Colloidal carbon nitride quantum dots with high‐temperature ultraviolet afterglow from oxygen pre‐doping induced spin–orbit coupling

Ultraviolet (UV) afterglow materials capable of operating at high temperatures and in dispersed states are highly desirable yet remain elusive due to severe thermal quenching and the reliance on rigid host matrices. Herein, we report an oxygen pre‐doping strategy that enables spin–orbit coupling (SOC) engineering in colloidal carbon nitride quantum dots (CNQDs), affording matrix‐free, high‐temperature, and dispersible UV afterglow. The resulting UV‐CNQDs exhibit a distinct UV‐A afterglow centered at 330 nm at room temperature, which blue‐shifts to 310 nm (UV‐B) upon heating and remains stable up to 150°C, even in aqueous dispersion. Mechanistic studies reveal that oxygen pre‐doping introduces abundant carbonyl groups, generating high‐density ( n , π*) configurations within the conjugated network. The resultant π–n orbital mixing enhances SOC, promotes intersystem crossing, and intrinsically stabilizes triplet excitons. Consequently, both phosphorescence and triplet–triplet annihilation‐driven delayed fluorescence (DF) operate in solid and dispersed states. At elevated temperatures, TTA‐driven DF dominates as phosphorescence is thermally suppressed, accounting for the robust UV afterglow. Leveraging these unique properties, we demonstrate the use of these CNQDs for antibacterial activity and real‐time visualization of microcracks in high‐temperature components.