Water-soluble phosphorescent iridium(iii) complexes as multicolor probes for imaging of homocysteine and cysteine in living cells

With the emergence of phosphorescent heavy-metal complexes as a class of attractive probes for bioimaging, there is a parallel need to develop new phosphorescent probes with complete solubility in pure water for phosphorescent staining in living cells. Herein, a convenient and general design strategy for realizing phosphorescent heavy-metal complexes with complete water-solubility is provided and a series of cationic iridium(III) complexes [Ir(C^N)2(N^N)]+PF6− (C^N = 2-(2,4-difluorophenyl)pyridine (dfppy), 2-(4-(tert-butyl)phenyl)pyridine(t-buppy), 2-(thiophen-2-yl)quinoline) (thq), 4-(pyridin-2-yl)benzaldehyde (pba)) are prepared. The water-solubility of the complexes was successfully realized through the quaternization of the tertiary amino group in the N^N ligand. By changing the C^N ligands, the luminescent emission colors of these complexes can be tuned from green to red. These cationic iridium(III) complexes are membrane-permeable and can be applied as phosphorescent dyes for cell imaging in phosphate buffer solution (PBS). Complexes Ir1–Ir3 displayed specific staining of the cytoplasm and complex Ir4 containing two aldehyde groups could detect the changes of cysteine/homocysteine concentration in living cells. These results demonstrated that our design strategy offers an effective way to develop excellent phosphorescent cellular probes for real applications.