Metal-ligand covalency enables room temperature molecular qubit candidates

Harnessing synthetic chemistry to design electronic spin-based qubits, the smallest unit of a quantum information system, enables us to probe fundamental questions regarding spin relaxation dynamics. We sought to probe the influence of metal-ligand covalency on spin-lattice relaxation, which comprises the upper limit of coherence time. Specifically, we studied the impact of the first coordination sphere on spin-lattice relaxation through a series of four molecules featuring V-S, V-Se, Cu-S, and Cu-Se bonds, the Ph₄P⁺ salts of the complexes [V(C₆H₄S₂)₃]^2- (1), [Cu(C₆H₄S₂)₂]^2- (2), [V(C₆H₄Se₂)₃]^2- (3), and [Cu(C₆H₄Se₂)₂]^2- (4). The combined results of pulse electron paramagnetic resonance spectroscopy and ac magnetic susceptibility studies demonstrate the influence of greater M-L covalency, and consequently spin-delocalization onto the ligand, on elongating spin-lattice relaxation times. Notably, we observe the longest spin-lattice relaxation times in 2, and spin echos that survive until room temperature in both copper complexes (2 and 4).