GB2235192A - Difluorophenyl esters and their use as liquid crystal materials - Google Patents

Abstract

Difluorophenyl esters of formula: <IMAGE> wherein R1 is C1-12 alkyl, perfluoroalkyl or hydrogen, each of A, B, D, E, is hydrogen or fluorine, rings X and Y are phenyl or pyrimidine, (F)2 indicates two fluorine substituents, R2 is C1-12 alkyl, perfluoroalkyl, alkoxy or perfluoroalkoxy. The esters have liquid crystalline properties and may be used as constituents of Sc liquid crystal mixtures.

Description

DIFLUOROPHENYL ESTERS AND THEIR USE AS LIQUID CRYSTAL MATERIALS.

This invention relates to liquid crystal mixtures and to compounds for use in them. The invention is particularly concerned with ferroeleotric smeotio liquid crystal mixtures and compounds.

The invention also relates to electro-optical devices incorporating these mixtures.

Perroeleotric smectic liquid crystal materials utilise the ferroelectrio properties of the chiral tilted smectic phase, ie the chiral smeotic C, F, G1 II, I, F and K phases (hereinafter designated Sc* etc, the asterisk * denoting chirality). The Sc* phase is most commonly sougllt fo: use in ebctro-optical devices as it is the most fluid, and it is also desirable that the material shows an SA phase at a temperature above the Sc* phase, to assist in surface alignment.

Ferroelectrio liquid crystal materials ideally have a low viscosity, a broad smectic liquid crystal temperature range, stability eot, and in particular should show a high spontaneous polarisation co-efficient (Ps, measured in nCom-2). Although some single component materials show these properties, it has become common practice to use n mixture which shows a smectic phase. At least one of the compounds iii the mixture in optically active (chiral) so as to induco tlio smectic phase shown by the mixture to be chiral esp ocinlly Sa.

An object of thin invention is to provide compounds which show advantageous SC phases.

Mnny compounds are known which show SC phases and are thus useful components of SC and SC compositions. For example WO 87/05291 describes inter alia fluorinated esters of general formula:

where R1 and R2 are alkyl or alkoxy and (F) indicates that the ring may be either unnubntituted phenyl or else carry one fluorine substituent and which show SC phases.

According to this invention, difluorophenol esters of general formula I below are provided:

wherein R1 is selected from C1-C12 alkyl, perfluoroalkyl, alkoxy, perfluoronlkoxy, or hydrogen, each of A, B, D, E is independently hydrogen or fluorine, rings X and Y are independently selected from phenyl or pyrimidyl, (F)2 indicates that the ring carries two fluorine substituents on any two of its 2, 3. 5 or 6 substitution positions, and R2 is selected from C1-C12 alkyl.

perfluoroalkyl, alkoxy or perfluoroalkoxy.

The structural preferences below are on the basis of inter alia the advantageous smectic phases shown and/or ease of preparation.

Preferably R1 is C3-C12 n-alkyl or n-alkoxy.

Preferably all of A, 13, C, D anl E are hydrogen or only one is fluorine.

Preferably rings X and Y are both phenyl Preferably R2 is C3-C12 n-alkyl, n-alkoxy, chiral alkyl or alkoxy, especially n-alkyl or alkoxy.

The two fluorine substituents (F)2 may be present in any of the (2, 3), (2, 5), (2,6) or (3, 5) combinations. but substitution in the (2, 3) positions, is:

is preferred.

A preferred overall structure for compounds of formula I is consequently IA below:

where Bl 1 and n2 in formula IA are independently selected from C3 - C12 n-alkyl or n-alkoxy. Compounds of formula IA show particularly beneficial smectic phases.

Compounds of formula I, including those of preferred formula IA may be prepared by a genoral route which involves an esterification reaction between the appropriate aryl-propionic acid:

where R1. A, B, U, E, X and Y are as designated above, and the appr- opriate difluorophenol:

The aryl-propionic acid may be prepared by the route A described in WO 87/05291, starting from an appropriately substituted carboxylic acid, such as a 4-alkyl or 4-alkoxy 4'-biphenylcarboxylic acid.

Methotln or preparing such acids, with or without lateral fluoro subs tituents will bo apparent to those skilled in the art of organic synth esis.

Suitable difluorophenols may be prepared by routes 1 to 7 shown in figures 1 to 6 of the accompanying drawings, by means of which the difluorophenols below, where R is alkyl, may be made:

The difluorinated starting compounds of routes 1 to 7 are known and commercially available (eg BDH Ltd., Fluorochem). The individual steps are well known literature reactions and suitable reaction conditions will bo apparent to those skilled in the art oJ organic synthesis.

The esterification reaction between the propionic acid and the difluorophenol sly be carried out using well known methods, for example esing N,N'-dicyclohexylcarbodiimide ('DCC') and 4-pyrrolidinopyridine ('4-PPy'). For example compounds of preferred formula IA may bn prepared by the reaction:

When R1 or fl2 in formula I is chiral, ie contains an asymmetrically substituted carbon atom. the compound may be in an optically active or racemic form.

The invention also provides a liquid crystalline material, being a mixture of components, at least one of which is a compound of formula I. The material may be one which shows an SC or SI phase, eg a chiral SC phone with ferroelectric properties. A compound of formula I for use in such a material may have R1 and/or R2 groups which are straight chain (eg n-alkyl or n-alkoxy) branched chain or chiral.

Preferred compounds of lormula I for use in such a material are those of formula IA A nmeotio, eg SC, liquid crystal material according to this aspect of thn invention will normally contain one or more other compounds which either singly or together show a smectic, eg SC, phase. Many such compounds aro known but noms preferred types of such compounds are: (a) The compounds disclosed in WO 86/04327, preferably of the formula:

where nA is n-alkyl or n-alkoxy and RB is n-alkyl. or mixtures thereof, especially eutectic mixtures.

(b) The terpllenyls disclosed in EPA 84304894-3 eg having a formula:

where RA and RB are independently selected from n-alkyl and n-alkoxy, or mixtures thereof, especially eutectic mixtures.

To induce a smectic, @g SC, phase shown by a material of this aspect of the invention to be chiral, the material must contain at least one optically active compound. Many such suitable optically active compounds are known but rome preferred types are Tho lactic acid derivatives described in wO 86/02398, preferably of formula:

or the compounds disclosed in EPA 0110299, eg:

Particularly prefbrrocl optically active compounds for use in such a material are the cyano-compounds described in WO 87/05896, eg:

where RA is n-alkyl or n-alkoxy (especially n-octyloxy or n-nonyloxy), RB is branched or chiral alkyl or C1 -4 4 n n-alkyl. and (F) indicates that the ring may carry a fluorine substituent.

If the mixture includes more than one optically active compound eg a compound of formula I or IA plus one or more of the optically active compounds referred to above, or two ore more of the optically active compounds referred to above1 helical twist senses of the S* phase induced by these compounds may be the same or opposite, eg by the use of both (+) and (-) optically active compounds. It is often desirable to include compounds which induce opposite twist senses of the 8* pliaso in order to increase the helical pitch length.

It is desirable that each optically active compound in the mixture induces the enme sense of Ps in ttio mixture.

The mixture mny also contain other known additives to improve the properties, eg Ps, SC phase breadth, viscosity etc or to induce the apparence of nn 6 phnna of n temperature above the S*c to assist alignment. An nxnmple of n class of compounds which may be used to broaden the SC phase is:

where RA and RB are independently C1-12 n-alkyl or n-alkoxy.

The mixture may also contain pleocliroic dyes.

Typlcalty but not exclusively a ferroelectric smectic liquids crystal mixture of the invention will have the following composition: One or more compounris of formula I 10 - 99 wt %

Onn or morn compounds which singly or together show a smeotic C liquid F 0 - 90 wt % crystal phase One or more optically active com- 1 - 25 wt ounds Additives and/or pleochroic dyes. 0 - 20 wt % Tile total boing 100 weight%. The nature and relative proportions of the various components of a liquid crystal material of the invention will depend upon the use for which the material is intended, and some experimentation may be necessary to suit a particular requirement0 but the basic principles of mixing and assessment of such materials is well known in the field.

Tile liquid crystal materials of tlie invention may be used in any of the known types of ferroelectric smectic liquid crystal display device, eg the "Clark-Lngerwall Device" described in Appl.

Phys Lott (1980), 36 899 and in Recent Developments in Condensed Matter Physics (1981), 4,309. The physics of this type of device and the method of constructing it are well known, and are described for example in WO 86/02398 and WO 86/04327 In practice such n device usually consists of two substrates, at leant onn nt wlllell in optically transparent, electrodes on the inner surfaces of the substrates by which a voltage may be applied and a layer of the liquid crystal materials sandwiched between the substrates. it in desirable that tiie helical pitch length of the SC* phase is comparable to the thickness of the material.

which is why long pitch mixtures are useful. The materials of the invention may be used in both the blrefrlngence type display mode and the guost-host type display mode of the Clark-Lagerwalldevice/ The device may for example be in the form of a clock, calculator or video screen display, and methods of making the device in this form will be well known to those skilled in the art.

The invention will now bn illuntrated by way of example only with reference to the accompanying figures 1 to 6 which show preparative routes 1 to 7 for preparation of compounds of the invention 1. rrjo:r'ati on or 3-(4'-alkoxybiphenyl-4-yl) propanoic acids Step 1.

Preparation of 4-hydroxy-4'-methoxy biphenyl.

4'-Methoxybiphenyl-4-carboxylic acid (2.85 g., 12.5 mmol), obtained by acid hydrolysis by known procedures from commercially available 4cyano-4'-methoxybiphenyl, was dissolved in dry tetrahydrofuran (THF) (20 ml). The solution was cooled to 0 C and a solution of borane, BH3, (17 ml, 17 mmol of n 1M solution in TlfF) was added through a syringe, under nitrogen, during about 20 minutes. The ice bath was removed and replaced by a water bath at 25 C, whereupon stirring was continued for 1 hr (reaction complete by tlc). The excess of borane was destroyed by careful addition of 10ml of a 1s1 mixture of TIIF : water. The reaction mixture was treated with chloroform and water, shaken briefly, and the organic layer separated off.After drying (Ng SO4) and removal of solvent, the product was crystallised from ethanol. The yield was quant itativo; mpt. 162-3 C (lit mpt. 161 - 2 C) Step 2.

Preparation of 4-bromomethyl-4'-methoxybiphenyl.

A solution of phosphorus tribromide (1.3 ml, 13.8 mmol) in benzene (3 ml) was added to 4-hydroxy-4'-methoxy biphenyl (1.5 g, 7 mmol) dissolved in benzene (6 ml) at room temperature. The reaction mixture was then boiled for 3 hr, with separation of a yellow solid. On being cooled, the reaction mixture was poured onto ice and ether. The ether eal solution obtained after shaking (the yellow solid was not soluble) was washed with water, aqueous sodium carbonate, then water, and was dried (Mg SO4). removal of solvent gave a white solid, yield: 1.7 g, 87.6% which was used in stop 1A(iii) below without purification.

Step .3, Preparation of 3-(4'-methoxybiphenyl-4-yl) propanoic acid.

To a solution of sodium (2.6g, 0.11 g atom) in absolute ethanol (55ml), diethyl malonate (42 ml., 0.27 mol) was added to yield a clear solution which was then evaporated to dryness.

4-Bromomethyl-4'-methoxybiphenyl (16.3 g, 0.059 mol) in dry benzene (125 ml) was added in one portion to the sodium salt of diethyl malonate and tiie mixture was heated at 750C for 51/2 hours.

The cooled mixture was pourod into water and the whole was shaken with ether. Separation of the ether layer and washing with water were followod by drying (MgSO4) and removal of the solvent. The oil obtained wan hydrolysed by boiling with a mixture of 10M NaOH (55 ml) and 111 water t ethanol (450 ml) for 2 hr. Removal of the ethanol by distillation was followed by addition of a large volume of water to obtain n solution or the solium salt of the dicarboxylic acid.

This solution was washed with ether to remove some insoluble oil.

The aqueous solution wan acidified (to pH 1) with conc. HCl. The precipitate of the di-acid (94 %) was filtered off and dried.

Decarboxylation was achieved by heating the white, powdered di-acid progressively to 180 C. The product (the mono-acid) was obtained in 91 - 92 % yield, mp 196-70C.

Step 4.

Preparation of 3-(4'-hydroxybiphenyl-4-yl) propanoic acid.

The methyl ether from Step 1A(iii) (10.24 g, 0.04 mmol) was suspended in dry dichloromethane (20 ml) and the mixture stirred as the temperature was lowored to -700C, when a solution of boron tribromide in dichloromethane (8 ml of an 0.06 M solution) was added dropwise. After stirring for 30 mln. the mixture was allowed to warm to room temperature, with stirring, which was continued for 4 hr.

The mixture was poured into ice water, a few drops of conc. HCl were added, and the whole was stirred overnight. The product (9.5g, 95%) mpt. ?.07 - 210 C, was obtained by filtration and drying. The occurencc of demethylation was checked by 1R spectroscopy. The product was difioult to crystallise and was used without purification in the further alkylation reactions, Step 1A(v).

Step 5.

Preparation of 3-(4'-alkoxybiphenyl-4-yl)propanoic acids.

3-(4'-Hydroxybiphenyl-4-yl) propanoic acid (4.84 g, 0.02 mol) was dissolved in 2-methoxyethanol (145 ml) with heating; to the hot solution was added (with vigorous stirring) a solution of KOII (2.24 g 0.04 mol) in water (7.5 ml) (overall mixture 95:5, 2-methoxyethanol water). After 10 min. the bromoalkane (0.024 mol) was added and the reaction mixture heated under reflux with stirring overnight. A solution of K()11 (2g) in water (20 ml) was then added and boiling was continued for two further hours.

l'lln cooled reaction mixture was poured into a large volume (1.5 L) or water, and acidified to Congo Red with conc. IIC1. The suspension or free acid was stirred for about 2 hours before the product wan filtered crf, washed and crystallised from 95% acetic acid, The prolluot acids were checked for identity and purity by IR spectroscopy, NMR spectrometry and mass spectrometry. Yields after crystallisation were 65 - 75 %.

Two examples of acids prepared in this way are characterised below: Pentyloyxy mpt. 196.7 - 197.4 C; m/e = 312 Nonyloxy mpt. 183.5 - 184.7 C; m/e = 368 2. route 2: preparation of 4-alkyl-2, 3-difluorophenol.

Step 2,1 Preparation oft

A solution of methyl iodide (8.0 g) in acetone (20 ml) was added dropwise to a stirred mixture of 2,3-difluorophenol (6.00g) and potassium carbonate (13.5g) in acetone (75 ml) at room temperature.

The stirred mixture was heated under reflux for 21 hr (ie until gic analysis revealed a complete reaction). The product was extracted into ether twice, and tile combined ether extracts were washed with water, 10% sodium hydroxide, water and dried (Mg SO4). The solvent was removed under vacuo and the residue was distilled.

Step 2.2 Preparation of

Pentanol chloride (2.sg)was added dropwise to a stirred, cooled (0 C) mixture of product 2.1 (5.0 g.) and aluminium chloride (1.0 g).

The mixture waa stirred at 0 C for 1 hr, heated at 800C for 2 hr, cooled and poured into 18% hydrochloric acid. The product was extracted into CHCl3 twice, the combined organic extracts were washed with water antl steam disilled to remove chloroform and the excess of bromobenzene. Tiie product was extracted into chloroform twice, the combined organic phases were washed with water and dried (Mg S04).

The solvent was removed in vacuo and the residue was distilled.

Step ?,3 Preparation of:

A mixture or 2.2( 4.0g), hydrazine hydrate (2.5 g) and KOH (30 g) in diethylene glycol ( so ml) was heated at 1300C for 2 hr, the excess of hydrazine hydrate was distilled off and the temperature was raised to 2000C for 2 hr. The cooled mixture was poured into 18% CL, the product wao extracted into ehter twice and the combined ethereal extracts were washed with water and dried (Mg SO4). The solvent was removed in vacuo and the residue was distilled Step 2.4 Preparation of:

A solution of product 2.3 (5 g) in dry chloroform (50 ml) was slowly added dropwise to a rolution of boron tribromide (10 g) in dry chloroform with stirring, at - 78 C under dry nitrogen. The mixture wan stirred for 24 hr and was then added to excess water (500ml) with vigorous sstlrrlng. allowing the mixture to warm to room temperature, The product was extracted into chloroform twice, driod (MgSO4). and the solvent removal under vacuo. The product was distilled.

By an exactly analogue route 4-n-nonyl-2, 3-difluorophenol was prepared.

3. Esterification The 3-(4'-alkoxybiphenyl-4-yl) propanoic acid (0.001 mol), the 4-alkyl-2,3-difluorophenol (0.0011mmol). N,N-dicyclohexylcarbo diimide (0.0011 mol) and 4-pyrrolidinepyridine (0.0001 mol) were mixed in dry dichloromethane (10 ml). The reaction mixture was stirred at room temperature for 15 - 20 hr, completion of reaction being checked by tlc.

Dicyclohexylurea was removed by filtration and the solution in dichloromethane was combined with dichloromethane washing of the uron derivative. Washing Or the solution with water (3 x 10 ml), acetic acid solution (3 x 10 ml) and water was followed by drying and removal nt the solvent by evaporation. Yields at this stage were hearly quantitative.

4. Liquid crystalline properties of esters prepared.

'liquid crystal 'Phases and transition temDeratures (Oc) 79. f + 67.3 \ 79 l7 n-C4H9- ~K S ~ S SA 5A 62,1 E 66. 6 76.2 105.2 66.2 X .5 ssS 8 106.2 n 6 13 K 2 33 |S 5C S = S 52.4 \ B 6ll.0 C 88.1 105.8 61F.8 8 5s3,w3 109.1 n-C8H17- K K I/F 57.9) C lo8.4 17 57.9 71.3 I , 62.1 SC 110.1 "'C1OITZ1 624} Sc 109 3 SC I 61.4 109.8 Abbreviations:K = solid crystal, SE = smectic E, SB = smectic B smectic A SI/F = smectic I or F I = isotropic liquid.

In figs 1 - 5, n-Bu = n butyl, OPr-i = isopropyloxy # = supercooling only.

5. Routes 1 and 7 The preparation of the difluorophenylboronic acides:

by the present inventors is described in WO 89/02425 on pages 37 - 42 as example 8C, 8E and 9D, These acids may be converted to the corres- ponding phenola by treatment with hydrogen peroxide in a modification of the method described in M F Hawthorne, J. Org. Chem. (1957), 22, 1001 in which the boronio acid group is replaced by a hydroxy group.

The preparation of 2, 3- difluorophenol by this method is described on page 40 of WO 89/02425. Esterification of these phenols may be carried out by the method of 3 above.

6. Comparative Examples.

The properties of a 2,3- difluorophenyl ester of the invention is compared below with those of monofluorinated analogues of the 'type exemplified in prior art WO 07/05291. The liquid crystal transitions ( C) obnerved on heating are listed;

The difluorinated compound (iii) therefore shows the following advantages over (i) and (ii): Absence or th SB phase shown by (i) Broader SC phase range than (i) or (ii), ie 44.30C Appearence of SC phase at a lowor temperature than (i) or (ii) Higher clearing point than (i) or (ii) 7, Liquid Crystal Mixtures A mixture (7A) was prepared containing a 1:1 ::1 weight mixture of compounds of formula IA:

where RO was C6H13O, C8H17O and C10H21O. This mixture showed phase transitions ( C): K 55 SC 103 I Thia mixture was mixed in a 1:1 weight ratio with a second SC material which wars itself a 1s1s1 weight mixture of compounds of formula:

where R was C8H17O , C8H17 and C711150 . This 1:1 mixture (7B) showed phase transitions: 27 S? 52SC 102SA 116 N 122 I Mixture 73 therefore showed the useful SC-SA-N sequence of transitions which io preferred for liquid crystal alignment.

To mixture 73 was added 5 weight % of an optically active compound of formula:

This produced a ferroeleotrio mixture (7C) having the liquid crystal phase transitions ( C): 48 SC 78 SA 112 N 116 I and the properties listed in the table below: Temp ( C) Ps (nC cm-2) Tilt Angle ( ) 75 3.0 5.0 70 6.4 10.0 65 9.0 11.0 60 10.5 12.5 55 12.0 13.-5 50 13.0 14.0 Using a 50 V monopolar pulse the response time at 50 C using a 2 m oell was found to be 5 sec.

A further mixture was prepared consisting of mixture 7B plus 20 weight 56 of a compound:

to this mixture 5 weight % of the optically active compound used in mixture 7C was added. The mixture had the following liquid crystal transitions ( C): 10 SC 43 SA 83 N 97 1 (ie again showing the usoful SC-SA-N phase sequence). The mixture had the properties: Temp (%o) 's (nC cm-2) Tilt Angle (0) 45 2.9 6.0 40 5.3 10.0 35 7.0 11.0 30 8,3 12.5 25 9.4 13.0 20 10.3 13.5 Using a 50 V monopolar pulse the response time at 300C was 18 sec.

The birefringence # n of this mixture was ca. 0.14.

Claims (11)

1. Difluorophenol esters of formula Is

wherein R1 is selected from C1-12 alkyl, alkoxy, perfluoroalkyl, perfluoroalkoxy or hydrogen, each of A, B, D and E is independently selected from hydrogen or fluorine, rings X and Y are independently selected from phonyl or pyrimidyl (F)2 indicates that the ring carries two fluorine substituents on any two of its 2, 3, 5 or 6 positions and R2 io selected from C1-12 alkyl, alkoxy, perfluoroalkyl or perfluoroalkoxy.

2. Difluorophenyl esters according to claim 1 characterised by a formula IA:

wherein R1 is C3-12 n-alkyl or n-alkoxy and R2 is C3-12 n-alkyl, nalkoxy or ohirnl alkyl or alkoxy.

3. Difluorophenyl esters according to claim 2 characterised in that the fluorine aulntituento (F)2 are in the 2, 3- positions relative to the ester link.

4. Difluorophenyl estero aocording to claim 2 characterised in that the fluorine substituents (F)2 are in the 2, 6- positions relative to the ester link,

5. Difluorophenyl esters according to claim 3 characterised in that Rj is n-alkyl or n-alkoxy and R2 is n-alkyl.

6. Difluorophonyl esters acoording to claim 3 characterised in that R1 is n-alkoxy and R2 is n-alkyl.

7. Difluorophenyl esters according to claim 4 oharacterised in that R1 is n-alkoxy and R2 is n-alkyl.

8. Liquid crystal material being a mixture of at least two compounds at least one having a formula I'as defined in claim 1.

9. Liquid crystal material according to claim 8, characterised in that the compound of formula I has a formula IA:

wherein R1 is C3-12 n-alkyl or n-alkoxy and R2 is C3~12 n-alkyl, n-alkoxy or choral alkyl or alkoxy.

10. Liquid crystal material according to claim 9 characterised in that substituents (F)2 are in the 2,3- positions relative to the ester link.

11. Liquid crystal material according to claim 9 or 10 characterised in that the material additionally contains at least one compound of formula:

where RA is n-alkyl or n-alkoxy and RB is n-alkyl.

12, Liquid crystal material according to claim 11 characterised in that the material additionally contains at least one optically active compound of formula:

where RA is n-nlkyl or n-alkoxy and RB is branched or chiral alkyl or C 1-4 n-alkyl and (F) indicates that the ring may carry a fluorine substituont.