Functional Nanostructured Molecular Materials

The Electrochemical Society Interface • Fall 2001 here has been tremendous recent interest in the design and assembly of porous nanostructured materials.1 Synthetic approaches range from coordination-polymer formation to surfactant-templated alumino-silicate growth to semiconductor nanoparticle aggregation, while functions include chemical catalysis, chemical sensing, energy storage, and energy conversion. A promising but thus far largely neglected synthetic approach is the assembly of porous materials from discrete molecular building blocks—especially building blocks consisting of cyclic, nanoscale cavity-containing coordination compounds. The molecular approach is especially useful for circumventing two common difficulties with many of the conventional approaches. The first is the collapse of channels upon removal of solvent from the porous solid. While the units comprising molecular solids are held in place only by weak forces (mainly dispersion interactions), molecules defining individual cavities are beneficiaries of substantially stronger interactions (covalent and coordinatecovalent bonds). Indeed, the interactions are strong enough to preclude cavity and channel collapse. The second complication is interpenetration such that vacancies—for example in a lattice polymer structure—are occupied by a geometrically orthogonal, but structurally identical structure (or in some cases, by multiple intersecting and interpenetrating structures). The lattice interpenetration phenomenon is generally thermodynamically driven, reflecting the free-energy gains achievable by maximizing van der Waals interactions. While an equivalent molecular phenomenon, cycle catenation, is occasionally encountered, the isolated assembly of molecular building blocks via intentionally substitution-inert coordination chemistry, precludes catenation and leads to open, high porosity, high void volume solids. Functional Nanostructured Molecular Materials