GB2205098A - Liquid crystal material - Google Patents

Abstract

A compound forming a tilted smectic liquid crystal phase has the general formula R<1>-O-CH2-CH2-O-{Z}-R<2> in which: R<1> is an aliphatic radical containing carbon and hydrogen atoms and optionally one or more oxygen atoms each linked to separate carbon atoms and having a total number of carbon or of carbon and oxygen atoms not less than 1 and not more than 15; R<2> is an aliphatic radical containing carbon and hydrogen atoms and optionally one or more oxygen atoms each linked to separate carbon atoms and having a total number of carbon or of carbon and oxygen atoms not less than 4 and not more than 18; and {Z} is an aromatic or alicyclic core of a type suitable for a liquid crystal.

Description

Liquid Crystal Material This invention relates to compounds forming a tilted smectic liquid crystal phase and to electrooptical devices containing a ferroelectric liquid crystal material.

Ferroelectric behaviour has been observed in liquid crystal materials which possess a tilted chiral smectic phase. The liquid crystal is desirably in the smectic C phase, although other tilted smectic phases such as smectic I and smectic F phases can also form a ferroelectri c system.

The chirality may be present either intrinsically in the liquid crystal molecule or in a chiral dopant which may be added at about 5 - 20% to the liquid crystal material. Examples of suitable chiral dopants include the oct-2-yl esters of 4-carboxyethyl -4' -heptyl oxybi phenyl, 6-(4-octylbenzoyloxy)naphth-2-oic acid and 6-octyloxynaphth-2-oic acid.

In either case the material should possess a high spontaneous polarisation in order to be used in a ferroelectric liquid crystal display device. Such a device containing for example a chiral smectic C (Sc*) liquid crystal should be constructed to permit the action of a direct electric field upon the ferroelectric dipole moment.

The molecules in liquid crystal materials generally have a rigid core of known type, to opposite ends of which are attached two groups of various kinds. In materials forming a smectic liquid crystal phase, each of these groups is usually a straight or branched alyky or alkoxy group.

Compounds having more than one oxygen atom in the groups attached to the core have been thought to be relatively unattractive as liquid crystal materials. The introduction of the second oxygen atom into the carbon chain has been found to cause a large reduction in the nematic to isotropic liquid phase transition temperature.

However, it has now been found that certain compounds of this type are surprisingly advantageous in tending to promote a tilted smectic liquid crystal phase.

According to the invention a compound forming a tilted smectic liquid crystal phase has the general formula R1 - 0 - CH2 - CH2-O - {Z} - R2 (I) in which: R1 is an aliphatic radical containing carbon and hydrogen atoms and optionally one or more oxygen atoms each linked to separate carbon atoms and having a total number of carbon or of carbon and oxygen atoms not less than 1 and not more than 15;; R2 is a straight-chain or branched alkyl or perfluoroakyl group with in each case 4 - 18 C atoms, in which one or more CH2 or CF2 groups can also be replaced by a grouping selected from the group comprising -0-, -S-, -CO-, -0-, -0-CO-, -S-CO-, -0-COO-, -CO-O-, -CH=CH-, -CHCF3, -CHhalogen- and -CHCN- or by a combination of two suitable groupings, two hetero atoms not being linked directly to one another, R2 is preferably an aliphatic radical containing carbon and hydrogen atoms and optionally one or more oxygen atoms each linked to separate carbon atoms and having a total number of carbon or of carbon and oxygen atoms not less than 4 and not more than 18; and (Z) is an aromatic or alicyclic core of known type for liquid crystals.

R1 is preferably an alkyl or alkoxyalkyl radical, with up to 7 carbon atoms, for example methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl methoxyethyl, ethoxyethyl, propoxyethyl or butoxethyl.

R1 can be straight-chained or branched. Straight-chained groups are preferred. If R1 is a branched radical containing an asymmetric carbon atom, such as but-2-yl, the compound may form a tilted chiral smectic phase and have ferroelectric properties without the need for admixture with optically active dopant.

R2 is preferably an alkyl, alkenyl, alkoxy, alkenyloxy, al koxyal koxy, al koxycarbonyl, al kanoyl oxy or al koxyal koxyal koxy radical. Preferably, they have 5, 6, 7, 8, 9 or 10 C atoms and accordingly preferably denote pentyl, hexyl, heptyl, octyl, nonyl, decyl, pentoxy, hexoxy, heptoxy, octoxy, nonoxy or decoxy, or furthermore also ethyl, propyl, butyl, undecyl, dodecyl, propoxy, ethoxy, butoxy, undecoxy, dodecoxy, 2-oxapropyl (=2-methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxypentyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl or 2-, 3-, 4-, 5- or 6-oxaheptyl. Preferably R2 is a straight-chained group.

If R2 contains an asymmetric carbon atom, the compound may similarly form a tilted chiral smectic phase without added dopant.

Preferred chiral groups R2 have the formula -X'-Q'-C*H-R5 I wherein X' is -CO-O-, -0-CO-, -O-CO-O-, -CO-, -0-, -S-, -CH=CH-, -CH=CH-COO- or a single bond, Q' is alkylene with 1 to 5 C atoms, wherein a CH2 group which is not linked to X' can also be replaced by -0-, -CO-, -0-CO-, -C0-O- or -CH=CH-, or a single bond, Y' is C halogen, methyl or methoxy, and R5 is an alkyl group with 1 to 15 C atoms which differs from Y, wherein one or two non-adjacent CH2 groups can also be replaced by -0-, -CO-, -0-CO-, -CO-O- and/or -CH=CH-.

X' is preferably -CO-0, -0-CO-, -CH=CH-COO- (trans) or a single bond. -CO-O-/-O-CO- or a single bond are particularly preferred.

Q' is preferably -CH2-, -CH2CH2-, -CH2CH2CH2- or a single bond, particularly preferably a single bond.

Y' is preferably CH3, -CN or Cl, particularly preferably CN.

R5 is preferably straight-chain or branched alkyl with 1 to 10, In particular with 1 to 7, C atoms.

Of the compounds of the formula I, those in which X' and Y' are not simulataneously methyl are preferred.

The core (Z} preferably contains at least two rings or ring systems, at least one of which is a carbocyclic or heterocyclic aromatic ring or ring system, the rings or ring systems being linked together by direct covalent bond or any of a wide range of linking groups known for the purpose, for example carbonyloxy and ethylene groups. Preferably It) is a residue of biphenyl, biphenyl-4-yl benzoate, phenyl 4-benzoyloxybenzoate or naphthalene.

Preferably (Z) is a divalent group of the formula -A4-Z1-A2- (Z2A3)n wherein A2, A3 and A4 are each 1,4-phenylene which is unsubstituted or substituted by one or two F and/or Cl atoms and/or CH3 groups and/or CN groups, wherein one or two CH groups can also be replaced by N, or 1,4-cyclohexylene, wherein one or two non adjacent CH2 groups can also be replaced by 0 atoms and/or S atoms, or piperidine-1,4-diyl, 1,4-bicyclo(2,2,2) octylene, naphthalene-2,6 diyl, decahydronaphthalene-2,6-diyl or 1,2,3,4, tetrahydronaphthalene-2,6-diyl groups, and z1 and z2 are each -CO-O-, -CO-S-, -0-CO-, -S-CO-, -CH2CH2-, -OCH2-, -CH20, -C-C or a single bond.

For simplicity, in the following text Ph is a 1.4-phenylene group, which may be substituted (e.g. by fluorine) and wherein one or two CH groups can also be replaced by N, Cy is a 1,4-cyclohexylene group, wherein one or two non-adjacent CH2 groups can also be replaced by 0 atoms, and Bi is a bicyclo (2,2,2) octylene group.

n is preferably 0.

A2, A3 and A4 are preferably Cy or Ph. In the compounds of the formulae above and below, Ph preferably denotes a 1,4-phenylene (Phe), a pyrimidine-2,5-diyl (Pyr), a pyridine-2,5-diyl (Pyn), a pyrazine-3,6-diyl or a pyridazine-2,5-diyl group, particularly preferably Phe, Pyr or Pyn. The compounds according to the invention preferably contain not more than one 1,4-phenylene group wherein one or two CH groups are replaced by N. Cy preferably denotes a trans-1,4-cyclohexylene group.

Particularly preferred compounds of the formula I and of the above part formulae are those which contain a grouping -Ph-Ph-.

-Ph-Ph- is preferably -Phe-Phe-, Phe-Pyr or Phe-Pyn. Particularly preferred groups are

And furthermore 4,4'-biphenylyl which is unsubstituted or monoor polysubstituted by fluorine.

z1 and z2 are preferably single bonds, and after these preferably -0-CO-, CO-O-, CC or -CH2CH2- groups, in particular the -COO- or the -OCO- group.

For the construction of an electrooptical device containing a ferroelectric smectic display, the liquid crystal material is contained in a cell between rubbed polymer surfaces separated by a short distance, preferably not more than Zum. Ferroelectric behaviour together with the high viscosity and long range order of the smectic liquid crystal phase permits both extremely short switching times under the action of a direct electric field upon the ferroelectric dipole moment and also bistability.

This combination of short addressing time and bistability opens up the possibility of a highly multiplexed flat panel display capable of video frame rates. The total number of addressed lines is limited by the response time of the optical effect which determines the frame time. Such a display has many possible applications ranging from an alphagraphic display for a word processor or portable computer to a full-colour flat panel television.

The compounds of the formula I are prepared by methods which are known per se, such as are described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der Organischen Chemie (Methods of Organic Chemistry), Georg-Thieme-Verlag, Stuttgart), and in particular under reaction conditions which are known and suitable for the reactions mentioned. Variants which are known per se and are not mentioned here in more detail can also be utilized.

If desired, the starting substances can also be formed in situ such that they are not isolated from the reaction mixture but are immediately reacted further to give the compounds of the formula I.

Suitable starting substances are available in a large variety, e.g.

The following examples illustrate the invention.

EXAMPLE 1 4,4'-Di (2-ethoxyethoxy)biphenyl A mixture of 4, 4'-dihydroxybiphenyl (1 mole), potassium carbonate (4 moles) and 1-bromo-2-ethoxyethane (4 moles) in butanone was heated under reflux for approximately 35 hours. The butanone was removed by rotary evaporation and the desired product extracted into ether. The ether extracts were washed with dilute hydrochloric acid and then with water. The ether was removed and the residue crystallized from ethanol. The product was purified by two recrystallisations from ethanol. Its structure was verified by nuclear-magnetic-resonance spectroscopy and mass spectroscopy.

The transition temperatures of the product were determined by optical microscopy and differential thermal analysis, with the following results:crystal to smectic K, 750C; smectic K to isotropic, 118"C.

The liquid crystal phase was proved to be smectic K by X-ray analysis.

EXAMPLE 2 4,4'-Di(2-methoxyethoxy)biphenyl The procedure of Example 1 was repeated using 1-bromo-2-methoxyethane instead of 1-bromo-2-ethoxyethane, and the structure of the product and its transition temperatures were determined in the same way:crystal to smectic K, 1270C; smectic K to isotropic, 139"C.

EXAMPLE 3 4-Octyl phenyl 4-(2-ethoxyethoxy)benzoate 4-(2-Ethoxyethoxy)benzoic acid was prepared as follows. A mixture of 4-hydroxybenzoic acid (13.8 g), 1-bromo-2-ethoxyethane (30.6 g), sodium hydroxide (8.8. g), ethanol (20 ml) and water (50 ml) was heated under reflux for 16 hours. Potassium hydroxide (5.6 g) in water (100 ml) was added to the reaction mixture and reflux continued.

After 1 hour, while the mixture was cooling, concentrated hydrochloric acid was added to pH 2 - 3. Crystals appeared on continued cooling and were filtered off and dissolved at boiling point in water (350 ml) to which ethanol (100 ml) was subsequently added. The milky mixture was filtered and cooled and crystallization occurred to giv.e white material m.p 133-135"C.

This acid (1 eq.) was suspended and stirred in dry dichloromethane under nitrogen. Trifluoroacetic anhydride (1.1 eq) was added and the suspended acid gradually dissolved. After 20 minutes complete solution had occurred. 4i0ctylphenol (0.95 eq) was added and the reaction mixture left overnight. The solvents were removed under reduced pressure and the crude product was subjected to flash column chromatography (silica, dichloromethane). The major band corresponding to the desired ester was collected and evaporated to give an oily solid as product. The product was recrystallized from methanol (with addition of water as necessary). Its structure was verified by nuclear-magnetic-resonance spectroscopy and mass spectroscopy.

Transition temperatures were determined as in previous examples (the compound may be polymorphic):solid to isotropic, 40.5 - 41.5"C; isotropic to nematic, 12.40C.

EXAMPLE 4 4-Nonylphenyl 4-(2-ethoxyethoxy)benzoate The procedure of Example 3 was repeated using 4-nonylphenol instead of 4-octylphenol to react with the acid. The structure of the product was determined in the same way.

Transition temperature:solid to isotropic, 35 - 37"C.

isotropic to nematic, 24.60C.

EXAMPLE 5 4'-Hexylbiphenyl-4-yl 4-(2-ethoxyethoxy)benzoate The procedure of Example 3 was repeated using 4-hexyl-4'hydroxybiphenyl instead of 4-octylphenol to react with the acid. The structure of the product was determined in the same way.

Transition temperatures:solid to nematic, 107.0 - 107.2"C; nematic to isotropic, 165.3 -166.3"C; nematic to smectic C 91 -900C.

Claims (11)

1. A compound forming a tilted smectic liquid crystal phase having the general formula R1 - 0 - CH2 - CH2 - O - {Z} - R2 in which: R1 is an aliphatic radical containing carbon and hydrogen atoms and optionally one or more oxygen atoms each linked to separate carbon atoms and having a total number of carbon or of carbon and oxygen atoms not less than 1 and not more than 15; R2 is an aliphatic radical containing carbon and hydrogen atoms and optionally one or more oxygen atoms each linked to separate carbon atoms and having a total number of carbon or of carbon and oxygen atoms not less than 4 and not more than 18; and (Z) is an aromatic or alicyclic core of known type for liquid crystals.

2. A compound as claimed in Claim 1, in which R1 is an alkyl or alkoxyalkyl radical.

3. A compound as claimed in Claim 2, in which R1 is methyl, ethyl or methoxyethyl.

4. A compound as claimed in Claim 1 or Claim 2, in which R1 is a radical containing an asymmetric carbon atom conferring chirality.

5. A compound as claimed in any one of Claims 1 to 4, in which R2 is an alkyl, alkoxy, alkoxyalkoxy or alkoxyalkoxyalkoxy radical.

6. A compound as claimed in Claim 5, in which R2 contains an asymmetric carbon atom conferring chirality.

7. A compound as claimed in any one of Claims 1 to 6, in which the core (Z) contains at least two rings or ring systems, at least one of which is a carbocyclic or heterocyclic aromatic ring or ring system, the rings or ring system being linked together by direct covalent bond or carbonyloxy or ethylene group.

8. A compound as claimed In Claim 7, in which {Z} is a residue of biphenyl, biphenyl-4-yl benzoate,phenyl 4-benzoyloxybenzoate or naphthal ene.

9. A ferroelectric liquid crystal material comprising a compound as claimed in any one of Claims 1 to 8 in which chirality is present either intrinsically in the liquid crystal molecule or in a chiral dopant and which possesses a high spontaneous polarisation.

10. An electrooptical device which contains a ferroelectric liquid crystal material as claimed in Claim 9 and is constructed to permit the action of a direct electric field upon the ferroelectric dipole moment.

11. A compound forming a tilted smectic liquid crystal phase, substantially as hereinbefore described.