  The advent of on-surface chemistry under vacuum has vastly increased our
capabilities to synthesize carbon-nanomaterials with atomic precision. Among
the types of target structures that have been synthesized by these means,
graphene nanoribbons (GNRs) have probably attracted the most attention. In this
context, the vast majority of GNRs have been synthesized from the same chemical
reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a
detailed study of the growth process of 5-atom-wide armchair GNRs starting from
dibromoperylene. Combining scanning probe microscopy with temperature-dependent
XPS measurements and theoretical calculations, we show that the GNR growth
departs from the conventional reaction scenario. Instead, precursor molecules
couple by means of a concerted mechanism whereby two covalent bonds are formed
simultaneously, along with a concomitant dehydrogenation. Indeed, this novel
reaction path is responsible for the straight GNR growth, something remarkable
considering the initial mixture of reactant isomers with irregular
metal-organic intermediates that we find. The provided insight will not only
help understanding the reaction mechanisms of other reactants, but also serve
as a guide for the design of new precursor molecules.
