Ferrielectric liquid crystal compound

Details Ferrielectric liquid crystal compound Ferrielectric liquid crystal compound

1999-07-14

2001-03-06

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299650, C349S143000, C349S184000, C560S076000, C560S083000

Reexamination Certificate

active

06197389

ABSTRACT:

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 of another, and to its use.
PRIOR ART
A liquid crystal display device (LCD) has been being widely used as a flat panel display as a substitute for a conventional display using a Braun tube (CRT), 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 is proposed and 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.
The above display method has problems in 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. Due to a high display quality, however, the above display method is about to surpass an STN display method which has been so far a mainstream and to overtake CRT.
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 millisecond (ms)). In the display of video rate, no good image quality can be obtained.
(2) A twisted state (twist alignment) of liquid crystal molecules is used for displaying, and the viewing angle is therefore narrow. In the display with a gray scale in particular, the viewing angle becomes sharply narrowed. That is, the contrast ratio, the color or the like change 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. 5-249502-A, 5-150257-A and 6-95080-A). However, at present, 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 hence, the driving is made difficult. In the display with a ferroelectric liquid crystal, it is very difficult to perform a gray-scale display since only a binary state 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 permanent spontaneous polarization.
However, in the AM driving, there is needed a liquid crystal material which can be at least driven at 10 V or less. But, 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.
An object of the present invention is 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.
A ferrielectric phase (Scy* 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, in 1989 (Japanese Journal of Applied Physics, Vol. 29, No. 1, 1990, pp. L131-137).
The chemical structural formula and phase transition temperatures (° C.) of the MHPOBC are as follows. Structural formula:
C
8
H
17
—O—Ph—Ph—COO—Ph—COO—C*H(CH
3
)—C
6
H
13
wherein Ph is a 1,4-phenylne group and C* is an asymmetric carbon atom.
Phase sequence:
Cr(30)SIA*(65)SCA*(118)SC&ggr;*(119)SC*(121)SC&agr;*(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&ggr;* is a chiral smectic C&ggr; phase (ferrielectric phase), SC* is a chiral smectic C phase (ferroelectric phase), SC&agr;* is a chiral smectic C&agr; phase, SA is a smectic A phase, and I is an isotropic phase.


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