An easy, large-scale synthesis of N-doped carbon quantum dots (CQDs) was developed by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation. The yield of raw N-doped CQDs was about 83%, which is suitable for industrial-scale production. A detailed formation mechanism for N-doped CQDs involving self-polymerization and condensation of IPDI was demonstrated. Moreover, the obtained N-doped CQDs can be homogeneously dispersed in various organic monomers and do not need toxic organic solvents as dispersing agents. This advantage expands the range of applications of CQDs in composites. The N-doped CQDs dispersed in polyurethane (PU) matrixes emit not only fluorescence but also phosphorescence and delayed fluorescence at room temperature upon excitation with ultraviolet (UV) light. Furthermore, the phosphorescence of CQD/PU composites is sensitive to oxygen and therefore, the obtained-CQDs could be exploited in the development of novel oxygen sensors.

Carbon quantum dots (CQDs) have been widely studied; however, nearly all CQDs can only be used either in aqueous phase or oil phase. There is much less attention on amphiphilic CQDs (ACDs) and their potential applications. In this work, ACDs were successfully prepared by a one-pot hydrothermal treatment of oil soluble N-doped CQDs. The obtained ACDs exhibit favorable aqueous solubility, high photoluminescence (PL) quantum yields, and excellent photostability, showing great potential in Fe³⁺ ion detection. Interestingly, up-conversion PL of the ACDs is also observed and successfully applied in two-photon cell imaging. In addition, the obtained ACDs can be homogeneously dispersed into polyvinyl alcohol (PVA) and polyurethane matrices. Importantly, a room temperature phosphorescence phenomenon can be detected in these ACD-based composites and the ACD/PVA composite films show an ultralong phosphorescence lifetime (450 ms), which expands the scope of research and applications of CQDs.

Carbon dots (CDs) have been promising candidates for metal-free room temperature phosphorescence (RTP) materials recently, but they generally exhibit extremely dim phosphorescence, mainly due to the lack of design guidelines. Herein, we present a new RTP emission enhancing strategy that induces the deprotonation of carboxyl groups on the surfaces of CDs by adjusting the pH to increase the degree of conjugation between the carboxyl group and the carbon core and decrease the surface defect. In addition, for the matrices, the C–O⁻⋯H hydrogen-bonding systems in cyanurate have stronger hydrogen-bonded networks than the CO⋯H hydrogen-bonding systems in cyanuric acid, minimizing nonradiative transitions of triplet excitons. Notably, their phosphorescence quantum yield (14%) at high pH is nearly 5 times higher than that of CD-based composites under acidic conditions. Furthermore, chemical sensors and light-emitting diodes based on CD-based composites are realized by taking advantage of their efficient dual emission feature.

Metal‐free room‐temperature phosphorescence (RTP) materials are of great significance for many applications; however, they usually exhibit low efficiency and weak intensity. This article reports a new strategy for the preparation of a high‐efficiency and strong RTP materials from crystalline thermal‐annealed carbon dots (CDs) and boric acid (BA) composite (g‐t‐CD@BA) through grinding‐induced amorphous to crystallization transition. Amorphous thermal‐annealed CDs and BA composite (t‐CD@BA) is prepared following a thermal melting and super‐cooling route, where the CDs are fully dispersed in molten BA liquid and uniformly frozen in an amorphous thermal annealed BA matrix after super‐cooling to room temperature. Upon grinding treatment, the fracture and fragmentation caused by grinding promote the transformation of the high‐energy amorphous state to the lower energy crystalline counterparts. As a result, the CDs are uniformly in situ embedded in the BA crystal matrix. This method affords maximum uniform embedding of the CDs in the BA crystals, decreases nonradiative decay, and promotes intersystem crossing by restraining the free vibration of the CDs, thus producing strong RTP materials with the highest reported phosphorescence quantum yield (48%). Remarkably, RTP from g‐t‐CD@BA powder is strong enough to illuminate items with a delay time exceeding 9 s.