JPH1082999A - Liquid crystal display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress production of stripe domain and hysteresis due to excessive twisting force in a liquid crystal bulk and to obtain a bright and sharp image display, by allowing an optically active compd. or a structural body having a helical structure to be locally present on the interface between a substrate surface and a liquid crystal layer. SOLUTION: A chiral highly-coordinated phosphorus compd. 3 adsorbs on the interface between a substrate 1 and a liquid crystal layer 2. When a N-type liquid crystal material or P-type dichroic dye is used, the initial state shows homeotropic orientation and gives a color-erased mode. When voltage is applied on the cell, the liquid crystal layer causes phase transition into a helical structure by the twisting force of the interface to have a colored state. Since the surface of the substrate 1 where the highly coordinated phosphorus compd. 3 adsorbs is made hydrophobic by alkyl substituents for the phosphorus atoms, the surface has a homeotropic orientation in the initial state. When voltage over the threshold is applied, the twisting force becomes dominant to cause phase transition into a helical structure. As for the helical structural body, helical polymers can be cited.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a guest-host type reflection type liquid crystal display device.

[0002]

2. Description of the Related Art A guest-host type liquid crystal display device in which a dye having a large dichroic ratio is dissolved in a liquid crystal has advantages such as a wide viewing angle and is one of the display systems expected in the future. 2. Description of the Related Art In recent years, demand for color displays has been increased, and color displays based on a guest-host system have been actively developed.

As an example of a guest-host type color liquid crystal display device, a nematic liquid crystal having a positive dielectric anisotropy is mixed with a positive type dichroic dye, and liquid crystal molecules are homogeneously aligned. A so-called Heilmeier type guest-host liquid crystal display device in which a polarizer having a polarization direction parallel to the alignment direction of liquid crystal molecules is arranged can be given.

In this type of liquid crystal display device, the dye molecules are arranged in the initial alignment direction when no voltage is applied, that is, in the polarization direction of light, and light is absorbed and colored in the visible region. On the other hand, when a voltage is applied, as the liquid crystal molecules rearrange, the dye molecules also have their molecular long axes oriented in the direction of the electric field, that is, in the direction of light propagation, so that the polarization direction of light becomes the short axis direction of the dye molecules, The absorption of transmitted light is reduced.

[0005] The contrast of the liquid crystal display element of this system is as follows.
It depends on the dichroic ratio of dichroic dye and the degree of orientational order. By using an N-type dichroic dye instead of a P-type dichroic dye with the same element configuration, a positive display is possible. Positive display is also possible by using a homeotropic alignment as an initial alignment by using a nematic liquid crystal having a negative dielectric anisotropy and a P-type dye.

However, in order to obtain a sufficient contrast in this type of guest-host cell, a polarizer is required in all cases, and as a result, only half of the incident light functions effectively, and the brightness at the time of ON is reduced. It will be lower. Especially,
When this method is applied to a reflective display element, darkness of display becomes a particular problem.

Therefore, in order to solve such a problem,
J. Appl. Phys. , 45 (1974) 4718
A phase change guest-host cell has been developed, such as that disclosed in US Pat. In this method, for example, when a positive dye is mixed into a homogeneously aligned cholesteric liquid crystal having a positive dielectric anisotropy, the dye molecule also rotates with its major axis in the cell thickness direction (light traveling direction). . Therefore, the incident light is absorbed and colored even if polarized in any direction. However, when a voltage is applied, the liquid crystal is changed to a homeotropically aligned nematic liquid crystal, and light absorption is reduced.

In this method, since a polarizer is not required to produce an electro-optic effect, the transmittance in the bright state is higher than that in the above-described method. However, since the applied voltage required for the phase transition is large, there is a problem that power consumption increases and a response speed is relatively slow.

[0009] As a reflection type color liquid crystal display device, SID 92 DIGEST P437. S. Mi
tsui et. all. As disclosed in U.S. Pat. I have.

In this method, there is a problem that the contrast and the reflectance are low. In other words, if the concentration of the black dichroic dye serving as a light valve is increased in order to enhance the display capability of black, the dichroic ratio decreases, and even when a voltage is applied, the absorption of visible light increases and the reflection increases. The rate decreases and the entire display becomes dark. This is a fatal defect for a reflective display element. On the other hand, if the concentration of the dichroic dye is kept low to increase the reflectance, the function as a light valve cannot be sufficiently achieved. In addition, the absorption of light by the color filters is a major factor in lowering the reflectance.

As a guest-host type reflection type color liquid crystal display device without using a color filter, (TU)
Tida: Proc. 3rd. Display Re
s. Conf. , P. 202, 1983), in which guest-host cells of three colors of yellow, cyan, and magenta are stacked.

However, since this method has a structure in which three layers of cells are deposited, the amount of visible light absorbed by the transparent electrode and the alignment film is not negligible, and as a result, the reflectance is lowered, and particularly, white display is performed. The problem of darkening occurs.

In order to increase the reflectance at the time of white display, a phase transition type in which the initial state is homeotropic orientation has been devised, but the response speed is slow, and the hysteresis is eliminated while maintaining good electro-optical characteristics. This has not yet been achieved. If the amount of the chiral agent is increased to improve the electro-optical properties, stripe domains are generated during the phase transition from homeotropic alignment to a helical structure, which makes it difficult to use for display.

[0014]

In order to realize a bright and clear color display in a color guest-host type liquid crystal display device, the absorbance of the colored state by the dichroic dye is sufficient, Is required to have high reflectance or transmittance. However, as described above, in the method using the color filter, the reflectance and the transmittance at the time of decoloring are suppressed to about one third due to absorption of visible light by the color filter, and a bright display with low power consumption is obtained. Can not do.

In a system in which guest-host cells of three colors of yellow, cyan, and magenta are stacked, the absorption of visible light by the three layers of transparent electrodes and the alignment film is not negligible, and furthermore, the remaining color at the time of decoloring Is a major factor in lowering the reflectance or transmittance.

A method in which the influence of the color residue is the least is a phase transition type guest in which the initial state is homeotropic alignment.
Host cell. However, this method has a problem in that the response speed is slow, and it has not yet been achieved to eliminate the hysteresis while maintaining good electro-optical characteristics.

The present invention has been made in view of the above points, and has as its object to provide an inexpensive liquid crystal display element which can display clear and bright colors, has a simple structure, and is suitable for mass production. I do.

[0018]

Means for Solving the Problems As a result of diligent studies on the above problems, the present inventors have found that by adsorbing a chiral agent on a substrate, the amount of the chiral agent in the bulk of the liquid crystal can be kept low and the chiral agent including the interface can be suppressed. It has been found that the effect of the agent can be obtained. The present invention is based on such findings. That is, the presence of an optically active compound or a helical structure at the interface between the substrate and the liquid crystal layer allows the helical structure-a homeotropic phase transition guest-
The present inventors have found that the host liquid crystal cell can eliminate the hysteresis while maintaining good electro-optical characteristics, and have accomplished the present invention.

That is, the present invention (Claim 1) comprises a pair of substrates disposed to face each other, and a liquid crystal layer disposed between the pair of substrates, wherein at least one of the pair of substrates is provided. At the interface with the liquid crystal layer, a liquid crystal display device characterized by the presence of a compound having an asymmetric center, an optically active compound selected from the group consisting of an axially asymmetric compound and a structurally asymmetric compound, or a helical structure. provide.

The present invention (claim 2) is characterized in that, in the above-mentioned liquid crystal display device (claim 1), the optically active compound is a phosphorus compound. The present invention (claim 3)
In the above liquid crystal display device (claim 1), the optically active compound is an organometallic complex.

The present invention (claim 4) is characterized in that, in the above-mentioned liquid crystal display device (claim 1), the helical structure is formed of a polymer chain. The present invention (Claim 5) is characterized in that, in the above-mentioned liquid crystal display element (Claim 1), the region where the helical structure exists is the entire liquid crystal layer.

According to the present invention (claim 6), in the above-mentioned liquid crystal display device (claim 1), the helical structure exists at both interfaces of the substrate sandwiching the liquid crystal layer, and the helical structure at one interface is formed. The existence region extends from the interface of the liquid crystal layer to the vicinity of the center, and the helical rotation direction of the cholesteric liquid crystal phase indicated by the liquid crystal layer is reversed near the center of the substrate.

According to the present invention (claim 7), in the above-mentioned liquid crystal display device (claim 1), the liquid crystal layer exhibits a cholesteric liquid crystal phase which reflects a specific visible light when no voltage is applied. I do.

According to the present invention (claim 8), in the above-mentioned liquid crystal display device (claims 5 and 6), the substrate having the helical structure at the interface forms a polymer layer containing a radical generator on the substrate. Then, the substrate is immersed in a medium containing a polymerizable optically active compound, radicals are generated, and a substrate structure is grown on the substrate surface.

Hereinafter, the liquid crystal display device of the present invention will be described in more detail. The liquid crystal display device of the present invention is characterized in that a chiral agent, that is, an optically active compound or a helical structure is present at the interface with the liquid crystal layer of the substrate. For example, as shown in FIG. 1, at the interface between the substrate 1 and the liquid crystal layer 2,
Chiral high coordination phosphorus compound 3 is adsorbed. When an N-type liquid crystal material or a P-type dichroic dye is used, the initial state is homeotropic alignment and the mode is a decoloring mode. To a colored state.

Substrate 1 on which highly coordinated phosphorus compound 3 is adsorbed
Is initially homeotropic because the surface is hydrophobicized by the alkyl substituent of the phosphorus atom.However, when a voltage exceeding the threshold is applied, the torsional force becomes dominant and the surface changes to a helical structure. It is thought that.
In order to further stabilize the helical structure, a chiral agent can be dissolved in the liquid crystal.

In FIG. 1, R ₁ and R ₂ represent a biphenyl group, a cyclohexylphenyl group, a phenyl group, an alkyl group, an alkoxy group, a halogen, an alkylsulfide group, and X represents O, S, or Se. . As the optically active compound usable in the present invention, the following general formula (1)
And a highly coordinated phosphorus compound shown in the following.

[0028]

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(Wherein R ₁ and R ₂ represent a biphenyl group, a cyclohexylphenyl group, a phenyl group, an alkyl group, an alkoxy group, a halogen, an alkylsulfide group,
X represents O, S, or Se. Particularly preferred substituents are biphenylcyclohexylphenyl and isomers thereof. Other examples of the optically active compound that can be used in the present invention include a metal complex represented by the following general formula (2).

[0030]

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The helical structure usable in the present invention includes a helical polymer having a side chain having dielectric anisotropy.
Can be mentioned. For example, a polymer obtained by polymerizing a methacrylic acid ester and a polymer obtained by polymerizing a cholesterol derivative. Examples of the side chain having dielectric anisotropy include mesogenic groups such as biphenyl and cyclohexylphenyl. Examples of the cholesterol derivative include those represented by the following general formula (2).

[0032]

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In the method according to the present invention, the torsional force giving the helical structure is localized at the interface and is absent or slight in the liquid crystal layer. It is possible to obtain excellent electro-optical characteristics that do not pass through the stripe domain and are steep and have no hysteresis.

The liquid crystal display element of the present invention comprises an N-type liquid crystal material and an N-type dichroic dye, a P-type liquid crystal material and a P-type dichroic dye, and a P-type liquid crystal material and an N-type dichroic dye. It can also be realized by a combination of sex dyes.

In the liquid crystal display device of the present invention, in order to increase the torsional force at the interface, it is more desirable that the density of the optically active compound or the helical structure localized at the interface is large. Further, in order to effectively apply the twisting force of the interface to the liquid crystal layer, it is more desirable that the thickness of the liquid crystal layer is small. That is, the thickness of the liquid crystal layer is preferably 10 μm.

The optically active compound or the helical structure may be localized only at the interface with the liquid crystal layer of one of the substrates, or may be localized at the interface with the liquid crystal layer of both substrates. As described above, the liquid crystal display element of the present invention avoids a stripe domain generated during the helical structure-homeotropic phase transition by localizing a torsional force at an interface with a liquid crystal layer on a substrate, It is intended to obtain excellent electro-optical characteristics with no steepness and no hysteresis. As described above, according to the present invention, a liquid crystal display device having high contrast and excellent color display performance can be realized.

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Various embodiments of the present invention will be described below, but the present invention is not limited to these embodiments. Example 1 Four glass substrates each having an ITO film sputtered on the surface thereof were subjected to ultrasonic cleaning using acetone and then water, and dried by heating. Then, after ultrasonic cleaning using water again, about 1%
For 30 minutes. As the phosphorus compound, a compound having biphenylcyclohexylphenyl was used. The optical purity of this phosphorus compound is 95
% Was confirmed by specific rotation.

Next, these glass substrates were lightly washed with a solvent (chloroform) and heated on a hot plate at 90 ° C. for 5 minutes. Subsequently, the resultant was washed with a solvent (chloroform), acetone and water, and then dried.

Using the four substrates 11a, 11b, 11c and 11d thus subjected to the hydrophobic treatment, a three-layer cell having a thickness of 10 μm as shown in FIG. 2 was produced. Yellow and magenta P-type dichroic dyes are dissolved in an N-type liquid crystal, and each is injected into a three-layer cell, and a yellow guest-host liquid crystal layer 12a, a cyan guest-host liquid crystal layer 12b, and a magenta guest-host The liquid crystal layers 12c were respectively formed. Reference numeral 13 indicates a reflection plate.

When a voltage was applied to the three-layer cell configured as described above, it was found that a wide color display range as shown by the XY chromaticity coordinates in FIG. 3 was obtained. Next, a single layer cell of each of yellow, cyan and magenta was produced under exactly the same conditions as the three-layer cell, and their electro-optical characteristics were examined. The results shown in FIG. 4 were obtained. FIG. 4 shows that the rise of the electro-optical characteristics was steep and no hysteresis was observed in any of the single-layer cells, indicating that the contrast was much larger than in the conventional example.

Example 2 Instead of a 1% alcoholic solution of a 1% phosphorus compound,
A three-layer cell was manufactured in the same manner as in Example 1, except that a 1% alcohol solution of the metal complex represented by the general formula (2) was used. When a voltage was applied to this three-layer cell, it was found that a wide color display range as shown by the XY chromaticity coordinates in FIG. 3 obtained in Example 1 was obtained.

Next, yellow, cyan, and magenta single-layer cells were manufactured under exactly the same conditions as the three-layer cell, and their electro-optical characteristics were examined. The results were the same as those shown in FIG. That is, in each of the single-layer cells, the rise of the electro-optical characteristics was steep, and no hysteresis was observed, indicating that the contrast was much larger than that of the conventional example.

Example 3 In the metal complex used in Example 2, a metal complex having a large volume alkyl substituent such as a steroid skeleton was used, and a P-type liquid crystal in which a yellow and magenta P-type dichroic dye was dissolved was used. A three-layer cell was produced in the same manner as in Example 1 except that the above-mentioned operation was performed. In this liquid crystal cell, the initial state is a helical structure due to a strong torsional force at the interface between the substrate and the liquid crystal layer. When a voltage exceeding a threshold value (2.0 V) is applied, the liquid crystal cell transitions to homeotropic alignment.

Next, single-layer cells of yellow, cyan, and magenta were manufactured under exactly the same conditions as in the three-layer cell, and their electro-optical characteristics were examined. The results shown in FIG. 5 were obtained. That is, for any single-layer cell,
The rise of the electro-optical characteristics was steep, no hysteresis was observed, and it was found that the contrast was much larger than that of the conventional example.

Example 4 A three-layer cell was prepared in the same manner as in Example 1 except that a helical polymer obtained by polymerizing methacrylic acid ester was used instead of the 1% alcohol solution of the phosphorus-containing compound. did.

When a voltage was applied to the three-layer cell, it was found that a wide color display range as shown by the XY chromaticity coordinates in FIG. Next, yellow, cyan, and magenta single-layer cells were manufactured under the exactly same conditions as in the three-layer cell, and their electro-optical characteristics were examined. The results were the same as those in FIG. That is,
Regarding any single-layer cell, the rise of the electro-optical characteristics is steep, and no hysteresis is observed.
It was found that the contrast was much higher than in the conventional example.

Next, as shown in FIG. 6, when a mesogen group 22 was introduced into the side chain of the helical polymer 21, the density of adsorption of the helical polymer on the substrate was about one third, and the same result was obtained. It turned out to be obtained.

Comparative Example 1 A glass substrate was hydrophobized using the racemic (inert) phosphorous compound used in Example 1, and parameters d / p (d: cell thickness, p: helical pitch) ) Is 0.4
Example 1 except that the chiral agent S811 (trade name: manufactured by Merck Japan) was dissolved in the liquid crystal so that
In the same manner as described above, single-layer cells of yellow, cyan, and magenta were produced.

When the electro-optical characteristics were examined, the results shown in FIG. 7 were obtained. As shown in the VT curve of FIG.
The rise of the electro-optical characteristics is gentle, and it can be seen that sufficient contrast cannot be obtained in this method.

Comparative Example 2 Using the racemic phosphorus compound used in Example 1, a glass substrate was subjected to a hydrophobic treatment, and the parameters d / p (d: cell thickness, p: helical pitch) were 1.0. Thus, single-layer cells of yellow, cyan and magenta were prepared in the same manner as in Example 1 except that the chiral agent S811 (trade name: manufactured by Merck Japan Ltd.) was dissolved in the liquid crystal.

When the electro-optical characteristics were examined, the results shown in FIG. 8 were obtained. As shown in the VT curve of FIG.
This cell cannot be used for display because hysteresis occurs via the stripe domain during the layer transition from the homeotropic alignment to the helical structure, and further.

Comparative Example 3 A glass substrate was subjected to a hydrophobic treatment using the racemic phosphorus compound used in Example 1, and the parameter d / p (d: cell thickness, p: spiral pitch) was 0.8. Monolayer cells of yellow, cyan and magenta were prepared in the same manner as in Example 1 except that the chiral agent S811 (trade name: manufactured by Merck Japan Ltd.) was dissolved in the liquid crystal.

When their electro-optical characteristics were examined, the results shown in FIG. 9 were obtained. From the comparison with the result of Comparative Example 1 shown in FIG. 7, it was confirmed that the steeperness of the VT curve was improved as the parameter d / p was increased, but from the result of Comparative Example 2 shown in FIG. It has been found that when the value exceeds a certain value, stripe domains and hysteresis are generated and cannot be used for display.

Incidentally, d / p = 0.8 in this comparative example.
Is an optimum value in a stripe domain and a region where hysteresis does not occur. The results of this comparative example are steeper than the results of comparative example 1 shown in FIG.
3, it was found that the steepness was much worse.

Example 5 A glass substrate having an ITO film formed on its surface was coated with a polymer containing 2,4,6-trimethylbenzoyldiphenylphosphine oxide as a photopolymerization initiator. A cell was constructed using this substrate, and a liquid crystal containing a chemically active photopolymerizable compound represented by the above formula (3) was injected. The cell was irradiated with ultraviolet light to grow a helical structure from the interface between the substrate and the liquid crystal layer.

In this case, the region where the helical structure having a liquid crystal side chain is present was adjusted so as to cover the entire liquid crystal layer as shown in FIG. As a result, the obtained liquid crystal element had excellent electrochemical characteristics, no stripe domain and no hysteresis were observed, and the same result as in FIG. 5 could be obtained.

That is, from this example, it can be seen that a more stable helical structure can be obtained by providing the helical structure not only at the interface but also at the entire liquid crystal layer as compared with the case where the helical structure is present only at the interface.

Example 6 A glass substrate having an ITO film formed on its surface was coated with a polymer containing 2,4,6-trimethylbenzoyldiphenylphosphine oxide as a photopolymerization initiator. This substrate was immersed in a solvent containing the chemically active photopolymerizable compound represented by the above formula (3), and irradiated with ultraviolet rays to form a helical structure on the substrate surface.

Next, the solvent was completely removed, and a pair of substrates were stuck together to form a cell as shown in FIG. 11 in which the spiral direction was reversed near the center of the cell. A liquid crystal was injected into this cell to form a liquid crystal cell. The reflectivity of this liquid crystal display device at the time of selective reflection was 60%, and a bright liquid crystal display device could be obtained.

[0060]

As described in detail above, according to the present invention,
Since the optically active compound or the helical structure is localized at the interface with the liquid crystal layer on the substrate surface, the torsional force is localized at the interface, so that the amount of the chiral agent in the liquid crystal does not increase. Therefore, the stripe domain and the hysteresis due to the excess twisting force in the liquid crystal bulk are suppressed, and a steep VT curve can be obtained. As a result, it is possible to realize a liquid crystal display element having a large contrast and displaying a bright and clear image, in particular, a reflective liquid crystal display element.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a glass substrate surface-treated with a chiral phosphorus compound.

FIG. 2 is a cross-sectional view showing a configuration of a three-layer guest-host liquid crystal cell used in an example.

FIG. 3 is a characteristic diagram showing a color display range of a three-layer guest-host liquid crystal cell of Example 1.

FIG. 4 is a characteristic diagram showing electro-optical characteristics of each layer of the three-layer guest-host liquid crystal cell according to Example 1.

FIG. 5 is a characteristic diagram showing electro-optical characteristics of each layer of the three-layer guest-host liquid crystal cell according to Example 2.

FIG. 6 is a diagram illustrating a helical polymer having a mesogain group in a side chain.

FIG. 7 is a characteristic diagram showing electro-optical characteristics of each layer of the three-layer guest-host liquid crystal cell according to Comparative Example 1.

FIG. 8 is a characteristic diagram showing electro-optical characteristics of each layer of the three-layer guest-host liquid crystal cell according to Comparative Example 2.

FIG. 9 is a characteristic diagram showing electro-optical characteristics of each layer of a three-layer guest-host liquid crystal cell according to Comparative Example 3.

FIG. 10 is a diagram showing a liquid crystal cell according to a fifth embodiment in which the region where the helical structure exists is the entire region of the liquid crystal layer.

FIG. 11 is a diagram illustrating a liquid crystal cell according to a sixth embodiment in which the rotation direction of the helix is reversed near the center of the cell.

[Explanation of symbols]

 1, 11a, 11b, 11c, 11d: glass substrate 2: liquid crystal layer 3: chiral molecule 12a: yellow guest-host layer 12b: cyan guest-host layer 12c: magenta guest-host layer 13: reflector.

Claims (8)

[Claims] And a liquid crystal layer disposed between the pair of substrates, wherein at least one of the pair of substrates has an asymmetric interface with the liquid crystal layer. A liquid crystal display device comprising an optically active compound selected from the group consisting of a compound having a center, an axially asymmetric compound and a structurally asymmetric compound, or a helical structure. 2. The liquid crystal display device according to claim 1, wherein the optically active compound is a phosphorus compound. 3. The liquid crystal display device according to claim 1, wherein the optically active compound is an organometallic complex. 4. The liquid crystal display device according to claim 1, wherein the helical structure is formed of a polymer chain. 5. The liquid crystal display device according to claim 1, wherein the region where the helical structure exists is the entire liquid crystal layer. 6. The helical structure exists at both interfaces of the substrate sandwiching the liquid crystal layer, and the helical structure at one interface extends from the liquid crystal layer interface to the vicinity of the center, and the liquid crystal layer indicates 2. The liquid crystal display device according to claim 1, wherein the helical rotation direction of the cholesteric liquid crystal phase is reversed near the center of the substrate. 7. The liquid crystal display device according to claim 1, wherein the liquid crystal layer exhibits a cholesteric liquid crystal phase that reflects a specific visible light when no voltage is applied. 8. A substrate having a helical structure at an interface,
Forming a polymer layer containing a radical generator on the substrate,
6. A substrate obtained by immersing the substrate in a medium containing a polymerizable optically active compound to generate radicals to generate a radical structure on the substrate surface.
Or the liquid crystal display element according to 6.