A compound of formula I (P-Sp) x -Ar- y I in which the individual radicals have the following meanings Ar denotes aryl or heteroaryl having 4 to 30 ring atoms, which is mono- or polycyclic and may also contain further substituents, M on each occurrence identically or differently denotes -G-NR y R z, G denotes -CO- or -SO 2 -, one of R y and R z denotes P-Sp- and the other denotes R x or P-Sp-, or R y and R z together with the N-atom form an N-heterocyclic group with 4 to 6 ring atoms that is substituted with at least one group P-Sp-, R x denotes H, F, Cl, CN, or straight chain, branched or cyclic alkyl having 1 to 25 C atoms, wherein one or more non-adjacent CH 2 -groups are optionally replaced by -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a manner that O- and/or S-atoms are not directly connected with each other, and wherein one or more H atoms are each optionally replaced by F, Cl, P- or P-Sp-, Sp on each occurrence identically or differently denotes a spacer group or a single bond, P on each occurrence identically or differently denotes a polymerisable group, x is 0 or an integer ≥ 1, y is an integer ≥ 1, and x+y being an integer ≥ 1, characterized in that Ar is selected from the following formulae wherein X is O, S, CH 2, CHR a or CHR a R b, R a and R b have independently of each other one of the meanings of R x, and each of the above structures is optionally substituted by one or more groups L, wherein L denotes P-, P-Sp-, F, Cl, -CN, -NO 2, -NCO, -NCS, -OCN,-SCN, -C(=O)N(R x) 2, -C(=O)Y 1, -C(=O)R x, -N(R x) 2, optionally substituted silyl, optionally substituted aryl or heteroaryl having 5 to 20 ring atoms, or straight-chain or branched alkyl having 1 to 25 C atoms, in which, in addition, one or more non-adjacent CH 2 groups may each be replaced, independently of one another, by -C(R 0)=C(R 00)-, -C≡C-, -N(R 0)-, -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a way that O and/or S atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by F, Cl, CN, P- or P-Sp-, R 0 and R 00 each, independently of one another, denote H or alkyl having 1 to 20 C atoms, and Y 1 is halogen, with the proviso that the following compound is excluded Ar denotes aryl or heteroaryl having 4 to 30 ring atoms, which is mono- or polycyclic and may also contain further substituents, M on each occurrence identically or differently denotes -G-NR y R z, G denotes -CO- or -SO 2 -, R x denotes H, F, Cl, CN, or straight chain, branched or cyclic alkyl having 1 to 25 C atoms, wherein one or more non-adjacent CH 2 -groups are optionally replaced by -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a manner that O- and/or S-atoms are not directly connected with each other, and wherein one or more H atoms are each optionally replaced by F, Cl, P- or P-Sp-, Sp on each occurrence identically or differently denotes a spacer group or a single bond, P on each occurrence identically or differently denotes a polymerisable group, x is 0 or an integer ≥ 1, y is an integer ≥ 1, L denotes P-, P-Sp-, F, Cl, -CN, -NO 2, -NCO, -NCS, -OCN,-SCN, -C(=O)N(R x) 2, -C(=O)Y 1, -C(=O)R x, -N(R x) 2, optionally substituted silyl, optionally substituted aryl or heteroaryl having 5 to 20 ring atoms, or straight-chain or branched alkyl having 1 to 25 C atoms, in which, in addition, one or more non-adjacent CH 2 groups may each be replaced, independently of one another, by -C(R 0)=C(R 00)-, -C≡C-, -N(R 0)-, -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a way that O and/or S atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by F, Cl, CN, P- or P-Sp-, R 0 and R 00 each, independently of one another, denote H or alkyl having 1 to 20 C atoms, and Y 1 is halogen,