DE4112003A1 - Liq. crystal mixt. with wide nematic phase range etc. - contg. one or more substd. alkyl bis:phenylene cpds. - Google Patents

Abstract

A liq. crystal medium based on a mixt. of polar cpds. with positive dielectric anisotropy contains one cpd. of formula (I) (claimed), where R = alkyl, oxa-alkyl, fluoroalkyl or alkenyl, each with up to 7C; X = F, Cl, CF3, OCF3 or OCHF2; and L1-4 = F or H. USE/ADVANTAGE - The medium can be used in matrix-, twisted nematic- or super-twisted nematic-cells and has, compared to prior art liq. crystal compsns., a broadened nematic phase range (esp. at low temps.), switchability at low temps. (e.g., for outdoor, automobile or avionic use), increased UV resistance, improved clear pt., lower viscosity at low temps. and improved heat stability. Viscosity at -30 deg.C is 600 mPa.s or less and at -40 deg.C 1800 mPa.s or less, while anisotropy value (delta epsilon) is 3.5 or above and clear pt. is above 65 deg.C. The nematic phase range is -30 to +70 deg.C. (21pp Dwg.No.0/0)

Description

The present invention relates to a liquid crystalline Medium, its use for electro-optical purposes and this medium containing ads.

Liquid crystals are mainly used as dielectrics in display devices used because of the optical properties of such substances is affected by an applied voltage can be. Electro-optical devices on the base of liquid crystals are well known to those skilled in the art and can be based on different effects. Such devices are for example cells with dynamic scattering, DAP cells (upright phase deformation), guest / host cells, TN cells with twisted nematic ("twisted nematic") Structure, super-twisted nematic (STN) cells, SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference "). The most common display devices are based on the Schadt-Helfrich effect and possess a twisted nematic structure.  

The liquid crystal materials need a good chemical and thermal stability and good stability over have electrical fields and electromagnetic radiation. Furthermore, the liquid crystal materials should be lower Have viscosity and in the cells short response times, low threshold voltages and high contrast. Furthermore, they should at normal operating temperatures, d. H. in as wide a range as possible below and above Room temperature have a suitable mesophase, for example for the above cells a nematic or cholesteric mesophase. Because liquid crystals usually be used as mixtures of several components, It is important that the components mix well with each other are. Other properties, such as electrical conductivity, the dielectric anisotropy and the optical Anisotropy, depending on the cell type and application meet different requirements. For example should materials for cells with twisted nematic Structure a positive dielectric anisotropy and a have low electrical conductivity.

For example, are for matrix liquid crystal displays with integrated non-linear elements for switching individual Pixels (MFK displays) Media with large positive dielectric Anisotropy, broad nematic phases, relative low birefringence, very high resistivity good UV and temperature stability of the resistor and low Steam pressure desired.  

Such matrix liquid crystal displays are known. When non-linear elements for individual switching of the individual For example, pixels can be active elements (i.e. Transistors) can be used. One speaks then of one "active matrix", where you can distinguish two types:

• 1. MOS (Metal Oxide Semiconductor) transistors on silicon wafers as a substrate.
• 2. Thin-film transistors (TFT) on a glass plate as Substrate.

The use of monocrystalline silicon as a substrate material Restricts the display size, as well as the modular Composition of various sub-displays at the joints too Problems leads.

In the more promising type 2, which is preferred is as electro-optical effect usually the TN effect used. There are two different technologies: TFT's Compound semiconductors such. CdSe or TFT's on the base of polycrystalline or amorphous silicon. At the latter Technology is being worked on worldwide with great intensity. The TFT matrix is on the inside of a glass plate the display is applied while the other glass plate is on the inside carries the transparent counter electrode. in the Compared to the size of the pixel electrode is the TFT very small and does not disturb the picture practically. This technology can also be used for fully color-capable image presentations  be extended, with a mosaic of red, green and blue filters is arranged such that each one filter element lies opposite a switchable picture element.

The TFT displays usually work as TN cells crossed polarizers in transmission and are from behind illuminated.

The term MFK displays here includes every matrix display integrated nonlinear elements, d. H. next to the active one Matrix also displays with passive elements like varistors or diodes (MIM = metal-insulator-metal).

Such MFK displays are particularly suitable for TV applications (eg pocket TV) or for highly informative Displays for computer applications (laptop) and in automotive or aircraft construction. In addition to problems with the angle dependence the contrast and the switching times result in MFK ads difficulties caused by insufficient high resistivity of the 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 Transistor for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris). With decreasing resistance the contrast of a MFK display deteriorates, and the problem of "after image elimination" can occur.  

As the resistivity of the liquid crystal mixture by interacting with the internal surfaces of the display generally decreases over the lifetime of a MFK display, a high (initial) resistance is very important to acceptable To obtain service life. Especially with low-volt mixtures it was not possible, very high specific To realize resistances. Furthermore, it is important that the specific resistance is the smallest possible increase rising temperature and after temperature and / or UV exposure shows. The MFK displays from the state of the art do not meet today's requirements.

So far, liquid-crystalline media could be used for the practical Application required values for birefringence and Phase range (eg clearing point 70 °) only with threshold voltages of about 1.8 volts are made, if converted to values about 98% for the holding ratio under extreme conditions (eg after UV exposure) value was placed.

There is thus still a great need for MFK ads with very high specific resistance at the same time large working temperature range, short switching times and low threshold voltage that does not have these disadvantages or only to a lesser extent.

For TN (Schadt-Helfrich) cells, media are desired allow the following advantages in the cells:

• - extended nematic phase range (in particular to low temperatures),  
• - Switchability at extremely low temperatures (out-door-use, automobile, avionics),
• - increased resistance to UV radiation (longer lifetime).

With the available from the prior art Media, it is not possible to take advantage of these at the same time Receiving the remaining parameters to realize.

For higher-twisted cells (STN), media are desired a higher multiplexability and / or lower threshold voltages and / or broader nematic phase ranges (in particular at low temperatures). This is a further extension of the available parameter space (Clearing point, transition smectic-nematic or melting point, Viscosity, dielectric sizes, elastic Sizes) urgently desired.

The invention is based on the object media in particular to provide for such MFK, TN or STN displays, the disadvantages mentioned above are not or only to a lesser extent Dimensions and preferably at the same time very high specific Resistances and low threshold voltages have.

It has now been found that this problem can be solved when used in ads according to the invention media.  

The invention thus relates to a liquid-crystalline Medium based on a mixture of polar compounds with positive dielectric anisotropy, characterized that there are one or more compounds of the general Formula I

wherein R is alkyl, oxaalkyl, fluoroalkyl or alkenyl with each up to 7 C-atoms, X is fluorine, chlorine, CF₃, OCF₃ or OCHF₂ and L¹ to L⁴ each independently of one another fluorine or Hydrogen means.

The invention also provides electro-optical displays (especially STN or MFK displays with two plane-parallel Support plates that form a cell with a border, integrated non-linear elements for switching individual Pixels on the carrier plates and one in the cell nematic liquid-crystal mixture with positive dielectric anisotropy and high resistivity), containing such media, as well as the use these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention make it possible a significant extension of the available Parameter space.  

The achievable combinations of clearing point, viscosity at low temperature, thermal and UV stability and dielectric Anisotropy or threshold voltage exceed far previous materials from the prior art.

The demand for high clearing point, nematic phase at -40 ° C and a high Δε was so far only insufficient be fulfilled. Systems such. Although ZLI-3119 have comparable Clarification point and comparable cheap viscosities but have a Δε of only +3.

Other mixing systems have comparable viscosities and values of Δε, however, have only clearing points in the area from 60 ° C to.

The liquid-crystal mixtures according to the invention make it possible it, at low viscosities at low temperatures (at -30 ° C 600, preferably 550 mPa · s; at -40 ° C 1800, preferably 1700 mPa · s) simultaneously dielectric anisotropy values Δε3,5, preferably 4.0, clearing points above 65 °, preferably above 70 ° and a high value for the to achieve specific resistance, resulting in excellent STN and MKF displays can be achieved.

It is understood that by suitable choice of the components the mixtures according to the invention also have higher clearing points (eg above 90 °) at higher threshold voltages or lower Clarification points at lower threshold voltages while preserving  the other advantageous properties are realized can. The MFK displays according to the invention preferably operate in the first transmission minimum after Gooch and Tarry (C.H.Gooch and H.A. Tarry, Electron, Lett., 10, 2-4, 1974; C.H. Gooch and H.A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975), whereby in addition to particularly favorable electro-optical Properties such. B. high slope of the characteristic and low angular dependence of the contrast (DE-PS 30 22 818) at the same threshold voltage as in an analog display in the second minimum a smaller dielectric anisotropy is sufficient. This can be done using the Mixtures according to the invention clearly in the first minimum realize higher specific resistances than with mixtures with cyano compounds. The skilled person can by suitable Choice of the individual components and their proportions by weight Simple routine methods for a given layer thickness Set the MFK display required birefringence.

The viscosity at 20 ° C is preferably 25 mPa · s. The nematic phase range is preferably at least 70 °, in particular at least 80 °. Preferably, it extends this range at least from -30 ° to + 70 °.

Measurements of the capacity holding ration (HR) (S. Matsumoto et al., Liquid Crystals 5, 1320 [1989]; K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304 [1984]; G. Weber et al., Liquid Crystals 5, 1381 [1989]) have revealed that mixtures according to the invention containing compounds of  Formula I, a much smaller decrease in HR with increasing Have temperature as analogous mixtures containing instead the compounds of formula I Cyanophenylcyclohexane the formula

The UV stability of the mixtures according to the invention is also considerably better, d. H. they show a much smaller one Decrease in HR under UV exposure.

The achieved threshold voltages V₁₀ _/ ₀ _/ ₂₀ are generally 1.6 volts and preferably in the range 1.4 to 1.6 volts.

The media of the invention are also unusual wide nematic phase range also by extraordinary high elastic constants with very favorable viscosity values which, in particular when used in STN ads clear advantages over media from the state of Technology result.

Preferably, the media of the invention are based on several (preferably two or more) compounds of the formula I, d. H. the proportion of these compounds is 25%, preferably 40%.

The individual compounds of formulas I to XII and their Sub-formulas used in the media of the invention are either known, or they can be analog be prepared to the known compounds.  

Preferred embodiments are given below:

• - Medium additionally contains one or more compounds selected from the group consisting of the general formulas II, III and IV: in which the individual radicals have the following meanings:
  R: alkyl, oxaalkyl, fluoroalkyl or alkenyl having in each case up to 7 C atoms,
  X: F, Cl, CF₃, OCF₃ or OCHF₂,
  Y¹ and Y² are each H or F,
  r: 0 or 1.
• - Media additionally contains one or more compounds selected from the group consisting of the general formulas V to VIII: wherein R, X, Y¹ and Y² each independently have one of the meanings given in claim 2.
• - Medium additionally contains one or more compounds selected from the group consisting of the general formulas IX to XII: wherein R, X, Y¹ and Y² each independently have one of the meanings given in claim 2.
• - The proportion of compounds of formulas I to IV together is at least 50% by weight in the total mixture,
• - The proportion of compounds of formula I is in Total mixture 10 to 50% by weight,
• the proportion of compounds of the formulas II to IV in the total mixture is 30 to 70% by weight,
• - The medium contains compounds of formulas II and III or IV,
• R is straight-chain alkyl or alkenyl with 2 to 7 C atoms,
• - The medium consists essentially of compounds of the formulas I to IV,
• the medium contains further compounds, preferably selected from the following group (Y = H or F):
• The weight ratio I: (II + III + IV) is preferred 1: 4 to 1: 1.
• - Medium consists essentially of compounds selected from the group consisting of the general Formulas I to XII.

It was found that already a relatively small proportion of Compounds of the formula I in admixture with customary liquid-crystal materials, but especially with one or several compounds of formula II, III and / or IV to a considerable improvement in response times and too low Threshold voltages leads, while wide nematic phases with low transition temperatures smectic-nematic to be observed. The compounds of the formulas I to IV are colorless, stable and with each other and with others Liquid crystal materials readily miscible.  

The term "alkyl" includes 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 of 2-5 carbon atoms are in the generally preferred.

The term "alkenyl" includes straight-chain and branched Alkenyl groups with 2-7 carbon atoms, in particular the straight-chain groups. Especially alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6 alkenyl, in particular C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4 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 of up to 5 carbon atoms are generally preferred.

The term "fluoroalkyl" preferably includes straight chain Fluorine-terminated groups, d. H. Fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. Other positions of the fluorine are but not excluded.

The term "oxaalkyl" preferably includes straight chain Remainders of the formula

C _n H₂ _n ®₁-O- (CH₂) _m

where n and m are each independently from 1 to 6 mean. Preferably, n = 1 and m 1 to 6.  

By suitable choice of the meanings of R, X and Y can the response times, the threshold voltage, the slope of the Transmission characteristics etc. modified in the desired manner become. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-Alkenyloxyreste and the like usually too shorter response times, improved nematic tendencies and a higher ratio of the elastic constant k₃₃ (bend) and k₁₁ (splay) compared to alkyl or alkoxy. 4-alkenyl radicals, 3-alkenyl radicals and the like generally lower threshold voltages and smaller ones Values of k₃₃ / k₁₁ compared to alkyl and alkoxy. A group -CH₂CH₂- in Z¹ or Z² generally leads to higher values of k₃₃ / k₁₁ compared to a simple Covalent bond. Higher values of k₃₃ / k₁₁ allow z. B. shallower transmission characteristics in TN cells with 90 ° twist (to achieve shades of gray) and steeper transmission characteristics in STN, SBE and OMI cells (higher multiplexability) and vice versa.

The optimum ratio of the compounds of the formulas I and II + III + IV largely depends on the desired properties, on the choice of the components of the formulas I, II, III and / or IV and the choice of any other existing ones Components off. Suitable proportions within of the above range may be easy from case to case be determined.  

The total amount of compounds of formulas I to XII in the mixtures according to the invention is not critical. The mixtures may therefore contain one or more other components for the purpose of optimizing various properties. He watched Effect on the response times and the threshold voltage However, the higher the total concentration, the larger the rule to compounds of formulas I to XII.

In a particularly preferred embodiment, the Media of the invention compounds of the formula II, III, V and / or VII (preferably II and / or III), wherein X is CF₃, OCF₃ or OCHF₂ means. A favorable synergistic effect with The compounds of formula I leads to particularly advantageous Properties.

For STN applications, the media preferably contain compounds selected from the group consisting of the formulas V to VIII, wherein X is preferably OCHF₂.

The media according to the invention may further comprise a component A. Contain one or more compounds with a dielectric anisotropy of -1.5 to +1.5 of the general Formula I '

wherein
R¹ and R² are each independently of one another n-alkyl, n-alkoxy, ω-fluoroalkyl or n-alkenyl having up to 9 C atoms,
the rings A¹, A² and A³ are each independently of one another 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene,
Z¹ and Z² are each independently -CH₂CH₂-, -C≡C-, -CO-O-, -O-CO- or a single bond and
m is 0, 1 or 2.

Component A preferably contains one or more compounds selected from the group consisting of II1 to II7:

wherein R¹ and R² have the meaning given in formula I '.

Preferably, component A additionally contains one or more Compounds selected from the group consisting of II8 to II20:

wherein R¹ and R² have the meaning given in formula I ' and the 1,4-phenylene groups in II8 to II17 each independently also substituted by fluorine mono- or polysubstituted could be.

Furthermore, component A preferably additionally contains one or several compounds selected from the group consisting of II21 to II25:

wherein R¹ and R² have the meaning given in formula I ' and the 1,4-phenylene groups in II21 to II25 each independently also substituted by fluorine mono- or polysubstituted could be.  

Finally, such mixtures are preferred, their component A one or more compounds selected from the Group consisting of II26 and II27 contains:

wherein C _r H₂ _r ®₁ is a straight-chain alkyl group having up to 7 carbon atoms.

In some cases, the addition of compounds proves the formula

wherein
R¹ and R² have the meaning given in formula I 'and
Z⁰ a single bond,

means
for the suppression of smectic phases as advantageous, although thereby the resistivity can be lowered. In order to obtain optimum parameter combinations for the application, the person skilled in the art can easily determine whether and, if so, in what quantity these compounds can be added. Normally, less than 15%, especially 5-10% are used.

Further preferred are liquid-crystal mixtures which have a or more compounds selected from the group from III 'or IV' contain:

wherein R¹ and R² have the meaning given in formula I '.

The type and amount of polar compounds with positive Dielectric anisotropy is not critical per se. The Specialist can be known among a wide range and in many cases also commercially available components and Basic mixtures in simple routine tests suitable materials choose. Preferably, the inventive Media one or more compounds of formula I ''

wherein
Z¹, Z² and m have the meaning given in formula I ', Q¹ and Q² each independently of one another are 1,4-phenylene, trans-1,4-cyclohexylene or 3-fluoro-1,4-phenylene or one of the radicals Q¹ and Q 2 is also trans-1,3-dioxane-2,5-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,4-cyclohexenylene,
R⁰ is n-alkyl, n-alkenyl, n-alkoxy or n-oxaalkyl each having up to 9 C atoms, Y is H or F and X 'is CN, halogen, CF₃, OCF₃ or OCHF₂.

In a preferred embodiment, the inventive Media for STN or TN applications on connections of formula I ", wherein X 'is CN. It is understood that also smaller or larger proportions of other compounds of formula I '' (X '≠ CN) come into question. For MFK applications preferably contain only the media of the invention to about 10% of nitriles of the formula I '' (preferably, however no nitriles of the formula I ", but compounds of the formula I 'with X' = halogen, CF₃, OCF₃ or OCHF₂). These media are preferably based on the compounds of the formulas II to XII.

The structure of the STN or MFK display according to the invention Polarizers, electrode base plates and electrodes with Surface treatment is the same as for such displays usual construction. The term is the usual construction here broad and includes also all variations and Modifications of the MFK display, in particular matrix display elements based on poly-Si TFT or MIM.  

An essential difference of the displays according to the invention to the hitherto usual on the basis of the twisted nematic Cell, however, is the choice of liquid crystal parameters the liquid crystal layer.

The preparation of the liquid crystal mixtures which can be used according to the invention takes place in a conventional manner. In the Usually, the desired amount will be in lesser quantity used components in the constituting the main component Components dissolved, expedient at elevated temperature. It is also possible to have solutions of the components in one organic solvents, for. In acetone, chloroform or Methanol, to mix and the solvent after mixing to remove again, for example by distillation.

The dielectrics may also be further known to those skilled in the art and additives described in the literature. For example can 0-15% pleochroic dyes or chiral dopants are added.

C is a crystalline, S is a smectic, S _{B is} a smectic B, N is a nematic and I is the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (viewing direction perpendicular to the disk surface) t _on denotes the switch-on time and t _off the switch-off time at an operating voltage corresponding to 2.5 times the value of V₁₀Δn denotes the optical anisotropy and n _o the refractive index. Δε denotes the dielectric anisotropy (Δε = ε _∥ -ε _⟂ , where ε _{∥ is} the dielectric constant parallel to the _longitudinal molecular _axes and ε _{⟂ is} the dielectric constant perpendicular thereto.) The electro-optical data were determined in a TN cell in a first minimum (ie at a d Δn value of 0.5) at 20 ° C unless expressly stated otherwise Optical data were measured at 20 ° C unless expressly stated otherwise.

The following examples are intended to illustrate the invention, without to limit them. Above and below are all temperatures indicated in ° C. The percentages are by weight.

In the present application and in the following examples, the structures of the liquid crystal compounds are indicated by acronyms, wherein the transformation into chemical formulas according to following Tables A and B takes place. All radicals C _n H₂ _n ®₁ are straight-chain alkyl radicals with n or m C atoms. The coding according to Table B is self-evident. In Table A, only the acronym for the main body is given. In the individual case, a code for the substituents R 1, R 2, L 1 and L 2 follows separately from the acronym for the main body with a dash:

Table A

Table B

example 1 B-30CF₃ 8th% B-50CF₃ 13% B-70CF₃ 13% CCP 20CF₃ 8th% CCP 30CF₃ 13% CCP 40CF₃ 5% CCP 50CF₃ 12% BCH-3F · F 14% BCH-5F · F 14% N 65 I @ Δn = 0.117 @ V₁₀ = 1.68 volts

Example 2 EPCH-3F.F 10% EPCH-5F · F 8th% B-3F · F 8th% B-5F · F 7% CCP-2F · F 15% CCP-3F · F 12% CCP-5F · F 12% BCH-3F · F 10% BCH-5F · F 10% CCEB-3F · F 4% CCEB-5F · F 4%

Example 3 B-3F · F 10% B-5F · F 8th% PCH-3-CI 8th% PCH 5Cl 10% CCP-2F · F 10% CCP-3F · F 12% CCP-5F · F 12% BCH-3 · FCl 8th% BCH-3 · FCl 10% CCB-3 · FF 3% CCB-5 · FF 3% CCP 30Cf₃ 6%

Example 4 PCH-3-CI 10% PCH 5Cl 10% B-3F · F 6% B-5F · F 6% CCP-2F · F 8th% CCP-3F · F 12% CCP-5F · F 12% BCH-3F · F 14% BCH-5F · F 10% CCEB-3F · F 4% CCEB-5-F · F 4% CCB-3 · FF 2% CCB-5 · FF 2%

Example 5 PCH-7F · F 10% PCH-5F · F 12% B-3F · F 6% B-5F · F 6% CCP-2F · F 10% CCP-3F · F 12% CCP-5F · F 12% BCH-3F · F 10% BCH-5F · F 10% CCEB-3F · F 3% CCEB-5F · F 3% CBC-33F 3% CBC-53F 3%

Example 6 B-3F · F 9% B-5F · F 6% PCH-3-CI 15% PCH 5Cl 15% CCP-2F 8th% CCP-3F 8th% CCP-2F · F 14% BCH-3F · F 15% CCEB-3F · F 5% CCEB-5F · F 5%

Example 7 B-3F · F 9% B-5F · F 6% PCH-3-CI 15% PCH 5Cl 15% CCP-2F · F 12% CCP-3F · F 10% CCP-5F · F 8th% BCH-3F · F 15% CCEB-3F · F 5% CCEB-2F · F 5%

Example 8 B-3Cl · F 8th% B-5Cl · F 6% PCH-5F 10% PCH-7F 12% CCP-2Cl · F 8th% CCP-3Cl · F 8th% CCP-5Cl · F 6% BCH-3 · FCF₃ 10% BCH-5 · FCF₃ 10% BCH-3 · FCl 7% CCB-2 · FF 5% CCB-33F 5% CCB-53F 5%

Example 9 substance tailor% B-3F · F 6:00 am B-5F · F 7:00 BCH-2F · F 8:00 BCH-3F · F 12:00 BCH-5F · F 10:00 BCH-2CL * F * F 5:00 BCH-3CL · F · F 9:00 BCH-5CL · F · F 11:00 BECH-3F · F 11:00 BECH-4F · F 10:00 BECH-5F · F 11:00 T _{(N, I)} = 85 ° @ V₂₀ = 22 cst @ Δn = 0.148 @ V₁₀ (2nd) = 2.2V

Example 10 substance tailor% B-3FCL 6:00 am B-5FCL 7:00 BCH-2F · F 8:00 BCH-3F · F 12:00 BCH-5F · F 10:00 BCH-2CL * F * F 5:00 BCH-3CL · F · F 9:00 BCH-5CL · F · F 11:00 BECH-3F · F 11:00 BECH-4F · F 10:00 BECH-5F · F 11:00 T _{(N, I)} = 83 ° @ V = 23 cst @ Δn = 0.158 @ V₁₀ (2nd) = 2.3V

Example 11 substance tailor% B-5 · FOCF₃ 8:00 B-7 · FOCF₃ 7:00 CCP 20CF₃ 13:00 CCP 30CF₃ 15:00 CCP 40CF₃ 13:00 CCP 50CF₃ 14:00 BCH-3F · F · F 9:00 CCP-3F · F · F 12:00 CCP-5F · F · F 9:00 T _{(N, I)} = 79 ° @ V = 16 cst @ Δn = 0.088 @ V₁₀ (1st) = 1.65V @ 0.5 μm

Claims (10)

1. Liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it comprises one or more compounds of general formula I. wherein R is alkyl, oxaalkyl, fluoroalkyl or alkenyl each having up to 7 carbon atoms, X is fluorine, chlorine, CF₃, OCF₃ or OCHF₂ and L¹ to L⁴ is independently fluorine or hydrogen. 2. Medium according to claim 1, characterized in that it additionally contains one or more compounds selected from the group consisting of the general formulas II, III and IV: in which the individual radicals have the following meanings:
R: alkyl, oxaalkyl, fluoroalkyl or alkenyl having in each case up to 7 C atoms,
X: F, Cl, CF₃ or OCHF₂,
Y¹ and Y² are each H or F,
r: 0 or 1. 3. Medium according to claim 1 or 2, characterized in that it additionally contains one or more compounds selected from the group consisting of the general formulas V to VIII: wherein R, X, Y¹ and Y² each independently have one of the meanings given in claim 2. 4. Medium according to at least one of claims 1 to 3, characterized in that it additionally contains one or more compounds selected from the group consisting of the general formulas IX to XII: wherein R, X, Y¹ and Y² each independently have one of the meanings given in claim 2. 5. Medium according to claim 2, characterized in that the Proportion of compounds of the formulas I to IV together in Total mixture is at least 50 wt .-%. 6. Medium according to claim 1 or 2, characterized that the proportion of compounds of formula I in the total mixture 10 to 50 wt .-% is. 7. Medium according to at least one of claims 2 to 4, characterized in that the proportion of compounds of the formulas II to IV in the total mixture 30 to 70% by weight is. 8. Medium according to at least one of claims 1 to 4, characterized in that it consists essentially of compounds selected from the group consisting of the general formulas I to XII.   9. Use of the liquid-crystalline medium according to Claim 1 for electro-optical purposes. 10. Electro-optical liquid crystal display containing a Liquid-crystalline medium according to Claim 1.