Compound of the formula where the following applies to the symbols and indices used: Ar 1, Ar 2, Ar 3 are on each occurrence, identically or differently, an aryl or heteroaryl group having 5 to 22 aromatic ring atoms, selected from benzene, naphthalene, anthracene, phenanthrene, fluoranthene, naphthacene, benzanthracene, chrysene, pyrene, benzofluoranthene, triphenylene, perylene, dibenzanthracene, benzopyrene, picene, pentacene, pentaphene, pyridine, pyrazine, pyrimidine, pyridazine, quinoline, isoquinoline, phenanthroline and acridine, which may be substituted by one or more radicals R 1, with the proviso that Ar 2 does not stand for anthracene, naphthacene or pentacene; X is on each occurrence, identically or differently, a group selected from C(R 2) 2; R 1 is on each occurrence, identically or differently, a group selected from H, D, F, straight-chain alkyl or alkoxy groups having 1 to 10 C atoms or branched or cyclic alkyl or alkoxy groups having 3 to 10 C atoms, which may in each case be substituted by one or more radicals R 3, where in each case one or more non-adjacent CH 2 groups may be replaced by R 3 C=CR 3 or O and where one or more H atoms may be replaced by F, or aromatic or heteroaromatic ring systems having 5 to 24 aromatic ring atoms; two or more substituents R 1 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; R 2 is on each occurrence, identically or differently, a group selected from H, straight-chain alkyl groups having 1 to 10 C atoms or branched or cyclic alkyl groups having 3 to 10 C atoms, where in each case one or more non-adjacent CH 2 groups may be replaced by -R 2 C=CR 2 - or -O- and where one or more H atoms may be replaced by F, or a monovalent aryl or heteroaryl group having 5 to 16 aromatic ring atoms, which may be substituted by one or more non-aromatic radicals R 2; two radicals R 2 which are bonded in the same group X may also form a ring system with one another; R 3 is on each occurrence, identically or differently, H or an aliphatic or aromatic hydrocarbon radical having 1 to 20 C atoms; m, n are 0 or 1, with the proviso that m + n = 1; p is 1; Ar 1, Ar 2 and X together form a five-membered ring or a six-membered ring, and Ar 2, Ar 3 and X together form a five-membered ring or a six-membered ring, with the proviso that either all symbols X in the compound of the formula are bound in a five-membered ring or all symbols X in the compound of the formula are bound in a six-membered ring; characterised in that the sum of all π electrons in groups Ar 1, Ar 2 and Ar 3 is at least 28: the following compounds are excluded from the invention: Ar 1, Ar 2, Ar 3 are on each occurrence, identically or differently, an aryl or heteroaryl group having 5 to 22 aromatic ring atoms, selected from benzene, naphthalene, anthracene, phenanthrene, fluoranthene, naphthacene, benzanthracene, chrysene, pyrene, benzofluoranthene, triphenylene, perylene, dibenzanthracene, benzopyrene, picene, pentacene, pentaphene, pyridine, pyrazine, pyrimidine, pyridazine, quinoline, isoquinoline, phenanthroline and acridine, which may be substituted by one or more radicals R 1, with the proviso that Ar 2 does not stand for anthracene, naphthacene or pentacene; X is on each occurrence, identically or differently, a group selected from C(R 2) 2; R 1 is on each occurrence, identically or differently, a group selected from H, D, F, straight-chain alkyl or alkoxy groups having 1 to 10 C atoms or branched or cyclic alkyl or alkoxy groups having 3 to 10 C atoms, which may in each case be substituted by one or more radicals R 3, where in each case one or more non-adjacent CH 2 groups may be replaced by R 3 C=CR 3 or O and where one or more H atoms may be replaced by F, or aromatic or heteroaromatic ring systems having 5 to 24 aromatic ring atoms; two or more substituents R 1 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; R 2 is on each occurrence, identically or differently, a group selected from H, straight-chain alkyl groups having 1 to 10 C atoms or branched or cyclic alkyl groups having 3 to 10 C atoms, where in each case one or more non-adjacent CH 2 groups may be replaced by -R 2 C=CR 2 - or -O- and where one or more H atoms may be replaced by F, or a monovalent aryl or heteroaryl group having 5 to 16 aromatic ring atoms, which may be substituted by one or more non-aromatic radicals R 2; two radicals R 2 which are bonded in the same group X may also form a ring system with one another; R 3 is on each occurrence, identically or differently, H or an aliphatic or aromatic hydrocarbon radical having 1 to 20 C atoms; m, n are 0 or 1, with the proviso that m + n = 1; p is 1; Ar 1, Ar 2 and X together form a five-membered ring or a six-membered ring, and Ar 2, Ar 3 and X together form a five-membered ring or a six-membered ring, with the proviso that either all symbols X in the compound of the formula are bound in a five-membered ring or all symbols X in the compound of the formula are bound in a six-membered ring; the following compounds are excluded from the invention: