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

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, KSV 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 (O-1(2)) by two-photon excitation at 740 nm resulting in photodynamic effects that lead to localised cell damage and death.