diphenyl compounds represented by the formula: ##STR4## wherein A represents a chemical bond or a divalent group selected from the group consisting of --O--, --S--, --SO2 --, --CO-- --CONH--, --COO--, --C(R7)(R8)-- and --N(R7)-- wherein R7 and R8 each is a hydrogen atom, an aliphatic group or an alicyclic group.