Liquid Crystalline Substances

The invention relates to nematic liquid crystalline substances as additives in mixtures for electro-optical systems, which by utilizing the possibility of changing optical characteristics of liquid crystals, such as transparency, dispersion of light and double refraction, make possible the rendition of numbers, symbols and images.

The possibility of producing opto-electronic components of very low dimensions by utilizing the optical and dielectrical anisotropy of liquid crystals has been in use to a great extent for a long time. Particularly indication systems, which function according to the principle of electro-elastic deformation of twisted homogeneous nematic layers (Schadt-Helfrich-effect), are presently of main interest.

There are always utilized liquid crystal mixtures, because no individual substance can meet the high requirements with respect to mesophase range, chemical stability, viscosity, dielectric and elastic constants, etc. The great interest in the synthesis of new liquid crystalline substances proves that with respect to their various fields of application, the utilized mixtures still remain worthy of improvement.

The object of the invention is to provide new liquid crystalline substances of broad mesophase ranges which improve the chemical stability of the mixtures.

This and other objects and advantages of the invention will become evident from the description which follows.

It has been found that liquid crystalline 5-alkyl-pyridine-2-carboxylic acid esters of the general formula ##STR2## are suitable for utilization in electro-optical displays for the rendition of numbers, symbols and images, because they increase the chemical stability of mixtures, which in particular contain hydrolysis-sensitive compounds, for instance, 1,3-dioxane derivatives.

The substances possess broad mesophase ranges, and thus allow to considerably increase the clarification points of mixtures.

The invention will hereafter be more closely explained with examples of the embodiments.

1.97 g (0.01 mol) of 5-n-butyl-pyridine-2-carboxylic acid chloride dissolved in chloroform are added to the corresponding 4-substituted phenols which are also dissolved in chloroform. The starting reaction substance is heated for 10 minutes in a water bath under exclusion of moisture. Subsequently the solvent is removed at the rotary evaporator and the reaction mixture is heated again to 80.degree. C. The cooled mass is dissolved in ether and the ether solution is washed with a 2.5% Na.sub.2 CO.sub.3 solution and water. After drying over Na.sub.2 SO.sub.4 and the removal of the solvent, the residue can be recrystallized out of a little methanol.

Examples for the utilized substances according to the invention which are obtained in this way are given in Tables 1 to 3.

By means of DSC, the melting enthalpies have been determined for a few substances.

For the presentation of the changes of the clarification temperatures, to a mixture 14 of the following composition ##STR10## substances according to the invention are added.

The clarification temperatures and the electro-optical parameters are summarized in the following chart:

wherein

Klp=clarification point

U.sub.o =threshold voltage

.tau..sub.E.sup.50 =switching-on period at 50% change of intensity, 20.degree. C., 500 Hz

.tau..sub.A.sup.50 =decay period at 50% change of intensity, 20.degree. C., 500 Hz

d=thickness of layer

U=operating voltage

There have also been performed measurements for the change of the viscosity at addition of substances according to the invention.

To a mixture having the composition ##STR11## have been added 20 mol-% of the substances according to the invention. The result was a great increase of the clarification point.

It thus will be seen that there are provided liquid crystalline substances of high chemical stability as additives in nematic mixtures for electro-optical systems which attain the various objects of the invention, and which are well adapted for the conditions of practical use. As numerous alternatives within the scope of the present invention will occur to those skilled in the art, besides those embodiments, equivalents, alternatives and variations mentioned supra, it is to be understood that the invention is to be limited only by the scope of the appended claims, and functional and structural equivalents thereof.