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.