The properties of bulk matter are determined by its structure over a number of length scales. In order to understand, design and prepare functional materials it is necessary to control this structure from the atomic level to the bulk. Examples of functionality are electrical conductivity, non-linear optical activity and mechanical properties such as toughness and strength. Modulation of a property in response to a stimulus, whether chemical or physical, may enable a material to find an application as a sensor.
The connectivity of atoms can be controlled by covalent synthesis. Discrete molecular components may be designed with ‘recognition’ sites that enable them to bind to one another specifically and with well defined geometry to create a supramolecular entity.1 Several recognition motifs, such as hydrogen bonding and metal ion coordination, may be engineered to operate independently of each other to yield supermolecules of increased complexity. Another level of assembly is the packing of molecules into a crystal, which has been regarded as the ultimate of supramolecular structures.2 Further structure, which may extend up to the macroscopic length scale, is the spatial distribution of the chemical components of heterogeneous systems. For example, different areas of a surface may have different chemical compositions. Within each area a further level of organization is provided by the supramolecular interactions between the molecules comprising the interface, which are in turn dictated by their covalent structure.
The coordination of organic molecules to metals provides a rich field in which to explore self assembly due to the diversity of binding selectivities, geometries and kinetics available from the different elements. Some of the principles by which chemists have created large organized assemblies from simpler components using coordination chemistry will be discussed in the following section with reference to some specific examples taken from the recent literature.