Compound of a formula where: Ar 1 is on each occurrence, identically or differently, an aryl or heteroaryl group having 6 to 18 aromatic ring atoms, which may be substituted by one or more radicals R 1; Ar 2 is on each occurrence, identically or differently, an aryl or heteroaryl group having 6 aromatic ring atoms, which may be substituted by one or more radicals R 2; X 1 is on each occurrence, identically or differently, BR 3, C(R 3) 2, -C(R 3) 2 -C(R 3) 2 -, -C(R 3) 2 -O-, -C(R 3) 2 -S-,-R 3 C=CR 3 -, -R 3 C=N-, Si(R 3) 2, -Si(R 3) 2 -Si(R 3) 2 -, C=O, O, S, S=O, SO 2, NR 3, PR 3 or P(=O)R 3; R 1, R 2, R 3 are on each occurrence, identically or differently, H, D, F, Cl, Br, I, C(=O)R 4, CN, Si(R 4) 3, N(R 4) 2, P(=O)(R 4) 2, OR 4, S(=O)R 4, S(=O) 2 R 4, a straight-chain alkyl or alkoxy group having 1 to 20 C atoms or a branched or cyclic alkyl or alkoxy group having 3 to 20 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms, where the above-mentioned groups may in each case be substituted by one or more radicals R 4 and where one or more CH 2 groups in the above-mentioned groups may be replaced by -R 4 C=CR 4 -, -C≡C-, Si(R 4) 2, C=O, C=NR 4, -C(=O)O-, -C(=O)NR 4 -, NR 4, P(=O)(R 4), -O-, -S-, SO or SO 2, or an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 4, where two or more radicals R 3 may be linked to one another and may form a ring; R 4 is on each occurrence, identically or differently, H, D, F, Cl, Br, I, C(=O)R 5, CN, Si(R 5) 3, N(R 5) 2, P(=O)(R 5) 2, OR 5, S(=O)R 5, S(=O) 2 R 5, a straight-chain alkyl or alkoxy group having 1 to 20 C atoms or a branched or cyclic alkyl or alkoxy group having 3 to 20 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms, where the above-mentioned groups may in each case be substituted by one or more radicals R 5 and where one or more CH 2 groups in the above-mentioned groups may be replaced by -R 5 C=CR 5 -, -C≡C-, Si(R 5) 2, C=O, C=NR 5, -C(=O)O-, -C(=O)NR 5 -, NR 5, P(=O)(R 5), -O-, -S-, SO or SO 2, or an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 5, where two or more radicals R 4 may be linked to one another and may form a ring; R 5 is on each occurrence, identically or differently, H, D, F or an aliphatic, aromatic or heteroaromatic organic radical having 1 to 20 C atoms, in which, in addition, one or more H atoms may be replaced by D or F; two or more substituents R 5 here may be linked to one another and may form a ring; where at least one of the two groups Ar 1 must contain 10 or more aromatic ring atoms; and where, if one of the two groups Ar 1 is a phenyl group, the other of the two groups Ar 1 must not contain more than 14 aromatic ring atoms. Ar 1 is on each occurrence, identically or differently, an aryl or heteroaryl group having 6 to 18 aromatic ring atoms, which may be substituted by one or more radicals R 1; Ar 2 is on each occurrence, identically or differently, an aryl or heteroaryl group having 6 aromatic ring atoms, which may be substituted by one or more radicals R 2; X 1 is on each occurrence, identically or differently, BR 3, C(R 3) 2, -C(R 3) 2 -C(R 3) 2 -, -C(R 3) 2 -O-, -C(R 3) 2 -S-,-R 3 C=CR 3 -, -R 3 C=N-, Si(R 3) 2, -Si(R 3) 2 -Si(R 3) 2 -, C=O, O, S, S=O, SO 2, NR 3, PR 3 or P(=O)R 3; R 1, R 2, R 3 are on each occurrence, identically or differently, H, D, F, Cl, Br, I, C(=O)R 4, CN, Si(R 4) 3, N(R 4) 2, P(=O)(R 4) 2, OR 4, S(=O)R 4, S(=O) 2 R 4, a straight-chain alkyl or alkoxy group having 1 to 20 C atoms or a branched or cyclic alkyl or alkoxy group having 3 to 20 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms, where the above-mentioned groups may in each case be substituted by one or more radicals R 4 and where one or more CH 2 groups in the above-mentioned groups may be replaced by -R 4 C=CR 4 -, -C≡C-, Si(R 4) 2, C=O, C=NR 4, -C(=O)O-, -C(=O)NR 4 -, NR 4, P(=O)(R 4), -O-, -S-, SO or SO 2, or an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 4, where two or more radicals R 3 may be linked to one another and may form a ring; R 4 is on each occurrence, identically or differently, H, D, F, Cl, Br, I, C(=O)R 5, CN, Si(R 5) 3, N(R 5) 2, P(=O)(R 5) 2, OR 5, S(=O)R 5, S(=O) 2 R 5, a straight-chain alkyl or alkoxy group having 1 to 20 C atoms or a branched or cyclic alkyl or alkoxy group having 3 to 20 C atoms or an alkenyl or alkynyl group having 2 to 20 C atoms, where the above-mentioned groups may in each case be substituted by one or more radicals R 5 and where one or more CH 2 groups in the above-mentioned groups may be replaced by -R 5 C=CR 5 -, -C≡C-, Si(R 5) 2, C=O, C=NR 5, -C(=O)O-, -C(=O)NR 5 -, NR 5, P(=O)(R 5), -O-, -S-, SO or SO 2, or an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms, which may in each case be substituted by one or more radicals R 5, where two or more radicals R 4 may be linked to one another and may form a ring; R 5 is on each occurrence, identically or differently, H, D, F or an aliphatic, aromatic or heteroaromatic organic radical having 1 to 20 C atoms, in which, in addition, one or more H atoms may be replaced by D or F; two or more substituents R 5 here may be linked to one another and may form a ring;