Three-photon absorption iridium(iii) photosensitizers featuring aggregation induced emission

Abstract

In this study, three-photon absorbing cyclometalated iridium(III) complexes (Ir-H, Ir-8C, and Ir-O) were rationally designed and synthesized through fine-tuning of an auxiliary ligand. The long alkoxy chains in Ir-8C and Ir-O allow maintaining the intramolecular mobility upon aggregation, resulting in the absence of emission in aggregated states. In comparison with Ir-8C and Ir-O, Ir-H exhibited excellent aggregation-induced emission properties in dimethyl sulfoxide/water (DMSO/H₂O) solution. The three-photon absorption cross section reached 300 × 10⁻⁹² cm⁶ s² photon⁻² in the aggregated state. Furthermore, the singlet oxygen (¹O₂) quantum yield of Ir-H could be increased to 0.87 through the synergistic effect of the strong spin–orbit coupling effect of the iridium center and the aggregation-induced intersystem crossing. Ir-H effectively induced cancer cell death both ex vivo and in vivo, with potential application in multiphoton imaging-guided photodynamic therapy.