First-principles investigation of ligand and host effects in Cr(IV)-based molecular color centers

Chromium(IV)-based molecular qubits represent a promising platforms for quantum technologies, and an alternative to solid state color centers. They can be designed following a bottom-up approach that allows for tunable properties, while still retaining the desirable optical-spin interface of solid-state spin defects. Notably, the properties of molecular qubits depend on the metal and ligand structure, as well as on the host crystalline diamagnetic matrix (usually Sn). Thus, precise strategies for the control of the properties of molecular qubits require both ligand and host effects to be elucidated. Here we combine several first-principles approaches to disentangle these effects in a series of Cr(IV)-aryl molecular qubits. We perform density functional theory calculations to optimize the geometry of in vacuo and embedded qubit structures and we evaluate their emissive properties using multireference approaches. Our calculations reveal that both the ligand and the host-induced distortions affect the emissive properties of the qubit. Particularly, we find that radiative decay processes are mainly affected by ligand and host-induced symmetry lowering of the qubit geometry; the host plays a major role in determining possible non-radiative decay pathways that could affect the qubit emissive behavior. We expect our findings to be useful in guiding the design of efficient molecular qubits, and in providing guidelines on how to accurately study these materials at the first-principles level.