Periodic Reporting for period 1 - TACY (Tackling the Cyclacene Challenge)
Reporting period: 2023-04-01 to 2024-09-30
Cyclacenes are polycyclic compounds with linearly fused aromatic moieties that form hoop-shaped ring structures. They represent the shortest cyclic cut-outs of zig-zag carbon nanotubes. The unique chemical, electronic and structural properties proposed for these ring-shaped carbon compounds make them attractive for use in organic electronics and spintronics. Despite decades of research efforts, it has not been possible to produce even a single representative of this substance class, let alone investigate their physical or chemical properties. Cyclacenes are assumed to be highly reactive, which likely explains the extraordinary challenges in their synthesis. The ERC-funded TACY project will pioneer new synthetic strategies to generate nanobelt precursors in high yields, enabling the formation of cyclacenes and more stable derivatives thereof. The proposed approach is highly modular, allowing to structurally vary the belt-type precursors, and thus the cyclacenes, by placing different substituents at the rims. Project research, combined with state-of-the-art computational activities and molecular characterisation techniques, will provide unprecedented insight into the structure-property relationship of the conjugated zigzag topology in cyclacenes.
The TACY project team made already substantial progress. This is best described by the first publications, which are summarized herein. The synthesis of conjugated nanobelts is a challenging task and we demonstrated for the first time that these can be made in 84% yield by dimerization reactions of chiral precursors (Angew. Chem. Int. Ed. 2024, e202414059). One further goal is to compare the spectroscopic and chemical properties of the linear longer acenes having the same number of rings as the targeted cyclacenes. In this respect, we demonstrated the feasibility of the on-surface single-molecule STM manipulation approach for the synthesis of unprecedentedly long acenes. We achieved the synthesis of the so far longest published acene, tridecacene, and characterized its structural and electronic properties in detail using scanning probe and density functional theory methods (J. Am. Chem. Soc. 2024, 146, 3700-3709). To understand the stability of cyclacenes more deeply, computational studies of their reactivity is needed. We computed that cyclacenes are much less stable towards dimerization than the corresponding acenes (J. Phys. Chem. A. 2024, 128, 6847–6852). This research supports the central tenet of the project that special reaction conditions and techniques are required for the generation of cyclacenes.