Anisotropy Of Molecular Polarizability Research Articles

Na3SiS3F: A Wide Bandgap Fluorothiosilicate with Unique [SiS3F] Unit and High Laser‐Induced Damage Threshold

AbstractThe exploration of antilaser damage wide bandgap infrared (IR) nonlinear optical (NLO) materials is urgent but challenging. Herein, by introducing the idea of fluorination into chalcogenides, a wide bandgap IR NLO material Na3SiS3F with unprecedented [SiS3F] tetrahedra is designed and synthesized. Na3SiS3F shows a wide bandgap of 4.75 eV (the largest one in the reported quaternary metal chalcogenides), resulting in a high laser damage induced threshold of ≈5 × AgGaS2(AGS). Meanwhile, the compound has a moderate NLO response (≈0.3 ×AGS) with phase‐matching behavior, large birefringence (0.15@1064 nm), and wide IR transparent region. The introduction of fluorine breaks the structural symmetry and broadens the highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) gap, polarizability anisotropy, and hyperpolarizability of the Si–S tetrahedral unit. The results indicate that Na3SiS3F is a promising IR NLO material for the high‐power laser application and open an avenue for the design of new wide bandgap IR NLO materials based on NLO‐active [SiSxF4−x] (x = 1, 2, 3) mixed anionic tetrahedral group.

New method to calculate the anisotropies of polarizability and thermal expansion of uniaxial liquid crystals.

The anisotropy of molecular polarizability in liquid crystals is linked to the birefringence in these substances. The classic methods to compute the polarizabilities of liquid crystals assume an average number density of molecules that is equal in all directions. In the present work, a new model is proposed for the anisotropic molar polarization based on a virtual anisotropy of the number density of molecules in the liquid-crystalline material. This new strategy hence allows for the computation of both the anisotropic polarizabilities and the anisotropic thermal-expansion coefficients of liquid crystals. The model is applied to the liquid crystals 4-n-pentyl-4'-cyanobiphenyl and N-(4-methoxybenzylidene)-4-butylaniline, yielding polarizabilities similar to those reported for these materials. For these nematic liquid crystals, the results imply the existence of a positive thermal-expansion coefficient in the direction perpendicular to the director vector throughout the entire nematic temperature range and a negative thermal-expansion coefficient parallel to the director vector near the temperature of the nematic-isotropic transition. At the isotropization temperature, there exists divergent and critical behavior of the anisotropic thermal-expansion coefficients, consistent with the typical discontinuity of volume in first-order transitions.

Methylguanidinium at the Air/Water Interface: A Simulation Study with the Drude Polarizable Force Field.

Methylguanidinium is an important molecular ion, which also serves as the model compound for the arginine side chain. We studied the structure and dynamics of the methylguanidium ion at the air/water interface by molecular dynamics simulations employing the Drude polarizable force field. We found out that methylguanidinium accumulated at the interface, with a majority adopting tilted conformations. We also demonstrated that methylguanidinium and guanidinium ions had different preference toward the air/water interface. Detailed analysis of induced dipole moments showed how ions adjusted their charge distribution at the interface and revealed how the anisotropy in molecular polarizability impacted the orientation of molecular ions. Our results illustrate the importance of explicitly including the electronic polarization effects in modeling interfacial properties.

Photoreversible Birefringence Change of Diarylethene Single Crystals as Revealed by Change in Molecular Polarizability Anisotropy.

Stimuli-responsive organic crystals represent a new frontier of material chemistry. Recently, we have reported photoreversible interference color change to multicolor in single crystals composed of a photochromic diarylethene derivative, 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a), accompanied by the photochromic reaction. The origin of the interference color change is due to the photoinduced birefringence change in the photoisomerization of diarylethenes. In this study, we newly found that single crystals composed of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (2a) also exhibit a photoreversible interference color change. The birefringence value for crystal 2a increased with the photocyclization conversion, while that for crystal 1a decreased. The relationship between the photoinduced birefringence changes for crystals 1a and 2a and their molecular structures was discussed based on the change in the molecular polarizability anisotropy accompanied by the photochromic reaction. These results would provide not only new opportunities for the application of photochromic crystals but also useful strategies for the design of crystalline materials that exhibit the desired birefringence change.

Molecular polarizability anisotropy of liquid water revealed by terahertz-induced transient orientation

Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules. The simulation of these interactions requires a parametrization of the permanent and induced dipole moments. However, the underlying molecular polarizability of water and its dependence on ions are partially unknown. Here, we apply intense terahertz pulses to liquid water, whose oscillations match the timescale of orientational relaxation. Using a combination of terahertz pump / optical probe experiments, molecular dynamics simulations, and a Langevin dynamics model, we demonstrate a transient orientation of their dipole moments, not possible by optical excitation. The resulting birefringence reveals that the polarizability of water is lower along its dipole moment than the average value perpendicular to it. This anisotropy, also observed in heavy water and alcohols, increases with the concentration of sodium iodide dissolved in water. Our results enable a more accurate parametrization and a benchmarking of existing and future water models.

The sign of the polarizability anisotropy of polar molecules is obtained from the terahertz Kerr effect

The terahertz Kerr effect (TKE) of polar molecular vapors is reported. The birefringence signal of fluoroform appears with opposite polarity compared to acetonitrile and water. This behavior is a hallmark of the opposite sign of a new molecular polarizability anisotropy ΔαTKE=αzz-(αxx+αyy)/2, with αzz being the polarizability along the permanent dipole moment. As the excitation of the rotational states orients the permanent dipoles along the terahertz electric field, the orientation is translated into an optical birefringence proportional to ΔαTKE. Thus, the sign of ΔαTKE is imprinted onto the TKE signal, providing novel insights into the polarizability tensor of water.

Structural Design Parameters for Highly Birefringent Coordination Polymers.

A series of coordination polymer materials incorporating the highly anisotropic 2-(2-pyridyl)-1,10-phenanthroline (phenpy) building block have been synthesized and structurally characterized. M(phenpy)[Au(CN)2]2 (M = Cd, Mn) are isostructural and form a 1-D chain through bridging [Au(CN)2](-) units and extend into a 2-D sheet through aurophilic interactions. M(phenpy)(H2O)[Au(CN)2]2·2H2O (M = Cd, Mn, and Zn) are also isostructural but differ from the first set via the inclusion of a water molecule into the coordination sphere, resulting in a 1-D topology through aurophilic interactions. In(phenpy)(Cl)2[Au(CN)2]·0.5H2O forms a dimer through bridging chlorides and contains a free [Au(CN)2](-) unit. In the plane of the primary crystal growth direction, the birefringence values (Δn) of 0.37(2) (Cd(phenpy)[Au(CN)2]2), 0.50(3) (In(phenpy)(Cl)2[Au(CN)2]·0.5H2O), 0.56(3) and 0.59(6) (M(phenpy)(H2O)[Au(CN)2]2·2H2O M = Cd and Zn, respectively) were determined. β, a structural parameter defined by phenpy units rotated in the A-C plane relative to the light propagation (C) direction, was found to correlate to Δn magnitudes. The addition of a carbon-carbon double bond to terpy has increased the molecular polarizability anisotropy of the building block, and all structures have reduced deviation from planarity in comparison to terpy and terpy derivative structures, leading to these higher Δn values, which are among the highest reported for crystalline solids.

Polarizability anisotropy relaxation in nanoconfinement: molecular simulation study of water in cylindrical silica pores.

We report the results of a molecular simulation study of polarizability anisotropy relaxation for water confined in approximately cylindrical silica pores, with diameters in the range from 20 to 40 Å. In our calculations, we use a polarizability model that includes molecular and interaction-induced components. In agreement with optical Kerr effect experimental data, we find strong confinement effects on the relaxation rate of water polarizability anisotropy. Given that water molecular polarizability anisotropy is small, much of the intensity of the polarizability anisotropy response comes from the interaction-induced component. However, we find that, at longer times, the relaxation properties of this component strongly resemble those of collective reorientation, the mechanism by which the molecular polarizability anisotropy relaxes. We also find that the relevant collective orientational relaxation differs considerably from single molecule reorientation and that this difference varies with the extent of confinement. Our investigation of the effects of axial-radial pore anisotropy indicates that these effects play a minor role in water polarizability anisotropy relaxation in this pore diameter range.

Distribution of Dye and Mesogen Molecules Orientation in Photoaligning Layer vs. Aperture of Polarized Light Beam

Orientation of dyes and liquid crystals molecules in the field of laser radiation is considered taking into account anisotropy of molecular optical polarizability. A dependence of dyes and liquid crystals clusters energy on the strength of laser radiation electrical field, aperture of the light beam, optical polarizability anisotropy, order parameter and the number of molecules in a cluster, is derived. The angular distribution function of clusters is derived depending on the aperture of the light beam.

Paper No P23: Distribution of Dye Molecules Orientation in Photoaligning Layer Versus Incidence Angle of Polarized Light Beam

AbstractA mechanism of dyes molecules orientation by laser radiation is proposed that takes into account anisotropy of molecular optical polarizability. A dependence of dye molecules energy on polarizability anisotropy, order parameter, quantity of molecules in a cluster, strength of light wave electrical field, light beam incidence angle is derived.

Measurement of molecular polarizability anisotropy via alignment-induced spatial focusing and defocusing

We demonstrate that the polarizability anisotropy of the acetylene molecule could be experimentally determined by molecular alignment-induced spatial focusing and defocusing effects under femtosecond impulsive excitation. The molecular alignment signals of oxygen and acetylene were measured under identical experimental conditions and the polarizability anisotropy of acetylene was quantitatively determined with oxygen as the reference molecule. Meanwhile, the Kerr coefficients of oxygen and acetylene were estimated. This intuitive and simple method is also applicable to other molecules.

Orientational order parameter estimated from molecular polarizabilities – an optical study

An optical study of N-(p-n-alkyloxybenzylidene)-p-n-butyloxyanilines, nO.O4 compounds with the alkoxy chain number n = 1, 3, 6, 7, and 10 has been carried out by measuring the refractive indices using modified spectrometer and direct measurement of birefringence employing the Newton's rings method. Further, the molecular polarizability anisotropies are evaluated using Lippincott δ-function model, the molecular vibration method, Haller's extrapolation method, and scaling factor method. The molecular polarizabilities α e and α 0 are calculated using Vuk's isotropic and Neugebauer anisotropic local field models. The order parameter S is estimated by employing the molecular polarizability values determined from experimental refractive indices and density data and the polarizability anisotropy values. Further, the order parameter S is also obtained directly from the birefringence data. A comparison has been carried out among the order parameter obtained from different ways and the results are compared with the body of the data available in the literature.

An Optical Study on Orientational Order Parameter and Molecular Polarizability in Benzylidene Anilines

Refractive indices measurements are carried out in four alkoxy benzylidene anilines viz., N-(p-n-methoxy and ethoxy benzaldehyde)-p-n-dodecyl and tetra decyl anilines, 1O.12, 1O.14, 2O.12 and 2O.14. First three compounds exhibit monovariant nematic phase while the last compound exhibits nematic and SmA phases with variable thermal ranges. The molecular polarizabilities are calculated from the refractive indices and the density using the well known internal field models by Vuks and Neugebauer. The molecular polarizability anisotropy is estimated from the Lippincott δ-function model as well as molecular vibrational methods. The order parameter is estimated using a) from the Haller’s extrapolation from molecular polarizabilities, b) from the scaling factors obtained from the equations by Vuks and Neugbeaur and c) directly from the birefringence - a method developed by Kuczynski et al without considering any internal field model to the liquid crystal molecule. The results evaluated from these methods are compared from one another and the limitations of the methods used are discussed.

Molecular polarizability anisotropy of some five-membered cyclic imides

Dipole moments ( μ), molar Kerr constants ( mK) and molecular polarizabilities of several five-membered ring imides and a number of model compounds were measured. Various models for determination of molecular polarizability and mK values were evaluated. It was shown that the atom–dipole interaction model does not always give satisfactory results consistent with the experimental data, even for molecules containing only 6–8 atoms. An alternative method for mK calculation was offered based on the concept of interacting molecular fragments. The method uses the differences between the experimental and additive mK values of model systems as the parameter characterizing the interaction of the fragments. Good agreement with experiment was obtained. Correlations of the differences between the experimental and additive values of mK and μ with the total number of n- and π-electrons conjugated with the imide group were established.

Study of Molecular Polarizabilities and Orientational Order Parameter in the Nematic Phase of 6.O12O.6 and 7.O12O.7

As a part of our systematic study of the physical properties of the compounds of symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4’-oxy) alkanes (m.OnO.m), we report here an optical study of two nematogenic compounds of this series viz., 6.O12O.6 and 7.O12O.7 by carrying out the measurement of extraordinary and ordinary refractive indices using modified spectrometer. The molecular polarizability anisotropies are evaluated using Lippincott δ-function model, the molecular vibration method, Haller’s extrapolation method and scaling factor method. The molecular polrizabilities, αe and αo are calculated using Vuks’ isotropic field model and Neugebauer’s anisotropic field model. The orientational order parameter, S, is estimated by using the molecular polarizability values calculated from refractive index and denstity data as well as polarizability anisotropy values calculated as a function of temperature in the nematic phase of both of the dimers. These values are discussed with reference to the order parameter values obtained directly from the optical birefringence, Δn, data without assuming any internal field. A comparison of the order parameter values from all of these methods was carried out in the light of available literature data.

Studies on binary mixtures of 4-pentyloxy-4′-cyanobiphenyl with three 4, 4′-di(alkoxy)azoxybenzenes

AbstractEighteen binary mixtures of 4-pentyloxy-4′-cyanobiphenyl (5OCB) and 4, 4′-di(alkoxy)azoxybenzenes (nOAB) with n = 5, 6, and 7 at different concentrations have been studied in detail. Over a wide concentration range, induced SmA phase was observed in all the mixtures except in one. In most of the mixtures, the refractive indices no, ne, and Δn lie between the values of pure components. The optical birefringence of the mixtures is found to decrease non-additively with concentration. The molecular polarizability anisotropy and the orientational order parameter <P2> of the mixtures were calculated. Nature of variation of polarizability anisotropy with concentration is similar to that of Δn. In allmixtures <P2> is more in smectic phase than in nematic phase and it also shows non-additive behaviour. Observed <P2> values of the mixtures were compared with McMillan’s and Maier-Saupe theoretical values and good agreement is found in some mixtures. The transition temperatures obtained by fitting <P2> data to McMillan’s theory agree with the observed values within a few degrees. Ratio of elastic constants K33 and K11 is determined for mixtures having nematic phase and found to diverge at TNA transition point.

Anisotropy of the local field of the light wave in cholesteric liquid crystals

The experimental values of the Lj components of the Lorentz tensor have been obtained for the first time for the quasinematic layer of the cholesteric phase and in the smectic phase A for homologs of cholesteric fatty ethers using the dispersion dependences of the refraction indices for the planar texture of cholesteric liquid crystals (CLCs). The dependence of the Lj components on the homolog number, mesophase temperature, the magnitude of birefringence, and the change in the orientational ordering of molecules in the cholesteric phase and at the cholesteric-smectic A phase transition was determined. Isotropization of the Lorentz tensor L and the local field tensor f was found for CLCs when the birefringence of LCs and the anisotropy of molecular polarizability decreased simultaneously. The anisotropy Δf was found to be negative for the quasinematic layer of CLCs and the smectic phase in the visible range of the spectrum. The values of Lj, obtained with known local field models for CLCs and smectics A, gave positive Δf irrespective of the chemical structure of molecules, optical anisotropy of LCs, and the spectral region, which contradicts to the experiment.

Dependence of molecular polarizability on the orientational order in nematic liquid crystals

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L j and the mean molecular polarizability \( \bar \gamma \) for five nematic liquid crystals belonging to two homological series. The dependence of L j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence \( \bar \gamma \)(S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences \( \bar \gamma \)(S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.

Calculation of the anisotropy of molecular polarizability of liquid n-alkanes and n-alcohols

Light scattering from liquid n-alkanes and n-alcohols in a strong electric field is measured by a photoelectric colorimeter at various wavelengths. The anisotropy of molecular polarizability of the substances is calculated by the Rayleigh formula.

A combustion temperature and species standard for the calibration of laser diagnostic techniques

Optical diagnostic thermometry techniques are not generally traceable to temperature standards and due to their complexity it is often difficult to assess their absolute accuracy. This work aims to address this by calibrating a McKenna premixed flat-flame burner with a high-accuracy Rayleigh scattering thermometry system. By applying corrections for the temperature dependence of the scattering cross section, it has been possible to determine the temperature of a premixed propane/air flame with an uncertainty of less than 1%. The temperature dependence of the differential Rayleigh scattering cross section arises from changes in the mean molecular polarizability and mean molecular anisotropy. Changes in the cross section of up to 2% between room and flame temperatures have been found. The burner and associated gas mixing system are described here as a combustion temperature and species standard allowing the calibration of optical diagnostic techniques.