(R???C???C???n???Ar???L???Ar???C???C???R)m]q wherein each Ar is an aromatic group or inertly-substituted aromatic group and each Ar comprises at least one aromatic ring; each R is independently hydrogen, an alkyl, aryl or inertly-substituted alkyl or aryl group; L is a covalent bond or a group which links one Ar to at least one other Ar; preferably a substituted or unsubstituted alkyl group, n and