  Excitonic emission from atomically precise graphene nanoribbons (GNRs)
synthesised on a metal surface is probed with atomic-scale spatial resolution
using a scanning tunneling microscopy (STM) approach. A STM-based strategy to
transfer the GNRs to a partially insulating surface is used to prevent light
emission quenching of the ribbons by the metal substrate. Sub-nanometer
resolved STM-induced fluorescence spectra reveal emission from localized dark
excitons build upon the topological end states of the GNRs. A low frequency
vibronic emission comb whose characteristics change with the GNR length is
attributed to longitudinal acoustic modes confined to a finite box. Overall,
our study provides a novel path to investigate the interplay between excitons,
vibrons and topology in atomically precise graphene nanostructures.
