Compound of the formula,, or, where the following applies to the symbols used: Y is CR or N; Z is CR or N; 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, Cl, Br, I, N(Ar') 2, N(R 1) 2, OAr`, SAr`, 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 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms or a branched or cyclic alkyl group having 3 to 20 C atoms, where the alkyl, alkenyl or alkynyl 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 Si(R 1) 2, C=O, NR 1, O, S or CONR 1, or an aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, preferably having 5 to 40 aromatic ring atoms, which may in each case be substituted by one or more radicals R 1; two radicals R may also form a ring system with one another; 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 1; R 1 is on each occurrence, identically or differently, 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 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms or a branched or cyclic alkyl group having 3 to 20 C atoms, where the alkyl, alkenyl or alkynyl 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 Si(R 2) 2, C=O, NR 2, O, S or CONR 2 and one or more H atoms in the alkyl, alkenyl or alkynyl group may be replaced by D, F, Cl, Br, I or CN, or an aromatic or heteroaromatic ring system having 5 to 40 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 a 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. Y is CR or N; Z is CR or N; 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, Cl, Br, I, N(Ar') 2, N(R 1) 2, OAr`, SAr`, 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 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms or a branched or cyclic alkyl group having 3 to 20 C atoms, where the alkyl, alkenyl or alkynyl 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 Si(R 1) 2, C=O, NR 1, O, S or CONR 1, or an aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, preferably having 5 to 40 aromatic ring atoms, which may in each case be substituted by one or more radicals R 1; two radicals R may also form a ring system with one another; 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 1; R 1 is on each occurrence, identically or differently, 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 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms or a branched or cyclic alkyl group having 3 to 20 C atoms, where the alkyl, alkenyl or alkynyl 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 Si(R 2) 2, C=O, NR 2, O, S or CONR 2 and one or more H atoms in the alkyl, alkenyl or alkynyl group may be replaced by D, F, Cl, Br, I or CN, or an aromatic or heteroaromatic ring system having 5 to 40 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 a 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.