Supramolecular Materials
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Jeffrey S. Moore, Guest Editor This issue of MRS Bulletin is devoted to the subject of supramolecular materials. The term supramolecular is widely used to describe the intentional use of noncovalent interactions to bring about a desired arrangement of molecules. As the articles in this issue illustrate, this type of molecular engineering can provide structural control on the nanoscale and beyond, broadly impacting the properties of the resulting materials. The goal is to create a collection of molecules in which the whole possesses characteristics that are different and unattainable from the individual components. The supramolecular approach to materials is, in fact, a subject that was practiced long before the term “supramolecular” became fashionable. The development of liquid crystals for use in display technology, the control of crystallinity in polymeric materials, and the modification of a coating’s viscosity are all examples that have emerged over the past several decades. In each of these examples, the desired result was achieved, directly or indirectly, by manipulating noncovalent interactions. The flurry of activity in recent years can be traced to the widely held belief that even the most sophisticated properties can be rationally designed from the bottom up. This motivation has been further fueled by increased synthetic capabilities, especially for constructing large and well-defined organic molecules. Enormous progress in spectroscopies and other characterization techniques has continued to keep pace and offer insight into the increasingly complex structures that have emerged in this field. All the while, our understanding of how to use weak molecular forces in a collective fashion to achieve a desired organization continues to grow from the myriad of systems that have appeared in the scientific literature. Supramolecular materials come about by the spontaneous action of selfassembly and self-organization. These autonomous processes remind us that the
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molecules of supramolecular materials must contain the instructions for their own fabrication. By analogy to the way a child puts together complex structures from modular building blocks, a molecule’s architecture governs which segments are available for interaction as well as the nature of the contacting interfaces through which noncovalent forces are transmitted. The term self-assembly will be used to describe the thermodynamically controlled ordering that involves atom-specific noncovalent interactions. A snapshot of a self-assembled structure would possess a defined connectivity, meaning that one could trace the network of covalent and noncovalent interactions. Self-assembly may lead to finite-sized assemblies (e.g., hydrogen-bonded dimers) or it may create extended structures (onedimensional chains, two-dimensional sheets, three-dimensional networks). Selforganization, on the other hand, describes the collective ordering of molecules or assemblies into larger ensembles, including equilibrium phases. The interactions responsible for self-organizat
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