Slow Magnetic Relaxation in a Californium Complex

We report the synthesis and characterization of the macrocyclic californium derivative Na[Cf(H₂O)(DOTA)] (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), <b>1-Cf</b>, which was studied in comparison to its dysprosium counterpart, Na[Dy(H₂O)(DOTA)], <b>1-Dy</b>. Divergent spectroscopic and magnetic behaviors were observed between <b>1-Cf</b> and <b>1-Dy</b>. Based upon spectroscopic measurements, we propose that accessible 5f → 6d transitions (potentially operating in tandem with charge-transfer transitions) are the major contributors to the observed broadband photoluminescence in <b>1-Cf</b>. Dc magnetic susceptibility data for <b>1-Cf</b> revealed lower magnetic moments than those previously observed for <b>1-Dy</b> and expected for an f⁹ free ion, which calculations suggest is the result of greater ligand field effects. Notably, <b>1-Cf</b> displays slow magnetic relaxation on the time scale of ac susceptibility measurements, making it the first example of a californium-based single-molecule magnet. A side-by-side comparison of the ac susceptibility data reveals magnetic relaxation properties that widely differ between <b>1-Cf</b> and <b>1-Dy</b>. This divergent relaxation behavior is attributed mainly to the inherent difference in spin-orbit coupling between Dy³⁺ and Cf³⁺.