Iridium(III) complexes based on 5-nitro-2-(2′,4′-difluorophenyl)pyridyl: Syntheses, structures and photoluminescence properties

A series of cyclometalated Ir(III) complexes [Ir(dFNppy)₂(PPh₃)L] (PPh₃ = triphenylphosphine, L = Cl^-, 1; NCS^-, 2; NCO^-, 3; N₃^-, 4) in which 5-nitro-2-(2′,4′-difluorophenyl)pyridyl (dFNppy) is used as the main ligand are synthesized and characterized. The crystal structures of 3 and 4 are determined by X-ray diffraction analyses. Photoluminescence spectra of 1-4 in CH₂Cl₂ solutions at room temperature show the maximum emission wavelengths (λ_max) in the range from 586 to 627 nm, corresponding to red-orange luminescence. HOMO-LUMO energy levels of 1-4 are estimated by cyclic voltammetry measurements. Compared with corresponding 2-phenylpyridyl (ppy)-containing Ir(III) complexes having same ancillary ligands, 1-4 have obviously smaller HOMO-LUMO energy gaps (E_g). Simultaneously, 1-4 show relatively bigger E_g compared with 2-phenyl-5-nitropyridyl (5-NO₂-ppy)-containing Ir(III) complexes possessing same ancillary ligands. Methods of E_g adjustment are also discussed.