Stepwise Synthesis of Low-Symmetry Hexacationic Pyridinium Organic Cages.

An efficient approach was developed for the synthesis of the well-known BlueCage by pre-bridging two 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) panels with one linker followed by cage formation in a much improved yield and shortened reaction time. Such a stepwise methodology was further applied to synthesize three new pyridinium organic cages, C2, C3, and C4, where the low-symmetry cages C3 and C4 with angled panels demonstrated better recognition properties toward 1,1'-bi-2-naphthol (BINOL) than the high-symmetry analogue C2 featuring parallel platforms.

54 K Spin Transition Temperature Shift in a Fe6L4 Octahedral Cage Induced by Optimal Fitted Multiple Guests.

Spin-crossover (SCO) coordination cages are at the forefront of research for their potential in crafting next-generation molecular devices. However, due to the scarcity of SCO hosts and their own limited cavities, the interplay between the SCO host and the multiple guests binding has remained elusive. In this contribution, we present a family of pseudo-octahedral coordination cages (M6L4, M = ZnII, CoII, FeII, and NiII) assembled from a tritopic tridentate ligand L with metal ions. The utilization of FeII ion leads to the successful creation of the Fe6L4-type SCO cage. Host-guest studies of these M6L4 cages reveal their capacity to encapsulate four adamantine-based guests. Notably, the spin transition temperature T1/2 of Fe6L4 is dependent on the multiple guests encapsulated. The inclusion of adamantine yields an unprecedented T1/2 shift of 54 K, a record shift in guest-mediated SCO coordination cages to date. This drastic shift is ascribed to the synergistic effect of multiple guests coupled with their optimal fit within the host. Through a straightforward thermodynamic cycle, the binding affinities of the high-spin (HS) and low-spin (LS) states are separated from their apparent binding constant. This result indicates that the LS state has a stronger binding affinity for the multiple guests than the HS state. Exploring the SCO thermodynamics of host-guest complexes allows us to examine the optimal fit of multiple guests to the host cavity. This study reveals that the T1/2 of the SCO host can be manipulated by the encapsulation of multiple guests, and the SCO cage is an ideal candidate for determining the multiple guest fit.

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.

Excited-Multimer Mediated Supramolecular Upconversion on Multicomponent Lanthanide-Organic Assemblies.

Upconversion (UC) is a fascinating anti-Stokes-like optical process with promising applications in diverse fields. However, known UC mechanisms are mainly based on direct energy transfer between metal ions, which constrains the designability and tunability of the structures and properties. Here, we synthesize two types of Ln8L12-type (Ln for lanthanide ion; L for organic ligand L1 or L2R/S) lanthanide-organic complexes with assembly induced excited-multimer states. The Yb8(L2R/S)12 assembly exhibits upconverted multimer green fluorescence under 980 nm excitation through a cooperative sensitization process. Furthermore, upconverted red emission from Eu3+ on the heterometallic (Yb/Eu)8L12 assemblies is also realized via excited-multimer mediated energy relay. Our findings demonstrate a new strategy for designing UC materials, which is crucial for exploiting photofunctions of multicomponent lanthanide-organic complexes.

Counteranion-mediated efficient iodine capture in a hexacationic imidazolium organic cage enabled by multiple non-covalent interactions.

Developing efficient adsorbents to capture radioactive iodine produced from nuclear wastes is highly desired. Here we report the facial synthesis of a hexacationic imidazolium organic cage and its iodine adsorption properties. Crucial role of counteranions has been disclosed for iodine capture with this cage, where distinct iodine capture behaviors were observed when different counteranions were used. Mechanistic investigations, especially with the X-ray crystallographic analysis of the iodine-loaded sample, allowed the direct visualization of the iodine binding modes at the molecular level. A network of multiple non-covalent interactions including hydrogen bonds, halogen bonds, anion···π interactions, electrostatic interaction between polyiodides and the hexacationic skeleton of the cage are found responsible for the observed high iodine capture performance. Our results may provide an alternative strategy to design efficient iodine adsorbents.

Controlled Self-Assembly, Isomerism, and Guest Uptake/Release of Charge-Reversible Lanthanide-Organic Octahedral Cages.

Charge plays a crucial role in the function of molecular and supramolecular systems, but coordination hosts capable of orthogonal charge regulation remain elusive so far. In this study, we report the condition-dependent self-assembly of charge-reversible lanthanide-organic tetra-capped octahedral cages, i.e., [Ln6(H3L)4]6+ and [Ln6L4]6-, from a series of lanthanide ions (Ln3+; Ln = Lu, Yb, Eu) and a tritopic tetradentate acylhydrazone ligand (H6L) featuring multiple deprotonation states and propeller conformations. While direct self-assembly under basic conditions produced a mixture of various ΔxΛ6-x-[Ln6L4]6- (x = 0-6) stereoisomers, racemic Δ6- and Λ6-[Ln6L4]6- could be exclusively obtained from the first self-assembly of Δ6- and Λ6-[Ln6(H3L)4]6+ under neutral conditions followed by post-assembly deprotonation. Rich isomerism on the tetra-capped octahedral cages arising from the coupling between the metal-centered Δ/Λ chirality and the ligand conformations has been discussed based on X-ray single-crystal structures of the C3-symmetric Δ3Λ3-Ln6L4 and T-symmetric Δ6/Λ6-Ln6L4 complexes. Host-guest studies confirmed that positively charged rac-Δ6/Λ6-[Ln6(H3L)4]6+ could bind anionic sulfonates, and negatively charged rac-Δ6/Λ6-[Ln6L4]6- exhibited strong encapsulation ability toward ammonium guests, where acid/base-triggered guest uptake/release could be realized taking advantage of the charge reversibility of the cage. Moreover, photophysical studies revealed visible-light-sensitized and guest-encapsulation-enhanced NIR emissions on the rac-Δ6/Λ6-Yb6L4 cage. This work not only enriches the library of functional lanthanide-organic cages but also provides a promising candidate with charge reversibility for the development of smart supramolecular materials.

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 ß-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.

Hierarchical subcomponent self-assembly of covalent triple-stranded complexes with 3d-4f vertices: luminescence and magnetic properties.

Well-defined 3d-4f heterometallic supramolecular architectures have attracted attention because of their applications in the field of luminescence and magnetism. However, covalent metallo-supramolecular discrete complexes, decorated with hetero-metallic vertices, have never been reported because of the difficulties in design and control. Herein, we report a series of covalent metallo-supramolecular discrete complexes with 3d-4f vertices synthesized by hierarchical subcomponent self-assembly of tris(2-aminoethyl)amine, 2,6-diformyl-p-cresol, and lanthanide ions (Ln) with different amines and transition metal ions. The programmable self-assembly process results in the formation of triple-stranded hetero-metallic covalent organic discrete complexes, namely 3a-3c-(Ln, Zn) (Ln = SmIII, EuIII, DyIII, YbIII and LuIII) and 3a'-(Dy, Co), which are characterized by nuclear magnetic resonance (NMR) analysis, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and single-crystal X-ray analysis. Photophysical investigations disclose that the organic skeleton of 3a-(Ln, Zn) exhibits an excellent sensitizing ability toward SmIII, EuIII, and YbIII ions, displaying characteristic luminescence emission in both the visible and near-infrared (NIR) regions. AC susceptibility measurements of 3a'-(Dy, Co) reveal the frequency-independent performance under zero dc field, suggesting the absence of slow relaxation of magnetization. This work offers a new approach for the fabrication of discrete metallic covalent architectures with 3d-4f vertices.

A Pd4L2 cage containing Brønsted-base active sites for the one-pot photooxidation/Knoevenagel condensation reaction.

Brønsted-base active sites on a Pd4L2 cage facilitates enhanced catalytic efficiency, wide substrate scope and high turnover number (TON) for the one-pot photooxidation/Knoevenagel condensation reaction under mild conditions.

Embedding Lanthanide Organic Polyhedra into Mesoporous Silica Nanoparticles for the Photocatalytic Degradation of Organic Dyes.

Organic pollutants cause severe environmental problems because of their damage to human health and ecological systems. Photocatalytic degradation of persistent organic pollutants is of great importance to address these hazards. Herein, we report a lanthanide organic polyhedra-based hybrid material Gd8 L12 ⊂MSN with the capability of photocatalytic dye degradation. Gd8 L12 ⊂MSN was prepared by embedding the Gd8 L12 complex into mesoporous silica nanoparticles (MSNs) using a "ship-in-a-bottle" strategy. Photocurrent response tests revealed that this hybrid material is a potential semiconductor and could generate a rapid and steady photocurrent upon irradiation. Further dye degradation experiments indicated that it could photocatalyze the degradation of familiar organic dyes. Thereinto, compared with the critical Gd8 L12 complex, the hybrid material exhibited an acceleration of 2.4 times and realized reusability. This not only offers a potential advanced photocatalyst for degrading persistent organic pollutants, but also provides a strategy for the application of supramolecular materials in environmental science.

Dinuclear mono-bridged or polymeric lanthanide complexes from one ligand: structural transformation and chiral induction.

We designed and synthesized a semi-rigid bis-tridentate ligand L which could undergo a trans-to-cis conformational transition when coordinated onto lanthanum ions through the rotation of the single bond on the central terphenyl bridge. By adjusting the metal/ligand ratio, a single-ligand bridged dinuclear complex La2L3 and an infinitely extending two-dimensional layered metal organic polymer (La2L2)n can be obtained. Through the induction of the chiral auxiliary ligand GR/S, these two achiral assemblies could both be transformed into the chiral mononuclear three-component complex LaLGR/S. In addition, we also realized a linear ee sensing for the auxiliary ligand by induced circular dichroism.

Stereocontrolled Self-Assembly of Ln(III)-Pt(II) Heterometallic Cages with Temperature-Dependent Luminescence.

Structurally well-defined discrete d/f heterometallic complexes show diverse application potential in electrooptic and magnetic materials. However, precise control of the component and topology of such heterometallic compounds with fine-tuned photophysical properties is still challenging. Herein, we report the stereocontrolled syntheses of a series of LnIII-PtII heterometallic cages through coordination-driven self-assembly of enantiopure alkynylplatinum-based metalloligands (L1R/S, L2R/S) with lanthanide ions (Ln = EuIII, YbIII, NdIII, LuIII). Taking advantage of the metal-to-ligand charge transfer (MLCT) excited state on the designed alkynylplatinum ligands, the excitation window for the sensitized near-infrared (NIR) luminescence on the YbIII- and NdIII-containing cages can be extended to the visible region (up to 500 nm). Linear temperature-dependent red and NIR emissions observed on the Ln4(L2R/S)6 (LnIII = EuIII and YbIII, respectively) complexes suggest their potential applications as luminescent temperature sensors, with sensitivities of -0.54% (LnIII = EuIII, 77-250 K) and -0.17% (LnIII = YbIII, 77-300 K) per K achieved. This work not only offers a good strategy to prepare new d/f heterometallic supramolecular cages but also paves the way for the design of stimuli-responsive luminescent materials.

An Organo-Palladium Host Built from a Dynamic Macrocyclic Ligand: Adaptive Self-Assembly, Induced-Fit Guest Binding, and Catalysis.

Artificial hosts with rich conformational dynamics are attractive to supramolecular chemists due to their adaptive guest-binding properties and enzyme-like catalytic functions. We report here the adaptive self-assembly and host-guest catalysis of a new water-soluble organo-palladium host (Pd2 L2 ) built from a pyridinium-bonded macrocyclic ligand (L) and cis-blocked palladium corners (Pd). While the direct self-assembly of L with Pd gives rise to a dynamic mixture of products, both neutral polyaromatic hydrocarbons and an anionic polyoxometalate cluster (W10 O32 4- ) can template the dominant formation of the Pd2 L2 host. Guest-adaptive conformational changes and induced-fit cavity deformation of the Pd2 L2 host have been clearly observed in the crystal structures. Moreover, the installation of the electron-rich W10 O32 4- cluster within the cationic redox-active host (W10 O32 ⊂Pd2 L2 ) facilitates the efficient and selective C-H photooxidation of toluene derivatives to aldehyde products under mild conditions, thus representing an ideal platform for green supramolecular catalysis.

Shape Complementary Coordination Self-Assembly of a Redox-Active Heteroleptic Complex.

We present here the coordination self-assembly of a new heteroleptic (bpyPd)4L1L22 coordination complex (1) from one novel pyridinium-functionalized bis-2,4,6-tris(pyridin-3-yl)-1,3,5-triazine (bis-3-TPT, L1) macrocyclic ligand, two separate 3-TPT (L2) ligands, and four cis-blocking bpyPd(NO3)2 (bpy = 2,2'-bipyridine). While homoleptic self-assemblies with either L1 or L2 gave dynamic mixtures of products, a single thermodynamic heteroleptic complex was obtained driven by the shape complementarity of building blocks. Moreover, the redox-active nature of the heteroleptic assembly facilitates the highly efficient catalytic aerobic photo-oxidation of aromatic secondary alcohols under mild conditions.

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.

Mesoporous silica nanoparticle-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 hybrid materials, i.e. Eu8L12@MSNs, Eu8L12@MSNs-NH2 and Eu8L12@MSNs-biotin, have been prepared and characterized by FT-IR, TGA, SEM and TEM. Photo-optical measurements confirmed that the photoluminescence quantum yields in water have been greatly improved from 5.50% for pristine Eu8L12 to 44.04% for Eu8L12@MSNs-biotin, along with fast disassembly for the former and the optical performance has been maintained for the latter under acidic conditions (pH = 4). Moreover, compared to Eu8L12, Eu8L12@MSNs and Eu8L12@MSNs-NH2, the biotin-modified hybrid material Eu8L12@MSNs-biotin has exhibited much enhanced fluorescence-imaging ability toward the MDA-MB-231 human breast cancer cells, with significantly reduced dosage of the complex. Our work provides a useful strategy for the functionalization of multinuclear lanthanide organic assemblies toward their biosocial applications.

Cation modulated spin state and near room temperature transition within a family of compounds containing the same [FeL2]2- center.

Spin-crossover (SCO) active compounds have received much attention due to their potential application in molecular devices. Herein, a family of solvent-free FeII compounds, formulated as (A)2[FeL2], (H2L = pyridine-2,6-bi-tetrazolate, A = (Me)4N+1, Et2NH2+2, iPr2NH2+3 and iPrNH3+4), were synthesized and characterized. Single-crystal X-ray diffraction studies reveal that 1-4 are all supramolecular frameworks containing the same [FeL2]2- center, which is arranged into two packing modes via inter-molecular interactions, that is, a 3D architecture in 1 and 1D chain in 2-4. The spin states of 1-4 at different temperatures are assigned on the basis of the single-crystal X-ray diffraction data. Solid state magnetic investigations indicate that 1 and 4 exhibit a low spin state (below 350 K) and high spin state (2-400 K), respectively. 2 and 3 display clear SCO behavior in the measured temperature, but with different profiles and critical temperatures. 2 undergoes a complete gradual SCO with a critical temperature of T1/2 = 260 K. 3 has an abrupt near room temperature transition between T1/2 cooling = 278 K and T1/2 warming = 286, centered at 282 K (9 °C). This study reveals the importance of organic cations in the modulation of SCO behavior and offers a new insight for the design of SCO compounds with near room temperature spin transitions.

Guest-Driven Self-Assembly and Chiral Induction of Photofunctional Lanthanide Tetrahedral Cages.

Chiral luminescent lanthanide-organic cages have many potential applications in enantioselective recognition, sensing, and asymmetric catalysis. However, due to the paucity of structures and their limited cavities, host-guest chemistry with lanthanide-organic cages has remained elusive so far. Herein, we report a guest-driven self-assembly and chiral induction approach for the construction of otherwise inaccessible Ln4L4-type (Ln = lanthanide ions, i.e., EuIII, TbIII; L = ligand) tetrahedral hosts. Single crystal analyses on a series of host-guest complexes reveal remarkable guest-adaptive cavity breathing on the tetrahedral cages, reflecting the advantage of the variation tolerance on coordination geometry of the f-elements. Meanwhile, noncovalent confinement of pyrene within the lanthanide cage not only leads to diminishment of its excimer emission but also facilitates guest to host energy transfer, opening up a new sensitization window for the lanthanide luminescence on the cage. Moreover, stereoselective self-assembly of either Λ4- or Δ4- type Eu4L4 cages has been realized via chiral induction with R/S-BINOL or R/S-SPOL templates, as confirmed by NMR, circular dichroism (CD), and circularly polarized luminescence (CPL) with high dissymmetry factors (glum) up to ±0.125.

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-Ln2nL3n1/2 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-Eu2nL3n1 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-Eu2nL3n1 hydrogel has also been observed. Our work provides potential strategies for the design of next-generation smart responsive hydrogel materials with variable structures.

Photooxidase Mimicking with Adaptive Coordination Molecular Capsules.

One important feature of enzyme catalysis is the induced-fit conformational change after binding substrates. Herein, we report a biomimetic water-soluble molecular capsule featuring adaptive structural change toward substrate binding, which offers an ideal platform for efficient photocatalysis. The molecular capsule was coordination-assembled from three anthracene-bridged bis-TPT [TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine] ligands and six (bpy)Pd(NO3)2 (bpy = 2,2'-bipyridine). Once substrates bind to its hydrophobic cavity, this capsule would undergo quantitative capsule-to-bowl transformation. Visible-light absorption brought about by both the anthracene units and the charge-transfer absorption on the late-formed quintuple π-π stacked host-guest complex efficiently facilitates aerobic photooxidation for the sulfide guests by visible-light irradiation under mild conditions. Desired turnover numbers and product selectivity (sulfoxide over sulfone) have been achieved by the transformable nature of the catalyst and the hydrophilicity of the sulfoxide product. Such a photocatalytic process enabled by an adaptive coordination capsule and substrates as the allosteric effector paves the way for constructing artificial systems to mimic enzyme catalysis.