A novel near monochromatic red emissive europium(III) metal-organic framework based on 1,2,4,5-benzenetetracarboxylate: From synthesis to photoluminescence studies

Rational Design of Dual-Emission Lanthanide Metal-Organic Framework for Visual Alkaline Phosphatase Activity Assay

The alkaline phosphatase (ALP) activity assay is very significant for disease diagnosis and biomedical research. Lanthanide metal-organic framework (Ln-MOF) based fluorescence sensors have great application potential in ALP activity assays. However, it is critical but challenging to investigate the emission law of Ln-MOFs for revealing rational design principles and selecting an appropriate MOF. Here, we describe a reasonable design strategy for dual-emission Ln-MOFs based on theoretical calculations. This strategy combines Reinhoudt empirical rule, intramolecular charge transfer theory, and aggregation/coordination-induced emission theory; reveals the luminescence law of Ln-MOFs; and provides theoretical guidance for the rational design of dual-emission Ln-MOFs. On the basis of this strategy, we create a dual-emission Tb-MOF fluorescent probe used for ALP activity assay and investigate the detection mechanism. The probe shows ultrasensitive (limit of detection 0.002 mU mL^-1) and selective response to ALP, and it suits for point-of-care visual detection coupled with a self-designed portable enzyme activity assay kit and smartphone-assisted visual device. The kit-based visual assay method can accurately quantify the activity of ALP in real serum samples (recovery >93%, and relative error is less than 6.8% compared with the results of fluorescence spectrometer-based method) and consumes only 25 μL of serum. In addition, a logical decoder based on the "dual-key unlocking strategy" is designed, providing a feasible solution for the development of intelligent ALP activity detection equipment. As far as we know, this is the first report of a theoretical calculation-guided versatile design strategy for dual-emission Ln-MOFs and a portable enzyme activity assay kit for visual detection.

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 [Eu_1-xTb_x(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.

Phase selectivity and tunable photophysical nature of rare earth metal-organic frameworks of EuxY1-x-PTC (H3PTC = 2,4,6-pyridine tricarboxylic acid; x = 0-1)

Two rare earth metal-organic frameworks (MOFs), [Y2(PTC)2(H2O)2]·3H2O (Y-PTC) and [Eu2(PTC)2(H2O)5]·H2O (Eu-PTC) together with the solid solutions [Eu2xY2(1-x)(PTC)2(H2O)5]·H2O (EuxY1-x-PTC, x = 0.013-0.82), were synthesized hydrothermally, and characterized by microanalysis, IR spectroscopy, TG, powder, and single crystal X-ray diffraction techniques. Eu-PTC and Y-PTC showed different crystal structures; however, all solid solutions were isomorphic to Eu-PTC even at x = 0.013, leading to the IR spectra and TG plots of the solid solutions to be similar to those of Eu-PTC but distinct from those of Y-PTC. DFT calculations for the crystal lattice energy demonstrated that the procedure for the crystallizing nucleation of Eu-PTC occurred prior to that of Y-PTC in the reaction solution, leading to the all solid solutions being isomorphic to Eu-PTC. The solid emission spectra at ambient condition showed that Y-PTC emitted ligand-based phosphorescence at 433 nm with a quantum yield (QY) of 27.02%, while Eu-PTC and EuxY1-x-PTC (x = 0.013-0.82) emitted the characteristic luminescence of Eu3+ ions, and most solid solutions showed higher QYs than Eu-PTC; in particular, the QY of Eu0.195Y0.805-PTC was up to 29.48%, i.e., increased by 10% regarding Eu-PTC (19.86%). Interestingly, solid solutions with x = 0.013-0.395 showed excitation-wavelength-dependent luminescence, and such type of luminescence MOFs have promising applications including the areas of precise temperature, gas sensing and information encryption or anti-counterfeiting materials.

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 (dcpa^2-) 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.

Synthesis, crystal structures and, magnetic and photoluminescence properties of lanthanide-based metal-organic frameworks constructed with 2,5-dihydroxybenzene-1,4-dicarboxylic acid

Four new lanthanide(iii) coordination polymers (1–4) have been synthesized by using 2,5-dihydroxybenzene-1,4-dicarboxylate (Dhbdc, C₈H₆O₆²⁻) as a ligand and characterized by elemental analysis, infrared spectroscopy, TGA-DSC (Thermogravimetric Analysis-Differential Scanning Calorimetry) and single crystal X-ray diffraction studies. The isolated complexes include; [Ce(Dhbdc)(H₂O)₃Cl]_n (1) and [Ln(Dhbdc)_1.5(H₂O)₂]_n·3n(C₂H₆O)·nH₂O [Ln = Eu (2), Gd (3) and Tb (4)]. The structural analysis reveals that compound 1 is a two-dimensional polymer in which the cerium atoms are coordinated by six oxygen and two bridging chloride ions adopting a dodecahedral geometry. The compounds 2–4 are isomorphous and their extended structures consist of three-dimensional supramolecular frameworks encapsulating [001] channels occupied by the guest water and ethanol molecules. The metal atoms in 2–4 exhibit a square antiprism geometry. All these compounds have O–H⋯O hydrogen bonds originating from the coordinated water and solvent molecules that help to consolidate the structures. The compounds 1, 3 and 4 were investigated for magnetic properties and they exhibited weak antiferromagnetic coupling interactions between the lanthanides as per the Curie Weiss law. Anisotropic nature of 3 and 4 has been depicted as per magnetization versus field plots. Complex 4 behaves as a soft magnetic material as compared to 3. The luminescence studies indicate that compounds 2, 3 and 4 show ligand-centred fluorescence, which is significantly enhanced in the case of 4.

Four new lanthanide(iii) coordination polymers (1–4) have been synthesized by using 2,5-dihydroxybenzene-1,4-dicarboxylate as a ligand and investigated for magnetic and photo-luminescence properties.