Lanthanide-Based Coordination Polymers Molecular Alloys Stability: A Thermochemical Approach.

In this study, we investigate the thermodynamics of lanthanide-based coordination polymer molecular alloys. We demonstrate that if lanthanide ions have many chemical similarities, the solubility of homo-lanthanide-based coordination polymers can vary significantly from one lanthanide ion to another. Indeed, we experimentally determine the solubility constants of a series of isostructural homo-lanthanide coordination polymers, with general chemical formula [Ln2(bdc)3(H2O)4]∞ with Ln = La-Er plus Y and where bdc2- symbolizes 1,4-benzene-di-carboxylate. Then, we extend the study to two series of isostructural molecular alloys with general chemical formula [Ln2xLn'2 -2x(bdc)3(H2O)4]∞ with 0 ≤ x ≤ 1 based either on heavy ([Eu2xTb2 - 2x(bdc)3(H2O)4]∞) or light ( [Nd2xSm2-2x(bdc)3(H2O)4]∞) lanthanide ions. We found that whatever the solubility difference of the homo-nuclear compounds is, the configurational entropy is the main driving force of the stabilization of molecular alloys.

Metallogels: a novel approach for the nanostructuration of single-chain magnets.

In this study we demonstrate that single-chain magnets (SCMs) can be assembled in gel phase and transferred intact on surface. We take advantage of a family of SCMs based on TbIII ions and nitronyl-nitroxides radicals functionalized with short alkyl chains known to form crystalline supramolecular nanotubes interacting with heptane acting as crystallizing solvent. When the radicals are functionalized with long aliphatic chains a robust gel is formed with similar structural and functional properties respect to its crystalline parent. Indeed, a small-angle X-ray scattering (SAXS) study unambiguously demonstrates that the gel is made of supramolecular nanotubes: the high stability of the gel allows the determination from SAXS data of precise nanotube metrics such as diameter, helical pitch and monoclinic cell of the folded 2D crystal lattice along the tube direction. Additionally, static and dynamic magnetic investigations show the persistence of the SCM behavior in the metallogel. Last, on-surface gelation provides thick films as well as sub-monolayer deposits of supramolecular nanotubes on surface as evidenced by atomic force microscopy (AFM) observations. This paves the road toward magnetic materials and devices made of SCMs profiting of their isolation on surface as individual chains.

Luminance and Brightness: Application to Lanthanide-Based Coordination Polymers.

Measuring the luminance of lanthanide-based coordination polymers under UV excitation is of prime importance for many technological applications. This study highlights that the quantum yield gives no information about the luminescence intensity of a solid-state compound. Indeed, compounds with high quantum yield can actually be poorly luminescent. Therefore, a brightness calculation or a luminance measurement are mandatory for a quantitative estimation of the luminescence intensity. The calculated brightness appears to be a convenient quantitative parameter for the estimation of the luminescence intensity in the infrared domain, in which luminance is senseless. It is also a useful parameter in the visible domain, but one must keep in mind that only compounds with similar colorimetric coordinates can be compared. For comparing the luminescence intensities of compounds that exhibit different emission colors, the luminance measurement seems to be the most efficient method. A home-made setup that allows this measurement with high reproducibility is described in detail. The luminance of several lanthanide-based coordination polymers with benzene-poly-carboxylate ligands is measured, and the results are compared with brightness and quantum yield measurements. A standard is suggested for calibration.

Investigation of Intermetallic Energy Transfers in Lanthanide Coordination Polymers Molecular Alloys: Case Study of Trimesate-Based Compounds.

Reactions in water at ambient temperature and pressure between a lanthanide ion and benzene-1,3,5-tricarboxylate (or trimesate) lead to two series of iso-structural coordination polymers. Their general chemical formula is [Ln(tma)(H2O)6]∞ for the lightest lanthanide ions (Ln = La-Dy except Pm), while it is [Ln(tma)(H2O)5·3.5H2O]∞ for the heaviest ones (Ho-Lu plus Y). For the heaviest lanthanide ions, reactions at 50 °C lead to a third structural family with the general chemical formula [Ln(tma)(H2O)3·1.5H2O]∞ with Ln = Ho-Lu plus Y. Homo-lanthanide coordination polymers that belong to the latter two families do not exhibit luminescence in the visible region. Therefore, we used a phase induction strategy to obtain molecular alloys that belong to these structural families and show sizeable emission. The random distribution of the lanthanide ions over the metallic sites has been investigated using 89Y and 139La solid-state NMR spectroscopy experiments. Luminescent properties of homo- and hetero-nuclear coordination polymers based on Eu3+ and Tb3+ have been studied in detail and compared. As a result, this study strongly suggests that exchange-based intermetallic energy transfer mechanisms play an important role in these systems. It also suggests the presence of an intermetallic exchange pathway through π-stacking interactions.

Synthesis, Crystal Structure, and Luminescence Properties of the Iso-Reticular Series of Lanthanide Coordination Polymers Synthesized from Hexa-Lanthanide Molecular Precursors.

Microwave-assisted reactions in DMSO, between a hexa-lanthanide octahedral complex ([Ln6(µ6-O)(µ3-OH)8(NO3)6(H2O)12·2NO3·2H2O] with Ln = Nd-Yb plus Y) and either 3-halogenobenzoic acid (hereafter symbolized by 3-xbH with x = f or c for fluoro or chloro, respectively) or 4-halogenobenzoic acid (hereafter symbolized by 4-xbH with x = f, c, or b for fluoro, chloro, or bromo, respectively), lead to 1D lanthanide coordination polymers. These coordination polymers are almost iso-reticular. The crystal structure is described on the basis of the coordination polymer with chemical formula [Tb(4-fb)3(DMSO)(H2O)2·DMSO]∞ obtained from 4-fluorobenzoic acid (4-fbH) and the Tb3+-based octahedral complex: It crystallizes in the triclinic system, space group P1̅ (n°2), with the following cell parameters: a = 9.8561(9) Å, b = 10.5636(9) Å, c = 15.1288(15) Å, α = 100.840(3)°, ß = 95.552(3)°, γ = 110.482(3)°, V = 1426.4(3) Å3, and Z = 2. It can be described on the basis of 1D molecular chains. Luminescence properties of the Tb and Eu derivatives have been measured and compared vs the halogeno-function and its position (meta or para). Some molecular alloys have also been prepared to estimate the strength of the intermetallic energy transfers. To confirm that the hexa-nuclear complexes (and not the halogenated ligand) have a structuring effect for the formation of the straight chain-like molecular motif, another coordination polymer with chemical formula [Tb(4-npa)3DMSO·DMSO·H2O]∞ where 4-npaH symbolizes 4-nitro-phenyl-acetic acid has been prepared. It crystallizes in the triclinic system, space group P1̅ (n°2) with the following cell parameters: a = 7.8784(8) Å, b = 14.8719(16) Å, c = 15.2753(17) Å, α = 73.612(4)°, ß = 86.406(4)°, γ = 83.104(4)°, V = 1703.8(3) Å3, and Z = 2. Its crystal structure can be described on the basis of a molecular motif that is similar to the one observed in the five previous crystal structures which confirms the structuring effect of the hexa-nuclear complexes.

Hexanuclear Molecular Precursors as Tools to Design Luminescent Coordination Polymers with Lanthanide Segregation.

Solvothermal reactions between hexanuclear complexes with the general chemical formula [Ln6(µ6-O)(µ3-OH)8(NO3)6(H2O)12]·2NO3·2H2O and 2-bromobenzoic acid (2-bbH) lead to a series of isostructural one-dimensional coordination polymers with the general chemical formula [Ln2(2-bb)6]∞ with Ln = Sm, Eu, Tb, Dy, and Y. These coordination polymers crystallize in the orthorhombic space group Fdd2 (No. 43) with the following cell parameters: a = 29.810(3) Å, b = 51.185(6) Å, c = 11.7913(14) Å, V = 17992(4) Å3, and Z = 16. The europium- and terbium-based derivatives show sizable luminescence intensities under UV excitation. Isostructural heterolanthanide coordination polymers have also been prepared. Their luminescent properties suggest that during the synthetic process the starting hexanuclear complexes are destroyed but strongly influence the distribution of the different lanthanide ions over the metallic sites of the crystal structure. Indeed, it is possible to prepare heterolanthanide coordination polymers in which lanthanide-ion segregation is controlled.

Single-chain magnet behavior in a finite linear hexanuclear molecule.

The careful monitoring of crystallization conditions of a mixture made of a TbIII building block and a substituted nitronyl-nitroxide that typically provides infinite coordination polymers (chains), affords a remarkably stable linear hexanuclear molecule made of six TbIII ions and five NIT radicals. The hexanuclear units are double-bridged by water molecules but ab initio calculations demonstrate that this bridge is inefficient in mediating any magnetic interaction other than a small dipolar antiferromagnetic coupling. Surprisingly the hexanuclears, despite being finite molecules, show a single-chain magnet (SCM) behavior. This results in a magnetic hysteresis at low temperature whose coercive field is almost doubled when compared to the chains. We thus demonstrate that finite linear molecules can display SCM magnetic relaxation, which is a strong asset for molecular data storage purposes because 1D magnetic relaxation is more robust than the relaxation mechanisms observed in single-molecule magnets (SMMs) where under-barrier magnetic relaxation can operate.

Highly Luminescent Europium-Based Heteroleptic Coordination Polymers with Phenantroline and Glutarate Ligands.

Isostructural lanthanide-based coordination polymers with general chemical formula [Ln(phen)(glu)(NO3)]∞ with Ln = La-Tm (except Ce and Pm) have been synthesized by hydrothermal methods (H2glu stands for glutaric acid and phen stands for 1,10-phenantroline). They crystallize in the monoclinic system with the P21/c (no. 14) space group. The crystal structure has been solved on the basis of the La derivative. It can be described as the superimposition of molecular chains of dimeric La(phen)(NO3)-La(phen)(NO3) units bridged by glutarate ligands. Luminescent properties have been explored and show that the Eu derivative exhibits the highest luminance observed for Eu-based coordination polymers (85 to 105 cd·m-2). Effects of the dilution of the Eu3+ and Tb3+ luminescent ions by Gd3+ optically inactive ions are unexpected and to the best of our knowledge unprecedented. This could be related to the different intermetallic energy-transfer mechanisms in competition and to the nonisotropic distribution of the lanthanide ions in these molecular alloys. The investigation of molecular alloys with general chemical formula [Eu1-xTbx(phen)(glu)(NO3)]∞ with 0 ≤ x ≤ 1 highlights a very sizable and constant Eu3+ luminescence whatever the x value, which further confirms the existence of very strong intermetallic energy transfers in this family of compounds. It is also noticeable that some coordination polymers based on weakly emissive lanthanide ions exhibit very well defined emission spectra.

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 ß-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.

Rational Design of Dual IR and Visible Highly Luminescent Light-Lanthanides-Based Coordination Polymers.

A series of isostructural homo- and heterolanthanide coordination polymers of formula [Ln2(dcpa)3(H2O)]∞ with Ln = La-Gd have been obtained by reactions in water between the lightest lanthanide chlorides and the disodium salt of 4,5-dichlorophthalic acid (H2dcpa). They present particularly high thermal stability for coordination compounds (up to 400 °C). Their luminescent properties have been studied in detail. Interestingly an insensitivity to water coordination as well as a very strong effect of optical dilution is observed. Therefore, molecular alloys with very high lanthanum concentration have been prepared. Some of them present highly tunable and very intense luminescence. For example, to the best of our knowledge, [Sm0.04La1.96(dcpa)3(H2O)]∞ presents one of the highest overall quantum yields measured so far for a Sm3+-based coordination compound (QSmLigand = 9.2%), and [Nd0.03Sm0.14Eu0.03La1.8(dcpa)3(H2O)]∞ is one of the brightest (12 Cd·m-2 under 0.75 mW·cm-2 UV flux) multiemissive visible and near-infrared lanthanide-coordination polymers reported to date.

High Luminance of Heterolanthanide-Based Molecular Alloys by Phase-Induction Strategy.

Reactions in water of lanthanide chlorides with the sodium salt of 4,5-dichlorophthalate (dcpa2-) lead to two families of isostructural coordination polymers: family F1 that gathers compounds with the general chemical formula [Ln2(dcpa)3(H2O)]∞ with Ln = La-Gd (except Pm) and family F2 that gathers compounds with general chemical formula [Ln2(dcpa)3(H2O)5·3H2O]∞ with Ln = Tb-Lu plus Y. Heterolanthanide molecular alloys that contain both Eu3+ and Tb3+ ions have been prepared in both structural families. Their luminescence properties have been studied, especially from the brightness point of view. This study revealed that structural family F1 provides molecular alloys that are much more luminescent than those of structural family F2. Therefore, a phase-induction strategy was followed that allowed the design of some molecular alloys (La/Tb/Eu and La/Dy) that are, to the best of our knowledge, among the most luminescent coordination polymers reported so far. This study opens the way to bright luminescent bar codes as well as to bright white luminescent lanthanide-based coordination polymers.

Sonocrystallization as an Efficient Way to Control the Size, Morphology, and Purity of Coordination Compound Microcrystallites: Application to a Single-Chain Magnet.

The size, morphology, and purity control of coordination compound powders is a key stage for their conversion into materials and devices. In particular, surface science techniques require highly pure bulk materials with a narrow crystallite-size distribution together with straightforward, scalable, and reproducible crystallization procedures. In this work we demonstrate how sonocrystallization, i.e. the application of ultrasound during the crystallization process, can afford very quickly powders made of crystallites with controlled size, morphology, and purity. We show that this process drastically diminishes the crystallite-size distribution (low polydispersity indexes, PDI) and crystallite aspect ratio. By comparing sonicated samples with samples obtained by various silent crystallization conditions, we unambiguously show that the improvement in the crystallite morphology and size distribution is not due to any thermal effect but to the sonication of the crystallizing media. The application of sonocrystallization on crystallization batches of single-chain magnets (SCMs) maintains the chemical integrity of the SCMs together with their original magnetic behavior. Moreover, luminescent measurements show that sonocrystallization induces an efficient micromixing that drastically enhances the purity of the SCM powders. We thus propose that sonocrystallization, which is already used on organic or MOF compounds, can be applied to (magnetic) coordination compounds to readily afford bulk powders for characterization or shaping techniques that require pure, morphology- and crystallite-size-controlled powder samples.

Chiral Supramolecular Nanotubes of Single-Chain Magnets.

We report a single-chain magnet (SCM) made of a terbium(III) building block and a nitronyl-nitroxide radical (NIT) functionalized with an aliphatic chain. This substitution is targeted to induce a long-range distortion of the polymeric chain and accordingly it gives rise to chains that are curled with almost 20 nm helical pitch. They self-organize as a chiral tubular superstructure made of 11 chains wound around each other. The supramolecular tubes have a 4.5 nm internal diameter. Overall, this forms a porous chiral network with almost 44 % porosity. Ab initio calculations highlight that each TbIII ion possesses high magnetic anisotropy. Indeed, notwithstanding the supramolecular arrangement each chain behaves as a SCM. Magnetic relaxation with both finite and infinite-size regimes is observed and confirms the validity of the Ising approximation. This is associated with quite strong coercive field and magnetic remanence (Hc =2400 Oe MR =2.09 µB at 0.5 K) for this class of compounds.

Hetero-hexalanthanide Complexes: A New Synthetic Strategy for Molecular Thermometric Probes.

Reactions in solvothermal or microwave-assisted conditions between a hexanuclear rare-earth entity ([Ln6] with Ln = Eu-Dy) and m-halogeno-benzoic acids lead to three series of isostructural complexes with respective chemical formulas [Ln6(µ3-OH)2(H2O)2(NO3)2(3-cb)14]·4CH3CN, [Ln6(µ3-OH)2(H2O)2(NO3)2(3-bb)14]·6CH3CN, and [Ln6(µ3-OH)2(H2O)2(NO3)2(3-ib)14]·6CH3CN, where 3-cb-, 3-bb-, and 3-ib- represent 3-chloro-, 3-bromo-, and 3-iodo-benzoate, respectively. These three series of compounds are almost isostructural. Their luminescent properties, in the solid and solution states, have been studied in detail and compared. This study shows that hexanuclear complexes own strong intermetallic energy transfers. This makes these complexes good candidates for thermometric probes in solid state or in solution state.

A supramolecular chain of dimeric Dy single molecule magnets decorated with azobenzene ligands.

We report the synthesis, ab initio calculations, magnetic and optical characterization of a DyIII-based dimeric compound named DyAZO. The dimers self-organize into a supramolecular chain decorated with photo-isomerizable azobenzene ligands. DyAZO displays single-molecule magnet (SMM) behavior. However, ab initio calculations highlight a quite strong admixture of MJ states of the 6H15/2 level of DyIII ions, the presence of low-lying excited MJ states and antiferromagnetic Dy-Dy dipolar coupling. This favors zero-field fast tunneling. Accordingly the Dy-doped analogue YDyAZO (5.5% Dy doping) displays enhanced magnetic relaxation with a hysteresis that is observed at 0.5 K. The influence of the cis- to trans-isomerization of the decorating azobenzene ligand on magnetic properties has been tested for both solid samples and solutions of DyAZO and YDyAZO. This provides hints for the synthesis of future Dy-based photo-isomerizable molecules.

Synthesis and crystal structure of a new coordination polymer based on lanthanum and 1,4-phenyl-enedi-acetate ligands.

Reaction in gel between the sodium salt of 1,4-phenyl-enedi-acetic acid (Na2C10O4H8-Na2 p-pda) and lanthanum chloride yields single crystals of the three-dimensional coordination polymer poly[[tetra-aqua-tris-(µ-1,4-phenyl-enedi-acetato)-dilanthanum(III)] octa-hydrate], {[La2(C10H8O4)3(H2O)4]·8H2O}∞. The LaIII coordination polyhedron can be described as a slightly distorted monocapped square anti-prism. One of the two p-pda2- ligands is bound to four LaIII ions and the other to two LaIII ions. Each LaIII atom is coordinated by five ligands, thereby generating a metal-organic framework with potential porosity properties.

Multi-Emissive Lanthanide-Based Coordination Polymers for Potential Application as Luminescent Bar-Codes.

Isostructural lanthanide-based coordination polymers that are obtained by reactions in water of a lanthanide chloride and the sodium salt of 5-methoxyisophthalate (mip2-) have the general chemical formula [Ln2(mip)3(H2O)8·4H2O]∞ with Ln = Nd-Er except Pm plus Y (symbolized by [Ln2(mip)3]∞). Some of these homo-lanthanide compounds present very high luminescence brightness. The weak intermetallic energy transfer between lanthanide ions observed in these compounds allows the design of hetero-lanthanide coordination polymers with tunable luminescence properties. A molecular alloy that involved six different lanthanide ions (Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+) has been prepared and its luminescent properties have been studied. This compound, under a unique irradiation wavelength (λexc = 325 nm), exhibits almost 20 emission peaks in both the visible and the NIR regions at room temperature. This unprecedented richness of the emission spectrum could be of great interest as far as luminescent bar-codes are targeted.

Microcrystalline Core-Shell Lanthanide-Based Coordination Polymers for Unprecedented Luminescent Properties.

Microcrystalline core-shell powders of lanthanide-based coordination polymers with general chemical formula ([Ln(cpbOH)]∞)1- x@([Ln'(cpbOH)]∞) x with Hcpb = 1,4-carboxyphenylboronic acid have been synthesized and structurally characterized. Their luminescent properties have been studied. They are drastically different from those of heterolanthanide coordination polymers, also called "molecular alloys", that present the same crystal structure and chemical composition. Study of the photophysical properties of core-shell lanthanide-based coordination polymers reveals that it is possible to control efficiently the intermetallic energy transfers between lanthanide ions. Furthermore, multiemissive compounds, under unique irradiation, in both visible and infrared regions are easily feasible. Core-shell microstructured lanthanide-based coordination polymers have also been prepared with terephthalic (H2bdc) and trimesic (H3tma) acids as ligands for evidencing that lanthanide-ion-based coordination compounds are excellent candidates for 3D molecular epitaxial growth.

Lanthanide-Based Coordination Polymers With 1,4-Carboxyphenylboronic Ligand: Multiemissive Compounds for Multisensitive Luminescent Thermometric Probes.

Reactions in water of lanthanide chlorides with the sodium salt of 1,4-carboxyphenylboronic acid lead to two series of isostructural compounds with respective general chemical formulas [Ln(cpb)3(H2O)2]∞ for Ln = La or Ce and [Ln(cpbOH)(H2O)2·(cpb)]∞ for Ln = Pr-Lu (except Pm) plus Y. Heterolanthanide coordination polymers that are isostructural to the second series have been synthesized, and their photophysical properties have been studied. This study evidences that it is possible to design multiemissive lanthanide-based coordination polymers that could find their application as multigauge luminescent thermometric probes.

Self-assembly of a terbium(III) 1D coordination polymer on mica.

The terbium(III) ion is a particularly suitable candidate for the creation of surface-based magnetic and luminescent devices. In the present work, we report the epitaxial growth of needle-like objects composed of [Tb(hfac)3·2H2O] n (where hfac = hexafluoroacetylacetonate) polymeric units on muscovite mica, which is observed by atomic force microscopy. The needle-like shape mimics the structure observed in the crystalline bulk material. The growth of this molecular organization is assisted by water adsorption on the freshly air-cleaved muscovite mica. This deposition technique allows for the observation of a significant amount of nanochains grown along three preferential directions 60° apart from another. The magnetic properties and the luminescence of the nanochains can be detected without the need of surface-dedicated instrumentation. The intermediate value of the observed luminescence lifetime of the deposits (132 µs) compared to that of the bulk (375 µs) and the CHCl3 solution (13 µs) further reinforces the idea of water-induced growth.