4.1. Aggregation induced fluorescence quenching and quench-resistant
Aggregation induced fluorescence quenching describes the phenomenon wherein a fluorophore has effective luminescence in dilute solution, but the fluorescence is reduced or even disappears completely in concentrated solution or the solid state.[10, 92]Most CDs show this phenomenon, due to the quenching effect and reabsorption effect at high concentrations. Hence, beyond some threshold (low) concentration, the PL intensity of CDs decreases with an increase of concentration. The aggregation induced fluorescence quenching effect of small organic luminescent molecules with planar aromatic ring structures is described as follows: when they are close enough, their large planar polycyclic aromatic structure encourages make them orientate due to strong intermolecular π−π stacking interactions. This superposition of molecules will allow the photoexcited state to fall back to the ground state via the nonradiative transition, resulting in luminescence quenching. It reasonable to assume that in CDs with similar planar aromatic ring structures, a similar quenching process occurs as the concentration increases. Due to the complex structure of CDs, multiple quenching mechanisms may be possible and the origin of the quenching difficult to identify.