Supramolecular Aufbau: Folded Polymers as Building Blocks for Adaptive Organic Materials

The design of inorganic and organic solids with novel structures and properties has long been the object of materials research. The classical examples of porous materials (i.e., involving reversible guest passage) are zeolites. First discovered in the mid-1700s, zeolites are hydrated, crystalline aluminosilicates that organize into stable, discrete frameworks. Basic structures employ tetrahedral atoms (silicon or aluminum) bridged by oxygen atoms, where each oxygen is shared between two metalloid tetrahedra. Resulting covalent lattices can be neutral or negatively charged (as a result of bridging oxides) and often employ alkali metal or alkalineearth counterions. As a consequence of this ordered structure, zeolites both benefit from and are limited by their highly geometrical nature: their rigid structures are inherently robust, yet they are difficult to process. In addition, while natural and unnatural zeolites have been characterized, harsh synthetic conditions are common to both and lead to limitations in design and processability. Even so, these impediments have not prevented constructing a myriad of architectures on zeolite host lattices. Strengths and weaknesses aside, zeolites demonstrate a major objective of materials chemistry: the ability to manifest macroscopic physical properties based on embedded microscopic structure.