Collaborators from Taiwan have synthesized and characterized an interesting example of a urea-based [2]rotaxane which behaves as both a molecular switch and an organogelator. They nicely demonstrated that both functions can be mediated under acid/base or anion control (see scheme of the anion-induced switching of the [2]rotaxane).

Chen, Chiu, and co-workers have carried out different kinds of 2D NMR experiments and X-ray analysis to characterize and study these switches based on interlocked molecules. They have presented a rare example of switchable sol-gels based on a rotaxane motif—where the gel form arises from intermolecular hydrogen bonds and the switching arises from the mechanical location of the ring component.

• Acid/Base- and Anion-Controllable Organogels Formed From a Urea-Based Molecular Switch
  S.-Y. Hsueh, C.-T. Kuo, T.-W. Lu, C.-C. Lai, Y.-H. Liu, H.-F. Hsu, S.-M. Peng, C.-H. Chen, S.-H. Chiu,
  Angew. Chem. Int. Ed. 2010, 49.
  DOI: 10.1002/anie.201004090
• S.-Y. Hsueh, C.-T. Kuo, T.-W. Lu, C.-C. Lai, Y.-H. Liu, H.-F. Hsu, S.-M. Peng, C.-H. Chen, S.-H. Chiu,
  Angew. Chem. 2010, 122.
  DOI: 10.1002/ange.201004090