DE2366470C2 - Liquid crystal materials and their use in display devices - Google Patents

Description

The invention relates to liquid crystal materials based on 4'-substituted 4'-cyanodiphenyl and their use in display devices.

In the field of display devices, there is a need for low power devices. Liquid crystal display devices meet this requirement because they have a very high electrical resistance, which is why there is great interest in such devices.

Numerous liquid crystal materials are already known. Liquid crystal materials are organic materials that have a liquid crystalline phase in which the molecules are arranged in an ordered structure over limited spatial areas. There are three known types of liquid crystal phases. One of these is known as the smectic mesophase, in which the molecules are arranged in a lamellar manner. Another is known as the nematic mesophase, in which there is a statistical arrangement of the molecules parallel to the long axes of the molecules. The third phase is known as the cholesteric mesophase, in which the molecules are arranged in a helical or spiral manner.

Liquid crystal materials consist of compounds with an elongated molecular structure. The following general formula can be given for the liquid crystal compounds that have been used to date: °=c:40&udf54;H&udf53;vu10&udf54;&udf53;vz3&udf54;&udf53;vu10&udf54;in which Q and Q¹ are terminal groups and A and B are bridging groups. With the exception of certain esters in which the A-B group represents the -CO-O- group, the A-B group generally contains a double or triple bond. These include, for example, the Schiff bases that are widely used in commercial liquid crystal devices, such as p-methoxybenzylidenebutylaniline (MBBA) or p-ethoxybenzylidenebutylaniline (EBBA), i.e. compounds that contain the C=N- group.

The presence of a double or triple bond in the A-B grouping leads to chemical and/or photochemical instability of the material and is therefore undesirable for this reason. For example, Schiff bases are easily hydrolyzed even by traces of water, possibly forming toxic amines.

It has previously been assumed that the double or triple bond in the A-B grouping is desirable to achieve a rigid molecule and an acceptably high transition temperature from the liquid crystalline to the isotropic liquid state together with an acceptably low transition temperature from the crystalline solid to the liquid crystalline state.

The liquid crystal materials according to the invention based on 4,4'-disubstituted diphenyl are characterized by at least one 4'-substituted 4-cyanodiphenyl of the formula I °=c:40&udf54;H&udf53;vu10&udf54;&udf53;vz3&udf54;&udf53;vu10&udf54;wherein R is methyl, ethyl or ethoxy.

It was already known that certain diphenyl compounds of the general formula II °=c:40&udf54;H&udf53;vu10&udf54;&udf53;vz3&udf54;&udf53;vu10&udf54; exhibit liquid-crystalline phases, although with the previous para-substituents Z and Z¹ the transitions from crystalline solid to liquid crystal occurred at very high temperatures. These known compounds were therefore unsuitable for display devices.

With the compounds according to the invention with the p-substituents R and CN, acceptably low transition temperatures from the crystalline solid to the liquid crystal and acceptably high transition temperatures from the liquid crystal to the isotropic liquid can be obtained, so that these liquid crystal materials in known display devices can be used.

Furthermore, since the liquid crystal materials of the present invention do not contain double or triple bonds in an A-B group of the above-mentioned liquid crystal molecular structure, their chemical and photochemical stability is improved.

The liquid crystal materials according to the invention can consist of the compounds of the formula I or represent a multicomponent mixture, optionally in the form of a corresponding solution, which contains at least one compound of the formula I.

The liquid crystal materials according to the invention can be used to build up nematic and smectic liquid crystal phases (mesophases). They can be used in optical and electro-optical display devices.

Electro-optical liquid crystal display devices in which the liquid crystal materials according to the invention can be used are devices known per se which have two glass plates lying opposite one another, at least one of which is optically transparent, a layer of a liquid crystal material provided in the space between the glass plates and a film of conductive material provided on the inner surface of the glass plates, with which an electric field can be applied to the layer. Such electro-optical display devices are suitable, for example, for instruments such as clocks, etc. The change in the molecular arrangement of the liquid crystal material caused by the electric field serves to influence the permeability of the material to electromagnetic radiation at any wavelength of interest. Instead of an electric field, a magnetic field or a device for achieving a temperature change can also be used for this purpose.

The invention is explained in more detail below with reference to the drawings; it shows

Fig. 1 is a sectional view of an electro-optical display device with schematic circuit;

Fig. 2 is a schematic view of a liquid crystal information storage device and

Fig. 3 is a block diagram of a clock with liquid crystal display.

In connection with investigations leading to the invention, it was found that materials containing compounds of the formula I °=c:40&udf54;H&udf53;vu10&udf54;&udf53;vz3&udf54;&udf53;vu10&udf54;with R = methyl, ethyl or ethoxy have at least one liquid crystal phase (mesophase). The phases can be nematic and/or smectic and exist at relatively low temperatures, which is extremely desirable for use in liquid crystal display devices.

The transition temperatures for the phase transition from the crystalline to the nematic liquid crystalline state and from the nematic liquid crystalline state to the isotropic liquid are given in the table below. Transition temperatures for the compounds of the formula °=c:40&udf54;H&udf53;vu10&udf54;H&udf53;vz3&udf54;&udf53;vu10&udf54;&udf53;vu10&udf54;&udf53;vz10&udf54;&udf53;vu10&udf54;

The following examples illustrate methods for preparing the compound of formula I.

example 1

Preparation of 4'-ethyl-4-cyanodiphenyl by the following route: °=c:120&udf54;H&udf53;vu10&udf54;&udf53;vz11&udf54;&udf53;vu10&udf54;

Level A1 Preparation of 4'-acetyl-4-bromodiphenyl by Friedel-Crafts acylation Example of implementation of this stage

To commercial 4-bromodiphenyl (0.08 mol) and anhydrous aluminum trichloride (0.1 mol) dissolved in dry nitrobenzene (85 ml) is added acetyl chloride (0.1 mol) dropwise, keeping the temperature below 20°C. The solution is stirred at room temperature for about 18 h and then poured onto a mixture of ice, water and concentrated hydrochloric acid. The mixture is then stirred for about 0.5 h, after which the nitrobenzene layer is separated. The nitrobenzene is then removed by steam distillation and the residue is recrystallized from absolute ethanol to constant melting point.

Level B1 Preparation of 4'-ethyl-4-bromodiphenyl by Huang-Minlon reduction Example of implementation of this stage

To the product of Step A1 (0.037 mol), dissolved with heating in dry diethylene glycol (60 ml), is added a solution of hydrazine hydrate (90%, 0.08 mol) in dry diethylene glycol (20 ml). Potassium hydroxide (0.09 mol) is then added to the warm solution and heated to about 100°C until most of the solid has dissolved; the reaction mixture is then heated at reflux for about 1 h. The temperature is then raised to about 180°C with distillation and maintained at this value for about 4 h. After cooling, the reaction mixture is shaken with benzene and the extract is dried. The solvent is then removed and the residue is recrystallized from absolute ethanol to constant melting point.

Level C1 Preparation of 4'-ethyl-4-cyanodiphenyl Example of implementation of this stage

A mixture of the product of Step B1 (0.021 mol) and copper(I) cyanide (0.031 mol) in dry dimethylformamide (75 ml) is heated at reflux for about 12 hours. After cooling, the reaction mixture is poured into a mixture of iron(III) chloride, concentrated hydrochloric acid and water with stirring. After heating for 20 minutes at about 60 to 70°C, the mixture is shaken with chloroform and the extract is washed with 5N hydrochloric acid, water, 10% aqueous sodium hydroxide solution and water and then dried. The solvent is then removed and the residue is purified by column chromatography on silica followed by crystallization from petroleum ether (bp. 40 to 60°C) at -78°C to constant melting point. Finally, the product is distilled under vacuum 0.7 kPa (0.5 mm Hg) at about 140 to 150°C.

Example 2

Preparation of 4-ethoxy-4'-cyanodiphenyl by the following route: °=c:160&udf54;H&udf53;vu10&udf54;&udf53;vz15&udf54;&udf53;vu10&udf54;

Level A2 Preparation of 4-bromo-4'-nitrodiphenyl Example of implementation of this stage

Commercially available 4-nitrodiphenyl is brominated to 4'-bromo-4-nitrodiphenyl according to the method of Le Fevre and Turner (J. Chem. Soc. 1926, p. 2045).

Level B2 Preparation of 4-bromo-4'-aminodiphenyl Example of implementation of this stage

Reduction of the nitro compound (81 g) prepared in step A2 is effected by stirring with a mixture of iron dust (58 g), ethanol (500 ml; 90%) and concentrated hydrochloric acid (29 ml) at 100°C for about 12 hours. Sodium carbonate (22 g) is then added and stirring is continued for 30 minutes, after which a slight excess of dilute aqueous ammonia is added. After cooling, the amine formed is extracted with ether. The combined ether/alcohol extract of the amine is concentrated to remove the ether and the residue is poured into water. The amine is precipitated, filtered and dried.

Level C2 Preparation of 4-bromo-4'-hydroxydiphenyl Example of implementation of this stage

Dissolve 4'-Amino-4-bromodiphenyl (60 g), prepared as in Step B2, in aqueous 95% acetic acid (360 ml) at 90°C; to the vigorously stirred solution is rapidly added 40% w/w sulphuric acid (360 ml) preheated to 90°C. Stirring and rapid cooling to 0°C gives finely divided amine sulphate which is diazotised at 0°C (±1°C) by slow addition of aqueous sodium nitrite (41.6 g) in water (100 ml). The excess sodium nitrite is then destroyed with urea. 95% acetic acid (400 ml) is then added to effect dissolution of the diazonium salt. The ice-cold solution of diazonium sulfate is then immediately poured into a boiling solution of 40% by mass sulfuric acid (200 ml) with vigorous stirring. The solution is initially added slowly until suspended particles are visible in the mixture, whereupon the rate of addition is increased. The time required for the addition is about 30 minutes. The solution is then heated at reflux for about 15 minutes and then cooled; dilution with the same volume of water gives a solid which is digested with hot 1N sodium hydroxide solution (3000 ml). After removal of insoluble material by filtration, the filtrate is acidified to give colorless 4-bromo-4'-hydroxydiphenyl.

Level D2 Preparation of 4'-ethoxy-4-bromodiphenyl Example of implementation of this stage

To a mixture of 4-bromo-4'-hydroxydiphenyl (0.033 mol) prepared according to step C2 and anhydrous potassium carbonate (0.132 mol) in cyclohexanone (70 ml) is added ethyl bromide (0.053 mol) and the mixture is heated under reflux with stirring for about 4 h. After cooling, the reaction mixture is filtered and the solvent is removed from the filtrate. The residue is extracted from a mixture of benzene and petroleum ether (bp. 40 up to 60°C) to a constant melting point.

Level E2 Preparation of 4'-ethoxy-4-cyanodiphenyl Example of implementation of this stage

A mixture of the 4'-ethoxy-4-bromodiphenyl (0.024 mol) prepared in step D2 with copper(I) cyanide (0.036 mol) is heated under reflux in dry dimethylformamide (100 ml) for about 12 hours. After cooling, the reaction mixture is poured into a mixture of iron(III) chloride, concentrated hydrochloric acid and water. After heating the mixture for 20 minutes at 60 to 70°C, it is shaken with chloroform; the extract is washed with 5N hydrochloric acid, water, 20% aqueous sodium hydroxide and water and then dried. The solvent is then removed and the residue is purified by column chromatography on silica and subsequent recrystallization from petroleum ether (bp. 40 to 60°C) at -78°C to a constant melting point. Finally, the product is distilled under vacuum (0.7 kPa; 0.5 mm Hg) at about 140°C.

The nematic liquid crystal materials according to the invention have a positive dielectric anisotropy (difference between the dielectric constants measured parallel to the molecular long axis and perpendicular to it).

• (I) Dem Schadt-Helfrich-Effekt, gemäß dem ein verdrillt- nematisches Flüssigkristallmaterial (normalerweise zwischen zwei transparenten parallelen Platten so angeordnet, daß der Richtungsvektor der Moleküle in der Ebene der Platten liegt, jedoch von einer Platte zur anderen über einen Winkel von 90° variiert), der zwischen einem optischen Polarisator und Analysator angeordnet ist, eine Änderung der optischen Durchlässigkeit ergibt, wenn ein elektrisches Feld daran angelegt wird;
• (II) dem Freedericks-Effekt, gemäß dem der Richtungsvektor der Moleküle eines Flüssigkristallmaterials von einer ersten Richtung zu einer zweiten, senkrecht zur ersten Richtung liegenden Richtung durch die Einwirkung eines elektrischen Feldes, das in der zweiten Richtung angelegt ist, verändert wird;
• (III) dem Phasenwechseleffekt, gemäß dem der Phasenzustand eines Flüssigkristallmaterials durch die Einwirkung eines elektrischen Feldes zwischen einer cholesterinischen und einer feldinduzierten nematischen Phase geändert werden kann. Hierzu wird das nematische Flüssigkristallmaterial zur Erzeugung eines cholesterinischen Materials mit einem optisch aktiven Material gemischt.

Nematic liquid crystal materials with a positive dielectric anisotropy can be used in display devices based on the following known effects:

• (I) The Schadt-Helfrich effect, according to which a twisted nematic liquid crystal material (normally placed between two transparent parallel plates so that the direction vector of the molecules lies in the plane of the plates but varies from one plate to the other over an angle of 90°) placed between an optical polarizer and analyzer gives a change in optical transmittance when an electric field is applied thereto;
• (II) the Freedericks effect, according to which the direction vector of the molecules of a liquid crystal material is changed from a first direction to a second direction perpendicular to the first direction by the action of an electric field applied in the second direction;
• (III) the phase change effect, according to which the phase state of a liquid crystal material can be changed between a cholesteric and a field-induced nematic phase by the action of an electric field. For this purpose, the nematic liquid crystal material is mixed with an optically active material to produce a cholesteric material.

Examples of display devices in which the liquid crystal materials according to the invention can be used are described below.

As a first example of the application of a nematic liquid crystal material according to the invention, a display device based on phase change will be described. It is known that in order to produce cholesteric liquid crystal materials for use in display devices based on phase change, an optically active liquid crystal material is mixed with a nematic liquid crystal material or a mixture of nematic liquid crystal materials. A liquid crystal material according to the invention is used as the nematic liquid crystal material.

Fig. 1 shows a cross-sectional view of an electro-optical display device based on phase change with a schematic circuit. A thin layer 17 of the liquid crystal material according to the invention is contained between a glass plate 19 and a glass plate 29. The glass plate 19 has a coating 23 of tin oxide on its inner side, which is in contact with the layer 17. The glass plate 19 has a region protruding from the end of the glass plate 29 in which an electrical connection 31 is provided on the coating 23 of tin oxide. Likewise, the glass plate 29 has a region protruding from the end of the glass plate 19 with an electrical connection 33 on the coating 25 of tin oxide provided on its inner side. A battery 37 can be connected to the connections 31 and 33 when a switch 35 in series with the battery 37 is closed. The voltage of the battery 37 is selected such that the electric field strength that can be achieved with it at the layer 17 is greater than the critical field strength for the phase change effect. The surface of the glass plate 19 that is not in contact with the layer 17 contains a silver coating as a mirror 39. The layer 17 can be sealed between the glass plates 19 and 29 with a ring 41 made of epoxy resin or another suitable sealing material.

A dark screen S is arranged at an acute angle to the glass plate 19 so that the reflection of the screen S in the mirror 39 can be seen by the eye E of the observer, as indicated in the drawing, when the layer 17 is transparent.

When the switch 35 is open, the image seen in the mirror 39 is bright because the layer 17 is opaque and reflects ambient light into the eye E. However, when the switch 35 is closed, the mirror image is dark because the layer 17 has become transparent and the image of the screen S is seen in the mirror 39. When the switch 35 is opened again, the image is bright because the layer 17 is opaque again.

Such display devices can be manufactured in a known manner and have characters or symbols which can be controlled electronically by a control logic (not shown) analogous to the switch 35 .

Alternatively, the applied field can be an alternating field (frequency up to about 50 kHz) instead of a equal field.

Fig. 2 is a schematic representation of a liquid crystal information storage device or display device of known type in which the liquid crystal materials of the invention can be used. A layer 63 of the liquid crystal material is arranged between glass plates 65, 67 having conductive coatings 69, 68 on their respective inner surfaces. A laser 71 which produces a beam 73 in the infrared part of the spectrum is arranged so that the beam 73 can scan the plate 65 with the coating 69 with a conventional laser deflection unit 75. The plate 65 is transparent to the beam 73 which is, however, absorbed by the coating 69. An alternating output voltage source 77 is electrically connected to the coating 68 and the coating 69 by a switch 70 .

The device operates in the following way: Information is recorded by the beam 73 onto the coating 69 and onto the layer 63 by heat conduction from the coating 69. The heat conducted to the layer 63 is sufficient to increase the temperature of selected local areas of the layer 63 and to cause a phase change of these areas. If the material of the layer 63 is originally in a cholesteric mesophase, the phase is changed to the isotropic liquid phase.

If the material is originally in a smectic mesophase, the phase is changed to a nematic mesophase, if the material has one, or to an isotropic liquid phase. After the beam 73 has heated a given region of the coating 69 and layer 63 and is deflected to another region, the given region rapidly cools and the material in that region returns to its original phase, but in a cloudy or scattering form of the original phase. Thus, if the regions of layer 63 not scanned by the beam 73 are in the clear or non-scattering form of the original phase, the scanned regions can be distinguished from the non-scanned regions by their optical scattering properties. Thus, when the layer 63 is visually viewed through the wafer 67 and coating 68 either after removal of the beam 73 or while it is in operation through a suitable filter, the recorded information can be seen. The coatings 69, 68 are made optically transparent for this purpose.

Erasing of stored or displayed information is accomplished by scanning layer 63 with beam 73 while applying an electric field to layer 63 by closing switch 70. The liquid crystal material of layer 63 is heated to change phase state and then cooled to return to its original phase; however, in this case, the applied voltage causes the liquid crystal material to return to the clear form of the original phase.

Information can optionally also be erased by scanning the layer 63 with the beam 73 without applying an electric field, provided that a sufficiently low scanning speed is used for the beam 73 .

Fig. 3 is a block diagram of a timepiece in which a liquid crystal material according to the invention can be used. A quartz oscillator 81 with an oscillation frequency of 32.168 kHz provides an output signal which is divided in a down counter 83 which provides an output signal with a frequency of 64 Hz. The output signal is applied to a logic unit 85 which performs a further reduction in the ratio 1:64. The logic unit 85 thus generates a single pulse every second. It counts these second pulses and generates a minute pulse every 60 second pulses. It also counts the minute pulses and generates an hour pulse every 60 minute pulses.

The second, minute and hour pulses are fed to a gate (not shown) within the logic unit 85 which allows drive signals to be applied to a liquid crystal display device via selected conductors of a conductor bundle 86. When a second, minute or hour pulse arrives at the gate of the logic unit 85 , it causes selected conductors 86 to be switched in an appropriate manner.

The display unit 87 is a digital display having three sections. The first section 88 displays the respective seconds, the second section 89 the respective minutes, and a third section the respective hours. The logic unit 85 gate selects the appropriate digits to be displayed in sections 88, 89 , and 90 by selecting the appropriate conductors 86 for applying drive signals. When it receives a second, minute, or hour pulse, the gate changes the selected portions of conductors 86 to those required for the next respective digits in chronological order.

The display device 87 operates in any known manner; for example, its digits may be operated by phase change effect in the manner described with reference to Figure 1. The liquid crystal material used for the display device 87 is selected to suit the method of operation and contains at least one compound of formula I.

Commercially available ICs can be used as the down counter 83 and logic unit 85 , e.g. the CMOS ICs SCL 5423 or 5425 or SCL 5424 or 5428 (digit/decoder control device).

Claims (2)

1. Liquid crystal materials based on 4,4'-di-substituted diphenyl,
marked by
at least one 4'-substituted 4-cyanodiphenyl of the formula I °=c:40&udf54;&udf53;vu10&udf54;&udf53;vz3&udf54;&udf53;vu10&udf54;wherein R is methyl, ethyl or ethoxy. 2. Use of the liquid crystal materials according to claim 1 in optical and electro-optical display devices.