Novel complex nanostructures in supramolecular systems

Over the last several years complex organic nanostructures have been produced synthetically, using building blocks such as dendrons and dendrimers. These tree-like molecules self-assemble prevalently into cylinders or spheres (micelles), which in turn pack on a variety of 2-d or 3-d periodic lattices.Recently a novel, and so far the most complex, mode of packing of micelles was observed in a number of self-assembled dendrimers. The phase is a liquid crystal as well as a quasicrystal (liquid quasicrystal or LQC), possessing the distinctive but crystallographically forbidden 12-fold symmetry. This is the first quasicrystalline structure found in systems other than metal alloys. Unlike conventional crystals, quasicrystals possess long range translational order without being periodic, and give rise to rotational symmetries other than 2-, 3-, 4-, and 6-fold, allowed in conventional crystallography. Compared to metallic quasicrystals, the characteristic length in LQC is increased by nearly two orders of magnitude, from a few angstroms to nearly 100 +. It has been established that the unusual symmetry of quasicrystals can be utilized to induce and widen the complete photonic bandgap. The discovery of LQC points the way to scaling up the structure still further toward self-assembled photonic quasicrystals.In the proposed project, transmission electron microscopy will be used to determine the structure of LQC. The fact that the diameter of the micelles in the LQC is ~40 + makes it possible to directly examine the packing of micelles under the electron microscope. Determination of atomic positions in a quasicrystal is extremely difficult for metal alloys, either by diffraction or by microscopy. With diffraction the problem lies intrinsically in the quasicrystalline symmetry: different packings of atoms can generate similar diffraction patterns. The problem with microscopy methods is their limited resolution: current experimental methods cannot reliably resolve structures at atomic scale. LQC thus provides a unique opportunity to determine unambiguously the positions of micelles in a quasicrystalline structure.Other related complex self-assembled nanostructures will be investigated, including possible quasicrystalline order in other liquid crystal systems, as well as the newly discovered triple network tricontinuous cubic liquid crystal, honeycomb columnar structures in tri-block amphiphiles, etc. Computer modelling will be used to further our understanding of the relationship between the mode of self-assembly and molecular structure. New molecular architectures, capable of forming previously unobserved structures, will be designed in collaboration with synthetic chemists. Efforts will also be made to construct nanostructures on still larger scales, with a view of constructing photonic band gap materials through self-assembly.Many of the above organic nanostructures present opportunities for engineering devices such as molecular ion-channel, molecular wires, membranes, molecular sieves, organic magnets and drug-release agents. Since similar principles guide structure formation in thermotropic dendrimers, lyotropic l.c.s and block copolymers, the new structures are likely to lead to the discovery of their equivalents in these latter systems, increasing the range of scales and applications. The elucidation of the structure of liquid quasicrystals is likely to lead to an improved understanding of quasicrystals in general. The developments in techniques involved in this project would also benefit researchers in the wider area of nanoscience.