Unravelling the Reason for Transition from the Field-Induced Single-Molecule Magnet to Zero-Field One in a Group of Dysprosium Complexes via Ab Initio Calculation.

Single-molecule magnets (SMM) hold the potential of increasing the information-storage density in a revolutionary way. However, the necessity of applying an external magnetic field in most SMMs is detrimental. Thus, the transition from field-induced SMMs to zero-field ones deserves intensive study. Here, we report an ab initio study of three dysprosium complexes, , , and . Although having the same coordination pocket, and are field-induced SMMs, but changes to be a zero-field one. Without applying a field, the quantum tunnelling of magnetization (QTM) rate, i.e., the lowest relaxation rate, of and is predicted to be higher than the upper limit of the experimental apparatus by at least 1 order of magnitude. Thus, the experimental apparatus is hardly capable of recording the SMM behaviors of and under zero field. The zero-field QTM rate of lies within the experimental detective range, and thus applying a field is not necessary. This difference arises mainly from their different magnetic axialities, of which the highest one comes from . Covalent interaction is usually assumed to be insignificant in lanthanide complexes. However, our analysis based on the crystal field model and perturbation theory indicates the crucial role of covalent interaction here. When the covalent effect is excluded via replacing all the ligand atoms by atomic charges, ab initio calculations indicate that the zero-field SMM will change to be and rather than . The change of the QTM rate, given by the covalent effect, could be as much as 3 to 4 orders of magnitude.