Simon Greenfield

Laterally fluorinated phenyl biphenylcarboxylates; versatile components for ferroelectric smectic C mixtures

Abstract Twenty-six fluoro-substituted 4-n-alkoxyphenyl 4′-n-alkyi-and 4′-n-alkoxybiphenyl-4-carboxylates are reported. The number and position of the fluoro groups influences the liquid crystal phase sequence and has a dramatic effect on the liquid crystal transition temperatures. Some compounds have low melting points and a very wide smectic C phase, whereas other compounds exhibit a very wide smectic A phase. Compounds containing a 2,3-difluorophenyl group provide useful materials which have a large negative dielectric anisotropy.

Lateral fluoro substituted 4-alkyl-4″-chloro-1,1′:4′,1″-terphenyls and derivatives. Useful high birefringence, high stability liquid crystals

Abstract By modification of the basic 4-alkyl-4″-chloro-1,1′:4′,1″-terphenyl structure with lateral fluoro substituents and ethyl linking groups, a wide range of mesogens has been synthesized. Control over the phase behaviour, especially optimization of nematic properties, can be achieved by the degree and position of lateral fluoro substitution and/or the presence and position of an ethyl linking group. The effect of these changes on the physical properties such as dielectric anisotropy (Δe) and birefringence (Δn) has also been studied. Compounds of low melting point, wide nematic range and high birefringence (>0·2) have been made. These materials are suitable for active matrix display applications where high chemical and photostability are necessary.

Liquid Crystals with a Thiomethyl End Group: Lateral Fluoro Substituted 4-(trans-4-(n-Propyl) Cyclohexylethyl-4′-Thiomethylbiphenyls and 4-n-Alkyl-4″-Thiomethylterphenyls

Abstract The synthesis and mesogenic properties of some lateral fluoro substituted 4-(trans-4-(n-propyl)cyclohexylethyl)-4′-thiomethylbiphenyls and 4-n-alkyl-4″-thiomethylterphenyls are described. Birefringence (Δn) and dielectric anisotropy (Δϵ) measurements for these compounds are also given and compared with analogous terminal chloro, cyano and methoxy compounds. It is found that the use of a terminal thiomethyl group produces compounds of intermediate Δn, Δϵ and clearing point than analogous terminal chloro, methoxy and cyano compounds. The terminal thiomethyl group is also more smectogenic than any of the other three terminal endgroups. In most cases, however, the melting points of the compounds are too high to be of practical use.

Recent developments in materials for TFT/PDLC devices

Liquid crystal mixtures for use over an active matrix substrate must have a high resistivity and, to maintain high value during the lifetime of the display, the materials must be very stable. Many such liquid crystal mixtures are known and used in twisted nematic displays but they are of low birefringence; very few high birefringence liquid crystals of this type are known. Several classes of liquid crystal which fulfill these criteria have been developed and formulated into low melting point mixtures suitable for use with UV curing prepolymers. The properties of PDLC films containing these new mixtures show that the high birefringence values now possible can lead to substantially increased contrast ratios.

Liquid crystal mixtures for polymer matrix displays

Chemically stable , wide temperature range nematic liquid crystal mixtures with birefringence values up to almost 0 .3 are described . For applications where high ultra-violet stability is required a range of mixtures optirnised for this application have been developed. Chemically stable mixtures with a lc*i threshold voltage have also been developed and are compared with standard mixtures.

Laterally fluorinated tolanes of low melting point

Abstract Tolanes having a terminal polar group and a lateral fluoro substituent are reported with the desirable features of high birefringence, wide nematic range and low melting point.

Nematogenic laterally fluorinated biphenyls with polar terminal groups

Abstract The physical properties are reported for a series of positive dielectric anisotropy (δe) alkylbiphenyls possessing a halogen-containing terminal group and incorporating varying degrees of lateral fluorination. The nematic range and dielectric and optical anisotropies can be varied by the appropriate choice of terminal group and extent of lateral substitution. These materials possess greater stability than the corresponding cyano compounds and are suitable for use in applications that require high stability.

Unusual Odd-Even Effects in Vinyl Ether and Acrylated Direactive Liquid Crystals

Abstract Four series of direactive liquid crystals are presented and their physical properties characterised. The core groups are based on either a lateral chloro or methyl terphenyl and the reactive groups are either acrylates or vinyl ethers. It is shown that reactive liquid crystals with vinyl ether reactive groups have higher transition temperatures, birefringence, and order parameter than the corresponding acrylates. Both the vinyl ether and acrylate series show an odd-even effect in transition temperatures, birefringence and order parameters. The odd-even effect in the vinyl ether series is particularly marked.