. . . . "A1 and A3 independently are a substituted or unsubstituted bridging group comprising either a cyclic group of 5 to 8 carbon atoms, a bridging chain of 2 to 5 carbon atoms, or a carbon, silicon, germanium, tin, boron, nitrogen, or phosphorus bridging atom, any substituents on A1 and A3 independently are a hydrogen atom, or a substituted or unsubstituted aliphatic, aromatic, or cyclic group, or a co" .