Novel Yellow Azo Dyes and Dichroic Liquid Crystal Composition Made Therewith

This invention relates to yellow azo dye compounds, and more particularly, to dichroic liquid crystal compositions containing the yellow azo dyes.

Yellow dyes having improved properties are desirable for various applications including the dyeing of textiles and the coloring of plastics and liquid crystal displays. Among those properties which are desirable for dyes, especially for dyes used in liquid crystal displays, are photostability of the dye, suitable extinction coefficient of the dye, solubility of the dye in liquid crystal hosts and light absorption in the desired spectral region. In liquid crystal displays, yellow dyes are especially useful for modifying the color of other dyes or as additives for block dichroic formulations. The fabrication of black liquid crystal displays poses a particular problem in that no black dichroic dyes are known, and black liquid crystal displays are made by mixing dichroic dyes of various colors in proper proportions to obtain a material which absorbs light radiation in the spectral range of about 400-700 nm resulting substantially in a black color. The dyes used in such mixtures must have a high order parameter, preferably at least 0.75, to ensure satisfactory brightness and contrast ratio. Heretofore, the only available yellow dyes of high order parameter contained Schiff base linkages within the dye molecule, and the presence of a double bond in the base unit of the dye (exclusive of substituted groups and derivatives), leads to chemical and/or photochemical instability of the material.

Dye compounds suitable for use in solution with a liquid crystal material and containing azo linkages rather than Schiff base linkages in the dye molecule, are well known in the prior art. In U.S. Pat. No. 4,145,114, there is a disclosure of a substantial number of dye compounds suitable for use in liquid crystal materials and having an azo or azoxy linkage in the following general formula: ##STR3## or a substituted derivative of the foregoing formula in which any one of the positions on any one of the benzene rings is substituted by halogen, methyl, halogen-substituted methyl, or methoxy; wherein A is an azo or azoxy linkage group; n.sub.1 and n.sub.2 are each integers in the inclusive range 0 to 4; and Y.sub.1 and Y.sub.2 are each one of the following groups: (i) hydrogen; (ii) OR where R is an alkyl or arylalkyl group. Although the general structure of the dyes of U.S. Pat. No. 4,145,144 avoid the Schiff base linkages and thereby eliminate the chemical and/or photochemical instability disadvantages of materials containing the Schiff base linkages, it is always desirable to improve the classes of azo dyes and to improve liquid cyrstal displays containing azo dyes.

Accordingly, it is the primary object of the present invention to provide yellow azo dyes having a novel chemical composition.

It is another object of the present invention to provide novel liquid crystal compositions for use in liquid crystal displays.

Another object of the present invention is to provide yellow azo dyes having improved chemical and/or photochemical stability for use in liquid crystal compositions.

Another object of the present invention is to provide improved black dichroic liquid crystal displays made by mixing dichroic dyes of various colors.

Yet another object of the present invention is to provide a method of improving the black color of dichroic dyes used in black liquid crystal displays.

Still another object of the present invention is to provide a class of yellow dyes devoid of Schiff base linkages.

Other objects and advantages of the invention will become apparent as the description thereof proceeds.

These and other objects of the invention are achieved by providing aliphatic and aromatic esters of 1,4-bis(2-methyl-4-hydroxyphenylazo)benzene. A class of yellow azo dyes comprising the general formula: ##STR4## wherein R and R' are acyl groups having the formula: ##STR5## and R.sub.2 is an alkyl or substituted alkyl wherein the alkyl has at least 4 carbon atoms or R.sub.2 is an aryl, arylalkyl, substituted aryl, or substituted arylalkyl, has been discovered in accordance with the present invention. The acyl groups in the general formula form terminal ester groups at the 4-hydroxy position of the bis(2-methyl-4-hydroxyphenylazo)benzene.

In accordance with the present invention, there is also provided a liquid crystal composition for use in a liquid crystal display, comprising:

a host liquid crystal material; and

a guest dichroic dye dissolved in said host liquid crystal material, said dichroic dye having the general formula: ##STR6## wherein R and R' are acyl groups and form an ester with the general formula, and R and R' have the formula: ##STR7##

wherein R.sub.2 is alkyl or substituted alkyl wherein the alkyl has at least 4 carbon atoms, or R.sub.2 is an aryl, arylalkyl, substituted alkyl or substituted arylalkyl.

In another embodiment of the present invention, it has been found that when a liquid crystal composition for use in liquid crystal display comprises a host liquid crystal material and a guest yellow dye dissolved in said host liquid crystal material in conjunction with other dichroic dyes of various colors which absorb radiation within the spectral range of about 400-700 nm, improved black liquid crystal compositions can be obtained when said yellow dye has the general formula: ##STR8## wherein R and R' are acyl groups and form an ester with the general formula, and R and R' have the formula: ##STR9## wherein R.sub.2 is alkyl or substituted alkyl, and the alkyl has at least 4 carbon atoms, or R.sub.2 is an aryl, arylalkyl, substituted alkyl or substituted arylalkyl. Thus, improved black liquid crystal compositions and a method for improving the quality of black liquid crystal compositions are possibly by using the yellow dichroic azo dyes having at least one terminal ester linkage in the para-position on an ortho-methyl substituted phenylazobenzene in a dye having two azo linkages. Thus, the invention embraces a method of improving the black color of dichroic dyes for use in black liquid crystal displays, comprising adding to a host liquid crystal material, a dye composition having a mixture of dichroic dyes of various colors in proper proportion to obtain a material which absorbs radiation in the spectral range of about 400-700 nm wherein at least one of the dichroic dyes of various colors is a yellow azo dichroic dye having the general formula: ##STR10## wherein R and R' are acyl groups and form an ester with the general formula, and R and R' have the formula: ##STR11## wherein R.sub.2 is alkyl or substituted alkyl and the alkyl has at least 4 carbon atoms, or R.sub.2 is an aryl, arylalkyl, substituted aryl or substituted arylalkyl; and heating the composition to dissolve the dyes in the liquid crystal host material.

In accordance with the present invention, the chemical and/or photochemical instability of the prior art has been substantially reduced by using the azo linkages in the yellow azo dichroic dyes having terminal ester groups. Furthermore, a new class of yellow dyes has been synthesized, and liquid crystal displays of various colors can be fabricated by mixing at least one of the new class of yellow azo dyes with at least one dichroic dye of a different color and dissolving the mixture in a host liquid crystal material.

These and various other objects, features and advantages of the invention can be best understood from the following detailed description.

The yellow azo dyes or guest yellow azo dyes of the present invention have been synthesized by a reaction mechanism which produced the following general formula representing the class of yellow azo dyes: ##STR12## having a relatively elongated and rigid molecular structure. The yellow dyes have two azo (--N.dbd.N--) linking groups joining a benzene ring at the para-position to a second and third benzene ring having a methyl group substituted in the orthoposition (relative to the azo linkage) and an ester group substituted in the para-position (relative to the azo linkage), so that the para-substituted ester forms the terminal portion of the yellow azo dye molecule. The ester end groups or terminal ester groups normally designated as auxochromes, e.g., electron-withdrawing groups or electron-repelling groups, enhance the characteristic yellow color of the dyes by enhancing absorption of the blue end of the spectrum, e.g., 380-410 nm. R and R' in the general formula for the yellow azo dyes and guest yellow azo dyes, represent acyl groups which form the ester. In accordance with preferred embodiments of the present invention, R.dbd.R'; however, it is within the scope of the present invention to provide dyes of the general formula wherein R and R' comprise different acyl groups, and in fact, R and R' may represent any combination of the alkyl, substituted alkyl, aryl, arylalkyl, substituted aryl or substituted arylalkyl as specified in this invention.

The acyl groups of the present invention represented by R and R' in the general formula, have the following formula: ##STR13## The acyl group is a functional group, and as used herein, is generally the functional group of an acid chloride. In various embodiments, the acyl group of the yellow azo dyes of the present invention, may be an aromatic (aryl) group or a substituted aromatic group or mixtures thereof. However, the acyl group may also be an aliphatic (alkyl), substituted aliphatic, arylalkyl or substituted arylalkyl or mixtures thereof. Thus, in accordance with the present invention, R.sub.2 in the formula representing the acyl groups, may be aryl, arylalkyl, substituted aryl, substituted arylalkyl, alkyl having at least 4 carbon atoms, or mixtures of the foregoing.

The dyes of the present invention may be synthesized by any suitable manner. In one preferred method of synthesis, bis-(2-methyl-4-hydroxyphenylazo)benzene, a commercially available material, is reacted with the appropriate acid chloride in a suitable solvent, for example, pyridine. The mixture is heated for an appropriate time and at a suitable temperature to complete the reaction, e.g., about 1-4 hours at about 80.degree. C.-120.degree. C. Upon cooling, the yellow dye precipitates and is collected. In certain instances, hexane or other suitable aliphatic hydrocarbons may be used to promote precipitation. If both acyl groups on the dye molecule are the same, a slight molar excess of the appropriate acid chloride, such as anisoyl chloride, may be used. If the acyl groups differ on the same molecule, the acylation is carried out in two separate steps. The first acylation is carried out with about one mole of the appropriate acid chloride per mole of bis-(2-methyl-4-hydroxyphenylazo)benzene in a basic solution, such as, an organic base (pyridine), and the solution is heated at a suitable temperature until the reaction is complete, e.g., it is heated for three hours at 110.degree. C. A second acylation is carried out with about one mole of the appropriate acide chloride, per mole of the foregoing acylated bis-(2-methyl-4-hydroxyphenylazo)benzene in a basic solution such as an organic base (pyridine), and the solution is heated at a suitable temperature until the reaction is complete, e.g., it is heated for three hours at 110.degree. C. Thereafter, the solution is cooled. The yellow dye precipitates and is collected. A precipitating agent, such as n-hexane, may be added to promote the precipitation of the dye from the solution.

In preferred embodiments, the acyl group represented by the formula: ##STR14## is an aromatic or substituted aromatic group designated herein as an aryl or substituted aryl group. As used herein, aryl generally refers to a phenyl group or a naphthyl group. Examples of several acyl groups which form the ester linkage with the bis-(2-methyl-4-hydroxyphenylazo)benzene starting material, are shown in Table 1 below where the illustrated acyl group represents any combination of R and R' in the general formula of this invention but preferably where R.dbd.R'. In Table 1 below, the corresponding acid chloride for preparing the particular dye from the bis-(2-methyl-4-hydroxyphenylazo)benzene as discussed above, is also shown along with the order parameter (S) in liquid crystal E-7 described infra. In all cases, the color of the dye was light yellow or yellow. In dye number 2, the color of the dye was yellow and had maximum absorption at a wavelength of 385 nm, and in dye number 3, the color of the dye was yellow and had maximum absorption at a wavelength of 390 nm.

Other aromatic (aryl) acyl groups can be used in the general formula and can be prepared from the corresponding acid chloride. Other aryl groups include benzene (phenyl) and naphthalene (naphthyl) and the halogen, nitro, cyano, amino, alkyl, aryloxy, biaryl, alkoxy derivatives substituted upon the aryl group, mixtures and combinations of the foregoing derivatives substituted upon the aryl group, and the like. Other exemplary aromatic acyl groups preferred in accordance with the present invention include para-n-decylbenzoyl, p-ethoxybenzoyl, p-butoxybenzoyl, p-hexyloxybenzoyl, p-N,N-dimethylaminobenzoyl, p-N, N-diethylaminobenzoyl, p-N,N-dibutylaminobenzyl, p-hydroxybenzoyl, p-chlorobenzoyl, p-bromobenzoyl, p-benzoyloxybenzoyl, p-phenoxybenzoyl, p-toluyl, p-ethylbenzoyl, and p-phenylbenzoyl. Naturally, it is within the purview of one skilled in the art to add a considerable number of acyl groups to the foregoing list. As explained above, dyes of the general formula with different R and R' groups can by synthesized by the reaction of one mole of the bis-(2-methyl-4-hydroxyphenylazo)benzene with one mole of the appropriate acyl chloride to provide a reaction product having one acyl group thereon, followed by the reaction of one mole of the reaction product having one acyl group thereon with one mole of another appropriate acyl chloride to form the yellow azo dye composition having two different acyl groups thereon.

For example, in a first reaction, about 1.0 mole of benzoyl chloride per mole of the bis-(2-methyl-4-hydroxyphenylazo)benzene is heated at about 110.degree. C. for about three hours to form the following ester: ##STR24## In a second reaction, about 1.0 mole of p-anisoyl chloride per mole of the ester of formula (I) above is heated at about 110.degree. C. for about three hours under the reaction conditions discussed above to form the following ester (II): ##STR25## The yellow azo dye (II) is 1-(2-methyl-4-benzoyloxyphenylazo)-4-(2-methyl-4-anisoyloxyphenylazo)benze ne.

The acyl groups of the present invention may also be alkyl or substituted alkyl acyl groups wherein the alkyl or substituted alkyl has at least 4 carbon atoms. The acyl groups of the present invention can also be arylalkyl or substituted arylalkyl acyl groups. Thus, in accordance with the general formula of this invention, R and R' may be the foregoing enumerated acyl groups of the formula: ##STR26## wherein R.sub.2 is an alkyl or substituted alkyl and the alkyl has at least 4 carbon atoms, or R.sub.2 can be arylalkyl or substituted arylalkyl. The substituted groups may be any of the conventional derivatives well known in the art including the halogens (e.g., chlorine and bromine), hydroxy, nitro, cyano, amino, alkyl, aryloxy, alkoxy, and the like, and various combinations and permutations of the foregoing derivatives. In preferred embodiments, the foregoing acyl derivatives can be obtained by using the corresponding acid chloride in the synthesis described above. In accordance with the present invention, the acyl groups on the yellow azo dye molecule can be the same or different groups including different alkyl or substituted alkyl or different arylalkyl or substituted arylalkyl groups and can also comprise the foregoing groups mixed with aromatic acyl groups.

When the invention has alkyl or substituted alkyl acyl groups, in preferred embodiments, the alkyl or substituted alkyl has about four to about 20 carbon atoms. Preferred alkyl groups include n-butyl, iso-butyl, n-amyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl and stearyl and the like and the halogen (e.g., chlorine and bromine), nitro, amino, hydroxy and cyano derivatives thereof.

Specific examples of alkyl acid chlorides which may be used in a reaction with the bis-(2-methyl-4-hydroxyphenylazo)benzene described above in the appropriate molar quantities include n-butyryl chloride, iso-butyric acid chloride, n-valeryl chloride, iso-valeryl chloride, n-caprylic acid chloride (octanoyl chloride), pelargonic acid chloride (pelargonyl chloride or nonanoyl chloride), n-caproic acid chloride (n-caproyl chloride), n-capric acid chloride, stearic acid chloride (stearyl chloride), palmityl chloride, and the like.

Examples of the above yellow azo dyes having alkyl acyl groups are shown in Table 2 below where the acyl group and the corresponding acid chloride are illustrated:

To prepare the yellow azo dyes of this invention having alkyl acyl groups, substituted alkyl acyl groups, arylalkyl acyl groups, or substituted arylalkyl acyl groups, the corresponding acid chloride (acyl chloride) is reacted with the bis-(2-methyl-4-hydroxyphenylazo)benzene intermediate in the same manner described above for the preparation of the aromatic acyl compounds. The bis-(2-methyl-4-hydroxyphenylazo)benzene starting material is reacted with the appropriate aliphatic acid chloride or arylaliphatic acid chloride as discussed above by using the Schotten Baumann technique in an organic base such as pyridine. A slight molar excess of the appropriate aliphatic acid chloride is added to the bis-(2-methyl-hydroxyphenylazo)benzene to produce the yellow azo dye product. For example, slightly over 2 moles of stearyl chloride is added to the bis-(2-methyl-hydroxyphenylazo)benzene in pyridine solvent, and the mixture is heated for about three hours to about 110.degree. C. to form the product. Upon cooling, the yellow azo dyes precipitate and may be filtered from the solution. Conventional precipitating promoters, such as n-hexane, can be added to promote precipitation.

The yellow azo dyes prepared in accordance with this invention can be used in any conventional manner. For example, they can be used alone or in combination with other dyes, modifiers or adjuvants to dye textiles or to color plastic materials. They can be used alone or in combination with other dyes, modifiers or adjuvants to color liquid crystal displays. In preferred embodiments, the yellow azo dyes are used with liquid crystal host materials well known in the art to improve the color of the liquid crystal composition for use in a liquid crystal display. When the yellow azo dyes of this invention are used with a liquid crystal host, they are called guest dyes, and they are dissolved in the host liquid crystal material, that is, the guest yellow azo dye is soluble in the host liquid crystal material. Many of the yellow azo dyes require mild heat, e.g., above ambient to 110.degree. C. to promote their dissolution in the host liquid crystal.

Any liquid crystal host material may be used in accordance with the present invention. In those embodiments of the present invention wherein the yellow diazo dichroic dye material is used as a great dye in a host liquid crystal material, it is not critical which of the many well known liquid crystal materials or combinations thereof may be used as long as the yellow azo dye materials of the present invention are soluble in the host liquid crystal material and as long as the yellow azo dye or dyes of the present invention are compatible with or have no adverse effect upon the host liquid crystal material. In preferred liquid crystal compositions of the present invention, the host liquid crystal material is a nematic liquid crystal of positive or negative dielectric anisotropy, a cholesteric liquid crystal of positive or negative dielectric anisotropy and mixtures thereof, that is, mixtures of nematic liquid crystals and cholesteric liquid crystals modified with other optically active compounds.

Any conventional liquid crystal host material can be used with the yellow azo dyes of the present invention. The liquid crystal hosts can be selected by one skilled in the art depending upon the electro-optic effect to be utilized. Nematic liquid crystals include the biphenyl liquid crystals such as E7 and E9 from BDH, Ltd., phenylcyclohexanes and azoxy mixtures available from Merck Co., alkoxybenzylidene anils such as those having the structural formula: ##STR37## wherein OR is an alkoxy radical preferably having from 1 to 7 carbon atoms or an acyloxy radical having from 2 to 7 carbon atoms, p-anisylidene-p'-n-butylaniline, p-anisylidene-p'-aminophenylbutyrate, p-(p'-methoxyphenylazoxy)butylbenzene, p-(p'-ethoxyphenylazo) phenylheptanoate, p-n-hexylbenozic acid-p'-n-hexyloxyphenyl ester, and other liquid crystal materials, such as the esters disclosed in U.S. Pat. No. 3,984,344, p-n-hexylbenzylidene-p'-aminobenzonitrile, p-capryloxybenzylidene-p'-aminobenzonitrile, p-cycanophenyl-p'-n-heptylbenzoate, p-cyanobenzylidine-p'-n-butoxyaniline, p-cycanobenzylidene-p'-aminophenylvalerate, p-azoxyanisole, butyl-p-(p'-ethoxyphenoxycarbonyl)phenylcarbonate, p(p'-ethoxyphenylazo)phenylheptanoate, and the like. Another class of liquid crystal mateials have the general formula: ##STR38## wherein R is an alkyl or alkoxy group and m=0 or 1, at least one of said compounds being cyanobiphenyl wherein m=0. Generally, nematic liquid crystals fall within the class of chemical compounds having the general formula: ##STR39## wherein X is an azomethine linkage of the Schiff base class, X is an ester, vinylene, acetylene, azo or azoxy, or X is merely a single bond connecting the two benzene rings; and A and B are C.sub.n H.sub.2n+1 (alkyl group); C.sub.n H.sub.2n+1 O (alkoxy group); or C.sub.n H.sub.2n+1 COO (ester group). Cholesteric liquid crystals can be pure compounds such as derivatives of cholesterol or noncholesteric materials. These are characterized by optical activity arising from the absence of molecular symmetry.

In accordance with the present invention, it is not critical how much of the dye is present in the host liquid crystal as long as the dye is soluble therein. In preferred embodiments, about 0.05% by weight to about 1.0% by weight, and more preferably about 0.1% by weight to about 0.5% by weight, (based upon the weight of the liquid crystal host material) of the dye of this invention or mixtures of dyes of this invention are present in the host liquid crystal. One skilled in the art can adjust the amount of yellow azo dye as desired and can determine the amount required for maximum absorption at a particular wavelength. The upper limit of the amount of dye varies with the solubility of the particular dye in the host liquid crystal. The amount of guest dichroic yellow azo dye of the present invention in the host liquid crystal material is that amount up to the limit of maximum solubility in the host of yellow dye required to color, tint or shade the host liquid crystal, required to add to the yellow color of the host liquid crystal material, or required to contribute to the color of a mixture of dyes used in the host liquid crystal material, e.g., the use of the yellow dye or dyes of this invention with dyes of other colors to improve the "blackness" of guest dyes in the host liquid crystal. The guest yellow azo dichroic dyes of this invention are particularly useful in black liquid crystal displays because improved black liquid crystal materials can be obtained by mixing dichroic dyes of various colors in proper proportions to obtain a material which absorbs radiation in the spectral range of about 400-700 nm, and the yellow azo dyes of this invention substantially improve the absorption in the 400 region (the blue region) of such dye mixtures. Thus, in accordance with the present invention, an improved method of making black dichroic dyes or black dichroic liquid crystal displays is achieved by providing a host liquid crystal material and mixing therewith dichroic dyes of various colors in suitable proportions to obtain a material which absorbs radiation in the spectral range of about 400-700 nm wherein the dichroic dyes of various colors include a dye having the general formula: ##STR40## wherein R and R' are acyl groups and form an ester with the general formula, and R and R' have the formula: ##STR41## wherein R.sub.2 is an alkyl or substituted alkyl, and alkyl or substituted alkyl having at least 4 carbon atoms, or R.sub.2 is an aryl, arylalkyl, substituted aryl, or substituted arylalkyl. The appropriate mixture of dichroic dyes of various colors in suitable proportions to obtain a material which absorbs radiation in the spectral range of about 400-700 nm, and which approaches a substantially black appearance in color, is within the purview of one skilled in the art, and the improvement of the present invention is directed to the use of one or more of the yellow azo dyes having the designated acyl groups to improve the "blackness" of the dichroic dye mixture made from various colors. For example, the yellow azo dyes of this invention, when mixed with appropriate blue dyes in a liquid crystal, produce an improved black display. When about 1% by weight of the yellow azo dye is used with about 0.5% to about 1.0% blue dichroic dye or dyes, the black displays are substantially improved in a biphenyl liquid crystal, and depending upon the relative amounts of dyes used, displays of various shades of blue and black are obtained including greenish-black, blue-black, grey-black and greenish-blue. The yellow azo dyes of this invention used in such mixtures have substantially high order parameters (S) generally at least 0.75 and above, to ensure satisfactory brightness and contrast ratio.

Two types of liquid crystal displays having two different liquid crystal hosts were prepared with the dyes of the present invention. In one of the types of liquid crystal display, the yellow azo dyes were dissolved at a concentration of about 0.5-1% (by weight of the liquid crystal host material) in a nematic liquid crystal such as E-7 (from BDH Co.), ROTN-404 (from Hoffman LaRoche Co.) or Licristal-1132 (from E. Merck Co.). The resulting solution was sandwiched between conductive glass plates in test cells whose surfaces are in contact with the solution, the surfaces having been previously rubbed unidirectionally. When illuminated with polarized light, the display switches from yellow in the absence of an applied voltage to substantially colorless in the presence of a voltage. In the other type of liquid crystal display used to demonstrate the present invention, the liquid crystal host was a cholesteric liquid crystal prepared by the addition of an optically active compound, such as CB-15 (from B.D.H. Co.) to one of the above-mentioned nematic liquid crystal materials. The concentration of the optically active compound was varied from about 1% to about 10%. The yellow azo dyes of this invention were dissolved at a concentration of about 0.5-1% in the cholesteric host. Upon application of a voltage, the yellow display became substantially colorless.

Examples of specific commercial nematic liquid crystals include E-7 from BDH Chemicals wherein the chemical structure is of the biphenyl type, and the temperature range is -10.degree. to 60.degree. C. with a dielectric anisotropy of 11; E-8 from BDH Chemicals having a basic chemical structure of the biphenyl type with a temperature range of about -10.degree. to 70.degree. C. with a dielectric anisotropy of 13; Licristal 1132 from E. Merck of the phenylcyclohexane type having a temperature range of -6.degree. to 70.degree. C. with a dielectric anisotropy of 10; Licristal 1221 from E. Merck of the phenylcyclohexane type having a temperature range of -10.degree. to 90.degree. C. with dielectric anisotropy of 8; ROTN-103 from Hoffman-LaRoche of the ester type having a temperature range from about -10.degree. to 80.degree. C. with dielectric anisotropy of 26; ROTN-200 from Hoffmann-LaRoche of the Schiff base type having a temperature range of about -15.degree. to 65.degree. C. with a dielectric anisotropy of 18; and ROTN-404 from Hoffman-LaRoche of the biphenyl-pyrimidine type having a temperature range from about -10.degree. to 105.degree. C. and a dielectric anisotropy of 21.

The following examples further illustrate the practice of the invention, and they are meant to be exemplary only and are not to be construed as to limiting the invention in any way.

Liquid crystal displays of various color shades were fabricated by mixing one of the yellow azo dyes of the present invention which a dichroic dye of a different color. The following examples illustrate the utility of the novel dyes of the present invention. Unless otherwise specified, the yellow azo dye of the present invention which was used in the following examples was dye No. 3 in Table 1 above, otherwise designated as 1-4-bis [2-methyl-(4-pentyloxybenzoxy)phenylazo]benzene. This yellow azo dye was obtained by reacting 1 mole of bis(2-methyl-4-hydroxyphenylazo)benzene with 2.0 moles (preferably an excess) of 4-pentoxybenzoyl chloride dissolved in pyridine. The solution was heated under reflux for about three hours at about 110.degree. C. Upon cooling a yellow dye precipitated and was collected by filtration. The dye had a peak absorption wavelength of 390 nm and an order parameter in liquid crystal E-7 of 0.78.

The yellow azo dye prepared above with variable amounts of a blue dichroic dye, was added in various concentrations to a liquid crystal host mixture of E-7 biphenyl liquid crystal from BDH, Ltd.

Biphenyl liquid crystal E-7 is a eutectic of the following general formula: ##STR42## wherein R is a mixture of C.sub.5 H.sub.11, C.sub.7 H.sub.15, C.sub.8 H.sub.17 O and ##STR43## The blue dichroic dye was 4,4'-bis-(4-N-ethylaminonaphthylazo)azobenzene having an order parameter of about 0.72 and a maximum absorption wavelength of about 598 nm as described in U.S. Pat. No. 4,122,027. The mixture of dyes was dissolved in the E-7 liquid crystal. The foregoing solution of E-7, blue dye and yellow dye was heated to 90.degree. C. to dissolve the dyes in the host, and was filtered. The liquid was placed in a test cell. The test cell was a conventional liquid crystal cell having a distance of 18 microns between the plates and a polarizer external to the cell. The cell was filled with the above dichroic liquid crystal composition containing varying concentrations of the dye mixture. The boundary condition inside the cell was homogeneous, and as indicated above, the spacing between the cell substrates was about 18 microns. In the unenergized state, the colors were greenish-black, blue-black and greenish-blue depending upon the concentrations of the dye in the E-7 liquid crystal. Typical dye concentrations are on the order of 0.1% to 1.0% by weight of the individual dyes with respect to E-7 liquid crystal. The ratio of the concentration of the yellow dye to the blue dye can be varied 1:1 to approximately 2:1. Upon applying a voltage, the display became substantially colorless.

The same general conditions set forth in Example 1 including the E-7 liquid crystal host material were used in this experiment. Various amounts of violet dye 4,4'-bis(4-N,N-dimethylaminonaphthylazo)azobenzene described in U.S. Pat. No. 4,122,027 were mixed with 1,4-bis[2-methyl-4-(4-pentyloxybenzoxy)phenylazo]benzene and dissolved in liquid crystal E-7 (nematic). The formulation was heated to promote dissolution of the dyes in the host. The formulation was filtered, and the filtrate was place in a test cell as described above and having a spacing between the cell substrates of about 18 microns. Shades of maroon and red were obtained, depending upon the relative amounts of dyes used in the host material. The display switched from a red or maroon color to a substantially colorless state upon application of the appropriate voltage.

The dyes of Example 1 were dissolved in substantially same concentrations in a different liquid crystal host material. In this experiment, the liquid crystal host material was Licristal 1132, a phenylcyclohexane nematic liquid crystal having a temperature range from -6.degree. C. to 70.degree. C. from E. Merck Co. The foregoing formulation was heated as in Example 1, filtered and placed in a conventional liquid crystal test cell having a spacing between the cell substrates of about 18 microns. Results similar to those set forth in Example 1 were obtained.

In a flask fitted with a reflux condenser was placed anisoyl chloride and bis-(2-methyl-4-hydroxyphenylazo)benzene in a molar ratio of 2.2:1 respectively. Pyridine was added as a solvent, and the mixture was heated for three hours at about 110.degree. C. Upon cooling a yellow precipitate formed and was collected by filtration. The yellow dye had a peak absorption wavelength of 385 nm and an order parameter in E-7 liquid crystal of 0.76.

The utility of the yellow azo-ester dyes of this invention has been demonstrated. When used alone, the dyes can be used to provide yellow displays, or they can be used as additives in conjunction with other dyes to provide improved black dyes for black dichroic displays when used with liquid crystals. The dyes of the present invention can also be used as additives with other dyes to obtain unusual color effects in liquid crystal displays. Furthermore, the yellow azo-ester dyes of the present invention have been shown to be useful with a wide variety of liquid crystal compositions and are compatible with known liquid crystals, such as Schiff base, esters, azoxy, biphenyl, phenylcyclohexane, biphenyl/pyrimidine-type liquid crystals and the like. The novel yellow azoester dyes of the present invention can also be used for the dyeing of textiles, coloring of plastics or the coloring of any other media conventionally colored by azo dyes.

While other modifications of the invention and variations thereof which may be employed within the scope of the invention, have not been described, the invention is intended to include such modifications as may be embraced within the following claims.