The bottom-up, atomically precise synthesis of carbon nanostructures enables the tailoring of their electronic properties at a molecular level. [n]cycloparaphenylenes ([n]CPPs)—closed rings of phenylenes linked in the para position—possess desirable π-electron delocalization along the carbon backbone, but the high strain in these systems prevents their π-extension into either larger or planar structures. Now, a collaboration led by Sabine Maier and Andreas Görling at Friedrich-Alexander-Universität Erlangen-Nürnberg and Konstantin Amsharov at Martin-Luther-Universität Halle-Wittenberg in Germany describes the on-surface synthesis of planar π-extended [12]CPP, featuring an all-armchair edge topology, whereby the peripheral phenylene units are solely para-conjugated (https://doi.org/10.1038/s41557-022-00968-3)1.

Planarizing CPPs requires the introduction of a strong in-plane bend in what would otherwise be a straight edge. The team achieved this by designing bowl-shaped dibrominated indacenopicene precursors that contain the appropriate curvature for the paraphenylene backbone. These precursors were covalently coupled on a Au(111) surface—where the surface acts as both a support and a catalyst—via an Ullmann-type dehalogenative coupling followed by cyclodehydrogenation. Trimers, tetramers ([12]CPP) and pentamers form via cis coupling, while dominant trans coupling leads to chain structures (Fig. 1).

Fig. 1: Atomically precise carbon nanostructures formed by Ullmann-type coupling of dibrominated indacenopicene precursors followed by cyclodehydrogenation on Au(111).
figure 1

Trans coupling leads to π-extended polyparaphenylene chains (a), while cis coupling affords bent armchair graphene nanoribbons (trimers), planar π-extended [12]CPPs (tetramers) and π-extended [15]CPP (pentamers) (b). Micrographs are high-resolution scanning tunneling microscopy images, with scale bars 1 nm. Reprinted by permission from Springer Nature: Nat. Chem., copyright 2022.

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“The exclusive para-conjugation at the periphery of planar π-extended [12]CPP yields delocalized electronic states and facilitates a strong electronic communication along an extended π-system.”, explains Maier. Furthermore, planarization maximizes p-orbital overlap, contributing to a reduced electronic bandgap in comparison to conventional CPPs. Density functional theory calculations additionally find that [12]CPP features ring currents in its doubly charge configuration, affording global aromaticity.

Looking to the future, Maier hopes to experimentally characterize the unique electronic properties of such systems: “Synthesis strategies on insulting surfaces should be explored to decouple the molecular systems from the metal substrate electronically. Scanning probe measurements in the presence of a magnetic field could then facilitate the first direct visualization of ring currents at the atomic scale.” Furthermore, these properties could serve to make planar π-extended CPPs promising quantum materials.