Alexander ReichmannMSc Student Karl-Franzens-Universität GrazInstitute of Physics – Solid State Theory
In this work the geometric, aromatic and electronic structure of the polycyclic hydrocarbon kekulene (C48 H24 ) is studied using density functional theory (DFT), the harmonic oscillation model of aromaticity (HOMA), the simple Hückel molecular orbital theory and the probe particle model.
Experimental results of kekulene adsorbed on the two copper substrates, Cu(111) and Cu(110) are compared to DFT optimized calculations regarding the same metal-organic interfaces as well as theoretical calculations of gas phase kekulene.
Firstly, the energetically most favorable adsorption position of kekulene on the copper surfaces is obtained. With the optimized configuration the aromatic structure is investigated via the HOMA. In order to further analyze the aromaticity of kekulene, various theoretical models of kekulene are constructed by fixing the geometric structure of kekulene and by fixing electronic structure via the Hückel molecular orbital model. These models, the DFT optimized structures of free kekulene as well as the kekulene/Cu interface are examined in terms of their photoemission intensity, in order to gain insights into their electronic structure. The photoemission momentum maps are compared with photoemission momentum maps gained from angle resolved photoemission spectroscopy measurements. Furthermore, the density of states is investigated for the DFT optimized calculations and the charge density distribution and real space distribution of the frontier orbital nodal structure are investigated of all theoretical configurations. Finally the probe particle model is used in order to simulate non-contact atomic force microscopy and inelastic tunneling spectroscopy measurements.
From the theoretical, as well as the experimental results, it is concluded that the aromatic Clar’s sextet model is a good predictor of kekulene’s aromatic structure.
A. Haags, A. Reichmann, Q. Fan, L. Egger, H. Kirschner, T. Naumann, S. Werner, T. Vollgraff, J. Sundermeyer, L. Eschmann, X. Yang, D. Brandstetter, F. C. Bocquet, G. Koller, A. Gottwald, M. Richter, M. G. Ramsey, M. Rohlfing, P. Puschnig, J. M. Gottfried, S. Soubatch, F. S. Tautz
In: ACS Nano, vol. 14, pp. 15766-15775, 2020.
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.