**4. Conclusions**

In summary, we investigated the binding interactions of Q[7] with a series of 4-pyrrolidinopyridinium guests (namely C0, C2, C4, C6) bearing aliphatic substituents at the pyridinium nitrogen by using 1H NMR and UV spectroscopy and mass spectrometry. The results herein revealed that for C0, the entire C0 molecule is encapsulated in the cavity of Q7. For C2, the pyridine and pyrrole rings are accommodated within the cavity of Q[7] and this is reminiscent of the situation observed for Q[8] and the same guest, but the difference between Q[7] and Q[8] is whether the alkyl chain is completely at its portal. For C4, this shuttling situation is reminiscent of that observed for Q[8] and the same guest. For C6, the shuttling situation herein is in contrast with that observed previously for Q[8], where the alkyl chain was twisted and buried in the Q[8] cavity; only part of pyrrole ring protruded from the portal. Based on this work and the previous results for the same guests with Q[8], we found that, for larger guests in particular, the size of the cavity of the cucuribit[*n*]uril dictates the interaction between the cucuribit[*n*]uril and guest. This is illustrated herein for C6, where there is insu fficient space in the Q[7] system to allow for the bending of the alkyl chain previously observed for the Q[8] system. Among the Q[*n*]-based rotaxane/pseudorotaxane systems, most of the axle molecules have been built from ammonium, pyridinium ions, and viologen derivatives [17–20]. In comparison with previously reported cucurbit[7]uril-based host–guest systems (such as pyridinium ions, viologen derivatives), the interaction mode is similar [6,25]. This provides more insight into the possible applications of systems such as switchable mechanically pseudorotaxane molecules, for instance, in the area of supramolecular materials science.

**Author Contributions:** The main work of W.X. and X.Z. was to complete the experiment; B.B. participated in the optimization of the MS experiment; Manuscript writing by X.X., Z.T., and C.R. All authors have read and agree to the published version of the manuscript.

**Funding:** The Natural Science Foundation of China (21861011), the Major Program for Creative Research Groups of Guizhou Provincial Education Department (2017-028), the Innovation Program for High-level Talents of Guizhou Province (No. 2016-5657), and the Science and Technology Fund of Guizhou Province (No. 2018-5781) are gratefully acknowledged for their financial support. CR thanks the EPSRC (Engineering and Physical Sciences Research Council) for a travel gran<sup>t</sup> (EP/L012804/1).

**Conflicts of Interest:** The authors declare no conflicts of interest.
