DE10152831B4 - Liquid-crystalline medium and its use - Google Patents

Abstract

worin R eine Alkyl-, Alkoxy- oder Alkenylgruppe mit 1 bis 12 C-Atomen, wobei auch ein oder zwei nicht benachbarte CH₂-Gruppen durch -O-, -CH=CH-, -CO-, -OCO- oder -COO- so ersetzt sein können, daß O-Atome nicht direkt miteinander verknüpft sind, bedeutet, und eine oder mehrere Verbindungen der Formel XIX und/oder XXIe

worin
R⁰ n-Alkyl, Alkoxy, Fluoralkyl oder Alkenyl mit jeweils bis zu 7 C-Atomen,
X⁰ F,
Y¹ und Y² F,
R^1a H, CH₃, C₂H₅ oder n-C₃H₇, und
alkyl eine Alkylgruppe mit 1 bis 7 C-Atomen
bedeuten.Liquid-crystalline medium based on a mixture of polar compounds with positive dielectric anisotropy, characterized in that it contains one or more compounds of the formula I.

wherein R is an alkyl, alkoxy or alkenyl group having 1 to 12 C atoms, wherein also one or two non-adjacent CH ₂ groups by -O-, -CH = CH-, -CO-, -OCO- or -COO - may be replaced so that O atoms are not directly linked together, and one or more compounds of formula XIX and / or XXIe

wherein
R ⁰ n-alkyl, alkoxy, fluoroalkyl or alkenyl, each having up to 7 C atoms,
X ⁰ F,
Y ¹ and Y ² F,
R ^{1a is} H, CH ₃ , C ₂ H ₅ or nC ₃ H ₇ , and
alkyl is an alkyl group having 1 to 7 carbon atoms
mean.

Description

The present invention relates to a liquid-crystalline medium and its use for electro-optical purposes.

Liquid crystals are mainly used as dielectrics in display devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects. Such devices include, for example, dynamic scattering cells, DAP (out-of-phase deformation) cells, guest / host cells, twisted nematic (TN) cells, supertwisted nematic (STN) cells, SBE cells. Cells ("superbirefringence effect") and OMI cells ("optical mode interference"). The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.

The liquid crystal materials must have good chemical and thermal stability and good stability against electric fields and electromagnetic radiation. Further, the liquid crystal materials should have low viscosity and provide short response times, low threshold voltages, and high contrast in the cells.

Furthermore, they should at normal operating temperatures, d. H. have a suitable mesophase in the broadest possible range below and above room temperature, for example, for the above-mentioned cells a nematic or cholesteric mesophase. Since liquid crystals are generally used as mixtures of several components, it is important that the components are readily miscible with each other. Other properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet different requirements depending on the type of cell and the field of application. For example, materials for cells of twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.

For example, for matrix liquid crystal displays with integrated non-linear elements for single pixel switching (MFK displays), media with high positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high resistivity, good UV and temperature stability, and low vapor pressure are desired ,

• 1. MOS (Metal Oxide Semiconductor) oder andere Dioden auf Silizium-Wafer als Substrat.
• 2. Dünnfilm-Transistoren (TFT) auf einer Glasplatte als Substrat.

Such matrix liquid crystal displays are known. As non-linear elements for individual switching of the individual pixels, for example, active elements (ie transistors) can be used. One speaks then of an "active matrix", whereby one can distinguish two types:

• 1. MOS (Metal Oxide Semiconductor) or other diodes on silicon wafer as a substrate.
• 2. Thin-film transistors (TFT) on a glass plate as a substrate.

The use of monocrystalline silicon as a substrate material limits the display size, since the modular composition of various partial displays on the joints leads to problems.

In the more promising Type 2, which is preferred, the TN effect is usually used as the electro-optic effect. A distinction is made between two technologies: TFTs made of compound semiconductors such as. B. CdSe or TFTs based on polycrystalline or amorphous silicon. The latter technology is being worked on worldwide with great intensity.

The TFT matrix is applied on the inside of one glass plate of the display, while the other glass plate on the inside carries the transparent counter electrode. Compared to the size of the pixel electrode, the TFT is very small and practically does not disturb the image. This technology can also be extended to fully color-capable image representations, wherein a mosaic of red, green and blue filters is arranged such that each one filter element is opposite to a switchable picture element.

The TFT displays usually operate as TN cells with crossed polarizers in transmission and are backlit.

The term MFK displays here includes any matrix display with integrated nonlinear elements, i. H. in addition to the active matrix also displays with passive elements such as varistors or diodes (MIM = metal-insulator-metal).

Such MFK displays are particularly suitable for TV applications (eg pocket televisions) or for high-information displays for computer applications (laptops) and in the automotive or aircraft industry. In addition to problems with regard to the angle dependence of the contrast and the switching times, difficulties arise in MFK displays due to insufficiently 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 Transistors for Matrix Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistance, the contrast of a MFK display deteriorates and the problem of "after image elimination" may occur. Since the resistivity of the liquid crystal mixture decreases by interaction with the internal surfaces of the display, generally over the lifetime of an MFK display, high (initial) resistance is very important in order to obtain acceptable service lives. In particular, in low-volt mixtures, it has not been possible to realize very high resistivities. Furthermore, it is important that the resistivity shows the smallest possible increase with increasing temperature and after temperature and / or UV exposure. Also particularly disadvantageous are the low-temperature properties of the mixtures of the prior art. It is required that no crystallization and / or smectic phases occur even at low temperatures and that the temperature dependence of the viscosity is as low as possible. The MFK displays from the prior art thus do not meet today's requirements.

Thus, there is still a great need for MFK displays with very high resistivity at the same time large working temperature range, short switching times even at low temperatures and low threshold voltage, which do not show these disadvantages or only to a lesser extent.

• – erweiterter nematischer Phasenbereich (insbesondere zu tiefen Temperaturen)
• – Schaltbarkeit bei extrem tiefen Temperaturen (out-door-use, Automobil, Avionik)
• – erhöhte Beständigkeit gegenüber UV-Strahlung (längere Lebensdauer)

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

• - extended nematic phase range (especially at low temperatures)
• - Switchability at extremely low temperatures (out-door-use, automotive, avionics)
• - increased resistance to UV radiation (longer life)

With the media available from the prior art, it is not possible to realize these advantages while maintaining the other parameters.

For higher-twisted cells (STN), media are desired which allow for higher multiplexability and / or lower threshold voltages and / or broader nematic phase ranges (especially at low temperatures). For this purpose, a further expansion of the available parameter space (clearing point, transition smectic-nematic or melting point, viscosity, dielectric values, elastic sizes) is urgently desired.

The invention has for its object to provide media especially for such MFK, TN or STN displays that do not have the disadvantages mentioned above, or only to a lesser extent, and preferably at the same time very high resistivities and low threshold voltages.

For some applications it would be particularly desirable to further lower the viscosity at low temperatures. As a result, even faster switching times can be achieved.

In the pamphlets WO 92/05230 A2 and DE 19541181 A1 3,4,5-trifluorophenyl-4-cyclohexylbenzoesäureester are disclosed as a liquid crystal component, in the latter document also also a mixture containing this component.

It has now been found that the objects described above can be achieved by using media according to the invention in displays.

worin R eine Alkylgruppe mit 1 bis 12 C-Atomen, wobei auch ein oder zwei nicht benachbarte CH₂-Gruppen durch -O-, -CH=CH-, -CO-, -OCO- oder -COO- so ersetzt sein können, daß O-Atome nicht direkt miteinander verknüpft sind, bedeutet,
und
eine oder mehrere Verbindungen der Formel XIX und/oder XXIe

The invention thus relates to a liquid-crystalline medium based on a mixture of polar compounds having a positive dielectric anisotropy, characterized in that it contains one or more compounds of the formula I.

where R is an alkyl group having 1 to 12 C atoms, whereby also one or two non-adjacent CH ₂ groups may be replaced by -O-, -CH = CH-, -CO-, -OCO- or -COO-, that O atoms are not directly linked means that
and
one or more compounds of the formula XIX and / or XXIe

In the media according to the invention, the compounds of the formula I bring about a reduction in the threshold voltage while at the same time optimizing the low-temperature behavior.

The compounds of the formula I have a wide range of applications. These compounds can serve as base materials from which liquid-crystalline media are predominantly composed; However, it is also possible to add compounds of the formula I to liquid-crystalline base materials from other classes of compounds in order, for example, to influence the dielectric and / or optical anisotropy of such a dielectric and / or to optimize its threshold voltage and / or its viscosity.

The compounds of the formula I are colorless in the pure state and form liquid-crystalline mesophases in a temperature range which is favorably located for the electro-optical use. Chemically, thermally and against light, they are stable. The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in the standard works such as Houben-Weyl, Methods of Organic Chemistry, Georg-Thieme-Verlag, Stuttgart), namely under Reaction conditions which are known and suitable for the said reactions. One can also make use of known per se, not mentioned here variants.

Also disclosed are electro-optical displays (in particular STN or MFK displays with two plane-parallel support plates which form a cell with a border, integrated non-linear elements for switching individual pixels on the support plates and a nematic liquid-crystal mixture having positive dielectric anisotropy in the cell and high resistivity) containing such media. The use of these media for electro-optical purposes is an object of the invention.

The liquid-crystal mixtures according to the invention enable a significant expansion of the available parameter space. Thus, the achievable combinations of clearing point, low temperature viscosity, thermal and UV stability and dielectric anisotropy far surpass previous prior art materials.

The requirement for a high clearing point, nematic phase at low temperature, high γε and, at the same time, low viscosity has so far been insufficiently fulfilled. Although previously known mixtures have comparably high values for the clearing point and for Δε and a favorable birefringence, they still do not have sufficiently low values for the rotational viscosity γ ₁ .

Other mixing systems have comparable viscosities and values of Δε, but have only clearing points in the region of 60 ° C.

The liquid-crystal mixtures according to the invention make it possible, while maintaining the nematic phase to -20 ° C and preferably to -30 ° C, particularly preferably to -40 ° C, clearing points above 80 °, preferably above 85 °, particularly preferably above 90 ° C, at the same time dielectric Anisotropiewerte Δε ≥ 5, preferably ≥ 7 and to achieve a high value for the resistivity, which excellent STN and MKF displays can be achieved. In particular, the mixtures are characterized by low operating voltages. The TN thresholds are below 2.0 V, preferably below 1.8 V, more preferably <1.6 V.

It is understood that by suitable choice of the components of the mixtures according to the invention also higher clearing points (eg above 110 °) can be realized at higher threshold voltage or lower clearing points at lower threshold voltages to obtain the other advantageous properties. Likewise, mixtures can be obtained with a larger Δε and thus lower thresholds at correspondingly little increased viscosities. The MFK displays preferably work in the first transmission minimum according to Gooch and Tarry [CH Gooch and HA Tarry, Electron. Lett. 10, 2-4, 1974; CH Gooch and HA Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], which in addition to particularly favorable electro-optical properties such. B. high slope of the characteristic and low angle 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. As a result, significantly higher specific resistances can be achieved using the mixtures according to the invention in the first minimum than in the case of mixtures with cyano compounds. By suitable choice of the individual components and their proportions by weight, the person skilled in the art can set the birefringence required for a given layer thickness of the MFK display with simple routine methods.

The rotational viscosity γ ₁ at 20 ° C is preferably <150 mPa · s, more preferably <130 mPa · s. The nematic phase range is preferably at least 90 °, in particular at least 100 °. Preferably, this range extends at least from -20 ° to + 80 °.

oder Ester der Formel

Measurements of the Capacity Holding Ratio (HR) [p. 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 shown that mixtures according to the invention comprising compounds of the formula I have a significantly smaller decrease in HR with increasing temperature than analogous mixtures comprising instead of the compounds of the formula I cyanophenylcyclohexanes of the formula

or esters of the formula

It has also been found that mixtures according to the invention comprising compounds of the formula I have a higher clearing point and higher Δε than analogous mixtures comprising cyanophenylcyclohexanes of the above-mentioned type. Formula. Compared to the latter mixtures, the mixtures according to the invention also have a smaller Δn, which is advantageous for many applications.

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

The proportion of the compounds of the formula I in the overall mixture of the media according to the invention is preferably 2-55%, preferably 3-35% and particularly preferably 5-15%.

The individual compounds of the formulas I to XII and their sub-formulas which can be used in the media according to the invention are either known or they can be prepared analogously to the known compounds.

• – Das Medium enthält zusätzlich eine oder mehrere Verbindungen ausgewählt aus der Gruppe bestehend aus den allgemeinen Formeln II bis VII:
  
  
  worin die einzelnen Reste die folgenden Bedeutungen haben R⁰ n-Alkyl, Alkoxy, Fluoralkyl, Alkenyl oder Oxaalkenyl mit jeweils bis zu 9 C-Atomen, Z¹ CF₂O, C₂F₄ oder eine Einfachbindung, Z² CF₂O, C₂F₄ oder C₂H₄, X⁰ F, Cl, halogeniertes Alkyl, Alkenyl oder Alkoxy mit 1 bis 6 C-Atomen, Y¹ und Y² jeweils unabhängig voneinander H oder F, und r 0, oder 1.
• – Die Verbindungen der Formel IV sind vorzugsweise ausgewählt aus der folgenden Gruppe
  
  
  
  worin R⁰ und X⁰ die in Anspruch 2 angegebene Bedeutung haben, R⁰ besonders bevorzugt n-Alkyl mit 1 bis 8 C-Atomen oder Alkenyl mit 2 bis 7 C-Atomen und X⁰ bevorzugt OCF₃ oder F, besonders bevorzugt F bedeuten. Besonders bevorzugt sind Verbindungen der Formel IVa und IVf;
• – Die Verbindungen der Formel VI sind vorzugsweise ausgewählt aus der folgenden Gruppe
  
  worin R⁰ und X⁰ die in Anspruch 2 angegebene Bedeutung haben, R⁰ besonders bevorzugt n-Alkyl mit 1 bis 8 C-Atomen oder Alkenyl mit 2 bis 7 C-Atomen und X⁰ bevorzugt OCF₃ oder F, besonders bevorzugt F bedeuten. Besonders bevorzugt sind Verbindungen der Formel Via, VIb und VIc;
• – Die Verbindungen der Formel VII sind vorzugsweise ausgewählt aus der folgenden Gruppe
  
  
  worin R⁰ und X⁰ die in Anspruch 2 angegebene Bedeutung haben, R⁰ besonders bevorzugt n-Alkyl mit 1 bis 8 C-Atomen oder Alkenyl mit 2 bis 7 C-Atomen und X⁰ bevorzugt OCF₃ oder F, besonders bevorzugt F bedeuten. Besonders bevorzugt sind Verbindungen der Formel VIIa und VIIb;
• – Die Verbindungen der Formel II sind vorzugsweise ausgewählt aus der folgenden Gruppe:
  
  worin R⁰ und X⁰ die in Anspruch 2 angegebene Bedeutung haben, R⁰ besonders bevorzugt n-Alkyl mit 1 bis 8 C-Atomen oder Alkenyl mit 2 bis 7 C-Atomen und X⁰ besonders bevorzugt F, Cl, CF₃, OCF₃ oder OCHF₂ bedeuten;
• – Das Medium enthält zusätzlich eine oder mehrere Verbindungen ausgewählt aus der folgenden Gruppe:
  
  
  worin R⁰, X⁰, Y¹ und Y² die in Anspruch 2 angegeben Bedeutung haben, Y³ H oder F und X⁰ vorzugsweise F, Cl, CF₃, OCF₃, OCHF₂ ist, und X⁰ zusätzlich auch Alkyl, Oxaalkyl, Fluoralkyl oder Alkenyl mit jeweils 1 bis 6 C-Atomen bedeuten kann.
• – Das Medium enthält weitere Verbindungen, vorzugsweise ausgewählt aus der folgenden Gruppe:
  
  worin R⁰ und X⁰ die in Anspruch 2 angegebene Bedeutung haben, X⁰ zusätzlich auch Alkyl, Oxaalkyl, Fluoralkyl oder Alkenyl mit jeweils 1 bis 6 C-Atomen bedeuten kann, und die 1,4-Phenylenringe durch CN, Chlor oder Fluor substituiert sein können. Vorzugsweise sind die 1,4-Phenylenringe ein- oder mehrfach durch Fluoratome substituiert;
• Besonders bevorzugt sind die Verbindungen ausgewählt aus der folgenden Gruppe
  
  worin R⁰ und X⁰ die in Formel XV und XVIII angegebene Bedeutung haben, L H oder F, vorzugsweise F, R⁰ besonders bevorzugt n-Alkyl mit 1 bis 8 C-Atomen oder Alkenyl mit 2 bis 7 C-Atomen, und X⁰ in Formel XVa, XVb und XVIIIa vorzugsweise F oder Cl, in Formel XVc und XVd vorzugsweise Alkyl oder Alkoxy mit 1 bis 6 C-Atomen, bedeutet;
• – Das Medium enthält zusätzlich eine oder mehrere Verbindungen ausgewählt aus der folgenden Gruppe:
  
  worin R⁰, Y¹, Y², X⁰ und r die in Anspruch 2 angegebene Bedeutung besitzen, X eine der für X⁰ angegebenen Bedeutungen besitzt oder CN bedeutet, und L¹ und L² jeweils unabhängig voneinander H oder F bedeuten.
• Besonders bevorzugt sind die Verbindungen ausgewählt aus der folgenden Gruppe:
  
  
  worin R⁰ und X⁰ die in Formel XIX und XX angegebene Bedeutung haben, R⁰ besonders bevorzugt n-Alkyl mit 1 bis 8 C-Atomen oder Alkenyl mit 2 bis 7 C-Atomen, und X⁰ besonders bevorzugt F, Cl, CF₃, OCF₃ oder OCHF₂, insbesondere F, in Formel XXd insbesondere OCF₃, bedeuten;
• – Das Medium enthält zusätzlich eine oder mehrere Verbindungen ausgewählt aus der folgenden Gruppe:
  
  worin R¹ und R² jeweils unabhängig voneinander Alkyl oder Alkoxy mit 1 bis 8 C-Atomen oder Alkenyl mit 2 bis 7 C-Atomen und L H oder F bedeuten. In den Verbindungen der Formel XXII, XXIII und XXIV bedeuten R¹ und R² vorzugsweise Alkyl oder Alkoxy mit 1 bis 8 C-Atomen.
• Besonders bevorzugt sind die Verbindungen ausgewählt aus der folgenden Gruppe:
  
  
  worin R^1a und R^2a jeweils unabhängig voneinander H, CH₃, C₂H₅ oder n-C₃H₇, alkyl eine Alkylgruppe mit 1 bis 7 C-Atomen, s 0 oder 1 und L H oder F bedeuten.
  
• – R⁰ ist geradkettiges Alkyl oder Alkenyl mit 2 bis 7 C-Atomen;
• – Das Medium enthält Verbindungen ausgewählt aus den Formeln II, III, IV, V, VI, VII, XIX, XX, XXI, XXII, XIII und XXIV;
• – Das Medium enthält eine oder mehrere Verbindungen ausgewählt aus den Formeln VIa, VIb, VIc, VIIa und IVf;
• – Das Gewichtsverhältnis I: (II + III + IV + V + VI + VII) ist vorzugsweise 1:10 bis 10:1;
• – Das Medium besteht im wesentlichen aus Verbindungen ausgewählt aus der Gruppe bestehend aus den allgemeinen Formeln I bis XIII und XIX bis XXIV;
• – Der Anteil an Verbindungen der Formel I beträgt im Gesamtgemisch 2 bis 55 Gew.-%, insbesondere 3 bis 35 Gew.-%, ganz besonders bevorzugt 5 bis 15 Gew.-%;
• – Der Anteil an Verbindungen der Formeln I bis VII zusammen beträgt im Gesamtgemisch mindestens 50 Gew.-%;
• – Der Anteil an Verbindungen der Formeln II bis VII und XIX bis XXIV im Gesamtgemisch beträgt 30 bis 95 Gew.-%;
• – Der Anteil an Verbindungen der Formel XIX beträgt im Gesamtgemisch weniger als 28%.

Preferred embodiments are given below:

• The medium additionally contains one or more compounds selected from the group consisting of the general formulas II to VII:
  
  
  in which the individual radicals have the following meanings R ⁰ n-alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl having in each case up to 9 C atoms, Z ¹ CF ₂ O, C ₂ F ₄ or a single bond, Z ² CF ₂ O, C ₂ F ₄ or C ₂ H ₄ , X ⁰ F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 C atoms, Y ¹ and Y ^{2 are} each independently H or F, and r 0, or 1.
• The compounds of the formula IV are preferably selected from the following group
  
  
  
  wherein R ⁰ and X ^{0 have} the meaning given in claim 2, R ⁰ particularly preferably n-alkyl having 1 to 8 carbon atoms or alkenyl having 2 to 7 carbon atoms and X ^{0 is} preferably OCF ₃ or F, particularly preferably F , Particular preference is given to compounds of the formulas IVa and IVf;
• The compounds of the formula VI are preferably selected from the following group
  
  wherein R ⁰ and X ^{0 have} the meaning given in claim 2, R ⁰ particularly preferably n-alkyl having 1 to 8 carbon atoms or alkenyl having 2 to 7 carbon atoms and X ^{0 is} preferably OCF ₃ or F, particularly preferably F , Particular preference is given to compounds of the formulas Via, VIb and VIc;
• The compounds of the formula VII are preferably selected from the following group
  
  
  wherein R ⁰ and X ^{0 have} the meaning given in claim 2, R ⁰ particularly preferably n-alkyl having 1 to 8 carbon atoms or alkenyl having 2 to 7 carbon atoms and X ^{0 is} preferably OCF ₃ or F, particularly preferably F , Particular preference is given to compounds of the formulas VIIa and VIIb;
• The compounds of the formula II are preferably selected from the following group:
  
  wherein R ⁰ and X ^{0 have} the meaning given in claim 2, R ⁰ particularly preferably n-alkyl having 1 to 8 carbon atoms or alkenyl having 2 to 7 carbon atoms and X ⁰ particularly preferably F, Cl, CF ₃ , OCF ₃ or OCHF ₂ ;
• - The medium additionally contains one or more compounds selected from the following group:
  
  
  wherein R ⁰ , X ⁰ , Y ¹ and Y ^{2 have} the meaning given in claim 2, Y ^{3 is} H or F and X ^{0 is} preferably F, Cl, CF ₃ , OCF ₃ , OCHF ₂ , and X ⁰ additionally also alkyl, Oxaalkyl, fluoroalkyl or alkenyl each having 1 to 6 carbon atoms.
• The medium contains further compounds, preferably selected from the following group:
  
  wherein R ⁰ and X ^{0 have} the meaning given in claim 2, X ⁰ additionally may also mean alkyl, oxaalkyl, fluoroalkyl or alkenyl each having 1 to 6 carbon atoms, and the 1,4-phenylene substituted by CN, chlorine or fluorine could be. Preferably, the 1,4-phenylene rings are mono- or polysubstituted by fluorine atoms;
• Particularly preferred are the compounds selected from the following group
  
  wherein R ⁰ and X ^{0 have} the meaning given in formula XV and XVIII, LH or F, preferably F, R ⁰ particularly preferably n-alkyl having 1 to 8 carbon atoms or alkenyl having 2 to 7 carbon atoms, and X ⁰ in formula XVa, XVb and XVIIIa preferably F or Cl, in formula XVc and XVd preferably alkyl or alkoxy having 1 to 6 C atoms;
• - The medium additionally contains one or more compounds selected from the following group:
  
  wherein R ⁰ , Y ¹ , Y ² , X ⁰ and r have the meaning given in claim 2, X has one of the meanings given for X ⁰ or CN, and L ¹ and L ^{2 are} each independently H or F.
• The compounds are particularly preferably selected from the following group:
  
  
  in which R ⁰ and X ^{0 have} the meaning given in formulas XIX and XX, R ⁰ particularly preferably n-alkyl having 1 to 8 C atoms or alkenyl having 2 to 7 C atoms, and X ⁰ particularly preferably F, Cl, CF ₃ , OCF ₃ or OCHF ₂ , in particular F, in formula XXd, in particular OCF ₃ , mean;
• - The medium additionally contains one or more compounds selected from the following group:
  
  wherein R ¹ and R ^{2 are} each independently alkyl or alkoxy having 1 to 8 carbon atoms or alkenyl having 2 to 7 carbon atoms and LH or F. In the compounds of the formulas XXII, XXIII and XXIV, R ¹ and R ^{2 are} preferably alkyl or alkoxy having 1 to 8 C atoms.
• The compounds are particularly preferably selected from the following group:
  
  
  wherein R ^1a and R ^{2a are} each independently H, CH ₃ , C ₂ H ₅ or nC ₃ H ₇ , alkyl is an alkyl group having 1 to 7 C atoms, s is 0 or 1 and L is H or F.
  
• - R ⁰ is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms;
• The medium contains compounds selected from the formulas II, III, IV, V, VI, VII, XIX, XX, XXI, XXII, XIII and XXIV;
• The medium contains one or more compounds selected from the formulas VIa, VIb, VIc, VIIa and IVf;
• The weight ratio I: (II + III + IV + V + VI + VII) is preferably 1:10 to 10: 1;
• The medium consists essentially of compounds selected from the group consisting of the general formulas I to XIII and XIX to XXIV;
• - The proportion of compounds of formula I is in the total mixture 2 to 55 wt .-%, in particular 3 to 35 wt .-%, most preferably 5 to 15 wt .-%;
• - The proportion of compounds of formulas I to VII together in the total mixture is at least 50 wt .-%;
• - The proportion of compounds of formulas II to VII and XIX to XXIV in the total mixture is 30 to 95 wt .-%;
• - The proportion of compounds of formula XIX is less than 28% in the total mixture.

It has been found that even a relatively small proportion of compounds of the formula I in admixture with conventional liquid crystal materials, but in particular with one or more compounds of the formula II, III, IV, V, VI and / or VII to a considerable lowering of the threshold voltage and resulting in low values of birefringence, while simultaneously observing broad nematic phases with low transition temperatures smectic-nematic, thereby improving storage stability. Particular preference is given to mixtures which, in addition to one or more compounds of the formula I, contain one or more compounds of the formula IV, in particular compounds of the formula IVa, in which X ⁰ denotes F or OCF ₃ . The compounds of the formulas I to VII are colorless, stable and readily miscible with one another and with other liquid crystal materials.

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

The term "alkenyl" includes straight-chain and branched alkenyl groups having 2-7 carbon atoms, especially the straight-chain groups. Especially alkenyl groups are C ₂ -C ₇ 1E-alkenyl, C ₄ -C ₇ 3E-alkenyl, C ₅ -C ₇ -alkenyl, C ₆ -C ₇ -5-alkenyl and C ₇ -6-alkenyl, in particular C ₂ -C ₇ -1E-alkenyl, C ₄ -C ₇ -EE-alkenyl and C ₅ -C ₇ -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 fluoro-end-capped groups, d. H. Fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. Other positions of the fluorine are not excluded.

The term "oxaalkyl" preferably includes straight-chain radicals of the formula C _n H _{2n + 1} -O- (CH ₂ ) _m , wherein n and m are each independently 1 to 6. Preferably, n = 1 and m is 1 to 6.

By suitably selecting the meanings of R ⁰ and X ⁰ , the response times, the threshold voltage, the transconductance of the transmission characteristics etc. can be modified in the desired manner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally lead to shorter response times, improved nematic tendencies and a higher ratio of the elastic constants k ₃₃ (bend) and k ₁₁ (splay) compared to alkyl or alkoxy. 4-Alkenyl radicals, 3-alkenyl radicals and the like generally give lower threshold voltages and smaller values of k ₃₃ / k ₁₁ compared to alkyl and alkoxy radicals.

A -CH ₂ CH ₂ group generally results in higher values of k ₃₃ / k ₁₁ compared to a single covalent bond. Higher values of k ₃₃ / k ₁₁ allow z. B. flatter 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 + V + VI + VI depends largely on the desired properties, on the choice of the components of the formulas I, II, III, IV, V, VI and / or VII and from the choice of other optional components. Suitable proportions within the range given above can be easily determined on a case-by-case basis.

The total amount of compounds of formulas I to XXIV in the mixtures according to the invention is not critical. The mixtures may therefore contain one or more other components to optimize various properties. However, the observed effect on the response times and the threshold voltage is generally greater the higher the total concentration of compounds of formulas I to XXIV.

In a particularly preferred embodiment, the media of the invention comprise compounds of Formula II to VII (preferably II, III and / or IV, in particular IVa) wherein X ⁰ is F, OCF _3, OCHF _2, F, OCH = CF _2, OCF = CF ₂ or OCF ₂ -CF ₂ H means. A favorable synergistic effect with the compounds of the formula I leads to particularly advantageous properties. In particular, mixtures comprising compounds of the formula I and of the formula IVa are distinguished by their low threshold voltages.

The structure of the MFK display consisting of polarizers, electrode base plates and electrodes with surface treatment corresponds to the construction customary for such displays. In this case, the term of the usual construction is broad and includes all modifications and modifications of the MFK display, in particular matrix display elements based on poly-Si TFT or MIM.

However, a significant difference of the displays to the usual on the basis of the twisted nematic cell is the choice of the liquid crystal parameters of the liquid crystal layer.

The preparation of the liquid crystal mixtures which can be used according to the invention is carried out in a conventional manner. In general, the desired amount of the components used in lesser amount is dissolved in the constituent of the main component, expediently at elevated temperature. It is also possible to use solutions of the components in an organic solvent, e.g. B. in acetone, chloroform or methanol, to mix and remove the solvent after mixing again, for example by distillation.

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

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 the following Tables A and B takes place. All radicals C _n H _{2n + 1} and C _m H _{2m + 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: Code for R ¹ , R ² , L ¹ , L ² R ¹ R ² L ¹ L ² nm C _n H _{2n + 1} C _m H _{2m + 1} H H nOM C _n H _{2n + 1} OC _m H _{2m + 1} H H NO.m OC _n H _{2n + 1} C _m H _{2m + 1} H H N C _n H _{2n + 1} CN H H NN.F C _n H _{2n + 1} CN H F NF C _n H _{2n + 1} F H H NOF OC _n H _{2n + 1} F H H NCI O _n H _{2n + 1} Cl H H NF.F O _n H _{2n + 1} F H F NF.FF C _n H _{2n + 1} F F F NCF ₃ C _n H _{2n + 1} CF ₃ H H NOCF ₃ C _n H _{2n + 1} OCF ₃ H H NOCF ₂ C _n H _{2n + 1} OCHF ₂ H H NS C _n H _{2n + 1} NCS H H RVSN C _r H _{2r + 1} -CH = CH-C _s H _2s - CN H H rESn C _r H _{2r + 1} -OC _S H _2s - CN H H NA m C _n H _{2n + 1} COOC _m H _{2m + i} H H NOCCF ₂ .FF C _n H _{2n + 1} OCH ₂ CF ₂ H F F

Tabelle B:

Preferred mixture components are given in Tables A and B. TABLE A

Table B:

The following examples are intended to illustrate the invention without limiting it. It uses the following abbreviations:
Mp means melting point, clp. = Clearing point. Furthermore, K = crystalline state, N = nematic phase, S = smectic phase and I = isotropic phase. The data between these symbols represent the transition temperatures [° C]. V _10,0,20 denotes the voltage [V] for 10% transmission at 0 ° viewing angle (viewing direction perpendicular to the plate surface) and 20 ° C. t _on denotes the switch-on time and t _off the switch-off time [ms] at an operating voltage corresponding to twice the value of V ₁₀ . Δn denotes the optical anisotropy and n _o the refractive index at 589 nm. Δε denotes the dielectric anisotropy at 1 kHz (Δε = ε _{|| -ε ⊥} , where ε _|| the dielectric constant parallel to the _longitudinal molecular _axes and ε _⊥ the dielectric constant perpendicular to it ). γ ₁ means the rotational viscosity [mPa · sec]. The electro-optical data were measured in a TN cell in the 1st minimum (ie at a d · Δn value of 0.5) at 20 ° C, if not expressly stated otherwise. All data above was measured at 20 ° C, unless expressly stated otherwise.

Above and below, percentages are by weight. All temperatures are in degrees Celsius. Example 1 Containing a liquid-crystalline medium CCP 2F.FF 8.00% No .: +72.5 CCP 3F.FF 10.00% SN transition: -30 CCP 20CF3.F 6:00 am% Δn: +0.0902 CCP 40CF3 3:00% V _10,0,20 : 1,04 CCP 30CF3 8.00% γ ₁ : 166 CGU-2-F 11.00% CGU-3-F 11.00% CGU-5-F 5.00% CCZU-2-F 5.00% CCZU-3-F 15.00% CCZU-5-F 5:00 CUZG-3-F 10:00 CC-3-V1 3:00 has a low threshold voltage and favorable low temperature behavior. Example 2 (not according to the invention) A liquid-crystalline medium containing PCH-5F 8:50% Cleavage: +87 PCH-6 6.80% SN transition: PCH 7 10.05% Δn: 0.0917 CCP 20CF3 6.80% Δε: +6.4 CCP 30CF3 10.20% CCP 40CF3 6:00 am% CCP 50CF3 30.09% BCH 3F.F 10.30% BCH 5F.F 8:50% ECCP 30CF3 04.02% ECCP 50CF3 20.04% CBC-33F 1.70% CBC-53F 1.70% CBC-55F 1.70% CUZG-3- 15.00% Example 3 (not according to the invention) A liquid-crystalline medium containing ME2N.F 4:00% ME3N.F 4:00% ME5N.F 15.00% ME7N.F 14.00% HP 3N.F 2:00% HP 4N.F 5.00% HP 5N.F 5.00% PCH-301 3:00% CCH 303 6:00 am% CCH 501 13.00% CCPC-33 5.00% CCPC-34 6:00 am% CCPC-35 6:00 am% CUZG-3- 12:00% Example 4 (not according to the invention) A liquid-crystalline medium containing FET 3F.F 13.00% Cleavage: 85.5 FET 5F.F 11.00% Δn: 0.1952 GGP-3-Cl 8.00% Δε: +11.2 GGP-5-Cl 14.00% V ₁₀ : 1.43 CUZG-2-F 12:00% γ ₁ : 382 CUZG-3-F 11.00% PGIGI-3-F 4:00% T-2FF3 12:00% GGP 5-3 10.00% CBC 33 5.00% Example 5 (not according to the invention) A liquid-crystalline medium containing CCH-34 5.00% Clp .: 71.0 CCP 2F.FF 11.00% Δn: 0.0910 CCP 3F.FF 11.00% V ₁₀ : 1.04 CCP 20CF3.F 10.00% γ ₁ : 158 CCP 30CF3 8.00% CCP 40CF3 5.00% CGU-2-F 9.00% CGU-3-F 9.00% CGU-5-F 4:00% CPZG-2-OT 2:00% CPZG-3-OT 4:00% CUZG-2-F 9.00% CUZG-3-F 11.00% CBC 33 2:00% Example 6 A liquid-crystalline medium containing CC-3-V1 1:50% Cleavage: 71.5 CCP 2F.FF 10.00% Δn: 0.0913 CCP 3F.FF 11.00% V ₁₀ : 1.03 CCP 20CF3.F 7:00% γ ₁ : 166 CCP 30CF3 8.00% CCP 40CF3 8.00% CGU-2-F 10.00% CGU-3-F 10.00% CGU-5-F 3:00% CCZU-3-F 9.00% CUZG-2-F 10.00% CUZG-3-F 11.00% CBC 33 1:50% Example 7 Containing a liquid-crystalline medium BCH-32 3:00% CCP 2F.FF 11.00% CCP 3F.FF 11.00% CCP 20CF3.F 3:00% BCH 3F.FF 3:00% CCP 30CF3 8.00% CCP 40CF3 5:00 CGU-2-F 9.00% CGU-3-F 9.00% CCZU-2-F 3:00% CCZU-3-F 12:00% CUZG-2-F 11.00% CUZG-3-F 12:00% Example 8 (not according to the invention) A liquid-crystalline medium containing BCH 3F.F 10.79% Cp .: 90.3 BCH 5F.F 9.00% Δn: 0.0976 ECCP 30CF3 4:50% Δε: +6.4 ECCP 50CF3 4:50% CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% PCH-6F 20.07% PCH-7F 5:40% CCP 20CF3 20.07% CCP 30CF3 10.79% CCP 40CF3 06.30% CCP 50CF3 9.89% PCH-5F 9.00% CUZG-5-F 10.05% Example 9 (not according to the invention) A liquid-crystalline medium containing BCW 3F.F 10.82% Cl: 86.6 BCH 5F.F 9:02% γ ₁ : 132 ECCP 30CF3 4:51% ECCP 50CF3 4:51% CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% PCH-6F 22.07% PCH-7F 5:41% CCP 20CF3 22.07% CCP 30CF3 10.82% CCP 40CF3 31.06% CCP 50CF3 9.92% PCH-5F 9:02% CUZG-2-F 9.80% Example 10 (not according to the invention) A liquid-crystalline medium containing BCH 3F.F 10.79% Cl: 86,7 BCH 5F.F 8.99% Δn: 0.0966 ECCP-300F3 4:50% Δε: +6.4 ECCF-50CF.3 4:50% CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% PCH-6F 19.07% PCN-7F 5:40% CCP 20CF3 19.07% CCP 30CF3 10.79% CCP 40CF3 29.06% CCP 50CF3 9.89% PCH-5F 8.99% CUZG-2-F 08.10% Example 11 (not according to the invention) A liquid-crystalline medium containing PCH-5F 5.00% Cl: 73.0 PCH-6F 13.00% Δn: 0.0924 PCH-7F 7:00% CCP 20CF3 10.00% CCP 30CF3 11.00% CCP 40CF3 10.00% CCP 50CF3 12:00% BCH 3F.F 12:00% BCH 5F.F 12:00% CUZG-3-F 8.00% Example 12 (not according to the invention) A liquid-crystalline medium containing PCH-5F 21.03% CCF 20CF2.FF 07.17% CCP 30CF2.FF 16.03% Δε: +10.8 CCP 50CF2.FF 07.17% CUP 2F.F 5:37% CUP 3F.F 5:37% CBC-33F 5:37% CBC-53F 5:37% CBC-55F 29.05% CUZG-3-F 19.87% Example 13 (not according to the invention) A liquid-crystalline medium containing BCH 3F.F 10.81% Cleavage: 89.2 BCH 5F.F 9:02% γ ₁ : 135 ECCP 30CF3 4:51% ECCP 50CF3 4:51% CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% PCH-6F 21.07% PCH-7F 5:41% CCP 20CF3 21.07% CCP 30CF3 10.81% CCP 40CF3 31.06% CCP 50CF3 9.91% PCH-5F 9:01% CUZG-3-F 9.88%

Claims (9)

Liquid-crystalline medium based on a mixture of polar compounds with positive dielectric anisotropy, characterized in that it contains one or more compounds of the formula I.

wherein R is an alkyl, alkoxy or alkenyl group having 1 to 12 C atoms, wherein also one or two non-adjacent CH ₂ groups by -O-, -CH = CH-, -CO-, -OCO- or -COO - may be replaced so that O atoms are not directly linked together, and one or more compounds of formula XIX and / or XXIe

wherein R ⁰ n-alkyl, alkoxy, fluoroalkyl or alkenyl each having up to 7 carbon atoms, X ⁰ F, Y ¹ and Y ² F, R ^{1a is} H, CH ₃ , C ₂ H ₅ or nC ₃ H ₇ , and Alkyl is an alkyl group having 1 to 7 carbon atoms. 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, IV, V, VI and VII:

wherein the individual radicals have the following meanings R ⁰ n-alkyl, alkoxy, fluoroalkyl or alkenyl having in each case up to 7 C atoms, Z ¹ CF ₂ O, C ₂ F ₄ or a single bond, Z ² CF ₂ O, C ₂ F ₄ or C ₂ H ₄ , X ⁰ F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 carbon atoms, Y ¹ and Y ^{2 are} each independently H or F, and r is 0 or 1. Medium according to claim 1 or 2, characterized in that it additionally contains one or more compounds selected from the following group:

wherein R ⁰ , X ⁰ , Y ¹ and Y ^{2 have} the meaning given in claim 2. Medium according to one or more of Claims 1 to 3, characterized in that it additionally contains one or more compounds of the formula XX:

wherein R ⁰ , Y ¹ , Y ² and r have the meaning given in claim 2, 1 ¹ and 1 ^{2 are} each independently H or F, and X has one of the meanings given for X ⁰ in claim 2 or CN. Medium according to one or more of Claims 1 to 4, characterized in that it additionally contains one or more compounds selected from the following group:

wherein R ¹ and R ^{2 are} each independently alkyl or alkoxy having 1 to 8 carbon atoms or alkenyl having 2 to 7 carbon atoms and LH or F. Medium according to one or more of claims 1 to 5, characterized in that the proportion of compounds of formula I in the total mixture is 2 to 55 wt .-%. Medium according to one or more of Claims 2 to 6, characterized in that the proportion of compounds of the formulas I to VII together in the total mixture is at least 50% by weight. Medium according to one or more of claims 2 to 7, characterized in that the proportion of compounds of the formulas II to VII, and XIX to XXIV in the total mixture is 30 to 95 wt .-%. Use of the liquid-crystalline medium according to one or more of Claims 1 to 8 for electro-optical purposes.