Studies Of Intermolecular/Intramolecular Interaction Influence On The Slow Relaxation Behavior Of Single-Molecule Magnets

Single-Molecule Magnets（SMMs）can maintain magnetization properties when they are away from the magnetic field and show slow magnetic relaxation behavior.Due to their broad application prospects in high density information storage,quantum computation,spintronics and so on,they have attracted much attention from scientists.Among them,the transition metal Co（II）and the rare earth metal Dy（III）have become the best two choices for constructing high-performance SMMs due to the unquenched orbital angular momentum and strong magnetic anisotropy.However,as more and more SMMs are synthesized and reported,various factors affecting their slow magnetic relaxation behavior are found in an endless stream.The coordination configuration,symmetry of the crystal field,and the charge effect of the ligands all have great influence on the properties of SMMs.These impacts have been continuously studied over the past several years.Herein,cobalt/dysprosium complexes with special configuration are synthesized,their structures and magnetic properties have been characterized in order to study the magneto-structural relationship and relaxation mechanism.The specific contents are as follows:1.Based on the 15-Crown-5 ligand,two Co（II）complexes 1 and 2 with pentagonal bipyramid configurations were synthesized.Complex 1 behaves as field-induced single-ion magnet（SIM）whose energy barrier is 72.9 K under low field.When the field improves to 1000 Oe,two relaxation peaks appears.The magnetic dilution experiment found that the intermolecular dipole-dipole interaction can cause double relaxation,thus affects the magnetic relaxation behavior.According to the crystal structure of complex 1,when the dilution ratio reaches 10 times,the dipole-dipole interaction can be completely inhibited,and its influence on the intrinsic properties of the SIM can be eliminated.2.Two ligands TPIP and PIP were synthesized by a one-step method,and they were used as auxiliary ligands together withβ-diketone to synthesize four eight-coordinated Dy（III）complexes 3-6 with D_4dor D_2d configuration.Complex 3 does not form aπ-πstacking due to the non-coplanarity of the TPIP ligand,and the hydrogen bond effect is simple,showing good magnetic relaxation behavior.The relaxation process of complex 4 became faster due to the strong hydrogen bond interactions.Complex 5 and6 shows double relaxation due to the existence ofπ-πinteraction.The relaxation of diluted sample 4a slowed down,its energy barrier improved and the double relaxation was absent,which confirmed the previous viewpoint.Also the theoretical calculation results are consistent with the above analysis.These results indicate that the interactions between molecules may cause double relaxation,and sometimes accelerate the relaxation process or increase the probability of quantum tunneling of magnetization（QTM）.3.One dinuclear dysprosium complex 7 was synthesized based on 4-hydroxy-3,5-dimethoxyacetophenone as bridging ligand.The crystal structure tests show that the two Dy（III）ions are both eight-coordinated D_2d structure.The magnetic tests show that it exhibits slow magnetic relaxation behavior under zero field and its dynamic magnetic properties are also tested under the optimal field of 1500 Oe.Magnetic dilution experiment shows that the intramolecular magnetic coupling has a certain inhibitory effect on the QTM.Based on the theoretical calculation results,the coupling factor between the two Dy（III）ion centers is J=-0.15 cm^-1.