Type A smectic liquid crystal having a positive dielectric anisotropy

A family of liquid crystals having a type A smectic mesophase with a positive dielectric anisotropy, which are particularly useful in the case of the thermo-optical effect. The substances of the family are in accordance with the general formula: ##STR1## in which R represents an organic group of the alkyl or alkoxy type containing 1 to 15 carbon atoms and in which X and Y designate either the bromine or the nitrile radical.

The present invention relates to a liquid crystal, whose general formula 
corresponds to a family of substances only differing from one another by 
an alkyl or alkoxy group and by a bromo or cyano radical. In certain 
temperature ranges, these substances have a type A smectic mesophase with 
a positive dielectric anisotropy. Mixtures of these substances with one 
another and with other liquid crystals are included within the scope of 
the invention. They can be used in visual displays where the advantage of 
the thermo-optical effect is utilized, with or without assistance of an 
electrical field. 
The liquid crystal according to the invention is in accordance with the 
general formula: 
##STR2## 
in which R represents an organic group of the alkyl or alkoxy type having 
1 to 15 carbon atoms and in which X and Y designate either a bromo or 
cyano radical. 
These substances are designated by: 
(1) p-bromodiphenyl-4-(alkyl or alkoxy)-3-bromobenzoate, if X=Y=Br 
(2) p-cyanodiphenyl-4-(alkyl or alkoxy)-3-bromobenzoate, if X=Br and Y=CN 
(3) p-cyanodiphenyl-4-(alkyl or alkoxy)-3-cyanobenzoate, if X=Y=CN. 
Hereinafter, the mechanism of the general production process of the 
invention is given, followed by a number of examples of operating 
procedures. 
GENERAL PRODUCTION PROCESS 
Firstly, 4-hydroxy-4'-bromodiphenyl and, as applicable, 
4-alkoxy-3-bromobenzoic acid or 4-alkyl-3-bromobenzoic acid are 
synthesized to obtain brominated benzoates. The bromine in the brominated 
compounds is replaced by nitrile to obtain CN-containing compounds. 
1. Synthesis of 4-hydroxy-4'-bromodiphenyl 
The starting substance is diphenyl acetate with which bromine is reacted at 
30.degree. C. in the presence of iodine as the catalyst. The reaction 
diagram is as follows: 
##STR3## 
The product obtained is saponified by potassium hydroxide in an ethanolic 
medium and then hydrolyzed by hydrochloric acid in accordance with the 
following diagram: 
##STR4## 
In the case of products X=Br and Y=CN it is necessary to synthesize 
4-hydroxy-4'-cyanodiphenyl. It is obtained from the aforementioned product 
by reacting cuprous cyanide in dimethyl formamide:(DMF) 
##STR5## 
2. Synthesis of 4-(alkyl or alkoxy)-3-bromobenzoic acid 
By bromination, the corresponding 4-substituted benzoic acid is obtained: 
##STR6## 
For R=C.sub.n H.sub.2n+1, reaction takes place in an aqueous solution of 
nitric and acetic acid in the presence of silver nitrate at 25.degree. C. 
For R=C.sub.n H.sub.2n+1 O, the reaction takes place in an aqueous medium 
at 55.degree. C. 
3. Synthesis of the chloride of the aforementioned acid 
The diagram is as follows: 
##STR7## 
4. Synthesis of diphenylbenzoates 
Three cases are to be envisaged: 
(1) Case in which X=Y=Br: 
4-hydroxy-4'-bromodiphenyl is esterified with the selected 3-bromo acid 
chloride at ambient temperature in a medium constituted by pyridine: 
##STR8## 
(2) Case where X=Br and Y=CN: 
Esterification takes place between the acid chloride and the 
4-hydroxy-4'-cyanodiphenyl in the same medium (pyridine): 
##STR9## 
(3) Case where X=Y=CN: 
The two Br groups are substituted by CN on products of type (1) in a medium 
containing cuprous cyanide and dimethyl formamide:

EXAMPLES OF OPERATING PROCEDURES (R=C.sub.7 H.sub.15, X=Br, Y=CN) 
(a) Synthesis of 4-n-heptyl-3-bromobenzoic acid 
8.8 g (0.04 mole) of 4-n-heptyl-3-bromobenzoic acid and 8 g (0.05 mole) of 
bromine are introduced, accompanied by stirring, into a mixture of 20 ml 
of soft water, 27 ml of nitric acid and 120 ml of glacial acetic acid 
heated to 25.degree. C. A solution of 6.8 g (0.04 mole) of silver nitrate 
in 20 ml of soft water are then added over a period of 30 minutes, whilst 
maintaining the temperature at 25.degree. C. Stirring is continued for 
31/2 hours. The solution is then poured into 200 ml of ice water and 
stirred for 30 minutes. The product in suspension is filtered and washed 
with soft water up to neutrality. The acid is then separated from the 
silver bromide formed by dissolving in ethanol. The ethanolic solution is 
evaporated. 10.5 g of crude product is obtained, which is recrystallized 
in 30 cc of hexane. 9.1 g of synthesized acid is recovered. The melting 
point observed in 80.degree. C. 
(b) Synthesis of 4-hydroxy-4'-cyanodiphenyl 
1 mole or 249 g of 4-hydroxy-4'-bromodiphenyl 1.25 mole or 113 g of cuprous 
cyanide and 1 liter of dimethyl formamide are introduced into a reactor. 
Refluxing, accompanied by stirring, takes place for 6 hours. The reaction 
mass is then poured into a reactor containing ethylene diamine and water. 
Extraction takes place with chloroform (2.times.1 liter), then the organic 
phase is washed to neutrality. After drying and evaporating the 
chloroform, 100 g of crude product are obtained. 47 g of product are 
obtained after chromatography on silica with benzene as the eluent. The 
synthesis yield is approximately 24%. 
(c) Synthesis of 4'-cyanodiphenyl-4-n-heptyl-3-bromobenzoate 
0.5 g (2.5.10.sup.-3 mole) of 4-hydroxy-4'-cyanodiphenyl are dissolved in 5 
ml of pyridine and 0.8 g (2.5.10.sup.-3 mole) of 4-heptyl-3-bromobenzoyl 
chloride is added thereto. The mixture is stirred for 3 days at ambient 
temperature and then poured into a mixture of 20 g of ice and 2 cc of 
concentrated sulphuric acid. After stirring for 4 hours, the product is 
extracted with benzene, washed in soft water to neutrality and dried. It 
is then purified by silica column chromatography by eluting with a hexane: 
benzene (50:50) mixture. The product obtained is dissolved in ethanol and 
treated with vegetable black, followed by recrystallization in 15 cc of 
ethanol. 300 mg of pure product are collected, giving a yield of 25%. The 
product has the following characteristics: K 89.5 S.sub.A 129 N 151 I 
(temperatures in .degree.C.) with the following conventions: 
K crystalline phase 
S.sub.A smectic phase A 
N nematic phase 
I isotropic liquid. 
Table I summarises the formulas and temperature ranges of a number of 
examples of substances according to the invention. 
TABLE I 
__________________________________________________________________________ 
Temperature ranges 
Example 
Formula and heat of fusion. 
__________________________________________________________________________ 
##STR11## K90.5S.sub.A 152.5 I and 4.8 Kcal/mole 
2 
##STR12## K121 S.sub.A 172 N 176 I 
3 
##STR13## K89.5 S.sub.A 129 N151 I and 6.5 
Kcal/mole 
4 
##STR14## K131 [S.sub.A ] 94 [N] 117 I and 9.0 
Kcal/mole 
5 
##STR15## K125 [N] 152 I 
__________________________________________________________________________ 
The transition points between the phases are in degrees Celsius. The phases 
in square brackets are of the metastable type. 
The dielectric properties of the substances of examples 1 and 3 were 
determined on the basis of measurements carried out on the mixture of each 
of the products with a nematic liquid crystal "F", p-pentyl phenol 
p-methoxybenzoate. "F" is a nematic of 29.degree. to 43.degree. C. The 
composition of the mixture is 90% of "F" and 10% of the compound according 
to the invention in molar fractions. The mixture is itself nematic. The 
measurements are performed at 22.degree. C., the magnetic orientation 
field is 10.sup.4 oersteds and the frequency of the electrical measuring 
field is 10 kHz. The substances of examples 1 and 3 are designated by 
letters A and C, giving the following results table: 
TABLE II 
______________________________________ 
Case of: .epsilon..sub..parallel. 
.epsilon..sub..perp. 
.epsilon..sub.a 
______________________________________ 
Pure F 5.7 5.6 +0.1 
Mixture of A + F 
5.1 4.8 +0.3 
Mixture of C + F 
6.8 5.1 +1.7 
______________________________________ 
The pure products A and C therefore have a positive dielectric anisotropy. 
On the basis of a linear variation law, .epsilon..sub.a as a function of 
the concentration the following values are obtained: .epsilon..sub.a 
(A)=2, .epsilon..sub.a (C)=16. Thus, A is slightly positive and C highly 
positive. 
Eutectic mixtures of compounds according to the invention with other type A 
smectic liquid crystals can be used in particular with p-cyanodiphenyls 
having the smectic phase A. 
The following table gives examples of mixtures: 
TABLE III 
______________________________________ 
Formulas of the ingredients and 
Molar fraction of 
temperature range each ingredient. 
______________________________________ 
##STR16## 0.82 
and 
##STR17## 0.18 
Range: K 15.5 S.sub.A 53 N 57.5 I 
##STR18## 0.89 
and 
##STR19## 0.11 
Range: K 18 S.sub.A 44 N 53 I 
##STR20## 0.64 
##STR21## 0.22 
and 
##STR22## 0.15 
Range: K 9.0 S.sub.A 54 N 57 I 
##STR23## 0.54 
##STR24## 0.18 
##STR25## 0.13 
and 
##STR26## 0.15 
Range: K 5 S.sub.A 57 N 63 I 
##STR27## 0.58 
##STR28## 0.19 
##STR29## 0.14 
and 
##STR30## 0.09 
Range: K 7 S.sub.A 63 N 69 I 
______________________________________ 
These mixtures can be directly used in thermo-optical devices such as 
matrix screens or laser addressing screens. They all have the requisite 
characteristics of a wide smectic A range including the ambient, a narrow 
nematic range, a positive dielectric anisotropy, a good diffusing power 
and therefore a good constant and a low operating power.