Compound of the formula, where the following applies to the symbols used: X is on each occurrence, identically or differently, CR or N or two adjacent X stand for a group of the following formula, where the dashed bonds denote the linking of this group in the formula, with the proviso that the compound of the formula contains one or two groups of the formula; X 1 is CH; Y is on each occurrence, identically or differently, CR or N; Z 1, Z 2, Z 3 are on each occurrence, identically or differently, O, S, N-Ar or CR 2; Ar is on each occurrence, identically or differently, an aromatic or heteroaromatic ring system having 5 to 40 aromatic ring atoms, which may be substituted by one or more radicals R; R is on each occurrence, identically or differently, H, D, F, N(Ar') 2, CN, OR 1, a straight-chain alkyl group having 1 to 10 C atoms or an alkenyl group having 2 to 10 C atoms or a branched or cyclic alkyl group having 3 to 10 C atoms, where the alkyl or alkenyl group may in each case be substituted by one or more radicals R 1, where one or more non-adjacent CH 2 groups may be replaced by O, or an aromatic or heteroaromatic ring system having 6 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 1; two radicals R may also form an aliphatic ring system with one another; Ar' is on each occurrence, identically or differently, an aromatic or heteroaromatic ring system having 6 to 30 aromatic ring atoms, which may be substituted by one or more radicals R 1; R 1 is on each occurrence, identically or differently, H, D, F, CN, OR 2, a straight-chain alkyl group having 1 to 10 C atoms or an alkenyl group having 2 to 10 C atoms or a branched or cyclic alkyl group having 3 to 10 C atoms, where the alkyl or alkenyl group may in each case be substituted by one or more radicals R 2, where one or more non-adjacent CH 2 groups may be replaced by O, or an aromatic or heteroaromatic ring system having 6 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 2; two or more radicals R 1 may form an aliphatic ring system with one another; R 2 is on each occurrence, identically or differently, H, D, F, CN or an aliphatic, aromatic or heteroaromatic organic radical, in particular a hydrocarbon radical, having 1 to 20 C atoms, in which, in addition, one or more H atoms may be replaced by F. X is on each occurrence, identically or differently, CR or N or two adjacent X stand for a group of the following formula, where the dashed bonds denote the linking of this group in the formula, with the proviso that the compound of the formula contains one or two groups of the formula; X 1 is CH; Y is on each occurrence, identically or differently, CR or N; Z 1, Z 2, Z 3 are on each occurrence, identically or differently, O, S, N-Ar or CR 2; Ar is on each occurrence, identically or differently, an aromatic or heteroaromatic ring system having 5 to 40 aromatic ring atoms, which may be substituted by one or more radicals R; R is on each occurrence, identically or differently, H, D, F, N(Ar') 2, CN, OR 1, a straight-chain alkyl group having 1 to 10 C atoms or an alkenyl group having 2 to 10 C atoms or a branched or cyclic alkyl group having 3 to 10 C atoms, where the alkyl or alkenyl group may in each case be substituted by one or more radicals R 1, where one or more non-adjacent CH 2 groups may be replaced by O, or an aromatic or heteroaromatic ring system having 6 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 1; two radicals R may also form an aliphatic ring system with one another; Ar' is on each occurrence, identically or differently, an aromatic or heteroaromatic ring system having 6 to 30 aromatic ring atoms, which may be substituted by one or more radicals R 1; R 1 is on each occurrence, identically or differently, H, D, F, CN, OR 2, a straight-chain alkyl group having 1 to 10 C atoms or an alkenyl group having 2 to 10 C atoms or a branched or cyclic alkyl group having 3 to 10 C atoms, where the alkyl or alkenyl group may in each case be substituted by one or more radicals R 2, where one or more non-adjacent CH 2 groups may be replaced by O, or an aromatic or heteroaromatic ring system having 6 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 2; two or more radicals R 1 may form an aliphatic ring system with one another; R 2 is on each occurrence, identically or differently, H, D, F, CN or an aliphatic, aromatic or heteroaromatic organic radical, in particular a hydrocarbon radical, having 1 to 20 C atoms, in which, in addition, one or more H atoms may be replaced by F.