GB2061256A - Liquid crystal ester compounds exhibiting a negative dielectric anisotropy and liquid crystal materials and devices containing such compounds - Google Patents

Abstract

A novel liquid crystal ester compound has the general formula: <IMAGE> where X and Y are both selected from CN, F, Cl, Br, CH3, CF3 and NO2 and where R1 and R2 are alkyl groups. It has a low or negative dielectric anistropy and may be used by itself or in admixture with other compounds (e.g. nematic or optically active) in electro-optic devices.

Description

SPECIFICATION Liquid crystal ester compounds exhibiting a negative dielectric anisotropy and liquid crystal materials and devices incorporating such compounds The present invention relates to liquid crystal ester compounds exhibiting a negative 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 (AE) of the material. This is the difference, for a given frequency and temperature between the average dielectric constant measured parallel (E11) 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 is one of the major parameters which determine the kinds of electro-optic devices in which thatmaterial may be used.

For example, materials having a positive dielectric anisotropy, 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 negative dielectric anisotropy may also be mixed with those having a positive dielectric anisotropy to produce mixtures whose overall dielectric anisotropy is positive (but reduced in value). Such mixtures may for example be particularly useful in known multiplexed twisted nematic effect devices or known two-frequency switching effect devices.

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

where X and Y are both selected from the following: CN, F, Cl, Br, CH3, CF NO2 and where R1 and R2 are alkyl groups.

Preferably R1 is the same as R2 and X is the same as Y.

Preferably X and Y are both CN.

The alkyl groups R1 and R2 preferably contains less than eighteen carbon atoms, eg one to ten carbon atoms, and may be normal or branched. If R1 or R2 is branched it may contain a chiral centre in which case the compound is optically active. If R1 and R2 are n-alkyl groups the compound is nematogenic. Preferably R1 = R2.

By a 'liquid crystal compound' is meant a compound in one of the following two known categories: 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 a temperature below the melting point of their solid phase. The monotropic or virtual transition may be detected respectively 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 be 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 according to formula (I), which are di-esters of 2,3 di-substituted hydroquinone and 4-alkyl-trans-cyclohexane carboxylic acids, may be prepared by conventional esterification reactions. For example, in the case of compounds having a symmetrical molecular structure, the appropriate 4-alkyl-transcyclohexane carboxylic acid may be converted into its corresponding acid chloride by a conventional reaction with thionyl chloride; the acid chloride may be converted into its corresponding di-ester by a conventional single esterification step with the appropriate substituted hydroquinone.

One or more of the compounds according to formula (I) may be used in any of the following applications (which are themselves known): together with a positive nematic material giving an overall positive material for use in twisted nematic effect devices particularly multiplexed devices; an example of such a device is given below.

ii either alone, or together with another nematic material, preferably negative, giving an overall negative material, preferably also with a pleochroic dye, in Freedicksz effect devices in which the molecular arrangement may be changed from the homeotropic texture (OFF state) to the homogeneous texture (ON state); an example of such a device is given below.

iii together with a nematic material giving an overall positive 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 homeotropictexture (OFF sfate) by an electric field.

iv together with an optically active material giving an overall negative material which is a cholestericof suitable resistivity (about 109 ohm-cm), in cholesteric memory mode devices in which the molecular arrangement may be changed from a twisted homogeneous texture (OFF state) to a turbulent scattering focal conic texture (ON state) by an electric field.

v together with an optically active material giving an overall 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 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 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.

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 multiplexability 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 (vii) above.

An example of a mixture according to the second aspect which may be used in multiplexed 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:

R=alkyl Preferably the 4'-alkyl- or 4'-alkoxy-4-cyanobiphenyls constitute between about 10 and 90% by weight of the mixture and the compounds selected from formulae (II) to (VII) constitute not more than about 10% by weight in total.

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.

As further examples, the mixture according to the second aspect may be a compound according to formula (I) above 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 properties):

where

is a cyclohexane ring,

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

,or a 4,4' biphenylyl group

or a 2,6 naphthyl group

and Y is CN, or R1, or OR1 or CO.O-X-Y1 where yl is tN, or R' or OR'; the definition of R' being the same as that of R.

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, a two-frequency switching effect device, a cholesteric memory mode device or a dynamic scattering effect device all constructed in a known manner. 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.

Examples of the preparation and properties of compounds according to formula (I) above will now be given.

Example 1 Preparation of symmetrical esters according to formula (I) above by the following route:

StepAl This conversion of the acid into its corresponding acid chloride is carried out under conditions well known for conversions of this kind, eg using thionyl chloride. An example where R = C7H15 and X = CN is as follows.

Trans-4-heptylcyclohexane carboxylic acid (6.8g) and thionyl chloride (10 ml) are boiled together under reflux on a steambath for one hour. The excess thionyl chloride is distilled off under high vacuum, affording trans-4-heptylcyclohexane carbonyl chloride as a light yellow oil.

Step A2 This esterification is also carried out under conditions which are well known per se. An example where R = C7H15 and X = CN is as follows.

A mixture of 2,3-dicyanohydroquinone (2.5g), triethylamine (12.5 ml) and dichloromethane (110 ml) is placed in a flask equipped with condenser and protected from atmospheric moisture with a drying tube containing calcium chloride. The crude trans-4-heptylcyclohexane carbonyl chloride, prepared in Step Al as described above, is added cautiously to the mixture through the condenser, with swirling of the flask contents. Heat is evolved, and frothing occurs. The reaction is boiled under reflux for 1 hour, cooled to 1 50C, and washed successively with water (100 ml), 20% aqueous hydrochloric acid (100 ml) and water (100 ml), dried (Na2SO4), filtered and distilled to dryness. The crude ester is chromatographed on a column of silica gel (120 g), and the column is eluted with petroleum spirit:dichloromethane (3:2).Fractions containing the pure ester are distilled to dryness, and the residual ester (5.7g) is recrystallised from boiling ethanol (450 ml).

The pure ester is recovered by filtration, washed with a little cold ethanol, and dried in vacuo at 40 .

A product yield of 4.7g has been obtained using this example.

The following Tables list properties of compounds of the form where R = alkyl.

In the Tables the following symbols are used: K-S: this denotes a crystalline solid to smectic liquid crystal transition.

ii K-N: this denotes a crystalline solid to nematic liquid crystal transition.

iii S-N: this denotes a smectic to nematic liquid crystal transition.

iv N-l: this denotes a nematic to isotropic liquid transition.

v S-l: this denotes a smectic to isotropic liquid transition.

TABLE 1 - TRANSITION TEMPERATURES ( C) R K-S K-N S-N N-l S-l C2H 138 162 C3H7 137 209 C4H9 1370 146 216 C5H11 136 176 217 * CsH13 1300 202 212 C7H15 1270 2050 2080* Samples turn yeliow if heated to about 210 C * Approximate.

TABLE 2 ANALYSIS RESULTS (weight percentages of constituent atoms) found calculated R C H N C H N C2H5 71.6 7.8 6.3 71.5 7.4 6.4 C3H7 71.8 7.9 6.0 72.4 7.8 6.0 C4Hs 72.8 8.7 5.6 73.1 8.2 5.7 C5H1l 73.2 8.8 5.4 73.8 8.5 5.4 C6H,3 75.4 9.4 5.1 74.4 8.8 5.1 C7H15 74.3 9.4 4.75 75.0 9.1 4.9 TABLE-3 - LATENT HEAT OF FUSION RESULTS R Temp "C AH Kcal mop~1 phase change C2H 140 7.67 K-N 159 0.14 N-l C3H7 132 1.03 K-K1 139 5.04 K1-N 212 0.15 N-l C4H9 139 9.92 K-S 144 0.41 S-N 207 0.51 N-l C5H11 138 11.1 K-S 207 0.3 S-l C6H13 132 5.79 K-S 198 0.16 S-N 202 0.28 N-l C7H15 124 7.29 K-S 184 0.29 202 2.03 N-l In the case of the compound having R = C3H7 and X = CN the dielectric constants were measured at 140"C using the pure compound. These were found to be s11 = +5.4 := -16.6 AE -11.2 These results illustrate that the compounds according to formula (I) have large negative dielectric anisotropies.

Examples of devices embodying the invention 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 Figure 1; Figure 3 shows a rear electrode configuration for Figure 1; Figure 4 shows a front electrode configuration for Figure 1; Figures 5, 6, 7 show schematic views of the device of Figures 1 to 4 with typical addressing voltages; Figure 8 is a graph of transmission v Voltage for a Freedericksz effect cell.

The display of Figures 1 to 4 comprises a cell 1, formed of two, front and back, glass slides 2,3 respectively, spaced about 7 lim apart by a spacer 4 all held together by an epoxy resin glue. A liquid crystal material 12 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 thick are deposited on the inner faces of the slides 2,3 as a complete layer and etched to the shapes shown in Figures 3,4. The display has seven bars per digit 10 plus a decimal point 11 between each digit. As shown in Figure 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 0.2% 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 12 is introduced between the slides 2, 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 Figure 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 12 loses its optical activity, the molecules being rearranged to lie perpendicular to the slides 2,3, ie along the axis Z. Thus light at that 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 Figures 5, 6 and 7 for three successive time intervals in a linescan fashion. An electric 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 they electrodes. A coincidence of 3V/2 and - 3V/2 at an intersection results in a voltage 3 V across the liquid crystal layer 12.

Elsewhere the voltage is V or -V. Thus by applying -3V/2 to appropriate y electrodes 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 Figures 1 to 7 is a multiplexed display because the electrodes are shared between ON and OFF intersections or display elements.

Each OFF intersection receives For the entire scan period whilst each ON intersection receives 3 V for one third of the scan period and V for the rest of the scan period making a rms value of 1.91 V. The material of the layer 12 needs to be such that the voltage required to achieve the ON state is less than 1.91 times the voltage giving the OFF state to provide a suitable contrast between the ON and OFF states. (The ratio of these two voltages for a given material may be designated as the parameter M20 which is defined below). This illustrates that the properties of the material are important in determining the multiplexability of the device.

For similar displays having more than three rows of electrodes the figure of 1.91 V is reduced accordingly.

A material embodying the present invention which is suitable for use as the material 12 in the above device is as follows (Mixture 1).

The following Table gives various electro-optical parameters which have been found for Mixture 1 in a twisted nematic cell constructed similarly to that shown in Figures 1 and 2 and using light of wavelength 589 nm. Corresponding measurements are given for the known mixture E7.

TABLE 4 Parameter Value for Mixture 1 ValueforE7 Vsat 1.78 V 1.96 V VT 1.34 V 1.485 V Tr 66 ms 60 ms Td 58 ms 50 ms M20 1.78 1.79 My 40 2.33 2.60 -dV -10.7 mv C- -10.5 mv C- dT M20 isthe ratio at a temperature of -20 C of V10 (D ) = V90(45 ), where Vr0(0 ) is the voltage giving 10% light transmission along the normal to cell glass slides and V90845 ) is the voltage giving 90% light transmission along a direction 45" to the normal to the cell glass slides, (that which gives the lowest value of V90).

My ,40 iS the ratio of V10(0") as defined above but measured at 0 C to V90(45") as defined above but measured at 40"C.

Tr = rise time required for the transmission to change from 100 to 10% (on application of the appropriate voltage).

td= decay time required for the transmission to change from 0 to 90% (on removal of the appropriate voltage).

dV/dT = rate of change of threshold voltage with temperature over the range 0 to 400C.

Table 4 shows how a number of the parameters of the known biphenyl/terphenyl mixture E7 which are

n-CgH11- 51% by weight n-C7H15 CN 25% by weight E7 ( ) 95% 25%byweight by weight (known) ( n CgH17 < CN 16% by weight n C5H11 < CN 8% by weight Compound s coo ?oc c2Hs 5% by weight tN A CN relevant to use in twisted nematic cells, particularly in multiplexed applications, may be significantly improved by the addition of only 5% by weight of Compound A.

An alternative biphenyl/terphenyl mixture which may be switched by lower voltages than -E7 is: Mixture 2

Component Weight percentage C2H5 XCN 30% n-C3H7(lCN 20% n < 4H9 Mmoo CN 30% C2HS} COO mThON 20% Compound A when added to this mixture in similar proportions will change its properties similarly.

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 appropriate 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 CB1 5 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 Application No. 42,810/77.

In another embodiment mixtures according to the second aspect of 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 oh 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 material 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 was made in the above way incorporating Mixture 3 below, the cell spacing being 101lem.

Mixture 3

Material Percentage by weight CH3O CO0 eCSH11-n ZLI 1052 1 213 by weight 94% n-C6H130COO C5H11-n 1e3 be weight Compound A H5 C0OOH c2H5 6% CNQJ 1.2% by weight of the known pleochroic dye 1 ,5-bis-4'-n-butylphenylaminoanthraquinone was added to Mixture 3 to give dyed Mixture 4.

When a voltage was applied across the cell, the colour changed from a weakly absorbing state to a strongly absorbing state. Figure 8 illustrates the transmission V voltage curve for the cell. The threshold voltage was found to be 1.8 volts at 90% transmission.

Claims (2)

1. A liquid crystal ester compound having the general formula:

where X and Y are both selected from CN, F, CI, Br, CH3 CF3 and NO2 and where R1 and R2 are alkyl groups.

2. A liquid crystal compound as claimed in claim 1 and wherein R1 and R2 are both C2H6 and X and Y are both CN.