Upon mixing and dehydration, 2,6-diformylpyridine and 2,2'-oxybis(ethylamine) form a dynamic combinatorial library of at least nine members. Through hydrogen bonding and other intermolecular interactions, templating dumbbell molecules select one macrocyclic member of the library, at the expense of all the others, to create [2]rotaxanes. These rotaxanes, however, retain the dynamic character of the library, since a diformylpyridine analogue can exchange with the macrocyclic component in solution. In addition, crystallization of the mixture surprisingly furnishes only the [24]crown-8-like macrocycle on its own--evidence of a kinetic selection process occurring between phase transitions.

As the complexity of mechanically interlocked molecular architectures increases, it is important to understand the underlying principles, such as molecular recognition and self-assembly processes, that govern the practice of template-directed synthesis necessary to create these particular compounds. In this review, we explain the importance of dynamic processes in the synthesis of mechanically interlocked compounds. We show how many different dynamic covalent bonds have been used in the synthesis of rotaxanes, catenanes, and other higher-order mechanically interlocked compounds, with the goal of revealing the state of the art in dynamic covalent chemistry.

The electrical properties of polyaniline changes by orders of magnitude upon exposure to analytes such as acids or bases, making it a useful material for detection of these analytes in the gas phase. The objectives of this lab are to synthesize different diameter polyaniline nanofibers and compare them as sensor materials. In this experiment polyaniline nanofibers are synthesized using a two-phase interfacial polymerization method that yields nanofibers with relatively narrow diameter distributions centered around 30, 50, and 120 nm. The sensors are then fabricated by drop-casting aqueous dispersions of nanofibers onto electrode arrays to form films and measuring their change in resistance upon exposure to acids or bases. The sensor response is dependent on the surface area, diameter, and porosity of the nanofiber films. The larger diameter nanofibers have slower response times because of the difficulty for gas to diffuse through more material. The advantages to this lab include simplicity and low cost, making it suitable for both high school and college students, particularly in departments with modest means.