Optically anisotropic sheet containing discotic liquid...

Details Optically anisotropic sheet containing discotic liquid... Optically anisotropic sheet containing discotic liquid...

2000-03-10

2001-06-12

Thomas, Alexander S. (Department: 1772)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With viewing layer of specified composition

C349S117000, C349S121000, C558S271000, C558S265000

Reexamination Certificate

active

06245398

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optically anisotropic sheet containing discotic liquid crystal molecules twisted with an optically active compound. The invention also relates to a liquid crystal display of an STN mode and an optically active triphenylene compound.
BACKGROUND OF THE INVENTION
A liquid crystal display of an STN mode comprises a liquid crystal cell of an STN (Super Twisted Nematic) mode, two polarizing elements placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets (phase retarders) placed between the liquid crystal cell and the polarizing element.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecules. The rod-like liquid crystal molecules are twisted by using a chiral agent. A twist angle of the molecules is in the range of 180° to 360°.
Without an optical compensatory sheet, a liquid crystal display of an STN mode gives an image colored blue or yellow because of birefringence of rod-like liquid crystal molecules. The blue or yellow color is inconvenient for not only a black and white image but also a color image. An optical compensatory sheet has a function of removing color from an image to display a bright and clear image. The optical compensatory sheet also has a function of enlarging a viewing angle of a liquid crystal cell. As the optical compensatory sheet, a stretched birefringent film has been usually used. Japanese Patent Provisional Publication Nos. 7(1995)-104284 and 7(1995)-13021 disclose an optical compensatory sheet made of a stretched birefringent film for a liquid crystal display of an STN mode.
In place of the stretched birefringent film, an optically anisotropic sheet comprising an optically anisotropic layer on a transparent substrate has been proposed as an optical compensatory sheet. The optically anisotropic layer is formed by aligning discotic liquid crystal molecules and fixing the aligned molecules. The discotic liquid crystal molecules usually have large birefringence, and they also have various alignment forms. Accordingly, an optical compensatory sheet obtained by using the discotic liquid crystal molecule has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film. The optical compensatory sheet using the discotic liquid crystal molecule is disclosed in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. However, the disclosed optical compensatory sheet is designed to be used in a liquid crystal display of a TN (Twisted Nematic) mode.
The optical compensatory sheet containing the discotic liquid crystal molecules should be redesigned to be used in a liquid crystal display of an STN mode. In a liquid crystal cell of the STN mode, rod-like liquid crystal molecules are aligned according to a super twisted birefringent mode at a twist angle of larger than 900. The liquid crystal display of the STN mode can give a clear image with a large capacity according to a time-sharing addressing method even though the display has a simple matrix electrode structure (having no active matrix such as a thin film transistor or a diode).
The discotic liquid crystal molecules should be oriented in twisted alignment at an average twist angle of 90° to 360° to optically compensate the liquid crystal cell of the STN mode. The oriented molecules form a chiral discotic phase, which is preferably a chiral discotic nematic (Np*) phase.
Further, the discotic liquid crystal molecules are preferably aligned at an average inclined angle of 50° to 90°. Japanese Patent Provisional Publication No. 9(1997)-26572 discloses an optical compensatory sheet in which discotic liquid crystal molecules are twisted. The drawings of Japanese Patent Provisional Publication No. 9(1997)-26572 further illustrate those discotic liquid crystal molecules are essentially vertically aligned.
SUMMARY OF THE INVENTION
It is technically difficult to align discotic liquid crystal molecules uniformly (i.e., to orient the molecules in monodomain alignment) from the interface facing an orientation layer to the opposite interface facing the air, according to the disclosures of Japanese Patent Provisional Publication No. 9(1997)-26572.
For orienting the discotic liquid crystal molecules in twisted alignment, an optically active compound is used as a chiral agent. Japanese Patent Provisional Publication No. 9(1997)-26572 discloses a discotic liquid crystal compound having asymmetric carbon, which works as a chiral agent.
An object of the present invention is to provide an optical compensatory sheet particularly suitable for a liquid crystal display of an STN mode.
Another object of the invention is to provide an optically anisotropic sheet in which discotic liquid crystal molecules are oriented uniformly (in stable monodomain alignment) in twisted alignment.
A further object of the invention is to provide a liquid crystal display of an STN mode that can display a clear image with a high contrast, in which blue or yellow color caused by birefringence is reduced.
A furthermore object of the invention is to provide a new optically active triphenylene compound.
The present invention provides an optically anisotropic sheet which comprises a transparent substrate and an optically anisotropic layer, said optically anisotropic layer comprising discotic liquid crystal molecules, and said discotic liquid crystal molecules being oriented in twisted alignment, wherein the optically anisotropic layer further contains an optically active triphenylene compound represented by the formula (I):
in which each of R
1
, R
2
, R
3
, R
4
, R
5
and R
6
is independently an optically active monovalent group.
The invention also provides a liquid crystal display of an STN mode comprising a liquid crystal cell of an STN mode, two polarizing elements placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets placed between the liquid crystal cell and one or both of the polarizing elements, wherein the optical compensatory sheet comprises a transparent substrate and an optically anisotropic layer comprising discotic liquid crystal molecules in this order from the side of the polarizing element, said discotic liquid crystal molecules being oriented in twisted alignment at a twist angle of 90° to 360°, and said optically anisotropic layer further containing an optically active triphenylene compound represented by the formula (I).
The invention further provides an optically active triphenylene compound represented by the formula (II):
in which each of R
11
, R
12
, R
13
, R
14
, R
15
and R
16
is independently an alkyl group having 4 to 40 carbon atoms or a cycloalkyl group having 4 to 40 carbon atoms, said carbon atoms including at least one asymmetric carbon atom.
In the present specification, the term “average inclined angle” means an average of angles between discotic planes of discotic liquid crystal molecules and a surface of a transparent substrate (or a surface of an orientation layer). The present specification refers to alignment of liquid crystal molecules at an average inclined angle in the range of 50° to 90° as essentially vertical alignment of the molecules.
The applicants have found that the optically active triphenylene compound represented by the formula (I) works as an excellent chiral agent for discotic liquid crystal molecules. A triphenylene compound itself is a discotic liquid crystal compound. In the present invention, the triphenylene compound of the formula (I) is used as a chiral agent, while another discotic liquid crystal compound is used in the optically anisotropic layer. The discotic liquid crystal molecules are o

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