Construction,structure and Magnetic Relaxation Dynamics Study of Dysprosium Single-molecule Magnets with D4d/D5h Configuration

Author:Jiang Zhi Jie

Supervisor:chen san ping


Degree Year:2019





The potential application value of single-molecule magnets(SMMs)in high-density information storage,quantum computing and molecular spintronics has aroused great interest.Compared to the transition metal ions,rare earth ions are suitable as spin carriers for the design and synthesis of high performance SMMs thanks to their large magnetic moments and strongly inherent magnetic anisotropies caused by strong spin-orbit coupling.Among them,Dy(III)ions having a high spin ground state value(6H15/2)and a strong Ising type magnetic anisotropy have been attracting attention.Studies have shown that Dy(III)ions have a high coordination number and a flexible coordination geometry.Therefore,magnetic relaxation dynamic behavior depends on the structure,and minor structural changes may cause significant differences in magnetic properties.It is found that Dy(III)ions in high symmetry crystal field environments(D4d,D5h,and D∞...)are beneficial for eliminating non-axial magnetic anisotropy and can largely suppress quantum tunneling effects(QTM).It is beneficial to increase the effective energy barrier(Ueff)and increase the blocking temperature(TB).Based on the above considerations,polydentate chelating ligands L1-L3 were used to construct eight dysprosium-based compounds with D4dd and D5h configuration by changing the self-assembly conditions.Combined with ab initio calculation,their magneto-structural correlations and magnetic relaxation mechanism were deeply studied.The research results will provide experimental support for the accurate interpretation of the relaxation mechanism of SMMs,and provide important theoretical value and application significance for its design,synthesis and magnetic chemistry.The specific research results and conclusions are as follows:1.Based on 2-(2′-hydroxy-5′-chlorophenyl)-1,3-bis[3′-aza-4′-(2″-hydroxy-5″-chlorophenyl)prop-4′-en-1′-yl]-1,3-imidazolidine(L1,H3Clapi),we prepared two eight-coordinated dinuclear dysprosium compounds with D4d symmetry,namely,[Dy(Clapi)]2·(CH2Cl2)2(1 and2),by changing the ratio of methanol and dichloromethane in the mixed solvent.The coordination configurations of compounds 1 and 2 are almost identical,the only difference being the orientation of the solvent molecule CH2Cl2 in the crystal lattice.Alternate current(AC)magnetic susceptibility measurements reveal that the two have completely different magnetic behavior.Among them,compound 1 did not exhibit zero field slow magnetic relaxation behavior,while compound 2 showed SMMs behavior under zero DC field.Ab initio calculations show that the significant magnetic differences between 1 and 2 are mainly due to different intra-molecular Dy(III)-Dy(III)exchange coupling(Jexchange).This work presents the rare example that the magnetic dynamic behaviors of Dy(III)compounds could be fine-tuned by the lattice orientation of solvent molecules,which offers an illuminating insight into the magneto-structural relationship of Dy-SMMs.2.Based on the previous work,a series of binuclear dysprosium compounds,namely,[Dyapi]2(3),[Dy(api)]2·2CH2Cl2(4),[Dy(Clapi)]2·2C4H8O(5),and[Dy(Clapi)]2·2C3H6O(6)(H3api:2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazoline),have been isolated by the reactions of salen-type ligands L1 and L2,DyCl3?6H2O in methanol,methanol/dichloromethane(1:2),methanol/tetrahydrofuran(1:2),methanol/acetone(1:2),respectively.The configuration calculations exhibit that Dy(III)ions in compounds 3-6 adopt distorted square antiprism geometry with D4d symmetry.Dynamic magnetic measurements showed that compounds 4,5 and 6 exhibited field-induced slow magnetic relaxation behavior with Ueff of 18.2 K(4),28.0 K(5)and 16.4 K(6),respectively.However,in the case of zero field or applied field,the compound 3 did not exhibit SMMs properties.Ab initio calculation is used to explain the difference in magnetic behavior between the compounds,and the results show that the dominant one is also the intra-molecular Dy(III)-Dy(III)exchange coupling.The comparison between the experimental and theoretical magnetostructural data for all compounds give a more thorough understanding of the effects of structural factors on the relaxation processes in the type of Dy2 SMMs.3.In this chapter,a large excess of electrostatic repulsion,arising from the axial ligands,over that from the equatorial ligands is taken as the design strategy for high performance pentagonal bipyramidal(D5h)Dy(III)single-ion magnets(SIMs).In this strategy,two PBP Dy-SIMs(7 and 8)[Dy(bbpen-CH3)X](X=Cl,7;Br,8;H2bbpen=N,N’-bis(2-hydroxybenzyl)-N,N’-bis(2-methylpyridyl)ethylenediamine)were synthesized and structurally characterized on the basis of a highly symmetrical ligand H2bbpen-CH3.Dynamic magnetic measurements verify the value of our design strategy since compound 8 exhibits high performance with large Ueff(above 1000 K)and a high TB(15 K).Ab initio calculations further verified the importance of the high excess of axial interaction which eventually leads to the special electronic structure possessing the desired magnetic properties.The search for excessive axial repulsion is not incompatible with the strategy based on local symmetry around the central ion since various high-performance D5h Dy-SIMs of both clearly distorted and nearly ideal geometries successfully acquire such a kind of excess.Apparently,this study presents an alternative to the designing strategy for promising SIMs.