Applications of Supramolecular Polymers Generated from Pillar[n]arene-Based Molecules

Supramolecular chemistry enables the manipulation of functional components on a molecular scale, facilitating a "bottom-up" approach to govern the sizes and structures of supramolecular materials. Using dynamic non-covalent interactions, supramolecular polymers can create materials with reversible and degradable characteristics and the abilities to self-heal and respond to external stimuli. Pillar[n]arene represents a novel class of macrocyclic hosts, emerging after cyclodextrins, crown ethers, calixarenes, and cucurbiturils. Its significance lies in its distinctive structure, comparing an electron-rich cavity and two finely adjustable rims, which has sparked considerable interest. Furthermore, the straightforward synthesis, uncomplicated functionalization, and remarkable properties of pillar[n]arene based on supramolecular interactions make it an excellent candidate for material construction, particularly in generating interpenetrating supramolecular polymers. Polymers resulting from supramolecular interactions involving pillar[n]arene find potential in various applications, including fluorescence sensors, substance adsorption and separation, catalysis, light-harvesting systems, artificial nanochannels, and drug delivery. In this context, we provide an overview of these recent frontier research fields in the use of pillar[n]arene-based supramolecular polymers, which serves as a source of inspiration for the creation of innovative functional polymer materials derived from pillar[n]arene derivatives.

Handcuff-like metallo-pseudorotaxanes consisting of tiara[5]arene wheels and dimeric silver trifluoroacetate axles

The complexation between tiara[5]arene (T[5]) and silver trifluoroacetate affords a binuclear metallo-pseudorotaxane on account of multiple endo-cavity Ag^I η²-arene interactions. Furthermore, two such enantiomeric [(CF₃CO₂Ag)₂⊂T[5]] complexes are bridged by an extra (CF₃CO₂Ag)₂ dimer through exo-wall Ag^I η²-arene coordination, resulting in a unique handcuff metallo-bis-pseudorotaxane structure in the solid state.

Solvent-Dependent Self-Assembly of a Pillar[5]arene-Based Poly-Pseudo-Rotaxane Linked and Threaded by Silver(I) Trifluoroacetate: A Double Role

In the supramolecule area, the fabrication of a new concept called polyrotaxanes or poly-pseudo-rotaxanes remains challenging. We herein report the formation of a poly-pseudo-rotaxane in which the same salt-type guest serves both linking and threading in the resulting structure. The combination of A1/A2-thiopyridyl pillar[5]arene (L) and silver(I) trifluoroacetate in CHCl₃/CH₃OH afforded a one-dimensional (1D) poly-pseudo-rotaxane. In this structure, to our surprise, the AgCF₃CO₂ guest not only links the di-armed L ligands via an infinite -L-Ag-L-Ag- arrangement but also threads into a pillar[5]arene cavity in a dimer form, (AgCF₃CO₂)₂. In contrast, the same reaction in CH₂Cl₂/CH₃OH yielded a simple 1D coordination polymer because an included CH₂Cl₂ molecule in the pillar[5]arene cavity prevents the threading of the silver(I) trifluoroacetate guest. Comparative ¹H- and ¹⁹F-NMR studies support the solvent-dependent poly-pseudo-rotaxane formation at a lower concentration of L.

Metallosupramolecules of Pillar[5]arene with Two Flexible Thiopyridyl Arms: A Heterochiral Cyclic Dimer and Organic Guest-Assisted Homochiral Poly-Pseudo-Rotaxanes

The formation of a cyclic dimer complex (1) and a poly-pseudo-rotaxane (2) of a racemic A1/A2-thiopyridyl pillar[5]arene (rac-L) with different chirality is reported. A one-pot reaction of rac-L with HgCl₂ afforded a heterochiral cyclic dimer complex, [Hg₂(pR-L)(pS-L)Cl₄]·8CH₂Cl₂ (1), in which two Hg²⁺ atoms and one (pR-L)/(pS-L) enantiomeric pair form a [2:2] metallacycle via a metal coordination-based cyclization. Interestingly, the same reaction in the presence of the linear dinitrile guest, CN(CH₂)₈CN (G), yielded a one-dimensional poly-pseudo-rotaxane, {[Hg(G@pR-L)Cl₂][Hg(G@pS-L)Cl₂]}_n (2), probably due to the rigidified ligand structure resulting from the dinitrile guest (G) threading. In 2, pR-L and pS-L generate two separated homochiral poly-pseudo-rotaxanes in a crystal. Both products are new members of the pillararene-derivative family. This study improves our understanding of self-assembly in nature and leads to this approach being an engineering tool for the construction of mechanically interlocked supramolecules.

Noncovalently bound and mechanically interlocked systems using pillar[n]arenes

Pillar[n]arenes are pillar-shaped macrocyclic compounds owing to the methylene bridges linking the para-positions of the units. Owing to their unique pillar-shaped structures, these compounds exhibit various excellent properties compared with other cyclic host molecules, such as versatile functionality using various organic synthesis techniques, substituent-dependent solubility, cavity-size-dependent host-guest properties in organic media, and unit rotation along with planar chiral inversion. These advantages have enabled the high-yield synthesis and rational design of pillar[n]arene-based mechanically interlocked molecules (MIMs). In particular, new types of pillar[n]arene-based MIMs that can dynamically convert between interlocked and unlocked states through unit rotation have been produced. The highly symmetrical pillar-shaped structures of pillar[n]arenes result in simple NMR spectra, which are useful for studying the motion of pillar[n]arene wheels in MIMs and creating sophisticated MIMs with higher-order structures. The creation and application of polymeric MIMs based on pillar[n]arenes is also discussed.

Macrocycle-Based Solid-State Supramolecular Polymers

Supramolecular polymers, generated by connecting monomers through noncovalent interactions, have received considerable attention over the past years, as they provide versatile platforms for developing diverse aesthetically pleasing polymeric structures with promising applications in a variety of fields, such as medicine, catalysis, and sensing. In the development of supramolecular polymers, macrocyclic hosts play a very important role. Benefiting from their abundant host-guest chemistry and self-assembly characteristics, macrocycles themselves or their host-guest complexes can self-assemble to form well-ordered supramolecular polymeric architectures including pseudopolyrotaxanes and polyrotaxanes. The integration of these topological structures into supramolecular polymeric materials also imbues them with some unforeseen functions. Current interest in macrocycle-based supramolecular polymers is mostly focused on the development of supramolecular soft materials in solution or gel-state, in which the dynamic nature of noncovalent interactions endows supramolecular polymers with a wealth of "smart" properties, such as multiresponsiveness and self-repair capabilities. While preparation of macrocycle-derived supramolecular polymers in the solid state is a relatively challenging but intriguing prospect, they are an important part of the field of supramolecular polymers. On one hand, the construction of macrocycle-based solid-state supramolecular polymers enables us to obtain new materials with novel properties and functions such as mechano-responsiveness. On the other hand, the molecular structures and arrangements in these materials are well-identified by X-ray crystallography techniques, offering a direct visual representation of the supramolecular polymerization process. The analysis of the role of noncovalent interactions in these architectures allows us to design more sophisticated and elegant supramolecular polymers in a highly rationalized and controllable manner. This Account serves to summarize the research progress on macrocycle-based solid-state supramolecular polymers (MSSPs), including the contributions toward this field made by our group. For constructing MSSPs, the key point is to control noncovalent interactions. Thus, in this Account, we primarily classify these MSSPs by different noncovalent interactions involved to connect the monomers, including metal-ligand interactions, host-guest interactions, π···π stacking, and halogen bonding. These noncovalent interactions are highly associated with the structures and functions of the resultant MSSPs. For instance, using metal-ligand interactions as driving forces, metal clusters can be introduced in MSSPs which afford systems with solid-state luminescence or proton conduction properties; supramolecular polymerization using macrocycle-based host-guest interactions can modulate the molecular arrangement of some specific molecules in the solid state, which further influences their solid-state properties; π···π stacking interactions and halogen bonding give chemists more choice to design MSSPs with various elements. The role of macrocyclic hosts in MSSPs is also revealed in these descriptions. Finally, the remaining challenges are identified for further development of future prospects. We hope that this Account can inspire new discoveries in the realm of supramolecular functional systems and offer new opportunities for the construction of supramolecular architectures and solid-state materials.

Rim-differentiated pillar[5]arene based nonporous adaptive crystals

The first rim-differentiated pillar[5]arene based nonporous adaptive crystals (NACs) were developed and used to separate dichloromethane from a halomethane mixture with 99.1% purity.

Stabilization of Antitumor Agent Busulfan through the Encapsulation within a Water-Soluble Pillar[5]arene

The complexation of antitumor agent busulfan by negatively charged carboxylatopillar[5]arenein water is reported. The encapsulation within carboxylatopillar[5]arenein reduces the hydrolytic degradation of busulfan from 90.7 % to 25.2 % after 24 days and accordingly enhances its stability by providing a hydrophobic shelter for busulfan in water. Moreover, the complexation result in 12 times improvement of water solubility for busulfan. Our result provides a supramolecular approach for stabilizing the anticancer agent busulfan.

Pillar[5]arene-derived covalent organic materials with pre-encoded molecular recognition for targeted and synergistic cancer photo- and chemotherapy

An efficient targeted and synergistic cancer photo- and chemotherapy platform was constructed from aldehyde-modified pillar[5]arene and tetra-(4-aminophenyl)porphyrin successfully.

GOx-assisted synthesis of pillar[5]arene based supramolecular polymeric nanoparticles for targeted/synergistic chemo-chemodynamic cancer therapy

Background

Cancer is the most serious world's health problems on the global level and various strategies have been developed for cancer therapy. Pillar[5]arene-based supramolecular therapeutic nano-platform (SP/GOx NPs) was constructed successfully via orthogonal dynamic covalent bonds and intermolecular H-bonds with the assistance of glucose oxidase (GOx) and exhibited efficient targeted/synergistic chemo-chemodynamic cancer therapy.

Methods

The morphology of SP/GOx NPs was characterized by DLS, TEM, SEM and EDS mapping. The cancer therapy efficinecy was investigated both in vivo and in vitro.

Results

SP/GOx NPs can load drug molecules (Dox) and modify target molecule (FA-Py) on its surface conveniently. When the resultant FA-Py/SP/GOx/Dox NPs enters blood circulation, FA-Py will target it to cancer cells efficiently, where GOx can catalyst the overexpressed glucose to generate H₂O₂. Subsequently, the generated H₂O₂ in cancer cells catalyzed by ferrocene unit to form •OH, which can kill cancer cells. Furthermore, the loaded Dox molecules released under acid microenvironment, which can further achieve chemo-therapy.

Conclusion

All the experiments showed that the excellent antitumor performance of FA-Py/SP/GOx/Dox NPs, which provided an new method for pillar[5]arene-based supramolecular polymer for biomedical applications.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01237-0.

Pillararenes: fascinating planar chiral macrocyclic arenes

Chiral macrocycles possess significant value in chiral science and supramolecular chemistry. Pillararenes, as a class of relatively young supramolecular macrocyclic hosts, have been widely used for host-guest recognition and self-assembly. Since the position of substituents on the benzene rings breaks the molecular symmetry (symmetric plane and symmetric center), pillararenes possess planar chirality. However, it is a great challenge to synthesize stable and resolvable enantiomers because of the easy rotation of the phenylene group. In this review, we summarize the construction methods of resolvable chiral pillararenes. We also focus on their applications in enantioselective recognition, chiral switches, chirality sensing, asymmetric catalysis, circularly polarized luminescence, metal-organic frameworks, and highly permeable membranes. Finally, we discuss the future research perspectives in this field of pillararene-based planar chiral materials. We hope that this review will encourage more researchers to work in this exciting field.

One-Pot and Shape-Controlled Synthesis of Organic Cages

Organic cages are fascinating because of their well-defined 3D cavities, excellent stability, and accessible post-modification. However, the synthesis is normally realized by fragment coupling approach in low yields. Herein, we report one-pot, gram-scale and shape-controlled synthesis of two covalent organic cages (box-shaped [4]cage and triangular prism-shaped [2]cage) in yields of 46 % and 52 %, involving direct condensation of triangular 1,3,5-tris(2,4-dimethoxyphenyl)benzene monomer with paraformaldehyde and isobutyraldehyde, respectively. The cages can convert into high-yielding per-hydroxylated analogues. The [2]cage can be utilized as gas chromatographic stationary phase for high-resolution separation of benzene/cyclohexane and toluene/methylcyclohexane. By changing the central moiety of the triangular monomer and/or aldehyde, this synthetic method would have the potential to be a general strategy to access diverse cages with tunable shape, size, and electronic properties.

Pillar[5]-bis-trithiacrown: Influence of Host-Guest Interactions on the Formation of Coordination Networks

A pillar[5]-bis-trithiacrown (L) capable of metal binding and organic guest threading simultaneously has been employed, and the influence of dinitrile guests [CN(CH₂)_nCN (n = 2-6: abbreviated C2-C6)] on the coordination behaviors has been investigated. When the ditopic ligand L was reacted with HgCl₂ in the presence of the C2-C6 guests, the shorter guests C2 and C3 afforded a two-dimensional coordination polymer [Hg₇Cl₁₄(C2@L)₂]_n (1) and a one-dimensional coordination polymer [(Hg₃Cl₆)₂(C3@L)₂]_n (2), respectively. In 1 and 2, each dinitrile guest threads into the pillararene cavity to form a C2@L or C3@L unit via the host-guest interaction. Further linking of these units by exocyclic Hg-S bonds and anion coordination lead to the formation of coordination products with different dimensionalities. While the use of the longer guests C4-C6 under the same reactions yielded a discrete dimercury(II) complex 3, [Hg₂Cl₄(CH₃CN@L)] which contains one acetonitrile solvent molecule because the longer dinitriles do not serve as effective guests. In the NMR and UV-vis studies, the association constants (log K_1:1) for the host-guest interactions of L with the dinitrile guests are C2 (4.75) > C3 (4.17) ≫ C4 (2.85) > C5 (2.45) > C6 (too small), indicating that the shorter guests C2 or C3 interact more strongly than longer ones due to the confined interior space of L. Taken collectively, the C2 and C3 guests with proper size-matching promote the formation of coordination polymers and vice versa, suggesting that the guest size could be a controlling factor.

Pillar[5]arene-Based 3D Hybrid Supramolecular Polymer for Green Catalysis in Water

Pillar[n]arene-based supramolecular polymers have attracted great interest because of their tunable morphologies and external stimuli responsiveness. However, most of the investigations of supramolecular polymers previously reported were focused on their formation and transformation, and investigations on their applications are rare. Herein, we designed and prepared hybrid polymeric materials by incorporating Pd nanoparticles into a supramolecular polymer, constructed from a pillar[5]arene dimer and a three-arm guest. The obtained hybrid polymer was fully characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy-energy-dispersive X-ray mapping, and X-ray diffraction technologies. Importantly, the hybrid supramolecular polymeric materials exhibited desirable catalytic activity for reductions of toxic nitroaromatics and C-C bond-forming Suzuki-Miyaura reaction in aqueous solution.

Incorporation of carboxylated pillar[5]arene and strontium(ii) into supramolecular coordination complexes of different nuclearities

The nuclearity of the coordination complexes of carboxylated pillar[5]arene and strontium(ii) can be varied with the aid of phenanthroline as a coligand.

Cationic Water-Soluble Pillar[5]arene-Modified Cu2-xSe Nanoparticles: Supramolecular Trap for ATP and Application in Targeted Photothermal Therapy in the NIR-II Window

With the rapid progress of nanotechnology, near-infrared (NIR), light-assisted phototherapy as a minimally invasive local cancer therapy, especially photothermal therapy (PTT), has captured broad research attention in recent years. However, combined target molecules with a PTT system through reversible supramolecular interactions has been reported rarely. In this work, we constructed a supramolecular nanosystem combining ATP capture and target PTT based on cationic pillar[5]arene (CWP5)-functionalized Cu_2-xSe nanoparticles (Cu_2-xSe@CWP5 NPs). Cu_2-xSe@CWP5 NPs, with an average diameter of approximately 100 nm and strong absorption in the near-infrared-II window, were prepared in water through a facile one-step in situ synthesis method, then (4-carboxybutyl)triphenylphosphonium bromide (TPP), a mitochondria-targeted molecule, was modified on the surface of the particles through the host-guest recognition. Upon irradiation with a 1064 nm laser, the obtained Cu_2-xSe@CWP5/TPP NPs showed remarkably photothermal ablation capability to HeLa cells. Importantly, our Cu_2-xSe@CWP5/TPP NPs exhibited excellent therapeutic effect due to the combination of inhibited hydrolysis of ATP and targeted photothermal therapy upon in vitro and in vivo studies. Significantly, through host-guest interactions, we can modify different types of target molecules within this PTT system at will.

Metallosupramolecules of pillar[5]-bis-trithiacrown including a mercury(ii) iodide ion-triplet complex

A combination of pillar[5]-bis-trithiacrown (L) and mercury(ii) halides afforded a monomer complex (Cl^--form), a 1-D coordination polymer (Br^--form) and a supramolecular ion-triplet complex [(I·Hg·I)@L] (I^--form). In the ion-triplet complex, the host encapsulates the (I^--Hg²⁺-I^-) entity via Hg²⁺π and C-HI^- interactions, reflecting geometrical complementarity.

Helic[1]triptycene[3]arene: Synthesis, Complexation, and Formation of [2]Rotaxane Shuttle

A new macrocyclic arene, helic[1]triptycene[3]arene H, was conveniently synthesized in 37% yield by a one-pot reaction starting from 2,6-dimethoxyl-3,7-dihydroxymethyltriptycene. Macrocycle H showed fixed conformation in solution and could form 1:1 complexes with a series of neutral guests, secondary ammonium salts, and tertiary ammonium salts in both solution and solid states. The association constants between H and the neutral guests were between (1.23 ± 0.10) × 10² and (4.70 ± 0.47) × 10³ M^-1, while the association constants between H and the ammonium guests were between (1.35 ± 0.12) × 10³ and (1.59 ± 0.14) × 10⁵ M^-1. Moreover, H showed bigger association constants with secondary ammonium salts than those with tertiary ammonium salts possibly because of the steric hindrance effect and multiple intermolecular interactions. The stimuli-responsive complexation between H and the ammonium salts could be controlled by the addition and removal of acids and bases as well. Based on the host-guest complexation between H and the secondary ammonium salt, [2]rotaxane was further synthesized, and its shuttling motion could be efficiently controlled by an acid and base.

The Role of Calix[n]arenes and Pillar[n]arenes in the Design of Silver Nanoparticles: Self-Assembly and Application

Silver nanoparticles (AgNPs) are an attractive alternative to plasmonic gold nanoparticles. The relative cheapness and redox stability determine the growing interest of researchers in obtaining selective plasmonic and electrochemical (bio)sensors based on silver nanoparticles. The controlled synthesis of metal nanoparticles of a defined morphology is a nontrivial task, important for such fields as biochemistry, catalysis, biosensors and microelectronics. Cyclophanes are well known for their great receptor properties and are of particular interest in the creation of metal nanoparticles due to a variety of cyclophane 3D structures and unique redox abilities. Silver ion-based supramolecular assemblies are attractive due to the possibility of reduction by “soft” reducing agents as well as being accessible precursors for silver nanoparticles of predefined morphology, which are promising for implementation in plasmonic sensors. For this purpose, the chemistry of cyclophanes offers a whole arsenal of approaches: exocyclic ion coordination, association, stabilization of the growth centers of metal nanoparticles, as well as in reduction of silver ions. Thus, this review presents the recent advances in the synthesis and stabilization of Ag (0) nanoparticles based on self-assembly of associates with Ag (I) ions with the participation of bulk platforms of cyclophanes (resorcin[4]arenes, (thia)calix[n]arenes, pillar[n]arenes).