Realizing atomic-level spatial control over qubits, the fundamental units of both quantum information processing systems and quantum sensors, constitutes a crucial cross-field challenge. Toward this end, embedding electronic-spin-based qubits within the framework of a crystalline porous material is a promising approach to create precise arrays of qubits. Realizing porous hosts for qubits would also impact the emerging field of quantum sensing, whereby porosity would enable analytes to infuse into a sensor matrix. However, building viable qubits into a porous material is an appreciable challenge because of the extreme sensitivity of qubits to local magnetic noise. To insulate these frameworks from ambient magnetic signals, we borrowed from atomic physics the idea to exploit clock transitions at avoided level crossings. Here, sensitivity to magnetic noise is inherently limited by the flat slope of the so-called clock transition. More specifically, we created an array of clocklike qubits within a metal-organic framework by combining coordination chemistry considerations with the fundamental concept of atomic clock transitions. Electron paramagnetic resonance studies verify a clocklike transition for the hosted cobalt(II) spins in the framework [(TCPP)CoZn][ZrO(OH)(HO)], the first demonstration in any porous material. The clocklike qubits display lifetimes of up to 14 μs despite abundant local nuclear spins, illuminating a new path toward proof-of-concept quantum sensors and processors with high inherent structural precision.

We describe and employ a high-throughput screening method to accelerate the synthesis and identification of pure-phase, nanocrystalline metal-organic frameworks (MOFs). We demonstrate the efficacy of this method through its application to a series of porphyrinic zirconium MOFs, resulting in the isolation of MOF-525, MOF-545, and PCN-223 on the nanoscale.

The porphyrinic metal-organic framework (MOF) PCN-224 is metalated with Fe(II) to yield a 4-coordinate ferrous heme-containing compound. The heme center binds O2 at -78 °C to give a 5-coordinate heme-O2 complex. For the first time, this elusive species is structurally characterized, revealing an Fe(III) center coordinated to superoxide via an end-on, η(1) linkage. Mössbauer spectroscopy supports the structural observations and indicates the presence of a low-spin electronic configuration for Fe(III). Finally, variable-temperature O2 adsorption data enable quantification of the Fe-O2 interaction, providing a binding enthalpy of -34(4) kJ/mol. This value is nearly half of that observed for comparable 6-coordinate, imidazole-bound heme-O2 complexes, a difference that further illustrates the importance of axial ligands in biological heme-mediated O2 transport and storage. These results demonstrate the ability of a MOF, by virtue of its rigid solid-state structure, to enable isolation and thorough characterization of a species that can only be observed transiently in molecular form.