Luminescent 3D Lanthanide–Cadmium Heterometal–Organic Frameworks with Chemical Stability and Selective Luminescent Sensing

Synthesis, structure and photoluminescence properties of heterometallic-based coordination polymers of trimesic acid

Reacting trimesic acid (H3TMA, C9H6O6) with CaCl2 and MCl2 at 110 °C under hydrothermal conditions gave the isostructural heterobimetallic coordination polymers (CPs) catena-poly[[tetraaquazinc(II)]-μ-5-carboxybenzene-1,3-dicarboxylato-[tetraaquacalcium(II)]-μ-5-carboxybenzene-1,3-dicarboxylato], [CaZn(HTMA)2(H2O)8]n, 1, and catena-poly[[tetraaquacobalt(II)]-μ-5-carboxybenzene-1,3-dicarboxylato-[tetraaquacalcium(II)]-μ-5-carboxybenzene-1,3-dicarboxylato], [CaCo(HTMA)2(H2O)8]n, 2. Compounds 1 and 2 crystallize in the monoclinic space group C2/c. The solid-state structures consist of eight-coordinate CaII ions and six-coordinate MII ions. These ions are connected by a doubly deprotonated HTMA2- ligand to create a one-dimensional (1D) zigzag chain. Poly[[decaaquabis(μ3-benzene-1,3,5-tricarboxylato)calcium(II)dizinc(II)] dihydrate], {[CaZn2(TMA)2(H2O)10]·2H2O}n, 3, was found incidentally as a minor by-product during the synthesis of 1 at a temperature of 140 °C. It forms crystals in the orthorhombic space group Ccce. The structure of 3 consists of a two-dimensional (2D) layer composed of [Zn(TMA)] chains that are interconnected by CaII ions. The presence of aromatic carboxylic acid ligands and water molecules, which can form numerous hydrogen bonds and π-π interactions, increases the stability of the three-dimensional (3D) supramolecular architecture of these CPs. Compounds 1 and 2 exhibit thermal stability up to 420 °C, as indicated by the thermogravimetric analysis (TGA) curves. The powder X-ray diffraction (PXRD) data reveal the formation of unidentified phases in methanol and dimethyl sulfoxide, while 1 exhibits chemical stability in a wide range of solvents. The luminescence properties of 1 dispersed in various low molecular weight organic solvents was also examined. The results demonstrate excellent selectivity, sensitivity and recyclability for detecting acetone molecules in aqueous media. Additionally, a possible sensing mechanism is also outlined.

Two AIE-Ligand-Based 2-D Luminescent Metal-Organic Frameworks as Fe3+ Sensors

The design and synthesis of high-performance sensors are very important but remain great challenges. In this work, a new aggregation-induced-emission (AIE) molecule 4,4'-(((9H-fluoren-9-ylidene)methylene)bis(4,1-phenylene))dipyridine (L) was successfully synthesized and first developed as a functional ligand to construct two isomorphic metal-organic frameworks (MOFs) [M(L)(OBBA)]n [M2+ = Cd2+ (1), Co2+ (2); H2OBBA = 4,4'-oxybisbenzoic acid]. They adopt [M2(COO)4] flywheel clusters, OBBA2- bridges, and terminal L ligands as building units to form isomorphic 2-D networks with Lewis base active cites (uncoordinated pyridyl N). Both 1 and 2 exhibit excellent water, pH, and thermal stabilities and extremely efficient Fe3+ sensing abilities in the water environment. The quenching constants and detection limits reach the best levels reported so far. The sensing mechanism of 1 and 2 toward Fe3+ is studied in depth, and the difference in their sensing performance is also explained.

One Luminescent Cadmium Iodide with Free Bifunctional Azole Sites as a Triple Sensor for Cu2+, Fe3+, and Cr2O72- Ions

The exploration of an excellent triple sensor for monitoring Cu²⁺, Fe³⁺, and Cr₂O₇^2- ions is of exceeding significance because of their serious effects on the human body. Herein, optically active 1H-3,5-bis(pyrazinyl)-1,2,4-triazole (Hbpt) with triazolyl and pyrazinyl groups was applied for the construction of a new type of organic hybrid cadmium iodide [Cd₆I₈(bpt)₄(H₂O)₄]·2H₂O (1) incorporating a hitherto-unknown [Cd₃I₄(H₂O)₂]²⁺ trimeric-cationic unit, which shows an orange light emission at 589 nm with a large Stokes shift of 246 nm. In virtue of the existence of free bifunctional azole sites as the receptors, 1 exhibits a highly selective and sensitive sensing property toward Cu²⁺, Fe³⁺, and Cr₂O₇^2- ions in aqueous solution with lower detection limits of 0.70∼4.46 ppm, which offers the sole example of cadmium iodide as an excellent triple sensor for detecting Cu²⁺, Fe³⁺, and Cr₂O₇^2- ions. Moreover, temperature-dependent luminescent determinations also reveal that 1 can be used as the potential luminescent molecular thermometer.

Luminescent Dimeric Oxalate-Bridged Eu3+/Tb3+-Implanted Arsenotungstates: Tunable Emission, Energy Transfer, and Detection of Ba2+ Ion in Aqueous Solution

Two cases of lanthanide (Ln)-implanted arsenotungstates, K₁₇Na₂H₅[{(As₂W₁₉O₆₇(H₂O))Ln(H₂O)₂}₂(C₂O₄)]·87H₂O (Ln = Eu (1), Ln = Tb (2)) and their codoped derivatives Eu_xTb_1-x-POM (x = 0.01 (3), x = 0.04 (4), x = 0.1 (5), x = 0.2 (6)) were prepared and further characterized by powder X-ray diffraction, infrared spectra, and thermogravimetric analyses. An X-ray structural analysis of 1 and 2 indicates that they both present a dimeric oxalate-bridged Ln³⁺-implanted lanthanide arsenotungstate polyanion structure. Under the O → W LMCT excitation at 265 nm of arsenotungstate polyanions, the emissions of Ln³⁺ ions in 1 and 2 are sensitized and the lifetimes are prolonged. Codoped compounds 3-6 demonstrate a color-tunable emission from green to red by adjusting the Eu³⁺/Tb³⁺ ratio. Emission spectra and time-resolved emission spectroscopic studies were performed for 3 to further authenticate the energy transfer processes from excited arsenotungstates to the Eu³⁺ and Tb³⁺ metal ions and also between the Eu³⁺ and Tb³⁺ centers. More interestingly, 1 is an effective fluorescent probe for the recognition and detection of Ba²⁺ ions in aqueous solution. The optical properties of the Ln-implanted arsenotungstate compounds not only expressly reveal distinctive energy transfer processes in those compounds but also broaden the application of POM-based materials in the fluorescence sensing field.

Deciphering supramolecular isomerization in coordination polymers: connected molecular squares vs. fused hexagons

The self-assembly of Mn(ii), bis(tridentate) ligands and bent dicarboxylate linkers under ambient conditions has been exploited to generate a series of 1D coordination polymers in good yields. For a set of seven compounds, structural isomerization of these architectures is demonstrated through the variation of length and nature of the spacer between the tridentate capping sites of the bis(tridentate) ligands, such as tpbn (N,N',N'',N'''-tetrakis-(2-pyridylmethyl)-1,4-diaminobutane), tphxn (N,N',N'',N'''-tetrakis-(2-pyridylmethyl)-1,6-diaminohexane), and tpxn (N,N',N'',N'''-tetrakis-(2-pyridylmethyl)-xylylamine) or by varying the bent dicarboxylate linker 4,4'-(dimethylsilanediyl)bis-benzoic acid (H2L1) or 4,4'-oxybis-benzoic acid (H2L2). These compounds have been structurally characterized by single-crystal and powder X-ray diffraction, FTIR, and thermogravimetric and elemental analyses. This study reveals that the supramolecular structural variation can be precisely controlled either by a judicious selection of reaction conditions or linker/ligand combinations. For example, the self-assembly of Mn(ii), tpbn and H2L1 in DMF/EtOH/water affords a mixture of products (1 and 1a) while changing the solvent combination to EtOH/water results in the generation of a single isomer (1a) in a highly selective manner. On the other hand, for the Mn(ii)-tphxn system, different structural isomers have been isolated by varying the dicarboxylates, H2L1 and H2L2 (2vs.5). Similarly, for the Mn(ii)-H2L2 system, a variation in the spacer chain length of bis(tridentate) ligands, tpbn and tphxn resulted in the formation of different structural isomers (4vs.5).

Efficient luminescence sensing in two lanthanide metal–organic frameworks with rich uncoordinated Lewis basic sites

Two novel Ln-MOFs containing uncoordinated Lewis basic sites for sensitive detection of Fe3+ ions and nitrobenzene through fluorescence quenching.

Luminescence tuning and sensing properties of stable 2D lanthanide metal–organic frameworks built with symmetrical flexible tricarboxylic acid ligands containing ether oxygen bonds

New Ln-MOFs were yielded by a flexible ligand and lanthanide ions. The colors can be regulated by adjusting the molar ratios of Eu³⁺/Tb³⁺, showing promising applications in NB sensing, tunable photoluminescence and pH sensing.

Structural Complexity and Magnetic Orderings in a Large Family of 3d-4f Heterobimetallic Sulfates

The synthesis of a large family of heterobimetallic lanthanide copper sulfates was realized via stoichiometric hydrothermal reactions among Ln₂O₃, CuO, and H₂SO₄, giving rise to four distinct phases, namely Ln₂Cu(SO₄)₂(OH)₄ (Ln = Sm-Ho) (LnCuSO₄-1), Ln₄Cu(SO₄)₂(OH)₁₀ (Ln = Tm-Lu) (LnCuSO₄-2), LnCu(SO₄)(OH)₃ (Ln = Nd-Gd, except Pm) (LnCuSO₄-3), and LnCu(SO₄)(OH)₃ (Ln = Dy-Lu) (LnCuSO₄-4), with completely different topologies. The passage from LnCuSO₄-1 and LnCuSO₄-3 to LnCuSO₄-2 and LnCuSO₄-4 across the 4f series, respectively, can be ascribed to the effect of lanthanide contraction, which progressively induces shrinking of the Ln-O distance, reduction in the Ln coordination number, and eventually structural transitions. The incorporation of identical 3d-4f metal ions into different spin-lattices, in conjunction with substitution of diverse Ln³⁺ cations within the same spin-lattice, gives rise to tunable magnetic properties varying from ferromagnetic ordering in GdCuSO₄-3 and HoCuSO₄-4 to antiferromagnetic ordering in YbCuSO₄-4, and to paramagnetic correlations found in GdCuSO₄-1 and YbCuSO₄-2.

A Rare 3D Porous Inorganic-Organic Hybrid Polyoxometalate Framework Based on a Cubic Polyoxoniobate-Cupric-Complex Cage with a High Water Vapor Adsorption Capacity

A rare 3D porous inorganic-organic polyoxoniobate framework based on the cubic polyoxoniobate-cupric-complex cage {[Cu(en)₂]@{[Cu₂(en)₂(trz)₂]₆(Nb₆₈O₁₈₈)}} (1a), has been successfully synthesized by a hydrothermal method. The cubic cages 1a are connected with 4-(tetrazol-5-yl)pyridine to form a 1D pillar-like chain structure, and every 1D pillar-like chain is further linked with four adjacent pillar-like chains by the [Cu(en)₂]²⁺ complex to form a 3D porous inorganic-organic polyoxoniobate framework with 4-connected CdSO₄-type topology. To our knowledge, it is the first time that three different types of organic ligands are simultaneously introduced into one polyoxoniobate. This material also exhibits a high vapor adsorption capacity and good ionic conductivity properties.

A Trinuclear Zinc Coordination Cluster Exhibiting Fluorescence, Colorimetric Sensitivity, and Recycling of Silver Ion and Detection of Cupric Ion

The detection and reusage of transition-metal ions play a crucial role in human health and environmental protection. Recently, various analytical methods and substances have been successfully applied to probe or sense silver ions; however, rare representative examples have been presented regarding the simultaneous detection of silver and silver recycling with the elemental silver powder form. Herein, an unparalleled sensing mechanism for silver ions and recycling silver in its elemental form is exemplified by a fluorescent trinuclear zinc coordination cluster possessing the dual function of colorimetric sensing of silver and responding cupric ions. A Schiff-base-based trinuclear zinc coordination cluster, 1, with formula Zn₃(L₁)₂(CH₃COO)₂(H₂O)₂, has been successfully synthesized by the initial exploration of multidentate ligand H₂L₁-((E)-2,4-di-tert-butyl-6-((2-hydroxy-3-methoxybenzy-lidene)amino)phenol) with various metal ions under self-assembly reactions. Complex 1 is highly fluorescent in solution and as a solid, in addition to acting as a fluorescence sensor toward Ag^I in ethanol media. Compound 1 displays distinctive sensing of Ag^I through the fluorescence quenching effect at 576 nm and signal augment at 446 nm over 11 kinds of cations in the absence of interference. The proposed sensing mechanism is attributed to the ligands in 1 which interact with Ag^I; the ligands undergo oxidation cyclization reaction, leading to the formation of 2 with the formula Zn₃(L₂)₄(CH₃COO)₂·2CH₃CH₂OH·H₂O, and Ag^I reduction to elemental Ag powder. Compound 1 presents specific selectivity and sensitivity for Ag^I in ethanolic solution with a detection limit of 0.1722 μM. The orange color of 1 changes to colorless during the mixing of a small amount of Ag^I, revealing its potential practical application in naked-eye detection of Ag^I. Furthermore, 2 exhibits obvious fluorescence emission at 448 nm (λ_ex = 380 nm) and selectively responds to Cu^II over 11 kinds of metal ions with the fluorescence "turn-off" owing to the formation of 3 in ethanolic solution; it also has a detection limit of 0.0226 μM.

Construction of 3D lanthanide based MOFs with pores decorated with basic imidazole groups for selective capture and chemical fixation of CO2

Rational construction of three new 3D lanthanide-based MOFs exhibiting selective CO₂ capture and conversion to value-added cyclic carbonates under mild conditions is reported.

A zinc2+-dpbt framework: luminescence sensing of Cu2+, Ag+, MnO4− and Cr(vi) (Cr2O72− and CrO42−) ions

We report here a zinc²⁺-dpbt MOF with luminescence sensing properties for the detection of Cu²⁺, Ag⁺, MnO₄⁻ and Cr(vi) (Cr₂O₇²⁻, CrO₄²⁻) ions.

Novel 3D anionic heterometallic frameworks based on trinuclear CoII and trinuclear LnIII motifs displaying slow magnetic relaxation and selective adsorption of methylene blue

3D CoLn heterometallic frameworks have been synthesized. CoDy and CoHo show slow magnetic relaxation behavior. CoTb exhibits excellent adsorption capacity for methylene blue.

CdII -Organic Frameworks Fabricated with a N-Rich Ligand and Flexible Dicarboxylates: Structural Diversity and Multi-Responsive Luminescent Sensing for Toxic Anions and Ethylenediamine

Three metal-organic frameworks {[Cd(L)(glu)]⋅3 H₂ O}_∞ (1), {[Cd₂ (L)₂ (adi)₂ ]⋅5 H₂ O}_∞ (2) and {[Cd(L)(sub)]⋅3 H₂ O⋅DMA }_∞ (3) (L=pyridine-3,5-bis(5-azabenzimidazole), H₂ glu=glutaric acid, H₂ adi=adipic acid and H₂ sub=suberic acid) were obtained under solvothermal conditions. Complex 1 shows a 2D (4,4) network constructing of Cd₂ -glu and Cd-L chains. Complex 2 presents a 2-fold interpenetrating 3D framework with pcu topology. Complex 3 is a 3D framework with cds topology. Three complexes with versatile structures were obtained by changing aliphatic dicarboxylate ligands with different lengths based on a N-rich ligand. Moreover, the fluorescence measurements indicate that complex 1 is a good multifunctional chemosensor for the detection of Cr₂ O₇ ^2- and MnO₄ ^- anions by fluorescence quenching effect, and ethylenediamine by fluorescence enhancement effect, with detection limits of 1.196 ppm, 0.551 ppm and 64.572 ppm, respectively. Both complexes 2 and 3 can selectively sense Cr₂ O₇ ^2- anion with detection limits of 1.126 ppm for 2 and 0.831 ppm for 3 by a fluorescence quenching effect.

A novel (3,6)-connected CdII coordination polymer based on an ether-linked tricarboxylate ligand: synthesis, topology and luminescence sensing properties in aqueous solution

A novel three-dimensional coordination polymer, namely, poly[[diaquabis(μ-4,4'-bipyridine)bis{μ₃-5-[(2-carboxyphenoxy)methyl]isophthalato}tricadmium(III)] dimethylformamide monosolvate 2.5-hydrate], {[Cd₃(C₁₆H₉O₇)₂(C₁₀H₈N₂)₂(H₂O)₂]·2C₃H₇NO·5H₂O}_n, was obtained by the reaction of ether-linked 5-[(2-carboxyphenoxy)methyl]isophthalic acid (H₃L) with Cd^II salts in the presence of 4,4'-bipyridine (bpy) under solvothermal conditions. In this complex, the Cd^II centres are connected by the carboxylate ligands to form two-dimensional wave-like layers, which are pillared by bpy ligands and extended into a rare three-dimensional (3,6)-connected sqc27 framework. The complex demonstrated good water stability and strong luminescence emissions. It not only possesses excellent luminescence sensing activities toward Fe³⁺ and Cr₂O₇^2- in aqueous solution, but can also distinguish between Cr₂O₇^2- and CrO₄^2- by luminescence. Furthermore, it could be simply and quickly regenerated at least five times. A study of the sensing mechanism indicated that luminescence quenching may be related to the energy competition between the complex and sensing analytes.

Tunable Light Emission and Multiresponsive Luminescent Sensitivities in Aqueous Solutions of Two Series of Lanthanide Metal-Organic Frameworks Based on Structurally Related Ligands

Two series of lanthanide metal-organic frameworks (Ln-MOFs) from two structurally related flexible carboxylate-based ligands were solvothermally synthesized. H₃L2 with additional -CH₂- group provides more flexibility and different coordination modes and conformations compared with H₃L1. As a result, 2-Ln MOFs are modulated from two-dimensional kgd of 1-Ln to three-dimensional rtl topological frameworks and further achieve enhanced chemical stability. The Eu- and Tb-MOFs exhibit strong fluorescent emission at the solid state because of the antenna effect of the ligands. Interestingly, the emissions can be tuned by simply doping Eu³⁺ and Tb³⁺ of different concentrations within the Eu _xTb_{1- x} MOFs. Notably, 2-Ln MOFs realize nearly white light emission by means of a trichromatic approach (red of Eu(III), green of Tb(III), and blue of the H₃L2 ligand). Furthermore, 2-Ln MOFs also exhibit water stability and demonstrate high selective and sensitive sensing activities toward Fe(III) and Cr(VI) in aqueous solutions. The results further highlight the importance of the ligand flexibility on tuning MOF structures with improved structural stability and ion-sensing properties.

Three novel topologically different metal–organic frameworks built from 3-nitro-4-(pyridin-4-yl)benzoic acid

The design and synthesis of metal–organic frameworks (MOFs) have attracted much interest due to the intriguing diversity of their architectures and topologies. However, building MOFs with different topological structures from the same ligand is still a challenge. Using 3-nitro-4-(pyridin-4-yl)benzoic acid (HL) as a new ligand, three novel MOFs, namely poly[[(N,N-dimethylformamide-κO)bis[μ2-3-nitro-4-(pyridin-4-yl)benzoato-κ3 O,O′:N]cadmium(II)] N,N-dimethylformamide monosolvate methanol monosolvate], {[Cd(C12H7N2O4)2(C3H7NO)]·C3H7NO·CH3OH} n , (1), poly[[(μ2-acetato-κ2 O:O′)[μ3-3-nitro-4-(pyridin-4-yl)benzoato-κ3 O:O′:N]bis[μ3-3-nitro-4-(pyridin-4-yl)benzoato-κ4 O,O′:O′:N]dicadmium(II)] N,N-dimethylacetamide disolvate monohydrate], {[Cd2(C12H7N2O4)3(CH3CO2)]·2C4H9NO·H2O} n , (2), and catena-poly[[[diaquanickel(II)]-bis[μ2-3-nitro-4-(pyridin-4-yl)benzoato-κ2 O:N]] N,N-dimethylacetamide disolvate], {[Ni(C12H7N2O4)2(H2O)2]·2C4H9NO} n , (3), have been prepared. Single-crystal structure analysis shows that the CdII atom in MOF (1) has a distorted pentagonal bipyramidal [CdN2O5] coordination geometry. The [CdN2O5] units as 4-connected nodes are interconnected by L − ligands to form a fourfold interpenetrating three-dimensional (3D) framework with a dia topology. In MOF (2), there are two crystallographically different CdII ions showing a distorted pentagonal bipyramidal [CdNO6] and a distorted octahedral [CdN2O4] coordination geometry, respectively. Two CdII ions are connected by three carboxylate groups to form a binuclear [Cd2(COO)3] cluster. Each binuclear cluster as a 6-connected node is further linked by acetate groups and L − ligands to produce a non-interpenetrating 3D framework with a pcu topology. MOF (3) contains two crystallographically distinct NiII ions on special positions. Each NiII ion adopts an elongated octahedral [NiN2O4] geometry. Each NiII ion as a 4-connected node is linked by L − ligands to generate a two-dimensional network with an sql topology, which is further stabilized by two types of intermolecular OW—HW...O hydrogen bonds to form a 3D supramolecular framework. MOFs (1)–(3) were also characterized by powder X-ray diffraction, IR spectroscopy and thermogravimetic analysis. Furthermore, the solid-state photoluminescence of HL and MOFs (1) and (2) have been investigated. The photoluminescence of MOFs (1) and (2) are enhanced and red-shifted with respect to free HL. The gas adsorption investigation of MOF (2) indicates a good separation selectivity (71) of CO2/N2 at 273 K (i.e. the amount of CO2 adsorption is 71 times higher than N2 at the same pressure).

Construction of a 3D porous Co(ii) metal–organic framework (MOF) with Lewis acidic metal sites exhibiting selective CO2 capture and conversion under mild conditions

The application of a 3D Co(ii)-MOF as a recyclable heterogeneous catalyst for conversion of CO₂ to cyclic carbonates is reported.

Construction of bifunctional 2-fold interpenetrated Zn(ii) MOFs exhibiting selective CO2 adsorption and aqueous-phase sensing of 2,4,6-trinitrophenol

Design of Zn(ii) MOFs, [{Zn(BINDI)_0.5(bpa)_0.5(H₂O)}·4H₂O]_n (MOF1) and [{Zn(BINDI)_0.5(bpe)}·3H₂O]_n (MOF2) for selective CO₂ storage and aqueous-phase detection of TNP is demonstrated.

Two luminescent transition-metal–organic frameworks with a predesigned ligand as highly sensitive and selective iron(iii) sensors

The water-stable imidazole dicarboxylated-based MOFs are excellent luminescent probes for the Fe³⁺ ion in an aqueous solution.