GB2037803A

GB2037803A - Liquid crystal mixture

Info

Publication number: GB2037803A
Application number: GB7943676A
Authority: GB
Original assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1978-12-21
Filing date: 1979-12-19
Publication date: 1980-07-16
Also published as: DE2903095A1; CH638828A5; GB2037803B; JPS55106280A; FR2444702B1; FR2444702A1; DD148060A5

Abstract

In a liquid crystal mixture which is intended for use in displays that have no polarizers and which is composed of a dye as a guest phase embedded in a host phase mixture consisting of a nematic liquid crystal with an optically active dopant, at least part of the dye is an anthraquinone dye having the general formula: <IMAGE> in which R<2>, R<3>, and R<4> are the same or are different and each is a hydrogen atom or a hydroxyl, amino or short- chain N-monoalkylamino group; Y and Z are the same or are different and each is a halogen atom, an amino or hydroxyl group or an alkyl, alkoxy or alkylamino groups containing from 1 to 16 carbon atoms; and m and n are each 0, 1, or 2. These dyes are commercially available and their dichroic ratios are nearly as good as the best anthraquinone dyes proposed for the cholesteric guest-host effect, which must be custom prepared. Furthermore, the dyes used have an adequate solubility in the host phase and are especially stable to the UV component found in natural daylight.

Description

SPECIFICATION Liquid crystal mixture The invention concerns liquid crystal (LC) mixtures that are intended for application in electrooptic display cells that have no polarizers.

In display cells with a liquid crystalline dielectric phase, it is known that an electrooptical effect is produced by embedding dichroic or pleochroic dydes as the so-called "guest phase" in a liquid crystalline matrix as the so-called "host phase" (see G.H. Heilmeier, et al, Molec. Cryst. Liq. Tryst. 8, 293-304 (1969)). The dye molecules of the guest phase are dissolved in the host phase and oriented by the application of an electric field. Because of their pleochroic properties, these dyes show a different amount of light absorption depending upon their orientation.In contrast to ordinary monochroic dyes, the amount of light absorbed by pleochroic dyes depends upon the orientation of their molecules with respect to the electric field vector of the incident light By application of an electric field to the LC mixture, distributed as a thin layer in a cell, the nematic liquid crystal of the host phase together with the embedded pleochroic dyes of the guest phase are reoriented, leading to a change in the amount of light absorbed. The practical application of this effect, known by the experts in the field as the "guest-host" effect, is described, for example, in German Offenlegungsschrift No. 1 928,003.

Indophenol blue, indigo derivatives, and azo dyes, among others, are described in this patent as examples of pleochroic or dichroic dyes. The display brightness and contrast ratios that can be obtained with these dyes, however, are relatively weak in comparison with those that can be obtained with the well-known twisted nematic liquid crystal display devices, in addition to requiring a polarizer.

According to a recent development described, for example, in German Offenlegungsschrift No.

2,410, 557, the polarizers can be eliminated if a relatively small amount of optically active material (e.g.

115% by weight) is added to the dielectric phase of the display which consists of nematic liquid crystals with positive dielectric ansiotropy (host phase) and embedded pleochroic dyes (guest phase).

This optically active material, also known as an optically active dopant, gives the bulk liquid crystal phase a twisted internal structure which is known as a "chrial" or "cholestoric" structure and is described, for example, in the publication by D.L. White and G.N. Taylor entitled "New Absorptive Mode Reflective Liquid Crystal Display Device" in J. Appl. Phys. 45, 4718-4723 (1974). An example of the construction and operation of a display cell employing this principle is described in German Offenlegungsschrift Nos. 2,639,675 and 2,658,568.

Liquid crystal display cells having a host phase with a cholesteric structure and an embedded dye as the guest phase have come to be known among experts as "cholesteric guest-host" (CGH) liquid crystal display cells. Such display cells have the advantages of being suitable for commercial manufacture, requiring no polarizers and making possible an adequate display contrast ratio with a simpler cell technology.

Experience has shown that the choice of suitable dyes for "cholesteric guest-host" cells is very difficult. First of all, the dichroic ratio of such a host phase embedded with dye as the guest phase must be large enough to give the display cell a sufficient brightness and a sufficient contrast ratio. The pleochroic or dichroic ratio of the system is defined as the ratio at/az of the absorbance (optical density) a, of the dye measured when the molecular orientation or optic axis of the nematic host phase with the embedded dye as the guest phase is parallel to the E-vector of the incident light to the absorbance a2 measured when the molecular orientation is perpendicular to the E-vector of the incident light.The values a7 and a2 can be measured by well-known photometric techniques described, for example, in the publication by White and Taylor mentioned earlier. Generally the a,/a2 value should be at least 5 in order to guarantee that the display will have a sufficient brightness and contrast ratio.

In addition to the dichroic ratio a,/a2, other parameters of the dye are also important for their application in chloesteric guest-host displays. The dye must have an absorption maximum in the visible region of the electromagnetic spectrum, i.e. between 400 and 700 nm, and the dye must be stable in the host phase up to 1000C (a) to the application of a.c. voltages up to 20 v, (b) to light, especially the UV light present in natural daylight, and (c) to the other components of the doped host mixture.

The dyes previously proposed for the cholesteric guest-host effect does not adequately fulfill all these requirements. A particular example is the azo dyes which are frequently proposed for cholesteric guest-host liquid crystal cells. They have suitable a1/a2 values and absorption maximae, but insufficient stability to light, especially ultra-violet light (too low a photochemical stability). Many of the azo dyes have also been found to be chemically labile. On the other hand, the generally higher photochemical stability of the anthraquinone dyes with the basic structure (30)

in comparison with the azo dyes is well-known, and in fact anthraquinone dyes have actually been proposed in the scientific literature for application in cholesteric guest-host liquid crystal display cells.

However, the majority of the anthraquinone dyes suggested up to now have too low an a,/a2 value, i.e.

too small a dichroic ratio, to permit a low operating voltage of 3 to 4.5 volts to be used on the display.

Various substituted anthraquinone dyes having the structures (31) and (32) were investigated by J.

Constant, et a in a paper entitled "Photostable Anthraquinone Pleochroic Dyes" presented at the 7th International Liquid Crystal Conference in 1978:

in which symbols R, R', and R" represent alkyl and alkoxy groups with up to 9 carbon atoms in the alkyl part, or a dimethylamino group.

The dichroic ratios of these dyes in a commercially available nomadic phase (product Ro-TN-403 from Hoffmann-La Roche, a mixture of biphenyl and phenyl-pyrimidine compounds) fall in the range from 5.6 to 7.0, which seems to confirm the suitability of phenylamino groups in the modifications of the anthraquinone dyes that are of interest here. However the phenylamino group also lowers the solubility of the dye in the nematic host phase, but this can be compensated for by introducing longer alkyl or alkoxy chains as R, R', and R". Because such chains are flexible, they have practically no effect upon the value of the dichroic ratio.A further disadvantage of the dyes of formulae (31) and (32) is that they are not the usual commercially available anthraquinone dyes and must be custom prepared just for utilization in liquid crystal mixtures.

The object of this invention is to devise anthraquinone dyes for cholesteric guest-host liquid crystal displays that, like the proposed dyes (31) and (32), also have dichroic ratios in the range 5-7, but have simpler structures and are commercially available. It was found that this goal could be achieved by the utilization of anthraquinone dyes having the general formula (1) given below.

The present invention provides a liquid crystal mixture intended for cholesteric guest-host liquid crystal displays of which essentially 80100% consists of (a) a nematic liquid crystal, preferably with positive dielectric ansiotropy, as the host phase, (b) an optically active additive to give the host phase a cholesteric structure, e.g. having a concentration of 0.5 to 1 5% by weight of the total mixture, and (c) at least one dissolved pleochroic anthraquinone dye as the guest phase which can be either a single component or a mixture with other dyes, preferably in concentrations from 0.1 to 5% by weight of the total mixture.

A distinguishing feature of the LC mixture of this invention is that at least some of the dissolved anthraquinone dyes have the general formula (1)

In formula (1) R2, R3, and R4 represent substituents which are the same or are different and are hydrogen atoms or hydroxyl, amino, or short-chain monoalkylamino groups.

The term short-chain used here means alkyl groups with 1 or 2 carbon atoms. In preferred embodiments of this dye, two of the substituents R2, R3, and R4 are amino and/or hydroxyl groups. Y and Z are optional substituents in positions 6 and/or 7 or in position 2 and/or 3, respectively. These substituents are halogen atoms such as chlorine, bromine and iodine, with bromine being preferred. The indices m and/or n are therefore zero, 1 or 2. Dyes having the general formula (1) are well-known anthraquinone dyes and are commercially available as such.

The mixtures of this invention can be produced in the usual way by mixing the components (a), (b) and (c) together and, if necessary, gently warming. Suitable display cells, for example, having a structure described in German Offenlegungsschrift No. 2,639,675 are then filled with the mixture.

Suitable components (a) and (b), as mentioned earlier, are welkknown and many are commercially available. Examples of component (a) are, besides the product"Ro-TN-403" already mentioned, the product "Ro-TN-103", "Merck ZLI 1211", and "Merck ZLI 1291". Examples of component (a) having negative dielectric anisotropy are the products "Merck Licristal 887" and "Chisso EN--18". Additional examples for suitable components (a) as well as (b) can be found in the publication by White and Taylor mentioned above as well as in German Offenlegungsschrift Nos.2,410,557 and 2,658,568.An example of component (b) that is commercially available is the product "CB 1 5" having the chemical formula:

which can be obtained from British Drug Houses Ltd.

In order that the invention may be well understood, the following Examples are given by way of illustration only.

EXAMPLES 1-18 Anthraquinone dyes of Formula (1) as identified in Table I below were employed.

TABLE I

Dye No. R2 R3 R4 Y Z m n n 2 H -NH2 H - - 0 0 3 OH NH2 -OH - - 0 0 4 -CH -OH -NH2 - - 0 0 5 -NH2 -NH2 -NH2 - - 0 0 6 -OH -NH2 -OH Br Br 1 1 The dyes were each mixed, as component (c) with commercially available nematic phases (with positive dielectric ansiotropy) as component (a) and the commercially available product "CB 15" as component (b).The dichroic ratios of the dyes (2)-(6) in the nematic phase "Ro-TN-403" are listed in Table II.

TABLE II

Dye No. Dichroic ratio 2 6.5, 3 5.7 4 5.3 5 5.7 6 6.2 The compositions of the resulting LC mixtures are summarized in the Table III.

Daylight UV tests of the liquid crystal mixtures of the above Examples 1-18 have shown that they are completely stable. The stability to a.c. voltages is also very good.

TABLE ill

(a) (b) (c) Dye Example Nematic phase CB 15 (No. and T No. (Wt.%) (Wt.%) Wt.%) ( C) 1 Ro-TN-403 96.5 3 (2) 0.5 78.0 2 .. 96.5 4 ,, 0.5 77.4 3 ,, 94.5 5 ,,0.5 76.4 4 " 96.5 3 (3) 0.5 78.1! 5 " 96.5 4 ,, 0.5 76.9 6 ,, 94.5 5 ,, 0.5 75.9 7 ,, 96 3 (4) 0.5 77.8 8 ,, 95 4 ,, 0.5 76.6 9 ,, 94 5 ,, 0.5 75.6 10 ,, 96.5 3 (5) 0.5 79.1 11 ,, 95.5 4 ,, 0.5 77.9 12 ,, 94.5 5 ,, 0.5 76.9 13 ,, 96.5 3 (6) 0.5 78.4 14 ,, 95.5 4 ,, 0.5 77.2 15 " 94.5 5 ,,' 0.5 76.2 16 ,, 96 3 (6)1 78.5 17 .. 95 4 " 1 77.3 18 ,, 94 5 ,, 1 76.3

Claims

1. A liquid crystal mixture Intended for electrooptical displays having no polarizers, consisting of: (a) a nematic liquid crystal as the host phase, (b) an optically active additive to give the host phase a cholesteric structure, and (c) at least one pleochroic anthraquinone dye as the guest phase embedded in the host phase, in which said anthraquinone dye has the general formula:

in which R2, R3 and R4 are the same or different and each is a hydrogen atom or a hydroxyl, amino, or short-chain N-mono-alkylamino group; Y and Z are the same or are different and each is a halogen atom or an amino group, a hydroxyl group or an alkyl, alkoxy or alkylamino group containing from 1 to 16 carbon atoms; and m and n are each 0, 1, or 2.

2. A liquid crystal mixture as claimed in claim 1, in which R2, R3 and R4 are each a hydrogen atom or a hydroxyl or amino group.

3. A liquid crystal mixture as claimed in claim 1 or claim 2 in which m and n are both zero.

4. A liquid crystal mixture as claimed in claim 1 or claim 2, in which X and/orY is a bromine atom.

5. A liquid crystal mixture as claimed in claim 1 in which R2 and R4 are both hydrogen atoms, R3 is an amino group and m and n are both zero.

6. A liquid crystal mixture as claimed in claim 1 in which R2 is an amino group and m and n are both zero.

7. A liquid crystal mixture as claimed in claim 1 in which R2 and R3 are both hydroxyl groups, R4 is an amino group and m and n are both zero.

8. A liquid crystal mixture as claimed in claim 1 in which R2, R3 and R4 are each amino groups and m and n are both zero.

9. A liquid crystal mixture as claimed in claim 1 in which R2 and R4 are both hydroxyl groups, R3 is an amino group, Y and Z are both bromine atoms and m and n are both 1.

10. A liquid crystal mixture as claimed in any one of the preceding claims in which the mixture is composed of: from 80 to 98.5 wt.% of host phase, from 1 to 1 5 wt.% of optically active additive, and from 0.5 to 5 wt.% of anthraquinone dye, and the solubility of the anthraquinone dye in the host phase at normal temperatures is at least 0.5 wt.%.

11. A liquid crystal mixture as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.