Liquid crystal phase

Details Liquid crystal phase Liquid crystal phase

1991-12-05

2001-07-10

Kelly, C. H. (Department: 1756)

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

C252S299010, C252S299500, C252S299650, C252S299630, C252S299640, C252S299660, C252S299670, C349S182000

Reissue Patent

active

RE037270

ABSTRACT:

The invention relates to liquid crystal phases (LC phases) having a particularly low threshold voltage and broad nematic phases.
BACKGROUND OF THE INVENTION
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 color 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 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 colorless.
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, the rule is to prepare mixtures of two to twenty-five, preferably three to fifteen, compounds 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 additionally mixed. 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 compound 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 to ZLI-1957/5 (commercially available mixture from E. Merck, Darmstadt, containing phenylcyclohexane, cyclohexylbiphenyl, biscyclohexylbiphenyl compounds and phenyl cyclohexylbenzoates), it has been possible to show (Hp. Schad and S. M. Kelly, 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 same time. In particular, for many applications their threshold voltages are still too high, and, in addition, the steepnesses of the electro-optic characteristic are not sufficient for highly informative displays owing to the relatively large ratio of the elastic constants for bending (K
3
) and spreading (K
1
)K
3
/K
1
. The hitherto disclosed mixtures therefore have an excessively high threshold voltage and/or an unacceptably poor steepness of characteristics (characterized by an excessively high K
3
/K
1
) and/or high temperature dependence of the threshold voltage dV/dT and/or large angular dependence of contrast.
There is thus still a great need for liquid crystal phases having high clear points, low melting points, a low viscosity (and hence short switching times) and a low threshold voltage, which at the same time have favorable electro-optic steepness and excellent viewing angle characteristics combined with a birefringence suitable for use in standard display cells.
SUMMARY OF THE INVENTION
The invention has for its object to prepare liquid crystal phases which have a nematic phase within the required temperature range and are completely or almost completely free from the abovementioned disadvantages.
It has now been found that the liquid crystal phase having particularly favorable combinations of material properties, in particular an acceptable combination of threshold steepness, viewing angle and temperature dependence at a low threshold voltage are obtained when they contain at least one component of the formula I
in which
R
1
is alkyl of 1 to 12 C atoms, in which in addition one or two non-adjacent CH
2
groups can be replaced by 0 atoms, —CO—, —O—CO—, —CO—O— and/or —CH═CH— groups,
A
o
and A
1
are each, independently of each other, Cy, Dio, Pyr or Phe,
Z is —CO—O—, —O—CO—, —CH
2
—O—, —O—CH
2
— or a single bond,
X
1
is F, Cl, —CN or —NCS,
X
2
is F, Cl and in the case of X
1
=NCS also H,
m is 0 or 1,
Cy is trans-1,4-cyclohexylene,
Pyr is pyrimidine-2,5-diyl or pyridine-2,5-diyl,
Dio is trans-1,3-dioxane-2,5-diyl and
Phe is 1,4-phenylene,
and at least one component of the formula III
R
1
—A
3
—COO—Phe—R
2
  III
in which
A
3
is Cy or Phe,
R
2
has one of the meaning of R
1
,
and R
1
, Cy and Phe have the meanings indicated above.
The invention thus provides the above-described liquid crystal phases which may also contain one or more pleochroic dyes (guest-host systems), and the use of these phases in liquid crystal display elements.
The invention further provides liquid crystal display elements, in particular electro-optical display elements which contain such phases.
The guest-host systems according to the invention generally contain 0.1 to 15, preferably 0.5 to 10, in particular 1 to 7, percent by weight of pleochroic dyes.
The pleochroic dyes of the guest-host system according to the invention are preferably chosen in such a way that they cover a suitable part of the visible spectrum and that the absorption in this range is more or less constant.
The preparation of the guest-host systems according to the invention is effected in conventional manner. In general, the desired amounts of the various pleochroic dyes are dissolved in the host material, preferably at elevated temperature.
However, it is also possible to mix solutions of the pleochroic dye and of the host material in a suitable organic solvent, for example acetone, chloroform or methanol, and to remove the solvent after thorough mixing, for example by distillation under reduced pressure. It is self-evident that with this method it is necessary to ensure that the solvent does not introduce any contaminants or undesirable dopants.
The individual compounds of the formula I to VI of the liquid crystal phases according to the invention are either known or preparable by methods which are easily derivable by those skilled in the art from the prior art, since they are based on standard methods described in the literature.
Corresponding compounds of the formula I are described for example in German patent applications P 34 05 914 and P 35 15 633; in European patent specification No. 0,019,665; in S. M. Kelly and Hp. Schad, Helvetica Chimica Acta, 67, 1580-1587 (1984); in S. M. Kelly, ibid, 67, 1572-1579 (1984); in Japanese Offenlegungsschrift 59-191,789 and European Offenlegungsschriften 0,099,099 and 0,119,756. Compounds of the formula II are described for example in German Offenlegungsschriften 29 44 905, 31 12 185, 31 46 249, 31 50 761, 31 50 763, 32 07 114, 32 27 916, 33 02 218, 33 34 054, in European Offenlegungsschrift 0,122,389, in U.S. Pat. No. 4,322,354 and in Japanese Offe

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