L.B.

Phonons are important in energy transfer for compensating the energy mismatch between a donor and an acceptor. In inorganic hosts doped with lanthanides, phonon-assisted energy transfer can lead to quenching by a direct transfer of the energy to the phonon mode of the acceptor lanthanide. We demonstrate that this also applies to lanthanide coordination polymers and is the reason for the weak concentration quenching in this type of material. [Yb_xGd_1–x(hfa)₃dpbp]_n [x = 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1; hfa: hexafluoroacetylacetonate, dpbp: 4,4′-bis(diphenylphosphoryl)biphenyl] coordination polymers have been synthesized and their photophysical properties investigated. The single-exponential emission lifetimes of Yb³⁺ gradually decreased with increasing concentration of Yb³⁺. Qualitative agreement was achieved between the experimental and theoretical lifetimes by using the phonon-assisted energy-transfer-induced quenching model, which indicates that the origin of the concentration quenching in the coordination polymers is phonon-assisted energy transfer.

Lanthanide coordination polymers with various concentrations of Yb³⁺ have been synthesized to investigate the concentration-quenching mechanism. Experimental and theoretical considerations showed that concentration quenching occurs by nonradiative phonon-assisted energy-transfer relaxation.

Taking advantage of the steric hindrance and charge-driving effects, four air-stable pentagonal bipyramidal mononuclear Dy^III compounds were hydrothermally synthesized. With a tetradentate ligand, N,N′-bis(2-methylenepyridinyl)ethylenediamine (Bpen), invariably coordinates to Dy^III in an equatorial plan, 1–3 achieve an orderly transformation of the ligand field by sequentially replacing the remaining sites of the Dy^III ion. Compound 4 possesses the same coordination atoms but a different peripheral coordination sphere with 3. Magnetic characterizations display that the compounds are field-induced single-ion magnets (SIM) with actually low barriers, even though 2 has both the same atoms and a similar geometry of the first sphere compared with [Dy(bbpen)Cl] (2′, H₂bbpen=N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)ethylenediamin), a high-performance SIM previously reported. Detailed ab initio calculations have been employed to further elucidate the electronic and magnetic structure of the low-lying energy levels of compounds 1–4 and 2′. The theoretical results indicate there is an apparent difference in the electronic structure for these compounds. The analysis on the electrostatic potential further demonstrates that although the pentagonal bipyramidal D_5h is one of the ideal configurations expected, the electron density of the donor atoms from the different hybridizations and other functional groups, outside the first sphere, should also be considered in the rational design of promising molecular magnets.

A detailed comparison of the structural and magnetic properties in a series of D5h symmetry DyIII SIMs suggests that the electron density affected by different hybridization of donor atoms and other functional groups outside the first sphere remarkably influence the axiality of the KDs and the magnetic relaxation behaviors (see figure)

Double-arm cyclen-based Gd³⁺ tags are shown to produce accurate nanometer scale Gd³⁺–Gd³⁺ distance measurements in double electron–electron resonance (DEER) experiments by confining the space accessible to the metal ion. The results show excellent agreement with predictions both for the maximum and width of the measured distance distributions. For distance measurements in proteins, the tags can be attached to two cysteine residues located in positions i and i+4, or i and i+8, of an α-helix. In the latter case, an additional mutation introducing an aspartic acid at position i+4 achieves particularly narrow distribution widths. The concept is demonstrated with cysteine mutants of T4 lysozyme and maltose binding protein. We report the narrowest Gd³⁺–Gd³⁺ distance distributions observed to date for a protein. By limiting the contribution of tag mobility to the distances measured, double-arm Gd³⁺ tags open new opportunities to study the conformational landscape of proteins in solution with high sensitivity.

Metal on a tether: Double-arm cyclen-based Gd³⁺ tags confine the space accessible to the metal ion (see figure) and produce accurate nanometer scale Gd³⁺–Gd³⁺ distance measurements by double electron-electron resonance (DEER) experiments, which show excellent agreement with predictions both for the maximum and width of the measured distance distributions. This research opens new opportunities to study the conformational landscape of proteins in solution.

Eu^II-containing complexes were studied with respect to properties relevant to their use as contrast agents for magnetic resonance imaging. The influences of molecular parameters and field strength on relaxivity were studied for a series of Eu^II-containing cryptates and their adducts with β-cyclodextrins, poly-β-cyclodextrins, and human serum albumin. Solid- and solution-phase characterization of Eu^II-containing complexes is presented that demonstrates the presence of inner-sphere molecules of water. Additionally, relaxivity, water-exchange rate, rotational correlation time, and electronic relaxation times were determined using variable-temperature ¹⁷O NMR, nuclear magnetic relaxation dispersion, and electron paramagnetic resonance spectroscopic techniques. These results are expected to be instrumental in the design of future Eu^II-based contrast agents.

Agents from the crypt: Eu^II-containing cryptates were studied as contrast agents. The relationship between relaxivity, field strength, and molecular parameters for Eu^II-containing cryptates are discussed using Solomon–Bloembergen–Morgan theory (see figure).

The right environment: The remarkable properties of a recently reported holmium(III)-based single-ion magnet have been ascribed to the hyperfine interactions with the half-integer nuclear spin in combination with the pentagonal-bipyramidal coordination environment. These results provide insight into the complicated magnetic properties of nanosized magnetic materials.

Abstraction of a chloride ligand from the dysprosium metallocene [(Cp^ttt)₂DyCl] (1_Dy Cp^ttt=1,2,4-tri(tert-butyl)cyclopentadienide) by the triethylsilylium cation produces the first base-free rare-earth metallocenium cation [(Cp^ttt)₂Dy]⁺ (2_Dy) as a salt of the non-coordinating [B(C₆F₅)₄]⁻ anion. Magnetic measurements reveal that [2_Dy][B(C₆F₅)₄] is an SMM with a record anisotropy barrier up to 1277 cm⁻¹ (1837 K) in zero field and a record magnetic blocking temperature of 60 K, including hysteresis with coercivity. The exceptional magnetic axiality of 2_Dy is further highlighted by computational studies, which reveal this system to be the first lanthanide SMM in which all low-lying Kramers doublets correspond to a well-defined M_J value, with no significant mixing even in the higher doublets.

SMMashing: A dysprosium(III) metallocenium cation is a single-molecule magnet (SMM) with a record anisotropy barrier of 1277 cm⁻¹ and record magnetic blocking up to 60 K, including hysteresis with coercivity.

Suppression of quantum tunneling at zero field and field-induced quantum tunneling of magnetization are observed in an extremely rare holmium(III) single-ion magnet and discussed by J.-L. Liu, M.-L. Tong, and co-workers in their Communication on page 4996 ff. These dramatic dynamics are attributed to the combination of a favorable pentagonal-bipyramidal crystal-field environment and the hyperfine interactions arising from ¹⁶⁵Ho (I=7/2) with a natural abundance of 100 %.

High-spin complexes act as polarizing agents (PAs) for dynamic nuclear polarization (DNP) in solid-state NMR spectroscopy and feature promising aspects towards biomolecular DNP. We present a study on bis(Gd-chelate)s which enable cross effect (CE) DNP owing to spatial confinement of two dipolar-coupled electron spins. Their well-defined Gd⋅⋅⋅Gd distances in the range of 1.2–3.4 nm allowed us to elucidate the Gd⋅⋅⋅Gd distance dependence of the DNP mechanism and NMR signal enhancement. We found that Gd⋅⋅⋅Gd distances above 2.1 nm result in solid effect DNP while distances between 1.2 and 2.1 nm enable CE for ¹H, ¹³C, and ¹⁵N nuclear spins. We compare 263 GHz electron paramagnetic resonance (EPR) spectra with the obtained DNP field profiles and discuss possible CE matching conditions within the high-spin system and the influence of dipolar broadening of the EPR signal. Our findings foster the understanding of the CE mechanism and the design of high-spin PAs for specific applications of DNP.

Poles apart: Bis(Gd-chelate)s are investigated as polarizing agents for dynamic nuclear polarization (DNP). On varying the length of the linker between two chelated Gd^III ions, a transition from the solid effect to the cross effect DNP mechanism is observed. The analysis of NMR signal enhancements for ¹H, ¹³C, and ¹⁵N sheds light on the distance dependence between two electron spins for the cross effect.

An extremely rare non-Kramers holmium(III) single-ion magnet (SIM) is reported to be stabilized in the pentagonal-bipyramidal geometry by a phosphine oxide with a high energy barrier of 237(4) cm⁻¹. The suppression of the quantum tunneling of magnetization (QTM) at zero field and the hyperfine structures originating from field-induced QTMs can be observed even from the field-dependent alternating-current magnetic susceptibility in addition to single-crystal hysteresis loops. These dramatic dynamics were attributed to the combination of the favorable crystal-field environment and the hyperfine interactions arising from ¹⁶⁵Ho (I=7/2) with a natural abundance of 100 %.

An extremely rare non-Kramers holmium(III) single-ion magnet is reported. The suppression of the quantum tunneling of magnetization at zero field and the hyperfine structures were observed in AC magnetic susceptibility measurements, and were attributed to the combination of a favorable crystal-field environment and the hyperfine interactions arising from ¹⁶⁵Ho (I=7/2) with a natural abundance of 100 %.