Aggregation-induced emission luminogens (AIEgens) are
of great
importance in optoelectronics and biomedical fields. However, the
popular design philosophy by combining rotors with traditional fluorophores
limits the imagination and structural diversity of AIEgens. Inspired
by the fluorescent roots of the medicinal plantToddalia asiatica, we discovered two unconventional rotor-free AIEgens, 5-methoxyseselin
(5-MOS) and 6-methoxyseselin (6-MOS). Interestingly, a slight structural
difference of the coumarin isomers leads to completely contrary fluorescent
properties upon aggregation in aqueous media. Further mechanism investigation
indicates that 5-MOS forms different extents of aggregates with the
assistance of protonic solvents, leading to electron/energy transfer,
which is responsible for its unique AIE feature, i.e., reduced emission
in aqueous media but enhanced emission in crystal. Meanwhile, for
6-MOS, the conventional restriction of the intramolecular motion (RIM)
mechanism is responsible for its AIE feature. More interestingly,
the unique water-sensitive fluorescence property of 5-MOS enables
its successful application for wash-free mitochondria imaging. This
work not only demonstrates an ingenious tactic to seek new AIEgens
from natural fluorescent species but also benefits the structure design
and application exploration of next-generation AIEgens.