A liquid-crystalline medium, in particular based on a mixture of polar compounds, having compounds of formulae Y, CY, PY and/or LY, and compounds of formula S1 and/or S2:

##STR00001##

It is an object of the present invention to provide a liquid crystal composition having positive Δ∈, having a liquid crystal phase in a wide temperature range, having high solubility at low temperatures, having high ODF process compatibility, having a high specific resistance and voltage holding ratio, and being insensitive to heat and light. In order to achieve this object, there is provided a liquid crystal composition containing at least one compound represented by the following general formula (i) and at least one compound represented by the following general formula (ii). ##STR00001## (wherein R.sup.i1 denotes an alkyl group having 2 to 5 carbon atoms) ##STR00002## (wherein R.sup.ii1 and R.sup.ii2 independently denote an alkyl group having 1 to 5 carbon atoms or an alkenyl group having 2 to 5 carbon atoms, and X.sup.ii1 and X.sup.ii2 independently denote a hydrogen atom or a fluorine atom)

The invention relates to a liquid-crystalline medium and to the use thereof for an active-matrix display, in particular based on the VA, PSA, PS-VA, PM-VA, SS-VA, PALC, IPS, PS-IPS, FFS or PS-FFS effect.

To provide a liquid crystal compound satisfying at least one of physical properties such as a high stability to heat, light and so forth, a high clearing point, a low minimum temperature of a liquid crystal phase, a small viscosity, a suitable optical anisotropy, a large dielectric anisotropy, a suitable elastic constant and an excellent compatibility with other liquid crystal compounds; a liquid crystal composition containing the compound; and a liquid crystal display device including the composition. The compound is represented by formula (1): ##STR00001##
wherein, for example, Ra and Rb are alkyl having 1 to 10 carbons, alkenyl having 2 to 10 carbons or alkoxy having 1 to 9 carbons; A.sup.1, A.sup.2 and A.sup.3 are 1,4-cyclohexylene or 1,4-phenylene; Z.sup.1 and Z.sup.2 are single bonds, —(CH.sub.2).sub.2—, —CH═CH—, —COO—, —OCO—, —CF.sub.2O—, —OCF.sub.2—, —CH.sub.2O— or —OCH.sub.2—; Y.sup.1 is —CF.sub.2H or —CF.sub.3; Y.sup.2 is fluorine; and a, b and d are 0, 1, 2 or 3, and c is 1 or 3.

The invention relates to a liquid-crystalline medium which comprises at least one compound of the formula I, ##STR00001##
and to the use thereof for an active-matrix display, in particular based on the VA, PSA, PS-VA, PALC, FFS or IPS effect.

In a process for the purification of a liquid-crystal mixture (7), the liquid-crystal mixture (7) is passed through a first electrodialysis cell (2) and a concentrate solution (14) is passed through a second electrodialysis cell (8) which is adjacent to the first electrodialysis cell (2) and is separated by an ion-exchanger membrane (9), and an electric field transverse to a direction of passage of the liquid-crystal mixture (7) through the first electrodialysis cell (2) is generated with the aid of an anode/cathode arrangement (15, 16) arranged outside the electrodialysis cells (2, 8) so that ionized constituents of the liquid-crystal mixture (7) are discharged at the ion-exchanger membrane (9) and removed from the liquid-crystal mixture (7).

The invention relates to a liquid-crystalline medium, preferably having a nematic phase and negative dielectric anisotropy, which comprises a) one or more compounds of formula I

##STR00001## and b) one or more compounds selected from the group of compounds of the formulae II and III

##STR00002##

in which the parameters have the respective meanings indicated in claim 1, to the use thereof in an electro-optical display, particularly in an active-matrix display based on the VA, ECB, PALC, IPS or FFS effect, to displays of this type which contain a liquid-crystalline medium of this type, and to the use of the compounds of formula I for reduction of the dispersion of the birefringence of a liquid-crystalline medium which comprises one or more compounds of the formulae II and/or III.

The invention relates to a liquid-crystalline medium having a nematic phase comprising one or more compounds of formula X

##STR00001##

wherein the parameters have the meaning given in the text, to the use thereof in an electro-optical display, particularly in an active-matrix display based on the IPS or FFS effect, to displays of this type which contain a liquid-crystalline medium of this type and to the use of the compounds of formula X for improvement of the transmission and/or response times of a liquid-crystalline medium which comprises one or more additional mesogenic compounds, as well as to the compounds of formula X.

The invention relates to a liquid-crystalline medium and to the use thereof for an active-matrix display, in particular based on the VA, PSA, PS-VA, PM-VA, SS-VA, PALC, IPS, PS-IPS, FFS or PS-FFS effect.

The present invention provides a liquid crystal composition for forming a polymer layer capable of keeping high voltage holding ratio. The liquid crystal composition in an aspect of the present invention contains a liquid crystal material and one or more kind monomers and, at least one of the monomers is a compound produced by further bonding a hydrocarbon group with 12 carbon atoms or more to a compound produced by bonding two polymerizable groups to a cyclic aliphatic compound or aromatic compound with 6 carbon atoms or more.