KR101208150B1 - Liquid crystal medium - Google Patents

Abstract

The present invention provides a liquid crystal medium based on a mixture of polar compounds characterized in that it comprises at least one compound of formula 1a and at least one compound of formula 1b, uses for electro-optics of this medium, and displays containing the medium It is about:

Where

, -CH=CH-, -C≡C-, -CO-, -CO-O-, -O-CO- 또는 -O-CO-O-로 대체되고;R ¹ and R ² are each independently of each other an alkyl radical having 1 to 12 carbon atoms which is monosubstituted or unsubstituted or substituted with CN or CF ₃ , or at least monosubstituted with halogen, wherein at least one CH ₂ group is optionally independently of each other, -O-, -S-,

, -CH = CH-, -C≡C-, -CO-, -CO-O-, -O-CO- or -O-CO-O-;

Y ¹ and Y ² are each independently H or F;

X is F or Cl or a halogenated alkyl, alkenyl, alkoxy or alkenyloxy radical having up to 6 carbon atoms;

A is independently of each other 1,4-phenylene, trans-1,4-cyclohexylene or 2,3-difluoro-1,4-phenylene;

One of B and C is 2,3-difluoro-1,4-phenylene and the other is 1,4-phenylene or 2,3-difluoro-1,4-phenylene;

r is 1 or 2.

Description

Liquid crystal medium {LIQUID CRYSTAL MEDIUM}

The present invention relates to liquid crystal media, their use for electro-optics, and displays containing the media.

Liquid crystals are mainly used as dielectrics in display devices because the optical properties of these materials can be modified by applied voltages. Liquid crystal based electro-optical devices are well known to those skilled in the art and can be based on various effects. Examples of such devices are cells with dynamic scattering, DAP (aligned phase strain) cells, guest / host cells, TN cells with twisted nematic structures, STN (over twisted nematic) cells , SBE (excess birefringence effect) cells and OMI (optical mode interference) cells. The most common display device is based on the Schatt-Helfrich effect and has a twisted nematic structure.

The liquid crystal material should have good stability against electric fields and electromagnetic radiation and good chemical and thermal stability. In addition, the liquid crystal material should have low viscosity and exhibit short addressing time, low threshold voltage and high contrast in the cell.                         

They should also have suitable mesophases, for example nematic or cholesteric boundary phases, at normal operating temperatures for the above-mentioned cells, ie at the widest possible temperature range above or below room temperature. Since liquid crystals are generally used as a mixture of a plurality of components, it is important that the components are easily miscible with each other. In addition to properties such as electrical conductivity, dielectric anisotropy and optical anisotropy must satisfy various requirements depending on cell type and application area. For example, materials for cells with twisted nematic structures should have positive dielectric anisotropy and low electrical conductivity.

For example, in matrix liquid crystal displays (MLC displays) with integrated nonlinear elements for switching individual pixels, large amounts of dielectric anisotropy, wide nematic phases, relatively low birefringence, very high resistivity, good UV and temperature stability And media having low vapor pressures are desired.

Matrix liquid crystal displays of this type are known. Nonlinear devices that can be used to individually switch individual pixels are, for example, active devices (ie transistors). The term "active matrix" is thus used where a distinction can be made between the following two types:

1. a MOS (metal oxide semiconductor) or other diode on a silicon wafer as a substrate,

2. A thin film transistor (TFT) on a glass plate as a substrate.

The use of single crystal silicon as the substrate material limits the display size, which causes problems in bonding even with modular assemblies of various part-displays.                         

The electrooptic effect used in the case of the preferred and more potent Type 2 is typically the TN effect. Two techniques are distinguished (compound semiconductors, for example TFTs comprising CdSe, or polycrystalline or amorphous silicon-based TFTs).

The TFT matrix is applied inside one glass plate of the display while the other glass plate has a transparent counter electrode inside. Compared with the size of the pixel electrode, the TFT is very small and substantially does not adversely affect the image. This technique can also be extended to displays capable of all colors, where the red, green and blue filter mosaics are arranged such that the filter elements are opposite to each switched pixel.

TFT displays are usually illuminated from the back, acting as TN cells with polarizers crossed in transmission.

The term MLC display herein includes any matrix display, including an integrated nonlinear element, in addition to the active matrix, and also a display comprising a passive element such as a varistor or diode (MIM = metal-insulator-metal).

This type of MLC display is particularly suitable for high-end information displays for TV applications (eg pocket TVs) or computers (laptop computers) and for automotive or aircraft construction. In addition to problems related to angular dependence of contrast and response time, difficulties also arise in MLC displays due to inadequately high resistivity of liquid crystal mixtures (TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA.E., WATANABE.H., SHIMIZU.H., Proc.Eurodisplay 84, Sept. 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings., P.141 ff , Paris; STROMER, M., Proc. Eurodisplay 84, Sept. 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145ff, Paris]. As the resistance decreases, the contrast of the MLC display is lowered and problems after image removal may occur. Since the resistivity of the liquid crystal mixture generally falls beyond the lifetime of the MLC display due to interaction with the inner surface of the display, high (initial) resistance is very important to achieve acceptable service life. In particular for low voltage mixtures it has not been possible to achieve very high resistivity values so far. It is even more important that the resistivity shows as small an increase as possible with increasing temperature after heating and / or UV exposure. The low temperature properties of the mixtures obtained from the prior art, in particular so-called "low temperature stability" (LTS), are also particularly disadvantageous. Even at low temperatures no crystallization and / or smectic phases occur, and the temperature dependence of the viscosity is as low as possible. Thus, MLC displays obtained from the prior art do not meet these today's requirements.

There is a continuing need for MLC displays that have a very high resistivity, which does not have these disadvantages but only reduces them, while having a wide operating temperature range, short response time even at low temperatures and low threshold voltages.

TN (Shaft-Helfried) cells require a medium that promotes the following benefits in the cell:

Extended nematic phase range, especially at low temperatures

-Ability to switch at extremely low temperatures (outdoor, automotive, avionics)                         

-Increased resistance to UV radiation (long service life)

Commercially available media from the prior art do not allow these benefits to be achieved while simultaneously maintaining other parameters.

For excessively twisted (STN) cells, a medium is needed that can impart greater multiplexing capability and / or lower threshold voltages and / or wider nematic phase ranges (especially at low temperatures). As a result, it is urgently needed to widen the range of available parameters (clearing point, smectic-nematic transition point or melting point, viscosity, dielectric parameter, elastic parameter).

For TV and monitor applications, a medium with a fast response time and a low threshold voltage is required, as well as good low temperature stability. In addition, high birefringence may be required depending on the thickness of the switchable LC layer.

The present invention does not have the above mentioned disadvantages or only to a reduced extent, and preferably provides a medium for MLC, TN or STN displays, which also preferably has very high resistivity, low threshold voltage, improved LTS and fast switching time simultaneously. I'm trying to do that.

It has now been found that this object can be achieved by using the medium according to the invention in a display.

Accordingly, the present invention relates to liquid crystal media based on polar compound mixtures comprising at least one compound of formula 1a and at least one compound of formula 1b:

Formula 1a

1b

Where

, -CH=CH-, -C≡C-, -CO-, -CO-O-, -O-CO- 또는 -O-CO-O-로 대체되고;R ¹ and R ² are each independently of each other an alkyl radical having 1 to 12 carbon atoms which is monosubstituted or unsubstituted or substituted with CN or CF ₃ , or at least monosubstituted with halogen, wherein at least one CH ₂ group is optionally independently of each other, -O-, -S-,

, -CH = CH-, -C≡C-, -CO-, -CO-O-, -O-CO- or -O-CO-O-;

Y ¹ and Y ² are each independently H or F;

X is F or Cl or a halogenated alkyl, alkenyl, alkoxy or alkenyloxy radical having up to 6 carbon atoms;

A is independently of each other 1,4-phenylene, trans-1,4-cyclohexylene or 2,3-difluoro-1,4-phenylene;

One of B and C is 2,3-difluoro-1,4-phenylene and the other is 1,4-phenylene or 2,3-difluoro-1,4-phenylene;

r is 1 or 2.

The compounds of formulas 1a and 1b in the pure state are colorless and form a liquid crystal boundary phase suitably positioned for electro-optics in a certain temperature range. They are chemically and thermally stable to light.

R ¹ and R ² in formulas 1a and 1b are preferably straight-chain alkyl or alkoxy having 1 to 12 carbon atoms, very preferably 2 to 7 carbon atoms.

X in formula 1a is preferably F, Cl, CF ₃ , CHF ₂ , OCF ₃ , OCHF ₂ , OCFHCF ₃ , OCFHCHF ₂ , OCF ₂ CH ₃ , OCF ₂ CHF ₂ , OCF ₂ CF ₂ CHF ₂ , OCFHCF ₂ CF ₃ , OCFHCF ₂ CHF ₂ , OCF ₂ CF ₂ CF ₃ , OCF ₂ CF ₂ CClF ₂ , OCClFCF ₂ CF ₃ or CH = CF ₂ , most preferably F or OCF ₃ .

Particularly preferred compounds of formula 1a are compounds selected from formulas 1aa-1ah:                     

Wherein R ¹ has the meaning given in Formula 1a. Very preferred are compounds of formula 1aa.

Particular preference is given to compounds of the formula 1b in which r is 1. More preferred are compounds of formula 1b, wherein A is 1,4-phenylene or trans-1,4-cyclohexylene. More preferred are compounds of formula 1b, wherein one of B and C is 1,4-phenylene and the other is 2,3-difluoro-1,4-phenylene.

Particularly preferred compounds of formula 1b are compounds selected from formulas 1ba-1bx:                     

Wherein R ¹ and R ² have the meanings given in Formula 1b. Very preferred are those compounds of formulas 1ba-1bf, particularly preferred are compounds of formula 1ba.

Compounds of formulas 1a and 1b are prepared for the reaction by methods known per se, as described in Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart]. Known and prepared correctly under appropriate reaction conditions. It is also possible to use variant methods known per se but not mentioned in more detail herein.

The invention also relates to an electro-optic display containing this type of medium (in particular two planes parallel to the outer plate forming the cell with the frame, integrated nonlinear elements for switching individual pixels on the outer plate, and the cell STN or MLC displays having a mixture of nematic liquid crystals having a positive dielectric anisotropy and high resistivity), and the use for electro-optics of these media. In addition to reflective displays, the mixtures according to the invention are also suitable for IPS (in-plane switching) applications, OCB (optically controlled birefringence) applications and VA (vertical alignment) applications.

The liquid crystal mixtures according to the invention can significantly broaden the range of parameters available. In particular, it has been found that by using liquid crystal mixtures comprising the compounds of formulas 1a and 1b, mixtures with low threshold voltages and improved LTS can be obtained.

The obtainable combination with clarity point, rotational viscosity (γ ₁ ), low Δn and dielectric anisotropy is far superior to previous materials obtained from the prior art.

To date, the demand for high clarity points, nematic phases at low temperatures and high amounts of Δε has only been achieved to an unsuitable degree. There are mixtures of the prior art with high amounts of Δε, for example MLC-6424, but they only have low values of clarity and high values of rotational viscosity γ ₁ . Other mixture systems have good flow viscosity (ν ₂₀ ) and Δε values but only a clear point of 60 ° C. in this region.

The liquid crystal mixtures according to the present invention also have a dielectric anisotropy value Δε of 4 or more, preferably 5 or more, and a resistivity, while maintaining the nematic phase to -20 ° C, preferably -30 ° C, particularly preferably -40 ° C. It is possible to achieve a high value and at the same time have a clear point above 80 ° C., preferably above 90 ° C., particularly preferably above 100 ° C., thus making it possible to obtain excellent STN and MLC displays. In particular, the mixture is characterized by a low operating voltage. The TN threshold voltage is less than 2.0V, preferably less than 1.7V, particularly preferably less than 1.5V.

Proper selection of the components of the mixtures according to the invention can achieve higher clearing points (eg above 110 ° C.) at higher threshold voltages or lower lower clearing points while retaining other advantageous properties at lower threshold voltages. Can be achieved. At only slightly correspondingly increased viscosities it is likewise possible to obtain mixtures with smaller thresholds with larger Δε. Preferably, the first Gooch and Tarry transmission minimums (CH Gooch and HA Tarry, Electron. Lett. 10, 2-4, 1974), the posterior and tarry [ Appl. Phys., Vol. 8, 1575-1584, 1975), using an MLC display according to the invention, with particularly advantageous electro-optic such as high feature slope and low contrast angle dependence, for example. In addition to the properties (German Patent No. 30 22 818), the low dielectric anisotropy is sufficient for the second minimum at the same threshold voltage as in similar displays. This makes it possible to achieve significantly higher resistivity using the mixture according to the invention than when using a mixture comprising a cyano compound as the first minimum. By appropriately selecting the individual components and their weight ratios, one skilled in the art can set the birefringence required for the predetermined layer thickness of the MLC display using simple conventional methods.

The flow viscosity ν ₂₀ at 20 ° C. is preferably less than 60 mm ² s ⁻¹ , particularly preferably less than 50 mm ² s ⁻¹ . The rotational viscosity γ ₁ at 20 ° C. of the mixture according to the invention is preferably less than 120 mPa · s, particularly preferably less than 100 mPa · s. The nematic phase range is preferably at least 90 °, particularly preferably at least 100 °. This range preferably extends at least from -20 ° to + 80 °.

의 사이아노페닐사이클로헥세인 또는 화학식

의 에스터를 포함한 유사한 혼합물보다 온도 증가에 따른 HR의 감소가 훨씬 더 적게 나타남을 보여주었다. Dose maintenance ratio (HR) (S. Matsumoto et al., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304 ( 1984); measurements of G. Weber et al. (Liquid Crystals 5, 1381 (1989)) indicate that mixtures according to the invention, including compounds of formula 1, may be substituted for compounds of formula 1 according to the invention. Chemical formula

RTI ID = 0.0 > cyanophenylcyclohexane < / RTI >

The decrease in HR with increasing temperature is much less than that of similar mixtures containing esters.

The UV stability of the mixtures according to the invention was also significantly better. That is, they showed much less reduction in HR upon exposure to UV.

The medium according to the invention preferably contains compounds containing only minor amounts (up to 10% by weight) of cyano groups, very preferably no compounds at all. The maintenance ratio value of the medium according to the invention is preferably above 98%, very preferably above 99% at 20 ° C.

Short switching times in liquid crystal display devices are particularly desirable when used for video and TV applications. For this purpose, a switching time (t _on + t _off ) of less than 25ms is required. The upper limit of the switching time is determined by the repetition rate of the screen.

Preferred embodiments of the invention are as follows:

The medium further comprises at least one compound selected from the group consisting of:

[In the above formula,                     

R ⁰ is n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl having 9 or less carbon atoms,

X ⁰ is F or Cl, halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms,

Z ⁰ is -C ₂ H ₄ -,-(CH ₂ ) _4- , -CH = CH-, -CF = CF-, -C ₂ F _4- , -CH ₂ CF _2- , -CF ₂ CH _2- It is or _{-CF 2 O-, -, -CH 2} O-, -OCH 2 -, -COO-, -OCF 2

Y ¹ to Y ⁴ are each independently H or F,

r is 0 or 1;

The compound of formula 4 is preferably selected from formulas 4a to 4e:

[Wherein, X ⁰ and R ⁰ have the meaning given in the formula (2), and X ⁰ is preferably F or OCF ₃ . Particular preference is given to the compounds of the formulas 4b and 4c.];

The compound of formula 7 is preferably selected from formulas 7a to 7e:

[Wherein, X ⁰ and R ⁰ have the meaning given in the formula (2), and X ⁰ is preferably F or OCF ₃ . Particular preference is given to compounds of the formula 7b.];

The LC medium further comprises at least one compound selected from formulas 8-13:                     

[Wherein, X ⁰ and R ⁰ have the meaning given in formula (2), and L is H, F or Cl. X ⁰ is preferably F or Cl. In the compounds of Formulas 8, 9 and 10, X ⁰ is preferably Cl or F and L is H. Very preferred are compounds of Formulas 9 and 10.];

The medium further comprises at least one compound selected from formulas 14 to 21:

[Wherein, R ⁰ , X ⁰ , Y ¹ , Y ² , Y ³ and Y ⁴ are each independently as defined in formulas (2) to (7). X ⁰ is preferably F, Cl, CF ₃ , OCF ₃ or OCHF ₂ . R ⁰ is preferably alkyl having 6 or less carbon atoms, oxaalkyl, fluoroalkyl or alkenyl. Particular preference is given to compounds of the formula (16) wherein Y ¹ , Y ² and Y ³ are F and Y ⁴ is H.];

The medium further comprises at least one compound of the formula

[Wherein, R ¹ and R ² are as defined in formula (1b), Y ¹ is as defined in formula (2). Particular preference is given to compounds of the formula 22 in which Y ¹ is F.];

The medium further comprises at least one alkenyl compound of the formula

[In the above formula,

A is 1,4-phenylene or trans-1,4-cyclohexylene,

a is 0 or 1,                     

R ³ is an alkenyl group having 2 to 9 carbon atoms,

R ⁴ is as defined for R ¹ in Formula 1a.];

Particularly preferred compounds of formula 23 are compounds selected from formulas 23a to 23i:

[In the above formula,

R ^3a and R ^4a are each independently H, CH ₃ , C ₂ H ₅ or nC ₃ H ₇ , and alkyl is an alkyl group having 1 to 8 carbon atoms. Particular preference is given to compounds of the formulas 23a, 23f and 23g, in which R ^3a is H or CH ₃ ];

The medium further comprises at least one alkenyl compound of formula 24:                     

[In the above formula,

R ³ is an alkenyl group having 2 to 7 carbon atoms,

Q is CF ₂ , OCF ₂ , CFH, OCFH or a single bond,

Y is F or Cl,

L ¹ and L ² are independently of each other H or F.

Especially preferred are compounds of formula 24, wherein L ¹ and / or L ² are F and QY is F or OCF ₃ . Preference is furthermore given to compounds of the formula 11 in which R ³ is 2 to 7, carbon atoms, preferably 2 to 4 1E-alkenyl or 3E-alkenyl.

Very preferred are compounds of formula 24a:

Wherein R ^3a is H, CH ₃ , C ₂ H ₅ or nC ₃ H ₇ , in particular H or CH _3. ];

The medium further comprises at least one compound selected from formulas 25 and 26:

[In the above formula,

R ¹ and R ² are as defined in Formula 1b.];

는 바람직하게는

-

Is preferably

이다;

to be;

The medium comprises at least one compound of the formula 2, 3, 4, 5, 6 or 7;

R ⁰ is straight chain alkyl or alkenyl of 2 to 7 carbon atoms;

The medium contains 1 to 4 compounds of the formula 1a, most preferably of the formula 1aa;

The medium contains 1 to 4, preferably 1 or 2 compounds of formula 1b, most preferably of formula 1ba;

The medium comprises at least one compound selected from formulas 1a, 1b, 4b, 4c, 7b, 9, 10, 22, 23 and 25;

The proportion of compounds of formula 1a in the medium is 1 to 35% by weight, in particular 1 to 25% by weight;

The proportion of the compound of formula 1b in the medium is 1 to 25% by weight, in particular 1 to 15% by weight, most preferably 1 to 8% by weight;

The medium consists essentially of compounds selected from the group consisting of the formulas 1a, 1b, and 2 to 26.

The term "alkyl" includes straight and branched chain alkyl groups having 1 to 7 carbon atoms, in particular straight chain groups, methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups of 2 to 5 carbon atoms are generally preferred.

The term "alkenyl" includes straight and branched chain alkenyl groups, especially straight chain groups, having from 2 to 7 carbon atoms. Specific alkenyl groups include 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, especially 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 refers to a straight-chain group having terminal fluorine, ie fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6- Fluorohexyl and 7-fluoroheptyl. However, other positions of fluorine are not excluded.

The term “oxaalkyl” is preferably of the formula C _n H _{2n + 1} -O- (CH ₂ ) _m , where n and m are each independently of each other 1 to 6, n is preferably 1, m Preferably 1 to 6).

A relatively low proportion of compounds of formulas 1a and 1b mixed with conventional liquid crystal materials, in particular one or more compounds of formulas 2, 3, 4, 5, 6 and / or 7, significantly lower the threshold voltage and give a low birefringence value , A wide nematic phase with a low smectic-nematic transition temperature was observed simultaneously and found to improve the shelf life. Particular preference is given to mixtures comprising at least one compound of formula (7), in particular at least one compound of formula (7b) wherein X ⁰ is F, OCHF ₂ or OCF _{3 in} addition to at least one compound of formula (la) and (lb). Compounds of formulas 1a, 1b, and 2-7 are colorless, stable, and readily miscible with each other and other liquid crystal materials. In addition, the mixtures according to the invention are distinguished by very high clarity and the rotational viscosity (γ ₁ ) values are also relatively low.

Through appropriate selection of the meanings of R ⁰ and X ⁰ , the addressing time, threshold voltage, slope of the transmission feature line, etc. can be modified in a preferred manner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals, etc. generally shorten the addressing time, improve the nematic tendency, and improve the elastic constant k ₃₃ ( Bend) and k ₁₁ (display). 4-alkenyl radicals, 3-alkenyl radicals and the like generally exhibit low threshold voltages and low k ₃₃ / k ₁₁ values compared to alkyl and alkoxy radicals.

—CH ₂ CH ₂ —groups generally exhibit large k ₃₃ / k ₁₁ values compared to single covalent bonds. Higher k ₃₃ / k ₁₁ values result in flatter transmission features (e.g. to achieve gray shading) in TN cells with 90 ° twist, and greater multiplexing capabilities (in STN, SBE and OMI cells). To facilitate steeper transmission features, and vice versa.

Optimal mixing ratios of the compounds of Formulas 1a, 1b, and 2, 3, 4, 5, 6, and 7 may be used to select the desired properties, components of Formulas 1a, 1b, 2, 3, 4, 5, 6, and / or 7. And the choice of any other component that may be present. Suitable mixing ratios within the ranges given above can be readily determined in each case.

The total amount of the compounds of formulas 1a, 1b and 2 to 26 in the mixture according to the invention is not critical. Thus, the mixture may include one or more additional ingredients to optimize various properties. However, the larger the observed effect on the addressing time and the threshold voltage is, in general, the greater the total concentration of the compounds of Formulas 1a, 1b and 2 to 26.

In a particularly preferred embodiment, the medium according to the invention is of formulas 2 to 7 (preferably of formulas 2, 3 and / or 7, in particular formula 7b), wherein X ⁰ is F, OCF ₃ , OCHF ₂ , OCH = CF ₂ , OCF = CF ₂ or OCF ₂ —CF ₂ H). Advantageous synergistic effects with compounds of Formulas 1a and 1b produce particularly beneficial properties. In particular, mixtures comprising the compounds of formulas 1a, 1b and 7b are distinguished by a low threshold voltage.

Liquid crystal media according to the invention comprising the compounds of formulas 1a and 1b and further comprising at least one compound of formulas 2 to 16 are characterized by low rotational viscosity values, high birefringence, good LTS, and show fast response times. They are particularly suitable for TV, video and monitor applications.

The individual compounds of formulas 1a, 1b and 2 to 26, and subformulae thereof, which can be used in the media according to the invention are known or can be prepared analogously to known compounds.

The structure of the MLC display according to the invention produced from polarizers, electrode base plates and surface treated electrodes corresponds to the conventional structure for this type of display. The term "conventional structure" is used broadly herein and also includes all variants and derivatives of MLC displays, including matrix display elements based on poly-Si TFTs or MIM in particular.

However, an important difference between the display according to the invention and the display based on conventional twisted nematic cells lies in the choice of liquid crystal parameters of the liquid crystal layer.

Liquid crystal mixtures that can be used according to the invention are prepared in a conventional manner. In general, the desired amount of the components used in smaller amounts is advantageously dissolved in the components making up the main component at elevated temperatures. In addition, a solution of the components in an organic solvent such as acetone, chloroform or methanol can be mixed and thoroughly mixed and then the solvent removed again, for example by distillation.

In addition, the dielectric may further comprise additives known to those skilled in the art and described in the literature. For example, 0 to 15% of a polychromatic dye, stabilizer or chiral dopant may be added. Suitable dopants and stabilizers are shown in Tables C and D below.

In the present and following examples, structural formulas of liquid crystal compounds are indicated in acronyms, and modifications to chemical formulas are shown in Tables A and B below. All radicals of C _n H _{2n + 1} and C _m H _{2m + 1} represent straight chain alkyl radicals having n and m carbon atoms, respectively, and n and m are preferably 0, 1, 2, 3, 4, 5, 6 or 7. In Table B, the encoding is self-explanatory. Table A shows only the acronyms for the parent structural formula. In each case the acronyms for the parent structural formulas are separated by dashes, followed by the symbols for the substituents R ¹ , R ² , L ¹ and L ² .

Preferred mixture components of the mixtures according to the invention are given in Tables A and B below.

Table A

TABLE B

Table C

Table C shows possible dopants which are generally added to the compounds according to the invention in a proportion of preferably 0.1 to 10% by weight, very preferably 0.1 to 6% by weight.

Table D

For example, stabilizers that can be added to the mixtures according to the invention are shown below.

In addition to one or more compounds of Formulas 1a and 1b, particularly preferred mixtures comprise one, two, three, four, five or more of the compounds of Table B.

The following examples illustrate the invention without limiting the invention. Above and below, percentages are percentages by weight. All temperatures are given in degrees Celsius. In addition, C represents a crystalline phase, N represents a nematic phase, S represents a smectic phase, and I represents an isotropic phase. The data between these symbols indicates the transition temperature. Δn represents optical anisotropy, and n ₀ represents a refractive index (589 nm, 20 ° C.). Flow viscosity ν ₂₀ (mm ² / sec) and rotational viscosity γ ₁ [mPa · s] were determined at 20 ° C., respectively. V ₁₀ is the voltage for 10% transmission (viewing angle perpendicular to the plate surface). t _on represents the switch-on time at the operating voltage corresponding to twice the value of V ₁₀ and t _off represents the switch-off time. Δε represents dielectric anisotropy (Δε = ε? -Ε⊥, where ε? Refers to the dielectric constant parallel to the longitudinal molecular axis, and ε⊥ refers to the dielectric constant perpendicular to the longitudinal molecular axis). Electro-optic data was measured in TN cells at 20 ° C. with a first minimum (ie, d? Δn value of 0.5 μm) unless otherwise specified. Optical data were measured at 20 ° C. unless otherwise specified.

Example 1

delete

delete

Example 2

Example 3

According to the present invention it is possible to provide a medium for MLC, TN or STN displays with very high resistivity, low threshold voltage, improved LTS and fast switching time.

Claims (10)

Liquid crystal medium based on a polar compound mixture comprising at least one compound of formula 1a and at least one compound of formula 1b: Formula 1a

1b

Where R ¹ and R ² are each independently of each other an alkyl radical having 1 to 12 carbon atoms which is monosubstituted or unsubstituted or substituted with CN or CF ₃ , or at least monosubstituted with halogen, wherein at least one CH ₂ group is optionally independently of each other, -O-, -S-,

, -CH = CH-, -C≡C-, -CO-, -CO-O-, -O-CO- or -O-CO-O-; Y ¹ and Y ² are each independently H or F; X is F or Cl or halogenated alkyl, halogenated alkenyl, halogenated alkoxy or halogenated alkenyloxy radical having up to 6 carbon atoms; A is 1,4-phenylene; B is 2,3-difluoro-1,4-phenylene; C is 1,4-phenylene; r is 1. The method of claim 1, Wherein the compound of Formula 1a is a compound of Formula 1aa: Formula 1aa

Where R &^lt; 1 > is as defined in claim 1. delete The method of claim 1, A liquid crystal medium further comprising at least one compound selected from the group consisting of (2)

(3)

Formula 4

Formula 5

6

Formula 7

Where R ⁰ is 9 or less carbon atoms, alkoxy, oxaalkyl, fluoroalkyl or alkenyl; X ⁰ is F or Cl or halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms; Z ⁰ is -C ₂ H ₄ -,-(CH ₂ ) _4- , -CH = CH-, -CF = CF-, -C ₂ F _4- , -CH ₂ CF _2- , -CF ₂ CH _{2- , -CH 2 O-, -OCH 2 -} , -COO-, -OCF 2 - or -CF ₂ O-, and; Y ¹ to Y ⁴ are each independently H or F; r is 0 or 1; The method of claim 1, A liquid crystal medium further comprising at least one compound of formula Formula 23

Where A is 1,4-phenylene or trans-1,4-cyclohexylene; a is 0 or 1; R ³ is an alkenyl group having 2 to 9 carbon atoms; R ⁴ is as defined for R ¹ in Formula 1a. The method of claim 1, A liquid crystal medium further comprising one or more compounds selected from formulas 4b, 4c, 7b, 9, 10, 16, 22, 23a, 23f, 23g, and 25: 4b

Formula 4c

Formula 7b

Formula 9

Formula 10

Formula 16

Formula 22

Formula 23a

Formula 23f

Formula 23g

Formula 25

Where R ¹ and R ² are as defined in claim 1; R ⁰ is 9 or less carbon atoms, alkoxy, oxaalkyl, fluoroalkyl or alkenyl; X ⁰ is F or Cl or halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms; Y ¹ to Y ⁴ are each independently H or F; R ^3a is H, CH ₃ , C ₂ H ₅ or nC ₃ H ₇ ; Alkyl is an alkyl group having 1 to 8 carbon atoms; L is H or F. The method of claim 1, A liquid crystalline medium in which the proportion of the compound of formula 1a in the mixture as a whole is 1-25% by weight. The method of claim 1, A liquid crystalline medium in which the proportion of the compound of formula 1b in the mixture as a whole is 1-25% by weight. delete An electro-optical liquid crystal display containing the liquid crystal medium according to any one of claims 1, 2 and 4-8.