Novel complex nanostructures in supramolecular systems

在过去的几年里，复杂的有机纳米结构已经被合成出来，使用的是树突和树状大分子等构建块。这些树状分子通常会自我组装成圆柱体或球体（胶束），它们又会堆积在各种二维或三维的周期晶格上。最近，在许多自组装的树状大分子中观察到一种新颖的、迄今为止最复杂的胶束堆积模式。该相是液晶和准晶体（液态准晶体或LQC），具有独特的但晶体学上禁止的12重对称。这是在金属合金以外的体系中发现的第一个准晶结构。与传统晶体不同，准晶体具有长程平动顺序而不具有周期性，并且产生旋转对称性，而不是传统晶体学中允许的2-，3-，4-和6-倍。与金属准晶体相比，LQC的特征长度增加了近两个数量级，从几埃增加到近100 +。结果表明，准晶体的不寻常对称性可用于诱导和加宽全光子带隙。LQC的发现指出了将结构进一步扩大到自组装光子准晶体的方法。在拟建的项目中，将使用透射电镜来确定LQC的结构。LQC中胶束的直径为~40 +，这使得在电子显微镜下直接观察胶束的堆积成为可能。无论是用衍射法还是用显微法，测定金属合金准晶中的原子位置都是极其困难的。衍射的问题本质上在于准晶体的对称性：不同的原子排列可以产生相似的衍射图样。显微镜方法的问题在于它们的分辨率有限：目前的实验方法不能可靠地分辨原子尺度的结构。因此，LQC提供了一个独特的机会来明确确定准晶体结构中胶束的位置。其他相关的复杂自组装纳米结构将被研究，包括其他液晶系统中可能的准晶秩序，以及新发现的三网络三连续立方液晶，三嵌段两亲体中的蜂窝柱状结构等。计算机建模将用于进一步理解自组装模式与分子结构之间的关系。新的分子结构，能够形成以前未观察到的结构，将与合成化学家合作设计。人们还将努力在更大的尺度上构建纳米结构，以期通过自组装构建光子带隙材料。上述许多有机纳米结构为诸如分子离子通道、分子线、膜、分子筛、有机磁铁和药物释放剂等工程器件提供了机会。由于类似的原理指导了热致树枝状大分子、溶致lcs和嵌段共聚物的结构形成，新的结构很可能导致在这些体系中发现它们的等效物，从而增加了规模和应用范围。对液体准晶体结构的阐明可能会导致对准晶体的一般认识的提高。该项目所涉及的技术发展也将使纳米科学更广泛领域的研究人员受益。

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.