GB2080820A - Liquid crystal composition containing a cyanobiphenyl and a phenyl benzoate - Google Patents

Abstract

A liquid crystal material suitable for a multiplexed twisted nematic device comprises a mixture of: (a) one or more cyanobiphenyl compounds having a formula: <IMAGE> where R1 is an n-alkyl group containing between 7 and 12 carbon atoms inclusive, and (b) one or more benzoate esters having a formula: <IMAGE> where R2 is an n-alkyl group containing 3, 5 or 7 carbon atoms and R3 is an n-alkyl group containing 5, 7 or 9 carbon atoms, the mixture containing between 53 and 90 per cent (inclusive) by weight of the compound(s) of formula (II). Preferably the mixture contains between 60 and 80 per cent by weight of the compound(s) of formula (II). The mixture may be further mixed with other compounds, eg other 4-n-alkyl- or -alkoxycyanobiphenyls or esters, eg having five or less carbon atoms in their n-alkyl group, the compound(s) of formula (II) being present as the major overall biphenyl constituent(s), or high clearing point nematics, ie materials with nematic-to-isotropic liquid (N-I) transition temperatures at over 150 DEG C.

Description

SPECIFICATION Liquid crystal composition containing a cyanobiphenyl and a phenyl benzoate This invention relates to liquid crystal materials and devices.

Liquid crystal devices are commonly formed by enclosing a layer typically 7 to 1 2 pm thick, of a liquid crystal material between optically transparent electrically insulating substrates, eg glass slides. Electrode structures on the substrates enable voltages to be applied across the layer and change the molecular arrangement of the liquid crystal molecules in selected regions. This causes an observable effect in those regions which can be used in displays of various types, for example, digital watches, small calculators, voltage meters, etc.

There are several different display effects which may be obtained from electrical rearrangement of liquid crystal molecules. These depend on the liquid crystal material employed and the initial molecular alignment (if any). This invention is concerned particularly with the twisted nematic effect in which an essentially nematic material is arranged in its OFF state (zero or low applied voltage) in an arrangement in which the molecules gradually twist, usually through 90 , from one substrate surface to the other as defined by fixed alignments at these surfaces. Such an arrangement is optically active, ie rotates the plane of polarised light in this state. An applied voltage greater than a threshold essentially causes the twist to be destroyed so that the molecular arrangement is no longer optically active.Thus, when placed between plane (iinear) polarisers a twisted nematic effect device provides one or more optical shutters which are switchable from a light state to a dark state or vice versa depending on whether the polarisers are in a parallel or cross relationship.

Such shutters may in a single device form the component elements of a series of letters, words or numerals, ie in an information display, as defined by the electrode structures on the substrates and the applied voltages. An example of a twisted nematic effect device is further described in UK Patent Number 1,478,592.

One method of arranging the electrodes and applying the voltages in a liquid crystal display is called multiplexing. This involves applying eiectric potentials to lines of electrodes connected in series, each region or display element, eg each individual shutter in a twisted nematic effect display, being defined by the intersection of the electrodes on the two substrates. Examples of multiplexed displays are the matrix displays described in UK Patent Number 1,458,045.

One of the major problems concerned with multiplexed displays is that all electrodes in the display receive a non-zero electrical potential, even when they are in the OFF state. Consequently it is difficult to provide sufficient contrast between the ON and OFF states. The contrast can vary with viewing angle, temperature and liquid crystal material as well as applied voltage.

As described in UK Patent Application Number 7931 563 a figure of merit which may be used to define the usefulness of any given material in a multiplexed twisted nematic effect display is the parameter M20. This is the ratio, at 20"C, of voltages V,,(O"): V90(45 ) which are voltages which may be designated as necessary to achieve the ON and OFF states respectively.In fact V,,(O") is the minimum voltage required to achieve along an axis normal to the display substrates 10% of the maximum light transmission along that axis for a twisted nematic effect cell: the incident light is plane polarised parallel to the surface alignment of the liquid crystal molecules upon which the light is first incident and the analyser is arranged with its polarising axis perpendicular to that alignment and parallel to the surface alignment of the molecules from which the light emerges.

V90(45 ) is the maximum voltage which gives along an axis at a viewing angle of 45 to the alignment directions on the substrate surfaces 90% of the maximum transmission along that direction for the same cell for which V,,(O") is measured. Of the four possible directions which are at 45 to the alignment directions the direction having the lowest value of V90(45 ) is chosen.

An ideal material would have M20 equal or just greater than unity so that a very small change in applied voltage could be used to switch between the OFF and ON states. In practice, other liquid crystal properties, eg the temperature range of the nematic mesophase, have to be taken into account as well.

Materials based on the cyanobiphenyls and terphenyls as described in UK Patent Number 1,433, 130 have found wide use in twisted nematic effect displays and have an excellent range of properties for many purposes. However, it is not easy to achieve multiplexing with such materials because they show relatively high values of M20. For example the well known material E7 marketed by BDH Chemicals Limited of Broom Road, Poole, Dorset, has a value of 1 81 for M20 UK Patent Application No 7931,563 describes how the value of M20 may be reduced by the addition of benzoate esters of the form

R, R' = alkyl or alkoxy, (...formula (1)) (as described and daimed. in in USA Patent Number 4,002,670) to mixtures of cyanobiphenyls and cyanoterphenyls or other compounds having similar properties.It is an object of the present invertitin to reduce the value of my stil further.

The said copending Application is particularly concemed with mixtures of benzoate esters and cyanobiphenyls containing essentially short-chained ail:yl groups (as well as other nematic materials which are bigh-melting, such as cyano-p4erphenyls). it has been hitherto belied highly desirable to use only short chained compounds to reduce operation voltages and to preserve the nematic properties of the liquid crystal material. Long-chained cyanobiphenyls have been ignored hitherto because of their tendency to form smectic mesophases.

According to the present invention in a first aspect a liquid crystal material suitable for a multiplexed twisted nematic device comprises a mixture of: (a) one or more cyanobiphenyl compounds having a formula:

formula (11) where R, is an n-alkyl group containing between 7 and 12 carbon atoms inclusive, and (b) one or more benzoate esters having a formula:

formula (111) where R2 is an n-alkyl group containing 3, 5 or 7 carbon atoms and R3 is an n-alkyl group containing 5, 7 or 9 carbon atoms, the mixture containing between 53 and 90 per cent (inclusive) by weight of the compound(s) of formula (II).

Preferably the mixture contains between 60 and 80 per cent by weight of the compound(s) of formula (II).

The mixture may be further mixed with other compounds, eg other 4-n-alkyl- or -alkoxycyanobiphenyls or esters, eg having five or less carbon atoms in their alkyl group, the compound(s) of formula (11) being present as the major overall biphenyl constituent(s), or high clearing point nematics, ie materials with nematic-to-isotropic liquid (N-l) transition temperatures at over 150'C, such as one or more of the compounds in the following list (totalling up to O% by weight of the total mixture):

where R = n-alkyl; or optically active additives (typically 1% by weight or less of the total).

By using cyanobiphenyls according to formula (11) in the materials according to the first aspect the multiplexibility may unexpectedly be improved in twisted nematic devices (ie M20 may be reduced). Surprisingly, using the longer chained nematics of formula (11) does not affect the usefulness of the materials because the benzoate esters with which they are mixed and the relative proportions of the compounds in the mixture are specially selected so that the materials are in a "nematic window'' of the overall mixture composition. This means that, although the phase diagram (a plot of temperature against material composition) shows room temperature smectic behaviour for most benzoate ester/longchain cyanobiphenyl compositions, there is a narrow range at selected compositions where no smectic mesophase is observed above-10 C.

Therefore at these compositions nematic mesophases prevailing over wide temperature bands can be obtained (as illustrated below).

Thus, the present invention represents a special selection of compositions from the broad class of materials disclosed in UK Patent Application No 7931 563.

In formula (II) above R, is preferably n-C7H,5 or n-CgH,g.

According to the present invention in a second aspect a liquid crystal twisted nematic effect device includes two dielectric substrates at least one of which is optically transparent, electrodes on the inner surfaces of the substrates in an arrangement such as to provide a multiplexed display, a nematic liquid crystal material between the substrates with a twisted molecular arrangement and characterised in that the nematic liquid crystal material is a mixture according to the first aspect as defined above.

The device of the second aspect may be arranged between crossed, or parallel, linear polarisers. A reflector may be located adjacent to one polariser so that the display may be observed by reflected light. The reflector may be only partly reflecting and a tungsten light (or other suitable light source) arranged to illuminate the display through the reflector as required.

Although the voltages required to drive the device of the second aspect are greater, because of the longer alkyl groups in the compounds of formula (II), than the corresponding voltages for devices using shorter-chained cyanobiphenyl compounds (eg those in UK Patent Application No 7931,563) the difference is not as great as might be expected because the use of a biphenylrich mixture as in the present invention partly compensates for the voltage increase. In any event the increase is less important in a multiplexed device than it would be in a nonmultiplexed display (eg a simple calculator or timepiece display having ON/OFF display bars) driven directly by a battery.

Compounds of Formulae (II) and (III) are known per se and may be produced by known methods (see UK Patent No 1,433,130 and USA Patent No 4,002,670).

Mixtures may be prepared in a known way, eg by adding the constituent compounds together in the appropriate proportions, heating the mixture with stirring into its isotropic liquid phase, maintaining it in that phase for a period of time, eg 30 minutes, with stirring and allowing it to cool to room temperature.

Examples of devices and materials 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 digital display; Figure 2 is a sectional view of 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, 7 show schematic views of the device of Figs. 1 to 4 with typical addressing voltages; Figure 8 is a phase diagram (graph of temperature v material composition) in part of which is a composition which may be used in the display of Figs. 1 and 2.

The display of Figs. 1 to 4 comprises a cell 1, formed of two, front and back, glass slides 2, 3 respectively, spaced 7lim apart by a spacer 4 all held together by an epoxy resin glue. In front of the front glass slide 2 is a front polariser 5 arranged with its axis of polarisation horizontal. A reflector 7 is arranged behind a back polariser 6 behind the slide 3.

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 point1, 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 electrode clear slides 2, 3 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. When a 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 90' 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 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 the electrode 8, 9 the liquid crystal layer 1 2 loses its rotary power, or optical activity, the molecules being re-arranged 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 1 0.

Voltages are applied as follows as shown in Figs. 5, 6 and 7 for three successive time intervals. an electric potential of 3V/2 is 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 3V 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 intrsections are turned ON as indicated by solid circles. The voltage V is an ac signal of eg 100 Hz square wave, and the sign indicates the phase.

Each OFF intersection receives V for the entire scan period whilst each ON intersection receives 3V for one third of the scan period and V for the rest of the scan period making an rms value of 1.91 V. Thus the material of layer 12 preferably has M20 less than 1.91 to give a reasonable contrast between the ON and OFF states.

For similar displays having more than three rows of electrodes the figure of 191 V is reduced accordingly. In other words M20 is desirably as small as possible.

Suitable nematic mixtures for use to form the layer 12 in the above device are described below.

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 to 0.5% by weight and CB 15 about 0.01 to 0.05% by weight.

The following mixtures are examples of nematic materials embodying the invention which may be used in the above device. In the properties of the mixtures (measured in a 7,um thick cell) M20 is as defined above, M'20 is a similarly measured ratio but is the voltage required to give 50% transmission at an angle of 10 to the normal to the twisted nematic cell divided by the voltage V90(45 ), ie M'20 = V50(10 )/V90(45 )/ #n is the birefringence of the material measured at 20 C and at 589 nm; N-I is the nematic-to-isotropic liquid transition temperature.

Suitable nematic mixtures for use to form the layer 12 in the above device are as follows: MIXTURE 1

Component Weight % "-C5 H11CN 10 n-C7HMff CN 39 fl (O) C9H1 < CN 21 n-C3H7 COO C7H-15H0 30 Properties of Mixture 1 N-l = 56 C #n = 0.18 M20 = 1.61 M120 = 1 -29 MIXTURE 2

Component Weight % n-C5H11ffDCN 100 n-C7 < CN 34 6 n 9H19Mm CN 24 0 n C3H7+COO+C7H15 n 31'4

= trans 1,4 disubstituted cyclohexane ring Properties of Mixture 2 N-1 = 54 C #n = 0.17 M20 =158 M'20= 1 26 MIXTURE 3

Component Weight % nC7smCOO mffCN 3 "-CSH11- CN 7 n C7H15 < CN 35 nC 9H1 < CN 25 n-C3H7 COO+C7H15 n 30 Properties of Mixture 3 N-1 = 56 C An =017 M20 = 1.59 M'20 = 1.25 MIXTURE 4

Component Weight % LJ --C00--CN 10 5 n C 5 H11OOOCN n-C5H11 < CN 15 n- C7 Hp;;-cN 40 n--C3H7 0 COO C7H1s 30 Properties of Mixture 4 N-1 = 58 C Birefringence (#n) = 0.18 N20 = 1.64 M'20 = 1.30 MIXTURE 5

Component Weight % C2H5ffThCN 15 n-7HMff CN 35 n C5H1 +CN 10 C2COOffCN 10 n-C3H7 COO C5H11-n 30 Properties of Mixture 5 N-1 = 62 C #n = 0.18 M20 = 1.68 M'20 = 1.32 MIXTURE 6

Component Weight % nC7H&num;;ff CN 58 n C5Hll > CN 10 n-C3H7COOC5H11-n 20 nC7Hi5COOC7H157n 12 Properties of Mixture 6 N-1 = 54 C Birefringence (#n) = 0.18 M20 = 1.65 M'20 = 1.29 MIXTURE 7

Component Weight % n C7 H15 CN con 59 n C5H19CN 10 n-C51COOC5H11n 31 Properties of Mixture 7 N-1 = 53 C #n = 0.17 M20 = 1.64 M'20 = 128 MIXTURE 8

Component Weight % n-C7 H15&num;;CN 57OO n-C5 H11- 9 950 95O CH3O COO CH11-n 14 25 C 7 H15+ COO C 7 H15n 19 - 25 Properties of Mixture 8 N-l = 55"C An =018 M20 = 1 65 M'20 = 1 26 Fig. 8 illustrates the effect of operating in the "nematic window" of the phase diagram of mixtures of long-chained cyanobiphenyls and benzoate esters. The particular phase diagram illustrated is that of the various compositions obtained by mixing the compounds K27, ie

and ME 3-7, ie

The two curves denoted by the symbols A and B are transitions from the smectic A mesophase to the nematic mesophase. The regions below these curves exhibit a smectic A mesophase as indicated by the symbol SA. The curve denoted by the symbol C is the transition from the nematic mesophase to the isotropic liquid phase. The region between the curves A, B and C is a nematic mesophase as indicated by the symbol N. A particular part of this region W exists as a nematic window between the curves A and B at temperatures close to 20"C for compositions of between approximately 12% and 42% by weight of K27 and it is in this compositional region where compositions embodying the invention are chosen to avoid the SA region at room temperature.

Other similar mixtures of long-chained cyanobiphenyls and benzoate esters exhibit nematic windows in a similar way at similar compositions.

Claims (9)

1. A liquid crystal material suitable for a multiplexed twisted nematic device comprising a mixture of: (a) one or more cyanobiphenyl compounds having a formula:

formula (II) where R, is an n-alkyl group containing between 7 and 12 carbon atoms inclusive, and (b) one or more benzoate esters having a formula:

formula (III) where R2 is an n-alkyl group containing 3, 5 or 7 carbon atoms and R3 is an n-alkyl group containing 5, 7 or 9 carbon atoms, the mixture containing between 53 and 90 per cent inclusive by weight of the compound or compounds of formula (II).

2. A material as claimed in claim 1 and wherein the mixture contains between 60 and 80 per cent inclusive by weight of the compound or compounds of formula (II).

3. A material as claimed in claim 1 and wherein the material includes a mixture as claimed in claim 1 or claim 2 which is further mixed with one or more other compounds.

4. A material as claimed in claim 3 and wherein the said other compound or compounds include one or more other 4-n-alkyl- or 4-n-alkoxy-4'-cyanobiphenyls or one or more other esters, wherein more than 50% by weight of the biphenyl content of the material is provided by the compound or compounds of formula (II).

5. A material as claimed in claim 3 or 4 and wherein the said other compound or compounds include one or more compounds, present in the liquid crystal material to the extent of 20% by weight or less in total, having the following formulae:

where the groups R are the same or different n-alkyl groups.

6. A material as claimed in claim 3, claim 4 or claim 5 and wherein the said other compound or compounds includes an optically active additive present in the liquid crystal material to the extent of about 1 % by weight or less.

7. A material as claimed in any one of the preceding claims and wherein the or each compound of formula (II) has seven or nine carbon atoms.

8. A liquid crystal material as claimed in claim 1 and which is substantially the same as any one of Mixtures 1 to 8 defined hereinbefore.

9. A liquid crystal twisted nematic effect device including two dielectric substrates at least one of which is optically transparent, electrodes on the inner surfaces of the substrates in an arrangement such as to provide a multiplexed display, a nematic liquid crystal material between the substrates with a twisted molecular arrangement and characterised in that the nematic liquid crystal material is a material as claimed in any one of the preceding claims.