Ferrielectric liquid crystal compound

A novel ferrielectric liquid crystal compound of the formula (1), ##STR1## wherein R is a linear alkyl group having 6 to 12 carbon atoms, X is a hydrogen atom or a fluorine atom, m is an integer of 1 to 3, n is an integer of 1 to 3, and C* is an asymmetric carbon atom. The ferrielectric liquid crystal compound of the invention characteristically has a ferrielectric phase in a broad temperature range and shows a high speed optical response in spite of a small spontaneous polarization.

DETAILED DESCRIPTION OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a novel ferrielectric liquid crystal
 compound suitable for use in an active matrix-type liquid crystal display
 device in which a liquid crystal is driven for each pixel independently,
 and to its use.
 2. Prior Art of the Invention
 A liquid crystal display device (LCD) has been being widely used as a flat
 panel display as a substitute for a conventional Braun tube (CRT) display,
 mainly in portable machines and equipment. Along with the recent expansion
 of the functions of personal computers and word processors and with the
 recent increase in the capacity of data processing, LCD is also required
 to have higher functions, that is, to have functions such as a large
 display capacity, a full-color display, a wide viewing angle, a high-speed
 response and a high contrast.
 As a liquid crystal display method (liquid crystal driving method) to
 comply with such requirements, there has been proposed and, in some areas,
 is practically used an active matrix (AM) display device which works by a
 method in which thin film transistors (TFT) or diodes (MIM) are formed
 such that one transistor or diode corresponds to one pixel on a display
 screen and a liquid crystal is driven for one pixel independently of
 another.
 Although the above display method has problems that it is difficult to
 decrease a cost due to a low production yield and that it is difficult to
 form a large-sized display screen, the above display method is about to
 surpass an STN display method which has been so far a mainstream and to
 overtake CRT due to its high display quality.
 Problems to be Solved by the Invention
 However, the above AM display device has the following problems due to the
 use of a TN (twisted nematic) liquid crystal as a liquid crystal material.
 (1) A TN liquid crystal is a nematic liquid crystal, and the response speed
 is generally low (several tens ms), so that no good image quality can be
 obtained in the display of video rate.
 (2) A twisted state (twist alignment) of liquid crystal molecules is used
 for displaying, and the viewing angle is therefore narrow. In displaying
 with a gray scale in particular, the viewing angle becomes sharply
 narrowed. That is, the contrast ratio, the color or the like changes
 depending upon viewing angles to a display screen.
 For overcoming the above problems, there have been, in recent years,
 proposed AM panels which use a ferroelectric liquid crystal or an
 anti-ferroelectric liquid crystal in place of the TN liquid crystal
 (Japanese Laid-open Patent Publications Nos. JP-A-5-249502, JP-A-5-150257
 and JP-A-6-95080). At present, however, the following problems remain to
 solve for the practical use of these liquid crystals.
 (3) A ferroelectric liquid crystal has spontaneous polarization. An image
 sticking is liable to occur due to constant presence of the spontaneous
 polarization, and the driving is hence made difficult. In displaying with
 a ferroelectric liquid crystal, it is very difficult to perform a
 gray-scale display since only a binary display of black and white is
 possible in principle.
 For the gray-scale display, a special devising is required (for example,
 use of a ferroelectric liquid crystal device using monostability; Keiichi
 NITO et al., SID '94, Preprint, p. 48), and it is required to develop a
 high technique for practical use.
 (4) An anti-ferroelectric liquid crystal is free from the image sticking
 problem described in the above (3) since it has no spontaneous
 polarization.
 In the AM driving, however, there is needed a liquid crystal material which
 can be at least driven at 10 V or lower. However, the anti-ferroelectric
 liquid crystal generally shows a high threshold voltage, and its driving
 at a low voltage is therefore difficult. Further, it has another problem
 that the gray-scale display is difficult to perform since its optical
 response involves a hysteresis.
 It is an object of the present invention to provide a novel material which
 can overcome the above problems and is suitable for use in AM driving, and
 a ferrielectric liquid crystal is thinkable as the above novel material.
 In 1989, a ferrielectric phase (Sc.gamma.* phase) was found for the first
 time in
 4-(1-methylheptyloxy-carbonyl)phenyl-4-(4'-octyloxybiphenyl)carboxylate
 (called "MHPOBC" for short) that is an anti-ferroelectric liquid crystal
 compound (Japanese Journal of Applied Physics, Vol. 29, No. 1, pp.
 L131-137 (1990)).
 The chemical structural formula and phase transition temperatures (.degree.
 C.) of the MHPOBC are as follows.
 Structural formula
EQU C.sub.8 H.sub.17 --O--Ph--Ph--COO--Ph--COO--C*H(CH.sub.3)--C.sub.6 H.sub.13
 wherein Ph is a 1,4-phenylne group and C* is an asymmetric carbon atom.
 Phase Sequence
EQU Cr(30)SIA*(65)SCA*(118)SC.gamma.*(119)SC*(121)SC.alpha.*(122)SA(147)I
 wherein Cr is a crystal phase, SIA* is a chiral smectic IA phase, SCA* is a
 chiral smectic CA phase (anti-ferroelectric phase), SC.gamma.* is a chiral
 smectic C.gamma. phase (ferrielectric phase), SC* is a chiral smectic C
 phase (ferroelectric phase), SC.alpha.* is a chiral smectic C.alpha.
 phase, SA is a smectic A phase, and I is an isotropic phase.
 Molecular arrangement states of a ferrielectric liquid crystal and an
 optical response of a ferrielectric phase to a triangular wave will be
 explained with reference to drawings hereinafter.

A ferrielectric phase has a molecular arrangement of FI(+) (a case where an
 applied voltage is positive) or a molecular arrangement of FI(-) (a case
 where an applied voltage is negative) as shown in FIG. 1. In a state free
 of an electric field, FI(+) and FI(-) are equivalent and are therefore
 co-present.
 In a state free of an electric field, therefore, average optic axes are in
 the direction of a layer normal, and the state is a dark state under the
 condition of a polarizer shown in FIG. 1. This state corresponds to a
 portion where a voltage is 0 in FIG. 2.
 Further, each of FI(+) and FI(-) has spontaneous polarization as is
 apparent from the molecular arrangement states. However, each spontaneous
 polarization is canceled by other in a state in which these are co-present
 and consequently, an average spontaneous polarization is zero. This shows
 that, like an anti-ferroelectric phase, a ferrielectric phase is free from
 an image sticking phenomenon observed in a ferroelectric phase.
 As an electric field is applied to a ferrielectric phase, a region (domain)
 having an extinguished position appears at a voltage lower than a voltage
 at which a ferroelectric phase is reached. This shows that the above
 domain has an optic axis in the direction that tilts from the direction of
 layer normal although the tilt is not so large as that in a ferroelectric
 state.
 The above intermediate state is considered to be FI(+) or FI(-).
 As far as the liquid crystal compound of the present invention is
 concerned, a liquid crystal phase which always shows the above
 intermediate state is called a ferrielectric phase, and a liquid crystal
 compound of which the ferrielectric phase is the broadest in its phase
 sequence is called a ferrielectric liquid crystal compound.
 When the applied voltage is further increased, the ferrielectric phase
 causes a phase transition to a ferroelectric phase FO(+) or FO(-) that is
 a stabilized state, depending upon a direction of the electric field. In
 FIG. 2, a phase in which the intensity of transmitted light is brought
 into a saturated state (flat portions on left and right sides) is FO(+) or
 FO(-).
 In the above ferroelectric state FO(+) or FO(-), there is exhibited a
 spontaneous polarization greater than that in the ferrielectric phase
 FI(+) or FI(-), as is seen in FIG. 1. The response speed increases with an
 increase in the spontaneous polarization, and as a result, the capability
 of high speed response is materialized.
 Both the ferroelectric states are in a light state under the condition of a
 polarizer shown in FIG. 1.
 A conventional ferroelectric phase provides a switching between FO(+) and
 FO(-), while the ferrielectric phase has a great characteristic feature
 that it permits switching among four states of FO(+), FI(+), FI(-) and
 FO(-).
 In the ferrielectric phase, therefore, not a continuous change in the
 intensity of transmitted light between voltages of 0 V and 4 V but a
 stepwise change in the intensity of transmitted light ought to be
 observed.
 In FIG. 2, however, a continuous change in the intensity of transmitted
 light is observed.
 It is assumed that the above occurs because the threshold voltage from the
 co-presence state of FI(+) and FI(-) to FO(+) via FI(+) or the threshold
 voltage from the co-presence state of FI(+) and FI(-) to FO(-) via FI(-)
 is not clear.
 As shown in FIG. 2, generally, a ferrielectric liquid crystal has a
 tendency that a difference between the voltage required for change from a
 ferrielectric state to a ferroelectric state and the voltage required for
 change from a ferroelectric state to a ferrielectric state is small, that
 is, the width of its hysteresis is very narrow. It characteristically
 shows an optical response having V-letter-shape and has properties
 suitable for an active matrix driving (AM driving) and a display with a
 gray scale in AM driving.
 Further, in the ferrielectric phase, the voltage (phase transition voltage)
 required for a phase change between a ferrielectric state and a
 ferroelectric state tends to be very small as compared with that of an
 anti-ferroelectric phase, and it can be therefore said that the
 ferrielectric phase is suitable for AM driving.
 In the ferrielectric phase, generally, the change between the co-presence
 state of FI(+) and FI(-) and a ferroelectric state (FO(+) or FO(-)) is
 continuous as shown in FIG. 2, and besides, the voltage required for the
 change is small. Further, the light transmittance in the co-presence state
 of FI(+) and FI(-) at an applied voltage of 0 can be further decreased by
 devising an alignment film.
 On the basis of these, in the ferrielectric phase, the co-presence state of
 FI(+) and FI(-) can be used as dark, the ferroelectric states FO(+) and
 FO(-), as light, and an intermediate state of these, as gray. The display
 principle thereof uses birefringence of a liquid crystal, and a display
 device having a decreased viewing angle dependency can be produced as the
 liquid crystal molecules are arranged in parallel with the substrate
 surface.
 However, the number of ferrielectric liquid crystal that have been
 synthesized so far is very small, and when application to an AM driving
 device is taken into account, few ferrielectric liquid crystal that have
 been already known are satisfactory in respect of hysteresis and a voltage
 in the phase transition from a ferrielectric phase to a ferroelectric
 phase (phase transition voltage).
 Further, in the active matrix driving device, it is an essential problem in
 practice how large or small the spontaneous polarization of the
 ferrielectric liquid crystal compound is.
 J. Funfscilling et al. show that in the AM driving, the degree of the
 voltage required for driving a liquid crystal having spontaneous
 polarization is in proportion to the spontaneous polarization (Jpn. J.
 Appl. Phy. Vol. 33, pp 4950 (1994)). It is desirable from the aspect of
 driving voltage, therefore, that the spontaneous polarization is as small
 as possible.
 On the other hand, it is thought that the speed (response speed) in the
 phase transition from a ferrielectric state to a ferroelectric state is
 largely in proportion to the degree of spontaneous polarization.
 It is therefore very advantageous in practice if there can be provided a
 ferrielectric liquid crystal having a small spontaneous polarization and
 having a high response speed.
 Means to Solve the Problems
 Under the circumstances, the present inventors have made studies to find a
 novel ferrielectric liquid crystal compound having a narrow width of
 hysteresis, small phase transition voltage, a high response speed and
 accordingly, excellent characteristic properties as an AM driving device
 and to find its use, and as a result, the present invention has been
 arrived at.
 That is, according to the present invention, there is provided a
 ferrielectric liquid crystal compound of the following general formula
 (1),
 ##STR2##
 wherein R is a linear alkyl group having 6 to 12 carbon atoms, X is a
 hydrogen atom or a fluorine atom, m is an integer of 1 to 3, n is an
 integer of 1 to 3, and C* is an asymmetric carbon atom.
 In the ferrielectric liquid crystal compound of the above general formula
 (1) in the present invention, R is a linear alkyl group having 6 to 12,
 preferably 9 to 12 carbon atoms. X is a hydrogen or fluorine atom,
 preferably a fluorine atom, m is an integer of 1 to 3, preferably 2 or 3,
 most preferably 2, and n is an integer of 1 to 3, preferably 1 or 2, most
 preferably 1.
 When the ferrielectric liquid crystal compound of the present invention is
 considered as a raw material for practical use, the temperature
 (transition temperature on the high-temperature side) for the transition
 from an isotropic phase, a smectic A phase or a chiral smectic C phase to
 a ferrielectric phase is preferably at least 40.degree. C. Further, it is
 preferable from the practical standpoint that the ferrielectric phase has
 a temperature range at or above 10.degree. C.
 Since the voltage in the phase transition from a ferrielectric state to a
 ferroelectric state is in proportion to a driving voltage, the above
 voltage is preferably 5 V/.mu.m or less, more preferably 3 V/.mu.m or less
 in view of the voltage proof of current driving ICs.
 Further, in the ferrielectric liquid crystal compound of the present
 invention, desirably, a difference between a voltage (phase transition
 voltage I) in phase transition from the ferrielectric state to the
 ferroelectric state and a voltage (phase transition voltage II) in phase
 transition from the ferroelectric state to the ferrielectric state is
 smaller, and it is preferably 0.5 V or lower.
 The ferrielectric liquid crystal provided by the present invention can give
 an active matrix liquid crystal display device by interposing it between
 substrates on which non-linear active devices such as thin film
 transistors or diodes are provided for individual pixels.
 An optically active alcohol CH.sub.3 CH(OH)(CH.sub.2).sub.m OCH(C.sub.n
 H.sub.2n+1).sub.2 used for the synthesis of the ferrielectric liquid
 crystal compound of the present invention can be easily produced by the
 method that the present inventors have already disclosed (Japanese
 Laid-open patent Publication No. JP-A-11-80054).
 The method of the production thereof, for example, when m is 2 and n is 1,
 is outlined as follows.
EQU CH.sub.3 CO(CH.sub.2).sub.2 OCH(CH.sub.3).sub.2
 +(NaBH.sub.4).fwdarw.CH.sub.3 CH(OH) (CH.sub.2).sub.2 OCH(CH.sub.3).sub.2
 (a)
EQU CH.sub.3 CH(OH)(CH.sub.2).sub.2 OCH(CH.sub.3).sub.2 +CH.sub.3 CH.sub.2
 COOCH.dbd.CH.sub.2 +(lipase).fwdarw.R-(-)--CH.sub.3 C*H(OCOC.sub.2
 H.sub.5)(CH.sub.2).sub.2 OCH (CH.sub.3).sub.2 +S-(+)--CH.sub.3
 C*H(OH)(CH.sub.2).sub.2 OCH(CH.sub.3).sub.2 (b)
EQU R-(-)--CH.sub.3 C*H(OCOC.sub.2 H.sub.5)(CH.sub.2).sub.2 OCH(CH.sub.3).sub.2
 +(KOH).fwdarw.R-(-)--CH.sub.3 C*H (OH) (CH.sub.2).sub.2 OCH
 (CH.sub.3).sub.2 (c)
 The above method for the production of the optically active alcohol will be
 briefly explained below.
 (a) shows the reduction of 4-isopropyloxybutan-2-on to an alcohol.
 (b) shows the formation of an R-configuration ester by an asymmetric
 trans-esterification between the alcohol (a) and vinyl propionate in the
 presence of lipase.
 (c) shows the hydrolysis of the optically resolved R-configuration ester
 (b) with an alkali.
 Effect of the Invention
 The novel ferrielectric liquid crystal compound provided by the present
 invention has a ferrielectric phase in a broad temperature range and shows
 a high speed response in spite of a small spontaneous polarization, so
 that it is remarkably useful as a practical raw material for liquid
 crystal display devices.
 EXAMPLES
 The present invention will be explained more in detail with reference to
 Example hereinafter, while the present invention shall not be limited
 thereto.
 Example 1
EQU (formula (1): R=C.sub.9 H.sub.19, X=F, m=2, n=1 (E1))
 Preparation of
 R-(-)-3-fluoro-4-(1-methyl-3-isopropyloxypropanecarbonyl)phenyl-4'-n-nonyl
 oxybiphenyl-4-carboxylate
 (1) Preparation of 4-(4'-n-nonyloxy)biphenylcarboxylic acid
 10.0 Grams of 4-(4'-hydroxy)biphenylcarboxylic acid, 9.8 g of n-nonyl
 bromide, 16 ml (millilitre) of triethyl amine and 1 g of
 dimethylaminopyridine were dissolved in 150 ml of dichloromethane, and the
 mixture was stirred at room temperature for one day and night.
 After completion of the reaction, 50 ml of a 10% hydrochloric acid was
 added to the reaction mixture and then, the resulting mixture was
 extracted with 100 ml of ether three times.
 An organic phase was washed with 100 ml of a sodium chloride aqueous
 solution three times and dried over anhydrous sodium sulfate. After the
 solvent was distilled off, the distillate was washed with 400 ml of hexane
 to obtain an end product.
 (2) Preparation of 4-acetoxy-2-fluorobenzoic acid
 4.3 Grams of 4-hydroxy-2-fluorobenzoic acid and 8.4 g of acetic anhydride
 were placed in a two-necked flask and mixed. While the mixture was cooled
 with water, 5 drops of sulfuric acid were added. After heat generation
 ended, the mixture was heated at 80.degree. C. for 30 minutes. Then, the
 reaction mixture was poured into cold water, and a precipitated crystal
 was recovered by filtration.
 The crystal was dried in vacuum and used in the next step.
 (3) Preparation of
 R-(-)-4-acetoxy-2-fluoro-1-(1-methyl-3-isopropyloxypropanecarbonyl)benzene
 1.0 Gram of 4-acetoxy-2-fluorobenzoic acid was added to 7 ml of thionyl
 chloride, and the mixture was allowed to react under reflux for 5 hours.
 Then, excessive thionyl chloride was distilled off, and a mixture
 containing 1 ml of pyridine, 4 ml of dry ether and 0.5 g of
 R-(-)-4-isopropyloxybutan-2-ol was added dropwise.
 After the addition, the mixture was stirred at room temperature for one day
 and night, and diluted with 200 ml of ether, and an organic layer was
 washed with diluted hydrochloric acid, with a 1N sodium hydroxide aqueous
 solution and with water in this order, and then dried over magnesium
 sulfate. The solvent was distilled off, and the resultant crude product
 was purified through silica gel column chromatography using hexane/ethyl
 acetate as a solvent, to give an end product.
 (4) Preparation of
 R-(-)-4-hydroxy-2-fluoro-1-(1-methyl-3-isopropyloxypropanecarbonyl)benzene
 1.0 Gram of the compound obtained in the above (3) was dissolved in 30 ml
 of ethanol, and 3 g of benzylamine was dropwise added thereto.
 Further, the mixture was stirred at room temperature for one day and night,
 diluted with 300 ml of ether, washed with diluted hydrochloric acid and
 then with water, and dried over magnesium sulfate.
 The solvent was distilled off, and the remainder was subjected to silica
 gel column chromatography for isolation and purification to give an end
 product.
 (5) Preparation of
 R-(-)-3-fluoro-4-(1-methyl-3-isopropyloxypropanecarbonyl)phenyl-4'-n-nonyl
 oxybipheyl-4-carboxylate
 To 1.0 g of the compound obtained in the above (1) was added to 10 ml of
 thionyl chloride, and the mixture was refluxed under heat for 10 hours.
 Excessive thionyl chloride was distilled off, and then, 10 ml of pyridine
 and 25 ml of toluene were added to the mixture. Then, a solution of 0.8 g
 of the compound obtained in the above (4) in 25 ml of benzene was dropwise
 added, and the mixture was allowed to react at room temperature for 10
 hours.
 After completion of the reaction, the reaction mixture was diluted with 300
 ml of ether and washed with diluted hydrochloric acid, with a 1N sodium
 carbonate aqueous solution and with water in this order, and an organic
 layer was dried over magnesium sulfate.
 Then, the solvent was distilled off, and the remainder was subjected to
 silica gel column chromatography for isolation.
 An isolated product was recrystallized from ethanol, to give an end
 product.
 Example 2
EQU (formula (1): R=C.sub.10 H.sub.21, X=F, m=2, n=1 (E2)
 Preparation of
 R-(-)-3-fluoro-4-(1-methyl-3-isopropyloxypropanecarbonyl)phenyl-4'-n-decyl
 oxybiphenyl-4-carboxylate
 An end product was prepared in the same manner as in Example 1 except for
 the use of 4-(4'-n-decyloxy)biphenylcarboxylic acid which was obtained in
 the same manner as in (1) of Example 1 except that the n-nonyl bromide was
 replaced with n-decyl bromide.
 Table 1 shows chemical formula and .sup.1 H-NMR spectrum data of the end
 products obtained in Examples 1 and 2.
 Table 2 shows results of identification of liquid crystal phases of the end
 products.
 The liquid crystal compounds were identified for liquid crystal phases by
 texture observation, conoscopic image observation and DSC (differential
 scanning calorimeter) measurement. The observation of a conoscopic image
 is an effective means for identifying a ferrielectric phase. The
 conoscopic image observation was conducted according to a literature (J.
 Appl. Phys. 31, 793 (1992)).
 Then, the ferrielectric liquid crystal compound obtained in Examples 1 and
 2 were measured for optical responses, and the results are shown also in
 Table 2. Cells were prepared by the following procedure.
 A pair of glass plates with insulating film (SiO.sub.2, film thickness; 50
 nm) and ITO electrodes were coated with polyimide (film thickness, about
 80 nm), and one of the pair of glass plates was rubbed.
 The pair of glass plates were attached to each other through a spacer
 having a particle diameter of 1.6 .mu.m to form a test cell. The cell had
 a thickness of 2 .mu.m.
 A liquid crystal was heated until the liquid crystal showed an isotropic
 phase, and the liquid crystal was then injected into the test cell by
 capillarity. Thereafter, the cell was gradually cooled at a rate of
 1.degree. C./minute to align the liquid crystal in parallel.
 The light transmittance was defined as follows. The lowest intensity of
 transmitted light was taken as 0% of light transmittance, and the highest
 intensity of transmitted light was taken as 100% of light transmittance.
 The phase transition voltage was defined to be a voltage found at a light
 transmittance of 90%.
 A triangular wave voltage of .+-.10 V, 5 Hz was applied to the test cell,
 and a voltage (phase transition voltage) in the transition from a
 ferrielectric phase to a ferroelectric phase was measured.
 The spontaneous polarization was determined by applying a triangular wave
 voltage of 10 V and measuring a polarization inversion current.
 Further, the response speed was defined to be a time required for a change
 of the light transmittance from 0% to 90% when a rectangular wave voltage
 of 8V, 10 Hz was applied, and the test cell was measured for a response
 speed.
 TABLE 1
 ##STR3##
 Chemical shift (ppm)
 Hydrogen atom No.
 1H 2H 3H 4H 5H 6H
 7H 8H 9H 10H
 Example 1 (E1) ( (ppm)) 4.0 7.0 7.5 7.7 8.1 7.1
 -- 7.1 8.0 5.3
 Example 2 (E2) ( (ppm)) 4.0 7.0 7.5 7.7 8.1 7.1
 -- 7.1 8.0 5.3
 TABLE 2
 Phase transition Response Spontaneous
 Phase sequence voltage (V/.mu.m) time (.mu.sec)
 polarization (nC/cm.sup.2)
 Example 1 Cr(60)SC.gamma.* (119)SA(135)I 1.6 140 46
 Example 2 Cr(14)SC.gamma.* (118)SA(136)I 1.7 161 37
 In the phase sequence of the above Table 2, parenthesized values show phase
 transition temperatures (.degree. C.), Cr is a crystal phase, SC.gamma.*
 is a ferrielectric phase, SA is a smectic A phase, and I is an isotropic
 phase.
 Further, phase transition voltages, response time periods and spontaneous
 polarization data were measured at 70.degree. C. in Example 1 and at
 50.degree. C. in Example 2.