endently a hydrocarbon grouphaving 1 to 20 carbon atoms or a halogenated hydrocarbon group having 1 to 20 carbon atoms; and Ar is an aryl group which may be substituted, with the proviso that at least one of the two 7-membered rings is bonded to the halogenated hydrocarbon grouphaving 1 to 20 carbon atoms.General formula (IV) ##STR5##wherein R.sup.1 and R.sup.4 are independently a hydrocarbon grou