Harnessing the Spin-Flip Radiative Lifetimes of Optically Addressable Molecular Qubits

Optically addressable molecular qubits based on spin-flip (SF) emissive transitions are promising candidates for quantum technologies, due to their sharp luminescence lines and tunable optical-spin interfaces. Yet, the microscopic mechanisms controlling the spin-flip radiative lifetime of SF emitters, a key property for efficient spin readout, remain largely unexplored. Here, we present a computational study of several Cr⁴⁺ and Mo⁴⁺ pseudo-tetrahedral molecular qubits, and we identify chemical and structural design rules to optimize the values of the transition dipole moment associated to the SF emission. We find that the magnitude of the dipole moment is governed by the multireference character of the spin-flip excited-state wavefunction, which can be engineered by tuning the energy separation between the d orbitals of the metal and the spin-pairing energy. Both parameters are sensitive to molecular symmetry, metal-ligand bond covalency and bond anisotropy and are thus efficiently tunable via ligand and metal design, as well as applied strain, which is relevant for sensing applications. Our findings provide a quantitative framework for engineering SF lifetimes in molecular qubits and SF emitters, offering valuable design principles for applications in quantum information science and molecular sensing and metrology.