GB2250748A

GB2250748A - Liquid crystal mixtures

Info

Publication number: GB2250748A
Application number: GB9027103A
Authority: GB
Inventors: David Coates; Mark Goulding; Shirley Maden; Damien Gerald Mcdonnell
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1990-12-13
Filing date: 1990-12-13
Publication date: 1992-06-17
Also published as: GB9027103D0

Abstract

A liquid crystal phase is disclosed comprising a nematic or cholesteric component and a component A consisting of one or more compounds of the formula I wherein <IMAGE> R' is alkyl of 1 to 12 C atoms or alkyl of 2 to 12 C atoms wherein one CH2 group is replaced by an oxygen atom, <IMAGE> or a covalent bond L<1> is H or F. having a lowered Vth as compared with the Vth of the nematic or cholesteric component.

Description

LIOUID CRYSTAL MIXES This invention relates to novel liquid crystalline mixtures.

Liquid crystal materials and devices exploit the electro-optical properties of nematic or cholesteric (N or N*), or smectic (S) in particular chiral smectic C (Sc*) or smectic A (SA) phases. The most common type of liquid crystal materials in use are those which show a nematic phase, and these are extensively used in for example watches, clocks, calculators, electronic displays etc.

Liquid crystal compounds and liquid crystal mixtures exhibit various liquid crystal phases at various temperatures and some are capable of being switched by applied voltages when incorporated into liquid crystal devices.

The present invention provides novel compounds and nematic or cholesteric liquid crystalline mixtures containing them which have a low threshold voltage (Vth).

The present invention comprises a liquid crystal phase comprising-a nematic or cholesteric component and a component A consisting of one or more compounds of formula I :

Formula I wherein R1 is alkyl of 1 to 12 C atoms or alkyl of 2 to 12 C atoms wherein one CH2 group is replaced by an oxygen atom,

or a covalent bond, and L is H or F, having a lowered Vth as compared with the Vth of the nematic or cholesteric component.

Preferred embodiments of the invention inter alia are chosen with respect to ease of preparation and their liquid crystal properties, particularly with respect to suitability for use in twisted nematic (TN) displays, for example, in digital watches, and supertwisted nematic (STN) displays, for example, in computer screens and dyed phase change displays.

Preferably, R1 contains between three and six carbon atoms.

Preferably, also, Q is a covalent bond.

Preferred overall structures for compounds of formula I are listed below TABLE I

Compounds of formula I may have nematic liquid crystalline properties. They are particularly valuable because they can lower the threshold voltage of nematic mixtures making them suitable for applications in which this characteristic is required, e.g. by lowering the threshold voltage of a liquid crystalline mixture so that it can be used in a digital watch, the digital watch will then be able to function at a much lower voltage, compatible with recent developments reducing (to 1.5 V) the voltage requirements of the timekeeping circuitry.

Compounds of Formula I are stable and colourless, and have a high dielectric anistropy compared to prior art compounds many of which are listed as component B.

Compounds of Formula I may be prepared by known methods described in WO 89/00178. A preferred route (Route I) is shown in Figure 1.

The liquid crystal phase of the present invention comprises a mixture of three main components; A, B and C.

Component A consists of one or more compounds of Formula I. The compounds of component A are present in the liquid crystal phase in an amount from 2 to 50 wt.%.

The liquid crystal phase also comprises a component B consisting of at least one compound selected from compounds of formulae II to XIII Formulae II to XIII

wherein ring A is 1,4-phenylene or trans-l,4-cyclohexy- lene, and R1 and Q0 have the meanings indicated above.

The compounds of component B are present in the liquid crystal phase in an amount from 10 to 40 wt.%.

Component C consists of at least one compound of formulae XIV to XXII.

Formulae XIV to XXII:

wherein R1 and ring A each independently have the meanings indicated above, R2 is n-alkyl or n-alkoxy 2 with in each case up to 7 carbon atoms and Q1 and Q2 each are independently

The compounds of component C are present in the liquid crystal phase in an amount from 3 to 30 wt.%.

Liquid crystal materials of the invention may be used in display devices of known construction and method of operation. Typically, an electrooptical display device will consist of two substrates between which a layer of the liquid crystal material may be sandwiched.

At least one of the substrates is optically transparent, and both have electrodes which are preferably made of a transparent material on their opposing faces. By applying an electric field across the layer of liquid crystal material via the electrodes an electrooptic effect is achieved which may be viewed directly or preferably through one or more polarising filters.

Example 1 is of a mixture according to the invention which has a low Vth suitable for use in a multiplexed twisted nematic displays operating from a low voltage power supply, e.g. a 1.5 V battery Example 1

Compound $ Component Formula R- COO- CN 9.45 B XI F R e COO 4 CN 8.49 B XI R- COO- CN 4.75 B XI C3H7 ::mCN CN 6.60 B III

C2H5MCN 12.75 B II C3H7 CN 10.39 B VIII CH3COOCsH 6.65 C XIV C3H COO- C5H ,, 8.03 C XIV C5HIl 9 COO CgH C5Hll 4.28 C XIV C5Htl C001gH,, 4.72 D XXI C5H7 COO- CN 18.89 B XII C*HqO e COO e CN CN 5.0 A I In a second example (Example II) the A component is 5 Wt%

Table 2 shows the threshold voltage (Vth) and nematic-isotropic transition temperature (N-I) of Examples I and II and, for comparison, of a mixture identical but without the component A.

TABLE 2 ComDarison ExamPle I Examnle II Vth (V) 0.87 0.73 0.69 N-I (OC) 60.6 58 55 It is clearly seen that the compounds of component A lower the threshold voltage to 0.69 volts, which is a significant reduction from the Vth of a mixture without a component A; a Vth of as little as 0.69 volts renders the mixture eminently suitable for low voltage operation.

Another advantage of the compounds is illustrated by the relatively small depression of N-I caused by these additives.

An example of the use of a mixture of compounds of components A, B and C in a liquid crystal device embodying the present invention will now be described with reference to Figure 2.

Figure 2 illustrates a liquid crystal cell comprising a layer 1 of liquid crystal material exhibiting a twisted nematic phase sandwiched between a glass slide 2 having a transparent conducting layer 3 on its surface, e.g. in tin oxide or indium oxide, and a glass slide 4 having a transparent conducting layer 5 on its surface. The slides 2,4 bearing the layers 3,5 are respectively coated by films 6,7 of a polyimide polymer. Prior to construction of the cell the films 6 and 7 are rubbed with a soft tissue in a given direction, the rubbing directions being arranged perpendicular upon construction of the cell. A spacer 8 e.g. of polymethylmethacrylate, separates the slides 2,4 to the required distance, e.g. 5 microns. The liquid crystal material 1 is introduced between the slides 2,4 to the required distance e.g. 5 microns. The liquid crystal material 1 is introduced between the slides 2,4 by filling the space between the slides 2,4 and spacer 8 and sealing the spacer 8 in d vacuum in a known way.

Preferably, the liquid crystal material is in the nematic or isotropic liquid phase (obtained by heating the material) when it is introduced between the slides 2,4 to facilitate alignment of the liquid crystal molecules with the rubbing directions on the slides 2,4.

A polarizer 9 is arranged with its polarization axis parallel to the rubbing direction on the films 6,7 and an analyzer (crossed polarizer) 10 is arranged with its polarization axis perpendicular to that rubbing direction.

In an alternative device (not shown) based on the cell construction shown in Figure 2 the layers 3 and 5 may be selectively shaped in a known way, e.g. by photoetching or deposition through a mask, e.g. to provide one or more display symbols, e.g. letters, numerals, words or graphics and the like as conventionally seen on displays. The electrode portions formed thereby may be addressed in a variety of ways which include multiplexed operation.

Claims

1. A liquid crystal phase comprising a nematic or cholesteric component and a component A consisting of one or more compounds of Formula I

or a covalent bond L1 is H or F, having a lowered Vth as compared with the Vth of the nematic or cholesteric component.

2. A liquid crystal phase according to claim 1, in which the compound(s) of component A is(are) present in an amount from 2 to 50 wt.%.

3. A liquid crystal phase according to claim 1 or claim 2, comprising a component B consisting of at least one compound selected from compounds for formulae II to VIII

whereby ring A is 1,4-phenylene or trans-l,4-cyclohexy- lene, R1 and Q1 have the meanings indicated above.

4. A liquid crystal phase according to claim 3, in which the compound(s) of component B is(are) present in an amount from 10 to 40 wt.%.

5. A liquid crystal phase according to any preceding claim, comprising a component C consisting of at least one compound selected from compounds of formulae XIV to XXII,

wherein R1 and ring A each independently have the meanings indicated above and R2 is n-alkyl or n-alkoxy with in each case up to 7 carbon atoms, and Q1 and 2 each are independently

6. A liquid crystal phase according to claim 5, in which the compound(s) of component C is(are) present in an amount from 3 to 30 wt.%.

7. A liquid crystal electro-optic display device characterised in that it incorporates a material as claimed in claims 1 to 6.