Paul Hindmarsh

Investigation of the effect of bulky lateral substituents on the discotic mesophase behaviour of triphenylene benzoates

Lateral substitution in the peripheral phenyl groups of the triphenylen-2,3,6,7,10,11-hexayl hexakis(4-alkoxybenzoate)s was investigated via substitution of methyl, ethyl, isopropyl and tert-butyl groups ortho to either the peripheral alkoxy chain or the internal ester linking group. The discotic mesogenic compounds were prepared by the esterification of 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) with the respective 3-alkyl- or 2-alkyl-4-decyloxybenzoyl chlorides (alkyl = methyl, ethyl, isopropyl or tert-butyl) in the presence of pyridine. Thermal polarized light microscopy and differential scanning calorimetry were used to investigate the liquid-crystalline behaviour of the materials prepared. Suppression of columnar mesophases was observed for both series. A general increase in the clearing temperatures was observed for the outer-substituted series when the size of the lateral substituent was increased, but a depression of the clearing points was found for the inner-substituted series. These observations were investigated through molecular modelling.

The Design, Synthesis and Structure-Property Relationships of Ferroelectric and Antiferroelectric Liquid Crystal Materials

After nearly 20 years of intense research, ferroelectric liquid crystals are now being used in commercial devices. This timely review will consider the structural units and their specific combinations that can be used to generate the required smectic C phase, and provide the necessary physical properties for optimum operation of devices. The design features required to generate the more recent antiferroelectric phase will also be considered.

Trans−1,2-Difluorostilbenes: Promising Materials for High Dielectric Biaxiality Ferroelectric Mixtures

Abstract A range of two- and three-ring liquid crystals have been synthesised that incorporate a trans−1,2-difluoroethene linking group. Some of the materials have surprisingly low melting points and high smectic C phase stability, and they exhibit the ideal phase sequence for ferroelectric hosts of smectic C to smectic A to nematic. The synthesis was accomplished in a convergent manner through the use of low-temperature lithiations to generate organometallics based on boron and tin. Palladium-catalysed cross-couplings and Michael-type reactions were used to generate the final mesogenic materials. The polar fluoro substituents enable the generation of ferroelectric mixtures with a high dielectric biaxiality which is very important in τV minimum driving schemes. The alkenic linking group in stilbenes normally confers instability, but the difluoro-substituted analogues are stable compounds.

Lateral substitution in the peripheral moieties of triphenylen-2,3,6,7,10,11-hexayl hexakis(4-alkoxybenzoate)s: dimethyl-substituted systems

Lateral substitution in the peripheral phenyl groups of the triphenylen-2,3,6,7,10,11-hexayl hexakis(4-alkoxybenzoate)s has been investigated via dimethyl substitution ortho to the terminal alkoxy chain or the ester linking group. Similarly, lateral methyl substitution in the terminal alkoxy chain was also investigated. The discotic mesogenic compounds were prepared by the esterification of 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) with the respective 4-alkoxy-3,5- or 4-alkoxy-2,6-dimethylbenzoyl chlorides in the presence of pyridine. Thermal polarized light microscopy and differential scanning calorimetry were used to investigate the liquid-crystalline behaviour of the materials prepared. The compounds with dimethyl substitution in the 3 and 5 positions in the peripheral phenyl rings were found to exhibit the hexagonal disordered (Dhd) columnar mesophase and the discotic nematic mesophase (ND), whereas the analogous compounds with substitution at the 2 and 6 positions only exhibited discotic nematic (ND) mesophases. Thus, methyl substitution was found to suppress totally columnar phase formation in the second series of compounds, where the methyl groups were positioned pointing in towards the triphenylene core.

The Synthesis and Transition Temperatures of Some Ortho-Dichloroterphenyls for Ferroelectric Mixtures

Abstract A range of ortho-dichloroterphenyls has been prepared and evaluated for their transition temperatures and physical properties. The analogous ortho difluoro-substituted materials have been very successful as ferroelectric host materials. It was expected that the dichloro-substituted compounds would provide a greater lateral dipole and confer of dielectric biaxiality. State of the art synthetic techniques were used. For example, low-temperature ortho directed metallations provided arylboronic acids which were used in palladium-catalysed cross-coupling reactions to provide final materials. Materials with low melting points that exhibit the SC phase have been produced.

Triphenylene Hexa-n-alkylcyclohexanoates: a new series of disc-like liquid crystals

A series of hexa-n-alkylcyclohexanoates of triphenylene were prepared by the esterification of hexahydroxytri-phenylene with a variety of 4-n-alkylcyclohexylcarbonyl chlorides in the presence of pyridine. The liquid-crystalline phase behaviour of the materials synthesized was investigated by a combination of thermal polarized light microscopy, differential scanning calorimetry and X-ray diffraction. All of the compounds were found to exhibit a single discotic mesophase that had hexagonal symmetry. The hexagonal columnar phase was found to occur over much wider temperature ranges in this series than was observed for the aromatic analogues.

New Coumarin Polymers as Non-Contact Alignment Layers for Liquid Crystals

Abstract The synthesis of polymethacrylate derivatives of 6- and 7-hydroxycoumarin is described and their use as photoinduced alignment layers for liquid crystals is discussed. Strong anchoring is found with azimuthal anchoring energies > 6.9 × 1 (10−5 J m−2. The liquid crystal alignment direction is parallel to the polarisation direction of the incident UV light when low fluences are incident. At a critical fluence threshold, a sharp change to perpendicular alignment is found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. These results are discussed in terms of a molecular model.

Alignment of coumarin-containing polymers for liquid crystal displays

In this paper, a range of polymethacrylate derivatives of hydroxycoumarin are investigated as photoinduced alignment layers for liquid crystals, and their performance is compared with polyvinyl cinnamate. For al the coumarin- containing polymers, the liquid crystal alignment direction is parallel to the polarization direction of the incident UV light at low fluences. At a critical fluence threshold, a sharp change to perpendicular alignment is found. Molecular models and spectroscopy are used to explain this phenomenon as well as the fact that only perpendicular alignment is observed for polyvinyl cinnamate. The azimuthal anchoring energies of the alignment layers are measured and values greater than 6 by 10-5 J m-2 are found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. Exposure of the coumarin-containing alignment layers at oblique incidence gives pretilted alignment with pretilt angles up to 5.1 degrees.

Grazing incidence X-ray reflectivity of coumarin side-chain polymers used for liquid crystal photoalignment layers

Specular X-ray reflectivity measurements are used to examine photoinduced surface roughness of side-chain coumarin polymers that form liquid crystal photoalignment layers. Thin films were irradiated with polarised or unpolarised ultraviolet light and the X-ray reflectivity at grazing incidence was measured for samples irradiated with different ultraviolet fluences. Kiessig fringes are observed over a large dynamic range and a comparison of the measured and calculated reflectivity gives a value for the surface roughness. The photoinduced roughness increases with ultraviolet fluence, from 5.5 to 20 A. Negligible photoinduced roughness is detected for a film, which was previously irradiated using conditions that give strong planar liquid crystal alignment. Specular reflectivity data taken with the plane of incidence of the X-ray parallel and perpendicular to the polarisation direction of the ultraviolet light are identical within experimental uncertainty. Hence in-plane anisotropic surface roughness is not observed.

The Synthesis and Characterisation of Nematic NLO Polymer Networks

Abstract Ultra-fast optical intensity modulators are required for high speed telecommunications and organic electro-optical materials offer the advantages of low cost and fast switching capability. We propose a new method to fabricate electro-optical waveguide modulators using low temperature processing and poling with low electric field strength. Our method uses nematic photoreactive mesogens with large second-order polarisability and low viscosity. They can be poled with an electric field of low strength and simultaneously photopolymerised to maintain polar order. A nematic mixture with a melting point of 30°C was successfully crosslinked when aligned by an electric field. New tolane based chromophores show second order polarisabilities up to 180 × 10−30 esu, ten times the value of para-nitroaniline.