Liquid crystal display devices have come to be used in wide-ranging applications including consumer products such as liquid crystal televisions, cellular phones, and personal computers and industrial equipment. Such products have a relatively long life span of several years to 10 or more years. In order to allow normal operation of display devices for the life span, liquid crystal materials used for liquid crystal display devices are required to have high stability. A representative index of stability of liquid crystal materials is resistivity. In order to allow normal operation of liquid crystal display devices, liquid crystal materials used for the liquid crystal display devices need to have a sufficiently high resistivity and deterioration over time also needs to be suppressed.
There are other various requests for liquid crystal materials, for example, liquid crystal materials that allow higher response speed and higher contrast. Such properties cannot be achieved by a single compound and hence a plurality of liquid crystal compounds are mixed to provide liquid crystal compositions that meet the requests. Accordingly, in order to obtain liquid crystal materials having high stability, liquid crystal compounds constituting the liquid crystal materials need to have high stability.
The stability of liquid crystal compounds is effectively enhanced by increasing the purity through removal of impurities or removing water, organic ions, inorganic ions, and the like from liquid crystal materials. In general, this is performed by a process of bringing compounds into contact with silica gel or alumina or a process of purifying compounds by recrystallization (Patent Literatures 1 and 2).
However, among compounds constituting liquid crystal materials, compounds having —CF2O— as a linking group have low chemical stability and may undergo, for example, oxidation with oxygen, hydrolysis with water, or dissociation of fluorine atoms. This results in a decrease in the purity, which may also cause a decrease in the resistivity. For this reason, in spite of purification of compounds by the above-described processes or the like, the compounds undergo oxidation reaction or the like upon contact with oxygen, water, or the like during the purification treatment; and the resultant compounds do not satisfy the required purity or resistivity, which has been problematic. There has been a strong demand for a simple and practical method in which compounds having —CF2O— as a linking group are purified, without deterioration of the compounds, to thereby increase the purity and resistivity of the compounds. However, specific methods meeting the demand have not been reported yet.