Recent Applications of Pillar[n]arene-Based Host-Guest Recognition in Chemosensing and Imaging

Pillar[n]arene is a novel kind of synthetic supramolecular macrocyclic host characterized by its particular pillar-shaped structure consisting of an electron-rich cavity and two finely adjustable rims. Benefiting from its rigid structure, facile synthesis, ease of functionalization, and outstanding host-guest chemistry, pillar[n]arene shows great potential for diverse applications. Significantly, the host-guest recognition of pillar[n]arene provides a novel approach for chemosensing and imaging. Herein, this Review critically and comprehensively reviews the applications of pillar[n]arene-based host-guest recognition in chemosensing and imaging. The sensing and imaging mechanisms as well as the unique roles and advantages of pillar[n]arene-based host-guest recognition are summarized. In addition, preparations of hybrid materials based on pillar[n]arene and inorganic materials are also introduced comprehensively in the light of chemosensing and imaging. Finally, current challenges and perspectives on pillar[n]arene-based host-guest recognition in chemosensing and imaging are outlined.

Construction of a pH-sensitive self-assembly in aqueous solutions based on a dansyl-modified β-cyclodextrin

Here we present a pH-responsive self-assembly based on a β-cyclodextrin (β-CD) derivative bearing a dansyl terminus (βCD-C₆-Dns). Vesicular structures were formed over the entire studied pH range (8.5-0.7); however, the molecular configuration and packing within the vesicles were different at different pH values. Intramolecular host-guest complexation occurred mainly between the dansyl group and β-CD at pH values where the dansyl group was not protonated. The alkyl chain also acted as a competitive guest to form host-guest inclusions as confirmed by 2D ¹H NMR measurements. The pH-responsive βCD-C₆-Dns vesicles have potential application prospects in pH-controlled drug release based on the low cytotoxicity of βCD-C₆-Dns.

Host-Guest Complexation of Monoanionic and Dianionic Guests with a Polycationic Pillararene Host: Same Two-Step Mechanism but Striking Difference in Rate upon Inclusion

Supramolecular dynamic studies provide the most direct information to elucidate the binding mechanisms of the systems and yet are underdeveloped in pillararene chemistry. Herein, we describe the first real-time study on the binding dynamics of a water-soluble per-substituted pillar[5]arene (H1) with pentanesulfonate (G1) and butane-1,4-disulfonate (G2). Both the host-guest complexes were formed via a two-step process. The first step, equilibrated within 1 ms for both guests, was associated with the formation of a 1:1 exclusion complex, and the second step was the conversion of this exclusion complex to the inclusion complex. Threading and dethreading processes in the second step for G2 were at least a million times slower than for G1. Kinetics results reveal that for H1, complexation with a charged guest may follow the same "two-step" mechanism regardless of the number of charged moieties in the guests and the rate of the complexation. This study may advance the mechanistic understanding necessary for further development of functional supramolecular systems.

A Binary Supramolecular Assembly with Intense Fluorescence Emission, High pH Stability, and Cation Selectivity: Supramolecular Assembly-Induced Emission Materials

We construct a fluorescent supramolecular system (TPE-Q₄ ⊂ DSP5) of excellent tolerance to a wide range of pH by the facile self-assembly of a new pillar[5]arene bearing disulfonated arms (DSP5) with an AIE-active tetraphenylethene-based tetratopic guest bearing four quaternary ammonium binding sites (TPE-Q₄), which exhibits strong blue emission even in dilute aqueous solutions along with much higher quantum yield and longer fluorescence lifetime than TPE-Q₄ itself. This appreciable property can be attributed to the supramolecular assembly-induced emission (SAIE) mechanism endowed by the host-guest inclusion complexation based on synthetic macrocycles. Remarkably, the enhanced fluorescence of the supramolecular assembly is quenched efficiently and exclusively by ferric ions in water with a high Stern-Volmer formula constant of 1.3 × 10⁵  mol^-1, demonstrating the excellent cation selectivity and visualized responsiveness in ion sensing and detection.

Thermoresponsive luminescence properties of polyfluorinated bistolane-type light-emitting liquid crystals

We developed and characterized four polyfluorinated bistolane derivatives. These compounds, which possess either two alkoxy substituents or an alkoxy group and a bromine atom in their two molecular terminals, were synthesized from readily available 4-alkoxy-1-ethynylbenzene with a facile three-step procedure. Their thermodynamic and photophysical properties were evaluated in detail, and they were found to display both liquid-crystalline (LC) and photoluminescence properties. Remarkably, the photoluminescence behaviors dramatically changed during the thermal phase transition between the crystal and LC phases. Thus, these polyfluorinated bistolanes may be promising candidates for thermoresponsive luminous molecules.

Self-locked dipillar[5]arene-based pseudo[1]rotaxanes and bispseudo[1]rotaxanes with different lengths of bridging chains

The relationships between lengths of bridging chains and self-locked behaviors of dipillar[5]arene-based pseudo[1]rotaxanes and bispseudo[1]rotaxanes were studied in detail.

Stronger host–guest binding does not necessarily give brighter particles: a case study on polymeric AIEE-tunable and size-tunable supraspheres

Compared with the tritopic guest without chains, the tritopic guest with flexible alkyl chains bound to the polymeric host more strongly and induced the formation of larger but duller supraspheres.

Bioinspired ultrasound-responsive fluorescent metal–ligand cross-linked polymer assemblies

We proposed a general strategy to construct bioinspired ultrasound-responsive fluorescent metal–ligand cross-linked polymer assemblies.

Pillararene-based fluorescent chemosensors: recent advances and perspectives

This feature article summarizes recent research in the pillararene-based fluorescent chemosensor field in terms of ion sensing, small molecule recognition, biomolecule detection, fluorescent supramolecular aggregates, and biomedical imaging.

The first water-soluble pillar[5]arene dimer: synthesis and construction of a reversible fluorescent supramolecular polymer network in water

The first water-soluble pillar[5]arene dimer was successfully designed and synthesized. It can complex with carboxylate anion functionalized tetraphenyl ethylene to form a reversible fluorescent supramolecular polymer network in water.

Host-Guest Complexes of Carboxylated Pillar[n]arenes With Drugs

Pillar[n]arenes are a new family of nanocapsules that have shown application in a number of areas, but because of their poor water solubility their biomedical applications are limited. Recently, a method of synthesizing water-soluble pillar[n]arenes was developed. In this study, carboxylated pillar[n]arenes (WP[n], n = 6 or 7) have been examined for their ability to form host-guest complexes with compounds relevant to drug delivery and biodiagnostic applications. Both pillar[n]arenes form host-guest complexes with memantine, chlorhexidine hydrochloride, and proflavine by ¹H nuclear magnetic resonance and modeling. Binding is stabilized by hydrophobic effects within the cavities, and hydrogen bonding and electrostatic interactions at the portals. Encapsulation within WP[6] results in the complete and efficient quenching of proflavine fluorescence, giving rise to "on" and "off" states that have potential in biodiagnostics. The toxicity of the pillar[n]arenes was examined using in vitro growth assays with the OVCAR-3 and HEK293 cell lines. The pillar[n]arenes are relatively nontoxic to cells except at high doses and after prolonged continuous exposure. Overall, the results show that there could be a potentially large range of medical applications for carboxylated pillar[n]arene nanocapsules.

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.

Fabrication of a cross-linked supramolecular polymer on the basis of cucurbit[8]uril-based host–guest recognition with tunable AIE behaviors

A photo-responsive cross-linked supramolecular polymer was prepared based on a ternary host–guest molecular recognition motif between cucurbit[8]uril and 1,1-dimethyl-4,4-bipyridinium dication and azobenzene derivative, and its AIE behavior was also investigated.

A diols-responsive triple-component supra-amphiphile constructed from pillar[5]arene-based recognition

A triple-component supra-amphiphile is constructed from dynamic covalent chemistry and pillar[5]arene-based host–guest recognition. The diol-responsive self-assembly of this ternary supra-amphiphile is investigated in aqueous solution.

A pillar[6]arene-based [2]pseudorotaxane in solution and in the solid state and its photo-responsive self-assembly behavior in solution

A pillar[6]arene-based [2]pseudorotaxane was constructed in solution and studied in the solid state, and its photo-responsive self-assembly behavior in solution was investigated.