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

Engineering the local dynamic environment is currently the major approach for preventing fast magnetization loss for single-molecule magnets (SMMs). It is hypothesized that the presence of fewer −CH3 which results in fewer C−H bonds, would reduce excitation energy loss via vibrations, but thus far, no experimental evidence clearly elaborates this effect. Moreover, although Gu and Wu proposed a vibronic barrier model to interpret the Raman process, the relationship between the barriers (\hbar\omega) and the molecular structure has not been explicitly correlated. Here, we use the trifluoromethyl group to systematically substitute the methyl groups in the axial position of the parental bis-butoxide pentagonal-bipyramidal dysprosium(III) SMM - [Dy(OtBu)2(py)5][BPh4]. The resulting complexes - [Dy(OLA)2py5][BPh4] (LA = CH(CF3)2− 1, CH2CF3− 2, CMe2CF3− 3)- show progressively enhanced TBhys (@100 Oe/s) from 17 K (for 3), 20 K (for 2) to 23 K (for 1). As the gradually enhanced averaged vibration energy generated by different axial ligands from 230 cm−1 (for 3), 257 cm−1 (for 2) to 321 cm−1 (for 1) was identified experimentally and theoretically to be the only variant that leads to this improvement, this finding unambiguously reveals, for the first time, the correlation between structural change and the multi two-phonon (Raman) relaxation processes in lanthanide-based SMMs and highlights the importance of controlling the relevant vibrations in building SMMs with higher blocking temperatures.