Luminescent iridium(III) polypyridine PEG complexes: Synthesis, photophysical, and biological properties

We report the synthesis, characterization, and photophysical properties of a class of luminescent cyclometalated iridium(III) polypyridine poly(ethylene glycol) (PEG) complexes [Ir(N⁁C)2(bpy-CONH-PEGn)](PF6) (HN⁁C = Hppy (1a), HN⁁C = Hpq (2a), HN⁁C = Hpba (3a)). The discrete PEG complexes [Ir(N⁁C)2(bpy-CONH-dPEG24)](PF6) (HN⁁C = Hppy (1b), HN⁁C = Hpq (2b), HN⁁C = Hpba (3b)) and the PEG-free complexes [Ir(N⁁C)2(bpy-CONH-Et)](PF6) (HN⁁C = Hppy (1c), HN⁁C = Hpq (2c), HN⁁C = Hpba (3c)) have also been prepared for comparison studies. The lipophilicity of the complexes has been determined by shake-flask method. We have also investigated the cytotoxicity and cellular uptake of these complexes by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay and laser-scanning confocal microscopy, respectively. The results illustrated that the nondiscrete PEG complexes can act as biological imaging reagents with low cytotoxicity. Co-localization experiments using an endocytic marker Alexa Fluor 633-labeled transferrin revealed the involvement of endosomes in the uptake of the complexes by the human cervix epithelioid carcinoma (HeLa) cells. Since the aldehyde groups of the pba complexes 3a-c are reactive toward primary amines, they have been conjugated to bovine serum albumin (BSA) and the resulting conjugates have been isolated, purified, and its photophysical properties studied. Complexes 3a and 3b serve as new luminescent PEGylation reagents.