Compounds and liquid-crystalline medium

A liquid-crystalline medium comprising:

• a) one or more compounds of formula I
• in which
• n denotes 2,
• m denotes (4−n),
• denotes an organic radical having 4 bonding sites,
• Z11 and Z12, independently of one another, denote —O—, —(C═O)—, —(N—R14)— or a single bond, but do not both simultaneously denote —O—,
• r and s, independently of one another, denote 0 or 1,
• Y11 to Y14 each, independently of one another, denote alkyl having 1 to 4 C atoms, or alternatively, independently from each other, the two pairs (Y11 and Y12) and (Y13 and Y14) may be connected by a bond to form together a divalent group having 3 to 6 C-atoms
• R11 denotes O•,
• R12 on each occurrence, independently of one another, denotes H, F, OR14, NR14R15, a straight-chain or branched alkyl chain having 1-20 C atoms, in which one —CH2— group or a plurality of —CH2— groups may each be replaced by —O— or —C(═O)—, but two adjacent —CH2— groups cannot be replaced by —O—, or denotes a hydrocarbon radical which contains a cycloalkyl or alkylcycloalkyl unit, and in which one —CH2— group or a plurality of —CH2— groups may each be replaced by —O— or —C(═O)—, but two adjacent —CH2— groups cannot be replaced by —O—, and in which one H atom or a plurality of H atoms may each be replaced by OR14, N(R14)(R15) or R16, or denotes an aromatic or heteroaromatic hydrocarbon radical, in which one H atom or a plurality of H atoms may each be replaced by OR14, N(R14)(R15) or R16,
• R14 on each occurrence, independently of one another, denotes a straight-chain or branched alkyl or acyl group having 1 to 10 C atoms or an aromatic hydrocarbon or carboxyl radical having 6-12 C atoms,
• R15 on each occurrence, independently of one another, denotes a straight-chain or branched alkyl or acyl group having 1 to 10 C atoms or an aromatic hydrocarbon or carboxyl radical having 6-12 C atoms,
• R16 on each occurrence, independently of one another, denotes a straight-chain or branched alkyl group having 1 to 10 C atoms, in which one —CH2— group or a plurality of —CH2— groups may each be replaced by —O— or —C(═O)—, but two adjacent —CH2— groups cannot be replaced by —O—;
• and
• b) one or more compounds of formula II
• in which
• R21 denotes an unsubstituted alkyl radical having 1 to 7 C atoms or an unsubstituted alkenyl radical having 2 to 7 C atoms, and
• R22 denotes an unsubstituted alkenyl radical having 2 to 7 C atoms, and optionally
• c) one or more compounds selected from formulae III-1 to III-4,
• in which
• R31 denotes an unsubstituted alkyl radical having 1 to 7 C atoms,
• R32 denotes an unsubstituted alkyl radical having 1 to 7 C atoms or an unsubstituted alkoxy radical having 1 to 6 C atoms, and
• m, n and o each, independently of one another, denote 0 or 1.