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-polarised by optical excitation, positioning them as long-lived, tunable, and scalable qubits. Recent advancements have utilised 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 minimise deleterious ISC steps through destructive interferences, which can be accentuated by populating the carbene non-bonding orbital. These chemically intuitive results not only inform qubit designs but also offer a minimal Hamiltonian framework for accelerating high-throughout 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 characterisation over different carbene bond angles.