Controlling self-assembly

As a result of cooperative noncovalent bonding interactions (namely, π-π stacking, [CH ··· O] hydrogen bonding, and [CH ··· π] interactions) supramolecular complexes and mechanically interlocked molecular compounds-in particular pseudorotaxanes (precatenanes) and catenanes-self-assemble spontaneously from appropriate complementary components under thermodynamic and kinetic control, respectively. The stereoelectronic information imprinted in the components is crucial in controlling the extent of the formation of the complexes and compounds in the first place; moreover, it has a very significant influence on the relative orientations and motions of the components. In other words, the noncovalent bonding interactions-that is, the driving forces responsible for the self-assembly processes-live on inside the final superstructures and structures, governing both their thermodynamic and kinetic behavior in solution. In an unsymmetrical [2]catenane, for example, changing the constitutions of the aromatic rings or altering the nature of substituents attached to them can drive an equilibrium associated with translational isomerism in the direction of one of two or more possible isomers both in solution and in the solid state. Generally speaking, the slower the components in mechanically interlocked compounds like catenanes and rotaxanes move with respect to each other, the easier it is for them to self-assemble.