| The advent of self-assembly and template-directed synthetic protocols has led to a tremendous surge in the number of mechanically-interlocked compounds being prepared and studied. As these investigations are being carried out, it is becoming increasingly apparent that the presence of a mechanical bond creates the possibility for novel dynamics processes---both degenerate and nondegenerate ones---in these systems. Unique forms of Euclidian chirality can also arise from the relative orientations of the components about the mechanical bond. The drive to create molecular switches and machines for nanotechnological applications has generated a desire to understand how to control these properties both in solution and in condensed phases. In this regard, 1H NMR spectroscopy has proven to be a powerful tool for probing both degenerate and nondegenerate dynamic processes of these compounds in solution, as well as for identifying the presence of stereoisomers. Exploration of the dynamic processes occurring in a series of bistable [2]catenanes by variable temperature 1H NMR spectroscopy, for example, led to the discovery of a novel form of helical chirality that contributes to these [2]catenanes existing as libraries of dynamically interconverting diastereoisomers. In addition to characterizing simple degenerate processes---such as shuttling in degenerate [2]rotaxanes---it was also possible to confirm the identity of the two states in bistable [2]rotaxanes by chemical oxidation and reduction. Having established the link between solution-phase properties and device characteristics, we can now test structural variants in solution before attempting to incorporate them into devices. Thus, a series of neutral bistable [2]rotaxanes were investigated to determine the nature of their dynamic processes and verify their switching behavior. Similarly, alternative recognition sites within the dumbbell component of [2]rotaxanes were probed to measure their impact on molecular properties. Although [2]catenanes and [2]rotaxanes have proven to be widely useful for construction of molecular machine-based devices, new structural motifs are needed to expand the set of available building blocks. Investigation of pretzelanes and self-complexed species---compounds closely related to catenanes and rotaxanes---in solution revealed complex dynamic and stereochemical attributes similar to those observed for their mechanically-interlocked relatives, creating promise for the next generation of molecular switches and machines. |
