  Graphene nanoribbons are one-dimensional stripes of graphene with width- and
edge-structure-dependent electronic properties. They can be synthesized
bottom-up in solution to obtain precise ribbon geometries. Here we investigate
the optical properties of solution-synthesized 9-armchair graphene nanoribbons
(9-aGNRs) that are stabilized as dispersions in organic solvents and further
fractioned by liquid cascade centrifugation (LCC). Absorption and
photoluminescence spectroscopy reveal two near-infrared absorption and emission
peaks whose ratios depend on the LCC fraction. Low-temperature
single-nanoribbon photoluminescence spectra suggest the presence of two
different nanoribbon species. Based on density functional theory (DFT) and
time-dependent DFT calculations, the lowest energy transition can be assigned
to pristine 9-aGNRs, while 9-aGNRs with edge-defects, caused by incomplete
graphitization, result in more blue-shifted transitions and higher Raman
D/G-mode ratios. Hole doping of 9-aGNR dispersions with the electron acceptor
F4TCNQ leads to concentration dependent bleaching and quenching of the main
absorption and emission bands and the appearance of redshifted, charge-induced
absorption features but no additional emission peaks, thus indicating the
formation of polarons instead of the predicted trions (charged excitons) in
doped 9-aGNRs.
