Construction Of Complex Rotaxanes And Pseudorotaxanes Based On Crown Ethers And Cucurbiturils

As an important and intriguing area in supramolecular chemistry, the construction of rotaxanes and pseudorotaxanes based on the macrocyclic hosts and the relative guests has attracted more and more attentions. The investigations on their structures and functions not only help us to understand the non-covalent bonds, but also promote the development of molecular machinery and nanoscience. Herein, we have designed and constructed some novel pseudorotaxanes and rotaxanes, based on the unique complexation behaviors of macrocyclic hosts, including crown ethers and cucurbiturils and the relative guests. Furthermore, their switchable self-assembly behaviors under specific external stimuli have also been investigated. The details are as follows:1. The general aspect of supramolecular chemistry was discussed briefly. Some representative results on rotaxanes and pseudorotaxanes based on crown ethers and cucurbiturils were reviewed.2. Two pseudorotaxanes could be combined to form a twin-axial hetero[7]rotaxane by using the copper-catalyzed alkyne-azide "click" reaction. The synthetic route, in which twin-axial and single-axial rotaxanes are formed simultaneously, combines self-assembly and the formation of covalent bonds to ensure the correct positioning of the two types of rings in the final product. This strategy allows precise positional control in the final [n]rotaxane product and will thus be beneficial for the construction of more complicated interlocked molecules with well-defined structures and functions.3. A series of styryl pyridinium and4,4’-bipyridinium derivatives were synthesized. The self-assembly behaviors between some of these organic cationic guests and cucurbit[7,8]urils and a-cyclodextrin were investigated.4. Two stable supramolecular loops were successfully constructed by the molecular recognition of cucurbit[8]uril (CB[8]) and the homoditopic bispyridinium derivatives. The interconversion between molecular loops and [2]pseudorotaxanes could be reversibly switched under neutral and acidic condition, while the CB[8] moved between the bispyridinium unit and the linker. The result provides a noncovalent and reversible way to efficiently control the molecular conformations by noncovalent interactions.5. An acceptor-donor-acceptor type linear molecule containing one electron-rich naphthoxy unit and two mono-charged viologen units was synthesized. Through the noncovalent interaction of equimolar CB[8] with the linear guest molecule, we firstly obtained a [2]pseudorotaxane, then added excessive CB[8] to form a [5]pseudorotaxane. Its transformation to [3]pseudorotaxane was achieved through detaching the two axle molecules in the presence of acid, which could be reversed by the addition of base. The result provides an unexplored approach for elongating reversibly the linear molecules by noncovalent interactions.