Insights into the Difference Between Rotaxane and Pseudorotaxane

Rotaxane and pseudorotaxane are two types of mechanically interlocked molecular architectures, and there is a clear topological difference and boundary between them. In this work, a suggested [2]rotaxane 1 subset of alpha-CD was constructed based on axle molecule 1 bearing two terminal ferrocene groups and a wheel component alpha-cyclodextrin (alpha-CD), but the result obtained indicated that the ferrocene group cannot prevent alpha-CD dethreading under UV irradiation. That is, 1 subset of alpha-CD is just a pseudo[2]rotaxane. Furthermore, the two ferrocene groups in 1 subset of alpha-CD were encapsulated by two cucurbit[7]uril (CB[7]) units to obtain a heteropseudo[4]rotaxane 1 subset of alpha-CD.2CB[7]. This heteropseudo[4]rotaxane displayed high stability towards harsh temperatures and the isomerization of azobenzene in 1, so it can be regarded as a [2]rotaxane. In this [2]rotaxane, the stoppers are not the bulky groups covalently bonded to the axle, but the cyclic CB[7] units connected through noncovalent interactions.