Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization
Anja Haags, Alexander Reichmann, Qitang Fan, Larissa Egger, Hans Kirschner, Tim Naumann, Simon Werner, Tobias Vollgraff, Jörg Sundermeyer, Lukas Eschmann, Xiaosheng Yang, Dominik Brandstetter, François C. Bocquet, Georg Koller, Alexander Gottwald, M. Richter, Michael G. Ramsey, Michael Rohlfing, Peter Puschnig, J. Michael Gottfried, Serguei Soubatch, F. Stefan Tautz
Abstract
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.