GB2077286A

GB2077286A - Nematic liquid crystal materials for display devices

Info

Publication number: GB2077286A
Application number: GB8117223A
Authority: GB
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1980-06-06
Filing date: 1981-06-05
Publication date: 1981-12-16
Also published as: JPS572386A; DE3122529A1; GB2077286B; DE3122529C2; HK24688A; JPS6124431B2

Abstract

A nematic liquid crystal composition comprises at least one compound of the formula: <IMAGE> and at least one compound of the formula: <IMAGE> wherein R1, R2 are alkyl, alkoxy, alkylcarbonyl and R3 and R4 are alkyl or alkoxy. At least one additional nematic liquid crystal material having positive or negative dielectric anisotropy and/or analogous compounds can be incorporated. The composition is suitable for use in liquid crystal display devices having multiplexing driving system because it provides low cross-talk and a fast response.

Description

SPECIFICATION Nematic liquid crystal materials for display devices This invention relates to a nematic liquid crystal composition used for display devices, particularly those of a multiplexing drive system.

Twisted nematic type (TN type) liquid crystal display elements which belong to field-effect type liquid crystal display elements and have a structure as shown in the attached Fig. 1 are widely used now. The liquid crystal display element shown in Fig. 1 comprises a first substrate 1 and a second substrate 2 both of which are made of transparent glass or other like material and arranged substantially parallel to each other with a predetermined spacing, for example 5 to 1 5 ,um, and sealed at the periphery with a sealant 3 such as glass frit or an organic adhesive, and a nematic liquid crystal 4 encapsulated therein. The predetermined spacing may be provided by placing a spacer 5 made of fiber glass, glass powder or such. The sealant 3 may be so designed as to double as spacer.

Electrodes 6 of a predetermined pattern are formed on the internal opposing sides of said first and second substrates 1 and 2, and the faces contacted with the liquid crystal are worked into liquid crystal controlling planes 7 and 8 where the liquid crystal molecules in the vicinity of these planes are oriented in a given direction. Such orientation controlling planes can be formed by coating the electrode-carrying side of each substrate with an oblique vacuum vaporization film of SiO or with an organic highmolecular film or a film of an inorganic material, and rubbing the coated surface in a given direction with cotton or other means.

The liquid crystal orientation controlling planes 7 and 8 of the first and second substrates 1 and 2 are differentiated in the liquid crystal orienting direction so that the molecules of the nematic liquid crystal 4 disposed between said both substrates 1 and 2 will be oriented twistedly from one direction (first direction on the controlling plane 7) to the other direction (second direction on the controlling plane 8). The angle made by the first and second directions, namely, the angle of twist of the liquid crystal molecules may be suitably selected, but usually such angle is defined to be about 900 as shown in Fig. 2.

A first polarizer 9 and a second polarizer 10 are disposed on the outside of the substrates 1 and 2, respectively. These two polarizers 9, 10 are usually arranged such that the angle made by their respective axes of polarization will be equal to the twist angle of the liquid crystal molecules (the angle made by said first and second orientation directions) or will be zero (in this case the respective axes of polarization are parallel to each other), and that the axis of polarization of each polarizer will be parallel to or cross at right angles with the liquid crystal orienting plane of the associated substrate.

Such display elements is widely utilized as a reflection type display element by providing a reflector 11 on the back side of the second polarizer 9 for effectuating normal display as seen from the first substrate side, or as a night-time display element by further incorporating a photoconductor made of an acrylic resin, glass or the like with a suitable thickness between the second polarizer 9 and reflector 11 and placing a light source on a suitable location on a side of said photoconductor.

Here, the operational principle of a reflection type liquid crystal display element arranged with 900 twist angle and 900 polarization axes crossing angle is described.

In case no electric field is present in the liquid crystal layer, the incoming light (ambient light incident upon the first polarizer 9 of the liquid crystal display element) is transmitted through the first polarizer 9 to become rectilinear polarized light running along the axis of polarization and enters the liquid crystal layer, but since the liquid crystal molecules are twisted 900 in said layer, the plane of polarization of the polarized light is optically rotated by 900 upon passage of the light through the liquid crystal layer, and then the polarized light passes the second polarizer 10. This polarized light is then reflected on the reflector 11 and transmitted reversely through the second polarizer 10, liquid crystal layer 4 and first polarizer 9 in that order and finally emanated out of the liquid crystal display element.

Thus, the observer can see the polarized light which has entered the liquid crystal display element and again comes out of said element after reflected by the reflector.

On the other hand, when a predetermined voltage is applied to a certain selected electrode 6 to give an electric field in a certain area of the liquid crystal layer in said display element, the liquid crystal molecules in that area are oriented in the direction of the electric field, and as a result, such area of the liquid crystal layer is deprived of its optical rotating capacity for the polarization plane and hence the plane of polarization in said area undergoes no optical rotation, so that the light polarized by the first polarizer 9 is intercepted by the second polarizer 10 and therefore that area looks dark to the observer.

It is thus possible to effect desired display by applying an electric voltage to a pertinent electrode.

It is said that most desirable ones of the liquid crystal materials (including compounds and compositions) used for field-effect type liquid crystal display elements mentioned above are those which meet the following three requirements: First requirement: good adaptability to the orientation controlling section.

Second requirement: operability over a wide temperature range.

Third requirement: good responsiveness over a wide temperature range, particularly at low temperatures.

Regarding the first requirement, it is of paramount importance for the structure of the display element to control the molecular arrangement such that the molecules of the liquid crystal compound 4 will be oriented parallel to each other and in one direction at the interfaces of the upper and lower plates which hold the molecules. Such control has been accomplished heretofore by forming a SiO film at the interface by obliquid vacuum vaporization, or by rubbing techniques.

As for the second requirement, it is the minimum requirement that the material is liquid crystal at around normal temperature (250C), but practically it is required that the material presents a liquid crystal condition in the temperature range of 0 C to about +6000 or higher.

In this invention, the transition temperature of a solid + a liquid crystal (or a smectic liquid crystal tut a nematic liquid crystal) is decided and defined according to the results of the following measurements. There are many occasions where the individual single liquid crystal compounds or mixed compositions thereof undergo over-cooling. In such a case, the compound (or composition) is cooled to a sufficiently low temperature (for example -400C) and then the transition temperature in the rising trend of temperature is measured by a micro melting-point measuring apparatus, and the thus measured temperature is given as transition temperature of the solid + liquid crystal compounds (or the smectic liquid crystal Hz the nematic liquid crystal).This second requirement is of great significance not only for ordinary static driving but also for driving by a so-called multiplexing drive system. The multiplexing drive system according to, for example, the voltage-averaging method is now predominantly employed in the liquid crystal display devices, particularly those requiring voluminous informations, such as for example table-type electronic computers or matrix displays. Low voltage driving is desirable for the table-type electronic computers or the like, and usually there is employed a 4.5 V driving system (using three 1.5 V cells) or 3 V driving system (using two 1.5 V cells) where the cells are connected in series to effect direct driving.This low voltage driving requires no boosting circuit since the cells are connected in series, and also the cell life can be prolonged to 500 to 2,000 hours by combination with C--MOS.

However, such multiplexing drive system is subject, in principle, to certain operational restrictions which are not seen in the static driving system. In the multiplexing drive display devices, it is required to prevent cross-talks in the picture element at each half-selection or non-selection point, and the voltageaveraging method is most popularly used for prevention of such cross-talks. This method was deviced for expanding the operational margin by averaging the cross-talk voltages to enlarge the difference from the selection voltage. This method is explained below by citing a typical case of application.

Described here is a case of application of the voltage-averaging method where the cross-talk voltages are averaged down to 1/3 of the selection voltage and the driving wave form is made alternating. The driving wave form of this system is shown in Fig. 3 in which Vx is selection voltage, Vy is signal voltage, and Vx-Vy is applied voltage. In Fig. 3, a voltage of +Vo is applied to the liquid crystal in the selected condition while a voltage of +(1/3)Vo is applied to the liquid crystal in the half-selected or non-selected condition. In this case, the effective voltage PsX applied to the display point (the point at which the liquid crystal is brought into a displaying condition) is given by the following formula:

wherein N: duty number.

On the other hand, effective voltage vS2 applied to the non-display point is given by: U52 = - Vo (2) 3 Here, in order to evolve a displaying mode at the display point, effective voltage U51 must be greater than or equal to threshold voltage Vth of the liquid crystal (Ps1 > = Vth), and in order to prevent cross-talks from being produced at the non-display point, effective voltage U52 must be smaller than or equal to Vth (v52 = < Vth). In other words, the following condition must be met for providing a cross-talk-free display according to this driving system:: vs2 # Vth # ws1 (3) Introducing the formulae (1) and (2) into the formula (3), Vo is defined as follows:

Measuring brightness at the display and non-display points by varying Vo, there are obtained the results such as shown in Fig. 8. At the display and non-display points exist the liquid crystal threshold voltages Vth, and Vth2 as converted to the Vo basis, and when the following condition is met: Vth1 # Vo # Vth2 (5) cross-talk-free display is made possible. From the formula (4), Vth1 and Vth2 can be given as floows:

To be more exact concerning the formula (5), the lower threshold value of voltage that allows display is not Vth, but should rather be the saturation voltage Vsat, shown in Fig. 8.In other words, the voltage range that allows cross-talk-free display is given by the following formula: Vsat1 # Vo # Vth2 (8) It may be said that the greater the range of fluctuation of Vo in the above-formula (8), the broader is the operational margin (M) of the display device. In the above-described derivation of formulae, v,, and v52 and hence Vth,, Vth2, and Vsati are all considered as constant, but these values are actually variable depending on the ambient temperature (T), viewing angles to the element (0 ) and other factors (Fig.

4). In the above explanations for the formula (1) through formula (8), viewing angle 0 defined in Fig. 4 is supposed to be 0, but actually such viewing angle may take a value within a limited range.

As viewed above, there are various factors that decide the operational margin (M). These factors are explained in due order hereinbelow, but it will be convenient for understanding such factors and the essence of the problem to give particular considerations to the following three essential elements: (i) variation of threshold voltage with change of temperature (ii) variation of threshold voltage with change of angle (iii) sharpness of voltage-brightness characteristic. The relation between (i)-(iii) and operational margin (M) will be clarified quantitatively by means of actual measurements.

The electro-optical characteristics of the multiplexing drive system are determined by the method illustrated in Fig. 5. A liquid crystal display element 51 is placed in a constant-temperature tank 53 with an inclination between 100 and 400 to the luminometer 52, and light is applied to said display element 51 through a heat-absorbing glass filter 55 from a tungsten lamp 54 disposed with an angle of 300 to the luminometer 52, and the brightness of said element 51 is measured by the luminometer 52.

The driving wave forms in the case of 1/3 bias, 1/3 duty and 1/2 bias, 1/2 duty multiplexing drive as measured by the above-said method are as shown in Figs. 6 and 7. Fig. 8 shows the voltage brightness characteristics as determined from these wave forms. In Fig. 8, region I is the area where the display is not lighted, and region II is the area where the display is lighted only at the selected segments Desired display of figures, letters, etc., can be made in the region II. Region III is the area where all the segments are lighted and no display function is performed, that is, cross-talk occurs.In the drawing, Vt is voltage at selected segment (ON mode) of 10% brightness, Vth2 is voltage at non-selected segment (OFF mode) of 10% brightness, Vsati is 50% brightness selected segment voltage, and Vsat2 is 50% brightness non-selected segment voltage.

The operational margin (M) is defined by the following formula:

wherein T = temperature ( C) 0--400C 0= viewing angle (0) 10--400 f = frequency (Hz) 100--550 Hz Therefore, "broad operational margin" is synonymous with "broad region II". Thus, the multiplexing drive system must be driven in a certain range of voltage margin.

Further analysis of the operational margin (M) given by the formula (9) shows that M is decided by the aforesaid three factors (i)-(iii), and these factors are quantitatively defined by the following formulae: (i) Temperature characteristic AT of Vth

The definition is made under the following conditions: T = 0400 C, (3=400, f=1 00 Hz.

(ii) Angle dependency A(3 of Vth; Vth2(# = 40 ) ## = (11) Vth2(# - 10 ) atT=400C,f= 100 Hz.

(iii) Sharpness y of voltage-brightness characteristic: Vsat1 # = (12) Although these three factors (i)-(iii) are the principal elements, usually frequency characterstic Af should be also taken into consideration as additional factor.

Vthi(f= 550) #@ - Vth1(f = 100) (@@) Af was defined under the conditions of T = 400C and (3=400.

The margin a of the voltage-averaging method is defined as follows for convenience of derivation of formulae: Vth2 (14) @@2 α = Vth1 Substituting the formulae (10)-(14) for the formula (9), operational margin M is given as:

1 = #T wherein A = 1 + #T Generally, #, ##, #T and #f may be defined as follows: y > 1 A(3 < 1 AT > 0andM < 1 The hereabove defined operational margin may vary over a wide range depending on the liquid crystal compound used, but it is noticed that the compound capable of giving a greater margin is suited for multiplexing drive.As apparent from the formula (1 5), it is required for enlarging the operational margin M to make temperature characteristic AT approach zero as much as possible and to make the angle dependency A(3, voltage-brightness sharpness and frequency characteristic Af approach as close to 1 as possible. In some cases, the effect of the temperature characteristic may be made almost ignorable in enlarging the operational margin by incorporating a temperature compensation circuit in the device. However, provision of such temperature compensation circuit necessarily leads to an elevated manufacturing cost of the device, so that it is desirable to employ the parts (elements) which can provide a wide operational margin with no extra condition such as provision of a compensation circuit, particularly in the case of the popularly used devices such as table-type electronic computers.

As for the third requirement, namely, good responsiveness over a wide temperature range, particularly at low temperatures, the following consideration will be instructive. Generally, responsiveness in the twisted nematic mode for multiplexing drive is given by the following formulae:

7}: viscosity K: elastic constant (refer to formula (43)" shown later) d: liquid crystal layer thickness It is noticed from the above formulae that liquid drystal responsiveness is decided mostly by viscosity of the liquid crystal material. It is said that these theoretical formulae well agree with actual measurements, and it is apparent to those skilled in the art that improvement of responsiveness can be attained by properly adjusting the viscosity of the liquid crystal compound used.

Thus, fulfillment of the third requirement depends on whether a liquid crystal compound having a low viscosity (and of course meeting the first and second requirements) can be found or not.

As for the third requirement, there has been studied to find out liquid crystal materials having a low viscosity as well as satisfying the first and second requirements mentioned above.

On one hand, various types of liquid crystal materials for display elements, particularly those of a multiplexing drive system, such as Schiff base type, ester type, biphenyl type, azoxy type, etc., have been proposed to date. The azoxy type liquid crystal materials have excellent temperature characteristics (small in AT), that is they are very limited in variation of threshold voltage with change of temperature and, as explained later, they provide an operational margin M of greater than 1 0% under the 1/3 bias and 1/3 duty multiplexing drive conditions. The azoxy liquid crystal materials are represented by the following general formula:

They possess per se weakly negative dielectric anisotropy and are usually used as a mixed system with a nematic liquid crystal compound having positive dielectric anisotropy (Np).But these azoxy type liquid crystal materials are colored (in yellow) as they absorb a part of visible light. Also, they show the maximum light absorption at 350 nm and undergo the following photochemical reaction owing to the wavelength around such level:

A non-liquid crystal compound is produced by such photochemical reaction and this new product changes the color of the liquid crystal from yellow into red. Usually, electric resistance of the liquid crystal is also sharply lowered. Therefore, in actual use of such azoxy type nematic liquid crystals, it needs to adapt a 500 nm cut filter in the device (element) so as to avoid photo-deterioration that might be caused by the sunlight or fluorescent light. This naturally complicates the mechanism of the device (element).

Other types of liquid crystals which are resistant to such photo-deterioration, such as Schiff base type, biphenyl type, ester type, etc., have been noticed for their availability as white display material and their adaptation to the display devices has been debated.

The biphenyl type liquid crystals are credited with high chemical stability as they are highly resistant to light, water and oxygen. However, most of the known biphenyl type materials which form liquid crystal at room temperature are the ones having positive dielectric anisotropy, and a few are known of the negative equivalents which are liquid crystal at room temperature and practically useful.

Therefore, there are only a few kinds of liquid crystal compounds which can form a mixed system with biphenyl type alone. Also, because of not so high value of positive dielectric anisotropy, wide range adjustment of threshold value is hardly possible with these materials, and further, such threshold voltage has high temperature dependency (large in AT), so that these materials are generally considered unsuited for multiplexing drive.

The ester type liquid crystal compounds have relatively good chemical stability and there are known many kinds of single liquid crystal compounds of positive or negative dielectric anisotropy.

However, threshold voltage of these compounds has relatively high temperature dependency and their viscosity is also considerably high, so that generally these compounds can hardly meet the afore-said second and third requirements.

The Schiff base type liquid crystal compounds have better properties than the ester type, but because of strong hydrolytic disposition, matching with the packing portion of the display element is often required for their use.

Individual liquid crystal materials are disclosed in, for example, U.S. Patent Nos. 4,137,192 and 4,147,651, Molecular Crystals and Liquid Crystals 22, 285-299 (1973), J. Org. Chem. 38, 3160--3164(1973), East German Patent No. 105,701, etc., but their special combinations are not known yet.

The present inventors have found that a system using a nematic liquid crystal with negative dielectric anisotropy (Nn type liquid crystal) as matrix and also incorporating a proper quantity of a nematic liquid crystal with positive dielectric anisotropy (Np type liquid crystal) and/or its analogue (a substance analogous to the positive nematic liquid crystal in molecular structure, hereinafter referred to as Np type liquid crystal analogue) can meet the second and third of the aforesaid three requirements and that such system can exist in the Schiff base type liquid crystal systems or cyclohexanecarboxylic acid-trans-4'-alkoxyphenyl ester type liquid crystal systems.

According to this invention, therefore, the said defects of the conventional liquid crystal materials are desirably eliminated, in other words, there is provided a liquid crystal composition which is capable of stable orientation in a wide temperature range, permits setting of the threshold voltage value over a wide range, is minimized in both temperature dependency and viewing angle dependency of such threshold value and also has high responsiveness.

The present invention provides a liquid crystal composition for display devices comprising at least one compound represented by the following general formula:

wherein R1 is n-CmH2m+t, n-C mH2m+1O or n-CmH2m+140, R2 is n-CqH2q+t, n-CqH2q+tO or n-C@H2+1-CO, and m and q are each an integer of 1 to 10, and at least one compound represented by the general formula::

wherein R3 is n-CrH2r+t or n-CrH2r+tO, R4 is n-CsH2s+t or n-C5H 25+1O, and r and s are each an integer of 1 to 1 0. In the above definitions, n is a symbol denoting that carbon is straight-chain, and this denotation applies throughout this specification. Also, the above-shown compounds, linkage of cyclohexane ring carbon to carboxyl group carbon and that of cyclohexane ring carbon to benzene group are supposed to be an equatorial bond.

The present invention also provides a liquid crystal composition best suited for multiplexing drive by combining said matric mixture system and a nematic liquid crystal with positive dielectric anisotropy (Np) and its analogue.

This invention further provides a liquid crystal composition capable of improving the various properties required for multiplexing drive by adding an Nn type liquid crystal and its analogue to the mixture system consisting of an Nn type mixture system comprising the compounds of the aboveshown formulae (18) and (19) and an Np type liquid crystal obtained from the above-said researches.

In the accompanying drawings, Fig. 1 is a sectional view showing an example of liquid crystal display element; Fig. 2 is a structural diagram showing a pattern of orientation of the liquid crystal molecules; Fig. 3 is a diagram exempiifying the multiplexing drive waveforms according to the voltageaveraging method (1/3 bias); Fig. 4 is a drawing illustrating the definition of viewing angle; Fig. 5 is a schematic illustration of an electro-optical properties measuring device; Fig. 6 shows the 1/3 bias and 1/3 duty driving waveforms; Fig. 7 shows the 1/2 bias and 1/2 duty driving waveforms; Fig. 7 is a graphic representation of the brightness-voltage characteristics in multiplexing drive; Fig. 9 is a characteristic diagram showing the relationship between Np/Nn mixing ratio and Vth; and Figs. 10 to 13 are characteristic diagrams of viscosity as observed when the compounds of the general formula:

were added to various matrix liquid crystals.

The invention is described in further detail hereinbelow while showing the preferred embodiments thereof.

Regarding first the compositions constituted by the respective liquid crystal compounds having the chemical structure shown by the formula 8), the most preferred constituent compounds are those of the formula:

wherein m and q are each an integer of 1 to 10, and the preferred combinations of (m and q) in the above formula (20) are: (3 and 5), (4 and 5), (5 and 5), (6 and 5), (4 and 6), (3 and 1), (3 and 2), (3 and 3), (3 and 4), (3 and 9), (4 and 1), (4 and 2), (4 and 3), (4 and 4), (4 and 6), (4 and 8), (5 and 1), (5 and 2), (5 and 3), (5 and 4), (5 and 6), and (5 and 7). In the case of the compounds of the formula:

wherein m and q are each an integer of 1 to 10, the (m and q) combinations of (5 and 2), (5 and 3) and (5 and 5) are preferred.As for the compounds of the formula:

wherein m and q are each an integer of 1 to 10, the combinations of (3 and 4), (4 and 4), (4 and 1), (5 and 4) and (5 and 9) are preferred, and for the compounds of the formula:

wherein m and q are each an integer of 1 to 10, the combinations of (5 and 3) and (5 and 5) are recommended.As regards the compounds represented by the formula (19), the following types are most preferred: (R3 = C2H5 and R4 = CH3), (R3 = C2H5 and R4 = n-C4Hg), (R3 = n-C3H7 and R4 = n-C5H,t), (R3 = n-C4Hg and R4 = n-C4H9), (R3 = n-C5H11 and R4 = n-C5H11), (R3 = n-C7H15 and R4 = n-C5H11), (R3 = CH3 and R4 = C2H50), (R3 = CH3 and R4 = n-C8H,70), (R3 = C2H and R4 = CH3O), (R3 = n-C4Hg and R4 = CH3O), (R3 = n-C4Hg and R4 = C2H50), (R3 = n-C4Hg and R4 = n-C6H,30), (R3 = n-C5H and R4 = n-C8H,70), (R3 = n-C6H,3 and Ra = n-C6Ht30), (R3 = CH3O and R4 = C2Hs), (R3 = CH3O and R4 = n-C3H7), (R3 = n-C5H11O and R4 = n-C3H7), (R3 = CH3O and R4 = n-C4Hg), (R3 = C2H5O and R4 = n-C4Hg), (R3 = n-CtOH2tO and R4 = n-C4Hg), (R3 = n-C4Hg and R4 = n-C5H11), (R3 = n-CsHt,O and R4 = n-C5H") (R3 = CH3O and R4 = n-C8H,7).

In the mixed system of at least one compound of the formula (1 8) and at least one compound of the formula (19), it is desirable that the respective compounds of the formulae (18) and (19) have per se a wide mesomorphic range (MR) so as to be a matrix system satisfying not only the first requirement but also the second and third requirements as well.

Table 1 below shows mesomorphic ranges (MR) of principal Nn type 4-n-alkylcyclohexanecarboxylic acid-trans-4'-alkoxyphenyl esters.

TABLE 1

(4-n-alkyl-cyclohexane R1 4 COO Q OR2 carboxylic acid-trans 4'-alkoxyphenyl esters) Symbol R1 R2 MR (OC) A C3H7 C5Htt 37 to 67 B C4H9 C5Htt 26 to 67 C C5Htt C5H11 31 to 77 D C6H13. C5Htt 44 to 52 E C4H9 C6Ht3 25 to 69 F C3H7 CH3 55 to 64 G C3H7 C3H, C7 to 65 H CAH9 CH3 42 to 61 C4H9 C2Hs 36 to 74 J C4H9 C6H,3 26 to 70 K C5H,t C2H5 56 to 86 L C5Htt 04H9 48 to 80 Proper mixing of these compounds gives mixed systems having fairly wide MR as shown in Table 2 below.

TABLE 2 Nn liquid crystals (figures in No. Symbol parentheses show % by weight) MR ( ) 1 1-1 A(50) + C(50) 13 to 70 2 1-2 C(50) + E(50) 17 to 71 3 1-3 A(50) + E(50) 12 to 65 4 1-4 A(50) + C(25) + E(25) 9 to 69.5 5 1-5 A(33.3) + C(33.3) + E(33.3) 11 to 70 6 1-6 A(33.3) + B(33.3) + C(33.3) 15 to 69 7 1-7 D(50) + K(50) 13 to 81 8 1-8 B(50) + K(50) 15 to 78 9 1-9 B(50) + D(50) 21 to 69 10 1-10 A(50) + K(50) 21 to 77 11 1-11 C(50) + K(50) 15 to 81 Table 3 shows mesomorphic ranges (MR) of typical examples of the compounds represented by the general formula: (see formula (19)).

TABLE 3

3 +) < R4 Crystal-smectic transition Symbol R3 R4 temp. (OC) MR (OC) 3-1 n-C5H11- C2H5- 143 146 to 163 3-2 n-C5H11- n-C4H9- 143 160 to 168 Table 4 shows the mesomorphic ranges (MR) of mixed systems of the compounds represented by the general formula:

and mixed systems of the compounds represented by the general formula:

TABLE 4 A B Symbol (wt%) (wt%) MR ( C) 4-1 100 0 4 to 62 4-2 95 5 -15to66 4-3 90 10 -13to71 4-4 85 15 -10to75 In the above table, A is composed of::

(25% by weight) 125% by weight) (25% by weight) and (25% by weight) and B is composed of:

As is clear from the above-shown tables, the upper limit of the mesomorphic range (MR) rises as a compound of the formula:

is added. Also, as shown in Table 3, some of the compounds of the formula:

have a smectic phase in a relatively high mesomorphic range, but when they are mixed in the matrix systems principally composed of the liquid crystals represented by the formula (1 8), the result is that either the smectic phase appears in a low temperature range alone or it does not appear at all.

In case these mixed system liquid crystals area used in the twisted nematic field-effect type liquid crystal display elements, it is essential that the mixed system liquid crystals used have positive dielectric anisotropy, that is, E Elt, ; (= ) iS positive.

It is very easy to impart positive dielectric anisotropy to said mixed liquid crystal systems by modifying the their quality. To be more definitely, although the mixed system of the liquid crystals of the formulae:

according to this invention has negative or very weakly positive dielectric anisotropy, such mode of dielectric anisotropy can be changed to positive by adding a relatively small quantity of one or more nematic liquid crystals having strongly positive dielectric anisotropy (Np) or an anlogue thereof without inducing any noticeable change in the desirable properties possessed by said mixed system such as, for example, wide mesomorphic range and low viscosity.

The present inventors have also discovered the substances of the following formulae as preferred Np type liquid crystals and/or analogues thereof for use in the mixed system of this invention:

In the above formulae (24) to (37), m is an integer of 1 to 1 0. Among the above-specified substances, those of the formulae (24) to (37) are preferred.

Favorable results are also obtained by adding the substances of the following formulae to the matrix system:

m: an integer of 1 to 8 m: an integer of 1 to 8 X: F, Br, Cl or I (halogen)

m: an integer of 1 to 10 X: F, Br, Cl or I (halogen)

m: an integer of 1 to 10 m: an integer of 1 to 10

In case of adding the substances of the above-shown formulae (24) to (42) or their optional mixed systems as the third component, the following general facts or rules will serve as the guiding principle for deciding the amount of such third component to be added.The amount ofthe Np type liquid crystal and/or its analogue to be mixed with the matrix Nn type liquid crystal is decided by the working threshold voltage required by the mixed system, but the relation between said amount to be mixed and working threshold voltage is determined substantially according to the following conceptions. The threshold voltage (Vth) of the twisted nematic liquid crystal element is given by the following formula:

wherein 4 is twist angle which is usually 7t/2, and K,, K22 and K33 are elastic constants of splay, twist and bend, respectively.The above formula (43) may be simplified as: wherein

K = K" + 4 (K33-2K22) (43)" It is possible, in principle, to obtain a liquid crystal with any desired value of AE by mixing the liquid crystals with different values of AE. Assuming here that dielectric constants of the two different kinds of liquid crystals A and B are ## A and ## , respectively, and their mixing ratio A/B = X/(1-X) and further supposing that additivity of the dielectric constants applies here, then #@ of the mixed system is given as follows:: AE = XAEA + (1-X)##B = X(AEA AEB) + AEB (44) Also assuming that same additivity applies for K, too, then K of the mixed liquid crystal is given by the following formula: KXKA+(1 X)KbX(KAKB)+KB (45) Substituting the formulae (44) and (45) for the formula (43)',

The threshold voltage may be calculated in the following way by giving the definite figures to the respective constants.

Assuming that AB of the Nn type liquid crystal is -0.3 and hEA of the Np type liquid crystal of the formula:

is 25 and further supposing KB = 4 x 10-7 dyne and KA = 1 7 x 10-17 dyne, then the formula (46) gives:

It will suggest itself to those skilled in the art that above assignment of figures to AEA, A'EB, KA and KB is not arbitrary but well conforms to the actual properties of the liquid crystals.

Fig. 9 shows the relationship between the mixing ratio and the value of Vth (static drive) when the Np and Nn type liquid crystals are mixed by using a substance of the formula:

as Np type liquid crystal and the matrix 1-4 shown in Table 2 as Nn type liquid crystal. The experimental results well agree with the product of the theoretical formula (calculation formula) (46) or (46)'.

However, the above-said combination alone, that is, the mere combination of the substances of the formulae:

and an Np type liquid crystal or its analogue, proves to be unsatisfactory regarding their mutual compatibility. It is therefore necessary to add an Nn type, particularly polar Nn type substance (a polar Nn type liquid crystal and/or its analogue) as the fourth component. The amount of such substance to be added may be properly selected in conformity to the amount of the Np type liquid crystal and/or its analogue. This will be exemplified by the embodiments shown later.

In order to enhance compatibility of the non-polar Nn type matrix liquid crystals with the polar Np type liquid crystals and/or analogues thereof and to also obtain a wide MR, it is recommended to add as the fourth component an Nn type liquid crystal system of the type other than the Nn type matrix mixed systems represented by the above-shown formula (18). The nematic liquid crystals or analogues thereof having electric polarity in their molecules and also having negative dielectric anisotropy are preferred for use as the fourth component since they are capable of providing said favorable properties, namely, increased compatibility of the mixed system and wide MR.Preferred examples of such Nn type liquid crystals and/or analogues thereof to be used as the fourth component are listed below:

In the foregoing formulae (47) to (59), m and q are each an integer of 1 to 10.

(60) R: CH2-O-CH2CH2 or CH3-O-(CH2)3-O

m: an integer of 1 to 9 R: (CH2)2-CH-O or (CH2)2-CH(CH2)2-O

m and q: each an integer of 1 to 10

R: CH3-O-CH2-O, CH3-O-(CH2)2-O, C2H5-O-(CH2)2-O, CH3-O-(CH2)3-O, C3H7-O-(CH2)2-O or C2H5-O-(CH2)3-O

In the foregoing formulae (64) to (74), m and q are each an integer of 1 to 10.

m: an integer of 1 to 10 q: an integer of 1 to 8

m: an integer of 1 to 12 q: an integer of 1 to 10

m, q: each an integer of 1 to 10

m: an integer of 1 to 18 q: an integer of 1 to 6

m, q: each an integer of 3 to 8

m, q: each an integer of 1 to 10 Shown here as an example using an Nn type liquid crystal is a mixed liquid crystal indicated by A in Table 4. This is a system prepared by adding a substance of the formula:

to a matrix liquid crystal of the formula (18), and this system has wide MR of from 40 to 62 C and is applicable to practical use. In comparison with this, MR of the system not added with an Nn type liquid crystal of the formula:

is in a higher temperature region, i.e., 13-70 C, as noted from Example 1 shown in Table 2.

Therefore, use of an Nn liquid crystal as the fourth component in an amount within the range of 240% by weight of the whole Nn liquid crystal system gives a favorable result. A particularly good result is obtained when said Nn liquid crystal as the fourth component is added in an amount of 30+10% by weight of the whole Nn liquid crystal system.

The examples indicated by 4-2, 4-3 and 4-4 in Table 4 are the systems formulated by adding the liquid crystal B composed of a compound of the formula:

in an amount of 5 to 15% by weight to an Nn type mixed liquid crystal A principally composed of a compound of the formula:

according to this invention. It will be seen that these systems have very wide MR and hence high practical utility. It was also found that the mixed system consisting of an Nn matrix liquid crystal given by removing the compound of the formula:

from A and a compound of the formula:

which constitutes the liquid crystal B, is inferior in MR to the mixed system of A and B.It was thus clarified that the compounds of the formula:

are well miscible with the Nn matrix liquid crystals mainly composed of the compounds of the formula (18) and give a wide MR.

Another significant fact concerning the effective properties derived from addition of a substance of the formula:

is that when this substance is added to an Nn matrix principally composed of a compound of the formula (18) or to such matrix system further added with an Np type liquid crystal, the whole system is appreciably suppressed in rise of viscosity.For example, when the system (B-i) formulated by adding a substance of the formula:

to the mixed system of a matrix liquid crystal C represented by the formula (18) and a phenylcyclohexane Np type mixed liquid crystal D was compared with the liquid crystal systems 5-2, 5-3 and 5-4 formulated by adding said substance to the other systems effective for expanding MR, i.e., the systems of the formulae:

respectively, the results showed that the rise of viscosity by 10% addition of the substance of the formula:

to the C + D mixed matrix system (having a viscosity of 23 CP at 250C) was on!y 2 CP as shown in - Table 5.Other addition systems caused a sharp rise of viscosity. TABLE 5

o E S z o o 1/3 bias, 1/3 duty In tr t Viscosity Mesomorphic Composition Margin voltage AT (ms) (ms (cp) range Symbol (Figures in parentheses are% by weight) M u) cM Oo y AO 250C 25C 25C MR(C) E C + D + C51111mffHHQQC2H5 9.32 3.38 7.51 1.13 < , 140 80 25 -11 to 65.6 o (10) 5-2 () t + c3H7CO0C3H7 6.94 3.24 8.80 1.13 0.830 COD 90 28 3 3 (Ii5)(5) (10) 5-3 C O D o CH9COOCOOCH9 6.80 3.09 9.15 1.16 0.850 160 105 o e to 66.0 (II5)(5) Cr D g C + D + cH9ffmHQC0OCN 5.56 3.06 8.35 1.16 0.833 160 110 33 -18 to 70.5 5 (10) In the above cs o C C5H11C0OOC5H11(LiO ,: g D C3H7mHHOCN tt wt%), s a) wt%) and C5H11MmHQCN p wt%) and n wt%). C7H1CN (30 wt%).

g(al C)- or ow W X O ro co u) Generally speaking, to obtain a mixed system which is capable of expanding the mesomorphic range (MR) while suppressing the rise of viscosity (71) means to obtain a liquid crystal system for multiplexing drive with excellent responsiveness and high working margin (M), and usually it is possible to reduce the temperature dependency (AT, see formula (10)) of the threshold value of MR by brinding the upper limit of MR to a higher temperature region in a mixed system. Thus, as apparent from the comparison with other addition systems 5-2, 5-3 and 5-4, the system 5-1 added with a substance of the formula:

is excellent in both responsiveness and working margin (M) as seen in Table 5.It is to be noted that the factor which causes rise of viscosity when adding a compound of the formula:

to other matrix liquid crystals does not originate in the intrinsic properties of the compound of said formula. As a result of minute studies on viscosity of the whole systems in case of adding a compound of the formula:

to other various mixed liquid crystal systems such as, for example, a biphenyl type liquid crystal (trade name:

50.5% by weight 27.5% by weight 14.6% by weight 7.4% by weight a phenylcyclohexane type liquid crystal (trade name, ZL@-1083):

36.6% by weight 36.4% by weight 27.0% by weight a mixed system of an Nn type ester and biphenyl (trade name, SP-21)::

1 5% by weight 2% by weight 10% by weight 10% by weight 30% by weight 15% by weight etc., the fact was disclosed that it is not that addition of these mixed liquid crystal systems does not cause rise.of viscosity for any matrix; More definitely, when the compound of the formula:

among the compounds represented by the general formula:

was added to a matrix liquid crystal of the following composition:

(20% by weight) (20% by weight) (30% by weight), (30% by weight) which constitutes the principal component of the liquid crystal mixture of this invention and which is represented by the formula (1 8), and to a mixed liquid crystal system of the following composition::

(40.8% by weight) (34.9% by weight) (23.3% by weight) which is principally composed of a phenylcyclohexane type liquid crystal, the resulting rise of viscosity at 250C was very slight in both cases as shown in Fig. 10 and Fig. 11, respectively. On the other hand, it was clarified that when the said compound of the formula:

is added to the biphenyl type liquid crystal (trade name: E-7) and to the mixed system of an Nn type ester and biphenyl (trade name: SP-21 ), there takes place a sharp rise of viscosity at 250C as shown in Fig. 1 2 and Fig. 13, respectively. Said mixed liquid crystal system of an Nn type ester and biphenyl has the following composition:

(22.2% by weight) (16.7% by weight) (1 % by weight)

(50% by weight) As described above, the nematic liquid crystal compositions for display devices according to this invention have high margin and fast responsiveness, so that they are best suited for use as material for the multiplexing drive systems. Also, the liquid crystal compositions of this invention are chemically stable and have high reliability as a liquid crystal material, so that they can find best application for the liquid crystal display devices.

Claims

1. A nematic liquid crystal composition for display devices comprising at least one compound of the formula:

wherein R1 is n-CmH2m+1, n-CmH2m+1-O or n-CmH2m+1-CO, R2 is n-CqH2q+1, n-CqH2q+1-O or n- CqH2q+1-CO, and m and q are each an integer of 1 to 10, and at least one compound of the formula:

wherein R3 is n-CrH2r+1 or n-CrH2r+1-O, R4 is n-CsH2s+1 or n-CsH2s+1-O, and r and s are each an integer of 1 to 10.

2. A nematic liquid crystal composition for display devices comprising a mixed liquid crystal system of at least one compound of the formula:

wherein R1 is n-CmH2m+1, n-CmH2m+1-O or n-CmH2m+1-CO, R2 is n-CqH2q*1, n-CqH2q+1-O or n- CqH2q+1-CO, and m and q are each an integer of 1 to 10, and at least one compound of the formula:

wherein R3 is n-CrH2r+1 or n-CrH2r+1-O, R4 is n-CsH2s+1-O,and r and s are each an integer of 1 to 10, said mixed liquid crystal system being further mixed with at least on nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound in an amount of 2% by weight or more.

3. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+1; and m is an integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

4. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+i-O; and m is an integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

5. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+1 or n-CmH2m1-O; and m is an integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

6. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

7. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+1 n-CmH2m+1O, n-CmH2m+i-CO or n-CmH2m+1-COO; and m is an integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

8. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+1; and m is an.integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

9. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+1, n-CmH2m+O, n-CmH2m+i-CO or n-CmH2m+i-COO; and m is an integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

10. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one of the compounds of the formula:

wherein R is n-CmH2m+,; and mis an integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

11. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+,; and m is an integer of 1 to 10, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

12. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

13. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+,; m is an integer of 1 to 8; and X is a halogen selected from F, Cl, Br and I, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

14. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

1 5. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+1; m is an integer of 1 to 8 and Xis a halogen selected from F, CI, Br and I, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

1 6. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

wherein R is n-CmH2m+1; and m is an integer of 1 to 8, is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

17. A nematic liquid crystal composition for display devices according to Claim 2, wherein at least one compound of the formula:

18. A nematic liquid crystal composition for display devices according to Claim 2, wherein a mixture comprising at least two of the compounds set forth in any of Claims 3 to 1 7 is used as said nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound.

19. A nematic liquid crystal composition for display devices according to any of Claims 2 to 18, containing at least one nematic liquid crystal having positive dielectric anisotropy and/or its analogous compound in an amount of 2 to 50% by weight.

20. A nematic liquid crystal composition for display devices according to any one of claims 2-1 9 additionally comprising at least one nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

21. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R1 is n-Cm H2m+i, n-CmH2m+1-O, n-CmH2m+i-CO, n-CmH2m+1-COO or n-Cm H 2m+1-O-COO; R2 is n-CqH2q+1, n-CqH2q+1-O, n-CqH2q+1-CO, n-CqH2q+1-COO or n-CqH2q+1-O-COO; and m and q are each an integer of 1 to 10, is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

22. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R1 is n-CmH2m+ or n-CmH2m+i-O; R2 is n-CqH2q+ or n-CqH2q+1-O; and m and q are each an integer of 1 to 10, is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

23. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R, is n-CmH2m+,, n-C mH 2m+1 -O or n-CmH2m+1-O-CO; R2 is n-CqH2q+, n-CqH2q+1-O, n CqH2q+1-CO, n-CqH2q+1-O-CO or n-CqH2q+1-O-COO; and m and q are each an integer of 1 to 10, is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

24. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R1 is n-CmH2m+i-O or n-CmH2m+1-COO; R2 is n-CqH2q+, or n-CqH2q+1-0; and m and q are each an integer of 1 to 10, is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

25. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R, is n-Cm H 2m+i' n-CmH2m+1-O or n-CmH2m+1-CO; R2 is n-CqH2q+1, n-C1H21+1-0,, n- C1H21+1-CO or n-CqH2q+1-COO; and m and q are each an integer of 1 to 10, is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

26. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R1 is n-CmH2m+i-O; R2 is n-CqH2q+1; and m and q are each an integer of 1 to 10, is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

27. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R, is n-CmH2m+,; R2 is n-CqH2q+,; and m and q are each an integer of 3 to 8, is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

28. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least one compound of the formula:

wherein R, is n-CrH2r+, or n-CrH2r+i-0; R2 is n-CsH2s+ or n-C5H25+1-O; and rand s are each an integer of 1 to 10 is used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

29. A nematic liquid crystal composition for display devices according to Claim 20, wherein at least two of the substances set forth in Claims 21 to 28 are used as said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound.

30. A nematic liquid crystal composition for display devices according to any of Claims 20 to 29, wherein the ratio of said nematic liquid crystal having negative dielectric anisotropy and/or its analogous compound to the whole composition is 50% by weight or less.

31. A composition according to any one of claims 20-30 wherein the nematic liquid crystal having negative dielectric anisotropy is at least one of the compounds 47-80 hereinbefore specifically mentioned for this purpose.

32. A composition according to any one of claims 2-31 wherein the nematic liquid crystal having positive dielectric anisotropy is at least one of the compounds 24-42 hereinbefore specifically mentioned for this purpose.

33. A composition according to any one of the preceding claims wherein the compound of formula

is one of those hereinbefore specifically mentioned.

34. A composition according to any one of the preceding claims wherein the compound of formula

is one of those hereinbefore specifically mentioned.

35. A composition according to claim 1 substantially as hereinbefore described.

36. A composition according to claim 2 substantially as hereinbefore described.

37. A composition according to claim 20 substantially as hereinbefore described.