Electro-optical liquid-crystal display

Details Electro-optical liquid-crystal display Electro-optical liquid-crystal display

1998-03-13

2001-06-19

Wu, Shean C. (Department: 1721)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C349S132000, C349S178000

Reexamination Certificate

active

06248410

ABSTRACT:

SUMMARY OF THE INVENTION
The invention relates to an electro-optical liquid-crystal display having a realignment layer for realigning the liquid crystals, where the field thereof has a significant component parallel to the liquid-crystal layer, comprising a liquid-crystalline medium having negative dielectric anisotropy, where the medium comprises at least four mesogenic compounds which contain a group of the formula A
In conventional liquid-crystal displays {TN, STN, OMI and AMD-TN}, the electric fields for realignment are produced essentially perpendicular to the liquid-crystal layer.
International Patent Application WO 91/10936 discloses a liquid-crystal display in which the electric signals are produced in such a way that the electric fields have a significant component parallel to the liquid-crystal layer (IPS, in-plane switching). The principles of operating such a display are described, for example, by R. A Soref in Journal of Applied Physics, Vol. 45, No. 12, pp. 5466-5468 (1974).
EP 0 588 568 discloses various ways of addressing such a display.
These IPS displays can be operated with liquid-crystalline materials either having a positive dielectric anisotropy or having a negative dielectric anisotropy (&Dgr;&egr;≠0). The specification describes, inter alia, an IPS display containing the mixture ZLI-2806 (E. Merck, Darmstadt) having negative dielectric anisotropy.
This mixture is based on compounds which contain a structural unit of the formula A However, IPS displays containing this material have relatively high threshold voltages and long response times. The object was therefore to indicate liquid-crystalline materials which are suitable for achieving relatively low threshold voltages and short response times in IPS displays.
Surprisingly, this object has been achieved by using liquid-crystalline materials which comprise at least four compounds which contain a group of the formula A
Such compounds are disclosed, for example, in EP 0 332 007 (formula A).
However, these specifications give no indication that these substances can be used to improve the threshold voltages and the response times of IPS displays.
The invention thus relates to an electro-optical liquid-crystal display having a realignment layer for realigning the liquid crystals, where the field thereof has a significant component parallel to the liquid-crystal layer, e.g., a parallel compound strong enough to force the switching of the molecules in the plane compared to the switching out of the plane as, e.g., in TN and STN or into the plane, as, e.g., in ECB, comprising a liquid- crystalline medium having negative dielectric anisotropy, where the medium comprises at least four mesogenic compounds which contain a group of the formula A
Preferred embodiments are IPS displays in which (a) the medium comprises at least one compound of the formula I
in each of which
R and R
a
are each an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF
3
or at least monosubstituted by halogen, where, in addition, one or more CH
2
groups in these radicals may each, independently of one another, be replaced by
in such a way that O atoms are not linked directly to one another,
A
1
and A
2
are each, independently of one another,
(a) a trans-1,4-cyclohexylene radical in which, in addition, one or more non-adjacent CH
2
groups may be replaced by —O— and/or —S—,
(b) a 1,4-phenylene radical in which, in addition, one or two CH groups may be replaced by N,
(c) a radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo-[2.2.2]octylene, piperidine- 1,4-diyl, naphthalene-2,6-diyl, decahydro-naphthal- ene-2,6-diyl and 1,2,3,4-tetra-hydronaphthalene-2,6-diyl,
where the radicals (a) and (b) may be substituted by one or two fluorine atoms,
Z
1
and Z
2
are each, independently of one another, —CO—O—, —O—CO—, —CH
2
O—, —OCH
2
—, —CH
2
CH
2
—, —CH═CH—, —C═C— or a single bond, or one of the radicals Z
1
and Z
2
is alternatively —(CH
2
)
4
— or —CH═CH—CH
2
CH
2
—,
m and n are 0, 1 or 2 and
m+n is 1, 2 or 3;
b) the medium has a dielectric anisotropy &Dgr;&egr; of <−4.6, preferably between −4.7 and −8; and/or
c) the medium comprises at least one compound of the formula II
R
3
-(A
3
-Z
3
)
o
-A
4
-R
4
  II
in which
R
3
and R
4
each, independently of one another, are as defined for R
1
and R
a
,
A
3
and A
4
each, independently of one another, are as defined for A
1
and A
2
,
Z
3
in each case independently of the others, is as defined for Z
1
and Z
2
, and
o is 1, 2 or 3.
Preference is even more given to an IPS display in which the pixels are addressed by means of an active matrix, and to mixtures wherein the four compounds of (A) are compounds of formula I.
The invention furthermore relates to a liquid-crystalline medium having negative dielectric anisotropy which comprises at least one compound of the formula I and at least one compound of the formula II, in particular which comprises
from 15 to 85 % by weight, preferably from 30 to 75 % by weight, of at least four compounds of the formula I, and
from 10 to 55 % by weight, preferably from 10 to 40 % by weight, of at least one compound of the formula II.
The novel liquid crystalline medium preferably comprises
at least one compound of the formula Ia, and
at least one compound of the formula IIb
at least one compound selected from the formulae IIa, IIb and IIc
in which
and R, R
a
, R
3
and R
4
are in each case as defined above.
The novel liquid-crystalline media generally have a birefrigence (&Dgr;n) of <0.10, preferably between 0.04 and 0.09, in particular between 0.05 and 0.07.
The bulk viscosity (at 20° C.) of the novel materials is generally less than 30 mm
2
s
−1
, in particular between 15 and 25 mm
2
·s
−1
. The resistivity of the novel materials is generally between 5×10
10
and 7×10
13
&OHgr;·cm
−1
at 20° C.
It has been found that even a relatively small proportion of compounds of the formula I mixed with conventional liquid-crystal materials, but in particular with one or more compounds of the formula II, results in a significant reduction in the threshold voltage and in fast response times, and at the same time broad nematic phases having low smectic-nematic transition temperatures are observed. The compounds of the formulae I and II are colorless, stable and readily miscible with one another and with other liquid-crystal materials.
The term “alkyl” covers straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atoms are generally preferred.
The term “alkenyl” covers straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. Particular alkenyl groups are C
2
-C
7
-1E-alkenyl, C
4
-C
7
3E-alkenyl, C
5
-C
7
-4-alkenyl, C
6
-C
7
-5-alkenyl and C
7
-6-alkenyl, in particular C
2
-C
7
-1E-alkenyl, C
4
-C
7
-3E-alkenyl and C
5
-C
7
-4-alkenyl. Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
The term “fluoroalkyl” preferably covers straight-chain groups containing terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. However, other positions of the fluorine are not excluded.
The term “oxaalkyl” preferably covers straight-chain radicals of the formula C
n
H
2n+1
—O—(CH
2
)
m
, in which n and m are each, independently of one another, from 1 to 6. n is preferably 1 and m is preferably from 1 to 6.
Through a suitable choice of the meanings of R, R
a
, R
3
and R
4
, the addressing times, the threshold voltage, the steepness of the transmission characteristic lines, etc., can be modified

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