Damien Gerard Mcdonnell

Dipole Moments and the Smectic C Phase

Abstract Current theories of the smectic C phase depend primarily on the presence of dipole moments. However, certain mesogens have now been prepared which suggest that an alternative view relating to the origin of these tilted lamellar phases will have to be considered.

Synthesis, mesomorphic behaviour and optical anisotropy of some novel materials for nematic mixtures of high birefringenceElectronic supplementary information (ESI) available: experimental procedures for all compounds not already given in this paper. See http://www.rsc.org/suppdata/jm/b4/b400630e/

Structural moieties including core units (such as phenyl, naphthyl and thiophenyl), linking groups (such as ethynyl), terminal substituents (such as cyano, isothiocyanato and fluoro), and lateral fluoro substituents have been incorporated into novel materials designed to confer a high birefringence on nematic mixtures. The materials have all been prepared through convergent syntheses involving palladium-catalysed cross-coupling reactions of arylboronic acids. The materials were examined for their mesomorphic behaviour as neat materials and as mixtures; some materials were found to have extremely high nematic phase stability, and others were non-mesogenic. The materials were evaluated in nematic host mixtures for their optical anisotropy, and they were all found to show promisingly high values.

The relationship between molecular structure and mesomorphic properties of 2,2′- and 3,2′-difluoroterphenyls synthesized by palladium-catalysed cross-couplings

Abstract Several homologues of two structurally different types of mesogenic difluoroterphenyls have been prepared. One set of compounds contains two inner-core fluoro-substituents and the other set possesses both an inner-core and an outer-core fluoro-substituent. The use of various types of palladium-catalysed cross-coupling reactions was invaluable in the synthesis (for example, selective couplings, and couplings to trifluoromethanesulphonate derivatives). Additionally, a novel selective lithiation is reported. The mesogenic materials are all low melting and the types of mesophase generated are considerably structure dependent. However, most of the materials support the tilted Sc phase which is required of ferroelectric host materials. The mesomorphic behaviour of these novel materials is discussed in terms of the molecular structure and the structure/property relationships are compared with those for difluoroterphenyls with different arrangements of lateral fluoro-substitution.

Synthesis, optical anisotropies, polarisabilities and order parameters of 4-cyanophenyl and 4-isothiocyanatophenyl 4'-butylsulfanylbenzoates with oxygen and sulfur substitution in the ester linkage

The optical anisotropies, polarisabilities, refractive indices and order parameters of 4-cyanophenyl and 4-isothiocyanatophenyl 4′-butylsulfanylbenzoates with oxy- and thin-substitution in all possible combinations in the ester link have been measured and the results are discussed in terms of the position and number of sulfur atoms in the ester linkage. The S-aryl thio- and dithio-benzoates have greater polarisabilities than the corresponding O-aryl esters; the differences between O-aryl and S-aryl thiobenzoates are discussed in terms of the electronegativity and structural changes caused by substitution with sulfur.

On the influence of short range order upon the physical properties of triphenylene nematic discogens

Abstract The dielectric and elastic properties of two novel triphenylene nematic discogens are measured and compared with those of calamitic nematics and the known material H7OBT. The new compounds have nematic phases at much lower temperatures than H7OBT. All three compounds have a small positive permittivity anisotropy, indicating the presence of a permanent dipole contribution parallel to the director. Measurement of the molecular moment dipole provides a value consistent with nearly-free rotation of the substituent groups. The temperature dependence of ∊ and the concentration dependence of the dipolar strength Δ show evidence of antiparallel dipole correlations in the ND phase. The strong temperature dependence of |k 33/Δχ|, the large value of k11/k 33 and the rise in k 11/k 33 as the temperature approaches the clearing point are believed to be due to short range columnar order. Theoretical study suggests that dipole correlations favour local columnar order, and we predict that antiparallel dipole cor...

The Influence of the Backbone on the Structure of Side-Chain Liquid Crystal Polymers

Abstract Structural investigations using X-ray diffraction on four side-chain liquid crystal polymers with the same mesogenic units but different backbones are described. An S*C phase was identified in all of them. Three of the polymers exhibit an unusual phase with a superlattice structure below this S*C phase. A simple model of the packing in this phase is proposed to explain the formation of the superlattice. The effects of varying the polymer backbone on the stability of this phase are also discussed.

The Physical Properties of the Cyanophenylcyclohexyl Ethanes (PECH)

Abstract The physical properties of a new homologous series of stable, low-melting nematic materials, the cyanophenylcyclohexyl ethanes (PECH) are reported and compared with those of other stable, nematic materials. A broad range mixture is reported with a low viscosity, medium birefringence and large positive dielectric anisotropy. This mixture shows a high order parameter which increases the contrast of displays using dissolved pleochroic dyes. A comparison is also made between the structural isomers of PECH and PCH which questions the validity of molecular theories of elastic constants.

Measurement of x(3) in novel liquid-crystal materials

In this paper we describe measurements of the third order nonlinear susceptibility and the response time for a large number of novel liquid crystal materials, in their isotropic phase, at temperatures close to the isotropic to nematic phase transition. By examining the dependence of the length of the alkyl chain part of the molecule, for the cyanobiphenyl and alkoxy- cyanobiphenyl homologous series, it is shown that the small alkyl chain materials have a superior nonlinear response. Unfortunately in these materials the large nonlinear effects close to the nematic phase are inaccessible because the solid phase is formed directly on cooling the isotropic liquid. An alternative route to large effects is to increase the molecular anisotropy in polarizability, while maintaining approximately the same molecular dimensions. It is shown that a significant increase in the size of the nonlinear response can be achieved by changing the various chemical constituents.

A Switchable Liquid Crystal Impregnated Porous Polymer Device

Abstract This paper describes the fabrication and performance of a novel device based on liquid crystals dispersed in a polymer matrix. A low molecular weight liquid crystal material is introduced into a pre-formed, thin, porous, low density polymer film. Application of an electric field across the thin film produces a change in the transmission of the film just as in conventional Polymer Dispersed Liquid Crystals (PDLC) due to the reorientation of the liquid crystal director. Thus the device acts as a shutter. One significant advantage of this type of device over conventional PDLC is that there is no intermixing of the liquid crystal and monomeric material. Also compressible porous polymer matrices can be formed1 giving the potential for controllably distorting the pores in a given direction. The formation and switching characteristics of a device using a porous polymer based on styrene and divinylbenzene is described.

Greyscale and color in ferroelectric liquid crystal displays

Liquid crystal displays by virtue of their flat profile, low volume and low power offer the user a number of advantages over CRTs, without compromising on the image quality. High definition (1000 line) flat panel displays enhance image recognition, which improves the viewers ability to resolve images. High definition displays are required over a wide spectrum of sizes for HMDs through HDDs, to multifunction consoles. These size and complexity requirements may be uniquely met with ferroelectric liquid crystal display (FLCD) technology. The fast line address times (l.a.t. approximately equals 20 microsecond(s) ) may be traded off to meet complexity, color and grayscale. This paper describes the latest developments in FLCD design to produce temporal and spatial dither grayscale. These exploit new developments in material response times and fast drive schemes developed at DRA Malvern. The increased speed provided by the Malvern drive schemes also allows color to be implemented in a temporal manner similar to temporal grayscale. A new method to add color to existing fast displays using a frame sequential technique is also described.