Controlling Zero-Field Splitting in Molecular Spin Qubits via Conjugation with Insights into Optical Addressability.

Molecular spin qubits based on transition-metal complexes offer scalable alternatives to spin-defect qubits with performance governed by zero-field splitting (ZFS) and singlet-triplet gaps (ΔEST). Here, we leverage the conjugation length of polyacene ligands by tuning the number of fused rings to tailor the ZFS of chromium-acene complexes. Multireference ab initio calculations show that the axial ZFS parameter (D) increases with an increase in the number of fused rings in the ligands, while the transverse component (E) remains negligible. This enhancement in D values originates from spin-spin correlations and amplified spin-orbit coupling, driven by larger effective spin-orbit coupling constants (ζeff), reduced ligand-field splitting (Δ), and diminished metal-ligand covalency. All complexes exhibit |D| < 9 GHz and ΔEST values (1.09-1.29 eV) in the near-IR range, ensuring that these molecular qubits are both optically addressable and compatible with the X-band frequencies employed in EPR spectroscopy for qubit operations. Analysis of d-d transition energies and simulated absorption spectra further reveals that all complexes possess a triplet ground state and well-separated excited singlet and triplet manifolds, enabling a robust spin-optical interface. Overall, our results demonstrate that the ligand conjugation length can be used as a molecular design strategy for tuning the magnetic anisotropy in molecular spin qubits.