GB2065158A

GB2065158A - Anthraquinone Dichroic Dyes

Info

Publication number: GB2065158A
Application number: GB7943075A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1979-12-13
Filing date: 1979-12-13
Publication date: 1981-06-24
Also published as: JPS56100856A; NL8006666A

Abstract

A class of anthraquinone dichroic dyes having the general formula <IMAGE> where R is an alkyl group or a simple alkyl derivative. Examples of these dyes show an improved solubility in certain liquid crystal media compared with other dichroic dyes having a similar general formula but with an oxygen atom substituted for the sulphur atom.

Description

SPECIFICATION Anthraquinone Dichroic Dye This invention relates to anthraquinone dichroic dyes.

One particular use for dichroic dyes is in the manufacture of liquid crystal display devices. For this purpose dyes are required which are chemically stable, have a high order parameter, and, on account oi the thinness of such cells, have a high extinction coefficient. One class of dye that satisfies these criteria reasonably well is that given by the general formula:

where R is a normal alkyl group. The preparation of one of such dyes is described by D. X. Klein in United States Patent Specification 2,419,405. Some of these dyes are marketed by British Drug Houses.An example is the dye marketed by them under the designation BDM 1 6. However we have found that the limited solubility of these dyes is a distinct drawback militating against their use in liquic crystal devices because they do not produce deep colours while still remaining in solution at low temperatures (e.g. -200C). This applies particularly to cells with liquid crystal layer thicknesses of 10 microns or less. We have found a class of related dyes that have an improved solubility.

According to the present invention there is provided an anthraquinone dichroic dye having the general formula:

where R is an alkyl group or simple alkyl derivative.

There follows a description of the manufacture of specific examples of such dyes. This manufacture involves the following four process steps:

80 OJOs NO2 b- STEPAI,8 ce SH SR STCP ci H + SR OoO STEP OH SR Step Al : The Production of 4-nitrothiophenol According to the standard method of C. C. Price and G. W. Stacey described in the Journal of the American Chemical Society, volume 68, page 498, (1946), commercially available 1 -chloro-4nitrobenzene is reacted in ethanol with a mixture of sulphur and sodium sulphide. After isolation and crystallisation from industrial methylated spirits the compound is found to have a melting point in the range 74 to 750C.

Step By : the Production of Snitrophenylalkyl Sulphide Using the method described by G. D. Hurd-K. Greengard in the Journal of the American Chemical Society volume 52, page 3356, (1930), a mixture of alkylbromide, sodium ethoxide, and 4nitrothiophenol in ethanol is stirred for 40 hours at room temperature. The required product is isolated by distillation under reduced pressure. Specific examples of 4-nitro-phenyl-alkylsulphide have been prepared using n-propylbromide and its normal homologues up to and including n-heptylbromide. The following table 1 lists the yields and the boiling point of the reaction products.

Table 1 Alkyl Group Boillng Point Yield n-Propyl 1220C at 0.4 mm Hg 66% n-Butyl 1 200C at 0.25 mm Hg 78% n-Pentyl 15700 at 0.4 mm Hg 65% n-Hexyl 15600 at 0.35 mm Hg 79% n-Heptyl 158 C at 0.35 mm Hg 71% Step Cl: The Production of 4alkyl Thioaniline 4-Nitrophenyl alkylsulphide is converted to 4-alkylthioaniline by reduction. One way of achieving this is the mild reaction procedure of J. Kunak and M. Semonsky in Chemicke Listy volume 47 page 598 (1953), in which a mixture of ethanol (120 ml), 4-nitrophenyl alkylsulphide (0.027 mole), iron pin dust (13 g, 0.23 m) and 20% acetic acid (25 ml) is heated under reflux and vigourously stirred for 4 hours.After cooling, sodium carbonate (9 g) is added and the mixture heated and stirred for 30 minutes. The hot solution is then filtered through a filtering aid sold under the designation 'Hyflow Supercell', and rotary evaporated to give a brown oil. This oil is taken up in ether, washed with water, dried over sodium sulphate and distilled. Table 2 lists the boiling points and yields obtained using the specific alkyl examples previously referred to.

Table 2 Alkyl Group Boiling Point Yield n-Propyl 100--1020C at 0.15 mm Hg 73% n-Butyl 120-121 OC at 0.5 mm Hg 70% n-Pentyl 126--1280C at 0.35 mm Hg 70% n-Hexyl 1350Cat0.4mmHg 71% n-Heptyl 1420C at 0.2 mm Hg 77% Step Dl: The Production of l-(4-alkylthioanilino)-4-hydroxy Anthraquinone The required anthraquinone dye is prepared using a method similar to that used by D. X. Klein in United States Patent No. 2,419,405, in which leucoquinizarin (0.75 g), quinizarin (2.6 g), 4-alkyl thioaniline (0.016 mole), boric acid (1.6 g) and ethanol (35 ml) are heated under reflux and stirred for 12 hours.A quantity of sodium (1 g) is added to dry ethanol (50 ml) to form a solution of sodium methoxide which is added to the cold mixture. This is then warmed for 30 minutes, and rotary evaporated to dryness. The solid residue is washed repeatedly with dichloromethane until the washings are colourless. The dry solid is then stirred with acetic acid (8 ml) and water (60 ml), filtered, and washed with water.

The required reaction product is extracted by column chromatography on silica gel using dichlommethane as the eluant, followed by crystallisation from industrial methylated spirits to yield dark needle-shaped crystals. Table 3 lists some of the properties of the products obtained using the specific alkyl examples referred to previously.

Table 3 Extinction Absorption Absorption Dye Alkyl Melting Coefficient Peak Amax Peak Amax Name Group Point (e) in CHCl3 in CHCl3 in BDH E8 DC/9 n-propyl 12600 0.89x104 580 nm 590 nm DC/4 n-butyl 1150C 1.37x104 580 nm 590 nm DC/6 n-pentyl 920C 1.00x 104 580 nm 590 nm DC/7 n-hexyl 720C 1.05x104 580 nm 590 nm DC/8 n-heptyl 790C 1.20x104 580 nm 590 nm Solubility Solubility Solubility (5) Dye (S) in Merck (S) in BDH in BDH K24 at Order Parameterrs) Name 1 132 at --23 OC E8 at --23 OC room temperature in Merck 1132 DC/9 s < 2.3% 0.62 DC/4 2.27% < s < 3.2% 2.9% < s < 3.7% s > 3.4% 0.64 DC/6 s > 2.27% 0.62 DC/7 s > 2.4% 0.625 DC/8 s > 2.1% 0.62 D16 s < 2.2% s < 2.9% s~3.4% 0.65 For the purpose of comparison, the last member of the table is the prior art BDH anthraquinone dye 1 6 to which earlier reference has been made. The solubility measurement quoted in the table are based on tests lasting two months.The first solubility column refers to solubilities in a eutectic mixture of phenylcyclohexanes marketed by Merck Sharpe 8 Dohme under the designation Merck 11 32, the second to solubilities in a nematic eutectic mixture of alkylcyanobiphenyls marketed by BDH under the designation E8, and the third to solubilities in the smectic A phase of 4-octyl-4-cyanobiphenyl marketed by BDH under the designation K24.

In a stability test using an ultra-violet lamp marketed by General Electric under the designation 'Black Light' the absorption of the dye DC/4 was found to be reduced by about 1 5% after an exposure of 1,000 hours (corresponding to 10,000 hours of sunlight). For the purposes of this test the dye was dissolved in a cholesteric mixture of the eutectic mixture E8 containing the cholesteric 4-(2-methylbutyl)-4'-cyanobiphenyl, and having a pitch of 3 microns. A ten micron thick layer of this dye was contained in a cell formed by two 1.5 mm thick glass plates and a perimeter seal. The inner surfaces of the plates were provided with alignment layers of silicon monoxide evaporated obliquely at 30 . This cell had an initial absorption of 0.38, and during the test was placed 1 5 cm from the lamp.

Dyes according to the present invention may be used on their own or in association with other dyes. For use in liquid crystal display devices one particular requirement is for a substantially black dichroic dye. Such a dye may be prepared by mixing one of the dyes according to the present invention with appropriate amounts of orange and red dichroic dyes. An example of a suitable red dye is given by the disazo dye

sold by BDH under the designation D2 for which peak absorption max is at 505 nm. An example of a suitable orange dye is given by the triazo dye

designated by us DC/17. This dye can be prepared by the general method described in German OLS 2,627,215. In solution in the phenylcyclohexane eutectic mixture Merck 11 32, the orange dye DC/17 exhibits an order parameter of 0.68, and its peak absorption A max is at 425 nm. Thus a substantially black dichroic dye is provided by mixing 9.5 parts by weight DC/4 with 0.7 parts D2 and 2.2 parts DC/17.

Claims

1. An anthraquinone dichroic dye having the general formula

where R is an alkyl group or a simple alkyl derivative.

2. An anthraquinone dye as claimed in claim 1 wherein R is a normal alkyl group.

3. The anthraquinone dye as claimed in claim 2 wherein R is n-butyl.

4. An anthraquinone dye as claimed in claim 2 wherein R is n-propyl, n-pentyl n-hexyl or n-heptyl.

5. A substantially black dichroic dye consisting of a mixture one component of which is an anthraquinone dye as claimed in claim 1 or 2.

6. A substantially black dichroic dye as claimed in claim 5 another component of which is the trisazo dye identified hereinbefore as DC/17.

7. A substantially black dichroic dye as claimed in claim 6 another component of which is the anthraquinone dye identified hereinbefore as D2.

8. A substantially black dichroic dye as claimed in claim 5, 6, or 7 wherein said one component is the anthraquinone dye as claimed in claim 3.

9. A substantially black dichroic dye as claimed in claim 8 consisting substantially of the mixture of 9.5 parts by weight DC/4 with 0.7 parts D2 and 2.2 parts IDC/17.

10. A substantially black dichroic dye as claimed in claim 5, 6, or 7 wherein said one component is an anthraquinone dye as claimed in claim 4.

11. A liquid crystal containing a dichroic dye as claimed in any preceding claim.