The inherent atomistic precision of synthetic chemistry enables bottom-up structural control over quantum bits, or qubits, for quantum technologies. Tuning paramagnetic molecular qubits that feature optical-spin initialization and readout is a crucial step toward designing bespoke qubits for applications in quantum sensing, networking, and computing. Here, we demonstrate that the electronic structure that enables optical-spin initialization and readout for S = 1, Cr(aryl)4, where aryl = 2,4-dimethylphenyl (1), o-tolyl (2), and 2,3-dimethylphenyl (3), is readily translated into Cr(alkyl)4 compounds, where alkyl = 2,2,2-triphenylethyl (4), (trimethylsilyl)methyl (5), and cyclohexyl (6). The small ground state zero field splitting values (<5 GHz) for 1-6 allowed for coherent spin manipulation at X-band microwave frequency, enabling temperature-, concentration-, and orientation-dependent investigations of the spin dynamics. Electronic absorption and emission spectroscopy confirmed the desired electronic structures for 4-6, which exhibit photoluminescence from 897 to 923 nm, while theoretical calculations elucidated the varied bonding interactions of the aryl and alkyl Cr4+ compounds. The combined experimental and theoretical comparison of Cr(aryl)4 and Cr(alkyl)4 systems illustrates the impact of the ligand field on both the ground state spin structure and excited state manifold, laying the groundwork for the design of structurally precise optically addressable molecular qubits.
Bibliographical noteFunding Information:
We acknowledge support from the US Army Research Office under award number W911NF2010088 for the synthesis (D.W.L. and D.E.F.) and calculation (A.K. and T.C.B.) of new molecular color centers. S.L.B., L.R.W., and D.D.A. acknowledge funding from ONR N00014-17-1-3026 and Tohoku University for execution of optical measurements. D.E.F. and D.D.A. acknowledge support for the integration of molecules into a larger quantum infrastructure from the U.S. Department of Energy Office of Science National Quantum Information Science Research Centers. T.G. acknowledges support from the NSF under award number CHE-1848369. S.M.G. acknowledges support from the Molecular Sciences Software Institute, which is funded by NSF under award number OAC-1547580. K.A.C. acknowledges support from the NSF GRFP through DGE-11842165. This work made use of the IMSERC Crystallography facility at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-2025633), and Northwestern University. Major funding for the Bruker EMXPlus was provided by National Science Foundation Award 1726244 (2017) to the School of Chemical Sciences EPR lab at the University of Illinois. This work also made use of the Caltech EPR facility, which is supported by the NSF (NSF-1531940) and the Dow Next Generation Educator Fund. Metal analysis was performed at the Northwestern University Quantitative Bioelement Imaging Center. The Flatiron Institute is a division of the Simons Foundation.
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