JPH0952852A - Fluorine-substituted biphenyl derivative and liquid crystal composition containing the derivative - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a new compound having large negative dielectric anisotropy, low viscosity and excellent chemical stability and useful as a liquid crystal display element. SOLUTION: The objective compound is expressed by formula I (R<1> and R<2> are each a 1-14C alkyl or alkoxy; Xa and Xb are each H or F and at least one of them is F), e.g. 2,2',3,3'-tetrafluoro-4,4'-dioctyloxybiphenyl. The compound of formula I causes an AC stabilizing effect on the development of a stable memory effect and remarkably lowers Vmin in the preparation of a ferroelectric liquid crystal composition for τ-Vmin mode to enable the driving of the memory at a low voltage. The compound of formula I can be produced by the cross- coupling reaction of a compound of formula II (R is an alkyl having a carbon number less than that of R<1> by 2; Z is single bond or O) with a compound of formula III in the presence of Pd(PPh3 )3.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a novel liquid crystal compound and a liquid crystal composition containing at least one kind of those liquid crystal compounds. More specifically,
The present invention relates to a liquid crystal composition, which contains a novel fluorine-substituted biphenyl derivative which is useful as a composition component of the liquid crystal composition when it is prepared and has excellent chemical stability, and at least one of them. And a liquid crystal composition.

[0002]

BACKGROUND ART A liquid crystal display device is of a light receiving type, so that eyes do not get tired.
Since it has excellent features such as low power consumption and thin shape, it has been widely used for clocks, calculators, word processors, pocket TVs, etc., and recently has a large screen size or high information. It has been attracting attention as a display device that is applied as a display of a quantity and replaces a CRT.

Most of these liquid crystal display devices utilize the electro-optical effect of a nematic liquid crystal phase, and as a display system thereof, a TN type (twisted nematic type), an STN type (super twisted nematic type), an ECB type ( Electrically controlled birefringence type), GH type (guest-host type) and the like. The characteristics required for a display element include a driving temperature range, threshold voltage (V _th ), response time, visual characteristics, contrast, etc., and must satisfy those characteristics required only by one liquid crystal compound. Therefore, it is difficult to meet the required performance by mixing various liquid crystal substances to prepare a liquid crystal composition that makes the best use of the characteristics of each substance. Therefore, in order to prepare a liquid crystal composition for practical use, various liquid crystal compounds having individual characteristics are required,
For example, to obtain a wide operating temperature range, a material having a nematic phase in a wide temperature range, a material having a low crystal-nematic phase transition temperature (T _CN ) or a material having a high nematic phase-isotropic liquid transition temperature (T _NI ). Therefore, a low viscosity material is effective for high-speed response, and a low threshold voltage material is effective for low voltage driving. A material having a large refractive index anisotropy (Δn) or a material having a small refractive index anisotropy (Δn) is required for the viewing angle and the contrast depending on the purpose. Further, a liquid crystal used in a driving system using a recent TFT requires a material having a very high resistivity.

On the other hand, research on a display device using a ferroelectric liquid crystal [chiral smectic C (chiral SmC) phase] rather than a nematic phase has been actively conducted. Ferroelectric liquid crystal [RB Meyer et al .; J. de Phys. 36 L
-69-71 (1975)] display method [NAClark et al .; Ap
plied Phys.lett., 36,899 (1980)] has excellent basic characteristics such as 100-1,000 times faster response than conventional nematic liquid crystal system and memory property. Therefore, the applications of liquid crystal display devices are expected to expand.

Liquid crystals used in ferroelectric liquid crystal display devices are currently used in SmC compositions (SmC host) obtained by mixing various smectic C (SmC) compounds with one to several optically active compounds (chiral SmC phase). It is better to have a compound, which is not necessarily required to have a compound called a chiral dopant), and to prepare a ferroelectric liquid crystal composition [LABersnev et al. Mol. Cryst. Liq.
Cryst. 89 327 (1982), HRBrand et al. J. Physiq
ue 44 (Lett.) L-771 (1983)] is the mainstream.

The response time in a ferroelectric liquid crystal is τ = η /
E · Ps (τ is response time, η is viscosity, E is electric field, Ps is spontaneous polarization), and τ can be shortened by increasing Ps and decreasing η.

However, practically, in addition to the response time, it is necessary to optimize various characteristics such as operating temperature range, viewing angle, contrast, etc. Therefore, as in the case of nematic liquid crystals, a large number of various compounds can be used. It has been attempted to optimize by mixing and utilizing the characteristics of individual compounds. Since the SmC host component occupies a large proportion in the ferroelectric liquid crystal composition and has a great influence on the characteristics of the ferroelectric liquid crystal composition, an excellent SmC host is required, and for example, the SmC temperature range is wide. It has a low viscosity, a tilt angle is 22.5 ° when a chiral dopant is added, and the layer structure is a bookshelf or chevron type and is not twisted.

As described above, in both the nematic liquid crystal composition and the ferroelectric liquid crystal composition, there is a demand for research and development of various compounds that can be effective components in producing them. In recent years, the τ-V _min mode method has been proposed as a useful method for realizing a practical surface-stabilized ferroelectric liquid crystal (hereinafter referred to as SSFLC) display (H. Oriha
ra et al., 1986, Jpn.J.appl.phys, 25, L839, PWHSurg
y et al., 1991, Ferroelectrics, 122, 63, JRHughes et
al., 1993, Liq.Crystal, 13,597, M.Koden et al., 1993,
Ferroelectrics, 149, 183). This mode method has features useful for realizing an SSFLC display such as an AC stabilization effect, a fast line address time due to a wide operating temperature range, and a high contrast.

In the above literature, P.W.H.Surgy and J.R.Hugh
es is τ-V_minSC manufactured by Merck as a modal material
E-8 is used, but it is practically high V_minBy value
The problem of high drive voltage remains, and this problem is solved
Low V to do_minAchiral liquid crystal compound showing value
Main composition Development is desired. P.W.H.Surg
y et al_minAs shown in the following equation,
Depending on the balance between (Ps) and dielectric anisotropy (Δε)
It is reported that it depends.

[Equation 1]

That is, a liquid crystal composition having a small spontaneous polarization and a large negative dielectric anisotropy is most effective for decreasing V _min . Generally, the spontaneous polarization of a liquid crystal composition is affected by a chiral dopant, and the dielectric anisotropy is affected by an achiral liquid crystal compound. Therefore, the present inventors have a relatively low viscosity and a negative dielectric anisotropy. We have started to develop a large achiral liquid crystal compound.

There are very few negative materials known to date with large Δε.

Embedded image It has been known. These have many drawbacks such as high viscosity, high melting point, chemically unstable and poor compatibility.

Further, the following tolan compounds (1) to (3) have been reported as materials for the τ-V _min mode (K.
Tanaka et al., 1994, Bull. Chem. Soc. Jpn, 67, 2550),

Embedded image Tolan compounds have problems in chemical stability against light and heat. Therefore, the present inventors have focused on biphenyl as a skeleton of a liquid crystal compound exhibiting low viscosity and good chemical stability, and by substituting a fluorine atom on the side of this biphenyl skeleton, a negative dielectric anisotropy can be obtained. I tried to make it bigger.

The present inventors have surprisingly found that as a result of designing a novel compound represented by the general formula (I) and synthesizing them, evaluating each compound alone and a composition containing them, It has been found that the compound is an effective achiral liquid crystal compound that significantly reduces V _min .

That is, the compound represented by the general formula (I) is τ
It was a very useful material as a liquid crystal composition component used in the -V _min mode SSFLC display. The compound represented by the general formula (I) having a large negative dielectric anisotropy is an ECB type display device [J. Rbert, F.
Clerc SID 80 Digest Techn. Paper 30 (1980), J. Duchen
e Display 73 (1986)], a material for adjusting the magnitude of the dielectric anisotropy of the liquid crystal composition used for the TN or STN type, or a ferroelectric liquid crystal display device by the high frequency superposition method [JMGeary SID 85 Digest Techn. Paper 128 (198
5), Y. Sato et al. SID 86 Digest Techn. Paper 128 (19
85)] can also be used as a material.

Some of the biphenyl liquid crystal compounds whose side is substituted with a fluorine atom are described in WO89 / 02425, but there is no description about applying these to the τ-V _min mode method. , Δε and τ-V
Physical properties such as characteristics have not been clarified. Further, in the examples, the number of fluorine atoms is up to 2, and the number of fluorine atoms of 3 or more is not described.

[0016]

DISCLOSURE OF THE INVENTION The present invention has the general formula,

Embedded image (In the formula, R ^{1 and} R ² each independently have 1 to 1 carbon atoms.
14 represents a linear or branched alkyl group or alkoxy group, X _{a and} X _b each independently represent a hydrogen atom or a fluorine atom, and at least one of them represents a fluorine atom) And a liquid crystal composition characterized by containing at least one of them.

The synthetic route of the liquid crystalline compound according to the present invention will be described below, and the present invention will be described in detail with reference to Examples and the like. The synthetic routes shown are by way of example, and the examples, together with the examples, are not meant to limit the invention. [Synthetic Route Diagram] In the following description, a compound is sequentially numbered with an underline, and the compound is denoted by the number according to a method commonly used in many chemical literatures.

[0018]

[Chemical 6] R is an alkyl group having two carbon atoms less than the carbon atoms of R ¹ , and Z is a single bond or an oxygen atom. Hereinafter, the same meanings will be applied to the routes 2 and 3.

[0019]

[Chemical 7]

[0020]

Embedded image Route 3 Synthetic route of the compound represented by the general formula (I)

The synthetic route represented by the formula will be described below. The compound represented by the route 1 Formula 1 1 ～1- 6 are commercially available. Using the general formula alkyl bromide compound represented by 1- ¹ (R 1 Br), compounds represented by the formula 1-7 is obtained by etherifying a conventional manner, a further periodate and iodine Iodination reaction using the general formula P
The compound represented by -1 can be obtained.

The compound represented by Formula 1-12, the compound represented by the general formula 12 by the action of an alkyl magnesium bromide (RMgBr) in, then the general formula 1-8 performs the dehydration reaction The compound is obtained by subjecting it to a hydrogenation reaction in the presence of palladium carbon (Pd / C). After lithiation by the formula compounds represented by the 1-12 n- butyl lithium (C ₄ H ₉ Li), a compound represented by the general formula P-1 is allowed to act with iodine is obtained.

[0023] After the general formula 1 compounds represented by 13 is expressed by the general formula 13 compound was brominated with bromine, obtained by cleavage of the methoxy group at boron tribromide. Compound represented by the general formula P-1 is obtained by further R ¹ Br and etherification of the compound represented by the general formula 1- 13.

[0024] Formula 1-3 with a compound under acid catalyst represented, after obtaining a compound represented by the general formula 1 10 is reacted with acetic anhydride, brominated by bromine (1- 14) further deacetylation To obtain a diazonium salt by reacting the compound represented by the general formula 1-15 obtained by the reaction with sodium nitrite (NaNO ₂ ), which is potassium iodide (KI).
It is reacted appear in the general formula 1 16 compound. This pyrrolidine solvent tetrakistriphenylphosphine Pas indium [Pd (PPh _{3) 4]} catalyst under reacted with 1 alkyne (RC≡CH) (1- 17), more usually performed Pd / C exists sewage hydrogenation reaction A compound of formula P-1 is obtained. Further, the compound represented by the general formula P-1 can be obtained by the etherification reaction of the compound represented by the general formula 1-5 with R ¹ Br.

The compound represented by Formula 1-11 compounds of the general formula 16 from formula 1 16 1 17
It is obtained by the same method as for obtaining the compound represented by. Further subjected to the 1-11 Pd / C exists sewage hydrogenation reaction on a compound represented by a compound represented by the general formula P-1 is obtained.

[0026] After the path 2 lithiation generally obtained by the route 1 formula 1-7 or 1-12 compounds represented by the compound with n-C ₄ H ₉ Li, trimethyl borate [B (OCH _{3) 3]} By acid-treating the borate ester derivative obtained by reacting with, the compound represented by the general formula P-2 is obtained.

Route 3 A synthetic route for a compound represented by the general formula (I): a compound represented by the general formula P-1 obtained in the route 1 and a compound represented by the general formula P-2 obtained in the route 2; Pd (P
By carrying out a cross coupling reaction in the presence of Ph ₃ ) ₄ , a compound represented by the general formula (I) can be obtained.

The present invention will be described in more detail with reference to the following examples. The abbreviations used in this specification have the following meanings. GTO: glass tube oven GLC: gas chromatography HPLC: high performance liquid chromatography TLC: thin layer chromatography IR: infrared absorption spectrum Mass: mass spectrometry b. p. : Boiling point C: Crystalline S _C : Smectic C phase Chiral S _C : Chiral Smectic C phase S _A : Smectic A phase Ne: Nematic phase Ch: Cholesteric phase I: Isotropic liquid? : Does not solidify below 0 ° C

[0029]

【Example】

 Example 1

Embedded image A mixture of 25 g of 2,3-difluorophenol, 52.1 g of n-octyl bromide, 49.1 g of potassium carbonate and 250 ml of methyl ethyl ketone was added to 3 parts of a mixture.
The mixture was stirred under reflux for 2 hours. Insoluble matter was removed from the reaction mixture by suction filtration, the filtrate was concentrated, extracted with ether, washed with water, dried over sodium sulfate, the solvent was distilled off, and the residue was distilled under reduced pressure to obtain 2,3-difluorooctyl. 44.1 g (94.8%) of oxybenzene was obtained. b. p 83
~ 84 ° C / 0.15 torr, GLC 95.1%

[0030]

Embedded image Acetic acid 23 ml, concentrated sulfuric acid 0.7 ml and water 4.5 m
10 g of 2,3-difluorooctyloxybenzene obtained in the above (a) and 4.2 g of iodine in a solution consisting of 1
And periodic acid dihydrate 1.98 g were added sequentially,
The mixture was heated and stirred at 5 to 80 ° C for 3 hours.

Sodium hydrogen sulfite was added to the reaction mixture, which was extracted with n-hexane and washed with water, dried over sodium sulfate and the solvent was distilled off. The residue was distilled under reduced pressure with GTO to give 2,3-difluoro. 12.1 g (80.0%) of -4-iodooctyloxybenzene was obtained. b. p.
110 ° C / 0.2 torr, GLC 98.6%

Embedded image A solution of 20 g of 2,3-difluorooctyloxybenzene obtained in (a) above in 100 ml of tetrahydrofuran was cooled to −70 ° C. under an argon atmosphere, and then 1.6
64 ml of a hexane solution of Mn-butyllithium was added dropwise, and the mixture was stirred at the same temperature for 90 minutes. To this reaction mixture, 10.6 g of boric acid trimethyl ester was added, and the mixture was slowly warmed to room temperature and stirred overnight. The reaction mixture was poured into a dilute aqueous hydrochloric acid solution, extracted with benzene, washed with water, dried over sodium sulfate, the solvent was distilled off, and the residue was recrystallized from n-hexane to give 2,3-difluoro-4. -Octyloxyphenylboronic acid 19.9 g (84.3%)
I got HPLC 99.9%

[0032]

Embedded imageIn an argon atmosphere, Pd (PPh_Three)_Four 0.29g, above
2,3-difluoro-4-iodoocty obtained in the above (b)
A solution of 1.8 g of luoxybenzene in 20 ml of benzene,
2,3-difluoro-4-octylio obtained in (c) above
Xyphenylboronic acid 1.54 g of ethanol 20
Consisting of a ml solution and 5 ml of 2M aqueous sodium carbonate solution
The mixture was stirred at reflux for 32 hours. Pour the reaction mixture into water
Then, extract with benzene, wash with water, and add sodium sulfate.
After drying, the solvent is distilled off and the residue is n-hexane-benzene.
Silica gel column chromatography with (10: 1) as the eluent
Purified by chromatography, then mixed with methanol-acetone
Recrystallize in a combined solvent to give 2,2 ', 3,3 ’-Tetra
Fluoro-4,4'-dioctyloxybiphenyl
1.3 g (55%) was obtained.

The purity of the obtained compound was 9 by HPLC.
It was 9.5%, and it was 1 spot by TLC. In addition, it was confirmed that the obtained substance was the title compound from the result of IR measurement and the fact that a molecular ion peak was observed at 482 in Mass analysis and the relation of the raw materials used. Using a METTLER HOT STAGE FP-82, this compound was observed for phase change under a polarizing microscope. Table 1 shows the results.

Embodiment 2

Embedded image In Example 1- (a), 2,3-difluorophenol (25 g) was replaced with 2-fluoro-4-bromophenol (34.1 g), and otherwise the same operation as in Example 1- (a) was performed. 38.5 g (96.9%) of -fluoro-4-bromo-octyloxybenzene was obtained. b.
p. 124-126 ° C / 0.5 torr, GLC 9
6.1%

[0035]

[Chemical 14]In Example 1- (d), 2,3-difluoro-4-yo
De-octyloxybenzene
2-Fluoro-4-bromo-octylio obtained in (a)
Example 1- (d) except that 1.48 g of xylbenzene was used.
Same as above, 2, 3, 3 ′ -Trifluoro-4,
4 '-Dioctyl oxybiphenyl 1.24 g (54.
6%).

The purity of the obtained compound was 9 by HPLC.
It was 9.5%, and it was 1 spot by TLC. In addition, it was confirmed that the obtained substance was the title compound from the result of IR measurement and the fact that a molecular ion peak was recognized at 464 by Mass analysis and the relation of the raw materials used. Using a METTLER HOT STAGE FP-82, this compound was observed for phase change under a polarizing microscope. Table 1 shows the results.

Embodiment 3

Embedded image 3-fluoroanisole 56 g chloroform 80
69 g of bromine was added dropwise to the ml solution over 6 hours with stirring at room temperature, and the mixture was further stirred under reflux for 1 hour. The reaction mixture was poured into dilute aqueous sodium hydroxide solution, the chloroform layer was washed with water,
After drying over sodium sulfate, the solvent was distilled off, and the residue was distilled under reduced pressure to give 3-fluoro-4-bromoanisole 7
7 g (84.7%) was obtained. b. p. 108-113 ° C
/ 21 torr, GLC 98%

[0038]

Embedded image 3-Fluoro-4 obtained in (a) above under an argon atmosphere
To a solution of 77 g of -bromoanisole in 400 ml of methylene chloride, 400 ml of 1M-boron tribromide-methylene chloride solution was added dropwise at 0 ° C or lower, and the mixture was further stirred at room temperature overnight. The reaction mixture was poured into ice-cold water, the methylene chloride layer was washed with water, dried over sodium sulfate, the solvent was distilled off, the residue was distilled under reduced pressure, and then recrystallized from hexane to give 3
-Fluoro-4-bromophenol 35.2 g (4
9.3%). m. p. 71.5-72.5 ° C, G
LC 99%

[0039]

Embedded image In place of 25 g of 2,3-difluorophenol in Example 1- (a), the 3-fluoro-obtained in (b) above was used.
Using 44.1 g of 4-bromophenol and otherwise operating as in Example 1- (a), 52.5 g (97.0%) of 3-fluoro-4-bromooctyloxybenzene.
I got b. p. 111-112 ° C / 0.15to
rr, GLC 96.5%

[0040]

[Chemical 18]In Example 1- (d), 2,3-difluoro-4-yo
Replaced with 1.8 g of deoctyloxybenzene,
3-fluoro-4-bromooctyloxy obtained in (c)
1.48 g of benzene was used, and the others were as in Example 1- (d).
Using the same operation, 2,2 ', 3-trifluoro-4,
4 '-Dioctyloxybiphenyl 0.83 g (3
6.6%).

The purity of the obtained compound was 9 by HPLC.
It was 9.9%, and it was 1 spot by TLC. In addition, it was confirmed that the obtained substance was the title compound from the result of IR measurement and the fact that a molecular ion peak was recognized at 464 by Mass analysis and the relation of the raw materials used. Using a METTLER HOT STAGE FP-82, this compound was observed for phase change under a polarizing microscope. Table 1 shows the results.

[0042]

[Table 1]

Embodiment 4

Embedded image

A liquid crystal composition (DM-B) composed of the above-mentioned five kinds of demass ester compounds was prepared. In this DM-B, the following three kinds of compounds of the present invention and comparative compounds were added.

Embedded image 30 wt% was added to each of the liquid crystal compositions DM-E1 and DM.
-E2, DM-E3 and DM-C were adjusted, and the dielectric anisotropy (Δε) was measured by the following method. The results are shown in Table 2.

Vertically Aligned Cell and Horizontally Aligned Cell (EHC
Manufactured by Hitachi Chemical Co., Ltd .: Hitachi Chemical Polyimide LX-1400), and the respective capacitances C _{1 and} C ₂ at the time of empty cells are Ge
It was measured with a 1620A type capacitance bridge manufactured by neral Radio (± 0.5V, 1KH sine wave). Then 2
The measurement sample was enclosed in a seed cell, and the electrostatic capacitances C _{3 and} C ₄ at 25 ° C. of the vertical alignment cell and the horizontal alignment cell were measured in the same manner as described above, and Δε was calculated by the following formula. Δε = [C ₃ / C ₁ ]-[C ₄ / C ₂ ] As shown in Table 2, the liquid crystal composition containing the compound of the present invention (DM-E system) has a liquid crystal composition containing the comparative compound. Compared with the product (DM-C), the Δε of DM-B is obviously increased to a negative value. In particular, the change was remarkable in DM-E1 to which the biphenyl compound substituted with four fluorine atoms was added. From the above, one of the objects of the present invention is
That is, a material having a large negative Δε is obtained. From this result, it can be said that the compound of the present invention is an effective material for reducing V _min as a liquid crystal material for the τ-V _min mode from the formula shown in the above [Background Art].

[0046]

[Table 2] Table 2 Dielectric anisotropy of liquid crystal composition

Example 5 90 wt% of a ferroelectric liquid crystal composition (SCE-8) manufactured by Merck & Co., Inc.
% And 10 wt% of each compound obtained in Examples 1 to 3 was prepared.

[Chemical 21]

These ferroelectric liquid crystal compositions and SC
E-8 was injected into a 2 μm gap cell (manufactured by EHC) consisting of a glass substrate with a transparent electrode, each of which was rubbed, heated to an isotropic liquid, and then cooled to 30 ° C. at 1 ° C./min. Therefore, one orientation state in the chevron structure, that is, the C2 orientation (M. Koden et al., 1
981, Jpn.J.Appl.phy.30, L1823) was prepared.

By applying a pulse waveform as shown in FIG. 1 to these ferroelectric liquid crystal elements and measuring the pulse width (τ) required for memory for each voltage (V), τ-V A curve (Fig. 2) was created, and V _min was determined from this curve. Table 3 shows the phase transition temperatures and V _min values of these ferroelectric liquid crystal devices. As shown in Table 3, in the ferroelectric liquid crystal compositions MM-E1 to 3 in which the compound of the present invention was added to SCE-8, the V _min value was lower than that of SCE-8 alone, and a good V _min value was obtained. You can see that it shows.

[0050]

[Table 3] Table 3 Properties of ferroelectric liquid crystal composition

[0051]

As described above, since the compound represented by the general formula (I) according to the present invention has a particularly large negative dielectric anisotropy, it exhibits a stable memory property. AC stabilizing effect for τ-V
It is a material that is useful for significantly lowering V _min when driving a ferroelectric liquid crystal composition for _min mode and for driving at a low voltage.

[Brief description of drawings]

FIG. 1 shows a pulse waveform applied to a ferroelectric liquid crystal device.

FIG. 2 shows a τ-V curve obtained by measuring the pulse width (τ) required for memory for each voltage (V) from the pulse waveform of FIG. 1.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenji Suzuki 1-7-1 Inari, Soka City, Saitama Kanto Chemical Co., Ltd. Central Research Laboratory

Claims (6)

[Claims] 1. A compound of the general formula (I) (In the formula, R ^{1 and} R ² each independently have 1 to 1 carbon atoms.
14 represents a linear or branched alkyl group or alkoxy group, X _{a and} X _b each independently represent a hydrogen atom or a fluorine atom, and at least one of them represents a fluorine atom) Biphenyl derivative. 2. The fluorine-substituted biphenyl derivative according to claim 1, wherein both X _{a and} X _b in the general formula (I) are fluorine atoms. 3. The fluorine-substituted biphenyl derivative according to claim 1, which is used in a τ-V _min mode. 4. A compound of the general formula (I) (In the formula, R ^{1 and} R ² each independently have 1 to 1 carbon atoms.
14 represents a linear or branched alkyl group or alkoxy group, X _{a and} X _b each independently represent a hydrogen atom or a fluorine atom, and at least one of them represents a fluorine atom) A liquid crystal composition comprising at least one biphenyl derivative. 5. A liquid crystal composition containing at least one fluorine-substituted biphenyl derivative according to claim 4, wherein both X _{a and} X _b in the general formula (I) are represented by fluorine atoms. 6. A liquid crystal composition containing at least one fluorine-substituted biphenyl derivative according to claim 4, which is used in a τ-V _min mode.