pH-triggered dethreading-rethreading and switching of cucurbituril on bistable pseudorotaxanes and rotaxanes
Chemistry a European Journal
4110 - 4116
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A series of water-soluble rotaxanes-(n+2) and pseudorotaxanes-(n+2) with short (propyl, n=1) and long (dodecyl, n=10) aliphatic spacers have been prepared in high yields by a 1,3-dipolar cycloaddition reaction catalyzed by cucurbituril (CB6). The pH-triggered dethreading and rethreading of CB6 on these pseudorotaxanes was monitored by 1H NMR spectroscopy. A previously reported rotaxane-12 that is known to behave as a bistable molecular switch has two recognition sites for CB6, that is, the diaminotriazole moieties and the dodecyl spacer. By changing the pH of the system, it is possible to observe more than one state in the shuttling process. At low pH values both CB6 units are located on the diaminotriazole moieties owing to an ion-dipole interaction, whereas at high pH values both of the CB6 units are located on the hydrophobic dodecyl spacer. Surprisingly, the CB6 units shuttle back to their initial state very slowly after reprotonation of the axle. Even after eighteen days at room temperature, only about 50 % of the CB6 units had relocated back onto the diaminotriazole moieties. The rate constants for the shuttling processes were measured as a function of temperature over the range from 313 to 333 K and the activation parameters (enthalpy, entropy, and free energy) were calculated by using the Eyring equation. The results indicate that this rotaxane behaves as a kinetically controlled molecular switch. The switching properties of rotaxane-3 have also been studied. However, even under extreme pH conditions this rotaxane has not shown any switching action, which confirms that the propyl spacer is too short to accommodate CB6 units.