Thiophene compounds for liquid-crystalline media

Compounds of the formula I

• in which
• R1 and R2 each, independently of one another, denote H, a halogenated or unsubstituted alkyl radical having 1 to 15 C atoms, where, in addition, one or more CH2 groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CH═CH—, —(CO)O—, —O(CO)—, —(CO)— or —O— in such a way that O atoms are not linked directly to one another, where R2 additionally denotes F, Cl, Br, CN, SCN, NCS or SF5,
• A1, A2, A3 and A4 each, independently of one another, identically or differently, denote:
• a) trans-1,4-cyclohexylene or cyclohexenylene, in which, in addition, one or more non-adjacent CH2 groups may be replaced by —O— and/or —S—, and in which H may be substituted by F,
• b) 1,4-phenylene, in which one or two CH groups may be replaced by N, and in which, in addition, one or more H atoms may be replaced by Br, Cl, F, CN, methyl, methoxy or a mono- or polyfluorinated methyl or methoxy group, or
• c) a radical from the group 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, cyclobut-1,3-diyl, spiro[3.3]heptane-2,6-diyl,
• in which one or more hydrogen atoms may be replaced by F, CN, SCN, SF5, CH2F, CHF2, CF3, OCH2F, OCHF2 or OCF3, one or more CH groups may be replaced by N,
• one or more double bonds may be replaced by single bonds,
• M denotes —O—, —S—, —CH2—, —CHY— or —CYY1—, and
• Y and Y1 denote Cl, F, CN, OCF3 or CF3,
• V denotes H or F,
• Z1, Z2 and Z3 each, independently of one another, identically or differently, denote a single bond, —CH2O—, —(CO)O—, —CF2O—, —CH2CH2CF2O—, —CF2CF2—, —CH2CF2—, —CH2CH2—, —(CH2)4—, —CH═CH—, —CH═CF—, —CF═CF— or —C≡C—, where asymmetrical bridges may be oriented to both sides,
• a denotes 0, 1 or 2,
• b denotes 0, 1, 2 or 3, and
• c denotes 0, 1 or 2,
• where a+b+c≦4.