Dendrimers and supramolecular chemistry
Dendrimers and supramolecular chemistry
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DOI：
10.1073/pnas.082013899
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发表时间：
2002-04-16
影响因子：
11.1
通讯作者：
Fréchet, JMJ
中科院分区：
文献类型：
--
作者：
Fréchet, JMJ
自从1985年由Toma-lia等人(1)和Newkome等人(2)提出以来，树枝状大分子因其迷人的结构和独特的性质而引起了人们的极大关注(3，4)。树枝状大分子是球形、大小的单分散大分子，其中所有的键都从一个中心焦点或核心径向出现，具有规则的分支图案和重复单元，每个单元都贡献了一个分支点。并不是所有的规则支化分子都是树枝状大分子，因为树枝状状态(4)的性质，如核心包裹(5，6)和溶液中异常低的特性粘度(7)，只有当球状达到一定的代数或大小阈值时才能达到。因此，许多低代树形分子或Vögtle和同事(8)的早期级联分子太小，不能表现出树枝状大分子的性质，但它们经常被用作支化低聚物构建块，并且与树枝状大分子的尺寸关系有点类似于低聚物和聚合物之间的关系。两种不同的合成方法被用于制备树枝状大分子：发散方法(1，2)，其中生长从核心开始，向外径向向树枝状大分子外围进行，以及收敛方法，(9，10)，其中生长从将成为树枝状大分子的外围开始向内进行。这两种方法是互补的，通常都不是更好的，选择合成方法通常是根据目标分子所需的特征、可用于生长的化学物质以及构建树枝状框架所使用的特定构建块来证明的。总体而言，收敛方法提供了更好的总体结构控制，部分原因是它在生长中期增强了净化潜力，部分原因是它固有的在树枝状大分子的焦点和外围引入不同功能的能力。相比之下，发散方法更难保持纯度和结构一致性，因为生长的每一步必须完成的反应数量呈指数级增加，需要大量过量的试剂，但该过程不仅更适合于更大规模的合成，而且更适合于制备高世代树枝状大分子。虽然到目前为止制备的大多数树枝状大分子都是由共价键(3，10)构建的，但也有许多非共价树状大分子(3，11)是通过各种自组装过程制备的，例如氢键(12)或超分子配位化学(13)。在本文中，将树枝状大分子和超分子化学的概念联系起来需要更多，而不仅仅是考虑由超分子结构产生的分子。树枝状大分子独特的层状结构、球状形状、高度可控的大小、径向可控的化学组成、多价外围、可变的内部体积和受控的分子内动力学赋予了树枝状大分子独特的特征，并使其能够响应外部刺激而变形为各种虚拟的超分子排列。与可能需要超分子组装才能提供功能的小分子集合不同，树枝状大分子可以简单地使用内部动力学来安排其多个相互连接的组件，以最大限度地减少自由能并提供特定的功能。这种分子内重组可能导致形状或体积的变化，内部微环境的创建，表面或内部功能的协同组织，底物的浓缩或从…中排除
Since their introduction in 1985 by Toma-lia et al.(1) and Newkome et al.(2), dendrimers have attracted much attention because of their fascinating structure and unique properties (3, 4). Dendrimers are globular, size monodisperse macromolecules in which all bonds emerge radially from a central focal point or core with a regular branching pattern and with repeat units that each contribute a branch point. Not all regularly branched molecules are dendrimers because properties of the dendritic state (4), such as core encapsulation (5, 6) and unusually low intrinsic viscosity in solution (7), are reached only when globularity is achieved at a certain generation or size threshold. Therefore, many lowgeneration dendrons or the early cascade molecules of Vögtle and coworkers (8) are too small to exhibit the properties of dendrimers, but they are frequently used as branched oligomeric building blocks in their construction, and have a size relationship to dendrimers somewhat akin to that between oligomers and polymers. Two distinct synthetic methodologies have been used for the preparation of dendrimers: the divergent approach (1, 2), in which growths starts at the core and proceeds radially outward toward the dendrimer periphery, and the convergent approach,(9, 10) in which growth starts at what will become the periphery of the dendrimer proceeding inward. The two methodologies are complementary and neither is generally better, the choice of synthetic approach being usually justified by the features desired for the target molecule, the chemistry available for growth, and the specific building blocks used in the construction of the dendritic framework. In general, the convergent approach provides better overall structural control, in part as a result of its enhanced potential for purification at intermediate stages of growth, and, in part, as a result of its innate ability to introduce differentiated functionalities at the focal point and the periphery of the dendrimer. In contrast, purity and structural uniformity are harder to maintain in the divergent approach, because the number of reactions that must be completed at each step of growth increases exponentially requiring large excesses of reagents, but the process is better suited not only for syntheses on a larger scale but also for the preparation of high-generation dendrimers. Although the majority of the dendrimers prepared to-date have been built of covalent bonds (3, 10), many noncovalent dendrimers (3, 11) have also been prepared by a variety of selfassembly processes involving, for example, hydrogen bonding (12) or supramolecular coordination chemistry (13). Relating the concepts of dendrimers and supramolecular chemistry (14) in this article requires more that just a consideration of molecules resulting from supramolecular construction. The unique layered architecture of dendrimers, their globular shape, highly controlled size, radially controlled chemical make-up, multivalent periphery, variable inner volume, and controlled intramolecular dynamics endow dendrimers with unique features and provide them with the ability to morph into a variety of virtual supramolecular arrangements in response to external stimuli. Unlike collections of small molecules, which might require supramolecular assembly to deliver function, dendrimers can simply use internal dynamics to arrange their multiple and interconnected components in ways that minimize free energy (15) and afford specific functions. Such intramolecular reorganizations may lead to shape or volume changes, the creation of internal microenvironments, the cooperative organization of surface or inner functionalities, the concentration or exclusion of substrate from …