Modification of luminescent iridium(III) polypyridine complexes with discrete polyethylene glycol (PEG) pendants: Synthesis, emissive behavior, intracellular uptake, and PEGylation properties

We report the synthesis, characterization, and photophysical properties of a new class of luminescent cyclometalated iridium(III) polypyridine poly(ethylene glycol) (PEG) complexes [Ir(N∧C) ₂(N∧N)](PF ₆) (HN∧C = Hppy (2-phenylpyridine), N∧N = bpy-CONH-PEG1 (bpy = 2,2′-bipyridine; la), bpy-CONH-PEG3 (1b); HN∧C = Hpq (2-phenylquinoline), N∧N = bpy-CONH-PEG1 (2a), bpyCONH-PEG3 (2b); HN∧C = Hpba (4-(2-pyridyl)benzaldehyde), N∧N = bpyCONH-PEG1 (3)) and their PEG-free counterparts (N∧N = bpy-CONH-Et, HN∧C = Hppy (1c); HN∧C = Hpq (2 c)). The cytotoxicity and cellular uptake of these complexes have been investigated by the MTT assay, ICPMS, laser-scanning confocal microscopy, and flow cytometry. The results showed that the complexes supported by the water-soluble PEG can act as biological probes and labels with considerably reduced cytotoxicity. Because the aldehyde groups of complex 3 are reactive toward primary amines, the complex has been utilized as the first luminescent PEGylation reagent. Bovine serum albumin (BSA) and poly(ethyleneimine) (PEI) have been PEGylated with this complex, and the resulting conjugates have been isolated, purified, and their photophysical properties studied. The DNA-binding and genedelivery properties of the luminescent PEI conjugate 3-PEI have also been investigated. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.