Solvent-dependent host-guest chemistry of polyhedral coordination cages

多面体配位笼是中空的胶囊状配合物，可根据用于制备它们的金属离子和配体的性质提供一系列形状和大小；四面体、立方体、截尾四面体和立方体等结构都是众所周知的例子。它们的一个吸引人的特点是它们有一个中心腔，能够容纳各种形状和大小的客人物种。这些客体可能是阴离子（因为笼是带正电的），或者，在主要的水性溶剂中，是中性有机分子，它们更喜欢笼的疏水内部，而不是它们亲和力较低的极性外部。我们计划准备一系列新的包含向内导向功能基团的笼子，这样我们就可以通过改变排列在中心腔内的群体的空间安排和性质来系统地改变笼子的内部环境，这些群体将与不同的客体物种相互作用。这将改变笼子“主人”对不同客人的亲和力。通过系统地改变寄主笼，以及寄主体内的空腔形状/大小和官能团的排列，将有可能分析导致寄主在一系列溶剂中有效结合寄主的因素，其中水是最重要的。对水系统中分子识别的理解是化学中尚未解决的突出问题之一，它将对其他领域产生重大影响，如生物学和医学，其中诸如药物/受体相互作用和蛋白质折叠等问题都是基于水介质中简单分子组分之间的识别。确定了适合不同笼子主人的客人，并分析了造成强结合的因素，应该可以使用这些客人作为“模板”来控制周围笼子的组装。这可能导致新的笼型，如果一个特定的客人选择了一个特定的配体组合，以前没有被研究过，或者它可能导致已知的笼组装得更快，因为客人的行动，把组件拉在一起。最后，我们将利用这样一个事实，即许多笼是荧光的，因为在用于制备它们的配体中存在荧光基团。在吸收光后，激发能从宿主笼分子转移到被捕获的客体将被研究，如果可能的话，我们将利用这一现象来触发客体中的异构化反应。因此，客体分子将进入宿主腔，并且在笼上照射光线将导致能量传递给客体，然后客体发生反应。因此，宿主笼充当“光化学微反应器”，既结合客体，又为其提供进行反应所需的能量。

Polyhedral coordination cages are hollow, capsule-like complexes available in a range of shapes and sizes according to the nature of the metal ion and ligand used to prepare them; examples are known with structures such as tetrahedral, cubic, truncated tetrahedral and cuboctahedral. An appealing feature of them is that they have a central cavity which is capable of accommodating guest species of various shapes and sizes. These guests may be anions (as the cages are positively charged) or, in predominantly aqueous solvents, neutral organic molecules which prefer the hydrophobic interior of the cage to the polar exterior for which they have less affinity.We plan to prepare a new series of these cages that contain inwardly-directed functional groups so that we can systematically change the internal environment of the cages by altering the spatial arrangement and nature of the groups lining the central cavity that will interact with different guest species. This will change the affinity the cage 'hosts' for different guests. By systematically varying the host cages, and the cavity shape/size and arrangement of functional groups in the guests, it will be possible to analyse the factors that are responsible for effective binding of guests by the hosts in a range of solvents, of which water is the most important. Understanding of molecular recognition in aqueous systems is one of the outstanding unsolved problems in chemistry that would have a major impact in other fields, like biology and medicine, where issues such as drug/receptoy interactions and protein folding are all based on recognition between simple molecular components in aqueous media.Having identified well-fitting guests for different cage hosts, and analysed the factors responsible for the strong binding, it should be possible to use these guests as 'templates' to control assembly of cages around them. This may lead to new types of cage if a particular guest selects a particular combination of ligands that had not previously been investigated, or it may lead to known cages assembling much more quickly because of the action of the guest in pulling the components together.Finally, we will exploit the fact that many of the cages are fluorescent because of the presence of fluorescent groups in the ligands used to prepare them. Transfer of excitation energy from a host cage molecule - after absorption of light - to a trapped guest will be investigated, and if possible we will use this phenomenon to trigger isomerisation reactions in the guest. Thus, a guest molecule will enter a host cavity, and shining light on the cage will result in energy transferred to the guest which then undergoes reaction. the host cage therefore acts as a 'photochemical microreactor', both binding the guest and supplying the energy necessary for it to undergo a reaction.