A novel pH-responsive [2]pseudorotaxane based on ethylated pillar[6]arene and bis(1,2,3-triazolium)butane cation was constructed. The dethreading and rethreading could be reversibly controlled by adding base and acid, i.e., the deprotonation and reprotonation of the guest. In contrast, such responsible switching behavior does not occur for the complexation between the guest and ethylated pillar[5]arene, since both the cationic axle and the deprotonated one can be strongly bound by ethylated pillar[5]arene. Furthermore, the bis(1,2,3-triazolium)butane⊂pillar[6]arene-[2]pseudorotaxane can also disassemble upon the addition of competitive ethylated pillar[5]arene.

The complexation of four neutral guest molecules containing carbon-oxygen double bonds, i.e., 1,6-diacetoxyhexane (1), dimethyl hexanedioate (2), hexanal (3), and 2-hexanone (4), by per-ethylated pillar[5]arene is reported. These guests show remarkably different binding affinities. For example, ester 1 could be fully engulfed by the host cavity to form a [2]pseudorotaxane-type inclusion complex, while for 2, no obvious host-guest interaction was observed.

To clarify the binding mechanisms and driving forces for the unique ability of pillar[5]arenes to recognize neutral molecules, the complexation of a series of nitrogen heterocycle substituted 1,4-butylene guests with pillar[5]arenes was studied. Guests molecules that have similar structures exhibit remarkably different association constants, and multiple weak C–H···N interactions between the alkyl groups of the pillar[5]arenes and the outer nitrogen atoms of the heterocycles of the guests are the dominant driving forces for complexation.