EP1141172A1

EP1141172A1 - Liquid crystal materials

Info

Publication number: EP1141172A1
Application number: EP99954274A
Authority: EP
Inventors: Alexander Beer; John William Goodby; Michael Hird; Stephen Malcolm Kelly
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 1998-11-17
Filing date: 1999-11-17
Publication date: 2001-10-10
Also published as: JP2003519244A; WO2000029505A1

Abstract

A liquid crystal material comprises a smectic host component and one or both of (a) a chiral dopant; and (b) either a monomer comprising two polymerisable portions either end of an intermediate portion, or a polymer formed from said monomer. The smectic host component is closely matched in length to the said chiral dopant and/or to the intermediate component. Preferably it is also closely matched in polarisability to one or other of components (a) and (b). In preferred ferroelectric compositions, both the components (a) and (b) are present, the chiral component in an amount of between 5 and 30 weight per cent relative to smectic plus chiral components. These can exhibit improved resistance to mechanical shock in surface stabilised display devices, together with good switching speeds, while not unduly depressing the SC/SA transition temperature.

Description

Liquid Crystal Materials

The present invention relates to liquid crystal materials, and has particular but not exclusive relevance to ferroelectric liquid crystal materials.

Since it was disclosed in 1980 in Applied Physics Letters, 1980, 36, 899, the surface stabilised ferroelectric crystal display has remained the state of the art in display applications requiring ferroelectric liquid crystal materials which are bistable, or have a long relaxation time, or can be made to act as though they were bistable.

Such materials include ferroelectric smectic liquid crystal materials which possess a smectic C* (S_c* or SmC*) phase. It will be understood by the skilled person that it is only the smectic C* phase, and not the smectic C (S_c or SmC) phase, which exhibits ferroelectric properties and which has optical activity, and that a common way of obtaining a smectic C* phase is to add an optically active dopant to an ordinary smectic C liquid crystal host material or composition.

It is known to dope smectic C host materials, particularly difluorinated terphcnyls which have a very low viscosity, with chiral cyanohydrincther dopants to obtain very fast optical switching. However, in so doing, the S_C*/S_Λ transition temperature is significantly depressed relative to the original S /S_Λ transition temperature, which is undesirable. From here onwards this transition temperature may be referred to as the S_C/S_Λ transition temperature irrespective of whether or not a ferroelectric material is involved.

A major problem which largely remains to be overcome is that known surface stabilised ferroelectric crystal displays tend to be excessively sensitive to mechanical shock, which degrades or destroys the liquid crystal alignment, and so gives rise to very adverse optical effects. Where speed of operation is not critical, the use of polymer ferroelectric liquid crystal display devices, as described in Ferroclectrics, 1991, 122, 53 offers a solution to this problem, but these devices are slow, lacking the necessary switching speed for applications such as television, and other forms of video, including real time displays.

Other solutions to this problem include the provision of combined polymer/low molar mass systems, for example polymer dispersed ferroclectrics as described in J. Appl. Phys., 1991, 9, 405 and polymer doped ferroelectrics as described in Liquid Crystals, 1992, 12, 319. Also the use of anisotropic network stabilised systems, previously used for ncmatic and cholesteric liquid crystal materials, has recently been suggested in Adv. Mat. 1995,7(3), 300 and Liquid Crystals, 1995, 19, 65. In the network stabilised systems, a stabilised uniformly aligned ferroelectric liquid crystal is formed in a display by creating a polymer network of anisotropic building blocks in an aligned sample. In all such cases the optical response is still too slow for many commercial applications.

It has now been found that careful selection of smectic host and chiral dopant can give ferroelectric smectic materials with very good switching speeds while not unduly depressing the S_C/S_Λ transition temperature. Furthermore, it has been found that the use of such materials together with selected anisotropic network building units should allow the build up of network stabilised ferroclectrics which have improved shock resistance while retaining fast switching.

In a first aspect, the present invention provides a liquid crystal material comprising a smectic host component and at least one further component selected from the group consisting of

(a) a chiral dopant; and

(b) a monomer comprising two polymerisable portions either end of an intermediate portion, or a polymer formed from said monomer; wherein the smectic host component is matched in length to the said chiral dopant or to said intermediate component. The smectic host component may be a single compound or may comprise a plurality of different compounds. Similarly the chiral dopant where present may be a single compound or may comprise a plurality of different compounds. Where present the monomer may be a single compound or may comprise a plurality of different compounds, and the latter may have or may not different intermediate portions; the polymer derived therefrom may correspondingly comprise the same or different intermediate portions.

When any component is not a single compound, or when the polymer comprises different intermediate portions, properties which are a function of individual molecules, for example length and number of rings in a chain, are determined as a mole per cent weighted average. Thus a smectic host component comprising equal mole amounts of two smectic compounds having 2 and 3 rings respectively will be regarded as having 2.5 rings, and the length will be the average of the lengths of the two molecules involved. However, other properties, such as polarisability may be determined in a similar manner where possible, or the property of the component overall may be used.

"Length" in each case is the (molecular or part molecular) length of the all-trans configuration, and is determined in the same manner, for example in each case by a computer simulation, or by the use of (physical) Drieding models. It excludes any terminal hydrogen atoms in each case, it being understood that liquid crystal and associated molecules commonly have a major long axis.

By the phrase "matched in length" as used herein is meant that the length of the first material referred to is within 30% of that of the second material being referred to.

By "matched in (terms of its) polarisability" as used herein is meant that the polarisability of the first material referred to is within 30% of that of the second material. This aspect, and how it may be achieved, will be discussed in more detail later. "Polarisability" in each case may be determined in the same known manner, for example by dielectric spectroscopy oτ ϋrom jptical measurements (birefringence/refractive index).

In this specification, by "polymerisable portions^ is meant that portion winc is necessary to the polymerisation, process. Thus_^ for example, if the monomer is a diacrylale of the general formula (1)

^■σ^

where. R_ is. H or lower alkyl (1 to 5 carbon atoms, preferably unbranched, and preferably methyl if not hydrogen), and A is a mesogenic core, the polymerisable portions are (2):

C2)

and the intermediate pσrtioiris the-whote of -O-A-O-. The use of a cϋacrylate^ monomer where R is H is more particularly- described later in the specification. As will be understood by the skilled person, although the carbonyl groups are not incorporated in the polymer chain, they are essential fn enabling polymerisation to take place, and hence are regarded as^~paιt ofthr "polymerisable portions" .

In a preferred material accorriingJa the invention both components (a) and (b) are present in addition to the smectic host. While matching in length may apply only

4

RECTIFIED SHEET (RULE 91) ISA/ EP between the smectic host and one such component, it is particularly preferred for the smectic host to be matched in length to each of components (a) and (b)

In preferred embodiments, the polarisability of the smectic host is also matched to the polarisability of a said other component. Where only one component (a) or (b) is present, this will then be matched for length and polarisability. Where both components (a) and (b) are present, it is particularly preferred but not necessary for the smectic host to be matched in polarisability to each of the components (a) and (b). Generally, where (a) and (b) are present and there is polarisability matching, one component is matched for length, or both length and polarisability, and the other is matched for length, or polarisability, or both length and polarisability. Both length and polarisability matching for both components (a) and (b) is most preferred.

One particular way in which matching in length and/or polarisability can be facilitated is to choose materials with molecules of formulae having a similar appearance. In particular, many common smectic hosts and dopant materials comprise a chain comprising a plurality of rings which may be carbocyclic or heterocyclic, aromatic or non-aromatic. Commonly but not necessarily such rings are six membered. It is also possible for the intermediate portion of the monomer or polymer which can be used according to the invention to comprise a plurality of rings in similar manner. One recognised requirement for fast switching is low viscosity, and the embodiments described later are based on difluoroterphenyl host smectic C materials which as stated above are known to have remarkably low viscosities.

Thus length matching may be achieved if the smectic host comprises a chain including a first number of rings in a chain and the dopant and/or the intermediate portion comprise a second number of rings which is within 30% of the first plurality (ring number matching). Where both components (a) and (b) are present in addition to the smectic host, while only one of such components may show such matching of ring numbers, it is particularly preferred if both components show ring matching within 30% of the smectic host. Furthermore, the rings of the first number arc preferably either all aromatic or all non-^~ aromatic. Preferably, both from the length and polarisability matching aspects, the rings of the second number as defined in the preceding paragraph are also all aromatic or all non-aromatic, and more preferably the rings of the first and second numbers taken together are cither all aromatic or all non-aromatic. Where both components (a) and (b) are present in addition to the smectic host, while either of these conditions may apply only in respect of one component, it is even more preferred if either of the conditions apply in respect of both components. In one preferred form of material according to the invention, tcrphcnyl chains arc present in all of the smectic host, all of the chiral dopant, and all of the intermediate porlion(s) of a monomer or derived polymer.

The invention extends to a method of making a liquid crystal material, to a liquid crystal display device and to a method of making a liquid crystal display device.

Where present, the amount of chiral dopant present in a liquid crystal material of the invention relative to (host plus dopant) is preferably no more than 30 mole per cent, more preferably no more than 20 mole per cent and even more preferably no more than 10 mole per cent. When provided, there is preferably at least 5 mole per cent of chiral dopant present.

However, the present invention is not to be regarded as being limited to the types of smectic host material exemplified above. More important in many applications arc the maintenance of a S_C/S_Λ transition temperature which is not unduly altered relative to the host material, optimising the switching speed whatever the viscosity of the starting host material, and, in the case of polymer network materials, the obtaining of a system which has improved resistance to mechanical shock. Naturally, materials of lower viscosity are desirable in the context of fast display applications. A useful class of smectic C materials comprises a central portion consisting of a plurality of rings linked in a chain, commonly from 2 to 4 rings. Each of the rings may be, for example, a 5, 6 or 7 membered heterocyclic or alicyclic ring, whether aromatic or not, such as pyridine, pyrimidine, pyrrole or cyclohexane. The chain of rings is commonly substituted at the para position of each terminal ring, and may be further substituted on any or all of the rings in the chain.

When seeking to match a dopant or monomer thereto in terms of polarisability, it is to be expected that the best match will normally be with a molecule having a very similar structure, particularly in terms of the number of rings, and/or whether the rings are or are not aromatic (as mentioned above) and, possibly to a lesser extent, the presence and type of substituents and/or heterocyclic atoms. Thus, in addition to the terphenyls particularly specified in the description of the embodiments, the invention is equally applicable to other known smectic materials, including, for example, other terphenyls, terphenyl analogues wherein one two or all of the phenyl rings is/are replaced by another 5, 6 or 7 membered heterocyclic or alicyclic ring, whether aromatic or not, such as pyridine, pyrimidine, pyrrole or cyclohexane; biphenyls and other polyphenyls, and analogues thereof as in the case of the terphenyls.

Thus the smectic host may be a terphenyl, e.g. a 4'4" substituted terphenyl, and more preferably a difluoroterphenyl of formula (3)

or formula (4)

SUBSTTT ΠE SHEET (RUU 26) where Rl and R2 are the same or different and are each an alkyl or alkoxy group comprising from 3 to 10 carbon atoms, branched or straight chain.

The chiral dopant may also be a teφhenyl, preferably a difluoroteφhenyl, for example of formula (5):

where R3 and R4 are alkyl, preferably n-alkyl, having from 3 to 10 carbon atoms.

The intermediate portion of the monomer may include a teφhenyl moiety, again preferably a difluoroterphenyl moiety. The polymerisable portions may be acrylic ester groups. In one preferred form, the monomer has the formula (6)

where A is -O- or, more preferably, -CH₂-, and n is from 5 to 8.

Other features and advantages of the invention can be gained from a consideration of the appended claims, to which the reader is referred, and from the following more detailed description in respect of combinations of smectic hosts, dopants and polymer network materials, made with reference to the accompanying drawings, in which:

Figure 1 is a plot of apparent tilt angle as a function of temperature for five mixtures of a smectic material to which different chiral dopants have been added;

Figure 2 is a plot of spontaneous polarisation against temperature for the same five mixtures of Figure 1;

SUBSTmJT! SHEEr<RULE26) Figure 3 is a plot of optical response time against temperature for the same five mixtures of Figure 1;

Figure 4 is a plot of apparent till angle as a function of temperature for three mixtures of a smectic material to which a chiral dopant has been added, two of the mixtures also comprising different polymer networks;

Figure 5 is a plot of optical response time against temperature for the same three mixtures of Figure 4;

Figure 6 is a plot of optical response time against temperature for three further mixtures based on a different chiral dopant;

Figure 7 illustrates the steps in the preparation of chiral dopants III and IV;

Figure 8 illustrates the steps in the preparation of chiral dopants V and VI.

Figure 9 illustrates the steps in the preparation of chiral dopant VII; and

Figures 10 and 11 illustrate the steps in the preparation of monomers M.l and M.2.

(A) Mixtures of smectic host and chiral dopant.

Smectic Host. The basic smectic C host used in this investigation, henceforth referred to as "S_c-host", consists of a mixture of:

50% of 2',3'-difluoro-4-heptyl-4"-pentyl-p-terphenyl (I); 25% of 2,3-difluoro-4-heptyl-4"-pcntyI-p-teφhenyl (Ha); and 25% of 2,3-difluoro-4"-heptyl-4-pentyl-p-tcrphenyl (lib)

exhibiting the phase sequence SmC/SmA/N/I with a room temperature SmC phase and a SmC/SmA transition temperature of 87.2 °C (Table 1).

π Oa: p = 6, q = 4 πb: p = 4, q = 6

Dopants Five chiral dopants III to VII were synthesised. Syntheses of these dopants are given at the end of the description.

vπ

10

SUBSTITUTE SHEET (RULE 2β) Dopants HI to VI are based on the 2'3'difluoroteφhenyl mesogen, whereas dopaiϊt VII contains the 2,3-difluorobiphenyl unit. The chiral moieties employed were either the (S)-2-hexylpropionic acid (III and V) or the (S)-α-fluorooctanoic acid (IV, VI, VII). Dopants HI, IV and VI contain a single chiral moiety, and were employed in a concentration of 10 mol percent of the total mixture with the S_c-host, whereas Dopants V and VI have two such moieties, and provided 5 mol percent of the total mixture with the S_c-host. These mixtures will henceforth be referred to as Mixtures III to VII, the numeral corresponding to the dopant employed.

Physical Properties of the Mixtures and Components.

Table 1 below shows the phase sequence and transition temperatures of the smectic C Host (S_c-host), the Dopants HI to VII, and the ferroelectric Mixtures III to VII as determined by microscopy and differential scanning calorimetry (DSC) on heating.

Table 1 As shown, Dopant VII is a liquid at room temperature, whereas the others arc crystalline. Dopants IV and VI containing the -fluorooctanoic acid group also have a smectic A phase.

All ferroelectric mixtures show the phase sequence S_C*/S_Λ/N/I, and in Mixtures III to VI containing the terphenyl dopants the S_C*/S_A transition temperatures arc only slightly reduced relative to the undoped host (87.2°C). This is believed to be due to their closely related structures. Mixture VII with the shorter biphenyl dopant has a lower melting point than the S_c-host, but at 71.9°C the S_C*/S_A transition temperature is considerably reduced.

Electro-optic Properties

Figure 1 shows the apparent tilt angle 0, measured with a low frequency rectangular wavefoπn field, typically OJ Hz, and the value extrapolated to zero field.

Figure 2 shows the spontaneous polarisation P_s as calculated from the current signals (current pulse technique) using triangular waveform fields of typically 80 to 100 Hz.

Figure 3 shows the optical response time τ, defined as the time for a 10 to 90 percent change in optical transmission with an applied rectangular wavefoπn field of 7.3 V per micron.

More detail regarding these measurements is given in the Experimental Section below.

All of the values of Figures 1 to 3 are shown as a function of temperature. The shape of the curves of Figure 1 suggests that the tilt angle is controlled mainly by the smectic C host (S_c-host). In Figure 3, for temperatures between 48°C and the S_C*/S_Λ transition, the mixtures exhibit very fast optical response in the microsecond range, and the similarity of the temperature dependence suggest that in this range the response times are dominated by the viscosity of the host material. Below 40°C Mixtures V and VI with dopants containing two ester groups are significantly slower, presumed to be due essentially to viscosity effects. The Mixtures IV and VII exhibit fast switching at room temperature with response times of around 70 and 110 microseconds respectively.

As shown in Figure 2, the tilt angles increase with decreasing temperature. However, for this parameter, the values differ substantially between Dopants III and V, which both comprises the 2-hexyloxypropionic acid unit, and the Dopants IV, VI and VII which comprises an α-fluoro acid group.

Thus, in this system, an important parameter, viz. the S_C*/S_Λ transition temperature is affected only to a very minor amount when the dopant is closely matched in length to the smectic host, as in Mixtures III to VI. Furthermore Mixtures III and IV comprising monoester dopants, which are better matched to the host material for polarisability, also give shorter optical switching times.

(B) Network Stabilised Materials.

Mixtures IV and VII were investigated, each in combination with Monomer M.l or M.2. Mixture III exhibits a spontaneous polarisation (P of 20 nC cm^"2, a tilt angle (θ) of 27° and an optical response time (τ) of 95 microseconds (8 V μm^"1) at 30 °C. At the same conditions Mixture VII shows a P_s of 13.5 nC cm^"2, a tilt angle of 27.5° and a response time of 85 microseconds.

Synthesis of Monomer M.l is given in the section labelled "Preparations". Synthesis of Monomer M.2 is by known procedures from 2',3'-difluoro-4,4"-dihydroxy-p- teφhenyl in 44 % overall yield (see Figure 8). It is considered that Monomer M.2 is better matched to the host material than Monomer M.l insofar as in Monomer M.2 the 4 and 4" positions are oxygen substituted, which inter alia is expected to affect matching of polarisability adversely.

Compositions of Mixtures IV and VII comprised 10 mole percent of the monomer. The phase properties of the host S_c-host, the Mixtures IV and VII (III/VII mix), the doped mixtures containing the monomer (IV/VII mix + 10%M.l/2) and the doped mixtures containing the polymer (IV/VII network 10% P.1/2) are summarised in Table 2 below. The monomer was polymerised in situ in an electro-optic cell as described in the Experimental Section below.

As is demonstrated by the thermal data in Table 2, the smectic C* to smectic A transition temperatures for the ferroelectric network systems based on the Monomer M.l are lower than those of the networks based on the Monomer M.2 and also lower than these of the original mixtures (IV/VII mix). As a consequence the tilt angle (0) is higher and the switching times (τ) are lower than those observed for less well matched systems (see Figures 4 and 5) and can be even lower than for the ferroelectric mixture without the polymer network (Figure 6). Thus, it is advantageous if all or most of the components of the monomer composition giving rise to the network are strongly similar, particularly in terms of length and polarisability.

14

SUBSTmfTi HEεT (RUlI 26)

Table 2

Further aspects are that it has been surprisingly found that a low viscosity of the monomer leads to fast switching times of the ferroelectric network, and that matching the achiral components of the smectic C* mixture, the chiral dopant and the monomer results in the avoidance of phase separation in the polymerised network.

Experimental

Transition temperatures were determined by optical microscopy on heating (polarising microscope Zeiss, calibrated hot stage Mettler FP 5 with FP 52). All transitions were checked by differential scanning calorimetry (Perkin Elmer DSC 7). Electro-optic measurements: samples were investigated in 11 μm ITO test cells (EHC, antiparallel rubbed PI). Tilt angles are optical tilt angles, typical parameters are 0,1 Hz, rectangular waveform AC field (waveform generator Hewlett Packard 33120A and custom made amplifier), all values extrapolated to zero field. Response times arc optical response times taken as 10-90%) change in optical transmission using a photodiode, signal amplifier and oscilloscope (Hewlett Packard HP 54600B); typical parameters are 80 Hz, rectangular waveform AC field. Spontaneous polarisation values were measured using the current pulse technique and the above oscilloscope with computer-aided signal integration based on the IIP program 34810A bench link.

Spectroscopic data

IR: Pcrkin Elmer 983G or Perkin Elmer 487G; 'II-NMR: JEOL JMN GX270 FT spectrometer (270 MHz), solvent CDC1₃; MS: Finπigan MAT 1020 GC/MS spectrometer, solvent acetone or ether; the purity of all target compounds was checked using TLC (silica gel F254 backed onto aluminium sheets), HPLC (Anachcm Microsorb 5 μm C 18, 25 cm/4.6 mm ID; Spectraflow 757 UV detector (254 nm); Perkin Elmer data station) and microscopy.

Preparations

Preparation of the network containing ferroelectric LCs

Test cells were filled with the polymerisable mixtures (III/VII mix + 10%M.l/2) containing the ferroelectric system, monomer and a photoinitiator (Irgacurc 184, 5 mol% with respect to the monomer). The filling was performed in the chiral ncmatic (cholesteric) phase of the mixture in the dark. The samples were aligned using a microscope (Olympus B1I2, red light) and a hot stage (Linkam PR 600 with THM 600) for temperature control. The mixtures were then exposed to a UV light source (Philips HB 171/A, UVΛ emission: 65 W/m², UVI3: 0.012 W/m⁷, distance 2 cm) for one hour at a specific temperature corresponding either to the SmC* phase, the SmA- phase or the high pitch chiral ncmatic phase of the mixture. Syntheses

Dopants

The synthesis of the dopants III and IV is illustrated in Figure 7. It consists of five linear steps providing not only for the above dopants but also intermediates for the synthesis of the dopants V and VI and for the monomer M.l.

The 2,3-difluorophenylboronic acid 1.7 was coupled with 4- tetrahydropyranyloxybromobenzene 2.7 according to a procedure published by M Hird et al, Liq. Cryst. 1993, 15, 123, employing tetrakis- (triphenylphosphine)palladium(O) as catalyst. The H-acidity at the 4-position of the resulting 2,3-difluorobiphenyl 3.7 was used to create the corresponding lithium salt at -70°C, which was then transformed into the boronic acid 4.7 in a one-pot reaction (J. Chem Soc Perkin Trans II, 1989, 2041). The aryl-aryl coupling of 4.7 with 1-bromo- 4-hexylbenzene 5.7 gave the difluoroteφhenyl 6.7. Acid catalysed cleavage of the THP-ether protecting group yielded the phenol 7.7, which was finally esterified with (S)-2-hexyloxypropionic acid 8.7 or S-α-fluorooctanoic acid 9.7 using the Steiglich method (Angewandte Chemie Int Ed Engl, 17, 1978, 522, to give the dopants HI and IV in overall yields of 38% and 30% respectively.

The synthesis of dopants V and VI is shown in Figure 8. The biphenyl boronic acid 4.7 is coupled with l-bromo-4-tetrahydropyranyloxybenzene 2.8 resulting in the symmetric teφhenyl 10.8. Cleavage of both the THP-ether protecting groups gave the dihydroxy terphenyl 11.8, which was esterified with the acid 8.7 or 9.7 yielding the dopants V and VI in overall yields of 38% and 58% respectively.

Figure 9 illustrates the preparation of dopant VII. 2,3-difluoro-4'-pentylbiphenyl-4- ylboronic acid 12.9 was transformed into the phenol 13.9 using a diluted hydrogen peroxide solution. The product was then esterified with the acid 9.8 to give dopant VII in 15% overall yield. Monomers

The synthesis of the monomer M.2 is illustrated in Figure 10. 8-Benzyloxyoctanal 18 was coupled with 4-bromobenzylphosphonium bromide 19 in a Wittig-reaction using potassium carbonate as base yielding a one to one mixture of the cis- and trans- configured 4-(l-alkenyl)-bromobenzene 20. Coupling of compounds 20 and 1.7 in a boronic acid coupling procedure gave the 2,3-difluorobiphenyl 21, which was transformed into the boronic acid 22 in a manner analogous to the formation of 4.7. The terphenyl derivative 23 was made by an aryl-aryl-coupling of 22 and a second equivalent of the above mentioned 20. Compound 23 consists of a mixture of the cis/cis, cisltrans and trans/trans isomers with an overall cis to trans ratio of ca. one to one by NMR analysis. Reaction of 23 with four equivalents of hydrogen employing palladium-on-charcoal as catalyst yielded the α,w-diol 24, which was finally esterified with acryloyl chloride 17 to give the monomer M.l in 12 % overall yield.

The synthesis of the monomer M.2 is illustrated in Figure 11. The difluorodihydroxyteφhenyl 11 was etherified with l-bromo-8-tetrahydropyranyl- octane 14 employing potassium carbonate as base. Both THP-ether protecting groups of the resulting liquid crystalline compound 15 were then cleaved to give the hardly soluble ,w-diol 16. Esterification of 16 with acryloyl chloride 17 gave the monomer M.2 in 44% yield overall.

Although matching of length, polarisability and ring number have been described as being within a 30% limit, a more preferred limit in each case, taken individually (i.e. whether or not the same limit applies to the other two of these parameters) is 20% and even more preferably 10%. For example, length matching between the smectic host and chiral dopant could be better than 20%, with length matching between the smectic host and intermediate portion better than 30%. Ring number matching could be better than 10 or 20% between the smectic host and both components (a) and (b), for example in this particular case, and possibly it could be perfectly matched, for example where all essential parts of the liquid crystal material are teφhenyl derivatives.

Claims

1. A liquid crystal material comprising a smectic host component and at least one further component selected from the group consisting of

(a) a chiral dopant; and

(b) a monomer comprising two polymerisable portions either end of an intermediate portion, or a polymer formed from said monomer; wherein the smectic host component is matched in length to the said chiral dopant or to said intermediate component.

2. A liquid crystal material according to claim 1 wherein the smectic host component is also matched in polarisability to a said at least one further component.

3. A liquid crystal material according to claim 1 or claim 2 and comprising both components (a) and (b).

4. A liquid crystal material according to claim 3 wherein the smectic host component is matched in length to both the said components (a) and (b).

5. A liquid crystal material according to claim 3 or claim 4 wherein the smectic host component is matched in polarisability to both the said components (a) and (b).

6. A liquid crystal material according to any preceding claim and comprising the said chiral dopant component (a) in an amount of no more than 30 mole per cent relative to the amount of dopant component (a) and smectic host component taken together.

7. A liquid crystal material according to any preceding claim wherein the smectic host comprises a first number of rings linked in a chain.

8. A liquid crystal material according to claim 7 wherein the said chiral dopant comprises a second number of rings in a chain or said intermediate component comprises a third number of rings linked in a chain.

9. A liquid crystal material according to claim 8 wherein said first number is ^" within 30 percent of said second or third number.

10. A liquid crystal material according to claim 9 and comprising both components (a) and (b) wherein said first number is within 30 percent of both said second and third numbers.

11. A liquid crystal material according to any one of claims 7 to 10 wherein all or none of the rings of the first number are aromatic.

12. A liquid crystal material according to claim 8 wherein all or none of the rings of the said second or said third number are aromatic.

13. A liquid crystal material according to claim 11 and claim 12 wherein all or none of the rings of the first number taken together with the rings of second and/or third number are aromatic.

14. A liquid crystal material according to any preceding claim wherein said intermediate portion includes a terphenyl moiety.

15. A liquid crystal material according to claim 14 wherein the said teφhenyl moiety is

20

SUBSTmm SHEET (RULE 26)

16. A liquid crystal material according to claim 15 and comprising a said monomer having the formula

where A is -O- or, more preferably, -CH₂-, and n is from 5 to 8.

17. A liquid crystal material according to any preceding claim wherein the smectic host comprises at least one 4, 4"-substituted terphenyl compound.

18. A liquid crystal material according to claim 17 wherein the said at least one 4, 4"-substituted terphenyl compound has the formula (3)

and/or formula (4)

where Rl and R2 are the same or different and are each an alkyl or alkoxy group comprising from 3 to 10 carbon atoms, branched or straight chain.

19. A liquid crystal material according to claim 18 wherein Rl and R2 are both n- alkyl, the total number of carbon atoms therein being 12.

20. A liquid crystal material according to any preceding claim wherein the chiral dopant component (a) is a terphenyl compound.

21. A liquid crystal material according to claim 20 wherein the dopant teφhenyl compound has a formula III, IV, V, VI or VII:

vπ

22. A liquid crystal display device comprising a liquid crystal material according to any preceding claim.

23. A method of making a liquid crystal cell wherein a material according to any one of claims 1 to 21, and comprising a said monomer component, is placed into a cell and subjected to ultra-violet light to polymerise the monomer.

24. A method of making a liquid crystal material according to any one of claims 1 to 21, and comprising a said monomer component, is subjected to ultra-violet light to polymerise the monomer.