Self-Assembly of Organic Nano-Objects into Functional Materials
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of Organic Nano-Objects into Functional Materials
Samuel I. Stupp, Martin U. Pralle, Gregory N. Tew, Leiming Li, Mehmet Sayar, and Eugene R. Zubarev Introduction One of the goals of contemporary science is the atomic or molecular design of materials in order to achieve specific properties. There is special interest in imitating with these designed materials the remarkable integrated functionality we see in biology. Soft matter offers a particularly good opportunity to realize these goals because of the vast structural space offered by organic systems. One subset of designed organic materials is that in which the constituent units are groups of molecules that achieve a specific shape and size (see Figure 1). We refer to these systems as supramolecular materials, since their rational synthesis goes beyond molecular structure and requires supramolecular chemistry. This branch of chemistry aims to control intermolecular, noncovalent interactions in the same way that synthetic organic chemists control the formation of covalent bonds.1–3 One could argue that common semicrystalline polymers, liquid crystals, and monolayers are supramolecular because groups of molecules in such systems are, after all, defined by defects or external boundaries. The materials of interest in this article are composed of molecular aggregates and are therefore inherently supramolecular in nature. In these systems, thermodynamically or kinetically defined defects and surfaces are not the controlling factors of supramolecular structure. Aggregate formation is controlled instead by molecular architecture. Supramolecular materials composed of large and regular molecular clusters can
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dimensionally amplify the lattices we know in crystalline materials into superlattices with large periodicities. This also suggests that efficient packing considerations would affect symmetry preferences and allow one to predict structure at
longer length scales than those characteristic of the clusters themselves. Good examples would include the tendency of flat, platelike groups of molecules to stack parallel to each other and the tendency of tubes or cylindrical structures to align along a common direction. This latter feature of objects with high aspect ratios is well known in liquid-crystalline phases.4 Our laboratory showed a few years ago that chiral and chemically reactive molecules could be designed to self-assemble into two-dimensional structures. In these structures, defects control the x-y dimensions of the system and molecular design determines the z coordinate (see Figure 2).5,6 Furthermore, the reactivity of these molecules made the layered assemblies convertible to two-dimensional polymers, a set of molecular objects predicted theoretically to have many interesting properties.7–12 The properties we found in such systems include the thermal stability of stacked plates5 and the spontaneous formation of nanoporous structures as a result of poor tiling among irregular twodimensional objects.13 In supramolecular form, these materials are examples in wh
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