GB2198743A

GB2198743A - Ferroelectric smectic liquid crystal mixtures containing terphenyls

Info

Publication number: GB2198743A
Application number: GB08725928A
Authority: GB
Inventors: Edward Peter Raynes; Madeline Joan Bradshaw
Original assignee: SECR DEFENCE; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; UK Secretary of State for Defence
Priority date: 1986-11-13
Filing date: 1987-11-05
Publication date: 1988-06-22
Anticipated expiration: 2007-11-05
Also published as: GB2198743B; GB8627107D0; GB8725928D0

Abstract

Ferroelectric smectic liquid crystal materials, being a mixture of compounds and containing at least one terphenyl of formula I: <IMAGE> where R1 is H, R or RO; R2 is H, R', OR', COOR', OOCR'; R and R' being C1-12 alkyl; one of X and Y being F, the other F or H, together with an optically active compound compatible with smectic phases. These materials may be used in electro- optical display devices.

Description

FERROELECTRIC SMECTIC LIQUID CRYSTAL MIXTURES CONTAINING TERPHENYLS This invention relates to ferroelectric smectic liquid crystal materials which are mixtures of compounds including terphenyls.

Liquid crystal materials are in widespread use in electrooptical displays, wherein an electric field induced change in the liquid crystalline structure leads .to a related change in the optical properties of the liquid crystal material.

Two important classes of liquid crystal material exist: those which use the electro-optical properties of the nematic state and those which use the electro-optical properties of the ferroelectric chiral smectic state. Generally in ferroelectric smectic liquid crystal materials the ferroelectric properties of the chiral smectic C (Sc) state are exploited, as this is the most fluid.

Ferroelectric smectic materials offer the possibility of very high switching speeds eg 1 msec - 100 psec compared to the nematic 10-100 msec6 and bistable memory effects. Although some single compounds are effective as ferroelectric smectic materials, generallly such materials comprise a mixture of compounds.

These mixtures nominally consist of a "host" component, which is a smectic material, usually Sc, with which is mixed an optically active (chiral) "dopant" which induces the for * mation of an Sc phase, preferably with a high spontaneous polarisation coefficient Ps. The dopant need not be a smectic material or compound, but should at least be compatible with hectic phases, ie the smectic phase shown by the host should not be destroyed, but preferably encouraged by the dopant. In some cases the host need not show a smectic phase until a dopant is added.

In preparing a useful ferroelectic smectic liquid crystal mixture of this type it is a primary requirement to identify a good host-dopant combination showing desirable properties such as low viscosity, high Ps broad smectic phases and high switching speeds.

It is an object of the present invention to provide novel and useful host materials and host-dopant combinations.

According to the present invention a ferroelectic smectic liquid crystal material is a mixture of compounds containing at least one terphenyl of formula I:

Formula I wherein R1 is slected from H, R and RO; R2 is selected from H, 1 1 1 1 1 R , OR , COOR and OOCR where R and R independently represent C1 - 12 alkyl1 and X and Y are independently selected from fluorine and hydrogen at least one being fluorine; and at least one other compound which is optically active and compatible with smectic liquid crystal phases.

By "compatible is meant that the optically active compound, the "dopant", does not destroy the smectic phase, and also induces the appearance of a chiral smectic phase.

Terphenyls of formula I are known, and are described for example in European Patent Application 8430494.3. They are described therein exclusively for use in nematic materials, and made into mixtures with the purpose of producing materials which show a substantially reduced tendancy to form smectic phases.

Their usefulness in ferroelectic smectic materials is thus wholly unexpected.

The liquid crystal materials of the invention may be used in liquid crystal electro-optical devices which use the ferroelectric effect of conventional design, such as the "Clark Lagerwall Device" described in Appl Phys Lett (1980), 36, 899, and their suitability for such use is among the reasons for the various preferred aspects of the invention discussed below.

Preferred structures for the compounds of forumla I are those in which R1 and R2 are both independently alkyl or alkoxy, and Y is fluorine, ie structures 1.1, 1.2, 1.3 and 1.4 below:

where R and R1 represent either the same or different alkyl groups and preferably each contain 3 - 12, especially 4 to 10 carbon atoms.

The alkyl groups'R and R1 may be straight chain (n-), branched chain, optically active or racemic mixtures of enantiomorphs. A useful ferroelectic smectic mixture of the invention may consist of one or more terphenyls of formuls I, eg of structures 1.1 - 1.4, in which both R and R1 are selected from n-alkyl or racemic alkyl plus a chiral dopant which induces a usefully high Ps.

Generally terphenyls of structure 1.1 and/or 1.2, or mixtures thereof are preferred as the principal constituents having a formula I used in a mixture of the invention.

It may be advantageous to include also one or more terphenyls of formula I, especially of formulae 1.1, 1.2, 1.3 or 1.4, partic ularly 1.1 or 1.2, in which one or both, but preferably one, of and R2 are optically active in the mixture of the invention. In such a case the mixture may also contain one or more non-optically act ive (which may be racemic) terphenyls of formula I.

It may be advantageous when the mixture contains optically ac tive terphenyls of formula I for the mixture to include one or more terphenyls of formula I where the twist sense of the helical Sc phase induced in the mixture by the optically active terphenyl of formula I is opposite to that induced by the dopant, which will lead to an unwinding" of the helical phase thus increasing the pitch) length. This is of value as it is desirable for the pitch length * of the Sc phase of a ferroelectric smectic liquid crystal material to approximate to the cell thickness of the device in which it is used. In practice it can be difficult to manufacture cells of very narrow spacing.This method of unwinding the pitch of an Sc phase is known as "pitch compensation" and when used it is preferred that the senses of Ps induced by the compensator and the dopant are the same.

Preferred alkyl groups which may exist in an optically active form or as racemic mixtures, from which R and R1 may be selected for use as outlined above are 2-methylbutyl, 3-methylpentyl, 4methylhexyl and 2-methylheptyl. For use in pitch compensation 2-methylbutyl is preferred.

Although the smectic C phase of a mixture according to the invention which contains an optically active terphenyl of structure 1.1, 1.2, 1.3 or 1.4 would be chiral, it is not likely to show a usefully high Ps, therefore the inclusion of a chiral dopant of a structure other than these is still necessary.

It is preferred that the mixture according to the invention contains two or more terphenyls of formuls I, for example the mixture may contain two or more compounds of structure 1.1, 1.2, 1.3 or 1.4. Such a mixture may for example contain two or more compounds of structure 1.1 where the combinations of R and R1 are different, together perhaps with one or more compounds of structure 1.2 or/and 1.3. Mixtures containing 1 to 4 terphenyls of formula I are generally preferred.

Whichever compounds of formula I are used in the mixture it is preferred that the proportions of the compounds in the mixture are selected such that the combination of compounds of formula I is a eutectic having a low melting point and a brood Sc temperature range.

The chiral dopant(s) used in a material of the invention may be one of those which are known to induce a usefully high Ps in smectic hosts. Although the dopant may be an optically active temphenyl of formula I, preferred dopants are the smectogenic esters of optically active 2-methylheptanol (2-octanol) for example those described in European Patent Application 0110299, eg the compounds of structure:

where 20ct is optically active 2-octyl, (2-methylheptyl), and where Ra is n-alkyl or n-alkoxy containing 1-12 carbon atoms.

Suitable chiral dopants also include the compounds described in PCT/GB85/0512 which are diesters of lactic acid, for example the compounds of general structure:

where R1 is C3-C12 n-alkyl or n-alkoxy and R2is C1-C5 n-alkyl preferably ethyl.

Other suitable chiral dopants include the terpenoid derivatives described in PCT/GB86/OD46, particularly those where the terpenoid group in the compound is a cyclic terpenoid. Other dopants include the compounds described in PCT/GB87/00058, PCT/GB87/00132, PCT/GB 87/ 00131 and PCT/GB87/00280.

Examples of compounds described in PCT/GB87/00280 include the following:

where R is C1-12 n-alkyl or n-alkoxy.

A particularly preferred class of chiral dopant is those described in PCT/GB87/00441 (claiming priority from GB-A-8615316), ie compounds of strucutre:

where X is a mesogenic (ie liquid crystal forming) group such as

where R is C1-C12 n-alkyl or n-alkoxy, and R' is cyclohexyl or alkyl, which may be n-alkyl containing 1 to 12 carbon atoms or a branched alkyl group of formula:

where t may be o or an integer 1-6 and n and m may be the same or different and have values 1 to 6. Preferably t is o or 1 and at least one of n or m is lg and whicm may be optically active.

R' may for example therefore be CH3, CH(CH3)2, CH(CH3)C2H5, CH2CH(CH3)2, C4 H9orC2H5.

Other examples of suitable chiral dopants include the compounds described in the following published European Patent Applications: 115693, 131373, 136845, 152217, 153826, 157519, 159872, 160416, 164,814, 167328, 168963, 174191, 175591, 188222, 191600, 192267 and 207712.

Other suitable compounds will be known to those skilled in the art.

As well as including the compounds mentioned above, materials according to the invention may also include other known additivies to usefully modify the properties of the material. These additives may include other known Smectic host materials such as the compounds described in PCT/GB86/0040, which are good host materials.

Additives may include compounds which broaden the Sc range, such as the known compounds:

where Ra is as defined above and Rb is n-alkyl or n-alkoxy containing 1-12 carbon atoms.

Additives may also include compounds containing a

group, for example compounds of formula

where R' and R are independently selected from n-alkyl containing 1 to 12 carbon atoms.

Additives may also include pitch compensators other than optically active terphenyls of formula I, for example the compounds of PCT/GB87/00280 (described above) or the amide compounds described in PCT/GB87/00223.

Particularly suitable pitch compensators include the terphenyls described in GB-A-870313, for example compounds of formula:

where Ra is C1C12 n-alkyl or n-alkoxy.

Typically but not exclusively a material of the invention will have the following composition in wieght percentages:

Non-optically active (may include racemic) terphenyls of Formula I. 3 3 95 % Cryptically active terphenyls of Formula I. # 0 - 50 % Chiral Dopants. i 1 - 20 % Other host materials. # 0 - 20 % Other additives, eg to extend 5c phase, or pitch compensators other than terphenyls of 5 O - 30 % formula I -) The total being 100 wt %.

The nature and relative proportions of the various components of a material of the invention will depend upon the practical requirements of the user, eg a liquid crystal device manufacturer. Some experimentation may be required to determine the most suitable composition for a particular requirement, but it is considered that the invention has at least identified a useful base material from which to start.

The materials of the invention may be used in any of the known types of ferroelectric smectic liquid crystal device. An example of such a device is the "Clark-Lagerwall Device" also described in "Recent Developments in Condensed Matter Physics" (1981), 4, 309. The physics of this device, and methods of constructing one are well known. In practice such a device usually consists of two substrates, at least one of which is optically transparent, electrodes on the inner surfaces of the substrates by which a voltage may be applied and a layer of the liquid crystal material sandwiched between the substrates.

The Clark-Lagerwall device uses a layer of liquid crystal material between the substrates of a thickness comparable to or * less than the helical pitch of the S configuation, which causes the helix to be unwound by surface interactions. In its unwound state the material has two surface stabilised states with director orientations (ie molecular tilt direction) at twice the tilt angle to one another, and also permanent dipole orientations perpendicular to the substrates but in opposite directions.

An alternative approach to providing cells for a Clark Lagerwall device having a thicker layer of liquid crystal material is to use an applied electric field to induce homogeneous alignment through interaction with the dielectric anisotropy of the liquid crystal material.

The Clark-Lagerwall device may operate in two display modes: a birefringence type using two polarisers, and the other a guesthost type using a dichroic dye. The Sc phase has spontaneous polarisation, hence when the polarity of the applied voltage is * reversed, the liquid crystal molecules in the Sc phase reverses around their helical axis as a rotating axis. This collective molecular rotation can occur very quickly making fast electrooptical sqitching possible, and generally requires a fairly high field, eg 10-15 volts applied across a lum sample.

For use in such a device the materials of the invention offer most of the advantageous properties generally required of ferroelectric smectic liquid crystal materials eg a high Ps. They offer a particular advantage in that certain of them have exceptionally high switching speeds, in some cases up to five times that of presently known ferroelectric smectic mixtures. Certain materials also possess the desirable phase sequence isotropicne matic - SA - Sc, which is advantageous in achieving surface alignment in a liquid crystal device as described above, for the reasons discussed inter alia in Mol Cryst Liq Cryst (1984), 114, 151-187.

Materials according to the invention may be prepared by the entirely convertial technique of mixing the various components described above in a suitable vessel, heating this mixture to a temperature of which it is an isotropic liquid, maintaining it at this temperature for a few minutes, and allowing it to cool to ambient temperatures.

The terphenyls of formulae I may be prepared by the preparative routes described in published European Patent Application 0132377. An alternative route is that shown in Fig 1 of the 11 1 accompanying drawings, where R and R may be alkyl or alkoxy, and in which the following steps are involved: (i) (a) n-Buli, either, < - 50C, 2 hours.

(b) ZnCl2, THF at room temp. 1 hour.

(ii) THFat room temp, 20 hours, PdO (P Ph3)4.

The PdO (P Ph3)4 is prepared in situ using the side route shown. (ref E Negishi, A O King, N Okukado, J.O.C (1977), 42, 1821 and A O King, E Negishi, JOC (1978), 43, 358).

The compound

is commercially available from Boots plc. The overall yield of this route is 40-60%. Although the individual steps are known1 the overall route is novel.

These preparative routes are equally suitable for the preparation of optically, non-optically active and racemic terphenyls of formula I.

The invention will now be described by way of example, referring to the accompanying figures: Fig 1 shows a preparative route for compounds of Formulae I and 11.

Fig 2 shows a graph of switching time against applied voltage for the materials of examples 1 and 4.

Fig 3 shows variation of Ps with temperature of materials of examples 1, 2 and 4.

Fig 4 shows variation of the tilt angle with temperature of materials of examples 1, 2 and 4.

Fig 5 shows variation of the tilt angle and Ps with temperature for the material of example 5.

Fig 6 shows the variation of switching time against applied voltage for the material of example 5.

Fig 7 shows a liquid crystal cell used for response time measurements.

Various materials according to the invention were prepared as exemplified below.

Example 1 (Mixture 123)

30.99 wt % 31.27 wt % 30.96 wt % 6.77% This material showed the following liquid crystal phase transitions, in OC.

Sx 30.5 Sc 44.8 SA 84.8 Ch 116.9 I Fig 2 shows the log-log plot of response time against applied voltage (peak to peak). This measurement was per formed using a 6u polyimide (PI) cell. Response time is expressed in terms of O - 100% transmission. At convenient working voltages the response time is seen to be substantially less than 1 msec.

FigN and 4 show the variation of Ps and tilt angle respectively with temperature for the material of example 1.

-2 Extrapolated Ps was 26.46 nCcm Example 2 (Mixture 121)

43.74 wt % 43.96 wt % 12.3 wt % Fig 3 and a show the variation of ts anì tilt angle resectively with temperature for the material of example 2.

The liquid crystal phase transitions were (OC) * S 31.0 Sc 44.8 SA 85.7 Ch 110.9 I.

Response time for the mixture was found to be 1 msec 0-100% transmission using a 6u PI cell at 35 C, applying 10 volts peak to peak.

-2 Extrapolated PS was 35.28 nC on

Example 3 (Mixture 132) 4 9 a+ Cg 11 31.39 wt % C7 15 mThF\OC3H? 12.63 wt % F C 87 C3H7 12.93 wt % F CH 3 C6 H13 oOQ-CH2%H5-i-(-) 43.06 wt % Phase transitions were ( C) * K 1.0 S 25.6 Sc 65.9 ch 139.9 I Example 4 (Mixture 133) This consisted of the material of Example 3 (91.5 wt %) plus 8.5 wt % of

Fig 2 shows the log - log plot of response time against applied voltage (peak to peak).

Phase transitions were ( C) * S 28.2 Sc 74.3 SA 86.4 Ch 128.1 I Fig 3 and 4 show the variation of Ps and the tilt angle with temperature of this material. Extrapolated Ps of this material -2 was 110.5 nC cm mle Example 5 (mixture 139)

80 wt %.

5 wt .

15 wt %.

The liquid crystal transitions were ( C): 3x 19.0 Sc 79 5A 94 Ch 115 1 Fig 5 shows the variation of tilt angle and Ps of this material with temperature.

Fig 6 shows the variation of response time with applied voltage (peak to peak) of this material at 250C and 400c, together with comparitive data for a 5 wt % solution of The compound identified as (D) in mixture 139 in the smectic host H1

1:1:1 by wt.

this mixture being identified as mixture H1D. Fig 7 shows that the mixture of the invention, 139, is consistently faster switching than mixture H1D.

The host H1 is a commercially available smectic host, and is widely used in ferroelectric smectic liquid crystal mixtures.

Example 6 A smectic host H2 was prepared having composition:

32.5 wt % 29.5 wt % 22.0 wt % 16.0 wt % A mixture according to the invention was made by having the composition: H2 75 wt %

5 wt % 20 wt % and had phase transitions (OC) I 128 N* 84 SA 64 S* < -20 C Bv2mnSe 7 H2 80 wt %

5.5 wt % 4.5 wt % 10 wt % The configuration of the two optically active compounds was chosen to be such that they induced the appearance of chiral smectic phases of opposite helical twist sense.

The mixture had phase transitions (OC) r 145 N* 114 5A 59.7 Sc 10 In examples 1 to 7 above the following terminology is used: K = solid crystal PI = polyimide (layer 3 in cell) S = smectic +E = asymmetric centre in structure Sc = chiral smectic C SA = smectic A Ch = cholesteric (chiral nematic) I = isotropic liquid Referring to Fig 7 a liquid crystal cell as used for response time measurements, and equally suitable for use as an electrooptic display device with the materials (4) of the invention, eg of examples 1 to 7, is shown.

The cell consists of two glass slides (1), arranged parallel.

On the inner face of each slide is a layer of a transparent conducting material eg tin oxide or indium tin oxide (2), coated with a transparent layer of a polyimide (3) to protect the conducting material (2) and to aid surface alignment of a layer of Sc liquid crystal material (4) sandwiched between the coated slides (1). The space between the polyimide layers defines the thickness of the layer of liquid crystal material (4), and its edges are sealed with a spacer (5) of nylon.

The cell is normally positioned between crossed polarisers (not shown) when being used in the birefringence mode referred to above.

Claims

1. A ferroelectric smectic liquid crystal material, being a mixture of compounds and containing at least one terphenyl of formula I:

Formula I wherein R1 is selected from H, R and RO: R2 is selected from H, R', OR', COOR' and OOCR' where R and R' independently represent C1 - C12 alkyl, and X and Y are independently selected from fluorine and hydrogen at least one being fluorine, together with at least one other compound which is optically active and compatible with smectic liquid crystal phases.

2. A material according to claim 1, wherein R1 and R2 are both independently alkyl or alkoxy each containing 3-12 carbon atoms, X is hydrogen and Y is fluorine.

3. A material according to claim 2, wherein the groups R1 and R2 are n-alkyl or n-alkoxy.

4. A material according to claim 2, wherein one or both of R1 and R2 are racemic alkyl or alkoxy groups.

5. A material according to claim 2, wherein one or both of R1 and R2 are optically active.

6. A material according to claim 3 or 4 containing in addition at least one terphenyl of formula I in which one or both of R1 and R2 are optically active alkyl or alkoxy groups.

7. A material according to claim 6 wherein only one of R1 or R2 are optically active alkyl or alkoxy.

8. A material according to any one of claims 5, 6 or 7 wherein the optically active alkyl or alkoxy is 2-methylbutyl or 2-methylbutyloxy.

9. A material according to any one of the preceding claims containing up to four terphenyls of formula I.

10. A material according to claim 9 wherein the combination of terphenyls of formula I is a eutectic.

11. A material according to any one of the preceding claims wherein at least one of the said other optically active compounds has a formula

where 2 Oct is optically active 2-octyl, (2-methylbeptyl) and where R is n-alkyl or n-alkoxy containing 1-12 carbon atoms.

12. A material according to any one of claims 1 tolO wherein at least one of the said other optically active compounds has a formula:

where X is a mesogenic group and R' is cyclohexyl or alkyl.

13. A material according to claim 12 wherein X is selected from

14. A material as claimed in any one of claims 12 or 13 wherein R' is n-alkyl containing 1 to 12 carbon atoms or a branched aklyl group of formula:

where t may be o or an integer 1-6 and n and m may be the same or different and have values 1 to 6.

15. A material according to claim 14 wherein R' is selected from CH3, CH(CH3)2, CH(CH3)C2Hs, CH2CH(CH3)2, C4H9 or C2H5.

16. A material according to any one of the preceding claims containing one or more compounds selected from:

where Ra and Rb are independently selected from C1 -C12 n-alkyl or n-alkoxy, or from:

where R' and R are independently selected from n-alkyl containing 1 to 12 carbon atoms or from:

17. A material as clained in any one of the preceding claims wherein the percentage of non-optically active (which may include racemic) terphenyls of formula I is in the inclusive range 30-95 weight %.

18. A material as claimed in any one of claims 6 to 17 wherein the percentage of optically active terphenyls of formula I is in the inclusive range 0 to 50 weight t.

19. A material as claimed in any one of the preceding claims in which the percentage of the said other optically active compound(s) is in the range 1 to 20 weight %.

20. A material according to any one of the preceding claims, substantially as described herein with reference to examples 1 to 7.

21. A liquid crystal electro-optical device which uses the ferroelectric effect in a smectic liquid crystal material, wherein the material is a material as claimed in any one of claims 1 to 20.