Design And Magnetic Investigation Of Bistable Molecular Magnets

Bistable molecular magnet is one of the center research areas in molecule-based materials due to its potential applications in high-density information storage,spintronics,quantum computation,and molecular mechanisms.To explore the magnetism mechanism of single-molecule toroics(SMTs)and spin-crossover(SCO),a series of mononuclear iron and multinuclear lanthanide complexes with magnetic bistability were successfully constructed.Several significant results have been obtained including the exploration of the counter anions effects of SCO properties,the attempt to combine of the SCO with SMTs,the construction of tetrahedron Dy4 and planar Tb4 SMTs,and the assemblies of macrocyclic Dy6 and Dy8 supramolecules.The details are as follows:1.Utilizing the 2,6-bis(pyrazol-1-yl)-4-pyridine-aldehyde(bppCHO)as ligands,two mononuclear iron complexes were isolated by changing the counter anion.Magnetic studies show that both complexes exhibit multistep thermal-induced SCO behavior with hysteresis loops and light-induced excited spin state trapping(LIESST)behavior,which were regulated by the crystal packing effect and the structure distortion related to the counter anion.The aldehyde part on bppCHO is beneficial for constructions of multifunctional complexes,such as fluorescence,SMTs,etc.through ligand modification.Indeed,a hydrazone ligand was obtained by the condensation of o-vanillyl hydrazide with bppCHO.What’s more,two Dy3 complexes based on this ligand have been obtained,exhibiting SMTs and single-molecule magnets(SMMs)properties under zero-field,which is suitable as a precursor for the further construction of heterometallic Dy-Fe complex with SMT and SCO properties.2.A tetranuclear Dy4 complex was obtained from Schiff base ligand that was formed by in situ reaction.Structural analyses show that the DyⅢ ions in the complex reside in the tetragonal antiprism coordination geometry that is beneficial for magnetic anisotropy,and are bridged by μ4-O forming a tetrahedral topology.Magnetic measurements indicate that the complex exhibit a diamagnetic ground state and typical single-molecule magnet behavior.Ab initio calculations show the strong magnetic anisotropy and the well-separated ground state are from the D4d symmetry of DyⅢ ions.In addition,the magnetic anisotropy axes of the spin center are arranged toroidally,confirming the toroidal magnetic moment properties of the complex,which represents the first tetrahedral SMT.3.With the use of the reduced and dimerized polydentate Schiff base ligands,three tetranuclear Tb4 complexes were constructed successfully,in which the TbⅢions are planar arranged forming a square topology.Micro-SQUID technique was carried out on a single-crystal sample and suggests the toroidal arrangement of the magnetic moments.Ab initio calculations are performed on these complexes and confirm the mixed-moment SMT properties.In addition,the differences between the ligand and the counter anion regulate the coordination and topology of the complexes,resulting in a larger split energy barrier and better SMT properties in the Tb4 complex with the reduced ligand.This allowed us to access a new strategy for generating single-molecule toroics in terbium-based molecules.4.Hexanuclear and octanuclear dysprosium macrocyclic complexes were synthesized by using two hydrazone ligands with flexible di-o-vanillin groups.In Dy6 complex,two Dy3 triangles are connected by a macrocyclic ligand in an"edge-to-edge" form.While,in Dy8 complex,four Dy2 units are connected by the ligands to form a macrocyclic topology.Magnetic measurements reveal that the Dy6 exhibits SMT and SMM properties,while Dy8 exhibits field-induced slow magnetic relaxation.Magnetic calculations indicate the SMT is from the sum toroics of the two Dy3 units in complex Dy6,while the DyⅢ ions of the Dy2 units in Dy8 complex are antiferromagnetically coupled resulting in the non-toroidal arrangement.