GB2155945A

GB2155945A - Substituted thioanthraquinone dyes

Info

Publication number: GB2155945A
Application number: GB08416656A
Authority: GB
Inventors: Dr David John Thompson
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1981-02-25
Filing date: 1984-06-29
Publication date: 1985-10-02
Also published as: GB8416656D0; GB2155945B

Abstract

Substituted anthroquinones having the formula: <IMAGE> Wherein Y<1>, Y<2> and Y<3> are each independently optionally substituted alkyl, aryl or cycloalkyl, provided that not more than two of Y1 to Y3 are identical, are useful as pleochroic dyes in liquid crystal mixtures.

Description

SPECIFICATION Substituted thioanthraquinone dyes The present invention is concerned with organic materials, in particular with pleochroic dyes for use with liquid crystal materials e.g. for electro-optic display applications.

Liquid crystal materials are well known organic materials which display phases, known as liquid crystal phases or mesophases, having a degree of molecular ordering intermediate between that of the fully ordered crystalline solid state and the fully disordered isotropic liquid state.

Electro-optical devices incorporating liquid crystal materials are well known and widely used as digital displays in such applications as watches, calculators and digital voltmeters. These devices utilisethe optical contrast when an electric field is applied across a thin insulating film of suitable liquid crystal material. The molecules of the material (in a liquid crystal phase at the temperature of operation) are re-orientated by the field causing a change in an optical property of the part of the film where the field is applied, e.g. a change in ambient light scattering or transmissivity.

Liquid crystal materials have the property that their molecules can impose their ordering upon the molecules of other suitable dopant materials incorporated within them. This property is the basis of so-called "guest-host" devices e.g. display devices in which the host liquid crystal material and its guest material have one molecular configuration in the absence of an applied electric field and another molecular configuration when an electric field is applied across the material. The guest material is usually a pleochroic dye, which is a dye whose molecular absorption properties vary with the orientation of the electric vector of light incident upon its molecules.

The presence of such a dye can be used to enhance the contrast between the off state (with no electric field applied) and the on state (with electric field applied) of a liquid crystal display because the orientation of the dye molecules is in effect switchable by the effect of the applied electric field on the liquid crystal molecules and by the consequent re-orientation of the dye molecules by the guest-host effect.

As discussed further below there are several kinds of liquid crystal effects which can make use of the guest-host effect in electro-optical displays. These vary according to the kind of liquid crystal material used and the configuration of its molecules in the off state (e.g. as determined by the surface treatments of the substrates employed to contain the film of liquid crystal material).

In order to provide maximum contrast between the on and off states of a guest-host liquid crystal display it is important that the guest molecules adopt as closely as possible the time averaged orientation of the host molecules. However this is achieved only to a limited degree because of random thermal fluctuations. The degree to which the orientation varies from the ideal is measured by a quantity known as the order parameter S which is given by the following equation: S = 1/2 (3 cos20 - 1 ) Equation (1) where cos20 is a time averaged term and H is the instantaneous angular orientation of the molecules with respect to the time averaged orientation of the host molecules.The determination of the value of the order parameter S is well understood in the art; see for example the paper "A new absorptive mode reflective liquid crystal display device" by D.L. White and G.N. Taylor in the Journal of Applied Physics, 1974, 45 pages 4718 to 4723.

For perfect orientation the order parameter S is unity (that is H is zero). Thus, pleochroic dyes for use in guest-host devices should have an order parameter in the liquid crystal host as high as possible (i.e. less than one but as near to one as possible). However they must also have adequate chemical, photochemical and electrochemical stability, e.g. stability when exposed to atmospheric contaminants, electric fields (as in device operation) and to ultra-voilet radiation. They should not be ionic or have any ionisable character (otherwise the liquid crystal material will lose its insulating nature and conduct making the device useless).

They must also have sufficient solubility in the host materials; although the concentration of guest pleochroic dye required for the desired effect is generally quite small (e.g. not more than a few per cent of dye) nevertheless many pieochroic dyes are unsuitable because they are essentially insoluble in liquid crystal materials.

In UK Patent Application GB 2043097A, liquid crystal compositions have been proposed which can contain symmetrical dyes of the

formula: 54^3~R o I Fc=ula A T?Ms Example 1 A mixture of 1.48g thiophenol, 2.24g 4-t-butylphenyl-thiol and 1.5g KOH were refluxed in 15 ml ethanol for I hour and cooled to ambient temperature. To the cooled mixture were added 1 .5g of 1,4,5-tricholoro-AQ and the mixture refluxed for 16 hours. After cooling to ambient temperature the mixture was worked up by the following procedure.After filtration and washing with a 50:50 mixture of 2N NaOH and ethanol the crude product was slurried in 30 ml of the same solvent mixture, stirred for 30 minutes, filtered, washed successively with the NaOH!ethanol solvent and water and dried at 80 C. The 2.3g of worked up material contained 62% unsymmetrical tri(substituted thio)AQ, i.e. di(phenylthio)-(4-t-butyl-phenylthio)AQ and phenylthio-di(4-t-butylphenylthio)AQ, the remainder being mainly the two symmetrical tri(substituted thio)AQs.

Example 2 A mixture of 0.127g ofthiophenol and 0.065g KOH in 15 ml ethanol was refluxed for 1 hour and cooled to ambient temperature. To the cooled mixture was added 0.66g of 1,5-di(4-t-butylphenyl-thio)-4-chloro-AQ and the mixture refluxed for 16 hours before cooling to ambient temperature. After working up as described in Example 1 the yield of dry material was 0.6g. This material contained 78% of 1,5-di(4-t-butyl 1,5-di(4-t-butylphenylthio)-4phenylthio-AQ.

The 1 ,5-di(4-t-butylphenylthio)-4-chloro-AQ was prepared as follows: A mixture of 2.98g of 4-t-butylphenylthiol and 0.51g KOH in 10 ml ethanol was refluxed for 1 hour and cooled to ambient temperature. To this was added 1.56g of 1 ,4,5-trichlor-AQ and and the m mixture stirred at 400C for 16 hours before cooling to ambient temperature. After working up the crude material is described in Example 1 it was further purified by dissolving in 100 ml 60-80 petroleum ether, screening and passage through a silica-packed chromatography column, eluting with the same solvent and collecting the middle orange band. After evaporation of the solvent, washing with methanol and drying the yield of purified material was 0.9g.

The compounds prepared according to Examples 1 and 2 have the properties identified in Table 1 below as solutions in the liquid crystal material E43 marketed by BDH Chemicals Ltd at Broom Road, Poole, Dorset, England, containing the compounds:

In Table 1 and elsewhere in this specification the symbols S and AmaX represent respectively order parameter and wavelength(s) (measured in nm) of maximum absorption both measured at 20"C.

TABLE 1 (Order Example No Solubility Amax(nm) Parameter) 1 2.0 520 0.77 2 14.3 520 0.81 Where the term 'solubility' is used in this specification in relation to a dyelliquid crystal solution this refers to the percentage by weight of dye in the solution measured at 20 C.

It should be noted that solubility figures are important in the production of materials for practical guest-host applications for the following reasons: a. the optical properties at 20 C are improved with greater dye content in the liquid crystalzdye solution at 20"C.

b. the optical properties at lower temperatures are improved with greater dye content in the liquid crystal/dye solution at 20"C because greater solubility of a given dye in a given host at 200C normally leads to greater solubility of the dye in that host at lower temperatures.

preparative or high pressure liquid chromatography. The main advantage of the pure unsymmetrical componds over compositions containing mixtures of these with the symmetrical compounds is the greater solubility ofthe former which, as noted above, is of importance in obtaining good contrast in a liquid crystal display.

Solubility is, however, only one of the factors affecting the achievement of good contrast in a liquid crystal display, another factor is the extinction conefficient of the dye in the liquid crystal material. One useful indication of the ability of a dye to give good contrast is the product of the molar extinction coefficient and the solubility (in molesilitre). Solutions of dyes in liquid crystal compositions for use in electronic display applications should have a value for this product which is preferably at least 500 cm ' and more preferably at least 750 cm . 1 As the molar extinction coefficient for a dye does not vary significantly from one liquid crystal material to another, the preferred value of the product can be used to calculate the preferred minimum solubility of a particular dye in any liquid crystal material in order to give good contrast. Thus for a dye having a molar extinction coefficient of 11000 cm2.moles-' the solubility should preferably be at least 4.5x10-2 moles/litre and more preferably at least 6.8x 10-2 moles/litre.

Dyes prepared for use in liquid crystal displays should preferably be as pure as possible in terms of their freedom from inorganic and other ionisable materials which can interfere with the operation of the display or products which are radiation sensitive and decompose within the display during operation. The dyes should also preferably be free from non- or inferior pleochroic materials, such as starting materials, intermediates and by-products, which do not contribute to the perceived contrast of the display. To obtain the dyes in a pure form, ie substantially free from interfering or deleterious matter, it is generally desirable to submit them to repeated recrystallisations from organic solvents, such as chloroform, and/or chromatographic separation procedures.

The compounds of the Formula I have very high stability in liquid crystal materials and high order parameters, generally greater than 0.7.

Where the material according to the first aspect is for use as in an electro-optical display the addition of the dye to the liquid crystal material raises the viscosity of the latter and thus tends to increase the response time of the display. It is therefore desirable to use as little dye as possible (but sufficient to give an adequate electro-optical contrast). In this respect the dyes of Formula I are of particular value because many of them have very high extinction coefficients and thus only small quantities, generally less than 7%, are required in the liquid crystal material.

It has been found that the dyes of Formula I show adequate order parameter and solubility in a variety of liquid crystal materials, including materials of both positive and negative dielectric anisotropy.

Preferably, the dye/liquid crystal solution contains at least 0.5% by weight of the dye and preferably between about 0.75% and 10% by weight of the dye, desirably between 1 and 5 percent. The exact amount of dye is not critical within the preferred range although the concentration is preferably not too low, in order to give a display whose contrast is enhanced as much as possible, and not too high in order to give a display whose electro-optical response is not slow.

Solutions of dye and liquid crystal material may be made in a conventional way simply by mixing the dye and the liquid crystal material together and then heating the mixture at about 80"C with stirring for about 10 minutes and then allowing the mixture to cool.

Pleochroic dyes of Formula I above may be mixed together with other dyes (which may or may not also be of Formula I) to extend their spectral absorption properties, when dissolved in liquid crystal material. For example where a dye of Formula I is yellow and/or orange it may be mixed with a blue dye and a red dye. The relative proportions of the dyes mixed together are determined by the desired spectral response. This is an absorption curve extending across the spectrum to give a grey colouration. The dye mixture is then used with liquid crystal material as above or as follows.

Use of the material defined in the invention is not limited to electro-optical displays. The material may, in fact, be used in any known application of a dyed liquid crystal material. As example of such a 'non electric-optical' application is a thermally addressed display in which a symbol or character is provided in a smectic or cholesteric material by selective heating of the material e.g. by a laser (eg He/Ne) beam, to produce a localised change in the molecular texture of the material. The dye enhances the contrast between the different regions of the display, ie between those which are selectively heated and those which are not heated.

The dyes of Formula I may be prepared by reacting two different substituted thiols with a trichioroanthraquinone, in the presence of an acid binding agent, to give a composition containing mainly the unsymmetrical compounds together with a smaller proportion of the symmetrical compounds.

Alternatively one thiol may be reacted with a nitrodichloroanthraquinone under mild conditions to give a dichloroanthraquinone and the second thiol reacted with the dichloroanthraquinone intermediate to give a single unsymmetrical tri(substituted thio)anthraquinone with only minor quantities of the symmetrical products.

The invention is further illustrated by the following Examples in which all parts and percentages are by weight and the letters "AQ" are used to represent anthraquinone.

Example 1 A mixture of 1.48g thiophenol, 2.24g 4-t-butylphenyl-thiol and 1.5g KOH were refluxed in 15 ml ethanol for I hour and cooled to ambient temperature. To the cooled mixture were added 1.5g of 1 ,4,5-tricholoro-AQ and the mixture refluxed for 16 hours. After cooling to ambient temperature the mixture was worked up by the following procedure.After filtration and washing with a 50:50 mixture of 2N NaOH and ethanol the crude product was slurried in 30 ml of the same solvent mixture, stirred for 30 minutes, filtered, washed successively with the NaOH!ethanol solvent and water and dried at 80"C. The 2.3g of worked up material contained 62% unsymmetrical tri(substituted thio)AQ, i.e. di(phenylthio)-(4-t-butyl-phenylthio)AQ and phenylthio-di(4-t-butylphenylthio)AQ, the remainder being mainly the two symmetrical tri(substituted thio)AQs.

Example 2 A mixture of 0.127g ofthiophenol and 0.065g KOH in 15 ml ethanol was refluxed for 1 hour and cooled to ambient temperature. To the cooled mixture was added 0.66g of 1,5-di(44-butylphenyl-thio)-4-chloro-AQ and the mixture refluxed for 16 hours before cooling to ambient temperature. After working up as described in Example 1 the yield of dry material was 0.6g. This material contained 78% of 1 ,5-di(4-t-butylphenylthio)-4phenylthio-AQ.

The 1,5-di(4-t-butylphenylthio)-4-chloro-AQwas prepared as follows: A mixture of 2.989 of 4-t-butylphenylthiol and 0.51g KOH in 10 ml ethanol was refluxed for 1 hour and cooled to ambient temperature. To this was added 1 .56g of 1,4,5-trichlor-AQ and a nd the m mixture stirred at 40"C for 16 hours before cooling to ambient temperature. After working up the crude material is described in Example lit was further purified by dissolving in 100 ml 60-80 petroleum ether, screening and passage through a silica-packed chromatography column, eluting with the same solvent and collecting the middle orange band. After evaporation of the solvent, washing with methanol and drying the yield of purified material was 0.9g.

TABLE 1 (Order Example No Solubility AmaxrnmJ Parameter) 1 2.0 520 0.77 2 14.3 520 0.81 Where the term 'solubility' is used in this specification in relation to a dye/liquid crystal solution this refers to the percentage by weight of dye in the solution measured at 20"C.

It should be noted that solubility figures are important in the production of materials for practical guest-host applications for the following reasons: a. the optical properties at 20oC are improved with greater dye content in the liquid crystal/dye solution at 20"C.

Other liquid crystal host materials which may be used with the dyes of the present invention include Hosts (1)to (12) listed below (1) Host 1, which is the material E7 supplies by BDH Chemicals Ltd having a composition:

51",, by ct4 51Ce bJ zit wt.

t C 5 C\j' 25who At? \\7 QN n~Cs tlts J 8% (2) Host 2, which is the material ZLI 1132 supplied by E Merck Co, which includes the following compounds:

(3) Host 3, which is the material ZLI 1695 supplied by E Merck Co., Darmstadt. This has a clearing point (nematic-to-isotropic transition temperature) of 72"C. It is a mixture including cyanophenylcyclohexane (PCH) compounds.

(4) Host 4, which is a commercially available material containing phenyl dioxans.

This has a clearing point of 87"C. It is a mixture.

(5) Host 5, which is the material ZLI 1565 supplied by E Merck Co. This has a clearing point of 85"C. It is a mixture including cyanocyclohexylcyclohexane (CCH) compounds.

(6) Host 6, which is the material ZLI 1624 supplied by E Merck Co. This has a clearing point of 87"C. It is a mixture including PCH compounds.

(7) Host 7, which is the material RO TN 403 supplied by F Hoffman La Roche Co., Basle. This is a clearing point of 82"C and is a mixture including cyanophenylpyrimidine (PPM) compounds.

(8) Host 8, which is the material RO TN 430 supplied by F Hoffman Lt Roche Co. This has a clearing point of 69"C and is a mixture including PPM compounds.

(9) Host 9, which is a mixture of the following bicyclo(2,2,2) octane derivatives (see UK Patent Application Number 7926902):

tZ- C,U, -MOCH 30% by weight p-C2ff1 H C 1 40% by weight C < 0S by weig!lt

Host Number 10 Compound Percentage of compound in composition k- CUII-cnl 38 3 H7ffThi4ocN 16 VL-CS ffThI4O 21 I'- CPI1ffThot z-Cy H,l çc,m' 10 c,H-coo--(o^l 4 sC7 Sw Hk This is an example of a host material which itself has a high order parameter.

(11) Host 11, which is a mixture of the following compounds in the stated percentages by weight.

Compound Percentage ofcompound in composition h- " Xfzf 33 3 7 ~ J 14 he 14 { 1 wcS 18 -c,CT < 7 n-CCI, A C{\J 10 C, Hg+ zW 6 k-C7 11 imǒoo#moc & 9+Cd 4 4 k-Cs 1" 43 87h 8 This is an example of a host material which itself has a high order parameter.

(12) Host 12, which is a mixture of the following compounds in the stated percentages by weight.

Component A: 90% by weight:

Compound Percentage C3 i \ CO O &commat;ts h " A 45 r C's Hre 8 XC5 55 Component B: 10% by weight.

This specification contains subject matter related to the subject matter of UK 2093475A.

Claims

1. An anthraquinone compound of the formula:

wherein yl, Y2 and Y3 are each independently alkyl, aryl or cycloalkyl provided that not more than two of Y1 toY3 are identical.

2. An anthraquinone compound according to claim 1 wherein Y1, Y2 and Y3 are each independently C1 to C20 alkyl, up to C15 aryl or up to C15 cycloalkyl.

3. An anthraquinone compound according to claim 1 or claim 2 wherein at least one of the alkyl, aryl and cycloalkyl radicals represented by Y', Y2 and Y3 carries a non-ionic substituent.

4. An anthraquinone compound according to claim 3 wherein the substituent on the aryl radical is selected from lower alkyl, lower alkoxy, halogeno, cycloalkyl and monocyclic aryl.

5. An anthraquinone compound according to claim 3 wherein the substituent on the alkyl radical is selected from C1 to C4 alkoxy, halogeno and monocyclic aryl.

6. An anthraquinone compound according to any one of claims 1 to 4 wherein Y', Y2 and Y3 are phenyl radicals two of which differ in the nature or the position of a substituent thereon.

7. An anthraquinone compound according to claim 1 or claim 4 wherein Y2 and Y3 are 4-t-butylphenyl and Y' is phenyl.

8. A composition comprising at least one unsymmetrical tri(alkyl,aryl or cycloalkyl substituted thio) anthraquinone compound according to any one of claims 1 to 7 and at least one symmetrical (as herein before defined) tri(alkyl, aryl or cycloalkyl substituted thio) compound, in which two or more of the 1, 4, 5 and 8 carbons on the anthraquinone ring are substituted by the substituted thio group.

9. A composition according to claim 8 containing at least 50%, by weight, of the unsymmetrical poly(substituted-thio)- anthraquinone compound.

10. An anthraquinone compound or composition according to any one of claims 1 to 9 wherein the product of the molar extinction coefficient of the dye or composition in solution in a liquid crystal material and the solubility of the dye or composition in the liquid crystal material (in moles/litre) is at least 500 cam~~1.

11. An anthraquinone compound according to claim 1 as described in Example 1 or Example 2.