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.