Designing chromium(IV) molecular color centers*

Transition metal-based molecular spins are a promising class of chemically tunable quantum bits (qubits), providing precise control over both the physical and electronic structure within a scalable qubit platform. However, molecular systems typically lack an optically addressable ground state spin. To introduce this valuable resource into molecular spins, we recently created a series of chromium(IV) compounds with the desired optical-spin interface through bottom-up design [1]. Here, we illustrate the versatility of this methodology to generate designer molecular color centers by modifying the organic moieties, or ligands, directly bound to the chromium(IV) site. Variation of the surrounding ligands across six unique systems modulates both the optical emission and ground state zero-field splitting (ZFS) energies. Moreover, the small ZFS values allowed for coherent spin manipulation at X-band microwave frequency, enabling temperature, concentration, and orientation dependent investigations of the spin dynamics. Combining the experimental results with electronic structure calculations, we outline how to control the optical-spin interface in such Cr(IV) systems, laying the framework for directed design of structurally precise, optically addressable molecular qubits.