AIE-active Ir(iii) complexes functionalised with a cationic Schiff base ligand: synthesis, photophysical properties and applications in photodynamic therapy

Abstract

Photodynamic therapy (PDT) is a promising cancer treatment method. Traditional small-molecule photosensitizers (PSs) suffer from low intersystem crossing (ISC) ability and aggregation-caused quenching (ACQ), which adversely affects the luminous efficiency and singlet oxygen (¹O₂) yield of PSs in the aggregated state. Ir(III) complexes are promising PSs with long excited-state lifetime, good photophysical and photochemical properties and large Stokes shifts. Aggregation-induced emission (AIE) characteristics could reduce the nonradiative recombination and improve the ISC ability of excited states through the restriction of the intramolecular motions in aggregated states. Accordingly, two AIE-active Ir(III) complexes Ir-1-N⁺ and Ir-2-N⁺ were successfully designed and obtained based on Schiff base ligands. Experimental results showed that Ir-1-N⁺ and Ir-2-N⁺ have good photophysical properties and the corresponding nanoparticles (NPs) have good water solubility and ¹O₂ generation ability. Notably, Ir-2-N⁺ NPs can be efficiently taken up by mouse breast cancer cells (4T1 cells) with good biocompatibility, low dark toxicity and excellent phototoxicity. This work demonstrates a versatile strategy for exploiting efficient transition metal PSs with a cationic ligand in PDT.