Lanthanide Single-molecule Magnets Research Articles

Pressure-Induced Structural, Optical and Magnetic Modifications in Lanthanide Single-Molecule Magnets.

Lanthanide Single-Molecule Magnets are fascinating objects that break magnetic performance records with observable magnetic bistability at the boiling temperature of liquid nitrogen, paving the way for potential applications in high-density data storage. The switching of lanthanide SMM has been successfully achieved using several external stimuli such as redox reaction, pH titration, light irradiation or solvation/desolvation thanks to the high sensitivity of the magnetic anisotropy to any structural change in the lanthanide surrounding. Nevertheless, the use of applied high pressure as an external stimulus is largely underused, especially considering that it can be combined with high pressure X-ray diffraction to establish a complementary structure-property relationship. This Concept article summarizes the few relevant examples of investigations of lanthanide SMMs under applied high pressure, provides conclusions on the effect of such stimulus on molecular structures and magnetic anisotropy, and finally draws perspective on the future development of magnetic measurements under applied pressure.

Development and Prospects in the Structure and Performance of Lanthanide Single-Molecule Magnets

Development and Prospects in the Structure and Performance of Lanthanide Single-Molecule Magnets

Solvato- and vapochromic exchange-coupled Dy2 single-molecule magnets achieved by attaching iron-cyanido metalloligands

DyIII2 molecules equipped with dicyanidoiron(ii) metalloligands exhibit the unique conjunction of single-molecule magnetism, supported by exchange coupling, with rich solvatochromism in solution and vapochromism in the solid state.

Dysprosium Clusters from 3-Ethoxysalicylidene Terminal-Decorated Acylhydrazone Ligands: In Situ Ligand Transformation-Assisted Assembly and Zero-Field Single-Molecule Magnet Behaviors

High-performance polynuclear lanthanide single-molecule magnets (Ln-SMMs) are still scarce now. Herein, we report the assembly of three novel DyIII clusters, where in situ ligand transformation-assisted assembly offered a system with high SMM performance successfully. These clusters, [Dy6(bepho)3(NO3)6(CH3OH)3(H2O)3]·5CH3OH·CH3CN·2H2O (1), [Dy4(bepho)2(H3epho)2 (CH3OH)2]·2CH3OH (2), and [Dy5(eehz)3(eqzl)(μ-CH3O)(μ3-CH3O)2(μ4-O)(CH3OH)]·2CH3OH (3), were all assembled from 3-ethoxysalicylidene terminal-decorated acylhydrazones. H4bepho, H5epho, H3eehz, and Heqzl represent N,N′-bis(3-ethoxysalicylicdene)pyridine-2,6-dicarbohydrazide N-oxide, N-(3-ethoxysalicylidene)pyridine-2,6-dicarbohydrazide N-oxide, N-(3-ethoxysalicylicdene)-2-(3-ethoxysalicylideneamino)benzohydrazide, and 3-(3-ethoxysalicylicdeneamino)quinazolinone, respectively. Cluster 1 is built by H4bepho directly, showing a hexanuclear zig-zag chain structure, while for 2 and 3, their original ligands all undergo unprecedented in situ transformations during the assembly. H5epho featuring asymmetric chelating arms and Heqzl bearing the quinazolinone parent are formed in 2 and 3, respectively, and participate in the assembly of clusters along with original ligands. In 2, H4bepho and H5epho construct a quasi-linear {Dy4} topology. A unique pentanuclear core of 3 is assembled by H3eehz and Heqzl. It consists of a [Dy4] tetrahedron and a [Dy3] triangle unit sharing an edge. Clusters 1–3 all show zero-field SMM behaviors, and 2 displays two distinct relaxation processes. Strikingly, the quantum tunneling of magnetization (QTM) effect in 3 might be suppressed by intramolecular magnetic interactions associated with its unique cluster topology. The zero-field energy barrier (Ueff) of 3 reaches up to 211.3 K, which is among the high values for all polynuclear Ln-SMM species and also the second largest one in the {Ln5}-SMM family.

Magnetic and Luminescent Dual Responses of Photochromic Hexaazamacrocyclic Lanthanide Complexes.

Herein, hexaazamacrocyclic ligand LN6 was employed to construct a series of photochromic rare-earth complexes, [Ln(LN6)(NO3)2](BPh4) [1-Ln, Ln = Dy, Tb, Eu, Gd, Y; LN6 = (3E,5E,10E,12E)-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane-3,5,10,12-tetraene]. The behavior of photogenerated radicals of hexaazamacrocyclic ligands was revealed for the first time. Upon 365 nm light irradiation, complexes 1-Ln exhibit photochromic behavior induced by photogenerated radicals according to EPR and UV-vis analyses. Static and dynamic magnetic studies of 1-Dy and irradiated product 1-Dy* indicate weak ferromagnetic interactions among DyIII ions and photogenerated LN6 radicals, as well as slow magnetization relaxation behavior under a 2 kOe applied field. Further fitting analyses show that the magnetization relaxation in 1-Dy* is markedly different from 1-Dy. Time-dependent fluorescence measurements reveal the characteristic luminescence quenching dynamics of lanthanide in the photochromic process. Especially for irradiated product 1-Eu*, the luminescence is almost completely quenched within 5 min with a quenching efficiency of 98.4%. The results reported here provide a prospect for the design of radical-induced photochromic lanthanide single-molecule magnets and will promote the further development of multiresponsive photomagnetic materials.

Strategies to quench quantum tunneling of magnetization in lanthanide single molecule magnets.

Enhancing blocking temperature (TB) is one of the holy grails in Single Molecule Magnets(SMMs), as any future potential application in this class of molecules is directly correlated to this parameter. Among many factors contributing to a reduction of TB value, Quantum Tunnelling of Magnetisation (QTM), a phenomenon that is a curse or a blessing based on the application sought after, tops the list. Theoretical tools based on density functional and ab initio CASSCF/RASSI-SO methods have played a prominent role in estimating various spin Hamiltonian parameters and establishing the mechanism of magnetization relaxation in this class of molecules. Particularly, various strategies to quench QTM effects go hand-in-hand with experiments, and different methods proposed to quell QTM effects are scattered in the literature. In this perspective, we have explored various approaches that are proposed in the literature to quench QTM effects, and these include the role of (i) local symmetry of lanthanides, (ii) super-exchange interaction in {3d-4f} complexes, (iii) direct-exchange interaction in {radical-4f} and metal-metal bonded complexes to suppress the QTM, (iv) utilizing external stimuli such as an electric field or pressure to modulate the QTM and (v) avoiding QTM effects by stabilising toroidal states in 4f and {3d-4f} clusters. We believe the strategies summarized here will help to design new-generation SMMs.

Isotopic enrichment in lanthanide coordination complexes: contribution to single-molecule magnets and spin qudit insights.

Lanthanide Single-Molecule Magnets (SMMs) fascinate the scientific community due to their plethora of potential applications ranging from data storage to spintronic devices and quantum computing. This review article proposes a comprehensive description of the influence of the nuclear spin, i.e. hyperfine interaction, on the magnetic properties of lanthanide SMMs and on quantum information processing of qudit. This influence is analysed for non-Kramers and Kramers lanthanide SMMs as well as for the electronic distribution of the electron in 4f orbitals i.e. oblate and prolate ions. Then the role of magnetic interactions in isotopically enriched polynuclear Dy(III) SMMs is discussed. Finally the possible effect of superhyperfine interaction due to the nuclear spin of elements originating from the surrounding of the lanthanide centre is analyzed. The effect of nuclear spin on the dynamics of the lanthanide SMMs is demonstrated using different techniques such as magnetometry, muon spectroscopy (μ-SR), and Mössbauer and Resonance Vibrational Spectroscopies.

Slow magnetic relaxation behavior of a {Dy2} complex based on a large π-conjugated bridging ligand

Binuclear lanthanide single molecule magnets (Ln-SMMs) based on large π-conjugated bridging ligands have shown potential in achieving excellent SMM performance. However, their members are still scarce until now. In this work, a novel binuclear Ln-SMM featuring a large π-conjugated bridging ligand, [Dy2(hfac)6(tpphz)]·CH2Cl2 (1), has been prepared. Its two DyIII centers are chelated and bridged by the large π-conjugated tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′′,3′′′-j]phenazine (tpphz) ligand. Three 1,1,1,5,5,5-hexafluoroacetylacetonate (hfac-) ions acting as terminal ligands further complete the square antiprism coordination geometry of each DyIII center. Magnetic measurements revealed that 1 exhibits zero field SMM behavior with a single magnetic relaxation process. The effective energy barrier (Ueff) is 13.79 K. Interestingly, dual relaxation behavior including well-separated slow relaxation (SR) and fast relaxation (FR) processes was further observed when a dc field was applied. It could be ascribed to the different single ion anisotropy of the DyIII centers, as well as a possible contribution of magnetic interactions in the system of 1. The Ueff values for the SR and FR processes are 27.3 and 1.30 K, respectively.

Chiral All-Nitrogen-Coordinated Dysprosium Single-Molecule Magnets.

Two pairs of chiral end-on azido-bridged dinuclear hexaazamacrocycles, [Dy2 (LN6 R/S )2 (N3 )2 Cl2 ](BPh4 )2 (1R/1S) and [Dy2 (LN6 R/S )2 (N3 )4 ]Cl2 (2R/2S) (LN6 R/S is hexaazamacrocyclic neutral Schiff base ligand derived from 2,6-diformylpyridine and (1R, 2R)/(1S, 2S)-diaminocyclohexane), were constructed by adjusting the molar ratio of sodium azide to Dy(III) macrocycle precursor. Structural analyses reveal that all Dy(III) centers in complexes 1R/1S and 2R/2S are nine-coordinate with hula-loop coordination geometry, and the differences between 1R/1S and 2R/2S are the terminal coordination anion and counter anion. Magnetic studies indicate that complex 2S displays typical SMM behaviors under a zero dc field, whereas 1S just shows slow relaxation of magnetization resulting from a relatively weak axial crystal field. Significantly, complex 2R/2S represents the first homochiral all-nitrogen-coordinated lanthanide single-molecule magnet.

Lanthanide single-molecule magnets with high anisotropy barrier: where to from here?

High-temperature lanthanide single-molecule magnets with large coercive fields are being pursued.

Lanthanide SMMs Based on Belt Macrocycles: Recent Advances and General Trends.

Enhancement of axial magnetic anisotropy is the central objective to push forward the performance of Single-Molecule Magnet (SMM) complexes. In the case of mononuclear lanthanide complexes, the chemical environment around the paramagnetic ion must be tuned to place strongly interacting ligands along either the axial positions or the equatorial plane, depending on the oblate or prolate preference of the selected lanthanide. One classical strategy to achieve a precise chemical environment for a metal centre is using highly structured, chelating ligands. A natural approach for axial-equatorial control is the employment of macrocycles acting in a belt conformation, providing the equatorial coordination environment, and leaving room for axial ligands. In this review, we present a survey of SMMs based on the macrocycle belt motif. Literature systems are divided in three families (crown ether, Schiff-base and metallacrown) and their general properties in terms of structural stability and SMM performance are briefly discussed.

Intuitive Control of Low-Energy Magnetic Excitations via Directed Dipolar Interactions in a Series of Er(III)-Based Complexes.

Dipolar coupling is rarely invoked as a driving force for slow relaxation dynamics in lanthanide-based single-molecule magnets, though it is often the strongest mechanism available for mediating inter-ion magnetic interactions in such species. Indeed, for multinuclear lanthanide complexes, the magnitude and anisotropy of the dipolar interaction can be considerable given their ability to form highly directional, high-moment ground states. Herein, we present a mono-, di-, and trinuclear erbium-based single-molecule magnet sequence, ([Er-TiPS2COT]+)n (n = 1-3), wherein a drastic reduction in the allowedness of magnetic relaxation pathways is rationalized within the framework of the dipole-dipole interactions between angular momentum quanta. The resulting design principles for multinuclear molecular magnetism arising from intramolecular dipolar coupling interactions between highly anisotropic magnetic states present a nuanced justification of the relaxation dynamics in complex manifolds of individual quantized transitions. Experimental evidence for the validity of this model is provided by coupling the relaxation dynamics to an AC magnetic field across an unprecedented frequency range for molecular magnetism (103-10-5 Hz). The combination of slow dynamics and multiple, low-energy transitions leads to a number of noteworthy phenomena, including a lanthanide single-molecule magnet with three well-defined relaxation processes observable at a single temperature.

Ligand Fluorination to Mitigate the Raman Relaxation of DyIII Single‐Molecule Magnets: A Combined Terahertz, Far‐IR and Vibronic Barrier Model Study

AbstractThe fast Raman relaxation process via a virtual energy level has become a puzzle for how to chemically engineer single‐molecule magnets (SMMs) with better performance. Here, we use the trifluoromethyl group to systematically substitute the methyl groups in the axial position of the parent bis‐butoxide pentapyridyl dysprosium(III) SMM. The resulting complexes—[Dy(OLA)2py5][BPh4] (LA=CH(CF3)2− 1, CH2CF3− 2, CMe2CF3− 3)—show progressively enhanced TBhys (@100 Oe s−1) from 17 K (for 3), 20 K (for 2) to 23 K (for 1). By experimentally identifying the varied under barrier relaxation energy in the 5–500 cm−1 regime, we are able to identify that the C−F bond related vibration energy of the axial ligand ranging from 200 to 350 cm−1 is the key variant for this improvement. Thus, this finding not only reveals a correlation between the structure and the Raman process but also provides a paradigm for how to apply the vibronic barrier model to analyze multi‐phonon relaxation processes in lanthanide SMMs.

Ligand Fluorination to Mitigate the Raman Relaxation of DyIII Single-Molecule Magnets: A Combined Terahertz, Far-IR and Vibronic Barrier Model Study.

The fast Raman relaxation process via a virtual energy level has become a puzzle for how to chemically engineer single-molecule magnets (SMMs) with better performance. Here, we use the trifluoromethyl group to systematically substitute the methyl groups in the axial position of the parent bis-butoxide pentapyridyl dysprosium(III) SMM. The resulting complexes-[Dy(OLA )2 py5 ][BPh4 ] (LA =CH(CF3 )2 - 1, CH2 CF3 - 2, CMe2 CF3 - 3)-show progressively enhanced TB hys (@100 Oe s-1 ) from 17 K (for 3), 20 K (for 2) to 23 K (for 1). By experimentally identifying the varied under barrier relaxation energy in the 5-500 cm-1 regime, we are able to identify that the C-F bond related vibration energy of the axial ligand ranging from 200 to 350 cm-1 is the key variant for this improvement. Thus, this finding not only reveals a correlation between the structure and the Raman process but also provides a paradigm for how to apply the vibronic barrier model to analyze multi-phonon relaxation processes in lanthanide SMMs.

Metal-metal bond in lanthanide single-molecule magnets.

Lanthanide (Ln) compounds represent a unique chemical platform for developing high-temperature single-molecule magnets (SMMs). The shift in research focus from increasing the magnetic anisotropy barrier (Ueff) to raising the blocking temperature (TB) has upgraded the design criteria from considering only the static crystal field (CF) to paying attention to the effects of molecular vibrations beyond the first coordination environment on the relaxation of magnetization. Although the realization of high working temperatures for Ln SMMs remains a formidable challenge, recent remarkable advances in dimetallofullerenes (di-EMFs) with Ln ions and mixed-valence dilanthanide complexes both feature single-electron Ln-Ln bonds to afford room-temperature molecular magnets. In this review, we provide critical discussion on the achievements of metal-metal (MM) bonded lanthanide SMMs, focusing on the effects of MM bonds on the magnetization dynamics, and shedding light on the future developments of high-temperature Ln SMMs.

A single-ion magnet building block strategy toward Dy2 single-molecule magnets with enhanced magnetic performance.

A molecular dysprosium(III) complex [Dy(DClQ)3(H2O)2] (1) was used as a building unit for the construction of lanthanide SMMs, leading to the isolation of two dinuclear Dy(III) complexes, namely [Dy2(DClQ)6(MeOH)2] (2) and [Dy2(DClQ)6(bpmo)2]·6MeCN (3) (DClQ = 5,7-dichloro-8-hydroxyquinoline, bpmo = 4,4'-dipyridine-oxide). Structural analyses revealed the same N3O5 coordination environment of the Dy(III) centers with a distorted biaugmented trigonal prism (C2V symmetry) and triangular dodecahedron (D2d symmetry) for 2 and 3, respectively. Magnetic studies revealed the presence of ferromagnetic and weak antiferromagnetic exchange interactions between the Dy3+ centers in 2 and 3, respectively. Interestingly, slow relaxation of magnetization at zero fields was evidenced with an Ueff of 51.4 K and 159.0 K for complexes 2 and 3, respectively. The detailed analysis of relaxation dynamics discloses that the Orbach process is dominant for 2 whereas Raman and QTM play an important role in 3. Theoretical calculations were carried out to provide insight into the magnetic exchange interactions and relaxation dynamics for the complexes. Due to a single-ion magnet (SIM) of 1, the foregoing results demonstrate a SIM modular synthetic route for the preparation of dinuclear lanthanide SMMs.

Effect of the Transition Metal Ions on the Single-Molecule Magnet Properties in a Family of Air-Stable 3d-4f Ion-Pair Compounds with Pentagonal Bipyramidal Ln(III) Ions.

Single-molecule magnets (SMMs) are expected to be promising candidates for the applications of high-density information storage materials and quantum information processing. Lanthanide SMMs have attracted considerable interest in recent years due to their excellent performance. It has always been interesting but not straightforward to study the relaxation and blocking mechanisms by embedding 3d ions into 4f SMMs. Here we report a family of air-stable 3d-4f ion-pair compounds, YFe (1), DyCr (2), DyFe (3), DyCo (4), and Dy0.04Y0.96Fe (5), composed of pentagonal bipyramidal (D5h) LnIII cations and transition metallocyanate anions. The ion-pair nature makes the dipole-dipole interactions almost the only component of the magnetic interactions that can be clarified and analytically resolved under proper approximation. Therefore, this family provides an intuitive opportunity to investigate the effects of 3d-4f and 4f-4f magnetic interactions on the behavior of site-resolved 4f SMMs. Dynamic magnetic measurements of 1 under a 4 kOe external field reveal slow magnetic relaxation originating from the isolated [FeIII]LS (S = 1/2) ions. Under zero dc field, compounds 2-5 show similar magnetic relaxation processes coming from the separated pentagonal bipyramidal (D5h) DyIII ions with high Orbach barriers of 592(5), 596(4), 595(3), and 606(4) K, respectively. Comparatively, both compounds 3 and 5 exhibit two distinct relaxation processes, respectively from the [FeIII]LS and DyIII [Ueff = 596(4) K for 3 and 610(7) K for 5] ions, under a 4 kOe dc field. The dipolar interactions between the neighboring TMIII (TM = transition metal, CrIII or [FeIII]LS) and DyIII ions were revealed to have little effect on the thermal relaxation in compounds 2, 3, and 5, or the coexistence of the two separate relaxation processes in compounds 3 and 5 under a 4 kOe dc field, but they significantly affect the quantum tunneling of magnetization and the magnetic hysteresis behavior of 2 and 3 at low temperatures compared to those of 4.

Precise measurement of angles between two magnetic moments and their configurational stability in single-molecule magnets

A key parameter for the low-temperature magnetic coupling of in dinuclear lanthanide single-molecule magnets (SMMs) is the barrier $U_{FA}$ resulting from the exchange and dipole interactions between the two $4f$ moments. Here we extend the pseudospin model previously used to describe the ground state of dinuclear endofullerenes to account for variations in the orientation of the single-ion anisotropy axes and apply it to the two SMMs Dy$_2$ScN@C$_{80}$ and Dy$_2$TiC@C$_{80}$. While x-ray magnetic circular dichroism (XMCD) indicates the same $J_z=15/2$ Dy groundstate in both molecules, the Dy-Dy coupling strength and the stability of magnetization is distinct. We demonstrate that both the magnitude of the barrier $U_{FA}$ and the angle between the two $4f$ moments are determined directly from precise temperature-dependent magnetization data to an accuracy better than $1^{\circ}$. The experimentally found angles between the $4f$ moments are in excellent agreement with calculated angles between the quantisation axes of the two Dy ions. Theory indicates a larger deviation of the orientation of the Dy magnetic moments from the Dy bond axes to the central ion in Dy$_2$TiC@C$_{80}$. This may explain the lower stability of the magnetisation in Dy$_2$TiC@C$_{80}$, although it exhibits a $\sim 49\%$ stronger exchange coupling than in Dy$_2$ScN@C$_{80}$.

Inside Cover: Magnetization Dynamics on Isotope‐Isomorphic Holmium Single‐Molecule Magnets (Angew. Chem. Int. Ed. 52/2021)

Nuclear-spin driven relaxation collaborating with the ligands in lanthanide single molecule magnets (SMMs) remains ambiguous. In their Communication on page 27282, Liviu Ungur, Jun-Liang Liu, and co-workers report two HoIII isotopic-isomorphic SMMs, whose magnetization dynamics reveals a switch in the relative relaxation rates at low/high temperatures. A theoretical analysis suggests that the noticeable isotope effect arises from the tunnel splitting and the super-hyperfine interaction related to the distinct nuclear spin numbers of 1H and 2H.

The Underexplored Field of Lanthanide Complexes with Helicene Ligands: Towards Chiral Lanthanide Single Molecule Magnets

The effective combination of chirality and magnetism in a single crystalline material can lead to fascinating cross-effects, such as magneto-chiral dichroism. Among a large variety of chiral ligands utilized in the design and synthesis of chiral magnetic materials, helicenes seem to be the most appealing ones, due to the exceptionally high specific rotation values that reach thousands of deg·cm3·g−1·dm−1, which is two orders of magnitude higher than for compounds with chiral carbon atoms. Despite the sizeable family of transition metal complexes with helicene-type ligands, there are only a few examples of such complexes with lanthanide ions. In this mini-review, we describe the most recent developments in the field of lanthanide-based complexes with helicene-type ligands and summarize insights regarding the further exploration of this family of compounds towards multifunctional chiral lanthanide single molecule magnets (Ln-SMMs).