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

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.