Toward Cove-Edged Low Band Gap Graphene Nanoribbons

Graphene
nanoribbons (GNRs), defined as nanometer-wide strips of
graphene, have attracted increasing attention as promising candidates
for next-generation semiconductors. Here, we demonstrate a bottom-up
strategy toward novel low band gap GNRs (Eg = 1.70 eV) with a well-defined cove-type periphery both in solution
and on a solid substrate surface with chrysene as the key monomer.
Corresponding cyclized chrysene-based oligomers consisting of the
dimer and tetramer are obtained via an Ullmann coupling followed by
oxidative intramolecular cyclodehydrogenation in solution, and much
higher GNR homologues via on-surface synthesis. These oligomers adopt
nonplanar structures due to the steric repulsion between the two C–H
bonds at the inner cove position. Characterizations by single crystal
X-ray analysis, UV–vis absorption spectroscopy, NMR spectroscopy,
and scanning tunneling microscopy (STM) are described. The interpretation
is assisted by density functional theory (DFT) calculations.