1. Field of the Invention
The present invention relates to a novel light-reaction type optically active compound, a light-reaction type chiral agent which changes the helical structure of a liquid crystal, a liquid crystal composition, an optical film, a liquid crystal color filter, a recording medium, and a method of changing the twist structure of helices of liquid crystals.
2. Description of the Related Art
Recently, liquid crystal materials such as cholesteric liquid crystals and the like having a helical structure and showing various selective reflected colors by twisting power (twist angle) of the helix have become prominent. Further, due to excellent selective reflecting property and excellent color purity of selective reflected light, the materials have become widely used in optical films, liquid crystal color filters, recording media and the like.
For example, a color filter used in a color liquid crystal display or the like is generally constituted of picture elements of red (R), green (G) and blue (B), and black matrices formed for the purpose of improving display contrast in gaps between the elements. Main types of such a color filter are conventionally those obtained by dispersing a pigment in a resin, or dyeing a resin with a dye. As a production method thereof, methods are usual in which a coloring resin solution is applied on a glass substrate by spin coating or the like to form a coloring resist layer, and patterning by a photolithography method is conducted to form color filter picture elements, or coloring picture elements are directly printed on a substrate.
However, for example, a production method by printing has problems in that resolution of picture elements is low and formation of an image pattern of high accuracy is difficult, and a production method by spin coating has problems in that material loss is large and application unevenness when applying on a substrate having large area is significant. Further, in a production method by electrodeposition, a color filter having relatively high resolution and having small unevenness of a colored layer can be obtained, but the production process is complicated and management of liquids is difficult.
As described above, there is a great desire for a method which can produce a color filter having high quality at high efficiency with small material loss and in a simple manner.
On the other hand, as characteristics of a color filter, high-percentage transmission and color purity are required. Recently, improvements corresponding to the above-mentioned requirements have been attempted by optimizing a dyeing resin and kind of dye in a method using a dye, and by using a more finely dispersed pigment in a method using a pigment. However, in recent liquid crystal display (LCD) panels, requirements for the percent transmission and color purity of a color filter are extremely high. Particularly in a color filter for a reflection type LCD, compatibility between white display of paper white, contrast and color reproducibility is difficult, while color filters produced by dyeing a resin with a dye or dispersing a pigment in a resin according to conventional production methods are all color filters of light absorption type. Therefore, improvement of color purity by a further increase in percent transmission is almost at an upper limit.
Against the above-mentioned situations, a color filter utilizing polarization, having a cholesteric liquid crystal as a main component, is known. This color filter utilizing polarization shows high efficiency of utilizing light and has more remarkable abilities also in percent transmission and color purity than a color filter of light absorption type, since a certain light amount is reflected and other light is transmitted to display images in the color filter utilizing polarization. On the other hand, as a method of producing the same filter, a method in which a film is formed on a substrate using a spin coat method or the like is usually conducted from the standpoint of uniform thickness. However, this method has a problem of large material loss and is disadvantageous in the point of cost.
As a means which can solve the above-mentioned problems, can secure uniformity of color purity and the like of a color filter film, and can also realize a reduction in the number of production steps, a method using a light-reaction type chiral compound is useful. This method uses the theory that when a liquid crystal composition containing a light-reaction type chiral compound is irradiated pattern-wise with light having a reaction wavelength of this chiral compound, the reaction of the chiral compound progresses depending on the strength of the irradiation energy, and helical pitch (twist angle of helix) of the liquid crystal compound changes. Consequently, a selective reflection color is formed for each picture element simply by pattern exposure including a difference between light amounts. Namely, there is a merit that patterning for forming a color filter can be completed by one-time mask exposure using a mask having different transmission light amounts.
Therefore, a film functioning as a color filter can be formed by effecting patterning by image-wise light irradiation, and then solidifying the patterned cholesteric liquid crystal compound. This can be applied also to films for optical use or recording of images, and the like.
Particularly when a color filter is produced by one-time mask exposure or the like, it is desired that three primary colors, B (blue), G (green) and R (red), can be formed with high color purity by one-time exposure. However, when a rate of change of twist of a liquid crystal is low, sufficient color purity is not obtained. Therefore, in view of displaying three primary colors having high color purity by one-time exposure, it is necessary to use a chiral compound (chiral agent) that is capable of changing twisting power of a helical structure of a liquid crystal compound and that has a high rate of change of twist as a light-reaction type chiral compound. Namely, a range of selectively reflected hues is enlarged due to change of light amount by using a chiral compound having a high rate of change of twist.
Further, Japanese Patent Application Laid-Open (JP-A) No. 11-248943 discloses use of a polymerizable mesogene compound having an isosorbide skeleton in a chiral mother nucleus as a chiral compound constituting a polarization plate of reflection type in a wide range. This polymerizable mesogene compound can act on a liquid crystal compound to cause change of the helical structure of the liquid crystal. However, when a reflection wavelength range of a polarization plate described in the above-mentioned publication is changed, namely, the helical structure (twisting power of helix) of a liquid crystal is changed so as to show the desired selective reflection, control of the change is carried out by a mixing ratio (of amounts used) of the mesogene compound to a non-chiral compound (liquid crystal), and light and light amount thereof are not correlated at all. That is, in this publication, there is utterly no suggestion that the helical structure (twisting power of helix) of a liquid crystal is changed by changing light amount, and there is also no suggestion that compounds exemplified in the publication are useful in the point of rate of change of twisting power in response to light.
As described above, a light-reaction type chiral agent as follows has not been proposed until now. This agent has light reactivity which can change orientation structures such as the helical pitch of a liquid crystal (twisting power, twist angle of helix) and the like by controlling light amounts used in irradiation. Further, in the case of a cholesteric liquid crystal phase containing a nematic liquid crystal compound, for example, the agent is able to provide significant changes of the helical pitch (twisting power), such as a wide range of selectively reflectable wavelengths, various selective reflections and, particularly, display of three primary colors (R, G, B) at high color purity, and the like.
An object of the present invention is to solve the above-mentioned various conventional problems and to attain the following objects.
Namely, a first object of the present invention is to provide a light-reaction type optically active compound which has light sensitivity and can be isomerized by light to reveal significant structural change. A second object of the present invention is to provide a light-reaction type chiral agent which can control the orientation of a liquid crystalline compound and manifests high rate of change of twisting power (twist angle, helical pitch) of a liquid crystal (hereinafter, sometimes referred to as twisting change rate) due to light, and in the case, for example, of a cholesteric liquid crystal phase, which can give selective reflection in a wide range containing three primary colors (B, G, R) and can display the three primary colors with high color purities.
A third object of the present invention is to provide a liquid crystal composition which contains a light-reaction type optically active compound or light-reaction type chiral agent that can change twisting power of a liquid crystal by light, and that has high twisting change rate, and which can change optical properties by sterically significantly controlling the orientation condition of a liquid crystal molecule by light, and in the case of, for example, a cholesteric liquid crystal, which shows selective reflection colors in a wide range containing three primary colors due to irradiation with light, and further, can display the three primary colors with excellent color purity.
Further, a fourth object of the present invention is to provide a liquid crystal color filter which contains a light-reaction type optically active compound or light-reaction type chiral agent that can significantly change twisting power of a liquid crystal when irradiated with light and has high color purity. A fifth object of the present invention is to provide an optical film which contains a light-reaction type optically active compound or light-reaction type chiral agent that can significantly change twisting power of a liquid crystal by irradiation with light, and in the case of, for example, a cholesteric liquid crystal phase, which manifests a wider selective reflection range and has high color purity. A sixth object of the present invention is to provide a recording medium which contains a light-reaction type optically active compound or light-reaction type chiral agent that can significantly change twisting power of a liquid crystal when irradiated with light, and that can form a bright image by image-wisely changing light amounts, and when, for example, the liquid crystal phase is a cholesteric liquid crystal phase, which can form images composed of selective reflection colors manifesting a wide hue range and having high color purity.
Further, a seventh object of the present invention is to provide a method of changing twist structure of a liquid crystal in which the twisting power (twist angle) of a liquid crystal can be changed significantly by irradiating with light a liquid crystal composition containing a light-reaction type optically active compound or light-reaction type chiral agent that reveals high twisting change rate.
A first aspect of the present invention is a light-reaction type optically active compound of the following general formula (I): 
In the formula, R1 represents a hydrogen atom, an alkoxy group having 1 to 15 carbon atoms, an acryloyloxyalkyloxy group having 3 to 15 carbon atoms in total or a methacryloyloxyalkyloxy group having 4 to 15 carbon atoms in total.
A second aspect of the present invention is a light-reaction type chiral agent which is composed of a compound of the general formula (I) or (II), and changes the twisting power of a liquid crystal by irradiation with light. 
wherein, R1 and R2 represent a hydrogen atom, an alkoxy group having 1 to 15 carbon atoms, anacryloyloxyalkyloxy group having 3 to 15 carbon atoms in total or a methacryloyloxyalkyloxy group having 4 to 15 carbon atoms in total.
A third aspect of the present invention is a liquid crystal composition containing the above-mentioned light-reaction type optically active compound.
A fourth aspect of the present invention is a liquid crystal composition containing the above-mentioned light-reaction type chiral agent.
A fifth aspect of the present invention is a liquid crystal color filter containing the above-mentioned light-reaction type optically active compound.
A sixth aspect of the present invention is an optical film containing the above-mentioned light-reaction type optically active compound.
A seventh aspect of the present invention is a recording medium containing the above-mentioned light-reaction type optically active compound.
An eighth aspect of the present invention is a method of changing the twist structure of a liquid crystal including a step of irradiating the above-mentioned liquid crystal composition with light to cause a change of the twisting power of the liquid crystal.
A ninth aspect of the present invention is a liquid crystal color filter containing the above-mentioned light-reaction type chiral agent.
A tenth aspect of the present invention is an optical film containing the above-mentioned light-reaction type chiral agent.
An eleventh aspect of the present invention is a recording medium containing the above-mentioned light-reaction type chiral agent.