Liquid crystal compounds, mixtures and devices

Details Liquid crystal compounds, mixtures and devices Liquid crystal compounds, mixtures and devices

1995-04-18

1997-01-21

Wu, Shean C.

Compositions

Liquid crystal compositions

25229961, 25229963, 25229964, 25229965, 25229966, 25229967, 349182, C09K 1952, C09K 1920, C09K 1912, G02F 113

Patent

active

055956823

DESCRIPTION:

BRIEF SUMMARY
This application is a 371 of PCT/GB93/01133 filed May 23, 1993.
This invention relates to novel liquid crystal compounds, liquid crystal materials containing them and their inclusion in liquid crystal devices.
Liquid crystals can exist in various phases. In essence there are three different classes of liquid crystalline material, each possessing a characteristic molecular arrangement. These classes are nematic, cholesteric and smectic. A wide range of smectic phases exists, for example smectic A and smectic C. Some liquid crystal materials possess a number of liquid crystal phases on varying the temperature, others have just one phase. For example, a liquid crystal material may show the following phases on being cooled from the isotropic phase:--isotropic--nematic--smectic A--smectic C--solid. If a material is described as being smectic A then it means that the material possesses a smectic A phase over a useful working temperature range.
Compounds such as: ##STR3## are described in J. Am. Chem. Soc., 1989, 111, 8119-8125 by J. W. Goodby et. al., where n ranges from 8 to 16. At long n-alkoxy chain lengths the series exhibits a novel variation of mesophase behaviour to which a smectic A* phase was assigned. Some of the materials also appeared to exhibit two further mesophases.
According to this invention there are provided compounds having a general formula I ##STR4## in which A.sub.1 is selected from alkyl, alkoxy or alkenyl; X.sub.1, X.sub.2, X.sub.3 and X.sub.4 are independently selected from the halogen group; m, n, p and q are independently 0, 1, 2, 3 or 4 such that m+n+p+q.noteq.0; Y is selected from O and COOO; A.sub.2 is an end group of formula II ##STR5## where Z is selected from halogen, CH.sub.3, CN, CF.sub.3, CHF.sub.2 ; R is a linear or branched alkyl group or H; R.sub.1 is a linear or branched alkyl group or H. preferably C.sub.9-15. preferably C.sub.5-7. m+n+p+q=1 or 2.
Compounds of Formula I can be included in a material, the material being a mixture of compounds.
The materials of this aspect of the invention may be used in many of the known forms of liquid crystal display devices, for example chiral smectic electrooptic devices. Such a device may comprise a layer of liquid structures and surface treated to align liquid crystal material molecules. The liquid crystal mixtures may have applications in ferroelectric, ferrielectric (M. Johno et al., Japan Display, 1989, 22), antiferroelectric, thermochromic and electroclinic devices; they may also lead to the formation of frustrated liquid crystal phases. Frustrated phases arise from mesophases possessing double twist structures which means that space cannot be filled uniformly and the phase is stabilised via a lattice of defects. Frustrated phases can also arise from the competition between helical and layer structures, an example of which is the smectic A* analogue of the Abrikson flux phase as described by J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak, Nature, 1989, 337, 449; A. J. Slaney and J. W. Goodby, J. Mater. Chem. 1991, 1, 5. The competition between helical and layer structures leads to defects being formed which again stabilise the phase.
Ferroelectric smectic liquid crystal materials, which can be produced by mixing an achiral host and a chiral dopant, use the ferroelectric properties of the tilted chiral smectic C, F, G, H, I, J and K phases. The chiral smectic C phase is denoted S.sub.c * with the asterisk denoting chirality. The S.sub.c phase is generally considered to be the most useful as it is the least viscous. Ferroelectric smectic liquid crystal materials should ideally posses the following characteristics: low viscosity, controllable spontaneous polarisation (Ps) and an S.sub.c phase that persists over a broad temperature range, which should include ambient temperature and exhibits chemical and photochemical stability. Materials which possess these characteristics offer the prospect of very fast switching liquid crystal containing devices. Some applications of ferroelectric liquid crystals are described by J. S

REFERENCES:
Liquid Crystals, vol. 11., No. 1, Jan. 1992, London GB pp. 135-143, J. W. Goodby et al `the effect of structural changes in the molecular core and periphery on the liquid-crystalline properties et al`.

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