WO2002054141A2

WO2002054141A2 - Discotic liquid crystal materials and o-plate compensation films made therefrom

Info

Publication number: WO2002054141A2
Application number: PCT/US2002/000059
Authority: WO
Inventors: Frank W. Harris; Stephen Z. D. Cheng
Original assignee: University of Akron
Current assignee: University of Akron
Priority date: 2001-01-02
Filing date: 2002-01-02
Publication date: 2002-07-11
Anticipated expiration: 2003-07-02
Also published as: WO2002054141A3; AU2002239812A1

Abstract

A photopolymerizable discotic liquid crystal compound is disclosed. The photopolymerizable discotic liquid crystal compound may be used as a component of a liquid crystal display optical compensator such as an O-plate compensator. A method of making a discotic liquid crystal compound is also disclosed.

Description

UA387

DISCOTIC LIQUID CRYSTAL MATERIALS AND O-PLATE COMPENSATION FILMS MADE THEREFROM

CROSS REFERENCE TO RELATED APPLICATIONS This application claims benefit of pending U.S. Provisional Application

Nos. 60/259,162 and 60/259,235, both filed on January 2, 2001.

BACKGROUND OF THE INVENTION

Liquid Crystal Displays (LCD) are currently used for a variety of display applications, such as watch faces, calculators, computer screens and other types of electronic equipment. LCD technology offers widely known advantages over traditional display technologies such as cathode ray tubes. Among these advantages are low weight and low power consumption.

Liquid crystal displays, however, have previously been considered to provide a narrower field of view than traditional display technologies, such as the previously mentioned cathode ray tubes.

Liquid crystal displays typically contain a plurality of liquid crystal cells. Each liquid crystal (LC) cell generally contains a liquid crystal material sandwiched between two substrates. Located on either side of the liquid crystal material is a set of electrodes which are typically indium-tin oxide (ITO) or tin oxide. A pair of polarizing filters is located outside of the substrates, with each filter on an opposite side of the liquid crystal cell. The polarizers are oriented at right angles relative to each other. The orientation of the liquid crystal material in the cell determines whether light passes through each polarizer in the absence of external influence such as an electric field, thereby giving a transparent appearance, or whether light is blocked by one of the polarizers, thereby giving the cell a darkened appearance. The orientation of the liquid crystal material is changed by the application of an electric field by the electrodes to alter light transmission through the cell. Typically, the liquid crystal material is aligned such that the cell appears opaque or transparent absent an application of an electric field through the electrodes. When an electric field is applied to such a cell, the orientation of the liquid crystal UA387 2 material is altered in such a way as to prevent the transmission of light through the cell, making the cell appear darkened.

The orientation of a liquid crystal material at its surface is dependent on the orientation of material it comes in contact with. It is known to coat the surface of a substrate with an agent which influences the orientation of a liquid crystal material that comes in contact with the coated substrate. Such coating agents are known as alignment layers. Optimally, an alignment layer material provides a uniform angle of orientation, also known as a pre-tilt angle. Various materials and methods have been used in establishing an alignment layer of a desired orientation. For example, it is known in the art that an alignment layer may comprise anisotropically absorbing molecules which can be oriented by exposure to polarized light. Inorganic thin films, such as metal oxide films, which have been deposited on a substrate at an oblique angle can also be used as alignment layers as disclosed in U.S. Patent No. 5,638,197. It is also known to use a polymeric alignment layer which can be oriented by means of a mechanical buffing process. In such a process, a polymer layer is applied to a substrate and is buffed with a cloth or other fibrous material. Liquid crystal material coming into contact with a surface treated in this way typically aligns itself parallel to the direction of buffing. Polyimides are frequently used as a polymeric alignment material for liquid crystal cells.

Polyimides may be used in optical compensator layers such as O-plate compensators. Polyimides generally display good chemical stability and are easily deposited on a substrate and rubbed. Polyimides are generally prepared by contacting a diamine with an acid anhydride, producing a polyamic acid. This polyamic acid may be coated onto a substrate and heat treated at about 150°- 230°C, converting the polyamic acid to a polyimide. The polyimide film is then mechanically rubbed as mentioned above.

Inducing the proper orientation of liquid crystal material is important in optical compensators. As mentioned above, LCDs frequently have a narrow field of view. It is frequently desirable to increase this field of view especially in applications such as computer displays, avionic displays and televisions. The viewing zone of an LCD that is not equipped with an optical compensator is narrow UA387 3

because light leaks through the liquid crystal material when viewed at angles other than those close to normal relative to the surface of the liquid crystal. Such light leakage degrades the image quality and can also cause color shifts in color LCDs. Optical compensators have been used to increase the viewable angle of LCDs without negatively affecting image quality when viewed normal to the surface of the LCD. Optical compensators typically take the form of an additional layer of liquid crystal material located between a polarizer and an analyzer within the LCD. This liquid crystal material may be given a specific orientation under the influence of an alignment layer material. O-plate compensation films, or O-plate compensators, are one type of optical compensator. O-plate compensators generally minimize reversal of gray levels and improve overall gray scale stability. O-plate compensators have been previously described as comprising a positive birefringent material which has a principle optic axis oriented at an oblique angle relative to the surface of the liquid crystal layer. An oblique angle includes any angle between 0° and 90°. In previous O-plate compensators, this angle has been provided in various ways. For example, U.S. Patent No. 5,619,352 describes an O-plate compensator which includes an alignment layer, a liquid crystal pretilt layer , and a liquid crystal compensator layer. The described O-plate compensator depends on the liquid crystal pre-tilt layer to provide an adequate pre-tilt angle for the liquid crystal compensator layer because the alignment layer produces only a 1° to 10° liquid crystal pretilt angle at the alignment layer/liquid crystal pre-tilt layer interface. The described O-plate compensator therefore depends on multiple layers of liquid crystal material to provide an adequate angle of orientation of the liquid crystal material. A similar O- plate compensator is also described in U.S. Patent No. 5,986,734 and PCT Application No. WO 96/10770.

U.S. Patent No. 5,583,679 also describes an O-plate compensator for a liquid crystal display. The O-plate compensator described in this patent comprises a transparent support and a discotic liquid crystal structural unit with a plane inclined from the plane of the support. This angle varies along the depth of the liquid crystal layer. It does not disclose an O-plate compensator with an angle of orientation that is independent of the thickness of the liquid crystal layer. It also UA387 4

does not disclose an O-plate compensator having a liquid crystal with a higher ordered columnar discotic phase.

U.S. Patent No.5,612,801 describes a "monolithic" O-plate compensator which comprises a plurality of layers that are deposited on a substrate. In this patent however, the term "monolithic" means that one layer is deposited on another in assembling the compensator. Therefore, this patent does not disclose an O-plate compensator that is effective with a single alignment layer and a single liquid crystal material layer.

It should be appreciated that the term "pre-tilt angle" has frequently been used in the prior art to describe a final angle provided by a combination of an alignment layer and a liquid crystal layer. Heretofore, no single polyimide alignment layer for a liquid crystal layer has provided a pre-tilt angle greater than about 15°.

O-plate compensators that provide a high angle of orientation in a single layer of liquid crystal material, independent of the thickness of that layer, have been previously unknown.

Therefore, there is a need for an O-plate compensator which provides an adequate angle of orientation of liquid crystal material in a single layer of liquid crystal material. There is also a need for an O-plate compensator which provides an angle of orientation of liquid crystal material that is independent of the thickness of the liquid crystal.

There is also a need for an O-plate compensator which is effective with a single alignment layer and a single liquid crystal material layer. There is also a need for an O-plate compensator which provides a liquid crystal layer which possesses a columnar discotic phase.

SUMMARY OF THE INVENTION

In general, the present invention provides a photopolymerizable discotic liquid crystal compound comprising the general structure: UA387 5

where n is a positive integer, and R' is hydrogen or a methyl group. A discotic liquid crystal compound having the above structure may be a component of a liquid crystal display optical compensator.

The present invention also provides a method of synthesizing a discotic liquid crystal compound. According to this method, a hydroxyalkyl benzoic acid is' alkylated with a halogenated alkyl alcohol to form a substituted benzoic acid. The substituted benzoic acid is then hydrolyzed to generate a hydroxyl-terminated benzoic acid, and the hydroxyl-terminated benzoic acid is contacted with an acid chloride to generate a hydroxyalkyloxyl benzoic acid. The hydroxyalkyloxyl benzoic acid is then contacted with a alkylhydroxy triphenylene compound to form a discotic liquid crystal compound.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The Figure is a schematic summary of a method of preparing a mesogenic group for attachment to a diamine to create a high pre-tilt polyimide alignment layer component of an O-plate compensator.

DETAILED DESCRIPTION OF THE INVENTION UA387 6

An optical compensator for liquid crystal displays eliminates reversal of grey levels and improves gray scale stability. The optical compensator of the present invention is comprised of an alignment layer that generates a high liquid crystal pretilt angle and a photopolymerizable discotic liquid crystal with the general structure shown below:

In this general formula, n is a positive integer. Preferably, n is between 6 and 11, and most preferably between 6 and 8. R' is hydrogen or a methyl group. It is believed that the discotic LC molecules of the present invention possess characteristic intrinsic uniaxial negative birefringence. It is also believed that the discotic LC molecules of the present invention may be useful as optical compensators in the absence of an alignment layer such as a C-plate compensator, for example. The discotic LC molecules of the present invention may also be used in conjunction with an alignment layer to form other types of optical compensators. Alignment layers that generate a high liquid crystal (LC) pre-tilt angle are believed to be particularly useful in such compensator.

In one embodiment, the alignment layer provides a pre-tilt angle between about 5° and about 90°. Preferably, the alignment layer provides a pre-tilt UA387

angle between about 10° and about 80°. More preferably, the alignment layer provides a pre-tilt angle between about 20° and about 80°. In one particular example, the alignment layer provides a pre-tilt angle between about 40° and about 70°. In one particular embodiment, the discotic liquid crystal material of the present invention is used in an O-plate compensator.

An O-plate compensator according to the present invention is constructed by applying an alignment layer onto a substrate. In the case of a polyimide alignment layer, the polyimide may be solution or spin cast as a film onto a substrate. Other alignment layer materials and methods of deposition may also be utilized. The pre-tilt angle provided by the alignment layer is preferably greater than 20° and most preferably greater than or equal to 40°. The discotic liquid crystal and a small amount of a photoinitiator is then solution or spin cast upon the alignment layer. After a heat treatment, the liquid crystal layer is crosslinked with UV light. The liquid crystal of O-plate compensator of the present invention possesses a columnar discotic phase.

A preferred alignment layer is a polyimide comprising a reaction product of at least one dianhydride and at least one diamine, wherein the at least one diamine contains a pendent mesogenic group, and wherein the pendent mesogenic group. In one example, the prndent mesogenic group is attached to the diamine by a linking group selected from the group consisting of an ester and an ether, and a methylene spacer. Polyimides may be schematically represented by the structure

wherein A is one or more residues from an acid dianhydride group and B is one or more residues from a diamine compound and n is a positive number. It has been known that the properties of the polyimide may be altered by varying the components "A" and "B" as listed above. However, the use of polyimides containing mesogenic substituents to prepare high pre-tilt alignment layers has not been previously known. In the present invention, mesogenic groups are contributed to the structure of a polyimide by the diamine component. Any acid dianhydride useful in the synthesis of polyimides may be utilized in the present invention. Such acid dianhydrides are commercially available.

As mentioned above, polyimide polymers are prepared from appropriately substituted diamines. Suitable diamines are represented by formulas I and II below.

(CH₂)_X— O — R₂

In formulas I and II, R^ is an ester or ether linking group, R2 is a mesogenic group or a functional group as described below, and x is a positive number. In formula I, R3 is hydrogen or a halogen. In one embodiment, x is between 6 and 18. In another example, x is between 6 and 11. In one particular example, x is 6. In another example, R3 is bromine. Mesogenic groups are groups with a rod-like molecular structure. That is, mesogenic groups, or simply mesogens, are groups UA387 9

with a length to width ratio of at least 5:1. Functional groups are those groups which allow one polyimide molecule to react with another molecule. Among preferred functional groups are groups which permit the crosslinking of polyimide molecules within a layer. Especially preferred functional groups include molecules which allow the photopolymerization of polyimide molecules, such as acrylate and methacrylate groups . Suitable mesogenic and functional groups include the following structures.

O CH₃

II I — c— C =CH_{2 (}πi)

In formula VI, X may be hydrogen or an organic group having from 1 to 20 carbon atoms and R4 may be an organic group selected from the group consisting of esters, ethers, groups containing a methylene subunit, groups containing a crosslinking subunit and groups containing a combination of any of these subunits. Groups containing acrylate or methacylate subunits may be crosslinked such as by photopolymerization, for example. In one example, when the at least one dianhydride is 2,2'-bis-(3,4-dicarboxyphenyl)-l,l,l,3,3,3-hexafluoropropane UA387 10 dianhydride or dibromo-biphenyltetracarboxylic dianhydride, the substituentis not represented by formula V.

By way of example and not of limitation, a mesogenic group shown in VI may be synthesized by the method as described below with reference to Figure 1. Ethyl 4-hydroxybenzoic ester is alkylated with 6-chlorohexanol to produce intermediate (1). Intermediate (1) is used in two different reactions. In the first reaction, the hydroxyl group of intermediate (1) is protected, using 3,4-dihydro-2- pyran (DHP) for example, forming intermediate (2). Intermediate (2) is then hydrolyzed to form a THP -benzoic acid derivative (4). In the second reaction, intermediate (1) is hydrolyzed to generate a hydroxy terminated benzoic acid (3) . The hydroxy terminated benzoic acid (3) is contacted with CH2CH2COCI in an organic solvent to form an intermediate (5). Tert-butyl dimethylsilyl chloride (TBDMS) may be used to protect methyl hydroquinone. The desired isomer is isolated, by chromatography for example, and is then reacted with the THP-benzoic acid derivative (4) forming intermediate (6). This reaction product is then selectively deprotected, for example, using tert-butylamonium fluoride (TBAF) in tetrahydrofuran (THF) , to form intermediate (7) . Intermediate (7) is esterified with intermediate (5) forming ester (8) and the resulting ester is deprotected, such as with AMBERLYSTR-15 in a mixture of methanol and THF, forming mesogenic group (9) . Various components used in this synthesis method may be varied to affect the composition of the resulting diamine without undue experimentation.

Mesogenic group (9) may be contacted with dinitro diphenic acid followed by tin (II) chloride reduction in ethanol to form a diamine of formula I. Such a diamine may be used to prepare a polyimide alignment layer in an optical compensator of the present invention.

Mesogens within the class encompassed by V may be synthesized in the following manner. 4-cyano-4'-hydroxybiphenyl may be contacted with a ω- bromoalkanol in an Sjψ2 reaction in refluxing acetone over 3-4 days. This results in the production of a 4-(ω-hydroxyalkoxy)-4'-cyanobiphenyl compound which may be further purified by recrystalization from ethanol. Alternatively, 4-cyano-4'- hydroxybiphenyl may be contacted with an ,(f> alkanediol in a Mitsunobu reaction to form a 4-(ω-hydroxyalkoxy)-4'-cyanobiphenyl compound. The product of the UA387 11

Mitsunobu reaction may be purified by flash chromatography. Mesogens within the class encompassed by IV may be synthesized by similar methods, by starting with a hydroxybiphenyl compound instead of 4-cyano-4'-hydroxybiphenyl.

These mesogens may be used to produce mesogen-containing diamine compounds of formula I by coupling the mesogen with a dinitro diphenic acid using the standard dicyclohexylcarbodiimide (DCC)/DMAP procedure to produce a dinitro intermediate compound. Alternatively, dinitro diphenic acid may be converted to 4,4'-dinitro-2,2'-biphenyl-carbonyl chloride by refluxing with thionyl chloride. The mesogen may be contacted with 4,4'-dinitro-2,2'-biphenyl-carbonyl chloride in an organic solvent such as triethylamine or methylene chloride to produce a dinitro intermediate. The dinitro intermediate may be reduced to form a diamine, by stannous chloride reduction or by reduction using hydrazine in an organic solvent at 80 °C, for example.

Mesogens of the present invention may also be coupled to brominated biphenylcarboxylic acids to produced brominated diamines of formula I. Cyanuric acid is contacted with bromine and the resulting compound was used to brominate 4,4'-dinitro-2,2'-biphenyl-carboxylic acid yielding 6,6'-dibromo-2,2'- biphenylcarboxylic acid. This brominated carboxylic acid may be coupled with a mesogen and reduced as described above to produce a brominated diamine. Diamines of formula II may be synthesized by the following technique.

3,5-dinitrobenzoic acid is esterified with n-octadecanol to afford n-octadecyl 3,5- dinitrobenzoate using DCC as a dehydration agent in dichloroethane. The dinitrobenzoate is reduced to n-octadecyl 3,5-diaminobenzoate using hydrazine as a reducing agent. By substituting other alcohols for n-octadecanol, the value of x in formula II may be varied.

Diamines of the present invention may be purified by chromatography on deactivated silica gel and subsequent recrystalization. Purified diamines may then be contacted with acid dianhydrides to produce polyimides. The synthesis of polyimides is known in the art. See for example, "Synthesis and Characterization of Aromatic Polyesters and Polyimides Containing Mesogenic Pendent Groups," PhD dissertation of Shyh-Yeu Wang, The University of Akron, December, 1995, the disclosure of which is herein incorporated by reference. Briefly summarized, UA387 12 polyimide precursors may be synthesized from dianhydrides and diamines by either a 2-step or a 1-step method. In the 2-step method, a soluble polyimide precursor, i.e., a polyamic acid, is prepared by the reaction of dianhydrate and diamine in a polar aprotic solvent at room temperature. The polyimide precursor is cyclodehydrated to form the corresponding polyimide either by thermal or chemical methods. The 2-step method gives high molecular weight polyimides if the diamine is highly reactive. However, when the diamine contains electron withdrawing groups such as CF3, CN and NO2, for example, the reactivity of the diamine is reduced and low molecular weight products result. When such electron withdrawing groups are present, the 1-step method is preferred. In the 1-step method, polymerization is carried out by heating the dianhydride and diamine at 180°-220°C in high boiling solvents, such as m-cresol and p-chlorophenol for example, in the presence of a tertiary amine catalyst. Under these conditions, polymerization and imidization occur essentially simultaneously. The water generated from imidization is continuously removed, such as by distillation for example.

The polyimide alignment layer of the present invention provides a high pre-tilt angle. In one embodiment, the alignment layer provides a pre-tilt angle between about 5° and about 90°. Preferably, the alignment layer provides a pre-tilt angle between about 10° and about 80°. More preferably, the polyimide layer provides a pre-tilt angle between about 20° and about 80°. In one particular example, the polyimide layer provides a pre-tilt angle between about 40° and about 70°. It will be appreciated that the pre-tilt angle described herein relates to the use of one single alignment layer with one single liquid crystal layer and not a plurality of layers to obtain the necessary or desired angle. A greater thickness of liquid crystal material is not required.

Further description of high pre-tilt polyimide alignment layers may be found in U.S. Provisional Application No. 60/259,235 "Polyimide -LCD Alignment Layers," and the corresponding non-provisional U.S. Pat. Application filed on the same day as the present application, the disclosure of which is hereby incorporated by reference. UA387 ' 3

By way of example and not of limitation, 2,3,6,7,10,11 hexa[4-(6- acryloyloxy-n-hexyloxy)benzo]triphenylene, a discotic liquid crystal material of the present invention, may be synthesized by the following method. 4- (6- hydroxyhexyloxyl) benzoic acid is synthesized by alkylating a commercially available ethyl 4-hydroxybenzoic ester with 6-chlorohexanol. Other halogenated alcohols may also be used in place of the 6-chlorohexanol to produce other discotic liquid crystal materials. The resulting intermediate is then hydrolyzed to generate a hydroxyl-terminated benzoic acid, which is treated with acryloyl chloride in dioxane in the presence of N,N-dimethylaniline to generate 4-(6-hydroxyhexyloxyl) benzoic acid. The 4-(6-hydroxyhexyloxyl) benzoic acid is then placed in THF and pyridine. The mixture is cooled to 0° and acryloyl chloride is added dropwise. Other acid chlorides may also be substituted for acryloyl chloride to produce other discotic liquid crystal compounds. The mixture is then stirred overnight and the solid product is removed by filtration. The filtrate is then poured into water and the resulting precipitate is collected by filtration and washed with water. The 4- (6- acryloyloxy)hexyloxyl benzoic acid product may then be purified by recrystalization from isopropanol. The 4-(6-acryloyloxy)hexyloxyl benzoic acid is then contacted with 2,3,6,7,10,11 hexahydroxytriphenylene in the presence of 4- (dimethylamino)pyridiniump-toluenesulfonate (DPTS) in acetone under nitrogen. The mixture is stirred for about 10 minutes and DGC is added. The mixture is then stirred for several days at room temperature and then filtered. The filtrate is then concentrated under reduced pressure to yield a milky viscous liquid. The 2,3,6,7,10,11 hexa[4-(6-acryloyloxy-n-hexyloxy)benzo]triphenylene product is purified by chromatography on silica gel using acetone/hexane (1:2, v:v) as the eluent.

In order to demonstrate the practice of the present invention, O-plate compensators were produced using high-pretilt polyimide alignment layers, as described below. The photopolymerizable liquid crystal components Cmallx, Cma6x and Ca6x were solution-cast or spin-cast onto alignment substrates. The structures of these liquid crystals are shown below. UA387 14

O CH₃ 11 1

Cmallx R= — C — ( ) ι >— O — (CH₂)n-O — C — C =CH₂

Cmaόx R=

The film thickness was controlled by adjusting the solution concentration and spinning rate. A heat treatment was carried out at a predetermined temperature after the coating had been deposited. The films, which contained UV initiators, were then crosslinked with UV irradiation.

Upon UV irradiation, the discotic LC molecules of the present invention undergo a crosslinking reaction to form uniaxially negative birefringence films, in which the in-plane refractive index (n(TE)) is different than that of the out-of- plane refractive index (n(TM)). Within the planar direction, the array of discotic LC molecules is randomly stacked with their normal axis perpendicular to the substrate. In order to achieve anisotropic orientation of in-plane discotic LC UA387 15 molecules, one must use surface alignment technology to align the discotic LC molecules. The use of rubbed polyimide alignment layers that generate high pretilt angles is one way to achieve an oblique optical axis within negative retardation films. Therefore, the in-plane optical properties parallel to the rubbing direction may be different from those perpendicular to the rubbing direction. It is also believed that this topological arrangement can be retained after UV crosslinking, and that the oblique optical axis and negative birefringence are stable with respect to the orientation direction. In the following examples, the anisotropic optical birefringence was measured using a Metricon prism coupler method based on different refractive indices of the in-plane n(TE) and out-of-plane n(TM) modes. The optical properties of the Cmallx films on a high pretilt angle alignment layer-coated glass substrate are summarized in Table 1. Cmallx films were formed on alignment layers providing a 10° or an 18° pre-tilt angle. The in- plane (n(TE)) and out-of-plane (n(TM)) optical responses of the films relative to the rubbing direction of the alignment layer were tested in duplicate for each direction. As shown in Table 1, the linear optical in-plane (n(TE)) responses along different directions in the films varied. The optical birefringence of the film, which is the difference between the out-of-plane refractive index and the in-plane refractive index, is expressed as Δn. In all cases, the films exhibited negative birefringence . The anisotropic optical properties are indicative of films with oblique optical axes.

UA387 16

Table 1

Anisotropic optical films of discotic liquid crystals (Cma 1 lx) oriented on high pretilt polyimide alignment layers.

A prototype O-plate compensation film of a Ca6x compound was also constructed on a high pretilt (39°) alignment substrate using solution casting. The in-plane (n(TE)) and out-of-plane (n(TM)) optical responses of the films relative to the rubbing direction of the alignment layer were tested in duplicate for each direction, as described above. The observed properties of the films at a thickness of 20 micrometers are summarized in Table 2. Along the rubbing direction, the optical birefringence (Δn) and retardation values (Δn x the thickness of the film) are larger than those along the other directions. However, the out-of-plane refractive index n(TM) is very similar along the different orientation directions.

The optical retardation values are between about 80 and about 240 nm, which is useful for optical applications. Additionally, after UV irradiation, an oblique optical axis could be seen by the naked-eye when viewing along the rubbing direction with respect to the substrate. UA387 17

Table 2

Anisotropic optical films of discotic liquid crystals (Ca6x) oriented on the high pretilt polyimide alignment layers with a thickness of 20 microns.

A prototype photo-polymerizable liquid crystal film was made using a high pretilt polyimide alignment layer to achieve the O-plate with Ca6x. The photopolymerizable LC compounds were spin-cast onto the polyimide alignment substrate with a pretilt angle of 39° and the thickness was controlled to approximately 4 μm. A heat treatment at a preset temperature of 100°C was carried out after the coating had been applied. The photopolymerizable LC films are generated by UV irradiation after the thermal treatment. The resulting film optical properties were observed as described above and are listed in Table 3. The optical data for commercially available (Fuji) retardation films are listed in Table 4 for comparison. After UV irradiation, the Ca6x discotic LC molecules are believed to undergo a crosslinking reaction to form uniaxially negative birefringence films, in which the in-plane refractive index (n(TE)) is larger than that of the out-of-plane refractive index (n(TM)). In this case, the optical birefringence parallel to the rubbing direction is larger than that perpendicular to the rubbing direction. The desired retardation values of the optical films ranged from about 100 nm to about 400 nm.

It is evident that the optical performance of films made according to the present invention and that of commercially available films are comparable. Untreated films of Ca6x are semi-solid at room temperature. They also show haziness when crosslinked at room temperature, which is believed to be caused by UA387 18 a highly ordered columnar or other ordered phase. However, at elevated temperature, the compound's viscosity is relatively low before UV curing. Thus, it can easily be oriented on alignment layers. Therefore, in some applications, it may be desirable to treat the films at about 100°C for 15 minutes, for example, and then photo-crosslink the LC compounds at that temperature to avoid the higher ordered phase structures.

As shown in Table 3, the results indicate that the linear optical in-plane responses of the films along the different directions with respect to the rubbing direction are anisotropic, which is the first sign of a negative birefringent film with an oblique optical axis. The optical birefringence is approximately 0.037-0.040 perpendicular to the rubbing direction, while the optical birefringence is 0.0612- 0.0635 in the direction parallel to the rubbing direction. These optical birefringence results on the alignment surface are obviously larger than those data without alignment substrates. This indicates that both the orientation along the out-of-plane and in-plane direction can be improved on the alignment layer substrates. Thus it is believed that the molecular packing arrangement of discotic LC molecules becomes more ordered in both of the directions.

In the commercial retardation films, the optical retardation values are between 0.0440 and 0.0458 perpendicular to the rubbing direction, and the retardation values are between 0.0617 and 0.0622 parallel to the rubbing direction (see Table 4) . Therefore, the optical birefringence and retardation values parallel to the rubbing direction are greater that those perpendicular to the rubbing direction. However, the out-of-plane refractive index is almost constant along the different orientation directions. The optical retardation values range from 150 nm perpendicular to the rubbing direction to 250 nm parallel to the rubbing direction. After UV irradiation, an oblique optical axis can be directly observed by the naked- eye from different angles, which correspond to different directions with respect to the rubbing direction. Since the thickness of the top-layer of the commercial films is difficult to measure, the retardation values of these films cannot be evaluated. Both the Ca6x films and the commercially available films show the similar intensity of the optical birefringence between two cross polarizers in a light box. UA387 19

Table 3

Negative optical films of discotic liquid crystals (Ca6x) oriented on the high pretilt polyimide alignment layers with a thickness of 4 μm.

Table 4

Optical properties of commercial retardation films along different directions.

It is also envisioned that the Cma6x crosslinkable compound is a highly desirable compound for use in O-plate compensators because it displays a nematic phase at room temperature with a low viscosity.

Based upon the foregoing disclosure, it should now be apparent that the novel discotic liquid crystal and O-plate compensator described herein will carry out the objects set forth hereinabove. It is, therefore, to be understood that any variations evident fall within the scope of the claimed invention and thus, the UA387 20 selection of specific component elements can be determined without departing from the spirit of the invention herein disclosed and described. In particular, high pre-tilt alignment layers used in an O-plate compensator according to the present invention are not necessarily limited to those having a polyimide composition. Thus, the scope of the invention shall include all modifications and variations that may fall within the scope of the description above.

Claims

UA387 21CLAIMS

We claim: 1. A photopolymerizable discotic liquid crystal compound comprising the general structure:

wherein n is a positive integer, and R' is hydrogen or a methyl group.

2. The photopolymerizable discotic liquid crystal compound according to claim 1, wherein n is between 6 and 11.

3. The photopolymerizable discotic liquid crystal compound according to claim 1, wherein R' is a methyl group.

4. The photopolymerizable discotic liquid crystal compound according to claim 1, wherein R' is hydrogen.

5. A liquid crystal display optical compensator comprising: a photopolymerizable discotic liquid crystal compound having the general structure: UA387 22

wherein n is a positive integer, and R' is hydrogen or a methyl group.

6. The liquid crystal display optical compensator according to claim 5, additionally comprising an alignment layer.

7. The liquid crystal display optical compensator according to claim 6, wherein the alignment layer is a polyimide alignment layer and wherein the alignment layer provides a pre-tilt angle of between about 5° and about 90°.

8. The liquid crystal display optical compensator according to claim 7, wherein the polyimide alignment layer is a reaction product of at least one acid dianhydride and at least one diamine, wherein the at least one diamine contains a pendent mesogenic group.

9. The liquid crystal display optical compensator according to claim 8, wherein the at least one diamine is selected from the group consisting of diamines represented by formulas I and II, UA387 23

(CH₂)_X— O -R?

wherein R is an ester or ether linking group, R2 is a mesogenic group or a functional group, R3 is hydrogen or a halogen, and x is a positive integer.

10. The liquid crystal display optical compensator according to claim 9, wherein x is between 6 and 18.

11. The liquid crystal display optical compensator according to claim 9, wherein R2 is selected from the group consisting of compounds containing one or more of the subunits represented by formulas III, IV, V, and VI,

Q CH₃

-C =CH, (HI) UA387 24

wherein R4 is selected from the group consisting of an ester, an ether, a methylene group, a vinyl group and combinations thereof, and X is selected from the group consisting of hydrogen and an organic group having from 1 to 20 carbon atoms.

12. A method of making a discotic liquid crystal compound comprising the steps of : alkylating a hydroxyalkyl benzoic acid with a halogenated alkyl alcohol to form a substituted benzoic acid; hydrolyzing the substituted benzoic acid to generate a hydroxyl- terminated benzoic acid; contacting the hydroxyl-terminated benzoic acid with an acid chloride to generate a hydroxyalkyloxyl benzoic acid; contacting the hydroxyalkyloxyl benzoic acid with a alkylhydroxy triphenylene compound to form a discotic liquid crystal compound.

13. The method of making a discotic liquid crystal compound according to claim 12, wherein the hydroxyalkyl benzoic acid is ethyl 4-hydroxybenzoic ester.

UA387 25 14. The method of making a discotic liquid crystal compound according to claim 12, wherein the halogenated alkyl alcohol is chlorohexanol.

15. The method of making a discotic liquid crystal compound according to claim 12, wherein the acid chloride is acryloyl chloride.

16. The method of making a discotic liquid crystal compound according to claim 12, wherein the alkylhydroxy triphenylene compound is 2,3,6,7,10,11 hexahydroxy triphenylene.