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 ) 2 py 5 ][BPh 4 ] (L A =CH(CF 3 ) 2 - 1, CH 2 CF 3 - 2, CMe 2 CF 3 - 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.