Inside Back Cover: Fabrication of Neutral Supramolecular Polymeric Films via Post‐electropolymerization of Discrete Metallacycles (Chin. J. Chem. 2/2018)

The inside back cover picture shows the construction of neutral supramolecular polymeric films containing well‐defined metallacycles as the main scaffolds through combination of coordination‐driven self‐assembly with post‐electropolymerization. A new 120° triphenylamine substituted dicarboxylate donor ligand and the complementary 120° triphenylamine functionalized di‐Pt(II) acceptor were employed to build neutral multi‐triphenylamine functionalized 2‐D metallacycles with the well‐defined shape and size via the formation of oxygen‐to‐platinum coordination bonds. Subsequent post‐electropolymerization of the obtained neutral multi‐triphenylamine containing metallacycles allowed for fabrication of a new type of neutral polymeric film with well‐controlled cavity sizes and thickness, which may have potential applications in neutral molecule detection, separation, and capture. More details are discussed in the article by Yang et al. on page 134–138.

     

Hierarchical Self‐Assembly of Supramolecular Hydrophobic Metallacycles into Ordered Nanostructures

We describe herein the hierarchical self‐assembly of discrete supramolecular metallacycles into ordered fibers or spherical particles through multiple noncovalent interactions. A new series of well‐defined metallacycles decorated with long alkyl chains were obtained through metal–ligand interactions, which were capable of aggregating into ordered fibroid or spherical nanostructures on the surface, mostly driven by hydrophobic interactions. In‐depth studies indicated that the morphology diversity was originated from the structural information encoded in the metallacycles, including the number of alkyl chains and their spatial orientation. Interestingly, the morphology of the metallacycle aggregates could be tuned by changing the solvent polarity. These findings are of special significance since they provide a simple yet highly controllable approach to prepare ordered and tunable nanostructures from small building blocks by means of hierarchical self‐assembly.     

Construction of Highly Emissive Pt(II) Metallacycles upon Irradiation

Photoswitchable or photoactivatable fluorescent species have been found wide applications within supramolecular chemistry and materials science. In this study, we successfully constructed two highly emissive Pt(II) metallacycles from the diarylethene ligands via coordination‐driven self‐assembly. Different from the most known fluorescent metallacycles, the obtained metallacycles have displayed “turn‐on” fluorescence switching. They are non‐fluorescent in solution, but they emit highly yellow or orange fluorescence under ultraviolet irradiation. The metallacycles were well characterized by ¹H NMR, ³¹P NMR and ESI‐TOF‐MS. The photochromic properties of the resultant metallacycles were investigated by ¹H NMR, ³¹P NMR, UV/Vis spectrum and fluorescence spectrum. Notably, NMR studies revealed that these two metallacycles featured excellent cyclization efficiency (90% conversion efficiency). Moreover, the closed‐ring isomers of the metallacycles displayed relatively high quantum yield (Φ_F = 0.5). DFT simulations demonstrated that the antiparallel configuration of the diarylethene ligand had an angle closed to 120°, which was more stable in energy compared to the parallel configuration, thus allowing for the facile construction of highly emissive metallacycles. We believe that such highly emissive metallacycles which are in‐situ prepared upon irradiation can be used as new fluorescence materials for sensing and bioimaging in the future.     

A Rapidly Self‐Healing Supramolecular Polymer Hydrogel with Photostimulated Room‐Temperature Phosphorescence Responsiveness

Development of self‐healing and photostimulated luminescent supramolecular polymeric materials is important for artificial soft materials. A supramolecular polymeric hydrogel is reported based on the host–guest recognition between a β‐cyclodextrin (β‐CD) host polymer (poly‐β‐CD) and an α‐bromonaphthalene (α‐BrNp) polymer (poly‐BrNp) without any additional gelator, which can self‐heal within only about one minute under ambient atmosphere without any additive. This supramolecular polymer system can be excited to engender room‐temperature phosphorescence (RTP) signals based on the fact that the inclusion of β‐CD macrocycle with α‐BrNp moiety is able to induce RTP emission (CD‐RTP). The RTP signal can be adjusted reversibly by competitive complexation of β‐CD with azobenzene moiety under specific irradiation by introducing another azobenzene guest polymer (poly‐Azo).     

Hierarchical assembly of uranyl metallacycles involving macrocyclic hosts

Actinide metallacycles are an emerging class of functional coordination assemblies, but multi-level assembly from metallacycle units toward hierarchical supramolecular structures are still rarely investigated. In this work, we put forward a novel supramolecular inclusion-based method through introducing two macrocyclic hosts, cucurbit[7]uril (CB[7]) and cucurbit[8]uril (CB[8]) to facilitate hierarchical assembly of uranyl metallacycles with higher complexity, and successfully prepare two different kinds of uranyl metallacycle-based complexes with intriguing hierarchical structures, a CB[7]-based four-member molecular necklace ([4]MN) and a CB[8]-involved ring-in-ring supramolecular polymer chain. The results obtained here prove the feasibility of supramolecular inclusion for regulating coordination assembly of uranyl metallacycles and related hierarchical structures. It is believed that this method can be used to achieve the construction of actinide coordination assemblies with higher structural complexity.     

Discrete Stimuli‐Responsive Multirotaxanes with Supramolecular Cores Constructed through a Modular Approach

The synthesis of discrete multirotaxanes with well‐defined structures remains a great challenge. Herein, we present the successful construction of diverse discrete multirotaxanes with well‐defined supramolecular metallacycles as cores by a modular approach. Moreover, these novel multirotaxanes featured a stimuli‐responsive property that enabled the introduction and removal of the bromide anion by taking advantage of dynamic nature of the supramolecular metallacycle scaffold. Through the combination of rotaxane‐containing prefunctionalized building blocks with the corresponding different organoplatinum(II) acceptor building blocks (60, 120, or 180°), diverse discrete multirotaxanes with well‐defined metallacycles (rhomboid or hexagon) as cores as well as certain numbers of rotaxane units were successfully obtained quantitatively by means of coordination‐driven self‐assembly. Furthermore, owing to the existence of a dynamic metallacycle as the supramolecular cores, the resultant multirotaxanes showed anion‐induced disassembly and reassembly properties, which allowed for the reversible transformation between multirotaxanes and the corresponding individual rotaxane‐containing building blocks. Therefore, this research not only enriches the family of discrete multirotaxanes, but also provides a novel strategy for the construction of “smart” stimuli‐responsive multirotaxane systems.     

Facile Construction of Structurally Defined Porous Membranes from Supramolecular Hexakistriphenylamine Metallacycles through Electropolymerization

The construction of well‐controlled porous materials is very challenging. Herein, we report the successful preparation of structurally defined porous membranes based on hexakistriphenylamine metallacycles through electropolymerization. The newly designed porous materials were characterized by the typical cyclic voltammograms, XPS, SEM, and TEM investigations. Further investigations revealed that the metallacycle‐based polymer films displayed a good size‐selective molecular‐sieving behavior.     

Polymer science with transition metals and main group elements: Towards functional,supramolecular inorganic polymeric materials

Inorganic polymers are relatively unexplored because the efficient formation of macromolecular chains from atoms of transition metals and main group elements has presented a synthetic challenge. Nevertheless, these materials offer exciting opportunities for accessing properties that are significantly different from and which therefore complement those available with the well‐established organic systems. Inorganic block copolymers are of particular interest for the generation of functional, nanoscale supramolecular architectures and hierarchical assemblies using self‐assembly processes. This article focuses on research in my group over the past decade, which has targeted the development of new and controlled routes to inorganic polymers and their subsequent use in forming supramolecular materials as well as studies of their properties and applications. The use of ring‐opening polymerization (ROP) and transition‐metal‐catalyzed polycondensation approaches are illustrated. Controlled ROP procedures have been developed that allow access to polyferrocene block copolymers that self‐assemble into interesting nanoscopic architectures such as cylinders and superstructures such as flowers. The future prospects for inorganic polymer science are discussed, and a growing emphasis on the study of supramolecular inorganic polymeric materials is predicted. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 179–191, 2002     

Supramolecular Alternating Polymer from Crown Ether and Pillar[5]arene‐Based Double Molecular Recognition for Preparation of Hierarchical Materials

A novel supramolecular alternating polymer is constructed based on double molecular recognition events of benzo‐21‐crown‐7 with a secondary ammonium salt and of pillar[5]arene with a neutral guest. The resulting polymer is utilized to prepare hierarchical materials with different dimensionalities for the first time. These materials included zero‐dimensional spherical aggregates, one‐dimensional nanofibers, two‐dimensional microstructured films, and three‐dimensional ordered glue. This development will be helpful for designing and preparing supramolecular hierarchical materials with different dimensionalities.     

Supramolecular Interfacial Polymerization: A Controllable Method of Fabricating Supramolecular Polymeric Materials

A new method of supramolecular polymerization at the water–oil interface is developed. As a demonstration, an oil‐soluble supramonomer containing two thiol end groups linked by two ureidopyrimidinone units and a water‐soluble monomer bearing two maleimide end groups are employed. Supramolecular interfacial polymerization can be implemented by a thiol–maleimide click reaction at the water–chloroform interface to obtain supramolecular polymeric films. The glass transition temperature of such supramolecular polymers can be well‐tuned by simply changing the polymerization time and temperature. It is highly anticipated that this work will provide a facile and general approach to realize control over supramolecular polymerization by transferring the preparation of supramolecular polymers from solutions to water–oil interfaces and construct supramolecular materials with well‐defined properties.     

Supramolecular Control of Photocycloadditions in Solution: In Situ Stereoselective Synthesis and Release of Cyclobutanes

A new class of supramolecular metallacycles capable of undergoing photochemical reactions and in situ release of cyclobutanes in solution is described. The molecular metallacycles were generated through coordination‐driven self‐assembly of dinuclear metal‐carbene complexes as organometallic clips with olefin‐functionalized bridging ligands. Photolysis of these molecular metallacycles in situ led to structural interconversion and release of the formed cyclobutane products with quantitative conversion. Further modifications of the obtained cyclobutanes provided a series of new species containing the cyclobutane skeleton.     

Renal Epithelial Monolayer Formation on Monomeric and Polymeric Catechol Functionalized Supramolecular Biomaterials

Induction of a functional, tight monolayer of renal epithelial cells on a synthetic membrane to be applied in a bioartificial kidney device requires for bio‐activation of the membrane. The current golden standard in bio‐activation is the combination of a random polymeric catechol (L‐DOPA) coating and collagen type IV (Col IV). Here the possibility of replacing this with defined monomeric catechol functionalization on a biomaterial surface using supramolecular ureido‐pyrimidinone (UPy)‐moieties is investigated. Monomeric catechols modified with a UPy‐unit are successfully incorporated and presented in supramolecular UPy‐polymer films and membranes. Unfortunately, these UPy‐catechols are unable to improve epithelial cell monolayer formation over time, solely or in combination with Col IV. L‐DOPA combined with Col IV is able to induce a tight monolayer capable of transport on electrospun supramolecular UPy‐membranes. This study shows that a random polymeric catechol coating cannot be simply mimicked by defined monomeric catechols as supramolecular additives. There is still a long way to go in order to synthetically mimic simple natural structures.     

Luminescent Metallacycle‐Cored Liquid Crystals Induced by Metal Coordination

Two rhomboidal metallacycles based on metal‐coordination‐driven self‐assembly are presented. Because metal‐coordination interactions restrict the rotation of phenyl groups on tetraphenylethene units, these metallacycles were emissive both in solution and in solid state, and their aggregation‐induced emission properties were well‐retained. Moreover, the rhomboidal metallacyclic structures offer a platform for intermolecular packing beneficial for the formation of liquid crystalline phases. Therefore, although neither of building blocks shows mesogenic properties, both thermotropic and lyotropic (in DMF) mesophases were observed in one of metallacycles, indicating that mesophases could be induced by metal‐coordination interactions. This study not only reveals the mechanism for the formation of cavity‐cored liquid crystals, but also provides a convenient approach to preparing supramolecular luminescent liquid crystals, which will serve as good candidates for chemo sensors and liquid crystal displays.     

Hierarchical Supramolecular Spinning of Nanofibers in a Microfluidic Channel: Tuning Nanostructures at a Dynamic Interface

One of the fundamental problems in supramolecular chemistry, as well as in material sciences, is how to control the self‐assembly of polymers on the nanometer scale and how to spontaneously organize them towards the macroscopic scale. To overcome this problem, inspired by the self‐assembly systems in nature, which feature the dynamically controlled self‐assembly of biopolymers, we have previously proposed a self‐assembly system that uses a dynamic liquid/liquid interface with dimensions in the micrometer regime, thereby allowing polymers to self‐assemble under precisely controlled nonequilibrium conditions. Herein, we further extend this system to the creation of hierarchical self‐assembled architectures of polysaccharides. A natural polysaccharide, β‐1,3‐glucan (SPG), and water were injected into opposite “legs” of microfluidic devices that had a Y‐shape junction, so that two solvents would gradually mix in the down stem, thereby causing SPG to spontaneously self‐assemble along the flow in a head‐to‐tail fashion, mainly through hydrophobic interactions. In the initial stage, several SPG nanofibers would self‐assemble at the Y‐junction owing to the shearing force, thereby creating oligomers with a three‐way junction point. This unique structure, which could not be created through conventional mixing procedures, has a divergent self‐assembly capability. The dynamic flow allows the oligomers to interact continuously with SPG nanofibers that are fed into the Y‐junction, thus amplifying the nanostructure along the flow to form SPG networks. Consequently, we were able to create stable, centimeter‐length macroscopic polysaccharide strands under the selected flow conditions, which implies that SPG nanofibers were assembled hierarchically in a supramolecular fashion in the dynamic flow. Microscopic observations, including SEM and AFM analysis, revealed the existence of clear hierarchical structures inside the obtained strand. The network structures self‐assembled to form sub‐micrometer‐sized fibers. The long fibers further entangled with each other to give stable micrometer‐sized fibers, which finally assembled to form the macroscopic strands, in which the final stabilities in the macroscopic regime were governed by that of the network structures in the nanometer regime. Thus, we have exploited this new supramolecular system to create hierarchical polymeric architectures under precisely controlled flow conditions, by combining the conventional supramolecular strategy with microfluidic science.     

Routes to Hydrogen Bonding Chain‐End Functionalized Polymers

The contribution of supramolecular chemistry to polymer science opens new perspectives for the design of polymer materials exhibiting valuable properties and easier processability due to the dynamic nature of non‐covalent interactions. Hydrogen bonding polymers can be used as supramolecular units for yielding larger assemblies that possess attractive features, arising from the combination of polymer properties and the responsiveness of hydrogen bonds. The post‐polymerization modification of reactive end‐groups is the most common procedure for generating such polymers. Examples of polymerizations mediated by hydrogen bonding‐functionalized precursors have also recently been reported. This contribution reviews the current synthetic routes toward hydrogen bonding sticker chain‐end functionalized polymers.     

Self‐Healing,Expansion–Contraction,and Shape‐Memory Properties of a Preorganized Supramolecular Hydrogel through Host–Guest Interactions

Supramolecular materials cross‐linked between polymer chains by noncovalent bonds have the potential to provide dynamic functions that are not produced by covalently cross‐linked polymeric materials. We focused on the formation of supramolecular polymeric materials through host–guest interactions: a powerful method for the creation of nonconventional materials. We employed two different kinds of host–guest inclusion complexes of β‐cyclodextrin (βCD) with adamantane (Ad) and ferrocene (Fc) to bind polymers together to form a supramolecular hydrogel (βCD‐Ad‐Fc gel). The βCD‐Ad‐Fc gel showed self‐healing ability when damaged and responded to redox stimuli by expansion or contraction. Moreover, the βCD‐Ad‐Fc gel showed a redox‐responsive shape‐morphing effect. We thus succeeded in deriving three functions from the introduction of two kinds of functional units into a supramolecular material.     

Construction of Alkynylplatinum(II) Bzimpy‐Functionalized Metallacycles and Their Hierarchical Self‐Assembly Behavior in Solution Promoted by Pt⋅⋅⋅Pt and π–π Interactions

A family of new alkynylplatinum(II) 2,6‐bis(benzimidazol‐2′‐yl)pyridine (bzimpy)‐functionalized supramolecular metallacycles with different shapes and sizes have been successfully prepared by coordination‐driven self‐assembly. The obtained metallacycles showed switchable emission and a strong tendency to form intermolecular Pt???Pt and π–π stacking interactions in solution that were not displayed by their individual precursors. Further investigation revealed that the existence of the metallacyclic scaffold at the core could facilitate the formation of intermolecular Pt???Pt and π–π stacking interactions of peripheral alkynylplatinum(II) bzimpy units. Moreover, the shapes and sizes of the metallacyclic scaffold have a significant influence on the hierarchical self‐assembly behavior. Among the three metallacycles, hexagonal metallacycle A , with a relatively small size, could spontaneously self‐assemble into an aromatic guest stimuli‐responsive metallogel at room temperature without a heating–cooling process.     

Supramolecular Proteoglycan Aggregate Mimics: Cyclodextrin‐Assisted Biodegradable Polymer Assemblies for Electrostatic‐Driven Drug Delivery

Self‐assembled, noncovalent polymeric biodegradable materials mimicking proteoglycan aggregates were synthesized from inclusion complexes of cationic surfactants with γ‐cyclodextrin and the natural anionic polymer hyaluronan. The amorphous structure of this ternary system was proven by X‐ray diffraction and thermal analysis. Light‐scattering measurements showed that there was a competition between hyaluronic acid and the surfactant for the cyclodextrin cavity. These self‐assembled supramolecular matrices were loaded with both hydrophilic and lipophilic drug substances for dissolution studies. The release of the entrapped drugs was found to be controlled by cations in the surrounding media and by biodegradation. Slow drug release in an ion‐free medium became faster in physiological salt solution in which the macroscopic polymer matrix was disassembled. In contrast, the enzymatic degradation of hyaluronan was hindered in the polymeric matrix. The supramolecular systems consisting of γ‐cyclodextrin as a macrocyclic host, a cationic surfactant guest, and hyaluronic acid as the anionic polymer electrostatically cross‐linked by the inclusion complex of the first two was found to be a novel drug‐delivery system for the controlled release of traditional drugs such as curcumin and ketotifen and proteins such as bovine serum albumin.     

Building nanostructures using RAFT polymerization

Reversible addition fragmentation chain transfer (RAFT) polymerization is one of the most extensively studied controlled/living radical polymerization methods that has been used to prepare well‐defined nanostructured polymeric materials. This review, with more 650 references illustrates the range of well‐defined functional nanomaterials that can be accessed using RAFT chemistry. The detailed syntheses of macromolecules with predetermined molecular weights, designed molecular weight distributions, controlled topology, composition and functionality are presented. RAFT polymerization has been exploited to prepare complex molecular architectures, such as stars, blocks and gradient copolymers. The self‐assembly of RAFT‐polymer architectures has yielded complex nanomaterials or in combination with other nanostructures has generated hybrid multifunctional nanomaterials, such as polymer‐functionalized nanotubes, graphenes, and inorganic nanoparticles. Finally nanostructured surfaces have been described using the self‐organization of polymer films or by the utilization of polymer brushes. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Self‐Healing,Expansion–Contraction,and Shape‐Memory Properties of a Preorganized Supramolecular Hydrogel through Host–Guest Interactions

Supramolecular materials cross‐linked between polymer chains by noncovalent bonds have the potential to provide dynamic functions that are not produced by covalently cross‐linked polymeric materials. We focused on the formation of supramolecular polymeric materials through host–guest interactions: a powerful method for the creation of nonconventional materials. We employed two different kinds of host–guest inclusion complexes of β‐cyclodextrin (βCD) with adamantane (Ad) and ferrocene (Fc) to bind polymers together to form a supramolecular hydrogel (βCD‐Ad‐Fc gel). The βCD‐Ad‐Fc gel showed self‐healing ability when damaged and responded to redox stimuli by expansion or contraction. Moreover, the βCD‐Ad‐Fc gel showed a redox‐responsive shape‐morphing effect. We thus succeeded in deriving three functions from the introduction of two kinds of functional units into a supramolecular material.