The invention relates to a liquid-crystalline medium which comprises at least one compound of the formula I,

in which

R ¹ and R ^1* each, independently of one another, denote H, an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH ₂ groups in these radicals may each be replaced, independently of one another,

in such a

way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, each, independently of one another, denote

) a 1 ,4-cyclohexenylene or 1 ,4-cyclohexylene radical, in which one or two non-adjacent CH2 groups may be replaced by -O- or -S-, b) a 1 ,4-phenylene radical, in which one or two CH groups may be replaced by N, c) a radical from the group piperidine-1 ,4-diyl, 1 ,4-bicyclo[2.2.2]- octylene, naphthalene-2,6-diyl, decahydronaphthalene-2,6- diyl, 1 ,2,3,4-tetrahydronaphthalene-2,6-diyl, phenanthrene- 2,7-diyl and fluorene-2,7-diyl, where the radicals a), b) and c) may be mono- or polysubstituted by halogen atoms, Z ¹ and Z ^{1 *} each, independently of one another, denote -CO-O-, -O-CO-, a, b independently 0 or 1 ,

X denotes -S- or -O-, and

L ¹ and L ² each, independently of one another, denote F, CI, CF3 or

CHF ₂ , and at least one compound of formula ST as below, or a polymer comprising its polymerized form, P-Sp-(A ² -Z ² -A ¹ )mi-Z ¹ -Pip ST wherein the individual radicals, independently of each other, and on each occurrence identically or differently, have the following meanings Pip a group selected from the following formulae

R ^g H or straight chain or branched alkyl or alkoxyalkyl with 1 to 10

C atoms, preferably with 1 to 6 C atoms, very preferably with 1 to 4 C atoms, or benzyl, most preferably H,

R ^a , R ^b , R ^c , R ^d

straight chain or branched alkyl with 1 to 10 C atoms, preferably with 1 to 6 C atoms, very preferably with 1 to 4 C atoms,

P vinyloxy, acrylate, methacrylate, fluoroacrylate, chloroacrylate, oxetane or epoxy, preferably acrylate, methacrylate,

fluoroacrylate, chloroacrylate, and more preferably acrylate or methacrylate, most preferably methacrylate,

Sp a spacer group or a single bond,

A ¹ , A ² an alicyclic, heterocyclic, aromatic or heteroaromatic group with

4 to 30 ring atoms, which may also contain fused rings, and is optionally substituted by one or more groups L or R-(A ³ -Z ³ ) _m 2-, and one of A ¹ and A ² may also denote a single bond,

A ³ an alicyclic, heterocyclic, aromatic or heteroaromatic group with 4 to 30 ring atoms, which may also contain fused rings, and is optionally substituted by one or more groups L,

Z ¹

or a single bond, with the proviso that, if ml is 0, Z ¹ is a single bond,

R ⁰⁰ , R ⁰⁰⁰ H or alkyl having 1 to 12 C atoms, R P-Sp-, H, F, CI, CN, or straight chain, branched or cyclic alkyl having 1 to 25 C atoms, wherein one or more non-adjacent CH2- groups are optionally replaced by -O-, -S-, -CO-, -CO-O-, -O- CO-, -O-CO-O- in such a manner that O- and/or S-atoms are not directly connected with each other, and wherein one or more H atoms are each optionally replaced by F, CI or P-Sp-, or a group Pip,

L P-Sp-, F, CI, CN, or straight chain, branched or cyclic alkyl having

1 to 25 C atoms, wherein one or more non-adjacent Ch -groups are optionally replaced by -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-

CO-O- in such a manner that O- and/or S-atoms are not directly connected with each other, and wherein one or more H atoms are each optionally replaced by F, CI or P-Sp-, or a group selected from formula 1 , 2 and 3, ml 0, 1 , 2, 3 or 4, m2 0, 1 , 2, 3 or 4, and n 1 , 2, 3 or 4.

Compounds of formula ST have been proposed as additives in

polymerizable liquid crystalline media in WO 2016/1 161 19 A1 .

Media of this type can be used, in particular, for electro-optical displays having active-matrix addressing based on the ECB effect and for IPS (in-plane switching) displays or FFS (fringe field switching) displays. The principle of electrically controlled birefringence, the ECB effect or also DAP (defornnation of aligned phases) effect, was described for the first time in 1971 (M.F. Schieckel and K. Fahrenschon, "Deformation of nematic liquid crystals with vertical orientation in electrical fields", Appl. Phys. Lett. 19 (1971 ), 3912). This was followed by papers by J.F. Kahn (Appl. Phys. Lett. 20 (1972), 1 193) and G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869).

The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) showed that liquid-crystalline phases must have high values for the ratio of the elastic constants K3/K1 , high values for the optical anisotropy Δη and values for the dielectric anisotropy of Δε < -0.5 in order to be suitable for use in high-information display elements based on the ECB effect. Electro-optical display elements based on the ECB effect have a homeotropic edge alignment (VA technology = vertically aligned).

Dielectrically negative liquid-crystal media can also be used in displays which use the so-called IPS or FFS effect. While the FFS type displays using dielectrically negative mixtures are usually referred to as UB-FFS, the IPS display with negative mixtures shall be referred to here as negative IPS (sometimes also U-IPS). Negative IPS displays have essentially the same electrode configuration as their counterpart IPS. However, initial alignment of the liquid crystal director is not parallel to the electrodes in the off state, but rectangular. Switching occurs like in the ordinary IPS display by a in-plane twisting of the director, resulting in alignment almost parallel to the electrodes.

Displays which use the ECB effect, as so-called VAN (vertically aligned nematic) displays, for example in the MVA (multi-domain vertical alignment, for example: Yoshide, H. et al., paper 3.1 : "MVA LCD for Notebook or Mobile PCs SID 2004 International Symposium, Digest of Technical Papers, XXXV, Book I, pp. 6 to 9, and Liu, C.T. et al., paper 15.1 : "A 46-inch TFT-LCD HDTV Technology SID 2004 International Sympo- sium, Digest of Technical Papers, XXXV, Book II, pp. 750 to 753), PVA (patterned vertical alignment, for example: Kim, Sang Soo, paper 15.4: "Super PVA Sets New State-of-the-Art for LCD-TV", SID 2004 International Symposium, Digest of Technical Papers, XXXV, Book II, pp. 760 to 763), ASV (advanced super view, for example: Shigeta, Mitzuhiro and Fukuoka, Hirofumi, paper 15.2: "Development of High Quality LCDTV", SID 2004 International Symposium, Digest of Technical Papers, XXXV, Book II, pp. 754 to 757) modes, have established themselves as one of the three more recent types of liquid-crystal display that are currently the most important, in particular for television applications, besides IPS (in-Djane switching) displays (for example: Yeo, S.D., paper 15.3: "An LC Display for the TV Application", SID 2004 International Symposium, Digest of Technical Papers, XXXV, Book II, pp. 758 & 759) and the long-known TN (twisted nematic) displays. The technologies are compared in general form, for example, in Souk, Jun, SID Seminar 2004, seminar M-6: "Recent

Advances in LCD Technology", Seminar Lecture Notes, M-6/1 to M-6/26, and Miller, Ian, SID Seminar 2004, seminar M-7: "LCD-Television", Seminar Lecture Notes, M-7/1 to M-7/32. Although the response times of mod- ern ECB displays have already been significantly improved by addressing methods with overdrive, for example: Kim, Hyeon Kyeong et al., paper 9.1 : "A 57-in. Wide UXGA TFT-LCD for HDTV Application", SID 2004 International Symposium, Digest of Technical Papers, XXXV, Book I, pp. 106 to 109, the achievement of video-compatible response times, in particular on switching of grey shades, is still a problem which has not yet been satisfactorily solved.

Industrial application of this effect in electro-optical display elements requires LC phases, which have to satisfy a multiplicity of requirements. Particularly important here are chemical resistance to moisture, air and physical influences, such as heat, infrared, visible and ultraviolet radiation and direct and alternating electric fields.

Furthermore, industrially usable LC phases are required to have a liquid- crystalline mesophase in a suitable temperature range and low viscosity. None of the hitherto-disclosed series of compounds having a liquid-crystalline mesophase includes a single compound which meets all these requirements. Mixtures of two to 25, preferably three to 18, compounds are there- fore generally prepared in order to obtain substances which can be used as LC phases. However, it has not been possible to prepare optimum phases easily in this way since no liquid-crystal materials having significantly negative dielectric anisotropy and adequate long-term stability were hitherto available.

Matrix liquid-crystal displays (MLC displays) are known. Non-linear elements which can be used for individual switching of the individual pixels are, for example, active elements (i.e. transistors). The term "active matrix" is then used, where a distinction can be made between two types:

1 . MOS (metal oxide semiconductor) transistors on a silicon wafer as substrate

2. thin-film transistors (TFTs) on a glass plate as substrate.

A distinction is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. The latter technology is being worked on intensively worldwide.

The TFT matrix is applied to the inside of one glass plate of the display, while the other glass plate carries the transparent counterelectrode on its inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully colour-capable displays, in which a mosaic of red, green and blue filters is arranged in such a way that a filter element is opposite each switchable pixel.

The term MLC displays here covers any matrix display with integrated non- linear elements, i.e. besides the active matrix, also displays with passive elements, such as varistors or diodes (MIM = metal-insulator-metal). MLC displays of this type are particularly suitable for TV applications (for example pocket TVs) or for high-information displays in automobile or aircraft construction. Besides problems regarding the angle dependence of the contrast and the response times, difficulties also arise in MLC displays due to insufficiently high specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORI- MACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, Sept. 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, pp. 141 ff., Paris; STROMER, M., Proc. Eurodisplay 84, Sept. 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, pp. 145 ff., Paris]. With decreasing resistance, the contrast of an MLC display deteriorates. Since the specific resistance of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the inside surfaces of the display, a high (initial) resistance is very important for displays that have to have acceptable resistance values over a long operating period.

There is thus still a great demand for MLC displays having very high spe- cific resistance at the same time as a large working-temperature range, short response times and a low threshold voltage, with the aid of which various grey shades can be generated.

The disadvantage of the MLC-TN displays frequently used is due to their comparatively low contrast, the relatively high viewing-angle dependence and the difficulty of generating grey shades in these displays.

The invention is based on the object of providing liquid-crystal mixtures, in particular for monitor and TV applications, based on the IPS, UB-FFS or ECB effect, which do not have the disadvantages indicated above, or only do so to a reduced extent. In particular, it must be ensured for monitors and televisions that they also work at extremely high and extremely low temperatures and at the same time have very short response times and at the same time have an improved reliability behaviour, in particular exhibit no or significantly reduced image sticking after long operating times. Surprisingly, it is possible to obtain fast response times of LC mixtures at the same time as good reliability through the use of the compounds of the formula I if suitable stabilisers are added. A reliability parameter which can be specifically influenced here is the voltage holding ratio after exposure to light, such as, for example, exposure to UV light (sun test) or exposure by the backlight of an LCD. The use of stabilisers of this type increases the voltage holding ratio after exposure to light. The invention thus relates to a liquid-crystalline medium which comprises at least one compound of the formula I, at least one compound from formula ST, and optionally, preferably obligatory, one or more compounds selected from formulae T and L

wherein

L ¹ , L ² independently are H or F, where at least one of L ¹ and L ² is F,

L ³ , L ⁴ independently are H or F, where at least one of L ³ and L ⁴ is F,

R each, independently of one another, denote H, an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more Ch groups in these radicals may each be replaced, independently of one another, by

such a way that O

atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, preferably a straight-chain alkyl or alkoxy radical having 1 -6 C atoms, alkyl a straight-chain or branched, preferably straight-chain alkyl radical having 1 -6 C atoms, n 0 or 1 , preferably 1 , and

(O) is -O- or a single bond, preferably -O-.

In a preferred embodiment the liquid-crystalline medium comprises at least one compound of the formula I, at least one compound from formula ST, and one or more compounds of formula T.

Preferred compounds of formula T are selected from the following subformulae:

in which R denotes a straight-chain alkyl or alkoxy radical having 1 -6 C atoms, and m = 0, 1 , 2, 3, 4, 5 or 6 . In these compounds, R preferably denotes alkyl, each having 1 -5 C atoms.

R preferably denotes methyl, ethyl, propyl, butyl, pentyl, hexyl, methoxy, ethoxy, propoxy, butoxy or pentoxy.

The medium according to the invention preferably comprises the

terphenyls of the formulae T-1 to T-5 in amounts of 2-30% by weight, in particular 3-20% by weight.

Particular preference is given to compounds of the formulae T-1 to T-3, and more particularly to the compounds of formula T-1 .

The terphenyls are preferably employed in the mixtures according to the invention if the Δη value of the mixture is to be > 0.1 . Preferred mixtures comprise 2-20% by weight of one or more terphenyl compounds selected from the group of the compounds T-1 to T-5.

The mixtures according to the invention preferably exhibit very broad nematic phase ranges with clearing points > 70°C, preferably > 75°C, in particular > 80°C, relatively high values of the holding ratio, very favourable values of the capacitive threshold and at the same time very good low- temperature stabilities at -20°C and -30°C, as well as very low rotational viscosity values and short response times. The mixtures according to the invention are furthermore distinguished by the fact that, in addition to the improvement in the rotational viscosity γι , relatively high values of the elastic constants K33 for improving the response times can be observed. The use of the compounds of the formula I in LC mixtures, preferably having negative dielectric anisotropy, the ratio of rotational viscosity γ-\ and elastic constants K is reduced.

The invention also relates to a process for the preparation of a liquid- crystalline medium according to the invention, characterised in that at least one compound of the formula I and at least one compound of formula ST is mixed with at least one further liquid-crystalline compound, and optionally one or more additives are added. Some preferred embodiments of the mixtures according to the invention are indicated below.

The compounds of formula ST are preferably employed in the liquid crystalline media in a concentration in the range from 0.0005 % by weight to 2 %, more preferably in the range from 0.001 % to 1 %, particularly preferably in the range from 0,005 % to 0.05 %, all % by weight.

The total content of polymerizable or polymerized components in the liquid crystalline medium according to the invention is preferably below 0.1 % by weight, more preferably below 0.05 %, and most preferably lower than 0.02 % (200 ppm).

In a preferred embodiment the liquid-crystalline medium comprises at least one compound of the formula I, at least one compound from formula ST, and one or more compounds of formula L. Most preferably the liquid- crystalline medium comprises at least one compound of the formula I, at least one compound from formula ST, one or more compounds of formula T, and one or more compounds of formula L. Preferred compounds of formula L are selected from the following subformulae L-1 and L-2:

which independently of one another, denote H, an alkyl or alkenyl radical having up to 15 C atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, where, in addition, one or more CH2 groups in these radicals may be replaced by -O-, in such a

way that O atoms are not linked directly to one another, and alkyl denotes an alkyl radical having 1 -6 C atoms.

In the compounds of the formula I, R ¹ and R ^1* preferably each, independently of one another, denote straight-chain alkoxy, in particular OCH3, furthermore alkenyl, in ,

R ¹ and R ^1* particularly preferably each, independently of one another, denote straight-chain alkoxy having 1 -6 C atoms, in particular methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy.

A ¹ and A ^1* preferably denote 1 ,4-cyclohexane or 1 ,4-phenylene.

Preferably the parameters a and b are both 0, or alternatively a is 1 and b is 0. Most preferably a = b = 0.

L ¹ and L ² in formula I preferably both denote F.

More preferred compounds of the formula I are the compounds of the formulae 1-1 to I-20,

in which alkyl and alkyl ^* each, independently of one another, denote a straight-chain alkyl radical having 1 -6 C atoms, alkenyl and alkenyl ^* each, independently of one another, denote a straight-chain alkenyl radical having 2-6 C atoms, alkoxy and alkoxy ^* each, independently of one another, denote a straight- chain alkoxy radical having 1 -6 C atoms, and L ¹ and L ² each, independently of one another, denote F or CI.

In the compounds of the formulae 1-1 to I-20, L ¹ and L ² preferably each, independently of one another, denote F or CI, in particular L ¹ = L ² = F. Particular preference is given to the compounds of the formulae I-2 and I-6. In the compounds of the formulae I-2 and I-6, preferably L ¹ = L ² = F.

The mixture according to the invention very particularly preferably comprises at least one compound selected from the group of the compounds of the formulae 1-1 A, I-2A, I-4A, I-6A, I-6B, 1-1 1A, 1-12-A, l-14A and I-16A shown below:

Very particularly preferred mixtures comprise at least one compound selected from the group of the compounds of the formulae below:

The compounds of the formula I can be prepared, for example, as described in US 2015/0259602 A, US 9,512,102 B, US 2005/0258399 A or US 2004/124399 A. The media according to the invention preferably comprise one, two, three, four or more, preferably one, two or three, compounds of the formula I.

The compounds of the formula I are preferably employed in the liquid- crystalline medium in amounts of > 1 , preferably > 3% by weight, based on the mixture as a whole. Particular preference is given to liquid-crystalline media which comprise 1 - 40% by weight, very particularly preferably 2 - 30% by weight, of one or more compounds of the formula I.

Of the compounds of the formula ST, especial preference is given to the compounds of the formulae wherein Pip is a group selected from the formulae

in which

a _re independently straight chain or branched alkyl with 1 to

10 C atoms.

Preferably Z ¹ in formula ST denotes or a single bond, very preferably or a single bond.

Preferably Z ² and Z ³ in formula ST denote or a single bond, very preferably a single bond.

Preferably P in formula ST is an acrylate or methacrylate group. Preferably Sp in formula ST is a single bond.

Preferably R ^g in formula I is H.

Preferably A ³ in formula ST denotes an aromatic or heteroaromatic group with 6 to 24 ring atoms, which may also contain fused rings, and is optionally substituted by one or more groups L. Very preferably A ³ in formula ST denotes benzene or naphthalene, which is optionally substituted by one or more groups L.

Preferably A ¹ and A ² in formula ST denote an aromatic or heteroaromatic group with 6 to 24 ring atoms, which may also contain fused rings, and is optionally substituted by one or more groups L or R-(A ³ -Z ³ ) _m 2-, or A ¹ is a single bond.

Very preferably A ¹ and A ² in formula ST denote benzene, cyclohexylene, naphthalene, phenanthrene or anthracene, which is optionally substituted by one or more groups L or R-(A ³ -Z ³ ) _m 2-, or A ¹ is a single bond.

Preferably -(A ² -Z ² -A ¹ ) _m i- in formula ST denotes benzene, biphenylene, p- terphenylene (1 ,4-diphenylbenzene), m-terphenylene (1 ,3- diphenylbenzene), naphthylene, 2-phenyl-naphthylene, phenanthrene or anthracene, all of which are optionally substituted by one or more groups L.

Very preferably -(A ² -Z ² -A ¹ ) _m i- denotes biphenylene, p-terphenylene or m- terphenylene, all of which are optionally substituted by one or more groups L.

Preferred groups -(A ² -Z ² -A ¹ ) _m i- are selected from the following formulae

A1



wherein L is as defined in fornnula ST or has one of the preferred meanings as described above and below, r is 0, 1 , 2, 3 or 4, s is 0, 1 , 2 or 3, t is 0, 1 or 2, and u is 0, 1 , 2, 3, 4 or 5.

Particular preference is given to the groups of formula A1 , A2, A3, A4 and A5.

Very particularly preferred mixtures according to the invention comprise one or more stabilisers selected from the following subformulae

-26-

-28-

35 -32-

wherein P, Sp, R ^a"d , Z ¹ , L and R are as defined in formula ST or have one of the preferred meanings as described above and below,

R ^e is alkyl having 1 to 12 C atoms,

r is 0, 1 , 2, 3 or 4 and

s is 0, 1 , 2 or 3.

Preferably Z ¹ in formulae ST and ST-1 to ST-45 is -CO-O-, -O-CO-, or a single bond, very preferably -CO-O- or a single bond. Preferably P in formulae ST and ST-1 to ST-45 is acrylate or methacrylate

Preferably Sp in formulae ST and ST-1 to ST-45 is a single bond.

Preferably R ^a , R ^b , R ^c and R ^d in formulae ST and ST-1 to ST-45 are methyl

Preferred structures among ST-1 to ST-45 are the structures ST-1 and ST 23.

Further preferred compounds of formula ST and its subformulae ST-1 to ST-45 are independently selected from the following preferred

embodiments, including any combination thereof:

The compounds contain exactly one polymerisable group

(represented by the group P),

Sp, when being different from a single bond, is selected from

-(CH ₂ ) _a -O-, -(CH ₂ ) _a -CO-O-, -(CH ₂ ) _a - and -(CH ₂ ) _a -O-CO-, wherein a is 2, 3, 4, 5 or 6, and the O-atom or the CO-group, respectively, is connected to the next ring A ² or the group Pip, as applicable,

R ^e is methyl, ethyl, n-propyl, iso-propyl, tert-butyl, n-butyl or n-pentyl, ml is 0, 1 or 2,

m2 is 0, 1 or 2,

Z ¹ denotes -CO-O-, -O-CO- or a single bond, preferably -CO-O-,

Z ² denotes -CO-O-, -O-CO- or a single bond, preferably a single bond,

L denotes F, CI, CN, or alkyl or alkoxy with 1 to 6 C atoms that is optionally fluorinated, very preferably

most preferably F,

one or more of L denote a group Pip,

r is 0 or 1 ,

s is 0,

t is O u is 0, 1 or 2.

Preferred embodiments of the liquid-crystalline medium according to the invention are indicated below: a) Liquid-crystalline medium which additionally comprises one or more compounds selected from the group of the compounds of the formu- lae MA, MB and IIC,

in which

R ^2A , R ^2B and R ^2C each, independently of one another, denote H, an alkyl or alkenyl radical having up to 15 C atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, where, in addition, one or more CH2 groups in these radicals may be replaced by -O-, -S-, , -C≡C-, -CF2O-, -OCF2-, -OC-O- or

-O-CO- in such a way that O atoms are not linked directly to one another, 1-4 each, independently of one another, denote F, CI,

Z ² and Z ^2' each, independently of one another, denote a

single bond,

P denotes 0, 1 or 2, denotes 0 or 1 , and denotes 1 to 6.

In the compounds of the formulae IIA and MB, Z ² may have identical or different meanings. In the compounds of the formula MB, Z ² and Z ^2' may have identical or different meanings.

In the compounds of the formulae IIA, MB and IIC, R ^2A , R ^2B and R ^2C each preferably denote alkyl having 1 -6 C atoms, in particular

In the compounds of the formulae IIA and MB, L ¹ , L ² , L ³ and L ⁴ preferably denote L ¹ = L ² = F and L ³ = L ⁴ = F, furthermore L ¹ = F and L ² = CI, L ¹ = CI and L ² = F, L ³ = F and L ⁴ = CI, L ³ = CI and L ⁴ = F. Z ² and Z ² ' in the formulae IIA and MB preferably each, independently of one another, denote a single bond, furthermore a -C ₂ H ₄ - bridge.

If, in the formula MB, Z ² = -C ₂ H ₄ - or -CH ₂ O-, Z ^2' is preferably a single bond or, if Z ^2' = -C ₂ H ₄ - or -CH ₂ O-, Z ² is preferably a single bond. In the compounds of the formulae IIA and MB, (O)C _v H _2v +i preferably denotes OC _v H _2v +i . In the compounds of the formula IIC, (O)C _v H _2v +i preferably denotes C _v H _2v +i . In the compounds of the formula IIC, L ³ and L ⁴ preferably each denote F.





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 2-6 C atoms.

Particularly preferred mixtures according to the invention comprise one or more compounds of the formulae IIA-2, IIA-8, IIA-14, IIA-26, II-28, IIA-33, IIA-39, IIA-45, IIA-46, IIA-47, IIA-50, IIB-2, IIB-1 1 , IIB-16 and IIC-1 .

The proportion of compounds of the formulae IIA and/or MB in the mixture as a whole is preferably at least 20% by weight.

Particularly preferred media according to the invention comprise at least one compound of the formula IIC-1 , in which alkyl and alkyl ^* have the meanings indicated above, preferably in amounts of > 3% by weight, in particular > 5% by weight and particularly preferably 5-25% by weight.

Liquid-crystalline medium which additionally comprises one or more compounds of the formula III, in which and R ³² each, independently of one another, denote a straight- chain alkyl, alkoxy, alkenyl, alkoxyalkyl or alkoxy radical having up to 12 C atoms, and

Preferred compounds of the formula III are indicated below:

alkyl and

alkyl ^* each, independently of one another, denote a straight- chain alkyl radical having 1 -6 C atoms.

The medium according to the invention preferably comprises at least one compound of the formula Ilia and/or formula 1Mb.

The proportion of compounds of the formula III in the mixture as a whole is preferably at least 5% by weight Liquid-crystalline medium additionally comprising a compound of the formula

preferably in total amounts of > 5% by weight, in particular > 10% by weight.

Preference is furthermore given to mixtures according to the invention comprising the compound (acronym: CC-3-V1 )

preferably in amounts of 2-1 5% by weight.

Preferred mixtures comprise 5-60% by weight, preferably 1 0-55% by weight, in particular 20-50% by weight, of the compound of the formula (acronym: CC-3-V)

Preference is furthermore given to mixtures which comprise a compound of the formula (acronym: CC-3-V)

and a compound of the formula (acronym: CC-3-V1 )

preferably in amounts of 1 0 - 60% by weight. Liquid-crystalline medium which additionally comprises one or more tetracyclic compounds of the formulae

in which each, independently of one another, have one of the meanings indicated for R ^2A in Claim 5, and w and x each, independently of one another, denote 1 to 6. Particular preference is given to mixtures comprising at least one compound of the formula V-9. Liquid-crystalline medium which additionally comprises one or more compounds of the formulae Y-1 to Y-6,

in which R ¹⁴ -R ¹⁹ each, independently of one another, denote an alkyl or alkoxy radical having 1 -6 C atoms; z and m each, independently of one another, denote 1 -6; x denotes 0, 1 , 2 or 3. The medium according to the invention particularly preferably comprises one or more compounds of the formulae Y-1 to Y-6, preferably in amounts of > 5% by weight. Liquid-crystalline medium additionally comprising one or more fluori- nated terphenyls of the formulae TF-1 to TF-21 ,

in which

R denotes a straight-chain alkyl or alkoxy radical having 1 -6 C atoms, and m = 0, 1 , 2, 3, 4, 5 or 6 and n denotes 0, 1 , 2, 3 or 4.

R preferably denotes methyl, ethyl, propyl, butyl, pentyl, hexyl, meth- oxy, ethoxy, propoxy, butoxy, pentoxy.

The medium according to the invention preferably comprises the terphenyls of the formulae TF-1 to TF-21 in amounts of 2-30% by weight, in particular 5-20% by weight.

Particular preference is given to compounds of the formulae TF-1 , TF-2, TF-4, TF-20 and TF-21 . In these compounds, R preferably denotes alkyl, furthermore alkoxy, each having 1 -5 C atoms. In the compounds of the formula TF-20, R preferably denotes alkyl or alkenyl, in particular alkyl. In the compound of the formula TF-21 , R preferably denotes alkyl.

The terphenyls are preferably employed in the mixtures according to the invention if the Δη value of the mixture is to be > 0.1 . Preferred mixtures comprise 2-20% by weight of one or more terphenyl compounds selected from the group of the compounds TF-1 to TF-21 .

Liquid-crystalline medium additionally comprising one or more bi- phenyls of the formulae PP-1 to PP-3,

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 2-6 C atoms.

The proportion of the biphenyls of the formulae PP-1 to PP-3 in the mixture as a whole is preferably at least 3% by weight, in particular > 5% by weight.

Of the compounds of the formulae PP-1 to PP-3, the compounds of the formula PP-2 are particularly preferred.

Particularly preferred biphenyls are

in which alkyl ^* denotes an alkyl radical having 1 -6 C atoms. The medium according to the invention particularly preferably comprises one or more compounds of the formulae PP-1 a and/or PP-2c. Liquid-crystalline medium comprising at least one compound of the formulae Z-1 to Z-7,

in which R and alkyl have the meanings indicated above. Liquid-crystalline medium additionally comprising at least one compound of the formulae O-1 to Ο-1 8,

-56-

in which R ¹ and R ² have the meanings indicated for R ^2A . R ¹ and R ² preferably each, independently of one another, denote straight-chain alkyl or alkenyl.

Preferred media comprise one or more compounds of the formulae and/or

Mixtures according to the invention very particularly preferably comprise the compounds of the formula O-10, O-12, O-16 and/or O-17, in particular in amounts of 5-50%. Preferred compounds of the formulae O-10 and 0-17 are indicated below:

The medium according to the invention particularly preferably comprises the tricyclic compounds of the formula 0-1 Oa and/or of the formula O-10b in combination with one or more bicyclic compounds of the formulae O-17a to 0-17i. The total proportion of the compounds of the formulae O-10a and/or O-10b in combination with one or more compounds selected from the bicyclic compounds of the formulae O-17a to 0-17i is 5-55%, very particularly preferably 15-40%.

Very particularly preferred mixtures comprise compounds 0-1 Oa and 0-17a:

The compounds 0-1 Oa and 0-17a are preferably present in the mixture in a concentration of 15-50%, particularly preferably 25-45% and especially preferably 25-35%, based on the mixture as a whole.

Very particularly preferred mixtures additionally comprise one or more of the compounds 0-17b and 0-17i:

The compounds 0-17b and 0-17i are preferably present in the mixture in a concentration of 5-20%, particularly preferably 8-15%, based on the mixture as a whole for both compounds.

Preferred liquid-crystalline media according to the invention comprise one or more substances which contain a tetrahydronaphthyl or naphthyl unit, such as, for example, the compounds of the formulae N-1 to N-5,

in which R ^{1 N} and R ^2N each, independently of one another, have the meanings indicated for R ^2A , preferably denote straight-chain alkyl, straight-chain alkoxy or straight-chain alkenyl, and

Z ¹ and Z ² each, independently of one another, denote -C2H ₄ -,

or a single bond.

Preferred mixtures comprise one or more compounds selected from the group of the difluorodibenzochroman compounds of the formula BC, chromans of the formula CR, fluorinated phenanthrenes of the formulae PH-1 and PH-2, fluorinated dibenzofurans of the formula BF-1 and BF-2,

in which

R ^B1 , R ^B2 , R ^CR1 , R ^CR2 , R ¹ , R ² each, independently of one another, have the meaning of R ^2A . c is 0, 1 or 2 and d denotes 1 or 2. R ¹ and R ² preferably, independently of one another, denote alkyl or alkoxy having 1 to 6 C atoms. The compounds of the formulae BF-1 and BF-2 should not be identical to one or more compounds of the formula I.

The mixtures according to the invention preferably comprise the compounds of the formulae BC, CR, PH-1 , PH-2 and/or BF in amounts of 3 to 20% by weight, in particular in amounts of 3 to 15% by weight.

Particularly preferred compounds of the formulae BC and CR are the compounds BC-1 to BC-7 and CR-1 to CR-5,

in which alkyl and alkyl ^* each, independently of one another, denote a

straight-chain alkyl radical having 1 -6 C atoms, and

each, independently of one another, denote a straight-chain alkenyl radical having 2-6 C atoms

Very particular preference is given to mixtures comprising one, two or three compounds of the formula BC-2, BF-1 and/or BF-2. Preferred mixtures comprise one or more indane compounds of the formula In,

In in which

R ¹¹ , R ¹² ,

_R 13 each, independently of one another, denote a straight- chain alkyl, alkoxy, alkoxyalkyi or alkenyl radical having 1 -6 C atoms, additionally denote halogen, preferably F,

denotes 0, 1 or 2.

Preferred compounds of the formula In are the compounds of the formulae ln-1 to ln-16 indicated below:



Particular preference is given to the compounds of the formulae ln-1 , ln-2, ln-3 and ln-4.

The compounds of the formula In and the sub-formulae ln-1 to ln-16 are preferably employed in the mixtures according to the invention in concentrations > 5% by weight, in particular 5 - 30% by weight and very particularly preferably 5 - 25% by weight.

Preferred mixtures additionally comprise one or more compounds of the formulae LY-1 to LY-1 1 , R, R ¹ and R ² each, independently of one another, have the meanings indicated for R ^2A in Claim 5 and alkyl denotes an alkyl radical having 1 -6 C atoms, s denotes 1 or 2. -(O)alkyl denotes -O-alkyl or -alkyl, preferably -O-alkyl.

Particularly preferred mixture concepts are indicated below: (the acronyms used are explained in Table A. n and m here each, independently of one another, denote 1 -15, preferably 1 -6).

The mixtures according to the invention preferably comprise

- one or more compounds of the formula I in which L ¹ = L ² = F and R ¹ = R ^1* = alkoxy;

- one or more compounds of the formula ST in which m is 0 or 1 ;

- one or more compounds of the formula L in which n is 1 ;

- one or more compounds of the formula CPY-n-Om, in particular CPY-2- 02, CPY-3-O2 and/or CPY-5-O2, preferably in concentrations > 5%, in particular 10-30%, based on the mixture as a whole, and

- one or more compounds of the formula CY-n-Om, preferably CY-3-O2, CY-3-O4, CY-5-O2 and/or CY-5-O4, preferably in concentrations > 5%, in particular 5-25%, based on the mixture as a whole, and/or

- one or more compounds of the formula CCY-n-Om, preferably CCY-4-O2, CCY-3-O2, CCY-3-O3, CCY-3-O1 and/or CCY-5-O2, preferably in concentrations > 5%, in particular 10-30%, based on the mixture as a whole. The invention furthermore relates to an electro-optical display having active-matrix addressing based on the ECB, VA, PS-VA, PA-VA, IPS, PS- IPS, FFS or PS-FFS effect, characterised in that it contains, as dielectric, a liquid-crystalline medium according to one or more of the claims.

The liquid-crystalline medium according to the invention preferably has a nematic phase from≤ -20°C to > 70°C, particularly preferably from≤ -30°C to≥ 80°C, very particularly preferably from≤ -40°C to≥ 90°C.

The expression "have a nematic phase" here means on the one hand that no smectic phase and no crystallisation are observed at low temperatures at the corresponding temperature and on the other hand that clearing still does not occur on heating from the nematic phase. The investigation at low temperatures is carried out in a flow viscometer at the corresponding temperature and checked by storage in test cells having a layer thickness corresponding to the electro-optical use for at least 100 hours. If the storage stability at a temperature of -20°C in a corresponding test cell is 1000 h or more, the medium is referred to as stable at this temperature. At tempera- tures of -30°C and -40°C, the corresponding times are 500 h and 250 h respectively. At high temperatures, the clearing point is measured by conventional methods in capillaries.

The liquid-crystal mixture preferably has a nematic phase range of at least 60 K and a flow viscosity V20 of at most 30 mm ² · s ^-1 at 20°C.

The values of the birefringence Δη in the liquid-crystal mixture are generally between 0.07 and 0.16, preferably between 0.08 and 0.13. The liquid-crystal mixture according to the invention has a Δε of -0.5 to -8.0, in particular -2.5 to -6.0, where Δε denotes the dielectric anisotropy. The rotational viscosity γ-\ at 20°C is preferably≤ 150 mPa s, in particular≤ 120 mPa-s. The liquid-crystal media according to the invention have relatively low values for the threshold voltage (Vo). They are preferably in the range from 1 .7 V to 3.0 V, particularly preferably≤ 2.5 V and very particularly preferably≤ 2.3 V. For the present invention, the term "threshold voltage" relates to the capa- citive threshold (Vo), also called the Freedericks threshold, unless explicitly indicated otherwise.

In addition, the liquid-crystal media according to the invention have high values for the voltage holding ratio in liquid-crystal cells.

In general, liquid-crystal media having a low addressing voltage or threshold voltage exhibit a lower voltage holding ratio than those having a higher addressing voltage or threshold voltage and vice versa.

For the present invention, the term "dielectrically positive compounds" denotes compounds having a Δε > 1 .5, the term "dielectrically neutral compounds" denotes those having -1 .5≤ Δε≤ 1 .5 and the term "dielectrically negative compounds" denotes those having Δε < -1 .5. The dielectric ani- sotropy of the compounds is determined here by dissolving 10% of the compounds in a liquid-crystalline host and determining the capacitance of the resultant mixture in at least one test cell in each case having a layer thickness of 20 μιτι with homeotropic and with homogeneous surface alignment at 1 kHz. The measurement voltage is typically 0.5 V to 1 .0 V, but is always lower than the capacitive threshold of the respective liquid-crystal mixture investigated.

All temperature values indicated for the present invention are in °C. The mixtures according to the invention are suitable for IPS (in-Djane switching) and UB-FFS (fringe field switching) applications having negative Δε. They are furthermore suitable for all VA-TFT applications, such as, for example, VAN, MVA, (S)-PVA, ASV, PSA (polymer sustained VA) and PS- VA (polymer stabilized VA). The nematic liquid-crystal mixtures in the displays according to the invention generally comprise two components A and B, which themselves consist of one or more individual compounds.

Component A has significantly negative dielectric anisotropy and gives the nematic phase a dielectric anisotropy of≤ -0.5. Besides one or more compounds of the formula I, it preferably comprises the compounds of the formulae IIA, MB and/or IIC, furthermore one or more compounds of the for- mula T.

The proportion of component A is preferably between 45 and 100%, in particular between 60 and 100%. For component A, one (or more) individual compound(s) which has (have) a value of Δε≤ -0.8 is (are) preferably selected. This value must be more negative, the smaller the proportion A in the mixture as a whole.

Component B has pronounced nematogeneity and a flow viscosity of not greater than 30 mm ² · s ^-1 , preferably not greater than 25 mm ² · s ^-1 , at 20°C.

A multiplicity of suitable materials is known to the person skilled in the art from the literature. Particular preference is given to compounds of the formula Ο-17.

Particularly preferred individual compounds in component B are extremely low-viscosity nematic liquid crystals having a flow viscosity of not greater than 18 mm ² · s ^-1 , preferably not greater than 12 mm ² · s ^-1 , at 20°C. Component B is monotropically or enantiotropically nematic, has no smectic phases and is able to prevent the occurrence of smectic phases down to very low temperatures in liquid-crystal mixtures. For example, if various materials of high nematogeneity are added to a smectic liquid- crystal mixture, the nematogeneity of these materials can be compared through the degree of suppression of smectic phases that is achieved. It goes without saying for the person skilled in the art that the IPS, FFS or VA mixture according to the invention may also comprise compounds in which, for example, H, N, O, CI and F have been replaced by the corresponding isotopes.

The mixtures according to the invention may furthermore comprise conventional additives, such as, for example, stabilisers, antioxidants, UV absorbers, nanoparticles, microparticles, etc.

The structure of the liquid-crystal displays according to the invention corresponds to the usual geometry. For example, the preferred IPS displays contain an LC layer between two substrates with planar orientation, where the two electrodes are arranged on only one of the two substrates and preferably have interdigitated, comb-shaped structures. On application of a voltage to the electrodes an electric field with a significant component parallel to the LC layer is generated between them. This causes

realignment of the LC molecules in the layer plane. For negative IPS the alignment direction of the alignment layer is preferably perpendicular to the interdigital electrodes. The so-called FFS ("fringe-field switching") displays have been reported (see, inter alia, S.H. Jung et al., Jpn. J. Appl. Phys., Volume 43, No. 3, 2004, 1028), which contain two electrodes on the same substrate, one of which is structured in a comb-shaped manner and the other is unstructured. Furthermore, FFS displays have been disclosed (see S.H. Lee et al., Appl. Phys. Lett. 73(20), 1998, 2882-2883 and S.H. Lee et al., Liquid Crystals 39(9), 2012, 1 141 -1 148), which have similar electrode design and layer thickness as FFS displays, but comprise a layer of an LC medium with negative dielectric anisotropy instead of an LC medium with positive dielectric anisotropy.

The following meanings apply above and below:

Unless stated otherwise, the term "polymerisable compound" as used herein will be understood to mean a polymerisable monomeric compound.

The term "spacer group" or "spacer" for short, also referred to as "Sp" above and below, is known to the person skilled in the art and is described in the literature, see, for example, Pure Appl. Chem. 73(5), 888 (2001 ) and C. Tschierske, G. Pelzl, S. Diele, Angew. Chem. 2004, 1 16, 6340-6368. Unless indicated otherwise, the term "spacer group" or "spacer" above and below denotes a flexible group which connects the mesogenic group and the polymerisable group(s) to one another in a polymerisable mesogenic compound. Whereas the mesogenic group generally contains rings, the spacer group is generally without ring systems, i.e. is in chain form, where the chain may also be branched. The term chain is applied, for example, to an alkylene group. Substitutions on and in the chain, for example by -O- or -COO-, are generally included. In functional terms, the spacer (the spacer group) is a linker between functional structural parts of a molecule which facilitates a certain spatial flexibility between these parts. In a preferred embodiment a spacer denotes an alkylene (like -( CH ₂ )n- and n= 1 to10) or alkyleneoxy group, preferably with 2 to 5 carbon atoms.

The term "halogen" refers to fluorine, chlorine or bromine, preferably fluorine or chlorine and in particular to fluorine. The term halogenated is used analogously.

Throughout the patent application, 1 ,4-cyclohexylene rings and 1 ,4- phenylene rings are depicted as follows:

The 1 ,4-substituted cyclohexylene rings are trans-1 ,4-cyclohexylene rings.

The following examples are intended to explain the invention without limiting it. Above and below, per cent data denote per cent by weight; all temperatures are indicated in degrees Celsius.

Throughout the patent application and in the working examples, the structures of the liquid-crystal compounds are indicated by means of acronyms. Unless indicated otherwise, the transformation into chemical formulae is carried out in accordance with Tables 1 -3. All radicals C _n H _2n+ i, C _m H _2m+ i and C _m 'H _2m '+i or C _n H _2n and C _m H _2m are straight-chain alkyl radicals or alkylene radicals respectively in each case having n, m, m' or z C atoms respectively, n, m, m', z each denote, independently of one another, 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 or 12, preferably 1 , 2, 3, 4, 5 or 6. In Table 1 the ring elements of the respective compound are coded, in Table 2 the bridging members are listed and in Table 3 the meanings of the symbols for the left-hand or right-hand side chains of the compounds are indicated.

Table 1 : Ring elements

Table 3: Side chains

Besides one or more compounds of the formula I, the mixtures according to the invention preferably comprise one or more compounds of the compounds mentioned below from Table A. Table A

The following abbreviations are used:

(n, m, m', z: each, independently of one another, 1 , 2, 3, 4, 5 or 6;









The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in lesser amount is dissolved in the components making up the principal constituent, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example by distillation, after thorough mixing.

By means of suitable additives, the liquid-crystal phases according to the invention can be modified in such a way that they can be employed in any type of, for example, ECB, VAN, IPS, GH or ASM-VA LCD display that has been disclosed to date.

The dielectrics may also comprise further additives known to the person skilled in the art and described in the literature, such as, for example, UV absorbers, antioxidants, nanoparticles and free-radical scavengers. For example, 0-15% of pleochroic dyes, stabilisers, such as, for example, phenols, HALS (hindered amine light stabilisers), or chiral dopants may be added. Suitable stabilisers for the mixtures according to the invention are, in particular, those listed in Table C.

For example, 0-15% of pleochroic dyes may be added, furthermore conductive salts, preferably ethyldimethyldodecylammonium 4-hexoxybenzo- ate, tetrabutylammonium tetraphenylboranate or complex salts of crown ethers (cf., for example, Haller et al., Mol. Cryst. Liq. Cryst., Volume 24, pages 249-258 (1973)), may be added in order to improve the conductivity or substances may be added in order to modify the dielectric anisotropy, the viscosity and/or the alignment of the nematic phases. Substances of this type are described, for example, in DE-A 22 09 127, 22 40 864, 23 21 632, 23 38 281 , 24 50 088, 26 37 430 and 28 53 728.

Table B

Table B indicates possible dopants which can be added to the mixtures according to the invention. If the mixtures comprise a dopant, it is added amounts of 0.01 -4% by weight, preferably 0.01 -3% by weight.

The mixtures according to the invention comprise at least one stabiliser from Table C given below.

Table C

Stabilisers which can be added, for example, to the mixtures according to the invention in amounts of 0-10% by weight, preferably 0.001 -5% by weight, in particular 0.001 -1 % by weight, are indicated below.





Working examples

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

The host mixture used for determination of the optical anisotropy Δη of the compounds of the formula I is the commercial mixture ZLI-4792 (Merck KGaA). The dielectric anisotropy Δε is determined using commercial mixture ZLI-2857. The physical data of the compound to be investigated are obtained from the change in the dielectric constants of the host mixture after addition of the compound to be investigated and extrapolation to 100% of the compound employed. In general, 10% of the compound to be investigated are dissolved in the host mixture, depending on the solubility. Unless indicated otherwise, parts or per cent data denote parts by weight or per cent by weight.

Above and below: Vo denotes threshold voltage, capacitive [V] at 20°C,

n _e denotes extraordinary refractive index at 20°C and 589 nm, n ₀ denotes ordinary refractive index at 20°C and 589 nm,

Δη denotes optical anisotropy at 20°C and 589 nm,

ε± denotes dielectric permittivity perpendicular to the director at

20°C and 1 kHz,

ε 11 denotes dielectric permittivity parallel to the director at 20°C and 1 kHz,

Δε denotes dielectric anisotropy at 20°C and 1 kHz,

cl.p., T(N,I) denotes clearing point [°C],

γ! denotes rotational viscosity measured at 20°C [mPa s], determined by the rotation method in a magnetic field, Ki denotes elastic constant, "splay" deformation at 20°C [pN],

K2 denotes elastic constant, "twist" deformation at 20°C [pN],

K3 denotes elastic constant, "bend" deformation at 20°C [pN], LTS denotes low-temperature stability (nematic phase), determined in test cells. Unless explicitly noted otherwise, all values indicated in the present application for temperatures, such as, for example, the melting point T(C,N), the transition from the smectic (S) to the nematic (N) phase T(S,N) and the clearing point T(N,I), are indicated in degrees Celsius (°C). M.p. denotes melting point, cl.p. = clearing point. Furthermore, Tg = glass state, C = crystalline state, N = nematic phase, S = smectic phase and I = isotropic phase. The numbers between these symbols represent the transition temperatures.

The term "threshold voltage" for the present invention relates to the capacitive threshold (Vo), also called the Freedericksz threshold, unless explicitly indicated otherwise. In the examples, as is generally usual, the optical threshold can also be indicated for 10% relative contrast (Vio).

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

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

The polymerisable compounds are polymerised in the display or test cell by irradiation with UVA light (usually 365 nm) of a defined intensity for a pre- specified time, with a voltage simultaneously being applied to the display (usually 10 V to 30 V alternating current, 1 kHz). In the examples, unless indicated otherwise, a 50 mW/cm ² mercury vapour lamp is used, and the intensity is measured using a standard UV meter (make Ushio UNI meter) fitted with a 365 nm band-pass filter. The tilt angle is determined by a rotational crystal experiment (Autronic- Melchers TBA-105). A low value (i.e. a large deviation from the 90° angle) corresponds to a large tilt here.

The VHR value is measured as follows: The liquid crystalline medium is introduced into TN-VHR test cells (alignment layer as indicated). The HR value is determined after 5 min at 60°C before and after UV exposure at 1 V, 60 Hz, 64 ps pulse (measuring instrument: Autronic-Melchers VHRM- 105).

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

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

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

The following polymerizable stabilizers (polymerizable piperidine derivatives) are used:

Source: Santa Cruz Biotechnology Inc. (CAS 31582-45-3)

Synthesis example for additives

Exemplary compounds of formula I are synthesized as follows or according to WO 2016/1 161 19 A1 (examples).

Synthesis Example

Compound STF-2 can be prepared as follows.

4-hydroxyl TEMPO (8.00 g, 45.5 mmol) and 4-(dimethylamino)pyridine (0.30 g, 2.46 mmol) is added into 100 ml DCM. After cooled down to 2°C, triethylamine (25.00 ml, 180.35 mmol) is added to the above solution, followed by dropwise addition of 3-bromo-propionyl chloride (6.00 ml, 50.6 mmol) in 50 ml DCM. After complete addition, the reaction mixture is allowed to warm up to room temperature. After complete conversion indicated by TLC, aqueous ammonium chloride solution is added. The aqueous phase is extracted with DCM. The organic phase is combined and dried over anhydrous sodium sulfate, and filtrated. After removing solvent in vacuo, the solid residue is purified by column chromatography on silica gel with DCM/methyl t-butyl ether (MTBE) 95:5 as eluent, and further recrystallization from heptane/MTBE to afford STF-2 as red crystal (m.p. 102 °C).

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

Mixture examples

Example M1

The following liquid-crystalline mixture

additionally comprises 0.01 % of STF-1 (Ex-1 ) and STF-2 (Ex.2). The mixtures of Example M1 comprising stabiliser is distinguished by very good reliability and high VHR values after stress tests. Comparative Example A

A comparative liquid-crystalline mixture (Ex.A) is prepared following Example M1 , but omitting the stabilizers STF-1/2.

VHR measurement: Effect of polvmerizable piperidine derivatives under backlight load

Test cells made of glass plates with a surface of rubbed polyimide are filled with the media of the preceding Example M1 and Comparative Example A. The voltage-holding ratio (VHR) of the test cells is measured before and after intensive light load (120 min). The irradiated light is equivalent to 500 h of a typical white CCFL backlight for displays.

^* BL = Backlight Load test; 120 h accelerated LED-based backlight