JPH02294388A - Liquid crystal mixture - Google Patents

Abstract

PURPOSE: To provide a liq. crystal mixture which can operate in a high multiplex mode, has a high contrast, and is used for a twisted nematic liq. crystal device, and comprises a mixture of liq. crystal compds., such as 2,5-disubstd. 1,3,4-thiadiazole compds.

CONSTITUTION: This liq. crystal mixture comprises at least a first liq. crystal material comprising a liq. crystal compd. represented by the formula {wherein X represents N or CH; R¹ and R² represent R or OR [wherein R represents a 1-12C straight-chain (substd.) alkyl]; A and Q₁ represent 1,4-phenylene or trans-1,4-cyclohexylene; Z¹ represents -COO-, -CH₂CH₂- or the like; and m is 0 to 2}, a second liq. crystal material comprising a liq. crystal compd. having positive dielectric anisotropy (Δε≥+3), and a third liq. crystal material comprising a 2,5-disubstd. 1,3,4-thiadiazole compd. and/or 3,6-disubstd. 1,2-difluorobenzene compd. having negative or neutral dielectric anisotropy, the liq. crystal mixture having an elastic constant ratio of less than 0.9 and a Δε/ε⊥ ratio of not more than 0.4.

COPYRIGHT: (C)1990,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in twisted nematic liquid crystal display devices that operate in a highly multiplexed mode and have very high contrast and particularly steep electro-optical properties. The device operates at the first minimum of Gooch-Tarry transparency.

The invention furthermore relates to novel liquid crystal dielectrics having an elastic constant ratio Ks/Kl as small as possible but not greater than 0.9 and a Δt/ε1 ratio of at most 0.4. Such a dielectric, according to the invention,
Used in matrix-controlled liquid crystal display devices driven by time division.

For liquid crystal display elements, the property of nematic or nematic-cholesteric liquid crystal materials is exploited, in that their optical properties, such as light transmission, light scattering, birefringence, reflection or color, change considerably under the influence of an electric field. be done. The operation of display elements of this type is based, for example, on the phenomenon of dynamic scattering, on the deformation of aligned phases or on the Schattor-Helfrich effect in twisted cells.

Among these common types of liquid crystal display elements, display elements based on twisted nematic cells (TN cells), in particular, have a relatively low control voltage, which allows them to be easily utilized even in small batteries. Recently, it has become especially important because it can be operated by

Furthermore, in the prior art, these display elements are constructed as matrix display elements, which makes it possible to provide high information density without using an unacceptably large number of control lines, lead-ins and lead-outs. It was the best that could be done.

However, when put into practical use, this TN cell, particularly in the form of a matrix display element, has
There are still considerable difficulties.

These difficulties include, among others, the more or less pronounced viewing angle dependence of contrast and limited multiplexing capacity.

The dielectric commonly used in TN cells is approximately 1.2 to 1.6
It has an elastic constant ratio Ks/K+ of 1.0 or more and Δε/c soil. Such conventional liquid crystal compositions are widely used in commercially available twisted nematic display devices and are described in many publications, such as JP-A-56-105510 and JP-A-56-105511; 4
．． No. 285.829. The design of these compositions is based on the knowledge that the proportion of liquid crystal compounds with distinctly positive dielectric anisotropy is generally 30% or more. These compositions are characterized in that their threshold voltage is in most cases in the range of 1 to 2 ports.

Here, surprisingly, △ε/g l of the liquid crystal dielectric
When the value and Ks/Kt value are extremely small in a time-divisionally driven liquid crystal display element, the response speed and γ physical property (sharpness in voltage-luminance physical property) are Gooch-Tarry.
d. of a range that meets the requirements for the first minimum value of. It was found that Δn was significantly improved. Such liquid crystal dielectrics operate in high multiplex mode,
Most suitable for use in liquid crystal display devices with surprisingly improved contrast.

The object of the invention is to provide an improved display device in which a particularly improved contrast and at the same time a high response speed are ensured by operating the display element at a first minimum value of Gooch-Tarry transparency. It's about doing.

Further objects and advantages of the invention will be made apparent to those skilled in the art from this specification and the claims.

It has now surprisingly been found that by appropriately selecting the liquid crystal parameters and operating the display element at the first minimum value of transparency, the desired physical properties can be obtained.

Therefore, the problems to be solved by the present invention are at least the following (a) and (b).
), (c), operates in a high multiplex mode, has high contrast, and has a ratio of elastic constants K3/Kl of the liquid crystal dielectric present in the device.
as small as possible, no larger than 0.9, and
In a twisted nematic liquid crystal display device whose Δε/ε1 ratio is at most +0.4: (a) General formula (I) [wherein, X represents N or CH, and Rl represents a group R or a group OR and R2 is a group R or a group OR
and each R is independently a straight-chain alkyl group having up to 12 carbon atoms, one CB, group or two non-adjacent CH,
The group may be replaced by 10- and/or -CH═CH-, provided that the two oxygen atoms are not directly bonded to each other, and A is 1.4-7 enylene or trans-1 .4-cyclohexylene, and zl is -CO-O-, . -O-CO-, -CH, -0-, -
0CH x-, -CH,CH,- or a single bond
01 is 1.4-7 enylene or trans-1.4-cyclohexylene, and m is 0, 1, or 2.
a first liquid crystal material containing seeds, at least 10% by weight; (b) a second liquid crystal material containing one or more liquid crystal compounds having a distinctly positive dielectric anisotropy (Δε≧+3); a third liquid crystal material containing at least l types of a liquid crystal material and (c) a liquid crystal compound having negative dielectric anisotropy or substantially neutral anisotropy: a third liquid crystal in the above liquid crystal material; 2,5-disubstituted 1,3,4-thiadiazole compound containing one or more kinds of materials and/or 3,6-disubstituted 1
．． The object of the present invention is to use an improved liquid crystal mixture containing a 2-difluorobenzene compound.

The structure of the liquid crystal display element according to the present invention consists of a polarizing plate, an electrode substrate, and an electrode, and the electrode is usually arranged between one electrode and another electrode such that a suitable orientation of specific liquid crystal molecules adjacent thereto is determined. The surface treatment is such that they are twisted by 90'' with respect to each other, and such a structure corresponds to a conventional type of structure for this type of display element.

The term conventional formal structure has a very broad meaning;
Also known from the literature, twisted nematic cells,
In particular, it also encompasses both variations and modified structures of matrix display elements. However, the fundamental difference between the display element according to the invention and the known and customary display elements based on twisted nematic cells lies in the selection of the liquid crystal parameters in the liquid crystal layer. For example, the K3/Kl ratio of a conventional liquid crystal phase for display elements of this type is at least >0.8
and the Δε/ε1 ratio is usually between 1.2 and 1.
7, whereas K in the display element according to the present invention is
3/κ1 ratio is ≦0.9, and also △ε/t l
is ≦0.4. Ks in the display element according to the present invention
/K+ ratio is preferably 0.4 to 0.9, and Δt/ε1 is preferably 0.4 to 0.2. Particularly preferred dielectrics have a Ks/Kr of 0.2 to
0.75, and Δε/tl is 0.3 to 0.
1, which is a dielectric material.

The parameters used to define the invention are well known to those skilled in the art. The meaning of these definitions is explained in many publications, for example W. H. Phys by DeJeu
ical Properties of Liquor
idCrystal Materials, Go
rdon and Breach, ■980 and E. Brochure by Merck, rLiqu
id Crystals, Measurement
of the Physical ProperLi
esJ, which are incorporated herein by reference.

The thickness of the liquid crystal layer of the display element according to the present invention and the optical anisotropy Δn of the liquid crystal material are expressed by the following formula λ d. Δ”2−·J. In this formula, the wavelength λ is the commonly used value of 550 nm, which is related to the highest sensitivity of the human eye. The layer thickness of the display element according to the invention is, for example, 2 ~
It can be within the range of lOμ shoulder. Preferably, the lowest available layer thickness is chosen, the lower limit of d being determined by the quality standards required for mass production. Therefore, a particularly preferred range of d is 3 to 8μ, particularly 4 to 7gII1.

Liquid crystal dielectrics whose material parameters are within the range according to the invention can be prepared from conventional liquid crystal substrates.

A large number of such materials are known from the literature.

Preferably, the dielectric material according to the present invention contains at least one liquid crystal compound represented by the general formula Ia. Preferably, the second liquid crystal material has a general formula 11a to mg [in each of the above formulas, Z! Ha-CH, CH! -, Y is F
, -CFs, -CHF,, -0CF,, -0CHF.
or -OCH*CFs, in which each R is independently a linear alkyl group having up to 12 carbon atoms, and one CO, group or adjacent The two CH, groups that are not
-CH- may be substituted] Contains at least one liquid crystal compound represented by the following.

The third liquid crystal material preferably contains a liquid crystal compound having dielectric anisotropy of -2 to -8.

Preferably, the third liquid crystal material has the general formula n [a to f, and two Cll groups that are not in contact with each other have a formula of 10- or -, assuming that two oxygen atoms do not bond directly to each other. 〇〇=CH- may be substituted] Contains at least one liquid crystal compound represented by the following.

Another preferred dielectric is that the third liquid crystal material has the general formula mi [In each of the above formulas, ring A is as defined in claim 1, X is N, and CH is , and R are each independently a straight-chain alkyl group having up to 12 carbon atoms, in which one CH, group or adjacent (where n is O or l,
and R1'' and R16 each independently represent R or OR) A liquid crystal compound represented by the general 2■2 (wherein, n is 0 or l, and R!I and R! 1 and each independently represent R or OR), a general liquid crystal compound represented by
! False represents R or OR, respectively, independently of each other) A liquid crystal compound represented by the general formula ■4 (where n is O or l, and R!s and R26 each independently represent respectively represent R) and a liquid crystal compound represented by the general formula (5) (wherein, n is 0 or l, and R1 and R! are each independently R); and n' is O or 1).

In the above general formulas 1111 to 5, R is each independently a linear alkyl group having up to 12 carbon atoms, and one CI {1 group or adjacent 10- or 1CH, assuming that the two oxygen atoms do not bond directly to each other.
=CI- may be replaced.

The second liquid crystal material contains a dielectrically positive compound. Many different compounds of this type are known and there is no limit to the selection of these compounds per se. However, preferably, for example F, -CF. , -CHF. , -0C
Select dielectrically positive compounds with polar groups such as HF, or 100F. A preferred compound is of formula I11
1, 2■2 and formula I1 [3 nioiite, R", R"oJ
: and R14 is soreso F, one CF,, -CHF t,
-0CHFt or 100F, the corresponding compound.

The following table shows suitable liquid crystal compounds along with their physical properties. These compounds are examples of compounds that can be used in the display element according to the invention. Table 1 also lists compounds of formula 1a, formula ib, formula Ic, formula Ie, formula If and formula 1g. R1 is preferably n-alkyl having 3 to 9 carbon atoms. R! is preferably ethyl or n-propyl, or R'
g is n-alkoxy having 5 to 9 carbon atoms.

Furthermore, pyrimidine compounds in formula 1a where Rt is methoxy are preferred.

At Ronto! Dcn flash ω(”l −t h■＋IP
Ll"+ 1'l') C'J W k h Co 6
Representative examples of compounds having clearly positive dielectric anisotropy (general formulas 11a to Ilh) of the second liquid crystal material are shown in Table 2 below.

/ As liquid crystal compounds having a negative or substantially 0 Δε value, phenylcyclohexane liquid crystal compounds generally represented by 2ma (these compounds are disclosed in West German Patent No. 2
No. 636684), phenyl hexane hydroxylate compound (described in No. 636684),
These compounds are described in German Patent No. 2429093), cyclohexylcyclohexane compounds of the general formula mc (these compounds are described in U.S. Pat. No. 4,622.164) , biphenyl hexane compounds (formula I111) (these compounds are described in Japanese Patent No. 1.311,608), phenyl hexyl hexane rupooxylate compounds (formula 11 [2) (these compounds are described in Japanese Patent No. 1.311,608), The compound is fully described in Japanese Patent No. 1.297, 121)
, l-7z-2-cyclohexylcyclohexylethane compounds (2m3) (these compounds are described in U.S. Pat. No. 4.606.845). Cyclohexyl-hexyl-hexane-rupoxylate compounds (2) (4) (These compounds are disclosed in Japanese Patent No. 1.
297,121), 4,4'-biscyclohexylbiphenyl compounds (these compounds are
No. 4,331,552) and 4,4'-hiscyclohexyl-2-7-fluorobiphenyl compound (2) (these compounds are described in U.S. Pat. No. 4,419. No. 264) can be used. These liquid crystal compounds are a low-viscosity liquid crystal compound for adjusting viscosity, a high-temperature liquid crystal for increasing the N-1 point, and a liquid crystal for suppressing smectic phase formation. As a low viscosity liquid crystal compound, about 6 to 20n+
Pa. s. The liquid crystal compounds shown in Table 3 below can be used.

The high temperature liquid crystal has an N-T point of about 130 to 330°C and η of about 13 to 65 mPa. s. The liquid crystal compounds shown in Table 4 can be used.

In all the components in the liquid crystal composition according to the present invention, R
is preferably alkyl having 1 to 10 carbon atoms. A particularly preferred range is 1 to 5 carbon atoms.

At least one of the liquid crystal compounds shown in Tables 3 and 4, or general formulas I [Id to mh and general formulas 6 to I [I17] At least one of the compounds shown can be used in the third liquid crystal material.

In preferred devices according to the invention, the liquid crystal dielectric present therein consists of at least five components.

Preferred embodiments are shown below: The liquid crystal mixture has a Gooch-Tarry transmittance of
Tailored for devices that operate at the lowest values.

A liquid crystal mixture is tailored for a device whose electro-optical characteristic curve is at least as steep as the curve at this second minimum value of transmission.

The liquid crystal mixture is tailored for a device in which the steepness of the characteristic curve at its first minimum value is greater than at its second minimum value.

The liquid crystal mixture exhibits K s / K r of 0.4-0.9 and Δt / g 1 of 0.2-0.4.

A liquid crystal mixture consists of at least five components.

- The first liquid crystal material contains at least l types of liquid crystal compounds represented by the general formula 1a.

A preferred dielectric has a first liquid crystal material, a second liquid crystal material, and a third liquid crystal material present therein in an amount of at least 30% by weight and less than 10% by weight, respectively;
and less than 70% by weight.

Particularly preferable dielectric materials have a compounding ratio of the first liquid crystal material, the second liquid crystal material, and the third liquid crystal material present therein in an amount of 30 to 65% by weight, less than 5% by weight, and 35 to 65% by weight, respectively. The amount is less than 70% by weight.

These dielectrics can further contain dyes and/or doping substances in customary amounts, provided that their liquid crystal parameters do not depart from the range according to the invention.

Mixing of the liquid crystal components to achieve the above-described parameters can be carried out in a conventional manner, taking into account conventional considerations, and by conventional elementary experimentation. A number of publications may contain details of suitable materials described herein; examples of such publications are provided below.

West German Patent Application Publication No. 2,257.588;2
．． 306.738; 2.017.727; 2.
321,632; 3.807,801; 3.31
7.597; 3.807.871; 3,315
．． 295 (see Japanese Unexamined Patent Publication No. 59-210070);
3.825.425; 3,732.284; European Published Patent Application Publication 0.126.883. USP
3.997.536; 4.062.798; 4
．． 462,923; 4,389,329; 4,
364.838; 4.066.570: 4,45
2,718. 4,419.262 ;4,510,
069; Japanese Patent Application Publication No. 144.770/8
4; 144.771/84; 144.772/
84; 43.961/83; International Patent Application No.
PCT/DE 88/00133 (Special Publication Heisei 15
(See Publication No. 02908); International Published Patent Application No. W.O.
87/06602; D. Fl by Demus et al.
tissige Krista11e in Tabe
llen, VEB DeuLscher Verla
g furGrundstoffindustrie
(Leipzig City, 1974) and D. Dem
Fltlssige Kristalle by us et al.
inTabellenll, VEB Deuts
cher Verlag f(ir Grun-dst
offindustrie (Leipzig, 19
1984). All of these components are known or can be prepared analogously to known compounds.

It is believed that without elaboration, one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. Accordingly, the preferred specific examples below are to be construed as illustrative only and are not intended to limit the remaining description in any way in any respect. In Tables 1-4 above and in the examples below, all temperatures are given in uncorrected degrees Celsius. All parts and percentages are by weight unless otherwise indicated.

Table 1 shows all liquid crystal compounds used in the following examples: Table 1: Lc Compound II 1st 2-p-ethylphenyl-5-propylbyrimidine I2 1st 2-p-probylphenyl- 5 -Propylpyrimidine ■3 1st 2-p-propyl 7enyl-5 -pentylpyrimidine 14 1st 2-p-ethyl 7enyl - 5-heptylpyrimidine ■5 1st 2-9-heptyl 7enyl enyl-5-hexylpyrimidine ■6 1st 2-p-nonyloxy7enyl-5-hexylpyrimidine ■7 1st 2-p-heptyloxyphenyl-5-heptylpyrimidine ■8 1st 2-p-nonyloxy7enyl -5-hemethylpyrimidine I9 1st 2-p-hexyloxy7enyl-5-1nonylpyrimidine 1st 2-p-1nonyloxyphenyl-5-1nonylpyrimidine I (continued) Table 1 (continued) Compound number Type of LC material ■l Second Second Second Second Second Second Second Structure of the second compound trans-1-p-difulmethoxy7enyl-4
-Petinolecitalohexane 1-(trans-4-pentylcyclohexyl)-2-(p-trifluoromethylphenyl)monoethane 1-(I-trans-4-bentylcyclohexyl)-
2-(p-) methoxyphenyl)-ethane [trans-4-(trans-4-propylcyclohexyl)-cyclohexyl)-2-(p-trifluoromethoxyphenyl)-ethane] trans-4-(trans-4-pentylcyclohexyl)monocyclohexyl)-2-(p-}riflemethoxy7enyl)monoethanep-t-ri7fluoromethoxy7enyltrans-4-7
2-(3-fluor-4-cyanophenyl)-5-heptylpyrimidine 2-(3-fluor-4-cyano-7enyl)-5-propylpyrimidine Compound number LC material type Iffl Third ■2 Third +1 [3 Third ■4 Third ■5 Third 1 [16 Third [117 Third ■8 Structure of the third compound 1-[trans-4-(trans-4-propylcyclohexyl )-1cyclohexyl)-2- (p-methyl7
(trans-4-(trans-4-propylcyclohexyl)-cyclohexyl)-2-(p-ethylphenyl)-ethane 1-[trans-4-(trans-4-propylcyclohexyl) 1-cyclohexyl)-2-(p-pentylphenyl)-ethane■-[trans-4-(trans-4-probylcyclohexyl)1cyclohexyl)-2-(p-propylphenyl)1ethanetrans-1-p -Methoxyphenyl-4-propylcyclohexane trans-1-p-propylphenyl-4-bethylcyclohexane trans, trans-4-methoxy 4'-propylcyclohexane trans, trans-4-propoxy-4 '-Flopyrcyclohexylcyclohexane l (Continued) Compound number Type of LC material ■9 3rd 3rd 11 [12 3rd I [[13 Structure of the 3rd compound l-(trans-4-(trans-4 -propylcyclohexyl)-cyclohexyl)-2-(2,3-difluor-4-ethoxyphenyl)-ethane-2,3-difluor-4-ethoxy-7enyltrans-
4-propylcyclohexane 2,3-difluor 4-ethoxyphenyltrans-
4-Pentylcyclohexane lupoxylate 2.3-difluoro-4-ethoxy4'-pentylutolane 2.3-difluoro-4-ethoxy4'-probilutolane 6 5.45 6 5.45 6 5.45 2,73 2.73 2.73 2.73 l0 10 9.09 9 8 7.27 7 8 10
10 8 10 117 6.36
7 8 10 10 8 10
Table 10 (continued) ■2 ■3 1+14 ■5 ■6 m7 ■8 111.9 ml! ! [112 7.27 6.36 9.11 10 9.09 10 1010 9.0
9 9 10 l0 9.09 l0

Claims (1)

[Claims] 1) (a) General formula (I) ▲There are mathematical formulas, chemical formulas, tables, etc.▼(I) [In the formula, X represents N or CH, and R^1 represents the group R
or represents the radical OR, and R^2 represents the radical R or the radical OR, and R is each independently a straight-chain alkyl radical having up to 12 carbon atoms, present in this radical. One CH_2 group or two non-adjacent CH_2 groups are
Assuming that the two oxygen atoms do not bond directly to each other,
optionally replaced by -O- and/or -CH=CH-, A is 1,4-phenylene or trans-1,4-cyclohexylene, Z1 is -CO
-O-, -O-CO-, -CH_2-O-, -OCH_
2-, -CH_2CH_2- or a single bond, Q^
1 is 1,4-phenylene or trans-1,4-cyclohexylene, and m is 0, 1, or 2] A first liquid crystal containing at least one liquid crystal compound represented by (b) a second liquid crystal material containing one or more liquid crystal compounds having a distinctly positive dielectric anisotropy (Δε≧+3); a third liquid crystal material containing at least one liquid crystal compound having dielectric anisotropy of or substantially neutral; Containing a liquid crystal mixture in which the ratio K_3/K_1 is as small as possible and not greater than 0.9 and the Δε/ε⊥ ratio is at most +0.4, operating in high multiflex mode and with high contrast. In the liquid crystal mixture used in a twisted nematic liquid crystal display device having
A liquid crystal mixture comprising at least one 2,5-disubstituted 1,3,4-thiadiazole compound and/or a 3,6-disubstituted 1,2-difluorobenzene compound. 2) Liquid-crystal mixture according to claim 1 in a device operating at the first minimum of the Gooch-Tarry transmission. 3) Liquid-crystal mixture according to claim 2, in a device whose electro-optical characteristic curve has a steepness at least as steep as the characteristic curve at the second minimum of the Gooch-Tarry transmission. 4) Liquid-crystal mixture according to claim 3, in a device in which the steepness of the characteristic curve at the first minimum value is greater than the steepness of the characteristic curve at the second minimum value. 5) K_3/K_1 is 0.4 to 0.9, and Δ
2. A liquid crystal mixture according to claim 1 in a device in which ε/ε⊥ is between 0.4 and 0.2. 6) In a device where K_3/K_1 is 0.2 to 0.75 and Δε/ε⊥ is 0.1 to 0.3,
A liquid crystal mixture according to claim 1. 7) Liquid crystal mixture according to claim 1, wherein said liquid crystal mixture contains at least three components. 8) The above-mentioned first liquid crystal material has the general formula I a ▲There are mathematical formulas, chemical formulas, tables, etc.▼ I a (In the formula, R^1 and R^2 each represent a group R or a group OR, and R is as defined in claim 1) The liquid crystal mixture according to claim 1, containing at least one liquid crystal compound represented by: 9) The second liquid crystal material is F, -CF_3, -CH
F_2, -OCF_3, -OCHF_2 and -OCH
The liquid crystal mixture according to claim 1, containing at least one liquid crystal compound having a terminal polar substituent selected from the group consisting of _2CF_3. 10) The above-mentioned second liquid crystal material has general formulas IIa to IIg ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ IIa ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ IIb ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ IIc ▲ There are mathematical formulas, chemical formulas, tables, etc.▼IId ▲There are mathematical formulas, chemical formulas, tables, etc.▼IIe ▲There are mathematical formulas, chemical formulas, tables, etc.▼IIf ▲There are mathematical formulas, chemical formulas, tables, etc.▼IIg [In each formula, Z^ 2 is -CH_2CH_2-, ▲There are mathematical formulas, chemical formulas, tables, etc.▼, ▲There are mathematical formulas, chemical formulas, tables, etc.▼ or a single bond, Y is F, -CF_3, -CHF_2, -OCF_3, -OCHF
_2 or -OCH_2CF_3, and R is
each independently a straight-chain alkyl group having up to 12 carbon atoms, in which one CH_
Two groups or two non-adjacent CH_2 groups are -O-, assuming that the two oxygen atoms do not bond directly to each other.
or -CH=CH-] The liquid crystal mixture according to claim 9, comprising at least one liquid crystal compound represented by the following. 11) The liquid crystal mixture according to claim 1, wherein the third liquid crystal material contains a liquid crystal compound having a dielectric anisotropy of -2 to -8. 12) The third liquid crystal material mentioned above has general formulas IIIa to IIIc: ▲There are mathematical formulas, chemical formulas, tables, etc.▼IIIa ▲There are mathematical formulas, chemical formulas, tables, etc.▼IIIb ▲There are mathematical formulas, chemical formulas, tables, etc.▼IIIc [In each formula, Ring A is as defined in claim 1, X is N or CH, and R is each independently a linear alkyl group having up to 12 carbon atoms. One CH_2 group or two non-adjacent CH_2 groups present in this group are -O- or -C, assuming that two oxygen atoms are not directly bonded to each other.
2. The liquid crystal mixture according to claim 1, comprising at least one liquid crystal compound represented by the following formula.