Construction of Metallacage-Cored Supramolecular Gel by Hierarchical Self-Assembly of Metal Coordination and Pillar[5]arene-Based Host-Guest Recognition

Developing Supramolecular Metallogel Derived from Pd2L4 Cage Molecule for Delivering an Anti-Cancer Drug to Melanoma Cell B16-F10

Supramolecular gels are an important class of materials that are promising for its wide range of applications including drug delivery. While supramolecular gels are intrinsically porous because of the 3D nano-matrix (gel matrix) that is being formed due to supramolecular self-assembly process involving the gelator molecules during gelation, additional nanopores can be introduced to the overall gel if the gelator molecule itself holds molecular cavity such as metal-organic-cage (MOC) molecules. A MOC having the molecular formula [(Pd2L24).4NO3].3H2O.2DMF.MeOH (Pd-cage) (L2=5-Azido-N,N'-di-pyridin-3-yl-isophthalamide) was successfully synthesized and characterized by FT-IR, 1H NMR, ESI-MS and single crystal X-ray diffraction. Stimuli-reversible supramolecular metallogel PdG could easily be formed from Pd-cage in DMSO/water mixture. The molecular cage of Pd-cage was demonstrated to be available for loading an anti-cancer drug namely doxorubicin (DOX). Subsequently, DOX was also loaded within PdG and delivered to melanoma cell line B16-F10 displaying significant anti-cancer activity as revealed by both MTT and scratch assay. Rheoreversibility of PdG and its ability to load and deliver DOX to cancer cells clearly raised hope for developing this metallogel further as topical anticancer gel.

Pillararene-Based Supramolecular Polymers for Cancer Therapy

Supramolecular polymers have attracted considerable interest due to their intriguing features and functions. The dynamic reversibility of noncovalent interactions endows supramolecular polymers with tunable physicochemical properties, self-healing, and externally stimulated responses. Among them, pillararene-based supramolecular polymers show great potential for biomedical applications due to their fascinating host-guest interactions and easy modification. Herein, we summarize the state of the art of pillararene-based supramolecular polymers for cancer therapy and illustrate its developmental trend and future perspective.

Controllable gelation of coordination nanocages from the physical interactions among surface grafted cholesteryl groups

Coordination nanocage (CNC) incorporated gels have attracted enormous attention for the effective integration of micro-porosity, mechanical flexibility and processability; however, the understanding of their microscopic structure-property relationships remains unclear. Herein, CNCs with 24 surface grafted cholesterol groups are constructed precisely and their gelation can be manipulated upon the tunning of solvent polarities. Optically homogeneous organogels can be formed by introducing a certain amount of bad solvents into the solutions of hairy CNCs and the gelation can be reversed through temperature variation. Suggested from scattering and molecular dynamics studies, the solvophobic interaction-driven aggregation of cholesterol units contributes to the physical crosslinking of CNCs and finally the gelation of CNC solutions. The mechanical strength of the obtained gels is observed to be highly dependent on the flexibility of the organic linkers that bond the cholesterol units on the CNC surface. The effective interaction and dense packing of the cholesterol units in their aggregates highly rely on the degree of freedom of the cholesterol, which is controlled by the flexibility of the organic linkers that bond them on the CNC surface. The observed viscoelastic performance accompanied by the well-controlled mechanical strength of the organogels unambiguously demonstrates the potential for exploiting the synergistic physical correlations to fabricate novel functional materials from CNCs.

Metal-ligand Interactions and Oligo(p-Phenylene Vinylene) Derivatives Based Supramolecular Polymer Possessing Variable Fluorescence Colors

Fluorescent supramolecular polymers combine the benefits of supramolecular polymers in terms of dynamic nature with the optoelectronic features of incorporated fluorophores. However, the majority of fluorescent supramolecular polymers can only exhibit a single fluorescent state, restricting their applications. Incorporating J-type dyes into supramolecular monomers is expected to impart supramolecular polymers with variable fluorescence colors, because the aggregation mode of J-type dyes is closely related to the formation of supramolecular polymers. Herein, we report a supramolecular polymer [M1·Zn(OTf)₂ ]_n , in which the monomer M1 contains a J-type dye, oligo(p-phenylene vinylene) (OPV) derivative, and two terpyridine ends. The M1 + Zn(OTf)₂ solutions exhibit fluorescence color changes varying from cyan to yellow-green in the monomer concentration ranging from 0.04 to 1.00 mM. Moreover, based on the outputs from laser scanning confocal microscopy (LSCM), the fluorescence color transition during the formation of supramolecular polymers is intuitively proven. Additionally, considering the close relationship between the supramolecular polymer structure and the fluorescence color, the fluorescence color can be regulated by introducing tetraethylammonium hydroxide (TBAOH) that can bind with Zn²⁺ competitively to break up the structure of the supramolecular polymer. This article is protected by copyright. All rights reserved.

Cross-Linked Supramolecular Polymer Networks Constructed by Pillar[5]arene-Based Host–Guest Recognition and Coordination/Oxidation of Catechol

In this work, two cross-linked supramolecular polymers are prepared by pillar[5]arene-based molecular recognition and coordination/oxidation of catechol. In addition, two supramolecular glues are obtained at high concentrations of the cross-linked...

Covalently bridged pillararene-based oligomers: from construction to applications

Covalently bridged pillararene-based oligomers (CBPOs) are formed by covalent bonding of pillararene monomers, and they play a critical role in expanding the multi-disciplinary application of pillararenes due to their excellent molecular complexing ability, specially designed geometry and multifunctional linking groups. This article provides a comprehensive review of the synthesis and applications of CBPOs. The design and synthetic strategies of a series of CBPOs (dimers, trimers, tetramers and others) are first introduced. Many CBPOs with multi-cavities and unique geometry are very attractive and efficient building blocks for constructing novel smart supramolecular polymers (SPs) with different topological structures through host-guest interactions. We describe the methods of constructing various SPs based on CBPOs in detail. Furthermore, the extensive applications of CBPOs and CBPO-based SPs in recognition and detection of ions and organic small molecules, selective adsorption and separation, artificial light-harvesting systems, catalysis, drug delivery systems, and others are systematically introduced. Finally, the future challenges and perspectives for CBPOs are also highlighted.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

A Stretchable Pillararene-Containing Supramolecular Polymeric Material with Self-Healing Property

Constructing polymeric materials with stretchable and self-healing properties arise increasing interest in the field of tissue engineering, wearable electronics and soft actuators. Herein, a new type of supramolecular cross-linker was constructed through host-guest interaction between pillar[5]arene functionalized acrylate and pyridinium functionalized acrylate, which could form supramolecular polymeric material via photo-polymerization of n-butyl acrylate (BA). Such material exhibited excellent tensile properties, with maximum tensile strength of 3.4 MPa and strain of 3000%, respectively. Moreover, this material can effectively dissipate energy with the energy absorption efficiency of 93%, which could be applied in the field of energy absorbing materials. In addition, the material showed self-healing property after cut and responded to competitive guest.

Spatiotemporal Control of Supramolecular Polymerization and Gelation of Metal-Organic Polyhedra

In coordination-based supramolecular materials such as metallogels, simultaneous temporal and spatial control of their assembly remains challenging. Here, we demonstrate that the combination of light with acids as stimuli allows for the spatiotemporal control over the architectures, mechanical properties, and shape of porous soft materials based on metal-organic polyhedra (MOPs). First, we show that the formation of a colloidal gel network from a preformed kinetically trapped MOP solution can be triggered upon addition of trifluoroacetic acid (TFA) and that acid concentration determines the reaction kinetics. As determined by time-resolved dynamic light scattering, UV-vis absorption, and ¹H NMR spectroscopies and rheology measurements, the consequences of the increase in acid concentration are (i) an increase in the cross-linking between MOPs; (ii) a growth in the size of the colloidal particles forming the gel network; (iii) an increase in the density of the colloidal network; and (iv) a decrease in the ductility and stiffness of the resulting gel. We then demonstrate that irradiation of a dispersed photoacid generator, pyranine, allows the spatiotemporal control of the gel formation by locally triggering the self-assembly process. Using this methodology, we show that the gel can be patterned into a desired shape. Such precise positioning of the assembled structures, combined with the stable and permanent porosity of MOPs, could allow their integration into devices for applications such as sensing, separation, catalysis, or drug release.

Nanosheets and Hydrogels Formed by 2 nm Metal-Organic Cages with Electrostatic Interaction

We report the mechanism of hydrogel formation in dilute aqueous solutions (>15 mg/mL) by 2 nm metal-organic cages (MOCs). Experiments and all-atom simulations confirm that with the addition of small electrolytes, the MOCs self-assemble into 2D nanosheets via counterion-mediated attraction because of their unique molecular structure and charge distribution as well as σ-π interactions. The stiff nanosheets are difficult to bend into 3-D hollow, spherical blackberry type structures, as observed in many other macroion systems. Instead, they stay in solution and their very large excluded volumes lead to gelation at low (∼1.5 wt %) MOC concentrations, with additional help from hydrophobic and partial π-π interactions similar to the gelation of graphene oxides.

Guest Binding Drives Host Redistribution in Libraries of CoII 4 L4 Cages

Two CoII 4 L4 tetrahedral cages prepared from similar building blocks showed contrasting host-guest properties. One cage did not bind guests, whereas the second encapsulated a series of anions, due to electronic and geometric effects. When the building blocks of both cages were present during self-assembly, a library of five CoII LA x LB 4-x cages was formed in a statistical ratio in the absence of guests. Upon incorporation of anions able to interact preferentially with some library members, the products obtained were redistributed in favor of the best anion binders. To quantify the magnitudes of these templation effects, ESI-MS was used to gauge the effect of each template upon library redistribution.

Pillararenes Trimer for Self-Assembly

Pillararenes trimer with particularly designed structural geometry and excellent capacity of recognizing guest molecules is a very efficient and attractive building block for the fabrication of advanced self-assembled materials. Pillararenes trimers could be prepared via both covalent and noncovalent bonds. The classic organic synthesis reactions such as click reaction, palladium-catalyzed coupling reaction, amidation, esterification, and aminolysis are employed to build covalent bonds and integrate three pieces of pillararenes subunits together into the “star-shaped” trimers and linear foldamers. Alternatively, pillararenes trimers could also be assembled in the form of host-guest inclusions and mechanically interlocked molecules via noncovalent interactions, and during those procedures, pillararenes units contribute the cavity for recognizing guest molecules and act as a “wheel” subunit, respectively. By fully utilizing the driving forces such as host-guest interactions, charge transfer, hydrophobic, hydrogen bonding, and C–H^…π and π–π stacking interactions, pillararenes trimers-based supramolecular self-assemblies provide a possibility in the construction of multi-dimensional materials such as vesicular and tubular aggregates, layered networks, as well as frameworks. Interestingly, those assembled materials exhibit interesting external stimuli responsiveness to e.g., variable concentrations, changed pH values, different temperature, as well as the addition/removal of competition guests and ions. Thus, they could further be used for diverse applications such as detection, sorption, and separation of significant multi-analytes including metal cations, anions, and amino acids.

Functional supramolecular gels based on pillar[n]arene macrocycles

Supramolecular gels constructed from low-molecular-weight gelators via noncovalent interactions have received increasing attention. The rapid development of stimuli-responsive supramolecular gels with attractive properties is highly desirable to meet the ever-growing demand of materials science and chemistry. The inherent reversible and dynamic nature of noncovalent interactions in supramolecular gels endows the materials with sensing, processing, and actuating functions in response to specific environmental changes and offers them great potential in flexible biomaterials and intelligent devices. In particular, pillar[n]arenes with symmetrical pillar-shaped architectures have been recognized as an emerging class of synthetic macrocycles after crown ethers, cyclodextrins, calixarenes, and cucurbiturils, and proven to be excellent candidates for the fabrication of functional supramolecular gels due to their many advantages including facile synthesis, diverse functionalization, and appealing host-guest properties. This review provides a comprehensive overview of recent progress in supramolecular gels involving pillar[n]arenes and their derivatives as synthetic macrocyclic arenes, from the viewpoints of the synthetic approach, controllable assembly, stimuli-responsiveness, and functions. Perspectives of this burgeoning field of research are also given at the end.

Solvent-controlled assembly of pillar[5]arene-based supramolecular networks via π–π interactions for white light modulation

A supramolecular network based on pyrene-containing pillar[5]arene and a red emissive Eu(iii) complex was constructed, whose assembly and emission can be controlled by solvent polarity, eventually achieving white light emission.

Photo-regulated supramolecular star with a pillar[6]arene-coated metal–organic polyhedron (MOP) core

We report a photo-regulated supramolecular star centered by a pillar[6]arene-coated metal–organic polyhedron (MOP) core.

Weak Links To Differentiate Weak Bonds: Size-Selective Response of π-Conjugated Macrocycle Gels to Ammonium Ions

Molecular-level host-guest interactions can drive gel-to-sol phase transitions of the bulk material. Using supramolecular gels constructed from π-conjugated aza-crown macrocycles, we have investigated the effects of guest chemical structures on the kinetics of gel disassembly. While ammonium ions bind only weakly to the individual macrocycles in solution, gel-to-sol transitions of self-assembled macrocycles occur readily under ambient conditions. This net signal amplification process was monitored conveniently by time-dependent spectroscopic studies to reveal a straightforward correlation between the response rate and shape/size of the guest species. Well-designed weak links thus respond to subtle differences in weak bonds and translate them into visually discernible macroscopic signaling events.

Soft Materials with Diverse Suprastructures via the Self-Assembly of Metal-Organic Complexes

Inspired by assemblies in the natural world, researchers have prepared diverse suprastructures with distinct spatial arrangements by artificial self-assembly, including micelles, vesicles, ribbons, films, fibers, and tubes. The field of assembly is undergoing a transition from single-component to multicomponent assembly and single-step to multistep processing. Control over the size, shape, and composition of these building blocks has enabled the formation of suprastructures with substantial structural diversity. More importantly, harnessing noncovalent interactions to create suprastructures in a controlled manner will lead to a better understanding of the formation of complex self-organized patterns. However, for the construction of multiscale self-assemblies with controllable shapes and functions, the selection of a suitable protocol remains challenging. Coordination-driven self-assembly provides a bottom-up approach to construct various metal-organic complexes (MOCs), which could be further used as building blocks with controllable shapes and sizes. Despite the tremendous progress made in the design of MOC-based supramolecular materials, most of these MOCs have dimensions of only several nanometers, and investigations of these structures rely on the characterization of their crystal structure. However, most of the functional suprastructures in living organisms have dimensions ranging from microns to centimeters and have the form of soft materials. Thus, obtaining MOC-based highly ordered materials of larger size remains a challenge. This Account focuses on our recent advances in the construction of soft suprastructure materials with MOCs. A series of functionalized MOCs was first constructed through coordination-driven self-assembly. Then, further self-assembly of the as-prepared MOCs gave rise to the formation of higher-order structures. By changing the functional groups in the acceptors and donors in the MOCs, different suprastructures, including nanospheres, nanodiamonds, nanorods, nanofibers, membranes, films, and gels, were prepared. These studies suggest that using MOCs as building blocks is a highly efficient strategy to achieve complex architectures and functional materials for the development of desired MOC-based soft materials with high precision and fidelity.

Multicomponent Porphyrin-Based Tetragonal Prismatic Metallacages and their Photophysical Properties

Multicomponent coordination-driven self-assembly has proved to be a convenient approach to prepare advanced supramolecular coordination complexes (SCCs), especially for those with three-dimensional structures. Herein, we report the preparation of three tetragonal prismatic cages via the self-assembly of Pt(PEt₃)₂(OTf)₂, three different linear dipyridyl ligands and porphyrin-based sodium benzoate ligands. Due to the efficient charge separation in the coordination process of Pt(PEt₃)₂(OTf)₂ with pyridine and carboxylic acid and the directionality of metal-coordination bonds, these cages were prepared in high isolated yields (more than 90%). The absorption and emission properties as well as the singlet oxygen quantum yields of these cages were also studied, showing their potential applications as contrast agents for bio-imaging and photosensitizers for photodynamic therapy.

Diffusion NMR for the characterization, in solution, of supramolecular systems based on calixarenes, resorcinarenes, and other macrocyclic arenes

The use of diffusion NMR in studying calixarenes and other arene-based supramolecular systems is described, emphasizing the pivotal role played by the calixarene community in transforming the methods into a routine tool used in supramolecular chemistry.

Recent developments in the construction of metallacycle/metallacage-cored supramolecular polymers via hierarchical self-assembly

Supramolecular polymers have received considerable attention during the last few decades due to their scientific value in polymer chemistry and profound implications for future developments of advanced materials. Discrete supramolecular coordination complexes (SCCs) with well-defined size, shape, and geometry have been widely employed to construct hierarchical systems by coordination-driven self-assembly with the spontaneous formation of metal-ligand bonds, which results in the formation of well-defined two-dimensional (2D) metallacycles or three-dimensional (3D) metallacages with high functionalities. The incorporation of discrete SCCs into supramolecular polymers by the orthogonal combination of metal-ligand coordination and other noncovalent interactions or covalent bonding could further facilitate the construction of novel supramolecular polymers with hierarchical architectures and multiple functions including controllable uptake and release of guest molecules, providing a flexible platform for the development of smart materials. In this review, the recent progress in metallacycle/metallacage-cored supramolecular polymers that were constructed by the combination of metal-ligand interactions and other orthogonal interactions (including hydrophobic or hydrophilic interactions, hydrogen bonding, van der Waals forces, π-π stacking, electrostatic interactions, host-guest interactions and covalent bonding) has been discussed. In addition, the potential applications of metallacycle/metallacage-cored supramolecular polymers in the areas of light emitting, sensing, bio-imaging, delivery and release, etc., are also presented.

A [2]pseudorotaxane based on a pillar[6]arene and its application in the construction of a metallosupramolecular polymer

A novel metallosupramolecular polypseudorotaxane was constructed by pillar[6]arene-based host–guest recognition and metal coordination.