Liquid Crystal Solvent Research Articles

Anisotropy of indirect couplings and accurate molecular structures of 1,2- and 1,3-difluorobenzenes by combined analysis of gas electron diffraction, rotational spectroscopy and liquid crystal NMR data

In the combined analysis of gas-phase electron-diffraction scattering data, rotation constants and dipolar coupling constants from NMR experiments in liquid crystal solvents, not only are high-accuracy molecular structures of 1,2- and 1,3-difluorobenzene obtained, but the anisotropic components of some of the CF and FF indirect couplings have been deduced directly from the experimental data. The benefits of combined analyses of data from several different experimental techniques, and factors influencing the accuracy of structures determined in this way, are discussed.

Phase Behavior of Mixtures of Low Molecular Weight Nematic Liquid Crystals and Photochemically Crosslinked Polyacrylates

The present work deals with theoretical and experimental studies to explore some physical properties of composite materials made of crosslinked poly(n-butylacrylate) networks and low molecular weight nematic liquid crystals (LCs). The chemically crosslinked polymers were obtained by exposure to UV radiation of initial solutions composed of a reactive monomer, n-butylacrylate, a small amount of a crosslinking agent, hexanedioldiacrylate, and a photoinitiator. To obtain different polymer network densities, the ratio of n-butylacrylate to hexanedioldiacrylate was varied prior to exposure to UV irradiation. Immersion in excess LC solvent of the obtained polymer networks allowed for the measurement of the solvent uptake by determining the size increase as function of temperature by polarized optical microscopy. A strong change of the swelling ratio was found around the nematic-isotropic transition temperatures of the two LCs involved, i.e., 4-cyano-4′-n-pentylbiphenyl known as 5CB, and the eutectic commercial mixture of cyanoparaphenylene derivatives (E7). The obtained swelling data were used to establish phase diagrams in terms of composition and temperature for three different network densities and two LCs. The phase diagrams exhibit upper criticial solution temperatures and isotropic network + nematic solvent (I + N), isotropic network + isotropic solvent (I + I), and isotropic swollen network (I) coexistence regions. The results were successfully analyzed within a theoretical model using a combination of the Flory-Rehner theory of isotropic mixing and the Maier-Saupe theory of nematic ordering.

The butane condensed matter conformational problem

From the dipolar couplings of orientationally ordered n-butane obtained by NMR spectroscopy we have calculated conformer probabilities using the modified Chord (Cd) and Size-and-Shape (CI) models to estimate the conformational dependence of the order matrix. All calculation methods make use of G aussian 03 structures for the gauche and trans conformers. Calculations were performed for both the Rotational Isomeric State (RIS) approximation, as well as a continuous gas-phase potential for the dihedral angle rotation. Conformational probability distribution functions for butane as a solute in the ordered liquid–crystal solvent are obtained.

Mechanistic Origins of Hierarchical Order in Organic Monolayers Deposited From Liquid Crystal Solvents

The orientational and chiral properties of monolayer films of the fluorescent dye 3-(2-benzothiazolyl)-7-octadecyloxy coumarin (BOC) on highly oriented pyrolytic graphite have been studied by scanning tunneling microscopy and polarized absorption spectroscopy. Films were deposited from two oriented liquid crystal solvents and one isotropic solvent using a sacrificial template method in which a solvent film applied in a first deposition step is displaced by BOC in a second deposition step. Fluid-phase orientational order parameters, monolayer order parameters, and short-range orientational and chiral autocorrelation were analyzed to determine the mechanisms by which liquid crystal solvents influence order in the deposited BOC films. By comparing BOC molecular packing characteristics and ordering statistics it is concluded that the substrate potential provides the dominant influence, with rotational and chiral symmetry among the six substrate-allowed configurations broken by much weaker liquid crystal forces. A combination of elastic and anchoring forces explains most of the observed long-range order, with templating effects making a small additional contribution at submicrometer length scales. The orientation of BOC molecules in solution appeared not to be a significant factor in affecting order in the adsorbed monolayer.

Scope and Limitations of Accurate Structure Determination of Solutes Dissolved in Liquid Crystals

The determination of accurate structures of relatively small molecules dissolved in liquid-crystal solvents is no trivial matter. Extensive vibrational corrections to the observed dipolar couplings are required. Vibrational force fields are often available, but the usual harmonic corrections are strictly limited to small-amplitude internal motions. Moreover, the need to also apply anharmonic corrections and to include the elusive vibration-reorientation interaction is problematic and can only be fulfilled for a very limited set of small molecules. In this paper we discuss the implications for the accuracy of the structure of larger molecules for which this information is not available. We discuss the examples of azulene and biphenylene, and set realistic limits on their proton structures, derived from (1)H dipolar couplings extracted from NMR spectra obtained in different liquid-crystal solvents and analyzed with sophisticated evolutionary algorithms.

3H NMR of the tritiated isotopologues of methane in nematic liquid-crystal solvents

The NMR spectra of the tritiated isotopologues of methane dissolved in several nematic liquid-crystalline solvents are measured. The spectral parameters obtained agree extremely well with those predicted from earlier NMR studies of the deuterated isotopologues, providing excellent confirmation of the theory for vibration–reorientation interaction developed earlier.

Entanglement-free fibrils of aligned polyacetylene films that produce single nanofibers

An aligned polyacetylene (PA) film was synthesized in a macroscopically-aligned nematic liquid crystal (N-LC) solvent using a gravity-flow method. Long and single nanofibers of less than 100 nm in radius were successfully prepared by ultrasonication of the aligned PA film immersed in ethanol. The usual PA film was synthesized in an isotropic solvent, such as toluene, only yielding short and non-dispersed fibers after the ultrasonication due to the entangled fibril morphology. Entanglement-free fibrils of the aligned PA were well-separated into single fibrils through ultrasonication, even without a surfactant.

Uniaxially ordered conjugated polymer film prepared by electrochemical polymerization in a nematic liquid crystal with rubbing orientation method showing redox-driven tunable dichroism

We demonstrate electrochemical polymerization of an aromatic conjugated monomer in a macroscopically aligned nematic liquid crystal. This method is a combination of employment of liquid crystal as the electrolyte solution, a rubbing technique, and electropolymerization. The resulting polymer film shows linear dichroism and anisotropy. Polarizing optical microscopy reveals that the polymer film is uniformly ordered on a macroscopic scale. These results indicate that the conjugated main chains grow along a director of the aligned nematic liquid crystal solvent.

Visualization of enantiomers using natural abundant 13C-filtered single and double quantum selective refocusing experiments: Application to small chiral molecules

The routine use of proton NMR for the visualization of enantiomers, aligned in the chiral liquid crystal solvent poly-γ-benzyl-l-glutamate (PBLG), is restricted due to severe loss of resolution arising from large number of pair wise interaction of nuclear spins. In the present study, we have designed two experimental techniques for their visualization utilizing the natural abundance 13C edited selective refocusing of single quantum (CH-SERF) and double quantum (CH-DQSERF) coherences. The methods achieve chiral discrimination and aid in the simultaneous determination of homonuclear couplings between active and passive spins and heteronuclear couplings between the excited protons and the participating 13C spin. The CH-SERF also overcomes the problem of overlap of central transitions of the methyl selective refocusing (SERF) experiment resulting in better chiral discrimination. Theoretical description of the evolution of magnetization in both the sequences has been discussed using polarization operator formalism.

Solute order parameters in liquid crystals from NMR spectra solved with evolutionary algorithms: Application of double Maier–Saupe Kobayashi–McMillan theory

We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multiple spin-systems simultaneously and automatically from the NMR spectra of several solutes in several nematic and smectic liquid crystal solvents. The order parameters obtained from the dipolar couplings are used to test a novel Hamiltonian that includes two Maier–Saupe nematic terms plus Kobayashi–McMillan smectic A terms. It is shown that this Hamiltonian can rationalize the NMR experiments with physically reasonable smectic order parameters and Hamiltonian prefactors.

Orientational order of near D3h solutes in nematic liquid crystals

Solutes that are similar in size, have a shape that is close to D(3h) symmetry but contain dissimilar substituent groups (methyl versus chloro, contributing different electrostatic interactions to the anisotropic intermolecular potential), are used to delineate the short- and long-range anisotropic intermolecular interactions that lead to solute orientational order in nematic liquid crystals. The short-range interactions should be similar for all solutes and for D(3h) symmetry should yield a single independent order parameter, whereas the long-range interactions are expected to differ with solute. Short-range size and shape mechanisms account for solute orientational order measured in magic mixtures (e.g., 55 wt % ZLI-1132/N-p-ethoxybenzylidene-p(')-n-butylaniline), whereas additional mechanisms are required in other nematic liquid-crystal solvents. The results obtained for long-range interactions cannot be rationalized in detail using simple mean-field models that incorporate solute dipoles, quadrupoles, or polarizabilities. The results suggest that details of the solute electrostatics may need to be incorporated into the description of the anisotropic intermolecular potential.

Application of a 1H δ‐resolved 2D NMR experiment to the visualization of enantiomers in chiral environment, using sample spatial encoding and selective echoes

We present the application of a 2D broadband homodecoupled proton NMR experiment to the visualization of enantiomers. In a chiral environment, the existence of diastereoisomeric intermolecular interactions can yield-generally slight-variations of proton chemical shifts from one enantiomer to another. We show that this approach, which relies on a spatial encoding of the NMR sample, is particularly well suited to the analysis of enantiomeric mixtures, since it allows, within one single 2D experiment, to detect subtle chemical shift differences between enantiomers, even in the presence of several couplings. This sequence, which uses semiselective radio-frequency (rf) pulses combined to a z-field gradient pulse, produces different selective echoes in various parts of the sample. The resulting homonuclear decoupling provides an original delta-resolved spectrum along the diagonal of the 2D map where it becomes possible to probe the chiral differentiation process through every proton site where the resulting variation in the chemical shift is detectable. We discuss the advantages and drawbacks of this approach, regarding other experiments which provide homodecoupled proton spectra. This methodology is applied to the observation of enantiomers of (1) ( +/- )2-methyl-isoborneol coordinated to europium (III) tris[3-(trifluoromethyl-hydroxymethylene)-(+)-camphorate] in isotropic solution, and (2) ( +/- )3-butyn-2-ol dissolved in a chiral liquid-crystal solvent, in order to show the robustness of this pulse sequence for a wide range of chiral samples.

Self-Assembly of Coil/Liquid-Crystalline Diblock Copolymers in a Liquid Crystal Solvent

Diblock copolymers having a random-coil polymer block (polystyrene, PS) connected to a side-group liquid crystal polymer (SGLCP) self-assemble in a nematic liquid crystal (LC), 4-pentyl-4′-cyanobiphenyl, into micelles with PS-rich cores and SGLCP-rich coronas. The morphologies of block copolymers with varying PS content are characterized as a function of temperature and concentration using small-angle neutron scattering, rheometry, and transmission electron microscopy. Unlike conventional solvents, the nematic LC can undergo a first-order transition between distinct fluid phases, accessing the regimes of both strong and slight selectivity in a single polymer/solvent pair. Micelles dissolve away above a microphase separation temperature (MST) that is often equal to the solution’s isotropization point, TNI. However, increasing or decreasing the polymer’s PS content can shift the MST to be above or below TNI, respectively, and in the former case, micelles abruptly swell with solvent at TNI. Comparable effect...

Use of inverse gas chromatography for the physicochemical characterisation of a new synthesised liquid crystal: (S)‐5‐(2‐methylbutoxy)‐2‐{[(4‐dodecyloxyphenyl)imino]methyl}phenol

A new liquid crystalline compound, i.e. (S)‐5‐(2‐methylbutoxy)‐2‐{[(4‐dodecyloxyphenyl)imino]methyl}phenol (MBDOPIMP), was prepared and characterised. Phase transition temperatures of MBDOPIMP were determined by polarising optical microscopy and differential scanning calorimetry. Trace amounts of ethyl acetate, butyl acetate, isobutyl acetate, hexane, heptane, octane, nonane and decane were passed through a chromatographic column loaded with MBDOPIMP coated on Chromosorb W. The retention diagrams of the solvents on MBDOPIMP were plotted using specific retention volumes at temperatures between 105 and 125°C. Thermodynamic parameters, such as Flory–Huggins liquid–solvent interaction, , equation‐of‐state liquid–solvent interaction, , effective exchange energy, X eff, the exchange enthalpy, X 12 and exchange entropy, Q 12, were determined for the liquid crystal–solvent systems. Subsequently, the partial molar heats of sorption, ΔH¯1,sorp and mixing, , were determined from the slopes of the plots of the logarithm of the specific retention volume, ln , versus 1/T and logarithm of the weight fraction activity coefficients, ln , versus 1/T, respectively.

Comparison of the Fluorescence Characteristics of Dye Molecules in Liquid Crystal and Conventional Solvents

We investigated the uorescence characteristics of hemicyanine dye molecules in conventional solvents and in the isotropic phase of liquid-crystal 4-pentyl-4'-cyanobiphenyl. Fluorescence decay lifetime of hemicyanine in liquid crystal decreased monotonically as the temperature was increased. As compared to glycerol, the uorescence decay lifetime of hemicyanine in the isotropic phase of liquid crystal was appreciably longer. This longer decay lifetime of hemicyanine in liquid crystal can be explained by a restricted internal rotational motion of the hemicyanine molecule due to strong interaction between the dye molecule and the liquid crystal molecules as compared to other solvents.

Solvent smectic order parameters from solute nematic order parameters

In liquid crystals, while the second and fourth rank orientational order parameters characterizing a nematic phase can be experimentally determined via several techniques, there is no straightforward experiment rendering the positional order parameters characterizing a smectic A phase. This work illustrates a novel method to estimate the positional order parameters of a smectogenic liquid crystal solvent from knowledge of the orientational order parameters of a number of solutes dissolved therein. The latter order parameters can be experimentally determined via liquid crystal NMR spectroscopy. These data can be then analyzed with a statistical-thermodynamic density functional theory, whose basic ingredient is a model for solute-solvent intermolecular interactions. Its parametrization and the subsequent fitting procedure eventually permit one to obtain the positional order parameters of the solvent besides the positional-orientational distribution function of the solutes. The method is applied to the smectogen 4,4(')-di-n-heptyl-azoxybenzene, in which the solutes 1,4-dichlorobenzene and naphthalene have been dissolved. With the help of this exploratory practical example, pros and cons of the method are pointed out and further developments prospected.

Chain-Length Dependent Nematic Ordering of Conjugated Polymers in a Liquid Crystal Solvent

In this communication we demonstrate the dependence of the solute order parameter on the solute molecular weight for polymer solutes dissolved in liquid crystalline solvents. Using ensemble absorption polarization spectroscopy together with single molecule fluorescence polarization measurements, we have determined the order parameter of the conjugated polymer MEH-PPV (poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]) in the liquid crystal 5CB (4-cyano-4-n-pentylbiphenyl) as a function of polymer chain length. Ensemble absorption polarization measurements agree well with results obtained by single molecule fluorescence polarization spectroscopy, indicating a large-scale ordering of the MEH-PPV solute in 5CB. These results demonstrate that the increasing number of defects for larger polymer weights inherently limits the alignment of the polymer solute.

Mechanisms of solute orientational order in nematic liquid crystals

The order parameters of a number of different solutes dissolved in a number of different nematic liquid-crystal solvents are measured and compared. It is shown that the order parameters can all be rationalized with the ansatz that there exist two independent ordering mechanisms operating in the liquid crystals employed. With this ansatz it is possible to fit the experimental order parameters to better than 5%. This opens the possibility of the accurate prediction of order parameters in ordered liquids.

Forces between Cylindrical Nanoparticles in a Liquid Crystal

Using classical density functional theory, the forces between two cylindrical nanoparticles in a liquid crystal solvent are calculated. Both the nematic and isotropic phases of the solvent are considered. In the nematic phase, the interaction is highly anisotropic. At short range, changes in the defect structure around the cylinders leads to a complex interaction between them. In the isotropic phase, an attractive interaction arises due to overlap between halos of ordered fluid adsorbed on the surfaces of the cylinders.

Accurate structures from combined gas electron diffraction and liquid crystal NMR data; the importance of anisotropy of indirect couplings for 1,4-difluorobenzene

Gas-phase electron-diffraction scattering data and dipolar couplings from NMR experiments in four different liquid crystal solvents have been combined to give a high-accuracy molecular structure of 1,4-difluorobenzene. The anisotropic components of the CF and FF indirect couplings have been deduced directly from the experimental data. The resultant structure has standard deviations of around 0.2 pm for interatomic distances and less than 0.20 for inter-bond angles.