Liquid crystal composition and liquid crystal display element

A chiral nematic liquid crystal composition comprising:

• at least one optically active compound represented by general formula (I-a):
• wherein * indicates a position of an asymmetric carbon atom;
• R1 represents an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, a cyano group, or an isothiocyanate group, said alkyl group or said alkenyl group being either unsubstituted or substituted with at least one fluorine atom, chlorine atom, cyano group, methyl group or trifluoromethyl group, and at least one methylene group within said alkyl group or said alkenyl group may be substituted with either —CO— group, or with oxygen atom or —COO— group, provided oxygen atoms are not bonded together directly;
• A1 represents a 1,4-phenylene group, 1,4-cyclohexylene group, 1,4-cyclohexenylene group, tetrahydropyran-2,5-diyl group, 1,3-dioxane-2,5-diyl group, tetrahydrothiopyran-2,5-diyl group, 1,4-bicyclo[2,2,2]octylene group, decahydronapthalene-2,6-diyl group, pyridine-2,5-diyl group, pyrimidine-2,5-diyl group, pyrazine-2,5-diyl group, 1,2,3,4-tetrahydronaphthalene-2,6-diyl group, 2,6-naphthylene group, phenanthrene-2,7-diyl group, 9,10-dihydrophenanthrene-2,7-diyl group, 1,2,3,4,4a,9,10a-octahydrophenanthrene-2,7-diyl group, or fluorene-2,7-diyl group, and said 1,4-phenylene group, 1,2,3,4-tetrahydronaphthalene-2,6-diyl group, 2,6-naphthylene group, phenanthrene-2,7-diyl group, 9,10-dihydrophenanthrene-2,7-diyl group, 1,2,3,4,4a, 9,10a-octahydrophenanthrene-2,7-diyl group, or fluorene-2,7-diyl group is either unsubstituted or is substituted with at least one fluorine atom, chlorine atom, trifluoromethyl group, trifluoromethoxy group, or methyl group;
• Z1 represents a single bond, or —CO—, —COO—, —OCO—, —CH═N—, —N═CH—, —C≡C—, —CH2CH2—, —CH2CH2CH2—, —CH2CH2CH2CH2—, —CH2O—, —OCH2—, —CF2O—, —OCF2—, —CH═N—N═CH—, —CF═CF—, —CH═CH—, —CH2CH2CH═CH—, —CH═CHCH2CH2—, or —CH2CH═CHCH2—;
• and
• Y1 represents a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, a cyano group, or an isothiocyanate group, said alkyl group or said alkenyl group being either unsubstituted or substituted with at least one fluorine atom, chlorine atom, cyano group, methyl group or trifluoromethyl group, and at least one methylene group within said alkyl group or said alkenyl group may be substituted with either —CO— groups, or with oxygen atoms or —COO— groups provided oxygen atoms are not bonded together directly, and
• wherein the natural pitch induced upon addition of the compound represented by general formula (I-a) to a nematic liquid crystal decreases with increasing temperature;
• and
• at least one optically active compound represented by general formula (II-a):
• R2P1-L1SP2-L2-P3—R3 (II-a)
• wherein R2 and R3 each independently represent a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, a cyano group, a phenyl group, an alkyl group having 1 to 10 carbon atoms, or an alkenyl group having 2 to 10 carbon atoms, said alkyl group or said alkenyl group being either unsubstituted or substituted with at least one fluorine atom, chlorine atom, cyano group, methyl group or trifluoromethyl group, and at least one methylene group within said alkyl group or said alkenyl group may be substituted with either —CO— group, or with oxygen atom or —COO— group, provided oxygen atoms are not bonded together directly;
• The meaning of each of P1 and P2 independently is the same as that of said group A1 in general formula (I-a);
• The meaning of P3 is the same as that of said group A1 in general formula (I-a), or P3 represents a 1,3-phenylene group, and said 1,3-phenylene group is either unsubstituted or is substituted with at least one fluorine atom, chlorine atom, trifluoromethyl group, trifluoromethoxy group, or methyl group;
• L1 and L2 each independently represent a single bond, or —CO—, —COO—,—OCO—, —C≡C—, —CH2CH2—, —CH2CH2CH2—, —CH2CH2CH2CH2—, —CH2COO—, —CH2OCO—, —CH2O—, —OCH2—, —CF2O—, —OCF2—, —CH═N—N═CH—, —CF═CF—, —CH═CH—, —CH2CH2CH═CH—, —CH═CHCH2CH2—, —CH2CH═CHCH2—, or —COOCH2CH2OCO—, and a hydrogen atom of a C—H linkage in —CH2CH2—, —CH2CH2CH2—, —CH2CH2CH2CH2—, —CH2COO—, —CH2OCO—, —CH2O—, —CH2CH2CH═CH—, —CH═CHCH2CH2—, or —COOCH2CH2OCO— may be substituted with an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom in said alkyl group may be substituted with a fluorine atom, or a phenyl group;
• and
• s represents 0, 1, or 2, and in a case in which s=2, a plurality of P1 and L1 groups represent either identical groups or different groups, although at least one of R2, R3, L1 and L2 must be an optically active group, with an identical helical twisting direction to said compound represented by general formula (I-a), a helical twisting power;
• HTP in 1/μm, as represented by a formula shown below,
• HTP=1/(P×0.01C)
• wherein C represents an amount of added optically active compound in weight %, and P represents a natural pitch in μm of at least 3, and
• wherein the natural pitch induced upon addition of the compound represented by general formula (II-a) to a nematic liquid crystal increases with increasing temperature.