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

基本上所有人类活动都需要受控运动。文明的关键进步与促进运动的技术突破有关。在微观尺度上，在分子水平上精确控制运动被用来调节重要的生物功能。目前，人们对合成和应用其运动可以由外部刺激控制的人造装置产生了极大的兴趣。三种类型的外部输入-即化学，电化学和光化学-已被用于在这些所谓的分子机器内诱导明确的旋转或平移运动。在迄今为止研究的所有纳米级器件中，最简单的可能是分子开关。分子开关在分子水平上的信息存储和检索新材料的开发中具有巨大的前景。现有类型的分子开关存在一些缺点（例如，复杂的合成、可靠性），因此非常需要发现和开发新类型的分子开关的工作。这项研究计划的重点是制造和研究新型的纳米开关，其基础是利用与金字塔氮反转（也称为原子或伞反转）相关的运动。在金字塔倒位中，线性运动来自氮原子顶端的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翻时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（称为反相体）的相对量可以通过外部刺激（例如光，添加的化学物质或电子）可逆地控制。在我们的实验室中进行的初步实验使用了一个系统，该系统响应于同时添加的电子和质子（氧化还原过程），为支持这一假设提供了强有力的证据。在这里，我们计划通过构建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。我们的重点将是基于一个众所周知的杂环系统称为氮丙啶。在开发能够控制运动的新分子器件的背景下，基于氮丙啶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，它们是组装简单的结构，可以将几个不同的组放置在靠近反转中心的位置，每个组在三维空间中具有非常可预测和精确的方向。使用NMR光谱学可以很容易地获得与它们的运动速度有关的定量数据。运动的速率可以通过改变杂环周围的取代基模式来微调。此外，关于反演过程的有价值的数据可以从使用基于计算机的方法执行的计算中获得，这可以极大地帮助设计过程。在这里学到的原则有关的控制运动与金字塔的氮丙啶反转可以很容易地外推到其他类别的N-杂环，和杂环含有其他非碳原子（如磷）拥有截然不同的开关速率。因此，关于利用原子反转构建分子器件的一般规则预计将从该计划中出现。

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.