Liquid crystal composition and liquid crystal display element

The present invention provides a liquid crystal composition comprising a component A being at least one compound selected from the group of compounds represented by Formulas (1-1) and (1-2) in the specification and a component B being at least one compound selected from the group of compounds represented by Formulas (2-1), (2-2), (2-3), (2-4) and (2-5) in the specification. This liquid crystal composition satisfies general characteristics required for a liquid crystal display element of an active matrix mode (AM-LCD).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a nematic liquid crystal composition. More
 specifically, the present invention relates to a liquid crystal
 composition for an active matrix (AM) mode and a liquid crystal display
 element (LCD) using this liquid crystal composition.
 2. Description of the Related Art
 A liquid crystal display element (AM-LCD) of an active matrix mode enables
 highly fine display, and therefore it attracts attentions as the most
 likely candidate of LCD and is applied to display faceplates for monitors,
 note type personal computers, digital still cameras, digital video cameras
 and the like. Characteristics required to a liquid crystal composition for
 AM-LCD include the following (1) to (5):
 (1) In order to expand a temperature range in which the liquid crystal
 display element can be used, the liquid crystal composition shows a
 nematic phase in as broad temperature range as possible (the upper limit
 temperature of the nematic phase is elevated as much as possible, and the
 lower limit temperature of the nematic phase is lowered as much as
 possible).
 (2) In order to accelerate the response speed of the liquid crystal display
 element, the liquid crystal composition is reduced in a viscosity as much
 as possible.
 (3) In order to raise the contrast of the liquid crystal display element,
 the optical anisotropy value (.DELTA.n) of the liquid crystal composition
 can take a suitable vale according to the cell thickness (d) of the liquid
 crystal display element.
 (4) In order to raise the contrast of the liquid crystal display element,
 the resistivity value of the liquid crystal composition is elevated, and
 the voltage-holding ratio of a cell into which the liquid crystal
 composition is charged is elevated. In particular, the voltage-holding
 ratio in a high temperature area is elevated. Measurement of the
 voltage-holding ratio in a high temperature area corresponds to an
 acceleration test for making sure of the durability of the liquid crystal
 composition.
 (5) In order to miniaturize a battery which is a driving power source for
 the liquid crystal display element, the liquid crystal composition is
 reduced in a threshold voltage.
 In light of such backgrounds, disclosed in Japanese Patent Application
 Laid-Open No. 73857/1996 is a liquid crystal composition which has a high
 voltage-holding ratio and is reduced in a threshold voltage and which has
 a suitably large optical anisotropy. Also, disclosed in Japanese Patent
 Application Laid-Open No. 31460/1997 is a liquid crystal composition
 which, in particular, has a low threshold voltage, an excellent
 compatibility at a low temperature and a broad temperature range of a
 nematic phase while satisfying various characteristics required to a
 liquid crystal composition for AM-LCD. Further, disclosed in International
 Publication WO96/11897 are a novel liquid crystal compound which has a
 large dielectric constant anisotropy and is notably low in a viscosity as
 a liquid crystal compound for low voltage driving in various modes
 including an AM mode and an STN mode, and a liquid crystal composition
 containing the same. A liquid crystal composition using a compound which
 is analogous to the compound of the present invention represented by
 Formula (1-1) is described in Japanese Patent Application Laid-Open No.
 251186/1998.
 Driving power sources for note type personal computers, digital still
 cameras, digital video cameras and the like depend on batteries. In order
 to use these batteries for long time by one charging, a power consumption
 of LCD has to be reduced. In recent years, these batteries have been
 further miniaturized and come to be elongated in use time by one charging.
 Accordingly, liquid crystal compositions have come to be desired to be
 reduced in a threshold voltage while maintaining the characteristics shown
 in the items (1) to (4) described above.
 In order to reduce a threshold voltage of a liquid crystal composition, a
 liquid crystal compound having a large dielectric constant anisotropy has
 to be used. If a liquid crystal compound having a large dielectric
 constant anisotropy is used to prepare a liquid crystal composition, the
 liquid crystal composition is increased in a viscosity. Accordingly, a
 liquid crystal display element using a liquid crystal composition having a
 low threshold voltage is slow as well in a response speed.
 As proposed by E. Jakeman et al [Phys. Lett., A, 39 (1972) 69], this is
 because a response speed is proportional to a square of a cell gap, and a
 cell gap of a cell constituting a liquid crystal display element can be
 reduced in order to accelerate the response speed. As shown in the item
 (3) described above, however, in a first minimum condition of a TN mode, a
 value shown by a product (.DELTA.n.multidot.d) of a cell gap of a cell
 constituting a liquid crystal display element and optical anisotropy of a
 liquid crystal composition is set to about 0.4 to about 0.5 in order to
 obtain a high contrast, and therefore if the cell gap is reduced, the
 optical anisotropy of the liquid crystal composition has to be inevitably
 increased.
 The composition disclosed in Japanese Patent Application Laid-Open No.
 73857/1996 described above has the defects that as shown in the
 comparative examples in the present invention, while the threshold voltage
 is low and the optical anisotropy is suitably large, the nematic phase has
 a too low upper limit temperature and the voltage-holding ratio in a high
 temperature area is low and the defects that while the optical anisotropy
 is suitably large and the nematic phase has a high upper limit
 temperature, the threshold voltage is too high and the voltage-holding
 ratio in a high temperature area is low.
 A composition disclosed in Japanese Patent Application Laid-Open No.
 31460/1997 has the defects that as shown in the comparative examples in
 the present invention, the optical anisotropy is small and the threshold
 voltage is high and that the nematic phase has a low upper limit
 temperature. Usually, a compound having a cyano group has a low
 voltage-holding ratio and therefore can not be used for a liquid crystal
 composition for AM-LCD.
 A liquid crystal composition for AM-LCD which does not contain a compound
 having a cyano group at a terminal is disclosed in International
 Publication WO96/11897. This composition has the defects that as shown in
 the comparative examples in the present invention, the threshold voltage
 is not sufficiently lower and the optical anisotropy is small.
 Compounds having three phenylene rings and one --CF.sub.2 O-- bonding group
 in a molecule are disclosed in Japanese Patent Application Laid-Open No.
 251186/1998, but among the physical property values described in the
 examples thereof in the compositions comprising the F base compound having
 a high holding rate, the threshold voltage falls in a range of 1.29 V to
 2.37 V and is relatively high.
 As described above, various investigations of liquid crystal compositions
 have been carried out, but satisfactory liquid crystal compositions for
 AM-LCD are not obtained. That is, required are liquid crystal compositions
 for AM-LCD which maintain the characteristics shown in the items (1) and
 (2) described above but have a low threshold voltage required for reducing
 a power consumption while maintaining a high voltage-holding ratio in a
 high temperature area and which are provided with a large optical
 anisotropy as a reduction in the gap of the cell comes to be required in
 order to accelerate the response speed.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a liquid crystal
 composition which satisfies general characteristics required to AM-LCD and
 which has particularly a high voltage-holding ratio in a high temperature
 area, a sufficiently low threshold voltage and a large optical anisotropy.
 Intensive investigations repeated by the present inventors in order to
 solve these problems have resulted in finding that a liquid crystal
 composition capable of achieving the object of the present invention can
 be obtained by combining a conventional compound having a --CF.sub.2 O--
 bonding group with a specific liquid crystal compound which is different
 in an F substitution number and a substitution position, and thus they
 have come to complete the present invention.
 The liquid crystal composition of the present invention is shown by the
 following items 1 to 2:
 1. A liquid crystal composition comprising a component A being at least one
 compound selected from the group of compounds represented by Formulas
 (1-1) and (1-2) and a component B being at least one compound selected
 from the group of compounds represented by Formulas (2-1), (2-2), (2-3),
 (2-4) and (2-5):
 ##STR1##
 wherein R.sub.1, R.sub.2, R.sub.3 R.sub.4, R.sub.5, R.sub.6 and R.sub.7
 each represent independently an alkyl group or alkoxy group having 1 to 10
 carbon atoms, or an alkenyl group or alkoxymethyl group having 2 to 10
 carbon atoms; A.sub.1, A.sub.2, A.sub.3, A.sub.4, A.sub.5 and A.sub.6 each
 represent independently a single bond, --C.sub.2 H.sub.4 -- or --COO--;
 X.sub.1, X.sub.2, X.sub.3, X.sub.4, X.sub.5 and X.sub.6 each represent
 independently H or F; B.sub.1 and B.sub.2 each represent independently a
 single bond, cyclohexylene, trans-1,3-dioxane-2,5-diyl, 1,4-phenylene or
 1,4-phenylene substituted by 1 to 4 fluorine atoms; B.sub.3 represents
 1,4-phenylene, cyclohexylene, or 1,4-phenylene or cyclohexylene
 substituted by 1 to 4 fluorine,; Y.sub.1 and Y.sub.2 each represent
 independently F, CF.sub.3, OCF.sub.3, CF.sub.2 H or Cl; and n is 0 or 1.
 2. The liquid crystal composition according to the above item 1, comprising
 the component A of 5 to 95% by weight and the component B of 5 to 95% by
 weight, respectively, based on the total quantity of the liquid crystal
 composition.
 The liquid crystal display element of the present invention is shown by the
 following item 3.
 3. A liquid crystal display element containing a liquid crystal composition
 as set forth in any one of items 1 and 2.
 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Compounds represented by the following Formulas (1-1-1) to (1-1-31) are
 preferably used as the compound represented by Formula (1-1) in the liquid
 crystal composition of the present invention.
 ##STR2##
 ##STR3##
 ##STR4##
 ##STR5##
 In these formulas, R's each represent independently an alkyl group or
 alkoxy group having 1 to 10 carbon atoms, or an alkoxymethyl group or
 alkenyl group having 2 to 10 carbon atoms.
 Compounds represented by the following Formulas (1-2-1) to (1-2-21) are
 preferably used as the compound of the present invention represented by
 Formula (1-2).
 ##STR6##
 ##STR7##
 ##STR8##
 In these formulas, R's each represent independently an alkyl group or
 alkoxy group having 1 to 10 carbon atoms, or an alkoxymethyl group or
 alkenyl group having 2 to 10 carbon atoms.
 Compounds represented by the following Formulas (2-1-1) to (2-1-3) are
 preferably used as the compound of the present invention represented by
 Formula (2-1).
 ##STR9##
 In these formulas, R's each represent independently an alkyl group or
 alkoxy group having 1 to 10 carbon atoms, or an alkoxymethyl group or
 alkenyl group having 2 to 10 carbon atoms.
 Compounds represented by the following Formulas (2-2-1) to (2-2-7): are
 preferably used as the compound of the present invention represented by
 Formula (2-2).
 ##STR10##
 In these formulas, R's each represent independently an alkyl group or
 alkoxy group having 1 to 10 carbon atoms, or an alkoxymethyl group or
 alkenyl group having 2 to 10 carbon atoms.
 Compounds represented by the following Formulas (2-3-1) or (2-3-2) is
 preferably used as the compound of the present invention represented by
 Formula (2-3).
 ##STR11##
 In these formulas, R's each represent independently an alkyl group or
 alkoxy group having 1 to 10 carbon atoms, or an alkoxymethyl group or
 alkenyl group having 2 to 10 carbon atoms.
 Compounds represented by the following Formulas (2-4-1) or (2-4-3) is
 preferably used as the compound of the present invention represented by
 Formula (2-4).
 ##STR12##
 In these formulas, R's each represent independently an alkyl group or
 alkoxy group having 1 to 10 carbon atoms, or an alkoxymethyl group or
 alkenyl group having 2 to 10 carbon atoms.
 A compound represented by the following Formula (2-5-1) or (2-5-2) is
 preferably used as the compound of the present invention represented by
 Formula (2-5).
 ##STR13##
 In these formulas, R's each represent independently an alkyl group or
 alkoxy group having 1 to 10 carbon atoms, or an alkoxymethyl group or
 alkenyl group having 2 to 10 carbon atoms.
 The functions and roles of the compounds constituting the liquid crystal
 composition of the present invention shall be explained.
 The component A in the liquid crystal composition of the present invention
 has the effects to maintain the viscosity relatively small, increase the
 optical anisotropy and significantly lower the threshold voltage while
 maintaining high the voltage-holding ratio of the liquid crystal
 composition in a high temperature area. Almost all tetracyclic compounds
 in the component A have a very high T.sub.NI (upper limit temperature of
 liquid crystal) of 80 to 180.degree. C., and therefore the liquid crystal
 composition having a high T.sub.NI can be prepared by using this
 tetracyclic compound.
 Such effects are considered attributable to that the compound represented
 by Formula (1-1) or (1-2) which is the component A is substituted with
 fluorine atoms in the 3-position and 5-position of a phenyl ring which is
 bonded to a carbon atom of a --CF.sub.2 O-- bonding group, so that it has
 as very large dielectric constant anisotropy as about 20 or more and that
 it has a relatively small viscosity, a large optical anisotropy and a high
 resistivity value.
 The compound represented by Formula (2-1) in the component B in the liquid
 crystal composition of the present invention has the effects to reduce the
 viscosity particularly at a low temperature and control T.sub.NI, the
 threshold voltage and the optical anisotropy while maintaining high the
 voltage-holding ratio of the liquid crystal composition in a high
 temperature area. This compound represented by Formula (2-1) is a dicyclic
 compound, and the dielectric constant anisotropic value is not so large as
 that of the compound of the component A described above but shows a
 positive value of about 6 to about 8. Further, it has the characteristics
 that it has a smaller viscosity than that of the compound of the component
 A described above, a very low T.sub.NI (the upper limit temperature of
 liquid crystal) of 0.degree. C. or lower and a small optical anisotropy.
 The compound of the present invention represented by Formula (2-2) has the
 effects to elevate the upper limit temperature of the liquid crystal
 composition, reduce the viscosity and control the threshold voltage and
 the optical anisotropy while maintaining high the voltage-holding ratio of
 the liquid crystal composition in a high temperature area. The compound of
 the present invention represented by Formula (2-2) is a tricyclic compound
 and does not show such level of a dielectric constant anisotropy as that
 of the compound of the component A but shows a positive value of about 9
 to about 13. It has the characteristics that it has a relatively high
 T.sub.NI of 50 to 120.degree. C., a smaller viscosity than that of the
 compound of the component A and a high resistivity value. Further, it
 shows a relatively smaller optical anisotropy than that of the compound of
 the component A.
 The compound of the present invention represented by Formula (2-3) or (2-4)
 has the effects to elevate the dielectric constant anisotropy, reduce the
 threshold voltage and control the optical anisotropy.
 The compound of the present invention represented by Formula (2-3) or (2-4)
 is a tricyclic compound and does not show such level of a dielectric
 constant anisotropy as that of the compound of the component A but shows a
 larger value than that of the compound represented by Formula (2-1) or
 (2-2). Further, it shows a very smaller optical anisotropy of about 0.04
 to about 0.07 than that of the component A.
 The compound of the present invention represented by Formula (2-5) has the
 effects to elevate the upper limit temperature and control the threshold
 voltage while maintaining high the voltage-holding ratio of the liquid
 crystal composition in a high temperature area. The compound of the
 present invention represented by Formula (2-5) is a tetracyclic compound
 and does not show such level of a dielectric constant anisotropy as that
 of the compound of the component A but shows a positive value. It has a
 very high T.sub.NI point of 180.degree. C. or higher and a high
 resistivity value.
 The preferred contents of the components constituting the liquid crystal
 composition of the present invention shall be explained.
 The component A in the liquid crystal composition of the present invention
 has preferably a content of 5 to 95% by weight based on the whole weight
 of the liquid crystal composition, and the component B has preferably a
 content of 5 to 95% by weight based on the whole weight of the liquid
 crystal composition. The threshold voltage is reduced very much by the
 component A, and the nematic phase range, the optical anisotropy and the
 threshold voltage are further controlled by the component B. To explain in
 further details, the threshold voltage can be reduced by blending the
 compound represented by Formula (1-1) or (1-2) which is the component A in
 as large amount as possible with the composition, but if a large amount
 thereof is added, the nematic phase lower limit temperature of the liquid
 crystal composition is elevated in a certain case. Accordingly, the
 preferred content of the component A is 95% by weight or less based on the
 whole weight of the liquid crystal composition. Further, in order to
 pretty reduce the threshold voltage while maintaining high the
 voltage-holding ratio of the liquid crystal composition in a high
 temperature area, the content of the component A is 5% by weight or more,
 preferably 15% by weight or more based on the whole weight of the liquid
 crystal composition.
 Further, when the tricyclic compound as the component A has a large
 content, the tricyclic compound has a relatively low T.sub.NI of
 50.degree. C. or lower in many cases, and therefore the content is more
 preferably 70% by weight or less based on the whole weight of the
 composition in order to turn the T.sub.NI of the composition to a
 practical temperature.
 The component B in the liquid crystal composition of the present invention
 has preferably a content of 5 to 95% by weight, more preferably 30 to 95%
 by weight based on the whole weight of the liquid crystal composition. If
 the content is less than 5% by weight, troubles are brought about in
 controlling particularly the nematic phase range and the optical
 anisotropy, and the content exceeding 95% by weight reduces the effect of
 the present invention that the threshold voltage is low.
 The compounds of Formulas (2-1) to (2-5) have more preferred contents
 falling in the following ranges respectively.
 The compound of the present invention represented by Formula (2-1) has
 preferably a content of 30% by weight or less, more preferably 25% by
 weight or less based on the whole weight of the liquid crystal
 composition. This compound of Formula (2-1) has a very low T.sub.NI, and
 therefore if a large amount thereof is added to the composition, the
 nematic phase lower limit temperature of the liquid crystal composition is
 lowered.
 The compound represented by Formula (2-2) in the liquid crystal composition
 of the present invention has preferably a content of 95% by weight or
 less, more preferably 85 to 5% by weight based on the whole weight of the
 composition. This compound represented by Formula (2-2) has a relatively
 high T.sub.NI, and therefore if a large amount thereof is added to the
 composition, the nematic phase lower limit temperature of the liquid
 crystal composition is elevated. On the other hand, if the content is very
 small, reduced are the effects of reducing the viscosity while maintaining
 the voltage-holding ratio of the liquid crystal composition in a high
 temperature area and the low threshold voltage, and the effect of
 elevating the upper limit temperature of the nematic phase is lowered as
 well.
 The compound represented by Formula (2-3) or (2-4) in the liquid crystal
 composition of the present invention has preferably a content of 50% by
 weight or less. If this compound is added in a large amount, the nematic
 phase lower limit temperature of the liquid crystal composition is
 elevated, and the resistivity value is lowered.
 The compound represented by Formula (2-5) in the liquid crystal composition
 of the present invention has preferably a content of 20% by weight or less
 based on the whole weight of the composition. If this compound has a large
 content, the nematic phase lower limit temperature of the liquid crystal
 composition is elevated.
 The synthetic processes of the compounds represented by Formula (1-1) or
 (1-2) in the present invention, for example, the compounds represented by
 Formulas (1-1-1), (1-1-16) and (1-2-12) are described in Japanese Patent
 Application Laid-Open No. 251186/1998. The synthetic processes of the
 compounds represented by Formulas (2-1-3), (2-2-1) or (2-2-4) are
 described in Japanese Patent Application Laid-Open No. 233626/1990. The
 synthetic processes of the compounds represented by Formula (2-4-1) are
 described in Japanese Patent Application Laid-Open No. 141206/1997. As
 described above, the compounds in the respective components constituting
 the composition of the present invention can be synthesized according to
 prior arts.
 Liquid crystal compounds other than the preceding compounds in the
 component A and the component B can be used for the liquid crystal
 composition of the present invention in a mixture as long as the object of
 the present invention is not damaged.
 The liquid crystal composition according to the present invention is
 prepared by conventional processes. In general, employed is a process in
 which various compounds are mixed and dissolved each other at a high
 temperature. In order to induce a helical structure of the liquid crystal
 molecules to control the required twist angle, cholesteryl nonanoate (CN)
 and a chiral dopant such as CM-43L represented by the following formula
 may be added:
 ##STR14##
 The liquid crystal composition of the present invention can be used as a
 liquid crystal composition of a guest-host mode by adding dichronic dyes
 of a merocyanine base, a styryl base, an azo base, an azomethine base, an
 azoxy base, a quinophthalone base, an anthraquinone base and a tetrazine
 base. Further, it can be used as a polymer dispersion type liquid crystal
 display element and liquid crystal compositions of a
 birefringence-controlling mode and a dynamic scattering mode. It can also
 be used as a liquid crystal composition of an in.plane.swtching mode.
 The present invention can provide a liquid crystal composition having
 particularly a high voltage-holding ratio in a high temperature area, a
 sufficiently low threshold voltage and a large optical anisotropy while
 satisfying general characteristics required to a liquid crystal display
 element (AM-LCD) of an active matrix mode.

EXAMPLES
 The present invention shall be explained below in detail with reference to
 examples.
 TABLE 1
 1) Left terminal group R-- Symbol 2) Bonding group --Z.sub.1 --,
 --Z.sub.n -- Symbol
 C.sub.n H.sub.2n+1 -- n- --C.sub.2 H.sub.4 -- 2
 C.sub.n H.sub.2n+1 O-- nO- --C.sub.4 H.sub.8 -- 4
 C.sub.n H.sub.2n+1 OC.sub.m H.sub.2m -- nOm- --COO--
 E
 CH.sub.2.dbd.CH-- V- --C.ident.C-- T
 CH.sub.2.dbd.CHC.sub.n H.sub.2n -- Vn- --CH.dbd.CH-- V
 C.sub.n H.sub.2n+1 CH.dbd.CHC.sub.m H.sub.2m -- nVm- --CF.sub.2 O--
 CF2O
 C.sub.n H.sub.2n+1 CH.dbd.CHC.sub.m H.sub.2m CH.dbd.CHC.sub.k H.sub.2k --
 nVmVk- --OCF.sub.2 -- OCF2
 3) Ring structure --(Al)--, --(An)-- Symbol 4) Right terminal group
 --X Symbol
 ##STR15## B --F --Cl --F
 --CL
 ##STR16## B(F) --CN --CF.sub.3 --C
 CF3
 ##STR17## B(2F, 3F) --OCF.sub.3 --OCF.sub.2 H
 --OCF3 --OCF2H
 ##STR18## B(F, F) --C.sub.n H.sub.2n+1
 --OC.sub.n H.sub.2n+1 --n --On
 ##STR19## H --COOCH.sub.3 --EMe
 ##STR20## Py --C.sub.n H.sub.2n
 CH.dbd.CH.sub.2 --C.sub.m H.sub.2m CH.dbd.CHC.sub.n H.sub.2n+1 --nV --mVn
 ##STR21## D --C.sub.m H.sub.2m
 CH.dbd.CHC.sub.n H.sub.2n F --CH.dbd.CF.sub.2 --mVnF --VFF
 ##STR22## Ch --C.sub.n H.sub.2n
 CH.dbd.CF.sub.2 --C.ident.C--CN --nVFF --TC
 5) Notation example
 Example 1 3-H2B(F, F)B(F)--F
 ##STR23##
 Example 2 3-HB(F)TB-2
 ##STR24##
 Example 3 1V2-BEB(F, F)--C
 ##STR25##
 The present invention shall not be restricted to the examples shown below.
 All composition ratios shown in the examples and the comparative examples
 were represented by weight %. Compounds used in the examples and the
 comparative examples were represented by codes based on definitions shown
 in Table 1.
 In the characteristics of the liquid crystal composition, represented were
 the upper limit temperature of nematic liquid crystal phase by T.sub.NI,
 the lower limit temperature of the nematic liquid crystal phase bt TC, the
 viscosity by .eta., the optical anistrophy by .DELTA.n, the threshold
 voltage by Vth, the voltage-holding ratio at 25.degree. C. by VHR
 (25.degree. C.), the voltage-holding ratio at 100.degree. C. by VHR
 (100.degree. C. ) and the response speed by .tau..
 T.sub.NI was determined by measuring the temperature observed when the
 composition was changed from a nematic phase to an isotropic phase liquid
 in a step of elevating the temperature by means of a polarizing
 microscope.
 TC was judged by the liquid crystal phases after the liquid crystal
 compositions were left standing for 30 days in the respective freezers of
 10.degree. C., 0.degree. C., -10.degree. C., -20.degree. C., -30.degree.
 C. and -40.degree. C. (for example, when one liquid crystal composition
 took a nematic phase state at -20.degree. C. and was crystallized or
 turned into smectic phase state at -30.degree. C., TC of the liquid
 crystal composition was expressed by &lt;-20.degree. C.).
 .eta. was measured at 20.degree. C.
 .DELTA.n was measured at 25.degree. C. using a light source lamp having a
 wavelength at 589 nm.
 Vth was measured at 25.degree. C. Vth was represented by a value of voltage
 applied when the transmission factor of light passing through the cell
 became 90%, wherein the cell having a cell gap of (0.5/.DELTA.n) .mu.m and
 a twist angle of 80.degree. was used, and a square wave having a frequency
 of 32 Hz was applied in a normally white mode. The voltage-holding ratio
 was determined by an area method.
 .tau.was defined by a value of the sum of .tau. on and .tau. off, wherein
 the liquid crystal composition to which cholesteric nonanoate was added so
 that the pitch of twist was controlled to 80 .mu.m was put in a sell
 having a cell gap of (0.5/.DELTA.n) .mu.m and a twist angle of 80.degree.;
 a 5 V square wave having a frequency of 1 kHz was applied thereto to
 determine the response speed (.tau. on) when the liquid crystal rose up
 and the response speed (.tau. off) when the liquid crystal fell down. In
 this case, the measuring temperature was 25.degree. C.
 Compounds used for the compositions shown in the examples (32 to 42, 44 and
 45) of Japanese Patent Application Laid-Open No. 251186/1998 are analogous
 to those of Formula (1-1) in the present invention but are not the same
 compounds.
 A difference between the compounds of the present invention represented by
 Formula (1-1) and the compounds of Japanese Patent Application Laid-Open
 No. 251186/1998 shall be shown below.
 A mother liquid crystal having the following composition was prepared:

3-HB--C 24% T.sub.NI = 71.7.degree. C.
 5-HB--C 36% .DELTA..epsilon. = 11.0
 7-HB--C 25% .DELTA.n = 0.137
 3-HBB--C 15% .eta. = 27.0 mPa .multidot. s
 and the following compounds a, b and c were added to this mother liquid
 crystal each in 15%:
 a: 3-BB (F, F) CF2OB (F, F) -F
 &lt;R in Formula (1-1-3) of the present invention: C.sub.3 H.sub.7 &gt;
 b: 3-B (F, F) CF2OBB (F, F) -F
 &lt;R in Formula (1-2-4) of the present invention: C.sub.3 H.sub.7 &gt;
 c: 3-B (F) CF2OBB (F, F) -F
 &lt;Japanese Patent Application Laid-Open No. 251186/1998 (compound
 described in Example 34)&gt;
 The physical properties thereof were measured, and the physical properties
 of the compounds a, b and c obtained from the measured values thereof and
 the physical properties of the mother liquid crystal by extrapolation
 shall be shown in the following Table 2.
 TABLE 2
 Extrapolated physical property value
 Compound T.sub.NI (.degree. C.) .DELTA..epsilon. .DELTA.n .eta.
 (mPa .multidot. s)
 a -5.0 29.7 0.110 40.2
 b -9.8 27.6 0.104 46.6
 c -6.3 21.1 0.104 53.1
 It is apparent from this Table 2 that the compound a or b of the present
 invention represented by Formula (1-1) has larger .DELTA.n and
 .DELTA..di-elect cons. and lower .eta. than those of the compound c
 described in Example 34 of Japanese Patent Application Laid-Open No.
 251186/1998. Further, the tetracyclic compounds had the same tendency.
 Further, detailed explanations shall be given with reference to the
 following examples and comparative examples.
 Example A
 A composition comprising
 as the component A:

3-H2HB (F, F)--F 12%
 4-H2HB (F, F)--F 10%
 5-H2HB (F, F)--F 10%
 3-HHB (F, F)--F 10%
 4-HHB (F, F)--F 5%
 3-HH2B (F, F)--F 10%
 3-HBB (F, F)--F 3%
 2-HHBB (F, F)--F 4%
 3-HHBB (F, F)--F 4%
 4-HHBB (F, F)--F 4%
 was prepared. The characteristics of the composition are shown in Table 3.
 Comparative Example A
 The same procedure as in Example A was repeated to prepare a liquid crystal
 composition, except that the following component D (a compound of Formula
 (1) described in Japanese Patent Application No. 251186/1998) was
 substituted for the component A. The characteristics of the composition
 are shown in Table 3.
 Component D

2-HB (F, F) CF2OB (F, F)--F 14%
 3-HB (F, F) CF2OB (F, F)--F 14%
 TABLE 3
 Composition Comparative
 Characteristics Example A Example A
 T.sub.NI .degree. C. 71.4 71.8
 .theta. mPa .multidot. s 31.6 30.8
 .DELTA.n 0.096 0.083
 Vth V 1.05 1.14
 .DELTA..epsilon. 11.8 10.0
 VHR (25.degree. C.) % 98.5 98.5
 VHR (100.degree. C.) % 95.3 95.4
 .tau. mS 33 40
 As apparent from Table 3, the composition of Example A has a larger optical
 anisotropy, a larger .DELTA..di-elect cons., a lower threshold voltage and
 a faster response speed as compared with those of the composition of
 Comparative Example A. This is considered attributable to that the
 compound in the component A of Example A has more phenylene rings as
 compared with that of the compound in the component D of Comparative
 Example A.
 Example 1
 A composition comprising
 as the component A:

T.sub.NI =74.5.degree. C.
 TC &lt;-20.degree. C.
 .theta. =27.8 mPa .multidot. s
 .DELTA.n =0.086
 Vth =1.21 V
 .DELTA..epsilon. =10.7
 VHR (25.degree. C.) =98.7%
 VHR (100.degree. C.) =95.1%
 This composition has a high VHR at a high temperature, a very small Vth and
 a small viscosity as compared with those of the compositions of the
 comparative examples described later. It has a high T.sub.NI a high
 holding rate and a low viscosity as compared with those of the composition
 of Comparative Example 2.
 Example 2
 A composition comprising
 as the component A:

T.sub.NI =78.7.degree. C.
 TC &lt;-20.degree. C.
 .DELTA.n =0.139
 Vth =1.00 V
 .DELTA..epsilon. =16.4
 VHR (25.degree. C.) =98.7%
 VHR (100.degree. C.) =95.2%
 This composition has a high VHR at a high temperature and a very small Vth
 as compared with those of the compositions of the comparative examples
 described later.
 Example 4
 A composition comprising
 as the component A:

T.sub.NI =76.1.degree. C.
 TC &lt;-20.degree. C.
 .DELTA.n =0.132
 Vth =0.96 V
 .DELTA..epsilon. =17.7
 VHR (25.degree. C.) =98.7%
 VHR (100.degree. C.) =95.0%
 This composition has a high VHR at a high temperature and a very small Vth
 as compared with those of the compositions of the comparative examples
 described later.
 Example 5
 A composition comprising
 as the component A: