Unconventional Nuclear-Spin-Dependent Toroidal Ground States in Isotopologue ^ADy₄ [2 × 2] Complexes

Understanding how nuclear spin influences electronic spin relaxation in single-molecule magnets (SMMs) is essential for advancing quantum technologies. Isotopologue coordination chemistry offers a promising route to probe nuclear-spin effects, yet its role in relaxation dynamics into Single Molecule Toroic (SMT) systems remains unexplored. Here, we study two isotopically enriched ^ADy₄ grid complexes, ¹⁶⁴Dy₄L₄ (with I = 0) and ¹⁶³Dy₄L₄ (with I = 5/2), combining μSQUID magnetometry down to 30 mK with ab initio calculations and spin-Hamiltonian modeling. Both isotopologues exhibit a pseudotoroidal ground state stabilized by noncollinear anisotropy axes and competing interactions, producing hexagonal angular maps and S-shaped hysteresis loops. The nuclear-spin-free ¹⁶⁴Dy₄L₄ displays sharp quantum tunneling of magnetization (QTM) transitions, whereas hyperfine coupling in ¹⁶³Dy₄L₄ generates a dense manifold of crossings, profoundly altering relaxation dynamics. Contrary to expectations, nuclear spins do not accelerate relaxation but promote hyperfine-mediated population transfer into the toroidal state, yielding slower dynamics and larger hysteresis openings. Temperature and sweep-rate dependences further reveal contrasting mechanisms: pure tunneling in ¹⁶⁴Dy₄L₄ versus thermally assisted processes in ¹⁶³Dy₄L₄. Overall, our findings demonstrate that nuclear spins exert a constructive and nonintuitive influence on toroidal SMMs, providing an additional lever for controlling low-temperature relaxation. Isotopically controlled lanthanide assemblies thus emerge as promising platforms for exploring quantum tunneling, spin-phonon coupling, and molecular-scale information storage/processing.