by enhancing laser parameter. Therefore, the optical properties of the differently shaped NSQWs could be tuned by proper tailoring of the heterostructure parameters (well shape, barrier asymmetry) and/or dielectric mismatch as well as by varying the laser field intensity.     

Kamlet-Taft and Catalan Studies of Some Novel Y-Shaped Imidazole Derivatives

Some novel Y-shaped imidazole derivatives were developed and characterized by NMR and mass spectral techniques. The photophysical properties of these imidazole derivatives were studied in several solvents. The Kamlet-Taft and Catalan’s solvent scales were found to be the most suitable for describing the solvatochromic shifts of the absorption and fluorescence emission. The adjusted coefficient representing the electron releasing ability or basicity of the solvent, C _β or C _SB has a negative value, suggesting that the absorption and fluorescence bands shift to lower energies with the increasing electron-donating ability of the solvent. This effect can be interpreted in terms of the stabilization of the resonance structures of the chromophore. The observed lower fluorescence quantum yield may be due to an increase in the non-radiative deactivation rate constant. This is attributed to the loss of planarity in the excited state provided by the non co-planarity of the cinnamaldehyde ring attached to C(2) atom of the imidazole ring. Such a geometrical change in the excited state leads to an important Stokes shift, reducing the reabsorption and reemission effects in the detected emission in highly concentrated solutions.     

Pressure broadening and shifting parameters for the spectral lines in the fundamental vibration-rotation bands of HBr and HI in mixtures with rare gases

We report measurements of the line broadening and shifting coefficients in the (1 ← 0) fundamental absorption bands of the HBr and HI molecules in mixtures with rare gases He, Ne, Ar, Kr, and Xe. Comparison is given with the published data on other HHal-Rg systems. The measured line shifts are separated into terms symmetric and asymmetric in the line number m. The magnitudes of the symmetric shifts change in a regular manner in the series of rare gases and reach perturber-specific asymptotic values at higher ∣m∣. It is found that the asymptotic values of the symmetric line shifts linearly correlate with the respective C₆ potential energy constants and that the slopes of these correlations are proportional to the vibrational ground state dipole moments squared of the hydrogen halide molecules.     

Synchrotron x-ray study of the smectic layer directional instability

We have investigated the phenomenon of field-induced smectic layer instability, as monitored by synchrotron x-ray scattering. This instability means that, upon application of time-asymmetric electric fields to chiral smectics, the layer direction seems to "rotate" locally around an axis given by the direction of the applied field. For moderate values of field amplitude and asymmetry, domains with a favored layer inclination grow at the expense of unfavored ones, while larger fields and asymmetries generally lead to a chaotic flow behavior. At moderate amplitudes, we have followed the process of the horizontal layer folding (or horizontal chevron domain formation) and the smectic C* layer reorientation of ferroelectric liquid crystals by applying symmetric and asymmetric wave forms, respectively, and performing time resolved x-ray measurements. The studies unambiguously show the formation of a horizontal (in-plane, i.e., in a plane parallel to the cell substrates) chevron domain structure from a nonoriented sample by application of a symmetric electric field of sufficient amplitude. It is then demonstrated that a transition from the horizontal chevron domain structure to an in-plane uniform smectic layer direction takes place on application of asymmetric electric wave forms. Reversal of the field asymmetry reverses the inclination direction and selects the other layer normal direction as the uniform end state. The in-plane smectic layer reorientation process is followed here as it evolves, and analyzed directly by means of x-ray scattering.     

Tuning of the photonic band gaps and the reflection spectra of a one-dimensional photonic crystal based on silicon and a liquid crystal

The reflection spectra and the photonic band gaps of a one-dimensional composite photonic crystal are calculated for different filling factors (the ratio of the thickness of a silicon layer to the lattice constant A). A change in the refractive index of the filler in a photonic crystal leads to the shift of both edges for certain side photonic bands. It is shown that, with a change in the refractive index from 1.49 to 1.69, the shift of the edges (the tuning effect) can reach the value A/λ = 0.04 for the main photonic band and 0.08 for a side band. An additional criterion for designing a photonic crystal is the estimation of the sharpness of the edges of photonic bands in the reflection spectra.     

Simulation of the influence of the polymer conformation,orientation, crystallinity,and stretching on its IR absorption spectrum

Using cis-1,4-polyisoprene as an example, the influence of the conformation and orientation of the polymer chain on its polarization absorption spectra has been simulated. It has been shown that if a change in the orientation of the polymer chain leads to a monotonic change in the intensities of the absorption bands with their maxima positions remaining unchanged, then changes in the polymer chain conformation additionally cause a shift and the appearance of new absorption bands. According to the results of the calculation of the vibrational spectra, the inversion of the IR dichroism of the absorption bands of polymers is associated with the conformation transition. We simulate the influence on the IR absorption spectrum of the degree of crystallinity of cis-1,4-polyisoprene, in which the amorphous and amorphous-crystalline states are considered as a mixture of noninteracting polymer chains, where each state is characterized by a certain ratio of conformers. Likewise, the changes in the IR absorption spectrum of cis-1,4-polyisoprene under its stretching have been investigated. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 727–733, November–December, 2005.     

UV–VIS and mid-IR ellipsometer characterization of layers used in OLED devices

We report on the analysis of layer materials commonly used for the fabrication of organic light-emitting diodes (OLEDs) by means of spectroscopic ellipsometry operating in the ultraviolet–visible (UV–VIS) and in the mid-infrared (mid-IR) range of the electromagnetic spectrum. On covering a wide spectral range, these non-destructive, non-contact techniques offer a huge potential in characterizing thin films in terms of layer thickness values, optical indices, absorption properties modified by incorporating optically active dopant molecules, and, finally, electrical layer properties. Individual absorption bands of single organic films can significantly affect the luminance and performance efficiency of OLEDs when building multi-layer stacks. Using mid-IR spectroscopic ellipsometry, the resistance of as-deposited and annealed indium–tin oxide (ITO) layers on glass is determined and compared with values obtained by four-point probe techniques.     

Humidity sensor based on localized surface plasmon resonance of multilayer thin films of gold nanoparticles linked with myoglobin

Layer-by-layer self-assembled multilayer thin films of gold nanoparticles (GNPs) linked with myoglobin (Mb) show substantial sensitivity to humidity at room temperature according to measurements of localized surface plasmon resonance (LSPR) absorption that relies on the interparticle interaction present in the film. The sensor response is reversible, with response and recovery times as low as 5 s. The sensing mechanism is as follows: as the ambient humidity changes, Mb molecules change their size, making the GNP-to-GNP spacing and thereby the interparticle interaction change; the change in the interparticle interaction causes a change in the LSPR absorption of the multilayer thin film. We found that the LSPR band of the multilayer thin film was almost insensitive to both the surrounding refractive index and the adlayer thickness, rendering the multilayer-film-based humidity sensor highly immune to ambient disturbances.     

一种基于十字镂空结构的低频超材料吸波体的设计与制备

设计了一种十字镂空的超材料结构,与传统的铁磁吸波材料相结合,实现了低频吸收频带的扩宽.仿真结果显示,吸波体在2-4 GHz范围可以实现-10 dB以下的吸收,相比于没有加载超材料的情况,吸收带宽扩展了0.5 GHz.实验结果在2.5-5.1 GHz范围内也显示了相似的吸收曲线,低于-9 dB的吸收频带有0.48 GHz的扩宽,扩展了23%.不同结构的能量损耗密度分布表明,相比于无镂空的十字结构,镂空十字结构可以增加磁场能量损耗,加强单元结构之间的耦合,降低超材料对传统吸波材料性能的破坏.探索了传统铁磁吸波材料厚度的变化对吸波体吸收性能的影响,发现由超材料引入的附加峰位置不随着吸波材料厚度的改变而明显地移动.根据这个结果,进一步设计了由两种超材料叠加组合而成的吸波体,仿真结果和实验结果均显示,在降低了1.1 mm铁磁吸波材料层厚度的情况下,使低频吸收带宽又扩展了0.9 GHz.     

Recent Studies of the Circular Dichroism and Optical Rotatory Dispersion of Biopolymers

When electromagnetic radiation interacts with matter, a number of changes result both in the light and in matter. The intensity of the light at given wavelength may be reduced due to absorption; the velocity of light may change, dependent on the wavelength, upon interaction with the medium due to refractive index dispersion (scattering). If the incident light is plane polarized, a change in the plane of polarization may occur, again dependent on wavelength [optical rotatory dispersion (ORD)]. Also, the radiation may become elliptically polarized at the absorption bands; this phenomenon is termed circular dichroism (CD). While normal absorption and dispersion occur with any molecule, ORD and CD require not only polarized light but also molecular systems with built-in asymmetry or dissymmetry.