The effect of magnetic coupling along the magnetic axis on slow magnetic relaxation in Dy^III complexes with D_5h configuration based on an aggregation-induced-emission-active ligand

The adjustment of crystal symmetry and intramolecular magnetic coupling is of great importance for the construction of high-performance single-molecule magnets. By using an aggregation-induced-emission-active pyridine-carbohydrazone-based Schiff base ligand and phosphine oxides, four dinuclear and one one-dimensional Dy-III-based complexes, [Dy-2(TPE-pc)2(Bu3PO)2Cl2]center dot 2CH(3)CN center dot 2H(2)O (1), [Dy-2(TPE-pc)2(Cy3PO)(2)Cl-2] (2), [Dy-2(TPE-pc)(2)(MePA)(2)Cl-2]center dot 2CH(3)OH (3), [Dy-2(TPE-pc)2(Ph3PO)(2)Cl-2](2 )(4) and [Dy-2(TPE-pc)(2)(DPPO)Cl2]n (5) (H2TPE-pc = (E)-N'-(2-hydroxy-5-(1,2,2-triphenylvinyl)benzylidene)picolinohydrazide, MePA = N-phenyl-N',N''-bis(morpholinyl) phosphoric triamide, DPPO = piperazine-1,4-diylbis(diphenyl phosphine oxide)), were isolated. All complexes are made up of an enol oxygen-bridged Dy-2 unit, where Dy-III ions possess a pentagonal bipyramidal geometry with pseudo D-5h symmetry. Magnetic measurements reveal that intramolecular Dy-III-Dy-III couplings are ferromagnetic and all complexes display a significant slow magnetic relaxation phenomenon below 30 K under a zero dc field. Ab initio calculations indicate that the anisotropic magnetic axes of all DyIII ions are approximately perpendicular to the higher-order symmetric axes in all complexes, and that DyIII-DyIII magnetic couplings along the magnetic axes effectively suppress the ground state quantum tunneling effect of magnetization and promote the occurrence of slow magnetic relaxation. Raman relaxation prevails in all complexes. In addition, the H2TPE-pc ligand shows an aggregation-induced emission (AIE) effect; however, all complexes exhibit an aggregation-caused quenching (ACQ) phenomenon.