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1. 31 P spin–lattice and singlet order relaxation mechanisms in pyrophosphate studied by isotopic substitution, field shuttling NMR, and molecular dynamics simulation

   https://doi.org/10.1039/D2CP03801C  

   Korenchan, David E. ; Lu, Jiaqi ; Sabba, Mohamed ; Dagys, Laurynas ; Brown, Lynda J. ; Levitt, Malcolm H. ; Jerschow, Alexej ( October 2022 , Physical Chemistry Chemical Physics)

   Nuclear spin relaxation mechanisms are often difficult to isolate and identify, especially in molecules with internal flexibility. Here we combine experimental work with computation in order to determine the major mechanisms responsible for 31 P spin–lattice and singlet order (SO) relaxation in pyrophosphate, a physiologically relevant molecule. Using field-shuttling relaxation measurements (from 2 μT to 9.4 T) and rates calculated from molecular dynamics (MD) trajectories, we identified chemical shift anisotropy (CSA) and spin–rotation as the major mechanisms, with minor contributions from intra- and intermolecular coupling. The significant spin–rotation interaction is a consequence of the relatively rapid rotation of the –PO 3 2− entities around the bridging P–O bonds, and is treated by a combination of MD simulations and quantum chemistry calculations. Spin–lattice relaxation was predicted well without adjustable parameters, and for SO relaxation one parameter was extracted from the comparison between experiment and computation (a correlation coefficient between the rotational motion of the groups). 

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2. Singlet excitation in the intermediate magnetic equivalence regime and field-dependent study of singlet–triplet leakage

   https://doi.org/10.1039/C8CP06883F  

   Kharkov, Boris ; Duan, Xueyou ; Tovar, Emily S. ; Canary, James W. ; Jerschow, Alexej ( January 2019 , Physical Chemistry Chemical Physics)

   The examination and optimized preparation of nuclear spin singlet order has enabled the development of new types of applications that rely on potentially long-term polarization storage. Lifetimes several orders of magnitude longer than T 1 have been observed. The efficient creation of such states relies on special pulse sequences. The extreme cases of very large and very small magnetic equivalence received primary attention, while relatively little effort has been directed towards studying singlet relaxation in the intermediate regime. The intermediate case is of interest as it is relevant for many spin systems, and would also apply to heteronuclear systems in very low magnetic fields. Experimental evidence for singlet–triplet leakage in the intermediate regime is sparse. Here we describe a pulse sequence for efficiently creating singlets in the intermediate regime in a broad-band fashion. Singlet lifetimes are studied with a specially synthesized molecule over a wide range of magnetic fields using a home-built sample-lift apparatus. The experimental results are supplemented with spin simulations using parameters obtained from ab initio calculations. This work indicates that the chemical shift anisotropy (CSA) mechanism is relatively weak compared to singlet–triplet leakage for the proton system observed over a large magnetic field range. These experiments provide a mechanism for expanding the scope of singlet NMR to a larger class of molecules, and provide new insights into singlet lifetime limiting factors. 

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3. Trends in trigonal prismatic Ln-[1]ferrocenophane complexes and discovery of a Ho 3+ single-molecule magnet

   https://doi.org/10.1039/D0SC01197E  

   Latendresse, Trevor P. ; Vieru, Veacheslav ; Upadhyay, Apoorva ; Bhuvanesh, Nattamai S. ; Chibotaru, Liviu F. ; Nippe, Michael ( April 2020 , Chemical Science)

   Lanthanide metallocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to transition metal ions. Herein, we present a systematic study of the structural and magnetic properties of the ferrocenophanes, [LnFc 3 (THF) 2 Li 2 ] − , of the late trivalent lanthanide ions (Ln = Gd ( 1 ), Ho ( 2 ), Er ( 3 ), Tm ( 4 ), Yb ( 5 ), Lu ( 6 )). One major structural trend within this class of complexes is the increasing diferrocenyl (Fc 2− ) average twist angle with decreasing ionic radius ( r ion ) of the central Ln ion, resulting in the largest average Fc 2− twist angles for the Lu 3+ compound 6 . Such high sensitivity of the twist angle to changes in r ion is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 Å). This geometry also allows the non-Kramers ion Ho 3+ to exhibit slow magnetic relaxation in the absence of applied dc fields, rendering compound 2 a rare example of a Ho-based single-molecule magnet (SMM) with barriers to magnetization reversal ( U ) of 110–131 cm −1 . In contrast, compounds featuring Ln ions with prolate electron density ( 3–5 ) don't show slow magnetization dynamics under the same conditions. The observed trends in magnetic properties of 2–5 are supported by state-of-the-art ab initio calculations. Finally, the magneto-structural relationship of the trigonal prismatic Ho-[1]ferrocenophane motif was further investigated by axial ligand (THF in 2 ) exchange to yield [HoFc 3 (THF*) 2 Li 2 ] − ( 2-THF* ) and [HoFc 3 (py) 2 Li 2 ] − ( 2-py ) motifs. We find that larger average Fc 2− twist angles (in 2-THF* and 2-py as compared to in 2 ) result in faster magnetic relaxation times at a given temperature. 

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4. Entangled spin-polarized excitons from singlet fission in a rigid dimer

   https://doi.org/10.1038/s41467-023-36529-6  

   Dill, Ryan D. ; Smyser, Kori E. ; Rugg, Brandon K. ; Damrauer, Niels H. ; Eaves, Joel D. ( March 2023 , Nature Communications)

   Singlet fission, a process that splits a singlet exciton into a biexciton, has promise in quantum information. We report time-resolved electron paramagnetic resonance measurements on a conformationally well-defined acene dimer molecule, TIPS-BP1', designed to exhibit strongly state-selective relaxation to specific magnetic spin sublevels. The resulting optically pumped spin polarization is a nearly pure initial state from the ensemble. The long-lived spin coherences modulate the signal intrinsically, allowing a measurement scheme that substantially removes noise and uncertainty in the magnetic resonance spectra. A nonadiabatic transition theory with a minimal number of spectroscopic parameters allows the quantitative assignment and interpretation of the spectra. In this work, we show that the rigid dimer TIPS-BP1' supports persistent spin coherences at temperatures far higher than those used in conventional superconducting quantum hardware.

    

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5. Probing the structure of vanadium tetracyanoethylene using electron energy-loss spectroscopy

   https://doi.org/10.1063/5.0087997  

   Trout, Amanda H. ; Kurfman, Seth W. ; Shi, Yueguang ; Chilcote, Michael ; Flatté, Michael E. ; Johnston-Halperin, Ezekiel ; McComb, David W. ( August 2022 , APL Materials)

   The molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene (V[TCNE] x , x [Formula: see text] 2) has garnered interest from the quantum information community due to its excellent coherent magnonic properties and ease of on-chip integration. Despite these attractive properties, a detailed understanding of the electronic structure and mechanism for long-range magnetic ordering have remained elusive due to a lack of detailed atomic and electronic structural information. Previous studies via x-ray absorption near edge spectroscopy and the extended x-ray absorption fine structure have led to various proposed structures, and in general, V[TCNE] x is believed to be a three-dimensional network of octahedrally coordinated V 2+ , each bonded to six TCNE molecules. Here, we elucidate the electronic structure, structural ordering, and degradation pathways of V[TCNE] x films by correlating calculations of density functional theory (DFT) with scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) of V[TCNE] x films. Low-loss EELS measurements reveal a bandgap and an excited state structure that agree quantitatively with DFT modeling, including an energy splitting between apical and equatorial TCNE ligands within the structure, providing experimental results directly backed by theoretical descriptions of the electronic structure driving the robust magnetic ordering in these films. Core-loss EELS confirms the presence of octahedrally coordinated V +2 atoms. Upon oxidation, changes in the C1s- π* peak indicate that C=C of TCNE is preferentially attacked. Furthermore, we identify a relaxation of the structural ordering as the films age. These results lay the foundation for a more comprehensive and fundamental understanding of magnetic ordering and dynamics in these classes of metal–ligand compounds. 

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