Liquid crystal composition

A liquid crystal composition includes at least one of specific pyrimidine compounds, and may be used to form a strong dielectric liquid crystal display element.

FIELD OF THE INVENTION 
This invention relates to a liquid crystal composition used in a strong 
dielectric liquid crystal display element, etc. 
BACKGROUND OF THE INVENTION 
Recently, various liquid crystal compositions exhibiting high 
dielectricities are proposed. However, there are very few liquid 
compositions that are acceptable in orientation and response, and have the 
lower limit of the temperature range of the chiral smectic C(Sc*) phase 
below the room temperature. 
For example, by mixing compounds A, B and C shown by the following formulas 
(A), (B) and (C) as proposed by Goodby, etc., a liquid crystal composition 
having acceptable orientation and response is obtained. (In the formulas 
below, (S) and (R) mean that the absolute configuration of an optical 
active group forming its side chain is of S type or R type.) 
##STR1## 
Analyzing the relationship between the transition point temperature and the 
formulating ratio of a liquid crystal composition consisting of these 
compounds A, B and C, it is recognized that the optimum temperature range 
representing the Sc* phase is about 13.degree. C. to 55.degree. C. 
The optimum temperature range of the Sc* phase of a liquid crystal 
composition obtained by mixing liquid crystal compounds D and E shown by 
the following formulas (D) and (E) is about 20.degree. C. to 65.degree. C. 
##STR2## 
On the other hand, there is a proposal to mix compounds having different 
skeltons to extend the Sc* phase temperature range. The optimum Sc* phase 
temperature range of a composition obtained by the foregoing compounds A 
to E as taught by the proposal is about 0.degree. to 63.degree. C. 
As studied above, prior art strong dielectric liquid crystal compositions 
did not have the Sc* phase lower limit temperature sufficiently low for a 
reliable practical use. 
OBJECT OF THE INVENTION 
It is therefore an object of the invention to provide a strong dielectric 
liquid crystal composition having excellent orientation and response and 
have the Sc* phase lower limit temperature below the room temperature. 
SUMMARY OF THE INVENTION 
An invention liquid crystal composition includes at least a pyrimidine 
compound I shown by the following general formula (I): 
##STR3## 
(where R.sub.1 is an alkyl group shown by CnH.sub.2 n+1, R.sub.2 is an 
alkyl group shown by CmH.sub.2 m+1, 4.ltoreq.n.ltoreq.8 and 
6.ltoreq.m.ltoreq.14). 
Among compounds (I) shown by the general formula (I), those wherein n=6 and 
m=12 are optimum. 
Addition of the pyrimidine compound I can drop the Sc* phase temperature 
range to obtain a liquid crystal composition which exhibits the Sc* phase 
below the room temperature. 
Although, various liquid compounds may be used to be mixed with the 
pyrimidine compound I, it is preferable that the liquid crystal 
composition is mixed with at least one sort of liquid crystal compounds 
having the ester linkage like the foregoing compounds A and B. The 
pyrimidine compound I to be added to the composition is preferably 70 mol 
% or less. If the added amount of the compound I exceeds 70 mol %, the Sc* 
phase upper limit temperature largely drops to the range of the room 
temperature.

DETAILED DESCRIPTION 
The invention is described below in detail, referring to preferred 
embodiments. 
(Embodiment 1) 
A pyrimidine compound II shown by the following formula (II) was added to 
the mixture of the compounds A to E to determine the relationship between 
the added amount of the compound II and the transition point temperature. 
The mixture ratio of the compounds A to E was A:B:C:D:E =27:40.5:7.5:20:5 
(mol ratio). 
##STR4## 
The results are shown in FIG. 1. 
For the purpose of comparison, the relationship between the mixture ratio 
and the transition point temperature was studied about some prior art 
liquid crystal compositions. 
As compared example 1, a liquid crystal composition consisting of compounds 
A, B and C was taken. The composition was prepared by first mixing 
compounds A and B by A:B=4:6 (mol ratio) into a resulting mixture Z and 
subsequently adding compound C threto. The results are shown in FIG. 2. 
Subsequently, a liquid crystal composition consisting of compounds D and E 
taken as compared example 2 was studied. The results are shown in FIG. 3. 
Further, a liquid crystal composition consisting of compounds A through E 
taken as compared example 3 was studied. The composition was prepared by 
first mixing compounds A, B and C by A:B:C=37:55.5:7.5 (mol ratio) into a 
resulting mixture Y, separately mixing compounds D and E by D:E=8:2 (mol 
ratio) into a resulting mixture Y, and finally mixing mixtures X and Y 
together. The results are shown in FIG. 4. 
It is recognized from the results of FIG. 1 that the Sc* phase temperature 
range, particularly the lower limit temperature of the liquid crystal 
composition according to embodiment 1 mixed with the pyrimidine compound 
II can be decreased more than those of compared examples 1 to 3. It is 
further recognized that the composition added with 10 mol % of the 
pyrimidine compound II was dropped in the Sc* phase temperature as much as 
-10.about.+60.degree. C. If the added amount of the pyrimidine compound II 
exceeds 70 mol %, the Sc* phase upper limit temperature largely drops to 
40.degree. C. or less. Therefore, a preferable added amount of compound II 
appears to be 70% or less. 
Next, a liquid crystal composition including 10 mol % of pyrimidine 
compound II was injected in a liquid crystal cell having a cell thickness 
of 2 .mu.m to study its orientation and response. The liquid crystal cell 
was prepared by spreading polyimide on a glass substrate having 
transparent electrodes of indium-tin oxide (ITO) and subsequently carrying 
out a rubbing treatment in one axial direction. 
Studying the prepared liquid crystal cell, it was recognized that the 
liquid crystal composition exhibits an excellent orientation and a 
bi-stability. Subsequently, a current voltage of -10V rectangular waveform 
was applied to it to determine its response speed, and it was recognized 
that it exhibits a high speed response below 200 .mu.msec below the room 
temperature. 
(Embodiments 2 through 4) 
The same experiment as embodiment 1 was carried out on compositions each 
prepared by adding a pyrimidine compound III, IV or V shown by the 
following general formulas, in lieu of the pyrimidine compound II used in 
embodiment 1. 
##STR5## 
As a result, it was recognized that addition of any of pyrimidine compounds 
III to V decreases the Sc* phase temperature range of the composition in 
the substantially same degree as embodiment 1 does. 
Further, either composition mixed with the compound III, IV or V exhibited 
an excellent orientation and response. 
As described above, since the inventive liquid crystal composition includes 
at least one pyrimidine compound shown by the general formula (I), the Sc* 
phase temperature range can be lowered. 
Therefore, the invention provides a strong dielectric liquid crystal 
composition having excellent orientation and response and having the lower 
limit temperature of the Sc* phase below the room temperature.