Liquid crystal compositions

Abstract

A liquid crystal composition having a high order, tilted ferro-electric achiral Smectic I phase exhibiting a relaxed cone angle of greater than 20.degree. in the absence of high surface tilt alignment layers at temperatures in the range of 5.degree. C. to 40.degree. C. and includes from 90% to 99.9% by weight of a mixture of at least two achiral host compounds and from 0.1% to 10% by weight of a chiral dopant compound.

Description

This invention relates to liquid crystal compositions and to devices incorporating such compositions. Such compositions are incorporated in liquid crystal devices (LCDs) and exhibit useful electro-optical effects, due to the fact that by application of an electric field across a thin layer of such a composition the molecules can be caused to switch between two possible states of molecular orientation and, as a result of the anisotropy of such a medium, the optical properties of the two states are different and the transmission and/or reflection of light is changed. One particular class of compositions in which both of the states are stable even in the absence of an applied field contains such a thin layer and exhibits ferro-electricity. It is with such compositions that the present invention is concerned.

It is desirable in such ferro-electric compositions to achieve a high contrast ratio and high transmissivity over as wide a temperature range as possible whilst utilising the lowest possible operating voltage. To achieve this it is believed that the molecules comprising the composition should be in as ordered a state as possible, a property which is often measured by the so-called "relaxed cone angle" which should ideally be 45.degree. . Some success in this regard has been obtained previously by modifying the environment containing the composition, which typically comprises two parallel flat Mass plates, for example by providing the so called "high surface tilt alignment layer" on the surfaces of the plates which come into contact with the composition. However, this is very difficult to apply sufficiently uniformly and consistently, which leads to low yields of product and corresponding high costs.

It is an object of the present invention to provide a ferro-electric liquid crystal composition which, in the absence of such expensive alignment layers, exhibits a high cone angle.

Accordingly we provide a liquid crystal composition which in use, comprises a high order, tilted ferro-electric achiral Smectic I phase exhibiting a relaxed cone angle of greater than 20.degree. in the absence of high surface tilt alignment layers, at temperatures in the range of 5.degree. C. to 40.degree. C. The composition comprises from 90% to 99.9% by weight of a mixture of at least two achiral host compound each of the Formula 1. ##STR1## wherein X is A.sub.1, or OA.sub.1,;

Y is A.sub.1 ; OA.sub.1 ; OCOA.sub.1,; or CO.sub.2 A.sub.1 ; wherein A.sub.1 is a straight or branched chain alkyl group containing from 1 to 18 carbons atoms, optionally substituted with one or more F or CN; Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 may each be H or F, but either one or two of Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 must be F; ##STR2## and from 0.1% to 10% by weight of a chiral-dopant compound of the Formula 2. ##STR3## wherein D.sup.1 is D (as Formula 1) or a single bond; Y.sup.1 is OA.sub.3, OCOA.sub.3 or C.sub.2 A.sub.4 ; A.sub.3 is an alkyl group containing from 1 to 18 carbon atoms, optionally substituted with one or more F or CN and containing at least one centre of asymetry; A.sub.4 can either be the same as A.sub.3 or a group of the formula --CH(R.sub.1)COA.sub.5 --. wherein R.sup.1 is an alkyl group containing from 1 to 5 carbon atoms and A.sub.5 is OR.sub.1 or --N(R.sub.2)R.sub.3 where R.sub.2 and R.sub.3 are hydrogen, R.sub.1, phenyl optionally substituted by R.sub.1, or together form a homocyclic or hetrocyclic ring optionally substituted by R.sub.1 provided that R.sub.2 and R.sub.3 are not the same; and D, X, Z.sub.1, Z.sub.2, Z.sub.3, and Z.sub.4 have the above significance, except that all of Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 can be hydrogen wherein the said mixture of at least two achiral host compound comprises a mixture of: between 40.0% and 60.0% wt of Component A between 10.0% and 25.4% wt of Component C between 10.0% and 25% wt of Component D (note compound D is not the same as Group D of formula 1) wherein component A consists of one of the members selected from the consisting of group: ##STR4## wherein Component C consists of one of the members selected from the group ##STR5## and wherein Component D consists of one of the members selected from the group consisting of: ##STR6##

By achiral compounds we mean compounds which are not optically active either because they possess no centre of asymmetry or because they contain a racemic mixture of the (+) and (-) forms of such enantiomers.

By chiral compounds we mean compounds which are optical active because they possess a centre of asymmetry.

Particularly useful achiral host compounds of formula 1 comprise those in which:

A.sub.1 is a straight chain alkyl containing from 3 to 12 carbon atoms; ##STR7## either Z.sub.1 is F and Z.sub.2, Z.sub.3, Z.sub.4, are H or Z.sub.1, Z.sub.2, are H and Z.sub.3, 7.sub.4 are F; Y is A.sub.1 or OA.sub.1

Particularly useful chiral dopant compounds of formula 2 comprise those in which: ##STR8## or a single bond; A.sub.1 is a straight chain alkyl containing from 3 to 12 carbon atoms; either Z.sub.1 is F and Z.sub.2, Z.sub.3, Z.sub.4 are H or Z.sub.1, Z.sub.2, are H and Z.sub.3, are F, or all of Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 are H;

Y.sup.1 is CO.sub.2 A.sub.4 A.sub.4 is a straight or branched chain alkyl group substituted by F or CN or a branched chain alkyl group possessing a centre of asymmetry or --CH(R.sub.1)--CON(R.sub.2)R.sub.3

Specific achiral host compounds of formula 1 which we have found to be particularly useful include: ##STR9## Specific chiral dopant compounds which we have found to be particularly useful include: ##STR10##

The compounds used as achiral hosts and as chiral dopants are known and most are commercially available.

The achiral host is formulated as a mixture of at least four compounds of formula 1 such that this mixture has suitable physical properties i.e. such that it remains in the liquid crystalline state between -25.degree. C. and 100.degree. C.

The compositions of our invention comprise the said mixture of achiral host compounds of formula 1 and at least one dopant of formula 2, and when mixed to form eutectic ferro-electric liquid crystal mixture, exhibit a high degree of ordering of their molecules, which is believed to be due to the composition being in the so-called chiral Smectic I phase (denoted as S*.sub.1 phase). Although the S*.sub.1 phase range exhibited, by most of the ferro-electric liquid crystal (FLC) mixtures are found to be in the range of 20.degree. C. to 40.degree. C., the higher ordered FLC phases such as S*.sub.j and S*.sub.K which occur at temperatures below the S*.sub.1 phase do not affect the alignment stability macroscopically. This is also true for the lower ordered FLC S*.sub.c phase which occurs at a higher temperature above the S*.sub.1. Hence, an FLC device containing such mixtures can be stored over a wide temperature of >-25.degree. C. up to 90.degree. C.

The relaxed cone angle (.theta.) of a ferro-electric liquid crystal device (FLCD) is measured in the absence of an applied field. It is normally carried out under crossed polarising microscope. A region in the FLCD cell containing the two domain states that are adjacent to each other (ie, the dark and bright states) is then selected and the cell is rotated until an extinction for one of the states is obtained. The position on the rotating stage of the microscope is noted as .theta..sub.1, The cell is then rotated in either clockwise or anticlockwise direction until an extinction for the other domain states has occurred and the new position on the stage is noted as .theta..sub.2. The relaxed cone angle, .theta., of the FLCD is taken as the difference of .theta..sub.2 -.theta..sub.1. The measurement is valid only when 0 is .ltoreq.45.degree. C.

As will be seen from the specific examples given hereafter, the compositions of our invention exhibit a relaxed cone angle of greater than 20.degree. over a range of 5.degree. C. to 30.degree. C. and, when it is believed that the S*.sub.1 phase is present, a cone angle of greater than 25.degree. over a range of 5.degree. C. to 45.degree. C. and, at the most common ambient working temperatures of 5.degree. C. to 40.degree. C., a cone angle of greater than 28.degree..

The consequence of this high cone angle is that the liquid crystal cells incorporating our compositions have a high contrast ratio of at least 10:1 over then temperature range (-25.degree. C. to 60.degree. C.) and of at least 50:1 within the usual working Lang(. of 5.degree. C. to 40.degree. C.

Devices made which contain cells employing our compositions (in the S*.sub.1 phase) also exhibit a high light transmission (between 15% to 20% or about twice as high as the FLCD using the S*.sub.c phase) and are more mechanically stable than devices employing previously known compositions containing the S*.sub.c phase.

The invention is illustrated, but in no way limited, by the following examples.

EXAMPLES 1 TO 5

Mixtures were prepared containing the percentages by weight ads shown in Table I of the achiral compounds A to F and of the chiral dopant XI. In the case of Examples 1 to 4, racemic XI (i.e. a mixture of the S-(+) and S-(-) enantiomers of XI) was also present.

Table II shows the properties of the resultant mixtures including the range of temperatures over which these exhibit S*.sub.I and the S*.sub.c phases; the relaxed cone angle as measured over a temperature range from 5.degree. C. to 40.degree. C.; and the contrast ratio over the same temperature range.

Additionally it may be noted that the relaxed cone angle of the mixture of Example 1 was between 28.degree. and 32.degree. over the temperature range 20.degree. to 35.degree. C. and 30.degree. at 25.degree. C. Even at 45.degree. C. (S*C phase) the relaxed cone angle was still 15.degree.. For the same mixture at temperatures above 45.degree. C., the contrast ratio was still between 10:1 and 30:1.

EXAMPLES 6 to 17

Mixtures comprising the same blend of host compounds as in Example 5 but, in the place of the dopant X2 using 2.0 per cent by weight of the dopants indicated in Table II were used, and this yielded mixtures having the properties indicated in table II.

TABLE I______________________________________% wt composition of mixtures EXAM- EXAM- EXAM- EXAM- EXAM-COMPOUND PLE 1 PLE 2 PLE 3 PLE 4 PLE 5______________________________________C1 12.5 12.0 11.4 7.5C2 12.7C3 12.5 12.0 11.4 7.5C4 12.7A1 25.3 6.7 6.5 6.1 8.9A2 20.3 15.7 15.1 14.3 20.9A3 15.7 15.1 14.3 20.9A4 5.0 5.0 4.8 4.5A5 6.7 6.5 6.1 8.9D1 12.0 16.0 15.4 14.6 14.5D2 4.0 2.0 1.9 1.8 2.2D3 4.0 2.0 1.9 1.8 1.9D4 1.2 2.4D5 1.2 2.4F1 1.0 1.9F2 1.0 1.9F3 0.5 1.0E1 1.2E2 1.2E3 1.2E4 1.2X1 CHIRAL 2.0 2.0 2.0 2.0 2.0X1 RACEMIC 2.0 3.0 2.0 2.0TOTAL 100.0 100.0 100.0 100.0 100.0______________________________________ TABLE II______________________________________ S.sub.I * Relaxed coneMixture range S.sub.C * range angle Contrast ratioexample Dopant .degree. C. .degree. C. 40 to 5.degree. C. 40 to 5.degree. C.______________________________________1 X1 <20-42 42-86 25-38 50:1 to >100:12 X1 <10-39 39-79 16-30 20:1 to >100:13 X1 <10-38 38-87 13-34 20:1 to >100:14 X1 <10-32 32-90 13-35 10:1 to >100:15 X1 <20-40 40-95 20-40 30:1 to >100:16 Y1 <20-47 47-93 25 to 40 50:1 to >100:17 Y2 <20-48 48-95 25 to 38 50:1 to >100:18 Z1 <20-44 44-96 24 to 36 50:1 to >100:19 Z2 <20-42 42-96 24 to 38 50:1 to >100:110 Y3 <20-45 45-97 24 to 40 50:1 to >100:111 N1 <20-44 44-95 23 to 38 50:1 to >100:112 N2 <20-42 42-93 22 to 38 50:1 to >100:113 N3 <20-45 45-96 25 to 39 50:1 to >100:114 N4 <20-46 46-96 25 to 38 50:1 to >100:115 N5 <20-45 45-94 25 to 39 50:1 to >100:116 N6 <20-43 43-96 23 to 38 50:1 to >100:117 N7 <20-43 43-90 24 to 38 50:1 to >100:1______________________________________

EXAMPLES 18 to 28

Mixtures were prepared containing the percentages by weight as shown in Table III of the achiral compounds A to F and of a combination of any one or more of dopants X1 chiral, X1 racemic, N8, X2 and N9.

TABLE III__________________________________________________________________________ Example Example Example Example Example Example Example Example Example Example ExampleCompound 18 19 20 21 22 23 24 25 26 27 28__________________________________________________________________________A1 4 7A2 23 25A3 18.2 20 25 24.74 24.74 17.48 17.48 17.13 17.13 16.95 16.59A4A5 5 6.2 11.34 11.34 11.11 11.11 10.99 10.76A6 25 24.74 24.74 17.48 17.48 17.13 17.13 16.95 16.59A7 9.6 9.5 9.5 7.35 7.35 7.2 7.2 7.13 6.98B1 4.9 4.9 4.75 4.65B2 4.9 4.9 4.75 4.65B3 4.8 4.8B4 4.8 4.8C1 6 5 5 4.95 4.95 4.5 4.5 4.41 4.41 4.37 4.27C2C3 6 5 5 4.95 4.95 4.5 4.5 4.41 4.41 4.37 4.27C4C5 6 5 5 4.95 4.95 4.5 4.5 4.41 4.41 4.37 4.27D1 10 7 14.5 14.35 14.35 9 9 8.81 8.81 8.72 8.54D2 1.9 1.88 1.88D3 5 3 2.2 2.18 2.18D4D5D6 3.88 3.88 3.8 3.8 3.76 3.68D7 10 10 3.88 3.88 3.8 3.8 3.76 3.68E1 1.2 1.2 1.2 1.19 1.19 1.08 1.08 1.06 1.06 1.05 1.02E2 1.2 1.2 1.2 1.19 1.19E3 1.2 1.2 1.2 1.19 1.19 1.08 1.08 1.06 1.06 1.05 1.02E4 1.2 1.2 1.2 1.18 1.19 1.08 1.08 1.06 1.06 1.05 1.02E5 1.08 1.08 1.06 1.06 1.05 1.02F1F2F3X1 chiral 1.7 1.7 2 2 3 5 7X1 racemic 0.3 0.3 1N8 3X2 2N9 3 2 2TOTAL 100 100 100 99.9 100 100 100.3 100.1 100.1 100.1 100__________________________________________________________________________

Claims

1. A liquid crystal composition which, in use, comprises a high order, tilted ferro-electric achiral Smectic I phase exhibiting a relaxed cone angle of greater than 20.degree. in the absence of high surface tilt alignment layers, at temperatures in the range of 5.degree. C. to 40.degree. C., comprising from 90% to 99.9% by weight of a mixture of at least two achiral host compounds each of the Formula I: ##STR11## wherein X is A.sub.1, or OA.sub.1 ;

Y is A.sub.1 ; OA.sub.1 ; OCOA.sub.1 ; or CO.sub.2 A.sub.1 ;

wherein

A.sub.1 is a straight or branched chain alkyl group containing from 1 to 18 carbon atoms, optionally substituted with one or more F or CN; Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 may each be H or F, but either one or two of Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 must be F; ##STR12## and from 0.1% to 10% by weight of a chiral dopant compound of the Formula 2: ##STR13## wherein D.sup.1 is D or a single bond;

Y.sup.1 is OA.sub.3, OCOA.sub.3 or CO.sub.2 A.sub.4 ;

A.sub.3 is an alkyl group containing from 1 to 18 carbon atoms, optionally substituted with one or more F or CN and containing at least one centre of asymmetry; A.sub.4 can either be the same as A.sub.3 or a group of the formula --CH(R.sub.1)COA.sub.5 --;

wherein

R.sub.1 is an alkyl group containing from 1 to 5 carbon atoms and

A.sub.5 is OR.sub.1 or --N(R.sub.2)R.sub.3 where R.sub.2 and R.sub.3 are hydrogen, R.sub.1, phenyl optionally substituted by

R.sub.1, or together form a homocyclic or heterocyclic ring optionally substituted by R.sub.1 provided that R.sub.2 and R.sub.3 are not the same;

and D, X, Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 have the above significance, except that all of Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 can be hydrogen;

wherein said mixture of at least two achiral host compounds comprises a mixture of:

between 40.0% and 60.0% wt of Component A

between 10.0% and 25.4% wt of Component C

between 10.0% and 25% wt of Component D

wherein component A comprises one of the members selected from the group consisting of: ##STR14## wherein component C comprises one of the members selected from the group consisting of: ##STR15## and wherein Component D comprises one of the members selected from the group consisting of: ##STR16##

2. A liquid crystal composition as in claim 1 wherein said mixture further comprises between 4% and 15% wt of component E, wherein component E comprises one of the members selected from the group consisting of:

3. A liquid crystal composition as in claim 1 wherein said mixture further comprises component F, wherein component F comprises one of the members selected from the group consisting of:

4. A liquid crystal composition as claimed in claim 1 wherein the said mixture further comprises component B:

5. A liquid crystal composition as claimed in claim 1 wherein the dopant compound consists of one of the members selected from the group consisting of: Chiral ##STR17## Racemic ##STR18##

6. A liquid crystal composition as claimed in claim 2, wherein said chiral dopant comprises: Chiral ##STR19## Racemic ##STR20##

7. A liquid crystal composition as claimed in claim 5 wherein said chiral dopant comprises: (a) X1 chiral and X1 racemic or (b) X1 chiral or (c) N8 or (d) N9 or combinations of any one or more said dopants.

8. A liquid crystal composition as claimed in claim 5 wherein said chiral dopant comprises (a) N9 or (b) X1 chiral or (c) X1 chiral and X1 racemic or (d) X2 and N9 or combinations of any one or more said dopants.

9. A liquid crystal composition as in claim 1 wherein said mixture includes:

______________________________________50.6% wt of component A25.4% wt of component C20.0% wt of component D.______________________________________

10. A liquid crystal composition as in claim 1 wherein said mixture includes:

______________________________________45.3%-49.8% wt of component A22.8%-24.05% wt of component C20.0%-23.0% wt of component D.______________________________________

11. A liquid crystal composition as in claim 1 wherein said mixture includes:

______________________________________ 50.2%-59.7% wt of component A14.85%-18.0% wt of component C18.41%-25.0% wt of component D04.75%-4.8% wt of component E______________________________________

wherein component E is a member selected from the group consisting of: ##STR21##

12. A liquid crystal composition as in claim 1 wherein said mixture includes:

______________________________________50.92%-53.65% wt of component A09.3%-09.8% wt of component B12.81%-13.5% wt of component C15.9%-16.76% wt of component D______________________________________ wherein component E is a member selected from the group consisting of: ##STR22## and wherein component B comprises: ##STR23##

13. A liquid crystal composition as claimed in claim 9 further comprising at least one additional dopant.

14. A liquid crystal composition as claimed in claim 10 further comprising at least one additional dopant.

15. A liquid crystal composition as claimed in claim 11 further comprising at least one additional dopant.

16. A liquid crystal composition as claimed in claim 12 further comprising at least one additional dopant.

17. A liquid crystal composition as claimed in claim 13, wherein said at least one additional dopant is selected from the group consisting of

18. A liquid crystal composition as claimed in claim 5, wherein said chiral dopant comprises X1 chiral and a compound selected from the group consisting of N8, N9, additional X1 chiral, and combinations of at least two of these compounds.

19. A liquid crystal composition as claimed in claim 5, wherein said chiral dopant comprises X1 racemic and a compound selected from the group consisting of N8, N9, X1 chiral, and combinations of at least two of these compounds.