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Molecular qubits are a promising platform for future quantum information science technologies; however, to find success in novel devices requires that the molecules exhibit long spin relaxation times. Understanding and optimizing these relaxation times has been shown to be challenging and much experimental work has been done to understand how various chemical features of the molecular qubit influence relaxation times. Here we have curated a data set of relaxation times of metal complex molecular qubits and formulated systems design charts to provide a hierarchical organization of how chemical variables affect relaxation times via known physical processes. We demonstrate the utility of the systems charts by combining examples from the literature with calculated descriptors for molecules in the dataset. This approach helps reduce the complexity associated with de novo molecular design by providing a map of interdependencies and identifying features to prioritize during synthesis. [ABSTRACT FROM AUTHOR]

Molecular qubits are a promising platform for future quantum information science technologies; however, to find success in novel devices requires that the molecules exhibit long spin relaxation times. Understanding and optimizing these relaxation times has been shown to be challenging and much experimental work has been done to understand how various chemical features of the molecular qubit influence relaxation times. Here we have curated a data set of relaxation times of metal complex molecular qubits and formulated systems design charts to provide a hierarchical organization of how chemical variables affect relaxation times via known physical processes. We demonstrate the utility of the systems charts by combining examples from the literature with calculated descriptors for molecules in the dataset. This approach helps reduce the complexity associated with de novo molecular design by providing a map of interdependencies and identifying features to prioritize during synthesis.

Creating the next generation of quantum systems requires control and tunability, which are key features of molecules. To design these systems, one must consider the ground-state and excited-state manifolds. One class of systems with promise for quantum sensing applications, which require water solubility, are d 8 Ni 2+ ions in octahedral symmetry. Yet, most Ni 2+ complexes feature large zero-field splitting, precluding manipulation by commercial microwave sources due to the relatively large spin-orbit coupling constant of Ni 2+ (630 cm -1 ). Since low lying excited states also influence axial zero-field splitting, D , a combination of strong field ligands and rigidly held octahedral symmetry can ameliorate these challenges. Towards these ends, we performed a theoretical and computational analysis of the electronic and magnetic structure of a molecular qubit, focusing on the impact of ligand field strength on D . Based on those results, we synthesized 1, [Ni(ttcn) 2 ](BF 4 ) 2 (ttcn = 1,4,7-trithiacyclononane), which we computationally predict will have a small D ( D calc = +1.15 cm -1 ). High-field high-frequency electron paramagnetic resonance (EPR) data yield spin Hamiltonian parameters: g x = 2.1018(15), g x = 2.1079(15), g x = 2.0964(14), D = +0.555(8) cm -1 and E = +0.072(5) cm -1 , which confirm the expected weak zero-field splitting. Dilution of 1 in the diamagnetic Zn analogue, [Ni 0.01 Zn 0.99 (ttcn) 2 ](BF 4 ) 2 (1') led to a slight increase in D to ∼0.9 cm -1 . The design criteria in minimizing D in 1 via combined computational and experimental methods demonstrates a path forward for EPR and optical addressability of a general class of S = 1 spins., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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 Cr 4+ 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.