4.3. Aggregation-induced room temperature phosphorescence
Aggregation often causes another special phenomenon to emerge, such as room temperature phosphorescence. Phosphorescence is more difficult to produce than fluorescence because it involves a triplet exciton transition and is spin-forbidden.[108, 109] At room temperature, the long lifetime of the triplet excitons means they are easily dissipated by nonradiative decay processes.[110] In order to produce phosphorescence, efficient intersystem crossing and suppression of nonradiative transitions are necessary. Hence, the room temperature phosphorescence of CDs is usually realized in the aggregate state, which can strongly limit fluorescence and promote radiative relaxation via phosphorescence. Lin et al. [105] prepared room temperature phosphorescence CDs via hydrothermal treatment of trimellitic acid (Figure 6E). Aggregates of the large conjugated structures created a triplet excited state resulting in aggregation-induced phosphorescence. The abundant sub-luminophores supply sufficient energy levels contributing to populate triplet states via intersystem crossing. Crosslinked polymer networks can also effectively protect fluorophores and inhibit nonradiative transitions. Yang et al.[106] demonstrated that the covalent crosslinking in the interior of CPDs can prevent the vibrations and rotations of polymer chains, thus providing favorable conditions for intersystem crossing (Figure 6F). Through hydrothermal addition polymerization with acrylamide as monomer, they synthesized ultrahigh-yield CPDs with ultralong phosphorescence lifetime. Carbonization changes the degree of crosslinking and forces the sub-luminophores to form aggregates that increase the degree of conjugation, leading to an emission red-shift. Likewise, inspired by the concept of CEE. Wang et al.[107] prepared CPDs with room temperature phosphorescence by self-crosslinking and carbonization (Figure 6G). The core-shell structure of CPDs enhanced the crosslinking of CPDs and boosted the phosphorescence, creating rich energy levels for intersystem crossover. They proposed a design rule that can be applied for adjust the quantum yields and phosphorescence lifetime of CPDs, based on the stabilization of triplet excited states through the degree of crosslinking.