Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Reexamination Certificate
1994-04-08
2004-02-10
Kelly, Cynthia H. (Department: 1774)
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
C252S299660, C349S182000
Reexamination Certificate
active
06689291
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid-crystalline medium, to the use thereof for electro-optical purposes, and to displays containing this medium, and to novel compounds.
The main use of liquid crystals is as dielectrics in display devices since the optical properties of such substances can be affected by an applied voltage. Electro-optical devices based on liquid crystals are extremely well known to persons skilled in the art and may be based on various effects. Examples of devices of this type are cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells (“supertwisted nematic”), SBE cells (“superbirefringence effect”) and OMI cells (“optical mode interference”). The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability towards electrical fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have low viscosity and give short addressing times, low threshold voltages and high contrast in the cells. Furthermore, at customary operating temperatures, i.e., in the broadest possible range above and below room temperature, they should have a suitable mesophase, for example, a nematic or cholesteric mesophase for the abovementioned cells. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, the media desired for matrix liquid-crystal displays containing integrated nonlinear elements for switching individual pixels (MLC displays) are those having high positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high specific resistance, good UV and temperature stability of the resistance and low vapor pressure.
Matrix liquid-crystal displays of this type are known. Examples of nonlinear elements which can be used to individually switch the individual pixels are active elements (i.e. transistors). This is then referred to as an “active matrix”, and a differentiation can be made between various types, for example:
1. MOS (metal oxide semiconductor) transistors and
2. thin-film transistors (TFTs).
The use of monocrystalline silicon as a substrate material limits the display size since even the modular assembly of the various part displays results in problems at the joints.
In the case of type 1, as in the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. In the case of type 2, a differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries the transparent counterelectrode. 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-compatible image displays, in which a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable image element.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC display here covers any matrix display containing integrated nonlinear elements, i.e., in addition to 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 TV sets) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. In addition to problems with respect to the angle dependence of the contrast and the response times, problems result in MLC displays due to inadequate 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, September 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 Adressing [sic] of Television Liquid Crystal Displays, p. 145 ff, Paris]. As the resistance decreases, the contrast of an MLC display worsens and the problem of “after image illumination” may occur. Since the specific resistance of liquid crystal mixture generally decreases over the life of an MLC display due to interaction with the internal surfaces of the display, a high (initial) resistance is very important to give acceptable service lives. In particular in the case of low-voltage mixtures, it was hitherto not possible to achieve very high specific resistances. It is furthermore important that the specific resistance increases as little as possible with increasing temperature and after heating and/or exposure to UV radiation.
In particular in the case of high-resolution MLC displays, the use of materials from the prior art can have a considerable adverse effect on the image quality due to the occurrence of reversed tilt domains [E. Takahashi et al., Proc. 16th Japan. Liq. Cryst. Conference (1990), 212-213]. The MLC displays of the prior art do not meet current requirements.
It has hitherto been possible to prepare liquid-crystalline media having birefringence and phase range values necessary for practical use (for example clearing point of ≧70°) and having threshold voltages of only about 1.8 volts if values of about 98% for the holding ratio under extreme conditions (for example after UV exposure) are desired.
Thus, there continues to be a great demand for MLC displays having very high specific resistance and at the same time a broad operating temperature range, short response times and low threshold voltage which do not have these disadvantages or only do so to a lesser extent.
For TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
broadened nematic phase range (in particular down to low temperatures),
switchability at extremely low temperatures (outdoor use, automobiles, avionics),
increased stability to UV radiation (longer life).
The media available from the prior art do not make it possible to achieve these advantages while simultaneously retaining the other parameters.
For supertwisted (STN) cells, media are desired which have a greater multiplexing ability and/or lower threshold voltages and low rotational viscosity and/or low frequency dependence of &egr; and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further extension of the parameter latitude available (clearing point, smectic-nematic transition or melting point, viscosity, dielectric values, elastic values) is urgently desired.
Fluorophenylcyclohexene derivatives of the formulae
are disclosed in JP 58/018326, JP 57/154136, JP 58/018326, JP 59/082323A and JP 58/198428A.
DE 41 11 765 describes tetra- and pentafluorophenylcyclohexene derivatives of the formula
DE 41 13 424 C1 mentions the compound
as an intermediate.
DE 40 35 509 mentions compounds of the formulae
Compounds of the formula
in which R
1
is trans-4-alkylcyclohexyl, and R
2
is H, CN, halogen or alkyl, are disclosed in JP 05/058928.
JP 04/099739 describes fluorine-containing cyclohexene derivatives of the formula
Pauluth Detlet
Plach Herbert
Reiffenrath Volker
Kelly Cynthia H.
Merck Patent Gesellschaft mit Beschrankter Haftung
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