Polyyne and cumulene rotaxanes: Toward insulated molecular wires

• Applicant: Professor Dr. Rik Tykwinski
• Subject Area: Organic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 244751608

Oligomeric molecules built from sp-hybridized carbon, namely polyynes and cumulenes, are fundamentally important due to their unique one-dimensional skeleton. They are also the key link to predicting the properties of the carbon allotrope carbyne. On an applied level, polyynes are ideal candidates for molecular wires, they show impressive nonlinear optical (NLO) properties, and they are invaluable building blocks to carbon-rich architectures. Conversely, the potential of cumulenes to function as conjugated materials remains essentially unexplored, due the limited number of known derivatives. The synthesis and chemistry of polyynes and cumulenes currently relies on a rather small toolbox of reactions and few have proven to be broadly applicable to extended polyynes and cumulenes. Beyond synthetic difficulties, however, lie significant challenges due to the instability/reactivity of polyynes and cumulenes, and this translates directly to the fact that very little has been accomplished toward the use of these unique molecules in real world devices. The research outlined in this proposal aims to solve the problem of polyyne and cumulene stability, through development of synthetic methods to form derivatives stabilized through mechanical bonds, i.e., rotaxanes. The thus formed rotaxanes will provide products for fundamental investigations, as well as for incorporation into nanoscale devices. The workplan consists of three areas of discovery. First, Work-Package 1 will optimize the synthetic methods for polyyne rotaxane, including substrates, reaction conditions, transition metal catalyst, solvent, and the size and functionality of the macrocyclic component. Next we strive to evaluate and refine parameters to form stabilized polyynes, in comparison to their naked, non-rotaxane analogs. The successful conclusion of this last point would offer polyynes of unprecedented length, to study the properties of carbyne. Work-Package 2 then explores stoppers that might be appended to the termini of the polyyne. The main goals here center on the formation of functionalized polyyne rotaxanes. Targeted groups include aromatic and heteroaromatic moieties that provide molecular wires for attachment to metal or graphene electrodes. Alternatively, donor and acceptor (D-A) groups will be attached to offer polarized, push-pull polyynes. D-A polyynes will be studied in comparison to known D-A polyenes, to determine the efficiency of triple bonds to mediate communication, as well as the formation of optimized NLO materials. Finally, protocols developed for polyyne rotaxanes will be adapted to form cumulene rotaxanes (Work-Package 3). To date, derivatives longer than [7]cumulenes are insufficiently stable for extensive studies in solution or the solid state. As a result, the compounds provided by the proposed research will give the first opportunity to probe the properties of longer members of this class of sp-carbon oligomers.

• DFG Programme: Research Grants