Water‐Soluble Pillar[n]arene Mediated Supramolecular Self‐Assembly: Multi‐Dimensional Morphology Controlled by Host Size

We report tunable supramolecular self‐assemblies formed by water‐soluble pillar[n]arenes ( WPn s, n=5–7) and bipyridinium‐azobenzene guests. Nanoscale or microscale morphology of self‐assemblies in water was controlled by the host size of WPn . Supramolecular self‐assemblies could undergo morphology conversion under irradiation with UV light.     

Cucurbit[8]uril (CB[8])‐Based Supramolecular Switches

The use of cucurbit[8]uril as a molecular host has emerged in the chemical literature as a reliable strategy for the creation of dynamic chemical systems, owing to its ability to form homo‐ and heteroternary complexes in aqueous media with appropriate molecular switches as guests. In this manner, CB[8]‐based supramolecular switches can be designed in a predictable and modular fashion, through the selection of appropriate guests able to condition the redox, photochemical, or pH‐triggered behavior of tailored multicomponent systems. Furthermore, CB[8] allows the implementation of dual/triple and linear/orthogonal stimuli‐dependent properties into these molecular devices by a careful selection of the guests. This versatility in their design gives these supramolecular switches great potential for the rational development of new materials, in which their function is not only determined by the custom‐made stimuli‐responsiveness, but also by the transient aggregation/disaggregation of homo‐ or heteromeric building blocks.     

Cucurbit[n]urils for Supramolecular Catalysis

The control over chemical reactivity and selectivity are always pursued. Using non-covalent interactions to achieve efficient and selective catalysis is an essential goal of supramolecular catalysis. Supramolecular catalysis based on cucurbit[n]urils (CB[n]s) possesses distinct characteristics for the unique structure of CB[n]s. CB[n]s are a family of pumpkin-shaped host molecules with various molecular sizes, rigid structures, electronegative portals and wealthy host-guest chemistry. Herein, we summarize the three major mechanisms of CB[n]s based supramolecular catalysis. Owing to the structural properties of CB[n]s, CB[n]s can serve as nanoreactors and steric hindrance to modulate the reactivity of substrates. They can also catalyze the reactions by modulating the reactivity of ionized intermediates. Recent progresses on the CB[n]s based supramolecular catalysis are introduced in this Minireview and the future development in this field is discussed. It is anticipated that this review provides insights into the mechanism of CB[n]s based supramolecular catalysis and may help scientists find new opportunities in this field.     

Reversible Shape‐Morphing and Fluorescence‐Switching in Supramolecular Nanomaterials Consisting of Amphiphilic Cyanostilbene and Cucurbit[7]uril

We demonstrate a reversible shape‐morphing with concurrent fluorescence switching in the nanomaterials which are complexed with cucurbit[7]uril (CB[7]) in water. The cyanostilbene derivative alone forms ribbon‐like two‐dimensional (2D) nanocrystals with bright yellow excimeric emission in water (λ_em=540 nm, Φ_F=42 %). Upon CB[7] addition, however, the ribbon‐like 2D nanocrystals immediately transform to spherical nanoparticles with significant fluorescence quenching and blue‐shifting (λ_em=490 nm, Φ_F=1 %) through the supramolecular complexation of the cyanostilbene and CB[7]. Based on this reversible fluorescence switching and shape morphing, we could demonstrate a novel strategy of turn‐on fluorescence sensing of spermine and also monitoring of lysine decarboxylase activity.     

Visible-Light-Excited Room-Temperature Phosphorescence in Water by Cucurbit[8]uril-Mediated Supramolecular Assembly

Solid-state materials with efficient room-temperature phosphorescence (RTP) emissions have found widespread applications in materials science, while liquid or solution-phase pure organic RTP emission systems has been rarely reported, because of the nonradiative decay and quenchers from the liquid medium. Reported here is the first example of visible-light-excited pure organic RTP in aqueous solution by using a supramolecular host-guest assembly strategy. The unique cucurbit[8]uril-mediated quaternary stacking structure allows tunable photoluminescence and visible-light excitation, enabling the fabrication of multicolor hydrogels and cell imaging. The present assembly-induced emission approach, as a proof of concept, contributes to the construction of novel metal-free RTP systems with tunable photoluminescence in aqueous solution, providing broad opportunities for further applications in biological imaging, detection, optical sensors, and so forth.     

New Wine in Old Bottles: Prolonging Room‐Temperature Phosphorescence of Crown Ethers by Supramolecular Interactions

Supramolecular macrocyclic hosts have long been used in smart materials. However, their triplet emission and regulation at crystal level is rarely studied. Herein, ultralong and universal room‐temperature phosphorescence (RTP) is reported for traditional crown ethers. A supramolecular strategy involving chain length adjustment and morphological locking through complexation with K⁺ was explored as a general method to tune the phosphorescence lifetime in the solid state. A maximum 10‐fold increase of lifetime after complex formation accompanied with by invisible to visible phosphorescence was achieved. A deep encryption based on this activated RTP strategy was also facilely fabricated. This work thus opens a new world for supramolecular macrocycles and their intrinsic guest responsiveness offers a new avenue for versatile smart luminescent materials.     

Stimuli‐Responsive Cucurbit[7]uril‐Mediated BSA Nanoassembly for Uptake and Release of Doxorubicin

We report the construction of a non‐toxic nanoassembly of bovine serum albumin (BSA) protein and the cucurbit[7]uril macrocycle as well as its stimuli‐responsive breakage with adamantylamine or pH, which restores the protein structure and recognition properties. The assembly showed efficient loading and controlled release of a standard drug, doxorubicin (DOX), and the same was validated in live cells. The cell viability studies documented that the DOX‐loaded assembly mask the cytotoxicity of DOX and the toxicity can be revived at the target on demand, triggering its therapeutic activation. This is found to be more effective in the cancer cells. In addition, such host‐assisted protein assemblies are also highly promising for stabilizing/protecting the native protein structure, a viable approach to prevent/inhibit protein misfolding and aggregation.     

Hierarchical Self‐Assembly of Poly‐Pseudorotaxanes into Artificial Microtubules

Hierarchical self‐assembly of building blocks over multiple length scales is ubiquitous in living organisms. Microtubules are one of the principal cellular components formed by hierarchical self‐assembly of nanometer‐sized tubulin heterodimers into protofilaments, which then associate to form micron‐length‐scale, multi‐stranded tubes. This peculiar biological process is now mimicked with a fully synthetic molecule, which forms a 1:1 host‐guest complex with cucurbit[7]uril as a globular building block, and then polymerizes into linear poly‐pseudorotaxanes that associate laterally with each other in a self‐shape‐complementary manner to form a tubular structure with a length over tens of micrometers. Molecular dynamic simulations suggest that the tubular assembly consists of eight poly‐pseudorotaxanes that wind together to form a 4.5 nm wide multi‐stranded tubule.     

Cucurbit[7]uril‐Dimethyllysine Recognition in a Model Protein

Here, we provide the first structural characterization of host–guest complexation between cucurbit[7]uril ( Q7 ) and dimethyllysine (KMe₂) in a model protein. Binding was dominated by complete encapsulation of the dimethylammonium functional group. While selectivity for the most sterically accessible dimethyllysine was observed both in solution and in the solid state, three different modes of Q7 ‐KMe₂ complexation were revealed by X‐ray crystallography. The crystal structures revealed also entrapped water molecules that solvated the ammonium group within the Q7 cavity. Remarkable Q7 ‐protein assemblies, including inter‐locked octahedral cages that comprise 24 protein trimers, occurred in the solid state. Cucurbituril clusters appear to be responsible for these assemblies, suggesting a strategy to generate controlled protein architectures.     

Supramolecular Self‐Assembly of Histidine‐Capped‐Dialkoxy‐Anthracene: A Visible‐Light‐Triggered Platform for Facile siRNA Delivery

Supramolecular self‐assembly of histidine‐capped‐dialkoxy‐anthracene (HDA) results in the formation of light‐responsive nanostructures. Single‐crystal X‐ray diffraction analysis of HDA shows two types of hydrogen bonding. The first hydrogen bond is established between the imidazole moieties while the second involves the oxygen atom of one amide group and the hydrogen atom of a second amide group. When protonated in acidic aqueous media, HDA successfully complexes siRNA yielding spherical nanostructures. This biocompatible platform controllably delivers siRNA with high efficacy upon visible‐light irradiation leading up to 90 % of gene silencing in live cells.     

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).     

Metalla‐Assembled Electron‐Rich Tweezers: Redox‐Controlled Guest Release Through Supramolecular Dimerization

Developing methodologies for on‐demand control of the release of a molecular guest requires the rational design of stimuli‐responsive hosts with functional cavities. While a substantial number of responsive metallacages have already been described, the case of coordination‐tweezers has been less explored. Herein, we report the first example of a redox‐triggered guest release from a metalla‐assembled tweezer. This tweezer incorporates two redox‐active panels constructed from the electron‐rich 9‐(1,3‐dithiol‐2‐ylidene)fluorene unit that are facing each other. It dimerizes spontaneously in solution and the resulting interpenetrated supramolecular structure can dissociate in the presence of an electron‐poor planar unit, forming a 1:1 host–guest complex. This complex dissociates upon tweezer oxidation/dimerization, offering an original redox‐triggered molecular delivery pathway.     

Uptake of Hydrocarbons in Aqueous Solution by Encapsulation in Acyclic Cucurbit[n]uril‐Type Molecular Containers

The ability of two water‐soluble acyclic cucurbit[n]uril (CB[n]) type containers, whose hydrophobic cavity is defined by a glycoluril tetramer backbone and terminal aromatic (benzene, naphthalene) sidewalls, to act as solubilizing agents for hydrocarbons in water is described. ¹H NMR spectroscopy studies and phase‐solubility diagrams establish that the naphthalene‐walled container performs as well as, or better than, CB[7] and CB[8] in promoting the uptake of poorly soluble hydrocarbons into aqueous solution through formation of host–hydrocarbon complexes. The naphthalene‐walled acyclic CB[n] container is able to extract large hydrocarbons from crude oil into aqueous solution.     

Highly Luminescent and Water‐Soluble Two‐Dimensional Supramolecular Organic Framework: All‐Organic Photosensitizer Template for Visible‐Light‐Driven Hydrogen Evolution from Water

A highly fluorescent (Φ_F=0.60) and water‐soluble two‐dimensional (2D) honeycomb‐shaped supramolecular organic framework (SOF) was successfully synthesized in pure aqueous solution via self‐assembly of novel cyanostilbene‐functionalized trilateral guest molecules and cucurbit[8]uril hosts. The size of this fluorescent 2D SOF was >500 nm in diameter, 1.7 nm in thickness, and 3.9 nm in the honeycomb pore diameter. This 2D SOF holds potential as a new all‐organic photosensitizer template for photocatalytic H₂ evolution from pure water.