Chemical tuning and on-chip tranmission experiments of clock transitions in molecular spin qubits*

We report a sizeable quantum tunnelling splitting for the mononuclear Ni(II) molecular complexes [Ni(Me₆tren)Cl](ClO₄) (1) and [Ni(2-Imdipa)(NCS)](NCS) (2). With their S = 1 ground state and strong anisotropy, these molecules provide a realization of the simplest non-Kramers system (integer spin). The “clock transition” between levels associated with superpositions of m_S = ±1 spin states, with its characteristic non-linear magnetic field dependence, has been directly monitored by heat capacity experiments. The comparison of complex 1 with a Co derivative (S = 3/2), for which tunnelling is forbidden, shows that the clock transition leads to an effective suppression of intermolecular spin–spin interactions. We also show that the splitting admits a chemical tuning via the modification of the ligand shell that determines the magnetic anisotropy. In particular, the weaker magnetic anisotropy of complex 2 makes its qubit frequency compatible with superconducting microwave circuits, and has allowed its direct detection by on-chip broadband transmission experiments.