Ionic Aggregation‐Induced Emission: Bulky Hydrophobic Counterions Light Up Dyes in Polymeric Nanoparticles

Abstract

Dye‐loaded polymeric nanoparticles emerge as powerful bioimaging tools, but their assembly is challenged by aggregation‐caused quenching (ACQ) of flat fluorophores. Aggregation‐induced emission (AIE) proposes an effective solution against ACQ by exploiting propeller‐shaped neutral fluorophores without polymeric blending. Fighting ACQ of ionic dyes in polymeric nanoparticles can be achieved by bulky hydrophobic counterions. Here, it is aimed to generate an AIE phenomenon in poorly emissive ionic dyes using bulky counterions. Three cationic hemicyanine dyes of styryl pyridinium family are synthesized: two planar fluorophores featuring ACQ and one propeller‐shaped AIE fluorophore. It is found that, in water, bulky fluorinated tetraphenylborates can light up all three dyes, including planar non‐AIE fluorophores. The described “ionic” AIE (iAIE) with bulky counterions enables preparation of 50‐nm dye‐loaded polymeric nanoparticles showing 40% quantum yield at 500 m m dye loading, whereas small anions favor poorly emissive large aggregates. Single‐particle microscopy reveals that these nanoparticles emit without blinking 50‐fold brighter than commercial quantum dots “Qdot 605” (at 470‐nm excitation). iAIE opens the route to assembling charged intrinsically non‐AIE fluorophores into bright fluorescent (nano)materials.