Four cyclometalated Ir(iii) complexes and insights into their luminescence, cytotoxicity and DNA/BSA binding performance

Abstract

Four cyclometalated Ir(III) complexes based on 4′-p-N,N-bis(2-hydroxyethyl)benzyl-2,2′:6′,2′′-terpyridine (TPYOH) and 4′-p-N,N-bis(2-hydroxyethyl)benzyl-6′-benzyl-2,2′-bipyridine (PhbpyOH) were synthesized and characterized. All the Ir(III) complexes exhibited strong MLCT absorption peaks at about 450 nm, broad emission bands in the range of 500–700 nm. Z-scan results revealed that only complex Ir1A could exhibit certain two-photon absorption with maximal cross section values of 215 GM at 890 nm. When excited by 700–850 nm femtosecond laser, complex Ir1A gave a TPEF peak around 567 nm. All four complexes exhibited enhanced cell growth inhibitory activity against MCF-7 tumour cells under light irradiation comparing to their dark toxicity, with Ir1B showing the highest PI value (>50). The pathways and efficiencies of ROS generation by Ir(III) complexes varied, with Ir2A being more effective in producing ¹O₂ while Ir1A mainly generating O₂˙⁻. The Ir(III) complexes undergo hydrogen bonding with DNA bases/phosphodiester through two O–H bonds on the bis(hydroxyethyl)amino group. The free pyridine-N atom in Ir1A forms additional hydrogen bond with DNA base, while the ligand TPYOH in Ir2A has better molecular planarity due to adopting {N, N, N} coordination mode, thus these two complexes show better DNA affinity. The complexes demonstrated weak interactions with BSA, through hydrogen bonding with amino acid residues at different regions of BSA molecule.