Interconversion between [5]Pseudorotaxane and [3]Pseudorotaxane by Pasting/Detaching Two Axle Molecules

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).

Stimuli-responsive mechanically interlocked molecules constructed from cucurbit[n]uril homologues and derivatives

Cucurbit[n]uril supramolecular chemistry has developed rapidly since 2001 when different cucurbit[n]uril homologues (Q[n]) were successfully separated in pure form. The combination of Q[n] cavity size and various types of external stimuli has given birth to numerous types of Q[n]-based mechanically interlocked molecules (MIMs), including (pseudo)rotaxanes, catenanes, dendrimers and poly(pseudo)rotaxanes. In this review article, the important advances in the field of Q[n]-based MIMs over the past two decades are highlighted. This review also describes examples of heterowheel (pseudo)rotaxanes and poly(pseudo)rotaxanes involving Q[n]s, and reflects on the opportunities and challenges of constructing Q[n]-based stimuli-responsive MIMs.

Near-Infrared Phosphorescent Switch of Diarylethene Phenylpyridinium Derivative and Cucurbit[8]uril for Cell Imaging

Near-infrared (NIR) pure organic room-temperature phosphorescence (RTP) materials have received growing research interest due to their wide application in the fields of high-resolution bioimaging and luminescent materials. In this work, the authors report a macrocycle-confined pure organic RTP supramolecular assembly, which is constructed by diarylethene phenylpyridinium derivative (DTE-TP) and cucurbit[8]uril (CB[8]). Compared with CB[6] and CB[7], the larger cavity of CB[8] induces molecular folding and enhances the intramolecular charge transfer interactions, which leads to the obtained assembly emitting efficient NIR phosphorescence at 700 nm. Due to the photochromism of the diarylethene core, the NIR phosphorescence is reversibly regulated by light irradiation at wavelengths of 365 and >600 nm. Furthermore, cell-based experiments show that this supramolecular assembly is located in the lysosomes and displays a NIR phosphorescence at 650-750 nm. In addition, by means of phosphorescence resonance energy transfer, the obtained assembly exhibits a red-shifted NIR emission at 817 nm. This supramolecular phosphorescent switch provides a convenient path for the modular design of water-soluble pure organic room-temperature NIR phosphorescent materials.

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.

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.

Facile Construction of a Supramolecular Organic Framework Using Naphthyl Viologen Guests and CB[8] Host via Charge-Transfer Complexation

Herein, we report the synthesis of guest-host systems comprising naphthyl-viologen-naphthyl (Np-Vio-Np) and viologen-naphthyl-viologen (Vio-Np-Vio) guest molecules and their subsequent supramolecular polymerization in the presence of a CB[8] host in water. In addition, the guest complexation of ethyl-terminated trimeric viologen (ETV) with Np-Vio-Np and CB[8] was investigated. As a result of supramolecular interactions, 2D supramolecular organic frameworks with high internal periodicity were constructed. 1H NMR studies clearly demonstrated the formation of a host-stabilized charge-transfer complex via folding back (Np-Vio-Np and Vio-Np-Vio) in the presence of CB[8]. In the case of ETV + Np-Vio-Np + CB[8], a large polymeric network was formed as indicated by the NMR titrations. UV-vis and fluorescence studies clearly confirm the formation of an inter/intra molecular CT complex upon complexation with cucurbit[8]uril. The size obtained using the dynamic light scattering (DLS) method pinpoints the formation of larger supramolecular aggregates in the order of μm through host-guest assembly, which is further complemented by FESEM and TEM. SAXS measurements indicate the formation of a 2D supramolecular polymer/polymer aggregate with long-range order.

Red-light-responsive molecular encapsulation in water: an ideal combination of photochemistry and host–guest interaction

Red-light-responsive CB[8]-mediated host–guest system featured with high photoisomerization ratio was fabricated which could be used as molecular container with red-light-activated release ability.

Cu(ii) templated formation of [n]pseudorotaxanes (n = 2, 3, 4) using a tris-amino ether macrocyclic wheel and multidentate axles

The systematic development of mono-, bi- and tri-nuclear [n]pseudorotaxanes (n = 2, 3, 4) via Cu(ii) templation and π–π stacking interactions.

Complexation Between (O-Methyl)6 -2,6-Helic[6]arene and Tertiary Ammonium Salts: Acid/Base- or Chloride-Ion-Responsive Host-Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O-methyl)₆ -2,6-helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹ H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base- and chloride-ion-responsive host-guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6-helic[6]arene-based [2]rotaxane was also synthesized from the condensation between the host-guest complex and isocyanate.

Five-Component Self-Assembly of Cucurbituril-Based Hetero-pseudorotaxanes

[5]Pseudorotaxanes can be obtained by self-sorting using heteroditopic guests and various cucurbituril homologues as hosts. The assembly and chemically induced disassembly of the pseudorotaxanes can be monitored by measuring the fluorescence of the anthracene guest in solution. Mass spectral evidence for the supramolecular assemblies is obtained in the gas phase. The disassembly in the gas phase can be achieved by collision-induced dissociation leading to the corresponding [2]- and [3]pseudorotaxanes.

Photolysis of polymeric self-assembly controlled by donor–acceptor interaction

A photodegradable polymer was built using a light-inert monomer through self-assembly-mediated donor–acceptor interaction.

The construction of a two-dimensional supramolecular organic framework with parallelogram pores and stepwise fluorescence enhancement

A novel single-layer two-dimensional (2D) supramolecular organic framework (SOF) with parallelogram pores has been assembled to turn on the fluorescence emission of a non-emissive building block, and the emission could be further enhanced by the aggregation of the as-prepared 2D monolayers.

Fluorescence enhancement through the formation of a single-layer two-dimensional supramolecular organic framework and its application in highly selective recognition of picric acid

A novel two-dimensional (2D) supramolecular organic framework (SOF) has been assembled, which was further used as a fluorescent chemosensor to detect picric acid with high selectivity and sensitivity.

Acid/base controllable complexation of a triptycene-derived macrotricyclic host and protonated 4,4′-bipyridinium/pyridinium salts

A new acid/base controllable host–guest system based on a triptycene-derived macrotricyclic host and protonated 4,4′-bipyridinium/pyridinium salts was developed.

Supramolecular chemistry: from aromatic foldamers to solution-phase supramolecular organic frameworks

This mini-review covers the growth, education, career, and research activities of the author. In particular, the developments of various folded, helical and extended secondary structures from aromatic backbones driven by different noncovalent forces (including hydrogen bonding, donor-acceptor, solvophobicity, and dimerization of conjugated radical cations) and solution-phase supramolecular organic frameworks driven by hydrophobically initiated aromatic stacking in the cavity of cucurbit[8]uril (CB[8]) are highlighted.

Photo-responsive pseudorotaxanes and assemblies

Optical responses for understanding the dynamic conformational change(s) in solution during the stimuli responsive complexation and decomplexation process(es) in a supramolecular assembly.

Highly thermally stable hydrogels derived from monolayered two-dimensional supramolecular polymers

Hydrogels have been constructed from monolayered two-dimensional (2D) supramolecular polymers in water. The as-prepared hydrogels exhibited extremely high thermal stabilities, which demonstrates how the 2D structure can impact the bulk properties of soft materials.

A thermally stable pH-responsive “supramolecular buckle” based on the encapsulation of 4-(4-aminophenyl)-N-methylpyridinium by cucurbit[8]uril

A CB[8]-based host-guest system forms a thermally stable 1 : 2 complex under neutral conditions and transforms into a 1 : 1 complex upon acidification, which makes it a “supramolecular buckle” to fabricate pH-responsive materials at elevated temperatures.

The construction of rigid supramolecular polymers in water through the self-assembly of rod-like monomers and cucurbit[8]uril

Two types of stick-like supramolecular polymers possessing rigid backbones have been fabricated through the self-assembly of rod-like monomers and cucurbit[8]uril in water.

Toward a single-layer two-dimensional honeycomb supramolecular organic framework in water

The self-assembly of well-defined 2D supramolecular polymers in solution has been a challenge in supramolecular chemistry. We have designed and synthesized a rigid stacking-forbidden 1,3,5-triphenylbenzene compound that bears three 4,4'-bipyridin-1-ium (BP) units on the peripheral benzene rings. Three hydrophilic bis(2-hydroxyethyl)carbamoyl groups are introduced to the central benzene ring to suppress 1D stacking of the triangular backbone and to ensure solubility in water. Mixing the triangular preorganized molecule with cucurbit[8]uril (CB[8]) in a 2:3 molar ratio in water leads to the formation of the first solution-phase single-layer 2D supramolecular organic framework, which is stabilized by the strong complexation of CB[8] with two BP units of adjacent molecules. The periodic honeycomb 2D framework has been characterized by various (1)H NMR spectroscopy, dynamic light scattering, X-ray diffraction and scattering, scanning probe and electron microscope techniques and by comparing with the self-assembled structures of the control systems.