Advances on the nanostructuration of magnetic molecules on surfaces: the case of single-molecule magnets (SMM)

All-electron APW+lo calculation of magnetic molecules with the SIRIUS domain-specific package

We report APW+lo (augmented plane wave plus local orbital) density functional theory (DFT) calculations of large molecular systems using the domain specific SIRIUS multi-functional DFT package. The APW and FLAPW (full potential linearized APW) task and data parallelism options and the advanced eigen-system solver provided by SIRIUS can be exploited for performance gains in ground state Kohn-Sham calculations on large systems. This approach is distinct from our prior use of SIRIUS as a library backend to another APW+lo or FLAPW code. We benchmark the code and demonstrate performance on several magnetic molecule and metal organic framework systems. We show that the SIRIUS package in itself is capable of handling systems as large as a several hundred atoms in the unit cell without having to make technical choices that result in the loss of accuracy with respect to that needed for the study of magnetic systems.

Tailored Lewis Acid Sites for High-Temperature Supported Single-Molecule Magnetism

Generating or even retaining slow magnetic relaxation in surface immobilized single-molecule magnets (SMMs) from promising molecular precursors remains a great challenge. Illustrative examples are organolanthanide compounds that show promising SMM properties in molecular systems, though surface immobilization generally diminishes their magnetic performance. Here, we show how tailored Lewis acidic Al(III) sites on a silica surface enable generation of a material with SMM characteristics via chemisorption of (Cp^ttt)₂DyCl ((Cp^ttt)^- = 1,2,4-tri(tert-butyl)-cyclopentadienide). Detailed studies of this system and its diamagnetic Y analogue indicate that the interaction of the metal chloride with surface Al sites results in a change of the coordination sphere around the metal center inducing for the dysprosium-containing material slow magnetic relaxation up to 51 K with hysteresis up to 8 K and an effective energy barrier (U_eff) of 449 cm^-1, the highest reported thus far for a supported SMM.

Realizing Two-Dimensional Supramolecular Arrays of a Spin Molecule via Halogen Bonding

Well-ordered spin arrays are desirable for next-generation molecule-based magnetic devices, yet their synthetic method remains a challenging task. Herein, we demonstrate the realization of two-dimensional supramolecular spin arrays on surfaces via halogen-bonding molecular self-assembly. A bromine-terminated perchlorotriphenylmethyl radical with net carbon spin was synthesized and deposited on Au(111) to achieve two-dimensional supramolecular spin arrays. By taking advantage of the diversity of halogen bonds, five supramolecular spin arrays form and are probed by low-temperature scanning tunneling microscopy at the single-molecule level. First-principles calculations verify that the formation of three distinct types of halogen bonds can be used to tailor supramolecular spin arrays via molecular coverage and annealing temperature. Our work suggests that supramolecular self-assembly can be a promising method to engineer two-dimensional molecular spin arrays.

Towards large area surface functionalization with luminescent and magnetic lanthanoid complexes

Homogeneous surface deposition of molecules over a large area of substrate is difficult to achieve but extremely important for proposed applications of magnetic molecules in data storage, information processing or...

A lanthanide(iii) dodecanuclear structure with a acylhydrazone Schiff-base ligand: slow magnetic relaxation and magnetocaloric effects

In the complex synthesis process, the design of new type structures of lanthanide clusters with high nuclearity provides an opportunity to understand the nature of magnetic dynamics.

Direct Imaging of Isolated Single-Molecule Magnets in Metal-Organic Frameworks

Practical applications involving the magnetic bistability of single-molecule magnets (SMMs) for next-generation computer technologies require nanostructuring, organization, and protection of nanoscale materials in two- or three-dimensional networks, to enable read-and-write processes. Owing to their porous nature and structural long-range order, metal-organic frameworks (MOFs) have been proposed as hosts to facilitate these efforts. Although probing the channels of MOF composites using indirect methods is well established, the use of direct methods to elucidate fundamental structural information is still lacking. Herein we report the direct imaging of SMMs encapsulated in a mesoporous MOF matrix using high-resolution transmission electron microscopy. These images deliver, for the first time, direct and unambiguous evidence to support the adsorption of molecular guests within the porous host. Bulk magnetic measurements further support the successful nanostructuring of SMMs. The preparation of the first magnetic composite thin films of this kind furthers the development of molecular spintronics.

Between single ion magnets and macromolecules: a polymer/transition metal-based semi-solid solution

The creation of functional magnetic materials for application in high-density memory storage or in the new field of molecular spintronics is a matter of widespread interest among the material research community. Herein, we describe a new approach that combines the qualities of single ion magnets, displaying slow magnetic relaxations, and the merits of polymers, being easy to process and widely used to produce thin films. Basing the idea on cobalt(ii) ions and pyridine-based single ion magnets, a new macromolecular magnetic material was obtained - a polymeric matrix of poly(4-vinylpyridine) (P4VP) cross-linked by a cobalt(ii) salt bound within it, effectively forming a network of single ion magnets, with field-induced magnetic relaxations preserved in both bulk and thin film forms. The binding of cobalt is confirmed by a series of methods, like secondary ion mass spectroscopy or high-resolution X-ray photoelectron spectroscopy. The magnetic relaxation times, up to 5 × 10-6 s, are controllable simply by dilution, making this new material a semi-solid solution. By this approach, a new path is formed to connect molecular magnetism and polymer science, showing that the easy polymer processing can be used in forming self-organizing functional magnetic thin films.

Design and synthesis of a family of 1D-lanthanide-coordination polymers showing luminescence and slow relaxation of the magnetization

We have designed and synthesized eight isostructural 1D coordination polymers (CPs) with the general formula {[Ln(aapc)3(DMF)]}n [where Ln(iii) = Y (2), La (3), Nd (4), Eu (5), Gd (6), Tb (7), Dy (8), Er (9); and aapc = 3-((anthraquinone-1-yl)amino)propanoate]. These CPs consist of Ln-carboxylate infinite rods in which the bulky anthraquinone scaffolds arise from it in such a way that the resulting supramolecular packing exhibits isolated 1D chains. Solution structures have been corroborated through NMR methods including PGSE and EXSY NMR studies and, due to the presence of lanthanide ions, magnetic and luminescence properties have been studied. Alternating current magnetic measurements of compound 8 show slow relaxation of the magnetization, a characteristic of single molecule magnets (SMMs). The evaluation of solid-state photophysical properties reveals that the aapc scaffold sensitizes lanthanide(iii) based emission of compounds 4-9 both in the visible and near-infrared (NIR) regions at 10 K.

Polymer-Encapsulated Lanthanide-Containing Clusters as Platforms for Fabricating Magnetic Soft Materials

Although many high-nuclearity lanthanide-containing clusters with aesthetical topological nanoarchitectures and unique magnetic properties have been synthesized, there was a big time lag to develop functional soft materials and devices on the basis of these clusters, because of their stability and processability. Herein, we report a universal strategy to fabricate lanthanide cluster based magnetic soft materials under ambient conditions. The prototypical cluster [Gd₅₂Ni₅₆(IDA)₄₈(OH)₁₅₄(H₂O)₃₈]¹⁸⁺ was encapsulated as an inorganic core by polymeric shells of sulfonate end functionalized poly(ethylene glycol) monoalkyl ethers through electrostatic interaction. The thickness of the shell was readily controlled by precisely tuning length of the polymer chain, leading to controllably reduced antiferromagnetic interactions between the clusters. The encapsulated hybrids can self-assemble to form vesicles in solution and can be used as an excellent agent for in vivo magnetic resonance imaging.

Magnetic properties and structure of tetranuclear lanthanide complexes based on 8-hydroxylquinoline Schiff base derivative and β-diketone coligand

Herein, we report the synthesis, structure, and magnetism of five tetranuclear Ln^III complexes encapsulated by 5-(4-pyridinecarboxaldehyde)amino-8-hydroxylquinoline and β-diketonate coligand.

Porous substrates as platforms for the nanostructuring of molecular magnets

This article highlights recent advances in the newly emerging field on the nanostructuration of molecular magnets using porous substrates.

Deposition of exchange-coupled dinickel complexes on gold substrates utilizing ambidentate mercapto-carboxylato ligands

The chemisorption of magnetically bistable transition metal complexes on planar surfaces has recently attracted increased scientific interest due to its potential application in various fields, including molecular spintronics. In this work, the synthesis of mixed-ligand complexes of the type [NiII2L(L')](ClO4), where L represents a 24-membered macrocyclic hexaazadithiophenolate ligand and L' is a ω-mercapto-carboxylato ligand (L' = HS(CH2)5CO2- (6), HS(CH2)10CO2- (7), or HS(C6H4)2CO2- (8)), and their ability to adsorb on gold surfaces is reported. Besides elemental analysis, IR spectroscopy, electrospray ionization mass spectrometry (ESIMS), UV-vis spectroscopy, and X-ray crystallography (for 6 and 7), the compounds were also studied by temperature-dependent magnetic susceptibility measurements (for 7 and 8) and (broken symmetry) density functional theory (DFT) calculations. An S = 2 ground state is demonstrated by temperature-dependent susceptibility and magnetization measurements, achieved by ferromagnetic coupling between the spins of the Ni(II) ions in 7 (J = +22.3 cm-1) and 8 (J = +20.8 cm-1; H = -2JS1S2). The reactivity of complexes 6-8 is reminiscent of that of pure thiolato ligands, which readily chemisorb on Au surfaces as verified by contact angle, atomic force microscopy (AFM) and spectroscopic ellipsometry measurements. The large [Ni2L] tail groups, however, prevent the packing and self-assembly of the hydrocarbon chains. The smaller film thickness of 7 is attributed to the specific coordination mode of the coligand. Results of preliminary transport measurements utilizing rolled-up devices are also reported.

A Dy4 single-molecule magnet and its Gd(iii), Tb(iii), Ho(iii), and Er(iii) analogues encapsulated by an 8-hydroxyquinoline Schiff base derivative and β-diketonate coligand

We report five tetranuclear Ln^IIIcomplexes encapsulated by 5-(benzylidene)amino-8-hydroxyquinoline and the β-diketonate coligand.

STM Study of Au(111) Surface-Grafted Paramagnetic Macrocyclic Complexes [Ni2L(Hmba)](+) via Ambidentate Coligands

Molecular anchoring and electronic properties of macrocyclic complexes fixed on gold surfaces have been investigated mainly by using scanning tunnelling microscopy (STM) and complemented with X-ray photoelectron spectroscopy (XPS). Exchange-coupled macrocyclic complexes [Ni2L(Hmba)](+) were deposited via 4-mercaptobenzoate ligands on the surface of a Au(111) single crystal from a mM solution of the perchlorate salt [Ni2L(Hmba)]ClO4 in dichloromethane. The combined results from STM and XPS show the formation of large monolayers anchored via Au-S bonds with a height of about 1.5 nm. Two apparent granular structures are visible: one related to the dinickel molecular complexes (cationic structures) and a second one related to the counterions ClO4(-) which stabilize the monolayer. No type of short and long-range order is observed. STM tip-interaction with the monolayer reveals higher degradation after 8 h of measurement. Spectroscopy measurements suggest a gap of about 2.5 eV between HOMO and LUMO of the cationic structures and smaller gap in the areas related to the anionic structures.

Metal-Organic Frameworks as Platforms for the Controlled Nanostructuring of Single-Molecule Magnets

The prototypical single-molecule magnet (SMM) molecule [Mn12O12(O2CCH3)16(OH2)4] was incorporated under mild conditions into a highly porous metal-organic framework (MOF) matrix as a proof of principle for controlled nanostructuring of SMMs. Four independent experiments revealed that the SMM clusters were successfully loaded in the MOF pores, namely synchrotron-based powder diffraction, physisorption analysis, and in-depth magnetic and thermal analyses. The results provide incontrovertible evidence that the magnetic composite, SMM@MOF, combines key SMM properties with the functional properties of MOFs. Most importantly, the incorporated SMMs exhibit a significantly enhanced thermal stability with SMM loading advantageously occurring at the periphery of the bulk MOF crystals with only a single SMM molecule isolated in the transverse direction of the pores.

Highly Ordered Surface Self-Assembly of Fe₄ Single Molecule Magnets

Single molecule magnets (SMMs) have attracted considerable attention due to low-temperature magnetic hysteresis and fascinating quantum effects. The investigation of these properties requires the possibility to deposit well-defined monolayers or spatially isolated molecules within a well-controlled adsorption geometry. Here we present a successful fabrication of self-organized arrays of Fe4 SMMs on hexagonal boron nitride (h-BN) on Rh(111) as template. Using a rational design of the ligand shell optimized for surface assembly and electrospray as a gentle deposition method, we demonstrate how to obtain ordered arrays of molecules forming perfect hexagonal superlattices of tunable size, from small islands to an almost perfect monolayer. High-resolution low temperature scanning tunneling microscopy (STM) reveals that the Fe4 molecule adsorbs on the substrate in a flat geometry, meaning that its magnetic easy axis is perpendicular to the surface. By scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations, we infer that the majority- and minority-spin components of the spin-split lowest unoccupied molecular orbital (LUMO) can be addressed separately on a submolecular level.

A salen-type Dy4 single-molecule magnet with an enhanced energy barrier and its analogues

A discrete hexadentate salen-type tetranuclear Dy complex with a unique {Ln₄O₈} core exhibits two slow magnetic relaxations with the highest energy barrier of 48.14 K among the reported tetranuclear salen-type dysprosium SMMs.

Synthesis, structure and magnetic properties of phenylhydroxamate-based coordination clusters

The strategic recombination of preformed coordination clusters in the presence of polymodal bridging ligands has successfully led to the characterisation of five new compounds of structural and magnetic interest. Indeed using the dinuclear complex [M2(H2O)(piv)4(Hpiv)4] (M = Co, Ni; Hpiv = pivalic acid) as starting material and reacting it with phenylhydroxamic acid (H2pha) has yielded the four tetrametallic coordination clusters [Co4(Hpha)2(piv)6(Hpiv)4] (1), [Ni4(Hpha)2(piv)6(Hpiv)2(DMF)2] (2), [Co4(Hpha)2(piv)6(EtOH)2(H2O)2] (3), [Ni4(Hpha)2(piv)6(EtOH)2(H2O)2] (4) and the hexanuclear complex [Co6(Hpha)4(piv)8(EtOH)2]·EtOH (5). All the compounds have been structurally characterised revealing a particular binding mode for the hydroxamate ligand. The study of their magnetic properties has been performed and the modelling of these properties has been done using the appropriate hamiltonians for each compound. The experimental data and their modelling show non-zero spin ground states for compounds 4 and 5.

Mn12 single molecule magnets deposited on μ-SQUID sensors: the role of interphases and structural modifications

Direct measurements of the linear ac susceptibility and magnetic relaxation of a few Mn12 monolayers deposited on a μ-SQUID sensor are reported. In order to integrate the molecules into the device, DPN has been the technique of choice. It enabled the structuration of the molecules on the most sensitive areas of the sensor without the need for any previous functionalization of the molecule or the substrate, while controlling the number of molecular units deposited on each array. The measurements reveal that their characteristic SMM behaviour is lost, a fact that is attributed to molecular distortions originated by the strong surface tensions arising at the molecular interphases.

Hybrid nanomaterials: anchoring magnetic molecules on naked gold nanocrystals

The pairing of molecular magnets and nanomaterials couples top-down and bottom-up approaches to nanotechnology; facilitating a unique methodology to the controlled study of interfacial magnetic properties. Attaching Single-Molecule Magnets (SMMs) to "naked" gold nanoparticles is a novel method of exploring various avenues of magnetic nanotechnology, such as drug delivery, information storage, catalysis, and assembly of magnetic-nanostructural motifs. Herein we report the successful capping of laser ablation synthesized "naked" gold nanoparticles with a dinuclear dysprosium complex, while introducing new information regarding the changes in molecular magnetic properties upon surface attachment. We anticipate that this methodology in producing these magneto-plasmonic nanostructures not only provides answers to fundamental questions but also has the potential to provide new avenues to applications including information storage, multimodal imaging, biomedicine, and optoelectronics.

Chemisorption of exchange-coupled [Ni2L(dppba)]+ complexes on gold by using ambidentate 4-(diphenylphosphino)benzoate co-ligands

A new strategy for the fixation of redox-active dinickel(II) complexes with high-spin ground states to gold surfaces was developed. The dinickel(II) complex [Ni2L(Cl)]ClO4 (1ClO4), in which L(2-) represents a 24-membered macrocyclic hexaaza-dithiophenolate ligand, reacts with ambidentate 4-(diphenylphosphino)benzoate (dppba) to form the carboxylato-bridged complex [Ni2L(dppba)](+), which can be isolated as an air-stable perchlorate [Ni2L(dppba)]ClO4 (2ClO4) or tetraphenylborate [Ni2L(dppba)]BPh4 (2BPh4) salt. The auration of 2ClO4 was probed on a molecular level, by reaction with AuCl, which leads to the monoaurated Ni(II)2Au(I) complex [Ni(II)2L(dppba)Au(I)Cl]ClO4 (3ClO4). Metathesis of 3ClO4 with NaBPh4 produces [Ni(II)2L(dppba)Au(I)Ph]BPh4 (4BPh4), in which the Cl(-) is replaced by a Ph(-) group. The complexes were fully characterized by ESI mass spectrometry, IR and UV/Vis spectroscopy, X-ray crystallography (2BPh4 and 4BPh4), cyclic voltammetry, SQUID magnetometry and HF-ESR spectroscopy. Temperature-dependent magnetic susceptibility measurements reveal a ferromagnetic coupling J = +15.9 and +17.9 cm(-1) between the two Ni(II) ions in 2ClO4 and 4BPh4 (H = -2 JS1S2). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0), which implies a bistable (easy axis) magnetic ground state. The binding of the [Ni2L(dppba)]ClO4 complex to gold was ascertained by four complementary surface analytical methods: contact angle measurements, atomic-force microscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The results indicate that the complexes are attached to the Au surface through coordinative Au-P bonds in a monolayer.

Self-assembled monolayer of Cr7Ni molecular nanomagnets by sublimation

We show, by complementary spectroscopic and STM analysis, that Cr(7)Ni derivatives are suitable to be sublimed in UHV conditions. Cr(7)Ni-bu weakly bonds to gold surface and can diffuse relatively freely on it, forming monolayers with hexagonal 2D packing. Conversely, by adding a functional thiol group to the central dibutylamine, a covalent bond between the molecule and surface gold adatoms is promoted, leading to a strong molecular grafting and the formation of a disordered monolayer. These two examples demonstrate the possibility to control the assembly of a large molecular complex, as rationalized by DFT calculations that establish different energy scales in the deposition processes. Moreover, low-temperature XMCD sprectra show that the magnetic features of Cr(7)Ni rings deposited in UHV on gold remain unchanged with respect to those of the corresponding bulk sample.

Mn12 single-molecule magnet aggregates as magnetic resonance imaging contrast agents

Mn(12) single-molecule magnets have been dispersed in water through an emulsion-assisted self-assembly method with an improved stability in water, in order to investigate the use of Mn(12) as MRI contrast agents.

Atomic-Resolution Kinked Structure of an Alkylporphyrin on Highly Ordered Pyrolytic Graphite

The atomic structure of the chains of an alkyl porphyrin (5,10,15,20-tetranonadecylporphyrin) self-assembled monolayer (SAM) at the solid/liquid interface of highly ordered pyrolytic graphite (HOPG) and 1-phenyloctane is resolved using calibrated scanning tunneling microscopy (STM), density functional theory (DFT) image simulations, and ONIOM-based geometry optimizations. While atomic structures are often readily determined for porphyrin SAMs, the determination of the structure of alkyl-chain connections has not previously been possible. A graphical calibration procedure is introduced, allowing accurate observation of SAM lattice parameters, and, of the many possible atomic structures modeled, only the lowest-energy structure obtained was found to predict the observed lattice parameters and image topography. Hydrogen atoms are shown to provide the conduit for the tunneling current through the alkyl chains.

An invitation to molecular magnetism

Molecular magnetism is a new and extremely fascinating field on the borders of chemistry, physics and materials science. The design and synthesis of molecule-based magnets requires the chemist to exert considerable control over the molecules to arrange them appropriately. It also demands the development of new theories to explain the complex magneto-structural behaviour of these intriguing solids. Molecular magnetism is still at a very early stage of development. The main challenge is to increase the strength of the magnetic interactions between spin carriers so the resulting materials can be usable at room temperature. However molecular magnets exhibit true potential to become multifunctional materials. They show some considerable advantages over conventional magnets: optical transparency, chemical sensitivity and low weight to name just a few. The following article is not a complete review of the field. Its aim is rather to show how beautiful and versatile magnetic molecular solids can be, and to encourage the in-depth study of the subject.

Self-organized honeycomb structures of Mn(12) single-molecule magnets

In this paper, Mn(12)-based ordered honeycomb structures were successfully constructed from a simple solution casting process at high relative humidity through the modification of fatty acids to Mn(12) clusters. Mn(12)-fatty acid complexes maintain typical features of a single-molecule magnet as confirmed by IR spectra and magnetization hysteresis studies. Investigation of the effects of concentration, velocity of humid airflow, solvent, substrate, and alkyl chain length of the Mn(12) complex on the morphology of the honeycomb structures demonstrated wide generality and high reproducibility of the formation of Mn(12)-based self-organized honeycomb-patterned films. Both two-dimensional and three-dimensional honeycomb structures were obtained by adjusting the concentration of the complex solution. Mn(12)-based, honeycomb-patterned films maintain a paramagnetic response at room temperature, and thus give rise to a spatially distributed magnetic pattern on the substrate, which can be imaged by magnetic force microscopy. Importantly, the single-molecule magnetic property of the Mn(12) complex at low temperature is well maintained in the honeycomb-patterned film, which represents a promising outlook for high-density information storage and quantum computing applications.