Liquid Crystalline Medium Research Articles

Long Pitch Orthoconic Antiferroelectric Binary Mixture for Display Applications

Orthoconic Antiferroelectric Liq. Cryst. (OAFLC) have been extensively studied due to their unique electrooptical properties [1,2]. Practical implementation of this kind of liquid crystalline medium experiences many obstacles including a number of parasitic effects deteriorating the electrooptical performance. In search for working OAFLC a mixture with the helical pitch longer than the typical cell gap was formulated. Using this mixture near perfect optical uniformity and excellent dark state at the zero electric field applied was obtained. The physical and structural properties of mixture are presented and their parameters are discussed. The electrooptical performance is presented and discussed.

Formation of a 1,8-Octanedithiol Self-Assembled Monolayer on Au(111) Prepared in a Lyotropic Liquid-Crystalline Medium

A characterization of the 1,8-octanedithiol (ODT) self-assembled monolayer (SAM) formed from a Triton X-100 lyotropic medium has been conducted by electrochemical techniques. It is found that an ODT layer of standing-up molecules is obtained at short modification time without removing oxygen from the medium. The electrochemical study shows that the ODT layer formed after 15 min of modification time has similar electron-transfer blocking properties to the layers formed from organic solvents at much longer modification times. On the basis of XPS data, it is demonstrated that the inability to bind gold nanoparticles (AuNPs) is due to the presence of extra ODT molecules either interdigited or on top of the layer. Treatment consisting of an acid washing step following the formation of the ODT-Au(111) SAM produces a layer that is able to attach AuNPs as demonstrated by electrochemical techniques and atomic force microscopy (AFM) images.

ChemInform Abstract: Liquid Crystals as Solvents in NMR Spectroscopy: Current Developments in Structure Determination

Nuclear Magnetic Resonance (NMR) spectroscopy of molecules aligned in liquid crystalline media provides information on molecular structure and order parameters. The partial alignment of the molecules in the anisotropic phase of the liquid crystals gives rise to residual intramolecular dipolar couplings which are dependent on the internuclear distances and therefore yield information on the geometry of molecules. The spectra of these aligned molecules become rapidly complex with the increase in the number of interacting spins and with decrease in the symmetry of the molecules. The spectra of these molecules are generally strongly coupled and are complex. Thus the first order analysis of the spectra as applicable to liquid state is generally not applicable and one should resort to numerical analysis using computers. In this report problems associated with the analyses of such complex spectra, methods developed to aid such analyses and the precision of the structural parameters derived are discussed. Use of mixed liquid crystals of opposite diamagnetic susceptibility anisotropies to extract information which is otherwise not possible to derive using single liquid crystals is highlighted. Current developments in the field with special emphasis on the discovery of a novel liquid crystal with low order parameter and the use of natural abundance deuterium NMR spectra of the probe molecules for the analyses of complex spectra are discussed.

Selective Homonuclear Decoupling in 1H NMR: Application to Visualization of Enantiomers in Chiral Aligning Medium and Simplified Analyses of Spectra in Isotropic Solutions

The proton NMR spectral complexity arising due to severe overlap of peaks hampers their analyses in diverse situations, even by the application of two-dimensional experiments. The selective or complete removal of the couplings and retention of only the chemical shift interactions in indirect dimension aids in the simplification of the spectrum to a large extent with little investment of the instrument time. The present study provides precise enantiodiscrimination employing more anisotropic NMR parameters in the chiral liquid crystalline medium and differentiates the overlapped peaks of many organic molecules and peptides dissolved in isotropic solvents.

Dielectric Properties of Strongly Polar Nematogens

Results of the static and dynamic dielectric investigations of four liquid crystalline compounds consisting of two or three phenyl rings with n-alkoxy and cyano terminals, carboxylic bridging groups, and fluoro lateral substituents in pure state and in the solution of low polar liquid crystalline medium, are presented. These compounds form the nematic phase being a monotropic one in the case of two ring compounds. The dielectric parameters are analyzed in relation to the dipolar structures of the molecules. The dielectric properties of pure substances are strongly influenced by conductivity and dimerization effects, while extracted from the data for solutions are consisted well with the structures. In the nematic phase the compounds exhibit a crossover of the principal permittivity components among the kilo and megahertz frequency range

Electric-Field Effects in Dilute Suspensions of Carbon Nanotubes Dispersed in Nematic Liquid Crystals

Colloids composed of liquid-crystal hydrosols exhibit a rich set of interesting phenomena. The coupling between liquid-crystalline media and colloidal particles plays an essential role leading to an abundant source of new physics. In the last few years, peculiar behaviors of carbon-nanotube-doped calamitic liquid crystals have attracted considerable attention. This paper provides a brief introduction to this alluring subject for its on-going research development in this laboratory. First presented are our current understandings of the nematic colloidal system comprising carbon nanotubes and of their possible orientation and dynamics under the application of an external field. Various electro-optical and electrical properties of a liquid-crystal display rectified by the nanoscale carbonaceous guest are then addressed to a larger extent. Dielectric relaxation obtained from a nematic impregnated with carbon nanotubes is also discussed. With historical significance for the dawn of the liquid-crystal–carbon-nanotube research, several important findings of enhanced nonlinear optical properties in typical nematic mesomaterials consisting of suspended nanotubes are delineated. With the new colloidal systems of elongated nanoscale solids dispersed in anisotropic fluids in the mesophase, many new intriguing phenomena are awaiting theoretical and experimental explorations. Collaborations are called to draw attention of interested theoretical physicists, in particular.

Design of 2D Binary Colloidal Crystals in a Nematic Liquid Crystal

In this paper, we examine directed self-assembly in a 2D binary system of dipolar and quadrupolar colloidal particles with normal surface boundary conditions, dispersed in the nematic liquid crystal. Using the laser tweezers, we assembled a large variety of stable 2D colloidal crystal structures. In all analyzed structures, the particles, their surface treatment and the cell conditions were the same, which gives us the ability to systematically follow the evolution of colloidal assembly when many particles are present. We present an analogy between molecular self-assembly and organization of colloidal microspheres in liquid crystalline medium to extend the strategy for designing colloidal crystalline structures of different level of complexity.

Dielectric spectroscopy of liquid crystals. Theoretical model of ITO electrodes influence on dielectric measurements

AbstractIndium tin oxide (ITO) layers are widely used to make electrodes in measuring cells, because these layers are transparent and electrooptical investigations can be performed using such prepared cells. It was found during the dielectric spectroscopy measurements, performed for smectic liquid crystalline mixture, that it is not possible to detect some important relaxation modes in paraelectric SmA*, ferroelectric SmC*, and antiferroelectric SmCA* phases for the frequencies higher than 300 kHz. The measuring cell does not allow to measure relaxations, because its own dielectric behaviour covers the dielectric response of a liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to the finite resistance of ITO layers. The theoretical model was introduced, which shows how to calculate relaxations related to liquid crystals from dielectric response of the empty and filled measuring cell. The proof of strong influence of cell properties on effective (measuring) values of dielectric permittivities was shown.

Cellulose-Based Anisotropic Composites

In an attempt to improve the mechanical properties of an all-cellulose based composite we made solid flexible films containing microcrystalline cellulose fibres produced by a shearing casting technique. To an anisotropic (liquid crystalline) solution of 2-hydroxypropylcellulose (HPC) in N,N – dimethylacetamide (60 w/w %) were added AVICEL fibres (0, 2, 4, 6, 10, 12 w/wHPC %). After homogenization these solutions were poured into a Teflon plate and sheared at a constant rate of 5 mm/s. The cellulosic microfibres in the liquid crystalline polymer medium were partially aligned by shear flow. Mechanical and morphological properties of the films were investigated using tensile tests, polarized optical microscopy and scanning electron microscopy. The mechanical properties of these films, as expected, are higher than those of the films produced from an isotropic solution of HPC. The results show that the orientation of the microfibres, and the mesoscopic properties, can be tuned by adjusting the topography of the nematic matrix in the micro-nano scale.

Technical aspects of dielectric spectroscopy measurements of liquid crystals

AbstractThe dielectric spectroscopy measurements were performed for antiferroelectric liquid crystalline mixture. For this purpose, the cells with ITO electrodes were prepared. It was found that it is not possible to detect some important relaxation modes in Sm A*, Sm C*, and Sm CA* phases. The own cell mode (related to cell properties, i.e., capacity and resistivity) covers the dielectric response of liquid crystalline medium. Dielectric measurements in cells with gold electrodes were done to show all possible relaxations in antiferroelectric liquid crystals (LCs).

On an effective model for ferronematic liquid crystals

AbstractThe tools of homogenization theory are used to develop an effective model for ferronematics–colloidal suspensions of small ferromagnetic particles in a nematic liquid crystalline medium. The results are presented for dilute suspensions of identical prolate spheroidal particles. For needle‐like particles of large eccentricity, the model reduces to a known expression [5]. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of alignment tensors generated for native tRNAVal using magnetic fields and liquid crystalline media

Residual dipolar couplings (RDCs) complement standard NOE distance and J-coupling torsion angle data to improve the local and global structure of biomolecules in solution. One powerful application of RDCs is for domain orientation studies, which are especially valuable for structural studies of nucleic acids, where the local structure of a double helix is readily modeled and the orientations of the helical domains can then be determined from RDC data. However, RDCs obtained from only one alignment media generally result in degenerate solutions for the orientation of multiple domains. In protein systems, different alignment media are typically used to eliminate this orientational degeneracy, where the combination of RDCs from two (or more) independent alignment tensors can be used to overcome this degeneracy. It is demonstrated here for native E. coli tRNA(Val) that many of the commonly used liquid crystalline alignment media result in very similar alignment tensors, which do not eliminate the 4-fold degeneracy for orienting the two helical domains in tRNA. The intrinsic magnetic susceptibility anisotropy (MSA) of the nucleobases in tRNA(Val) was also used to obtain RDCs for magnetic alignment at 800 and 900 MHz. While these RDCs yield a different alignment tensor, the specific orientation of this tensor combined with the high rhombicity for the tensors in the liquid crystalline media only eliminates two of the four degenerate orientations for tRNA(Val). Simulations are used to show that, in optimal cases, the combination of RDCs obtained from liquid crystalline medium and MSA-induced alignment can be used to obtain a unique orientation for the two helical domains in tRNA(Val).

The mutual influence of a liquid crystalline medium and chiral dopants of 1R,4R-n-mentane-3-one 2-arylidene derivatives dissolved in it

Molecular dynamics computer simulation and quantum chemistry methods were used to study the influence of nematic liquid crystalline 4-n-pentyl-4′-cyanobiphenyl on the conformations of chiral dopant molecules of 1R,4R-n-menthane-3-one 2-arylidene derivatives and the influence of these chiral dopants on the conformation of the liquid crystal. The simulation results are compared with the available experimental data.

Modulating Protein Alignment in a Liquid-Crystalline Medium through Conservative Mutagenesis

The Saupe matrix describing protein alignment in a liquid-crystalline medium contains five independent elements, enabling the generation of up to five linearly independent alignment conditions. Measurement of internuclear residual dipolar couplings by NMR spectroscopy under these conditions, orthogonal in five-dimensional alignment space, provides access to the amplitude, asymmetry, and direction of motions of the internuclear vector. It is demonstrated for the small protein domain GB3 (56 residues) that suitably orthogonal alignment conditions can be generated in a single liquid-crystalline medium of Pf1 phage, by generating a series of conservative mutants that have negligible impact on the time-averaged backbone structure of the domain. Mutations involve changes in the charge of several solvent-exposed side chains, as well as extension of the protein by either an N- or C-terminal His-tag peptide, commonly used for protein purification.

Different reorientational regimes in a liquid crystalline medium undergoing multiple irradiation

We present a numerical approach to the nemato-elasticity differential equation in a nematic liquid crystal cell when irradiated with multiple gaussian beams. Solutions have been carried out on a configuration with two coplanar beams illuminating the sample in order to compare it with particular nonlinear phenomena experimentally studied in the past. A new set of experimental measures were realized confirming the validity of the numerical model. Solutions for an instable case showing nonlocal effects are also presented as an example of the broader class of systems this approach can describe.

13C– 1H HSQC experiment of probe molecules aligned in thermotropic liquid crystals: Sensitivity and resolution enhancement in the indirect dimension

The spectra of molecules oriented in liquid crystalline media are dominated by partially averaged dipolar couplings. In the 13C– 1H HSQC, due to the inefficient hetero-nuclear dipolar decoupling in the indirect dimension, normally carried out by using a π pulse, there is a considerable loss of resolution. Furthermore, in such strongly orienting media the 1H– 1H and 13C– 1H dipolar couplings leads to fast dephasing of transverse magnetization causing inefficient polarization transfer and hence the loss of sensitivity in the indirect dimension. In this study we have carried out 13C– 1H HSQC experiment with efficient polarization transfer from 1H to 13C for molecules aligned in liquid crystalline media. The homonuclear dipolar decoupling using FFLG during the INEPT transfer delays and also during evolution period combined with the π pulse heteronuclear decoupling in the t 1 period has been applied. The studies showed a significant reduction in partially averaged dipolar couplings and thereby enhancement in the resolution and sensitivity in the indirect dimension. This has been demonstrated on pyridazine and pyrimidine oriented in the liquid crystal. The two closely resonating carbons in pyrimidine are better resolved in the present study compared to the earlier work [H.S. Vinay Deepak, Anu Joy, N. Suryaprakash, Determination of natural abundance 15N– 1H and 13C– 1H dipolar couplings of molecules in a strongly orienting media using two-dimensional inverse experiments, Magn. Reson. Chem. 44 (2006) 553–565].

Electron-Transfer Studies in a Lyotropic Columnar Hexagonal Liquid Crystalline Medium

We have studied the electron-transfer properties of some redox systems on a gold electrode in a lyotropic hexagonal columnar liquid crystalline phase (H1 phase) using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The liquid crystalline medium consists of the nonionic surfactant Triton X-100 and water. The redox reactions that have been studied are ferrocene/ferricenium, [Fe(CN)6]3-/4-, and [Ru(NH3)6]3+/2+. We have confirmed by polarizing optical microscopy that the liquid crystalline nature of the medium is maintained even after the addition of the redox species and the supporting electrolyte. The CV studies show a significant shift in the half-peak potentials of these redox reactions in the liquid crystalline medium. From the EIS studies, we have measured the diffusion coefficients and the kinetic parameters for the redox species. These results are discussed and compared with the values obtained in the conventional solvent medium. The CV and impedance studies demonstrate that the hexagonal columnar phase provides a novel controlled environment for the study of electron-transfer reactions in biological and physiological media.

Self-assembled monolayers (SAMs) of alkoxycyanobiphenyl thiols on gold surface using a lyotropic liquid crystalline medium

We have studied the self-assembled monolayers (SAMs) of alkoxycyanobiphenyl thiols on gold prepared in a lyotropic liquid crystalline medium using electrochemical techniques. We have used thiols of different alkyl chain lengths viz., C 5, C 8 and C 10 which exhibit nematic liquid crystalline order in bulk. They are dispersed in a hexagonal lyotropic liquid crystalline phase, consisting of a non-ionic surfactant, Triton X-100 and water. This medium provides a highly hydrophobic environment to solubilize the thiols and later facilitate their delivery to the gold surface to form a monolayer. The electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy were used to evaluate the barrier property and ionic permeability of these monolayers on gold surface. We have compared our results with that of the corresponding monolayers prepared using dichloromethane as a solvent. We find from our studies that the monolayers prepared using the hexagonal liquid crystalline phase show a better electrochemical blocking ability towards the redox reactions and exhibit very low ionic permeability. From the impedance studies, we have determined a surface coverage value of >99.9% for the monolayer on Au surface for all the thiol molecules studied in this work. We have also proposed a model for the likely processes involved in the formation of monolayer from the liquid crystalline medium. This is the first example in literature of self-assembled monolayer formation on Au by a compound exhibiting bulk nematic phase that is dispersed in a lyotropic liquid crystalline medium.

NMR assessment of the global shape of a non‐labelled DNA dodecamer containing a tandem of GT mismatches

We have carried out a solution study of two non-labelled self-complementary DNA dodecamers d(GACTGTACAGTC)2 and d(GACTGTGCAGTC)2 by NMR, the second sequence composed of two G-T mismatches. Structures were determined using distances extracted from NOE effects alone or using both NOE and RDC constraints, measured in three different liquid crystalline media. We ensured that our data on the influence of the mesogen on the DNA structures, and the way in which the RDCs were incorporated as constraints in the protocol refinement, were consistent. We also tested the influence of different sets of RDCs and the best means of optimizing the calculation of D(a) and R. Resolution and accuracy of the ten best energy final structures were compared. The addition of a small set of RDC constraints significantly improves the final determined structures. We took advantage of the specificity of the RDC, i.e. it contains orientational information, and explored the global shape of the DNA duplexes; it was found that the duplexes do not have a large curvature. For the G-T base pair, we observed, in this particular sequence (tandem of G-T mismatches), a new pattern of base pairing, which involved the formation of a bifurcated hydrogen bond.

Laser manipulation in liquid crystals: an approach to microfluidics and micromachines

Laser trapping of particles in three dimensions can occur as a result of the refraction of strongly focused light through micrometre-sized particles. The use of this effect to produce laser tweezers is extremely common in fields such as biology, but it is only relatively recently that the technique has been applied to liquid crystals (LCs). The possibilities are exciting: droplets of LCs can be trapped, moved and rotated in an isotropic fluid medium, or both particles and defects can be trapped and manipulated within a liquid crystalline medium. This paper considers both the possibilities. The mechanism of transfer of optical angular momentum from circularly polarized light to small droplets of nematic LCs is described. Further, it is shown that droplets of chiral LCs can be made to rotate when illuminated with linearly polarized light and possible mechanisms are discussed. The trapping and manipulation of micrometre-sized particles in an aligned LC medium is used to provide a measure of local shear viscosity coefficients and a unique test of theory at low Ericksen number in LCs.