Compounds of the formula I: in which m and n are each, independently of one another, 0, 1 or 2, R 1 and R 2, independently of one another, denote an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH 2 groups in these radicals may each be replaced, independently of one another, by -C≡C-, -CF 2 O-, -OCF 2 -, -CH=CH-, -O-, -CO-O- or -O-CO- in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, A 1 and A 2, independently of one another, denote a radical selected from the following groups: a) 1,4-phenylene, in which, in addition, one or two CH groups may be replaced by N and in which, in addition, one or more H atoms may be replaced by a group L, b) the group consisting of trans-1,4-cyclohexylene and 1,4-cyclohexenylene, in which, in addition, one or more non-adjacent CH 2 groups may be replaced by -O- and/or -S- and in which, in addition, one or more H atoms may be replaced by F or Cl, and c) the group consisting of tetrahydropyran-2,5-diyl, 1,3-dioxane-2,5-diyl, tetrahydrofuran-2,5-diyl, cyclo-butane-1,3-diyl, piperidine-1,4-diyl, thiophene-2,5-diyl and selenophene-2,5-diyl, each of which may also be mono- or polysubstituted by a group L, L on each occurrence, independently, denotes F, Cl, CN, SCN, SF 5 or straight-chain or branched, in each case optionally fluorinated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 12 C atoms, and Z 1 and Z 2, independently of one another, denote a single bond, -CF 2 O-, -OCF 2 -, -CH 2 O-, -OCH 2 -, - O-, -O -, -(CH 2) 4 -, -CH 2 CH 2 -, -CF 2 -CF 2 -, -CF 2 -CH 2 -, -CH 2 -CF 2 -, -CH=CH-, -CF=CF-, -CF=CH-, -CH=CF-, -(CH 2) 3 O-, -O(CH 2) 3 -, -C≡C-, -O-, -CH 2 -, -(CH 2) 3 - or -CF 2 -, preferably a single bond. m and n are each, independently of one another, 0, 1 or 2, R 1 and R 2, independently of one another, denote an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH 2 groups in these radicals may each be replaced, independently of one another, by -C≡C-, -CF 2 O-, -OCF 2 -, -CH=CH-, -O-, -CO-O- or -O-CO- in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, A 1 and A 2, independently of one another, denote a radical selected from the following groups: a) 1,4-phenylene, in which, in addition, one or two CH groups may be replaced by N and in which, in addition, one or more H atoms may be replaced by a group L, b) the group consisting of trans-1,4-cyclohexylene and 1,4-cyclohexenylene, in which, in addition, one or more non-adjacent CH 2 groups may be replaced by -O- and/or -S- and in which, in addition, one or more H atoms may be replaced by F or Cl, and c) the group consisting of tetrahydropyran-2,5-diyl, 1,3-dioxane-2,5-diyl, tetrahydrofuran-2,5-diyl, cyclo-butane-1,3-diyl, piperidine-1,4-diyl, thiophene-2,5-diyl and selenophene-2,5-diyl, each of which may also be mono- or polysubstituted by a group L, a) 1,4-phenylene, in which, in addition, one or two CH groups may be replaced by N and in which, in addition, one or more H atoms may be replaced by a group L, b) the group consisting of trans-1,4-cyclohexylene and 1,4-cyclohexenylene, in which, in addition, one or more non-adjacent CH 2 groups may be replaced by -O- and/or -S- and in which, in addition, one or more H atoms may be replaced by F or Cl, and c) the group consisting of tetrahydropyran-2,5-diyl, 1,3-dioxane-2,5-diyl, tetrahydrofuran-2,5-diyl, cyclo-butane-1,3-diyl, piperidine-1,4-diyl, thiophene-2,5-diyl and selenophene-2,5-diyl, each of which may also be mono- or polysubstituted by a group L, L on each occurrence, independently, denotes F, Cl, CN, SCN, SF 5 or straight-chain or branched, in each case optionally fluorinated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 12 C atoms, and Z 1 and Z 2, independently of one another, denote a single bond, -CF 2 O-, -OCF 2 -, -CH 2 O-, -OCH 2 -, - O-, -O -, -(CH 2) 4 -, -CH 2 CH 2 -, -CF 2 -CF 2 -, -CF 2 -CH 2 -, -CH 2 -CF 2 -, -CH=CH-, -CF=CF-, -CF=CH-, -CH=CF-, -(CH 2) 3 O-, -O(CH 2) 3 -, -C≡C-, -O-, -CH 2 -, -(CH 2) 3 - or -CF 2 -, preferably a single bond.