A compound of formula I in which the individual radicals have the following meanings A 1, A 2 are selected from benzene and naphthalene which are unsubstituted or are substituted by one or more groups L, R a, R b denote, independently of each other, P-Sp-, 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 or P-Sp-, Sp denotes, on each occurrence identically or differently, a spacer group or a single bond, wherein those groups Sp that are different from a single bond, are selected from -(CH 2) p1 -,-(CH 2) p1 -O-, -(CH 2) p1 -O-CO-, -(CH 2) p1 -CO-O-, -(CH 2) p1 -O-CO-O-, in which p1 is an integer from 1 to 12, and wherein at least one group Sp is different from a single bond, P denotes, on each occurrence identically or differently, a polymerisable group selected from the group consisting of acrylate, methacrylate and oxetane, 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, particularly preferably 1 to 10, 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, Y 1 is halogen, a is 0 and b is 1, or a is 1 and b is 0, and the compound contains exactly three polymerisable groups. A 1, A 2 are selected from benzene and naphthalene which are unsubstituted or are substituted by one or more groups L, R a, R b denote, independently of each other, P-Sp-, 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 or P-Sp-, Sp denotes, on each occurrence identically or differently, a spacer group or a single bond, wherein those groups Sp that are different from a single bond, are selected from -(CH 2) p1 -,-(CH 2) p1 -O-, -(CH 2) p1 -O-CO-, -(CH 2) p1 -CO-O-, -(CH 2) p1 -O-CO-O-, in which p1 is an integer from 1 to 12, and wherein at least one group Sp is different from a single bond, P denotes, on each occurrence identically or differently, a polymerisable group selected from the group consisting of acrylate, methacrylate and oxetane, 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, particularly preferably 1 to 10, 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, Y 1 is halogen, a is 0 and b is 1, or a is 1 and b is 0, and the compound contains exactly three polymerisable groups.