Cyclometalated Iridium(III) Bipyridyl-Phenylenediamine Complexes with Multicolor Phosphorescence: Synthesis, Electrochemistry, Photophysics, and Intracellular Nitric Oxide Sensing

We present a new class of phosphorescent cyclometalated iridium(III) bipyridyl-phenylenediamine complexes [Ir(N<^>C)2(bpy-DA)](PF6) (bpy-DA=4-(N-(2-amino-5-methoxyphenyl)aminomethyl)-4-methyl-2,2-bipyridine; HN<^>C=2-(2,4-difluorophenyl)pyridine (Hdfppy) (1a), 2-phenylpyridine (Hppy) (2a), 2-phenylquinoline (Hpq) (3a), 2-phenylcinchoninic acid methyl ester (Hpqe) (4a)) and their triazole counterparts [Ir(N<^>C)2(bpy-T)](PF6) (bpy-T=4-((6-methoxybenzotriazol-1-yl)methyl)-4-methyl-2,2-bipyridine; HN<^>C=Hdfppy (1b), Hppy (2b), Hpq (3b), Hpqe (4b)). Upon photoexcitation, the diamine complexes exhibited fairly weak green to red phosphorescence under ambient conditions whereas the triazole derivatives emitted strongly. The photophysical properties of complexes 2a and 2b have been studied in more detail. Upon protonation, the diamine complex 2a displayed increased emission intensity, but the emission properties of its triazole counterpart complex 2b were independent on the pH value of the solution. Also, complex 2a was found to be readily converted into complex 2b upon reaction with NO under aerated conditions, resulting in substantial emission enhancement of the solution. The reaction was highly specific toward NO over other reactive oxygen and nitrogen species (RONS) as revealed by spectroscopic analyses. The lipophilicity and cellular uptake efficiency of the diamine complexes have been examined and correlated to their molecular structures. Also, cell-based assays showed that these complexes were noncytotoxic toward human cervix epithelioid carcinoma (HeLa) cells (at 10M, 4h, percentage survival approximate to 80-95%). Additionally, the diamine complexes have been used to visualize intracellular NO generated both exogenously in HeLa cells and endogenously in RAW264.7 murine macrophages by laser-scanning confocal microscopy.