Xiaodong Chi

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.

A Multiresponsive, Shape‐Persistent, and Elastic Supramolecular Polymer Network Gel Constructed by Orthogonal Self‐Assembly

A cross-linked supramolecular polymer network gel is designed and prepared, which shows reversible gel-sol transitions induced by changes in pH, temperature, cation concentration, and metal co-ordination. The gel pore size is controlled by the amount of cross-linker added to the system, and the material can be molded into shape-persistent, free-standing objects with elastic behavior. These features are all due to the dynamically reversible host-guest complexation and good mechanical properties of the cross-linked polymer network. No single organogel has previously been reported to possess all of these features, making this supramolecular gel an unprecedentedly intelligent soft material.

Development of Pseudorotaxanes and Rotaxanes: From Synthesis to Stimuli-Responsive Motions to Applications

Stimuli-Responsive Motions to Applications Min Xue,† Yong Yang,‡ Xiaodong Chi,† Xuzhou Yan,† and Feihe Huang*,† †State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China

A Dual-Responsive Supra-Amphiphilic Polypseudorotaxane Constructed from a Water-Soluble Pillar[7]arene and an Azobenzene-Containing Random Copolymer

Macromolecular supra-amphiphiles refer to a kind of macromolecular amphiphiles whose hydrophlic and hydrophobic parts are connected by noncovalent forces. They have applications in various fields, such as drug delivery, sensor systems, and biomedical materials. Here we report a novel molecular recognition motif between a new thermoresponsive water-soluble pillar[7]arene (WP7) and an azobenzene derivative. Furthermore, we utilized this recognition motif to construct the first pillararene-based supra-amphiphilic polypseudorotaxane which can self-assemble to form vesicles in water. Due to the dual-responsiveness of the molecular recognition motif (the thermoresponsiveness of WP7 and photoresponsiveness of azobenzene), the reversible transformations between solid nanospheres based on the self-assembly of the polymer backbone and vesicles based on the self-assembly of the supra-amphiphilic polypseudorotaxane were achieved by adjusting the solution temperature or UV-visible light irradiation. These dual-responsive aggregation behaviors were further used in the controlled release of water-soluble dye calcein molecules.

per-Hydroxylated Pillar[6]arene: Synthesis, X-ray Crystal Structure, and Host–Guest Complexation

A per-hydroxylated pillar[6]arene was prepared. Single-crystal X-ray analysis demonstrated that its molecules are arranged in an up-to-down manner to form infinite channels in the solid state. Its host-guest complexation with a series of bispyridinium salts in solution was further investigated. It was found that the per-hydroxylated pillar[6]arene could form a 1:1 complex with paraquat in acetone with an association constant of 2.2 × 10(2) M(-1). This complex is a [2]pseudorotaxane as shown by its crystal structure, which is the first pillar[6]arene-based host-guest complex crystal structure.

Supramolecular polymer nanofibers via electrospinning of a heteroditopic monomer.

Driven by the benzo-21-crown-7/secondary ammonium salt recognition motif, a linear supramolecular polymer was formed from self-organization of a low-molecular-weight self-complementary monomer in chloroform. From this supramolecular polymer, nanofibers were obtained successfully via electrospinning.

A bola-type supra-amphiphile constructed from a water-soluble pillar[5]arene and a rod–coil molecule for dual fluorescent sensing

A bola-type supra-amphiphile was successfully prepared and demonstrated to act as a dual fluorescent sensor. It was constructed from a water-soluble pillar[5]arene (WP5) and an imidazolium functionalized rod–coil molecule (1) driven by the WP5–imidazolium molecular recognition. Compared with the rod–coil molecule, the bola-type supra-amphiphile has strong fluorescence due to the influence of two bulky WP5 rings at its two ends, which can suppress the electronic coupling of the quinquephenyl aromatic rings, thus leading to the enhanced fluorescence. Thanks to the dual stimuli-responsiveness of the host–guest interactions, the fluorescence intensity of the bola-type supra-amphiphile was weakened by two types of signals, addition of paraquat and decrease in pH. Hence, this bola-type supra-amphiphile can serve as a paraquat sensor and a pH sensor.

A Dual-Thermoresponsive Gemini-Type Supra-amphiphilic Macromolecular [3]Pseudorotaxane Based on Pillar[10]arene/Paraquat Cooperative Complexation

Herein, first we report the preparation of a thermoresponsive [3]pseudorotaxane from cooperative complexation between a water-soluble pillar[10]arene and a paraquat derivative in water. Then we successfully construct the first pillararene-based gemini-type supra-amphiphilic [3]pseudorotaxane from the water-soluble pillar[10]arene and a paraquat-containing poly(N-isopropylacrylamide) based on this new molecular recognition motif in water. This macromolecular [3]pseudorotaxane shows unique dual-thermoresponsiveness. Furthermore, it can self-assemble into polymeric vesicles at 37 °C in water. These vesicles can be further used in the controlled release of small molecules induced by cooling to 25 °C or heating to 60 °C.

pH-responsive assembly and disassembly of a supramolecular cryptand-based pseudorotaxane driven by π–π stacking interaction

Driven by π-π stacking interaction, a supramolecular cryptand-based [2]pseudorotaxane was formed and its formation was demonstrated to be pH-responsive.

[2]Pseudorotaxanes based on the recognition of cryptands to vinylogous viologens.

Host-guest complexation between two crown ether-based cryptands and two vinylogous viologens has been studied. Formation of [2]pseudorotaxanes from a dibenzo-24-crown-8-based cryptand and these vinylogous viologens can be reversibly controlled by adding and removing potassium cation in acetone. Furthermore, the complexation between a bis(m-phenylene)-32-crown-10-based cryptand and a vinylogous viologen exhibits a high association constant, 1.18 × 10(6) M(-1) in acetone, and leads to the formation of a supramolecular poly[2]pseudorotaxane in the solid state.