Host-Guest Complexation And The Self-Assembly Behaviors Based On Pillar [5]Arenes

The emergence of crown ethers as the first generation of macrocyclic host has promoted the development of supramolecular chemistry. From then on, different kinds of macrocyclic hosts, for instance, cyclodextrins, calixarenes and cucurbiturils, came into our view, which has been widely used in construction of host-guest systems and functional supramolecular assemblies. Pillararenes, a new class of macrocycles with symmetric pillar structure and electron-donating cativites, which were first reported in 2008, have shown interesting host-guest properties with different guests and proven to be versatile macrocycles in the fabrication of dynamic supramolecular assemblies, mechanically interlocked structures and controlled release systems, and so forth. In this dissertation, we investigated the host-guest properties, the preparation of [3]pseudorotaxane and supramolecular polymers and the self-assembly behaviors based on pillar[5]arenes.In the first part, we designed a new pillar[5]arene-based heteroditopic receptor containing a urea anion recognition site. It was demonstrated that the introduction of the urea group remarkably promoted the binding affinity to n-octyltriethyl ammonium salts with different counterions in chloroform since the corresponding monotopic host 1,4-dimethoxypillar[5]arene showed weak complexation with these guests. The important role of the urea unit was investigated and it was indicated that the urea group and the macrocyclic unit not only provided positioned binding sites for the guests but also acted cooperatively in binding these guests.In the second part, we reported a novel [3]pseudorotaxane based on a simple copillar[5]arene and its monomer. Actually, it was serendipitously obtained through synthesis of copillar[5]arene. Single crystal X-ray analysis revealed that this [3]pseudorotaxane was made up of the copillar[5]arene and its monomer with a molar ratio of 2:1.In the third part, a muscle-like metallo-supramolecular polymer based on a solvent-driven [c2]daisy chain was prepared from an amino-modified pillar[5]arene. The integration of terpyridine moieties on both ends of the [c2]daisy chain and the stiff architecture of pillar[5]arene units facilitated the efficient formation of the metallo-supramolecular polymer. UV/Vis absorption spectroscopy, dynamic light scattering, transmission electron microscopy and scanning electron microscopy were used to characterize the self-assembly behavior of the resulting polymer chains. From proton NMR studies, we confirmed that the pillar[5]arene-based [c2]daisy chain could change its length continuously in response to changes in the polarity of the solvent so the metallo-supramolecular polymer could change its length continuously by varying the solvent polarity based on the individual contraction and extension of each daisy chain repeating unit.In the fourth part, a novel bolaamphiphilic pillar[5]arene was prepared. It spontaneously formed responsive reverse multilamellar giant vesicles in chloroform and a gel in water/tetrahydrofuran (5:1, v/v). Dynamic light scattering, transmission electron microscopy, scanning electron microscopy, optical microscopy, and fluorescent microscopy were used to characterize its self-assembly in different solvents and aggregation behavior under external stimuli. These vesicles were not only stable for weeks, but also showed reversible thermal and dynamic properties under external physical stimuli. KPF6 and benzo-18-crown-6 could also be used as switches to turn-off and re-turn-on the assembly of vesicles. Moreover, the aggregation behavior of this bolaamphiphilic pillar[5]arene could be changed by simply introducing host-guest chemistry. Besides, the gel formed from it showed reversible gel-sol phase transitions by heating and cooling, or adding and removing potassium cations.In the fifth part, we reported an enzyme-responsive drug delivery system constructed from a pillar[5]arene-based polymeric amphiphile which could self-assemble into micelles in water with relatively low critical micelle concentration. Encapsulation and controlled drug release were demonstrated with the chemotherapeutic drug doxorubicin (DOX). By employing activating enzymes, PEG chains were cleaved from the pillar[5]arene framework, which changed the amphiphilicity of the polymeric amphiphile and resulted in the disintegration of the aggregates, finally leading to the release of encapsulated DOX. Moreover, cell cytotoxicity studies revealed that drug-free micelles were biocompatible and loaded DOX in the micelles exhibited improved inhibition to MCF-7/ADR cell proliferation in comparison with free-form DOX.