Biphenyl esters and liquid crystalline mixtures comprising them

Biphenyl esters of the formula ##STR1## wherein X is --CO--O-- or --O--CO-- and R.sub.1 and R.sub.2 each are alkyl or alkoxy of 1 - 8 carbon atoms, enlarge significantly the temperature range of the nematic phase of liquid-crystalline compositions without adversely affecting other properties.

BACKGROUND OF THE INVENTION 
This invention relates to novel biphenyl esters, to liquid crystalline 
mixtures comprising them, and to processes for their preparation and their 
use as components in such mixtures, which are useful as dielectrics for 
liquid crystal display elements. 
To an increasing extent the changes of the optical properties, such as 
light scattering, birefringence, reflecting power or color, of nematic or 
nematic-cholesteric liquid crystalline materials under the influence of an 
electric field are employed for converting electrical voltages pulses into 
optical signals. The function of such liquid crystal display elements 
thereby depends, for example, upon the phenomenon of dynamic scattering, 
the deformation of aligned phases or the Schadt-Helfrich effect in the 
twisted cell. 
For the technical application of these effects in electronic constructional 
elements, liquid crystalline materials are required which satisfy a 
plurality of requirements. Especially important are chemical stability 
towards moisture, air and physical influences, such as heat, radiation in 
the infrared, visible and untra-violet range and electrical direct and 
alternating fields. Furthermore, liquid crystalline materials, to be 
useable, should have a liquid crystal mesophase in the temperature range 
of at least +10.degree. C. to +60.degree. C., preferably 0.degree. C. to 
60.degree. C., and a viscosity at room temperature of not more than 70 cP. 
Finally, they must not exhibit any inherent absorption in the range of 
visible light, i.e., they must be colorless. 
Colorless liquid crystalline compounds are known which satisfy the 
stability requirements demanded of dielectrics for electronic 
constructional elements, e.g., the p,p'-disubstituted benzoic acid phenyl 
esters described in published German Specification No. 2,139,628 (U.S. 
application Ser. No. 277,502, filed Aug. 1, 1972, now U.S. Pat. No. 
4,002,670) and the p,p'-disubstituted biphenyl derivatives described in 
published German Specification No. 2,356,085 (U.S. application Ser. No. 
413,247, filed Nov. 6, 1973, now U.S. Pat. No. 3,947,375). In both of 
these classes of compounds, as in the case of other known series of 
compounds with a liquid crystalline mesophase, there are no individual 
compounds which form a liquid crystalline nematic mesophase in the 
temperature range of 10.degree. C. to 60.degree. C. Therefore, as a rule, 
there are employed mixtures of two or more compounds which are useable as 
liquid crystalline dielectrics, usually a mixture of at least one compound 
with low melting and clear point and one having a considerably higher 
melting and clear point. Normally, there is obtained a mixture whose 
melting point lies below that of the lower melting component thereof and 
whose clear point lies between the clear points of the components. Optimal 
dielectrics cannot be prepared in this manner because the component with 
the high melting and clear point almost always also imparts a high 
viscosity to the mixture. The switch times of the electro-optical 
constructional elements produced therewith are thereby prolonged in an 
undesirable manner. 
It has now been found that the biphenyl esters of this invention are 
exceptionally suitable as components of liquid crystalline dielectrics, 
imparting thereto the requisite properties without imparting thereto an 
undesirably high viscosity. 
SUMMARY OF THE INVENTION 
In a composition aspect, this invention relates to novel biphenyl esters of 
the general Formula I 
##STR2## 
wherein X is --CO--O-- or --O--CO-- and R.sub.1 and R.sub.2, which can be 
alike or different, are alkyl or alkoxy of 1-8 carbon atoms in a straight 
or branched chain. 
In other composition aspects, this invention relates to nematic mixtures of 
liquid crystalline compounds comprising at least one biphenyl ester of 
this invention which are useful as dielectrics for electronic 
constructional elements. 
In process aspects, this invention relates to methods of making and using 
the compositions of this invention. 
DETAILED DISCUSSION 
The biphenyl derivatives of the general Formula I are biphenyl-carboxylic 
acid phenyl esters of the general Formula II 
##STR3## 
and benzoic acid biphenyl esters of the general Formula III 
##STR4## 
wherein R.sub.1 and R.sub.2 have the values given above. 
These compounds exhibit a nematic and, in some cases, an additional smectic 
mesophase within certain temperature ranges for the same purposes as known 
liquid crystalline substances. 
Although almost all of the compounds of the general Formula I possess such 
high melting (above 90.degree. C.) and clear points (above 170.degree. 
C.), that they alone are unsuited as dielectrics for electronic indicator 
elements which are to be operated at room temperature, these compounds 
admixed with other liquid crystalline compounds achieve a markedly lower 
melting point thereof, as well as favorably improving the clear point, 
without, at the same time, causing an undesirably large increase in 
viscosity. 
It has been found that those compounds of Formula I are especially well 
suited as components of liquid crystalline dielectrics which form both a 
nematic phase and a smectic phase. This is especially surprising because, 
as is known, smectic phases possess a substantially higher viscosity than 
nematic phases. Heretofore, no strict rules have been derived according to 
which, in the case of the compounds according to the invention, certain 
structural elements are responsible for the existence of a smectic 
mesophase. In the case of the compounds of Formula II, smectic mesophases 
are observed more often than in the case of those of Formula III. For this 
reason, the compounds of Formula II according to the invention are 
preferred as components of liquid crystalline dielectrics. 
In the compounds of Formula I, the R.sub.1 and R.sub.2 can be the same or 
different. Especially valuable as components of the liquid crystalline 
compositions of this invention are those biphenyl esters of Formula I in 
which at least one of R.sub.1 and R.sub.2 is alkyl. Especially preferred 
are those in which both R.sub.1 and R.sub.2 are alkyl of 1-8 C-atoms, 
e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, n-pentyl, 
2-methyl-butyl, 3-methyl-butyl, n-hexyl, 1-methyl-pentyl, n-heptyl, 
n-octyl and 2-ethyl hexyl. Most preferred of this latter group are those 
compounds of Formula I in which on of R.sub.1 and R.sub.2 is a 
straight-chain alkyl group of 1-7 carbon atoms and the other is 
straight-chain or branched alkyl of 3-8 carbon atoms. 
Another important class of biphenyl esters of Formula I are those in which 
one of R.sub.1 and R.sub.2 is an alkyl, preferably a straight-chain alkyl 
group and most preferably of 3-8 carbon atoms, and the other is straight 
chain alkoxy, preferably of 1-6 carbon atoms, i.e., methoxy, ethoxy, 
n-propyloxy, n-butyloxy, n-pentyloxy or n-hexyloxy. 
In a process aspect, this invention relates to the preparation of the 
biphenyl esters of Formula I employing reactions, such as are frequently 
described in the literature for the preparation of aromatic carboxylic 
acid esters, preferably by reacting a biphenyl compound of the general 
Formula IV 
##STR5## 
wherein Z is OH, OMe, wherein Me is an equivalent of a metal cation, or 
COOH or a reactive derivative thereof, at a temperature from -50.degree. 
C. to +250.degree. C., optionally in the presence of an organic solvent 
and/or a conventional esterification catalyst, with a compound of the 
general Formula V wherein Z' is COOH or a reactive derivative thereof 
when Z is OH or OMe, and is OH or OMe when Z is COOH or a reactive 
derivative thereof. There are thus obtained compounds of the general 
Formula II when Z is COOH or a reactive derivative thereof, preferably 
--CO-halogen and especially --COCl, --COO-lower alkyl, preferably 
--COOCH.sub.3, or an anhydride group, preferably a mixed anhydride, e.g., 
--COOCOCH.sub.3. In a compound of Formula V, Z' is a phenolic hydroxyl 
group or a phenolate group, preferably an alkali metal or alkaline earth 
metal phenolate group. The reaction conditions for the process according 
to the invention are substantially determined by the nature of the Z and 
Z' groups. Thus, a carboxylic acid is, as a rule, reacted with a phenol 
(Z, Z' = COOH, OH) in the presence of a strong acid, for example, a 
mineral acid, such as hydrochloric acid or sulfuric acid. Preferred is the 
reaction of an acid anhydride or more preferably an acid chloride with a 
phenol (Z, Z' = COCl, OH). These esterification reactions are preferably 
carried out in a basic medium, employing as base, e.g., an alkali metal 
hydroxide, such as sodium or potassium hydroxide, an alkali metal 
carbonate or hydrogen carbonate, such as sodium carbonate, sodium hydrogen 
carbonate, potassium carbonate or potassium hydrogen carbonate, an alkali 
metal acetate, such as sodium or potassium acetate, an alkaline earth 
metal hydroxide, such as calcium hydroxide, or an organic base, such as 
triethylamine, pyridine, lutidine, collidine or quinoline. 
The esterification is advantageously carried out in the presence of an 
inert solvent, e.g., an ether, such as diethyl ether, di-n-butyl ether, 
tetrahydrofuran, dioxane or anisole, a ketone, such as acetone, butanone, 
pentan-3-one or cyclohexanone, an amide, such as dimethylformamide or 
hexamethyl phosphoric acid triamide, a hydrocarbon, such as benzene, 
toluene or xylene, a halogenated hydrocarbon, such as carbon 
tetrachloride, or tetrachloroethylene, or a sulphoxide, such as dimethyl 
sulphoxide or sulfolane. Solvents which are not miscible with water can 
advantageously be employed simultaneously for azeotropically distilling 
off any water formed by the esterification. Sometimes, an excess of the 
organic base used, for example, pyridine, quinoline or triethylamine, can 
be employed as solvent for the esterification. The esterification reaction 
can also be carried out in the absence of a solvent, for example, by 
simple heating of the components in the presence of sodium acetate. 
The reaction temperature usually is from -50.degree. C. to +250.degree. C., 
preferably -20.degree. C. to +80.degree. C. At these temperatures, the 
esterification reaction is usually complete after 15 minutes to 48 hours. 
In a further preferred embodimental of this process aspect of the 
invention, the phenol to be esterified of the formula IV or V (Z or Z' = 
OH) is first converted into its sodium or potassium salt, for example, by 
treatment with ethanolic sodium or potassium hydroxide solution, the thus 
produced salt is isolated and, together with sodium hydrogen carbonate or 
potassium carbonate, is suspended, with stirring, in acetone or diethyl 
ether, the resulting suspension is mixed dropwise, with stirring, with a 
solution of an acid chloride or anhydride in diethyl ether, acetone or 
dimethylformamide. The reaction mixture is thereby maintained at a 
temperature between -25.degree. C. and +20.degree. C., preferably at 
-10.degree. C. to -20.degree. C. In this method, the esterification 
reaction is usually complete after 15 to 50 minutes. 
Some of the starting materials for the process of this invention are known, 
such as, for example, the phenols and benzoic acids of Formula V 
substituted in the para-position. The others can be prepared without 
difficulty according to standard processes of organic chemistry from 
compounds known from the literature. Thus, for example, carboxylic acids 
of Formula IV (Z = COOH) are obtained by hydrolysis of 4-cyanobiphenyls 
substituted in the 4'-position, which are described in published German 
Specification No. 2,356,085. The alkoxy-hydroxy-biphenyls of the general 
Formula IV (Z = OH, R.sub.1 = alkoxy) can be obtained, for example, by 
partial etherification of 4,4'-dihydroxybiphenyl. The 
alkyl-hydroxy-biphenyls of Formula IV (Z = OH, R.sub.1 = alkyl) can be 
prepared from the 4-alkyl-4'-nitrodiphenyls described in published German 
Specification No. 2,356,085 by reduction to the corresponding 
4'-aminodiphenyls, followed by diazotization and heating the resulting 
diazonium salt with water. 
The compounds of Formula I can be mixed with known liquid crystalline 
compounds or mixture of compounds, e.g., in amounts of 0.5-40 mole 
percent, preferably 1-20 mole percent, more preferably of 5-15 mole 
percent. 
Known liquid crystalline materials whose properties can be improved by 
their admixture with one or more compounds of this invention include those 
which have been employed as dielectrics in electrooptical indicator 
devices or which are suitable for this purpose, the most common of which 
being mixtures of derivatives of the azobenzene, azoxybenzene, biphenyl, 
Schiff base, especially benzylidene derivative, phenyl benzoate, 
optionally halogenated stilbene, diphenyl-acetylene derivative, 
diphenyl-nitrone and substituted cinnamic acid series. Frequently, isomer 
pairs and/or eutectic mixtures are employed. 
The most important components of known nematic compositions are compounds 
of the general Formula VI: 
______________________________________ 
##STR6## VI 
wherein A is 
CHCH 
##STR7## 
CX'CH 
##STR8## 
CHCX' CHN 
CC NCH 
NN 
##STR9## 
##STR10## 
##STR11## 
##STR12## or a CC single bond; 
OCO 
COO 
______________________________________ 
x' is halogen, preferably Cl; and R.sub.3 and R.sub.4 are alike or 
different and are cyano, nitro or isonitrile, or alkyl, alkoxy or 
alkanoyloxy of up to 18, preferably up to 8 carbon atoms. In most of these 
compounds, R.sub.3 and R.sub.4 are preferably different, one usually being 
alkyl or alkoxy. However, all other variants are also suitable. A whole 
series of such nematic compositions are commercially available. 
The nematic compositions can sometimes be modified by the addition thereto 
of cholesteric compounds, to achieve memory effects, usually in an amount 
of up to about 10 mole percent. A large number of nematic substances is 
described e.g., in published German Specifications Nos. 1,951,092; 
2,014,989 (U.S. Pat. No. 3,773,747); 2,139,628; 2,201,122; and 2,356,085, 
whose disclosures are incorporated by reference. 
By the addition of 0.5 to 40, preferably of 1-20, mole percent of one or 
more compounds of Formula I to a known liquid crystalline compound or 
mixture of compounds with a melting point in the range of room temperature 
or below and clear points of 50.degree. C. or above, the range of the 
liquid crystalline mesophase is, surprisingly, widened. Frequently, this 
widening takes place at both ends, i.e., the melting point, even of low 
melting mixtures, is further lowered and the clear point is further 
raised. In this way, many liquid crystalline compositions which, because 
of an unfavorable position or limited breadth of the temperature range of 
their nematic mesophase, were not suitable for use as dielectrics, are now 
suitable for this purpose. The influence of the compounds of this 
invention on the viscosity of the dielectrics is especially favorable for 
this purpose. Heretofore, the viscosity was frequently increased too much 
by the addition of a compound with a high clear point, for example, of the 
p-benzoyloxybenzoic acid phenyl ester series, so that mixtures of this 
type in displays exhibited switch times which were too long to be 
technically useable to a great extent. Surprisingly, such a large increase 
in viscosity does not occur by the addition of the biphenyl esters of this 
invention. 
The liquid crystalline compositions of this invention are suitable for use 
as dielectrics in electrooptical displays used at room temperature, have a 
liquid crystal mesophase from at least 10.degree. to 60.degree., 
preferably at least 0.degree. to 60.degree. C., and comprise 
A. 99.5-60, preferably 99-80, more preferably 95-85, mole percent of at 
least one compound, e.g., one or a mixture of two, three, four, five or 
more compounds having a nematic phase, or forming such phase in admixture 
with one another, at least one preferably being a compound of Formula VI, 
having, in the absence of a compound of Formula I, a liquid crystal 
mesophase beginning above about 45.degree., preferably about 20.degree. 
C., and ending in the range of about 40.degree. C., preferably about 
55.degree. C., and 
B. 0.5-40, preferably 1-20, more preferably 95-85, mole percent of at least 
one compound, e.g., one or a mixture of two, three or more compounds, of 
Formula I. 
Liquid crystalline mixtures according to the invention containing at least 
one compound of the general Formula I can be employed as dielectrics in 
all types of liquid crystal displays which have hitherto been known. By 
means of the additions of the new biphenyl esters, the values of the 
dielectric anisotropy of the liquid crystalline basis substances are only 
insubstantially changed. Therefore, by a suitable choice of the basis 
substances, it is possible to prepare dielectrics according to the 
invention with positive and negative dielectric anisotropy. Furthermore, 
the dielectrics according to the invention can contain additives which 
influence the conductivity, the ability of orientation and/or the sign and 
value of the dielectric anisotropy additives of this kind are described, 
for example, in published German Specifications Nos. 2,209,127, 2,240,864 
and 2,321,632.