Calix-Based Molecular Machines and Devices

Because of its cuplike structure and the possibility to gain full control over its regio- and stereoselective functionalization, the calix [4]arene represents the most extensively employed molecular platform to anchor and orient in space functional groups and binding sites for the selective and efficient recognition of neutral and charged species in the solid state, in solution, and in the gas phase. The interest in the calixarene platform is still very high, and extensive studies focused on the transfer of its binding properties to the development of working devices like switches, sensors, supramolecular catalysts, or active coating of solid surfaces are currently undergoing. Relatively less explored has been the exploitation of the larger members of the calixarene series for which the synthetic procedures for the selective (partial) functionalization are usually low yielding and because of the difficulties to fix their conformation through functionalization. Nevertheless, calix[5]arene and calix[6]arene are recently emerging as very convenient tools to further expand the scopes of calixarene chemistry toward nanoscience and nanotechnology. Quite amazingly, their larger size, which for some decades was somehow considered a drawback for their employment inmolecular recognition processes, is instead the reason for the growing interest in their chemistry. In fact, it was recently discovered that a guest having an axial symmetry and suitable chemical information can thread their annulus to yield interwoven supramolecular complexes belonging to the class of pseudorotaxanes or interlocked systems like rotaxanes and catenanes. These achievements disclose the possibility to use these larger members of the calixarene series for the construction of new generation of calixarene-based devices and prototypes of molecular machines. The discussion that follows deals with the achievements gained over the last decade or so on the use of calixarene derivatives, with emphasis on the larger calix[5]arene and calix[6]arene, for the construction of molecular devices and machines having as the common feature that of belonging to the class of interwoven species like pseudorotaxanes or interlocked systems like rotaxanes or catenanes. Resorcinarenes, thiacalixarenes, oxacalixarenes, azacalixarenes, nor calixarene-containing polymers will be discussed in this article.