Revealing graphitic nitrogen participating in p–π conjugated domain as emissive center of red carbon dots and applied to red room-temperature phosphorescence

Synthesizing high-efficiency red carbon dots (R-CDs) and exploring their photoluminescence mechanism remain challenges as well as critical issues hindering the development of red room-temperature phosphorescence (RTP) materials of CDs. Herein, a series of highly efficient R-CDs are prepared from citric acid and formamide by solvothermal method on account of the protective and destructive effects of solvents. The larger sp2 conjugated domains of the carbon core and the high content of graphitic N are identified as the key factors for R-CDs to exhibit red fluorescence. Surface groups also contribute to red emission by means of the formation of the N, O-related defect state. By establishing N-doping models and using DFT calculation, it is further demonstrated that the strong p–π conjugation effect caused by the non-bonding electrons of graphitic N would facilitate HOMO–LUMO splitting of the carbon core, thus leading to a significant decrease in the HOMO–LUMO gap. Energy level splitting could lead to a certain degree of energy level crossing, which could contribute to effective intersystem crossing and produce red RTP. From this, the potential applications of R-CDs in the information encryption field are proposed and demonstrated.