Richard E. P. Winpenny

The synthesis and crystal structure of a 7-spin supramolecular poly-rotaxane is reported. The [4]rotaxane structure involves a trigonal thread centred on a {CrNi2} core, with each of the three arms terminated by Cu(II) centres and bearing a {Cr7Ni} metallocycle. The macromolecule brings together three distinct electron spin centers. Continuous-wave electron paramagnetic resonance spectroscopy confirms the molecule remains intact in solution, with the three types of spin center each uniquely addressable. Orientation-selective Double Electron Electron Resonance (DEER) and Relaxation Induced Dipolar Modulation Enhancement (RIDME) spectroscopies are used to selectively probe the different spin-spin interactions in the frozen solution phase and to define differences from the solid-state crystal structure.

Attachment of molecular nanomagnets (MNMs) to surfaces is an essential step toward multi‐qubit assemblies for the implementation of quantum information technologies. Here, thiol‐monofunctionalized {Cr ₇ Ni} molecular antiferromagnetic rings that have potential as electron spin qubits have been synthesized and successfully attached to the surface of a highly luminescent core‐shell CdSe/ZnS quantum dot (QD) nanoparticle via a ligand exchange process. The successful grafting of eleven molecular rings per QD was confirmed by high‐angle annular dark field (HAADF) scanning transmission electron microscope (STEM) imaging, energy dispersive x‐ray (EDX) imaging, and dynamic light scattering (DLS) measurements. Photoluminescence (PL) and pulse electron paramagnetic resonance (EPR) spectroscopies indicated that the resulting hybrid nano‐objects display the bright optical emission provided by CdSe/ZnS QDs and long‐lived quantum coherence associated with eleven grafted {Cr ₇ Ni} molecular qubits. Thus, grafting of MNMs to nanoparticle surface provides an elegant method for generating hybrid assemblies with magnetic optical dual functionality and a facile approach to scale electron spin qubits.

We report a new class of heterovalent, homometallic ring complexes, with {Ti ^IV ₇ Ti ^III } cores: [py‐CH ₂ NH ₂ Et][Ti ₇ TiO ₈ (O ₂ CR) ₁₆ ] (R = ^t Bu and cyclopropyl). The compounds are made from [Ti(O ⁱ Pr) ₄ ] reacted with carboxylic acid and py‐CH ₂ NHEt. We also show that {Ti ^IV ₇ Ti ^III } rings can be generated by photoreduction of {Ti ^IV ₈ } precursors in the presence of a suitable sacrificial electron donor, of relevance to photocatalysts. UV–vis/NIR spectroscopy of the {Ti ^IV ₇ Ti ^III } rings reveal Robin and Day Class II mixed‐valent behavior at room temperature, while EPR spectroscopy is consistent with a localized 3d ¹ ( s = 1/2) configuration, with C ₂ local symmetry, at low temperatures. We compare the EPR behavior of these new rings with the isostructural heterometallic {Cr ^III ₇ Ni ^II } rings [py‐CH ₂ NH ₂ Et][Cr ₇ NiF ₈ (O ₂ CR) ₁₆ ], which also have s = ½ ground states, due to strong intramolecular antiferromagnetic exchange interactions. We demonstrate that the electron spin lattice relaxation time constants ( T ₁ ) for {Ti ^IV ₇ Ti ^III } are over two orders of magnitude longer at low temperatures than for the {Cr ₇ Ni} equivalents, and that phase memory time constants ( T _m ) are maintained to much higher temperatures.

Metallosupramolecular assemblies are promising platforms for selective extraction, catalysis, and molecular recognition, yet their solution-phase synthesis often relies on complex self-assembly pathways that can be difficult to control. Here, using ion mobility mass spectrometry supported by density functional theory calculations, we introduce a gas-phase approach for directing topology in polymetallic complexes based on the encapsulation of coordination metal cations within a homometallic {Cr₈} ring. Upon inclusion, the resulting host-guest complexes undergo collision-induced dissociation to produce distinct polymetallic fragments whose topologies, ring, host-guest complex or open chain, depend on the identity of the guest cation. This topology switching enables the targeted synthesis of new polymetallic rings, host-guest complexes and open chains in vacuo and provides insight into the stoichiometric and conformational preferences of such assemblies.

A new heterometallic ring, [Me ₄ N][(Me ₄ N) ₂ Cr ₅ Ni ₃ F ₈ (BTFB) ₁₆ ] (BTFB = 3,5‐bis(trifluoromethyl)benzoate) is reported, templated about tetramethylammonium (Me ₄ N) ⁺ . This ring features five Cr(III) centers and three Ni(II) centers at the vertices of an octagon. Each edge of the octagon is bridged by internal fluoride and two external BTFB ligands. In the crystal structure, two of the three ammonium cations are at the center of the octagonal ring, while the third cation is outside. X‐ray single crystal diffraction does not distinguish between five possible isomers, but a single isomer, with nickel sites at the 1, 3, and 6 positions of the octagon, can be deduced by a combination of collision‐induced dissociation mass spectrometry (CID‐MS), and EPR spectroscopy and magnetometry, which show a ground state spin of ¹ / ₂ . The same reaction carried out with cobalt(II) or zinc(II) in place of nickel(II) leads to ordered [(Me ₄ N) ₂ Cr ₆ M ₂ F ₈ (BTFB) ₁₆ ] (M = Co or Zn) in which the divalent metals are placed at the 1,5‐positions in the metal octagon.

We report ligand manipulation of high-nuclearity, paramagnetic transition metal clusters based on heterometallic {Cr₇Ni} rings to significantly prolong electron spin phase memory time constants. This enables the application of Double Electron-Electron Resonance (DEER) spectroscopy methods to measure cluster···cluster interactions to longer distances. Specifically, replacing bridging pivalates by cyclopropyl-carboxylates enables DEER up to 4.2 nm. This is applied to four new hybrid [2]- and [3]rotaxanes based on hetero- and homodimers, respectively, of {Cr₇Ni} rings about organic threads. The crystal structures of these supramolecules are reported and are in excellent agreement with the DEER results.

Molecular magnets are attractive as spin qubits due to their chemical tunability, addressability through electron-spin resonance techniques, and long coherence times. Clock transitions (CTs), for which the system is immune to the effect of magnetic-field fluctuations to first order, provide a method to enhance the coherence time T ₂, and to reveal mechanisms of decoherence that are not due to such fluctuations. Here we investigate two variants of Cr₇Mn, a spin-1 molecular nanomagnet, at fields near a zero-field CT. We find that at temperatures ≤2 K, T ₂ ∼ 1 μs at the CT using a Hahn-echo pulse sequence. Away from the CT, electron-spin–echo envelope modulation (ESEEM) oscillations due to coupling to nuclear spins are observed and have a T ₂ as high as 1.35 μs, indicating a distinct mechanism of coherence preservation. Dynamical decoupling with the CPMG pulse sequence yields T ₂ ∼ 2.8 μs at the CT and up to ∼3.6 μs in the ESEEM regime along with a demodulation of the oscillatory behavior. The experimental values of T ₂ are largely independent of the degree of dilution of the molecules in solvent or whether the solvent is deuterated, indicating that much of the decoherence and ESEEM arises from sources within the molecules themselves. To account for decoherence, we develop a model that includes not only field fluctuations but also fluctuations in the CT transition frequency itself. Our results can be well explained by treating the environment as a combination of noise at the nuclear Larmor precession frequency and 1/f noise in the transverse anisotropy parameter E. Such information about the microscopic origins of decoherence can aid the rational design of molecular-based spin qubits.

Molecular assemblies are commonly found in biological systems, and designing synthetic mimics of these is a challenge for modern chemistry. Here, we apply ion mobility mass spectrometry (IM‐MS), density functional theory (DFT), and mass‐selective electrospray ion beam deposition followed by low‐temperature scanning tunneling microscopy (STM) to decipher the self‐assembly, rearrangement, and disassembly processes of {Cr ₆ } _n horseshoe oligomers ( n = 1–5). Activated tandem IM‐MS reveals the oligomer disassembly in detail, highlighting the stability of the dimer unit. When n = 2 and n = 3 oligomers are deposited on surfaces, we observe the rearrangement of dimers and trimers to dimers of dimers, and at higher coverages, the formation of an unexpected hexagonal‐like network. In its entirety, the experimental and computational data provide a convincing framework for the analysis of supramolecular assembly processes in noncrystalline phases that could be used in future design strategies.