Synthesis of versatile fluorescent isoquinolinium salts and their applications

Isoquinolinium salts are well-known N-heterocyclic cationic compounds that have been applied in various research fields, such as materials chemistry and pharmaceutical applications. In particular, isoquinolinium salts with polyaromatic structures have been widely investigated due to their intrinsic photophysical properties, including fluorescence emission maxima, fluorescence lifetime, and quantum yield. Notably, fluorescent isoquinolinium salts have been synthesized via various synthetic strategies, such as alkylation of isoquinoline, oxidation of dihydropyridines, rearrangement reactions, and transition-metal-catalyzed C–H activation. In this review, we summarize the synthetic methodologies for diverse fluorescent isoquinolinium salts and their applications in pharmaceutical applications, materials chemistry, theranostics, DNA binding, fluorescent sensing, and bioimaging.