LIQUID CRYSTAL DISPLAY DEVICES AND LIQUID CRYSTAL MEDIUM TO BE USED THEREIN

Abstract: The invention concerns a liquid crystal display device containing a liquid crystalline medium which comprises a host material and at least one fluorescent dye, characterized in that the host material is composed of at least two liquid crystalline compounds, one having a negative or a positive dielectric anistropy, and one having a neutral dielectric anisotropy, further characterized in that the host mixture has an extinction coefficient between 300 and 400 nm of less than 2 x 10<2> and in that the liquid crystal medium further comprises an efficient amount of an energy transfer agent having an extinction coefficient between 300 and 400 nm of at least 10<4>, a solubility in the liquid crystal medium of at least 0.1 % and a fluorescence quantum yield of at least 0.2.

Description: Liquid crystal display devices and liquid crystal medium to be used therein The invention refers to a liquid crystal display device containing a liquid crystal medium which comprises a host material and at least one fluorescent dye. Fluores¬ cent liquid crystal display devices of this type are in principle known, e.g. from U.S. patents 4,208,106, 4,211,473 and 4,556,287, from GB patent 21 69 092 and from the publications in Electronic Letters Vol. 22, No. 18, pp. 962-963 of August 28, 1986, and Proceedings Eurodisplay 1987, pp. 149-151. In these display devices in a nematic and/or cholesteric liquid crystal host material a pleochroic fluorescent dye is dissolved such that by a guest-host interaction the molecules of the pleochroic dye may align with those of the liquid crystal material. As a rule, for these devices especially cyanobiphenyls or other liquid crystal materials containing conjugated aromatic ring systems are employed. These aromatic liquid crystal materials have the advantage that invisible UV light is absorbed by the host and in the presence of a suitable fluorescent dye an energy transfer process can occur during which the dye molecule obtains energy from the excited host and can then emit light in the visible region of the spectrum. The broad application of such fluorescent display devices fails, however, by the fact that the in principle good display characteristics suffer from a gradual fading of the fluorescent " emission, possibly a result of chemical degradation of the dye caused by the inci¬ dent UV light. It has, therefore, been a task of the present invention to find fluorescent liquid crystal display devices which do not only possess good display characteristics but also an improved stability and a longer lifetime. This task could be solved by the present invention. Sur¬ prisingly it was found that the stability and lifetime of the fluorescent dye is dependent on the nature of the host material and that especially a host having a low extinction coefficient between 300 and 400 nm and being composed both of compounds having a neutral dielectric anisotropy and of compounds having a positive or a negative dielectric anisotropy, possesses especially,., advantageous properties when it is used in connection with a compound having a high extinction coefficient between 300 and 400 nm and which acts as an energy- transfer agent for the fluorescent dye. Subject of the present invention is therefore a liquid crystal display device containing a liquid crystalline medium which comprises a host material and at least one fluorescent dye, characterized in that the host material is composed of at least two liquid crystalline compounds, one having a negative or a positive dielec¬ tric anisotropy and one having a neutral dielectric anisotropy, further characterized in that the host mix¬ ture has an extinction coefficient between 300 and 400 nm 2 of less than 2 x 10 and m that the liquid crystal medium further comprises an efficient amount of an energy- transfer agent having an extinction coefficient between 4 300 and 400 nm of at least 10 , a solubility in the liquid crystal medium of at least 0.1 % and a fluores- cence quantum yield of at least 0.2. A further subject of the present invention is the liquid crystalline medium for use in this display. The liquid crystal compounds which can be used as the host material are known and the skilled worker can choose an appropriate material according to the prin¬ ciples given above. To fulfill the demand for a low extinction coefficient between 300 and 400 nm at least i. one of the rings usually present in liquid crystalline compounds must be hydrogenated, preferably fully hydro- genated compounds are used and preferably the extinc¬ tion coefficient is less than 50. Examples for the liquid crystalline compounds with a positive dielectric anisotropy are R-Cy-Cy-CN, R-Cy-CH 2 -CH 2 -Cy-CN and R-Cy-Cy-Cy-CN, Cy in each case being 1,4-cyclohexylene. Examples for liquid crystalline compounds with a nega- CN tive dielectric anisotropy are R-Cy-Cy (CN being axially oriented) and R-Cy-CH-CH 2 -Cy-R' . I CN Preferred compounds with neutral dielectricity can be characterized by the formula R 1 -(Cy)„-Z-Cy-R 2 , wherein R 1 and R2 are alkyl, alkenyl, alkoxy or alkenyloxy, n is 1 or 2, Z is a single bond, -COO- or -CH ? -CH 2 and Cy is 1,4-cyclohexylene or l,4-cyclohexen-(2)-ylene. Other suitable liquid crystalline compounds can be taken from "Flύssige Kristalle in Tabellen" (liquid crystals in tables), VEB Deutscher Verlag fur Grundstoffindustrie, 2nd edition, 1976 and "Flϋssige Kristalle in Tabellen II", ibid, 1st edition (1984). Compounds with negative or positive dielectric anisotropy are defined as having jΔε|> 2.0, whereas compounds with neutral dielectricity have Δε in the range from -2.0 to +2.0. In the host mixture the neutral compound(s) as a rule amount to about 10 to 90 % by weight, mixtures contain¬ ing about 20 to 70 % being preferred. Due to the low extinction coefficient in the range from 300 to 400 nm this host mixture is not suited for use in an indirectly excited display. This can be rectified, how¬ ever, by addition of an effective amount of a suitable material which possesses a high extinction coefficient between 300 and 400 nm and which acts as an energy-transfer agent which transmits the absorbed energy to the fluores- cent dye. As an energy-transfer agent in principle all compounds are suitable which possess an extinction coefficient be- tween 300 and 400 nm of at least 10 4, preferably of at least 15000, which are sufficiently soluble in the liquid crystalline host mixture and which possess an emission wavelength which overlaps with the excitation wavelength of the fluorophor. The energy-transfer agent thereby allows not only indirect excitation of the fluorophor to be used but surprisingly acts also as UV-absorbing stabilizer leading to further enhancement in photostabi- lity of the liquid crystal medium. Suitable energy- ransfer agents possess as a rule a linear aromatic or heteroaro atic structure as it is exemplified by the following compounds: * Excitation maximum Basically, however, a large variety of compounds is suitable, especially compounds which are known as fluorescent whitening agents. A survey on such compounds is found in Rev. Prog. Coloration Vol. 17 (1987), pp. 39-55 and the references cited therein. From this large number of known compounds the skilled worker can chose easily suitable compounds according to the above men¬ tioned principles, like e.g. excitation and emission wavelength, solubility and effectivity. Preferably the fluorescence quantum yield in the solution of the liquid crystal medium should be higher than 0.2. There¬ fore, as a rule, aldehyde and ketone groups which usually lower the quantum yield and which also may cause acceler¬ ated photodegradation are not present in preferred struc¬ tures and it is also preferred that the compounds are as planar as possible to ensure efficient fluorescence. The energy-transfer agents are used in an effective amount which may vary depending on the nature of the agent but which can be easily optimized by a skilled worker. As a rule amounts of about 0.05 to about 10 weight %, preferably of about 0.1 to about 6 % are used. The fluorescent dyes, capable of being used in the liquid crystal medium of the present invention, are dyes that exhibit anisotropic absorption and emission, i.e. molecules which have the property that the absorption coefficient and consequently the fluorescence intensity of the dye molecule is strongly dependent on the align¬ ment between the plane of polarization of the incident radiation and the molecular axis of the dye molecule. Basically, these are rigid, lath-shaped fluorescent dyes possessing a uniform major polarization axis which are well known to those skilled in the art. Examples of fluorescent dyes useful in this invention are usually aromatic compounds such as those named in U.S. patents 4,556,287, 4,211,473 and 4,208,106, GB patent 2,169,092, EP patent 4655 as well as in the references cited in these patents, all of these publi- cations being incorporated by reference into this appli¬ cation. As a further preferred embodiment of the present invention it has been found that cyano-coumarin-dyes of formula I are especially well suited for the use in these display devices. Preferred are dyes wherςin R is a benzoxazoly] and especially a 5-chloro-l,3-benzoxazol-2-yl radical. The emission wavelength of these dyes is in any case in the visible range of the spectrum. Preferred is a wavelength in the range of from 550 to 620 nm. The dyes as a rule are used in concentrations with regard to the complete liquid crystal medium of about 0.05 to about 2 % by weight, concentrations of about 0.1 to about 1 % being preferred. It is also possible to use different dyes of similar or different structures and emission wavelengths. In some cases it has been found that by the use of a mixed dye system, e.g. a green and a red fluorophor, not only the brightness of the display can be increased but that there is a further gain in photostability. This is there¬ fore a further preferred aspect of this invention. Among these mixed dye systems with a green and a red dye are especially preferred mixtures containing the green perylene dye of formula II (isomer mixture) and its homologues. For an additional improvement of the light stability of the inventive liquid crystal medium small amounts of con¬ ventional UV stabilizers can be added as they are des- cribed in European patent application 19 58 98, British patent application 2117390 and the references cited therein. Amounts of about 0.05 to 1 % of these stabilizers are preferred. In the examples dyes of the following structures are used: Dye 1 (a green perylene dye) (isomer mixture) Dye 2 (a red benzopyran dye) Dye 3 (a red benzanthrone dye) Dye 4 (a red cyanocoumarin dye ) Dye 5 ( a red cyanocoumarin dye) OL In the examples as energy- ransfer agents are used T15, POPOP, DFQP7.7 and BOXT, the structures of which are given above. In the examples the following host mixtures are used: ZLI 2585 composed of: 34 % rτl-cyan-cis-4-(trans-4-butylcyclohexyl)-l-heptyl- cyclohexan 29.1 % r-l-cyan-cis-4-(trans-4-pentylcyclohexyl)-l- pentyl-cyclohexan 10.8 % trans,trans-4-methoxy-4'-propylcyclohexylcyclo- hexan 9.7 % trans,trans-4-ethoxy-4 , -propylcyclohexylcyclo- hexan 4.8 % trans,trans-4-propylcyclohexyl-cyclohexan-4*- carbonsaure-trans-4-propylcyclohexylester 3.9 % trans,trans-4-propylcyclohexyl-cyclohexan-4'- carbonsaure-trans-4-pentylcyclohexylester 3.9 % trans,trans-4-butylcyclohexyl-cyclohexan-4'- carbonsaure-trans-4-propylcyclohexylester 3.9 % trans,trans-4-butylcyclohexyl-cyclohexan-4'- carbonsaure-trans-4-pentylcyclohexylester ZLI 5310 composed of: 7 % trans,trans-4-ethylcyclohexylcyclohexan-4'- carbonitril 9 % trans,trans-4-propylcyclohexylcyclohexan-4'- carbonitril 15 % trans,trans-4-butylcyclohexylcyclohexan-4 ■ - carbonitril 9 % trans,trans-4-pentylcyclohexylcyclohexan-4'- carbonitri1 11 % trans,trans-4-ethoxy-4'-propylcyclohexylcyclo- hexan 20 % trans,trans-4-propoxy-4'-propylcyclohexyl- cyclohexan 15 % trans,trans-4-methoxy-4'-pentylcyclohexyl- cyclohexan 6 % trans,trans-4-propylcyclohexyl-cyclohexan-4*- carbonsaure-trans-4-pentylcyclohexylester 8 % trans,trans-4-methoxy-4'-propylcyclohexyl- cyclohexan ZLI 2359 composed of: 18 % trans,trans-4-ethylcyclohexylcyclohexan-4'- carbonitril 5 % trans,trans-4-propylcyclohexylcyclohexan-4'- carbonitril 15 % trans,trans-4-butylcyclohexylcyclohexan-4'- carbonitril 5 % trans,trans-4-pentylcyclohexylcyclohexan- f - carbonitril 5 % trans,trans-4-heptylcyclohexylcyclohexan-4'- carbonitril 7 % trans-4-propylcyclohexancarbonsaure-(trans-4- propylcyclohexylester) 8 % trans-4-propylcyclohexancarbonsaure-(trans-4- pentylcyclohexylester) 5 % trans,trans-4-propylcyclohexyl-cyclohexan-4'- carbonsaure-trans-4-pentylcyclohexylester 6 % trans,trans-4-butylcyclohexyl-cyclohexan-4'- carbonsaure-trans-4-propylcyclohexylester 6 % trans,trans-4-butylcyclohexyl-cyclohexan-4'- carbonsaure-trans-4-pentylcyclohexylester Example 1 A mixture was made containing Dye 1 0.8 % ZLI 2585 98.95 % POPOP 0.25 % When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a green fluorescent display with dark non-emissive segments was obtained. Example 2 A mixture was made containing When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a green fluorescent display with dark non-emissive segments was obtained. Example 3 A mixture was made containing When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a green fluorescent display with dark non-emissive segments was obtained. Example 4 A mixture was made containing When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a green fluorescent display with dark non-emissive segments was obtained. Example 5 A mixture was made containing When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a green fluorescent display with dark non-emissive segments was obtained. Example 6 A mixture was made containing When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a green fluorescent display with dark non-emissive segments was obtained. Example 7 A mixture was made containing When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a green fluorescent display with dark non-emissive segments was obtained. Examples 8-14 Mixtures analogous to examples 1-7 are used containing 0.15 % of Dye 2 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a red fluorescent display with dark non-emissive segments was obtained. Examples 15-21 Mixtures analogous to examples 1-7 are used containing 0.20 % of Dye 3 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a red fluorescent display with dark non-emissive segments was obtained. Examples 22-28 Mixtures analogous to examples 1-7 are used containing 0.30 % of Dye 4 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a red fluorescent display with dark non-emissive segments was obtained. Examples 29-35 Mixtures analogous to examples 1-7 are used containing 0.30 % of Dye 5 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a red fluorescent display with dark non-emissive segments was obtained. Examples 36-42 Mixtures analogous to examples 1-7 are used containing 0.15 % of Dye 1 and 0.15 % of Dye 2 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a orange- red fluorescent display with dark non-emissive segments was obtained. Examples 43-49 Mixtures analogous to examples 1-7 are used containing 0.20 % of Dye 1 and 0.20 % of Dye 3 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a orange- red fluorescent display with dark non-emissive segments was obtained. Examples 50-56 Mixtures analogous to examples 1-7 are used containing 0.30 % of Dye 1 and 0.30 % of Dye 4 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a orange- red fluorescent display with dark non-emissive segments was obtained. Examples 57-63 Mixtures analogous to examples 1-7 are used containing 0.30 % of Dye 1 and 0.30 % of Dye 5 instead of Dye 1. When filled into an electro-optic display cell and excited with a lower power ultra violet lamp a orange- red fluorescent display with dark non-emissive segments was obtained. Comparative examples The following liquid crystal media contain each 0.8 % of dye 1. The other components of these media, i.e. the liquid crystal host and the energy-transfer agent (ETA) and its percentage as well as the fluorescent half-life of each of these media are compiled in the following: Medium No. Host ETA Fluorescent half-life It is clearly apparent from the above data that the media according to the present invention (media 4, 5 and 6) show markedly better life-times than the media 1, 2 and 3, the host of which is based on liquid crystals having an extinction coefficient between 300 and 400 nm of considerably more than 2 x 10 2- E7 is based on cyano- biphenyls, whereas ZLI-1083 is based on phenylcyclohexanes. The lifetests were carried out by filling a commercially produced polyimide aligned display cell, 7 microns thick, with the medium under test. The cells were exposed to filtered Xenon arc radiation in a Heraeus Sun Test machine, followed by measurement of the fluorescence intensity in a Perkin Elmer LS5 spectrofluorimeter.

Claims: Patent Claims 1. Liquid crystal display device containing a liquid crystalline medium which comprises a host material and at least one fluorescent dye, characterized in that the host material is composed of at least two liquid crystalline compounds, one having a negative or a positive dielectric anisotropy, and one having a neutral dielectric anisotropy, further characterized in that the host mixture has an extinction coef icient between 300 and 400 nm of less than 2 x 10 and in that the liquid crystal medium further comprises an efficient amount of an energy transfer agent having an extinction coefficient between 300 and 400 nm of at least 10 4, a solubility iri : the liquid crystal medium of at least 0.1 % and a fluorescence quantum yield of at least 0.2. 2. Display device according to Claim 1, characterized in that the host material is composed of fully saturated liquid crystal compounds. 3. Display device according to Claim 1 or 2, characterized in that the energy transfer agent comprises at least one compound having a structure element of formula 3 2 3 wherein Ar , Ar and Ar are identical or different and are selected from the group consisting of unsubstituted 1,4-phenylene, 1,4-phenylene substituted by 1 or 2 fluorine atoms, and heteroaromatic ring structures. 4. Display device according to Claim 3, characterized in that the energy transfer agent comprises at least one aniso- tropic compound. 5. Display device according to one of Claims 1 to 4, charac¬ terized in that the liquid crystalline medium comprises a mixed dye system. 6. Display device according to Claim 5, characterized in that an energy transfer from a green to a red fluorophor is used. 7. Display device according to one of Claims 1 to 6, charac¬ terized in that the liquid crystal medium comprises a fluorescent dye of the general formula I wherein R 1, R and R3 are each independently alkyl with 1 to 6 carbon atoms R 4 is alkyl or cycloalkyl with up to 7 carbon atoms possibly substituted once or twice with hydroxy, cyano, alkoxy with 1 to 3 carbon atoms or alkoxycarbonyl with 1 to 3 carbon atoms is hydrogen, chlorine, bromine, cyano, alkyl, alkoxy, alkoxycarbonyl or carbalkox each with up to 7 carbon atoms in the alkyl chain X is O or N-R 8 o R is hydrogen or alkyl with 1 to 3 carbon atom . 8. Liquid crystal medium containing at least one fluorescent dye and a host material, characterized in that the host material is composed of at least two liquid crystalline compounds, one having a negative or a positive dielectric anisotropy, and one having a neutral dielectric aniso¬ tropy, further characterized in that the host mixture has an extinction coefficient between 300 and 400 nm 2 of less than 2 x 10 , and in that it further comprises an efficient amount of an energy transfer agent having an extinction coefficient between 300 and 400 nm of 4 . . . at least 10 , a solubility in the liquid crystal medium of at least 0.1 %, and a fluorescence quantum yield of at least 0.2. 9. Liquid crystal medium according to Claim 8, characterized in that the host material is composed of fully saturated liquid crystal compounds. 10. Liquid crystal medium according to Claim 8 or 9, charac¬ terized in that the energy transfer agent comprises at least one compound having a unit of formula wherein Ar 1, Ar2 and Ar3 are identical or different and are selectred from the group consisting of unsubstituted 1,4-phenylen, 1,4-phenylen substituted by 1 or 2 fluorine atoms, and heteroaromatic ring structures. 11. Liquid crystal medium according to Claim 10 characterized in that the energy transfer agent comprises at least one anisotropic compound. 12. Liquid crystal medium according to one of Claims 8 to 11 characterized in that it comprises a mixed dye system. 13. Liquid crystal medium according to one of Claims 8 to 12 characterized in that it comprises a fluorescent dye of the general formula I wherein R 1, R2 and R3 are each independently alkyl with 1 to 6 carbon atoms R 4 is alkyl or cycloalkyl with up to 7 carbon atoms possibly substituted once or twice with hydroxy, cyano, alkoxy with 1 to 3 carbon atoms or alkoxycarbony with 1 to 3 carbon atoms R 6 and R7 is hydrogen, chlorine, bromine, cyano, alkyl, alkoxy, alkoxycarbonyl or carbalkox each with up to 7 carbon atoms in the alkyl chain X is O or N-R 8 o R is hydrogen or alkyl with 1 to 3 carbon atoms. 14. Liquid crystal medium according to Claim 12, charac¬ terized in that it comprises a green fluorescent dye of formula II and a red fluorescent dye.

Inventors: GRIFFITHS JOHN, LEWIS MARTIN, SAGE IAN CHARLES, STANFORD JOHN EDWARD, TAILOR SURESH, WILTSHIRE MICHAEL, GRIFFITHS, JOHN, LEWIS, MARTIN, SAGE, IAN, CHARLES, STANFORD, JOHN, EDWARD, TAILOR, SURESH, WILTSHIRE, MICHAEL
Applicants: MERCK PATENT GMBH, MERCK PATENT GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG
Application year: 1989
Main classification: C09K19
IPC codes: C09B3, C09B5, C09B57, C09K19, G02F1