The slow magnetic relaxation regulated by the coordination, configuration and intermolecular dipolar field in two mononuclear DyIII single-molecule magnets (SMMs)

Abstract

Tuning the magnetic dynamics of single-molecule magnets (SMMs) is a crucial challenge for chemists. Some feasible approaches have been developed to understand parts of the magneto-structural correlations and regulate the relaxation behaviors via rational design. Herein, the syntheses, structures and magnetic properties of two mononuclear Dy^III SMMs are reported. The first structural motif reveals a trigonal dodecahedron (D_2d) N₂O₆ coordination environment in 1, while the second one displays a square antiprismatic configuration (D_4d). A Dy⋯Dy distance of 8.589 Å in 1 is clearly shorter than that of 2 (10.433 Å) because of the existence of π⋯π stacking between benzene rings from two adjacent dbpy molecules in 1. The temperature and frequency-dependent out-of-phase ac susceptibility peaks were observed in the absence of a static dc field for 1 and 2. Two distinct thermal relaxation processes were observed in 1, while 2 exhibits one thermal relaxation process. It is interesting that the quantum tunneling of magnetization (QTM) was suppressed when optimum dc fields (1000 Oe) were applied. From ab initio calculations, the energies of the first excited state (KD₁) are indeed close to the experimental relaxation energy barrier (U_eff) under zero dc field, which also reveals the typical features associated with the SMM behavior. In detail, the U_eff values are 103.62 cm⁻¹ (149.87 K) as well as 55.10 cm⁻¹ (79.69 K) for 1 and 116.07 cm⁻¹ (167.87 K) for 2. The KD₁ of 1 (133.82 cm⁻¹) is slightly higher than that of 2 (129.31 cm⁻¹). Comparing 1 and 2, this discrepancy from KD₁ and the experimental U_eff might come from the apparent difference in the magnitude of tunneling probability between the two compounds. In other words, the intermolecular dipolar field plays an important role in their magnetic properties.