Quantum coherence enhancement through control of metal-ligand covalency: modulating spin-orbit coupling in...

Manipulation of quantum systems for sensing and transduction rely on controlling the interactions between a quantum system and the many degrees of freedom of the bath. In molecular spin quantum systems, spin-orbit coupling serves as a conduit for energy dissipation via vibrational and phonon modes, which in turn are dictated by changes in oxidation state, metal-ligand covalency, and symmetry of the coordination sphere. The confluence of these factors complicate design strategies however for manipulation of spin qubits for quantum sensing and transduction strategies. Here, we report an investigation of the spin dynamics in isostructural S = 1/2 first-row transition metal complexes in which the spin-orbit coupling is varied between a ls-Co(ii)N₄Phen (1-Co) and Cu(ii)N₄Phen (1-Cu) complex. Based on free-ion spin-orbit coupling parameters (528 cm^-1 for Co(ii) and 829 cm^-1 for Cu(ii)), faster spin-lattice relaxation rates (1/T ₁) are initially expected for 1-Cuvs.1-Co. However, X-band pulsed EPR and AC susceptibility reveal that both complexes have nearly identical slow spin-lattice relaxation processes. Notably, decoherence (phase memory times, T _m) at 60 K is longer for 1-Cu (0.63(1) µs) than for 1-Co (0.56(1) µs). Direct observation of d-d splittings, and determination of anisotropic g-values by EPR spectroscopy reveals an effective decrease in spin-orbit coupling for 1-Cu (λ' = 400-435 cm^-1) relative to 1-Co (λ' = 370-400 cm^-1) due to greater metal-ligand covalency in the Cu(ii) complex. Computational modelling of spin density distributions (DFT) and the excited state manifolds (CASSCF) support the differences in excited state energies and spin densities that dictate spin dynamics in these complexes. Two sets of nearly degenerate low-frequency modes were identified as possible vibrational relaxation channels via a two-phonon (Raman) process, consistent with contributions from spin-vibrational orbit interactions. This study provides fundamental insight into the role of metal-ligand covalency in modulating spin-orbit coupling contributions to spin-lattice relaxation and decoherence processes. Increased metal-ligand covalency reduces effective spin-orbit coupling, thereby increasing both spin-lattice and coherence time in molecular spin qubits, providing an important strategy for controlling quantum states and spin-vibrational energy transfer processes in molecular qubit platforms for quantum information processing.