Liquid Crystal Medium and Liquid Crystal Device

The invention relates to a medium comprising one or more bimesogenic compounds and one or more liquid crystalline compounds having a high positive value for dielectrical anisotropy. Further, the invention relates to a method of production of such LC media, to the use of such media in LC devices, in particular, in flexoelectric LC devices and to a flexoelectric LC device comprising a LC medium according to the present invention. Background and Prior Art

The flexoelectric effect is described, for example, by Chandrasekhar, "Liquid Crystals", 2 ^nd edition, Cambridge University Press (1992) and P.G. deGennes et al., "The Physics of Liquid Crystals", 2 ^nd edition, Oxford Science Publications (1995).

Flexoelectric devices utilizing the flexoelectric effect, for example ULFI devices and liquid crystal media that are especially suitable for flexoelectric devices and are known from EP 0 971 016, GB 2 356 629 and Coles, H.J., Musgrave, B., Coles, M.J. and Willmott, J., J. Mater. Chem., 11 , p. 2709-2716 (2001 ).

The Uniform Lying Hel ix (ULFI) has high potential as a fast switching liquid crystal display mode. It is capable of sub millisecond switching at 35°C and possesses an intrinsically high aperture ratio, resulting in an energy efficient display mode.

The materials commonly used in media suitable for the ULFI mode are typically bimesogens. Due to the size of these materials and the presence of polar groups, such as, for example terminal cyano groups, they normally have high rotational viscosities (gi) in the order of many thousands mPa.s at 35°C. The high values for gi are not problematical at increased temperatures of, for example, 35°C, since the switching speed is directly proportional to gi. On the other hand, the values for yl are also proportional to the chiral pitch squared. Since the chiral pitch is normally in the region of 300nm this means that, the switching speeds are still very fast, in the region of 1 millisecond or a few milliseconds.

However, upon reaching lower temperatures, such as room temperature at which the ULH devices are typically operated, the value for gi increases exponentially and even with a short pitch material the switching speeds increase beyond favourable levels.

In order to maintain fast switching speeds at temperatures below 35°C, the value gi of the LC mixtures needs to be reduced and therefore mixture components with lower gi need to be identified.

Accordingly, there is a great demand for new bimesogenic compounds, which exhibit favourable low gi values while preferably at the same time exhibiting:

• favourable e/K (V ¹ ) values,

• favourable broad nematic phase ranges and

• high clearing points. In addition to those requirements, the corresponding LC media should exhibit favourable low gi values while preferably at the same time exhibiting:

• low melting points,

• high clearing points,

· broad chiral nematic phase ranges,

• short temperature independent pitch lengths,

• high flexoelectric coefficients and

• a favourable low temperature stability without crystallization

effects in cells as well as in the bulk.

Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.

Surprisingly, the inventors have found out that one or more of the above- mentioned aims can be achieved by providing a compound according to claim 1. Terms and Definitions

The term "liquid crystal", "mesomorphic compound” or "mesogenic compound" (also shortly referred to as "mesogen") means a compound that under suitable conditions of temperature, pressure and concentration can exist as a mesophase (nematic, smectic, etc.) or in particular as a LC phase. Non-amphiphilic mesogenic compounds comprise for example one or more calamitic, banana-shaped or discotic mesogenic groups. The term "mesogenic group" means in this context, a group with the ability to induce liquid crystal (LC) phase behaviour. The compounds comprising mesogenic groups do not necessarily have to exhibit an LC phase themselves. It is also possible that they show LC phase behaviour only in mixtures with other compounds. For the sake of simplicity, the term "liquid crystal" is used hereinafter for both mesogenic and LC materials.

Throughout the application, unless stated explicitly otherwise, the term “aryl and heteroaryl groups” encompass groups, which can be

monocyclic or polycyclic, i.e. they can have one ring (such as, for example, phenyl) or two or more rings, which may also be fused (such as, for example, naphthyl) or covalently linked (such as, for example, biphenyl) or contain a combination of fused and linked rings. Heteroaryl groups contain one or more heteroatoms, preferably selected from O, N, S and Se. Particular preference is given to mono-, bi- or tricyclic aryl groups having 6 to 25 C atoms and mono-, bi- or tricyclic heteroaryl groups having 2 to 25 C atoms, which optionally contain fused rings and which are optionally substituted. Preference is furthermore given to 5 , 6 or 7-membered aryl and heteroaryl groups, in which, in addition, one or more CH groups may be replaced by N, S or O in such a way that O atoms and/or S atoms are not linked directly to one another. Preferred aryl groups are, for example, phenyl, biphenyl, terphenyl,

[1 ,T:3',1 "]terphenyl-2'-yl, naphthyl, anthracene, binaphthyl,

phenanthrene, pyrene, dihydropyrene, chrysene, perylene, tetracene, pentacene, benzopyrene, fluorene, indene, indenofluorene, spirobifluorene, more preferably 1 ,4- phenylene, 4,4’-biphenylene, 1 , 4- tephenylene.

Preferred heteroaryl groups are, for example, 5 membered rings, such as pyrrole, pyrazole, imidazole, 1 ,2,3-triazole, 1 ,2,4-triazole, tetrazole, furan, thiophene, selenophene, oxazole, isoxazole, 1 ,2 thiazole, 1 ,3-thiazole,

1.2.3-oxadiazole, 1 ,2,4 oxadiazole, 1 ,2,5-oxadiazole, 1 ,3,4-oxadiazole,

1.2.3-thiadiazole, 1 ,2,4-thiadiazole, 1 ,2,5-thiadiazole, 1 ,3,4-thiadiazole, 6 membered rings, such as pyridine, pyridazine, pyrimidine, pyrazine, 1 ,3,5-triazine, 1 ,2,4-triazine, 1 ,2,3-triazine, 1 ,2,4,5-tetrazine, 1 , 2,3,4- tetrazine, 1 ,2,3,5-tetrazine or condensed groups, such as indole, iso- indole, indolizine, indazole, benzimidazole, benzotriazole, purine, naphth- imidazole, phenanthrimidazole, pyridimidazole, pyrazinimidazole, quinoxalinimidazole, benzoxazole, naphthoxazole, anthroxazole, phen- anthroxazole, isoxazole, benzothiazole, benzofuran, isobenzofuran, dibenzofuran, quinoline, isoquinoline, pteridine, benzo-5, 6-quinoline, benzo-6,7-quino 1ine, benzo-7, 8-quinoline, benzoisoquinoline, acridine, phenothiazine, phenoxazine, benzopyridazine, benzopyrimidine, quinoxaline, phenazine, naphthyridine, azacarbazole, benzocarboline, phenanthridine, phenanthroline, thieno[2,3b]thiophene, thieno[3,2b]- thiophene, dithienothiophene, isobenzothiophene, dibenzothiophene, benzothiadiazothiophene or combinations of these groups. The heteroaryl groups may also be substituted by alkyl, alkoxy, thioalkyl, fluorine, fluoroalkyl or further aryl or heteroaryl groups.

In the context of this application, the term“(non-aromatic) alicyclic groups” encompass saturated rings and“heterocyclic groups”

encompass both saturated rings, i.e. those that contain exclusively single bonds and partially unsaturated rings, i.e. those that may also contain multiple bonds. Heterocyclic rings contain one or more heteroatoms, preferably selected from Si, O, N, S and Se. The (non-aromatic) alicyclic and heterocyclic groups can be monocyclic, i.e. contain only one ring (such as, for example, cyclohexane) or polycyclic, i.e. contain a plurality of rings (such as, for example, decahydro-naphthalene or bicyclooctane). Preference is furthermore given to mono-, bi- or tricyclic groups having 3 to 25 C atoms, which optionally contain fused rings and that are optionally substituted. Preference is furthermore given to 5-, 6-, 7- or 8- membered carbocyclic groups in which, in addition, one or more C atoms may be replaced by Si and/or one or more CH groups may be replaced by N and/or one or more non-adjacent CFh groups may be replaced by - O- and/or -S-. Preferred alicyclic and heterocyclic groups are, for example, 5-membered groups, such as cyclopentane, tetrahydrofuran, tetrahydrothiofuran, pyrrolidine, 6-membered groups, such as

cyclohexane, silinane, tetrahydropyran, tetrahydrothiopyran, 1 ,3- dioxane, 1 ,3-dithiane, piperidine, 7-membered groups, such as

cycloheptane and fused groups, such as tetrahydronaphthalene, decahydronaphthalene, indane, bicyclo[1.1.1 ]->pentane-1 ,3-diyl, bicyclo[2.2.2]octane-1 ,4-diyl, spiro[3.3]heptane-2,6-diyl, octa hydro-4, 7- methanoindane-2,5-diyl, more preferably 1 ,4-cyclohexylene 4,4’- bicyclohexylene, 3,17-hexadecahydro-cyclopenta[a]phenanthrene, optionally being substituted by one or more identical or different groups L. Especially preferred aryl-, heteroaryl-, alicyclic- and heterocyclic groups are 1 ,4-phenylene, 4,4’-biphenylene, 1 , 4-terphenylene, 1 ,4- cyclohexylene, 4,4’- bicyclohexylene and 3,17-hexadecahydro- cyclopenta[a]-phenanthrene, optionally being substituted by one or more identical or different groups L.

Preferred substituents of the above-mentioned aryl-, heteroaryl-, alicyclic- and heterocyclic groups (L) are, for example, solubility-promoting groups, such as alkyl or alkoxy and electron-withdrawing groups, such as fluorine, nitro or nitrile.

Particularly preferred substituents are, for example, halogen, CN, NO ₂ , CHs, C2H5, OCHs, OC2H5, COCH3, COC2H5, COOCHs, COOC2H5, CFs, OCFs, OCHF2 or OC2F5.

Above and below "halogen" denotes F, Cl, Br or I.

Above and below, the terms "alkyl", "aryl", "heteroaryl", etc., also encompass polyvalent groups, for example alkylene, arylene,

heteroarylene, etc. The term "aryl" denotes an aromatic carbon group or a group derived there from.

The term "heteroaryl" denotes "aryl" in accordance with the above definition containing one or more heteroatoms.

Preferred alkyl groups are, for example, methyl, ethyl, n propyl, isopropyl, n butyl, isobutyl, s butyl, t butyl, 2 methylbutyl, n pentyl, s pentyl, cyclo- pentyl, n hexyl, cyclohexyl, 2 ethylhexyl, n heptyl, cycloheptyl, n octyl, cyclooctyl, n nonyl, n decyl, n undecyl, n dodecyl, dodecanyl, trifluoro- methyl, peril uoro-n-butyl, 2,2,2-trifluoroethyl, peril uorooctyl, perfluoro- hexyl, etc.

Preferred alkoxy groups are, for example, methoxy, ethoxy, 2-methoxy- ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy, t-butoxy, 2- methylbutoxy, n-pentoxy, n-hexoxy, n-heptoxy, n-octoxy, n-nonoxy, n- decoxy, n-undecoxy, n-dodecoxy.

Preferred alkenyl groups are, for example, ethenyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, and octenyl.

Preferred alkynyl groups are, for example, ethynyl, propynyl, butynyl, pentynyl, hexynyl, octynyl. Oxaalkyl, i.e. where one Chte group is replaced by -0-, 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.

Preferred amino groups are, for example, dimethylamino, methylamino, methylphenylamino, phenylamino. The term“chiral” in general is used to describe an object that is non- superimposable on its mirror image. “Achiral” (non- chiral) objects are objects that are identical to their mirror image. The terms“chiral nematic” and“cholesteric” are used synonymously in this application, unless explicitly stated otherwise.

The term“bimesogenic compound” relates to compounds comprising two mesogenic groups in the molecule. Just like normal mesogens, they can form many mesophases, depending on their structure. In particular, bimesogenic compound may induce a second nematic phase, when added to a nematic liquid crystal medium. Bimesogenic compounds are also known as“dimeric liquid crystals”. The term "director" is known in prior art and means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axes (in case of discotic compounds) of the liquid-crystalline molecules. In case of uniaxial ordering of such anisotropic molecules, the director is the axis of anisotropy.

The term“alignment” or“orientation” relates to alignment (orientation ordering) of anisotropic units of material such as small molecules or fragments of big molecules in a common direction named“alignment direction”. In an aligned layer of liquid-crystalline material, the liquid- crystalline director coincides with the alignment direction so that the alignment direction corresponds to the direction of the anisotropy axis of the material.

The term "planar orientation/alignment", for example in a layer of an liquid-crystalline material, means that the long molecular axes (in case of calamitic compounds) or the short molecular axes (in case of discotic compounds) of a proportion of the liquid-crystalline molecules are oriented substantially parallel (about 180°) to the plane of the layer. The term "homeotropic orientation/alignment", for example in a layer of a liquid-crystalline material, means that the long molecular axes (in case of calamitic compounds) or the short molecular axes (in case of discotic compounds) of a proportion of the liquid-crystalline molecules are oriented at an angle Q ("tilt angle") between about 80° to 90° relative to the plane of the layer.

The terms "uniform orientation" or "uniform alignment" of an liquid- crystalline material, for example in a layer of the material, mean that the long molecular axes (in case of calamitic compounds) or the short molecular axes (in case of discotic compounds) of the liquid-crystalline molecules are oriented substantially in the same direction. In other words, the lines of liquid-crystalline director are parallel.

The wavelength of light generally referred to in this application is 550 nm, unless explicitly specified otherwise. The birefringence Dh herein is defined by the following equation

D ^h = n _e - n ₀ wherein n _e is the extraordinary refractive index and n ₀ is the ordinary refractive index and the effective average refractive index n _av. is given by the following equation nav. = [(2 n ₀ ² + n _e ² )/3] ^1/2 The extraordinary refractive index n _e and the ordinary refractive index n ₀ can be measured using an Abbe refractometer.

In the present application, the term“dielectrically positive” is used for compounds or components with De > 3.0,“dielectrically neutral” with -1.5 < De < 3.0 and“dielectrically negative” with De < -1.5. De is determined at a frequency of 1 kHz and at 20°C. The dielectric anisotropy of the respective compound is determined from the results of a solution of 10 % of the respective individual compound in a nematic host mixture. In case the solubility of the respective compound in the host medium is less than 10 % its concentration is reduced by a factor of 2 until the resultant medium is stable enough at least to allow the determination of its properties. Preferably, the concentration is kept at least at 5 %, however, in order to keep the significance of the results a high as possible. The capacitance of the test mixtures is determined both in a cell with homeo- tropic and with homogeneous alignment. The cell gap of both types of cells is approximately 20 pm. The voltage applied is a rectangular wave with a frequency of 1 kHz and a root mean square value typically of 0.5 V to 1.0 V; however, it is always selected to be below the capacitive threshold of the respective test mixture. De is defined as ( | | - _± ), whereas _av. is ( | | + 2 e _± ) / 3. The dielectric permittivity of the compounds is determined from the change of the respective values of a host medium upon addition of the compounds of interest. The values are extrapolated to a concentration of the

compounds of interest of 100 %. A typical host medium is ZLI-4792 or BL-087 both commercially available from Merck, Darmstadt.

For the present invention, denote trans-1 ,4-cyclohexylene, denote 1 ,4-phenylene.

For the present invention the groups -COO- -C(=0)0- or -CO2- denote an o ester group of formula , and the groups -OCO-, -OC(=0)-, -O2C-

O or -OOC- denote an ester group of formula Furthermore, the definitions as given in C. Tschierske, G. Pelzl and S. Diele, Angew. Chem. 2004, 116, 6340-6368 shall apply to non-defined terms related to liquid crystal materials in the instant application. Detailed description

The invention relates to a medium comprising one or more compounds or of formula I,

R ^{1 1} _A ^{1 1} (-Z ^{1 1} -A ¹² -) _P -X ^{1 1} -Sp ^{1 1} -X ⁱ² -(A ¹³ -Z ¹² -) _q A ¹⁴ -R ¹² wherein

R ¹¹ and R ¹² denotes independently from another H, F, Cl, CN, NO2,

NCO, NCS or a straight-chain or branched alkyl group, which may be unsubstituted, mono- or polysubstituted by halogen or CN and in which one or more non- adjacent and non-terminal CFI2 groups may be replaced, in each occurrence independently from one another, by -0-, -S-, -NH-, -N(CH ₃ )-, -CO-, -COO-, - OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -CºC- in such a manner that oxygen atoms are not linked directly to one another, A ¹¹ to A ¹⁴ each independently in each occurrence denote, 1 ,4- phenylene, wherein in addition one or more CFI groups may be replaced by N, trans-1 ,4-cyclo- hexylene in which, in addition, one or two non- adjacent CFI2 groups may be replaced by O and/or S, 1 ,4-cyclohexylene, naphthalene-2, 6-diyl, decahydro- naphthalene-2,6-diyl, 1 ,2,3,4-tetrahydro-naphthalene- 2, 6-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, wherein one or more FI atoms may be substituted by F or Cl, Z ^{1 1} and Z ¹² are, independently of each other in each occurrence, a single bond, -COO-, -OCO-, -0-C0-0-, -OCH2-, -CH2O-, -OCF2-, -CF2O-, -CH2CH2-, -(CH ₂ ) ₄ -, -CF2CF2-, -CH=CH-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or

-CºC-, optionally substituted with one or more of F, S and/or Si, p and q is each and independently 0, 1 , 2, 3 or 4,

Sp ^{1 1} is a spacer group comprising 1 , 3 or 5 to 40 C atoms, wherein one or more non-adjacent and non-terminal CFI2 groups may also be replaced by -0-, -S-, -NH-, -N(CH ₃ )-, -CO-, -O-CO-, -S-CO-, -O-COO-, -CO-S-, -CO-O-, -CF2-, -CF2O-, -OCF2- -C(OH)-, -CH(alkyl)-, -CH(alkenyl)-,-CH(alkoxyl)-, -CH(oxaalkyl)-, -CH=CH- or -CºC-, however in such a way that no two O-atoms are adjacent to one another and no two groups selected from -O-CO-, -S-CO-, -O-COO-, -CO-S-, -CO-O- and -CFI=CFI- are adjacent to each other,

X ^{1 1} and X ¹² are independently from one another selected from a single bond, -CO-O-, -O-CO-, -O-COO-, -0-,

-CH=CH-, -CºC-, -CF2-O-, -O-CF2-, -CF2-CF2-,

-CH2-O-, -O-CH2-, -CO-S-, -S-CO-, -CS-S-, -S-CS-,

-S-CSS- and -S-, wherein in -X ^{1 1} -Sp ¹ -X ¹² - respectively two O atoms, two -CFI=CFI- groups and two groups selected from -O-CO-, -S-CO-, -O-COO-, -CO-S- and -CO-O- are not linked directly to one another, and one or more compounds of formula A or F, wherein

R ²¹ denotes alkyl, alkoxy, oxaalkyl or alkoxyalkyl having 1 to 9 C atoms or alkenyl or alkenyloxy having 2 to 9 C atoms, all of which are optionally fluorinated,

Z ²¹ denotes -CH2CH2-, -CF2CF2-, -COO-, trans- CFI=CFI-, trans-CF=CF~, -CFI2O- or a single bond,

preferably -CFI2CFI2-, -COO-, trans- CFI=CFI- or a single bond, particularly preferably -C00-, frans-CH=CH- or a single bond,

X° F, Cl, CN, halogenated alkyl or alkoxy having 1 to 6 C atoms or halogenated alkenyl or alkenyloxy having 2 to 6 C atoms, and

L ²¹ to L ²⁴ each, independently of one another denotes H or F.

In case of compounds of formula I with one non-polar groups, R ¹¹ and/or R ¹² is preferably each and independently, alkyl, alkoxy, alkenyl or alkinyl, preferably alkyl, alkoxy, with up to 15 C atoms, preferably 2 to 10 C atoms, more preferably 2 to 5 C atoms.

In addition, compounds of formula I containing an achiral branched group R ¹¹ and R 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.

In case of a compounds of formula I with a terminal polar group, R ¹¹ and/or R ¹² is, preferably each and independently, selected from CN, NO2, NCS, NCO, halogen, OCH3, OCN, SCN, COR ^x , COOR ^x or a mono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 C atoms. R ^x is

optionally fluorinated alkyl with 1 to 4, preferably 1 to 3 C atoms. Halogen is preferably F or Cl.

Especially preferred are compounds of formula I wherein the respective pairs of mesogenic groups -A ¹¹ (-Z ¹¹ -A ¹² -) _p - or -(A ¹³ -Z ¹² -) _q A ¹⁴ - each comprise one, two or three six-atomic rings, more preferably -A ¹¹ (-Z ¹¹ - A ¹² -) _p - or -(A ¹³ -Z ¹² -) _q A ¹⁴ - each comprise two six-atomic rings or -A ¹¹ (-Z ¹¹ -A ¹² -) _p - comprises two six-atomic rings and -(A ¹³ -Z ¹² -) _q A ¹⁴ - comprises three six-atomic rings or -A ¹¹ (-Z ¹¹ -A ¹² -) _p - comprises three six-atomic rings and or -(A ¹³ -Z ¹² -) _q A ¹⁴ - comprises two six-atomic rings. In a preferred embodiment, -A ¹¹ (-Z ¹¹ -A ¹² -) _p - or -(A ¹³ -Z ¹² -) _q A ¹⁴ - of the compounds of formula I are selected as such that both mesogenic groups are mirror images of each other, for example if -A ¹¹ (-Z ¹¹ -A ¹² -) _p - denotes -Phe-PheL- then

-(A ¹³ -Z ¹² -) _q A ¹⁴ -denotes -PheL-Phe-.

In another preferred embodiment, -A ¹¹ (-Z ¹¹ -A ¹² -) _p - or -(A ¹³ -Z ¹² -) _q A ¹⁴ - of the compounds of formula I are selected as such that both mesogenic groups are different. Further preferred are compounds of formula I wherein

Sp ¹¹ denotes -(CH2) _n - with n an integer from 1 to 15, wherein one or more -CFte- groups may be replaced by -CO-, preferably an uneven integer, more preferably 3, 5, 7, 9, 11 or 13,

Further preferred compounds of formula I are those wherein

-X ¹¹ -Sp ¹¹ -X ⁱ² _ is -Sp ¹¹ -, -Sp ¹¹ -0-, -Sp ¹¹ -C0-0-, -Sp ¹¹ -0-C0-,

-Sp ¹¹ -CF ₂ 0-, -CO-O-Sp ¹¹ , -0-C0-Sp ¹¹ ,-0-Sp ¹¹ -,

-OCF ₂ -Sp ¹¹ -, -0-Sp ¹¹ -0-, -0CF ₂ -Sp ¹¹ -CF ₂ 0-,

-0-Sp ¹¹ -C0-0-, -0-Sp ¹¹ -0-C0-, -0-C0-Sp ¹¹ -0-, -O- C0-Sp ¹¹ -0-C0-, -CO-O-Sp ¹¹ -0- or -CO-O-Sp ¹¹ -CO- O-, however under the condition that in -X ¹¹ -Sp ¹¹ -X ¹² - no two O-atoms are adjacent to one another, no two -

CFI=CFI- groups are adjacent to each other and no two groups selected from -O-CO-, -S-CO-, -O-COO-, -CO- S-, -CO-O- and -CFI=CFI- are adjacent to each other. In a preferred embodiment, compounds of formula I are selected from the compounds of formula IA,

R ^H -A ^{1 1} -A ¹² -(CH ₂ ) _a -A ^{1 3} -A ¹⁴ -R ¹² IA wherein

R ^{1 1} and R ¹² denote independently FI, F, Cl, CN, or a straight-chain or branched alkyl group, which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CFh groups to be replaced, in each occurrence independently from one another, by -0-, -S-,

-NH-, -N(CH ₃ )-, -CO-, -COO-, -OCO-, -0-C0-0-,

-S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -CºC- in such a manner that oxygen atoms are not linked directly to one another,

preferably F, Cl, CN, a straight-chain or branched alkyl or alkoxy group which may be unsubstituted, mono- or polysubstituted by halogen or CN,

more preferably F, CN or OCF ₃ ,

A ^{1 1} to A ¹⁴ denote independently in each occurrence a aryl-,

heteroaryl-, alicyclic- and heterocyclic group,

preferably 1 ,4-phenylene, wherein in addition one or more CH groups may be replaced by N, 1 ,4-bicyclo- (2,2,2)-octylene, 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, wherein one or more H atoms may be substituted by F or Cl,

more preferably each independently in each

occurrence 1 ,4-phenylene, wherein in addition one or more CH groups may be replaced by N or trans-1 ,4- cyclohexylene in which, in addition, one or two non- adjacent Chte groups may be replaced by O and/or S, it being possible for both ring groups to be

unsubstituted, mono-, di-, tri- or tetrasubstituted with

F, Cl, CN or alkyl, alkoxy, alkylcarbonyl or

alkoxycarbonyl groups, wherein one or more H atoms may be substituted by F or Cl, a denotes an integer from 1 to 15, preferably an odd (i.e.

uneven) integer and, more preferably 3, 5, 7, 9 or 1 1 .

Preferred compounds of formula IA are selected from compounds in which the groups (-A ¹¹ -A ¹² -) and (-A ¹³ -A ¹⁴ -) are each and independently selected from the following groups

-Phe-Phe- MG1

-PheL-PheL- MG2

-Phe-PheL- MG3 -PheL-Phe- MG4 wherein,

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, NO2 or an optionally fluorinated alkyl, alkoxy or alkanoyl group with 1 to 7 C atoms, very preferably F, Cl, CN, OH, NO2, CH3, C2H5, OCHs, OC2H5, COCH3, COC2H5, COOCHs,

COOC2H5, CF3, OCF3, OCHF2, OC2F5, in particular F, Cl, CN, CH3, C2H5, OCH3, COCH3 and OCF3, most preferably F, Cl, CHs, OCHs and COCHs and

Cyc is 1 ,4-cyclohexylene.

Preferred are compounds of formula IA wherein the groups (R ¹¹ -A ¹¹ -A ¹² -) and (-A ¹³ -A ¹⁴ -R ¹² ) in formula IA are identical or mirror images. Likewise preferred are compounds of formula IA wherein (R ¹¹ -A ¹¹ -A ¹² -) and (-A ¹³ -A ¹⁴ -R ¹² ) in formula IA are different.

Further preferred compounds of formula IA are indicated below:

wherein

n denotes an integer from 1 to 15, preferably an odd (i.e.

uneven) integer and, more preferably 3, 5, 7, 9 or 11.

In the mixture according to the present invention, the utilization of compounds of formula IA, is especially useful in order to further improve the switching speeds whilst maintaining a good phase range and a favorable value for e/K.

In a preferred embodiment, the compounds of formula I are preferably selected from the compounds of formula IB,

R ^{i i} -A ¹¹ -A ¹² -(A ¹³ ) _b -X ¹¹ -(CH ₂ ) _C -X ¹² -A ¹⁴ -A ¹⁵ -A ¹⁶ - R ¹² IB wherein R ^{1 1} and R ¹² have each and independently from another one of the meanings as given for R ^{1 1} and R ¹² under formula IA,

A ^{1 1} to A ¹⁶ have each and independently from another one of the meanings as given for A ^{1 1} to A ¹⁴ under formula IA,

X ^{1 1} and X ¹² are each independently in each occurrence,

-COO-, -OCO-, -0-C0-0-, -CF2-O-, -O-CF2-, -OCH2-, - CH2O-, -CH2CH2-, -(CH ₂ ) ₄ -, -CF2CF2-, -CH=CH-, -

CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or -CºC-, optionally substituted with one or more of F,

preferably -COO-, -OCO- or -O-CO-O-,

more preferably -COO- or -OCO-, b denotes an integer from 1 to 15, preferably an odd (i.e.

uneven) integer and, more preferably 3, 5, 7 or 9 and c denotes 0 or 1 , preferably 0. Preferred compounds of formula IB are selected from compounds in which c denotes 0 and the group (-A ^{1 1} -A ¹² -) is selected from the groups MG1 to MG4 as given above.

Further preferred compounds of formula IB are selected from compounds in which c denotes 1 and the groups (-A ¹⁴ -A ¹⁵ -A ¹⁶ -) and (-A ^{1 1} -A ¹² -A ¹³ -) are each and independently selected from the following groups

-Phe-Phe-Phe- MG5

-Phe-Phe-PheL- MG6

-Phe-PheL-Phe- MG7

-PheL-Phe-Phe- MG8

-PheL-Phe-PheL- MG9

-PheL-PheL-Phe- MG10

-PheL-PheL-PheL- MG1 1 wherein Phe, PheL an L have one of the meanings given above for the groups MG-1 to MG-4.

Further preferred compounds of formula IB are selected from compounds in which c denotes 0 and the group (-A ¹¹ -A ¹² -) is selected from the groups MG1 to MG4 as given above and in which the group (-A ¹⁴ -A ¹⁵ - A ¹⁶ -) is selected from the groups MG5 to MG1 1 .

Especially preferred compounds of formula IB are selected from the group of compounds of the following formulae,

In the mixture according to the present invention, the utilization of compounds of formula IB, is especially useful in order to achieve high stabilities, favourable high clearing points and broad phase ranges, as well as, low appearances of the nematic twist-bend phase.

In a further preferred embodiment, the compounds of formula I are selected from the compounds of formula IC,

R ^{i i} -A ^{1 1} -A ¹² -X ^{1 1} -(CH ₂ ) _d -X ¹² -A ¹³ -A ¹⁴ -R ¹² IC wherein

R ^{1 1} and R ¹² have each and independently one of the meanings as given above for R ^{1 1} under formula IA,

A ^{1 1} to A ¹⁴ have each and independently one of the meanings as given above for A ^{1 1} under formula IA, X ^{1 1} and X ¹² are each independently in each occurrence,

-COO-, -OCO-, -0-C0-0-, -OCH2-, -CH2O-, -CH2CH2-, -(CH ₂ ) ₄ -, -CF2-0-, -0-CF ₂ -,-CF ₂ CF ₂ -, -CH=CH-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or -CºC-, optionally substituted with one or more of F,

preferably -COO-, -OCO- or -O-CO-O-,

more preferably -COO- or -OCO-. denotes an integer from 1 to 15, preferably an odd (i.e. uneven) integer and, more preferably 3, 5, 7 or 9. Preferred compounds of formula IC are selected from compounds in which the groups (-A ¹¹ -A ¹² -) and (-A ¹³ -A ¹⁴ -) are each and independently selected from the groups of MG1 to MG4 as given above.

Especially preferred compounds of formula IC are selected from the group of compounds of the following formulae:

- symmetrical ones (IC-a and IC-b):

- non-symmetrical ones (IC-c):

In the mixture according to the present invention, the utilization of compounds of formula IC, is especially useful in order to reduce the nematic twist bend phase whilst maintaining favorable values for e/K.

In further preferred embodiment, the compounds or formula I are selected from the compounds of formula ID,

R ⁱ 1 -A ¹¹ -X ¹¹ -(CH ₂ ) _e -X ¹² -A ¹² -(A ¹³ ) _f -R ¹² ID wherein

R ¹¹ and R ¹² have each and independently one of the meanings as given above for R ¹¹ under formula IA,

A ¹¹ to A ¹³ have each and independently one of the meanings as given above for A ¹¹ under formula IA,

X ¹¹ and X ¹² are each independently in each occurrence,

-COO-, -OCO-, -0-C0-0-, -OCH2-, -CH2O-, -CH2CH2-, -(CH ₂ ) ₄ -, -CF2-O-, -O-CF2-, -CF2CF2-, -CH=CH-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or -CºC-, optionally substituted with one or more of F,

preferably -COO-, -OCO- or -O-CO-O-,

more preferably -COO- or -OCO-, e denotes an integer from 1 to 15, preferably an odd (i.e.

uneven) integer and, more preferably 3, 5, 7 or 9 and f denotes 0 or 1 .

Especially preferred are compounds of formula ID wherein the A ¹¹ is selected from the following group of formulae ID-a’ to ID-e’ and the mirror images of formulae ID-c’ and ID-d’

ID-a’

ID-b’

ID-c’

I D-d’

ID-e’

Preferably R ¹¹ and R ¹² in formula ID are selected of H, F, Cl, CN, NO ₂ , OCHs, COCH3, COC2H5, COOCHs, COOC2H5, CFs, C2F5, OCFs, OCHF2 and OC2F5, in particular of H, F, Cl, CN, OCFI3 and OCF3, especially of H, F, CN and OCF _3.

Preferred compounds of formula ID are selected from the group of compounds of formulae ID-A to ID-D, preferably of formulae ID-A and/or ID-C, most preferably of formula ID-C,

wherein

LG ^{1 1} is X ^{1 1} -(CH ₂ ) _z -X ¹² ,

(F)o denotes FI and

(F)i denotes F. and the other parameters have the respective meanings given above including the preferred meanings. Preferably X ¹¹ -(CH2) _Z -X ¹² in the compounds of formula ID denotes -O- C0-(CH ₂ ) _n -C0-0-, -0-(CH ₂ ) _n -0- or -(Chtej _n -, more preferably -O-CO- (CH ₂ ) _n -C0-0-, wherein n denotes 3, 5, 7 or 9,

Particularly preferred compounds of formula ID-A are selected from the group of compounds of formulae ID-A-1 to ID-A-3

wherein the parameters have the respective meanings given above including the preferred meanings.

Particularly preferred compounds of formula ID-B are selected from the group of compounds of formulae ID-B-1 to ID-B-3

wherein the parameters have the respective meanings given above including the preferred meanings.

Compounds of formula ID-C are very much preferred. And of these particularly preferred compounds are selected from the group of compounds of formulae ID-C-1 to ID-C-3

wherein the parameters have the respective meanings given above including the preferred meanings. In a further preferred embodiment, the compounds of formula I are selected from the group of compound of formula IE, R ^{i i} -A ^{1 1} -A ¹² -(CH ₂ ) _g -X ¹² -A ¹³ -A ¹⁴ -(A ¹⁵ ) _h -R ¹² IE wherein

R ¹¹ and R ¹² have each and independently one of the meanings as given above for R ¹¹ under formula IA,

A ¹¹ to A ¹⁵ have each and independently one of the meanings as given above for A ¹¹ under formula IA,

X ¹² denotes -0-, -COO-, -OCO-, -0-C0-0-, -OCH2-, -CH2O,

-CH2CH2-, -(CH ₂ ) ₄ -, -CF2-O-, -O-CF2-, -CF2CF2-, -CH=CH-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or -CºC-, optionally substituted with one or more of F, S and/or Si,

preferably -0-, -COO-, -OCO- or -O-CO-O-,

more preferably -0-, -COO- or -0C0-, most preferably

-COO- or -0C0-, h denotes 0 or 1 and denotes an integer from 1 to 15, preferably an odd (i.e. uneven) integer and, more preferably 3, 5, 7 or 9.

Preferred compounds of formula IE are selected from compounds in which the groups (-A ¹¹ -A ¹² -) and (-A ¹³ -A ¹⁴ -) are each and independently selected from the groups of MG1 to MG4 as given above.

Further preferred are compounds of formula IE wherein h denotes 0 and the groups (-A ^{1 1} -A ¹² -) and (-A ¹³ -A ¹⁴ -(A ¹⁵ )h) in formula IE are not identical or not mirror images or wherein h denotes 1 ln particular preferred compounds of formula IE are selected from the group of compounds of the following formulae,

In the mixture according to the present invention, the utilization of compounds of formula IE is especially useful in order to achieve high clearing points and also favorable values for e/K. In a further preferred embodiment, the compounds of formula I are selected from the group of compounds of formula IF,

R ^{i i} -A ^{1 1} -Z ^{1 1} -A ¹² -(Z ¹² -A ¹³ )i-(CH ₂ )r(A ¹⁴ -Z ¹³ -)k-A ¹⁵ -Z ¹⁴ -A ¹⁶ -R ¹² IF wherein

R ^{1 1} and R ¹² have each and independently one of the meanings as given above for R ^{1 1} under formula IA,

A ^{1 1} to A ¹⁶ have each and independently one of the meanings as given above for A ^{1 1} under formula IA,

Z ^{1 1} to Z ¹⁴ each and independently denotes -COO-, -OCO-,

-0-C0-0-, -OCH2-, -CH2O-, -OCF2-, -CF2O-, -CH2CH2-, -(CH ₂ ) ₄ -,-CF ₂ CF ₂ -, -CH=CH-, -CF=CF-, -CH=CH-COO-, -OCO-CFI=CFI- or -CºC-, optionally substituted with one or more of F, S and/or Si or a single bond,

preferably -COO-, -OCO-, -O-CO-O-, -OCF2-, -CF2O- or a single bond

more preferably -COO-, -OCO-, -OCF2-, -CF2O- or a single bond,

with the proviso that at least one of Z ^{1 1} to Z ¹⁴ is not a single bond,

J denotes an integer from 1 to 15, preferably an odd (i.e.

uneven) integer and, more preferably 3, 5, 7 or 9 and i and k each and independently denotes 0 or 1 .

Preferred compounds of formula IF are selected from compounds in which at least one of the groups -A ^{1 1} -Z ^{1 1} -A ¹² -(Z ¹² -A ^{1 3} )i-,

-(A ¹⁴ -Z ^{1 3} -) _k -A ^{1 5} -Z ¹⁴ -A ¹⁶ - are is selected from the groups of MGa to MGn and their mirror images wherein

L is in each occurrence independently of each other preferably F, Cl, CN or an optionally fluorinated alkyl, alkoxy or alkanoyl group with 1 to 7 C atoms, very preferably F, Cl, CN, CFb, C2FI5, OCFI3, OC2FI5, COCFI3, COC2H5, COOCH3, COOC2H5, CF3, OCF3, OCHF2, OC2F5, in particular F, Cl, CN, CFI3, C2FI5, OCFI3, COCFI3 and OCF3, most preferably F, Cl, CFI3, OCFI3 and COCFI3 and r is in each occurrence independently of each other 0, 1 , 2, 3 or 4, preferably 0, 1 or 2.

or

wherein L is preferably F, Cl, CFI3, OCFI3 and COCFI3.

Further preferred are compounds of formula IF wherein the groups -A ¹¹ - Z ^{1 1} -A ¹² -(Z ¹² -A ^{1 3} )i- and -(A ¹⁴ -Z ¹³ -) _k -A ¹⁵ -Z ¹⁴ -A ¹⁶ - in formula IF are identical or mirror images with the proviso that at least one of Z ^{1 1} to Z ¹⁴ is not a single bond.

Further preferred are compounds of formula IF, wherein i and k both denote 1 , more preferably one of i and k denotes 0 and the other 1 , most preferably i and k both denote 0.

Especially preferred compounds of formula IF are selected from the group of compounds of the following formulae,

wherein R ⁷¹ and R ⁷² , each and independently denote F or CN.

The compounds of formula I and their subformulae IA to IF 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, Flouben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart. In a preferred embodiment, the LC medium comprises in addition to one or more compounds of formula I, one or more compounds of formula A that are selected from the group consisting of the following formulae:

in which A ²¹ , R ²¹ , X°, L ²¹ and L ²² have the meanings given in formula A, L ²³ and L ²⁴ each, independently of one another, are FI or F, and X° is preferably F, CF3, OCF3 or CN. Particularly preferred are compounds of formulae A1 and A2. Particularly preferred compounds of formula A1 are selected from the group consisting of the following subformulae:

in which R ²¹ , X°, L ²¹ and L ²² have the meaning given in formula A1 , L ²³ , L ²⁴ , L ²⁵ and L ²⁶ are each, independently of one another, H or F, and X° is preferably F.

Very particularly preferred compounds of formula A1 are selected from the group consisting of the following subformulae:

In which R ²¹ is as defined in formula A1.

Particularly preferred compounds of formula A2 are selected from the group consisting of the following subformulae:

in which R ²¹ , X°, L ²¹ and L ²² have the meaning given in formula A2, L ²³ , L ²⁴ , L ²⁵ and L ²⁶ each, independently of one another, are FI or F, and X° is preferably F or CN.

Very particularly preferred compounds of formula A2 are selected from the group consisting of the following subformulae: in which R ²¹ is as defined in formula A2.

Particularly preferred compounds of formula A3 are selected from the group consisting of the following subformulae:

in which R ²¹ , X°, L ²¹ and L ²² have the meaning given in formula A3, and X° is preferably F. Particularly preferred compounds of formula A4 are selected from the group consisting of the following subformulae:

in which R ²¹ is as defined in formula A4.

Particularly preferred compounds of formula F are selected from the group consisting of the following formulae:

in which R ²¹ , X°, L ²¹ and L ²² have the meaning given in formula F, L ²⁵ and L ²⁶ are each, independently of one another, H or F, and X° is preferably F, CFs or OCF3.

Very particularly preferred compounds of formula F1 -F3 are selected from the group consisting of the following subformulae:

In which R ²¹ is as defined in formula F1 .

In a further preferred embodiment, the medium in accordance with the present invention optionally comprises one or more chiral dopants, especially when utilized in a flexoelectric device.

The chiral compounds induce a chiral nematic texture with a pitch (Po), which is in a first approximation inversely proportional to the

concentration (c) of the chiral material used. The constant of

proportionality of this relation is called the helical twisting power (FITP) of the chiral substance and defined by the following equation

HTP º 1 / (c Po) (1 ) wherein

c is concentration of the chiral compound. For example, a uniform lying helix texture is realized using a chiral nematic liquid crystal with a short pitch, typically in the range from 0.2 pm to 1 pm, preferably of 1.0 pm or less, in particular of 0.5 pm or less, which is unidirectional aligned with its helical axis parallel to the substrates, e. g. glass plates, of a liquid crystal cell. In this configuration, the helical axis of the chiral nematic liquid crystal is equivalent to the optical axis of a birefringent plate. Preferred are chiral dopants with a high helical twisting power (HTP), in particular those disclosed in WO 98/00428.

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).

In another preferred embodiment, the chiral dopants are preferably selected from formula CD-1 ,

including the respective (S,S) enantiomer wherein E and F are each independently 1 ,4-phenylene or trans-1 ,4- cyclohexylene, v is 0 or 1 , Z° is -COO-, -OCO-, -CH2CH2- or a single bond and R is alkyl, alkoxy or alkanoyl with 1 to 12 C atoms. The compounds of formula CD-1 and their synthesis are described in WO 98/00428. The compounds of formula CD-2 and their synthesis are described in GB 2,328,207.

The above-mentioned chiral dopants R/S-5011 and the compounds of formula CD-1 and CD-2 exhibit a very high helical twisting power (HTP) and are therefore particularly useful for the purpose of the present invention.

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 CD-1 , and/or formula CD-2 and/or

R-5011 or S-5011 , very preferably, the chiral compound is R-5011 , or S-5011. The amount of chiral compounds in the liquid crystalline medium is preferably from 0.1 to 15 %, in particular from 0.5 to 10 %, very preferably 1 to 5 % by weight of the total mixture.

In a further preferred embodiment, the LC medium comprises, in addition to the compounds of formulae I and A and/or B, one or more compounds of formula B,

in which the individual radicals have, independently of each other and on each occurrence identically or differently, the following meanings: each, independently

and

of one another, and on each occurrence, identically or differently

R ³¹ alkyl, alkoxy, oxaalkyl or alkoxyalkyl having 1 to 9 C

atoms or alkenyl or alkenyloxy having 2 to 9 C atoms, all of which are optionally fluorinated,

X° F, Cl, CN, halogenated alkyl or alkoxy having 1 to 6 C atoms or halogenated alkenyl or alkenyloxy having 2 to 6 C atoms,

Z ³¹ -CH2CH2-, -CF2CF2-, -COO-, trans- CFI=CFI-, trans- CF=CF-, -CFI2O- or a single bond, preferably - CH2CH2-,

-COO-, trans- CFI=CFI- or a single bond, particularly preferably -COO-, trans- CFI=CFI- or a single bond,

L ³¹ , L ³² each, independently of one another, FI or F,

9 0, 1 , 2 or 3. ln the compounds of formula B, X° is preferably F, Cl, CF3, CFIF ₂ , OCF3,

OCHF2, OCFHCFs, OCFHCHF2, OCFHCHF2, OCF2CH3, OCF2CHF2,

OCF2CHF2, OCF2CF2CHF2, OCF2CF2CHF2, OCFHCF2CF3,

OCFHCF2CHF2, OCF2CF2CF3, OCF2CF2CCIF2, OCCIFCF2CF3 or

CFI=CF ₂ , very preferably F or OCF3, most preferably F.

In the compounds of formula B, R ³¹ is preferably selected from straight- chain alkyl or alkoxy with 1 , 2, 3, 4, 5 or 6 C atoms, and straight-chain alkenyl with 2, 3, 4, 5, 6 or 7 C atoms.

In the compounds of formula B, g is preferably 1 or 2. In the compounds of formula B, Z ³¹ is preferably COO, trans-CFI=CFI or a single bond, very preferably COO or a single bond.

The compounds of formula B are preferably selected from the group consisting of the following formulae:

in which g, A ³¹ , A ³² , R ³¹ , X°, L ³¹ and L ³² have the meanings given in formula B, and X° is preferably F or CN. Particularly preferred are compounds of formulae B1 and B2.

Particularly preferred compounds of formula B1 are selected from the group consisting of the following subformulae:

in which R ³¹ , X°, L ³¹ and L ³² have the meaning given in formula B1 , and X° is preferably F.

Very particularly preferred compounds of formula B1 a are selected from the group consisting of the following subformulae: in which R ³¹ is as defined in formula B1 .

Very particularly preferred compounds of formula B1 b are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B1 .

Particularly preferred compounds of formula B2 are selected from the group consisting of the following subformulae:

35 in which R ³¹ , X°, L ³¹ and L ³² have the meaning given in formula B2, L ³³ , L ³⁴ , L ³⁵ and L ³⁶ are each, independently of one another, H or F, and X° is preferably F or CN.

Very particularly preferred compounds of formula B2 are selected from the group consisting of the following subformulae: in which R ³¹ is as defined in formula B2.

Very particularly preferred compounds of formula B2b are selected from the group consisting of the following subformulae

B2b1

in which R ³¹ is as defined in formula B2.

Very particularly preferred compounds of formula B2c are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B2.

Very particularly preferred compounds of formula B2d and B2e are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B2.

Very particularly preferred compounds of formula B2f are selected from the group consisting of the following subformulae: in which R ³¹ is as defined in formula B2.

Very particularly preferred compounds of formula B2g are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B2.

35 Very particularly preferred compounds of formula B2h are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B2.

Very particularly preferred compounds of formula B2i are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B2. Very particularly preferred compounds of formula B2k are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B2.

Very particularly preferred compounds of formula B2I are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B2. Alternatively to, or in addition to, the compounds of formula B1 and/or B2, the LC medium may also comprise one or more compounds of formula B3 as defined above.

Particularly preferred compounds of formula B3 are selected from the group consisting of the following subformulae:

in which R ³¹ is as defined in formula B3.

In a further preferred embodiment, the LC medium comprises in addition to the compounds of formulae I and A and/or B, one or more compounds

in which the individual radicals have the following meanings: each, independently of one another, and

on each occurrence, identically or differently

R ⁴¹ , R42 each, independently of one another, alkyl, alkoxy,

oxaalkyl or alkoxyalkyl having 1 to 9 C atoms or alkenyl or alkenyloxy having 2 to 9 C atoms, all of which are optionally fluorinated,

Z ⁴¹ , Z ⁴² each, independently of one another, -CH2CH2-, -COO-, frans-CH=CH-, trans-CF=CF-, -CFI2O-, -CF2O-, -C=C- or a single bond, preferably a single bond, h 0, 1 , 2 or 3.

In the compounds of formula C, R ⁴¹ and R ⁴² are preferably selected from straight-chain alkyl or alkoxy with 1 , 2, 3, 4, 5 or 6 C atoms, and straight- chain alkenyl with 2, 3, 4, 5, 6 or 7 C atoms.

In the compounds of formula C, h is preferably 0, 1 or 2.

In the compounds of formula C, Z ⁴¹ and Z ⁴² are preferably selected from

COO, trans-CFI=CFI and a single bond, very preferably from COO and a single bond.

Preferred compounds of formula C are selected from the group

consisting of the following subformulae: wherein R ⁴¹ and R ⁴² have the meanings given in formula C, and preferably denote each, independently of one another, alkyl, alkoxy fluorinated alkyl or fluorinated alkoxy with 1 to 7 C atoms, or alkenyl, alkenyloxy, alkoxyalkyl or fluorinated alkenyl with 2 to 7 C atoms.

In a further preferred embodiment, the LC medium comprises in addition to the compounds of formulae I and A and/or B, one or more compounds of formula D

in which A ⁴¹ , A ⁴² , Z ⁴¹ , Z ⁴² , R ⁴¹ , R ⁴² and h have the meanings given in formula C or one of the preferred meanings given above.

Preferred compounds of formula D are selected from the group consisting of the following subformulae:

in which R ⁴¹ and R ⁴² have the meanings given in formula D and R ⁴¹ preferably denotes alkyl, and in formula D1 R ⁴² preferably denotes alkenyl, particularly preferably -(CH ₂ ) ₂ -CH=CH-CH3, and in formula D2 R ⁴² preferably denotes alkyl, -(CH ₂ )2-CH=CH ₂ or -(CH ₂ ) ₂ -CH=CH-CH ₃ . Preferably, the LC medium comprises, in addition to the compounds of formula I and A and/or B, one or more compounds of formula E containing an alkenyl group in which the individual radicals, on each occurrence identically or differ- ently, each, independently of one another, have the following meaning:

R ^A1 alkenyl having 2 to 9 C atoms or, if at least one of the rings X,

Y and Z denotes cyclohexenyl, also one of the meanings of

R ^A2 , R ^A2 alkyl having 1 to 12 C atoms, in which, in addition, one or two non-adjacent Chte groups may be replaced by -0-, -CH=CH-, -CO-, -OCO- or -COO- in such a way that O atoms are not linked directly to one another, x 1 or 2. R ^A2 is preferably straight-chain alkyl or alkoxy having 1 to 8 C atoms or straight-chain alkenyl having 2 to 7 C atoms.

Preferred compounds of formula E are selected from the following sub- formulae:

in which alkyl and alkyl ^* each, independently of one another, denote a straight-chain alkyl radical having 1 -6 C atoms, and alkenyl and alkenyl ^* each, independently of one another, denote a straight-chain alkenyl radical having 2-7 C atoms. Alkenyl and alkenyl ^* preferably denote

CH ₂ =CH-, CH ₂ =CHCH ₂ CH ₂ -, CH ₃ -CH=CH-, CH ₃ -CH ₂ -CH=CH-, CH ₃ -

(CH ₂ ) ₂ -CH=CH-, CH ₃ -(CH ₂ ) ₃ -CH=CH- or CH ₃ -CH=CH-(CH ₂ ) ₂ -.

Very preferred compounds of the formula E are selected from the following sub-formulae:

E1 a in which m denotes 1 , 2, 3, 4, 5 or 6, i denotes 0, 1 , 2 or 3, and R ^b1 denotes H, Chb or C2H5.

Very particularly preferred compounds of the formula E are selected from the following sub-formulae:

Most preferred are compounds of formula E1 a2, E1 a5, E3a1 and E6a1 .

In a further preferred embodiment, a small amount (for example 0.3% by weight, typically < 8% by weight per compound type) of a polymerisable compound is optionally added to the above described liquid-crystalline medium and, after introduction into the light modulation element, is polymerised or cross-linked in situ, usually by UV photopolymerisation.

The addition of polymerisable mesogenic or liquid-crystalline compounds, also known as "reactive mesogens" (RMs), to the LC mixture has been proven particularly suitable in order further to stabilise the ULH texture (eg. Lagerwall et al., Liquid Crystals 1998, 24, 329 - 334.).

Suitable polymerisable liquid-crystalline compounds are preferably selected from the group of compounds of formula RM,

P-Sp-MG-R ⁰ RM wherein

P is a polymerisable group,

Sp is a spacer group or a single bond,

MG is a rod-shaped mesogenic group, which is

preferably selected of formula M,

M is -(A ^D21 -Z ^D21 ) _k -A ^D22 -(Z ^D22 -A ^D23 )r,

A ⁰²¹ to A ⁰²³ are in each occurrence independently of one another an aryl-, heteroaryl-, heterocyclic- or alicyclic group optionally being substituted by one or more identical or different groups L, preferably 1 ,4-cyclohexylene or 1 ,4- phenylene, 1 ,4 pyridine, 1 , 4-pyrimidine, 2,5-thiophene, 2,6-dithieno[3,2-b:2',3'-d]thiophene, 2,7- fluorine, 2,6- naphtalene, 2,7-phenanthrene optionally being

substituted by one or more identical or different groups L,

Z ^D21 and Z ^D22 are in each occurrence independently from each other,

-0-, -S-, -CO-, -COO-, -OCO-, -S-CO-, -CO-S-, -O-COO-, -CO-NR ⁰¹ -, -NR ⁰¹ -CO-, -NR ⁰¹ -CO-NR ⁰² , -NR ⁰¹ -CO-O-, -O-CO-NR ⁰¹ -, -OCH ₂ -, -CH ₂ O-, -SCH ₂ -, -CH ₂ S-, -CF ₂ O-, -OCF2-, -CF2S-, -SCF2-, -CH2CH2-, -(CH ₂ ) ₄ -, -CF2CH2-, -CH ₂ CF ₂ -, -CF ₂ CF ₂ -, -CH=N-, -N=CH-, -N=N-, - CH=CR ⁰¹ -, -CY ⁰¹ =CY ⁰² -, -CºC-, -CH=CH-COO-, -

OCO-CFI=CFI-, or a single bond, preferably -COO-, - OCO-, -CO-O-, -O-CO-, -OCH ₂ -, -CH ₂ O-, - _, -

CH2CH2-, -(CH ₂ ) ₄ -, -CF2CH2-, -CH2CF2-, -CF2CF2-, - CºC-, -CFI=CFI-COO-, -OCO-CFI=CFI-, or a single bond,

L is in each occurrence independently of each other F,CI or optionally fluorinated alkyl, alkoxy, thioalkyl,

alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy with 1 to 20 C atoms more,

R° is H, alkyl, alkoxy, thioalkyl, alkylcarbonyl,

alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy with 1 to 20 C atoms more, preferably 1 to 15 C atoms which are optionally fluorinated, or is Y ^D0 or P-Sp-,

Y ^D0 is F, Cl, CN, N0 ₂ , OCHS, OCN, SCN, optionally

fluorinated alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy with 1 to 4 C atoms, or mono- oligo- or polyfluorinated alkyl or alkoxy with 1 to 4 C atoms, preferably F, Cl, CN, N0 ₂ , OCFI3, or mono- oligo- or polyfluorinated alkyl or alkoxy with 1 to 4 C atoms

Y ⁰¹ and Y ⁰² each, independently of one another, denote FI, F, Cl or

CN, R ⁰¹ and R ⁰² have each and independently the meaning as defined above R°, and k and I are each and independently 0, 1 , 2, 3 or 4, preferably 0,

1 or 2, most preferably 1 .

Preferred polymerisable mono-, di-, or multireactive liquid

crystalline compounds are disclosed for example in WO 93/22397, EP 0 261 712, DE 195 04 224, WO 95/22586, WO 97/00600, US 5,518,652, US 5,750,051 , US 5,770, 107 and US 6,514,578.

Preferred polymerisable groups are selected from the group consisting of

(0)k2-, CH ₂ =CH-(C0)ki-Phe-(0)k2-, Phe-CH=CH-, HOOC-, OCN- and W ⁴ W ⁵ W ⁶ Si-, in which W ¹ denotes H, F, Cl, CN, CF3, phenyl or alkyl having 1 to 5 C atoms, in particular H, F, Cl or CFI3, W ² and W ³ each, independently of one another, denote FI or alkyl having 1 to 5 C atoms, in particular FI, methyl, ethyl or n-propyl, W ⁴ , W ⁵ and W ⁶ each,

independently of one another, denote Cl, oxaalkyl or oxacarbonylalkyl having 1 to 5 C atoms, W ⁷ and W ⁸ each, independently of one another, denote FI, Cl or alkyl having 1 to 5 C atoms, Phe denotes 1 ,4-phenylene, which is optionally substituted by one or more radicals L as being defined above but being different from P-Sp, and ki, k ₂ and k3 each,

independently of one another, denote 0 or 1 , k3 preferably denotes 1 , and k ₄ is an integer from 1 to 10. Particularly preferred groups P are CH2=CH-COO-, CH2=C(CH3)-COO-,

particular vinyloxy, acrylate, methacrylate, fluoroacrylate, chloroacrylate, oxetane and epoxide.

In a further preferred embodiment of the invention, the polymerisable compounds of the formulae I ^* and II ^* and sub-formulae thereof contain, instead of one or more radicals R-Sp-, one or more branched radicals containing two or more polymerisable groups P (multifunctional polymerisable radicals). Suitable radicals of this type, and polymerisable compounds containing them, are described, for example, in

US 7,060,200 B1 or US 2006/0172090 A1 . Particular preference is given to multifunctional polymerisable radicals selected from the following for- mulae:

-X-alkyl-CHP ¹ -CH ₂ -CH ₂ P ² l ^* a

-X-alkyl-C(CH ₂ P ¹ )(CH ₂ P ² )-CH ₂ P ³ l ^* b

-X-alkyl-CHP ¹ CHP ² -CH ₂ P ³ l ^* c

-X-alkyl-C(CH ₂ P ¹ )(CH ₂ P ² )-CaaH2aa _+i l ^* d

-X-alkyl-CHP ¹ -CH ₂ P ² l ^* e

-X-alkyl-CHP ¹ P ² l ^* f

-X-alkyl-CP ¹ P ² -CaaH _2aa+i i ^* g

-X-alkyl-C(CH ₂ P ¹ )(CH2P ² )-CH20CH2-C(CH2P ³ )(CH2P ⁴ )CH ₂ P ⁵ l ^* h

-X-alkyl-CH((CH ₂ )aaP ¹ )((CH ₂ )bbP ² ) l ^* i

-X-alkyl-CHP ¹ CHP ² -C _aa H _2aa+i l ^* k in which alkyl denotes a single bond or straight-chain or branched alkylene having 1 to 12 C atoms, in which one or more non-adjacent Chte groups may each be replaced, independently of one another, by -C(R ^X )=C(R ^X )-, -CºC-, -N(R ^X )-, -0-, -S-, -CO-, -C0-0-, -0-C0-, -O-CO-O- in such a way that O and/or S atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by F, Cl or CN, where R ^x has the above-mentioned meaning and preferably denotes R° as defined above, aa and bb each, independently of one another, denote 0, 1 , 2, 3, 4, 5 or 6,

X has one of the meanings indicated for X', and

P ^{1 5} each, independently of one another, have one of the meanings indicated above for P.

Preferred spacer groups Sp are selected from the formula Sp'-X', so that the radical "P-Sp-" conforms to the formula "P-Sp'-X'-", where

Sp' denotes alkylene having 1 to 20, preferably 1 to 12 C

atoms, which is optionally mono- or polysubstituted by F, Cl, Br, I or CN and in which, in addition, one or more non- adjacent CFh groups may each be replaced, independently of one another, by -O-, -S-, -NH-, -NR ^X -, -SiR ^x R ^xx -, -CO-, -COO-, -OCO-, -0C0-0-, -S-CO-, -CO-S-, -NR ^x -C0-0-, -0-C0-NR ^x -, -NR ^x -CO-NR ^x -, -CH=CH- or -CºC- in such a way that O and/or S atoms are not linked directly to one another,

X' denotes -0-, -S-, -CO-, -COO-, -OCO-, -0-C00-,

-CO-NR ^x -, -NR ^x -CO-, -NR ^x -CO-NR ^x -, -OCH ₂ -, -CH ₂ 0-, -SCH ₂ -, -CH ₂ S-, -CF ₂ O-, -OCF ₂ -, -CF ₂ S-, -SCF ₂ -, -CF ₂ CH ₂ -, -CH2CF2-, -CF2CF2-, -CH=N-, -N=CH-, -N=N-, -CH=CR ^X - -CY ² =CY ³ -, -CºC-, -CH=CH-COO-, -OCO-CH=CH- or a single bond, R ^x and R ^xx each, independently of one another, denote FI or alkyl

having 1 to 12 C atoms, and

Y ² and Y ³ each, independently of one another, denote H, F, Cl or CN.

X' is preferably

-0-, -S-, -CO-, -COO-, -OCO-, -O-COO-, -CO-NR ^x - -NR ^x -CO-, -NR ^x -CO-NR ^x - or a single bond.

Typical spacer groups Sp' are, for example, -(CFhJpi-, -(CFhCF OJqi- CH2CH2-, -CH2CH2-S-CH2CH2-, -CH2CH2-NH-CH2CH2- or -(SiR ^x R ^xx -0) _pi -, in which p1 is an integer from 1 to 12, q1 is an integer from 1 to 3, and R ^x and R ^xx have the above-mentioned meanings.

Particularly preferred groups -X'-Sp'- are -(CFhJpi-, -0-(CFl2)pi-, -OCO- (CH ₂ ) _P I-, -OCOO-(CH ₂ ) _P I-.

Particularly preferred groups Sp' are, for example, in each case straight- chain ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, octadecylene, ethyleneoxyethylene, methyleneoxybutylene, ethylenethioethylene, ethyl- ene-N-methyliminoethylene, 1 -methylalkylene, ethenylene, propenylene and butenylene.

Further preferred polymerisable mono-, di-, or multireactive liquid crystalline compounds are shown in the following list:

P°-(CH ₂ ) _X (0) v Vcoo RM-2

19 wherein

P° is, in case of multiple occurrence independently of one

another, a polymerisable group, preferably an acryl, methacryl, oxetane, epoxy, vinyl, vinyloxy, propenyl ether or styrene group,

A ⁰ is, in case of multiple occurrence independently of one

another, 1 ,4-phenylene that is optionally substituted with 1 , 2,

3 or 4 groups L, or trans-1 ,4-cyclohexylene,

is, in case of multiple occurrence independently of one another, -COO-, -OCO-, -CH2CH2-, -CºC-, -CH=CH-, - CH=CH-COO-, -OCO-CH=CH- or a single bond, r is 0, 1 , 2, 3 or 4, preferably 0, 1 or 2,

t is, in case of multiple occurrence independently of one

another, 0, 1 , 2 or 3,

u and v are independently of each other 0, 1 or 2,

w is O or l ,

x and y are independently of each other 0 or identical or different

integers from 1 to 12,

z is 0 or 1 , with z being 0 if the adjacent x or y is 0,

in addition, wherein the benzene and naphthalene rings can additionally be substituted with one or more identical or different groups L and the parameter R° , Y°, R ⁰¹ , R ⁰² and L have the same meanings as given above in formula RM.

The total concentration of these polymerisable compounds of formula RM is in the range of 0.1 % to 25 %, preferably 0.1 % to 15 %, more preferably 0.1 % to 10 % based on the total mixture.

The polymerisable compounds are polymerised or cross-linked (if a corn- pound contains two or more polymerisable groups) by in-situ polymerisa- tion in the LC medium between the substrates of the LC display. Suitable and preferred polymerisation methods are, for example, thermal or photopolymerisation, preferably photopolymerisation, in particular UV photopolymerisation. If necessary, one or more initiators may also be added here. Suitable conditions for the polymerisation, and suitable types and amounts of initiators, are known to the person skilled in the art and are described in the literature. Suitable for free-radical polymerisation are, for example, the commercially available photoinitiators Irgacure651 ^® , Irgacure184 ^® , Irgacure907®, Irgacure369 ^® or Darocurel 173 ^® (Ciba AG). If an initiator is employed, its proportion in the mixture as a whole is preferably 0.001 to 5% by weight, particularly preferably 0.001 to 1 % by weight. However, the polymerisation can also take place without addition of an initiator. In a further preferred embodiment, the LC medium does not comprise a polymerisation initiator. The polymerisable component or the cholesteric liquid-crystalline medium may also comprise one or more stabilisers in order to prevent undesired spontaneous polymerisation of the RMs, for example during storage or transport. Suitable types and amounts of stabilisers are known to the person skilled in the art and are described in the literature.

Particularly suitable are, for example, the commercially available stabilisers of the Irganox ^® series (Ciba AG). If stabilisers are employed, their proportion, based on the total amount of RMs or polymerisable compounds, is preferably 10 - 5000 ppm, particularly preferably 50 - 500 ppm.

The above-mentioned polymerisable compounds are also suitable for polymerisation without initiator, which is associated with considerable advantages, such as, for example, lower material costs and in particular less contamination of the LC medium by possible residual amounts of the initiator or degradation products thereof.

The polymerisable compounds can be added individually to the liquid- crystalline medium, but it is also possible to use mixtures comprising two or more polymerisable compounds. On polymerisation of mixtures of this type, copolymers are formed. The invention furthermore relates to the polymerisable mixtures mentioned above and below.

The liquid crystal media may contain further additives like for example stabilizers, inhibitors, 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.

In general, the total concentration of all compounds in the media according to this application is 100 %. The liquid crystal media according to the present invention consists of several compounds, preferably of 2 to 40, more preferably of 3 to 30 and most preferably of 4 to 25 compounds.

The media in accordance with the present invention exhibit high values of the elastic constant kn and a high flexoelectric coefficient e. The liquid crystal media preferably exhibit a kn < 100 pN, preferably < 20 pN.

The liquid crystal media preferably exhibit a k ₃ 3 £ 100 pN, preferably < 15 pN.

The liquid crystal media preferably exhibit a flexoelectric coefficient | en | > 0.2 pC/m, preferably > 1 pC/m.

The liquid crystal media preferably exhibit a flexoelectric coefficient | e331 ³ 0.2 pC/m, preferably > 2 pC/m.

The liquid crystal media preferably exhibit a flexo-elastic ratio (e / K) in the range from 1 to 10 V ¹ , preferably in the range from 1 to 7 V ¹ , more preferably in the range from 1 to 5 V ¹ .

The media in accordance with the present invention exhibit high clearing points up to 60°C and higher, preferably up 65°C and higher and more preferably up to 70°C and higher. The media in accordance with the present invention exhibit broad nematic phases of 30°C and more, preferably 35°C and more or even 40°C or more. The media in accordance with the present invention exhibit high stabilities against crystallization at room temperature of more than 100 h, preferably more than 250 h or more than 1000 h.

The media in accordance with the present invention exhibit high stabilities against crystallization even at low temperatures (LTS).

Accordingly, the media do not crystallize even at temperatures down to 0°C, preferably down to -10°C, more preferably down to -20°C.

In a preferred embodiment, the LC medium comprises:

• 1 to 10, preferably 1 to 5, more preferably 1 , 2 or 3, most

preferably 1 or 2 compounds of formula IA, preferably selected from compounds compounds of formula IA wherein (-A ¹¹ -A ¹² -) and (-A ¹³ -A ¹⁴ -) in formula IA are identical or mirror images, more preferably of compounds of formulae IA-a-5 and/or IA-a-6. The amount of compounds of formula IA in the liquid crystalline medium, if present, is preferably in the range from 0 to 30 %, more preferably in the range from 1 to 20 %, even more preferably in the range from 2 to 10 % by weight of the total mixture, and/or

• 1 to 10, preferably 1 to 5, more preferably 1 , 2 or 3, most

preferably 1 or 2 compounds of formula IB, preferably selected from symmetrical compounds of the above formulae IB-c-2 and/or

IB-c-3. The amount of compounds of formula IB in the liquid crystalline medium, if present, is preferably in the range from 1 to 50 %, more preferably in the range from 5 to 30 %, even more preferably in the range from 10 to 20 % by weight of the total mixture, and/or

• 1 to 10, preferably 1 to 5 compounds of formula IC, preferably selected from the symmetrical ones IC-b and/or non-symmetrical ones IC-c, more preferably from formulae IC-b-5, IC-c-2, IC-c-3,

IC-c-12 and or IC-c-15. The amount of compounds of formula IC in the liquid crystalline medium, if present, is preferably in the range from 1 to 98 %, more preferably in the range from 20 to 80 %, even more preferably in the range from 30 to 60 % by weight of the total mixture, and/or

• 1 to 6, in particular 1 to 5 compounds of formula ID, preferably selected from the above formulae ID-A-1 , ID-C-2 and/or ID-C-3.

The amount of compounds of formula ID in the liquid crystalline medium , if present, is preferably in the range from 1 to 70 %, more preferably in the range from 10 to 60 %, even more preferably in the range from 20 to 50 % by weight of the total mixture, and/or

• 1 to 10, preferably 1 to 5, more preferably 1 or 3, most preferably 1 or 2 compounds from the above formulae IE, preferably form compounds of formula IE-4, IE-5, IE-7 and/or IE-8. The amount of compounds of formula IE in the liquid crystalline medium, if present, is preferably from 1 to 40 %, in particular from 5 to 25 %, very preferably 10 to 15 % by weight of the total mixture, and/or

• 1 to 10, preferably 1 to 5, more preferably 1 or 3, most preferably 1 or 2 compounds from the above formulae IF, preferably form compounds of formula IF-4, IF-5 and/or IF-8. The amount of compounds of formula IF in the liquid crystalline medium, if present, is preferably from 1 to 35 %, in particular from 5 to 25 %, very preferably 10 to 15 % by weight of the total mixture, and/or · 1 to 25, in particular 2 to 20, very preferably 3 to 15, especially 4 to 10 different compounds selected from compounds of formulae A and/or F and their respective subformulae. The amount of compounds of formulae A and/or F in the liquid crystalline medium as a whole, is preferably from 1 to 50 %, in particular from 2 to 40 %, very preferably 5 to 25 % by weight of the total mixture, and

• optionally, 1 to 10, in particular 2 to 8, very preferably 3 to 5,

different compounds selected from compounds of formula B, preferably selected from compounds of formulae B2c1 and/or B2g3 and/or B2I1. The amount of compounds of formulae B in the liquid crystalline medium as a whole, is preferably from 0.1 to 25 %, in particular from 0.5 to 20 %, very preferably 1 to 15 % by weight of the total mixture, and/or

• optionally, 1 to 10, in particular 1 to 5, very preferably 1 or 2,

compounds selected from compounds of formula D, preferably selected from compounds of formula D2. The amount of compounds of formulae D in the liquid crystalline medium as a whole, is preferably from 0.1 to 25 %, in particular from 0.5 to 20 %, very preferably 1 to 15 % by weight of the total mixture, and/or

• optionally, 1 to 10, in particular 1 to 5, very preferably 1 , 2, 3 or 4 compounds selected from compounds of formula E, preferably selected from compounds of formula E1 and/or E6. The amount of compounds of formulae E in the liquid crystalline medium as a whole, is preferably from 0.1 to 30 %, in particular from 0.5 to 25 %, very preferably 1 to 20 % by weight of the total mixture,

• optionally, 1 to 5, in particular 1 to 3, very preferably 1 or 2 chiral dopants, preferably selected from the above formula CD-1 and/or formula CD-2 and/or R-5011 or S-5011 , very preferably, the chiral compound is R-5011 or S-5011. The amount of chiral compounds in the liquid crystalline medium, if present, is preferably from 1 to 15 %, in particular from 0.5 to 10 %, very preferably 0.1 to 5 % by weight of the total mixture, and/or

• optionally further additives, such as for example stabilizers,

antioxidants, etc. in usual concentrations. The total concentration of these further constituents, if present, 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 compounds forming the LC medium in accordance with the present invention 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 media of compounds or using so-called multi-bottle-systems, the constituents of which are ready to use media themselves. Thus, the invention also relates to a process for the production of an LC medium as described above and below. In particular, the invention relates to a process for the production of an LC medium comprising the steps of mixing one or more compounds of formula I, with at least one compound selected from compounds of formulae A and/or F.

The liquid crystalline media in accordance with the present invention can be used in electro optic devices, for example liquid crystal devices, such as STN, TN, AMD-TN, VA, PS-VA, FFS, IPS, temperature compensation, guest-host, phase change or surface stabilized or polymer stabilized cholesteric texture (SSCT, PSCT) displays, in active and passive optical elements like polarizers, compensators, reflectors, alignment layers, colour 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. Thus, another aspect of the present invention is the use of a LC medium, comprising at least one compound of formula I and one or more compounds of formulae A and/or F in electro optic devices.

Since the media in accordance with the present invention are particularly beneficially for flexoelectric liquid crystal display applications, such as, for example, devices of the ULFI or USFI mode.

Thus, another object of the present invention is a liquid crystal device, preferably a flexoelectric device, comprising a medium, which comprises one or more compounds of formula I and one or more compounds of formulae A and/or F.

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, optionally alignment layers and a medium comprising one or more compounds of formula I, and one or more compounds of formulae A and/or F and a chiral dopant as described above and below. If an electrical field is applied to this configuration normal to the helical axis, the optical axis is rotated in the plane of the cell, similar as the director of a ferroelectric liquid crystal rotate as in a surface stabilized ferroelectric liquid crystal display.

The field induces a splay bend structure in the director, which is accommodated by a tilt in the optical axis. The angle of the rotation of the axis is in first approximation directly and linearly proportional to the strength of the electrical field. The optical effect is best seen when the liquid crystal cell is placed between crossed polarizers with the optical axis in the unpowered state at an angle of 22.5° to the absorption axis of one of the polarizers. This angle of 22.5° is also the ideal angle of rotation of the electric field, as thus, by the inversion the electrical field, the optical axis is rotated by 45° and by appropriate selection of the relative orientations of the preferred direction of the axis of the helix, the absorption axis of the polarizer and the direction of the electric field, the optical axis can be switched from parallel to one polarizer to the centre angle between both polarizers. The optimum contrast is then achieved when the total angle of the switching of the optical axis is 45°. In that case, the arrangement can be used as a switchable quarter wave plate, provided the optical retardation, i. e. the product of the effective birefringence of the liquid crystal and the cell gap, is selected to be the quarter of the wavelength. In this context, the wavelength referred to is 550 nm, the wavelength for which the sensitivity of the human eye is highest, unless explicitly stated otherwise. The angle of rotation of the optical axis (F) is given in good

approximation by the formula: tan F = e Po E / (2 p K)

wherein

Po is the undisturbed pitch of the cholesteric liquid crystal, e is the average [e = ½ (en + e33)] of the splay flexoelectric

coefficient (en) and the bend flexoelectric coefficient (e33),

E is the electrical field strength and

K is the average [K = ½ (kn + k33)] of the splay elastic constant (kn) and the bend elastic constant (k33)

and wherein e / K is called the flexo-elastic ratio.

This angle of rotation is half the switching angle in a flexoelectric switching element.

The response time (t) of this electro-optical effect is given in good approximation by the formula: t = [Po/(2 p)] ² g / K wherein

g is the effective viscosity coefficient associated with the distortion of the helix. The flexoelectric effect is characterized by fast response times (T _0n+ T _0ff at 35°C) typically ranging from 1 ms to 10 ms, preferably < 5ms and even more preferably < 3ms. It further features excellent grey scale capability.

There is a critical field (E _c ) to unwind the helix, which can be obtained from equation

Ec = (p ² / Ro) [I<22/(eo De)] ^1/2 wherein

|<22 is the twist elastic constant,

so is the permittivity of vacuum and

De is the dielectric anisotropy of the liquid crystal.

The inventive media in accordance with the present invention 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. 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.

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.

The switching between different states of orientation according to a preferred embodiment of the present invention is exemplarily described below in detail.

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.

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 flow alignment by lightly pressing the cell.

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. 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 _rms /pm, 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 electric field. In the homeotropic state, the sample is transmissive when viewed in normal daylight and appears black when being put between crossed polarizers.

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.

A sample of a medium in accordance with the present invention in 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.

In a sample of a medium in accordance with the present invention, 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.

Starting from the ULH texture, the inventive 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.

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.

Apart from the use in flexoelectric devices, the media in accordance with the present invention are also suitable for other types of displays and other optical and electro optical applications, such as optical

compensation or polarizing films, colour filters, reflective cholesterics, optical rotatory power and optical information storage.

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

whatsoever.

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.

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. 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 C=C or C=0 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 CºC or in a CºN triple bond or in an allylic position C=C=C are 180°, unless these angles are otherwise restricted, e.g. like being part of small rings, like 3-, 4- or 5-atomic rings, notwithstanding that in some instances in some structural formulae these angles are not represented exactly.

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.

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).

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.

The total concentration of all compounds in the media according to this application is 100 %. All concentrations are given in % w/w, unless explicitly stated otherwise. 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. It goes without saying to the person skilled in the art that the LC media may also comprise compounds in which, for example, H, N, O, Cl, F have been replaced by the corresponding isotopes.

The following abbreviations are used to illustrate the liquid crystalline phase behaviour of the compounds: T N,I = clearing point; K = crystalline; N = nematic; NTB = second nematic or twist-bend nematic phase; S or Sm = smectic; Ch = cholesteric; I = isotropic; Tg = glass transition. The numbers between the symbols indicate the phase transition temperatures in °C.

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.

All groups CnFten+i , CmFtem+i and C1H21+1 are preferably straight chain alkyl groups with n, m and I C-atoms, respectively, all groups CnFten, CmFtem and C1H21 are preferably (CFteJn, (CFteJm and (CFte)!, respectively and -CH=CH- preferably is trans- respectively E vinylene. Preferably n, m and I denote an integer between 1 and 12.

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. Table A: Ring Elements

Table B: Linking Groups n (-CH ₂ -)n “n” is an integer between 1 and 13 except 2

E -CH2-CH2-

V -CH=CH- T -CºC-

W -CF2-CF2-

B -CF=CF-

Z -CO-O- Zl -O-CO-

X -CF=CH- XI -CH=CF-

10 -CH ₂ -O- 01 -O-CH2- Q -CF2-O- Ql -O-CF2- Table C: End Groups

Left hand side, used alone or in Right hand side, used alone or combination with others _ in combination with others

-P- CnH2n ₊₁ - -n -CnFten _+l

-nO- C _n H _2n+1 -0- -nO -O-CnFten _+i

-V- CH ₂ =CH- -V -CH=CH ₂

-nV- C _n H _2n+i -CH=CH- -nV -C _n H _2n -CH=CH ₂

-Vn- CH ₂ =CH- C _n H _2n - -Vn -CH=CH-CnH _2n+i

-nVm- CnH2n ₊₁ -CH=CH-C _m H2m- -nVm -C _n H2n-CH = CH-C _m H2m ₊₁

-N- NºC- -N -CºN

-S- S=C=N- -S -N=C=S

-F- F- -F -F

-CL- Cl- -CL -Cl

-M- CFH ₂ - -M -CFH ₂

-D- CF ₂ H- -D -CF ₂ H

-T- CFs- -T -CFs

-MO- CFH2O- -OM -OCFH2

-DO- CF2HO- -OD -OCF2H

-TO- CFsO- -OT -OCFs

-A- H-CºC- -A -CºC-H

-nA- C _n H _2n+1 -C=C- -An -C=C-C _n H2n _+i

-NA- NºC-CºC- -AN -CºC-CºN

Left hand side, used in Right hand side, used in

combination with others only combination with others only

-...n...- (-CH ₂ -) _n -...n... (-CH ₂ -) _n

-...M...- -CFH- -...M... -CFH- -...D...- -CF ₂ - -...D... -CF ₂ - -...V...- -CH=CH- -...V... -CH=CH- -...Z...- -CO-O- -...Z... -CO-O- -...Zl...- -O-CO- -...Zl... -O-CO- -...K...- -CO- -...K... -CO- -...W...- -CF=CF- -...W... -CF=CF- wherein n und m each are integers and three points indicate a space for other symbols of this table. Examples

Test cells and methods Typically a 3 mhh thick cell, having an anti-parallel rubbed PI alignment layer on their mutually opposite substrate, is filled on a hotplate at a temperature at which the flexoelectric mixture is in the isotropic phase.

Before filling the cell, typically, the phase transitions including clearing point and the crystallization behavior are determined using Differential Scanning Calorimetry (DSC). In addition, 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.

Wires are then attached to the ITO electrodes of the cell using indium metal. The cell is secured in a Linkam TFIMS600 hot-stage connected to a Linkam TMS93 hot-stage controller. The hot-stage is secured to a rotation stage in an Olympus BX51 microscope.

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.

In order to measure the flexoelectric response of the material, the change in the size of the tilt of the optic axis is measured as a function of increasing voltage at a temperature of 25°C and 35°C, unless stated explicitly otherwise. This is achieved by using the equation: tan <p = (R ₀ /2p) (e/K) E wherein f 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 Ki and

K ₃ ) and e is the flexoelectric coefficient (where e = ei + e3). 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, Po, 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.

The media shown in the following examples are well suitable for use in ULH-displays. To that end, an appropriate concentration of the chiral dopant or dopants used has to be applied in order to achieve a typical cholesteric pitch of 350 to 275 nm.

Mixture examples

Bimesoqenic mixture

The following mixture BM-1 is prepared.

The mixture has the following chracteristics:

Calamitic mixture

The following mixture H-1 is prepared

The mixture H-1 has a dielectric anisotropy (De) of +202.1.

The following mixture H-2 is prepared.

The mixture H-2 has a dielectric anisotropy (De) of +54.4. The following mixture H-3 is prepared.

The mixture H-3 has a dielectric anisotropy (De) of +15.3.

The following mixture H-4 is prepared.

The mixture H-4 has a dielectric anisotropy (De) of +12.9. The following mixture H-5 is prepared.

The mixture H-5 has a dielectric anisotropy (De) of +3.5.

- H O -

Mixture example M-1

14.7 % w/w of mixture H-1 and 2 % w/w R-5011 are added to 83.3 % w/w of mixture BM-1. The resulting mixture M-1 is homogenized and filled into a test cell as described above.

Measurements with respect to the switching performance(T _0n ⁺ T _0ff ), the TNI (clearing point), the e/K (flexo elastic constant) and the NTB (transition temperature to the second nematic phase or nematic twist bend phase) are performed and the results are summarized in the following table.

Mixture example M-2

14.7 % w/w of mixture H-2 and 2 % w/w R-5011 are added to 83.3 % w/w of mixture BM-1.The resulting mixture M-2 is homogenized and filled into a test cell as described above.

Measurements with respect to the switching performance (T _0n +T _0ff ), the TNI (clearing point), the e/K (flexo elastic constant) and the NTB (transition temperature to the second nematic phase or nematic twist bend phase) are performed and the results are summarized in the following table.

Mixture example M-3

14.7 % w/w of mixture H-3 and 2 % w/w R-5011 are added to 83.3 % w/w of mixture BM-1.The resulting mixture M-3 is homogenized and filled into a test cell as described above.

Measurements with respect to the switching performance (T _0n +T _0ff ), the TNI (clearing point), the e/K (flexo elastic constant) and the NTB (transition temperature to the second nematic phase or nematic twist bend phase) are performed and the results are summarized in the following table.

Mixture example M-4

14.7 % w/w of mixture H-4 and 2 % w/w R-5011 are added to 83.3 % w/w of mixture BM-1.The resulting mixture M-4 is homogenized and filled into a test cell as described above.

Measurements with respect to the switching performance (T _0n +T _0ff ), the TNI (clearing point), the e/K (flexo elastic constant) and the NTB (transition temperature to the second nematic phase or nematic twist bend phase) are performed and the results are summarized in the following table.

Mixture example M-5

14.7 % w/w of mixture H-5 and 2 % w/w R-5011 are added to 83.3 % w/w of mixture BM-1.The resulting mixture M-5 is homogenized and filled into a test cell as described above.

5

Measurements with respect to the switching performance (T _0n +T _0ff ), the TNI (clearing point), the e/K (flexo elastic constant) and the NTB (transition temperature to the second nematic phase or nematic twist bend phase) are performed and the results are summarized in the following table.

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Example summary

Of)

From the mixture examples M-1 to M-5 is clear that the mixtures comprising one or more bimesogenic compounds and one or more compounds having a high value for dielectric anisotropy show an advantage in terms of e/K of the mixture, thus resulting in a lower operating voltage when compared to a standard concept.

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In the test cells having a 3 pm cell gap, the reduction in operating voltage is as much as 12.5% at 35°C for the range of e/K values of 2.52 to 2.97

V ¹ . This difference in operating voltage increase to an 18% difference for the 25°C measurements.

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