CONTROLLING THE MOLECULAR MOTION ASSOCIATED WITH PYRAMIDAL INVERSION: TOWARDS NEW TYPES OF NANOSCALE SWITCHES

基本上所有的人类活动都需要有控制的运动。文明的关键进步与促进流动的技术突破联系在一起。在微观尺度上，在分子水平上对运动的精确控制被用来调节重要的生物功能。目前，可以通过外部Stimulii控制其运动的人造装置的合成和应用受到了极大的关注。三种类型的外部输入--即化学输入、电化学输入和光化学输入--被用来在这些所谓的分子机器中诱导明确定义的旋转或平移运动。到目前为止，在所有研究的纳米设备中，最简单的可能是分子开关。分子开关在开发用于分子水平的信息存储和检索的新材料方面有着巨大的前景。现有的分子开关存在一些缺点(如合成复杂、可靠性差等)，因此迫切需要发现和开发新型的分子开关。这项研究计划致力于基于与金字塔氮反转(也称为原子反转或伞形反转)相关的运动来制造和研究新型纳米级开关。在金字塔倒置中，线性运动来自氮原子上的顶端取代基从分子的一侧横向移动到另一侧(这种运动与雨伞被强风吹倒时所看到的运动没有什么不同)。我们认为，与这种运动相关的运动速度，以及两种形式的分子(称为反相子)的相对量，可以由外部刺激(如光、添加的化学物质或电子)可逆地控制。我们实验室使用一种对电子和质子同时添加(氧化还原过程)做出反应的系统进行的初步实验为这一假说提供了强有力的证据。在这里，我们计划通过建立和研究广泛的分子开关来进一步发展这些想法，这些分子开关旨在利用我们对这种运动施加控制的能力。我们的重点将放在基于一种著名的杂环体系的体系上，该体系称为氮杂环丙烷。在开发能够控制运动的新的分子装置的背景下，基于氮杂环丙烷的系统提供了几个独特的特征。在没有强酸或亲核试剂的情况下，N-烷基氮杂环丙烷是相当稳定的分子。此外，它们是易于组装的简单结构，可以在反转中心附近放置几个不同的组，每个组在三维空间中具有非常可预测和精确的方向。利用核磁共振波谱可以很容易地获得与它们的运动速度有关的定量数据。可以通过改变杂环周围的取代基模式来微调运动速度。此外，有关反演过程的有价值的数据可以从使用基于计算机的方法进行的计算中获得，这将极大地辅助设计过程。这里所学到的与氮杂环中的金字塔倒置有关的受控运动原理可以很容易地外推到其他类型的N-杂环，以及含有其他非碳原子(例如磷)的具有极大不同开关速率的杂环。因此，关于利用原子反转建立分子装置的一般规则预计将从该计划中产生。

Controlled motion is required for essentially all human activities. Key advances in civilisation have been associated with technological breakthroughs that have facilitated movement. At the microscopic scale, precise control of motion at the molecular level is used to regulate important biological functions. Currently, there is enormous interest in the synthesis and application of man-made devices whose motion can be controlled by external stimulii. Three types of external inputs - that is chemical, electrochemical and photochemical - have been used to induce well-defined rotational or translation movements within these so-called molecular machines. Of all the nanoscale devices studied to date, the simplest is perhaps the molecular switch. Molecular switches hold enormous promise in the development of new materials for information storage and retrieval at the molecular level. Existing classes of molecular switches suffer from several drawbacks (e.g. complex synthesis, reliability), so work to discover and develop new types of molecular switch is much needed. This research proposal is focused on making and studying new types of nanoscale switches based upon exploiting the motion associated with pyramidal nitrogen inversion (also called atomic or umbrella inversion). In pyramidal inversion, the linear movement comes from the apical substituent on the nitrogen moving laterally from one side of the molecule to the other (a movement not dissimilar to that witnessed when an umbrella is blown inside out by strong winds). We suggest that the speed of motion associated with this movement, and the relative amounts of the two forms of the molecule (called the invertomers) can be reversibly controlled by external stimuli (e.g. light, added chemicals or electrons). Initial experiments conducted in our laboratories using a system that responds to the simultaneous addition of electrons and protons (a redox process) provides strong evidence in support of this hypothesis. Here, we plan to further develop these ideas by building and studying a broad range of molecular switches that are designed to exploit our ability to exert control over this type of motion. Our focus will be on systems based upon a well-known heterocyclic ring system called an aziridine. In the context of developing new molecular devices capable of controlled motion, aziridine based systems offer several unique features. In the absence of strong acid or nucleophiles, N-alkyl aziridines are rather stable molecules. Furthermore, they are simple structures to assemble, with the possibility of placing several different groups close to the inversion centre, each with very predictable and precise orientations in three dimensional space. Quantitative data relating to their speed of motion can easily be obtained using NMR spectroscopy. The rate of motion can be fine tuned by altering the substituent pattern around the heterocyclic ring. Moreover, valuable data concerning the inversion process can be obtained from calculations performed using computer based methods which can greatly aid the design process. The principles learnt here concerning controlled motion associated with pyramidal inversion in aziridines could readily be extrapolated to other classes of N-heterocycles, and to heterocycles containing other non-carbon atoms (e.g. phosphorus) possessing vastly different switching rates. Hence, general rules concerning building molecular devices based on exploiting atomic inversion are expected to emerge from this programme.