DD282543A5 - FERROELECTRIC LIQUID CRYSTAL DISPLAY CELL - Google Patents

Abstract

The invention relates to a liquid crystal cell having a chiral ferroelectric smectic liquid crystal layer whose helix structure is influenced by the action of an electric field to change its optical anisotropy, a pair of transparent plates enclosing the liquid crystal, having a surface structure aligning the molecules of the liquid crystal, and electrodes are provided for generating an electric field in liquid crystal and each with a polarizer in front of and behind the liquid crystal. According to the invention, the ratio d / p of the thickness d of the liquid crystal layer to the pitch p of the helical twist is greater than 5, the smectic tilt angle Uo is between 22.5 and 50 and the product dU2 oDnll (phase factor of the thickness d, the square of the tilt angle Uo , the value of the birefringence Dn and the reciprocal of the optical wavelength l is greater than 0.45 mm) {deformation of helical structure in electric field; Application as light valve or indicator cell}

Description

10. Chiral additive for liquid crystal mixtures, characterized by the following formula

COO-C * H-R

wherein R ¹ and R ^{2 are} independently alkyl having at least 2 carbon atoms and C ^x denotes a chiral carbon atom.

For this 1 page drawings

Field of application of the invention

The present invention relates to a liquid crystal display cell having a chiral ferroelectric smectic liquid crystal layer whose helical structure is affected by an electric field to change its optical anisotropy, a pair of liquid crystal enclosing transparent plates having a surface structure aligning the molecules of the liquid crystal are provided with electrodes for generating an electric field in the liquid crystal, and a respective polarizer in front of and behind the liquid crystal. The invention further relates to chiral or optically active compounds and liquid-crystalline mixtures for use in this cell as well as the use of these compounds for electro-optical purposes.

-2- 282 543 characteristic of the known prior art

The effect of deformation of a helical structure of the voi standing type by an electric field is described in Ostrovski, BI, Advances in Liquid Cystal Research and Applications, Oxford / Budapest, 1980, pages 469ff. described. The effect is sometimes referred to in the literature as DHF for "Distorted Helix Ferroelectric." In the article mentioned, the possibility of modulating a light beam at a frequency of 1 kHz and with an electric field of less than 6 x 1 (T ³ As a result, however, it appeared that no practically useful electro-optic cell can be realized on the basis of this effect.

A well-known, but different, effect is the so-called Surface Stabilized Ferroelectric Liquid (SSFLC) effect of Lagerwall and Clark, in which ferroelectric liquid crystals are used in an untwisted configuration. This effect is in Mol. Cryst. Liq. Cryst. 94 (1983), 213-234 and 114 (1984), 151-187.

Object of the invention

The object of the invention is to provide a DHF cell which, in addition to the known advantages of the DHF effect, has the properties required for practical use as a light valve or display cell.

Explanation of the essence of the invention

The invention has for its object to make the liquid crystal display cell with respect to the dimensions and arrangement of their components so that it has the desired properties.

According to the invention, this is achieved by a DHF cell of the type specified in the introduction, which is characterized in that the ratio d / p of the thickness d of the liquid crystal layer to the pitch ρ of the helical twist is greater than 5, the smectic tilt angle G ₀ / wipe 22, 5 ° and 50 ° and the product d · Θ? · Δη · Ι / λ is greater than 0.1 pm. Δη is the birefringence and λ di'3 wavelength of the light.

The liquid crystal display cell according to the invention has particularly favorable properties when d / p is greater than 10 when Θο = 29 ° and when the phase factor is greater than 1.

For an operating mode of the cell in which the intensity of the transmitted light is proportional to the applied voltage, according to the invention the angle β between the helical axis and the polarization device of the input-side polarizer is 22.5 ° or 67.5 °.

For a mode of operation of the cell in which the intensity of the transmitted light is proportional to the square of the applied voltage, the angle β between the helical axis and the polarization direction of the polarity-side polarizer is 0 °,

In accordance with the invention, the surface structure of the plates has a planar aligning effect on the liquid molecules.

According to the invention, the polarizers are crossed.

embodiment

In the following, embodiments of the invention will be described with reference to the accompanying drawings. Show it:

1 is a schematic perspective view of a small section of a liquid crystal cell after a

preferred embodiment of the invention; 2 shows a graphic representation of the dependence of the storage time on the applied voltage.

As shown in Fig. 1, the present liquid crystal cell consists of a liquid crystal layer 1 sandwiched between two substantially planar plates 2,3 of transparent material, e.g. glass, acrylic glass, plastic films, etc.

is arranged. In front of the front plate 2 is a polarizer 4, which is preferably connected to the plate 2, for example glued. Accordingly, the rear plate 3 is assigned a polarizer 5. The plates 2; 3 have a distance d from each other.

On their surfaces facing the liquid crystal 1, the plate η 2; 3 shows the usual electrode coatings that have been blended to display characters or pixels. In the section of a cell shown here, only a single electrode segment 6 on the plate 2 and an opposite electrode segment 7 on the rear plate 3 is shown.

The liquid crystal 1 facing surfaces are also treated so that sia exert on the adjacent ι liquid crystal molecules a directivity and thus determine the direction of the director. For example, this treatment involves rubbing the surface in one direction to achieve a planar orientation. Other possibilities are the oblique evaporation of orienting layers, etc. The result of this treatment is referred to for the purpose of this specification as the surface orientation. The surface orientation on the two plates is shown by arrows. On the back plate 3, the surface orientation is parallel to that on the front plate. Apart from planar surface orientation, homeotropic orientation or combinations of both can also be considered.

The polarizer 4 belonging to the front plate 2 is arranged so as to enclose an angle β with the direction of the surface orientation or with the helical axis. The belonging to the rear plate 3 polarizer 5 is rotated against the front Pf 'arisator 4 by an angle of 90 °. This arrangement of the polarizers 4; 5 illustrates a preferred embodiment. Other polarizer arrangements are possible with which good results can also be achieved.

The appropriate polarizer arrangements are readily determinable by simple optimization.

The liquid crystal 1 is a chiral smectic ferroelectric, often referred to in the Litei atur as smectic C.

Liquid crystal. Such liquid crystals are known from the literature. They are characterized by the fact that their molecules are not perpendicular to the smectic planes, but at a certain angle, the smectic tilt angle Q ₀ angeordiiet. The chirality is that the molecular directions are not parallel to each other in all layers, but from layer to layer are rotated against each other, so that a total of a helical twist arises. With planar surface orientation, the axis of this helical twist is aligned parallel to the plates. In FIG. 1, one molecule 8 per smectic layer and the helical twisting of the molecules continuing from one layer to another along a helix axis 9 are shown. The smectic tilt angle Θο is the angle between the molecular longitudinal axis and the helix axis 9. The pitch of the helix in the field-free state of the liquid crystal is p ₀ . If a voltage is applied to the electrodes 6 and 7, the result is an electric field in the liquid crystal 1 which causes a reorientation of the molecules. From the symbolically indicated voltage source 10 come in the practical case periodic signals or drive pulses, which in the

present cell, as explained in more detail below, opposite polarity can have.

At rest, i. without applied electric field, the cell shows a certain Lichtmittlässigktit. By applying an electric field, the helical arrangement of the molecules is deformed, which results in a change in the light transmittance. There is an optical contrast between the two states. The deformation of the screw structure caused by the electric field essentially causes a change in the pitch.

Depending on the dependence of the intensity of the light on the applied voltage, two different modes are possible, namely the linear, in which the intensity is proportional to the applied voltage, and the square, in which the intensity is proportional to the square of the voltage.

The effect occurs when the light beam passing through the liquid-crystalline structure has an aperture a,

² ^ the multiple periods D of the helical structure

'o

equivalent. In this way the initial electro-optical effect along the helical direction is averaged.

In contrast to effects in which the helical structure is completely removed, e.g. the so-called SSFLC effect, there are a number of important differences.

1. The operating voltage depends on the smectic tilt angle θ, the polarization P in the untwisted state, the

Place ^{0 C}

Ratio of layer thickness to pitch d / p and is generally lower than the voltage required to cancel the helix U. By increasing the ratio of layer thickness d to pitch ρ and increasing the smectic tilt angle θ, it is possible to expand the operating voltage range.

2. The effect has a good modulation depth and a high contrast ratio as well as an approximately proportional to the applied voltage gray scale.

3. The effect allows storage controlled by the applied voltage amplitude. The time t until the structure returns to the original undisturbed Helixforra depends on the viscosity and the elastic constants of the liquid crystal material, as well as on the helical pitch. In addition, the storage time is influenced by the surface conditions. For low values of pitch, the storage time can be very small values of about 10-100 μsec. up to big values of 5-10 see. be varied.

4. The effect is based on minor variations of director

Orientation and does not change the desinting walls in the display area. As a result, delay times are not observed in the display mode.

5. The effect is not sensitive to wall orientation in a planar configuration. The usual methods of wall orientation as in nematic liquid crystals can be used.

6. The effect allows the application of bipolar signals

as well as harmonic fields with a characteristic frequency f.

7. With suitable control signals, the effect can also cause optical phase differences in the liquid-crystal layer such that color switching is possible.

As already mentioned, preferred parameters are a ratio of thickness d to pitch ρ of less than 5, particularly preferably more than 10. The smectic tilt angle should preferably be 22.5 to 50 ° and particularly preferably 25 °

-C-

to 40 °. Furthermore, the value for the so-called phase factor should be

greater than 0.45, more preferably greater than 1.

The liquid crystal in the present embodiment consists of the following mixture: 26.1% by weight of 5-octyl-2- [p- (octyloxy) phenyl] pyrimidine, 17.1% by weight of 5-octyl 2- [p- (nonyloxy) phenyl ] pyrimidine, 24.5% by weight of 5-octyl-2- [p- (decyloxy) phenyl] pyrimidine and 32.3% by weight of the chiral additive

Generally, as chiral additives, compounds of the general formula

CH ₃

• 2 COO-C * H-R

wherein R and R are independently alkyl of at least two carbon atoms and C * is a chiral carbon atom.

The compounds of the formula I are novel and likewise the subject of the present invention. They induce in

Liquid crystal mixture a very high spontaneous polarization, are chemically stable, colorless and readily soluble in conventional liquid crystal materials, especially in ferroelectric smectic liquid crystals.

The term "alkyl having at least 2 carbon atoms" includes straight-chain and branched groups, especially straight-chain alkyl having at least 2 carbon atoms and isoalkyl such as ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, hexyl, heptyl, octyl, nonyl and the like. The

-ι ο

¹ ^ alkyl groups R and R preferably have 2-15 each,

1 2 particularly preferably 2-9 carbon atoms. R and R may have different or preferably identical meanings.

The configuration at the chiral centers in formula II is preferably such that the chiral groups induce a co-directed spontaneous polarization. Preferably, both chiral carbon atoms are in the S configuration or both in the R configuration, especially when R and R have the same meaning.

The compounds of the formula XI can be prepared in a manner known per se by customary esterification methods from 4,4 "-p-terephenyl dicarboxylic acid and 2-alkanols.The starting materials are known or analogous known compounds.

The compounds of the formula II are particularly suitable as chiral dopants for liquid-crystalline mixtures. The invention therefore also relates to a liquid-crystalline mixture having at least two components, characterized in that at least one component is an optically active compound of the formula II. Preferably, the mixture contains a liquid crystal material having a wetted smectic phase and one or more optically active compounds of the formula II.

-r-

The proportion of compounds of the formula II in the mixtures according to the invention can vary within wide limits and be, for example, about 0.5-30% by weight. In general, a range of about 1-20% by weight, and more preferably a range of about 3-10% by weight, is preferred. 5

The liquid crystal material with a tipped smectic phase may consist of conventional materials. Preferred are materials having a smectic C phase, such as derivatives of phenylpyrimidine, phenylpyridine. Phenyl benzoate, Benzoesäurebiphenylester, 4-Biphenylcarbonsäure-phenylester with a smectic C phase.

In the cell with this liquid-crystal mixture, the pitch is ρ 0.3 to 0.4 μ at 25 ° C, the birefringence Δη = 0.25. The smectic tilt angle θ is

carries about 29 ° at room temperature The spontaneous polarization

-8 2 P is about 7 · 10 c / cm at 25 ° C. The rotational viscosity is about 1 poise. The liquid crystal layer in the cell has a thickness of 3.6 μm.

The . oaanung U for the complete removal of the screw structure is about 2 V. For this cell, the response time is about 200 μsec. The response time is approximately the same for the entire range of the applied voltage 0 <U <U.

The preferred values for the angle β are β = 22.5 °

ir 3-ir

( ₈ ) or ß = 67,5 ° ( ₈ ) door the linear

Operating mode and ß = 0 °. β = 45 ° (£). ß = 90 °

(-) for the quadratic mode. 30

The cell thus constructed shows a contrast of more than 100: 1. The following table shows the intensity values I for a range of voltage amplitudes. The voltage consists of rectangular pulses. The values apply to the linear mode.

table

U [V] I [%]

O 1

0.15 10

0.25 20

0.45 35

0.6 55

\ .O 90

1.5 99

1.75 98

2.0 97

2.45 80

2.6 65

2.95 50

3.3 40

4.0 35

5.15 35

The maximum intensity value is 100%. The first values in the voltage range from 0 to 2 volts give the achievable gray scale. Above 2 volts, the contrast decreases, which is due to the higher smectic tilt angle. At voltages above 4.0 volts, the helix is completely unwound.

The following is the preparation of a compound of

Formula II described using the example

 30

Example 1

A solution of 2 g of 4,4 "-p-Terphenyldicarbonsäuredichlorid (prepared by reaction of 4.4" -p-terphenyl- ^ ⁵ dicarboxylic acid with thionyl chloride in pyridine at 80 ⁰ C) in ml of dry pyridine was treated with 1.59 g of S- ( +) - 2-octanol

"70-

added and stirred at 20 ⁰ C for 24 hours. Subsequently, the reaction mixture was poured onto a mixture of ice and dilute hydrochloric acid. The crystalline product was filtered off, washed with water and dried under high vacuum. Chromatographic purification and recrystallization gave pure 4,4 "di- [2 (SJ-octyloxycarbonyl-p-terphenyl, m.p. 81- R2 ° C.

In an analogous manner, the following compounds can be prepared: 10

4,4 "-Di- [2 (S) -exyloxycarbonyl] -p-terphenyl; 4,4" -Di- [2 (R) -xyloxycarbonyl] -p-terphenyl; 4.4 "di- [2 (S) -heptyloxycarbonyl] -p-terphenyl; 4: ^/ i" -Di- [2 (R) -heptyloxycarbonyl] -p-tet; phenyl; 4,4 "di- [2 (R) octyloxycarbonyl] -p-terphenyl; m.p.

81-82 ° C;

4,4 "-di [2 (S) -nonyloxycarbonyl] -p-terphenyl ;, 4,4" -di [2 (R) -nonyloxycarbonyl] -p-terphenyl;

4.4 "-Di- [2 (S) -decyloxycarbonyl] -p-terphenyl ;, ² ^ 4,4" -di [2 (R) -decyloxycarbonyl] -p-terphenyl;

Spontaneous polarization Ps was assayed on a mixture of 5% by weight of 4.4% di- [2 (S) -oxyloxycarbonyl] -p-terphenyl and 95% by weight of 4-octyloxybenzoic acid 4-hexyloxyphenyl ester at 40 ⁰ C a Ps value

2 of 8.8 nC / cm. The extrapolated Ps value for 4.4 "-

Di- [2 (S) -Octyloxycarbonyl] -p-terphenyl is at 40 ⁰ C

176 nC / cm.

Claims (9)

A liquid crystal display cell having a chiral-salt-electrochemical liquid crystal layer whose helical structure is affected by an electric field to change its optical anisotropy, a pair of liquid crystal-enclosing transparent plates having a surface structure aligning the liquid crystal molecules and Electrodes for generating an electric field in the liquid crystal, and each having a polarizer in front of and behind the liquid crystal, characterized in that the ratio d / p of the thickness of the liquid crystal layer to the pitch of the helical twist is greater than 5, the smectic tilt angle O ₀ between 22.5 ° and 50 ° and the product d · ·ο · Δη · Ι / λ (phase factor) from the thickness d, the square of the tilt angle Θ _ο , the value of the birefringence Δη and the reciprocal of the light wavelength λ is greater than 0.45 microns. 2. Liquid crystal display cell according to claim 1, characterized in that the ratio d / p is greater than 10. 3. Liquid crystal display _cell according to claim 1, characterized in that that Θ _ο = 29 °. 4. Liquid crystal display cell according to claim 1, characterized in that the phase factor is greater than 1. 5. Liquid crystal, eigezelle according to claim 1, characterized in that the angle β between the helical axis and the polarization direction of the input-side polarizer 22.5 ° or 67.5 ° is for a mode of operation of the cell, wherein the intensity of the transmitted light is proportional to the applied voltage. 6. A liquid crystal display cell according to claim 1, characterized in that the angle β between the helix axis and the polarization direction of the input-side polarizer 0 °, 45 ° or 90 ° is for a mode of operation of the cell, wherein the intensity of the r quitting light proportional to Square of the applied voltage is. 7. A liquid crystal display cell according to claim 1, characterized in that the surface structure of the plates has a liquid crystal molecules planar aligning Wiikunp. 8. Liquid crystal display cell according to claim 1, characterized in that the polarizers are crossed. 9. Liquid crystal display cell according to one of the preceding claims, characterized in that the liquid crystal consists of the following mixture: 26.1 wt .-% 5-octyl-2 - [(octyloxy) phenyllpyrimidine, 17.1 wt .-% 5-octyl-2 - [(nonyloxy) phenyl] pyrimidine, 24.5% by weight of 5-octyl-2 - [(decyloxy) phenyl] pyrimidine and 32.3% by weight of the chiral additive