Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2003-09-16

2004-09-14

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299620, C252S299630

Reexamination Certificate

active

06790489

ABSTRACT:

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of negative dielectric anisotropy which comprises at least one compound of the formula I
in which
R
11
is an alkyl or alkenyl radical having up to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF
3
or at least monosubstituted by halogen, where, in addition, one or more CH
2
groups in these radicals may be replaced by —O—, —S—,
 —C≡C—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another,
A
1
a) is a 1,4-cyclohexenylene or 1,4-cyclohexylene radical, in which one or two non-adjacent CH
2
groups may be replaced by —O— or —S—,
b) a 1,4-phenylene radical, in which one or two CH groups may be replaced by N,
c) a radical from the group consisting of piperidine-1,4-diyl, 1,4-bicyclo[2.2.2]octylene, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl, phenanthrene-2,7-diyl and fluorene-2,7-diyl,
 where the radicals a), b) and c) may be monosubstituted or polysubstituted by halogen atoms,
Z
1
is —CO—O—, —O—CO—, —CF
2
O—, —OCF
2
—, —CH
2
O—, —OCH
2
—, —CH
2
CH
2
—, —(CH
2
)
4
—, —C
2
F
4
—, —CH
2
CF
2
—, —CF
2
CH
2
—, —CF═CF—, —CH═CF—, —CF═CH—, —CH═CH—, —C≡C— or a single bond,
R
12
is alkenyl having 2-7 carbon atoms, and
m is 0, 1 or 2.
Media of this type can be used, in particular, for electro-optical displays having active matrix addressing based on the ECB effect and for IPS (in-plane switching) displays.
The principle of electrically controlled birefringence, the ECB effect or DAP (deformation of aligned phases) effect, was described for the first time in 1971 (M. F. Schieckel and K. Fahrenschon, “Deformation of nematic liquid crystals with vertical orientation in electrical fields”, Appl. Phys. Lett. 19 (1971), 3912). This was followed by papers by J. F. Kahn (Appl. Phys. Lett. 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869).
The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) have shown that liquid-crystalline phases must have high values for the ratio of the elastic constants K3/K1, high values for the optical anisotropy &Dgr;n, and values for the dielectric anisotropy &Dgr;&egr; of from −0.5 to about −5 in order to be suitable for high-information display elements based on the ECB effect. Electro-optical display elements based on the ECB effect have a homeotropic edge alignment. Dielectrically negative liquid-crystal media can also be used in displays which use the so-called IPS effect.
Industrial use of this effect in electro-optical display elements requires LC phases which have to meet a multiplicity of requirements. Particularly important here are the chemical resistance to moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet region and direct and alternating electric fields.
LC phases that can be used in industry are furthermore required to have a liquid-crystalline mesophase in a suitable temperature range and low viscosity.
None of the series of compounds having a liquid-crystalline mesophase that have been disclosed hitherto includes an individual compound which meets all these requirements. In general, therefore, mixtures of from two to 25, preferably from three to 18, compounds are prepared in order to obtain substances that can be used as LC phases. However, it has not been possible to prepare optimum phases easily in this way, since no liquid-crystal materials of significantly negative dielectric anisotropy and adequate stability were available hitherto.
Matrix liquid-crystal displays (MLC displays) of this type are known. Non-linear elements which can be used for individual switching of the individual pixels are, for example, active elements (i.e. transistors). The term “active matrix” is then used, where a distinction can be made between two types:
1. MOS (metal oxide semiconductor) transistors on a silicon wafer as substrate.
2. Thin-film transistors (TFTs) on a glass plate as substrate.
In type 1, the electro-optical effect used is usually dynamic scattering or the guest-host effect. The use of single-crystal silicon as substrate material restricts the display size, since even modular assembly of various part-displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect.
A distinction is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. The latter technology is being worked on intensively worldwide.
The TFT matrix is applied to the inside of one glass plate of the display, while the other glass plate carries the transparent counterelectrode on its inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully color-capable displays, in which a mosaic of red, green and blue filters is generally arranged in such a way that a filter element is opposite each switchable pixel.
The TFT displays known to date usually operate as TN cells with crossed polarizers in transmission and are back-lit.
The term MLC displays here covers any matrix display with integrated non-linear elements, i.e., besides the active matrix, also displays with passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TVs) or for high-information displays in automobile or aircraft construction. Besides problems regarding the angle dependence of the contrast and the response times, difficulties also arise in MLC displays due to insufficiently high specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, Sep. 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September. 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistance, the contrast of an MLC display deteriorates. Since the specific resistance of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the interior surfaces of the display, a high (initial) resistance is very important for displays that have to have acceptable resistance values over a long operating period.
The disadvantage of the MLC-TN displays disclosed hitherto is based on their comparatively low contrast, the relatively high viewing-angle dependence and the difficulty in producing gray shades in these displays.
EP 0 474 062 discloses MLC displays based on the ECB effect. The LC mixtures described therein are based on 2,3-difluorophenyl derivatives which contain an ester, ether or ethyl bridge and have low values for the voltage holding ratio (HR) after exposure to UV.
There thus continues to be a great demand for MLC displays having very high specific resistance at the same time as a large working-temperature range, short response times and low threshold voltage with the aid of which various gray shades can be produced.
The invention has an object of providing MLC displays which are based on the ECB or IPS effect, which do not have the above-mentioned disadvantages or only do so to a reduced extent, and at the same time have very high specific resistance values.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that these objects can be achieved if nematic liquid-crystal mixtures which comprise at least one compound of the formula I are used in t

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