DE10008712B4 - Liquid-crystalline medium and its use - Google Patents

Abstract

Liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it contains one or more compounds of general formula I wherein R is H, an unsubstituted alkyl group which is monosubstituted by CN or CF 3 or at least monosubstituted by halogen. or alkenyl radical having 1 to 15 C atoms, where in these radicals also one or more CH 2 groups are each independently denoted by -O-, -S-, -CO-, -CO-O-, -O-CO- or - O-CO-O- can be replaced so that O atoms are not directly linked, LH or F, trans-1,4-cyclohexylene ring, Z is a single bond, and n is 1, and in each case one or more compounds of the general Formulas IIb and IVa, wherein the individual radicals have the following meanings: R0: n-alkyl, oxaalkyl, fluoroalkyl or alkenyl each having up to 7 carbon atoms, and X0: F, Cl, halogenated alkyl, alkenyl or alkoxy with 1 to 6 C atoms.

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 nonlinear 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-optical 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, MFK displays result in difficulties due to the insufficiently high specific resistance 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.

In addition to liquid crystal displays which use backlighting, ie are operated transmissively and optionally transflectively, reflective liquid crystal displays are also of particular interest. These reflective liquid crystal displays use the ambient light for information presentation. Thus, they consume significantly less energy than backlit liquid crystal displays of appropriate size and resolution. Since the TN effect is characterized by a very good contrast, such reflective displays are still easy to read even in bright ambient conditions. This is already from simple reflective TN displays, as they are in z. As watches and calculators are used known. However, the principle is also on high-quality, higher-resolution active matrix driven displays such. B. TFT displays applicable. Here, as with the general conventional transmissive TFT-TN displays, the use of liquid crystals with low birefringence (Δn) is necessary in order to achieve a small optical delay (d · Δn). This small optical delay leads to a mostly acceptable low viewing angle dependence of the contrast (cf. DE 30 22 818 ). For reflective displays, the use of liquid crystals with low birefringence is even more important than for transmissive displays, because with reflective displays the effective layer thickness through which the light passes is approximately twice that of transmissive displays with the same layer thickness.

Advantages of reflective displays over transmissive displays are, in addition to lower power consumption (no backlighting required), the space savings that result in a very low overall depth and the mitigation of temperature gradient problems due to different backlight heating.

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)
• – niedrige Schwellen-(Ansteuer-)spannung
• – niedrige Doppelbrechung für verbesserten Beobachtungswinkelbereich

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)
• - low threshold (drive) voltage
• Low birefringence for improved viewing angle range

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 for such MFK, TN or STN displays, in particular for reflective MFK displays, not or only to a lesser extent, the above-mentioned disadvantages very high specific resistances and low threshold voltages exhibit.

It has now been found that this object can be achieved when used in ads according to the invention media. The mixtures according to the invention are distinguished in particular by their excellent low-temperature behavior.

In the pamphlets DE 4112025 A1 . DE 4111824 A1 and DE 4111805 A1 are disclosed with the compounds "ECCP-3OCF ₃ .F" and "ECCP-5OCF ₃ .F" compounds which may be contained in the liquid-crystalline media according to the claims of the present invention. In the pamphlets EP 0844229 A1 and EP 0761799 A2 Similar compounds are disclosed with an ethylene bridge between two cyclohexane rings, but having no end group of the formula -OCF ₃ , but a fluorine substituent. EP 0761799 A1 also discloses corresponding compounds having two ethylene bridges. The publication EP 0568040 A1 discloses compound corresponding to those in the above DE 4112025 A1 but with a butylene bridge instead of the ethylene bridge. The cited publications each disclose further liquid-crystalline compounds as components of liquid-crystalline media.

enthält,
worin
R H, einen unsubstituierten, einen einfach durch CN oder CF₃ oder einen mindestens einfach durch Halogen substituierten Alkyl- oder Alkenylrest mit 1 bis 15 C-Atomen, wobei in diesen Resten auch eine oder mehrere CH₂-Gruppen jeweils unabhängig voneinander durch -O-, -S-,

L H oder F,

trans-1,4-Cyclohexylenring Z eine Einfachbindung, und
n 1
bedeuten,
und jeweils eine oder mehrere Verbindungen der Formeln IIb und IVa,

worin die einzelnen Reste die folgenden Bedeutungen haben:
R⁰: n-Alkyl, Oxaalkyl, Fluoralkyl oder Alkenyl mit jeweils bis zu 7 C-Atomen, und
X⁰: F, Cl, halogeniertes Alkyl, Alkenyl oder Alkoxy mit 1 bis 6 C-Atomen.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 general formula I.

contains
wherein
R is an unsubstituted, an alkyl or alkenyl radical having 1 to 15 C atoms which is monosubstituted by CN or CF ₃ or at least monosubstituted by halogen, where one or more CH ₂ groups in these radicals are each independently of one another denoted by -O- , -S-,

LH or F,

trans-1,4-cyclohexylene ring Z is a single bond, and
n 1
mean,
and in each case one or more compounds of the formulas IIb and IVa,

in which the individual radicals have the following meanings:
R ⁰ : n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 7 carbon atoms, and
X ⁰ : F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 carbon atoms.

The compounds of the formula I have a wide range of applications. Depending on the choice of substituents, these compounds may 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.

Particular preference is given to compounds of the formula I in which L is F.

If R is an alkyl radical and / or an alkoxy radical, this may be straight-chain or branched. Preferably, it is straight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and thus preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore methyl , Octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (= methoxymethyl), 2 - (= ethoxymethyl) or 3-oxabutyl (= 2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5- Oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxo-octyl, 2-, 3-, 4-, 5-, 6 -, 7- or 8-oxanonyl, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R is an alkyl radical in which a CH ₂ group is replaced by -CH = CH-, this may be straight-chain or branched. It is preferably straight-chain and has 2 to 10 C atoms. It therefore means especially vinyl, prop-1, or prop-2-enyl, but-1, 2- or but-3-enyl, pent-1, 2-, 3- or pent-4-enyl, hex 1-, 2-, 3-, 4- or hex-5-enyl, hept-1, 2-, 3-, 4-, 5- or hept-6-enyl, Oct-1, 2-, 3-, 4-, 5-, 6- or oct-7-enyl, non-1-, 2-, 3-, 4-, 5-, 6-, 7- or non-8-enyl, Dec-1 -, 2-, 3-, 4-, 5-, 6-, 7-, 8- or dec-9-enyl.

If R is an alkyl radical in which one CH ₂ group has been replaced by -O- and one by -CO-, these are preferably adjacent. Thus, they include an acyloxy group -CO-O- or an oxycarbonyl group -O-CO-. Preferably, these are straight-chain and have 2 to 6 carbon atoms.

They therefore particularly mean acetyloxy, propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetyloxymethyl, propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl, 2-acetyloxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetyloxypropyl, 3-propionyloxypropyl, 4-acetyloxybutyl, methoxycarbonyl, ethoxycarbonyl, Propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, Ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2- (methoxycarbonyl) ethyl, 2- (ethoxycarbonyl) ethyl, 2- (propoxycarbonyl) ethyl, 3- (methoxycarbonyl) propyl, 3- (ethoxycarbonyl) propyl, 4- (methoxycarbonyl) -butyl.

If R is an alkyl radical in which a CH ₂ group is replaced by unsubstituted or substituted -CH = CH- and an adjacent CH ₂ group by CO or CO-O or O-CO, this may be straight-chain or branched. Preferably, it is straight-chain and has 4 to 13 carbon atoms. It therefore particularly means acryloyloxymethyl, 2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl, 8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl, 2-methacryloyloxyethyl, 3 Methacryloyloxypropyl, 4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl, 7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl, 9-methacryloyloxynonyl.

If R is an alkyl or alkenyl radical which is monosubstituted by CN or CF ₃ , this radical is preferably straight-chain. The substitution by CN or CF ₃ is in any position.

If R is an alkyl or alkenyl radical which is at least monosubstituted by halogen, this radical is preferably straight-chain and halogen is preferably F or Cl. In the case of multiple substitution, halogen is preferably F. The resulting radicals also include perfluorinated radicals. For single substitution, the fluoro or chloro substituent may be in any position, but preferably in the ω position.

Compounds of the formula I which have wing groups R suitable for polymerization reactions are suitable for the preparation of liquid-crystalline polymers.

Compounds of the formula I with branched wing groups R may occasionally be of importance because of their better solubility in the usual liquid-crystalline base materials, but in particular as chiral dopants if they are optically active. Smectic compounds of this type are useful as components of ferroelectric materials.

Compounds of the formula I with S _A phases are suitable, for example, for thermally addressed displays.

Branched groups of this type usually contain no more than one chain branch. Preferred branched radicals R are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl), 2-methylbutyl, isopentyl (= 3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2 Propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, 1-methylheptoxy.

If R is an alkyl radical in which two or more CH ₂ groups have been replaced by -O- and / or -CO-O-, this may be straight-chain or branched. Preferably, it is branched and has 3 to 12 carbon atoms. It therefore particularly denotes bis-carboxy-methyl, 2,2-bis-carboxy-ethyl, 3,3-bis-carboxy-propyl, 4,4-bis-carboxy-butyl, 5,5-bis-carboxy-pentyl, 6,6-bis-carboxy-hexyl, 7,7-bis-carboxy-heptyl, 8,8-bis-carboxy-octyl, 9,9-bis-carboxy-nonyl, 10,10-bis-carboxy-decyl, Bis (methoxycarbonyl) methyl, 2,2-bis (methoxycarbonyl) ethyl, 3,3-bis (methoxycarbonyl) propyl, 4,4-bis (methoxycarbonyl) butyl, 5,5-bis (methoxycarbonyl) pentyl, 6,6-bis (methoxycarbonyl) hexyl, 7,7-bis (methoxycarbonyl) heptyl, 8,8-bis (methoxycarbonyl) octyl, bis (ethoxycarbonyl) methyl, 2,2-bis (ethoxycarbonyl) ethyl, 3,3-bis (ethoxycarbonyl) propyl, 4,4-bis (ethoxycarbonyl) butyl, 5,5-bis (ethoxycarbonyl) hexyl.

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. Furthermore, the compounds of the formula I, as described in the EP 0 334 911 B1 described, are produced.

Also described 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 a positive dielectric anisotropy in the cell and high resistivity) containing such media. The invention also relates to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable a significant expansion of the available parameter space.

The achievable combinations of clearing point, low temperature viscosity, thermal and UV stability, and optical anisotropy and threshold voltage far surpass existing prior art materials.

The demand for high clearing point, nematic phase at low temperature and at the same time a low threshold voltage has so far been met only inadequate. Liquid crystal mixtures, such as. Although MLC-6476 and MLC-6625 (Merck KGaA, Darmstadt, FRG) have comparable clearing points and low-temperature stabilities, they have both much higher Δn values of about 0.075 and much higher threshold voltages of about ≥ 1.7 V.

The liquid-crystal mixtures according to the invention make it possible, while maintaining the nematic phase, to -20 ° C. and preferably to -30 ° C., more preferably -40 ° C., clearing points above 70 ° C., preferably above 80 ° C., particularly preferably above 90 ° C. simultaneously achieve birefringence of ≤ 0.100, preferably ≤ 0.095, in particular ≤ 0.090 and a low threshold voltage, whereby excellent STN and MFK displays, in particular reflective MFK displays, can be achieved. In particular, the mixtures are characterized by low operating voltages. The TN thresholds are 1.9 V, preferably less than 1.7 V, more preferably ≦ 1.5 V. In particular, reflective MFK mixtures are characterized by TN thresholds of <1.5 V.

It will be appreciated that by appropriate choice of the components of the blends of the invention, higher clearing points (e.g., above 110 ° C) at lower dielectric anisotropy values and thus higher threshold voltages or lower clearing points at higher dielectric anisotropy levels (eg,> 12) and Thus, lower threshold voltages (eg, <1.5V) can be realized while preserving the other advantageous characteristics. 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 skilled person can set the birefringence required for a given layer thickness of the MFK display with simple routine methods. The requirements for reflective MFK displays have been In the Digest of Technical Papers, SID Symposium 1998.

The rotational viscosity at 20 ° C is preferably <220 mPa · s, more preferably <190 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 °.

Measurements of the Capacity Holding Ratio also Voltage 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 sufficient HR for MFK displays.

Preferably, the media of the invention obtain several (preferably two or more) compounds of formula I, d. H. the proportion of these compounds is 5-50%, preferably 5-40%, and more preferably in the range of 5-35%.

The individual compounds of the formulas I to XV 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.

• – Mischung enthaltend ein oder mehrere Verbindungen der Formeln Ia bis Id:
  
  
  worin Alkyl ein geradkettiger Alkylrest mit 1-8 C-Atomen und Alkenyl ein 1-E- bzw. 3-E-Alkenylrest mit 2-8 C-Atomen ist;
• – Das Medium enthält ein oder mehrere Verbindungen der Formel Ia, und/oder Ib:
• – Das Medium enthält zusätzlich eine oder mehrere Verbindungen ausgewählt aus der Gruppe bestehend aus den allgemeinen Formeln II bis VIII:
  
  
  worin die einzelnen Reste die folgenden Bedeutungen haben: R⁰: n-Alkyl, Oxaalkyl, Fluoralkyl oder Alkenyl mit jeweils bis zu 9 C-Atomen; X⁰: F, Cl, halogeniertes Alkyl oder Alkoxy mit 1 bis 6 C-Atomen, oder halogeniertes Alkenyl mit 2 bis 6 C-Atomen; Z⁰: -C₄H₈-, -CF₂O-, -OCF₂- oder -C₂F₄-; Y¹, Y², Y³: jeweils unabhängig voneinander H oder F; und Y⁴ r: 0 oder 1, wobei die Formel V nicht mit der Formel I identisch ist.
• – Das Medium enthält vorzugsweise zwei, drei, vier oder fünf Verbindungen der Formel II;
• – Das Medium enthält vorzugsweise ein oder mehrere Verbindungen der Formeln IIa bis Iih (L: H oder F):
  
  
• – Die Verbindung der Formel IV ist vorzugsweise
  
  
• – Das Medium enthält zusätzlich eine oder mehrere Verbindungen ausgewählt aus der Gruppe bestehend aus den allgemeinen Formeln IX bis XV:
  
  
  worin R⁰, X⁰, Y¹ und Y² jeweils unabhängig voneinander eine der in Anspruch 2 angegebene Bedeutung haben. Vorzugsweise bedeutet X⁰ F, Cl, CF₃, OCF₃, OCHF₂. R⁰ ist vorzugsweise Alkyl, Oxaalkyl, Fluoralkyl oder Alkenyl mit jeweils bis zu 6 C-Atomen.
• – Das Medium enthält zusätzlich eine oder mehrere Verbindungen der Formel
  
  worin R⁰ und X⁰ die oben angegebenen Bedeutungen haben,
• – Das Medium enthält zusätzlich ein oder mehrere Ester-Verbindungen der Formeln E1 bis E5:
  
  worin R⁰, L, X⁰, Y¹, Y² die oben angegebenen Bedeutungen haben und L H oder F bedeutet.
• – Medium enthält zusätzlich ein oder mehrere Verbindungen der Formeln Xa bis Xd:
  
• – Medium enthält ein oder mehrere Verbindungen der Formel E1a,
  
  worin R⁰ die oben angegebene Bedeutung hat.
• – Der Anteil an Verbindungen der Formeln 1 bis VIII zusammen beträgt im Gesamtgemisch mindestens 50 Gew.-%;
• – Der Anteil an Verbindungen der Formel I beträgt im Gesamtgemisch 5 bis 50 Gew.-%;
• – Der Anteil an Verbindungen der Formeln II bis VIII im Gesamtgemisch beträgt 20 bis 80 Gew.-%
  
• – Das Medium enthält Verbindungen der Formeln II, III, IV, V, VI, VII oder VIII;
• – R⁰ ist geradkettiges Alkyl oder Alkenyl mit 2 bis 7 C-Atomen;
• – Das Medium besteht im wesentlichen aus Verbindungen der Formeln I bis VIII;
• – Das Medium enthält weitere Verbindungen, vorzugsweise ausgewählt aus der folgenden Gruppe bestehend aus den allgemeinen Formeln XVI bis XIX:
  
  
  worin R⁰ und X⁰ die oben angegebene Bedeutung haben 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.
• – Das Gewichtsverhältnis I: (II + III + IV + V + VI + VII + VIII) 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 XV.
• – Der Anteil an Verbindungen der Formeln Xa bis Xd beträgt im Gesamtgemisch 3–45 Gew.-%, vorzugsweise 5–40 Gew.-%, insbesondere 5–30 Gew.-%.
• – Der Anteil der Verbindungen der Formel E1 beträgt im Gesamtgemisch 10–60 Gew.-%, vorzugsweise 10–45 Gew.-%, insbesondere 15–40 Gew.-%.
• – Der Anteil der Verbindungen der Formeln E2 und/oder E3 im Gesamtgemisch beträgt 1–30 Gew.-%, vorzugsweise 3–20 Gew.-% und insbesondere 3–15 Gew.-%.
• – Der Anteil der Verbindungen der Formel E4 ist vorzugsweise ≤ 20 Gew.-%, insbesondere ≤ 10 Gew.-%.

Preferred embodiments are given below:

• Mixture containing one or more compounds of the formulas Ia to Id:
  
  
  wherein alkyl is a straight-chain alkyl radical having 1-8 C atoms and alkenyl is a 1-E or 3-E-alkenyl radical having 2-8 C atoms;
• The medium contains one or more compounds of the formula Ia, and / or Ib:
• The medium additionally contains one or more compounds selected from the group consisting of the general formulas II to VIII:
  
  
  wherein the individual radicals have the following meanings: R ⁰ : n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9 C atoms; X ⁰ : F, Cl, halogenated alkyl or alkoxy having 1 to 6 C atoms, or halogenated alkenyl having 2 to 6 C atoms; Z ⁰ : -C ₄ H ₈ -, -CF ₂ O-, -OCF ₂ - or -C ₂ F ₄ -; Y ¹ , Y ² , Y ³ : each independently is H or F; and Y ⁴ r: 0 or 1, wherein the formula V is not identical to the formula I.
• The medium preferably contains two, three, four or five compounds of the formula II;
• The medium preferably contains one or more compounds of the formulas IIa to Iih (L: H or F):
  
  
• The compound of formula IV is preferred
  
  
• The medium additionally contains one or more compounds selected from the group consisting of the general formulas IX to XV:
  
  
  wherein R ⁰ , X ⁰ , Y ¹ and Y ² each independently have one of the meaning given in claim 2. Preferably, X ^{0 is} F, Cl, CF ₃ , OCF ₃ , OCHF ₂ . R ⁰ is preferably alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 6 C atoms.
• - The medium additionally contains one or more compounds of the formula
  
  wherein R ⁰ and X ^{0 have} the meanings given above,
• The medium additionally contains one or more ester compounds of the formulas E1 to E5:
  
  wherein R ⁰ , L, X ⁰ , Y ¹ , Y ^{2 have} the meanings given above and L is H or F.
• - Medium additionally contains one or more compounds of the formulas Xa to Xd:
  
• Medium contains one or more compounds of the formula E1a,
  
  wherein R ^{0 has} the meaning given above.
• - The proportion of compounds of formulas 1 to VIII together in the total mixture is at least 50 wt .-%;
• - The proportion of compounds of formula I is in the total mixture 5 to 50 wt .-%;
• The proportion of compounds of the formulas II to VIII in the total mixture is from 20 to 80% by weight
  
• - The medium contains compounds of the formulas II, III, IV, V, VI, VII or VIII;
• R ⁰ is straight-chain alkyl or alkenyl having 2 to 7 C atoms;
• - The medium consists essentially of compounds of formulas I to VIII;
• The medium contains further compounds, preferably selected from the following group consisting of the general formulas XVI to XIX:
  
  
  wherein R ⁰ and X ^{0 have} the abovementioned meaning and the 1,4-phenylene rings may be substituted by CN, chlorine or fluorine. Preferably, the 1,4-phenylene rings are mono- or polysubstituted by fluorine atoms.
• The weight ratio I: (II + III + IV + V + VI + VII + VIII) is preferably 1:10 to 10: 1.
• The medium consists essentially of compounds selected from the group consisting of the general formulas I to XV.
• - The proportion of compounds of the formulas Xa to Xd is in the total mixture 3-45 wt .-%, preferably 5-40 wt .-%, in particular 5-30 wt .-%.
• - The proportion of compounds of formula E1 is in the total mixture 10-60 wt .-%, preferably 10-45 wt .-%, in particular 15-40 wt .-%.
• - The proportion of compounds of formulas E2 and / or E3 in the total mixture is 1-30 wt .-%, preferably 3-20 wt .-% and in particular 3-15 wt .-%.
• - The proportion of compounds of formula E4 is preferably ≤ 20 wt .-%, in particular ≤ 10 wt .-%.

It has been found that even a relatively low proportion of compounds of the formula I mixed with customary liquid crystal materials, but in particular with one or more compounds of the formula II, III, IV, V, VI, VII and / or VIII to a lowering of the threshold voltage and to low birefringence values, while simultaneously observing broad nematic phases with low transition temperatures smectic-nematic, thereby dramatically 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 VIII are colorless, stable and readily miscible with one another and with other liquid crystal materials.

The term "alkyl" or "alkyl *" embraces 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 generally preferred.

The term "alkenyl" or "alkenyl *" embraces straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular 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 a suitable choice of the meanings of R ⁰ and X ⁰ , the switching times, the threshold voltage, the steepness of the transmission characteristics, etc. can be modified as desired. 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.

The optimum ratio of the compounds of formulas I and II + III + IV + V + VI + VII + VIII depends largely on the desired properties of the choice of components of the formulas I, II, III, IV, V, VI, VII and / or VIII and from the choice of further 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 the formulas I to XV 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 switching times and the threshold voltage is generally greater the higher the total concentration of compounds of formulas I to XV.

In a particularly preferred embodiment, the media of the invention comprise compounds of the formula II to VIII (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 STN or MFK display consisting of polarizers, electrode base plates and electrodes with surface treatment corresponds to the construction customary for such displays. The term of the usual construction is here broadly and includes all modifications and modifications of the MFK display, in particular matrix display elements based on poly-Si TFT or MIM and very particularly reflective displays.

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. Furthermore, it is possible the mixtures in other conventional ways, eg. B. by using premixes, z. B. homolog mixtures or using so-called "multi-bottle" systems to produce.

The dielectrics may also contain further additives known to the person skilled in the art and described in the literature. For example, 0-15%, preferably 0-10%, pleochroic dyes and / or chiral dopants may be added. The individual added compounds are used in concentrations of 0.01 to 6%, preferably from 0.1 to 3%. However, the concentration data of the other constituents of the liquid-crystal mixtures ie the liquid-crystalline or mesogenic compounds are given, without taking into account the concentration of these additives.

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

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 Tabelen 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; Each of m and n is independently of one another preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12. The coding according to Table B is self-evident. In Table A, only the Acronym indicated for the main body. 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 n Om 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 n Cl C _n H _{2n + 1} Cl H H nF.F C _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 VT CH ₂ = CH CF ₃ H H V2 T CH ₂ -CH-C ₂ H ₄ CF ₃ H H 1V OT CH ₃ -CH = CH OCF ₃ H H rESn C _r H _{2r + 1} -OC _s H _2s - CN H H nAm C _n H _{2n + 1} COOC _m H _{2m + 1} 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. Above and below, percentages are by weight. All temperatures are in degrees Celsius. Mp means melting point, bp = 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. The optical anisotropy (589 nm, 20 ° C.) and the flow viscosity ν ₂₀ (mm ² / sec) and the rotational viscosity γ ₁ (mPa · s) were each determined at 20 ° C.

V ₁₀ denotes the voltage for 10% transmission (viewing direction perpendicular to the plate surface). t _on denotes the switch-on time and t _off the switch-off time at an operating voltage corresponding to twice the value of V ₁₀ . Δn denotes the optical anisotropy and n _o the refractive index. Δε denotes the dielectric anisotropy (Δε = ε _|| - ε _⊥ , where ε _|| the dielectric constant parallel to the _longitudinal molecular _axes and ε _⊥ the dielectric constant perpendicular to it). 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, unless expressly stated otherwise. The optical data were measured at 20 ° C unless expressly stated otherwise.

Examples mix

Example A - TFT mixture CCP 2F.FF 7.0% S → N <-40 ° C CCP 3F.FF 9.0% clearing point: 71.5 ° C CCP 5F.FF 6.0% Δn [589 nm, 20 ° C]: +0.0906 CECG-3-OT 7.0% γ ₁ [mPa.s, 20 ° C]: 185 CCP-3OCF ₃ 7.0% d · Δn [μm, 20 ° C]: 0.5 CGU-2-F 11.0% twist: 90 ° CGU-3-F 11.0% V1 _0,0,20 [V]: 1.07 CGU-5-F 10.0% BCH 3F.FF 4.0% CCZU-2-F 7.0% CCZU-3-F 14.0% CCZU-5-F 7.0%

Claims (9)

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

wherein R is H, an unsubstituted, an alkyl or alkenyl radical having 1 to 15 carbon atoms, which is monosubstituted by CN or CF ₃ or at least monosubstituted by halogen, wherein one or more CH ₂ groups in these radicals are each independently of one another -O-, -S-,

-CO-, -CO-O-, -O-CO- or -O-CO-O- may be replaced so that O atoms are not directly linked to one another, LH or F,

trans-1,4-cyclohexylene ring, Z is a single bond, and n is 1, and in each case one or more compounds of the general formulas IIb and IVa,

in which the individual radicals have the following meanings: R ⁰ : n-alkyl, oxaalkyl, fluoroalkyl or alkenyl having in each case up to 7 C atoms, and X ⁰ : F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 C 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, VII and VIII:

in which the individual radicals have the following meanings: R ⁰ : n-alkyl, oxaalkyl, fluoroalkyl or alkenyl having in each case up to 7 C atoms, X ⁰ : F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 C atoms , Y ¹ and Y ² : each independently of one another are H or F, r: 0 or 1. Medium according to Claim 2, characterized in that the proportion of compounds of the formulas I to VIII together in the total mixture is at least 50% by weight. Medium according to Claim 1 or 2, characterized in that the proportion of compounds of the formula I in the total mixture is 5 to 50% by weight. Medium according to at least one of Claims 2 to 3, characterized in that the proportion of compounds of the formulas II to VIII in the total mixture is from 20 to 80% by weight. Medium according to one of Claims 1 to 5, characterized in that it additionally contains one or more compounds of the formula E1

in which R ⁰ , X ⁰ and V ^{2 have} the meanings given in claim 2. Medium according to Claim 6, characterized in that X ⁰ denotes F or OCF ₃ and Y ² denotes H or F. Medium according to one of claims 1 to 7, characterized in that the compound of the formula I is selected from the group of the compounds Ia to Id:

where alkyl is a straight-chain alkyl radical having 1-8 C atoms and alkenyl is a 1E or 3-E-alkenyl radical having 2-8 C atoms. Use of the liquid-crystalline medium according to one of Claims 1 to 8 for electro-optical purposes.