Ar1 is optionally substituted phenyl or optionally substituted monocyclic heteroaryl having 5 or 6 ring atoms； Ar2 is optionally substituted phenyl, optionally substituted monocyclic heteroaryl having 5 or 6 ring atoms or optionally substituted fused bicyclic heteroaryl having 5 or 6 ring atoms in each fused ring； and Ar3 is a divalent radical selected from optionally substituted phenylene and optionally substituted monocyclic heteroarylene radicals having 5 or 6 ring atoms.
