Chiral, tilted, smectic liquid-crystal phases containing at least two liquid-crystal components and at least one chiral doping agent, at least one liquid-crystal component being a compound having the structural unit 2,3-difluoro-1,4-phenylene, are distinguished by a very low viscosity and advantageous switching times.

The invention relates to chiral, tilted, smectic liquid-crystal media 
having at least two liquid-crystal components and at least one chiral 
doping agent, at least one compound containing the structural element 
2,3-difluoro-1,4-phenylene, excepting fluorinated oligophenyls of the 
formula 1 
##STR1## 
wherein a is 0 or 1 and wherein the terminal substituents R.sub.1, R.sub.2 
and R.sub.3 independently of one another are each alkyl or alkenyl radical 
which has up to 15 C. atoms and is optionally substituted by CN or by at 
least one halogen atom and in which one or more non-adjacent CH.sub.2 
groups of these radicals can also be replaced by --O--, --S--, --CO--, 
--O--CO--, --CO--O--, --O--CO--O-- or --C.tbd.--, one of these radicals 
R.sub.1 and R.sub.2 is also a group of formula, 
##STR2## 
in one of the following pairs of lateral substituents the two substituents 
are fluorine 
(A,B), (C,D), (C',D') 
and all the remaining lateral substituents are hydrogen or fluorine. 
The invention relates in particular to media which contain at least one 
compound of the formula I 
##STR3## 
wherein R.sup.1 and R.sup.2 are alkyl having 1 to 15 C atoms or alkenyl 
having 3-15 C atoms, each of which, independently of one another, is 
unsubstituted, monosubstituted by cyano or at least monosubstituted by 
fluorine or chlorine, it being also possible for a CH.sub.2 group in these 
radicals to be replaced in each case by --O--, --CO--, --O--CO--, 
--CO--O-- or --O--CO--O--, and one of the radicals R.sup.1 and R.sup.2 
also denotes an organic radical Q* which induces chirality and which has 
an asymmetric carbon atom, 
A.sup.1 and A.sup.2, independently of one another, are each 1,4-phenylene 
wherein one or two CH groups can also be replaced by N, 1,4-cyclohexylene 
wherein one or two non-adjacent CH.sub.2 groups can also be replaced by O 
atoms and/or S atoms, piperidine-1,4-diyl, 1,4-bicyclo(2,2,2)octylene, 
1,3,4-thiadiazole-2,5-diyl, naphthalene-2,6-diyl or 
1,2,3,4-tetrahydronaphthalene-2,6-diyl, each of which is unsubstituted or 
substituted by one or two F and/or Cl atoms and/or CH.sub.3 groups and/or 
CN groups, 
Z.sup.1 and Z.sup.2 are each --CO--O--, --O--CO--, --CH.sub.2 CH.sub.2 --, 
--OCH.sub.2 --, --CH.sub.2 O--, --C.tbd.C-- or a single bond, 
m and n are each 0, 1 or 2 and (m+n) is 1 or 2, subject to the proviso that 
one or two of the groups Z.sup.1 and/or Z.sup.2 present in the molecule of 
the formula I are --CO--O--, --O--CO--, --CH.sub.2 CH.sub.2 --, 
--OCH.sub.2 --, --CH.sub.2 O-- or --C.tbd.C--, if A.sup.1 and A.sup.2 
independently of one another are each 1,4-phenylene which is unsubstituted 
or substituted by one or two F atoms. 
Compounds of the formula 1 and/or chiral, tilted, smectic, liquid-crystal 
media containing oligophenyls of the formula 1 form the subject of 
International Patent Application PCT/EP 88/00724. 
Chiral, tilted, smectic liquid-crystal media having ferroelectric 
properties can be prepared by adding a suitable chiral doping agent to 
base mixtures containing one or more tilted smectic phases (L. A. Beresnev 
et al., Mol. Cryst. Liq. Cryst. 89, 327 (1982); H. R. Brand et al., J. 
Physique 44 (lett.), L-771 (1983)). Phases of this type can be used as 
dielectrics for high-speed displays based on the principle of SSFLC 
technology described by Clark and Lagerwall (N. A. Clark and S. T. 
Lagerwall, Appl. Phys. Lett. 36, 899 (1980); U.S. Pat. No. 4,367,924), 
based on the ferroelectric properties of the chiral, tilted medium. In 
this medium, the longitudinally extended molecules are arranged in layers 
and the molecules have an angle of tilt to the layer perpendicular. In 
proceeding from layer to layer, the direction of tilt changes by a small 
angle in respect to an axis vertical to the layers, so that a helical 
structure is formed. In displays based on the SSFLC technology, the 
smectic layers are arranged perpendicularly to the plates of the cell. The 
helical arrangement of the directions of tilt of the molecules is 
suppressed by means of a very small distance between the plates (approx. 
1-2 .mu.m). By this means, the longitudinal axes of the molecules are 
forced to arrange themselves in a plane parallel to the plates of the 
cell, as a result of which two distinct tilt orientations are formed. By 
applying a suitable electric alternating field, it is possible to switch 
backwards and forwards between these two states in the liquid-crystal 
medium having a spontaneous polarization. This switching process is 
appreciably faster than in the case of conventional twisted cells 
(TN-LCDs) based on nematic liquid crystals. 
A great disadvantage for many applications of the materials having chiral, 
tilted smectic phases (such as, for example, Sc*, but also S.sub.H *, 
S.sub.I *, S.sub.J *, S.sub.K *, S.sub.G *, S.sub.F *) available at the 
present time is their low stability to chemicals, heat and light. A 
further disadvantageous property of displays based on chiral, tilted, 
smectic media available at the present time is that the spontaneous 
polarization has excessively low values, so that the switching time 
behavior of the displays is unfavourably affected and/or the pitch and/or 
the tilt and/or the viscosity of the phases does not meet the requirements 
of display technology. In addition, in most cases the temperature range of 
the ferroelectric media is too small and is situated for the most part at 
excessively high temperatures. 
It has now been found that the use of compounds of formula I as components 
of chiral, tilted, smectic media can appreciably reduce the disadvantages 
mentioned. The compounds of formula I are thus excellently suitable for 
use as components of chiral, tilted, smectic liquid-crystal media. In 
particular it is possible with their aid to prepare chiral, tilted, 
smectic liquid-crystal media which are particularly stable to chemicals 
and have advantageous ferroelectric phase ranges and advantageous widths 
of viscosity, in particular having broad Sc* phase ranges and an excellent 
capacity for being supercooled down to temperatures below 0.degree. C. 
without crystallization occurring, and high values of spontaneous 
polarization for phases of this type. P is the spontaneous polarization in 
nC/cm.sup.2. 
The compounds of formula I have a wide range of applications. Depending on 
the selection of the substituents, these compounds can be used as the base 
materials of which liquid-crystal, smectic media are mostly composed; it 
is also possible, however, to add compounds of the formula I to 
liquid-crystal base materials belonging to other classes of compounds in 
order, for example to vary the dielectric and/or optical anisotropy and/or 
the viscosity and/or the spontaneous polarization and/or the phase range 
and/or the tilt angle and/or the pitch of such a dielectric. 
The invention therefore relates to a chiral, tilted, smectic liquid-crystal 
medium containing at least two liquid-crystal components and at least one 
chiral doping agent, this medium containing at least one compound 
containing the structural unit 2,3-difluoro-1,4-phenylene, and to the use 
of compounds of this type, in particular those of the formula I, as 
components of chiral, tilted, smectic liquid-crystal media. The invention 
also relates to electrooptical display elements, in particular 
ferroelectric, electrooptical display elements, containing media of this 
type. 
The media according to the invention preferably contain at least two, 
especially at least three compounds of formula I. Chiral, tilted, smectic 
liquid-crystal media according to the invention which are particularly 
preferred are those in which an achiral base mixture contains, as well as 
compounds of the formula I, at least one other component having a 
dielectric anisotropy of low value, a low viscosity and a broad S phase 
range. These further component(s) of the achiral base mixture can amount, 
for example, to 40 to 90%, preferably 50 to 80%, of the base mixture. 
Possible suitable components are, in particular, compounds of the partial 
formulae IIa to IIh: 
##STR4## 
R.sup.4 and R.sup.5 are each preferably alkyl, alkoxy, alkanoyloxy or 
alkoxycarbonyl having in each case 3 to 12 C atoms. X is preferably 0. In 
the compounds of the formulae IIa to IIg, a 1,4-phenylene group can also 
be substituted laterally by halogen, particularly preferably by fluorine. 
Preferably, one of the groups R.sup.4 or R.sup.5 is alkyl and the other 
group is alkoxy. The Ls independently of one another are each H or F. In 
IIa the F atom can be in the ortho-position or meta-position relative to 
R.sup.5. 
The compounds of the partial formulae IIa to IIh wherein R.sup.4 and 
R.sup.5 are each linear alkyl or alkoxy having in each case 5 to 10 C 
atoms are particularly preferred. 
Media according to the invention which are also preferred are those which, 
as well as components of the formulae IIa to IIg, also contain at least 
one component having a markedly negative dielectric anisotropy 
(.DELTA..epsilon. .ltoreq. -2). Compounds of the formulae IIIa to IIIc 
##STR5## 
wherein R.sup.4 and R.sup.5 have the general and preferred meanings 
indicated in the formulae IIa to IIg are particularly suitable in this 
regard. In the compounds of the formulae IIIa, IIIb and IIIc, a 
1,4-phenylene group can also be substituted laterally by halogen, 
preferably fluorine. 
The compounds of the formula I embrace, in particular dinuclear and 
trinuclear materials. Of the dinuclear materials, which are preferred, 
preferred materials are those wherein R.sup.1 is n-alkyl or n-alkoxy 
having 7 to 12, in particular 7 to 9, C atoms. 
Compounds of the formula I wherein R.sup.1 is n-alkyl having 7 to 10 C 
atoms and R.sup.2 is n-alkanoyloxy, n-alkoxycarbonyl or n-alkylthio having 
in each case 5 to 10 C atoms are also preferred. 
The media according to the invention preferably contain at least one 
trinuclear compound of the formula I. These media are distinguished by 
particularly high S.sub.C /S.sub.A transition temperatures. 
In formula I R.sup.1 and R.sup.2, independently of one another, are each 
preferably alkyl or alkoxy having 5 to 15 C atoms. 
A.sup.1 and A.sup.2 are preferably Cy or Ph. In the compounds of the 
preceding and following formulae, Ph is preferably a 1,4-phenylene (Phe) 
group, a pyrimidine-2,5-diyl (Pyr) group, a pyridine-2,5-diyl (Pyn) group, 
a pyrazine-3,6-diyl group or a pyridazine-2,5-diyl group, particularly 
preferably Phe, Pyr or Pyn. The compounds according to the invention 
preferably contain not more than one 1,4-phenylene group, wherein one or 
two CH groups have been replaced by N. Cy is preferably a 
1,4-cyclohexylene group. Compounds of the formula I, which are 
particularly preferred are, however, those wherein one of the groups 
A.sup.2, A.sup.3 and A.sup.4 is a 1,4-cyclohexylene group which is 
substituted in the 1-position or 4-position by CN and the nitrile group is 
also located in an axial position, i.e. the group A.sup.2, A.sup.3 or 
A.sup.4, as in the following configuration: 
##STR6## 
Compounds of the formula I and of the above partial formulae containing a 
grouping --Ph--Ph-- are particularly preferred. --Ph--Ph-- is preferably 
--Phe--Phe--, --Phe--Pyr or Phe--Pyn. The groups 
##STR7## 
and also 4,4'-biphenylyl which is unsubstituted or monosubstituted or 
polysubstituted by fluorine are particularly preferred. 
Z.sup.1 and Z.sup.2 are preferably single bonds; --O--CO--, --CO--O--, 
--C.tbd.C-- or --CH.sub.2 CH.sub.2 -- groups are a second preference. 
Preferably, only one of the groups Z.sup.1 and Z.sup.2 present in the 
molecule is other than a single bond. Z.sup.1 =Z.sup.2 =single bond is 
particularly preferred. 
Preferred branched radicals R.sup.1 or R.sup.2 are isopropyl, 2-butyl 
(=1-methylpropyl), isobutyl (=2-methylpropyl), tert.-butyl, 2-methylbutyl, 
isopentyl (=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 
4-methylpentyl, 2-ethylhexyl, 5-methylhexyl, 2-propylpentyl, 
6-methylheptyl, 7-methyloctyl, isopropoxy, 2-methylpropoxy, 
2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 
2-ethylhexoxy, 1-methylhexoxy, 1-methylheptoxy, 2-oxa-3-methylbutyl, 
3-oxa-4-methylpentyl. 
In the compounds of the formula I and also in the preceding and following 
partial formulae, --(A.sup.1 --Z.sup.1).sub.m --PheF.sub.2 --(Z.sup.2 
--A.sup.2).sub.n -- is preferably a group of the formulae 1 to 16 below or 
a mirror image thereof: 
##STR8## 
Groups of the formulae 1, 3, 4, 5, 7, 8, 10, 11, 12 and 13, particularly 
those of the formulae 1, 3 and 10-13, are particularly preferred. L is H 
or F. 
The radical R.sup.1 can also be an optically active organic radical 
containing an asymmetric carbon atom. The asymmetric carbon atom is then 
preferably attached to two differently substituted C atoms, an H atom and 
a substituent selected from the group comprising halogen (especially F, Cl 
or Br), alkyl or alkoxy having in each case 1 to 5 C atoms and CN. The 
optically active organic radical R.sup.1 or Q* preferably has the formula 
##STR9## 
wherein X' is --CO--O, --O--CO--, --O--CO--O--, --CO--, --O--, --S--, 
--CH.dbd.CH--, --CH.dbd.CH--COO-- or a single bond, Q' is alkylene which 
has 1 to 5 C atoms and in which a CH.sub.2 group not attached to X' can be 
replaced by --O--, --CO--, --O--CO--, --CO--O-- or --CH.dbd.CH--, or is a 
single bond, Y' is CN, halogen, methyl or methoxy, and R.sup.5 is an alkyl 
group different from Y which has 1 to 15 C atoms and in which one or two 
non-adjacent CH.sub.2 groups can be replaced by --O--, --CO--, --O--CO--, 
--CO--O-- and/or --CH.dbd.CH--. 
X' is preferably --CO--O--, --O--CO--, --CH.dbd.CH--COO--(trans) or a 
single bond. --CO--O/--O--CO-- or a single bond is particularly preferred. 
Q' is preferably --CH.sub.2 --, --CH.sub.2 CH.sub.2 --, --CH.sub.2 CH.sub.2 
CH.sub.2 -- or a single bond, particularly preferably a single bond. 
Y' is preferably CH.sub.3, --CN, F or Cl, particularly preferably CN or F. 
R.sup.5 is preferably linear or branched alkyl having 1 to 10, in 
particular 1 to 7, C atoms. 
Compounds of the formula I' 
##STR10## 
wherein Q.sup.1, Q.sup.2, R.sup.o and X have the meanings indicated in 
claim 2 are particularly preferred. R.sup.o is an alkyl group different 
from X and Q.sup.2 --R.sup.2 which preferably has 1 to 5 C atoms. Methyl 
and ethyl, especially methyl, are particularly preferred. R.sup.2 is 
preferably an alkyl group having 2 to 10, in particular 2 to 6, C atoms. 
Q.sup.1 and Q.sup.2, independently of one another, are each preferably 
--O--CO-- (the carbonyl carbon atom being attached to the asymmetric C 
atom C*), --O--CH.sub.2 -- (the methylene group being attached to the 
asymmetric C atom C*), --CH.sub.2 CH.sub.2 --, --CH.sub.2 -- or a single 
bond (--). Combinations of Q.sup.1 and Q.sup.2 which are particularly 
preferred are indicated in the table below: 
__________________________________________________________________________ 
Q.sup.1 
--O--CO-- 
--O--CH.sub.2 -- 
--CH.sub.2 -- 
--CH.sub.2 CH.sub.2 -- 
--CH.sub.2 -- 
--CH.sub.2 CH.sub.2 -- 
Q.sup.2 
-- -- --CO--O-- 
--CO--O-- 
--CH.sub.2 --O-- 
--CH.sub.2 --O-- 
__________________________________________________________________________ 
In the preferred compounds of the preceding and following formulae, it is 
possible for the alkyl radicals, in which a CH.sub.2 group (alkoxy or 
oxaalkyl) can also be replaced by an O atom, to be linear or branched. 
They preferably have 5, 6, 7, 8, 9 or 10 C atoms and accordingly are 
preferably pentyl, hexyl, heptyl, octyl, nonyl, decyl, pentoxy, hexoxy, 
heptoxy, octoxy, nonoxy or decoxy, and also ethyl, propyl, butyl, undecyl, 
dodecyl, propoxy, ethoxy, butoxy, undecyloxy, dodecyloxy, 2-oxapropyl 
(=2-methoxymethyl), 2-oxabutyl (=ethoxymethyl) or 3-oxabutyl 
(=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl or 2-, 
3-, 4-, 5- or 6-oxaheptyl. 
A medium according to the invention which is particularly preferred is one 
containing at least 25% of one or more compounds of the formula II 
##STR11## 
wherein R.sup.3 and R.sup.4, independently of one another, are each alkyl 
which has 1 to 15 C atoms or alkenyl which has 3 to 15 C atoms, each of 
which is unsubstituted, monosubstituted by cyano or at least 
monosubstituted by fluorine or chlorine, it being also possible for a 
CH.sub.2 group to be replaced by --O--, --O--CO--, --CO--O-- or 
--O--CO--O-- in each of these radicals, and Ring A is pyrimidine-2,5-diyl 
or pyridine-2,5-diyl. 
Compounds of the formulae IIe, and/or IIh wherein R.sup.4 and R.sup.5 are, 
independently of one another, each alkyl, alkoxy, alkanoyloxy or 
alkoxycarbonyl having in each case 3 to 12 C atoms are preferably present. 
A medium which contains one or more compounds of the formulae IId and/or 
IIh and, at the same time, one or more compounds of the formula IIg is 
particularly preferred. A preferred medium contains 2 to 25% of a chiral 
doping agent which preferably has an S.sub.c *phase. 
A preferred medium contains 8 to 50% of one or more compounds which have a 
2,3-difluoro-1,4-phenylene structural unit and which preferably correspond 
to the formula I. 
Compounds of the formula Ia' 
##STR12## 
wherein R.sup.1 and R.sup.2 have the meaning indicated are preferred. 
Q* is preferably a radical of formula 
EQU --Q.sup.1 --C*R.sup.o X--Q.sup.2 --R.sup.2 
wherein Q.sup.1 and Q.sup.2 independently of one another are each alkylene 
which has 2 to 4 C atoms and in which a CH.sub.2 group can also be 
replaced by --O--, --S--,--CO--, --O--CO--, --CO--O--, --S--CO--, 
--CO--S--, --CH.dbd.CH--COO--, --CH.dbd.CH--, --CHHalogen and/or --CHCN--, 
or are each a single bond, X is halogen, CN, CH.sub.3, CH.sub.2 CN or 
OCH.sub.3, R.sup.o is H or an alkyl group having 1 to 10 C atoms which is 
different from X and --Q.sup.2 --R.sup.2, and C* is a carbon atom attached 
to four different substituents. 
A preferred medium contains one or more compounds of the formulae Ia, Ib or 
Ic 
##STR13## 
wherein R.sup.3, and R.sup.4 ', independently of one another, are each 
alkyl having 1 to 15 C atoms and o, p and q are each 0 or 1. 
Media according to the invention containing only compounds of the formula I 
wherein m =0 are also preferred. These media are distinguished by a 
particularly advantageous low temperature behavior and particularly low 
viscosity values. Media according to the invention containing compounds of 
formula I wherein at least one group R.sup.1 or R.sup.2 is a 
branched-chain, alkyl or alkoxy radical are also preferred. These media 
also exhibit an advantageous low temperature behavior. 
R.sup.1 and R.sup.2, independently of one another, are each preferably 
alkyl, alkoxy, alkanoyl, alkanoyloxy, alkoxycarbonyl or alkoxycarbonyloxy, 
preferably having in each case 5 to 12, in particular 6 to 10 C atoms. 
Alkyl and alkoxy are particularly preferred. One of the groups R.sup.1 and 
R.sup.2 is preferably alkyl. A particularly preferred combination is 
R.sup.1 =alkyl and R =alkoxy, and also R.sup.1 =alkoxy and R.sup.2 =alkyl. 
R.sup.1 and R.sup.2 groups having a linear alkyl radical are particularly 
preferred. 
Media which, in addition to one or more compounds of the formula I, contain 
one or more oligophenyl compounds of the formula 1 are also preferred. 
The media according to the invention contain one or more chiral components. 
The present invention also relates to the corresponding base mixtures 
(i.e., the achiral part) which contain a tilted, smectic phase (for 
example S.sub.C). By adding one or more doping agents those skilled in the 
art can obtain chiral, tilted, smectic media without inventive assistance. 
Preferred doping agents are those corresponding to WO 86/06373, WO 
87/05018, German Offenlegungsschrift 3,638,026, German Offenlegungsschrift 
3,807,802, German Offenlegungsschrift 3,843,128 and British Patent 
Applications 8,615,316, 8,629,322, 8,724,458, 8,729,502, 8,729,503, 
8,729,865 and 8,729,866. 
All the components of the media according to the invention are either known 
or can be prepared in a manner known per se analogously to known 
compounds. 
The compounds having the structural unit 2,3-difluoro-1,4-phenylene are 
prepared by methods known per se, such as are described in the literature 
(for example in the standard works such as Houben-Weyl, Methoden der 
Organischen Chemie "Methods of Organic Chemistry", Georg-ThiemeVerlag, 
Stuttgart), specifically under reaction conditions which are known and 
suitable for the reactions mentioned. In this regard, it is also possible 
to make use of variants which are known per se but are not mentioned here 
in detail. 
If desired, the starting materials can also be formed in situ by a process 
in which they are not isolated from the reaction mixture, but are 
immediately reacted further to give the target compounds. 
Compounds of the formula I are accessible using 1,2-difluorobenzene as 
starting material. This is metalated according to a known process (for 
example A. M. Roe et al., J. Chem. Soc. Comm., 22, 582 (1965)) and reacted 
with the appropriate electrophile. Using the 1-substituted 
2,3-difluorobenzene thus obtained, this reaction sequence can be carried 
out a second time with a suitable electrophile, thus affording the 
1,4-disubstituted 2,3-difluorobenzenes which are suitable for the 
synthesis of the heterocyclic compounds. 1,2-Difluorobenzene or 
1-substituted 2,3-difluorobenzene is reacted, in an inert solvent, such as 
diethyl ether, tetrahydrofuran, dimethoxyethane, tert.-butyl methyl ether 
or dioxane, hydrocarbons, such as hexane, heptane, cyclohexane, benzene or 
toluene or mixtures of these solvents, if appropriate with an addition of 
a complexing agent such as tetramethylethylenediamine (TMEDA) or 
hexamethylphosphoric triamide, with phenyllithium, lithium 
tetramethylpiperidine or n-, sec.- or tert.-butyllithium at temperatures 
from -100.degree. C. to +50.degree. C., preferably -78.degree. C. to 
0.degree. C. 
The lithium-2,3-difluorophenyl compounds are reacted with the corresponding 
electrophiles at -100.degree. C. to 0.degree. C., preferably at 
-50.degree. C. Suitable electrophiles are aldehydes, ketones, nitriles, 
epoxides, carboxylic acid derivatives, such as esters, anhydrides or 
halides, halogenoformic acid esters or carbon dioxide. 
For reaction with aliphatic or aromatic halogen compounds, the 
lithium-2,3-difluorophenyl compounds are transmetalated and coupled by 
catalysis with transition metals. The zinc-2,3-difluorophenyl (compare 
German Offenlegungsschrift 3,632,410) or the titanium-2,3-difluorophenyl 
(compare German Offenlegungsschrift 3,736,489) compounds are particularly 
suitable for this reaction. 
The new compounds of the formula I are the subject of the following German 
patent applications of the same application date: 
______________________________________ 
Derivatives of 2,3-difluorohydroquinone 
(P 3,807,801) 
Derivatives of 2,3-difluorobenzoic acid 
(P 3,807,823) 
Derivatives of 2,3-difluorophenol 
(P 3,807,803) 
Phenyl difluorobenzoates 
(P 3,807,870) 
Derivatives of 2,3-difluorophenol 
(P 3,807,819) 
2,3-Difluorobiphenyls (P 3,807,861) 
Heterocyclic derivatives of 
(P 3,807,871) 
1,2-difluorobenzene 
Chiral derivatives of (P 3,807,802) 
1,2-difluorobenzene 
______________________________________ 
The compounds described therein are preferred compounds of the formula I. 
The preparation of the media according to the invention is effected in a 
manner customary per se. As a rule, the components are dissolved in one 
another, preferably at an elevated temperature. 
By means of suitable additives, it is possible to modify the liquid-crystal 
media according to the invention in such a way that they can be used in 
all types of liquid-crystal display elements hitherto disclosed. 
The following examples are intended to illustrate the invention without 
limiting it. In the preceding and following text, percentages are percent 
by weight; all the temperatures are quoted in degrees centigrade. The 
values of spontaneous polarization apply to room temperature. The 
following abbreviations are also used: C: crystalline-solid state, S: 
smectic phase (the index characterizes the type of phase), N: nematic 
state, Ch: cholesteric phase, I: isotropic phase. The number between two 
symbols indicates the transition temperature in degrees centigrade.