Novel complex nanostructures in supramolecular systems

在过去的几年里，人们利用树枝和树枝状大分子等构建块合成了复杂的有机纳米结构。这些树状分子自组装成圆柱体或球体(胶束)，进而堆积在各种二维或三维周期晶格上。最近，人们在一些自组装的树枝状大分子中观察到了一种新的、也是迄今为止最复杂的胶束堆积方式。该相既是液晶又是准晶(液体准晶或LQC)，具有独特的但在结晶学上被禁止的12倍对称性。这是在非金属合金系统中发现的第一个准晶结构。与传统晶体不同，准晶具有长程平移有序性而不是周期性的，并产生传统结晶学中允许的2、3、4和6倍以外的旋转对称性。与金属准晶相比，LQC的特征长度增加了近两个数量级，从几埃增加到近100+。准晶的反常对称性可以被用来诱导和加宽完整的光子带隙。LQC的发现为进一步放大自组装光子准晶的结构指明了方向。在拟议的项目中，将使用透射电子显微镜来确定LQC的结构。由于LQC中胶束的直径为~40+，使得在电子显微镜下直接观察胶束的堆积成为可能。对于金属合金来说，确定准晶中的原子位置是极其困难的，无论是用衍射法还是用显微镜。对于衍射，问题本质上在于准晶对称性：不同的原子堆积可以产生类似的衍射图。显微镜方法的问题在于其有限的分辨率：目前的实验方法不能可靠地解析原子尺度的结构。因此，LQC提供了一个明确确定胶束在准晶结构中位置的独特机会。我们还将研究其他相关的复杂自组装纳米结构，包括其他液晶系统中可能的准晶有序结构，以及新发现的三重网络三连续立方液晶、三嵌段两亲分子中的蜂窝状柱状结构等。计算机模拟将用于加深我们对自组装方式与分子结构之间关系的理解。新的分子结构，能够形成以前没有观察到的结构，将与合成化学家合作设计。此外，还将努力构建更大规模的纳米结构，以期通过自组装来构建光子带隙材料。上述许多有机纳米结构为分子离子通道、分子线、膜、分子筛、有机磁体和药物释放剂等工程器件提供了机会。由于相似的原理指导了热致树枝状大分子、溶致液晶聚合物和嵌段共聚物的结构形成，因此新的结构很可能导致在这些后一种体系中发现它们的等价物，从而扩大规模和应用范围。对液体准晶结构的阐明可能有助于从总体上加深对准晶的理解。该项目所涉及技术的发展也将使更广泛的纳米科学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.