Representative examples of liquid crystal display cells are a field effect mode cell proposed by M. Schadt et al., APPLIED PHYSICS LETTERS, 18, 127-128 (1971), a dynamic scattering mode cell proposed by G. H. Heilmeier et al., PROCEEDING OF THE I.E.E.E., 56, 1162-1171 (1968), a guest-host mode cell proposed by G. H. Heilmeier, APPLIED PHYSICS LETTERS, 13, 91 (1968) and E. L. White et al., JOURNAL OF APPLIED PHYSICS, 45, 4718 (1974).
Of those liquid crystal cells, the field effect mode cell is particularly excellent and is widely used. The field effect mode cell requires various characteristics, and a quick response time and a sharp transmittance-voltage are the important characteristics required in order to produce highly multiplexed TN-LCD. It is known that a compound having a low viscosity represented by ##STR1## (hereinafter referred to as "Compound (a)" for simplicity) is useful as a nematic liquid crystal material for quick response time and a compound having a small K.sub.33 /K.sub.11 value (ratio of elastic constants of bent to spray) represented by ##STR2## (hereinafter referred to as "Compound (b)" for simplicity) is useful as a nematic liquid crystal material having a sharp transmittance-voltage characteristics. The excellent characteristics of those compounds, i.e., low viscosity and small K.sub.33 /K.sub.11 value, are mainly due to the skeletons ##STR3## of respective compound.
In general, the liquid crystal material used in liquid crystal display cells must have various characteristics, and it is the important characteristic for all display cells to have a nematic phase over a wide temperature range including room temperature. Many liquid crystal cells practically used having such characteristics are generally prepared by mixing at least one compound having the nematic phase in the vicinity of room temperature and at least one compound having the nematic phase at a temperature region higher than room temperature. Many of mixed liquid crystals practically used, for example, in the production of TN-LCD must have the nematic phase over a temperature range of from -30.degree. C. to 65.degree. C.
Therefore, a liquid crystal compound containing Compound (a) having excellent quick response time or Compound (b) having sharp transmittance-voltage characteristic is used in combination with a liquid crystal compound having three rings which has a high nematic phase-isotropic liquid phase transition point (hereinafter referred to as "N-I point" for simplicity) in order to elevate the N-I point of the mixed liquid crystal. The N-I point is elevated by mixing such compound having three rings, but excellent characteristics Compound (a) or Compound (b) inherently has tend to be deteriorated. Therefore, compounds having a high N-I point are demanded.
Further, in order to satisfy the requirement that the mixed crystal liquid must have the nematic phase over the temperature of from -30.degree. C. to 65.degree. C., a compound having a crystal phase-nematic phase transition point (hereinafter referred to as "C-N point" for simplicity) of about 100.degree. C. and the N-I point of about 200.degree. C., such as 4,4'-substituted terphenyl, 4,4'-substituted biphenylcyclohexane, or 4,4'-substituted benzoyloxybenzoic acid phenyl ester, is occasionally used as the compound having the nematic phase in the temperature range higher than room temperature.
However, in the case where those compounds are mixed in amounts sufficient to make the N-I point of the mixed liquid crystal at 65.degree. C. or more, the compounds exhibit undesirable properties to increase the viscosity of the mixed liquid crystal obtained, thereby decreasing the response time.