Compound of formula where the variables that occur are as follows: Y is the same or different at each instance and is selected from CR 1 and N; Z is C if an X bridge is bonded to the Z group, and is Y if no X bridge is bonded to the Z group; X is the same or different at each instance and is a bivalent bridge selected from C(R 1) 2 and Si(R 1) 2; R 1 is the same or different at each instance and is selected from H, D, F, C(=O)R 2, CN, Si(R 2) 3, N(R 2) 2, P(=O)(R 2) 2, OR 2, S(=O)R 2, S(=O) 2 R 2, straight-chain alkyl or alkoxy groups having 1 to 20 carbon atoms, branched or cyclic alkyl or alkoxy groups having 3 to 20 carbon atoms, alkenyl or alkynyl groups having 2 to 20 carbon atoms, aromatic ring systems having 6 to 40 aromatic ring atoms, and heteroaromatic ring systems having 5 to 40 aromatic ring atoms; where two or more R 1 radicals may be joined to one another and may form a ring; where the alkyl, alkoxy, alkenyl and alkynyl groups mentioned and the aromatic ring systems and heteroaromatic ring systems mentioned may each be substituted by one or more R 2 radicals; and where one or more CH 2 groups in the alkyl, alkoxy, alkenyl and alkynyl groups mentioned may be replaced by -R 2 C=CR 2 -, -C≡C-, Si(R 2) 2, C=O, C=NR 2, -C(=O)O-, C(=O)NR 2 -, NR 2, P(=O)(R 2), -O-, -S-, SO or SO 2; R 2 is the same or different at each instance and is selected from H, D, F, CN, alkyl groups having 1 to 20 carbon atoms, aromatic ring systems having 6 to 40 aromatic ring atoms and heteroaromatic ring systems having 5 to 40 aromatic ring atoms; where two or more R 2 radicals may be joined to one another and may form a ring; and where the alkyl groups, aromatic ring systems and heteroaromatic ring systems mentioned may be substituted by F or CN; a, b, c, d is the same or different at each instance and is 0 or 1, with the proviso that a + b = 1 and c + d = 1, where a = 0, b = 0, c = 0 and d = 0 respectively means that the corresponding X bridge is absent; where there is at least one Y in which a unit of the formula is bonded instead of R 1, in which the bond identified by an asterisk marks the bond to Y, and where the formula corresponds to one of the following formulae (N-2) to (N-12): where the units of the formula may be substituted at all positions shown as unsubstituted by R 1 radicals; E is the same or different at each instance and is selected from the following divalent groups: -C(R 1) 2 -C(R 1) 2 - (E-2) -C(R 1) 2 -C(R 1) 2 - (E-3) 2 -C(R 1)-CR 1 =CR 1 - (E-4) where R 1 is as defined above. Y is the same or different at each instance and is selected from CR 1 and N; Z is C if an X bridge is bonded to the Z group, and is Y if no X bridge is bonded to the Z group; X is the same or different at each instance and is a bivalent bridge selected from C(R 1) 2 and Si(R 1) 2; R 1 is the same or different at each instance and is selected from H, D, F, C(=O)R 2, CN, Si(R 2) 3, N(R 2) 2, P(=O)(R 2) 2, OR 2, S(=O)R 2, S(=O) 2 R 2, straight-chain alkyl or alkoxy groups having 1 to 20 carbon atoms, branched or cyclic alkyl or alkoxy groups having 3 to 20 carbon atoms, alkenyl or alkynyl groups having 2 to 20 carbon atoms, aromatic ring systems having 6 to 40 aromatic ring atoms, and heteroaromatic ring systems having 5 to 40 aromatic ring atoms; where two or more R 1 radicals may be joined to one another and may form a ring; where the alkyl, alkoxy, alkenyl and alkynyl groups mentioned and the aromatic ring systems and heteroaromatic ring systems mentioned may each be substituted by one or more R 2 radicals; and where one or more CH 2 groups in the alkyl, alkoxy, alkenyl and alkynyl groups mentioned may be replaced by -R 2 C=CR 2 -, -C≡C-, Si(R 2) 2, C=O, C=NR 2, -C(=O)O-, C(=O)NR 2 -, NR 2, P(=O)(R 2), -O-, -S-, SO or SO 2; R 2 is the same or different at each instance and is selected from H, D, F, CN, alkyl groups having 1 to 20 carbon atoms, aromatic ring systems having 6 to 40 aromatic ring atoms and heteroaromatic ring systems having 5 to 40 aromatic ring atoms; where two or more R 2 radicals may be joined to one another and may form a ring; and where the alkyl groups, aromatic ring systems and heteroaromatic ring systems mentioned may be substituted by F or CN; a, b, c, d is the same or different at each instance and is 0 or 1, with the proviso that a + b = 1 and c + d = 1, where a = 0, b = 0, c = 0 and d = 0 respectively means that the corresponding X bridge is absent; E is the same or different at each instance and is selected from the following divalent groups: -C(R 1) 2 -C(R 1) 2 - (E-2) -C(R 1) 2 -C(R 1) 2 - (E-3) 2 -C(R 1)-CR 1 =CR 1 - (E-4)