A highly luminescent tetrahydrocurcumin Ir(III) complex with remarkable photoactivated anticancer activity

Curcumin has chemopreventive properties against a variety of tumours, but it has poor bioavailability. Two new bis-cyclometallated iridium(III) complexes have been prepared, featuring the natural product curcumin (CUR) or its reduced form, tetrahydrocurcumin (THC), as bidentate, anionic O^O-binding ligands. The iridium THC complex is highly luminescent in deoxygenated solution and efficiently generates singlet oxygen under aerated conditions, whereas in the curcumin analogue, other non-radiative decay pathways are competitive. The complexes are rapidly taken up into a variety of human tumour cell lines from solutions of micromolar concentration. They have negligible cytotoxicity in the absence of irradiation. When briefly irradiated by visible light, Ir-THC becomes highly phototoxic, inducing rapid apoptosis within 2 h. The results show the high potential of such complexes as sensitizers in photodynamic therapy (PDT).