Subtle adjustments for constructing multi-nuclear luminescent lanthanide organic polyhedra with triazole-based chelates

Controlled self-assembly of predetermined multi-nuclear lanthanide organic polyhedra (LOPs) still presents a challenge, primarily due to the unpredictable coordination numbers and labile coordination geometries of lanthanide ions. In this study, through introducing triazole-based chelates to increase the chelating angle of C2-symmetric linear ligands and stabilize the coordination geometry of Eu(III) centers, M4L6-type (M = EuIII, L = ligand) tetrahedra were efficiently synthesized, especially a biphenyl-bridged ligand which is well known to form M2L3-type helicates. A series of LOPs were formed and characterized by high-resolution electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS) and X-ray crystallography. Moreover, the europium complexes exhibit bright emission (luminescence quantum yield up to 42.4%) and circularly polarized luminescence properties (|glum| up to 4.5 × 10-2). This study provides a feasible strategy for constructing multi-nuclear luminescent LOPs towards potential applications.

Self-Assembly of Triple-Stranded Lanthanide Molecular Quasi-Lantern Containing 2,2'-Bipyridine Receptor: Luminescence Sensing and Magnetic Property

Ln₂L₃-type supramolecular architectures have received significant attention recently due to their unique magnetism and optical properties. Herein, we report the triple-stranded Ln₂L₃-type lanthanide molecular quasi-lanterns, which are fabricated by the deprotonation self-assembly of a linear ligand featuring a β-diketone chelating claw and 2,2'-bipyridine (bpy) moiety with lanthanide ions (Ln = Eu³⁺ and Dy³⁺). The crystal structure analysis indicates that Eu³⁺ and Dy³⁺ ions are all coordinated by eight oxygen donors but in different coordination geometries. The eight oxygen donors in Eu₂L₃ and Dy₂L₃ are arranged in a square antiprism and triangular dodecahedron geometry, respectively. Taking into account the fact that the bpy moiety has a strong coordination affinity for transition metal ions, luminescence sensing toward Cu²⁺ ions has been demonstrated with Eu₂L₃, bearing a detection of limit as low as 2.84 ppb. The luminescence sensing behavior of Eu₂L₃ is ascribed to the formation host-guest complex between Eu₂L₃ and Cu²⁺ ions with a 1:2 binding ratio. Dynamic AC susceptibility measurements for Dy₂L₃ reveal the relaxation of magnetization in it. This work provides a potential way for design and fabrication of lanthanide-based molecular materials with functions endowed by the ligands.

Selective Supramolecular Recognition of Nitroaromatics by a Fluorescent Metal-Organic Cage Based on a Pyridine-Decorated Dibenzodiaza-Crown Macrocyclic Co(II) Complex

Two isomorphous fluorescent (FL) lantern-shaped metal-organic cages 1 and 2 were prepared by coordination-directed self-assembly of Co(II) centers with a new aza-crown macrocyclic ligand bearing pyridine pendant arms (L_py). The cage structures were determined using single-crystal X-ray diffraction analysis, thermogravimetric, elemental microanalysis, FT-IR spectroscopy, and powder X-ray diffraction. The crystal structures of 1 and 2 show that anions (Cl^- in 1 and Br^- in 2) are encapsulated within the cage cavity. 1 and 2 bear two coordinated water molecules that are directed inside the cages, surrounded by the eight pyridine rings at the "bottom" and the "roof" of the cage. These hydrogen bond donors, π systems, and the cationic nature of the cages enable 1 and 2 to encapsulate the anions. FL experiments revealed that 1 could detect nitroaromatic compounds by exhibiting selective and sensitive fluorescence quenching toward p-nitroaniline (PNA), recommending a limit of detection of 4.24 ppm. Moreover, the addition of 50 μL of PNA and o-nitrophenol to the ethanolic suspension of 1 led to a significant large FL red shift, namely, 87 and 24 nm, respectively, which were significantly higher than the corresponding values observed in the presence of other nitroaromatic compounds. The titration of the ethanolic suspension of 1, with various concentrations of PNA (>12 μM) demonstrated a concentration-dependent emission red shift. Hence, the efficient FL quenching of 1 was capable of distinguishing the dinitrobenzene isomers. Meanwhile, the observed red shift (10 nm) and quenching of this emission band under the influence of a trace amount of o- and p-nitrophenol isomers also showed that 1 could discriminate between o- and p-nitrophenol. Replacement of the chlorido with a bromido ligand in 1 generated cage 2 which was a more electron-donating cage than 1. The FL experiments showed that 2 was partially more sensitive and less selective toward NACs than 1.

Lanthanide-organic pincer hosts with allosteric-controlled metal ion binding specificity

A series of lanthanide-organic pincer hosts were synthesized, which showed allosteric-controlled metal ion binding selectivities due to the lanthanide-induced subtle changes of the central vacant binding site.

Nonaqueous Emulsion Polycondensation Enabled by a Self‐Assembled Cage‐like Surfactant

Nonaqueous emulsions are crucial for a range of applications based on water‐sensitive systems such as controlled polymerizations requiring anhydrous reaction conditions and the stabilization of readily hydrolyzable reagents or pharmacologically active components. However, defined molecular surfactants to stabilize such nonaqueous emulsions are scarce. We introduce a self‐assembled coordination cage, decorated with cholesterol functionalities, to serve as a molecular surfactant for various oil‐in‐oil emulsions of immiscible organic solvents. While the positively charged cage forms the amphiphile's polar moiety, the non‐polar cholesterol appendices can bend in a common direction to stabilize the emulsion. Templated by the droplets, polycondensation reactions were carried out to produce microstructured polyurethane and polyurea materials of different particle sizes and morphologies. Further, the amphiphilic cage can encapsulate a guest molecule and the resulting host‐guest assembly was also examined as a surfactant. In addition, the aggregation behavior of the amphiphilic cage in an aqueous medium was examined.

Molecular Assemblies Offering Hydrogen-Bonding Cavities: Influence of Macrocyclic Cavity and Hydrogen Bonding on Dye Adsorption

This work presents a set of Hg macrocycles of amide-phosphine-based ligands offering H-bonding cavities of different dimensions. Such macrocycles are shown to selectively adsorb anionic dyes followed by neutral dyes as well as Prontosil, a biologically relevant antibiotic, within their cavities with the aid of H-bonding-assisted encapsulation. Kinetic experiments supported by spectroscopic and docking studies illustrate the importance of the cavity structure as well as H-bonds for the selective adsorption of dyes.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Regulatable Detection of Antibiotics Based on a Near-IR-Luminescent Tubelike Zn(II)-Yb(III) Nanocluster

A tubelike Zn(II)-Yb(III) cluster, [Zn₆Yb₅L₅(HL)(NO₃)₄(DMF)₆(EtOH)₄(H₂O)₄] (1; DMF = N,N-dimethylformamide and EtOH = ethanol; molecular size 1.5 × 1.8 × 2.9 nm), was synthesized from a new long-chain Schiff base ligand. 1 exhibits a regulatable near-IR-luminescent response to nitrofuran antibiotics (NFAs) and fluoroquinolones with high sensitivity, which is not influenced by other antibiotics. The quenching constants of NFAs and fluoroquinolones range from 0.55 × 10⁴ to 8.8 × 10⁴ M^-1, and the detection limits of 1 to them are from 4.2 × 10^-4 to 2.6 × 10^-5 M. It also shows a luminescent response to real antibiotic drugs containing NFAs and fluoroquinolones.

Mesoporous Silica Nanoparticles-Embedded Lanthanide Organic Polyhedra for Enhanced Stability, Luminescence and Cell Imaging

We report here a simple but efficient “ship-in-a-bottle” synthetic strategy for increasing the stability and luminescence performance of LOPs by embedding them into mesoporous silica nanoparticles (MSNs). Three types of...

A high-nuclearity Cd(ii)–Nd(iii) nanocage for the rapid ratiometric fluorescent detection of quercetin

A 32-metal Cd(ii)–Nd(iii) nanocage was constructed, and it shows rapid and stable ratiometric fluorescent response to quercetin.

Construction of a Cd8Tb4 nanoring for luminescence response to 2,6-dipicolinic acid as an anthrax biomarker

One 12-metal Cd(ii)–Tb(iii) nanoring (1.2 × 2.8 × 2.8 nm) was constructed from a flexible Schiff base ligand, and it shows luminescent response to 2,6-dipicolinic acid with high sensitivity and selectivity.

Coordination-Assembly of Lanthanide Supramolecular Hydrogels with Luminescent Multi-stimulus Response

Luminescent supramolecular hydrogels have shown extensive potential for a variety of applications due to their unique optical properties and biocompatibility. Coordination self-assembly provides a promising strategy for the preparation of supramolecular hydrogels. In this contribution, a series of luminescent lanthanide (Ln) supramolecular hydrogels HG-Ln_2nL_3n^1/2 are synthesized by coordination self-assembly of Ln ions and V shaped bis-tetradentate ligands (H₄L¹ and H₄L²) with different bent angles (∠B). Two rigid conjugated ligands H₄L¹ and H₄L² with bent angles (∠B ≈ 150°) featuring a 2,6-pyridine bitetrazolate chelating moiety were designed and synthesized, which generated hydrogels via the deprotonation self-assembly with lanthanide ions. Characteristic Eu³⁺ and Yb³⁺ emissions were realized in the corresponding hydrogels, with intriguing multi-stimulus response behaviors. The luminescence of the HG-Eu_2nL_3n¹ hydrogel can be enhanced or quenched when stimulated by diverse metal ions, attributed to the replacement of the coordinated lanthanide ions and changes in the intersystem crossing efficiency of the ligand. Furthermore, pH-responsive emission of the HG-Eu_2nL_3n¹ hydrogel has also been observed. Our work provides potential strategies for the design of next-generation smart responsive hydrogel materials with variable structures.

Ratiometric fluorescent detection of dipicolinic acid as an anthrax biomarker based on a high-nuclearity Yb18 nanoring

An 18-metal lanthanide nanoring [Yb₁₈(L¹)₈(HL²)₂(OAc)₂₀(MeOH)₈(EtOH)₆(H₂O)₄] (1), which shows a ratiometric fluorescent response to DPA, was constructed through the strategy of using two types of polydentate organic ligands. The addition of DPA increases the visible ligand-centered emission, but decreases the NIR lanthanide luminescence of 1. The limit of luminescent detection of 1 for DPA is 1.5 μM. The high fluorescence sensitivity of 1 to DPA is not affected by the existence of interferents such as aromatic carboxylates and ions.

Helicate-to-tetrahedron transformation of chiral lanthanide supramolecular complexes induced by ionic radii effect and linker length

Controlled formation of desired lanthanide supramolecular complexes is challenging because of the difficulties in predicting coordination geometry, as well as a labile coordination number. Herein, we explore the effect of ionic radii and linker length on supramolecular species formation. A helicate-to-tetrahedron transformation occurred between [Ln₂L1₃] and [Ln₄L1₆] (Ln = La, Sm, Eu, Gd, Tb and Lu). For six lanthanide ions, the unfavored tetrahedron [La₄L1₆] can only be observed in a concentrated mixture with the helicate [La₂L1₃] where no pure [La₄L1₆] species was isolated via crystallization. For Sm, Eu, Gd, Tb, the [Ln₄L1₆] supramolecular tetrahedron can be isolated via crystallization from diisopropyl ether. A similar result was also observed for Lu, but the tetrahedral structure was found to be relatively stable and transformed back to [Lu₂L1₃] much slower upon dissolution.  No tetrahedron formation was observed with L3 giving rise to only [Ln₂L3₃] species, in which L3 contains a longer and more flexible linker compared with that of L1. Results show that the supramolecular transformation in these systems is governed by both the ionic radii as well as the ligand design. Special focus is on both [Eu₂L1₃] and [Eu₄L1₆] which form chiral entities and exhibit interesting circular polarized luminescence.

One high-nuclearity Eu18 nanoring with rapid ratiometric fluorescence response to dipicolinic acid (an anthrax biomarker)

One 18-metal Eu(iii) nanoring (size: 1.0 × 2.7 × 2.7 nm) was constructed as a rapid ratiometric fluorescent probe for the detection of dipicolinic acid with high sensitivity and selectivity, by using two types of polydentate organic ligands.

Water-stable lanthanide-organic macrocycles from a 1,2,4-triazole-based chelate for enantiomeric excess detection and pesticide sensing

Water-stable anionic Ln2L2-type (Ln = LaIII and EuIII) lanthanide-organic macrocycles have been constructed by deprotonation self-assembly of a bis-tridentate ligand consisting of two 2,6-bis-(1,2,4-triazole)-pyridine chelation arms bridged by a dibenzofuran chromophore, of which the luminescent Eu2L2 macrocycle can be used for enantiomeric excess (ee) detection toward pybox-type chiral ligands and selective colorimetric sensing of omethoate (OMA) in water.

Structural variation of self-assembled lanthanide supramolecular complexes induced by reaction conditions

The structural variation of self-assembled lanthanide supramolecular complexes which can be induced by different factors such as concentration, anion and solvent, cationic radii, stoichiometric ratio and light.

One Nanoscale Zn(II)-Nd(III) Complex With Schiff Base Ligand: NIR Luminescent Sensing of Anions and Nitro Explosives

One Zn-Nd complex [Zn₂Nd₄L₂(OAc)₁₀(OH)₂(CH₃OH)₂] (1) was synthesized from Schiff base ligand bis(3-methoxysalicylidene)ethylene-1,2-phenylenediamine (H₂L). 1 shows nanoscale rectangular structure with sizes of about 0.8 × 1.1 × 2.8 nm. 1 exhibits typical near-infrared luminescence of Nd(III) under the excitation of UV-visible light. Further study shows that the complex displays luminescent response behavior to anions and nitro explosives, especially with high sensitivity to H₂ PO 2 - and 2,4,6-trinitrophenol.

Construction of a High-Nuclearity Elliptical Yb(III) Nanoring: NIR Luminescent Response to Metal Ions and Nitro Explosives

One 14-metal Yb(III) nanoring [Yb₁₄(HL)₂L₂₀(DMF)₈(H₂O)₈] (1) with a size of about 1.1 × 2.5 × 2.7 nm was synthesized from a tridentate ligand. Under the excitation of ligand absorption bands, 1 exhibits the NIR luminescence of Yb(III) and displays high luminescence sensitivity and selectivity to Co(II), Cu(II), and 2,4,6-trinitrophenol (PA) at the parts per million level. The K_SV values of 1 to Co(II), Cu(II), and PA are 6.0 × 10⁴ M^-1, 3.8 × 10⁴ M^-1, and 6.9 × 10⁴ M^-1, respectively. 1 exhibits high luminescent sensitivity to PA even in the presence of other explosives.

Radii-dependent self-assembly of chiral lanthanide complexes: synthesis, chirality, and single-molecule magnet behavior

A pair of 3-methoxysalicylhydrazone-based homochiral ligands constructed chiral trinuclear and pentanuclear complexes with La^III and Dy^III ions, respectively, which indicates that the radii controlled the self-assembled structures. Chiral transfer during the self-assembly processes was confirmed by crystal structure analysis and CD spectroscopy. Then, magnetic investigations demonstrated that the chiral Dy₅ complexes exhibited typical single-molecule magnet behavior.

Construction of 14-metal lanthanide nanorings with NIR luminescence response to ions

Two 14-metal lanthanide nanorings [Ln14(HL)2L20(DMF)8(H2O)8] (Ln = Nd(1) and Gd(2)) were constructed from a tridentate ligand. 1 displays NIR luminescence sensing properties towards metal cations and anions, especially Cu2+, Co2+, H2PO4- and F- at a ppm level.

Construction of Chiral "Triple-Decker" Nd(III) Nanocluster with High NIR Luminescence Sensitivity toward Co(II)

One Nd(III) complex [Nd₃L₃(OAc)₃] (1) was synthesized from a conjugate Schiff base ligand H₂L. It shows a chiral "triple-decker" structure (1.1 × 1.2 × 1.8 nm) with Nd(III) ions sandwiched between the Schiff base ligands. 1 exhibits NIR Nd(III) luminescence, and the LMET efficiency is calculated to be 13.8%. It displays high luminescence sensitivity and selectivity to Co(II). The K_SV value and LOD of 1 to Co(II) are 9.96 × 10⁴ M^-1 and 0.97 μM, respectively.

Eu(III) Tetrahedron Cage as a Luminescent Chemosensor for Rapidly Reversible and Turn-On Detection of Volatile Amine/NH3

Because of the involvement of the gas-solid diffusion, device fabrication, and the relatively complex photophysical process, the lanthanide complexes are rarely exploited as fluorescence sensors for volatile compound (VC) detection. Herein, we report the first example of a discrete 3D Ln-based architecture as a sensor for VCs. The designed Eu₄L₄ tetrahedral cage shows highly selective, rapidly reversible, and turn-on emissive responses toward volatile amines/NH₃ in a spin-coated film. Through the comprehensive spectral characteristic and density functional theory calculation, an intermolecular weak nucleophilic interaction is proposed for this response mechanism. Combining this weak interactions with the permeability of the cage, the film presents subsecond to second timescales rapid response; combining the fitting electrophilic capability of the β-diketonate units to amine nitrogen with the tunable intramolecular charge-transfer feature, the cage shows excellent selectivity and turn-on emissive response. This work provides a new clue to develop the lanthanide complexes as luminescence probes for VCs.

A 42-metal Yb(iii) nanowheel with NIR luminescent response to anions

One Yb42 nanowheel [Yb42L14(OH)28(OAc)84] was constructed using a tridentate vanillin ligand. The external diameter of the wheel-like structure is about 3.6 nm, which allows direct visualization by TEM. It shows interesting NIR lanthanide luminescence sensing towards anions, especially to fluoride at the ppm level.

Precise Self-Assembly of Molecular Four- and Six-Pointed Stars

A novel metal-organic ligand (MOL 2) has been prepared by linking two V-shaped bis-terpyridines with one X-shaped tetrakis-terpyridine through the stable <tpy-Ru²⁺-tpy> connectivity. The complexation between MOL 2, X-shaped tetrakis-terpyridine, and Zn²⁺ gave rise to a supramolecular C₆-symmetrical six-pointed star quantitatively. In addition, a mixture of MOL 2, K-shaped tetrakis-terpyridine, and Zn²⁺ afforded a C₂-symmetrical four-pointed star. These metallo-supramolecular architectures were adequately characterized by NMR, ESI-MS, TWIM-MS, and TEM analyses.

Two [Ln4] molecular rings folded as compact tetrahedra

A new highly photo-switchable ligand furnishes supramolecular tetrahedral nanomagnets with Ln(iii) ions (Ln = Dy, Tb). Intramolecular weak interactions define the conformation of the ligand, quenching the photochromic activity.

Metallopolymers cross-linked with self-assembled Ln4L4 cages

Herein, we report a metallopolymer based on a polydivinylbenzene (PDVB) matrix cross-linked by tetranuclear Ln₄L₄ cages.

Construction of a crystalline 14-metal Zn–Nd rectangular nanocluster with a dual-emissive response towards metal ions

A crystalline 14-metal Zn–Nd cluster [Zn₆Nd₈L₂(OAc)₂₀(O)₂(NO₃)₄(OC₂H₅)₄] (1) was constructed using a flexible Schiff base ligand with long-chain (CH₂)₂O(CH₂)₂O(CH₂)₂ backbone. 1 exhibits a nanoscale rectangular structure (10 × 14 × 18 Å). Besides the visible ligand-centered emission, 1 displays typical NIR luminescence of Nd³⁺. Interestingly, 1 shows both a ligand-centered and lanthanide fluorescence response towards metal ions, especially to K⁺ and Co²⁺ at the ppm level.

A crystalline 14-metal Zn–Nd nanocluster was constructed, and it shows ligand-centered and lanthanide fluorescence response towards metal ions.

Strongly luminescent 5d/4f heterometal–organic macrocycles with open metal sites: post-assembly modification and sensing

Strongly luminescent 5d/4f heterometal–organic macrocycles featuring open metal sites have been constructed, along with their post-assembly modification and sensing properties.

Stereocontrolled self-assembly and photochromic transformation of lanthanide supramolecular helicates

Stereocontrolled self-assembly of a dinuclear triple-stranded europium helicate (Eu2L3) based on DTE-functionalized ligands has been achieved via the chiral-induction strategy. The point-chirality of the ligands is transferred to give either Δ or Λ metal-centers and hence leads to the overall P or M helical senses. CD spectroscopy and NMR enantiomeric differentiation experiments have confirmed the formation of enantiomers. Moreover, the helicates in solution feature reversible photocyclization and cycloreversion, offering an opportunity to develop as chiroptical switches.

The effect of the linker size in C2-symmetrical chiral ligands on the self-assembly formation of luminescent triple-stranded di-metallic Eu(iii) helicates in solution

Chiral lanthanide-based supramolecular structures have gained significant importance in view of their application in imaging, sensing and other functional purposes. We have designed chiral C2-symmetrical ligands (L) based on the use of two 2,6-pyridine-dicarboxylic-amide moieties (pda), that differ from one another by the nature of the diamine spacer groups (from 1,3-phenylenedimethanamine (1(S,S), 2(R,R)) and benzene-1,3-diamine (3(S,S), 4(R,R)) to much bulkier 4,4'-(cyclohexane-1,1-diyl)bis(2,6-dimethylaniline) (5(S,S), 6(R,R))) between these two pda units. The self-assembly between L and Eu(iii) ions were investigated in CH3CN solution at low concentration whereby the changes in the absorbance, fluorescence and Eu(iii)-centred emission spectra allowed us to model the binding equilibria occurring in the solution to the presence of [Eu:L2], [Eu2:L2], [Eu2:L3] assemblies and reveal their high binding constant values. The self-assembly in solution were also studied at higher concentration by following the changes in the 1H NMR spectra of the ligands upon Eu(iii) addition, as well as by using MALDI-MS of the isolated solid state complexes. The chiroptical properties of the ligands were used in order to study the structural changes upon self-assembly between the ligands and Eu(iii) ions using circular dichroism (CD) and circularly polarised luminescence (CPL) spectroscopies. The photophysical properties of [Eu2:L3] complexes were evaluated in solution and showed a decrease of luminescence quantum yield when going from the ligand with smaller (1(S,S)) to bulkier (5(S,S)) linker from ∼5.8% to ∼2.6%. While mass-spectrometry revealed the possible formation of trinucler assemblies such as [Eu3:L3] and [Eu3:L2].