Electrooptical properties of mesogenic chain molecules in solution and in nematic state

Electrooptical characteristics of mesogenic chain molecules in solution and in mesophase can be described in terms of intra- and intermolecular orientational orders. The value and sign of electric birefringence δn in a solution of kinetically rigid chain molecules are determined by the combination of two factors: intramolecular orientational order which depends on the dipolar and anisotropic architecture of the molecule and intermolecular orientational order caused by the action of the external electric field E. The value and sign of the dielectric anisotropy δε of the polymer nematic phase are also determined by the combination of intra- and intermolecular orders. However, in this case the latter is not maintained by the external field but by the nematic potential of the mesophase. Therefore, comparative investigations of electrooptical properties of polymers in solutions and in nematic melts make it possible to obtain information about the intra- and intermolecular orientational orders of the molecules under investigation in these two states. These investigations were carried out using the method of electric birefringence in solutions and the method of orientational deformations of nematic textures in an electric field. The objects being investigated were nematogenic dimers and trimers. Experimental data obtained for these compounds showed the presence of intramolecular order in their molecules, which is manifested in the odd-even oscillations of the value and sign of Kerr constant K≈δn/E² in solution and δε in the nematic phase when the number of C-C bonds in the methylene spacers of these molecules is varied. This effect is particularly dramatic in the mesophase where it is enhanced by intermolecular nematic potential.     

Refractometric studies of mesogenic molecules in isotropic solutions

The Lorenz-Lorentz equation for binary isotropic mixtures, consisting of anisotropic molecules, has been derived using the point-dipole approximation taking into account only pair molecular correlations. This permits the calculation of the effect of molecular correlations on the refractive index. Special attention has been paid to the case of infinite dilution in the solvents consisting of isotropic molecules, where an experimental check of the equations is possible. The specific refraction of some solutes with different molecular polarizability anisotropy and polarity has been studied in various solvents. It has been shown, that the main theoretically predicted features are observed in these experiments, but for a quantitative comparison information on the two-particle distribution function is needed.     

Highly polarised electroluminescence in low aspect ratio mesogenic molecules

We have synthesised the low aspect ratio mesogenic compounds containing one fluorene and two biphenyl moieties, and investigate their mesomorphic properties and the resultant anisotropic properties in both photoluminescence and electroluminescence. We introduced pentyloxy as a terminal flexible group in order to guarantee formation of mesophase and varied the lateral flexible group from propyl to octyl to control the mesogenic transition temperature. Eventually, in spite of the low aspect ratio mesogen, high polarisation ratio (PR) was achieved by spin-coating of luminescent nematogenic molecules. Furthermore, the PR was significantly enhanced by the additional thermal annealing process.     

Photoluminescence of polysiloxane containing mesogenic 4-cyanobiphenyl in solution

Photoluminescent characteristics of polysiloxane containing pendant mesogenic 4-cyanobiphenyl (CB) and its monomeric analogues were studied in THF solution. An excimer formation was observed, and the increase in CB content in polysiloxane induced the increase in the fluorescence intensity from the excimers. The fluorescence decays were analyzed by triexponential function. A high CB content induced small decay rates, indicating the decrease of nonradiational decay. Such suppression of the nonradiation decay in higher CB content was explained by a strong mesogenic interaction between CB groups in solution. © 1996 John Wiley & Sons, Inc.     

Films of novel mesogenic molecules at air-water and air-solid interfaces

Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide counterion (Im-Tp) at A-W and air-solid interfaces. The monolayer phases were investigated at the A-W interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different surface pressures by Langmuir-Blodgett technique were studied by employing atomic force microscopy. The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate a transformation of the monolayer from face-on to edge-on structure.     

Light scattering and dielectric studies of molecular association in mesogenic solutions

Results are reported of light scattering and dielectric measurements on solutions of 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-pentyl-4'-cyanobicyclo-hexane (5CCH). Correlation factors g₁ and g₂ are deduced from the measurements, and are discussed in terms of a model for molecular association. It is concluded that at low concentrations the association between 5CB molecules is substantially greater than that between 5CCH molecules.     

Dielectric studies on some mesogenic,non-mesogenic and organic molecules

Two important molecular parameters, namely, the dielectric relaxation time τ and electric dipole moment are highly useful in having insight into the molecular structure, size, shape, apart from the inter- and intra-molecular forces, etc. With these in view dielectric measurements in benzene at room temperature on the pure samples of o-ethyl phenol, 2-n-butylphenol, 4-n-butylphenol, 2,6-dimethoxyphenol and 3,4-difluorophenol were carried out at a frequency of 9.98 GHz by employing concentration variation method. Similar measurements, on a single weight fraction of each of them at 9.98 GHz and also at 8.74 GHz are carried out. Measurements on a single weight fraction in benzene of each of the liquid crystal samples, namely, EPCP.car (Butyl-p-(p-ethoxy phenoxy corbonyl)-phenyl carbonate), PPPB (p-pentylphenyl-p-propyl benzonate), EPAP.Hp (p-(p-ethoxyphenyl azo) phenyl heptenate) and EPAP. Und (p-(p-ethoxyphenyl azo) phenyl undecylenate were also carried out at the said two frequencies. Using the obtained values of ε′, ε′′ relaxation time and dipole moment were determined using different methods. The obtained results are compared with earlier results wherever available.     

Investigation of the association of aromatic dyestuff molecules in aqueous solution by1H NMR

We have studied experimentally the proton chemical shifts of the molecules of Acridine Orange and proflavine in aqueous solution as a function of the.concentration of the aromatic ligands. A method is proposed for determining the chemical shifts of the protons in associations from the observed concentration dependence of the proton chemical shifts of dyestuff molecules in solution, which can be used at fairly high concentrations of the interacting molecules. The association constants of the dyestuff molecules and the proton chemical shifts in the association have been calculated. The proton shifts obtained have been used toether with a model of the ring currents to determine the most probable structure for the 1:1 molecular complexes of Acridine Organce and proflavine in aqueous solution.Sevastopol Institute for Instrument Construction. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, No. 1, pp. 83–89, January–February, 1991. Original article submitted February 2, 1988.     

X-ray diffraction studies of binary mixtures of rod-like and biforked mesogenic molecules

Two binary mixtures of a liquid-crystalline biforked molecule with derivatives of the p-terephtaldehyde-bis-alkyl-anilines have been studied. The phase diagrams as determined by differential thermal analysis and optical observations show a continuous miscibility in the nematic and smectic C phases. Information on the molecular conformation of the biforked molecules is derived from X-ray diffraction studies of the binary mixtures.     

Dynamics of the self-assembling of mesogenic molecules in the prenematic region of isotropic liquid

The linear and nonlinear dielectric relaxation spectra, recorded in the isotropic phase of mesogenic 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT), reveal an equilibrium between the molecules that are involved and non-involved in the pseudo-nematic domains, which are spontaneously formed in the prenematic region. A perturbation of the equilibrium by a static electric field of high strength (E0 approximately 10(7) V/m) is followed by a nonlinear dielectric relaxation process that, measured with the probing electric field E(omega) of small amplitude and variable high frequency (up to 100 MHz), reflects the rate of the domains formation. The fraction of the mesogenic molecules involved in the prenematic domains at different temperatures is estimated.     

Self-assembly of azo molecules to mesogenic phasmid-like materials through inter-molecular hydrogen bonding

A series of acid-functionalised new azo compounds was synthesised and characterised. The constituent molecules self-organise to form dimers through inter-molecular hydrogen bonding, resulting in phasmid-like compounds. This was once considered to be a promising molecular architecture to form biaxial nematic phases. The mesomorphic properties of these new azo compounds were studied using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. Investigations revealed that these compounds form nematic and columnar mesophases. A few more compounds were synthesised by replacing the??N=N- linkage in these compounds with??CH=N- and??COO- to study the effect of different linkages on the mesomorphic properties in such molecular systems. All were found to be liquid crystalline. The compounds with a??N=N- linkage are more conducive to mesomorphism and are thermally very stable. The effect of number of alkoxy chains on the mesomorphic properties of this system was also studied. To the best of our knowledge, at present, only a handful of phasmid-like mesogenic compounds, formed by the intermolecular hydrogen bonding, are known.     

Thermalization of photoexcited molecules in solution

Analysis of the steady-state absorption/fluorescence spectra of several laser dyes in room temperature solution suggests that the fluorescing molecules, if initially formed in a vibrationally excited state, lose their excess energy surprisingly slowly, remaining significantly warm on a nanosecond time scale. A similar analysis of the steady-state absorption/fluorescence spectra of GaAs confirms that in this case the carriers are fully thermalized, possessing no excess energy when they recombine on a microsecond time scale. A classical model which accounts qualitatively for slow molecular cooling is presented. We conclude that rapid molecular photoconversion processes are likely to involve incompletely cooled (thermalized) excited states.     

The average shapes of flexible mesogenic molecules On the choice of reference frame

The flexibility of most mesogenic molecules creates a problem when we wish to consider their shape, especially as the anisotropy in this is often invoked to explain liquid-crystalline behaviour. It has been suggested that images constructed from the average positions of atoms in the molecule with superimposed ellipsoids representing the mean square displacements of the atoms could present one solution to the problem. However, the nature of the image necessarily depends on the choice of the reference frame set in the molecule. Here we show how the images change as the atoms used to define the reference frame move along a single heptyl chain attached to a mesogenic core in both the isotropic and nematic phases. The appearance of the images and how they change with phase and reference frame are discussed in general terms. In addition, the possible relationship between the images and the order parameters for axes set in the same reference frame as that used to construct the image is explored. The significance of the fluctuational ellipsoids is also considered although they appear only to influence the order parameters at second order.     

Poloxamer association in aqueous solution

Aqueous solutions of the surface active poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers (poloxamers) were studied using photon correlation spectroscopy (quasi-elastic light scattering) and viscosity measurements. Poloxamers 184 and 237 showed detectable aggregates at 25° only at concentrations above about 6% with size increasing with concentration and with significant polydispersity, probably indicating a multiple association process. At 35°, however, essentially invariant values for the hydrodynamic radius were found over a wide concentration range and the systems were essentially monodisperse: these systems are more likely to be represented by a closed association model. The more hydrophilic poloxamer 188, however, retained its concentration dependence of aggregate size up to 55°. The variation with temperature of both the hydrodynamic radius of aggregates and the intrinsic viscosity of several poloxamers was rationalized by relating the temperature-dependency curves to the cloud point of the poloxamer. In some cases only certain sections of the curve are observable when the cloud point is high, e.g., >100°, or low, e.g., <40°.     

Modeling absorption spectra of molecules in solution

The presence of solvent tunes many properties of a molecule, such as its ground and excited state geometry, dipole moment, excitation energy, and absorption spectrum. Because the energy of the system will vary depending on the solvent configuration, explicit solute–solvent interactions are key to understanding solution-phase reactivity and spectroscopy, simulating accurate inhomogeneous broadening, and predicting absorption spectra. In this tutorial review, we give an overview of factors to consider when modeling excited states of molecules interacting with explicit solvent. We provide practical guidelines for sampling solute–solvent configurations, choosing a solvent model, performing the excited state electronic structure calculations, and computing spectral lineshapes. We also present our recent results combining the vertical excitation energies computed from an ensemble of solute–solvent configurations with the vibronic spectra obtained from a small number of frozen solvent configurations, resulting in improved simulation of absorption spectra for molecules in solution.     

Aggregation of rose bengal molecules in solution

Absorption, emission and excitation spectral data support the thesis that rose bengal forms H-type aggregates in water and polar, protic solvents.     

Conformational energies of para-aromatic and aryl-aliphatic polyamides: Relation to mesogenic character in solution

PCILO conformational calculations have been carried out on several model compounds of aromatic polyamide oxamides and polyamide hydrazides. The comparison of the entropy of the equilibrium conformations allows a classification of the various constitutive units into “rigid” segments, and “flexible” ones having respectively low or high entropies. From the results, it is possible to predict the mesogenic nature of solutions of a large number of aromatic polyamides, polyamide hydrazides, and polyamide oxamides.     

Theoretical study of the association of glycine molecules on ionic crystals NaCl,KI, LiF in aqueous solution

Summary The adsorption of glycine on different crystals (namely NaCl, KI, LiF) is studied in an isolated state and in water by methods of theoretical chemistry in view of determining the preferential adsorption sites, the modifications of adsorbing crystal properties, and the role played by the electronic structure of the adsorbed molecule, by the solvent and by the parametric concordance on various sites of the structure.     

Sub-picosecond relaxation of large organic molecules in solution

Using sub-picosecond ultraviolet and visible pulses we have studied relaxations from highly-excited molecular vibronic states with a resolution of 2 × 10^?13 s.