Supramolecular polymer gels: from construction methods to functionality

Supramolecular polymer gels (SPGs) are precisely designed gels brought together by noncovalent interactions to form three-dimensional network structures of polymers. SPGs combine the merits of supramolecular polymers and gels, such as stimuli-responsiveness, self-healing, and self-adaptation, which endows SPGs with potential application value in the fields of biomaterials, etc. Recently, much effort has been made to design new SPGs and related materials with high performance. Herein, we review the research endeavor and future directions of SPGs depending on the construction methods, topological structures, stimuli-responsiveness, and functionality. We hope that the review will provide reference values for the researchers working in supramolecular chemistry and gels.

Functional supramolecular gels based on pillar[n]arene macrocycles

Supramolecular gels constructed from low-molecular-weight gelators via noncovalent interactions have received increasing attention. The rapid development of stimuli-responsive supramolecular gels with attractive properties is highly desirable to meet the ever-growing demand of materials science and chemistry. The inherent reversible and dynamic nature of noncovalent interactions in supramolecular gels endows the materials with sensing, processing, and actuating functions in response to specific environmental changes and offers them great potential in flexible biomaterials and intelligent devices. In particular, pillar[n]arenes with symmetrical pillar-shaped architectures have been recognized as an emerging class of synthetic macrocycles after crown ethers, cyclodextrins, calixarenes, and cucurbiturils, and proven to be excellent candidates for the fabrication of functional supramolecular gels due to their many advantages including facile synthesis, diverse functionalization, and appealing host-guest properties. This review provides a comprehensive overview of recent progress in supramolecular gels involving pillar[n]arenes and their derivatives as synthetic macrocyclic arenes, from the viewpoints of the synthetic approach, controllable assembly, stimuli-responsiveness, and functions. Perspectives of this burgeoning field of research are also given at the end.

A dual redox-responsive supramolecular amphiphile fabricated by selenium-containing pillar[6]arene-based molecular recognition

A dual redox-responsive pillar[6]arene-based supramolecular amphiphile was fabricated in water. The self-assembly behavior of this supramolecular amphiphile in response to dual redox stimuli was investigated.

Multi-stimuli-responsive supramolecular gel constructed by pillar[5]arene-based pseudorotaxanes for efficient detection and separation of multi-analytes in aqueous solution

Here, a novel pseudorotaxanes-type crosslinker of a supramolecular polymer network (WP5-PN) has been constructed from a host water-soluble pillar[5]arene (WP5) and a guest naphthalene dimethylamine derivative (PN) via a stepwise process involving multiple non-covalent interactions. The obtained supramolecular polymers were able to transform into a supramolecular polymer gel (WP5-PN-G) and show AIE properties in DMSO-H2O binary solution. Interestingly, due to the dynamic and reversible nature of non-covalent interactions, the resultant supramolecular polymer gels exhibited external stimuli-responsiveness to different parameters, such as temperature, acid-base, competitive guest and mechanical stress. Moreover, WP5-PN-G showed fluorescent response for Fe3+ and Cu2+, while its xerogel showed excellent recyclable separation properties for these metal ions with adsorption rates up to 98.07% and 95.38%, respectively. Moreover, by rational introduction of these metal ions into the WP5-PN-G, corresponding metal ion coordinated metallogels, such as WP5-PN-FeG and WP5-PN-CuG were obtained. These metallogels could selectively and sensitively sense F- and CN-, respectively. The detection limits of these metallogels for F- and CN- were about 1 × 10-8 M. The WP5-PN-G has potential applications in multi-analytes detection and separation as well as fluorescent display materials.

Construction of Metallacage-Cored Supramolecular Gel by Hierarchical Self-Assembly of Metal Coordination and Pillar[5]arene-Based Host-Guest Recognition

In this work, a Pd₂ L₄ metallacage 2•([BF₄ ]^- )₄ with four pillar[5]arene units is first prepared and characterized by 1D multinuclear NMR (¹ H, ¹¹ B, and ¹⁹ F NMR), 2D ¹ H-¹ H correlation spectra, ¹ H-¹³ C heteronuclear single quantum coherence, and diffusion-ordered NMR spectroscopy, and electrospray ionization time-of-flight mass spectrometry. By the introduction of a ditopic guest molecule 3 into a chloroform solution of 2•([BF₄ ]^- )₄ , a supramolecular polymer network gel is successfully constructed based on the metal coordination interactions and host-guest recognition between the pillar[5]arene units of 2•([BF₄ ]^- )₄ and neutral ditopic guest molecule 3. The temperature and pH responsivenesses of the supramolecular gel are studied, which are further employed for the controlled release of different cargos. As a demonstration, emodin and methylene blue are trapped in the cavities of the metallacage and in the pores of the supramolecular gel, respectively. Methylene blue is first released along with the gel-sol transition while emodin is then released by the further addition of acid to destroy the metallacage. This study explores the use of metallacage-cored supramolecular network gels for sequential controlled release and contributes to the development of smart and adaptive materials.

A novel pillar[5]arene-based supramolecular organic framework gel to achieve an ultrasensitive response by introducing the competition of cationπ and ππ interactions

Ultrasensitive response properties are an intriguing concern for stimuli-responsive materials. Herein, we report a novel method to achieve an ultrasensitive response by introducing the competition of cationπ and ππ interactions into a pillar[5]arene-based supramolecular organic framework (SOF-AMP). SOF-AMP was constructed with a novel bis-naphthalimide functionalized pillar[5]arene, which was able to form a stable supramolecular gel (SOF-AMP-G) in cyclohexanol. Interestingly, SOF-AMP-G shows an ultrasensitive response to Fe3+ through the competition of cationπ and ππ interactions. Meanwhile, the Fe3+ coordinated SOF (MSOF-Fe) shows an ultrasensitive response to H2PO4-. SOF-AMP-G displayed yellow fluorescence whereas, after the addition of 0.5 equiv. of Fe3+ to SOF-AMP-G, the yellow fluorescence was quenched. The detection limit of SOF-AMP-G for Fe3+ is 7.54 × 10-9 M. More interestingly, the Fe3+ coordinated SOF gel (MSOF-Fe-G) could sense H2PO4- with a fluorescence "turn-on". The detection limit of MSOF-Fe-G for H2PO4- is 4.21 × 10-9 M. Simultaneously, the Fe3+ and H2PO4- responsive thin films based on these SOF gels were prepared. Moreover, these SOF gels could be used as ultrasensitive ion sensors, fluorescent display materials and sensitive logic gates.

A dual-responsive cross-linked supramolecular polymer network gel: hierarchical supramolecular self-assembly driven by pillararene-based molecular recognition and metal–ligand interactions

A novel 2,2′-bipyridine-bridged pillar[5]arene dimer was synthesized and further applied for constructing linear supramolecular polymers and supramolecular polymer network gels.

A multi-stimuli responsive metallosupramolecular polypseudorotaxane gel constructed by self-assembly of a pillar[5]arene-based pseudo[3]rotaxane via zinc ion coordination and its application for highly sensitive fluorescence recognition of metal ions

A novel pillar[5]arene-based metallosupramolecular polypseudorotaxane gel has been successfully prepared.

Pillar[5]arene-Based Supramolecular Organic Framework with Multi-Guest Detection and Recyclable Separation Properties

The selective detection and separation of target ions or molecules is an intriguing issue. Herein, a novel supramolecular organic framework (SOF-THBP) was constructed by bis-thioacetylhydrazine functionalized pillar[5]arenes. The SOF-THBP shows a fluorescent response for Fe³⁺ , Cr³⁺ , Hg²⁺ and Cu²⁺ ions. The xerogel of SOF-THBP shows excellent recyclable separation properties for these metal ions and the absorption rates were up to 99.29 %. More interestingly, by rationally introducing these metal ions into the SOF-THBP, a series of metal-ion-coordinated SOFs (MSOFs) such as MSOF-Fe, MSOF-Hg and MSOF-Cu were constructed. These metal ions coordinated MSOFs could selectively sense F^- , Br^- , and l-Cys, respectively. The detection limits of these MSOFs for F^- , Br^- and l-Cys were about 10^-8  m.

A novel supramolecular organogel based on acylhydrazone functionalized pillar[5]arene acts as an I- responsive smart material

A novel organic gelator (PZ) has been synthesized by rationally connecting a pillar[5]arene moiety and a bis(hexadecyloxy)phenyl functionalized acylhydrazone moiety. PZ could self-assemble into a supramolecular polymer and form a stable organogel (OPZ) in cyclohexanol by multi-self-assembly driving forces such as C-Hπ, ππ, vdW and hydrogen bonding interactions. The organogel (OPZ) shows blue aggregation-induced emission (AIE). Interestingly, the organogel OPZ could sense iodide ions (I^-) in the gel-gel state with high selectivity and sensitivity. The detection limit of OPZ for I^- is 9.4 × 10^-8 M, indicating high sensitivity to I^-. Furthermore, a thin film based on OPZ was prepared, which could be used as a smart material for the detection of I^- as well as a fluorescent security display material.

A pillar[5]arene-based multiple-stimuli responsive metal-organic gel was constructed for facile removal of mercury ions

A thioacetohydrazide functionalized pillar[5]arene was synthesized, which could further assemble into a linear supramolecular metal-organic polymer upon adding Zn²⁺. Furthermore, the obtained linear supramolecular metal-organic polymer could self-assemble to form a fluorescent supramolecular metal-organic gel at high concentration. When TBAOH was added to the viscous solution at high temperature, the obtained solution could not form a supramolecular metal-organic gel upon cooling. More importantly, when Hg²⁺ ions are added to the metal-organic gel, the strong blue fluorescence is clearly quenched, and this metal-organic gel (xerogel) could effectively remove Hg²⁺ from water. Simultaneously, a thin film based on the metal-organic gel was prepared, which was confirmed to be a convenient test kit for detecting Hg²⁺.

A novel iodination-triggered competitive coordination mechanism: indirect detection of Hg2+and I−using a simple copillar[5]arene-based fluorometric sensor

Proposed mechanism for the detection of Hg²⁺and I⁻byDBP5.

A redox-responsive supramolecular amphiphile fabricated by selenium-containing pillar[5]arene-based host–guest recognition

A novel redox-responsive molecular recognition motif was built between a neutral selenium-containing pillar[5]arene and a pyridinium bromide salt in water. It was further used to construct the first pillararene-based selenium-containing supramolecular amphiphile with application in controlled release.

Construction of stimuli-responsive supramolecular gel via bispillar[5]arene-based multiple interactions

A linear supramolecular polymer has been constructed from host–guest recognition. Furthermore, the linear supramolecular polymer could self-assemble to form a supramolecular gel at high concentration, which exhibited external stimuli-responsiveness.

An efficient iodide ion chemosensor and a rewritable dual-channel security display material based on an ion responsive supramolecular gel

The ion stimuli-responsive metallogel PbG could act as a highly selective and sensitive I⁻ sensor and a rewritable dual-channel security display material.

Two novel supramolecular metallogels constructed by platinum(ii) coordination and pillar[5]arene-based host–guest interactions

Two multi-responsive supramolecular metallogels were constructed by coordination-driven self-assembly, host–guest interaction and supramolecular polymerization.

A supramolecular polymer network gel with stimuli-responsiveness constructed by orthogonal metal ion coordination and pillar[5]arene-based host–guest recognition

A novel external stimuli-responsive supramolecular polymer network gel was fabricated by orthogonal Ag-coordination and pillar[5]arene-based host–guest interactions.

Host–guest interaction enhanced aggregation-induced emission and its application in cell imaging

A host–guest inclusion complex based on a monofunctionalized pillar[5]arene and a tetraphenylethene derivative was prepared, resulting in an enhanced emission from the tetraphenylethene-based guest, which was applied in cell imaging.

A cationic water-soluble biphen[3]arene: synthesis, host–guest complexation and fabrication of a supra-amphiphile

The first cationic water-soluble biphen[3]arene was synthesized, which was used to change the aggregation of the amphiphilic guest in water.

Fluorescent cross-linked supramolecular polymers constructed from a novel self-complementary AABB-type heteromultitopic monomer

A novel AABB-type heteromultitopic monomer (APOPV), having a self-complementary perpendicular structure, could solely self-assemble to fluorescent cross-linked supramolecular polymers.

Novel functionalized pillar[5]arene: synthesis, assembly and application in sequential fluorescent sensing for Fe3+ and F− in aqueous media

The synthesis, assembly and applications of novel functionalized pillar[5]arene in the sequential fluorescent sensing of Fe³⁺ and F⁻ in aqueous media is reported.

The synthesis, structure, and molecular recognition properties of a [2]calix[1]biphenyl-type hybrid[3]arene

The first [2]calix[1]biphenyl-type hybrid[3]arene was synthesized. Its molecular recognition was investigated by using 1-dihexylammonium hexafluorophosphate as a guest.

Supramolecular main-chain polycatenanes formed by orthogonal metal ion coordination and pillar[5]arene-based host–guest interaction

A novel supramolecular main-chain polycatenane was constructed by coordination-driven self-assembly, host–guest interaction and supramolecular polymerization.

A water-soluble biphen[3]arene: synthesis, host–guest complexation, and application in controllable self-assembly and controlled release

The first water-soluble biphen[3]arene was synthesized. Its pH-responsive host–guest complexation with secondary ammonium salts in water was investigated. This novel recognition motif was further used in controllable self-assembly and controlled release.

A “keto–enol tautomerization”-based response mechanism: a novel approach to stimuli-responsive supramolecular gel

By “keto–enol tautomerization”, gelator G3 can be self-assembled into a stable organogel (OG3) accompanied by strong aggregation induced emission (AIE). OG3 could dual-channel sense S²⁻ with specific selectivity via reversible sol–gel transition and fluorescent changes.

A dual-responsive supra-amphiphile based on a water-soluble pillar[7]arene and a naphthalene diimide-containing guest

A dual-responsive supra-amphiphile was firstly constructed based on a water-soluble pillar[7]arene and a naphthalene diimide-containing guest with a long alkyl chain and a trimethylammonium group. The morphology of the self-assembly formed from this supra-amphiphile could be adjusted by changing the solution pH or adding α-cyclodextrin.

A non-symmetric pillar[5]arene based on triazole-linked 8-oxyquinolines as a sequential sensor for thorium(iv) followed by fluoride ions

A novel non-symmetric pillar[5]arene bearing triazole-linked 8-oxyquinolines at one rim was synthesized and demonstrated as a sequential fluorescence sensor for thorium(iv) followed by fluoride ions with high sensitivity and selectivity.

Copillar[5]arene-based supramolecular polymer gel: controlling stimuli–response properties through a novel strategy with surfactant

Cartoon representation of self-assembly pathway of gelator DCP5-16 with G in CD₃CN.

A novel supramolecular polymer gel constructed by crosslinking pillar[5]arene-based supramolecular polymers through metal–ligand interactions

A novel heteroditopic A–B monomer was synthesized and used to construct linear supramolecular polymers utilizing pillar[5]arene-based host–guest interactions. Upon addition of Cu²⁺ ions, a supramolecular polymer gel formed through metal–ligand interactions.

An anthracene-appended 2:3 copillar[5]arene: synthesis, computational studies, and application in highly selective fluorescence sensing for Fe(iii) ions

A novel anthracene-appended 2:3 copillar[5]arene was prepared and it was further used as a Fe³⁺-selective fluorescent chemosensor over a wide range of metal ions. The binding mode was modeled by DFT and molecular dynamics computational calculations.

Formation of fluorescent supramolecular polymeric assemblies via orthogonal pillar[5]arene-based molecular recognition and metal ion coordination

A fluorescent supramolecular polymer was efficiently constructed by pillar[5]arene-based host–guest molecular recognition and zinc ion coordination.

Competitive coordination control of the AIE and micro states of supramolecular gel: an efficient approach for reversible dual-channel stimuli-response materials

An organogelator (G2) based on multi self-assembly driving forces, fluorescent signal groups and coordination binding sites was designed and synthesized. G2 could form a stable Cd(2+)-coordinated supramolecular metallogel (CdG) accompanied by strong brilliant blue aggregation-induced fluorescence emission (AIE). By the competitive coordination of Cd(2+) with gelator and I(−), the AIE of CdG could be reversibly switched "on-off-on" under gel–gel states via alternative adding I(−) and Cd(2+) into CdG. Interestingly, because of the competitive coordination of Cd(2+) with I(−), the micro structure of the CdG xerogel carried out dramatic changes and formed lots of micro cavities. These micro cavities could absorb iodine vapour and caused the color of CdG xerogel change from white to brown. The CdG could not only act as a convenient high selective and sensitive I(-) detection test kit (detection limit for I(-) is 1.0 × 10(-7) M) but also as rewritable dual-channel security display materials.

A water-soluble supramolecular polymer constructed by pillar[5]arene-based molecular recognition

A water-soluble linear supramolecular polymer was efficiently constructed driven by pillar[5]arene-based host–guest molecular recognition.

A novel strategy for the design of smart supramolecular gels: controlling stimuli-response properties through competitive coordination of two different metal ions

With the competitive coordination, the stimuli-response properties of the supramolecular gels are accurately controlled. The bimetal–gel CaFeG could act as H₂PO₄⁻ test kit and rewritable security display materials.