LIQUID-CRYSTALLINE MEDIUM

Abstract

A liquid-crystalline medium which comprises at least one compound of the formula I,

##STR00001## and one or more compounds of the formula ST

##STR00002## and to the use thereof for an active-matrix display, in particular based on the VA, PSA, PA-VA, SS-VA, SA-VA, PS-VA, PALC, IPS, PS-IPS, UB-FFS, U-IPS, FFS or PS-FFS effect.

Claims

1. A liquid-crystalline medium which comprises one or more compounds of the formula I, ##STR00302## in which R.sup.1 and R.sup.1* each, independently of one another, denote H, an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH.sub.2 groups in these radicals may each be replaced, independently of one another, by C?C, CF.sub.2O, OCF.sub.2, CH?CH, ##STR00303## O, COO, OCO in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, A.sup.1 and A.sup.1* each, independently of one another, denote a) a 1,4-cyclohexenylene or 1,4-cyclohexylene radical, in which one or two non-adjacent CH.sub.2 groups may be replaced by O or S, b) a 1,4-phenylene radical, in which one or two CH groups may be replaced by N, c) a radical from the group piperidine-1,4-diyl, 1,4-bicyclo-[2.2.2]octylene, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl, phenanthrene-2,7-diyl and fluorene-2,7-diyl, wherein the radicals a), b) and c) may be mono- or polysubstituted by halogen atoms, Z.sup.1 and Z.sup.1* each, independently of one another, denote COO, OCO, CF.sub.2O, OCF.sub.2, CH.sub.2O, OCH.sub.2, CH.sub.2, CH.sub.2CH.sub.2, (CH.sub.2).sub.4, CH?CHCH.sub.2O, C.sub.2F.sub.4, CH.sub.2CF.sub.2, CF.sub.2CH.sub.2, CF?CF, CH?CF, CF?CH, CH?CH, C?C or a single bond, and L.sup.1 and L.sup.2 each, independently of one another, denote F, Cl, CF.sub.3 or CHF.sub.2, a and b each, independently of one another, are 0 or 1, and one or more compounds the formula ST, ##STR00304## in which G denotes a single bond or a divalent aliphatic or cycloaliphatic radical having 1 to 20 C atoms.

2. A liquid-crystalline medium according to claim 1, wherein the medium comprises one or more compounds of the formulae I-1 to I-10, ##STR00305## in which alkyl and alkyl* each, independently of one another, denote a straight-chain alkyl radical having 1-6 C atoms, alkenyl and alkenyl* each, independently of one another, denote a straight-chain alkenyl radical having 2-6 C atoms, alkoxy and alkoxy* each, independently of one another, denote a straight-chain alkoxy radical having 1-6 C atoms, and L.sup.1 and L.sup.2 each, independently of one another, denote F, Cl, CF.sub.3 or CHF.sub.2.

3. A liquid-crystalline medium according to claim 12, which comprises one or more compounds of formula I-6 ##STR00306## in which each alkoxy, independently of one another, denote a straight-chain alkoxy radical having 1-6 C atoms.

4. A liquid-crystalline medium according to claim 1, wherein L.sup.1 and L.sup.2 in the formula I each denote F.

5. A liquid-crystalline medium according to claim 1, which comprises one or more compounds of the formula ST-1 ##STR00307## in which R.sup.ST denotes H or alkyl having 1 to 6 C atoms, t is 0 or 1, and q is O, 1, 2, 3, 4, 5, 6, 7, 8, or 9.

6. A liquid-crystalline medium according to claim 1, which additionally comprises one or more compounds selected from the group of the compounds of the formulae IIA, IIB and IIC, ##STR00308## in which (O)C.sub.vH.sub.2v+1 is OC.sub.vH.sub.2v+1 or CH.sub.2v+1, R.sup.2A, R.sup.2B and R.sup.2C each, independently of one another, denote H, an alkyl or alkenyl radical having up to 15 C atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 mono- or poly-substituted by halogen, where, in addition, one or more CH.sub.2 groups in these radicals may be replaced by O, S, ##STR00309## C?C, CF.sub.2O, OCF.sub.2, OCO or OCO in such a way that O atoms are not linked directly to one another, L.sup.1-4 each, independently of one another, denote F or Cl, Z.sup.2 and Z.sup.2 each, independently of one another, denote a single bond, CH.sub.2CH.sub.2, CH?CH, CF.sub.2O, OCF.sub.2, CH.sub.2O, OCH.sub.2, COO, OCO, C.sub.2F.sub.4, CF?CF, CH?CHCH.sub.2O, p denotes 0, 1 or 2, q denotes 0 or 1, and v denotes 1 to 6.

7. A liquid-crystalline medium according to claim 1, which additionally comprises one or more compounds of the formulae L-1 to L-11, ##STR00310## in which (O)alkyl is Oalkyl or -alkyl, R, R.sup.1 and R.sup.2 each, independently of one another, denote H, an alkyl or alkenyl radical having up to 15 C atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 mono- or poly-substituted by halogen, where, in addition, one or more CH.sub.2 groups in these radicals may be replaced by O, S, ##STR00311## C?, CF.sub.2O, OCF.sub.2, OCO or OCO in such a way that O atoms are not linked directly to one another, and alkyl denotes an alkyl radical having 1-6 C atoms, and s denotes 1 or 2.

8. A liquid-crystalline medium according to claim 1, which additionally comprises one or more compounds of the formulae O-1 to O-18, ##STR00312## ##STR00313## in which R.sup.1 and R.sup.2 each, independently of one another, denote H, an alkyl or alkenyl radical having up to 15 C atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 mono- or poly-substituted by halogen, where, in addition, one or more CH.sub.2 groups in these radicals may be replaced by O, S, ##STR00314## C?C, CF.sub.2O, OCF.sub.2, OCO or OCO in such a way that O atoms are not linked directly to one another.

9. A liquid-crystalline medium according to claim 1, which comprises one or more compounds of the formulae O-6, O-7 and O-17, ##STR00315## in which R.sup.1 denotes alkyl or alkenyl having 1-6 or 2-6 C atoms, respectively, and R.sup.2 denotes alkenyl having 2-6 C atoms.

10. A liquid-crystalline medium according to claim 1, wherein the proportion of compounds of the formula I in the mixture as a whole is 1-40% by weight.

11. A liquid-crystalline medium according to claim 1, wherein the proportion of compounds of the formula ST in the mixture as a whole is in the range of from 0.005% to 1%.

12. A process for the preparation of a liquid-crystalline medium according to claim 1, wherein one or more compounds of the formula I of claim 1 are mixed with one or more further liquid-crystalline compounds, and one or more compounds of the formula ST of claim 1 are added.

13. A method which comprises including a liquid-crystalline medium according to claim 1 in an electro-optical display.

14. An electro-optical display having active-matrix addressing, which contains, as dielectric, a liquid-crystalline medium according to claim 1.

15. An electro-optical display according to claim 15, wherein the electro-optical display is a VA, PSA, PA-VA, SS-VA, SA-VA, PS-VA, PALC, IPS, PS-IPS, FFS, UB-FFS, U-IPS or PS-FFS display.

Description

WORKING EXAMPLES

[0271] The following examples are intended to explain the invention without limiting it. In the examples, m.p. denotes the melting point and C denotes the clearing point of a liquid-crystalline substance in degrees Celsius; boiling temperatures are denoted by m.p. Furthermore: C denotes crystalline solid state, S denotes smectic phase (the index denotes the phase type), N denotes nematic state, Ch denotes cholesteric phase, I denotes isotropic phase, T.sub.g denotes glass-transition temperature. The number between two symbols indicates the conversion temperature in degrees Celsius an.

[0272] The host mixture used for determination of the optical anisotropy ?n of the compounds of the formula I is the commercial mixture ZLI-4792 (Merck KGaA). The dielectric anisotropy ?? is determined using commercial mixture ZLI-2857. The physical data of the compound to be investigated are obtained from the change in the dielectric constants of the host mixture after addition of the compound to be investigated and extrapolation to 100% of the compound employed. In general, 10% of the compound to be investigated are dissolved in the host mixture, depending on the solubility.

[0273] Unless indicated otherwise, parts or percent data denote parts by weight or percent by weight.

[0274] Above and below: [0275] V.sub.0 denotes threshold voltage, capacitive [V] at 20? C., [0276] n.sub.e denotes extraordinary refractive index at 20? C. and 589 nm, [0277] n.sub.0 denotes ordinary refractive index at 20? C. and 589 nm, [0278] ?n denotes optical anisotropy at 20? C. and 589 nm, [0279] ?.sub.? denotes dielectric permittivity perpendicular to the director at 20? C. and 1 kHz, [0280] ?.sub.? denotes dielectric permittivity parallel to the director at 20? C. and 1 kHz, [0281] ?? denotes dielectric anisotropy at 20? C. and 1 kHz, [0282] cl.p., T(N,I) denotes clearing point [? C.], [0283] ?.sub.1 denotes rotational viscosity measured at 20? C. [mPa.Math.s], determined by the rotation method in a magnetic field, [0284] K.sub.1 denotes elastic constant, splay deformation at 20? C. [pN], [0285] K.sub.2 denotes elastic constant, twist deformation at 20? C. [pN], [0286] K.sub.3 denotes elastic constant, bend deformation at 20? C. [pN], [0287] LTS denotes low-temperature stability (nematic phase), determined in test cells,

[0288] Unless explicitly noted otherwise, all values indicated in the present application for temperatures, such as, for example, the melting point T(C,N), the transition from the smectic (S) to the nematic (N) phase T(S,N) and the clearing point T(N,I), are indicated in degrees Celsius (? C.). M.p. denotes melting point, cl.p.=clearing point. Furthermore, Tg=glass state, C=crystalline state, N=nematic phase, S=smectic phase and I=isotropic phase. The numbers between these symbols represent the transition temperatures.

[0289] The term threshold voltage for the present invention relates to the capacitive threshold (V.sub.0), also called the Freedericksz threshold, unless explicitly indicated otherwise. In the examples, as is generally usual, the optical threshold can also be indicated for 10% relative contrast (V.sub.10).

[0290] The display used for measurement of the capacitive threshold voltage consists of two plane-parallel glass outer plates at a separation of 20 m, which each have on the insides an electrode layer and an unrubbed polyimide alignment layer on top, which cause a homeotropic edge alignment of the liquid-crystal molecules.

[0291] The display or test cell used for measurement of the tilt angle consists of two plane-parallel glass outer plates at a separation of 4 ?m, which each have on the insides an electrode layer and a polyimide alignment layer on top, where the two polyimide layers are rubbed antiparallel to one another and cause a homeotropic edge alignment of the liquid-crystal molecules.

[0292] Unless indicated otherwise, the VHR is determined at 20? C. (VHR.sub.20) and after 5 minutes in an oven at 100? C. (VHR.sub.100) in a commercially available instrument Model 6254 from TOYO Corporation, Japan. The voltage used has a frequency of in a range from 1 Hz to 60 Hz, unless indicated more precisely.

[0293] The accuracy of the VHR measurement values depends on the respective value of the VHR. The accuracy decreases with decreasing values. The deviations generally observed in the case of values in the various magnitude ranges are compiled in their order of magnitude in the following table.

TABLE-US-00007 Deviation VHR range (relative) VHR values ?.sub.GVHR/VHR/% from to Approx. 99.6%.sup. 100% +/?0.1 99.0%.sup. 99.6%.sup. +/?0.2 98% 99% +/?0.3 95% 98% +/?0.5 90% 95% +/?1.sup. 80% 90% +/?2.sup. 60% 80% +/?4.sup. 40% 60% +/?8.sup. 20% 40% +/?10 10% 20% +/?20

[0294] The stability to UV irradiation is investigated in a Suntest CPS, a commercial instrument from Heraeus, Germany. The sealed test cells are irradiated for between 30 min and 2.0 hours, unless explicitly indicated, without additional heating. The irradiation power in the wavelength range from 300 nm to 800 nm is 765 W/m.sup.2 V. A UV cut-off filter having an edge wavelength of 310 nm is used in order to simulate the so-called window glass mode. In each series of experiments, at least four test cells are investigated for each condition, and the respective results are indicated as averages of the corresponding individual measurements.

[0295] The decrease in the voltage holding ratio (?VHR) usually caused by the exposure, for example by UV irradiation or by LCD backlighting, is determined in accordance with the following equation (1):

?VHR(t)=VHR(t)?VHR(t=0)(1).

[0296] In order to investigate the low-temperature stability, also known as LTS, i.e. the stability of the LC mixture to spontaneous crystallisation of individual components at low temperatures, bottles containing 1 g of LC/RM mixture are stored at ?10? C., and it is regularly checked whether the mixtures have crystallised out.

[0297] The ion density from which the resistivity is calculated is measured using the commercially available LC Material Characteristics Measurement System Model 6254 from Toyo Corporation, Japan, using VHR test cells with AL16301 Polyimide (JSR Corp., Japan) having a 3.2 ?m cell gap. The measurement is performed after 5 min of storage in an oven at 60? C. or 100? C.

[0298] The so-called HTP denotes the helical twisting power of an optically active or chiral substance in an LC medium (in ?m). Unless indicated otherwise, the HTP is measured in the commercially available nematic LC host mixture MLD-6260 (Merck KGaA) at a temperature of 20? C.

[0299] Unless explicitly noted otherwise, all concentrations in the present application are indicated in percent by weight and relate to the corresponding mixture as a whole, comprising all solid or liquid-crystalline components, without solvents. All physical properties are determined in accordance with Merck Liquid Crystals, Physical Properties of Liquid Crystals, Status November 1997, Merck KGaA, Germany, and apply for a temperature of 20? C., unless explicitly indicated otherwise.

[0300] The following mixture examples having negative dielectric anisotropy are suitable, in particular, for liquid-crystal displays which have at least one planar alignment layer, such as, for example, IPS and FFS displays, in particular UB-FFS (=ultra-bright FFS), and for VA displays.

MIXTURE EXAMPLES

[0301] Comparative Mixture C1 is prepared as follows:

TABLE-US-00008 CCY-3-1 7.5% Clearing point [? C.]: 85.5 CCY-3-O1 5.0% ?n (589 nm, 20? C.): 0.1053 CLY-3-O2 9.0% ?? (1 kHz, 20? C.): ?3.4 CPY-2-O2 10.0% ?.sub.? (1 kHz, 20? C.): 3.4 CPY-3-O2 10.0% ?.sub.? (1 kHz, 20? C.): 6.8 PYP-2-3 6.0% K.sub.1 [pN], (20? C.): 14.4 CC-3-V 25.0% K.sub.3 [pN], (20? C.): 17.4 CC-3-V1 8.5% V.sub.0 [V], (20? C.): 2.41 CC-2-3 3.0% ?.sub.1 [mPa .Math. s], (20? C.): 117 CY-3-O2 13.0% CY-5-O2 3.0%

Example C2

[0302] The mixture C2 is prepared from 99.9% of mixture C1 and 0.1% of a stabiliser of formula ST-1a-1.

[0303] A nematic host mixture N1 is prepared as follows:

TABLE-US-00009 CCY-3-O2 10.00% Clearing point [? C.]: 85 CCY-5-O2 7.00% ?n (589 nm, 20? C.): 0.1047 CPY-2-O2 10.00% ?? (1 kHz, 20? C.): ?3.4 CPY-3-O2 10.00% ?.sub.? (1 kHz, 20? C.): 3.5 PYP-2-3 5.50% ?.sub.? (1 kHz, 20? C.): 6.9 B-2O-O5 4.00% K.sub.1 [pN], (20? C.): 14.6 CC-3-V 32.00% K.sub.3 [pN], (20? C.): 17.4 CC-3-V1 10.00% V.sub.0 [V], (20? C.): 2.37 CY-3-O2 10.00% ?.sub.1 [mPa .Math. s], (20? C.): 108 CY-5-O2 1.50%

Example Mixtures M1 to M4

[0304] To the host mixture N1, the stabilisers ST-1a-1 and ST-1b-1 are added in the amounts indicated in table 1.

TABLE-US-00010 TABLE 1 ST-1a-1 [00300] ST-1b-1 [00301] concentration [%] Example Mixture N1 ST-1a-1 ST-1b-1 1 N1 100.00 2 M1 99.90 0.10 3 M2 99.97 0.03 4 M3 99.95 0.05 5 M4 99.90 0.10

[0305] The Voltage Holding Ratio (VHR) of the above mixtures was measured after storage at 60? C. in an oven for 5 min, as described above. The following values were obtained (table 2):

TABLE-US-00011 TABLE 2 VHR values VHR [%] VHR [%] Mixture at 60 Hz at 3 Hz N1 97.1 78.2 M1 99.2 90.4 M2 99.1 89.3 M3 99.2 89.5 M4 99.0 87.3 all values measured at 1 V, 60? C.

[0306] As can be seen from table 2, the VHR is significantly improved for the mixtures M1 to M4 containing stabilisers ST-1a-1 or ST-1b-1 compared to the unstabilized host mixture N1.

[0307] Long term reliability tests are performed with the above mixtures in LC TV panels: The long-term reliability test includes two parts: one is the rolling pattern test and the other is the so-called and more severe NDS test. For the rolling pattern test, various alternating images are displayed on the screen for 1000 h at a storage temperature of 60? C.

[0308] For the NDS test, images of a mosaic pattern is displayed on one half of the screen and a full white image is displayed on the other half of the screen for 2100 h at an ambient temperature of 60? C. After the test, the panel is visually inspected for changes of the image quality (table 3).

TABLE-US-00012 TABLE 3 Panel test results. Mixture C1 C2 N1 M1 Short term image sticking N/A ? N/A ? Rolling pattern test ? ? ? ? NDS test X ? X ? N/A not available X not good ? moderate ? good (passed)

[0309] Mixture C1 without stabiliser does not pass the NDS test. Addition of stabiliser ST-1a-1 to mixture C1 gives mixture C2 which passes the NDS test but has only moderate properties in terms of image sticking. While the host mixture N1 also does not pass the NDS test, the mixture M1 according to the invention including a stabiliser of formula ST and a compound of formula I passes the rolling pattern test as well as the NDS test and also has improved image sticking properties compared to mixture C2 without a compound of formula I.

[0310] In addition, resistivity measurements were performed. The results are summarised in table 4.

TABLE-US-00013 TABLE 4 Resistivity values Resistivity [G?] Resistivity [G?] Mixture at 60? C. at 100? C. N1 2.66 0.55 C2 1.35 0.09 M1 1.15 0.08 M2 1.11 0.10 M3 1.02 0.09 M4 0.76 0.07 all values measured at ?10 V, 1 Hz

[0311] For liquid crystalline media it is basically desirable to achieve a high resistivity. From table 4 it can be seen that all mixtures have a sufficiently high resistivity in the GO range, which is why they are all very well suitable for the operation of a liquid crystal display. The resistivity is lower for the mixtures M1 to M4 containing stabilisers ST-1a-1 or ST-1b-1 compared to the unstabilized host mixture N1. As expected, the resistivity decreases with increased temperatures.

[0312] Without wishing to be bound by theory, and although this has not yet been fully evaluated, it is assumed that the combination of

[0313] 1) a VHR value that is on a similar high level or higher than the VHR of a corresponding medium without a compound of formula I and a stabiliser according to the invention,

[0314] with

[0315] 2) a resistivity value that is lower than the resistivity value of a corresponding medium without compound of formula I and a stabiliser according to the invention,

[0316] leads to an improvement (lower degree) of image sticking or even to the total absence of image sticking.

[0317] As a result, the data above show that the liquid crystalline media according to the invention, comprising a compound of formula I and a stabiliser of formula ST, have much improved reliability compared to the unstabilized host mixture. While the host N1 without stabiliser fails under the long term NDS pattern test conditions, unexpectedly, under the same conditions the mixture M1 pass the Rolling Pattern Test as well as the NDS Pattern Test and also shows improved short term image sticking.