GB2089345A

GB2089345A - Liquid crystal 4-(4'-alkyl or -alkoxy-benzyloxy) Alkoxybenzenes

Info

Publication number: GB2089345A
Application number: GB8136154A
Authority: GB
Original assignee: SECR DEFENCE; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; UK Secretary of State for Defence
Priority date: 1980-12-02
Filing date: 1981-12-01
Publication date: 1982-06-23
Also published as: GB2089345B

Abstract

A liquid crystal phenyl ether compound exhibiting a small dielectric anisotropy and suitable for adding to liquid crystal materials for use in twisted nematic (including multiplexed) devices, cholesteric to nematic phase change effect devices and Fréedericksz effect devices has a formula: <IMAGE> where R1 is an alkyl or alkoxy group and R2 is an alkoxy group.

Description

SPECIFICATION Liquid crystal materials The present invention relates to liquid crystal compounds exhibiting a small (negative or positive) dielectric anisotropy and liquid crystal materials and devices incorporating such compounds.

The use of liquid crystal materials to exhibit electro-optic effects in display devices such as digital calculators or watches is now well known. One of the parameters of a liquid crystal material which is important in relation to electro-optical operation is the dielectric anisotropy () of the material. This is the difference, for a given frequency and temperature between the average dielectric constant measured parallel (e"1) to the molecules of the material, eg when aligned together, less the average dielectric constant measured perpendicular (E,) to the molecules.

The sign and magnitude of the dielectric anisotropy of a given liquid crystal material are amongst the major parameters which determine the kinds of electro-optic devices in which that material may be' used.

For example, materials having a positive dielectric an isotropy, herein referred to as 'positive' materials, eg mixtures of 4-alkyl-or-alkoxy-4'-cyanobiphenyls and a 4' '-alkyl-or-alkoxy-4-cyano-p- terphenyl may be used in known twisted nematic effect devices (if nematic) or known cholesteric-to-nematic phase change effect devices (if cholesteric) in which the molecular arrangement is changed from the focal conic to the homeotropic texture.

Materials having a negative dielectric anisotropy herein referred to as 'negative' materials (of suitable resistivity) may be used in known dynamic scattering effect devices (if nematic) or cholesteric memory mode devices (if cholesteric).

Materials having a small dielectric anisotropy (negative or positive), may also be mixed with those having a large positive or negative dielectric anisotropy to produce mixtures whose overall dielectric anisotropy is positive or negative as appropriate (but reduced in magnitude).

According to the present invention in a first aspect there is provided a novel liquid crystal compound exhibiting a small dielectric anisotropy having a structural formula:

formula (I) where R1 is an alkyl or alkoxy group and R2 is an alkoxy group.

The groups R, and R2 preferably contain eighteen or less carbon atoms, desirably ten or less carbon atoms. These groups may be normal or branched. If branched, one or both may contain a chiral centre in which case the compound may be optically active. If R, and R2 are normal or not chiral groups then the compound is nematogenic.

By a 'liquid crystal compound' is meant a compound in one of the following two known categories: (i) Compounds which normally exhibit a liquid crystal phase; (ii) Compounds which do not normally exhibit a liquid crystal phase but which nevertheless usefully affect some aspect of liquid crystal behaviour when dissolved in other liquid crystal compounds.

Compounds in category (ii) show a 'monotropic' or a 'virtual' liquid crystal to isotropic liquid transition at at temperature below the melting point of their solid phase. The monotropic or virtual transition may be detected respectevely by rapid cooling of the liquid phase or by dissolving the compound in a material exhibiting a liquid crystal phase, observing the change in the transition to the isotropic liquid phase of the material by the addition and calculating the virtual transition temperature by extrapolation.

Compounds in category (ii) might for example by usefully dissolved in other liquid crystal compounds to extend or vary the liquid crystal temperature ranges of the compounds or to vary the molecular helical pitch (in the case of cholesteric liquid crystals).

The compounds of formula (I) are ethers which may be prepared by known procedures. For example, an appropriate 4-alkyl-or-alkoxy-benzyl halide

where X is a halogen, eg

is reacted with the appropriate 4-alkoxyphenol

or a salt, eg sodium salt, thereof, under known conditions to give the ether.

One or more compounds according to formula (I) may be used in any one of the following applications: together with a positive nematic material giving an overall positive nematic material for use in twisted nematic effect devices particularly multiplexed devices; an example of such a device is given below; ii together with another nematic material, preferably strongly negative, giving an overall negative material preferably also with a pleochroic dye, in Freedericksz effect devices in which the molecular arrangement may be changed from the homeotropic texture (OFF state) to the homogeneous texture (ON state) by an electric field; an example of such a device is given below; iii together with a positive nematic material giving an overall positive nematic material, preferably also with a pleochroic dye, in Freedericksz effect devices in which the molecular arrangement may be changed from the homogeneous texture (OFF state) to the homeotropic texture (ON state) by an electric field; iv together with an optically active and a negative material giving an overall negative material which is a cholesteric of suitable resistivity (about 109 ohm-cm), in cholesteric memory mode devices in which the molecular arrangement may be changed from a homogeneous texture (OFF state) to a turbulent scattering focal conic texture (ON state) by an electric field;; v together with an optically active material and a strongly negative nematic material giving an overall strongly negative material which is a cholesteric, preferably together also with a pleochroic dye, in cholesteric-to-nematic phase change effect devices in which the molecular arrangement may be changed from a weakly scattering, ie clear, surface aligned homeotropic texture (OFF state) to a strongly scattering twisted homogeneous texture (ON state) by an electric field; vi together with an optically active material and a positive nematic material giving an overall positive material which is a cholesteric, preferably together also with a pleochroic dye, in cholesteric-to-nematic phase change effect devices in which the molecular arrangement may be changed from a scattering focal conic texture (OFF state) to a clear homeotropic texture (ON state) by an electric field;; vii together with a nematic material giving an overall negative nematic material of suitable resistivity (about 109 ohm-cm), in dynamic scattering effect devices in which the molecular arrangement may be changed from a clear homeotropic texture (OFF state) to a turbulent scattering texture (ON state) by an electric field; viii together with a positive nematic material giving an overall positive nematic material in two frequency switching effect devices in which the dielectric anisotropy of the material may be ,changed from (at low frequency) positive (OFF state) to negative (ON state) by the application of a high frequency electric field.

The construction and operation of the above devices and the general kinds of material which are suitable for use in them are themselves known.

It will be apparent to those skilled in the art that in the above applications where mixtures are formed these mixtures can have the value and sign of their dielectric anisotropy controlled as required by control of the proportions of the materials blended together to form them.

Where a material is added to one or more compounds according to formula (I) the material may itself be a mixture of 2 or more compounds.

Mixtures may be formed in a known way, eg simply by heating the constituent compounds to form an overall isotropic liquid, stirring the liquid and allowing it to cool.

The compounds according to formula (I) may usefully reduce operating voltages in the above applications as well as enhance negativity or reduce positivity of the dielectric anisotropy; this may usefully affect other properties, eg multiplexibility in twisted nematic effect devices or switching frequency in two-frequency switching effect devices.

According to the present invention in a second aspect there is provided a mixture of liquid crystal compounds including at least one compound according to formula (I) above. Such a mixture may, depending on its selected composition and properties, be used in any one of the applications (i) to (viii) above.

An example of a mixture according to the second aspect which may be used in twisted nematic devices is one including, in addition to at least one compound according to formula (I) above, one or more 4'-alkyl-or 4'-alkoxy-4-cyanobiphenyls and one or more of the following compounds:

Formula (II) Formula (III) Formula (IV) Formula (V) Formula (Vl) Formula (VII) Formula (VIII) For multiplexed twisted nematic devices one or more other compounds having a small dielectric anisotropy may be added also, eg one of the following compounds:

Formula (IX) Formula (X) Formula (Xl) Formula (XII) or an analogue of one or more of the compounds of Formulae (IX) to (XII) in which the 2 or 3 position of the right hand 1,4 disubstituted benzene ring as shown carries a fluorine substituent; R,R' = alkyl.The total content of compounds having a small dielectric anisotropy is preferably between 20 and 80 per cent by weight of the mixture with the one or more biphenyls.

Preferably the 4'-alkyl-or 4'-alkoxy -4-cynaobiphenyls constitute between about 20 and 80%, desirably between 30 and 70% by weight of the total mixture and the compounds selected from formulae (Il) to (VIII) constitute not more than about 20% by weight in total, the remainder being one or more compounds according to formula (I) and formulae (VIII) to (Xl).

Preferably each 4'-alkyl- or 4'-alkoxy -4-cyanobiphenyl incorporated in the mixture has five or less carbon atoms in its alkoxy or alkyl group. Preferably this number of carbon atoms is odd when the group is an alkoxy group and is even when the group is an alkyl group.

To provide more general examples of a mixture according to the second aspect at least one compound according to formula (I) above may be mixed together with one or more compounds in the following known families for use in one or more of the applications given above (the actual application(s) depending on the mixture's properties):

where

is a cyclohexane ring,

is a bicyclo (2.2.2) octane ring, X is a 1,4 phenylene group,

or a 4,4' biphenylyi group

or a ;e,6 naptthyl group

and Y1 is CN, or R' or CO.o-X-Y' where Y1 is CN, or R' or OR'; where R and R' are alkyl groups.

Preferably, the compound(s) according to formula (i) comprises between 5 and 80%, preferably not more than 20%, by weight of the mixture.

According to the present invention in a third aspect a liquid crystal device includes two dielectric substrates at least one of which is optically transparent, a layer of liquid crystal material sandwiched between the substrates and electrodes on the inner surfaces of the substrates to enable an electric field to be applied across the layer of liquid crystal material to provide an electro-optic effect therein, characterised in that the liquid crystal material consists of or includes a compound according to formula (I) above.

The device according to the third aspect may be a twisted nematic effect device, which may or may not be operated in a multiplexed fashion, a cholesteric-to-nematic phase change effect device, a Freedericksz effect device or a two-frequency switching effect device all constructed in a known manner or any of the other devices mentioned above. The various ways in which compounds according to formula (I) may be used in these devices are outlined above and will be further apparent to those skilled in the art.

EXAMPLE 1 This example describes the preparation of 4-(4'-alkylbenzyloxy) alkoxybenzenes and 4-(4'alkoxybenzyloxy)alkoxybenzenes by the following route:

CO2H STEP Al H CH20H S STEP B1 RimHfmo% CH2 C where, R, is an alkyl or alkoxy group as defined above, and R2 is an alkoxy group as defined above.

STEP Al: The production of 4-alkylbenzyl alcohols and 4-alkoxybenzyl alcohols.

STEP B1: The production of 4-alkylbenzyl chlorides and 4-alkoxybenzyl chlorides.

STEP C1: The production of 4-(4'-alkylbenzyloxy)alkoxybenzenes and 4-(4'-alkoxybenzyloxy)alkoxybenzenes.

STEP Al The production of 4-alkylbenzyl alcohols and 4-alkoxybenzyl alcohols from the known 4alkylbenzoic acids and 4-alkoxybenzoic acids respectively.

This step may be carried out essentially by the same method as that described in published UK Patent Application No 7917145 (Specification No 20231 36A) for the treatment of trans-4 alkyl-cyclohexane-l-carboxylic acids (step D1 of Example 1 of that Application).

An example of a product of the present step is 4-methyl-benzyl alcohol, yield 90%. mp 61 C and 4-methoxybenzyl alcohol yield 94%, bp 95"C at 1.5 mm Hg.

STEP B1 The production of 4-alkylbenzyl chlorides and 4-alkoxy-benzyl chlorides from 4-alkylbenzyl alcohols and 4-alkoxybenzyl alcohols respectively.

4-alkylbenzyl alcohol (0.02 mole) is added to chloroform (60 cm3) and thionyl chloride (2.86 g) and the mixture heated for 90 min at 80"C. The solvent is removed in vacuo and the residual oil taken up in ether (50 cm3). The solution is washed with 10% aqueous sodium bicarbonate (2 X 30 cm3), dried (Na2SO4), and the solvent removed in vacuo. The residual product is purified by distillation.

Examples of such products are 4-methylbenzyl chloride, yield 90%, bp 1 20 C at 4 mm Hg and 4-methoxybenzyl chloride, yield 90%, bp 110"C at 3 mm Hg.

STEP C1 The production of 4-(4'-alkylbenzyloxy)alkoxybenzenes and 4-(4'-alkoxybenzyloxy)alkoxybenzenes.

The sodium salt of the 4-alkoxyphenol

is formed by adding a solution of the phenol (0.05 mole) in super-dry ethanol (20 cm3) to a solution of sodium ethoxide (prepared from sodium (0.05 g atom) and super-dry ethanol (100 cm3)). The excess of ethanol is removed in vacuo, and the salt dried in vacuo. The salt is taken up in 1,2-dimethoxythane (60 cm3), the 4-alkylbenzyl chloride or 4-alkoxybenzyl chloride (0.04 mole) is added, and the whole heated under reflux for 6 hr. The reaction reaction mixture is poured into water (120 cm3); the resulting precipitate is filtered off, taken up in ether (100 cm3), and the solution dried (Na2SO4). The solvent is removed in vacuo and the crude material is crystallised from light petroleum (bp 40-60"C). The pure material is obtained by recrystallisation from the same solvent.

Examples of such products are:

Yield 32%; C-l, 82"C; N-l, [ - 6"C]

Yield 21%; C-l, 76"C; N-l, [19 C]

Yield 5%; C-l, 93 C; N-l, r38'CJ [J represents a virtual transition temperature C-I represents crystalline solid to isotropic liquid transition temperature N-l represents nematic liquid crystal to isotropic liquid transition temperature.

Examples of materials and devices embodying the invention (in the second and third aspects above) will now be described by way of example only with reference to the accompanying drawings wherein: Figure 1 is a sectional view of a twisted nematic digital display; Figure 2 is a sectional view of the display shown in Fig. 1; Figure 3 shows a rear electrode configuration for Fig. 1; Figure 4 shows a front electrode configuration for Fig. 1; Figures 5, 6, 7show schematic views of the device of Figs. 1 to 4 with typical addressing voltages; The display of Figs. 1 to 4 comprises a cell 1, formed of two, front and back, glass slides 2, 3 respectively, spaced about 7,um apart by a spacer 4 all held together by an epoxy resin glue.

A liquid crystal material 1 2 fills the gap between the slides 2, 3 and the spacer 4. In front of the front glass slide 2 is a front polariser 5 arranged with its axis of polarisation axis horizontal.

A reflector 7 is arranged behind the slide 3. A rear polariser 6 or analyser is arranged between the slide 3 and reflector 7.

Electrodes 8, 9 of tin oxide typically 100 A thick are deposited on the inner faces of the slides 2, 3 as a complete layer and etched to the shapes shown in Figs. 3, 4. The display has seven bars per digit 10 plus a decimal point 11 between each digit. As shown in Fig. 3 the rear electrode structure is formed into three electrodes x1, x2, x3. Similarly the front electrode structure is formed into three electrodes per digit and decimal point y1, y2, y3 Examination of the six electrodes per digit shows that each of the eight elements can independently have a voltage applied thereto by application of suitable voltage to appropriate x, y electrodes.

Prior to assembly the slides 2, 3 bearing the electrodes are cleaned then dipped in a solution of 0.02% by weight of poly-vinyl alcohol (PVA) in water. When dry, the slides are rubbed in a single direction with a soft tissue then assembled with the rubbing directions orthogonal to one another and parallel to the optical axis of the respective adjacent polarisers, ie so that the polarisers are crossed. When the nematic liquid crystal material 1 2 is introduced between the slides 2, 3 the molecules at the slide surfaces lie along the respective rubbing directions with a progressive twist between the slides.

When zero voltage is applied to the cell 1 light passes through the front polariser 5, through the cell 1 (whilst having its plane of polarisation rotated 90 ) through its rear polariser 6 to the reflector 7 where it is reflected back again to an observer (shown in Fig. 1 at an angle of 45 to the axis Z normal to axes X and Y in the plane of the slides 2, 3). When a voltage above a threshold value is applied between two electrodes 8, 9 the liquid crystal layer 1 2 loses its optical activity, the molecules being re-arranged to lie perpendicular to the slides 2, 3 ie along the axis Z. Thus light at the position does not reach the reflector 7 and does not reflect back to the observer who sees a dark display of one or more bars of a digit 10.

Voltages are applied as follows as shown in Figs. 5, 6 and 7 for three successive time intervals in a linescan fashion. An electrical potential of 3V/2 is applied to, ie scanned down, each x electrode in turn whilst - V/2 is applied to the remaining x electrodes. Meanwhile 3V/2 or V/2 is applied to the y electrodes. A coincidence of 3V/2 and - 3V/2 at an intersection results in a voltage 3 V across the liquid crystal layer 1 2. Elsewhere the voltage is V or - V. Thus by applying - 3V/2 to appropriate y electodes as 3V/2 is scanned down the x electrodes selected intersections are turned ON as indicated by solid circles. The electric voltage V is an ac signal of eg 100 Hz square wave, and the sign indicates the phase.

It will be apparent to those skilled in the art that the device shown in Figs. 1 to 7 is a multiplexed display because the electrodes are shared between ON and OFF intersections or display elements.

A material embodying the second aspect of the invention which is suitable for use as the material 1 2 in the above device is in Table 3 as follows (Mixture 1).

Table 3: Mixture 1

Compound Weight percentage E2H5 MffEN 1 5 n-C4H9MCN 23 c2HS {) EOODffCN 12 EsHiffThmcN 10 CH3&commat;CH20{+ O - OE4H-n 5 n~C6H13{0}C SC6H13-n 15 n-C5H11iC00 OE4H9-n 20 Small amounts of a cholesteric material may be added to the nematic material to induce a preferred twist in the molecules in the liquid crystal layer. This and the use of eppropriate slide surface treatment removes the problems of display patchiness as taught in UK Patent Serial Numbers 1,472,247 and 1,478,592.

Suitable cholesteric materials are: C 15 about 0.1 - 0.5% by weight and CB 15 about 0.01% to 0.05% by weight.

Small amounts of pleochroic dye may be added to enhance the display contrast, eg one of the anthraquinone dyes described in UK Patent Specification No 2011 94or.

In another embodiment mixtures embodying the second aspect af the invention may be used in a Freedericksz effect cell. Such a cell may be constructed by sandwiching the liquid crystal material between glass slides having electrode films deposited on their inner surfaces as in the above device. However, in this case the polarisers are not necessary, the glass slide inner surfaces are treated with a coating of lecithin and the liquid crystal material is a negative whose molecules are aligned in the OFF state perpendicular to the slide substrates (homeotropic texture) by the lecithin coating. Application of an appropriate electric field across the material in the ON state re-arranges the molecules parallel to the slide surfaces (homogeneous texture). A pleochroic dye may be incorporated in the liquid crystal material to enhance the ,contrast between the ON and OFF states.

A Freedericksz effect cell made in the above way may incorporate Mixture 2 below, the cell spacing being 10 ym.

TABLE 4: Mixture 2

CH30 C00 CSH11-n ZLI 1052 2/3 by weight 88% n-C6H13O coo -CgH11-n 1/3 by weight CH3CH20oC4H9-n 6% Compound A C2H5CO000CC2H5 24 6% CN CN The preparation of Compound A is described in UK Patent Application No 7934129. 1.2% by weight of a known pleochroic dye eg 1,5-bis-4'-n-butylphenylaminoanthraquinone may be added to Mixture 2 to give a dyed mixture. (Mixture 2A) When a voltage is applied across the cell, the colour changes from a weakly absorbing state to a strongly absorbing state.

In an alternative embodiment of the invention a (cholesteric-to-nematic) phase change effect device incorporates a material as defined above.

A cell is prepared containing a long helical pitch cholesteric material sandwiched between electrode-bearing glass slides as in the twisted nematic cell described above. However the polarisers and surface preparations for homogeneous alignment, eg treatment of the glass slide surfaces with SiO, are not used in this case.

If the glass slides are untreated and the liquid crystal material has a positive dielectric anisotropy (AC) the liquid crystal material is in a twisted focal conic molecular texture in the OFF state which scatters light. The effect of an electric field applied between a pair of electrodes on the respective inner surface of the glass slides is to convert the region of liquid crystal material between the electrodes into the ON state which is a homeotropic nematic texture which is less scattering than the OFF state. This is a 'negative contrast' type of phase change effect device.

If the inner glass slide surfaces are treated, eg with a coating of lecithin, to give alignment perpendicular to those surfaces, and the liquid crystal material has negative the material in the OFF state is in a homeotropic texture which has little scattering effect on incident light. If an electric field is applied between a pair of electrodes on the respective inner surfaces of the glass slides the region of liquid crystal material between the electrodes is converted to a twisted homogeneous texture which scatters light (the ON state). This is a 'positive contrast' type of phase change effect device.

The contrast between the two states in each case may be enhanced by the addition of a small amount of a suitable pleochroic dye (eg 1% by weight of 1,5-bis-4'n-butylphenylaminoanthraquinone in the case where ae is positive) to the liquid crystal material.

A suitable positive dielectric anisotropy material embodying the invention for use in a phase change effect device is: TABLE 5: Mixture 3

Material Weight percentage t C2Hs < CN 37 5% Mixture B g n-C4H9## CN 37.5% ; 800% l nC3H70 < CN 25% C 15 RCOThmOOCN (Rc = 2-methylbutyl) 10% (known) /*\ CM3OCH2OOC4H9-n 10% A suitable negative dielectric anisotropy material embodying the invention for use in a plane change effect device, Mixture 4, is as follows: TABLE 6: Mixture 4

Compound Weight percentage ZLI 1052 (see above) 88% CHCH2OOC4H9-n 5% C2Hg- Orooc-C2Hg 6% CN CN Rc#ffOOCOOc 1% R, = 2-methylbutyl

Claims

1. A liquid crystal compound exhibiting a small dielectric anisotropy and having a structural formula:

where R, is an alkyl or alkoxy group and R2 is an alkoxy group.

2. A compound as claimed in claim 1 and wherein the groups R, and R2 have at most 18 carbon atoms.

3. A compound as claimed in claim 2 and wherein the group R, and R2 have at most 10 carbon atoms.

4. A compound as claimed in any one of claims 1 to 3 and wherein the groups are straight chained groups.

5. A compound as claimed in claim 1 and wherein R, is CH3- and R2 is -OC4Hg-n.

6. A compound as claimed in claim 1 and wherein R,'is CH3O- and R2 -OC5H"-n.

7. A compound as claimed in any one of the preceding claims and which has been made by a method substantially the same as that described in Example 1 herein.

8. A liquid crystal material comprising a mixture of compounds at least one of which is a compound as claimed in claim 1.

9. A liquid crystal material as claimed in claim 8 and which is suitable for use in twisted nematic effect devices.

1 0. A liquid crystal material as claimed in claim 8 and which is suitable for use in a Fréedericksz effect device.

11. A liquid crystal material as claimed in claim 8 and which is suitable for use in a cholesteric to nematic phase change effect device.

1 2. A liquid crystal device including two dielectric substrates as least one of which is optically transparent, a layer of liquid crystal material sandwiched between the substrates and electrodes on the inner surfaces of the substrates to enable an electric field to be applied across the layer of liquid crystal material to provide an electro-optic effect therein, characterised in that the liquid crystal material consists of or includes a compound as claimed in claim 1.

1 3. A device as claimed in claim 1 3 and wherein the device is a twisted nematic effect device, the device having a arrangement of electrodes such as to enable electric fields to be applied across the layer of liquid crystal material in a multiplexed fashion.