TWI848928B - Cholesteric liquid crystal composition and liquid crystal display element - Google Patents

Abstract

The present invention provides a cholesteric liquid crystal composition having positive dielectric anisotropy and comprising a compound represented by the general formula (1-1), a compound represented by the general formula (1-2), a compound represented by the general formula (2-1) as a second component having neutral dielectric property, and a third component containing at least two optically active substances having different temperature dependencies of chiral pitch lengths, and a liquid crystal display device using the same.

Description

Cholesteric liquid crystal composition and liquid crystal display element

This application discloses a cholesteric liquid crystal composition useful as a display element material and a display device using the same.

Liquid crystal display elements using cholesteric (chiral nematic) liquid crystal compositions that impart twisted orientation to the liquid crystal phase by adding optically active compounds to the liquid crystal composition have bistability, and can therefore be used in electronic books, electronic labels, display panels, etc. that require particularly low power consumption. The bistability of cholesteric liquid crystal can be achieved by switching between the planar state and the focal conic state by applying a pulse voltage. Each state of the planar and focal conic can be maintained even after the pulse voltage, so the bistability is also called memory.

Liquid crystal display elements using cholesteric phase liquid crystals show a specific selective reflection wavelength that depends on the chiral pitch when the helical axis of the cholesteric phase is perpendicular to the substrate (planar state). The selective reflection wavelength (λ) is determined by the product of the average refractive index (n) of the liquid crystal composition and the chiral pitch (P). Therefore, when the refractive index of the liquid crystal composition is fixed, the desired selective reflection wavelength can be obtained by adjusting the concentration of the chiral compound and adjusting the chiral pitch P. At this time, the half-width of the selective reflection light (△λ) is proportional to the product of the refractive index anisotropy of the liquid crystal (△n=ne-no) and the chiral pitch, but in order to achieve a bright display, a certain degree of half-width is required, so a liquid crystal composition with a large △n must be used. In the above case, the average refractive index of the liquid crystal composition also increases, so in order to adjust to the desired λ, it is necessary to further add a chiral agent to shorten P, but if the amount of chiral compound added is excessively increased, the liquid crystal temperature range of the host liquid crystal will deteriorate and the viscosity will increase. That is, for the design of cholesteric phase liquid crystal, it is necessary to have a chiral compound that has a high liquid crystal twisting force that induces the chiral pitch and has a small effect on the host liquid crystal, and a host liquid crystal composition that has excellent compatibility with such a compound and a high △n is required. Furthermore, it is required to have a sufficiently large dielectric constant anisotropy (△ε) that can be driven at a low voltage as a liquid crystal display element, and good storage properties such as no precipitation in a low temperature environment. For example, Patent Document 1 discloses several cholesteric liquid crystal compositions to solve the above-mentioned problem.

The chiral pitch length of cholesteric liquid crystal depends on the type or concentration of chiral compounds added, and can be expressed by P=1/(β.c). Here, c is the concentration of chiral compounds in the cholesteric liquid crystal composition, and β is the force of the chiral compound to twist the liquid crystal composition, called helical twisting power (HTP), which can be calculated using the formula HTP=n/(λ×0.01×c). Here, n is the average refractive index of the liquid crystal composition. The HTP can be used to provide the desired pitch to the liquid crystal composition.

Chiral compounds generally reduce the temperature range of the liquid crystal phase and increase the viscosity. Furthermore, it is known that the chiral pitch length generated by adding chiral compounds has temperature dependence. For example, the compound represented by formula (Chiral-1), which is a chiral compound with asymmetric carbon, shows the following characteristics: when added to a normal liquid crystal composition, the pitch length increases as the temperature changes from room temperature to high temperature, and the change also varies greatly depending on the liquid crystal composition that serves as the host.

Due to the temperature dependence of the chiral pitch of the cholesteric phase liquid crystal, the cholesteric phase liquid crystal composition has the following disadvantages: due to the change of the ambient temperature when driven as an element, the selected reflection wavelength λ is shifted and the color changes. That is, considering the influence on the physical properties or stability of the main liquid crystal composition and the ambient temperature in which it is used, the chiral compound and the main liquid crystal composition must be selected in a way that does not cause the above disadvantages as much as possible. However, there is no specific main liquid crystal composition that satisfies the various characteristics required of the cholesteric phase liquid crystal composition, that is, large dielectric constant anisotropy, wide operating temperature range, stability at low temperature, desired selective reflection wavelength, high reliability against external light, etc., and can solve the above disadvantages at the same time.

[Prior art literature] [Patent Literature]

[Patent document 1] Japanese Patent Publication No. 2010-275463

The problem to be solved by the present invention is to provide a cholesteric liquid crystal composition, which has high dielectric constant anisotropy (△ε) and refractive index anisotropy (△n), a wide temperature range of cholesteric liquid crystal, stable at low temperatures, and high reliability to external stimuli such as heat or light, and small temperature changes in the selected reflection wavelength. In addition, a liquid crystal display element is provided, which uses such cholesteric liquid crystal, has small color changes with temperature, has sufficient brightness, a wide operating temperature range, low driving voltage and high reliability.

As a result of diligent research, the inventors and others found that the above-mentioned problem can be solved by the following cholesteric liquid crystal composition, thereby completing the invention of this application, wherein the cholesteric liquid crystal composition comprises: a first component with positive dielectricity, a second component with neutral dielectricity, and a third component containing at least two optically active substances with different temperature dependencies of chiral pitch lengths, and contains one or more compounds represented by general formula (1-1) as the first component, one or more compounds represented by general formula (1-2) as the first component, and one or more compounds represented by general formula (2-1) as the second component, and the cholesteric liquid crystal composition has positive dielectric constant anisotropy.

(R ¹ , R ² , R ³ and R ⁴ independently represent an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms or an alkenyl group having 2 to 7 carbon atoms, Ring A represents 1,4-cyclohexylene or 1,4-phenylene, Y ¹ to Y ⁵ independently represent a hydrogen atom or a fluorine atom, Y ⁶ to Y ¹¹ independently represent a hydrogen atom, a fluorine atom or a methyl group, Z ¹ represents a single bond, -COO- or -CH ₂ CH ₂ -, and a, b and c independently represent 0 or 1).

The present invention can provide a cholesteric liquid crystal composition having superior properties compared to existing cholesteric liquid crystals from the perspective of having multiple physical properties. That is, it can provide a cholesteric liquid crystal composition having superior physical properties such as temperature change of the selective reflection wavelength, dielectric constant anisotropy (△ε) and refractive index anisotropy (△n), cholesteric liquid crystal temperature range, low temperature stability, stability to external stimuli such as heat or light, etc. In addition, especially in the practical temperature range of 0°C to 50°C, the chiral pitch of the liquid crystal composition of the present invention hardly changes, and therefore the selective reflection wavelength depending on the chiral pitch also does not change. This makes it possible to provide a high-quality cholesteric liquid crystal display element that meets the various characteristics of the previously required cholesteric liquid crystal display element and whose color in the reflective state does not substantially change.

The present invention is described in detail below.

The so-called "positive dielectric property" of a liquid crystal compound means that the Δε of the compound is above +3.0, and the so-called "neutral dielectric property" means that the Δε of the compound is -1.5~+1.5. The Δε of each compound can be obtained by extrapolating the Δε value obtained when a certain amount of each compound is added to a certain liquid crystal composition. In addition, "%" in the present invention refers to mass % unless otherwise specified.

The liquid crystal composition of the present invention contains a compound with positive dielectric properties as the first component. The preferred content of the compounds represented by general formula (1-1) and general formula (1-2) is 40% to 80%, more preferably 45% to 75%, and particularly preferably 50% to 70%. The compound represented by general formula (1-1) has a large △ε and a moderate △n, and the compound represented by general formula (1-2) has a large △ε and a high △n. By using these together, the composition can be given very large △ε and △n while maintaining the liquid crystal temperature range or low temperature stability.

In addition, the compound represented by the general formula (1-3) may be added for the purpose of increasing △ε and △n.

( ^R7 represents an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms, or an alkenyl group having 2 to 7 carbon atoms, ^Y12 to ^Y23 independently represent a hydrogen atom or a fluorine atom, ^X1 represents a fluorine atom, a chlorine atom, -CN, _-NCS , _-CF3 or _-OCF3 , and ^Z3 and ^Z4 independently represent a _single bond, _-CH2CH2- , -COO-, -OCO-, -CH2O-, _{-OCH2- ,} -CF2O-, _-OCF2- or -C≡C-).

In the above case, the preferred content of the compound of general formula (1-3) is 8% or more, more preferably 12% or more. The preferred total content of the first component is 45% to 80%, more preferably 50% to 75%, and most preferably 55% to 65%. As compounds represented by general formula (1-1), formula (1-2) and formula (1-3), preferred examples are compounds represented by the following general formulas (1-1-a) to (1-1-d), general formulas (1-2-a) to (1-2-d) and general formulas (1-3-a) to (1-3-e).

[Chemistry 4]

[Chemistry 5]

(wherein, ^R1 has the same meaning as ^R1 in formula (1-1), ^R2 has the same meaning as ^R2 in formula (1-1), and ^R7 has the same meaning as ^R7 in formula (1-3))

The liquid crystal composition of the present invention contains a dielectrically neutral compound as the second component. The preferred content of the compound represented by the general formula (2-1) is 15% to 45%, more preferably 20% to 40%, and particularly preferably 25% to 35%. The compound represented by the general formula (2-1) can not only adjust the viscosity or upper limit temperature of the liquid crystal phase of the liquid crystal composition and give the composition a high △n, but also show extremely good solubility with the first component.

If a large amount of compounds in which Y ⁶ to Y ¹¹ in the general formula (2-1) are hydrogen atoms are used, the viscosity of the composition can be suppressed to a low level. If a compound in which at least one atom is substituted with a methyl group or a fluorine atom is used, the stability of the liquid crystal phase can be further improved.

The preferred content of the compound represented by general formula (2-1) is 40% to 80%, more preferably 45% to 75%, and particularly preferably 50% to 70%. The compound represented by general formula (2-1) has a large Δε and a moderate Δn, and the compound represented by general formula (1-2) has a large Δε and a high Δn. By using these together, the composition can be given very large Δε and Δn while maintaining the liquid crystal temperature range or low temperature stability.

Preferred examples of the compound represented by the general formula (2-1) are the following general formulas (2-1-a) to (2-1-e).

(wherein, ^R3 has the same meaning as ^R3 in formula (2-1), and ^R4 has the same meaning as ^R4 in formula (2-1))

In addition, the compound represented by the general formula (2-2) may be added as a second component for the purpose of reducing viscosity or expanding the liquid crystal temperature range.

( ^R5 and ^R6 independently represent an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms, or an alkenyl group having 2 to 7 carbon atoms, Ring B and Ring C independently represent a 1,4-cyclohexylene group or a 1,4-phenylene group in which the hydrogen atom may be substituted by a fluorine atom, ^Z2 represents a single bond, -COO-, _-CH2CH2- , _-CH =CH-, -FC=CF-, or -C=NN=C-, d represents 1, 2, or 3, and when there are a plurality of Ring B and ^Z2, they may be the same or different).

Preferred examples of compounds represented by general formula (2-2) are the following general formulas (2-2-a) to (2-2-k).

[Chemistry 8]

(wherein, R ⁵ has the same meaning as R ⁵ in formula (2-2), and R ⁶ has the same meaning as R ⁶ in formula (2-2))

More preferred examples of the compound represented by the general formula (2-2) are compounds represented by the formula (2-2-a), the formula (2-2-b), the formula (2-2-c) or the formula (2-2-d), and particularly preferred are compounds selected from the group of compounds in which R ⁵ is an alkenyl group in the formula (2-2-a) and/or the formula (2-2-d).

The preferred content of the compound represented by general formula (2-2) is 3% or more, more preferably 5% or more, and particularly preferably 7% or more.

The preferred total content of the second component is 18% to 52%, more preferably 25% to 45%, and particularly preferably 32% to 42%.

The preferred total content of the compounds represented by general formula (2-1) and general formula (2-2) is 18% to 52%, more preferably 25% to 45%, and particularly preferably 32% to 42%.

The liquid crystal composition of the present invention contains at least two optically active substances (chiral compounds) with different temperature dependencies of chiral pitch length as the third component. These compounds preferably have a positive chiral pitch temperature dependency on the one hand and a negative chiral pitch temperature dependency on the other hand at the usual temperature of the liquid crystal display element, i.e. preferably in the range of 0°C to 50°C. The total content of the third component varies depending on the target selected reflection wavelength, preferably less than 10%, more preferably less than 8%, and particularly preferably less than 6%. There is no particular restriction on the preferred lower limit, but in the case where the liquid crystal torsional force is significantly large, there is a situation where the amount of the third component added to the composition is trace, and the selected reflection wavelength or its temperature dependency deviates due to a slight deviation in the mixing ratio. It will also be affected by the weighing accuracy of the manufacturing equipment, but if the concentration of the third component is roughly 0.1% or more, the composition can be manufactured with high precision regardless of the manufacturing scale.

As the chiral compound, it is preferred to contain one or more compounds represented by the general formula (3).

[Chemistry 9]

( ^R8 and ^R9 independently represent an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms, or an alkenyl group having 2 to 7 carbon atoms, Ring D, Ring E, Ring F, and Ring G independently represent 1,4-cyclohexylene or 1,4-phenylene, ^Z5 and ^Z6 independently represent a single bond or -COO-, and e and f independently represent 0 or 1).

As the compound represented by the general formula (3), preferred examples are the compounds represented by the following formulas (3-1) to (3-4).

The compounds represented by formula (3-1) and formula (3-2) usually have a positive temperature dependence of the chiral pitch length. The compounds represented by formula (3-3) and formula (3-4) usually have a negative temperature dependence of the chiral pitch length. In addition, the compounds represented by formula (3-1) and formula (3-3) are dextrorotatory substances, and the compounds represented by formula (3-2) and formula (3-4) are levorotatory substances. The combination of preferred chiral compounds in the present invention is provided according to the following table. Dextrorotatory and levorotatory can be appropriately selected according to the required specifications of the display element. When adjacent to cholesteric phase liquid crystal regions adjusted to different selected reflection wavelengths, it is preferred to make the respective optical rotations opposite in advance for the purpose of avoiding color mixing.

The preferred physical property values of the cholesteric phase liquid crystal composition of the present invention are described. In addition, △n and △ε are the values of the main liquid crystal composition before adding the chiral compound.

The preferred range of △n is 0.10~0.40, more preferably 0.15~0.35, and particularly preferably 0.20~0.30.

The preferred range of △ε is 10~50, more preferably 15~45, and particularly preferably 20~40.

The preferred range of Tni is 60℃~120℃, more preferably 70℃~110℃, and particularly preferably 80℃~110℃.

The lower the flow viscosity, the better. There is no special lower limit. If it is below 100mPa.s, a sufficient response speed can be achieved in practical use.

The cholesteric liquid crystal composition of the present invention may further contain additives as needed. For example, ultraviolet (UV) absorbers or antioxidants, hindered amine light stabilizers (HALS), etc. may be added for the purpose of improving stability against external stimuli. The structure is not particularly limited, and various compounds commonly known to be added to liquid crystal compositions, such as benzotriazole-based UV absorbers or hindered phenol-based antioxidants, may be used. In addition, various polymerizable compounds for the purpose of fixing the cholesteric liquid crystal phase may also be included. In the above case, the polymerizable compound is preferably a substance that exhibits liquid crystal properties in the stage before curing. It can also be polymerized and cured by irradiating it with ultraviolet rays or heating it, and it can be made into a layer that has no fluidity and changes to a state where the orientation state does not change by external force.

[Implementation example]

The cholesteric phase liquid crystal described in Example 1 was prepared, and the temperature dependence of various physical properties and the selected reflection wavelength was measured.

The compositions of the following examples and comparative examples contain each compound in the proportions in the table, and the content is recorded in "mass %". The following abbreviations are used to record the compounds in the examples.

(Ring structure)

Unless otherwise specified, the trans isomer is used.

(Side chain structure and connection structure)

[Table 2]

(Physical property value)

Transparent point (℃): The temperature at which the composition transforms into the isotropic phase (Tni)

Melting point (℃): The temperature at which the composition returns from the solid phase to the cholesteric phase

△n: Refractive index anisotropy of the host liquid crystal composition at 25°C and 589nm

△ε: Dielectric anisotropy of the host liquid crystal composition at 25°C and 1kHz

η(mPa.s): Flow viscosity at 20°C after adding chiral agent

Vth(V): Threshold voltage in the twisted nematic (TN) unit of the host liquid crystal composition at 25°C

The compositions of Example 1 and Comparative Example 1 are shown.

The physical property values of each composition are shown below.

It can be seen that: compared with the cholesteric liquid crystal composition of Comparative Example 1, the cholesteric liquid crystal composition of Example 1 has a lower melting point (= a wider liquid crystal temperature range), a smaller viscosity (η), can be driven by a lower voltage, and furthermore, the temperature dependence of the cell selection reflection wavelength is small, and it is more suitable as a cholesteric material for display elements. Here, the cholesteric liquid crystal compositions of Example 1 and Comparative Example 1 were injected into a glass cell, the temperature was changed from 0°C to 50°C, and the color change was visually observed. As a result, the cell injected with the composition of Example 1 did not change, while the cell injected with the composition of Comparative Example 1 changed color as the temperature increased. Furthermore, the low-temperature storage stability of the compositions of Example 1 and Comparative Example 1 was compared. It was found that precipitation was observed in the composition of Comparative Example 1 after 48 hours at -20°C. In contrast, the composition of Example 1 still maintained a liquid crystal phase after 240 hours, and its storage stability was also excellent.

The following main liquid crystal composition A and main liquid crystal composition B were prepared in the same manner as in Example 1, and the physical property values were measured. In addition, 1.6 parts by mass of the compound represented by formula (3-1) and 2.0 parts by mass of the compound represented by formula (3-3) were added to 100 parts by mass of these main liquid crystal compositions as chiral compounds, and a cholesteric phase liquid crystal composition was prepared in such a way that the selective reflection wavelength at 25°C was about 540 nm.

Example 2 using main liquid crystal composition A and Example 3 using main liquid crystal composition B are similar to Example 1. They can obtain a cholesteric liquid crystal display element with a wide liquid crystal temperature range, low viscosity, low voltage drive, low temperature dependence of the selected reflection wavelength (below 1nm/℃), excellent low-temperature storage stability, and extremely small temperature change of color when the unit is made.

Claims (6)

A cholesteric liquid crystal composition comprises: a first component with positive dielectricity, a second component with neutral dielectricity, and a third component containing at least two optically active substances whose chiral pitch lengths have temperature dependencies different from each other, and contains one or more compounds represented by the general formula (1-1) as the first component, one or more compounds represented by the general formula (1-2) as the first component, and one or more compounds represented by the general formula (2-1) as the second component, and the cholesteric liquid crystal composition has a positive dielectric anisotropy, wherein the dielectric anisotropy (Δε) of the first component with positive dielectricity is greater than or equal to +3.0, and the dielectric anisotropy (Δε) of the second component with neutral dielectricity is in the range of -1.5 to +1.5, wherein based on Based on 100 mass% of the cholesteric liquid crystal composition, the content of the compounds represented by the general formula (1-1) and the general formula (1-2) is 50 mass% to 70 mass%, based on 100 mass% of the cholesteric liquid crystal composition, the content of the compound represented by the general formula (2-1) is 20 mass% to 40 mass%, based on 100 mass% of the cholesteric liquid crystal composition, and the content of the third component is 6 mass% or less, the compound represented by the general formula (1-1) includes one or more compounds represented by the general formula (1-1-a), and the compound represented by the general formula (1-1) includes one or more compounds represented by the general formula (1-1-b), and the compound represented by the general formula (1-2) includes one or more compounds represented by the general formula (1-2-c) The compound represented by the general formula (2-1) includes one or more compounds represented by the general formula (2-1-a), and the compound represented by the general formula (2-1) includes one or more compounds represented by the general formula (2-1-c),

R ¹ , R ² , R ³ and R ⁴ independently represent an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms or an alkenyl group having 2 to 7 carbon atoms, Ring A represents a 1,4-cyclohexylene group or a 1,4-phenylene group, Y ¹ to Y ⁵ independently represent a hydrogen atom or a fluorine atom, Y ⁶ to Y ¹¹ independently represent a hydrogen atom, a fluorine atom or a methyl group, Z ¹ represents a single bond, -COO- or -CH ₂ CH ₂ -, and a, b and c independently represent 0 or 1; the third component comprises a combination of compounds represented by the general formula (3-1) and the general formula (3-3), or a combination of compounds represented by the general formula (3-2) and the general formula (3-4),

The cholesteric liquid crystal composition as described in claim 1 further contains one or more compounds represented by the general formula (2-2) as a second component,

^R5 and ^R6 independently represent an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms, or an alkenyl group having 2 to 7 carbon atoms, Ring B and Ring C independently represent a 1,4-cyclohexylene group or a 1,4-phenylene group in which the hydrogen atom may be substituted by a fluorine atom, ^Z2 represents a single bond, -COO-, _-CH2CH2- , _-CH =CH-, -FC=CF-, or -C=NN=C-, d represents 1, 2, or 3, and when there are a plurality of Ring B and ^Z2 , they may be the same or different. The cholesteric liquid crystal composition as described in claim 1 or 2 further comprises one or more compounds represented by general formula (1-3) as a first component,

^R7 represents an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms, or an alkenyl group having 2 to 7 carbon atoms, ^Y12 to ^Y23 independently represent a hydrogen atom or a fluorine atom, ^X1 represents a fluorine atom, a chlorine atom, -CN, -NCS, _-CF3 , or _-OCF3 , and ^Z3 and ^Z4 independently represent a single bond, _-CH2CH2- , _-COO- , -OCO-, -CH2O-, -OCH2- _{, -CF2O-} , _{-OCF2- ,} or -C≡C-. The cholesteric liquid crystal composition as described in claim 1 or 2, wherein the compound represented by the general formula (1-2) contains one or more compounds selected from the group of compounds represented by the general formula (1-2-a) and the general formula (1-2-b),

^R2 represents the same meaning as ^R2 described in item 1 of the patent application. A cholesteric liquid crystal composition as described in item 1 or 2 of the patent application, wherein the temperature variation of the chiral pitch length in the range of 0°C to 50°C is above -1nm/°C and below +1nm/°C. A liquid crystal display element comprises a cholesteric liquid crystal composition as described in any one of items 1 to 5 of the patent application scope.