DE19611096A1 - Electro-optical LCD for in-plane switching display - Google Patents

Abstract

An electro-optical liquid crystal display, having a layer for re-orienting the liquid crystals that has a field with a significant component parallel to the liquid crystal layer. The display contains a liquid crystal medium with a positive dielectric anisotropy, containing at least one compound of formula (I) and at least one compound of formula (II): (where, R1, R1' = H or 1-15C alk(en)yl optionally substituted with a single CN or CF3 group or with at least one halogen atom, and optionally containing one or more non-adjacent CH2 groups replaced by O, S, 1,3-cyclobutyl, CO, -CO-O-, -O-CO- or -O-CO-O- ; A1-A2' = trans-1,4-cyclohexylene (optionally containing one or more non-adjacent CH2 groups replaced by O and/or S, and optionally substituted with 1-2 F atoms), 1,4-phenylene (optionally containing 1-2 CH groups replaced by N and optionally substituted with 1-2 F atoms), 1,4-cyclohexenylene, 1,4-bicyclo(2,2,2)-octylene, piperidine-1,4-diyl, (decahydro)naphthalene-2,6-diyl or 1,2,3,4-tetrahydronaphthalene-2,6-diyl ; Z1-Z2' = -CO-O-, -O-CO-, -CH2-O-, -OCH2-, -CH2CH2-, -CH=CH-, -C=CH- (alkyne bond) or a single bond, and one of Z1-Z2' may also be -(CH2)4- or -CH=CH-CH2CH2- ; X, Y = F, Cl, fluoroalk(en)yl or fluoroalkoxy; and l, m = 0-2). Also claimed other are liquid crystal mediums with positive dielectric anisotropy.

Description

The invention relates to an electro-optical liquid crystal display with a Reorientation layer for reorientation of the liquid crystals whose Field a significant component parallel to the liquid crystal layer comprising a liquid-crystalline medium having positive dielectric Anisotropy, wherein the medium comprises at least one compound of the formula I,

and
at least one compound of the formula II

contains
wherein
R¹ and R¹⁰ are each independently H, an unsubstituted, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is monosubstituted by CN or CF₃ or at least monosubstituted by halogen, one or more CH₂ groups in each of these radicals also being independent of one another other,

can be replaced so that C atoms are not directly linked to each other,
A¹, A¹ ', A² and A²' each independently

• (a) trans-1,4-cyclohexylene radical, wherein also one or several non-adjacent CH₂ groups represented by -O- and / or -S- can be replaced,
• (b) 1,4-phenylene radical, wherein also one or two CH groups can be replaced by N,
• (c) radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo- (2,2,2) -octylene, Piperidine-1,4-diyl, naphthalene-2,6-diyl, Decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydro-naphthalene-2,6-diyl,

where the radicals (a) and (b) can be substituted by one or two fluorine,
Z¹, Z¹ ', Z² and Z²' are each independently -CO-O-, -O-CO-, -CH₂O-, -OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C- or a single Bindung, one of the radicals Z¹ or Z¹ 'and Z² or Z²' also - (CH₂) ₄- or -CH = CH-CH₂CH₂-, and
X and Y are each independently of one another F, Cl, fluorinated alkyl, alkoxy or alkenyl radical,
l and m are each independently 0, 1 or 2
mean.

In conventional liquid crystal displays (TN, STN, OMI, AMD-TN) For example, the electric fields for reorientation become substantially generated perpendicular to the liquid crystal layer.  

International Patent Application WO 91/10936 discloses a liquid crystal display disclosed in which the electrical signals are generated so who that the electric fields are a significant component parallel to Liquid crystal layer have (IPS, in-plane switching). The principles, to operate such a display, z. As described by R.A. Soref in Journal of Applied Physics, Vol. 45, No. 12, p. 5466-5468 (1974).

In EP 0 588 568 various possibilities for driving are provided such a display.

These IPS displays can work with liquid crystalline materials either with a positive or a negative dielectric anisotropy (Δε ≠ 0) are operated. With the hitherto known materials are in However, IPS displays have relatively high threshold voltages and long switching achieved times. It was therefore the object of liquid-crystalline materials which are suitable for IPS displays relatively low threshold to achieve voltages and short switching times.

This problem has surprisingly been solved by the use of liquid crystalline materials of the formulas I and II.

Such compounds are for. B. known from EP 0387032 and the WO 91/03450.

There is, however, no indication that one can do so with the help of these two Substances the threshold voltages and the switching times of Can improve IPS ads.

The invention thus relates to an electro-optical liquid crystal Display with a reorientation layer for reorientation of the liquid crystals whose field is a significant component parallel to the liquid crystal having stable layer containing a liquid crystalline medium with positive dielectric anisotropy, wherein the medium comprises at least one compound of the formula I and II.  

Preferred embodiments are IPS displays, wherein

• a) the medium at least one compound of formula I1 and at least one compound of formula II1 contains
  in which mean;
• b) the medium contains at least one compound which has a group of formula 2: wherein
  L 1 and L 2 are each independently H or F
  mean,
  in particular wherein the medium contains at least one compound of the formula III, wherein
  L¹ and L² have the meaning indicated,
  R² has the meaning given for R¹ and R¹⁰,
  Each of A³ and A⁴ independently has the meaning given for A¹, A¹ ', A² and A²',
  Z 3 and Z⁴ each independently have the meaning given for Z 1, Z 1 ', Z 2 and Z 2', and
  n is 0, 1 or 2;
• c) the medium has a dielectric anisotropy Δε 7.5 before preferably between 7.5 and 14;
• d) the medium contains at least one compound of the formula IV, R³- (A⁵-Z⁵) ₀-A⁶-R⁴ IVworin
  R³ and R⁴ each independently of one another have the meaning given for R¹ and R¹ ',
  A⁵ and A⁶ each independently have the meaning given for A¹, A¹ ', A² and A²',
  each independently of one another has the meaning given for Z¹, Z¹ ', Z² and Z²', and
  o is 1, 2 or 3;
• e) the medium contains at least one compound of the formula V, wherein
  R⁵ has the meaning given for R¹ and R¹ ',
  A⁷ and A⁸ each independently of one another have the meaning given for A¹, A¹ ', A² and A²',
  Z⁷ and Z⁸ each independently of one another have the meaning given for Z¹, Z¹ ', Z² and Z²', and
  Q is a polyfluoroalkylene radical of the formula - (O) _q - (CH₂) _r - (CF₂) _s -, in which
  q is 0 or 1,
  r is 0, or an integer between 1 and 6,
  s is an integer between 0 and 6,
  ZH, F or Cl is, and
  p is 0, 1 or 2, mean.

In the compounds of the formulas I and II, X and Y are preferably F, OCF₃, OCHF₂, CF₃, OCHFCF₃, OCF₂CHFCF₃, OC₂F₅, OC₃F₇, OCH = CF₂, OCF = CF₂, OCF = CHF, especially F, OCF₃ and OCHF₂.

Further preferred is an IPS display, wherein the picture elements by means of Active matrix to be controlled.

Another object of the invention is a liquid-crystalline medium with positive dielectric anisotropy, which

• - 25 to 60 wt .-%, preferably 30 to 50 wt .-% of at least one Compound of the formula I,
• - 25 to 60 wt .-%, preferably 35 to 50 wt .-% of at least one Compound of the formula II,
• - 0 to 30 wt .-%, preferably 5 to 15 wt .-% of at least one Compound of formula III, and  
• - 0 to 30 wt .-%, preferably 0 to 20 wt .-% of a compound of formula IV,
• - 5 to 30 wt .-%, preferably 5 to 15 wt .-% of a compound the formula V,

contains.

The liquid-crystalline medium according to the invention preferably contains:

• - contains at least one compound of formula I1a and II1a
• contains at least one compound selected from the formulas IIIa, IIIb, IIIc and IIId,
• optionally a compound selected from the formulas IVa, IVb and IVc,

such as

• at least one compound of the formula Va, wherein and R¹, R¹ ', R², R²', R⁴, R⁵, L¹, and L⁴ are each as defined.
  In the compounds of formula Va, X * is preferably F; furthermore fluorinated alkyl, alkoxy or alkenyl radical.

The liquid-crystalline media according to the invention generally have a birefringence (Δn) <0.12, preferably Δn is between 0.07 and 0.1, in particular between 0.07 and 0.09.

The viscosity (at 20 ° C.) of the materials according to the invention is generally less than 30 mm 2 s ^-1 . The specific resistance of the materials according to the invention is usually between 5 × 10¹⁰ and 5 × 10¹³ Ω · cm at 20 ° C., more preferably between 5 × 10 11 and 5 × 10 12 Ω · cm, more preferably between 5 x 10¹¹ and 5 x 10¹² Ω · cm.

It has been found that even a relatively low proportion of Verbindun gene of formulas I and II in admixture with conventional Flüssigkristallmateria but in particular with one or more compounds of the Formula III, IV and / or V to a considerable lowering of the threshold voltage and leads to fast switching times, while at the same time wide nematic phases with low transition temperatures smectic-nema be observed. The compounds of formulas I to V are colorless, stable and with each other and with other Flüssigkristallmate good miscible.

The term "alkyl" includes straight-chain and branched alkyl groups with 1-7 carbon atoms, in particular the straight-chain groups methyl, Ethyl, propyl, butyl, pentyl, hexyl and heptyl. Group with 2-5 carbon Atoms are generally preferred.

The term "alkenyl" includes straight 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 groups terminal fluorine, 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- (CH₂) _m , wherein n and m are each independently 1 to 6. Preferably, n = 1 and m is 1 to 6.

By suitable choice of the meanings of R¹ ', R¹ to R⁵ can the An Speech times, the threshold voltage, the steepness of the transmissions characteristics, etc. are modified in the desired manner. For example lead 1E-alkenyl, 3E-alkenyl, 2E-Alkenyloxyreste and the like, usually to shorter response times, improved nematic tendencies and a higher ratio of elastic Constants k₃₃ (bend) and k₁₁ (splay) compared to alkyl or Alkoxy. 4-alkenyl radicals, 3-alkenyl radicals and the like result in general lower threshold voltages and smaller values of k₃₃ / k₁₁ in comparison to alkyl and alkoxy radicals.

A group -CH₂CH₂- in Z¹, Z¹ ', Z² or Z²' generally leads to higher values of k₃₃ / k₁₁ compared to a simple covalency binding. Higher values of k₃₃ / k₁₁ allow z. B. shallower trans Mission characteristics in TN cells with 90 ° twist (to obtain Gray tones) 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 and / or IV largely depends on the desired property of the choice of the components of the formulas I, II and / or III to V and from the choice of other optional components. Suitable proportions within the above Range can be easily determined on a case-by-case basis.

The total amount of compounds of formulas I to V in the inventions The mixtures according to the invention are not critical. The mixtures can therefore one or more other components included for optimization different properties. The observed effect on the response times and the threshold voltage, however, is generally greater higher is the total concentration of compounds of formulas I and II.

In a particularly preferred embodiment, the inventions contain According to the invention media compounds of the formula I and II, wherein X = OCF₂ and Y = OCF₃ means.

The liquid-crystalline media of the invention preferably contain in addition to one or more compounds of the formulas I, II and III and optionally IV and / or V as further constituents 2 to 40, in particular from 4 to 30 components. Most preferably, these contain Media in addition to one or more compounds 7 according to the invention up to 25 components. These other ingredients are preferably selected from nematic or nematogenic (monotropic or isotropic) substances, in particular substances from the classes of Azoxybenzenes, Benzylidenanilines, biphenyls, terphenyls, phenyl or Cyclohexyl benzoates, cyclohexane-carboxylic acid phenyl or cyclohexyl ester, phenyl or cyclohexyl ester of cyclohexylbenzoic acid, phenyl or cyclohexyl esters of cyclohexylcyclohexanecarboxylic acid, cyclohexyl phenylester of benzoic acid, the cyclohexanecarboxylic acid, or the Cyclohexylcyclohexanecarboxylic acid, phenylcyclohexanes, cyclohexyl biphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, Cyclohexylcyclohexylcyclohexenes, 1,4-biscyclohexylbenzenes, 4,4'-bis- cyclohexylbiphenyls, phenyl or cyclohexylpyrimidines, phenyl or Cyclohexylpyridines, phenyl or cyclohexyldioxanes, phenyl or  Cyclohexyl-1,3-dithiane, 1,2-diphenylethane, 1,2-dicyclohexylethane, 1-phenyl-2-cyclohexylethane, 1-cyclohexyl-2- (4-phenyl-cyclohexyl) -ethanes, 1-Cyclohexyl-2-biphenylylethane, 1-phenyl-2-cyclohexylphenyl ethane, optionally halogenated stilbenes, benzylphenyl ethers, Tolans and substituted cinnamic acids. The 1,4-phenylene groups in these Compounds may also be fluorinated.

The most important as further constituents of inventive media in Questioning compounds can be represented by the formulas 1, 2, 3, 4 and 5 characterize:

R'-L-E-R "1

R'-L-COO-E-R "2

R'-L-OOC-E-R "3

R'-L-CH₂CH₂-E-R "4

R'-L-C≡C-E-R "5

In the formulas 1, 2, 3, 4 and 5, L and E denote the same or ver can be separate, each independently a bivalent The residue of -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio, -G-Phe and -G-Cyc and their mirror images formed group, where Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc trans-1,4-cyclohexylene or 1,4-cyclohexylene, Pyr pyrimidine-2,5-diyl or pyridine-2,5-diyl, dio-1,3-dioxane-2,5-diyl and G 2- (trans-1,4-cyclo hexyl) -ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl mean.

Preferably, one of L and E is Cyc, Phe or Pyr. E is present preferably Cyc, Phe or Phe-Cyc. Preferably, the inventions contain According to the media one or more components selected from the compounds of formulas 1, 2, 3, 4 and 5 wherein L and E are selected are from the group Cyc, Phe and Pyr and at the same time one or more Components selected from the compounds of the formulas 1, 2, 3, 4 and 5, wherein one of the radicals L and E is selected from the group Cyc, Phe and Pyr and the other rest is selected from the group -Phe-Phe, -Phe-Cyc, -Cyc-Cyc, -G-Phe and -G-Cyc-, and given  if one or more components selected from the compounds of the formulas 1, 2, 3, 4 and 5, wherein the radicals L and E are selected from the group -Phe-Cyc, -Cyc-Cyc, -G-Phe and -G-Cyc-.

R 'and R "in a smaller subgroup of the compounds mean Formulas 1, 2, 3, 4 and 5 are each independently alkyl, alkenyl, Alkoxy, alkoxyalkyl, alkenyloxy or alkanoyloxy of up to 8 carbon atoms. In the following, this smaller subgroup is called group A. and the compounds are described with sub-formulas 1a, 2a, 3a, 4a and 5a designated. For most of these compounds, R 'and R "are of one different, one of these radicals usually alkyl, alkenyl, alkoxy or alkoxyalkyl.

In a smaller subgroup of compounds of formulas 1, 2, 3, 4 and 5, other than group B, R '' is -F, -Cl, -NCS or - (O) _i CH ₃ - _{(k + 1)} F _k Cl₁, where i is 0 or 1 and k + 1 is 1, 2 or 3; the compounds in which R "has this meaning are designated by sub-formulas 1b, 2b, 3b, 4b and 5b. Particular preference is given to those compounds of the sub-formulas 1b, 2b, 3b, 4b and 5b in which R "has the meaning -F, -Cl, -NCS, -CF₃, -OCHF₂ or -OCF₃.

In the compounds of sub-formulas 1b, 2b, 3b, 4b and 5b R 'has the at the meaning of the compounds of sub-formulas 1a-5a and is preferably alkyl, alkenyl, alkoxy or alkoxyalkyl.

In a further smaller subgroup of the compounds of the formulas 1, 2, 3, 4 and 5 are R "-CN; this subgroup is hereafter referred to as Group C and become the compounds of this subgroup according to sub-formulas 1c, 2c, 3c, 4c and 5c described. In the Compounds of sub-formulas 1c, 2c, 3c, 4c and 5c have R 'in the Compounds of the subformulae 1a-5a given meaning and is present preferably alkyl, alkoxy or alkenyl.  

In addition to the preferred compounds of groups A, B and C are also other compounds of formulas 1, 2, 3, 4 and 5 with other variants the proposed substituents in use. All these substances are by literature methods or in analogy available.

In addition to compounds of the formula I according to the invention, the media according to the invention preferably contain one or more compounds which are selected from group A and / or group B and / or group C. The mass fractions of the compounds from these groups on the media according to the invention are preferably
Group A: 0 to 90% 1, preferably 20 to 90%, in particular 30 to 90%
Group B: 0 to 80%, preferably 10 to 80%, in particular 10 to 65%
Group C: 0 to 80%, preferably 5 to 80%, in particular 5 to 50%
wherein the sum of the mass fractions of the compounds of the groups A and / or B and / or C contained in the respective inventive media is preferably 5% to 90% and in particular 10% to 90%.

The media according to the invention preferably contain 1 to 40%, ins particular preferably 5 to 30% of the invention verbin fertilize. Further preferred are media containing more than 40%, in particular 45 to 90% of compounds according to the invention. The Media preferably contain three, four or five inventive Links.

The structure of the IPS display according to the invention corresponds to that for Such ads usual construction, such. B. described in the WO 91/10936 or EP 0 588568. The term is the usual Construction here broad and includes all modifications and Modifications of the IPS display, in particular z. B. also matrix display elements based on poly-Si TFT or MIM.  

A significant difference of the displays according to the invention to the hitherto usual, however, consists in the choice of liquid crystal parameters the liquid crystal layer.

The preparation of the liquid crystal which can be used according to the invention Mixtures takes place in a conventional manner. In general, the ge desired amount of components used in lesser amount the components constituting the main component are solved, purpose moderately at elevated temperature. It is also possible solutions of Components in an organic solvent, eg. In acetone, Chloroform or methanol, and the solvent after Remove again mixing, for example by distillation.

The dielectrics may also other, known in the art and in the Literature described additives contain. For example, 0-15% pleochroic dyes or chiral dopants are added.

C means a crystalline, S a smectic, SB a smectic B, N a nematic and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (view perpendicular right 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 V₁₀. Δn denotes the optical anisotropy and n₀ the refractive index (each at 589 nm). Ae denotes the dielectric (anisotropy (Δε = ε _∥ -ε _⟂ , where ε _{∥ denotes} the dielectric constant parallel to the _longitudinal molecular _axes and ε _⟂ the dielectric constant perpendicular thereto.) The electro-optical data were measured in an IPS cell at 20 ° C., provided The optical data were measured at 20 ° C unless expressly stated otherwise.

It became an IPS test cell according to WO 91/10936 with a comb structure used, wherein the electrodes and the electrode interspaces a Distance of 20 microns have.  

The layer thickness d of the liquid crystal material is 5 μm. The cell also states:

Output twist angle: 0 °
Alignment angle: 5 °
Tilt angle: 3 °

The cells are dark in the "off" state.

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 H _{2n + 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

An IPS display contains a nematic mixture with

Clarification point: 84 ° C
Δn [589 nm, 20 ° C]: +0.0792
n₀: 1.4636
Δε (1 kHz, 20 ° C): 7.6
ε _⟂ : 4.0
Viscosity (20 ° C): 24 mm² · s ^-1

consisting of

PCH 7F | 12.80% CCP 2OCF2.F.F 14.90% CCP 3OCF2.F.F 13.80% CCP 5OCF2.F.F 15.90% CCP 2OCF3.F 17.00% CCP 3OCF3.F 12.80% CCP 5OCF3.F 12.80%

and has high contrast and low switching times.

V₁₀: 4.0V
V _max : 8.2 V.

Example 2

An IPS display contains a nematic mixture with

Clarification point: 77 ° C
Δn [589 nm, 20 ° C]: + 0.0837
n₀: 1.4666
Δε (1 kHz, 20 ° C): 10.4
ε _⟂ : 4,7

consisting of

PCH 7F | 12.0% CCP 2OCF2.F.F 14.0% CCP 3OCF2.F.F 13.0% CCP 5OCF2.F.F 15.0% CCP 2OCF3.F 16.0% CCP 3OCF3.F 12.0% CCP 5OCF3.F 12.0% ME 2N.F 4.0% ME 3N.F 2.0%

and has high contrast and low switching times.

V₁₀: 3.5V
V _max : 7.0 V.

Example 3

An IPS display contains a nematic mixture with

Clarification point: 74 ° C
Δn [589 nm, 20 ° C]: +0.0895
n₀: 1.4687
Δε (1 kHz, 20 ° C): 12.9
ε _⟂ : 5.3
Viscosity (20 ° C): 27 mm² · s ^-1

consisting of

PCH 7F | 10.0% CCP 2OCF2.F.F 10.0% CCP 3OCF2. F. F 12.0% CCP 5OCF2.F.F 16.0% CCP 2OCF3.F 16.0% CCP 3OCF3.F 12.0% CCP 5OCF3.F 12.0% ME2N.F 5.0% ME3N.F 5.0% ME5N.F 2.0%

has a high contrast and low switching times.

V _(10,0,20) : 2,7 V
V _max : 5.7 V.

Claims (17)

1. Electro-optical liquid crystal display

• with a reorientation layer for reorienting the liquid crystals whose field has a significant component parallel to the liquid crystal layer,
• containing a liquid-crystalline medium having a positive dielectric anisotropy, characterized in that the medium comprises at least one compound of the formula I,

and at least one compound of formula II contains
wherein
R¹ and R¹ 'are each independently H, an unsubsti tuierten, a simply by CN or CF₃ or at least one halogen-substituted alkyl or alkenyl radical having 1 to 15 carbon atoms, wherein in these radicals also one or more CH₂ groups each independently through can be replaced so that C atoms are not directly linked to each other,
A¹, A¹ ', A² and A²' each independently

• (a) trans-1,4-cyclohexylene radical, in which also one or more non-adjacent CH₂ groups may be replaced by -O- and / or -S-,
• (b) 1,4-phenylene radical, in which also one or two CH groups may be replaced by N,
• (c) radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo (2,2,2) octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6- diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
  where the radicals (a) and (b) can be substituted by one or two fluorine,

Z¹, Z¹ ', Z² and Z²' are each independently -CO-O-, -O-CO-, -CH₂O-, -OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C- or a single bond , one of Z¹ or Z¹ 'and Z² or Z²' also - (CH₂) ₄- or -CH = CH-CH₂CH₂-, and
X and Y are each independently of one another F, Cl, fluorinated alkyl, alkoxy or alkenyl radical,
l and m are each independently 0.1 or 2
mean. 2. Liquid crystal display according to claim 1, characterized in that the medium contains at least one compound which has a group of formula 2: wherein
L 1 and L 2 are each independently H or F
mean. 3. Liquid crystal display according to one of claims 1 to 2, characterized in that the medium comprises at least one compound of formula III, wherein
L 1 and L 2 are each independently H or F,
R² is H, an unsubstituted, an alkyl or alkenyl radical having 1 to 15 carbon atoms, which is monosubstituted by CN or CF₃ or at least monosubstituted by halogen, in which radicals one or more CH₂ groups are each independently of one another may be replaced so that O atoms are not directly linked to each other,
A³ and A⁴ each independently

• (a) trans-1,4-cyclohexylene radical, in which also one or more non-adjacent CH₂ groups may be replaced by -O- and / or -S-,
• (b) 1,4-phenylene radical, in which also one or two CH groups may be replaced by N,
• (c) radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo (2,2,2) octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6- diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
  where the radicals (a) and (b) can be substituted by one or two fluorine,

Z³ and Z⁴ are each independently -CO-O-, -O-CO-, -CH₂O-, -OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C- or a single bond, one of the radicals Z³ or Z⁴ also - (CH₂) ₄- or -CH = CH-CH₂CH₂-, and
n 0, 1 or 2
mean,
contains. 4. Liquid crystal display according to one of claims 1 to 3, characterized characterized in that the medium has a dielectric anisotropy Δε 7.5.   5. Liquid crystal display according to one of claims 1 to 4, characterized in that the medium additionally contains a compound of formula IV, R³- (A⁵-Z⁵) _o -A⁶-R⁴ IVworin
R³ and R⁴ are each independently H, an unsubstituier th, a simple by CN or CF₃ or at least one halogen-substituted alkyl or alkenyl radical having 1 to 15 carbon atoms, wherein in these radicals also one or more CH₂ groups each independently from each other may be replaced so that O atoms are not directly linked to each other,
A⁵ and A⁶ each independently

• (a) trans-1,4-cyclohexylene radical, in which also one or more non-adjacent CH₂ groups may be replaced by -O- and / or -S-,
• (b) 1,4-phenylene radical, in which also one or two CH groups may be replaced by N,
• (c) radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo (2,2,2) octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6- diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
  where the radicals (a) and (b) can be substituted by one or two fluorine,

Z⁵-CO-O-, -O-CO-, -CH₂O-, -OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C- or a single invention,
o is 1, 2 or 3,
mean. 6. Liquid crystal display according to one of claims 1 to 5, characterized in that the medium contains at least one compound of formula V, wherein
R⁵ H, an unsubstituted, a simply by CN or CF₃ or at least one substituent by halogen sub stituted alkyl or alkenyl radical having 1 to 15 carbon atoms, wherein in these radicals also one or more CH₂ groups each independently may be replaced so that O atoms are not directly linked to each other,
A⁷ and A⁸ each independently

• (a) trans-1,4-cyclohexylene radical, in which also one or more non-adjacent CH₂ groups may be replaced by -O- and / or -S-,
• (b) 1,4-phenylene radical, in which also one or two CH groups may be replaced by N,
• (c) radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo (2,2,2) octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6- diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
  where the radicals (a) and (b) can be substituted by one or two fluorine,

Z⁷ and Z⁸-CO-O-, -O-CO-, -CH₂O-, -OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C- or a single finding, one of the radicals Z⁷ and Z⁸ also - (CH₂) ₄ or -CH = CH-CH₂CH₂-,
Q is a polyfluoroalkylene radical of the formula - (O) _q - (CH₂) _r - (CF₂) _s -worin
q is 0 or 1
r is 0, or an integer between 0 and 6 is and
s is an integer between 0 and 6,
ZH, F or Cl, and
p is 0, 1 or 2. 7. Display according to one of claims 1 to 6, characterized gekennzeich net, that the picture elements are controlled by means of active matrix. 8. Liquid-crystalline medium with positive dielectric anisotropy, characterized in that it consists of the compounds I, II and III or I, II, III, IV and / or V according to claims 1 to 6 composed.   9. Liquid-crystalline medium according to claim 8, characterized in that it

• From 25 to 60% by weight, preferably from 30 to 50% by weight, of at least one compound of the formula I,
• From 25 to 60% by weight, preferably from 35 to 50% by weight, of at least one compound of the formula II,
• - 0 to 30 wt .-%, preferably 5 to 15 wt .-% of at least one compound of formula III, and
• From 0 to 30% by weight, preferably from 0 to 20% by weight, of a compound of the formula IV,
• From 5 to 30% by weight, preferably from 5 to 15% by weight, of a compound of the formula V

contains. 10. Liquid-crystalline medium according to claim 8 or 9, characterized in that it comprises:

• - contains at least one compound of formula Ila and IIa in which mean,
• contains at least one compound selected from the formulas IIIa, IIIb, IIIc and IIId,
• optionally contains a compound selected from the formulas IVa, IVb and IVc,

such as

• contains at least one compound of formula Va, wherein X * F, halogenated alkyl, alkoxy or alkenyl radical, and R¹, R¹ ', R², R³, R⁴, R⁵, L¹ and L², each having the meaning indicated.