JP2008174747A - Liquid crystal medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal medium appropriate for an AMD display. <P>SOLUTION: The liquid-crystal medium based on a mixture of polar compounds of positive dielectric anisotropy is characterized in that it comprises a fluorinated terphenyl compound having formula I and a specific fluorinated compound having a cyclohexane ring-benzene ring-benzene ring wherein R is independently an alkyl or alkenyl radical having up to 12 C atoms, these radicals being unsubstituted or substituted by halogens, it also being possible for one or more CH<SB>2</SB>groups in these radicals to be replaced, in each case independently of one another, by -O-, -CO-O- or -O-CO- in such a manner that oxygen atoms are not linked directly to one another; X is independently F, Cl, CF<SB>3</SB>, OCF<SB>3</SB>or OCHF<SB>2</SB>; and L<SP>1</SP>and L<SP>2</SP>are independently H or F. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a liquid crystal medium based on a mixture of polar compounds having positive dielectric anisotropy and a matrix liquid crystal display having high temperature and UV stability.

Active matrix displays (AMD) are particularly preferred as interesting displays in the market with high information capacity. Such AMDs are used for TV applications (eg projection devices) and also as displays for laptops, automobiles and aircraft, for example.
AMD has non-linear electrical switching elements integrated in each pixel. Non-linear drive elements include thin film transistors (TFTs) [SID 82 Digest by Okubo, U. et al., Pages 40-41 (1982)] or diodes (for example, metal insulator metal: MIM) [Niwa, K. et al. SID 84 Digest, pages 304-307 (1984)]. These non-linear drive elements allow the use of an electro-optic effect with a rather flat electro-optic characteristic curve if good visual angle characteristics can be obtained.

  Thus, a TN type LC cell [Appl. Phys. Lett., 18, 127 (1971) by Schadt, M. and Helfrich, W.] having a twist angle of 90 ° can be used. In order to obtain good contrast over a wide viewing angle, it is required to operate at a minimum of the first light transmittance [Pohl, L., Eidenschink, R., Pino, F.del., And Weber, G German Patent DBP 3022818 (1980) and U.S. Patent 4990803 (1981); Pohl, L., Weber, G., Eidenschink, R., Baur, G. and Fehrenbach, W. Appl. Phys. Lett., 38,497 (1981); Weber, G., Finkenzeller, U., Geelhaar, T., Plach, HJ, Rieger, B. and Pohl, L., Int. Symp. On Liq. Cryst. Freiburg (1988)]. These AMDs are particularly well suited for TV applications and are therefore attracting particular attention in the market.

  For these applications, several properties of the liquid crystal are even more important than in the case of passive TV displays. Several physical properties that are decisive for AMD performance include the resistance and stability of liquid crystals [Togashi, S., Sekiguchi, K., Tanabe, H., Yamamoto, E., Sorimachi, K., Kajima. Proc. Eurodisplay by S., E., Watanabe, H., Shimizu, H. 84, September 1984: A210-288; Matrix LCD Controlled by Double Stage Diode Rings, p. 144 et seq., Proc. 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, page 145 and later, Paris].

A problem that often arises is the detrimental effect of UV radiation on the resistivity value and thus on the general performance of the liquid crystal mixture in the display. In AMD, nonlinear switching elements are addressed in a multiplex manner. These devices charge the pixel electrodes within the time limit during which they are active. These elements are then deactivated until the next time they are addressed again. Therefore, it is also a very important feature of such displays that the voltage change for activated (charged) pixels is undesirable. The discharge of this pixel is determined by two factors. These factors are the capacitance of the pixel including the liquid crystal and the dielectric material between the electrodes, that is, the resistance value of the liquid crystal. The characteristic time constant of the voltage decay at the pixel must be significantly longer than the time between two address cycles ( _tadr ^. ). A parameter often used to describe the performance of AMD is the pixel voltage holding ratio (HR):

画素における電圧は、この保持率が増加するに従い指数的に減衰されるから、格別に大きい抵抗値を有する液晶が必要である。
ディスプレイ内部の液晶の抵抗値にとって重要な数種の問題点があり、この問題点には、例えば配向層、配向層材料の硬化条件などがある。あまり重要ではないものの、使用する液晶の電気的性質も問題である。特に、ディスプレイ内部の液晶の抵抗値は、画素における電圧低下の大きさを決定する。

Since the voltage at the pixel is exponentially attenuated as the retention rate increases, a liquid crystal having a particularly large resistance value is required.
There are several problems that are important for the resistance value of the liquid crystal inside the display. Examples of these problems include alignment layers and curing conditions for alignment layer materials. Although not so important, the electrical properties of the liquid crystals used are also a problem. In particular, the resistance value of the liquid crystal inside the display determines the magnitude of the voltage drop in the pixel.

  In previous studies using materials with small Δn values, the requirement for resistance and UV stability for TFT applications and the requirement for temperature dependence of this resistance contain cyano groups as end groups. It proved that some substances could not be adapted. Non-cyano materials containing halogenated end groups can exhibit better resistance and UV stability than conventional cyano materials, and can also exhibit superior viscosity values. In general, however, these non-cyano materials unfortunately exhibit a strong tendency to form crystalline phases and / or smectic phases, especially at low temperatures. In addition, the clearing point and dielectric anisotropy value of non-cyano substances having halogenated end groups are exceptionally low.

Modern commercial mixtures must operate over a wide temperature range; therefore, low temperature crystallization or smectic phase formation must be eliminated. Concerning the usefulness of liquid crystal materials in the development of nematic mixtures, good solubility is one of the most important preconditions. For this reason, compounds having a high melting temperature or a tendency to form a smectic phase are not suitable.
By very careful selection of the components and the corresponding mixture design, it was possible to find a non-cyano mixture with a wide nematic temperature range for the first local minimum application and also with a small birefringence [B Proc. By Rieger et al., 18. Freiburger Arbeitstagung Fleussigkristalle, Freiburg, 1989, 16 (1989)]. Non-cyano materials with large birefringence, which are essential for the target mixture of the present invention, unfortunately quite often, have higher melting points and / or stronger smectics compared to similar materials with lower birefringence. It may even exhibit less desirable properties such as phase formation tendency:

The broad general formula of WO 90/09240 includes fluoroterphenyl compounds of formula I. However, this patent does not describe one example for these compounds, nor does it describe one example for mixtures containing fluorinated fluoro- and chloro-terphenyl compounds.
Prior art mixtures with birefringence suitable for operation at the second light transmission minimum or third and higher light transmission minimum on the Gooch-Tarry curve are acceptable for active matrix applications. Absent.
Therefore, a liquid crystal composition having a high resistance value and other material properties suitable for use in AMD is still required.

As mentioned above, the chloroterphenyl compound (Compound No. 3) is known from JP 60-056932-A. As noted above, these compounds meet stringent requirements from the electronics industry, especially in view of limited solubility in different types of liquid crystal materials, their high melting points and their distinct smectic phase formation. I can't. Accordingly, there is a need in the art for improved non-cyano LC compounds with high birefringence.
The chloroterphenyl compound of formula I has been published at the International LC Conference (Pisa / Italy; 1992).

An object of the present invention is to provide a liquid crystal medium which is a medium for an AMD display in particular, and which does not have the above-mentioned drawbacks or has only a small extent.
Another object of the present invention is to provide a matrix liquid crystal display having high temperature stability and UV stability.

  It has been found that this object is achieved when the liquid crystal medium contains at least one compound of the formula I below and at least one compound of the formula II below.

を有するフッ素化ターフェニル化合物の１種または２種以上および式ＩＩ： That is, the present invention is a liquid crystal medium based on a mixture of polar compounds having positive dielectric anisotropy, which has the formula I:

One or more fluorinated terphenyl compounds having the formula II and

で表わされる化合物の１種または２種以上を含有することを特徴とする液晶媒体を提供するものである（各式中、Ｒ、Ｒ´、Ｘ、Ｘ´、Ｌ¹およびＬ²は、請求項１に示されている意味を有する）。
式Ｉおよび式ＩＩで表わされる化合物において、ＲおよびＲ´は好ましくは、アルキルまたはアルコキシであり、ＸおよびＸ´は好ましくは、ＣｌまたはＦである。

The liquid crystal medium is characterized by containing one or more of the compounds represented by the formula (wherein R, R ′, X, X ′, L ¹ and L ² are claims) Having the meaning shown in item 1).
In the compounds of formula I and formula II, R and R ′ are preferably alkyl or alkoxy, and X and X ′ are preferably Cl or F.

The matrix liquid crystal display of the present invention is
-Two plane-parallel support substrates which, together with the frame, form cells of thickness d;
An integrated non-linear element present on this support substrate to drive each pixel, and a nematic liquid crystal mixture present in the cell and having positive dielectric anisotropy and birefringence Δn,
And the display operates at a Gooch-Tariy second light transmission minimum or a third or higher light transmission minimum by appropriately selecting d · Δn, The quotient of the voltage holding ratio HR ₂₀ after exposure to UV light (280-400 nm, 12 mW / cm ² ) for 20 hours and HR ₀ before exposure to UV light is greater than or equal to 98% , And also having a very high resistance value that meets other requirements.

It has been found according to the invention that such HR values are possible even in mixtures with a large birefringence by using laterally fluorinated and / or ethyl-bonded non-cyano substances. It was done. Very large RC time values can be obtained with AMD. These mixtures also exhibit reduced viscosity and also allow short switching times at reasonable threshold voltages.
The thickness of this AMD is preferably in the range of 3-10 μm. A particularly preferable range is a range of 3 to 7 μm.

The following preferred embodiments relate to nematic liquid crystal mixtures that are present in AMD:
This liquid crystal mixture preferably contains one or more compounds of the formula III below:

In which R ^* is an alkyl or alkenyl group having up to 15 carbon atoms, which groups are unsubstituted or substituted by halogen and are also present in these groups One or more CH ₂ groups are each independently replaced by —O—, —CO—O— or —O—CO— as oxygen atoms are not directly bonded to each other. Well,
r is 0 or 1;
X ^* is F, Cl, CF ₃ , OCF ₃ or OCHF ₂ , and L ^* , Y ^* and Z ^* are each H or F, and ^one of Q ¹ and Q ² is 1, 4 -Phenylene or 3-fluoro-1,4-phenylene and the other is a single bond.

The compounds of the following formula IIIa are particularly preferred:

The liquid crystal mixture of the present invention preferably contains one or more compounds selected from the following formulas IV to VII:

In each formula, R is an alkyl or alkenyl group, each having up to 12 carbon atoms, which groups are unsubstituted or substituted by halogen and are also present in these groups One or more CH ₂ groups are each independently replaced by —O—, —CO—O— or —O—CO—, as oxygen atoms are not directly bonded to each other. And the benzene ring may be further substituted with fluorine,
X ¹ is F, Cl, CF ₃ , OCF ₃ or OCHF ₂ , or a fluoroalkyl or fluoroalkoxy group having up to 7 carbon atoms, respectively
Y ¹ , Y ² and Y ³ are each H or F;
Q is —C ₂ H ₄ —, —C ₄ H ₈ — or —CO—O—,

The liquid crystal mixture of the present invention preferably contains one or more of the compounds represented by the following formulas VIII to XIII:

In each formula, R, r, X ¹ , Y ¹ , Y ² , Y ³ are each independently as defined in claim 4 and R ′ has the same meaning as described for R Has meaning.
The mixtures according to the invention basically consist of compounds selected from the group consisting of the general formulas I to VII.
Preferred embodiments are listed below:
A mixture containing one or more compounds of the formula I in a proportion of at least 10% by weight.
A mixture containing one or more compounds of the formula II in a proportion of at least 5% by weight.

-A mixture which may further contain at least one compound represented by the following formula:

  Examples of the compound include DOS 3042391, DOS 3902328, DOS 3913554, DOS 3909802, WO 89/02884, WO 90/15113, WO 90/09420, International Patent Application No.PCT / EP90 / 01292, No.PCT / EP91 / No. PCT / EP90 / 01471, No. PCT / EP90 / 02109, and European Patent Application No. 91100675.7, or can be prepared in the same manner as known compounds.

The mixtures according to the invention are usually based on moderately polar compounds having an indicator skeleton structure and other non-cyano components. However, if a particularly large HR value is not required, for example when used with TN or STN, such a mixture also contains further known cyano LC components, preferably compounds of the formula Of course you can do:

The resulting mixture is important for obtaining an exceptionally wide nematic phase range that encompasses very low temperatures (field use).
The mixtures according to the invention are preferably based on moderately polar halogenated components and / or are substantially free of cyano components. The novel compounds of the formula I can be prepared in the same way as described in EP 0439089A1, WO 90/09420, WO 90/15113 and WO 91/13850, WO 91/00411. And incorporate it here.
In the components of the formulas I to XIII, R, R ′ and R ^* are preferably straight chain alkyl groups having 1 to 7 carbon atoms, or straight chain methoxyalkyl groups (methoxymethyl, methoxyethyl). , Methoxypropyl, methoxybutyl, methoxypentyl, methoxyhexyl, methoxyheptyl).

The mixture according to the invention is produced in a conventional manner. In general, the desired amount of the components used in the lesser amount is dissolved in the components making up the major component, preferably at elevated temperatures. The completion of the dissolution process can be seen particularly easily if this temperature is chosen to be above the clearing point of the main component.
However, it is also possible to mix a solution in a suitable organic solvent, such as acetone, chloroform or methanol, which does not introduce any contaminants or undesirable doping substances of the components, and then remove the solvent.
By using suitable additives, the liquid crystal phase according to the invention can be modified so that it can be used in any of the previously disclosed AMD.

The following examples are illustrative of the invention and are not intended to limit the invention. In each example, the melting point and clearing point of the liquid crystal material are indicated in degrees Celsius. Percentages are by weight.
Measurement of HR was carried out as disclosed by S. Matsumoto et al. [Liquid Crystals, 5, 1320 (1989)] on a standard 6 μm TN-display without the use of spacers. Standard float glass (Balzers) with a conductive ITO layer and rubbed polyimide (AL-1051 from Nippon Synthetic Rubber) were used as alignment layers. The cell was sealed with a UV curable adhesive (Norland NOA-61) and filled under standard conditions. The liquid crystal mixture consisted of components that were carefully purified by standard methods. UV exposure was carried out with a Heraeus-Suntest (1.1 kW, 0.082 W / cm ² , UV block 310 nm) equipped with a xenon lamp.

In the present specification and in the following examples, the chemical structure of the liquid crystal compound is indicated by an abbreviation for the initial letter. This abbreviation means a chemical formula as shown below. The groups C _n H _{2n + 1} and C _m C _{2m + 1} are straight chain alkyl groups each having n or m carbon atoms. The codes in Table B are self-explanatory. In Table A, only the initial letters relating to the basic structure are shown. In each case, the code for the substituents R ¹ , R ² , L ¹ , L ² and L ³ is shown, separated by a hyphen after the initial letter associated with this basic structure:

Table A:

Table B;

Claims (7)

A liquid crystal medium based on a mixture of polar compounds having positive dielectric anisotropy, which has the formula I:

One or more fluorinated terphenyl compounds having the formula II and

One or more of the compounds represented by:
In each formula, R and R ′, independently of one another, are alkyl or alkenyl groups having up to 12 carbon atoms, which groups are unsubstituted or substituted by halogen, One or two or more CH ₂ groups present in these groups are each independently independent of the other of oxygen atoms not directly bonded to each other as —O—, —CO—O— or — May be replaced by O—CO—, and X and X ′ are each independently of each other F, Cl, CF ₃ , OCF ₃ or OCHF ₂ , and L ¹ and L ² are each Independently H or F.
A liquid crystal medium comprising:   The mixture according to claim 1, wherein X and X ′ are chlorine or fluorine. The liquid crystal mixture is of formula III:

In which R ^* is an alkyl or alkenyl group having up to 15 carbon atoms, which groups are unsubstituted or substituted by halogen and are also present in these groups One or more CH ₂ groups are each independently replaced by —O—, —CO—O— or —O—CO— as oxygen atoms are not directly bonded to each other. Well,
r is 0 or 1;
X ^* is F, Cl, CF ₃ , OCF ₃ or OCHF ₂ ,
L ^* , Y ^* and Z ^* are each H or F, and ^one of Q ¹ and Q ² is 1,4-phenylene or 3-fluoro-1,4-phenylene and the other is a single bond Is,
The mixture according to claim 1 or 2, further comprising one or more of the compounds represented by formula (1). The liquid crystal mixture is represented by formulas IV to VII:

In each formula, R is an alkyl or alkenyl group, each having up to 12 carbon atoms, which groups are unsubstituted or substituted by halogen and are also present in these groups One or more CH ₂ groups are each independently replaced by —O—, —CO—O— or —O—CO—, as oxygen atoms are not directly bonded to each other. And the benzene ring may be further substituted with fluorine,
X ¹ is F, Cl, CF ₃ , OCF ₃ or OCHF ₂ , or a fluoroalkyl or fluoroalkoxy group having up to 7 carbon atoms, respectively
Y ¹ , Y ² and Y ³ are each H or F;
Q is —C ₂ H ₄ —, —C ₄ H ₈ — or —CO—O—,

r is 0 or 1;
The mixture according to at least one of claims 1 to 3, comprising one or more compounds selected from the group consisting of: The liquid crystal mixture has the formula:

The mixture according to claim 1, further comprising one or more of the compounds represented by formula (1):   5. A mixture according to at least one of claims 1 to 4, essentially comprising a compound selected from the group consisting of general formulas I to VII.   An electro-optic liquid crystal display containing the liquid crystal mixture according to claim 1.