5,7-difluoro-1,2,3,4-tetrahydronaphthalene derivatives and their

Details 5,7-difluoro-1,2,3,4-tetrahydronaphthalene derivatives and their 5,7-difluoro-1,2,3,4-tetrahydronaphthalene derivatives and their

1999-08-20

2000-12-12

Kelly, C. H.

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229961, 25229963, 546339, 544298, 570127, 349184, C09K 1932, C09K 1934, C09K 1902, C09K 1930, C07C 1908

Patent

active

061593923

DESCRIPTION:

BRIEF SUMMARY
5,7-Difluoro-1,2,3,4-tetrahydronaphthalene derivatives and their use in liquid-crystalline mixtures.
In addition to nematic and cholesteric liquid crystals, recent times have also seen the use of optically active tilted smectic (ferroelectric) liquid crystals in commercial display devices.
Clark and Lagerwall showed that the use of ferroelectric liquid crystals (FLCs) in very thin cells leads to optoelectric switching or display elements having response times faster by a factor of up to 1000 than those of the conventional TN (twisted nematic) cells (see, for example, EP-A 0 032 362). On the basis of these and other favorable properties, for example the possibility of bistable switching and the contrast, which is virtually independent of viewing angle, FLCs are in principle highly suited to applications such as computer displays.
For the use of FLCs in electrooptical or completely optical assemblies there is a need either for compounds which form tilted or orthogonal smectic phases and which are themselves optically active, or else for compounds which, although forming such smectic phases are not themselves optically active, can be doped with optically active compounds to induce ferroelectric smectic phases. The desired phase should at the same time be stable over as wide as possible a temperature range.
Obtaining a good contrast ratio in electrooptical assemblies necessitates a uniform planar orientation of the liquid crystals. Good orientation in the S.sub.A and S*.sub.C phase can be achieved, for example, when the phase sequence of the liquid-crystal mixture with decreasing temperature is as follows: (greater than 10 .mu.m) or, even better, is fully compensated (see, for example, T. Matsumoto et al., Proc. of the 6th Int. Display Research Conf., Japan Display, Sep. 30-Oct. 2, 1986, Tokyo, Japan, pp. 468-470; M. Murakami et al., ibid. p. 344-p. 347). This is done, for example, by adding one or more optically active dopes which induce, say, a right-handed helix to the chiral liquid-crystal mixture which in the N* phase has a left-handed helix, in amounts such that the helix is compensated.
For the use of the SSFLCD effect (Surface Stabilized Ferroelectric Liquid Crystal Display) of Clark and Lagerwall for uniform planar orientation a further precondition is that the pitch in the smectic C* phase is substantially greater than the thickness of the display element (Mol. Cryst. Liq. Cryst. 1983, 94, 213 and 1984, 114, 151.
The optical response time .tau. [.mu.s] of ferroelectric liquid-crystal systems, which should be as short as possible, depends on the rotational viscosity of the system .gamma.[mPas], on the spontaneous polarization P.sub.s [nC/cm.sup.2 ] and on the electric field strength E [V/m] in accordance with the relationship ##EQU1## Since the field strength E is determined by the electrode separation in the electrooptical component and by the applied voltage, the ferroelectric display medium must be of low viscosity and must have a high spontaneous polarization in order for a short response time to be obtained.
Finally, requirements in addition to thermal, chemical and photochemical stability are for a small optical anisotropy .DELTA.n and a small positive or, preferably, negative dielectric anisotropy .DELTA..epsilon. (see, for example, S. T. Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, October Meeting 1985, San Diego, Calif., USA).
The entirety of these requirements can only be met with mixtures of two or more components. The basis of these mixtures (or matrix) is preferably formed by compounds which as far as possible themselves already have the desired phase sequence I.fwdarw.N.fwdarw.S.sub.A .fwdarw.S.sub.C. Further components are often added to the mixture in order to lower the melting point and to broaden the S.sub.C phase and usually the N phase as well, for inducing the optical activity, for pitch compensation and for adapting the optical and dielectric anisotropy, as far as possible without increasing the rotational viscosity, for example.
Ferroelectric li

REFERENCES:
patent: 4386007 (1983-05-01), Krause et al.
patent: 4391731 (1983-07-01), Boller et al.
patent: 6019911 (2000-02-01), Hirano et al.

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