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.