Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2001-12-06

2003-09-02

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299630, C252S299660, C252S299670

Reexamination Certificate

active

06613401

ABSTRACT:

The invention relates to a liquid-crystalline medium and to twisted nematic (TN) and supertwisted nematic (STN) liquid crystal displays having very short switching times and good steepness and angular dependence and to the novel nematic liquid crystal mixtures employed therein.
TN displays are known, e.g. from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, e.g. from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17
th
Freiburg Conference on Liquid Crystals (Aug.-Nov. 4, 1987); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, AppI. Phys. Lett. 50 (5), 236 (1987), and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN herein comprises any relatively strongly twisted display element having a twist angle amounting to between 160° and 360°, for example the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3
rd
Intern. Display Conference, Kobe, Japan), the STN LCDs (DE-A-35 03 259), SBE LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST LCDs (EP-A 0 246 842) or BW STN LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
STN displays in particular are distinguished, compared with standard TN displays, by considerably better slopes of the electro-optical characteristic curve and the attendant better contrast values and by a significantly reduced angular dependence of the contrast.
Of interest are TN and STN displays having a very short switching time, especially at lower temperatures. To achieve short switching times it has hitherto been the practice to optimize the rotational viscosities of the liquid crystal mixtures by employing usually monotropic additives having a relatively high vapor pressure. The switching times achieved were not, however, adequate for all applications.
To achieve a steep electro-optical characteristic curve in the displays according to the invention, the liquid crystal mixtures should have relatively large values of K
33
/K
11
and relatively small values of &Dgr;&egr;/&egr;
⊥
, &Dgr;&egr; being the dielectric anisotropy and &egr;
⊥
being the dielectric constant perpendicular to the longitudinal axis of the molecules.
Beyond optimized contrast and optimized switching times, such mixtures are subject to further important requirements:
1.
wide d/p window
2.
high chemical long-term stability
3.
high electrical resistance
4.
low frequency dependence and temperature dependence of the
threshold voltage.
The parameter combinations achieved are still inadequate by a long shot, especially for high-multiplex STN displays (with a multiplex rate in the range of about 1/400), but also for low- and medium-multiplex STN displays (with multiplex rates in the range of from about 1/64 and 1/16, respectively), and TN displays. One reason for this is that the effects of material parameters on the various requirements pull in opposite directions.
There is therefore still a great need for TN and STN displays, especially for low- and medium-multiplex STN displays, having very short switching times in conjunction with a wide operating temperature range, a steep characteristic curve, good angular dependence of contrast and low threshold voltage, which comply with the above-specified requirements.
SUMMARY OF THE INVENTION
An object of the invention is to provide liquid-crystalline media, especially for TN and STN displays, which do not have the above-specified drawbacks or which have them only to a lesser extent and at the same time have short switching times, in particular at low temperatures, and have very good steepness.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects can be achieved by employing liquid-crystal mixtures which comprise one or more compounds of formula A
and at least one compound of formula B
where
R
a
, R
b
and R
bb
are each, independently, an alkyl radical having 1 to 12 C atoms which is unsubstituted, singly substituted by CN or CF
3
or at least singly substituted by halogen, with the additional option of one or more CH
2
groups in the radicals, independently of one another in each case, being replaced by —O—, —S—,
—CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —C—CO—C— in such a way that O atoms are not directly linked together,
L
1
is H or F,
Q is CF
2
, OCF
2
, CFH, OCFH or a single bond
Y is F or Cl, and
Q—Y alternatively also is NCS, SCN, OCHFCF
3
or SF
5
.
The use of the compounds of formulae A and B in mixtures for TN and STN displays according to the invention results in
a steeply sloping electro-optical characteristic curve,
a low temperature dependence of the threshold voltage, and
very rapid switching times, especially at low temperatures.
The compounds of formula A and B, in particular, distinctly shorten the switching times of TN and STN mixtures, while at the same time result in an increase in the steepness and a low temperature dependence of the threshold voltage.
The mixtures according to the invention are further distinguished by the following advantages:
they have low viscosity,
they have a low threshold voltage and operating voltage,
they result in long storage times in the display at low temperatures.
The invention further relates to a liquid crystal display comprising
two substrates which, together with an edging, form a cell,
contained in the cell, a nematic liquid crystal mixture having positive dielectric anisotropy,
electrode layers having alignment layers on the insides of the substrates,
a pre-tilt angle between the longitudinal axis of the molecules at the surface of the substrates and the substrates of from 0 to 30 degrees, and
a twist angle of the liquid crystal mixture in the cell which, from alignment layer to alignment layer, is 22.5° to 600°,
a nematic liquid crystal mixture comprising
a)
15-75 wt % of a liquid-crystalline component A, containing one or
more compounds having a dielectric anisotropy of more than +1.5;
b)
25-85 wt % of a liquid-crystalline component B, containing one or
more compounds having a dielectric anisotropy of −1.5 to +1.5;
c)
0-20 wt % of a liquid-crystalline component D, containing one or
more compounds having a dielectric anisotropy of less than −1.5, and
d)
optionally an optically active component C in such an amount that the
ratio between layer thickness (spaqing of the substrates) and natural
pitch of the chiral nematic liquid crystal mixture is about 0.2 to 1.3,
characterized in that component A comprises at least one compound of formula A,
and component B comprises at least one compound of formula B
where
where R
a
, R
b
, R
bb
, L
1
and Q-Y have the meanings specified previously.
The invention also relates to TN and STN displays, especially in medium- and low-multiplexed STN displays, which contain the liquid-crystal mixture according to the invention.
In particular, formula A comprises compounds of the subformulae A-1 to A-8,
where R
a
and L
1
have the meanings specified previously.
Preferred are mixtures according to the invention which comprise at least one compound of formula A-1 and/or A-2, more preferably at least one compound of formula A-1 where L
1
=F.
In the formulae A and A-1 to A-8, R
a
preferably is straight-chain alkyl or alkoxy, or 1E-alkenyl or 3E-alkenyl having 2 to 7 C atoms.
Formula B comprises compounds of the subformulae B-1 to B-5,
where alkyl or alkyl
*
each, independently of one another, is a straight-chain or branched alkyl radical having from 1 to 12 C atoms, an

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