Lanthanide-Based Metal–Organic Frameworks with Single-Molecule Magnet Properties

Lanthanide metal-organic frameworks (Ln-MOFs) showing single-molecule magnet (SMM) properties are an ever-growing family of materials where the magnetic properties can be tuned by various interrelated parameters, such as the coordinated solvent, temperature, organic linkers, lanthanide ions and their coordination environment. An overview of the general synthetic methodologies to access MOFs/Ln-MOFs and the peculiarities and parameters to control and/or fine-tune their SMM behavior is herein presented. Additionally, diverse challenging strategies for inducing SMM/SIM behavior in an Ln-MOF are discussed, involving redox activity and chirality. Furthermore, intriguing physical phenomena such as the CISS effect and CPL are also highlighted

Designing Magnetic NanoMOFs for Biomedicine: Current Trends and Applications

Metal–organic frameworks (MOFs) have shown a great potential in biomedicine due to their promising applications in different fields, including drug delivery, thermometry, theranostics etc. In this context, the development of magnetic sub-micrometric or nanometric MOFs through miniaturization approaches of magnetic MOFs up to the nanoscale still represents a crucial step to fabricate biomedical probes, especially in the field of theranostic nanomedicine. Miniaturization processes have to be properly designed to tailor the size and shape of particles and to retain magnetic properties and high porosity in the same material, fundamental prerequisites to develop smart nanocarriers integrating simultaneously therapeutic and contrast agents for targeted chemotherapy or other specific clinical use. An overview of current trends on the design of magnetic nanoMOFs in the field of biomedicine, with particular emphasis on theranostics and bioimaging, is herein envisioned

Reversible Tuning of Luminescence and Magnetism in a Structurally Flexible Erbium-Anilato MOF

By combining 3,6-N-ditriazolyl-2,5-dihydroxy-1,4-benzoquinone (H2trz2An) with NIR-emitting ErIII ion, two different 3D neutral polymorphic frameworks (1a and 1b), differing for the number of uncoordinated water molecules, formulated as [Er2(trz2An)3(H2O)4]n·xH2O (x = 10, a; x = 7, b), have been obtained. The structure of 1a shows layers with (6,3) topology forming six-membered rings with distorted hexagonal cavities along the bc plane. These 2D layers are interconnected through the N4 atoms of the two pendant arms of the trz2An linkers, leading to a 3D framework, where neighboring layers are eclipsed along the a axis, with hexagonal channels filled with water molecules. In 1b, layers with (6,3) topology in the [101] plane are present, each ErIII ion being connected to three other ErIII ions through bis-bidentate trz2An linkers, forming rectangular six-membered cavities. 1a and 1b are multifunctional materials showing coexistence of NIR emission and fieldinduced slow relaxation of the magnetization. Remarkably, 1a is a flexible MOF, showing a reversible structural phase transition involving shrinkage/expansion from a distorted hexagonal 2D framework to a distorted 3,6-brickwall rectangular 3D structure in [Er2(trz2An)3(H2O)2]n·2H2O (1a_des). This transition is triggered by a dehydration/hydration process under mild conditions (vacuum/heating to 360 K). The partially dehydrated compound shows a sizeable change in the emission properties and an improvement of the magnetic blocking temperature with respect to the hydrated compound, mainly related to the loss of one water coordination molecule. Theoretical calculations support experimental findings, indicating that the slight improvement observed in the magnetic properties has its origin in the change of ligand field around the ErIII ion due to the loss of a water molecule

Insights into NdIII to YbIII Energy Transfer and Its Implications in Luminescence Thermometry

This work challenges the conventional approach of using NdIII 4F3/2 lifetime changes for evaluating the experimental NdIII → YbIII energy transfer rate and efficiency. Using near-infrared (NIR) emitting Nd:Yb mixed-metal coordination polymers (CPs), synthesized via solvent-free thermal grinding, we demonstrate that the NdIII [2H11/2 → 4I15/2] → YbIII [2F7/2 → 2F5/2] pathway, previously overlooked, dominates energy transfer due to superior energy resonance and J-level selection rule compatibility. This finding upends the conventional focus on the NdIII [4F3/2 → 4I11/2] → YbIII [2F7/2 → 2F5/2] transition pathway. We characterized Nd0.890Yb0.110(BTC)(H2O)6 as a promising cryogenic NIR thermometry system and employed our novel energy transfer understanding to perform simulations, yielding theoretical thermometric parameters and sensitivities for diverse Nd:Yb ratios. Strikingly, experimental thermometric data closely matched the theoretical predictions, validating our revised model. This novel perspective on NdIII → YbIII energy transfer holds general applicability for the NdIII/YbIII pair, unveiling an important spectroscopic feature with broad implications for energy transfer-driven materials design

Luminescent platforms for metal ions sensing

This PhD thesis reports on the strategies for the design of novel luminescent chemosensors as valid alternative to conventional optical sensors, to recognize environmental pollutants, mainly heavy metal ions and nitroaromatics. Luminescent lab-on-chip platforms, molecular and Hybrid-based chemosensors, and luminescent Nanosheets are the three platforms developed for sensing applications. Fluorescent dyes of the family of Xhantenes, Rhodamine (RhO) and Fluorescein (Fluo), Diketopyrrolopyrroles (DPPs) and 2,5 -dihydroxy-1,4-benzoquinone derivatives are the old-fashioned and new building blocks selected, respectively, to fabricate these optical sensors, because of their unique luminescent properties. Particularly, 2,3-dicyano-5,6-dichloro-1,4-benzoquinone, was selected for its capability i) to produce two dimensional (2D) Coordination Polymers/Metal-Organic Frameworks (CPs/MOFs) and ii) to act as a good antenna ligand towards NIR and Vis. emitting Lanthanide ions. 2D CPs/MOFs, based on this linker, were obtained and miniaturized to 2D Nanosheets (NS), as challenging sensing platforms

Redox-active benzoquinones as challenging “non-innocent” linkers to construct 2D frameworks and nanostructures with tunable physical properties

Exploiting redox activity in supramolecular frameworks such as coordination polymers, metal-organic frameworks and related nanostructures is of paramount importance both at the molecular level and for their technological applications, since the modulation of their redox states is an emerging strategy to enhance their physical properties. In the plethora of organic linkers, quinone derivatives are excellent redox-active ligands, widely used for various applications such as electrode materials, flow batteries, pseudocapacitors, etc. Benzoquinones undergo a one-electron reversible reduction to form a semiquinone radical species that can be further reduced to form hydroquinone. Remarkably, the quinoid ring can be functionalized with various functional groups making these systems excellent linkers to construct supramolecular frameworks as well as challenging platforms to tune the redox potential and therefore the stability of radical anions and electrochemical performances of the obtained materials. The recent advances on benzoquinone-based 2D CPs/MOFs and related nanostructures are reported, highlighting the extreme versatility of this class of redox-active linkers in tailoring the physical properties of the obtained materials. The current/future potential of these materials in electrochemical and technologically relevant applications will also be envisioned

Tuning the slow magnetic relaxation with the substituents in anilate bridged bis(dysprosium) complexes

International audienceDinuclear lanthanide complexes [((HB(pz)3)2Dy)2(μ-Th2An)] (1Dy) and [((HB(pz)3)2Dy)2(μ-ClCNAn)] (2Dy), based on the hydrotris(pyrazol-1-yl)borate (HBpz3-) scorpionate capping ligand and anilate (An2-) bridging linkers, namely homosubstituted dithiophene- and heterosubstituted chlorocyanoanilate, bearing electron-donating and withdrawing substituents at the 3,6-positions of the benzoquinone core, are reported. 1Dy shows an octacoordinated {N6O2} DyIII ion within a D4h distorted square antiprismatic coordination, an ideal geometry for Single-Molecule Magnet (SMM) behavior, given its oblate nature, whereas in 2Dy the octacoordinated DyIII ion adopts a D2d triangular dodecahedron geometry, while maintaining the same {N6O2} coordination sphere. Both complexes show field-induced single molecule magnet (SMM) behaviour, with tuning of the slow magnetic relaxation as a function of the nature of the substituents at the 3,6-positions of the anilate moiety. A comparison of the Arrhenius fitting parameters for 1Dy and 2Dy supports the hypothesis that square antiprismatic DyIII complexes, as 1Dy, exhibit higher energy barriers. This interpretation is supported by ab initio calculations that also shed light on the crucial role of intermolecular dipolar interactions

Terbium and Europium Chlorocyananilate-Based 2D Coordination Polymers

Two-dimensional layered coordination polymers based on the hetero-substituted 3-chloro-6-cyano-2,5-dihydroxybenzoquinone ligands, hereafter ClCNAn2− anilate, and LnIII ions (Tb and Eu) are reported. Compounds 1 and 2, formulated as Ln2(ClCNAn)3(DMSO)6 (LnIII = Tb, 1; Eu, 2), and their related intermediates 1′ and 2′, formulated as Ln2(ClCNAn)3(H2O)x·yH2O (x + y likely = 12, Ln = Tb, 1′; and Eu, 2′), were prepared by a conventional one-pot reaction (the latter) and recrystallized from DMSO solvent (the former). Polyhydrated intermediates 1′ and 2′ show very similar XRPD patterns, while, despite their common stoichiometry, 1 and 2 are not isostructural. Compound 1 consists of a 2D coordination framework of 3,6 topology, where [Tb(DMSO)3]III moieties are bridged by three bis-chelating ClCNAn2− ligands, forming distorted hexagons. Ultrathin nanosheets of 1 were obtained by exfoliation via the liquid-assisted sonication method and characterized by atomic force microscopy, confirming the 2D nature of 1. The crystal structure of 2, still showing the presence of 2D sheets with a “hexagonal” mesh and a common (3,6) connectivity, is based onto flat, non-corrugated slabs. Indeed, at a larger scale, the different “rectangular tiles” show clear roofing in 1, which is totally absent in 2. The magnetic behavior of 1 very likely indicates depopulation of the highest crystal-field levels, as expected for TbIII compounds

Dysprosium Chlorocyanoanilate-Based 2D-Layered Coordination Polymers

A series of two-dimensional (2D)-layered coordination polymers (CPs) based on the heterosubstituted anilate ligand ClCNAn2– derived from 3-chloro-6-cyano-2,5-dihydroxybenzoquinone and DyIII are reported. By changes in the synthetic methods (layering technique, solvothermal or conventional one-pot reactions) and conditions (solvent, concentration, etc.), different types of 2D extended networks could be prepared and structurally characterized. Compounds 1 and 1′, two polymorphs with the formula [Dy2(ClCNAn)3(DMSO)6]n·(H2O)x [x = 7 (1), 0 (1′)], were prepared by a conventional one-pot reaction and recrystallized at different concentrations. Compound 2, formulated as [Dy2(ClCNAn)3(DMF)6]n, was prepared by a layering technique, while compound 3, formulated as {(Me2NH2)2[Dy2(ClCNAn)4(H2O)2]·(DMF)2·(H2O)5}n, was obtained by a solvothermal method. Compounds 1 and 2 are neutral 2D CPs of the ClCNAn2– ligand and DyIII ions, while 3 presents 2D anionic layers of [Dy2(ClCNAn)4(H2O)2]2– alternating with cationic layers of Me2NH2+ ions. These compounds show very diverse networks, with compound 1 forming 2D (8,3) and (4,3) topology with eight- and four-membered rings with square cavities, 1′ and 2, respectively, a 2D (6,3) topology with six-membered rings (a rectangular cavity for 1′ and a regular hexagonal cavity for 2), and 3 a 2D (4,4) topology with distorted square cavities. In this respect, 1 and 1′ represent the first examples of polymorphism in the family of anilate-based CPs. Thermal analysis measurements (differential scanning calorimetry and thermogravimetry) show an exothermic polymorphic transformation from the kinetically stable 1′ phase to the thermodynamically stable phase 1. The magnetic behavior of 1–3 very likely indicates depopulation of the mJ levels, while the presence of weak antiferromagnetic coupling between the DyIII centers mediated by the anilate bridge cannot be excluded