  In this work, we report on the exciton radiative lifetimes of graphitic
carbon nitride monolayers in the triazine- (gC$_3$N$_4$-t) and heptazine-based
(gC$_3$N$_4$-h) forms, as obtained by means of ground- plus excited-state ab
initio calculations. By analysing the exciton fine structure, we highlight the
presence of dark states and show that the photo-generated electron-hole pairs
in gC$_3$N$_4$-h are remarkably long-lived, with an effective radiative
lifetime of 260 ns. This fosters the employment of gC$_3$N$_4$-h in
photocatalysis and makes it attractive for the emerging field of exciton
devices. Although very long intrinsic radiative lifetimes are an important
prerequisite for several applications, pristine carbon nitride nanosheets show
very low quantum photo-conversion efficiency, mainly due to the lack of an
efficient e-h separation mechanism. We then focus on a vertical heterostructure
made of gC$_3$N$_4$-t and gC$_3$N$_4$-h layers which shows a type-II band
alignment and looks promising for achieving net charge separation.
