Liquid Crystal Composition

This invention relates to a novel nematic composition for use in a liquid crystal display device.

The liquid crystal display device is an electro-optical display device which diffuses or modulates light rays from a light source by utilizing a thin layer of a nematic state composition in which the molecules are arranged in a specific pattern. This device comprises a first light-transmissible substrate material, a first light-transmissible and electrically conductive coating applied on the first substrate material, a second substrate material, and a second light-transmissible and electrically conductive coating applied on the second substrate material. The device, as generally known, is so constructed that when a voltage is applied between said conductive coatings placed facing each other with a gap between them, said gap having been filled with a nematic liquid crystal composition, the latter undergoes a change in its molecular arrangement, resulting in a velocity difference between the ordinary ray and the extraordinary ray of an incident light, a change in the amount of light absorbed, and scattering of light due to the current flowing through said nematic light crystal composition.

An optical display device, among these known examples, has employed a nematic liquid crystal which was developed for use in an electro-optical liquid crystal display device operable at temperatures (about 25.degree. C.) prevailing usually in residential structures.

There has been a strong demand, on the other hand, for a nematic liquid crystal material which may remain in the nematic state in a wider temperature range, because such a material would enlarge the operable temperature range of an optical display device, leading to exploitation of new application fields for the electro-optical light crystal display device.

This invention is based on the finding that a number of specific compounds among bis(4'-n-alkylbenzal)-2-methyl-1,4-phenylenediamines, ##STR2## and bis(4'-n-alkylbenzal)-2-chloro-1,4-phenylenediamines, ##STR3## (R represents an alkyl group) remain in the nematic state at temperatures over wide ranges and that a composition containing as one of the components at least one of said specific compounds also remains in the nematic state over a wide temperature range.

Thus, according to the present invention there is provided a nematic liquid crystal composition comprising at least one of the compounds of the formula, ##STR4## wherein X is methyl or chlorine, Y is a normal -C.sub.n H.sub.2n+1 group wherein n is an integer of 1 to 8, provided that when X is methyl n is an integer of 2 to 8, and when X is chlorine n is an integer of 1 to 6. Such a nematic liquid crystal composition is very useful for the electro-optical liquid crystal display device.

Before disclosing the invention in further detail, the procedure for synthesizing the present novel compounds is given below.

In 500 ml of benzene, were dissolved 0.2 mole of distilled and dried p-ethylbenzaldehyde, ##STR5## and 0.1 mole of distilled and dried 2,5-diaminotoluene, ##STR6## The resulting solution was refluxed for 4 hours, while removing the azeotropically distilled water by means of a Dean-Stark trap. Thereafter, the solvent was removed from the reaction mixture by distillation by use of a rotary evaporator. The distillation residue was collected and purified by repeated recrystallization from ethyl alcohol until a constant melting point was attained to obtain bis(4'-ethylbenzal)-2-methyl-1,4-phenylenediamine, ##STR7## (Compound A), which remained in a nematic state in the temperature range of 93.8.degree. - 182.5.degree. C. (hereinafter said mesomorphic temperature range is referred to as M.R.).

In a manner similar to that mentioned above, the following compounds were prepared. ##STR8##

2-Chloro-1,4-phenylenediamine sulfate, ##STR9## was dissolved in water, neutralized with an aqueous sodium hydroxide solution, and extracted with ethyl ether. The ether extract layer, after removal of the ether, was purified by recrystallization, repeated twice, from chlorobenzene, and dried to obtain 2-chloro-1,4-phenylenediamine, ##STR10## In 500 ml of benzene, were dissolved 0.1 mole of said 2-chloro-1,4-phenylenediamine and 0.2 mole of distilled and dried p-methylbenzaldehyde. The resulting solution was refluxed for about 4 hours, while removing the azeotropically distilled water by means of a Dean-Stark trap. Thereafter, the solvent was removed from the reaction mixture by distillation by use of a rotary evaporator. The distillation residue was collected and purified by repeated recrystallization from isopropyl alcohol until a constant melting point was attained to obtain bis(4'-methylbenzal)-2-chloro-1,4-phenylenediamine, ##STR11## (Compound H), which showed M.R. of 144.4.degree. - 196.6.degree. C.

In a manner similar to that mentioned above, the following compounds were prepared. ##STR12##

There is scarcely any single compound which remains in nematic liquid crystal state at temperatures in a wide range including those in the neighborhood of room temperature. Accordingly, attempts have been made to achieve the object by a mixture of two or more compounds.

According to this invention, there is provided a composition which remains in nematic state at temperatures over a wide range and, in addition, at temperatures in the neighborhood of room temperature, by mixing the present novel nematic liquid crystal compound with other compounds in a suitable proportion. Such a composition is very useful as a liquid crystal material for use in an electro-optical liquid crystal display device.

By way of illustration, but not by way of limitation, Examples of compositions according to this invention are listed below.