Supramolecular chemistry and dimensonality

Key point: Supramolecular chemistry is regarded as a most promising method for designing and controlling the bottom-up assembly of nanometre-scale objects with rationally designed dimensionality. A general concept for generating ordered structures is based on the recognition-derived spontaneous assembly of complementary subunits. In the field of supramolecular chemistry, one of the key challenges is the characterization of the supramolecules themselves, in particular the characterization of discrete, highly symmetrical, large, self-assembled entities. A great number of biological systems are formed as a result of the formation of many weak hydrogen bonds and van der Waals interactions. Synthetic systems can take advantage not only of these bonds but also of metal–ligand bonding interactions, the latter being much stronger and generally highly directional. There are some important examples of biological systems using coordination chemistry for self-assembly, such as the metal-ion activated proteins, including Ca²⁺ signalling proteins and Zn fingers (Sections 27.4 and 27.5). As a result of various d-metal coordination geometries, metal complexes provide a pool of different acceptor subunits that can be linked together by donor building blocks to form rigid frameworks. The final shape of the self-assembled entity is defined by both the metal atom coordination geometry and the orientation of the interaction sites in a given ligand. This methodology has been successfully used to synthesize the cuboctahedron shown in Fig. 25.13. Many self-assembled nanostructures use similar approaches to take advantage of encoded information within the components of the nanostructure building block. Molecular simulations have been performed to study the self-assembly of nanoparticles functionalized with oligomeric tethers attached to specific locations on the nanoparticle surface (Fig. 25.14). These results suggest that tethered nanobuilding blocks constitute a unique class of macromolecule that can be used to assemble novel nanomaterials.

Figure 25.13 A cuboctahedron synthesized through self-assembly of two distinct building blocks. (Adapted from S. Leininger, et al., Chem. Rev., 2000, 100, 853.)

Figure 25.14 Representative tethered nanobuilding blocks: (a–d) tethered nanospheres; (e–g) tethered nanorods; (h–p) tethered nanoplates, including circular, triangular, and rectangular plates; (q, r) tethered nanowheels; (s, t) tethered nanocubes; (u–w) triblock nanobuilding blocks created by joining two nanoobjects with a polymer tether. (Adapted from Z. Zhang, et al., Nano Lett., 2003, 3, 1341.)

Figure 25.15 Effects of confinement of charge carriers on the density of states for nanomaterials with specific dimensionality. (Adapted from A. Mrumjadar, Nonoscience Boot Camp at Northwestern University 2001.) Specific tethering groups influence the nature of the self-assembly of the nanobuilding blocks into larger ordered nanostructures. Control over dimensionality in materials can yield unique control over their physical properties. For example, dimensionality has a noted influence on the density of electronic states (Fig. 25.15). In the next few sections we examine specific examples of how that control has been achieved and the novel properties that have been reported.

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مواضيع ذات صلة

Quantum wells

Two-dimensional control: quantum wells and solid-state superlattices

One-dimensonal control: carbon nanotubes and inorganic nanowires

Morphosynthesis

Self-assembly

Chemical vapour deposition

Physical vapour deposition

Nanosized reaction vessels

Vapour-phase synthesis of nanoparticles

Black, absorbing, and specialist pigments

White pigments

Other inorganic hydrogen-storage materials