4.2. Aggregation-induced emission
Aggregation-induced emission (AIE) refers to fluorophores that have
negligible light emission in the dilute solution, but strong emission in
aggregated form.[100] In studies of small organic
molecules, AIE can be achieved in many ways, especially in cases where
aggregation limits intramolecular rotation, aggregation limits
intramolecular vibration, or highly twisted aggregated structures weaken
intermolecular π−π stacking.[92] In CDs, phenomena
similar to AIE have been reported, specifically aggregation-induced
fluorescence enhancement and aggregation-induced wavelength-redshifts.
In a large degree, concentration-dependent PL is a reflection of AIE in
CDs. Liu et al. [101] prepared CDs using tannic
acid, which exhibit visual aggregation induced emission enhancement. The
enhancement was attributed to surface groups on the CDs, such as
aromatic rings and phenolic hydroxyl (Figure 6C), interacting to hinder
rotation in the fluorophores. Many researchers have proposed that
aggregation increases the conjugation degree of surface luminophores,
leading to a decrease of band gap. Hence, emission wavelengths gradually
red shift with increasing CDs concentration. Yang et al.[102] designed CPDs from dithiosalicylic
acid/acetic acid and melamine (Figure 6D). The CPDs contained two PL
centers, π-conjugated domains in the nucleus, and S–S bond on the
surface. In the dispersed state, the S–S bond did not possess
fluorescence emission due to intramolecular rotation, with the blue
emission of π-conjugated domains dominating the emission spectrum. When
the CPDs aggregate, the π-conjugated domains undergo π–π stacking,
quenching the blue emission. However, aggregation causes rotation
restriction of S–S bonds, resulting in a red emission. Unlike extremely
low quantum yields caused by H-type aggregation, J-type aggregation
usually results in a red-shift of the emission wavelength. Chen et al.[103] and Li et al. [104]both developed GQDs exhibiting red-shift emissions at high
concentration, involving self-assembled J-type π−π aggregates. They
indicated that the large number of carboxyl groups at the edge of GQDs
are limit π−π intermolecular interactions, thus leading to J-type
aggregation rather than H-type aggregation.