Ligand Fluorination to Mitigate the Raman Relaxation of Dy^III Single-Molecule Magnets: A Combined Terahertz, Far-IR and Vibronic Barrier Model Study

The fast Raman relaxation process via a virtual energy level has become a puzzle for how to chemically engineer single-molecule magnets (SMMs) with better performance. Here, we use the trifluoromethyl group to systematically substitute the methyl groups in the axial position of the parent bis-butoxide pentapyridyl dysprosium(III) SMM. The resulting complexes—[Dy(OL^A)₂py₅][BPh₄] (L^A=CH(CF₃)₂⁻ 1, CH₂CF₃⁻ 2, CMe₂CF₃⁻ 3)—show progressively enhanced T_B^hys (@100 Oe s⁻¹) from 17 K (for 3), 20 K (for 2) to 23 K (for 1). By experimentally identifying the varied under barrier relaxation energy in the 5–500 cm⁻¹ regime, we are able to identify that the C−F bond related vibration energy of the axial ligand ranging from 200 to 350 cm⁻¹ is the key variant for this improvement. Thus, this finding not only reveals a correlation between the structure and the Raman process but also provides a paradigm for how to apply the vibronic barrier model to analyze multi-phonon relaxation processes in lanthanide SMMs.