Ancillary Ligands In Complexes Research Articles

Controlling the Excited State and Photosensitizing Property of a 2-(2-Pyridyl)benzo[b]thiophene-Based Cationic Iridium Complex through Simple Chemical Modification

Bis-cyclometalated cationic iridium (Ir) complexes 1-4 comprising two 2-(2-pyridyl)benzo[b]thiophene (btp) ligands and one 2,2'-bipyridyl (bpy) ancillary ligand with different substituents were prepared as new visible light-absorbing sensitizers and examined for their photophysical and electrochemical properties. Complex 1 was prepared as a parent complex without any substituents. Complexes 2-4 contained methyl-, methoxy-, and trifluoromethyl groups at 4,4'-positions on the bpy ancillary ligand. Systematic investigation of these complexes revealed that such a simple chemical modification selectively controls the excited-state lifetime, while the absorption and emission spectral features remain unchanged. Specifically, the phosphorescence lifetimes of complexes 2 and 3 with electron-donating groups (τ = 3.50 μs, 3.90 μs) were found to be much longer than that of complex 1 (τ = 0.273 μs), and complex 4, possessing strong electron-withdrawing trifluoromethyl groups, did not exhibit detectable phosphorescence at room temperature. The large differences in excited-state lifetimes of complexes 1-3, as well as the nonemissive character of complex 4, are attributed to a strong influence of the substituents on the ligand field strength. The increased σ-donating ability of the ancillary ligand in complexes 2 and 3 destabilizes a short-lived, nonemissive triplet metal-centered ((3)MC) state and increases the energy separation between the (3)MC state and emissive triplet ligand-centered ((3)LC) state based on the btp ligand. For complex 4, however, the (3)MC state is close in energy to the (3)LC state because of the decreased σ-donating ability of the ancillary ligand. Additional evidence of the (3)MC state associated with the changeable excited state was also provided via low-temperature phosphorescence measurements and density functional theory calculations. Ir complexes 1-4 were tested as sensitizers in photoinduced electron-transfer reaction of triethanolamine and methylviologen chloride (MVCl2). As a result, complexes 2 and 3 exhibited much better photosensitizing property compared to complex 1 since their long-lived excited states promoted an oxidative quenching pathway. This Study has first demonstrated that simple substitution on the diimine ancillary ligand can control the (3)MC state of the bis-cyclometalated cationic Ir complex to finely tune the excited-state lifetime and photosensitizing property.

DNA-binding and cytotoxicity studies of ruthenium(II) mixed-ligand complexes containing two intercalative ligands

New Ru(II) polypyridyl complexes containing two intercalative ligands, [Ru(dppz)2phen]2+ (1) (dppz=dipyrido-[3,2-a:2′,3′-c]phenazine, phen=1,10-phenanthroline) and [Ru(dppz)2dpq]2+ (2) (dpq=di-pyrido[3,2-f:2′,3′-h]quinoxaline) have been synthesized and characterized. The binding of both complexes with calf thymus DNA (CT-DNA) has been investigated by spectroscopic and viscosity measurement. Results indicate that complexes 1 and 2 bind to DNA through intercalation, and the DNA binding affinity of complex 2 is much stronger than that of complex 1. However, the equilibrium binding constants of both complexes with DNA are smaller than that of [Ru(phen)2(dppz)]2+ and [Ru(bpy)2(dppz)]2+ (bpy=2,2´-bipyridine) with one intercalative ligand, which may be interpreted by the steric hindrance effect of the two intercalated ligand dppz. On the other hand, comparing complex 1 with complex 2, the former can serve as a molecular “light switch” for DNA, and this difference may be due to the different ancillary ligands in complexes 1 and 2. In addition, both complexes demonstrate different antitumor activities against selected tumor cell lines in vitro, and can make the cells apoptosis.