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

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 photoelectron 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.<br>

Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography

“…Indeed, POT has been demonstrated to shed light on subtle questions such as the degree of aromaticity in kekulene. 26 POT has also been used successfully to detect and quantify geometric changes of psexiphenyl, namely the planarization of the molecule upon adsorption on metals and oxide thin films, when there is charge transfer to the molecule, thereby removing the torsional angle between the phenyl rings around the nominal single bonds of the molecule. 24,27,28 In this work, we show that POT can be used to detect and quantify even more subtle geometric changes, namely the bend of the molecule PTCDA.…”