Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2001-04-13

2004-07-20

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660

Reexamination Certificate

active

06764722

ABSTRACT:

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds having negative dielectric anisotropy, which comprises at least one compound of the formula I1
and at least one compound of the formula I2
in which
R
11
, R
12
and R
21
are each, independently of one another, 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 one or more CH
2
groups in these radicals may also, in each case independently of one another, be replaced by —O—, —S—,
—C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another,
Z is —C
2
H
4
—, —CH═CH—, —CF
2
O—, —OCF
2
— or a single bond, and
alkenyl is a straight-chain alkenyl radical having 2-6 carbon atoms.
Such media are particularly suitable for electro-optical displays with 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 alternatively DAP effect (deformation of aligned phases), 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 between the elastic constants K
3
/K
1
, high values for the optical anisotropy &Dgr;n and values for the dielectric anisotropy &Dgr;&egr; of from −0.5 to −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 utilizing the so-called IPS effect.
Technical use of this effect in electro-optical display elements requires LC phases which must satisfy a multiplicity of requirements. Particularly important here are chemical resistance to moisture, air and physical effects, such as heat, radiation in the infrared, visible and ultraviolet regions and direct and alternating electric fields.
Technically suitable LC phases 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 which have been disclosed hitherto includes a single compound which meets all these requirements, In general, therefore, mixtures of from 2 to 25, preferably from 3 to 18, compounds are prepared in order to obtain substances which can be used as LC phases. However, optimum phases could not be prepared easily in this way, since no liquid-crystalline materials of significantly negative dielectric anisotropy were hitherto available.
Matrix liquid-crystal displays are known. Non-linear elements which can be used for individual switching of the individual pixels are, for example, active elements (i.e. transistors). This is then referred to as an “active matrix”, and 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 the case of type 1, the electro-optical effect used is usually dynamic scattering or the guest-host effect. The use of single-crystal silicon as the substrate material limits the display size, since even modular assembly of various part-displays results in problems at the joints.
In the case of more promising type 2, which is prefer-red, the electro-optical effect used is usually the TN effect.
A distinction is made between two technologies: TFTs comprising compound semiconductors, for example CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive work is being carried out worldwide on the latter technology.
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 expanded to fully colour-compatible displays, in which a mosaic of red, green and blue filters is arranged in such a way that each filter element is located opposite a switchable pixel.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are backlit.
The term MLC displays here covers any matrix display containing integrated non-linear elements, i.e., besides the active matrix, also displays containing 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 inadequate resistivity of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, Sept. 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 drops. Since the resistivity 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 which must have acceptable resistance values over a long service life.
The disadvantage of the MLC-TN displays disclosed hitherto is due to their comparatively low contrast, relatively high viewing-angle dependence and the difficulty of generating grey shades in these displays.
EP 0 474 062 discloses MLC displays based on the ECB effect. However, the LC mixtures described therein, which are based on 2,3-difluorophenyl derivatives containing an ester, ether or ethyl bridge, have low “voltage holding ratio” (HR) values after UV exposure.
There thus continues to be a great demand for MLC displays which have very high resistivity at the same time as a broad operating temperature range, short response times and a low threshold voltage which can be used to produce various grey shades.
It is an object of the invention to provide MLC displays based on the ECB effect which do not have the abovementioned disadvantages, or only do so to a lesser extent, and at the same time have very high resistivities.
It has now been found that this object can be achieved if nematic liquid-crystal mixtures comprising at least one compound of the formula I1 and one compound of the formula I2 are used in these display elements.
The invention thus relates to a liquid-crystalline medium based on a mixture of polar compounds having negative dielectric anisotropy which comprises at least one compound of the formula I1 and at least one compound of the formula I2.
The mixture according to the invention has very favorable values for the capacitive threshold, relatively high values for the holding ratio and at the same time very good low-temperature stability.
Some preferred embodiments are mentioned below:
a) A medium which additionally comprises one or more compounds of the formula II:
 in which
R
2
is independently as defined for R
11
, R
12
and R
21
,
p is 1 or 2, and
v is from 1 to 6.
b) A medium which additionally comprises one or more compounds of the formula III:
 in which
R
31
and R
32
are each, i

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