Synthesis, physicochemical properties, thermal analysis and biological application of phosphorescent cationic iridium(III) complexes

Abstract

Cyclometalating iridium(III) complexes with formula, [Ir(cpy)2(biim)Cl] 1, [Ir(cpy)2(bpy)Cl] 2 and [Ir(cpy)2(dppe)Cl] 3 (where, cpy=9-(pyridine-2-yl)-9H-carbazole, biim=biimidazole, bpy=2,2′-bipyridine, dppe=1,2-bis(diphenylphosphino)ethane) were synthesized and characterized by elemental analysis, 1H, 13C NMR, and ESI-Mass spectral studies. The complex 1–3 show their emissions in green emitting region (λPL,max=536–580nm), and the emission maxima can be tuned by changing the ancillary ligand. The phosphorescent line shape indicates that the emissions originate predominantly from 3MLCT states with little admixture of ligand-based 3(π-π∗) excited states. These complexes exhibited biexponential phosphorescence decays with relatively short lifetimes of 0.40–12.35µs in solution state at room temperature and the relaxation dynamics of 1–3 are O2 dependent. The electrochemical and thermal properties were systematically evaluated. Binding of these complexes (1–3) with calf thymus DNA was investigated by UV–Vis, fluorescence, circular dichroic spectroscopy, viscosity measurements and computer aided molecular docking studies. The binding constant of complexes (1–3) with ct-DNA obtained by UV–Vis absorption studies were 2.28×104, 1.98×104 and 2.98×104M−1, respectively. The result indicated that the complexes (1–3) were able to bind to DNA with different binding affinity in the order: 3>1>2. Complexes (1–3) also exhibit a good binding propensity to bovine serum albumin (BSA). Gel electrophoresis assay demonstrated that complexes (1–3) cannot promote the cleavage ability of the pBR322 plasmid DNA or plasmid pBluescript II KS+ DNA or pUC19 DNA in the presence of the reducing agent 3-mercaptopropionic acid. The cytotoxicity studies of complexes 1–3 were tested in vitro on in human cervical cancer cell line (HeLa) and they found to be active. We propose that the phosphorescent iridium(III) complexes (1–3) are possible candidates for the green phosphors in organic light emitting diodes (OLEDs) applications as well as better anticancer agents.