Compositions – Liquid crystal compositions – Containing pleochroic dye
Patent
1986-09-12
1989-04-04
Gron, Teddy S.
Compositions
Liquid crystal compositions
Containing pleochroic dye
25229961, 2522995, 25229963, 25229967, 350349, 350350R, C09K 1934, C09K 1954, C09K 1900, G02F 113
Patent
active
048184284
DESCRIPTION:
BRIEF SUMMARY
The invention relates to liquid crystal phases (LC phases) having a particularly low threshold voltage, a particularly low ratio K.sub.3 /K.sub.1 of the elastic constants for bending (K.sub.3) and spreading (K.sub.1) and broad nematic phase ranges.
Liquid crystal display elements (LC display elements) increasingly utilize the properties of nematic or nematic-cholesteric liquid crystal materials of significantly changing their optical properties such as light absorption, light scattering, birefringence, reflectance or colour under the influence of electric fields. The function of such display elements is based for example on the phenomena of dynamic scattering, the deformation of aligned phases, the guest-host effect, the Schadt-Helfrich effect in the twisted cell, or the cholesteric-nematic phase transition.
The industrial application of these effects in electronic components requires liquid crystal phases which must meet a large number of requirements. Of particular importance here are the chemical resistance to moisture, air and physical factors such as heat, radiation in the infrared, visible and ultraviolet region and continuous and alternating electric fields. Industrially utilizable liquid crystal phases are also required to have a liquid crystal mesophase within a suitable temperature range, a very low viscosity, a relatively low optical anisotropy, a high steepness of the electro-optical characteristic and adequate dissolving power for pleochroic dyes. Finally, they must not have any self-absorption in the region of visible light, i.e. they must be colourless.
None of the hitherto disclosed series of compounds with a liquid crystal mesophase contain an individual compound which meets all these requirements. For that reason, mixtures of two to twenty-five, preferably three to fifteen, compounds are generally prepared in order to obtain substances which are usable as liquid crystal phases. Usually at least one compound having a low melting and clear point is mixed with at least one compound having a higher clear point. The result is normally a mixture whose melting point is below that of the lower-melting component, while the clear point is between the clear points of the components. However, optimal phases cannot be easily prepared in this way, since the components having the high melting and clear points frequently also confer on the mixtures a high viscosity. As a result the switching times of the electro-optical display elements prepared therewith are changed in an undesirable manner.
To reduce the threshold voltage, hitherto disclosed LC phases have added to them strongly polar nematic compounds having a terminal cyano group. The effective dipole moment of these compounds, however, is distinctly reduced by a more or less strong antiparallel association of these molecules, so that relatively large amounts of polar compounds need to be added. This in turn gives rise to various disadvantages, such as unfavorable elastic properties of the LC phases, and high viscosity. On addition of 4-cyano-3-fluorophenyl p-alkylbenzoates as strongly polar components of ZLI-1975/5 (commercially available mixture from E. Merck, Darmstadt, containing phenylcyclohexane, cyclohexylbiphenyl, bis-cyclohexylbiphenyl compounds and phenyl cyclohexylbenzoates), it has been possible to show (Hp. Schad and S. M. Kelley, J. Chem. Phys. 81 (3), 1514-15 (1984)) that the threshold voltage is reduced, which is explained in terms of a reduced degree of association of the added compounds. However, even these LC phases do not meet all the abovementioned requirements at the ame time. In particular, for many applications their threshold voltages are still too high, and, in addition, the steepnesses of the characteristic are not sufficient for highly informative displays owing to the relatively large ratio of the elastic constants for bending (K.sub.3) and spreading (K.sub.1) K.sub.3 /K.sub.1). The hitherto disclosed mixtures therefore have an excessively high threshold voltage and/or an unacceptable poor steepness of characteristic (cha
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Krause Joachim
Scheuble Bernhard
Weber Georg
Gron Teddy S.
Merck Patent Gesellschaft mit beschrankter Haftung
Thomas J. E.
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