Construction and Magnetic Relaxation Dynamics Study of Dysprosium Single-molecule Magnets Based on Acylhydrazone Schiff Base

Author:Sun Lin

Supervisor:chen san ping


Degree Year:2018





Single-molecule magnets(SMMs)have potential applications in the areas of spintronic devices,ultra-high-density information storage and quantum information processing,which sparks increasing interest at home and abroad.Rare earth ions,especially dysprosium ions,are suitable as spin carriers for the design and synthesis of high performance SMMs due to their high spin ground state as well as strong spin orbit coupling and magnetic anisotropy.It is a frontier topic for the research of lanthanide-based SMMs:exploring the self-assembly mechanism of lanthanide-based SMMs and obtaining regularity of magnetic anisotropy,magnetic easy axis orientation and magnetic interactions,thereby realizing the precise and controllable preparation of such materials.Based on the tridentate acylhydrazone Schiff base ligands,ten mononuclear and dinuclear dysprosium-based complexes were constructed by changing the assembly conditions.Combined with ab initio calculation,their magneto-structural correlations and magnetic relaxation mechanism were deeply studied.The specific research results and conclusions are as follows:1.Based on N3-(2-pyridoyl)-3-pyridinecarboxamidrazone(2,3’-Hpcad),we prepared three nine-coordinated mononuclear dysprosium complexes,namely,[Dy(2,3 ’-pcad)(NO3)2(CH3OH)2](1),[Dy(2,3 ’-Hpcad)2(H2O)3]· 3Cl·5H20(2)and[Dy(2,3’-pcad)(NO3)(H2O)4]·NO3·H2O(3),respectively,by changing anions and solvents.Thermodynamic studies on the formation reaction of 1-3 were carried out by the isothermal titration calorimetry(ITC)to elucidate the self-assembly regularity of Dy(III)-based SMMs in solution systems,which can provide thermodynamic guidance for directional synthesis.Alternate current(AC)magnetic susceptibility measurements reveal that 2 exhibits single-molecule magnetic behavior under zero DC field,while 1 and 3 show field-induced slow magnetic relaxation behaviour.The magnetic anisotropy of 1-3 was calculated by ab initio calculation with the following order:2>1>3,which is consistent with the order of the effective energy barrier fitted by the experimental data.2.Based on the above work.we selected the isomeric ligand NS-(2-pyridoyl)-3-pyridinecarboxamidrazone(2,4’-Hpcad)and prepared two nine-coordinated mononuclear Dy(III)complexes,namely,[(2,4’-pcad)Dy(NO3)2(H2O)2]·C2H5OH(4)and[(2,4’-pcad)Dy(NO3)2(C2H5OH)2]·H2O(5)under different solvent ratios.The configuration calculations show that compared with 4,the coordination configuration of each Dy(III)ion in 5 is closer to a monocapped square-antiprism.Dynamic magnetic measurements show that 4 does not exhibit zero-field slow magnetic relaxation behavior,while 5 shows zero-field single-molecule magnetic behavior with an effective energy barrier of 203 K.Combined with ab initio calculation and charge analysis,it was further verified that the large magnetic differences between 4 and 5 were mainly attributed to the axis symmetry.A possible coordination model was constructed to realize strong axis symmetry of nine-coordinated mononuclear Dy(III)complexes.In 4 and 5,the interchange between coordination solvents and the lattice solvents promotes the coordination configuration of each Dy(III)ion close to a single ideal configuration,thereby obtaining Dy(III)-SMMs with high axis symmetry,which provides a new model to control the magnetic relaxation behavior.3.Two isomeric three-dentate Schiff base compounds were used as ligands(2,3’-Hpcad and 2,4’-Hpcad).Based on anion regulation,three carboxylate-bridged centrosymmetric dinuclear Dy(Ⅲ)complexes,namely,[Dy2(2,4’-pcad)2(C2H3O2)4(H2O)2]·4H2O(6),[Dy2(2,3’-pcad)2(C2H3O2)4(H2O)2](7)and[Dy2(2,3’-pcad)2(C6H5COO)4(H2O)2](8),respectively.AC susceptibility measurements show that 6-8 exhibit SMM behavior under zero DC field.The experimental data was fitted to obtain effective energy barriers with the the following order:7>8>6.Ab initio calculations show that the ground state gz values of Dy(III)in 6-8 are 19.617,19.581 and 19.688,respectively,indicating that 6-8 have strong single ion axis symmetry.The magnetic exchange coupling constants were obtained by fitting the DC magnetic susceptibility data with the following order:8>7>6,indicating that the higher effective energy barrier values of 7 and 8 are mainly attributed to the stronger Dy(III)-Dy(III)exchange coupling.The antiferromagnetic exchange coupling can effectively suppress the fast quantum tunneling of magnetization(QTM).4.Based on the above study of Dy2-SMMs,we further explore the exchange coupling effect on the magnetic regulation of Dy2-SMMs.Based on N3-(2-pyrazinyl)-3-pyridinecarboxamidrazone(2,3’-Hppcad),we prepared two centersymmetric Dy2 complexes,namely,[Dy2(2,3’-ppcad)2(C2H3O2)4(H2O)2](9)and[Dy2(2,3’-ppcad)2(C2H3O2)4(H2O)2]-6H2O(10)by changing the assembly environment(the molar amount of LiOH·H2O).The configuration calculations show that each nine-coordinated Dy(III)ion in 9 and 10 exhibits a distorted monocapped square-antiprism configuration.Compared with 9,the distortion degree of 10 is smaller.AC magnetic susceptibility measurements reveal that both 9 and 10 show SMM behavior under zero DC field.Ab initio calculations show that the single ion axial symmetry of 10 is lower,while the magnetic exchange coupling effect is stronger.Moreover,the effective energy barrier for the experimental fitting of 10 is higher than the theoretically calculated energy difference between the ground Kramers doublets(KDs)and the first excited KDs,which is due to the antiferromagnetic exchange coupling effectively suppressing QTM.The introduction of lattice water changes the exchange coupling between Dy(III)-Dy(III),thereby optimizing the relaxation process,which provides a strategy for the synthesis of high-performance Dy2-SMMs.