Generation and Characterization of Cyclocarbons

The family of carbon allotropes was expanded by tip-induced synthesis and on-surface characterization of cyclo[N]carbons, CN, molecular rings composed of N carbon atoms [1-3]. Cyclocarbons possess two orthogonal delocalized pi-systems and can undergo different Jahn-Teller distortions rendering them excellent systems for benchmarking theory [4-6]. Because they are monocyclic, they are valuable in the context of aromaticity. Even-N cyclocarbons can be doubly aromatic [1, 3] or doubly anti-aromatic [2], and can exhibit polyynic [1, 2] or cumulenic [3] structures. Odd-N cyclocarbons [5], e.g., C13 [7] cannot adopt a complete polyynic structure, might be open-shell systems and might feature localized carbene centres.

Cyclo[26]carbon, C26, on ML NaCl/Au(111), generated from two C13 precursors. (A) AFM and (B) Laplace-filtered AFM data, with the positions of short bonds marked. (C-H) gas-phase DFT calculations. (I, J) STM at V = 2.0 V, (K, L) STM at V = –2.75 V. (J, L) Laplace-filtered images of (I, K) with the positions of the short bonds (from B) marked. CO tip. Scale bars 5 Å. [7]

References [1] K. Kaiser et al. Science 365, 1299-1301 (2019) [2] Y. Gao et al. Nature 623, 977–981 (2023) [3] L. Sun et al. Nature 623, 972–976 (2023) [4] G. V. Baryshnikov et al. J. Phys. Chem. A 124, 10849–10855 (2020) [5] G. V. Baryshnikov et al. J. Phys. Chem. A 126, 2445–2452 (2022) [6] I. Rončević et al. J. Am. Chem. Soc. 145, 26962–26972 (2023) [7] F. Albrecht et al. preprint: 10.26434/chemrxiv-2023-ddrh7-v2 (2024)