Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Patent
1985-12-16
1987-12-01
Gron, Teddy S.
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
2522996, 2522995, 350350R, 558414, 558419, 558423, 558425, 560138, 560141, 568642, 568643, 568645, 568647, 568660, 568661, 570129, 570182, 570184, C09K 1930, C07C12100, C07C12175, C07C12160, C07C 43225, C07C 2518
Patent
active
047103159
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
In electrooptical displays based on liquid crystals, which are also referred to as liquid crystal displays or LCDs, the liquid crystal phase is present as a dielectric between two sheetlike electrodes which are spaced from each other and are constructed in patterns or grids; selective excitation of predetermined electrode field regions has the effect of changing the position or orientation of the molecules of the dielectric within the regions of the selective excitation and thereby of bringing about a visible selective local modification of the dielectric.
This effect requires potentials whose size is dependent inter alia on the degree of anisotropy of the liquid crystal phase; this degree of anisotropy is expressed in general for a given liquid crystal substance by the difference (.DELTA.) of the dielectric constants (.epsilon.) parallel to the electric field (.epsilon..sub..parallel.) minus that perpendicular thereto (.epsilon..sub..perp.), i.e. by the relationship referred to in short as dielectric constant anisotropy or DCA .DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp..
In liquid crystal displays which are based on the principle of the twisted nematic cell (TN=twisted nematic; see W. Helfrich, and M. Schadt, Appl. Phys. Lett. 18 (1971) 127), a property which is dependent upon the DCA is the so-called threshold voltage, which in turn must not be excessively high for various reasons, such as CMOS compatibility, and which in practice can be the smaller, the higher the value of .DELTA..epsilon. is.
Since .DELTA..epsilon. is the difference of two values, the same difference can be the result of very different pairs of values, for example the resulting DCA being the same for
For a completely different reason, namely because of the matrix addressing, which is essential in particular for displays having high information content (for example within the range of up to 10.sup.6 image dots per display), and the attendant desirability of a very steep and high shoulder of discontinuity in the voltage/contrast diagram of the liquid crystal dielectric of the liquid crystal display, the value of the ratio .DELTA..epsilon./.epsilon..sub..perp. should in addition be small; from the above numerical example it will be readily understood that this ideal can be approached in two ways, namely: strongly positive or weakly negative DCA (see for example A. R. Kmetz, SID Digest, Tech. Papers IX (1078) 70); this has in particular the disadvantage that the mixtures tend to form smectic phases; not only in the longitudinal direction but also in the transverse direction, relative to the generally "stretched" form of the liquid crystal molecules (M. A. Osman, European Patent Application Publication No. 0,019,665); this has the advantage that the ratio .DELTA..epsilon./.epsilon..sub..perp. can be reduced by increasing the contribution and not or not necessarily by reducing the .epsilon..sub..parallel. contribution of the same molecule. The disadvantages of the compounds described in the citeed European patent application reside in their relatively high viscosity or relatively high melting points and in the usually difficult synthesis and/or low stability.
SUMMARY OF THE INVENTION
It is an object of the invention to specify a new class of anisotropic compounds which are suitable for liquid crystal displays or for liquid crystal mixtures and permit a substantially free choice of the .DELTA..epsilon. and .epsilon. contributions by reducing or eliminating the disadvantages of the known compounds.
This object is achieved according to the invention with anisotropic compounds of the formula (1) ##STR1## in which R is an alkyl, alkoxy, oxaalkyl, alkenyl or alkanoyloxy group which has in each case 1-12 C atoms in the alkyl moiety and which has a straight or branched and chiral or non-chiral chain, X.sup.2 and X.sup.3 are selected from hydrogen, halogen and nitrile, X.sup.4 is halogen or nitrile, Z is the methylene group --CH.sub.2 -- or the ether oxygen bridge --O-- and m and n are each 0, 1 or
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Kelly Stephen M.
Schad Hans P.
Gron Teddy S.
Merck Patent Gesellschaft mit beschrankter Haftung
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