BIMESOGENIC COMPOUNDS AND MESOGENIC MEDIA

Abstract

The invention relates to bimesogenic compounds of formula I

##STR00001##

wherein R.sup.11, R.sup.12, MG.sup.11, MG.sup.12, X.sup.11, X.sup.12 and Sp.sup.1 have the meaning given in claim 1, to the use of bimesogenic compounds of formula I in liquid crystal media and particular to flexoelectric liquid crystal devices comprising a liquid crystal medium according to the present invention.

Claims

1. Bimesogenic compounds of formula I ##STR00152## wherein R.sup.11 and R.sup.12 are each independently H, F, Cl, CN, NCS or a straight-chain or branched alkyl group with 1 to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH.sub.2 groups to be replaced, in each occurrence independently from one another, by O, S, NH, N(CH.sub.3), CO, COO, OCO, OCOO, SCO, COS, CHCH, CHCF, CFCF or CC in such a manner that oxygen atoms are not linked directly to one another, preferably a polar group, more preferably F, Cl, CN, OCF.sub.3, CF.sub.3, MG.sup.11 and MG.sup.12 are each independently a mesogenic group and at least one of MG.sup.11 and MG.sup.12 each comprises one, two or more 6-atomic rings, and Sp.sup.1 is a spacer group comprising 1, 3 or 5 to 40 C atoms, wherein one or more non-adjacent and non-terminal CH.sub.2 groups may also be replaced by O, S, NH, N(CH.sub.3), CO, OCO, SCO, OCOO, COS, COO, CH(halogen)-, CH(CN), CHCH or CC, however in such a way that no two O-atoms are adjacent to one another, now two CHCH groups are adjacent to each other and no two groups selected fromOCO, SCO, OCOO, COS, COO and CHCH are adjacent to each other, X.sup.11 and X.sup.12 are different from each another and otherwise independently from one another are a linking group selected from COO, O CO, CHCH, CC, O, SCO, COS, S, and CO, or a single bond, however under the condition that in X.sup.11-Sp.sup.1X.sup.12 no two O atoms are adjacent to one another, no two CHCH groups are adjacent to each other and no two groups selected from OCO, SCO, OCOO, COS, COO and CHCH are adjacent to each other.

2. Bimesogenic compounds according to claim 1, characterized in that MG.sup.11 and MG.sup.12 are independently of each other selected of partial formula II
-A.sup.11-(Z.sup.11-A.sup.12).sub.k- II wherein Z.sup.11 are, independently of each other in each occurrence, a single bond, COO, OCO, OCOO, OCH.sub.2, CH.sub.2O, OCF.sub.2, CF.sub.2), CH.sub.2CH.sub.2, (CH.sub.2).sub.4, CF.sub.2CF.sub.2, CHCH, CFCF, CHCHCOO, OCOCHCH or CC, optionally substituted with one or more of F, S and/or Si, A.sup.11 and A.sup.12 are each independently in each occurrence 1,4-phenylene, wherein in addition one or more CH groups may be replaced by N, trans-1,4-cyclo-hexylene in which, in addition, one or two non-adjacent CH.sub.2 groups may be replaced by O and/or S, 1,4-cyclohexenylene, 1,4-bicyclo-(2,2,2)-octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydro-naphthalene-2,6-diyl, 1,2,3,4-tetrahydro-naphthalene-2,6-diyl, cyclobutane-1,3-diyl, spiro[3.3]heptane-2,6-diyl or dispiro[3.1.3.1] decane-2,8-diyl, it being possible for all these groups to be unsubstituted, mono-, di-, tri- or tetrasubstituted with F, Cl, CN or alkyl, alkoxy, alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H atoms may be substituted by F or Cl, and k is 0, 1, 2, 3 or 4.

3. Bimesogenic compounds according to claim 1, characterized in that MG.sup.11 and MG.sup.12 are independently of one another selected from the group of formulae II-1 to II-26
-Phe-Z-Phe- II-1
-Phe-Z-Cyc- II-2
-Cyc-Z-Cyc- II-3
-Phe-Z-PheL- II-4
-PheL-Z-Phe- II-5
-PheL-Z-Cyc- II-6
-PheL-Z-PheL- II-7
-Phe-Z-Phe-Z-Phe- II-8
-Phe-Z-Phe-Z-Cyc- II-9
-Phe-Z-Cyc-Z-Phe- II-10
-Cyc-Z-Phe-Z-Cyc- II-11
-Phe-Z-Cyc-Z-Cyc- II-12
-Cyc-Z-Cyc-Z-Cyc- II-13
-Phe-Z-Phe-Z-PheL- II-14
-Phe-Z-PheL-Z-Phe- II-15
-PheL-Z-Phe-Z-Phe- II-16
-PheL-Z-Phe-Z-PheL- II-17
-PheL-Z-PheL-Z-Phe- II-18
-PheL-Z-PheL-Z-PheL- II-19
-Phe-Z-PheL-Z-Cyc- II-29
-Phe-Z-Cyc-Z-PheL- II-21
-Cyc-Z-Phe-Z-PheL- II-22
-PheL-Z-Cyc-Z-PheL- II-23
-PheL-Z-PheL-Z-Cyc- II-24
-PheL-Z-Cyc-Z-Cyc- II-25
-Cyc-Z-PheL-Z-Cyc- II-26 wherein Cyc is 1,4-cyclohexlene, preferably trans-1,4-cyclohexlene, Phe is 1,4-phenylene, PheL is 1,4-phenylene, which is substituted by one, two or three fluorine atoms, by one or two Cl atoms or by one Cl atom and one F atom, and Z is a single bond, COO, OCO, OCOO, OCH.sub.2, CH.sub.2O, OCF.sub.2, CF.sub.2O, CH.sub.2CH.sub.2, (CH.sub.2).sub.4, CF.sub.2CF.sub.2, CHCH, CFCF, CHCHCOO, OCOCHCH or CC, optionally substituted with one or more of F, S and/or Si.

4. Bimesogenic compounds according to claim 1, characterized in that R.sup.12 is selected from OCF.sub.3, CF.sub.3, F, Cl and CN.

5. Bimesogenic compounds according to claim 1, characterized in that Sp.sup.1 is (CH.sub.2).sub.o and o is 1, 3 or an integer from 5 to 15.

6. (canceled)

7. Liquid-crystalline medium, characterised in that it comprises one or more bimesogenic compounds according to claim 1.

8. Liquid-crystalline medium according to claim 7, characterised in that it additionally comprises one or more compounds selected from the group of the compounds of the formulae III R.sup.31-MG.sup.31-X.sup.31-Sp.sup.3-X.sup.32-MG.sup.32-R.sup.32 III wherein R.sup.31 and R.sup.32 are each independently H, F, Cl, CN, NCS or a straight-chain or branched alkyl group with 1 to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH.sub.2 groups to be replaced, in each case independently from one another, by O, S, NH, N(CH.sub.3), CO, COO, OCO, OCOO, SCO, COS, CHCH, CHCF, CFCF or CC in such a manner that oxygen atoms are not linked directly to one another, MG.sup.31 and MG.sup.32 are each independently a mesogenic group, Sp.sup.3 is a spacer group comprising 5 to 40 C atoms, wherein one or more non-adjacent CH.sub.2 groups may also be replaced by O, S, NH, N(CH.sub.3), CO, OCO, SCO, OCOO, COS, COO, CH(halogen)-, CH(CN), CHCH or CC, and X.sup.31 and X.sup.32 are each independently O, S, CO, COO, OCO, OCOO, CONH, NHCO, CH.sub.2CH.sub.2, OCH.sub.2, CH.sub.2O, SCH.sub.2, CH.sub.2S, CHCH, CHCHCOO, OCOCHCH, CC or a single bond, and with the condition that compounds of formula I are excluded.

9. (canceled)

10. Liquid crystal device comprising a liquid crystalline medium comprising two or more components, one or more of which is a bimesogenic compound of formula I according to claim 1.

11. Liquid crystal device according to claim 10, characterized in that it is a flexoelectric device.

Description

COMPOUND AND SYNTHESIS EXAMPLES

Synthesis Example 1: Preparation of: FPGIZI-9-OPPN

[0235] ##STR00131##

[0236] Reaction scheme for synthesis example 1

##STR00132##

Step 1.1

[0237] ##STR00133##

[0238] 4-Cyano hydroxybiphenyl (12.5 g, 64 mmol) is dissolved in acetone (300 ml) in a 500 ml flask. Potassium carbonate (19.0 g, 137 mmol) is added in one portion under stirring and the solution turns yellow. The bromoundecanoic acid (16.1 g, 64 mmol) is added in one portion. The reaction mixture is stirred and heated under reflux for 16 h under an atmosphere of an inert gas (N.sub.2). Then the reaction mixture is cooled to ambient temperature. Throughout this application ambient temperature is used for a temperature of approximately 22 C. The reaction solution is then filtered to remove any solids and the filter pad is washed first with acetone, then tetrahydrofuran and finally methanol to extract the product. The filtrate is concentrated under reduced pressure to yield the crude product. Purification is carried out by column chromatography through silica gel, eluting with a mixture of dicloromethane and methanol (1:1. The product is obtained as a white powder. It is identified by nmr 8818.

Step 1.2

[0239] ##STR00134##

[0240] The 4-fluorophenylboronic acid (35.0 g, 0.25 mol) and 4-bromo-3-fluorophenol (46.8 g, 0.245 mol are charged to a 3-neck 2 L roundbottomed flask and dissolved in 1,4-dioxane(1,000 ml), the palladium catalyst (2.0 g, 2.4 mmol) is added, followed by the addition of sodium carbonate (53 g, 0.50 mol) and water (250 ml). The reaction mixture is vigorously stirred at a temperature of 80 C. for 96 h before it is cooled to ambient temperature and neutralized with dilute hydrochloric acid. The phases are separated, washed with dilute acid, brine and water in this sequence. The organic contents of the aqueous phases are extracted with ethyl acetate three times and the organic phases combined and concentrated under reduced pressure. The crude product is purified by recrystallization from a mixture of 50 ml ethylacetate and 300 ml DCM after treatment with charcoal.

Step 1.3

[0241] ##STR00135##

[0242] The acid, the intermediate product from step 1.2, (5.0 g, 13.7 mmol) is added into a reaction flask with toluene (75 ml). The flask is filled with an atmosphere of inert gas (N.sub.2) and cooled to a temperature of 2 C. Then tetrahydrofuran is added to enable dissolving of the compounds. 2-4-difluorobiphenol (2.8 g, 13.7 mmol) is added followed by 4-(dimethyl-amino)pyridine (0.49 g, 4.0 mmol) under constant stirring. A slight precipitate is formed. A solution of N,N-dicyclohexylcarbodiimide (3.1 g, 15 mmol) in toluene is slowly added, while the temperature of the reaction mixture is always maintained in the range from 0 to 5 C. After the addition is complete, the reaction mixture is heated to a temperature of 35 C. and stirred for 16 h at this temperature. The reaction mixture then is cooled to ambient temperature and the precipitate filtered off under vacuum. The filtrate is concentrated to yield a yellow liquid. The crude product is purified by successive passing over a chromatography column to obtain the pure product.

##STR00136##

[0243] The product has the following phase range: K 106 N 111 I and an e/K of 1.80 Cm.sup.1N.sup.1. The e/K has been measured for mixture M-1 as specified below.

Synthesis Example 2: Synthesis of: NPGIZI-5-GPN

[0244] ##STR00137##

Step 2.1

[0245] ##STR00138##

(Wherein n=3.)

[0246] 4-bromo-3-fluoroiodobenzene (14.3 g, 47 mmol) is added into a round bottom flask with tetrahydrofuran (30 ml) and the mixture is stirred under nitrogen atmosphere until it is fully dissolved. Then diisopropylamine (30 ml) is added and the reaction placed in an ultrasonic bath for 10 minutes. Catalysts, bis(triphenylphosphine)palladium(II) dichloride (0.9 g, 1.28 mmol) and copper (I) iodide (0.2 g, 1.05 mmol) are added and the reaction mixture is cooled in a water bath to 20 C. Methyl-5-hex-ynoate (n=3) (5.9 g, 0.047 mol) is slowly added to the reaction mixture and this is stirred for a further 20 hours. The reaction mixture is cooled and filtered under vacuum to remove precipitates. The filtrate is acidified with dilute hydrochloric acid and extracted with diethyl ether. The organic phase is washed with water before concentrating to yield the crude product. The material is purified by column chromatography, eluting the product using a mixture of dichloromethane in petrol obtain the desired product.

Step 2.2

[0247] ##STR00139##

[0248] The intermediate product form step 2.1 (10.5 g, 0.035 mol) is dissolved in methanol (200 ml). Platinum on Carbon catalyst (6 g) is added and the reaction mixture is and stirred under a hydrogen atmosphere for 72 hours until no further hydrogen was up taken. The material is filtered and concentrated to yield the product as a pale coloured solid.

Step 2.3

[0249] ##STR00140##

[0250] The intermediate product from step 2.2 (10.0 g, 33 mmol) is stirred with sodium hydroxide (2.0 g, 50 mmol) in IMS (50 ml) under nitrogen atmosphers. After the reaction is completed, the mixture is poured into a mixture of ice/HCl and then dichloromethane (200 ml) is added. The two layers are separated and the solvent from the organic layer removed in vacuo to give the crude product. This is dissolved in a minimum of DCM and applied to a column of silica eluting with petrol:DCM, 1:1 to give the desired product.

Step 2.4

[0251] ##STR00141##

[0252] The intermediate product from stage 2.3 (7.6 g, 26 mmol) is added to a round bottom flask with 4-bromo-3-fluorophenol (5.3 g, 28 mmol) along with dichloromethane (30 ml). The reaction is stirred until dissolved and placed under an inert nitrogen atmosphere. Dicyclohexylcarbodiimide (6.0 g, 29 mmol) and dimethylaminopyridine (3.5 g, 29 mmol) are added before stirring for 4 hours at room temperature. The reaction is filtered and the filter pad washed well with dichloromethane, before concentrating the filtrate in vacuo. The crude product was purified by column chromatography through silica gel, eluting with DCM/petrol (1:5 ratio) and combining fractions which contained the target compound.

Step 2.5

[0253] ##STR00142##

[0254] Three intermediate product from step 2.4 (9.10 g, 19.6 mmol), 4-cyanophenylboronic acid (4.48 g, 19.6 mmol), potassium phosphate (6.4 g, 30 mmol), dioxane (40 ml) and water (18.7 ml) are sonicated in an ultrasonic bath for 30 minutes under a nitrogen atmosphere. The mixture is stirred at ambient temperature and Pd(DPPF)Cl.sub.2:DCM complex (65 mg) is added. The reaction mixture is heated to 90 C. for 2 hours. The mixture is cooled. The two layers are separated and the solvent from the organic layer removed in vacuo to give a black oil. This is dissolved in a minimum of DCM and applied to a column of silica eluting with petrol:DCM, 1:1 to give the desired product.

Step 2.6

[0255] ##STR00143##

[0256] The intermediate product from step 2.4 (9.10 g, 19.6 mmol), 4-cyanophenylboronic acid (4.48 g, 19.6 mmol), potassium phosphate (6.4 g, 30 mmol), dioxane (40 ml) and water (18.7 ml) are sonicated in an ultrasonic bath for 30 minutes under a nitrogen atmosphere. The reaction mixture is stirred at room temperature and Pd(DPPF)Cl.sub.2:DCM complex (65 mg) are added. The mixture is heated to 90 C. for 2 hours. The mixture is cooled. The two layers are separated and the solvent from the organic layer removed in vacuo to give a black oil. This is dissolved in a minimum of DCM and applied to a column of silica and eluted with petrol:DCM, 1:1 to give the desired product.

##STR00144##

[0257] Phase sequence: K 111 (N 48) I; e/K=2.09 Cm.sup.1N.sup.1.

Compound Examples 3 and Following

[0258] The following compounds of formula I are prepared analogously.

##STR00145## ##STR00146##

[0259] Phase sequence: K 109 (N 90) I; e/K=2.15 Cm.sup.1N.sup.1.

##STR00147##

[0260] Phase sequence: K 45 I; e/K=1.93 Cm.sup.1N.sup.1.

##STR00148##

[0261] The materials in the above table generally showed increased performance in the screening mixtures, as compared to known, more conventional bimesogenic compounds as e.g. those shown in the table below.

Comparative Compound Examples

[0262] ##STR00149##

[0263] Phase sequence: K 137 N 181 I.

##STR00150##

[0264] Phase sequence: K 88 (N 64) I.

##STR00151##

[0265] Phase sequence: K 98 (N 82.5) I.

Use Examples, Mixture Examples

[0266] Typically a 5.6 m thick cell, having an anti-parallel rubbed PI alignment layer, is filled on a hotplate at a temperature at which the flexoelectric mixture in the isotropic phase.

[0267] After the cell has been filled phase transitions, including clearing point, are measured using Differential Scanning Calorimetry (DSC) and verified by optical inspection. For optical phase transition measurements, a Mettler FP90 hot-stage controller connected to a FP82 hot-stage is used to control the temperature of the cell. The temperature is increased from ambient temperature at a rate of 5 degrees C. per minute, until the onset of the isotropic phase is observed. The texture change is observed through crossed polarizers using an Olympus BX51 microscope and the respective temperature noted.

[0268] Wires are then attached to the ITO electrodes of the cell using indium metal. The cell is secured in a Linkam THMS600 hot-stage connected to a Linkam TMS93 hot-stage controller. The hot-stage is secured to a rotation stage in an Olympus BX51 microscope.

[0269] The cell is heated until the liquid crystal is completely isotropic. The cell is then cooled under an applied electric field until the sample is completely nematic. The driving waveform is supplied by a Tektronix AFG3021B arbitrary function generator, which is sent through a Newtons4th LPA400 power amplifier before being applied to the cell. The cell response is monitored with a Thorlabs PDA55 photodiode. Both input waveforms and optical response are measured using a Tektronix TDS 2024B digital oscilloscope.

[0270] In order to measure the flexoelastic response of the material, the change in the size of the tilt of the optic axis is measured as a function of increasing voltage. This is achieved by using the equation:

[00001]$\tan .Math..Math.=\frac{P_0}{2.Math.}.Math.\frac{e}{K}.Math.\underset{\_}{E}$

wherein is the tilt in the optic axis from the original position (i.e. when E=0), E is the applied field, K is the elastic constant (average of K.sub.1 and K.sub.3) and e is the flexoelectric coefficient (where e=e.sub.1+e.sub.3). The applied field is monitored using a HP 34401A multimeter. The tilt angle is measured using the aforementioned microscope and oscilloscope. The undisturbed cholesteric pitch, P.sub.0, is measured using an Ocean Optics USB4000 spectrometer attached to a computer. The selective reflection band is obtained and the pitch determined from the spectral data.

[0271] The mixtures shown in the following examples are well suitable for use in USH-displays. To that end an appropriate concentration of the chiral dopant or dopants used has to be applied in order to achieve a cholesteric pitch of 200 nm or less.

Comparative Mixture Example 1

Host Mixture HO

[0272] The host mixture HO is prepared and investigated.

TABLE-US-00006 Composition Compound No. Abbreviation Conc./% 1 F-PGI-O-9-O-GP-F 25.0 2 F-PGI-O-9-O-PP-N 25.0 3 F-PGI-ZI-9-Z-GP-F 25.0 4 F-PGI-ZI-9-Z-PP-N 25.0 100.0

[0273] 2% of the chiral dopant R-5011 are added to the mixture HO leading to the mixture H-1, which is investigated for its properties.

TABLE-US-00007 Composition Compound No. Abbreviation Conc./% 1 R-5011 2.0 2 F-PGI-O-9-O-GP-F 24.5 3 F-PGI-O-9-O-PP-N 24.5 4 F-PGI-ZI-9-Z-GP-F 24.5 5 F-PGI-ZI-9-Z-PP-N 24.5 100.0

[0274] The mixture H-1 may be used for the ULH-mode. It has a clearing point of 82 C. and a lower transition temperature of 33 C. It has a cholesteric pitch of 291 nm at 25 C. The e/K of this mixture is 1.80 Cm.sup.1N.sup.1 at a temperature of 0.9T(N,I).

Mixture Examples 1.1 to 1.3: Mixtures M-1.1 to M-1.4

Mixture Example 1: Mixture M-1

[0275]

TABLE-US-00008 Composition Compound No. Abbreviation Conc./% 1 R-5011 2.0 2 F-PGI-O-9-O-GP-F 22.0 3 F-PGI-O-9-O-PP-N 22.0 4 F-PGI-ZI-9-Z-GP-F 22.0 5 F-PGI-ZI-9-Z-PP-N 22.0 6 F-PGI-ZI-9-O-PP-N 10.0 100.0

[0276] This mixture (M-1) is prepared and investigated. It is well suitable for the ULH-mode.

[0277] It has a cholesteric pitch of 328 nm at 35 C. The e/K of this mixture is 1.80 Cm.sup.1N.sup.1 at a temperature of 50 C. The investigation described above is performed with 10% each of several compounds of formula I instead of that of synthesis example 1 used in host mixture HO, together with 2% R-5011. The results are shown in the following table.

TABLE-US-00009 T(N, e/K/ Ex. Mixture Compound I)/ C. T.sub.low/ C. P/nm V.sup.1 C1.1 H-1.0 None 82 33 291 1.80 C1.2 H-1.1 N-PP-9-PP-N t.b.d. 42 t.b.d. t.b.d. C1.3 H-1.2 F-PGI-O-7-O-GP-F 108 26.5 332 1.70 E1.1 M-1.1 F-PGI-ZI-9-O-PP-N t.b.d. t.b.d. 328 1.80 E1.2 M.1.2 N-PGI-ZI-4-GP-N t.b.d. t.b.d. t.b.d. t.b.d. E1.3 M-1.3 N-PGI-ZI-6-GP-N t.b.d. t.b.d. t.b.d. t.b.d. E1.4 M-1.4 F-PGI-O-6-GP-F t.b.d. t.b.d. t.b.d. t.b.d. Remarks: t.b.d.: to be determined the cholesteric pitch (P) is given at 0.9 T(N, I) and e/K is given V.sup.1 (i.e. Cm.sup.1N.sup.1) at 0.9 T(N, I).

Comparative Mixture Example 1.2: Mixture H-1.1

[0278] The following mixture is prepared (Mixture H-1.1) and investigated.

TABLE-US-00010 Composition Compound No. Abbreviation Conc./% 1 R-5011 2.0 2 F-PGI-O-9-O-GP-F 22.0 3 F-PGI-O-9-O-PP-N 22.0 4 F-PGI-ZI-9-Z-GP-F 22.0 5 F-PGI-ZI-9-Z-PP-N 22.0 6 N-PP-9-PP-N 10.0 100.0

[0279] This mixture, mixture H-1.1, shows an N to N2 transition at 42 C.

Comparative Mixture Example 1.3: Mixture H-1.2

[0280] The following mixture is prepared (Mixture H-1.1) and investigated.

TABLE-US-00011 Composition Compound No. Abbreviation Conc./% 1 R-5011 2.0 2 F-PGI-O-9-O-GP-F 22.0 3 F-PGI-O-9-O-PP-N 22.0 4 F-PGI-ZI-9-Z-GP-F 22.0 5 F-PGI-ZI-9-Z-PP-N 22.0 6 F-PGI-O-7-O-GP-F 10.0 100.0

[0281] This mixture, mixture H-1.2, has a clearing point of 108 C. and shows an N to N2 transition at 26.5 C. It has a cholesteric pitch of 332 nm at 0.9T(N,I). The e/K of this mixture is 1.70 Cm.sup.1N.sup.1 at 0.9T(N,I), i.e. at a temperature of 70 C.

Mixture Example 2: Mixture M-2

[0282] The following mixture (Mixture M-2) is prepared and investigated.

TABLE-US-00012 Composition Compound No. Abbreviation Conc./% 1 R-5011 2.0 2 N-PP-ZI-9-O-Z-GP-F 13.5 3 F-PGI-ZI-7-Z-PP-N 20.5 4 F-PGI-ZI-9-Z-GUU-N 12.0 5 N-PGI-ZI-9-Z-GU-F 22.0 6 N-PGI-ZI-5-GP-N 15.0 7 N-PGI-O-6-GP-N 15.0 100.0

[0283] This mixture, mixture M-2, shows an N to N2 transition [T(N,N2)] at 44.5 C., and a clearing point [T(N,I)] at 96 C. This mixture (M-2) is well suitable for the ULH-mode.

[0284] It has a cholesteric pitch of 330 nm at 35 C. The e/K of this mixture is 4.2 Cm.sup.1N.sup.1 at a temperature of 35 C.

TABLE-US-00013 TABLE Response times of Mixture M-2 U.sub.rms/V E/V/m .sub.on/ms .sub.off/ms 5.0 0.89 3.8 4.3 7.0 1.24 3.2 4.2 9.0 1.60 3.0 4.2

[0285] With an appropriate adjustment of the concentration of the chiral dopant, e.g. to achieve a cholesteric pitch of 200 nm or less, the mixtures of the examples are suitable for use in the USH (uniformly standing helix) mode, and not only in the ULH (uniformly lying helix) mode.