Red emitting cyclometallated iridium(III) complexes: Synthesis, characterization and evaluation of biological activities

Abstract

A series of phosphorescent heteroleptic iridium(III) complexes containing two cyclometallating 6-chloro-2-(4-methoxyphenyl)-3-phenylquinoline (p-OMepq) ligands and one chromophoric ancillary ligand were synthesized and characterized: [(p-OMepq)2Ir(prz)] (1), [(p-OMepq)2Ir(biim)]Cl (2), [(p-OMepq)2Ir(bpy)]Cl (3) and [(p-OMepq)2Ir(dppe)]Cl (4) (where prz=pyrazine-2-carboxylic acid, biim=biimidazole, bpy=2,2′-bipyridine, dppe=1,2-bis(diphenylphosphino)ethane). The absorption, emission, lifetime measurements, cyclic voltammetry, molecular docking studies and thermal analysis of these complexes were systematically investigated. Binding of complexes 1–4 with calf thymus DNA was investigated by UV–Vis, fluorescence, circular dichroic spectroscopy and viscosity measurements. The intrinsic binding constants Kb of complexes 1–4 with CT-DNA, obtained from UV–Vis absorption studies, were 2.3×104, 4.7×104, 4.3×104 and 5.0×104M−1, respectively. The results indicated that the four complexes are able to bind to DNA with different binding affinities in the order 4>2>3>1. Complexes 1–4 exhibit a good binding propensity to bovine serum albumin (BSA) proteins, having relatively high binding constant values. A gel electrophoresis assay demonstrated that complexes 1–4 cannot promote the pBR322 plasmid DNA, plasmid pBluescript II KS+ DNA or pUC19 DNA cleavage in the presence of the reducing agent 3-mercaptopropionic acid. The cytotoxic activity of iridium(III) complexes 1–4 were tested with the human cervical cancer cell line (HeLa) and complex 4 was found to be more active compared to the other complexes.