the centrosymmetric compounds (BzA)2PbI4 and (4-FBzA)2PbI4 exhibit only a weak SHG signal, which we attribute to surface effects, while the non-centrosymmetric compounds (4-ClBzA)2PbI4 and (4-BrBzA)2PbI4 show 2 orders of magnitude higher SHG intensities