Zibin Zhang

Pillararenes, A New Class of Macrocycles for Supramolecular Chemistry

Because of the importance of novel macrocycles in supramolecular science, interest in the preparation of these substances has grown considerably. However, the discovery of a new class of macrocycles presents challenges because of the need for routes to further functionalization of these molecules and good host-guest complexation. Furthermore, useful macrocylic hosts must be easily synthesized in large quantities. With these issues in mind, the recently discovered pillararenes attracted our attention. These macrocycles contain hydroquinone units linked by methylene bridges at para positions. Although the composition of pillararenes is similar to that of calixarenes, they have different structural characteristics. One conformationally stable member of this family is pillar[5]arene, which consists of five hydroquinone units. The symmetrical pillar architecture and electron-donating cavities of these macrocycles are particularly intriguing and afford them with some special and interesting physical, chemical, and host-guest properties. Due to these features and their easy accessibility, pillararenes, especially pillar[5]arenes, have been actively studied and rapidly developed within the last 4 years. In this Account, we provide a comprehensive overview of pillararene chemistry, summarizing our results along with related studies from other researchers. We describe strategies for the synthesis, isomerization, and functionalization of pillararenes. We also discuss their macrocyclic cavity sizes, their host-guest properties, and their self-assembly into supramolecular polymers. The hydroxyl groups of the pillararenes can be modified at all positions or selectively on one or two positions. Through a variety of functionalizations, researchers have developed many pillararene derivatives that exhibit very interesting host-guest properties both in organic solvents and in aqueous media. Guest molecules include electron acceptors such as viologen derivatives and (bis)imidazolium cations and alkyl chain derivatives such as n-hexane, alkanediamines, n-octyltrimethyl ammonium, and neutral bis(imidazole) derivatives. These host-guest studies have led to the fabrication of (pseudo)rotaxanes or poly(pseudo)rotaxanes, supramolecular dimers or polymers, artificial transmembrane proton channels, fluorescent sensors, and other functional materials.

Pillar[6]arene-Based Photoresponsive Host–Guest Complexation

The trans form of an azobenzene-containing guest can complex with a pillar[6]arene, while it cannot complex with pillar[5]arenes due to the different cavity sizes of the pillar[6]arene and the pillar[5]arenes. The spontaneous aggregation of its host-guest complex with the pillar[6]arene can be reversibly photocontrolled by irradiation with UV and visible light, leading to a switch between irregular aggregates and vesicle-like aggregates. This new pillar[6]arene-based photoresponsive host-guest recognition motif can work in organic solvents and is a good supplement to the existing widely used cyclodextrin/azobenzene recognition motif.

Pillar[6]arene/Paraquat Molecular Recognition in Water: High Binding Strength, pH-Responsiveness, and Application in Controllable Self-Assembly, Controlled Release, and Treatment of Paraquat Poisoning

The complexation between a water-soluble pillar[6]arene (WP6) and paraquat (G1) in water was investigated. They could form a stable 1:1 [2]pseudorotaxane with an extremely high association constant of (1.02 ± 0.10) × 10(8) M(-1) mainly driven by electrostatic interactions, hydrophobic interactions, and π-π stacking interactions. This molecular recognition has not only high binding strength but also pH-responsiveness. The threading and dethreading processes of this [2]pseudorotaxane could be reversibly controlled by changing the solution pH. This novel recognition motif was further used to control the aggregation of a complex between WP6 and an amphiphilic paraquat derivative (G2) in water. The reversible transformations between micelles based on G2 and vesicles based on WP6⊃G2 were realized by adjusting the solution pH due to the pH-responsiveness of WP6. The controlled release of water-soluble dye molecules from the vesicles could be achieved by the collapse of the vesicles into the micelles upon changing the solution pH to acidity. Additionally, the high binding affinity between WP6 and paraquat could be utilized to efficiently reduce the toxicity of paraquat. After the formation of a stable host-guest complex between WP6 and paraquat, less opportunity was available for paraquat to interact with the reducing agents in the cell, which made the generation of its radical cation more difficult, resulting in the efficient reduction of paraquat toxicity.

A Water-Soluble Pillar[6]arene: Synthesis, Host–Guest Chemistry, and Its Application in Dispersion of Multiwalled Carbon Nanotubes in Water

The first water-soluble pillar[6]arene was synthesized. Its water solubility can be reversibly controlled by changing the pH. This solubility control was used in reversible transformations between nanotubes and vesicles and dispersion of multiwalled carbon nanotubes in water.

Syntheses of copillar[5]arenes by co-oligomerization of different monomers.

Three copillar[5]arenes, pillar[5]arenes containing different repeating units, were successfully prepared by co-oligomerization of different monomers. It was demonstrated that the yields of pillararenes could be improved by using hydroquinone diethers with appropriate aliphatic chain lengths. Pseudorotaxane-type threaded structures were obtained in the solid state by the inclusion of an n-hexane molecule into the cavity of either a homopillar[5]arene, a pillar[5]arene containing only one repeating unit, or a copillar[5]arene.

DIBPillar[n]arenes (n = 5, 6): Syntheses, X-ray Crystal Structures, and Complexation with n-Octyltriethyl Ammonium Hexafluorophosphate

DIBPillar[n]arenes (n = 5, 6) were synthesized. They showed different host-guest properties with n-octyltriethyl ammonium hexafluorophosphate G due to their different cavity sizes. DIBpillar[5]arene showed no complexation with G, while DIBpillar[6]arene formed a 1:1 complex with G with an association constant of 334 (±24) M(-1) in chloroform. In this letter, the first pillar[6]arene crystal structure and the first investigation of the host-guest chemistry of pillar[6]arenes are reported.

A solvent-driven molecular spring

A solvent-driven doubly threaded rotaxane dimer based on an amino-modified copillar[5]arene was prepared using bis(trifluoromethyl)phenyl isocyanate as stoppers. By comparison of proton NMR spectra of the rotaxane dimer and the control compound, the inclusion-induced shielding effects of the decyl protons of the dumbbell compound were estimated. From the crystal structures of previously reported analogous pillar[5]arene/alkane pseudorotaxanes, we know that four methylenes can be totally encapsulated in the pillar[5]arene cavity. When a pillar[5]arene is swaying along a guest with a long linear alkyl chain (more than four methylenes), its cavity statistically locates on the four methylenes whose protons showed relatively larger upfield shifts. Based on this, the length of the rotaxane dimer can be estimated. In CDCl3, it was in a contracted state with a length of 31 A. In DMSO-d6, it was in a extended state with a length of 37 A. Moreover, as the polarity of the solvent is changing, the length of the rotaxane dimer can change continuously as the contraction/stretching systems work in living organisms. Therefore, we can control the length of this molecular spring as needed.

A non-symmetric pillar[5]arene-based selective anion receptor for fluoride

A novel non-symmetric pillar[5]arene-based anion receptor containing multiple triazole anion-binding sites was designed and synthesized. It has high affinity and selectivity for the fluoride anion.

Formation of a Cyclic Dimer Containing Two Mirror Image Monomers in the Solid State Controlled by van der Waals Forces

Two new copillar[5]arenes were prepared. They are arranged in two completely different motifs, a cyclic dimer containing two monomers with two different conformations that are mirror images of each other and linear supramolecular polymers in the solid state. Not only has it been shown that to form this kind of dimer is a unique feature associated with pillar[5]arene macrocycles but also it was demonstrated that weak van der Waals forces can be used to control the self-organization of monomers during their supramolecular polymerization process.

Gold nanoparticles stabilized by an amphiphilic pillar[5]arene: preparation, self-assembly into composite microtubes in water and application in green catalysis

Gold nanoparticles stabilized by an amphiphilic pillar[5]arene (AP5) are prepared. They can be decorated on the surfaces of microtubes prepared from self-assembly of AP5 to form template composite microtubes (TCMTs). They can also be used in the fabrication of self-assembled composite microtubes (SCMTs) in water without any external assistance. A combination of UV-Vis spectra, Fourier transform-infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, X-ray diffraction measurements, and thermogravimetric analysis is used to confirm the formation of these composite microtubes. The SCMTs are stable at high temperature, with strong acid, with strong base, and under sonication. Interestingly, the properties of SCMTs are different from those of TCMTs and SCMTs can be used in green catalysis with a yield loss within 3% for 20 cycles. This work provides a new method to create amphiphilic nanoparticles for the fabrication of novel functional materials and devices by self-assembly.