Compound of formula where the symbols used are as follows: Z is a group of the following formula where the dotted bonds indicate the linkage of this group to X and the carbon atom in formula; X is the same or different at each instance and is CR or N; Ar is an aromatic or heteroaromatic ring system which has 5 to 40 aromatic ring atoms and may be substituted by one or more R radicals; R is the same or different at each instance and is H, D, F, Cl, Br, I, N(Ar') 2, N(R 1) 2, CN, NO 2, OR 1, SR 1, COOR 1, C(=O)N(R 1) 2, Si(R 1) 3, B(OR 1) 2, C(=O)R 1, P(=O)(R 1) 2, S(=O)R 1, S(=O) 2 R 1, OSO 2 R 1, a straight-chain alkyl group having 1 to 20 carbon atoms or an alkenyl or alkynyl group having 2 to 20 carbon atoms or a branched or cyclic alkyl group having 3 to 20 carbon atoms, where the alkyl, alkenyl or alkynyl group may in each case be substituted by one or more R 1 radicals, where one or more nonadjacent CH 2 groups may be replaced by Si(R 1) 2, C=O, NR 1, O, S or CONR 1, or an aromatic or heteroaromatic ring system which has 5 to 60 aromatic ring atoms, preferably 5 to 40 aromatic ring atoms, and may be substituted in each case by one or more R 1 radicals; at the same time, two R radicals together may also form a ring system; Ar' is the same or different at each instance and is an aromatic or heteroaromatic ring system which has 5 to 40 aromatic ring atoms and may be substituted by one or more R 1 radicals; R 1 is the same or different at each instance and is H, D, F, Cl, Br, I, N(R 2) 2, CN, NO 2, OR 2, SR 2, Si(R 2) 3, B(OR 2) 2, C(=O)R 2, P(=O)(R 2) 2, S(=O)R 2, S(=O) 2 R 2, OSO 2 R 2, a straight-chain alkyl group having 1 to 20 carbon atoms or an alkenyl or alkynyl group having 2 to 20 carbon atoms or a branched or cyclic alkyl group having 3 to 20 carbon atoms, where the alkyl, alkenyl or alkynyl group may in each case be substituted by one or more R 2 radicals, where one or more nonadjacent CH 2 groups may be replaced by Si(R 2) 2, C=O, NR 2, O, S or CONR 2, or an aromatic or heteroaromatic ring system which has 5 to 40 aromatic ring atoms and may be substituted in each case by one or more R 2 radicals; at the same time, two or more R 1 radicals together may form a ring system; R 2 is the same or different at each instance and is H, D, F or an aliphatic, aromatic or heteroaromatic organic radical, especially a hydrocarbyl radical, having 1 to 20 carbon atoms, in which one or more hydrogen atoms may also be replaced by F; where the following compounds are excluded from the invention: Z is a group of the following formula where the dotted bonds indicate the linkage of this group to X and the carbon atom in formula; X is the same or different at each instance and is CR or N; Ar is an aromatic or heteroaromatic ring system which has 5 to 40 aromatic ring atoms and may be substituted by one or more R radicals; R is the same or different at each instance and is H, D, F, Cl, Br, I, N(Ar') 2, N(R 1) 2, CN, NO 2, OR 1, SR 1, COOR 1, C(=O)N(R 1) 2, Si(R 1) 3, B(OR 1) 2, C(=O)R 1, P(=O)(R 1) 2, S(=O)R 1, S(=O) 2 R 1, OSO 2 R 1, a straight-chain alkyl group having 1 to 20 carbon atoms or an alkenyl or alkynyl group having 2 to 20 carbon atoms or a branched or cyclic alkyl group having 3 to 20 carbon atoms, where the alkyl, alkenyl or alkynyl group may in each case be substituted by one or more R 1 radicals, where one or more nonadjacent CH 2 groups may be replaced by Si(R 1) 2, C=O, NR 1, O, S or CONR 1, or an aromatic or heteroaromatic ring system which has 5 to 60 aromatic ring atoms, preferably 5 to 40 aromatic ring atoms, and may be substituted in each case by one or more R 1 radicals; at the same time, two R radicals together may also form a ring system; Ar' is the same or different at each instance and is an aromatic or heteroaromatic ring system which has 5 to 40 aromatic ring atoms and may be substituted by one or more R 1 radicals; R 1 is the same or different at each instance and is H, D, F, Cl, Br, I, N(R 2) 2, CN, NO 2, OR 2, SR 2, Si(R 2) 3, B(OR 2) 2, C(=O)R 2, P(=O)(R 2) 2, S(=O)R 2, S(=O) 2 R 2, OSO 2 R 2, a straight-chain alkyl group having 1 to 20 carbon atoms or an alkenyl or alkynyl group having 2 to 20 carbon atoms or a branched or cyclic alkyl group having 3 to 20 carbon atoms, where the alkyl, alkenyl or alkynyl group may in each case be substituted by one or more R 2 radicals, where one or more nonadjacent CH 2 groups may be replaced by Si(R 2) 2, C=O, NR 2, O, S or CONR 2, or an aromatic or heteroaromatic ring system which has 5 to 40 aromatic ring atoms and may be substituted in each case by one or more R 2 radicals; at the same time, two or more R 1 radicals together may form a ring system; R 2 is the same or different at each instance and is H, D, F or an aliphatic, aromatic or heteroaromatic organic radical, especially a hydrocarbyl radical, having 1 to 20 carbon atoms, in which one or more hydrogen atoms may also be replaced by F;