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

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.