The present invention relates to a nematic liquid crystal composition and, more specifically, relates to a liquid crystal composition for active matrix (AM) mode and to a liquid crystal display device (LCD) using the liquid crystal composition.
An active matrix mode-liquid crystal display devices (AM-LCD) is attracting a great deal of attention as the likely winner among LCD""s as it enable to display with extreme precision, and is applied for displays such as monitors, note-type personal computers, digital still cameras and digital video cameras. Characteristics required for the liquid crystal composition of AM-LCD are shown in the items (1) to (5) below.
(1) A liquid crystal composition should show a nematic phase in the range of temperature as wide as possible in order to widen the usable temperature range of the liquid crystal display devices (elevating the upper temperature limit of a nematic phase as high as possible, and lowering the lower temperature limit of a nematic phase as low as possible).
(2) Viscosity of the liquid crystal composition should be as low as possible in order to accelerate the response speed of the liquid crystal display device.
(3) Optical anisotropy (xcex94n) of the liquid crystal composition should be able to have a proper value depending on the cell thickness (d) in order to enhance the contrast of the liquid crystal display device.
(4) Values of specific resistivity (specific resistance) on the liquid crystal composition should be increased and the voltage holding ratio of the cell containing the liquid crystal composition should be large, in order to enhance the contrast of the liquid crystal display device. The voltage holding ratio should be large especially at a high temperature region.
(5) Threshold voltage of the liquid crystal composition should be lowered in order to downsize the battery which is the power source to drive liquid crystal display device.
Recently it has been requested keenly to apply for an animation in the LCD and various LCD display modes have been studied. The addressing mode of the AM-LCD employs TN display mode in which the orientation of liquid crystal molecules between the upper and lower substrates is twisted by 900xc2x0. In the TN display mode, it is necessary to keep the product, xcex94nxc2x7d, of optical anisotropy (xcex94n) of a liquid crystal material to be filled in a cell and cell thickness (d xcexcm) at a certain value (for example xcex94nxc2x7d=0.5 etc.) in order to obtain an optimum contrast and to avoid coloration by the interference of the liquid crystal cell when no voltage is applied, as reported by G.Bauer (Cryst. Liq., 63, 45 (1981)). Therefore, if the liquid crystal material having large xcex94n is used, the value of d can be decreased. Response speed (xcfx84) is proportional to viscosity (xcex7) of the liquid crystal material and to d squared, as proposed by E. Jakeman et al. (Phys. Lett. , A, 39 (1972) 69). When the liquid crystal material having large xcex94n is used, the thickness of cell constituting liquid crystal display device can be decreased, and thus xcfx84 can be increased. As such, the liquid crystal composition having large xcex94n and low viscosity is very useful for the liquid crystal display devices.
Also the development of the display intended for outdoor use has been requested with the increase of a portable type display. For being bearable in the outdoor use, materials are requested to have a nematic phase over the wide range of temperature exceeding the temperature range-of environment in use. To widen a nematic phase range of the liquid crystal composition, it is necessary to use liquid crystal compounds having a high clearing point and good miscibility with other liquid crystal compounds. Generally, a compound having a high clearing point, namely having many six member-rings in the chemical structure may be used for elevating the clearing point. However, miscibility at a low temperature region may tend to be a problem in such case.
It is also requested the liquid crystal composition with high reliability such as a high voltage holding ratio (V.H.R.) or large specific resistivity in order to keep a high contrast especially in the AM-LCD.
Based on the background described above, the specification of WO 96/11897 discloses a novel liquid crystal compound having large dielectric anisotropy (xcex94n) together with extremely low viscosity and disclosed a liquid crystal composition comprising the compound for low voltage driving in various modes such as a AM mode or a super twisted nematic mode (STN mode). JP 10-251186 A describes a compound similar to the compound of formula (1-3) in the present invention as the compound having large xcex94xcex5 and small temperature dependence.
The liquid crystal composition for the AM-LCD disclosed in WO 96/11897 described above (the composition does not contain compounds having cyano in the terminal, and compounds having cyano can not be used for the liquid crystal composition of the AM-LCD because of its low voltage holding ratio) has drawbacks that the V.H.R. is low and optical anisotropy is low as is shown in the Comparative Example of the present invention.
JP 10-251186 A discloses a composition using the compound similar to that of formulae (1-1) to (1-3) of the present invention, however, the similar compound has large and positive xcex94xcex5. When such compound is used, viscosity of the composition is increased. Then, the compound has a drawback of low response speed in the LCD. The similar compound has a drawback that xcex94n is small and that a high voltage holding ratio required for the composition of TFT can not be attained by the combination of a cyano compound.
Although liquid crystal compositions were studied, the liquid crystal composition for AM-LCD has been required especially to keep a high voltage holding ratio at a high temperature region in order to enhance the contrast of the liquid crystal display device, to have a wide range of a liquid crystal phase in order to have a wide addressing temperature range, and to have low viscosity in order to accelerate response speed, while maintaining the characteristics (1) to (5) described above. Enlarging xcex94n has also been requested by the requirement being smaller cell gap.
The liquid crystal composition has been always required to be improved though enthusiastic efforts are made for various purposes.
The object of the present invention is to provide a liquid crystal composition especially having a high voltage holding ratio at a high temperature region in order to enhance the contrast of a liquid crystal display device, having a wide range of a liquid crystal phase which means a high clearing point and good miscibility at low temperature, having low viscosity in order to accelerate response speed of the liquid crystal display device, and having properly large xcex94n in order to enhance the contrast in the liquid crystal display device, while satisfying general characteristics required for the liquid crystal composition of the AM-LCD.
The present inventors have made enthusiastic efforts to achieve the above object, have found the liquid crystal composition described below can achieve the purpose of the present invention, and have completed the present invention.
The liquid crystal composition of the present invention is described in the items 1 to 4 below.
1. A liquid crystal composition characterized by comprising at least one compound selected from the group of compounds expressed by formulae (1-1), (1-2) and (1-3) as a first component and comprising at least one compound selected from the group of compounds expressed by formulae (2-1), (2-2), (2-3), (2-4), (2-5), and (2-6) as a second component. 
xe2x80x83wherein R1 to R9 each independently represents alkyl or alkoxy having 1 to 10 carbon, alkenyl or alkoxymethyl having 2 to 10 carbon; R10 to R12 each independently represents alkyl or alkoxy having 1 to 10 carbon, alkenyl or alkoxymethyl having 2 to 10 carbon, F, Cl, CF3, OCF3, or OCF2H; X1 to X6 each independently represents F, CF3, OCF3, OCF2H or Cl; Y1 to Y11 each independently represents H or F; Z1 to Z4 each independently represents a single bond or xe2x80x94C2H4xe2x80x94.
2. The liquid crystal composition described in the item 1 described above characterized by comprising 3 to 45% by weight of the first component, and comprising 25 to 97% by weight of a second component each based on the total weight of the liquid crystal composition.
3. A liquid crystal composition comprising at least one compound selected from the group of compounds expressed by formulae (1-1) to (1-3) as a first component, comprising at least one compound selected from the group of compounds expressed by formulae (2-1) to (2-6) as a second component, and comprising at least one compound selected from the group of compounds expressed by formula (3) as a third component 
xe2x80x83wherein R13 and R14 each independently represents alkyl having 1 to 10 carbon.
4. The liquid crystal composition in the item 3 described above characterized by comprising 3 to 45% by weight of the first component, comprising 25 to 97% by weight of the second component, and comprising 25% and less by weight of the third component each based on the total weight of the liquid crystal composition.
The liquid crystal display device of the present invention is described in the item 5 below.
5. The liquid crystal display device composed by using the liquid crystal composition in any one of the items 1 to 4 described above.
Followings are explanation of preferable embodiments of compounds constituting the liquid crystal composition of the present invention.
Among the compounds expressed by formula (1-1) as the first component of the liquid crystal composition of the present invention, the compounds expressed by formulae (1-1-1) to (1-1-12) in the following are preferably used. In these formulae, R and Rxe2x80x2 each independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (1-2) as the first component of the liquid crystal composition of the present invention, the compounds expressed by formulae (1-2-1) to (1-2-12) in the following are preferably used. In these formulae, R and Rxe2x80x2 each independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (1-3) as the first component of the liquid crystal composition of the present invention, the compounds expressed by formulae (1-3-1) to (1-3-12) in the following are preferably used. In these formulae, R and Rxe2x80x2 each independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (2-1) as the second component of the liquid crystal composition of the present invention, the compounds expressed by formulae (2-1-1) to (2-1-30) in the following are preferably used. In these formulae, each R independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (2-2) as the second component of the liquid crystal composition of the present invention, the compounds expressed by formulae (2-2-1) to (2-2-40) in the following are preferably used. In these formulae, each R independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (2-3) as the second component of the liquid crystal composition of the present invention, the compounds expressed by formulae (2-3-1) to (2-3-20) are preferably used. In these formulae, each R independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (2-4) as the second component of the liquid crystal composition of the present invention, the compounds expressed by formulae (2-4-1) to (2-4-9) are preferably used. In these formulae, each R independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (2-5) as the second component of the liquid crystal composition of the present invention, the compounds expressed by formulae (2-5-1) to (2-5-9) in the following are preferably used. In these formulae, each R independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Among the compounds expressed by formula (2-6) as the second component of the liquid crystal composition of the present invention, the compounds expressed by formulae (2-6-1) to (2-6-9) are preferably used. In these formulae, each R independently represents alkyl or alkoxy having 1 to 10 carbon, or alkoxymethyl or alkenyl having 2 to 10 carbon. 
Followings are explanation on the role of compounds constituting the liquid crystal composition of the present invention.
The compounds expressed by formulae (1-1) to (1-3), which are the first component of the liquid crystal composition of the present invention, has features of a very large xcex94n being about 0.13 to 0.25, of relatively small xcex7 being 20 to 60 mPaxc2x7s compared with other three or four ring-compounds, and of a large value of specific resistivity. For this reason, the compounds expressed by formulae (1-1) to (1-3) of the present invention are used for the purpose to enlarge xcex94n, to minimize xcex7, and to adjust threshold voltage, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region. As the compounds expressed by formulae (1-1) and (1-2) have four rings and have very high TNI (the upper temperature limit of a liquid crystal phase) being 100xc2x0 C. to 180xc2x0 C., the liquid crystal compositions having a high TNI can be prepared by the use of these four ring-compounds.
The compounds expressed by formulae (2-1) to (2-6), which are the second component of the present invention, have features of large xcex94n being about 0.03 to 0.18, of large xcex94xcex5 being about 6 to 32, and of a large value of specific resistivity. For this reason, the compounds expressed by formulae (2-1) to (2-6) of the present invention are used for the purpose to keep large xcex94n, to widen a range of a liquid crystal phase, and to adjust threshold voltage lower, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
For further detailed explanation, the compounds expressed by formula (2-1), which are the second component of the present invention, have three rings and have xcex94n being about 0.03 to 0.10 and a large xcex94xcex5 value being about 6 to 13. They also have features of relatively small xcex7 and high specific resistivity. For this reason, the compounds expressed by formula (2-1) of the present invention are used for the purpose to decrease xcex7, and especially to adjust threshold voltage and xcex94n, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
The compounds expressed by formula (2-2), which are the second component of the present invention, have three rings and have relatively large xcex94n being about 0.10 to 0.14 and a large value of xcex94xcex5 being about 9 to 18. They also have features of relatively small xcex7 and high specific resistivity. For this reason, the compounds expressed by formula (2-2) of the present invention are used for the purpose to minimize xcex7, to keep An relatively large, and especially to adjust threshold voltage being small, while maintaining a high voltage holding ratio of liquid crystal composition at a high temperature region.
The compounds expressed by formula (2-3), which are the second component of the present invention, have four rings and have extremely a high TNI (the upper temperature limit of a liquid crystal phase) of 180xc2x0 C. or higher. They also have relatively large xcex94n being about 0.13 to 0.16 and relatively large xcex94xcex5 being about 10 to 14. Further they have features of relatively small xcex7 and high specific resistivity. For this reason, the compounds expressed by formula (2-3) of the present invention are used for the purpose to elevate the upper temperature limit of a liquid crystal phase of the liquid crystal composition, to keep xcex94n relatively large, and especially to adjust threshold voltage, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
The compounds expressed by formula (2-4), which are the second component of the present invention, have three rings and have considerably large xcex94xcex5 being 15 to 32 and xcex94n being about 0.05 to 0.09. They also have features of a large value of specific resistivity. For this reason, the compounds expressed by formula (2-4) of the present invention are used especially to reduce threshold voltage, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
The compounds expressed by formula (2-5), which are the second component of the present invention, have three rings and have considerably large xcex94xcex5 of about 15 to 32 and relatively a large value of xcex94n being about 0.1 to 0.15. They also have features of high specific resistivity. For this reason, the compounds expressed by formula (2-5) of the present invention are used especially to enlarge xcex94n and to reduce threshold voltage, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
The compounds expressed by formula (2-6), which are the second component of the present invention, have four rings and have extremely high TNI being about 100xc2x0 C. to 130xc2x0 C., considerably large xcex94xcex5 being about 15 to 30 and relatively a large value of xcex94n being about 0.14 to 0.18. They also they have features of a large value of specific resistivity. For this reason, the compounds expressed by formula (2-6) of the present invention are used to elevate the upper temperature limit of a liquid crystal phase, and especially to enlarge xcex94n and to reduce threshold voltage, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
The compounds expressed by formula (3), which are the third component of the present invention, have features of xcex94xcex5 being close to 0, of a large value of specific resistivity, of extremely large xcex94n of 0.2 or more which is extremely large, and of a high upper temperature limit of a nematic phase being 250xc2x0 C. or more. For this reason, the compounds expressed by formula (3) of the present invention are used for the purpose to elevate the upper temperature limit of a nematic phase, to adjust threshold voltage, and to enlarge xcex94n considerably, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
Summarizing the above, the liquid crystal composition of the present invention has features widening the range of a liquid crystal phase, especially minimizing xcex7 and enlarging xcex94n, while keeping a high upper temperature limit of the liquid crystal phase of the liquid crystal composition based on the first component, and further adjusting the range of the liquid crystal phase, xcex94n, and threshold voltage based on the second component.
Followings are explanation on preferable ratios in the contents of components which constitute the liquid crystal composition of the present invention.
The content of the first component in the liquid crystal composition of the present invention is preferably 3 to 45% by weight and the content of the second component therein is preferably 25 to 97% by weight each based on the total weight of the liquid crystal composition.
For further detailed explanation, the compounds expressed by formulae (1-1) to (1-3) are desirable to be mixed into the composition as much as possible to enlarge xcex94n and to minimize xcex7. However, they may elevate the lower temperature limit of a nematic phase of the liquid crystal composition if they are contained too much in the composition. For this reason, the compounds expressed by formulae (1-1) to (1-3), which are the first component of the present invention, are preferably 45% by weight and less based on the total weight of the liquid crystal composition. The ratio of the first component of the present invention in the liquid crystal composition is preferably 3% or more by weight to enlarge xcex94n, to widen the range of a liquid crystal phase, and to minimize xcex7, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
The compounds expressed by formulae (2-1) to (2-6), which are the second component of the present invention, may lower the upper temperature limit of a nematic phase of the liquid crystal composition, increase xcex7, and lower xcex94n, when they are contained in the composition in large quantities. For this reason, the ratio of the compounds expressed by formulae (2-1) to (2-6), which are the second component of the present invention, is preferably 97% and less by weight based on the total weight of the liquid crystal composition. The ratio of the second component in the liquid crystal composition is preferably 25% or more by weight to keep low threshold voltage and to lower the lower temperature limit of a nematic phase, while maintaining a high voltage holding ratio of the liquid crystal composition at a high temperature region.
The compounds expressed by formula (3), which are the third component of the present invention, may elevate the lower temperature limit of a nematic phase of the liquid crystal composition and may increase the threshold voltage because their xcex94xcex5 are close to 0, when they are contained in the composition in large quantities. For this reason, the ratio of compounds expressed by formula (3) of the present invention is preferably 25% and less by weight based on the total weight of the liquid crystal composition.
The method of the preparation on the compounds expressed by formula (1-3) of the present invention, for example, that on the compounds expressed by formula (1-3-2) is described in JP 10-251186 A. On the compounds expressed by formulae (2-1), (2-2) and (2-3), for example, those expressed by formulae (2-1-16), (2-2-11) and (2-3-12), the method of their preparation is described in JP 2-233626 A. On the compounds expressed by formulae (2-4), (2-5) and (2-6) of the present invention, for example, those expressed by formulae (2-4-5), (2-5-5) and (2-6-5), the method of their preparation is described JP 10-251186 A. On the compounds expressed by formula (3), the method of their preparation is described in JP 2-237949 A. Then, each compound constituting the composition of the present invention can be prepared based on prior arts.
To the liquid crystal composition of the present invention, liquid crystal compounds other than those expressed by formulae described above can be mixed in the amount not affecting the object of the present invention. The liquid crystal composition used in the present invention can be prepared by the conventional methods. In general, the method is that various compounds are mixed and dissolved each other at a high temperature. To the liquid crystal composition of the present invention, chiral doping agents such as cholesteryl nonanoate (CN) or CM-43L expressed by the following formula may be added in order to adjust to the required twist angle by inducing the spiral structure to liquid crystal molecules. 
The liquid crystal composition of the present invention can also be used for a guest-host mode by adding dichroic dyes such as phthalocyanine type, styril type, azo type, azomethine type, azoxy type, quinophthalone type, anthraquinone type and tetrazine type. It can also be used for a polymer-dispersed type liquid crystal display device, a birefringence control mode or a dynamic scattering mode. It can also be used for in-plane switching mode. Further, it is preferable for OCB mode utilizing its large xcex94n.