wherein Ar1 and Ar2 are independently a substituted or unsubstituted aromatic ring group having 6 to 50 nucleus carbon atoms, and m and n are each an integer of 1 to 4, provided that in the case where m=n=1 and Ar1 and Ar2 are symmetrically bonded to the benzene rings, Ar1 and Ar2 are not the same, and in the case where m or n is an integer of 2 to 4, m is different from n,