Abstract: Compounds of formula (I) wherein A, C and D represent (1) or (2), B represents (3) or (4) wherein at least one of the substituents is other than hydrogen and also at least one of the phenylene rings may be replaced by a 1,4-phenylene ring in which one or two non-adjacent CH groups have been replaced by nitrogen; k, l, m are 0 or 1, wherein k+l+m must be equal to 1 or 2; Z1, Z2, Z3 each independently of the others represents a single bond, —CH2CH2—, —CH2O—, —OCH2—, —COO—, —OOC—, —CH═CH—COO—, —OOC—CH═CH—, —(CH2)4—, —O(CH2)3—, —(CH2)3O— or —C≡C—; R1, R2 represent crosslinkable groups, and S1, S2 represent spacer units.