Created Ex Nihilo: A Near-Infrared Afterglow Luminescent Framework of Imidazole Derivatives for Bioimaging

Afterglow luminescence probes are limited by (i) a lack of new afterglow scaffolds; (ii) a dependence on initial external light activation, and (iii) aggregation-caused quenching. Herein, we demonstrate for the first time that Ir(III) complexes of general formula Ir(C^N)₂(N^N)⁺ PF₆ ^- with an imidazole-based N^N ligand exhibit bright afterglow luminescence in the presence of reactive oxygen and nitrogen species (RONS) or under ultrasound (US) irradiation. A series of Ir(III) complexes (Ir-1-Ir-4) establish a new afterglow luminescence mechanism based on experimental data and theoretical calculations. The key feature of the present mechanism is the reaction of the ─N═C─N─H unit of the imidazole ligands with various RONS. After modification of the luminescent framework by quinolyl-triphenylamine C^N ligands, Ir-4 is endowed with aggregation induced emission (AIE) properties and with enhanced near-infrared afterglow luminescence. When assembled into water-soluble nanoparticles (NPs) with the aid of an amphiphilic polymer, Ir-4@R NPs demonstrated excellent afterglow luminescence imaging and effective sonodynamic therapy in vitro and in vivo. These Ir(III) complexes represent the first examples of an afterglow luminescent molecular framework based on readily available and versatile imidazole ligands, breaking through the existing limitation of molecular structures with afterglow luminescence.