Compound containing structures of the formula where the following applies to the symbols used: X is O; Ar is on each occurrence, identically or differently, an aryl group having 6 to 40 C atoms or a heteroaryl group having 3 to 40 C atoms, which may in each case be substituted by one or more radicals R 1; R 1 is on each occurrence, identically or differently, H, D, F, Cl, Br, I, a straight-chain alkyl group having 1 to 40 C atoms or an aromatic ring system having 6 to 40 aromatic ring atoms, which may in each case be substituted by one or more radicals R 2; R 2 is on each occurrence, identically or differently, H, D, F, Cl, Br, I, CHO, C(=O)Ar 1, P(=O)(Ar 1) 2, S(=O)Ar 1, S(=O) 2 Ar 1, CN, NO 2, Si(R 3) 3, B(OR 3) 2, OSO 2 R 3, a straight-chain alkyl, alkoxy or thioalkoxy group having 1 to 40 C atoms or a branched or cyclic alkyl, alkoxy or thioalkoxy group having 3 to 40 C atoms, which may in each case be substituted by one or more radicals R 3, where one or more non-adjacent CH 2 groups may be replaced by C=C, Si(R 3) 2, Ge(R 3) 2, Sn(R 3) 2, C=O, C=S, C=Se, P(=O)(R 3), SO, SO 2, O, S or CONR 3 and where one or more H atoms may be replaced by D, F, Cl, Br, I, CN or NO 2, or a combination of these systems; two or more adjacent substituents R 2 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; Ar 1 is on each occurrence, identically or differently, an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms, which may be substituted by one or more radicals R 2; two radicals Ar 1 which are bonded to the same phosphorus atom may also be linked to one another by a single bond or a bridge selected from B(R 3), C(R 3) 2, Si(R 3) 2, C=O, C=NR 3, C=C(R 3) 2, O, S, S=O, SO 2, N(R 3), P(R 3) and P(=O)R 3; R 3 is on each occurrence, identically or differently, H, D, F or an aliphatic, aromatic and/or heteroaromatic hydrocarbon radical having 1 to 20 C atoms, in which, in addition, H atoms may be replaced by F; two or more adjacent substituents R 3 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; n is on each occurrence, identically or differently, 0, 1, 2, 3 or 4; m is on each occurrence, identically or differently, 0, 1, 2 or 3; o is on each occurrence, identically or differently, 0, 1 or 2; q is 1; p, r, s, t, u are on each occurrence equal to 0; E is an electron-transporting group, which may in each case be substituted by one or more radicals R 1; R a is R 1; R b is R 1; with the proviso that the structure of the formula contains precisely one electron-transporting group E; the sum of all the indices m, s and t is less than 7; the sum of all indices n, q and u is less than 9; and where the sum of the indices r and o is less than 3. X is O; Ar is on each occurrence, identically or differently, an aryl group having 6 to 40 C atoms or a heteroaryl group having 3 to 40 C atoms, which may in each case be substituted by one or more radicals R 1; R 1 is on each occurrence, identically or differently, H, D, F, Cl, Br, I, a straight-chain alkyl group having 1 to 40 C atoms or an aromatic ring system having 6 to 40 aromatic ring atoms, which may in each case be substituted by one or more radicals R 2; R 2 is on each occurrence, identically or differently, H, D, F, Cl, Br, I, CHO, C(=O)Ar 1, P(=O)(Ar 1) 2, S(=O)Ar 1, S(=O) 2 Ar 1, CN, NO 2, Si(R 3) 3, B(OR 3) 2, OSO 2 R 3, a straight-chain alkyl, alkoxy or thioalkoxy group having 1 to 40 C atoms or a branched or cyclic alkyl, alkoxy or thioalkoxy group having 3 to 40 C atoms, which may in each case be substituted by one or more radicals R 3, where one or more non-adjacent CH 2 groups may be replaced by C=C, Si(R 3) 2, Ge(R 3) 2, Sn(R 3) 2, C=O, C=S, C=Se, P(=O)(R 3), SO, SO 2, O, S or CONR 3 and where one or more H atoms may be replaced by D, F, Cl, Br, I, CN or NO 2, or a combination of these systems; two or more adjacent substituents R 2 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; Ar 1 is on each occurrence, identically or differently, an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms, which may be substituted by one or more radicals R 2; two radicals Ar 1 which are bonded to the same phosphorus atom may also be linked to one another by a single bond or a bridge selected from B(R 3), C(R 3) 2, Si(R 3) 2, C=O, C=NR 3, C=C(R 3) 2, O, S, S=O, SO 2, N(R 3), P(R 3) and P(=O)R 3; R 3 is on each occurrence, identically or differently, H, D, F or an aliphatic, aromatic and/or heteroaromatic hydrocarbon radical having 1 to 20 C atoms, in which, in addition, H atoms may be replaced by F; two or more adjacent substituents R 3 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; n is on each occurrence, identically or differently, 0, 1, 2, 3 or 4; m is on each occurrence, identically or differently, 0, 1, 2 or 3; o is on each occurrence, identically or differently, 0, 1 or 2; q is 1; p, r, s, t, u are on each occurrence equal to 0; E is an electron-transporting group, which may in each case be substituted by one or more radicals R 1; R a is R 1; R b is R 1;