  We analyse the electronic and optical properties of graphene quantum dots
(GQD) using accurate \textit{ab initio} many-body $GW$ and Bethe-Salpeter
calculations. We show that most pristine GQD, including structures with
irregular shapes, are characterized by dark low energy singlet excitations that
quench fluorescence. We rationalizqe this property by exploiting the chiral
symmetry of the low energy electronic states in graphene. Edge \textit{sp}$^3$
functionalization is shown to efficiently brighten these low lying excitations
by distorting the \textit{sp}$^2$ backbone planar symmetry. Such findings
reveal an original indirect scenario for the influence of functionalization on
the photoluminescence properties.
