Liquid-crystalline medium

A liquid-crystalline medium comprising a mixture of polar compounds having positive dielectric anisotropy, wherein said mixture comprises one or more compounds of formula I,

• in which
• R is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another by —O—
• —CO—, —CO—O—, —O—CO—, —S— or —O—CO—O— in such a way that O atoms are not linked directly to one another,
• A1 (a) is a trans-1,4-cyclohexylene radical in which one CH2 group is replaced by —O— or —S—, where the radical (a) may be monosubstituted or polysubstituted by CN, CH3 or F,
• Z1 is —CO—O—, —O—CO—, —CH2—O—, —OCH2—, —CH2CH2—, —CF2O—, —OCF2—, —CH═CH—, —C≡C—, —(CH2)4—, —CH═CH—CH2CH2— or a single bond,
• Y is F, Cl, halogenated alkyl having 1 to 6 carbon atoms, halogenated alkenyl having up to 6 carbon atoms, or halogenated alkoxy having 1 to 6 carbon atoms,
• L is H or F, and
• m is 1.