Surface Stabilization to Enhance Single Molecule Toroidal Behavior in {Dy₃} Molecules: the Impact of Au(111), MgO, and Graphene Surfaces

Single-molecule toroids (SMTs), with vortex-like magnetic anisotropy axes, hold promise for quantum technologies, but controlling their toroidal states on the surface remains challenging. To address this, the SMT behavior of [Dy₃(µ₃-OH)₂L₃Cl(H₂O)₅]Cl₃ (where L = ortho-vanillin) grafted onto Au(111), MgO has been studied, and graphene surfaces in pristine form (1) and with pyrene (2) and (CH₂)₈S (3) linkers, using periodic density functional theory and ab initio CASSCF/RASSI-SO methods. Both pristine and chemically functionalized molecules are stable on Au(111) and graphene surfaces; however, functionalization provides higher binding energies and, in some cases, enhances the SMT properties. The MgO surface, however, is found to be unsuitable as it abstracts an H atom from the molecule, leading to the loss of its SMT characteristics. The energy gap (ΔE) between the toroidal (nonmagnetic) and spin-flip (magnetic) states in complex 1 on Au(111) and graphene surfaces are 6.9 and 6.6 cm^-1, respectively. Complexes 2 on Au(111) and 3 on graphene exhibit ΔE and toroidal blocking fields of 9.8 cm^-1/1.2 T and 6.8 cm^-1/0.83 T, respectively, representing the highest recorded values for this class of SMTs. These findings demonstrate the potential of surface stabilization to improve the functionality and applicability of SMTs in advanced quantum technologies.