DE19720296A1 - Super:twist liquid crystal display - Google Patents

Abstract

Supertwist nematic liquid crystal displays (STN displays) consist of a cell with electrodes and orientation films, filled with a nematic liquid crystal (LC) mixture with positive dielectric anisotropy (at 20 deg C), tilt angle of 1-30 deg and twist angle of 100-600 deg . The nematic LC mixture consists of: (a) 20-90 wt.% LC component (A) based on mesogenic compound(s) (I) with 4-cyano-3-fluoro-phenyl or 4-cyano-3,5-difluorophenyl benzoate groups of formula (A) and mesogenic compound(s) (II) with a terminal 3,4,5-trifluorophenyl group; in which L = hydrogen (H) or fluorine (F), and having a dielectric anisotropy greater than +1.5; (b) 10-65 wt.% LC component B containing compound(s) with a dielectric anisotropy of -1.5 to +1.5; and (c) an optically active component D in such an amount that the ratio between the film thickness and natural pitch of the chiral nematic LC mixture is 0.2-1.3, the nematic phase range is not less than 60 deg C, the viscosity is not greater than 35 mPa, and the threshold potential is less than 1.5 V.

Description

The invention relates to supertwist Liquid crystal displays (STN displays) with short Switching times, good slopes and low Threshold voltages as well as the new ones used therein nematic liquid-crystal mixtures.

STN displays according to the preamble are known, for. B. from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Workshop Liquid crystals (8-10.04.87); 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, L1784-L1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); HA. 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, Appl. Phys. Lett. 50 (5), 236 (1987) and E.P. Raynes, Mol. Cryst. Liq. Cryst. Letters, Vol. 4 (1), pages 1-8 (1986). The term STN here includes any relatively high twist Display element with a twist angle in the amount between 160 ° and 720 °, such as the Display elements according to Waters et al. (C.M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), the STN-LCDs (DE-OS 35 03 259), SBE-LCDs (T.J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984)), OMI LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987)), DST-LCDs (EP-OS 0 246 842) or BW-STN-LCDs (K.Kawasaki et al., SID 87 Digest 391 (20.6)).

Such STN displays stand out in comparison to Standard TN displays with significantly better slopes the electro-optical characteristic and associated better contrast levels as well as through a significant lower angle dependence of the contrast. From Of particular interest are STN ads with short ones  Switching times, especially at relatively low Temperatures. To achieve short switching times have been So far in particular the viscosities of Liquid crystal mixtures optimized using mostly optimized combinations of Liquid crystal components and optionally also monotropic additives with relatively high vapor pressure. The However, achieved switching times were not for each Application sufficient.

Shorter switching times can also be achieved Reduction of the layer thickness of the liquid crystal layer the STN display and by using Liquid crystal mixtures with higher birefringence Δn achieve.

As a rule, compositions with short Switching times comparatively high threshold voltages.

All these approaches to achieve shorter switching times Ultimately, however, still lead to mixtures that are not were suitable for any application, especially those with high threshold voltages.

In International Patent Application WO 91/00898 are STN displays with comparatively short switching times, but high threshold voltages are described on Compounds with a terminal one or two times fluorinated benzene ring based.

From European Patent Application EP 0 681 022 are liquid crystalline compositions are known, the Compounds of the formula

10% of a compound of the formula

as well as other compounds with a dielectric Anisotropy of more than 1.5 included.

However, these compositions have Threshold voltages of more than 1.3 V.

Thus, there is still the demand for one Improving the threshold voltages of STN displays.

Further requirements for an STN display are one higher multiplexability, lower threshold voltages and a steep characteristic.

Optimal parameters can be, however, due to the counteracting different material parameters such as dielectric and elastic properties not for achieve all of the above properties simultaneously.

There is thus still a great need for improved STN displays with short switching times at the same time large working temperature range, high Slope slope, good angle dependence of Contrast and low threshold voltage.

The invention is based on the object, STN displays to provide that does not have the disadvantages mentioned above or only to a small extent and at the same time very useful Have overall properties.

It has now been found that this problem can be solved can, if you nematic liquid crystal mixtures used, whose component A consists essentially of at least one compound with a terminal 3,4,5-trifluorophenyl group and at least one compound with a structural element of formula 1,

where L is H or F.

Preferably, the nematic Liquid crystal mixture has a nematic phase range of at least 60 ° C, a viscosity of not more than 35 mPa · s and a dielectric anisotropy of at least +8 on, wherein the dielectric anisotropies of the compounds and those on the nematic liquid crystal mixture referenced parameters to a temperature of 20 ° C. are.

The invention thus relates to an STN display with

• - two plane-parallel support plates, with a Border form a cell,
• - a nematic one in the cell Liquid crystal mixture with positive dielectric anisotropy
• - Electrode layers with superimposed Orientation layers on the inside of the Carrier plates,
• - An angle between the longitudinal axis of the on the surface of the carrier plates located molecules and the carrier plates of about 1 degree to 30 degrees such as
• a twist angle of the liquid-crystal mixture in the cell from orientation layer too Orientation layer in the amount between 100 ° and 600 °, wherein the nematic liquid crystal mixture
• a) 20-90 wt .-% of a liquid-crystalline component A. containing one or more compounds with a dielectric anisotropy greater than +1.5,
• b) 10-65% by weight of a liquid-crystalline component B containing one or more compounds with a  dielectric anisotropy of -1.5 to +1.5,
• c) an optically active component C in such an amount contains that the ratio between the layer thickness (Distance between the plane-parallel carrier plates) and the natural pitch of the chiral nematic Liquid crystal mixture is about 0.2 to 1.3, and
• d) has a nematic phase range of at least 60 ° C, a viscosity of not more than 35 mPa · s, and a threshold voltage of less than 1.5V, the dielectric anisotropies of the compounds and the parameters related to the nematic liquid crystal mixture being at a temperature of 20 ° C, characterized in that component A consists essentially of at least one mesogenic compound with a structural element of formula 1 in particular wherein R¹ is alkyl having 1 to 8 C atoms, m is 0 or 1 and LH or F, and at least one mesogenic compound having a terminal 3,4,5-trifluorophenyl group, in particular a compound of formula II, wherein
  R is alkyl, alkoxy, oxaalkyl, alkenyl or alkenyloxy having up to 12 C atoms, each independently L 1 and L 2 are each independently H or F,
  Z¹ and Z² are each independently -COO-, -CH₂CH₂-, -C≡C- or a single bond and
  ○ 0 or 1,
  in particular those selected from the formulas IIa to IIh: wherein R has the meaning given.

Preferably, component B contains one or several compounds selected from the group consisting from the formulas III and IV:

wherein
R⁵ and R⁶ have the meaning given for R,

and L¹ have the meaning given for formula II,
Z 3 is -COO-, -CH 2 CH 2 -, -CH = CH- or a single bond,
Z⁴ -COO- or a single bond and
r is 1, 2 or 3.

In a particularly preferred embodiment contains Component B at least one compound of formula III and at least one compound of the formula IV1,

wherein
R ^{5 '} and R ^{6' are} each independently of the other alkyl of 1 to 8 carbon atoms,
L¹ H or F and
each k and l are independently 0 or 1,
especially those
wherein
L¹ F,
k 1 and
l 0 mean
and / or at least one compound of formula IV1, wherein
L 1 H,
k 0 and
l 0 or 1
mean.

The invention also relates Liquid crystal mixtures.

The individual compounds of the formulas I, II, III and IV and others in the STN displays of the invention usable compounds are either known or can be prepared analogously to the known compounds.

The compounds of the formula I are known from EP 0 019 665, the compounds of formula II z. B. from DE 39 29 525 and the compounds of formula IV1 from the German DE-OS 29 33 563 and DE 32 11 306, but there is no indication in these publications that the slope and the threshold voltages improve STN ads using these connections to let.

STN displays can be used with the mixtures according to the invention with high multiplexing ratios, large Operating temperature range, low threshold voltages (below 1.5 volts, preferably between 1.00 and 1.40 Volts, in particular between 1.05 and 1.30 volts) and steep Achieve characteristic curves.

The compounds of the formulas I, II and III confer improved STN compositions in particular higher Clarification points, wherein the compounds of the formula II are high values for the ratio of the elastic constants (K₃ / K₁) and therefore give high slopes.

The mixtures according to the present invention have an optical anisotropy (Δn) of more than 0.100, preferably from 0.100-0.140.

Preferably, the inventive Mixtures one or more compounds of formula II three rings selected from the following group:

wherein alkyl is an alkyl group having up to 12 carbon atoms means.

Preferred mixtures contain two, three or more Compounds of the formulas I, IIc, III and IV.

Preferred liquid-crystal mixtures which can be used according to the invention, contain a or more compounds from group A, preferably in a share of 20% to 90%, in particular 35% to 80%. This compound or compounds of group A have a dielectric anisotropy greater than +8 (preferably greater than +12) and represent the component A of the liquid crystal mixtures according to the invention.

Preferred liquid crystal mixtures contain two or more compounds from group B, preferably in one Share from 10% to 40%. This connection or connections  from group B have either low values for the Rotational viscosity (γ₁) <150 mPa · s or a clearing point greater than 120 ° C and are dielectrically neutral ((Δε) <2) or of medium polarity (Δε in the range of +2 to +10, preferably +4 to +8) and provide component B of Liquid crystal mixtures according to the invention.

Preferably, group B contains besides the compounds of the formulas I and II one or more compounds selected from the group consisting of III1 to III8 with two rings:

wherein R⁵ and R⁶ have the meaning given for R, and / or one or more compounds selected from Group consisting of III9 to III25 with three rings:

wherein R⁵ and R⁶ have the meaning given for R, and the 1,4-phenylene groups in the formulas III9 to III18 and III22 and III23 each independently also by Fluorine can be monosubstituted or polysubstituted, and / or one or more compounds selected from Group consisting of III26 to III31:

wherein R⁵ and R⁶ have the meaning given for R, and the 1,4-phenylene groups in III26 to III31, respectively independently also by fluorine simple or can be substituted several times.

The liquid crystal mixtures also contain a optically active component C, in such an amount, that the ratio between layer thickness (distance of the plane parallel support plates) and natural pitch of the chiral nematic liquid-crystal mixture in adaptation at the desired twist angle is more than 0.2.

Suitable dopants can be made of a large  Diversity of known chiral substances and in commerce available dopants such as cholesteryl nonanoate, S-811 (E. Merck, Darmstadt, FRG) and CB 15 (BDH, Poole, United Kingdom). Their choice is not in itself critical.

In a further preferred embodiment contain the liquid crystal mixtures one or more Compounds selected from group T consisting of the Compounds of formulas T1 and T2:

wherein Q³

preferably

L³ to L⁶ are each independently H or F,
R⁹ and R¹⁰ are each independently of one another R,
XF, Cl or OCF₃ and
Z⁰ is -COO-, -CH₂CH₂- or a single bond.

The proportion of component (s) in Group T is preferably 5% to 30%, in particular 5% to 20%.

Preferably, group T contains one or more Compounds selected from the group consisting of X to XII:

wherein R⁹ and R¹⁰ have the meaning given, especially those in which R⁹ is alkyl of 1 to 4, completely particularly preferably 1 or 2 C atoms, and R¹⁰ alkoxy with 1 to 4, very particularly preferably 1 or 2 C atoms.

Particularly preferred compounds of the formulas III2 and III4 are those of the following sub-formula:

wherein
Ring A is 1,4-phenylene or trans-1,4-cyclohexylene,
R¹³ CH₃- (CH₂) _n -O- or trans-H- (CH₂) _r -CH = CH- (CH₂-CH₂) _s -CH₂-O- and
R¹⁴ is CH₃- (CH₂) _t -, where
n 1, 2, 3 or 4,
r 0, 1, 2 or 3,
s 0 or 1 and
t is 1, 2, 3 or 4.

The proportion of these compounds or compounds of formula III2 and / or III4 is preferably about 2% to 15%, more preferably about 4% to 10%.

Inventive mixtures containing at least one Compound of formula II and at least two compounds of the formula I are particularly preferred.

The proportion of the compounds of the formulas I and II is preferably about 20% to 75%, especially 40% to 70%.

The proportion of the compounds of the formula I with m is the same 0 is preferably 10 to 35%, in particular 15 to 30%. The proportion of the compounds of formula II is preferably 10% to 30%, in particular 15 to 25%.

In a particularly preferred embodiment the mixtures contain about 5% to 35%, in particular preferably about 10% to 20% of liquid-crystalline Tolan compounds. This can at lower Layer thicknesses (about 5-6 microns) are worked, whereby the Switching times are significantly shorter. Especially preferred Tolans are given below:

R⁹ is preferably n-alkyl having 1 to 7 carbon atoms,
Z⁰ is preferably -CH₂CH₂- or a single bond,

preferably

Q3 is preferred

in which
R¹⁰ is preferably n-alkyl or n-alkoxy each having 1 to 7 carbon atoms or n-alkenyl or n-alkenyloxy each having 3 to 7 carbon atoms.

Preferably, component A contains one or several compounds of formula T1b

wherein

n is an integer from 1 to 15 and
Each of L⁵ and L⁶ independently represents H or F.

In a preferred embodiment, the nematic mixture contains
20-65% of one or more compounds of the formula I,
10-30% of two or more compounds selected from the formula II,
20-40% of one or more compounds of the formula III and
5-20% of two or more compounds of formula IV.

These mixtures show very short switching times of less than 450 ms, a slope of 1.05 to 1.20, a Birefringence between 0.10 and 0.14 and one Threshold voltage between 1.0 and 1.4 volts.

In further particularly preferred embodiments, the mixtures contain
10-35% of one or more compounds of the formula I where m is 0,
8-30% of two or more compounds selected from the formula I with m equal to 1,
10-30% of one or more compounds of the formula II,
20-40% of two or more compounds of formula III and
5-20% of one or more compounds of formula IV,
at least one connection from the following group:

wherein alkyl is a straight-chain alkyl group having 2-7 C atoms,
one or more compounds in which R is a trans-alkenyl group or a trans-alkenyloxy group,
one or more compounds selected from the following group:

wherein R⁵ and R⁶ are those given for component B. have preferred meanings, wherein the proportion of these compounds 0% to 50%, preferably about 20% up to 40%.

In general, component B consists of 3 to 10, preferably from 4 to 9 different compounds.

In a preferred embodiment, component B consists essentially of:
at least one compound selected from the following formulas:

In these formulas, C _n H _{2n + 1} and C _m H _{2m + 1} each independently represent alkyl having up to 12 C atoms.

As a rule, component A consists of 2 to 9, preferably 4 to 7 compounds. In a preferred Embodiment, the component A consists essentially from at least one compound selected from the formulas I and II, in particular selected from the following formulas:

In these formulas, C _n H _{2n + 1 means} alkyl having the meaning given in formulas I and II.

The structure of the liquid crystal display elements according to the invention from polarizers, electrode base plates and electrodes having such a surface treatment that the preference orientation (director) of each adjacent liquid crystal molecules from one to the other other electrode usually by amount 160 ° to 720 ° rotated against each other, corresponds to that for such Display elements usual construction. Ads with one Twist angles between 220 ° and 270 ° are preferred. The term of the usual construction is here far and includes all modifications and modifications the supertwist cell, especially Matrix display elements. The surface angle at the both carrier plates may be the same or different. Same tilt angles are preferred.

An essential difference of the invention Display elements to the usual on the basis of However, twisted nematic cell exists in the choice the liquid crystal components of the liquid crystal layer.

The preparation of the invention used Liquid crystal mixtures are carried out in a conventional manner Wise. In general, the desired amount of in relatively small amount of components used in solved the components constituting the main component,  expedient at elevated temperature. It is also possible, Solutions of components in an organic Solvent, for. In acetone, chloroform or methanol mix and the solvent after mixing again remove, for example by distillation.

The dielectrics can also be further, the expert known additives described in the literature contain. For example, 0-15% pleochroic Dyes are added.

Even without further explanation, it is assumed that the expert with reference to the preceding description the maximum advantage of the present invention. The preferred ones specific embodiments are therefore only as Illustrative and in no way as a limitation specific.

The following examples are intended to illustrate the invention explain without limiting it.

The abbreviations have the following meanings:

Kp .: clearing point

T _on : time from switching on until reaching 90% of maximum contrast,
T _off : time from switching off to reaching 10% of maximum contrast,
V₁₀: threshold voltage (volts)
V₉₀: saturation voltage (volts)
V₉₀ / V₁₀: slope.

The STN display is addressed in multiplexing mode (Multiplexing ratio 1: 240, bias 1:15).

Above and below are all temperatures in ° C specified. Percentages mean weight percent. The Values for the switching times and viscosities are at 20 ° C based.  

In the present patent application and in the following examples, all chemical structures of the liquid crystal compounds are indicated by acronyms, wherein the transformation into chemical formulas is carried out as shown below. All radicals C _n H _{2n + 1} and C _m H _{2m + 1} are straight-chain alkyl groups with n or m carbon atoms. The coding according to Table B is self-evident. In Table A, only the acronym for the main body is indicated. In individual cases, followed by the acronym for the main body with a dash followed by a code for the substituents R¹, R², L¹, L² and L³:

Table A

Table B

example 1

An STN ad with the following parameters:

Twist angle: 240 °
Bias: 1:16
Multiplexing ratio: 1: 240
Frame rate: 80 Hz
Tilt angle: 5 °
d: 7 μm

and a liquid crystalline medium having the following Properties:

Clarification point: + 89 ° C
Δn: 0.1101

and consisting of an achiral base mixture:

ME2N.F | 8.0% ME3N.F 8.0% ME4N.F 8.0% MESN.F 3.0% HP 3N.F 5.0% HP 4N.F 5.0% HP 5N.F 5.0% CCH 301 17.0% CCP 3F.F.F 12.0% CCP 5F.F.F 8.0% CP-33F 4.0% CP-35F 4.0% CH-33 5.0% CH-35 4.0% CH-43 4.0%

and doped with 0.7%

shows the following switching properties:

V₁₀: 1.1V
V₉₀ / V₁₀: 1.10

Example 2

An STN ad with the following parameters:

Twist angle: 220 °
d: 8 μm

and a liquid crystalline medium having the following Properties:

Clarification point: 100 ° C
Δn: 0.1142

and consisting of:

ME2N.F | 7.0% ME3N.F 7.0% ME4N.F 10.0% ME5N.F 10.0% PCH-301 3.0% CCH 301 6.0% CCZU-2F 4.0% CCZU-3F 6.0% CCZU-5F 6.0% CCP 3F.F.F 6.0% CVCP V 01 6.0% CP-33F 8.0% CP-35F 8.0% CP-55F 8.0% CCPC-34 5.0%

and doped with S-811, shows the following switching properties:

V₁₀: 1.20 V
V₉₀ / V₁₀: 1.067

Claims (8)

1. Supertwist liquid crystal display with

• two plane-parallel carrier plates, which form a cell with a border,
• an in-cell nematic liquid-crystal mixture having positive dielectric anisotropy,
• Electrode layers with superimposed orientation layers on the inside of the carrier plates,
• - An angle between the longitudinal axis of the molecules located on the surface of the carrier plates and the carrier plates of about 1 degree to 30 degrees and
• a twist angle of the liquid crystal mixture in the cell from orientation layer to orientation layer in the amount between 100 ° and 600 °, the nematic liquid crystal mixture
• a) 20-90 wt .-% of a liquid-crystalline component A, containing one or more compounds having a dielectric anisotropy of more than +1.5,
• b) 10-65% by weight of a liquid-crystalline component B containing one or more compounds having a dielectric anisotropy of from -1.5 to +1.5,
• c) contains an optically active component C in such an amount that the ratio between the layer thickness (distance of the plane-parallel support plates) and the natural pitch of the chiral nematic liquid-crystal mixture is about 0.2 to 1.3, and
• d) has a nematic phase range of at least 60 ° C, a viscosity of not more than 35 mPa · s, and a threshold voltage of less than 1.5V, the dielectric anisotropies of the compounds and the parameters related to the nematic liquid crystal mixture being at a temperature of 20 ° C, characterized in that component A consists essentially of at least one mesogenic compound having a structural element of formula 1, wherein L is H or F, and at least one mesogenic compound having a terminal 3,4,5-trifluorophenyl group.

2. A display according to claim 1, characterized in that component A contains at least one compound of formula I, wherein R¹ is alkyl having 1 to 8 C atoms, m is 0 or 1 and L is H or F. 3. A display according to claim 1 or 2, characterized in that component A contains at least one compound of formula I and at least one compound of formula II, wherein
R is alkyl, alkoxy, oxaalkyl, alkenyl or alkenyloxy having up to 12 C atoms, each independently L 1 and L 2 are each independently H or F,
Z¹ and
Each Z² is independently -COO-, -CH₂CH₂-, -C≡C- or a single bond and
○ 0 or 1 mean. 4. A display according to claim 3, characterized in that the component A contains at least one compound selected from the formulas IIa to IIh: wherein R has the meaning given. 5. A display according to claims 1 to 4, characterized in that component B contains one or more compounds selected from the group consisting of compounds of formulas III and IV: wherein R⁵ and R⁶ have the meaning given for R, and L¹ have the meaning given for formula II,
Z 3 is -COO-, -CH 2 CH 2 -, -CH = CH- or a single bond,
Z⁴ -COO- or a single bond and
r is 1, 2 or 3. 6. A display according to at least one of claims 1 to 5, characterized in that component B contains one or more compounds selected from the group consisting of X to XII: wherein R⁹ and R¹⁰ have the meaning given for R. 7. Display according to claims 1 to 6, characterized in that the nematic mixture
10-35% of one or more compounds of the formula I where m is 0,
8-30% of two or more compounds selected from the formula I with m equal to 1,
10-30% of one or more compounds of the formula II,
20-40% of two or more compounds of formula III and
Contains 5-20% of one or more compounds of formula IV. 8. Liquid crystal mixture with the in one of the claims 2 to 7 defined composition.