re symmetrically bonded to the benzene rings, Ar.sup.1 and Ar.sup.2 are not the same, and in the case where m or n is an integer of 2 to 4, m is different from n.R.sup.31 to R.sup.40 are independently a hydrogen atom, a substituted or unsubstituted aromatic ring group having 6 to 50 ring carbon atoms, a substituted or unsubstituted aromatic heterocyclic group having 5 to 50 ring atoms, a substitut