THE ART AND SCIENCE OF SELF-ASSEMBLING MOLECULAR MACHINES

In this review, we show how noncovalent bonding interactions between p i-electron rich aromatic ring systems (e.g. hydroquinone) and the pi-e lectron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenyl ene) can be used to self-assemble novel molecular architectures which are not only interesting to us, because of their fascinating topologie s, but also because they have the potential to be developed into molec ular structures with switchable properties on the nanometre scale. The high efficiency observed in the self-assembly of a [2]catenane, and i ts dynamic properties in solution, represent the first step in the des ign and self-assembly of other molecular assemblies better suited for the study of molecular switching processes. Therefore, a series of [2] rotaxanes, mechanically-interlocked molecular compounds, consisting of a linear pi-electron rich dumbbell-shaped component and the pi-electr on deficient tetracationic cyclophane as the cyclic component, have be en self-assembled and evaluated. All of the so-called molecular shuttl es show translational isomerism and one of them, comprising benzidine and biphenol recognition sites as the non-degenerate pi-electron rich sites, shows molecular switching properties when it is perturbed by ex ternal stimuli, such as electrons and protons. The versatility of our approach to nanoscale molecular switches is proven by the description of a series of molecular assemblies and supramolecular arrays, consist ing of pi-electron rich and pi-electron deficient components, which di splay molecular switching properties when they are influenced by exter nal stimuli that are photochemical, electrochemical and/or chemical in nature. However, the molecular switching phenomena take place in the solution state. Therefore, finally we describe how simple molecular st ructures can be ordered on to a solid support at the macroscopic level using Langmuir-Blodgett techniques. This is a necessary condition whi ch must be fulfilled if we wish to construct supramolecular structures with device-like properties at the macroscopic level.