  Polymeric carbon nitrides (PCNs) exhibit intriguing optical properties and
exceptional performance in (photo)catalysis, optoelectronics, and energy
storage. Nevertheless, the intricate phenomena involving light absorption
extending to the visible range, long-lived excitons, photo-charging, and
photochemical processes observed in PCN derivatives remain poorly understood.
Our theoretical investigation elucidates the origin of distinct dark and bright
excitons, their stability and lifetimes, and their correlation with the
microstructural attributes of PCNs. Furthermore, we identify the decisive role
of dark excitons in catalytic reactivity, which underlies the differences in
the photocatalytic performance of different PCN derivatives.
