Alkyl chain-modified cyclometalated iridium complexes as tunable anticancer and imaging agents

Five mononuclear cyclometalated iridium complexes [1](PF₆)-[5](PF₆) were prepared using imidazole-based ligands of varying alkyl chain length. The complexes were characterised by various analytical techniques. The single crystal X-ray structures of [2](PF₆), [3](PF₆) and [4](PF₆) revealed the expected distorted O_h structures around the metal centre; however, the chain length was found to play a crucial role in deciding the overall geometry. Theoretical investigations demonstrated that the HOMOs were mainly contributed by iridium and cyclometalated ligands, whereas the LUMOs were constituted from bpy/phen units. The complexes were found to be luminescent with a moderate emission quantum yield and lifetime in CH₃CN. The in vitro growth inhibition assay of the complexes with a shorter alkyl chain ([4]⁺ and [5]⁺) displayed higher anticancer activity (IC₅₀ < 0.5 μM) compared to the complexes with a longer alkyl chain ([1]⁺-[3]⁺) (IC₅₀ < 30 μM) against human breast cancer (MCF-7) cells. The complexes [4]⁺ and [5]⁺ also displayed moderate cancer cell selectivity (∼3 times) over normal breast (MCF-10) cells. The flow cytometry assay and fluorescence microscopy analysis suggested that cellular accumulation was primarily responsible for the variation in anticancer activity. Interestingly, without possessing any anticancer activity or toxicity ((IC₅₀ > 50 μM), the complex [1]⁺ mainly accumulated near the cell membrane outside the cell and displayed a clear image of the cell membrane. The light microscopy images and western blot analysis reveal that complex [4]⁺ induced combined apoptosis and paraptosis. Thus, tuning the anticancer activity and cellular imaging property mediated by the alkyl chain would be of great importance and would be useful in anticancer research.