Compound of formula where the symbols and indices used are as follows: A is O or S; characterized in that, for the compound of the formula: X two adjacent X are a group of the formula where ^ indicates the corresponding adjacent X groups in formula, and the two remaining X groups are CR; Z is CR; or two adjacent Z are a group of the formula and the two other Z are CR, where ^ indicates the corresponding adjacent Z groups in formula; W here is O, S, NR or CR 2; 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 selected from the group consisting of H, D, F, Cl, Br, I, CN, NO 2, N(Ar 1) 2, N(R 1) 2, C(=O)Ar 1, C(=O)R 1, P(=O)(Ar 1) 2, P(Ar 1) 2, B(Ar 1) 2, Si(Ar 1) 3, Si(R 1) 3, a straight-chain alkyl, alkoxy or thioalkyl group having 1 to 40 carbon atoms or a branched or cyclic alkyl, alkoxy or thioalkyl group having 3 to 40 carbon atoms or an alkenyl or alkynyl group having 2 to 40 carbon atoms, each of which may be substituted by one or more R 1 radicals, where one or more nonadjacent CH 2 groups may be replaced by R 1 C=CR 1, C≡C, Si(R 1) 2, C=O, C=S, C=NR 1, P(=O)(R 1), SO, SO 2, NR 1, O, S or CONR 1 and where one or more hydrogen atoms may be replaced by D, F, Cl, Br, I, CN or NO 2, an aromatic or heteroaromatic ring system which has 5 to 60 aromatic ring atoms and may be substituted in each case by one or more R 1 radicals, an aryloxy or heteroaryloxy group which has 5 to 40 aromatic ring atoms and may be substituted by one or more R 1 radicals; at the same time, it is optionally possible for two adjacent R substituents to form a monocyclic or polycyclic, aliphatic, aromatic or heteroaromatic ring system which may be substituted by one or more R 1 radicals; Ar 1 is the same or different at each instance and is an aromatic or heteroaromatic ring system which has 5 to 30 aromatic ring atoms and may be substituted by one or more nonaromatic R 1 radicals; at the same time, two Ar 1 radicals bonded to the same nitrogen, phosphorus, boron or silicon atom may also be bridged to one another by a single bond or a bridge selected from N(R 1), C(R 1) 2, O and S; R 1 is the same or different at each instance and is selected from the group consisting of H, D, F, CN, an aliphatic hydrocarbyl radical having 1 to 20 carbon atoms, or an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms in which one or more hydrogen atoms may be replaced by D, F, CN or an alkyl group having 1 to 10 carbon atoms; at the same time, two or more adjacent R 1 substituents together may form a mono- or polycyclic, aliphatic ring system; p is the same or different at each instance and is 0, 1, 2, 3 or 4. A is O or S; X two adjacent X are a group of the formula Z is CR; or two adjacent Z are a group of the formula and the two other Z are CR, 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 selected from the group consisting of H, D, F, Cl, Br, I, CN, NO 2, N(Ar 1) 2, N(R 1) 2, C(=O)Ar 1, C(=O)R 1, P(=O)(Ar 1) 2, P(Ar 1) 2, B(Ar 1) 2, Si(Ar 1) 3, Si(R 1) 3, a straight-chain alkyl, alkoxy or thioalkyl group having 1 to 40 carbon atoms or a branched or cyclic alkyl, alkoxy or thioalkyl group having 3 to 40 carbon atoms or an alkenyl or alkynyl group having 2 to 40 carbon atoms, each of which may be substituted by one or more R 1 radicals, where one or more nonadjacent CH 2 groups may be replaced by R 1 C=CR 1, C≡C, Si(R 1) 2, C=O, C=S, C=NR 1, P(=O)(R 1), SO, SO 2, NR 1, O, S or CONR 1 and where one or more hydrogen atoms may be replaced by D, F, Cl, Br, I, CN or NO 2, an aromatic or heteroaromatic ring system which has 5 to 60 aromatic ring atoms and may be substituted in each case by one or more R 1 radicals, an aryloxy or heteroaryloxy group which has 5 to 40 aromatic ring atoms and may be substituted by one or more R 1 radicals; at the same time, it is optionally possible for two adjacent R substituents to form a monocyclic or polycyclic, aliphatic, aromatic or heteroaromatic ring system which may be substituted by one or more R 1 radicals; Ar 1 is the same or different at each instance and is an aromatic or heteroaromatic ring system which has 5 to 30 aromatic ring atoms and may be substituted by one or more nonaromatic R 1 radicals; at the same time, two Ar 1 radicals bonded to the same nitrogen, phosphorus, boron or silicon atom may also be bridged to one another by a single bond or a bridge selected from N(R 1), C(R 1) 2, O and S; R 1 is the same or different at each instance and is selected from the group consisting of H, D, F, CN, an aliphatic hydrocarbyl radical having 1 to 20 carbon atoms, or an aromatic or heteroaromatic ring system having 5 to 30 aromatic ring atoms in which one or more hydrogen atoms may be replaced by D, F, CN or an alkyl group having 1 to 10 carbon atoms; at the same time, two or more adjacent R 1 substituents together may form a mono- or polycyclic, aliphatic ring system; p is the same or different at each instance and is 0, 1, 2, 3 or 4.