Compounds of the general structure: Z-A-(S-B)n where A represents a linear aromatic molecular core, S represents a flexible spacer unit, B represents a crosslinking group such as a methacrylate group, n equals 1 to 3, and Z represents any entity compatible with the B groups, and in which A is a substantially aromatic nucleus containing fluorene ring structures substituted at the 9-position and in which the 9-positions of the fluorenes are not susceptible to oxidation.