Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization

• Published in: ACS nano Vol. 14; no. 11; pp. 15766 - 15775
• Main Authors: Haags, Anja, Reichmann, Alexander, Fan, Qitang, Egger, Larissa, Kirschner, Hans, Naumann, Tim, Werner, Simon, Vollgraff, Tobias, Sundermeyer, Jörg, Eschmann, Lukas, Yang, Xiaosheng, Brandstetter, Dominik, Bocquet, François C, Koller, Georg, Gottwald, Alexander, Richter, Mathias, Ramsey, Michael G, Rohlfing, Michael, Puschnig, Peter, Gottfried, J. Michael, Soubatch, Serguei, Tautz, F. Stefan
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 24-11-2020
• Subjects: kekulene; photoemission; DFT; STM; ARPES; aromaticity
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/acsnano.0c06798

We revisit the question of kekulene’s aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7­(2,7)-triphen­anthrena­cyclo­nonaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene’s highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C–C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.