The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Non-Covalent Dimer as Donor Chromophore for Constructing Artificial Light-Harvesting System in Water

Dynamic emissive materials in aqueous media have received much attention owing to their ease of preparation, tunable luminescence and environmental friendliness. However, hydrophobic fluorophores usually suffer from aggregation-caused quenching in water. In this work, we constructed an artificial light-harvesting system by using a non-covalent aggregation-induced emission dimer as antenna and energy donor. The dimer is quadruple hydrogen bonded from a ureidopyrimidinone derivative (M) containing a tetraphenylethylene group. The dispersed nano-assemblies based on the dimer in aqueous media were fabricated with the help of surfactant. By loading a hydrophobic acceptor molecule DBT into the nano-assemblies, man-made light-harvesting nanoparticles were fabricated, showing considerable energy transfer efficiency and a relatively high antenna effect. Additionally, the fluorescence color of the system can be gradually tuned by varying the content of the acceptors. This study provides a general way for the construction of an aqueous light-harvesting system based on a supramolecular dimer, which is important for potential application in luminescent materials.

A Review on Synthesis Methods of Phyllosilicate- and Graphene-Filled Composite Hydrogels

This review discusses, in brief, the various synthetic methods of two widely-used nanofillers; phyllosilicate and graphene. Both are 2D fillers introduced into hydrogel matrices to achieve mechanical robustness and water uptake behavior. Both the fillers are inserted by physical and chemical gelation methods where most of the chemical gelation, i.e., covalent approaches, results in better physical properties compared to their physical gels. Physical gels occur due to supramolecular assembly, van der Waals interactions, electrostatic interactions, hydrophobic associations, and H-bonding. For chemical gelation, in situ radical triggered gelation mostly occurs.

A calix[4]pyrrole-based linear supramolecular polymer constructed by orthogonal self-assembly

A calix[4]pyrrole having a ureidopyrimidinone unit was successfully synthesized from its alcohol-functionalized congener and further used for the first time to obtain a thermo- and chemical-responsive linear supramolecular polymer via orthogonal self-assembly comprising a combination of quadruple hydrogen bonding and anion recognition.

Synthesis, structure elucidation and functionalization of sulfonamide [2]catenanes

A pyrene-functionalized [2]catenane with switchable optical output was constructed through a novel sulfonamide [2]catenane synthesized by a self-templation approach.

Coordination-Driven Poly[2]Pseudorotaxanes in Highly Polar Organic Solvent

Self-assembly of polypseudorotaxanes in high-polar organic solvents is difficult due to remarkably weak interactions between macrocycles and axles. Reported here is a novel metal-coordinated poly[2]pseudorotaxane constructed by pillar[5]arene, 1,4-bis(4-pyridyl pyridinium)butane, and [PdCl₂(PhCN)₂] in highly polar organic solvent of dimethyl sulfoxide (DMSO). Utilizing a combination of ¹H NMR, NOESY, DOSY, DLS, SEM, and viscosity measurements, the formation of polypseudorotaxane was shown to be dependent on the concentration of [2]pseudorotaxanes/[PdCl₂(PhCN)₂] and temperature. Furthermore, a temperature-responsive supramolecular gel with reversibly gel-sol transformation was obtained via spontaneous assembly of the polypseudorotaxanes at high concentrations.

Ring-opening supramolecular polymerization controlled by orthogonal non-covalent interactions

The π–π interaction has been successfully utilized to orthogonally regulate the supramolecular polymerization driven by quadruple hydrogen bonding.

Guidelines for the assembly of hydrogen-bonded macrocycles

This article highlights selected examples on the synthesis of hydrogen-bonded macrocycles from ditopic molecules and analyze the main factors, often interrelated, that influence the equilibrium between ring and chain species.

Biomimetic folding of small organic molecules driven by multiple non-covalent interactions

The supramolecular self-folding of UPy-based monomers with low molecular weight driven by multiple non-covalent interactions has been developed.

Catenane Crosslinked Mechanically Adaptive Polymer Gel

A new strategy is introduced to prepare an adaptive polymer gel that has a unique adaptability in response to environmental stimuli. This gel is prepared by the thiol-ene "click" reaction between a bisvinyl [2]catenane and a poly(ethylene glycol) derivative containing multiple thiol groups. The catenane crosslinker is responsive to external stimuli due to the existence of intercomponent hydrogen bonding (IHB). The strong IHB restricts the rotation and movement of the crosslinker, giving it a rigid feature; however, the crosslinker becomes flexible when the IHB is destroyed. In consequence, the resulting gel can be reversibly switched between tough and soft states under stimulations.

Supramolecular polymerization of a ureidopyrimidinone-based [2]catenane prepared via ring-closing metathesis

The synthesis and supramolecular polymerization of a ureidopyrimidinone‐based Sauvage‐type [2]catenane is reported. The monomer synthesis explores many routes using the elegant metathesis catalysts of Bob Grubbs, yielding a catenane with one ureidopyrimidinone in each cycle. The supramolecular polymer obtained features both mechanical bonds and quadruple hydrogen bonding connections.

Improved in vivo tumor therapy via host–guest complexation

A decomposable and intracellular pH-responsive drug delivery system by immobilizing a water-soluble pillar[5]arene onto hollow mesoporous nanoparticles through host–guest complexation was successfully prepared and its application in controlled drug delivery in vitro and in vivo was also investigated.

Multi-stimuli responsive supramolecular polymers and their electrospun nanofibers

A novel type of multi-stimuli responsive supramolecular polymer was successfully constructed and its electrospun nanofibers exhibited cation-, pH-, anion-, and thermo-responsiveness.

Supramolecular Polymerization Controlled by Reversible Conformational Modulation

We report a new method for fabricating supramolecular polymers with controlled structure and molecular weight through reversible conformational modulation. To this end, the crown-ether-based "taco complex" was introduced. We prepared a monomer containing a bis(m-phenylene)-32-crown-10 in the core, which can supramolecularly polymerize efficiently in solution. When the conformation of the crown ether core was folded into a taco complex, the linear supramolecular polymerization could be significantly depressed, thus decreasing the molecular weight of the supramolecular polymer. In addition, extracting the depolymerizing agent with aqueous solution of cucurbit[7]uril could disassociate the taco complex and regenerate the supramolecular polymer with molecular weight as high as before. It is anticipated that this study can provide a facile and general methodology for controllable supramolecular polymerization.

Functional supramolecular polymers for biomedical applications

As a novel class of dynamic and non-covalent polymers, supramolecular polymers not only display specific structural and physicochemical properties, but also have the ability to undergo reversible changes of structure, shape, and function in response to diverse external stimuli, making them promising candidates for widespread applications ranging from academic research to industrial fields. By an elegant combination of dynamic/reversible structures with exceptional functions, functional supramolecular polymers are attracting increasing attention in various fields. In particular, functional supramolecular polymers offer several unique advantages, including inherent degradable polymer backbones, smart responsiveness to various biological stimuli, and the ease for the incorporation of multiple biofunctionalities (e.g., targeting and bioactivity), thereby showing great potential for a wide range of applications in the biomedical field. In this Review, the trends and representative achievements in the design and synthesis of supramolecular polymers with specific functions are summarized, as well as their wide-ranging biomedical applications such as drug delivery, gene transfection, protein delivery, bio-imaging and diagnosis, tissue engineering, and biomimetic chemistry. These achievements further inspire persistent efforts in an emerging interdisciplin-ary research area of supramolecular chemistry, polymer science, material science, biomedical engineering, and nanotechnology.

The construction of supramolecular polymers through anion bridging: from frustrated hydrogen-bonding networks to well-ordered linear arrays

The construction of supramolecular polymers has been realized by converting random hydrogen-bonding networks into well-ordered linear hydrogen-bonding arrays through an anion-bridging strategy.

Supramolecular polymers as surface coatings: rapid fabrication of healable superhydrophobic and slippery surfaces

Supramolecular polymerization for non-wetting surface coatings is described. The self-assembly of low-molecular-weight gelators (LMWGs) with perfluorinated side chains can be utilized to rapidly construct superhydrophobic, as well as liquid-infused slippery surfaces within minutes. The lubricated slippery surface exhibits impressive repellency to biological li-quids, such as human serum and blood, and very fast self-healing.

Dynamic supramolecular complexes constructed by orthogonal self-assembly

CONSPECTUS: Supramolecular complexes, including various low-molecular-mass structures and large molecular aggregates that are assembled by reversible and highly directional noncovalent interactions, have attracted more and more attention due to their fascinating and unconventional chemical and physical properties that are different from those of traditional architectures encountered by covalently linked backbones. Supramolecular complexes are by nature dynamic architectures considering the reversibility of noncovalent interactions by which small molecular monomers can assemble into specific architectures that are able to be repeatably reorganized through the assembly/disassembly processes under certain environmental factors such as temperature, concentration, and solvent conditions. The construction of supramolecular complexes by orthogonal self-assembly with different types of highly specific, noninterfering interactions is currently attracting considerable interest since they not only can dynamically self-assemble, but also can be tuned by various external stimuli through addressing each type of noncovalent interaction separately. Therefore, these dynamic supramolecular complexes, especially with external responsiveness, represent the most outstanding candidates for the future development of functional and smart materials, and even mimic the assembling process of natural systems. In this Account, we will summarize the recent advances of dynamic supramolecular complexes constructed by orthogonal self-assembly in soluiton in two sections: (1) Construction strategies for supramolecular complexes based on orthogonal self-assembly, whose dynamic behaviors with external responsiveness were not experimentally investigated but potentially existed due to the intrinsic reversibility of noncovalent bonds; (2) dynamic behaviors of multiresponsive supramolecular complexes, which were experimentally reported to exhibit reversible multi-responsiveness to external stimuli. Dynamic nature is one of intrinsic properties of supramolecular complexes constructed by self-assembly. Therefore, in the first section, we will describe the dynamic self-assembly in the construction of supramolecular complexes, but will focus on their external responsive dynamic behaviors in the second section. In addition, considering that an increasing number of supramolecular complexes constructed by biological building blocks through bio-orthogonal assembly as mimics of biological systems have been reported in recent years, in the second section we will also present some typical examples on such special dynamic biological supramolecular complexes. The final part of this Account is devoted to foreseeing the rapid development of dynamic supramolecular complexes toward applications in functional and smart materials and fundamental questions facing dynamic supramolecular complexes in the future.

Supramolecular autoregulation

Enzyme activity in biological systems is often governed by control mechanisms in which the catalytic properties are made sensitive or insensitive to differences in enzyme or substrate concentration. Here, we report the first supramolecular system where the catalytic activity is made concentration independent through the use of newly designed inhibitor molecules. The precise concentration dependence of coupled supramolecular equilibriums between free catalyst, inhibited catalyst, active inhibitor, and inactive inhibitor allows to keep the concentration of free catalyst at 1 mM in a broad concentration range, yielding an autoregulated catalytic system.