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

Four cationic Ir(III) complexes in the form of [Ir(C^N)2(N^N)]PF6 (C^N = 2-(thiophen-2-yl)pyridine or 2-(benzo[b]thiophen-2-yl)pyridine, N^N = 4,4′-dibromo-2,2′-bipyridine or 5,5′-dibromo-2,2′-bipyridine) were synthesized based on electron-withdrawing groups (-Br) on neutral ligands. The introduction of electron-withdrawing groups on neutral ligands can turn the photoluminescence spectra of Ir(III) complexes towards the red region and stabilize both the LUMO and HOMO energy levels. [Display omitted] •Four cationic Ir(III) complexes with electron-withdrawing groups were synthesized.•The cationic Ir(III) complexes exhibit strong red phosphorescence.•The property tuning is dependent on the substituent nature of neutral ligands. Four cationic iridium(III) complexes Ir1–Ir4 with the general form of [Ir(C^N)2(N^N)]PF6 (where C^N = 2-(thiophen-2-yl)pyridine or 2-(benzo[b]thiophen-2-yl)pyridine, N^N = 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 (λem = 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.