Piperidine derivatives and liquid-crystalline medium

A compound of formula I

• where
• Sp1 and Sp2 are each independently alkylene having 1 to 12 carbon atoms which is optionally mono or polysubstituted by F, Cl or CN, and in which one or more nonadjacent CH2 groups are optionally each independently replaced by —O—, —S—, —NH, —COO—, —OCO—, —OCO—O—, —CH═CH—or —C≡C—,
• A and B are each independently:
• a) trans-1,4-cyclohexylene, 1,4-cyclohexenylene or 1,4′-bicyclohexylene, in which one or two nonadjacent CH2 groups are optionally replaced by —O—and/or —S—and in which one or more hydrogen atoms are optionally replaced by F, or
• b) 1,4-phenylene or 1,3-phenylene, in which one or two nonadjacent CH groups are optionally replaced by N and in which one or more hydrogen atoms are optionally replaced by L, or
• c)
• in which one or more hydrogen atoms are optionally replaced by L, and/or one or more double bonds are optionally replaced by single bonds, and/or one or more CH groups are optionally replaced by N,
• i and j are each independently 0 or 1,
• Z0 is a single bond, —CH2—, —CF2—, —CO—, —O—, —NH—, —NH—(CO)—, —CH2CH2—, —CH═CH—, —CF2O—, —OCF2—, —CH2O—, —OCH2—, —COO—, —OCO—, —C2F4—or —CF═CF—,
• and
• L is the same or different at each instance and is F, Cl, CN or a straight-chain or branched, in each case optionally fluorinated alkyl, alkoxy, arylalkyl or alkylarylalkyl having 1 to 15 carbon atoms, in which one or more nonadjacent CH2 groups are optionally replaced by —O—and/or —S—,
• with the proviso that the following compound is not included