Mesogenic media and liquid crystal display

Abstract

The invention relates to mesogenic media comprising one or more bimesogenic compounds, one or more nematogenic compound, one or more chiral compound and a compound of formula I,
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.m-AGI
in which, the parameter A.sup.11, A.sup.12, Z.sup.12 and R.sup.11 have one of the meanings as given in claim 1, and to the use of these mesogenic media in liquid crystal devices and in particular in flexoelectric liquid crystal devices, as well as to liquid crystal devices comprising a liquid crystal medium according to the present invention.

Claims

1. Medium comprising one or more bimesogenic compounds, one or more nematogenic compound, one or more chiral compound and one or more compounds of formula I,
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.m-AGI in which, A.sup.11 and A.sup.12 each, independently of one another, denote an aryl-, heteroaryl-, heterocyclic- or alicyclic group optionally being substituted by one or more identical or different groups L, L in each case, independently of one another, denotes, halogen, CN, NO.sub.2, NCO, NCS, OCN, SCN, straight-chain or branched alkyl or alkoxy having 1 to 5 C atoms, where, in addition, one or more non-terminal CH.sub.2 groups in these radicals may each be replaced, independently of one another, by CC, CF.sub.2O, CHCH, ##STR00116## 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, Z.sup.12 in each case, independently of one another, denotes O, S, CO, COO, OCO, SCO, COS, OCOO, CONR.sup.0, NR.sup.0-CO, NR.sup.0-CONR.sup.00, NR.sup.0-COO, OCONR.sup.0, OCH.sub.2, CH.sub.2O, SCH.sub.2, CH.sub.2S, CF.sub.2O, OCF.sub.2, CF.sub.2S, SCF.sub.2, CH.sub.2CH.sub.2, (CH.sub.2).sub.4, CF.sub.2CH.sub.2, CH.sub.2CF.sub.2, CF.sub.2CF.sub.2, CHN, NCH, NN, CHCR.sup.0, CY.sup.01CY.sup.02, CC, CHCHCOO, OCOCHCH, or a single bond, Y.sup.01 and Y.sup.02 each, independently of one another, denote H, F, Cl or CN, R.sup.0 and R.sup.00 each, independently of one another, denote H or alkyl having 1 to 12 C atoms, R.sup.11 denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more non-terminal CH.sub.2 groups in these radicals may each be replaced, independently of one another, by CC, CF.sub.2O, CHCH, ##STR00117## 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, AG denotes -Sp-X.sup.11, Sp denotes (CH.sub.2), p denotes 0, 1 or 2, X.sup.11 denotes a group NH.sub.2, SH, OH, (CO)OH or a group of the formulae ##STR00118## and m denotes 0, 1 or 2; wherein: the one or more bimesogenic compound is selected from the group of compounds of formulae A-I to A-III, ##STR00119## wherein R.sup.1 and R.sup.12, R.sup.21 and R.sup.22, and 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 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, MG.sup.11 and MG.sup.12, MG.sup.21 and MG.sup.22, and MG.sup.31 and MG.sup.32 are each independently a mesogenic group, Sp.sup.1, Sp.sup.2 and Sp.sup.3 are each independently a spacer group comprising 5 to 40 C atoms, wherein one or more non-adjacent CH.sub.2 groups, with the exception of the CH.sub.2 groups of Sp.sup.1 linked to O-MG.sup.11 and/or O-MG.sup.12, of Sp.sup.2 linked to MG.sup.21 and/or MG.sup.22 and of Sp.sup.3 linked to X.sup.31 and X.sup.32, 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, in such a way that 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, and X.sup.31 and X.sup.32 are independently from one another a linking group selected from COO, OCO, CHCH, CC or S, and, alternatively, one of them may also be either O or a single bond, and, again alternatively, one of them may be O and the other one a single bond; the one or more nematogenic compound is selected from the group of compounds of formulae B-I to B-III, ##STR00120## wherein R.sup.B1, R.sup.B21, R.sup.B22, R.sup.B31 and R.sup.B32 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, X.sup.B1 is F, Cl, CN, NCS, Z.sup.B1, Z.sup.B2 and Z.sup.B3 are in each occurrence independently CH.sub.2CH.sub.2, COO, OCO, CF.sub.2-O, O CF.sub.2, CHCH or a single bond, ##STR00121## are in each occurrence independently ##STR00122## alternatively one or more of ##STR00123## are ##STR00124## and n is 1, 2 or 3; and the one or more chiral compound is selected from the group of compounds of formulae C-I to C-III, ##STR00125## the latter ones including the respective (S,S) enantiomers, and wherein E and F are each independently 1,4-phenylene or trans-1,4-cyclohexylene, v is 0 or 1, Z.sup.0 is COO, OCO, CH.sub.2CH.sub.2 or a single bond, and R is alkyl, alkoxy or alkanoyl with 1 to 12 C atoms.

2. Medium according to claim 1, characterized in that the compounds of formula I are selected from the group of compounds wherein the anchor group AG in formula I denotes NH.sub.2, SH, OH or (CO)OH.

3. Medium according to claim 1, characterized in that the compounds of formula I are selected from the group of compounds of the following sub formulae,
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.m-Sp-NH.sub.2Ia
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.mSp-SHIb
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.mSp-OHIc
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.mSp-(CO)OHId in which, the groups R.sup.11, A.sup.11, A.sup.12, Z.sup.12, Sp and parameter m have one of the meanings as indicated in claim 1.

4. Medium according to claim 1, characterized in that the compounds of formula I are selected from the following sub formulae, ##STR00126## ##STR00127## in which R.sup.11 is a straight chain or branched alkyl, having 1 to 25 C atoms.

5. Medium according to claim 1, characterized in that the total concentration of compounds of formula I in the medium is from 0.01 to 10% by weight.

6. Medium according to claim 1, characterized in that the amount of chiral compounds in the medium is from 1 to 20% by weight.

7. Electro-optical device comprising a medium according to claim 1.

8. Electro-optical device according to claim 7, characterized in that it is a flexoelectric device.

9. Electro-optical device according to claim 7, characterized in that it comprises two plane parallel electrodes the inner surfaces of which exhibit planar, anti-parallel alignment conditions.

Description

DETAILED DESCRIPTION

(1) In a preferred embodiment of the present invention, the compounds of formula I are selected from the group of compounds wherein the anchor group AG in formula I denotes NH.sub.2, SH, OH or (CO)OH.

(2) More preferably, the compounds of formula I are selected from the group of compounds of the following formulae,
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.m-Sp-NH.sub.2Ia
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.m-Sp-SHIb
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.m-Sp-OHIc
R.sup.11-A.sup.11-(Z.sup.12-A.sup.12).sub.m-Sp-(Co)OHId
in which, the groups R.sup.11, A.sup.11, A.sup.12, Z.sup.12, Sp and parameter m have one of the meanings as indicated under formula I.

(3) Even more preferred are the compounds of formula I selected from the group of compounds in which m denotes 0.

(4) Accordingly, preferred compounds are the compounds of formula I selected from formulae Ia and Ic, such as

(5) ##STR00004## ##STR00005##

(6) in which R.sup.11 is a straight chain or branched alkyl, having 1 to 25 C atoms.

(7) Especially preferred are the compounds wherein no group Sp is present, such as the compounds of formulae Ia-1 to Ia-5, Ia-10, Ic-1 to Ic-6 and Ic-11, and in particular the compounds of formulae Ic-3, Ic-4, Ic-5 and Ic-6, most preferably the compounds of formula Ic-3.

(8) The compounds of formula I are either known to the expert and can be synthesized according to or in analogy to methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.

(9) Typically, the total concentration of compounds of formula I range from approximately 0.01 to approximately 10% by weight, preferably from approximately 0.01 to approximately 5% by weight, and more preferably from approximately 0.01 to approximately 3% by weight.

(10) Preferably, the bimesogenic compounds are selected from the group of compounds of formulae A-I to A-III,

(11) ##STR00006## wherein R.sup.11 and R.sup.12, R.sup.21 and R.sup.22, and 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 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, MG.sup.11 and MG.sup.12, MG.sup.21 and MG.sup.22, and MG.sup.31 and MG.sup.32 are each independently a mesogenic group, Sp.sup.1, Sp.sup.2 and Sp.sup.3 are each independently a spacer group comprising 5 to 40 C atoms, wherein one or more non-adjacent CH.sub.2 groups, with the exception of the CH.sub.2 groups of Sp.sup.1 linked to O-MG.sup.11 and/or O-MG.sup.12, of Sp.sup.2 linked to MG.sup.21 and/or MG.sup.22 and of Sp.sup.3 linked to X.sup.31 and X.sup.32, 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, 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 and X.sup.31 and X.sup.32 are independently from one another a linking group selected from COO, OCO, CHCH, CC or S, and, alternatively, one of them may also be either O or a single bond, and, again alternatively, one of them may be O and the other one a single bond,

(12) Preferably used are compounds of formulae A-I to A-III wherein Sp.sup.1, Sp.sup.2 and Sp.sup.3 are each independently (CH.sub.2).sub.n with n an integer from 1 to 15, most preferably an uneven integer, wherein one or more CH.sub.2 groups may be replaced by CO.

(13) Especially preferably used are compounds of formula A-III wherein X.sup.31Sp.sup.3-X.sup.32 is -Sp.sup.3-O, -Sp.sup.3-COO, -Sp.sup.3-OCO, O-Sp.sup.3-, O-Sp.sup.3-COO, O-Sp.sup.3-OCO, OCO-Sp.sup.3-O, OCO-Sp.sup.3-OCO, COO-Sp.sup.3-O or COO-Sp.sup.3-COO, however under the condition that in X.sup.31-Sp.sup.3-X.sup.32 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.

(14) Preferably used are compounds of formula A-I in which MG.sup.11 and MG.sup.12 are independently from one another -A.sup.11-(Z.sup.1-A.sup.12).sub.m- wherein Z.sup.1 is COO, OCO, OCOO, OCH.sub.2, CH.sub.2O, CH.sub.2CH.sub.2, (CH.sub.2).sub.4, CF.sub.2CF.sub.2, CHCH, CFCF, CHCHCOO, OCOCHCH, CC or a single bond, 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 m is 0, 1, 2 or 3.

(15) Preferably used are compounds of formula A-II in which MG.sup.21 and MG.sup.22 are independently from one another -A.sup.21-(Z.sup.2-A.sup.22).sub.m- wherein Z.sup.2 is COO, OCO, OCOO, OCH.sub.2, CH.sub.2O, CH.sub.2CH.sub.2, (CH.sub.2).sub.4, CF.sub.2CF.sub.2, CHCH, CFCF, CHCHCOO, OCOCHCH, CC or a single bond, A.sup.21 and A.sup.22 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 m is 0, 1, 2 or 3.

(16) Most preferably used are compounds of formula A-III in which MG.sup.31 and MG.sup.32 are independently from one another -A.sup.31-(Z.sup.3-A.sup.32).sub.m- wherein Z.sup.3 is COO, OCO, OCOO, OCH.sub.2, CH.sub.2O, CH.sub.2CH.sub.2, (CH.sub.2).sub.4, CF.sub.2CF.sub.2, CHCH, CFCF, CHCHCOO, OCOCHCH, CC or a single bond, A.sup.31 and A.sup.32 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 m is 0, 1, 2 or 3.

(17) Preferably, the compounds of formula A-III are asymmetric compounds, preferably having different mesogenic groups MG.sup.31 and MG.sup.32.

(18) Generally preferred are compounds of formulae A-I to A-III in which the dipoles of the ester groups present in the mesogenic groups are all oriented in the same direction, i.e. all COO or all OCO.

(19) Especially preferred are compounds of formulae A-I and/or A-II and/or A-III wherein the respective pairs of mesogenic groups (MG.sup.11 and MG.sup.12) and (MG.sup.21 and MG.sup.22) and (MG.sup.31 and MG.sup.32) at each occurrence independently from each other comprise one, two or three six-atomic rings, preferably two or three six-atomic rings.

(20) A smaller group of preferred mesogenic groups of formula II is listed below. For reasons of simplicity, Phe in these groups is 1,4-phenylene, PheL is a 1,4-phenylene group which is substituted by 1 to 4 groups L, with L being preferably F, Cl, CN, OH, NO.sub.2 or an optionally fluorinated alkyl, alkoxy or alkanoyl group with 1 to 7 C atoms, very preferably F, Cl, CN, OH, NO.sub.2, CH.sub.3, C.sub.2H.sub.5, OCH.sub.3, OC.sub.2H.sub.5, COCH.sub.3, COC.sub.2H.sub.5, COOCH.sub.3, COOC.sub.2H.sub.5, CF.sub.3, OCF.sub.3, OCHF.sub.2, OC.sub.2F.sub.5, in particular F, Cl, CN, CH.sub.3, C.sub.2H.sub.5, OCH.sub.3, COCH.sub.3 and OCF.sub.3, most preferably F, Cl, CH.sub.3, OCH.sub.3 and COCH.sub.3 and Cyc is 1,4-cyclohexylene. This list comprises the sub-formulae shown below as well as their mirror images
-Phe-Z-Phe-II-1
-Phe-Z-Cyc-II-2
-Cyc-Z-Cyc-II-3
-PheL-Z-Phe-II-4
-PheL-Z-Cyc-II-5
-PheL-Z-PheL-II-6
-Phe-Z-Phe-Z-Phe-II-7
-Phe-Z-Phe-Z-Cyc-II-8
-Phe-Z-Cyc-Z-Phe-II-9
-Cyc-Z-Phe-Z-Cyc-II-10
-Phe-Z-Cyc-Z-Cyc-II-11
-Cyc-Z-Cyc-Z-Cyc-II-12
-Phe-Z-Phe-Z-PheL-II-13
-Phe-Z-PheL-Z-Phe-II-14
-PheL-Z-Phe-Z-Phe-II-15
-PheL-Z-Phe-Z-PheL-II-16
-PheL-Z-PheL-Z-Phe-II-17
-PheL-Z-PheL-Z-PheL-II-18
-Phe-Z-PheL-Z-Cyc-II-19
-Phe-Z-Cyc-Z-PheL-II-20
-Cyc-Z-Phe-Z-PheL-II-21
-PheL-Z-Cyc-Z-PheL-II-22
-PheL-Z-PheL-Z-Cyc-II-23
-PheL-Z-Cyc-Z-Cyc-II-24
-Cyc-Z-PheL-Z-Cyc-II-25

(21) Particularly preferred are the sub formulae II-1, II-4, II-6, II-7, II-13, II-14, II-15, II-16, II-17 and II-18.

(22) In these preferred groups, Z in each case independently has one of the meanings of Z.sup.1 as given in formula II. Preferably Z is COO, OCO, CH.sub.2CH.sub.2, CC or a single bond, especially preferred is a single bond.

(23) Very preferably the mesogenic groups MG.sup.11 and MG.sup.12, MG.sup.21 and MG.sup.22 and MG.sup.31 and MG.sup.32 are each and independently selected from the following formulae and their mirror images

(24) Very preferably, at least one of the respective pairs of mesogenic groups MG.sup.11 and MG.sup.12, MG.sup.21 and MG.sup.22 and MG.sup.31 and MG.sup.32 is, and preferably, both of them are each and independently, selected from the following formulae IIa to IIn (the two reference Nos. II i and II l being deliberately omitted to avoid any confusion) and their mirror images

(25) ##STR00007## ##STR00008##

(26) wherein

(27) L is in each occurrence independently of each other F or Cl, preferably F and

(28) r is in each occurrence independently of each other 0, 1, 2 or 3, preferably 0, 1 or 2.

(29) The group

(30) ##STR00009##
in these preferred formulae is very preferably denoting

(31) ##STR00010##
furthermore

(32) ##STR00011##

(33) Particularly preferred are the sub formulae IIa, IId, IIg, IIh, IIi, IIk and IIo, in particular the sub formulae IIa and IIg.

(34) In case of compounds with a non-polar group, R.sup.11, R.sup.12, R.sup.21, R.sup.22, R.sup.31, and R.sup.32 are preferably alkyl with up to 15 C atoms or alkoxy with 2 to 15 C atoms.

(35) If R.sup.11 and R.sup.12, R.sup.21 and R.sup.22 and R.sup.31 and R.sup.32 are an alkyl or alkoxy radical, i.e. where the terminal CH.sub.2 group is replaced by O, this may be straight chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8 carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, or octoxy, furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy, for example.

(36) Oxaalkyl, i.e. where one CH.sub.2 group is replaced by O, 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-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl, for example.

(37) In case of a compounds with a terminal polar group, R.sup.11 and R.sup.12, R.sup.21 and R.sup.22 and R.sup.31 and R.sup.32 are selected from CN, NO.sub.2, halogen, OCH.sub.3, OCN, SCN, COR.sup.X, COOR.sup.X or a mono-oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 C atoms. R.sup.X is optionally fluorinated alkyl with 1 to 4, preferably 1 to 3 C atoms. Halogen is preferably F or Cl.

(38) Especially preferably R.sup.11 and R.sup.12, R.sup.21 and R.sup.22 and R.sup.31 and R.sup.32 in formulae A-I, A-II, respectively A-III are selected of H, F, Cl, CN, NO.sub.2, OCH.sub.3, COCH.sub.3, COC.sub.2H.sub.5, COOCH.sub.3, COOC.sub.2H.sub.5, CF.sub.3, C.sub.2F.sub.5, OCF.sub.3, OCHF.sub.2, and OC.sub.2F.sub.5, in particular of H, F, Cl, CN, OCH.sub.3 and OCF.sub.3, especially of H, F, CN and OCF.sub.3.

(39) In addition, compounds of formulae A-I, A-II, respectively A-III containing an achiral branched group R.sup.11 and/or R.sup.21 and/or R.sup.31 may occasionally be of importance, for example, due to a reduction in the tendency towards crystallization. Branched groups of this type generally do not contain more than one chain branch. Preferred achiral branched groups are isopropyl, isobutyl (=methylpropyl), isopentyl (=3-methylbutyl), isopropoxy, 2-methyl-propoxy and 3-methylbutoxy.

(40) The spacer groups Sp.sup.1, Sp.sup.2 and Sp.sup.3 are preferably a linear or branched alkylene group having 5 to 40 C atoms, in particular 5 to 25 C atoms, very preferably 5 to 15 C atoms, in which, in addition, one or more non-adjacent and non-terminal CH.sub.2 groups may be replaced by O, S, NH, N(CH.sub.3), CO, OCO, OCOO, COS, CH(halogen)-, CH(CN), CHCH or

(41) Terminal CH.sub.2 groups are those directly bonded to the mesogenic groups. Accordingly, non-terminal CH.sub.2 groups are not directly bonded to the mesogenic groups R.sup.11 and R.sup.12, R.sup.21 and R.sup.22 and R.sup.31 and R.sup.32.

(42) Typical spacer groups are for example (CH.sub.2).sub.o, (CH.sub.2CH.sub.2O).sub.pCH.sub.2CH.sub.2, with o being an integer from 5 to 40, in particular from 5 to 25, very preferably from 5 to 15, and p being an integer from 1 to 8, in particular 1, 2, 3 or 4.

(43) Preferred spacer groups are pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, octadecylene, diethyleneoxyethylene, dimethyleneoxybutylene, pentenylene, heptenylene, nonenylene and undecenylene, for example.

(44) Especially preferred are compounds of formulae A-I, A-II and A-III wherein Sp.sup.1, Sp.sup.2, respectively Sp.sup.3 are alkylene with 5 to 15 C atoms. Straight-chain alkylene groups are especially preferred.

(45) Preferred are spacer groups with even numbers of a straight-chain alkylene having 6, 8, 10, 12 and 14 C atoms.

(46) In another embodiment of the present invention are the spacer groups preferably with odd numbers of a straight-chain alkylene having 5, 7, 9, 11, 13 and 15 C atoms. Very preferred are straight-chain alkylene spacers having 5, 7, or 9 C atoms.

(47) Especially preferred are compounds of formulae A-I, A-II and A-III wherein Sp.sup.1, Sp.sup.2, respectively Sp.sup.3 are completely deuterated alkylene with 5 to 15 C atoms. Very preferred are deuterated straight-chain alkylene groups. Most preferred are partially deuterated straight-chain alkylene groups.

(48) Preferred are compounds of formula A-I wherein the mesogenic groups R.sup.11-MG.sup.11- and R.sup.12-MG.sup.1- are different. Especially preferred are compounds of formula A-I wherein R.sup.11-MG.sup.11- and R.sup.12-MG.sup.12- in formula A-I are identical.

(49) Preferred compounds of formula A-I are selected from the group of compounds of formulae A-I-1 to A-I-3

(50) ##STR00012##

(51) wherein the parameter n has the meaning given above and preferably is 3, 5, 7 or 9, more preferably 5, 7 or 9.

(52) Preferred compounds of formula A-II are selected from the group of compounds of formulae A-II-1 to A-II-4

(53) ##STR00013##

(54) wherein the parameter n has the meaning given above and preferably is 3, 5, 7 or 9, more preferably 5, 7 or 9.

(55) Preferred compounds of formula A-III are selected from the group of compounds of formulae A-III-1 to A-III-11

(56) ##STR00014## ##STR00015##

(57) wherein the parameter n has the meaning given above and preferably is 3, 5, 7 or 9, more preferably 5, 7 or 9.

(58) Particularly preferred exemplary compounds of formulae A-I are the following compounds:

(59) symmetrical ones:

(60) ##STR00016##

(61) and non-symmetrical ones:

(62) ##STR00017##

(63) Particularly preferred exemplary compounds of formulae A-II are the following compounds:

(64) symmetrical ones:

(65) ##STR00018##

(66) and non-symmetrical ones:

(67) ##STR00019##

(68) Particularly preferred exemplary compounds of formulae A-III are the following compounds:

(69) symmetrical ones:

(70) ##STR00020##

(71) and non-symmetrical ones:

(72) ##STR00021## ##STR00022## ##STR00023##

(73) The compounds of formulae A-I to A-III can be synthesized according to or in analogy to methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.

(74) Compounds of formula A-II, when added to a nematic liquid crystalline mixture, produce a phase below the nematic. In this context, a first indication of the influence of bimesogenic compounds on nematic liquid crystal mixtures was reported by Barnes, P. J., Douglas, A. G., Heeks, S. K., Luckhurst, G. R., Liquid Crystals, 1993, Vol. 13, No. 4, 603-613. This reference exemplifies highly polar alkyl spacered dimers and perceives a phase below the nematic, concluding it is a type of smectic.

(75) A photo evidence of an existing mesophase below the nematic phase was published by Henderson, P. A., Niemeyer, O., Imrie, C. T. in Liquid Crystals, 2001, Vol. 28, No. 3, 463-472, which was not further investigated.

(76) In Liquid Crystals, 2005, Vol. 32, No. 11-12, 1499-1513 Henderson, P. A., Seddon, J. M. and Imrie, C. T. reported, that the new phase below the nematic belonged in some special examples to a smectic C phase. An additional nematic phase below the first nematic was reported by Panov, V. P., Ngaraj, M., Vij, J. K., Panarin, Y. P., Kohlmeier, A., Tamba, M. G., Lewis, R. A. and Mehl, G. H. in Phys. Rev. Lett. 2010, 105, 1678011-1678014.

(77) In this context, liquid crystal mixtures comprising the new and inventive bimesogenic compounds of formulae A-I and/or A-II and/or A-III show also a novel mesophase that is being assigned as a second nematic phase. This mesophase exists at a lower temperature than the original nematic liquid crystalline phase and has been observed in the unique mixture concepts presented by this application.

(78) Accordingly, the bimesogenic compounds of formula A-II according to the present invention allow the second nematic phase to be induced in nematic mixtures that do not have this phase normally. Furthermore, varying the amounts of compounds of formula A-II allow the phase behaviour of the second nematic to be tailored to the required temperature.

(79) In a preferred embodiment, the nematogenic compounds are preferably selected from the group of compounds of formulae B-I to B-III

(80) ##STR00024## wherein R.sup.B1, R.sup.B21 and R.sup.B22 and R.sup.B31 and R.sup.B32 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, X.sup.B1 is F, Cl, CN, NCS, preferably CN, Z.sup.B1, Z.sup.B2 and Z.sup.B3 are in each occurrence independently CH.sub.2CH.sub.2, COO, OCO, CF.sub.2O, O CF.sub.2, CHCH or a single bond, preferably CH.sub.2CH.sub.2, COO, CHCH or a single bond, more preferably CH.sub.2CH.sub.2 or a single bond, even more preferably one of the groups present in one compound is CH.sub.2CH.sub.2 and the others are a single bond, most preferably all are a single bond,

(81) ##STR00025## are in each occurrence independently

(82) ##STR00026## preferably

(83) ##STR00027## most preferably

(84) ##STR00028##
alternatively one or more of

(85) ##STR00029## are

(86) ##STR00030##
and

(87) n is 1, 2 or 3, preferably 1 or 2.

(88) Further preferred are liquid crystalline media comprising one or more nematogens of formula B-I selected from the from the group of formulae B-I-1 to B-I-, preferably of formula B-I-2 and/or B-I-4, most preferably B-I-4

(89) ##STR00031## wherein the parameters have the meanings given above and preferably R.sup.B1 is alkyl, alkoxy, alkenyl or alkenyloxy with up to 12 C atoms, and L.sup.B1 and L.sup.B1 are independently H or F, preferably one is H and the other H or F and most preferably both are H.

(90) Further preferred are liquid crystalline media comprising one or more nematogens of formula B-II selected from the from the group of formulae B-II-1 and B-II-2, preferably of formula B-II-2 and/or B-II-4, most preferably of formula B-II-1

(91) ##STR00032## wherein the parameters have the meanings given above and preferably R.sup.B21 and R.sup.B22 are independently alkyl, alkoxy, alkenyl or alkenyloxy with up to 12 C atoms, more preferably R.sup.B21 is alkyl and R.sup.B22 is alkyl, alkoxy or alkenyl and in formula B-II-1 most preferably alkenyl, in particular vinyl or 1-propenyl, and in formula B-II-2, most preferably alkyl.

(92) Further preferred are liquid crystalline media comprising one or more nematogens of formula B-III, preferably selected from the group compounds of formulae B-III-1 to B-III-3

(93) ##STR00033## wherein the parameters have the meanings given above and preferably R.sup.B31 and R.sup.B32 are independently alkyl, alkoxy, alkenyl or alkenyloxy with up to 12 C atoms, more preferably R.sup.B31 is alkyl and R.sup.B32 is alkyl or alkoxy and most preferably alkoxy, and L.sup.B31 and L.sup.B32 are independently H or F, preferably one is F and the other H or F and most preferably both are F.

(94) The compounds of formulae B-I to B-III are either known to the expert and can be synthesized according to or in analogy to methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.

(95) The chiral compounds are preferably selected from the group of compounds of formulae C-I to C-III,

(96) ##STR00034##

(97) the latter ones including the respective (S,S) enantiomers,

(98) wherein E and F are each independently 1,4-phenylene or trans-1,4-cyclo-hexylene, v is 0 or 1, Z.sup.0 is COO, COO, CH.sub.2CH.sub.2 or a single bond, and R is alkyl, alkoxy or alkanoyl with 1 to 12 C atoms.

(99) Particularly preferred media according to the invention comprise at least one or more chiral dopants which themselves do not necessarily have to show a liquid crystalline phase and give good uniform alignment themselves.

(100) The compounds of formula C-II and their synthesis are described in WO 98/00428. Especially preferred is the compound CD-1, as shown in table D below. The compounds of formula CIII and their synthesis are described in GB 2 328 207.

(101) Especially preferred are chiral dopants with a high helical twisting power (HTP), in particular those disclosed in WO 98/00428.

(102) Further, typically used chiral dopants are e.g. the commercially available R/S-5011, CD-1, R/S-811 and CB-15 (from Merck KGaA, Darmstadt, Germany).

(103) The above mentioned chiral compounds R/S-5011 and CD-1 and the (other) compounds of formulae C-I, C-II and C-III exhibit a very high helical twisting power (HTP), and are therefore particularly useful for the purpose of the present invention.

(104) The liquid crystalline medium preferably comprises preferably 1 to 5, in particular 1 to 3, very preferably 1 or 2 chiral dopants, preferably selected from the above formula C-II, in particular CD-1, and/or formula C-III and/or R-5011 or S-5011, very preferably, the chiral compound is R-5011, S-5011 or CD-1.

(105) The amount of chiral compounds in the liquid crystalline medium is preferably from 1 to 20%, more preferably from 1 to 15%, even more preferably 1 to 10%, and most preferably 1 to 5%, by weight of the total mixture.

(106) The liquid crystal media according to the present invention may contain further additives like for example stabilizers, inhibitors, chain-transfer agents, co-reacting monomers, surface-active compounds, lubricating agents, wetting agents, dispersing agents, hydrophobing agents, adhesive agents, flow improvers, defoaming agents, deaerators, diluents, reactive diluents, auxiliaries, colourants, dyes, pigments or nanoparticles in usual concentrations. The total concentration of these further constituents is in the range of 0.1% to 10%, preferably 0.1% to 6%, based on the total mixture. The concentrations of the individual compounds used each are preferably in the range of 0.1% to 3%. The concentration of these and of similar additives is not taken into consideration for the values and ranges of the concentrations of the liquid crystal components and compounds of the liquid crystal media in this application. This also holds for the concentration of the dichroic dyes used in the mixtures, which are not counted when the concentrations of the compounds respectively the components of the host medium are specified. The concentration of the respective additives is always given relative to the final doped mixture.

(107) The liquid crystal media according to the present invention consists of several compounds, preferably of 3 to 30, more preferably of 4 to 20 and most preferably of 4 to 16 compounds. These compounds are mixed in conventional way. As a rule, the required amount of the compound used in the smaller amount is dissolved in the compound used in the greater amount. In case the temperature is above the clearing point of the compound used in the higher concentration, it is particularly easy to observe completion of the process of dissolution. It is, however, also possible to prepare the media by other conventional ways, e.g. using so-called pre-mixtures, which can be e.g. homologous or eutectic mixtures of compounds or using so-called multi-bottle-systems, the constituents of which are ready to use mixtures themselves.

(108) Particularly preferred mixture concepts are indicated below: (the acronyms used are explained in Table A).

(109) The mixtures according to the invention preferably comprise one or more compounds of formula I, preferably selected from the group of compounds of formulae Ia to Ic, preferably compounds wherein no group Sp is present, such as the compounds of formulae Ia-1 to Ia-5, Ia-10, Ic-1 to Ic-5 and Ic-10, and in particular the compounds of formulae Ic-3, Ic-4, Ic-5 and Ic-6, most preferably the compounds of formula Ic-3, preferably, the total concentration of compounds of formula I range from approximately 0.01 to approximately 10% by weight, preferably from approximately 0.01 to approximately 5% by weight, and more preferably from approximately 0.01 to approximately 3% by weight, based on the mixture as a whole, one or more compounds, preferably two, three, four, five or more compounds selected from the group of formulae A-I to A-III, preferably one, two, three or more compounds of formula A-I and one or more compounds of formula A-II, or one, two, three or more compounds of formula A-I and one or more compounds of formula A-III, or one, two, three or more compounds of formula A-II and one or more compounds of formula A-III, or, most preferred, one, two, three or more compounds of formula A-I and one or more compounds of formula A-II and one, two, three or more compounds of formula A-III, preferably in a total concentration of 95% or less, more preferably in the range from 50 to 95%, more preferably from 60 to 95%, and most preferably from 70 to 90% by weight of the total mixture, preferably these compounds are selected from one or more compounds of formula A-I (i.e. ether-linked dimers), preferably in a concentration of 40% or less, more preferably of 30% or less, based on component A, particularly preferred one or more compounds of formula A-I-1 to A-I-3, preferably in concentration >5%, in particular from 10 to 30%, based on the mixture as a whole, and/or one or more compounds of formula A-II (i.e. methylene-linked dimers), preferably in a concentration of 40% or less, more preferably of 20% or less, particularly preferred one or more compounds of formula A-II-1 to A-II-4, and especially preferred selected from the group of formulae A-II-1, preferably in concentrations of 5% or more, in particular of 5 to 10% based on the mixture as a whole, and/or one or more compounds of formula A-III (e.g. ester-linked dimers), preferably in a concentration of 90% or less, more preferably of 80% or less, particularly preferred one or more compounds of formula A-III-1 to A-III-11, and especially preferred selected from the group of formulae A-III-5, A-III-7, A-III-8 and A-III-11 preferably in concentrations of 5% or more, in particular of 10 to 30% per compound, based on the mixture as a whole, one, two, three or more compounds selected from the group of formulae B-I to B-III, preferably one or more compounds of formula B-I and one or more compounds of formula B-II, or one or more compounds of formula B-I and one or more compounds of formula B-III, or one or more compounds of formula B-II and one or more compounds of formula B-III, or one or more compounds of formula B-I and one or more compounds of formula B-II and one or more compounds of formula B-III, preferably in a total concentration of 40% or less, preferably in the range from 1 to 40%, more preferably from 3 to 25%, and most preferably from 5 to 15% by weight of the total mixture, preferably these compounds are selected from formulae B-I and/or BII and/or B-III, and especially preferred selected from the group of formulae PP-n-N, PPP-n-N, CC-n-V, CC-n-V1, CEPGI-n-m, PY-n-Om, CCY-n-Om, CPY-n-Om and PYP-n-(O)m, preferably PP-5-N and/or PPP-3-N and/or CC-3-V and/or CC-4-V and/or CC-5-V and/or CC-3-V1 and/or CC-4-V1 and/or CEPGI-3-2 and/or CEPGI-5-2 and/or PY-3-04, preferably in concentrations of 0.1% or more, in particular in the range from 0.1 to 10% per compound, based on the mixture as a whole, and one or more chiral compounds preferably in a total concentration in the range from 0.1 to 20%, in particular from 0.5 to 10%, very preferably 1 to 5% by weight of the total mixture, preferably these compounds are selected from formulae C-I, C-II, and C-III, in particular R-5011 or S-5011 or CD-1, especially preferred they comprise R-5011, S-5011 or CD-1, preferably in a concentration of 1% or more based on the mixture as a whole particularly preferred between 1 and 3%, in particular between 1 and 2%, of R-5011 or S-5011, or another chiral material in a concentration leading to the same cholesteric pitch as R-5011 or S-5011 in the preferred concentrations mentioned.

(110) Further preferred conditions for the mesogenic media are the following. They are fulfilled independently from one another and from the conditions mentioned above. Preferably, however, two, three four or more of these conditions and of the conditions mentioned above are fulfilled simultaneously.

(111) A further, especially preferred condition is that the mixture has a low absolute value of , but preferably is dielectrically positive, especially at the temperatures between T(N,I) and 0.8 T(N,I). Preferably preferably is dielectrically positive at the temperatures from T(N,I) to the temperatures at which the ULH texture is still stable, preferably at least down to 40 C. Preferably, the value of at these temperatures is 3 or less, more preferably in the range from 0 or more to 2 or less. In this respect it is not very important, if the value of becomes negative at lower temperatures, then it preferably is the in the range from between 1 or more to 0 or less.

(112) The liquid crystalline media according to the present invention can be used in liquid crystal displays, such as STN, TN, AMD-TN, temperature compensation, guest-host, phase change or surface stabilized or polymer stabilized cholesteric texture (SSCT, PSCT) displays, in particular in flexoelectric devices, in active and passive optical elements like polarizers, compensators, reflectors, alignment layers, color filters or holographic elements, in adhesives, synthetic resins with anisotropic mechanical properties, cosmetics, diagnostics, liquid crystal pigments, for decorative and security applications, in nonlinear optics, optical information storage or as chiral dopants.

(113) The liquid crystalline media according to the present invention are particularly useful for flexoelectric liquid crystal display. Thus, another object of the present invention is a flexoelectric display comprising a liquid crystal medium according to the present invention.

(114) The inventive mesogenic mixtures can be aligned in their cholesteric phase into different states of orientation by methods that are known to the expert, such as surface treatment or electric fields. For example, they can be aligned into the planar (Grandjean) state, into the focal conic state or into the homeotropic state. Inventive compounds of formula I comprising polar groups with a strong dipole moment can further be subjected to flexoelectric switching, and can thus be used in electro optical switches or liquid crystal displays.

(115) The switching between different states of orientation according to a preferred embodiment of the present invention is exemplarily described below in detail for a sample of an inventive mixture.

(116) The total concentration of all compounds in the media according to this application is 100%.

(117) According to this preferred embodiment, the sample is placed into a cell comprising two plane-parallel glass plates coated with electrode layers, e.g. ITO layers, and aligned in its cholesteric phase into a planar state wherein the axis of the cholesteric helix is oriented normal to the cell walls. This state is also known as Grandjean state, and the texture of the sample, which is observable e.g. in a polarization microscope, as Grandjean texture. Planar alignment can be achieved e.g. by surface treatment of the cell walls, for example by rubbing and/or coating with an alignment layer such as polyimide.

(118) A Grandjean state with a high quality of alignment and only few defects can further be achieved by heating the sample to the isotropic phase, subsequently cooling to the chiral nematic phase at a temperature close to the chiral nematic-isotropic phase transition, and rubbing the cell.

(119) In the planar state, the sample shows selective reflection of incident light, with the central wavelength of reflection depending on the helical pitch and the mean refractive index of the material.

(120) When an electric field is applied to the electrodes, for example with a frequency from 10 Hz to 1 kHz, and an amplitude of up to 12 V.sub.rms/m, the sample is being switched into a homeotropic state where the helix is unwound and the molecules are oriented parallel to the field, i.e. normal to the plane of the electrodes. In the homeotropic state, the sample is transmissive when viewed in normal daylight, and appears black when being put between crossed polarizers.

(121) Upon reduction or removal of the electric field in the homeotropic state, the sample adopts a focal conic texture, where the molecules exhibit a helically twisted structure with the helical axis being oriented perpendicular to the field, i.e. parallel to the plane of the electrodes. A focal conic state can also be achieved by applying only a weak electric field to a sample in its planar state. In the focal conic state, the sample is scattering when viewed in normal daylight and appears bright between crossed polarizers.

(122) The sample in the different states of orientation exhibits different transmission of light. Therefore, the respective state of orientation, as well as its quality of alignment, can be controlled by measuring the light transmission of the sample depending on the strength of the applied electric field. Thereby it is also possible to determine the electric field strength required to achieve specific states of orientation and transitions between these different states.

(123) In the sample, the above-described focal conic state consists of many disordered birefringent small domains. By applying an electric field greater than the field for nucleation of the focal conic texture, preferably with additional shearing of the cell, a uniformly aligned texture is achieved where the helical axis is parallel to the plane of the electrodes in large, well-aligned areas. In accordance with the literature on state of the art chiral nematic materials, such as P. Rudquist et al., Liq. Cryst. 23 (4), 503 (1997), this texture is also called uniformly lying helix (ULH) texture. This texture is required to characterize the flexoelectric properties of the inventive compound.

(124) The sequence of textures typically observed in a sample on a rubbed polyimide substrate upon increasing or decreasing electric field is given below:

(125) ##STR00035##

(126) Starting from the ULH texture, the inventive mesogenic media can be subjected to flexoelectric switching by application of an electric field. This causes rotation of the optic axis of the material in the plane of the cell substrates, which leads to a change in transmission when placing the material between crossed polarizers. The flexoelectric switching of inventive materials is further described in detail in the introduction above and in the examples.

(127) It is also possible to obtain the ULH texture, starting from the focal conic texture, by applying an electric field with a high frequency, of for example 10 kHz, to the sample whilst cooling slowly from the isotropic phase into the cholesteric phase and shearing the cell. The field frequency may differ for different compounds.

(128) The bimesogenic compounds of formula A-I to A-III are particularly useful in flexoelectric liquid crystal displays as they can easily be aligned into macroscopically uniform orientation, and lead to high values of the elastic constant k.sub.11 and a high flexoelectric coefficient e in the liquid crystal medium according to the present invention.

(129) The liquid crystal medium preferably exhibits a k.sub.11<110.sup.10 N, preferably <210.sup.11 N and an average flexoelectric coefficient e>110.sup.11 C/m, preferably >110.sup.1 C./m.

(130) Apart from the use in flexoelectric devices, the inventive media are also suitable for other types of displays and other optical and electro-optical applications, such as optical compensation or polarizing films, color filters, reflective cholesterics, optical rotatory power and optical information storage.

(131) A further aspect of the present invention relates to a display cell wherein the cell walls exhibit hybrid alignment conditions. The term hybrid alignment or orientation of a liquid crystal or mesogenic material in a display cell or between two substrates means that the mesogenic groups adjacent to the first cell wall or on the first substrate exhibit homeotropic orientation and the mesogenic groups adjacent to the second cell wall or on the second substrate exhibit planar orientation.

(132) A flexoelectric display according to a preferred embodiment of the present invention comprises two plane parallel substrates, preferably glass plates covered with a transparent conductive layer such as indium tin oxide (ITO) on their inner surfaces, and a flexoelectric liquid crystalline medium provided between the substrates, characterized in that one of the inner substrate surfaces exhibits homeotropic alignment conditions and the opposite inner substrate surface exhibits planar alignment conditions for the liquid crystalline medium.

(133) Planar alignment can be achieved e.g. by means of an alignment layer, for example a layer of rubbed polyimide or sputtered SiO.sub.x, that is applied on top of the substrate.

(134) Alternatively, it is possible to directly rub the substrate, i.e. without applying an additional alignment layer. For example, rubbing can be achieved by means of a rubbing cloth, such as a velvet cloth, or with a flat bar coated with a rubbing cloth. In a preferred embodiment of the present invention rubbing is achieved by means of a at least one rubbing roller, like e.g. a fast spinning roller that is brushing across the substrate, or by putting the substrate between at least two rollers, wherein in each case at least one of the rollers is optionally covered with a rubbing cloth. In another preferred embodiment of the present invention rubbing is achieved by wrapping the substrate at least partially at a defined angle around a roller that is preferably coated with a rubbing cloth.

(135) Homeotropic alignment can be achieved e.g. by means of an alignment layer coated on top of the substrate. Suitable aligning agents used on glass substrates are for example alkyltrichlorosilane or lecithine, whereas for plastic substrate thin layers of lecithin, silica or high tilt polyimide orientation films as aligning agents may be used. In a preferred embodiment of the invention silica coated plastic film is used as a substrate.

(136) Further suitable methods to achieve planar or homeotropic alignment are described for example in J. Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1, 1-77 (1981).

(137) By using a display cell with hybrid alignment conditions, a very high switching angle of flexoelectric switching, fast response times and a good contrast can be achieved.

(138) The flexoelectric display according to present invention may also comprise plastic substrates instead of glass substrates. Plastic film substrates are particularly suitable for rubbing treatment by rubbing rollers as described above.

(139) Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following examples are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever.

(140) Unless the context clearly indicates otherwise, as used herein plural forms of the terms herein are to be construed as including the singular form and vice versa.

(141) The parameter ranges indicated in this application all include the limit values including the maximum permissible errors as known by the expert. The different upper and lower limit values indicated for various ranges of properties in combination with one another give rise to additional preferred ranges.

(142) Throughout this application, the following conditions and definitions apply, unless expressly stated otherwise. All concentrations are quoted in percent by weight and relate to the respective mixture as a whole, all temperatures are quoted in degrees Celsius and all temperature differences are quoted in differential degrees. All physical properties are determined in accordance with Merck Liquid Crystals, Physical Properties of Liquid Crystals, Status November 1997, Merck KGaA, Germany, and are quoted for a temperature of 20 C., unless expressly stated otherwise. The optical anisotropy (n) is determined at a wavelength of 589.3 nm. The dielectric anisotropy () is determined at a frequency of 1 kHz or if explicitly stated at a frequency 19 GHz. The threshold voltages, as well as all other electro-optical properties, are determined using test cells produced at Merck KGaA, Germany. The test cells for the determination of have a cell thickness of approximately 20 m. The electrode is a circular ITO electrode having an area of 1.13 cm.sup.2 and a guard ring. The orientation layers are SE-1211 from Nissan Chemicals, Japan, for homeotropic orientation (e) and polyimide AL-1054 from Japan Synthetic Rubber, Japan, for homogeneous orientation (.sub.). The capacitances are determined using a Solatron 1260 frequency response analyser using a sine wave with a voltage of 0.3 V.sub.rms. The light used in the electro-optical measurements is white light. A set-up using a commercially available DMS instrument from Autronic-Melchers, Germany, is used here.

(143) Throughout the description and claims of this specification, the words comprise and contain and variations of the words, for example comprising and comprises, mean including but not limited to, and are not intended to (and do not) exclude other components. On the other hand, the word comprise also encompasses the term consisting of but is not limited to it.

(144) It will be appreciated that many of the features described above, particularly of the preferred embodiments, are inventive in their own right and not just as part of an embodiment of the present invention. Independent protection may be sought for these features in addition to, or alternative to any invention presently claimed.

(145) Throughout the present application it is to be understood that the angles of the bonds at a C atom being bound to three adjacent atoms, e.g. in a CC or CO double bond or e.g. in a benzene ring, are 120 and that the angles of the bonds at a C atom being bound to two adjacent atoms, e.g. in a CC or in a CN triple bond or in an allylic position CCC are 180, unless these angles are otherwise restricted, e.g. like being part of small rings, like 3-, 5- or 5-atomic rings, notwithstanding that in some instances in some structural formulae these angles are not represented exactly.

(146) It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Alternative features serving the same, equivalent or similar purpose may replace each feature disclosed in this specification, unless stated otherwise. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(147) All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).

(148) In the foregoing and in the following examples, unless otherwise indicated, all temperatures are set forth uncorrected in degrees Celsius and all parts and percentages are by weight.

(149) The following abbreviations are used to illustrate the liquid crystalline phase behavior of the compounds: K=crystalline; N=nematic; N2=second nematic; S=smectic; Ch=cholesteric; I=isotropic; Tg=glass transition. The numbers between the symbols indicate the phase transition temperatures in C.

(150) In the present application and especially in the following examples, the structures of the liquid crystal compounds are represented by abbreviations, which are also called acronyms. The transformation of the abbreviations into the corresponding structures is straightforward according to the following three tables A to C.

(151) All groups C.sub.nH.sub.2n+1, C.sub.mH.sub.2m+1, and C.sub.lH2.sub.l+1 are preferably straight chain alkyl groups with n, m and I C-atoms, respectively, all groups C.sub.nH.sub.2n, C.sub.mH.sub.2m and C.sub.lH.sub.2l are preferably (CH.sub.2).sub.n, (CH.sub.2).sub.m and (CH.sub.2).sub.l, respectively and CHCH preferably is trans-respectively E vinylene.

(152) Table A lists the symbols used for the ring elements, table B those for the linking groups and table C those for the symbols for the left hand and the right hand end groups of the molecules.

(153) Table D lists exemplary molecular structures together with their respective codes.

(154) TABLE-US-00002 TABLE A Ring Elements C P D DI A 0 AI G GI G(CI) GI(CI) G(1) GI(1) U UI Y 0 M MI N NI np n3f n3fI th thI th2f 0 th2fI o2f o2fI dh K KI L LI F FI 0

(155) TABLE-US-00003 TABLE B Linking Groups n (CH.sub.2).sub.n n is an integer except 0 and 2 E CH.sub.2CH.sub.2 V CHCH T CC W CF.sub.2CF.sub.2 B CFCF Z COO ZI OCO X CFCH XI CHCF O CH.sub.2O OI OCH.sub.2 Q CF.sub.2O QI OCF.sub.2

(156) TABLE-US-00004 TABLE C End Groups Left hand side, used alone or Right hand side, used alone or in combination with others in combination with others -n- C.sub.nH.sub.2n+1 -n C.sub.nH.sub.2n+1 -nO- C.sub.nH.sub.2n+1O -nO OC.sub.nH.sub.2n+1 -V- CH.sub.2CH -V CHCH.sub.2 -nV- C.sub.nH.sub.2n+1CHCH -nV C.sub.nH.sub.2nCHCH.sub.2 -Vn- CH.sub.2CHC.sub.nH.sub.2n -Vn CHCHC.sub.nH.sub.2n+1 -nVm- C.sub.nH.sub.2n+1CHCHC.sub.mH.sub.2m -nVm C.sub.nH.sub.2nCHCHC.sub.mH.sub.2m+1 -N- NC -N CN -S- SCN -S NCS -F- F -F F -CL- Cl -CL Cl -M- CFH.sub.2 -M CFH.sub.2 -D- CF.sub.2H -D CF.sub.2H -T- CF.sub.3 -T CF.sub.3 -MO- CFH.sub.2O -OM OCFH.sub.2 -DO- CF.sub.2HO -OD OCF.sub.2H -TO- CF.sub.3O -OT OCF.sub.3 -A- HCC -A CCH -nA- C.sub.nH.sub.2n+1CC -An CCC.sub.nH.sub.2n+1 -NA- NCCC -AN CCCN Left hand side, used in Right hand side, used in combination with others only combination with others only - . . . n . . . - C.sub.nH.sub.2n - . . . n . . . C.sub.nH.sub.2n - . . . M . . . - CFH - . . . M . . . CFH - . . . D . . . - CF.sub.2 - . . . D . . . CF.sub.2 - . . . V . . . - CHCH - . . . V . . . CHCH - . . . Z . . . - COO - . . . Z . . . COO - . . . ZI . . . - OCO - . . . ZI . . . OCO - . . . K . . . - CO - . . . K . . . CO - . . . W . . . - CFCF - . . . W . . . CFCF

(157) wherein n and m each are integers and three points . . . indicate a space for other symbols of this table.

(158) Preferably, the liquid crystalline media according to the present invention comprise, besides the compound(s) of formula I one or more compounds selected from the group of compounds of the formulae of the following table.

(159) TABLE-US-00005 TABLE D 0 0 00 01 02 03 04 05 06 07 08 09 0

EXAMPLES

(160) The measurement cells have soda-lime glass substrates and are constructed with polyimide alignment layers (AL-3046 from Japan Synthetic Rubber, Japan) with a layer thickness of 50 nm, which have been rubbed perpendicularly to one another. The layer thickness is a uniform 3.0 m. The surface area of the transparent ITO electrodes is 1 cm.sup.2. Cells were filled vertically on a hot plate in the isotropic phase. In order to obtain ULH alignment the cells were cooled from 100 C. to 85 C. at 3 C./min under an applied electric field of 12V.sub.pk-pk at 30 Hz.

Comparative Example 1

(161) The following mixture CM-1 is prepared:

(162) TABLE-US-00006 Compound wt. % R5011 1.7 F-PGI-ZI-7-Z-PP-N 23.8 F-PGI-ZI-9-Z-PU-N 15.5 F-PGI-ZI-9-Z-PUU-N 13.7 F-UIGI-ZI-9-Z-GP-N 25.8 N-PZIP-7-PZP-N 7.6 CGP-32 1.6 CPPC -33 0.5 CC-3-V 3.5 CCY-3-O2 0.4 CCY-4-O2 1.6 CPY-2-O2 1.3 CPY-3-O2 1.3 PY-3-O2 1.7

Example 1

(163) To the mixture CM-1 the compound SA-1 is added in an amount of 2 wt. % resulting in mixture M-1:

(164) ##STR00115##

(165) The mixture M-1 shows in comparison to the mixture CM-1 without SA-1 improved textures in the test cells.

(166) Furthermore, in comparison to CM-1 the amount of alignment defects observed in the test cells is drastically reduced and the dark state is significantly improved.

(167) In summary, SA-1 has a positive effect on the alignment of ULH-mixtures and results in a higher quality dark state in the corresponding test cells.