A liquid crystal composition for twist nematic mode

Abstract: A liquid crystal composition for twist nematic mode having improved reduction in the threshold voltage and the temperature dependency of threshold voltage in the liquid crystal display element using the composition, and a liquid crystal display element are provided, which composition comprises a compound of the formula wherein R₁ is 1-8C alkyl, R₂ is F or CN and n is 1 or 2; or comprises the compound of the formula (I) as a first component and at least one member as a second component selected from compounds of the following formulas (II), (III) and (IV); wherein R₃ is 1-8C alkyl and R₄ is H or F, wherein R₅ is 1-8C alkyl, R₆ is 1-8C alkoxy or F, m is 1 or 2 and X is single bond or -COO-, and wherein R₇ is 1-8C alkyl, R₈ is 1-8C alkyl or alkoxy, F or CN and

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a liquid crystal composition for twist nematic (hereinafter abbreviated to TN) mode and a liquid crystal display element of TN mode using the above-mentioned composition. More particularly it relates to a liquid crystal composition capable of reducing the threshold voltage (hereinafter abbreviated to Vth) of TN mode liquid crystal display elements and improving the temperature dependency thereof. 2. Description of the Related Art In recent years, the performances of liquid crystal display devices have been improved and their application fields have been being broadened. At the initial period of liquid crystal display devices, the devices have been mainly used for handy type electronic calculators, watches, clocks, etc., making use of the specific feature of small power consumption thereof. Recently, however, the application fields of the devices, in combination with development of liquid crystal compositions, have come to be rapidly broadened toward those of large-size devices such as outdoor display devices, instrument panels for automobiles, and further, display devices for hand-held computers, terminals of computers, etc. From the viewpoint of the driving mode of display devices relative to the foregoing, since the information quantity has increased and the number of parts has been reduced, the 3V drive static mode at the initial period has come to be transferred into 3V drive, 1 / 2 duty in the case of watches, etc., 3V drive, 1 / 3 to 1 / 4 duty in the case of electronic calculators, and 4.5V drive, 1 / 16 and ten and several V drive, 1 / 64 duty or further, highly functional display having higher driving voltage and multiplex number in the case of graphic displays such as those of word processors, computer terminals, etc. Further, in addition to these movements, need for low voltage drive as one more specific feature of liquid crystal display devices, has come to be intensified in order to realize making LCD smaller and more lightweight. For example, 1 / 2 to 1 / 4 duty in the case of watches, electronic calculators, etc. has been desired to be driven at 1.5 V, 1 / 16 duty in the case of word processors, etc., at 3 V, and 1 / 64 duty in the case of computer terminals, etc., at 10 V or lower. Namely, it has been desired to reduce the threshold voltage in the voltage-brightness characteristic of liquid crystal compositions, but it has been very difficult in the case of conventional materials to effect low voltage drive, while satisfying specific features for securing a display quality as mentioned later, i.e. a sufficiently low optical anisotropy value for securing a broad viewing angle and a small temperature-dependency value of threshold voltage. Usually, in the case of multiplex drive mode employing a voltage-leveling method, there is an operational restriction which does not occur at all in principle in the case of static mode. Namely, cross-talk is liable to occur in a half-selected element or unselected element. Furthermore, the higher the multiplex, the less the allowance of the operating voltage, in short, the operating margin; hence cross-talk is more liable to occur. The operating margin depends on the following three factors: 1 ○ temperature dependency of threshold voltage, 2 ○ viewing angle dependency of threshold voltage and 3 ○ steepness of threshold voltage. As to the factor 2 ○ among these three, a product of the thickness of display cell (hereinafter abbreviated to d) by the optical anisotropy value (hereinafter abbreviated to Δn) i.e. a light path length of birefringence (hereinafter abbreviated to d·Δn) has a great influence upon the factor 2 ○. For example, it has generally been carried out to obtain a display element of broad viewing angle, by satisfying the following conditional equation (1) derived from Gooch-Tarry's equation (J. Phys. D.: Appl. Phys., vol. 8, 1975) wherein T: light transmittance, ϑ: twist angle, u: πd·Δn/ϑλ , and λ: wavelength: d·Δn = 550 nm (1) In general, it is difficult in the aspects of cell preparation and product yield to make the cell thickness (d) 5 µm or less; hence the Δn value of liquid crystal materials is preferred to be less than 0.11 in view of the above equation (1). The above factor 3 ○ is specific of TN mode liquid crystal display devices, and no notable difference in the factor (3) occurs depending on materials. The above factor 1 ○ has a relation also with the operating temperature range, and is most affected by materials. In general, as a countermeasure to liquid crystal compositions in order to improve the above factor 1 ○, it has been carried out to increase the proportion of a material having a negative dielectric anisotropy value (hereinafter abbreviated to Δε) to thereby reduce the temperature dependency of threshold voltage. However, increase in the proportion of a material having a negative Δε raises the threshold voltage; hence a sufficiently low voltage drive could not have been achieved. Further, in the case of display devices, the response rate is a nonnegligible and important factor. It is well known that the response rate of liquid crystal display devices has a correlationship with the viscosity of liquid crystal compositions relative thereto. EP-A-258868 reveals a nematic liquid crystal composition comprising at least two components at least one of which is a compound expressed by the formula wherein R₁ represents an alkyl group or an alkoxy group each of 1 to 10 carbon atoms, each independently represent a benzene ring or a cyclohexane ring and n represents an integer of 0 or 1, said compound being contained in 10 to 100 % by weight in said composition. EP-A-281065 discloses a liquid crystal composition comprising: (a) 20 to 40 % by weight first component consisting of at least one compound expressed by the general formula: wherein R¹ is an alkyl group of 1 to 8 carbon atoms; and R² is a hydrogen atom or a fluorine atom; (b) 10 to 40 % by weight second component consisting of at least one compound expressed by the general formula: wherein R¹ is as defined above; and R³ is an alkyl group or an alkoxy group each of 1 to 8 carbon atoms; and (c) 20 to 50 % by weight third component consisting of at least one compound expressed by the general formula: wherein R¹ is as defined above; and R⁴ is a fluorine atom, a cyano group, or an alkyl group or an alkoxy group each of 1 to 8 carbon atoms; said first, second and third components being contained in a total amount of 70 % by weight or more in said composition. Finally, EP-A-232052 includes a nematic liquid crystal composition comprising 10 to 80 % by weight of a first component comprising at least one compound expressed by the formula wherein m represents 0 or 1, R¹ represents F or H and R² represents an alkyl group of 1 to 8 carbon atoms, and 10 to 40 % by weight of a second component comprising at least one compound expressed by the formula wherein ℓ represents 1 or 2 and R² is as defined above. SUMMARY OF THE INVENTION As apparent from the foregoing, an object of the present invention is to provide a nematic liquid crystal composition which has a sufficiently low optical anisotropy α value and also a sufficiently low threshold voltage value for effecting a broad viewing angle, while keeping various characteristics required for liquid crystal display devices such as operating temperature range, response rate, etc. in well-balanced manner, and has an improved temperature dependency. Another object of the present invention is to provide a liquid crystal display element which can be driven at low voltages, has a broad viewing angle and has a small temperature dependency. The present invention in a first aspect resides in a liquid crystal composition for twist nematic mode comprising a compound expressed by the formula wherein R₁ represents an alkyl group of 1 to 8 carbon atoms, R₂ represents F or CN and n represents 1 or 2 including about 13 to about 40 %, by weight, of at least one compound in which n=1 and about 14.2 to about 60 %, by weight, of at least one compound in which n=2, the total proportion of said compounds of said component of formula (I) being about 42.7 to about 100 %, by weight. According to a preferred embodiment of the above composition, a liquid crystal composition is provided comprising a first component comprising at least one member of the compound expressed by the above formula (I) and a second component comprising at least one member selected from the group consisting of a compound expressed by the formula wherein R₃ represents an alkyl group of 1 to 8 carbon atoms and R₄ represents H or F, a compound expressed by the formula wherein R₅ represents an alkyl group of 1 to 8 carbon atoms, R₆ represents an alkoxy group of 1 to 8 carbon atoms or F, m represents 1 or 2 and X represents a single bond or -COO-, and a compound expressed by the formula wherein R₇ represents an alkyl group of 1 to 8 carbon atoms, R₈ represents an alkyl group or alkoxy group each of 1 to 3 carbon atoms, F or CN group and represents The present invention in the second aspect resides in a liquid crystal display element using a liquid crystal composition for twist nematic mode as defined above. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the liquid crystal composition of the present invention, the content of the compound of the formula (I) wherein n=1, i.e. a two-ring compound is preferably 15 to 40% by weight and the content of the compound of the formula (I) wherein n=2, i.e. a three-ring compound is preferably 15 to 60% by weight. The content of the compound expressed by the formula (II) is preferably 25% by weight or less, that of the compound expressed by the formula (III) is preferably 30% by weight or less and that of the compound expressed by the formula (IV) is preferably 35% by weight or less. Besides the above compounds of the formulas (I) to (IV), the liquid crystal composition of the present invention may further contain other nematic liquid crystals or liquid crystalline compounds for adjusting threshold voltage, mesomorphic range, viscosity, etc. in a suitable quantity, within a range where the object of the present invention is not damaged. Preferred representative examples of such other compounds are as follows: 4-(trans-4-alkylcyclohexyl)benzonitriles, 4′-alkyl-4-cyanobiphenyls, trans-2-(4-cyanophenyl)-5-alkyl-1,3-dioxanes, 4-(trans-4-alkoxymethylcyclohexyl)benzonitriles, etc. In the liquid crystal composition of the present invention, compounds of the formula (I) wherein n=1, i.e. 4-(trans-4-alkylcyclohexyl)-1,2-difluorobenzenes and 4-(trans-4-alkylcyclohexyl)-2-fluorobenzonitriles mostly exhibit no liquid crystal phase at room temperature, but these compounds have an effectiveness of increasing the Δε value of compositions obtained by using them, toward positive values and reducing the Δn value thereof. The content of these compounds are preferably 15 to 40% by weight. If it is less than about 15% by weight, contribution of Δε is small and reduction in the threshold voltage value is insufficient, whereas if it exceeds about 40% by weight, they mostly exhibit no liquid crystal phase at room temperature, as described above; thus the clearing point of the resulting composition often lowers and hence such higher contents are undesirable. Further, compounds of the formula (I) wherein n=2, i.e. 4-{trans-4-(trans-4-alkylcyclohexyl)cyclohexyl}-1,2-difluorobenzenes, exhibit a high nematic-isotropic liquid phase transition point (hereinafter referred to as clearing point) of 85.4°C or higher, a small Δn value, a low viscosity and a positive Δε value. Further, 4-{trans-4-(trans-4-alkylcyclohexyl)cyclohexyl}-2-fluorobenzonitriles exhibit a clearing point of 176.5°C or higher and a positive large Δε value. These compounds contribute to a high clearing point and reduction in the threshold voltage without increasing Δn, in the resulting composition. The content of these compounds is preferably 15 to 60% by weight. If it is less than 15% by weight, the above-mentioned contribution to Δn, clearing point and threshold voltage is small, whereas if it exceeds 60% by weight, the lower limit temperature of the nematic phase of the resulting composition rises and the operation temperature range on the lower temperature side is narrowed. In the liquid crystal composition of the present invention, compounds expressed by the formula (II), i.e. 2-(4-fluorophenyl)-5-alkylpyrimidines and 2-(3,4-difluorophenyl)-5-alkylpyrimidines exhibit no liquid crystal phase at room temperature, but they contribute to increase in the Δε value, reduction in the Δn value and reduction in the viscosity in the compositions obtained by using these compounds. Further, the content of these compounds is preferably about 25% by weight or less. If it exceeds 25% by weight, no liquid crystal phase is exhibited at room temperature; hence the clearing point of the resulting liquid crystal composition often lowers. In the liquid crystal composition of the present invention,compounds expressed by the formula (III), i.e. 4″-(trans-4-alkylcyclohexyl)alkoxybenzenes, 4-alkoxyphenyl trans-4-alkylcyclohexanecarboxylates, 4-fluorophenyl trans-4-alkylcyclohexanecarboxylates, 4-fluorophenyl trans-4-(trans-4-alkylcyclohexyl)cyclohexanecarboxylates, etc., have a low viscosity and a small Δn value, but exhibit a positive or negative Δε value close to zero; hence they often raise the threshold voltage of the resulting liquid crystal composition depending on blending thereof. The content of these compounds is preferably 25% by weight or less. In the liquid crystal composition of the present invention, compounds expressed by the formula (IV), i.e. 4-{trans-4-(trans-4-alkylcyclohexyl)cyclohexyl}alkylbenzenes, 4-{trans-4-(trans-4-alkylcyclohexyl)cyclohexyl}benzonitriles, 4′-(trans-4-alkylcyclohexyl)-4-alkylbiphenyls, etc. exhibit a low viscosity for three-ring compounds, but exhibit a high clearing point of 146.3°C or higher; hence they contribute to viscosity reduction, retention of clearing point, etc. in the resulting liquid crystal composition, but the lower limit temperature of nematic phase rises depending on blending thereof with other compounds and the operation temperature range on the lower temperature side is narrowed; hence the content of these compounds is preferably 10% by weight or less. The present invention will be described in more detail by way of Examples, but it should not be construed to be limited thereto. In order to show the effectiveness of the liquid crystal composition of the present invention and the display element using the composition, a manner expressing the characteristic values of the composition will be described. In addition, the threshold voltage and the temperature dependency of the threshold voltage in the present invention are defined as follows: In the voltage-brightness characteristic, a voltage at which the transmittance of a light in the light axis direction perpendicular to the display surface reaches 10% is referred to as threshold voltage and expressed by Vth. Further, when the respective threshold voltages at two different temperatures t₁ (°C) and t₂ (°C) are expressed by Vth (t₁) and Vth (t₂), the temperature dependency of the threshold voltage dV/dt is determined by the following equation (1): Thus, this equation shows that the nearer the absolute value of the parameter dV/dt is to zero, the more the temperature dependency is improved. In the following Examples and Comparative examples, the compositions are expressed in terms of % by weight. Example 1 A liquid crystal composition consisting of the following 5 compounds of the formula (I) was prepared: 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 20% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 20% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 20% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 20% and 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 20%. This liquid crystal composition exhibited a clearing point of 60.4°C, an optical anisotropy value of 0.111 and a viscosity at 20°C of 33.0 cp. This liquid crystal composition was filled in a 90° twist TN cell having a rubbed film of a polyimide on the respective opposed transparent electrodes under a condition of d·Δn=550 nm , followed by measuring various characteristics. Here, d refers to the thickness of the cell i.e. a distance between the electrodes. The threshold voltage at 25°C was 1.05 V and the temperature dependency of the threshold voltages at from 0°C to 40°C (dV/dt) was -0.40%/°C. Example 2 A liquid crystal composition consisting of three compounds of the formula (I), 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 14.3% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 14.2% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 14.2%; two compounds of the formula (II), 2-(3,4-difluorophenyl)-5-propylpyrimidine 9.5% and 2-(4-fluorophenyl)-5-ethylpyrimidine 14.3%; and three compounds of the formula (IV), 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-toluene 9.5% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-propylbenzene 19% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-benzonitrile 5% was prepared, followed by measuring its characteristics in the same manner as in Example 1. The results are shown in Table 1. Example 3 A liquid crystal composition consisting of six compounds of the formula (I), 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 15% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 10% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 15% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 15%; and two other compounds, 4-(trans-4-ethylcyclohexyl)benzonitrile 10% and 4′-ethyl-4-cyanobiphenyl 5% was prepared, followed by measuring its characteristics in the same manner as in Example 1. The results are shown in Table 1. Example 4 A liquid crystal composition consisting of seven compounds of the formula (I), 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 7% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 6% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 7% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 7% 4-{trans-4-(trans-4-pentylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 3% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 13% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 13%; a compound of the formula (II), 2-(3,4-difluorophenyl)-5-propylpyrimidine 11% three compounds of the formula (III), 4-ethoxyphenyl trans-4-propylcyclohexanecarboxylate 8% 4-ethoxyphenyl trans-4-butylcyclohexanecarboxylate 8% and 4-butoxyphenyl trans-4-butylcyclohexanecarboxylate 9%; and another compound, trans-2-(4-cyanophenyl)-5-propyl-1,3-dioxane 8% was prepared, followed by measuring its characteristics in the same manner as in Example 1. The results are shown in Table 1. Comparative example 1 A liquid crystal composition consisting of the following component A, component B and component C (seven compounds in total)was prepared: component A: cyanophenylmetadioxanes which, when dissolved in 15% by weight to a so far known composition of cyanophenylcyclohexanes (Zℓi-1083, tradename of product made by Merck Company), the resulting compositions exhibit an exceptionally low Δn and an exceptionally large Δε, concretely a clearing point of 26.0° to 41.3°C, a Δn value of 0.09 to 0.100 and a Δε value of 25.6 to 26.9 in terms of extrapolated values: trans-2-(4-cyanophenyl)-5-propyl-1,3-dioxane 19.6% trans-2-(4-cyanophenyl)-5-butyl-1,3-dioxane 19.6% and trans-2-(4-cyanophenyl)-5-pentyl-1,3-dioxane 19.6%; component B : phenyl cyclohexanecarboxylates which exhibit specific characteirstic values of a Δn value of 0.087 and a Δε value of -1.2 and have been generally known to persons skilled in the art to be suitable for reducing the temperature dependency of threshold voltage: 4-ethoxyphenyl trans-4-propylcyclohexanecarboxylate 10% 4-ethoxyphenyl trans-4-butylcyclohexanecarboxylate 10% and 4-butoxyphenyl trans-4-butylcyclohexanecarboxylate 13.3%; and Component C : a so far known biphenyl cyclohexane generally used by persons skilled in the art for securing transparent point, 4′-(trans-4-pentylcyclohexyl)-4-cyanobiphenyl 7.9%, followed by measuring its characteristics in the same manner as in Example 1. The results are shown in Table 1. Example 5 A liquid crystal composition consisting of seven compounds of the formula (I): 4-(trans-4-pentylcyclohexyl)-1,2-difluorobenzene 10% 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 10% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 10% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 10% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 10%; a compound of the formula (IV), 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-propylbenzene 10%; and another compound, 4-(trans-4-methoxymethylcyclohexyl)benzonitrile 10%, was prepared, followed by measuring is characteristics in the same manner as in Example 1. The results are shown in Table 2. Example 6 A liquid crystal composition consisting of six compounds of the formula (I), 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 12.2% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 14.6% 4-(trans-4-pentylcyclohexyl)-2-fluorobenzonitrile 12.2% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 13.4% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 13.4% and 4-{trans-4-(trans-4-pentylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 13.4%; two compounds of the formula (III), 4-fluorophenyl trans-4-(trans-4-propylcyclohexyl)-cyclohexanecarboxylate 4.9% and 4-fluorophenyl trans-4-(trans-4-pentylcyclohexyl)-cyclohexanecarboxylate 4.9%; and two compounds of the formula (IV), 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-fluorobenzene 6.1% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-methoxybenzene 4.0%, was prepared, followed by measuring its characteristics in the same manner as in Example 1. The results are shown in Table 2. Example 7 A liquid crystal composition consisting of five compounds of the formula (I), 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 12% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 12% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-2-fluorobenzonitrile 8%; one compound of the formula (II), 2-(4-fluorophenyl)-5-ethylpyrimidine 15%; and two compounds of the formula (IV), 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-toluene 9% and 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-propylbenzene 14% was prepared, followed by measuring its characteristics in the same manner as in Example 1. The results are shown in Table 2. Example 8 A liquid crystal composition consisting of two compounds of the formula (I), 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 18% and 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 18%, five compounds of the formula (III), 4-ethoxyphenyl trans-4-propylcyclohexanecarboxylate 10% 4-ethoxyphenyl trans-4-butylcyclohexanecarboxylate 10% 4-butoxyphenyl trans-4-butylcyclohexanecarboxylate 7% 4-fluorophenyl trans-4-(trans-4-propylcyclohexyl)-cyclohexanecarboxylate 4.5% and 4-fluorophenyl trans-4-(trans-4-pentylcyclohexyl)-cyclohexanecarboxylate 4.5%; three compounds of the formula (IV), 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-benzonitrile 5% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-benzonitrile 5% and 4′-(trans-4-pentylcyclohexyl)-4-ethylbiphenyl 9%; and another compound, trans-2-(4-cyanophenyl)-5-propyl-1,3-dioxane 9% was prepared, followed by measuring its characteristics in he same manner as in Example 1. The results are shown in Table 2. Example 9 A liquid crystal composition consisting of five compounds of the formula (I), 4-(trans-4-ethylcyclohexyl)-2-fluorobenzonitrile 15% 4-(trans-4-propylcyclohexyl)-2-fluorobenzonitrile 15% 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 15% and 4-{trans-4-(trans-4-pentylcyclohexyl)cyclohexyl}-1,2-difluorobenzene 10%; one compound of the formula (III), 4-(trans-4-propylcyclohexyl)ethoxybenzene 8%; two compounds of the formula (IV), 4-{trans-4-(trans-4-propylcyclohexyl)cyclohexyl}-benzonitrile 6% and 4-{trans-4-(trans-4-ethylcyclohexyl)cyclohexyl}-benzonitrile 6%; and another compound, trans-2-(4-cyanophenyl)-5-propyl-1,3-dioxane 10% was prepared, followed by measuring its characteristics in the same manner as in Example 1. The results are shown in Table 2. Comparative example 2 A liquid crystal composition consisting of the following components A, B and C used in Comparative example 1 and further the following component D, that is, a so far known compound, 4′-(trans-4-pentylcyclohexyl)-4-cyanobiphenyl having a clearing point of 280.0°C was prepared: component A trans-2-(4-cyanophenyl)-5-propyl-1,3-dioxane 19.6% trans-2-(4-cyanophenyl)-5-butyl-1,3-dioxane 19.6% and trans-2-(4-cyanophenyl)-5-pentyl-1,3-dioxane 19.6%; component B 4-ethoxyphenyl trans-4-propylcyclohexanecarboxylate 9% 4-ethoxyphenyl trans-4-butylcyclohexanecarboxylate 9% and 4-butoxyphenyl trans-4-butylcyclohexanecarboxylate 11.3%; component C 4′-(trans-4-pentylcyclohexyl)-4-cyanobiphenyl 7.9%; and component D 4′-cyano-4-biphenylyl trans-4-(trans-4-pentylcyclohexyl)cyclohexanecarboxylate 4%. The characteristics of the composition was then measured in the same manner as in Example 1. The results are shown in Table 2. The liquid crystal composition and the liquid crystal display element of the present invention have various characteristics required for liquid crystal materials used for twist nematic (TN) mode liquid crystal display elements, particularly multiplex drive TN elements, namely a sufficiently low viscosity for securing response properties, a sufficiently low Δn value for obtaining a broad viewing angle, particularly a low threshold voltage for low voltage drive and its low temperature dependency, and these various characteristics have been notably improved as compared with those of conventional compositions and display elements. As to the temperature dependency of threshold voltage defined by the above equation (1), the nearer the dependency is to zero, the more it is desired, and in the present invention, a further less value has been achieved. It has been well known for persons skilled in the art that as the threshold voltage lowers, the temperature dependency thereof increases, that is, this has a bad influence upon display elements, but as directly shown in comparison of Comparative example 1 with Example 1, the compound of the formula (I) of the present invention has almost the same clearing point and the same low Δn value as those of Comparative example 1 and nevertheless affords particularly a lower threshold voltage and a less temperature dependency of threshold voltage. Further, as seen from comparisons of Comparative example 1 with Examples 2 to 4 and those of Comparative example 2 with Examples 5 to 9 in Table 2, particularly a lower threshold voltage and a less temperature dependency of threshold voltage are exhibited while clearing point, Δn and viscosity values are kept in well-balanced manner. It is considered that the compounds of the formulas (I) and (II) greatly contribute to the above facts and particularly the compound of the formula (I) does. Since the nematic liquid crystal composition of the present invention has the above-mentioned characteristics, when the composition is used, a broad viewing angle and a low-voltage multiplex drive (e.g. 1.5 V, 1 / 2 - 1 / 3 duty drive) is possible.

Claims: A liquid crystal composition for twisted nematic mode comprising a first component expressed by the formula wherein R₁ represents an alkyl group of 1 to 8 carbon atoms, R₂ represents F or CN and n represents 1 or 2 including about 13 to about 40 %, by weight, of at least one compound in which n=1 and about 14.2 to about 60 %, by weight, of at least one compound in which n=2, the total proportion of said compounds of said component of formula (I) being about 42.7 to about 100 %, by weight. A liquid crystal composition for twisted nematic mode according to claim 1 wherein said compounds represented by the formula (I) comprises a first component and further comprises a second component including at least one member selected from the group consisting of a compound expressed by the formula wherein R₃ represents an alkyl group of 1 to 8 carbon atoms and R₄ represents H or F; a compound expressed by the formula wherein R₅ represents an alkyl group of 1 to 8 carbon atoms, R₆ represents an alkoxy group of 1 to 8 carbon atoms or F, m represents 1 or 2 and X represents a single bond or -COO-, and a compound expressed by the formula wherein R₇ represents an alkyl group of 1 to 8 carbon atoms, R₈ represents an alkyl group or alkoxy group each of 1 to 8 carbon atoms, F or CN group and represents A liquid crystal composition for twisted nematic mode according to claim 2, wherein the proportion of the compound of the formula (II) is 25 % by weight or less, that of the compound of the formula (III) is 30 % by weight or less and that of the compound of the formula (IV) is 35 % by weight or less. A liquid crystal display element using a liquid crystal composition for twisted nematic mode as set forth in claim 1. A liquid crystal display element using a liquid crystal composition for twisted nematic mode as set forth in claim 2. A liquid crystal display element using a liquid crystal composition for twisted nematic mode as set forth in claim 3.

Inventors: ISOYAMA TOYOSHIRO, MATSUSHITA TETSUYA, SAITO HIDEO, SAWADA SHINICHI, ISOYAMA, TOYOSHIRO, MATSUSHITA, TETSUYA, SAITO, HIDEO, SAWADA, SHINICHI
Applicants: CHISSO CORP, CHISSO CORPORATION
Application year: 1988
Main classification: C09K19
IPC codes: G02F1, C09K19