Quantifying the Spin-Optical Properties of Bent π-Carbene Molecular Qubits

Through spin-selective intersystem crossings (ISCs) among photoexcited electronic states, organic π-diradicals can be spin-polarized by optical excitation, positioning them as long-lived, tunable, and scalable qubits. Recent advancements have utilized triplet π-carbenes of bent structures, which unlock an additional spin-selective ISC channel between the ground states, improving the qubit's spin-optical properties. However, bent π-carbenes also lose spin selectivity in the excited-state ISC step and a compromise has yet to be found. In this work, we balance the two effects by developing a minimal Hamiltonian for bent π-carbenes, the first of its kind, and solving its spin-optical interactions analytically. Our approach is a valuable alternative to large-scale ab initio screenings, which are highly intensive given the costs of existing open-shell ab initio methods. We find the ideal π-carbene to have a high spin density on its central atom and excitations dominated by HOMO-to-SOMO transitions. These strategies minimize deleterious ISC steps through destructive interferences, which can be accentuated by populating the carbene nonbonding orbital. These chemically intuitive results not only inform qubit designs but also offer a minimal Hamiltonian framework for accelerating high-throughput discovery of optically addressable π-carbenes. As an illustration, we exemplified our design rules on a realistic molecular prototype, completed by a rigorous ab initio characterization over different carbene bond angles.