Proton transfer over extended molecular architectures

Content: The tremendous increase in computational efficiency seen in the past three decades is due to the miniaturisation of semiconductor-based components. However, drastic limitations in their performance are expected when approaching atomic dimensions, due to atomic defects and quantum phenomena such as tunneling. Using single functional molecules as electronic components is a conceptually novel way of circumventing this limitation. Molecular switches represent key components in such Molecular Electronics, either as storage units that encode binary bits or to control electric currents. The most promising candidates for molecular switches are based on tautomerism, that is, the transfer of a single proton within the structure of a molecule, since such molecules can reversibly change their physical and chemical properties without altering their conformation. Moreover, investigation of tautomerisation gives fundamental insight into basic chemical processes involving proton transfer that are important in photochromism and biological reactions. Hypotheses and methods: This project will focus on the tautomerisation of aminotroponimine derivatives, which contain a molecular motif that has recently been identified as switching unit in a very simple form. A systematic modification of the molecular skeleton with differing functional groups shall allow controlled tuning of the energies required to activate the process and to impose a predefined asymmetry in the reversible switching. Specifically, the elemental composition, atomic arrangement, and electronic structure will be varied within the same molecule that also contains the active tautomeric centre. From such a controlled modulation of the potential energy landscape along the tautomerisation pathway, detailed understanding of the proton transfer and its important parameters is expected. Innovative character: Furthermore, the possibility of proton transfer over relatively long distances within extended molecular architectures will be investigated by the controlled covalent coupling of discrete switching units. This should result in molecular structures with multiple switching moieties, independent from each other, something that has not yet been achieved, although representing a key challenge in the field. Through careful selection of the reactive sites of the monomers, one- and two-dimensional architectures with active switching sites will be produced. Concerted proton transfer within these structures will be attempted with the central vision of long range proton transport. Such systems could be of great interest for future applications in high density data storage. Potential for success: The applicant (Dr. Grant Simpson) studied a molecular prototype of tautomerisation-based molecular switches (Ph.D. at University of St. Andrews, UK) and hence has expert knowledge on both tautomerisation and scanning probe methods. The host (Prof. Leonhard Grill) is a leading researcher in the manipulation of single functional molecules by scanning tunneling microscopy. The combined expertise and ideal experimental facilities available at the University of Graz gives the project the very best chance of success.

This project aimed to address aminotroponimine (ATI) derivatives on the single-molecule scale to investigate their tautomerisation properties. Tautomerisation is the transfer of individual hydrogen atoms, or protons, within a single molecule from one location to another. This process renders such a molecule bistable and, with a suitable method such as scanning tunnelling microscopy (STM), these states can be selected and resolved. The switching unit within ATI molecules was previously identified to be active and consists of an amino (R- CNH2) group in close proximity to an imine group (R-C=NH). In the first part of the project, ATI molecules were successfully deposited on metallic surfaces such as silver or gold and imaged individually using the STM. With the topography of the molecule matching closely to what was hypothesised, excitations were delivered to the molecule from the STM tip to induce tautomerisation. Bistability in the tunnel current measured during these excitations suggest that two stable tautomeric states were active in the molecule. However, the resolution of these two states through STM imaging was not successful and requires more work. A further task of the project was to investigate the long-range transfer of protons within molecular architectures. To this end, extended cyclopyrrole derivatives were investigated using STM. Many reports in the literature have shown that porphyrin-type molecules, which have a central macrocycle containing four pyrrole units, are tautomerically active. In this project the much larger cyclooctopyrroles were used. Due to their increased size and complexity, sample preparation of this compound posed initial difficulties and a procedure was developed to spray the molecules into the ultra-high vacuum chamber where a metal substrate was located. STM imaging revealed that the molecules were intact and it was possible to identify the locations of specific functional groups within the molecules. No immediate signs of tautomerisation were present in these molecules were found, likely due to the purity of the compound and the chemical environment within the central pyrrole cavity. Further work continues to investigate the possibility of realising a molecule with up to eight stable tautomeric forms.