Liquid Crystal Dyestuffs and Electro-Optic Devices Incorporating Same

This invention relates to novel liquid crystal electro-optic devices. More particularly, this invention relates to liquid crystal devices containing new benzene-fused heterocyclo-substituted phenylazo dyes.

The most popular liquid crystal cells, particularly for applications such as watch displays, are the so-called field effect liquid crystal devices. These devices employ nematic liquid crystals having positive dielectric anisotropy which align in a direction parallel to the cell's major walls in the off-state and realign in the perpendicular direction in the presence of an electric field. This effect can be viewed through crossed polarizers.

One way to improve the contrast between the on and off states is to add a liquid crystal dye that will change color when an electric field is applied. Several dyes that are soluble in nematic liquid crystals are known; 4"-cyano- or 4"-nitrobenzylidene-4'-(N,N-dialkylamino)-4-aminoazobenzenes have been disclosed in our copending application, Ser. No. 741,538, filed Nov. 12, 1976, U.S. Pat. No. 4,105,654, issued Aug. 8, 1978. These dyes have colors in the orange to red to brown hues.

Cholesteryl p-phenylazophenyl carbonates and carbamates can also be added to positive anisotropy nematic liquid crystals to impart yellow to red colors, as disclosed in our copending application, Ser. No. 642,728, filed Dec. 22, 1975, U.S. Pat. No. 4,032,470, issued June 28, 1977.

2,6-Substituted azonaphthalenes as liquid crystal dyes have been disclosed in copending application of Bloom, Ser. No. 757,374, filed Jan. 6, 1977.

White and Taylor have disclosed various pleochroic dyes that can be added to cholesteric liquid crystal hosts which undergo a phase transition to the nematic state when an electric field is applied, in particular thiazole-substituted phenylazo dyes, see Journal of Applied Physics, Vol. 45, 4718 (1974).

Liquid crystal dyes that can be added to nematic liquid crystals useful in field effect cells having improved contrast and in particular that have colors in the red to blue range, would be highly desirable.

We have found certain azophenyl dyes of the formula ##STR4## wherein R.sub.1 -R.sub.4 independently at each occurrence can be hydrogen, fluoro, chloro, bromo, alkoxy, cyano, nitro and dialkylamino or any two of R.sub.1 -R.sub.4 together can be part of an aromatic ring; X can be --NH, --NCH.sub.3, --S or --O; R.sub.5 and R.sub.6 independently can be hydrogen, fluoro, chloro, bromo or a lower alkyl; and R.sub.7 can be hydrogen, fluoro, chloro, bromo, alkoxy, cyano, nitro, dialkylamino, ##STR5## have the meanings given above and R.sub.8 is hydrogen, fluoro, chloro, bromo, alkoxy, cyano, nitro, dialkylamino, ##STR6## These dyes can be added in small amounts to nematic liquid crystals having positive dielectric anisotropy to impart colors in the magenta to blue range for liquid crystal cells having improved contrast.

The FIGURE is a cross-sectional view of an electro-optic device embodying the invention.

The azophenyl dyes of the invention are deeply colored dyes which are not liquid crystalline in themselves, but are soluble in nematic liquid crystals and can impart color and improved contrast to liquid crystal devices. Moreover, they have high order parameters which result in improved displays.

The above described dyes are prepared by diazotizing a benzoheterocycle and reacting with an appropriate substituted benzene compound. The product is isolated in known manner, as by column chromatography and recrystallization from a suitable solvent.

The dyes can be mixed with nematic liquid crystal compositions to impart a color characteristic of each dye. Low melting point nematic liquid crystal compositions, particularly p-alkoxybenzylidene-p'-alkylanilines and their mixtures with other liquid crystals and mixtures of p-alkoxy- or -acyloxybenzylidene-p'-cyanoanilines, have a transition temperature range that encompasses room temperature and are particularly suitable. The amount of dye to be added depends on the solubility of the dye in the particular liquid crystal composition employed and also on the color desired. In general, from about 0.05% by weight up to about 2% by weight of the liquid crystal mixture of the dye will be employed.

P-alkoxybenzylidene-p'-butylanilines are known liquid crystal compounds and are described for example in U.S. Pat. No. 3,829,491 which issued Aug. 13, 1974 to Strebel. Mixtures of p-methoxybenzylidene-p'-n-butylaniline (hereinafter referred to as MBBA) and p-ethoxybenzylidene-p'-n-butylaniline (hereinafter referred to as EBBA) have particularly broad and low useful temperature ranges that encompass room temperature. Mixtures containing about 35 to about 70% by weight of MBBA are preferred.

P-alkoxybenzylidene-p'cyanoanilines of the formula ##STR7## wherein R is lower alkyl are also known and are described in U.S. Pat. No. 3,499,702, issued Mar. 10, 1970, to Goldmacher et al. A mixture comprising about 85 weight percent of a 70:30 mixture of MBBA and EBBA and about 15% by weight of p-ethoxybenzylidene-p'-cyanoaniline has been described by Tarry, Services Electronic Research Laboratory Technical Journal, Vol. 23, No. 1, 1973. This mixture is particularly appropriate for field effect liquid crystal devices. P-acyloxy-benzylidene-p'-cyanoanilines have been disclosed by Castellano in U.S. Pat. No. 3,597,044.

In preparing an electro-optic device, liquid crystal compounds should be rigorously purified to remove ionic and nonionic impurities which may react to degrade the liquid crystal compounds either by decomposition, transubstitution reactions and the like.

In addition to the liquid crystal and dye, other additives such as homeotropic or chiral aligning agents and ionic organic compounds can also be added to the liquid crystal mixture, as is known.

Referring to the FIGURE, a liquid crystal cell is constructed from two glass plates 2 and 4 having conductive indium-doped tin oxide electrodes or coatings 6 and 8, respectively, on facing surfaces thereof. Thin silicon oxide layers 10 and 12 are evaporated onto the conductive layers 6 and 8, respectively, at an angle of 30 degrees. One-half mil (25.4 microns) thick glass frit spacers 14 and 16 maintain the coated glass plates 2 and 4 apart to complete the cell components. The cell is baked at 525.degree. C. to melt the glass frit and seal the cell except for a fill port. The cell is filled with the desired liquid crystal composition 18 in the isotropic state and hermetically sealed with solder.

The order parameter, S, is a measure of how well a particular dyestuff aligns in a liquid crystal host and is determined according to the following equation, wherein "OD off" is the optical density in the off state and "OD on" is the optical density in the on state at an absorption maxima of the dye in a liquid crystal cell: ##EQU1## When S is 1, the alignment would be perfect; thus the closer to 1, the better the alignment and the greater is the contrast between the on and off states of the liquid crystal cell.

The invention will be further described in the following examples, but the invention is not meant to be limited to the details disclosed therein. In the examples percent is by weight.

Part A

A solution of nitrosyl sulfuric acid was prepared by admixing 0.78 gram (11 mmols) of sodium nitrite in 10 ml of concentrated sulfuric acid which was cooled to 5.degree. C. and added slowly to a solution, cooled to 0.degree. C., of 1.95 grams (10 mmols) of 2-amino-6-nitrobenzothiazole in a mixture of 6:1 by volume of glacial acetic acid and propionic acid. The mixture was stirred at 0.degree.-5.degree. C. for one hour and the excess nitrosyl sulfuric acid destroyed with sulfamic acid. The cold diazonium salt solution was added slowly to 1.5 grams (10 mmols) of N,N-diethylaniline in 40 ml of the acetic acid-propionic acid mixture at 0-5.degree. C.

Anhydrous sodium acetate was aded to neutralize the excess mineral acid and the reaction mixture was stirred for two hours at 0.degree.-5.degree. C., when it was poured into 400 ml of ice water.

The resultant precipitate was filtered, washed with hot water and dried. The solid was dissolved in 50 ml of boiling benzene, filtered and the solution concentrated and chromatographed on a 250.times.40 mm column of silica gel using benzene as the eluant.

The product was further purified by repeated column chromatography using chloroform and ethylacetate/hexane as the eluants and then recrystallized from chloroform/hexane and ethylacetate/hexane.

A yield of 0.38 gram (11%) of the product, 2-(4'-N,N-diethylaminophenylazo)-6-nitrobenzothiazole of the formula ##STR8## was obtained as a black solid. The structure was confirmed by elemental analysis:

Theoretical, C, 57.46%; H, 4.79%.

Found, C, 57.25%; H, 4.21%.

Part B

A positive dielectric anisotropy liquid crystal mixture was prepared by stirring together 85 weight percent of a 70:30 molar mixture of EBBA and MBBA and 15 weight percent of p-ethoxy-benzylidene-p'-cyanoaniline. The product of Part A was added to the liquid crystal to make a 0.167% by weight solution of the dye.

The liquid crystal molecules were aligned parallel to the coated glass plates of the cell. The optical density, as seen through plane polarized light parallel to the liquid crystal orientation, was measured at the visible absorption maximum (550 nm) as a function of applied voltage. At 0 volts, the optical density was 0.224. At 10 volts the optical density was 0.041.

The dye order parameter therefore was 0.598.

The procedure of Example 1, Part A was followed except substituting 2-amino-6-ethoxybenzothiazole as the starting material.

2-(4'-N,N-diethylaminophenylazo)-6-ethoxybenzothiazole of the formula ##STR9## was obtained. The structure was confirmed by elemental analysis:

Theoretical, C, 64.40%; H, 6.21%,

Found, C, 63.84%; H, 6.24%.

The above dye was added to the liquid crystal mixture of Example 1, Part B as a 0.18% solution.

The optical density at teh visible absorption maximum (522 nm) was 0.599 at 0 volts and 0.122 at 10 volts.

The order parameter thus was 0.565.

The procedure of Example 1 was followed except substituting 2-amino-6-chloroimidazole as the starting material.

2-(4'-N,N-diethylaminophenylazo)-6-chlorimidazole having the formula ##STR10## was obtained. The structure was confirmed by elemental analysis:

Theoretical, C, 62.1%; H, 5,48%,

Found, C, 62.82%; H, 5.60%.

The above dye was added to the liquid crystal mixture of Example 1, Part B as a 0.192% solution.

The optical density at the visible absorption maximum (505 nm) was 0.574 at 0 volts and 0.156 at 10 volts.

The order parameter was 0.471.

The procedure of Example 1 was followed except substituting 2-amino-naptho[1,2]thiazole as the starting material.

2-(4'-N,N-diethylaminophenylazo)naphto[1,2]thiazole having the formula ##STR11## was obtained. The structure was confirmed by elemental analysis:

Theoretical, C, 70.0%; H, 5.55%.

Found, C, 69.78%; H, 5.49%.

The above dye was added to the liquid crystal mixture of Example 1, Part B as a 0.178% solution.

The optical density at the visible absorption maximum (539 nm) was 0.575 at 0 volts and 0.119 at 10 volts.

The order parameter was 0.561.

As a control, liquid crystal cells were prepared as in Example 1, Part B except substituting different dyes and the order parameters were measured.

2-(4'-N,N-diethylaminophenylazo)-5-chlorothiazole had an order parameter of 0.46 and 2-(4'-N,N-diethylaminophenylazo)-5-bromothiazole had an order parameter of 0.45, both less than the benzothiazole dye of Example 3.