Dynamic supramolecular polymers built from cucurbit[n]urils and viologens

A Review of Macrocycles Applied in Electrochemical Energy Storge and Conversion

Macrocycles composed of diverse aromatic or nonaromatic structures, such as cyclodextrins (CDs), calixarenes (CAs), cucurbiturils (CBs), and pillararenes (PAs), have garnered significant attention due to their inherent advantages of possessing cavity structures, unique functional groups, and facile modification. Due to these distinctive features enabling them to facilitate ion insertion and extraction, form crosslinked porous structures, offer multiple redox-active sites, and engage in host-guest interactions, macrocycles have made huge contributions to electrochemical energy storage and conversion (EES/EEC). Here, we have summarized the recent advancements and challenges in the utilization of CDs, CAs, CBs, and PAs as well as other novel macrocycles applied in EES/EEC devices. The molecular structure, properties, and modification strategies are discussed along with the corresponding energy density, specific capacity, and cycling life properties in detail. Finally, crucial limitations and future research directions pertaining to these macrocycles in electrochemical energy storage and conversion are addressed. It is hoped that this review is able to inspire interest and enthusiasm in researchers to investigate macrocycles and promote their applications in EES/EEC.

Internal Dynamics and Modular Peripheral Binding in Stimuli-Responsive 3 : 2 Host:Guest Complexes

Now that the chemistry of 1 : 1 host:guest complexes is well-established, it is surprising to note that higher stoichiometry (oligomeric) complexes, especially those with excess host, remain largely unexplored. Yet, proteins tend to oligomerize, affording new functions for cell machinery. Here, we show that cucurbit[n]uril (CB[n]) macrocycles combined with symmetric, linear di-viologens form unusual 3 : 2 host:guest complexes exhibiting remarkable dynamic properties, host self-sorting, and external ring-translocation. These results highlight the structural tunability of cucurbit[8]uril (CB[8]) based 3 : 2 host:guest complexes in water and their responsiveness toward several stimuli (chemicals, pH, redox).

Ni-Centered Coordination-Induced Spin-State Switching Triggered by Electrical Stimulation

We herein report the synthesis and magnetic properties of a Ni(II)-porphyrin tethered to an imidazole ligand through a flexible electron-responsive mechanical hinge. The latter is capable of undergoing a large amplitude and fully reversible folding motion under the effect of electrical stimulation. This redox-triggered movement is exploited to force the axial coordination of the appended imidazole ligand onto the square-planar Ni(II) center, resulting in a change in its spin state from low spin (S = 0) to high spin (S = 1) proceeding with an 80% switching efficiency. The driving force of this reversible folding motion is the π-dimerization between two electrogenerated viologen cation radicals. The folding motion and the associated spin state switching are demonstrated on the grounds of NMR, (spectro)electrochemical, and magnetic data supported by quantum calculations.

Self-Healing Thiolated Pillar[5]arene Films Containing Moxifloxacin Suppress the Development of Bacterial Biofilms

Polymer self-healing films containing fragments of pillar[5]arene were obtained for the first time using thiol/disulfide redox cross-linking. These films were characterized by thermogravimetric analysis and differential scanning calorimetry, FTIR spectroscopy, and electron microscopy. The films demonstrated the ability to self-heal through the action of atmospheric oxygen. Using UV–vis, 2D 1H-1H NOESY, and DOSY NMR spectroscopy, the pillar[5]arene was shown to form complexes with the antimicrobial drug moxifloxacin in a 2:1 composition (logK11 = 2.14 and logK12 = 6.20). Films containing moxifloxacin effectively reduced Staphylococcus aureus and Klebsiella pneumoniae biofilms formation on adhesive surfaces.

A cucurbit[8]uril-stabilized 3D charge transfer supramolecular polymer with a remarkable confinement effect for enhanced photocatalytic proton reduction and thioether oxidation

A water-soluble porous supramolecular polymer is assembled through a CB[8]-based 2 + 2 host–guest binding motif, which can greatly increase the efficiency of photocatalysis.

Light-Controlled Aggregation and Gelation of Viologen-Based Coordination Polymers

Ditopic bis-(triazole/pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate π-radicals that can self-associate to form π-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense, polydisperse, micron-sized, spindle-shaped particles which grow in time. Using microscopy and time-resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process, a new scenario to supramolecular systems.

Potential Applications of Cucurbit[n]urils and Their Derivatives in the Capture of Hazardous Chemicals

Cucurbit[n]urils (Q[n]s) are a relatively young family of macrocycles, consisting of glycoluril units bridged by methylene groups, and their unique structures have attracted extensive attention from chemists in recent decades. Due to the presence of a rigid hydrophobic inner cavity and two polar outer portals lined with carbonyl groups, Q[n]s not only encapsulate guest species into the cavity, but also coordinate with metal ions/clusters. Considerable achievements have been obtained in the fields of Q[n]s-based host-guest chemistry, coordination chemistry, as well as the combination of host-guest and coordination chemistry. Furthermore, the outer surface of Q[n]s has been demonstrated to be capable of interacting with definite species to generate supramolecular architectures in recent years. With more in-depth research into Q[n]s, their application studies have also emerged as a hot topic. This Minireview focuses on recent advances in the potential applications of solid-state materials based on Q[n]s and their derivatives for the capture and adsorption of hazardous chemicals from a solution or a gas mixture.

Electron-Triggered Metamorphism in Palladium-Driven Self-Assembled Architectures

A metal-induced self-assembly strategy is used to promote the π-dimerization of viologen-based radicals at room temperature and in standard concentration ranges. Discrete box-shaped 2:2 (M:L) macrocycles or coordination polymers are formed in solution by self-assembly of a viologen-based ditopic ligand with cis-[Pd(en)(NO₃)₂], trans-[Pd(CH₃CN)₂(Cl)₂], or [Pd(CH₃CN)₄(BF₄)₂]. Changing the redox state of the bipyridium units involved in the tectons, from their dicationic state to their radical cation state, results in a reversible "inflation/deflation" of the discrete 2:2 (M:L) macrocyclic assemblies associated to a large modification in the size of their inner cavity. Viologen-centered electron transfer is also used to trigger a dissociation of the coordination polymers formed with tetrakis(acetonitrile)Pd(II), the driving force of the disassembling process being the formation of discrete box-shaped 2:2 (M:L) assemblies stabilized by π-dimerization of both viologen cation radicals.

Physical and Chemical Factors Influencing the Printability of Hydrogel-based Extrusion Bioinks

Bioprinting researchers agree that "printability" is a key characteristic for bioink development, but neither the meaning of the term nor the best way to experimentally measure it has been established. Furthermore, little is known with respect to the underlying mechanisms which determine a bioink's printability. A thorough understanding of these mechanisms is key to the intentional design of new bioinks. For the purposes of this review, the domain of printability is defined as the bioink requirements which are unique to bioprinting and occur during the printing process. Within this domain, the different aspects of printability and the factors which influence them are reviewed. The extrudability, filament classification, shape fidelity, and printing accuracy of bioinks are examined in detail with respect to their rheological properties, chemical structure, and printing parameters. These relationships are discussed and areas where further research is needed, are identified. This review serves to aid the bioink development process, which will continue to play a major role in the successes and failures of bioprinting, tissue engineering, and regenerative medicine going forward.

Oligomeric Cucurbituril Complexes: from Peculiar Assemblies to Emerging Applications

Proteins are an endless source of inspiration. By carefully tuning the amino-acid sequence of proteins, nature made them evolve from primary to quaternary structures, a property specific to protein oligomers and often crucial to accomplish their function. On the other hand, the synthetic macrocycles cucurbiturils (CBs) have shown outstanding recognition properties in water, and a growing number of (host)_n :(guest)_n supramolecular polymers involving CBs have been reported. However, the burgeoning field of discrete (n:n) host:guest oligomers has just started to attract attention. While 2:2 complexes are the major oligomers, 3:3 and up to 6:6 oligomers have been described, some associated with emerging applications, specific to the (n:n) arrangements. Design rules to target (n:n) host:guest oligomers are proposed toward new advanced host:guest systems.

Facile synthesis of crystalline viologen-based porous ionic polymers with hydrogen-bonded water for efficient catalytic CO2 fixation under ambient conditions

In this work, we report a series of crystalline viologen-based porous ionic polymers (denoted VIP-X, X = Cl or Br), that have in situ formed dicationic viologens paired with halogen anions and intrinsic hydrogen-bonded water molecules, towards metal-free heterogeneous catalytic conversion of carbon dioxide (CO₂) under mild conditions. The targeted VIP-X materials were facilely constructed via the Menshutkin reaction of 4,4′-bipyridine with 4,4′-bis(bromomethyl)biphenyl (BCBMP) or 4,4′-bis(chloromethyl)biphenyl (BBMBP) monomers. Their crystalline and porous structures, morphological features and chemical structures and compositions were fully characterized by various advanced techniques. The optimal catalyst VIP-Br afforded a high yield of 99% in the synthesis of cyclic carbonate by CO₂ cycloaddition with epichlorohydrin under atmospheric pressure (1 bar) and a low temperature (40 °C), while other various epoxides could be also converted into cyclic carbonates under mild conditions. Moreover, the catalyst VIP-Br could be separated easily and reused with good stability. The remarkable catalytic performance could be attributed to the synergistic effect of the enriched Br⁻ anions and available hydrogen bond donors –OH groups coming from H-bonded water molecules.

Viologen-based porous ionic polymers with halogen anions and hydrogen-bonded water were constructed for efficient catalytic CO₂ fixation under mild conditions.

A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift

A viologen derivative carrying a benzimidazole group (V-P-I ²⁺ ; viologen-phenylene-imidazole V-P-I) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pK_a , by more than 1 pH unit, contrasting with the positive pK_a shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I³⁺ indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I ³⁺ ⋅CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pK_a shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.

A single-crystal-to-single-crystal transformation affording photochromic 3D MORF crystals

Metal–organic framework (MOF) type crystals in which rigid, viologen-based pillars are surrounded by cucurbit[7]uril (CB[7]) macrocycles are described.