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

控制运动基本上是所有人类活动所必需的。人类文明的重大进步与技术突破有关，这些突破促进了人类的迁徙。在微观尺度上，精确控制分子水平上的运动被用来调节重要的生物功能。目前，人们对人造装置的合成和应用产生了极大的兴趣，这些装置的运动可以由外部刺激控制。三种类型的外部输入——化学的、电化学的和光化学的——已经被用来在这些所谓的分子机器中诱导明确的旋转或平移运动。在迄今为止研究过的所有纳米级器件中，最简单的可能是分子开关。分子开关在开发用于分子水平信息存储和检索的新材料方面具有巨大的前景。现有类型的分子开关存在一些缺陷（例如复杂的合成，可靠性），因此发现和开发新型分子开关的工作非常必要。本研究计划的重点是制造和研究基于利用与锥体氮反转（也称为原子或伞反转）相关的运动的新型纳米级开关。在金字塔反转中，线性运动来自于氮的顶端取代基从分子的一边向另一边横向移动（这种运动与强风将雨伞从里面吹到外面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（称为反相物）的相对数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响应电子和质子同时添加（氧化还原过程）的系统，为支持这一假设提供了强有力的证据。在这里，我们计划通过建立和研究广泛的分子开关来进一步发展这些想法，这些开关旨在利用我们对这种运动的控制能力。我们的重点将是基于一个众所周知的杂环系统的系统，称为叠氮嘧啶。在开发能够控制运动的新分子装置的背景下，基于叠氮吡啶的系统提供了几个独特的功能。在没有强酸或亲核试剂的情况下，n -烷基氮嘧啶是相当稳定的分子。此外，它们是简单的结构组装，可以将几个不同的组放置在反转中心附近，每个组在三维空间中都具有非常可预测和精确的方向。利用核磁共振光谱学可以很容易地获得与它们运动速度有关的定量数据。通过改变杂环周围取代基的模式可以微调运动速率。此外，利用基于计算机的方法进行计算可以获得有关反演过程的有价值的数据，这可以极大地帮助设计过程。这里学到的有关控制运动的原理与叠氮嘧啶的锥体反转有关，可以很容易地外推到其他类别的n-杂环，以及含有其他非碳原子（如磷）的杂环，它们具有截然不同的开关速率。因此，关于建立基于利用原子反转的分子装置的一般规则有望从这个程序中出现。

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.