Coordination-driven Self-assembly Research Articles

Intraligand Charge Transfer Sensitization on Self-Assembled Europium Tetrahedral Cage Leads to Dual-Selective Luminescent Sensing toward Anion and Cation

Luminescent supramolecular lanthanide edifices have many potential applications in biology, environments, and materials science. However, it is still a big challenge to improve the luminescent performance of multinuclear lanthanide assemblies in contrast to their mononuclear counterparts. Herein, we demonstrate that combination of intraligand charge transfer (ILCT) sensitization and coordination-driven self-assembly gives birth to bright EuIII tetrahedral cages with a record emission quantum yield of 23.1%. The ILCT sensitization mechanism has been unambiguously confirmed by both time-dependent density functional theory calculation and femtosecond transient absorption studies. Meanwhile, dual-responsive sensing toward both anions and cations has been demonstrated making use of the ILCT transition on the ligand. Without introduction of additional recognition units, high sensitivity and selectivity are revealed for the cage in both turn-off luminescent sensing toward I- and turn-on sensing toward Cu2+. This study offers important design principles for the future development of luminescent lanthanide molecular materials.

Coordination-Driven Self-Assembly of Ionic Irregular Hexagonal Metallamacrocycles via an Organometallic Clip and Their Cytotoxicity Potency.

Two new irregular hexagons (6 and 7) were synthesized from a pyrazine motif containing an organometallic acceptor clip [bearing platinum(II) centers] and different neutral donor ligands (4,4'-bipyridine or pyrazine) using a coordination-driven self-assembly protocol. The two-dimensional supramolecules were characterized by multinuclear NMR, mass spectrometry, and elemental analyses. Additionally, one of the macrocycles (6) was characterized by single-crystal X-ray analyses. Macrocycles are unique examples of [2 + 2] self-assembled ensembles that are hexagonal but irregular in shape. These hexagon frameworks require the assembly of only four tectons/subunits. The cytotoxicity of platinum(II)-based macrocycles was studied using various cell lines such as A549 (human lung carcinoma), KB (human oral cancer), MCF7 (human breast cancer), and HaCaT (human skin keratinocyte) cell lines, and the results were compared with those of cisplatin. The smaller macrocycle (7) exhibited a higher cytotoxic effect against all cell types, and its sensitivity was found to be comparable with that of cisplatin for A549 and MCF7 cells. Cell cycle analysis and live propidium iodide staining suggest that the macrocycles 6 and 7 induced a loss of membrane integrity that ultimately might lead to necrotic cell death.

A novel electrochemical chiral sensor for tyrosine isomers based on a coordination-driven self-assembly

Alfa-cyclodextrin (α-CD) based chiral sensing device is proposed for electrochemical recognition of tyrosine (Tyr) isomers. α-CD was first modified with Cu2+, and then the obtained Cu2+-modified α-CD (Cu2-α-CD) was integrated to chitosan (CS) to form a coordination-driven self-assembly. The amino group and phenolic hydroxyl group of Tyr could form three intermolecular H-bonds with the high-energy water molecules confined in the cavity of Cu2-α-CD. The host (self-assembly) −guest (Tyr isomers) interactions are different for D- and L-Tyr due to the opposite steric configurations of the Tyr isomers, resulting in significantly discernable electrochemical differences in both peak currents and peak potentials and consequently effective chiral recognition of Tyr isomers. The temperature-sensitivity and chiral specificity of the proposed self-assembly are also investigated in this work. Finally, the percentage of D-Tyr in a racemic mixture can be accurately predicted with the CS/Cu2-α-CD self-assembly, showing its potential in practical applications.

A Self-Assembled Trigonal Prismatic Molecular Vessel for Catalytic Dehydration Reactions in Water.

A water-soluble Pd6 trigonal prism (A) was synthesized by two-component coordination-driven self-assembly of a PdII 90° acceptor with a tetraimidazole donor. The walls of the prism are constructed by three conjugated aromatic building blocks, which means that the confined pocket of the prism is hydrophobic. In addition to the hydrophobic cavity, large product egress windows make A an ideal molecular vessel to catalyze otherwise challenging pseudo-multicomponent dehydration reactions in its confined nanospace in aqueous medium. This study is an attempt at selective generation of the intermediate tetraketones and xanthenes by fine-tuning the reaction conditions employing a supramolecular molecular vessel. Moreover, either poor or no yield of the dehydrated products in the absence of A under similar reaction conditions supports the ability of the confined space of the barrel to promote such reactions in water. Furthermore, we focused on the rigidification of the tetraphenylethylene-based tetraimidazole unit anchored within the PdII coordination architecture; enabling counter-anion dependent aggregation induced emission in the presence of water.

Coordination Polymer Gels with Modular Nanomorphologies, Tunable Emissions, and Stimuli-Responsive Behavior Based on an Amphiphilic Tripodal Gelator.

The recent upsurge in research on coordination polymer gels (CPGs) stems from their synthetic modularity, nanoscale processability, and versatile functionalities. Here we report self-assembly of an amphiphilic, tripodal low-molecular weight gelator (L) that consists of 4,4',4-[1,3,5-phenyl-tri(methoxy)]-tris-benzene core and 2,2':6',2″-terpyridyl termini, with different metal ions toward the formation of CPGs that show controllable nanomorphologies, tunable emission, and stimuli-responsive behaviors. L can also act as a selective chemosensor for ZnII with very low limit of detection (0.18 ppm) in aqueous medium. Coordination-driven self-assembly of L with ZnII in H2O/MeOH solvent mixture results in a coordination polymer hydrogel (ZnL) that exhibits sheet like morphology and charge-transfer emission. On the other hand, coordination of L with TbIII and EuIII in CHCl3/tetrahydrofuran solvent mixture results in green- and red-emissive CPGs, respectively, with nanotubular morphology. Moreover, precise stoichiometric control of L/EuIII/TbIII ratio leads to the formation of bimetallic CPGs that show emissions over a broad spectral range, including white-light-emission. We also explore the multistimuli responsive properties of the white-light-emitting CPG by exploiting the dynamics of LnIII-tpy coordination.

Metal-Folded Single-Chain Nanoparticle: Nanoclusters and Self-Assembled Reduction-Responsive Sub-5-nm Discrete Subdomains.

Easy access to discrete nanoclusters in metal-folded single-chain nanoparticles (metal-SCNPs) and independent ultrafine sudomains in the assemblies via coordination-driven self-assembly of hydrophilic copolymer containing 9% imidazole groups is reported herein. 1 H NMR, dynamic light scattering, and NMR diffusion-ordered spectroscopy results demonstrate self-assembly into metal-SCNPs (>70% imidazole-units folded) by neutralization in the presence of Cu(II) in water to pH 4.6. Further neutralization induces self-assembly of metal-SCNPs (pH 4.6-5.0) and shrinkage (pH 5.0-5.6), with concurrent restraining residual imidazole motifs and hydrophilic segment, which organized into constant nanoparticles over pH 5.6-7.5. Atomic force microscopy results evidence discrete 1.2 nm nanoclusters and sub-5-nm subdomains in metal-SCNP and assembled nanoparticle. Reduction of metal center using sodium ascorbate induces structural rearrangement to one order lower than the precursor. Enzyme mimic catalysis required media-tunable discrete ultrafine interiors in metal-SCNPs and assemblies have hence been achieved.

Real-Time Monitoring the Dynamics of Coordination-Driven Self-Assembly by Fluorescence-Resonance Energy Transfer

It is quite challenging to investigate the dynamics of coordination-driven self-assembly due to the existence of multiple intermediates and many possible processes. By taking advantage of the high sensitivity and efficiency of fluorescence-resonance energy transfer (FRET), FRET was successfully employed to real-time monitor the dynamic behavior of coordination-driven self-assembly. The Förster energy transfer efficiencies and kinetic aspects of a series of discrete, well-defined metallacycles have been determined. Moreover, the dynamic characteristics of these supramolecular assemblies, such as the dynamic ligand exchange, anion-induced disassembly and reassembly, and stability in different solvents, have been investigated by using FRET.

Coordination-Driven Self-Assembly Using Ditopic Pyridyl-Pyrazolyl Donor and p-Cymene Ru(II) Acceptors: [2]Catenane Synthesis and Anticancer Activities.

Coordination-driven self-assembly of m-bis[3-(4-pyridyl)pyrazolyl]xylene (L) and [(p-cymene)2Ru2(OO∩OO)2(OTf)2] (A1) (OO∩OO = 6,11-dioxido-5,12-naphthacenedione) in methanol resulted in a mixture of [2]catenane 1 and macrocycle 2, and self-assembly in nitromethane resulted in pure macrocycle 2, whereas the coordination-driven self-assembly of L and similar acceptors [(p-cymene)2Ru2(OO∩OO)2(OTf)2] [OO∩OO = 5,8-dioxido-1,4-naphthoquinonnato (A2); 2,5-dioxido-1,4-benzoquinonato (A3); oxalato (A4)] resulted in the formations of monomeric macrocycles 3-5, respectively. All self-assembled macrocycles were obtained in excellent yields (>90%) as triflate salts and were fully characterized by multinuclear NMR, elemental analysis, and electrospray ionization mass spectrometry (ESI-MS). The structures of [2]catenane 1 and macrocycles 5 were confirmed by single-crystal X-ray diffraction analysis. The X-ray structure of 1 confirmed an edge-to-face interaction between the tetracene moiety in parallel-displaced π-π stacks (3.5 Å), and CH···π (2.5 Å) stabilizes the [2]catenane topology. Macrocycles 2-5 were assessed for anticancer activities using human cancer cell lines of different origins, and the macrocycle 3 was found to exhibit the best inhibitory effect and to do so in a dose-dependent manner. Further examination with the Tali apoptosis assay suggested the growth inhibitory effect of 3 involved the induction of the programmed cell death, and this suggestion was supported by observations of PARP and caspase 3 cleavage after treating cells with 3. In addition, exposure to 3 increased the expression of Bax and repressed the expression of Bcl-2, thus indicating the involvement of macrocycle 3 upstream of Bax and Bcl-2 in the apoptotic signaling pathway. Macrocycle 3 also tended to repress metastasis as evidenced by changes in the transcriptional expressions E- and N-cadherin (markers of metastasis). Furthermore, a stability assay demonstrated macrocycle 3 remained stable at high concentration.

Phosphorescent Decanuclear Bimetallic Pt6M4 (M = Zn, Fe) Tetrahedral Cages.

Coordination-driven self-assembly delivers discrete, nanoscopic architectures that may preserve or enhance the physicochemical properties of their parent building blocks. Herein, we report the syntheses, characterization, and photophysical properties of two tetrahedral cages, [ZnII4L6](PF6)8 (C1) and [FeII4L6](OTf)8 (C2), where L = PtII(PEt3)2(C≡C-bpy)2 (PEt3 = triethylphosphine; C≡C-bpy = 5-ethynyl-2,2'-bipyridine) and OTf = trifluoromethanesulfonate. C1 and C2 were assembled in isolated yields of 72% and 81% by treating 2 equiv of Zn(NO3)2·6H2O or Fe(OTf)2 with 3 equiv of L, respectively. Both cages were fully characterized by NMR, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction (SCXRD). The local D3 symmetry at each polypyridyl metal node raises the possibility of a number of isomeric cages; however, only the homochiral enantiomers (ΔΔΔΔ and ΛΛΛΛ) are formed based on 1H NMR and SCXRD. C1 exhibits phosphorescence centered at 545 nm with a quantum yield of 10% in N2-degassed acetonitrile at 25 °C. The quantum yield of C2 is significantly lower due to a nonradiative relaxation from 5MC (MC = metal-centered) states introduced by the FeII nodes.

Versatility of Two Diimidazole Building Blocks in Coordination-Driven Self-Assembly.

A series of discrete molecular architectures was synthesized via self-assembly of two "linear" diimidazole donors [L1 = 1,4-di(1H-imidazol-1-yl)benzene and L2 = 1,1'-(2,5-dimethyl-1,4-phenylene)bis(1H-imidazole)] independently with cis-[(tmeda)Pd(NO3)2] [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine], cis-[(tmeda)Pt(NO3)2], a diplatinum(II) acceptor (bisPt), and Pd(NO3)2. The donors L1 and L2 are isostructural with four hydrogens in the benzene ring of L1, whereas in L2 two of such hydrogens are substituted by methyl groups. The assemblies were characterized by NMR spectroscopy and electrospray ionization mass spectrometry analyses along with single-crystal structure determination of three of them. While the self-assembly of L1 with cis-[(tmeda)Pd(NO3)2] solely formed a [3 + 3] self-assembled molecular triangle (1), L2 with the same acceptor predominantly resulted in the formation of a [4 + 4] molecular square (3). Such a dramatic change in the final outcome in the coordination-driven self-assembly by simple alkyl substitution of isostructural donors is remarkable. Interestingly, self-assembly of L1 and L2 with analogous Pt(II) acceptor cis-[(tmeda)Pt(NO3)2] yielded mixtures of [3 + 3] triangle and [4 + 4] square, where the molecular triangles (4 and 6) were the predominant products in both the cases. The same donors in combination with a 0° acceptor bisPt independently formed the expected [2 + 2] metallo-macrocycles (8 and 9). Surprisingly, the macrocycle (9) involving L2 is found to exist in more than one conformation at room temperature. Moreover, the diimidazole donors formed unprecedented Pd6L12 molecular spheres (10 and 11) when they were separately treated with Pd(NO3)2. The imidazole moieties in the ligands are found to appear in versatile orientations in the synthesized molecules due to their rotational flexibility to produce required bite angles for the particular architecture.

A Self-Assembled Cofacial Cobalt Porphyrin Prism for Oxygen Reduction Catalysis.

Herein we report the first study of the oxygen reduction reaction (ORR) catalyzed by a cofacial porphyrin scaffold accessed in high yield (overall 53%) using coordination-driven self-assembly with no chromatographic purification steps. The ORR activity was investigated using chemical and electrochemical techniques on monomeric cobalt(II) tetra(meso-4-pyridyl)porphyrinate (CoTPyP) and its cofacial analogue [Ru8(η6-iPrC6H4Me)8(dhbq)4(CoTPyP)2][OTf]8 (Co Prism) (dhbq = 2,5-dihydroxy-1,4-benzoquinato, OTf = triflate) as homogeneous oxygen reduction catalysts. Co Prism is obtained in one self-assembly step that organizes six total building blocks, two CoTPyP units and four arene-Ru clips, into a cofacial motif previously demonstrated with free-base, Zn(II), and Ni(II) porphyrins. Turnover frequencies (TOFs) from chemical reduction (66 vs 6 h-1) and rate constants of overall homogeneous catalysis (kobs) determined from rotating ring-disk experiments (1.1 vs 0.05 h-1) establish a cofacial enhancement upon comparison of the activities of Co Prism and CoTPyP, respectively. Cyclic voltammetry was used to initially probe the electrochemical catalytic behavior. Rotating ring-disk electrode studies were completed to probe the Faradaic efficiency and obtain an estimate of the rate constant associated with the ORR.

Compartmentalization of an ABC triblock copolymer single-chain nanoparticle via coordination-driven orthogonal self-assembly

We present coordination-driven intramolecular orthogonal self-assembly of ABC triblock copolymer into protein-like compartmentalized SCNP, whose sub-10 nm ultrafine subdomains are discrete and can respond to aqueous surroundings individually.

Novel magnesium-seamed organic nanocapsules with hierarchical structural complexity.

Enhancing hierarchical structural complexities of metal-organic nanocapsules is an effective way of introducing new functions and properties. Herein, we present the synthesis of novel magnesium-seamed C-alkylpyrogallol[4]arene nanocapsules with hierarchical structural complexity via both exterior and endohedral functionalization. The exterior functionalization is realized by using a variety of covalently modified pyrogallol[4]arene ligands, while the endohedral functionalization is achieved via coordination-driven self-assembly. Especially, functionalization of nanocapsule interior by attaching a proline hexad via metal coordination will shed light to the design and synthesis of metal-organic species with heteroligated architectures.

Two novel supramolecular metallogels constructed by platinum(ii) coordination and pillar[5]arene-based host–guest interactions

Two multi-responsive supramolecular metallogels were constructed by coordination-driven self-assembly, host–guest interaction and supramolecular polymerization.

Selective synthesis of iridium(iii)-derived molecular Borromean rings, [2]catenane and ring-in-ring macrocycles via coordination-driven self-assembly.

This paper reports the formation of unprecedented iridium(iii)-derived topological macrocycles. Discrete molecular Borromean rings 1 and 3 in pure form are synthesized via coordination-driven self-assembly of an acceptor [(Cp*Ir)2(OO∩OO)](OTf)2 (Cp* = pentamethylcyclopentadienyl, OO∩OO = 6,11-dioxido-5,12-naphthacenedione) (A) with dipyridyl donors 1,4-bis(4-pyridinylethynyl)benzene (L1) and 2,5-bis(4-pyridinylethynyl)thiophene (L2) respectively in methanol. Self-assembly using the same acceptor under similar conditions with two other donors 9,10-bis(4-pyridinylethynyl)anthracene (L3) and 1,4-di(4-pyridinylethynyl)buta-1,3-diyne (L4) resulted in [2]catenane 5 and non-catenane ring-in-ring topological macrocycle 7 respectively. Rectangular macrocycles 2, 4, 6, and 8 were respectively obtained when the self-assembly of acceptor A with one of the donors L1-L4 was carried out under dilute conditions in nitromethane or methanol. All these new macrocycles were characterized by 1H and 13C NMR, 2D NMR, ESI-MS and elemental analyses. Single crystal X-ray structures of Borromean rings 1 and 3, and ring-in-ring macrocycle 7 revealed that the length and functionality of donors enabling CHπ and ππ interactions govern the topology.

Integrative self-sorting of coordination cages based on 'naked' metal ions.

Coordination-driven self-assembly of metal ions and organic ligands has been extensively utilised over the past four decades to access a variety of nano-sized cage assemblies, with functions ranging from sensing and catalysis to drug delivery. Many of the reported examples, however, are highly symmetric architectures that contain one type of organic ligand carrying not more than a single functionality. This contrasts significantly with the level of structural and functional complexity encountered in biological macromolecular hosts, which are able to bind and chemically convert smaller molecules in their highly-decorated internal cavities. To address this disparity, rational approaches that facilitate heteroleptic assembly by regulating integrative self-sorting of metal ions and multiple ligand components have emerged. Among these, routes to access coordination cages from 'naked' metal cations that offer more than two coordination sites are still in early development, as the complexity of the self-sorted products in terms of composition and stereochemistry presents an entropic challenge. This feature article highlights recent progress in controlling integrative self-sorting of multi-component cage systems with a focus on structures composed of 'naked' metal cations and two different ligands. Once heteroleptic self-assembly strategies find a wider implementation in supramolecular design, the resultant interplay between tailored combinations of precisely positioned substituents promises enhanced functionality in nanoscale structures.

Photophysical Properties of Organoplatinum(II) Compounds and Derived Self-Assembled Metallacycles and Metallacages: Fluorescence and its Applications.

Over the past couple of decades, coordination-driven self-assembly has evolved as a broad multidisciplinary domain that not only covers the syntheses of aesthetically pleasing supramolecular architectures but also emerges as a method to form new optical materials, chemical sensors, theranostic agents, and compounds with light-harvesting and emissive properties. The majority of these applications depend upon investigations that reveal the photophysical nature and electronic structure of supramolecular coordination complexes (SCCs), including two-dimensional (2D) metallacycles and three-dimensional (3D) metallacages. As such, well-defined absorption and emission spectra are important for a given SCC to be used for sensing, bioimaging, and other applications with molecular fluorescence being an important component. In this Account, we summarize the photophysical properties of some bis(phosphine)organoplatinum(II) compounds and their discrete SCCs. The platinum(II) based organometallic precursors typically display spectral red-shifts and have low fluorescence quantum yields and short fluorescence lifetimes compared to their organic counterparts because the introduction of metal centers enhances both intersystem crossing (ISC) and intramolecular charge transfer (ICT) processes, which can compete with the fluorescence emissions. Likewise ligands with conjugation can also increase the ICT process; hence the corresponding organoplatinum(II) compounds undergo a further decrease in fluorescence lifetimes. The use of endohedral amine functionalized 120°-bispyridyl ligands can dramatically enhance the emission properties of the resultant organoplatinum(II) based SCCs. As such these SCCs display emissions in the visible region (ca. 400-500 nm) and are significantly red-shifted (ca. 80-100 nm) compared to the ligands. This key feature makes them suitable as supramolecular theranostic agents wherein these unique emission properties provide diagnostic spectroscopic handles and the organoplatinum(II) centers act as potential anticancer agents. Using steady state and time-resolved-spectroscopic techniques and quantum computations in concert, we have determined that the emissive properties stem from the ligand-centered transitions involving π-type molecular orbitals with modest contributions from the metal-based orbitals. The self-assembly and the photophysics of organoplatinum(II) ← 3-substituted pyridyl based SCCs are highly diverse. Subtle changes in the ligands' structures can form molecular congener systems with distinct conformational and photophysical properties. Furthermore, the heterometallic SCCs described herein possess rich photophysical properties and can be used for sensing based applications. Tetraphenylethylene (TPE) based SCCs display emissions in the aggregated state as well as in dilute solutions. This is a unique phenomenon that bridges the aggregation caused quenching (ACQ) and aggregation induced emission (AIE) effects. Moreover, a TPE based metallacage exhibits solvatoluminescence, including white light emission in THF solvent, and can act as a fluorescence-sensor for structurally similar ester compounds.

Coordination-Driven Self-Assembly and Anticancer Potency Studies of Ruthenium-Cobalt-Based Heterometallic Rectangles.

Three new cobalt-ruthenium heterometallic molecular rectangles, 1-3, were synthesized through the coordination-driven self-assembly of a new cobalt sandwich donor, (η5 -Cp)Co[C4 -trans-Ph2 (4-Py)2 ] (L; Cp: cyclopentyl; Py: pyridine), and one of three dinuclear precursors, [(p-cymene)2 Ru2 (OO∩OO)2 Cl2 ] [OO∩OO: oxalato (A1 ), 5,8-dioxido-1,4-naphthoquinone (A2 ), or 6,11-dioxido-5,12-naphthacenedione (A3 )]. All of the self-assembled architectures were isolated in very good yield (92-94 %) and were fully characterized by spectroscopic analysis; the molecular structures of 2 and 3 were determined by single-crystal X-ray diffraction analysis. The anticancer activities of bimetallic rectangles 1-3 were evaluated with a 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay, an autophagy assay, and Western blotting. Rectangles 1-3 showed higher cytotoxicity than doxorubicin in AGS human gastric carcinoma cells. In addition, the autophagic activities and apoptotic cell death ratios were increased in AGS cells by treatment with 1-3; the rectangles induced autophagosome formation by promoting LC3-I to LC3-II conversion and apoptotic cell death by increasing caspase-3/7 activity. Our results suggest that rectangles 1-3 induce gastric cancer cell death by modulating autophagy and apoptosis and that they have potential use as agents for the treatment of human gastric cancer.

Construction of multiferrocenes end-capped metallodendrimers via coordination-driven self-assembly and their electrochemical behavior

The construction of a new family of multiferrocenes end-capped metallodendrimers H1–4 with various generations via coordination-driven self-assembly was described. The structures of these metallodendrimers H1–4 were characterized by multinuclear NMR {1H and 31P} spectroscopy, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and the PM6 semiempirical molecular orbital method. Insight into the structure and electronic properties of these supramolecules was obtained through electrochemical studies.

Microfibres and macroscopic films from the coordination-driven hierarchical self-assembly of cylindrical micelles.

Anisotropic nanoparticles prepared from block copolymers are of growing importance as building blocks for the creation of synthetic hierarchical materials. However, the assembly of these structural units is generally limited to the use of amphiphilic interactions. Here we report a simple, reversible coordination-driven hierarchical self-assembly strategy for the preparation of micron-scale fibres and macroscopic films based on monodisperse cylindrical block copolymer micelles. Coordination of Pd(0) metal centres to phosphine ligands immobilized within the soluble coronas of block copolymer micelles is found to induce intermicelle crosslinking, affording stable linear fibres comprised of micelle subunits in a staggered arrangement. The mean length of the fibres can be varied by altering the micelle concentration, reaction stoichiometry or aspect ratio of the micelle building blocks. Furthermore, the fibres aggregate on drying to form robust, self-supporting macroscopic micelle-based thin films with useful mechanical properties that are analogous to crosslinked polymer networks, but on a longer length scale.