Anisotropic compounds having nematic phase and liquid crystal mixtures

Anisotropic compounds having a nematic phase have the formula ##STR1## wherein A is a cyclic radical selected from the group consisting of ##STR2## Z is selected from the group consisting of a single bond, a --CH2CH2-- group, a methyleneoxy group and an oxymethylene group, PA0 n is 1 or 2, PA0 p is 0, 1, or 2, PA0 r is 0 or 1, PA0 X is selected from the group consisting of fluorine, chlorine, bromine, iodine, nitrile and methyl, PA0 R.sup.1 and R.sup.2 are selected from the group consisting of hydrogen, C.sub.1 -C.sub.12 -alkyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.1 -C.sub.12 -alkanoyloxy and C.sub.1 -C.sub.12 -alkylamino and cyclic groups of the formulas ##STR3## wherein R.sup.3 through R.sup.6 are selected from the group consisting of hydrogen, C.sub.1 -C.sub.12 -alkyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.1 -C.sub.12 -alkanoyloxy, and C.sub.1 -C.sub.12 -alkylamino, and Z.sup.2 through Z.sup.6 represent a Z group, with the provisos that: PA1 (A) none of the above aromatic radicals 10), (11), (13), (14) or (15) is directly bonded to the C-atom of a methyleneoxy or oxymethylene group; and PA1 (B) at least one X group is methyl when only one or none of the groups R.sup.1 and R.sup.2 is a cyclic radical of formulas (13) or (16), A has the formula (12), Z is a single bond, and any of Z.sup.3 or Z.sup.6, if present, is a single bond. Compounds having the defined lateral substituents on the rings have a reduced tendency to form smectic phases as compared with the compounds not having such lateral substituents.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to compounds having a negative or slightly positive 
dielectric anisotropy which have liquid-crystalline properties, and more 
particularly to such compounds which have a nematic mesophase. 
2. Description of the Prior Art 
Liquid crystal displays using twisted nematic cells, also known as 
TN-displays require, as is well-known, nematic liquid-crystalline 
materials having a positive anisotropy of the dielectric constant, also 
indicated as a positive DCA. The DCA, or .DELTA..epsilon., defined as 
.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. wherein 
.epsilon..sub..parallel. is the dielectric constant (DC) parallel to the 
long axis of the molecule and .epsilon..sub..perp. is the DC perpendicular 
to the molecular axis. 
For multiplexed operation of these cells it is also desirable that the 
LC-material have a characteristic curve exhibiting the steepest possible 
contrast, i.e., the greatest possible change of the light absorption 
properties with change of the applied electric field. This can be attained 
by optimizing two criteria used in choosing the available anisotropic 
materials, namely (a) the smallest possible value of the ratio of the 
elastic constants of bending and of expansion, also indicated as k.sub.33 
/k.sub.11, and (b) by the smallest possible value of the ratio of the DCA 
(.DELTA..epsilon.) to the DC perpendicular to the nematic axis 
(.epsilon..sub..perp.), i.e., of .DELTA..epsilon./.epsilon..sub..perp.. 
The particulars of these requirements can be found in the literature: 
Gharadjedagji, F. et al., Rev. Phys. Appl., 11 (1976), 467; Metz, A. R., 
SID Digest, Techn. Papers IX (1978), 70 and Alt, P. M. et al., IEEE Trans. 
El. Dev. Ed. (1974), 146. 
At present, two types of anisotropic compounds are available from which to 
select suitable anisotropic compounds by the above criteria. Each type may 
contain two or three rings, and they have the following structure: 
##STR4## 
wherein a.sup.1, a.sup.2 and a.sup.3 are aromatic, heteroaromatic or 
cycloaliphatic rings, such as benzene, pyrimidine, and trans-cyclohexane 
rings; X.sup.a and Y.sup.a are so-called bridge groups such as --COO--, 
--CH.dbd.N--, --CH.sub.2 O--, --N.dbd.N--, --CH.sub.2 CH.sub.2 -- or a 
single bond and R.sup.a, R.sup.b are so-called terminal groups, such as 
alkyl, alkoxy, alkanoyloxy, or alkylamino groups, which contain an alkyl 
portion having in general 1 to 12 C atoms in a straight or branched chain. 
Type I substances, because of their relatively large polarity R.sup.a /CN 
in the direction of the long axis of the molecule (the nematic axis or 
director) offer a high positive DCA-value and a correspondingly low 
threshold voltage. However, they have high 
.DELTA..epsilon./.epsilon..sub..perp. -values, which is disadvantageous 
for multiplex operation and also have a comparatively high k.sub.33 
/k.sub.11 -value which is also disadvantageous for this purpose. The 
contrast characteristic curve of these substances is relatively weak. 
Type II substances are slightly polar or nonpolar and have 
.DELTA..epsilon./.epsilon..sub..perp. -values necessary for a steep 
characteristic curve, and k.sub.33 /k.sub.11 -values in comparison with 
substances of Type I (at comparable chain length). Furthermore, the 
viscosity of Type II substances is less than that of Type I substances. 
For all these reasons, anisotropic substances of Type II are indispensible 
components of LC-mixtures for multiplex operation of TN-displays. 
However, a significant disadvantage of the known anisotropic substances of 
Type II is their pronounced tendency to form smectic phases, especially 
when these materials have relatively long terminal groups, e.g., when they 
have alkyl groups of a certain length. This can be seen by referring to 
the values in the literature, e.g. in "Liquid Crystals in Tables," VEB 
Deutscher Verlag, Leipzig, 1974. 
The appearance of smectic phases in anisotropic compounds narrows the range 
of the nematic phase which alone is suitable for TN-displays or completely 
eliminates it. 
Therefore, a need has continued to exist for anisotropic compounds of Type 
II having a reduced tendency to form smectic phases. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide anisotropic 
compounds of Type II wherein the tendency to form smectic phases is 
decreased or eliminated. 
A further object is to provide compounds of this type which in general 
possess a nematic phase or which have a greater temperature range of the 
nematic phase for comparable structures. 
Further objects of the invention will become apparent from the description 
of the invention which follows. 
It has now surprisingly been found that the objects of the invention can be 
attained by an accurately defined group of Type II compounds containing at 
least one so-called lateral substituent on the ring in the fundamental 
structural formula. 
The compounds of the invention are anisotropic compounds having a nematic 
phase having the formula 
##STR5## 
wherein Z is selected from the group consisting of a single bond and a 
--CH.sub.2 CH.sub.2 -- group, 
n is 1 or 2, 
p is 0, 1, or 2, 
X is selected from the group consisting of fluorine, chlorine, bromine, 
iodine, nitrile and methyl, 
R.sup.1 and R.sup.2 are selected from the group consisting of C.sub.1 
-C.sub.12 -alkyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.1 -C.sub.12 
-alkanoyloxy and C.sub.1 -C.sub.12 -alkylamino and cyclic groups of the 
formulas 
##STR6## 
wherein R.sup.3 through R.sup.6 are selected from the group consisting of 
C.sub.1 -C.sub.12 -alkyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.1 -C.sub.12 
-alkanoyloxy, and C.sub.1 -C.sub.12 -alkylamino, r is 0 or 1, and Z.sup.2 
through Z.sup.6 are selected from the group consisting of a single bond, a 
--CH.sub.2 CH.sub.2 -- group, a methyleneoxy group and an oxymethylene 
group, with the provisos that 
(A) none of the above aromatic radicals (13), (14) or (15) is directly 
bonded to the C-atom of a methyleneoxy or oxymethylene group; and 
(B) at least one X group is methyl when one or both of the group R.sup.1 
and R.sup.2 are a cyclic radical of formula (16) and Z and Z.sup.6 each 
are a single bond. 
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The new Type II anisotropic compounds of the invention having a nematic 
phase have the formula (22) 
##STR7## 
wherein Z is selected from the group consisting of a single bond and a 
--CH.sub.2 CH.sub.2 -- group, 
n is 1 or 2, 
p is 0, 1, or 2, 
Y is selected from the group consisting of fluorine, chlorine, bromine, 
iodine, nitrile and methyl, 
R.sup.1 and R.sup.2 are selected from the group consisting of C.sub.1 
-C.sub.12 -alkyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.1 -C.sub.12 
-alkanoyloxy and C.sub.1 -C.sub.12 -alkylamino and cyclic groups of the 
formulas 
##STR8## 
wherein R.sup.3 through R.sup.6 are selected from the group consisting of 
C.sub.1 -C.sub.12 -alkyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.1 -C.sub.12 
-alkanoyloxy, and C.sub.1 -C.sub.12 -alkylamino, r is 0 or 1, and Z.sup.2 
through Z.sup.6 are selected from the group consisting of a single bond, a 
--CH.sub.2 CH.sub.2 -- group, a methyleneoxy group and an oxymethylene 
group, with the provisos that 
(A) none of the above aromatic radicals (13), (14) or (15) is directly 
bonded to the C-atom of a methyleneoxy or oxymethylene group; and 
(B) at least one X group is methyl when one or both of the group R.sup.1 
and R.sup.2 are a cyclic radical of formula (16) and Z and Z.sup.6 each 
are a single bond. 
The cyclic radical of formula (12) always has the trans-configuration as 
shown by the dot, provided that R.sup.6 is not a hydrogen atom, which 
would eliminate the possibility of cis/trans-isomers. 
The bridge member Z.sup.3 through Z.sup.6 in formula (22) indicate a single 
bond, an ethylene group (--CH.sub.2 CH.sub.2 --), a methyleneoxy 
(--CH.sub.2 O--), or an oxymethylene group (--COH.sub.2 --), insofar as 
these are not excluded by the conditions which follow. 
The numerical indices have the following significance: n is 1 or 2; p is 0, 
1 or 2; r is 0 or 1. 
The most important groups for the lateral substituent X (one or more) are 
halogen, i.e., fluorine, chlorine, bromine or iodine, or methyl 
(--CH.sub.3) or nitrile (--CN). If the molecule of formula (22) has 
several lateral substituents X, these can be the same or different. 
Thus, the following conditions apply for compounds of formula (22) of the 
invention: 
(A) When an aromatic radical of the formula (13), (14) or (15) is present 
in a molecule of formula (22), a methyleneoxy or oxymethylene group is 
never directly bonded to a C-atom of one of these radicals; in other 
words, the molecule of formula (22) should not contain any group of the 
formula 
##STR9## 
wherein Ar represents a cyclic radical of the formula (13), (14) or (15). 
It has been determined that benzylethers of this type tend to produce 
instability when used as liquid crystals or in mixtures of liquid 
crystals. 
(B) Furthermore the new compounds of formula (22) of the invention have the 
proviso that at least one X in the molecule of formula (1) is methyl, when 
one or both of the groups R.sup.1 and R.sup.2 are a cyclic radical of 
formula (16) and Z and Z.sup.6 each is a single bond. 
For many purposes it is further preferred that the cyclic radical of the 
formula (16) should be not at the same time be bonded on one side to an 
oxygen atom and on the other side to an oxygen atom or nitrogen atom. 
Finally it is preferred for especially stable compounds of the formula 
(22) that the aromatic rings not lie between two oxygen atoms which are 
directly bonded to a ring, especially when a lateral substituent X.dbd.CN 
is attached to this ring. 
R.sup.1 and R.sup.2 can be the same or different and signify alkyl, alkoxy, 
alkanoyloxy (Alk--C(O)--O--) or alkylamino (Alk--N(H)--) having 1 to 12 
C-atoms in the alkyl portion; the alkyl portion can be linear or branched 
and may optionally be chiral; R.sup.1 and/or R.sup.2 can also be cyclic 
groups of the formula (13) to (16): 
##STR10## 
In the formulas (13) to (16) the terminal groups R.sup.3 to R.sup.6 have 
the significance given above for the noncyclic R.sup.1 and R.sup.2 ; X, p 
and r are as given above and the bridge groups X.sup.3 through X.sup.6 
have one of the meanings given for Z, whereby in a molecule of formula 
(22) the significance of Z on the one hand and Z.sup.3 through Z.sup.6 on 
the other can be the same or different. 
Z.sup.3, Z.sup.4 and Z.sup.5 are each preferably a --CH.sub.2 CH.sub.2 -- 
group or a single bond, in particular a single bond. Z.sup.6 is preferably 
a --CH.sub.2 CH.sub.2 -- group, a methyleneoxygroup or a oxymethylene 
group, in particular a --CH.sub.2 CH.sub.2 -- group. 
For many puroses it is further preferred that only one of R.sup.1 and 
R.sup.2 is a cyclic group of the formula (13), (14), (15) or (16), 
preferably of formula (16). 
The new compounds of formula (22) of the invention can in general be 
dinuclear, trinuclear or tetranuclear. Preferred compounds of the 
invention have the formulas (26) to (33) as illustrated and defined in 
Claims 3-10. 
##STR11## 
The invention in particular relates to liquid crystal mixtures which 
contain at least one compound of formula (22), e.g., one or more compounds 
of formulas (26) to (33). This will be further explained below. 
In a preferred group of compounds of the invention X is a methyl group. In 
another preferred group of compounds of the invention X is fluorine, 
chlorine, bromine or iodine, wherein fluorine is especially preferred. In 
another preferred group of compounds of the invention X is a nitrile 
group. 
If the compound of formula (22) has a large transverse dipole moment, which 
can result, e.g., when two or more X's, such as nitrile and/or halogen are 
present, the compound generally also has a strongly negative 
.DELTA..epsilon. and can then be used, e.g., for liquid crystal displays 
which operate by the so-called inverse guest/host-effect, or for certain 
types of dynamic scattering cells, DAP-devices and HN-devices. The 
compounds of the invention contain at least one lateral substituent, X; 
however they also can have two or more lateral substituents X, wherein the 
particular molecule of formula (22) can bear different or identical 
lateral substituents X, e.g., methyl and halogen, methyl and nitrile, 
halogen and nitrile or methyl and halogen and nitrile. 
The new compounds of formula (22) of the invention are based on the 
discovery that it is important for avoiding or suppressing the tendency to 
form smectic phases in certain compounds of Type II that the lateral 
substituent or substituents must have a certain volume but not too great a 
volume. The lower limit of the volume of the lateral substituents suitable 
for use according to the invention depends on the tendency of the 
fundamental structure II (i.e., formula (22) without lateral substituents 
X) to form smectic phases. In general, the introduction of one or more 
lateral substituents X having relatively large volumes in comparison to 
the corresponding fundamental structure without lateral substituents 
produces a lowering of the clearing point; for dinuclear compounds of 
formula (22) the lateral substituents having lesser volumes, namely 
fluorine, chlorine and methyl are accordingly preferred. The volumes of 
the substituents X increases in approximately the following sequence: 
EQU F&lt;Cl&lt;CH.sub.3 .apprxeq.Br&lt;J&lt;CN 
Two synthetic procedures are especially suitable for preparing the 
compounds (22) or the invention. On the one hand a desired substituent(s) 
X may be introduced into the corresponding parent compound, e.g., by 
chlorination or bromination, optionally with subsequent replacement of a 
substituent X, such as chlorine, by another, such as fluorine, by halogen 
exchange or by reaction of a corresponding halogen compound with a metal 
cyanide to introduce the nitrile group. In analogous fashion a carbonyl 
group can be introduced into the corresponding parent compound, e.g., by 
catalytic reaction with a formylating reagent, such as oxalyl chloride, 
and then be reduced to a methyl group. 
The other method of synthesis relies on the fact that for construction of 
the compound (22) component compounds already bearing the corresponding 
substituents X are used and are linked together, e.g., by condensation. 
Furthermore, a compound of the invention can be transformed by conventional 
methods into another compound of the invention by modification of the 
terminal groups R.sup.1 through R.sup.6. 
Suitable parent or starting compounds are known or can be obtained by 
methods analogous to those used for known compounds. Suitable starting 
compounds having pyrimidine rings are, e.g., described in East German Pat. 
No. 95,892 and West German OS No. 26 41 724. The following reaction scheme 
is presented as a general example of the preparation of the novel 
anisotropic compounds. The characters have the same general significance 
as in formula (22). 
##STR12## 
The compounds of formula (22) according to the invention can be used 
analogously to the known compounds of Type II for LC-mixtures in 
TN-displays with multiplex operation. They offer the advantage that the 
problem of smectic phases can be eliminated or significantly reduced. 
LC-mixtures according to the invention can be comprised of one or more 
compounds of the formula (22), e.g., in a total amount up to 60 mole 
percent, wherein the proportion of individual compounds of formula (22) 
can comprise up to 30 mole percent of the mixture. The remaining portion 
of the LC-mixtures according to the invention is for the most part one or 
preferably more known compounds of Type I. 
Having generally described the invention, a more complete understanding can 
be obtained by reference to certain specific examples which are provided 
herein for purposes of illustration only and are not intended to be 
limiting unless otherwise specified. In the following examples the 
temperature properties of the mesophases are indicated in the conventional 
way by giving the corresponding temperatures (.degree. C.) between the 
symbols K (crystalline), S (smectic), N (nematic) and I (isotropic); 
m.p.=melting point; c.p.=clearing point; .DELTA.=optical anisotropy.