Chiral Dysprosium-[7]Helicene Macrocycles Showing Record Single-Molecule Magnet Properties in the Lanthanide–Helicene Family

Chiral helicene-based metal complexes have emerged as an extremely promising class of multifunctional molecules for a wide range of applications. Despite significant progress in the synthesis of helicene-based transition-metal complexes in recent decades, lanthanide species lag far behind and their number is very limited. Compared to the widely studied optical activity and magneto-chiroptical effects, the single-molecule magnet (SMM) properties of lanthanide-helicene compounds, in particular their spin relaxation dynamics, are still largely underexplored. In this work, starting from the chiral Dy(III) hexaazamacrocycles, we prepared the first lanthanide-[7]helicene enantiomers using [7]helicene-2-hydroxy as an axial ligand. The introduction of [7]helicene not only endows the compounds of relatively large molar absorptivities but also land them extremely high thermal stability with decomposition temperature beyond 320 °C. Remarkably, in terms of the traditional metrics of SMMs, these molecules are the best lanthanide-helicene SMMs to date, showing an effective energy barrier (<i>U</i>_eff) exceeding 600 cm^-1 and open hysteresis loops at zero field up to 12 K using a sweep rate of 200 Oe/s. Ab initio spin dynamics calculations and phonon analysis reveal that vibrations of the equatorial benzene rings that are in <i>trans</i>-diaxial manner and the two nearest nitrogen atoms dominantly contribute to the Orbach relaxation, driving the relaxation of the magnetization via the third Kramers doublet. Raman relaxation, responsible for the hysteresis closure at 12 K, is instead driven by delocalized phonons involving rigid movements of the ligands, counterions, and solvent. These findings pave a new way toward preparing and modulating magnetodynamics of lanthanide-helicene compounds.