Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Utility Patent
1999-09-10
2001-01-02
Kelly, C. H. (Department: 1756)
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
C252S299610, C252S299630, C252S299660, C585S023000, C585S025000
Utility Patent
active
06168728
ABSTRACT:
TECHNICAL FIELD
The present invention relates to liquid crystalline compounds, liquid crystal materials using the compounds and liquid crystal display devices. More particularly, the present invention relates to new liquid crystalline compounds, which are preferable as components of liquid crystal materials having compatibility with many liquid crystalline compounds and high optical anisotropy (may be abbreviated as &Dgr;n, hereinafter), liquid crystal materials having high &Dgr;n and excellent compatibility with the other liquid crystal compounds, and liquid crystal display devices using them.
BACKGROUND ART
Liquid crystal display devices are used for watches, desk-top calculators, miscellaneous measuring apparatus, metering panels for automobiles, word processors, electronic notebooks, printers, computers, television sets and the like.
Liquid crystal display devices are obtained by using liquid crystal materials having properties of optical anisotropy and dielectric anisotropy (may be abbreviated as &Dgr;&egr;, hereinafter). The display modes are principally dynamic scattering (DS) mode, guest-host (G.H) mode, twist nematic (TN) mode, super twist nematic (STN) mode, thin film transistor (TFT) mode, surface stabilized ferroelectric liquid crystal (SSFLC) mode and the like. Driving modes are static driving mode, time-division driving mode, active matrix driving mode, two-frequency driving mode and the like.
In these modes, STN mode is now the most broadly used for the high display quality and low production cost. Various characteristics are required in the liquid crystal materials used for the STN mode, and the following is generally important:
(1) Quick electric-optical response time,
(2) Steep transparency per applied voltage,
(3) Broad driving temperature range, and
(4) Low driving voltage.
For showing the above characteristics, the liquid crystalline compounds for constituting liquid crystal materials are required to have the following characteristics. Namely:
(1) high optical anisotropy,
(2) when the liquid crystal compound is added to a liquid crystal composition, the temperature region of the nematic phase does not reduce, and the phase separation such as crystal deposition is difficult in the low temperature region,
(3) low viscosity,
(4) chemical stability, and
(5) high bend to splay elasticity constant ratio (K
33
/K
11
).
It is described more specifically hereinafter. It has been known that the viscosity of liquid crystals is an element that controls the response speed of liquid crystal molecules aligning in a display device to the electric field applied (Phys. Lett., 39A, 69 (1972). Namely, to prepare the liquid crystal composition showing high-speed response, it is preferable to prepare liquid crystal compositions by using liquid crystalline compounds having very low viscosity in large quantities.
The liquid crystalline compounds having low viscosity have been earnestly studied. Such studies have brought bicyclic compounds having simple structures, such as bicyclohexanes and phenyl cyclohaxanes having the lowest viscosity. However, it is very difficult to find out the compounds having simpler structure, or lower viscosity than the compounds known now. A method for resolving the problems is to reduce the thickness of a liquid crystal cell (it may be called as “d” hereinafter). By reducing the cell thickness in comparison with that of conventional liquid crystal cell, high-speed response is obtained.
However, considering the display quality, the adequate retardation value (d·&Dgr;n), which is the first minimum condition described in Appl. Phys. Lett. 38(7), 497, should not deviate from the optimum value. Namely, it is needed to maintain a constant product of optical anisotropy and cell thickness and to reduce the cell thickness. Accordingly, it needs to make higher the &Dgr;n of liquid crystal materials by using as a component the liquid crystalline compound having a very higher optical anisotropy value than that of conventional compounds.
The steepness of applied voltage to transparency characteristics (may be abbreviated as V-T characteristics, hereinafter) is highly caused by the bend to splay elasticity constant ratio K
33
/K
11
(Proc. of Japan Display, 388 (1986)). According to the report, the liquid crystal having a high elasticity constant ratio has steeper V-T characteristics. By using liquid crystal materials having good V-T characteristics, liquid crystal display having higher display quality can be attained.
For the availability in a broader temperature region, liquid crystal compositions having a nematic phase at a low temperature are especially demanded. At the low temperature region, liquid crystal compositions, which do not deposit crystals and do not show a smectic phase, are naturally demanded. To fit liquid crystal materials with many characteristics required of each display device, a mixture of several kinds or twenties of liquid crystalline compounds constitutes a liquid crystal material. Accordingly, it is very important that the liquid crystalline compounds used have high compatibility with the other liquid crystalline compounds at the low temperature region.
It becomes important problem to reduce consumption power of liquid crystal display devices recently, so that it is necessary to lower the driving power of the devices. To lower the driving power, it is needed to reduce the threshold voltage (may be abbreviated as Vth, hereinafter). The threshold voltage is a function of a dielectric anisotropy value, and the following relation is reported in Mol. Cryst. Liq. Cryst., 12, 57 (1970).
V
th
=&pgr;(K/&egr;0&Dgr;&egr;)
½
,
wherein K is an elastic constant and &egr;0 is a dierectric constant in vacuum. In the relation, when the liquid crystal materials having low driving power is demanded, it is needed to use liquid crystal materials having a high dielectric anisotropy value.
Since liquid crystal display devices are frequently used at a high temperature, for example, under severe conditions such as outdoors, the liquid crystalline compounds used for liquid crystal compositions should have sufficiently high chemical stability.
It is further important to have a little temperature dependence as an important factor in several characteristics. The practical liquid crystal display devices must be maintained with various surroundings, at an especially broad temperature region (for example, −20~120° C.) not so as to largely change the display quality. To satisfy the demand, it is necessary to use liquid crystal materials having very little temperature dependence of various characteristic values.
The liquid crystal materials for STN display devices have been studied, and many excellent compounds have been developed. As a compound having a particularly high elasticity constant ratio K
33
/K
11
, an alkenyl compound has been obtained and a liquid crystal material having steep V-T characteristics has been practically used.
However, at the present time, only a few example of liquid crystalline compound having remarkably high &Dgr;n has been known and practically used.
Compounds having high &Dgr;n that have been first practically used are tolan compounds as shown by formula (10), which have been disclosed by J. Malthete et al (Mol. Cryst. Liq. Cryst., 23, 233 (1973)). The tolan compounds have relatively high &Dgr;n and now are practically used. However, the optical anisotropy value is 0.20 and it is not enough for practical use.
To resolve the problem, the tolan derivative having alkyl groups as shown by formula (11) has been developed (Japanese Patent Application Laid-open No. sho-64-879) for increasing the polarization ratio. However, in spite of the increased optical anisotropy value (>0.20), the compatibility with the other many liquid crystal compounds is very low, and what is more the practical use of the compounds is limited by lack of chemical stability.
To obtain a compound having nematic phase at a high temperature, a compound represented by formula (12) has been prepared (Japanese Patent Application Laid-open No. hei 2-83340
Hachiya Norihisa
Koga Kouji
Matsui Shuichi
Miyazawa Kazutoshi
Nakagawa Etsuo
Browdy and Neimark
Chisso Corporation
Kelly C. H.
LandOfFree
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