G. W. Gray

Physical properties of liquid crystals

Introduction and Historical Development. Guide to the Nomenclature and Classification of Liquid Crystals. Theory of the Liquid Crystalline State. Physical Properties.

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.

Liquid crystal materials and devices

A novel liquid crystal material contains a compound having a molecular structure such that the material exhibits a liquid crystal phase, the molecular structure of the compound being ##SPC1## Or an orthosubstituted derivative thereof, where X and Y are different parasubstituents of the kind which promote liquid crystal behaviour, where the para substituent is a group selected from the following list; an alkyl group, an acyloxy group, an unsaturated group or an alkoxy group having more than one carbon atom; and where the para-substituent Y is a group selected from the following list; a cyano group, a nitro group, a group which includes a terminal cyano group, a group which includes a terminal nitro group and an acyloxy group. The material may be the said compound or it may be a multi-component solution or mixture containing it. The liquid crystal material may be nematic, smectic or cholesteric depending on the selection of the substituents X and Y. A device such as a twisted nematic effect device, incorporating the material may be operated at low temperatures. The material does not suffer from significant chemical and/or photochemical decomposition when used in a liquid crystal device.

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 dynamics of liquid crystalline side-chain polymers: The dielectric relaxation behaviour of a siloxan polymer in the nematic and isotropic phases

Abstract The dielectric relaxation behaviour of a liquid crystalline side-chain polymer having a siloxan backbone to which is attached an aromatic ester group via a C 5 methylene spacer has been studied in the nematic and isotropic phases. One process was observed in the isotropic phase and was characteristic of that of a glass-forming liquid. A composite process was observed in the liquid crystalline phase. In the transition range the dielectric behaviour changed continuously from that of the liquid crystal to the isotropic material. The characteristic features of the relaxations are described and their mechanisms are discussed in relation to earlier dielectric data for small molecule nematogens and for liquid crystalline side-chain polymers based on acrylate and methacrylate monomers.

Advances in Synthesis and the Role of Molecular Geometry in Liquid Crystallinity

Abstract Although publications concerning advances in synthesis and the role of molecular geometry in liquid crystallinity have not been numerous since the last conference in 1965, I have nevertheless had to be somewhat selective in the material which I have chosen to review in the available time. Broadly speaking, I have selected those areas of study which have given rise to new results which can be logically related to past experience in the field of molecular structure and the properties of liquid crystals. To this end, I have divided the lecture into three parts, of which the first is largely introductory.

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.

The Raman Spectra of 4-Cyano- 4′ -pentylbiphenyl and 4-Cyano-4′-pentyl-d 11-biphenyl

Abstract The Raman spectra of the solid, nematic, and isotropic liquid phases of 4-cyano-4′-pentyl-biphenyl (5CB) and 4-cyano-4′-pentyl-d 11-biphenyl (5CB-d 11) are reported together with the method of synthesis of 5CB-d 11. A comparison of the spectra of the isotropic liquid phases of 5CB and 5CB-d 11 has led to a partial assignment of these spectra. For each compound, the spectra of the fluid phases resembled each other, but differed from the corresponding spectrum of the solid phase. In particular, the cyano streching vibration occured as a doublet in the spectra of the solid phases of 5CB and 5CB-d 11, but as a single band in spectra of the fluid phases. The nematic to isotropic liquid transition temperature of 5CB-d 11 was 1[ddot] lower than that of 5CB.