Influence of the ligand shell on the surface orientation of Mn12 single molecule magnets

Here we report the synthesis, investigation as well as surface deposition of a truly axial symmetry Mn₁₂-diphenylphosphinate (Mn₁₂-phn) single molecule magnet. Out of 16 acetate ligands encapsulating the Mn₁₂O₁₂ core, 12 ligands were exchanged by diphenylphosphinate in this compound. Mn₁₂-phn shows well-defined magnetic hysteresis curves indicating a very high crystal quality. A monolayer of Mn₁₂-phn was chemically grafted on a functionalized Au(1 1 1) surface via ligand exchange reaction and studied by means of scanning tunneling microscopy and spectroscopy. Via distance–voltage spectroscopy we determine the real-space height of the Mn₁₂-phn molecules with high accuracy. A large spread in the measured molecular heights obtained from the distance–voltage spectra indicates the absence of preferential orientation of Mn₁₂-phn molecules with respect to the surface which we attribute to the equal anchoring probability of all diphenylphosphinate ligands in Mn₁₂-phn while none of the four acetate ligands are exchanged. These results are compared with the experimental data obtained from a different Mn₁₂ derivative containing 16 thiophenecarboxylate ligands. In general, we show that the substitution of the ligand shell may have a major impact on the surface orientation of the Mn₁₂ clusters deposited on Au, i.e. on the orientation of the easy magnetization axis.