Tailoring the electronic configurations of YPc₂ on Cu(111): decoupling strategies for molecular spin qubit platforms

Molecule-based spin architectures have been proposed as promising platforms for quantum computing. Among the potential spin qubit candidates, yttrium phthalocyanine double-decker (YPc₂) features a diamagnetic metal ion core that stabilizes the molecular structure, while its magnetic properties arise primarily from an unpaired electron (S = 1/2) delocalized over the two phthalocyanine (Pc) ligands. Understanding its properties in the proximity of metal electrodes is crucial to assess its potential use in molecular spin qubit architectures. Here, we investigated the morphology and electronic structure of this molecule adsorbed on a Cu(111) surface using scanning tunneling microscopy (STM). On Cu(111), YPc₂ adsorbs flat, with isolated molecules showing a preferred orientation along the 〈111〉 crystal axes. Moreover, we observed two different types of self-assembly patterns when growing molecular patches. For YPc₂ in direct contact with Cu(111), STM revealed widely separated highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO), suggesting the quenching of the unpaired spin. Conversely, when YPc₂ is separated from the metal substrate by a few-layer thick diamagnetic zinc phthalocyanine (ZnPc) layer, we found the HOMO to split into singly occupied and singly unoccupied molecular orbitals (SOMO/SUMO). We observed that more than 2 layers of ZnPc are needed to avoid intermixing between the two molecules and spin quenching in YPc₂. Density functional theory (DFT) calculations reveal that spin quenching is due to the hybridization between YPc₂ and Cu(111) states, confirming the importance of using suitable decoupling layers to preserve the unpaired molecular spin. Our results suggest the potential of YPc₂/ZnPc heterostructures as a stable and effective molecular spin qubit platform and validate the possibility of integrating this molecular spin qubit candidate in future quantum logic devices.