Polymer carbon dots (PCDs) are proposed as a new class of room-temperature phosphorescence (RTP) materials. The abundant energy levels in PCDs increase the probability of intersystem crossing (ISC) and their covalently crosslinked framework structures greatly suppress the nonradiative transitions. The efficient methods allow the manufacture of PCDs with unique RTP properties in air without additional metal complexation or complicated matrix composition. They thus provide a route towards the rational design of metal-free RTP materials that may be synthesized easily. Furthermore, we find that RTP is associated with a crosslink-enhanced emission (CEE) effect, which provides further routes to design improved PCDs with diverse RTP performance. Our results show the potential of PCDs as a universal route to achieve effective metal-free RTP.

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Carbon dots (CDs), as emerging luminescent nanomaterials, possess excellent but complex properties, bringing about extensive attention and a lot of controversy. In this Perspective, we put forward the concept of “carbonized polymer dots” and emphasize the important role of polymerization and carbonization during the formation of CDs. We explore the common characters and clarify the complicated relationship of CDs, based on the reasonable classification of graphene quantum dots, carbon quantum dots, and carbonized polymer dots. Moreover, different perspectives are provided for comprehensive analysis about the essence of CDs, including quantum dots, molecules, and polymers. The photoluminescence mechanism has been classified into molecule state, carbon core state, surface/edge state, and cross-link enhanced emission effect for further understanding of complicated phenomena.

Polymer carbon dots (PCDs) represent a new class of carbon dots (CDs) possessing sub‐fluorophores and unique polymer‐like structures. However, like small molecule dyes and traditional CDs, PCDs often suffer from self‐quenching effect in solid state, limiting their potential applications. Moreover, it is hard to prepare PCDs that have the same chemical structure, exhibiting full‐color emission under one fixed excitation wavelength by only modulating the concentration of the PCDs. Herein, self‐quenching‐resistant solid‐state fluorescent polymer carbon dots (SSFPCDs) are prepared, which exhibit strong red SSF without any other additional solid matrices, while having a large production yield (≈89%) and a considerable quantum yield of 8.50%. When dispersed in water or solid matrices in gradient concentrations, they can exhibit yellow, green, and blue fluorescence, realizing the first SSFPCDs with the same chemical structure emitting in full‐color range by changing the ratio of SSFPCDs to the solid matrices.

The crosslink‐enhanced emission effect was first proposed to explore the strong luminescence of nonconjugated polymer dots possessing only either non‐emissive or weakly emissive sub‐luminophores. Interesting phenomena in recent research indicate such enhancement caused by extensive crosslinking appears in diverse luminescent polymers with sub‐luminophores (electron‐rich heteroatomic moieties) or luminophores (conjugated π domains). This enhancement can promote the emission from nonluminous to luminous, from weakly luminous to strongly luminous, and even convert the pathway of radiative transitions. The concept of the crosslink‐enhanced emission effect should be updated and extended to an in‐depth spatial effect, such as electron overlap and energy splitting in confined domains by effective crosslinking, more than initial immobilization. This Minireview outlines the development of the crosslink‐enhanced emission effect from the perspective of the detailed classification, inherent mechanism and applicable systems. An outlook on the further exploration and application of this theory are also proposed.