Michael Hird

Fluorinated liquid crystals – properties and applications

This critical review begins with a brief, but essential, introduction to the special nature of liquid crystal materials, their peculiar properties, and their commercial applications, followed by an introductory insight into the remarkable nature of the fluoro substituent, and its fascinating influence on the properties of organic compounds. However, the main focus of the review is to discuss the enormous amount of exciting research on fluorinated liquid crystals that has been reported. The small size of the fluoro substituent enables its incorporation into all types of liquid crystal, including calamitic, discotic, banana, lyotropic, and polymers, without ruining the liquid crystalline nature of the material. However the fluoro substituent is larger than hydrogen, and hence causes a significant steric effect, which combined with the high polarity, confers many fascinating, and often remarkable, modifications to melting point, mesophase morphology and transition temperatures, and the many other very important physical properties, such as dielectric anisotropy, optical anisotropy, and visco-elastic properties. There are many different positions within a liquid crystal structure where a fluoro substituent can be located, including (i) a terminal position, (ii) within a terminal chain, as a semi-fluorinated or as a perfluorinated chain, or as one fluoro substituent at a chiral centre, (iii) as part of a linking group, and (iv) a lateral position in the core section. Such variety enables the interesting and advantageous tailoring of properties, both for the fundamental purposes of establishing structure-property relationships, and for materials targeted towards commercially-successful liquid crystal display applications.

Palladium-catalysed cross-coupling reactions in the synthesis of some high polarizability materials

Abstract Liquid crystal materials of high optical anisotropy consist of moieties of high electron density in conjugation with each other along the molecular length. Such structures are conducive to convergent synthesis methods. Here we report the synthesis of a range of novel materials by the strategic use of palladium-catalysed cross-coupling methods. In addition to the traditional use of bromide and iodide leaving groups, invaluable use of the triflate leaving group and the importance of selective cross-coupling methods using both arylboronic acids and alkynylzinc chloride derivatives is illustrated. This systematic methodology allows the separate synthesis of the appropriate sub-units that can be efficiently coupled together to provide high overall product yields.

The synthesis and transition temperatures of some 4,4″-dialkyl- and 4,4″-alkoxyalkyl-1,1′ :4′,1″-terphenyls with 2,3- or 2′,3′-difluoro substituents and of their biphenyl analogues

The tetrakis(triphenylphosphine) palladium (0)-catalysed coupling of arylboronic acids with aryl halides is used to prepare several 4,4″-dialkyl- and 4,4″-alkoxyalkyl-1,1′:4′,1″-terphenyls with 2,3- or 2′,3′-difluoro substituents and their related biphenyl systems. Lithiation ortho to a 1,2-difluoroaromatic unit provides the route to the 2,3-difluoroarylboronic acids.The 2,3-difluoro substituted terphenyls are low-melting liquid crystals with wide-range Sc phases and no underlying smectic phase; these compounds are excellent hosts for ferroelectric systems. The compounds with widest Sc ranges are those with the difluoro substituents in an end ring and the compounds with difluoro substituents in the central ring show more nematic character and so are useful for ECB devices.

Iron(III) salen-type catalysts for the cross-coupling of aryl Grignards with alkyl halides bearing β-hydrogens

Iron(III) salen and related complexes are active catalysts for the coupling, under mild and simple reaction conditions, of aryl Grignard reagents with primary and secondary alkyl halide substrates bearing beta-hydrogens.

Cross-coupling reactions in the synthesis of liquid crystals

Abstract The synthesis of structurally complex liquid crystals by traditional linear methods is often difficult, if not impossible. When many functional groups of a specific substitution pattern are required it is very likely that the sequence of reactions will interfere with functional groups already present or the directing effect of such functional groups may lead to the wrong substitution pattern. Syntheses using small, more easily managed units of the correct substitution patterns in high-yielding coupling reactions are a much more viable approach; the possible use of the sub-units in the synthesis of many different final compounds leads to systematic and economic syntheses. Suitable methods of synthesis of liquid crystals by cross-coupling reactions of various types are discussed, examples are provided and issues of selectivity are considered. The methods based on arylboronic acids are particularly useful.

Molecular complexity and the control of self-organising processes

In this article we investigate the complexity of the molecular architectures of liquid crystals based on rod-like mesogens. Starting from simple monomeric systems founded on fluoroterphenyls, we first examine the effects of aromatic core structure on mesophase formation from the viewpoint of allowable polar interactions, and then we model these interactions as a function of terminal aliphatic chain length. By incorporating a functional group at the end of one, or both, of the aliphatic chains we study the effects caused by intermolecular interfacial interactions in lamellar phases, and in particular the formation of synclinic or anticlinic modifications. We then develop these ideas with respect to dimers, trimers, tetramers, etc. We show, for dendritic systems, that at a certain level of molecular complexity the local mesogenic interactions become irrelevant, and it is gross molecular shape that determines mesophase stability. The outcome of these studies is to link the complexity of the molecular interactions at the nanoscale level, which lead to the creation of the various liquid-crystalline polymorphs, with the formation of mesophases that are dependent on complex shape dependencies for mesoscopic supermolecular architectures.

Ferroelectricity in liquid crystals—materials, properties and applications

Molecular chirality confers the necessary reduced symmetry to tilted smectic liquid crystals and disordered crystals to generate ferroelectric properties. By definition, ferroelectrics are materials that possess a spontaneous polarisation in the absence of an external field, and on the application of an electric field, the polarisation can be reoriented. Combine the fluidity of liquid crystals with ferroelectric properties and a fast-switching electro-optic device is possible. However, the road was very rocky in developing ferroelectric technology into commercially viable devices, despite the many advantages over the existing nematic-based devices. Indeed, research and development is on-going to enable ferroelectric technology to be more widely adopted. This review introduces the phenomenon of ferroelectric liquid crystals and charts the development of the technology to commercially viable devices, with a specific focus on the development of suitable materials in terms of design, synthesis and properties.

Fluoro Substitution in Thermotropic Liquid Crystals

Abstract Fluoro substitution in thermotropic liquid crystals provides a general way of modifying the properties of a parent system. Melting point, transition temperatures, mesophase types and other physical properties are affected by fluoro substitution so that frequently the behaviour of the parent compound can be manipulated and improved in a predictable manner. This review discusses the effects of fluoro substitution at different positions in a variety of core systems and briefly considers the problems arising in the synthesis of fluoro-substituted liquid crystals.

The synthesis and transition temperatures of some trans-4-alkylcyclohexylethyl-substituted 2,3-difluorobiphenyls

Abstract Terphenyls with two lateral ortho-fluoro-substituents have proved to be excellent host materials for ferroelectric (SC∗) mixtures. The compounds reported here are biphenyls with the same arrangement of lateral substituents but with a trans-4-alkylcyclohexylethyl moiety as one of the terminal substituents. Such three ring systems retain the ability to generate the SC mesophase and have low melting points. Low temperature lithiation procedures were used to prepare phenylboronic acids, which were then used in palladium catalysed cross-coupling procedures to prepare the desired compounds. The effect of molecular structure on the mesophase types and thermal stabilities is discussed and comparisons are made with analogous terphenyls and biphenyls with open chain terminal substituents.

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.