Medium comprising at least 60% of two or more compounds of formula I wherein R 11 and R 12 are each independently H, F, Cl, CN, NCS or a straight-chain or branched alkyl group with 1 to 25 C atoms, which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH 2 groups to be replaced, in each occurrence independently from one another, by -O-, -S-, -NH-, -N(CH 3)-, -CO-, -COO-, -OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -C≡C- in such a manner that oxygen atoms are not linked directly to one another, at least one of R 11 and R 12 is an alkyl group, i.e. a straight-chain or branched alkyl group with 1 to 25 C atoms, which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH 2 groups to be replaced, in each occurrence independently from one another, by -O-, -S-, -NH-, -N(CH 3)-, -CO-, -COO-, -OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -C≡C- in such a manner that oxygen atoms are not linked directly to one another, MG 11 and MG 12 are independently from one another a group of (partial) formula II -A 11 -(Z 11 -A 12) k - II wherein Z 11 are, independently of each other in each occurrence, a single bond, -COO-, -OCO-, -O-CO-O-, -OCH 2 -, -CH 2 O-, -OCF 2 -, -CF 2 O-, -CH 2 CH 2 -, -(CH 2) 4 -, -CF 2 CF 2 -, -CH=CH-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or -C≡C-, optionally substituted with one or more of F, S and/or Si, one or more of A 11 and A 12 are each independently in each occurrence 1,4-phenylene, wherein in addition one or more CH groups are optionally replaced by N, trans-1,4-cyclo-hexylene in which, in addition, one or two non-adjacent CH 2 groups are optionally replaced by O and/or S, 1,4-cyclohexenylene, 1,4-bicyclo-(2,2,2)-octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydro-naphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl, cyclobutane-1,3-diyl, spiro[3.3]heptane-2,6-diyl or dispiro[3.1.3.1] decane-2,8-diyl, it being possible for all these groups to be unsubstituted, mono-, di-, tri- or tetrasubstituted with F, Cl, CN or alkyl, alkoxy, alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H atoms are optionally substituted by F or Cl, and k is 0, 1, 2, 3 or 4, Sp 1 is -(CH 2) o -, o is 1, 3 or an integer from 5 to 15, and X 11 and X 12 are independently from one another a linking group selected from -CO-O-, -O-CO-, -O-, -CH=CH-, -C≡C-, -CF 2 -O-, -O-CF 2 -, -CF 2 -CF 2, -CH 2 -O-, -O-CH 2 -, -CO-S-, -S-CO-, -CS-S-, -S-, and a single bond, under the condition that in -X 11 -Sp 1 -X 12 - no two O atoms are adjacent to one another, no two -CH=CH- groups are adjacent to each other and no two groups selected from -O-CO-, -S-CO-, -O-COO-, -CO-S-, -CO-O- and -CH=CH- are adjacent to each other. R 11 and R 12 are each independently H, F, Cl, CN, NCS or a straight-chain or branched alkyl group with 1 to 25 C atoms, which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH 2 groups to be replaced, in each occurrence independently from one another, by -O-, -S-, -NH-, -N(CH 3)-, -CO-, -COO-, -OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -C≡C- in such a manner that oxygen atoms are not linked directly to one another, R 11 and R 12 is an alkyl group, i.e. a straight-chain or branched alkyl group with 1 to 25 C atoms, which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH 2 groups to be replaced, in each occurrence independently from one another, by -O-, -S-, -NH-, -N(CH 3)-, -CO-, -COO-, -OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -C≡C- in such a manner that oxygen atoms are not linked directly to one another, MG 11 and MG 12 are independently from one another a group of (partial) formula II Z 11 are, independently of each other in each occurrence, a single bond, -COO-, -OCO-, -O-CO-O-, -OCH 2 -, -CH 2 O-, -OCF 2 -, -CF 2 O-, -CH 2 CH 2 -, -(CH 2) 4 -, -CF 2 CF 2 -, -CH=CH-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or -C≡C-, optionally substituted with one or more of F, S and/or Si, A 11 and A 12 are each independently in each occurrence 1,4-phenylene, wherein in addition one or more CH groups are optionally replaced by N, trans-1,4-cyclo-hexylene in which, in addition, one or two non-adjacent CH 2 groups are optionally replaced by O and/or S, 1,4-cyclohexenylene, 1,4-bicyclo-(2,2,2)-octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydro-naphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl, cyclobutane-1,3-diyl, spiro[3.3]heptane-2,6-diyl or dispiro[3.1.3.1] decane-2,8-diyl, it being possible for all these groups to be unsubstituted, mono-, di-, tri- or tetrasubstituted with F, Cl, CN or alkyl, alkoxy, alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H atoms are optionally substituted by F or Cl, and k is 0, 1, 2, 3 or 4, Sp 1 is -(CH 2) o -, o is 1, 3 or an integer from 5 to 15, and X 11 and X 12 are independently from one another a linking group selected from -CO-O-, -O-CO-, -O-, -CH=CH-, -C≡C-, -CF 2 -O-, -O-CF 2 -, -CF 2 -CF 2, -CH 2 -O-, -O-CH 2 -, -CO-S-, -S-CO-, -CS-S-, -S-, and a single bond, under the condition that in -X 11 -Sp 1 -X 12 - no two O atoms are adjacent to one another, no two -CH=CH- groups are adjacent to each other and no two groups selected from -O-CO-, -S-CO-, -O-COO-, -CO-S-, -CO-O- and -CH=CH- are adjacent to each other.