Spectral characteristics and nonlinear studies of crystal violet dye

Solid-state dye-doped polymer is an attractive alternative to the conventional liquid dye solution. In this paper, the spectral characteristics and the nonlinear optical properties of the dye crystal violet are studied. The spectral characteristics of crystal violet dye doped poly(methylmethacrylate) modified with additive n-butyl acetate (nBA) are studied by recording its absorption and fluorescence spectra and the results are compared with the corresponding liquid mixture. The nonlinear refractive index of the dye in nBA and dye doped polymer film were measured using z-scan technique, by exciting with He-Ne laser. The results obtained are intercompared. Both the samples of dye crystal violet show a negative nonlinear refractive index. The origin of optical nonlinearity in the dye may be attributed due to laser-heating induced nonlinear effect.     

Ionic impurities in nematic liquid crystal displays

This paper shows that the dielectric anisotropy of conductivity in cells composed of nematic liquid crystal E7 (NLC-E7) is related to the fact that the diffusion constant (D) is greater in a cell with homeotropic alignment than in one with homogeneous alignment (D _∥ > D _⊥). This behaviour can be understood by the study of the dielectric properties of the NLC based on the ionic hopping behaviour and on the analysis of the electrical conductivity in relation to the voltage applied.     

Charge-transfer exciton transitions in anthracene-PMDA crystal

The reflection spectrum of the first singlet charge transfer transition of anthracene-PMDA (pyromellitic dianhydride) crystal has been obtained at temperatures close to 2 K. The spectrum is polarized along the stack axis and consists of a series of vibronic bands, approximately 500 cm^?1 wide, with impressed fine structure arising from intermolecular phonon modes. This fine structure is particularly prominent for the 00 band which starts with a zero-phonon line which is the most dominant feature of the entire spectrum. The zero-phonon line has a reflectivity of approximately 0.37 and a line width less than 5 cm^?1. This is the first time that an absolute reflection spectrum of a charge transfer transition with such vibronic details has been obtained. A preliminary discussion of the nature of the transition is presented.     

Analytical approach to V-shaped characteristics in antiferroelectric liquid crystal displays

Not only in ferroelectric liquid crystal displays (LCDs), but also in antiferroelectric LCDs, grey levels are possible by actively addressing the ‘continuous director rotation mode’. For ferroelectric LCDs this was shown qualitatively and quantitatively in previous articles. In this article it is shown that an exact analytical approach is also possible for antiferroelectric LCDs. In two consecutive layers the director orientations are symmetric, and at zero voltage they are in a splayed state. The conditions on alignment layer thickness and interaction coefficients are related to those of ferroelectric liquid crystal but are easier to fulfill.     

Analytical approach to V-shaped characteristics in antiferroelectric liquid crystal displays

Not only in ferroelectric liquid crystal displays (LCDs), but also in antiferroelectric LCDs, grey levels are possible by actively addressing the 'continuous director rotation mode'. For ferroelectric LCDs this was shown qualitatively and quantitatively in previous articles. In this article it is shown that an exact analytical approach is also possible for antiferroelectric LCDs. In two consecutive layers the director orientations are symmetric, and at zero voltage they are in a splayed state. The conditions on alignment layer thickness and interaction coefficients are related to those of ferroelectric liquid crystal but are easier to fulfill.     

A positron annihilation study on phase transitions in trans-stilbene single crystal

Positron lifetime spectroscopic and Dopplerbroadening measurements have been carried out on trans-stilbene single crystals. The results (including the complementary DSC measurement) give new evidence for the existence of phase transitions in trans-stilbene at temperatures between 20 and 300 K.     

Dielectric studies of the hexatic B-smectic A and crystal B-smectic A transitions

Static dielectric constants, ε_∥ and ε_⊥ as well as the low frequency dielectric dispersion of ε_∥ have been studied in the vicinity of the hexatic B-smectic A transition in n-hexyl-4'-pentyloxybiphenyl-4-carboxylate (65OBC) and n-butyl-4'-hexyloxybiphenyl-4-carboxylate (46OBC) and also near the crystal B-smectic A transition in n-(4-n-butyloxybenzylidene)-4-n-octylaniline (4O. 8). While the behaviour of the dielectric anisotropy is essentially similar for both transitions, the dispersion results show significant differences.     

A complete diagrammatic construction of PCILO method

In the present short communication, the diagrammatic many-body perturbation theory for the rederivation of PCILO method (perturbation configuration interaction using localized orbitals, see Theoret. Chim. Acta (Berl.)13, 1 (1969) and15, 100 (1969)) is consequently used. Using the Goldstone-Hugenholtz linked-cluster theorem, the diagrammatic expression for the exact ground-state energy is obtained. An application of the present approach to the low-lying excited and/or ionized states is also discussed.     

Spectral locations of low-energy electronic transitions in some fluorene derivatives

The low energy singlet—singlet (S-S) and triplet—triplet (TT) absorption spectra of fluorene and 2-amino fluorene were measured. Also, the spectral locations of low-energy SS and TT intensity maxima of 2-methoxy, 2-hydroxy, 2,7-diamino, and 2-dimethylamino fluorene were recorded.When expressed in wavenumbers (energy) and within measurement accuracy, the following holds for the 2-position substituted fluorene: (a) the red shift caused by a certain substituent is about the same in both (singlet and triplet) manifolds; (b) the energy difference between SS and TT absorption maxima in fluorene as well as in all the substituted compounds is about the same and (c) the spectral red shifts observed in fluorene in the singlet and triplet manifold due to substitution by auxochromic groups can probably be used in fair approximation to estimate anticipated red shifts in other chromophores.For 2,7-diamino fluorene; p-terphenyl, p,p′-diamino, p,p′-diethylamino terphenyl, p-quaterphenyl; p,p′-diamino, p,p−diethylamino quaterphenyl, spectral relations (a) to (c) also hold rather well.     

Interdependence of liquid crystal phase transitions in mixed systems

From calorimetric measurements, the phase transition enthalpies and entropies in mixed systems of each of two cholesteryl esters with a nematogenic compound PCPB have been determined as a function of composition. The calorimetric data for the LC-I transition, as well as the slope (dp/dT)_t of the phase boundary line LC-I, show a conspicuous discontinuity at the critical mole fraction of PCPB where the blue phases disappear. The enthalpy of the SmA-N* transition becomes zero at the same critical mole fraction. Possible reasons for the observed transition interdependence are discussed.     

Interdependence of liquid crystal phase transitions in mixed systems

From calorimetric measurements, the phase transition enthalpies and entropies in mixed systems of each of two cholesteryl esters with a nematogenic compound PCPB have been determined as a function of composition. The calorimetric data for the LC-I transition, as well as the slope (dp/dT)_t of the phase boundary line LC-I, show a conspicuous discontinuity at the critical mole fraction of PCPB where the blue phases disappear. The enthalpy of the SmA-N* transition becomes zero at the same critical mole fraction. Possible reasons for the observed transition interdependence are discussed.     

Spectral characteristics of C-methylflavones

A study has been made of natural C-methylflavones and their dealkylated analogues: 5-hydroxy-4,7-dimethoxy-7-methylflavone (noreucalyptin); 4,5,7-trihydroxy-6-methylflavone; 5-hydroxy-4,7-dimethoxy-6,8-dimethylflavone (eucalyptin); 4,5-dihydroxy-7-methoxy-6,8-dimethylflavone (sideroxylin); 4,5,7-trihydroxy-6,8-dimethylflavone; 4,5,6-trihydroxy-3-methoxy-8-methylflavone (silpin) and 3,4,5,6-tetrahydroxy-8-methylflavone.All-Union Scientific-Research Institute of Medicinal Plants, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 306–309, May–June, 1984.     

Gravity-induced liquid crystal phase transitions of colloidal platelets

The influence of gravity on a suspension of sterically stabilized colloidal gibbsite platelets is studied. An initially isotropic-nematic biphasic sample of such a suspension develops a columnar phase on the bottom on prolonged standing. This phenomenon is described using a simple osmotic compression model. We performed Monte Carlo simulations of cut spheres with aspect ratio L/D=1/15 and took data from the literature to supply the equations of state required for the model. We find that the model describes the observed three-phase equilibrium quite well.     

Polymorphic transitions mediated by surfactants in liquid crystal nanodroplet

Recently, various techniques have been developed using photonic crystals. Liquid crystals (LC) confined in a nanodroplet mimicked photonic crystals, such as those of opal. Therefore, investigating the phase behaviour of LC molecules in nanodroplets is very important in the next-generation optical field. In this study, the chemical interaction between surfactants and LCs in nanodroplets is reproduced using a dissipative particle dynamics method. We identify the phase behaviour of LCs and investigate how the chemical interaction affect on the orientation of LCs. In particular, by adding surfactant molecules, various morphological behaviours were observed in the LC nanodroplet. The phase transition temperature varied depending on R_ND (amount of surfactant molecules). Furthermore, difference of the self-assembly structure also appeared inside the droplet depending on R_ND. Our simulation offers a theoretical guide to control morphologies of self-assembled LCs inside a nanodroplet, a novel system that may find applications in nanofluidic devices or in photonic crystal technology.     

A diagrammatic construction of the direct configuration interaction method

A simple diagrammatic formalism useful for describing and comparing theories of electronic structure is described. The formalism is applied to derive the direct singles and doubles configuration interaction (CI) method. The resultant equations have the form of an iterative perturbation theory and execute more rapidly on a computer than conventional CI. Examples are given of the convergence of the method.     

Infrared study of phase transitions in thermotropic liquid crystal polyesters

Three samples of semiflexible thermotropic liquid-crystalline polyesters based on mesogenic aromatic triads and a decamethylene spacer have been investigated by IR spectroscopy at various temperatures between room temperature and 290°C. The crystal-to-liquid crystal (either nematic or smectic) phase transition was accompanied by fairly strong spectral variations, whereas slight, but significant, changes in the IR profiles were detected at the liquid crystal-to-isotropic transition. By comparing the results obtained with the spectral behavior recorded for corresponding structural analogs of low molar mass, it was possible to attribute the spectral variations observed in the polymer samples to a decrease in intermolecular interactions rather than to conformational changes. The thermal transitions indicated by IR spectra were in good agreement with the analogous data obtained by calorimetric or optical microscopy techniques.     

A new approach to calculations of intensities of vibrational overtone transitions

A new approach to the theory of intensities of vibrational overtone transitions is formulated in terms of vibrational coupling of the molecular ground state to excited electronic states. Model calculations indicate an important role of nuclear geometry of the excited electronic states in determining overtone spectra. It is shown that the observed overtone spectrum of the CH stretching mode in benzene can be reproduced theoretically with the assumption that CH bond lengths in the e_lu electronic state are shortened relative to the ground configuration. A simple rule for qualitative prediction of the overtone spectra for diatomic molecules is proposed.     

From crystal to micelle: A new approach to the micellar structure

The structure of sodium and rubidium deoxycholate micellar aggregates in aqueous solutions was found to be helical and to be stabilized mainly by polar interactions. Astonishingly, the lateral surface of the helix is covered by nonpolar groups and the interior part is filled with cations surrounded by water molecules, as in the case of an inverted micelle. This helical model was inferred from the crystal structures of sodium and rubidium deoxycholates and proved by spectroscopic and diffractometric experimental data. The strategy of the approach to the determination of the micellar structure and the comparison with another model, previously proposed for the bile salt micelles, are reported. On the basis of some results obtained for sodium tauro- and glyco-deoxycholates, micellar models are suggested which could account for the biological function of these important conjugated bile salts.