Breaking the Spin-Forbidden Restriction to Achieve Long Lifetime Room-Temperature Phosphorescence of Carbon Dots

Room-temperature phosphorescent (RTP) carbon dots (CDs) demonstrate significant potential applications in the field of information anticounterfeiting due to their excellent optical properties. However, RTP emission of CDs remains significantly limited due to the spin-forbidden properties of triplet exciton transitions. In this work, an in situ nitrogen doping strategy was employed to design and construct strong spin–orbit coupling nitrogen-doped CDs with mesoporous silica with alumina (N-CDs@MS@Al2O3) RTP composites. Both experimental results and theoretical calculations confirmed that the formation of 1(n, π*) following the introduction of nitrogen breaks the spin-forbidden restriction from 1(π, π*) to 3(π, π*), thereby enhancing spin–orbit coupling, which further promotes intersystem crossing and leads to the effective population of triplet excitons. The designed N-CDs@MS@Al2O3 benefiting from an impressive long lifetime of 3.18 s demonstrates potential application prospects in the field of multilevel information encryption. This work provides a new concept to boost the RTP lifetime of CDs.