Kenneth J. Toyne

The synthesis and high optical birefringence of nematogens incorporating 2,6-disubstituted naphthalenes and terminal cyano-substituent

Abstract A range of nematogenic materials which incorporate a 2,6-disubstituted naphthyl moiety and a terminal cyano-substituent have been synthesized by using palladium-catalysed cross-coupling procedures involving arylboronic acids and alkynylzinc reagents with aryl iodides, bromides and trifluoromethanesulphonates (triflates). The compounds have very high nematic phase stability, but their melting points are also quite high. The birefringences were measured using an extrapolation technique and the values were found to be between 0·26 and 0·42.

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.

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.

Chirality and Frustration in Ordered Fluids

Abstract Chirality in liquid crystal systems is a complex and sometimes difficult concept to understand and appreciate. In this article we review some of the reduced symmetry aspects of calamitic liquid crystal phases with reference to point symmetry, space symmetry, and helicity. We utilize these concepts in the discussion of new results obtained on Twist Grain Boundary phases, inversions in chiral dependent properties such as the helicity in cholesteric and smectic C* phases and the spontaneous polarization in ferroelectric liquid crystals, and antiferroelectric behaviour in low molar mass systems. Our observations suggest that many novel effects found in chiral liquid crystals are a result of conflicts between the effects of reduced symmetry and the desire to form normal liquid-crystalline structures, i.e., a form of frustration exists.

Chirality in liquid crystals. The remarkable phenylpropiolates

Abstract The biphenylyl esters of the 4-n-alkoxyphenylpropiolic acids are a unique family of liquid-crystalline materials. In particular, when the biphenyl moiety of the compounds carries a chiral end-group, many optically active mesophases are created which exhibit unusual structures and physical properties. For instance, when the chiral group attached to the biphenyl moiety is 1-methylheptyl then Abrikosov, twist grain boundary smectic A* and antiferroelectric smectic C* phases are observed. The wide variety of chiral phases and electrochiral properties exhibited by this family of materials makes them ideal candidates for exploring chirality in the liquid-crystalline state. These investigations allow us to contrast and compare chirality dependent phenomena in liquid crystals, thereby producing a broader view of the concept of chirality in organized fluids than is traditionally presented.

The Synthesis and Transition Temperatures of Some Lateral Monofluoro-substituted 4,4″- Dialkyl- and 4,4″-Alkoxyalkyl- 1, 1′: 4′,1″-terphenyls

Abstract The tetrakis(triphenylphosphine)palladium(0)-catalysed cross-coupling of arylboronic acids with aryl halides has been used to prepare several lateral fluoro-substituted 4,4″-dialkyl- and 4.4″-alkoxyalkyl- 1,1′: 4′,1″-terphenyls. The melting points, mesophase types and transition temperatures of these compounds vary considerably depending on where the lateral fluoro-substituent is positioned. All the compounds have a strong tendcncy to form tilted sniectic mesophases.

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.