Towards multifunctional lanthanide-based metal-organic frameworks

Holmium-Containing Metal-Organic Frameworks as Modifiers for PEBA-Based Membranes

Recently, there has been an active search for new modifiers to create hybrid polymeric materials for various applications, in particular, membrane technology. One of the topical modifiers is metal-organic frameworks (MOFs), which can significantly alter the characteristics of obtained mixed matrix membranes (MMMs). In this work, new holmium-based MOFs (Ho-MOFs) were synthesized for polyether block amide (PEBA) modification to develop novel MMMs with improved properties. The study of Ho-MOFs, polymers and membranes was carried out by methods of X-ray phase analysis, scanning electron and atomic force microscopies, Fourier transform infrared spectroscopy, low-temperature nitrogen adsorption, dynamic and kinematic viscosity, static and dynamic light scattering, gel permeation chromatography, thermogravimetric analysis and contact angle measurements. Synthesized Ho-MOFs had different X-ray structures, particle forms and sizes depending on the ligand used. To study the effect of Ho-MOF modifier on membrane transport properties, PEBA/Ho-MOFs membrane retention capacity was evaluated in vacuum fourth-stage filtration for dye removal (Congo Red, Fuchsin, Glycine thymol blue, Methylene blue, Eriochrome Black T). Modified membranes demonstrated improved flux and rejection coefficients for dyes containing amino groups: Congo Red, Fuchsin (PEBA/Ho-1,3,5-H3btc membrane possessed optimal properties: 81% and 68% rejection coefficients for Congo Red and Fuchsin filtration, respectively, and 0.7 L/(m2s) flux).

Biomineralized Nano-Assemblies of Poly(Ethylene Glycol) Derivative with Lanthanide Ions as Ratiometric Fluorescence Sensors for Detection of Water and Fe3+ Ions

An effective strategy was developed to fabricate novel lanthanide ions–pyromellitic acid–methoxy poly(ethylene glycol) (Ln-PMA-MPEG) nano-assemblies. The amphiphilic partially esterified derivative (PMA-MPEG) of pyromellitic acid with methoxy poly(ethylene glycol) was designed and synthesized via the coupling reaction. Ln-PMA-MPEG nano-assemblies were rapidly fabricated using PMA-MPEG as a polymer ligand with Eu³⁺ ions or mixed Eu³⁺/Tb³⁺ ions through biomimetic mineralization in neutral aqueous systems. The size of the as-prepared materials could be designed in the range 80–200 nm with a uniform distribution. The materials were readily dispersed in various solvents and displayed visible color variations and different photoluminescent properties for solvent recognition. The mixed Eu/Tb-PMA-MPEG nanomaterials were investigated as ratiometric sensors for the detection of trace water in DMF and Fe³⁺ ions in aqueous solutions. The sensor materials can quantitatively detect trace water in DMF from 0% to 10% (v/v). The resultant materials also display a strong correlation between the double luminescence intensity ratios (I_Tb/I_Eu) and Fe³⁺ concentration, with a good linear detection concentration in the range of 0–0.24 mM and a limit of detection of 0.46 μM, and other metal ions did not interfere with the sensing mechanism for Fe³⁺ ions. The novel nano-assemblies have potential applications as ratiometric fluorescent nanosensors in the chemical industry as well as in biomedical fields.

2D Porphyrinic Metal-Organic Frameworks Featuring Rod-Shaped Secondary Building Units

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called 'rod MOFs', which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2L-CuII, where H2 refers to the ligand's carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D MnII- or CoII-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

Highlights of the development and application of luminescent lanthanide based coordination polymers, MOFs and functional nanomaterials

The development of lanthanide based coordination polymer and metal-organic framework (CPs and MOFs) nanomaterials as novel functional (e.g. luminescent and magnetic) materials has attracted significant attention in recent times. This is in part due to the wide, but yet unique coordination requirements that the f-metal ions possess, as well as their attractive physical properties, which are often transferred to the bulk material. Hence, there is no surprise, that the design, synthesis and characterisation of lanthanide based CP/MOF materials (featuring either 'pure' lanthanides, or a mixture of both f- and d-metal ions) for applications in gas and small molecule absorption, storage, conversion/catalysis, chemical sensing, bio-imaging, drug delivery, etc. has been a prominent feature in the scientific literature. In this review, we give a selected overview of some of the recent developments in the area of Ln CP/MOF based nanomaterials for sensing, optical materials and bio-medicine research, as well as making reference to some more established examples, with the view of introducing, particularly to new researchers to the field, the powerful and attractive features of lanthanide based materials.

Rare-earth metal–organic frameworks: from structure to applications

In the past 30 years, rare-earth metal–organic frameworks (MOFs) have been gaining attention owing to their diverse chemical structures, and tunable properties.

Color-tunable X-ray scintillation based on a series of isotypic lanthanide–organic frameworks

X-ray excited luminescence from lanthanide–organic frameworks are reported for the first time, enabling color-tunable visualization of X-ray radiation by changing the lanthanide moiety in the crystals.

Ln-MOFs using a compartmental ligand with a unique combination of hard–soft terminals and their magnetic, gas adsorption and luminescence properties

The present work is part of our ongoing quest for developing multifunctional lanthanide–organic hybrid materials using non-conventional soft N-rich pyridyl–pyrazole-based ligands.

Near-infrared-emissive metal–organic frameworks

We describe the recent progress in near-infrared-emissive metal–organic frameworks, and especially highlight their appealing applications in bio-imaging, sensing and barcoding.

A highly augmented, (12,3)-connected Zr-MOF containing hydrated coordination sites for the catalytic transformation of gaseous CO2 to cyclic carbonates

A Zr-MOF, which contains partially hydrated, 12-connected {Zr₆} nodes and extended carboxylate ligands was synthesized, characterised and utilised in CO₂ cycloaddition catalysis.

Tetraarylpyrrolo[3,2-b]pyrroles as versatile and responsive fluorescent linkers in metal-organic frameworks

The first examples of crystalline coordination polymers containing the tetraarylpyrrolo[3,2-b]pyrrole (TPP) fluorophore are presented. We have prepared three new TPP ligands L1, H2L2 and H2L3, containing nitrile, carboxylate and mixed imidazole-carboxylate donor functionality, respectively. The ligands themselves each show significant fluorescence in the solution phase, with the nitrile species exhibiting solvatofluorochromism and the two carboxylate-containing compounds exhibiting concentration-dependent emission colour suggesting aggregation processes in solution. Three 3-dimensional polymeric structures are then presented. The compound poly-[AgL12]SbF6·3THF·2H2O 1 is an eightfold-interpenetrated diamondoid material, while poly-[Zn4O(L2)3]·20DMA·10H2O 2 is a porous Metal-Organic Framework with pcu topology, and both 1 and 2 show notable luminescence in the solid state. Complex 2 readily undergoes guest exchange accompanied by a reversible switching in emission colour with no change in chemical structure. While complex poly-[CdL3]·2.5DMA·3.5H2O 3 is non-emissive, it displays a twofold interpenetrated pts topology with hexagonal symmetry and an extremely long hexagonal pitch of 100.3 Å, and shows an impressive 22 wt% CO2 uptake capacity at 278 K and 1 bar.

Exploring the reversible host–guest chemistry of a crystalline octanuclear Ag(i) metallosupramolecular macrocycle formed from a simple pyrazinylpyridine ligand

High nuclearity Ag(i) assemblies are prepared from simple polytopic ligands, including an octanuclear metallomacrocycle which exhibits reversible and selective guest exchange.

A supramolecular Tröger's base derived coordination zinc polymer for fluorescent sensing of phenolic-nitroaromatic explosives in water

A V-Shaped 4-amino-1,8-napthalimide derived tetracarboxylic acid linker (L; bis-[N-(1,3-benzenedicarboxylic acid)]-9,18-methano-1,8-naphthalimide-[b,f][1,5]diazocine) comprising the Tröger's base (TB) structural motif was rationally designed and synthesised to access a nitrogen-rich fluorescent supramolecular coordination polymer. By adopting the straight forward precipitation method, a new luminescent nanoscale Zn(ii) coordination polymer (TB-Zn-CP) was synthesized in quantitative yield using Zn(OAc)2·2H2O and tetraacid linker L (1 : 0.5) in DMF at room temperature. The phase-purity of as-synthesised TB-Zn-CP was confirmed by X-ray powder diffraction analysis, infra-red spectroscopy, and elemental analysis. Thermogravimetric analysis suggests that TB-Zn-CP is thermally stable up to 330 °C and the morphological features of TB-Zn-CP was analysed by SEM and AFM techniques. The N2 adsorption isotherm of thermally activated TB-Zn-CP at 77 K revealed a type-II reversible adsorption isotherm and the calculated Brunauer-Emmett-Teller (BET) surface area was found to be 72 m2 g-1. Furthermore, TB-Zn-CP displayed an excellent CO2 uptake capacity of 76 mg g-1 at 273 K and good adsorption selectivity for CO2 over N2 and H2. The aqueous suspension of as-synthesized TB-Zn-CP showed strong green fluorescence (λmax = 520 nm) characteristics due to the internal-charge transfer (ICT) transition and was used as a fluorescent sensor for the discriminative sensing of nitroaromatic explosives. The aqueous suspension of TB-Zn-CP showed the largest quenching responses with high selectivity for phenolic-nitroaromatics (4-NP, 2,4-DNP and PA) even in the concurrent presence of other potentially competing nitroaromatic analytes. The fluorescence titration studies also provide evidence that TB-Zn-CP detects picric acid as low as the parts per billion (26.3 ppb) range. Furthermore, the observed fluorescence quenching responses of TB-Zn-CP towards picric acid were highly reversible. The highly selective fluorescence quenching responses including the reversible detection efficiency make the nanoscale coordination polymer TB-Zn-CP a potential material for the discriminative fluorescent sensing of nitroaromatic explosives.

Two 2-D multifunctional cobalt(ii) compounds: field-induced single-ion magnetism and catalytic oxidation of benzylic C–H bonds

Two 2-D multifunctional cobalt(ii) compounds were reported with both field-induced single-ion magnetism and catalytic oxidation of benzylic C–H bonds.

Supramolecular coordination polymers using a close to ‘V-shaped’ fluorescent 4-amino-1,8-naphthalimide Tröger's base scaffold

A novel 4-amino-1,8-naphthalimide derived Tröger's base ligand L has been synthesised and subsequently used in the formation of two new supramolecular coordination polymers TB-Co-CP and TB-Cd-CP.

Lanthanide-directed synthesis of luminescent self-assembly supramolecular structures and mechanically bonded systems from acyclic coordinating organic ligands

Herein some examples of the use of lanthanide ions (f-metal ions) to direct the synthesis of luminescent self-assembly systems (architectures) will be discussed. This area of lanthanide supramolecular chemistry is fast growing, thanks to the unique physical (magnetic and luminescent) and coordination properties of the lanthanides, which are often transferred to the resulting supermolecule. The emphasis herein will be on systems that are luminescent, and hence, generated by using either visibly emitting ions (such as Eu(III), Tb(III) and Sm(III)) or near infrared emitting ions (like Nd(III), Yb(III) and Er(III)), formed through the use of templating chemistry, by employing structurally simple ligands, possessing oxygen and nitrogen coordinating moieties. As the lanthanides have high coordination requirements, their use often allows for the formation of coordination compounds and supramolecular systems such as bundles, grids, helicates and interlocked molecules that are not synthetically accessible through the use of other commonly used templating ions such as transition metal ions. Hence, the use of the rare-earth metal ions can lead to the formation of unique and stable species in both solution and in the solid state, as well as functional and responsive structures.

Luminescent lanthanide–organic polyrotaxane framework as a turn-off sensor for nitrobenzene and Fe3+

Three isomorphous lanthanide–organic polyrotaxane frameworks [Ln(mtpc)_1.5(DMA)(H₂O)]·2H₂O [Ln = Tb (1); Dy (2); Er (3)] were assembled and characterized. 1 acts as a fluorescent probe for nitrobenzene and iron(iii) ions, and the detection limit of nitrobenzene is 15–150 ppm.