A tolan derivative useful as a component of liquid crystal materials and being superior to conventional tolan derivatives in the characteristics of .DELTA.n and viscosity, and a liquid crystal composition containing the same are provided, which tolan derivative is expressed by the formula ##STR1## wherein R.sup.1 and R.sup.2 each represent an alkyl group of 1 to 10 carbon atoms.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a 
1-4-(alkylphenyl-ethynyl)-4-(alkylphenylethynyl)benzene as a substituted 
benzene derivative used as a component of liquid crystal materials and a 
liquid crystal composition containing the derivative. 
2. Description of the Related Art 
Display devices having applied liquid crystals utilize an electrooptical 
effect based on the anisotropies of the dielectric constant and electric 
conductivity of liquid crystal substances. Liquid crystal display modes 
include various ones such as dynamic scattering mode, twisted nematic 
mode, super-twist nematic mode, phase transition mode, DAP mode, 
guest-host mode, etc. Properties required for liquid crystal substances 
used for liquid crystal display vary depending on the respective liquid 
crystal display modes, but a broad mesomorphic range, good stabilities to 
moisture, air, light, electricity, etc. and the like are required in 
common to any of the display modes. Further, it is also required that when 
display elements are used in the liquid crystal display devices, the 
response of the display elements is quick and the devices can be driven at 
a voltage as low as possible. At present, however, there is no single 
compound satisfying all of these requirements; hence, practically, liquid 
crystal compositions obtained by blending several kinds of liquid crystal 
compounds or these compounds with compounds similar to liquid crystals 
have been used. 
Recently, in order to provide a liquid crystal display element having a 
good image quality even in a multiplex number of 100 or more, it has been 
proposed to change the cell structure wherein the twist angle of the 
helical structure of liquid crystal molecule is combined with a polarizing 
plate, into a novel mode (e.g. Japanese patent application laid-open No. 
Sho 60-50511 (1985), etc.). 
The case of a liquid crystal display element of a cell structure having an 
increased the twist angle of the liquid crystal molecule exhibits an 
entirely different tendency from that of the case of conventional 
90.degree. twist, in the effect on physical properties obtained by 
choosing liquid crystal materials. 
FIGS. 1a and 1b illustrate the characteristics of a liquid crystal display 
element having an increased twist angle of a liquid crystal molecule as 
compared with those of conventional 90.degree. twist, in terms of the 
angle of view and the angle dependency of voltage-transmittance 
characteristics. FIGS. 1a and 1b illustrate a case of 90.degree. twist and 
a case of 180.degree. twist, respectively. As seen from FIGS. 1a and 1b, 
the element having a structure of 180.degree. twist is steep in the rise 
characteristic (Y characteristic) of transmittance depending on voltage, 
and this characteristic is evidently improved as compared with the case of 
conventional 90.degree. twist. This tendency becomes more notable with 
increase of twist angle. As described above, since the element having an 
element structure increased in the twist angle is steep in the rise 
characteristic brought about by voltage, the transmittance difference 
between voltage impression and nonimpression at the time of multiplex 
drive increases so that a multiplex drive higher than a conventional one 
becomes possible. 
However, as to the relationship between the .gamma. characteristic in the 
case of 90.degree. twist and that in the case of about 200.degree. twist, 
the same tendency is not exhibited depending upon liquid crystal 
materials; thus there has come to appear a tendency which does not apply 
to the general report that, in the case of 90.degree. twist, materials 
having a good .gamma. characteristic are superior when the ratio of 
elastic constants (K.sub.33 /K.sub.11) is low (Gunter Baur, Euro Display, 
84 "Liquid crystal properties in relation to multiplexing requirements"). 
This is presumed to be related to the elastic constant of twist and other 
factors, but it has not yet been elucidated. FIG. 2 shows a relationship 
between the ratio of elastic constants and the .gamma. characteristic in 
the cases of 180.degree. twist and 230.degree. twist as to two kinds of 
compounds, i.e. pyrimidines and PCHs (phenylcyclohexanes). As seen from 
FIG. 2, pyrimidines having smaller elastic constants are inferior to PCHs 
in the .gamma. characteristic. In FIG. 2, .gamma. 10.degree., 80-20% 
refers to a .gamma. characteristic in terms of the ratio of a voltage at 
80% transmittance to that at 20% transmittance in the case of an angle of 
10.degree. against normal. As described above, in the case of about 
200.degree. twist, a conventional way of thinking does not apply, and it 
is necessary to choose liquid crystal materials on a different basis from 
that in the case of the cell structure of 90.degree. twist. As described 
above, by increasing the twist angle or researching liquid crystal 
materials, a liquid crystal display element having a good .gamma. 
characteristic is obtained. However, a new phenomenon has been clarified 
by improving the .gamma. characteristic the .gamma. characteristic of 
liquid crystal materials, the lower the response rate, as shown in FIG. 3. 
When the response properties are taken into consideration, choice a of 
only a material having a good .gamma. characteristic is not sufficient, 
but in order to obtain a liquid crystal display element having good 
response properties, while maintaining .gamma. characteristic to a certain 
extent, a method of reducing the thickness of the liquid crystal layer is 
proposed. 
Thus, with accompaniment of reducing the thickness of the liquid crystal 
layer in order to improve the response properties, the optical anisotropy 
value .DELTA.n of the material should be varied. In the case of 
200.degree. C. twist, the product of the .DELTA.n of the liquid crystal 
material by the thickness d of the liquid crystal layer (.DELTA.n.times.d) 
is best in the vicinity of 0.96 .mu.m. When the thickness d of the liquid 
crystal layer is 7 .mu.m, the .DELTA.n of the material should be adjusted 
to 0.137. In order to make the thickness of the liquid crystal layer 5 
.mu.m for improving the response rate, it is necessary to increase the 
.DELTA.n to 0.192. As described above, in order to correspond to thinning 
of the liquid crystal layer, the .DELTA.n of the material should be 
increased, but in this case, there is a problem of viscosity. As to the 
relationship between the .DELTA.n and viscosity of liquid crystal 
materials so far reported, a tendency that the viscosity increases with 
increase in the .DELTA.n has been clarified. Namely, conventional 
materials having a large .DELTA.n and yet a low viscosity have been very 
few. 
As examples of so far known compounds having a large optical anisotropy 
value .DELTA.n when used as a component of liquid crystal materials, 
compounds expressed by the following formulas (1)-(4) are disclosed in (1) 
French patent application laid-open No. 2,141,438, (2) Japanese patent 
application laid-open No. Sho 60-152427/1985, (3) Japanese patent 
application laid-open No. Sho 61-260031/1986 and (4) Japanese patent 
application laid-open No. Sho 60-204731/1985, respectively: 
##STR2## 
wherein R.sup.3 and R.sup.4 each represent an alkyl group or an alkoxy 
group; 
##STR3## 
and R.sup.5 and R.sup.6 each represent a linear alkyl group; 
##STR4## 
wherein R.sup.7 represents a linear alkyl group and X represents a halogen 
atom; and 
##STR5## 
wherein R.sup.8 and R.sup.9 each represent a linear alkyl group. 
These tolan derivatives have been said to have a large .DELTA.n and a 
relatively low viscosity, but compounds far exceeding the characteristics 
of these compounds have recently been required. 
SUMMARY OF THE INVENTION 
The present invention resides in 
a 1-(4-alkylphenylethynyl)-4-(alkylphenylethynyl)-benzene expressed by the 
formula 
##STR6## 
wherein R.sup.1 and R.sup.2 each represent an alkyl group of 1 to 10 
carbon atoms, and a liquid crystal mixture containing the same.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Examples of the compound provided by the present invention are as follows: 
1,4-bis-(4-methylphenylethynyl)benzene, 
1,4-bis-(4-ethylphenylethynyl)benzene, 
1,4-bis-(4-propylphenylethynyl)benzene, 
1,4-bis-(4-butylphenylethynyl)benzene, 
1,4-bis-(4-pentylphenylethynyl)benzene, 
1,4-bis-(4-hexylphenylethynyl)benzene, 
1,4-bis-(4-heptylphenylethynyl)benzene, 
1,4-bis-(4-octylphenylethynyl)benzene, 
1,4-bis-(4-nonylphenylethynyl)benzene, 
1,4-bis-(4-decylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-ethylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-propylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-butylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-pentylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-hexylphenylethynyl)benzene 
1-(4-methylphenylethynyl)-4-(4-heptylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-octylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-nonylphenylethynyl)benzene, 
1-(4-methylphenylethynyl)-4-(4-decylphenylethynyl)benzene, 
1-(4-ethylphenylethynyl)-4-(4-propylphenylethynyl)benzene, 
1-(4-ethylphenylethynyl)-4-(4-butylphenylethynyl)benzene 
1-(4-ethylphenylethynyl)-4-(4-pentylphenylethynyl)benzene, 
1-(4-ethylphenylethynyl)-4-(4-hexylphenylethynyl)benzene, 
1-(4-ethylphenylethynyl)-4-(4-heptylphenylethynyl)benzene, 
1-(4-ethylphenylethynyl)-4-(4-octylphenylethynyl)benzene, 
1-(4-ethylphenylethynyl)-4-(4-nonylphenylethynyl)benzene, 
1-(4-ethylphenylethynyl)-4-(4-decylphenylethynyl)benzene, 
1-(4-propylphenylethynyl)-4-(4-butylphenylethynyl)benzene, 
1-(4-propylphenylethynyl)-4-(4-pentylphenylethynyl)benzene, 
1-(4-propylphenylethynyl)-4-(4-hexylphenylethynyl)benzene, 
1-(4-propylphenylethynyl)-4-(4-heptylphenylethynyl)benzene, 
1-(4-propylphenylethynyl)-4-(4-octylphenylethynyl)benzene 
1-(4-propylphenylethynyl)-4-(4-nonylphenylethynyl)benzene, 
1-(4-propylphenylethynyl)-4-(4-decylphenylethynyl)benzene, 
1-(4-butylphenylethynyl)-4-(4-pentylphenylethynyl)benzene, 
1-(4-butylphenylethynyl)-4-(4-hexylphenylethynyl)benzene, 
1-(4-butylphenylethynyl)-4-(4-heptylphenylethynyl)benzene, 
1-(4-butylphenylethynyl)-4-(4-octylphenylethynyl)benzene, 
1-(4-butylphenylethynyl)-4-(4-nonylphenylethynyl)benzene, 
1-(4-butylphenylethynyl)-4-(4-decylphenylethynyl)benzene, 
1-(4-pentylphenylethynyl)-4-(4-hexylphenylethynyl)benzene, 
1-(4-pentylphenylethynyl)-4-(4-heptylphenylethynyl)benzene, 
1-(4-pentylphenylethynyl)-4-(4-octylphenylethynyl)benzene, 
1-(4-pentylphenylethynyl)-4-(4-nonylphenylethynyl)benzene, 
1-(4-pentylphenylethynyl)-4-(4-decylphenylethynyl)benzene, 
1-(4-hexylphenylethynyl)-4-(4-heptylphenylethynyl)benzene, 
1-(4-hexylphenylethynyl)-4-(4-octylphenylethynyl)benzene, 
1-(4-hexylphenylethynyl)-4-(4-nonylphenylethynyl)benzene, 
1-(4-hexylphenylethynyl)-4-(4-decylphenylethynyl)benzene, 
1-(4-heptylphenylethynyl)-4-(4-octylphenylethynyl)benzene, 
1-(4-heptylphenylethynyl)-4-(4-nonylphenylethynyl)benzene, 
1-(4-heptylphenylethynyl)-4-(4-decylphenylethynyl)benzene, 
1-(4-octylphenylethynyl)-4-(4-nonylphenyl-ethynyl)benzene, 
1-(4-octylphenylethynyl)-4-(4-decylphenyl-ethynyl)benzene and 
1-(4-nonylphenylethynyl)-4-(4-decylphenyl-ethynyl)benzene. 
The compound provided by the present invention has specific features 
desired as liquid crystal component a in well-balanced manner; namely the 
compound has an optical anisotropy .DELTA.n as very large as about 0.4, a 
viscosity at 20.degree. C. of about 20 cp which is low for a compound of 
three-ring structure, a high clearing point, etc. 
PREATION OF COMPOUND 
Next, an embodiment of preparation of the compound of the present invention 
will be illustrated. 
A symmetric compound may be prepared according to the following equation: 
##STR7## 
Namely, when an alkylphenylacetylene (II) is reacted with 1,4-diiodobenzene 
(III) in a solvent of diethylamine in the presence of Pd catalyst and 
copper iodide according to a method in the literature (TetraL hedron 
Letters No. 50, pp. 4467-4470, 1975), it is possible to prepare the 
objective compound (I). 
Further, an asymmetric compound may be prepared according to the following 
equation: 
##STR8## 
Namely, when a 4-alkylphenylacetylene (II) is reacted with a 
4-alkyl-4'-iodotolan (IV) which can be prepared in a conventional manner, 
according to the above-mentioned method, it is possible to obtain the 
objective asymmetric compound (I) (R.sup.1 and R.sup.2 in the above 
equation being as defined above). 
The liquid crystal composition comprises at least two kinds of liquid 
crystals or compounds similar to liquid crystals and at least one of them 
is a liquid crystal compound expressed by the above formula (I), the 
content of which is preferably in the range of 2 to 30% by weight. 
Further, the liquid crystal composition containing the above liquid crystal 
compound (I) is preferably used in a liquid crystal element wherein a 
nematic liquid crystal having an optically active substance added thereto 
and having a positive dielectric anisotropy is filled between a pair of 
opposed upper and lower electrode substrates; the liquid crystal molecule 
forms a helical structure of 160.degree.-270.degree. twist in the 
direction of its thickness; the polarizing axis of polarizing plates 
provided so as to place the helical structure therebetween is deviated in 
the range of 20.degree.-70.degree. against the aligning direction of 
liquid crystal molecules between the electrode substrates; and the product 
of the thickness d of the liquid crystal layer by the dielectric 
anisotropy .DELTA.n of the liquid crystal layer, .DELTA.n.times.d, is 
0.7-1.2 .mu.m. 
As to a component of the liquid crystal composition of the present 
invention, and as compounds used in admixture with the compound of the 
formula (I), the following compound groups (i)-(xxxiii) are enumerated: 
##STR9## 
In the formulas (i)-(xxxiii), X represents 
##STR10## 
.gamma. represents --CN, halogen atom, R.sup.1 or --OR.sup.1 and R 
.DELTA.and R.sup.1 each represent an alkyl group. 
The present invention will be described in more detail by way of Examples, 
but it should not be construed to be limited thereto. 
In the Examples, C-N point represents liquid crystalnematic phase 
transition point and N-I point represents nematic phase-isotropic liquid 
phase transition point. 
EXAMPLE 1 
Preparation of 1,4-bis-(4-propylphenylethynyl)-benzene 
1,4-Diiodobenzene (2.5 g, 7.57 mmol) was added to diethylamine (80 ml) in 
nitrogen gas stream, followed by agitating the mixture at room 
temperature, successively adding copper iodide (60 mg) and 
dichlorobis(triphenylphosphine)palladium(II) (100 mg), adding 
4-propylphenylacetylene (4.37 g, 30 mmol) in several steps, agitating the 
reaction mixture at room temperature overnight, adding water (200 ml), 
extracting the resulting deposited crystals with toluene (100 ml), washing 
the toluene solution with dilute hydrochloric acid, water, an aqueous 
solution of sodium hydrogen sulfate and water in this order, drying the 
solution on anhydrous sodium sulfate, passing it through a short column of 
activated alumina, distilling off toluene and recrystallizing the residue 
from ethyl acetate to obtain the objective 
1,4-bis-(4-propylphenyl-ethynyl)benzene (1.92 g, 5.3 mmol). This product 
exhibited liquid crystal phases. C-N point: 179.1.degree. C. N-I point: 
245.5.degree. C. 
EXAMPLE 2, 3 and 4 
The following compounds were obtained in the same manner as in Example 1: 
1,4-bis-(4-ethylphenylethynyl)benzene 
(C-N point: 208.9.degree. C., N-I point: 235.8.degree. C.). 
1,4-bis-(4-butylphenylethynyl)benzene 
(C-N point: 151.8.degree. C., N-I point: 215.9.degree. C.). 
1,4-bis-(4-pentylphenylethynyl)benzene 
(C-N point: 151.1.degree. C., N-I point: 212.3.degree. C.). 
USE EXAMPLE 1 
A liquid crystal composition A consisting of 
trans-4-propyl-(4-cyanophenyl)cyclohexane 
30 parts by weight, 
trans-4-pentyl-(4-cyanophenyl)cyclohexane 
40 parts by weight .DELTA.nd 
trans-4-heptyl-(4-cyanophenyl)cyclohexane 
30 parts by weight, 
had a N-I point of 52.1.degree. C., a dielectric anisotropy 
.DELTA..epsilon. of 10.7, a viscosity at 20.degree. C. of 22.4 cp and an 
optical anisotropy .DELTA.n of 0.118. When 
1,4-bis-(4-butylphenylethynyl)-benzene of the present invention shown in 
Example 3 (5 parts by weight) was added to the above liquid crystal 
composition A (95 parts by weight), the resulting liquid crystal 
composition exhibited a N-I point raised up to 57.2.degree. C., a 
viscosity at 20.degree. C. of 22.5 cp (unchanged) and an optical 
anisotropy .DELTA.n raised up to 0.131. 
USE EXAMPLE 2 
To the liquid crystal composition A used in Use example 1 (90 parts by 
weight) were added 1,4-bis-(4-propylphenylethynyl)benzene (2 parts by 
weight), [,4-bis-(4-butylphenylethynyl)benzene (5 parts by weight) and 
1,4-bis-(4-pentylphenylethynyl)benzene (2 parts by weight), shown in 
Examples 1, 3 and 4, respectively. The resulting liquid crystal 
composition exhibited a N-I point of 63.1.degree. C., a viscosity at 
20.degree. C. of 22.6 cp and an optical anisotropy .DELTA.n of 0.151. 
Thus, the usefulness of the compound of the present invention is evident 
from these examples.