Breaking the\nSpin-Forbidden Restriction to Achieve\nLong Lifetime Room-Temperature Phosphorescence of Carbon Dots

Room-temperature phosphorescent (RTP)\ncarbon dots (CDs)\ndemonstrate\nsignificant potential applications in the field of information anticounterfeiting\ndue to their excellent optical properties. However, RTP emission of\nCDs remains significantly limited due to the spin-forbidden properties\nof triplet exciton transitions. In this work, an in situ nitrogen\ndoping strategy was employed to design and construct strong spin–orbit\ncoupling nitrogen-doped CDs with mesoporous silica with alumina (N-CDs@MS@Al<sub>2</sub>O<sub>3</sub>) RTP composites. Both experimental results and\ntheoretical calculations confirmed that the formation of <sup>1</sup>(n, π*) following the introduction of nitrogen breaks the spin-forbidden\nrestriction from <sup>1</sup>(π, π*) to <sup>3</sup>(π,\nπ*), thereby enhancing spin–orbit coupling, which further\npromotes intersystem crossing and leads to the effective population\nof triplet excitons. The designed N-CDs@MS@Al<sub>2</sub>O<sub>3</sub> benefiting from an impressive long lifetime of 3.18 s demonstrates\npotential application prospects in the field of multilevel information\nencryption. This work provides a new concept to boost the RTP lifetime\nof CDs.