A cyclometallated fluorenyl Ir(iii) complex as a potential sensitiser for two-photon excited photodynamic therapy (2PE-PDT)

Abstract

A new Ir(III) cyclometallated complex bearing a fluorenyl 5-substituted-1,10-phenanthroline ligand ([Ir(ppy)₂(L1)][PF₆], ppy = 2-phenylpyridine) is presented which exhibits enhanced triplet oxygen sensing properties. The efficacy of this complex to act as a photosensitiser for altering the morphology of C6 Glioma cells that represent malignant nervous tumours has been evaluated. The increased heavy metal effect and related spin–orbit coupling parameters on the photophysical properties of this complex are evidenced by comparison with Ru(II) analogues. The complex [Ir(ppy)₂(L1)][PF₆] is shown to exhibit relatively high two-photon absorption efficiencies for the lowest energy MLCT electronic transitions with two-photon absorption cross sections that range from 50 to 80 Goeppert–Mayer units between 750 to 800 nm. Quantum yields for the complex were measured up to 23% and the Stern–Volmer quenching constant, K_SV was determined to be 40 bar⁻¹ in acetonitrile solution, confirming the high efficiency of the complex as a triplet oxygen sensitiser. Preliminary in vitro experiments with C6 Glioma cells treated with [Ir(ppy)₂(L1)][PF₆], show that the complex is an efficient sensitizer for triplet oxygen, producing cytotoxic singlet oxygen (¹O₂) by two-photon excitation at 740 nm resulting in photodynamic effects that lead to localised cell damage and death.