Suppression of Magnetic Quantum Tunneling in a Chiral Single-Molecule Magnet by Ferromagnetic Interactions

Single-molecule magnets (SMMs) retain a magnetization without applied magnetic field for a decent time due to an energy barrier U for spin-reversal. Despite the success to increase U, the difficult to control magnetic quantum tunneling often leads to a decreased effective barrier U_eff and a fast relaxation. Here, we demonstrate the influence of the exchange coupling on the tunneling probability in two heptanuclear SMMs hosting the same spin-system with the same high spin ground state S_t = 21/2. A chirality-induced symmetry reduction leads to a switch of the Mn^III-Mn^III exchange from antiferromagnetic in the achiral SMM [Mn^III₆Cr^III]³⁺ to ferromagnetic in the new chiral SMM ^RR[Mn^III₆Cr^III]³⁺. Multispin Hamiltonian analysis by full-matrix diagonalization demonstrates that the ferromagnetic interactions in ^RR[Mn^III₆Cr^III]³⁺ enforce a well-defined S_t = 21/2 ground state with substantially less mixing of M_S substates in contrast to [Mn^III₆Cr^III]³⁺ and no tunneling pathways below the top of the energy barrier. This is experimentally verified as U_eff is smaller than the calculated energy barrier U in [Mn^III₆Cr^III]³⁺ due to tunneling pathways, whereas U_eff equals U in ^RR[Mn^III₆Cr^III]³⁺ demonstrating the absence of quantum tunneling.