New 2-phenyl-5-nitropyridyl containing iridium(III) cyclometalated complexes: syntheses, structures, electrochemistry and photophysical properties

A series of new 2-phenyl-5-nitropyridyl containing iridium(III) cyclometalated complexes ([Ir(5-NO2-ppy)2Cl]2 (1, 5-NO2-ppy = 2-phenyl-5-nitropyridyl), Ir(5-NO2-ppy)2(PPh3)Cl (2, PPh3 = triphenylphosphine), Ir(5-NO2-ppy)2(pic) (3, pic = picolinic acid)) have been synthesized and unambiguously characterized. The crystal structures of 2 and 3 have been determined by X-ray diffraction analyses. The HOMO–LUMO energy gaps of 1–3 estimated by the cyclic voltammetry (CV) show values in the order of 2 > 3 > 1. The UV–vis absorption and the solid photoluminescence (PL) spectra of 1–3 have also been measured, exhibiting the changing orders being consistent with that of the HOMO–LUMO energy gaps. Moreover, the UV–vis absorption spectra of 1–3 show obvious red-shifts compared with those of the corresponding 2-phenylpyridyl (ppy) containing Ir(III) complexes ([Ir(ppy)2Cl]2, Ir(ppy)2(PPh3)Cl and Ir(ppy)2(pic)) in which no strongly electron-withdrawing nitro groups are introduced to the pyridyl rings of the ppy ligands. Thus, the HOMO–LUMO energy gaps of 1–3 should be narrower than those of [Ir(ppy)2Cl]2, Ir(ppy)2(PPh3)Cl and Ir(ppy)2(pic).