Molecular electronic spins represent one of the most promising building blocks for the design of quantum computing architectures. However, the advancement of this technology requires the increase of spin lifetime at ambient temperature. Spin-phonon coupling has been recognized as the key interaction dictating spin relaxation at high temperature in molecular crystals and the search for chemical-design principles to control such interaction are a fundamental challenge in the field. Here we present a multi-reference first-principles analysis of the g-tensor and the spin-phonon coupling in a series of four exa-coordinate Vanadium(IV) molecular complexes, where the catecholate ligand donor atom is progressively changed from Oxygen to Sulphur, Selenium and Tellurium. A ligand field interpretation of the multi-reference electronic structure theory results made it possible to rationalize the correlation between the molecular g-shifts and the average spin-phonon coupling coefficients, revealing the role of spin-orbit coupling, chemical bond covalency and energy splitting of d-like orbitals in spin relaxation. Our study reveals the simultaneous increase of metal-ligand covalency and electronic excited state energy separation as key elements of an optimal strategy towards long spin-lattice lifetimes in molecular qubits.
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