1. Field of the Invention
The invention relates to a liquid crystal compound, a liquid crystal composition and a liquid crystal display device. More specifically, it relates to a chlorofluorobenzene derivative, which is a liquid crystal compound, a liquid crystal composition having a nematic phase and containing the compound, and a liquid crystal display device containing the composition.
2. Related Art
A liquid crystal display device, which is represented by a liquid crystal display panel and a liquid crystal display module, utilizes optical anisotropy and dielectric anisotropy of a liquid crystal compound (which is a generic term for a compound having a liquid crystal phase, such as a nematic phase, a smectic phase and so forth, and also for a compound having no liquid crystal phase but being useful as a component of a composition), and as an operation mode of a liquid crystal display device, various modes have been known, such as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (OPS) mode, a vertical alignment (VA) mode, and so forth.
Among these modes, an ECB mode, an IPS mode, a VA mode and so forth are operation modes utilizing vertical orientation property of a liquid crystal molecule. In particular, it has been known that an IPS mode and a VA mode can eliminate a narrow viewing angle, which is a defect of the conventional modes, such as a TN mode and an STN mode.
As a component of a liquid crystal composition having a negative dielectric anisotropy, which can be used in a liquid crystal display device of these modes, various kinds of liquid crystal compounds, in which hydrogen on a benzene ring is replaced by fluorine, have been investigated (as described, for example, in WO 98/23561, JP H10-158652 A/1998, JP H02-4723 A/1990, JP 2002-193853 A, JP H10-237075 A/1998, JP H02-4725 A/1990, WO 89/08633, WO 89/08687 and EP 1333017).
For example, a compound (A) represented by the following structural formula has been investigated (as described in WO 98/23561). A compound, in which hydrogen on a benzene ring is replaced by chlorine and fluorine, represented by the compound (A) does not have a high clearing point and has a high viscosity. The compound is poor in stability to an ultraviolet ray.
As a dopant for a liquid crystal medium used in an electrochemical display, a compound (B), in which hydrogen on a benzene ring is replaced by chlorine and fluorine, and a terminal group has a branched structure, has been disclosed (for example, in JP H10-158652A/1998). The mesophase range where the compound exhibits liquid crystallinity is significantly narrow.
As a compound having cyclohexenylene, in which hydrogen on a benzene ring is replaced by fluorine, a compound (C) and a compound (D) have been reported (as described, for example, in JP H02-4723 A/1990 and JP 2002-193853 A). However, the compound (C) and the compound (D) are poor in compatibility in a low temperature range as compared to the other liquid crystal compounds.
Compounds having cyclohexenylene, in which hydrogen on a benzene ring is replaced by fluorine, have been reported as intermediates in, for example, WO 89/08633, WO 89/08687 and EP 1333017.

Accordingly, a liquid crystal display device having an operation mode, such as an IPS mode or a VA mode, still has problems as a display device compared to CRT, and for example, need improved response time and contrast, and decreased driving voltage.
The display device driven in an IPS mode or a VA mode is constituted mainly by a liquid crystal composition having a negative dielectric anisotropy, and in order to improve the aforementioned properties, a liquid crystal compound contained in the liquid crystal composition necessarily has the following properties (1) to (8):
(1) The compound is chemically stable and physically stable;
(2) The compound has a high clearing point (transition temperature from a liquid crystal phase to an isotropic phase);
(3) The compound has a low minimum temperature of a liquid crystal phase (such as a nematic phase and a smectic phase), particularly has a low minimum temperature of a nematic phase;
(4) The compound has a low viscosity;
(5) The compound has a suitable optical anisotropy;
(6) The compound has a suitable negative dielectric anisotropy;
(7) The compound has suitable elastic constants K33 and K11 (K33: bend elastic constant, K11: splay elastic constant); and
(8) The compound is excellent in compatibility with other liquid crystal compounds.
In the case where a composition containing a liquid crystal compound that is chemically and physically stable as in (1) is used in a display device, the voltage holding ratio can be increased.
In the case where a composition containing a liquid crystal compound having a high clearing point or a low minimum temperature of a liquid crystal phase as in (2) and (3) is used, the temperature range of a nematic phase can be enhanced, and a display device can be used in a wide temperature range.
In the case where a composition containing a compound having a small viscosity as in (4) or a large elastic constant K33 as in (7) is used in a display device, the response time can be improved. In the case where a composition containing a compound having a suitable optical anisotropy as in (5) is used in a display device, a contrast of the display device can be improved. Liquid crystal compounds having optical anisotropy varying over a wide range are necessary depending on design of a display device. In recent years, there is a tendency that the cell thickness is decreased for improving the response time, and accordingly, a composition having a large optical anisotropy is demanded.
Furthermore, in the case where a liquid crystal compound has a negatively large dielectric anisotropy, a liquid crystal composition containing the compound can have a low threshold voltage, and accordingly, a display device using a composition containing a suitable negative dielectric anisotropy as in (6) can have a low driving voltage and a small electric power consumption. In the case where a composition containing a compound having a suitable elastic constant K33 as in (7) is used in a display device, the driving voltage of the display device can be controlled, and the electric power consumption can also be controlled.
A liquid crystal compound is generally used as a composition by mixing with other various liquid crystal compounds for obtaining properties that cannot be exhibited with a sole compound. Accordingly, a liquid crystal compound used in a display device preferably has good compatibility with other liquid crystal compounds as in (8). Furthermore, a display device may be used over a wide temperature range including a temperature below freezing point, and therefore, the compound preferably exhibits good compatibility from a low temperature range.