JPS647116B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in liquid crystal display devices using nematic liquid crystals containing dichroic dyes and having negative dielectric anisotropy. For example, for a so-called positive type guest-host liquid crystal display device that is colored when an electric field is applied, a method using a dye whose transition moment direction is perpendicular to the molecular axis (Japanese Patent Application Laid-open No. 111847/1984), and a method to devise a panel structure. There are various methods, but the most commonly produced combination is a nematic liquid crystal with negative dielectric anisotropy and a dichroic dye with a transition moment parallel to the molecular axis. However, if only a nematic liquid crystal with negative dielectric anisotropy is used, the absolute value of the dielectric anisotropy is small, and a high driving voltage of 10 V or more is required. As a method of lowering the drive voltage, published patent publications, No. 67578, 1982, and 1983-
A method of incorporating an additive with a large negative dielectric anisotropy, as shown in No. 72156, has been considered.
These additives are required to have a large negative dielectric anisotropy, not to impair the responsiveness and liquid crystallinity of the liquid crystal to which they are added, and not to impair the stability of the liquid crystal to be added. At present, no material that completely satisfies these requirements has been obtained, but only a few types of materials that can be used are found in the above-mentioned published patent publications. Although efforts are being made to develop such excellent additive substances, the interface treatment of the liquid crystal to be added and the substrate must also be considered. An object of the present invention is to provide a liquid crystal display device that is stable and has sufficient responsiveness even at low voltages. In order to achieve the above-mentioned object, the present invention provides that the general formula is A novel compound (wherein R ₃ represents a straight-chain alkyl group having 4 or 5 carbon atoms, and R ₄ represents a straight-chain alkyl group having 5 or 7 carbon atoms when the general formula is Added to a phenylcyclohexyl _{carbonate liquid} crystal given by This liquid crystal composition uses a liquid crystal composition. The present invention will be described in detail below based on Examples. FIG. 1 shows the alignment state of the dichroic dye 3 and liquid crystal molecules 4 in a panel in which two opposing electrodes are subjected to vertical alignment treatment using a chromium complex or the like. In state a where no electric field is applied, both the liquid crystal molecules and the dichroic dye are aligned perpendicularly to the substrate. This state is a lightly colored OFF state. When voltage is applied to this panel, the liquid crystal has negative dielectric anisotropy, so
It is oriented parallel to the substrate as shown in FIG. 1b. This state is a colored ON state. FIG. 2 shows the relationship between the transmittance T and the applied voltage V when a voltage is applied to such a panel. The transmittance decreases with increasing voltage. The voltage when the transmittance changes up to 10% is V _th , 90%
If the voltage when the voltage changes to V sat is V _sat , it is desirable that the drive voltage at the time of ON is greater than V _sat . Conventional liquid crystals with negative dielectric anisotropy have a high V _sat , and even phenylcyclohexyl carbonate liquid crystals have a high V sat.
It is about 7V (gap between panels 8μ). Table 1 shows the composition of typical liquid crystal compositions based on the present invention.

【table】

[Table] The voltage-transmittance characteristics of the compositions in Table 1 are shown by solid lines in FIG. Compared to the phenylcyclohexyl carbonate liquid crystal shown by the broken line, V _th and V _sat
There is a significant decrease in Table 2 shows the melting point T _np and the transition temperature T _c at which the liquid crystal phase transitions to an isotropic liquid for the liquid crystal and compound contained in the liquid crystal composition.

【table】

[Table] Figure 3 This shows the changes in V _th and V _sat when the concentration of addition of is changed.
The liquid crystals to be added are phenylcyclohexyl carbonate liquid crystals shown in Table 3.

[Table] Figure 3 also shows cases where the number of carbon atoms in the alkyl group was changed. Almost the same characteristics as in the case of . Furthermore, the dielectric anisotropy of the additive alone extrapolated from the dielectric anisotropy when 10 wt% was added was about Δε=ε ₁₁ −ε⊥−15 for any number of carbon atoms. Table 4 shows the response speeds of the compositions shown in Table 1 at 20°C and 0°C. The applied voltage is as shown in Figure 2.
V _sat and panel gap are both 8μ. For comparison, the phenylcyclohexyl carbonate liquid crystal shown in Table 3 and the phenyltel liquid crystal for low voltage driving are shown. The liquid crystal composition used in the present invention is a phenylcyclohexyl carbonate-based liquid crystal composition, and although it cannot surpass the responsiveness of liquid crystals, it has a value that is sufficiently practical.

[Table] In addition, by adding an optically active substance and cholesteric liquid crystal so that pitch P and panel gap d<p, contrast can be improved, V _th ,
A reduction in V _sat is possible. FIG. 4 shows the decrease in V _th and V _sat when CB-15 manufactured by BDH Co., Ltd. was added as an optically active substance to the composition shown in Table 1.
The parameter in the figure is the weight percent of CB-15 added. The liquid crystal display device according to the present invention has the general formula: And the general formula is By using a liquid crystal composition whose main component is a _compound having a predetermined alkyl group represented by Although it was thought that it would be difficult to drive the device, it has been significantly improved, making it possible to drive at a low voltage, and the response speed is sufficient for normal use without significantly reducing the response speed. Since it can be used for many purposes, it is effective as a display device for wristwatches, calculators, etc. that require low voltage drive.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a panel and the alignment state of liquid crystal molecules and dichroic dyes within the panel. A is a state in which there is no electric field, and b is a state in which a sufficient voltage is applied. 1, 2...Glass substrate with vertically aligned transparent electrodes, 3...Dichroic dye molecules, 4...Liquid crystal molecules, 5...Incoming light, 6...Transmitted light. FIG. 2 is a diagram showing the relationship between applied voltage and transmittance. The solid line is for the composition in Table 1, and the broken line is for the composition in Table 3. Note that the temperature was 20°C. Figure 3 shows phenylcyclohexyl carbonate liquid crystal. A diagram showing changes in V _th and V _sat when adding . The vertical axis is voltage, and the horizontal axis is added weight %. FIG. 4 is a diagram showing changes in applied voltage-transmittance characteristics when CB-15 is added to the liquid crystal compositions shown in Table 1. Note that the arrow represents the voltage sweep direction.

Claims (1)

[Claims] 1. The general formula is (However, R ₁ represents a straight chain alkyl group having 3 to 5 carbon atoms, and R ₂ represents an alkyl group having 1 or 2 carbon atoms or 4
represents a straight-chain alkyl group. ) and the general formula is (However, R ₃ represents a straight chain alkyl group having 4 or 5 carbon atoms, and R ₄ represents a straight chain alkyl group having 5 or 7 carbon atoms.) A liquid crystal display device characterized by: