Liquid crystal composition

Details Liquid crystal composition Liquid crystal composition

2000-10-27

2003-08-19

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299010, C252S299610, C252S299630, C252S299670

Reexamination Certificate

active

06607791

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a nematic liquid crystal composition useful as an electro-optical liquid crystal display material and a liquid crystal display device comprising such a nematic liquid crystal composition.
BACKGROUND OF THE INVENTION
Since its appearance as a display for desk electronic calculator, a liquid crystal display device (LCD) has met the demand for display capacity, i.e., from TN-LCD (twisted-nematic liquid crystal display device) to STN-LCD, with the progress of development of computer. STN-LCD was developed Scheffer et al. [SID '85 Digest, page 120 (1985)] or Kinugawa et a. [SID '86 Digest, page 122 (1986)] and has been widely spread for display for high data processors such as word processor and personal computer. An active addressing drive system has been recently proposed for the purpose of improving the response in STN-LCD (see Proc. 12th International Display Research Conference p. 503 (1992)). Good display properties over a wider temperature range have been required for portable terminal display (Personal Digital Assistance). Such a liquid crystal material is required to have a low viscosity and a low constant driving voltage over a wide temperature range. Such a liquid crystal material is also required to have no fluctuations in driving voltage within a frequency range corresponding to various time-division multiplexes. However, the background art liquid crystal material leaves something to be desired in response (switching time) and contrast developed when assembled into display device. Thus, new liquid crystal compounds or liquid crystal compositions are still proposed.
As mentioned above, one of the important problems TN-LCD and STN-LCD are facing is enhancement of contrast. With the rapid expansion of its use, LCD has been used more and more for indoor use but also for outdoor use under severe temperature conditions as in the case of portable terminal display for computer and display for on-board meter and outdoor measuring instrument. Thus, problem of deterioration of display quality have arisen due to the reduction of display contrast caused by the change of temperature in the atmosphere when LCD is installed and the reduction of response at low temperatures. LCD has also been required to have a high reliability in outdoor use.
The reduction of LCD display quality due to the change of ambient temperature is attributed to various causes. As these causes there can be considered the change of elastic constant and dielectric constant of nematic liquid crystal with temperature and the change of threshold voltage Vth with temperature due to the change of inherent pitch of a chiral material with temperature. Accordingly, a proposal for the improvement of temperature dependence of threshold voltage by the control of the inherent pitch of a chiral material has been already known (JP-A-55-38869 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”)). Problems have arisen that different combination of a matrix liquid crystal and a chiral material have different effects and the increase of the content of a chiral material has an adverse effect on the display properties such as response.
However, the change of mobility of an ionic material contained in the liquid crystal with temperature causes an increase of current flow. Accordingly, no countermeasures have been known against phenomena caused by the consumption of the effective voltage applied to the liquid crystal by ions resulting in the deterioration of contrast and reliability. From this standpoint of view, it is necessary that the amount of a compound having ester bond which has heretofore been widely used be reduced. However, the resulting rise of threshold voltage has caused a problem.
As a material which is free of ester bond but reduces the threshold voltage of liquid crystal there has already been known a material having 2,6-difluorobenzonitrile skeleton. However, the use of such a material having a large dielectric anisotropy has an adverse effect on response. Therefore, such a material must be used in a limited amount that often makes it impossible to provide a sufficient effect. As a material for improving response there has been known a compound having an alkenyl group incorporated in its side chain (JP-A-4-296387). However, 1-alkenyl group, which has a great effect, cannot be used from the standpoint of stability unless it is connected to cyclohexane ring. Further, 3-alkenyl group and alkyl-substituted 1-alkenyl group provide a great elastic constant ratio and thus have insufficient effect of lowering threshold voltage.
SUMMARY OF THE INVENTION
An object of the invention is to provide a liquid crystal composition which allows inhibited current flow and exhibits a high reliability without deteriorating various properties of liquid crystal such as threshold voltage and response.
A further object of the invention is to attain desired display properties such as high stabilized contrast (steepness) over a wider temperature range by improving the temperature dependence of threshold voltage.
A still further object of the invention is to provide a 2,6-difluorobenzonitrile derivative compound useful as a constituent material of such a liquid crystal composition. The use of such a liquid crystal composition for liquid crystal display device provides an effect of improving the properties of display, e.g., on {fraction (1/32)} to {fraction (1/400)} duty, preferably {fraction (1/80)} to {fraction (1/250)} duty, making it possible to provide a liquid crystal display device (STN-LCD) having enhanced contrast against increase of data or color display.
In order to solve the foregoing problems, the inventors made studies of liquid crystal compositions comprising various liquid crystal compounds. As a result, the following liquid crystal compositions were found.
Invention 1:
A liquid crystal composition comprising as a first liquid crystal component at least one compound selected from the group consisting of compounds represented by the following general formula (I):
wherein R
1
represents a C
2-16
alkenyl group or C
3-16
alkenyloxy group
and as a second liquid crystal component at least one compound selected from the group consisting of compounds represented by the following general formulae (II) and (III):
wherein R
2
, R
3
and R
4
each independently represents a C
1≠
alkyl or alkoxyl group which may be substituted by a fluorine atom, a C
2-16
alkenyl group which may be substituted by a fluorine atom, a C
3-16
alkenyloxy group which may be substituted by a fluorine atom or a C
1-10
alkoxyl group-substituted C
1-12
alkyl group which may be substituted by a fluorine atom; the rings A, B, C, D and E each independently represents a 1,4-phenylene group, 2-methyl-1,4-phenylene group, 3-methyl-1,4-phenylene group, naphthalene-2,6-diyl group, phenanthrene-2,7-diyl group, fluorene-2,7-diyl group, trans-1,4-cyclohexylene group, 1,2,3,4-tetrahydronaphthalene-2,6-diyl group, decahydronaphthalene-2,6-diyl group, trans-1,3-dioxane-2,5-diyl group, pyridine-2,5-diyl group, pyrimidine-2,5-diyl group, pyrazine-2,5-diyl group or pyridazine-2,5-diyl group, each of which may be substituted by a fluorine atom; l and m each independently representss 0, 1 or 2; Z
1
, Z
2
, Z
3
and Z
4
each independently represents a single bond, —CH
2
CH
2
—, —(CH
2
)
4
—, —OCH
2
—, —CH
2
O—, —CH=CH—, —CH=N—N=CH— or —C=C—; X
2
represents a cyano group, a fluorine atom, a chlorine atom, a trifluoromethoxy group, a trifluoromethyl group, a difluoromethoxy group, a hydrogen atom, a 3,3,3-trifluoroethoxy group, R′ or —OR′ in which R′ represents a C
1-12
straight-chain alkyl group or a C
2-12
straight-chain alkenyl group; and X
1
and X
3
each independently representss a hydrogen atom, a fluorine atom or a chlorine atom, with the proviso that when R
4
is an alkenyl or alkenyloxy group, X
2
is a cyano group, the ring D is a trans-1,4-cyclohexylene group, m is 0, and Z
4
is a single bond,

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