Synthesis, structures, photophysical properties, and theoretical study of four cationic iridium(III) complexes with electron-withdrawing groups on the neutral ligands

Four cationic iridium(III) complexes Ir1-Ir4 with the general form of [Ir((CN)-N-boolean AND)(2)((NN)-N-boolean AND)]PF6 (where (CN)-N-boolean AND = 2-(thiophen-2-yl)pyridine or 2-(benzo[b]thiophen-2-yl) pyridine, (NN)-N-boolean AND = 4,4'-dibromo-2,2'-bipyridine or 5,5'-dibromo-2,2'-bipyridine) have been synthesized and characterized. X-ray diffraction analysis of complexes Ir1-Ir4 indicate the coordination of the iridium atoms are distorted octahedral geometry. All complexes exhibit strong red phosphorescence (lambda(em) = 599-633 nm) with quantum efficiencies of 0.16-0.37 in oxygen-free dichloromethane solution at room temperature. The introduction of electron-withdrawing groups on neutral ligands was found to be useful in red-shifting the emission maxima of the complexes compared to that of the complexes with electron-donating substituents on neutral ligands. Electrochemical study shows a quasireversible, metal-centered oxidation with potentials at 0.76-0.90 V (vs. Ag+/Ag). Density functional theory (DFT) calculation shows that the LUMOs are mainly localized on the neutral ligands and the electron-withdrawing groups on neutral ligands can stabilize both the LUMO and HOMO though the former is affected to a much more appreciable degree than the latter.