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 ##STR00139## 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 comprises one, two or more 5-atomic rings, Sp.sup.1 is a spacer group comprising 1, 3 or 5 to 40 C atoms, wherein one or more non-adjacent 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 from OCO, SCO, OCOO, COS, COO and CHCH are adjacent to each other, X.sup.11 is a group selected from COS, SCO, CSS, SCS, CSO, OCS, SCOS, SCSS and S, X.sup.12 independently from X.sup.11 has one of the meanings given for X.sup.11 or is a group selected from COO, OCO, CHCH, CC and O or a single bond, however under the condition that in -X.sup.11-Sp.sup.1-X.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 at least one of MG.sup.11 and MG.sup.12 comprises one, two or more 5-atomic rings, and one or more 6-atomic rings, and at least two of these are optionally linked by a 2-atomic.

3. Bimesogenic compounds according to claim 1, characterized in that both MG.sup.11 and MG.sup.12 comprise one, two or more 5-atomic rings.

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.32III 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

[0059] In a first preferred embodiment [0060] X.sup.11 and X.sup.12 are different from each another and otherwise independently from one another are a linking group as defined above, preferably selected from COO, OCO, CHCH, CC, O, SCO, COS, S, and CO, or a single bond, preferably X.sup.11 is COO or O and X.sup.12 is a single bond or X.sup.11 is COO and X.sup.12 is O, most preferably X.sup.11 is COO and X.sup.12 is a single bond.

[0061] In a further preferred embodiment, which may be different from or identical with any of the preferred embodiments,

[0062] at least one of [0063] MG.sup.11 and MG.sup.12 comprises two or more 5- or 6-atomic rings, at least two of which are linked by a 2-atomic linking group, preferably selected from the group of linking groups COO, OCO, CH.sub.2O, OCH.sub.2, CF.sub.2O and OCF.sub.2.

[0064] In a further preferred embodiment, which may be different from or identical with any of the preferred embodiments, [0065] X.sup.11 is a group selected from COS, SCO, CSS, SCS, COS, SCO, SCOS, SCSS and S, preferably COS, SCO or S, most preferably COS and SCO. [0066] X.sup.12 independently from X.sup.11 has one of the meanings given for X.sup.11 or is a linking group selected from COO, OCO, CHCH, CC and O or a single bond, preferably it is COS or S, most preferably COS and SCO and preferably [0067] -X.sup.11-Sp.sup.1-X.sup.12- is SCO-Sp.sup.1-COS, SCO-Sp.sup.1-S or S-Sp.sup.1-S, more preferably SCO-Sp.sup.1-COS. [0068] Sp.sup.1 is (CH.sub.2).sub.n with [0069] n 1, 3 or an integer from 5 to 15, most preferably an odd (i.e. uneven) integer and, most preferably 7 or 9,

[0070] wherein one or more H atoms in (CH.sub.2).sub.n may independently of each other optionally be replaced by F or CH.sub.3.

[0071] In a further preferred embodiment, which may be different from or identical with any of the preferred embodiments, [0072] X.sup.11 and X.sup.12 both are a single bond and [0073] Sp.sup.1 is (CH.sub.2).sub.n with [0074] n 1, 3 or an integer from 5 to 15, most preferably an odd (i.e. uneven) integer and, most preferably 7 or 9,

[0075] wherein one or more H atoms in (CH.sub.2).sub.n may independently of each other optionally be replaced by F or CH.sub.3.

[0076] Preferred compounds of formula I are compounds in which [0077] MG.sup.11 and MG.sup.12 are independently from one another a group of (partial) formula II

-A.sub.11-(Z.sup.11-A.sup.12).sub.k-II

[0078] wherein [0079] 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.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, preferably a single bond, [0080] one or more of A.sup.11 and A.sup.12 are each independently in each occurrence a comprise a 5-atomic ring, and are preferably selected from thiophene-2,5-diyl, furane-2,5-diyl, thiazole-diyl, thiadiazole-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, preferably F, Cl, CH.sub.3 or CF.sub.3, and the other [0081] 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, preferably F, Cl, CH.sub.3 or CF.sub.3, and [0082] k is 0, 1, 2, 3 or 4, preferably 1, 2 or 3 and, most preferably 1 or 2.

[0083] Especially preferred are compounds of formula I wherein the mesogenic groups MG.sup.11 and MG.sup.12 at each occurrence independently from each other comprise one, two or three rings, which are five- or six-membered rings, preferably one of both or of MG.sup.11 and MG.sup.12 comprise one or more, preferably one, five-membered rings.

[0084] Preferably this five-membered ring, respectively these five-membered rings, are independently of each other selected from the following group of partial formulae

##STR00004##

[0085] wherein preferably [0086] L in each occurrence independently of each other is H, F or Cl, and [0087] r is 1, 2, 3 or 4, preferably 1 or 2.

[0088] A smaller group of preferred mesogenic groups of formula II comprising only 6-membered rings 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 subformulae shown below as well as their mirror images

-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-20

-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

[0089] wherein [0090] Cyc is 1,4-cyclohexene, preferably trans-1,4-cyclohexene, [0091] Phe is 1,4-phenylene, [0092] 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 [0093] Z has one of the meanings of Z.sup.11 as given under partial formula II, at least one is preferably selected from COO, OCO, OCOO, OCH.sub.2, CH.sub.2O, OCF.sub.2 or CF.sub.2O.

[0094] Particularly preferred are the sub-formulae II-1, II-4, II-5, II-7, II-8, II-14, II-15, II-16, II-17, II-18 and II-19.

[0095] In these preferred groups Z in each case independently has one of the meanings of Z.sup.11 as given under formula I. Preferably one of Z is COO, OCO, CH.sub.2O, OCH.sub.2, CF.sub.2O or OCF.sub.2, more preferably COO, OCH.sub.2 or CF.sub.2O, and the others preferably are a single bond.

[0096] Very preferably at least one of the mesogenic groups MG.sup.11 and MG.sup.12 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

##STR00005## ##STR00006##

[0097] wherein

[0098] L is in each occurrence independently of each other F or Cl, preferably F and

[0099] r is in each occurrence independently of each other 0, 1, 2 or 3, preferably 0, 1 or 2.

[0100] The group

##STR00007##

[0101] in these preferred formulae is very preferably denoting

##STR00008##

[0102] furthermore

##STR00009##

[0103] L is in each occurrence independently of each other F or Cl, F.

[0104] In case of compounds with a unpolar group, R.sup.11 and R.sup.12 are preferably alkyl with up to 15 C atoms or alkoxy with 2 to 15 C atoms.

[0105] If R.sup.11 or R.sup.12 is 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.

[0106] 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.

[0107] In case of a compounds with a terminal polar group, R.sup.11 and R.sup.12 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.

[0108] Especially preferably R.sup.11 and R.sup.12 in formula I 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.

[0109] In addition, compounds of formula I containing an achiral branched group R.sup.11 and/or R.sup.12 may occasionally be of importance, for example, due to a reduction in the tendency towards crystallisation. 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.

[0110] The spacer group Sp.sup.1 is preferably a linear or branched alkylene group having 1, 3 or 5 to 40 C atoms, in particular 1, 3 or 5 to 25 C atoms, very preferably 1, 3 or 5 to 15 C atoms, and most 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, SCO, OCOO, COS, COO, CH(halogen)-, CH(CN), CHCH or CC.

[0111] 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 MG.sup.11 and MG.sup.12.

[0112] 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.

[0113] Preferred spacer groups are pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, octadecylene, diethyleneoxyethylene, dimethyleneoxybutylene, pentenylene, heptenylene, nonenylene and undecenylene, for example.

[0114] Especially preferred are inventive compounds of formula I wherein Sp is denoting alkylene with 5 to 15 C atoms. Straight-chain alkylene groups are especially preferred.

[0115] Preferred are spacer groups, which are straight-chain alkylene with odd numbers of C atoms, preferably a having 5, 7, 9, 11, 13 or 15 C atoms, very preferred are straight-chain alkylene spacers having 7, 9, and 11 C atoms.

[0116] In another embodiment of the present invention the spacer groups are straight-chain alkylenes with even numbers of C atoms, preferably having 6, 8, 10, 12 and 14 C atoms. This embodiment is particularly preferred if one of X.sup.11 and X.sup.12 consists of one atom, i.e. is S or O, or of three atoms, e.g. is SCO, SCOS or SCSS, and the other does not consist of one or three C atoms.

[0117] Especially preferred are inventive compounds of formula I wherein Sp is denoting complete 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.

[0118] Preferred are compounds of formula I wherein the mesogenic groups R.sup.11-MG.sup.11-X.sup.11- and R.sup.12-MG.sup.12-X.sup.12- are different from each other. In another embodiment compounds of formula I wherein R.sup.11-MG.sup.11-X.sup.11- and R.sup.12-MG.sup.12-X.sup.12- in formula I are identical to each other.

[0119] Preferred compounds of formula I are selected from the group of compounds of formulae IAs to IHs and IAo to IHo,

##STR00010## ##STR00011## ##STR00012##

[0120] wherein the alkylene spacers (CH.sub.2).sub.n shown are exemplary only and n therein is an integer of 3 or from 5 to 15, preferably 5, 7 or 9, more as shown,

[0121] R.sup.11 and R.sup.12 are independently from each other as defined above, including the preferred meanings of these groups, preferably R.sup.11 is F or CN, preferably R.sup.12 is OCF.sub.3, CF.sub.3, F or CN, more preferably F or CN and most preferably CN and wherein L is in each occurrence independently of each other F, Cl or preferably F or Cl, most preferably F.

[0122] Particularly preferred compounds are selected from the group of formulae given above, which bear 0, 2 or 4 F atoms in lateral positions (i.e. as L).

[0123] In a preferred embodiment of the present invention R.sup.11 is OCF.sub.3 and R.sup.12 is OCF.sub.3, F or CN, preferably OCF.sub.3 or CN and most preferably CN.

[0124] The compounds of formula I 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. A preferred method of preparation can be taken from the following synthesis schemes.

[0125] The compounds of formula I are preferably accessible according to the following general reaction scheme(s).

##STR00013##

[0126] wherein the alkylene spacers (CH.sub.2).sub.n shown are exemplary only and n therein is an integer of 3 or from 5 to 15, preferably 5, 7 or 9, more as shown,

[0127] R independently in each occurrence has one of the meanings given for R.sup.11 and in the second occurrence alternatively may have one of the additional meanings given for R.sup.12 including the preferred meanings of these groups,

[0128] L is F or Cl, preferably F and

[0129] the conditions of the successive reactions are as exemplified under Synthesis Example 1.

[0130] All phenylene moieties shown in this scheme (and in any of the following schemes) may independently of each other be optionally bearing one, two or three, preferably one or two, F atoms or one Cl atom or one Cl and one F atom.

[0131] Another object of the invention is the use of bimesogenic compounds of formula I in liquid crystalline media.

[0132] Compounds of formula I, when added to a nematic liquid crystalline mixture, producing 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.

[0133] 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.

[0134] 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. A 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.

[0135] In this context, liquid crystal mixtures comprising the new and inventive bimesogenic compounds of formula I 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.

[0136] Accordingly, the bimesogenic compounds of formula I 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 I allow the phase behaviour of the second nematic to be tailored to the required temperature.

[0137] The invention thus relates to a liquid-crystalline medium which comprises at least one compound of the formula I.

[0138] Some preferred embodiments of the mixtures according to the invention are indicated below.

[0139] Preferred are compounds of formula I wherein the mesogenic groups MG.sup.11 and MG.sup.12 at each occurrence independently from each other comprise one, two or three six-membered rings, preferably two or three six-membered rings.

[0140] Particularly preferred are the partial formulae II-1, II-4, II-6, II-7, II-13, II-14, II-15, II-16, II-17 and I-18.

[0141] Preferably R.sup.11 and R.sup.12 in formula I 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.

[0142] Typical spacer groups (Sp.sub.1) are for example (CH.sub.2).sub.o, (CH.sub.2CH.sub.2O).sub.pCH.sub.2CH.sub.2, with o being 1, 3 or an integer from 5 to 40, in particular from 1, 3 or 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.

[0143] Preferred are compounds of formula I wherein R.sup.11-MG.sup.11-X.sup.11- and R.sup.12-MG.sup.12-X.sup.12- in formula I are identical.

[0144] The media according to the invention preferably comprise one, two, three, four or more, preferably one, two or three, compounds of the formula I.

[0145] The amount of compounds of formula I in the liquid crystalline medium is preferably from 1 to 50%, in particular from 5 to 40%, very preferably 10 to 30% by weight of the total mixture.

[0146] In a preferred embodiment the liquid crystalline medium according to the present invention comprises additionally one or more compounds of formula III, like those or similar to those known from GB 2 356 629.

R.sup.31-MG.sup.31-X.sup.31-Sp.sup.3-X.sup.32-MG.sup.32-R.sup.32III

[0147] wherein [0148] 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, [0149] MG.sup.31 and MG.sup.32 are each independently a mesogenic group, [0150] 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 [0151] 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

[0152] with the condition that compounds of formula I are excluded.

[0153] The mesogenic groups MG.sup.31 and MG.sup.32 are preferably selected of formula II.

[0154] Especially preferred are compounds of formula III wherein R.sup.31-MG.sup.31-X.sup.31- and R.sup.32-MG.sup.32-X.sup.32- are identical.

[0155] Another preferred embodiment of the present invention relates to compounds of formula III wherein R.sup.31-MG.sup.31-X.sup.31- and R.sup.32-MG.sup.32-X.sup.32- are different.

[0156] Especially preferred are compounds of formula III wherein the mesogenic groups MG.sup.31 and MG.sup.32 comprise one, two or three six-membered rings very preferably are the mesogenic groups selected from formula II as listed below.

[0157] For MG.sup.31 and MG.sup.32 in formula III are particularly preferred are the subformulae II-1, II-4, II-6, II-7, II-13, II-14, II-15, II-16, II-17 and II-18. 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.

[0158] Very preferably the mesogenic groups MG.sup.31 and MG.sup.32 are selected from the formulae IIa to IIo and their mirror images.

[0159] In case of compounds with a non-polar group, R.sup.31 and R.sup.32 are preferably alkyl with up to 15 C atoms or alkoxy with 2 to 15 C atoms.

[0160] If R.sup.31 or R.sup.32 is 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.

[0161] 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.

[0162] In case of a compounds with a terminal polar group, 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.

[0163] Especially preferably R.sup.31 and R.sup.32 in formula III are selected of 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 F, Cl, CN, OCH.sub.3 and OCF.sub.3.

[0164] As for the spacer group Sp.sup.3 in formula III all groups can be used that are known for this purpose to the skilled in the art. The spacer group Sp is 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 CH.sub.2 groups may be replaced by O, S, NH, N(CH.sub.3), CO, OCO, SCO, OCOO, COS, COO, CH(halogen)-, CH(CN), CHCH or CC.

[0165] Typical spacer groups are for example (CH.sub.2).sub.o, (CH.sub.2CH.sub.2O).sub.pCH.sub.2CH.sub.2, CH.sub.2CH.sub.2SCH.sub.2CH.sub.2 or CH.sub.2CH.sub.2NHCH.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.

[0166] Preferred spacer groups are pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, octadecylene, diethyleneoxyethylene, dimethyleneoxybutylene, pentenylene, heptenylene, nonenylene and undecenylene, for example.

[0167] Especially preferred are inventive compounds of formula III wherein Sp.sup.3 is denoting alkylene with 5 to 15 C atoms. Straight-chain alkylene groups are especially preferred.

[0168] In another preferred embodiment of the invention the chiral compounds of formula III comprise at least one spacer group Sp.sup.1 that is a chiral group of the formula IV.

[0169] X.sup.31 and X.sup.32 in formula III denote preferably O, CO, COO, OCO, OCOO or a single bond. Particularly preferred are the following compounds selected from formulae III-1 to III-4:

##STR00014##

[0170] wherein R.sup.31, R.sup.32 have the meaning given under formula III, Z.sup.31 and Z.sup.31-l are defined as Z.sup.31 and Z.sup.32 and Z.sup.32-l are respectively the reverse groups of Z.sup.31 and Z.sup.32-l in formula III and o and r are independently at each occurrence as defined above, including the preferred meanings of these groups and wherein L is in each occurrence independently of each other 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 from which compounds of formula I are excluded.

[0171] Particularly preferred mixtures according to the invention comprise one or more compounds of the formulae III-1 a to III-1e and III-3a to III-3b.

##STR00015##

[0172] wherein the parameters are as defined above.

[0173] In a preferred embodiment of the invention the liquid crystalline medium is consisting of 2 to 25, preferably 3 to 15 compounds of formula III.

[0174] The amount of compounds of formula III in the liquid crystalline medium is preferably from 10 to 95%, in particular from 15 to 90%, very preferably 20 to 85% by weight of the total mixture.

[0175] Preferably, the proportion of compounds of the formulae III-1 a and/or III-1b and/or III-1c and/or III-1e and or III-3a and/or III-3b in the medium as a whole is preferably at least 70% by weight.

[0176] 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.

[0177] Especially preferred are chiral dopants selected from formula IV

##STR00016##

[0178] and formula V

##STR00017##

[0179] including the respective (S,S) enantiomer, wherein E and F are each independently 1,4-phenylene or trans-1,4-cyclo-hexylene, v is 0 or 1, Z 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.

[0180] The compounds of formula IV 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 V and their synthesis are described in GB 2,328,207.

[0181] Especially preferred are chiral dopants with a high helical twisting power (HTP), in particular those disclosed in WO 98/00428.

[0182] 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).

[0183] The above mentioned chiral compounds R/S-5011 and CD-1 and the compounds of formula IV and V exhibit a very high helical twisting power (HTP), and are therefore particularly useful for the purpose of the present invention.

[0184] 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 IV, in particular CD-1, and/or formula V and/or R-5011 or S-5011, very preferably the chiral compound is R-5011, S-5011 or CD-1.

[0185] The amount of chiral compounds in the liquid crystalline medium is preferably from 1 to 20%, in particular from 1 to 15%, very preferably 1 to 10% by weight of the total mixture.

[0186] Further preferred are liquid crystalline media comprising one or more additives selected from the following formula VI

##STR00018##

[0187] wherein

[0188] R.sup.5 is alkyl, alkoxy, alkenyl or alkenyloxy with up to 12 C atoms,

##STR00019##

[0189] L.sup.1 through L.sup.4 are each independently H or F,

[0190] Z.sup.2 is COO, CH.sub.2CH.sub.2 or a single bond,

[0191] m is 1 or 2

[0192] Particularly preferred compounds of formula VI are selected from the following formulae

##STR00020##

[0193] wherein, R has one of the meanings of R.sup.5 above and L.sup.1, L.sup.2 and L.sup.3 have the above meanings.

[0194] The liquid crystalline medium preferably comprises preferably 1 to 5, in particular 1 to 3, very preferably 1 or 2, preferably selected from the above formulae VIa to VIf, very preferably from formulae VIf.

[0195] The amount of suitable additives of formula VI in the liquid crystalline medium is preferably from 1 to 20%, in particular from 1 to 15%, very preferably 1 to 10% by weight of the total mixture.

[0196] The liquid crystal media according to the present invention may contain further additives 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.

[0197] 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.

[0198] Particularly preferred mixture concepts are indicated below: (the acronyms used are explained in Table A).

[0199] The mixtures according to the invention preferably comprise [0200] one or more compounds of formula I in a total concentration in the range from 1 to 50%, in particular from 5 to 40%, very preferably 10 to 30% by weight of the total mixture and/or [0201] one or more compounds of formula III in a total concentration in the range from 10 to 95%, in particular from 15 to 90%, very preferably 20 to 85% by weight of the total mixture, preferably these compounds are selected from formulae III-1a to III-1e and III-3a to III-3b especially preferred they comprise [0202] N-PGI-ZI-n-Z-GP-N, preferably N-PGI-ZI-7-Z-GP-N and/or N-PGI-ZI-9-Z-GP-N preferably in concentrations >5%, in particular 10-30%, based on the mixture as a whole, and/or [0203] F-UIGI-ZI-n-Z-GU-F, preferably F-UIGI-ZI-9-Z-GU-F, preferably in concentrations >5%, in particular 10-30%, based on the mixture as a whole, and/or [0204] F-PGI-O-n-OPPN, preferably F-PGI-O-9-O-PP-, preferably in concentrations of >1%, in particular 1-20%, based on the mixture as a whole, and/or [0205] NPP-O-n-O-PG-OT, preferably NPP-O-7-O-PG-OT, preferably in concentrations of >5%, in particular 5-30%, based on the mixture as a whole, and/or [0206] NPP-O-n-O-GU-F, preferably NPP-O-9-O-GU-F, preferably in concentrations of >1%, in particular 1-20%, based on the mixture as a whole, and/or [0207] F-PGI-O-n-O-GP-F, preferably F-PGI-O-7-O-GP-F and/or F-PGI-O-9-O-GP-F preferably in concentrations of >1%, in particular 1-20%, based on the mixture as a whole, and/or [0208] N-GIGIGI-n-GGG-N, in particular N-GIGIGI-9-GGG-N, preferably in concentration >5%, in particular 10-30%, based on the mixture as a whole, and/or [0209] N-PGI-n-GP-N, preferably N-PGI-9-GP-N, preferably in concentrations >5%, in particular 15-50%, based on the mixture as a whole, and/or [0210] one or more suitable additives of formula VI in a total concentration in the range from 1 to 20%, in particular from 1 to 15%, very preferably 1 to 10% by weight of the total mixture, preferably are these compounds selected from formula VIa to VIf, especially preferred they comprise [0211] PP-n-N, preferably in concentrations of >1%, in particular 1-20%, based on the mixture as a whole, and/or [0212] one or more chiral compounds preferably in a total concentration in the range from 1 to 20%, in particular from 1 to 15%, very preferably 1 to 10% by weight of the total mixture, preferably these compounds are selected from formula IV, V, and R-5011 or S-5011, especially preferred they comprise [0213] R-5011, S-5011 or CD-1, preferably in a concentration of >1%, in particular 1-20%, based on the mixture as a whole.

[0214] The bimesogenic compounds of formula I and the liquid crystalline media comprising them 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.

[0215] The compounds of formula I and the mixtures obtainable thereof are particularly useful for flexoelectric liquid crystal display. Thus, another object of the present invention is a flexoelectric display comprising one or more compounds of formula I or comprising a liquid crystal medium comprising one or more compounds of formula I.

[0216] The inventive bimesogenic compounds of formula I and the mixtures thereof 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 electrooptical switches or liquid crystal displays.

[0217] 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 compound of formula I.

[0218] 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.

[0219] 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.

[0220] 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.

[0221] 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.

[0222] 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.

[0223] A sample of an inventive compound 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.

[0224] In a sample of an inventive compound of formula I, 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.

[0225] The sequence of textures typically observed in a sample of an inventive compound of formula I on a rubbed polyimide substrate upon increasing or decreasing electric field is given below:

##STR00021##

[0226] Starting from the ULH texture, the inventive flexoelectric compounds and mixtures 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.

[0227] 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.

[0228] The bimesogenic compounds of formula I 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.

[0229] The liquid crystal medium preferably exhibits a k.sub.11<110.sup.10 N, preferably <210.sup.11 N, and a flexoelectric coefficient e>110.sup.11 C/m, preferably >110.sup.10 C/m.

[0230] Apart from the use in flexoelectric devices, the inventive bimesogenic compounds as well as mixtures thereof are also suitable for other types of displays and other optical and electrooptical applications, such as optical compensation or polarizing films, color filters, reflective cholesterics, optical rotatory power and optical information storage.

[0231] 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.

[0232] The term homeotropic alignment or orientation of a liquid crystal or mesogenic material in a display cell or on a substrate means that the mesogenic groups in the liquid crystal or mesogenic material are oriented substantially perpendicular to the plane of the cell or substrate, respectively.

[0233] The term planar alignment or orientation of a liquid crystal or mesogenic material in a display cell or on a substrate means that the mesogenic groups in the liquid crystal or mesogenic material are oriented substantially parallel to the plane of the cell or substrate, respectively.

[0234] 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.

[0235] 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.

[0236] 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.

[0237] 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 lecithine, 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.

[0238] 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).

[0239] 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.

[0240] 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.

[0241] Another object of the present invention is that compounds of formula I, when added to a nematic liquid crystalline mixture, produce a phase below the nematic.

[0242] Accordingly, the bimesogenic compounds of formula I according to the present invention allow the second nematic phase to be induced in nematic mixtures that do not show evidence of this phase normally. Furthermore, varying the amounts of compounds of formula I allow the phase behaviour of the second nematic to be tailored to the required temperature.

[0243] Examples for this are given and the mixtures obtainable thereof are particularly useful for flexoelectric liquid crystal display. Thus, another object of the present invention is liquid crystal media comprising one or more compounds of formula I exhibiting a second nematic phase.

[0244] 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.

[0245] 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.

[0246] 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.

[0247] 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.

[0248] 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. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0249] 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).

[0250] The total concentration of all compounds in the media according to this application is 100%.

[0251] 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.

[0252] The following abbreviations are used to illustrate the liquid crystalline phase behavior of the compounds: K=crystalline; N=nematic; N2=second nematic; S or Sm=smectic; Ch=cholesteric; I=isotropic; Tg=glass transition. The numbers between the symbols indicate the phase transition temperatures in C.

[0253] 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 straight forward according to the following three tables A to C.

[0254] 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 l 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.

[0255] 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.

[0256] Table D lists exemplary molecular structures together with their respective codes.

TABLE-US-00002 TABLE A Ring Elements C [00022] P [00023] D [00024] DI [00025] A [00026] AI [00027] G [00028] GI [00029] G(CI) [00030] GI(CI) [00031] G(1) [00032] GI(1) [00033] U [00034] UI [00035] Y [00036] M [00037] MI [00038] N [00039] NI [00040] np [00041] n3f [00042] n3fI [00043] th [00044] thI [00045] th2f [00046] th2fI [00047] o2f [00048] o2fI [00049] dh [00050] K [00051] KI [00052] L [00053] LI [00054] F [00055] FI [00056]

TABLE-US-00003 TABLE B Linking Groups n (CH.sub.2).sub.n n is an integer E CH.sub.2CH.sub.2 except 0 and 2 V CHCH T CC W CF.sub.2CF.sub.2 B CFCF Z COO ZI OCO X CFCH XI CHCF 1O CH.sub.2O O1 OCH.sub.2 Q CF.sub.2O QI OCF.sub.2

TABLE-US-00004 TABLE C End Groups Left hand side, used alone or in Right hand side, used alone or 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...- (CH.sub.2).sub.n -...n... (CH.sub.2).sub.n -...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

[0257] wherein n and m each are integers and three points . . . indicate a space for other symbols of this table.

[0258] 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.

TABLE-US-00005 TABLE D In this table n is an integer selected from 3 and 5 to 15, preferably from 3, 5, 7 and 9, unless explicitly defined otherwise. [00057] F-PGI-O-n- O-GP-F [00058] F-PG-O-n- O-GIP-F [00059] N-PP-O-n- O-GU-F [00060] N-PP-O-n- O-PG-OT [00061] F-PGI-O-n- O-PP-N [00062] R-5011 respectively S-5011 [00063] CD-1 [00064] PP-n-N (n {2; 3; 4; 5; 6; 7}) [00065] PPP-n-N (n {2; 3; 4; 5; 6; 7}) [00066] CC-n-V (n {2; 3; 4; 5; 7}) [00067] CEPGI-n-m (n {2; 3; 4; 5;}; m {2; 3; 4; 5}) [00068] F-UIGI-ZI-n- Z-GU-F [00069] N-PGI-ZI-n- Z-GP-N [00070] F-PGI-ZI-n- Z-GP-F [00071] N-GIGIGI-n- GGG-N [00072] N-PGIUI-n- UGP-N [00073] N-GIUGI-n- GUG-N [00074] N-GIUIP-n- PUG-N [00075] N-PGI-n- GP-N [00076] N-PUI-n- UP-N [00077] N-UIUI-n- UU-N [00078] N-GIGI-n- GG-N [00079] N-PGI(1)-n- G(1)P-N [00080] F-UIGI-n- GU-F [00081] UIP-n-PU [00082] N-PGI-n- GP-N [00083] N-PG(1)-n- GI(1)P-N [00084] N-PZIP-n- PZP-N [00085] N-GIZIP-n- PZG-N [00086] N-PZIGI-n- GZP-N [00087] N-GIZIGI-n- GZG-N [00088] N-UIZIP-n- PZU-N [00089] N-UIZIGI-n- GZU-N [00090] F-PZIP-n- PZP-F [00091] F-GIZIP-n- PZG-F [00092] F-PZIGI-n- GZP-F [00093] F-GIZIGI-n- GZG-F [00094] F-UIZIP-n- PZU-F [00095] F-UIZIGI-n- GZU-F [00096] N-PGIZIP-n- PZGP-N [00097] N-GIGIZIP- n-PZGG-N [00098] N-PGIZIGI- n-GZGP-N [00099] N-GIGIZIGI- n-GZGG-N [00100] N-PO1GI-n- GO1P-N [00101] N-PQIGI-n- GQP-N [00102] F-PO1P-n- PO1P-N (= N-PO1P-n- PO1P-F) [00103] F-PO1GI-n- GO1P-N (= N-PO1GI-n- GO1P-F) [00104] 2(CHCH3) 1-PPZIP-O-n- O-PP-N [00105] TO-GIP-ZI-n- Z-PG-OT [00106] TO-PGI-ZI-n- Z-GP-OT [00107] TO-GIGI -ZI- n-Z-GG-OT [00108] TO-GIP-ZI-n- Z-PP-N [00109] TO-PGI-ZI-n- Z-PP-N [00110] TO-GIGI-ZI-n- Z-GP-N [00111] TO-GIP-O-n- O-PG-OT [00112] TO-P-T-n- T-P-OT [00113] F-PGI-O-n- O-PG-OT [00114] TO-GIP-O-n- O-PP-N [00115] TO-PGI -ZI-n- Z-GP-T [00116] T-PGI -n-GP-T [00117] T-UIQIUI-n- UQU-T [00118] F-PGI-ZI-n- O-PP-N [00119] N-PGI-ZI-n- O-GP-N [00120] N-PGI-ZI-n- GP-N [00121] N-PP-ZI-n- PP-N [00122] N-PGI-ZI-n- GP-N [00123] F-PGI-ZI-n- GP-F [00124] N-PP-SCO-n- COS-PP-N [00125] N-PGI- SCO-n- COS-GP-N [00126] F-PGI- SCO-n- COS-GP-F [00127] F-GIP- SCO-n- COS-PG-F [00128] F-GIP- SCO-n- COS-PP-N [00129] F-GIP-SCO- n-COS- PGG-N [00130] N-PP-S-9- S-PP-N [00131] N-PGI-S-n- S-GP-N

COMPOUND AND SYNTHESIS EXAMPLES

Synthesis Example 1

Preparation of

[0259] ##STR00132##

[0260] The compound of interest is prepared according to the following scheme.

##STR00133##

[0261] Stage 1

[0262] 1-bromo-3-fluoroiodobenzene (27.5 g, 0.092 mol) is added into a round bottom flask with tetrahydrofuran (30 ml) and the mixture is stirred under nitrogen atmosphere until comlegte dissolution. Then diisopropylamine (30 ml) is added and the reaction mixture is placed in an ultrasonic bath for 10 minutes. As catalysts, bis(triphenyl-phosphine)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 a temperature of 20 C. 1,8-nonadiyne (5.0 g, 0.041 mol) is added slowly to the reaction mixture, which is stirred for a further 20 hours. The reaction mixture then is cooled and filtered under vacuum to remove the precipitates. The filtrate is acidified with dilute hydrochloric acid and extracted with diethyl ether. The organic material is separated and washed with water before concentrating it to yield the product as a black solid (19 g). The crude material is purified by column chromatography, eluting the product using a mixture of dichloromethane and petrol. This yields the desired product.

[0263] Stage 2

[0264] 21.5 g (0.040 mol) of the product of the previous stage (Example 1, Stage 1) are dissolved in tetrahydrofuran (600 ml) and passed through a Thalesnano hydrogenator. The conditions for the reactions are 70 bar pressure at a temperature of 60 C.-. The product is obtained as a pale coloured solid.

[0265] Stage 3

[0266] The product from the precious stage (Example 1, Stage 2, 10.0 g, 21.1 mmol), bis-pinacolatodiboron (11.8 g, 46.5 mmol) and dioxane (350 ml) are stirred together. Then potassium acetate (3.1 g, 31.4 mmol) and dichlorobis(tricyclohexylphosphine)palladium (0.77 g, 1.05 mmol) are added. Then the reaction mixture is heating to a temperature of 120 C. for 20 hours. The reaction mixture is then cooled and dilute hydrochloric acid (100 ml) is added. The resultant phases (layers) are separated. The organic material is washed with brine and water before it is dried over magnesium sulphate and concentrated in vacuo. The crude product is purified by column chromatography through silica gel, eluted with a mixture of dichloromethane and petroleum ether (1:1 ratio). The fractions comprising the product are combined and used in the next stage.

[0267] Stage 4

[0268] The product from the previous stage (Example 1, Stage 3, 4.2 g, 7.4 mmol) and 2-bromo-5-cyanothiophene (2.9 g, 15.5 mmol) are stirred together in tetrahydrofuran (35 ml). Then water (3.5 ml) is added and subsequently sodium metaborate octahydrate (6.1 g, 22 mmol) is added. The reaction vessel is evacuated and refilled with nitrogen before adding bis-triphenylphosphinepalladium dichloride (0.26 g, 0.37 mmol) and heating the reaction mixture to a temperature of to 90 C. for 125 hours. Then the reaction mixture is cooled and poured into water (50 ml). The resultant phases are separated and the organic phase is washed first with brine and then with water. The aqueous material is re-extracted with ethyl acetate (three times each time with 50 ml) before combining the organic material. After drying over magnesium sulphate, filtering and concentration in vacuo, the crude product is obtained as a brown solid. The crude product is purified by column chromatography through silica gel, eluting with a mixture of dichloromethane and petroleum ether (3:7 ratio). The fractions containing the product are combined and re-crystallised from isopropyl alcohol to yield the product as a white solid.

##STR00134##

[0269] The product has the following phase range: K 65 N 110 I and an e/K of 2.1 Cm.sup.1N.sup.1 (=2.1 V.sup.1). The e/K has been determined for mixture M-1 as specified below. These properties make the material very useful for mixtures comprising bimesogenic compounds, in particular for use in the USH and in the ULH mode.

[0270] Compound Examples 2 and following.

[0271] The following compounds of formula I are prepared analogously.

##STR00135## ##STR00136## ##STR00137##

[0272] The materials in the above table generally show 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 Example 1

[0273] ##STR00138##

[0274] Phase sequence: K 98 (N 83) I, e/K=2.25 V.sup.1.

USE EXAMPLES, MIXTURE EXAMPLES

[0275] 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.

[0276] 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.

[0277] 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.

[0278] 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 AFG3021 B 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.

[0279] 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]$\tan .Math..Math.=\frac{P_0}{2.Math.}.Math.\frac{e}{K}.Math.\underset{\_}{E}$

[0280] 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.

[0281] 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 1.1

Host Mixture H-0

[0282] The host mixture H-0 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

[0283] 2% of the chiral dopant R-5011 are added to the mixture H-0 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

[0284] The mixture H-1 may be used for the USH-mode. It has a clearing point of 82 C. and a lower transition temperature [T(N2,N)] of 33 C. It has a cholesteric pitch of 301 nm at 35 C. The e/K of this mixture is 1.9 Cm.sup.1N.sup.1 at a temperature of 34.8 C.

Mixture Examples 1.1 to 1.4

[0285] 2% of the chiral dopant R-5011 and 10% of the compound of synthesis example 1 are added to the mixture H-0 leading to the mixture M-1.1, which is investigated for its properties.

Mixture Example 1

Mixture M-1

[0286]

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 Compound 1* 10.0 100.0 Remark: *Compound of Synthesis Example 1.

[0287] This mixture (M-1) is prepared and investigated. It is well suitable for the ULH-mode. It shows an N to N2 transition [T(N,N2)] at 34 C. It has a transition from the nematic phase to the isotropic phase [T(N,I)] at 80.0 C. This mixture (M-1) is well suitable for the USH-mode. It has a cholesteric pitch of 299 nm at 47 C. The e/K of this mixture is 2.1 Cm.sup.1N.sup.1 at a temperature of 47 C.

[0288] 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 H-0, together with 2% R-5011. The results are shown in the following table.

TABLE-US-00009 T(N, I)/ T.sub.low/ P/ e/K/ Ex. Mixt. Compound C. C. nm V.sup.1 C1.1 H-1.0 None 82 33 291 1.80 C1.2 H-1.1 N-PGI-9-GP-N t.b.d. t.b.d. t.b.d. 2.25 C1.3 H-1.21 N-PP-9-PP-N t.b.d. 42 t.b.d. t.b.d. C1.4 H-1.3 F-PGI-O-7-O-GP-F 108 26.5 332 1.70 E1.1 M-1.1 Compound 1* 87 39 301 2.1 E1.2 M-1.2 Compound 2* 82 27 283 2.1 E1.3 M-1.3 Compound 3* t.b.d. 36 289 1.9 E1.4 M-1.4 Compound 4* t.b.d. t.b.d. t.b.d. t.b.d. E1.5 M-1.5 Compound 5* t.b.d. t.b.d. t.b.d. t.b.d. Remarks: *compound n: of Synthesis Example No. n, t.b.d.: to be determined the cholesteric pitch (P) is given at 0.9T(N, I) and e/K is given in V.sup.1 (i.e. Cm.sup.1N.sup.1) at 0.9T(N, I).

[0289] The mixture, mixture M-2, shows an N to N2 transition [T(N,N2)] at 27 C. It has a transition from the nematic phase to the isotropic phase [T(N,I)] at 82.0 C. This mixture (M-2) is well suitable for the USH-mode.

[0290] It has a cholesteric pitch of 283 nm at 47 C.

[0291] The e/K of this mixture is 2.1 Cm.sup.1N.sup.1 at a temperature of 47 C.

[0292] The mixture, mixture M-3, shows an N to N2 transition [T(N,N2)] at 36 C. It has a transition from the nematic phase to the isotropic phase [T(N,I)] at 76 C. This mixture (M-2) is well suitable for the USH-mode.

[0293] It has a cholesteric pitch of 289 nm at 50 C.

[0294] The e/K of this mixture is 2.09 Cm.sup.1N.sup.1 at a temperature of 50 C.