Cyclometalated Iridium(III) Polyamine Complexes with Multicolor Phosphorescence

We have synthesized and characterized a new class of luminescent cyclometalated iridium(III) polyamine complexes [{Ir(N^C)2}n(bPEI)](PF6)n (bPEI = branched poly(ethyleneimine), average Mw » 25 kDa) and [Ir(N^C)2(en)](PF6) (en = ethylenediamine). The electrochemical and photophysical properties of these complexes can be readily tuned using different cyclometalating ligands. We also found that the polyamine and cyclometalating ligands both played a role in the lipophilicity and cellular uptake properties of the complexes. Upon internalization, the bPEI complexes were localized in the lysosomal compartments, whereas the en complexes were diffusely distributed in the cytoplasmic region and were mainly localized in the mitochondria. Additionally, the cytotoxicity of the complexes toward HeLa and HEK293T cell lines has been evaluated by the MTT assay. To improve its biocompatibility and transfection efficiency, one of the bPEI complexes has been grafted with poly(ethylene glycol) (PEG, average Mw » 2 kDa) to different extents, yielding the copolymers PEG12.3-g-5a, PEG25.4-g-5a, and PEG62.1-g-5a. These copolymers not only retained the interesting photophysical properties of the complex, but also exhibited enhanced in vitro gene delivery capability as revealed by in vitro transfection experiments using tissue culture-based luciferase assays.