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⊂α‐CD” was constructed based on axle molecule 1 bearing two terminal ferrocene groups and a wheel component α‐cyclodextrin (α‐CD), but the result obtained indicated that the ferrocene group cannot prevent α‐CD dethreading under UV irradiation. That is, 1⊂α‐CD is just a pseudo[2]rotaxane. Furthermore, the two ferrocene groups in 1⊂α‐CD were encapsulated by two cucurbit[7]uril (CB[7]) units to obtain a heteropseudo[4]rotaxane 1⊂α‐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. Wheels of change: Pseudo[2]rotaxane 1⊂α‐CD and pseudo[3]rotaxane 1⊂2CB[7] were constructed by using axle molecule 1 with α‐cyclodextrin (α‐CD) and cucurbit[7]uril (CB[7]), respectively. When the CB[7] units were capped at the terminal of 1⊂α‐CD, the heteropseudo[4]rotaxane 1⊂α‐CD⋅2CB[7] was successfully obtained. Significantly, it displayed high stability towards UV irradiation and harsh temperatures, hence, it can be regarded as a [2]rotaxane (see figure).