Hypoxia-Active\nIridium(III) Bis-terpyridine Complexes\nBearing Oligothienyl Substituents: Synthesis, Photophysics, and Phototoxicity\ntoward Cancer Cells

In an effort to develop hypoxia-active iridium(III) complexes\nwith\nlong visible-light absorption, we synthesized and characterized five\nbis(terpyridine) Ir(III) complexes bearing oligothienyl substituents\non one of the terpyridine ligands, i.e., <i><b>n</b></i><b>T-Ir</b> (<i>n</i> = 0–4). The UV–vis\nabsorption, emission, and transient absorption spectroscopy were employed\nto characterize the singlet and triplet excited states of these complexes\nand to explore the effects of varied number of thienyl units on the\nphotophysical parameters of the complexes. <i>In vitro</i> photodynamic therapeutic activities of these complexes were assessed\nwith respect to three melanoma cell lines (SKMEL28, A375, and B16F10)\nand two breast cancer cell lines (MDA-MB-231 and MCF-7) under normoxia\n(∼18.5% oxygen tension) and hypoxia (1% oxygen tension) upon\nbroadband visible (400–700 nm), blue (453 nm), green (523 nm),\nand red (633 nm) light activation. It was revealed that the increased\nnumber of thienyl units bathochromically shifted the low-energy absorption\nbands to the green/orange spectral regions and the emission bands\nto the near-infrared (NIR) regions. The lowest triplet excited-state\nlifetimes and the singlet oxygen generation efficiency also increased\nfrom <b>0T</b> to <b>2T</b> substitution but decreased\nin <b>3T</b> and <b>4T</b> substitution. All complexes\nexhibited low dark cytotoxicity toward all cell lines, but <b>2T-Ir–4T-Ir</b> manifested high photocytotoxicity for all cell lines upon visible,\nblue, and green light activation under normoxia, with <b>2T-Ir</b> showing the strongest photocytotoxicity toward SKMEL28, MDA-MB-231,\nand MCF-7 cells, and <b>4T-Ir</b> being the most photocytotoxic\none for B16F10 and A375 cells. Singlet oxygen, superoxide anion radicals,\nand peroxynitrite anions were found to likely be involved in the photocytotoxicity\nexhibited by the complexes. <b>4T-Ir</b> also showed strong\nphotocytotoxicity upon red-light excitation toward all cell lines\nunder normoxia and retained its photocytotoxicity under hypoxia toward\nall cell lines upon visible, blue, and green light excitation. The\nhypoxic activity of <b>4T-Ir</b> along with its green to orange\nlight absorption, NIR emission, and low dark cytotoxicity suggest\nits potential as a photosensitizer for photodynamic therapy applications.