Interpenetrating nets: Ordered, periodic entanglement

Taking inspiration from the examples biology provides for the organization of simple units into a vast range of intricate and beautiful structures with complex and wonderfully efficient functions, many chemists have been turning their attention to the deliberate design of self-assembling aggregates of molecular building blocks with some specific structural or functional purpose in mind. This is supramolecular chemistry. It embraces systems ranging upwards in size and complexity to include infinite solid arrays. Molecular entanglement is common in biology. Entanglement that is inextricable, in the sense that disentanglement can be achieved only by breaking internal connections, has been one of the major themes of supramolecular chemistry. Such inextricable entanglements - as seen in catenanes, rotaxanes, and molecular knots - have provided a long-standing fascination for chemists, and many beautiful examples of ingenious design have been constructed. In the area of solids, the deliberate control of structure, and therefore properties and function, is a long-term objective being vigorously pursued by numerous groups. Increasingly, solid structures are being invented and created rather than discovered. The potential for technologically useful electronic, optical, electrochemical, and catalytic applications is an obvious driving force for much of this work, but the area is also of great fundamental structural interest and importance. Framework solids, which are often accessible by self-assembly under exceedingly mild conditions, are particularly attractive in this context because of their chemical and structural diversity and aesthetic appeal and because of the control potentially achievable in their construction. One aspect of framework solids which becomes increasingly apparent as more of them are discovered or invented is their tendency to form entangled structures in which two or more independent infinite networks interpenetrate each other. Polycatenane or polyrotaxane associations are an inherent feature of these structures. Examples of interpenetration were relatively rare until recently, but they are now being reported with increasing frequency. This is an opportune time to review this expanding area.