Journal
ACS NANO
Volume 14, Issue 11, Pages 15766-15775Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.0c06798
Keywords
kekulene; aromaticity; STM; photoemission; ARPES; DFT
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Funding
- Deutsche Forschungsgemeinschaft (DFG) [Po 2226/2-1, Ri 804/8-1, Go 1812/2-1, 223848855-SFB 1083]
- Austrian Science Fund (FWF) [P27649-N20, P27427-N20, I3731]
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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 pi-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 pi-conjugated molecule.
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