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.

Single-Molecule Magnet Behavior of Confined Dy(III) in a Mixed Heteroatom-Substituted Polyoxotungstate

Two heteroatom-templated Dy(III)-confined polyoxotungstates [H2N(CH3)2]7Na7[Dy2(H2O)7(W4O9)(HPSeW15O54)(α-SeW9O33)2]·31H2O (1) and [H2N(CH3)2]14K2Na18{[Dy2(H2O)13W14O40]2[α-SeW9O33]4[HPSeW15O54]2}·44H2O (2) were synthesized by a one-pot aqueous reaction and structurally characterized. The most distinctive structural feature of complexes 1 & 2 is the simultaneous presence of both trivacant Keggin [α-SeW9O33]8- and Dawson [HPSeW15O54]10- building blocks containing P(III)-Se(IV) heteroatoms. The trimeric polyanion of 1 can be represented as a fusion of two trivacant Keggin [α-SeW9O33]8- and Dawson [HPSeW15O54]10- building units encapsulating the [Dy2(H2O)7(W4O9)]12+ cluster. On the other hand, hexameric polyoxoanions of 2 are described as four trivacant Keggin [α-SeW9O33]8- and two Dawson [HPSeW15O54]10-, building units anchoring a [Dy4(H2O)26W28O80]20+ cluster. The magnetic investigation revealed the presence of significant magnetic anisotropy and slow relaxation of magnetization behavior for complex 1 with a phenomenological energy barrier, Ueff = 13.58 K in the absence of an external magnetic field, and Ueff = 24.57 K in the presence of a 500 Oe external dc magnetic field. On the other hand, complex 2 favors the QTM relaxation process in the absence of an external magnetic field and shows field-induced slow relaxation of magnetization with Ueff = 11.11 K at 1500 Oe applied dc field. The in-depth analysis of magnetic relaxation dynamics shows that the relaxation process follows the Orbach as well as Raman relaxation pathways. Further, the ab initio calculation of the studied complexes confirms that the highly axial ground and first excited energy states (containing pure highest mJ states) are responsible for the observed single-molecule magnet (SMM) behavior. Remarkably, this is the first example of a mixed heteroatom-based Dy(III)-substituted polyoxotungstate with both trimeric Keggin [α-SeW9O33]8- and Dawson [HPSeW15O54]10- building units showing SMM behavior.

Vibronic effects on the quantum tunnelling of magnetisation in Kramers single-molecule magnets

Single-molecule magnets are among the most promising platforms for achieving molecular-scale data storage and processing. Their magnetisation dynamics are determined by the interplay between electronic and vibrational degrees of freedom, which can couple coherently, leading to complex vibronic dynamics. Building on an ab initio description of the electronic and vibrational Hamiltonians, we formulate a non-perturbative vibronic model of the low-energy magnetic degrees of freedom in monometallic single-molecule magnets. Describing their low-temperature magnetism in terms of magnetic polarons, we are able to quantify the vibronic contribution to the quantum tunnelling of the magnetisation, a process that is commonly assumed to be independent of spin-phonon coupling. We find that the formation of magnetic polarons lowers the tunnelling probability in both amorphous and crystalline systems by stabilising the low-lying spin states. This work, thus, shows that spin-phonon coupling subtly influences magnetic relaxation in single-molecule magnets even at extremely low temperatures where no vibrational excitations are present.

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.

Synthesis and characterization of two self-assembled [Cu6Gd3] and [Cu5Dy2] complexes exhibiting the magnetocaloric effect, slow relaxation of magnetization, and anticancer activity

Two new paths for coordination driven self-assembly reactions under the binding support of 2-((1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-methoxyphenol (H₂L) have been discovered from the reactions of Cu(ClO₄)₂·6H₂O, NEt₃ and GdCl₃/DyCl₃·6H₂O in MeOH/CHCl₃ (2 : 1) medium. A similar synthetic protocol is useful to provide two different types of self-aggregated molecular clusters [Cu₆Gd₃(L)₃(HL)₃(μ₃-Cl)₃(μ₃-OH)₆(OH)₂]ClO₄·4H₂O (1) and [Cu₅Dy₂(L)₂(HL)₂(μ-Cl)₂(μ₃-OH)₄(ClO₄)₂(H₂O)₆](ClO₄)₂·2NHEt₃Cl·21H₂O (2). The adopted reaction procedure established the importance of the HO^- and Cl^- ions in the mineral-like growth of the complexes, derived from solvents and metal ion salts. In the case of complex 1, one Gd^III center has been trapped at the central position of the core upheld by six μ₃-OH and three μ₃-Cl groups, whereas for complex 2 one Cu^II center was trapped using four μ₃-hydroxo and two μ-chlorido groups. The magnetothermal behavior of 1 has been examined for a magnetocaloric effect of -ΔS_m = 11.3 J kg^-1 K^-1 at 2 K for ΔH = 7 T, whereas the magnetic susceptibility measurements of 2 showed slow magnetic relaxation with U_eff = 15.8 K and τ₀ = 9.8 × 10^-7 s in zero external dc field. Cancer cell growth inhibition studies proved the potential of both the complexes with interestingly high activity for the Cu₆Gd₃ complex against human lung cancer cells. Both complexes 1 and 2 also exhibited DNA and human serum albumin (HSA) binding abilities in relation to the involved binding sites and thermodynamics.

Slow magnetic relaxation in a homoaxially phosphine oxide coordinated pentagonal bipyramidal Dy(III) complex

We report the synthesis of [(L)Dy^III(Cy₃PO)₂]·[BPh₄] (1-Dy) (where H₂L = 2,6-diacetylpyridine bis-benzoylhydrazone and Cy = cyclohexyl) which crystallized in the triclinic, P1̄ space group. The local geometry around Dy(III) in 1-Dy was found to be pentagonal bipyramidal (pseudo-D_5h). The AC magnetic susceptibility measurements performed on 1-Dy and on its diluted 1-Y(Dy) samples showed a typical single-molecule magnet signature revealed by the appearance of AC-frequency dependent out-of-phase susceptibility signals in the absence of a static magnetic field. The out-of-phase AC susceptibility signals were well resolved on the application of a small magnetic field (H_DC = 500 Oe) and yielded an energy barrier for magnetization flipping of U_eff/k_B = 50 K for the diluted derivative. The magnetic studies on 1-Dy and 1-Y(Dy) and data analysis further confirm that Raman and QTM under-barrier magnetic relaxations play a crucial role in lowering U_eff despite the almost axial nature of the Dy(III) ion in 1-Dy. We have rationalized these observations through detailed ab initio calculations performed on the X-ray crystal structure of 1-Dy.

3-Pyridylacetic-Based Lanthanide Complexes Exhibiting Magnetic Entropy Changes, Single-Molecule Magnet, and Fluorescence

Four complexes from lanthanides, 3-pyridylacetate, and 1,10-phenanthroline, formulated as [Ln2(3-PAA)2(μ-Cl)2(phen)4](ClO4)2 [Ln = Gd(1), Dy(2), Eu(3), Tb(4), 3-PAA = 3-pyridylacetic acid, phen = 1,10-phenanthroline], were obtained. The four compounds were characterized by IR spectra, thermogravimetric analyses, powder X-ray diffraction, and single-crystal X-ray diffraction. Compounds 1-4 are isomorphous, and they have a dinuclear structure. Magnetic studies reveal that 1 shows the magnetocaloric effect with -ΔS m max = 19.03 J kg-1 K-1 at 2 K for ΔH = 5 T, and 2 displays a field-induced single-molecule magnet with U eff = 19.02 K. The photoluminescent spectra of 3 and 4 exhibit strong characteristic emission, which demonstrate that the ligand-to-EuIII/TbIII energy transfer is efficient.

Single-Molecule Magnets beyond a Single Lanthanide Ion: The Art of Coupling

The promising future of storing and processing quantized information at the molecular level has been attracting the study of Single-Molecule Magnets (SMMs) for almost three decades. Although some recent breakthroughs are mainly about the SMMs containing only one lanthanide ion, we believe SMMs can tell a much deeper story than the single-ion anisotropy. Here in this Perspective, we will try to draw a unified picture of SMMs as a delicately coupled spin system between multiple spin centres. The hierarchical couplings will be presented step-by-step, from the intra-atomic hyperfine coupling, to the direct and indirect intra-molecular couplings with neighbouring spin centres, and all the way to the inter-molecular and spin–phonon couplings. Along with the discussions on their distinctive impacts on the energy level structures and thus magnetic behaviours, a promising big picture for further studies is proposed, encouraging the multifaceted developments of molecular magnetism and beyond.

In this Perspective, we draw a unified picture for single-molecule magnets as delicately coupled spin systems, discuss the hierarchical couplings (from intra-atomic to inter-molecular) and their distinctive impacts on the magnetic behaviours.

Magnetization Dynamics on Isotope-Isomorphic Holmium Single-Molecule Magnets

Here we reported the deuteration of the metal-binding equatorial water molecules in a reported Ho^III single-molecule magnet (SMM) with pentagonal-bipyramidal geometry, from [Ho(CyPh₂ PO)₂ (H₂ O)₅ ]³⁺ to [Ho(CyPh₂ PO)₂ (D₂ O)₅ ]³⁺ . The hyperfine structures originating from the nuclear spin of ¹⁶⁵ Ho^III can be clearly observed. Moreover, the resulting magnetization dynamics revealed the switch of the relative relaxation rates for the two isotope-isomorphic complexes-respectively faster/slower at low/high temperature. The noticeable isotope effect arises from not only the paramagnetic metal center but also the diamagnetic ligands, which can be explained by the ab initio calculated tunnel splitting and the involvement of the super-hyperfine interaction related to the difference in the nuclear spin number of protium (¹ H, I=¹ /₂ ) and deuterium (² H, I=1).

Study of the influence of nuclear spin and dilution over the slow relaxation in a 3d4f heterobimetallic single-molecule magnet

The effects of isotopic enrichment and magnetic dilution have been investigated in a heterobimetallic complex of formula [Zn₂L₂^ADyCl₃]·2H₂O (^ADyZn₂) (A = 162 and 163) presenting slow relaxation of the magnetization. Isotopic substitution for ¹⁶²Dy (I = 0) and ¹⁶³Dy (I = 5/2) leads to a shift in the relaxation times depending on the suppression or enhancement of the hyperfine interactions. The release of the dipolar interactions through magnetic dilution in a Y(iii)-based matrix enhances the slow relaxation of the magnetization and the visibility of the nuclear spin effect. A comparison of the hysteresis loop at 0.5 K for bulk and diluted analogues of pure isotopically enriched complexes suggested a role of the nuclear spin in the interaction between the active system and the matrix.

A Journey in Lanthanide Coordination Chemistry: From Evaporable Dimers to Magnetic Materials and Luminescent Devices

ConspectusLanthanide ions are prime ingredients for the design of compounds, materials, and devices with unique magnetic and optical properties. Accordingly, coordination chemistry is one of the best tools for building molecular edifices from these ions because it allows careful control of the ions' environment and of the dimensionality of the final compound.In this Account, we review our results on lanthanide-based dimers. We show how a pure fundamental study on lanthanide coordination chemistry allows the investigation of a full continuum of results from the compound to materials and then to devices. The conversion of molecules into materials is a tricky task because it requires strong molecular robustness toward the surface deposition processes as well as the preservation and detectability of the molecular properties in the material. Additionally, the passage of a material toward a device implies a material with a given function, for example, a tailored response to an external stimulus.To do so, we targeted neutral and isolated molecules whose transfer on surfaces by chemi- or physisorption is much easier than that of charged molecules or extended coordination networks. Then, we focused on molecules with very strong evaporability to avoid wet chemistry deposition processes that are more likely to damage the molecules and/or distort their geometries.We thus designed lanthanide dimers based on fluorinated β-diketonates and pyridine-N-oxide ligands. As expected, they show remarkable evaporability but also strong luminescence and interesting magnetic behavior because they behave as single-molecule magnets (SMMs). Ligand substitutions and stoichiometric modifications allow the optimization of the geometric organization of the dimers in the crystal packing as well as their evaporability, SMM behavior, luminescent properties, or their ability to be anchored on surfaces. Most of all, this family of molecules shows a strong ability to form thick films on various substrates. This allows converting these molecules to magnetic materials and luminescent devices.Magnetic materials can be designed by creating thick films of the dimers deposited on gold. These films have been designed and investigated with the most advanced techniques of on-surface imaging (atomic force microscopy, AFM), on-surface physicochemical characterization (X-ray photoelectron spectroscopy (XPS), time of flight-secondary ion mass spectroscopy (Tof-SIMS)), and on-surface magnetic investigation (low-energy muon spin relaxation (LE-μSR)). Contrary to what was previously observed on other SMM films, no depth dependence of the SMM behavior was observed. This means that the dimers do not suffer from the vacuum or substrate interface and behave similarly, whatever their localization. This exceptional magnetic robustness is a key ingredient in the creation of materials for molecular magnetic data storage.Luminescent devices can be obtained by layering molecular films of the dimers with a copper-rich solid-state electrolyte between ITO/Pt electrodes. The electromigration of Cu²⁺ ions into films of Eu³⁺, Tb³⁺, and Dy³⁺ dimers quenches their luminescence. This luminescence tuning by electromigration is reversible, and this setup can be considered to be a proof of concept of full solid-state luminescent device where reversible coding can be tailored by an electric field. It is envisioned for optical data storage purposes. In the future, it could also benefit from the SMM properties of the molecules to pave the way toward multifunctional molecular data storage devices.

Ytterbium-Centered Isotopic Enrichment Leading to a Zero-Field Single-Molecule Magnet

An unprecedented combination of isotopic enrichment and magnetic dilution approaches for a prolate ytterbium(III)-based complex was performed. It results in the appearance of the first observations of a nuclear spin effect on both quantum tunneling of magnetization and slow magnetic relaxation for an ytterbium complex under a zero applied field.

Tuning magnetic anisotropy via terminal ligands along the Dy⋯Dy orientation in novel centrosymmetric [Dy2] single molecule magnets

The SMM properties of four dinuclear Dy^III complexes can be effectively tuned by the appropriate alteration of terminal ligands and lattice guests.

Blocking like it's hot: a synthetic chemists' path to high-temperature lanthanide single molecule magnets

Progress in the synthesis, design, and characterisation of single-molecule magnets (SMMs) has expanded dramatically from curiosity driven beginnings to molecules that retain magnetization above the boiling point of liquid nitrogen. This is in no small part due to the increasingly collaborative nature of this research where synthetic targets are guided by theoretical design criteria. This article aims to summarize these efforts and progress from the perspective of a synthetic chemist with a focus on how chemistry can modulate physical properties. A simple overview is presented of lanthanide electronic structure in order to contextualize the synthetic advances that have led to drastic improvements in the performance of lanthanide-based SMMs from the early 2000s to the late 2010s.

Slow magnetic relaxation in a homo dinuclear Dy(iii) complex in a pentagonal bipyramidal geometry

We hereby report a dinuclear Dy(iii) complex, [Dy(LH₃)Cl₂]₂·2Et₂O (1) (LH₄ = 2,3-dihydroxybenzylidene)-2-(hydroxyimino)propanehydrazide where both the metal centres are in a pentagonal bipyramidal (PBP) geometry with the axial positions being occupied by negatively charged Cl^- ions. The complex as well as it's 10% diluted analogue (1₁₀) do not show zero-field SMM behaviour. However, in the presence of small optimum dc fields the slow relaxation of magnetization was displayed. The effective energy barrier for 1₁₀ at 800 Oe of applied field was extracted as 83(17) K with τ₀ = 2(4) × 10^-12 s. Through a combined experimental and ab initio electronic structure calculations studies we have thoroughly analysed the role of the ligand field around the Dy(iii), present in pentagonal bipyramidal geometry, in contributing to the slow relaxation of magnetization.

Redox- and solvato-magnetic switching in a tetrathiafulvalene-based triad single-molecule magnet

Simultaneous redox and solvato-magnetic switching was achieved for a dinuclear dysprosium single-molecule magnet involving an extended tetrathiafulvalene fused semiquinone based triad.

Structurally modulated single-ion magnets of mononuclear β-diketone dysprosium(iii) complexes

Four perturbed eight-coordinated mononuclear β-diketone based Dy(iii) SIMs with distinct hydrogen bond interactions and electron delocalization are noteworthily modulated by the aromatic groups of auxiliary ligands.

Redox-Active Dysprosium Single-Molecule Magnet: Spectro-Electrochemistry and Theoretical Investigations

The mononuclear single-molecule magnet (SMM) [Dy(tta)3(L)]⋅C6H14 (1) (where tta− = 2-thenoyltrifluoroacetonate and L = 4,5-bis(propylthio)-tetrathiafulvalene-2-(2-pyridyl)benzimidazole-methyl-2-pyridine) was studied by spectro-electrochemistry. The resulting electronic spectra of the three oxidation states 1, 1+∙, and 12+ were rationalized by time-dependent density functional theory (TD-DFT) calculations starting from the DFT optimized structures. The modulation of the magnetic anisotropy of the DyIII center upon oxidation was also inspected at the Complete Active Space Self-Consistent Field (CASSCF) level of calculation.

Quantum tunnelling of the magnetisation in single-molecule magnet isotopologue dimers

Quantum tunnelling of the magnetisation plays a major role in the magnetic properties of lanthanide Single-Molecule Magnets: while it is considered a problem for data storage device applications since it leads to information loss, it is an essential pre-requisite for the read-out and manipulation of the nuclear states in Quantum Information Processing schemes. Here we describe two isotopologue dysprosium dimers, i.e. [(163Dy(tmhd)3)2(bpym)] and [(164Dy(tmhd)3)2(bpym)] (tmd = tris(tetramethylheptanedionato) and bpym = bipyrimidine), where the nuclear spin presence or absence clearly affects the magnetic properties of the systems. Through μ-SQUID studies at milli-Kelvin temperatures and alternating current magnetic measurements, we find significant differences in the magnetic behaviour of both complexes. While simulation of the hysteresis loops at 30 mK reveals that the presence of nuclear spin does not influence the tunnelling rate, we find that it facilitates the coupling to the phonon bath enhancing the direct relaxation process; an observation reflected in the temperature and field dependence of the relaxation rates.

Series of Chloranilate-Bridged Dinuclear Lanthanide Complexes: Kramers Systems Showing Field-Induced Slow Magnetic Relaxation

A series of chloralilate-bridged dinuclear lanthanide complexes of formula [{LnIII(Tp)2}2(μ-Cl2An)]·2CH2Cl2, where Cl2An2− and Tp− represent the chloranilate and hydrotris (pyrazolyl)borate ligands, respectively, and Ln = Gd (1), Tb (2), Ho (3), Er (4), and Yb (5) was synthesized. All five complexes were characterized by an elemental analysis, infrared spectroscopy, single crystal X-ray diffraction, and SQUID measurements. The complexes 1–5 in the series were all isostructural. A comparison of the temperature dependence of the dc magnetic susceptibility data of these complexes revealed clear differences depending on the lanthanide center. Ac magnetic susceptibility measurements revealed that none of the five complexes exhibited a slow magnetic relaxation under a zero applied dc field. On the other hand, the Kramers systems (complexes 4 and 5) clearly displayed a slow magnetic relaxation under applied dc fields, suggesting field-induced single-molecule magnets that occur through Orbach and Raman relaxation processes.

Studies of hysteresis and quantum tunnelling of the magnetisation in dysprosium(iii) single molecule magnets

We report a study of quantum tunneling of the magnetisation in three Dy(iii) single-molecule magnets.

Electro-activity and magnetic switching in lanthanide-based single-molecule magnets

The present work reviews switching of single-molecule magnetic behaviour achieved through various stimuli such as temperature, light irradiation, redox processes, solvation/desolvation, and magnetic field.