Pillar[n]arene-Based Supramolecular Polymers

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This entry is adapted from the peer-reviewed paper 10.3390/s24051530

The field of fluorescence sensing, leveraging various supramolecular self-assembled architectures constructed from macrocyclic pillar[n]arenes, has seen significant advancement in recent decades.

fluorescence sensing environmental pollutants pillar[n]arene supramolecular

1. Introduction

Through noncovalent interactions, supramolecular chemistry unites components with different structures and functionalities ^[1]^[2]^[3]^[4]^[5]^[6]^[7]^[8], enabling the construction of dynamically reversible smart composites ^[9]^[10]^[11]^[12]. Increasing attention has been given to intelligent supramolecular structures formed by host–guest interactions, particularly for their stimuli-responsive properties and expansive application potential. As a vital segment of supramolecular chemistry, macrocyclic hosts have found extensive use across various domains. Presently, there are five prominent macrocyclic hosts, including cyclodextrins, calix[n]arenes, crown ethers, cucurbit[n]urils, and pillar[n]arenes. Each host can specifically accommodate different guest molecules by offering suitable cavities. Pillar[n]arenes, in particular, stand out due to their electron-rich and rigid cavities, symmetrical frame structure, and exceptional host–guest properties ^[13]^[14]^[15]^[16]^[17]^[18]^[19]^[20]^[21]. The positional modification of pillar[n]arenes endows them with distinct characteristics ^[22]^[23]^[24]^[25]^[26]^[27]^[28].

The coordination metal ions discussed in this review include Ag⁺, Hg²⁺, Cu²⁺, Zn²⁺, Pt²⁺, Fe³⁺, Al³⁺, Eu³⁺, and Tb³⁺. Host–guest complexes with metal coordination sites offer considerable scientific value, characterized by a structure resembling a pillar with many self-assembly driving forces, including cation–π, hydrophobic/hydrophilic, C–H–π, and π–π interactions, among others. Researchers categorize two types of supramolecular architectures with metal ion coordination sites: (i) functional pillar[n]arenes with metal coordination sites are initially designed and synthesized. The existing studies on pillar[n]arenes derive from the successful synthesis of pillar[5]arenes ^[29], followed by pillar[6]arenes ^[30]. Additionally, pillar[n]arenes with larger cavities (n = 6–15) can be synthesized from pillar[5]arenes by ring expansion, a process driven by kinetic control ^[31]. Due to the unique advantages of pillar[n]arenes, many novel functional pillar[n]arenes have emerged one after another ^[32]^[33]. The key factors for the design and synthesis of functional pillar[n]arene molecules mainly lie in the following two aspects: Firstly, the reaction time, concentration, temperature, feeding order, and other external conditions should be strictly controlled to minimize the production of various polymer byproducts when synthesizing pillar[n]arenes. Secondly, considering that the kind, position, and quantity of substituents on both sides of pillar[n]arenes have significant effects on their solubility, conformation, and host–guest properties, which directly influences the practical application of pillar[n]arenes, it is also challenging to design functional pillar[n]arenes. Pillar[n]arene derivatives reviewed in this paper mostly concentrate on the single- or dual-functional modifications of the macrocyclic host, including mono, double, and total substitution of the benzene rings. Pillar[n]arenes before functionalization often contain halogen atoms, azide groups, hydroxyl groups, or amino groups as the modification sites. Different reaction sites can adopt specific synthesis strategies to achieve high yield, easy separation, green synthesis, and other goals. The pillar[n]arenes can be also substituted by molecules (e.g., adenin, quinoline, acylhydrazone, thymine, pyridine, triazole, 1,3,4-oxadiazole, bis-2,2′:6′,2″-terpyridin, carboxyl, naphthalimide, thioacetylhydrazine, et al.) to possess the metal coordination ability. These functional pillar[n]arenes can further coordinate with metal ions in the presence or absence of guest molecules, generating supramolecular architectures with specific sensing capacities through multiple noncovalent interaction forces. (ii) Guest molecules with both host–guest recognition abilities and metal coordination sites are firstly designed and synthesized. The host–guest recognition groups are generally composed of quaternary ammonium groups, cyanogen groups, halogen atoms, or nitrogen heterocycles. Additionally, the involved coordination sites mainly consist of phenazine imidazole, thienyl functionalized diketopyrrolopyrrole, or multiple amide groups. After combining these functional groups into a small molecule, these guests can form complexes through host–guest interactions with pillar[n]arenes, which further coordinate with metal ions to generate supramolecular architectures with specific sensing capacities.

2. Single-Stimulus Responsive Sensors

Scheme 1 lists the molecular structures of the guest molecules and pillar[n]arenes that are discussed in this section.

Scheme 1. Molecular structures of pillar[n]arenes and guests in the section of single-stimulus responsive fluorescence sensors.

A prime example is the widespread concern over environmental pollution caused by heavy metal ions. Silver(I) (Ag⁺) ions are known for their excellent electrical and thermal conductivity, ductility, and stability. However, excessive silver can disrupt active enzymes by binding to sulfhydryl groups in proteins, leading to their accumulation in the food chain. Therefore, developing economical, simple, environmentally friendly, and efficient materials for detecting and removing Ag⁺ ions is crucial.

In 2022, Wang W.M. and Yang Y.W. designed a stable supramolecular system utilizing host–guest interactions and coordination effects ^[34]. They employed two adenine binding sites in pillar[5]arene (P1) and tetraphenylvinyl (TPE, G1) functionalized with cyano groups, which possess AIE properties (Figure 1). Spherical supramolecular aggregates formed via 1:1 coordination between P1’s adenines and Ag⁺ ions. The synergistic effect of coordination between P1 and Ag⁺, coupled with the host–guest interaction between P1 and G1, resulted in a crosslinked P1⊂G1@Ag⁺ assembly. This assembly triggered the restriction of intramolecular rotation (RIR) and SAIEE mechanisms. The fluorescence lifetime and fluorescence quantum yield of P1⊂G1@Ag⁺ were 4.60 ns and 58.38%, respectively, with the rate constant of nonradiative decay being half that of P1⊂G1. Under 365 nm UV lamp irradiation, the assembly displayed bright blue-green fluorescence upon Ag⁺ ion addition, permitting effective Ag⁺ ion adsorption and sensitive detection. Moreover, the supramolecular assembly could be easily processed without a decline in activity, offering a useful tool for practical applications.

Figure 1. Schematic diagram showing how to detect and remove Ag⁺ ions from water using the supramolecular assembly and the assembly regeneration process.

The need for effective devices for sensitive toxic gas detection is also paramount. For instance, hydrazine hydrate (DH) is extensively used in synthesizing pesticides and medicines, fuels for satellites and rockets, and as a preservation agent in nuclear and power facilities. However, DH is very harmful, causing severe damage to the skin and central organs upon human absorption. Consequently, developing flexible, fast, and sensitive materials and methods for DH detection is critically important.

A year later, Lin Q.’s group developed a metallic gel by coordinating the nitrogen atom of 5-hydroxyquinoline functionalized pillar[5]arene (P2) with Ag⁺ ions ^[35]. P2 formed a one-dimensional coordination polymer through interaction with Ag⁺ ions via the N atoms on the quinoline groups (Figure 2). Adjacent coordination polymers were interconnected through π–π interactions between the P2 groups, facilitating the metallogel’s formation. The mean squared displacement of the gel P2 was lower than that of the P2-Ag gel, indicating that the inclusion of Ag⁺ improved the flexibility of the P2-Ag gel. The supramolecular assembly was further analyzed from a microscopic morphology perspective. P2 alone exhibited a lamellar structure. After adding Ag⁺ ions, the resultant metallogel P2-Ag transformed into a folded membrane structure, which is attributable to the coordination bond between Ag⁺ ions and P2. When exposed to DH vapor, the folded membrane structure of P2-Ag altered into a microspherical structure, suggesting the disruption of Ag⁺ ion coordination and the formation of a microspherical structure by P2 based on the hydrophobic effect. This gel enabled multichannel sensitive detection of DH through visual, fluorescence, and electrochemical means. DH disrupted the coordination by reducing Ag⁺ ions under DMSO/H₂O conditions (f_w = 20%), leading to the collapse and fluorescence quenching of the metallogel. The lowest critical gelation concentration (CGC) was 8% (w/v, 10 mg/mL = 1%). The gel–sol transition temperature was 62–63 °C. Multichannel detection of DH could be conveniently and efficiently realized in both water and air through sound and light alarms. The LOD reached 0.10 mg/m³ in air and 2.68 × 10⁻⁸ mol/L in water, below the US Environmental Protection Agency’s standard for drinking water.

Figure 2. (a) A cartoon depicting the metallo-supramolecular polymer gel’s assembly method, (b) the PQ-Ag’s multichannel DH detection, and (c) the DH alarm circuit schematic diagram (1: electric source; 2: switch; 3: resistance; 4: electric capacity; 5: NPN triode; 6: resistance; 7: bulb; 8: PNP triode; 9: buzzer; 10: gel probe).

Mercury (Hg²⁺), among the most hazardous heavy metals, poses significant health and environmental risks. Exposure to Hg²⁺ ions, even in minute concentrations, presents a potential hazard to humans. For instance, in 2018, the group led by Lin Q. designed and synthesized another sensor based on pillararene AIEgens, which utilized bi-pillar[5]arene-based assemblies incorporating advanced AIEgens (P3) ^[36]. The assembly was driven by intermolecular hydrogen bonding (such as –N–H⋯C=O– and –C–H⋯N=C–), π–π stacking interactions, and hydrophobic effects. The Tyndall effect was observed in a 30% aqueous solution, with the critical aggregation fraction of water being 24% for P3, which exhibited a fluorescence quantum yield of 21%. P3 formed a sharp rod-shaped structure in DMSO/H₂O (f_w = 50%). The aggregated P3 was disassembled at low concentrations or high temperatures. A 1:2 complex was formed by binding P3 with Hg²⁺ ions (binding constant: 2.50 × 10³ L²/mol²). The coordination between the Hg²⁺ ions and the P3 acylhydrazone group served as the basis for the sensing mechanism. To explore practical applications, a glass sheet was submerged in a high concentration of P3 to generate a film. This film enabled the convenient detection of Hg²⁺ ions in water, effectively separating and sensitively detecting them with an LOD of 4.30 × 10⁻⁸ mol/L. This innovative bi-pillar[5]arene AIEgen could pave the way for new designs and developments in pillar[n]arene AIEgens.

The pillar[n]arenes include two types: pillar[5]arene and pillar[6]arene ^[37]. The latter was developed by Yang Y.W.’s group. They utilized the strong interactions between thymine (T) and Hg²⁺ ions. Building on this, Dai D.H. and Yang Y.W. constructed a crosslinked supramolecular polymer through host–guest interactions, utilizing a TPE-bridged bis(quaternary ammonium) guest (G2) with AIE characteristics and a newly constructed [2]biphenyl-extended pillar[6]arene with two thymine sites as arms (P4) (Figure 3). The thymine groups’ close T-Hg²⁺-T coupling with the Hg²⁺ ions led to the formation of spherical assemblies with an average diameter of 164 nm. Fluorescent emission occurred immediately upon the addition of Hg²⁺ ions. The introduction of Hg²⁺ ions into the supramolecular system initiated supramolecular SAIEE. With its integrated mode of operation, great selectivity, and a quick adsorption rate (removal efficiency: 90%), this supramolecular polymer effectively accomplished the real-time detection and removal of Hg²⁺ ions from water. The assembly of P4 and G2 demonstrated excellent recyclability, maintaining effectiveness over more than five cycles of the removal process.

Figure 3. Schematic illustration showing how to detect and remove Hg²⁺ ions from water using the regeneration–recycling process and the “switch-on” fluorescence of supramolecular polymers.

Beyond the contributions of the previously mentioned research groups, a self-assembly consisting of carboxylatepillar[5]arene sodium salts (P5) and a diketopyrrole-bridged bis-quaternary ammonium guest (G3) was reported by Jiang X.M. and Cao D.R. ^[38]. Adding P5 to a G3 aqueous solution resulted in the morphological transformation of multilayer nanostructures (Figure 4). This host–guest complex exhibited multiple forces, including electrostatic, hydrophobic interactions, and π–π stacking interactions in aqueous solutions. The developed supramolecular system effectively detected and removed Hg²⁺ in real environmental water samples. The Hg²⁺ ions demonstrated synergistic interactions, including coordination with G3 and P5 and the Hg²⁺-cavity, forming a crosslinked network of P5⊂G3@Hg²⁺. The method exhibited good selectivity with a low LOD of 7.17 × 10⁻⁷ mol/L. Furthermore, the quenched fluorescence could be recovered post-treatment with Na₂S, exhibiting a reversible process.

Figure 4. Schematic representations of the self-assembly of carboxylatopillar[5]arene sodium salts in the presence and absence of Hg²⁺ ions in water, as well as chemical structures and cartoon representations of the diketopyrrolopyrrole-bridged bis(quaternary ammonium) guest.

Copper ions (Cu²⁺) are also critical in various industries, and can result in soil contamination, bioaccumulation, and decreased agricultural production. Therefore, developing a selective and sensitive Cu²⁺ ion fluorescent chemical sensor, preferably a proportional chemical sensor, remains crucial in ion sensing research.

Pillar[5]arene was modified by Chang R. and Chang K.C. with five neighboring naphthalimide groups to form a new ligand, P6, for metal ion coordination, effectively serving as a ratiometric fluorescence sensor for Cu²⁺ ions in a CH₂Cl₂/CH₃CN = 1/1 system ^[39]. This sensitivity was also observed in 10% aqueous methanol solutions. The introduction of naphthalimide groups enhanced intramolecular π–π stacking. P6 exhibited dual emission, comprising both the monomer and excimer emissions of the naphthalimide moieties. A synergistic interaction occurred between Cu²⁺ ions and the triazole groups on P6. P6 rapidly bound with Cu²⁺, maintaining stable fluorescence intensity. Upon complexing with Cu²⁺ ions, the excimer emission of ligand P6 was weakened, while the monomer emission intensified. The binding complexation ratio of P6 with Cu²⁺ ions was 1:1, with a binding constant of (3.39 ± 0.40) × 10⁵ L²/mol² and an LOD of 1.85 × 10⁻⁶ mol/L. Particles varying in diameters from (192 ± 65) to (206 ± 67) nm were produced. The relative fluorescence quantum yields of P1 and P1-Cu²⁺ were 0.13 and 0.11, respectively. The pillar[5]arene framework may be further functionalized in future studies to improve its selectivity for particular metal ions, or to modify it for use with different sensing platforms.

Liu S.Y. and Han J. developed another sensor for Cu²⁺ ions ^[40]. They designed a pillar[5]arene framework by functionalizing it with a cyanobutoxy moiety (P7) and a 1,3,4-oxadiazole subunit. This structure facilitated host–guest interactions between the electron-rich pillar[5]arene cavities and appropriately-sized neutral cyanobutoxy moieties, resulting in brush supramolecular polymers. Notably, the larger electron-deficient 1,3,4-oxadiazole groups remained outside the pillar[n]arene (P7) cavity after the creation of the host–guest inclusions, acting as a “brush” and enhancing the ability of the self-assembled supramolecular materials to interact with metal ions, such as Cu²⁺ ions. The critical aggregation concentration of P7 was 6.00 × 10⁻² mol/L. These supramolecular brush-polymer architectures displayed distinct structural changes in response to fluorescence quenching after adding Cu²⁺ ions, suggesting a potential transformation into a crosslinked supramolecular network. The Irving–Williams order of stability might provide an explanation for the particular recognition of Cu²⁺. Therefore, this supramolecular brush polymer holds potential for application in metal cationic fluorescent chemical sensors.

In 2021, Chong H. et al. prepared a “three-component” supramolecular assembly by combining terpyridine attached pillar[5]arene (P8), cyano- and triazole-bearing alkyl chain (G4), and Zn²⁺ ions in a CHCl₃ and CH₃CN solvent system ^[41]. Terpyridine’s strong affinity for a variety of transition metal ions makes it an adaptable building block. The resulting composition exhibited a closely crosslinked porous morphology with nanoscale pore size. The P8 segment was incorporated into the cyano and triazole segment upon encapsulation. An organogel formed at a concentration of 1.00 mol/L. The polymerization was driven by host–guest interaction and metal-chelate cooperative forces. The P8 and Zn²⁺ assembly had a flake-like morphology. Nitrobenzene (picric acid, o-nitrobenzene, and phenol) was used as a sample. Of the three analytes, the assembly exhibited the highest sensitivity to picric acid, with an LOD of 1.66 × 10⁻⁴ mol/L. The quenching mechanism was believed to involve mixed processes of photo-induced electron transfer (PET) and fluorescence resonance energy transfer (FRET). This study represents a practical illustration of creating functional “multi-component” supramolecular systems with capabilities for explosive detection.

The role of Fe³⁺ ions as an essential metal element in human physiology is well-recognized. However, both deficiency and excess of Fe³⁺ ions beyond permissible limits can lead to severe health issues, such as anemia, tumorigenesis, organ dysfunction, and neurodegenerative diseases such as Alzheimer’s and Parkinson’s. Consequently, sensitive detection of Fe³⁺ ions is a critical concern in environmental and health monitoring. In 2018, Zhang Y.M. and Qi L. discovered that the pillar[5]arene structure (P9) could self-assemble into high-molecular-weight supramolecular π-gels with AIE properties ^[42]. Notably, the simplicity of the supramolecular system components enhanced its practical application value. Through π–π stacking interactions, P9 self-assembled into one-dimensional linear supramolecular π–gel chains. These π–gel chains interacted with the pillar[5]arene and naphthalimide to produce a two-dimensional supramolecular network. Subsequently, hydrophobic interactions compressed the two-dimensional network into microspheres. The CGC was 5% (w/v, 10 mg/mL = 1%). The gel–sol transition temperature was 43–45 °C. The LOD was 6.06 × 10⁻⁸ mol/L for Fe³⁺ ions. The gel demonstrated excellent recyclability and a 99.80% removal rate for Fe³⁺ ions in aqueous solutions. Additionally, the Fe³⁺ ion-coordinated supramolecular gel selectively sensed L-Cys with an LOD of 1.00 × 10⁻⁸ mol/L. This gel could be utilized in logic gates, offering significant advantages over previous logic devices in terms of reversibility, sensitivity, and prospective uses for ongoing transition metal and amino acid detection.

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1. Introduction

2. Single-Stimulus Responsive Sensors

References